Physical determinants of strong voltage sensitivity of [K.sup.+] channel block

Strong voltage sensitivity of inward-rectifier [K.sup.+] (Kir) channels has been hypothesized to arise primarily from an intracellular blocker displacing up to five [K.sup.+] ions from the wide, intracellular part of the ion conduction pore outwardly across the narrow ion-selectivity filter. The validity of this hypothesis depends on two assumptions: (i) that five ion sites are located intracellular to the filter and (ii) that the blocker can force essentially unidirectional [K.sup.+] movement in a pore region generally wider than the combined dimensions of the blocker plus a [K.sup.+] ion. Here we present a crystal structure of the cytoplasmic portion of a Kir channel with five ions bound and demonstrate that a constriction near the intracellular end of the pore, acting as a gasket, prevents [K.sup.+] ions from bypassing the blocker. This heretofore unrecognized 'gasket' ensures that the blocker can effectively displace [K.sup.+] ions across the selectivity filter to generate exceedingly strong voltage sensitivity.