GEL PHASE PREFERENCE OF GANGLIOSIDE GM1 AT LOW CONCENTRATION IN 2-COMPONENT, 2-PHASE PHOSPHATIDYLCHOLINE BILAYERS DEPENDS UPON THE CERAMIDE MOIETY

In two-component phosphatidylcholine bilayers with coexisting liquid and P-beta' gel phases, the distribution between phases of low concentrations of glycosphingolipids can be determined by freeze-etch electron microscopy after labeling the glycolipid with a suitable protein. We have found that the distribution depends upon the glycosphingolipid species (Rock, P. et al., (1991) Biochemistry 30, 19-25). Using this technique with cholera toxin as the protein label and bilayers formed from dipalmitoyl- and dielaidoylphosphatidylcholine (1:1) containing < 1 mol% GM1, we have studied the distribution of a family of GM1 homologues differing in the acyl chain and sphingoid base moieties. The GM1 preference for the P-beta' ripple phase decreases with decreasing acyl chain length and increasing unsaturation. GM1 with either a C18:1 or C20:1 sphingoid base shows similar distributions in liquid and gel phases. When the molecules are preferentially found in the P-beta' phase, they are positioned along unique loci in both Lambda and Lambda/2 forms of the ripple structure. This localization and acyl chain dependence reflect the volume, shape and localization of molecular packing defects in the P-beta' phase. The ganglioside inclusions stabilize the P beta' phase and form compositional domains of unique topography.