The structural mechanism of KCNH-channel regulation by the eag domain

The KCNH voltage-dependent potassium channels (ether-á-go-go, EAG; EAG-related gene, ERG; EAG-like channels, ELK) are important regulators of cellular excitability(1-3) and have key roles in diseases such as cardiac long QT syndrome type 2 (LQT2)(4), epilepsy(5), schizophrenia(6) and cancer(7). The intracellular domains of KCNH channels are structurally distinct from other voltage-gated channels. The amino-terminal region contains an eag domain, which is comprised of a Per-Arnt-Sim (PAS) domain and a PAS-cap domain(8), while the carboxy-terminal region contains a cyclic nucleotide-binding homology domain (CNBHD) which is connected to the pore through a C-linker domain. Many disease-causing mutations localize to these specialized intracellular domains, which underlie the unique gating and regulation of KCNH channels(9). It has been suggested that the eag domain may regulate the channel by interacting with either the S4-S5 linker or the CNBHD(8,10). Here we present a 2-Å resolution crystal structure of the eag domain-CNBHD complex of the mouse EAG1 (mEAG1) channel. It displays extensive interactions between the eag domain and the CNBHD, indicating that the regulatory mechanism of the eag domain involves primarily the CNBHD. Surprisingly, the structure reveals that a number of LQT2 mutations at homologous positions in hERG, and cancer-associated mutations in EAG channels, localize to the eag domain-CNBHD interface. Furthermore, mutations at the interface produced dramatic effects on channel gating demonstrating the important physiological role of the eag domain-CNBHD interaction. Our structure of the eag domain-CNBHD complex of mEAG1 provides unique insights into the physiological and pathophysiological mechanisms of KCNH channels.