To locate sites important for the structure and function of the haemagglutinin—neuraminidase glycoprotein (HN) of Sendai virus, the biological characteristics of antibody-selected escape mutants were correlated with mutations in the primary HN amino acid sequence. An escape mutant virus deficient only in neuraminidase function but with an HN content equal to that of the wild-type virus had an amino acid change at residue 184, implying that this position may be important for maintaining a functionally active enzymic site. In contrast, other escape mutant viruses with reductions in haemagglutination (eightfold) and neuraminidase activities (70 to 80%) had a sharply diminished HN content and substitutions either at residue 375, or double mutations at residues 279 and 461. The loss of biological activity with the concomitant loss of HN content suggests that these sites may be important for the processing and transport of HN, or in maintaining a structure resistant to proteolytic degradation; residue 451 was shown to have an undefined role in fusion activity. The monoclonal antibodies (MAbs) used to isolate the mutant viruses included those of the IgA and IgG classes and were divided into four operational groups based on their haemagglutination-inhibition pattern against the selected mutants. MAbs of the IgA class recognized epitopes overlapping with (group A) as well as epitopes distinct from (groups C and D) those recognized by the IgG class; group B included only IgG antibodies. The epitopes recognized by IgA antibodies may identify residues important for the secretory immune response to the HN molecule.


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