SHAKER POTASSIUM CHANNEL GATING .1. TRANSITIONS NEAR THE OPEN STATE

Kinetics of single voltage-dependent Shaker potassium channels express ed in Xenopus oocytes were studied in the absence of fast N-type inact ivation. Comparison of the single-channel first latency distribution a nd the time course of the ensemble average current showed that the act ivation time course and its voltage dependence are largely determined by the transitions before first opening. The open dwell time data are consistent with a single kinetically distinguishable open state. Once the channel opens, it can enter at least two closed states which are n ot traversed frequently during the activation process. The rate consta nts for the transitions among these closed states and the open state a re nearly voltage-independent at depolarized voltages (> -30 mV). Duri ng the deactivation process at more negative voltages, the channel can close directly to a dosed state in the activation pathway in a voltag e-dependent fashion.