AN EARLY-STAGE OF MEMBRANE-FUSION MEDIATED BY THE LOW PH CONFORMATIONOF INFLUENZA HEMAGGLUTININ DEPENDS UPON MEMBRANE-LIPIDS

While the specificity and timing of membrane fusion in diverse physiol ogical reactions, including virus-cell fusion, is determined by protei ns, fusion always involves the merger of membrane lipid bilayers. We h ave isolated a lipid-dependent stage of cell-cell fusion mediated by i nfluenza hemagglutinin and triggered by cell exposure to mildly acidic pH. This stage preceded actual membrane merger and fusion pore format ion but was subsequent to a low pH-induced change in hemagglutinin con formation that is required for fusion. A low pH conformation of hemagg lutinin was required to achieve this lipid-dependent stage and also, d ownstream of it, to drive fusion to completion. The lower the pH of th e medium applied to trigger fusion and, thus, the more hemagglutinin m olecules activated, the less profound was the dependence of fusion on lipids. Membrane-incorporated lipids affected fusion in a manner that correlated with their dynamic molecular shape, a characteristic that d etermines a lipid monolayer's propensity to bend in different directio ns. The lipid sensitivity of this stage, i.e., inhibition of fusion by inverted cone-shaped lysophosphatidylcholine and promotion by cone-sh aped oleic acid, was consistent with the stalk hypothesis of fusion, s uggesting that fusion proteins begin membrane merger by promoting the formation of a bent, lipid-involving, stalk intermediate.