Polyglutamyl peptides: a new class of inhibitors of type-2 casein kinases

Mechanism of CK2 Inhibition by a Ruthenium-Based Polyoxometalate

CK2 is a Ser/Thr protein kinase involved in many cellular processes such as gene expression, cell cycle progression, cell growth and differentiation, embryogenesis, and apoptosis. Aberrantly high CK2 activity is widely documented in cancer, but the enzyme is also involved in several other pathologies, such as diabetes, inflammation, neurodegeneration, and viral infections, including COVID-19. Over the last years, a large number of small-molecules able to inhibit the CK2 activity have been reported, mostly acting with an ATP-competitive mechanism. Polyoxometalates (POMs), are metal-oxide polyanionic clusters of various structures and dimensions, with unique chemical and physical properties. POMs were identified as nanomolar CK2 inhibitors, but their mechanism of inhibition and CK2 binding site remained elusive. Here, we present the biochemical and biophysical characterizing of the interaction of CK2α with a ruthenium-based polyoxometalate, [Ru₄(μ-OH)₂(μ-O)₄(H₂O)₄ (γ-SiW₁₀O₃₆)₂]¹⁰⁻ (Ru₄POM), a potent inhibitor of CK2. Using analytical Size-Exclusion Chromatography (SEC), Isothermal Titration Calorimetry (ITC), and SAXS we were able to unravel the mechanism of inhibition of Ru₄POM. Ru₄POM binds to the positively-charged substrate binding region of the enzyme through electrostatic interactions, triggering the dimerization of the enzyme which consequently is inactivated. Ru₄POM is the first non-peptide molecule showing a substrate-competitive mechanism of inhibition for CK2. On the basis of SAXS data, a structural model of the inactivated (CK2α)₂(Ru₄POM)₂ complex is presented.

Chemical probes targeting the kinase CK2: a journey outside the catalytic box

CK2 is a protein kinase that plays important roles in many physio-pathological cellular processes. As such, the development of chemical probes for CK2 has received increasing attention in the past decade with more than 40 lead compounds developed. In this review, we aim to provide the reader with a comprehensive overview of the chemical probes acting outside the highly-conserved ATP-site developed to date. Such probes belong to different classes of molecules spanning from small molecules to peptides, act with a range of mechanisms of action and some of them present themselves as promising tools to investigate the biology of CK2 and therefore develop therapeutics for many disease areas including cancer and COVID-19.

Acetoxymethyl Ester of Tetrabromobenzimidazole-Peptoid Conjugate for Inhibition of Protein Kinase CK2 in Living Cells

CK2 is a ubiquitous serine/threonine protein kinase, which has the potential to catalyze the generation of a large proportion of the human phosphoproteome. Due to its role in numerous cellular functions and general anti-apoptotic activity, CK2 is an important target of research with therapeutic potential. This emphasizes the need for cell-permeable highly potent and selective inhibitors and photoluminescence probes of CK2 for investigating the protein phosphorylation networks in living cells. Previously, we had developed bisubstrate inhibitors for CK2 (CK2-targeted ARCs) that showed remarkable affinity (KD < 1 nM) and selectivity, but lacked proteolytic stability and plasma membrane permeability. In this report, the structures of CK2-targeted ARCs were modified for the application in live cells. Based on structure-activity studies, proteolytically stable achiral oligoanionic peptoid conjugates of 4,5,6,7-tetrabromo-1H-benzimidazole (TBBz) were constructed. Affinity of the conjugates toward CK2 reached subnanomolar range. Acetoxymethyl (AM) prodrug strategy was applied for loading TBBz-peptoid conjugates into living cells. The uptake of inhibitors was visualized by live cell imaging and the reduction of the phosphorylation levels of two CK2-related phosphosites, Cdc37 pSer13 and NFκB pSer529, was demonstrated by Western blot analysis.

Evaluation of 3-carboxy-4(1H)-quinolones as inhibitors of human protein kinase CK2

Due to the emerging role of protein kinase CK2 as a molecule that participates not only in the development of some cancers but also in viral infections and inflammatory failures, small organic inhibitors of CK2, besides application in scientific research, may have therapeutic significance. In this paper, we present a new class of CK2 inhibitors-3-carboxy-4(1H)-quinolones. This class of inhibitors has been selected via receptor-based virtual screening of the Otava compound library. It was revealed that the most active compounds, 5,6,8-trichloro-4-oxo-1,4-dihydroquinoline-3-carboxylic acid (7) (IC(50) = 0.3 microM) and 4-oxo-1,4-dihydrobenzo[h]quinoline-3-carboxylic acid (9) (IC(50) = 1 microM), are ATP competitive (K(i) values are 0.06 and 0.28 microM, respectively). Evaluation of the inhibitors on seven protein kinases shows considerable selectivity toward CK2. According to theoretical calculations and experimental data, a structural model describing the key features of 3-carboxy-4(1H)-quinolones responsible for tight binding to CK2 active site has been developed.

Eukaryotic translation-initiation factor eIF2beta binds to protein kinase CK2: effects on CK2alpha activity

eIF2 (eukaryotic translation-initiation factor 2) is a substrate and an interacting partner for CK2 (protein kinase CK2). Co-immuno-precipitation of CK2 with eIF2beta has now been observed in HeLa cells, overexpressing haemagglutinin-tagged human recombinant eIF2beta. A direct association between His6-tagged human recombinant forms of eIF2beta subunit and both the catalytic (CK2alpha) and the regulatory (CK2beta) subunits of CK2 has also been shown by using different techniques. Surface plasmon resonance analysis indicated a high affinity in the interaction between eIF2beta and CK2alpha, whereas the affinity for the association with CK2beta is much lower. Free CK2alpha is unable to phosphorylate eIF2beta, whereas up to 1.2 mol of phosphate/mol of eIF2beta was incorporated by the reconstituted CK2 holoenzyme. The N-terminal third part of eIF2beta is dispensable for binding to either CK2alpha or CK2beta, although it contains the phosphorylation sites for CK2. The remaining central/C-terminal part of eIF2beta is not phosphorylated by CK2, but is sufficient for binding to both CK2 subunits. The presence of eIF2beta inhibited CK2alpha activity on calmodulin and beta-casein, but it had a minor effect on that of the reconstituted CK2 holoenzyme. The truncated forms corresponding to the N-terminal or central/C-terminal regions of eIF2beta were much less inhibitory than the intact subunit. The results demonstrate that the ability to associate with CK2 subunits and to serve as a CK2 substrate are confined to different regions in eIF2beta and that it may act as an inhibitor on CK2alpha.

Drosophila melanogaster casein kinase II interacts with and phosphorylates the basic helix-loop-helix proteins m5, m7, and m8 derived from the Enhancer of split complex

Drosophila melanogaster casein kinase II (DmCKII) is composed of catalytic (alpha) and regulatory (beta) subunits associated as an alpha2beta2 heterotetramer. Using the two-hybrid system, we have screened a D. melanogaster embryo cDNA library for proteins that interact with DmCKIIalpha. One of the cDNAs isolated in this screen encodes m7, a basic helix-loop-helix (bHLH)-type transcription factor encoded by the Enhancer of split complex (E(spl)C), which regulates neurogenesis. m7 interacts with DmCKIIalpha but not with DmCKIIbeta, suggesting that this interaction is specific for the catalytic subunit of DmCKII. In addition to m7, we demonstrate that DmCKIIalpha also interacts with two other E(spl)C-derived bHLH proteins, m5 and m8, but not with other members, such as m3 and mC. Consistent with the specificity observed for the interaction of DmCKIIalpha with these bHLH proteins, sequence alignment suggests that only m5, m7, and m8 contain a consensus site for phosphorylation by CKII within a subdomain unique to these three proteins. Accordingly, these three proteins are phosphorylated by DmCKIIalpha, as well as by the alpha2beta2 holoenzyme purified from Drosophila embryos. In line with the prediction of a single consensus site for CKII, replacement of Ser(159) of m8 with either Ala or Asp abolishes phosphorylation, identifying this residue as the site of phosphorylation. We also demonstrate that m8 forms a direct physical complex with purified DmCKII, corroborating the observed two-hybrid interaction between these proteins. Finally, substitution of Ser(159) of m8 with Ala attenuates interaction with DmCKIIalpha, whereas substitution with Asp abolishes the interaction. These studies constitute the first demonstration that DmCKII interacts with and phosphorylates m5, m7, and m8 and suggest a biochemical and/or structural basis for the functional equivalency of these bHLH proteins that is observed in the context of neurogenesis.

Cloning and disruption of CKB1, the gene encoding the 38-kDa beta subunit of Saccharomyces cerevisiae casein kinase II (CKII). Deletion of CKII regulatory subunits elicits a salt-sensitive phenotype

Saccharomyces cerevisiae casein kinase II (CKII) contains two distinct catalytic (alpha and alpha') and regulatory (beta and beta') subunits. We report here the isolation and disruption of the gene, CKB1, encoding the 38-kDa beta subunit. The predicted Ckb1 sequence includes the N-terminal autophosphorylation site, internal acidic domain, and potential metal binding motif (CPX3C-X22-CPXC) present in other beta subunits but is unique in that it contains two additional autophosphorylation sites as well as a 30-amino-acid acidic insert. CKB1 is located on the left arm of chromosome VII, approximately 33 kilobases from the centromere and does not correspond to any previously characterized genetic locus. Haploid and diploid strains lacking either or both beta subunit genes are viable, demonstrating that the regulatory subunit of CKII is dispensable in S. cerevisiae. Such strains exhibit wild type behavior with regard to growth on both fermentable and nonfermentable carbon sources, mating, sporulation, spore germination, and resistance to heatshock and nitrogen starvation, but are salt-sensitive. Salt sensitivity is specific for NaCl and LiCl and is not observed with KCl or agents which increase osmotic pressure alone. These data suggest a role for CKII in ion homeostasis in S. cerevisiae.

pH-dependent changes in structure and RNA-binding activity of casein kinase 2 from Rana temporaria oocytes

It is demonstrated by filter-binding assay that casein kinase 2 from Rana temporaria oocytes binds rRNA in vitro with high affinity. Ligand-blotting shows that rRNA-binding activity is inherent to alpha and alpha' subunits of the enzyme. Increase of pH from 6.5 to 7.5 has little effect on casein kinase but completely suppresses rRNA-binding activity of the enzyme. Sedimentation coefficient of casein kinase 2 also depends on pH: at pH 7.5 it is mainly 10 S, and at pH 6.5-18 S. At pH 6.95 the amounts of both forms are equal. The heavy form of casein kinase 2 practically lacks rRNA-binding activity.

Synthetic peptides including acidic clusters as substrates of yeast casein kinase-2

The synthesis is reported of a series of glutamyl peptide analogs of the model substrate H-Ser-Glu-Glu-Glu-Glu-Glu-OH of casein kinase-2 (CK-2). A convenient HPLC method for the separation of slightly different acidic peptides is also reported. The site specificity of yeast casein kinase-2 (Y-CK2) is examined with the aid of synthesized peptide substrates.

Copolymers of glutamic acid and tyrosine are potent inhibitors of oocyte casein kinase II

Polypeptides rich in glutamic acid are strong inhibitors purified from isolated nuclei of Xenopus laevis oocytes of casein kinase II. The presence of tyrosine in these peptides greatly enhances their inhibitory capacity. Using casein as a substrate, copolyglu:tyr (4:1) has an I50 value of 20 nM, 250 fold lower than that of polyglutamic acid which is 5 microM. A similar large difference is observed when a synthetic peptide is used as substrate. The inhibition of copolyglu:tyr is competitive with casein and can be completely reversed by high ionic strength. The relative inhibitory capacity of the polypeptides tested, in descending order, is copolyglu:tyr (4:1) greater than copolyglu:tyr (1:1) greater than polyglu greater than copolyglu:phe (4:1) greater than copolyglu:ala (6:4) greater than copolyglu:leu (4:1). The high affinity for tyrosine-containing acid peptides is shared by rat liver and yeast casein kinase II so that it seems to be a general property of these enzymes.

Aberrant casein kinase II in Alzheimer's disease

Abnormal protein phosphorylation has been identified in Alzheimer's disease (AD) for several proteins including a Mr 60,000 protein, a Mr 86,000 protein and a microtubule-associated protein tau. The Mr 86,000 protein is phosphorylated by protein kinase C, whereas protein kinases responsible for other aberrant phosphorylation reactions are not known. In addition to protein kinase C, another kinase, casein kinase II (CK-II), has now been shown to be aberrant in AD. The spermine-dependent CK-II activity is reduced by 84% in AD and the amount of CK-II as determined by its immunoreactivity on a Western blot is reduced by 63%. Furthermore, the distribution of CK-II in AD is altered. Although the neuronal cell body reacts well with CK-II antisera in the normal cortex, the non-tangle-bearing neurons in the AD cortex showed a 15-30% decrease in anti-CK-II immunoreactivity. The neurofibrillary tangles, on the other hand, stain very strongly with rabbit anti-CK-II and indicates that CK-II may be involved in the pathology of AD. The study of CK-II immunoreactivity for dementing diseases other than AD revealed a similar reduction, suggesting the CK-II involvement in the common process of neurodegeneration.

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.

Random tyrosine and glutamic acid-containing polymers are very powerful inhibitors of casein kinase-2

The random heteropolymers Glu/Tyr(4:1) and Glu/Tyr(1:1) that are widely used as substrates for tyrosine protein kinases, are very powerful competitive inhibitors of casein kinase-2, but not of casein kinase-1, with respect to the protein substrate, their Ki values being one to two orders of magnitude lower than those of polyglutamates of similar size. The inhibitory power is reduced if tyrosine is partially replaced by alanine, as in the polymer Glu/Ala/Tyr(6:3:1) and it disappears upon inclusion of lysine, the polymer Glu/Ala/Lys/Tyr(2:6:5:1) actually behaving as a stimulator. These data indicate that non-phosphorylatable hydroxylic residues in addition to acidic ones are required in order to optimize the binding of pseudo-substrates to the catalytic site of casein kinase-2.

Phosphorylation of protein B-50 (GAP-43) from adult rat brain cortex by casein kinase II

The phosphoprotein B-50 (GAP-43) was purified from adult rat brain cortex and phosphorylated by casein kinase II. Phosphorylation of B-50 by casein kinase II approached 1.2 mol phosphate/mol B-50. The apparent Km of casein kinase II for B-50 was 4 microM with an apparent Vmax of 13 nmol.min-1.mg-1. A tryptic phosphopeptide map on reversed phase HPLC and phosphoamino acid analysis of [32P]B-50 showed that casein kinase II phosphorylated in serine residue(s) which were located in a single tryptic peptide. Phosphorylation of B-50 by casein kinase II was inhibited more than 90% by 5 micrograms heparin/ml or 2.4 mM peptide substrate specific for casein kinase II (RRREEETEEE). The initial phosphorylation rate was increased about 2-fold by 1 mM spermine.

The phosphorylation of nucleoplasmin by casein kinase-2 is resistant to heparin inhibition

Highly purified preparations of casein kinase-2 from the nuclei of Xenopus laevis oocytes and from calf thymus can phosphorylate in vitro purified nucleoplasmin from X. laevis oocytes and eggs. The phosphorylation of nucleoplasmin by both kinase preparations is quite insensitive to heparin in contrast with casein phosphorylation which is completely abolished by heparin concentrations above 10 micrograms/ml. However, the phosphorylation of nucleoplasmin and casein are inhibited in a very similar fashion by 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB), a well characterized specific inhibitor of casein kinase-2. Similarly, nucleoplasmin phosphorylation by the oocyte enzyme can be stimulated several-fold by spermine, another characteristic of this enzyme. These findings indicate that the phosphorylation of nucleoplasmin by purified casein kinase-2, while showing typical response to DRB and spermine, exhibits anomalous behavior in its resistance to heparin inhibition. It is possible that the large clusters of acidic amino acids in nucleoplasmin permit this substrate to interact with the enzyme more efficiently than other protein substrates. Heparin is generally considered a potent and specific inhibitor of casein kinase-2. This study, however, questions the validity of utilizing heparin inhibition as a criterion for casein kinase-2 involvement.

Regulation of casein kinase 2 by phosphorylation/dephosphorylation

The effects of various polycation-stimulated (PCS) phosphatases and of the active catalytic subunit of the ATPMg-dependent (AMDc) protein phosphatase on the activity of casein kinase 2 (CK-2) were investigated by using the synthetic peptide substrate Ser-Glu-Glu-Glu-Glu-Glu, whose phosphorylated derivative is entirely insensitive to these protein phosphatases. Previous dephosphorylation of native CK-2 enhances its specific activity 2-3-fold. Such an effect, accounted for by an increase in Vmax, is more readily promoted by the PCS phosphatases than by the AMDc phosphatase. The phosphate incorporated by autophosphorylation could not be removed by the protein phosphatases, suggesting the involvement of phosphorylation site(s) other than the one(s) affected by intramolecular autophosphorylation. The activation of CK-2 by the phosphatase pretreatment is neutralized during the kinase assay; the mechanism of this phenomenon, which is highly dependent on the kinase concentration, is discussed.

Structure and properties of casein kinase-2 from Saccharomyces cerevisiae. A comparison with the liver enzyme

A type-2 casein kinase (YCK-2), lacking the 25-kDa autophosphorylatable beta subunit characteristic of animal casein kinases-2, has been obtained in a nearly pure form from Saccharomyces cerevisiae and was compared with liver casein kinase-2 (LCK-2). A 22-kDa phosphorylatable protein, copurifying with YCK-2, can be removed by ultracentrifugation at low ionic strength and is shown by several criteria to be unrelated to the beta subunit of LCK-2. The native Mr of YCK-2, deprived of the 22-kDa phosphoprotein, is about 150 000. Limited proteolysis experiments show that YCK-2 included 37-kDa catalytic subunits, which can be converted into still active 35-kDa proteolytic derivatives. These data are consistent with a homotetrameric quaternary structure as opposed to the heterotetrameric subunit composition alpha 2 beta 2 of LCK-2 and other animal casein kinases-2. Although many properties of YCK-2 and LCK-2, including substrate specificity, inhibition by heparin, polyglutamic acid and quercetin and stimulation by polyamines, are similar; their stability under denaturing and dissociating conditions and their response to polybasic peptides are quite different. In particular YCK-2 is more readily denatured than LCK-2 by heating and exposure to urea, sodium dodecylsulphate and deoxycholate while its activity is inhibited by 100-150 mM NaCl, which conversely stimulates LCK-2 activity 2-3-fold. The Km value of the synthetic peptide substrate Ser-(Glu)5 for YCK-2 is not significantly changed by the addition of polylysine. On the contrary the Km value of the same peptide substrate for LCK-2 decreases approximately tenfold upon addition of polylysine, which also prevents the fast autophosphorylation of the kinase at its beta subunit. These data suggest that the beta subunit of animal CK-2 may play a role in determining both the stability of the enzyme and its regulation and that, consequently, the different properties of YCK-2 may be at least in part accounted for by its lack of beta subunits.

Identification of the substrates of the casein kinase II associated with non-polysomal messenger ribonucleoproteins of A. salina cryptobiotic embryos

The association of a protein kinase with cytoplasmic non-polysomal messenger ribonucleoproteins is demonstrated by chromatography on oligo(dT)-cellulose and sucrose gradient centrifugation. The cAMP-independent enzyme is inhibited by caffeine and poly(L)-glutamic acid and is classified as a casein kinase II. Among the exogenous proteins initiation factor eIF2 is the best substrate and is 7.8 times more efficiently phosphorylated than casein. Endogenous mRNP protein substrates have a Mr of 125 000, 65 000, 38 000, 26 000 and 23 500. The main phosphate acceptor is the Mr 38 000 poly(A)-binding protein. Dephosphorylation of the poly(A)-binding protein by protein phosphatases decreases its RNA binding property. The effect of phosphorylation-dephosphorylation of mRNP proteins on the initiation of protein synthesis is discussed.

Ribosome-associated cyclic nucleotide-independent protein kinase of Artemia salina cryptobiotic gastrulae

An extra-ribosomal cAMP-independent protein kinase from cryptobiotic embryos of Artemia salina has been purified to near homogeneity by gel filtration on Bio-Gel A-0.5 m, ion-exchange chromatography on DEAE-cellulose and phosphocellulose P11 and affinity chromatography on casein-Sepharose 4B and ATP-agarose. The enzymatic activity has a broad optimum at pH 7-8. Maximal activity is obtained in the presence of 5-6 mM MgCl2. The activity is inhibited by Mn2+, Ca2+ and K+. The enzyme has an Mr of 127 000, utilizes both ATP and GTP as phosphoryl donors and is completely inhibited by heparin and poly(L-glutamic acid). According to its properties, the enzyme can be classified as a casein kinase type II. Although the enzyme is associated with ribosomes, ribosomal proteins are not among the main substrates. The kinase is able to phosphorylate both the alpha and the beta subunits of initiation factor eIF2 using ATP or GTP as phosphoryl donors. The function of phosphorylation in the initiation of protein synthesis is discussed.

A 25 000 dalton inhibitor of cAMP independent protein kinases present in rat liver HMG protein preparations

A protein kinase inhibitor was found in rat liver cells as a component of HMG proteins. It is located in cytosol as well as in nuclei. It inhibits all tested cAMP independent protein kinases and has no effect on cAMP dependent protein kinases. This inhibitor is a 25 000 Da protein. It has no ATPase, phosphoprotein phosphatase or proteinase activity and is heat unstable.

A type-1 casein kinase from yeast phosphorylates both serine and threonine residues of casein. Identification of the phosphorylation sites

A protein kinase (casein kinase 1A) active on casein and phosvitin but not on histones has been purified to near homogeneity from yeast cytosol and meets most criteria for being considered a type-1 casein kinase: it is a monomeric enzyme exhibiting an Mr of about 27 kDa by sucrose gradient centrifugation: it is not affected by inhibitors of type-2 casein kinases, such as heparin and polyglutamate, and shows negligible affinity for GTP. It also readily phosphorylates the residue Ser-22 of beta-casein located within the sequence -Ser(P)-Ser(P)-Ser(P)-Glu-Glu-Ser22-Ile-Thr-Arg- which is typically affected by casein kinases of the first class. On the other hand, casein kinase 1A displays the unusual property of phosphorylating threonine residue(s) in both whole casein and alpha s1-casein. The threonine residue phosphorylated in alpha s1-casein and accounting for most of the 32P incorporated into this protein by casein kinase 1A has been identified as Thr-49, which occurs in the sequence -Ser(P)-Glu-Ser(P)-Thr(P*)49-Glu-Asp-Gln-, whose two Ser(P) residues are already phosphorylated in the native protein. It is concluded that some type-1 casein kinases can also phosphorylate threonine residues provided they fulfil definite structural requirements, probably an acidic cluster near their N-terminal side.

The primary structure of avian phosvitins. Contributions through the Edman degradation of methylmercaptovitins prepared from the constituent phosphoproteins

Methylmercaptovitins were prepared from the constituent phosvitin phosphoproteins as well as from the CNBr cleavage peptides derived from the methionine-containing phosphoproteins. Edman degradation of these methylmercaptovitins has afforded partial sequence information and identifies the serine phosphorylation sites in the phosphoprotein. Primary structure, determined for approximately seventy-seven residues from the amino terminus of the hen methionine-containing phosphoprotein, agrees fully with that deduced from recently published nucleotide sequence data, and provides corrections to results of earlier work on enzymatically dephosphorylated samples. Partial sequence data, together with corrections to earlier results, are also provided for phosphoproteins from duck and turkey phosvitins, as well as for the methionine-free phosphoprotein from hen phosvitin. All phosphoproteins have N-terminal leader sequences of low serine content. Sequences of the methionine-containing group are homologous, as are the sequences of the methionine-free group, but the groups differ significantly from one another. Unphosphorylated sites appear to have a fractional distribution over all available serine residues.