Rab3, a neural/neuroendocrine-specific member of the Rab family, is involved in Ca2+-regulated exocytosis (1-2). Rab3 functions in an inhibitory capacity by controlling the recruitment of secretory vesicles into a releasable pool at the plasma membrane. Rim (rab3 interacting molecule), a putative effector protein for Rab3s, is composed of an amino-terminal zinc-finger motif and carboxy-terminal PDZ and C2 domains. Rim exists as two variants, Rim1 and Rim2, produced by alternative splicing (3). Rim1 is expressed near the active zone at the synapse, where it interacts in a GTP-dependent manner with Rab3, located on synaptic vesicles (4). Therefore, Rim serves as a Rab3-dependent regulator of synaptic-vesicle fusion by forming a GTP-dependent complex between synaptic plasma membranes and docked synaptic vesicles (5). Both Rim1 and Rim2 can bind to cAMP-GEFII, which is a direct target of cAMP in regulated exocytosis and is responsible for cAMP-dependent, PKA-dependent exocytosis (3). Rim also localizes on the plasma membrane of INS-1E cells and pancreatic beta-cells. Rab3 binding domain of Rim enhances glucose-stimulated secretion in intact cells and Ca2+-stimulated exocytosis in permeabilized cells, suggesting that Rim may also play a regulatory role in insulin secretion (6).