Coordination of a Single Calcium Ion in the EF-hand Maintains the Off State of the Stromal Interaction Molecule Luminal Domain. Issue 2 (17th January 2020)

• Record Type: Journal Article
• Authors: Enomoto, Masahiro; Nishikawa, Tadateru; Back, Sung-In; Ishiyama, Noboru; Zheng, Le; Stathopulos, Peter B.; Ikura, Mitsuhiko
• Is Part Of: Journal of molecular biology. Volume 432:Issue 2(2020)
• Page Start: 367
• Page End: 383
• Publication Date: 2020-01-17
• Subjects: Calcium binding stoichiometry -- Stromal interaction molecule -- EF–SAM -- Crystal structure -- Caenorhabditis elegans; Molecular biology -- Periodicals; Biology -- Periodicals; Biochemistry -- Periodicals; Bacteriology -- Periodicals; Molecular Biology -- Periodicals; Biochemistry -- Periodicals; Biologie moléculaire -- Périodiques; Biologie -- Périodiques; Biochimie -- Périodiques; Moleculaire biologie; Biochemistry; Biology; Molecular biology; Periodicals; 572.805
• DOI: 10.1016/j.jmb.2019.10.003
• Languages: English
• ISSNs: 0022-2836

Abstract

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 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. Graphical abstract: Image 1 Highlights: C aenorhabditis elegans and human STIMs activate SOCE after sensing decreases in ER Ca 2+ . A crystal structure revealed pentagonal bipyramidal coordination of a single Ca 2+ . An ancillary α-helix formed in the variable N-terminal region stabilizes EF–SAM. Ca 2+ depletion causes unfolding and conformational exchange of the EF-hand. The terminal α-helix transitions to β-strand after Ca 2+ depletion of EF–SAM.