Kinetic regulation of β₃ integrin tyrosine phosphorylation

Tyrosine phosphorylation of β₃ integrins is a permissive stage in the activation of α_IIbβ₃ and α_vβ₃ in platelets and leukocytes, respectively. In this study we demonstrated direct phosphorylation of β₃ integrins as a result of interaction with soluble monomeric ligand, and we characterized the differential kinetics of β₃ phosphorylation as a consequence of α subunit pairing. We found that 133 phosphorylation is initiated by RGD peptide binding in a dose-dependent and saturable fashion with α_IIbβ₃ becoming phosphorylated and dephosphorylated more rapidly than α_vβ₃. Site mapping of phosphate incorporation reveals significant phosphorylation at Tyr-747 in both β3 integrin species with incorporation at Tyr-759 found at significant levels only in α_IIbβ₃. Mutation of cytoplasmic β₃ tyrosine residues in a transfection model prevents cell adhesion via these integrins. These data demonstrate that recognition of ligand is sufficient to induce 133 tyrosine phosphorylation and suggests that this event is regulated by the α subunit pairing of β₃.