A Novel Peptidoglycan-Associated Lipoprotein (PAL) Found in the Outer Membrane of Proteus mirabilis and Other Gram-Negative Bacteria

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Abstract
A novel peptidoglycan-associated lipoprotein (PAL) was found in the cell envelope of Proteus mirabilis. This protein showed the following properties: (1) The apparent molecular weight in sodium dodecyl sulfate (SDS) polyacrylamide gel was 18, 000. (2) The protein was present in the cell envelope in a form very closely, but not covalently, associated with the peptidoglycan layer. (3) The protein was recovered predominantly from the outer membrane fraction after separation of the cell envelopes. (4) [1-14C]Palmitic acid and [2-3H]glycerol were incorporated into the protein. (5) The protein contained covalently linked fatty acids (about 3 mol of fatty acid per mol of protein). (6) An unidentified compound was present in the hydrolysate of the protein. These properties, except for molecular weight and non-covalent association with the peptidoglycan, showed resemblance to those of Braun's lipoprotein. However the protein was distinct from Braun's lipoprotein in regard to amino acid composition. A similar peptidoglycan-associated lipoprotein (PAL) was present widely in the cell envelopes of various Gram-negative bacteria. P. mirabilis contains about twelve times as much PAL as Escherichia coil. Antiserum against PAL of P. mirabilis was cross-reactive against PAL of E. coli but not against Braun's lipoprotein of E. coli.