Pathways & interactions
AP complex, mu/sigma subunit (IPR022775)
Short name: AP_mu_sigma_su
Overlapping homologous superfamilies
- Longin-like domain superfamily (IPR011012)
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. This traffic is bidirectional, to ensure that proteins required to form vesicles are recycled. Vesicles have specific coat proteins (such as clathrin or coatomer) that are important for cargo selection and direction of transfer [PMID: 15261670].
Clathrin coats contain both clathrin 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 [PMID: 17449236]. All AP complexes are heterotetramers composed of two large subunits (adaptins), a medium subunit (mu) and a small subunit (sigma). Each subunit has a specific function. Adaptin subunits recognise and bind to clathrin through their hinge region (clathrin box), and recruit accessory proteins that modulate AP function through their C-terminal appendage domains. By contrast, GGAs are monomers composed of four domains, which have functions similar to AP subunits: 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 GAE domain is similar to the AP gamma-adaptin ear domain, being responsible for the recruitment of accessory proteins that regulate clathrin-mediated endocytosis [PMID: 12858162].
While clathrin mediates endocytic protein transport from ER to Golgi, coatomers (COPI, COPII) primarily mediate intra-Golgi transport, as well as the reverse Golgi to ER transport of dilysine-tagged proteins [PMID: 14690497]. Coatomers reversibly associate with Golgi (non-clathrin-coated) vesicles to mediate protein transport and for budding from Golgi membranes [PMID: 17041781]. Coatomer complexes are hetero-oligomers composed of at least an alpha, beta, beta', gamma, delta, epsilon and zeta subunits.
This entry represents the small sigma and mu subunits of various adaptins from different AP clathrin adaptor complexes (including AP1, AP2, AP3 and AP4), and the zeta and delta subunits of various coatomer (COP) adaptors. The small sigma subunit of AP proteins have been characterised in several species [PMID: 8157009, PMID: 8373805, PMID: 8157009, PMID: 9002613]. The sigma subunit plays a role in protein sorting in the late-Golgi/trans-Golgi network (TGN) and/or endosomes. The zeta subunit of coatomers (zeta-COP) is required for coatomer binding to Golgi membranes and for coat-vesicle assembly [PMID: 8276893, PMID: 14729954].
- PF01217 (Clat_adaptor_s)