Component of the BLOC-3 complex, a complex that acts as a guanine exchange factor (GEF) for RAB32 and RAB38, promotes the exchange of GDP to GTP, converting them from an inactive GDP-bound form into an active GTP-bound form. The BLOC-3 complex plays an important role in the control of melanin production and melanosome biogenesis and promotes the membrane localization of RAB32 and RAB38. Component of the biogenesis of lysosome-related organelles complex-3 (or BLOC-3), a heterodimer of HPS1 and HPS4. HPS1 cannot but BLOC-3 complex (heterodimer of HPS1 and HPS4) can interact with the GTP-bound form of RAB9A and RAB9B. HPS1 and BLOC-3 complex do not interact with the GDP-bound form of RAB9A and RAB9B. Expressed in all tissues examined with the possible exception of skeletal muscle. The highest expression was observed in lung, liver, kidney and spleen. Defects in Hps1 are the cause of the pale ear (ep) mutant which exhibits hypopigmentation associated with defects of multiple cytoplasmic organelles, including melanosomes, lysosomes, and granular elements of platelets (PubMed:9256466).