Functional stoichiometry of Shaker potassium channel inactivation.

Shaker potassium channels from Drosophila are composed of four identical subunits. The contribution of a single subunit to the inactivation gating transition was investigated. Channels carrying a specific mutation in a single subunit can be labeled in a heterogeneous population and studied quantitatively with scorpion toxin sensitivity as a selection tag. Linkage within a single subunit of a mutation that removes the inactivation gate to a second mutation that affects scorpion toxin sensitivity demonstrates that only a single gate is necessary to produce inactivation. The inactivation rate constant for channels with a single gate was one-fourth that of channels with four gates. In contrast, the rate of recovery from inactivation was independent of the number of gates. It appears that each of the four open inactivation gates in a Shaker potassium channel is independent, but only one of the four gates closes in a mutually exclusive manner.