Integrins mediate signal transduction through interaction with multiple cellular or extracellular matrix ligands. Integrin αvβ3 recognizes fibrinogen, von Willebrand factor, and vitronectin, while αvβ1 does not. We studied the mechanisms for defining ligand specificity of these integrins by swapping the highly diverse sequences in the I domain-like structure of the β1 and β3 subunits. When the sequence CTSEQNC (residues 187-193) of β1 is replaced with the corresponding CYDMKTTC sequence of β3, the ligand specificity of αvβ1 is altered. The mutant (αvβ1-3-1), like αvβ3, recognizes fibrinogen, von Willebrand factor, and vitronectin (a gain-of- function effect). The αvβ1-3-1 mutant is recruited to focal contacts on fibrinogen and vitronectin, suggesting that the mutant transduces intracellular signals on adhesion. The reciprocal β3-1-3 mutation blocks binding of αvβ3 to these multiple ligands and to LM609, a function-blocking anti-αvβ3 antibody. These results suggest that the highly divergent sequence is a key determinant of integrin ligand specificity. Also, the data support a recent hypothetical model of the I domain of β, in which the sequence is located in the ligand binding site.
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