Trapping of a methanesulfonanilide by closure of the HERG potassium channel activation gate

Deactivation of voltage-gated potassium (K+) channels can slow or prevent t he recovery from block by charged organic compounds, a phenomenon attribute d to trapping of the compound within the inner vestibule by closure of the activation gate. Unbinding and exit from the channel vestibule of a positiv ely charged organic compound should be favored by membrane hyperpolarizatio n if not impeded by the closed gate. MK-499, a methanesulfonanilide compoun d, is a potent blocker (IC50 = 32 nM) of HERG K+ channels. This bulky compo und (7 x 20 Angstrom) is positively charged at physiological pH. Recovery f rom block of HERG channels by MK-499 and other methanesulfonanilides is ext remely slow (Carmeliet, 1992; Ficker et al., 1998), suggesting a trapping m echanism. We used a mutant HERG (D540K) channel expressed in Xenopus oocyte s to test the trapping hypothesis. D540K HERG has the unusual property of o pening in response to hyperpolarization, in addition to relatively normal g ating and channel opening in response to depolarization (Sanguinetti and Xu , 1999). The hyperpolarization-activated state of HERG was characterized by long bursts of single channel reopening. Channel reopening allowed recover y from block by 2 mu M MK-499 to occur with time constants of 10.5 and 52.7 s at -160 mV. In contrast, wild-type HERG channels opened only briefly aft er membrane hyperpolarization, and thus did not permit recovery from block by MK-499. These findings provide direct evidence that die mechanism of slo w recovery from HERG channel block by methanesulfonanilides is due to trapp ing of the compound in the inner vestibule by closure of the activation gat e. The ability of HERG channels to trap MK-499, despite its large size, sug gests that the vestibule of this channel is larger than the well studied Sh aker K+ channel.