Two naturally occurring inhibitors of nuclear protein kinase

2Two types of protein kinase inhibitors present in rat liver nuclei have been partially purified and characterized. They are specific for the nuclear enzymes, being inert toward the protein kinases in the cytosol. One inhibitor is a 150,000 dalton, heat-labile, acidic protein; the other is a family of two oligonucleotides. Inhibitory activity in crude extracts becomes measurable only after complete removal of protein kinase activity by affinity chromatography (Farron-Furstenthal, F., and Lightholder, J.R. (1977) FEBS Lett. 84, 313). Initial separation of the inhibitor protein from the oligonucleotide inhibitors was achieved by filtration through an Amicon pressure cell. Further purification of the inhibitor protein was obtained by chromatography on ion exchangers and Bio-Gel. The oligonucleotides were purified by DEAE-cellulose chromatography and paper electrophoresis. The effects of the two types of inhibitors are additive. The 170 to 200% recovery of protein kinase activity after removal of the inhibitors from the initial extracts suggests that the inhibitors contribute in a quantitatively significant measure to the regulation of nuclear protein phosphorylation.