Retrovirus Cell Invasion
Viruses are small submicroscopic parasites that rely on their hosts for energy
production, replication, and release to infect further cells. Since viruses lack the ability to produce ATP, entry into a living cell is a necessity for the
use of the host cell's metabolic machinery for DNA, RNA, and protein synthesis.
The type of cell or tissues that a virus can infect is called the virus' host range.
An example of virus with a very narrow host rage would be Human Papilloma virus (HPV),
which can only infect human epithelial cells while herpes simplex virus (HSV)
has a broad range and is able to infect multiple cell and tissue types. For
further information on virus classifications and host ranges please look at the
following site:
Necessary Steps for Cell Invasion
Cell Recognition
The first step in cell invasion is recognition of the Host's cell through surface receptors located on
the cell's surface. Viruses will either use a receptor that is found on many cells
(such as sialic-acid-containing oligosaccharides, which are used by the influenza
virus) or one that is found only on the surface of the type of cell that is needed
for replication (such as the nicotinic acetylcholine receptor or the cell-cell
adhesion protein N-CAM which are used by the rabies virus). Some viruses such
as HIV require a co-receptor also be present. HIVs primary receptor is the CD4
receptor found on the surface of T-cells and macrophages and the co-receptor CCR5.
Recently a mutated strain also uses the CXCR4 receptor as its co-receptor. Once
the correct receptor is found, the virus will attach to the cell surface.
Receptor mediated endocytosis
In most enveloped viruses, receptor mediated endocytosis is followed by an
uncoating of the viral envelope to expose the DNA/RNA core. This step is
accomplished as the environment in the newly formed endosome becomes acidic.
The acidic environment causes conformational changes in the viral envelope and
fusion with the endosomal membrane. The fusion of the viraland endocytic membranes
allows for the release of the viral genome into the cytosol.
Fusion
Viruses such as HIV will fuse directly with the plasma membrane and release
into the cytosol. This is accomplished as the CD4 and chemokine receptors
interact releasing the anchoring protein Gp120. Once inserted into the plasma
membrane, the anchoring proteins undergo changes facilitating fusion of the
virus with the plasma membrane resulting in the release of the viral genome
directly into the cytosol.
The picture below gives a comparison of different strategies used by viruses to
invade the cell. Each is different but the end result is the same. Infection of
the cell, with the genomic contents of the virus, where the business of replication begins.
References