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

Cowpea lipid transfer protein 1 regulates plant defense by inhibiting the cysteine protease of cowpea mosaic virus

Many virus genomes encode proteases that facilitate infection. The molecular mechanism of plant recognition of viral proteases is largely unexplored. Using the system of Vigna unguiculata and cowpea mosaic virus (CPMV), we identified a cowpea lipid transfer protein (LTP1) which interacts with CPMV-encoded 24KPro, a cysteine protease, but not with the enzymatically inactive mutant 24KPro(C166A). Biochemical assays showed that LTP1 inhibited 24KPro proteolytic cleavage of the coat protein precursor large coat protein-small coat protein. Transient overexpression of LTP1 in cowpea reduced CPMV infection, whereas RNA interference-mediated LTP1 silencing increased CPMV accumulation in cowpea. LTP1 is mainly localized in the apoplast of uninfected plant cells, and after CPMV infection, most of the LTP1 is relocated to intracellular compartments, including chloroplast. Moreover, in stable LTP1-transgenic Nicotiana benthamiana plants, LTP1 repressed soybean mosaic virus (SMV) nuclear inclusion a protease activity, and accumulation of SMV was significantly reduced. We propose that cowpea LTP1 suppresses CPMV and SMV accumulation by directly inhibiting viral cysteine protease activity.

Proteolytic processing of plant proteins by potyvirus NIa proteases

The NIa protease of potyviruses is a chymotrypsin-like cysteine protease related to the picornavirus 3C protease. It is also a multifunctional protein known to play multiple roles during virus infection. Picornavirus 3C proteases cleave hundreds of host proteins to facilitate virus infection. However, whether or not potyvirus NIa proteases cleave plant proteins has so far not been tested. Regular expression search using the cleavage site consensus sequence [EQN]xVxH[QE]/[SGTA] for the plum pox virus (PPV) protease identified 90 to 94 putative cleavage events in the proteomes of Prunus persica (a crop severely affected by PPV), Arabidopsis thaliana, and Nicotiana benthamiana (two experimental hosts). In vitro processing assays confirmed cleavage of six A. thaliana and five P. persica proteins by the PPV protease. These proteins were also cleaved in vitro by the protease of turnip mosaic virus (TuMV), which has a similar specificity. We confirmed in vivo cleavage of a transiently expressed tagged version of AtEML2, an EMSY-like protein belonging to a family of nuclear histone readers known to be involved in pathogen resistance. Cleavage of AtEML2 was efficient and was observed in plants that coexpressed the PPV or TuMV NIa proteases or in plants that were infected with TuMV. We also showed partial in vivo cleavage of AtDUF707, a membrane protein annotated as lysine ketoglutarate reductase trans-splicing protein. Although cleavage of the corresponding endogenous plant proteins remains to be confirmed, the results show that a plant virus protease can cleave host proteins during virus infection and highlight a new layer of plant-virus interactions.

IMPORTANCE Viruses are highly adaptive and use multiple molecular mechanisms to highjack or modify the cellular resources to their advantage. They must also counteract or evade host defense responses. One well-characterized mechanism used by vertebrate viruses is the proteolytic cleavage of host proteins to inhibit the activities of these proteins and/or to produce cleaved protein fragments that are beneficial to the virus infection cycle. Even though almost half of the known plant viruses encode at least one protease, it was not known whether plant viruses employ this strategy. Using an in silico prediction approach and the well-characterized specificity of potyvirus NIa proteases, we were able to identify hundreds of putative cleavage sites in plant proteins, several of which were validated by downstream experiments. It can be anticipated that many other plant virus proteases also cleave host proteins and that the identification of these cleavage events will lead to novel antiviral strategies.

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.

ClCPI, a cysteine protease inhibitor purified from Cassia leiandra seeds has antifungal activity against Candida tropicalis by inducing disruption of the cell surface

Infections caused by Candida tropicalis have increased significantly worldwide in parallel with resistance to antifungal drugs. To overcome resistance novel drugs have to be discovered. The objective of this work was to purify and characterize a cysteine protease inhibitor from the seeds of the Amazon rainforest tree Cassia leiandra and test its inhibitory effect against C. tropicalis growth. The inhibitor, named ClCPI, was purified after ion exchange and affinity chromatography followed by ultrafiltration. ClCPI is composed of a single polypeptide chain and is not a glycoprotein. The molecular mass determined by SDS-PAGE in the absence or presence of β-mercaptoethanol and ESI-MS were 16.63 kDa and 18.362 kDa, respectively. ClCPI was stable in the pH range of 7.0-9.0 and thermostable up to 60 °C for 20 min. ClCPI inhibited cysteine proteases, but not trypsin, chymotrypsin neither alpha-amylase. Inhibition of papain was uncompetitive with a Ki of 4.1 × 10^-7 M and IC₅₀ of 8.5 × 10^-7 M. ClCPI at 2.6 × 10^-6 M reduced 50% C. tropicalis growth. ClCPI induced damages and morphological alterations in C. tropicalis cell surface, which led to death. These results suggest that ClCPI have great potential for the development of an antifungal drug against C. tropicalis.

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.

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.