Glu106 in the Orai1 pore contributes to fast Ca2+-dependent inactivation and pH dependence of Ca2+ release-activated Ca2+ (CRAC) current

FCDI (fast Ca2+-dependent inactivation) is a mechanism that limits Ca2+ entry through Ca2+ channels, including CRAC (Ca2+ release-activated Ca2+) channels. This phenomenon occurs when the Ca2+ concentration rises beyond a certain level in the vicinity of the intracellular mouth of the channel pore. In CRAC channels, several regions of the pore-forming protein Orai1, and STIM1 (stromal interaction molecule 1), the sarcoplasmic/endoplasmic reticulurn Ca2+ sensor that communicates the Ca2+ load of the intracellular stores to Orai1, have been shown to regulate fast Ca2+-dependent inactivation. Although significant advances in unravelling the mechanisms of CRAC channel gating have occurred, the mechanisms regulating fast Ca2+-dependent inactivation in this channel are not well understood. We have identified that a pore mutation, E106D Orai1, changes the kinetics and voltage dependence of the I-CRAC (CRAC current), and the selectivity of the Ca2+-binding site that regulates fast Ca2+-dependent inactivation, whereas the V102I and E190Q mutants when expressed at appropriate ratios with STIM1 have fast Ca2+-dependent inactivation similar to that of WT (wild-type) Orai1. Unexpectedly, the E106D mutation also changes the pH dependence of I-CRAC. Unlike WT I-CRAC, E106D-mediated current is not inhibited at low pH, but instead the block of Na+ permeation through the E106D Orai1 pore by Ca2+ is diminished. These results suggest that Glu(106) inside the CRAC channel pore is involved in co-ordinating the Ca2+-binding site that mediates fast Ca2+-dependent inactivation.