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 sequence 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 beta subunit is associated with the alpha subunit RGTA1 (PubMed:26589801). Required for male fertility and root tip growth. geranylgeranyl diphosphate + L-cysteinyl-[protein] = diphosphate + S-geranylgeranyl-L-cysteinyl-[protein] Binds 1 zinc ion per subunit. 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. No visible phenotype under normal growth conditions. The double mutant plants rgtb1 and rgtb2 are male sterile, due to shrunken pollen with abnormal exine structure, and strong disorganization of the endoplasmic reticulum membranes. Belongs to the protein prenyltransferase subunit beta family.