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RESEARCH PRODUCT

Extracellular Structural and Secretory Proteins

subject

description

The extracellular matrices of animal tissues consist of networks of collagens and elastins, the meshes of which are filled with structural glycoproteins and glycosaminoglycans. The cells themselves are in direct contact with the basement membranes, which are specific regions of the extracellular matrix with a thickness of 20200 nm. The very varied compositions of the different extracellular matrices determine their mechanical characters and, in particular, their interactions with the cells [73]. The binding of cells to various matrix components is mediated by specific receptors on the cell surface and these recognize certain peptide sequences in the matrix proteins [141]. Several particularly important and well-known protein types of the extracellular matrix will be dealt with in Sections 11.1 to 11.4, and the glycosaminoglycans will be discussed in Chapter 13. There are very few comparative biochemical.data on the other matrix components. Next to the collagens, the most predominant cartilage protein is the cartilage matrix protein (CMP), a homotrimer with subunits of 470 amino acids, the gene for which has been sequenced in the chicken [69]. Tendons, skin and cornea contain the collagen-binding fibromodulin, and the gut wall includes the homologous proteoglycans PG-S1 and PG-S2 (decorin); the 357- to 375- amino-acid polypeptide chains of these latter proteins are mainly built up of 23-residue repeats [92]. In addition to fibronectin, the other cell-binding proteins of the extracellular matrix are cytotactin and tenascin, which are mainly produced during the phases of histo- and organogenesis. In contrast to the dimeric fibronectin molecule, these two proteins form oligomers with a six-armed structure (hexabrachions). Their sequences of 1777 and 2203 amino acids, respectively, are highly repetitive; some of the repeats resemble the type-III repeats of fibronectin, and the others are similar to the repeats of the epidermal growth factor (EGF) [19, 90]. Endothelial cells, fibroblasts and smooth-muscle cells produce the large glycoprotein thrombospondin, which is able to bind to fibronectin, collagen and heparin in the extracellular matrix [25].