CHM and CHML are the substrate-binding subunits of the RAB geranylgeranyltransferase (GGTase) complex. CHMs, also known as RAB escort proteins (REPs) bind to unprenylated RAB proteins in the GDP bound state (Seabra, 1996). In the classical model of RAB recruitment, CHM proteins first bind the unprenylated RAB alone and then present it to the catalytic dimer of the RAB GGTase, while in the alternative model, depicted here, RAB recruitment occurs after the GGPP-dependent formation of a highly stable trimeric GGTase complex (Andres et al, 1993; Thoma et al, 2001a; Thoma et al 2001b; Baron and Seabra, 2008). After geranylgeranylation, binding of additional GGPP to the GGTase promotes release of the CHM:RAB complex, possibly through an allosteric mechanism (Baron and Seabra, 2008). CHM proteins remain in complex with the RABs after geranylgeranylation, dissociating after the RAB has been transferred to the target membrane (Alexandrov et al, 1994; Shen and Seabra, 1996; Baron and Seabra, 2008).