1. National Centre for Biological Sciences, TIFR Centre, IISc Campus, Post Box 1234, Bangalore 560012, India

Correspondence to: Satyajit Mayor¹ Correspondence and requests for materials should be addressed to S.M. (e-mail: Email: mayor@ncbs.tifrbng.res.in).

Abstract

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 proteins¹. These rafts dictate the sorting of associated proteins and/or provide sites for assembling cytoplasmic signalling molecules². However, there is no direct evidence that rafts exist in living cells³,⁴. 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 membranes⁵. 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.