Regulation of Orai1/STIM1 mediated ICRAC by intracellular pH

Ca2+ release activated Ca2+ (CRAC) channels composed of two cellular proteins, Ca2+-sensing stromal interaction molecule 1 (STIM1) and pore-forming Orai1, are the main mediators of the Ca2+ entry pathway activated in response to depletion of intracellular Ca2+ 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 pHi affects not only the amplitude of the current, but also Ca2+ dependent gating of CRAC channels. Intracellular acidification changes the kinetics of ICRAC, 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 pHi dependence of I-CRAC amplitude, implying that more than one mechanism is involved in CRAC channel regulation by intracellular pH.