Title: STIM Is a Ca2+ Sensor Essential for Ca2+-Store-Depletion-Triggered Ca2+ Influx
Abstract: Ca2+ signaling in nonexcitable cells is typically initiated by receptor-triggered production of inositol-1,4,5-trisphosphate and the release of Ca2+ from intracellular stores [1Berridge M.J. Bootman M.D. Roderick H.L. Calcium signalling: Dynamics, homeostasis and remodelling.Nat. Rev. Mol. Cell Biol. 2003; 4: 517-529Crossref PubMed Scopus (3825) Google Scholar]. An elusive signaling process senses the Ca2+ store depletion and triggers the opening of plasma membrane Ca2+ channels [2Putney Jr., J.W. A model for receptor-regulated calcium entry.Cell Calcium. 1986; 7: 1-12Crossref PubMed Scopus (2023) Google Scholar, 3Mikoshiba K. Hattori M. IP3 receptor-operated calcium entry.Sci. STKE. 2000; 2000: PE1Google Scholar, 4Putney Jr., J.W. Broad L.M. Braun F.J. Lievremont J.P. Bird G.S. Mechanisms of capacitative calcium entry.J. Cell Sci. 2001; 114: 2223-2229Crossref Google Scholar, 5Prakriya M. Lewis R.S. CRAC channels: Activation, permeation, and the search for a molecular identity.Cell Calcium. 2003; 33: 311-321Crossref Scopus (140) Google Scholar]. The resulting sustained Ca2+ signals are required for many physiological responses, such as T cell activation and differentiation [6Lewis R.S. Calcium signaling mechanisms in T lymphocytes.Annu. Rev. Immunol. 2001; 19: 497-521Crossref PubMed Scopus (676) Google Scholar]. Here, we monitored receptor-triggered Ca2+ signals in cells transfected with siRNAs against 2,304 human signaling proteins, and we identified two proteins required for Ca2+-store-depletion-mediated Ca2+ influx, STIM1 and STIM2 [7Oritani K. Kincade P.W. Identification of stromal cell products that interact with pre-B cells.J. Cell Biol. 1996; 134: 771-782Crossref Scopus (155) Google Scholar, 8Sabbioni S. Barbanti-Brodano G. Croce C.M. Negrini M. GOK: A gene at 11p15 involved in rhabdomyosarcoma and rhabdoid tumor development.Cancer Res. 1997; 57: 4493-4497Google Scholar, 9Williams R.T. Manji S.S. Parker N.J. Hancock M.S. Van Stekelenburg L. Eid J.P. Senior P.V. Kazenwadel J.S. Shandala T. Saint R. et al.Identification and characterization of the STIM (stromal interaction molecule) gene family: Coding for a novel class of transmembrane proteins.Biochem. J. 2001; 357: 673-685Crossref Scopus (251) Google Scholar]. These proteins have a single transmembrane region with a putative Ca2+ binding domain in the lumen of the endoplasmic reticulum. Ca2+ store depletion led to a rapid translocation of STIM1 into puncta that accumulated near the plasma membrane. Introducing a point mutation in the STIM1 Ca2+ binding domain resulted in prelocalization of the protein in puncta, and this mutant failed to respond to store depletion. Our study suggests that STIM proteins function as Ca2+ store sensors in the signaling pathway connecting Ca2+ store depletion to Ca2+ influx.