REGULATION OF DEACTIVATION BY AN AMINO-TERMINAL DOMAIN IN HUMAN ETHER-A-GO-GO-RELATED GENE POTASSIUM CHANNELS

Abnormalities in repolarization of the cardiac ventricular action pote ntial can lead to life-threatening arrhythmias associated with long QT syndrome. The repolarization process depends upon the gating properti es of potassium channels encoded by the human ether-a-go-go-related ge ne (HERG), especially those governing the rate of recovery from inacti vation and the rate of deactivation. Previous studies have demonstrate d that deletion of the NH2 terminus increases the deactivation rate, b ut the mechanism by which the NH2 terminus regulates deactivation in w ild-type channels has not been elucidated. We tested the hypothesis th at the HERG NH2 terminus slows deactivation by a mechanism similar to N-type inactivation in Shaker channels, where it binds to the internal mouth of the pore and prevents channel closure. We found that the reg ulation of deactivation by the HERG NH, terminus bears similarity to S haker N-type inactivation in three respects: (a) deletion of the NH2 t erminus slows C-type inactivation; (b) the action of the NH2 terminus is sensitive to elevated concentrations of external K+, as if its bind ing along the permeation pathway is disrupted by K+ influx; and (c) N- ethylmaleimide, covalently linked to an aphenotypic cysteine introduce d within the S4-S5 linker, mimics the N deletion phenotype, as if the binding of the NH2 terminus to its receptor site were hindered. In con trast to N-type inactivation in Shaker, however, there was no indicati on that the NH2 terminus blocks the HERG pore. In addition, we discove red that separate domains within the NH2 terminus mediate the slowing of deactivation and the promotion of C-type inactivation. These result s suggest that the NH2 terminus stabilizes the open state and, by a se parate mechanism, promotes C-type inactivation.