Effect of the Structure of Natural Sterols and Sphingolipids on the Formation of Ordered Sphingolipid/Sterol Domains (Rafts)

Abstract

Ordered lipid domains enriched in sphingolipids and cholesterol (lipid rafts) have been implicated in numerous functions in biological membranes. We recently found that lipid domain/raft formation is dependent on the sterol component having a structure that allows tight packing with lipids having saturated acyl chains (Xu, X., and London, E. (2000) Biochemistry 39, 844 -849). In this study, the domain-promoting activities of various natural sterols were compared with that of cholesterol using both fluorescence quenching and detergent insolubility methods. Using model membranes, it was shown that, like cholesterol, both plant and fungal sterols promote the formation of tightly packed, ordered lipid domains by lipids with saturated acyl chains. Surprisingly ergosterol, a fungal sterol, and 7-dehydrocholesterol, a sterol present in elevated levels in Smith-Lemli-Opitz syndrome, were both significantly more strongly domain-promoting than cholesterol. Domain formation was also affected by the structure of the sphingolipid (or that of an equivalent "saturated" phospholipid) component. Sterols had pronounced effects on domain formation by sphingomyelin and dipalmitoylphosphatidylcholine but only a weak influence on the ability of cerebrosides to form domains. Strikingly it was found that a small amount of ceramide (3 mol %) significantly stabilized domain/raft formation. The molecular basis for, and the implications of, the effects of different sterols and sphingolipids (especially ceramide) on the behavior and biological function of rafts are discussed.There is strong evidence supporting a model of eukaryotic plasma membrane structure in which domains (rafts) rich in cholesterol and lipids with relatively saturated acyl chains (i.e. sphingolipids) co-exist with domains rich in phospholipids attached to unsaturated acyl chains (1-6). Rafts have a distinct protein composition that is especially enriched in proteins anchored to membranes by saturated acyl chains while relatively depleted of most transmembrane proteins. Rafts have been implicated in numerous cellular processes, including signal transduction, protein and lipid sorting, cellular entry by toxins and viruses, and viral budding (4, 6 -15). Thus, an understanding of the principles that underlie raft formation is important with regard to the study of eukaryotic membrane function.We have shown that sphingolipid/cholesterol domains are likely to exist in the tightly packed liquid-ordered (L o ) state, whereas unsaturated phospholipid-rich domains are more likely to be in the less ordered liquid crystalline (L ␣ ) phase even if they contain large amounts of cholesterol (16 -18). The tight packing of lipids in the L o state gives rafts their characteristic resistance to solubilization by Triton X-100 (1, 18).Cholesterol can promote separation of lipid mixtures into co-existing L ␣ and L o domains (17,19) and is critical for raft formation in cells (2-4, 6). We recently found that the ability of cholesterol to pack tightly with saturated lipids is a key to its abilit...

Yeast Lipids Can Phase-separate into Micrometer-scale Membrane Domains

“…(i) In the presence of SM, replacement of cholesterol by ergosterol results in a membrane order that is as high as the order of membranes of the well characterized raft mixture SM/PC/cholesterol. This result confirms previous reports showing the property of ergosterol to have a condensing effect on glycerophospholipids (43)(44)(45)(46). (ii) Yeast PI increases the order of membranes containing IPC, irrespective of whether cholesterol or ergosterol is used as the sterol in these mixtures.…”

Lipid Rafts in Higher Plant Cells

“…Such a result is somewhat surprising if one considers the very different efficiencies of sitosterol and stigmasterol in ordering phospholipid acyl chains of soybean phosphatidylcholine bilayers (40). However, the present results are entirely consistent with a recent work demonstrating a similar ability of both sitosterol and stigmasterol to promote domain formation (41).…”

Resistance of cell membranes to different detergents

“…We found that sphingomyelinase treatment augments the effect of cyclodextrin, consistent with the idea that stability of detergent-resistant domains requires both cholesterol and sphingomyelin. The cleavage product of sphingomyelin, ceramide, appears to be unable to substitute for the cleaved sphingomyelin, even though it strongly supports the formation of l o domains in model membranes (36). However, sphingomyelinase on its own did not reduce DRM association of the proteins tested, nor did it enhance the effect of saponin.…”