title: Calcium sensitive ring-like oligomers formed by synaptotagmin.
abstract: The synaptic vesicle protein synaptotagmin-1 (SYT) is required to couple calcium influx to the membrane fusion machinery. However, the structural mechanism underlying this process is unclear. Here we report an unexpected circular arrangement (ring) of SYT's cytosolic domain (C2AB) formed on lipid monolayers in the absence of free calcium ions as revealed by electron microscopy. Rings vary in diameter from 18-43 nm, corresponding to 11-26 molecules of SYT. Continuous stacking of the SYT rings occasionally converts both lipid monolayers and bilayers into protein-coated tubes. Helical reconstruction of the SYT tubes shows that one of the C2 domains (most likely C2B, based on its biochemical properties) interacts with the membrane and is involved in ring formation, and the other C2 domain points radially outward. SYT rings are disrupted rapidly by physiological concentrations of free calcium but not by magnesium. Assuming that calcium-free SYT rings are physiologically relevant, these results suggest a simple and novel mechanism by which SYT regulates neurotransmitter release: The ring acts as a spacer to prevent the completion of the soluble N-ethylmaleimide-sensitive factor activating protein receptor (SNARE) complex assembly, thereby clamping fusion in the absence of calcium. When the ring disassembles in the presence of calcium, fusion proceeds unimpeded.
authors.affiliation: Departments of Cell Biology.
authors.forename: Jing
authors.initials: J
authors.lastname: Wang
authors.affiliation: Departments of Cell Biology.
authors.forename: Oscar
authors.initials: O
authors.lastname: Bello
authors.affiliation: Departments of Cell Biology.
authors.forename: Sarah M
authors.initials: SM
authors.lastname: Auclair
authors.affiliation: Chemistry, and.
authors.forename: Jing
authors.initials: J
authors.lastname: Wang
authors.affiliation: Departments of Cell Biology.
authors.forename: Jeff
authors.initials: J
authors.lastname: Coleman
authors.affiliation: Departments of Cell Biology, Laboratoire de Physique Statistique, Unité Mixte de Recherche, Centre National de la Recherche Scientifique, 8550 Associée aux Universités Paris 6 et Paris 7, Ecole Normale Supérieure, 75005 Paris, France.
authors.forename: Frederic
authors.initials: F
authors.lastname: Pincet
authors.affiliation: Departments of Cell Biology.
authors.forename: Shyam S
authors.initials: SS
authors.lastname: Krishnakumar
authors.affiliation: Molecular Biophysics and Biochemistry, Yale University School of Medicine, New Haven, CT 06520; and charles.sindelar@yale.edu james.rothman@yale.edu.
authors.forename: Charles V
authors.initials: CV
authors.lastname: Sindelar
authors.affiliation: Departments of Cell Biology, Chemistry, and charles.sindelar@yale.edu james.rothman@yale.edu.
authors.forename: James E
authors.initials: JE
authors.lastname: Rothman
mesh_terms: D002118:Calcium / Q000737:chemistry*; D006801:Humans; D008051:Lipid Bilayers / Q000737:chemistry*; D046912:Multiprotein Complexes / Q000737:chemistry* / Q000648:ultrastructure; D017434:Protein Structure, Tertiary; D050600:SNARE Proteins / Q000737:chemistry*; D050863:Synaptotagmin I / Q000737:chemistry*
chemical_list: D008051:Lipid Bilayers; D046912:Multiprotein Complexes; D050600:SNARE Proteins; C494310:SYT1 protein, human; D050863:Synaptotagmin I; D002118:Calcium