Differential Calcium Ca²⁺ signals drive alternate cell fates such as proliferation, apoptosis or tolerance. In immune cells one major entry pathway for Ca²⁺ into the cells are store-operated Ca²⁺ channels. In response to store depletion ER-resident Ca²⁺ sensor molecules STIM translocate to regions near the plasma membrane and interact with Orai1 proteins. These form the major ion conduction units mediating the Ca²⁺ release activated I_CRAC (CRAC) current. Reactive oxygen species (ROS) interacting with cysteine residues can alter protein function. Pretreatment of Orai1 with the oxidant H₂O₂ reduces I_CRAC . Our results demonstrate a novel mechanistic model for ROS-mediated inhibition of Orai1 and identify a candidate residue for pharmaceutical intervention.

Preincubation with ROS prevents activation of Orai1, but is unable to inhibit the channel complex once it is activatied [1]. The inhibition is mainly mediated through the reactive cysteine C195 at the exit of transmembrane region 3 of Orai1, a residue that is not conserved in the paralouge Orai3, which currents are not inhibited. We combine experimental and theoretical approaches and show that oxidation of Orai1 leads to reduced subunit interaction, slow diffusion and that either oxidized C195 or its oxidomimetic mutaion C195D virtually eliminates channel activation by intramolecular interaction with S239.

To assess whether the observed effects are sufficient to explain the dramatic oxidant inhibition of Orai1 mediated currents, we implemented the altered diffusion and reaction rates into a modified version of the stochastic reaction diffusion model that we have previously reported [2,3]. In addition, the model was modified to introduce and account for the locked channel subunits. With this the model also allows us to predict different degrees of inhibition depending on the fraction of inhibited channels.