Recombinant Vaccinia virus Late protein H2 (MVA092R, ACAM3000_MVA_092)

What is the functional role of H2 protein in Vaccinia virus replication?

H2 protein is absolutely essential for Vaccinia virus replication. Studies using a recombinant vaccinia virus with regulated H2 expression (vH2i) have demonstrated that without H2 expression, the virus cannot replicate or form plaques, despite producing morphologically normal virions . The critical function of H2 is specifically in virus entry—virions lacking H2 (-H2) can bind to cells, but their cores cannot penetrate into the cytoplasm .

Functionally, H2:

• Serves as a component of the Entry Fusion Complex (EFC)

• Interacts directly with A28 protein to form a crucial part of the fusion machinery

• Mediates membrane fusion between the viral and cellular membranes during entry

• Is required for cell-to-cell spread of the virus

Importantly, while H2-deficient virions can complete all stages of morphogenesis and form extracellular virions at the tips of actin tails, they cannot enter neighboring cells nor form syncytia after low-pH treatment .

How does the H2 protein interact with other components of the Entry Fusion Complex?

H2 interacts specifically with the A28 protein, forming a critical component of the EFC. Unlike other known viruses that contain a single fusion protein, poxviruses harbor a multimeric protein complex of 11 subunits (the EFC) to mediate fusion with host membranes .

Specific interaction sites have been identified through:

• Coimmunoprecipitation experiments that confirmed H2-A28 binding

• Alanine-mutagenesis screening in transient complementation systems

• Isothermal titration calorimetry measurements of binding affinity

The surface of the H2 ectodomain contains two loop regions that constitute the A28-binding interface:

• 170LGYSG174 loop region

• 125RRGTGDAW132 loop region

These regions form a broad A28-binding surface that is essential for EFC formation and function. The interaction between H2 and A28 is likely crucial for the conformational changes needed during the fusion process.

Expression and Purification Methods

Purification of recombinant H2 protein typically involves multiple chromatography steps to achieve high purity (>85-90%):

• For His-tagged constructs:

  • Initial capture using Ni-NTA affinity chromatography

  • Tag removal with specific proteases (ULP1 for SUMO-tagged proteins)

  • Reverse affinity chromatography to remove the cleaved tag

• Further purification:

  • Size exclusion chromatography (SEC) to separate monomeric protein from aggregates

  • Ion exchange chromatography to remove impurities with different charge properties

  • Optional hydrophobic interaction chromatography based on the protein's hydrophobicity

Recommended storage conditions include:

• Buffer: Tris-based buffer, pH 8.0, with 6% trehalose or 50% glycerol

• Temperature: -20°C/-80°C for long-term storage

• Avoiding repeated freeze-thaw cycles

• Working aliquots can be stored at 4°C for up to one week

How can researchers verify proper folding and functionality of recombinant H2 protein?

Verification of proper folding and functionality involves multiple complementary approaches:

• Structural integrity assessment:

  • Circular dichroism (CD) spectroscopy to analyze secondary structure content

  • Thermal shift assays to evaluate protein stability

  • Limited proteolysis to assess compact folding

  • Dynamic light scattering to check for aggregation

• Functional verification:

  • Binding assays with A28 protein (co-immunoprecipitation, pull-down assays)

  • Isothermal titration calorimetry to measure binding constants

  • Trans-complementation assays using H2-deficient virus (vH2i)

• Activity confirmation:

  • MV-induced membrane fusion assays

  • Cell entry assays using H2-deficient virions complemented with recombinant H2

A critical functional verification method involves the trans-complementation assay where 293T or BSC40 cells are transfected with wild-type or mutant H2R plasmids, then infected with vH2i (H2-deficient virus) in the absence of the inducer IPTG. Restoration of virus yield indicates functional H2 protein expression .

What mutagenesis approaches are most informative for studying H2 structure-function relationships?

Site-directed mutagenesis has proven highly informative for dissecting H2 protein domains critical for function:

• Alanine-scanning mutagenesis:

  • Systematically replacing residues with alanine to identify functional hotspots

  • Particularly effective for studying surface-exposed residues in the ectodomain

  • Has successfully identified key regions like 170LGYSG174 and 125RRGTGDAW132 as A28-binding sites

• Charge-reversal mutations:

  • Replacing charged residues with oppositely charged amino acids

  • Effective for studying the N-terminal helical region (64RIK66, 83ESDRGR88)

  • Disrupts electrostatic interactions important for function

• Cysteine mutations:

  • Modifying the four conserved cysteines to evaluate disulfide bond importance

  • Can be combined with chemical modification techniques to map structural features

Experimental workflow for mutagenesis studies typically includes:

• QuikChange Lightning Site-Directed Mutagenesis for generating mutants

• Sequencing verification of mutations

• Expression in appropriate systems

• Trans-complementation assays to evaluate function

• Binding assays to assess interactions with A28

What methods can effectively analyze the involvement of H2 in viral entry and membrane fusion?

Several complementary approaches can assess H2's role in viral entry and fusion:

• Cell-cell fusion assays:

  • HeLa cells expressing fluorescent proteins (GFP/RFP) are infected with H2-containing or H2-deficient virions

  • Cells are exposed to pH 5.0 buffer to trigger fusion

  • Syncytia formation is quantified using fluorescence microscopy

  • The percentage of cell fusion can be calculated using image analysis software like Fiji

• Core penetration assays:

  • Cells are infected with purified +H2 or -H2 virions

  • Immunofluorescence microscopy detects viral cores in the cytoplasm

  • Successful penetration is indicated by the presence of core proteins in the cytoplasm away from the cell membrane

• Virus binding assays:

  • Radiolabeled or fluorescently labeled virions are incubated with cells

  • Unbound virus is washed away

  • Bound virus is quantified by measuring radioactivity or fluorescence

  • Comparison between +H2 and -H2 virions reveals differences in binding vs. entry

• Electron microscopy:

  • Ultrastructural analysis of virus-cell interactions

  • Can visualize virions at different stages of entry

  • Distinguishes between surface-bound and internalized virus particles

How can researchers effectively study the interaction between H2 and A28 proteins?

Several techniques have been employed to characterize the H2-A28 interaction:

• Co-immunoprecipitation (Co-IP):

  • Cells are transfected with tagged versions of H2 and A28

  • Cell lysates are immunoprecipitated with antibodies against one protein

  • Western blotting detects the co-precipitated partner protein

  • This approach confirmed the interaction between H2 and A28

• Isothermal Titration Calorimetry (ITC):

  • Provides quantitative binding parameters (Kd, ΔH, ΔS)

  • Purified recombinant H2 and A28 proteins are used

  • Has confirmed direct binding between H2 ectodomain and A28

• Pull-down assays:

  • His-tagged H2 is immobilized on Ni-NTA resin

  • Incubated with A28-containing lysates or purified A28

  • Bound proteins are eluted and analyzed by SDS-PAGE

  • Can be used to test wild-type and mutant proteins

• Biolayer interferometry or surface plasmon resonance:

  • Real-time measurement of binding kinetics

  • One protein is immobilized on a sensor chip

  • The other protein flows over the surface

  • Association and dissociation rates are measured

These methods have revealed that the ectodomain of H2 protein, particularly the loop regions 170LGYSG174 and 125RRGTGDAW132, constitutes the A28-binding interface .

How does H2 protein contribute to the multi-subunit Entry Fusion Complex mechanism?

The vaccinia virus Entry Fusion Complex (EFC) represents a unique fusion machinery that differs from all other known viral fusion proteins (type I, II, and III). The EFC consists of 11 viral proteins that work together to mediate membrane fusion during virus entry .

Current understanding of H2's role in the EFC:

• Structural contribution:

  • H2 provides a transmembrane anchor to the complex

  • The ectodomain interacts with A28, potentially stabilizing the EFC structure

  • The N-terminal helical region (amino acids 51-90) may form an extended structure connecting the ectodomain to the transmembrane region

• Mechanistic involvement:

  • H2 and A28 together form a subcomplex that is essential for fusion

  • This subcomplex likely undergoes conformational changes during the fusion process

  • The interaction between H2 and A28 may trigger or regulate these conformational changes

• Research gaps:

  • The complete structure of the EFC remains undetermined

  • The precise sequence of conformational changes during fusion is unknown

  • How the 11 EFC components coordinate their functions remains to be elucidated

  • Whether H2 interacts with host cell receptors is unclear

Addressing these questions will require advanced structural studies (cryo-EM of the entire EFC), real-time imaging of the fusion process, and comprehensive protein-protein interaction mapping within the EFC .

What experimental approaches can determine if H2 is directly involved in membrane fusion or serves a regulatory role?

Distinguishing between direct and regulatory roles in membrane fusion requires sophisticated approaches:

• Reconstitution experiments:

  • Purification of all 11 EFC components

  • Reconstitution into liposomes

  • Liposome fusion assays with fluorescent lipid mixing

  • Systematic omission of components to determine minimum fusion requirements

• Trap fusion intermediates:

  • Use temperature-sensitive mutants or small molecule inhibitors

  • Arrest fusion at different stages

  • Analyze the conformational state of H2 at each stage

  • Cross-linking studies to capture transient interactions

• Single-particle tracking:

  • Label H2 in virions with quantum dots or fluorescent proteins

  • Real-time imaging during virus-cell interaction

  • Track conformational changes using FRET-based sensors

  • Correlate H2 dynamics with fusion events

• Structural studies of fusion intermediates:

  • Use cryo-electron tomography to visualize virus-cell contact sites

  • Image the EFC during different stages of entry

  • Determine if H2 undergoes conformational changes typical of fusion proteins

These approaches would help determine whether H2 directly mediates membrane merger or plays a regulatory/structural role in supporting the function of other fusion effectors within the EFC.

How can the crystal structure of H2 ectodomain inform antiviral drug development?

The crystal structure of the H2 ectodomain provides valuable insights for structure-based drug design approaches:

• Targeting the A28-binding interface:

  • The identified loop regions (170LGYSG174 and 125RRGTGDAW132) form a druggable surface

  • Small molecules or peptides can be designed to disrupt the H2-A28 interaction

  • Virtual screening against this interface can identify initial hit compounds

  • Fragment-based approaches can explore binding pockets within this region

• Allosteric inhibition strategies:

  • The central five-stranded β-sheet structure may contain allosteric sites

  • Compounds binding to these sites could prevent conformational changes needed for function

  • Molecular dynamics simulations can identify potential allosteric pockets

• Protein-protein interaction disruptors:

  • Peptidomimetics based on the A28-binding regions

  • Stapled peptides to mimic the binding loops

  • Small molecules identified through high-throughput screening

• Structure-based vaccine design:

  • The H2 ectodomain contains neutralizing epitopes

  • Recombinant H2 or modified versions could serve as vaccine antigens

  • Structure-guided immunogen design to expose critical epitopes

The H2 protein is an attractive target because it is essential for virus entry, highly conserved across poxviruses, and has no homology to human proteins, potentially reducing off-target effects .

What approaches can evaluate H2's potential immunogenicity for vaccine development?

Investigating H2's potential as a vaccine antigen requires multifaceted approaches:

• Epitope mapping:

  • Peptide scanning to identify immunodominant regions

  • Phage display to map conformational epitopes

  • X-ray crystallography of H2-antibody complexes

  • Computational prediction of B-cell and T-cell epitopes

• Animal immunization studies:

  • Vaccination with recombinant H2 protein (full-length or ectodomain)

  • Analysis of antibody responses (neutralizing titers, epitope specificity)

  • T-cell responses (CD4+ and CD8+)

  • Challenge studies to assess protection

• Viral vector-based delivery:

  • Using adeno-associated virus (AAV) vectors for H2 expression

  • Evaluating different promoters for optimal expression

  • Assessing immune responses to vectored H2

  • Comparison with other poxvirus antigen candidates

• Adjuvant formulation studies:

  • Testing different adjuvants with recombinant H2

  • Optimizing immune response quality and durability

  • Measuring neutralizing vs. binding antibodies

  • Assessing T-cell response quality (Th1/Th2 balance)

What strategies can overcome difficulties in expressing full-length H2 protein with its transmembrane domain?

Membrane proteins like H2 present unique expression challenges:

• Optimized expression systems:

  • Cell-free expression systems supplemented with detergents or nanodiscs

  • Specialized E. coli strains (C41, C43) designed for membrane protein expression

  • Insect cell systems for better folding of membrane proteins

  • Yeasts (Pichia pastoris) for high-density cultivation and proper folding

• Fusion partners and solubility tags:

  • SUMO tag to enhance folding and solubility

  • MBP (maltose-binding protein) as a large solubility enhancer

  • Truncation strategies to express soluble domains separately

• Membrane mimetics for purification:

  • Detergent screening (DDM, LDAO, OG) for optimal solubilization

  • Nanodiscs or styrene-maleic acid copolymer lipid particles (SMALPs)

  • Amphipols for stabilizing the transmembrane region

• Co-expression approaches:

  • Co-expression with A28 to stabilize the protein through natural interactions

  • Expression in vaccinia-infected cells to provide viral chaperones

  • Co-expression with molecular chaperones

These strategies have been successfully applied to various membrane proteins and could be adapted specifically for the challenges of H2 expression and purification .

How can researchers design experiments to study H2's role in different poxvirus species?

Cross-species analysis of H2 function requires several approaches:

• Comparative genomics and sequence analysis:

  • Alignment of H2 homologs across poxvirus species

  • Identification of conserved vs. variable regions

  • Phylogenetic analysis to understand evolutionary relationships

  • Prediction of species-specific functional differences

• Cross-complementation studies:

  • Express H2 homologs from different poxviruses in the vaccinia vH2i system

  • Evaluate the ability of heterologous H2 proteins to rescue infectivity

  • Identify species-specific functional constraints

• Chimeric protein analysis:

  • Create chimeric H2 proteins with domains from different poxvirus species

  • Map the domains responsible for species-specific functions

  • Identify regions critical for EFC formation across species

• Structural comparison:

  • Solve structures of H2 homologs from various poxviruses

  • Compare binding interfaces with A28 homologs

  • Identify structural features that contribute to species specificity

These approaches would provide insights into the evolution of the poxvirus entry machinery and potentially reveal adaptations to different host ranges and entry mechanisms.

What emerging technologies could advance our understanding of H2 protein dynamics during viral entry?

Several cutting-edge approaches show promise:

• Cryo-electron tomography:

  • Visualize the EFC in its native state within virions

  • Capture different conformational states during fusion

  • Determine the arrangement of all 11 components in 3D

• Single-molecule FRET:

  • Monitor real-time conformational changes in H2 during fusion

  • Measure distances between labeled domains

  • Correlate structural changes with fusion events

• In situ structural techniques:

  • Correlative light and electron microscopy to study H2 during entry

  • Cross-linking mass spectrometry to map interaction networks

  • Hydrogen-deuterium exchange mass spectrometry to identify dynamic regions

• Advanced imaging:

  • Super-resolution microscopy to track H2 during entry

  • Lattice light-sheet microscopy for 3D visualization of the entry process

  • Live-cell imaging with genetically encoded sensors

These technologies would help answer fundamental questions about how the EFC orchestrates membrane fusion, potentially revealing novel mechanisms distinct from the well-characterized class I, II, and III viral fusion proteins.

How might comprehensive mutagenesis techniques inform structure-function relationships in H2 protein?

Advanced mutagenesis approaches can provide deeper insights:

• Deep mutational scanning:

  • Create libraries of thousands of H2 mutants

  • Screen for functional variants using virus-based selection systems

  • Map the entire mutational landscape of H2

  • Identify permissive vs. critical regions of the protein

• CRISPR-based mutagenesis:

  • Apply in viral genomes to create diverse mutation libraries

  • Parallel functional screening of many variants

  • Identify unexpected functional determinants

• Computational protein design:

  • Use Rosetta or AlphaFold-based design to create optimized H2 variants

  • Test predictions about structure-stabilizing mutations

  • Engineer H2 proteins with enhanced properties

• Ancestral sequence reconstruction:

  • Infer ancestral H2 sequences

  • Express and characterize these proteins

  • Understand evolutionary trajectories and functional constraints

These approaches would provide a comprehensive map of H2 protein function and potentially reveal novel functional surfaces beyond those already identified in the A28-binding regions and N-terminal helical domain .

How does the H2-containing Entry Fusion Complex compare with other viral fusion mechanisms?

The poxvirus EFC represents a unique fusion system:

• Structural comparison:

  Fusion System	Components	Mechanism	Trigger
  Poxvirus EFC	11 proteins including H2 and A28	Complex multi-protein machinery	Low pH
  Class I (Influenza HA)	Single trimeric protein	Spring-loaded conformational change	Low pH
  Class II (Flavivirus E)	Dimeric proteins form trimers	Domain rearrangement	Low pH
  Class III (VSV G)	Trimeric proteins	Reversible conformational change	Low pH
  Herpesvirus	Multiple glycoproteins (gB, gH/gL)	Regulated multi-step process	Receptor binding

• Functional distinctions:

  • Unlike single fusion proteins, the EFC requires coordination of multiple components

  • H2 and A28 appear functionally analogous to fusion proteins but with distributed functions

  • The poxvirus system may represent a more primitive or evolutionarily distinct solution to membrane fusion

• Evolutionary considerations:

  • The EFC may represent an ancestral form of viral fusion machinery

  • The distributed function across multiple proteins suggests a different evolutionary path

  • No sequence or structural homology exists between EFC components and other viral fusion proteins

Understanding these differences could reveal novel principles of protein-mediated membrane fusion and potentially inform biotechnological applications or antiviral strategies.

What lessons from other viral fusion systems might inform new experimental approaches for studying H2?

Advances in other viral fusion systems suggest promising approaches:

• Single-particle fusion assays:

  • Techniques developed for influenza and HIV could be adapted

  • Fluorescently labeled virions on supported bilayers

  • Real-time visualization of individual fusion events

  • Correlation with EFC conformational changes

• Lipid mixing assays:

  • Fluorescently labeled virus membranes

  • Content mixing assays to distinguish hemifusion from full fusion

  • Systematic variation of lipid composition to identify requirements

• Intermediates trapping:

  • Temperature arrested states (as used for influenza)

  • Lipid composition modifications to trap hemifusion

  • Small molecule inhibitors that arrest fusion at different stages

• Structural biology approaches:

  • Pre-fusion and post-fusion structure determination (as done for class I-III)

  • Antibody-trapped intermediate states

  • Cryo-EM of fusion intermediates

These approaches have revealed detailed mechanisms for other viral fusion systems and could be adapted to address the unique challenges of the multi-component poxvirus EFC, potentially revealing new principles of protein-mediated membrane fusion .

What are the most significant unanswered questions about H2 protein function?

Despite substantial progress, several fundamental questions remain:

• Mechanistic unknowns:

  • Does H2 undergo significant conformational changes during fusion?

  • What is the exact sequence of molecular events during EFC-mediated fusion?

  • How do the 11 components of the EFC coordinate their functions?

  • Does H2 directly interact with host cell components?

• Structural questions:

  • What is the structure of full-length H2 in the membrane context?

  • How does the EFC assemble, and what is the spatial arrangement of components?

  • What intermediate structures form during the fusion process?

• Evolutionary puzzles:

  • Why did poxviruses evolve a complex multi-component system rather than a single fusion protein?

  • How has the H2-A28 interaction evolved across different poxvirus species?

  • Are there functional homologs in other virus families?