The adaptor protein complexes mediate both the recruitment of clathrin to membranes and the recognition of sorting signals within the cytosolic tails of transmembrane cargo molecules. Adaptor protein complex 3 (AP-3) appears to be involved in the sorting of a subset of transmembrane proteins targeted to lysosomes and lysosome-related organelles. 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). The subunits of non-clathrin- and clathrin-associated adaptor protein complex 3 play a role in protein sorting in the late-Golgi/trans-Golgi network (TGN) and/or in endosomes. This group represents an adaptor protein complex AP-3, delta subunit.
