Gel phase preference of ganglioside GM1 at low concentration in two-component, two-phase phosphatidylcholine bilayers depends upon the ceramide moiety

Abstract

In two-component phosphatidylcholine bilayers with coexisting liquidand P β, 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 β 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 β, phase, they are positioned along unique loci in both Λ and Λ/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 β, phase. The ganglioside inclusions stabilize the P β, phase and form compositional domains of unique topography.