An oligosaccharide at the C-terminus of the F-specific domain in the stalk of the human parainfluenza virus 3 hemagglutinin-neuraminidase modulates fusion.

The promotion of membrane fusion by the fusion (F) protein of human parainfluenza virus 3 (hPIV3) is dependent on a virus-specific contribution from the hemagglutinin-neuraminidase (HN) protein. By evaluation of chimeric hPIV3-Newcastle disease virus (NDV) HN proteins, we have previously shown that hPIV3-F-specificity is determined by a domain that extends from the middle of the membrane anchor to the 82nd residue in the ectodomain [Virology 209, (1995) 457; Arch. Virol. 13 (1997) 115]. If the corresponding NDV-derived residues replace the two C-terminal residues in this domain, no fusion is detected. However, these substitutions restore a glycosylation site present in NDV HN, but not in hPIV3 HN. Deletion of this site from a nested set of chimeras with hPIV3-derived N-terminal portions of decreasing length partially restores fusion, suggesting that an oligosaccharide near the top of hPIV3 HN stalk modulates fusion. In addition, further mutational analyses show that a chimera with only 125 N-terminal hPIV3-derived residues (72 in the stalk) actually promotes fusion more efficiently than the wt protein. These findings localize the C-terminus of the F-specific domain in hPIV3 HN a full 10 residues closer to the membrane than previously shown.