Regulatory 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. Rab3 GTPase-activating complex 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 (By similarity). Interacts with LMAN1 (By similarity). In neurons, it is enriched in the synaptic soluble fraction. Belongs to the Rab3-GAP regulatory subunit family. The sequence differs from that shown because it seems to be derived from a pre-mRNA.