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The fusion (F) protein of parainfluenza virus 5 (PIV-5) strain W3A is able to induce cell fusion when it is expressed alone in baby hamster kidney cells, whilst the F protein of PIV-5 strain WR induces cell fusion only when co-expressed with the haemagglutinin-neuraminidase (HN) protein. It has been shown previously that when Leu-22 of the WR F protein is replaced with the W3A F counterpart (Pro-22), the resulting mutant L22P exhibits HN-independent fusion activity. Furthermore, previous chimeric analysis between L22P and the F protein of PIV-5 strain T1 has suggested that Glu-132 also contributes to the HN-independent fusion activity of L22P. It was shown here that substitution of Glu-132 of L22P with various amino acids including the T1 F protein counterpart (Lys-132) resulted in a reduction in fusion activity, whereas substitution with Asp was the exception in being tolerated. Interestingly, reduced fusion activity of an L22P mutant that harboured the E132K substitution could be restored by an additional D416K substitution but not by a D416E mutation, suggesting that the presence of the same charge at positions 132 and 416 is important for the HN-independent fusion activity. In contrast, substitution of Leu-22 of the WR F protein with various amino acids except those with aliphatic side chains resulted in acquisition of fusion activity, suggesting that the HN dependence of the WR F protein in the induction of cell fusion is attributable to the hydrophobicity of Leu-22. These results indicate that at least three amino acids are involved in the HN-independent fusion activity of the PIV-5 F protein.

Effects of multiple amino acids of the parainfluenza virus 5 fusion protein on its haemagglutinin–neuraminidase-independent fusion activity