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

An X-ray crystal structure of two intracellular domains of a mouse KCNH voltage-dependent potassium channel, EAG1, is presented: the structure reveals that the eag domain interacts directly with the cyclic nucleotide-binding homology domain (CNBHD), and that many KCNH-associated disease mutations are localized to the eag domain–CNBHD interface. KCNH channels, a group that includes the well-known hEAG1 (human ether-a-go-go-related gene 1) potassium channel, are among the most important ion channels in human health and disease. These voltage-dependent channels are widely expressed, and are crucial for repolarization of the cardiac action potential and regulation of neuronal excitability. Their dysfunction is responsible for long QT syndrome type 2 (LQT2), epilepsy, schizophrenia and some cancers. This manuscript presents the X-ray crystal structure of two intracellular domains of the mouse EAG1 voltage-dependent potassium channel — the N-terminal eag domain and the C-terminal cyclic nucleotide-binding homology domain. The structures reveal that the eag domain regulates the ion channel through direct interaction with the cyclic nucleotide-binding homology domain, not through the S4–S5 linker as has been suggested. The authors also determined that several LQT2- and cancer-causing mutations localize to the eag domain–cyclic nucleotide-binding homology domain interface. The KCNH voltage-dependent potassium channels (ether-a-go-go, EAG; EAG-related gene, ERG; EAG-like channels, ELK) are important regulators of cellular excitability1,2,3 and have key roles in diseases such as cardiac long QT syndrome type 2 (LQT2)4, epilepsy5, schizophrenia6 and cancer7. The intracellular domains of KCNH channels are structurally distinct from other voltage-gated channels. The amino-terminal region contains an eag domain, which is composed of a Per-Arnt-Sim (PAS) domain and a PAS-cap domain8, whereas 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 channels9. It has been suggested that the eag domain may regulate the channel by interacting with either the S4–S5 linker or the CNBHD8,10. Here we present a 2 A resolution crystal structure of the eag domain–CNBHD complex of the mouse EAG1 (also known as KCNH1) channel. It displays extensive interactions between the eag domain and the CNBHD, indicating that the regulatory mechanism of the eag domain primarily involves the CNBHD. Notably, the structure reveals that a number of LQT2 mutations at homologous positions in human ERG, in addition to cancer-associated mutations in EAG channels, localize to the eag domain–CNBHD interface. Furthermore, mutations at the interface produced marked effects on channel gating, demonstrating the important physiological role of the eag domain–CNBHD interaction. Our structure of the eag domain–CNBHD complex of mouse EAG1 provides unique insights into the physiological and pathophysiological mechanisms of KCNH channels.