Three Orai (Orai1, Orai2 and Orai3) and two STIM (STIM1 and STIM2; stromal interaction molecule) mammalian protein homologues constitute major components of the store-operated Ca²⁺ entry mechanism. When co-expressed with STIM1, Orai1, Orai2 and Orai3 form highly selective Ca²⁺ channels with properties of Ca²⁺ release activated Ca²⁺ (CRAC) channels. Despite the high level of homology between Orai proteins, CRAC channels formed by different Orai isoforms have distinctive properties, particularly with regards to Ca²⁺ dependent inactivation, inhibition/potentiation by 2-APB and sensitivity to reactive oxygen species. This study characterises and compares the regulation of Orai1, Orai2- and Orai3-mediated CRAC current (ICRAC) by intracellular pH. Using whole-cell patch clamping of HEK293T cells heterologously expressing Orai and STIM1 we show that ICRAC formed by each Orai homologue has a unique sensitivity to changes in intracellular pH (pHi). Orai1-mediated ICRAC exhibits a strong dependence on pHi of both current amplitude and the kinetics of Ca²⁺ dependent inactivation. In contrast, Orai2 amplitude, but not kinetics, depends on pHi, whereas Orai3 shows no dependence on pHi at all. Investigation of different Orai1-Orai3 chimeras suggests that pHi dependence of Orai1 resides in both, the N-terminus and intracellular loop 2, and may also involve pH- dependent interactions with STIM1.

中文翻译：

---

Orai1和Orai2，但不是Orai3介导的ICRAC受细胞内pH调节

三个Orai（Orai1，Orai2和Orai3）和两个STIM（STIM1和STIM2；基质相互作用分子）哺乳动物蛋白同源物构成了存储操纵的Ca ²⁺进入机制的主要成分。当与STIM1，Orai1，Orai2和Orai3形式具有高度选择性的Ca共表达²⁺与钙的性质通道²⁺释放活化的Ca ²⁺（CRAC）通道。尽管Orai蛋白之间具有很高的同源性，但由不同Orai亚型形成的CRAC通道具有独特的特性，尤其是Ca ²⁺依赖的灭活，2-APB的抑制/增强作用以及对活性氧的敏感性。这项研究表征和比较细胞内pH对Orai1，Orai2和Orai3介导的CRAC电流（ICRAC）的调节。使用异源表达Orai和STIM1的HEK293T细胞的全细胞膜片钳，我们显示了由每个Orai同源物形成的ICRAC对细胞内pH（pHi）的变化具有独特的敏感性。Orai1介导的ICRAC对电流振幅和Ca ²⁺动力学的pHi表现出强烈的依赖性依赖性失活。相反，Orai2的幅度而不是动力学取决于pHi，而Orai3则完全不依赖pHi。对不同Orai1-Orai3嵌合体的研究表明，Orai1的pHi依赖性存在于N末端和细胞内环2中，并且还可能涉及与STIM1的pH依赖性相互作用。