Lipid raft idea is floated

T he introduction of a fl uorescent lipid probe, specifi cally fl uorescent ceramide, in the mid-1980s (Lipsky and Pagano, 1983; Lipsky and Pagano, 1985) gave Gerrit van Meer and Kai Simons just the tool they needed to attack a nagging question. Lipid locations For a decade, it had been known that the apical and basolateral membranes of epithelial cells had different lipid compositions (Kawai, 1974), and specifi cally that glycolipids are enriched apically. In 1981, the tight junction was proposed as the barrier that kept these two membrane populations distinct (Dragsten et al., 1981). Playing off a fi nding that different viruses budded from the different poles of cultured epithelial cells (Boulan and Sabatini, 1978), van Meer and Simons showed in 1982 that the envelopes of those viruses contained different lipid compositions (van Meer and Simons, 1982). At about the same time, others had shown that heterogeneous lipid domains existed (Karnovsky et al., 1982) and that glycosphingolipids clustered (Thompson and Tillack, 1985) in both model membrane systems and biological membranes. This suggested that lipid domains might affect membrane functions and structure, but real evidence of a biological role was lacking. Because plasma membrane lipids are synthesized in-tracellularly, van Meer and Simons reasoned that lipid sorting must take place to set up the epithelial cell membrane domains. The NBD–ceramide probe offered a handy way to start on the project because, once it was inside cells, it would be converted into two distinct lipid probes: an NBD– sphingomyelin and an NBD–glucosylceramide, analogues of a basolateral and apical lipid, respectively. The conversion occurred in the Golgi and then the fl uorescent probes could be followed to plasma membrane destinations. While he was a post-doc with Simons at the EMBL in Heidelberg, Germany, van Meer quantifi ed the sorting using the NBD-labeled probes. Using Madin-Darby canine kidney (MDCK) epithelial cells grown on fi lters, he used " back exchange " with serum albumin applied to either side of the fi lter to extract and measure the fl uorescent lipids that sorted to either the apical or basolateral side (van Meer et al., 1987). He found that the NBD–glucosylceramide was enriched two to four times on the apical membrane, whereas the NBD–sphingomyelin was equally distributed between the apical and basolateral sides. This process quantitatively accounted for the in vivo lipid distribution. If transport, then rafts " This was the fi rst piece of evidence …