STUDIES OF THE MEMBRANE-FUSION ACTIVITIES OF FUSION PEPTIDE MUTANTS OF INFLUENZA-VIRUS HEMAGGLUTININ

Influenza virus hemagglutinin (HA) fuses membranes at endosomal pH by a process which involves extrusion of the NH2-terminal region of HA(2) , the fusion peptide, from its buried location in the native trimer. W e have examined the amino acid sequence requirements for a functional fusion peptide by determining the fusion capacities of site-specific m utant HAs expressed by using vaccinia virus recombinants and of synthe tic peptide analogs of the mutant fusion peptides. The results indicat e that for efficient fusion, alanine cab to some extent substitute for the NH2-terminal glycine of the wild-type fusion Peptide but that ser ine, histidine, leucine, isoleucine, or phenylalanine cannot. In addit ion, mutants containing shorter fusion peptides as a result of single amino acid deletions are inactive, as is a mutant containing an alanin e instead of a glycine at HA(2) residue 8. Substitution of the glycine at HA(2) residue 4 with an alanine increases the pH of fusion, and va line-for-glutamate substitutions at HA(2) residues 11 and 15 are witho ut effect. We Confirm previous reports on the need for specific HA(0) cleavage to generate functional HAs, and we show that both inappropria tely cleaved HA and mutant HAs, irrespective of their fusion capacitie s, upon incubation at low pH undergo the structural transition require d for fusion.