Quantification of group A colicin import sites

Colicins and Microcins Produced by Enterobacteriaceae: Characterization, Mode of Action, and Putative Applications

Enterobacteriaceae are widely present in many environments related to humans, including the human body and the food that they consume, from both plant or animal origin. Hence, they are considered relevant members of the gastrointestinal tract microbiota. On the other hand, these bacteria are also recognized as putative pathogens, able to impair human health and, in food, they are considered indicators for the microbiological quality and hygiene status of a production process. Nevertheless, beneficial properties have also been associated with Enterobacteriaceae, such as the ability to synthesize peptides and proteins, which can have a role in the structure of microbial communities. Among these antimicrobial molecules, those with higher molecular mass are called colicins, while those with lower molecular mass are named microcins. In recent years, some studies show an emphasis on molecules that can help control the development of pathogens. However, not enough data are available on this subject, especially related to microcins. Hence, this review gathers and summarizes current knowledge on colicins and microcins, potential usage in the treatment of pathogen-associated diseases and cancer, as well as putative applications in food biotechnology.

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.

In vivo processing of DNase colicins E2 and E7 is required for their import into the cytoplasm of target cells

DNase colicins E2 and E7, both of which appropriate the BtuB/Tol translocation machinery to cross the outer membrane, undergo a processing step as they enter the cytoplasm. This endoproteolytic cleavage is essential for their killing action. A processed form of the same size, 18.5 kDa, which corresponds to the C-terminal catalytic domain, was detected in the cytoplasm of bacteria treated with either of the two DNase colicins. The inner-membrane protease FtsH is necessary for the processing that allows the translocation of the colicin DNase domain into the cytoplasm. The processing occurs near residue D420, at the same position as the FtsH-dependent cleavage in RNase colicins E3 and D. The cleavage site is located 30 amino acids upstream of the DNase domain. In contrast, the previously reported periplasm-dependent colicin cleavage, located at R452 in colicin E2, was shown to be generated by the outer-membrane protease OmpT and we show that this cleavage is not physiologically relevant for colicin import. Residue R452, whose mutated derivatives led to toxicity defect, was shown to have no role in colicin processing and translocation, but it plays a key role in the catalytic activity, as previously reported for other DNase colicins. Membrane associated forms of colicins E2 and E7 were detected on target cells as proteinase K resistant peptides, which include both the receptor-binding and DNase domains. A similar, but much less proteinase K-resistant form was also detected with RNase colicin E3. These colicin forms are not relevant for colicin import, but their detection on the cell surface indicates that whole nuclease-colicin molecules are found in a stable association with the outer-membrane receptor BtuB of the target cells.

Hijacking cellular functions for processing and delivery of colicins E3 and D into the cytoplasm

The mechanisms for importing colicins from the extracellular medium into Escherichia coli target cells implicate a complex cascade of interactions with host proteins. It is known that colicins interact with membrane receptors, and they may appropriate them structurally, but not functionally, as a scaffold on the surface of the target cell so that they can be translocated across the outer membrane. During the import into the periplasm, colicins parasitize functionally membrane porins and energy-transducers by mimicking their natural substrates or interacting partners. Such structural or functional parasitism also takes place during the late molecular events responsible for the processing and translocation of nuclease colicins across the inner membrane. Two different RNase colicins (D and E3) require an endoproteolytic cleavage, dependent on the inner membrane ATPase/protease FtsH, in order to transfer their C-terminal toxic domain into the cytoplasm. Moreover, the processing of colicin D necessitates a specific interaction with the signal peptidase LepB, but without appropriating the catalytic activity of this enzyme. A comparison of the differences in structural and functional organizations of these two colicins, as well as the pore-forming colicin B, is discussed in the present paper in connection with the sequential steps of their import mechanisms and the exploitation of the machinery of the target cell.

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.

Protein expression profile of Gluconacetobacter diazotrophicus PAL5, a sugarcane endophytic plant growth-promoting bacterium

This is the first broad proteomic description of Gluconacetobacter diazotrophicus, an endophytic bacterium, responsible for the major fraction of the atmospheric nitrogen fixed in sugarcane in tropical regions. Proteomic coverage of G. diazotrophicus PAL5 was obtained by two independent approaches: 2-DE followed by MALDI-TOF or TOF-TOF MS and 1-DE followed by chromatography in a C18 column online coupled to an ESI-Q-TOF or ESI-IT mass spectrometer. The 583 identified proteins were sorted into functional categories and used to describe potential metabolic pathways for nucleotides, amino acids, carbohydrates, lipids, cofactors and energy production, according to the Enzyme Commission of Enzyme Nomenclature (EC) and Kyoto Encyclopedia of genes and genomes (KEGG) databases. The identification of such proteins and their possible insertion in conserved biochemical routes will allow comparisons between G. diazotrophicus and other bacterial species. Furthermore, the 88 proteins classified as conserved unknown or unknown constitute a potential target for functional genomic studies, aiming at the understanding of protein function and regulation of gene expression. The knowledge of metabolic fundamentals and coordination of these actions are crucial for the rational, safe and sustainable interference on crops. The entire dataset, including peptide sequence information, is available as Supporting Information and is the major contribution of this work.

Colicin E2 is still in contact with its receptor and import machinery when its nuclease domain enters the cytoplasm

Colicins reach their targets in susceptible Escherichia coli strains through two envelope protein systems: the Tol system is used by group A colicins and the TonB system by group B colicins. Colicin E2 (ColE2) is a cytotoxic protein that recognizes the outer membrane receptor BtuB. After gaining access to the cytoplasmic membrane of sensitive Escherichia coli cells, ColE2 enters the cytoplasm to cleave DNA. After binding to BtuB, ColE2 interacts with the Tol system to reach its target. However, it is not known if the entire colicin or only the nuclease domain of ColE2 enters the cell. Here I show that preincubation of ColE2 with Escherichia coli cells prevents binding and translocation of pore-forming colicins of group A but not of group B. This inhibition persisted even when cells were incubated with ColE2 for 30 min before the addition of pore-forming colicins, indicating that ColE2 releases neither its receptor nor its translocation machinery when its nuclease domain enters the cells. These competition experiments enabled me to estimate the time required for ColE2 binding to its receptor and translocation.

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.

Release of immunity protein requires functional endonuclease colicin import machinery

Bacteria producing endonuclease colicins are protected against the cytotoxic activity by a small immunity protein that binds with high affinity and specificity to inactivate the endonuclease. This complex is released into the extracellular medium, and the immunity protein is jettisoned upon binding of the complex to susceptible cells. However, it is not known how and at what stage during infection the immunity protein release occurs. Here, we constructed a hybrid immunity protein composed of the enhanced green fluorescent protein (EGFP) fused to the colicin E2 immunity protein (Im2) to enhance its detection. The EGFP-Im2 protein binds the free colicin E2 with a 1:1 stoichiometry and specifically inhibits its DNase activity. The addition of this hybrid complex to susceptible cells reveals that the release of the hybrid immunity protein is a time-dependent process. This process is achieved 20 min after the addition of the complex to the cells. We showed that complex dissociation requires a functional translocon formed by the BtuB protein and one porin (either OmpF or OmpC) and a functional import machinery formed by the Tol proteins. Cell fractionation and protease susceptibility experiments indicate that the immunity protein does not cross the cell envelope during colicin import. These observations suggest that dissociation of the immunity protein occurs at the outer membrane surface and requires full translocation of the colicin E2 N-terminal domain.

The Tol proteins of Escherichia coli and their involvement in the translocation of group A colicins

The Tol proteins are involved in outer membrane stability of Gram-negative bacteria. The TolQRA proteins form a complex in the inner membrane while TolB and Pal interact near the outer membrane. These two complexes are transiently connected by an energy-dependent interaction between Pal and TolA. The Tol proteins have been parasitized by group A colicins for their translocation through the cell envelope. Recent advances in the structure and energetics of the Tol system, as well as the interactions between the N-terminal translocation domain of colicins and the Tol proteins are presented.

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.

Analysis of the Escherichia coli Tol-Pal and TonB systems by periplasmic production of Tol, TonB, colicin, or phage capsid soluble domains

The aim of this review is to describe an in vivo assay of the interactions taking place in the Tol-Pal or TonB-ExbB-ExbD envelope complexes in the periplasm of Escherichia coli and between them and colicins or g3p protein of filamentous bacteriophages. Domains of colicins or periplasmic soluble domains of Tol or TonB proteins can be artificially addressed to the periplasm of bacteria by fusing them to a signal sequence from an exported protein. These domains interact specifically in the periplasm with the Tol or TonB complexes and disturb their function, which can be directly detected by the appearance of specific tol or tonB phenotypes. This technique can be used to detect new interactions, to characterize them biochemically and to map them or to induce tol or tonB phenotypes to study the functions of these two complexes.

Macromolecular import into Escherichia coli: the TolA C-terminal domain changes conformation when interacting with the colicin A toxin

Various macromolecules such as bacteriotoxins and phage DNA parasitize some envelope proteins of Escherichia coli to infect the bacteria. A two-step import mechanism involves the primary interaction with an outer membrane receptor or with a pilus followed by the translocation across the outer membrane. However, this second step is poorly understood. It was shown that the TolA, TolQ, and TolR proteins play a critical role in the translocation of group A colicins and filamentous bacteriophage minor coat proteins (g3p). Translocation of these proteins requires the interaction of their N-terminal domain with the C-terminal domain of TolA (TolAIII). In this work, short soluble TolAIII domains were overproduced in the cytoplasm and in the periplasm of E. coli. In TolAIII, the two cysteine residues were found to be reduced in the cytoplasmic form and oxidized in the periplasmic form. The interaction of TolAIII with the N-terminal domain of colicin A (ATh) is observed in the presence and in the absence of the disulfide bridge. The complex formation of TolAIII and ATh was found to be independent of the ionic strength. An NMR study of TolAIII, both free and bound, shows a significant structural change when interacting with ATh, in the presence or absence of the disulfide bridge. In contrast, such a structural modification was not observed when TolAIII interacts with g3p N1. These results suggest that bacteriotoxins and Ff bacteriophages parasitize E. coli using different interactions between TolA and the translocation domain of the colicin and g3p protein, respectively.

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.

Molecular mechanisms of bacteriocin evolution

Microorganisms are engaged in a never-ending arms race. One consequence of this intense competition is the diversity of antimicrobial compounds that most species of bacteria produce. Surprisingly, little attention has been paid to the evolution of such extraordinary diversity. One class of antimicrobials, the bacteriocins, has received increasing attention because of the high levels of bacteriocin diversity observed and the use of bacteriocins as preservatives in the food industry and as antibiotics in the human health industry. However, little effort has been focused on evolutionary questions, such as what are the phylogenetic relationships among these toxins, what mechanisms are involved in their evolution, and how do microorganisms respond to such an arsenal of weapons? The focus of this review is to provide a detailed picture of our current understanding of the molecular mechanisms involved in the process of bacteriocin diversification.

Colicin N interaction with sensitive Escherichia coli cells: in situ and kinetic approaches

The kinetics of colicin N binding to the Escherichia coli surface, comprising the recognition of and association with cell-surface-exposed sites of OmpF porin, is a rapid event taking place during the first seconds of incubation. Immunogold labelling demonstrates the membrane localization of the colicin N bound to sensitive cells. Analyses of colicin-induced efflux indicate a short lag before the onset of cytoplasmic K+ release. This delay reflects the time necessary for translocation from the external side and pore-forming insertion into the cytoplasmic membrane.

Ion-channel-forming colicins

Several features of ion-channel-forming colicins have been illuminated by recent revelations: its four-domain structure, the mechanism and thermodynamics of binding to the gating loop of outer membrane porins, the mechanism of translocation, competition for the transperiplasmic excursion facilitated by the Tol or Ton transperiplasmic proteins, and the formation of a waisted, funnel-shaped transmembrane channel of well-characterized shape.

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.

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.

Crystal structure of colicin Ia

The ion-channel forming colicins A, B, E1, Ia, Ib and N all kill bacterial cells selectively by co-opting bacterial active-transport pathways and forming voltage-gated ion conducting channels across the plasma membrane of the target bacterium. The crystal structure of colicin Ia reveals a molecule 210 A long with three distinct functional domains arranged along a backbone of two extraordinarily long alpha-helices. A central domain at the bend of the hairpin-like structure mediates specific recognition and binding to an outer-membrane receptor. A second domain mediates translocation across the outer membrane via the TonB transport pathway; the TonB-box recognition element of colicin Ia is on one side of three 80 A-long helices arranged as a helical sheet. A third domain is made up of 10 alpha-helices which form a voltage-activated and voltage-gated ion conducting channel across the plasma membrane of the target cell. The two 160 A-long alpha-helices that link the receptor-binding domain to the other domains enable the colicin Ia molecule to span the periplasmic space and contact both the outer and plasma membranes simultaneously during function.

Dynamic aspects of colicin N translocation through the Escherichia coli outer membrane

Colicin N is a bacteriocin that kills sensitive Escherichia coli cells. After binding to the cell surface-exposed receptor, a short period exists when a significant number of the cell-associated colicin N molecules are sensitive to external enzymes. Two colicin N populations are discriminated by proteases: the susceptible pool bound to OmpF porin on the cell surface and another population corresponding to protease-inaccessible colicin N. During translocation, colicin N reaches the periplasmic space and proteolytic cleavage of the colicin occurs only when the outer membrane barrier is permeabilized.

Characterization of the tol-pal region of Escherichia coli K-12: translational control of tolR expression by TolQ and identification of a new open reading frame downstream of pal encoding a periplasmic protein

The TolQ, TolR, TolA, TolB, and Pal proteins appear to function in maintaining the integrity of the outer membrane, as well as facilitating the uptake of the group A colicins and the DNA of the infecting filamentous bacteriophages. Sequence data showed that these genes are clustered in a 6-kb segment of DNA with the gene order orf1 tolQ tolR tolA tolB pal orf2 (a newly identified open reading frame encoding a 29-kD9 protein). Like those containing orf1, bacteria containing an insertion mutation in this gene showed no obvious phenotype. Analysis of beta-galactosidase activity from fusion constructs in which the lac operon was fused to various genes in the cluster showed that the genes in this region constitute two separate operons: orf1 tolQRA and tolB pal orf2. In the orf1 tolQRA operon, translation of MR was dependent on translation of the upstream tolQ region. Consistent with this result, no functional ribosome-binding site for TolR synthesis was detected.

Membrane topology of the colicin A pore-forming domain analyzed by disulfide bond engineering

Four colicin A double-cysteine mutants possessing a disulfide bond in their pore-forming domain were constructed to study the translocation and the pore formation of colicin A. The disulfide bonds connected alpha-helices 1 and 2, 2 and 10, 3 and 9, or 3 and 10 of the pore-forming domain. The disulfide bonds did not prevent the colicin A translocation through the Escherichia coli envelope. However, the mutated colicins were able to exert their in vivo channel activity only after reduction of their disulfide bonds. In vitro studies with brominated phospholipid vesicles and planar lipid bilayers revealed that the disulfide bond that connects the alpha-helices 2 and 10 prevented the colicin A membrane insertion, whereas the other double-cysteine mutants inserted into lipid vesicles. The disulfide bonds that connect either the alpha-helices 1 and 2 or 3 and 10 were unable to prevent the formation of a conducting channel in presence of membrane potential. These results indicate that alpha-helices 1, 2, 3, and 10 remain at the membrane surface after application of a membrane potential.

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.

Major transmembrane movement associated with colicin Ia channel gating

Colicin Ia, a bacterial protein toxin of 626 amino acid residues, forms voltage-dependent channels in planar lipid bilayer membranes. We have exploited the high affinity binding of streptavidin to biotin to map the topology of the channel-forming domain (roughly 175 residues of the COOH-terminal end) with respect to the membrane. That is, we have determined, for the channel's open and closed states, which parts of this domain are exposed to the aqueous solutions on either side of the membrane and which are inserted into the bilayer. This was done by biotinylating cysteine residues introduced by site-directed mutagenesis, and monitoring by electrophysiological methods the effect of streptavidin addition on channel behavior. We have identified a region of at least 68 residues that flips back and forth across the membrane in association with channel opening and closing. This identification was based on our observations that for mutants biotinylated in this region, streptavidin added to the cis (colicin-containing) compartment interfered with channel opening, and trans streptavidin interfered with channel closing. (If biotin was linked to the colicin by a disulfide bond, the effects of streptavidin on channel closing could be reversed by detaching the streptavidin-biotin complex from the colicin, using a water-soluble reducing agent. This showed that the cysteine sulfur, not just the biotin, is exposed to the trans solution). The upstream and downstream segments flanking the translocated region move into and out of the bilayer during channel opening and closing, forming two transmembrane segments. Surprisingly, if any of several residues near the upstream end of the translocated region is held on the cis side by streptavidin, the colicin still forms voltage-dependent channels, indicating that a part of the protein that normally is fully translocated across the membrane can become the upstream transmembrane segment. Evidently, the identity of the upstream transmembrane segment is not crucial to channel formation, and several open channel structures can exist.