STUDIES USING DOUBLE MUTANTS OF THE CONFORMATIONAL TRANSITIONS IN INFLUENZA HEMAGGLUTININ REQUIRED FOR ITS MEMBRANE-FUSION ACTIVITY

Amino acid substitutions widely distributed throughout the influenza h emagglutinin (HA) influence the pH of its membrane fusion activity. We have combined a number of these substitutions in double mutants and d etermined the effects on the pH of fusion and on the pH at which the r efolding of HA required for fusion occurs, By analyzing combinations o f mutations in three regions of the metastable neutral-pH HA that are rearranged at fusion pH we obtain evidence for both additive and nonad ditive effects and for an apparent order of dominance in the effects o f amino acid substitutions in particular regions on the pH of fusion. We conclude that there are at least three components in the structural transition required for membrane fusion activity and consider possibl e pathways for the transition in relation to the known differences bet ween neutral and fusion pH HA structures.