SHAKER POTASSIUM CHANNEL GATING .2. TRANSITIONS IN THE ACTIVATION PATHWAY

Voltage-dependent gating behavior of Shaker potassium channels without N-type inactivation (ShB Delta 6-46) expressed in Xenopus oocytes was studied. The voltage dependence of the steady-state open probability indicated that the activation process involves the movement of the equ ivalent of 12-16 electronic charges across the membrane. The sigmoidal kinetics of the activation process, which is maintained at depolarize d voltages up to at least. +100 mV indicate the presence of at least f ive sequential conformational changes before opening. The voltage depe ndence of the gating charge movement suggested that each elementary tr ansition involves 3.5 electronic charges. The voltage dependence of th e forward opening rate, as estimated by the single-channel first laten cy distribution, the final phase of the macroscopic ionic current acti vation, the ionic current reactivation and the ON gating current time course, showed movement of the equivalent of 0.3 to 0.5 electronic cha rges were associated with a large number of the activation transitions . The equivalent charge movement of 1.1 electronic charges was associa ted with the closing conformational change. The results were generally consistent with models involving a number of independent and identica l transitions with a major exception that the first closing transition is slower than expected as indicated by tail current and OFF gating c harge measurements.