A Role for the S0 Transmembrane Segment in Voltage-dependent Gating of BK Channels

BK (Maxi-K) channel activity is allosterically regulated by a Ca²⁺ sensor, formed primarily by the channel's large cytoplasmic carboxyl tail segment, and a voltage sensor, formed by its transmembrane helices. As with other voltage-gated K channels, voltage sensing in the BK channel is accomplished through interactions of the S1–S4 transmembrane segments with the electric field. However, the BK channel is unique in that it contains an additional amino-terminal transmembrane segment, S0, which is important in the functional interaction between BK channel α and β subunits. In this study, we used perturbation mutagenesis to analyze the role of S0 in channel gating. Single residues in the S0 region of the BK channel were substituted with tryptophan to give a large change in side chain volume; native tryptophans in S0 were substituted with alanine. The effects of the mutations on voltage- and Ca²⁺-dependent gating were quantified using patch-clamp electrophysiology. Three of the S0 mutants (F25W, L26W, and S29W) showed especially large shifts in their conductance–voltage (G-V) relations along the voltage axis compared to wild type. The G-V shifts for these mutants persisted at nominally 0 Ca²⁺, suggesting that these effects cannot arise simply from altered Ca²⁺ sensitivity. The basal open probabilities for these mutants at hyperpolarized voltages (where voltage sensor activation is minimal) were similar to wild type, suggesting that these mutations may primarily perturb voltage sensor function. Further analysis using the dual allosteric model for BK channel gating showed that the major effects of the F25W, L26W, and S29W mutations could be accounted for primarily by decreasing the equilibrium constant for voltage sensor movement. We conclude that S0 may make functional contact with other transmembrane regions of the BK channel to modulate the equilibrium between resting and active states of the channel's voltage sensor.