Structure and function of membrane rafts

Lipids do not always mix uniformly in membranes, but can cluster to form microdomains. We will consider one type of microdomain that can form in cell membranes. These are enriched in cholesterol and sphingolipids, and are referred to as rafts. Rafts probably exist in membranes in the liquid-ordered phase or a phase with similar properties. We will briefly review membrane lipid phase behavior, and the differences between liquid-crystalline, liquid-ordered, and gel-phase membrane bilayer domains. We will present evidence suggesting that phospholipid-rich, liquid-crystalline phase domains and sphingolipid-rich, liquid-ordered phase domains (rafts) can exist in equilibrium in biological membranes, especially the plasma membrane. Preferential partitioning of membrane proteins into rafts can affect function. Among the proteins that are targeted to rafts are those anchored in the outer leaflet of the membrane through covalent attachment to a special glycolipid, glycosyl phosphatidylinositol (GPI). Other proteins that are linked to saturated acyl chains, such as those that are directly acylated with two or more palmitate chains, or a palmitate and a myristate chain, are also targeted to rafts. Targeting of GPI-anchored proteins and other proteins to rafts plays a role in signal transduction in hematopoietic cells, and possibly also in sorting in intracellular membranes and regulation of cell-surface proteolysis in other mammalian cells.