Post-translational cleavage of the influenza viral glycoprotein HA occurs to different extents in different systems, varying not only for a particular virus strain grown in different host cells, but also for two strains of virus grown in the same host cell. Preparations of virus in which the HA is not substantially cleaved contain hemagglutinating and infectious virions. Cleavage occurs at different sites in the HA molecule of different virus strains. The hemagglutinin glycoprotein HA is always found in association with cytoplasmic membranes and becomes rapidly incorporated into plasma membranes. Following a 10-min pulse-label, there is already about half as much HA in preparations of plasma membranes as eventually accumulates there during a 90-min chase. Membrane preparations which appear to be mainly composed of smooth endoplasmic reticulum are greatly enriched for HA while plasma membranes contain HA and the other major viral proteins. At 24 hr after infection, the amount of HA or its cleavage products in BHK21 cells infected with Bel or WSN represents a much smaller proportion of the total viral protein than the proportion of HA in purified virions. The same is true for the membrane protein, M, whereas NP is present in excess in the infected cell. Inhibition of protein synthesis by puromycin stops the incorporation of glucosamine into Bel-infected HeLa cells almost immediately, suggesting that glycosylation of HA occurs quickly. However, fucose continues to be incorporated for apporoximately 10-15 min after protein synthesis has been blocked by puromycin or after glycosiliation has been inhibited by glucosamine hydrochloride.
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