General Structural Features of Enveloped Viruses

The acquisition of a lipid bilayer from the infected host cell is the defining feature of enveloped viruses. However, the origin and composition of the bilayer can differ from virus to virus. Viruses are known to acquire a membrane, or bud, from various cellular compartments that include the nucleus, the endoplasmic reticulum (ER), the Golgi, and the plasma membrane. As the Upid contents of those membranes differ, so do the contents of the viral membranes. In some, if not all, cases the composition of the lipid bUayer is an important aspect of the viral assembly pathway. 

In addition to the lipid bilayer, enveloped viruses generally have two or more distinct layers of protein that are organized across the membrane. Thus, most viruses have an outer layer of proteins, usually glycoproteins, which are anchored in the membrane as integral membrane proteins. These proteins function to attach the virion to target host cell receptors and facilitate the entry or fusion of the viral membrane with that of the host cell. In addition, some viruses also contain enzymatic activities associated with this outer layer of protein. For example, influenza virus carries with it a neuraminidase that is responsible for cleaving sialic acid residues on host cells. 

On the interior of the lipid bilayer, a complex of protein and nucleic acid is found. This complex is usually referred to as a nucleocapsid core, and in many instances has an organized protein shell within which nucleic acid, possibly in complex with additional proteins, is found. These additional proteins may be nucleic acid-binding proteins as well as proteins necessary for genome replication. Some of the larger DNA viruses include many additional proteins within this core. 

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