A novel casein kinase 2 alpha-subunit regulates membrane protein traffic in the human hepatoma cell line HuH-7

A previously isolated endocytic trafficking mutant (TRF1) isolated from HuH-7 cells is defective in the distribution of subpopulations of cell-surface receptors for asialoorosomucoid (asialoglycoprotein receptor (ASGR)), transferrin, and mannose-terminating glycoproteins. The pleiotropic phenotype of TRF1 also includes an increased sensitivity to Pseudomonas toxin and deficient assembly and function of gap junctions. HuH-7xTRF1 hybrids exhibited a normal subcellular distribution of ASGR, consistent with the TRF1 mutation being recessive. A cDNA expression library derived from HuH-7 mRNA was transfected into TRF1 cells, which were subsequently selected for resistance to Pseudomonas toxin. Sequence analysis of a recovered cDNA revealed a unique isoform of casein kinase 2 (CK2), CK2alpha". Western blot analysis of TRF1 proteins revealed a 60% reduction in total CK2alpha expression. Consistent with this finding, the hybrids HuH-7xHuH-7 and HuH-7xTRF1 expressed equivalent amounts of total CK2alpha. Immunoblots using antibodies against peptides unique to the previously described CK2 isoforms CK2alpha and CK2alpha' and the novel CK2alpha" isoform showed that, although TRF1 and parental HuH-7 cells expressed comparable amounts of CK2alpha and CK2alpha', the mutant did not express CK2alpha". Based on the genomic DNA sequence, RNA transcripts encoding CK2alpha" apparently originate from alternative splicing of a primary transcript. Protein overexpression following transfection of TRF1 cells with cDNAs encoding either CK2alpha or the newly cloned CK2alpha" restored the parental HuH-7 phenotype, including Pseudomonas toxin resistance, cell-surface ASGR binding activity, phosphorylation, and the assembly of gap junctions. This study suggests that HuH-7 cells express at least three CK2alpha isoforms and that the pleiotropic TRF1 phenotype is a consequence of a reduction in total CK2 expression.

The gain-of-function Chinese hamster ovary mutant LEC11B expresses one of two Chinese hamster FUT6 genes due to the loss of a negative regulatory factor

The LEC11 Chinese hamster ovary (CHO) gain-of-function mutant expresses an alpha(1,3)fucosyltransferase (alpha(1,3)Fuc-T) activity that generates the LeX, sialyl-LeX, and VIM-2 glycan determinants and has been extensively used for studies of E-selectin ligand specificity. In order to identify regulatory mechanisms that control alpha(1,3)Fuc-T expression in mammals, mechanisms of FUT gene expression were investigated in LEC11 cells and two new, independent mutants, LEC11A and LEC11B. Northern and ribonuclease protection analyses, using probes that span the coding region of a cloned CHO FUT gene, detected transcripts in each LEC11 mutant but not in CHO cells or other gain-of-function CHO mutants that express a different alpha(1,3)Fuc-T activity. Coding region sequence analysis and alpha(1,3)Fuc-T acceptor specificity comparisons with recombinant human Fuc-TV and Fuc-TVI showed that the cloned FUT gene is orthologous to the human FUT6 gene. Southern analyses identified two closely related FUT6 genes in the Chinese hamster, whose evolutionary relationships are discussed. The blots showed that rearrangements had occurred in LEC11A and LEC11 genomic DNA, consistent with a cis mechanism of FUT6 gene activation in these mutants. By contrast, somatic cell hybrid analyses revealed that LEC11B cells express FUT6 gene transcripts due to the loss of a trans-acting, negative regulatory factor. Sequencing of reverse transcriptase-polymerase chain reaction products identified unique 5'- and 3'-untranslated region sequences in FUT6 gene transcripts from each LEC11 mutant. Northern and Southern analyses with gene-specific probes showed that LEC11A cells express only the cgFUT6A gene (where cg is Cricetulus griseus), whereas LEC11 and LEC11B cells express only the cgFUT6B gene. In LEC11A x LEC11B hybrid cells, the cgFUT6A gene was predominantly expressed, as predicted if a trans-acting negative regulatory factor functions to suppress cgFUT6B gene expression in CHO cells. This factor is predicted to be a cell type-specific regulator of FUT6 gene expression in mammals.

Mammalian cytidine 5'-monophosphate N-acetylneuraminic acid synthetase: a nuclear protein with evolutionarily conserved structural motifs

Sialic acids of cell surface glycoproteins and glycolipids play a pivotal role in the structure and function of animal tissues. The pattern of cell surface sialylation is species- and tissue-specific, is highly regulated during embryonic development, and changes with stages of differentiation. A prerequisite for the synthesis of sialylated glycoconjugates is the activated sugar-nucleotide cytidine 5'-monophosphate N-acetylneuraminic acid (CMP-Neu5Ac), which provides a substrate for Golgi sialyltransferases. Although a mammalian enzymatic activity responsible for the synthesis of CMP-Neu5Ac has been described and the enzyme has been purified to near homogeneity, sequence information is restricted to bacterial CMP-Neu5Ac synthetases. In this paper, we describe the molecular characterization, functional expression, and subcellular localization of murine CMP-Neu5Ac synthetase. Cloning was achieved by complementation of the Chinese hamster ovary lec32 mutation that causes a deficiency in CMP-Neu5Ac synthetase activity. A murine cDNA encoding a protein of 432 amino acids rescued the lec32 mutation and also caused polysialic acid to be expressed in the capsule of the CMP-Neu5Ac synthetase negative Escherichia coli mutant EV5. Three potential nuclear localization signals were found in the murine synthetase, and immunofluorescence studies confirmed predominantly nuclear localization of an N-terminally Flag-tagged molecule. Four stretches of amino acids that occur in the N-terminal region are highly conserved in bacterial CMP-Neu5Ac synthetases, providing evidence for an ancestral relationship between the sialylation pathways of bacterial and animal cells.

Molecular cloning of human GDP-mannose 4,6-dehydratase and reconstitution of GDP-fucose biosynthesis in vitro

We have cloned the cDNA encoding human GDP-mannose 4,6-dehydratase, the first enzyme in the pathway converting GDP-mannose to GDP-fucose. The message is expressed in all tissues and cell lines examined, and the cDNA complements Lec13, a Chinese Hamster Ovary cell line deficient in GDP-mannose 4,6-dehydratase activity. The human GDP-mannose 4,6-dehydratase polypeptide shares 61% identity with the enzyme from Escherichia coli, suggesting broad evolutionary conservation. Purified recombinant enzyme utilizes NADP+ as a cofactor and, like its E. coli counterpart, is inhibited by GDP-fucose, suggesting that this aspect of regulation is also conserved. We have isolated the product of the dehydratase reaction, GDP-4-keto-6-deoxymannose, and confirmed its structure by electrospray ionization-mass spectrometry and high field NMR. Using purified recombinant human GDP-mannose 4,6-dehydratase and FX protein (GDP-keto-6-deoxymannose 3,5-epimerase, 4-reductase), we show that the two proteins alone are sufficient to convert GDP-mannose to GDP-fucose in vitro. This unequivocally demonstrates that the epimerase and reductase activities are on a single polypeptide. Finally, we show that the two homologous enzymes from E. coli are sufficient to carry out the same enzymatic pathway in bacteria.

Lec32 is a new mutation in Chinese hamster ovary cells that essentially abrogates CMP-N-acetylneuraminic acid synthetase activity

LEC29.Lec32 is a glycosylation mutant that was isolated from a selection of mutagenized Chinese hamster ovary (CHO) cells for lectin resistance. Compared with LEC29 CHO cells, the double mutant exhibited an unusually high sensitivity to the toxic lectin, ricin, indicating increased exposure of galactose residues on cell surface carbohydrates. Structural analysis of LEC29.Lec32 cellular glycoproteins showed a nearly complete lack of sialic acid residues. Genetic analysis demonstrated that the lec32 mutation is recessive and novel. Biochemical analysis showed that the mutant cells contained less than 5% of the cytidine 5'-monophosphate N-acetylneuraminic acid (CMP-NeuAc) present in parental CHO cells (1.6 nmol/mg of cell protein). A sensitive radiochemical assay used to measure CMP-NeuAc synthetase activity showed that the properties of this enzyme in parental CHO cells were essentially identical to those of CMP-NeuAc synthetase in various mammalian tissues. However, no CMP-NeuAc synthetase activity was detected in LEC29.Lec32 extracts. Mixing experiments provided no evidence for an inhibitor in the mutant CHO cells, and two revertants, which expressed only the LEC29 phenotype, had normal CMP-NeuAc synthetase levels. The combined evidence indicates that the lec32 mutation resides in either the structural gene encoding CMP-NeuAc synthetase or in a gene that regulates the production of active enzyme.

Human hepatoma cell mutant defective in cell surface protein trafficking

To isolate a mutant liver cell defective in the endocytic pathway, a selection strategy using toxic ligands for two distinct membrane receptors was devised. Ovalbumin-gelonin and asialoorosomucoid (ASOR)-gelonin were incubated with mutagenized HuH-7 cells, and a rare survivor termed trafficking mutant 1 (Trf1) was isolated. Trf1 cells were stably 3-fold more resistant than the parental HuH-7 to both toxic conjugates. The anterograde steps of intracellular endocytic processing of ASOR, including internalization, endosomal acidification, and ligand degradation, were unaltered in Trf1 cells. In contrast, retrograde diacytosis of asialoglycoprotein receptor (ASGR).ASOR complex back to the cell surface was enhanced by about 250%. Selective labeling revealed an approximately 46% reduction in cell surface-associated ASGR in Trf1 cells, although their total cellular ASGR content was essentially equivalent to that in HuH-7. Similar results were obtained with the transferrin receptor. Binding of 125I-ASOR and 125I-transferrin was reduced in Trf1 cells to 49 +/- 2.5% and 30 +/- 2%, respectively, of HuH-7 cells. The methionine transporter was also reduced in Trf1 cells, as revealed by a 2-fold reduction in Vmax with no change in apparent Km. Pretreatment with monensin, sodium azide, or colchicine reduced surface binding of 125I-ASOR in HuH-7 cells by 50% but had no effect on binding to Trf1 cells. This result is predicted for a cell that expresses only State 1 ASGRs, which are resistant to modulation by metabolic and cytoskeletal inhibitors in contrast to State 2, which are responsive to these agents (Weigel, P. H., and Oka, J. A. (1984) J. Biol. Chem. 259, 1150-1154). The Trf1 mutant, having lost the ability to express State 2 receptors, provides genetic evidence for the existence of these two receptor subpopulations and an approach to identifying the biochemical mechanism by which they are generated.

Differential expression of an E-selectin ligand (SLex) by two Chinese hamster ovary cell lines transfected with the same alpha (1,3)-fucosyltransferase gene (ELFT)

The mammalian cDNA encoding alpha (1,3)-fucosyltransferase (alpha (1,3)Fuc-T) termed ELAM-1 ligand fucosyltransferase (ELFT) or Fuc-TIV was previously cloned by three groups who reported different results from transfection studies Goelz et al. (Goelz, S. E., Hession, C., Goff, D., Griffiths, B., Tizard, R., Newman, B., Chi-Rosso, G., and Lobb, R. (1990) Cell 63, 1349-1356) found that Chinese hamster ovary (CHO) cells expressing the ELFT cDNA had alpha (1,3)Fuc-T activity and were able to bind to E-selectin. In contrast, Lowe et al. (Lowe, J. B., Kukowska-Latallo, J. F., Nair, R. P., Larsen, R. D., Marks, R. M., Macher, B. A., Kelly, R. J., and Ernst, L. K. (1991) J. Biol. Chem. 266, 17467-17477) and Kumar et al. (Kumar, R., Potvin, B., Muller, W. A., and Stanley, P. (1991) J. Biol. Chem. 266, 21777-21783) found no binding to E-selectin of CHO transfectants expressing the same alpha (1,3)Fuc-T gene; nor did the latter transfectants synthesize a known E-selectin ligand, sialylated Lex (SLex), although they had substantial alpha (1,3)Fuc-T activity. We now show that these discrepant results were due to a difference between the parental CHO cell lines. Following transfection of ELFT cDNA into Pro-5 or dihydrofolate reductase (DHFR)- CHO cells, only the DHFR- transfectants expressed SLex and bound to E-selectin. Indirect evidence from monoclonal antibody and lectin binding studies indicates that the range of carbohydrate structures synthesized by the Pro-5 and DHFR- CHO cell lines differs. Since DHFR-/ELFT transfectants expressed cell surface SLex but transferred fucose poorly to sialylated substrates in vitro, ELFT may be able to fucosylate a complex carbohydrate missing from Pro-5 cells. Alternatively, either CHO line may have an activity (such as an alpha (2,3)-sialyltransferase), that modifies alpha (1,3)-fucosylated lactosamines.

Activation of two new alpha(1,3)fucosyltransferase activities in Chinese hamster ovary cells by 5-azacytidine

Several mammalian alpha(1,3)fucosyltransferases (alpha[1,3]Fuc-T) that synthesize carbohydrates containing alpha(1,3)fucosylated lactosamine units have been identified. Although Chinese hamster ovary (CHO) cells do not express alpha(1,3)Fuc-T activity, the rare mutants LEC11 and LEC12, isolated after mutagenesis or DNA transfection, each express an alpha(1,3)Fuc-T that may be distinguished by several criteria. Two new CHO mutants possessing alpha(1,3)Fuc-T activity (LEC29 and LEC30) have now been isolated after treatment of a CHO cell population with 5-azacytidine (5-AzaC), ethylnitrosourea (ENU), or 5-AzaC followed by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). Like LEC12, both mutants possess an N-ethylmaleimide-resistant alpha(1,3)Fuc-T activity that can utilize a variety of acceptors and both express the Lewis X (Lex) determinant (Gal beta[1,4](Fuc alpha[1,3])GlcNAc beta 1)) but not the sialyl alpha(2,3)Lex determinant on cell-surface carbohydrates. However, LEC29 and LEC30 may be distinguished from LEC11 and LEC12, as well as from each other, on the basis of their unique patterns of lectin resistance and their abilities to bind the VIM-2 monoclonal antibody that recognizes carbohydrates terminating in NeuNAc alpha(2,3)Gal beta(1,4)GlcNAc beta(1,3)Gal beta(1,4)(Fuc alpha[1,3])GlcNAc beta and also by the different in vitro substrate specificities and kinetic properties of their respective alpha(1,3)Fuc-T activities. The combined data provide good evidence that the LEC29 and LEC30 alpha(1,3)Fuc-Ts are novel transferases encoded by distinct gene products.

Cloning of a human alpha(1,3)-fucosyltransferase gene that encodes ELFT but does not confer ELAM-1 recognition on Chinese hamster ovary cell transfectants

In previous studies, Chinese hamster ovary (CHO) cell genomic DNA transfectants that expressed a human alpha(1,3)-fucosyltransferase (alpha(1,3)Fuc-T) gene were isolated and shown to possess a common approximately 7.5-kilobase (kb) EcoRI fragment that hybridized to an Alu probe (Potvin, B., Kumar, R., Howard, D. R., and Stanley, P. (1990) J. Biol. Chem. 265, 1615-1622). One of these transfectants was used to make a genomic DNA library in lambda ZAP-II from EcoRI-digested, size-selected (6-8 kb) DNA, and plaques that hybridized to an Alu probe were purified. After in vivo excision, two plasmids with DNA inserts greater than or equal to 6 kb were obtained and one of these (D2.1) conferred human alpha(1,3)-Fuc-T activity on CHO transfectants. A partial restriction map of this clone revealed an approximately 3.6-kb PstI fragment that contained an Alu sequence. This fragment was subcloned into pGEM-3Zf(+) and compared by restriction analyses with a previously described approximately 3.6-kb PstI DNA fragment isolated from a human peripheral blood lymphocyte library and shown to encode an alpha(1,3)-Fuc-T gene (Lowe, J. B., Stoolman, L. M., Nair, R. P., Larsen, R. D., Berhend, T. L., and Marks, R. M. (1990) Cell 63, 475-484). Both approximately 3.6-kb fragments gave identical restriction patterns. In addition, they both caused CHO transfectants to synthesize the Lex determinant Gal beta(1,4)[Fuc alpha(1,3)]GlcNAc beta 1 but not the alpha(2,3)-sialyl-Lex determinant. As expected, these transfectants did not bind to ELAM-1 on activated endothelial cells, since sialyl-Lex is a carbohydrate ligand recognized by ELAM-1. Surprisingly, however, an open reading frame encoded within the approximately 3.6-kb PstI fragment had a sequence identical to that of ELFT, an alpha(1,3)-Fuc-T previously reported to confer ELAM-1 binding on a previously reported to confer ELAM-1 binding on a CHO transfectant (Goelz, S. E., Hession, C., Goff, D., Griffiths, B., Tizard, R., Newman, B., Chi-Rosso, G., and Lobb, R., (1990) Cell 63, 1349-1356). Possible explanations for these apparently disparate results are discussed.

Transfection of a human alpha-(1,3)fucosyltransferase gene into Chinese hamster ovary cells. Complications arise from activation of endogenous alpha-(1,3)fucosyltransferases

In order to isolate a human gene encoding an alpha-(1,3)fucosyltransferase (alpha-(1,3)Fuc-T), genomic DNA from HL-60 cells was transfected by several methods into Chinese hamster ovary (CHO) cells. Colonies expressing alpha-(1,3)Fuc-T activity were identified by their ability to bind a monoclonal antibody (anti-SSEA-1) that recognizes the carbohydrate product of alpha-(1,3)Fuc-T action. CHO cells do not express alpha-(1,3)Fuc-T activity but contain at least two, silent alpha-(1,3)Fuc-T genes previously identified by their activation in the rare, dominant mutants LEC11 and LEC12. These CHO enzymes were shown to be distinguishable from the alpha-(1,3)Fuc-T activity of HL-60 cells by the latter's comparative inability to transfer fucose to paragloboside and fetuin. Based on these criteria, only 11 isolates from more than 70 putative transfectants examined were found to stably express an alpha-(1,3)Fuc-T activity typical of HL-60 cells. Genomic DNA from two of these isolates was used to generate five independent secondary transfectants with HL-60-like alpha-(1,3)Fuc-T activity. Southern analysis revealed a common DNA fragment that hybridized to an Alu probe in each secondary, providing evidence that a human alpha-(1,3)Fuc-T gene had been transfected. However, in all transfection experiments, isolates that expressed alpha-(1,3)Fuc-T activities similar to CHO-encoded enzymes were also obtained. Several lines of evidence indicated that these cells arose from activation of endogenous CHO alpha-(1,3)Fuc-T genes as a consequence of DNA transfection. These false positives complicated the identification of transfectants expressing a human alpha-(1,3)Fuc-T gene and represent an important consideration in experiments to transfect other glycosyltransferase genes.

Carbamyl phosphate synthesis in Bacillus subtilis

In vitro and "in situ" assays have been developed to test the carbamyl phosphate synthetase (CPSase) activity of a series of pyrimidine-requiring mutants of Bacillus subtilis. The enzyme has been shown to be highly unstable, and was successfully extracted only in the presence of 10% glycerol and 1 mM dithiothreitol (Cleland's reagent). It loses activity rapidly when sonicated or when treated with lysozyme. Genetic studies, using mutants, indicate that B. subtilis may possess two CPSases. This possibility and its physiological consequences were probed enzymatically. CPSase activity has been shown to undergo inhibition by both uridine triphosphate and dihydroorotate; activation has been demonstrated in response to phosphoribosyl pyrophosphate (PRPP) and (to a lesser extent) ornithine.