Catalyzes the transfer of a geranylgeranyl moiety from geranylgeranyl diphosphate to both cysteines of Rab proteins with the C-terminal sequence -CCXX, CXXX, -XCCX and -XCXC, such as RABA1A, RABA2A, RABF2A and RABG2 (By similarity). Does not seem to be a functional Rab-GGT alpha subunit in vitro (PubMed:26589801). geranylgeranyl diphosphate + L-cysteinyl-[protein] = diphosphate + S-geranylgeranyl-L-cysteinyl-[protein] The enzymatic reaction requires the aid of the Rab escort protein REP. Heterotrimer composed of the alpha subunit RGTA, the beta subunit RGTB and REP; within this trimer, RGTA and RGTB form the catalytic component, while REP mediates peptide substrate binding. Belongs to the protein prenyltransferase subunit alpha family.