Lipid and protein segregation in Escherichia coli membrane: morphological and structural study of different cytoplasmic membrane fractions.

Lipid and protein segregations can be induced in E. coli cytoplasmic membranes by conformational transitions of their lipid hydrocarbon chains from a disordered to an ordered state. For E. coli strain K 1059 (an unsaturated fatty acid auxotroph) supplemented with linolenic acid, the segregation leads to large areas of membrane surfaces having distinctly different morphological characteristics (smooth compared with strongly particulated fracture faces, as visualized by freeze fracture electron microscopy). The different regions are physically separated by osmotic lysis of spheroplasts at temperatures below those of the order-disorder transition of the lipid hydrocarbon chains. The analysis of the different cytoplasmic membrane fractions provides a direct demonstration and allows a direct analysis of the segregation. As compared to the nonfractionated membranes, the membrane regions corresponding to the smooth fracture surfaces are poor in proteins, rich in lipids, and enriched in saturated fatty acids, while the membrane regions corresponding to the strongly particulated fracture surfaces are rich in proteins, poor in lipids, and enriched in unsaturated fatty acids. Quantitative information about the extent of these segregations is obtained from high-angle x-ray diffraction of the different membrane fractions and of the corresponding total lipid extracts.