Structural Basis of Action for a Novel hERG1 Channel Activator

Abstract

The rapid delayed rectifier K+ current (IKr) is conducted by human ether-a-go-go-related gene 1 (hERG1) channels and plays a predominant role in the repolarization of cardiac action potentials. Long QT syndrome caused by loss-of-function mutations in hERG1, or unintended blockade of hERG1 channels by commonly used drugs, can lead to severe arrhythmia and sudden death. Here, we investigated the structural basis of action of ICA-105574 (ICA), a novel hERG1 agonist previously reported to activate current by causing a large shift (>180 mV) in the V0.5 for P-type inactivation. Here we show that ICA is a partial agonist and has decreased activity in inactivation-impaired mutants of hERG1. Alanine-scanning mutagenesis of the S5, pore-helix and S6 segments of hERG1 and voltage clamp analysis of mutant channels expressed in Xenopus oocytes was used to define residues critical for current activation by ICA. The effects of 30 µM ICA were tested on 43 different mutant channels. Nine mutations attenuated ICA-mediated hERG1 activation by >10-fold. Two mutations, F557L (S5) and L622 (pore helix) completely prevented hERG1 activation by ICA. Residues identified as critical for ICA activity form a cluster and define a putative binding pocket for ICA that was corroborated by molecular docking simulations.