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. Selectively localized to the brain (at protein level). Negligible levels at embryonic stages E9 and E15. RAB3IL1 levels increase progressively at postnatal ages P3, P7 and P13 with maximal levels in adult brain. Belongs to the SEC2 family.