Flab geranylgeranyl transferase (GG transferase) from rat brain contains two components, A and B. Component B comprises polypeptides of 60 and 36 kd. Here we report the purification of component A, a single 95 kd polypeptide. The holoenzyme attaches 3H-geranylgeranyl to cysteines in two GTP-binding proteins, Rab3A and RablA. The reaction is abolished when both cystelnes in the COOH-terminal CysCys sequence of RablA are mutated to serines. The mutant protein inhibits transfer of 3H-geranylgeranyl to wildtype RablA and RabBA, suggesting that the enzyme recognizes conserved sequences distinct from the COOH-terminus. Six peptides from rat component A show striking similarity to the product of the defective gene in choroideremia, an X-linked retinal degeneration disease. The choroideremia protein resembles Rab3A GDI, which binds Rab3A. We hypothesize that component A binds conserved sequences in Rab and that component B transfers geranylgeranyl. A defect in this reaction may cause choroideremia.