Regulation of Orai1/STIM1 mediated I_CRAC 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 pH_i affects not only the amplitude of the current, but also Ca²⁺ 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.