Specificity determinants of maize casein kinase-IIB are related to but distinct from those of rat liver casein kinase-2

Purification and characterization of casein kinase II from lactating rabbit mammary gland

1. Highly purified 200 kDa casein kinase II from rabbit lactating mammary gland (MG-CK II) was obtained by means of a new purification procedure consisting of one phosphocellulose and three Monó Q steps. 2. Its Km for ATP was 2.22 microM and 0.57 mg/ml and 0.13 mg/ml for partially dephosphorylated casein and phosvitin respectively. Stathmine was also suitable as substrate. 2-aminopurine and 6-dimethylaminopurine inhibited efficiently MG-CK II (Ki = 5 and 1 mM respectively). 3. MG-CK II autophosphorylated on its alpha-, alpha'- and beta-subunits. The beta-subunit autophosphorylation was enhanced in presence of exogenous substrate. Its modulation was highly dependent on ATP concentration. 4. The effects of basic compounds which affected dramatically the phosphorylation of dephosphorylated casein in presence of various ATP concentrations were reported.

Expression and characterization of a recombinant maize CK-2 alpha subunit

CKIIB, one of the CK-2 like enzymes which have been isolated from maize, has been shown to be a monomeric enzyme that cross-reacts with anti CK-2 alpha specific antibodies suggesting a possible relationship between the two proteins (Dobrowolska et al. (1992) Eur. J. Biochem. 204, 299-303). In order to support the immunological data also by biochemical and biophysical experiments the availability of a recombinant CK-2 alpha from maize was a prerequisite. A maize cDNA clone of maize CK-2 alpha was expressed in the bacterial strain BL21 (DE3). The recombinant protein was purified to homogeneity; its molecular mass on one-dimensional SDS PAGE was estimated to be 36.5 kDa. The calculated molecular mass according to the amino acid composition is 39,228 Da (332 amino acids). The recombinant maize CK-2 alpha (rmCK-2 alpha) exhibited mostly the same properties as the recombinant human CK-2 alpha (rhCK-2 alpha). In several respects it behaved differently from CKIIB, thus supporting the notion that either CKIIB is encoded by another gene or it undergoes extensive posttranscriptional and/or posttranslational alterations. Three observations in particular disprove any close relatedness between CKIIB and rmCK-2 alpha, namely: (a) the phosphorylation of calmodulin by CKIIB is dramatically stimulated by polylysine, whereas polylysine inhibits rather than stimulating the phosphorylation of calmodulin by rmCK-2 alpha (and by rhCK-2 alpha). (b) Addition of rhCK-2 beta has no significant influence on the stimulation of the calmodulin phosphorylation by CKIIB whereas in the case of rmCK-2 alpha and rhCK-2 alpha addition of rhCK-2 beta is required for optimal stimulation by polylysine. (c) CKIIB does not self-assemble with rhCK-2 beta to form a high molecular mass complex as it is demonstrated for rmCK-2 alpha.

Identification of protein phosphatase activities in maize seedlings

Three phosphatases active on phosphocasein (PhosphoCasein Phosphatases) termed PCP-I, PCP-II and PCP-III were isolated from maize seedlings by DEAE-cellulose chromatography and were shown to display a different specificity toward a variety of phosphorylated substrates including pNPP, phosphohistones, phosphorylase a and several phosphopeptides containing either phosphoserine or phosphothreonine. PCP-I and PCP-II bind to heparin-Sepharose, retain a remarkable pNPP activity, are uncapable to dephosphorylate phosphorylase a, and display striking activity toward the acidic phosphopeptide AS[32P]EEEEE. They also by far prefer phosphoseryl peptide RRAS[32P]VA over its phosphothreonyl derivative and are unsensitive to okadaic acid up to 1 microM. These properties are not consistent with the belonging of PCP-I and -II to any of the known classes of protein phosphatases and suggest that they are acidic phosphatases. Conversely, PCP-III is essentially free of pNPP activity; it readily dephosphorylates phosphohistone H1 and phosphorylase a and it displays a striking preference toward the phosphothreonyl peptides (RRAT[32P]VA and RRREEET[32P]EEEAA), while the phosphoseryl peptides (RRAS[32P]VA and AS[32P]EEEEE) are very poor substrates of the enzyme. These properties together with the findings that PCP-III does not bind to heparin-Sepharose and is highly sensitive to okadaic acid (IC50 = 0.2 nM) allow to identify PCP-III with a protein phosphatase of the PP-2A class.

Purification and characterization of maize seedling casein kinase IIB, a monomeric enzyme immunologically related to the alpha subunit of animal casein kinase-2

Casein kinase IIB (CKIIB), a protein kinase related to animal casein kinase-2 (CK2), has been purified to homogeneity. It appears to be a monomeric enzyme, composed by an individual 39 kDa subunit, homologous to the alpha/alpha' subunits of animal CK2 and devoid of the autophosphorylatable 25-kDa alpha subunit of animal CK2, which display an heterotetrameric alpha 2 beta 2/alpha alpha' beta 2 structure. Such a conclusion is supported by the following lines of evidence: (1) CKIIB displays an apparent 39,000 Mr by gel filtration on Ultrogel AcA 34 and it gives rise to a single prominent protein band of similar Mr (38,000) upon SDS/PAGE; (2) upon incubation of the enzyme with [32P]ATP, no radiolabeled bands are detectable which might be attributable to either canonical or atypical beta subunits; (3) the 39-kDa band immunoreacts with antisera that recognize the alpha subunit of rat and chicken CK2; (4) conversely, no component immunologically related with the beta subunit could be detected in CKIIB by Western-blot analyses with antisera that recognize animal beta subunits; (5) the recombinant beta subunit of human CK2 is readily phosphorylated by CKIIB, the reaction being prevented, rather than stimulated, by polylysine, a behaviour typical of animal CK2 autophosphorylation. While the responsiveness of CKIIB to either heparin inhibition or polylysine stimulation are reminiscent of those of animal CK2, its peptide substrate specificity is significantly different and its thermolability is increased. Altogether these data would indicate that maize seedling CKIIB represents a naturally occurring monomeric form of CK2 devoid of non-catalytic subunits. Its properties, compared to those of animal CK2, suggest that the beta subunits of animal CK2 may be responsible for structural modifications conferring an altered specificity and an increased stability to the catalytic subunit.

Cloning and sequencing of the casein kinase 2 alpha subunit from Zea mays

The nucleotide sequence of the cDNA coding for the alpha subunit of casein kinase 2 of Zea mays has been determined. The cDNA clone contains an open reading frame of 996 nucleotides encoding a polypeptide comprising 332 amino acids. The primary amino acid sequence exhibits 75% identity to the alpha subunit and 71% identity to the alpha' subunit of human casein kinase 2.

Benzimidazole nucleoside analogues as inhibitors of plant (maize seedling) casein kinases

Halogeno benzimidazole and benzimidazole nucleoside analogues have been screened for inhibitory activity vs. purified plant (maize seedling) casein kinases I, IIA and IIB, and the results compared with those previously reported for some of the compounds as inhibitors of the corresponding mammalian CK-1 and CK-2 (Meggio et al. (1990) Eur. J. Biochem. 187, 89-94). One new analogue, the riboside of 5,7-dibromobenzimidazole, which is sterically constrained to the anti conformation about the glycosidic bond, and is a good inhibitor, exhibited appreciable (5-7-fold) discrimination between the type I and type II enzymes. An increase in the number of halogen substituents on the benzene ring of benzimidazole from two to three led to marked enhancement of inhibitory activity, particularly against the type II enzymes, with a decrease in Ki from 24 to 4 microM. The 2-aza analogue of 5,6-dichlorobenzimidazole, i.e. 5,6-dichlorobenzotriazole, as the free base, even more effectively discriminated between the two types of plant casein kinases, with Ki approximately 100 microM for CK-I, and Ki approximately 9 microM for CK-IIA and CK-IIB. Inhibition in all instances was competitive with respect to ATP (for CK-I), and ATP and GTP (for CK-IIA and CK-IIB). The results are compared with those for halogenated isoquinolinesulfonamide inhibitors reported by Chijiwa et al. (J. Biol. Chem. (1989) 264, 4924-4927), leading to proposals for the synthesis of potentially more effective and more discriminating inhibitors. Attention is drawn to the significant role of the halogen substituents in the mechanism(s) of action of the structurally related benzimidazole, benzotriazole and naphthalene and isoquinoline, inhibitors of protein kinases.

Ribofuranosyl-benzimidazole derivatives as inhibitors of casein kinase-2 and casein kinase-1

5,6-Dichloro-1-(beta-D-ribofuranosyl)benzimidazole (DiCl-RB) is a powerful inhibitor of casein kinase-2 (CK-2) [Zandomeni, R. et al. (1986) J. Biol. Chem. 261, 3414-3420]. Here a series of 17 analogues of DiCl-RB has been employed for studying the specificity and the mode of action of this family of CK-2 inhibitors. The two halogen substituents on the benzene ring are shown to play a prominent role in inhibition, the 5,6-dibromo derivative (DiBr-RB) being fivefold more effective than DiCl-RB (Ki = 2 microM, with GTP as substrate), whereas the difluoro derivative (DiF-RB) is nearly as ineffective as unsubstituted 1-(beta-D-ribofuranosyl)benzimidazole. On the other hand, although some modifications of the ribose group significantly decrease the inhibitory efficiency, the sugar moiety is not strictly required, since dichlorobenzimidazole itself (DiCl-Bz) is an inhibitor almost as good as DiCl-RB. Inhibition of CK-2 by DiCl-RB and by its analogues, DiCl-Bz included, is of the competitive type with respect to the nucleotide substrate, the Ki values being lower with GTP than with ATP. The Ki values of the most potent inhibitor, DiBr-RB, with ATP and GTP, are 6 microM and 2 microM, respectively, denoting an affinity for the enzyme higher than that of the physiological substrates, ATP and GTP. DiBr-RB has been assayed for its inhibitory capacity toward several protein kinase other than CK-2. Protein kinase-C, cAMP-dependent protein kinase, the Ser/Thr protein kinase expressed by Pseudorabies virus, and four different tyrosine protein kinases from spleen, proved insensitive to DiBr-RB concentrations capable of almost entirely suppressing the activity of rat liver and maize seedling CK-2. Casein kinase-1 however is nearly as sensitive as CK-2 to DiBr-RB. Inhibition of CK-1 is also of the competitive type with respect to ATP (Ki = 14 microM). Although the inhibitory spectrum of CK-1 by the various analogues is reminiscent of that observed with CK-2, a remarkable difference is revealed by 5'-phosphorylation of ribose which increases the Ki with CK-2 while decreasing that with CK-1.