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Integrin Associated Protein (IAP) is a 50-kD membrane protein which copurifies with the integrin alpha v beta 3 from placenta and coimmunoprecipitates with beta 3 from platelets. IAP also is functionally associated with signal transduction from the Leukocyte Response Integrin. Using tryptic peptide sequence, human and murine IAP cDNAs have been isolated. The protein has an extracellular amino- terminal immunoglobulin domain that binds all monoclonal anti-IAP antibodies. The carboxy-terminal region is highly hydrophobic with three or five membrane-spanning segments and a short hydrophilic tail. Immunofluorescence microscopy suggests that this hydrophilic tail is located on the inside of the cytoplasmic membrane. Monoclonal anti-IAP antibody inhibits the binding of vitronectin-coated beads to alpha v beta 3 on human erythroleukemia cells, and polyclonal anti-IAP recognizes hamster IAP on CHO cells and inhibits vitronectin bead binding. When CHO cells are transfected with human IAP, monoclonal anti- human antibody completely inhibits vitronectin bead binding. These data suggest a model in which ligand binding by alpha v beta 3 is regulated by IAP.