Phosphorylation of high mobility group protein 14 by casein kinase II

Functional analysis of protein kinase CK2 of the human malaria parasite Plasmodium falciparum

Protein kinase CK2 (casein kinase 2) is a eukaryotic serine/threonine protein kinase with multiple substrates and roles in diverse cellular processes, including differentiation, proliferation, and translation. The mammalian holoenzyme consists of two catalytic alpha or alpha' subunits and two regulatory beta subunits. We report the identification and characterization of a Plasmodium falciparum CK2alpha orthologue, PfCK2alpha, and two PfCK2beta orthologues, PfCK2beta1 and PfCK2beta2. Recombinant PfCK2alpha possesses protein kinase activity, exhibits similar substrate and cosubstrate preferences to those of CK2alpha subunits from other organisms, and interacts with both of the PfCK2beta subunits in vitro. Gene disruption experiments show that the presence of PfCK2alpha is crucial to asexual blood stage parasites and thereby validate the enzyme as a possible drug target. PfCK2alpha is amenable to inhibitor screening, and we report differential susceptibility between the human and P. falciparum CK2alpha enzymes to a small molecule inhibitor. Taken together, our data identify PfCK2alpha as a potential target for antimalarial chemotherapeutic intervention.

High mobility group proteins and their post-translational modifications

The high mobility group (HMG) proteins, including HMGA, HMGB and HMGN, are abundant and ubiquitous nuclear proteins that bind to DNA, nucleosome and other multi-protein complexes in a dynamic and reversible fashion to regulate DNA processing in the context of chromatin. All HMG proteins, like histone proteins, are subjected to extensive post-translational modifications (PTMs), such as lysine acetylation, arginine/lysine methylation and serine/threonine phosphorylation, to modulate their interactions with DNA and other proteins. There is a growing appreciation for the complex relationship between the PTMs of HMG proteins and their diverse biological activities. Here, we reviewed the identified covalent modifications of HMG proteins, and highlighted how these PTMs affect the functions of HMG proteins in a variety of cellular processes.

Recruitment of ikaros to pericentromeric heterochromatin is regulated by phosphorylation

Ikaros encodes a zinc finger protein that is involved in heritable gene silencing. In hematopoietic cells, Ikaros localizes to pericentromeric heterochromatin (PC-HC) where it recruits its target genes, resulting in their activation or repression via chromatin remodeling. The function of Ikaros is controlled by post-translational modifications. CK2 kinase has been shown to phosphorylate Ikaros at its C terminus, affecting cell cycle progression. Using in vivo labeling of murine thymocytes followed by phosphopeptide mapping, we identified four novel Ikaros phosphorylation sites. Functional analysis of phosphomimetic mutants showed that the phosphorylation of individual amino acids determines the affinity of Ikaros toward probes derived from PC-HC. In vivo experiments demonstrated that targeting of Ikaros to PC-HC is regulated by phosphorylation. The ability of Ikaros to bind the upstream regulatory elements of its known target gene terminal deoxynucleotidyltransferase (TdT) was decreased by phosphorylation of two amino acids. In thymocytes, Ikaros acts as a repressor of the TdT gene. Induction of differentiation of thymocytes with phorbol 12-myristate 13-acetate plus ionomycin results in transcriptional repression of TdT expression. This process has been associated with increased binding of Ikaros to the upstream regulatory element of TdT. Phosphopeptide analysis of in vivo-labeled thymocytes revealed that Ikaros undergoes dephosphorylation during induction of thymocyte differentiation and that dephosphorylation is responsible for increased DNA binding affinity of Ikaros toward the TdT promoter. We propose a model whereby reversible phosphorylation of Ikaros at specific amino acids controls the subcellular localization of Ikaros as well as its ability to regulate TdT expression during thymocyte differentiation.

Phosphorylation of human high mobility group N1 protein by protein kinase CK2

High mobility group (HMG) N1 protein, formerly known as HMG 14, is a member of the chromosomal HMG protein family. Protein kinase CK2 was previously reported to be able to phosphorylate bovine HMGN1 in vitro; Ser89 and Ser99, corresponding to Ser88 and Ser98 in human HMGN1, were shown to be major and minor recognition sites, respectively. In this report, we employed mass spectrometry and examined both the extent and the sites of phosphorylation in HMGN1 protein catalyzed by recombinant human protein kinase CK2. We found that five serine residues, i.e., Ser6, Ser7, Ser85, Ser88, and Ser98, in HMGN1 can be phosphorylated by the kinase in vitro. All five sites were previously shown to be phosphorylated in MCF-7 human breast cancer cells in vivo. Among these five sites, Ser6, Ser7, and Ser85 were new sites of phosphorylation induced by protein kinase CK2 in vitro.

Tandem Mass Spectrometry for the Examination of the Posttranslational Modifications of High-Mobility Group A1 Proteins: Symmetric and Asymmetric Dimethylation of Arg25 in HMGA1a Protein

High-mobility group (HMG) A1a and A1b proteins are among a family of HMGA proteins that bind to the minor groove of AT-rich regions of DNA. Here we employed tandem mass spectrometry and determined without ambiguity the sites of phosphorylation and the nature of methylation of HMGA1 proteins that were isolated from the PC-3 human prostate cancer cells. We showed by LC-MS/MS that Ser101 and Ser102 were completely phosphorylated in HMGA1a protein, whereas only a portion of the protein was phosphorylated at Ser98. We also found that the HMGA1b protein was phosphorylated at the corresponding sites, that is, Ser90, Ser91 and Ser87. In addition, Arg25, which is within the first DNA-binding AT-hook domain of HMGA1a, was both mono- and dimethylated. Moreover, both symmetric and asymmetric dimethylations were observed. The closely related HMGA1b protein, however, was not methylated. The unambiguous identification of the sites of phosphorylation and the nature of methylation facilitates the future examination of the biological implications of the HMGA1 proteins.

Identification of novel in vivo phosphorylation sites in high mobility group N1 protein from the MCF-7 human breast cancer cells

High mobility group N1 (HMGN1) protein is a member of nonhistone chromosomal proteins that binds more strongly with nucleosomes than with DNA. Here we report the identification of the sites of in vivo phosphorylation of HMGN1 isolated from the MCF-7 human breast cancer cells. Our results showed that four serine residues, i.e., Ser6, Ser85, Ser88, and Ser98, can be phosphorylated in this protein. To our knowledge, this is the first demonstration that each of the three serine residues in the acidic C-terminal region of human HMGN1 can be phosphorylated. The additional negative charge resulting from the phosphorylation of the C-terminal serine residues is expected to modulate the interaction between HMGN1 and other proteins, which may enhance transcription and facilitate other cellular functions. In addition, the phosphorylation of HMGN1 at Ser85, which precedes Pro86, might play an important role in cellular signaling.

Phosphorylation and subcellular redistribution of high mobility group proteins 14 and 17, analyzed by mass spectrometry

High mobility group (HMG) proteins 14 and 17 are nonhistone nuclear proteins that have been implicated in control of transcription and chromatin structure. To examine the posttranslational modifications of HMG-14 and -17 in vivo, HMG proteins were prepared from nuclear vs. cytosolic fractions of human K562 cells treated with 12-O-tetradecanoylphorbol 13-acetate (TPA) or okadaic acid (OA) and examined by electrospray mass spectrometry. Analysis of full-length masses demonstrated mono-, di-, and triphosphorylation of HMG-14 and mono- and diphosphorylation of HMG-17 from OA treated cells, whereas HMG-14 and -17 from TPA treated cells were monophosphorylated. Peptide mass and sequence analysis showed major and minor phosphorylation sites, respectively, at Ser24 and Ser28 in HMG-17, and Ser20 and Ser24 in HMG-14. These sites were found in the consensus sequence RRSARLSAK, within the nucleosomal binding domain of each protein. A third phosphorylation site in HMG-14 was located at either Ser6 or Ser7. Interestingly, the proportion of HMG-14 and -17 found in cytosolic pools increased significantly after 1 h of treatment compared to control cells and showed preferential phosphorylation compared with proteins from nuclear fractions. These results suggest that phosphorylation of HMG-14 and -7 interferes with nuclear localization mechanisms in a manner favoring release from nuclei.

Protein kinases phosphorylating acidic ribosomal proteins from yeast cells

Phosphorylation of ribosomal acidic proteins of Saccharomyces cerevisiae is an important mechanism regulating a number of active ribosomes. The key role in the regulatory mechanism is played by specific phosphoprotein kinases and phosphoprotein phosphatases. Three different cAMP-independent protein kinases phosphorylating acidic ribosomal proteins have been identified and characterized. The protein kinase 60S (PK60S), RAP kinase, and casein kinase type 2 (CK2). All three protein kinases phosphorylate serine residues which are localized in the C-terminal end of phosphoproteins. Synthetic peptides were used to determinate the amino acid sequence of phosphoacceptor site for PK60S. Peptide AAEESDDD derived from phosphoproteins YP1 beta/beta' and YP2 alpha turned out to be the best substrate for PK60S. A number of halogenated benzimidazoles and 2-azabenzimidazoles were tested as inhibitors of the three protein kinases. 4,5,6,7-Tetrabromo-2-azabenzimidazole inhibits phosphorylation only of these polypeptides phosphorylated by protein kinase 60S, namely YP1 beta/beta' and YP2 alpha, but not the other, YP1 alpha and YP2 beta phosphorylated by protein kinases RAP and CK2. RAP kinase has been found in an active form in the soluble fraction of S. cerevisiae. The enzyme uses ATP as a phosphate donor and is less sensitive to heparin than casein kinase 2. RAP kinase monophosphorylates the four acidic proteins. The ribosome-bound proteins are a better substrate for the enzyme. Multifunctional CK2 kinase phosphorylate all four acidic proteins. The kinase phosphorylates preferentially serine or threonine residues surrounded by cluster of acidic residues. The enzyme activity is stimulated in vitro by the presence of polylysine and inhibited by heparin.

Insulin-like growth factor-binding protein-5 (IGFBP-5) stimulates phosphorylation of the IGFBP-5 receptor

The finding that insulin-like growth factor (IGF)-binding protein-5 (IGFBP-5) binding to mouse osteoblasts was capable of being downregulated by IGFBP-5 suggested that the 420-kDa membrane protein, which interacted with IGFBP-5, may be a signaling receptor (Andress, D. L. J. Biol. Chem. 270: 28289-28296, 1995). In the current study, a carboxy-terminal IGFBP-5 peptide, IGFBP-5-(201-218), which was found to competitively inhibit 125I-IGFBP-5 binding and to specifically bind to osteoblast monolayers, was used to affinity-purify the 420-kDa membrane protein. Co-incubation of the affinity-purified membrane protein with [32P]ATP resulted in autophosphorylation at serine residues. Serine phosphorylation of the 420-kDa protein was enhanced by intact IGFBP-5, IGFBP-5-(1-169), and IGFBP-5-(201-218). When the IGFBP-5 receptor was incubated with dephosphorylated casein in the presence of [32P]ATP, casein became phosphorylated on serine residues. These data indicate that IGFBP-5 stimulates the phosphorylation of the IGFBP-5 receptor and suggest that serine/threonine kinase activation may be important in mediating some of the IGF-independent effects of IGFBP-5.

A 180-kDa protein kinase seems to be responsible for the phosphorylation of prothymosin alpha observed in proliferating cells

Prothymosin alpha (ProTalpha) is an acidic protein involved in cell proliferation. Its phosphorylation status is correlated with proliferative activity. Here we report the isolation and characterization of a ProTalpha-phosphorylating kinase (ProTalphaK) from mouse splenocytes that seems to be responsible for the in vivo phosphorylation of ProTalpha and that differs from other protein kinases reported to date. This enzyme, mainly located in the cytosol, has an molecular mass of 180 kDa and appears to be made up of two proteins of 64 and 60 kDa. Its activity was markedly enhanced by mitogenic activation of cells. The ProTalpha residues phosphorylated by the enzyme in vitro are a Thr at position 7 and another Thr at positions 12 or 13, both located within casein kinase 2 (CK-2) consensus motifs; these are the same residues as are phosphorylated in vivo. The new enzyme shows a number of clear structural and catalytic differences from CK-2. It phosphorylates histones H2B and H3, although with weaker activity than ProTalpha. An enzyme with the same characteristics was also found in other murine tissues and cell lines.

Acidic pentapeptide phosphorylated in vitro by calf thymus protein kinase NII binds to DNA in the presence of Mg2+ cations

The pentapeptide pyroGlu-Ala-Glu-Ser-Asn has been synthetized and phosphorylated in vitro at level of serine by protein kinase NII isolated from calf thymus chromatin. It is noteworthy that the calf thymus kinase NII shows a remarkable affinity for this peptide. The [32P]peptide is able to bind to several DNAs in the presence of Mg2+ (lambda phage, calf thymus, pBR540 plasmid). This binding appears not specific with regard to the type of DNA and its base sequence. These data support the hypothesis that phosphorylated acidic domains of nuclear nonhistone proteins could bind directly to DNA in the presence of Mg2+ cations.

Role of polyamine in the regulation of RNA synthesis in uterine nucleoli

Administration of estradiol (E2) to ovariectomized mature rats has been shown to result in synthesis of uterine polyamines in the same temporal manner as E2 regulation of nucleolar transcription. Data is presented on the in vivo and in vitro effects of polyamines on uterine nucleolar RNA synthesis. Transcervical intrauterine administration of putrescine (100 micrograms), spermidine (100 micrograms), or spermine (100 micrograms) resulted in an increased transcriptional activity of 93 and 82% in uterine nucleoli isolated from putrescine and spermidine treated animals, respectively. Spermine administration was without effect on uterine nucleolar transcription. The polyamine-induced increase in transcription was totally accounted for by an increased rate of elongation of previously initiated RNA chains. No effect on the number of nucleolar RNA chains in the act of synthesis was observed. Preincubation of uterine nucleoli, isolated from control animals (no E2) with putrescine, spermidine, or spermine in the presence, but not in the absence of ATP, resulted in 44, 83 and 31% increased nucleolar RNA synthesis, respectively. In vitro polyamine-induced nucleolar RNA synthesis was correlated with a polyamine activated phosphorylation of nucleolar proteins of 110,000 24,000, 18,000 and 14,000 Da. Results suggest that early E2 action may result in activation of the polyamine pathway which modulates nucleolar protein kinase activity; initiating an increase in nucleolar transcription.

Mouse ornithine decarboxylase is phosphorylated by casein kinase-II at a predominant single location (serine 303)

Ornithine decarboxylase (ODC), a key enzyme in the biosynthetic pathway of polyamines in mammalian cells is characterized by an extremely short half-life and by a rapid induction following stimulation with growth-promoting agents. Inspection of its deduced amino acid sequence revealed the presence of sequences that may serve as targets for phosphorylation by casein kinase II (CK-II). In the present study we demonstrate that ODC serves as a substrate for phosphorylation by CK-II in vitro and that it is phosphorylated in intact mammalian cells. One-dimensional phosphopeptide analysis demonstrated that all the phosphopeptides generated by V8 protease digestion of in vivo phosphorylated ODC correspond to the major phosphopeptides of ODC phosphorylated in vitro by CK-II. Phosphopeptide analysis of wild-type ODC and of a mutant in which serine 303 was converted to alanine demonstrated that the latter lacks the phosphopeptides that correspond to those detected in ODC phosphorylated in vivo. In addition, no incorporation of phosphate into the alanine 303 mutant was observed when it was expressed in transfected cos cells. Based on these observations, we conclude that in mammalian cells serine 303 is the major (if not the only) phosphorylated residue of ODC and that CK-II or another cellular kinase with very similar sequence specificity is responsible for manifestation of this modification. The unphosphorylated alanine 303 mutant retained enzymatic activity, which decayed at a similar rate to that of the wild-type enzyme. We therefore conclude that phosphorylation is not essential for maintaining enzymatic activity or regulating ODC turnover.

Isolation and partial characterization of a protein kinase NII from wheat germ chromatin

A protein kinase, type NII, has been purified from wheat germ chromatin. The enzyme, which uses both ATP and GTP as phosphoryl donors, catalyzes the phosphorylation of casein, phosvitin and E. coli RNA polymerase, but not of histone proteins. Polypeptide bands at 46 kDa, 37 kDa and 25 kDa were estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Autophosphorylation of the 25 kDa subunit was observed following incubation of the purified kinase with (gamma-32P)ATP and (gamma-32P)GTP.

Antagonistic effect of the antiestrogen 4-hydroxytamoxifen on estradiol-stimulated acetylation of nuclear high mobility group (HMG) proteins in the uterus of newborn guinea-pigs

The effect of the antiestrogen 4-hydroxytamoxifen on the estradiol-stimulated acetylation of nuclear high mobility group (HMG) proteins was studied in the uterus of newborn (3-day-old) guinea-pig. 4-Hydroxytamoxifen (10(-6) M) selectively inhibits the stimulatory effect of estradiol (5 x 10(-8) M) on the acetylation of HMG-14 proteins 30 min after incubation with uterine tissue slices. No effect of 4-hydroxytamoxifen was observed on HMG-1 + HMG-2 or HMG-17 proteins. The data suggest that the blockage of HMG-14 acetylation is an early event in gene expression which is in relation to the antagonistic effect of the antiestrogen.

Phosphorylation sites for type N II protein kinase in DNA-topoisomerase I from calf thymus

1. Calf thymus DNA-topoisomerase I has been isolated, in an improved preparation, nearly to SDS-PAGE homogeneity, as a single major protein (100 kDa). 2. In vitro labeling experiments, which employed the purified enzyme [gamma-32P]ATP and N II protein kinase, also showed that the calf thymus topoisomerase I became phosphorylated. 3. Phosphorylation was accompanied by an increase in topoisomerase I activity. 4. Phosphoaminoacid analysis indicated that only serine residues became phosphorylated. 5. Tryptic peptides mapping, by HV electrophoresis, identified five major [32P]peptides. This number is higher than that reported for topoisomerase I from Novikoff hepatoma cells. 6. Separation of each spot, by reverse phase HPLC, resulted in their elution at fractions 1, 2, 3, 4 and 5 with 9, 11, 16, 27 and 28% acetonitrile, respectively. 7. Isolated phosphopeptides will be subjected to sequencing, to DNA-binding and transcription regulation tests; then, it will be speculated whether type N II protein kinase may contribute to the physiological regulation of DNA topoisomerase I activity from calf thymus, as well.

Identification of a novel casein kinase activity in HeLa cell nuclei

Three casein kinase activities have been resolved by column chromatography of HeLa cell nuclear extracts. In addition to casein kinases NI and NII, which have been described in other cell types, HeLa nuclei contain a third casein kinase activity which we have named NIII. NIII is a cyclic nucleotide-independent casein kinase which uses either Mg2+ or Mn2+ as a divalent cation, but is inhibited by increasing NaCl concentrations in the presence of Mg2+ and has optimal activity at 50 mM NaCl in the presence of Mn2+. In Mg2+, NIII uses only ATP as a phosphate donor, but in Mn2+ NIII transfers phosphate from either ATP or GTP. NIII phosphorylates the serine and threonine residues of casein, but does not phosphorylate phosvitin or calf thymus histones.

Casein kinase II from Rana temporaria oocytes. Intracellular localization and activity during progesterone-induced maturation

A homogeneous preparation of casein kinase II has been isolated from the ribosome-free extracts of Rana temporaria oocytes by means of chromatography on heparin-Sepharose, phosphocellulose and mono Q. The enzyme consists of three subunits with molecular mass of 43 kDa, 41 kDa and 29 kDa. The protein kinase was labelled with radioactive iodine and injected back into oocytes. As shown by histoautoradiography the enzyme forms a diffuse ring around the nucleus in the oocyte cytoplasm. A part of casein kinase II is found in informosomes. During oocytes maturation casein kinase II activity increases 7 h after progesterone administration and at the final stages of maturation (20-23 h). Cycloheximide blocks the second augmentation of kinase activity and does not influence the first one.

A one-step preparative method for separating SER 6-phosphorylated HMG 14 from unphosphorylated HMG 14 and in vitro phosphorylation reaction components

While clear evidence exists for the regulation of the phosphorylation of the very basic high mobility group (HMG) and histone chromatin proteins, the physiological role of their phosphorylation remains poorly understood. Elucidation of these roles has been difficult, in part, because of the inability to obtain sufficient quantities of purified phosphorylated derivatives. We have used Mono S cation-exchange chromatography to prepare milligram quantities of pure Ser 6-phosphorylated HMG 14 (Ser 6-PO4-HMG) from unphosphorylated Mono S-purified calf thymus HMG 14 following in vitro phosphorylation with cAMP-dependent protein kinase (A-kinase). In one step, this technique separates the phosphorylated derivative from A-kinase, ATP, unphosphorylated HMG 14, and a minor phosphorylated by-product which evidence suggests may be the previously reported Ser 6, 24-diphospho-HMG 14. Mono S chromatography also enhances the purity of calf thymus HMG 14 prepared by perchloric acid extraction, acetone and ethanol precipitations, and CM-Sephadex chromatography. In addition, it permits the detection of apparent microheterogenous forms of both unphosphorylated and Ser 6-PO4-HMG 14. The significant reductions in binding affinity resulting from the incorporation of phosphate groups into HMG 14 suggest that Mono S chromatography could have more general application in the isolation of phosphorylated derivatives of other basic proteins, including other chromatin-associated DNA-binding proteins which are known to undergo specific phosphorylation. It would especially be useful when the proteins and their phosphorylated derivatives bind more tightly to Mono S than the kinases used for their phosphorylation.

Binding of high-mobility-group proteins HMG 14 and HMG 17 to DNA and histone H1 as influenced by phosphorylation

We have used affinity chromatography to study the effects of phosphorylation of calf thymus high-mobility-group proteins HMG 14 and HMG 17 on their binding properties towards calf thymus single- and double-stranded DNA and histone H1. Without in vitro phosphorylation, HMG 14 and HMG 17 eluted from double-stranded DNA-columns at 200 mM NaCl. HMG 14 was released from single-stranded DNA-column at 300 mM NaCl and from H1-column at 130 mM NaCl, whereas the corresponding values for HMG 17 were 230 mM and 20 mM, respectively. Phosphorylation of HMG 14 and HMG 17 by cAMP-dependent protein kinase (A-kinase) decreased markedly their affinity (270 mM and 200 mM NaCl, respectively) for single-stranded DNA, whereas HMG 14 phosphorylated by nuclear protein kinase II (NII-kinase) eluted only slightly (290 mM NaCl) ahead of the unphosphorylated protein. HMG 14 phosphorylated by both A-kinase and NII-kinase eluted from double-stranded DNA-columns almost identically (190 mM NaCl) with the unphosphorylated protein. Interestingly, phosphorylation of HMG 14 by NII-kinase increased considerably its affinity for histone H1 and the phosphorylated protein eluted at 200 mM NaCl. Phosphorylation of HMG 14 by A-kinase did not alter its interaction towards histone H1. These results indicate that modification of HMG 14 by phosphorylation at specific sites may have profound effects on its binding properties towards DNA and histone H1, and that HMG 17 has much weaker affinity for single-stranded DNA and histone H1 than HMG 14.

Developmental changes in rat hepatic casein kinases 1 and 2

Cytosolic histone kinase and casein kinase activities varied considerably in the late fetal and postnatal periods of liver development. Both activities showed a maximum at day 21 of gestation and decreased at birth to values close to those of adult rats. The changes in total casein kinase activity were due to variations of casein kinase 1 and casein kinase 2. Similarly the activities of both the cyclic-AMP-dependent protein (histone) kinase and the cyclic-AMP-independent histone kinase varied during development. Besides the changes in total activity, the affinity of casein kinases 1 and 2 for casein also varied in fetal and postnatal development. The Km values of casein kinase 2 increased from day 18, reached a maximum at day 20 of gestation and then started to decrease until one day after birth. In contrast the Km values of casein kinase 1 decreased from day 18, reached its lowest value at day 21 of gestation and attained values similar to those in the adult at the day of birth. Changes in this parameter were also observed when insulin (3 IU/kg) was administered by intraperitoneal injection to one-day-old rats. The Km values of casein kinase 1 decreased while those of casein kinase 2 increased after administration of this hormone. On the other hand, the Km values for ATP of casein kinases 1 and 2 as well as their apparent molecular masses and sensitivity to heparin and GTP did not significantly change during ontogeny of rat liver.

Phosphorylation of high-mobility-group chromatin proteins by protein kinase C from rat brain

Chromosomal high-mobility-group (HMG) proteins have been examined as substrates for calcium/phospholipid-dependent protein kinase C. Protein kinase C from rat brain phosphorylated efficiently both HMG 14 and HMG 17 derived from calf thymus and the reactions were calcium/phospholipid-dependent. About 1 mol of 32P was incorporated per mol of HMG 14 and HMG 17. Phosphopeptide mapping suggested that the same major site was phosphorylated in both proteins at serine. The apparent Km values for HMG 14 and HMG 17 were about 5 microM. HMG 14, HMG 17 and the five histone H1 subtypes prepared from rat thymus, liver and spleen were phosphorylated by the kinase. HMG 14 and HMG 17 from transformed human lymphoblasts (Wi-L2) were also phosphorylated in a calcium/phospholipid-dependent manner. HMG 1 and HMG 2 from the tissues examined were found to be poor substrates for the kinase.

Purification of substrate proteins of casein kinases from the cytosol fraction of AH-66 hepatoma cells

We have attempted to purify endogenous substrate proteins for casein kinases I and II from the cytosol of AH-66 hepatoma cells. Utilizing the fact that only a few substrates are concentrated in the fraction eluted from DEAE-cellulose between 0.3 and 0.6 M NaCl, two substrates were purified from this fraction by DEAE-cellulose chromatography, hydroxyapatite chromatography, and HPLC on a DEAE-5PW column. The purified substrate proteins had molecular masses of 30.5 kDa and 31 kDa. The 31-kDa protein substrate was markedly phosphorylated by casein kinase II, but only slightly by casein kinase I. The radioactive phosphate incorporated into 31-kDa substrate by casein kinase II was 0.2 mol/mol of the protein and phosphorylation occurred on both threonine and serine residues. The 30.5 kDa protein was only slightly phosphorylated by casein kinase II, but not at all by casein kinase I.

Phosphorylation of high-mobility-group protein 14 by two specific kinases modifies its interaction with histone oligomers in free solution

Chromosomal protein HMG14 can be specifically phosphorylated by the cyclic AMP-dependent protein kinase at the N-terminus and by casein kinase 2 at the acidic C-terminus. Under the same conditions used for HMG14, HMG17 is not significantly phosphorylated by either of the two kinases. Further, we have studied the effect of phosphorylation by these kinases on the interaction of HMG14 with histone oligomers, using chemical cross-linking. Our results indicate that the phosphorylation of HMG14 by casein kinase 2 enhances its interaction with histone oligomers in free solution, whereas a minor effect was observed by phosphorylation with cyclic AMP-dependent protein kinase. In contrast, HMG17 does not interact at all with any histone oligomer in free solution under the conditions used. To gain insight into the possible effect that phosphorylation may play in vivo, the pattern of distribution among different chromatin fractions was analysed. It was found that, although phosphorylation of HMG14 by both kinases allowed reconstitution of HMG14 to chromatin, the patterns obtained showed some slight differences.

Polyamine regulation of protein phosphorylation in the brain of the tobacco hornworm, Manduca sexta

An analysis of the effects of polyamines on protein phosphorylation in cytosolic fractions of the pupal brain of Manduca sexta showed that spermine elicited an increase in casein phosphorylation in a dose-dependent manner (maximum three- to fourfold at 2.0 mM), whereas spermidine was less effective and putrescine was without effect. In contrast, with phosvitin as the exogenous substrate, higher doses of polyamines, especially spermine, inhibited phosphorylation. High salt conditions abolished the polyamine response. Cytosol protein kinase activity eluted from DEAE-cellulose at 0.2-0.3 M NaCl. This activity was enhanced in the presence of spermine, and inhibited in the presence of heparin (IC50 approximately equal to 30 ng/ml). The enzyme was characterized by a sedimentation coefficient of 6.5S, and a Stokes radius of 49 A, consistent with a Mr of 130,000. Both GTP (Km, 55 microM) and ATP (Km, 34 microM) were utilized as phosphoryl donors (Vmax for ATP being four-fold higher than that observed for GTP). These results indicate the presence in the insect brain of an enzyme very similar to vertebrate casein kinase II. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography demonstrated that low concentrations of spermine (100 microM) strongly enhanced the phosphorylation of three high-molecular-weight cytosolic proteins (305,000, 340,000, and 360,000) localized in the insect nervous system.

Catalytic and molecular properties of highly purified phosvitin/casein kinase type II from human epithelial cells in culture (HeLa) and relation to ecto protein kinase

Phosvitin/casein type II kinase was purified from HeLa cell extracts to homogeneity and characterized. The kinase prefers phosvitin over casein (Vmax phosvitin greater than Vmax casein; apparent Km 0.5 microM phosvitin and 3.3 microM casein) and utilizes as cosubstrate ATP (apparent Km 3-4 microM), GTP (apparent Km 4-5 microM) and other purine nucleoside triphosphates, including dATP and dGTP but not pyrimidine nucleoside triphosphates. Enzyme reaction is optimal at pH 6-8 and at 10-25 mM Mg2+.Mg2+ cannot be replaced by, but is antagonized by other divalent metal ions. The kinase is stimulated by polycations (spermine) and monovalent cations (Na+,K+), and is inhibited by fluoride, 2,3-diphosphoglycerate, and low levels of heparin (50% inhibition at 0.1 microgram/ml). The HeLa enzyme is composed of three subunits with Mr of approximately 43,000 (alpha), 38,000 (alpha'), and 28,000 (beta) forming alpha alpha'beta 2 and alpha'2 beta 2 structures with obvious sequence homology of alpha with alpha' but not with beta. Photoaffinity labeling with [alpha-32P]- and [gamma-32P]8-azido-ATP revealed high affinity binding sites on subunits alpha and alpha' but not on subunit beta. The kinase autophosphorylates subunit beta and, much weaker, subunits alpha and alpha'. Ecto protein kinase, detectable only by its enzyme activity but not yet as a protein (J. Biol. Chem. 257, 322-329), was characterized in cell-bound form and in released form, and the released form both with and without prior separation from phosvitin which was employed to induce the kinase release from intact HeLa cells (Proc. Natl. Acad. Sci. U.S.A. 80, 4021-4025). Ratios of phosvitin/casein phosphorylation (greater than 2) and of ATP/GTP utilization (1.5-2.1), inhibition by heparin (50% inhibition at 0.1 microgram/ml), and amino-acid side chains phosphorylated in phosvitin and casein (serine, threonine) are comparable for cell-bound and released form. These properties resemble those of type II kinase as does Mr of released ecto kinase (120-150,000). Consistently, a protein with Mr 125,000 in calf serum and a protein (possibly two) with Mr greater than 300,000 in calf plasma which are selectively phosphorylated by the ecto kinase are also substrates of the type II kinase. Thus, nearly all properties examined of the ecto kinase are characteristic for a type II kinase.

Site specificity of casein kinase-2 (TS) from rat liver cytosol. A study with model peptide substrates

The factors determining the site recognition and phosphorylation by rat liver casein kinase-2 (CK-2) have been explored with a set of 14 related hexapeptides each including a single phosphorylatable amino acid and five acidic plus neutral residues. Such peptides are different from each other in the following features: the nature of the phosphorylatable amino acid, if any; its position relative to the critically required acidic residues; the extension and the structure of the acidic cluster. All of them were tested as substrate and/or competitive inhibitors of CK-2, and their kinetic and inhibition constants were determined. The results suggest the following conclusions. Under strictly comparable conditions Ser is by far preferred over Thr. Tyr not being affected at all. In order to carry out its role of structural determinant the critical acidic cluster must be located on the C-terminal side of the target residue, though not necessarily adjacent to it. The affinity for the protein-binding site, as deduced from Km and/or Ki values, is largely dependent on the number of acidic residues but it is also significantly enhanced if a hydroxylic residue is located on their N-terminal side. An acidic residue at position +3 relative to serine plays an especially important role for triggering phosphorylation, the peptide Ser-Glu-Glu-Ala-Glu-Glu having similar Km but negligible Vmax compared to Ser-Glu-Ala-Glu-Glu-Glu and Ser-Glu-Glu-Glu-Ala-Glu. These data provide a rationale for the substrate specificity of CK-2 and will give a helpful insight into the structure of the protein-binding site of this enzyme.

Phosphorylation of a low Mr high mobility group protein in Chinese hamster ovary cells

Phosphorylation of high mobility group (HMG) chromatin proteins was studied both in intact Chinese hamster ovary cells (strain CHO-P22) and in vitro conditions using isolated HMG proteins from the same cells and purified protein kinases. Prominent phosphorylation of serine in a low Mr HMG protein designated as HMG P was observed in unsynchronized cells. Of the three protein kinases tested, only nuclear type II protein kinase phosphorylated HMG P in vitro. The phosphorylated amino acid was phosphoserine. Cyclic nucleotide dependent protein kinases did not phosphorylate HMG P but phosphorylated HMG 14 with a preference for cGMP-dependent protein kinase. 32P-labeling of HMG 17 was not observed in intact cells or in vitro.

Mechanisms and significance of polyamine stimulation of various protein kinase reactions

An overview of the work on polyamine effects on certain protein kinase reactions is presented. In general, the reactions catalyzed by the messenger-independent protein kinases but not by cyclic nucleotide-, Ca2+-, Ca2+-calmodulin-, and Ca2+-anionic lipid-dependent protein kinases, are markedly enhanced by polyamines. The extent of this stimulation depends critically on the nature of the protein substrate and several other factors. A variety of other polycationic compounds including Co3+(NH3)6, polybrene, and certain aminoglycoside antibiotics exert polyamine-like effects in the same reactions. These observations suggest that the charge properties rather than any strict chemical structure play a role in the action of polyamines. Available data do not support a specific "cofactor" function of these amines for the protein kinases involved in the polyamine-stimulable reactions. It appears that the action of polyamines is mediated via their influence on the conformational status of the protein substrates thereby altering the availability of the phosphorylatable sites to the active sites on the protein kinases. Although this notion is supported by several lines of evidence, at present a role of the influence of polyamines on both the substrate and enzyme cannot be ruled out. Possible physiological relevance of the polyamine-stimulable protein kinase reactions observed in the in vitro experiments remains problematic in the absence of precise knowledge on the "effective" or free concentrations of intracellular polyamines.

Phosphorylation alters the affinity of high mobility group protein HMG 14 for single-stranded DNA

The effect of phosphorylation on the affinity of HMG 14 from calf thymus for single-stranded DNA (ssDNA) was studied, using a cyclic GMP-dependent protein kinase from bovine lung and a nuclear protein kinase II from rat liver. When phosphorylated by G-kinase, HMG 14 eluted at 0.27 M NaCl from the ssDNA-column, whereas the native protein eluted at 0.30 M salt concentration. In contrast, phosphorylation by nuclear protein kinase II did not alter dissociation of HMG 14 from ssDNA and the phosphoprotein consequently coeluted with the native HMG 14. Thus, addition of a negative charge by phosphorylation of the Ser-6 residue by G-kinase presumably weakens the interaction between the DNA-binding amino acids of HMG 14 and the negatively charged phosphate groups of DNA.