The second RNA segment of the influenza virus genome encodes two proteins, PB1 and PB1-F2. The latter protein is believed to be an important determinant of virulence of influenza virus. Can we learn anything about the virulence of the new influenza virus H1N1 strains from a study of this protein?
During influenza virus infection, PB1-F2 is targeted to the mitochondria, where it induces a form of cell death known as apoptosis. Experiments in a mouse model of influenza virus infection have shown that PB1-F2 regulates lethality of the virus. By comparing the infection of mice with two strains of influenza virus, one of which produces much lower levels of the PB1-F2 protein, it was found that the protein enhances inflammation and increases frequency and severity of secondary bacterial pneumonia. A specific amino acid at position 66 of this protein appears to be an important determinant of viral virulence. This amino acid is a serine in the 1918 H1N1 influenza virus, in a 1997 avian H5N1 isolate from the Hong Kong outbreak, and in the H2N2 (1957) and H3N2 (1968) pandemic strains. Other less pathogenic influenza virus isolates have an asparagine at this position. Two viruses were constructed which differ at amino acid 66 of the PB1-F2 protein, and the virulence of these viruses was determined in mice. The influenza virus with a serine at amino acid 66 was pathogenic in mice, while the virus with an asparagine was significantly less virulent. Increased pathogenicity of the virulent virus was associated with higher levels of virus replication in the lungs. The results of these studies show that the PB1-F2 protein affects pathogenicity in a mouse model, and that position 66 plays an important role.
Because the amino acid change N66S of PB1-F2 is present in the three previous pandemic influenza virus strains – 1918 H1N1, 1957 H2N2, and 1968 H3N2 – it would be of interest to determine which amino acid, N or S, is present in the new H1N1 influenza virus strain that is spreading globally. However, examination of the nucleotide sequence of RNA from the current H1N1 isolates shows that these viruses do not even produce a PB1-F2 protein – a stop codon is present after amino acid 11 (see figure). In fact, many other influenza virus strains do not produce the protein. While the PB1-F2 protein is not the only determinant of influenza virus virulence, we can at least eliminate any contribution of this viral protein to increased lethality. As Peter Palese has written in today’s Wall Street Journal, “If this virulence marker is necessary for an influenza virus to become highly pathogenic in humans or in chickens, then the current swine virus doesn’t have what it takes to become a major killer.”
Conenello, G., Zamarin, D., Perrone, L., Tumpey, T., & Palese, P. (2007). A Single Mutation in the PB1-F2 of H5N1 (HK/97) and 1918 Influenza A Viruses Contributes to Increased Virulence PLoS Pathogens, 3 (10) DOI: 10.1371/journal.ppat.0030141
MCAULEY, J., HORNUNG, F., BOYD, K., SMITH, A., MCKEON, R., BENNINK, J., YEWDELL, J., & MCCULLERS, J. (2007). Expression of the 1918 Influenza A Virus PB1-F2 Enhances the Pathogenesis of Viral and Secondary Bacterial Pneumonia Cell Host & Microbe, 2 (4), 240-249 DOI: 10.1016/j.chom.2007.09.001