No abstract

The existence of sphingolipid-and sterol-enriched microdomains, known as lipid rafts, in the plasma membrane (PM) of eukaryotic cells is well documented. To obtain more insight into the lipid molecular species required for the formation of microdomains in plants, we have isolated detergent (Triton X-100)-resistant membranes (DRMs) from the PM of Arabidopsis (Arabidopsis thaliana) and leek (Allium porrum) seedlings as well as from Arabidopsis cell cultures. Here, we show that all DRM preparations are enriched in sterols, sterylglucosides, and glucosylceramides (GluCer) and depleted in glycerophospholipids. The GluCer of DRMs from leek seedlings contain hydroxypalmitic acid. We investigated the role of sterols in DRM formation along the secretory pathway in leek seedlings. We present evidence for the presence of DRMs in both the PM and the Golgi apparatus but not in the endoplasmic reticulum. In leek seedlings treated with fenpropimorph, a sterol biosynthesis inhibitor, the usual D 5 -sterols are replaced by 9b,19-cyclopropylsterols. In these plants, sterols and hydroxypalmitic acid-containing GluCer do not reach the PM, and most DRMs are recovered from the Golgi apparatus, indicating that D 5 -sterols and GluCer play a crucial role in lipid microdomain formation and delivery to the PM. In addition, DRM formation in Arabidopsis cells is shown to depend on the unsaturation degree of fatty acyl chains as evidenced by the dramatic decrease in the amount of DRMs prepared from the Arabidopsis mutants, fad2 and Fad31, affected in their fatty acid desaturases.Despite the ongoing debate on the size, lifespan, and dynamics of lipid microdomains (Munro, 2003;Pike, 2004;Nichols, 2005;Hancock, 2006), the existence of sterol-and sphingolipid-enriched membrane microdomains in the plasma membrane (PM) of eukaryotic cells is now well recognized (Simons and Ikonen, 1997;Brown and London, 2000;Simons and Toomre, 2000;Bagnat and Simons, 2002;Simons and Vaz, 2004;Hancock, 2006). Evidence for microdomains comes in part from examination of membrane constituents that are resistant to solubilization by nonionic detergents at low temperature, giving rise to the concept of lipid rafts (Simons and Ikonen, 1997;Brown and London, 1998;Rö per et al., 2000;Hancock, 2006). Such steroland sphingolipid-enriched microdomains can be isolated as detergent-resistant membranes (DRMs). In the following, the terms of DRMs and microdomains will be assigned to respectively indicate the isolated entities and the corresponding membrane entities. Lipid properties of microdomains are similar to liquid-ordered domains, which are characterized by tightly packed hydrocarbon tails but with a high degree of lateral mobility (Brown and London, 2000;Simons and Vaz, 2004). Cholesterol is thought to contribute to the tight packing of lipids in liquid-ordered domains by filling interstitial spaces between lipid molecules (Brown, 1998)

We report the synthesis of methionine-containing recombinant elastin-like polypeptides (ELPs) of different lengths that contain periodically spaced methionine residues. These ELPs were chemoselectively alkylated at all methionine residues to give polycationic derivatives. Some of these samples were found to possess solubility transitions in water, where the temperature of these transitions varied with ELP concentration, nature of the methionine alkylating group, and nature of the sulfonium counterions. These studies show that introduction and controlled spacing of methionine sulfonium residues into ELPs can be used as a means both to tune their solubility transition temperatures in water using a variety of different parameters and to introduce new side-chain functionality.