An amino acid in the heptad repeat 1 domain is important for the haemagglutinin-neuraminidase-independent fusing activity of simian virus 5 fusion protein.

A canine isolate (strain T1) of simian virus 5 (SV-5) performed multiple replication in BHK cells but did not induce cell fusion for up to 3 days. In contrast, a prototype strain (WR) provoked extensive cell fusion within 2 days during the course of its replication. Accordingly, the fusion (F) protein of the T1 strain did not cause cell fusion even when co-expressed with the SV-5 haemagglutinin-neuraminidase (HN) protein, whereas the WR F protein induced cell fusion in the presence of the HN protein. Differences in the predicted amino acid sequences of the T1 and WR F proteins were found at 12 positions and it was proved that the T1 F protein had a longer cytoplasmic tail than the WR F protein. By reducing the length of the cytoplasmic tail or by replacing the tail with the WR F counterpart, the T1 F protein partly restored its HN-dependent fusing activity. Chimeric and mutational analyses between the T1 F protein and the mutant F protein (L22P) suggested that Glu-132 in the heptad repeat 1 domain was involved in the HN-independent fusing activity in addition to the previously identified Pro-22 at the F(2) N terminus. It was also shown that Ala-290 in the heptad repeat 3 domain contributed to the HN-independent fusing activity to some extent.