Organization of GalCer in the plasma membrane : rafts

Membrane lipids are amphiphilic molecules. This means that they are formed of two parts, one hydrophobic (apolar) and the other hydrophilic (polar). The polar part interacts with water, whereas the apolar part has to find an apolar phase to avoid contact with water. In other words, membrane lipids have no other choice than to self-organize into supramolecular complexes in which only their polar part interacts with water. Biological membranes are formed by a bilayer of lipids, with a hydrophobic core formed by the hydrocarbon chains of lipids. In the very popular fluid mosaic model of biological membranes, lipids form a homogeneous two-dimensional solvent phase for membrane proteins. Yet membrane lipids comprise several hundreds ofdistinct molecules that exist in different physical states controlled by several physicochemical parameters such as the temperature, presence of cholesterol and chemical nature of the hydrocarbon chains.

Thus, biological membranes are probably better described as a ‘mosaic of lipid domains’ rather than a homogeneous fluid mosaic. Membrane cholesterol, for instance, is unevenly distributed into cholesterol-rich and cholesterol-poor domains, consistent with the notion that specialized lipid domains with specific biochemical composition and physicochemical properties do exist in membranes. Among these domains, those containing sphingolipids and cholesterol, referred to as lipid rafts or caveolae (when associated with the integral membrane protein caveolin), have been extensively studied.

Why do sphingolipids and cholesterol self-associate and segregate into specific membrane domains? The answer to this question may be given by the biochemical structure of membrane lipids. Glycerophospholipids such as phosphatidylcholine (PC) are rich in kinked unsaturated acyl chains (with C=C double bonds in the cis configuration), whereas the hydrophobic part of sphingolipids such as sphingomyelin or glycosphingolipids (GSL) contain a saturated acyl chain and sphingosine. Introducing a C=C double bond of cis geometric configuration results in a bending of the chain. This change from the linearity impairs the tight packing of lipid chains, so that glycerophospholipids have more mobile hydrophobic chains than sphingolipids. Since the mobility of the hydrophobic lipid anchor in the apolar phase of the membrane interferes with the packing capacity of lipid molecules, the energy required to separate two adjacent sphingolipid molecules is significantly higher than for glycerophospholipids. This energy can be quantified by measuring the temperature required to induce the solid–liquid phase transition of a lipid, i.e. the melting temperature (Tm). The Tm of PC is as low as -5°C,compared to 83°C for GalCer (purified from bovine brain). Because cholesterol also has a highTm, it has more affinity for sphingolipids than for glycerophospholipids. Consequently, at 37°C, sphingolipids and cholesterol segregate into specific microdomains,the so-called lipid rafts. Since the sphingolipid headgroups occupy a large volume, cholesterol functions as a molecular spacer, filling the voids between two adjacent sphingolipids. From a biophysical point of view, lipid rafts are in a liquid-ordered (Lo) phase floating in the more liquid glycerophospholipid-rich/cholesterol-poor bulk (liquid-disordered phase Ld) of the plasma membrane (for recent reviews, see 14-16).

GalCer, like most sphingolipids, is found in the external leaflet of the plasma membrane within lipid rafts. This unique localization has important consequences for the biological functions of GalCer.