Epitope Mapping of a Function-blocking β1 Integrin Antibody by Phage Display

Integrins are a major class of cell surface receptors involved in cell-cell and cell-matrix adhesion and communication. Ha2/11 is a function-blocking anti-rat β1 integrin hamster IgM that should be a useful reagent for understanding β1 integrin function. We demonstrate that Ha2/11 cross reacts with human, Xenopus, and Drosophila β1 integrins, and use phage display to map the epitope for Ha2/11 to residues within the sequence LRSGEPQTF which lies 18 amino acids proximal to the putative I domain in β1 integrins. Monoclonal antibody mapping experiments, mutational analyses, and direct binding assays have implicated integrin I domains in both cation and ligand binding. Our data therefore suggest that Ha2/11 blocks β1 integrin function by interfering with I domain-mediated ligand binding.