Structure of the C-Terminal Region of a KCNH Channel

Abstract

The KCNH family of ion channels comprises Ether-a -go-go (EAG), EAG-related gene (ERG), and EAG-like (ELK) K+ channel subfamilies. KCNH channels are best known for their regulatory function in tumor progression, repolarization of the cardiac action potential and regulation of neuronal excitability. The C-terminus of KCNH channels contains a cyclic nucleotide-binding homology domain (CNBHD) coupled to the pore with the C-linker. Mutations in the C-linker/CNBHD affect KCNH channel function and, in ERG channels, can lead to Long-QT Syndrome. The CNBHD shares sequence similarity with the cyclic nucleotide-binding domain (CNBD) of hyperpolarization-activated cyclic nucleotide-modulated (HCN) and cyclic nucleotide-gated (CNG) channels. However, unlike the HCN and CNG channels that are regulated by direct binding of cyclic nucleotides to the CNBDs, KCNH channels are not directly regulated by cyclic nucleotides and their CNBHDs do not bind cyclic nucleotides. The structural basis of the regulation of the KCNH channels by the C-linker/CNBHD and the failure of cyclic nucleotides to regulate the channels are not clear. Here we present the crystal structure of the C-linker/CNBHD of zebrafish ELK (zELK) channels at 2.2 A resolution. The structure revealed unique features of the KCNH channels including: 1) a negatively charged electrostatic profile that explains the lack of binding and regulation of ion channels in the KCNH family by cyclic nucleotides, 2) intersubunit interactions that produce a two-fold symmetry in the C-terminal region as opposed to the four-fold symmetry of HCN channels, and 3) a novel β-strand occupying the pocket where cyclic nucleotides bind to HCN channels. Mutations in the β-strand shifted the half-maximal activation voltage by ∼15 mV, implicating the β-strand as a regulatory element in KCNH channels. These findings provide a structural framework for understanding the regulation of ion channels in the KCNH family by the C-linker/CNBD.