Enables Rab geranylgeranyltransferase activity. Acts upstream of or within protein geranylgeranylation. Predicted to be located in nucleoplasm. Predicted to be part of Rab-protein geranylgeranyltransferase complex. Predicted to be active in cytoplasm. Used to study Hermansky-Pudlak syndrome. Orthologous to human RABGGTA (Rab geranylgeranyltransferase subunit alpha). [provided by Alliance of Genome Resources, Apr 2022]