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 (PubMed:26589801). In vitro, can prenylate PGGTI targets with the C-terminal Cys-aliphatic-aliphatic-X (CaaX) with leucine in the terminal position. Substrates with the C-terminal sequence -CSIL such as ARAC11/ROP1 or GG2/AGG2 are prenylated independently of REP and when the alpha subunit is associated with a beta subunit (RGTB1 or RGTB2) (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.