Colicin A immunity protein interacts with the hydrophobic helical hairpin of the colicin A channel domain in the Escherichia coli inner membrane

Channel domain of colicin A modifies the dimeric organization of its immunity protein

Proteins conferring immunity against pore-forming colicins are localized in the Escherichia coli inner membrane. Their protective effects are mediated by direct interaction with the C-terminal domain of their cognate colicins. Cai, the immunity protein protecting E. coli against colicin A, contains four cysteine residues. We report cysteine cross-linking experiments showing that Cai forms homodimers. Cai contains four transmembrane segments (TMSs), and dimerization occurs via the third TMS. Furthermore, we observe the formation of intramolecular disulfide bonds that connect TMS2 with either TMS1 or TMS3. Co-expression of Cai with its target, the colicin A pore-forming domain (pfColA), in the inner membrane prevents the formation of intermolecular and intramolecular disulfide bonds, indicating that pfColA interacts with the dimer of Cai and modifies its conformation. Finally, we show that when Cai is locked by disulfide bonds, it is no longer able to protect cells against exogenous added colicin A.

Colicin biology

Colicins are proteins produced by and toxic for some strains of Escherichia coli. They are produced by strains of E. coli carrying a colicinogenic plasmid that bears the genetic determinants for colicin synthesis, immunity, and release. Insights gained into each fundamental aspect of their biology are presented: their synthesis, which is under SOS regulation; their release into the extracellular medium, which involves the colicin lysis protein; and their uptake mechanisms and modes of action. Colicins are organized into three domains, each one involved in a different step of the process of killing sensitive bacteria. The structures of some colicins are known at the atomic level and are discussed. Colicins exert their lethal action by first binding to specific receptors, which are outer membrane proteins used for the entry of specific nutrients. They are then translocated through the outer membrane and transit through the periplasm by either the Tol or the TonB system. The components of each system are known, and their implication in the functioning of the system is described. Colicins then reach their lethal target and act either by forming a voltage-dependent channel into the inner membrane or by using their endonuclease activity on DNA, rRNA, or tRNA. The mechanisms of inhibition by specific and cognate immunity proteins are presented. Finally, the use of colicins as laboratory or biotechnological tools and their mode of evolution are discussed.

Integration of a transposon Tn1-encoded inhibitor-resistant beta-lactamase gene, bla(TEM-67) from Proteus mirabilis, into the Escherichia coli chromosome

Proteus mirabilis NEL-1 was isolated from a urine sample of a patient hospitalized in a long-term care facility. Strain NEL-1 produced a beta-lactamase with a pI of 5.2 conferring resistance to amoxicillin and amoxicillin-clavulanic acid. Sequencing of a PCR amplicon by using TEM-specific primers revealed a novel bla(TEM) gene, bla(TEM-67). TEM-67 was an IRT-1-like TEM derivative related to TEM-65 (Lys39, Cys244) with an additional Leu21Ile amino acid substitution in the leader peptide. The biochemical properties of TEM-67 were equivalent to those described for TEM-65. Analysis of sequences surrounding bla(TEM-67) revealed that it was located on a transposon, Tn1, which itself was located on a 48-kb non-self-transferable plasmid, pANG-1. Electroporation of plasmid pANG-1 into Escherichia coli DH10B resulted in the integration of bla(TEM-67) into the chromosome, whereas it remained episomal in the P. mirabilis CIP103181 reference strain. Further characterization of pANG-1 revealed the presence of two identical sequences on both sides of Tn1 that contained an IS26 insertion sequence followed by a novel colicin gene, colZ, which had 20% amino acid identity with other colicin genes. The characterization of this novel TEM derivative provides further evidence for the large diversity of plasmid-encoded beta-lactamases produced in P. mirabilis and for their spread to other enterobacterial species through transposable-element-mediated events.

Colicin crystal structures: pathways and mechanisms for colicin insertion into membranes

The X-ray structures of the channel-forming colicins Ia and N, and endoribonucleolytic colicin E3, as well as of the channel domains of colicins A and E1, and spectroscopic and calorimetric data for intact colicin E1, are discussed in the context of the mechanisms and pathways by which colicins are imported into cells. The extensive helical coiled-coil in the R domain and internal hydrophobic hairpin in the C domain are important features relevant to colicin import and channel formation. The concept of outer membrane translocation mediated by two receptors, one mainly used for initial binding and second for translocation, such as BtuB and TolC, respectively, is discussed. Helix elongation and conformational flexibility are prerequisites for import of soluble toxin-like proteins into membranes. Helix elongation contradicts suggestions that the colicin import involves a molten globule intermediate. The nature of the open-channel structure is discussed.

Ton-dependent colicins and microcins: modular design and evolution

Ton-dependent colicins and microcins are actively taken up into sensitive cells at the expense of energy which is provided by the proton motive force of the cytoplasmic membrane. The Ton system consisting of the proteins TonB, ExbB and ExbD is required for colicin and microcin import. Colicins as well as the outer membrane transport proteins contain proximal to the N-terminus a short sequence, called TonB box, which interacts with TonB and in which point mutants impair uptake. No TonB box is found in microcins. Colicins are composed of functional modules which during evolution have been interchanged resulting in new colicins. The modules define sites of interaction with the outer membrane transport genes, TonB, the immunity proteins, and the activity regions. Six TonB-dependent microcins with different primary structures are processed and exported by highly homologous proteins. Three of these microcins are modified in an unknown way and they have in common specificity for catecholate siderophore receptors.

High level expression of His-tagged colicin pore-forming domains and reflections on the sites for pore formation in the inner membrane

There exists ample evidence for the assumption that pore-forming colicins cannot exert their toxicity within the producing cell and that they must gain access to the outer face of the cytoplasmic membrane to achieve this. We wished to construct pET-vectors to produce pore-forming domains of colicin A and N with N-terminal hexa-histidine tags under the control of a T7 promoter. This was only possible when the correct immunity protein was also present. Hence it appears that this system exhibits the peculiarity that there is a toxicity associated with the over produced pore-forming domain. However, when the ratio of colicin to immunity protein is compared it is still clear that direct insertion into the cytoplasmic membrane does not occur and that membrane translocation of the colicin at limited sites may be occurring. This article reviews previous literature on the subject in terms of a model for limited sites of colicin action.

The pore-forming domain of colicin A fused to a signal peptide: a tool for studying pore-formation and inhibition

Pore-forming colicins are plasmid-encoded bacteriocins that kill Escherichia coli and closely related bacteria. They bind to receptors in the outer membrane and are translocated across the cell envelope to the inner membrane where they form voltage-dependent ion-channels. Colicins are composed of three domains, with the C-terminal domain responsible for pore-formation. Isolated C-terminal pore-forming domains produced in the cytoplasm of E. coli are inactive due to the polarity of the transmembrane electrochemical potential, which is the opposite of that required. However, the pore-forming domain of colicin A (pfColA) fused to a prokaryotic signal peptide (sp-pfColA) is transported across and inserts into the inner membrane of E. coli from the periplasmic side, forming a functional channel. Sp-pfColA is specifically inhibited by the colicin A immunity protein (Cai). This construct has been used to investigate colicin A channel formation in vivo and to characterise the interaction of pfColA with Cai within the inner membrane. These points will be developed further in this review.