Membrane lipids are not randomly distributed (evenly mixed) in each leaflet of a bilayer. One hint that lipids may be organized within the leaflets was the discovery that the lipids remaining after the extraction (solubilization) of plasma membranes with non-ionic detergents such as Triton X-100 predominantly contain two species: cholesterol and sphingomyelin. Because these two lipids are found in more ordered, less fluid bilayers, researchers hypothesized that they form microdomains, termed lipid rafts, surrounded by other, more fluid phospholipids that are more readily extracted by non-ionic detergents. (We discuss more fully the role of ionic and non-ionic detergents in extracting membrane proteins in Section 7.2.)
Some biochemical and microscopic evidence supports the existence of lipid rafts, which in natural membranes are typically 50 nm in diameter. Rafts can be disrupted by methyl-β-cyclodextrin, which specifically extracts cholesterol from membranes, or by antibiotics such as filipin that sequester cholesterol into aggregates within the membrane. Such findings indicate the importance of cholesterol in maintaining the integrity of lipid rafts. These raft fractions, defined by their insolubility in non-ionic detergents, contain a subset of plasma-membrane proteins, many of which are implicated in sensing extracellular signals and transmitting them into the cytosol. Because raft fractions are enriched in glycolipids, an important tool for microscopic visualization of raft-type structures in intact cells is the use of fluorescently labeled cholera toxin, a protein that specifically binds to certain gangliosides. By bringing many key proteins into close proximity and stabilizing their interactions, lipid rafts may facilitate signaling by cell-surface receptors and the subsequent activation of cytosolic events. However, much remains to be learned about the structure and biological function of lipid rafts. The unique properties of some of the raft-associated lipids, such as glycolipids, may permit interactions of their tails across the hydrophobic core and help organize lipids of the cytosolic leaflet in the formation of signaling platforms.