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

Plant Ribosome-Inactivating Proteins: Progesses, Challenges and Biotechnological Applications (and a Few Digressions)

Plant ribosome-inactivating protein (RIP) toxins are EC3.2.2.22 N-glycosidases, found among most plant species encoded as small gene families, distributed in several tissues being endowed with defensive functions against fungal or viral infections. The two main plant RIP classes include type I (monomeric) and type II (dimeric) as the prototype ricin holotoxin from Ricinus communis that is composed of a catalytic active A chain linked via a disulphide bridge to a B-lectin domain that mediates efficient endocytosis in eukaryotic cells. Plant RIPs can recognize a universally conserved stem-loop, known as the α-sarcin/ ricin loop or SRL structure in 23S/25S/28S rRNA. By depurinating a single adenine (A4324 in 28S rat rRNA), they can irreversibly arrest protein translation and trigger cell death in the intoxicated mammalian cell. Besides their useful application as potential weapons against infected/tumor cells, ricin was also used in bio-terroristic attacks and, as such, constitutes a major concern. In this review, we aim to summarize past studies and more recent progresses made studying plant RIPs and discuss successful approaches that might help overcoming some of the bottlenecks encountered during the development of their biomedical applications.

Pokeweed antiviral protein: its cytotoxicity mechanism and applications in plant disease resistance

Pokeweed antiviral protein (PAP) is a 29 kDa type I ribosome inactivating protein (RIP) found in pokeweed plants. Pokeweed produces different forms of PAP. This review focuses on the spring form of PAP isolated from Phytolacca americana leaves. PAP exerts its cytotoxicity by removing a specific adenine from the α-sarcin/ricin loop of the large ribosomal RNA. Besides depurination of the rRNA, PAP has additional activities that contribute to its cytotoxicity. The mechanism of PAP cytotoxicity is summarized based on evidence from the analysis of transgenic plants and the yeast model system. PAP was initially found to be anti-viral when it was co-inoculated with plant viruses onto plants. Transgenic plants expressing PAP and non-toxic PAP mutants have displayed broad-spectrum resistance to both viral and fungal infection. The mechanism of PAP-induced disease resistance in transgenic plants is summarized.

Pokeweed antiviral protein, a ribosome inactivating protein: activity, inhibition and prospects

Viruses employ an array of elaborate strategies to overcome plant defense mechanisms and must adapt to the requirements of the host translational systems. Pokeweed antiviral protein (PAP) from Phytolacca americana is a ribosome inactivating protein (RIP) and is an RNA N-glycosidase that removes specific purine residues from the sarcin/ricin (S/R) loop of large rRNA, arresting protein synthesis at the translocation step. PAP is thought to play an important role in the plant's defense mechanism against foreign pathogens. This review focuses on the structure, function, and the relationship of PAP to other RIPs, discusses molecular aspects of PAP antiviral activity, the novel inhibition of this plant toxin by a virus counteraction-a peptide linked to the viral genome (VPg), and possible applications of RIP-conjugated immunotoxins in cancer therapeutics.

An N-terminal fragment of yeast ribosomal protein L3 inhibits the cytotoxicity of pokeweed antiviral protein in Saccharomyces cerevisiae

We have previously shown that ribosomal protein L3 is required for pokeweed antiviral protein (PAP), a type I ribosome inactivating protein, to bind to ribosomes and depurinate the α-sarcin/ricin loop (SRL) in yeast. Co-expression of the N-terminal 99 amino acids of yeast L3 (L3Δ99) with PAP in transgenic tobacco plants completely abolished the toxicity of PAP. In this study, we investigated the interaction between PAP and L3Δ99 in Saccharomyces cerevisiae. Yeast cells co-transformed with PAP and L3Δ99 showed markedly reduced growth inhibition and reduced rRNA depurination by PAP, compared to cells transformed with PAP alone. Co-transformation of yeast with PAP and L3Δ21 corresponding to the highly conserved N-terminal 21 amino acids of L3Δ99, reduced the cytotoxicity of PAP. PAP mRNA and protein levels were elevated and L3Δ99 or L3Δ21 mRNA and protein levels were reduced in yeast co-transformed with PAP and L3Δ99 or with PAP and L3Δ21, respectively. PAP interacted with L3Δ21 in yeast cells in vivo and by Biacore analysis in vitro, suggesting that the interaction between L3Δ21 and PAP may inhibit PAP-mediated depurination of the SRL, leading to a reduction in the cytotoxicity of PAP.

Depurination of Brome mosaic virus RNA3 inhibits its packaging into virus particles

Packaging of the segmented RNA genome of Brome mosaic virus (BMV) into discrete particles is an essential step in the virus life cycle; however, questions remain regarding the mechanism of RNA packaging and the degree to which the viral coat protein controls the process. In this study, we used a plant-derived glycosidase, Pokeweed antiviral protein, to remove 14 specific bases from BMV RNA3 to examine the effect of depurination on virus assembly. Depurination of A771 within ORF3 and A1006 in the intergenic region inhibited coat protein binding and prevented RNA3 incorporation into particles. The disruption of interaction was not based on sequence identity, as mutation of these two purines to pyrimidines did not decrease coat protein-binding affinity. Rather, we suggest that base removal results in decreased thermodynamic stability of local RNA structures required for packaging, and that this instability is detected by coat protein. These results describe a new level of discrimination by coat protein, whereby it recognizes damage to specific viral RNA elements in the form of base removal and selects against incorporating the RNA into particles.