LIPID DOMAINS IN THE MEMBRANE - THERMOTROPIC PROPERTIES OF SPHINGOMYELIN VESICLES CONTAINING GM1 GANGLIOSIDE AND CHOLESTEROL

The thermotropic behavior of palmitoylsphingomyelin vesicles containin g GM1 ganglioside and cholesterol has been investigated by high-sensit ivity differential scanning calorimetry. The thermograms exhibited by binary palmitoylsphingomyelin/GM1 mixtures are resolvable into two com ponents. The relative contribution of the minor component, undetectabl e in the absence of ganglioside, to the total enthalpy and its transit ion temperature (>40 degrees C) increase with the concentration of the glycolipid embedded in the vesicles. These data suggest the occurrenc e of lateral phase separation and that more ordered, higher melting GM 1 ganglioside-enriched domains are present within the sphingomyelin bi layer. Studies on binary sphingomyelin/cholesterol mixtures confirmed the known tendency of the sterol to decrease the total enthalpy of sph ingomyelin, forming cholesterol-enriched domains. The thermograms exhi bited by ternary sphingomyelin/ganglioside/cholesterol mixtures in var iable proportions (up to 20% molar GM1 or Chol) displayed, on increasi ng the content of either the sterol or the ganglioside, features addre ssable to sphingomyelin/cholesterol (peaks centered at temperature 140 degrees C, decrease of enthalpy) or to sphingomyelin/GM1 mixtures (pe aks centered at a temperature >40 degrees C), respectively. This trend was confirmed by deconvolution analysis, showing that the thermograms are resolvable into components addressable to GM1-enriched and to cho lesterol-enriched domains. Taken all together, the results shout that the architectural features of sphingomyelin bilayers are strongly depe ndent on the presence of GM1 ganglioside and cholesterol, whose presen ce is leading to the formation of separate, GM1-enriched and cholester ol-enriched distinct domains. Ganglioside-sphingomyelin and sphingomye lin-cholesterol, together with mutual ganglioside-ganglioside, interac tions could contribute to maintain a network of bonds extending to pro teins, forming specialized membrane domains, such as caveolae, or othe rs, whose experimental clues are the glycolipid-enriched detergent-ins oluble fractions that can be isolated from cell membranes.