Scientific Progress in understanding Packaging

• For the virus to leave its host cell it must first package its 8 segments into a new virus.

• The mechanism of how the virus is able to ensure the correct segments is still unknown.

• However, virologists have been investigating this process and have made some progress in understanding how Influenza is able to do this.

• Liang and colleagues were able to determine that for the virus to incorporate the PA, PB1, and PB2 proteins, the distal coding regions were necessary. For efficient packaging of PA or PB1 vRNAs require 40 bases of 5’ and 66 bases of 3’ coding sequences. For packaging of the PB2 segment requires at least 80 bases of 5’ coding region, and does not depend on the 3’ coding sequence. Another interesting finding was that the two ends of any given vRNA may collaborate in forming specific structures, which can be recognized by the viral packaging machinery.

• Liang Y., Hong Y., & Parslow T.G., cis-Acting packaging signlas in the influenza virus PB1, PB2, and PA genomic RNA segments. J Virol. 2005 Aug;79(16):10348-55 PMID 16051827

• McCown and Pekosz suggest that the C termini of the M2 protein is a key player in producing infectious viruses. They were able to determine this by generating influenza A viruses that encoded M2 proteins with truncated C termini. The deletion of the 28 amino acids (M2Stop70) resulted in a virus that made fourfold-fewer particles but made less than 1000-fold-fewer infectious particles in comparison to a wild type virus. This indicated a defect in the vRNP packaging. Therefore, it was concluded that M2 cytoplasmic tail plays a role in the process of efficiently packaging genome segments.

• McCown M.F., & Pekosz A., the influenza A M2 cytoplasmic tail is required for infectious virus production and efficient genome packaging J Virol.2005 Mar;79(6):3595-605.

• Dos Santos Afonso and colleagues generated a recombinant A/WSn/33 influenza virus that expressed a PB2 protein fused to a flag epitope at the N-(Flag-PB2) or C terminius (PB2-Flag). Rescue experiments were performed and it was determined that the PB2-Flag viruses required the 5’ end of the PB2 segment to be kept intact beyond the 34 noncoding terminal nucletides. In the PB2 mini-genomes rescue experiments both the 5’ and 3’ coding ends of the PB2 segment were found to promote the incorporation of the mini-genomes. On the other hand, the presence of the Flag sequence at the junction between the 3’NCR and the coding sequence did not prevent the rescue of Flag PB2 viruses.

• Dos Santos afonso E., Escriou N., Leclerecq I., van der Werf S., & Naggakh N., The generation of recombinant influenza A viruses expressing a PB2 fusion protein requires the conservation of a packaging signal overlapping the coding and noncoding regions at the 5' end of the PB2 segment. Virology 2005 Oct 10;341(1):34-46