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. {ECO:0000269|PubMed:12356470, ECO:0000269|PubMed:15186776, ECO:0000269|PubMed:8294464}. This is the function of CHML (CHM like Rab escort protein, ENSG00000203668).