Substrate-binding subunit of the Rab geranylgeranyltransferase (GGTase) complex. Binds unprenylated Rab proteins and presents the substrate peptide to the catalytic component B composed of RABGGTA and RABGGTB, and remains bound to it after the geranylgeranyl transfer reaction. The component A is thought to be regenerated by transferring its prenylated Rab back to the donor membrane. Besides, a pre-formed complex consisting of CHM and the Rab GGTase dimer (RGGT or component B) can bind to and prenylate Rab proteins; this alternative pathway is proposed to be the predominant pathway for Rab protein geranylgeranylation. Monomer (PubMed:8513495, PubMed:1525821). Heterotrimer composed of RABGGTA, RABGGTB and CHM; within this trimer, RABGGTA and RABGGTB form the catalytic component B, while CHM (component A) mediates Rab protein binding (PubMed:12620235). Can associate with the Rab GGTase dimer (RGGT or component B) prior to Rab protein binding; the association is stabilized by geranylgeranyl pyrophosphate (GGpp). The CHM:RGGT:Rab complex is destabilized by GGpp (By similarity). Interacts with RAB1A, RAB1B, RAB7A and RAB27A and mediates their prenylation (PubMed:11389151, PubMed:12356470, PubMed:15186776, PubMed:8513495). Interacts with RAB5A (By similarity). Interacts with the non-phosphorylated forms of RAB3A, RAB3B, RAB3C, RAB3D, RAB5B, RAB5C RAB8A, RAB8B, RAB10, RAB12, RAB35, and RAB43 (By similarity). Most abundant in the heart, brain, and spleen. Lower levels seen in the lung, liver, muscle and kidney. Extremely low levels seen in the testis. Belongs to the Rab GDI family.