The NH2 terminus regulates voltage-dependent gating of CALHM ion channels

Abstract

Calcium homeostasis modulator protein-1 (CALHM1) and its Caenorhabditis elegans (ce) homolog, CLHM-1, belong to a new family of physiologically important ion channels that are regulated by voltage and extracellular Ca2+ (Ca2+o) but lack a canonical voltage-sensing domain. Consequently, the intrinsic voltage-dependent gating mechanisms for CALHM channels are unknown. Here, we performed voltage-clamp experiments on ceCLHM-1 chimeric, deletion, insertion, and point mutants to assess the role of the NH2 terminus (NT) in CALHM channel gating. Analyses of chimeric channels in which the ceCLHM-1 and human (h)CALHM1 NH2 termini were interchanged showed that the hCALHM1 NT destabilized channel-closed states, whereas the ceCLHM-1 NT had a stabilizing effect. In the absence of Ca2+o, deletion of up to eight amino acids from the ceCLHM-1 NT caused a hyperpolarizing shift in the conductance-voltage relationship with little effect on voltage-dependent slope. However, deletion of nine or more amino acids decreased voltage dependence and induced a residual conductance at hyperpolarized voltages. Insertion of amino acids into the NH2-terminal helix also decreased voltage dependence but did not prevent channel closure. Mutation of ceCLHM-1 valine 9 and glutamine 13 altered half-maximal activation and voltage dependence, respectively, in 0 Ca2+. In 2 mM Ca2+o, ceCLHM-1 NH2-terminal deletion and point mutant channels closed completely at hyperpolarized voltages with apparent affinity for Ca2+o indistinguishable from wild-type ceCLHM-1, although the ceCLHM-1 valine 9 mutant exhibited an altered conductance-voltage relationship and kinetics. We conclude that the NT plays critical roles modulating voltage dependence and stabilizing the closed states of CALHM channels.