Membrane interactions of mutated forms of the influenza fusion peptide

We have studied a group of fusion peptides of influenza hemagglutinin in wh ich the N-terminal amino acid, Gly (found in the wild-type peptide), has be en systematically substituted with Ala, Ser, Val, or Glu. The activity of t he intact hemagglutinin protein with these same substitutions has already b een reported. As a measure of the extent of modulation of intrinsic membran e curvature by these peptides, we determined their effects on the polymorph ic phase transition of dipalmitoleoylphosphatidylethanolamine. The wild-typ e peptide is the only one that. at pH 5, can substantially decrease the tem perature of this transition. This is also the only form in which the intact protein promotes contents mixing in cells. The Ala and Ser mutant hemagglu tinins exhibit a hemifusion phenotype, and their fusion peptides have littl e effect on lipid polymorphism at low pH. The two mutant proteins that are completely fusion inactive are the Val and Glu mutant hemagglutinins. The f usion peptides from these forms significantly increase the polymorphic phas e transition temperature at low pH. We find that the effect of the fusion p eptides on membrane curvature, as monitored by a shift in the temperature o f this polymorphic phase transition, correlates better with the fusogenic a ctivities of the corresponding protein than do measurements of the isotropi c P-31 NMR signals or the ability to induce the fusion of liposomes. The in activity of the hemagglutinin protein with the hydrophobic Val mutation can be explained by the change in the angle of membrane insertion of the helic al fusion peptide as measured by polarized FTIR. Thus, the nature of the in teractions of the fusion peptides with membranes can, in large part, explai n the differences in the fusogenic activity of the intact protein.