Role of overlapping glycosylation sequons in antigenic properties, intracellular transport and biological activities of influenza A/H2N2 virus haemagglutinin.

The haemagglutinin (HA) protein of influenza A/H2N2 virus possesses five oligosaccharide attachment sites, two of which have overlapping glycosylation sequons at positions 20-23 (NNST) and 169-172 (NNTS). Here, the role of these two oligosaccharide attachment sites is investigated with regard to antigenic property, intracellular transport and biological activity of the HA protein. Glycosylation-site HA mutants with mutation(s) in their overlapping glycosylated sequons, each of which had one or two oligosaccharide attachment sites removed, were constructed. Comparison of electrophoretic mobility between the wt and mutant HA proteins showed that both Asn residues 20 and 21 and Asn residues 169 and 170 could be used for glycosylation. Analysis of reactivity of the mutants with anti-HA monoclonal antibodies suggested that amino acid changes at these two positions result in a conformational change of the HA molecule. Even if oligosaccharide chains linked to Asn 20 or 21 and Asn 169 or 170 are eliminated, the antigenic properties, intracellular transport and biological activities are not influenced strongly. Thus it is reasonable to conclude that the two overlapping glycosylation sequons at positions 20-23 and 169-172 are conserved among all of the HAs of influenza A/H2N2 viruses because conservation of the amino acid sequence itself rather than that of N-glycosylation is essential for the formation of the proper conformation, intracellular transport and biological activities of the H2 subtype HA.