Substrate-binding subunit (component A) of the Rab geranylgeranyltransferase (GGTase) complex. Binds unprenylated Rab proteins and presents the substrate peptide to the catalytic component B. The component A is thought to be regenerated by transferring its prenylated Rab back to the donor membrane. Less effective than CHM in supporting prenylation of Rab3 family. Monomer (PubMed:8294464). Heterotrimer composed of RABGGTA, RABGGTB and CHML; within this trimer, RABGGTA and RABGGTB form the catalytic component B, while CHML (component A) mediates Rab protein binding (PubMed:12356470). Interacts with RAB1A, RAB7A and RAB27A, but has much lower affinity for RAB1A, RAB7A and RAB27A than CHM (PubMed:15186776). Interacts with the non-phosphorylated forms of RAB3A, RAB3B, RAB3C, RAB3D, RAB5B, RAB5C, RAB8A, RAB8B, RAB10, RAB12, RAB35, and RAB43 (PubMed:26824392, PubMed:29125462). Substitutes for REP-1 thereby preventing widespread tissue abnormalities in patients with choroideremia who lack REP-1. Belongs to the Rab GDI family.