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 (Cys(393)) in S6 of the voltage-gated K+ channel Kv2.1 predictably alter the stabil ity of the open state and the conductances for K+ and Rb+. The polarit y of the side chain at position 393 determines the stability of the op en state, probably by interaction of S6 with the narrow part of the io n-conduction pathway; however, the substitutions at position 393 have no effect on the stability of the closed state. An increase in side-ch ain volume leads to greater K+ conductance; in contrast, gradual decre ases in side-chain volume lead to progressively smaller K+ conductance s concomitantly with larger Rb+ conductances. Although the substitutio ns for Cys(393) alter open-state stability and ion permeation, they ha ve no effect on block by external or internal tetraethylammonium (TEA) . Our data indicate that molecular determinants that are involved in c onformational transitions between the open state and the brief closed state (i.e., voltage-independent gating) and ion selectivity are locat ed within the sphere of influence of the conserved Cys(393) in, S6. Th is region is physically separated from the voltage-controlled activati on gate located on the intracellular side of the K+ channel.