Adaptins are heterotetrameric subunits of adaptors, which are complexes involved in the formation of Clathrin-coated pits for vesicle-mediated endocytosis. Clathrin and its associated heterotetrameric protein complexes make up the main protein components of the coat surrounding the cytoplasmic face of coated vesicles. The Adaptin family, comprising a, b, b' and g classes, is also responsible for the transport of ligand-receptor complexes from plasma membranes and the trans-Golgi network to lysosomes. Two main types of adaptor proteins (APs), AP-1 and AP-2, are found in Clathrin-coated structures located at the Golgi complex and the plasma membrane of mammalian cells, respectively. Adaptor protein complex 2 (AP-2) is composed of two large Adaptins (a1A/AP2A1 and b1/AP2B1), a medium Adaptin (m2/AP-2m1) and a small Adaptin (s2 long/AP2S1). AP-2m1, a 435 amino acid protein, links Clathrin to receptors in coated vesicles.
