6pw7

Publication Abstract from PubMed

Store operated calcium (Ca(2+)) entry (SOCE) is the process whereby endoplasmic reticulum (ER) Ca(2+) store depletion causes Orai1-composed Ca(2+) channels on the plasma membrane (PM) to open, mediating a rise in cytosolic Ca(2+) levels. Stromal interaction molecules (STIM)s are the proteins that directly sense ER Ca(2+) content and gate Orai1 channels due to store depletion. The trigger for STIM activation is Ca(2+) unbinding from the ER lumen-oriented domains, which consist of a non-conserved amino (N) terminal region and EF-hand and sterile alpha motif (SAM) domains (EF-SAM), highly conserved from humans to Caenorhabditis elegans. Solution NMR structures of the human EF-SAM domains have been determined at high Ca(2+) concentrations; however, no direct structural view of the Ca(2+) binding mode has been elucidated. Further, no atomic resolution data currently exists on EF-SAM at low Ca(2+) levels. Here, we determined the X-ray crystal structure of the C. elegans STIM luminal domain, revealing that EF-SAM binds a single Ca(2+) ion with pentagonal bipyramidal geometry and an ancillary alpha-helix formed by the N-terminal region acts as a brace to stabilize EF-SAM. Using solution NMR, we observed EF-hand domain unfolding and a conformational exchange between folded and unfolded states involving the ancillary alpha-helix and the canonical EF-hand in low Ca(2+). Remarkably, we also detected an alpha-helix (+Ca(2+)) to beta-strand (-Ca(2+)) transition at the terminal SAM domain alpha-helix. Collectively, our analyses indicate that one canonically bound Ca(2+) ion is sufficient to stabilize the quiescent luminal domain structure, precluding unfolding, conformational exchange and secondary structure transformation.

Coordination of a single calcium ion in the EF-hand maintains the off state of the stromal interaction molecule luminal domain.,Enomoto M, Nishikawa T, Back SI, Ishiyama N, Zheng L, Stathopulos PB, Ikura M J Mol Biol. 2019 Oct 15. pii: S0022-2836(19)30594-7. doi:, 10.1016/j.jmb.2019.10.003. PMID:31626806^[3]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.