Regulation of Orai1/STIM1 mediated ICRAC by intracellular pH

Ca ²⁺ release activated Ca ²⁺ (CRAC) channels composed of two cellular proteins, Ca ²⁺ -sensing stromal interaction molecule 1 (STIM1) and pore-forming Orai1, are the main mediators of the Ca ²⁺ entry pathway activated in response to depletion of intracellular Ca ²⁺ stores. Previously it has been shown that the amplitude of CRAC current (I _CRAC ) strongly depends on extracellular and intracellular pH. Here we investigate the intracellular pH (pH _i ) dependence of I _CRAC mediated by Orai1 and STIM1ectopically expressed in HEK293 cells. The results indicate that pH _i affects not only the amplitude of the current, but also Ca ²⁺ dependent gating of CRAC channels. Intracellular acidification changes the kinetics of I _CRAC , introducing prominent re-activation component in the currents recorded in response to voltage steps to strongly negative potentials. I _CRAC with similar kinetics can be observed at normal pH _i if the expression levels of Orai1 are increased, relative to the expression levels of STIM1. Mutations in the STIM1 inactivation domain significantly diminish the dependence of I _CRAC kinetics on pH _i , but have no effect on pH _i dependence of I _CRAC amplitude, implying that more than one mechanism is involved in CRAC channel regulation by intracellular pH.