Partial purification and properties of a chromatin-associated phosphoprotein kinase from rat liver nuclei

Four casein kinase I isoforms are differentially partitioned between nucleus and cytoplasm

The casein kinase I (CKI) family consists of at least seven vertebrate genes, some of which can be alternatively spliced. Previously, we have studied the four splice variants of the chicken CKIalpha gene. The four proteins differ only by the presence or absence of two peptides, a 28-amino-acid "L" insert in the catalytic domain and a 12-amino-acid "S" insert near the extreme C-terminus. Here cells were transfected with DNA encoding all four isoforms fused to the green fluorescent protein (GFP) and the localization of each protein was examined. We noted that the L insert includes the sequence PVGKRKR, which has the characteristics of a nuclear localization signal (NLS), and we show that the CKIalphaL and CKIalphaLS isoforms which contain this sequence are targeted to the nucleus, where a fraction becomes associated with nuclear speckles. In contrast the two isoforms lacking the L insert remain predominantly cytoplasmic. Mutation of the first lysine in the putative NLS to asparagine prevented the nuclear entry of GFP-CKIalphaL. Therefore different CKIalpha isoforms are targeted to different cellular compartments in a fashion modulated by alternate transcription and in these locations presumably phosphorylate and regulate different cellular substrates.

Soluble cAMP-independent protein kinase from human spleen

A protein kinase (EC 2.7.1.37) was purified 2000-fold, from the soluble protein fraction of human spleen cells, using ion-exchange chromatography, ammonium sulfate fractionation, and gel filtration. This rapid procedure yielded 30% of the initial activity and an enzyme preparation with specific activity of 62 nmol min-1 mg-1 of protein. On the basis of disc gel electrophoresis in sodium dodecyl sulfate acrylamide gels and isoelectric focusing the enzyme preparation appears homogeneous and to consist of one polypeptide with a molecular weight of 43,000 and having a pI of 7.1. The purified enzyme activity is cyclic AMP and cGMP independent phosphorylates both alpha-casein and phosvitin, and uses Mg2+ ATP and Mg2+ GTP as phosphate donors, exhibiting an apparent Km of 2.0 and 6.6 X 10(-5)m, respectively. Furthermore, the enzyme activity is strongly inhibited by heparin (K50 = 0.1 micrograms/ml). These catalytic properties are characteristic of the enzyme casein kinase II, as described in several eukaryotic cells.

Rat liver nuclear protein kinases NI and NII. Purification, subunit composition, substrate specificity, possible levels of regulation

Rat liver nuclear protein kinases NI and NII have been purified to homogeneity by an improved method. This method includes a casein-phosvitin-Sepharose column step, which separates the enzymes from the other chromosomal non-histone proteins, and a gel filtration at high ionic strength in the presence of a high concentration of protease inhibitors to separate the two enzymes from each other. NI has an apparent molecular mass of approximately 50 kDa and is composed of a single subunit. NII has an apparent molecular mass of 133 kDa and is composed of two subunits of identical molecular mass. The V and the Km of the two enzymes were determined for several substrates. Both enzymes phosphorylate chromosomal non-histone proteins with partly different specificities as shown by two-dimensional electrophoreses. When incubated in the absence of protease inhibitors, the enzymes were degraded into discrete polypeptides. Autophosphorylation of a polypeptide derived from NII was observed after incubation of the enzyme with ATP. This phosphorylation stimulated the enzyme activity. Several chromosomal proteins coeluted with NII from the casein-phosvitin-Sepharose column. They remained associated with the enzyme in sucrose gradients, during gel filtration performed at physiological ionic strength, and are dissociated at high ionic strength. These proteins were highly phosphorylated when the protein-NII complex was incubated with ATP.

Polyamine-sensitive protein kinase from chick intestinal mucosa

A cyclic nucleotide-independent protein kinase which phosphorylates preferentially acidic proteins such as casein or phosvitin was isolated from cytosol of chick duodenal mucosa. The enzyme was purified more than 633 fold to apparent homogeneity by ammonium sulfate fractionation, column chromatography on DEAE-cellulose, phosphocellulose, hydroxylapatite and by sucrose density gradient centrifugation. The native enzyme has a molecular weight of 131 000 as measured by gel filtration. The enzyme is a complex protein containing three polypeptides of molecular weight of 39 000, 36 000 and 27 000. It behaves as a complex throughout its purification and gel filtration but its components are readily separated by electrophoresis in denaturing buffer. The 27 000 molecular weight band was selectively autophosphorylated when the enzyme was incubated in the presence of [gamma-32P]ATP. When casein was used as substrate, physiological concentrations of naturally occurring polyamines such as spermine and spermidine markedly stimulated enzyme activity. However with phosvitin as substrate polyamines were strong inhibitors of the enzyme activity. This contrasting effect on intestinal kinase activity was also apparent using cytoplasmic proteins as endogenous phosphate acceptors. A characterization of this differential effect is presented and some possible physiological implications are discussed.

Occurrence of protein kinases NI and NII in human and porcine thyroids

Cyclic nucleotide-independent protein kinases that preferentially phosphorylated casein and phosvitin as substrate were detected in the nuclei of human and porcine thyroid tissues, and compared with those from rat liver. Enzymes were extracted from the isolated nuclei with a buffer solution containing 0.4 M NaCl, and analyzed by DEAE-Sephadex and phosphocellulose column chromatographies. The chromatographies, together with the characterization of the enzymes, demonstrated that human and porcine thyroid tissues contained two major casein kinases in the cell nuclei, the properties of which revealed that they are to be identified as protein kinases NI and NII.

Preferential phosphorylation of basic non-histone proteins by nuclear protein kinase NII from rat liver

Rat liver nuclear protein kinase NII, which is independent of cyclic nucleotides, phosphorylated both the acidic protein casein and the basic histone preparation, soluble in 0.25 M HCl, which was extracted from the cell nuclei. When the isolated nuclei were incubated with [32P]ATP in the presence of protein kinase NII, more than 95% of the 32P-labelled proteins was recovered in the 0.25 M HCl extract. In order to analyze the substrate proteins of protein kinase NII, a histone preparation free from the endogenous protein kinase activities was used as substrate. The results demonstrated that histones were not phosphorylated, except for a faint 32P incorporation into the H3 fraction. Instead, multiple non-histone proteins were highly phosphorylated, which were present in a minor quantity in the histone preparation. The molecular weights of the main phosphorylated proteins were 72000, 68000, 56000, 47000, 46000, 43000, 38000 and 32000. Isoelectric focusing demonstrated the basic nature of the phosphorylated proteins, and as much as 72% of 32P radioactivity was distributed in the pH region higher than 7.0. Furthermore, many proteins phosphorylated in the isolated nuclei with added protein kinase NII were also found to be basic non-histone proteins. These results indicate that protein kinase NII preferentially phosphorylates in vitro a set of nuclear basic non-histone proteins.

Effect of cyclic AMP on chromatin-bound protein kinases in WI-38 fibroblasts stimulated to proliferate

When resting confluent monolayers of WI-38 fibroblasts are stimulated to proliferate by serum, DNA synthesis begins to increase between 15-18 h after stimulation. Chromatin-bound protein kinase activity increases in stimulated cells within 1 h after the nutritional change, concomitant with an increase in the template activity of nuclear chromatin. Addition of dibutyryl 3' : 5'-cyclic adenosine monophosphate (dibutyryl cyclic) AMP to the stimulating medium inhibits the entrance of cells into S phase, but only if dibutyryl cyclic AMP (5-10(-4) M) is added before the onset of DNA synthesis. The increases in chromatin template activity and in the chromatin-bound kinase activity are not inhibited by dibutyryl cyclic AMP in the early hours after stimulation, but are completely inhibited after the 5th hour from the nutritional change. This seems to indicate that in stimulated WI-38 cells, dibutyryl cyclic AMP exerts its inhibitory action somewhere between 5 and 12 h after stimulation. A number of protein kinase activities were extracted from chromatin with 0.3 M NaCl and partially resolved on a phosphocellulose column. Two distinct peaks of protein kinase activity appeared to be markedly increased in WI-38 cells 6 h after serum stimulation. Both peaks of increased activity were inhibited by dibutyryl cyclic AMP in vivo. Adenosine, sodium butyrate and adenosine 5'-monophosphate (AMP) do not inhibit the increase in DNA synthesis nor the increase in protein kinase activity. The results suggest that stimulation of cell proliferation in confluent monolayers of WI-38 cells causes an increase (or the new appearance) of certain chromatin-bound protein kinases, and that this increase is inhibited by cyclic AMP in vivo.