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, S.I., Ishiyama, N., Zheng, L., Stathopulos, P.B., Ikura, M.
(2020) J Mol Biol 432: 367-383
- PubMed: 31626806
- DOI: 10.1016/j.jmb.2019.10.003
- Structures With Same Primary Citation
- PubMed Abstract:
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 cyto ...
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 (STIMs) 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 nonconserved amino (N) terminal region and EF-hand and sterile α 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 α-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 α-helix and the canonical EF-hand in low Ca 2+ . Remarkably, we also detected an α-helix (+Ca 2+ ) to β-strand (-Ca 2+ ) transition at the terminal SAM domain α-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.
Princess Margaret Cancer Centre, Department of Medical Biophysics, University Health Network, University of Toronto, Toronto, ON M5G 1L7, Canada. Electronic address: email@example.com.