Adaptins are components 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. Beta adaptin is a subunit of the plasma membrane adaptor. Adaptor protein complex 2 (AP-2) is a heterotetramer composed of two large adaptins (alpha-type subunit APL3 and beta-type subunit APL1), a medium chain (mu-type subunit APM4) and a small adaptin (sigma-type subunit APS2). Interacts with APS2. Component of the coat surrounding the cytoplasmic face of coated vesicles in the plasma membrane. Present with 1670 molecules/cell in log phase SD medium. Belongs to the adaptor complexes large subunit family.