Biogenesis of lysosome-related organelles complex-1 (BLOC-1) is a ubiquitously expressed multisubunit
protein complex required for the normal biogenesis of specialized organelles of the endosomal-lysosomal system, such as melanosomes and platelet dense granules. The complex is known to contain the coiled-coil-forming
proteins, Pallidin, Muted, Cappuccino, and
Dysbindin. The genes encoding these
proteins are defective in inbred mouse strains that serve as models of
Hermansky-Pudlak syndrome (HPS), a
genetic disorder characterized by
hypopigmentation and
platelet storage pool deficiency. In addition, mutation of human
Dysbindin causes HPS type 7. Here, we report the identification of another four subunits of the complex. One is Snapin, a coiled-coil-forming
protein previously characterized as a binding partner of synaptosomal-associated
proteins 25 and 23 and implicated in the regulation of membrane fusion events. The other three are previously uncharacterized
proteins, which we named BLOC subunits 1, 2, and 3 (BLOS1, -2, and -3). Using specific
antibodies to detect endogenous
proteins from human and mouse cells, we found that Snapin, BLOS1, BLOS2, and BLOS3 co-immunoprecipitate, and co-fractionate upon size exclusion chromatography, with previously known BLOC-1 subunits. Furthermore, steady-state levels of the four
proteins are significantly reduced in cells from pallid mice, which carry a mutation in Pallidin and display secondary loss of other BLOC-1 subunits. Yeast two-hybrid analyses suggest a network of binary interactions involving all of the previously known and newly identified subunits. Interestingly, the HPS mouse model strain, reduced pigmentation, carries a
nonsense mutation in the gene encoding BLOS3. As judged from size exclusion chromatographic analyses, the reduced pigmentation mutation affects BLOC-1 assembly less severely than the pallid mutation. Mutations in the human genes encoding Snapin and the BLOS
proteins could underlie novel forms of HPS.