Glu ¹⁰⁶in the Orai1 pore contributes to fast Ca ²⁺ -dependent inactivation and pH dependence of Ca ²⁺ release-activated Ca ²⁺ (CRAC) current

FCDI (fast Ca ²⁺ -dependent inactivation) is a mechanism that limits Ca ²⁺ entry through Ca ²⁺ channels, including CRAC (Ca ²⁺release-activated Ca ²⁺) channels. This phenomenon occurs when the Ca ²⁺ 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 reticulum Ca ²⁺ sensor that communicates the Ca ²⁺ load of the intracellular stores to Orai1, have been shown to regulate fast Ca ²⁺ -dependent inactivation. Although significant advances in unravelling the mechanisms of CRAC channel gating have occurred, the mechanisms regulating fast Ca ²⁺ -dependent inactivation in this channel are not well understood. We have identified that a poremutation, E106D Orai1, changes the kinetics and voltage dependence of the I _CRAC (CRAC current), and the selectivity of the Ca ²⁺ -binding site that regulates fast Ca ²⁺ - dependent inactivation, whereas the V102I and E190Q mutants when expressed at appropriate ratios with STIM1 have fast Ca ²⁺ -dependent inactivation similar to that of WT (wild-type) Orai1. Unexpectedly, the E106D mutation also changes the pH dependence of I _CRAC. Unlike WTI _CRAC, E106D-mediated current is not inhibited at low pH, but instead the block of Na ⁺ permeation through the E106D Orai1 pore by Ca ²⁺ is diminished. These results suggest that Glu ¹⁰⁶ inside the CRAC channel pore is involved in co-ordinating the Ca ²⁺ -binding site that mediates fast Ca ²⁺ -dependent inactivation.