Short-chain alcohols promote an early stage of membrane hemifusion

Hemifusion, the linkage of contacting lipid monolayers of two membranes bef ore the opening of a fusion pore, is hypothesized to proceed through the fo rmation of a stalk intermediate, a local and strongly bent connection betwe en membranes, When the monolayers' propensity to bend does not support the stalk (e.g., as it is when lysophosphatidylcholine is added), hemifusion is inhibited. In contrast, short-chain alcohols, reported to affect monolayer bending in a manner similar to that of lysophosphatidylcholine, were here found to promote hemifusion between fluorescently labeled liposomes and pla nar lipid bilayers, Single hemifusion events were detected by fluorescence microscopy. Methanol or ethanol (1.2-1.6 w/w %) added to the same compartme nt of the planar bilayer chamber as liposomes caused a 5-50 times increase in the number of hemifusion events. Alcohol-induced hemifusion was inhibite d by lysophosphatidylcholine. Promotion of membrane hemifusion by short-cha in alcohol was also observed for cell-cell fusion mediated by influenza vir us hemagglutinin (HA). Alcohol promoted a fusion stage subsequent to the lo w pH-dependent activation of HA. We propose that binding of short-chain alc ohol to the surface of membranes promotes hemifusion by facilitating the tr ansient breakage of the continuity of each of the contacting monolayers, wh ich is required for their subsequent merger in the stalk intermediate.