LOSS OF SHAKER K CHANNEL CONDUCTANCE IN 0 K- ROLE OF THE VOLTAGE SENSOR( SOLUTIONS )

In potassium-free solutions some types of K channels enter a long-last ing nonconducting or ''defunct'' state. It is known that Shaker K chan nels must open in K+-free solutions to become defunct. Gating-current studies presented here indicate an abnormal conformation in the defunc t state that restricts S4 movement and alters its kinetics. Thus an ab normality initiated in the P region spreads to the gating apparatus. W e find that channels most readily become defunct on repolarization to an intermediate voltage, thus prolonging occupancy of one of the sever al intermediate closed states. The state dependence of becoming defunc t was further dissected by using the gating mutant L382A. Simply closi ng this channel at 0 mV (reversing the last activation step) does not make the mutant channel defunct. Instead, It is necessary to move furt her left (more fully closed) in the activation sequence. this was conf irmed with ShlR experiments showing that channels become defunct only if there is inward gating charge movement. Rapid transit through the i ntermediate states,achieved at very negative voltage, is relatively in effective at making channels defunct. Several mutations that removed C -type inactivation also made the channels resistant to becoming defunc t. Our results show that normal gating current cannot be stably record ed in the absence of K+.