Abstract
Compartmentalization of proteins within the cell plays a fundamental role in the spatial and temporal organization of intracellular signaling systems. Although protein phosphorylation has long been known to be involved in this process, lipid microdomains enriched in sphingolipids and cholesterol, also known as lipid rafts, have recently been identified as regions within plasma membranes that are important for numerous cellular processes, including signal transduction, membrane trafficking, molecular sorting, and cell adhesion (Harder et al., 1998; Dermine et al., 2001). The unique lipid composition of rafts creates a more ordered lipid environment than is found in the rest of the plasma membrane (Simons and Ikonen, 1997; Brown and London, 2000), conferring to these specialized structures resistance to non-ionic detergent extraction at 4°C using Triton X-100 and giving rise to their alternative name of detergent-resistant membranes (DRMs). However, one should be cautious in assuming that lipid rafts can be isolated in their native state and that the relationship between their operational definition, namely detergent-insolubility at 4°C, flotation at a certain buoyancy, and cholesterol-dependency, fully reflects their state in vivo (for review, see Lai, 2003). A difficulty with the study of lipid rafts is that they may be too small (a few tens to hundreds of nanometers in diameter) (Brown and London, 2000; Abrami et al., 2001; Brown, 2001; Pierini and Maxfield, 2001) and too highly dispersed to be directly observed in unperturbed living cells. However, upon stimulation of raft-inserted receptors or antibody clustering, disperse rafts can aggregate to form large domains of several micrometers (Pierini and Maxfield, 2001), like flotillas (Harder et al., 1998; Brown and London, 2000; Dermine et al., 2001; Pierini and Maxfield, 2001), thereby inducing clustering of membrane components as a prerequisite for signal transduction.