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. 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 (PubMed:22337587). In neurons, it is enriched in the synaptic soluble fraction. Ubiquitous. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. Belongs to the Rab3-GAP regulatory subunit family.