Figure 5. A diagram depicting the biogenesis of melanosomes and protein complexes involved in Hermansky-Pudlak syndrome (HPS). The four stages of melanosome maturation (I-IV) are regulated by four protein complexes (AP-3, BLOC-1, BLOC-2, and BLOC-3). Each protein complex has between 2 and 4 subunit components. When these subunit components are mutated, they result in HPS. BLOC-3 is required for activation of Rabs through conversion of GDP to GTP in the final step of melanosome maturation. The location of the 11 HPS loci in the four protein complexes are indicated in red text.