Lipid modification of bacterial prolipoprotein. Transfer of diacylglyceryl moiety from phosphatidylglycerol.
The peptide, MKATKLVLGAVILGSTLLAGCSSN, corresponding to the N-terminal 24 amino acids of Braun's prolipoprotein, was used to study the lipid modification of prolipoprotein in Escherichia coli by measuring the rate of incorporation of either [2-3H]glycerol or [9,10-3H]palmitate from the corresponding labeled phosphatidylglycerol into the peptide. Using E. coli strains containing varying levels of prolipoprotein diacylglyceryl modification activities due to mutations in or overexpression of the gene involved in diacylglyceryl modification (lgt), we have shown that the activities based on the peptide assay correlated well with the prolipoprotein-based assay. Further, we have followed the fate of the lipid substrate, phosphatidylglycerol, during the modification reaction and found that lipid modification of prolipoprotein involves the transfer of diacylglyceryl moiety from phosphatidylglycerol to the sulfhydryl group of the cysteine residue with the concomitant formation of sn-glycerol 1-phosphate. This mechanism is contrary to the previously proposed two-step mechanism of an initial glyceryl transferase followed by O-acyl transfer (Chattopadhyay, P.K., and Wu, H.C. (1977) Proc. Natl. Acad. Sci. U. S. A. 74, 5318-5322). Accordingly, the enzyme that catalyzes this activity has been named phosphatidylglycerol-prolipoprotein diacylglyceryl transferase. The revised pathway for the lipoprotein biogenesis in bacteria consists of three successive reactions catalyzed by prolipoprotein diacylglyceryl transferase, signal peptidase II, and apolipoprotein N-acyltransferase.