| Abstract | The membranes of extremely halophilic archaea are characterized by abundance of a diacidic phospholipid, archaetidylglycerol methylphosphate (PGP-Me), which accounts for 50-80 mol% of the polar lipids, and by the absence of phospholipids with choline, ethanolamine, inositol and serine head groups. These membranes are stable in concentrated 3-5M NaCl solutions, while membranes of non-halophilic archaea, which do not contain PGP-Me, are unstable and leaky in such conditions. By means of X-ray diffraction and vesicle permeability measurements we demonstrate that PGP-Me contributes in an essential way to membrane stability in hypersaline environments. Large unilamellar vesicles (LUV) prepared from the polar lipids of extreme halophiles, H. halobium and H. salinarum, retain entrapped carboxyfluorescein and resist aggregation in the whole range 0-4M NaCl, similarly to LUV prepared from purified PGP-Me. By contrast, LUV made of polar lipid extracts from moderately halophilic and non-halophilic archaea (M. jannaschii, M. mazei, M. smithii) are leaky and aggregate at high salt concentrations. However, adding PGP-Me to M. mazei lipids results in gradual enhancement of the LUV stability, in correlation with the PGP-Me content. The LUV data are substantiated by the X-ray results, which show that H. halobium and M. mazei lipids have dissimilar phase behavior and form different structures at high NaCl concentrations. H. halobium lipids maintain an expanded lamellar structure with spacing of 8.5-9 nm, which is stable up to at least 100 degrees C in 2M NaCl and up to ~60 degrees C in 4M NaCl, while M. mazei lipids form non-lamellar structures, represented by the Pn3m cubic phase and the inverted hexagonal HII phase. From these data, the forces preventing membrane aggregation in halophilic archaea appear to be steric repulsion due to the large head group of PGP-Me or, possibly, out-of-plane bilayer undulations rather than electrostatic repulsion due to the doubly charged PGP-Me head group |
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