Activation of p72syk by thrombin in a cell-free system.

Thrombin dramatically activated p72syk in a time- and dose- dependent fashion in extracts of resting porcine platelets in the presence of EDTA. Separation analysis using Sephacryl S-300 column chromatography has demonstrated that p72syk may exist as large (complex) and small (monomer) forms in resting platelets, and activation of p72syk was only observed in the fraction of large form. Pretreatment with ATP scavenger, GDP beta S and protein phosphatase inhibitors had no effect on this activation. Furthermore, washed immuno-precipitates of large form p72syk were also activated by thrombin or fibrinogen. These results suggest that p72syk may associate with thrombin receptor or other agonist receptors and there may be a novel activation mechanism of non-receptor type protein-tyrosine kinase, which does not require the modification by other protein kinases, protein phosphatases and GTP binding proteins.