enveloped and nonenveloped viruses

All viruses can be divided into nonenveloped or enveloped, depending upon whether they have a lipid bilayer surrounding their nucleocapsid. This characteristic affects the mode of entry of the virus, since enveloped viruses can fuse to cells and nonenveloped viruses cannot.

Enveloped viruses obtain a lipid bilayer envelope around their nucleocapsid by budding from host cells. Prior to budding, viral glycoproteins are targeted to the cell membrane. When the virus buds, it takes with it a portion of the cellular membrane containing viral proteins. These proteins are necessary for the infectivity of the virus as they mediate cell attachment and fusion.

Nonenveloped viruses do not obtain a lipid bilayer upon exit from host cells. They usually exit the cell by lysis, rupturing the membrane and releasing virus. Thus, nonenveloped viruses consist of a naked nucleocapsid with no lipid bilayer.

Enveloped viruses enter cells by fusion of their envelope with cellular membranes. Fusion does not occur spontaneously: there is a thermodynamic barrier to the mixing of lipid bilayers that must be overcome. In the case of enveloped viruses, their envelope glycoproteins mediate the fusion reaction. For Influenza, the hemagluttinin (HA) glycoprotein mediates both the receptor-binding and fusion events via separate domains on the protein. Upon the complete fusion of the viral envelope to the cellular membrane, the nucleocapsid is released into the cell and viral replication can proceed.

Since they do not have a lipid envelope, nonenveloped viruses cannot enter cells by fusion. They can be taken up by endocytosis and then penetrate the cellular membrane to get into the cell. In the case of Reovirus, the virus binds to a cell receptor and is taken up by receptor-mediated endocytosis. It undergoes structural changes during its passage through the endosome and lysosome. It finally passes through the lysosomal membrane into the cytoplasm, possibly by making anionic channels in the membrane.