Subunit of clathrin-associated adaptor protein complex 1 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. Belongs to the adaptor complexes small subunit family. 4 isoforms of the human protein are produced by alternative splicing. Note: This description may include information from UniProtKB.