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). 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. The Rab GGTase dimer (RGGT) interacts with CHM (component A) prior to Rab protein binding; the association is stabilized by geranylgeranyl pyrophosphate (GGpp). The CHM:RGGT:Rab complex is destabilized by GGpp. 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). Belongs to the protein prenyltransferase subunit beta family.