(6, 9). Whether the interaction of apoA-I with cell membrane is mediated by a specific receptor and the mechanism of its loading with PL and FC is still a matter of debate (7). The recent discovery that a defective ATP-binding cassette transporter (ABCA1) leads to Tangier disease (10, 11) has prompted the study of the role of this protein as a candidate apoA-I receptor (12-17). A direct molecular interaction between ABCA1 and apoA-I is indicated by some of these studies (12, 13). However, other studies (15, 16) do not support this interaction and suggest a role for ABCA1 in modifying the lipid distribution in the membrane and generating the biophysical microenvironment required for the docking of apoA-I to the cell surface. A high conformational flexibility in apoA-I is needed for its existence in different states: lipid-free, lipid-poor, and discoidal or spherical lipoproteins of different size. Mature human apoA-I contains 243 amino acid residues (18) with 11-and 22-mer homologous repeats (19) that are predicted to form amphipathic ␣-helices (20, 21) that interact with lipids through their hydrophobic face. The helices are linked by short and flexible ␤-turns usually containing a proline residue. Lipid-free apoA-I is thought to be a bundle of helices (22) in a molten globular-like state (23), and sedimentation velocity experiments indicate significant conformational heterogeneity (24), supporting a flexible structure. In the spherical ␣-HDL, it has been proposed (25) that the amphipathic helices are oriented parallel to the surface of the phospholipid monolayer with the hydrophobic faces embedded into the hydrocarbon region and with the hydrophilic faces interacting with the phospholipid

The maximum growth temperature, the optimal growth temperature, and the estimated normal physiological range for growth of Shewanella gelidimarina are functions of water activity (a w ), which can be manipulated by changing the concentration of sodium chloride. The growth temperatures at the boundaries of the normal physiological range for growth were characterized by increased variability in fatty acid composition. Under hyper-and hypoosmotic stress conditions at an a w of 0.993 (1.0% [wt/vol] NaCl) and at an a w of 0.977 (4.0% [wt/vol] NaCl) the proportion of certain fatty acids (monounsaturated and branched-chain fatty acids) was highly regulated and was inversely related to the growth rate over the entire temperature range. The physical states of lipids extracted from samples grown at stressful a w values at the boundaries of the normal physiological range exhibited no abrupt gel-liquid phase transitions when the lipids were analyzed as liposomes. Lipid packing and adaptational fatty acid composition responses are clearly influenced by differences in the temperature-salinity regime, which are reflected in overall cell function characteristics, such as the growth rate and the normal physiological range for growth.Antarctic sea ice is characterized by the presence of a unique bacterial community that is dominated by psychrophilic bacteria (2,3,12). Nichols et al. (25) have discussed the potential role of the temperature-salinity regime in selecting psychrophilic bacteria in sea ice. It has been suggested that understanding the physiological response of psychrophilic bacteria to combined temperature-salinity stress is very important for understanding the bacterial sea ice community. The importance of fluctuations in salinity that affect the composition of microbial populations in estuarine and brackish water ecosystems has been recognized (7, 30), and such fluctuations are very important when the survival and viability of psychrophilic marine bacteria are considered (12,20,21,34). Changes in salinity similar to those observed in estuarine and brackish water environments also occur in Antarctic coastal waters and sea ice and are associated with the annual ice formation and melting cycle. However, researchers have attempted to relate changes in physicochemical parameters to a physiological mechanism for growth in only a few studies of psychrophilic bacteria (8,11,21,26,35).Workers have proposed a number of models to describe the effect of temperature and/or salinity on bacterial growth. Most of these models are empirical and seek solely to summarize observations of bacterial growth under various conditions. Many are based on the Arrhenius equation and utilize Arrhenius kinetics to describe bacterial growth in response to temperature. The concept of a normal physiological range (NPR) for bacterial growth is derived from modelling of the bacterial growth rate by using Arrhenius models. Over a defined range of temperature, the growth rates of all bacteria obey Arrhenius kinetics, and this temperature range is desig...

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Apolipoprotein A-I (apoA-I) interaction with specific cell lipid domains was suggested to trigger cholesterol and phospholipid efflux. We analyzed here apoA-I interaction with dimyristoylphosphatidylcholine/distearoylphosphatidylcholine (DMPC/DSPC) bilayers at a temperature showing phase coexistence. Solid and liquid-crystalline domains were visualized by two-photon fluorescence microscopy on giant unilamellar vesicles (GUVs) labeled with 6-dodecanoyl-2-dimethylamino-naphthalene (Laurdan). A decrease of vesicle size was detected as long as they were incubated with lipid-free apoA-I, together with a shape deformation and a relative enrichment in DSPC. Selective lipid removal mediated by apoA-I from different domains was followed in real time by changes in the Laurdan generalized polarization. The data show a selective interaction of apoA-I with liquid-crystalline domains, from which it removes lipids, at a molar ratio similar to the domain compositions. Next, apoA-I was incubated with DMPC/DSPC small unilamellar vesicles, and products were isolated and quantified. Protein solubilized both lipids but formed complexes relatively enriched in the liquid component. We also show changes in the GUV morphology when cooling down. Our results suggest that the most efficient reaction between apoA-I and DMPC/DSPC occurs in particular bilayer conditions, probably when small fluid domains are nucleated within a continuous gel phase and interfacial packing defects are maximal.

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An excess of intracellular free Cholesterol (Chol) is cytotoxic, and its homeostasis is crucial for cell viability. Apolipoprotein A–I (apoA-I) is a highly efficient Chol acceptor as it activates complex cellular pathways that tend to mobilize and export Chol from cellular depots. Here we hypothesize that membrane composition and/or organization is strongly involved in Chol homeostasis. To test this hypothesis, we constructed a cell line over expressing Stearoyl CoA desaturase (SCD-cells), which modifies plasma membrane (PM) composition by the enrichment of monounsaturated fatty,acids and determined this effect on membrane properties, cell viability and cholesterol homeostasis. PM in SCD-cells has a higher phospholipids/sphingomyelin ratio and is slightly enriched in Chol. These cells showed an increase in the cholesteryl esters/free Chol ratio, they were more resistant to Chol toxicity and in addition, they exported more caveolin than Control cells. The data suggest that cell functionality is preserved by regulating membrane fluidity and Chol exportation and storage.