Membrane fusion protein, p10, of 95 aas and 1 central TMS. It has a cytoplasmic basic region and an N-terminal hydrophobic domain (HD) that has been hypothesized to function as a fusion peptide. Bulky aliphatic residues were found to be essential for optimal fusion, and an aromatic or highly hydrophobic side chain was found to be required at position 12 (Cheng et al. 2005). The requirement for hydrophilic residues within the HD was also examined: substitution of 10-Ser or 14-Ser with hydrophobic residues was found to reduce cell surface expression of p10 and delayed the onset of syncytium formation. Nonconservative substitutions of charged residues in the HD did not have an effect on fusion activity (Cheng et al. 2005).