Feedback modulation of Orai1α and Orai1β protein content mediated by STIM proteins

Store-operated Ca ²⁺ entry (SOCE) is a mechanism controlled by the filling state of the intracellular Ca ²⁺ stores, predominantly the endoplasmic reticulum (ER), where ER-resident proteins STIM1 and STIM2 orchestrate the activation of Orai channels in the plasma membrane, and Orai1 playing a predominant role. Two forms of Orai1, Orai1α and Orai1β, have been identified, which arises the question whether they are equally regulated by STIM proteins. We demonstrate that STIM1 preferentially activates Orai1α over STIM2, yet both STIM proteins similarly activate Orai1β. Under resting conditions, there is a pronounced association between STIM2 and Orai1α, implying their significance in maintaining basal Ca ²⁺ levels within the cytosol and ER. STIM1 and STIM2 are also shown to influence the protein levels of the Orai1 variants, independent of Ca ²⁺ influx, via lysosomal degradation. Interestingly, Orai1α and Orai1β appear to selectively regulate the protein level of STIM1, but not STIM2. These observations offer crucial insights into the regulatory dynamics between STIM proteins and Orai1 variants, enhancing our understanding of the intricate processes that fine-tune intracellular Ca ²⁺ signaling.