Our discussion of influenza virus replication has so far brought us to the stage of viral RNA synthesis. Last time we discussed the formation of viral RNAs, an event which takes place in the cell nucleus. Now we’ll consider how these RNAs participate in the assembly of new infectious viral particles, as illustrated in the following figure.
For simplicity, the nucleus is not shown. But remember that the viral RNAs have to be exported from the nucleus to the cytoplasm, where viral assembly occurs. First, the viral mRNAs are translated to produce all the proteins needed to synthesize a new virus particle. The mRNAs encoding the HA and NA glycoproteins are translated by ribosomes that are bound the the endoplasmic reticulum – the membranous organelle that assists in transporting certain proteins to the plasma membrane. As the HA and NA proteins are produced, they are inserted into the membrane of the endoplasmic reticulum as shown. These proteins are then transported to the cell surface via small vesicles that eventually fuse with the plasma membrane. As a result, the HA and NA are inserted in the correct direction in the lipid membrane of the cell. The M2 protein is sent to this location in a similar way.
The (-) strand viral RNAs that will be packaged into new virus particles are produced in the cell nucleus, then exported to the cytoplasm. These RNAs are joined with the viral proteins PA, PB1, PB2, and NP. Viral proteins other than HA, NA, and M2 are produced by translation on free ribosomes, as shown for M1. The latter protein binds to the membrane where HA, NA, and M2 have been inserted. The assembly consisting of viral RNAs and viral proteins – called a ribonucleoprotein complex or RNP – travels to the site of assembly. The virion then forms by a process called budding, during which the membrane bulges from the cell and is eventually pinched off to form a free particle.
As new virions are produced by budding, they would immediately bind to sialic acid receptors on the cell surface, were it not for the action of the viral NA glycoprotein. This enzyme removes sialic acids from the surface of the cell, so that newly formed virions can be released. This requirement explains how the neuraminidase inhibitors Tamiflu and Relenza function: they prevent cleavage of sialic acid from the cell surface. In the presence of these inhibitors, virions bud from the cell surface, but they remain firmly attached. Therefore Tamiflu and Relenza block infection by preventing the spread of newly synthesized virus particles to other cells.