Résumé
The lipid rafts membrane microdomains, enriched in sphingolipids and cholesterol, are implicated in numerous functions of biological membranes. Using atomic force microscopy, we have examined the effects of cholesterol-loaded methyl-β-cyclodextrin (MβCD-Chl) addition to liquid disordered (ld)-gel phase separated dioleoylphosphatidylcholine (DOPC)/sphingomyelin (SM) and 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC)/SM supported bilayers. We observed that incubation with MβCD-Chl led to the disappearance of domains with the formation of a homogeneously flat bilayer, most likely in the liquid-ordered (lo) state. However, intermediate stages differed with the passage through the coexistence of lo-ld phases for DOPC/SM samples and of lo-gel phases for POPC/SM bilayers. Thus, gel phase SM domains surrounded by a lo matrix rich in cholesterol and POPC could be observed just before reaching the uniform lo state. This suggests that raft formation in biological membranes could occur not only via liquid-liquid but also via gel-liquid immiscibility. The data also demonstrate that MβCD-Chl as well as the unloaded cyclodextrin MβCD make holes and preferentially extract SM in supported bilayers. This strongly suggests that interpretation of MβCD and MβCD-Chl effects on cell membranes only in terms of cholesterol movements have to be treated with caution.