Effect of Syk mutation on its intrinsic kinase activity.

Syk was immunoprecipitated from antigen-stimulated (Ag) or nonstimulated cells and tested by the in vitro immune-complex kinase (IVK) assay. Radiolabeled proteins were detected by autoradiography (top, autophosphorylation of Syk; middle, phosphorylation of the substrate cdb3). Amounts of precipitated proteins were confirmed by anti-Syk immunoblotting (bottom). Similar results were obtained when the other cloned lines were tested in 4 independent experiments.