The fusion promotion activity of the NDV HN protein does not correlate with neuraminidase activity

Three activities, attachment, neuraminidase, and fusion promotion, have been associated with the hemagglutinin-neuraminidase (HN) protein encoded by paramyxoviruses such as Newcastle disease virus. The fusion promotion activity of the HN protein can be separated from its attachment activity by mutation (Sergel et al., 1993, Virology 193, 717-726). To determine if neuraminidase activity of the HN protein has any role in fusion promotion, two sets of mutants were characterized. First, a change of amino acid 193 from a serine to a proline and a change of amino acid 175 from isoleucine to a methionine diminished neuraminidase activity as previously reported. However, these mutant proteins retained fusion promotion activity. In addition, mutation of amino acid 200 from a histidine to a proline resulted in nearly twice the neuraminidase activity of wild-type as previously reported. This mutant also had wild-type levels of fusion promotion activity. Second, substitution of three leucine residues at amino acids 94, 96, and 97 with three alanines resulted in a mutant protein with full neuraminidase as well as full attachment activity but no fusion promotion activity. Thus, two sets of HN protein mutants demonstrate that the fusion promotion activity does not correlate with the level of neuraminidase activity.