Molecular Determinants of Voltage-Dependent Gating in hERG Potassium Channels

Abstract

Shaker channel voltage sensor activation is rapid and pore opening is limited by concerted rearrangements of the intracellular gate. In contrast, hERG channel activation and deactivation gating kinetics are slow and appear to be limited by movement of the S4 voltage sensor. The mechanistic basis for these slow gating kinetics remains unclear. Recent findings in Shaker suggest that interaction of positive S4 charges (i.e. R1, K5) with a gating charge transfer centre, formed by F290 and E293 in S2 and D316 in S3 regulate voltage-dependent S4 movement and gating. Whether such a transfer centre exists in hERG channels has not been tested. Although the relevant residues are somewhat conserved, the positive charges are reversed, i.e. K1 and R5, compared to R1 and K5 in Shaker. Moreover, E293 is D466 in hERG. We investigated whether these differences in the nature of the charges contribute to the unique gating characteristics of hERG. The hERG K525R (K1R) mutation left-shifted the G-V relationship by ∼50mV. This is consistent with results in Shaker and suggest that the closed state is stabilized in hERG channels as a result of the presence of K1 rather than R1. hERG D466E alone had no effect on the G-V relation, but the mutation restored WT-like gating to K1R channels, suggesting that the introduced glutamate modifies the stability of the resting state of the voltage sensor. The R537K (R5K) mutation stabilized the closed state. This is in contrast to what the Shaker model would predict and suggests that the open state of the channels may be stabilized by different interactions within or beyond the charge transfer centre. Taken together, these findings suggest that charge-conserving sequence divergence in the voltage sensing unit contributes to the unique gating properties of hERG channels.