A targeted approach to identification of vaccinia virus postreplicative transcription elongation factors: Genetic evidence for a role of the H5R gene in vaccinia transcription

Poxviruses replicate within the cytoplasm and encode proteins for DNA and mRNA synthesis. To investigate poxvirus replication and transcription from a new perspective, we incorporated 5-ethynyl-2=-deoxyuridine (EdU) into nascent DNA in cells infected with vaccinia virus (VACV). The EdU-labeled DNA was conjugated to fluor-or biotin-azide and visualized by confocal, superresolution, and transmission electron microscopy. Nuclear labeling decreased dramatically after infection, accompanied by intense labeling of cytoplasmic foci. The nascent DNA colocalized with the VACV single-stranded DNA binding protein I3 in multiple puncta throughout the interior of factories, which were surrounded by endoplasmic reticulum. Complexes containing EdU-biotin-labeled DNA cross-linked to proteins were captured on streptavidin beads. After elution and proteolysis, the peptides were analyzed by mass spectrometry to identify proteins associated with nascent DNA. The known viral replication proteins, a telomere binding protein, and a protein kinase were associated with nascent DNA, as were the DNA-dependent RNA polymerase and intermediate-and late-stage transcription initiation and elongation factors, plus the capping and methylating enzymes. These results suggested that the replicating pool of DNA is transcribed and that few if any additional viral proteins directly engaged in replication and transcription remain to be discovered. Among the host proteins identified by mass spectrometry, topoisomerases II␣ and II␤ and PCNA were noteworthy. The association of the topoisomerases with nascent DNA was dependent on expression of the viral DNA ligase, in accord with previous proteomic studies. Further investigations are needed to determine possible roles for PCNA and other host proteins detected. IMPORTANCE Poxviruses, unlike many well-characterized animal DNA viruses, replicate entirely within the cytoplasm of animal cells, raising questions regarding the relative roles of viral and host proteins. We adapted newly developed procedures for click chemistry and iPOND (Isolation of proteins on nascent DNA) to investigate vaccinia virus (VACV), the prototype poxvirus. Nuclear DNA synthesis ceased almost immediately following VACV infection, followed swiftly by the synthesis of viral DNA within discrete cytoplasmic foci. All viral proteins known from genetic and proteomic studies to be required for poxvirus DNA replication were identified in the complexes containing nascent DNA. The additional detection of the viral DNAdependent RNA polymerase and intermediate and late transcription factors provided evidence for a temporal coupling of replication and transcription. Further studies are needed to assess the potential roles of host proteins, including topoisomerases II␣ and II␤ and PCNA, which were found associated with nascent DNA.

Section: Discussionmentioning

confidence: 99%

Identification of Vaccinia Virus Replisome and Transcriptome Proteins by Isolation of Proteins on Nascent DNA Coupled with Mass Spectrometry

Senkevich

Katsafanas

Weisberg

et al. 2017

“…IBT leads to the synthesis of run-on transcripts, which in turn generate excessive dsRNA that leads to the arrest of infection by the antiviral response. In addition to mapping IBT resistance to a variety of proteins known to be directly involved in transcription, they identified a mutation in the H5 gene (D 36 V) that conferred IBT resistance (61). Some years ago our laboratory isolated a virus (tsH5-4) with a mutated H5 allele (EEE 73-75 AAA) that conferred a dominant, temperature-sensitive phenotype (26).…”

Section: Discussionmentioning

confidence: 99%

Genetic Confirmation that the H5 Protein Is Required for Vaccinia Virus DNA Replication

Boyle

Greseth

Traktman

2015

The duplication of the poxvirus double-stranded DNA genome occurs in cytoplasmic membrane-delimited factories. This physical autonomy from the host nucleus suggests that poxvirus genomes encode the full repertoire of proteins committed for genome replication. Biochemical and genetic analyses have confirmed that six viral proteins are required for efficient DNA synthesis; indirect evidence has suggested that the multifunctional H5 protein may also have a role. Here we show that H5 localizes to replication factories, as visualized by immunofluorescence and immunoelectron microscopy, and can be retrieved upon purification of the viral polymerase holoenzyme complex. The temperature-sensitive (ts) mutant Dts57, which was generated by chemical mutagenesis and has a lesion in H5, exhibits defects in DNA replication and morphogenesis under nonpermissive conditions, depending upon the experimental protocol. The H5 variant encoded by the genome of this mutant is ts for function but not stability. For a more precise investigation of how H5 contributes to DNA synthesis, we placed the ts57 H5 allele in an otherwise wild-type viral background and also performed small interfering RNA-mediated depletion of H5. Finally, we generated a complementing cell line, CV-1-H5, which allowed us to generate a viral recombinant in which the H5 open reading frame was deleted and replaced with mCherry (v⌬H5). Analysis of v⌬H5 allowed us to demonstrate conclusively that viral DNA replication is abrogated in the absence of H5. The loss of H5 does not compromise the accumulation of other early viral replication proteins or the uncoating of the virion core, suggesting that H5 plays a direct and essential role in facilitating DNA synthesis. IMPORTANCEVariola virus, the causative agent of smallpox, is the most notorious member of the Poxviridae family. Poxviruses are unique among DNA viruses that infect mammalian cells, in that their replication is restricted to the cytoplasm of the cell. This physical autonomy from the nucleus has both cell biological and genetic ramifications. Poxviruses must establish cytoplasmic niches that support replication, and the genomes must encode the repertoire of proteins necessary for genome synthesis. Here we focus on H5, a multifunctional and abundant viral protein. We confirm that H5 associates with the DNA polymerase holoenzyme and localizes to the sites of DNA synthesis. By generating an H5-expressing cell line, we were able to isolate a deletion virus that lacks the H5 gene and show definitively that genome synthesis does not occur in the absence of H5. These data support the hypothesis that H5 is a crucial participant in cytoplasmic poxvirus genome replication. S mallpox has plagued humans throughout history. The etiological agent of this deadly disease is variola virus, a member of the Poxviridae family of viruses. Smallpox was declared eradicated as a natural pathogen in 1980 after a global vaccination campaign that utilized a closely related poxvirus, vaccinia virus. Vaccinia virus is now the protot...

“…An inability to form the viral membrane is associated with conditional lethal mutants of several nonmembrane proteins, namely, A11 (VACWR130) (29), F10 (VACWR049) (40,42), G5 (VACWR082) (9), H5 (VACWR103) (13), and H7 (VACVWR105) (33). The G5 and H5 proteins have other roles (8,12,21,35), however, and the phenotypes in which viral membranes fail to form have been demonstrated only with elevated-temperature-sensitive alanine substitution mutants, raising the possibility of indirect effects. In cells infected with inducible mutants of two late genes that encode the membrane proteins A14 (VACWR133) (32,43) and A17 (VACWR137) (31,45), vesicular or tubular structures accumulate at the boundaries of dense viroplasm under nonpermissive conditions.…”

mentioning

confidence: 99%

Participation of Vaccinia Virus L2 Protein in the Formation of Crescent Membranes and Immature Virions

Maruri-Avidal

Domi

Weisberg

et al. 2011

Morphogenesis of vaccinia virus begins with the appearance of crescent-shaped membrane precursors of immature virions in cytoplasmic factories. During the initial characterization of the product of the L2R reading frame, we discovered that it plays an important role in crescent formation. The L2 protein was expressed early in infection and was associated with the detergent-soluble membrane fraction of mature virions, consistent with two potential membrane-spanning domains. All chordopoxviruses have L2 homologs, suggesting an important function. Indeed, we were unable to isolate an infectious L2R deletion mutant. Consequently, we constructed an inducible mutant with a conditional lethal phenotype. When L2 expression was repressed, proteolytic processing of the major core proteins and the A17 protein, which is an essential component of the immature virion membrane, failed to occur, suggesting an early block in viral morphogenesis. At 8 h after infection in the presence of inducer, immature and mature virions were abundantly seen by electron microscopy. In contrast, those structures were rare in the absence of inducer and were replaced by large, dense aggregates of viroplasm. A minority of these aggregates had short spicule-coated membranes, which resembled the beginnings of crescent formation, at their periphery. These short membrane segments at the edge of the dense viroplasm increased in number at later times, and some immature virions were seen. Although the L2 protein was not detected under nonpermissive conditions, minute amounts could account for stunted and delayed viral membrane formation. These findings suggested that L2 is required for the formation or elongation of crescent membranes.