Structural features unique to each of the three antigenic sites on the hemagglutinin-neuraminidase protein of Newcastle disease virus.

Antigenic variants of D26 strain of Newcastle disease virus (NDV) were selected with monoclonal antibodies directed to the three nonoverlapping antigenic sites on the hemagglutinin-neuraminidase (HN) protein, and their HN genes were sequenced to identify the amino acids important for the integrity of each site. Seven variants for site I, which is immunodominant and conserved among NDV strains, had a change of glutamic acid at position 347, mostly to lysine, and in a single case, to glycine. In the second group of two variants for site IV, a change of asparagine to aspartic acid was found at position 481. This resulted in elimination of the oligosaccharide attached to this asparagine residue of the parental virus. Together with the finding that the site IV was destroyed by treatment with endoglycosidase F, it was suggested that the oligosaccharide is important for maintaining the structure of site IV. The oligosaccharide appeared to contribute to exposing a nearby determinant by conferring hydrophilicity on it. A variant for site II had also a nonconservative mutation resulting in the change of glutamic acid to valine at position 495. The site I recognized by antibodies which inhibit neuraminidase activity with a small substrate neuraminlactose was located closer to the predicted sialic acid-binding site than to the other sites recognized by antibodies lacking the enzyme-inhibiting capacity. The sequence of the parental virus HN gene revealed that the HNo precursor for the HN protein is an extra-long protein whose C terminus is elongated by 45 amino acids, compared with the usual HN protein sequenced in parallel.