Egg-grown virus of an influenza A strain (virus N), produces highly infectious particles in this host with its haemagglutinin glycoprotein present in the cleaved form. It also contains relatively large amounts of mucopolysaccharide. This substance cannot be detected in virus derived from cultures of chick embryo cells which has uncleaved haemagglutinin and reduced infectivity. These observations indicate that the host-dependent differences in infectivity cannot be attributed to the presence of mucopolysaccharide as a masking substance at the virus surface, and further substantiate the essential role of the cleavage of the haemagglutinin for activation of infectivity.

Cleavage of the precursor HA into fragments HA and HA can be accomplished by a variety of proteases. However, only cleavage by trypsin or a trypsin-like enzyme results in formation of highly infectious virus. Activation of infectivity therefore requires cleavage of a specific peptide bond with arginine or lysine in carboxyl linkage.

Double infection experiments demonstrate that virus N displays neither cleavage of the haemagglutinin nor formation of highly infectious virus under conditions where both phenomena are observed with fowl plague virus, another influenza A strain. This observation demonstrates that the relative resistance of the haemagglutinin to cleavage, and thus the resistance of the virus to activation, is a genuine structural property of this glycoprotein rather than the consequence of a low level of proteolysis in infected cells.


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