A side chain in S6 influences both open-state stability and ion permeation in a voltage-gated K+ channel.

Conservative substitutions of the conserved cysteine 393 (Cys393) in S6 of the voltage-gated K+ channel Kv2.1 predictably alter the stability of the open state and the conductances for K+ and Rb+. The polarity of the side chain at position 393 determines the stability of the open state, probably by interaction of S6 with the narrow part of the ion-conduction pathway; however, the substitutions at position 393 have no effect on the stability of the closed state. An increase in side-chain volume leads to greater K+ conductance; in contrast, gradual decreases in side-chain volume lead to progressively smaller K+ conductances concomitantly with larger Rb+ conductances. Although the substitutions for Cys393 alter open-state stability and ion permeation, they have no effect on block by external or internal tetraethylammonium (TEA). Our data indicate that molecular determinants that are involved in conformational transitions between the open state and the brief closed state (i.e., voltage-independent gating) and ion selectivity are located within the sphere of influence of the conserved Cys393 in S6. This region is physically separated from the voltage-controlled activation gate located on the intracellular side of the K+ channel.