iso-DGR sequences do not mediate binding of fibronectin N-terminal modules to adherent fibronectin-null fibroblasts

Fibronectin (FN) without an RGD sequence (FN-RGE), and thus lacking the principal binding site for alpha5beta1 integrin, is deposited into the extracellular matrix of mouse embryos. Spontaneous conversion of (263)NGR and/or (501)NGR to iso-DGR possibly explains this enigma, i.e. ligation of iso-DGR by alphavbeta3 integrin may allow cells to assemble FN. Partial modification of (263)NGR to DGR or iso-DGR was detected in purified plasma FN by mass spectrometry. To test functions of the conversion, one or both NGR sequences were mutated to QGR in recombinant N-terminal 70-kDa construct of FN (70K), full-length FN, or FN-RGE. The mutations did not affect the binding of soluble 70K to already adherent fibroblasts or the ability of soluble 70K to compete with non-mutant FN or FN-RGE for binding to FN assembly sites. Non-mutant FN and FN-N263Q/N501Q with both NGRs mutated to QGRs were assembled equally well by adherent fibroblasts. FN-RGE and FN-RGE-N263Q/N501Q were also assembled equally well. Although substrate-bound 70K mediated cell adhesion in the presence of 1 mm Mn(2+) by a mechanism that was inhibited by cyclic RGD peptide, the peptide did not inhibit 70K binding to cell surface. Mutations of the NGR sequences had no effect on Mn(2+)-enhanced cell adhesion to adsorbed 70K but caused a decrease in cell adhesion to reduced and alkylated 70K. These results demonstrate that iso-DGR sequences spontaneously converted from NGR are cryptic and do not mediate the interaction of the 70K region of FN with the cell surface during FN assembly.