Crystal structure of PD-L1, a ribosome inactivating protein fromPhytolacca dioicaL. Leaves with the property to induce DNA cleavage

Structure and Biological Properties of Ribosome-Inactivating Proteins and Lectins from Elder (Sambucus nigra L.) Leaves

Ribosome-inactivating proteins (RIPs) are a group of proteins with rRNA N-glycosylase activity that catalyze the removal of a specific adenine located in the sarcin–ricin loop of the large ribosomal RNA, which leads to the irreversible inhibition of protein synthesis and, consequently, cell death. The case of elderberry (Sambucus nigra L.) is unique, since more than 20 RIPs and related lectins have been isolated and characterized from the flowers, seeds, fruits, and bark of this plant. However, these kinds of proteins have never been isolated from elderberry leaves. In this work, we have purified RIPs and lectins from the leaves of this shrub, studying their main physicochemical characteristics, sequences, and biological properties. In elderberry leaves, we found one type 2 RIP and two related lectins that are specific for galactose, four type 2 RIPs that fail to agglutinate erythrocytes, and one type 1 RIP. Several of these proteins are homologous to others found elsewhere in the plant. The diversity of RIPs and lectins in the different elderberry tissues, and the different biological activities of these proteins, which have a high degree of homology with each other, constitute an excellent source of proteins that are of great interest in diagnostics, experimental therapy, and agriculture.

Strategies to Improve the Clinical Utility of Saporin-Based Targeted Toxins

Plant Ribosome-inactivating proteins (RIPs) including the type I RIP Saporin have been used for the construction of Immunotoxins (ITxs) obtained via chemical conjugation of the toxic domain to whole antibodies or by generating genetic fusions to antibody fragments/targeting domains able to direct the chimeric toxin against a desired sub-population of cancer cells. The high enzymatic activity, stability and resistance to conjugation procedures and especially the possibility to express recombinant fusions in yeast, make Saporin a well-suited tool for anti-cancer therapy approaches. Previous clinical work on RIPs-based Immunotoxins (including Saporin) has shown that several critical issues must be taken into deeper consideration to fully exploit their therapeutic potential. This review focuses on possible combinatorial strategies (chemical and genetic) to augment Saporin-targeted toxin efficacy. Combinatorial approaches may facilitate RIP escape into the cytosolic compartment (where target ribosomes are), while genetic manipulations may minimize potential adverse effects such as vascular-leak syndrome or may identify T/B cell epitopes in order to decrease the immunogenicity following similar strategies as those used in the case of bacterial toxins such as Pseudomonas Exotoxin A or as for Type I RIP Bouganin. This review will further focus on strategies to improve recombinant production of Saporin-based chimeric toxins.

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.

Extensive Evolution of Cereal Ribosome-Inactivating Proteins Translates into Unique Structural Features, Activation Mechanisms, and Physiological Roles

Ribosome-inactivating proteins (RIPs) are a class of cytotoxic enzymes that can depurinate rRNAs thereby inhibiting protein translation. Although these proteins have also been detected in bacteria, fungi, and even some insects, they are especially prevalent in the plant kingdom. This review focuses on the RIPs from cereals. Studies on the taxonomical distribution and evolution of plant RIPs suggest that cereal RIPs have evolved at an enhanced rate giving rise to a large and heterogeneous RIP gene family. Furthermore, several cereal RIP genes are characterized by a unique domain architecture and the lack of a signal peptide. This advanced evolution of cereal RIPs translates into distinct structures, activation mechanisms, and physiological roles. Several cereal RIPs are characterized by activation mechanisms that include the proteolytic removal of internal peptides from the N-glycosidase domain, a feature not documented for non-cereal RIPs. Besides their role in defense against pathogenic fungi or herbivorous insects, cereal RIPs are also involved in endogenous functions such as adaptation to abiotic stress, storage, induction of senescence, and reprogramming of the translational machinery. The unique properties of cereal RIPs are discussed in this review paper.

Biological activities of ribosome-inactivating proteins and their possible applications as antimicrobial, anticancer, and anti-pest agents and in neuroscience research

Ribosome-inactivating proteins (RIPs) are enzymes which depurinate ribosomal RNA (rRNA), thus impeding the process of translation resulting in inhibition of protein synthesis. They are produced by various organisms including plants, fungi and bacteria. RIPs from plants are linked to plant defense due to their antiviral, antifungal, antibacterial, and insecticidal activities in which they can be applied in agriculture to combat microbial pathogens and pests. Their anticancer, antiviral, embryotoxic, and abortifacient properties may find medicinal applications. Besides, conjugation of RIPs with antibodies or other carriers to form immunotoxins has been found useful to research in neuroscience and anticancer therapy.

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.