Bidirectional sensitivity of CALHM1 channel to protons from both sides of plasma membrane.

Calcium homeostasis modulator 1 (CALHM1), a newly discovered voltage-dependent nonselective ion channel, has drawn attention for its role in neuronal activity and taste sensation. Its sluggish voltage-dependent activation is facilitated by lowering extracellular Ca concentration ([Ca]). Here, we investigated the effects of extracellular and intracellular pH (pH and pH) on human CALHM1. When normalized to the amplitude of the CALHM1 current () under whole cell patch clamp at symmetrical pH 7.4, decreased at acidic pH or pH, whereas it sharply increased at alkaline pH or pH. The effects of pH were preserved in the inside-out configuration. The voltage dependence of showed leftward and rightward shifts at alkaline and acidic pH and pH, respectively. Site-directed mutagenesis of the water-accessible charged residues of the pore and nearby domains revealed that E17, K229, E233, D257, and E259 are nonadditively responsible for facilitation at alkaline pH. Identification of the pH-sensing residue was not possible because mutation of putative residues impaired membrane expression, resulting in undetectable . Alkaline pH-dependent facilitation appeared gradually with depolarization, suggesting that the sensitivity to pH might be due to H diffusion through the open-state CALHM1. At pH 6.2, decreased [Ca] could not recover the inhibited but further augmented the increased at pH 8.6, suggesting that unidentified common residues might contribute to the [Ca] and acidic pH. This study is the first, to our knowledge, to demonstrate the remarkable pH sensitivity of CALHM1, which might contribute to the pH-dependent modulation of neuronal excitability or taste sensation.