ACTIVATION OF P72(SYK) BY THROMBIN IN A CELL-FREE SYSTEM

Thrombin dramatically activated p72(syk) in a time- and dose-dependent fashion in extracts of resting porcine platelets in the presence of E DTA. Separation analysis using Sephacryl S-300 column chromatography h as demonstrated that p72(syk) may exist as large (complex) and small ( monomer) forms in resting platelets, and activation of p72(syk) was on ly observed in the fraction of large form. Pretreatment with ATP scave nger, GDP beta S and protein phosphatase inhibitors had no effect on t his activation. Furthermore, washed immune-precipitates of large form p72(syk) were also activated by thrombin or fibrinogen. These results suggest that p72(syk) may associate with thrombin receptor or other ag onist receptors and there may be a novel activation mechanism of non-r eceptor type protein-tyrosine kinase, which does not require the modif ication by other protein kinases, protein phosphatases and GTP binding proteins. (C) 1994 Academic Press, Inc.