Abstract
Escherichia coli strain K12 was grown at 17, 27, and 37 degrees C. The acyl chain composition of the membrane lipids varied with the growth temperature; the fraction of cis-vaccenoyl chains decreased, and the fraction of palmitoyl chains increased, when the growth temperature was increased. However, the polar head group composition did not change significantly. The equilibria between lamellar and reversed non-lamellar phases of lipids extracted from the inner membrane (IM), and from both the membranes (IOM), were studied with NMR and x-ray diffraction. At temperatures above the growth temperature the lipid extracts formed a reversed hexagonal phase, or a bicontinuous cubic phase, depending on the degree of hydration of the lipids. It was observed that: 1) at equal elevations above the growth temperature, IM lipid extracts, as well as IOM lipid extracts, have a nearly equal ability to form non-lamellar phases; 2) IM extracts have a stronger tendency than IOM extracts to form non-lamellar phases; 3) non-lamellar phases are formed under conditions that are relatively close to the physiological ones; the membrane lipid monolayers are thus "frustrated"; and 4) as a consequence of the change of the acyl chain structures, the temperature for the lamellar gel to liquid crystalline phase transition is changed simultaneously, and in the same direction, as the temperature for the lamellar to non-lamellar phase transition. With a too large fraction of saturated acyl chains the membrane lipids enter a gel state, and with a too large fraction of unsaturated acyl chains the lipids transform to non-lamellar phases. It is thus concluded that the regulation of the acyl chain composition in wild-type cells of E. coli is necessary for the organism to be able to grow in a "window" between a lamellar gel phase and reversed non-lamellar phases.
Highlights
All membranes contain significant amounts of at least one lipid species that forms non-lamellar phases under physiological conditions (Ansell et al, 1973; Goldfine, 1982)
The Gram-positive bacterium Clostridium butyricum has been used to investigate the regulation of the membrane lipid composition and, in resemblance with A. laidlawii, these cells can be grown under conditions where the regulation of the composition is primarily confined to the structures of the polar head groups (Goldfine et al, 1987a, 1987b)
It was suggested that one of the remaining lipids, diphosphatidylglycerol (DPG), together with divalent cations can replace the role of PE and that E. coli exhibits a polymorphic regulation of its membrane lipid composition (Killian et al, 1994; Rietveld et al, 1993, 1994)
Summary
All membranes contain significant amounts of at least one lipid species that forms non-lamellar phases under physiological conditions (Ansell et al, 1973; Goldfine, 1982). A. laidlawii strain A-EF22 synthesizes eight membrane lipids of which three are able to form non-lamellar phases: monoglucosyldiacylglycerol (MGlcDAG), and the monoacylated derivatives of MGlcDAG and diglucosyldiacylglycerol (DGlcDAG) (Lindblom et al, 1993; Rilfors et al, 1993). In the present work we have studied the metabolic regulation and the phase equilibria of the membrane lipids from wild-type cells of the Gram-negative bacterium Escherichia coli. A lipid biosynthetic mutant of E. coli was recently used to study the polymorphic regulation of the membrane lipid composition (Rietveld et al, 1993, 1994) This mutant lacks the ability to synthesize PE which constitutes about 75– 80 mol % of the membrane lipids in wild-type E. coli cells and has the ability to form non-lamellar phases (Cullis and de Kruijff, 1978; Rilfors et al, 1984). In this work we have studied the lamellar to non-lamellar phase transitions of lipids extracted from the inner membrane and from both the membranes of E. coli
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