Featured Arcle Coordination of a Single Calcium Ion in the EF-hand Maintains the Off State of the Stromal Interaction Molecule Luminal Domain Masahiro Enomoto 1,*, Tadateru Nishikawa 1,*, y, Sung-In Back 1, Noboru Ishiyama 1,‡, Le Zheng 1, Peter B. Stathopulos 2 and Mitsuhiko Ikura 1 1 - Princess Margaret Cancer Centre, Department of Medical Biophysics, University Health Network, University of Toronto, Toronto, ON M5G 1L7, Canada 2 - Department of Physiology and Pharmacology, Schulich School of Medicine and Dentistry, London, ON, Canada N6A 5C1 Correspondence to Mitsuhiko Ikura and Peter B. Stathopulos:
[email protected],
[email protected] https://doi.org/10.1016/j.jmb.2019.10.003 Edited by Ichio Shimada Abstract Store operated calcium (Ca2þ) entry (SOCE) is the process whereby endoplasmic reticulum (ER) Ca2þ store depletion causes Orai1-composed Ca2þ channels on the plasma membrane (PM) to open, mediating a rise in cytosolic Ca2þ levels. Stromal interaction molecules (STIMs) are the proteins that directly sense ER Ca2þ content and gate Orai1 channels due to store depletion. The trigger for STIM activation is Ca2þ unbinding from the ER lumen-oriented domains, which consist of a nonconserved amino (N) terminal region and EF-hand and sterile a motif (SAM) domains (EFeSAM), highly conserved from humans to Caenorhabditis elegans. Solution NMR structures of the human EFeSAM domains have been determined at high Ca2þ concentrations; however, no direct structural view of the Ca2þ binding mode has been elucidated. Further, no atomic resolution data currently exists on EFeSAM at low Ca2þ levels. Here, we determined the X-ray crystal structure of the C.