Every so often an issue emerges that splits the community ofcell biologists into believers and nonbelievers. The issue of lipid rafts falls into this category. When membrane lipids areextracted from cells and used to prepare artificiallipid bilayers,cholesterol and sphingolipids tend to self-assemble into microdomains that are more gelated and highly ordered thansurrounding regions consisting primarily of phosphoglycerides. Because of their distinctive physical properties, suchmicrodomains tend to float within the more fluid and disordered environment of the artificial bilayer (Figure 4.24a). As aresult, these patches of cholesterol and sphingolipid are referred to as lipid rafts. When added to these artificial bilayers,certain proteins tend to become concentrated in the lipidrafts, whereas others tend to remain outside their boundaries.GPI-anchored proteins show a particular fondness for the ordered regions of the bilayer (Figure 4.24a). Translating
The controversy arises over whether similar types osphingolipid- and cholesterol-rich lipid rafts, such as that illustrated in Figure 4.24b, exist within living cells. Most of theevidence in favor of lipid rafts is derived from studies that employ unnatural treatments, such as detergent extraction orcholesterol depletion, which makes the results difficult to interpret. Attempts to demonstrate the presence of lipid rafts inliving cells have generally been unsuccessful, which can eithermean that such rafts do not exist or they are so small (5 to 25 nmdiameter) and short-lived as to be difficult to detect with current techniques. The concept of lipid rafts is very appealingbecause it provides a means to introduce order into a seemingly random sea of lipid molecules. Lipid rafts are postulatedto serve as floating platforms that concentrate particular proteins, thereby organizing the membrane into functional compartments (Figure 4.24b). For example, lipid rafts are thought
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