TYROSINE PHOSPHORYLATION OF THE BETA(3) CYTOPLASMIC DOMAIN MEDIATES INTEGRIN-CYTOSKELETAL INTERACTIONS

Tyrosine phosphorylation of the beta(3) subunit of the major platelet integrin alpha(IIb)beta(3) has been shown to occur during thrombin-ind uced platelet aggregation (1), We now show that a wide variety of plat elet stimuli induced beta(3) tyrosine phosphorylation, but that this p hosphorylation occurred only following platelet aggregation. Several l ines of evidence suggest that the beta(3) cytoplasmic domain tyrosine residues and/or their phosphorylation function to mediate interactions between beta(3) integrins and cytoskeletal proteins. First, phospho-b eta(3) was retained preferentially in a Triton X-100 insoluble cytoske letal fraction of thrombin aggregated platelets. Second, in vitro expe riments show that the cytoskeletal protein, myosin, associated in a ph osphotyrosine-dependent manner with a diphosphorylated peptide corresp onding to residues 740-762 of beta(3), Third, mutation of both tyrosin es in the beta(3) cytoplasmic domain to phenylalanines markedly reduce d beta(3)-dependent fibrin clot retraction. Thus, our data indicate th at platelet aggregation is both necessary and sufficient for beta(3) t yrosine phosphorylation, and this phosphorylation results in the physi cal linkage of alpha(IIb)beta(3) to the cytoskeleton. We hypothesize t hat this linkage may involve direct binding of the phosphorylated inte grin to the contractile protein myosin in order to mediate transmissio n of force to the fibrin clot during the process of clot retraction.