Inhibitory effects of human cystatin C on plum pox potyvirus proteases

The Potyviral Protein 6K1 Reduces Plant Proteases Activity during Turnip mosaic virus Infection

Potyviral genomes encode just 11 major proteins and multifunctionality is associated with most of these proteins at different stages of the virus infection cycle. Some potyviral proteins modulate phytohormones and protein degradation pathways and have either pro- or anti-viral/insect vector functions. Our previous work demonstrated that the potyviral protein 6K1 has an antagonistic effect on vectors when expressed transiently in host plants, suggesting plant defenses are regulated. However, to our knowledge the mechanisms of how 6K1 alters plant defenses and how 6K1 functions are regulated are still limited. Here we show that the 6K1 from Turnip mosaic virus (TuMV) reduces the abundance of transcripts related to jasmonic acid biosynthesis and cysteine protease inhibitors when expressed in Nicotiana benthamiana relative to controls. 6K1 stability increased when cysteine protease activity was inhibited chemically, showing a mechanism to the rapid turnover of 6K1 when expressed in trans. Using RNAseq, qRT-PCR, and enzymatic assays, we demonstrate TuMV reprograms plant protein degradation pathways on the transcriptional level and increases 6K1 stability at later stages in the infection process. Moreover, we show 6K1 decreases plant protease activity in infected plants and increases TuMV accumulation in systemic leaves compared to controls. These results suggest 6K1 has a pro-viral function in addition to the anti-insect vector function we observed previously. Although the host targets of 6K1 and the impacts of 6K1-induced changes in protease activity on insect vectors are still unknown, this study enhances our understanding of the complex interactions occurring between plants, potyviruses, and vectors.

Molecular Plant-Plum Pox Virus Interactions

Plum pox virus, the agent that causes sharka disease, is among the most important plant viral pathogens, affecting Prunus trees across the globe. The fabric of interactions that the virus is able to establish with the plant regulates its life cycle, including RNA uncoating, translation, replication, virion assembly, and movement. In addition, plant-virus interactions are strongly conditioned by host specificities, which determine infection outcomes, including resistance. This review attempts to summarize the latest knowledge regarding Plum pox virus-host interactions, giving a comprehensive overview of their relevance for viral infection and plant survival, including the latest advances in genetic engineering of resistant species.

Functional characterization of Cystatin C in orange-spotted grouper, Epinephelus coioides

Cystatin C is an endogenous inhibitor of cysteine proteases and widely exist in organisms. Several studies in mammals have showed that Cystatin C plays critical role in the immune defense against microorganisms. It is also well known that some fish Cystatin C have important immune regulation functions in inflammatory responses. However, the function of fish Cystatin C in virus infection as well as its underlying molecular mechanisms remain to be elucidated. In the present study, a Cystatin C gene termed Ec-CysC was identified from orange-spotted grouper, Epinephelus coioides. The full-length of Ec-CysC cDNA was 817 bp with a 387 bp open reading frame (ORF) that encoded a 129-amino acid (aa) protein, including 18-aa signal peptide and 111-aa mature polypeptide. The deduced amino acid of Ec-CysC shared three conserved domains containing Glycine at the N-terminus region, QVVAG motif in the middle and PW motif near the C-terminus region. Transcription analysis of the Ec-CysC gene showed its expression in all twelve examined tissues including liver, spleen, kidney, brain, intestine, heart, skin, muscle, fin, stomach, gill and head kidney. Its expression following stimulation with Singapore grouper iridovirus (SGIV) was further tested in spleen, the relative expression of Ec-CysC was significantly up-regulated at 12 h post-infection. The subcellular localization experiment revealed that Ec-CysC was mainly distributed in the cytoplasm in Grouper Spleen (GS) cells. In vitro, Overexpression of Ec-CysC in GS cells significantly reduced the expression of viral genes, namely, ORF162, ORF049 and ORF072. Meanwhile, we found that overexpression of Ec-CysC resulted in upward trend of expression of inflammatory cytokines TNF-a, IL-1β and IL8 during SGIV infection. Further, SGIV-inducible apoptosis and Caspase-3 activity were also weakened by overexpression Ec-CysC in fathead minnow (FHM) cells. These results indicated that Ec-CysC might have a deeper involvement in fish immune defense, and played important roles in inflammation and apoptosis induced by SGIV.

Plant Viral Proteases: Beyond the Role of Peptide Cutters

Almost half of known plant viral species rely on proteolytic cleavages as key co- and post-translational modifications throughout their infection cycle. Most of these viruses encode their own endopeptidases, proteases with high substrate specificity that internally cleave large polyprotein precursors for the release of functional sub-units. Processing of the polyprotein, however, is not an all-or-nothing process in which endopeptidases act as simple peptide cutters. On the contrary, spatial-temporal modulation of these polyprotein cleavage events is crucial for a successful viral infection. In this way, the processing of the polyprotein coordinates viral replication, assembly and movement, and has significant impact on pathogen fitness and virulence. In this mini-review, we give an overview of plant viral proteases emphasizing their importance during viral infections and the varied functionalities that result from their proteolytic activities.

The HCPro from the Potyviridae family: an enviable multitasking Helper Component that every virus would like to have

RNA viruses have very compact genomes and so provide a unique opportunity to study how evolution works to optimize the use of very limited genomic information. A widespread viral strategy to solve this issue concerning the coding space relies on the expression of proteins with multiple functions. Members of the family Potyviridae, the most abundant group of RNA viruses in plants, offer several attractive examples of viral factors which play roles in diverse infection-related pathways. The Helper Component Proteinase (HCPro) is an essential and well-characterized multitasking protein for which at least three independent functions have been described: (i) viral plant-to-plant transmission; (ii) polyprotein maturation; and (iii) RNA silencing suppression. Moreover, multitudes of host factors have been found to interact with HCPro. Intriguingly, most of these partners have not been ascribed to any of the HCPro roles during the infectious cycle, supporting the idea that this protein might play even more roles than those already established. In this comprehensive review, we attempt to summarize our current knowledge about HCPro and its already attributed and putative novel roles, and to discuss the similarities and differences regarding this factor in members of this important viral family.

Plum pox virus and sharka: a model potyvirus and a major disease

TAXONOMIC RELATIONSHIPS

Plum pox virus (PPV) is a member of the genus Potyvirus in the family Potyviridae. PPV diversity is structured into at least eight monophyletic strains.

GEOGRAPHICAL DISTRIBUTION

First discovered in Bulgaria, PPV is nowadays present in most of continental Europe (with an endemic status in many central and southern European countries) and has progressively spread to many countries on other continents.

GENOMIC STRUCTURE

Typical of potyviruses, the PPV genome is a positive-sense single-stranded RNA (ssRNA), with a protein linked to its 5' end and a 3'-terminal poly A tail. It is encapsidated by a single type of capsid protein (CP) in flexuous rod particles and is translated into a large polyprotein which is proteolytically processed in at least 10 final products: P1, HCPro, P3, 6K1, CI, 6K2, VPg, NIapro, NIb and CP. In addition, P3N-PIPO is predicted to be produced by a translational frameshift.

PATHOGENICITY FEATURES

PPV causes sharka, the most damaging viral disease of stone fruit trees. It also infects wild and ornamental Prunus trees and has a large experimental host range in herbaceous species. PPV spreads over long distances by uncontrolled movement of plant material, and many species of aphid transmit the virus locally in a nonpersistent manner.

SOURCES OF RESISTANCE

A few natural sources of resistance to PPV have been found so far in Prunus species, which are being used in classical breeding programmes. Different genetic engineering approaches are being used to generate resistance to PPV, and a transgenic plum, 'HoneySweet', transformed with the viral CP gene, has demonstrated high resistance to PPV in field tests in several countries and has obtained regulatory approval in the USA.

Differential gene expression in response to Papaya ringspot virus infection in Cucumis metuliferus using cDNA-amplified fragment length polymorphism analysis

A better understanding of virus resistance mechanisms can offer more effective strategies to control virus diseases. Papaya ringspot virus (PRSV), Potyviridae, causes severe economical losses in papaya and cucurbit production worldwide. However, no resistance gene against PRSV has been identified to date. This study aimed to identify candidate PRSV resistance genes using cDNA-AFLP analysis and offered an open architecture and transcriptomic method to study those transcripts differentially expressed after virus inoculation. The whole genome expression profile of Cucumis metuliferus inoculated with PRSV was generated using cDNA-amplified fragment length polymorphism (cDNA-AFLP) method. Transcript derived fragments (TDFs) identified from the resistant line PI 292190 may represent genes involved in the mechanism of PRSV resistance. C. metuliferus susceptible Acc. 2459 and resistant PI 292190 lines were inoculated with PRSV and subsequently total RNA was isolated for cDNA-AFLP analysis. More than 400 TDFs were expressed specifically in resistant line PI 292190. A total of 116 TDFs were cloned and their expression patterns and putative functions in the PRSV-resistance mechanism were further characterized. Subsequently, 28 out of 116 candidates which showed two-fold higher expression levels in resistant PI 292190 than those in susceptible Acc. 2459 after virus inoculation were selected from the reverse northern blot and bioinformatic analysis. Furthermore, the time point expression profiles of these candidates by northern blot analysis suggested that they might play roles in resistance against PRSV and could potentially provide valuable information for controlling PRSV disease in the future.

Mechanistic divergence between P1 proteases of the family Potyviridae

P1a and P1b are two serine proteases of Cucumber vein yellowing virus (an ipomovirus). They belong to the group of P1 factors present at the N terminus of the polyproteins of most members of the family Potyviridae. The present work compares the protease activities of P1a and P1b in different experimental systems. The findings made regarding how these two proteases work, such as the requirement for a host factor by P1a but not by P1b, underscore important differences in their catalytic activity that point towards their undergoing divergent evolution involving the acquisition of mechanistic variations. The expression of several truncated forms of P1b in bacteria and in planta helped define the protease domain of P1b, along with other important features such as its apparently in cis mode of action. Recent phylogenetic data, together with the present results, allow an appealing hypothesis to be proposed regarding P1 evolution and its involvement in potyvirid speciation.

Specific and efficient cleavage of fusion proteins by recombinant plum pox virus NIa protease

Site-specific proteases are the most popular kind of enzymes for removing the fusion tags from fused target proteins. Nuclear inclusion protein a (NIa) proteases obtained from the family Potyviridae have become promising due to their high activities and stringencies of sequences recognition. NIa proteases from tobacco etch virus (TEV) and tomato vein mottling virus (TVMV) have been shown to process recombinant proteins successfully in vitro. In this report, recombinant PPV (plum pox virus) NIa protease was employed to process fusion proteins with artificial cleavage site in vitro. Characteristics such as catalytic ability and affecting factors (salt, temperature, protease inhibitors, detergents, and denaturing reagents) were investigated. Recombinant PPV NIa protease expressed and purified from Escherichia coli demonstrated efficient and specific processing of recombinant GFP and SARS-CoV nucleocapsid protein, with site F (N V V V H Q▾A) for PPV NIa protease artificially inserted between the fusion tags and the target proteins. Its catalytic capability is similar to those of TVMV and TEV NIa protease. Recombinant PPV NIa protease reached its maximal proteolytic activity at approximately 30 °C. Salt concentration and only one of the tested protease inhibitors had minor influences on the proteolytic activity of PPV NIa protease. Recombinant PPV NIa protease was resistant to self-lysis for at least five days.

Inhibitory effects of cystatins on proteolytic activities of the Plum pox potyvirus cysteine proteinases

In an effort to develop new antiviral strategies effective against potyviruses, several cystatins were evaluated for their ability to inhibit the cysteine proteinases of Plum pox potyvirus (PPV) using in vitro proteolytic assays. The following cystatins were purified as GST fusion proteins and shown to be active against papain:oryzacystatins I and II (OCI and OCII), corn cystatin II (CCII), human stefin A (HSA), the domain 8 of tomato multicystatin (TMC-8) and a large 24kDa tomato cystatin (LTCyst). These cystatins did not inhibit the activity of purified recombinant PPV NIa proteinase, a serine-like cysteine proteinases related to the 3C proteinases of picornaviruses and to chymotrypsin. The cystatins were shown to inhibit slightly the activity of the PPV HC-Pro proteinase with CCII being the best inhibitor. However a large excess of the cystatins was required to observe any inhibition. Based on these results and on the documented pleiotropic effects of cystatins on the metabolism of plants, we conclude that they are not the best candidates for antiviral strategies targeted to viral cysteine proteinases. The availability of soluble active recombinant PPV NIa proteinase will be instrumental for the selection of other proteinase inhibitors with increased affinity and specificity for this proteinase.

Identification of an N-terminal domain of the plum pox potyvirus CI RNA helicase involved in self-interaction in a yeast two-hybrid system

Potyvirus CI RNA helicase is a protein involved in RNA genome replication and virus movement. The protein aggregates in the cytoplasm of infected cells to form typical cylindrical inclusions. A yeast two-hybrid system was used to analyse interactions of the CI RNA helicase from plum pox potyvirus (PPV) with itself and with other viral proteins. No interactions could be detected of full-length CI protein with itself or with PPV P3/6K1, NIa, NIb or CP proteins. However, positive self-interactions were detected for N-terminal fragments of the CI protein, allowing the mapping of a CI-CI binding domain to the N-terminal 177 aa of the protein. Further deletion analysis suggested that several regions of this domain contribute to the interaction. Moreover, pull-down experiments demonstrate that, at least in vitro, full-length PPV CI protein is able to self-interact in the absence of other virus or plant factors.