Fusion between influenza virus and cell membranes is mediated by a major acid-induced conformational change of the spike glycoprotein of the viral envelope, hemagglutinin (HA). The conformational change of HA is commonly believed to be a kinetically controlled irreversible process, although the experimental evidence for this is controversial. Here we show by polarized infrared spectroscopy that the previously described acid-induced inclination of HA reconstituted in supported phospholipid bilayers is reversible in the absence, but irreversible in the presence, of bound target membranes. We also demonstrate reversible pH-dependent changes in the capability of reconstituted HA to bind target membranes. These results support a thermodynamically controlled mechanism of the conformational change of HA and provide new insight into the understanding of the energetics of influenza-mediated membrane fusion.
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