Role of Mg2+ and pH in the modification of Salmonella lipid A after endocytosis by macrophage tumour cells

Henry S. Gibbons, Suzanne R. Kalb, Robert J. Cotter, Christian R.H. Raetz

Research output: Contribution to journalArticlepeer-review

Abstract

Lipid A of Salmonella typhimurium is covalently modified with additional acyl and/or polar substituents in response to activation of the PhoP/PhoQ and/or PmrA/PmrB signalling systems, which are induced by growth at low Mg 2+ concentrations and mild acid pH respectively. Although these conditions are thought to exist within macrophage phagolysosomes, no direct evidence for lipid A modification after endocytosis has been presented. To address this issue, we grew S. typhimurium inside RAW264.7 cells in the presence 32Pi, and then isolated the labelled lipid A fraction, which was found to be extensively derivatized with phosphoethanolamine, aminoarabinose, 2-hydroxymyristate and/or palmitate moieties. S. typhimurium grown in tissue culture medium synthesized lipid A molecules lacking all these substituents with the exception of the 2-hydroxymyristate chain, which was still present. Using defined minimal media to simulate the intracellular pH and Mg2+ concentrations of endosornes, we found that lipid A of S. typhimurium grown in an acidic, low-Mg2+ medium closely resembled lipid A isolated from bacteria internalized by RAW264.7 cells. A subset of S. typhimurium lipid A modifications were induced by low Mg2+ alone. Escherichia coli K-12 W3110 modified its lipid A molecules in response to growth under acidic but not low-Mg2+ conditions. Growth in a high-Mg 2+, mildly alkaline medium resulted in suppression of most lipid A modifications with the exception of the 2-hydroxymyristate in S. typhimurium. Although IpxO transcription was stimulated by growth on low Mg2+, the biosynthesis of lipid A species containing 2-hydroxymyristate was independent of PhoP/PhoQ and PmrA/PmrB in S. typhimurium. Our labelling methods should be applicable to studies of lipid A modifications induced by endocytosis of diverse bacteria.

Original languageEnglish (US)
Pages (from-to)425-440
Number of pages16
JournalMolecular Microbiology
Volume55
Issue number2
DOIs
StatePublished - Jan 2005

ASJC Scopus subject areas

  • Microbiology
  • Molecular Biology

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