Knowledge of the structure of viral membrane bilayers has come chiefly from the study of a few viruses that are easily grown in large quantities in the laboratory, namely the orthomyxo-, paramyxo-, rhabdo- and togaviruses. In these, the bilayer arrangement of the lipids has been directly demonstrated using physical methods, and the lipid composition of various viruses grown under different conditions has been described in detail. Since all of these viruses acquire their bilayer by budding through the host cell's plasma membrane, the viral membrane contains the lipids present there. Wide variations of lipid composition are tolerated, and most of the lipids display the properties characteristic of lipids in a bilayer, not those of protein-bound lipids. The precise content of each individual phospholipid or glycolipid in a viral membrane does not always reflect the bulk composition of the host cell membrane from which it was derived, however. This difference may arise from interactions of lipids with the viral membrane proteins, or from inhomogeneity in the host cell membrane.
Intact virions are impermeant to proteases and other enzymes. Indeed, virions can swell and shrink in response to changes in osmolarity, showing that the viral membrane is impermeant to small molecules and ions as well as large proteins. This property indicates that the viral membrane consists of an intact bilayer, completely surrounding the encapsidated viral gen ome. It is generally assumed that intact bilayers are characteristic of all enveloped viruses, and not just for those few for which this property has actually been demonstrated.
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