Viral protein

A viral protein is both a component and a product of a virus. Viral proteins are grouped according to their functions, and groups of viral proteins include structural proteins, nonstructural proteins, regulatory, and accessory proteins. Viruses are non-living and they do not have the means to reproduce on their own. They depend on their host cell's metabolism for energy, enzymes, and precursors, in order to reproduce. As such, viruses do not code for many of their own viral proteins, but rather, they use the host cell's machinery to produce the viral proteins they require for replication.

Most viral structural proteins are components for the capsid and the envelope of the virus.

The genetic material of a virus is stored within a viral protein structure called the capsid. The capsid is a "shield" that protects the viral nucleic acids from getting degraded by host enzymes. It also functions to attach the virion to its host, and enable the virion to penetrate the host cell membrane. Many copies of a single viral protein or a number of different viral proteins make up the capsid, and each of these viral proteins are coded for by one gene from the viral genome. The structure of the capsid allows the virus to use a small number of viral genes to make a large capsid.

Several protomers, oligomeric (viral) protein subunits, combine to form capsomeres, and capsomeres come together to form the capsid. Capsomeres can arrange into an icosahedral, helical, or complex capsid, but in many viruses, such as the herpes simplex virus, an icosahedral capsid is assembled. Three asymmetric and nonidentical viral protein units make up each of the twenty identical triangular faces in the icosahedral capsid.

The capsid of some viruses are enclosed in a membrane called the viral envelope. In most cases, the viral envelope is obtained by the capsid from the host cell's plasma membrane when a virus leaves its host cell through a process called budding. The viral envelope is made up of a lipid bilayer embedded with viral proteins, including viral glycoproteins. These viral glycoproteins bind to specific receptors and coreceptors on the membrane of host cells, and they allow viruses to attach onto their target host cells. Some of these glycoproteins include:

Viral glycoproteins play a critical role in virus-to-cell fusion. Virus-to-cell fusion is initiated when viral glycoproteins bind to cellular receptors.

The fusion of the viral envelope with the cellular membrane requires high energy to occur. Viral membrane fusion proteins act as catalysts to overcome this high energy barrier. Following viral glycoprotein binding to cellular receptors, viral membrane fusion proteins undergo a change in structure conformation. This change in conformation then facilitates the destabilization and fusion of the viral envelope with the cellular membrane by allowing fusion loops (FLs) or hydrophobic fusion peptides (FPs) on the viral envelope to interact with the cell membrane. Most viral membrane fusion proteins would end up in a hairpin-like conformation after fusion, in which FLs/FPs and the transmembrane domain are all on the same side of the protein.

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