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9.8D: Viral Exit

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    Viruses are released from the host cell following assembly.

    Learning Objectives
    • Explain how viruses exit host cells

    Key Points

    • Once replication has been completed and the host cell is exhausted of all resources in making viral progeny, the viruses may begin to leave the cell by several methods.
    • Viral exit methods include budding, exocytosis, and cell lysis.
    • Budding through the cell envelope, in effect using the cell’s membrane for the virus itself is most effective for viruses that need an envelope.
    • This process will slowly use up the cell membrane and eventually lead to the demise of the cell.

    Key Terms

    • exocytosis: The secretion of substances through cellular membranes, either to excrete waste products or as a regulatory function
    • lysogenic: Of, relating to, or causing lysis.
    • virions: An entire virus particle, consisting of an outer protein shell called a capsid and an inner core of nucleic acid.

    Viral populations do not grow through cell division because they are acellular. Instead, they use the machinery and metabolism of a host cell to produce multiple copies of themselves, and they assemble in the cell. The life cycle of viruses differs between species, but follows the same basic stages.

    Viruses can be released from the host cell by lysis, a process that kills the cell by bursting its membrane and cell wall if present. This is a feature of many bacterial and some animal viruses. Some viruses undergo a lysogenic cycle where the viral genome is incorporated by genetic recombination into a specific place in the host’s chromosome. The viral genome is then known as a ” provirus ” or, in the case of bacteriophages a “prophage. ” Whenever the host divides, the viral genome is also replicated. The viral genome is mostly silent within the host. However, at some point, the provirus or prophage may give rise to active virus, which may lyse the host cells. Enveloped viruses (e.g., HIV) typically are released from the host cell by budding. During this process the virus acquires its envelope, which is a modified piece of the host’s plasma or other, internal membrane.

    Viral shedding refers to the successful reproduction, expulsion, and host-cell infection caused by virus progeny. Once replication has been completed and the host cell is exhausted of all resources in making viral progeny, the viruses may begin to leave the cell by several methods.

    Different types of virus have varying sites of synthesis and replication. For example, synthesis and replication for DNA viruses occur in the cell’s nucleus while it is usually the cytoplasm for RNA viruses. Virus assembly depends on the site of synthesis and such sites are the nucleus, endoplasmic reticulum, and the Golgi apparatus aka Golgi body. Aside from this, assembly also occurs in the viroplasm which is an inclusion body in a cell. When the virus has replicated and multiplied, they would want to leave the infected cell and infect other cells. However, they require an envelope to enclose the DNA as well as to bind with the other healthy cells so that they can infect. The viral envelope is the typical lipid bilayer, derived from the host cell itself and sources usually come from the nuclear membrane, endoplasmic reticulum, Golgi apparatus/body, and plasma membrane. It also depends on where the virus ‘bud’ off from the host.

    Budding is a method which viruses use to exit the cell. “Budding” through the cell envelope, in effect using the cell’s membrane for the virus itself is most effective for viruses that need an envelope in the first place. These include enveloped viruses such as HSV, SARS, or smallpox. Prior to budding, the virus may put its own receptor onto the surface of the cell in preparation for the virus to bud through, forming an envelope with the viral receptors already on it. This process will slowly use up the cell membrane and eventually lead to the demise of the cell. This is also how antiviral responses are able to detect virus infected cells.

    Other methods for exit would be cell lysis. This method releases the virus from the infected cell by bursting its membrane and this kills the cell as well. Another method is by accumulating the virus particles in vesicles and releasing them via exocytosis. Exocytosis is the process where vesicles containing the virus are secreted/excreted out of the infected cell.

    Positive-strand RNA mature virions are infectious. Virions are released following cell lysis. Excess capsids are formed and inclusion bodies may be seen in the cytoplasm.

    image
    Figure: Generic Viral Budding: Viral budding uses the host’s cell membrane eventually causing cell death.

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