Omnitemporal choreographies of all five STIM/Orai and IP3Rs underlie the complexity of mammalian Ca2+ signaling. Emrich et al.

Stromal-interaction molecules (STIM1/2) sense endoplasmic reticulum (ER) Ca2+ depletion and activate Orai channels. However, the choreography of interactions between native STIM/Orai proteins under physiological agonist stimulation is unknown. We show that the five STIM1/2 and Orai1/2/3 proteins are non-redundant and function together to ensure the graded diversity of mammalian Ca2+ signaling. Physiological Ca2+ signaling requires functional interactions between STIM1/2, Orai1/2/3 and IP3Rs, ensuring that receptor-mediated Ca2+ release is tailored to Ca2+ entry and nuclear factor of activated T-cells (NFAT) activation. The N-terminal Ca2+-binding ER-luminal domains of unactivated STIM1/2 inhibit IP3R-evoked Ca2+ release. Gradual increase in agonist intensity and STIM1/2 activation relieves IP3R inhibition. Concomitantly, activated STIM1/2 C-termini differentially interact with Orai1/2/3 as agonist intensity increases. Thus, coordinated and omnitemporal functions of all five STIM/Orai and IP3Rs translate the strength of agonist stimulation to precise levels of Ca2+ signaling and NFAT induction, ensuring the fidelity of complex mammalian Ca2+ signaling.

基质相互作用分子（Stromal-interaction molecules, STIM1/2）可感知内质网（endoplasmic reticulum, ER）内Ca²+的耗竭，并激活Orai通道（Orai channels）。然而，生理激动剂刺激下，天然STIM/Orai蛋白之间相互作用的动态调控过程仍未明确。本研究发现，STIM1、STIM2以及Orai1、Orai2、Orai3这五种蛋白并非功能冗余，而是协同行使功能，以保障哺乳动物Ca²+信号通路的分级多样性。生理状态下的Ca²+信号传导需要STIM1/2、Orai1/2/3与肌醇三磷酸受体（inositol trisphosphate receptors, IP3Rs）之间形成功能性相互作用，确保受体介导的Ca²+释放适配于Ca²+内流及活化T细胞核因子（nuclear factor of activated T-cells, NFAT）的激活。未激活状态下的STIM1/2，其N端Ca²+结合内质网腔结构域可抑制IP3R介导的Ca²+释放。随着激动剂作用强度逐渐升高，STIM1/2被激活，从而解除对IP3R的抑制作用。与此同时，随着激动剂强度的提升，激活后的STIM1/2的C端会与Orai1、Orai2、Orai3产生差异化的相互作用。综上，五种STIM/Orai蛋白与IP3Rs的协同且全时相的功能，可将激动剂的刺激强度转化为精准的Ca²+信号水平与NFAT诱导程度，从而保障复杂哺乳动物Ca²+信号传导的保真度。