Detergents induce raft-like domains budding and fission from giant unilamellar heterogeneous vesicles: a direct microscopy observation.

The effect of detergents on giant unilamellar vesicles (GUVs) composed of phosphatidylcholine, sphingomyelin and cholesterol and containing liquid-ordered phase (l(o)) domains was investigated. Such domains have been used as models for the lipid rafts present in biological membranes. The studied detergents included lyso-phosphatidylcholine, the product of phospholipase A2 activity, as well as Triton X-100 and Brij 98, i.e. detergents used to isolate lipid rafts as DRMs. Local external injection of each of the three detergents at subsolubilizing amounts promoted exclusion of l(o) domains from the GUV as small vesicles. The budding and fission processes associated with this vesiculation were interpreted as due to two distinct effects of the detergent. In this framework, the budding is caused by the initial incorporation of the detergent in the outer membrane leaflet which increases the spontaneous curvature of the bilayer. The fission is related to the inverted-cone molecular shape of the detergent which stabilizes positively curved structures, e.g. pores involved in vesicle separation. On the other hand, we observed in GUVs neither domain formation nor domain coalescence to be induced by the addition of detergents. This supports the idea that isolation of DRM from biological membranes by detergent-induced extraction is not an artifact. It is also suggested that the physico-chemical mechanisms involved in l(o) domain budding and fission might play a role in rafts-dependant endocytosis in cells.