Phosphatidylethanolamine is not essential for the N-acylation of apolipoprotein in Escherichia coli

It has been postulated that the N-acyl fatty acid attached to the amino terminus of the major Escherichia coli lipoprotein is derived from the fatty acid at the 1-position of phosphatidylethanolamine (PtdEtn) (Jackowski, S., and Rock, C.O. (1986) J. Biol. Chem. 261, 11328-11333). To ascertain the role of PtdEtn in the conversion of apolipoprotein to the mature lipoprotein, the lipoprotein from E. coli strain AH930 (pss::kan) containing a null mutation in the phosphatidylserine synthase gene (pss) was studied. Pulse labeling with [35S]methionine for 30 s or 5 min revealed the formation of mature lipoprotein in both wild-type (W3110) and mutant (AH930) cells. [3H]Palmitate-labeled lipoproteins from both the mutant and wild-type cells were found to contain nearly identical amounts of alkali-resistant (amide-linked, 41-42%) and alkali-labile (ester-linked, 58-59%) fatty acids. Edman degradation and dansylation of the immuno-affinity-purified [35S]cysteine-labeled lipoprotein showed that the NH2 terminus of the lipoprotein in the mutant was blocked as in the wild type. In vitro assay of apolipoprotein N-acyltransferase using membranes either from the mutant or the wild-type strain as the source of both the enzyme and the acyl donor revealed that both membranes were equally active in the conversion of [35S]methionine-labeled apolipoprotein to lipoprotein. These data strongly suggest that PtdEtn is not essential for the N-acylation of apolipoprotein to form lipoprotein, and other major phospholipids such as phosphatidylglycerol and cardiolipin can serve as the donor of fatty acid in the N-acylation of apolipoprotein.