Identification of Snapin and Three Novel Proteins (BLOS1, BLOS2, and BLOS3/Reduced Pigmentation) as Subunits of Biogenesis of Lysosome-related Organelles Complex-1 (BLOC-1)

Abstract

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.