Catalyzes the transfer of a geranylgeranyl moiety from geranylgeranyl diphosphate to both cysteines of Rab proteins with the C-terminal sequence -XXCC, -XCXC and -CCXX, such as RAB1A, RAB3A, RAB5A and RAB7A. geranylgeranyl diphosphate + L-cysteinyl-[protein] = diphosphate + S-geranylgeranyl-L-cysteinyl-[protein] Binds 1 zinc ion per subunit. The enzymatic reaction requires the aid of a Rab escort protein (also called component A), such as CHM. Heterotrimer composed of RABGGTA, RABGGTB and CHM; within this trimer, RABGGTA and RABGGTB form the catalytic component B, while CHM (component A) mediates peptide substrate binding (PubMed:10745007, PubMed:12620235, PubMed:18399557, PubMed:18756270, PubMed:19894725, PubMed:22480322, PubMed:8505342, PubMed:22963166). The Rab GGTase dimer (RGGT) interacts with CHM (component A) prior to Rab protein binding; the association is stabilized by geranylgeranyl pyrophosphate (GGpp) (PubMed:11675392). The CHM:RGGT:Rab complex is destabilized by GGpp (PubMed:11675392). Interaction of RABGGTB with prenylated PTP4A2 precludes its association with RABGGTA and inhibits enzyme activity (By similarity). Interacts with CHODL (By similarity). Interacts with non-phosphorylated form of RAB8A; phosphorylation of RAB8A at 'Thr-72' disrupts this interaction (By similarity). Most abundant in the heart, brain, spleen and liver. Less in the lung, muscle, kidney and testis; in these tissues, more abundant than the subunit alpha. Belongs to the protein prenyltransferase subunit beta family.