Human- and plant-pathogenic Pseudomonas species produce bacteriocins exhibiting colicin M-like hydrolase activity towards peptidoglycan precursors

Toxicity of the colicin M catalytic domain exported to the periplasm is FkpA independent

Colicin M (ColM) is a bactericidal protein that kills sensitive cells by hydrolyzing lipid II, involved in the biosynthesis of cell wall peptidoglycan. It recognizes FhuA on the outer leaflet, and its translocation through the outer membrane depends on the energized Ton complex in the inner membrane. To be active in the periplasm, ColM must be translocated through the outer membrane and then interact with FkpA, a periplasmic protein that exhibits both cis- and trans-peptidylprolyl isomerase (PPiase) and chaperon activities. In an attempt to directly target ColM to the periplasm of the producing bacteria, we fused the presequence of OmpA to ColM (sp-ColM). We found that expression of this hybrid protein in an Escherichia coli strain devoid of ColM immunity protein (Cmi) was bactericidal. We showed that sp-ColM was correctly expressed, processed, and associated with the inner membrane. sp-ColM toxicity was related to its enzymatic activity and did not rely on the TonB import proteins or the FhuA receptor. The presence of both activity domains of FkpA was still required for sp-ColM activity. Analyses of deletion mutants of sp-ColM show that the domain required for toxicity corresponds to the C-terminal last 153 amino acids of ColM. Like the full-length protein, this domain is not active in the presence of the immunity protein Cmi. On the other hand, it does not require FkpA for toxic activity.

A predicted S-type pyocin shows a bactericidal activity against clinical Pseudomonas aeruginosa isolates through membrane damage

The nucleic acid sequence at the positions 1067817-1066321 of Pseudomonas aeruginosa PAO1 genome was predicted to encode a novel S-type pyocin, designated S5, based on the genome sequence. However, its antimicrobial spectrum, activity and mechanism have not been investigated. Herein, we report that pyocin S5 has an antimicrobial activity against seven clinical P. aeruginosa isolates (DWW3, InA, InB, In3, In4, In7, and In8). Among them, DWW3 is most sensitive with a minimum inhibitory concentration of 12.6 microg/ml and a killing percentage of 95.7 at 225 microg/ml. Further, we demonstrated that the antimicrobial mechanism of pyocin S5 is membrane damage, evidenced by the leakage of intracellular materials, the increase of membrane permeability, and cell surface disruption.

Deciphering the catalytic domain of colicin M, a peptidoglycan lipid II-degrading enzyme

Colicin M inhibits Escherichia coli peptidoglycan synthesis through cleavage of its lipid-linked precursors. It has a compact structure, whereas other related toxins are organized in three independent domains, each devoted to a particular function: translocation through the outer membrane, receptor binding, and toxicity, from the N to the C termini, respectively. To establish whether colicin M displays such an organization despite its structural characteristics, protein dissection experiments were performed, which allowed us to delineate an independent toxicity domain encompassing exactly the C-terminal region conserved among colicin M-like proteins and covering about half of colicin M (residues 124-271). Surprisingly, the in vitro activity of the isolated domain was 45-fold higher than that of the full-length protein, suggesting a mechanism by which the toxicity of this domain is revealed following primary protein maturation. In vivo, the isolated toxicity domain appeared as toxic as the full-length protein under conditions where the reception and translocation steps were by-passed. Contrary to the full-length colicin M, the isolated domain did not require the presence of the periplasmic FkpA protein to be toxic under these conditions, demonstrating that FkpA is involved in the maturation process. Mutational analysis further identified five residues that are essential for cytotoxicity as well as in vitro lipid II-degrading activity: Asp-229, His-235, Asp-226, Tyr-228, and Arg-236. Most of these residues are surface-exposed and located relatively close to each other, hence suggesting they belong to the colicin M active site.