Fibronectins are high molecular mass glycoproteins that circulate as soluble molecules in the blood, and are also found in an insoluble, multimeric form in extracellular matrices throughout the body. Soluble fibronectins are polymerized into insoluble extracellular matrix (ECM) fibrils via a cell-dependent process. Recent studies indicate that the interaction of cells with the ECM form of fibronectin promotes actin organization and cell contractility, increases cell growth and migration, and enhances the tensile strength of artificial tissue constructs; ligation of integrins alone is insufficient to trigger these responses. Evidence suggests that the effect of ECM fibronectin on cell function is mediated in part by a matricryptic heparin-binding site within the first III 1 repeat (FNIII 1 ). In this study, we localized the heparin-binding activity of FNIII 1 to a cluster of basic amino acids, Arg 613 , Trp 614 , Arg 615 , and Lys 617 . Site-directed mutagenesis of a recombinant fibronectin construct engineered to mimic the ECM form of fibronectin demonstrates that these residues are also critical for stimulating cell spreading and increasing cell proliferation. Cell proliferation has been tightly correlated with cell area. Using integrin-and heparin-binding fibronectin mutants, we found a positive correlation between cell spreading and growth when cells were submaximally spread on ECM protein-coated surfaces at the time of treatment. However, cells maximally spread on vitronectin or fibronectin still responded to the fibronectin matrix mimetic with an increase in growth, indicating that an absolute change in cell area is not required for the increase in cell proliferation induced by the matricryptic site of FNIII 1 .

Fibronectin is a principal component of the extracellular matrix. Soluble fibronectin molecules are assembled into the extracellular matrix as insoluble, fibrillar strands via a cell-dependent process. In turn, the interaction of cells with the extracellular matrix form of fibronectin stimulates cell functions critical for tissue repair. Cross-talk between cell-cell and cellextracellular matrix adhesion complexes is essential for the organization of cells into complex, functional tissue during embryonic development and tissue remodeling. Here, we demonstrate that fibronectin matrix assembly affects the organization, composition, and function of N-cadherin-based adherens junctions. Using fibronectin-null mouse embryonic myofibroblasts, we identified a novel quaternary complex composed of N-cadherin, ␤-catenin, tensin, and actin that exists in the absence of a fibronectin matrix. In the absence of fibronectin, homophilic N-cadherin ligation recruited both tensin and ␣5␤1 integrins into nascent cell-cell adhesions. Initiation of fibronectin matrix assembly disrupted the association of tensin and actin with N-cadherin, released ␣5␤1 integrins and tensin from cell-cell contacts, stimulated N-cadherin reorganization into thin cellular protrusions, and decreased N-cadherin adhesion. Fibronectin matrix assembly has been shown to recruit ␣5␤1 integrins and tensin into fibrillar adhesions. Taken together, these studies suggest that tensin serves as a common cytoskeletal link for integrin-and cadherin-based adhesions and that the translocation of ␣5␤1 integrins from cell-cell contacts into fibrillar adhesions during fibronectin matrix assembly is a novel mechanism by which cell-cell and cell-matrix adhesions are coordinated.Tissue development, remodeling, and homeostasis are governed by adhesion of cells to extracellular matrix (ECM) 3 and to neighboring cells (1). Tight coordination between integrinmediated cell-ECM adhesion and cadherin-mediated cell-cell adhesion is required during tissue morphogenesis to facilitate the formation of functional, multicellular structures (1, 2). Cell adhesion to ECM proteins has been shown to affect the expression, localization, composition, and function of cell-cell adhesions (2). However, neither the intracellular nor extracellular mechanisms that serve to coordinate the activities of these two adhesion systems are well understood. The adhesive functions of both integrins and cadherins require interactions with the actin cytoskeleton (3, 4). Further, the intracellular protein complexes that link integrins and cadherins to the actin cytoskeleton share several molecular components, including ␣-actinin and vinculin (3). As such, the exchange or sequestration of molecular constituents that are common to both adhesion systems provides an attractive mechanism by which cell-ECM adhesion can influence cell-cell adhesion and vice versa.Cadherins are a family of transmembrane receptors that mediate calcium-dependent cell-cell adhesion by homophilic association of their ectodomains (3). The cadher...

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