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. 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 (By similarity).