Ca²⁺-dependent exocytosis in mast cells is stimulated by the Ca²⁺ sensor, synaptotagmin I

Mast cells secrete a variety of biologically active substances that mediate inflammatory responses. Synaptotagmin(s)-(Syts) are a gene family of proteins that are implicated in the control of Ca²⁺-dependent exocytosis. In the present study, we investigated the possible occurrence and functional involvement of Syt in the control of mast cell exocytosis. Here, we demonstrate that both connective tissue type and mucosal-like mast cells express Syt-immunoreactive proteins, and that these proteins are localized almost exclusively to their secretory granules. Furthermore, expression of Syt I, the neuronal Ca²⁺ sensor, in rat basophilic leukemia cells (RBL- 2H3), a tumor analogue of mucosal mast cells, resulted in prominent potentiation and acceleration of Ca²⁺-dependent exocytosis. Therefore, these findings implicate Syt as a Ca²⁺ sensor that mediates regulated secretion in mast cells to calcium ionophore.