Tight coupling of rubidium conductance and inactivation in human KCNQ1 potassium channels.

KCNQ1 K+ channels in humans are important for repolarization of cardiac action potentials and for K+ secretion in the inner ear. The pore-forming channel subunits form heteromeric complexes with small regulatory subunits of the KCNE family, in particular with KCNE1 to form channels that conduct a slow delayed rectifier K+ current, IKs. This association leads to alteration of biophysical properties, including a slowing of activation, a suppression of inactivation and an increase of the apparent single-channel conductance. In addition, inward Rb+ currents conducted by homomeric KCNQ1 channels are about threefold larger than K+ currents, whereas heteromeric KCNQ1-KCNE1 channels have smaller inward Rb+ currents compared to K+ currents. We determined inactivation properties and compared K+ vs. Rb+ inward currents for channels formed by co-assembly of KCNQ1 with KCNE1, KCNE3 and KCNE5, and for homomeric KCNQ1 channels with point mutations in the pore helix S5 or S6 transmembrane domains. Several of the channels with point mutations eliminated the apparent inactivation of KCNQ1, as described previously (Seebohm et al. 2001). We found that the extent of inactivation and the ratio of Rb+/K+ currents were positively correlated. Since the effect of Rb+ on the current size has been shown previously to be related to a fast 'flickery' process, our results suggest that inactivation of KCNQ1 channels is related to a fast flicker of the open channel. A kinetic model incorporating two open states, no explicit inactivated state and a fast flicker that is different for the two open states is able to account for the apparent inactivation and the correlation of inactivation and large Rb+ currents. We conclude that an association between KCNQ1 and KCNE subunits or removal of inactivation by mutation of KCNQ1 stabilizes the open conformation of the pore principally by altering an interaction between the pore helix and the selectivity filter and with S5/S6 domains.