Studies of subunit structure of phosvitin kinase FII from bovine brain

Kinetics of inhibition by 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole on calf thymus casein kinase II

The adenosine analogue 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB) is a specific inhibitor for RNA polymerase II transcription in vivo and in vitro [Tamm + Sehgal (1978) Adv. Virus Res. 22, 187-258; Zandomeni & Weinmann (1984) J. Biol. Chem. 259, 14804-14811]. The effect on RNA polymerase II-specific transcription seems to be mediated by its inhibition of nuclear casein kinase II [Zandomeni, Carrera-Zandomeni, Shugar & Weinmann (1986) J. Biol. Chem. 261, 3414-3419]. Inhibition studies indicated that DRB acted as a mixed-type inhibitor with respect to casein and as a competitive inhibitor with respect to the nucleotide phosphate donor substrates. The DRB inhibition constant is 7 microM for the calf thymus casein kinase II, with regard to both ATP and GTP.

An improved purification procedure and properties of casein kinase II from brain

A simple and short purification procedure applicable to casein kinase II has been developed, for fully characterizing the enzyme from calf cerebral cortex cytosol. The procedure consists of four chromatographic steps: DEAE-cellulose, phosphocellulose, phosvitin-Sepharose and ATP-agarose which yields 87% pure casein kinase II. The purified enzyme shows three major bands with apparent molecular masses of 42, 38, and 27 kDa by polyacrylamide gel electrophoresis in sodium dodecyl sulfate and is self-autophosphorylated on its 27 kDa polypeptide. The enzyme shows all the characteristics described for casein kinase II from other sources: it is independent of cyclic nucleotides, calcium/phospholipids, and double-stranded poly(I).poly(C); it can utilize both ATP and GTP as phosphoryl donors and can phosphorylate both casein and phosvitin but not histone. The kinetic studies establish that the Km for ATP is 12.5 microM and 25.1 microM when using phosvitin and casein respectively as phosphoryl acceptors. The Km for phosvitin is 0.91 mg/ml and for casein 1.43 mg/ml, while the Vmax is 315 nmol/min/per mg protein and 479 nmol/min/per mg protein for phosvitin and casein respectively. The activity of the kinase is highly stimulated by KCl or NaCl, and almost completely inhibited by heparin concentrations of 1 microgram/ml (92%). This inhibition is reduced to only 33% in the presence of optimal KCl concentrations (150 mM). Spermine stimulates enzyme activity, whilst hemin produces a slight inhibition.

A rapid purification method for calf thymus casein kinase II

We report here the rapid purification to homogeneity of a cyclic nucleotide-independent protein kinase sensitive to 5'6-dichloro-1-beta-D-ribofuranozylbenzimidazole (DRB), identical to the previously described casein kinase II, from lyophilized calf thymus by chromatography on phosphocellulose and Mono-Q FPLC columns.

Identification of calcium-independent myosin kinase with casein kinase II

A crude myosin fraction from bovine brain has been found to contain a Ca2+-independent myosin kinase that catalyzes the phosphorylation of 20,000-Da light chain of gizzard myosin. The myosin kinase has been separated from the myosin by Sepharose CL-4B gel filtration and purified further by chromatography on phosphocellulose, Sephacryl S-300, and hydroxylapatite. The myosin kinase was found to copurify with casein kinase II and show the same substrate specificity with the casein kinase. These results indicate that the myosin kinase is identical to casein kinase II. The purified myosin kinase catalyzed the preferential phosphorylation of the threonyl residues of 20,000-Da light chains of gizzard and brain myosins. The 17,000-Da light chains of these myosins and the mixed light chains of skeletal and cardiac muscle myosins were not phosphorylated by the enzyme to an appreciable extent.

Purification and partial characterization of two cyclic amp-independent protein kinases from AH-66 hepatoma cells

Two cyclic AMP-independent protein kinases (CK 1 and CK 2) in the cytosol fraction of AH-66 hepatoma cells have been purified to homogeneity. CK 1 and CK 2 had molecular weights of 36,000 and 340,000, respectively, as determined by gel filtration. CK 1 consists of a single polypeptide and CK 2 is composed of two subunits of 42,000 and 26,000 molecular weights. CK 1 and the 26,000 molecular weight subunit of CK 2 were autophosphorylated. Both enzymes catalyzed the phosphorylation of cytosolic proteins including those with molecular weights of 125,000, 95,000, 56,000, and 40,000.

Reversibility of the phosphate transfer between ATP and phosphoproteins catalysed by a cyclic nucleotide independent (G type) casein kinase

Purified casein kinase G was found able to catalyse the synthesis of [gamma-32P]ATP in the presence of ADP, phosphocasein (previously 32P-labeled by the forward kinase reaction) and magnesium. Apparent Km values of approx. 0.5 mM for phosphocasein and 7.5 mM for ADP were calculated, these values indicating low affinities for the substrates as compared to those exhibited for casein and ATP in the forward reaction. The reverse casein kinase G activity appeared to prefer ADP and GDP as phosphate acceptors. Whereas the casein kinase G reverse reaction could be supported by casein, phosvitin and histone previously phosphorylated by the enzyme, the same proteins could not serve as a phosphate source when previously phosphorylated by the cAMP-dependent protein kinase. Forward and reverse casein kinase G reactions exhibited different optimal pH values (8.5 and 7.2, respectively) and a different sensitivity to Mg2+. Spermine, which activated the kinase activity, blocked the reverse reaction at millimolar concentrations. Although the biological significance of the casein kinase G reverse activity remains to be assessed in intact cell, the process may be useful as a tool in the characterization of phosphorylatable sites in phosphoproteins.

Identification of the catalytic subunit of an oligomeric casein kinase (G type). Affinity labeling of the nucleotide site using 5'-[p-(fluorosulfonyl)benzoyl]adenosine

Identification of the catalytic subunit of a G type [using guanosine 5'-triphosphate (GTP) as well as adenosine 5'-triphosphate (ATP) as phosphate donor], oligomeric, cyclic nucleotide independent casein kinase purified from bovine lung was carried out after reaction with 5'-[p-(fluorosulfonyl)-benzoyl]adenosine (FSBA) and isolation of the subunit components of the enzyme. FSBA exhibited the major characteristics of an affinity label reacting at the nucleotide (ATP, GTP) site of the casein kinase. FSBA acted as a competitive inhibitor of ATP (and GTP), led to complete inactivation of the enzyme in a reaction showing two kinetic steps, and became irreversibly bound to the protein. After being labeled with FSBA, the casein kinase (apparent molecular weight of 140 000) was separated into its two monomeric components of apparent molecular weights 38 000 (alpha) and 27 000 (beta), respectively, after sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Use of radioactive FSBA showed that specific affinity labeling was limited to the alpha casein kinase subunit. This result was in agreement with the fact that casein kinase activity was found associated with the alpha monomer after electrophoretic separation of the alpha and beta subunits. It may thus be concluded that the largest (alpha) subunit contains the catalytic site of the casein kinase G. Electrophoretic analysis of purified protein kinase under denaturing conditions suggested an alpha 3 beta 2 combination for an apparent molecular weight of 130 000-140 000. However, a maximum of 2 mol of FSBA could be specifically bound to the alpha subunit per mol of enzyme, with a concomitant complete inactivation. These data would be in agreement with an alpha 2 beta 2 subunit composition for casein kinase G, as proposed by other research groups for a similar type of protein kinase of different sources. These observations suggest that the alpha subunits are functionally similar, each of them containing a nucleotide (ATP, GTP) binding site. The possible role of the beta subunit in the enzyme activity remains to be established.

Catalytic and molecular properties of a highly purified G type casein kinase from bovine lung tissue

A preparation procedure has been worked out to obtain a highly purified G type (using GTP as well as ATP) casein kinase from large quantities of bovine lung tissue. It included ion-exchange (DEAE and phosphocellulose) and affinity (casein and ATP-Sepharose) chromatography combined with a flocculation step, and yielded an apparently homogeneous preparation with a 16% yield and a purification factor of more than 1400. The purified lung casein kinase used GTP (Km 16 microM) almost as well as ATP (Km 6.7 microM) and exhibited the major catalytic properties of the casein kinase G previously described in bovine adrenal cortex (Cochet, C., Job, D., Pirollet, F. and Chambaz, E.M. (1981) Biochim. Biophys. Acta 658, 191-201). Mg2+ (30-50 mM) and spermine (2 mM) were potent activators of lung casein kinase G activity, whereas the enzyme was inhibited by heparin and quercetin. The purified enzyme underwent self-phosphorylation in the presence of ATP or GTP, serine being the only target amino acid under these conditions, whereas both serine and threonine were phosphorylated by the enzyme in casein. Lung casein kinase G exhibited an apparent molecular weight between 140 000-160 000 upon gel filtration and appeared formed by the association of two different subunits upon SDS-polyacrylamide gel electrophoresis. The two subunits of Mr 38 000 (alpha) and 27 000 (beta) exhibited a 2:1 ratio upon quantitative scanning, suggesting an alpha 3 beta 2 combination in the oligomeric native enzyme structure. Peptide mapping of the two isolated subunits following 125I-labeling and papain digestion did not disclose any common fragment. The casein kinase catalytic activity was found associated with the alpha (38 kDa) enzyme subunit after recovery from gel electrophoresis in the presence of SDS, whereas the 27 kDa (beta) subunit was the major target of the enzyme self-phosphorylation reaction. alpha and beta subunits appeared strongly associated in the oligomeric enzyme and the possible role of the beta subunit in the casein kinase G activity remains to be examined. The purified casein kinase G, which can be obtained by the present procedure, should facilitate the study of the biological significance of this phosphorylation system in the intact cell.

Cyclic nucleotide independent casein kinase (G type) in bovine adrenal cortex: purification and properties of two molecular forms

Two soluble cyclic nucleotide independent protein kinase (ATP: protein-phosphotransferase, EC 2.7.1.37) activities have been purified from bovine adrenal cortex cytosol. Both purified enzymes exhibit the best affinity for acidic substrates such as casein and can use GTP as well as ATP as phosphoryl donor. They can thus be classified as casein kinase of the G type as previously proposed (Cochet C. et al., (1980) Endocrinology 106, 750-757). Whereas the two moieties could be separated using their different affinities toward a phosphocellulose resin, both purified enzymes appeared indistinguishable on the basis of several molecular and catalytic properties. Both G type casein kinase moieties have an identical sedimentation behavior (5.5 S in the presence of 0.5 M NaCl), yield similar patterns upon electrophoresis under denaturing conditions with three major protein components (42 000, 38 000 and 27 000), and show an ability to undergo self-phosphorylation mostly on the 27 000 component. Both enzymes have the same protein and nucleotide (ATP and GTP) substrate specificity, show similar increases in activity in the presence of polyamines and Mg2+ (optimum at 50 mM) and similar inhibition by NaCl above 0.2 M. The only difference between the two forms of casein kinase (i.e., affinity for phosphocellulose) could not be explained by a different degree of self-phosphorylation or by a limited proteolytic process during handling and purification. These results suggest that the two active moieties may represent isoenzymatic forms of the G type casein kinase activity in bovine adrenal cortex cytosol.

The ATP substrate site of a cyclic-nucleotide-independent protein kinase from porcine liver nuclei

The ATP substrate site of a second messenger-independent protein kinase of the type NII from porcine liver nuclei was mapped using a series of 30 ATP derivatives with modifications at the base, ribose or triphosphate moiety. Ki values for these derivatives were determined by competition with [gamma-32P]ATP; they range from 4 microM to 1.5 mM. For a comparison with data previously reported for the catalytic subunit of cAMP-dependent protein kinase I from rabbit skeletal muscle, the Ki values were transformed into delta delta values. These values are related to the Ki value of unsubstituted ATP and indicate the decrease of affinity caused by the different substitutions. With both enzymes the major binding affinity is derived from the interaction of the adenine base. The contributions of the two ribosyl OH groups are marginal and the triphosphate moiety interacts most strongly with its beta-phosphoryl group. Between the two enzymes the most striking differences, however, were observed for the specificity of the nucleobase interaction. While an unmodified N-6 amino group is required in the case of the cAMP-dependent protein kinase, the nuclear enzyme seems to tolerate extensive modification at this position, such as the introduction of a keto group or a bulky benzyl residue. Obviously, the ATP site of the nuclear kinase has an open cleft next to the N-6 of the adenine and binding of the adenine occurs by hydrophobic interaction without the formation of hydrogen bonds to any of the adenine nitrogens.