The coronavirus peplomer protein S is responsible for attachment and fusion 
during viral entry as well as for the induction of cell to cell fusion. While 
several regions within S have been shown to influence the ability to induce 
fusion, the region of the protein actually responsible for fusion, the fusion 
peptide, has not yet been identified. We identified two hydrophobic peptides 
(peptides #1 and #2) within MHV-A59 S2 as possible fusion domains. This was 
based on hydrophobicity, conservation among coronavirus S proteins and the 
prediction of a sided helix conformation. Using site directed mutagenesis and an 
in vitro cell to cell fusion assay we showed that substitution of hydrophobic 
amino acids with charged amino acids, within the predicted hydrophobic face of 
either of these two peptides eliminated fusion. Within peptide #1 substitution 
of the same hydrophobic amino acids with other hydrophobic amino acids or 
substitution of polar amino acids with charged or polar amino acids had little 
effect on fusion. Thus peptides #1 and #2 remain likely candidates for the MHV 
fusion peptide. A third previously identified peptide within S2 (Chambers et 
al., 1990) is unlikely as a fusion peptide as it is not well conserved among 
coronaviruses and substitution within the hydrophobic face with charged amino 
acids does not effect fusion.