Triskelion

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. 

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. 

Clathrin-coated vesicles mediate transport from the Golgi apparatus to endosomes, and from the plasma membrane to endosomes. A multi-subunit protein, clath-rin, constitutes the major protein of this vesicle type (see Ch. 2). Clathrin is composed of three large and three small polypeptide chains, which assemble to form a triskelion (Fig. 9-2). Regulatory mechanisms control the assembly and formation of a convex, polyhexa-pentagonal basketlike structure by these triskelions [5], This structure is responsible for the formation of coated pits on the cytosolic face of plasma membranes. 

F1GURE 27-40 Clathrin. (a) Three light (L) chains (Mr 35,000) and three heavy (H) chains (Mr 180,000) of the (HL)3 clathrin unit, organized as a three-legged structure called a triskelion. (b) Triskelions... 

Once inside a cell the vesicles lose their coats to become endosomes which may then fuse with lysosomes or with Golgi membranes. The removal of a clathrin coat requires ATP as well as the chaperonin Hsp 70 (Chapter 10) and a coat protein called auxilin.568 Triskelion is distorted and displaced from the clathrin cage. The interior of the newly formed endosome is quickly acidified by the action of a proton pump in the vesicle walls.554 569 This sometimes leads to dissociation of enclosed receptors from their ligands and permits recycling of receptors and lipids of the vesicle membranes to the cell surface. This is the case for the low-density lipoprotein receptor.570 571... 

Both endocytosis of material at the plasma membrane and exocytosis from the Golgi apparatus involve the formation of clathrin-coated pits and vesicles. On the cytosolic side of the membrane these structures have an electron-dense coat consisting mainly of the protein clathrin, the polypeptides of which form a three-legged structure known as a triskelion. The clathrin triskelions assemble into a basket-like convex framework that causes the membrane to invaginate at that point and eventually to pinch off and form a vesicle. In endocytosis these clathrin-coated vesicles migrate into the cell where the clathrin coats are lost before delivering their contents to the lysosomes. 

Clathrins, first observed by Pearse, are structures with pentagonal and hexagonal faces that can form three dimensional structures with the same truncated icosahedral morphology as a C6o molecule or a soccer ball [1], The vertices of the clathrin coating are formed by three intertwined proteins composed of heavy and light chains that emanate from a central hub in a structure known as a triskelion, shown in Fig. 3.1. 

The latter is another complex protein that instead of adopting rigid helices forms a three legged, or triskelion, structure. In this it is not unlike Mao s three pointed DNA stars, however, each triskelion arm is bent allowing molecules to self assemble. The result is a lattice that can fold in on itself to form a truncated icosahedron ... 

Another activity associated with a stress-70 protein is the in vitro disassembly of clathrin cages into triskelions (see Section I V,D). Rothman and colleagues isolated and characterized a protein from bovine brain that facilitated the disassembly of clathrin cages in a reaction that required hydrolysis of ATP they referred to the protein as a clathrin-uncoating ATPase (Schlossman et ai, 1984). Subsequently, it was realized that this protein was a constitutively expressed member of the stress-70 family (HSC70) (Chappell et al 1986). 

Schmid, S. L., and Rothman, J. E. (1985b). Two classes of binding sites for uncoating protein in clathrin triskelions. y. Biol. Chem. 260, 10050-10056. 

The best-characterized vesicles that bud from the trans-Golgi network (TGN) have a two-layered coat an outer layer composed of the fibrous protein clathrin and an inner layer composed of adapter protein (AP) complexes. Purified clathrin molecules, which have a three-limbed shape, are called triskelions from the Greek for three-legged (Figure 17-19a). Each limb contains one clathrin heavy chain (180,000 MW) and one clathrin light chain... 

MW). Triskelions polymerize to form a polygonal lattice with an intrinsic curvature (Figure... 

Triskelion cross-section n. A trilobal cross-section in which the radiating arms are curved or bent. 

Under appropriate conditions, isolated triskelions spontaneously reassociate in vitro to form a range of open and closed baskets of various sizes. In vivo assembly to a clathrin basket of the correct size and structure involves participation of additional proteins known as assembly proteins, e.g. monomeric clathrin assembly protein (APj, 180,000 different from the large triskelion polypeptide), monomeric auxilin M, 90,000), and a relatively small (M, 20,000) clathrin assembly protein that is similar or identical to myelin basic protein, as well as the multimeric adaptors. All of these proteins promote the in vitro assembly of triskelions into homogeneous populations of clathrin cages. In vivo, assembly proteins are thought to be located between the membrane of the C-c.v. and its clathrin coat. 

Diagrammatic representation of a triskelion, showing how it forms the sides of polyhedra in the ciathrin iattice surrounding a clathrin-coated vesicie. 

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