Clathrin coats contain both clathrin and clathrin adaptor proteins. While clathrin acts as the scaffold, the clathrin adaptors bind to protein and lipid cargo. Specific cargos are recruited into clathrin-coated vesicles with the aid of CLASP proteins (clathrin-associated sorting proteins), such as ARH and Dab2. Clathrin-associated protein complexes are believed to interact with the cytoplasmic tails of membrane proteins, leading to their selection and concentration. At least twenty clathrin adaptors have been identified, which share a common design composed of a compact domain plus a long unstructured region that binds the clathrin beta-propeller. The two major types of clathrin adaptor complexes are: heterotetrameric adaptor protein (AP) complexes, and the monomeric GGA (Golgi-localising, Gamma-adaptin ear domain homology, ARF-binding proteins) adaptors.
AP (adaptor protein) complexes are found in coated vesicles and clathrin-coated pits. AP complexes connect cargo proteins and lipids to clathrin at vesicle budding sites, as well as binding accessory proteins that regulate coat assembly and disassembly (such as AP180, epsins and auxilin). There are different AP complexes in mammals. AP1 is responsible for the transport of lysosomal hydrolases between the TGN and endosomes. AP2 associates with the plasma membrane and is responsible for endocytosis. AP3 is responsible for protein trafficking to lysosomes and other related organelles. AP4 is less well characterised.
AP complexes are heterotetramers composed of two large subunits (adaptins), a medium subunit (mu) and a small subunit (sigma). For example, in AP1 these subunits are gamma-1-adaptin, beta-1-adaptin, mu-1 and sigma-1, while in AP2 they are alpha-adaptin, beta-2-adaptin, mu-2 and sigma-2. Each subunit has a specific function. Adaptins recognise and bind to clathrin through their hinge region (clathrin box), and recruit accessory proteins that modulate AP function through their C-terminal ear (appendage) domains. Mu recognises tyrosine-based sorting signals within the cytoplasmic domains of transmembrane cargo proteins. One function of clathrin and AP2 complex-mediated endocytosis is to regulate the number of GABA(A) receptors available at the cell surface.
GGAs (Golgi-localising, Gamma-adaptin ear domain homology, ARF-binding proteins) are a family of monomeric clathrin adaptor proteins that are conserved from yeasts to humans. GGAs regulate clathrin-mediated the transport of proteins (such as mannose 6-phosphate receptors) from the TGN to endosomes and lysosomes through interactions with TGN-sorting receptors, sometimes in conjunction with AP-1. GGAs bind cargo, membranes, clathrin and accessory factors. GGA1, GGA2 and GGA3 all contain a domain homologous to the ear domain of gamma-adaptin. GGAs are composed of a single polypeptide with four domains: an N-terminal VHS (Vps27p/Hrs/Stam) domain, a GAT (GGA and Tom1) domain, a hinge region, and a C-terminal GAE (gamma-adaptin ear) domain. The VHS domain is responsible for endocytosis and signal transduction, recognising transmembrane cargo through the ACLL sequence in the cytoplasmic domains of sorting receptors. The GAT domain (also found in Tom1 proteins) interacts with ARF (ADP-ribosylation factor) to regulate membrane trafficking, and with ubiquitin for receptor sorting. The hinge region contains a clathrin box for recognition and binding to clathrin, similar to that found in AP adaptins. The GAE domain is similar to the AP gamma-adaptin ear domain, and is responsible for the recruitment of accessory proteins that regulate clathrin-mediated endocytosis.