Properties and proteolysis of ferric enterobactin outer membrane receptor in Escherichia coli K12

ROS-based lethality of Caenorhabditis elegans mitochondrial electron transport mutants grown on Escherichia coli siderophore iron release mutants

Caenorhabditis elegans consumes bacteria, which can supply essential vitamins and cofactors, especially for mitochondrial functions that have a bacterial ancestry. Therefore, we screened the Keio Escherichia coli knockout library for mutations that induce the C. elegans hsp-6 mitochondrial damage response gene, and identified 45 E. coli mutations that induce hsp-6::gfp We tested whether any of these E. coli mutations that stress the C. elegans mitochondrion genetically interact with C. elegans mutations in mitochondrial functions. Surprisingly, 4 E. coli mutations that disrupt the import or removal of iron from the bacterial siderophore enterobactin were lethal in combination with a collection of C. elegans mutations that disrupt particular iron-sulfur proteins of the electron transport chain. Bacterial mutations that fail to synthesize enterobactin are not synthetic lethal with these C. elegans mitochondrial mutants; it is the enterobactin-iron complex that is lethal in combination with the C. elegans mitochondrial mutations. Antioxidants suppress this inviability, suggesting that reactive oxygen species (ROS) are produced by the mutant mitochondria in combination with the bacterial enterobactin-iron complex.

The regulation of Moco biosynthesis and molybdoenzyme gene expression by molybdenum and iron in bacteria

The regulation of the operons involved in Moco biosynthesis is dependent on the availability of Fe–S clusters in the cell.

Recognition of ferric catecholates by FepA

Escherichia coli FepA transports certain catecholate ferric siderophores, but not others, nor any noncatecholate compounds. Direct binding and competition experiments demonstrated that this selectivity originates during the adsorption stage. The synthetic tricatecholate Fe-TRENCAM bound to FepA with 50- to 100-fold-lower affinity than Fe-enterobactin (FeEnt), despite an identical metal center, and Fe-corynebactin only bound at much higher concentrations. Neither Fe-agrobactin nor ferrichrome bound at all, even at concentrations 10(6)-fold above the Kd. Thus, FepA only adsorbs catecholate iron complexes, and it selects FeEnt among even its close homologs. We used alanine scanning mutagenesis to study the contributions of surface aromatic residues to FeEnt recognition. Although not apparent from crystallography, aromatic residues in L3, L5, L7, L8, and L10 affected FepA's interaction with FeEnt. Among 10 substitutions that eliminated aromatic residues, Kd increased as much as 20-fold (Y481A and Y638A) and Km increased as much as 400-fold (Y478), showing the importance of aromaticity around the pore entrance. Although many mutations equally reduced binding and transport, others caused greater deficiencies in the latter. Y638A and Y478A increased Km 10- and 200-fold more, respectively, than Kd. N-domain loop deletions created the same phenotype: Delta60-67 (in NL1) and Delta98-105 (in NL2) increased Kd 10- to 20-fold but raised Km 500- to 700-fold. W101A (in NL2) had little effect on Kd but increased Km 1,000-fold. These data suggested that the primary role of the N terminus is in ligand uptake. Fluorescence and radioisotopic experiments showed biphasic release of FeEnt from FepA. In spectroscopic determinations, k(off1) was 0.03/s and k(off2) was 0.003/s. However, FepAY272AF329A did not manifest the rapid dissociation phase, corroborating the role of aromatic residues in the initial binding of FeEnt. Thus, the beta-barrel loops contain the principal ligand recognition determinants, and the N-domain loops perform a role in ligand transport.

Spectroscopic observations of ferric enterobactin transport

We characterized the uptake of ferric enterobactin (FeEnt), the native Escherichia coli ferric siderophore, through its cognate outer membrane receptor protein, FepA, using a site-directed fluorescence methodology. The experiments first defined locations in FepA that were accessible to covalent modification with fluorescein maleimide (FM) in vivo; among 10 sites that we tested by substituting single Cys residues, FM labeled W101C, S271C, F329C, and S397C, and all these exist within surface-exposed loops of the outer membrane protein. FeEnt normally adsorbed to the fluoresceinated S271C and S397C mutant FepA proteins in vivo, which we observed as quenching of fluorescence intensity, but the ferric siderophore did not bind to the FM-modified derivatives of W101C or F329C. These in vivo fluorescence determinations showed, for the first time, consistency with radioisotopic measurements of the affinity of the FeEnt-FepA interaction; K(d) was 0.2 nm by both methods. Analysis of the FepA mutants with AlexaFluor(680), a fluorescein derivative with red-shifted absorption and emission spectra that do not overlap the absorbance spectrum of FeEnt, refuted the possibility that the fluorescence quenching resulted from resonance energy transfer. These and other data instead indicated that the quenching originated from changes in the environment of the fluor as a result of loop conformational changes during ligand binding and transport. We used the fluorescence system to monitor FeEnt uptake by live bacteria and determined its dependence on ligand concentration, temperature, pH, and carbon sources and its susceptibility to inhibition by the metabolic poisons. Unlike cyanocobalamin transport through the outer membrane, FeEnt uptake was sensitive to inhibitors of electron transport and phosphorylation, in addition to its sensitivity to proton motive force depletion.

Surface loop motion in FepA

Using a lysine-specific cleavable cross-linking reagent ethylene glycolbis(sulfosuccimidylsuccinate) (Sulfo-EGS), we studied conformational motion in the surface loops of Escherichia coli FepA during its transport of the siderophore ferric enterobactin. Site-directed mutagenesis determined that Sulfo-EGS reacted with two lysines, K332 and K483, and at least two other unidentified Lys residues in the surface loops of the outer membrane protein. The reagent cross-linked K483 in FepA L7 to either K332 in L5, forming a product that we designated band 1, or to the major outer membrane proteins OmpF, OmpC, and OmpA, forming band 2. Ferric enterobactin binding to FepA did not prevent modification of K483 by Sulfo-EGS but blocked its cross-linking to OmpF/C and OmpA and reduced its coupling to K332. These data show that the loops of FepA undergo conformational changes in vivo, with an approximate magnitude of 15 A, from a ligand-free open state to a ligand-bound closed state. The coupling of FepA L7 to OmpF, OmpC, or OmpA was TonB independent and was unaffected by the uncouplers CCCP (carbonyl cyanide m-chlorophenylhydrazone) and DNP (2,4-dinitrophenol) but completely inhibited by cyanide.

Mechanisms of colicin binding and transport through outer membrane porins

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

Neisserial TonB-dependent outer-membrane proteins: detection, regulation and distribution of three putative candidates identified from the genome sequences

Computer searches were carried out of the gonococcal and meningococcal genome databases for previously unknown members of the TonB-dependent family (Tdf) of outer-membrane receptor proteins. Seven putative non-contiguous genes were found and three of these (identified in gonococcal strain FA1090) were chosen for further study. Consensus motif analysis of the peptide sequences was consistent with the three genes encoding TonB-dependent receptors. In view of the five previously characterized TonB-dependent proteins of pathogenic neisseriae, the putative genes were labelled tdfF, tdfG and tdfH. TdfF had homology with the siderophore receptors FpvA of Pseudomonas aeruginosa and FhuE of Escherichia coli, whereas TdfG and TdfH had homology with the haemophore receptor HasR of Serratia marcescens. The aim of this project was to characterize these proteins and determine their expression, regulation, distribution and surface exposure. Strain surveys of iron-stressed commensal and pathogenic neisseriae revealed that TdfF is unlikely to be expressed, TdfG is expressed by gonococci only and that TdfH is expressed by both meningococci and gonococci. Expression of TdfH was unaffected by iron availability. Susceptibility of TdfH to cleavage by proteases in live gonococci was consistent with surface exposure of this protein. TdfH may function as a TonB-dependent receptor for a non-iron nutrient source. Furthermore, TdfH is worthy of future investigation as a potential meningococcal vaccine candidate as it is a highly conserved, widely distributed and surface-exposed outer-membrane protein.

Effect of loop deletions on the binding and transport of ferric enterobactin by FepA

The siderophore ferric enterobactin enters Escherichia coli through the outer membrane (OM) porin FepA, which contains an aqueous transmembrane channel that is normally occluded by other parts of the protein. After binding the siderophore at a site within the surface loops, FepA undergoes conformational changes that promote ligand internalization. We assessed the participation of different loops in ligand recognition and uptake by creating and analysing a series of deletions. We genetically engineered 26 mutations that removed 9-75 amino acids from nine loops and two buried regions of the OM protein. The mutations had various effects on the uptake reaction, which we discerned by comparing the substrate concentrations of half-maximal binding (Kd) and uptake (Km): every loop deletion affected siderophore transport kinetics, decreasing or eliminating binding affinity and transport efficiency. We classified the mutations in three groups on the basis of their slight, strong or complete inhibition of the rate of ferric enterobactin transport across the OM. Finally, characterization of the FepA mutants revealed that prior experiments underestimated the affinity of FepA for ferric enterobactin: the interaction between the protein and the ferric siderophore is so avid (Kd < 0.2 nM) that FepA tolerated the large reductions in affinity that some loop deletions caused without loss of uptake functionality. That is, like other porins, many of the loops of FepA are superficially dispensable: ferric enterobactin transport occurred without them, at levels that allowed bacterial growth.

Protonmotive force, ExbB and ligand-bound FepA drive conformational changes in TonB

TonB couples the cytoplasmic membrane protonmotive force (pmf) to active transport across the outer membrane, potentially through a series of conformational changes. Previous studies of a TonB transmembrane domain mutant (TonB-delta V17) and its phenotypical suppressor (ExbB-A39E) suggested that TonB is conformationally sensitive. Here, two new mutations of the conserved TonB transmembrane domain SHLS motif were isolated, TonB-S16L and -H20Y, as were two new suppressors, ExbB-V35E and -V36D. Each suppressor ExbB restored at least partial function to the TonB mutants, although TonB-delta V17, for which both the conserved motif and the register of the predicted transmembrane domain alpha-helix are affected, was the most refractory. As demonstrated previously, TonB can undergo at least one conformational change, provided both ExbB and a functional TonB transmembrane domain are present. Here, we show that this conformational change reflects the ability of TonB to respond to the cytoplasmic membrane proton gradient, and occurs in proportion to the level of TonB activity attained by mutant-suppressor pairs. The phenotype of TonB-delta V17 was more complex than the -S16L and -H20Y mutations, in that, beyond the inability to be energized efficiently, it was also conditionally unstable. This second defect was evident only after suppression by the ExbB mutants, which allow transmembrane domain mutants to be energized, and presented as the rapid turnover of TonB-delta V17. Importantly, this degradation was dependent upon the presence of a TonB-dependent ligand, suggesting that TonB conformation also changes following the energy transduction event. Together, these observations support a dynamic model of energy transduction in which TonB cycles through a set of conformations that differ in potential energy, with a transition to a higher energy state driven by pmf and a transition to a lower energy state accompanying release of stored potential energy to an outer membrane receptor.

Immunization of cows with ferric enterobactin receptor from coliform bacteria

The serum and milk immunoglobulin (Ig) G responses of lactating dairy cows were determined following immunization with ferric enterobactin receptor FepA. Escherichia coli 471 was cultured in iron-depleted medium, and outer membrane proteins were extracted by 2% N-lauroylsarcosine sodium salt and 2% Triton X-100. The FepA was isolated from the outer membrane proteins by ion-exchange chromatography. Twenty cows were assigned to four treatment groups of 5 cows blocked by breed and days in milk. Treatment groups were vaccinated with 100 micrograms of FepA, 500 micrograms of FepA, Escherichia coli J5 bacterin, or sterile phosphate-buffered saline. Primary immunization was at approximately 200 d in milk, and booster immunizations were given 14 and 28 d later. Serum and whey IgG titers to FepA in cows vaccinated with FepA were significantly higher than those from cows vaccinated with either E. coli J5 bacterin or phosphate-buffered saline. Serum and whey IgG titers to FepA were elevated by 14 d in cows vaccinated with FepA. Significant differences were not observed between doses of FepA. The degree of cross-reactivity of purified IgG from cows vaccinated with FepA to E. coli and Klebsiella pneumoniae isolates was significantly higher than that to a control isolate that lacked FepA production. Immunization with FepA elicited an immunological response in serum and milk.

Attacin--an insect immune protein--binds LPS and triggers the specific inhibition of bacterial outer-membrane protein synthesis

Attacin is a 20 kDa antibacterial protein, originally isolated from the immune haemolymph of Hyalophora cecropia. It has been demonstrated previously that attacin causes increased permeability of the outer membrane of Escherichia coli and inhibition of outer-membrane protein synthesis at the transcriptional level. This is accompanied by inhibition of growth. Here, LPS is shown to serve as the receptor for attacin and evidence is presented that attacin does not need to enter the cell to exert its activity. The increase in outer-membrane permeability precedes any increase in inner-membrane permeability by at least one generation time (approximately 45 min), and the inhibiting effect of attacin on synthesis of outer-membrane proteins is detectable after only 10 min. It is also shown that attacin causes induction of several stress proteins and increased synthesis of LPS within, respectively, 25 and 60 min of treatment. Based on the results presented, it is proposed that attacin has the unique ability to specifically interfere with synthesis of outer-membrane proteins without entering the inner membrane or cytoplasm.

Guanidine hydrochloride unfolding of a transmembrane beta-strand in FepA using site-directed spin labeling

We have used the electron spin resonance (ESR) site-directed spin-labeling (SDSL) technique to examine the guanidine hydrochloride (Gdn-HCl) induced denaturation of several sites along a transmembrane beta-strand located in the ferric enterobactin receptor, FepA. In addition, we have continued the characterization of the beta-strand previously identified by our group (Klug CS et al., 1997, Biochemistry 36:13027-13033) to extend from the periplasm to the extracellular surface loop in FepA, an integral membrane protein containing a beta-barrel motif comprised of a series of antiparallel beta-strands that is responsible for transport of the iron chelate, ferric enterobactin (FeEnt), across the outer membrane of Escherichia coli and many related enteric bacteria. We have previously shown that a large surface loop in FepA containing the FeEnt binding site denatures independently of the beta-barrel domain (Klug CS et al., 1995, Biochemistry 34:14230-14236). The SDSL approach allows examination of the unfolding at individual residues independent of the global unfolding of the protein. This work shows that sites along the beta-strand that are exposed to the aqueous lumen of the channel denature more rapidly and with higher cooperativity than the surface loop, while sites on the hydrophobic side of the beta-strand undergo a limited degree of noncooperative unfolding and do not fully denature even at high (e.g., 4 M) Gdn-HCl concentrations. We conclude that, in a transmembrane beta-strand, the local environment of a given residue plays a significant role in the loss of structure at each site.

Biphasic binding kinetics between FepA and its ligands

The Escherichia coli FepA protein is an energy- and TonB-dependent, ligand-binding porin that functions as a receptor for the siderophore ferric enterobactin and colicins B and D. We characterized the kinetic and thermodynamic parameters associated with the initial, energy-independent steps in ligand binding to FepA. In vivo experiments produced Kd values of 24, 185, and 560 nM for ferric enterobactin, colicin B, and colicin D, respectively. The siderophore and colicin B bound to FepA with a 1:1 stoichiometry, but colicin D bound to a maximum level that was 3-fold lower. Preincubation with ferric enterobactin prevented colicin B binding, and preincubation with colicin B prevented ferric enterobactin binding. Colicin B release from FepA was unexpectedly slow in vivo, about 10-fold slower than ferric enterobactin release. This slow dissociation of the colicin B.FepA complex facilitated the affinity purification of FepA and FepA mutants with colicin B-Sepharose. Analysis of a fluorescent FepA derivative showed that ferric enterobactin and colicin B adsorbed with biphasic kinetics, suggesting that both ligands bind in at least two distinct steps, an initial rapid stage and a subsequent slower step, that presumably establishes a transport-competent complex.

Double mutagenesis of a positive charge cluster in the ligand-binding site of the ferric enterobactin receptor, FepA

Siderophores and colicins enter bacterial cells through TonB-dependent outer membrane proteins. Using site-directed substitution mutagenesis, we studied ligand recognition by a prototypic Escherichia coli siderophore receptor, FepA, that binds the iron chelate ferric enterobactin and colicins B and D. These genetic experiments identified a common binding site for two of the three ligands, containing multiple positive charges, within cell surface residues of FepA. Elimination of single residues in this region did not impair the adsorption or transport of ferric enterobactin, but double mutagenesis in the charge cluster identified amino acids (Arg-286 and Arg-316) that participate in siderophore binding and function in FepA-mediated killing by colicins B and D. Ferric enterobactin binding, furthermore, prevented covalent modification of FepA within this domain by either a fluorescent probe or an arginine-specific reagent, corroborating the involvement of this site in ligand recognition. These results identify, for the first time, residues in a TonB-dependent outer membrane protein that participate in ligand binding. They also explain the competition between ferric enterobactin and the colicins on the bacterial cell surface: all three ligands interact with the same arginine residues within FepA during their penetration through the outer membrane.

Binding characterization of the iron transport receptor from the outer membrane of Escherichia coli (FepA): differentiation between FepA and FecA

The dissociation constants for the binding of ferric enterobactin with FepA and FecA are quantitated with displacement experiments. It is found that Kd for FepA is 12 times lower than the one for FecA. This indicates that FepA is an high-affinity receptor while FecA binds ferric enterobactin with a lower affinity. Monoclonal antibodies specific for binding epitopes of FepA inhibit the binding of ferric enterobactin with purified FepA. These same antibodies do not inhibit the binding of ferric enterobactin with purified FecA. This indicates that the binding epitopes in FecA and FepA are different.

A site-directed spin-labeling study of ligand-induced conformational change in the ferric enterobactin receptor, FepA

The ferric enterobactin receptor, FepA, is a TonB-dependent gated porin that transports the siderophore ferric enterobactin across the outer membrane of gram-negative bacteria. We have created two site-directed mutants of Escherichia coli FepA, in both cases introducing a cysteine residue into the putative ligand-binding domain. The introduced cysteines were then modified with nitroxide spin labels for structural and dynamic studies using electron spin resonance (ESR) spectroscopy. The mutants were fully functional, as indicated by their ability to grow under iron-limiting conditions, their uptake of [59Fe]enterobactin, and their sensitivity to colicin B. Labeling of the mutant FepA receptors proceeded easily upon incubation with sulfhydryl-specific spin labels, e.g. MTSL, (1-oxy-2,2,5,5-tetramethylpyrrolidin-3-yl)methyl methanethiosulfonate. In contrast, spin labeling of the two native cysteines (Cys486 and Cys493) within wild-type FepA occurred only after treatment with a thiol reducing agent and partial denaturation in urea, suggesting that the native cysteines are disulfide-linked. ESR spectra showed a high degree of motional restriction for all three sites. Continuous wave (CW) saturation studies indicated that one of the mutationally introduced sites (Cys280) was surface-localized as evidenced by its exposure to the aqueous paramagnetic relaxation agent chromium oxalate and its low accessibility to O2. The other (Cys310) apparently occupies a site near the membrane/aqueous interface. The native cysteines occupy a site tightly packed within the protein structure with low accessibility to both CROX and O2. A shift in both conventional and saturation-transfer ESR spectra of MTSL-labeled E280C and E310C (but not MTSL-labeled wild type) FepA was observed upon addition of ferric enterobactin. The ESR spectral shift was dependent on ferric enterobactin concentration and did not occur with siderophores not recognized by FepA. Ferric enterobactin binding did not alter the CW saturation properties of MTSL bound to these sites, but did influence their accessibility to O2. These results provide consistent evidence for a ligand-specific conformational change in the surface peptides of FepA upon the binding of ferric enterobactin.

Permeability properties of a large gated channel within the ferric enterobactin receptor, FepA

FepA is an Escherichia coli outer membrane receptor protein for the siderophore ferric enterobactin. Prior studies conducted in vivo suggested that FepA and other TonB-dependent outer membrane proteins transport ligands by a gated-channel mechanism. To corroborate and extend these findings we have determined the permeability properties of the FepA channel in vitro, by measuring the diffusion rates of hydrophilic nonelectrolytes through the FepA channel in liposome swelling experiments. Like porins, the FepA deletion mutant delta RV showed a size-dependent permeability to oligosaccharides, indicating that it forms a nonspecific, hydrophilic pore. Unlike OmpF and other E. coli porins, however, delta RV proteoliposomes transported stachyose (666 Da) and ferrichrome (740 Da). These data, and other uptake results with a series of maltodextrins of increasing size, confirm the existence of a channel domain within FepA that is considerably larger than OmpF-type pores. These results represent a reconstitution of the channel function of a TonB-dependent receptor protein and establish that FepA contains the largest channel that has been characterized in the E. coli outer membrane.

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.

A novel purification of ferric citrate receptor (FecA) from Escherichia coli UT5600 and further characterization of its binding activity

In our earlier paper, it was demonstrated that the FecA receptor protein from Escherichia coli UT5600/pBB2 (leu-, proC-, trpE-, entA-, rpsl-, delta (ompT-fepA)-/Ampr, fepA) binds with ferric enterobactin. In order to explore this further the outer membrane receptor protein, FecA, has been isolated from UT5600 (fepA-) and purified to homogeneity by DE-52-cellulose anion exchange chromatography followed by MonoPFPLC chromatofocusing. Partially purified FecA and homogeneous FecA show binding activity to [55Fe]ferric enterobactin and the binding is specific. Binding activity of FecA can be enhanced by ferric citrate. Lipopolysaccharide-free FecA as ascertained by silver staining and the endotoxin test still retains the same activity. In vivo uptake studies using different strains of E. coli suggest that FecA in E. coli plays an important role in ferrienterobactin transport.

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.

Surface topology of the Escherichia coli K-12 ferric enterobactin receptor

Monoclonal antibodies (MAb) were raised to the Escherichia coli K-12 ferric enterobactin receptor, FepA, and used to identify regions of the polypeptide that are involved in interaction with its ligands ferric enterobactin and colicins B and D. A total of 11 distinct FepA epitopes were identified. The locations of these epitopes within the primary sequence of FepA were mapped by screening MAb against a library of FepA::PhoA fusion proteins, a FepA deletion mutant, and proteolytically modified FepA. These experiments localized the 11 epitopes to seven different regions within the FepA polypeptide, including residues 2 to 24, 27 to 37, 100 to 178, 204 to 227, 258 to 290, 290 to 339, and 382 to 400 of the mature protein. Cell surface-exposed epitopes of FepA were identified and discriminated by cytofluorimetry and by the ability of MAb that recognize them to block the interaction of FepA with its ligands. Seven surface epitopes were defined, including one each in regions 27 to 37, 204 to 227, and 258 to 290 and two each in regions 290 to 339 and 382 to 400. One of these, within region 290 to 339, was recognized by MAb in bacteria containing intact (rfa+) lipopolysaccharide (LPS); all other surface epitopes were susceptible to MAb binding only in a strain containing a truncated (rfaD) LPS core, suggesting that they are physically shielded by E. coli K-12 LPS core sugars. Antibody binding to FepA surface epitopes within region 290 to 339 or 382 to 400 inhibited killing by colicin B or D and the uptake of ferric enterobactin. In addition to the FepA-specific MAb, antibodies that recognized other outer membrane components, including Cir, OmpA, TonA, and LPS, were identified. Immunochemical and biochemical characterization of the surface structures of FepA and analysis of its hydrophobicity and amphilicity were used to generate a model of the ferric enterobactin receptor's transmembrane strands, surface peptides, and ligand-binding domains.

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.

Antigenically related iron-regulated outer membrane proteins produced by different somatic serotypes of Pasteurella multocida

An 84-kilodalton outer membrane protein was expressed when Pasteurella multocida, somatic serotype 3, was grown in brain-heart infusion broth containing the iron chelator dipyridyl but not in brain-heart infusion broth alone. Antigenically related outer membrane proteins of various molecular masses were also expressed by P. multocida strains belonging to all of the other 15 somatic serotypes (somatic serotype 12 being the possible exception) as well as by isolates expressing somatic antigens representative of multiple somatic serotypes when grown under the same conditions of iron deprivation.

Demonstration of ferric L-parabactin-binding activity in the outer membrane of Paracoccus denitrificans

Under low-iron conditions, Paracoccus denitrificans excretes a catecholamine siderophore, L-parabactin, to sequester and utilize iron. In this report, we demonstrate the presence of stereospecific high-affinity ferric L-parabactin-binding activity associated with P. denitrificans membranes grown in low-iron medium. Isolated outer membrane components were shown to be three to four times higher in specific activity for ferric L-parabactin. The same amount of binding activity existed whether or not the radiolabel was present in the metal (55Fe) or the ligand (3H) portion of ferric parabactin chelate, suggesting that binding was to the intact complex. Ion-exchange chromatography of a Triton X-100-solubilized outer membrane mixture on DEAE-cellulose resulted in a 10-fold increase in binding activity relative to that present in whole membranes. Polypeptide profiles by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the products of each stage of the purification showed that binding activity copurified with one or more of the low-iron-induced outer membrane proteins in the 80-kilodalton (kDa) region. Membrane proteins and [55Fe]ferric L-parabactin electrophoresed in nondenaturing gels demonstrated the presence of membrane component(s) which stereo-specifically bound ferric L-parabactin, thus providing independent confirmation of the binding assay results. Moreover, when the band labeled by [55Fe]ferric L-parabactin was excised and profiled by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, 80-kDa polypeptides were the major components present. These results demonstrate the presence of a high-affinity ferric L-parabactin receptor in P. denitrificans membranes and suggest that one or more of the 80-kDa low-iron-induced polypeptides are components of the ferric L-parabactin receptor.

Plasmid-determined cloacin DF13-susceptibility in Enterobacter cloacae and Klebsiella edwardsii; identification of the cloacin DF13/aerobactin outer membrane receptor proteins

Both Enterobacter cloacae H478 and Klebsiella edwardsii S15 were shown to harbour a relatively large conjugative plasmid that coded for cloacin DF13-susceptibility and the production and uptake of a hydroxamate iron chelator, most probably aerobactin. Protein-blotting experiments with antiserum raised against the purified cloacin DF13/aerobactin receptor protein from Escherichia coli (Co1V-K30) revealed that the corresponding outer membrane receptor proteins of Ent. cloacae H478 and K. edwardsii S15 had apparent mol wts of 85 000 and 76 000, respectively. E. coli transconjugants harbouring either the plasmid from Ent. cloacae H478 or K. edwardsii S15 expressed a cloacin DF13/aerobactin outer membrane receptor protein with a mol wt of 74 000. The receptor protein encoded by the Ent. cloacae and K. edwardsii plasmids were immunologically more related to each other than to the pCo1V-K30-encoded receptor protein.

Siderophore-mediated uptake of iron in Azotobacter vinelandii

Azotobacter vinelandii produces two siderophores, N,N'-bis-(2,3-dihydroxybenzoyl)-L-lysine (azotochelin) and a yellow-green fluorescent peptide (azotobactin), under iron-limited growth conditions. 55Fe uptake was not observed until the substantial nonspecific binding of 55Fe to the cell surface was eliminated by the addition of 10 mM sodium citrate to the uptake medium. Citrate alone did not promote rapid 55Fe uptake in A. vinelandii, nor did it induce Fe-repressible outer membrane proteins. Siderophore-mediated 55Fe uptake appeared biphasic, with both the initial rapid and ensuing slower uptake being energy dependent. The purified siderophores demonstrated the same uptake pattern as the Fe-limited culture supernatant fluid, but either individually or in combination accounted for less than the total 55Fe uptake activity found in the latter. The purified siderophores appeared to be sensitive to acid, but the inhibition of 55Fe uptake was in fact caused by salt generated during neutralization. Similar 60% inhibition of 55Fe uptake activity was caused by the addition of 40 mM Na+, K+, Li+, or Mg2+ salts to the uptake medium. Ammonium was less inhibitory than the latter ions. 55Fe uptake mediated by azotobactin was more sensitive to added NaCl than was that mediated by azotochelin. Neither the chelation of iron nor the stability of the ferrisiderophore was affected by added NaCl.

Demonstration of an iron-siderophore-binding protein in the outer membrane of Pseudomonas aeruginosa

Outer membranes prepared from Pseudomonas aeruginosa grown in low-iron medium bind three times as much [59Fe]pyochelin as do membranes from cells grown in high-iron medium. The deletion of pyochelin reduced 59Fe binding to background levels. Autoradiographic analysis of sodium dodecyl sulfate polyacrylamide gel electrophoretograms of outer membrane preparations previously incubated with [59Fe]pyochelin revealed that the iron-siderophore complex bound to a low-molecular-weight protein of approximately 14,000.