Viral entry is the earliest stage of infection in the viral life cycle, as the virus comes into contact with the host cell and introduces viral material into the cell. The major steps involved in viral entry are shown below. Despite the variation among viruses, the generalities are quite similar. However, the specifics are varied.
Reducing cellular proximity
A virus floating around an enclosed space with possible host cells faces a large hurdle, the thermodynamics of diffusion. Because neutrally charged objects do not naturally clump around each other, the virus must find a way to move even near a host cell. It does this by attachment -- or adsorption --- onto a susceptible cell; a cell which holds a receptor that the virus can bind to. The receptors on the viral envelope effectively become connected to complementary receptors on the cell membrane. This attachment causes the two membranes to remain in mutual proximity, favoring further interactions between surface proteins. This is also the first requisite that must be satisfied before a cell can become infected. Satisfaction of this requisite makes the cell susceptible. Viruses that exhibit this behavior include many enveloped viruses such as HIV and Herpes simplex virus
This basic idea extends to viruses that do not contain an envelope. Well studied examples are the viruses that infect bacteria, known as bacteriophages or simply phages. Typical phages have long tails used to attach to receptors on the bacterial surface.
A virus must now enter the cell, which is covered by a phospholipid bilayer, a cell's natural barrier to the outside world. The process by which this barrier is breached depends upon the virus. Types of entry are:
- Attachment or Viral Adsorption: Receptors on the viral envelope become connected to complementary receptors on the cell membrane.
- Membrane Fusion or Hemifusion State: The cell membrane is punctured and made to further connect with the unfolding viral envelope.
- Entry Pore formation: An opening is established for the stabilization of an opening for which viral particles can then enter.
- Viral Penetration: The viral capsid or genome is injected into the host cell's cytoplasm.
Through the use of green fluorescent protein (GFP), virus entry and infection can be visualized in real-time. Once a virus enters a cell, replication is not immediate and indeed takes some time (seconds to hours).
Entry via Membrane Fusion
The most well-known example is through membrane fusion. In this case, viral receptors attach to the receptors on the surface of the cell and secondary receptors may be present to initiate the puncture of the cell membrane or fusion with the host cell, followed by the unfolding of the viral envelope. In essence, the virus's envelope blends with the cell membrane, releasing its contents into the cell. Obviously, this can only be done with viruses that have an envelope. Examples include HIV and herpes simplex virus.
Entry via Endocytosis
Another example is the triggered endocytosis of a cell, in which the virus tricks the cell into thinking that the virus knocking at the door is nothing more than nutrition or harmless goods. A cell naturally takes in resources from the environment and one of the ways it does so is by attaching goods onto surface receptors and engulfing them into the cell. Once inside the cell within the vacuole by which it was taken up, the virus must now break out of the vacuole to gain access to the cytoplasm. Examples include the poliovirus, Hepatitis C virus and Foot-and-mouth disease virus.
Entry via Genetic Injection
A third and more specific example, is by simply attaching to the surface of the cell via receptors on the cell, and injecting only its gene into the cell, leaving the rest of the virus on the surface. This is restricted to viruses in which only the gene is required for infection of a cell (most positive-sense, single-stranded RNA viruses) and further restricted to viruses that actually exhibit this behavior. The best studied example includes the phages.
Once a virus is in a cell, it will activate formation of proteins (either by itself or using the host) to gain full control of the host cell, if it is able to. Control mechanisms include the suppression of intrinsic cell defenses, suppression of cell signaling and suppression of host cellular transcription and translation. Often, it is these cytotoxic effects that lead to the death and decline of a cell infected by a virus.
A cell is classified as susceptible to a virus if the virus is able to enter the cell. After the introduction of the viral particle, unpacking of the contents (viral proteins in the tegument and the viral genome via some form of nucleic acid) occurs as preparation of the next stage of viral infection: viral replication.
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