It is a fundamental question whether sphingomyelin (SM)- and cholesterol (Chol)- driven nanodomains exist in cells and in model membranes. Studies on model membranes revealed SM and Chol driven micrometer-sized liquid-ordered domains. Although the existence of such microdomains has not been proven for the plasma membrane, such lipid mixtures have been used as a model system for ‘rafts’ . On the other hand, super resolution results indicate that the plasma membrane might organize into nanocompartments. However, due to the limited resolution of those techniques the characterization of such nanodomains is still missing.
In this lecture, a combination of Förster resonance energy transfer and Monte Carlo simulations (MC-FRET)  identifies directly 10 nm large nanodomains in liquid-disordered model membranes composed of lipid mixtures containing SM and Chol . That MC-FRET approach can determine the sizes and concentrations of nanodomains down to 2 nm and enables studying the nanodomain inter-leaflet coupling [4,5]. Combining MC-FRET with solid-state wide-line and high resolution magic angle spinning NMR  as well as with fluorescence correlation spectroscopy  we demonstrate that these nanodomains containing hundreds of lipid molecules are fluid and interleaflet coupled [3,4].
Addition of GM1 ganglioside, a molecule which forms already at concentrations of 2% fluid nanometer sized clusters in phosphatidylcholine bilayers [7,8], leads to growth of those SM/ Chol nanodomains while preserving the fluidity. We examine the importance of the bulky ganglioside headgroup to the nanoscopic segregation of gangliosides by reducing the size of the headgroup from ganglioside GM1, to ganglioside GM2 and GM3. In this way, we identified the key molecular factors that drive nanoscopic segregation of gangliosides in lipid bilayers .
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