Purification and characterization of two cyclic AMP-independent casein/glycogen synthase kinases from rat liver cytosol

Characterization of insulin-like growth factor binding protein-1 kinases from human hepatoma cells

The phosphorylation of insulin-like growth factor binding protein-I (IGFBP-1) alters its binding affinity for insulin-like growth factor I (IGF-I) and thus regulates the bioavailability of IGF-I for binding to the IGF-I receptor. The kinase(s) responsible for the phosphorylation of IGFBP-1 has not been identified. This study was designed to characterize the IGFBP-1 kinase activity in HepG2 human hepatoma cells, a cell line that secretes IGFBP-1 primarily as phosphorylated isoforms. IGFBP-1 kinase activity was partially purified from detergent extracts of the cells by phosphocellulose chromatography and gel filtration. Two kinases of approximate M(r) 150,000 (peak I kinase) and M(r) 50,000 (peak II kinase) were identified. Each kinase phosphorylated IGFBP-1 at serine residues that were phosphorylated by intact HepG2 cells. The kinases were distinct based on their differential sensitivity to inhibition by heparin (IC50 = 2.5 and 16.5 micrograms/ml, peak I and II kinase, respectively) and inhibition by the isoquinoline sulfonamide CKI-7 (IC50 = 50 microM and 100 microM, peak I and II kinase, respectively). In addition, a tenfold molar excess of nonradioactive GTP relative to [gamma-32P]ATP lowered the incorporation of 32P into IGFBP-1 by 80% when the reaction was catalyzed by the peak I kinase, whereas GTP had no effect on the reaction catalyzed by the peak II kinase. In the presence of polylysine, IGFBP-1 was radiolabeled by the partially purified kinase activity when [gamma-32P]GTP served as the phosphate donor indicating the presence of casein kinase II activity. Furthermore, IGFBP-1 was phosphorylated by purified casein kinase I and casein kinase II at sites phosphorylated by the peak I and II kinases. Our data suggest that at least two kinases could be responsible for the phosphorylation of IGFBP-1 in intact HepG2 cells and that the kinases are related to the casein kinase family of protein kinases.

Phosphorylation of rat liver heterogeneous nuclear ribonucleoproteins A2 and C can be modulated by calmodulin

It was previously reported that the phosphorylation of three proteins of 36, 40 to 42, and 50 kDa by casein kinase 2 is inhibited by calmodulin in nuclear extracts from rat liver cells (R. Bosser, R. Aligué, D. Guerini, N. Agell, E. Carafoli, and O. Bachs, J. Biol. Chem. 268:15477-15483, 1993). By immunoblotting, peptide mapping, and endogenous phosphorylation experiments, the 36- and 40- to 42-kDa proteins have been identified as the A2 and C proteins, respectively, of the heterogeneous nuclear ribonucleoprotein particles. To better understand the mechanism by which calmodulin inhibits the phosphorylation of these proteins, they were purified by using single-stranded DNA chromatography, and the effect of calmodulin on their phosphorylation by casein kinase 2 was analyzed. Results revealed that whereas calmodulin inhibited the phosphorylation of purified A2 and C proteins in a Ca(2+)-dependent manner, it did not affect the casein kinase 2 phosphorylation of a different protein substrate, i.e., beta-casein. These results indicate that the effect of calmodulin was not on casein kinase 2 activity but on specific protein substrates. The finding that the A2 and C proteins can bind to a calmodulin-Sepharose column in a Ca(2+)-dependent manner suggests that this association could prevent the phosphorylation of the proteins by casein kinase 2. Immunoelectron microscopy studies have revealed that such interactions could also occur in vivo, since calmodulin and A2 and C proteins colocalize on the ribonucleoprotein particles in rat liver cell nuclei.

Casein kinases: pleiotropic mediators of cellular regulation

The present review on casein kinases focuses mainly on the possible metabolic role of CK-2, with special emphasis on its behavior in pathological tissues. From these data at least three ways to regulate CK-2 activity emerge: (i) CK-2 activity changes during embryogenesis, being high at certain stages of development and showing basal activity values at others; (ii) CK-2 activity can be enhanced in vitro by treatment of tissue culture cells with various growth factors and serum and (iii) CK-2 activity is constitutively enhanced in rapidly proliferating cells. The regulated CK-2 activity changes during embryogenesis cannot be explained as yet. In the case of the constitutive high expression of CK-2 in tumors, genetic changes may be responsible, e.g. through alterations of the regulatory genetic elements and/or regulation by specific transcription factors. In the case of serum induction, no genetic changes are necessarily involved; the observed changes may be entirely due to a signal transduction pathway where CK-2 could be phosphorylated by another kinase(s). CK-2 cDNAs from various organisms have been isolated and characterized. From the deduced amino acid sequence it turns out that CK-2 subunits are highly conserved during evolution. The relationship between CK-2 alpha from humans and plants is still 73%. Similar relationships are reported for the beta-subunit. Chromosomal assignment of CK-2 alpha shows two gene loci, one of which is a pseudogene. They are located on different chromosomes. Expression of the CK-2 subunits in Escherichia coli and the Baculo expression system is shown. The recombinant subunits can self-assemble to a functional holoenzyme in vitro. Biochemical and biophysical analysis of the recombinant beta-subunit suggests it to be trifunctional in association with the alpha-subunit affecting: (i) stability, (ii) enzyme specificity and (iii) enzyme activity. The question where CK-2 and its subunits are located throughout the cell cycle has also been addressed, mainly because of the large discrepancies that still exist between results obtained by different investigators. Tissue-specific expression of CK-2 at the mRNA and at the protein level has also been given attention. The fact that the enzyme activity is surprisingly high in brain and low in heart and lung may be indicative of involvement of CK-2 in processes other than proliferation.(ABSTRACT TRUNCATED AT 400 WORDS)

Heterogeneity of rat liver cytosol casein kinase 2. Association between the alpha/alpha' -subunits of casein kinase 2 and the phosphorylatable protein pp49

Casein kinase 2 activity could be resolved into three peaks by chromatography on DEAE-Sepharose. The peak eluted at high salt concentrations (casein kinase 2b) showed molecular and kinetic properties typical of the heterotetramer composed of alpha-(or alpha'-) and beta-subunits. In contrast, the peak that was eluted at low salt concentrations (casein kinase 2a) contained no beta-subunit but a phosphorylatable protein of 49 kDa (pp49), in addition to the alpha/alpha'-subunits. The presence of alpha/alpha'/alpha"-subunits in preparations of casein kinases 2a and 2b was confirmed by immunological assays. Casein kinase 2a had low specific activity and a very high apparent Km for beta-casein. The peak eluted at intermediate ionic strength contained the alpha/alpha'-subunits and variable amounts of beta-subunit and pp49, and had kinetic properties intermediate between those of casein kinases 2a and 2b. Experiments based on heat inactivation, inhibition by low concentrations of heparin and ability to use GTP as substrate suggested that phosphorylation of pp49 was catalysed by the alpha/alpha'-subunits of casein kinase 2. No similarities were observed in the phosphopeptide maps of pp49 and beta-subunit. These results show that the alpha/alpha'-subunits of rat liver cytosol casein kinase 2 can form complexes not only with the beta-subunit but also with pp49, and that the complexes containing pp49 have a reduced affinity for the exogenous protein substrate beta-casein.

Characterization of neurofilament-associated protein kinase activities from bovine spinal cord

1. A neurofilament-enriched preparation from bovine spinal cord contains endogenous protein kinases that phosphorylate high, middle, and low molecular weight neurofilament subunits (NF-H, NF-M, and NF-L), as well as certain other endogenous and exogenous substrates. 2. Most of this associated kinase activity can be separated from the neurofilament subunits and the bulk of the protein by extraction of the neurofilament preparation with 0.8 M KCl. Assays using specific exogenous substrates, activators, and inhibitors for known kinases reveal significant levels of Ca2(+)-calmodulin-dependent, cyclic nucleotide-dependent, Ca2(+)-phosphatidylserine diglyceride-dependent, and regulator-independent kinase activities in the high-salt extract. 3. Fractionation of the salt extract on a gel filtration column resolves a regulator-independent kinase activity identified by its ability to phosphorylate purified NF-M. This preparation can phosphorylate all three neurofilament proteins either in purified form or in the assembled form, as well as alpha-casein. Only the regulator-independent kinase activity in this fraction is responsible for the phosphorylation of neurofilament proteins. 4. While this partially purified kinase activity does not show a strong substrate specificity between the three neurofilament subunits, the phosphorylation pattern it produces upon incubation with salt-extracted neurofilaments is similar to the regulator-independent phosphorylation pattern found in the original neurofilament preparation and, thus, represents a useful starting point for the further purification of this neurofilament-associated kinase activity.

Effects of vanadate on protein kinases in rat hepatocytes

In rat hepatocytes, vanadate modifies neither the intracellular concentration of cyclic AMP nor the --cyclic AMP/+cyclic AMP activity ratio for cyclic AMP-dependent protein kinase. Vanadate can, however, counteract the increase in cyclic AMP and the increase in the --cyclic AMP/+cyclic AMP activity ratio of cyclic AMP-dependent protein kinase induced by glucagon. On the other hand, vanadate treatment of hepatocytes can produce a time- and concentration-dependent increase in cyclic AMP- and Ca2+-independent casein kinase activity. Maximal activation at the optimal time with 5 mM-vanadate was about 70% over control. A clear relationship was observed between the activation of casein kinase and the inactivation of glycogen synthase after vanadate treatment. These results suggest that casein kinase activity may be involved in vanadate actions in rat hepatocytes.

Polyamines stimulate the activity of glycogen synthase (casein) kinase-1 from bovine kidney and different rat tissues

Previous studies have established that casein kinase-2 (CK-2) is stimulated by polyamines. In this study it is shown that glycogen synthase (casein) kinase-1 (CK-1) can be activated similarly. Using casein as the substrate, bovine kidney CK-1 was stimulated 7-, 2-, and 0.5-fold by spermine, spermidine, and putrescine, respectively. Half-maximal activation of CK-1 by these polyamines was observed at 0.25, 0.70, and 0.50 mM, respectively. CK-1 was optimally activated by spermine at low ionic strength and low Mg2+ concentrations (1-3 mM). Using phosvitin as the substrate, CK-1 was stimulated at low concentrations (0-0.8 mM) and inhibited at higher concentrations of spermine. By contrast CK-2 was inhibited at all concentrations of spermine when phosvitin was used as substrate. Using calcineurin (not a substrate for CK-2) as a substrate, CK-1 from bovine kidney or from three rat tissues (liver, kidney, and testis) was stimulated greater than 2-fold by spermine. It is further shown that heparin inhibits CK-1 and this inhibition can be reversed by spermine. The Vmax of CK-1 for both casein and ATP is increased by spermine while the Km remains unchanged by the polyamine. These studies indicate that CK-1, like CK-2, is a heparin-inhibited and polyamine-activated protein kinase. The results also suggest that CK-1 may be activated by spermine in vivo.

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.

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.

Role of protein kinase C in the regulation of rat liver glycogen synthase

Rat liver glycogen synthase was phosphorylated by purified protein kinase C in a Ca2+- and phospholipid-dependent fashion to 1-1.4 mol PO4/subunit. Analysis of the 32P-labeled tryptic peptides derived from the phosphorylated synthase by isoelectric focusing and two-dimensional peptide mapping revealed the presence of a major radioactive peptide. The sites in liver synthase phosphorylated by protein kinase C appears to be different from those phosphorylated by other kinases. Prior phosphorylation of the synthase by protein kinase C has no significant effect on the subsequent phosphorylation by glycogen synthase (casein) kinase-1 or kinase Fa, but prevents the synthase from further phosphorylation by cAMP-dependent protein kinase, Ca2+/calmodulin-dependent protein kinase, phosphorylase kinase, or casein kinase-2. Additive phosphorylation of liver glycogen synthase can be observed by the combination of protein kinase C with the former set of kinases but not with the latter. Phosphorylation of liver synthase by protein kinase C alone did not cause an inactivation nor did the combination of this kinase with glycogen synthase (casein) kinase-1 or kinase Fa produce a synergistic effect on the inactivation of the synthase. Based on these findings we conclude that the phorbol ester-induced inactivation of glycogen synthase previously observed in hepatocytes cannot be accounted for entirely by the activation of protein kinase C.

Phosphorylation of rabbit skeletal muscle glycogen synthase by casein kinase 1: evidence of phosphorylation sites specific for casein kinase 1

Casein kinase 1 phosphorylated rabbit skeletal muscle glycogen synthase at both seryl and threonyl residues. With glycogen synthase phosphorylated up to 7.5 mol phosphate/mol subunit, about 26% of the phosphate was present in the N-terminal cyanogen bromide fragment (CB1) and 74% in the C-terminal fragment (CB2). Both fragments contained phosphothreonine (11 to 14%) in addition to phosphoserine. When 32P-labeled glycogen synthase was totally digested with trypsin and chromatographed on reverse-phase high-performance liquid chromatography, seven phosphopeptides were observed. Peptide I eluted in the vicinity of the peptide containing site 1a, peptide II coincided with sites 4 + 5, peptides III and IV eluted in the region corresponding to sites 3a + 3b + 3c, peptide V appeared slightly after the peptide containing site 1b and peptide VII behaved as the peptide containing site 2, whereas peptide VI did not coincide with any of the known phosphopeptides. Limited trypsinization prior to analysis by HPLC led to the disappearance of peaks V and VI without altering peaks I to IV and VII. Only peaks I and VII remained when limited chymotrypsinization was performed prior to HPLC analysis. Chromatography on HPLC of the fragments derived from complete trypsinization of CB2 showed the presence of peaks II to VI. Phosphoamino acid analysis of the different peptides demonstrated the presence of quantitative amounts of phosphothreonine in peptides V, VI, and VII. These results indicate that multiple phosphorylation sites for casein kinase 1 must exist in both the N-terminal and C-terminal regions of glycogen synthase, some of which would only be labeled by casein kinase 1.

Effect of phosphorylation by different protein kinases on the behaviour of glycogen synthase as a substrate for hepatic synthase phosphatases

Glycogen synthase a from skeletal muscle was phosphorylated in vitro and then used as substrate for the two major synthase phosphatases from liver. Synthase phosphorylated by cAMP-dependent protein kinase (1.4-1.7 P/subunit) was preferentially activated by the cytosolic S-component; in contrast, progressive phosphorylation by casein kinase-1 (0.9-6.5 P/subunit) yielded substrates that were always better dephosphorylated and activated by the glycogen-bound G-component. We have previously isolated from dog liver several types of synthase b that differ by their need for the S- and/or G-component for prompt activation. After additional phosphorylation by a mixture of synthase kinases the activation of these enzyme preparations required the presence of both components.

Phosphorylation of isolated plasma membranes of AH-66 hepatoma ascites cells by casein kinase 1

The isolated plasma membranes of AH-66 hepatoma cells were phosphorylated by casein kinase 1 purified from the cytosol fraction of AH-66 cells. Casein kinase 2 purified from the same source had little effect on the phosphorylation of the plasma membranes. Two-dimensional gel electrophoresis and autoradiography showed that casein kinase 1 enhanced the phosphorylation of approx. 10 plasma membrane proteins that are phosphorylated only faintly in the isolated plasma membranes by endogenous protein kinase. Among these phosphoproteins, tubulin was identified as judged from their molecular weights and isoelectric points. These results suggest that one of the physiological functions of casein kinase 1 is phosphorylation of plasma membrane and plasma membrane-associated proteins.

Phosphorylation of fibrinogen by casein kinase 2

Casein kinase 2 from rat liver cytosol phosphorylated human fibrinogen in a reaction that was not stimulated by Ca2+ or cyclic AMP, but was markedly inhibited by heparin, and proceeded at a similar rate when either ATP or GTP was used as phosphate donor. Analysis of casein kinase 2 by glycerol-density-gradient centrifugation showed that the activities towards fibrinogen, casein, phosvitin, high-mobility-group protein 14 and glycogen synthase coincided. Maximal incorporation into fibrinogen by casein kinase 2 averaged 1 mol of phosphate/mol of protein substrate, most of it in the alpha-chain, although some phosphorylation of the beta-chain was also detected. Analysis of phosphorylated alpha-chain revealed that most of the phosphate was incorporated on serine. Phosphorylation of human fibrinogen was also performed by casein kinase 2 from human polymorphonuclear leucocytes, lymphocytes and platelets.

Binding of HMG14 non-histone protein to histones H2A, H2B, H1 and DNA in reconstituted chromatin

The interaction between calf thymus HMG14 and rat liver chromatin components has been studied via reconstitution and chemical cross-linking. Selective labeling of HMG14 with photoactivable reversible heterobifunctional reagents has allowed a clear identification of the histones interacting with it (histones H2A, H2B and H1). These results are not dependent on whether the chromatin samples used were bulk chromatin, mononucleosomes, or core particles (for H2A and H2B). In addition to histone proteins, DNA also seems to be involved in HMG14 attachment to nucleosome.

Glycogen synthase (casein) kinase-1: tissue distribution and subcellular localization

The distribution of glycogen synthase (casein) kinase-1 (CK-1) among different rat tissues and subcellular fractions was investigated. Using casein, glycogen synthase and phosphorylase kinase as substrates, CK-1 activity was detected in kidney, spleen, liver, testis, lung, brain, heart, skeletal muscle and adipose tissue. The distribution of CK-1 among different subcellular fractions of rat liver was; cytosol (72.1%), microsome (17.6%), mitochondria (9.6%) and nuclei (0.7%). CK-1 from rat tissues was shown to have a similarly wide substrate specificity as highly purified CK-1 from rabbit skeletal muscle. Such wide substrate specificity and distribution among different mammalian tissues and subcellular organelles indicate that CK-1 may be involved in the regulation of diverse cellular functions.

Threonine phosphorylation of rat liver glycogen synthase

32P-labeled glycogen synthase specifically immunoprecipitated from 32P-phosphate incubated rat hepatocytes contains, in addition to [32P] phosphoserine, significant levels of [32P] phosphothreonine (7% of the total [32P] phosphoaminoacids). When the 32P-immunoprecipitate was cleaved with CNBr, the [32P] phosphothreonine was recovered in the large CNBr fragment (CB-2, Mapp 28 Kd). Homogeneous rat liver glycogen synthase was phosphorylated by all the protein kinases able to phosphorylate CB-2 "in vitro" (casein kinases I and II, cAMP-dependent protein kinase and glycogen synthase kinase-3). After analysis of the immunoprecipitated enzyme for phosphoaminoacids, it was observed that only casein kinase II was able to phosphorylate on threonine and 32P-phosphate was only found in CB-2. These results demonstrate that rat liver glycogen synthase is phosphorylated at threonine site(s) contained in CB-2 and strongly indicate that casein kinase II may play a role in the "in vivo" phosphorylation of liver glycogen synthase. This is the first protein kinase reported to phosphorylate threonine residues in liver glycogen synthase.

Effect of bivalent cations on rat liver cytosol casein (glycogen synthase) kinases 1 and 2. Influence of the protein substrate

Rat liver cytosol casein kinases 1 and 2 were stimulated by free Mg2+, but the optimal concentration of cation varied with both the casein kinase and the protein substrate used. Mn2+, but neither Ca2+ nor Zn2+, could efficiently substitute for Mg2+ in forming the bivalent-cation-ATP complex used as substrate, but free Mn2+ was inhibitory. The magnitude of these effects depended on the type of casein kinase and the protein substrate used. These results support the idea that, besides the effects of Mg2+ as a component of the Mg-ATP complex, or through interaction with the protein substrate, free Mg2+ is an allosteric effector of both casein kinases.

Protein kinases in human leukemic cells

Protein kinase activities and cyclic AMP binding capacity were investigated in human peripheral blood cells from leukemic patients and normal controls. Using [gamma 32P] ATP as phosphoryldonor, the phosphorylating activities were not found to be significantly different in either normal or leukemic cells when measured on both artificial basic and acidic substrates. In contrast, the GTP-dependent casein kinase activity, CK2, which is almost undetectable in normal granulocytes, was markedly increased in highly proliferating myeloblastic cells from patients with acute myelogenous leukemia (AML) or with chronic myelogenous leukemia in blastic crisis (BC-CML). Levels of endogenous phosphotyrosine were not higher in leukemic cells than in normal peripheral lymphocytes or granulocytes. Finally, cAMP binding capacity was found to be increased in several types of proliferating leukemic cells, due to a higher amount of the R1-type regulatory subunit of the cAMP-dependent protein kinases. Specific patterns of cAMP binding proteins observed in the different types of normal blood cells were rather blurred in leukemic cells. In conclusion, modifications observed in human leukemic cells seem to be more related to proliferation or blockage in normal differentiation than to their cellular origin.

Some properties of a liver protein that activates glycogen synthase b

A soluble protein has been identified in rat liver that increases the activity of glycogen synthase without causing synthase b to a conversion. The effect of the activator is to increase synthase b activity in the presence of saturating amounts of UDP-Glc and Glc-6-P. The activator is heat sensitive and does not have protease activity. The effect of the activator is linearly proportional to the amount assayed to a saturable level and its effect is not mimicked by other proteins associated with the control of glycogen metabolism (e.g., phosphorylase).

Effect of starvation, diabetes and insulin on the casein kinase 2 from rat liver cytosol

Starvation, diabetes and insulin did not alter the concentration of casein kinases in rat liver cytosol. However, the Km for casein of casein kinase 2 from diabetic rats was about 2-fold lower than that from control animals. Administration of insulin to control rats did not alter this parameter, but increased the Km for casein of casein kinase 2 in diabetic rats. Starvation did not affect the kinetic constants of casein kinases. The effect of diabetes on casein kinase 2 persisted after partial purification of the enzyme by glycerol-density-gradient centrifugation and affected also its activity on other protein substrates such as phosvitin, high-mobility-group protein 14 and glycogen synthase. The results indicate that rat liver cytosol casein kinase 2 is under physiological control.

Cyclic nucleotide- and Ca2+-independent phosphorylation of tubulin and microtubule-associated protein-2 by glycogen synthase (casein) kinase-1

MAP-2 and tubulin are both shown to be substrates for glycogen synthase (casein) kinase-1 (CK-1). Greater than 40 mol 32P is incorporated into MAP-2 by CK-1 compared to only 14 mol 32P observed when cyclic AMP-dependent protein kinase (A-kinase) is the catalyst. Peptide mapping shows that CK-1 and A-kinase recognize a few common sites; the majority of the sites phosphorylated on MAP-2 by CK-1 are quite distinct. Up to 4 mol 32P can be incorporated into the tubulin dimer by CK-1 compared to only 0.9 mol 32P by A-kinase. The preferred substrate for both kinases is beta-tubulin.

Purification and characterization of a protein phosphatase from rat liver acting on key enzymes of glucose metabolism

A phosphoprotein phosphatase has been purified from rat liver cytosol. The purification involved chromatography on DEAE-cellulose. Sephacryl S-200, fast protein liquid chromatography (FPLC) and sucrose density gradient centrifugation. It resulted in an almost homogeneous enzyme with a relative molecular mass, Mr, of 90 000 by gel filtration and sucrose gradient centrifugation and Mr = 44 500 by sodium dodecyl sulfate/polyacrylamide gel electrophoresis (SDS-PAGE). Therefore it seems to be a dimeric enzyme. This protein phosphatase (termed PFK-phosphatase) is completely dependent on Mg2+, which can be replaced partly by Mn2+. It can be eluted from DEAE-cellulose with 120 mM NaCl, is not affected by Ca2+, 100 microM trifluoperazine or the heat-stable inhibitor-2. Inhibition occurs with phosphate, ammonium sulfate and fluoride. PFK-phosphatase dephosphorylates preferentially the alpha subunit of phosphorylase kinase (alpha/beta dephosphorylation ratio 5-10). Phosphorylase a, mixed histone and casein do not serve as substrates. The enzyme dephosphorylates effectively the key enzymes of glucose metabolism 6-phosphofructo-1-kinase, fructose 1,6-bisphosphatase, pyruvate kinase and 6-phosphofructo-2-kinase. Using this protein phosphatase and the catalytic subunit of cAMP-dependent protein kinase, a complete phosphorylation, dephosphorylation and rephosphorylation cycle was possible with 6-phosphofructo-1-kinase as substrate.

Phosphorylation of fibrinogen by casein kinase 1

Casein kinase 1 phosphorylated human fibrinogen, in a reaction that did not use GTP as phosphoryl donor and was neither stimulated by cyclic AMP or Ca2+, nor inhibited by the cyclic AMP-dependent protein kinase inhibitor protein. Maximal incorporation averaged 4 mol of phosphate per mol of fibrinogen, most of it in the largest CNBr-fragment of the alpha-chain. Phosphoamino acid analysis revealed that phosphorylation occurred only at seryl residues. The phosphorylation of fibrinogen by casein kinase 1 was reverted by alkaline phosphatase.

Protein kinase patterns in hyperplastic rat thyroids and in human non-toxic nodular goitres

Patterns of cAMP-dependent protein kinases and cAMP-independent histone and casein kinases of hyperplastic rat thyroid glands and of human nodular non-toxic goitres have been analysed in two subcellular compartments, cytosol and particulate fraction. In hyperplastic rat glands the different compartmentalization of the two cAMP-dependent isoenzymes was preserved and the pattern of cAMP-independent protein kinases was not changed qualitatively, but the activities of the three classes of protein kinases were enhanced to different degrees: the highest increase was observed for the cAMP-dependent enzymes and the lowest for the cAMP-independent casein kinases. Analysis of individual peaks of cAMP-dependent kinases showed selective stimulation of the cytosolic Type II form and independent control of the Type I activity in the two subcellular compartments. Among cAMP-independent protein kinases only two histone kinase peaks were selectively enhanced; other kinase entities were changed to a lesser degree. During the involution of hyperplastic glands, a transient but differential enhancement of nearly all kinases was noted at first, which was followed later by a strong decrease, more or less rapid, of almost all kinase activities. In the regressed glands, neither the thyroid weight nor the pattern of different kinase entities corresponded to those of control, untreated glands, indicating that some of the protein kinase alterations in hyperplastic tissues are partly irreversible. In spite of great similarities, human and rat goitre tissues are not identical. The most important difference was the loss of compartmentalization of the cAMP-dependent isoenzymes in human tissue. The different ratios of the light and heavy peaks of cytosolic cAMP-independent histone kinases was the second characteristic which distinguished human and rat glands.

Protein kinases in normal human blood cells

Protein kinases active on basic and acidic artificial substrates were investigated in normal human erythrocytes, platelets, polymorphonuclear and mononuclear cells. These two types of protein kinases were partially purified by affinity chromatography, then assayed for their enzymatic activity using [gamma-32P]ATP or GTP as phosphoryl donor. Partially purified kinases active on acidic substrates were subjected to high-performance liquid chromatography (HPLC). Protein kinases active on basic substrates were analyzed by cellulose acetate electrophoresis of crude cellular extracts and the influence of 3'5' cyclic AMP was studied. Three forms of casein-phosvitin kinases could be distinguished according to their molecular weight (165 K, 38 K, and 31 K). The 165 K species, in contrast to the light species, can use GTP instead of ATP as phosphoryl donor and corresponds to the "so-called" casein kinase 2. This form is very sensitive to proteolysis and, when partial purification is performed without the addition of various antiproteolytic agents, it is degraded into 120-135 K and 105-115 K active species; this artefactual degradative process is especially active in platelet extracts. As many as eight different active bands of histone and protamine kinases can be separated by cellulose acetate electrophoresis, several of them being stimulated by cyclic AMP. Isozymic patterns of protein kinases, levels of activity on the different substrates, and utilization of ATP and GTP were found to be specific for each cell type. These results suggest the possibility of using protein kinases as markers for cell differentiation.

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.

The protein phosphatases involved in cellular regulation. 2. Glycogen metabolism

The nature of protein phosphatases that are active against the phosphorylated proteins of glycogen metabolism was investigated in rabbit skeletal muscle and liver. Six 32P-labelled substrates corresponding to the major phosphorylation sites on glycogen phosphorylase, phosphorylase kinase, glycogen synthase and inhibitor-1 were used in these studies. The results showed that the four protein phosphatases defined in the preceding paper, namely protein phosphatases-1, 2A, 2B and 2C [Ingebritsen, T. S. and Cohen, P. (1983) Eur. J. Biochem. 132, 255-261] were the only significant enzymes acting on these substrates. The four enzymes can be conveniently separated and identified by a combination of ion-exchange chromatography and gel filtration and by the use of specific inhibitors. Three species of protein phosphatase-2A were resolved on DEAE-cellulose, termed protein phosphatases-2Ao (0.12 M NaCl), 2A1 (0.2 M NaCl) and 2A2 (0.28 M NaCl) that had apparent molecular weights of 210000, 210000 and 150000 respectively. Protein phosphatase-2Ao was a completely inactive enzyme whose activity was only expressed after dissociation to a 34000-Mr(app) catalytic subunit by freezing and thawing in 0.2 M 2-mercaptoethanol. This treatment also dissociated protein phosphatases 2A1 and 2A2 to more active 34000-Mr(app) catalytic subunits. The catalytic subunits derived from protein phosphatases-2Ao, 2A1 and 2A2 possessed identical substrate specificities, preferentially dephosphorylated the alpha-subunit of phosphorylase kinase, were unaffected by inhibitor-1 and inhibitor-2 and were inhibited by similar concentrations of ATP. The properties of protein phosphatases-2A1 and 2A2 were very similar to those of the catalytic subunits, except that they were less sensitive to inhibition by ATP. Protein phosphatase-2B was eluted from DEAE-cellulose in the same fraction as protein phosphatase-2Ao. These activities were resolved by gel filtration, the Mr(app) of protein phosphatase-2B being 98000. Protein phosphatase-2B was completely inhibited by 100 microM trifluoperazine, which did not affect the activity of protein phosphatase-2Ao or any other protein phosphatase. Freezing and thawing in 0.2 M 2-mercaptoethanol resulted in partial inactivation of protein phosphatase-2B. Protein phosphatase-2C was eluted from DEAE-cellulose at the leading edge of the peak of protein phosphatase-2A1. These activities were completely resolved by gel filtration, since the Mr(app) of protein phosphatase-2C was 46000. Two forms of protein phosphatase-1 can be identified by chromatography on DEAE-cellulose, namely protein phosphatase-1 itself and the Mg X ATP-dependent protein phosphatase. Both these species were eluted at 0.16 M NaCl just ahead of protein phosphatases-2C and 2A1. These enzymes did not interfere with measurements of type-2 protein phosphatases, since it was possible to block their activity with inhibitor-2...

Different effects of glucagon and epinephrine on the kinetic properties of liver glycogen synthase

Kinetic constants of glycogen synthase (M0.5 for glucose-6-P and S0.5 for UDP-glucose) were determined after hepatocytes isolated from starved rats were incubated with either glucagon or epinephrine. Incubation with these hormones resulted in an increase in both S0.5 and M0.5. However, the action of glucagon resulted in great modifications on S0.5 whereas epinephrine affected mainly M0.5. Therefore, glucagon and epinephrine alter the kinetic properties of glycogen synthase provoke the phosphorylation of glycogen synthase at different site(s) acting through different mechanisms.