Substrate-binding subunit of the Rab geranylgeranyltransferase (GGTase) complex. Binds unprenylated Rab proteins and presents the substrate peptide to the catalytic component composed of the alpha subunit RGTA and the beta subunit RGTB (Probable). Preferentially binds the GDP-bound form of Rab and stimulates geranylgeranylation of various Rab GTPases in vitro (PubMed:15854662). 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. Expressed in roots, leaves and flowers. Belongs to the Rab GDI family.