Negative tension induced by lipid uptake.

Membrane fusion is an important process in cell biology. While the molecular mechanisms of fusion are actively studied at a very local scale, the consequences of fusion at a larger scale on the shape and stability of the membrane are still not explored. In this Letter, the evolution of the membrane tension during the fusion of positive small unilamellar vesicles with a negative giant unilamellar vesicle has been experimentally investigated and compared to an existing theoretical model. The tension has been deduced using videomicroscopy from the measurement of the fluctuation spectrum and of the time correlation function of the fluctuations. We show that fusion induces a strong decrease in the effective tension of the membrane which eventually reaches negative values. Under these conditions, we show that localized instabilities appear on the vesicle. The membrane finally collapses, forming dense lipid structures.