PMID: 20829353

Authors:

Xu X, Vysotskaya ZV, Liu Q, Zhou L

Title:

Structural basis for the camp-dependent gating in human HCN4 channel.

Journal:

J Biol Chem. 2010 Sep 9.

Abstract:

Hyperpolarization-activated cAMP-regulated (HCN) channels play important physiological roles in both cardiovascular and central nervous systems. Among the four HCN isoforms, HCN2 and HCN4 show high expression levels in the human heart, with HCN4 being the major cardiac isoform. Previously published crystal structure of mouse HCN2 (mHCN2) C-terminal fragment, including the C-linker and the cyclic-nucleotide binding domain (CNBD), has provided many insights into the cAMP-dependent gating in HCN channels. However, structures of other mammalian HCN channel isoforms have been lacking. Here we used a combination of approaches including structural biology, biochemistry, and electrophysiology, to study the cAMP-dependent gating in HCN4 channels. First we solved the crystal structure of the C-terminal fragment of human HCN4 (hHCN4) channel at 2.4A. Overall we observed a high similarity between mHCN2 and hHCN4 crystal structures. Functional comparison between two isoforms revealed that compared to mHCN2, the hHCN4 protein exhibited marked different contributions to channel function, such as an approximately 3-fold reduction in the response to cAMP. Guided by the structural differences in the loop region between beta4 and beta5 strands, we identified residues that could be partially accounted for the differences in the response to cAMP between mHCN2 and hHCN4 proteins. Moreover, upon cAMP binding, hHCN4 C-terminal protein exerts a much prolonged effect in channel deactivation which could have significant physiological contributions.