Life cycle of the influenza virus in detail - formation and distribution of virus proteins, assembly of new M1-RNP cores and budding of new virus particles
Following the transcription and the post-transcriptional modifications, the viral mRNA is transported into the cytoplasm, where it is translated into the viral proteins on the ribosomes of the host cell. After the viral mRNA in the cell nucleus has been prepared in a suitable manner, e.g. by attaching the cap structure, translation can now proceed according to the usual cellular mechanisms and with the help of cellular tRNA (transfer RNA).
According to their later localization in the virus and their functions in the further course of the life cycle, the virus proteins migrate to different locations in the host cell after their synthesis. The distribution of the virus proteins hemagglutinin (HA), matrix protein (M1) and nucleocapsid protein (NP) and their time course have been examined very carefully using immunofluorescence microscopic methods.
The first virus proteins are produced about 30 minutes after the infection, that is to say about 10 minutes after the start of the synthesis of the viral mRNA. Of the three proteins examined, HA, M1 and NP, only NP is initially formed. Since NP can initially only be detected in the cell nucleus, it is evidently transported quickly into the cell nucleus through an active process after translation. The synthesis of the proteins M1 and HA does not begin until about 2-2.5 hours after infection.
As expected from a membrane protein, HA is only detectable in the endoplasmic reticulum, in the Golgi apparatus and on the cell surface (cell membrane), so it is not transported into the cell nucleus. M1, on the other hand, is distributed both in the cytoplasm and in the cell nucleus. This obviously does not happen through active transport processes, but rather through diffusion, whereby the nuclear pores for the relatively small M1 or for the M1 monomer seem to be sufficiently large. Shortly after the start of the M1 synthesis, NP can also be detected in the cytoplasm, which indicates suggests a dependence of NP transport into the cytoplasm on M1.
Based on these and other experimental results, the following further course of the life cycle of the influenza virus is assumed (see film below):
- First, the proteins NP, PA, PB1 and PB2 are synthesized and then undergo active transport into the cell nucleus induced by corresponding signal sequences.
- There they are assembled with the new vRNA that has now been formed to form new RNP cores. Little is known about the mechanism and control of this process. In particular, it is still a mystery how it is ensured that the correct set of the eight individual RNA strands is incorporated into each RNP core. At the same time, the NS proteins should also be formed and develop some of the effects described in the previous chapter (not shown in the film).
- After some time (see above) the synthesis of the matrix proteins begins and M1 also reaches the cell nucleus through diffusion.
- There, due to the electrostatic interactions with the vRNA of the RNP cores, it forms the shell around the RNP cores relatively quickly.
- The thus enveloped RNP nuclei are transported through the nuclear membrane into the cytoplasm and migrate to the cell membrane.
- Almost parallel to the synthesis of the matrix proteins, the formation of the viral membrane proteins hemagglutinin (HA) and neuraminidase (NA) by ribosomes on the rough endoplasmic reticulum (RER) began. HA and NA are transported to the cell membrane after post-translational modifications (e.g. glycosylation) in the endoplasmic reticulum (ER) and in dictyosomes of the Golgi apparatus (not shown in the film), presumably in clathrin-covered vesicles, into which they are built in large numbers.
- When a new M1-enveloped RNP core reaches the cell membrane, the M1 molecules enter into a binding interaction with both the lipid bilayer and with the cytoplasmic domains of HA and NA. In addition to an enrichment of HA and NA in this membrane area, this finally leads to an envelope of the M1-RNP core with the cell membrane.
- The result of this process is the constriction (engl. budding) of a new virus particle from the host cell.
The following film clarifies the processes explained in points 1 to 8:
In addition to HA, the membrane of the new virus particles also contains some surface receptors of the former host cell, so that the newly formed influenza viruses would have to clump together. It is the job of neuraminidase to prevent this clumping by cleaving the sialic acid residues from these receptors.
The viruses can now make their way to new host cells, thereby closing the life cycle.
The death of the host cell does not ultimately occur through lysis (dissolution), but is a consequence of the suppression of some normal metabolic processes in the cell, such as the synthesis of cellular proteins. Before it finally dies, however, each host cell has contributed to the production of up to 100,000 (!) New influenza viruses.
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