FUNCTION: 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. SUBUNIT: Adaptor protein complex 1 (AP-1) is an heterotetramer composed of two large adaptins (gamma1/AP1G1 or gamma2/AP1G2 and beta1A/AP1B1 or beta1B/AP1B1), a medium adaptin (mu1A/AP1M1 or mu1B/AP1M2) and a small adaptin (sigma1A/AP1S1 or sigma1B/AP1S2 or sigma1C/AP1S3). SUBCELLULAR LOCATION: Golgi apparatus. Cytoplasmic vesicle; cytoplasmic vesicle membrane; peripheral membrane protein; cytoplasmic side. Note=Component of the coat surrounding the cytoplasmic face of coated vesicles located at the Golgi complex. ALTERNATIVE PRODUCTS: 2 named isoforms produced by alternative splicing. TISSUE SPECIFICITY: Widely expressed. DISEASE: Deletion of the AP1B1 gene may play a role in the tumorigenesis of meningiomas. SIMILARITY: Belongs to the adaptor complexes large subunit family.