Regulation of Hyperpolarization-Activated Hcn Channel Gating and Camp Modulation Due to Interactions of Cooh Terminus and Core Transmembrane Regions

Members of the hyperpolarization-activated cation (HCN) channel family generate HCN currents (I_h) that are directly regulated by cAMP and contribute to pacemaking activity in heart and brain. The four different HCN isoforms show distinct biophysical properties. In cell-free patches from Xenopus oocytes, the steady-state activation curve of HCN2 channels is 20 mV more hyperpolarized compared with HCN1. Whereas the binding of cAMP to a COOH-terminal cyclic nucleotide binding domain (CNBD) markedly shifts the activation curve of HCN2 by 17 mV to more positive potentials, the response of HCN1 is much less pronounced (4 mV shift). A previous deletion mutant study suggested that the CNBD inhibits hyperpolarization-gating in the absence of cAMP; the binding of cAMP shifts gating to more positive voltages by relieving this inhibition. The differences in basal gating and cAMP responsiveness between HCN1 and HCN2 were proposed to result from a greater inhibitory effect of the CNBD in HCN2 compared with HCN1. Here, we use a series of chimeras between HCN1 and HCN2, in which we exchange the NH₂ terminus, the transmembrane domain, or distinct domains of the COOH terminus, to investigate further the molecular bases for the modulatory action of cAMP and for the differences in the functional properties of the two channels. Differences in cAMP regulation between HCN1 and HCN2 are localized to sequence differences within the COOH terminus of the two channels. Surprisingly, exchange of the CNBDs between HCN1 and HCN2 has little effect on basal gating and has only a modest one on cAMP modulation. Rather, differences in cAMP modulation depend on the interaction between the CNBD and the C-linker, a conserved 80–amino acid region that connects the last (S6) transmembrane segment to the CNBD. Differences in basal gating depend on both the core transmembrane domain and the COOH terminus. These data, taken in the context of the previous data on deletion mutants, suggest that the inhibitory effect of the CNBD on basal gating depends on its interactions with both the C-linker and core transmembrane domain of the channel. The extent to which cAMP binding is able to relieve this inhibition is dependent on the interaction between the C-linker and the CNBD.