Guanyl-nucleotide exchange factor that regulates small GTPases of the Rab family (PubMed:18559336, PubMed:11359932). Converts GDP-bound inactive form of RAB27A and RAB27B to the GTP-bound active forms (PubMed:18559336). Converts GDP-bound inactive form of RAB3A, RAB3C and RAB3D to the GTP-bound active forms, GTPases involved in synaptic vesicle exocytosis and vesicle secretion (By similarity). Plays a role in synaptic vesicle formation and in vesicle trafficking at the neuromuscular junction (PubMed:11359932, PubMed:12388783, PubMed:18849981). Involved in up-regulating a post-docking step of synaptic exocytosis in central synapses (PubMed:12388783). Probably by binding to the motor proteins KIF1B and KIF1A, mediates motor-dependent transport of GTP-RAB3A-positive vesicles to the presynaptic nerve terminals (PubMed:18849981). Plays a role in TNFA-mediated activation of the MAPK pathway, including ERK1/2 (By similarity). May link TNFRSF1A with MAP kinase activation (By similarity). May be involved in the regulation of TNFA-induced apoptosis (By similarity). Interacts (via death domain) with TNFRSF1A (via death domain) (By similarity). Interacts with PIDD1 (By similarity). Interacts with YWHAZ (By similarity). Interacts (via death domain) with KIF1B (PubMed:18849981). Interacts with KIF1A (PubMed:18849981). Interacts (via uDENN domain) with RAB3A, RAB3B, RAB3C and RAB3D; the GTP-bound form of the Rab proteins is preferred for interaction (PubMed:18849981). Expressed in the brain. Apparently normal in utero development but immediate death after birth, probably due acute respiratory failure (PubMed:11359932). Absent GDP/GTP exchange activity of Rab3A (PubMed:11359932). Apparently normal development of the central nervous system during embryonic stages (PubMed:11359932). At 18.5 dpc, no evoked action potentials of the diaphragm and gastrocnemius muscles in response to electrical stimulation of the phrenic and sciatic nerves, respectively. In contrast, axonal conduction of the spinal cord and the phrenic nerve are not impaired. Reduced total numbers of synaptic vesicles at the neuromuscular junction, especially those docked at the presynaptic plasma membrane, whereas postsynaptic structures and functions appear normal (PubMed:11359932). Reduction of excitatory postsynaptic current amplitude in neurons (PubMed:12388783). Synaptic vesicles locate remote from the presynaptic membrane in the central region instead of at the active zones of the presynaptic terminal (PubMed:12388783). RNAi-mediated knockdown results in reduced levels of Rab27a in the activated GTP-bound form (PubMed:18559336). In melanocytes, results in aggregation and perinuclear clustering of melanosomes (PubMed:18559336). RNAi-mediated knockdown in hippocampal neurons leads reduced interaction between Kif1b and Rab3 and to reduced axonal transport of Rab3 (PubMed:18849981). Belongs to the MADD family. Extended N-terminus.