Mechanism of Assembly of the Outer Membrane of Salmonella typhimurium

Abstract The cell envelope of Salmonella typhimurium has been separated into cytoplasmic and outer membrane fractions of relatively high purity by a new method based on isopycnic sucrose density gradient centrifugation of the total membrane fraction obtained by lysis of lysozyme-EDTA spheroplasts. The outer membrane fraction contained approximately 60% of the protein of the total membranes, 50% of the phospholipid, and 90% of the lipopolysaccharide. The cytoplasmic and outer membrane fractions differed markedly in over-all protein composition as determined by polyacrylamide gel electrophoresis in sodium dodecyl sulfate as well as in specific enzyme activities. Total cytochromes, DPNH oxidase, succinate dehydrogenase, d-lactate dehydrogenase, and Enzyme II of the α-methylglucoside phosphotransferase system were recovered essentially exclusively in cytoplasmic membrane; the specific activities of these in the outer membrane were 1 to 5% of the values obtained for cytoplasmic membrane fractions. In contrast, a phospholipase activity tentatively identified as a mixture of phospholipase A and lysophospholipase, appeared to be localized primarily in the outer membrane. UDP-sugar hydrolase, ribonuclease I and endonuclease I activities were associated with both membrane fractions. Preliminary analysis of the phospholipid composition of the isolated fractions also showed significant quantitative differences in the relative distribution of the major glycerophosphatides. The ratios of phosphatidylglycerol to phosphatidylethanolamine and of cardiolipin to phosphatidylethanolamine in the outer membrane were approximately one-half and one-quarter those of the cytoplasmic membrane.