| Clathrin adaptor complex small chain | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Identifiers | |||||||||
| Symbol | Clat_adaptor_s | ||||||||
| Pfam | PF01217 | ||||||||
| InterPro | IPR022775 | ||||||||
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Clathrin adaptor proteins, also known asadaptins, arevesicular transport adaptor proteins associated withclathrin. The association between adaptins and clathrin are important for vesicularcargo selection and transporting.[1] Clathrin coats contain both clathrin (acts as a scaffold) and adaptor complexes that link clathrin to receptors in coatedvesicles. Clathrin-associated protein complexes are believed to interact with the cytoplasmic tails ofmembrane proteins, leading to their selection and concentration. Therefore, adaptor proteins are responsible for the recruitment of cargo molecules into a growing clathrin-coated pits.[2] The two major types of clathrin adaptor complexes are theheterotetrameric vesicular transport adaptor proteins (AP1-5), and the monomericGGA (Golgi-localising, Gamma-adaptin ear homology,ARF-binding proteins) adaptors.[3][4] Adaptins are distantly related to the other main type of vesicular transport proteins, thecoatomer subunits, sharing between 16% and 26% of their amino acid sequence.[5]
Adaptor protein (AP) 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 asbinding accessory proteins that regulate coat assembly and disassembly (such asAP180,epsins andauxilin). There are different AP complexes in mammals. AP1 is responsible for the transport oflysosomalhydrolases between the trans-Golgi network, andendosomes.[6]AP2 adaptor complex associates with the plasma membrane and is responsible forendocytosis.[7] AP3 is responsible for protein trafficking to lysosomes and other relatedorganelles.[8] 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 oftransmembrane cargo proteins.[9] One function of clathrin and AP2 complex-mediated endocytosis is to regulate the number ofGABAA receptors available at the cell surface .[10]