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Lateral heterogeneities in the classical fluid-mosaic model of cell membranes are now envisaged as domains or 'rafts' that are enriched in (glyco) sphingolipids, cholesterol, specific membrane proteins and glycosylphosphatidylinositol (GPI)-anchored proteins [1]. These rafts dictate the sorting of associated proteins and/or provide sites for assembling cytoplasmic signalling molecules [2]. However, there is no direct evidence that rafts exist in living cells [3,4]. We have now measured the extent of energy transfer between isoforms of the folate receptor bound to a fluorescent analogue of folic acid, in terms of the dependence of fluorescence polarization on fluorophore densities in membranes [5]. We find that the extent of energy transfer for the GPI-anchored folate-receptor isoform is density-independent, which is characteristic of organization in sub-pixel-sized domains at the surface of living cells; however, the extent of energy transfer for the transmembrane-anchored folate-receptor isoform was density-dependent, which is consistent with a random distribution. These domains are likely to be less than 70 nm in diameter and are disrupted by removal of cellular cholesterol. These results indicate that lipid-linked proteins are organized in cholesterol-dependent submicron-sized domains. Our methodology offers a new way of monitoring nanometre-scale association between molecules in living cells.

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