Exocytosis—the release of the contents of a vesicle—proceeds by two mechanisms1,2,3,4,5,6. Full fusion occurs when the vesicle and plasma membranes merge. Alternatively, in what is termed kiss-and-run, vesicles can release transmitter during transient contacts with the plasma membrane. Little is known at the molecular level about how the choice between these two pathways is regulated. Here we report amperometric recordings of catecholamine efflux through individual fusion pores. Transfection with synaptotagmin (Syt) IV increased the frequency and duration of kiss-and-run events, but left their amplitude unchanged. Endogenous Syt IV, induced by forskolin treatment, had a similar effect. Full fusion was inhibited by mutation of a Ca2+ ligand in the C2A domain of Syt I; kiss-and-run was inhibited by mutation of a homologous Ca2+ ligand in the C2B domain of Syt IV. The Ca2+ sensitivity for full fusion was 5-fold higher with Syt I than Syt IV, but for kiss-and-run the Ca2+ sensitivities differed by a factor of only two. Syt thus regulates the choice between full fusion and kiss-and-run, with Ca2+ binding to the C2A and C2B domains playing an important role in this choice.
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We thank L. A. Schuler for support and access to facilities; R. Jahn and S. Engers for antibodies against Syt I; R. Grishanin, C. A. Earles, J. A. Kowalchyk, and X. Han for discussions; and B. August of the University of Wisconsin Electron Microscopy Facility for performing electron microscopy. This work was supported by grants from the NIH to E.R.C., T.F.J.M. and M.B.J. E.R.C. is a Pew Scholar.
The authors declare that they have no competing financial interests.
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