Assembly of lipid‐containing viruses

Enveloped viruses which form by budding at the cell surface possess a membrane consisting of a lipid bilayer and a small number of virus‐coded polypeptides. Since viral polypeptides become integral components of the plasma membrane during assembly, the process of synthesis and incorporation into membranes of these proteins may reflect the pathway of plasma membrane assembly. Electron microscopic studies have suggested that viral envelope proteins are incorporated into discrete, localized regions of the plasma membrane which serve as recognition sites for the viral nucleocapsid. In influenza virus‐infected cells, viral polypeptides are associated with cytoplasmic membranes as well as the plasma membrane. The major envelope glycoprotein appears to be synthesized in rough endoplasmic reticulum, and to migrate to smooth membranes after synthesis. Glycosylation is initiated in rough membranes and progresses further in smooth membranes. Unlike the glycoproteins, the major nonglycosylated polypeptide appears to be inserted directly into the plasma membrane. In the presence of 2‐deoxyglucose or high concentrations of glucosamine, aberrant viral glycoproteins are detected which appear to be unglycosylated or partially glycosylated; these are associated with membranes and incorporated into virus particles of reduced infectivity. Therefore normal glycosylation is not essential for incorporation of viral glycoproteins into cellular membranes or virus particles, but is required for normal biological activity. The role of the viral neuraminidase in assembly and release has been studied using mutants defective in neuraminidase at restrictive temperature. Under these conditions virus formation occurs, but the progeny form large aggregates at the cell surface. Colloidal iron hydroxide staining indicates that such virus particles contain neuraminic acid, and these residues appear to serve as receptors leading to the extensive aggregation.