A calcium-accumulating region, CAR, in the channel Orai1 enhances Ca2+ permeation and SOCE-induced gene transcription

The Ca 2+ release–activated Ca 2+ channel mediates Ca 2+ influx in a plethora of cell types, thereby controlling diverse cellular functions. The channel complex is composed of stromal interaction molecule 1 (STIM1), an endoplasmic reticulum Ca 2+ -sensing protein, and Orai1, a plasma membrane Ca 2+ channel. Channels composed of STIM1 and Orai1 mediate Ca 2+ influx even at low extracellular Ca 2+ concentrations. We investigated whether the activity of Orai1 adapted to different environmental Ca 2+ concentrations. We used homology modeling and molecular dynamics simulations to predict the presence of an extracellular Ca 2+ -accumulating region (CAR) at the pore entrance of Orai1. Furthermore, simulations of Orai1 proteins with mutations in CAR, along with live-cell experiments, or simulations and electrophysiological recordings of the channel with transient, electrostatic loop3 interacting with loop1 (the site of CAR) determined that CAR enhanced Ca 2+ permeation most efficiently at low external Ca 2+ concentrations. Consistent with these results, cells expressing Orai1 CAR mutants exhibited impaired gene expression stimulated by the Ca 2+ -activated transcription factor nuclear factor of activated T cells (NFAT). We propose that the Orai1 channel architecture with a close proximity of CAR to the selectivity filter, which enables Ca 2+ -selective ion permeation, enhances the local extracellular Ca 2+ concentration to maintain Ca 2+ -dependent gene regulation even in environments with relatively low Ca 2+ concentrations.