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 (By similarity). Monomer. 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. Interacts with RAB1A, RAB7A and RAB27A, but has much lower affinity for RAB1A, RAB7A and RAB27A than CHM (By similarity). Interacts with the non-phosphorylated forms of RAB3A, RAB3B, RAB3C, RAB3D, RAB5B, RAB5C, RAB8A, RAB8B, RAB10, RAB12, RAB35, and RAB43 (By similarity). Belongs to the Rab GDI family.