GLYCOPROTEIN IB VON-WILLEBRAND-FACTOR INTERACTIONS ACTIVATE TYROSINE KINASES IN HUMAN PLATELETS

von Willebrand factor (vWF) in the presence of botrocetin induces p72( syk) activation, assessed as its autophosphorylated level and in vitro kinase assays, the transient association of p72(syk) with p60(c-src), and the translocation of p60(c-src) and p54/58(lyn) to cytoskeletal f ractions. Jararaca glycoprotein Ib-binding protein (GPIb-BP), which sp ecifically binds to GPIb, abolished these phenomena, suggesting that t hey are mediated by the vWF-GPIb interaction. These tyrosine kinase-re lated events were not inhibited by GRGDS peptide (plus EGTA), indicati ng that GPIIb/IIIa is not involved in the observed responses. Shc, an adaptor protein, was also tyrosine phosphorylated by the botrocetin-vW F activation. When GPIb was immunoprecipitated with nonfunctional mono clonal antibodies (MoAbs) directed against GPIb, a kinase activity was found to associate with GPIb upon botrocetin-vWF activation. On the o ther hand, anti-GPIb MoAbs that inhibit the vWF-GPIb interaction did n ot coprecipitate a kinase activity. Because the recovery of GPIb did n ot differ significantly, it is suggested that the excessive presence o f inhibitory anti-GPIb MoAb dissociated a kinase activity from GPIb. P hosphoamino acid analysis showed that the kinase activity was that of a tyrosine kinase. The identity of the tyrosine kinase and the mode of interaction with the cytoplasmic region of GPIb await to be determine d. Our findings suggest that the tyrosine kinase associated with GPIb serves at a most proximal step in the signal transduction pathway invo lved in the vWF-GPIb-induced platelet activation, which leads to other tyrosine kinase-related intracellular signals. (C) 1997 by The Americ an Society of Hematology.