Expression in Escherichia coli and purification of biologically active L proteinase of foot-and-mouth disease virus

MDA5 cleavage by the Leader protease of foot-and-mouth disease virus reveals its pleiotropic effect against the host antiviral response

The RIG-I-like receptor (RLR) melanoma differentiation-associated gene 5 (MDA5) plays a key role in triggering innate antiviral response during infection by RNA viruses. MDA5 activation leads to transcription induction of type-I interferon (IFN) and proinflammatory cytokines. MDA5 has also been associated with autoimmune and autoinflammatory diseases by dysfunctional activation of innate immune response in the absence of infection. Here, we show how foot-and-mouth disease virus (FMDV) counteracts the specific antiviral effect exerted by MDA5 targeting the protein for cleavage by the viral Leader protease (Lpro). MDA5 overexpression had an inhibitory effect on FMDV infection in IFN-competent cells. Remarkably, immunostimulatory viral RNA co-immunoprecipitated with MDA5 in infected cells. Moreover, specific cleavage of MDA5 by Lpro was detected in co-transfected cells, as well as during the course of FMDV infection. A significant reduction in IFN induction associated with MDA5 cleavage was detected by comparison with a non-cleavable MDA5 mutant protein with preserved antiviral activity. The Lpro cleavage site in MDA5 was identified as the RGRAR sequence in the conserved helicase motif VI, coinciding with that recently reported for Lpro in LGP2, another member of the RLRs family involved in antiviral defenses. Interestingly, specific mutations within the MDA5 Lpro target sequence have been associated with immune disease in mice and humans. Our results reveal a pleiotropic strategy for immune evasion based on a viral protease targeting phylogenetically conserved domains of immune sensors. Identification of viral strategies aimed to disrupt MDA5 functionality may also contribute to develop new treatment tools for MDA5-related disorders.

Innate immune sensor LGP2 is cleaved by the Leader protease of foot-and-mouth disease virus

The RNA helicase LGP2 (Laboratory of Genetics and Physiology 2) is a non-signaling member of the retinoic acid-inducible gene-I (RIG-I)-like receptors (RLRs), whose pivotal role on innate immune responses against RNA viruses is being increasingly uncovered. LGP2 is known to work in synergy with melanoma differentiation-associated gene 5 (MDA5) to promote the antiviral response induced by picornavirus infection. Here, we describe the activity of the foot-and-mouth disease virus (FMDV) Leader protease (Lpro) targeting LGP2 for cleavage. When LGP2 and Lpro were co-expressed, cleavage products were observed in an Lpro dose-dependent manner while co-expression with a catalytically inactive Lpro mutant had no effect on LGP2 levels or pattern. We further show that Lpro localizes and immunoprecipitates with LGP2 in transfected cells supporting their interaction within the cytoplasm. Evidence of LGP2 proteolysis was also detected during FMDV infection. Moreover, the inhibitory effect of LGP2 overexpression on FMDV growth observed was reverted when Lpro was co-expressed, concomitant with lower levels of IFN-β mRNA and antiviral activity in those cells. The Lpro target site in LGP2 was identified as an RGRAR sequence in a conserved helicase motif whose replacement to EGEAE abrogated LGP2 cleavage by Lpro. Taken together, these data suggest that LGP2 cleavage by the Leader protease of aphthoviruses may represent a novel antagonistic mechanism for immune evasion.

Insights into Jumonji C-domain containing protein 6 (JMJD6): a multifactorial role in foot-and-mouth disease virus replication in cells

The Jumonji C-domain containing protein 6 (JMJD6) has had a convoluted history, and recent reports indicating a multifactorial role in foot-and-mouth disease virus (FMDV) infection have further complicated the functionality of this protein. It was first identified as the phosphatidylserine receptor on the cell surface responsible for recognizing phosphatidylserine on the surface of apoptotic cells resulting in their engulfment by phagocytic cells. Subsequent study revealed a nuclear subcellular localization, where JMJD6 participated in lysine hydroxylation and arginine demethylation of histone proteins and other non-histone proteins. Interestingly, to date, JMDJ6 remains the only known arginine demethylase with a growing list of known substrate molecules. These conflicting associations rendered the subcellular localization of JMJD6 to be quite nebulous. Further muddying this area, two different groups illustrated that JMJD6 could be induced to redistribute from the cell surface to the nucleus of a cell. More recently, JMJD6 was demonstrated to be a host factor contributing to the FMDV life cycle, where it was not only exploited for its arginine demethylase activity, but also served as an alternative virus receptor. This review attempts to coalesce these divergent roles for a single protein into one cohesive account. Given the diverse functionalities already characterized for JMJD6, it is likely to continue to be a confounding protein resulting in much contention going into the near future.

Interaction between FMDV Lpro and transcription factor ADNP is required for optimal viral replication

The foot-and-mouth disease virus (FMDV) leader protease (L^pro) inhibits host translation and transcription affecting the expression of several factors involved in innate immunity. In this study, we have identified the host transcription factor ADNP (activity dependent neuroprotective protein) as an L^pro interacting protein by mass spectrometry. We show that L^pro can bind to ADNP in vitro and in cell culture. RNAi of ADNP negatively affected virus replication and higher levels of interferon (IFN) and IFN-stimulated gene expression were detected. Importantly, infection with FMDV wild type but not with a virus lacking Lpro (leaderless), induced recruitment of ADNP to IFN-α promoter sites early during infection. Furthermore, we found that L^pro and ADNP are in a protein complex with the ubiquitous chromatin remodeling factor Brg-1. Our results uncover a novel role of FMDV L^pro in targeting ADNP and modulation of its transcription repressive function to decrease the expression of IFN and ISGs.

Distance effects during polyprotein processing in the complementation between defective FMDV RNAs

Passage of foot-and-mouth disease virus (FMDV) in BHK-21 cells resulted in the segmentation of the viral genome into two defective RNAs lacking part of either the L- or the capsid-coding region. The two RNAs are infectious by complementation. Electroporation of L-defective RNA in BHK-21 cells resulted in the accumulation of the precursor P3 located away from the deleted sequence. Expression of L in trans led to the processing of P3, indicating that there is a connection between L protease activity and the secondary cleavages carried out by 3C protease within P3. These results suggest that the complementation mechanism between defective RNAs is not restricted to supplying the L and capsid proteins but that distance effects on polyprotein processing events are also implicated.

The nuclear protein Sam68 is cleaved by the FMDV 3C protease redistributing Sam68 to the cytoplasm during FMDV infection of host cells

Picornavirus infection can lead to disruption of nuclear pore traffic, shut-off of cell translation machinery, and cleavage of proteins involved in cellular signal transduction and the innate response to infection. Here, we demonstrated that the FMDV 3C(pro) induced the cleavage of nuclear RNA-binding protein Sam68 C-terminus containing the nuclear localization sequence (NLS). Consequently, it stimulated the redistribution of Sam68 to the cytoplasm. The siRNA knockdown of Sam68 resulted in a 1000-fold reduction in viral titers, which prompted us to study the effect of Sam68 on FMDV post-entry events. Interestingly, Sam68 interacts with the internal ribosomal entry site within the 5' non-translated region of the FMDV genome, and Sam68 knockdown decreased FMDV IRES-driven activity in vitro suggesting that it could modulate translation of the viral genome. The results uncover a novel role for Sam68 in the context of picornaviruses and the proteolysis of a new cellular target of the FMDV 3C(pro).

Introduction of tag epitopes in the inter-AUG region of foot and mouth disease virus: effect on the L protein

Foot-and-mouth disease virus (FMDV) initiates translation from two in-frame AUG codons producing two forms of the leader (L) proteinase, Lab (starting at the first AUG) and Lb (starting at second AUG). In a previous study, we have demonstrated that a cDNA-derived mutant FMDV (A24-L1123) containing a 57-nucleotide transposon (tn) insertion between the two AUG initiation codons (inter-AUG region) was completely attenuated in cattle, suggesting that this region is involved in viral pathogenesis. To investigate the potential role of the Lab protein in attenuation, we have introduced two epitope tags (Flag: DYKDDDK and HA: YPYDVPDYA) or a small tetracysteine motif (tc: CCGPCC) into the pA24-L1123 infectious DNA clone. Mutant viruses with a small plaque phenotype similar to the parental A24-L1123 were recovered after transfection of constructs encoding the Flag tag and the tc motif. However, expression of the Flag- or tc-tagged Lab protein was abolished or greatly diminished in these viruses. Interestingly, the A24-L1123/Flag virus acquired an extra base in the inter-AUG region that resulted in new AUG codons in-frame with the second AUG, and produced a larger Lb protein. This N terminal extension of the Lb protein in mutant A24-L1123/Flag did not affect virus viability or L functions in cell culture.

Conservation of L and 3C proteinase activities across distantly related aphthoviruses

The foot-and-mouth disease virus (FMDV) leader (L) proteinase is an important virulence determinant in FMDV infections. It possesses two distinct catalytic activities: (i) C-terminal processing at the L/VP4 junction; and (ii) induction of the cleavage of translation initiation factor eIF4G, an event that inhibits cap-dependent translation in infected cells. The only other member of the Aphthovirus genus, equine rhinitis A virus (ERAV), also encodes an L protein, but this shares only 32% amino acid identity with its FMDV counterpart. Another more distantly related picornavirus, equine rhinitis B virus (ERBV), which is not classified as an aphthovirus, also encodes an L protein. Using in vitro transcription and translation analysis, we have shown that both ERAV and ERBV L proteins have C-terminal processing activity. Furthermore, expression of ERAV L, but not ERBV L, in BHK-21 cells resulted in the efficient inhibition of cap-dependent translation in these cells. We have shown that the ERAV and FMDV L proteinases induce cleavage of eIF4GI at very similar or identical positions. Interestingly, ERAV 3C also induces eIF4GI cleavage and again produces distinct products that co-migrate with those induced by FMDV 3C. The ERBV L proteinase does not induce eIF4GI cleavage, consistent with its inability to shut down cap-dependent translation. We have also shown that another unique feature of FMDV L, the stimulation of enterovirus internal ribosome entry site (IRES) activity, is also shared by the ERAV L proteinase but not by ERBV L. The functional conservation of the divergent ERAV and FMDV proteinases indicates the likelihood of a similar and important role for these enzymes in the pathogenesis of infections caused by these distantly related aphthoviruses.

Gene gun-mediate DNA vaccination against foot-and-mouth disease virus

Foot-and-mouth disease (FMD) is one of the most dangerous diseases of cloven-hoofed animals and is a constant threat in the Middle-East and other regions throughout the world despite intensive vaccination programs. In this work, we describe the ability of FMDV expression constructs to protect pigs from FMDV challenge when used as a vaccine. The construct consists of encephalomyocarditis virus (EMCV) internal ribosome entry site (IRES), the entire P1 and 2A together with 3CD sequences, all in the same reading frame. Another plasmid that was tested, carries the serotype O1 (G) VP1, Asia1 VP1 and O1 (G) 3C. Between each of the genes the 3C cleavage sequences were inserted. All constructs carried the cytomegalo virus (CMV) promoter. Using immunofluorescent and immunoblot techniques, we could show the expression and processing of viral proteins. Following the application of FMDV expression constructs into pigs skin by 'Gene Gun', pigs were partially protected from FMDV challenge.

Structure of the foot-and-mouth disease virus leader protease: a papain-like fold adapted for self-processing and eIF4G recognition

The leader protease of foot-and-mouth disease virus, as well as cleaving itself from the nascent viral polyprotein, disables host cell protein synthesis by specific proteolysis of a cellular protein: the eukaryotic initiation factor 4G (eIF4G). The crystal structure of the leader protease presented here comprises a globular catalytic domain reminiscent of that of cysteine proteases of the papain superfamily, and a flexible C-terminal extension found intruding into the substrate-binding site of an adjacent molecule. Nevertheless, the relative disposition of this extension and the globular domain to each other supports intramolecular self-processing. The different sequences of the two substrates cleaved during viral replication, the viral polyprotein (at LysLeuLys/GlyAlaGly) and eIF4G (at AsnLeuGly/ArgThrThr), appear to be recognized by distinct features in a narrow, negatively charged groove traversing the active centre. The structure illustrates how the prototype papain fold has been adapted to the requirements of an RNA virus. Thus, the protein scaffold has been reduced to a minimum core domain, with the active site being modified to increase specificity. Furthermore, surface features have been developed which enable C-terminal self-processing from the viral polyprotein.

Equine rhinovirus 1 is more closely related to foot-and-mouth disease virus than to other picornaviruses

Equine rhinovirus 1 (ERhV1) is a respiratory pathogen of horses which has an uncertain taxonomic status. We have determined the nucleotide sequence of the ERhV1 genome except for a small region at the 5' end. The predicted polyprotein was encoded by 6741 nucleotides and possessed a typical picornavirus proteolytic cleavage pattern, including a leader polypeptide. The genomic structure and predicted amino acid sequence of ERhV1 were more similar to those of foot-and-mouth disease viruses (FMDVs), the only members of the aphthovirus genus, than to those of other picornaviruses. Features which were most similar to FMDV included a 16-amino acid 2A protein which was 87.5% identical in sequence of FMDV 2A, a leader (L) protein similar in size to FMDV Lab and the possibility of a truncated L protein similar in size to FMDV Lb, and a 3C protease which recognizes different cleavage sites. However, unlike FMDV, ERhV1 had only one copy of the 3B (VPg) polypeptide. The phylogenetic relationships of the ERhV1 sequence and nucleotide sequences of representative species of the five genera of the family Picornaviridae were examined. Nucleotide sequences coding for the complete polyprotein, the RNA polymerase, and VP1 were analyzed separately. The phylogenetic trees confirmed that ERhV1 was more closely related to FMDV than to other picornaviruses and suggested that ERhV1 may be a member, albeit very distant, of the aphthovirus genus.

The foot-and-mouth disease virus leader proteinase gene is not required for viral replication

The foot-and-mouth disease virus (FMDV) leader (L) proteinase has only two known functions: (i) autocatalytic removal from the N terminus of the viral polyprotein and (ii) cleavage of the p220 subunit of the eukaryotic initiation factor 4F complex, which helps to shut off host protein synthesis. Cleavage of p220 appears to be important for picornavirus replication, since rhinoviruses and enteroviruses utilize a different proteinase (2A) to cleave p220. To explore the role of L in FMDV replication, we generated synthetic FMDV genomes lacking the L gene and tested their viability in cells. Genomes were constructed with the N-terminal Gly codon of VP4 positioned directly following either the first (Lab) or second (Lb) Met codon of the L protein. Cells transfected with synthetic RNAs lacking L and initiating with the Lab Met codon failed to produce viable virus, but cells transfected with RNAs that utilized the second AUG to drive translation of the viral polyprotein produced viable viruses. These leader-deleted viruses produced plaques on BHK cells that were slightly smaller than those produced by wild-type (WT) virus, grew to slightly lower titers than WT virus in BHK cells, shut off host protein synthesis more slowly than WT virus, and were slightly attenuated in mice. These studies indicate that the L proteinase is not essential for FMDV replication and show that in the cells and animals tested the L gene has a limited effect on virus replication.

Identification of the active-site residues of the L proteinase of foot-and-mouth disease virus

The foot-and-mouth disease virus (FMDV) leader (L) protein is involved in autocatalytic cleavage at the L/P1 junction and in the cleavage of translation initiation factor p220, a subunit of the cap-binding protein complex. It has been suggested that this proteinase has homology to the papain-like family of cysteine proteinases, and from this information, we have investigated the active-site residues by introducing specific mutations into the L gene. Mutations of Cys-23 to Ala or His-120 to Leu resulted in enzymes that lacked cis activity at the L/VP4 cleavage site, trans activity on a truncated L-P1 substrate, and p220 cleavage activity. Mutations of Cys-23 to ser or His-110 to Leu resulted in enzymes that retained some or all cis activity and had reduced p220 cleavage. These mutations were introduced separately into a full-length FMDV cDNA, and RNA transcripts derived from these cDNAs were translated in a cell-free system and transfected into cells. The C23S mutant inefficiently cleaved at the L/P1 junction and within P1, and virus obtained from transfected cells reverted to wild type. The H110L mutant cleaved the L/P1 junction almost as well as the wild-type enzyme, and virus recovered from transfected cells retained the mutation and displayed wild-type viral protein synthesis and host shut-off kinetics.