Purification and characterization of a wheat germ protein kinase

Abstract

A cyclic AMP-independent protein kinase has been purified from wheat germ extracts. The enzyme catalyzes the phosphorylation of casein and phosvitin but not protamine, histone, or bovine serum albumin. However, the best substrate for the kinase appears to be that of an endogenous wheat germ protein. The kinase can utilize both ATP and GTP as phosphoryl donors. A molecular weight of 36,000-38,000 had been estimated for the kinase by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and by glycerol density gradient centrifugation and gel filtration in the presence of 0.5 M KCl. In the presence of low salt, however, the molecular weight of the kinase appears to double. In isoelectrofocusing, the kinase exhibits a pI of about 6.5. The activity of the kinase is strongly inhibited by spermine and heparin. Spermidine is slightly stimulatory at low concentrations but inhibitory at high concentrations. High concentrations of putrescine also inhibit the kinase activity, but not to the extent observed with the other polyamines. Both spermine and spermidine appear to enhance the kinase activity at low Mg2+ concentrations. The result suggests that these polyamines could partially replace Mg2+ for kinase activity.