Nanodomains can persist at physiologic temperature in plasma membrane vesicles and be modulated by altering cell lipids

The formation and properties of liquid-ordered (Lo) lipid domains (rafts) in the plasma membrane are still poorly understood. This limits our ability to manipulate ordered lipid domain-dependent biological functions. Giant plasma membrane vesicles (GPMVs) undergo large-scale phase separations into coexisting Lo and liquid-disordered lipid domains. However, large-scale phase separation in GPMVs detected by light microscopy is observed only at low temperatures. Comparing Förster resonance energy transfer-detected versus light microscopy-detected domain formation, we found that nanodomains, domains of nanometer size, persist at temperatures up to 20°C higher than large-scale phases, up to physiologic temperature. The persistence of nanodomains at higher temperatures is consistent with previously reported theoretical calculations. To investigate the sensitivity of nanodomains to lipid composition, GPMVs were prepared from mammalian cells in which sterol, phospholipid, or sphingolipid composition in the plasma membrane outer leaflet had been altered by cyclodextrin-catalyzed lipid exchange. Lipid substitutions that stabilize or destabilize ordered domain formation in artificial lipid vesicles had a similar effect on the thermal stability of nanodomains and large-scale phase separation in GPMVs, with nanodomains persisting at higher temperatures than large-scale phases for a wide range of lipid compositions. This indicates that it is likely that plasma membrane nanodomains can form under physiologic conditions more readily than large-scale phase separation. We also conclude that membrane lipid substitutions carried out in intact cells are able to modulate the propensity of plasma membranes to form ordered domains. This implies lipid substitutions can be used to alter biological processes dependent upon ordered domains.

Phosphorus starvation induces membrane remodeling and recycling in Emiliania huxleyi

Nutrient availability is an important factor controlling phytoplankton productivity. Phytoplankton contribute c. 50% of the global photosynthesis and possess efficient acclimation mechanisms to cope with nutrient stress. We investigate the cellular response of the bloom-forming coccolithophore Emiliania huxleyi to phosphorus (P) scarcity, which is often a limiting factor in marine ecosystems. We combined mass spectrometry, fluorescence microscopy, transmission electron microscopy (TEM) and gene expression analyses in order to assess diverse cellular features in cells exposed to P limitation and recovery. Early starvation-induced substitution of phospholipids in the cells' membranes with galacto- and betaine lipids. Lipid remodeling was rapid and reversible upon P resupply. The PI3K inhibitor wortmannin reduced phospholipid substitution, suggesting a possible involvement of PI3K- signaling in this process. In addition, P limitation enhanced the formation and acidification of membrane vesicles in the cytoplasm. Intracellular vesicles may facilitate the recycling of cytoplasmic content, which is engulfed in the vesicles and delivered to the main vacuole. Long-term starvation was characterized by a profound increase in cell size and morphological alterations in cellular ultrastructure. This study provides cellular and molecular basis for future ecophysiological assessment of natural E. huxleyi populations in oligotrophic regions.

Reconstitution of a calcium pump using defined membrane components

A (Mg(2+) + Ca(2+))ATPase (ATP phosphohydrolase, EC 3.6.1.3) has been purified from sarcoplasmic reticulum using a single step centrifugation procedure. The preparation is >95% pure by weight and contains only 25-30% of the lipid associated with the enzyme in native sarcoplasmic reticulum. The purified enzyme is unable to accumulate Ca(2+). Using a sedimentation-substitution technique, >98% of the lipid associated with the purified enzyme can be replaced by dioleoyl lecithin without grossly affecting the ATPase activity of the enzyme. The Ca(2+) pump can be restored to this dioleoyl lecithin-substituted enzyme by addition of excess sarcoplasmic reticulum lipids in the presence of cholate. Removal of the cholate by dialysis generates a system which accumulates Ca(2+) at a rate and to a level comparable to native sarcoplasmic reticulum. Significant reconstitution of the Ca(2+) pump can also be achieved using excess dioleoyl lecithin, but since the full expression of the capacity to accumulate Ca(2+) requires the presence of oxalate, these vesicles would appear to be more leaky than those reconstituted with an excess of sarcoplasmic reticulum lipids. Of about 90 lipid molecules which are associated with one molecule of ATPase in native sarcoplasmic reticulum, an average of less than one lipid molecule remains in these reconstituted systems. We have therefore achieved a fully functional Ca(2+) pump containing essentially a single protein and exogenous lipid.