Subunit of clathrin-associated adaptor protein complex 1 that plays a role in protein sorting in the trans-Golgi network (TGN) and endosomes. The AP complexes mediate the recruitment of clathrin to membranes and the recognition of sorting signals within the cytosolic tails of transmembrane cargo molecules. Adaptor protein complex 1 (AP-1) is a heterotetramer composed of two large adaptins (gamma-type subunit AP1G1 and beta-type subunit AP1B1), a medium adaptin (mu-type subunit AP1M1 or AP1M2) and a small adaptin (sigma-type subunit AP1S1 or AP1S2 or AP1S3). Interacts with P2X4 (By similarity). Component of the coat surrounding the cytoplasmic face of coated vesicles located at the Golgi complex. Phosphorylation of membrane-bound AP1M1/AP1M2 increases its affinity for sorting signals. Belongs to the adaptor complexes medium subunit family.