Topics in Depth
The Theme of Viral Assembly and Exit in Microorganisms: Viruses
If the conditions are right for making viruses, virus proteins are made, but must be assembled into virus particles before being released in the cold dark world. This is a process called "assembly," and you can think of it like "virus manufacturing." In the cell are all these virus "parts," and assembly is the process of putting the parts together. Virus parts include:
- Virus genome
- Accessory proteins (if a virus needs an enzyme immediately in a new cell)
- Capsid/structural proteins
- Envelope proteins (if an enveloped virus)
The "parts" described above are the minimum requirements for a functional virus. Virus assembly occurs through capsid proteins forming a scaffold for the virus genome to enter. The next step of virus replication (virus escape) depends on whether the virus has an envelope or not.
FIGURE 4: Virus budding
Naked virions usually just replicate to the point that the cell explodes from having too many viruses in it. This is called cell lysis, aka virus hulk mode. Enveloped viruses use a more elegant way to get out of the cell. They bud out of the cell, which is a process opposite to membrane fusion, called membrane "fission."
Just because a virus has escaped the wreckage it left in the cell after budding or lysis, doesn't mean it's ready to infect another cell. Many viruses have to undergo a process called "maturation," meaning they read the New York Times more often. It's as if a virus leaves an infected cell as a young dumb teenager, and is not ready to infect a new cell until it doesn't giggle every time it hears the word "poop."
Ed. We at Shmoop would like to apologize for the last joke. The writer of that joke has been fired and now is writing jokes for Now & Later wrappers.
Maturation is a process where proteases cleave the protein coat or membrane fusion glycoprotein of viruses that make it more capable of infecting a new cell. These proteases can be included in the virus particle (as happens with retroviruses), or can be found in the newly infected cell (as happens when reoviruses infect cells in low pH endosomes), or as the virus is being assembled in the previously infected cell (for certain influenza-like viruses).
FIGURE 5: Counting virus plaques
We talk a lot in this chapter about how much virus is made from an infection. How do we know how much virus there is? We can calculate how much virus is in infected samples by diluting the samples and infecting cells in culture. Once the virus has been diluted so that there are more cells on the dish than virus, the virus will infect one cell and kill the surrounding cells. This clearing out of killed cells on the dish is called a plaque. You can count the number of plaques, where each plaque represents an infectious plaque forming unit (pfu). The number of plaque forming units in a given infected sample tells you the amount of virus in the sample, or titre.
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