Insights into the structure and function of H_V1 from a meta-analysis of mutation studies

Abstract

The voltage-gated proton channel (H_V1) is a widely distributed, proton-specific ion channel with unique properties. Since 2006, when genes for H_V1 were identified, a vast array of mutations have been generated and characterized. Accessing this potentially useful resource is hindered, however, by the sheer number of mutations and interspecies differences in amino acid numbering. This review organizes all existing information in a logical manner to allow swift identification of studies that have characterized any particular mutation. Although much can be gained from this meta-analysis, important questions about the inner workings of H_V1 await future revelation.

Introduction

Voltage-gated proton channels are found in highly diverse species, from unicellular marine creatures such as dinoflagellates, diatoms, and coccolithophores (Smith et al., 2011; Taylor et al., 2011, 2012) to insects (Chaves et al., 2016), snails (Thomas and Meech, 1982; Byerly et al., 1984; Doroshenko et al., 1986), and human beings, where they are found in a variety of cells and perform many disparate functions (DeCoursey, 2013). Their unique properties (perfect H⁺ selectivity, ΔpH-dependent gating, extreme temperature dependence, and the ability to shift into a strikingly enhanced gating mode) are paralleled by a unique structure, the reconciliation of which is a goal of this review. Most voltage-gated ion channels are tetramers or quasi-tetramers of monomers comprising a voltage-sensing domain (VSD) S1­–S4 (transmembrane [TM] segments 1–4) and a pore domain S5–S6, four of which combine to produce a single central conduction pathway. In contrast, H_V1 consists of S1–S4 alone, a VSD without an explicit pore domain (Ramsey et al., 2006; Sasaki et al., 2006). In mammals and many other species (Koch et al., 2008; Lee et al., 2008; Tombola et al., 2008; Smith and DeCoursey, 2013), H_V1 forms a dimer in cell membranes. However, each monomer, or protomer, has its own pore and other necessary parts and can function as a monomer (Koch et al., 2008; Tombola et al., 2008). The properties of monomeric constructs are similar in most respects to those of the dimer, but monomeric constructs open faster (Koch et al., 2008; Tombola et al., 2008; Musset et al., 2010b,c; Fujiwara et al., 2012). Several lines of evidence indicate that the two protomers comprising the H_V1 dimer do not function independently, but gate cooperatively in the sense that each must undergo a voltage-dependent conformational change before either can conduct current (Gonzalez et al., 2010; Musset et al., 2010b; Tombola et al., 2010; Smith and DeCoursey, 2013).

Fig. 1 illustrates schematically the entire 273–amino acid sequence of hH_V1. The signature sequence that has been used successfully to identify H_V1 in new species is RxWRxxR in the S4 helix (Smith et al., 2011; Rodriguez et al., 2015; Chaves et al., 2016). This sequence also identifies c15orf27 proteins, of unknown function, but these all lack Asp¹¹² in S1, which is required for proton selectivity (Table 1). Another reported conserved motif in S2, [F,Y,W]xx[E,D]xxx[R,K], identifies some H_V1 channels but is not specific to H_V1, and instead identifies VSDs in general (Kang and Baker, 2016). This motif is not present in all H_V1; for example, it is not present in several unicellular marine species (Taylor et al., 2011), including kH_V1, which was identified by using the S4 motif (Smith et al., 2011).

Figure 1.

The amino acid sequence and schematic topology of the human voltage-gated proton channel, hH_V1. Within the TM domain, acidic residues are red, basic residues are blue, aromatic residues are orange, and polar residues are gray. Specific amino acids of note, beginning at the N terminus: deletion of 1–20 (green) produces a “short” isoform common in malignant B cells (Hondares et al., 2014); Thr²⁹ is a PKC phosphorylation site responsible for enhanced gating (Musset et al., 2010a); M91T is the first identified hH_V1 mutation (Iovannisci et al., 2010); Asp¹¹² is crucial to H⁺ selectivity (Musset et al., 2011); His¹⁴⁰ and His¹⁹³ coordinate Zn²⁺ binding (Ramsey et al., 2006); the three Arg in S4 are thought to open the conductance pathway in response to voltage (Ramsey et al., 2006; Sasaki et al., 2006; Gonzalez et al., 2013); and the C terminus has an extensive coiled-coil region (black) that holds the dimer together (Koch et al., 2008; Lee et al., 2008; Tombola et al., 2008; Fujiwara et al., 2014). The image was drawn with TOPO2 (Johns, 2016).

Table 1.

Numerical key to H_V1 in species verified by heterologous expression

Species are indicated by one- or two-letter abbreviations: h, human; m, mouse; Ci, Ciona intestinalis; Np, Nicoletia phytophila; Sp, Strongylocentrotus purpuratus; k, Karlodinium veneficum; Pt, Phaeodactylum tricornutum; Cb, Coccolithus braarudii; Eh, Emiliania huxleyi. HG, hydrophobic gasket residues (gray); Sel, selectivity filter (yellow). Zn²⁺, one of the two Zn²⁺-binding His (in some species). Green-shaded residues sense voltage. Okamura et al. (2015) propose a slightly different hydrophobic plug based on the mH_V1 crystal structure: F146, M147, L150, and F178. Several alignments of the S4 helix have been produced, which result in shifts in the register of the basic residues. Kv1.2 is human, NCBI Reference Sequence accession no. NP_004965.1; Shaker is UniProt/Swiss-Prot accession no. P08510.3.

Many reviews describe in detail the biological functions proposed for H_V1 (Eder and DeCoursey, 2001; DeCoursey, 2010, 2012, 2013; Capasso et al., 2011; Demaurex, 2012; Fischer, 2012; Lishko et al., 2012; Taylor et al., 2012; Smith and DeCoursey, 2013; DeCoursey and Hosler, 2014; Seredenina et al., 2015). Here we will summarize a few aspects of functions that are relevant to the analysis that follows. The main function of H_V1 in most cells is acid extrusion, although the specific consequences in each cell vary drastically. For example, H_V1-mediated acid extrusion triggers capacitation in human sperm (Lishko et al., 2010), enables histamine release by basophils (Musset et al., 2008b), and exacerbates several cancers (Wang et al., 2012, 2013; Hondares et al., 2014). Acid extrusion requires extreme proton selectivity because the concentration of H⁺ in biological solutions is a million-fold lower than that of other major ions. H_V1 are brilliantly designed and extremely efficient acid extrusion devices, changing pH_i at least an order of magnitude faster than other H⁺ exporters (DeCoursey and Cherny, 1994), mainly because of their unique ΔpH-dependent gating mechanism (Cherny et al., 1995), which is discussed in detail below in the section Table entries defined. A second type of function of H_V1 in many cells reflects the electrical consequences of the charge movement that occurs during H⁺ extrusion. For example, in phagocytes and certain other cells, H⁺ efflux serves to compensate electrically for the electron extrusion that occurs as a direct consequence of the electrogenic activity of nicotinamide adenine dinucleotide phosphate (NADPH; reduced form) oxidase and related NOX (cytochrome subunit of NADPH oxidase) isoforms (Henderson et al., 1987, 1988; DeCoursey et al., 2003). In dinoflagellates, H_V1 are thought to mediate the action potential that triggers the flash in bioluminescent species (Smith et al., 2011; Taylor et al., 2012).

The respiratory burst of phagocytes, which can be elicited by pathogenic microbes, chemotactic peptides, or the phorbol ester PMA, is the manifestation of NADPH oxidase activation. During the respiratory burst, H_V1 properties undergo a drastic transformation (Bánfi et al., 1999; DeCoursey et al., 2000; Murphy and DeCoursey, 2006; DeCoursey, 2016), resulting in a much higher level of activity in what has been called the enhanced gating mode (DeCoursey, 2003b). The bulk of evidence indicates that enhanced gating results from phosphorylation of H_V1 (Morgan et al., 2007, Musset et al., 2010a; DeCoursey, 2016); mutations to putative phosphorylation sites are listed in Table 2.

Table 2. Changes in H_V1 properties in N-terminal (1–100) mutants versus WT channels .

Mutant	Species	Expr. system	I?	τ act	τ tail	ΔV thr	ΔpH slope	Selectivity	Other	Reference
	hHV1S	LK35.2 wc	Yes	2.5	0.75	5.4	43.2	H+	More profound enhanced gating	Hondares et al., 2014
T9A	hHV1S	LK35.2 pp	Yes	0.28	0.9	−18.2			Enhanced gating lost	Hondares et al., 2014
S77A	hHV1S	LK35.2 pp	Yes	1.75	1.1	−3.6			nc	Hondares et al., 2014
T9A/S77A	hHV1S	LK35.2 pp	Yes	0.63	0.8	−5.8			Enhanced gating lost	Hondares et al., 2014
T29A	hHV1	LK35.2 pp	Yes	0.23	1.8	−27.6			Enhanced gating lost	Musset et al., 2010a
T29D	hHV1	LK35.2 pp	Yes	0.41	1.0	−12.5			Enhanced gating lost	Musset et al., 2010a
M91T	hHV1	COS wc	Yes			20	47		First identified naturally occurring hHV1 mutation	Iovannisci et al., 2010
S97A	hHV1	LK35.2 pp	Yes	0.96	1.2	3.9			nc	Musset et al., 2010a
S97D	hHV1	LK35.2 pp	Yes	0.34	0.9	−3.0			nc	Musset et al., 2010a
T29A/S97A	hHV1	LK35.2 pp	Yes	0.25	1.7	−19.4			Enhanced gating lost	Musset et al., 2010a
S98A	hHV1	HEK wc	Yes			−7				Ramsey et al., 2010
H99A	hHV1	HEK wc	Yes			13				Ramsey et al., 2010
R100A	hHV1	HEK wc	Yes			−7				Ramsey et al., 2010
ΔN (1–96 deleted)	hHV1	HEK wc	Yes			15	40			Ramsey et al., 2010
ΔN/ΔC	mHV1 (1–77 deleted, V216stop)	HEK wc	Yes	0.20		nc	nc		Loss of dimer formation	Koch et al., 2008
ΔN/ΔC	mHV1 (1–77 deleted, V216stop)	HEK i-o	Yes						Weaker Zn2+ effects	Musset et al., 2010b

That numerical entries are shown does not imply that any given change was significant. The entries for hH_V1_S are in a short isoform and are compared with full-length hH_V1. HEK, HEK-293, HEK-293T, tsA, or HM1; COS, COS-7; pp, perforated patch; wc, whole cell; i-o, inside-out patch configuration. Blank entries indicate that the parameter was not examined. nc, measured, but no change. Parameters are given relative to WT in each study. For I?, yes means currents are detectable. Time constants are ratios of mutant/WT. The ΔV_threshold value is the change in absolute position of the g_H–V relationship versus WT. The ΔpH slope is the slope in millivolts of the relationship between V_threshold (or other parameters reflecting the absolute position of the g_H–V relationship) and V_rev or E_H (which are not identical; see section Table entries defined). When C-terminal truncations (ΔC) are indicated as XNNNstop, this means STOP replaces X at position NNN; hence, position NNN and all subsequent residues are truncated, and the last position remaining is NNN-1. The mouse N-terminal deletions (ΔN) were done by replacing P78M to initiate translation at that position.

Table 1 presents the position numbers of several key amino acids in H_V1 from the nine species in which the channel has been verified by electrophysiological studies in heterologous expression systems, and the corresponding positions in two closely related molecules (c15orf27 and CiVSP) as well as in two exhaustively studied K⁺ channels. H_V1 contains two highly conserved Asp that other voltage-gated ion channels lack, Asp¹¹² and Asp¹⁸⁵, as well as an anomalously located Trp²⁰⁷. However, H_V1 lack the equivalent of Glu²⁸³ of Shaker and Na⁺ channels, having Ser¹⁴³ instead.

Fig. 2 shows where several key amino acids are located in the closed crystal structure of a mouse H_V1 (mH_V1) chimera (Fig. 2 A; Takeshita et al., 2014) and an open state model of hH_V1 (Fig. 2 B; Kulleperuma et al., 2013). Numbering for both corresponds to hH_V1, although the closed structure is of mH_V1.

Figure 2.

Location of key amino acids. Location of some key amino acids in the crystal structure of the mH_V1 chimera (Takeshita et al., 2014), labeled with hH_V1 numbers (A), and in an open state model of hH_V1, R2D (B; Kulleperuma et al., 2013). The channels are viewed from the side with the extracellular end at the top. An EPR study of hH_V1 generally agreed with the structure of the mH_V1 chimera, except in the EPR study the S2 helix (with F150 and E153) was one turn of the helix lower and S3 (with D174 and D185) was one turn higher relative to S1 and S4 (DeCoursey, 2015a; Li et al., 2015). E153 is the first amino acid replaced by the spliced-in CiVSP segment, and is actually D in the crystal. The images were produced with the PyMOL Molecular Graphics System (version 1.8; Schrödinger, LLC).

We hope that this assembly of information will in itself allow some general conclusions about structure–function relationships. Most existing data appear reasonably consistent, but in some instances, there appear to be species differences. It is unclear whether these are real or simply examples of laboratory to laboratory variation; such observations are inevitably somewhat anecdotal. We also identify examples of qualitatively different outcomes for the same mutation studied in different expression systems. In addition to listing the outcomes of mutations, the motivation for several strategies for generating mutants is discussed. The mutation studies have resulted in rapid progress in understanding how H_V1 works, but many important questions remain. One word of caution must be stated: mutations are designed to test the effect of changing one or more amino acids, usually with the assumption that the rest of the protein will assemble and function exactly as the WT does. This assumption can be tested rigorously only by determining the structure of every mutant. Although this procedure became almost routine for the bacterial reaction center (Xu et al., 2004), it is not remotely possible for H_V1. Nevertheless, many mutants function with little overt change beyond what might be imagined. On the other hand, mutations that alter charge may have powerful effects on structure and hence function at least locally if not globally.

Data organization and exclusions

The information in this review is organized according to the amino acid numbering of the human voltage-gated proton channel, hH_V1. To make the tables more manageable, we present the mutants in numerical order (with a few exceptions whose logic may or may not become apparent) and include separate tables for the N terminus, for each TM helix (S1–S4), and for the C terminus. The boundaries of the TM helices are defined according to the electron paramagnetic resonance (EPR) study of Li et al. (2015). Double or triple mutants are listed according to the first (i.e., nearest the N terminus) position mutated (again with a few exceptions whose rationale may become apparent). The intention is to make these tables exhaustive as far as is practical, but our bias is toward electrophysiological descriptions of individual point mutants. Thus, we do not list all 109 mutants studied by Cys scanning and assessed for accessibility by PEGylation protection (Sakata et al., 2010; Kurokawa and Okamura, 2014), for example. Nor do we list all 149 positions at which Cys was introduced for EPR measurements (Li et al., 2015). These blanket mutations are interpretable mainly within the context of the entire study. We exclude most mutations examining the link between S4 and the C terminus, which involve a large variety of deletions and insertions (Fujiwara et al., 2012, 2014). Mutations to the coiled-coil region of the C terminus resulting in trimeric and tetrameric channels (Fujiwara et al., 2013a) are not discussed here. Only a fraction of a large series of Trp scanning mutants in both monomeric and dimeric constructs is listed (Okuda et al., 2016). A series of mutants and tandem constructs in which one or both of the His that bind Zn²⁺ (His¹⁴⁰ and His¹⁹³) were replaced (Musset et al., 2010b) is not included in the tables. A series of Cys cross-linking mutations aimed at identifying the dimer interface (Lee et al., 2008) is also omitted. Finally, we do not include domain-swap mutants, such as those of Alabi et al. (2007), in part because chimerae do not logically fit into the format of the tables.

Table entries defined

The first column lists the mutations as per the usual convention (single-letter amino acid abbreviations: WT, position counting from the N terminus, and replacement). When a study used a nonhuman species, the hH_V1 equivalent is given in italics in the first column, and the actual mutation is listed in the second column. The third column gives the expression system and the voltage-clamp method used. To our knowledge, no studies exist in which different properties were observed in HEK versus COS cells (Musset et al., 2008a). However, mammalian versus amphibian studies sometimes differ. For example, D112S from three different species all expressed well and exhibited anion permeation in mammalian cells (Musset et al., 2011; Smith et al., 2011; Chaves et al., 2016), whereas currents were not observed in Xenopus laevis oocytes (Berger and Isacoff, 2011). Some proteins function better in certain expression systems: CiH_V1 works well in Xenopus oocytes, whereas mH_V1 does not and prefers mammalian (HEK) cells (Okuda et al., 2016). Among mammalian cells, hH_V1 expressed in the B cell–related LK35.2 cell line exhibits an enhanced gating response to stimulation with PMA (Musset et al., 2010a; Hondares et al., 2014), whereas hH_V1 expressed in HEK or COS cells did not respond (Musset et al., 2008a).

The fourth column (I?) simply reports whether interpretable currents were observed. A positive answer means the mutant protein is produced, reaches the plasma membrane, and functions. A negative result may have various undetermined explanations (protein misfolding, failure to traffic to the plasma membrane, disruption of gating or permeation) but is nevertheless potentially important because amino acids that play crucial roles in function may be difficult to replace without disrupting molecular function. For example, Asp¹¹² and Arg²⁰⁸ form a crucial nexus that in general cannot be meddled with without altering or eliminating function. However, different laboratories may have different criteria for deciding whether small currents are “real,” small enough to be negligible, or nonexistent. This evaluation is complicated by the fact that all common mammalian expression systems (HEK, Chinese hamster ovary [CHO], and COS cells) frequently display (typically small) native voltage-gated proton currents (Cherny et al., 1997; Musset et al., 2011). To address this concern, we often introduce mutations into hH_V1 in a Zn²⁺-insensitive background, meaning the two His primarily responsible for Zn²⁺ inhibition are mutated (H140A/H193A; Ramsey et al., 2006; Musset et al., 2010b). Then, if we see small currents that might be due either to native currents or to expression of a poorly conducting mutant, we add 10 µM Zn²⁺, which profoundly inhibits WT hH_V1, but will have negligible effects on a Zn²⁺-insensitive mutant. This approach cannot be applied to H_V1 from species lacking these His (Table 1).

The gating kinetics columns are self-evident: τ_act is the activation (channel opening) time constant, and τ_tail is the tail current or deactivation (channel closing) time constant. These are expressed as the ratio τ_mutant/τ_WT so that 1 means no change, a ratio <1 means faster than WT, and a ratio >1 means slower than WT. Because H_V1 gating kinetics depends very strongly on temperature, with Q₁₀ 6–9 (DeCoursey and Cherny, 1998; Kuno et al., 2009), and is also influenced profoundly by experimental artifacts including proton depletion-induced current decay (“droop”) and pH changes, anything less than a twofold change should be viewed with skepticism.

The columns labeled ΔV_threshold (ΔV_thr) and ΔpH slope embody one of the crucial and unique properties of this channel, namely its ΔpH-dependent gating. Decreasing pH_i or increasing pH_o shifts the g_H–V relationship negatively by roughly 40 mV/U of change in pH, as originally described by Cherny et al. (1995):

$$V_{\mathrm{threshold}}=\mathrm{ 2}0-\mathrm{4}0\mathrm{\Delta pH}$$ (1)

or, generalized:

$$V_{\mathrm{threshold}}=V_0–V_{\mathrm{slope}}\mathrm{\Delta pH,}$$ (2)

where V_threshold is the most negative voltage at which detectable current can be elicited, V₀ is V_threshold at symmetrical pH (roughly 20 mV), V_slope is the steepness (in millivolts/unit pH) of the relationship (nominally 40 mV), and ΔpH = pH_o − pH_i. The precise value for V_slope depends on whether the abscissae are E_H (the Nernst potential for H⁺ based on the nominal pH of the solutions) or the measured V_rev. Because V_rev often changes by less than a Nernstian amount in real experiments (almost certainly as a result of our inability to perfectly control pH, in combination with pH changes due to the measurement itself), plotting V_threshold against the measured V_rev usually produces a larger slope. Thus, the shift of V_threshold (vs. V_rev) for native proton currents measured in rat alveolar epithelial cells was 44 mV/U (Cherny et al., 1995). The mean shift reported in 15 types of cells was 46 mV/U (DeCoursey, 2003b). The slope for hH_V1 transfected into HEK or COS cells was 39–43 mV/U (Musset et al., 2008a). There are essentially no reports of mutants in which V_slope departs convincingly from this range. Two nominal deviations from this rule are R211A (53 mV) and ΔN (28 mV; Ramsey et al., 2010), but their P-values versus WT are 0.04 and 0.02, and 2/31 values in this study could easily fall just under the arbitrary P = 0.05 cutoff by chance.

Rather than give an absolute value for the position of the g_H–V relationship, such as using the parameter V₀ in Eq. 2 above (which is not in common parlance), we list the change of V_threshold (V_thr) or an equivalent parameter from control values in each study. Actual numbers will depend on conventions in each laboratory. When data were reported for asymmetrical pH, the V_threshold value was “corrected” by shifting it by 40 mV/U change in ΔpH (Cherny et al., 1995). For reasons discussed at length elsewhere (Musset et al., 2008a), we consider it a highly questionable practice to fit whole-cell g_H–V data with a Boltzmann function, as is routinely done for other voltage-gated channels. Because the distortion of current amplitudes and kinetics resulting from proton depletion are ubiquitous and profound, we prefer to quantify absolute voltage dependence by V_threshold (the most negative voltage at which discernable time-dependent H⁺ currents are detected) or V_gH,max/10 (the voltage at which the g_H is 10% of its maximal value), both measured during small depolarizations and thus minimizing depletion. The absolute position of the g_H–V relationship appears quite mutable with mutation; the extensive study of Ramsey et al. (2010) produced examples spanning >200 mV for various mutants. Given the technical difficulty and intrinsic variability of V_threshold estimation, it would be dangerous to draw conclusions about shifts of less than ∼20 mV. There is a 30-mV range of values reported for WT hH_V1 (see Table 3 in DeCoursey, 2013).

Selectivity is given only when it was explicitly evaluated. The WT channel is perfectly selective for protons, so H⁺ is entered. The entry Cl^– means that the channel is permeable to Cl^– and likely to other anions as well; Na⁺ means the channel is permeable to Na⁺ and likely other cations besides H⁺. The column Other simply provides concise information that does not fit elsewhere in the tables.

Table 2: The N terminus (positions 1–100 in hH_V1)

N terminus

The N terminus of hH_V1 comprises 100 amino acids and is intracellular. The effects of truncating the entire N terminus (ΔN) are not dramatic. Deleting both the N and C termini (ΔN/ΔC) simultaneously results in five- to sixfold faster activation, presumably because these truncations result in monomeric constructs. Although coiled-coil interactions in the C terminus are generally considered to be the main interaction that stabilizes the dimer (Koch et al., 2008; Lee et al., 2008; Tombola et al., 2008; Fujiwara et al., 2013b, 2014; Smith and DeCoursey, 2013), deleting both N and C termini (ΔN/ΔC) appeared to produce monomers more reliably (Koch et al., 2008). Nevertheless, even when both N and C termini are deleted, the VSD-only construct spontaneously dimerizes with a K_d of ∼3 µM (Li et al., 2015).

The short isoform, hH_V1_S

Some B cells, especially B lymphocytes from chronic lymphocytic leukemia patients or malignant B cell lines (Hondares et al., 2014), express a short isoform of hH_V1 (hH_V1_S) that lacks the first 20 amino acids (Capasso et al., 2010). Fig. 1 shows that the 21st amino acid is Met (ATG), which acts as an alternative start site (Hondares et al., 2014). Compared with the full-length protein, hH_V1_S opens more slowly, and its enhanced gating response to PMA is more profound. Furthermore, hH_V1_S interacts less with the B cell receptor, resulting in less internalization. Together, its properties suggest that its expression may contribute to the pathogenesis of B cell malignancies (Hondares et al., 2014).

The phosphorylation site responsible for enhanced gating

When phagocytes are stimulated to undergo the respiratory burst (i.e., activation of NADPH oxidase, or NOX2), the properties of proton channels change so dramatically (Bánfi et al., 1999; DeCoursey et al., 2000; Musset et al., 2009) that at first, the appearance of a second, distinct type of proton channel (proposed to be a component of the active NADPH oxidase complex) was hypothesized (Henderson et al., 1995; Henderson and Chappell, 1996; Bánfi et al., 1999). A decade of controversy ensued (Henderson et al., 1997; Henderson, 1998; Henderson and Meech, 1999, 2002; DeCoursey et al., 2001b, 2002, 2003; Maturana et al., 2002; Touret and Grinstein, 2002; DeCoursey, 2003a,b, 2016). The idea that the gp91^phox component of NADPH oxidase could function as a proton channel was not dispelled completely until well after the HVCN1 gene was identified (Ramsey et al., 2006; Sasaki et al., 2006) when the HVCN1 knockout mouse was developed, which provided the final nail in the coffin (Morgan et al., 2009; El Chemaly et al., 2010). In activated phagocytes, four characteristics of H⁺ currents change, all in the direction of increasing proton flux: the maximum g_H increases two- to fourfold, the g_H–V relationship shifts negatively by 30–40 mV, τ_act becomes two to five times faster, and τ_tail slows two- to sixfold (DeCoursey et al., 2000, 2001a,b; Cherny et al., 2001; DeCoursey, 2003a; Musset et al., 2009). This is referred to as the enhanced gating mode to emphasize that the properties of the H_V1 channel change as a result of phosphorylation, as opposed to a second type of channel appearing.

The original mechanism proposed for enhanced gating of H_V1 was not phosphorylation, but rather modulation of the channel by arachidonic acid generated by cPLA₂α (Henderson and Chappell, 1992). Although arachidonic acid does enhance H_V1 gating by a direct pharmacological effect (DeCoursey and Cherny, 1993; Kapus et al., 1994; Suszták et al., 1997; Kawanabe and Okamura, 2016), neither specific cPLA₂α inhibition nor genetic knockout of cPLA₂α affects the activation of NADPH oxidase or the enhanced gating of H_V1 channels during the respiratory burst (Morgan et al., 2007).

Two predicted PKC phosphorylation sites, Thr²⁹ and Ser⁹⁷, were studied in hH_V1 expressed in the B cell–related LK35.2 cell line (Musset et al., 2010a). Although both were detectably phosphorylated, mutation of Thr²⁹ but not Ser⁹⁷ abolished the enhanced gating response, implicating Thr²⁹ as the key PKC phosphorylation site in hH_V1 (Musset et al., 2010a). Analogous studies of the short isoform identified the same residue, Thr⁹, as the main phosphorylation site (Hondares et al., 2014).

The first identified human hH_V1 mutation

The first naturally occurring hH_V1 mutation from a human subject, M91T (Table 2), was identified by Iovannisci et al. (2010), who cloned HVCN1 genes from primary human airway tissue cultures. Unfortunately, the mutation was discovered only after the death of the donor, who consequently had no opportunity to mourn the defective nature of his/her proton channels, and we lack the opportunity to evaluate the effects of the mutation on his/her quality of life. The main effect of the mutation on H_V1 expressed in COS cells is to decrease the likelihood of channel opening. It requires ∼20 mV more depolarization or ∼0.5 U of greater ΔpH (for airway epithelia, this likely means a higher pH_o) to open mutant M91T channels (Iovannisci et al., 2010).

Table 3: The S1 helix (positions 101–125) and the S1–S2 linker (126–133)

Table 3. Changes in H_V1 properties in S1 (101–125) and S1–S2 linker (126–133) mutants versus WT channels .

Mutant	Species	Expr. system	I?	τ act	τ tail	ΔV thr	ΔpH slope	Selectivity	Other	Reference
Q102C	CiHV1 [H150C]	Xenopus i-o	Yes						MTSi access	Mony et al., 2015
V103C	CiHV1 [V151C]	Xenopus i-o	Yes						MTSi open> closed	Mony et al., 2015
I105C	CiHV1 [I153C]	Xenopus i-o	Yes						MTSi open> closed	Mony et al., 2015
I106C	CiHV1 [I154C]	Xenopus i-o	Yes						MTSi open> closed	Mony et al., 2015
C107A	hHV1	HEK wc	Yes			−17				Ramsey et al., 2010
C107S	hHV1								90% dimer	Li et al., 2015
V109C	CiHV1 [V157C]	Xenopus i-o	Yes						MTSi open> closed	Mony et al., 2015
V109A	hHV1	Xenopus i-o	Yes			−11				Hong et al., 2014
D112A	hHV1	HEK wc	Yes			59	38			Ramsey et al., 2010
D112Aa	hHV1	COS/HEK wc	Yes	2.2	3.0	41b	43	Cl–		Musset et al., 2011
D112A	hHV1	Vesicle flux							Slows H+ flux	Letts, 2014
D112A	hHV1	Xenopus i-o	No							Hong et al., 2014
D112A	CiHV1 [D160A]	Xenopus TEVC	No							Chamberlin et al., 2015
D112A	kHV1 [D51A]	COS/HEK wc	Yes					Cl–		Smith et al., 2011
D112A	NpHv1 [D66A]	HEK wc	Yes					Cl–		Chaves et al., 2016
D112C	CiHV1 [D160C]	Xenopus TEVC	No							Chamberlin et al., 2015
D112C	NpHv1 [D66C]	HEK wc	No							Chaves et al., 2016
D112C/R211C	CiHV1 [D160C/ R261C]	Xenopus TEVC	Yes				nc	Na+		Chamberlin et al., 2015
D112N	hHV1	HEK wc	Yes			31	42			Ramsey et al., 2010
D112Na	hHV1	COS/HEK wc	Yes	2.4	3.0	23b	35	Cl–		Musset et al., 2011
D112N/D185A	hHV1	HEK wc	Yes			103				Ramsey et al., 2010
D112A	hHV1	Xenopus i-o	No							Hong et al., 2014
D112E	hHV1	Xenopus i-o	Yes			−13				Hong et al., 2014
D112E	hHV1	COS/HEK wc	Yes	0.18	7.4/.085	−11b	34	H+	Biexponential tails	Musset et al., 2011
D112E	hHV1	Xenopus i-o	Yes			−15				Berger and Isacoff, 2011
D112E	kHV1 [D51E]	COS/HEK wc	Yes					H+		Smith et al., 2011
D112E	NpHv1 [D66E]	HEK wc	Yes					H+		Chaves et al., 2016
D112E/I127C	CiHV1 [D160A/I175C]	Xenopus i-o	Yes	nc	4.0					Mony et al., 2015
D112H	hHV1	COS/HEK wc	Yes	2.0	0.85	13b	38	Cl–		Musset et al., 2011
D112H	kHV1 [D51H]	COS/HEK wc	Yes					Cl–		Smith et al., 2011
D112H	NpHv1 [D66H]	HEK wc	Yes					Cl–		Chaves et al., 2016
D112Ka	hHV1	COS/HEK wc	Yes	0.8	0.22	46b	40	Cl–		Musset et al., 2011
D112S	hHV1	COS/HEK wc	Yes	2.0	4.1	25b	38	Cl–		Musset et al., 2011
D112S	hHV1	Xenopus i-o	No							Berger and Isacoff, 2011
D112S	hHV1	Vesicle flux							Slows H+ flux	Letts, 2014
D112S	kHV1 [D51S]	COS/HEK wc	Yes					Cl–		Smith et al., 2011
D112S	NpHv1 [D66S]	HEK wc	Yes					Cl–		Chaves et al., 2016
D112S/R211S	hHV1	Xenopus i-o	Yes			24	44	Gu+	At pH 8//8	Berger and Isacoff, 2011
D112Fa	hHV1	COS/HEK wc	Yes	1.6	0.03	44b	38	Cl–		Musset et al., 2011
D112R/R211D	hHV1	Xenopus i-o	Yes					H+		Berger and Isacoff, 2011
D112V	hHV1	COS/HEK wc	No							Musset et al., 2011
D112L	hHV1	Vesicle flux							Slows H+ flux	Letts, 2014
D112Ia	hHV1	COS/HEK wc	No							DeCoursey, 2015b
D112Q	hHV1	Xenopus i-o	No							Hong et al., 2014
D112A/L108Da	hHV1	COS/HEK wc	No							Morgan et al., 2013
D112V/V109Da	hHV1	COS/HEK wc	No							Morgan et al., 2013
D112A/V109Da	hHV1	COS/HEK wc	Yes					Cl–		Morgan et al., 2013
D112A/V110Da	hHV1	COS/HEK wc	No							Morgan et al., 2013
D112A/L111Da	hHV1	COS/HEK wc	No							Morgan et al., 2013
D112A/A113Da	hHV1	COS/HEK wc	No							Morgan et al., 2013
D112A/L114Da	hHV1	COS/HEK wc	No							Morgan et al., 2013
D112A/L115Da	hHV1	COS/HEK wc	No							Morgan et al., 2013
D112A/V116Da	hHV1	COS/HEK wc	Yes					H+		Morgan et al., 2013
D112V/V116Da	hHV1	COS/HEK wc	Yes					H+		Morgan et al., 2013
D112V/V116Ea	hHV1	COS/HEK wc	Yes					H+		Morgan et al., 2013
D112V/V116Sa	hHV1	COS/HEK wc	Yes					Cl–		Morgan et al., 2013
D112V/V116Na	hHV1	COS/HEK wc	Yes					Cl–		Morgan et al., 2013
D112A/L117Da	hHV1	COS/HEK wc	No							Morgan et al., 2013
D112A/A118Da	hHV1	COS/HEK wc	No							Morgan et al., 2013
D112N/I127C	CiHV1 [D160N/I175C]	Xenopus i-o	No							Mony et al., 2015
D112N/R211S/I127C	CiHV1 [D160N/R261S/I175C]	Xenopus i-o	Yes			15				Mony et al., 2015
D112N/R211S/G199C	CiHV1 [D160N/R261S/G249C]	Xenopus i-o	Yes			44				Mony et al., 2015
D112N/G199C	CiHV1 [D160N/G249C]	Xenopus i-o	No							Mony et al., 2015
A113Da	hHV1	COS/HEK wc	Yes					H+		Morgan et al., 2013
E119A	hHV1	HEK wc	Yes			20	47			Ramsey et al., 2010
E119L	hHV1	Vesicle flux							nc H+ flux	Letts, 2014
E119S	mHV1 [E115S]	HEK wc	Yes						nc Zn2+ inhibition	Takeshita et al., 2014
E119S/D123S	mHV1 [E115S/D119S]	HEK wc	Yes						Weaker Zn2+ inhibition	Takeshita et al., 2014
E119A	CiHV1 [E167A]	Xenopus TEVC	Yes			4				Chamberlin et al., 2014
E119C/R205C	CiHV1 [E167C/R255C]	Xenopus TEVC	Yes			−2				Chamberlin et al., 2014
E119C/R208	CiHV1 [E167C/R258C]	Xenopus TEVC	Yes			−52				Chamberlin et al., 2014
D123A	hHV1	HEK wc	Yes			20	48			Ramsey et al., 2010
D123S	mHV1 [D119S]	HEK wc	Yes						nc Zn2+ inhibition	Takeshita et al., 2014
D123C	CiHV1 [D171C]	Xenopus i-o	Yes						MTSo open> closed	Mony et al., 2015
D123A	CiHV1 [D171A]	Xenopus TEVC	Yes			72				Chamberlin et al., 2014
D123A/R205N	CiHV1 [D171A/R255N]	Xenopus TEVC	Yes			11				Chamberlin et al., 2014
K125A	hHV1	HEK wc	Yes			19	47			Ramsey et al., 2010
K125C	CiHV1 [K173C]	Xenopus i-o	Yes						MTSo access	Mony et al., 2015
I127C	CiHV1 [I175C]	Xenopus i-o	Yes			0				Mony et al., 2015
D130A	hHV1	HEK wc	Yes			13				Ramsey et al., 2010
K131A	hHV1	HEK wc	Yes			33				Ramsey et al., 2010

That numerical entries are shown does not imply that any given change was significant. Italicized mutant entries from nonhuman species show the hH_V1 equivalent. HEK, HEK-293, HEK-293T, tsA, or HM1; COS, COS-7; Xenopus, Xenopus laevis oocyte; wc, whole cell; i-o, inside-out patch configuration; TEVC, two-electrode voltage clamp. Blank entries indicate that the parameter was not examined. nc, measured, but no change. Parameters are given relative to WT in each study. For I?, yes means currents are detectable. Time constants are ratios of mutant/WT. The ΔV_threshold value is the change in absolute position of the g_H–V relationship versus WT. The ΔpH slope is the slope in millivolts of the relationship between V_threshold (or other parameters reflecting the absolute position of the g_H–V relationship) and V_rev or E_H (which are not identical; see section Table entries defined). For column Other, MTS access from inside or outside (MTS_i or MTS_o, respectively) is listed as open>closed if the open channel was more accessible.

^aIn an H140A/H193A (Zn²⁺ insensitive) background.

^bPreviously unpublished, analyzed from data for Musset et al. (2011).

The selectivity filter, Asp¹¹²

The most intensively studied position in hH_V1 is Asp¹¹², which was implicated in proton flux (Letts, 2014) and was identified as a crucial part of the selectivity filter (Musset et al., 2011). An indication of the importance of this position is that most mutants malfunction or fail to function altogether, although with some unexplained apparent variability between species or expression systems. Mutation of Asp¹¹² to a neutral residue in most cases results in anion conduction. Specifically, replacing the large anion methanesulfonate^– in the external solution with the smaller Cl^– shifts V_rev negatively, demonstrating permeability to Cl^– (Musset et al., 2011). Lowering the ionic strength by 90% shifts V_rev positively, confirming anion over cation permeation (Musset et al., 2011). Quite similar phenomenology supports an identical role for the analogous Asp in the middle of the S1 helix in two evolutionarily distant species, Karlodinium veneficum (Smith et al., 2011) and Nicoletia phytophila (Chaves et al., 2016), which respectively are only 15% and 33% identical to hH_V1. The conductance of Asp¹¹² mutants appears to vary inversely with the hydrophobicity of the substituent at position 112, with two of the most hydrophobic amino acids tested, Val and Ile, eliminating current flow altogether (Musset et al., 2011; DeCoursey, 2015b). The conservative Asp→Glu mutant retains proton selectivity.

Clearly, Asp¹¹² is crucial to the proton selectivity of hH_V1. However, other Asp are present in the presumed conduction pathway, such as Asp¹⁸⁵ (Fig. 2), but Table 5 shows that its mutation does not impair H⁺ flux (Letts, 2014) or H⁺ selectivity (Musset et al., 2011). In an attempt to determine what other requirements exist for selectivity, Asp was moved along the S1 helix to each position from 108 to 118 (Morgan et al., 2013). At most positions where Asp faced away from the pore, no current was observed. Asp produced proton selectivity at just one other position, 116. Molecular dynamics (MD) simulations suggest that the D112V/V116D construct is proton selective only when Asp¹¹⁶ interacts with one or more S4 Arg residues (Morgan et al., 2013). This result shows that not every near-neighbor interaction of Asp¹¹² in its native position is necessary, but there are clearly strong constraints. The requirements for proton selectivity in hH_V1 deduced from these and many other mutations include the following: (a) a carboxyl group (Asp or Glu) is required; (b) it must face the pore; (c) it must be located at a narrow point in the channel; and (d) it must be able to interact with a basic group (Arg or Lys).

Table 5. Changes in H_V1 properties in S3 (166–188) and S3–S4 linker (189–196) mutants versus WT channels .

Mutant	Species	Expr. system	I?	τ act	τ tail	ΔV thr	ΔpH slope	Selectivity	Other	Reference
E164A/E171A	hHV1	HEK wc	Yes			−7				Ramsey et al., 2010
H167N/H168V/K169N	hHV1	HEK wc	Yes			−13	44			Ramsey et al., 2010
E171A/D174A	hHV1	HEK wc	Yes			−116	39			Ramsey et al., 2010
E171A	hHV1	Vesicle flux							Δ H+ fluxa	Letts, 2014
D174A	hHV1	HEK wc	Yes			−111	38			Ramsey et al., 2010
D174A	hHV1	Vesicle flux							Δ H+ fluxa	Letts, 2014
D174N	hHV1	HEK wc	Yes			−142	36			Ramsey et al., 2010
D174H	hHV1	HEK wc	Yes			−136	37			Ramsey et al., 2010
D174E	hHV1	HEK wc	Yes			−52	46			Ramsey et al., 2010
D174A	CiHV1 [D222A]	Xenopus TEVC	Yes			−111				Chamberlin et al., 2014
D174C/R205C	CiHV1 [D222C/R255C]	Xenopus TEVC	Yes			−95				Chamberlin et al., 2014
D174C/R208C	CiHV1 [D222C/R258C]	Xenopus TEVC	Yes			48				Chamberlin et al., 2014
V178A	hHV1	Xenopus i-o	Yes			−27				Hong et al., 2014
D112V/V178D	hHV1	COS/HEK wc	No							Morgan et al., 2013
S181A	hHV1	HEK wc	Yes			18	46			Ramsey et al., 2010
S181A	hHV1	Xenopus i-o	Yes			0				Hong et al., 2014
D112V/S181D	hHV1	COS/HEK wc	No							Morgan et al., 2013
F182A	hHV1	Xenopus i-o	Yes			−9				Hong et al., 2014
D185A	hHV1	HEK wc	Yes			58	47			Ramsey et al., 2010
D185M	hHV1	COS/HEK wc	Yes					H+		Musset et al., 2011
D185V	hHV1	COS/HEK wc	Yes			20b	43b	H+		Musset et al., 2011
D185A	hHV1	COS/HEK wc	Yes			42b	40b	H+		Musset et al., 2011
D185A	hHV1	Vesicle flux							nc H+ flux	Letts, 2014
D185N	hHV1	COS/HEK wc	Yes			36b	47b	H+		Musset et al., 2011
D185C	CiHV1 [D233C]	Xenopus TEVC	Yes			76				Chamberlin et al., 2014
E185C/R208C	CiHV1 [D233C/R258C]	Xenopus TEVC	Yes			4				Chamberlin et al., 2014
E192A/E196A	hHV1	HEK wc	Yes			13				Ramsey et al., 2010
H193A	hHV1	HEK wc	Yes					H+	39 × Kd Zn2+	Ramsey et al., 2006
H193A	hHV1	Vesicle flux							nc H+ flux	Letts, 2014
H140A/H193A	hHV1	HEK wc	Yes			−12	46	H+	2,000 × Kd Zn2+	Ramsey et al., 2006

That numerical entries are shown does not imply that any given change was significant. Italicized mutant entries from nonhuman species show the hH_V1 equivalent. In the Expression system column: HEK, HEK-293, HEK-293T, tsA, or HM1; COS, COS-7; Xenopus, Xenopus laevis oocyte; wc, whole cell; i-o, inside-out patch configuration; TEVC, two-electrode voltage clamp. Blank entries indicate that the parameter was not examined. nc, measured, but no change. Parameters are given relative to WT in each study. For I?, yes means currents are detectable. Time constants are ratios of mutant/WT. The ΔV_threshold value is the change in absolute position of the g_H–V relationship versus WT. The ΔpH slope is the slope in millivolts of the relationship between V_threshold (or other parameters reflecting the absolute position of the g_H–V relationship) versus V_rev or E_H (which are not identical; see section Table entries defined).

^aNormal initial H⁺ flux followed by recovery ascribed to leak induced in vesicles.

^bPreviously unpublished, analyzed from data for Musset et al. (2011).

These conditions apparently exist only in the outer vestibule of hH_V1. Furthermore, several attempts to reposition Asp into S2 or S3 failed to produce a proton-selective conductance, suggesting that, for reasons that are not at all clear, the carboxyl group must be on S1. A quantum model of the selectivity filter of hH_V1 illustrates how interacting Asp and Arg side chains can selectively conduct protons while excluding other ions (Dudev et al., 2015).

Intriguingly, proton-selective conduction is preserved when Asp¹¹² is replaced by Glu¹¹² (Musset et al., 2011) or when Arg²⁰⁸ is replaced by Lys²⁰⁸ (Dudev et al., 2015). Clearly, this critical interaction has leeway with respect to chain length. The F₁F_o ATP synthase (H⁺ translocating ATPase) remarkably parallels H_V1 in that the proton pathway in its c subunit has an essential Asp⁶¹–Arg²¹⁰ pair and Asp⁶¹ can be moved to a different location or replaced by Glu with only partial loss of function (Miller et al., 1990). It is noteworthy that in several other molecules with critical proton transport pathways, analogous substitutions impair function: Asp→Glu (Chen et al., 2000; Ruivo et al., 2012; Luoto et al., 2013), Glu→Asp (Thorndycroft et al., 2007; Cornish et al., 2011), and Lys→Arg (Balashov et al., 2013). Li et al. (2015) found that hH_V1 is more mobile and dynamic than VSDs of other voltage-gated ion channels.

The series of mutations to Asp¹¹² nicely illustrates the difficulty of interpreting mutations. For example, D112A, D112S, and D112N (Table 3) all open and close more slowly than WT. So is the function of Asp¹¹² to speed gating? No, because D112K produced faster kinetics, and D112F changed activation and deactivation in opposite directions. So does Asp¹¹² regulate gating kinetics? Not really, because practically every mutation for which kinetics were reported alters kinetics, by up to 100-fold. Considering that gating reflects large conformational changes and perhaps other subtler changes that involve a large fraction of the amino acids in the protein, and H_V1 is a compact molecule, it is not surprising that most mutations affect gating kinetics. Their interpretation requires semantic judiciousness. Not every position whose mutation affects gating can reasonably be said to regulate gating kinetics. Nevertheless, the effects of mutations are real and suggest involvement in the process, whose mechanism may, however, be difficult to disentangle.

Countercharges in S1

A fundamental principle in the conception of how voltage gating works is that the periodically spaced cationic residues in S4 (Arg and sometimes Lys) that sense voltage interact electrostatically with anionic amino acids elsewhere in the channel protein to stabilize both closed and open states (Papazian et al., 1995; Tiwari-Woodruff et al., 1997; Lecar et al., 2003). In the General conclusions section below, we discuss alternative interpretations. Within the charge/countercharge conceptual framework, externally accessible acidic residues stabilize the open state, and internally accessible acidic groups stabilize the closed state, presumably by interacting with the cationic groups in S4. If this is the case, a neutralizing mutation to such an externally accessible amino acid should shift the g_H–V relationship positively because the mutant will lose open state stabilization. Conversely, an internal acidic residue would normally stabilize the closed state, and its mutation should promote channel opening, thus shifting the g_H–V relationship negatively. By these criteria, three acidic amino acids in S1 in the outer vestibule, Asp¹¹², Glu¹¹⁹, and Asp¹²³, and possibly Lys¹²⁵ may be considered weak stabilizers of the open state because their neutralization by mutation produces modest positive shifts. Both Asp¹¹² and Glu¹¹⁹ interact with S4 Arg residues in MD simulations of open state homology models of hH_V1 (Wood et al., 2012; Kulleperuma et al., 2013) and CiH_V1 (Chamberlin et al., 2014).

Table 4: The S2 helix (positions 134–156) and the S2–S3 linker (157–165)

Table 4. Changes in H_V1 properties in S2 (134–156) and S2–S3 linker (157–165) mutants versus WT channels .

Mutant	Species	Expr. system	I?	τ act	τ tail	ΔV thr	ΔpH slope	Selectivity	Other	Reference
Y134A	hHV1	HEK wc	Yes			3				Ramsey et al., 2010
H140A	hHV1	HEK wc	Yes					H+	9 × Kda Zn2+	Ramsey et al., 2006
H140A	hHV1	Vesicle flux							nc H+ flux	Letts, 2014
H193A	hHV1	HEK wc	Yes					H+	39 × Kda Zn2+	Ramsey et al., 2006
H140A/ H193A	hHV1	HEK wc	Yes			−12	46	H+	2,000 × Kda Zn2+	Ramsey et al., 2006
H140A/ H193A	hHV1	COS/HEK wc	Yes					H+		Musset et al., 2011
Y141A	hHV1	HEK wc	Yes			−27				Ramsey et al., 2010
S143A	hHV1	HEK wc	Yes			11	41			Ramsey et al., 2010
S143A	hHV1	Vesicle flux							nc H+ flux	Letts, 2014
D112V/S143D	hHV1	COS/HEK wc	Yes					Cl–		Morgan et al., 2013
D112V/I146D	hHV1	COS/HEK wc	No							Morgan et al., 2013
D112V/L147D	hHV1	COS/HEK wc	No							Morgan et al., 2013
F150A	hHV1	Xenopus i-o	Yes			−24				Hong et al., 2013
F150C	hHV1	Xenopus i-o	Yes			−22				Hong et al., 2013
F150W	hHV1	Xenopus i-o	Yes			−55				Hong et al., 2013
E153A	hHV1	HEK wc	Yes			−55	42			Ramsey et al., 2010
E153A	CiHV1 [E201A]	Xenopus TEVC	No							Chamberlin et al., 2014
E153G	CiHV1 [E201G]	Xenopus TEVC	No							Chamberlin et al., 2014
E153N	hHV1	HEK wc	Yes			−1.17	45			Ramsey et al., 2010
E153A	hHV1	Vesicle flux							nc H+ flux	Letts, 2014
E153D	hHV1	HEK wc	Yes			−23	37			Ramsey et al., 2010
E153D/D174E	hHV1	HEK wc	Yes			−102	40			Ramsey et al., 2010
E153C	hHV1	Xenopus i-o	Yes			−55				Tombola et al., 2010
E153C	CiHV1 [E201C]	Xenopus TEVC	Yes			−101				Chamberlin et al., 2014
E153C/R205C	CiHV1 [E201C/R255C]	Xenopus TEVC	Yes			−43				Chamberlin et al., 2014
E153C/R208	CiHV1 [E201C/R258C]	Xenopus TEVC	Yes			37				Chamberlin et al., 2014
E153C/R211C	CiHV1 [E201CR261C]	Xenopus TEVC	Yes			−60				Chamberlin et al., 2014
K157A	hHV1	HEK wc	Yes			1	39			Ramsey et al., 2010
K157A	hHV1	Vesicle flux							nc H+ flux	Letts, 2014
R162A	hHV1	HEK wc	Yes			13				Ramsey et al., 2010

That numerical entries are shown does not imply that any given change was significant. Italicized mutant entries from nonhuman species show the hH_V1 equivalent. HEK, HEK-293, HEK-293T, tsA, or HM1; COS, COS-7; Xenopus, Xenopus laevis oocyte; wc, whole cell; i-o, inside-out patch configuration; TEVC, two-electrode voltage clamp. Blank entries indicate that the parameter was not examined. nc, measured, but no change. Parameters are given relative to WT in each study. For I?, yes means currents are detectable. The ΔV_threshold value is the change in absolute position of the g_H–V relationship versus WT. The ΔpH slope is the slope in millivolts of the relationship between V_threshold (or other parameters reflecting the absolute position of the g_H–V relationship) and V_rev or E_H (which are not identical; see section Table entries defined).

^aNominal K_d values are nearly meaningless for Zn²⁺ inhibition of H_V1 because its main effects are slowing activation and shifting the g_H–V relationship positively (Cherny and DeCoursey, 1999). As demonstrated in the Appendix of DeCoursey et al. (2001a), the apparent K_d derived from the ratio I_H(Zn²⁺)/I_H can vary more than three orders of magnitude depending on the test potential selected. If all measurements are done the same way, relative K_d values have meaning.

The Zn²⁺-binding site

The most potent inhibitor of voltage-gated proton currents is Zn²⁺ (Mahaut-Smith, 1989; DeCoursey, 2003b). Unlike traditional channel blockers that occlude the pore, Zn²⁺ shifts the g_H–V relationship positively and slows activation (Cherny and DeCoursey, 1999). These effects were strongly inhibited at low pH_o, indicating competition between Zn²⁺ and H⁺ for a binding site. To model the competition between H⁺ and Zn²⁺ quantitatively required assuming that Zn²⁺ prevents channel opening by binding to an externally accessible site on the closed channel comprising at least two titratable groups with a pK_a of 6.2–6.6 (near that of His; Cherny and DeCoursey, 1999). Seven years later, the identification of the hH_V1 gene confirmed this deduction because two His residues, His¹⁴⁰ in S2 and His¹⁹³ in the S3–S4 linker, were found to comprise the main sites at which Zn²⁺ binds to inhibit proton currents (Ramsey et al., 2006). The single mutants H140A and H193A each have diminished sensitivity to Zn²⁺, and the double mutant is nearly impervious. The Zn²⁺ sensitivity of a series of mutants in which one or both of these His were mutated to Ala, including various tandem dimers, is described elsewhere (Musset et al., 2010b) and is not included in the tables. Remarkably, the crystal structure of the closed mH_V1 channel contained a Zn²⁺ atom tetrahedrally coordinated by the corresponding two His in the mouse (His¹³⁶ and His¹⁸⁹ in mH_V1), with weaker binding to Glu¹¹⁵ and Asp¹¹⁹ (Glu¹¹⁹ and Asp¹²³ in hH_V1).

Countercharges in S2

S2 contains an important countercharge, Glu¹⁵³, which, as seen in Table 1, is highly conserved among VSD-containing molecules (Smith et al., 2011). In neutral mutants, V_threshold is shifted consistently negatively, in some cases by >100 mV, suggesting that this internal acidic residue stabilizes the closed state.

Charge transfer center or hydrophobic gasket

The S2 helix contains Phe¹⁵⁰, another highly conserved residue among VSD-containing molecules (Tao et al., 2010; Smith et al., 2011) whose K⁺ channel correlate was described as the outer limit of the charge transfer center (Tao et al., 2010). As the positively charged Arg residues in S4 move outwards during a depolarization that opens the channel, they move past Phe¹⁵⁰, which serves as a delimiter of internal and external accessibility. Bezanilla and colleagues include Phe¹⁵⁰ along with two other hydrophobic residues, Val¹⁰⁹ and Val¹⁷⁸, in a hydrophobic gasket (or “plug”) that functions similarly (labeled HG in Table 1; Lacroix et al., 2014; Li et al., 2014, 2015; DeCoursey, 2015a). The global purpose of having a narrow isthmus of protein between the two aqueous vestibules is to focus the electric field (Yang et al., 1996, 1997; Starace and Bezanilla, 2001, 2004). This means that each gating charge (e.g., Arg) needs to move only a small distance to effectively cross the entire membrane electrical field. Mutations to Phe¹⁵⁰ in hH_V1, like those of the corresponding Phe in K⁺ channels, shift the g_H–V relationship (Tao et al., 2010; Hong et al., 2013).

Table 5: The S3 helix (positions 166–188) and the S3–S4 linker (189–196)

Countercharges in S3

S3 contains two important countercharges. Asp¹⁷⁴ is internally accessible and stabilizes the closed state, and neutral mutants shift the g_H–V relationship strongly negatively. In the closed structure of mH_V1, the Asp¹⁷⁴ equivalent appears to interact with Arg²¹¹ in an internal pocket (Takeshita et al., 2014; Cherny et al., 2015). Conversely, Asp¹⁸⁵ (which Table 1 shows is unique to the H_V1 family) is externally accessible and stabilizes the open state, and neutral mutants shift the g_H–V relationship positively. The milder effect of the Asp¹⁸⁵ mutation mirrors its moderate interaction with Arg²⁰⁵ observed in MD simulations of an open state model of hH_V1 (Kulleperuma et al., 2013) and with all three Arg in a model of CiH_V1 (Chamberlin et al., 2014).

Table 6: The S4 helix (positions 197–218)

Table 6. Changes in H_V1 properties in S4 (197–218) mutants versus WT channels .

Mutant	Species	Expr. system	I?	τ act	τ tail	ΔV thr	ΔpH slope	Selectivity	Other	Reference
E196C	CiHV1 [A246C]	Xenopus TEVC	Yes						MTSo open>closed	Gonzalez et al., 2010
L198C	CiHV1 [I248C]	Xenopus TEVC	Yes						MTSo open>closed	Gonzalez et al., 2010
G199C	CiHV1 [G249C]	Xenopus i-o	Yes			2				Mony et al., 2015
L200W	CiHV1 [L250W]	Xenopus TEVC	Yes	0.27	0.67					Okuda et al., 2016
L200W/ΔC	CiHV1 [L250W/ΔC]	Xenopus TEVC	Yes	0.20	0.019					Okuda et al., 2016
I202C	CiHV1 [V252C]	Xenopus TEVC	Yes						MTSo open>closed	Gonzalez et al., 2010
R205A	hHV1	HEK wc	Yes	0.0053	0.086	−1	48		0.63 × WT gating chargea	Ramsey et al., 2006, 2010
R205A	hHV1	Vesicle flux							Δ H+ fluxb	Letts, 2014
R205Hc/T222stop	hHV1	COS wc	Yes					H+	Accessible to external Zn2+	Kulleperuma et al., 2013
R205Q	Mouse [R201Q]	HEK wc	Yes	Faster		−50			1.36 × WT gating chargea	Sasaki et al., 2006
R205N	CiHV1 [R255N]	Xenopus i-o	Yes						0.33 × WT gating charged	Gonzalez et al., 2013
R205N	CiHV1 [R255N]	Xenopus TEVC	Yes			−36			0.78 × WT gating chargea	Chamberlin et al., 2014
R205C	CiHV1 [R255C]	Xenopus TEVC	Yes			−37			1.3 × WT gating chargea	Chamberlin et al., 2014
R205A/R208A	hHV1	HEK wc	Yes			128	51			Ramsey et al., 2010
R205A/R211A	hHV1	HEK wc	Yes			96	45			Ramsey et al., 2010
L206C	CiHV1 [L256C]	Xenopus TEVC	Yes						No MTSo/i access	Gonzalez et al., 2010
W207A,c W207S,c or W207Fc	hHV1	HEK/COS wc	Yes	0.01	0.034	−17.9	40	H+	Loss of selectivity at pHo > 8	Cherny et al., 2015
W207I	Mouse [W203I]	HEK wc	Yes	0.019	0.059				Tandem dimer	Okuda et al., 2016
W207A, W207S, or W207F	kHV1 [W176A, W176S, W176F]	HEK/COS wc	Yes	0.025			40	H+		Cherny et al., 2015
W207A, W207S, or W207F	EhHV1 [W278A, W278S, W278F]	HEK/COS wc	Yes	0.2		−28.2	50	H+		Cherny et al., 2015
W207I	CiHV1 [W257I]	Xenopus TEVC	Yes	0.29	0.030					Okuda et al., 2016
W207I/A210A	CiHV1 [W257I/F260A]	Xenopus TEVC	Yes	0.23	0.026					Okuda et al., 2016
R208A	hHV1	HEK wc	Yes	0.0965	0.075	7	45			Ramsey et al., 2006, 2010
R208A	hHV1	Vesicle flux							Δ H+ fluxb	Letts, 2014
R208A	hHV1	Xenopus i-o	No							Hong et al., 2014
R208K	hHV1	Xenopus i-o	Yes			−40			1.7 × WT gating chargea	Hong et al., 2014
R208K	hHV1	HEK/COS wc	Yes					H+		Dudev et al., 2015
R208Hc/T222stop	hHV1	COS wc	Yes					H+	Accessible to external & maybe internal Zn2+	Kulleperuma et al., 2013
R208Q	hHV1	Xenopus i-o	No							Hong et al., 2014
R208Q	Mouse [R204Q]	HEK wc	No							Sasaki et al., 2006
R208N	hHV1	Xenopus i-o	No							Hong et al., 2014
R208N	CiHV1 [R258N]	Xenopus i-o	Yes						0.50 × WT gating charged	Gonzalez et al., 2013
R208C	CiHV1 [R258C]	Xenopus TEVC	Yes			−10			0.87 × WT gating chargea	Chamberlin et al., 2014
V209C	CiHV1 [V259C]	Xenopus TEVC	Yes						No MTSo/i access	Gonzalez et al., 2010
R211A	hHV1	HEK wc	Yes	2.24	0.092	70	53			Ramsey et al., 2006, 2010
R211A	hHV1	Vesicle flux							Δ H+ fluxb	Letts, 2014
R211S	hHV1	Xenopus i-o	Yes			35			0.72 × WT gating chargea	Hong et al., 2014
R211S	hHV1	Xenopus i-o	Yes			87	49	Gu+	at pH 8//8	Berger and Isacoff, 2011
R211S/I127C	CiHV1 [R261S/I175C]	Xenopus i-o	Yes			15				Mony et al., 2015
R211Hc/T222stop	hHV1	COS wc	Yes					H+	Accessible to internal Zn2+ when open	Kulleperuma et al., 2013
R211H/D112V/V116Dc	hHV1	COS/HEK wc	Yes						Accessible to internal Zn2+ when open	Morgan et al., 2013
R211Q	mHV1 [R207Q]	HEK wc	Yes			nc				Sasaki et al., 2006
R211N	CiHV1 [R261N]	Xenopus i-o	Yes						0.38 × WT gating charged	Gonzalez et al., 2013
R211C	CiHV1 [R261C]	Xenopus TEVC	Yes			54		H+	0.78 × WT gating chargea	Chamberlin et al., 2014
R211C	NpHv1 [R163C]	HEK wc	Yes					H+		Chaves et al., 2016
I212C	CiHV1 [I262C]	Xenopus TEVC	Yes						MTSi closed>open	Gonzalez et al., 2010
N214K	hHV1	HEK wc	Yes			−3	43		Inward rectification	Ramsey et al., 2010
N214R	hHV1	HEK wc	Yes			10	40		Inward rectification	Ramsey et al., 2010
N214R	hHV1	Xenopus i-o	No							Tombola et al., 2008
N214A	hHV1	HEK wc	Yes			−3	42			Ramsey et al., 2010
N214A	hHV1	Vesicle flux							nc H+ flux	Letts, 2014
N214R	mHV1 [N210R]	HEK wc	Yes	Slow	Very slow	−V		H+		Sakata et al., 2010
N214D	hHV1	COS/HEK wc	Yes					H+		Musset et al., 2011
N214C	CiHV1 [N264C]	Xenopus TEVC	Yes						MTSi closed>open	Gonzalez et al., 2010
G215A	hHV1	COS/HEK wc	Yes		Fast			H+		Musset et al., 2011
I217stop	mHV1 [I213stop]	HEK wc	Yes							Sakata et al., 2010
G215stop	mHV1 [G211stop]	HEK wc	Yes							Sakata et al., 2010
I213stop	mHV1 [I209stop]	HEK wc	Yes							Sakata et al., 2010
A210stop	mHV1 [A206stop]	HEK wc	Yes			+V		H+	τact has weak V dependence	Sakata et al., 2010
L204stop	mHV1 [L200stop]	HEK wc	No							Sakata et al., 2010

That numerical entries are shown does not imply that any given change was significant. Italicized mutant entries from nonhuman species show the hH_V1 equivalent. HEK, HEK-293, HEK-293T, tsA, or HM1; COS, COS-7; Xenopus, Xenopus laevis oocyte; wc, whole cell; i-o, inside-out patch configuration; TEVC, two-electrode voltage clamp. Blank entries indicate that the parameter was not examined. nc, measured, but no change. Parameters are given relative to WT in each study. For I?, yes means currents are detectable. Time constants are ratios of mutant/WT. The ΔV_threshold value is the change in absolute position of the g_H–V relationship versus WT. The ΔpH slope is the slope in millivolts of the relationship between V_threshold (or other parameters reflecting the absolute position of the g_H–V relationship) and V_rev or E_H (which are not identical; see section Table entries defined). When C-terminal truncations are indicated as XNNNstop, this means STOP replaces X at position NNN; hence, position NNN and all subsequent residues are truncated, and the last position remaining is NNN-1. For column Other, MTS access from inside or outside (MTS_i or MTS_o, respectively) is listed as open>closed if the open channel was more accessible.

^aSlope factor of g_H–V relationship.

^bNormal initial H⁺ flux followed by recovery ascribed to leak induced in vesicles.

^cIn an H140A/H193A (Zn²⁺ insensitive) background.

^dBy limiting slope method.

Cys scanning reveals aqueous accessibility

A now standard approach to demonstrate aqueous accessibility of amino acids in a protein is to convert the target to Cys and then challenge the mutant with MTS reagents that react with Cys sulfhydryl groups. Whatever functional effect this reaction produces can be examined as a function of the time of exposure to determine accessibility of the Cys. Gonzalez et al. (2010) identified E196C, L198C, and I202C (all external to R1) that were accessible externally preferentially in the open state, suggesting that S4 moves outward and/or rotates. However, a smaller probe for accessibility, n-ethylmaleimide (NEM) in a PEGylation protection assay, revealed that all S4 residues external to position 203 (including the three mentioned above) are accessible, presumably in the closed state (Kurokawa and Okamura, 2014). It should be noted that the former study examined kinetics of MTS effects under voltage clamp, so the gating state was well defined. In the latter study, voltage clamp was not involved, and although at 0 mV most WT channels are closed, many mutations shift the g_H–V relationship, and thus mutant channels could be open at 0 mV.

Accessibility assays are limited by the size of the probe but will also be influenced by charge. For example, aqueous accessibility determined by Cys scanning with NEM as a probe revealed greater accessibility than when using the larger AMS (4-acetamido-4′-maleimidylstilbene-2,2′-disulfonic acid) as a probe (Kurokawa and Okamura, 2014). Furthermore, Ag⁺ as a probe revealed much greater accessibility than the larger NEM (Fillingame et al., 2002). Fillingame et al. (2002) point out that because Ag⁺ has an ionic radius like H₃O⁺, it is ideal for probing proton pathways. Zn²⁺ has a smaller ionic radius than Ag⁺ (Robinson and Stokes, 1959) but is divalent. As a probe of H_V1, it reveals greater accessibility than the bulkier MTS reagents (Kulleperuma et al., 2013; Morgan et al., 2013). In bulk solution, the proton diffuses almost exclusively as protonated buffer (DeCoursey, 1991; DeCoursey and Cherny, 1994, 1996). One expects that the proton permeates the aqueous vestibules of H_V1 as H₃O⁺ (DeCoursey, 2003b) and the selectivity filter as H⁺ (Dudev et al., 2015).

Internal accessibility assessed rigorously by Cys scanning and MTS reaction kinetics under voltage clamp indicated that I212C and N214C were both more accessible at negative voltages, indicating greater accessibility in closed channels (Gonzalez et al., 2010). The residues with state-dependent accessibility in S4 thus span two positions internally and four externally. These results strongly support the idea that S4 moves outwardly during opening, but the extent of movement could be one turn of the helix, consistent with the “one-click” model (Li et al., 2015). Also consistent with a small excursion of S4 during opening are studies using Zn²⁺ to probe for accessibility of Arg→His mutants. In hH_V1, R205H was externally accessible and R208H was accessible externally and possibly also internally (currents were tiny), whereas R211H was accessible only internally even in the open state (Kulleperuma et al., 2013; Morgan et al., 2013).

Accessibility of S1 was explored by Cys scanning, and five residues were found to be more accessible at positive (open) voltages (Mony et al., 2015). One was external, and the rest were internal. Although the internal residues span seven positions, the fact that both the external and internal residues were more exposed in open channels suggests a widening of the vestibules rather than a large inward translational movement.

Gating charge

Numerous mutations have been performed in the S4 helix with the goal of determining the extent to which channel opening involves outward movement of positively charged groups in S4 during depolarization, as is thought to occur in most other voltage-gated ion channels. Each of the three Arg in S4 has been mutated, and the effect on gating charge was evaluated in various ways. Unfortunately, the methods for estimating gating charge are challenging, and several problems unique to H_V1 make the task even more difficult. A direct approach is to measure the integral of the gating current and divide by the number of channels. However, it is nearly impossible to measure gating current in H_V1 because the permeant ion cannot be removed, simple blockers have not been identified, and gating is extremely slow. The most potent inhibitor, Zn²⁺, does not occlude the pore, as would be required to reveal gating charge, but rather shifts the g_H–V relationship positively and slows activation (Cherny and DeCoursey, 1999). Block by guanidine derivatives is also state dependent (Hong et al., 2013). Another procedure that is technically straightforward but of limited information value is to determine the slope factor of a Boltzmann function fit to the g_H–V relationship. The slope factor does include the effective gating charge, but in a highly model-dependent manner; nevertheless, the steepness of the voltage dependence should diminish if gating charges are removed by mutation. Such estimates are indicated in Table 6 by footnote b. There are numerous pitfalls in this measurement, a major one being proton depletion, which produces artificial saturation of current (DeCoursey, 1991; DeCoursey and Cherny, 1994; Musset et al., 2008a), and at best, the slope provides a model-dependent estimate that almost always underestimates the true gating charge (Bezanilla and Villalba-Galea, 2013). Early studies reported slope factors corresponding to a gating charge of 1.4 e₀ (Sasaki et al., 2006) for WT mH_V1 or 0.9 e₀ for WT hH_V1 (Ramsey et al., 2006). A more meaningful approach for H_V1 has been the limiting slope method, devised by Wolf Almers (Almers, 1978). This method works for a wide range of gating models, but not all (Sigg and Bezanilla, 1997), and provides gating charge estimates of ∼6 e₀ for native rat proton currents (DeCoursey and Cherny, 1996, 1997), for CiH_V1 (Gonzalez et al., 2010), and for hH_V1 (Musset et al., 2008a) and 4 e₀ for mH_V1 (Fujiwara et al., 2012). The difficulty is mainly that measurements need to be extended to large negative voltages to achieve sufficiently low g_H values to determine the limiting slope. When critical amino acids such as the Arg in S4 are mutated, the resulting currents are often quite small, which leads to underestimates of the gating charge. Another vexing source of error is that mutation, for example, of Arg²⁰⁵ (R255N in CiH_V1) appears to reduce the extent of S4 movement during gating (Gonzalez et al., 2013), which in retrospect is not a very surprising result of removing one of the charges in the protein thought to move in response to voltage changes!

Three studies reported lower gating charge (Ramsey et al., 2006; Gonzalez et al., 2013; Chamberlain et al., 2014) and two reported higher gating charge (Sasaki et al, 2006; Chamberlain et al, 2014) when Arg²⁰⁵ was neutralized (Table 6). Two studies reported lower gating charge when Arg²⁰⁸ was neutralized (Gonzalez et al., 2013; Chamberlain et al., 2014). The conservative Arg→Lys mutation increased gating charge by 70% (Hong et al., 2014). Finally, both studies of neutralized Arg²¹¹ reported lower gating charge (Gonzalez et al., 2013; Chamberlain et al., 2014). It might be noted that all studies reporting higher gating charge were based on the slope of Boltzmann functions. The tables do not list a number of studies of monomeric constructs, which consistently exhibit only half the gating charge of native dimers, a manifestation of cooperative gating (Gonzalez et al., 2010, 2013; Fujiwara et al., 2012; Okuda et al., 2016).

Why is there a Trp in the middle of S4?

Tryptophan prefers the interfacial environment near membrane lipid head groups (Landolt-Marticorena et al., 1993; Killian and von Heijne, 2000), but in H_V1, a perfectly conserved Trp residue (Trp²⁰⁷ in hH_V1) is located right in the middle of the S4 TM helix. Two studies have been conducted to determine why it is there, exploring mutations in multiple species (Table 1). The most prominent effect of Trp mutation was drastic acceleration of channel gating. In hH_V1, activation and deactivation were 100 times and 30 times faster, respectively (Cherny et al., 2015), whereas in CiH_V1, deactivation was more profoundly accelerated (Okuda et al., 2016). Effects of Trp mutation differ dramatically among species. Table 6 shows that the acceleration of channel opening was 100-fold in hH_V1, 5-fold in EhH_V1, 40-fold in kH_V1 (Cherny et al., 2015), 3.4-fold in CiH_V1, and 53-fold in mH_V1 (Okuda et al., 2016). It is noteworthy that the properties of W207A, W207S, and W207F (three substituents with quite different properties) were all modified identically in hH_V1 and in kH_V1, which strongly implicates a unique property of Trp at this location (Cherny et al., 2015). Two mechanisms have been proposed to explain how Trp slows gating. Based on the proximity of Trp²⁰⁷ and Arg²¹¹ in the closed crystal structure (Takeshita et al., 2014), cation–π interaction between Trp²⁰⁷ and Arg²¹¹ was postulated to stabilize closed hH_V1 channels, with this interaction broken during channel opening (Cherny et al., 2015). In CiH_V1, π stacking of Trp from each protomer at the dimer interface was proposed to slow deactivation (Okuda et al., 2016). This proposal was supported by Trp slowing deactivation preferentially in dimeric versus monomeric constructs (Okuda et al., 2016). Mutant cycle analysis supported the idea that π stacking of Trp at the dimer interface contributed to slow deactivation, but the slowing of activation was independent of channel dimerization. Consistent with this latter conclusion, kH_V1 lacks predicted coiled coil in its C terminus and thus is likely a monomer (Smith and DeCoursey, 2013), and τ_act was two orders of magnitude faster in Trp mutants of kH_V1.

Trp²⁰⁷ mutants not only opened and closed faster, but the Q₁₀ of their gating kinetics dropped from the astronomical 6–9 of WT H_V1 (DeCoursey and Cherny, 1998; Ramsey et al., 2006; Kuno et al., 2009) into the realm of ordinary voltage-gated ion channels, 3.5–4.0 (Cherny et al., 2015). Trp²⁰⁷ is a key component in several of the unique properties of H_V1 (Cherny et al., 2015).

Does Arg²¹¹ contribute to proton selectivity?

One study concluded that Arg²¹¹ is part of the selectivity filter because 15 R211x mutants were permeable to guanidinium, Gu⁺, at symmetrical pH 8, whereas outward current in WT hH_V1 expressed in Xenopus oocytes was blocked (Berger and Isacoff, 2011). However, this result could not be reproduced in hH_V1 expressed in HEK cells, suggesting that the expression system alters this property. Large outward currents were seen in both WT and R211A channels in HEK cells at high pH (DeCoursey, 2013). Neither WT nor R211A was detectably permeant to smaller cations (unpublished data), so the Gu⁺ result appears anomalous, perhaps related to the ability of this chaotropic ion to disrupt hydrogen bonds, interact with hydrophobic regions of proteins, bind to sites normally occupied by water, and denature proteins (Makhatadze and Privalov, 1992; Courtenay et al., 2001; England and Haran, 2011). Although molar Gu⁺ is typically required for wholesale denaturation, 50 mM Gu⁺ sufficed to perturb the permeation pathway of voltage-gated K⁺ channels (Kalia and Swartz, 2011). It is likely that ions are highly concentrated in the pores of channels just as they are in the active sites of enzymes (Jimenez-Morales et al., 2012) because of the high charge density in the protein. We imagine that Gu⁺ tunnels through the pore in a manner that no physiological ion can reproduce, perhaps by breaking the hydrogen bonds between Asp¹¹² and Arg²⁰⁸ that prevent ions other than H₃O⁺ from entering the selectivity filter (Dudev et al., 2015).

It is difficult to imagine a role for Arg²¹¹ in selectivity because the C terminus can be truncated along with the inner part of S4 (between Arg²⁰⁸ and Arg²¹¹) without loss of selectivity (Sakata et al., 2010). One might argue that when Arg²¹¹ is removed, Arg²⁰⁸ may take over its function. However, R211H in hH_V1 (Kulleperuma et al., 2013) and R211C in NpH_V1 (Chaves et al., 2016) and CiH_V1 (Chamberlin et al., 2015) are all proton selective.

What about Asn²¹⁴?

In S4, where many VSD-containing proteins have a fourth Arg or Lys (Table 1), H_V1 has Asn²¹⁴. Noting that the N214R mutant did not conduct, an early suggestion was that, assuming that S4 in H_V1 moves outward as it does in other channels, Asn²¹⁴ might occupy a narrow constriction where it would allow protons to pass (Tombola et al., 2008). Two other groups reported that N214R did conduct (Ramsey et al., 2010; Sakata et al., 2010), but both used mammalian expression systems, as opposed to Xenopus. This may be another example in which the expression system alters the outcome. However, the currents in N214R mutants were small, so this may be a case of different laboratories having different definitions of what comprises a detectable current. Given evidence that Arg²¹¹ remains internally accessible in open hH_V1 (Kulleperuma et al., 2013; Morgan et al., 2013; Li et al., 2015), Asn²¹⁴ most likely remains well inside the internal vestibule.

Table 7: The C terminus (positions 219–273)

Table 7. Changes in H_V1 properties in C-terminal (219–273) mutants versus WT channels .

Mutant	Species	Expr. system	I?	τ act	τ tail	ΔV thr	ΔpH slope	Selectivity	Other	Reference
S219P	hHV1	COS/HEK wc	Yes					H+		Musset et al., 2011
C249S	hHV1								Loss of dimer formation	Li et al., 2015
ΔC (T222stop)	hHV1	HEK wc	Yes			1	28			Ramsey et al., 2010
ΔC (T222stop)	hHV1		Yes	0.15					Weaker Zn2+ effects	Musset et al., 2010b
ΔC	mHV1 (V216stop)	HEK wc	Yes	0.34		nc	nc		Loss of dimer formation	Koch et al., 2008
ΔC	mHV1 (V216stop)	HEK wc	Yes	0.15	0.16					Okuda et al., 2016
ΔC	CiHV1 (D275stop)	Xenopus TEVC	Yes	0.14	0.075					Okuda et al., 2016
V220G/ K221G/T222G	mHV1 V216G/K217G/T218G	HEK wc							Loss of cooperative gating	Fujiwara et al., 2012

That numerical entries are shown does not imply that any given change was significant. HEK, HEK-293, HEK-293T, tsA, or HM1; COS, COS-7; Xenopus, Xenopus laevis oocyte; wc, whole cell; i-o, inside-out patch configuration; o-o, outside-out patch configuration; TEVC, two-electrode voltage clamp. Blank entries indicate that the parameter was not examined. nc, measured, but no change. Parameters are given relative to WT in each study. For I?, yes means currents are detectable. Time constants are ratios of mutant/WT. The ΔV_threshold value is the change in absolute position of the g_H–V relationship versus WT. The ΔpH slope is the slope in millivolts of the relationship between V_threshold (or other parameters reflecting the absolute position of the g_H–V relationship) and V_rev or E_H (which are not identical; see section Table entries defined). When C-terminal truncations are indicated as XNNNstop, this means STOP replaces X at position NNN; hence, position NNN and all subsequent residues are truncated, and the last position remaining is NNN-1. The mouse N-terminal deletions were done by replacing P78M to initiate translation at that position. Note that a large number of mutations to the linker between S4 and the C terminus have been studied (Fujiwara et al., 2012, 2014), but their results are beyond the scope of this table and thus are not included.

The coiled-coil region holds the dimer together

The C terminus of hH_V1 contains extensive typical coiled-coil sequences. When it was learned that mammalian and some other H_V1 assemble into dimers in cell membranes, the coiled-coil regions were implicated in dimerization (Koch et al., 2008; Lee et al., 2008; Tombola et al., 2008; Li et al., 2010). A cross-linking study with 15 strategically located Cys mutants confirmed that the C terminus was a major point of attachment of the dimer (Lee et al., 2008). By modifying the C terminus, Okamura’s group was able to generate functioning trimers and tetramers (Fujiwara et al., 2013a). A crystal structure of the C terminus revealed that the lone Cys (Cys²⁴⁹ in hH_V1) in the C terminus forms a disulfide bond, increasing the stability of the dimer (Fujiwara et al., 2013b). This result was confirmed by mutation of the two native Cys in hH_V1; C107S was still 90% dimer, whereas C249S was <5% dimer (Li et al., 2015).

C terminus truncation was determined to produce mainly monomeric constructs (Koch et al., 2008; Tombola et al., 2008). The monomers behave differently electrophysiologically because the dimer exhibits cooperative gating (Gonzalez et al., 2010; Tombola et al., 2010). The dimer opens with sigmoid kinetics, like a classical Hodgkin-Huxley n² mechanism (Hodgkin and Huxley, 1952). The monomer opens approximately five times faster and with exponential kinetics (Koch et al., 2008; Musset et al., 2010b,c; Tombola et al., 2010; Fujiwara et al., 2012). Because both monomers must move in response to voltage before either can open, the gating charge is twice as large in the dimer as in the monomer (Gonzalez et al., 2010, 2013; Fujiwara et al., 2012; Okuda et al., 2016).

Does the C terminus modulate gating?

The Okamura group has studied the C terminus extensively using creative approaches. They conclude that the C terminus and the S4 helix form a single rigid monolithic rod, which moves during cooperative gating (Fujiwara et al., 2012, 2014). Two types of evidence support this conclusion. First, when a rigid (AAA) or flexible (GGG) triplet was inserted between S4 and the C terminus (at 220–222, replacing VKT; Table 7), the rigid linker behaved like a WT dimer (sigmoid kinetics, slow activation, and full gating charge), whereas the floppy linker produced monomer-like behavior (exponential kinetics, fast activation, and half the gating charge). This showed that simply being a dimer was insufficient to produce cooperative gating; a continuous α helix from the C terminus through S4 was required (Fujiwara et al., 2012). In another intriguing study, 1–10 amino acids were inserted or deleted from the connection between the S4 TM segment and the C terminus. Gating kinetics exhibited a periodic dependence on the linker length, presenting slower WT-like kinetics when the whole S4-C domain was in register (Fujiwara et al., 2014). In this context, even though the VSD-only construct (lacking both N and C termini) spontaneously associates into dimers in liposomes (Li et al., 2015), it likely still functions as two independent monomers.

There is also evidence that interactions at the extracellular end of the S1 segment contribute to the dimer interface (Lee et al., 2008; Qiu et al., 2013; Hong et al., 2015). To enable this to occur, the outer ends of S4 were proposed to relax or unwind (Hong et al., 2015).

General conclusions and remaining questions

A large number of mutations result in functional proton channels. This might be inferred from the fact that there are sequence differences at most positions in H_V1 among different species (Smith et al., 2011). Perhaps not surprisingly, mutations that produce nonfunctional protein tend to occur at highly conserved positions and often involve changes in charge. Both Arg²⁰⁸ mutation (Table 6) and Asp¹¹² mutation (Table 3) are severely detrimental to H_V1 function, which we ascribe to both their central location and essential roles in proton permeation (Kulleperuma et al., 2013; Morgan et al., 2013; Dudev et al., 2015).

Several examples exist in which H_V1 channels apparently behave differently in mammalian and amphibian expression systems. Whether these discrepancies reflect differences in as yet unknown posttranslational modification can only be speculated.

Charge, countercharge, and contra-countercharge

Electrostatics is clearly of central importance in ion channel function. Voltage sensing almost certainly involves charged groups within the membrane electrical field. The idea that interaction of charge pairs (oppositely charged) helps stabilize open or closed states is well entrenched among enthusiasts of H_V1 as well as other voltage-gated ion channels (Papazian et al., 1995; Tiwari-Woodruff et al., 1997). It seems reasonable to conclude that mutations that neutralize a charge will preclude this element from performing this function. The extensive study by Ramsey et al. (2010) together with other table entries is consistent with the interpretation that Asp¹¹², Glu¹¹⁹, Asp¹²³, and Asp¹⁸⁵ in the outer vestibule stabilize the open state; internally accessible acidic groups (Glu¹⁵³ and Asp¹⁷⁴) more strongly stabilize the closed state. Interacting external and internal charge clusters have been observed consistently in MD simulations of H_V1 (Ramsey et al., 2010; Kulleperuma et al., 2013; Morgan et al., 2013; Chamberlin et al., 2014).

However, large shifts of the g_H–V relationship can also be observed upon replacing one uncharged residue with another one (e.g., F150W in Table 4). In fact, almost every mutation that has been examined changes V_threshold one way or the other. Pless et al. (2011) showed convincingly that two highly conserved acids in the Shaker K⁺ channel VSD (equivalent to Glu¹⁵³ and Asp¹⁷⁴ in hH_V1; Table 1) could be neutralized with little effect on the g_K–V relationship. Four external and two internal acidic residues were identified here as counterchanges based on the observation that in neutral mutants the g_H–V relationship shifted in the predicted direction; thus, by this definition, the mechanism appears to involve charge. Alternatively, these charged groups might function primarily to create an aqueous vestibule and consequently a highly focused electrical field (Pless et al., 2011). We cannot resolve the mechanism without strategically designed experiments. If the function of the acidic residues is to create aqueous vestibules, one might predict that polar substituents that replace charges should produce less drastic effects than nonpolar ones. Mining the tables, we find that existing data are neither extensive nor self-consistent enough to provide a clear answer. The reported g_H–V relationship shifts (polar vs. nonpolar) are as follows: Asp¹¹² (31, 23, 13, and 25 vs. 59, 41, and 44); Asp¹⁸⁵ (36 vs. 58, 20, 42, and 76); Glu¹⁵³ (−117 vs. −55, −55, and −101), and Asp¹⁷⁴ (−142 and −136 vs. −111 and −111). Another point is that Lys¹⁵⁷ is predicted by MD to be involved in an internal salt bridge in closed H_V1 (Chamberlin et al., 2014), yet its neutralization has little effect on the g_H–V relationship (Table 4). This question clearly requires future study.

Outstanding remaining problems

It seems surprising that the N terminus contains several sites that influence gating, despite being topologically remote from S4 (at least in the image in Fig. 1). It also seems paradoxical that truncation of the entire N terminus has less overt effect than point mutations within the N terminus. However, the N terminus is disordered, and its tertiary structure is unknown. How it interacts with the rest of the molecule is completely unknown. For example, how does phosphorylation of Thr²⁹ alter gating so profoundly? Why does the innocuous-appearing M91T mutation impede hH_V1 activation?

Voltage gating is a difficult structure–function problem because it is dynamic, not to mention three-dimensional. Despite Cys scanning and other studies designed to examine state-dependent accessibility and molecular movement, it remains unclear which parts of the H_V1 molecule move during channel opening (S4, S1?), in which direction, and how far. For example, accessibility may change as a result of movement “up” or “down,” or into or out of a hydrophobic region, but it can also change simply by expansion or contraction of an aqueous vestibule, or by the helix rotating toward or away from an aqueous space.

Although much has been learned about selectivity and a model has been proposed (Dudev et al., 2015), questions still remain. Is the model correct? Do other parts of the channel contribute to selectivity? Where is the rate-limiting point of the permeation pathway?

Without a doubt, the most important question remains completely unsolved; namely, the mechanism of ΔpH-dependent gating. This unique ΔpH dependence is crucial to all of the known biological functions of H_V1.