A novel function for the second C2 domain of synaptotagmin. Ca2+-triggered dimerization.
Synaptotagmin serves as the major Ca2+ sensor for regulated exocytosis from neurons. While the mechanism by which synaptotagmin regulates membrane fusion remains unknown, studies using Drosophila indicate that the molecule functions as a multimeric complex and that its second C2 domain is essential for efficient excitation-secretion coupling. Here we describe biochemical data that may account for these phenomena. We report that Ca2+ causes synaptotagmin to oligomerize, primarily forming dimers, via its second C2 domain. This effect is specific for divalent cations that can stimulate exocytosis of synaptic vesicles (Ca2+ >> Ba2+, Sr2+ >> Mg2+) and occurs with an EC50 value of 3-10 microM Ca2+. In contrast, a separate Ca2+-dependent interaction between synaptotagmin and syntaxin, a component of the fusion apparatus, occurs with an EC50 value of approximately 100 microM Ca2+ and involves the synergistic action of both C2 domains of synaptotagmin. We propose that Ca2+ triggers two consecutive protein-protein interactions: the formation of synaptotagmin dimers at low Ca2+ concentrations followed by the association of synaptotagmin dimers with syntaxin at higher Ca2+-concentrations. Our findings, in conjunction with physiological studies, indicate that the Ca2+-induced dimerization of synaptotagmin is important for the efficient regulation of exocytosis by Ca2+.
Chapman
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ER
Howard Hughes Medical Institute, Department of Pharmacology, Boyer Center for Molecular Medicine, Yale University School of Medicine, New Haven, Connecticut 06510, USA.
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Edwardson
J M
JM
Jahn
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D000818:Animals; D001921:Brain / Q000378:metabolism*; D002118:Calcium / Q000494:pharmacology*; D002135:Calcium-Binding Proteins; D002413:Cations, Divalent / Q000494:pharmacology; D002499:Centrifugation, Density Gradient; D004330:Drosophila; D007700:Kinetics; D046911:Macromolecular Substances; D008562:Membrane Glycoproteins / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism*; D008565:Membrane Proteins / Q000302:isolation & purification / Q000378:metabolism; D008961:Models, Structural; D009419:Nerve Tissue Proteins / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism*; D050765:Qa-SNARE Proteins; D051381:Rats; D011994:Recombinant Proteins / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism; D013574:Synaptosomes / Q000378:metabolism; D050857:Synaptotagmins
D002135:Calcium-Binding Proteins; D002413:Cations, Divalent; D046911:Macromolecular Substances; D008562:Membrane Glycoproteins; D008565:Membrane Proteins; D009419:Nerve Tissue Proteins; D050765:Qa-SNARE Proteins; D011994:Recombinant Proteins; D050857:Synaptotagmins; D002118:Calcium