The catalytic domain of p72syk kinase (CDp72syk) was purified from a 30000 g particulate fraction of rat spleen. The purification procedure employed sequential chromatography on columns of DEAE-Sephacel and Superdex-200, and elution from HA-Ultrogel by chloride. The analysis of the final CDp72syk preparation by SDS/PAGE revealed a major silver-stained 40 kDa protein. The kinase was identified by covalent modification of its ATP-binding site with [14C]5´-fluorosulphonylbenzoyladenosine and by immunoblotting with a polyclonal antibody against the ‘linker ’ region of p72syk. By using poly(Glu4, Tyr1) as a substrate, the specific activity of the enzyme was determined as 18.5 nmol Pi/min per mg. Casein, histones H1 and H2B and myelin basic protein were efficiently phosphorylated by CDp72syk. The kinase exhibited a limited ability to phosphorylate random polymers containing tyrosine residues. CDp72syk autophosphorylation activity was associated with an activation of the kinase towards exogenous substrates. The extent of activation was dependent on the substrates added. CDp72syk was phosphorylated by protein kinase C (PKC) on serine and threonine residues. With a newly developed assay method, we demonstrated that the PKC-mediated phosphorylation had a strong activating effect on the tyrosine kinase activity of CDp72syk. Studies extended to conventional PKC isoforms revealed an isoform-dependent manner (α > βI = βII > γ) of CDp72syk phosphorylation. The different phosphorylation efficiencies of the PKC isoforms closely correlated with the ability to enhance the tyrosine kinase activity.

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