A decrease of the absorbance at 578 nm of a cell wall suspension of mid log phase E. coli occurs when the suspension is incubated with trypsin. The reaction is so rapid that 55% of the total decrease is obtained within the first 2 min [ratio of enzyme to total cell wall protein = 1: 50 (w/w), room temperature]. The rate of the reaction is specific for trypsin when compared with other proteases, different lipases, lysozyme and other glycosidases. A peptide bond especially sensitive to trypsin could be localized within the complex cell wall by the demonstration that the decrease of the absorbance is paralleled by the splitting of the protein from the murein.
This protein could be shown to be a lipoprotein with a part of the lipid probably covalently bound to the protein. It is called murein-lipoprotein. The link between the lipoprotein and the murein is -lysine. After trypsin digestion lysine is the only additional amino acid remaining at the murein. The ratio of the amount of lysine to the known constituents of the murein demonsstrates that on the average one lipoprotein molecule is covalently bound to every tenth repeating unit of the murein (N-acetylglucosamine–N-acetylmuramic acid–l-alanine-d-glutamic acid–meso-diaminopimelic acid–d-alanine). After 3 min incubation with trypsin, the isolated lipoprotein molecules have a lysine to arginine ratio of 4:4 as compared with 5:4 in the native molecule. The lipoprotein has an unusual amino acid comosition since it contains about 65% polar amino acids and no glycine, cysteine, proline, phenylalanine, and histidine could be found. On a weight basis the lipoprotein accounts for more than 40% of the rigid layer. Since the murein is held together exclusively by covalent bonds one can get a fairly accurate idea of the distribution of the lipoprotein molecules in the cell wall. About 105 lipoprotein molecules per cell should be distributed 103 Å apart along the glycosidic chains of the murein.
The lipoprotein has an important function in stabilizing the total structure of the cell wall. It seems that cleavage of only one peptide bond adjacent to the lysine link between the lipoprotein and the murein causes the rapid decrease of the absorbance and as shown by electron microscopic exmination of ultrathin sections of trypsin treated cell walls, two separated membrane structures appear which otherwise are closely adjacent to one another.