fii, a bacterial locus required for filamentous phage infection and its relation to colicin-tolerant tolA and tolB

Selectively-infective phage (SIP): a mechanistic dissection of a novel in vivo selection for protein-ligand interactions

Selectively-infective phage (SIP) is a novel methodology for the in vivo selection of interacting protein-ligand pairs. It consists of two components, (1) a phage particle made non-infective by replacing its N-terminal domains of geneIII protein (gIIIp) with a ligand-binding protein, and (2) an "adapter" molecule in which the ligand is linked to those N-terminal domains of gIIIp which are missing from the phage particle. Infectivity is restored when the displayed protein binds to the ligand and thereby attaches the missing N-terminal domains of gIIIp to the phage particle. Phage propagation is thus strictly dependent on the protein-ligand interaction. We have shown that the insertion of beta-lactamase into different positions of gIIIp, mimicking the insertion of a protein-ligand pair, led to highly infective phage particles. Any phages lacking the first N-terminal domain were not infective at all. In contrast, those lacking only the second N-terminal domain showed low infectivity irrespective of the presence or absence of the F-pilus on the recipient cell, which could be enhanced by addition of calcium. An anti-fluorescein scFv antibody and its antigen fluorescein were examined as a protein-ligand model system for SIP experiments. Adapter molecules, synthesized by chemical coupling of fluorescein to the purified N-terminal domains, were mixed with non-infective anti-fluorescein scFv-displaying phages. Infection events were strictly dependent on fluorescein being coupled to the N-terminal domains and showed a strong dependence on the adapter concentration. Up to 10(6) antigen-specific events could be obtained from 10(10) input phages, compared to only one antigen-independent event. Since no separation of binders and non-binders is necessary, SIP is promising as a rapid procedure to select for high affinity interactions.

Characterization of the tol-pal and cyd region of Escherichia coli K-12: transcript analysis and identification of two new proteins encoded by the cyd operon

Sequence analysis showed that the cyd operon is immediately upstream of the tol-pal region. Northern (RNA) blot analysis demonstrated that the transcript for the cyd operon terminates just before the promoter for transcription of the tol genes. The cyd transcript contains cydA cydB followed by two open reading frames: orfC, encoding a 37-residue peptide, and orfD, encoding a 97-residue peptide. Both OrfC and OrfD are synthesized in minicells.

Filamentous phage IKe mRNAs conserve form and function despite divergence in regulatory elements

As a means of determining whether there has been selection to conserve the basic pattern of filamentous phage mRNAs, the major mRNAs representing genes II to VIII have been defined for a phage distantly related to the Ff group specific for Escherichia coli hosts bearing F pili. Phage IKe has a genome with 55% identity with the Ff genome and infects E. coli strains bearing N pili. The results reveal a remarkably similar pattern of overlapping polycistronic mRNAs with a common 3' end and unique 5' ends. The IKe mRNAs, like the Ff phage mRNAs, represent a combination of primary transcripts and processed RNAs. However, examination of the sequences containing the RNA endpoint positions revealed that effectively the only highly conserved regulatory element is the rho-independent terminator that generates the common 3' end. Promoters and processing sites have not been maintained in identical positions, but frequently are placed so as to yield RNAs with similar coding function. By conserving the pattern of transcription and processing despite divergence in the regulatory elements and possibly the requirements for host, endoribonucleases, the results argue that the pattern is not simply fortuitous.

Identification and characterization of the tolQRA genes of Pseudomonas aeruginosa

The tolQ, tolR, and tolA genes from Pseudomonas aeruginosa PAO were cloned using degenerate oligonucleotide PCR primers designed based on conserved transmembrane regions of Escherichia coli TolQ and TolR and E. coli and Pseudomonas putida ExbB and ExbD. The resulting PCR product was used as a probe to isolate a 6.5-kb DNA fragment containing P. aeruginosa tolQ, tolR, and tolA. The nucleotide sequence of a 2.9-kb DNA fragment containing the tolQ, tolR, and tolA genes was determined. The DNA sequence predicts TolQ to be a 25,250-Da protein exhibiting 53% identity to E. coli TolQ. TolR is predicted to be a 15,788-Da protein, sharing 38% identity with the E. coli TolR protein. The P. aeruginosa tolA sequence predicts a 37,813-Da protein with 27% identity to the E. coli TolA. The P. aeruginosa TolQRA proteins were expressed in E. coli minicells. Analysis of plasmid-encoded tolQ::lacZ and tolA::lacZ promoter fusions in E. coli indicated that these genes are expressed at different levels, suggesting transcription from different promoters. Transcriptional analysis of the tol genes in P. aeruginosa revealed that the tolQ and tolR genes are cotranscribed as an approximately 1.5-kb transcript and that tolA is transcribed from its own promoter as an approximately 1.2-kb transcript. The P. aeruginosa Tol proteins were functionally unable to complement E. coli tol mutants, although P. aeruginosa TolQ was able to complement the iron-limited growth of an E. coli exbB mutant. Introduction of the tolQRA genes in the tol-like mutant PAO 1652 restored pyocin AR41 killing, indicating that the Tol proteins are involved in the uptake of pyocin AR41 in P. aeruginosa. Attempts to inactivate the chromosomal copy of the tolA or tolQ gene in the parent strain PAO proved to be unsuccessful, and we propose that inactivation of these genes in P. aeruginosa results in a lethal phenotype.

Isolation and characterization of the Haemophilus influenzae tolQ, tolR, tolA and tolB genes

The tolQ, R, A and B genes have been isolated from the DNA of Haemophilus influenzae and sequenced. The deduced amino acid (aa) sequence of the H. influenzae TolQ, TolR, TolA and TolB show 67, 63, 41 and 62% identity with Escherichia coli TolQRAB proteins, respectively. These four proteins are involved in transport of colicins and phages across the cell envelope. The translational stop codon of TolB (the last gene in the cluster) is 23 bases upstream of the start codon of the P6 lipoprotein gene. Primer extension and Northern blot analysis revealed that the start of the P6 transcript is within the tolB gene. Nucleotide sequence (nt) analysis of the entire tolQRABP6 region shows a transcriptional terminator immediately downstream of the P6 gene. The tolQRABP6 gene cluster of H. influenzae may thus constitute an operon.

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.

Energy-coupled transport across the outer membrane of Escherichia coli: ExbB binds ExbD and TonB in vitro, and leucine 132 in the periplasmic region and aspartate 25 in the transmembrane region are important for ExbD activity

Ferric siderophores, vitamin B12, and group B colicins are taken up through the outer membranes of Escherichia coli cells by an energy-coupled process. Energy from the cytoplasmic membrane is transferred to the outer membrane with the aid of the Ton system, consisting of the proteins TonB, ExbB, and ExbD. In this paper we describe two point mutations which inactivate ExbD. One mutation close to the N-terminal end of ExbD is located in the cytoplasmic membrane, and the other mutation close to the C-terminal end is located in the periplasm. E. coli CHO3, carrying a chromosomal exbD mutation in which leucine at position 132 was replaced by glutamine, was devoid of all Ton-related activities. A plasmid-encoded ExbD derivative, in which aspartate at position 25, the only changed amino acid in the predicted membrane-spanning region of ExbD, was replaced by asparagine, failed to restore the Ton activities of strain CHO3 and negatively complemented ExbD+ strains, indicating an interaction of this mutated ExbD with wild-type ExbD or with another component. This component was shown to be ExbB. ExbB that was labeled with 6 histidine residues at its C-terminal end and that bound to a nickel-nitrilotriacetic acid agarose column retained ExbD and TonB specifically; both were eluted with the ExbB labeled with 6 histidine residues, demonstrating interaction of ExbB with ExbD and TonB. These data further support the concept that TonB, ExbB, and ExbD form a complex in which the energized conformation of TonB opens the channels in the outer membrane receptor proteins.

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.

Characterization of the exbBD operon of Escherichia coli and the role of ExbB and ExbD in TonB function and stability

TonB protein appears to couple the electrochemical potential of the cytoplasmic membrane to active transport across the essentially unenergized outer membrane of gram-negative bacteria. ExbB protein has been identified as an auxiliary protein in this process. In this paper we show that ExbD protein, encoded by an adjacent gene in the exb cluster at 65', was also required for TonB-dependent energy transduction and, like ExbB, was required for the stability of TonB. The phenotypes of exbB exbD+ strains were essentially indistinguishable from the phenotypes of exbB+ exbD strains. Mutations in either gene resulted in the degradation of TonB protein and in decreased, but not entirely absent, sensitivities to colicins B and Ia and to bacteriophage phi 80. Evidence that the absence of ExbB or ExbD differentially affected the half-lives of newly synthesized and steady-state TonB was obtained. In the absence of ExbB or ExbD, newly synthesized TonB was degraded with a half-life of 5 to 10 min, while the half-life of TonB under steady-state conditions was significantly longer, approximately 30 min. These results were consistent with the idea that ExbB and ExbD play roles in the assembly of TonB into an energy-transducing complex. While interaction between TonB and ExbD was suggested by the effect of ExbD on TonB stability, interaction of ExbD with TonB was detected by neither in vivo cross-linking assays nor genetic tests for competition. Assays of a chromosomally encoded exbD::phoA fusion showed that exbB and exbD were transcribed as an operon, such that ExbD-PhoA levels in an exbB::Tn10 strain were reduced to 4% of the levels observed in an exbB+ strain under iron-limiting conditions. Residual ExbD-PhoA expression in an exbB::Tn10 strain was not iron regulated and may have originated from within the Tn10 element in exbB.

Protein complex within Escherichia coli inner membrane. TolA N-terminal domain interacts with TolQ and TolR proteins

The TolA, TolB, TolQ, and TolR proteins are involved in maintaining the integrity of the Escherichia coli outer membrane and in the import of group A colicins and filamentous phage DNA. TolA, TolQ, and TolR are localized in the inner membrane while TolB is periplasmic, although a small amount of membrane-associated TolB is always found. In vivo cross-linking experiments with formaldehyde were performed in order to determine the proteins interacting with TolA. In wild-type strains, two specific complexes of 65 and 71 kDa, comprising TolA, were identified. These complexes were absent in a tolQ strain, while only the 65-kDa complex was absent in a tolR strain. When the tol strains were transformed with plasmids encoding TolR or TolQ, the specific complexes were restored. Moreover, immunoprecipitation experiments with the antiserum directed against TolA indicated that TolQ and TolR were co-immunoprecipitated with TolA after cross-linking. These data demonstrate that TolA interacts directly with TolR and TolQ. Two truncated TolA proteins devoid of either the C-terminal or the central domains of the protein were subjected to in vivo cross-linking. Since these two TolA derivatives still formed specific complexes with TolR derivatives still formed specific complexes with TolR and TolQ, we concluded that the TolA N-terminal domain interacted with these proteins.

Maturation and localization of the TolB protein required for colicin import

The tolB gene has been shown previously to encode two proteins of 47.5 kDa (TolB) and 43 kDa (TolB*). To explain the presence of these two forms, two hypotheses have been proposed: TolB might be posttranslationally processed to TolB*, or an internal in-frame translation initiation resulting in TolB* may occur (S. K. Levengood and R. E. Webster, J. Bacteriol. 171:6600-6609, 1989). To address this question, TolB was tagged by inserting in its C-terminal region an epitope recognized by monoclonal antibody 1C11 without altering the function of TolB. It was then demonstrated that the functional protein corresponded to TolB*, the mature periplasmic protein, and that TolB was its precursor form, which was observed only when the protein was overexpressed. These two forms were purified by immunoprecipitation, and their N-terminal sequences were determined. An antibody directed against TolB was raised, which confirmed the results obtained with the tagged TolB.

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.

Partial suppression of an Escherichia coli TonB transmembrane domain mutation (delta V17) by a missense mutation in ExbB

Active transport of vitamin B12 and Fe(III)-siderophore complexes across the outer membrane of Escherichia coli appears to be dependent upon the ability of the TonB protein to couple cytoplasmic membrane-generated protonmotive force to outer membrane receptors. TonB is supported in this role by an auxiliary protein, ExbB, which, in addition to stabilizing TonB against the activities of endogenous envelope proteases, directly contributes to the energy transduction process. The topological partitioning of TonB and ExbB to either side of the cytoplasmic membrane restricts the sites of interaction between these proteins primarily to their transmembrane domains. In this study, deletion of valine 17 within the aminoterminal transmembrane anchor of TonB resulted in complete loss of TonB activity, as well as loss of detectable in vivo crosslinking into a 59 kDa complex believed to contain ExbB. The delta V17 mutation had no effect on TonB export. The loss of crosslinking appeared to reflect conformational changes in the TonB/ExbB pair rather than loss of interaction since ExbB was still required for some stabilization of TonB delta V17. Molecular modeling suggested that the delta V17 mutation caused a significant change in the predicted conserved face of the TonB amino-terminal membrane anchor. TonB delta V17 was unable to achieve the 23 kDa proteinase K-resistant form in lysed sphaeroplasts that is characteristic of active TonB. Wild-type TonB also failed to achieve the proteinase K-resistant configuration when ExbB was absent. Taken together these results suggested that the delta V17 mutation interrupted productive TonB-ExbB interactions. The apparent ability to crosslink to ExbB as well as a limited ability to transduce energy were restored by a second mutation (A39E) in or near the first predicted transmembrane domain of the ExbB protein. Consistent with the weak suppression, a 23 kDa proteinase K-resistant form of TonB delta V17 was not observed in the presence of ExbBA39E. Neither the ExbBA39E allele nor the absence of ExbB affected TonB or TonB delta V17 export. Unlike the tonB delta V17 mutation, the exbBA39E mutation did not greatly alter a modelled ExbB transmembrane domain structure. Furthermore, the suppressor ExbBA39E functioned normally with wild-type TonB, suggesting that the suppressor was not allele specific. Contrary to expectations, the TonB delta V17, ExbBA39E pair resulted in a TonB with a greatly reduced half-life (approximately 10 min). These results together with protease susceptibility studies suggest that ExbB functions by modulating the conformation of TonB.

Role of the TonB amino terminus in energy transduction between membranes

Escherichia coli TonB protein is an energy transducer, coupling cytoplasmic membrane energy to active transport of vitamin B12 and iron-siderophores across the outer membrane. TonB is anchored in the cytoplasmic membrane by its hydrophobic amino terminus, with the remainder occupying the periplasmic space. In this report we establish several functions for the hydrophobic amino terminus of TonB. A G-26-->D substitution in the amino terminus prevents export of TonB, suggesting that the amino terminus contains an export signal for proper localization of TonB within the cell envelope. Substitution of the first membrane-spanning domain of the cytoplasmic membrane protein TetA for the TonB amino terminus eliminates TonB activity without altering TonB export, suggesting that the amino terminus contains sequence-specific information. Detectable TonB cross-linking to ExbB is also prevented, suggesting that the two proteins interact primarily through their transmembrane domains. In vivo cleavage of the amino terminus of TonB carrying an engineered leader peptidase cleavage site eliminates (i) TonB activity, (ii) detectable interaction with a membrane fraction having a density intermediate to those of the cytoplasmic and outer membranes, and (iii) cross-linking to ExbB. In contrast, the amino terminus is not required for cross-linking to other proteins with which TonB can form complexes, including FepA. Additionally, although the amino terminus clearly is a membrane anchor, it is not the only means by which TonB associates with the cytoplasmic membrane. TonB lacking its amino-terminal membrane anchor still remains largely associated with the cytoplasmic membrane.

The conserved proline-rich motif is not essential for energy transduction by Escherichia coli TonB protein

TonB protein functions as an energy transducer, coupling cytoplasmic membrane electrochemical potential to the active transport of vitamin B12 and Fe(III)-siderophore complexes across the outer membrane of Escherichia coli and other Gram-negative bacteria. Accumulated evidence indicates that TonB is anchored in the cytoplasm, but spans the periplasmic space to interact physically with outer membrane receptors. It has been presumed that this ability is caused by a conserved (Glu-Pro)n-(Lys-Pro)m repeat motif, predicted to assume a rigid, linear conformation of sufficient length to reach the outer membrane. Based on in vitro studies with synthetic peptides and purified FhuA outer membrane receptor, it has been suggested that this region contains a site that directly binds outer membrane receptors and is essential for energy transduction. We have found a TonB lacking the (Glu-Pro)n-(Lys-Pro)m repeat motif (TonB delta(66-100)). TonB delta(66-100) is fully capable of irreversible phi 80 adsorption, except under physiological circumstances where the periplasmic space is expanded. Based on the ability of TonB delta(66-100) to interact with outer membrane receptors and components of the energy transduction apparatus under normal physiological conditions, it is evident that the TonB proline-rich region has no role in energy transduction other than to provide a physical extension sufficient to reach the outer membrane.

Membrane topology of the Escherichia coli TolR protein required for cell envelope integrity

TolR is a 142-amino-acid protein required for the import of colicins and bacteriophage and for maintenance of cell envelope integrity. The topology of TolR in the inner membrane was analyzed by two methods. First, bacteria expressing a series of TolR-beta-galactosidase, TolR-alkaline phosphatase, and TolR-beta-lactamase fusions were assayed for the appropriate enzymatic activity. Second, the accessibility of TolR to proteinase K was determined in permeabilized cells and everted vesicles with an antibody elicited against the carboxyl-terminal 70% of TolR. The results are consistent with TolR spanning the inner membrane once via residues 23 to 43 and with the carboxyl-terminal moiety being exposed to the periplasm. Quantitative studies with the anti-TolR antibody indicated the presence of 2 x 10(3) to 3 x 10(3) TolR molecules per cell.

Membrane topologies of the TolQ and TolR proteins of Escherichia coli: inactivation of TolQ by a missense mutation in the proposed first transmembrane segment

The TolQ and TolR proteins of Escherichia coli are required for the uptake of group A colicins and for infection by filamentous phages. Their topology in the cytoplasmic membrane was determined by cleavage with aminopeptidase K, proteinase K, and trypsin in spheroplasts and cell lysates. From the results obtained, it is proposed that the N terminus of TolQ is located in the periplasm and that it contains three transmembrane segments (residues 9 to 36, 127 to 159, and 162 to 191), a small periplasmic loop, and two large portions in the cytoplasm. The N terminus of TolR is located in the cytoplasm and is followed by a transmembrane segment (residues 21 to 40), and the remainder of the protein is located in the periplasm. A tolQ mutant, which rendered cells resistant to group A colicins and sensitive to cholate, had alanine 13 replaced by glycine and was lacking serine 14 in the first transmembrane segment. The membrane topologies of TolQ and TolR are similar to those proposed for ExbB and ExbD, respectively, which is consistent with the partial functional substitution between ExbB and TolQ and between ExbD and TolR. The amino acid sequences of these proteins display the highest homology in the transmembrane segments, which indicates that the membrane-spanning regions play an important role in the activities of the proteins.

Activity domains of the TonB protein

Escherichia coli and related Gram-negative bacteria contain an energy-coupled transport system through the outer membrane which consists of the proteins TonB, ExbB, ExbD anchored in the cytoplasmic membrane and receptors in the outer membrane. Differences in the activities of the Escherichia coli and the Serratia marcescens TonB proteins were used to identify TonB functional domains. In E.coli TonB segments were replaced by equivalent fragments of S. marcescens TonB and the activities of the resulting chimaeric proteins were determined. In addition, E. coli TonB was truncated at the C-terminal end, and point mutants were generated using bisulphite. From the results obtained we draw the following conclusions: an important site of interaction between TonB and ExbB is located in the N-terminal region of TonB within or close to the cytoplasmic membrane since an N-terminal 44-residue fragment of TonB was stabilized by ExbB and interfered with wild-type TonB activity. In addition, the activity of a TonB derivative in which histidine residue 20 was replaced by arginine was strongly reduced, and a double mutant containing arginine-7 to histidine and alanine-22 to threonine substitutions displayed an impaired uptake of ferrichrome. Furthermore, the domain around residue 160 is involved in TonB activity. S. marcescens TonB segments of this region in E. coli TonB conferred S. marcescens TonB activities, and E. coli TonB point mutants displayed strongly impaired activities for the uptake of colicin B and M and ferric siderophores. Plasmid-encoded tonB mutants of this region showed negative complementation of chromosomal wild-type tonB, and certain tonB mutants suppressed colicin B TonB-box mutants. Uptake of colicins required different domains in TonB, for colicin B and M around residue 160 and for colicin Ia, a domain closer to the C-terminal end. Tandem duplication of the E. coli (EP)X(KP) region by insertion of the S. marcescens (EP)X(KP) region (38 residues) and replacement of lysine residue 91 by glutamate did not alter TonB activity so that no evidence was obtained for this region to be implicated in receptor binding. The aberrant electrophoretic mobility of TonB was caused by the proline-rich sequence since its removal resulted in a normal mobility.

Evolutionary relationship of uptake systems for biopolymers in Escherichia coli: cross-complementation between the TonB-ExbB-ExbD and the TolA-TolQ-TolR proteins

Escherichia coli possesses two energy-coupled import systems through which substances of low concentration and of a size too large to permit diffusion through the porins are translocated across the outer membrane. Group B colicins, ferric siderophores and vitamin B12 are taken up via the TonB-ExbB-ExbD, group A colicins via the TolA-TolQ-TolR system. Cross-complementation between the two systems was demonstrated in that tolQ tolR mutants transformed with plasmids carrying exbB exbD became sensitive to group A colicins, and exbB exbD mutants transformed with plasmid-encoded tolQ tolR became sensitive to group B colicins. TolQ-TolR interacted through TonB, and ExbB-ExbD interacted through TolA with the outer membrane receptors and colicins. Activity of ExbB ExbD via TolA was higher in cells lacking TonB, and activity of TolQ TolR via TonB was increased when TolA was missing. The very distinct TolA and TonB proteins mediate exclusive interaction with group A and group B receptors, respectively. ExbB-TolR and ExbD-TolQ mixtures showed little if any complementation of exbB exbD and tolQ tolR mutants indicating coevolution of ExbB with ExbD and TolQ with TolR. Sequence homology and mutual functional substitution of ExbB-ExbD and TolQ-TolR suggest the evolution of the two import systems from a single import system.

ExbB acts as a chaperone-like protein to stabilize TonB in the cytoplasm

The TonB protein is required to transduce energy from the cytoplasmic membrane to outer membrane transport proteins of Gram-negative bacteria. Two accessory proteins, ExbB and ExbD, are required for TonB function and it has been suggested that TonB and ExbBD form a complex in the membrane. In this paper we demonstrate that there are two spatially distinct, functional interactions between ExbBD and TonB. First, there is an interaction between ExbBD and the N-terminal signal-like peptide of TonB, probably the formation of a stable complex in the membrane. Second, ExbB interacts with TonB in the cytoplasm. This interaction involves the domain of TonB that is normally periplasmic. Thus, this is a transient interaction which occurs during the synthesis and/or localization of TonB, implying a chaperone-like role for ExbB. The transmembrane topology of ExbB was shown to be consistent with this role.

A sequence-specific function for the N-terminal signal-like sequence of the TonB protein

TonB is a proline-rich protein which provides a functional link between the inner and outer membranes of Gram-negative bacteria. TonB is anchored to the inner membrane via an N-terminal signal-like sequence and spans the periplasm, interacting with transport receptors in the outer membrane. We have investigated the role of the N-terminal signal-like peptide in TonB function. Replacement of the N-terminal sequence with heterologous sequences indicates that it has at least three distinct roles in TonB function: (i) to facilitate translocation of TonB across the cytoplasmic membrane; (ii) to anchor TonB to the cytoplasmic membrane; (iii) a sequence-specific functional interaction with the ExbBD proteins.

Role of the carboxyl-terminal domain of TolA in protein import and integrity of the outer membrane

The TolA protein is involved in maintaining the integrity of the outer membrane of Escherichia coli, as mutations in tolA cause the bacteria to become hypersensitive to detergents and certain antibiotics and to leak periplasmic proteins into the medium. This protein also is required for the group A colicins to exert their effects and for many of the filamentous single-stranded bacteriophage to infect the bacterial cell. TolA is a three-domain protein, with the amino-terminal domain anchoring it to the inner membrane. The helical second domain is proposed to span the periplasmic space to allow the carboxyl-terminal third domain to interact with the outer membrane. A plasmid that allowed the synthesis and transport of the carboxyl-terminal third domain into the periplasmic space was constructed. The presence of an excess of this domain in the periplasm of a wild-type cell resulted in an increased sensitivity to deoxycholate, the release of periplasmic alkaline phosphatase and RNase into the medium, and an increased tolerance to colicins E1, E2, E3, and A. There was no effect on the cells' response to colicin D, which depends on TonB instead of TolA for its action. The presence of the free carboxyl-terminal domain of TolA in the periplasm in a tolA null mutation did not restore the wild-type phenotype, suggesting that this domain must be part of the intact TolA molecule to perform its function. Our results are consistent with a model in which the carboxyl-terminal domain of TolA interacts with components in the periplasm or on the inner surface of the outer membrane to function in maintaining the integrity of this membrane.

Role of tol genes in cloacin DF13 susceptibility of Escherichia coli K-12 strains expressing the cloacin DF13-aerobactin receptor IutA

IutA is the outer membrane protein receptor for ferric aerobactin and the bacteriocin cloacin DF13. Although the same receptor is shared, ferric aerobactin transport across the outer membrane in Escherichia coli is TonB dependent, whereas cloacin DF13 transport is not. We have recently observed that tolQ is required for cloacin DF13 susceptibility (J.A. Thomas and M.A. Valvano, FEMS Microbiol. Lett. 91:107-112, 1992). In this study, we demonstrate that the genes tolQ, tolR, and tolA, but not tolB, tolC, and ompF, are required for the internalization of cloacin DF13 and they are not involved in the transport of ferric aerobactin.

Conserved structural and regulatory regions in the Salmonella typhimurium btuB gene for the outer membrane vitamin B12 transport protein

The Salmonella typhimurium btuB gene encodes an outer membrane protein that is necessary for the transport of vitamin B12 and the uptake of the E colicins and bacteriophage BF23. The sequence of this gene showed 87% identity of the deduced polypeptide to its Escherichia coli homolog, and its product was found to function in transport as effectively in cells of E. coli as did the native protein. The extensive sequence conservation within the first 300 transcribed nucleotides, which include the leader and early part of the coding sequence, supports the proposed role of this region in the regulation of btuB gene expression.

The excC gene of Escherichia coli K-12 required for cell envelope integrity encodes the peptidoglycan-associated lipoprotein (PAL)

The excC mutants of Escherichia coli are hypersensitive to drugs such as cholic acid and release periplasmic proteins into the extracellular medium. A 1884 bp fragment carrying the excC gene was isolated and sequenced. It contains the 3' end of the tolB gene which maps at min 17 on the E. coli map and an open reading frame which encodes the 18,748 Da ExcC protein. The protein is composed of a hydrophobic region of 22 residues and displayed an overall hydrophilic configuration. It was shown that the ExcC protein is indeed the PAL (peptidoglycan-associated lipoprotein) described by Mizuno (1979). The pal gene had not yet been characterized on the E. coli linkage map since no obvious phenotype could be identified for mutations in this gene. A topologic analysis of the PAL protein using PAL-PhoA translational fusions showed that PAL is associated with the outer membrane only by its N-terminal moiety. The carboxy-terminal part of the protein is necessary for correct interaction of PAL with the peptidoglycan layer.

Evidence for a TonB-dependent energy transduction complex in Escherichia coli

Escherichia coli TonB protein is required for the active transport of vitamin B12 and Fe(III)-siderophore complexes across the outer membrane, infection by bacteriophages T1 and phi 80, and sensitivity to B-group colicins. TonB appears to function as an energy transducer in these processes, coupling cytoplasmic membrane electrochemical potential to receptors in the outer membrane. Previous reports have demonstrated that chromosomally encoded TonB is functionally unstable in the absence of protein synthesis (half-life approximately 15-30 minutes) and have shown that plasmid-encoded, overexpressed TonB is chemically unstable (half-life approximately 5 minutes). In contrast, this study has shown that chromosomally encoded TonB was chemically stable for greater than 90 minutes while maintaining its functional instability. These data suggest that proteolytic degradation of TonB protein is not the basis of its functional instability. Auxiliary proteins such as ExbB also play a role in TonB-dependent energy transduction. In this study, we have shown that the chemical half-life of chromosomally encoded TonB in an exbB::Tn10 mutant was reduced at least 18-fold, suggesting that TonB is a part of a cytoplasmic membrane complex that includes, at the minimum, ExbB.(ABSTRACT TRUNCATED AT 250 WORDS)

TolA: a membrane protein involved in colicin uptake contains an extended helical region

The group A colicins and the DNA of many single-stranded filamentous bacteriophage are able to use combinations of the Tol proteins to gain entrance into or across the membrane of Escherichia coli. The TolA protein is a 421-amino acid residue integral membrane protein composed of three domains. Domain I, consisting of the amino-terminal 47 amino acids, contains a 21-residue hydrophobic segment that anchors the protein in the inner membrane. The remaining 374 amino acids, containing the other two domains, reside in the periplasmic space. Domain III, consisting of the carboxyl-terminal 120 residues, is considered to be the functional domain based on the location of the tolA592 deletion mutation. The internal 262 amino acids comprise domain II, which connects domains I and III together via short regions of polyglycine. It contains a large number of 3- to 5-residue polyalanine stretches, many of which have a repeat of the sequence Lys-Ala-Ala-Ala-(Glu/Asp). Circular dichroism analysis of different portions of TolA show domain II to be predominantly alpha-helical in structure while domain III contains approximately 10% helical structure.

The tol gene products and the import of macromolecules into Escherichia coli

Genetic studies have identified a number of genes whose products appear to be required for the transport of the group A colicins and the single-stranded DNA of certain filamentous bacteriophages into Escherichia coli. Mutations in these genes allow normal binding of the colicins to their outer-membrane receptors and of the bacteriophage of the tip of specific conjugative pili, but do not allow translocation of the macromolecules to their target. These mutations have been designed 'tolerant' (tol) mutations and the protein products specified by these genes appear to comprise part of a transport system known as the Tol import system. Some of these genes have been isolated, sequenced and their protein products localized to the membranes or periplasm of E. coli. Information is also available regarding the domains of the colicins or phage proteins which interact with the Tol proteins. A preliminary model of the location and possible interactions of the Tol proteins is presented.

Construction, expression and release of hybrid colicins

Colicins A and E1 are two pore-forming colicins sharing homology in their C-terminal domains but not in their N-terminal or central domains. Using site-directed mutagenesis, restriction sites were inserted at the proper locations to allow recombination of these domains. Six different constructs were obtained. All these proteins were expressed in Escherichia coli and properly recognized by monoclonal antibodies directed against epitopes located in different domains of colicin A. Out of the six hybrids, only two were released to the extracellular medium. Immunocytolocalization indicated that some of the hybrids aggregated within the cytoplasm. With some hybrids, the defect in release was related to a defect in synthesis of the lysis protein that normally promotes release.

Phospholipase-A-independent damage caused by the colicin A lysis protein during its assembly into the inner and outer membranes of Escherichia coli

The requirement for the activation of phospholipase A by the colicin A lysis protein (Cal) in the efficient release of colicin A by Escherichia coli cells containing colicin A plasmids was studied. In particular, we wished to determine if this activation is the primary effect of Cal or whether it reflects more generalized damage to the envelope caused by the presence of large quantities of this small acylated protein. E. coli tolQ cells, which were shown to be leaky for periplasmic proteins, were transduced to pldA and then transformed with the recombinant colicin A plasmid pKA. Both the pldA and pldA+ strains released large quantities of colicin A following induction, indicating that in these cells phospholipase A activation is not required for colicin release. This release was, however, still dependent on a functioning Cal protein. The assembly and processing of Cal in situ in the cell envelope was studied by combining pulse-chase labelling with isopycnic sucrose density gradient centrifugation of the cell membranes. Precursor Cal and lipid-modified precursor Cal were found in the inner membrane at early times of chase, and gave rise to mature Cal which accumulated in both the inner and outer membrane after further chase. The signal peptide was also visible on these gradients, and its distribution too was restricted to the inner membrane. Gradient centrifugation of envelopes of cells which were overproducing Cal resulted in very poor separation of the membranes. The results of these studies provide evidence that the colicin A lysis protein causes phospholipase A-independent alterations in the integrity of the E. coli envelope.(ABSTRACT TRUNCATED AT 250 WORDS)

Involvement of OmpF during reception and translocation steps of colicin N entry

[125I]-colicin N binds to OmpF receptor sites (70,000 per cell) with an average Kassoc of 3.2 x 10(6) M-1 at 23 degrees C. Monoclonal antibody directed against a cell-surface-exposed epitope of OmpF is able to complete with the binding of the colicin in vitro and also to protect against colicin N in vivo. OmpF is an absolute requirement for colicin N uptake. OmpC cannot serve as a substitute for OmpF during translocation across the outer membrane under receptor bypass conditions, which is in contrast to colicin A. Colicin N does not cross-react with various monoclonal antibodies directed against colicin A.

Import-defective colicin B derivatives mutated in the TonB box

The pore-forming colicin B is taken up into Escherichia coli by a receptor and TonB-dependent process. The receptor and colicin B both contain a similar amino acid sequence, close to the N-terminal end, termed the TonB box. Point mutations were introduced into the TonB-box region of the colicin B structural gene cba resulting in colicin B derivatives which were partially or totally inactive against E. coli cells. All derivatives still bound to the receptor. An inactive derivative killed cells when translocated across the outer membrane by osmotic shock treatment, and formed pores in planar lipid bilayer membranes identical to the wild-type colicin. Some of the mutations were partially suppressed by mutations in the tonB structural gene. It was concluded that the TonB-box mutations define a region that is involved in the uptake of colicin B across the outer membrane.

Characterization of ompF domains involved in Escherichia coli K-12 sensitivity to colicins A and N

Various ompF-ompC, ompC-ompF, and ompF-ompC-ompF chimeric genes were used to locate the domains of the OmpF protein involved in cellular sensitivity to colicins. Various parts of the porin participate in the entry of colicins. Colicin N receptor activity was found to require three regions: RN1, located between residues 1 and 63; RN2, located between residues 115 and 262; and RN3, located between residues 279 and 297. The central domain from residues 143 to 262 is involved during the translocation step after the binding step. A large region, including residues 1 to 262, is necessary during colicin A entry. The locations and interactions between these domains specifically required for the uptake of colicins to occur are described and discussed with regard to the homology and topology of the OmpC, OmpF, and PhoE porins.

Dissection of functional domains in phage fd adsorption protein. Discrimination between attachment and penetration sites

We constructed a set of deletion mutants in the attachment protein of phage fd. These mutants lack sequences coding for sections in the amino-terminal half. All the mutants that comprise a leader sequence are incorporated into phage particles. Our data strongly suggest a bipartite organization of the amino-terminal domain with (1) a region for receptor recognition and (2) a region that is necessary for penetration of the DNA into the host cell. These regions were mapped. Some evidence suggesting different roles for gene 3 protein in penetration of the outer and inner membrane are discussed. We demonstrate that the phenotypes caused by gene 3 protein in host cells can be subdivided into two groups with different sequence requirements: (1) phenotypes related to outer membrane disturbance; and (2) phenotypes related to the tolQRA transport system.

Interaction of mitochondrial porin with cytosolic proteins

Intracellular phosphorylation is an important step in active uptake and utilization of carbohydrates. For example glucose and glycerol enter the liver cell along the extra intracellular gradient by facilitated diffusion through specific carriers and are concentrated inside the cell by phosphorylation via hexokinase or glycerol kinase. Depending on the function of the respective tissue the uptake of carbohydrates serves different metabolic purposes. In brain and kidney medulla cells which depend on carbohydrates, glucose and glycerol are taken up according to the energy demand. However, in tissues such as muscle which synthesize glycogen or like liver which additionally produce fat from glucose, the uptake of carbohydrates has to be regulated according to the availability of glucose and glycerol. How the reversible coupling of the kinases to the outer membrane pore and the mitochondrial ATP serves to fulfil these specific requirements will be explained as well as how this regulates the carbohydrate uptake in brain according to the activity of the oxidative phosphorylation and how this allows glucose uptake in liver and muscle to persist in the presence of high glucose 6-phosphate without activating the rate of glycolysis.

Uptake across the cell envelope and insertion into the inner membrane of ion channel-forming colicins in E coli

Pore-forming colicins exert their lethal effect on E coli through formation of a voltage-dependent channel in the inner (cytoplasmic-membrane) thus destroying the energy potential of sensitive cells. Their mode of action appears to involve 3 steps: i) binding to a specific receptor located in the outer membrane; ii) translocation across this membrane; iii) insertion into the inner membrane. Colicin A has been used as a prototype of pore-forming colicins. In this review, the 3 functional domains of colicin A respectively involved in receptor binding, translocation and pore formation, are defined. The components of sensitive cells implicated in colicin uptake and their interactions with the various colicin A domains are described. The 3-dimensional structure of the pore-forming domain of colicin A has been determined recently. This structure suggests a model of insertion into the cytoplasmic membrane which is supported by model membrane studies. The role of the membrane potential in channel functioning is also discussed.

In vitro peptidoglycan synthesis by envelopes from Escherichia coli tolM mutants is inhibited by colicin M

An in vitro peptidoglycan synthesis reaction was employed to further characterize the role of the tolM product in colicin M-induced inhibition of peptidoglycan synthesis. It was found that the tolM product is not the colicin M target and that this gene product does not play a role in the interaction of the colicin with its target. Colicin M remained associated with envelopes prepared from colicin-treated tolM mutants. These findings suggested that the tolM product most likely is involved with the internalization of colicin M.

Nucleotide sequences of the tolA and tolB genes and localization of their products, components of a multistep translocation system in Escherichia coli

Various mutations in the tolQRAB gene cluster of Escherichia coli render the bacteria tolerant to high concentrations of the E, A, or K colicins as well as tolerant to infection by the single-stranded filamentous bacteriophage. The nucleotide sequence of a 2.8-kilobase fragment containing the tolA and tolB genes was determined. This sequence predicts TolA to be a 421-amino-acid protein of molecular mass 44,190 daltons. Studies using minicells show it to be associated with the inner membrane, presumably via a 21-amino-acid hydrophobic sequence between residues 13 and 35. The remaining 387 residues on the carboxyl side of this region are located in the periplasm. Within this region of TolA is a 230-residue portion that is predicted to form a very long helical segment. This region is rich in alanine, lysine, and glutamic and aspartic acids. The TolB protein is predicted to contain 431 amino acids. Localization studies using minicells show two proteins encoded by this open reading frame. The larger protein of 47.5 kilodaltons appears to be associated with the membrane fractions. The smaller protein is 43 kilodaltons in size and is found with the periplasmic components of the cell.

Pore-forming properties of the adsorption protein of filamentous phage fd

The gene 3-encoded adsorption protein (g3p) of filamentous phage fd has been purified to homogeneity by using high-performance liquid chromatography. Removal of SDS from the SDS-solubilized g3p results in spontaneous oligomerization of the g3p. Reconstitution into artificial lipid bilayer membranes shows that the oligomer forms large aqueous pores that remain open for seconds and are insensitive to changes in membrane potential. The estimated diameter of the pores suggest that they are large enough to allow passage of phage single-stranded DNA. The implications of these findings for phage infection are discussed.

The structurally related exbB and tolQ genes are interchangeable in conferring tonB-dependent colicin, bacteriophage, and albomycin sensitivity

Double exbB tolQ mutants of Escherichia coli were completely resistant to bacteriophages T1 and phi 80, in contrast to strains with exbB or tolQ mutations, which were sensitive. Cells carrying mutations in exbB were partially tolerant to colicins B, D, and M and became fully tolerant by the introduction of tolQ mutations. This suggested involvement of both exbB and tolQ in tonB-dependent uptake.

Cloning of the excC and excD genes involved in the release of periplasmic proteins by Escherichia coli K12

Strains of Escherichia coli K12 carrying a tolA, tolB, lky or exc mutation located at min 16.5 on the genetic map released periplasmic proteins into the extracellular medium. Wild-type genes defined by these mutations have been cloned from E. coli genomic bank made with plasmid pBR328. Subcloning experiments and complementation studies showed that lky and exc mutations were located either in the previously described tolA and tolB genes or in the newly characterized excC and excD genes. Using minicells, excC and excD gene products were identified as proteins with a molecular mass of 19 and 21 kDa, respectively.

Localization and assembly into the Escherichia coli envelope of a protein required for entry of colicin A

Mutations in tolQ, previously designated fii, render cells tolerant to high concentrations of colicin A. In addition, a short deletion in the amino-terminal region of colicin A (amino acid residues 16 to 29) prevents its lethal action, although this protein can still bind the receptor and forms channels in planar lipid bilayers in vitro. These defects in translocation across the outer membrane in the tolQ cells or the colicin A mutant cannot be bypassed by osmotic shock. The TolQ protein, which is constitutively expressed at a low level, was studied in recombinant plasmid constructs allowing the expression of various TolQ fusion proteins under the control of the inducible caa promoter. The TolQ protein was thus "tagged" with an epitope from the colicin A protein for which a monoclonal antibody is available. A fusion protein containing the entire TolQ protein plus the 30 N-terminal residues of colicin A was shown to complement the tolQ mutation. Pulse-chase labeling followed by gradient fractionation indicated that the bulk of the overproduced fusion protein was rapidly incorporated into the inner membrane, with small amounts localized to regions corresponding to the attachment sites between inner and outer membranes and to the outer membrane itself. However, most of the protein was rapidly degraded, leaving only that localized to the attachment sites and the outer membrane remaining at very late times of chase.

Isolation and molecular and functional properties of the amino-terminal domain of colicin A

A plasmid was constructed which allowed easy and efficient production and purification of the NH2-terminal domain of colicin A. In only three steps, an homogenous 18-kDa polypeptide was obtained. The NH2- and COOH-terminal sequences of the protein were determined and showed that it corresponded to the NH2-terminal 171 amino acid residues of the 63-kDa colicin A. Although colicin A is a highly asymmetric protein, hydrodynamic studies indicated that the NH2-terminal domain (designated AT) has a globular structure. This fragment is not the receptor-binding domain of colicin A but is required for the transfer of colicin A across the outer membrane of sensitive cells. However, it has a low affinity for phospholipid films and this affinity is not pH-dependent, in contrast to that of colicin A.