The influence of membrane bilayer thickness on KcsA channel activity

Atomic resolution structures have provided significant insight into the gating and permeation mechanisms of various ion channels, including potassium channels. However, ion channels may also be regulated by numerous factors, including the physiochemical properties of the membrane in which they are embedded. For example, the matching of the bilayer’s hydrophobic region to the hydrophobic external surface of the ion channel is thought to minimize the energetic penalty needed to solvate hydrophobic residues or exposed lipid tails. To understand the molecular basis of such regulation by hydrophobic matching requires examining channels in the presence of the lipid membrane. Here we examine the role of hydrophobic matching in regulating the activity of the model potassium channel, KcsA. (86)Rb(+) influx assays and single-channel recordings indicate that the non-inactivating E71A KcsA channel is most active in thin bilayers (<diC18:1PC). Bilayer thickness affects the open probability of KcsA and not its unitary conductance. Molecular dynamics simulations indicate that the bilayer can sufficiently modify its dimensions to accommodate KcsA channels without major perturbations in the protein helical packing within the nanosecond timescale. Based on experimental results and MD simulations, we present a model in which bilayer thickness influences the stability of the open and closed conformations of the intracellular gate of KcsA, with minimal impact on the stability of the selectivity filter of the non-inactivating mutant, E71A.