The structure of the lipid modification in the case of major outer membrane protein of E. coli was elucidated chemically in 1973. It was shown that Sulphydryl group of N-terminal Cysteine was modified with a diacylglyceryl group attached through a thioether linkage and the amino group was acylated with a fatty acid [1] (shown in figure). The acyl group composition was found to be same as that of membrane phospholipids [1] and therefore membrane phospholipids were suspected to be the lipid donors for the modification.

In another important finding it was shown that the precursor for this outer membrane lipoprotein was actually a pre-protein with an N-terminal extension of 20 amino acids possessing the characteristics of a typical signal peptide [2]. Naturally, the interest in the biosynthesis of this lipoprotein started immediately. Firstly the origin of the lipid in the molecule was traced to phospholipids as suspected earlier [3]. Phosphatidylglycerol was shown to be specifically required for the initial modification of Cys of the lipoproteins with diacylglycerol, catalyzed by the enzyme phosphatidylglycerol-prolipoprotein diacylglyceryl transferase[4,5]. N-acylation was shown to be achieved with any of the phospholipids [4]. Discovery of Globomycin, a cyclic pentapeptide antibiotic that specifically inhibited the maturation of lipoproteins led to the identification of a signal peptidase specific for lipoproteins [6]. This signal peptidase called signal peptidase II required diacylglyceryl modification prior to cleavage [7,8]. This meant that diacylglyceryl modification preceded cleavage of the signal peptide. The final modification i.e. after the cleavage of the signal peptide, fatty acylation of the amino group of the N-terminal diacylglyceryl modified Cys to form N-acyl diacylglyceryl Cysteine was subsequently identified [9,10]. The pathway [5] is shown in the figure below. The genes for these enzymes have been identified by Wu et. al. in a variety of bacteria and they are found to be highly conserved [11, 12].

The Biosynthetic Pathway

References

1) Hantke, K. and Braun, V., Covalent binding of lipid to protein. Diglyceride and amide-linked fatty acid at the N-terminal end of the murien lipoprotein of the Eschericia coli outer membrane, Eur. J. Biochem., 34, 284, 1973.

2) Inouye, S., Wang, S., Sekizawa, J., Halegoua, S., and Inouye, M., Amino acid sequence for the peptide extension on the prolipoprotein of the Eschericia coli outer membrane, Proc. Natl. acad. Sci. U. S. A., 74, 1004, 1977.

3) Chattopadhyay, P. K. and Wu, H. C., Biosynthesis of the covalently linked diglyceride in murein lipoprotein of Escherichia coli, Proc. Natl. Acad. Sci. U. S. A., 74, 5318, 1977.

4) Lai, J. S., Philbrick, W, M., and Wu, H. C. Acyl moieties in phospholipids are the procursers for the fatty acids in lipoprotein of Escherichia coli , J. Biol. Chem., 255, 5384, 1980.

5) Sankaran, K. and Wu, H. C. Lipid Modification of Bacterial Prolipoproteins, J. Biol. Chem., 269, 19701, 1994

6) Inukai, M., Takeuchi, M., Shimizu, K., and Arai, M., Mechanism of action of globomycin, J. Antibiot. (Tokyo), 31, 1203, 1978.

7) Hussain, M., Ichihara, S., and Mizushima, S., Accumlation of glyceride containing precursor of the outer membrane lipoprotein in the cytoplasmic membrane of E.coli treated with globomycin, J. Biol. Chem., 255, 3707, 1980.

8) Dev, I. K. and Ray, P. H., Rapid assay and purification of a unique signal peptidase that processes the prolipoprotein from E.coli, J. Biol. Chem., 259, 11114, 1984.

9) Gupta, S. D. and Wu, H. C. Identification and sub cellular localization of apolipoprotein N-acyltransferase in E.coli, FEMS Microbiol . Lett., 78, 37, 1991.

10) Sankaran, K., Gupta, S. D. and Wu, H. C., Modification of Bacterial Lipoproteins, Methods in Enzymol. 250, 683, 1995

11) Qi, H-Y., Sankaran, K., Gan, K., and Wu, H. C., Structure-Function relationship of Bacterial Prolipoprotein Diacylglyceryl Transferase: Functionally significant Conserved Regions, J. Bacteriol, 177, 6820, 1995

12) Sankaran, K., and Wu, H. C., Signal Peptidase II - Specific Signal Peptidase for Bacterial Lipoproteins, in Signal peptidases Ed. Heijine G. V., R.G. Landes Company, Austin. Ch. 3, P.17.

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