Sialic acid is incorporated into influenza hemagglutinin glycoproteins in the absence of viral neuraminidase.

We have analyzed the pronase-derived glycopeptides of the hemagglutinin glycoproteins expressed from SV40 vectors carrying cloned cDNA copies of the HA gene and of HA isolated from influenza virions (A/Jap/305/57). The glycopeptides derived from he HA glycoprotein obtained from cloned genes were heterogeneous, ranging in size from 3800 to 2800 daltons. Upon treatment with neuraminidase, sialic acid was released from the glycopeptides and their size was reduced to 2900-2400 daltons. However, under the same conditions, no sialic acid was detected in the virion HA. The presence of sialic acid was confirmed by monosaccharide analysis of the HA glycoprotein derived from products of cloned genes. These results support the idea that during replication of influenza virus, the viral neuraminidase cleaves sialic acid from the HA glycoprotein in infected cells.