Murein-lipoprotein

Figure 13.9 9.4 T MALDI-FTMS spectrum of E. coli JM109 whole cells showing both Braun s and Murein lipoproteins.

Although ribosomal proteins are readily observed as in Figures 13.7 and 13.8 altered matrix conditions can alter the relative ionization of bacterial whole-cell compounds. A systematic analysis involving laser power/fluence and sample preparation conditions reveals that if the concentrated trifluo-roacetic acid is added and the laser power increased above optimal conditions, ionization of bacterial surface compounds can be enhanced. Figure 13.9 is the resulting 9.4 T MALDI-FTMS, seen are both the Braun s lipoprotein56,57 and the Murein lipoprotein. Both of these compounds are complex combinations of hydrocarbon lipids attached to a protein base. This is the first MALDI-FTMS observation of surface proteins desorbed directly from whole cells by influencing ionization conditions. 

Braun, V. and Rehn, K. (1969). Chemical characterization, spatial distribution and function of a lipoprotein (murein-lipoprotein) of the E. coli cell wall. The specific effect of trypsin on the membrane structure, Eur. J. Biochem., 10, 426-438. 

Yem DW, Wu HC (1978) Physiological characterization of an Escherichia coli mutant altered in the structure of murein lipoprotein. J Bacteriol 133 1419-1426... 

The fatty acid chains are evidently embedded in the outer membrane as an anchor. About one-third of the lipoprotein molecules are attached covalently to the peptidoglycan through an amide linkage between the side chain amino group of the C-terminal lysine of the protein and a diaminopimelic acid residue of the peptidoglycan (Fig. 8-29). Thus, the protein replaces one of the terminal D-alanine residues of about one in ten of the murein peptides. There are 2.5 x 105 molecules of the bound form of the lipoprotein per cell spread over a surface area of peptidoglycan of 3 pm2. They appear to be associated as trimers located primarily in the periplasmic space.589... 

The wall of Gram-negative bacteria is more complicated. Freeze etching, in combination with electron microscopy, reveals 5 or even 6 concentric layers of which murein (only 2 nm thick) is the innermost (murein forms only 5-20% of the wall mass). The surrounding layers are lipoprotein and lipo-polysaccharide, but teichoic acid is absent. The integrity of these layers depends on the presence of magnesium and calcium. 

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