Preparation of the FhuA (TonA) receptor protein from cell envelopes of an overproducing strain of Escherichia coli K-12

Redesign of a plugged beta-barrel membrane protein

The redesign of biological nanopores is focused on bacterial outer membrane proteins and pore-forming toxins, because their robust β-barrel structure makes them the best choice for developing stochastic biosensing elements. Using membrane protein engineering and single-channel electrical recordings, we explored the ferric hydroxamate uptake component A (FhuA), a monomeric 22-stranded β-barrel protein from the outer membrane of Escherichia coli. FhuA has a luminal cross-section of 3.1 × 4.4 nm and is filled by a globular N-terminal cork domain. Various redesigned FhuA proteins were investigated, including single, double, and multiple deletions of the large extracellular loops and the cork domain. We identified four large extracellular loops that partially occlude the lumen when the cork domain is removed. The newly engineered protein, FhuAΔC/Δ4L, was the result of a removal of almost one-third of the total number of amino acids of the wild-type FhuA (WT-FhuA) protein. This extensive protein engineering encompassed the entire cork domain and four extracellular loops. Remarkably, FhuAΔC/Δ4L forms a functional open pore in planar lipid bilayers, with a measured unitary conductance of ∼4.8 nanosiemens, which is much greater than the values recorded previously with other engineered FhuA protein channels. There are numerous advantages and prospects of using such an engineered outer membrane protein not only in fundamental studies of membrane protein folding and design, and the mechanisms of ion conductance and gating, but also in more applicative areas of stochastic single-molecule sensing of proteins and nucleic acids.

An 8-A projected structure of FhuA, A "ligand-gated" channel of the Escherichia coli outer membrane

The structure of FhuA, a siderophore and phage receptor in the outer membrane of Escherichia coli, has been investigated by electron crystallography. Bidimensional crystals of hexahistidine-tagged FhuA protein solubilized in N,N-dimethyldodecylamine-N-oxide were produced after detergent removal with polystyrene beads. Frozen-hydrated crystals (unit cell dimensions of a = 124 A, b = 98 A, gamma = 90 degrees ) exhibited a p22121 plane group symmetry. A projection map at 8 A resolution showed the presence of dimeric ring-like structures with an elliptical shape (48 x 40 A). Each monomer was composed of a ring of densities with a radial width of 8-10 A corresponding to a cylinder of beta sheets. Few densities are present inside the barrel, leaving a central channel approximately 25 A in diameter. A projection map of FhuA at 15 A resolution, which was calculated from negatively stained preparations, demonstrated that most of the central channel was masked by extramembrane domains. This map also revealed an asymmetric distribution of extramembrane domains in FhuA, with large domains located mainly on one side of the molecule. Comparison with density maps derived from recent atomic structure allowed further interpretation of the electron microscopy projection structures with regard to long hydrophilic loops governing the selectivity and opening of the channel.

Colicin U, a novel colicin produced by Shigella boydii

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

Oligomeric states and siderophore binding of the ligand-gated FhuA protein that forms channels across Escherichia coli outer membranes

The channel-forming FhuA protein, which translocates ferrichrome across Escherichia coli outer membranes, binds 1 mol ligand/mol monomer in detergent solution. The protein is homogenous and migrates as a single band with a mobility corresponding to 77 kDa in SDS/PAGE electrophoresis. Analytical ultracentrifugation revealed a monodisperse species (s(20,w) = 3.8 S) with a mass of 77,800 +/- 3200 Da. The properties of ligand binding, determined by two independent methods, revealed one binding site/monomer, but are complicated by a pronounced convexity of the Scatchard plot and a Hill coefficient calculated to be 2.5. This strongly suggests that oligomeric species are present. Cross-linking agents revealed the existence of possibly transient, mostly dimeric and trimeric species. The difference between the FhuA protein in detergent solution and in its native membrane environment may be related to the removal of lateral pressure that exists in situ.

Reconstitution of FhuA, an Escherichia coli outer membrane protein, into liposomes. Binding of phage T5 to Fhua triggers the transfer of DNA into the proteoliposomes

The Escherichia coli outer membrane protein FhuA catalyzes the transport of ferrichrome and is the receptor of bacteriophage T5. Purified FhuA was reconstituted into liposomes. The size of the proteoliposomes and the distribution of the proteins in the vesicles were determined by freeze fracture electron microscopy. Unilamellar vesicles with a diameter larger than 200 nm were observed frequently. FhuA was symetrically oriented in the proteoliposomes. Reconstituted FhuA was functional as binding of phage T5 induced the release of phage DNA and its transfer inside the vesicles.

Purification and structural and functional characterization of FhuA, a transporter of the Escherichia coli outer membrane

The Escherichia coli outer membrane ferrichrome transporter FhuA was purified chromatographically in a neutral detergent (octyl glucoside or dodecyl maltoside). The amount of dodecyl maltoside bound to the protein (1.2 +/- 0.15 g/g of FhuA) and the Stokes radius of the FhuA-dodecyl maltoside complex (Rs = 4.2 nm) were determined using size exclusion chromatography. Sedimentation equilibrium and velocity experiments indicated that the FhuA preparation was monodisperse and that the protein was monomeric. The value found for the frictional coefficient of the protein-detergent complex (1.18) suggested a globular shape for the complex. Sedimentation experiments gave values for the molecular mass of the FhuA-dodecyl maltoside complex (180 kDa) and for the Stokes radius in complete agreement with those calculated from size exclusion chromatography. The circular dichroism spectrum indicated a 51% beta-sheet content. Functionality of the purified protein was assessed from fluorescence measurements using the DNA probe YO-PRO-1. Interaction of nM concentrations of FhuA with bacteriophage T5 resulted in the release of 90 +/- 8% of the phage DNA. The limiting step in DNA ejection was binding of the phage to its receptor. Release of DNA took place in a few seconds. Ferrichrome (0.8 microM) competed with the phage for binding to FhuA and prevented DNA ejection.

FhuA, a transporter of the Escherichia coli outer membrane, is converted into a channel upon binding of bacteriophage T5

The Escherichia coli outer membrane protein FhuA catalyzes the transport of Fe3+(-)ferrichrome and is the receptor of phage T5 and phi 80. The purified protein inserted into planar lipid bilayers showed no channel activity. Binding of phage T5 and FhuA resulted in the appearance of high conductance ion channels. The electrophysiological characteristics of the channels (conductance, kinetic behavior, substates, ion selectivity including the effect of ferrichrome) showed similarities with those of the channel formed by a FhuA derivative from which the 'gating loop' (delta 322-355) had been removed. binding of phage T5 to FhuA in E.coli cells conferred SDS sensitivity to the bacteria, suggesting that such channels also exist in vivo. These data suggest that binding of T5 to loop 322-355 of FhuA, which constitutes the T5 binding site, unmasks an inner channel in FhuA. Both T5 and ferrichrome bind to the closed state of the channel but only T5 can trigger its opening.

Topological analysis of the Escherichia coli ferrichrome-iron receptor by using monoclonal antibodies

Ferrichrome-iron transport in Escherichia coli is initiated by the outer membrane receptor FhuA. Thirty-five anti-FhuA monoclonal antibodies (MAbs) were isolated to examine the surface accessibility of FhuA sequences and their contribution to ligand binding. The determinants of 32 of the MAbs were mapped to eight distinct regions in the primary sequence of FhuA by immunoblotting against (i) five internal deletion FhuA proteins and (ii) four FhuA peptides generated by cyanogen bromide cleavage. Two groups of MAbs bound to FhuA in outer membrane vesicles but not to intact cells, indicating that their determinants, located between residues 1 and 20 and 21 and 59, are exposed to the periplasm. One of the 28 strongly immunoblot-reactive MAbs bound to FhuA on intact cells in flow cytometry, indicating that its determinant, located between amino acids 321 and 381, is cell surface exposed. This MAb and four others which in flow cytometry bound to cells expressing FhuA were tested for the ability to block ligand binding. While no MAb inhibited growth promotion by ferrichrome or cell killing by microcin 25, some prevented killing by colicin M and were partially able to inhibit the inactivation of T5 phage. These data provide evidence for spatially distinct ligand binding sites on FhuA. The lack of surface reactivity of most of the immunoblot-reactive MAbs suggests that the majority of FhuA sequences which lie external to the outer membrane may adopt a tightly ordered organization with little accessible linear sequence.

Overproduced and purified receptor binding protein pb5 of bacteriophage T5 binds to the T5 receptor protein FhuA

A promotor-less oad gene of bacteriophage T5, encoding the receptor binding protein pb5, was cloned into pT7-3 under the control of phage T7 promoter phi 10. Induction with IPTG resulted in enhanced production of pb5. Upon fractionation of the producing cells, most of the overproduced pb5 was found in the membrane fraction, which was most likely due to aggregation of the protein. The minor, soluble fraction of pb5 specifically inhibited adsorption of T5 to its FhuA receptor protein. Inhibition was also seen with trace amounts of pb5, and binding of pb5 to FhuA appeared to be almost irreversible. Purification of pb5 from the cytosolic fraction was performed by FPLC using a MonoQ column. pb5, which did not bind to the matrix of the column, was obtained in almost pure form. The purified protein also inhibited T5 adsorption.

Identification of receptor binding sites by competitive peptide mapping: phages T1, T5, and phi 80 and colicin M bind to the gating loop of FhuA

Previously we proposed a transmembrane model of the FhuA receptor protein in the outer membrane of Escherichia coli. Removal of the largest loop at the cell surface converted the FhuA transport protein into an open channel and rendered cells resistant to the FhuA-specific phages T1, T5, and phi 80 and to colicin M. In the present study we employed acetylated hexapeptide amides covering the entire surface loop to investigate binding of the phages and of colicin M. Competitive peptide mapping proved to be a powerful technique to uncover three ligand binding sites within a region of 34 amino acid residues. Hexapeptides derived from three specific regions of the surface loop inhibited infection of cells by the phages and killing by colicin M. Two of these regions were common among all four FhuA ligands. Electron microscopy of phage T5 revealed that one inhibitory peptide triggered a strong conformational change leading to the release of DNA from the phage head. These results suggest that the FhuA gating loop is the target for specific binding of phages T1, T5, and phi 80 and colicin M.

Inactivation of FhuA at the cell surface of Escherichia coli K-12 by a phage T5 lipoprotein at the periplasmic face of the outer membrane

Inactivation of phage T5 by lysed cells after phage multiplication is prevented by a phage-encoded lipoprotein (Llp) that inactivates the FhuA outer membrane receptor protein (K. Decker, V. Krauel, A. Meesmann, and K. Heller, Mol. Microbiol. 12:321-332, 1994). Using FhuA derivatives carrying insertions of 4 and 16 amino acid residues and point mutations, we determined whether FhuA inactivation is caused by binding of Llp to FhuA and which regions of FhuA are important for inactivation by Llp. Cells expressing Llp were resistant not only to phage T5 but to all FhuA ligands tested, such as phage phi 80, colicin M, and albomycin, and they were strongly reduced in the uptake of ferrichrome. Most of the FhuA derivatives which were not affected by Llp were, according to a previously published FhuA transmembrane topology model, located in periplasmic turns and in the TonB box close to the periplasm. Since the ligands bind to the cell surface, interaction of FhuA with Llp in the periplasm may induce a FhuA conformation which impairs binding of the ligands. This conclusion was supported by the increase rather than decrease of colicin M sensitivity of two mutants in the presence of Llp. The only Llp-resistant FhuA derivatives with mutations at the cell surface contained insertions of 16 residues in the loop that determines the permeability of the FhuA channel and serves as the principal binding site for all FhuA ligands. This region may be inactivated by steric hindrance in that a portion of Llp penetrates into the channel. Outer membranes prepared with 0.25% Triton X-100 from cells expressing Llp contained inactivated FhuA, suggesting Llp to be an outer membrane protein whose interaction with FhuA was not abolished by Triton X-100. Llp solubilized in 1.1% octylglucoside prevented T5 inactivation by FhuA dissolved in octylglucoside.

Lytic conversion of Escherichia coli by bacteriophage T5: blocking of the FhuA receptor protein by a lipoprotein expressed early during infection

The nucleotide sequence of the region between the oad gene, encoding the host specificity protein, and the right-terminal repetition of bacteriophage T5 DNA was determined. Five small open reading frames, the first of which was called llp, were detected, which apparently formed an operon transcribed from a promoter that overlapped the oad promoter. Both promoters were confirmed by primer extension assays. Using mRNA isolated at different times after T5 infection, the llp and oad promoters were identified as early and late promoters, respectively. The N-terminus of the llp gene product possess a signal sequence and a processing site characteristic of lipoproteins. After subcloning and expression of llp, its product Llp was identified as a 7.8 kDa polypeptide. Acylation of Llp was confirmed by addition of globomycin, which resulted in the accumulation of the unprocessed precursor form. FhuA+ cells synthesizing Llp were resistant to phage T5. Resistance was caused by inhibition of adsorption of T5 to its FhuA receptor protein. Resistance could be overcome by derepression of fhuA transcription, suggesting a blocking of FhuA by direct interaction with Llp. Since Llp-mediated T5 resistance has several aspects in common with the phenomenon of lysogenic conversion, we suggest that it should be called lytic conversion.

Purification of outer membrane iron transport receptors from Escherichia coli by fast protein liquid chromatography: FepA and FecA

Fast protein liquid chromatography (FPLC) with DEAE-Sepharose Fast Flow, PBE-94 and Q-Sepharose Fast Flow columns are applied to the purification of the ferric enterobactin protein receptor (FepA). The apparent single band of FepA on SDS-PAGE is isolated and purified into two proteins with very similar molecular weights. The two proteins are identified to be FepA and ferric citrate protein receptor (FecA) by N-terminus amino acid determination and a computer search with the Gene Bank file. The assay of binding activities of these proteins shows that both FepA and FecA bind ferric enterobactin, with the former having about double the activity of the latter. Competition studies shows that Fe-MECAM is competitively bound to both proteins and that ferric parabactin only slightly competes with [55Fe]ferric enterobactin. It is found that ferrichrome A has no effect on the binding of the receptor proteins with ferric enterobactin.

Insertion derivatives containing segments of up to 16 amino acids identify surface- and periplasm-exposed regions of the FhuA outer membrane receptor of Escherichia coli K-12

The FhuA receptor in the outer membrane of Escherichia coli K-12 is involved in the uptake of ferrichrome, colicin M, and the antibiotic albomycin and in infection by phages T1, T5, and phi 80. Fragments of up to 16 amino acid residues were inserted into FhuA and used to determine FhuA active sites and FhuA topology in the outer membrane. For this purpose antibiotic resistance boxes flanked by symmetric polylinkers were inserted into fhuA and subsequently partially deleted. Additional in-frame insertions were generated by mutagenesis with transposon Tn1725. The 68 FhuA protein derivatives examined contained segments of 4, 8, 12, 16, and 22 additional amino acid residues at 34 different locations from residues 5 to 646 of the mature protein. Most of the FhuA derivatives were found in normal amounts in the outer membrane fraction. Half of these were fully active toward all ligands, demonstrating proper insertion into the outer membrane. Seven of the 12- and 16-amino-acid-insertion derivatives (at residues 378, 402, 405, 415, 417, 456, and 646) were active toward all of the ligands and could be cleaved by subtilisin in whole cells, suggesting a surface location of the extra loops at sites which did not affect FhuA function. Two mutants were sensitive to subtilisin (insertions at residues 511 and 321) but displayed a strongly reduced sensitivity to colicin M and to phages phi 80 and T1. Four of the insertion derivatives (at residues 162, 223, 369, and 531) were cleaved only in spheroplasts and probably form loops at the periplasmic side of the outer membrane. The number and size of the proteolytic fragments indicate cleavage at or close to the sites of insertion, which has been proved for five insertions by amino acid sequencing. Most mutants with functional defects were affected in their sensitivity to all ligands, yet frequently to different degrees. Some mutants showed a specifically altered sensitivity to a few ligands; for example, mutant 511-04 was partially resistant only to colicin M, mutant 241-04 was reduced in ferrichrome and albomycin uptake and showed a reduced colicin M sensitivity, and mutant 321-04 was fully resistant to phage T1 and partially resistant to phage phi 80. The altered residues define preferential binding sites for these ligands. Insertions of 4 to 16 residues at positions 69, 70, 402, 530, 564, and 572 resulted in strongly reduced amounts of FhuA in the outer membrane fraction, varying in function from fully active to inactive. These results provide the basis for a model of FhuA organization in the outer membrane.

Cloning and characterization of the ferric enterobactin receptor gene (pfeA) of Pseudomonas aeruginosa

Pseudomonas aeruginosa K407, a mutant lacking a high-affinity 80,000-molecular-weight ferric enterobactin receptor protein (80K protein), exhibited poor growth (small colonies) on iron-deficient succinate minimal medium containing ethylenediamine-di(o-hydroxyphenylacetic acid) (EDDHA) and enterobactin. The gene encoding the ferric enterobactin receptor was cloned by complementation of this growth defect. The complementing DNA was subsequently localized to a 7.1-kilobase-pair (kb) SstI-HindIII fragment which was able to restore synthesis of the 80K protein in strain K407 and also to direct the synthesis of high levels of a protein of the same molecular weight in the outer membranes of Escherichia coli fepA strains MT912 and IR20. Moreover, the fragment complemented the fepA mutation in MT912, restoring both growth in EDDHA-containing medium and enterobactin-dependent uptake of 55Fe3+. Expression of the P. aeruginosa receptor in E. coli IR20 was shown to be regulated by both iron and enterobactin. The complementing DNA was further localized to a 5.3-kb SphI-SstI fragment which was then subjected to deletion analysis to obtain the smallest fragment capable of directing the synthesis of the 80K protein in the outer membrane of strain K407. A 3.2-kb DNA fragment that restored production of the receptor in strain K407 was subsequently isolated. The fragment also directed synthesis of the protein in E. coli MT912 but at levels much lower than those previously observed. Nucleotide sequencing of the fragment revealed an open reading frame (designated pfeA for Pseudomonas ferric enterobactin) of 2,241 bp capable of encoding a 746-amino-acid protein with a molecular weight of 80,967. The PfeA protein showed more than 60% homology to the E. coli FepA protein. Consistent with this, the two proteins showed significant immunological cross-reactivity.

Structure and function of X-Pro dipeptide repeats in the TonB proteins of Salmonella typhimurium and Escherichia coli

The TonB protein is required for several outer membrane transport processes in bacteria. A short 33-residue peptide segment of TonB has been studied by 1H and 13C nuclear magnetic resonance spectroscopy. The sequence of this peptide segment contains multiple Glu-Pro and Lys-Pro dipeptide repeats that maintain rigid, elongated structures and flank a short connecting segment that adopts a beta-strand configuration. This TonB peptide is shown to interact specifically with the FhuA protein, the outer membrane receptor for ferrichrome-iron, providing the first direct evidence that the TonB protein interacts with outer membrane receptors. Interaction with the FhuA protein involves the extended structural element containing positively charged Lys-Pro repeats, and suggests a functional role for this segment of the TonB protein. As TonB is anchored in the cytoplasmic membrane the protein must, uniquely, span the periplasm. These data, together with studies described in the accompanying paper, suggest a model by which TonB serves to transduce conformational information over extended distances, from the cytoplasmic membrane to the outer membrane.

TonB protein of Salmonella typhimurium. A model for signal transduction between membranes

The tonB gene product is required for several outer membrane transport processes in bacteria. The tonB gene from Salmonella typhimurium was sequenced and found to be similar to that of Escherichia coli. The TonB protein is highly proline-rich and includes an unusual segment consisting of multiple X-Pro dipeptide repeats. A synthetic peptide corresponding to this segment has been used to raise anti-TonB antibodies. TonB was shown to be associated with the cytoplasmic membrane, apparently anchored via a single hydrophobic N-terminal segment. Protease accessibility studies, and the use of a series of TonB-beta-lactamase fusions, showed that the rest of the TonB protein is periplasmic. Unusually, export of TonB is not accompanied by cleavage of the N-terminal signal peptide. In the accompanying paper, we show that TonB interacts directly with the outer membrane FhuA (TonA) receptor. Thus, TonB must span the periplasm, providing a link between the cytoplasmic membrane and receptors in the outer membrane. On the basis of these data, and those published by other laboratories, we propose a model whereby TonB serves as a "mechanical" linkage that, by transmitting protein conformational changes from the cytoplasmic membrane across the periplasm, acts as a means of coupling energy to outer membrane transport processes. Such a mechanism has general implications for signal transduction within and between proteins.

Insertion mutagenesis of the gene encoding the ferrichrome-iron receptor of Escherichia coli K-12

The ferrichrome-iron receptor of Escherichia coli K-12 encoded by the fhuA gene is a multifunctional outer membrane receptor with an Mr of 78,000. It is required for the binding and uptake of ferrichrome and is the receptor for bacteriophages T5, T1, phi 80, and UC-1 as well as for colicin M. The fhuA gene was cloned into pBR322, and the recombinant plasmid pGC01 was mutagenized by the insertion of 6-base-pair TAB (two amino acid Barany) linkers into CfoI and HpaII restriction sites distributed throughout the coding region. A library of 18 TAB linker insertions in fhuA was generated; 8 of the mutations were at CfoI sites and 10 were at HpaII sites. All mutations inserted a hexamer that encoded a unique SacI site. A large deletion in fhuA was also isolated by TAB linker mutagenesis. Except for the deletion mutant, all of the linker insertion mutant FhuA proteins were found in the outer membrane in amounts similar to those found in the wild type. Five of the linker insertion mutants were susceptible to cleavage by endogenous proteolytic activity: a second FhuA-related band that migrated at approximately 72 kilodaltons could be detected on Coomassie blue-stained gels and on Western blots (immunoblots) by using a carboxy terminus-specific anti-peptide antibody. Receptor functions were measured with the mutated genes present in a single copy on the chromosome. Some of the receptors conferred wild-type phenotypes: they demonstrated growth promotion by ferrichrome and the same efficiency of plating as that of wild-type FhuA; killing by colicin M was also unaffected. Several mutants were altered in their sensitivities to the lethal agents. TAB linker insertions after amino acids 69 and 128 abolished all receptor functions. Phage T5 id not bind to these mutant FhuA proteins in detergent extracts. The deletion mutant was also defective in all FhuA functions. Sensitivity to the lethal agents of cellsl that expressed mutant FhuAs with insertions after amino acids 59 and 135 was reduced by several orders of magnitude. Insertion at other selected sites decreased some or all receptor functions only slightly. An insertion after amino acid 321 selectively eliminated ferrichrome growth promotion. Finally, a strain carrying a mutant fhuA gene on the chromosome in which the linker insertion occurred after amino acid 82 showed a tonB phenotype. These subtle perturbations that were introduced into the FhuA protein resulted in changes in its stability and in the binding and uptake of its cognate ligands.

Irreversible binding of bacteriophage T5 to its FhuA receptor protein is associated with covalent cross-linking of 3 copies of tail protein pb4

Irreversible binding of bacteriophage T5 to its FhuA receptor protein is characterized by a high activation energy, typical for reactions where covalent bonds are formed [Zarnitz, M.L. and Weidel, W. (1963) Z. Naturforsch. 18b, 276-280]. Upon binding of radiolabeled T5 phages to FhuA formation of a new protein of 250 kDa was observed. Using electrophoretical and Western blotting techniques this protein was shown to be formed by cross-linking of 3 copies of tail protein pb4, rather than by cross-linking of FhuA and the receptor-binding protein.

Involvement of ExbB and TonB in transport across the outer membrane of Escherichia coli: phenotypic complementation of exb mutants by overexpressed tonB and physical stabilization of TonB by ExbB

The exb locus in Escherichia coli consists of two genes, termed exbB and exbD. Exb functions are related to TonB function in that most TonB-dependent processes are enhanced by Exb. Like tonB mutants, exb mutants were resistant to colicin M and albomycin but, in contrast to tonB mutants, showed only reduced sensitivity to colicins B and D. Overexpressed tonB on the multicopy vector pACYC177 largely restored the sensitivity of exb mutants to colicins B, D, and M but only marginally increased sensitivity to albomycin. Suppression of the btuB451 mutation in the structural gene for the vitamin B12 outer membrane receptor protein by a mutation in tonB occurred only in an exb+ strain. Degradation of the unstable overproduced TonB protein was prevented by overproduced ExbB protein. The ExbB protein also stabilized the ExbD protein. Pulse-chase experiments with radiolabeled ferrichrome revealed release of ferrichrome from exbB, tonB, and fhuC mutants, showing that ferrichrome had not crossed the cytoplasmic membrane. It is concluded that the ExbB and ExbD proteins contribute to the activity of TonB and, like TonB, are involved in receptor-dependent transport processes across the outer membrane.

Iron-hydroxamate transport into Escherichia coli K12: localization of FhuD in the periplasm and of FhuB in the cytoplasmic membrane

The fhuB, fhuC and fhuD genes encode proteins which catalyze transport of iron(III)-hydroxamate compounds from the periplasm into the cytoplasm of Escherichia coli. The fhuB, C, D genes were cloned downstream of a strong phage T7 promoter and transcribed by T7 RNA polymerase. The overexpressed FhuD protein appeared in two forms of 31 and 28 kDa and was released upon conversion of vegetative cells into spheroplasts, suggesting synthesis of FhuD as a precursor and export into the periplasm. The very hydrophobic FhuB protein was found in the cytoplasmic membrane. These properties, together with the previously found homologies in the FhuC protein to ATP-binding proteins, display the characteristics of a periplasmic binding protein dependent transport system across the cytoplasmic membrane. The molecular weight of FhuB and the sequence of fhuC, as previously published by us, was confirmed. FhuB exhibited double the size of most hydrophobic proteins of such systems and showed homology between the amino- and carboxy-terminal halves of the protein, indicating duplication of an original gene and subsequent fusion of the two DNA fragments.

Export and activity of hybrid FhuA'-'Iut receptor proteins and of truncated FhuA' proteins of the outer membrane of Escherichia coli

The FhuA protein in the outer membrane of Escherichia coli serves as a multifunctional receptor for the phages T5, T1, phi 80, for colicin M, for ferrichrome (Fe3+-siderophore) and for the structurally related antibiotic, albomycin. To determine structural domains required for these receptor functions and for export, a fusion protein between FhuA and Iut (receptor for Fe3+-aerobactin and cloacin DF13) was constructed. In the FhuA'-'Iut hybrid protein, 24 amino acids of FhuA were replaced by 19 amino acids, 18 of which were from Iut. The number of plaque forming units of phage T5 and T1 on cells expressing FhuA'-'Iut was nearly as high as on cells expressing plasmid-encoded wild-type FhuA. However, 10(7)-fold higher concentrations of phage phi 80 and 10(3) times more colicin M were required to obtain a zone of growth inhibition. Truncated FhuA' proteins in which the last 24 amino acids at the carboxy-terminus were replaced by 16 (FhuA'2) or 3 (FhuA'T) amino acids could hardly be detected on polyacrylamide electrophoretograms of outer membrane proteins, due to proteolytic degradation. Sensitivity of cells expressing FhuA'2 to phage T5 and T1 was reduced by several orders of magnitude and sensitivity to phage phi 80 and colicin M was totally abolished. In contrast, cells expressing FhuA'T were nearly as sensitive to pahge T5, T1, and phi 80 and to colicin M as cells containing FhuA'-'Iut. None of the constructs could grow on ferrichrome as sole iron source and none was sensitive to albomycin.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification and crystallization of ferric enterobactin receptor protein, FepA, from the outer membranes of Escherichia coli UT5600/pBB2

The ferric enterobactin receptor protein, FepA, was isolated and purified from the outer membranes of a genetically transformed strain of Escherichia coli (UT5600/pBB2) using anion-exchange chromatography, chromatofocusing and gel filtration. The purified protein was found to crystallize from 25 mM sodium phosphate buffer in the presence of 0.8% beta-D-octylglucoside under a range of conditions. The protein formed mostly small rods and needle-shaped crystals in the hanging drop method.

Probing FhuA'-'PhoA fusion proteins for the study of FhuA export into the cell envelope of Escherichia coli K12

The FhuA protein (formerly TonA) is located in the outer membrane of Escherichia coli K12. Fusions between fhuA and phoA genes were constructed. They determined proteins containing a truncated but still active alkaline phosphatase of constant size and a variable FhuA portion which ranged from 11%-90% of the mature FhuA protein. The fusion sites were nearly randomly distributed along the FhuA protein. The FhuA segments directed the secretion of the truncated alkaline phosphatase across the cytoplasmic membrane. The fusion proteins were proteolytically degraded up to the size of alkaline phosphatase and no longer reacted with anti-FhuA antibodies. The fusion proteins were more stable in lon and pep mutants lacking cytoplasmic protease and peptidases, respectively. The larger fusion proteins above a molecular weight of 64,000 dalton were predominantly found in the outer membrane fraction. They were degraded by trypsin when cells were converted to spheroplasts so that trypsin gained access to the periplasm. In contrast, FhuA protein in the outer membrane was largely resistant to trypsin. It is concluded that the larger FhuA'-'PhoA fusion proteins were associated with, but not properly integrated into, the outer membrane.

Nucleotide sequence of the fhuC and fhuD genes involved in iron (III) hydroxamate transport: domains in FhuC homologous to ATP-binding proteins

The transport of Fe3+ into cells of Escherichia coli occurs via siderophores and the uptake through the outer membrane of three Fe3+-siderophore compounds containing hydroxamate residues requires three specific receptor proteins. In contrast, transport through the cytoplasmic membrane is catalysed by three common proteins encoded by the fhuB, fhuC and fhuD genes. The nucleotide sequence of a DNA fragment containing the fhuC and fhuD genes has been determined: the open reading frame of fhuC contains 795 nucleotides which encode a polypeptide with a molecular weight of 29,255 and the largest open reading frame of the fhuD region comprises 888 nucleotides. However, we propose that translation of fhuD initiates at the fourth potential start codon resulting in a polypeptide with a molecular weight of 28,282. Both proteins are moderately nonpolar and membrane-bound. They lack obvious signal sequences. Segments of the FhuC protein display strong homology to ATP-binding proteins, suggesting a function in Fe3+ uptake similar to the ATP-binding proteins of transport systems that depend on periplasmic proteins. This study completes the nucleotide sequence of the fhu operon which consists of the four genes fhuA fhuC fhuD fhuB arranged in this order on the E. coli chromosome and transcribed from fhuA to fhuB.

Overproduction of the proFhuA outer membrane receptor protein of Escherichia coli K-12: isolation, properties, and immunocytochemical localization at the inner side of the cytoplasmic membrane

The fhu operon of Escherichia coli K-12 comprises four genes, termed fhuA,C,D,B, which are involved in the uptake of iron-hydroxamate compounds. The fhuA gene encodes the outer membrane receptor protein. Cells that contained three copies of the fhuACD fragment on the thermoamplifiable plasmid pHK232 accumulated at 37 degrees C large amounts of the proFhuA protein. Most of the overproduced proFhuA protein was not translocated into the outer membrane but instead precipitated at the cytoplasmic side of the inner membrane, presumably at the sites of synthesis. Despite inhibition of export proFhuA synthesis continued. The precipitate formed was sedimented by centrifugation at 8,000 x g. The proFhuA protein could be solubilized in 1% sodium dodecyl sulfate. Replacement of sodium dodecyl sulfate by Triton X-100 resulted in a proFhuA protein which exhibited 10% of the phage T5 binding activity of renatured mature FhuA protein. Binding of the phage T5 was inhibited by the FhuA-specific ligands ferrichrome, albomycin and colicin M. Limited proteolysis of the isolated pro- and mature form of the FhuA protein with trypsin yielded similar oligopeptide patterns. Addition of ferrichrome affected trypsin cleavage of both proteins in the same way. The common proteolytic intermediates together with phage inactivation indicate a similar conformation of the pro- and mature form.