Amino Acid Substitutions at Position 481 Differently Affect the Ability of the Measles Virus Hemagglutinin to Induce Cell Fusion in Monkey and Marmoset Cells Co-expressing the Fusion Protein

The hemagglutinin (H) protein of the measles virus (MV) Edmonston strain induced cell fusion in Cos (monkey) and B95a (marmoset) cells, when co-expressed with the fusion (F) protein, whereas the H protein of the wild-type KA strain induced fusion in B95a cells, but not in Cos cells. Asparagine residue at position 481 of the KA H protein was replaced by various amino acids through site-directed mutagenesis. Substitution with tyrosine, which was found at position 481 of the Edmonston H protein, enabled the mutant KA H protein (N481Y) to induce cell fusion in Cos cells co-expressing the F protein, which could be completely blocked by anti-CD46 antibody. This mutant, however, did not cause CD46 downregulation, unlike the Edmonston H protein. The other H protein mutants (N481S, N481T, N481D, N481H, N481F) did not produce syncytia in Cos cells. On the other hand, all of the mutants retained the ability to induce cell fusion in B95a cells. Thus, while tyrosine at position 481 was indispensable for the MV H protein's interaction with CD46, the residue at this position does not appear to be critically involved in the interaction with the receptor for wild-type strains present on B95a cells.