Kinetic Regulation of 3 Integrin Tyrosine Phosphorylation*

Tyrosine phosphorylation of 3 integrins is a permissive stage in the activation of IIb 3 and v 3 in platelets and leukocytes, respectively. In this study we demonstrated direct phosphorylation of 3 integrins as a result of interaction with soluble monomeric ligand, and we characterized the differential kinetics of 3 phosphorylation as a consequence of subunit pairing. We found that 3 phosphorylation is initiated by RGD peptide binding in a dose-dependent and saturable fashion with IIb 3 becoming phosphorylated and dephosphorylated more rapidly than v 3. Site mapping of phosphate incorporation reveals significant phosphorylation at Tyr747 in both 3 integrin species with incorporation at Tyr-759 found at significant levels only in IIb 3. Mutation of cytoplasmic 3 tyrosine residues in a transfection model prevents cell adhesion via these integrins. These data demonstrate that recognition of ligand is sufficient to induce 3 tyrosine phosphorylation and suggests that this event is regulated by the subunit pairing of 3.