*Zentrum für Molekulare Biologie der Universität Heidelberg (ZMBH), DKFZ-ZMBH Alliance, University of Heidelberg, Im Neuenheimer Feld 282, 69120 Heidelberg, Germany.
†Heidelberg University Biochemistry Center, Im Neuenheimer Feld 328, 69120 Heidelberg, Germany.
Biosci Rep. 2013 Oct 31;33(5):e00077. doi: 10.1042/BSR20130089.
Ca2+ (calcium) homoeostasis and signalling rely on physical contacts between Ca2+ sensors in the ER (endoplasmic reticulum) and Ca2+ channels in the PM (plasma membrane). STIM1 (stromal interaction molecule 1) and STIM2 Ca2+ sensors oligomerize upon Ca2+ depletion in the ER lumen, contact phosphoinositides at the PM via their cytosolic lysine (K)-rich domains, and activate Ca2+ channels. Differential sensitivities of STIM1 and STIM2 towards ER luminal Ca2+ have been studied but responses towards elevated cytosolic Ca2+ concentration and the mechanism of lipid binding remain unclear. We found that tetramerization of the STIM1 K-rich domain is necessary for efficient binding to PI(4,5)P2-containing PM-like liposomes consistent with an oligomerization-driven STIM1 activation. In contrast, dimerization of STIM2 K-rich domain was sufficient for lipid binding. Furthermore, the K-rich domain of STIM2, but not of STIM1, forms an amphipathic α-helix. These distinct features of the STIM2 K-rich domain cause an increased affinity for PI(4,5)P2, consistent with the lower activation threshold of STIM2 and a function as regulator of basal Ca2+ levels. Concomitant with higher affinity for PM lipids, binding of CaM (calmodulin) inhibited the interaction of the STIM2 K-rich domain with liposomes in a Ca2+ and PI(4,5)P2 concentration-dependent manner. Therefore we suggest that elevated cytosolic Ca2+ concentration down-regulates STIM2-mediated ER-PM contacts via CaM binding.
钙离子（Ca2+）稳态和信号依赖于内质网（ER）中的 Ca2+传感器和质膜（PM）中的 Ca2+通道之间的物理接触。STIM1（基质相互作用分子 1）和 STIM2 Ca2+传感器在 ER 腔中的 Ca2+耗竭时寡聚化，通过其细胞质富含赖氨酸（K）的结构域与 PM 上的磷酸肌醇（PI）相互作用，并激活 Ca2+通道。已经研究了 STIM1 和 STIM2 对 ER 腔中 Ca2+的不同敏感性，但对细胞溶质 Ca2+浓度升高的反应和脂质结合的机制仍不清楚。我们发现，STIM1 K-富含结构域的四聚化对于与含有 PI(4,5)P2 的 PM 样脂质体的有效结合是必需的，这与寡聚化驱动的 STIM1 激活一致。相比之下，STIM2 K-富含结构域的二聚化足以结合脂质。此外，STIM2 的 K-富含结构域，但不是 STIM1 的，形成一个两亲性的 α-螺旋。STIM2 K-富含结构域的这些独特特征导致与 PI(4,5)P2 的亲和力增加，与 STIM2 的较低激活阈值和作为基础 Ca2+水平调节剂的功能一致。与 PM 脂质结合的亲和力增加相一致，CaM（钙调蛋白）的结合以 Ca2+和 PI(4,5)P2 浓度依赖性的方式抑制了 STIM2 K-富含结构域与脂质体的相互作用。因此，我们认为升高的细胞溶质 Ca2+浓度通过 CaM 结合下调 STIM2 介导的 ER-PM 接触。
