T. Clementz, Zhimin Zhou, C. Raetz
Apr 18, 1997
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Journal
The Journal of Biological Chemistry
Abstract
Overexpression of the Escherichia coli msbB gene on high copy plasmids suppresses the temperature-sensitive growth associated with mutations in thehtrB gene. htrB encodes the lauroyl transferase of lipid A biosynthesis that acylates the intermediate (Kdo)2-lipid IVA (Brozek, K. A., and Raetz, C. R. H. (1990) J. Biol. Chem. 265, 15410–15417). SincemsbB displays 27.5% identity and 42.2% similarity tohtrB, we explored the possibility that msbBencodes a related acyltransferase. In contrast to htrB, extracts of strains with insertion mutations in msbB are not defective in transferring laurate from lauroyl acyl carrier protein to (Kdo)2-lipid IVA. However, extracts ofmsbB mutants do not efficiently acylate the product formed by HtrB, designated (Kdo)2-(lauroyl)-lipid IVA. Extracts of strains harboring msbB + bearing plasmids acylate (Kdo)2-(lauroyl)-lipid IVAvery rapidly compared with wild type. We solubilized and partially purified MsbB from an overproducing strain, lacking HtrB. MsbB transfers myristate or laurate, activated on ACP, to (Kdo)2-(lauroyl)-lipid IVA. Decanoyl, palmitoyl, palmitoleoyl, and (R)-3-hydroxymyristoyl-ACP are poor acyl donors. MsbB acylates (Kdo)2-(lauroyl)-lipid IVA about 100 times faster than (Kdo)2-lipid IVA. The slow, but measurable, rate whereby MsbB acts on (Kdo)2-lipid IVA may explain why overexpression of MsbB suppresses the temperature-sensitive phenotype ofhtrB mutations. Presumably, the acyloxyacyl group generated by excess MsbB substitutes for the one normally formed by HtrB.