The function of RAB3GAP1 (RAB3 GTPase activating protein catalytic subunit 1, Ensembl gene identifier ENSG00000115839) is as follows. Catalytic subunit of the Rab3 GTPase-activating (Rab3GAP) complex composed of RAB3GAP1 and RAB3GAP2, which accelerates the otherwise slow GTP hydrolysis catalyzed by Rab proteins (PubMed:9030515, PubMed:10859313, PubMed:39779760). Has GTPase- activating protein (GAP) activity towards various Rab3 subfamily members (RAB3A, RAB3B, RAB3C and RAB3D), RAB5A and RAB43 (PubMed:10859313, PubMed:9030515, PubMed:39779760). Additionally, it has guanine nucleotide exchange factor (GEF) activity towards RAB18, promoting GDP release from RAB18 and the conversion of inactive RAB18- GDP to the active form RAB18-GTP (PubMed:24891604, PubMed:39779760). Recruits and stabilizes RAB18 at the cis-Golgi membrane in fibroblasts where RAB18 is most likely activated (PubMed:26063829). Also involved in RAB18 recruitment at the endoplasmic reticulum (ER) membrane where it maintains proper ER structure (PubMed:24891604). Required for normal eye and brain development (PubMed:15696165, PubMed:23420520). May participate in neurodevelopmental processes such as cell proliferation, migration and differentiation before synapse formation, and is involved in regulating non-synaptic vesicular release of neurotransmitters (PubMed:9030515, PubMed:9852129). {ECO:0000269|PubMed:10859313, ECO:0000269|PubMed:15696165, ECO:0000269|PubMed:23420520, ECO:0000269|PubMed:24891604, ECO:0000269|PubMed:26063829, ECO:0000269|PubMed:39779760, ECO:0000269|PubMed:9030515, ECO:0000269|PubMed:9852129}.