1. Department of Physiology, University of Wisconsin–Madison, 1300 University Avenue, Madison, Wisconsin 53706, USA
2. Department of Comparative Biosciences, Endocrinology-Reproductive Physiology Program, University of Wisconsin–Madison, 1300 University Avenue, Madison, Wisconsin 53706, USA
3. Biophysics Program, University of Wisconsin–Madison, 1300 University Avenue, Madison, Wisconsin 53706, USA
4. Department of Biochemistry, University of Wisconsin–Madison, 1300 University Avenue, Madison, Wisconsin 53706, USA

Correspondence to: Meyer B. Jackson¹ Email: mjackson@physiology.wisc.edu

Abstract

Exocytosis—the release of the contents of a vesicle—proceeds by two mechanisms^{1, 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.