The Covalent Rigid‐Layer Lipoprotein in Cell Walls of Proteus mirabilis

The covalent‐linked lipoprotein was isolated from purified peptidoglycan sacculi of early stationary phase Proteus mirabilis cells after digestion with Chalaropsis lysozyme and gel filtration. Only 1.7% of the peptidoglycan subunits were found to be linked to lipoprotein. Further purification steps included chromatography on Sephadex G75 in 1.5% sodium dodecyl sulfate solution, removal of the detergent by acetone precipitation, and finally ultracentrifugation. The lipoprotein is soluble in water (about 1 μmol per ml) giving a slightly opalescent solution. Amino acid analysis in comparison with the covalent lipoprotein from Escherichia coli , kindly provided by V. Braun, revealed that firstly, the same set of amino acids is present except for methionine which is absent in the P. mirabilis lipoprotein, secondly the ratio of lipophilic and hydrophilic amino acids is essentially the same although P. mirabilis lipoprotein contains more acidic and fewer basic residues, thirdly, the P. mirabilis lipoprotein is composed of 60 amino acid residues as compared with 58 residues of the E. coli lipoprotein, fourthly, 55% and 59% of the lipoprotein molecules from E. coli and P. mirabilis respectively, are linked to peptide crosslinked peptidoglycan dimers. Fatty acid analysis of the P. mirabilis lipoprotein gave 1.71 mol ester‐linked and 1.14 mol amide‐linked (mainly palmitic acid) fatty acids per mol lipoprotein. Although the lipoproteins from E. coli and P. mirabilis have similar compositions and molecular weights they behave differently in sodium dodecyl sulfate – polyacrylamide gel electrophoresis under various conditions.