(HIV). Viral glycoproteins play a critical role in virus-to-cell fusion. Virus-to-cell fusion is initiated when viral glycoproteins bind to cellular receptors. 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 Viral nonstructural proteins are proteins coded for by the genome of the virus and are expressed in infected cells. However, these proteins are not assembled in the virion. During the replication of viruses, some viral nonstructural proteins carry out important functions that affect the replication process itself. Similarly, during the assembly of viruses, some of these proteins also carry out important functions