Recombinant Varicella-zoster virus Envelope protein US9 (ORF65)

What is the basic structure of VZV ORF65 protein?

VZV ORF65 is encoded in the unique short region of the Varicella-zoster virus genome. It is predicted to encode an 11-kDa protein with a hydrophobic carboxyl-terminus, but immunoprecipitation studies reveal it migrates as a 16-kDa protein in infected cells. The protein contains approximately 20% serine and threonine residues in its amino half, making it susceptible to phosphorylation. ORF65 is a type II membrane protein that localizes to the Golgi apparatus in infected cells and is incorporated into the viral envelope. The protein shares structural features with its pseudorabies virus (PRV) homolog, including dileucine and acidic domain endocytosis motifs that may facilitate protein trafficking between the plasma membrane and Golgi apparatus .

How is ORF65 localized within infected cells and virions?

Within infected cells, ORF65 primarily localizes to the Golgi apparatus, as demonstrated by co-staining with Golgi markers such as adaptin AP-1. This localization can be disrupted by brefeldin A treatment, which causes redistribution of ORF65 from its perinuclear location to a more diffuse cytoplasmic pattern. When brefeldin A is removed, ORF65 returns to its perinuclear pattern, confirming its association with the Golgi apparatus. In virions, ORF65 behaves as a type II membrane protein. When virions are treated with NP-40 detergent, a substantial portion of ORF65 protein is released into the supernatant, unlike tegument proteins such as ORF10 which remain in the virion pellet. This behavior is consistent with ORF65 being an envelope protein rather than a tegument component .

What are the phosphorylation characteristics of VZV ORF65?

VZV ORF65 is phosphorylated in infected cells, as demonstrated by [33P]orthophosphate labeling experiments. Interestingly, phosphorylation of ORF65 occurs independently of the viral ORF47 and ORF66 serine-threonine protein kinases. In vitro kinase assays reveal that ORF65 can be phosphorylated by cellular casein kinase II but not by casein kinase I. The phosphorylation status can be confirmed by alkaline phosphatase treatment, which results in a slight reduction in the size of immunoprecipitated ORF65 protein. This phosphorylation may be functionally significant, particularly as key serine residues (S46, S48) in VZV ORF65 are conserved with similar residues in PRV US9 that are essential for neuronal spread .

How can researchers generate antibodies against VZV ORF65?

To generate antibodies against VZV ORF65, researchers can amplify the coding region (from codons 5 to 102) using PCR with appropriate primers containing restriction sites. This amplified fragment can be cloned into a bacterial expression vector such as pGEX to create a GST-ORF65 fusion protein. After expression in bacteria, the fusion protein can be purified and used to immunize rabbits to generate polyclonal antibodies. The resulting antibodies can be validated by immunoprecipitation assays using VZV-infected cell lysates, where they should detect a 16-kDa protein in infected cells but not in uninfected controls. Further validation can include immunofluorescence assays to confirm the expected Golgi localization pattern .

What methods are effective for studying ORF65 localization in cells?

Multiple complementary approaches can be used to study ORF65 localization:

• Subcellular fractionation: Separate cytosolic and membrane fractions from infected cells, then perform immunoprecipitation with ORF65 antibodies to determine which fraction contains the protein.

• Immunofluorescence microscopy: Co-stain VZV-infected cells with antibodies against ORF65 and known organelle markers (e.g., adaptin AP-1 for Golgi). This can establish colocalization patterns.

• Pharmacological disruption: Treat infected cells with agents like brefeldin A that disrupt specific organelles (Golgi in this case) and observe changes in ORF65 distribution.

• Recovery experiments: After disruption with brefeldin A, wash and incubate cells in drug-free media to observe reorganization of ORF65 to its normal location, confirming the specificity of localization .

How can researchers confirm ORF65 presence in virions and determine its specific location?

To confirm ORF65 presence in virions and determine its location, researchers can use several complementary approaches:

• Virion purification: Isolate cell-free virions through a multi-step process involving differential centrifugation and sucrose or Ficoll gradients.

• Proteinase K protection assays: Treat purified virions with proteinase K in the presence or absence of detergent (NP-40). Proteins on the virion surface will be degraded by proteinase K alone, while internal proteins require both detergent and proteinase K for degradation.

• Detergent fractionation: Treat virions with NP-40 followed by ultracentrifugation to separate solubilized envelope components (supernatant) from capsid/tegument components (pellet).

• Western blot analysis: Use antibodies against ORF65 and control proteins (known envelope proteins like gC and tegument proteins like ORF10) to analyze the fractions from the above experiments .

How does ORF65 deletion affect VZV replication in vitro versus in vivo?

In vitro studies demonstrate that ORF65 is dispensable for VZV replication in cell culture. When comparing recombinant parent Oka strain (rP-Oka) with an ORF65 deletion mutant (rP-OkaΔ65), no significant differences in replication kinetics are observed in cultured cells. Both viruses show equivalent VZV protein expression when analyzed by immunoblotting.

In vivo studies using the SCID-hu mouse model reveal more nuanced findings:

*Virus detectable in only 2/6 implants
**Virus detectable in only 1/4 implants

These data indicate that while ORF65 is not essential for replication in either environment, there may be subtle effects on viral persistence in thymic/liver tissue at later timepoints. Both viruses caused similar pathological changes, including marked necrosis and lymphocyte depletion in thy/liv implants by day 18 .

What is the significance of ORF65 phosphorylation, and how can it be experimentally manipulated?

The phosphorylation of ORF65 by casein kinase II, but not by VZV-encoded protein kinases, suggests it plays a role in the protein's function that is independent of viral kinase activity. This phosphorylation may be critical for proper localization, trafficking, or interactions with other viral or cellular proteins.

To experimentally manipulate and study ORF65 phosphorylation, researchers can:

• Create phosphorylation site mutants: Using site-directed mutagenesis to change specific serine/threonine residues to alanine (preventing phosphorylation) or to aspartic/glutamic acid (phosphomimetic).

• Use kinase inhibitors: Apply specific inhibitors of casein kinase II (such as TBB or CX-4945) to infected cells and observe changes in ORF65 localization or function.

• In vitro phosphorylation assays: Immunoprecipitate ORF65 from infected cells and subject it to in vitro kinase assays with purified casein kinase II in the presence of [γ-32P]ATP.

• Phosphopeptide mapping: After phosphorylation, digest the protein and separate peptides to identify specific phosphorylation sites.

The conservation of key serine residues (S46, S48) between VZV ORF65 and PRV US9, where these residues are essential for neuronal spread, suggests potential functional significance. Experimental manipulation of these specific residues might reveal their role in VZV pathogenesis, particularly in neurotropism .

How does VZV ORF65 compare structurally and functionally to its homologs in other alphaherpesviruses?

Functionally, VZV ORF65 appears more similar to PRV US9 than to HSV US9, based on its localization and structural features. Both VZV ORF65 and PRV US9 are type II membrane proteins found in the virion envelope and Golgi apparatus, while HSV US9 is reported to be located in the tegument and nucleus. The conserved tyrosine and serine residues between VZV and PRV (but not HSV) suggest potential functional similarities, particularly in neuronal spread .

What can we learn about ORF65 function from studies of US9 in other alphaherpesviruses?

Studies of US9 homologs in other alphaherpesviruses provide valuable insights into potential functions of VZV ORF65, particularly in neuronal systems:

• Neuronal spread: PRV US9 null mutants show impaired anterograde spread in visual and cortical pathways in rats and reduced neurovirulence when inoculated into the eye. The tyrosine (Y49, Y50) and serine (S51, S53) residues in PRV US9 are essential for neuronal spread. These residues are conserved in VZV ORF65 (Y44, Y45, S46, S48), suggesting a potential role in neuronal transport.

• Protein trafficking: PRV US9 shuttles between the plasma membrane and the Golgi apparatus. VZV ORF65 shares the dileucine and acidic domain endocytosis motifs present in PRV US9, suggesting it might undergo similar trafficking.

• Dispensability in cell culture: US9 homologs in HSV-1, PRV, and EHV-1 are all dispensable for replication in cell culture, consistent with findings for VZV ORF65.

• Differential neurovirulence effects: Unlike PRV, HSV-1 lacking US9 is unimpaired for neurovirulence or latency in mice. This suggests that the neuronal functions of US9 proteins may vary among alphaherpesviruses, and VZV ORF65 might align with either pattern .

What are the methodological approaches for generating VZV ORF65 deletion mutants?

To generate VZV ORF65 deletion mutants, researchers typically use cosmid-based mutagenesis approaches. The process involves:

• Construct preparation: Modify the cosmid containing the ORF65 gene region (e.g., MstII A cosmid) by deleting all or part of the ORF65 sequence. This can be accomplished through restriction enzyme digestion followed by religation, or by PCR-based methods.

• Transfection: Co-transfect melanoma cells with the complete set of cosmids needed to reconstitute the VZV genome, including the modified cosmid containing the ORF65 deletion, along with a plasmid expressing an immediate-early transactivator (e.g., pCMV62).

• Virus recovery: Monitor transfected cells for cytopathic effect indicative of virus replication. Harvest infected cells when plaques are observed.

• Verification: Confirm the deletion by PCR, restriction enzyme analysis, and/or sequencing. Additionally, verify the absence of ORF65 protein expression by immunoblotting or immunoprecipitation using ORF65-specific antibodies.

• Growth characterization: Establish growth curves by infecting cells with a known quantity of virus (e.g., 200 PFU) and measuring viral titers at various time points (typically days 1-5) post-infection .

What experimental systems are most informative for studying ORF65 function in vivo?

Based on the available research, several experimental systems provide valuable insights into ORF65 function in vivo:

• SCID-hu mouse model: This model uses human tissue xenografts (skin, thymus/liver) in SCID mice to study VZV pathogenesis in human tissues. This system allows investigation of ORF65's role in replication within different human tissue environments and over time.

• Neuronal culture systems: Given the potential role of ORF65 in neuronal spread (based on PRV US9 homology), primary neuronal cultures or neuronal cell lines would be informative for studying axonal transport and neurotropism.

• Compartmentalized neuronal chambers: Systems like microfluidic chambers or modified Campenot chambers that separate neuronal cell bodies from axon terminals can specifically assess anterograde (soma to axon) versus retrograde (axon to soma) transport and spread.

• Ex vivo human ganglia: Where ethically and technically feasible, studying ORF65 mutants in ex vivo human ganglia cultures could provide direct evidence of its role in human sensory neurons.

• Animal models with VZV-permissive modifications: While standard rodent models don't support VZV replication, humanized mouse models or genetically modified animals with human receptors might allow for assessment of ORF65's role in pathogenesis and neurotropism .

How can researchers quantitatively assess differences between wild-type and ORF65-deleted VZV?

Researchers can employ multiple quantitative approaches to assess differences between wild-type and ORF65-deleted VZV:

• Growth kinetics: Perform multi-step growth curves in various cell types, harvesting virus at defined intervals (e.g., days 1-5 post-infection) and determining titers through plaque assays. Statistical analysis of replication rates and peak titers can identify subtle growth defects.

• Plaque size measurements: Quantify and compare plaque sizes between wild-type and mutant viruses, which can indicate differences in cell-to-cell spread efficiency.

• Quantitative PCR: Measure viral genome copies in infected cells or tissues to assess replication independent of infectious virus production.

• Protein expression profiling: Use quantitative immunoblotting with antibodies against various viral proteins to determine if ORF65 deletion affects expression kinetics or levels of other viral gene products.

• In vivo replication in SCID-hu model: Harvest infected human tissue implants at defined timepoints and quantify infectious virus by plaque assay. As demonstrated in the literature, this approach can reveal differences in tissue-specific replication or persistence over time.

• Electron microscopy with quantitative analysis: Count virions at different stages of assembly and in different cellular compartments to identify potential defects in virion assembly, envelopment, or egress .

What are the key unanswered questions about VZV ORF65 that warrant further investigation?

Several critical questions about VZV ORF65 remain unanswered and represent important areas for future research:

• Neurotropism role: Does ORF65 deletion affect VZV neurotropism or spread within the nervous system, similar to the PRV US9 phenotype? This is particularly relevant given the conserved tyrosine and serine residues between VZV ORF65 and PRV US9.

• Protein interactions: What viral or cellular proteins interact with ORF65, and how do these interactions contribute to its function? Proteomic approaches could identify binding partners.

• Trafficking mechanisms: Does ORF65 shuttle between the plasma membrane and Golgi, like its PRV counterpart? If so, what cellular machinery facilitates this movement?

• Phosphorylation significance: What is the functional significance of ORF65 phosphorylation by casein kinase II? Creating phosphorylation site mutants could address this question.

• Role in pathogenesis: Does ORF65 contribute to VZV pathogenesis in specific tissues or cell types that haven't been thoroughly investigated? Studies in additional human tissue types or specialized cell cultures might reveal context-dependent functions.

• Latency and reactivation: Does ORF65 play any role in the establishment, maintenance, or reactivation from latency? This is particularly relevant given VZV's propensity for neuronal latency .

How might advanced genome editing techniques improve our understanding of ORF65 function?

Advanced genome editing techniques offer new opportunities to study ORF65 function with greater precision:

• CRISPR/Cas9 editing: Direct editing of the VZV genome using CRISPR/Cas9 can create precise mutations or deletions without the need for traditional cosmid-based methods. This approach allows for:

  • Single amino acid changes to test specific functional domains

  • Introduction of fluorescent tags for live imaging studies

  • Creation of conditional expression systems

• BAC mutagenesis: Bacterial artificial chromosome (BAC) systems for VZV allow more efficient mutagenesis and can accommodate larger genomic fragments than cosmids.

• In situ tagging: Adding epitope or fluorescent protein tags to the endogenous ORF65 locus could enable visualization of the protein in living cells without overexpression artifacts.

• Inducible systems: Creating viruses with inducible expression or repression of ORF65 would allow temporal control of its expression to determine when it functions during the viral life cycle.

• Base editing technologies: New base editing approaches could introduce specific mutations without requiring double-strand breaks, potentially increasing the efficiency of generating point mutations in conserved residues.

These advanced techniques would enable more sophisticated functional analysis of ORF65, particularly in neuronal systems where its role may be most significant based on homology to PRV US9 .