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.