Subunit of clathrin-associated adaptor protein complex 1 that plays a role in protein sorting at the trans-Golgi network and early endosomes (TGN/EE). The AP complexes mediate the recruitment of clathrin to membranes and the recognition of sorting signals within the cytosolic tails of transmembrane cargo molecules. Functions redundantly with AP1M2 in multiple post-Golgi trafficking pathways leading from the TGN to the vacuole, the plasma membrane, and the cell-division plane. Adaptor protein complex 1 (AP-1) is a heterotetramer composed of two large adaptins (gamma-type subunit and beta-type subunit), a medium adaptin (mu-type subunit) and a small adaptin (sigma-type subunit). Growth retardation phenotype. Belongs to the adaptor complexes medium subunit family.