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Fibronectin matrix assembly is a complex cellular process initiated by specific fibronectin-binding cell surface receptors. Although the integrin alpha 5 beta 1 has been implicated in the assembly of fibronectin matrices, fibroblastic cells derived from alpha 5 integrin null mutant embryos assemble a fibronectin matrix. Thus, alternative receptors must support this process. Although the platelet integrin alpha IIb beta 3 supports fibronectin matrix assembly, its expression is restricted to platelets. We report that alpha v beta 3 integrin, a fibronectin receptor expressed on many cell types provides an alternative pathway for the assembly of soluble fibronectin into the extracellular matrix. This process occurs independent of alpha 5 beta 1, is also modulated by activation, and the resulting matrix is biochemically indistinguishable from that assembled under the control of alpha 5 beta 1. Matrix assembly requires binding to the RGD site in the 10th type III repeat of fibronectin, as well as the participation of the amino-terminal matrix assembly domain. The participation of two distinct integrins in fibronectin matrix assembly suggests a model for the involvement of integrins in a dual system of extracellular matrix assembly and recognition controlled by intracellular activation of extracellular receptors.
Wu et al. (Mon,) studied this question.