Component of the biogenesis of lysosome-related organelles complex-1 (BLOC-1) involved in pigment granule biogenesis and membrane trafficking in synapses (PubMed:20015953, PubMed:28317021). In response to high synaptic activity at neuromuscular junctions, stabilizes Pldn protein levels and, together with Pldn, plays a role in promoting efficient synaptic vesicle recycling and re-formation through early endosomes (PubMed:28317021). Component of the biogenesis of lysosome-related organelles complex-1 (BLOC-1) composed of Blos1, Blos2, Blos3, Blos4, Dysb, Muted, Pldn and Snapin. Interacts with Pldn. Show reduced eye pigmentation. Belongs to the BLOC1S1 family.