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The alpha 2 beta 1 integrin serves as either a specific cell surface receptor for collagen or as both a collagen and laminin receptor depending upon the cell type. Recently we established that the alpha 2 beta 1 integrin binds to a site within the alpha 1 (I)-CB3 fragment of type I collagen (Staatz, W. D., Walsh, J. J., Pexton, T., and Santoro, S. A. (1990) J. Biol. Chem. 265, 4778-4781). To define the alpha 2 beta 1 recognition sequence further we have prepared an overlapping set of synthetic peptides which completely spans the 148-amino acid alpha 1(I)-CB3 fragment and tested the peptides for ability to inhibit cell adhesion to collagen and laminin substrates. The minimal active recognition sequence defined by these experiments is a tetrapeptide of the sequence Asp-Gly-Glu-Ala (DGEA) corresponding to residues 435-438 of the type I collagen sequence. The DGEA-containing peptides effectively inhibited alpha 2 beta 1-mediated Mg2(+)-dependent adhesion of platelets, which use the alpha 2 beta 1 integrin as a collagen-specific receptor, to collagen but had no effect on alpha 5 beta 1-mediated platelet adhesion to fibronectin or alpha 6 beta 1-mediated platelet adhesion to laminin. In contrast, with T47D breast adenocarcinoma cells, which use alpha 2 beta 1 as a collagen/lamin receptor, adhesion to both collagen and laminin was inhibited by DGEA-containing peptides. Deletion of the alanine residue or substitution of alanine for either the glutamic or aspartic acid residues in DGEA-containing peptides resulted in marked loss of inhibitory activity. These results indicate that the amino acid sequence DGEA serves as a recognition site for the alpha 2 beta 1 integrin complex on platelets and other cells.