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The monolayers are well-suited to characterizing the roles of peptide ligands that have been identified from ECM proteins and for discovering new peptide ligands that participate in cell-ECM interactions. One system that we have studied involves the role of the PHSRN peptide from fibronectin.94 This peptide is referred to as a synergy peptide because it has been reported to have no activity when presented alone, but to enhance cell spreading when presented with RGD. Many current models have suggested that PHSRN acts by binding the integrin receptor at a different site than does RGD. We used monolayers presenting either PHSRN or RGD to find that PHSRN is able to mediate cell adhesion on its own and that PHSRN and RGD are competitive binders. This latter result is supported by the finding that either peptide in solution can inhibit cell adhesion to substrates presenting either peptide, and clearly necessitates a revised model for the relationship between the PHSRN and RGD peptides. This work is also important in that it serves as an early example of the value of model substrates in cell biology, and for employing bottom-up approaches to isolate ligand-receptor interactions in experimental models of adhesion.
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