Structure of the C-terminal Region of a KCNH Channel

The KCNH family of ion channels, comprising ether-à-go-go (EAG), EAG-related gene (ERG), and EAG-like (ELK) K(+) channel subfamilies, is crucial for repolarization of the cardiac action potential(1), regulation of neuronal excitability(2), and proliferation of tumor cells(3). The C-terminal region of KCNH channels contains a cyclic nucleotide-binding homology domain (CNBHD) and C-linker that couples the CNBHD to the pore(4). The C-linker/CNBHD is essential for proper function and trafficking of ion channels in the KCNH family(5–9). However, despite the importance of the C-linker/CNBHD for the function of KCNH channels, the structural basis of ion channel regulation by the C-linker/CNBHD is unknown. Here we report the crystal structure of the C-linker/CNBHD of zebrafish ELK channels at 2.2 Å resolution. While the overall structure of the C-linker/CNBHD of zELK channels is similar to the cyclic nucleotide-binding domain (CNBD) structure of the related HCN channels(10), there are dramatic differences. Unlike the CNBD of HCN, the CNBHD of zELK displays a negatively charged electrostatic profile that explains the lack of binding and regulation of KCNH channels by cyclic nucleotides(4,11). Instead of cyclic nucleotide, the binding pocket is occupied by a short β-strand. Mutations of the β-strand shift the voltage dependence of activation to more depolarized voltages, implicating the β-strand as an intrinsic ligand for the CNBHD of zELK channels. In both zELK and HCN channels the C-linker is the site of virtually all of the intersubunit interactions in the C-terminal region. However, in the zELK structure there is a reorientation in the C-linker so the subunits form dimers instead of tetramers as observed in HCN channels. These results provide a structural framework for understanding the regulation of ion channels in the KCNH family by the C-linker/CNBHD and may guide the design of specific drugs.