Clathrin adaptor proteins, also known as adaptins, are
vesicular transport adaptor protein
Vesicular transport adaptor proteins are proteins involved in forming complexes that function in the trafficking of molecules from one subcellular location to another. These complexes concentrate the correct cargo molecules in vesicles that bud o ...
s associated with
clathrin. These proteins are synthesized in the
ribosome
Ribosomes ( ) are macromolecular machines, found within all cells, that perform biological protein synthesis (mRNA translation). Ribosomes link amino acids together in the order specified by the codons of messenger RNA (mRNA) molecules to ...
s, processed in the
endoplasmic reticulum
The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum ( ...
and transported from the
Golgi apparatus to the trans-Golgi network, and from there via small carrier vesicles to their final destination compartment. The association between adaptins and clathrin are important for vesicular
cargo selection and transporting.
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. Therefore, adaptor proteins are responsible for the recruitment of cargo molecules into a growing clathrin-coated pits. The two major types of clathrin adaptor complexes are the
heterotetrameric vesicular transport adaptor proteins (AP1-5), and the monomeric
GGA (Golgi-localising, Gamma-adaptin ear homology,
ARF-binding proteins) adaptors.
Adaptins are distantly related to the other main type of vesicular transport proteins, the
coatomer subunits, sharing between 16% and 26% of their amino acid sequence.
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 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 trans-Golgi network, and
endosomes.
AP2 adaptor complex The AP2 adaptor complex is a multimeric protein that works on the cell membrane to internalize cargo in clathrin-mediated endocytosis. It is a stable complex of four adaptins which give rise to a structure that has a core domain and two appendage d ...
associates with the plasma membrane and is responsible for
endocytosis
Endocytosis is a cellular process in which substances are brought into the cell. The material to be internalized is surrounded by an area of cell membrane, which then buds off inside the cell to form a vesicle containing the ingested material. E ...
.
AP3 is responsible for protein trafficking to lysosomes and other related
organelle
In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' the ...
s.
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
GABAA receptors available at the cell surface .
See also
*
List of adaptins
References
External links
*
*
{{InterPro content, IPR008968
Peripheral membrane proteins
Protein families