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

A chromatin-associated casein-type protein kinase has been purified 500-fold from soybean (Glycine max, var. Wayne) tissue. The enzyme can be completely dissociated from isolated chromatin in 250 millimolar (NH(4))(2)SO(4). After purification, the kinase preparation is stable for at least 6 months at 0 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 K(m) of 8 micromolar. Five millimolar Mg(2+) is required for maximal activity, but Mn(2+) will support phosphorylation at a lower level. The average molecular weight as determined by sucrose gradient sedimentation and gel filtration is approximately 55,000. Under conditions of low ionic strength [less than 250 millimolar (NH(4))(2)SO(4)] soybean casein kinase forms higher molecular weight 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.