National Institute of Environmental Health Sciences-NIH, Department of Health and Human Services, Research Triangle Park, NC 27709, USA.
J Cell Sci. 2012 Sep 15;125(Pt 18):4354-61. doi: 10.1242/jcs.104919. Epub 2012 May 28.
Store-operated calcium entry is an almost ubiquitous signaling pathway in eukaryotic cells. The plasma membrane store-operated channels are comprised of subunits of the recently discovered Orai proteins, the major one being Orai1.We have discovered that native Orai1, as well as expressed Orai1, exists in two forms in similar quantities: a longer form (Orai1α) of approximately 33 kDa, and a shorter form (Orai1β) of approximately 23 kDa. The Orai1β form arises from alternative translation initiation from a methionine at position 64, and possibly also 71, in the longer Orai1α form. In the sequence upstream of the initiation site of Orai1β, there is a poly-arginine sequence previously suggested to be involved in interaction of Orai1 with plasma membrane phosphatidylinositol-4,5-bisphosphate. The loss of this phospholipid binding domain would be expected to influence the mobility of Orai1 protein in the plasma membrane. Indeed, experiments utilizing fluorescence recovery after photobleaching (FRAP) revealed that the recovery half-time for Orai1β was significantly faster than for Orai1α. Since Orai1 must diffuse to sites of interaction with the Ca(2+) sensor, STIM1, these two mobilities might provide for efficient recruitment of Orai1 subunits to sites of store-operated Ca(2+) entry during agonist-induced Ca(2+) signaling.
钙库操纵性钙内流是真核细胞中一种几乎普遍存在的信号通路。质膜钙库操纵性通道由最近发现的 Orai 蛋白亚基组成，其中主要的是 Orai1。我们发现，天然 Orai1 以及表达的 Orai1 以相似的数量存在两种形式：一种较长的形式（Orai1α）约 33 kDa，另一种较短的形式（Orai1β）约 23 kDa。Orai1β 形式来源于较长的 Orai1α 形式中第 64 位和可能还有第 71 位甲硫氨酸的选择性翻译起始。在 Orai1β 起始位点的上游序列中，有一个以前被认为参与 Orai1 与质膜磷脂酰肌醇-4,5-二磷酸相互作用的多精氨酸序列。失去这个磷脂结合域预计会影响 Orai1 蛋白在质膜中的流动性。事实上，利用荧光恢复后漂白（FRAP）实验发现，Orai1β 的恢复半衰期明显快于 Orai1α。由于 Orai1 必须扩散到与 Ca（2+）传感器 STIM1 相互作用的部位，这两种流动性可能为 Orai1 亚基在激动剂诱导的 Ca（2+）信号期间向钙库操纵性 Ca（2+）内流的部位有效募集提供了条件。
