Expression and secretion of Mirabilis antiviral protein in Escherichia coli and its inhibition of in vitro eukaryotic and prokaryotic protein synthesis

Hosts and Heterologous Expression Strategies of Recombinant Toxins for Therapeutic Purposes

The production of therapeutic recombinant toxins requires careful host cell selection. Bacteria, yeast, and mammalian cells are common choices, but no universal solution exists. Achieving the delicate balance in toxin production is crucial due to potential self-intoxication. Recombinant toxins from various sources find applications in antimicrobials, biotechnology, cancer drugs, and vaccines. "Toxin-based therapy" targets diseased cells using three strategies. Targeted cancer therapy, like antibody-toxin conjugates, fusion toxins, or "suicide gene therapy", can selectively eliminate cancer cells, leaving healthy cells unharmed. Notable toxins from various biological sources may be used as full-length toxins, as plant (saporin) or animal (melittin) toxins, or as isolated domains that are typical of bacterial toxins, including Pseudomonas Exotoxin A (PE) and diphtheria toxin (DT). This paper outlines toxin expression methods and system advantages and disadvantages, emphasizing host cell selection's critical role.

Digestion of chrysanthemum stunt viroid by leaf extracts of Capsicum chinense indicates strong RNA-digesting activity

KEY MESSAGE

CSVd could not infect Nicotiana benthamiana when the plants were pretreated with crude leaf extract of Capsicum chinense 'Sy-2'. C. chinense leaves were revealed to contain strong RNA-digesting activity. Several studies have identified active antiviral and antiviroid agents in plants. Capsicum plants are known to contain antiviral agents, but the mechanism of their activity has not been determined. We aimed to elucidate the mechanism of Capsicum extract's antiviroid activity. Chrysanthemum stunt viroid (CSVd) was inoculated into Nicotiana benthamiana plants before or after treating the plants with a leaf extract of Capsicum chinense 'Sy-2'. CSVd infection was determined using quantitative reverse transcription-polymerase chain reaction (qRT-PCR) 3 weeks after inoculation. When Capsicum extract was sprayed or painted onto N. benthamiana before inoculation, it was effective in preventing infection by CSVd. To evaluate CSVd digestion activity in leaf extracts, CSVd was mixed with leaf extracts of Mirabilis, Phytolacca, Pelargonium and Capsicum. CSVd-digesting activities were examined by quantifying undigested CSVd using qRT-PCR, and RNA gel blotting permitted visualization of the digested CSVd. Only Capsicum leaf extract digested CSVd, and in the Capsicum treatment, small digested CSVd products were detected by RNA gel blot analysis. When the digesting experiment was performed for various cultivars and species of Capsicum, only cultivars of C. chinense showed strong CSVd-digesting activity. Our observations indicated that Capsicum extract contains strong RNA-digesting activity, leading to the conclusion that this activity is the main mechanism for protection from infection by CSVd through spraying or painting before inoculation. To our knowledge, this is the first report of a strong RNA-digesting activity by a plant extract.

Engineering a switchable toxin: the potential use of PDZ domains in the expression, targeting and activation of modified saporin variants

A critical problem in studying ribosome-inactivating proteins (RIPs) lies in the very limited possibility to produce them in heterologous systems. In fact, their inherent toxicity for the producing organism nearly always prevents their recombinant expression. In this study, we designed, expressed and characterized an engineered form of the RIP saporin (SapVSAV), bearing a C-terminal extra sequence that is recognized and bound by the second PDZ domain from murine PTP-BL protein (PDZ2). The co-expression of SapVSAV and PDZ2 in Escherichia coli BL21 cells greatly enhances the production of the toxin in a soluble form. The increase of production was surprisingly not due to protection from bacterial intoxication, but may arise from a stabilization effect of PDZ2 on the toxin molecule during biosynthesis. We found that once purified, SapVSAV is stable but is not toxic to free ribosomes, while it is fully active against human cancer cells. This strategy of co-expression of a toxin moiety and a soluble PDZ domain may represent a new system to increase the production of recombinant toxic proteins and could allow the selection of new extra sequences to target PDZ domains inside specific mammalian cellular domains.

Improvement of downstream processing of recombinant proteins by means of genetic engineering methods

The rapid advancement of genetic engineering has allowed to produce an impressive number of proteins on a scale which would not have been achieved by classical biotechnology. At the beginning of this development research was focussed on elucidating the mechanisms of protein overexpression. The appearance of inclusion bodies may illustrate the success. In the meantime, genetic engineering is not only expected to achieve overexpression, but to improve the whole process of protein production. For downstream processing of recombinant proteins, the synthesis of fusion proteins is of primary importance. Fusion with certain proteins or peptides may protect the target protein from proteolytic degradation and may alter its solubility. Intracellular proteins may be translocated by means of fusions with signal peptides. Affinity tags as fusion complements may render protein separation and purification highly selective. These methods as well as similar ones for improving the downstream processing of proteins will be discussed on the basis of recent literature.

Purification and properties of growth stage-dependent antiviral proteins from the leaves of Celosia cristata

Two antiviral glycoproteins, active against mechanical transmission of two tobamoviruses, tobacco mosaic virus and sunnhemp rosette virus, and citrus ring spot virus (ungrouped), were purified from the dried leaves of Celosia cristata. These proteins, called CCP-25 and CCP-27, have M(r) 25 and 27 kDa, respectively. Their concentration was found to vary between the pre-flowering and post-flowering stages of C. cristata90% lesion formation at a concentration of 20-30 µg ml(-1). They were resistant to proteases in the native state, but were readily digested when denatured. Both of them imparted actinomycin D sensitive resistance by inhibiting local lesions on Nicotiana tabacum cv. Samsun NN by tobacco mosaic virus. Their application, individually, also resulted in high resistance in systemic hosts to sunnhemp rosette virus, and citrus ring spot virus, respectively.

Shiga toxin attacks bacterial ribosomes as effectively as eucaryotic ribosomes

Several pathogenic bacteria, including Shigelladysenteriae and certain strains of Escherichia coli, produce potent class 2 ribosome inhibiting proteins (RIPs) termed Shiga toxins (Stx). The toxins are bipartite molecules composed of a single A chain (StxA) noncovalently associated with a pentamer of receptor-binding B subunits (StxB). StxA and Stx1A from E. coli are protoxins. Proteolysis generates an A1 enzyme (28 kDa) and an A2 fragment (3 kDa), which remain bound, inactivating the enzyme, until a disulfide bond linking them is reduced. Efforts to express active recombinant Stx1A1 in the cytoplasm of E. coli were very difficult and led to the hypothesis that Stx1A1 is toxic to E. coli. We created the gene for a His-tagged Stx1A1 (cStx1A1) and expressed it in E. coli from a tightly controlled expression vector. About 1-2 mg of protein can be purified in a one-step isolation from 1 L of culture. cStx1A1, RTA, and PAP exhibited similar high toxicity against the Artemia ribosomes with IC50 values near 1 nM. Surprisingly, Stx1A1 had an IC50 of 0.8 nM against E. coli ribosomes, about the same as it had for Artemia ribosomes. This is about 250 times more active than PAP against bacterial targets, making Stx1A1 the most powerful RIP toxin presently known against E. coli ribosomes.

Production of functional chick liver HMG 2a protein in Escherichia coli

An efficient Escherichia coli system for the production of a variant form of high-mobility group-2a protein (HMG 2a), having the additional 5 amino acid residues (Ala-Pro-Thr-Leu-Glu) at the NH2-terminal, has been constructed. cDNA encoding HMG 2a was ligated with the Omp A signal peptide sequence and was inserted into an inducible bacterial expression vector pSH-L. After the plasmid introduced into E. coli was expressed by temperature shift, the recombinant product was purified by trichloacetic acid precipitation followed by Bio-Rex 70 column chromatography. The purified product showed the expected NH2-terminal sequence and the superhelical activity of circular DNA similar to the authentic HMG 2a isolated from chick liver.

Isolation and characterization of two new N-glycosidase type-1 ribosome-inactivating proteins, unrelated in amino-acid sequence, from Petrocoptis species

Two new N-glycosidase type-1 ribosome-inactivating proteins (RIPs), denoted petroglaucin 1 and petrograndin, respectively, were isolated from the plants Petrocoptis glaucifolia (Lag.) Boiss sp. viscosa (Rothm.) Lainz and Petrocoptis grandiflora Rothm. These new RIPs do not share H2N-terminal amino-acid sequence homology with petroglaucin (now denoted as petroglaucin 2), the only other type-1 RIP to be isolated from P. glaucifolia (Arias et al. (1992) Planta 186, 532-540). Petroglaucin 1 shares amino-acid sequence homology with RIPs from Cucurbitaceae while petroglaucin 2 and petrograndin do so with saporins and dianthin 30 (Caryophyllaceae). The new RIPs strongly inhibited protein synthesis at subnanomolar concentrations in rabbit reticulocyte lysates and other eukaryotic cell-free systems, but they were inactive on bacterial ribosomes.

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.

Nucleotide sequence of a genomic gene encoding tritin, a ribosome-inactivating protein from Triticum aestivum

A genomic gene of tritin, a ribosome-inactivating protein (RIP) from Triticum aestivum, was cloned using a barley RIP gene as a probe. The 5'-non-coding region has potential TATA boxes and three sequences homologous to the binding sequence of the transcriptional activator protein Opaque-2 which activates maize RIP gene expression. The cloned DNA encoded tritin consists of 275 amino acids with no secretion signal sequence. The coding region of tritin was expressed in Escherichia coli using lac promoter and yielded a protein similar to the native one, as determined by SDS-polyacrylamide gel electrophoresis and immunological analysis.

Expression of a pokeweed antiviral protein in Escherichia coli and its characterization

Two expression vectors were constructed to produce a putative mature alpha-pokeweed antiviral protein (alpha-PAP) in Escherichia coli with its NH2- and COOH-terminal extrapeptides excised. One was for its intracellular expression with a methionine at its NH2-terminal. The other was for its secretion using an ompA signal peptide. The former product was purified from the total soluble proteins of the transformant with a yield of 1.74 mg/liter and the latter had a yield of 5.55 mg/liter. Both products exhibited RNA N-glycosidase activity on wheat ribosomes and inhibitory activity to protein synthesis in a rabbit reticulocyte system.

Parameters influencing the productivity of recombinant E. coli cultivations

In the past 10 to 15 years, many of the promises of microbial genetic engineering have been realized: the use of recombinant Escherichia coli has moved from the laboratory to the production facility, and the manufacture of therapeutic recombinant proteins such as human growth hormone and interleukins is a rapidly growing industry. Along with this progress, however, have come new problems to solve: bioreactor operators have discovered that large-scale cultivations of plasmid-containing bacteria do not behave in exactly the same way as those of plasmid-free cells, plasmid stability has been recognized as a major hurdle, and the protein product might not be present in a soluble form but rather as intracellular granules that resist solubilization. These and other difficulties represent a new generation of challenges for genetic engineering. However, genetic engineering can do more than solve these problems. Molecular biological techniques also have the ability to create new opportunities: to produce new compounds, to use cheaper substrates, to facilitate downstream processing, and to optimize production in new ways. The productivity of a cultivation can generally be expressed as the product of the cell density and the specific biological activity. Both of these parameters are influenced by a variety of factors. For recombinant cultivations, though, the level of biological activity, a reflection of the plasmid copy number and expression efficiency, is the more interesting and important consideration and will therefore be given more attention in our review. In this contribution, our general goal is to discuss the factors that influence the productivity of recombinant E. coli cultivations, covering parameters relating to DNA; parameters relating to protein synthesis; parameters relating to proteins; and parameters relating to downstream processing. The object is not to tell the reader how to choose the perfect plasmid, host, and cultivation conditions, but to make known the many variables involved in designing a recombinant process and to point out recent and potential advances made possible by genetic engineering. The discussion focuses on the production of a protein, but many of the same concepts apply to other cultivations of recombinant E. coli, including cases in which the desired product is not a protein or the cells have been designed for a special metabolic capability such as pollutant biodegradation.

Adenine depurination and inactivation of plant ribosomes by an antiviral protein of Mirabilis jalapa (MAP)

Mirabilis antiviral protein (MAP) is a single-chain ribosome-inactivating protein (RIP) isolated from Mirabilis jalapa L. It depurinates the 28S-like rRNAs of prokaryotes and eukaryotes. A specific modification in the 25S rRNA of M. jalapa was found to occur during isolation of ribosomes by polyacrylamide/agarose composite gel electrophoresis. Primer extension analysis revealed the modification site to be at the adenine residue corresponding to A4324 in rat 28S rRNA. The amount of endogenous MAP seemed to be sufficient to inactivate most of the homologous ribosomes. The adenine of wheat ribosomes was also found to be removed to some extent by an endogenous RIP (tritin). However, the amount of endogenous tritin seemed to be insufficient for quantitative depurination of the homologous ribosomes. Endogenous MAP could shut down the protein synthesis of its own cells when it spreads into the cytoplasm through breaks of the cells. Therefore, we speculate that MAP is a defensive agent to induce viral resistance through the suicide of its own cells.

Isolation and analysis of a genomic clone encoding a pokeweed antiviral protein

Partial cDNAs encoding a pokeweed antiviral protein were obtained by polymerase chain reaction from the poly(A)+ RNA of seeds, leaves, and roots using two specific primers based on the amino acid sequence of a pokeweed antiviral protein from the seeds (PAP-S). Using the cDNAs as a radioactive probe, 17 and 39 positive plaques were isolated from libraries containing the genomic DNA of Phytolacca americana digested with Bam HI partially and completely, respectively. The plaques were grouped into nine types by Southern hybridization. The type alpha genomic clone encodes a protein of 294 amino acids. Its amino acid sequence is similar but not identical to that of PAP-S. A comparison of the two amino acid sequences suggested that the deduced protein contains extrapeptides of 24 and 9 amino acids at the NH2 and the COOH terminals, respectively. The putative protein was expressed in Escherichia coli and shown to depurinate the specific adenine of wheat 25S rRNA, indicating that the protein encoded by a type alpha genomic clone is a functional protein exhibiting RNA N-glycosidase activity.

Ribosome–Inactivating Proteins from Plants: Present Status and Future Prospects

Plant ribosome-inactivating proteins (RIPs) are N-glycosidases which cleave the N-glycosidic bond of adenine in a specific ribosomal RNA sequence. Most commonly RIPs are single-chain proteins (type 1 RIPs), but some (type 2 RIPs) possess a galactose-specific lectin domain that binds to cell surfaces. The latter RIPs are potent toxins, the best known of which is ricin. RIPs have antiviral and abortifacient activities, and, in a widespread application, can also be linked to antibodies or ligands to form immunotoxins or conjugates specifically toxic to a given type of cell.

Pokeweed antiviral protein: ribosome inactivation and therapeutic applications

Pokeweed antiviral protein (PAP) is a ribosome-inactivating protein (RIP) that inactivates ribosomes by the removal of a single adenine from ribosomal RNA. The studies summarized in our review concern the nature and application of this novel therapeutic agent. We describe how researchers continue to elucidate the structure and biologic activity of RIPs. Pokeweed antiviral protein is among the RIPs that have been conjugated to selective monoclonal antibodies for the treatment of several human cancers and viral diseases. Clinical trials using PAP immunotoxins for the treatment of leukemia have been particularly encouraging.

Escherichia coli ribosome is inactivated by Mirabilis antiviral protein which cleaves the N-glycosidic bond at A2660 of 23 S ribosomal RNA

Ribosome-inactivating proteins (RIPs) are known to inactivate eukaryotic ribosomes, which results in the inhibition of protein synthesis, but there has been no evidence that they inactivate the ribosomes of Escherichia coli. Recently, Mirabilis antiviral protein (MAP), a RIP, has been shown to inhibit the protein synthesis of E. coli as well as eukaryotes. To elucidate its mechanism, E. coli ribosomes treated with MAP were analyzed by polyacrylamide/agarose composite gel electrophoresis and RNA sequencing using reverse transcriptase with DNA primer. The 23 S rRNAs, with an A260 value for ribosomes of 15, were completely cleaved in vitro by a 30 minute treatment with MAP at a concentration of 100 nM at 37 degrees C and a subsequent treatment with aniline. However, they were not affected by ricin A-chain under the same conditions. The primer extension of DNA polymerization stopped before A2660 of 23 S rRNA in RNA sequencing. Furthermore, both 16 S and 23 S rRNAs were cleaved by the MAP and aniline treatments when naked E. coli rRNAs were used as substrates, and the primer extension stopped before bases A2660 and A1014, respectively, in RNA sequencing. As the A2660 region has been shown to interact with the elongation factors EF-Tu and EF-G these results indicate that MAP cleaves the N-glycosidic bond at A2660 in E. coli 23 S RNA resulting in the inactivation of the ribosome.

Single-chain ribosome inactivating proteins from plants depurinate Escherichia coli 23S ribosomal RNA

The rRNA N-glycosidase activities of the catalytically active A chains of the heterodimeric ribosome inactivating proteins (RIPs) ricin and abrin, the single-chain RIPs dianthin 30, dianthin 32, and the leaf and seed forms of pokeweed antiviral protein (PAP) were assayed on E. coli ribosomes. All of the single-chain RIPs were active on E. coli ribosomes as judged by the release of a 243 nucleotide fragment from the 3' end of 23S rRNA following aniline treatment of the RNA. In contrast, E. coli ribosomes were refractory to the A chains of ricin and abrin. The position of the modification of 23S rRNA by dianthin 32 was determined by primer extension and found to be A2660, which lies in a sequence that is highly conserved in all species.

Preproabrin: genomic cloning, characterisation and the expression of the A-chain in Escherichia coli

Synthetic oligonucleotides representing all possible sequences of an N-terminal and an internal region of the A-chain of abrin C were used to generate a probe specific for abrin-related sequences using the polymerase chain reaction on Abrus precatorius genomic DNA. A lambda phage library constructed from genomic DNA isolated from leaf tissue of A. precatorius was screened and positive hybridising clones were characterised by restriction enzyme analysis. The coding regions of unique clones were characterised by DNA sequencing. One clone encodes a preproprotein closely related to abrin C with 83% similarity between the A-chain sequences. Based on similarity with the ricin toxins and Ricinus communis agglutinin, the preproabrin consists of an A-chain of 251 amino acids preceded by 34 amino acids containing an N-terminal signal peptide, followed by a 14-amino-acid linker and a B-chain of 263 amino acids. The mature A-chain of the preproabrin has been expressed cytoplasmically in Escherichia coli and the soluble recombinant protein was produced at levels exceeding 6% of total cell protein. The recombinant A-chain has been purified to homogeneity and its ability to depurinate 28S rRNA in rat liver ribosomes has been demonstrated in vitro.