Relaxed acyl chain specificity of Bordetella UDP-N-acetylglucosamine acyltransferases

Lipid A (endotoxin) is a major structural component of Gram-negative outer membranes. It also serves as the hydrophobic anchor of lipopolysaccharide and is a potent activator of the innate immune response. Lipid A molecules from the genus Bordetella are reported to exhibit unusual structural asymmetry with respect to the acyl chains at the 3- and 3'-positions. These acyl chains are attached by LTDP-N-acetylglucosamine acyltransferase (LpxA). To determine the origin of the acyl variability, the single lpxA ortholog present in each of the genomes of Bordetella bronchiseptica (lpxA(Br)), Bordetella parapertussis (lpxA(Pa)), and Bordetella pertussis (lpxA(Pe)) was cloned and expressed in Escherichia coli. In contrast to all LpxA proteins studied to date, LpxA]3, and LpxA(Pe) display relaxed acyl chain length specificity in vitro, utilizing C10OH-ACP, C12OH-ACP, and C14OH-ACP at similar rates. Furthermore, hybrid lipid A molecules synthesized at 42 degreesC by an E. coli lpxA mutant complemented with lpxA(Pe) contain C10OH, C12OH, and C14OH at both the 3- and 3'-positions, as determined by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. In contrast, LpxA from B. parapertussis did not display relaxed specificity but was selective for C10OH-ACP. This study provides an enzymatic explanation for some of the unusual acyl chain variations found in Bordetella lipid A.