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Figure 3 Domain structure of one of the subunits of fibronectin. FN type I repeats (□), FN type II repeats (■) and the FN type III repeats (B) are shown. The three type III repeats that undergo alternative splicing are indicated. The number of additional amino acids (aa) encoded by splice variants of the third FN type III segment are indicated.

major influences on adhesion, growth, migration and differentiation of cells. Laminins are composed of three chains (a, (3 and y) with molecular weights of 140-400 kDa. At present eight genetically distinct laminin chains and seven different assemblies are known. Chain association occurs through an ct-heli-cal coiled coil near the C-terminus of each chain. The specific binding of laminins to perlecan, entactin, type IV collagen and other basement membrane components suggests that laminins play a crucial role in the supramolecular organization and assembly of basement membranes.

The type III repeat of fibronectin containing the RGDS cell-binding motif is found in a number of matrix glycoproteins, including thrombospondin and tenascin, and is probably a recurring motif for cell binding. Another common domain in matrix glycoproteins is the epidermal growth factor (EGF) repeat. In addition to laminin, entactin, thrombospondin and tenascin, it is found in the proteoglycans aggre-can and versican. These repeats may mediate two functions: first, facilitate protein-protein interactions; and second, interact with EGF receptors and control cellular activity. The occurrence of multiple domains in the glycoproteins suggests multiple functions, and during evolution this has been achieved by exon shuffling, which has produced the variety of matrix glycoproteins.

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