AP2S1 is a component of the adaptor protein complex 2 (AP-2). AP complexes are cytosolic heterotetramers that mediate the sorting of membrane proteins in the secretory and endocytic pathways. AP complexes are involved in the formation of clathrin-coated vesicles (CCVs) by recruiting the scaffold protein, clathrin. AP complexes also play a pivotal role in the cargo selection by recognizing the sorting signals within the cytoplasmic tail of integral membrane proteins. AP-2 is composed of two large adaptins (alpha-type subunit AP2A1 or AP2A2 and beta-type subunit AP2B1), a medium adaptin (mu-type subunit AP2M1) and a small adaptin (sigma-type subunit AP2S1). It works on the plasma membrane to internalize cargo in clathrin-mediated endocytosis.