Viral envelope

A viral envelope is the outermost layer of many types of viruses.[1] It protects the genetic material in their life cycle when traveling between host cells. Not all viruses have envelopes.

Schematic of a Cytomegalovirus, coat = envelope

The envelopes are typically derived from portions of the host cell membranes (phospholipids and proteins), but include some viral glycoproteins. Being made up mostly of host membrane, the viral envelope can also have the proteins associated with the host cell within their membrane after budding.[2] They may help viruses avoid the host immune system. Glycoproteins on the surface of the envelope serve to identify and bind to receptor sites on the host's membrane. The particular set of viral proteins are engaged in a series of structural changes. When these changes are set/finished, there is then and only then, fusion with the host membrane.[3] These glycoproteins mediate the interaction between virion and host cell, typically initiating the fusion between the viral envelope and the host's cellular membrane.[4] In some cases, the virus with an envelope will form an endosome within the host cell.[5] There are three main types of viral glycoproteins: Envelope proteins, membrane proteins, and spike proteins (E, M, and S).[6] The viral envelope then fuses with the host's membrane, allowing the capsid and viral genome to enter and infect the host.

All enveloped viruses also have a capsid, another protein layer, between the envelope and the genome.[1] The capsid, having a focused role of protecting the genome in addition to immune recognition evasion.[7] The viral capsid is known for its protection of RNA before it is inserted into the host cell, unlike the viral envelope which protects the protein capsid.[8]

The cell from which a virus buds often dies or is weakened, and sheds more viral particles for an extended period. The lipid bilayer envelope of these viruses is relatively sensitive to desiccation, heat, and amphiphiles such as soap and detergents, therefore these viruses are easier to sterilize than non-enveloped viruses, have limited survival outside host environments, and typically must transfer directly from host to host. Viral envelope persistence, wither it be enveloped or naked, are a factor in determining longevity of a virus on inanimate surfaces.[9] Enveloped viruses possess great adaptability and can change in a short time in order to evade the immune system. Enveloped viruses can cause persistent infections.

Vaccination against enveloped viruses can function by neutralizing the glycoprotein activity with antibodies.[10]