Substrate-binding subunit of the Rab geranylgeranyltransferase (GGTase) complex (By similarity). Binds unprenylated Rab proteins and presents the substrate peptide to the catalytic component B and remains bound to it after the geranylgeranyl transfer reaction (By similarity). The component A is thought to be regenerated by transferring its prenylated Rab back to the donor membrane (By similarity). Plays a role in neurotransmitter release from presynaptic terminals at neuromuscular junctions (PubMed:19090809). Positively regulates the function of rab-27 in synaptic transmission most likely through mediating rab-27 prenylation (PubMed:19090809). May interact with rab-5, rab-7 and rab-11 (PubMed:19090809). Does not interact with rab-3, rab-27 and rab-10 (PubMed:19090809). Expressed in several neurons including head neurons, motor neurons located in the ventral nerve cord, HSN and CAN neurons, and tail neurons, and in muscles such as body-wall, pharyngeal, intestinal and anal sphincter (PubMed:19090809). Also expressed in seam cells, the hypodermis and the intestine (PubMed:19090809). RNAi-mediated knockdown results in mild defecation defects (PubMed:19090809). RNAi-mediated knockdown increases the levels of unprenylated rab-27 (PubMed:19090809). RNAi-mediated knockdown disrupts the localization of rab-5 and rab-7 (PubMed:19090809). Belongs to the Rab GDI family.