Viral envelope

A viral envelope is a lipid membrane surrounding the internal structures of some virus particles. It contains host-derived lipids together with viral membrane proteins, often glycoproteins. It is structurally distinct from a capsid, the protein-based packaging associated with the viral genome.^[ref]

Formation of the envelope connects viral assembly to the organization of a host biological cell. In a 1978 experiment with polarized epithelial cells, influenza particles emerged from the free apical surface, whereas vesicular stomatitis virus acquired its envelope at the basolateral membrane. The same cellular setting therefore supported different sites of viral budding for different viruses.^[ref]

This observation makes the envelope a useful probe of membrane organization. Its host-derived material does not mean it is an arbitrary piece of unchanged host membrane: the completed particle has an organized set of viral components. Identifying where budding occurs and identifying which components enter the particle are related but different experimental questions.

Viral budding is a mode of particle assembly and release in which a host membrane curves around viral components and becomes the viral envelope of a departing virion. It connects the assembly of internal viral structures to organization of the membrane at the release site. The envelope is consequently derived from a particular cellular membrane region, rather than added later as an unrelated coat.^[ref]

A 1978 experiment used polarized epithelial cells grown as monolayers. Influenza particles budded from the apical surface, while vesicular stomatitis virus acquired its envelope from the basolateral surface. Those contrasting locations within the same type of cell made viral release a probe of membrane organization.^[ref]

The example separates three measurable events: producing viral components, assembling a particle and releasing it at a particular surface. More components inside a biological cell do not automatically demonstrate more released virions. Likewise, a change in release location need not mean that the viral genome changed. Structural and molecular measurements must be connected to the particular step being tested.