Clathrin adaptor proteins, also known as adaptins, are proteins that mediate the formation of vesicles for intracellular trafficking and secretion. Adaptins are clustered subunits of adaptor protein (AP) complexes. [1] There are several types of adaptin, each related to a different AP complex.
Adaptins show sequence similarity to some COPI subunits, thus they are thought to have a common evolutionary origin. [1] The adaptin is a heterotetramer consisting of two large adaptins (beta and one other depending on the complex), a medium adaptin (mu), and a small adaptin (sigma):
A diagram of the 5 complexes is shown here
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 or extrude off of one organelle and travel to another location, where the cargo is delivered. While some of the details of how these adaptor proteins achieve their trafficking specificity has been worked out, there is still much to be learned.
The coatomer is a protein complex that coats membrane-bound transport vesicles. Two types of coatomers are known:
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 domains attached to the core domain by polypeptide linkers. These appendage domains are sometimes called 'ears'. The core domain binds to the membrane and to cargo destined for internalisation. The alpha and beta appendage domains bind to accessory proteins and to clathrin. Their interactions allow the temporal and spatial regulation of the assembly of clathrin-coated vesicles and their endocytosis.
AP-2 complex subunit mu is a protein that in humans is encoded by the AP2M1 gene.
AP-2 complex subunit alpha-1 is a protein that in humans is encoded by the AP2A1 gene.
AP-1 complex subunit mu-1 is a protein that in humans is encoded by the AP1M1 gene.
ADP-ribosylation factor-binding protein GGA3 is a protein that in humans is encoded by the GGA3 gene.
AP-1 complex subunit gamma-1 is a protein that in humans is encoded by the AP1G1 gene.
AP-2 complex subunit alpha-2 is a protein that in humans is encoded by the AP2A2 gene.
AP-1 complex subunit beta-1 is a protein that in humans is encoded by the AP1B1 gene.
AP-2 complex subunit beta is a protein that in humans is encoded by the AP2B1 gene.
AP-3 complex subunit delta-1 is a protein that in humans is encoded by the AP3D1 gene.
AP-1 complex subunit mu-2 is a protein that in humans is encoded by the AP1M2 gene.
AP-3 complex subunit beta-1 is a protein that in humans is encoded by the AP3B1 gene.
AP-1 complex subunit sigma-1A is a protein that in humans is encoded by the AP1S1 gene.
AP-3 complex subunit sigma-2 is a protein that in humans is encoded by the AP3S2 gene.
AP-3 complex subunit sigma-1 is a protein that in humans is encoded by the AP3S1 gene.
AP-4 complex subunit mu-1 is a protein that in humans is encoded by the AP4M1 gene.
Clathrin adaptor proteins, also known as adaptins, are vesicular transport adaptor proteins associated with clathrin. These proteins are synthesized in the ribosomes, processed in the 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 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 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.
The C-terminal domain ofBeta2-adaptin is a protein domain is involved in cell trafficking by aiding import and export of substances in and out of the cell.