Identification and subcellular localization of apolipoprotein N-acyltransferase in Escherichia coli.

Apolipoprotein N-acyltransferase, the enzyme catalyzing the conversion of apolipoprotein to mature lipoprotein, was detected by an in vitro assay using [35S]methionine-labeled apolipoprotein as the substrate. Triton X-100 solubilized the enzyme, and was required for its activity. The enzyme showed a broad pH optimum (pH 6.5-7.5). N-Acylation of apolipoprotein with ethanol-washed membranes was dependent on exogenous phospholipids, with phosphatidylethanolamine, phosphatidylglycerol and cardiolipin all showing about 10- to 20-times enhancement of the enzyme activity in the delipidated membranes. Incubation of apolipoprotein with [3H]palmitate-labeled membranes resulted in the incorporation of [3H]palmitate into lipoprotein. The enzyme was found to be enriched in the inner membrane and in the inner membrane/outer membrane mixed fractions of the E. coli cell envelope.