Molecular basis of pH sensitivity of Orai1/stim1 channels

Changes of intracellular and extracellular pH are involved in a variety of physiological and pathological processes, in which regulation of the Ca 2+ release activated Ca 2+ channel (I CRAC ) by pH has been implicated. Ca 2+ entry mediated by I CRAC as well as native I CRAC currents have been shown to be regulated by acidic or alkaline pH. However, the mechanism by which the CRAC channel is regulated by internal pH (pH i ) and external pH (pH o ) has remained elusive. By generating a series of mutations in the channel pore region, intracellular and extracellular loops, transmembrane domains, and N‐ and C‐termini, we unexpectedly identified that the mutant E190D lost sensitivity to both acidic and alkaline pH o , and the mutant H155F markedly diminished the response to acidic and alkaline pH i . Thus, it appears that E190 in the TM3 is the external pH sensor, and H155 in the intracellular loop is the internal pH i sensor of Orai1/Stim1 channels. Our results provide a novel mechanism that changes of external and internal pH modulate I CRAC activity by directly regulating channel gating of the pore‐forming subunit Orai through residues E190 and H155. Given that pH changes can influence a variety of physiological/pathological functions, Orai/Stim channels may be an important mediator for various physiological and pathological processes associated with acidosis and alkalinization.