Recombinant Vaccinia virus Myristoylated protein G9 (G9R)

Evolutionary Conservation

The high degree of conservation of G9 across the poxvirus family indicates its essential role in the viral life cycle. No non-poxvirus homologs have been detected through position-specific iterative BLAST searches, suggesting that G9 represents a unique adaptation specific to poxviruses . This evolutionary conservation underscores the importance of G9 in poxvirus biology and potentially identifies it as a distinctive target for antiviral strategies.

Essential Role in Viral Replication and Entry

Experimental evidence strongly supports the critical importance of G9 in vaccinia virus replication. Various approaches have been employed to elucidate its functional significance, including the construction of inducible mutants and gene deletion attempts.

Evidence for Essential Function

Attempts to isolate a mutant virus lacking the G9R gene have been unsuccessful, suggesting that the protein is essential for virus replication . To further investigate its role, researchers constructed a recombinant vaccinia virus (vG9i) in which G9R expression is regulated by an inducer (isopropyl-β-d-thiogalactopyranoside, IPTG). This inducible system allowed precise control over G9 expression levels during infection.

When G9 expression was repressed (by omitting IPTG), viral replication was severely compromised, with infectious virus yield reduced by approximately 1.5 logs compared to conditions where G9 expression was induced . This finding provided compelling evidence for the essential nature of G9 in the viral life cycle.

Role in Viral Entry and Membrane Fusion

G9-deficient virions demonstrated a dramatic reduction in specific infectivity, retaining less than 5% of normal levels. While these virions could still bind to host cells, their ability to penetrate the cytoplasm and initiate early viral RNA synthesis was severely impaired . Furthermore, G9-deficient virions failed to trigger cell-cell fusion under low pH conditions, a characteristic typically observed with wild-type vaccinia virus.

These observations position G9 as a critical component of the entry-fusion complex (EFC), a multi-protein assembly required for poxvirus entry into host cells. Of the identified components of this complex, G9 is the sixth that has been demonstrated to be essential for entry and membrane fusion processes .

Construction and Characterization of Recombinant G9R

The development of recombinant vaccinia viruses with regulated G9R expression has been instrumental in elucidating the protein's functions. These experimental systems have allowed researchers to manipulate G9 levels and analyze the resulting effects on viral replication and morphogenesis.

Engineering the vG9i Recombinant Virus

The construction of vG9i involved sophisticated genetic engineering techniques. This recombinant virus contains:

1. An N-terminal HA epitope-tagged G9R open reading frame regulated by the bacteriophage T7 RNA polymerase promoter

2. An open reading frame encoding enhanced green fluorescent protein (GFP) regulated by the vaccinia virus P11 late promoter

3. Sequences flanking G9R for homologous recombination with the vaccinia virus genome

To prevent RNA polymerase read-through from neighboring genes, the G9R open reading frame was inserted in its natural site but in the opposite ("L") orientation. This design ensured that G9 expression would be tightly controlled by the inducer .

Attempts at G9R Deletion

Researchers attempted to delete the G9R open reading frame using the vaccinia virus bacterial artificial chromosome (BAC) system. This approach involved replacing the G9R gene with an ampicillin resistance gene in the VAC-BAC plasmid through recombination in Escherichia coli. Although ampicillin-resistant colonies were successfully isolated and the deletion of G9R was verified by sequencing, attempts to rescue the mutated VAC-BACΔG9 virus were unsuccessful .

This failure to isolate a G9R deletion mutant provided additional evidence for the essential nature of the protein. Only when intact G9R DNA was supplied along with the VAC-BACΔG9 DNA during transfection could infectious virus be rescued, confirming that G9 is either absolutely essential or that virus replication is too severely compromised without it to allow isolation by conventional methods .

Association with Viral Membranes and Surface Exposure

The localization and membrane association of G9 have been investigated using various biochemical and microscopic techniques, revealing important insights into its distribution within infected cells and mature virions.

Enrichment in Mature Virions

Western blot analysis of purified virions from cells infected with vG9i in the presence of IPTG revealed that G9 is significantly enriched in mature virions (MVs) compared to whole-cell extracts. Specifically, G9 was found to be enriched more than eightfold in MVs relative to the amount in whole-cell extracts. By comparison, A16 (another component of the entry-fusion complex) showed an even higher enrichment of more than 16-fold .

The relatively lower incorporation of G9 into MVs compared to A16 may reflect differences in expression kinetics or efficiency, possibly influenced by the inducible T7 promoter system used in the recombinant virus .

Surface Exposure on Mature Virions

To investigate the membrane association and surface exposure of G9, researchers employed a biotinylation approach. Mature virions purified from cells infected with vG9i were treated with sulfo-NHS-SS-biotin, a membrane-nonpermeating reagent that can only label proteins exposed on the virion surface.

After biotinylation, the virions were lysed, and biotinylated proteins were separated from non-biotinylated ones using NeutrAvidin beads. Western blot analysis revealed that in biotinylated MVs, G9 was almost entirely associated with the bound fraction, indicating its exposure on the virion surface . This finding confirms that G9 is not only incorporated into mature virions but is also accessible on their surface, where it can potentially participate in virus-host cell interactions during entry.

Impact on Viral Morphogenesis

Despite its essential role in viral entry, G9 appears to be dispensable for virion morphogenesis. Detailed microscopic and biochemical analyses have revealed that virion assembly proceeds normally even when G9 expression is repressed.

Electron Microscopic Evidence

Transmission electron microscopy of thin sections from cells infected with vG9i, either with or without IPTG, provided direct visual evidence that all stages of virion assembly occur normally regardless of G9 expression levels. The microscopic images revealed the presence of:

1. Viral crescents and immature virions

2. Mature virions (MVs)

3. Cell-associated extracellular virions (EVs)

These observations confirm that while G9 is critical for viral entry, it does not significantly impact the morphogenesis pathway. This functional specialization may reflect the compartmentalization of different stages in the viral life cycle, with G9 playing a specific role in entry rather than assembly.

Adaptive Mutations in G9 and Their Functional Significance

Recent research has identified specific mutations in G9 that can enhance viral infection under certain conditions, particularly in cells expressing the fusion regulatory proteins A56 and K2. These findings provide valuable insights into the functional regions of G9 and its interaction with host cell factors.

Identification of Adaptive Mutations

Through serial passage of wild-type vaccinia virus in cells expressing A56/K2 (which normally imposes a block to viral entry), researchers identified several adaptive mutations in G9 that overcome this restriction . These mutations were concentrated near the N-terminus of G9 and included:

1. Histidine to tyrosine substitution at position 44 (H44Y)

2. Histidine to arginine substitution at position 44 (H44R)

3. Tyrosine to cysteine substitution at position 42 (Y42C)

4. Duplication of amino acids 26 to 39

Table 1 summarizes the frequency of these mutations observed across multiple independent passage series:

The H44Y mutation was by far the most frequent, appearing in 41 out of 45 isolates across all passage series. This suggests that this particular mutation confers a significant selective advantage under the experimental conditions employed.

Emergence and Enrichment of Adaptive Mutations

Whole-genome sequencing of viral DNA from different rounds of passage revealed the timeline of emergence for these adaptive mutations. The H44Y mutation was not detected at round 1 but reached frequencies of 63-76% by round 5 and 85-98% by round 9, indicating rapid selection once it appeared . In contrast, the H44R mutation and the amino acid duplication were found in only one of the four independent passages each, with frequencies of 9% and 10% respectively by round 9.

Table 2 shows the frequency of these mutations across different passage rounds:

This pattern of emergence and enrichment suggests that these mutations arose or were significantly enriched at different rates during the passages, with H44Y showing the strongest selective advantage.

Functional Impact of G9 Mutations on Viral Spread

The adaptive mutations identified in G9 significantly enhanced viral replication and spread in cells expressing A56/K2 fusion regulatory proteins. These effects were quantified through plaque size measurements and virus yield determinations.

Effects on Plaque Size

Plaque size analysis revealed that each of the G9 mutants produced significantly larger plaques than wild-type virus in A56/K2 cells. These differences in mean plaque sizes were statistically significant (p < 0.001) across three independent experiments . This increased plaque size indicates enhanced cell-to-cell spread of the mutant viruses, suggesting that the mutations enable more efficient viral entry or post-entry steps in A56/K2-expressing cells.

Interestingly, the G9 mutants also formed slightly larger plaques than wild-type virus in parent HEK-293 cells, although this effect was not consistently observed in BS-C-1 cells . This observation suggests that the mutations may confer a general replication advantage under certain cellular conditions, even in the absence of A56/K2 expression.

Impact on Virus Yield

When infected at low multiplicities of infection, the G9 mutants produced significantly higher virus yields than wild-type virus in A56/K2 cells after 24 hours . This finding provided further evidence that the identified mutations enhance viral replication and spread in cells expressing the fusion regulatory proteins.

The ability of these mutants to overcome the replication block imposed by A56/K2 suggests that the N-terminal region of G9 plays a critical role in mediating interactions with these fusion regulatory proteins. By altering these interactions, the mutations likely prevent the inhibitory effect of A56/K2 on viral entry and fusion, allowing more efficient infection and spread.

Interaction of G9 with Fusion Regulatory Proteins

The interaction between G9 and the fusion regulatory proteins A56 and K2 represents a critical aspect of poxvirus entry regulation. This interaction appears to be modulated by specific regions near the N-terminus of G9, as revealed by the adaptive mutations discussed above.

G9/A16 Subcomplex and A56/K2 Interaction

Previous studies showed that A56/K2 interacts with the G9/A16 entry-fusion complex subcomplex in detergent-treated cell extracts . This interaction is believed to regulate fusion activity, with A56/K2 potentially preventing superinfection and cell-cell fusion by blocking the function of the entry-fusion complex on newly infected cells.

The fact that adaptive mutations in G9, particularly near its N-terminus, can overcome the entry block imposed by A56/K2 provides functional evidence for the importance of this interaction. These findings suggest that the N-terminal region of G9 is directly involved in the interaction with A56/K2, and that modifications to this region can modulate the strength or nature of this interaction .

Significance for Viral Entry Regulation

The interaction between G9 and A56/K2 appears to represent an important regulatory mechanism for poxvirus entry. By expressing A56/K2 on the surface of infected cells, poxviruses may prevent superinfection and limit cell-cell fusion, which could be beneficial for viral spread and persistence in the host.

The adaptive mutations identified in G9 likely disrupt this regulatory mechanism, allowing the virus to enter cells despite the presence of A56/K2. This ability may represent an adaptation that enhances viral replication under specific conditions, such as in tissues where A56/K2 expression is high or in response to host defense mechanisms that upregulate these fusion regulatory proteins .