Properties of a Polycation-stimulated Protein Kinase Associated with Purified Avena Phytochrome

ATP-dependent polycation-stimulated phosphorylation of highly purified phytochrome preparations from etiolated Avena seedlings has been reported previously (Y-S Wong, H-C Cheng, DA Walsh, JC Lagarias [1986] J Biol Chem 261: 12089-12097). In this study, we present a more detailed description of the properties of this protein kinase based on the analysis of over 30 different Avena phytochrome preparations. ATP-dependent phosphorylation of phytochrome was strongly stimulated by a wide range of polycationic molecules, including synthetic and natural polypeptides as well as nonpeptide cationic polymers. Many of the compounds known to stimulate other known protein kinases (i.e., cyclic nucleotides, Ca(2+), calmodulin, diacylglycerol, phospholipids) were either inhibitory or nonstimulatory. Among the polycations, histone H1, polylysine, and polybrene were the most effective, giving average stimulations of four- to sevenfold. Polycation-stimulated protein phosphorylation was inhibited by elevated ionic strength; of the salts examined, magnesium pyrophosphate was a particularly potent inhibitor of the kinase activity. MgATP was preferred as the phosphoryl donor to either MgGTP or magnesium pyrophosphate. The K(m) for MgATP was estimated to be 30 micromolar when histone H1 was used as a protein substrate. The Pr form of phytochrome was always a better substrate than the Pfr form regardless of the polycation present. Polylysine-stimulated, phytochrome(preparation)-dependent phosphorylation of purified maize phosphoenolpyruvate carboxylase was observed, as well as phosphorylation of a number of polypeptides in crude soluble protein extracts from etiolated Avena seedlings.