Stimulates GDP/GTP exchange reaction of a group of small GTP-binding proteins (G proteins) including Rap1a/Rap1b, RhoA, RhoB and KRas, by stimulating the dissociation of GDP from and the subsequent binding of GTP to each small G protein. Interacts with ralB (PubMed:9753634). Probably interacts with the post-translationally isoprenylated (geranyl-geranylation) forms of ral proteins. Interacts with both GDP-bound and GTP-bound forms of ralA, but interaction is much stronger with ralA-GDP (PubMed:9753634). Weakly expressed in adult tissues with highest levels found in spleen, kidney, skin and A6 cells. Expressed both maternally and zygotically from oogenesis through to late embryogenesis at a constant level.