Mount Sinai School of Medicine
September 13, 2011
Are Anomalous effects of genetically deleting a sodium/proton antiporter of Staphylococcus aureaus due to loss of cell-wide stress response?
Sodium/proton antiporters of bacteria catalyze exchange cytoplasmic sodium ions (Na+) for external protons (hydrogen ions, H+). These "secondary transporters" are energized by the transmembrane gradient of protons, i.e. the proton motive force (PMF), which is generated by membrane associated "primary transporters" that pump protons out of the cell such as those of the respiratory chain. Sodium/proton antiporters support resistance of bacterial to cytotoxic levels of sodium and both sodium/proton and potassium/proton antiporters can support alkali-resistance when the antiport turnover is electrogenic, with the number of H+ entering > than the number of Na+ or K+ exiting the cell. Most sodium (potassium)/proton antiporters are products of a single gene that encodes the hydrophobic trasnporter protein.
By contrast, one family of sodium (potassium)/proton antiporters, Cation: Proton Antiporter-3 (CPA3 or Mrp-type antiporters) requires a hetero-oligomeric complex of 6-7 hydrophobic products of distinct genes. Moreover, three of the proteins have strong sequence homology to the membrane-embedded domain of the L-shaped NADH: quinone Oxidoreductase (Complex I), a primary proton pump of respiratory chains. Bayer et al.¹ proposed that the CPA3 antiporter, called Mnh (for multi-subunit Na/H) in Staphylococcus aureus recruits an electron transferring module of some sort and actually functions as a primary pumping complex. The basis for this suggestion includes the observation that a putative deletion mutant in the mnh gene locus exhibited a lower transmembrane electrical potential (the component of the PMF that is present at pH 7.5 and above) than the wild-type strain, which seemed inconsistent with Mnh being a PMF-consuming secondary antiporter. My lab's data² strongly support a secondary antiport mechanism for Mnh and provide indications that a systems reaction occurs in bacterial cells in which Mnh and other CPA3 systems are active, such that "low PMF stress" leads to a compensatory response that leads to a transmembrance of electrical potential that is higher than the pre-stress level. A model has been proposed for this response.
The talk will review the background, the two sets of data that lead to different interpretations of how Mnh and other CPA3 antiporters work, as well as recent experimental data on this project.
¹Bayer, A.S., McNamara, P., Yeamen, M.R., Lucindo, N., Jones, T., Cheung, A.L., Sahl, H.G., Proctor, R.A., 2006, Transposon disruption of the compelx I NADH oxidoreductase gene (snoD) in Staphylococcus aereus is associated with reduced susceptibility to the microbicidal activity of thrombin-induced platelet microbicidal protein 1. J. Bacteriol. 188: 211-222.
²Swartz, T.H., Ito, M., Ohira, T., Natsui, S., Hicks, D.B., Krulwich, T.A., 2007, Catalytic properties of Staphylococcus aureus and Bacillus members of the secondary cation/proton antiporter-3 (Mrp) family are revealed by an optimized assay in an Escherichia coli host. J. Bacteriol. 189: 3081-3090.