Component of the adaptor complexes which link clathrin to receptors in coated vesicles. Clathrin-associated protein complexes are believed to interact with the cytoplasmic tails of membrane proteins, leading to their selection and concentration. AP50 is a subunit of the plasma membrane adaptor. Adaptor protein complex 2 (AP-2) is a heterotetramer composed of two large adaptins (alpha-type and beta-type subunits), a medium adaptin (mu-type subunit AP50) and a small adaptin (sigma-type subunit AP17). Component of the coat surrounding the cytoplasmic face of coated vesicles in the plasma membrane. Phosphorylated. Belongs to the adaptor complexes medium subunit family.