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Rabbit haemorrhagic disease virus (RHDV) is a calicivirus that infects and frequently kills rabbits. Previously, we showed that the RHDV RNA-dependent RNA polymerase (RdRp) is associated with distinct, but yet uncharacterised subcellular structures and is capable of inducing a redistribution of Golgi membranes. In this study, we identified a partially hidden hydrophobic motif that determines the subcellular localisation of recombinant RHDV RdRp in transfected cells. This novel motif, 189LLWGCDVGVAVCAAAVFHNICY210, is located within the F homomorph, between the conserved F3 and A motifs of the core RdRp domain. Amino acid substitutions that decrease the hydrophobicity of this motif reduced the ability of the protein to accumulate in multiple subcellular foci and to induce a rearrangement of the Golgi network. Furthermore, preliminary molecular dynamics simulations suggest that the RHDV RdRp could align with the negatively charged surfaces of biological membranes and undergo a conformational change involving the F homomorph. These changes would expose the newly identified hydrophobic motif so it could immerse itself into the outer leaflet of intracellular membranes.

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Rabbit haemorrhagic disease virus (RHDV) is a calicivirus that causes acute infections in both domestic and wild European rabbits (Oryctolagus cuniculus). The virus causes significant economic losses in rabbit farming and reduces wild rabbit populations. The recent emergence of RHDV variants capable of overcoming immunity to other strains emphasises the need to develop universally effective antivirals to enable quick responses during outbreaks until new vaccines become available. The RNA-dependent RNA polymerase (RdRp) is a primary target for the development of such antiviral drugs. In this study, we used cell-free in vitro assays to examine the biochemical characteristics of two rabbit calicivirus RdRps and the effects of several antivirals that were previously identified as human norovirus RdRp inhibitors. The non-nucleoside inhibitor NIC02 was identified as a potential scaffold for further drug development against rabbit caliciviruses. Our experiments revealed an unusually high temperature optimum (between 40 and 45 °C) for RdRps derived from both a pathogenic and a non-pathogenic rabbit calicivirus, possibly demonstrating an adaptation to a host with a physiological body temperature of more than 38 °C. Interestingly, the in vitro polymerase activity of the non-pathogenic calicivirus RdRp was at least two times higher than that of the RdRp of the highly virulent RHDV.

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The genome region encoding the RNA-dependent RNA polymerase 3CD-like precursor from rabbit hemorrhagic disease virus (RHDV) (isolate AST/89) was cloned and expressed in Escherichia coli using polyhistidine fusion-based vectors. The full-length recombinant 3CD-like precursor polypeptide could not be purified as a consequence of its autoproteolytic processing. A Cys-->Gly substitution of the 3C-like catalytic cysteine (C1212) impeded the cleavage and allowed the purification of the precursor at high yields using a polyhistidine fusion expression vector. Equimolar amounts of purified recombinant precursor (C1212G mutant) and mature 3D-like polymerase showed significant activity differences in genome-linked protein (VPg) uridylylation and RNA polymerization using in vitro assays. The data indicated that the precursor was more active than the mature polymerase in catalysing RHDV VPg uridylylation, whereas the latter enzyme form had higher activity than its precursor in RNA polymerization in vitro assays using a heteropolymeric RNA template.

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The structure of the RNA-dependent RNA polymerase (RdRP) from the rabbit hemorrhagic disease virus has been determined by x-ray crystallography to a 2.5-A resolution. The overall structure resembles a "right hand," as seen before in other polymerases, including the RdRPs of polio virus and hepatitis C virus. Two copies of the polymerase are present in the asymmetric unit of the crystal, revealing active and inactive conformations within the same crystal form. The fingers and palm domains form a relatively rigid unit, but the thumb domain can adopt either "closed" or "open" conformations differing by a rigid body rotation of approximately 8 degrees. Metal ions bind at different positions in the two conformations and suggest how structural changes may be important to enzymatic function in RdRPs. Comparisons between the structures of the alternate conformational states of rabbit hemorrhagic disease virus RdRP and the structures of RdRPs from hepatitis C virus and polio virus suggest novel structure-function relationships in this medically important class of enzymes.

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All positive-strand RNA viruses encode a RNA-dependent RNA polymerase which in most cases has been only identified on the basis of its sequence conservation. Catalytic activity has been experimentally demonstrated in only a handful of these viral proteins, including that from Rabbit hemorrhagic disease virus. Studies from our laboratory have reported that RHDV RNA polymerase produced in Escherichia coli was enzymatically active showing poly(A)-dependent poly(U) polymerase as well as RNA polymerase activity on heteropolymeric substrates. In this work, we have investigated the in vitro activity of the recombinant 3Dpol from RHDV, including ion requirements, resistance to inhibitors, substrate specificity as well as data on the initiation mechanism of the template-linked products derived from heteropolymeric RNA substrates. Our study demonstrates that in an in vitro reaction recombinant RHDV RNA polymerase generated the minus strand of the heteropolymeric RNA substrates by a "copy-back" mechanism that initiated at the template 3'-terminal OH.

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The calicivirus rabbit haemorrhagic disease virus (RHDV) possesses a 3C-like protease which processes the RHDV polyprotein. In order to monitor the proteolytic activity of the RHDV 3C-like protease on its putative target sequences, i.e. the 10 EG dipeptide bonds distributed along the large polyprotein, a new approach was carried out. Preliminary experiments showed that the luciferase gene when fused in-frame with a long gene yielded a fusion protein almost devoid of luciferase activity. This reporter system was used to test which EG dipeptide bonds were cleaved by the RHDV protease when the coding sequences of the dipeptides and their flanking sequences were inserted at the junction between the protease and luciferase genes. The coding sequences of the fusion proteins were cloned downstream of the T7 promoter and the proteolytic activity was evaluated by measuring the luciferase activity in both in vitro and 'in vivo' systems. The EG dipeptide bonds at positions 718-719, 1108-1109 and 1767-1768 were confirmed as cleavage sites and a new cleavage site EG (143-144) was identified.

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The rabbit hemorrhagic disease virus (RHDV) (isolate AST/89) RNA-dependent RNA-polymerase (3Dpol) coding region was expressed in Escherichia coli by using a glutathione S-transferase-based vector, which allowed milligram purification of a homogeneous enzyme with an expected molecular mass of about 58 kDa. The recombinant polypeptide exhibited rifampin- and actinomycin D-resistant, poly(A)-dependent poly(U) polymerase. The enzyme also showed RNA polymerase activity in in vitro reactions with synthetic RHDV subgenomic RNA in the presence or absence of an oligo(U) primer. Template-size products were synthesized in the oligo(U)-primed reactions, whereas in the absence of added primer, RNA products up to twice the length of the template were made. The double-length RNA products were double stranded and hybridized to both positive- and negative-sense probes.

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Rabbit hemorrhagic disease virus, a positive-stranded RNA virus of the family Caliciviridae, encodes a trypsin-like cysteine protease as part of a large polyprotein. Upon expression in Escherichia coli, the protease releases itself from larger precursors by proteolytic cleavages at its N and C termini. Both cleavage sites were determined by N-terminal sequence analysis of the cleavage products. Cleavage at the N terminus of the protease occurred with high efficiency at an EG dipeptide at positions 1108 and 1109. Cleavage at the C terminus of the protease occurred with low efficiency at an ET dipeptide at positions 1251 and 1252. To study the cleavage specificity of the protease, amino acid substitutions were introduced at the P2, P1, and P1' positions at the cleavage site at the N-terminal boundary of the protease. This analysis showed that the amino acid at the P1 position is the most important determinant for substrate recognition. Only glutamic acid, glutamine, and aspartic acid were tolerated at this position. At the P1' position, glycine, serine, and alanine were the preferred substrates of the protease, but a number of amino acids with larger side chains were also tolerated. Substitutions at the P2 position had only little effect on the cleavage efficiency. Cell-free expression of the C-terminal half of the ORF1 polyprotein showed that the protease catalyzes cleavage at the junction of the RNA polymerase and the capsid protein. An EG dipeptide at positions 1767 and 1768 was identified as the putative cleavage site. Our data show that rabbit hemorrhagic disease virus encodes a trypsin-like cysteine protease that is similar to 3C proteases with regard to function and specificity but is more similar to 2A proteases with regard to size.

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Expression studies conducted in vitro and in Escherichia coli led to the identification of a protease from rabbit hemorrhagic disease virus (RHDV). The gene coding for this protease was found to be located in the central part of the genome preceding the putative RNA polymerase gene. It was demonstrated that the protease specifically cuts RHDV polyprotein substrates both in cis and in trans. Site-directed mutagenesis experiments revealed that the RHDV protease closely resembles the 3C proteases of picornaviruses with respect to the amino acids directly involved in the catalytic activity as well as to the role played by histidine as part of the substrate binding pocket.

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