Rim4 (Rab 3 interacting molecule 4), also known as Rim4 or regulating synaptic membrane exocytosis protein 4, is a 269 amino acid protein that localizes to the cell junction and regulates synaptic membrane exocytosis. Rab 3, a neural/neuroendocrine-specific member of the Rab family, is involved in Ca2+-regulated exocytosis. Rab 3 functions in an inhibitory capacity by controlling the recruitment of secretory vesicles into a releasable pool at the plasma membrane. Rim (Rab 3 interacting molecule), a putative effector protein for Rab 3 proteins, is thought to regulate neutrotransmitter release through its interaction with Rab 3 and other synaptic proteins.