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 (By similarity). 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. Belongs to the adaptor complexes large subunit family. The gene for this protein is duplicated in strains AX3 and AX4. These strains contain a duplication of a segment of 750 kb of chromosome 2 compared to the corresponding sequence in strain AX2.