1.A.1.5.29
spHCN1 is a pacemaker hyperpolarization-activated cyclic nucleotide-gated (HCN) non-selective cation channel of 767 aas and 6 TMSs that opens due to inward movement of the positive charges in the fourth TMS (S4). This channel is similar to a COOH-terminal-deleted HCN1 channel, suggesting that the main functional differences between spHCN and HCN1 channels are due to differences in their COOH termini (Vemana et al. 2004). These channels open after only two S4s have moved, and S4 motion is rate limiting during voltage activation of spHCN channels (Bruening-Wright et al. 2007). HCN channels regulate electrical activity in the heart and brain. Distinct from mammalian isoforms, the sea urchin (spHCN) channel exhibits strong voltage-dependent inactivation in the absence of cAMP (Idikuda et al. 2018). The voltage sensor undergoes a large downward motion during hyperpolarization (Dai et al. 2019). Sea urchin HCN1 and 2 (TC# 1.A.1.5.33) (spHCN) channels undergo inactivation with hyperpolarization which occurs only in the absence of cyclic nucleotide (Dai et al. 2021). Removing cAMP produces a largely rigid-body rotation of the C-linker relative to the transmembrane domain, bringing the A' helix of the C-linker in close proximity to the voltage-sensing S4 helix. In addition, rotation of the C-linker is elicited by hyperpolarization minus cAMP. Thus, in contrast to electromechanical coupling for channel activation - the A' helix serves to couple the S4-helix movement for channel inactivation, which is likely a conserved mechanism for CNBD-family channels (Dai et al. 2021).