<p>Proteins synthesized on the ribosome and processed in the endoplasmic reticulum are transported from the Golgi apparatus to the trans-Golgi network (TGN), and from there via small carrier vesicles to their final destination compartment. These vesicles have specific coat proteins (such as clathrin or coatomer) that are important for cargo selection and direction of transport [<cite idref="PUB00035769"/>]. Clathrin coats contain both clathrin (acts as a scaffold) and adaptor complexes that link clathrin to receptors in coated vesicles. Clathrin-associated protein complexes are believed to interact with the cytoplasmic tails of membrane proteins, leading to their selection and concentration. The two major types of clathrin adaptor complexes are the heterotetrameric adaptor protein (AP) complexes, and the monomeric GGA (Golgi-localising, Gamma-adaptin ear domain homology, ARF-binding proteins) adaptors [<cite idref="PUB00035753"/>, <cite idref="PUB00035765"/>].</p><p>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 [<cite idref="PUB00035754"/>]. AP2 associates with the plasma membrane and is responsible for endocytosis [<cite idref="PUB00035755"/>]. AP3 is responsible for protein trafficking to lysosomes and other related organelles [<cite idref="PUB00035756"/>]. 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 [<cite idref="PUB00010644"/>]. One function of clathrin and AP2 complex-mediated endocytosis is to regulate the number of GABA(A) receptors available at the cell surface [<cite idref="PUB00035757"/>]. </p><p>This entry represents the mu subunit of various clathrin adaptors (AP1, AP2 and AP3) [<cite idref="PUB00025539"/>]. The mu subunit regulates the coupling of clathrin lattices with particular membrane proteins by self-phosphorylation via a mechanism that is still unclear [<cite idref="PUB00001404"/>]. The mu subunit possesses a highly conserved N-terminal domain of around 230 amino acids, which may be the region of interaction with other AP proteins; a linker region of between 10 and 42 amino acids; and a less well-conserved C-terminal domain of around 190 amino acids, which may be the site of specific interaction with the protein being transported in the vesicle [<cite idref="PUB00001404"/>].</p><p>More information about these proteins can be found at Protein of the Month: Clathrin [<cite idref="PUB00035905"/>].</p>