Recombinant Influenza B virus Glycoprotein NB (NB)

What is the Influenza B Virus Glycoprotein NB?

NB is a type III integral membrane protein that was first identified in 1983 as a previously unrecognized glycoprotein of Influenza B virus. It is encoded by RNA segment 6 of the Influenza B virus genome, which unusually contains a bicistronic mRNA that also encodes the viral neuraminidase (NA) . The protein is expressed abundantly on the surface of virus-infected cells and is incorporated into virions. NB consists of 100 amino acids with a molecular weight of approximately 11,242 daltons before post-translational modifications . Following glycosylation, the apparent molecular weight increases to approximately 17,700 daltons, as observed in infected cells .

What is the structural composition of the NB protein?

The NB protein has a distinctive tripartite structure consisting of an 18-residue N-terminal ectodomain, a 22-residue transmembrane domain, and a 60-residue cytoplasmic tail . The protein contains seven cysteine residues and 18 isoleucine residues, making these amino acids particularly useful as radioisotopic precursors for detection in laboratory settings . Additionally, the protein contains four potential glycosylation sites (Asn-X-Thr or Ser), which are utilized in infected cells, causing an increase in the observed molecular weight from the predicted 11,242 daltons to approximately 17,700 daltons .

How is the NB gene organized within the viral genome?

NB is encoded by RNA segment 6 of the Influenza B virus genome through a remarkable bicistronic arrangement. The reading frame for NB begins with the first AUG codon on the mRNA and overlaps with the reading frame for the viral neuraminidase (NA) by 292 nucleotides . This overlapping reading frame structure is a unique feature not found in influenza A virus NA genes. Analysis of the mRNAs derived from RNA segment 6 indicates the presence of only a single nucleotide species, which appears to be bicistronic, allowing for the translation of both NA and NB proteins from the same mRNA .

What experimental approaches can be used to detect NB protein in infected samples?

The detection of NB protein in infected samples can be achieved through several methodological approaches. Early research successfully detected NB using radioactive labeling with [³H]isoleucine or [³⁵S]cysteine rather than [³⁵S]methionine, as NB contains only one methionine residue (the initiator methionine that is normally cleaved from mature polypeptides) . Immunological methods employing polyclonal antibodies against NB protein, such as the rabbit polyclonal VNB antibody, can be utilized for ELISA, Western blotting (WB), and immunohistochemistry on paraffin-embedded sections (IHC-P) . Tryptic peptide mapping has also been demonstrated as an effective method to distinguish NB from other viral proteins, particularly NA .

How can recombinant NB protein be expressed for research purposes?

Recombinant NB protein can be expressed using several systems, though specific methodological details must be adapted from general viral protein expression approaches. Based on the available research:

• Reverse genetics systems: Researchers have successfully used reverse genetics systems to generate influenza B viruses with modifications to the NB gene, including complete knockout viruses . This approach allows for the study of NB function in the context of viral infection.

• In vitro translation: Wheat germ extracts have been used for in vitro synthesis of unglycosylated NB (designated NBₒ), using [³H]isoleucine as a labeled precursor . This approach yields a protein of approximately 11,000-12,000 daltons.

• Cell culture expression: NB can be detected in various cell culture systems infected with influenza B virus, including HeLa cells and the HKCC line of hamster kidney cells .

What methods can verify the identity and purity of expressed NB protein?

Several analytical techniques can be employed to verify the identity and purity of expressed NB protein:

• Tryptic peptide mapping: This technique has been effectively used to distinguish NB from NA and confirm the identity of the protein. The method can compare in vitro synthesized NBₒ with NB synthesized in vivo to establish their relationship .

• Western blotting: Antibodies specifically targeting the NB protein, such as the rabbit polyclonal VNB antibody, can be used in Western blots to confirm protein identity .

• Glycosylation analysis: Treatment with endoglycosidases such as endo-β-N-acetylglucosaminidase H (endo H) can be used to remove N-linked oligosaccharides, reducing the apparent molecular weight from approximately 17,700 to 11,000-12,000 daltons, which is the expected size of unglycosylated NB .

How does NB protein contribute to viral pathogenesis in animal models?

Studies in mice have provided important insights into the role of NB protein in viral pathogenesis. When comparing the median lethal dose (MLD₅₀) of wild-type and NB knockout viruses, researchers found that the MLD₅₀ values for NB knockout viruses were at least 1 log higher than the value for wild-type virus, indicating reduced virulence . Additionally, examination of virus replication in the lungs and nasal turbinates of mice infected with 10⁴ PFU of virus revealed that while wild-type virus grew well in both sites, the growth of mutant viruses lacking NB was restricted, showing viral titers generally more than 1 log lower than those of wild-type virus . This evidence suggests that NB protein enhances viral pathogenesis by promoting efficient replication in host tissues.

How does NB protein compare functionally to the M2 protein of Influenza A virus?

The NB protein of influenza B virus shares some functional similarities with the M2 protein of influenza A virus, though with distinct differences. Both proteins are implicated in promoting efficient viral replication in vivo, but the requirement for NB during in vivo replication appears less stringent than that for the M2 protein . An A/WSN/33 influenza A virus mutant lacking the transmembrane and cytoplasmic domains of M2 was severely attenuated in mice, and a mutant of A/Udorn/72 (H3N2) lacking nucleotides encoding amino acid residues 29 to 31 of the M2 protein was attenuated even in cell culture . In contrast, NB knockout influenza B viruses replicate efficiently in cell culture and show only moderate attenuation in mice.

While the ion channel activity of M2 is experimentally well established, such activity has not been unequivocally demonstrated for the NB protein . This functional distinction suggests either that influenza B virus does not depend as much on ion channel activity as influenza A virus does, or that NB serves functions beyond or distinct from ion channel activity .

What is the evolutionary relationship between NB and other viral membrane proteins?

The NB protein represents a unique evolutionary adaptation specific to influenza B viruses. The bicistronic arrangement of RNA segment 6 encoding both NA and NB proteins is not found in influenza A virus NA genes . This distinctive genomic organization highlights the evolutionary divergence between influenza A and B viruses. The overlapping reading frame strategy employed by influenza B virus for NB expression is similar to the coding strategy used by influenza viruses on RNA segments 7 and 8 for the synthesis of other polypeptides .

How can reverse genetics be used to study NB protein function?

Reverse genetics systems provide powerful tools for studying NB protein function by enabling the generation of influenza B viruses with targeted modifications to the NB gene. This approach allows researchers to directly assess the biological consequences of NB alterations in both in vitro and in vivo settings. The methodology involves:

Table 1: Comparative growth characteristics of wild-type and NB knockout influenza B viruses based on data from research studies .

What research approaches can clarify the potential ion channel activity of NB?

To resolve the ongoing questions regarding NB's potential ion channel activity, several complementary research approaches can be employed:

• Electrophysiological studies: Improved methodologies for measuring membrane currents in cellular systems rather than artificial lipid bilayers may provide more physiologically relevant data on potential ion channel activity.

• Mutagenesis of transmembrane domain: Systematic alterations to the 22-residue transmembrane domain of NB to identify residues critical for putative ion channel function.

• Inhibitor studies: Testing of ion channel inhibitors beyond amantadine, which is known to be ineffective against influenza B virus replication, to identify compounds that specifically target NB and correlate with inhibition of viral replication.

• Cross-complementation assays: Experiments testing whether NB can functionally substitute for the M2 protein of influenza A virus, which has established ion channel activity, in chimeric viruses.

What are the implications of NB protein research for antiviral development?

Understanding the structure and function of NB protein opens several avenues for antiviral development:

• Novel drug targets: If NB functions as an ion channel or has other essential functions in vivo, it could represent a target for developing influenza B-specific antivirals, potentially addressing the current limitations of drugs like amantadine that are ineffective against influenza B.

• Attenuated vaccine development: The finding that NB knockout viruses replicate efficiently in cell culture but show attenuated growth in vivo suggests potential for developing live attenuated influenza B vaccines with modified NB genes.

• Broad-spectrum antivirals: Comparative analysis of NB with functionally similar proteins from other viruses could lead to identification of conserved mechanisms that might be targeted by broad-spectrum antivirals.

What are the most pressing unanswered questions about NB protein?

Several critical questions remain to be addressed in future research:

• Definitive function: The precise biological function of NB remains unclear. While it promotes efficient viral growth in vivo, the molecular mechanism behind this effect is not fully understood. Additional studies are needed to determine whether it functions primarily as an ion channel, has immunomodulatory effects, or serves other roles in the viral lifecycle.

• Structural analysis: Detailed structural studies of NB, particularly through methods like X-ray crystallography or cryo-electron microscopy, could provide insights into its function and potential as a drug target.

• Host interactions: Identifying host cell proteins that interact with NB could illuminate its role in viral pathogenesis and potentially reveal new therapeutic approaches.

• Evolution and adaptation: Further analysis of NB sequence conservation across influenza B virus strains and the selective pressures acting on the gene could provide insights into its importance for viral fitness in different host environments.