The activity of CK2 in the extracts of COS-7 cells transfected with wild type and mutant subunits of protein kinase CK2

Protein kinase CK2 is ubiquitous in eukaryotes and is known to phosphorylate many protein substrates. The enzyme is normally a heterotetramer composed of catalytic ( and ) and regulatory () subunits. The physiological regulation of the enzyme is still unknown but one of the factors that may play an important role in this regulation is the ratio of the catalytic and regulatory subunits present in cells. The possible existence of free CK2 subunits, not forming part of the holoenzyme, may be relevant to the physiological function of the enzyme in substrate selection or in the interaction of the subunits with other partners. The objective of this work was to study in COS-7 cells the effects of transient expression of CK2 subunits and mutants of the catalytic subunit on the CK2 phosphorylating activity of the extracts of these cells. Using pCEFL vectors that introduce hemaggutinin (HA) or a heptapeptide (AU5) tags in the expressed proteins, COS-7 cells were transfected with and subunits of Xenopus CK2, with the subunit of D. rerio, and with Xl CK2A¹⁵⁶, which although inactive can bind tightly to CK2, and with Xl CK2E⁷⁵E⁷⁶, which is resistant to heparin and polyanion inhibition. The efficiency of transient transfection was of 10–20% of treated cells.Expression of CK2 or CK2E⁷⁵E⁷⁶ in COS-7 cells caused an increase of 5–7-fold of the CK2 activity in the soluble cell extracts. If these catalytic subunits were cotransfected with CK2, the activity increased further to 15–20-fold of the controls. Transfection of CK2 alone also increase the activity of the extracts about 2-fold. Transfection with the inactive CK2A¹⁵⁶ yielded extracts with CK2 activities not significantly different from those transfected with the empty vectors. However, cotransfection of CK2 or CK2E⁷⁵E⁷⁶ with CK2A¹⁵⁶ caused a 60–70% decrease in the CK2 activity as compared to those of cells transfected with only the active CK2 subunits. These results can be interpreted as meaning that CK2A¹⁵⁶ is a dominant negative mutant that can compete with the other catalytic subunits for the CK2 subunit. Addition of recombinant CK2 to the assay system of extracts of cells transfected with catalytic subunits causes a very significant increase in their CK2 activity, demonstrating that CK2 subunit is limiting in the extracts and that an excess of free CK2 has been produced in the transfected cells. Transfection of cells with CK2E⁷⁵E⁷⁶ results in a CK2 activity of extracts that is 90% resistant to heparin demonstrating that a very large proportion of the CK2 activity is derived from the expression of the exogenous mutant. In both the in vivo and in vitro systems, the sensitivity of CK2E⁷⁵E⁷⁶ to heparin increases considerably when it forms part of the holoenzyme CK2₂₂.     

Multiple forms of protein kinase CK2 present in leukemic cells: In vitro study of its origin by proteolysis

Human recombinant CK2 subunits were incubated for different times with the two main cytosolic proteases m-calpain and 20 S proteasome. Both, m-calpain in a calcium dependent manner and the 20 S proteasome, were able to degrade CK2 subunits in vitro. In both cases, CK2 was more resistant to these proteases than CK2. When these proteases were assayed on the reconstituted (22 holoenzyme, a 37 kDa -band, analogous to that observed in AML extracts, was generated which was resistant to further degradation. No degradation was observed when the 26 S proteasome was assayed on free subunits. Studies with CK2 deletion mutants showed that m-calpain and the 20 S proteasome acted on the C-terminus end of CK2. These results pointed to cytosolic proteases as agents involved in the control of the amount of free CK2 subunits within the cell, which becomes evident when CK2 is overexpressed as in AML cells.     

Characterization,subcellular localization and nuclear targeting of casein kinase 2 from Zea mays

We have isolated and characterized the genomic clone of maize casein kinase 2 (CK2) subunit using the previously described CK2-1 cDNA clone as a probe. The genomic clone is 7.5 kb long and contains 10 exons, separated by 9 introns of different size, two larger than 1.5 kb and the others around 100–150 bp. The sequence of the exons is 100% homologous to the sequence of the CK2-1 cDNA. Southern hybridization of total genomic DNA from maize embryos with CK2 cDNA indicated that the CK2-1 gene is part of a multigenic family. We also isolated a new embryo cDNA clone coding for an CK2-2 subunit. We studied the regulation of the enzyme in embryos at the mRNA level, at the protein level and by activity testing. By using immunocytochemistry the CK2 protein was localized in several types of cells of mature embryos. Particularly strong signals were visible in the cytoplasm of epidermis and meristematic cells. Decoration of nuclei of root cortex and scutellum cells was also observed suggesting that CK2 can shift from the cytoplasm into nuclei in specific cell types. We examined whether CK2 contained specific protein domains which actively target the protein to the nucleus by using in-frame fusions of the maize CK2 subunit to the reporter gene encoding -glucuronidase (GUS) which were assayed in transiently transformed onion epidermal cells. Analysis of chimeric constructs identified one region containing a nuclear localization signal (NLS) that is highly conserved in other CK2 proteins.     

Localization of individual subunits of protein kinase CK2 to the endoplasmic reticulum and to the Golgi apparatus

The protein kinase CK2 is composed of two catalytic - or - and two regulatory -subunits. In mammalian cells there is ample evidence for the presence of individual CK2 subunits beside the holoenzyme. By immunofluorescence studies using peptide antibodies which allow us to detect the CK2-, - and -subunits we found all three subunits to be co-localized with a 58 KDa Golgi protein which is specific for the Golgi complex. Subfractionation studies using dog pancreas cells revealed the presence of all three subunits of CK2 at the smooth endoplasmic reticulum (sER)/Golgi fraction whereas the rough endoplasmic reticulum (rER) harboured only the catalytic - and -subunits. We found that the microsomal preparation from dog pancreas cells contained CK2 which phosphorylated a CK2 specific synthetic peptide and which was heparin sensitive. Furthermore, we could immunoprecipitate the CK2-subunit that exhibited a kinase activity which phosphorylated a CK2 specific substrate and which was heparin sensitive. Protease digestion experiments revealed that the CK2 subunits were located on the cytosolic side of the rER and the sER/Golgi complex. Thus, we could demonstrate an asymmetric distribution of the CK2 subunits at the rER and sER/Golgi complex. Since the CK2- and -subunits exhibit a substrate specificity which is different from the CK2 holoenzyme one might speculate that the asymmetric distribution of the CK2 holoenzyme and the CK2 catalytic subunits may have regulatory functions.     

α-Subunit Composition of Nicotinic Cholinoreceptors of Neurons of the Inferior Mesenteric Ganglion of the Guinea Pig: an Electrophysiological Study

We studied the subunit composition of nicotinic cholinoreceptors (nChR) of neurons of the inferior mesenteric ganglion (IMG) of the guinea pig using antibodies against the ₃, ₄, ₅, and ₇ nChR subunits and a standard technique of intracellular recording. Application of the ₃ subunit-specific antibodies resulted in a decrease in the EPSP amplitude by 29.7 ± 1.8%, on average, in 17 of 20 studied neurons. The ₄ subunit-specific antibodies evoked no changes in the amplitude of EPSP recorded from IMG neurons (n = 10). Effects of the ₅ subunit-specific antibodies were studied in 20 neurons, where the EPSP amplitude dropped after application of these antibodies by 40.0 ± 1.8%, on average. Superfusion of the neurons with a solution containing the ₃ and ₅ subunit-specific antibodies completely suppressed synaptic responses (n = 3). The ₇ subunit-specific antibodies provided an increase in the EPSP amplitude in 13 of 15 studied IMG neurons (by 37 ± 6%, on average); this fact allows us to suppose that ₇-containing nChR are present in the IMG neurons and are indirectly involved in the processes of synaptic transmission. Application of the antibodies evoked no significant shifts in the membrane potential in IMG neurons. Our findings demonstrate that synaptic transmission through the guinea pig IMG is realized mostly with the involvement of the ₃- and ₅-containing nChR; the ₄-containing receptors are not engaged in this process.     

Cation effects on the conformations of muscle and non-muscle α-actinins

We examined the effects of changing KCl concentration on the secondary structures of -actinins using circular dichroism (CD), 1,1-bis(4-anilino) naphthalene-5,5-disulfonic acid (bisANS) fluorescence and proteolysis experiments. Under near-physiological conditions, divalent cations also were added and changes in conformation were investigated. In 25 mm KH₂PO₄, pH 7.5, increasing KCl from 0 to 120 mm led to decreases in -helix conformation for brain, platelet and heart -actinins (40.5-30.2%, 65.5-37.8% and 37.5-27.8%, respectively). In buffered 120 mm KCl, 0.65 mm calcium produced small changes in the CD spectra of both brain and platelet -actinin, but had no effect on heart -actinin. bisANS fluorescence of all three -actinins also showed significant changes in conformation with increasing KCl. However, in buffered 120 mm KCl increasing concentrations of Ca²⁺ or Mg²⁺ did not have significant effects on the bisANS fluorescence of any -actinin. Digestion of brain, platelet and heart -actinins with -chymotrypsin showed an increase of proteolytic susceptibility in 120 mm KCl. These experiments also showed that increasing the concentration of Ca²⁺ or Mg²⁺ led to greater changes in digestion fragment patterns in the absence of KCl than in the presence of 120 mm KCl. The results suggest that -actinins exist in different conformations depending on the ionic strength of the medium, which could explain the differences in calcium and F-actin binding results obtained from different -actinins.     

Alpha-adrenergic reactivity of the microcirculation in conscious spontaneously hypertensive rats

The goal of this study was to determine the functional distribution of ₁- and ₂-adrenoceptors in the striated muscle microcirculation. Experiments were performed in intact conscious spontaneously hypertensive rats (SHR) that were provided with a dorsal microcirculatory chamber to allow microvascular diameter measurements. Administration of selective ₁- and ₂-agonists, phenylephrine and azepexole, respectively, induced different patterns of microvascular constriction. ₁-Adrenoceptor stimulation showed a preferential constriction of large arteries and venules. The entire arteriolar microvasculature was sensitive to ₂-adrenoceptor stimulation, whereas the venular vessels did not respond to azepexole. The selective ₁- and ₂-antagonists prazosin and yohimbine showed patterns of vasodilator activity comparable to those of the corresponding agonists. The specificity of the drug-induced effects was verified by comparing their effects with those of graded hemorrhage, a non-pharmacological method for blood pressure lowering. In the range of blood pressure decreases comparable to that obtained by -adrenoceptor antagonists, graded hemorrhage did not influence microvascular diameters. These results show a differential functional distribution of ₁- and ₂-adrenoceptors along the microvascular tree in striated muscle of conscious SHR.     

Pseudo vitamin B12 or 5-hydroxybenzimidazolyl-cobamide are the corrinoids found in methanogenic bacteria

Thirteen species of methanogenic bacteria were analyzed for corrinoids. Pseudo vitamin B₁₂ (Co-[-(7-adenyl)]-cobamide) was the predominant cobamide of methanococcales and Methanoplanus. All other methanogens contained factor III (Co-[-(5-hydroxybenzimidazolyl)]-cobamide). Vitamin B₁₂ (Co-[-(5,6-dimethylbenzimidazolyl)]-cobamide) was not detected in any of these archaebacteria. Their cobamide content was 100 to 1400 nmol per gram cell dry weight, indicating that abundant cobamides are essential for methanogens.     

Interactions of protein kinase CK2 subunits

Several approaches have been used to study the interactions of the subunits of protein kinase CK2. The inactive mutant of CK2 that has Asp 156 mutated to Ala (CK2A156) is able to bind the CK2 subunit and to compete effectively in this binding with wild-type subunits and . The interaction between CK2A156 and CK2 was also demonstrated by transfection of epitope-tagged cDNA constructs into COS-7 cells. Immunoprecipitation of epitope-tagged CK2A156 coprecipitated the subunit and vice-versa. The assay of the CK2 activity of the extracts obtained from cells transiently transfected with these different subunits yielded some surprising results: The CK2 specific phosphorylating activity of these cells transfected with the inactive CK2A156 was considerably higher than the control cells transfected with vectors alone. Assays of the immunoprecipitated CK2A156 expressed in these cells, however, demonstrated that the mutant was indeed inactive. It can be concluded that transfection of the inactive CK2A156 affects the endogenous activity of CK2. Transfection experiments with CK2 and subunits and CK2A156 were also used to confirm the interaction of CK2 with the general CDK inhibitor p21WAF1/CIP1 co-transfected into these cells. Finally a search in the SwissProt databank for proteins with properties similar to those derived from the amino acid composition of CK2 indicated that CK2 is related to protein phosphatase 2A and to other phosphatases as well as to a subunit of some ion-transport ATPases.     

Angiotensin II-induced changes in G-protein expression and resistance of renal microvessels in young genetically hypertensive rats

Altered regulation of cAMP may contribute to enhanced renal reactivity to angiotensin II (Ang II) in spontaneously hypertensive rats (SHR). Such a phenomenon may occur in renal preglomerular arterioles and may involve changes in expression of GTP-binding regulatory proteins. We have examined the effects of Ang II on steady state levels of G_i-1,2, G_i-3 G_s and G_q in preglomerular arterioles from young marginally hypertensive SHR and on mean arterial pressure (MAP), renal vascular resistance (RVR) and renal cAMP excretion (UcAMP.V). Young (5-6 week old) SHR and Wistar Kyoto (WKY) rats received Ang II (35 ng/kg/min, s.c.) or vehicle for 7 days via osmotic minipumps. Urine was collected over the last 24 h. On day seven, MAP and renal blood flow were measured in anesthetized rats and RVR was determined. Preglomerular arterioles were isolated by perfusing the kidneys with iron oxide and using a series of mechanical steps coupled with the use of a magnet to retain iron-laden vessels. Membranes were prepared and the expressions of G_i-1,2, G_i-3, G_s and G_q were evaluated by Western immunoblotting. Baseline MAP (124 ± 6 mmHg) was only marginally (p > 0.05) higher in SHR when compared with WKY rats (110 ± 4 mmHg). RBF (3.04 ± 0.16 mL/min) was significantly lower and RVR (41.10 ± 1.37 mmHg.min/mL) was significantly higher in SHR when compared to age-matched WKY rats (4.36 ± 0.30 mL/min and 25.79 ± 1.58 mmHg.min/mL, respectively). Ang II significantly increased MAP in SHR (17 mmHg) but not in WKY rats. These increases in MAP were accompanied by significant increases in RVR in SHR (48% over control) but not in WKY rats. Compared to WKY rats, preglomerular arterioles from SHR exhibited significantly higher basal expression of G_i-1,2 (11- fold), G_i-3 (13-fold) and G_s (3-fold). Chronic infusion of Ang II, however, downregulated the expression of G_s (by 53%; p < 0.05), G_i-1,2 ( by 72%; p < 0.05) and G_i-3 (by 35%; p > 0.05) in SHR preglomerular arterioles but significantly upregulated the expression of these proteins in WKY by 3-, 8- and 15-fold, respectively. Basal levels of G_q were not different in preglomerular arterioles from the two strains but were downregulated by Ang II in both WKY (74% of basal) and SHR (52% of control). Baseline UcAMP.V was significantly lower in SHR (31.22 ± 6.51 nmol/24 h) compared with WKY rats (65.33 ± 3.60 nmol/24 h). Chronic Ang II infusion significantly increased UcAMP.V in SHR as well as WKY rats. These data clearly demonstrate that expressions of G_i isoforms as well as G_s in renal microvessels are elevated during early stages of hypertension and suggest that the elevated levels of G_i proteins may be directly associated with a blunted adenylyl cyclase-cAMP cascade in the renal microvasculature. Furthermore, Ang II appears to directly downregulate the expression of G_s in young SHR but not in young WKY renal microvessels. Such diversity in its effect on G-protein expression may be important for enhanced renal sensitivity to Ang II in SHR.     

Synthesis and binding properties of endomorphin-2analogs containing α-hydroxymethyl amino acids

Novel endomorphin-2 analogs containing the unusual amphiphilic amino acid (R)- and (S)--hydroxymethyltyrosine in position 1 and (R)- and (S)--hydroxymethylphenylalanine in the positions 3 and 4 were synthesized via the solid-phase method. The binding characteristics of the synthetic analogs may suggest that -hydroxymethyl substitution of aminoacid residues influences the conformation of a peptide much more than simply increasing the local amphiphilic character of the peptide.     

Acyl chain conformational ordering in liquid-crystalline bilayers: comparative FT-IR and 2H-NMR studies of phospholipids differing in headgroup structure and chain length

FT-IR spectroscopy has been used to evaluate the acyl chain conformational ordering of DMPC, DMPE, DMPA (pH 6 and 12), DMPG (pH 1 and 7), and DPPC, DPPE, DPPA (pH 6). The frequencies of the symmetric and antisymmetric methylene stretching vibrations were determined as a function of temperature. In the liquid-crystalline phase the frequencies show a qualitative dependence on the amount of chain disorder. Quantitative data for trans-gauche isomerization were obtained from the integral intensities of the conformation sensitive methylene wagging absorptions at ca. 1368 cm^–1 (gtg and gtg sequences), 1356 cm ^–1 (double gauche) and 1342 cm^–1 (end gauche). The integral band intensities were converted to the number of gauche conformers per acyl chain using the calibration factors published by Senak et al. (1991). At 69°C the highest number of gauche conformers excluding contributions from single gauche conformers and jogs (gtttg) are found for PCs (DMPC: 2.6; DPPC: 2.4), followed by DMPG^– (2.0), phosphatidylethanolamines (DMPE: 1.4; DPPE: 2.0), protonated DMPG (1.5), and phosphatidic acids (DPPA^–: 1.7; DMPA^–: 1.4, DMPA^2–: 1.7). From ²H-NMR measurements of perdeuterated samples of DMPC, DMPA, DPPC, and DPPA the quadrupolar splittings _Qi and the order parameter S _CDi of the CD₂-segments close to the chain ends could be determined whereas splittings in the plateau region of the chains could not be resolved. The quadrupolar splittings are affected by trans-gauche isomerization, long axis rotation, and restricted wobbling motions of the acyl chains. In the simplest assumption, the order parameter S_CD can be expressed as a product of a segmental order parameter S and a lhain order parameter S . For comparison of the different lipids we used average order parameters S_CD, obtained by averaging over all values, and S determined from the total number of gauche conformers per chain by FT-IR-spectroscopy, to calculate an empirical average chain order parameter S. The combination of ²H-NMR and FT-IR results allows the estimation of the relative extent of chain wobbling for the different lipid molecules. S is lowest for PCs (S 0.475) while PEs (S 0.51) and PAs (S0.52) show less chain wobbling.Abbreviations FT-IR Fourier transform infrared - ²H-NMR deuterium nuclear magnetic resonance - DMPC(–d₅₄) (perdeuterated) dimyristoyl-phosphatidylcholine - DMPE(–d₅₄) (perdeuterated) dimyristoyl-phosphatidylethanolamine - DMPA(–d₅₄) (perdeuterated) dimyristoyl-phosphatidic acid - DMPG dimyristoyl-phosphatidylglycerol - DPPC(–d₆₂) (perdeuterated) dipalmitoyl-phosphatidylcholine - DPPE(–d₆₂) (perdeuterated) dipalmitoyl-phosphatidylethanolamine - DPPA(–d₆₂) (perdeuterated) dipalmitoyl-phosphatidic acid - gtg gauche ^±-trans-gauche^± - gtg gauche^±-trans-gauche^± - dg double gauche - eg end gauche Correspondence to: A. Blume     

A new non-equilibrium enzyme linked immunosorbent assay for a glycogen-derived urinary tetrasaccharide

The tetrasaccharide, Glc1-6Glc1-4Glc1-4Glc, denoted (Glc)₄, is a limit dextrin formed by amylolytic degradation of glycogen. In order to evaluate the possible clinical importance of (Glc)₄ excretion as an indicator of certain physiological and pathological conditions, we have developed a new rapid and inexpensive immunoassay using a monoclonal antibody raised against (Glc)₄ glycosidically-linked to a carrier protein. As the antibody is highly specific, it can be used to measure native (Glc)₄ directly, without the chemical reduction step required in previous methods. A new type of non-equilibrium ELISA inhibition test was developed based on the capacity of free (Glc)₄ to decrease initial rates of antibody binding to (Glc)₄-coated microtiter wells. The method is highly reproducible and is as sensitive and accurate as the gas chromatography method or radioimmunoassay used previously.Abbreviations (Glc)₄ Glc1-6Glc1-4Glc1-4Glc - KLH keyhole limpet hemacyanin - BSA bovine serum albumin - PEG polyethylene glycol     

Functional specialization of CK2 isoforms and characterization of isoform-specific binding partners

In mammals, protein kinase CK2 has two isozymic forms of its catalytic subunit, designated CK2gr; and CK2. CK2 and CK2 exhibit extensive similarity within their catalytic domains but have completely unrelated C-terminal sequences. To systematically examine the cellular functions of each CK2 isoform in mammalian cells, we have generated human osteosarcoma U2-OS cell lines with the expression of active or inactive versions of each CK2 isoform under the control of an inducible promoter [22]. Examination of these cell lines provides evidence for functional specialization of CK2 isoforms at the cellular level in mammals with indications that CK2 is involved in the control of proliferation and/or cell survival. To understand the molecular basis for functional differences between CK2 and CK2, we have undertaken studies to identify proteins that interact specifically with each isoform of CK2 and could contribute to the regulation of their independent functions. A novel pleckstrin-homology domain containing protein, designated CK2-interacting protein 1 (i.e. CKIP-1) was isolated using the yeast two hybrid system as a protein that interacts with CK2 but not CK2 [23]. When expressed in cells as a fusion with green fluorescent protein, CKIP-1 localizes to the cell membrane and to the nucleus. In this study, we present evidence from deletion analysis of CKIP-1 suggesting that a C-terminal region containing a putative leucine zipper has a role in regulating its nuclear localization. Collectively, our data supports a model whereby CKIP-1 is a non-enzymatic regulator of CK2 that regulates the cellular functions of CK2 by targeting or anchoring CK2 to specific cellular localization or by functioning as an adapter to integrate CK2-mediated signaling events with components of other signal transduction pathways.     

C-Terminal exchange between Gα o and Gα i3 proteins differently modulates α2A-adrenoceptor activation

Chimeric G proteins, obtained by exchanging their C-terminal portion for that of a G protein from an unrelated class, drive the receptor selectivity to that corresponding to the introduced G protein domain. The _2A-adrenoceptor (_2AAR), which yielded an efficacious and weak [³⁵S]GTPS binding response by respectively G _o and G _i3 protein, was investigated in CHO-K1 cells co-expressing chimeric G proteins for which the six last C-terminal amino acids between G _o and G _i3 proteins, and reciprocally, were permuted. Activation of the chimeric G _{o / i3} protein was highly efficient whereas the G _{i3 / o} protein yielded a weak stimulation. These [³⁵S]GTPS binding responses were not different from their parental wild-type G _o and G _i3 proteins. Similar results were obtained with an _2AAR carrying a facilitating Thr³⁷³Lys mutation in a putative G protein interaction domain. These data indicate that the six terminal G _o protein amino acids do not constitute a major _2AAR interaction domain for G protein activation.     

CK2α - protein phosphatase 2A molecular complex: Possible interaction with the MAP kinase pathway

Despite its wide range of known substrates, the signaling function of protein kinase CK2 is still enigmatic. Mounting evidence suggests that CK2, the catalytic subunit of holoenzymic CK2, may exist free of its usual regulatory partner CK2, raising the possibility that free CK2 has regulation and function distinct from those of the holoenzyme. We previously reported that CK2 could bind to the core dimer of protein phosphatase 2A, and indirectly cause down-regulation of the PP2A substrate MEK1, possibly via activation of PP2A and/or targeting of PP2A to some element of the Ras/Raf/MEK pathway. Here, these results are confirmed and extended. By using transfection experiments and immune kinase assays, we show that endogenous PP2Ac and CK2 are the only major substrates associating with epitope-tagged CK2, and that expression of activated Raf results in disruption of the CK2- PP2A association. Such disruption might be a necessary step for maximal activation of the MAP kinase pathway by Raf. In keeping with this idea, overexpression of CK2 dose-dependently inhibits the mitogen-induced activation of cotransfected, epitope-tagged MAP kinase. We suggest that the CK2 free form of CK2 is both a target and a regulator of Raf/MAPK signaling.     

Soluble Expression and Affinity Purification of Functional Domain of Human Acetylcholine Receptor <Emphasis Type="Italic">α</Emphasis>-subunit by the Modulation of Maltose Binding Protein

An open reading frame of the -subunit 1-205 residues (₂₀₅) of human acetylcholine receptor (AchR) was amplified by PCR with pUC-AChR₂₀₅ as the template and inserted into vector pMAL-c2X. The constructed pMAR₂₀₅ was transferred into E. coli BL21 which were then grown in LB medium. The amount of soluble MBP-AChR₂₀₅ protein reached about 25% of total soluble proteins from the cell lysate. Using amylose-affinity chromatography, about 35 mg MBP-AChR₂₀₅ could be obtained from 1 l culture. Western blot analysis and ELISA showed that immunoreactivities of both MBP-AChR₂₀₅ and AChR₂₀₅ were similar to that of AChR -subunit from Torpedo.Revisions received 23 September 2004     

Enhanced expression of GTP-binding proteins in differentiated U937 monocytic cells: Possible involvement of tyrosine kinase and protein kinase C

Monocytic U937 cells were differentiated into mature macrophages in the presence of 100 nM phorbol 12-myristate 13-acetate (PMA) for 24 h at 37°C. We investigated the alterations in the expression of GTP-binding proteins that take place during differentiation of these cells. A 40 KDa -subunit of the inhibitory G-protein was identified by specific antibodies to G_i-1/2 and G_i-3 on Western blots and also by ADP-ribosylation catalyzed by pertussis toxin. The expression of the 40 KDa G_i subunit was increased 3.4 fold in differentiated cells. The expression of a 43 kDa G_s subunit identified by Western blotting using specific antibody to G_s and by ADP-ribosylation in the presence of cholera toxin was increased approximately 2 fold in differentiated cells. A faintly recognizable 46 KDa G_s subunit was also increased but to a lesser extent (1.3 fold). Small molecular weight GTP-binding proteins identified by [³⁵S]GTPS binding on nitrocellulose blots were also increased significantly. The PMA-induced expression of G_i-1/2 and G_s subunits was blocked to control level by both genistein and staurosporine, inhibitors of protein tyrosine kinase and protein kinase C, respectively. However, staurosporine was unable to block the PMA-induced expression of G_i-3; this was blocked only by genistein. These data suggest a role for tyrosine kinase and protein kinase C in the expression of G-proteins during differentiation of U937 cells.     

Expression and regulation of protein kinase CK2 during the cell cycle

There are indications from genetic, biochemical and cell biological studies that protein kinase CK2 (formerly casein kinase II) has a variety of functions at different stages in the cell cycle. To further characterize CK2 and its potential roles during cell cycle progression, one of the objectives of this study was to systematically examine the expression of all three subunits of CK2 at different stages in the cell cycle. To achieve this objective, we examined levels of CK2, CK2 and CK2 on immunoblots as well as CK2 activity in samples prepared from: (i) elutriated populations of MANCA (Burkitt lymphoma) cells, (ii) serum-stimulated GL30-92/R (primary human fibroblasts) cells and (iii) drug-arrested chicken bursal lymphoma BK3A cells. On immunoblots, we observed a significant and co-ordinate increase in the expression of CK2 and CK2 following serum stimulation of quiescent human fibroblasts. By comparison, no major fluctuations in CK2 activity were detected during any other stages during the cell cycle. Furthermore, we did not observe any dramatic differences between the relative levels of CK2 to CK2 during different stages in the cell cycle. However, we observed a significant increase in the amount of CK2 relative to CK2 in cells arrested with nocodazole. We also examined the activity of CK2 in extracts or in immunoprecipitates prepared from drug-arrested cells. Of particular interest is the observation that the activity of CK2 is not changed in nocodazole-arrested cells. Since CK2 is maximally phosphorylated in these cells, this result suggests that the phosphorylation of CK2 by p34cdc2 does not affect the catalytic activity of CK2. However, the activity of CK2 was increased by incubation with p34cdc2 in vitro. Since this activation was independent of ATP we speculate that p34cdc2 may have an associated factor that stimulates CK2 activity. Collectively, the observations that relative levels of CK2 increase in mitotic cells, that CK2 and CK2 are phosphorylated in mitotic cells and that p34cdc2 affects CK2 activity in vitro suggest that CK2 does have regulatory functions associated with cell division.