Type 1 ribosome-inactivating proteins from Phytolacca dioica L. leaves: differential seasonal and age expression, and cellular localization

Ribosome-Inactivating Proteins of Bougainvillea glabra Uncovered Polymorphism and Active Site Divergence

Ribosome-inactivating proteins (RIPs) are toxic proteins that can inhibit protein synthesis. RIPs purified from Bougainvillea have low nonspecific toxicity, showing promise for processing applications in the agricultural and medical fields. However, systematic research on the polymorphism of Bougainvillea RIPs is lacking, and it is worth exploring whether different isoforms differ in their active characteristics. The transcriptional and translational expression of type I RIPs in Bougainvillea glabra leaves was investigated in this study. Seven RIPs exhibited seasonal variation at both the mRNA and protein levels. The isoforms BI4 and BI6 showed the highest transcriptional expression in both the summer and autumn samples. Interestingly, BI6 was not detected in the protein level in any of the samples. However, the bioinformatics analysis showed that RIPs derived from the same species were gathered in a different cluster, and that the active sites changed among the isoforms during evolution. The significant discrepancy in Bougainvillea RIPs mainly locates at both termini of the amino acid sequence, particularly at the C terminus. Post-translational modifications may also exist in Bougainvillea RIPs. It is concluded that the reason for the polymorphism of Bougainvillea RIPs may be that these proteins are encoded by multiple genes due to genetic processes such as gene duplication and mutation. According to the results of sequence analysis, the possible functional differences of B. glabra RIP isoforms are discussed with regard to the observed discrepancy in both active sites and structures.

Antiviral Activity of PD-L1 and PD-L4, Type 1 Ribosome Inactivating Proteins from Leaves of Phytolacca dioica L. in the Pathosystem Phaseolus vulgaris-Tobacco Necrosis Virus (TNV)

Using the pathosystem Phaseolus vulgaris-tobacco necrosis virus (TNV), we demonstrated that PD-L1 and PD-L4, type-1 ribosome inactivating proteins (RIPs) from leaves of Phytolacca dioica L., possess a strong antiviral activity. This activity was exerted both when the RIPs and the virus were inoculated together in the same leaf and when they were inoculated or applied separately in the adaxial and abaxial leaf surfaces. This suggests that virus inhibition would mainly occur inside plant cells at the onset of infection. Histochemical studies showed that both PD-L1 and PD-L4 were not able to induce oxidative burst and cell death in treated leaves, which were instead elicited by inoculation of the virus alone. Furthermore, when RIPs and TNV were inoculated together, no sign of H₂O₂ deposits and cell death were detectable, indicating that the virus could have been inactivated in a very early stage of infection, before the elicitation of a hypersensitivity reaction. In conclusion, the strong antiviral activity is likely exerted inside host cells as soon the virus disassembles to start translation of the viral genome. This activity is likely directed towards both viral and ribosomal RNA, explaining the almost complete abolition of infection when virus and RIP enter together into the cells.

Ebulin-RP, a novel member of the Ebulin gene family with low cytotoxicity as a result of deficient sugar binding domains

BACKGROUND

Sambucus ebulus is a rich source of ribosome-inactivating proteins (RIPs) and RIP-related lectins generated from multiple genes. These proteins differ in their structure, enzymatic activity and sugar binding specificity.

METHODS

We have purified and characterized ebulin-RP from S. ebulus leaves and determined the amino acid sequence by cDNA cloning. Cytotoxicity was studied in a variety of cancer cells and a comparative study of the ability of ebulin-RP to bind sugars using "in vitro" and "in silico" approaches was performed.

RESULTS

Ebulin-RP is a novel heterodimeric type 2 RIP present in S. ebulus leaves together with the type 2 RIP ebulin l, which displayed rRNA N-glycosidase activity but unlike ebulin l, lacked functional sugar binding domains. As a consequence of changes in its B-chain, ebulin-RP displayed lower cytotoxicity than ebulin l towards cancer cells and induced apoptosis as the predominant pattern of cell death.

CONCLUSIONS

Ebulin-RP is a novel member of the ebulin gene family with low cytotoxicity as a result of deficient sugar binding domains. Type 2 RIP genes from Sambucus have evolved to render proteins with different sugar affinities that may be related to different biological activities and could result in an advantage for the plant.

GENERAL SIGNIFICANCE

The ebulin family of RIPs and lectins can serve as a good model for studying the evolutionary process which may have occurred in RIPs. The lack of cytotoxicity of ebulin-RP makes it a good candidate as a toxic moiety in the construction of immunotoxins and conjugates directed against specific targets.

Ribosome-Inactivating Proteins from Plants: A Historical Overview

This review provides a historical overview of the research on plant ribosome-inactivating proteins (RIPs), starting from the first studies at the end of eighteenth century involving the purification of abrin and ricin, as well as the immunological experiments of Paul Erlich. Interest in these plant toxins was revived in 1970 by the observation of their anticancer activity, which has given rise to a large amount of research contributing to the development of various scientific fields. Biochemistry analyses succeeded in identifying the enzymatic activity of RIPs and allowed for a better understanding of the ribosomal machinery. Studies on RIP/cell interactions were able to detail the endocytosis and intracellular routing of ricin, thus increasing our knowledge of how cells handle exogenous proteins. The identification of new RIPs and the finding that most RIPs are single-chain polypeptides, together with their genetic sequencing, has aided in the development of new phylogenetic theories. Overall, the biological properties of these proteins, including their abortifacient, anticancer, antiviral and neurotoxic activities, suggest that RIPs could be utilized in agriculture and in many biomedical fields, including clinical drug development.

A small RNA targets pokeweed antiviral protein transcript

Ribosome-inactivating proteins (RIPs) are a class of plant defense proteins with N-glycosidase activity (EC 3.2.2.22). Pokeweed antiviral protein (PAP) is a Type I RIP isolated from the pokeweed plant, Phytolacca americana, thought to confer broad-spectrum virus resistance in this plant. Through a combination of standard molecular techniques and RNA sequencing analysis, we report here that a small RNA binds and cleaves the open reading frame of PAP mRNA. Additionally, sRNA targeting of PAP is dependent on jasmonic acid (JA), a plant hormone important for defense against pathogen infection and herbivory. Levels of small RNA increased with JA treatment, as did levels of PAP mRNA and protein, suggesting that the small RNA functions to moderate the expression of PAP in response to this hormone. The association between JA and PAP expression, mediated by sRNA299, situates PAP within a signaling pathway initiated by biotic stress. The consensus sequence of sRNA299 was obtained through bioinformatic analysis of pokeweed small RNA sequencing. To our knowledge, this is the first account of a sRNA targeting a RIP gene.

Antifungal activity of the ribosome-inactivating protein BE27 from sugar beet (Beta vulgaris L.) against the green mould Penicillium digitatum

The ribosome-inactivating protein BE27 from sugar beet (Beta vulgaris L.) leaves is an apoplastic protein induced by signalling compounds, such as hydrogen peroxide and salicylic acid, which has been reported to be involved in defence against viruses. Here, we report that, at a concentration much lower than that present in the apoplast, BE27 displays antifungal activity against the green mould Penicillium digitatum, a necrotrophic fungus that colonizes wounds and grows in the inter- and intracellular spaces of the tissues of several edible plants. BE27 is able to enter into the cytosol and kill fungal cells, thus arresting the growth of the fungus. The mechanism of action seems to involve ribosomal RNA (rRNA) N-glycosylase activity on the sarcin-ricin loop of the major rRNA which inactivates irreversibly the fungal ribosomes, thus inhibiting protein synthesis. We compared the C-terminus of the BE27 structure with antifungal plant defensins and hypothesize that a structural motif composed of an α-helix and a β-hairpin, similar to the γ-core motif of defensins, might contribute to the specific interaction with the fungal plasma membranes, allowing the protein to enter into the cell.

Ribosome-inactivating and related proteins

Ribosome-inactivating proteins (RIPs) are toxins that act as N-glycosidases (EC 3.2.2.22). They are mainly produced by plants and classified as type 1 RIPs and type 2 RIPs. There are also RIPs and RIP related proteins that cannot be grouped into the classical type 1 and type 2 RIPs because of their different sizes, structures or functions. In addition, there is still not a uniform nomenclature or classification existing for RIPs. In this review, we give the current status of all known plant RIPs and we make a suggestion about how to unify those RIPs and RIP related proteins that cannot be classified as type 1 or type 2 RIPs.

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.

Differential expression of saporin genes upon wounding, ABA treatment and leaf development

Saporins are type 1 ribosome-inactivating proteins (RIPs: EC 3.2.2.22) produced in various organs of Saponaria officinalis L. Two distinct saporin types, saporin-L and saporin-S isoforms, were respectively purified from the intra- and extra-cellular fractions of soapwort leaves. The saporin-L isoform was lowly identical, differed for toxicity, molecular mass and amino acid composition from saporin-S proteins forming a new monophyletic group. Genes encoding both L- and S-type isoforms were cloned from leaf-specific cDNA library; the encoded products included the N-terminal diversity observed by protein sequencing and showed compatible weights with those from mass spectra. These genes were intron-less belonging to small gene families. Reverse transcription polymerase chain reaction/quantitative reverse transcription polymerase chain reaction experiments evidenced their differential expression during leaf development, wounding and abscisic acid treatment. These results suggest that the saporin-L and -S proteins may play diversified roles during stress responses.