Emerging Concepts in Ion Channel Biophysics

Emerging Concepts in Ion Channel Biophysics

Wednesday Speaker Abstracts

Molecular Mechanisms of Regulation of Ion Channels by Intracellular Domains William N. Zagotta . University of Washington, Seattle, WA, USA. The family of cyclic nucleotide-binding domain (CNBD)-containing ion channels includes CNG, HCN, and KCNH channels. While these channels all contain a C-linker and CNBD in their carboxy-terminal region and are structurally very similar, they are functionally quite diverse. Their ion selectivity ranges from strongly potassium selective (KCNH) to weakly potassium selective (HCN) to cation nonselective (CNG), and their voltage-dependence ranges from depolarization activated (KCNH) to hyperpolarization activated (HCN) to voltage independent (CNG). In addition, while the CNG and HCN channels are activated by the direct binding of cyclic nucleotide, the KCNH channels do not bind and are not regulated by cyclic nucleotides. Using a combination of X-ray crystallography and electrophysiology we have shown that this lack of regulation of KCNH channels by cyclic nucleotide is because the would-be binding pocket of KCNH channels is occupied by a segment of the channel itself, we call the intrinsic ligand. Furthermore, we have shown that the cyclic nucleotide-binding homology domain (CNBHD) of KCNH channels directly interacts with the amino-terminal eag domain of these channels. Recently, using a combination of transition metal ion FRET (tmFRET), patch-clamp fluorometry (PCF), and a fluorescent noncanonical amino acid (Anap), we have shown that there is a slow rearrangement of the eag domain/CNBHD interaction associated with the voltage- dependent activation of KCNH channels. This rearrangement produces a large voltage-dependent potentiation of the channel, similar to prepulse facilitation in other channels, which is thought to regulate cardiac and neuronal excitability. We propose that instead of cyclic nucleotide- regulation, the CNBHD of KCNH channels has evolved to produce voltage-dependent potentiation.

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