AP180, also known as AP-3, F1-20, NP185, and pp155, is one of four assembly proteins which are involved in the organization and assembly of clathrin triskelia in clathrin-coated vesicles. Two of these proteins, AP-1 and AP-2, exist as tetramers, while AP180 and auxilin are monomeric assembly proteins. The clathrin binding potential of AP180 is defined by several regions of the molecule, including the 30 kDa N-terminal domain, a central domain, and a 58 kDa C-terminal domain. Although clathrin binding occurs throughout the molecule, only the C-terminal domain is associated with both binding of clathrin and assembly of clathrin cages. The highly acidic central domain, which contains an uncharged alanine-rich segment, is thought to impart the irregular physical properties to this protein. AP180 is the only clathrin assembly protein specific for synapses and is thought to be involved in synaptic vesicle biogenesis and recycling. Furthermore, AP180 is bound by inositol-6-phosphate which has been shown to be closely regulated in neuronal cells by external stimuli.