Subunit of non-clathrin- and clathrin-associated adaptor protein complex 3 (AP-3) that plays a role in protein sorting in the late-Golgi/trans-Golgi network (TGN) and/or endosomes. The AP complexes mediate both the recruitment of clathrin to membranes and the recognition of sorting signals within the cytosolic tails of transmembrane cargo molecules. AP-3 appears to be involved in the sorting of a subset of transmembrane proteins targeted to lysosomes and lysosome-related organelles. In concert with the BLOC-1 complex, AP-3 is required to target cargos into vesicles assembled at cell bodies for delivery into neurites and nerve terminals. Adaptor protein complex 3 (AP-3) is a heterotetramer composed of two large adaptins (delta-type subunit AP3D1 and beta-type subunit AP3B1 or AP3B2), a medium adaptin (mu-type subunit AP3M1 or AP3M2) and a small adaptin (sigma-type subunit APS1 or AP3S2) (PubMed:21998198). AP-3 associates with the BLOC-1 complex (PubMed:21998198). Interacts with KIF3A; interaction is direct; interaction is impaired by pyrophosphorylation of AP3B1 (By similarity). Component of the coat surrounding the cytoplasmic face of coated vesicles located at the Golgi complex. Ubiquitously expressed. Phosphorylated on serine residues. Pyrophosphorylated by 5-diphosphoinositol pentakisphosphate (5-IP7) (PubMed:17873058). Pyrophosphorylation impairs interaction with KIF3A (By similarity). Serine pyrophosphorylation is achieved by Mg(2+)-dependent, but enzyme independent transfer of a beta-phosphate from a inositol pyrophosphate to a pre-phosphorylated serine residue (PubMed:17873058). Defects in Ap3b1 are the cause of the autosomal recessive phenotype 'pearl' (pe). Pearl mice exhibit hypopigmentation, lysosomal secretion abnormalities, and platelet-dense granules with reduced levels of adenine nucleotides and serotonin. The changes in platelets lead to prolonged bleeding. Additionally, pearl mice exhibit reduced sensitivity in the dark-adapted state (PubMed:9931340). Belongs to the adaptor complexes large subunit family.