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] 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:7991565). 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 (PubMed:18532927). Interacts with non-phosphorylated form of RAB8A; phosphorylation of RAB8A at 'Thr-72' disrupts this interaction (PubMed:26824392). Belongs to the protein prenyltransferase subunit alpha family.