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 2+ concentration ([Ca 2+ ] e ). Here, we investigated the effects of extracellular and intracellular pH (pH e and pH i ) on human CALHM1. When normalized to the amplitude of the CALHM1 current ( I CALHM1 ) under whole cell patch clamp at symmetrical pH 7.4, I CALHM1 decreased at acidic pH e or pH i , whereas it sharply increased at alkaline pH e or pH i . The effects of pH were preserved in the inside-out configuration. The voltage dependence of I CALHM1 showed leftward and rightward shifts at alkaline and acidic pH e and pH i , 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 i . Identification of the pH e -sensing residue was not possible because mutation of putative residues impaired membrane expression, resulting in undetectable I CALHM1 . Alkaline pH e -dependent facilitation appeared gradually with depolarization, suggesting that the sensitivity to pH e might be due to H + diffusion through the open-state CALHM1. At pH e 6.2, decreased [Ca 2+ ] e could not recover the inhibited I CALHM1 but further augmented the increased I CALHM1 at pH e 8.6, suggesting that unidentified common residues might contribute to the [Ca 2+ ] e and acidic pH e . 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.