Role of an invariant cysteine in gating and ion permeation of the voltage-sensitive K+ channel Kv2.1.

We examined the role of two invariant cysteines, one in S2 and one in S6, of the voltage-gated K+ channel Kv2.1 (DRK1) by site-directed mutagenesis and subsequent channel expression in Xenopus oocytes. Despite the conserved nature of the side chain, substitutions in S2 were generally tolerated. Fourteen of 17 substitutions for Cys 232 in S2 resulted in voltage-sensitive K(+)-selective channels, for the most part with minor changes in voltage dependence and channel kinetics. In contrast, only 7 of 19 substitutions for Cys 393 in S6 preserved channel function. Furthermore, the side chain at this position influenced deactivation kinetics, inactivation kinetics, and ion-permeation properties. The chemical nature but not the volume of the side chain governed the rate constants of deactivation and inactivation. In contrast, changes of the volume of the side chain but not of its chemical properties correlated with changes in ion conductance. Our results indicate that the side chain at position 393 in S6 is involved in conformational changes during transitions between open and closed states and that it also contributes to the control of ion permeation.