Figure 8.

Model and relevance of S3-S4 linker length and the relaxed state. (A) We provide evidence that shortening the S3-S4 linker in Shaker K⁺ channels increases the stability of voltage sensors in the relaxed state. The model shows long, WT S3-S4 linker (black), intermediate linker (dark gray), and short linker (light gray). During derelaxation, the shorter linker acts as a mechanical constraint, slowing the voltage sensor as it moves from the relaxed to the resting state, where the distance between S3 and S4 is larger. Additionally, shortening the linker may increase the rate at which voltage sensors enter the relaxed state. During relaxation, the shorter linker may reduce the degrees of freedom available to the voltage sensor, speeding the transition from the resting to the relaxed state. As this could be occurring due to destabilization of the active state, reduction of the barrier between the active and relaxed state, or both, these changes are denoted using dashed lines. (B) The leftward Q-V shift obtained upon derelaxation of short linker Shaker constructs. N is between 3 and 8. (C) Sequence homology of the S3-S4 linker of Shaker and other voltage-gated ionic channels. Shaded in light gray are apparently conserved negatively charged residues in the N-terminal portion of the linker. (D and E) The activation Q-V_1/2 from voltage-gated potassium channels (4,37–42) is shown to have no obvious correlation with linker length, similar to what was found with mutant Shaker constructs in Fig. 1, D. Similar to B, the leftward Q-V shift obtained upon derelaxation of voltage-gated ionic channels taken from the literature (4,37,43–45). The linker length of Nav is the average from the four domains of the sodium channel from Doryteuthis opalescens; the linker length of Cav1.2 is the average of the S3-S4 linker length from the first three domains from Cavia porcellus, as the fourth likely undergoes splicing of an undetermined amount in this model organism.