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. 2021 Aug 10;10:e71188. doi: 10.7554/eLife.71188

Analysis of the mechanosensor channel functionality of TACAN

Yiming Niu 1, Xiao Tao 1,2, George Vaisey 1,2, Paul Dominic B Olinares 3, Hanan Alwaseem 4, Brian T Chait 3, Roderick MacKinnon 1,2,✉
Editors: Kenton J Swartz5, Kenton J Swartz6
PMCID: PMC8376246  PMID: 34374644

Abstract

Mechanosensitive ion channels mediate transmembrane ion currents activated by mechanical forces. A mechanosensitive ion channel called TACAN was recently reported. We began to study TACAN with the intent to understand how it senses mechanical forces and functions as an ion channel. Using cellular patch-recording methods, we failed to identify mechanosensitive ion channel activity. Using membrane reconstitution methods, we found that TACAN, at high protein concentrations, produces heterogeneous conduction levels that are not mechanosensitive and are most consistent with disruptions of the lipid bilayer. We determined the structure of TACAN using single-particle cryo-electron microscopy and observed that it is a symmetrical dimeric transmembrane protein. Each protomer contains an intracellular-facing cleft with a coenzyme A cofactor, confirmed by mass spectrometry. The TACAN protomer is related in three-dimensional structure to a fatty acid elongase, ELOVL7. Whilst its physiological function remains unclear, we anticipate that TACAN is not a mechanosensitive ion channel.

Research organism: Human, Mouse

Introduction

Mechanosensitive ion channels (MSCs) open in response to mechanical forces (Guharay and Sachs, 1984; Guharay and Sachs, 1985; Kung, 2005; Sachs, 2010). When the channels open, ions flow across the cell membrane, triggering subsequent biochemical processes that ultimately represent a cellular response to the applied mechanical force. This coupling of transmembrane (TM) ion flow to mechanical forces underlies some forms of osmoregulation, cell and organ growth, blood pressure regulation, touch, and hearing (Chalfie, 2009; Coste et al., 2010; Pan et al., 2013; Peyronnet et al., 2012; Woo et al., 2015). Several MSCs have been discovered and characterized (Kefauver et al., 2020). Recently, a new MSC in mammals called TACAN was reported and proposed to mediate mechanical pain (Beaulieu-Laroche et al., 2020). TACAN, originally identified in a proteomics screen and called TMEM120A, was categorized as a nuclear envelope protein (NET29) that participates in lipid metabolism (Batrakou et al., 2015; Byerly et al., 2010; Haakonsson et al., 2013; Lee et al., 2005; Rosell et al., 2014). Adipocyte-specific TMEM120A knockout mice exhibited a lipodystrophy syndrome similar to human familial partial lipodystrophy FPLD2 (Czapiewski et al., 2021).

As our laboratory studies the biophysical mechanisms by which MSCs transduce mechanical forces and conduct ions across membranes, we were intrigued by TACAN’s potential role as an MSC and set out to examine this function and report our findings here.

Results

Functional analysis in cells and reconstituted membranes

We sought to reproduce the mechanically evoked currents reported when TACAN is expressed in cells (Beaulieu-Laroche et al., 2020). Using CHO cells, similar to those used in the original study, we did not observe pressure-evoked currents in excised membrane patches (Figure 1A,B). Background channels that were not sensitive to the pressure steps were observed in CHO cells expressing either TACAN or the M2 muscarinic receptor as a control. Similarly, TACAN expressed in a Piezo1 knockout HEK cell line did not elicit pressure-activated channels (Figure 1C,D). Purified TACAN protein reconstituted into giant unilamellar vesicles (GUVs) of soy L-α-phosphatidylcholine (soy-PC) also did not yield pressure-activated channels in membrane patches isolated from the GUVs (Figure 1E). We note that previously we have successfully recorded mechanosensitive TRAAK channels in GUVs using the identical approach (Brohawn et al., 2014).

Figure 1. TACAN does not produce mechanically evoked currents.

Figure 1.

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(A, B) Representative excised inside-out patch recordings of M2 muscarinic receptor (M2R, A) and TACAN (B) transfected into CHO-K1 cells. (C, D) Representative excised inside-out patch recordings of M2R (C) and TACAN (D) transfected into piezo-1 knockout HEK-293T cells. (E) Representative excised inside-out patch recording of TACAN reconstituted in giant unilamellar vesicles (GUVs). All recordings were performed with identical pipette and bath solution containing 10 mM HEPES pH 7.4, 140 mM KCl, and 1 mM MgCl2 (~300 Osm/L). Traces were obtained holding at –80 mV with a pressure pulse protocol shown at the bottom: 0 to –80 mmHg with 10 mmHg step. Traces colored in red represent the observed currents with –80 mmHg pressure pulse.

When TACAN was expressed, purified, and reconstituted into both GUVs and planar lipid bilayers at high protein-to-lipid ratios (≥1:100, m:m), transient currents were observed, as shown in Figure 2. These currents were insensitive to pressure applied to patches isolated from the GUVs (Figure 1E) and heterogeneous in amplitude (Figure 2A–C). These properties do not resemble aspects of currents from known ion channels but might suggest that TACAN renders the membrane transiently leaky when reconstituted at high protein concentrations.

Figure 2. TACAN produces heterogenous currents in reconstituted systems.

Figure 2.

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(A, B) Representative recordings of TACAN from excised giant unilamellar vesicle (GUV) patches. Symmetrical buffers (10 mM HEPES pH 7.4, 140 mM KCl, 1 mM MgCl2) were used in pipette and bath. The dashed red lines indicate the baseline currents. (A) Traces from GUVs at 1:20 protein-to-lipid ratio (w/w) holding at +80 mV and –60 mV. (B) Traces from GUVs at 1:20 and 1:100 protein-to-lipid ratio (w/w) holding at +80 mV. (C, D) Representative traces of TACAN reconstituted in a lipid bilayer. Symmetrical buffers (10 mM HEPES pH 7.4, 150 mM KCl) were used in top and bottom chambers. The dashed red lines indicate the baseline currents. (C) Traces while holding at +60 mV and –60 mV. (D) Traces recorded during a voltage family from –80 to +80 mV in 20 mV increment.

Structural analysis of TACAN

Alongside the functional characterization, we analyzed the structure of TACAN determined at 3.5 Å resolution using single-particle cryo-EM. Details of the structure determination are given in Materials and methods and Table 1 (Figure 3—figure supplements 1 and 2). TACAN is an α-helical TM protein that forms a symmetric dimer (Figure 3A). The orientation of the protein with respect to the cytoplasm is unknown; however, the charge distribution on TACAN (von Heijne, 1986) as well as the possible presence of an enzyme active site exposed to the cytoplasm (discussed below) suggests the orientation shown (Figure 3B). Each protomer consists of six TM helices (S1–S6), which form a barrel surrounding a tunnel open to the cytoplasm (Figure 3C). Non-continuous density was observed inside the tunnel, suggesting the presence of a small, non-protein molecule (Figure 3—figure supplement 3A). The two protomers of the TACAN dimer bury an extensive surface area of 3049 Å2, mediated through the TM domain as well as two long N-terminal helices that form a coiled coil (Figure 3A, Figure 3—figure supplement 3B).

Table 1. Cryo-EM data collection and refinement statistics, related to Figures 3 and 4.

TACANWT TACANH196A H197A
EMDB ID EMD-24107 EMD-24108
PDB ID 7N0K 7N0L
Data collection
Microscope Titan Krios
Detector K2 summit K3 summit
Voltage (kV) 300 300
Pixel size (Å) 1.03 0.515
Total electron exposure(e-/Å2) 75.4 56.6
Defocus range (μm) 0.7–2.1 0.8–2.2
Micrographs collected 2,071 10,541
Reconstruction
Final particle images 110,090 155,946
Pixel size (Å) 1.03 1.03
Box size (pixels) 256 256
Resolution (Å)(FSC = 0.143) 3.5 2.8
Map sharpening B-factor (Å2) –20 –3.4
Model composition
Non-hydrogen atoms 5,156 5,272
Protein residues 626 626
Ligands 0 2
Metals 0 0
Refinement
Model-to-map CC (mask) 0.77 0.80
Model-to-map CC (volume) 0.73 0.81
R.m.s deviations
Bond length (Å) 0.003 0.003
Bond angles (°) 0.54 0.52
Validation
MolProbity score 2.09 2.22
Clash score 7.86 9.10
Ramachandran plot
Outliers (%) 0 0
Allowed (%) 0.98 1.95
Favored (%) 99.02 98.05
Rotamer outliers (%) 7.46 9.23
C-beta deviations (%) 0 0
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FSC: Fourier shell correlation.

Figure 3. Overall structure of TACAN.

(A) Cartoon representation of the TACAN dimer with each protomer colored uniquely. (B) Surface charge distribution and the possible orientation of TACAN, blue and red representing the positive and negative charges, respectively. The membrane is demarcated by dashed lines. (C) Tertiary structure of TACAN protomer viewed from the side and the cytoplasmic side. The protein is colored rainbow from N-terminus (blue) to C-terminus (red). The six transmembrane helices (S1–S6), two horizontal helices (H1 and H2), as well as a short helix (H3) in between are labeled.

Figure 3.

Figure 3—figure supplement 1. Cryo-EM analysis of wild-type TACAN.

Figure 3—figure supplement 1.

(A) Size-exclusion chromatography of TACAN on a Superdex 200 Increase 10/300 GL column. (B) SDS- PAGE of fractions from size-exclusion chromatography between the red vertical lines shown in (A). (C) Representative cryo-EM image of the TACANWT, selected particles are circled in green, and the scale bar is 50 nm. (D) Cryo-EM data processing workflow for TACANWT. (E) Gold-standard Fourier shell correlation (FSC) curve after correction for masking effects. The resolution was estimated based on the FSC = 0.143 criterion. (F) FSC curve of the refined model versus EM map. The resolution was estimated based on the FSC = 0.5 criterion.
Figure 3—figure supplement 2. Representative density in the cryo-EM map of wild-type TACAN.

Figure 3—figure supplement 2.

(A) The angular distribution of final reconstruction. (B) Local resolution map of TACANWT. (C) Cryo-EM densities for selected regions of TACANWT (contour level 4.4 in COOT).
Figure 3—figure supplement 3. Structural analysis of wild-type TACAN.

Figure 3—figure supplement 3.

(A) The non-protein density (green mesh) observed in TACANWT is shown at different contour levels (UCSF Chimera). The surrounding protein density is shown as gray surface. (B) Interactions of TACAN protomers at the dimer interface. The protomers are colored uniquely, and the interfacial residues are shown in spheres.
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The DALI three-dimensional structure comparison server (Holm and Rosenström, 2010) identified a homologous protein called ELOVL7, a long-chain fatty acid (FA) elongase (Figure 4; Nie et al., 2021). This enzyme catalyzes the first step in the FA elongation cycle by transferring an acetyl group from malonyl-CoA onto long-chain FA-CoA (Naganuma et al., 2011). As shown in Figure 4A, the TM domain in TACAN is indeed similar to ELOVL7. The tunnel in ELOVL7 is lined by catalytically important histidine residues and contains a covalently linked eicosanoyl-CoA molecule (Figure 4B,C). TACAN conserves two of the four histidine residues (Figure 4B,D). To determine the identity of the small molecule implied by the broken density in the tunnel of TACAN (Figure 4E and Figure 3—figure supplement 3A), we determined the structure of TACAN with His196 and His197 mutated to alanine at 2.8 Å resolution (Table 1, Figure 4—figure supplement 1). Our rationale was that if the His residues are catalytically important – by analogy to ELOVL7 – then their mutation might influence the occupancy of a potential cofactor. The map showed clearer density consistent with a coenzyme A molecule (CoASH) (Figure 4F). Native mass spectrometry (nMS) was used to confirm the identity as CoASH (Figure 5). As shown in Figure 5B, the purified TACANH196A H197A sample contains a mixture of the 83,237 Da, +767 Da, and +1535 Da mass species, corresponding to an apo form, one and two CoASH bound forms, respectively. After incubation with CoASH, some fraction of the apo form shifts to one and two CoASH bound forms. Additionally, the +767 Da and +1535 Da species are replaced by +795 Da and +1591 Da or +811 Da and +1622 Da species after incubation with the two CoASH analogues S-ethyl-CoA or Acetyl-CoA, corresponding to the CoASH analogue bound forms. In the purified TACANWT sample, the apo form is dominant and incubation with S-ethyl-CoA shifts it to one and two analogue bound forms (Figure 5A). Together, these data are consistent with our cryo-EM results and indicate that TACAN is co-purified with endogenous coenzyme A.

Figure 4. TACAN shares structural homology to the fatty acid elongase ELOVL7.

(A) Superposition between TACAN (blue) and ELOVL7 (green) protomers. Transmembrane helices are labeled to correspond with the topology of TACAN. The extra transmembrane helix in ELOVL7 is labeled as S0. (B) Sequence alignment of TACAN from different species and human ELOVL7 with conserved residues highlighted. The catalytically important HxxHH motif (His147, His150, and His151) and His181 in ELOVL7 are underlined. (C) Structure details of the interactions between the HxxHH motif, His181 (sidechains shown as sticks), and eicosanoyl-CoA (shown as sticks) in ELOVL7 (PDB: 6Y7F). His150 and His181 are covalently linked to eicosanoyl-CoA. (D) Zoom-in view of the ELOVL7 (green) catalytic center with TACAN (blue) superimposed. (E, F) The non-protein density (green mesh) in the narrow tunnel of wild-type (E) and His196Ala, His197Ala mutant of TACAN (F). Protein density is represented as transparent surface (gray) with protein shown as lines and ribbons. The two maps are shown at the same contour level. CoASH in mutant TACAN is shown as sticks and colored according to atom type.

Figure 4.

Figure 4—figure supplement 1. Cryo-EM analysis of the His196Ala His197Ala mutant TACAN.

Figure 4—figure supplement 1.

(A) Representative cryo-EM image of the TACANH196A H197A, selected particles are circled in green, and the scale bar is 50 nm. (B) Gold-standard Fourier shell correlation (FSC) curve after correction for masking effects. The resolution was estimated based on the FSC = 0.143 criterion. (C) FSC curve of the refined model versus EM map. The resolution was estimated based on the FSC = 0.5 criterion. (D) The angular distribution of final reconstruction. (E) Local resolution map of TACANH196A H197A. (F) Cryo-EM densities for selected regions of TACANH196A H197A (contour level 5.2 in COOT). The CoASH molecule is colored according to atom type.
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Figure 5. Native mass spectrometry indicates the presence of coenzyme A in the mutant TACAN sample.

(A, B) Mass species detected in purified wild-type (A) and His196Ala, His197Ala mutant (B) TACAN protein without treatment (“untreated”), or incubated with CoASH (MW = 767.5 Da), S-ethyl-CoA (MW = 795.6 Da), or acetyl-CoA (MW = 809.6 Da).

Figure 5.

Figure 5—figure supplement 1. TACAN is co-purified with coenzyme A molecules.

Figure 5—figure supplement 1.

(A) Binding details of CoASH in TACANH196A H197A. Surface charge is represented from blue (positive charges) to red (negative charges). Neighboring residues are shown as sticks, and the hydrogen bonding is indicated by a black dash. A CoASH molecule is shown as sticks and colored according to atom type. (B) Conformation comparison of eicosanol-CoA in ELOVL7 (green) and CoASH in TACAN (blue) based on the alignment in Figure 4A. (C) CoASH releasing activity of ELOVL7 and TACAN. Proteoliposomes of TACAN and ELOVL7 reconstituted in soy L-α-phosphatidylcholine (soy-PC) at 1:50 protein-to-lipid ratio (w/w) with 10 μg protein and 500 μg soy-PC were used. Empty proteoliposomes made of soy-PC were used as control. ELOVL7 showed significant activity, while neither the wild-type nor His196Ala His197Ala mutant of TACAN showed any activity.
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It is noteworthy that CoASH binds with different conformations in TACAN from ELOVL7 (Figure 5—figure supplement 1A,B; Nie et al., 2021). In addition, no enzymatic activity was observed for TACAN using a free-CoA detection assay (for details, see Materials and methods), which demonstrated robust activity for ELOVL7 (Figure 5—figure supplement 1C), thus TACAN does not appear to catalyze the same reaction as ELOVL7. If TACAN is a coenzyme A-dependent enzyme, its substrate is unknown.

Discussion

We undertook this study to understand how TACAN functions as an MSC, but have been unable to replicate evidence of MSC activity. We observe no channel activity in the plasma membrane of cells expressing TACAN and the heterogeneous-in-amplitude currents (without mechanosensitive properties) that we observe when we reconstitute TACAN at high protein concentrations are not consistent with other native biological channels that we have studied.

Structurally, TACAN is related to coenzyme A-dependent FA elongases; however, without further data we cannot conclude that TACAN itself functions as an enzyme. It also remains to be determined which membranes in a cell express TACAN.

In conclusion, we do not find evidence that TACAN is a mechanosensitive ion channel. The strength of this conclusion is in the electrophysiological interrogation. The structure, because it looks like a known enzyme, is compatible with the ‘not a channel’ conclusion, but the structure alone would not make a strong argument. A number of ion channels, including CLC channels (Dutzler et al., 2002; Dutzler et al., 2003; Feng et al., 2012; Park et al., 2017; Park and MacKinnon, 2018), TMEM16 (Dang et al., 2017; Paulino et al., 2017), and CFTR (Liu et al., 2017; Zhang et al., 2017; Zhang et al., 2018), are not obviously ion channels based on their structures and indeed each are fairly indistinguishable from proteins exhibiting non-channel functions.

Materials and methods

Key resources table.

Reagent type (species) or resource Designation Source or reference Identifiers Additional information
Gene (Mus musculus TMEM120A) M. musculus TACAN Synthetic Synthesized at GeneWiz.
Gene (Homo sapiens TMEM120A) H. sapiens TACAN Synthetic Synthesized at GeneWiz.
Gene (Homo sapiens ELOVL7) H. sapiens ELOVL7 Synthetic Synthesized at GeneWiz.
Strain, strain background (Escherichia coli) DH10Bac Thermo Fisher Scientific 10361012  
Recombinant DNA reagent TACAN-eGFP BacMam This study  
Recombinant DNA reagent ELOVL7-eGFP BacMam This study  
Recombinant DNA reagent Halo-M2R-eGFP BacMam This study  
Cell line (Spodoptera frugiperda) Sf9 ATCC CRL-1711 Cells purchased from ATCC, and we have confirmed there is no mycoplasma contamination
Cell line (Chinese hamster) CHO-K1 ATCC CRL-9618 Cells purchased from ATCC, and we have confirmed there is no mycoplasma contamination
Cell line (Homo sapiens) HEK293S GnTI- ATCC CRL-3022 Cells purchased from ATCC, and we have confirmed there is no mycoplasma contamination
Cell line (Homo sapiens) Piezo1 knockout HEK293T https://digitalcommons.rockefeller.edu/cgi/viewcontent.cgi?article=1422&context=student_theses_and_dissertations We have confirmed there is no mycoplasma contamination
Chemical compound, drug SF-900 II SFM medium Gibco 11330-032  
Chemical compound, drug L-Glutamine (100×) Gibco 25030-081  
Chemical compound, drug Pen Strep Gibco 15140-122  
Chemical compound, drug Grace’s insect medium Gibco 11605-094  
Chemical compound, drug Freestyle 293 medium Gibco 12338-018  
Chemical compound, drug DMEM/F-12 medium Gibco 11605-094  
Chemical compound, drug DMEM Gibco 11965-118  
Chemical compound, drug Fetal bovine serum Gibco 16000-044  
Chemical compound, drug Cellfectin II reagent Invitrogen 10362100  
Chemical compound, drug FuGENE HD transfection reagent Promega E2312  
Chemical compound, drug Cholesteryl hemisuccinate (CHS) Anatrace CH210  
Chemical compound, drug n-Decyl-β-D-maltopyranoside (DM) Anatrace D322S  
Chemical compound, drug Lauryl maltose neopentyl glycol (LMNG) Anatrace NG310  
Chemical compound, drug Digitonin Millipore Sigma 300410  
Chemical compound, drug Coenzyme A trilithium salt (CoASH) Sigma-Aldrich C3019  
Chemical compound, drug Acetyl coenzyme A sodium salt (acetyl-CoA) Sigma-Aldrich A2056  
Chemical compound, drug S-Ethyl-coenzyme A sodium salt (S-ethyl-CoA) Jena-Biosciences NU-1168  
Chemical compound, drug Malonyl coenzyme A lithium salt (malonyl-CoA) Sigma-Aldrich M4263  
Chemical compound, drug Stearoyl coenzyme A lithium salt (stearoyl-CoA) Sigma-Aldrich S0802  
Chemical compound, drug (1H, 1H, 2H, 2H-Perfluorooctyl)phosphocholine (FFC8) Anatrace F300F  
Commercial assay or kit CNBr-activated Sepharose beads GE Healthcare 17-0430-01  
Commercial assay or kit Superdex 200 Increase 10/300 GL GE Healthcare Life Sciences 28990944  
Commercial assay or kit R1.2/1.3 400 mesh Au holey carbon grids Quantifoil 1210627  
Commercial assay or kit Coenzyme A (CoA) Assay Kit Sigma-Aldrich MAK034  
Software, algorithm RELION 3.0 https://doi.org/10.7554/eLife.42166.001 http://www2.mrc-lmb.cam.ac.uk/relion  
Software, algorithm RELION 3.1 https://doi.org/10.1101/798066 http://www2.mrc-lmb.cam.ac.uk/relion  
Software, algorithm MotionCor2 https://doi.org/10.1038/nmeth.4193 http://msg.ucsf.edu/em/software/motioncor2.html  
Software, algorithm Gctf 1.0.6 https://doi.org/10.1016/j.jsb.2015.11.003 https://www.mrc-lmb.cam.ac.uk/kzhang/Gctf/  
Software, algorithm CtfFind4.1.8 https://doi.org/10.1016/j.jsb.2015.08.008 http://grigoriefflab.janelia.org/ctffind4  
Software, algorithm CryoSPARC 2.9.0 https://doi.org/10.7554/eLife.46057.001 https://cryosparc.com/  
Software, algorithm COOT https://doi.org/10.1107/S0907444910007493 http://www2.mrc-lmb.cam.ac.uk/personal/pemsley/coot  
Software, algorithm PHENIX https://doi.org/10.1107/S0907444909052925 https://www.phenix-online.org  
Software, algorithm Adobe Photoshop version 16.0.0 (for figure preparation) Adobe Systems, Inc.  
Software, algorithm GraphPad Prism version 8.0 GraphPad Software  
Software, algorithm MacPyMOL: PyMOL v2.0 Enhanced for Mac OS X Schrodinger LLC https://pymol.org/edu/?q=educational/  
Software, algorithm Chimera https://doi.org/10.1002/jcc.20084 https://www.cgl.ucsf.edu/chimera/download.html  
Software, algorithm Serial EM https://doi.org/10.1016/j.jsb.2005.07.007 http://bio3d.colorado.edu/SerialEM  
Software, algorithm pClamp Axon Instruments, Inc  
Software, algorithm Thermo Xcalibur Qual Browser (v. 4.2.47) Thermo Fisher Scientific  
Software, algorithm UniDec v. 4.2.0 Marty et al., 2015; Reid et al., 2018 https://github.com/michaelmarty/UniDec/releases  
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Protein expression and purification

Homo sapiens or Mus musculus full-length TMEM120A (TACAN, residues 1–343) were cloned into pEG BacMam (Goehring et al., 2014). The C-terminus of the TACAN construct contains a PreScission protease cleavage site and an enhanced green fluorescent protein (eGFP) for purification (TACAN-eGFP). Briefly, bacmid carrying TACAN was generated by transforming Escherichia coli DH10Bac cells with the corresponding pEG BacMam construct according to the manufacturer’s instructions (Bac-to-Bac; Invitrogen). Baculoviruses were produced by transfecting Spodoptera frugiperda Sf9 cells with the bacmid using Cellfectin II (Invitrogen). Baculoviruses, after two rounds of amplifications, were used for cell transduction. HEK293S GnTl- cells (ATCC, CRL-3022) grown in suspension at a density of ~3 × 106 cells/mL were transduced with P3 BacMam virus of TACAN-eGFP, and inoculated at 37°C. 8–12 hr post-transduction, 10  mM sodium butyrate was added to the culture and cells were further inoculated for 40–48 hr at 30°C. Cells were then harvested by centrifugation, frozen in liquid N2, and stored at –80°C until needed.

Frozen cells (from 1 L cell cultures) were resuspended in 200 mL hypotonic lysis buffer containing 50 mM Tris-HCl pH 8.0, 3 mM dithiothreitol (DTT), 1 mM ethylenediaminetetraacetic acid (EDTA), 0.1 mg/mL DNase I, and a protease inhibitor cocktail (1  mM PMSF, 0.1 mg/mL trypsin inhibitor, 1 µg/mL pepstatin, 1 µg/mL leupeptin, and 1  mM benzamidine) for 30 min and centrifuged at 37,500 g for 30 min. The pellets were then homogenized in 20  mM Tris-HCl pH 8.0, 300  mM NaCl, 0.1 mg/mL DNase I, a protease inhibitor cocktail followed by addition of 10 mM lauryl maltose neopentyl glycol (LMNG), 2 mM cholesteryl hemisuccinate (CHS) (for cryo-EM samples), or 1% n-decyl-β-D-maltopyranoside (DM), 0.2% CHS (w/v) (for reconstitution and mass spectrometry samples) to solubilize for 2 hr. The suspension was then centrifuged at 37,500 g for 30 min and the supernatant incubated with 5 mL GFP nanobody-coupled CNBr-activated Sepharose resin (GE Healthcare) for 2 hr (Kubala et al., 2010). The resin was subsequently washed with 10 column volumes of wash buffer containing 20 mM HEPES pH 7.4, 250 mM NaCl, and 0.06% digitonin (w/v) (for cryo-EM samples) or 0.25% DM, 0.05% CHS (w/v) (for reconstitution and mass spectrometry samples). The washed resin was incubated overnight with PreScission protease at a protein to protease ratio of 40:1 (w:w) to cleave off GFP and release the protein from the resin. The protein was eluted with wash buffer, concentrated using an Amicon Ultra centrifugal filter (MWCO 100 kDa), and then injected onto a Superdex 200 increase 10/300 GL column (GE Healthcare) equilibrated with the wash buffer. Peak fractions corresponding to the TACAN dimer were pooled. For cryo-EM study, the pooled fractions were concentrated to 6–7 mg/mL using an Amicon Ultra centrifugal filter (MWCO 100 kDa). All the purification steps were carried out at 4°C.

H. sapiens full-length ELOVL7 (residues 1–281) was cloned into the same vector, expressed, and purified with the same protocol as TACAN in DM/CHS. The final protein concentration was ~2 mg/mL.

Proteoliposome reconstitution

Dialysis-mediated reconstitution of H. sapiens TACAN and ELOVL7 into liposome was accomplished according to published protocols with minor modifications (Brohawn et al., 2012; Heginbotham et al., 1999; Long et al., 2007; Tao and MacKinnon, 2008; Wang et al., 2014). Briefly, 20 mg of soy L-α-phosphatidylcholine (soy-PC) was dissolved in 1 mL chloroform in a glass vial and dried to a thin film under argon, rehydrated in reconstitution buffer (10 mM HEPES pH 7.4, 450 mM NaCl, and 2 mM DTT) to 20 mg/mL by rotating for 20 min at room temperature, followed by sonication with a bath sonicator until translucent. 1% DM was then added, and the lipid detergent mixture was rotated for 30 min and sonicated again until clear. Purified TACAN (~3 mg/mL) or ELOVL7 (~2 mg/mL) in DM/CHS and DM-solubilized lipids (20 mg/mL) were mixed at protein-to-lipid (w:w) ratios of 1:20, 1:50, and 1:100, incubated for 2 hr, and then dialyzed against 4 L reconstitution buffer for 4 days with daily exchange at 4°C. Biobeads (Bio-Rad) were added to the reconstitution buffer for the last 12 hr. The resulting proteoliposomes were flash frozen in liquid N2 and stored at −80°C.

GUV formation

The dehydration/rehydration-mediated blister formation technique was used for generation of GUVs as previously reported (Brohawn et al., 2014). In brief, an aliquot of reconstituted H. sapiens TACAN proteoliposome was thawed at room temperature and spotted onto the 14 mm glass coverslip inside a 35 mm glass-bottomed Petri dish (Mattek; P35G-1.5-14C) as 4–6 similar-sized drops. Spotted proteoliposomes were then dried under vacuum at 4°C for 6 hr followed by rehydration with ~20 μL rehydration buffer (10 mM HEPES pH 7.4, 140 mM KCl). The rehydration was done by sitting the 35 mm Petri dish inside a 15 cm Petri dish lined with wet filter paper overnight at 4°C (~16 hr). 3 mL bath solution (10 mM HEPES pH 7.4, 140 mM KCl, 1 mM MgCl2) was then added to the 35 mm dish before recording. Blisters were visible after ~10 min and were competent to form high-resistance seals for at least 2 hrs.

Cell culture and transfection for patch recordings

CHO-K1 cells (ATCC) and piezo-1 knockout HEK-293T cells (established in this lab) were used for electrophysiology experiments because they have low endogenous mechanosensitive currents (Brohawn et al., 2014; del Marmol, 2016).

Cells were cultured in DMEM-F12 (Gibco) (CHO cells) or DMEM (Gibco) (HEK-293T cells) supplemented with 10% FBS, 2 mM L-glutamine, 100 units/mL penicillin, and 100 μg/mL streptomycin. Cells were plated in 35 mm plastic dishes and grown to ~50–60% confluency at 37°C. Right before transfection, culture media was replaced by DMEM-F12 or DMEM with 10% FBS and 2 mM L-glutamine. 1 μg of H. sapiens TACAN-eGFP or the M2 muscarinic receptor (Halo-M2R-eGFP) plasmid (previously established in this lab) was transfected into the cells using FugeneHD (Promega) following the manufacturer’s protocol. Cells were transferred to 30°C after transfection and recordings were carried out 16–18 hr post-transfection. Immediately before recording, media were replaced by the bath solution (10 mM HEPES pH 7.4, 140 mM KCl, 1 mM MgCl2).

Excised inside-out patch recordings

Pipettes of borosilicate glass (Sutter Instruments; BF150-86-10) were pulled to ~2–6 MΩ resistance with a micropipette puller (Sutter Instruments; P-97) and polished with a microforge (Narishige; MF-83). Recordings were obtained with an Axopatch 200B amplifier (Molecular Devices) using excised inside-out patch techniques. Recordings were filtered at 1 kHz and digitized at 10 kHz (Digidata 1440A; Molecular Devices). Pressure application through patch pipettes was performed with a high-speed pressure clamp (ALA Scientific) controlled through the Clampex software. Pressure application velocity was set to the maximum rate of 8.3 mmHg/ms. All recordings were performed at room temperature. Pipette and bath solutions were identical unless otherwise stated: 10 mM HEPES pH 7.4, 140 mM KCl, and 1 mM MgCl2 (~300 Osm/L).

Planar lipid bilayer recordings

The bilayer experiments were performed as previously described with minor modifications (Ruta et al., 2003; Wang et al., 2014). A piece of polyethylene terephthalate transparency film separated the two chambers of a polyoxymethylene block, filled with symmetrical buffer containing 10 mM HEPES pH 7.4, 150 mM KCl unless otherwise stated. A lipid mixture of DPhPC (1,2-diphytanoyl-sn-glycero-3-phosphocholine, Avanti, Cat# 850356):POPA (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphate, Avanti, Cat# 840857) (3:1, w:w) dissolved in decane (20 mg/mL) was pre-painted over an ∼100 μm hole on the transparency film. Voltage was controlled with an Axopatch 200B amplifier in whole-cell mode. The analog current signal was low-pass filtered at 1 kHz (Bessel) and digitized at 10 kHz with a Digidata 1550A digitizer (Molecular Devices). Digitized data were recorded on a computer using the software pClamp (Molecular Devices, Sunnyvale, CA). Experiments were performed at room temperature.

Cell lines

All the cell lines except for Piezo1 knockout HEK-293T, which was previously generated in the lab, were purchased from ATCC, and we have confirmed there is no mycoplasma contamination for all of them.

Cryo-EM sample preparation and data collection

For both the WT and His196Ala, His197Ala mutant of M. musculus TACAN, purified protein at a concentration of 6–7 mg/mL was mixed with 2.9 mM Fluorinated Fos-Choline-8 (FFC8; Anatrace) immediately prior to grid preparation. 3.5 μL of the mixture was applied onto a glow-discharged Quantifoil R1.2/1.3 400 mesh Au grid (Quantifoil), blotted for 4 s at room temperature (RT) with a blotting force of 2–4 and humidity of 100%, and plunge-frozen in liquid ethane using a Vitrobot Mark IV (FEI).

Cryo-EM data were collected on a 300-kV Titan Krios electron microscope (Thermo Fisher Scientific) equipped with a K2 Summit (TACANWT), or a K3 Summit (TACANH196A H197A) direct electron detector and a GIF Quantum energy filter set to a slit width of 20  eV. Images were automatically collected using SerialEM in super-resolution mode. After binning over 2 × 2 pixels, the calibrated pixel size was 1.03 Å with a preset defocus range from 0.7 to 2.1 μm (TACANWT), or 0.515 Å with a preset defocus range from 0.8 to 2.2 μm (TACANH196A H197A), respectively. Each image was acquired as either a 10 s movie stack of 50 frames with a dose rate of 7.54 e/Å2/s, resulting in a total dose of about 75.4 e−/Å2 (TACANWT), or a 1.5 s movie stack of 38 frames with a dose rate of 37.7 e/Å2/s, resulting in a total dose of about 56.6 e/Å2 (TACANH196A H197A).

Image processing

For TACANWT, the image processing workflow is illustrated in Figure 3—figure supplement 1D. A total of 2,071 super-resolution movie stacks were collected. Motion-correction, twofold binning to a pixel size of 1.03  Å, and dose weighting were performed using MotionCor2 (Zheng et al., 2017). Contrast transfer function (CTF) parameters were estimated with Gctf (Zhang, 2016). Micrographs with ice contamination were removed manually, resulting in 1,982 micrographs for further processing. A total of 583,766 particles were auto-picked using Relion 3.1 (Scheres, 2020; Scheres, 2012; Zivanov et al., 2018; Zivanov et al., 2020) and windowed into 256  ×  256-pixel images. Reference-free 2D classification was performed to remove contaminants, yielding 383,719 particles. These particles were subjected to ab initio reconstruction in cryoSPARC-2.9.0 (Punjani et al., 2017), specifying four output classes. The best class with 245,031 particles was selected, then subjected to a resolution-based classification workflow similar to a previous study (Kang et al., 2020). In brief, 40 iterations of global search 3D classification (K = 1) in Relion 3.1 with an angular sampling step of 7.5° was performed to determine the initial alignment parameters using the initial model generated from cryoSPARC. For each of the last five iterations of the global search, a K  =  6 multi-reference local angular search 3D classification was performed with an angular sampling step of 3.75° and a search range of 30°. The multi-reference models were generated using reconstruction at the last iteration from global search 3D classification low-pass filtered to 8, 15, 25, 35, 45, and 55  Å, respectively. The classes that showed obvious secondary structure features were selected and combined. Duplicated particles were removed, yielding 130,491 particles in total. These particles were subsequently subjected to non-uniform refinement with C2 symmetry in cryoSPARC, which resulted in a map with a resolution of 4.5 Å. Iterative cycles of non-uniform refinement in cryoSPARC with C2 symmetry and Bayesian polishing in Relion 3.1 with new training parameters were performed until no further improvement, resulting in a 3.7 Å map. The refined particles were further cleaned up with one round of ab initio reconstruction (K = 4) in cryoSPARC and 110,090 particles remained. Finally, these particles were subjected to the non-uniform refinement with C2 symmetry in cryoSPARC, which yielded the final map at 3.5 Å resolution.

For TACANH196A H197A, 10,541 super-resolution movie stacks were collected. Motion-correction, twofold binning to a pixel size of 0.515 Å, and dose weighting were performed using MotionCor2 (Zheng et al., 2017). CTF parameters were estimated with CTFFind4 (Rohou and Grigorieff, 2015). Micrographs with ice contamination were removed manually, resulting in 9,600 micrographs for further processing. A total of 1,474,917 particles were auto-picked using Relion 3.1 and windowed into 400  ×  400-pixel images, then binned two times and subjected to 2D classification, yielding 975,636 particles. The following image processing workflow is identical to TACANWT sample. Briefly, these particles were subjected to ab initio reconstruction in cryoSPARC-2.9.0 (Punjani et al., 2017), specifying four output classes. The best class with 607,159 particles was selected, then subjected to the resolution-based classification, yielding 391,137 particles. Subsequent non-uniform refinement with C2 symmetry in cryoSPARC was performed, resulting in a map with a resolution of 3.8 Å and the resolution was further improved to 3.3 Å by iterative Bayesian polishing and non-uniform refinement cycles. Particles were further cleaned up with one round of ab initio reconstruction with 155,946 particles remaining. Finally, these particles were subjected to the non-uniform refinement with C2 symmetry in cryoSPARC, which yielded the final map at 2.8 Å resolution.

The mask-corrected Fourier shell correlation (FSC) curves were calculated in cryoSPARC 2.9.0, and reported resolutions were based on the 0.143 criterion. Local resolutions of the final maps were estimated by Relion 3.1 (Scheres, 2020; Zivanov et al., 2020). A summary of reconstructions is shown in Table 1 and Figure 3—figure supplement 1E,F, Figure 3—figure supplement 2A,B, Figure 4—figure supplement 1B–E.

Model building and refinement

For TACANWT, the 3.5 Å resolution map was subjected to Buccaneer in the CCP-EM suite (Burnley et al., 2017; Wood et al., 2015) to generate the de novo model. This initial model was further improved using phenix.sequence_from_map in Phenix (Adams et al., 2010). Several iterative cycles of refinement using the phenix.real_space_refine with secondary structure and NCS restraints and manual adjustments in COOT yielded the final model for the TACANWT containing residues 9–72, 76-250 and 262–335 (Adams et al., 2010; Emsley et al., 2010).

For TACANH196A H197A, model of TACANWT was placed into the 2.8 Å map using UCSF Chimera (Pettersen et al., 2004) and manually adjusted in COOT Emsley et al., 2010 followed by iterative refinement cycles using the phenix.real_space_refine in Phenix with secondary structure and NCS restraints and manual adjustments in COOT. The final model for TACANH196A H197A contained residues 9–72, 76–250 and 262–335 as well as 2 CoASH molecules bound.

Refinement statistics are summarized in Table 1. Structural model validation was done using Phenix and MolProbity based on the FSC = 0.5 criterion (Chen et al., 2010). Figures were prepared using PyMOL (https://pymol.org/2/) and UCSF Chimera (Pettersen et al., 2004). Representative densities of TACANWT and TACANH196A H197A are shown in Figure 3—figure supplement 2C and Figure 4—figure supplement 1F, respectively.

Native MS analysis

The purified wild-type and mutant M. musculus TACAN samples were incubated with excess (0.7–0.9 mM) ligand (CoASH, S-ethyl-CoA, or acetyl-CoA) for 1 hr on ice. The incubated samples were then buffer exchanged into 200 mM ammonium acetate, 0.002% LMNG (2 CMC) using a Zeba microspin desalting column with a 40 kDa MWCO (Thermo Fisher Scientific). For nMS analysis, a 2–3 µL aliquot of each buffer-exchanged sample was loaded into a gold-coated quartz capillary tip that was prepared in-house and then electrosprayed into an Exactive Plus with extended mass range (EMR) instrument (Thermo Fisher Scientific) using a modified static direct infusion nanospray source (Olinares and Chait, 2020). The MS parameters used include spray voltage, 1.22–1.25 kV; capillary temperature, 125–200 °C; in-source dissociation, 125–150 V; S-lens RF level, 200; resolving power, 8,750 or 17,500 at m/z of 200; AGC target, 1 × 106; maximum injection time, 200 ms; number of microscans, 5; injection flatapole, 8 V; interflatapole, 7 V; bent flatapole, 5 V; high-energy collision dissociation (HCD), 200 V; ultrahigh vacuum pressure, 6–7 × 10−10 mbar; and total number of scans, at least 100. The instrument was mass calibrated in positive EMR mode using cesium iodide.

For data processing, the acquired MS spectra were visualized using Thermo Xcalibur Qual Browser (v. 4.2.47). MS spectra deconvolution was performed either manually or with UniDec v. 4.2.0 (Marty et al., 2015; Reid et al., 2018). The UniDec parameters used included m/z range, 1500–5000; mass range, 20,000–100,000 Da; peak shape function, Gaussian; and smooth charge state distribution, on.

From their primary sequences, the expected masses for the proteins are TACANWT monomer: 41,770 Da, TACANWT dimer: 83,539 Da, TACANH196A H197A monomer: 41,637 Da, and TACANH196A H197A dimer: 83,275 Da. Experimental masses were determined as the average mass± standard deviation (SD) across all the calculated mass values in the relevant peak series (n ≥ 5 charge states) with typical SDs of ±1 Da.

Enzymatic activity assay to measure coenzyme A release

Coenzyme A releasing activity was measured using a fluorescence-based coupled-enzyme assay (Sigma-Aldrich, Cat. MAK034) in a 96-well microplate (Costar) at 37°C. The reaction was monitored with an Infinite-M1000 spectrofluorometer (Tecan) with 535 nm excitation and 587 nm emission. The reconstituted proteoliposomes of ELOVL7 and TACAN at 1:50 protein-to-lipid ratio were used (10 µg total protein), supplemented with 100 µM malonyl-CoA (Sigma-Aldrich, Cat. M4263) and 50 µM stearoyl-CoA (Sigma-Aldrich, Cat. S0802). Reaction mixtures were incubated at 37°C for 0, 0.5, 1, 2, 4, 8, 24, and 48 hr, frozen in liquid N2, and stored at –80°C. The mixtures were then centrifuged at 20,817 g for 10 min, and supernatants were used to perform the enzymatic assay following the manufacturer’s protocol.

Acknowledgements

We thank Mark Ebrahim, Johanna Sotiris, and Honkit Ng at the Evelyn Gruss Lipper Cryo-EM Resource Center at Rockefeller University for assistance in data collection; Dr. Chia-Hsueh Lee (St. Jude Children’s Research Hospital) for critical reading of the manuscript and suggestions for image analysis; Dr. Yixiao Zhang (Interdisciplinary Research Center on Biology and Chemistry) for advice and help on data collection; and members of the MacKinnon lab and Chen lab (Rockefeller University) for assistance. This work was supported in part by GM43949 (to RM) and GM109824 and GM103314 (to BTC). RM is an investigator in the Howard Hughes Medical Institute.

Funding Statement

The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication.

Contributor Information

Roderick MacKinnon, Email: mackinn@mail.rockefeller.edu.

Kenton J Swartz, National Institute of Neurological Disorders and Stroke, National Institutes of Health, United States.

Kenton J Swartz, National Institute of Neurological Disorders and Stroke, National Institutes of Health, United States.

Funding Information

This paper was supported by the following grants:

  • National Institutes of Health GM43949 to Roderick MacKinnon.

  • National Institutes of Health GM109824 to Brian Chait.

  • National Institutes of Health GM103314 to Brian Chait.

  • Howard Hughes Medical Institute to Roderick MacKinnon.

  • Rockefeller University to Hanan Alwaseem.

Additional information

Competing interests

none.

None.

Author contributions

Data curation, Formal analysis, Investigation, Methodology, Software, Writing – review and editing, Writing – review and editing.

Data curation, Investigation, Methodology, Funding acquisition, Software, Writing – review and editing, Writing – review and editing.

Data curation, Investigation, Methodology, Software, Writing – review and editing, Writing – review and editing.

Data curation, Investigation, Methodology, Writing – review and editing.

Data curation, Formal analysis, Methodology, Writing – review and editing.

Project administration, Investigation, Supervision, Funding acquisition, Validation, Writing – review and editing, Resources.

Project administration, Supervision, Validation, Resources, Writing – review and editing, Supervision, Investigation, Funding acquisition, Writing – review and editing.

Additional files

Transparent reporting form

Data availability

The B-factor sharpened 3D cryo-EM density map and atomic coordinates of wild-type TACAN have been deposited in Worldwide Protein Data Bank (wwPDB) under accession codes 7N0K and EMD-24107. The B-factor sharpened 3D cryo-EM density map and atomic coordinates of His196Ala, His197Ala mutant TACAN have been deposited in Worldwide Protein Data Bank (wwPDB) under accession codes 7N0L and EMD-24108.

The following dataset was generated:

Niu Y, Tao X, MacKinnon R. 2021. Cryo-EM structure of TACAN in the apo form (TMEM120A) RCSB Protein Data Bank. 7N0K

Niu Y, Tao X, MacKinnon R. 2021. Cryo-EM structure of TACAN in the apo form (TMEM120A) Electron Microscopy Data Bank. EMD-24107

Niu Y, Tao X, MacKinnon R. 2021. Cryo-EM structure of TACAN in the H196A H197A mutant form (TMEM120A) RCSB Protein Data Bank. 7N0L

Niu Y, Tao X, MacKinnon R. 2021. Cryo-EM structure of TACAN in the H196A H197A mutant form (TMEM120A) Electron Microscopy Data Bank. EMD-24108

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Decision letter

Editor: Kenton J Swartz1
Reviewed by: Sudha Chakrapani2

In the interests of transparency, eLife publishes the most substantive revision requests and the accompanying author responses.

Acceptance summary:

Tmem120a/TACAN was recently identified as a potential mechanosensitive ion channel that mediates mechanical pain. Here, the authors use electrophysiology and structural biology to show that Tmem120a/TACAN does not display mechanically activated currents and does not structurally resemble an ion channel. Instead, this work shows that Tmem120a has structurally homology to a fatty acid elongase, ELOVL thus providing clues to this molecule's true physiological function.

Decision letter after peer review:

Thank you for submitting your article "Analysis of the Mechanosensor Channel Functionality of TACAN" for consideration by eLife. Your article has been reviewed by 3 peer reviewers, one of whom is a member of our Board of Reviewing Editors, and the evaluation has been overseen by Kenton Swartz as the Senior Editor. The following individual involved in the review of your submission as agreed to reveal their identity: Sudha Chakrapani (Reviewer #3).

The reviewers have discussed their reviews with one another, and the Reviewing Editor has drafted this to help you prepare a revised submission. Overall, this is a timely and well-executed study that convincing shows that Tacan/Tmem120a is not a mechanosensitive ion channel. The reviewers reached the consensus that the following minor revisions are needed before publication:

1) Please provide an expanded introduction (see comments by reviewer #1).

2) Please provide more details/quantification of the electrophysiology data (see detailed comments by reviewer #2).

3) Additionally, if you have the data, all reviewers felt seeing other examples of heterogeneous conductance produced when different (non-ion channel) membrane molecules are reconstituted? This would support the general idea of that recordings reflect non-specific disruptions of the lipids by high concentrations of membrane proteins.

4) Please explain the rationale behind the His mutations a bit more (see comment by reviewer #3).

5) Although not required, please consider expanding the discussion to include any thoughts about the physiological role of Tacan/Tmem120a in nociceptors (see comment by reviewer #1).

Reviewer #1:

Tmem120a was initially identified as a nuclear localized enzyme important for adipocyte differentiation and lipogenesis. More recently this molecule was proposed to function as a mechanically-gate ion channel important for the transduction of mechanical forces in nociceptors. That study provides two key pieces of evidence to support their conclusions: that expression of Tmem120a was sufficient to endow cells with mechanically gated currents and hat this function could be reconstitution cell-free in proteo-liposomes. These data are challenged in the current study from Niu and colleagues. They are unable reproduce these findings and instead propose the high concentrations of Tmem120a cause heterogeneous membrane disruptions that do not resemble any known type of ion channel gated. More importantly, they have solved the cryo-EM structure which convincing shows that Tmem120a is not an ion channel but instead has homology to a fatty acid elongase, ELOVL. Other interesting features of the structure include the binding pocket for coenzyme-A co-factor that may be helpful to future work focused on better elucidated the physiological function of this molecule. Overall, the manuscript is very straight forward and convincing. Future work is need to elucidate the functional role(s) Tmem120a may play in pain sensation and other physiological processes.

Specific comments to the authors:

Because I am not an expert in cryo-EM, I am focusing my comments on the background, context and electro-physiological data:

The authors need to expand the introduction, although this is a short report, more context is needed as to what was previously known/unknown about Tmem120a. Perhaps, it would be helpful to include some information about the other molecules proposed to be involved in mechano-sensation and perhaps the criteria previously used to support various claims.

It would have been nice to include some positive control data from a bonafide mechanosensitive ion channel, if the authors already have some.

Do other transmembrane proteins (that are not ion channels) also cause non-selective conductances when reconstituted?

Although it is clear that Tacan is not an ion channel, the authors have not addressed the in vitro or behavioral aspects of the Beaulieu-Laroche study. Is it possible that the enzymatic activity of Tmem120a could indirectly effect mechano-transduction?

Reviewer #2:

A recent study had identified TACAN (TMEM120A) as a putative mechanosensitive channel with important physiological implications. In order to better understand the mechanisms underlying the mechanosensitivity of TACAN, the authors set out to functionally and structurally characterize this ion channel. Surprisingly, they were unable to detect mechanically activated currents in cellular and reconstituted contexts. The authors also used cryo-electron microscopy to determine a structure of TACAN. Rather than resembling an ion channel, the protein shared structural homology to a fatty acid elongase (ELOVL7). The authors found density for coenzyme-A in the homologous catalytic cleft and confirmed the identity of the ligand using mass spectrometry.

The results in this study clearly support the authors' claim that TACAN is not a mechanosensitive ion channel. Structural biology is a particularly useful tool in this case as the structural similarity to ELOVL7 and the presence of a similar coenzyme in the catalytic site are striking. The functional data appear to be convincing as well; however, only representative traces are shown, and more rigorous quantification of the data is necessary.

Overall, this study is of great importance to the field. The discovery and molecular characterization of bonafide mechanosensitive channels is of high interest due to the relative lack of identified proteins. This report succinctly and convincingly demonstrates that this proposed candidate is likely not actually a mechanically activated channel.

As presented, the manuscript concisely makes a strong case that TACAN is not actually a mechanosensitive channel. As stated in the public review, this kind of study is very important for the field to corroborate the findings of a high-profile publication. Although there are a few experiments that could potentially strengthen the study, these are likely not necessary for the main point of the paper. However, prior to acceptance of this paper in eLife, quantitative analyses of the electrophysiology data are necessary.

– In figure 1, it would be good to show open probability values of all traces in the presence and absence of pressure to convincingly show that there is no significant change in activation in response to mechanical stimuli.

– The representative traces in figure 2 do suggest that currents are heterogeneous in amplitude as stated in the text; however, amplitude histograms for single channel data would be helpful to further illustrate this point.

– IV curves in figures 2C and 2F appear to be based on n of 1. If this is the case, a higher n is necessary. It might also be helpful to clearly state in the text that the shift in reversal potential indicates that the larger NMDG cation is less permeable through the proposed "leaky membrane".

– In figure panels 2A and 2D, it would be helpful to indicate where in the trace "closed" is indicated, since it appears that some of the traces have inward and outward currents.

Other experiments to consider, but are not necessary for acceptance:

– Reconstituting other membrane proteins that are known to not be ion channels (such as the M2R control used in HEK and CHO) into GUVs at similarly high concentrations that caused leaks in TACAN would help determine whether the leaky membrane effect is TACAN-specific.

– The orientation of the protein with respect to the cytoplasm (line 62) could be determined with a tagging approach in cells.

Reviewer #3:

The manuscript by Niu et al., reports the functional properties and molecular architecture of TMEM120A/TACAN. TMEM120A has generated enormous interest due to its potentially divergent functional roles. It was originally identified as a transmembrane nuclear envelope protein expressed in adipocytes, and involved in fat metabolism. More recently, renamed as TACAN, TMEM120A was reported to be a mechanically-activated ion channel expressed in a subset of nociceptor neurons, and a potential therapeutic target in the management of chronic pain. The latter study showed slowly adapting, stretch-induced currents in excised patches from cells heterologously expressing TACAN as well as from reconstituted liposomes containing purified TACAN. Some intriguing behavior of TACAN were that there was no response to poking-stimulation in whole-cell configuration and the currents had markedly different single-channel conductance in cells vs liposomes.

In this work, the authors carried out electrophysiological recordings under similar conditions as in the previous study. Contrary to the previous findings, here they did not observe any stretch-induced currents in excised patches from cells (CHO and HEK) expressing TACAN or in patches from reconstituted proteoliposomes. Under conditions of very high protein-to-lipid ratio, highly heterogenous single-channel behavior were observed that were non-selective to permeant ions and unresponsive to mechanical stimuli. The authors conclude that these currents are likely due to membrane leak artifacts. Although in the previous study it was reported that currents (including those from liposomes) were blocked by Gd3+ and GsMTx4 toxin.

The authors solved cryo-EM structures of TACAN WT and a double mutant (to improve the ligand density) which revealed a dimeric architecture, with each monomer consisting of 6 TMs and two N-terminal helices that form a coiled-coil. Overall, the architecture bears close resemblance to ELOVL7, a member of the long chain fatty acid elongase family. With elaborate native mass-spec analysis, they establish the identity of the co-purified ligand (coenzyme-A) and its stoichiometry.

Overall, this is a well-executed study and a timely report.

eLife. 2021 Aug 10;10:e71188. doi: 10.7554/eLife.71188.sa2

Author response

The reviewers have discussed their reviews with one another, and the Reviewing Editor has drafted this to help you prepare a revised submission. Overall, this is a timely and well-executed study that convincing shows that Tacan/Tmem120a is not a mechanosensitive ion channel. The reviewers reached the consensus that the following minor revisions are needed before publication:

1) Please provide an expanded introduction (see comments by reviewer #1).

We have expanded the Introduction to provide additional background. We also revised the Discussion to address an issue raised in the review. In our opinion the electrophysiological experiments provide the primary evidence that TACAN is not an MSC and the structural experiments lend support to that conclusion. In the revised discussion we explain this view.

2) Please provide more details/quantification of the electrophysiology data (see detailed comments by reviewer #2).

The absence of a discernable response to patch pressurization is so clear that open probability graphs would look strange in our opinion. Papers often publish processed versions of data with insufficient primary data. Here we show primary data. For the same reason we do not think that amplitude histograms, Fourier transforms or any other such analysis will add to the obvious and only conclusion we are trying to make, that we do not see mechanically activated currents.

We removed the I-V curves in Figure 2 because they are unnecessary. We have carried these out many times and thought it worth showing an example of a conductance (not mechanosensitive) that exhibits almost no selectivity (weak selectivity for a metal cation over a larger organic cation). Now we realize this will serve a distraction, especially if we provide a table with statistics on this weak selectivity. Those who use techniques of ion channel reconstitution will recognize the conductance behavior we show here as “junk channels”.

On page 3 (lines 60-62) we added a sentence on TRAAK channels in GUVs using the same methods.

3) Additionally, if you have the data, all reviewers felt seeing other examples of heterogeneous conductance produced when different (non-ion channel) membrane molecules are reconstituted? This would support the general idea of that recordings reflect non-specific disruptions of the lipids by high concentrations of membrane proteins.

We do not have data on whether other proteins produce heterogeneous conductance in reconstituted membranes. We do not know whether they do.

4) Please explain the rationale behind the His mutations a bit more (see comment by reviewer #3).

A sentence was added to page 4 (lines 94-95) explaining the rationale behind the histidine to alanine mutations.

5) Although not required, please consider expanding the discussion to include any thoughts about the physiological role of Tacan/Tmem120a in nociceptors (see comment by reviewer #1).

It is possible that TACAN/Tmem120a affects nociceptor function. We did not study this.

Associated Data

    This section collects any data citations, data availability statements, or supplementary materials included in this article.

    Data Citations

    1. Niu Y, Tao X, MacKinnon R. 2021. Cryo-EM structure of TACAN in the apo form (TMEM120A) RCSB Protein Data Bank. 7N0K
    2. Niu Y, Tao X, MacKinnon R. 2021. Cryo-EM structure of TACAN in the apo form (TMEM120A) Electron Microscopy Data Bank. EMD-24107
    3. Niu Y, Tao X, MacKinnon R. 2021. Cryo-EM structure of TACAN in the H196A H197A mutant form (TMEM120A) RCSB Protein Data Bank. 7N0L
    4. Niu Y, Tao X, MacKinnon R. 2021. Cryo-EM structure of TACAN in the H196A H197A mutant form (TMEM120A) Electron Microscopy Data Bank. EMD-24108

    Supplementary Materials

    Transparent reporting form
    elife-71188-transrepform1.docx (111KB, docx)

    Data Availability Statement

    The B-factor sharpened 3D cryo-EM density map and atomic coordinates of wild-type TACAN have been deposited in Worldwide Protein Data Bank (wwPDB) under accession codes 7N0K and EMD-24107. The B-factor sharpened 3D cryo-EM density map and atomic coordinates of His196Ala, His197Ala mutant TACAN have been deposited in Worldwide Protein Data Bank (wwPDB) under accession codes 7N0L and EMD-24108.

    The following dataset was generated:

    Niu Y, Tao X, MacKinnon R. 2021. Cryo-EM structure of TACAN in the apo form (TMEM120A) RCSB Protein Data Bank. 7N0K

    Niu Y, Tao X, MacKinnon R. 2021. Cryo-EM structure of TACAN in the apo form (TMEM120A) Electron Microscopy Data Bank. EMD-24107

    Niu Y, Tao X, MacKinnon R. 2021. Cryo-EM structure of TACAN in the H196A H197A mutant form (TMEM120A) RCSB Protein Data Bank. 7N0L

    Niu Y, Tao X, MacKinnon R. 2021. Cryo-EM structure of TACAN in the H196A H197A mutant form (TMEM120A) Electron Microscopy Data Bank. EMD-24108

    Articles from eLife are provided here courtesy of eLife Sciences Publications, Ltd

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