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Mechanisms of Virus Entry Into Target Cells


Negative Stain TEM of Polio Virus, Bar = 50 nm

Negative Stain TEM of Polio Virus, Bar = 50 nm (Photo credit: Wikipedia)

Viruses are everywhere, and so tiny that a million could fit on the tip of a pin. Little can be done to protect us against viruses and modern medicine has only begun to explore the ways viruses works, or their mechanisms. It has been discovered that certain mechanisms are used to infect, or gain entry into, certain host cell types. Target cells are selected by viruses based on what kind of cell type it is.

Mechanisms of Virus Entry into Target Cells 

  • Membrane Fusion- This mechanism is only used by viruses that have envelops, a special barrier surrounding certain viruses. In this particular mechanism, the envelope of the virus fuses with the cellular membrane of the target host cells. Membrane fusion can be triggered in two different ways.
    • From outside- Virus interacts with specific receptor molecules/co-receptors on the host cell surface.
    • From inside- Receptor-mediated endocytosis tricks the cell’s surface into opening and helping the virus inside. After entering the endosome the viral membrane fuses with the host cell membrane.

Two examples commonly referred to are HIV and the Human Influenza Virus, which is also known as the flu virus. HIV and the Human Influenza virus, or the flu virus, have been widely studied till date. HIV infects the host cell through interactions with cell surface receptors whereas the flu virus infects host cells after entering the endosome.

HIV Infection Mechanism

An HIV virion binds to a CD4+ human cell. The ...

An HIV virion binds to a CD4+ human cell. The two bottom pictures depict two proposed models of HIV fusion with the cell. (Photo credit: Wikipedia)

  1. The virus first establishes contact with specific receptor molecules known as CD4 present on cell surface; the co-receptors being CCR5 or CXCR4;
  2. The viral entry protein is gp120 or gp160, and the target cells for HIV are the T-helper cells and macrophages.
  3. The gp120 of virus interacts with CD4 antigens on the target cells. This causes a conformational change in gp120,
  4. This exposes the viruses hydrophobic regions that in turn embeds within the host cell membrane. This causes the viral membrane to fuse with that of host,
  5.  This cause infection by releasing the nucleocapsid containing the RNA genome into the cytoplasm of the host cell.

The Flu Virus Infection Mechanism

A diagram of influenza viral cell invasion and...

A diagram of influenza viral cell invasion and replication. (Photo credit: Wikipedia)HIV Infection Mechanism

  1. The flu virus first establishes contact with target epithelial cells through the receptor molecule called sialic aicd.
  2. The viral entry protein is known as Haemagglutinin or HAgp, a bilayer membrane which covers the nucleocapsid core, and a haemagglutinin (HAgp) spike lying within the membrane.
  3. The HAgp spike has four major antigenic variable regions and a receptor site which both adheres to the cell surface and protects the fusion peptides used after entering the endosome.
  4.  After attachment to the cells, the viral particle is taken up into the host cell, a mechanism known as endocytosis. This new environment has a higher pH causing HAgp to undergo a conformational change releasing the fusion peptides.
  5. The fusion peptides then helps in the fusing process of the viral membrane and the target cell membrane.
  6. Finally, the nuclear material is released into the host epithelial cell.

Written by:

Soumashree Basu

Email: soumashreebasu@gmail.com

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