The external proteins of enveloped virions are virus-encoded proteins that are anchored in the lipid bilayer of the virus or whose precursors are anchored in the lipid bilayer. In the vast majority of cases these proteins are glycopro-teins, although examples are known that do not contain bound carbohydrate. These proteins are translated from viral mRNAs and transported by the usual cellular processes to reach the membrane at which budding will occur. When budding is at the cell plasma membrane, the glycoproteins are transported via the Golgi apparatus to the cell surface. Some enveloped viruses mature at intracellular membranes, and in these cases the glycoproteins are directed to the appropriate place in the cell. Both Type I integral membrane proteins, in which the N terminus of the protein is outside the lipid bilayer and the C terminus is inside the bilayer, and Type II integral membrane proteins, which have the inverse orientation with the C terminus outside, are known for different viruses.
Following synthesis of viral glycoproteins, during which they are transported into the lomen of the endoplasmic reticulum in an unfolded state, they must fold to assume their proper conformation, and assume their proper oxidation state by formation of the correct disulfide bonds. This process often occurs very quickly, but for some viral glyco-proteins it can take hours. Folding is often assisted by chap-eronins present in the endoplasmic reticulum. It is believed that at least one function of the carbohydrate chains attached to the protein is to increase the solubility of the unfolded glycoproteins in the lumen of the ER, so that they do not aggregate prior to folding. During folding, the solubility of the proteins is increased by hiding hydrophobic domains within the interior of the protein and leaving hydrophilic domains at the surface.
The glycoproteins possess a number of important functions in addition to their structural functions. They carry the attachment domains by which the virus binds to a susceptible cell. This activity is thought to be related to the ability of many viruses, nonenveloped as well as enveloped, to bind to and agglutinate red blood cells, a process called hemag-glutination. The protein possessing hemagglutinating activity is often called the hemagglutinin or HA. The viral glycoproteins also possess a fusion activity that promotes the fusion of the membrane of the virus with a membrane of the cell. The protein possessing this activity is sometimes called the fusion protein, or F. The glycoproteins, being external on the virus, are also primary targets of the humoral immune system, in which circulating antibodies are directed against viruses; many of these are neutralizing antibodies that inactivate the virus.
The glycoproteins of some enveloped viruses also contain enzymatic activities. Many orthomyxoviruses and paramyxoviruses possess a neuraminidase that will remove sialic acid from glycoproteins. The primary receptor for these viruses is sialic acid. The neuraminidase may allow the virus to penetrate through mucus to reach a susceptible cell. It also removes sialic acid from the viral glycoproteins, so that these glycoproteins or the mature virions do not aggregate, and from the surface of an infected cell, preventing virions from binding to it. The viral protein possessing neuraminidase activity may be called NA, or in the case of a protein that is both a neuraminidase and hemagglutinin, HN.
The structure of most enveloped viruses is not as rigorously constrained as that of icosahedral virus particles. The glycoproteins are not required to form an impenetrable shell, which is instead a function of the lipid bilayer. They appear to tolerate mutations more readily than do proteins that must form a tight icosahedral shell and appear to evolve rapidly in response to immune pressure. However, the integrity of the lipid bilayer is essential for virus infectivity, and enveloped viruses are very sensitive to detergents.
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