Isolation and Characterization of a Chromatin-associated Protein Kinase from Soybean

Abstract

A chromatin-associated casein-type protein kinase was purified 500-fold from soybean (Glycine max, var. Wayne) tissue. The enzyme can be completely dissociated from isolated chromatin in 250 mm (NH4)2SO4. After purification, the kinase preparation is stable for at least 6 mo. at 0.degree. C. The enzyme will phosphorylate casein, phosvitin and denatured chromatin proteins, but not histones. Only ATP will serve as a phosphate donor with an apparent Km of 8 .mu.mol. Five millimolar Mg2+ is required for maximal activity, but Mn2+ will support phosphorylation at a lower level. The average MW as determined by sucrose gradient sedimentation and gel filtration is approximately 55,000. Under conditions of low ionic strength [less than 250 mm (NH4)2SO4] soybean casein kinase forms higher MW aggregates with other chromosomal proteins in the preparation. The enzyme activity is not affected by cyclic AMP. Casein kinase shows a broad optimum between 7 and 8 and the isoelectric point is approximately 9. Preliminary data indicate that soybean casein kinase will not phosphorylate soybean RNA polymerases I or II, nor does it have any obvious effect on in vitro chromatin transcription by endogenous RNA polymerases.