Question

This is the time in the year where people are encouraged to receive a vaccination for influenza (flu). Flu in humans is caused by the influenza virus of which there are several strains. Strains A and B are the most important in causing flu in humans. Since strain A has been studied more, and A and B are similar, use strain A for your answers.

How does the flu virus enter human cells? This question refers to entering individual cells rather than entering the body.

How are the viral genes expressed? How does the virus get its own genes transcribed and translated in the host cell?

How do new progeny viral particles leave the host cell?

Answer 1

Influenza viruses bind its haemagglutinin to receptors containing sialic acid found on glycoproteins or glycolipid receptors on the cell surface of the host. Virus particles are then endocytosed and enter early endosomes.

Influenza viruses are negative-sense, single-stranded, segmented RNA viruses. In the host nucleus, the virus does primary transcription to produce necessary proteins for replication. The primary transcription involves "cap snatching." In this process, the viral endonuclease (PB2) cuts the 5' methylguanosine cap as well as ten to thirteen nucleotides from the RNA. This is then used as the primer for the transcription of the protein PB1, a viral transcriptase. In influenza A and B, ten proteins result from the translation of the eight segments of the genome, including hemagglutinin, neuraminidase, PB1, PB2, nucleoprotein, another RNA polymerase complex, two matrix proteins, and two NS proteins. Once the initial proteins are made, then eight complementary positive-sense RNA strands are made from the eight negative-sense RNA segments. From this cRNA, a negative-sense RNA is produced. Various proteins then help this negative sense RNA exit the nucleus and into the cytoplasm of the host.

In the cytoplasm, the hemagglutinin and neuraminidase have undergone glycosylation, polymerization, and acylation. The hemagglutinin, neuraminidase, and the matrix protein two (M2) all travel together to the plasma membrane. There the proteins meet with the other matrix protein (M1), and begin the budding process. At least eight RNA segments come to the site and the virus buds. The neuraminidase finally destroys the sialic acid receptors on the membrane, thus allowing the virus to leave the cell.