The active form of fibronectin is its extracellular matrix form, which allows for the attachment of cells and influences both the growth and migration of cells. The matrix form is assembled by cells; however, many cells are defective in this regard. Several regions within fibronectin have been shown to play a role in matrix assembly by cells. One such region has been localized into the first type III repeat of fibronectin (Chernousov, M. A., F. J. Fogerty, V. E. Koteliansky, and D. F. Mosher. J. Biol. Chem. 266:10851-10858). We have identified this site as a fibronectin-fibronectin binding site and reproduced it as a synthetic peptide. This site is contained in a 14-kD fragment that corresponds to portions of the first two type III repeats. The 14-kD fragment was found to bind to cell monolayers and to inhibit fibronectin matrix assembly. The 14-kD fragment only slightly reduced the binding of fibronectin to cell surfaces but it significantly inhibited the subsequent incorporation of fibronectin into the extracellular matrix. The 14-kD fragment also bound to purified fibronectin and inhibited fibronectin-fibronectin binding. A synthetic 31-amino acid peptide (P1) representing a segment of the 14-kD fragment retained the ability to inhibit fibronectin-fibronectin binding. Peptide P1 specifically bound fibronectin from plasma in affinity chromatography, whereas a column containing another peptide from the 14-kD fragment did not. These results define a fibronectin-fibronectin binding site that appears to promote matrix assembly by allowing the assembly of fibronectin molecules into nascent fibrils. The 14-kD fragment and the P1 peptide that contain this site inhibit matrix assembly by competing for the fibronectin-fibronectin binding.

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