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 (PubMed:23084991). Component of the biogenesis of lysosome-related organelles complex-3 (or BLOC-3), a heterodimer of HPS1 and HPS4 (PubMed:20048159, PubMed:23084991). HPS4 and the BLOC-3 complex interact with the GTP-bound form of RAB9A and RAB9B but not with the GDP-bound form of RAB9A and RAB9B. The BLOC-3 complex does not interact with RAB5A, RAB7A and RAB27A (PubMed:20048159). The disease is caused by variants affecting the gene represented in this entry. Intron retention. Intron retention. Retina International's Scientific Newsletter