Clathrin

Arrestins act as adaptor proteins that bind to phos-phorylated G protein-coupled receptors (GPCR) and link the receptors to clathrin-coated pits. (3-Arrestins are essential in the internalization of many GPCRs. 

Clathrin is a protein complex composed of three heavy and three light chains, which assemble in a so-called triskelion. Clathrin is a major constituent of endocytic vesicles. 

Specialized regions of internalization from the plasma membrane, coated with a polyhedral lattice of the protein clathrin. It is in these regions that the first step of the process of endocytosis takes place, with the formation of clathrin-coated endocytic vesicles. 

Clathrin-coated vesicles mediate transport within the late secretory and the endocytic pathways. Their major coat constituents are clathrin and various adaptor complexes. 

A process in which a substance gains entry into a cell. Endocytic mechanisms are crucial for a variety of cellular functions such as the uptake of nutrients, regulation of cell surface expression of receptors, maintenance of cell polarity, and more. Receptor-mediated endocytosis via clathrin-coated pits is the most studied endocytic process, which is important for regulation of the time and magnitude of signals generated by a variety of cell-surface receptors. 

Along their route through the Golgi, secretory and membrane proteins destined for the various post-Golgi pathways are intermixed. Thus, proteins of distinct routes, i.e. the endosomal and the secretory route, are sorted into individual types of transport vesicles at the TGN. Among the best characterized types of TGN-derived vesicles are clathrin-coated vesicles. In addition, several types of non-clathrin-coated vesicles have been identified but their specific functions remain to be characterized. 

Biochemical characterization of clathrin-coated vesicles revealed that their major coat components are clathrin and various types of adaptor complexes. Clathrin assembles in triskelions that consist of three heavy chains of approximately 190 kDa and three light chains of 30 40 kDa. Four types of adaptor complexes have been identified to date, AP-1, AP-2, AP-3 and AP-4 (AP for adaptor protein). Whereas AP-1, AP-3 and AP-4 mediate sorting events at the TGN and/or endosomes, AP-2 is involved in endocytosis at the plasma membrane. Each adaptor complex is a hetero-tetrameric protein complex, and the term adaptin was extended to all subunits of these complexes. One complex is composed of two large adaptins (one each of y/a/S/s and [31-4, respectively, 90-130 kDa), one medium adaptin (pi -4, <50 kDa), and one small adaptin (ol-4, <20 kDa). In contrast to AP-1, AP-2 and AP-3, which interact directly with clathrin and are part of the clathrin-coated vesicles, AP-4 seems to be involved in budding of a certain type of non-clathrin-coated vesicles at the TGN. 

A pre-requisite for clathrin-coat assembly is the recruitment to the membrane of an adaptor complex. Similar to what has been observed for the recruitment of coatomer to Golgi membranes, adaptor binding is dependent on the presence of ARF-GTP. However, in contrast to COPI vesicle formation, ARF-GTP is suggested to act in a process before budding and not as a stoichiometric coat component. Other differences between COP-coated and clathrin-coated vesicles concern their uncoating mechanism. Disassembly of clathrin-coated vesicles is believed to depend on the chaperoneHSC 70 and on auxilin. 

Traub LM (2005) Common principles in clathrin-mediated sorting at the Golgi and the plasma membrane. Biochim Biophys Acta 1744(3) 415-37... 

The mannose 6-phosphate receptor is the cargo/coat-receptor for trans-Golgi network (TGN)-derived cla-thrin vesicles. The receptor recognizes the mannose 6-phosphate tag of lysosomal hydrolases on the luminal side and the adaptor-1 complex of clathrin on the cytoplasmic face. 

NHE5. The distribution of this isoform is distinct, being in neuronal-rich areas of the central nervous system. Low levels have also been found in testis, spleen and skeletal muscle. Like the preceding isoforms, NHE5 is found in the plasma membrane and is internalised by clathrin-associated endocytosis into recycling endosomes. The normal role of NHE5 is unknown but its malfunction is speculated to contribute to the development of neurodegenerative disease. 

Clathrin-coated Pits Clathrin-coated Vesicle CLC... 

Braell, W.A., Schlossman. D.M., Schmid, S.L., Rothman, J. (1984). Dissociation of clathrin coats coupled to hydrolysis of ATP Role of an uncoating ATPase. J. Cell Biol. 99, 734-741. 

Pelkmans L, Fava E, Grabner H, Hannus M, Habermann B, Krausz E, Zerial M (2005) Genomewide analysis of human kinases in clathrin- and caveolae/raft-mediated endocytosis. Nature 436 78-86... 

Ungewickell, E. (1985). The 70-kd mammalian heat shock proteins are structurally and functionally related to the uncoating protein that releases clathrin triskelia from coated vesicles. EM BO Journal, 4, 3385-91. 

LDL (apo B-lOO, E) receptors occur on the cell surface in pits that are coated on the cytosolic side of the cell membrane with a protein called clathrin. The glycoprotein receptor spans the membrane, the B-lOO binding region being at the exposed amino terminal end. After binding, LDL is taken up intact by endocytosis. The apoprotein and cholesteryl ester are then hydrolyzed in the lysosomes, and cholesterol is translocated into the cell. The receptors are recycled to the cell surface. This influx of cholesterol inhibits in a coordinated manner HMG-CoA synthase, HMG-CoA reductase, and, therefore, cholesterol synthesis stimulates ACAT activ-... 

Figure 41 -15. Two types of endocytosis. An endocytotic vesicle (V) forms as a result of invagination of a portion of the plasma membrane. Fluid-phase endocytosis (A) is random and nondirected. Receptor-mediated endocytosis (B) is selective and occurs in coated pits (CP) lined with the protein clathrin (the fuzzy material). Targeting is provided by receptors (black symbols) specific for a variety of molecules. This results in the formation of a coated vesicle (CV).

Most proteins that are synthesized on membrane-bound polyribosomes and are destined for the Golgi apparatus or plasma membrane reach these sites inside transport vesicles. The precise mechanisms by which proteins synthesized in the rough ER are inserted into these vesicles are not known. Those involved in transport from the ER to the Golgi apparatus and vice versa—and from the Golgi to the plasma membrane— are mainly clathrin-free, unlike the coated vesicles involved in endocytosis (see discussions of the LDL receptor in Chapters 25 and 26). For the sake of clarity, the non-clathrin-coated vesicles will be referred to in... 

A Model of Non-Clathrin-Coated Vesicles Involves SNAREs Other Factors... 

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