Difference in molecular mechanism between envelope-cell fusion and cell-cell fusion.

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1.By replacing Leu-22 of the F protein of simian virus 5 (WR strain) with Pro, a mutant F protein, L22P, was obtained which induced the HN-independent cell-cell fusion. Mutational analysis of L22P has suggested that either the hydrophobic interaction between the F2 N-terminus and fusion peptide or the electrostatic interaction between the HR1 domain and a region downstream of the Cys-rich domain affects the fusing activity of L22P.2.Se-L22P is a mutant F protein, in which the cleavage site of L22P is replaced with that of the Sendai virus F protein. As the result, Se-L22P can be cleaved and can induce cell-cell fusion only when treated with trypsin. We found that Se-L22P located in the nonraft domains, but not in the raft domains, of the plasma membrane undergoes a conformational change upon treatment with trypsin and that cell-cell fusion is induced after this conformational change.3.By propagating human parainfluenza virus type 2 (PIV2:Toshiba strain) in the presence of an anti-HN monoclonal antibody (M1-1A), an escape mutant, F13, was selected which exhibited very low pathogenicity (or cell-cell fusion). Analysis employing a set of mutant viruses that were produced by reverse genetics, it has been shown that mutations at positions 83 and 186 of the F13 HN protein are both required for the low cytopathogenicity, whereas these mutations do not affect virus-cell fusion. These results suggest that there is difference in molecular mechanism between cell-cell fusion and virus-cell fusion and that an unknown cellular factor in the infected cell is involved. In contrast to PIV3, no correlation was observed between the neuraminidase activity and the cytopathogenicity of PIV2, suggesting that the difference in virulence between PIV2 and PIV3 may reflect the difference in neuraminidase function.