Sphingolipid partitioning into ordered domains in cholesterol-free and cholesterol-containing lipid bilayers

We have used fluorescence-quenching measurements to characterize the partitioning of a variety of indolyl-labeled phospho- and sphingolipids between gel or liquid-ordered and liquid-disordered lipid domains in several types of lipid bilayers where such domains coexist. In both cholesterol-free and cholesterol-containing lipid mixtures, sphingolipids with diverse polar headgroups (ranging from sphingomyelin and monoglycosylceramides to ganglioside GM1) show a net preference for partitioning into ordered domains, which varies modestly in magnitude with varying headgroup, structure. The affinities of different sphingolipids for ordered lipid domains do not vary in a consistent manner with the size or other simple structural properties of the polar headgroup, such that for example ganglioside GM1 partitions between ordered and disordered lipid domains in a manner very similar to sphingomyelin. Ceramide exhibits a dramatically higher affinity for ordered lipid domains in both cholesterol-free and cholesterol-containing bilayers than do other sphingolipids. Our findings suggest that sphingolipids with a variety of headgroup structures will be enriched by substantial factors in liquid-ordered versus liquid-disordered regions of membranes, in a manner that is only modestly dependent on the nature of the polar headgroup. Ceramide is predicted to show a very strong enrichment in such domains, supporting previous suggestions that ceramide-mediated signaling may be compartmentalized to liquid-ordered (raft and raft-related) domains in the plasma membrane.