Outer Membrane Proteins of Escherichia coli III. Evidence that the Major Protein of Escherichia coli O111 Outer Membrane Consists of Four Distinct Polypeptide Species

Previous studies have shown that the outer membrane of Escherichia coli O111 gives a single, major, 42,000-dalton protein peak when analyzed by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis at neutral pH. Further studies have shown that this peak consists of more than a single polypeptide species, and on alkaline SDS-gel electrophoresis this single peak is resolved into three subcomponents designated as proteins 1, 2, and 3. By chromatography of solubilized, outer membrane protein on diethylaminoethyl-cellulose followed by chromatography on Sephadex G-200 in the presence of SDS, it was possible to separate the 42,000-dalton major protein into four distinct protein fractions. Comparison of cyanogen bromide peptides derived from these fractions indicated that they represented at least four distinct polypeptide species. Two of these proteins migrated as proteins 1 and 2 on alkaline gels. The other two proteins migrated as protein 3 on alkaline gels and cannot be separated by SDS-polyacrylamide gel electrophoresis. In purified form, these major proteins do not contain bound lipopolysaccharide, phospholipid, or phosphate. These proteins may contain a small amount of carbohydrate, as evidenced by the labeling of these proteins by glucosamine, and to a lesser extent by glucose, under conditions where the metabolism of these sugars to amino acids and lipids is blocked. All of the proteins were labeled to the same extent by these sugars. Thus, it was concluded that there are at least four distinct polypeptide species with apparent molecular masses of about 42,000 daltons in the outer membrane of E. coli O111.