The Measles Virus Fusion Protein Transmembrane Region Modulates Availability of an Active Glycoprotein Complex and Fusion Efficiency

The glycoprotein complex of paramyxoviruses mediates receptor binding and membrane fusion. In particular, the measles virus (MV) fusion (F) protein executes membrane fusion, after receptor binding by the hemagglutinin (H) protein. Structures and single amino acids influencing fusion function have been identified in the F-protein ectodomain and cytoplasmic tail, but not in its transmembrane (TM) region. Since this region influences function of the envelope proteins of other viruses, we examined its role in the MV F protein. Alanine-scanning mutagenesis revealed that an F protein with a single mutation of a central TM region leucine (L507A) was more fusogenic than the unmodified F protein while retaining similar kinetics of proteolytic processing. In contrast, substitution of residues located near the edges of the lipid bilayer reduced fusion activity. This was true not only when the mutated F proteins were coexpressed with H but also in the context of infections with recombinant viruses. Analysis of the H-F complexes with reduced fusion activities revealed that more precursor (F-0) than activated (F1+2) protein coprecipitated with H. In contrast, in complexes with enhanced fusion activity, including H-F-L507A, the F-0/F1+2 ratio shifted toward F1+2. Thus, fusion activity correlated with an active F-H protein complex, and the MV F protein TM region modulated availability of this complex.