Viral envelope

A viral envelope is a lipid bilayer that forms the outer layer of an enveloped virus, surrounding the protein shell, or capsid, that holds the genome. The lipids come from the host: as new virus particles leave an infected cell, they bud through one of its membranes and wrap themselves in a piece of it. Which membrane depends on the virus. Influenza and HIV take theirs from the plasma membrane, herpesviruses from the nuclear membrane and coronaviruses from internal membranes near the endoplasmic reticulum. The proteins studding the envelope, however, are made from the virus's own genes.

These envelope glycoproteins, often seen as spikes, do the key work of infection. They bind receptors on target cells and then fuse the envelope with a cell membrane, releasing the capsid inside; examples are influenza hemagglutinin, the HIV envelope protein and the coronavirus spike. Matrix proteins line the inner face of the envelope and help assemble new particles. Because they sit on the surface, envelope proteins are the main targets of neutralizing antibodies and of many vaccines.

The envelope is fragile. Soap, detergents, alcohol, drying and heat disrupt it, so enveloped viruses such as influenza, HIV and SARS-CoV-2 are easier to inactivate outside the body than naked, or nonenveloped, viruses such as norovirus, poliovirus and adenovirus, which survive longer on surfaces.