Exocytotic fusion is activated by Rab3a peptides

STUDIES of intracellular traffic in yeast and mammalian systems have implicated members of the Rab family of small GTP-binding proteins as regulators of membrane fusion^1-10. We have used the patch clamp technique to measure exocytotic fusion events directly and investigate the role of GTP-binding proteins in regulating exocytosis in mast cells. Intracellular perfusion of mast cells with GTP-γS is sufficient to trigger complete exocytotic degranulation in the absence of other intracellular messengers ¹¹. Here we show that GTP is a potent inhibitor of GTP-γS-induced degranulation, indicating that sustained activation of a GTP-binding protein is sufficient for membrane fusion. We have found that synthetic oligopeptides, corresponding to part of the effector domain of Rab3a¹², stimulate complete exocytotic degranulation, similar to that induced by GTP-γS. The response is selective for Rab3a sequence and is strictly dependent on Mg²⁺ and ATP. This suggests that sustained activation of a Rab3 protein causes exocytotic fusion. The peptide response can be accelerated by GDP-βS, suggesting that Rab3a peptides compete with endogenous Rab3 proteins for a binding site on a target effector protein, which causes fusion on activation.