Characterization of new proteins maintaining the integrity of the Escherichia coli envelope : a step towards the development of new antibiotics

The cell envelope of Gram-negative bacteria is an essential organelle that is important for cell shape and protection from toxic compounds. Proteins involved in envelope biogenesis are therefore attractive targets for the design of new antibacterial agents. We set out to search for uncharacterized proteins that could potentially play a role in the assembly of the cell envelope by screening a collection of Escherichia coli deletion mutants for sensitivity to detergents and hydrophobic antibiotics, a phenotype indicative of defects in the cell envelope. We found that strains lacking yciM, a gene present in a conserved operon with yciS, were among the most sensitive strains of the mutant collection. Therefore, the main objective of my thesis was to clarify the role played by YciM and YciS in E. coli. First, the characterization of yciM and yciS mutants revealed that yciM mutants, but not yciS mutants, have a pleiotropic phenotype that is consistent with a defective cell envelope. The deletion of yciM leads to a thermosensitive growth defect on low-osmolarity medium and to a significantly altered cell morphology. At elevated temperatures, yciM mutants form bulges containing cytoplasmic material and subsequently lyse suggesting defects in the peptidoglycan layer. We also discovered that yciM genetically interacts with envC, a gene encoding a regulator of the activity of peptidoglycan amidases. Second, the biochemical characterization of the YciS and YciM proteins showed that they both localize to the inner membrane. While YciS is an integral membrane protein, YciM is anchored to the membrane via its N-terminus, the rest of the protein being exposed to the cytoplasm. Two CXXC motifs are present at the C-terminus of YciM and serve to coordinate a redox-sensitive iron center of the rubredoxin-type. Both the N-terminal membrane anchor and the C-terminal iron center of YciM are required for function. Altogether, my results reveal YciM as a protein playing an important role in the maintain of envelope integrity.