Acyl chain specificity of the acyltransferases lpxA and lpxD and substrate availability contributes to lipid A fatty acid heterogeneity in Porphyromonas gingivalis

Dept. of Periodontics and Oral Biology and Medicine, University of Washington, D-652 Health Sciences Building, 1959 NE Pacific St, Seattle, WA 98195-7444

^* To whom correspondence should be addressed. Email: rdarveau{at}u.washington.edu.

	Abstract

The lipid A of Porphyromonas gingivalis is heterogeneous with regard to the number, type, and placement of fatty acids. Analysis of lipid A by MALDI-TOF mass spectrometry reveals clusters of peaks differing by 14 mass units indicative of an altered distribution of the fatty acids generating different lipid A structures. To examine whether the transfer of hydroxy fatty acids with different chain lengths could account for the clustering of lipid A structures, lpxA_Pg and lpxD_Pg were cloned and expressed in Escherichia coli strains mutant for the homologous gene. Lipid A from strains expressing either of the P. gingivalis transferases was found to contain sixteen carbon hydroxy fatty acids in addition to the normal E. coli fourteen carbon hydroxy fatty acids demonstrating that these acyltranferases display a relaxed acyl chain length specificity. Both lpxA and lpxD, from either E. coli or P. gingivalis, were also able to incorporate odd chain fatty acids into lipid A when grown in the presence of 1% propionic acid. This indicates that E. coli lipid A acyltransferases do not have an absolute specificity for fourteen carbon hydroxy fatty acids, but can transfer fatty acids differing by one carbon unit if the fatty acid substrates are available. We conclude that the relaxed specificity of the P. gingivalis lipid A acyltransferases and the substrate availability account for the lipid A structural clusters that differ by 14 mass units observed in P. gingivalis lipopolysaccharide preparations.