The inactivation gate of the Shaker K+ channel behaves like an open-channel blocker

Following voltage-dependent activation, Drosophila Shaker K+ channels enter a nonconducting, inactivated state. This process has been proposed to occur by a "ball-and-chain" mechanism, in which the N-terminus of the protein behaves like a blocker tethered to the cytoplasmic side of the channel and directly occludes the pore to cause inactivation. To complement the ample evidence for the involvement of the N-terminus, we sought evidence that it blocks the pore directly. We found that inactivation exhibits several distinctive properties of pore blockade. First, recovery was speeded by increased external K+ concentrations, just as blockade can be relieved by trans-permeant ions. Second, single-channel experiments show that the channel reopens from the inactivated state upon repolarization. These openings were usually required for recovery, as though the blocking particle must exit the pore before the channel can close.