An alpha-helical hydrophobic hairpin as a specific determinant in protein-protein interaction occurring in Escherichia coli colicin A and B immunity systems

Colicin U from Shigella boydii Forms Voltage-Dependent Pores

Colicin U is a protein produced by the bacterium Shigella boydii (serovars 1 and 8). It exerts antibacterial activity against strains of the enterobacterial genera Shigella and Escherichia Here, we report that colicin U forms voltage-dependent pores in planar lipid membranes; its single-pore conductance was found to be about 22 pS in 1 M KCl at pH 6 under 80 mV in asolectin bilayers. In agreement with the high degree of homology between their C-terminal domains, colicin U shares some pore characteristics with the related colicins A and B. Colicin U pores are strongly pH dependent, and as we deduced from the activity of colicin U in planar membranes at different protein concentrations, they have a monomeric pore structure. However, in contrast to related colicins, we observed a very low cationic selectivity of colicin U pores (1.5/1 of K⁺/Cl^- at pH 6) along with their atypical voltage gating. Finally, using nonelectrolytes, we determined the inner diameter of the pores to be in the range of 0.7 to 1 nm, which is similar to colicin Ia, but with a considerably different inner profile.IMPORTANCE Currently, a dramatic increase in antibiotic resistance is driving researchers to find new antimicrobial agents. The large group of toxins called bacteriocins appears to be very promising from this point of view, especially because their narrow killing spectrum allows specific targeting against selected bacterial strains. Colicins are a subgroup of bacteriocins that act on Gram-negative bacteria. To date, some colicins are commercially used for the treatment of animals (1) and tested as a component of engineered species-specific antimicrobial peptides, which are studied for the potential treatment of humans (2). Here, we present a thorough single-molecule study of colicin U which leads to a better understanding of its mode of action. It extends the range of characterized colicins available for possible future medical applications.

Purification and characterization of the colicin A immunity protein in detergent micelles

The immunity proteins against pore-forming colicins represent a family of integral membrane proteins that reside in the inner membrane of producing cells. Cai, the colicin A immunity protein, was characterized here in detergent micelles by circular dichroism (CD), size exclusion chromatography, chemical cross-linking, nuclear magnetic resonance (NMR) spectroscopy, cysteine accessibility, and colicin A binding in detergent micelles. Bile-salt derivatives induced extensive protein polymerization that precluded further investigation. The physical characterization of detergent-solubilized protein indicates that phosphate-containing detergents are more efficient in extracting, solubilizing and maintaining Cai in a monomeric state. Yet, their capacity to ensure protein activity, reconstitution, helix packing, and high-quality NMR spectra was inferior to that of milder detergents. Solvent ionic strength and composition greatly modified the solubilizing capacity of milder detergents. Most importantly, binding to the colicin A pore-forming domain (pf-ColA) occurred almost exclusively in sugar-derived detergents. The relative performance of the different detergents in each experiment depends on their impact not only on Cai structure, solubility and oligomerization state, but also on other reaction components and technical aspects. Thus, proteoliposomes were best obtained from protein in LDAO micelles, possibly also due to indirect effects on the lipidic bilayer. The compatibility of a detergent with Cai/pf-ColA complex formation is influenced by its effect on the conformational landscape of each protein, where detergent-mediated pf-ColA denaturation could also lead to negative results. The NMR spectra were greatly affected by the solubility, monodispersity, fold and dynamics of the protein-detergent complexes, and none of those tested here provided NMR spectra of sufficient quality to allow for peak assignment. Cai function could be proven in alkyl glycosides and not in those detergents that afforded the best solubility, reconstitution efficiency or spectral quality indicating that these criteria cannot be taken as unambiguous proof of nativeness without the support of direct activity measurements.

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.

Microbially-produced peptides having potential application to the prevention of dental caries

Strategies advanced to decrease the occurrence of dental caries have in the past typically focussed upon attempting to reduce plaque accumulation by application of broad-spectrum antibacterial agents. In recent years however there has been growing interest in the application of a more targeted approach to the selective elimination from plaque of those bacterial species that are specifically implicated as the aetiological agents of this disease. This review focuses upon a number of the small bacterially-produced peptide antibiotics known as bacteriocins that are currently being explored for their potential role in the treatment and prevention of dental caries.

Inactivation of colicin Y by intramembrane helix-helix interaction with its immunity protein

The construction of hybrids between colicins U and Y and the mutagenesis of the colicin Y gene (cya) have revealed amino acid residues important for interactions between colicin Y and its cognate immunity protein (Cyi). Four such residues (I578, T582, Y586 and V590) were found in helices 8 and 9 of the colicin Y pore-forming domain. To verify the importance of these residues, the corresponding amino acids in the colicin B protein were mutated to the residues present in colicin Y. An Escherichia coli strain with cloned colicin Y immunity gene (cyi) inactivated this mutant, but not the wild-type colicin B. In addition, interacting amino acid pairs in Cya and Cyi were identified using a set of Cyi point mutant strains. These data are consistent with antiparallel helix-helix interactions between Cyi helix T3 and Cya helix 8 of the pore-forming domain as a molecular mechanism of colicin Y inactivation by its immunity protein.

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.

Recognition of pore-forming colicin Y by its cognate immunity protein

Construction of hybrid immunity genes between colicin U (cui) and Y (cyi) immunity genes and site-directed mutagenesis of cyi were used to identify amino-acid residues of the colicin Y immunity protein (Cyi) involved in recognition of colicin Y. These amino-acid residues were localized close to the cytoplasmic site of the Cyi transmembrane helices T3 (S104, S107, F110, A112) and T4 (A159). Mutations in cui, which converted Cui sequence to Cyi sequence in positions 104, 107, 110, 112 and 159, resulted in an immunity gene that also conferred (besides immunity to colicin U) a high degree of immunity to colicin Y.

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.

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

The colicin A pore-forming domain (pfColA) was fused to a bacterial signal peptide (sp-pfColA). This was inserted into the Escherichia coli inner membrane in functional form and could be coimmunoprecipitated with epitope-tagged immunity protein (EpCai). We constructed a series of fusion proteins in which various numbers of sp-pfColA alpha-helices were fused to alkaline phosphatase (AP). We showed that a fusion protein made up of the hydrophobic alpha-helices 8 and 9 of sp-pfColA fused to AP was specifically coimmunoprecipitated with EpCai produced in the same cells. This is the first biochemical evidence that Cai recognizes and interacts with the colicin A hydrophobic helical hairpin.

Import of colicins across the outer membrane of Escherichia coli involves multiple protein interactions in the periplasm

Several proteins of the Tol/Pal system are required for group A colicin import into Escherichia coli. Colicin A interacts with TolA and TolB via distinct regions of its N-terminal domain. Both interactions are required for colicin translocation. Using in vivo and in vitro approaches, we show in this study that colicin A also interacts with a third component of the Tol/Pal system required for colicin import, TolR. This interaction is specific to colicins dependent on TolR for their translocation, strongly suggesting a direct involvement of the interaction in the colicin translocation step. TolR is anchored to the inner membrane by a single transmembrane segment and protrudes into the periplasm. The interaction involves part of the periplasmic domain of TolR and a small region of the colicin A N-terminal domain. This region and the other regions responsible for the interaction with TolA and TolB have been mapped precisely within the colicin A N-terminal domain and appear to be arranged linearly in the colicin sequence. Multiple contacts with periplasmic-exposed Tol proteins are therefore a general principle required for group A colicin translocation.

Pore-forming colicins and their relatives

The pore-forming colicins, the first proteins that were capable of forming voltage-dependent ion channels to be sequenced, have turned out to be both less tractable and more mysterious than imagined; yet they have proved interesting at every step of their short journey from producing cell to vanquished target cell. Starting out as a remarkably extended water-soluble protein, the colicin molecule is designed to interact simultaneously with several components of the complex membrane of the target cell, transform itself into a membrane protein, and become an ion channel with inscrutable properties. Unraveling how it does all this appears to be leading us into the dark recesses of protein/protein and protein/membrane interaction, where lurk fundamental processes reluctantly waiting to be revealed.

Cleavage of colicin D is necessary for cell killing and requires the inner membrane peptidase LepB

Colicin D is known to kill target cells by cleaving tRNA(Arg). A colicin D-resistant mutant was selected that was altered in the inner membrane leader peptidase, LepB. The substituted residue (Asn274Lys) is located close to the catalytic site. The mutation abolishes colicin D cleavage but not the processing of exported proteins. LepB is required for colicin D cleavage, releasing a small C-terminal fragment that retains full tRNase activity. The immunity protein was found to prevent colicin D processing and furthermore masks tRNase activity, thus protecting colicin D against LepB-mediated cleavage during export. Catalytic colicins share a consensus sequence at their putative processing site. Mutations affecting normal processing of colicin D abolish cytotoxicity without affecting the in vitro tRNase activity.

Identification of specific residues in colicin E1 involved in immunity protein recognition

The basis of specificity between pore-forming colicins and immunity proteins was explored by interchanging residues between colicins E1 (ColE1) and 10 (Col10) and testing for altered recognition by their respective immunity proteins, Imm and Cti. A total of 34 divergent residues in the pore-forming domain of ColE1 between residues 419 and 501, a region previously shown to contain the specificity determinants for Imm, were mutagenized to the corresponding Col10 sequences. The residue changes most effective in converting ColE1 to the Col10 phenotype are residue 448 at the N terminus of helix VI and residues 470, 472, and 474 at the C terminus of helix VII. Mutagenesis of helix VI residues 416 to 419 in Col10 to the corresponding ColE1 sequence resulted in increased recognition by Imm and loss of recognition by Cti.

Identification and properties of the genes encoding microcin E492 and its immunity protein

The gene coding for the immunity protein (mceB) and the structural gene of microcin E492 (mceA), a low-molecular-weight channel-forming bacteriocin produced by a strain of Klebsiella pneumoniae, have been characterized. The microcin gene codes for a precursor protein of either 99 or 103 amino acids. Protein sequencing of the N-terminal region of microcin E492 unequivocally identified this gene as the microcin structural gene and indicated that this microcin is synthesized as a precursor protein that is cleaved at either amino acid 15 or 19, at a site resembling the double-glycine motif. The gene encoding the 95-amino-acid immunity protein (mceB) was identified by cloning the DNA segment that encodes only this polypeptide into an expression vector and demonstrating the acquisition of immunity to microcin E492. As expected, the immunity protein was found to be associated with the inner membrane. Analysis of the DNA sequence indicates that these genes belong to the same family as microcin 24, and they do not share structural motifs with any other known channel-forming bacteriocin. The organization of the microcin- and immunity protein-encoding genes suggests that they are coordinately expressed.

The tip of the hydrophobic hairpin of colicin U is dispensable for colicin U activity but is important for interaction with the immunity protein

The hydrophobic C terminus of pore-forming colicins associates with and inserts into the cytoplasmic membrane and is the target of the respective immunity protein. The hydrophobic region of colicin U of Shigella boydii was mutated to identify determinants responsible for recognition of colicin U by the colicin U immunity protein. Deletion of the tip of the hydrophobic hairpin of colicin U resulted in a fully active colicin that was no longer inactivated by the colicin U immunity protein. Replacement of eight amino acids at the tip of the colicin U hairpin by the corresponding amino acids of the related colicin B resulted in colicin U(575-582ColB), which was inactivated by the colicin U immunity protein to 10% of the level of inactivation of the wild-type colicin U. The colicin B immunity protein inactivated colicin U(575-582ColB) to the same degree. These results indicate that the tip of the hydrophobic hairpin of colicin U and of colicin B mainly determines the interaction with the corresponding immunity proteins and is not required for colicin activity. Comparison of these results with published data suggests that interhelical loops and not membrane helices of pore-forming colicins mainly interact with the cognate immunity proteins and that the loops are located in different regions of the A-type and E1-type colicins. The colicin U immunity protein forms four transmembrane segments in the cytoplasmic membrane, and the N and C termini face the cytoplasm.

Immunity proteins and their specificity for endonuclease colicins: telling right from wrong in protein-protein recognition

Immunity proteins inhibit colicins, protein toxins released by bacteria during times of environmental stress, by binding and inactivating their cytotoxic domains. This protects the producing organism as it attempts to kill off competing bacteria. The cytotoxic domains of related colicins share a high degree of sequence identity, as do their corresponding immunity proteins, yet specificity and affinity are also high, with little non-cognate biological cross-protection evident under physiological conditions. We review recent work on DNase-specific immunity proteins, which shows that, although both cognate and non-cognate proteins can bind a single toxin, their affinities can differ by as much as 12 orders of magnitude. We have termed this mode of binding dual recognition, because the DNase-binding surface of an immunity protein is made up of two components, one conserved and the other variable. The strength of the binding interaction is dominated by the conserved residues, while neighbouring variable residues control specificity. Similar dual recognition systems may exist in other biological contexts, particularly where a protein must discriminate the right binding partner from numerous, structurally homologous alternatives.

Intracellular immunization of prokaryotic cells against a bacteriotoxin

Intracellularly expressed antibodies have been designed to bind and inactivate target molecules inside eukaryotic cells. Here we report that an antibody fragment can be used to probe the periplasmic localization of the colicin A N-terminal domain. Colicins form voltage-gated ion channels in the inner membrane of Escherichia coli. To reach their target, they bind to a receptor located on the outer membrane and then are translocated through the envelope. The N-terminal domain of colicins is involved in the translocation step and therefore is thought to interact with proteins of the translocation system. To compete with this system, a single-chain variable fragment (scFv) directed against the N-terminal domain of the colicin A was synthesized and exported into the periplasmic space of E. coli. The periplasmic scFv inhibited the lethal activity of colicin A and had no effect on the lethal activity of other colicins. Moreover, the scFv was able to specifically inactivate hybrid colicins possessing the colicin A N-terminal domain without affecting their receptor binding. Hence, the periplasmic scFv prevents the translocation of colicin A and probably its interaction with import machinery. This indicates that the N-terminal domain of the toxin is accessible in the periplasm. Moreover, we show that production of antibody fragments to interfere with a biological function can be applied to prokaryotic systems.

Distinct regions of the colicin A translocation domain are involved in the interaction with TolA and TolB proteins upon import into Escherichia coli

Group A colicins need proteins of the Escherichia coli envelope Tol complex (TolA, TolB, TolQ and TolR) to reach their cellular target. The N-terminal domain of colicins is involved in the import process. The N-terminal domains of colicins A and E1 have been shown to interact with TolA, and the N-terminal domain of colicin E3 has been shown to interact with TolB. We found that a pentapeptide conserved in the N-terminal domain of all group A colicins, the 'TolA box', was important for colicin A import but was not involved in the colicin A-TolA interaction. It was, however, involved in the colicin A-TolB interaction. The interactions of colicin A N-terminal domain deletion mutants with TolA and TolB were investigated. Random mutagenesis was performed on a construct allowing the colicin A N-terminal domain to be exported in the bacteria periplasm. This enabled us to select mutant protein domains unable to compete with the wild-type domain of the entire colicin A for import into the cells. Our results demonstrate that different regions of the colicin A N-terminal domain interact with TolA and TolB. The colicin A N-terminal domain was also shown to form a trimeric complex with TolA and TolB.

Colicins--exocellular lethal proteins of Escherichia coli

Colicins are toxic exoproteins produced by bacteria of colicinogenic strains of Escherichia coli and some related species of Enterobacteriaceae, during the growth of their cultures. They inhibit sensitive bacteria of the same family. About 35% E. coli strains appearing in human intestinal tract are colicinogenic. Synthesis of colicins is coded by genes located on Col plasmids. Until now more than 34 types of colicins have been described, 21 of them in greater detail, viz. colicins A, B, D, E1-E9, Ia, Ib, JS, K, M, N, U, 5, 10. In general, their interaction with sensitive bacteria includes three steps: (1) binding of the colicin molecule to a specific receptor in the bacterial outer membrane; (2) its translocation through the cell envelope; and (3) its lethal interaction with the specific molecular target in the cell. The classification of colicins is based on differences in the molecular events of these three steps.

Analysis of the gene cluster involved in production and immunity of the peptide antibiotic AS-48 in Enterococcus faecalis

A region of 7.8 kb of the plasmid pMB2 from Enterococcus faecalis S-48 carrying the information necessary for production and immunity of the peptide antibiotic AS-48 has been cloned and sequenced. It contains the as-48A structural gene plus five open reading frames (as-48B, as-48C, as-48C1, as-48D and as-48D1). Besides As-48D, all the predicted gene products are basic hydrophobic proteins with potential membrane-spanning domains (MSDs). None of them shows any homology with protein sequences stored in databanks, except for As-48D, which shows similarity to the C-terminal domain of ABC transporters and contains a highly conserved ATP-binding site. The gene products of as-48B, as-48C, as-48C1 and as-48D are thought to be involved in AS-48 production and secretion. The only gene able to provide resistance to AS-48 by itself is as-48D1. Immunity also seems to be enhanced at least by the products of as-48B, as-48C1 and as-48D genes. Transcription analysis using probes derived from the different ORFs revealed two large (3.5 and 2.7kb) mRNAs, suggesting that the different genes are organized in two constitutive operons.

Colicin U, a novel colicin produced by Shigella boydii

A novel colicin, designated colicin U, was found in two Shigella boydii strains of serovars 1 and 8. Colicin U was active against bacterial strains of the genera Escherichia and Shigella. Plasmid pColU (7.3 kb) of the colicinogenic strain S. boydii M592 (serovar 8) was sequenced, and three colicin genes were identified. The colicin U activity gene, cua, encodes a protein of 619 amino acids (Mr, 66,289); the immunity gene, cui, encodes a protein of 174 amino acids (Mr, 20,688); and the lytic protein gene, cul, encodes a polypeptide of 45 amino acids (Mr, 4,672). Colicin U displays sequence similarities to various colicins. The N-terminal sequence of 130 amino acids has 54% identity to the N-terminal sequence of bacteriocin 28b produced by Serratia marcescens. Furthermore, the N-terminal 36 amino acids have striking sequence identity (83%) to colicin A. Although the C-terminal pore-forming sequence of colicin U shows the highest degree of identity (73%) to the pore-forming C-terminal sequence of colicin B, the immunity protein, which interacts with the same region, displays a higher degree of sequence similarity to the immunity protein of colicin A (45%) than to the immunity protein of colicin B (30.5%). Immunity specificity is probably conferred by a short sequence from residues 571 to residue 599 of colicin U; this sequence is not similar to that of colicin B. We showed that binding of colicin U to sensitive cells is mediated by the OmpA protein, the OmpF porin, and core lipopolysaccharide. Uptake of colicin U was dependent on the TolA, -B, -Q, and -R proteins. pColU is homologous to plasmid pSB41 (4.1 kb) except for the colicin genes on pColU. pSB41 and pColU coexist in S. boydii strains and can be cotransformed into Escherichia coli, and both plasmids are homologous to pColE1.

Membrane insertion: The strategies of toxins (review)

Protein toxins are soluble molecules secreted by pathogenic bacteria which act at the plasma membrane or in the cytoplasm of target cells. They must therefore interact with a membrane at some point, either to modify its permeability properties or to reach the cytoplasm. As a consequence, toxins have the built-in capacity to adopt two generally incompatible states: water-soluble and transmembrane. Irrespective of their origin or function, the membrane interacting domain of most protein toxins seems to have adopted one out of two structural strategies to be able to undergo this metamorphosis. In the first group of toxins the membrane interacting domain has the structural characteristics of most known membrane proteins, i.e. it contains hydrophobic and amphipathic alpha-helices long enough to span a membrane. To render this 'membrane protein' water-soluble during the initial part of its life the hydrophobic helices are sheltered from the solvent by a barrel of amphipathic helices. In the second group of toxins the opposite strategy is adopted. The toxin is an intrinsically soluble protein and is composed mainly of beta-structure. These toxins manage to become membrane proteins by oligomerizing in order to combine amphipathic beta-sheet to generate sufficient hydrophobicity for membrane insertion to occur. Toxins from this latter group are thought to perforate the lipid bilayer as a beta-barrel such as has been described for bacterial porins, and has recently been shown for staphylococcal alpha-toxin. The two groups of toxins will be described in detail through the presentation of examples. Particular attention will be given to the beta-structure toxins, since four new structures have been solved over the past year: the staphyloccocal alpha-toxin channel, the anthrax protective antigen protoxin, the anthrax protective antigen-soluble heptamer and the CytB protoxin. Structural similarities with mammalian proteins implicated in the immune response and apoptosis will be discussed. Peptide toxins will not be covered in this review.

Bacteriocins: nature, function and structure

Bacteriocins are extracellular substances produced by different types of bacteria, including both Gram positive and Gram negative species. They can be produced spontaneously or induced by certain chemicals such as mitomycin C. They are biologically one of the important substances, and have been found to be useful in membrane studies and also in typing pathogenic microorganisms causing serious nosocomial infections. Bacteriocins are a heterogeneous group of particles with different morphological and biochemical entities. They range from a simple protein to a high molecular weight complex: the active moiety of each molecule in all cases seems to be protein in nature. The genetic determinants of most of the bacteriocins are located on the plasmids, apart from few which are chromosomally encoded. These bactericidal particles are species specific. They exert their lethal activity through adsorption to specific receptors located on the external surface of sensitive bacteria, followed by metabolic, biological and morphological changes resulting in the killing of such bacteria. This review summarises the classification, biochemical nature, morphology and mode of action of bacteriocins as well as their genetic determinants and the microbiological relevance of these bactericidal agents.

The channel domain of colicin A is inhibited by its immunity protein through direct interaction in the Escherichia coli inner membrane

A bacterial signal sequence was fused to the colicin A pore-forming domain: the exported pore-forming domain was highly cytotoxic. We thus introduced a cysteine-residue pair in the fusion protein which has been shown to form a disulfide bond in the natural colicin A pore-forming domain between alpha-helices 5 and 6. Formation of the disulfide bond prevented the cytotoxic activity of the fusion protein, presumably by preventing the membrane insertion of helices 5 and 6. However, the cytotoxicity of the disulfide-linked pore-forming domain was reactivated by adding dithiothreitol into the culture medium. We were then able to co-produce the immunity protein with the disulfide linked pore-forming domain, by using a co-immunoprecipitation procedure, in order to show that they interact. We showed both proteins to be co-localized in the Escherichia coli inner membrane and subsequently co-immunoprecipitated them. The interaction required a functional immunity protein. The immunity protein also interacted with a mutant form of the pore-forming domain carrying a mutation located in the voltage-gated region: this mutant was devoid of pore-forming activity but still inserted into the membrane. Our results indicate that the immunity protein interacts with the membrane-anchored channel domain; the interaction requires a functional membrane-inserted immunity protein but does not require the channel to be in the open state.

Quantification of group A colicin import sites

Pore-forming colicins are soluble bacteriocins which form voltage-gated ion channels in the inner membrane of Escherichia coli. To reach their target, these colicins first bind to a receptor located on the outer membrane and then are translocated through the envelope. Colicins are subdivided into two groups according to the envelope proteins involved in their translocation: group A colicins use the Tol proteins; group B colicins use the proteins TonB, ExbB, and ExbD. We have previously shown that a double-cysteine colicin A mutant which possesses a disulfide bond in its pore-forming domain is translocated through the envelope but is unable to form a channel in the inner membrane (D. Duché, D. Baty, M. Chartier, and L. Letellier, J. Biol. Chem. 269:24820-24825, 1994). Measurements of colicin-induced K+ efflux reveal that preincubation of the cells with the double-cysteine mutant prevents binding of colicins of group A but not of group B. Moreover, we show that the mutant is still in contact with its receptor and import machinery when it interacts with the inner membrane. From these competition experiments, we conclude that each Escherichia coli cell contains approximately 400 and 1,000 colicin A receptors and translocation sites, respectively.

Colicin import and pore formation: a system for studying protein transport across membranes?

Pore-forming colicins are a family of protein toxins (M(r) 40-70 kDa) produced by Escherichia coli and related bacteria. They are bactericidal by virtue of their ability to form ion channels in the inner membrane of target cells. They provide a useful means of studying questions such as toxin action, polypeptide translocation across and into membranes, voltage-gated channels and receptor function. These colicins bind to a receptor in the outer membrane before being translocated across the cell envelope with the aid of helper proteins that belong to nutrient-uptake systems and the so-called 'Tol' proteins, the function of which has not yet been properly defined. A distinct domain appears to be associated with each of three steps (receptor binding, translocation and formation of voltage-gated channels). The Tol-dependent uptake pathway is described here. The structures and interactions of TolA, B, Q and R have by now been quite clearly defined. Transmembrane alpha-helix interactions are required for the functional assembly of the E. coli Tol complex, which is preferentially located at contact sites between the inner and outer membranes. The number of colicin translocation sites is about 1000 per cell. The role and the involvement of the OmpF porin (with colicins A and N) have been described in a recent study on the structural and functional interactions of a colicin-resistant mutant of OmpF. The X-ray crystal structure of the channel-forming fragment of colicin A and that of the entire colicin la have provided the basis for biophysical and site-directed mutagenesis studies.(ABSTRACT TRUNCATED AT 250 WORDS)

Novel colicin 10: assignment of four domains to TonB- and TolC-dependent uptake via the Tsx receptor and to pore formation

Uptake of a new colicin, colicin 10 (Col10), into cells of Escherichia coli required TonB, ExbBD (Ton system), but its cognate receptor, Tsx, functioned independently of Ton and TolQRAB (Tol system). Uptake of Col10 also required TolC which is unique for a Ton-coupled translocation through the outer membrane. A 2470 bp DNA fragment from the natural plasmid pCol10 encoding the Col10 activity (cta), immunity (cti) and lysis (ctl) genes was sequenced. The Cta, Cti and Ctl proteins, as deduced from the nucleotide sequences, consisted of 490 (M(r) 53,342), 96 (M(r) 11,586) and 43 (M(r) 4484) amino acid residues, respectively. Col10 (Cta) was highly homologous to colicin E1 in two regions which determined the common TolC requirement for uptake and the pore-forming activity. Col10 and E1 differed entirely in the regions which are predicted to determine the Ton dependence of Col10 and the Tol dependence of E1, and binding to the receptors Tsx and BtuB, respectively. The region responsible for the Ton-dependent uptake of Col10 was localized in the sequence ranging from residues 1 to 43 (Ton region), and the region responsible for the Tol-dependent uptake of colicin E1 extended from residues 1 to 34 (Tol region). Each Tol-dependent colicin contained a pentapeptide homologous to the sequence DGSGS in the Tol region of E1 which is proposed to be implicated in Tol-dependent uptake (TolA box). After the exchange of the Ton and the Tol regions between Col10 and E1, the Col10-E1 fusion protein was carried into cells via the Ton system and BtuB, whereas the E1-Col10 fusion protein was imported via the Tol system and Tsx. Although the immunity proteins of Col10 and E1 displayed a low homology, Cti conferred full immunity to E1, in contrast to the immunity protein of E1 which did not protect cells against Col10. It is proposed that Col10 belongs to the colicin E1, Ia, Ib group as opposed to the colicin A, B, N group of pore-forming colicins. Col10 consists of 4 domains of which two are very similar and two are very different to E1, supporting our previous proposal that colicins evolved by recombination of DNA fragments which encode uptake and activity domains.

Mode of action of LciA, the lactococcin A immunity protein

Monoclonal antibodies were raised against a fusion between the Escherichia coli maltose-binding protein and LciA, the immunity protein that protects Lactococcus lactis against the effects of the bacteriocin lactococcin A. One of the antibodies directed against the LciA moiety of the fusion protein was used to locate the immunity protein in the L. lactis producer cell. LciA was present in the cytosolic, the membrane-associated, and the membrane fractions in roughly equal amounts, irrespective of the production by the cells of lactococcin A. The monoclonal antibody specifically reacted with right-side-out vesicles obtained from a strain producing the immunity protein. It did not react with inside-out vesicles of the same strain, or with right-side-out vesicles obtained from a strain producing both LciA and lactococcin A. Also, externally added lactococcin A blocked the interaction between the antibody and right-side-out vesicles obtained from a strain producing only LciA. The epitope in LciA was localized between amino acid residues 60 and 80. As the epitope could be removed from right-side-out vesicles by proteinase K, it is located at the outside of the cell. The immunity protein contains a putative alpha-amphiphilic helix from residue 29 to 47. A model is proposed in which this helix is thought to traverse the membrane in such a way that the C-terminal part of the protein, containing the epitope, is on the outside of the cell. Vesicle-fusion studies together with leucine-uptake experiments suggest that the immunity protein interacts with the putative receptor for lactococcin A, thus preventing pore formation by the bacteriocin.

The colicin A pore-forming domain fused to mitochondrial intermembrane space sorting signals can be functionally inserted into the Escherichia coli plasma membrane by a mechanism that bypasses the Tol proteins

Colicin A is a pore-forming bacteriocin that depends upon the Tol proteins in order to be transported from its receptor at the outer membrane surface to its target, the inner membrane. The presequence of yeast mitochondria cytochrome c1 (pc1) as well as the first 167 amino acids of cytochrome b2 (pb2) were fused to the pore-forming domain of colicin A (pfColA). Both hybrid proteins (pc1-pfCoIA and pb2-pfColA) were cytotoxic for Escherichia coli strains devoid of colicin A immunity protein whereas the pore-forming domain without presequence had no lethal effect. The entire precursors and their processed forms were found entirely associated with the bacterial inner membrane and their cytotoxicities were related to their pore-forming activities. The proteins were also shown to kill the tol bacterial strains, which are unable to transport colicins. In addition, we showed that both the cytochrome c1 presequence fused to the dihydrofolate reductase (pc1-DHFR) and the cytochrome c1 presequence moiety of pc1-pfCoIA were translocated across inverted membrane vesicles. Our results indicated that: (i) pc1-pfCoIA produced in the cell cytoplasm was able to assemble in the inner membrane by a mechanism independent of the tol genes; (ii) the inserted pore-forming domain had a channel activity; and (iii) this channel activity was inhibited within the membrane by the immunity protein.

Immunity proteins to pore-forming colicins: structure-function relationships

Colicin A and B immunity proteins (Cai and Cbi, respectively) are homologous integral membrane proteins that interact within the core of the lipid bilayer with hydrophobic transmembrane helices of the corresponding colicin channel. By using various approaches (exchange of hydrophilic loops between Cai and Cbi, construction of Cbi/Cai hybrids, production of Cai as two fragments), we studied the structure-function relationships of Cai and Cbi. The results revealed unexpectedly high structural constraints for the function of these proteins. The periplasmic loops of Cai and Cbi did not carry the determinants for colicin recognition although most of these loops were required for Cai function; the cytoplasmic loop of Cai was found to be involved in topology and function of Cai. The immunity function did not seem to be confined to a particular region of the immunity proteins.

Colicins: structures, modes of action, transfer through membranes, and evolution

This article intends to inform a broader audience on a fascinating class of protein toxins (bacteriocins) which usually kill only cells of the same species. Those who gained a deeper interest in bacteriocins can find a comprehensive description of the field in a recent book based on a conference (James et al. 1992), and in more specialized review articles dealing with certain aspects (Pugsley 1984a, b), or certain colicins (De Graaf and Oudega 1986; Harkness and Olschläger 1991; Lazdunski et al. 1988). The older literature has been reviewed by Brandis and Smarda (1971), Reeves (1972), Hardy (1975) and Konisky (1982).

All in the family: the toxic activity of pore-forming colicins

Colicins are unusual bacterial toxins because they are directed against close relatives of the producing strain. They kill their targets in one of three distinct ways; via a ribonuclease or deoxyribonuclease activity or by forming pores in the target cell's membrane. This review deals with the steps involved in pore-forming colicin activity including, initial synthesis of the toxin, toxin release, receptor binding, translocation across the periplasm and pore formation in the cytoplasmic membrane. Special reference is made to the role of colicin in vivo, the structural changes occurring during pore formation and the role of the immunity protein.