The Covalent Murein‐Lipoprotin Structure of the Escherichia coli Cell Wall

After pronase treatment of the murein‐lipoprotein complex (rigid layer) of the cell wall of E. coli B or E. coli K12 (W 945), lysine and arginine remain as the sole amino acids covalently bound to the murein (peptidoglycan, glycopeptide). These amino acids occur in equimolar amounts, each equal to the amount of lysine remaining with the murein after trypsin digestion of the murein‐lipoprotein complex. From partial acid hydrolysates of such a murein, prepared by pronase digestion of the mureinlipoprotein complex, the following peptides have been isolated: (1) diaminopimelyl‐lysyl‐arginine; (2) alanyl‐glutamyl‐diaminopimelyl‐lysyl‐arginine; (3) glucosaminyl‐muramyl‐alanyl‐glutamyl‐diaminopimelyl‐lysyl‐arginine. Peptide 1 shows that the lipoprotein is bound by the α‐amino group of the presumbaly N‐terminal lysine to the carboxyl group of diaminopimelic acid. Peptide 2 consists of a peptide side chain of the murein to which the two amino acids of the N‐terminal end of the lipoprotein, lysine and arginine, are attached. Peptide 3 constitutes a repeating unit of the murein to which the peptide lysyl‐arginine of the lipoprotein is bound. The lipoprotein cleaved from the murein by a short trypsin digestion had an amino acid composition similar to the lipoprotein in the untreated murein‐lipoprotein complex and arginine as N‐terminal amino acid. The following structure is proposed: murein‐lysyl‐arginyl‐lipoprotein. In the rapid reaction of trypsin with the cell wall the enzyme apparently cleaves at the C‐terminal end of the lysime of the murein‐lipoprotein linkage which results in murein‐lysine and arginyl‐lipoprotein. Some cleavage at the C‐terminal end of arginine gives rise to free arginine (14%). These results support the supramolecular structure of the murein‐lipoprotein complex previously proposed. On the average one lipoprotein molecule is covalently bound to every tenth repeating unit of the murein from which an average distance of 103 Å between two lipoprotein molecules along the polysaccharide chains of the murein is deduced.