Mechanistic Insight into hERG Channel Deactivation Gating from the Solution Structure of the eag Domain

Abstract

hERG (Kv11.1) potassium channels deactivate slowly and this is critical for regulating the time course and amplitude of repolarising current during the cardiac action potential. Inherited mutations that alter deactivation and reduce hERG current are linked to long QT syndrome, a potentially lethal type of cardiac arrhythmia. The N-terminus is known to have a key role in regulating the slow deactivation and its removal dramatically accelerates deactivation. However, the precise molecular mechanism remains poorly understood. Deletion of residues 2 to 26 accelerates deactivation to a similar extent as removing the whole of the N-terminus. Slow deactivation can be restored to the N-truncated channel by the addition of a soluble peptide corresponding to residues 2-16. Residues 1 - 135 form the ‘eag domain’ which is conserved in the ether-a-go-go family of potassium channels. A crystal structure for part of the eag domain (Arg27 - Lys135) of hERG has been solved and shown to contain a PAS domain, however the functionally critical first 26 residues (NT1-26) were not resolved. Here we present an NMR solution structure of the whole of the eag domain. This new structure reveals a previously undiscovered amphipathic α-helix from residues Gln11 to Gly24, while the first 10 residues are extended and highly dynamic. Electrophysiological and mutagenesis data show that the helix is functionally important. Breaking it by Pro substitution accelerated deactivation to rates similar to the N-truncated channel. One face of NT1-26 is positively charged. Alanine substitution of basic residues (R4A:R5A and R20A:K21A) also profoundly accelerates hERG current deactivation. Our results indicate that slow deactivation of hERG involves NT1-26 binding to the channel via electrostatic interactions that stabilise the open conformation of the pore.