Characterization of the stop codon readthrough signal of Colorado tick fever virus segment 9 RNA.

Termination codon readthrough is utilized as a mechanism of expression of a growing number of viral and cellular proteins, but in many cases the mRNA signals that promote readthrough are poorly characterized. Here, we investigated the readthrough signal of Colorado tick fever virus (CTFV) segment 9 RNA (Seg-9). CTFV is the type-species of the genus Coltivirus within the family Reoviridae and is a tick-borne, double-stranded, segmented RNA virus. Seg-9 encodes a 36-kDa protein VP9, and by readthrough of a UGA stop codon, a 65-kDa product, VP9'. Using a reporter system, we defined the minimal sequence requirements for readthrough and confirmed activity in both mammalian and insect cell-free translation systems, and in transfected mammalian cells. Mutational analysis revealed that readthrough was UGA specific, and that the local sequence context around the UGA influenced readthrough efficiency. Readthrough was also dependent upon a stable RNA stem-loop structure beginning eight bases downstream from the UGA codon. Mutational analysis of this stem-loop revealed a requirement for the stem region but not for substructures identified within the loop. Unexpectedly, we were unable to detect a ribosomal pause during translation of the CTFV signal, suggesting that the mechanism of readthrough, at least at this site, is unlikely to be dependent upon RNA secondary-structure induced ribosomal pausing at the recoded stop codon.

Echovirus 11 infection induces dramatic changes in the actin cytoskeleton of polarized Caco-2 cells.

Binding of echovirus 11 strain 207 (EV11-207) to Caco-2 monolayers results in rapid transfer of the virus to tight junctions prior to uptake. Using a confocal microscopy based-method, this study quantified the spatiotemporal distribution of actin during the time course of infection by EV11-207 in Caco-2 polarized cells. It was found that binding of EV11-207 to the apical surface resulted in rapid rearrangement of the actin cytoskeleton, concomitant with transport of the virus particles to tight junctions. By interfering with the actin network dynamics, the virus remained trapped at the cell surface, leading to abortion of infection. In addition, it was observed that at 4 h post-infection, concomitant with the detection of virus replication, actin filament was depolymerized and degraded. Finally, it was shown that the mechanisms leading to loss of actin were independent of viral genome synthesis, indicating a potential role for the viral protein synthesis seen in late infection. These data confirmed a previous study on the requirement for an intact actin cytoskeleton for EV11-207 to infect cells and reinforce the notion of actin cytoskeleton subversion by picornaviruses during infection in polarized epithelial cells.

Decay-accelerating factor binding determines the entry route of echovirus 11 in polarized epithelial cells.

The interaction between echovirus 11 strain 207 (EV11-207) and decay-accelerating factor (DAF or CD55) at the apical surface of polarized Caco-2 cells results in rapid transport of the virus to tight junctions and in its subsequent uptake. A virus mutant (EV11-207R) which differs at 6 amino acids and whose affinity for DAF is apparently significantly lower remains at the apical surface, from where its uptake occurs. Binding of EV11-207 to DAF and its transport to tight junctions result in a loss of function of the junctions. In contrast, the mutant virus EV11-207R is not transferred to tight junctions, nor does it impair the integrity of these junctions. Cholesterol depletion from the apical membrane leads to DAF aggregation and, presumably, internalization and inhibits infection by EV11-207. However, infection by EV11-207R is significantly less sensitive to cholesterol depletion than infection by EV11-207, confirming the DAF requirement for EV11-207, but not EV11-207R, to infect cells. These data strongly indicate that in the case of infection of polarized epithelial cells by echovirus 11, DAF binding appears be a key determinant in the choice of entry pathway, at least in cell culture.

Characterization of the termination-reinitiation strategy employed in the expression of influenza B virus BM2 protein.

Coupled expression of the M1 and BM2 open-reading frames (ORFs) of influenza B from the dicistronic segment 7 mRNA occurs by a process of termination-dependent reinitiation. The AUG start codon of the BM2 ORF overlaps the stop codon of the upstream M1 ORF in the pentanucleotide UAAUG, and BM2 synthesis is dependent upon translation of the M1 ORF and termination at the stop codon. Here, we have investigated the mRNA sequence requirements for BM2 expression. Termination-reinitiation is dependent upon 45 nucleotide (nt) of RNA immediately upstream of the UAAUG pentanucleotide, which includes an essential stretch complementary to 18S rRNA helix 26. Thus, similar to the caliciviruses, base-pairing between mRNA and rRNA is likely to play a role in tethering the 40S subunit to the mRNA following termination at the M1 stop codon. Consistent with this, repositioning of the M1 stop codon more than 24 nt downstream from the BM2 start codon inhibited BM2 expression. RNA structure probing revealed that the RNA upstream of the UAAUG overlap is not highly structured, but upon encountering the M1 stop codon by the ribosome, a stem-loop may form immediately 5' of the ribosome, with the 18S rRNA complementary region in the apical loop and in close proximity to helix 26. Mutational analysis reveals that the normal requirements for start site selection in BM2 expression are suspended, with little effect of initiation codon context and efficient use of noncanonical initiation codons. This suggests that the full complement of initiation factors is not required for the reinitiation process.

Novel virus discovery and genome reconstruction from field RNA samples reveals highly divergent viruses in dipteran hosts.

We investigated whether small RNA (sRNA) sequenced from field-collected mosquitoes and chironomids (Diptera) can be used as a proxy signature of viral prevalence within a range of species and viral groups, using sRNAs sequenced from wild-caught specimens, to inform total RNA deep sequencing of samples of particular interest. Using this strategy, we sequenced from adult Anopheles maculipennis s.l. mosquitoes the apparently nearly complete genome of one previously undescribed virus related to chronic bee paralysis virus, and, from a pool of Ochlerotatus caspius and Oc. detritus mosquitoes, a nearly complete entomobirnavirus genome. We also reconstructed long sequences (1503-6557 nt) related to at least nine other viruses. Crucially, several of the sequences detected were reconstructed from host organisms highly divergent from those in which related viruses have been previously isolated or discovered. It is clear that viral transmission and maintenance cycles in nature are likely to be significantly more complex and taxonomically diverse than previously expected.

Further characterisation of the translational termination-reinitiation signal of the influenza B virus segment 7 RNA.

Termination-dependent reinitiation is used to co-ordinately regulate expression of the M1 and BM2 open-reading frames (ORFs) of the dicistronic influenza B segment 7 RNA. The start codon of the BM2 ORF overlaps the stop codon of the M1 ORF in the pentanucleotide UAAUG and ∼10% of ribosomes terminating at the M1 stop codon reinitiate translation at the overlapping AUG. BM2 synthesis requires the presence of, and translation through, 45 nt of RNA immediately upstream of the UAAUG, known as the 'termination upstream ribosome binding site' (TURBS). This region may tether ribosomal 40S subunits to the mRNA following termination and a short region of the TURBS, motif 1, with complementarity to helix 26 of 18S rRNA has been implicated in this process. Here, we provide further evidence for a direct interaction between mRNA and rRNA using antisense oligonucleotide targeting and functional analysis in yeast cells. The TURBS also binds initiation factor eIF3 and we show here that this protein stimulates reinitiation from both wild-type and defective TURBS when added exogenously, perhaps by stabilising ribosome-mRNA interactions. Further, we show that the position of the TURBS with respect to the UAAUG overlap is crucial, and that termination too far downstream of the 18S complementary sequence inhibits the process, probably due to reduced 40S tethering. However, in reporter mRNAs where the restart codon alone is moved downstream, termination-reinitiation is inhibited but not abolished, thus the site of reinitiation is somewhat flexible. Reinitiation on distant AUGs is not inhibited in eIF4G-depleted RRL, suggesting that the tethered 40S subunit can move some distance without a requirement for linear scanning.

Expression of the VP2 protein of murine norovirus by a translation termination-reinitiation strategy.

BACKGROUND: Expression of the minor virion structural protein VP2 of the calicivirus murine norovirus (MNV) is believed to occur by the unusual mechanism of termination codon-dependent reinitiation of translation. In this process, following translation of an upstream open reading frame (ORF) and termination at the stop codon, a proportion of 40S subunits remain associated with the mRNA and reinitiate at the AUG of a downstream ORF, which is typically in close proximity. Consistent with this, the VP2 start codon (AUG) of MNV overlaps the stop codon of the upstream VP1 ORF (UAA) in the pentanucleotide UAAUG. PRINCIPAL FINDINGS: Here, we confirm that MNV VP2 expression is regulated by termination-reinitiation and define the mRNA sequence requirements. Efficient reintiation is dependent upon 43 nt of RNA immediately upstream of the UAAUG site. Chemical and enzymatic probing revealed that the RNA in this region is not highly structured and includes an essential stretch of bases complementary to 18S rRNA helix 26 (Motif 1). The relative position of Motif 1 with respect to the UAAUG site impacts upon the efficiency of the process. Termination-reinitiation in MNV was also found to be relatively insensitive to the initiation inhibitor edeine. CONCLUSIONS: The termination-reinitiation signal of MNV most closely resembles that of influenza BM2. Similar to other viruses that use this strategy, base-pairing between mRNA and rRNA is likely to play a role in tethering the 40S subunit to the mRNA following termination at the VP1 stop codon. Our data also indicate that accurate recognition of the VP2 ORF AUG is not a pre-requisite for efficient reinitiation of translation in this system.

Translational termination-re-initiation in viral systems.

Viruses have evolved a number of translational control mechanisms to regulate the levels of expression of viral proteins on polycistronic mRNAs, including programmed ribosomal frameshifting and stop codon readthrough. More recently, another unusual mechanism has been described, that of termination-dependent re-initiation (also known as stop-start). Here, the AUG start codon of a 3' ORF (open reading frame) is proximal to the termination codon of a uORF (upstream ORF), and expression of the two ORFs is coupled. For example, segment 7 mRNA of influenza B is bicistronic, and the stop codon of the M1 ORF and the start codon of the BM2 ORF overlap in the pentanucleotide UAAUG (stop codon of M1 is shown in boldface and start codon of BM2 is underlined). This short review aims to provide some insights into how this translational coupling process is regulated within different viral systems and to highlight some of the differences in the mechanism of re-initiation on prokaryotic, eukaryotic and viral mRNAs.

Alpha2,6-linked sialic acid acts as a receptor for Feline calicivirus.

Feline calicivirus (FCV) is a major causative agent of respiratory disease in cats. It is also one of the few cultivatable members of the family Caliciviridae. It has recently been reported that FCV binding is in part due to interaction with junction adhesion molecule-A. This report describes the characterization of additional receptor components for FCV. Chemical treatment of cells with sodium periodate showed that FCV recognized carbohydrate moieties on the surface of permissive cells. Enzymic treatment with Vibrio cholerae neuraminidase demonstrated that sialic acid was a major determinant of virus binding. Further characterization using linkage-specific lectins from Maackia amurensis and Sambucus nigra revealed that FCV recognized sialic acid with an alpha2,6 linkage. Using various proteases and metabolic inhibitors, it was shown that alpha2,6-linked sialic acid recognized by FCV is present on an N-linked glycoprotein.

Entry of feline calicivirus is dependent on clathrin-mediated endocytosis and acidification in endosomes.

Feline calicivirus is a major causative agent of respiratory disease in cats. It is also one of the few cultivatable members of Caliciviridae. We have examined the entry process of feline calicivirus (FCV). An earlier study demonstrated that acidification in endosomes may be required. We have confirmed this observation and expanded upon it, demonstrating, using drugs to inhibit the various endocytic pathways and dominant-negative mutants, that FCV infects cells via clathrin-mediated endocytosis. We have also observed that FCV permeabilizes cell membranes early during infection to allow the co-entry of toxins such as alpha-sarcin. Inhibitors of endosome acidification such as chloroquine and bafilomycin A1 blocked this permeabilization event, demonstrating that acidification is required for uncoating of the genome and access to the cytoplasm.

Determination of the structure of a decay accelerating factor-binding clinical isolate of echovirus 11 allows mapping of mutants with altered receptor requirements for infection.

We have used X-ray crystallography to determine the structure of a decay accelerating factor (DAF)-binding, clinic-derived isolate of echovirus 11 (EV11-207). The structures of the capsid proteins closely resemble those of capsid proteins of other picornaviruses. The structure allows us to interpret a series of amino acid changes produced by passaging EV11-207 in different cell lines as highlighting the locations of multiple receptor-binding sites on the virion surface. We suggest that a DAF-binding site is located at the fivefold axes of the virion, while the binding site for a distinct but as yet unidentified receptor is located within the canyon surrounding the virion fivefold axes.