School of Biological Sciences, University of Auckland, Auckland, New Zealand.
J Biol Chem. 2011 May 6;286(18):16174-85. doi: 10.1074/jbc.M110.206946. Epub 2011 Mar 7.
Calcium (Ca(2+)) influx through the plasma membrane store-operated Ca(2+) channel ORAI1 is controlled by Ca(2+) sensors of the stromal interaction molecule (STIM) family. STIM1 responds to endoplasmic reticulum (ER) Ca(2+) store depletion by redistributing and activating ORAI1 from regions of the ER juxtaposed to the plasma membrane. Unlike STIM1, STIM2 can regulate ORAI1 in a store-dependent and store-independent manner, but the mechanism by which this is achieved is unknown. Here we find that STIM2 is translated from a highly conserved methionine residue and is directed to the ER by an incredibly long 101-amino acid signal peptide. We find that although the majority of the total STIM2 population resides on the ER membrane, a second population escapes ER targeting to accumulate as a full-length preprotein in the cytosol, signal peptide intact. Unlike STIM2, preSTIM2 localizes to the inner leaflet of the plasma membrane where it interacts with ORAI1 to regulate basal Ca(2+) concentration and Ca(2+)-dependent gene transcription in a store-independent manner. Furthermore, a third protein comprising a fragment of the STIM2 signal peptide is released from the ER membrane into the cytosol where it regulates gene transcription in a Ca(2+)-independent manner. This study establishes a new model for STIM2-mediated regulation of ORAI1 in which two distinct proteins, STIM2 and preSTIM2, control store-dependent and store-independent modes of ORAI1 activation.
钙（Ca(2+)）通过质膜储存操作 Ca(2+)通道 ORAI1 的内流受基质相互作用分子（STIM）家族的 Ca(2+)传感器控制。STIM1 通过重新分布和激活与质膜相邻的内质网（ER）中的 ORAI1，对 ER Ca(2+)库耗竭作出反应。与 STIM1 不同，STIM2 可以以依赖和不依赖储存的方式调节 ORAI1，但实现这一目标的机制尚不清楚。在这里，我们发现 STIM2 是从高度保守的蛋氨酸残基翻译而来的，并通过一个难以置信的长 101 个氨基酸的信号肽被引导到 ER。我们发现，尽管 STIM2 的大部分总群体存在于 ER 膜上，但第二个群体逃避 ER 靶向，以完整的信号肽形式在细胞质中积累为全长前体蛋白。与 STIM2 不同，preSTIM2 定位于质膜的内叶，在那里它与 ORAI1 相互作用，以不依赖储存的方式调节基础 Ca(2+)浓度和 Ca(2+)-依赖性基因转录。此外，由 STIM2 信号肽片段组成的第三种蛋白质从 ER 膜释放到细胞质中，以不依赖 Ca(2+)的方式调节基因转录。本研究建立了一个新的 STIM2 介导的 ORAI1 调节模型，其中两种不同的蛋白质，STIM2 和 preSTIM2，控制 ORAI1 激活的依赖储存和不依赖储存模式。
