Structure and Function of Sphingolipid- and Cholesterol-rich Membrane Rafts

Abstract

It is well known that separate domains with different lipid compositions can exist in liposomes containing mixtures of different phospholipids. The question of whether cellular membranes contain similar lipid domains has intrigued workers for many years. One type of domain, sphingolipid and cholesterol-based structures called membrane rafts, has received much attention in the last few years. We will review the evidence that rafts exist in cells and focus on their structure, or the organization of raft lipids and proteins. Our discussion of function will focus on the role of rafts in signaling in hematopoietic cells, a particularly well developed area that has provided insights into raft organization in the membrane. Several reviews of rafts (1-4) and of related structures called caveolae (5-7) have appeared recently. Lipid Phase Behavior and Raft FormationSphingolipids differ from most biological phospholipids in containing long, largely saturated acyl chains. This allows them to readily pack tightly together, a property that gives sphingolipids much higher melting temperatures (T m ) 1 than membrane (glycero)phospholipids, which are rich in kinked unsaturated acyl chains. It is now clear that tight acyl chain packing is a key feature of raft lipid organization (3,8,9). In fact, the differential packing ability of sphingolipids and phospholipids probably leads to phase separation in the membrane. Thus, sphingolipid-rich rafts co-exist with phospholipid-rich domains that are in the familiar, loosely packed disordered state (variously abbreviated as L␣, l c , or l d ). Phase separation between lipids in different physical states, most often the l c and the solid-like gel phases, has been well characterized in model membranes. Indeed, the gel phase is the most familiar state in which acyl chains are highly ordered.However, because of the high concentration of cholesterol in the plasma membrane and other membranes in which rafts form, raft lipids do not exist in the gel phase. Cholesterol has important effects on phase behavior. It is well known that addition of cholesterol to a pure phospholipid bilayer abolishes the normal sharp thermal transition between gel and l c phases, giving the membrane properties intermediate between the two phases. This effect initially suggested that domains in ordered and disordered states cannot co-exist at high cholesterol levels. However, further work showed that a different kind of phase separation can occur in binary mixtures of individual phospholipids with cholesterol. In these mixtures, domains in an l c -like phase co-exist with domains in a new state, the liquid-ordered (l o ) phase. Acyl chains of lipids in the l o phase are extended and tightly packed, as in the gel phase, but have a high degree of lateral mobility (3).Rafts probably exist in the l o phase or a state with similar properties. In support of this model, detergent-insoluble membranes that can be isolated from cell lysates and are likely to be derived from rafts (discussed below) are in the l o phase (10, 11). M...

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