Guanine nucleotide exchange factor (GEF) which may activate RAB3A, a GTPase that regulates synaptic vesicle exocytosis. Promotes the exchange of GDP to GTP, converting inactive GDP-bound Rab proteins into their active GTP-bound form. May also activate RAB8A and RAB8B (By similarity). Interacts with RAB3A and IHPK1 through the coiled-coil domain. This interaction is competitive. IHPK1 kinase activity is not required for this interaction (By similarity). Belongs to the SEC2 family.