Most viral structural proteins are components for the capsid and the envelope of the virus.
Capsid 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 or other types of pesticides or pestilences. 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. The viral envelope is made up of a
lipid bilayer embedded with viral proteins, including viral
glycoproteins.
Viral membrane fusion proteins 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. Viral glycoproteins and their three-dimensional structures, before and after fusion, have allowed a wide range of structural conformations to be discovered. Viral membrane fusion proteins have been grouped into four different classes, and each class is identified by characteristic structural conformations: • Class I: Post-fusion conformation has a distinct central coiled-coil structure composed of signature trimer of α-helical hairpins. An example of a Class I viral fusion protein is the HIV glycoprotein, gp41. • Class II: Protein lacks the central coiled-coil structure. Contains a characteristic elongated β- sheet ectodomain structure that refolds to give a trimer of hairpins. Examples of class II viral fusion proteins include the dengue virus E protein, and the west nile virus E protein. • Class III: Structural conformation is a combination of features from Class I and Class II viral membrane fusion proteins. An example of a Class III viral fusion protein is the rabies virus glycoprotein, G. • Class IV: Class IV viral fusion proteins are fusion-associated small transmembrane (FAST) proteins. They do not form trimers of hairpins or hairpin structures themselves, and they are the smallest known viral fusion proteins. FAST proteins are coded for by members of the nonenveloped
reoviridae family of viruses. == Viral nonstructural proteins ==