Isolation of Chinese hamster ovary ribosomal mutants differentially resistant to ricin, abrin, and modeccin

Absence of the dolichol synthesis gene DHRSX leads to N-glycosylation defects in Lec5 and Lec9 Chinese hamster ovary cells

Glycosylation-deficient Chinese hamster ovary cell lines have been instrumental in the discovery of N-glycosylation machinery. Yet, the molecular causes of the glycosylation defects in the Lec5 and Lec9 mutants have been elusive, even though for both cell lines a defect in dolichol formation from polyprenol was previously established. We recently found that dolichol synthesis from polyprenol occurs in three steps consisting of the conversion of polyprenol to polyprenal by DHRSX, the reduction of polyprenal to dolichal by SRD5A3, and the reduction of dolichal to dolichol, again by DHRSX. This led us to investigate defective dolichol synthesis in Lec5 and Lec9 cells. Both cell lines showed increased levels of polyprenol and its derivatives, concomitant with decreased levels of dolichol and derivatives, but no change in polyprenal levels, suggesting DHRSX deficiency. Accordingly, N-glycan synthesis and changes in polyisoprenoid levels were corrected by complementation with human DHRSX but not with SRD5A3. Furthermore, the typical polyprenol dehydrogenase and dolichal reductase activities of DHRSX were absent in membrane preparations derived from Lec5 and Lec9 cells, while the reduction of polyprenal to dolichal, catalyzed by SRD5A3, was unaffected. Long-read whole genome sequencing of Lec5 and Lec9 cells did not reveal mutations in the ORF of SRD5A3, but the genomic region containing DHRSX was absent. Lastly, we established the sequence of Chinese hamster DHRSX and validated that this protein has similar kinetic properties to the human enzyme. Our work therefore identifies the basis of the dolichol synthesis defect in Chinese hamster ovary Lec5 and Lec9 cells.

The N-glycosylation defect in Lec5 and Lec9 CHO cells is caused by absence of the DHRSX gene

Glycosylation-deficient Chinese hamster ovary (CHO) cell lines have been instrumental in the discovery of N-glycosylation machinery. Yet, the molecular causes of the glycosylation defects in the Lec5 and Lec9 mutants have been elusive, even though for both cell lines a defect in dolichol formation from polyprenol was previously established. We recently found that dolichol synthesis from polyprenol occurs in three steps consisting of the conversion of polyprenol to polyprenal by DHRSX, the reduction of polyprenal to dolichal by SRD5A3 and the reduction of dolichal to dolichol, again by DHRSX. This led us to investigate defective dolichol synthesis in Lec5 and Lec9 cells. Both cell lines showed increased levels of polyprenol and its derivatives, concomitant with decreased levels of dolichol and derivatives, but no change in polyprenal levels, suggesting DHRSX deficiency. Accordingly, N-glycan synthesis and changes in polyisoprenoid levels were corrected by complementation with human DHRSX but not with SRD5A3. Furthermore, the typical polyprenol dehydrogenase and dolichal reductase activities of DHRSX were absent in membrane preparations derived from Lec5 and Lec9 cells, while the reduction of polyprenal to dolichal, catalyzed by SRD5A3, was unaffected. Long-read whole genome sequencing of Lec5 and Lec9 cells did not reveal mutations in the ORF of SRD5A3, but the genomic region containing DHRSX was absent. Lastly, we established the sequence of Chinese hamster DHRSX and validated that this protein has similar kinetic properties to the human enzyme. Our work therefore identifies the basis of the dolichol synthesis defect in CHO Lec5 and Lec9 cells.

Biological activities of the antiviral protein BE27 from sugar beet (Beta vulgaris L.)

The ribosome inactivating protein BE27 displays several biological activities in vitro that could result in a broad action against several types of pathogens. Beetin 27 (BE27), a ribosome-inactivating protein (RIP) from sugar beet (Beta vulgaris L.) leaves, is an antiviral protein induced by virus and signaling compounds such as hydrogen peroxide and salicylic acid. Its role as a defense protein has been attributed to its RNA polynucleotide:adenosine glycosidase activity. Here we tested other putative activities of BE27 that could have a defensive role against pathogens finding that BE27 displays rRNA N-glycosidase activity against yeast and Agrobacterium tumefaciens ribosomes, DNA polynucleotide:adenosine glycosidase activity against herring sperm DNA, and magnesium-dependent endonuclease activity against the supercoiled plasmid PUC19 (nicking activity). The nicking activity could be a consequence of an unusual conformation of the BE27 active site, similar to that of PD-L1, a RIP from Phytolacca dioica L. leaves. Additionally, BE27 possesses superoxide dismutase activity, thus being able to produce the signal compound hydrogen peroxide. BE27 is also toxic to COLO 320 cells, inducing apoptosis in these cells by either activating the caspase pathways and/or inhibiting protein synthesis. The combined effect of these biological activities could result in a broad action against several types of pathogens such as virus, bacteria, fungi or insects.

Ribosomal versus non-ribosomal cellular antigens: factors determining efficiency of indirect presentation to CD4+ T cells

Proteins released from dying cells can be taken up and presented by antigen-presenting cells (APC) to T cells. While the presentation of such self antigens may lead to beneficial anti-tumour responses, in autoimmune disease it leads to pathological immune responses. The sub-set of self proteins targeted in autoimmune disease is circumscribed, and certain cellular components such as ribonucleoprotein (RNP) complexes are often targeted. Although explanations for this antigen selectivity have been proposed, there has been little direct testing of these hypotheses. We and others previously showed that ribosomal proteins, targeted in autoimmune disease, are also targets of anti-tumour T-cell responses. We asked whether particular properties of ribosomal proteins such as incorporation into RNP complexes or sub-cellular localization enhance ribosomal protein presentation by APC to CD4(+) T cells. Ribosomal protein antigens within purified intact ribosomes or free of the ribosomes were equally well taken up and presented by APC, demonstrating that inclusion of ribosomal proteins into an RNP complex does not confer an advantage. However, antigens localized to ribosomes within apoptotic cells were less efficiently taken up and presented by APC than the same antigens localized diffusely throughout the cell. This suggests that presentation of ribosomal proteins is somehow down-regulated, possibly to facilitate presentation of other less-abundant intracellular proteins. Consequently, the explanation for the frequent targeting of ribosomal proteins by both autoimmune and anti-tumour T-cell responses is not at the level of uptake from apoptotic cells and must be sought elsewhere.

Point mutation in essential genes with loss or mutation of the second allele: relevance to the retention of tumor-specific antigens

Antigens that are tumor specific yet retained by tumor cells despite tumor progression offer stable and specific targets for immunologic and possibly other therapeutic interventions. Therefore, we have studied two CD4(+) T cell-recognized tumor-specific antigens that were retained during evolution of two ultraviolet-light-induced murine cancers to more aggressive growth. The antigens are ribosomal proteins altered by somatic tumor-specific point mutations, and the progressor (PRO) variants lack the corresponding normal alleles. In the first tumor, 6132A-PRO, the antigen is encoded by a point-mutated L9 ribosomal protein gene. The tumor lacks the normal L9 allele because of an interstitial deletion from chromosome 5. In the second tumor, 6139B-PRO, both alleles of the L26 gene have point mutations, and each encodes a different tumor-specific CD4(+) T cell-recognized antigen. Thus, for both L9 and L26 genes, we observe "two hit" kinetics commonly observed in genes suppressing tumor growth. Indeed, reintroduction of the lost wild-type L9 allele into the 6132A-PRO variant suppressed the growth of the tumor cells in vivo. Since both L9 and L26 encode proteins essential for ribosomal biogenesis, complete loss of the tumor-specific target antigens in the absence of a normal allele would abrogate tumor growth.

The immunoprotective MHC II epitope of a chemically induced tumor harbors a unique mutation in a ribosomal protein

CD4(+) T lymphocyte clones, generated from mice immunized with the methylcholanthrene-induced fibrosarcoma Meth A (H-2(d)), are restricted by I-E(d) and recognize a unique antigen on Meth A. The antigen has been purified and characterized as the ribosomal protein L11. The antigenic epitope is contained within the sequence EYELRKHNFSDTG and is generated by substitution of Asn by His (italic) caused by a single point mutation. The tumor contains the wild-type and the mutated alleles. Immunization of BALB/cJ mice with the mutated epitope but not with the wild-type epitope protects mice against a subsequent challenge with the Meth A sarcoma. Adoptive transfer of CD4(+) clones into BALB/c mice renders the mice specifically resistant to Meth A sarcoma. The mutated L11 epitope is thus shown to be an immunoprotective epitope in vivo by several criteria.

Ribosome-inactivating proteins (RIPs) of wild Oregon cucumber (Marah oreganus)

Two type 1 RIPs, designated as MOR-I and MOR-II, have been isolated from Marah oreganus (manroot) seed extract. They are similar but not identical to trichosanthin, a type 1 RIP in the same family. MOR-I and MOR-II are monomeric proteins with molecular weights of 27989.0 and 27632.8 respectively and have pI values greater than 8.8. MOR-I and MOR-II inhibit cell-free protein synthesis with IC50s of 0.063 and 0.071 nM, respectively, and are relatively stable with respect to temperature and pH variations. They share a conserved N-terminal amino acid sequence (D-SF-LS) and cross-react with goat anti-trichosanthin polyclonal serum.

Isolation, cDNA cloning, biological properties, and carbohydrate binding specificity of sieboldin-b, a type II ribosome-inactivating protein from the bark of Japanese elderberry (Sambucus sieboldiana)

A type II ribosome-inactivating protein (RIP) was isolated from the bark tissue of Japanese elderberry (Sambucus sieboldiana) and named sieboldin-b. Sieboldin-b is a heterodimeric protein consisting of 27- and 33-kDa subunits and showed strong ribosome-inactivating activity in vitro but did not show in vivo toxicity. The amino acid sequence of sieboldin-b deduced from the structure of the cDNA showed that both subunits of sieboldin-b are encoded on a single precursor polypeptide. Sieboldin-b has a structure homologous with the Neu5Ac(alpha 2-6)Gal/GalNAc-specific bark lectin from S. sieboldiana (SSA) and also typical type II RIPs such as ricin and abrin. Detailed analyses of carbohydrate binding properties of sieboldin-b revealed that sieboldin-b binds to Gal/GalNAc, similar to ricin/abrin, in spite of its highly homologous structure with SSA. The biological properties of these toxins/lectins are compared, and the possible explanation for such diversity is discussed.

Investigation of ribosome binding by the Shiga toxin A1 subunit, using competition and site-directed mutagenesis

The enzymatic subunit of Shiga toxin (StxA1) is a member of the ribosome-inactivating protein (RIP) family, which includes the ricin A chain as well as other examples of plant toxins. StxA1 catalytically depurinates a well-conserved GAGA tetra-loop of 28S rRNA which lies in the acceptor site of eukaryotic ribosomes. The specific activities of native StxA1, as well as mutated forms of the enzyme with substitutions in catalytic site residues, were measured by an in vitro translation assay. Electroporation was developed as an alternative method for the delivery of purified A1 polypeptides into Vero cells. Site-directed mutagenesis coupled with N-bromosuccinimide modification indicated that the sole tryptophan residue of StxA1 is required for binding it to the 28S rRNA backbone. Northern analysis established that the catalytic site substitutions reduced enzymatic activity by specifically interfering with the capacity of StxA1 to depurinate 28S rRNA. Ribosomes were protected from StxA1 by molar excesses of tRNA and free adenine, indicating that RIPs have the capacity to enter the acceptor site groove prior to binding and depurinating the GAGA tetra-loop.

A unique tumor antigen produced by a single amino acid substitution

Mice immunized against a cancer recognize antigens unique to that cancer, but the molecular structures of such antigens are unknown. We isolated CD4+ T cell clones recognizing an antigen uniquely expressed on the UV-induced tumor 6132A; some clones inhibited the growth of tumors bearing the specific antigen. A T cell hybridoma was used to purify this antigen from nuclear extracts by RP-HPLC and SDS-PAGE using T cell immunoblot assays. A partial amino acid sequence was nearly identical to a sequence in ribosomal protein L9. The cDNA sequence of L9 from 6132A PRO cells differed from the normal sequence at one nucleotide; this mutation encoded histidine instead of leucine at position 47. A synthetic peptide containing this mutation was over 1000-fold more stimulatory of T cells than was the wild-type peptide. These results indicate that this unique tumor antigen is derived from a single amino acid substitution in a cellular protein.

Characterization of 3'-azido-3'-deoxythymidine inhibition of ricin and Pseudomonas exotoxin A toxicity in CHO and Vero cells

Ricin (RIC), modeccin (MOD), Pseudomonas exotoxin A (PE), and diphtheria toxin (DT) are protein toxins that enter cells by receptor-mediated endocytosis. After intracellular transport and membrane translocation to the cytosol, these toxins inhibit protein synthesis by enzymatically removing a specific adenine residue from ribosomal RNA (RIC, MOD), or by ADP-ribosylation of elongation factor-2 (PE, DT). Recently, Thompson and Pace (1992) reported that AZT (3'-azido-3'-deoxythymidine) inhibited RIC toxicity in Vero cells, and this inhibition was not due to a block of RIC enzymatic activity. This paper extends these findings and examines the effects of AZT treatment on the toxicities of other protein toxins in Chinese hamster ovary (CHO) and Vero cell lines. AZT treatment did not significantly alter the toxicity of DT or MOD in either cell line, but it markedly reduced RIC and PE toxicity in both cell lines. The ID50 values (concentration of toxin required to inhibit protein synthesis by 50%) for RIC and PE in CHO cells increased approximately 6.5- and 12.5-fold, respectively; while in Vero cells the ID50 values increased ca. 8.5- and 4.5-fold, respectively. Results of further studies revealed differences in the mechanisms by which AZT inhibited RIC and PE toxicity. Results of cell-free translation indicated that, unlike its effects on RIC, AZT blocked the ability of PE to perform its enzymatic activity. As AZT did not block RIC enzymatic activity, we examined the effects of AZT on earlier steps in the RIC intoxication process. AZT treatment did not inhibit cell-surface binding or internalization of [125I]-RIC. Results of kinetic studies showed that when AZT was incubated with cells at the time of RIC exposure, it caused no major change in the lag phase, during which RIC reaches the site of translocation. However, it clearly reduced the subsequent first-order reduction in the rate of protein synthesis, suggesting an effect on translocation. Monensin (an ionophore that perturbs intracellular trafficking and increases the toxicities of RIC and PE) reduced AZT protection against both toxins. Nocodazole and colchicine (agents that disrupt microtubules and some routes of intracellular trafficking) reduced the ability of AZT to inhibit RIC, but not PE, toxicity. In summary, our results suggest that (1) AZT acts within the cytosol to inhibit (directly or indirectly) the enzymatic action of PE, and (2) the AZT inhibition of RIC cytotoxicity does not involve perturbations of RIC cell-surface binding, internalization, or enzymatic activity but might result from an alteration in RIC translocation.

Cusativin, a new cytidine-specific ribonuclease accumulated in seeds of Cucumis sativus L

Dry seeds of Cucumis sativus L. were found to contain a heat-sensitive endoribonuclease of a novel type which we have named cusativin. It was purified to apparent electrophoretic homogeneity by chromatography through S-Sepharose Fast Flow, Sephadex G-75, CM-Sepharose, Superdex 75-FPLC (fast protein liquid chromatography) and Mono S-FPLC. It is a single unglycosylated polypeptide chain with an apparent molecular mass (M(r)) of 22900. Polyclonal anti-cusativin antibodies raised in rabbits only reacted with melonin, the translation inhibitor from Cucumis melo L. Functional, Western blot and enzyme-linked immunosorbent assay (ELISA) analyses indicated that cusativin is present in the coat and cotyledons of dry seeds, but not in embryonic axes. Cusativin is accumulated in maturing seeds. By contrast, after seed germination there is degradation of the cusativin present in cotyledons but not that present in the seed coat. The preference of cusativin for polynucleotide cleavage was poly(C) >> poly(A) acids, poly(U) and poly(G) being unaffected by cusativin. Under the denaturing conditions used for RNA sequencing, cusativin acted only on poly(C). Cusativin proved to be useful for RNA sequencing, in particular, complementing the data obtained with RNase CL3. Cusativin represents a new class of plant RNase and, as far as we are aware, is the first plant enzyme that shows cleavage specificity for cytidine under the denaturing conditions of RNA sequencing.

Effects of ribosome-inactivating proteins on Escherichia coli and Agrobacterium tumefaciens translation systems

The effects of 30 type 1 and of 2 (ricin and volkensin) type 2 ribosome-inactivating proteins (RIPs) on Escherichia coli and Agrobacterium tumefaciens cell-free translation systems were compared with the effects on a rabbit reticulocyte translation system. The depurinating activity of RIPs on E. coli ribosomes was also evaluated. Only six type 1 RIPs inhibited endogenous mRNA-directed translational activity of E. coli lysates, with submicromolar 50% inhibitory concentrations. Four RIPs had similar activities on poly(U)-directed phenylalanine polymerization by E. coli ribosomes, and three RIPs inhibited poly(U)-directed polyphenylalanine synthesis by A. tumefaciens ribosomes, with submicromolar 50% inhibitory concentrations.

Isolation and partial characterization of nigrin b, a non-toxic novel type 2 ribosome-inactivating protein from the bark of Sambucus nigra L

The bark of Sambucus nigra L. contains a non-toxic novel type 2 ribosome-inactivating protein that we named nigrin b. In vitro, nigrin b strongly inhibited mammalian protein synthesis but did not affect plant nor bacterial protein synthesis. The protein (M(r) 58,000) contains two subunits, A (M(r) 26,000) and B (M(r) 32,000); linked by disulphide bridge(s). Nigrin b was found to be an rRNA N-glycosidase of the rRNA of intact mammalian ribosomes and shares a very good N-terminal amino-acid sequence homology with the anti-HIV-1 proteins TAP 29 and trichosanthin.

Molecular action of the type 1 ribosome-inactivating protein saporin 5 on Vicia sativa ribosomes

The type 1 ribosome-inactivating protein (RIP) saporin 5 isolated from seeds of Saponaria officinalis L. strongly inhibited translation carried out by Vicia sativa L. purified ribosomes. The toxin multidepurinated V. sativa rRNA, which upon treatment with acid aniline releases several RNA fragments including an RNA fragment of approximately 370 nucleotides the 5'-end sequence of which was 5'-GAGGAACG-3'.

High sensitivity of cultured human trophoblasts to ribosome-inactivating proteins

Many plant proteins possessing abortifacient activities were identified as ribosome-inactivating proteins (RIPs). The effect of several ribosome-inactivating proteins (saporin 6, dianthin 32, pokeweed antiviral protein from seeds, gelonin, bryodin-R, and momordin) on primary cultures of human trophoblasts and human embryonal fibroblasts and on choriocarcinoma (JAR and BeWo) and ovarian carcinoma (TG) cell lines was studied. Protein synthesis of human trophoblasts and BeWo cells was lowered by RIPs more than that of other cells. The trophoblastic receptors for estradiol were not affected by treatment of the cells with momordin. The binding and uptake of saporin 6 and momordin by BeWo and HeLa cells were not correlated to cell toxicity.

A ribosomal protein is specifically recognized by saporin, a plant toxin which inhibits protein synthesis

Many plants express enzymes which specifically remove an adenine residue from the skeleton of the 28 S RNA in the major subunit of the eukaryotic ribosome (ribosome inactivating proteins, RIPs). The site of action of RIPs (A4324 in the rRNA from rat liver) is in a loop structure whose nucleotide sequence all around the target adenine is also conserved in those species which are completely or partially insensitive to RIPs. In this paper we identify a covalent complex between saporin (the RIP extracted from Saponaria officinalis) and ribosomal proteins from yeast (Saccharomyces cerevisiae), by means of chemical crosslinking and immunological or avidin-biotin detection. The main complex (mol. wt. congruent to 60 kDa) is formed only with a protein from the 60 S subunit of yeast ribosomes, and is not detected with ribosomes from E. coli, a resistant species. This observation supports the hypothesis for a molecular recognition mechanism involving one or more ribosomal proteins, which could provide a 'receptor' site for the toxin and favour optimal binding of the target adenine A4324 to the active site of the RIP.

Alteration of an amino acid residue outside the active site of the ricin A chain reduces its toxicity towards yeast ribosomes

Yeast transformants containing integrated copies of a galactose-regulated, ricin toxin A chain (RTA) expression plasmid were constructed and used in an attempt to isolate RTA-resistant yeast mutants. Analysis of RNA from mutant strains demonstrated that approximately half contained ribosomes that had been partially modified by RTA, although all the strains analysed transcribed full-length RTA RNA. The mutant strains could have mutations in yeast genes giving rise to RTA-resistant ribosomes or they could contain alterations within the RTA-encoding DNA causing production of mutant toxin. Ribosomes isolated from mutant strains were shown to be susceptible to RTA modification in vitro suggesting that the strains contain alterations in RTA. This paper describes the detailed analysis of one mutant strain which has a point mutation that changes serine 203 to asparagine in RTA protein. Although serine 203 lies outside the proposed active site of RTA its alteration leads to the production of RTA protein with a greatly reduced level of ribosome modifying activity. This decrease in activity apparently allows yeast cells to survive expression of RTA as only a proportion of the ribosomes become modified. We demonstrate that the mutant RTA preferentially modifies 26S rRNA in free 60S subunits and has lower catalytic activity compared with native RTA when produced in Escherichia coli. Such mutations provide a valuable means of identifying residues important in RTA catalysis and of further understanding the precise mechanism of action of RTA.

Genetic analysis of a vital mammalian housekeeping locus using CHO cells that express a transfected mutant allele

We describe a novel approach for the isolation of null mutations in a vital Chinese hamster ovary (CHO) cell housekeeping gene. Our experimental strategy required introduction of an expressible DNA clone encoding a recessive emetine-resistance allele of ribosomal protein S14 into wild-type CHO cells. Transgene heterozygote (TGH) cell lines, which harbor multiple emetine-resistance S14 transgenes, survive mutations that inactivate the CHO RPS14 locus by virtue of the transgenes' biological function. Null mutations in RPS14 yield TGH clones that display the transgene's drug-resistance phenotype. A large collection of emetine-resistant clones was isolated from one TGH cell line and shown to consist of three types of S14 mutations: (1) nonsense null mutations in the RPS14 protein coding sequence; (2) missense null mutations that affect S14 amino acid residues that have been conserved stringently during eukaryotic evolution; and (3) a recurrent missense mutation that results in a new, functional RPS14 emetine-resistance allele.

Lectin-resistant CHO cells: selection of seven new mutants resistant to ricin

In attempts to isolate new CHO glycosylation mutants, selection protocols using plant lectins that bind galactose residues of cell surface carbohydrates were applied to mutagenized CHO populations. The lectins were used alone or in combination to obtain seven ricin-resistant phenotypes. Each mutant had distinctive properties compared with previously described ricin-resistant CHO cells. One of the new phenotypes was dominant in somatic cell hybrids, and the others were recessive. Complementation analyses between related lectin-resistant (LecR) phenotypes indicated that each new isolate represented a novel genotype. Five of the mutants had properties typical of new CHO glycosylation mutants. The remaining two mutants were not readily categorized. Although they did not appear to be ricin-internalization or protein-synthesis mutants, they also did not display the marked alterations in sensitivity to several lectins of different sugar specificity expected for glycosylation mutants. The seven new LecR mutants described in these studies brings the total number of different LecR CHO mutants isolated by this and other laboratories to about 40. Criteria for identifying new LecR mutations in CHO cells are discussed.