Department of Molecular Neurobiology, Max Planck Institute of Experimental Medicine, Göttingen, Germany
Paulo S Pinheiro
Department of Neuroscience, University of Copenhagen, Copenhagen, Denmark; Center for Neuroscience and Cell Biology, University of Coimbra, Coimbra, Portugal
Sonja M Wojcik
Department of Molecular Neurobiology, Max Planck Institute of Experimental Medicine, Göttingen, Germany
Nils Brose
Department of Molecular Neurobiology, Max Planck Institute of Experimental Medicine, Göttingen, Germany
Benjamin H Cooper
Department of Molecular Neurobiology, Max Planck Institute of Experimental Medicine, Göttingen, Germany
Synaptotagmins confer calcium-dependence to the exocytosis of secretory vesicles, but how coexpressed synaptotagmins interact remains unclear. We find that synaptotagmin-1 and synaptotagmin-7 when present alone act as standalone fast and slow Ca2+-sensors for vesicle fusion in mouse chromaffin cells. When present together, synaptotagmin-1 and synaptotagmin-7 are found in largely non-overlapping clusters on dense-core vesicles. Synaptotagmin-7 stimulates Ca2+-dependent vesicle priming and inhibits depriming, and it promotes ubMunc13-2- and phorbolester-dependent priming, especially at low resting calcium concentrations. The priming effect of synaptotagmin-7 increases the number of vesicles fusing via synaptotagmin-1, while negatively affecting their fusion speed, indicating both synergistic and competitive interactions between synaptotagmins. Synaptotagmin-7 places vesicles in close membrane apposition (<6 nm); without it, vesicles accumulate out of reach of the fusion complex (20–40 nm). We suggest that a synaptotagmin-7-dependent movement toward the membrane is involved in Munc13-2/phorbolester/Ca2+-dependent priming as a prelude to fast and slow exocytosis triggering.
