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. AP2S1/AP17 is a subunit of the plasma membrane adaptor. The complex binds polyphosphoinositides. 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. Widely expressed in the embryo, endosperm, leaf and root. Belongs to the adaptor complexes small subunit family.