Lateral phase separations in Escherichia coli membranes

Membranes from unsaturated fatty acid auxotrophs of Escherichia coli were studied by spin labeling and freeze-fracturing. From measurements of the partition of the spin label TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) between the aqueous phase and fluid lipids in isolated membranes, temperatures, corresponding to the onset and completion of a lateral phase separation of the membrane phospholipids were determined. By freeze-fracture electron microscopy a change in the distribution of particle in the membrane was observed around the temperature of the onset of the lateral phase separation. When cells were frozen from above that temperature a netlike distribution of particles in the plasma membrane was observed for unfixed preparations. When frozen after fixing with glutaraldehyde the particle distribution was random. In membranes of cells frozen with or without fixing from a temperature below the onset of the phase separation, the particles were aggregated and large areas void of particles were present. This behavior can be understood in terms of the freezing rate with the aid of phase diagrams.