Gating kinetics and underlying thermodynamic properties of human ether-a-go-go-related gene (HERG) K(+) channels expressed in Xenopus oocytes were studied using protocols able to yield true steady-state kinetic parameters. Channel mutants lacking the initial 16 residues of the amino terminus before the conserved eag/PAS region showed significant positive shifts in activation voltage dependence associated with a reduction of z(g) values and a less negative DeltaG(o), indicating a deletion-induced displacement of the equilibrium toward the closed state. Conversely, a negative shift and an increased DeltaG(o), indicative of closed-state destabilization, were observed in channels lacking the amino-terminal proximal domain. Furthermore, accelerated activation and deactivation kinetics were observed in these constructs when differences in driving force were considered, suggesting that the presence of distal and proximal amino-terminal segments contributes in wild-type channels to specific chemical interactions that raise the energy barrier for activation. Steady-state characteristics of some single point mutants in the intracellular loop linking S4 and S5 helices revealed a striking parallelism between the effects of these mutations and those of the amino-terminal modifications. Our data indicate that in addition to the recognized influence of the initial amino-terminus region on HERG deactivation, this cytoplasmic region also affects activation behavior. The data also suggest that not only a slow movement of the voltage sensor itself but also delaying its functional coupling to the activation gate by some cytoplasmic structures possibly acting on the S4-S5 loop may contribute to the atypically slow gating of HERG.

Adamantane derivative IEM-1676 (Ad-N+H2-(CH2)5-N+Me3) causes open-channel block of Ca2+ permeable AMPA receptors when applied externally, but internal application results in both closed- and open-channel block. The relationships between blocking action of externally and internally applied IEM-1676 were studied using patch clamp technique. Extracellular action of IEM-1676 is decreased by its intracellular application, thus indicating overlap of the binding site at external and internal application. We demostrated that internal closed-channel block is voltage-dependent and occurs at the site where externally applied drug is trapped. We conclude that the selectivity filter of the closed AMPA receptor channel is not occluded and remains permeable for organic cations.
J Mol Neurosci. 39(1-2): 169-74 (2009)