Cellular role of the inhibitor-chaperone CpxP in Escherichia coli

• Author / Creator

  Wong, Julia L.

• Bacteria must sense stress signals and adapt accordingly in order to survive. In Gram-negative bacteria, the envelope is the first to encounter adverse environmental conditions and contains signal transduction systems to relay information from the periplasm to the cytoplasm. The Cpx two-component system consists of the inner membrane sensor kinase CpxA and the cytoplasmic response regulator CpxR. In the presence of envelope stress, CpxA autophosphorylates and phosphorylates CpxR. Phosphorylated CpxR modulates the transcription of over 100 genes, including the chaperone CpxP. CpxP is a small periplasmic protein that inhibits the Cpx pathway when over-expressed and exhibits weak chaperone activity in vitro. In the absence of stress, CpxP is thought to interact directly with the sensing domain of CpxA and prevent activation of the Cpx pathway. In the presence of stress such as misfolded pilin proteins, CpxP is titrated away from CpxA and degraded, freeing CpxA to sense stress and activate the pathway. Paralogues of CpxP bind metal ions and modulate signal transduction from the periplasm to the cytoplasm in response to metal-binding. Excess zinc and copper induce the Cpx pathway in E. coli and Salmonella enterica but iron chelation induces the Cpx pathway in Vibrio cholerae. We sought to test whether the Cpx system in E. coli could also respond to metal limitation. We show that the expression of cpxP is elevated when cells are grown in metal-limited conditions and this increase is reversed by the addition of exogenous zinc. The sensor kinase CpxA and CpxP are dispensable for the activation of cpxP expression by chelation or its reversal by zinc. We assessed the impact of Cpx mutations on agar containing toxic levels of zinc and discovered that deletion of cpxP in the laboratory strain of E. coli results in zinc-resistance that is complemented by the over-expression of cpxP in trans. Over-expression of cpxP strongly decreased the expression of the zinc efflux protein zntA, which remained high at toxic levels of zinc in the cpxP mutant. Deletion of either cpxA or cpxR did not phenocopy the zinc resistance of the cpxP mutant, though the expression of cpxP is low in these genetic backgrounds. Mutational analysis of CpxP demonstrated that residues that mediate the interaction between CpxP and CpxA are also important for the zinc-related function of CpxP. Unexpectedly, activation of the Cpx pathway by over-expressing NlpE in enteropathogenic E. coli conferred zinc resistance but insertional inactivation of cpxP did not. Therefore, the expression of cpxP appears to be toxic in the presence of high levels of zinc and the expression of Cpx-regulated factors other than cpxP may be required to mediate zinc resistance under these conditions. We hypothesized that CpxP binds zinc to alter intracellular zinc trafficking. To test our hypothesis, we performed inductively-coupled plasma mass spectrometry on purified CpxP protein and showed that CpxP binds zinc in non-stoichiometric amounts. This observation confirmed that CpxP, as the prototypical member of a family of extracytoplasmic signal transduction proteins, is a metal-binding protein like its paralogues. Finally, to gain a better understanding of how inhibition of the Cpx pathway is maintained, we performed a genetic screen for Cpx-regulated envelope-localized genes that inhibit the Cpx pathway upon over-expression. We identified nuoF, the soluble NADH-binding component of ComplexI and efeB, a heme peroxidase. NuoF and EfeB are novel cpxA-, cpxR-, and cpxP-dependent inhibitors of the Cpx pathway. NuoF did not require a functional Complex I and EfeB did not require the EfeU heme permease to inhibit the Cpx pathway. We speculate that the over-expression of NuoF and EfeB alter the accumulation of an unidentified cellular metabolite that induces the Cpx response. Our work shows that the non-essential auxiliary regulator CpxP integrates protein-folding, metal, and metabolic signals and relays them to the sensor kinase CpxA to modulate Cpx activity, promoting bacterial adaptation to multiple stresses.

• Subjects / Keywords

  • Two-component systems
  • Periplasmic chaperone
  • Auxiliary regulators

• Graduation date

  Spring 2015

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R30K26Q1V

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.