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The C Terminus of the Herpes Simplex Virus UL25 Protein Is Required for Release of Viral Genomes from Capsids Bound to Nuclear Pores.

ABSTRACT: The herpes simplex virus (HSV) capsid is released into the cytoplasm after fusion of viral and host membranes, whereupon dynein-dependent trafficking along microtubules targets it to the nuclear envelope. Binding of the capsid to the nuclear pore complex (NPC) is mediated by the capsid protein pUL25 and the capsid-tethered tegument protein pUL36. Temperature-sensitive mutants in both pUL25 and pUL36 dock at the NPC but fail to release DNA. The uncoating reaction has been difficult to study due to the rapid release of the genome once the capsid interacts with the nuclear pore. In this study, we describe the isolation and characterization of a truncation mutant of pUL25. Live-cell imaging and immunofluorescence studies demonstrated that the mutant was not impaired in penetration of the host cell or in trafficking of the capsid to the nuclear membrane. However, expression of viral proteins was absent or significantly delayed in cells infected with the pUL25 mutant virus. Transmission electron microscopy revealed capsids accumulated at nuclear pores that retained the viral genome for at least 4 h postinfection. In addition, cryoelectron microscopy (cryo-EM) reconstructions of virion capsids did not detect any obvious differences in the location or structural organization for the pUL25 or pUL36 proteins on the pUL25 mutant capsids. Further, in contrast to wild-type virus, the antiviral response mediated by the viral DNA-sensing cyclic guanine adenine synthase (cGAS) was severely compromised for the pUL25 mutant. These results demonstrate that the pUL25 capsid protein has a critical role in releasing viral DNA from NPC-bound capsids.IMPORTANCE Herpes simplex virus 1 (HSV-1) is the causative agent of several pathologies ranging in severity from the common cold sore to life-threatening encephalitic infection. Early steps in infection include release of the capsid into the cytoplasm, docking of the capsid at a nuclear pore, and release of the viral genome into the nucleus. A key knowledge gap is how the capsid engages the NPC and what triggers release of the viral genome into the nucleus. Here we show that the C-terminal region of the HSV-1 pUL25 protein is required for releasing the viral genome from capsids docked at nuclear pores. The significance of our research is in identifying pUL25 as a key viral factor for genome uncoating. pUL25 is found at each of the capsid vertices as part of the capsid vertex-specific component and implicates the importance of this complex for NPC binding and genome release.

SUBMITTER: Huffman JB

PROVIDER: S-EPMC5512264 | biostudies-literature | 2017 Aug

REPOSITORIES: biostudies-literature

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The C Terminus of the Herpes Simplex Virus UL25 Protein Is Required for Release of Viral Genomes from Capsids Bound to Nuclear Pores.

Huffman Jamie B JB Daniel Gina R GR Falck-Pedersen Erik E Huet Alexis A Smith Greg A GA Conway James F JF Homa Fred L FL

Journal of virology 20170712 15

The herpes simplex virus (HSV) capsid is released into the cytoplasm after fusion of viral and host membranes, whereupon dynein-dependent trafficking along microtubules targets it to the nuclear envelope. Binding of the capsid to the nuclear pore complex (NPC) is mediated by the capsid protein pUL25 and the capsid-tethered tegument protein pUL36. Temperature-sensitive mutants in both pUL25 and pUL36 dock at the NPC but fail to release DNA. The uncoating reaction has been difficult to study due t ...[more]

PMID: 28490590

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Project description:Herpes simplex virus replicates in the nucleus, where new capsids are assembled. It produces procapsids devoid of nucleic acid but containing the preVP22a scaffold protein. These thermo-unstable particles then mature into A-, B- or C-nuclear icosahedral capsids, depending on their ability to shed the proteolytically processed scaffold and incorporation of the viral genome. To study how these viral capsids differ, we performed proteomics studies of highly enriched HSV-1 A-, B- and C-nuclear capsids, relying in part on a novel and powerful flow virometry approach to purify C-capsids. We found that the viral particles contained the expected capsid components and identified several tegument proteins in the C-capsid fraction (pUL21, pUL36, pUL46, pUL48, pUL49, pUL50, pUL51 and pUS10). Moreover, numerous ribosomal, hnRNPs and other host proteins, absent from the uninfected controls, were detected on the capsids with some of them seemingly specific to C-capsids (glycogen synthase, four different keratin-related proteins, fibronectin 1 and PCBP1). A subsequent proteomics analysis was performed to rule out the presence of protein complexes that may share similar density as the viral capsids but do not otherwise interact with them. Using pUL25 or VP5 mutant viruses incapable of assembling C-nuclear or all nuclear capsids, respectively, we confirmed the bulk of our initial findings. Naturally, it will next be important to address the functional relevance of these proteins.IMPORTANCE Much is known about the biology of herpesviruses. This includes their unique ability to traverse the two nuclear envelopes by sequential budding and fusion steps. For HSV-1, this implies the pUL31/pUL34 and pUL17/pUL25 complexes that may favor C-capsid egress. However, this selection process is not clear, nor are all the differences that distinguish A-, B- and C-capsids. The present study probes what proteins compose these capsids, including host proteins. This should open up new research avenues to clarify the biology of this most interesting family of viruses. It also reiterates the use of flow virometry as an innovative tool to purify viral particles.

Project description:Much of the HSV-1 life cycle is carried out in the cell nucleus, including the expression, replication, repair, and packaging of viral genomes. Viral proteins, as well as cellular factors, play essential roles in these processes. Isolation of proteins on nascent DNA (iPOND) was developed to label and purify cellular replication forks. We adapted aspects of this method to label viral genomes to both image, and purify replicating HSV-1 genomes for the identification of associated proteins. Many viral and cellular factors were enriched on viral genomes, including factors that mediate DNA replication, repair, chromatin remodeling, transcription, and RNA processing. As infection proceeded, packaging and structural components were enriched to a greater extent. Among the more abundant proteins that copurified with genomes were the viral transcription factor ICP4 and the replication protein ICP8. Furthermore, all seven viral replication proteins were enriched on viral genomes, along with cellular PCNA and topoisomerases, while other cellular replication proteins were not detected. The chromatin-remodeling complexes present on viral genomes included the INO80, SWI/SNF, NURD, and FACT complexes, which may prevent chromatinization of the genome. Consistent with this conclusion, histones were not readily recovered with purified viral genomes, and imaging studies revealed an underrepresentation of histones on viral genomes. RNA polymerase II, the mediator complex, TFIID, TFIIH, and several other transcriptional activators and repressors were also affinity purified with viral DNA. The presence of INO80, NURD, SWI/SNF, mediator, TFIID, and TFIIH components is consistent with previous studies in which these complexes copurified with ICP4. Therefore, ICP4 is likely involved in the recruitment of these key cellular chromatin remodeling and transcription factors to viral genomes. Taken together, iPOND is a valuable method for the study of viral genome dynamics during infection and provides a comprehensive view of how HSV-1 selectively utilizes cellular resources.

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Dataset Information

The C Terminus of the Herpes Simplex Virus UL25 Protein Is Required for Release of Viral Genomes from Capsids Bound to Nuclear Pores.

Publications

The C Terminus of the Herpes Simplex Virus UL25 Protein Is Required for Release of Viral Genomes from Capsids Bound to Nuclear Pores.

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