Structural stability and domain organization of colicin E1

X-ray crystallography shines a light on pore-forming toxins

A common form of cellular attack by pathogenic bacteria is to secrete pore-forming toxins (PFTs). Capable of forming transmembrane pores in various biological membranes, PFTs have also been identified in a diverse range of other organisms such as sea anemones, earthworms and even mushrooms and trees. The mechanism of pore formation by PFTs is associated with substantial conformational changes in going from the water-soluble to transmembrane states of the protein. The determination of the crystal structures for numerous PFTs has shed much light on our understanding of these proteins. Other than elucidating the atomic structural details of PFTs and the conformational changes that must occur for pore formation, crystal structures have revealed structural homology that has led to the discovery of new PFTs and new PFT families. Here we review some key crystallographic results together with complimentary approaches for studying PFTs. We discuss how these studies have impacted our understanding of PFT function and guided research into biotechnical applications.

Similarities and Differences between Colicin and Filamentous Phage Uptake by Bacterial Cells

Gram-negative bacteria have evolved a complex envelope to adapt and survive in a broad range of ecological niches. This physical barrier is the first line of defense against noxious compounds and viral particles called bacteriophages. Colicins are a family of bactericidal proteins produced by and toxic to Escherichia coli and closely related bacteria. Filamentous phages have a complex structure, composed of at least five capsid proteins assembled in a long thread-shaped particle, that protects the viral DNA. Despite their difference in size and complexity, group A colicins and filamentous phages both parasitize multiprotein complexes of their sensitive host for entry. They first bind to a receptor located at the surface of the target bacteria before specifically recruiting components of the Tol system to cross the outer membrane and find their way through the periplasm. The Tol system is thought to use the proton motive force of the inner membrane to maintain outer membrane integrity during the life cycle of the cell. This review describes the sequential docking mechanisms of group A colicins and filamentous phages during their uptake by their bacterial host, with a specific focus on the translocation step, promoted by interactions with the Tol system.

On mechanisms of colicin import: the outer membrane quandary

Current problems in the understanding of colicin import across the Escherichia coli outer membrane (OM), involving a range of cytotoxic mechanisms, are discussed: (I) Crystal structure analysis of colicin E3 (RNAase) with bound OM vitamin B₁₂ receptor, BtuB, and of the N-terminal translocation (T) domain of E3 and E9 (DNAase) inserted into the OM OmpF porin, provide details of the initial interaction of the colicin central receptor (R)- and N-terminal T-domain with OM receptors/translocators. (II) Features of the translocon include: (a) high-affinity (K _d ≈ 10^-9 M) binding of the E3 receptor-binding R-domain E3 to BtuB; (b) insertion of disordered colicin N-terminal domain into the OmpF trimer; (c) binding of the N-terminus, documented for colicin E9, to the TolB protein on the periplasmic side of OmpF. Reinsertion of the colicin N-terminus into the second of the three pores in OmpF implies a colicin anchor site on the periplasmic side of OmpF. (III) Studies on the insertion of nuclease colicins into the cytoplasmic compartment imply that translocation proceeds via the C-terminal catalytic domain, proposed here to insert through the unoccupied third pore of the OmpF trimer, consistent with in vitro occlusion of OmpF channels by the isolated E3 C-terminal domain. (IV) Discussion of channel-forming colicins focuses mainly on colicin E1 for which BtuB is receptor and the OM TolC protein the proposed translocator. The ability of TolC, part of a multidrug efflux pump, for which there is no precedent for an import function, to provide a trans-periplasmic import pathway for colicin E1, is questioned on the basis of an unfavorable hairpin conformation of colicin N-terminal peptides inserted into TolC.

Membrane topology of the colicin E1 channel using genetically encoded fluorescence

The membrane topology of the colicin E1 channel domain was studied by fluorescence resonance energy transfer (FRET). The FRET involved a genetically encoded fluorescent amino acid (coumarin) as the donor and a selectively labeled cysteine residue tethered with DABMI (4-(dimethylamino)phenylazophenyl-4'-maleimide) as the FRET acceptor. The fluorescent coumarin residue was incorporated into the protein via an orthogonal tRNA/aminoacyl-tRNA synthetase pair that allowed selective incorporation into any site within the colicin channel domain. Each variant harbored a stop (TAG) mutation for coumarin incorporation and a cysteine (TGT) mutation for DABMI attachment. Six interhelical distances within helices 1-6 were determined using FRET analysis for both the soluble and membrane-bound states. The FRET data showed large changes in the interhelical distances among helices 3-6 upon membrane association providing new insight into the membrane-bound structure of the channel domain. In general, the coumarin-DABMI FRET interhelical efficiencies decreased upon membrane binding, building upon the umbrella model for the colicin channel. A tentative model for the closed state of the channel domain was developed based on current and previously published FRET data. The model suggests circular arrangement of helices 1-7 in a clockwise direction from the extracellular side and membrane interfacial association of helices 1, 6, 7, and 10 around the central transmembrane hairpin formed by helices 8 and 9.

Inhibitory activity of colicin E1 against Listeria monocytogenes

Colicins are gram-negative bacteriocins produced by and effective against Escherichia coli and related species. Colicin E1 (ColE1) is composed of three functional domains, which collectively have a pore-forming effect on targeted bacteria. ColE1 binding and translocation domains are highly specific in contrast to the pore-forming domain, implying that ColE1 could be broadly effective. In this study, the activity of ColE1 against Listeria monocytogenes was evaluated in broth and on surfaces of ready-to-eat products. Individual strains of L. monocytogenes were examined in broth containing ColE1 at 0, 0.1, 1, or 10 microg/ml. Although strain differences in sensitivity to ColE1 existed, growth was significantly reduced in all strains at doses as low as 0.1 microg/ml. Sterilized ham slices were submerged in a five-strain L. monocytogenes cocktail (either 7 or 4 log CFU/ ml) and placed in vacuum packages containing 0, 1, 5, 10, 25, or 50 microg of ColE1. Ham slices were then stored at 4 or 10 degrees C, and samples were removed and examined for L. monocytogenes after 1, 3, 7, and 14 days. Reduction of L. monocytogenes by ColE1 was dependent on initial inoculum concentration and storage temperature. For slices stored at 4 degrees C, treatment with 25 microg reduced Listeria growth below detection limits for the slices inoculated with 4 log CFU/ml for the entire 14 days, whereas for the 7-log CFU/ml slices, growth was detected at 7 days postinoculation. For slices stored at 10 degrees C, 10 microg/ml ColE1 significantly inhibited growth of L. monocytogenes for up to 3 days for both inoculation groups. These data indicate that ColE1 is highly effective against Listeria.

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.

Studies on the region involved in the transport activity of Escherichia coli TolC by chimeric protein analysis

Gram-negative bacteria possess the outer membrane protein TolC which acts as an exit duct across the outer membrane. However, the region involved in the transport activity of TolC has remained unclear. We analyzed this region by creating chimeric TolCs. First, we expressed the genes for TolCs of Vibrio parahaemolyticus (vp-tolC) and Salmonella typhimurium (sal-tolC) in Escherichia coli. The levels of sequence identity in the mature region of VP-TolC/EC-TolC and Sal-TolC/EC-TolC with maximum matching are 43% and 90%, respectively. We found that the transport activity of VP-TolC was weak compared with that of TolC of E. coli (EC-TolC) although the transport activity of Sal-TolC was similar to that of EC-TolC. A comparison of the sequence of the three tolCs showed that the sequence around the periplasmic region covering Asn-188 to Lys-214 of EC-TolC is lowly identical to that of VP-TolC although the region of EC-TolC is almost identical to that of Sal-TolC. We think, therefore, that the region covering Asn-188 to Lys-214 of EC-TolC may have an important role to express its transport activity in E. coli. To examine the possibility, we divided the region of EC-TolC into three and exchanged the gene for each portion with that of vp-tolC. These mutant ec-tolCs were expressed in E. coli and the activity of each chimeric TolC was measured. The results showed that the portion covering Val-198 to Lys-214 of EC-TolC is deeply involved in the transport activity.

Initial steps of colicin E1 import across the outer membrane of Escherichia coli

Data suggest a two-receptor model for colicin E1 (ColE1) translocation across the outer membrane of Escherichia coli. ColE1 initially binds to the vitamin B(12) receptor BtuB and then translocates through the TolC channel-tunnel, presumably in a mostly unfolded state. Here, we studied the early events in the import of ColE1. Using in vivo approaches, we show that ColE1 is cleaved when added to whole cells. This cleavage requires the presence of the receptor BtuB and the protease OmpT, but not that of TolC. Strains expressing OmpT cleaved ColE1 at K84 and K95 in the N-terminal translocation domain, leading to the removal of the TolQA box, which is essential for ColE1's cytotoxicity. Supported by additional in vivo data, this suggests that a function of OmpT is to degrade colicin at the cell surface and thus protect sensitive E. coli cells from infection by E colicins. A genetic strategy for isolating tolC mutations that confer resistance to ColE1, without affecting other TolC functions, is also described. We provide further in vivo evidence of the multistep interaction between TolC and ColE1 by using cross-linking followed by copurification via histidine-tagged TolC. First, secondary binding of ColE1 to TolC is dependent on primary binding to BtuB. Second, alterations to a residue in the TolC channel interfere with the translocation of ColE1 across the TolC pore rather than with the binding of ColE1 to TolC. In contrast, a substitution at a residue exposed on the cell surface abolishes both binding and translocation of ColE1.

Electrostatic interactions of colicin E1 with the surface of Escherichia coli total lipid

The surface properties of colicin E1, a 522-amino acid protein, and its interaction with monolayers of Escherichia coli (E. coli) total lipid and 1,2-Dimyristoyl-sn-Glycero-3-Phosphocholine (DOPC) were studied using the Langmuir-Blodgett (LB) technique. Colicin E1 is amphiphilic, forming a protein monolayer at the air/buffer interface. The protein is thought to interact with the E. coli total lipid head groups through electrostatic interactions, followed by its insertion into the lipid monolayers. Supported lipid bilayers (SLBs) of E. coli total lipid and DOPC, deposited onto mica at the cell membrane equivalence pressure for E. coli and incubated with colicin E1, were imaged by contact mode atomic force microscopy (CM-AFM). Colicin E1 formed protein aggregates on DOPC SLBs, while E. coli total lipid SLB was deformed following its incubation with colicin E1. Corresponding lateral force images, along with electrostatic surface potentials for colicin E1 P190, imply a direct interaction of colicin E1 with lipid head groups facilitating their charge neutralization.

Energetics of membrane protein folding and stability

The critical role of membrane proteins in a myriad of biological and physiological functions has spawned numerous investigations over the past several decades with the long-term goal of identifying the molecular origins and energetic forces that stabilize these proteins within the membrane. Parallel structural and thermodynamics studies on several systems have provided significant insight regarding the driving forces governing folding, assembly, insertion, and translocation of membrane proteins. The present review surveys families of membrane-associated proteins including alpha-helical and beta-barrel structures, viral surface receptors, and pore-forming toxins, citing representative proteins within each of these classes for further scrutiny in terms of structure-function relationships and global conformational stability. This overview presents seminal findings from pioneering studies on the energetics of membrane protein folding and stability to modern techniques that are exploiting the use of molecular genetics and single molecule studies. An overall consensus regarding the molecular origins of membrane protein stability is that a number of intrinsic properties resemble features of soluble proteins, yet there are distinct energetic differences arising from specific intra- and intermolecular interactions within the membrane. The combined efforts from structural, energetics, and dynamics approaches offer unique insights and improve our fundamental understanding of the driving forces dictating membrane protein folding and stability.

Cell entry mechanism of enzymatic bacterial colicins: porin recruitment and the thermodynamics of receptor binding

Binding of enzymatic E colicins to the vitamin B12 receptor, BtuB, is the first stage in a cascade of events that culminate in the translocation of the cytotoxic nuclease into the Escherichia coli cytoplasm and release of its tightly bound immunity protein. A dogma of colicin biology is that the toxin coiled-coil connecting its functional domains must unfold or unfurl to span the periplasm, with recent reports claiming this reaction is initiated by receptor binding. We report isothermal titration calorimetry data of BtuB binding the endonuclease toxin ColE9 and a disulfide form (ColE9S-S) where unfolding of the coiled-coil is prevented and, as a consequence, the toxin is biologically inactive. Contrary to expectation, the thermodynamics of receptor binding, characterized by large negative values for TDeltaS, are identical for the two colicins, arguing against any form of BtuB-induced unfolding. We go on to delineate key features of the "colicin translocon" that assembles at the cell surface after BtuB binding by using a complex of histidine-tagged Im9 bound to ColE9S-S. First, we show that the porin OmpF is recruited directly to the BtuB.colicin complex to form the translocon. Second, recruitment is through the natively unfolded region of the colicin translocation domain, with this domain likely having two contact points for OmpF. Finally, the immunity protein is not released during its assembly. Our study demonstrates that although colicin unfolding is undoubtedly a prerequisite for E. coli cell death, it must occur after assembly of the translocon.

Amino-acid residues involved in the expression of the activity of Escherichia coli TolC

The Escherichia coli TolC, composed of 471 amino-acid residues, functions as a channel tunnel in the transport of various molecules across the outer membrane. We found previously that Leu-412, the 60th amino-acid residue from the carboxy terminal end, was crucial to the transport activity of TolC. Leu-412 is located in a domain which protrudes from the main body of TolC into the periplasm. Subsequent study indicated that the hydrophobicity generated by Leu-412 played an important role in the activity of TolC (H. Yamanaka, T. Nomura, N. Morisada, S. Shinoda, and K. Okamoto, Microb. Pathog. 33: 81-89, 2002). We predicted that other hydrophobic amino-acid residues around Leu-412 were also involved in the expression of the activity of TolC. To test this possibility, we substituted several hydrophobic residues around Leu-412, (Leu-3, Val-6, Leu-212, Leu-213, Leu-223, and Leu-224), with serine and examined the activity of these mutant TolCs. The result showed that Leu-3 is involved in the activity of TolC, but the other residues are not. The involvement of Leu-3 was confirmed by the residue deletion experiment. A subsequent point-mutational analysis of the residue showed that a hydrophobic side chain is required at position 3 for TolC to express its activity. As the distance between the alpha-carbons of Leu-3 and Leu-412 is just 7.45 angstroms, hydrophobic interaction between the two leucine residues might be involved in the activity of TolC.

Colicin occlusion of OmpF and TolC channels: outer membrane translocons for colicin import

The interaction of colicins with target cells is a paradigm for protein import. To enter cells, bactericidal colicins parasitize Escherichia coli outer membrane receptors whose physiological purpose is the import of essential metabolites. Colicins E1 and E3 initially bind to the BtuB receptor, whose beta-barrel pore is occluded by an N-terminal globular "plug". The x-ray structure of a complex of BtuB with the coiled-coil BtuB-binding domain of colicin E3 did not reveal displacement of the BtuB plug that would allow passage of the colicin (Kurisu, G., S. D. Zakharov, M. V. Zhalnina, S. Bano, V. Y. Eroukova, T. I. Rokitskaya, Y. N. Antonenko, M. C. Wiener, and W. A. Cramer. 2003. Nat. Struct. Biol. 10:948-954). This correlates with the inability of BtuB to form ion channels in planar bilayers, shown in this work, suggesting that an additional outer membrane protein(s) is required for colicin import across the outer membrane. The identity and interaction properties of this OMP were analyzed in planar bilayer experiments.OmpF and TolC channels in planar bilayers were occluded by colicins E3 and E1, respectively, from the trans-side of the membrane. Occlusion was dependent upon a cis-negative transmembrane potential. A positive potential reversibly opened OmpF and TolC channels. Colicin N, which uses only OmpF for entry, occludes OmpF in planar bilayers with the same orientation constraints as colicins E1 and E3. The OmpF recognition sites of colicins E3 and N, and the TolC recognition site of colicin E1, were found to reside in the N-terminal translocation domains. These data are considered in the context of a two-receptor translocon model for colicin entry into cells.

Peptide signal molecules and bacteriocins in Gram-negative bacteria: a genome-wide in silico screening for peptides containing a double-glycine leader sequence and their cognate transporters

Quorum sensing (QS) in Gram-negative bacteria is generally assumed to be mediated by N-acyl-homoserine lactone molecules while Gram-positive bacteria make use of signaling peptides. We analyzed the occurrence in Gram-negative bacteria of peptides and transporters that are involved in quorum sensing in Gram-positive bacteria. Many class II bacteriocins and inducing factors produced by lactic acid bacteria (LAB) and competence stimulating peptides (CSPs) synthesized by streptococci are processed by their cognate ABC-transporters during their secretion. During transport, a conserved leader sequence, termed the double-glycine motif (GG-motif), is cleaved off by the N-terminal domain of the transporter, which belongs to the Peptidase C39 protein family. Several peptides containing a GG-motif were recently described in Gram-negative bacteria (Trends Microbiol 2001;9:164-8). To screen for additional putative GG-motif containing peptides, an in silico strategy based on MEME, HMMER2.2 and Wise2 was designed. Using a curated training set, a motif model of the leader peptide was built and used to screen over 120 fully sequenced bacterial genomes. The screening methodology was applied at the nucleotide level as probably many small peptide genes have not been annotated and may be absent from the non-redundant databases. It was found that 33% of the screened genomes of Gram-negative bacteria contained one or more transporters carrying a Peptidase C39 domain, compared to 44% of the genomes of Gram-positive bacteria. The transporters can be subdivided into four classes on the basis of their domain organization. Genes coding for putative peptides containing 23-142 amino acids and a GG-motif were found in close association with genes coding for Peptidase C39 domain containing proteins. These peptides show structural similarity to bacteriocins and peptide pheromones of Gram-positive bacteria. The possibility of signal transduction based on peptide signaling in Gram-negative bacteria is discussed.

Flexibility in the receptor-binding domain of the enzymatic colicin E9 is required for toxicity against Escherichia coli cells

The events that occur after the binding of the enzymatic E colicins to Escherichia coli BtuB receptors that lead to translocation of the cytotoxic domain into the periplasmic space and, ultimately, cell killing are poorly understood. It has been suggested that unfolding of the coiled-coil BtuB receptor binding domain of the E colicins may be an essential step that leads to the loss of immunity protein from the colicin and immunity protein complex and then triggers the events of translocation. We introduced pairs of cysteine mutations into the receptor binding domain of colicin E9 (ColE9) that resulted in the formation of a disulfide bond located near the middle or the top of the R domain. After dithiothreitol reduction, the ColE9 protein with the mutations L359C and F412C (ColE9 L359C-F412C) and the ColE9 protein with the mutations Y324C and L447C (ColE9 Y324C-L447C) were slightly less active than equivalent concentrations of ColE9. On oxidation with diamide, no significant biological activity was seen with the ColE9 L359C-F412C and the ColE9 Y324C-L447C mutant proteins; however diamide had no effect on the activity of ColE9. The presence of a disulfide bond was confirmed in both of the oxidized, mutant proteins by matrix-assisted laser desorption ionization-time of flight mass spectrometry. The loss of biological activity of the disulfide-containing mutant proteins was not due to an indirect effect on the properties of the translocation or DNase domains of the mutant colicins. The data are consistent with a requirement for the flexibility of the coiled-coil R domain after binding to BtuB.

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.

Structure of the periplasmic domain of Pseudomonas aeruginosa TolA: evidence for an evolutionary relationship with the TonB transporter protein

The crystal structure of the C-terminal domain III of Pseudomonas aeruginosa TolA has been determined at 1.9 A resolution. The fold is similar to that of the corresponding domain of Escherichia coli TolA, despite the limited amino acid sequence identity of the two proteins (20%). A pattern was discerned that conserves the fold of domain III within the wider TolA family and, moreover, reveals a relationship between TolA domain III and the C-terminal domain of the TonB transporter proteins. We propose that the TolA and TonB C-terminal domains have a common evolutionary origin and are related by means of domain swapping, with interesting mechanistic implications. We have also determined the overall shape of the didomain, domains II + III, of P.aeruginosa TolA by solution X-ray scattering. The molecule is monomeric-its elongated, stalk shape can accommodate the crystal structure of domain III at one end, and an elongated helical bundle within the portion corresponding to domain II. Based on these data, a model for the periplasmic domains of P.aeruginosa TolA is presented that may explain the inferred allosteric properties of members of the TolA family. The mechanisms of TolA-mediated entry of bateriophages in P.aeruginosa and E.coli are likely to be similar.

Structural dynamics of the membrane translocation domain of colicin E9 and its interaction with TolB

In order for the 61 kDa colicin E9 protein toxin to enter the cytoplasm of susceptible cells and kill them by hydrolysing their DNA, the colicin must interact with the outer membrane BtuB receptor and Tol translocation pathway of target cells. The translocation function is located in the N-terminal domain of the colicin molecule. (1)H, (1)H-(1)H-(15)N and (1)H-(13)C-(15)N NMR studies of intact colicin E9, its DNase domain, minimal receptor-binding domain and two N-terminal constructs containing the translocation domain showed that the region of the translocation domain that governs the interaction of colicin E9 with TolB is largely unstructured and highly flexible. Of the expected 80 backbone NH resonances of the first 83 residues of intact colicin E9, 61 were identified, with 43 of them being assigned specifically. The absence of secondary structure for these was shown through chemical shift analyses and the lack of long-range NOEs in (1)H-(1)H-(15)N NOESY spectra (tau(m)=200 ms). The enhanced flexibility of the region of the translocation domain containing the TolB box compared to the overall tumbling rate of the protein was identified from the relatively large values of backbone and tryptophan indole (15)N spin-spin relaxation times, and from the negative (1)H-(15)N NOEs of the backbone NH resonances. Variable flexibility of the N-terminal region was revealed by the (15)N T(1)/T(2) ratios, which showed that the C-terminal end of the TolB box and the region immediately following it was motionally constrained compared to other parts of the N terminus. This, together with the observation of inter-residue NOEs involving Ile54, indicated that there was some structural ordering, resulting most probably from the interactions of side-chains. Conformational heterogeneity of parts of the translocation domain was evident from a multiplicity of signals for some of the residues. Im9 binding to colicin E9 had no effect on the chemical shifts or other NMR characteristics of the region of colicin E9 containing the TolB recognition sequence, though the interaction of TolB with intact colicin E9 bound to Im9 did affect resonances from this region. The flexibility of the translocation domain of colicin E9 may be connected with its need to recognise protein partners that assist it in crossing the outer membrane and in the translocation event itself.

Using disulfide bond engineering to study conformational changes in the beta'260-309 coiled-coil region of Escherichia coli RNA polymerase during sigma(70) binding

RNA polymerase of Escherichia coli is the sole enzyme responsible for mRNA synthesis in the cell. Upon binding of a sigma factor, the holoenzyme can direct transcription from specific promoter sequences. We have previously defined a region of the beta' subunit (beta'260-309, amino acids 260 to 309) which adopts a coiled-coil conformation shown to interact with sigma(70) both in vitro and in vivo. However, it was not known if the coiled-coil conformation was maintained upon binding to sigma(70). In this work, we engineered a disulfide bond within beta'240-309 that locks the beta' coiled-coil region in the coiled-coil conformation, and we show that this "locked" peptide is able to bind to sigma(70). We also show that the locked coiled-coil is capable of inducing a conformational change within sigma(70) that allows recognition of the -10 nontemplate strand of DNA. This suggests that the coiled-coil does not adopt a new conformation upon binding sigma(70) or upon recognition of the -10 nontemplate strand of DNA.

Mechanisms of colicin binding and transport through outer membrane porins

To kill Escherichia coli, toxic proteins, called colicins, pass through the permeability barrier created by the outer membrane (OM) of the bacterial cell envelope. We consider a variety of different colicins, including A, B, D, E1, E3, Ia, M and N, that penetrate through the porins OmpF, FepA, BtuB, Cir and FhuA, to subsequently interact with a few targets in the periplasm, including TolA, TolB, TolC and TonB. We review the mechanisms, demonstrated and postulated, by which such toxins enter bacterial cells, from the initial binding stage on the cell surface to the internalization reaction through the OM bilayer. Our discussions endeavor to answer two main questions: what is the origin of colicin-binding affinity and specificity, and after adsorption to OM porins, do colicin polypeptides translocate through porin channels, or enter by another, currently unknown pathway?

The role of the TolC family in protein transport and multidrug efflux. From stereochemical certainty to mechanistic hypothesis

Gram-negative bacteria are enveloped by a system of two membranes, and they use specialized multicomponent, energy-driven pumps to transport molecules directly across this double-layered partition from the cell interior to the extra-cellular environment. One component of these pumps is embedded in the outer-membrane, and the paradigm for its structure and function is the TolC protein from Escherichia coli. A common component of a wide variety of efflux pumps, TolC and its homologues are involved in the export of chemically diverse molecules ranging from large protein toxins, such as alpha-hemolysin, to small toxic compounds, such as antibiotics. TolC family members thus play important roles in conferring pathogenic bacteria with both virulence and multidrug resistance. These pumps assemble reversibly in a transient process that brings together TolC or its homologue, an inner-membrane-associated periplasmic component, an integral inner-membrane translocase and the substrate itself. TolC can associate in this fashion with a variety of different partners to participate in the transport of diverse substrates. We review here the structure and function of TolC and the other components of the efflux/transport pump.