Probable catalytic subunit of a GTPase activating protein that has specificity for Rab3 subfamily (RAB3A, RAB3B, RAB3C and RAB3D). Rab3 proteins are involved in regulated exocytosis of neurotransmitters and hormones. Specifically converts active Rab3-GTP to the inactive form Rab3-GDP. Required for normal eye and brain development. May participate in neurodevelopmental processes such as proliferation, migration and differentiation before synapse formation, and non-synaptic vesicular release of neurotransmitters (By similarity). The Rab3 GTPase-activating complex is a heterodimer composed of RAB3GAP and RAB3-GAP150. The Rab3 GTPase-activating complex interacts with DMXL2. Interacts with LMAN1. In neurons, it is enriched in the synaptic soluble fraction. In the eye, it is highly expressed within the lens, particularly in the anterior lens epithelium and in a ring corresponding to the equatorial region where anterior cells are differentiating into lens fibers. Also highly expressed in the retina. From 10 dpc to 12 dpc, it is weakly expressed throughout the embryo. At 14.5 dpc, it is predominantly expressed in a number of organ systems, including the central and peripheral nervous systems. Belongs to the Rab3-GAP catalytic subunit family.