Periplasmic superoxide dismutase SodCI of Salmonella binds peptidoglycan to remain tethered within the periplasm

Abstract

Summary: Salmonellae survive and propagate in macrophages to cause serious systemic disease. Periplasmic superoxide dismutase plays a critical role in this survival by combating phagocytic superoxide. Salmonella Typhimurium strain 14028 produces two periplasmic superoxide dismutases: SodCI and SodCII. Although both proteins are produced during infection, only SodCI is functional in the macrophage phagosome. We have previously shown that SodCI, relative to SodCII, is both protease resistant and tethered within the periplasm and that either of these properties is sufficient to allow a SodC to protect against phagocytic superoxide. Tethering is defined as remaining cell-associated after osmotic shock or treatment with cationic antimicrobial peptides. Here we show that SodCI non-covalently binds peptidoglycan. SodCI binds to Salmonella and Bacillus peptidoglycan, but not peptidoglycan from Staphylococcus. Moreover, binding can be inhibited by a diaminopimelic acid containing tripeptide, but not a lysine containing tripeptide, showing that the protein recognizes the peptide portion of the peptidoglycan. Replacing nine amino acids in SodCII with the corresponding residues from SodCI confers tethering, partially delineating an apparently novel peptidoglycan binding domain. These changes in sequence increase the affinity of SodCII for peptidoglycan fragments to match that of SodCI and allow the now tethered SodCII to function during infection. Many Salmonella strains produce two periplasmic superoxide dismutases, SodCI and SodCII, but only SodCI functions to protect the bacterium from phagocytic superoxide. SodCI is both protease resistant and "tethered" within the periplasm and these properties independently allow SodCI to function during infection. Here we show that SodCI non-covalently binds peptidoglycan to remain tethered within the periplasm and, thus, the protein remains cell-associated after osmotic shock or treatment with antimicrobial peptide.