Atomic force microscopy studies of ganglioside GM1 domains in phosphatidylcholine and phosphatidylcholine/cholesterol bilayers

The distribution of ganglioside in supported lipid bilayers has been studie d by atomic force microscopy. Hybrid dipalmitoylphosphatidylcholine (DPPC)/ dipalmitoylphosphatidylethanolamine (DPPE) and (2:1 DPPC/cholesterol)/DPPE bilayers were prepared using the Langmuir Blodgett technique. Egg PC and DP PC bilayers were prepared by vesicle fusion. Addition of ganglioside GM1 to each of the lipid bilayers resulted in the formation of heterogeneous surf aces that had numerous small raised domains (30-200 nm in diameter). Incuba tion of these bilayers with cholera toxin B subunit resulted in the detecti on of small protein aggregates, indicating specific binding of the protein to the GM1-rich microdomains. Similar results were obtained for DPPC, DPPC/ cholesterol, and egg PC, demonstrating that the overall bilayer morphology was not dependent on the method of bilayer;reparation or the fluidity of th e lipid mixture. However, bilayers produced by vesicle fusion provided evid ence for asymmetrically distributed GM1 domains that probably reflect the p resence of ganglioside in both inner and outer monolayers of the initial ve sicle. The results are discussed in relation to recent inconsistencies in t he estimation of sizes of lipid rafts in model and natural membranes. It is hypothesized that small ganglioside-rich microdomains may exist within lar ger ordered domains in both natural and model membranes.