Synthetic peptides of the rab3 effector domain stimulate a membrane fusion event involved in regulated exocytosis.

We have developed a system in which the fusion of pancreatic zymogen granules with plasma membranes can be studied in vitro. Here we show that this membrane fusion event is stimulated specifically by peptides of the effector domain of rab3, a small, monomeric GTP-binding protein. In addition, we demonstrate that the stimulatory effect of the peptides involves their binding to a target on the plasma membrane, and is both qualitatively and quantitatively different from the effect of GTP gamma S, which also enhances membrane fusion. We suggest that regulated exocytosis in the pancreatic acinar cell may be under the control of more than one type of GTP-binding protein.