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. 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. Interacts with non-phosphorylated form of RAB8A; phosphorylation of RAB8A at 'Thr-72' disrupts this interaction. Defects in Rabggta are the cause of the gunmetal (gm) phenotype. Mice homozygous for gm have prolonged bleeding, thrombocytopenia and reduced platelet alpha- and delta-granule contents. Belongs to the protein prenyltransferase subunit alpha family.