Evidence for retro-translocation of pokeweed antiviral protein from endoplasmic reticulum into cytosol and separation of its activity on ribosomes from its activity on capped RNA

Pokeweed antiviral protein (PAP) is a single-chain ribosome inactivating protein (RIP) that binds to ribosomes and depurinates the highly conserved alpha-sarcin/ricin loop (SRL) of the large subunit rRNA. Catalytic depurination of a specific adenine has been proposed to result in translation arrest and cytotoxicity. Here, we show that both precursor and mature forms of PAP are localized in the endoplasmic reticulum (ER) in yeast. The mature form is retro-translocated from the ER into the cytosol where it escapes degradation unlike the other substrates of the retro-translocation pathway. A mutation of a highly conserved asparagine residue at position 70 (N70A) delays ribosome depurination and the onset of translation arrest. The ribosomes are eventually depurinated, yet cytotoxicity and loss of viability are markedly absent. Analysis of the variant protein, N70A, does not reveal any decrease in the rate of synthesis, subcellular localization, or the rate of transport into the cytosol. N70A destabilizes its own mRNA, binds to cap, and blocks cap dependent translation, as previously reported for the wild-type PAP. However, it cannot depurinate ribosomes in a translation-independent manner. These results demonstrate that N70 near the active-site pocket is required for depurination of cytosolic ribosomes but not for cap binding or mRNA destabilization, indicating that the activity of PAP on capped RNA can be uncoupled from its activity on rRNA. These findings suggest that the altered active site of PAP might accommodate a narrower range of substrates, thus reducing ribotoxicity while maintaining potential therapeutic benefits.     

Correlation between the activities of five ribosome-inactivating proteins in depurination of tobacco ribosomes and inhibition of tobacco mosaic virus infection

The rRNA depurination activities of five ribosome-inactivating proteins (RIPs) were compared in vitro using yeast and tobacco leaf ribosomes as substrates. All of the RIPs (pokeweed antiviral protein (PAP), dianthin 32, tritin, barley RIP and ricin A-chain) were active on yeast ribosomes. PAP and dianthin 32 were highly active and ricin A-chain weakly active on tobacco ribosomes, whereas tritin and barley RIP were inactive. PAP and dianthin 32 were highly effective in inhibiting the formation of local lesions caused by tobacco mosaic virus (TMV) on tobacco leaves, whereas tritin, barley RIP and ricin A-chain were ineffective. The apparent anomaly between the in vitro rRNA depurination activity, but lack of antiviral activity of ricin A-chain was further investigated by assaying for rRNA depurination in situ following the topical application of the RIP to leaves. No activity was detected, a finding consistent with the apparent lack of antiviral activity of this RIP. Thus, it is concluded that there is a positive correlation between RIP-catalysed depurination of tobacco ribosomes and antiviral activity which gives strong support to the hypothesis that the antiviral activity of RIPs works through ribosome inactivation.     

The C-terminus of pokeweed antiviral protein has distinct roles in transport to the cytosol, ribosome depurination and cytotoxicity

Pokeweed antiviral protein (PAP) produced by pokeweed plants is a single-chain (type I) ribosome-inactivating protein (RIP) that depurinates ribosomes at the alpha-sarcin/ricin loop of the large rRNA, resulting in inhibition of translation. Unlike the type II RIPs, which have an active and a binding moiety, PAP has only the active moiety. The mechanism by which toxins without a binding moiety gain access to cytosolic ribosomes is not known. We set up yeast as a simple and genetically tractable system to investigate how PAP accesses ribosomes and showed that the mature form of PAP is targeted to the cytosol from the endomembrane system in yeast. In the present study, we performed a systematic deletion analysis to identify the signal required for transport of PAP to the cytosol. We demonstrate here that processing of the C-terminal extension and sequences at the C-terminus of the mature protein are critical for its accumulation in the cytosol. Using a series of PAP mutants, we identified the C-terminal signal and demonstrated that it is distinct from the sequences required for ribosome depurination and cytotoxicity. The C-terminal motif showed sequence similarity to type II RIPs that retrotranslocate from the endoplasmic reticulum to the cytosol. These results demonstrate that a conserved sequence at the C-terminus of a type I RIP mediates its transport to the cytosol and suggest that type I and II RIPs may use a common signal to enter the cytosol.     

Inhibition of Pokeweed Antiviral Protein (PAP) by Turnip Mosaic Virus Genome-linked Protein (VPg)

Pokeweed antiviral protein (PAP) from Phytolacca americana is a ribosome-inactivating protein (RIP) and an RNA N-glycosidase that removes specific purine residues from the sarcin/ricin loop of large rRNA, arresting protein synthesis at the translocation step. PAP is also a cap-binding protein and is a potent antiviral agent against many plant, animal, and human viruses. To elucidate the mechanism of RNA depurination, and to understand how PAP recognizes and targets various RNAs, the interactions between PAP and turnip mosaic virus genome-linked protein (VPg) were investigated. VPg can function as a cap analog in cap-independent translation and potentially target PAP to uncapped IRES-containing RNA. In this work, fluorescence spectroscopy and HPLC techniques were used to quantitatively describe PAP depurination activity and PAP-VPg interactions. PAP binds to VPg with high affinity (29.5 nm); the reaction is enthalpically driven and entropically favored. Further, VPg is a potent inhibitor of PAP depurination of RNA in wheat germ lysate and competes with structured RNA derived from tobacco etch virus for PAP binding. VPg may confer an evolutionary advantage by suppressing one of the plant defense mechanisms and also suggests the possible use of this protein against the cytotoxic activity of ribosome-inactivating proteins.     

Generation of pokeweed antiviral protein mutations in Saccharomyces cerevisiae: evidence that ribosome depurination is not sufficient for cytotoxicity

Pokeweed antiviral protein (PAP) is a ribosome-inactivating protein that depurinates the highly conserved α-sarcin/ricin loop in the large rRNA. Here, using site-directed mutagenesis and systematic deletion analysis from the 5′ and the 3′ ends of the PAP cDNA, we identified the amino acids important for ribosome depurination and cytotoxicity of PAP. Truncating the first 16 amino acids of PAP eliminated its cytotoxicity and the ability to depurinate ribosomes. Ribosome depurination gradually decreased upon the sequential deletion of C-terminal amino acids and was abolished when a stop codon was introduced at Glu-244. Cytotoxicity of the C-terminal deletion mutants was lost before their ability to depurinate ribosomes. Mutations in Tyr-123 at the active site affected cytotoxicity without altering the ribosome depurination ability. Total translation was not inhibited in yeast expressing the non-toxic Tyr-123 mutants, although ribosomes were depurinated. These mutants depurinated ribosomes only during their translation and could not depurinate ribosomes in trans in a translation-independent manner. A mutation in Leu-71 in the central domain affected cytotoxicity without altering the ability to depurinate ribosomes in trans and inhibit translation. These results demonstrate that the ability to depurinate ribosomes in trans in a catalytic manner is required for the inhibition of translation, but is not sufficient for cytotoxicity.     

Pokeweed antiviral protein depurinates the sarcin/ricin loop of the rRNA prior to binding of aminoacyl-tRNA to the ribosomal A-site

Ribosome-inactivating proteins, such as the pokeweed antiviral protein (PAP), inhibit translation by depurinating the conserved sarcin/ricin loop of the large ribosomal RNA. Depurinated ribosomes are unable to bind elongation factor 2, and, thus, the translocation step of the elongation cycle is inhibited. Though the consequences of depurination are well characterized, the ribosome conformation required for depurination to take place has not been described. In this report, we correlate biochemical and genetic data to conclude that pokeweed antiviral protein depurinates the sarcin/ricin loop when the A-site of the ribosomal peptidyl-transferase center is unoccupied. We show that prior incubation of ribosomes with puromycin, an analog of the 3'-terminus of aminoacyl-tRNA, inhibits both binding and depurination by PAP in a concentration-dependent manner. Expression of PAP in the yeast strain mak8-1 results in little depurination unless the cells are lysed, a process that would promote loss of aminoacyl-tRNA from the ribosome. The mak8-1 strain is known to exhibit a higher affinity for aminoacyl-tRNA compared with wild-type cells, and therefore, its ribosomes are more resistant to PAP in vivo. These data contribute to the mechanism of action of pokeweed antiviral protein; specifically, they have uncovered the ribosomal conformation required for depurination that leads to subsequent translation inhibition.     

Depurination within the intergenic region of Brome mosaic virus RNA3 inhibits viral replication in vitro and in vivo

Pokeweed antiviral protein (PAP) is a glycosidase of plant origin that has been shown to depurinate some viral RNAs in vitro. We have demonstrated previously that treatment of Brome mosaic virus (BMV) RNAs with PAP inhibited their translation in a cell-free system and decreased their accumulation in barley protoplasts. In the current study, we map the depurination sites on BMV RNA3 and describe the mechanism by which replication of the viral RNA is inhibited by depurination. Specifically, we demonstrate that the viral replicase exhibited reduced affinity for depurinated positive-strand RNA3 compared with intact RNA3, resulting in less negative-strand product. This decrease was due to depurination within the intergenic region of RNA3, between ORF3 and 4, and distant from the 3′ terminal core promoter required for initiation of negative-strand RNA synthesis. Depurination within the intergenic region alone inhibited the binding of the replicase to full-length RNA3, whereas depurination outside the intergenic region permitted the replicase to initiate negative-strand synthesis; however, elongation of the RNA product was stalled at the abasic nucleotide. These results support a role of the intergenic region in controlling negative-strand RNA synthesis and contribute new insight into the effect of depurination by PAP on BMV replication.     

Characterization of pokeweed antiviral protein binding to mRNA cap analogs: competition with nucleotides and enhancement by translation initiation factor iso4G

Pokeweed antiviral protein (PAP) is a type I ribosomal inactivating protein (RIP). PAP binds to and depurinates the sarcin/ricin loop (SRL) of ribosomal RNA resulting in the cessation of protein synthesis. PAP has also been shown to bind to mRNA cap analogs and depurinate mRNA downstream of the cap structure. The biological role of cap binding and its possible role in PAP activity are not known. Here we show the first direct quantitative evidence for PAP binding to the cap analog m(7)GTP. We report a binding affinity of 43.3+/-0.1 nM at 25 degrees C as determined by fluorescence quenching experiments. This is similar to the values reported for wheat cap-binding proteins eIFiso4E and eIFiso4F. van't Hoff analysis of m(7)GTP-PAP equilibrium reveals a binding reaction that is enthalpy driven and entropy favored with TDeltaS degrees contributing 15% to the overall value of DeltaG degrees . This is in contrast to the wheat cap-binding proteins which are enthalpically driven in the DeltaG degrees for binding. Competition experiments indicate that ATP and GTP compete for the cap-binding site on PAP with slightly different affinities. Fluorescence studies of PAP-eIFiso4G binding reveal a protein-protein interaction with a K(d) of 108.4+/-0.3 nM. eIFiso4G was shown to enhance the interaction of PAP with m(7)GTP cap analog by 2.4-fold. These results suggest the involvement of PAP-translation initiation factor complexes in RNA selection and depurination.     

First structural evidence of sequestration of mRNA cap structures by type 1 ribosome inactivating protein from Momordica balsamina

This is the first structural evidence of recognition of mRNA cap structures by a ribosome inactivating protein. It is well known that a unique cap structure is formed at the 5′ end of mRNA for carrying out various processes including mRNA maturation, translation initiation, and RNA turnover. The binding studies and crystal structure determinations of type 1 ribosome inactivating protein (RIP‐1) from Momordica balsamina (MbRIP‐1) were carried out with mRNA cap structures including (i) N7‐methyl guanine (m7G), (ii) N7‐methyl guanosine diphosphate (m7GDP), and (iii) N7‐methyl guanosine triphosphate (m7GTP). These compounds showed affinities to MbRIP‐1 at nanomolar concentrations. The structure determinations of the complexes of MbRIP‐1 with m7G, m7GDP, and m7GTP at 2.65, 1.77, and 1.75 Å resolutions revealed that all the three compounds bound to MbRIP‐1 in the substrate binding site at the positions which are slightly shifted towards Glu85 as compared to those of rRNA substrates. In this position, Glu85 forms several hydrogen bonds with guanine moiety while N‐7 methyl group forms van der Waals contacts. However, the guanine rings are poorly stacked in these complexes. Thus, the mode of binding by MbRIP‐1 to mRNA cap structures is different which results in the inhibition of depurination. Since some viruses are known to exploit the capping property of the host, this action of MbRIP‐1 may have implications for the antiviral activity of this protein in vivo. The understanding of the mode of binding of MbRIP‐1 to cap structures may also assist in the design of anti‐viral agents. Proteins 2012. Proteins 2013. © 2012 Wiley Periodicals, Inc.     

Depurination of plant ribosomes by pokeweed antiviral protein

Mammalian ribosomes have been shown to be enzymatically modified by ribosomal inactivating protein (RIPs) via specific depurination of rRNA. Here we report that ribosomes isolated from wheat germ contain intact and undepurinated rRNA and are depurinated by pokeweed antiviral protein (PAP). Pokeweed ribosomes isolated under the same conditions are depurinated. Total RNA isolated from pokeweed in the presence of strong denaturants was found to pbe partially depurinated. We conclude that wheat germ ribosomes are resistant to the endogenous RIP, tritin, but are sensitive to PAP and that pokeweed ribosomes can be depurinated by the N-glycosidase activity of endogenous PAP during isolation.     

Expression of a truncated form of ribosomal protein L3 confers resistance to pokeweed antiviral protein and the Fusarium mycotoxin deoxynivalenol

The contamination of important agricultural products such as wheat, barley, or maize with the trichothecene mycotoxin deoxynivalenol (DON) due to infection with Fusarium species is a worldwide problem. Trichothecenes inhibit protein synthesis by targeting ribosomal protein L3. Pokeweed antiviral protein (PAP), a ribosome-inactivating protein binds to L3 to depurinate the alpha-sarcin/loop of the large rRNA. Plants transformed with the wild-type PAP show lesions and express very low levels of PAP because PAP autoregulates its expression by destabilizing its own mRNA. We show here that transgenic tobacco plants expressing both the wild-type PAP and a truncated form of yeast L3 (L3delta) are phenotypically normal. PAP mRNA and protein levels are very high in these plants, indicating that L3delta suppresses the autoregulation of PAP mRNA expression. Ribosomes are not depurinated in the transgenic plants expressing PAP and L3delta, even though PAP is associated with ribosomes. The expression of the endogenous tobacco ribosomal protein L3 is up-regulated in these plants and they are resistant to the Fusarium mycotoxin DON. These results demonstrate that expression of an N-terminal fragment of yeast L3 leads to trans-dominant resistance to PAP and the trichothecene mycotoxin DON, providing evidence that both toxins target L3 by a common mechanism.     

Depurination of ribosomal RNA and inhibition of viral RNA translation by an antiviral protein of Celosia cristata

An antiviral protein (25 kD) isolated from leaves of Celosia cristata (CCP 25) was tested for depurination study on ribosomal RNA from yeast. Ribosomal RNA yielded 360 nucleotide base fragment after treatment with CCP 25 indicating that CCP 25 was a ribosome inactivating protein. CCP 25 also inhibited translation of brome mosaic virus (BMV) and pokeweed mosaic virus (PMV) RNAs in rabbit reticulocyte translation system. The radioactive assay showed that incorporation of [35S]-methionine was less in translation proteins of BMV nucleic acid when CCP 25 was added to translation system. This indicated that antiviral protein from Celosia cristata not only depurinated ribosomal RNA but also inhibited translation of viral RNA in vitro.     

Isolation and characterization of an RIP (ribosome-inactivating protein)-like protein from tobacco with dual enzymatic activity

Ribosome-inactivating proteins (RIPs) are N-glycosidases that remove a specific adenine from the sarcin/ricin loop of the large rRNA, thus arresting protein synthesis at the translocation step. In the present study, a protein termed tobacco RIP (TRIP) was isolated from tobacco (Nicotiana tabacum) leaves and purified using ion exchange and gel filtration chromatography in combination with yeast ribosome depurination assays. TRIP has a molecular mass of 26 kD as evidenced by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and showed strong N-glycosidase activity as manifested by the depurination of yeast rRNA. Purified TRIP showed immunoreactivity with antibodies of RIPs from Mirabilis expansa. TRIP released fewer amounts of adenine residues from ribosomal (Artemia sp. and rat ribosomes) and non-ribosomal substrates (herring sperm DNA, rRNA, and tRNA) compared with other RIPs. TRIP inhibited translation in wheat (Triticum aestivum) germ more efficiently than in rabbit reticulocytes, showing an IC50 at 30 ng in the former system. Antimicrobial assays using highly purified TRIP (50 microg mL(-1)) conducted against various fungi and bacterial pathogens showed the strongest inhibitory activity against Trichoderma reesei and Pseudomonas solancearum. A 15-amino acid internal polypeptide sequence of TRIP was identical with the internal sequences of the iron-superoxide dismutase (Fe-SOD) from wild tobacco (Nicotiana plumbaginifolia), Arabidopsis, and potato (Solanum tuberosum). Purified TRIP showed SOD activity, and Escherichia coli Fe-SOD was observed to have RIP activity too. Thus, TRIP may be considered a dual activity enzyme showing RIP-like activity and Fe-SOD characteristics.     

Homodimerization of pokeweed antiviral protein as a mechanism to limit depurination of pokeweed ribosomes

Ribosome inactivating proteins are glycosidases synthesized by many plants and have been hypothesized to serve in defence against pathogens. These enzymes catalytically remove a conserved purine from the sarcin/ricin loop of the large ribosomal RNA, which has been shown in vitro to limit protein synthesis. The resulting toxicity suggests that plants may possess a mechanism to protect their ribosomes from depurination during the synthesis of these enzymes. For example, pokeweed antiviral protein (PAP) is cotranslationally inserted into the lumen of the endoplasmic reticulum and travels via the endomembrane system to be stored in the cell wall. However, some PAP may retrotranslocate across the endoplasmic reticulum membrane to be released back into the cytosol, thereby exposing ribosomes to depurination. In this work, we isolated and characterized a complexed form of the enzyme that exhibits substantially reduced activity. We showed that this complex is a homodimer of PAP and that dimerization involves a peptide that contains a conserved aromatic amino acid, tyrosine 123, located in the active site of the enzyme. Bimolecular fluorescence complementation demonstrated that the homodimer may form in vivo and that dimerization is prevented by the substitution of tyrosine 123 for alanine. The homodimer is a minor form of PAP, observed only in the cytosol of cells and not in the apoplast. Taken together, these data support a novel mechanism for the limitation of depurination of autologous ribosomes by molecules of the protein that escape transport to the cell wall by the endomembrane system.     

Pokeweed antiviral protein isoforms PAP-I, PAP-II, and PAP-III depurinate RNA of human immunodeficiency virus (HIV)-1.

Pokeweed antiviral protein (PAP) is a naturally occurring broad-spectrum antiviral agent with potent anti-human immunodeficiency virus (HIV)-1 activity by an as yet undeciphered molecular mechanism. In the present study, we sought to determine if PAP is capable of recognizing and depurinating viral RNA. Depurination of viral RNA was monitored by directly measuring the amount of the adenine base released from the viral RNA species using quantitative high-performance liquid chromatography. Our findings presented herein provide direct evidence that three different PAP isoforms from Phytolacca americana (PAP-I from spring leaves, PAP-II from early summer leaves, and PAP-III from late summer leaves) cause concentration-dependent depurination of genomic RNA (63 to 400 pmols of adenine released per micrograms of RNA) purified from human immunodeficiency virus type-I (HIV-I), plant virus (tobacco mosaic virus (TMV), and bacteriophage (MS 2). In contrast to the three PAP isoforms, ricin A chain (RTA) failed to cause detectable depurination of viral RNA even at 5 microM, although it was as effective as PAP in inhibiting protein synthesis in cell-free translation assays. PAP-I, PAP-II, and PAP-III (but not RTA) inhibited the replication of HIV-1 in human peripheral blood mononuclear cells with IC(50) values of 17 nM, 25 nM, and 16 nM, respectively. These findings indicate that the highly conserved active site residues responsible for the depurination of rRNA by PAP or RTA are not sufficient for the recognition and depurination of viral RNA. Our study prompts the hypothesis that the potent antiviral activity of PAP may in part be due to its unique ability to extensively depurinate viral RNA, including HIV-1 RNA.