Subunit of the adaptor protein complex 2 (AP-2). Adaptor protein complexes function in protein transport via transport vesicles in different membrane traffic pathways. Adaptor protein complexes are vesicle coat components and appear to be involved in cargo selection and vesicle formation. AP-2 is involved in clathrin-dependent endocytosis in which cargo proteins are incorporated into vesicles surrounded by clathrin (clathrin-coated vesicles, CCVs) which are destined for fusion with the early endosome. The complex binds polyphosphoinositides (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) and a small adaptin (sigma-type subunit). Component of the coat surrounding the cytoplasmic face of coated vesicles in the plasma membrane. Belongs to the adaptor complexes small subunit family.