Closed State Structure of the Pore Revealed by Uncoupled Shaker K+ Channel.

Voltage gated potassium (Kv) channels play key roles in physiological processes, from cellular excitability to immune response and are among the most important pharmaceutical targets. Despite recent advances in the structural determination of Kv channels, the closed state structure of strictly coupled Kv1 family remains elusive. Here, we captured the structure of Shaker potassium channel with a closed pore by uncoupling its voltage sensor domains from the pore domains. Structural determination of the uncoupled I384R mutant by single particle cryoEM revealed a fully closed pore in the presence of activated, non-relaxed voltage sensors. Putative conformational transitions estimated from a fully open pore domain indicates a "roll and turn" movement along the length of the pore-forming S6 helices, in sharp contrast to canonical gating models based on limited movements of S6 . These rotational and translational movement place two hydrophobic residues, one at inner cavity and the other at the bundle crossing region, directly at the permeation pathway, limiting the pore radius to less than 1 Å. Surprisingly, the selectivity filter was captured in a noncanonical state, partially expanded at G446, unlike previously described dilated or pinched filter conformations. Based on the present data we propose a reinterpretation of the mechanism of activation gating for strictly coupled Kv1 channels and the strictly coupled interactions that underlie different functional states.