Transport of Vitamin B12 in Escherichia coli

Vitamin B12-peptide nucleic acids use the BtuB receptor to pass through the Escherichia coli outer membrane

Short modified oligonucleotides that bind in a sequence-specific way to messenger RNA essential for bacterial growth could be useful to fight bacterial infections. One such promising oligonucleotide is peptide nucleic acid (PNA), a synthetic DNA analog with a peptide-like backbone. However, the limitation precluding the use of oligonucleotides, including PNA, is that bacteria do not import them from the environment. We have shown that vitamin B₁₂, which most bacteria need to take up for growth, delivers PNAs to Escherichia coli cells when covalently linked with PNAs. Vitamin B₁₂ enters E. coli via a TonB-dependent transport system and is recognized by the outer-membrane vitamin B₁₂-specific BtuB receptor. We engineered the E. coli ΔbtuB mutant and found that transport of the vitamin B₁₂-PNA conjugate requires BtuB. Thus, the conjugate follows the same route through the outer membrane as taken by free vitamin B₁₂. From enhanced sampling all-atom molecular dynamics simulations, we determined the mechanism of conjugate permeation through BtuB. BtuB is a β-barrel occluded by its luminal domain. The potential of mean force shows that conjugate passage is unidirectional and its movement into the BtuB β-barrel is energetically favorable upon luminal domain unfolding. Inside BtuB, PNA extends making its permeation mechanically feasible. BtuB extracellular loops are actively involved in transport through an induced-fit mechanism. We prove that the vitamin B₁₂ transport system can be hijacked to enable PNA delivery to E. coli cells.

Synthesis of Vitamin B12-Antibiotic Conjugates with Greatly Improved Activity against Gram-Negative Bacteria

There is an urgent need to discover new antibiotics and improve the efficacy of known antibiotics against Gram-negative bacteria. "Trojan horse" conjugates are novel and promising antibiotics. Herein we report the design and synthesis of vitamin-B₁₂-ampicillin conjugates, which exhibited more than 500 times improved activity against Escherichia coli compared with ampicillin itself. Our studies demonstrate that the vitamin-B₁₂ uptake pathway could be employed for effective antibiotic delivery and efficacy enhancement.

Extracellular loops of BtuB facilitate transport of vitamin B12 through the outer membrane of E. coli

Vitamin B12 (or cobalamin) is an enzymatic cofactor essential both for mammals and bacteria. However, cobalamin can be synthesized only by few microorganisms so most bacteria need to take it up from the environment through the TonB-dependent transport system. The first stage of cobalamin import to E. coli cells occurs through the outer-membrane receptor called BtuB. Vitamin B12 binds with high affinity to the extracellular side of the BtuB protein. BtuB forms a β-barrel with inner luminal domain and extracellular loops. To mechanically allow for cobalamin passage, the luminal domain needs to partially unfold with the help of the inner-membrane TonB protein. However, the mechanism of cobalamin permeation is unknown. Using all-atom molecular dynamics, we simulated the transport of cobalamin through the BtuB receptor embedded in an asymmetric and heterogeneous E. coli outer-membrane. To enhance conformational sampling of the BtuB loops, we developed the Gaussian force-simulated annealing method (GF-SA) and coupled it with umbrella sampling. We found that cobalamin needs to rotate in order to permeate through BtuB. We showed that the mobility of BtuB extracellular loops is crucial for cobalamin binding and transport and resembles an induced-fit mechanism. Loop mobility depends not only on the position of cobalamin but also on the extension of luminal domain. We provided atomistic details of cobalamin transport through the BtuB receptor showing the essential role of the mobility of BtuB extracellular loops. A similar TonB-dependent transport system is used also by many other compounds, such as haem and siderophores, and importantly, can be hijacked by natural antibiotics. Our work could have implications for future delivery of antibiotics to bacteria using this transport system.

Vitamin B12 transports modified RNA into E. coli and S. Typhimurium cells

Specifically designed, antisense oligonucleotides are promising candidates for antibacterial drugs. They suppress the correct expression of bacterial genes by complementary binding to essential sequences of bacterial DNA or RNA. The main obstacle in fully utilizing their potential as therapeutic agents comes from the fact that bacteria do not uptake oligonucleotides from their environment. Herein, we report that vitamin B12 can transport oligonucleotides into Escherichia coli and Salmonella typhimurium cells. 5'-Aminocobalamin with an alkyne linker and azide-modified oligonucleotides enabled the synthesis of vitamin B12-2'OMeRNA conjugates using an efficient "click" methodology. Inhibition of protein expression in E. coli and S. Typhimurium cells indicates an unprecedented transport of 2'OMeRNA oligomers into bacterial cells via the vitamin B12 delivery pathway.

Does a Conjugation Site Affect Transport of Vitamin B12 -Peptide Nucleic Acid Conjugates into Bacterial Cells?

Gram-negative bacteria develop specific systems for the uptake of scarce nutrients, including vitamin B₁₂ . These uptake pathways may be utilized for the delivery of biologically relevant molecules into cells. Indeed, it was recently reported that vitamin B₁₂ transported an antisense peptide nucleic acid (PNA) into Escherichia coli and Salmonella Typhimurium cells. The present studies indicate that the conjugation site of PNA to vitamin B₁₂ has an impact on PNA transport into bacterial cells. Toward this end, a specifically designed PNA oligomer has been tethered at various positions of vitamin B₁₂ (central Co, R_5' -OH, c and e amide chains, meso position, and at the hydroxy group of cobinamide) by using known or newly developed methodologies and tested for the uptake of the synthesized conjugates by E. coli. Compounds in which the PNA oligonucleotide was anchored at the R_5' -OH position were transported more efficiently than that of other compounds tethered at the peripheral positions around the corrin ring. Of importance is the fact that, contrary to mammalian organisms, E. coli also takes up cobinamide, which is an incomplete corrinoid. This selectivity opens up ways to fight bacterial infections.

Supramolecular assemblies underpin turnover of outer membrane proteins in bacteria

Gram-negative bacteria inhabit a broad range of ecological niches. For Escherichia coli, this includes river water as well as humans and animals, where it can be both a commensal and a pathogen. Intricate regulatory mechanisms ensure that bacteria have the right complement of β-barrel outer membrane proteins (OMPs) to enable adaptation to a particular habitat. Yet no mechanism is known for replacing OMPs in the outer membrane, an issue that is further confounded by the lack of an energy source and the high stability and abundance of OMPs. Here we uncover the process underpinning OMP turnover in E. coli and show it to be passive and binary in nature, in which old OMPs are displaced to the poles of growing cells as new OMPs take their place. Using fluorescent colicins as OMP-specific probes, in combination with ensemble and single-molecule fluorescence microscopy in vivo and in vitro, as well as molecular dynamics simulations, we established the mechanism for binary OMP partitioning. OMPs clustered to form ∼0.5-μm diameter islands, where their diffusion is restricted by promiscuous interactions with other OMPs. OMP islands were distributed throughout the cell and contained the Bam complex, which catalyses the insertion of OMPs in the outer membrane. However, OMP biogenesis occurred as a gradient that was highest at mid-cell but largely absent at cell poles. The cumulative effect is to push old OMP islands towards the poles of growing cells, leading to a binary distribution when cells divide. Hence, the outer membrane of a Gram-negative bacterium is a spatially and temporally organized structure, and this organization lies at the heart of how OMPs are turned over in the membrane.

Transmembrane gate movements in the type II ATP-binding cassette (ABC) importer BtuCD-F during nucleotide cycle

ATP-binding cassette (ABC) transporters are ubiquitous integral membrane proteins that translocate substrates across cell membranes. The alternating access of their transmembrane domains to opposite sides of the membrane powered by the closure and reopening of the nucleotide binding domains is proposed to drive the translocation events. Despite clear structural similarities, evidence for considerable mechanistic diversity starts to accumulate within the importers subfamily. We present here a detailed study of the gating mechanism of a type II ABC importer, the BtuCD-F vitamin B(12) importer from Escherichia coli, elucidated by EPR spectroscopy. Distance changes at key positions in the translocation gates in the nucleotide-free, ATP- and ADP-bound conformations of the transporter were measured in detergent micelles and liposomes. The translocation gates of the BtuCD-F complex undergo conformational changes in line with a "two-state" alternating access model. We provide the first direct evidence that binding of ATP drives the gates to an inward-facing conformation, in contrast to type I importers specific for maltose, molybdate, or methionine. Following ATP hydrolysis, the translocation gates restore to an apo-like conformation. In the presence of ATP, an excess of vitamin B(12) promotes the reopening of the gates toward the periplasm and the dislodgment of BtuF from the transporter. The EPR data allow a productive translocation cycle of the vitamin B(12) transporter to be modeled.

TonB-dependent maltose transport by Caulobacter crescentus

We have shown previously that Caulobacter crescentus grows on maltodextrins which are actively transported across the outer membrane by the MalA protein. Evidence for energy-coupled transport was obtained by deletion of the exbB exbD genes which abolished transport. However, removal of the TonB protein, which together with the ExbB ExbD proteins is predicted to form an energy-coupling device between the cytoplasmic membrane and the outer membrane, left transport unaffected. Here we identify an additional tonB gene encoded by the cc2334a ORF, which when deleted abolished maltose transport. MalA contains a TonB box that reads EEVVIT and is predicted to interact with TonB. Replacement of valine number 15 in the TonB box by proline abolished maltose transport. Maltose was transported across the cytoplasmic membrane by the MalY protein (CC2283). Maltose transport was induced by maltose and repressed by the MalI protein (CC2284). In addition to MalA, MalY and MalI, the mal locus encodes two predicted cytoplasmic alpha-amylases (CC2285 and CC2286) and a periplasmic glucoamylase (CC2282). The TonB dependence together with the previously described ExbB ExbD dependence demonstrates energy-coupled maltose transport across the outer membrane. MalY is involved in maltose transport across the cytoplasmic membrane by a presumably ion-coupled mechanism.

NagA-dependent uptake of N-acetyl-glucosamine and N-acetyl-chitin oligosaccharides across the outer membrane of Caulobacter crescentus

Among the 67 predicted TonB-dependent outer membrane transporters of Caulobacter crescentus, NagA was found to be essential for growth on N-acetyl-beta-D-glucosamine (GlcNAc) and larger chitin oligosaccharides. NagA (93 kDa) has a predicted typical domain structure of an outer membrane transport protein: a signal sequence, the TonB box EQVVIT, a hatch domain of 147 residues, and a beta-barrel composed of 22 antiparallel beta-strands linked by large surface loops and very short periplasmic turns. Mutations in tonB1 and exbBD, known to be required for maltose transport via MalA in C. crescentus, and in two additional predicted tonB genes (open reading frames cc2327 and cc3508) did not affect NagA-mediated GlcNAc uptake. nagA is located in a gene cluster that encodes a predicted PTS sugar transport system and two enzymes that convert GlcNAc-6-P to fructose-6-P. Since a nagA insertion mutant did not grow on and transport GlcNAc, diffusion of GlcNAc through unspecific porins in the outer membrane is excluded. Uptake of GlcNAc into tonB and exbBD mutants and reduction but not abolishment of GlcNAc transport by agents which dissipate the electrochemical potential of the cytoplasmic membrane (0.1 mM carbonyl cyanide 3-chlorophenylhydrazone and 1 mM 2,4-dinitrophenol) suggest diffusion of GlcNAc through a permanently open pore of NagA. Growth on (GlcNAc)(3) and (GlcNAc)(5) requires ExbB and ExbD, indicating energy-coupled transport by NagA. We propose that NagA forms a small pore through which GlcNAc specifically diffuses into the periplasm and functions as an energy-coupled transporter for the larger chitin oligosaccharides.

ExbBD-dependent transport of maltodextrins through the novel MalA protein across the outer membrane of Caulobacter crescentus

Analysis of the genome sequence of Caulobacter crescentus predicts 67 TonB-dependent outer membrane proteins. To demonstrate that among them are proteins that transport nutrients other than chelated Fe(3+) and vitamin B(12)-the substrates hitherto known to be transported by TonB-dependent transporters-the outer membrane protein profile of cells grown on different substrates was determined by two-dimensional electrophoresis. Maltose induced the synthesis of a hitherto unknown 99.5-kDa protein, designated here as MalA, encoded by the cc2287 genomic locus. MalA mediated growth on maltodextrins and transported [(14)C]maltodextrins from [(14)C]maltose to [(14)C]maltopentaose. [(14)C]maltose transport showed biphasic kinetics, with a fast initial rate and a slower second rate. The initial transport had a K(d) of 0.2 microM, while the second transport had a K(d) of 5 microM. It is proposed that the fast rate reflects binding to MalA and the second rate reflects transport into the cells. Energy depletion of cells by 100 microM carbonyl cyanide 3-chlorophenylhydrazone abolished maltose binding and transport. Deletion of the malA gene diminished maltose transport to 1% of the wild-type malA strain and impaired transport of the larger maltodextrins. The malA mutant was unable to grow on maltodextrins larger than maltotetraose. Deletion of two C. crescentus genes homologous to the exbB exbD genes of Escherichia coli abolished [(14)C]maltodextrin binding and transport and growth on maltodextrins larger than maltotetraose. These mutants also showed impaired growth on Fe(3+)-rhodotorulate as the sole iron source, which provided evidence of energy-coupled transport. Unexpectedly, a deletion mutant of a tonB homolog transported maltose at the wild-type rate and grew on all maltodextrins tested. Since Fe(3+)-rhodotorulate served as an iron source for the tonB mutant, an additional gene encoding a protein with a TonB function is postulated. Permeation of maltose and maltotriose through the outer membrane of the C. crescentus malA mutant was slower than permeation through the outer membrane of an E. coli lamB mutant, which suggests a low porin activity in C. crescentus. The pores of the C. crescentus porins are slightly larger than those of E. coli K-12, since maltotetraose supported growth of the C. crescentus malA mutant but failed to support growth of the E. coli lamB mutant. The data are consistent with the proposal that binding of maltodextrins to MalA requires energy and MalA actively transports maltodextrins with K(d) values 1,000-fold smaller than those for the LamB porin and 100-fold larger than those for the vitamin B(12) and ferric siderophore outer membrane transporters. MalA is the first example of an outer membrane protein for which an ExbB/ExbD-dependent transport of a nutrient other than iron and vitamin B(12) has been demonstrated.

Performance of standard phenotypic assays for TonB activity, as evaluated by varying the level of functional, wild-type TonB

The ability of gram-negative bacterial cells to transport cobalamin and iron-siderophore complexes and their susceptibility to killing by some bacteriophages and colicins are characteristics routinely used to assay mutations of proteins in the TonB-dependent energy transduction system. These assays vary greatly in sensitivity and are subject to perturbation by overexpression of TonB and, perhaps, other proteins that contribute to the process. Thus, the choice of assay and the means by which a potential mutant is expressed can greatly influence the interpretation and recognition of a given mutant. In the present study, we expressed TonB at several different quantified levels in cells that were then subjected to a panel of assays. Our results suggest that it is reasonable to regard the assays as having windows of sensitivity. Thus, while no single assay satisfactorily spans the potential range of TonB activity, it is evident that certain assays are better suited for resolving small deviations from wild-type levels of activity, with others most useful when activity levels are very low. It is apparent from the results that the application of all possible assays to the characterization of new mutants will yield the most meaningful results.

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?

Quantification of known components of the Escherichia coli TonB energy transduction system: TonB, ExbB, ExbD and FepA

The TonB-dependent energy transduction system couples cytoplasmic membrane proton motive force to active transport of iron-siderophore complexes across the outer membrane in Gram-negative bacteria. In Escherichia coli, the primary players known in this process to date are: FepA, the TonB-gated transporter for the siderophore enterochelin; TonB, the energy-transducing protein; and two cytoplasmic membrane proteins with less defined roles, ExbB and ExbD. In this study, we report the per cell numbers of TonB, ExbB, ExbD and FepA for cells grown under iron-replete and iron-limited conditions. Under iron-replete conditions, TonB and FepA were present at 335 +/- 78 and 504 +/- 165 copies per cell respectively. ExbB and ExbD, despite being encoded from the same operon, were not equimolar, being present at 2463 +/- 522 and 741 +/- 105 copies respectively. The ratio of these proteins was calculated at one TonB:two ExbD:seven ExbB under all four growth conditions tested. In contrast, the TonB:FepA ratio varied with iron status and according to the method used for iron limitation. Differences in the method of iron limitation also resulted in significant differences in cell size, skewing the per cell copy numbers for all proteins.

A 76-residue polypeptide of colicin E9 confers receptor specificity and inhibits the growth of vitamin B12-dependent Escherichia coli 113/3 cells

The mechanism by which E colicins recognize and then bind to BtuB receptors in the outer membrane of Escherichia coli cells is a poorly understood first step in the process that results in cell killing. Using N- and C-terminal deletions of the N-terminal 448 residues of colicin E9, we demonstrated that the smallest polypeptide encoded by one of these constructs that retained receptor-binding activity consisted of residues 343-418. The results of the in vivo receptor-binding assay were supported by an alternative competition assay that we developed using a fusion protein consisting of residues 1-497 of colicin E9 fused to the green fluorescent protein as a fluorescent probe of binding to BtuB in E. coli cells. Using this improved assay, we demonstrated competitive inhibition of the binding of the fluorescent fusion protein by the minimal receptor-binding domain of colicin E9 and by vitamin B12. Mutations located in the minimum R domain that abolished or reduced the biological activity of colicin E9 similarly affected the competitive binding of the mutant colicin protein to BtuB. The sequence of the 76-residue R domain in colicin E9 is identical to that found in colicin E3, an RNase type E colicin. Comparative sequence analysis of colicin E3 and cloacin DF13, which is also an RNase-type colicin but uses the IutA receptor to bind to E. coli cells, revealed significant sequence homology throughout the two proteins, with the exception of a region of 92 residues that included the minimum R domain. We constructed two chimeras between cloacin DF13 and colicin E9 in which (i) the DNase domain of colicin E9 was fused onto the T+R domains of cloacin DF13; and (ii) the R domain and DNase domain of colicin E9 were fused onto the T domain of cloacin DF13. The killing activities of these two chimeric colicins against indicator strains expressing BtuB or IutA receptors support the conclusion that the 76 residues of colicin E9 confer receptor specificity. The minimum receptor-binding domain polypeptide inhibited the growth of the vitamin B12-dependent E. coli 113/3 mutant cells, demonstrating that vitamin B12 and colicin E9 binding is mutually exclusive.

A new class of cobalamin transport mutants (btuF) provides genetic evidence for a periplasmic binding protein in Salmonella typhimurium

No periplasmic binding protein has been demonstrated for the ATP-binding cassette (ABC)-type cobalamin transporter BtuCD. New mutations (btuF) are described that affect inner-membrane transport. The BtuF protein has a signal sequence and resembles the periplasmic binding proteins of several other ABC transporters.

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.

Cobalamin (coenzyme B12): synthesis and biological significance

This review examines deoxyadenosylcobalamin (Ado-B12) biosynthesis, transport, use, and uneven distribution among living forms. We describe how genetic analysis of enteric bacteria has contributed to these issues. Two pathways for corrin ring formation have been found-an aerobic pathway (in P. denitrificans) and an anaerobic pathway (in P. shermanii and S. typhimurium)-that differ in the point of cobalt insertion. Analysis of B12 transport in E. coli reveals two systems: one (with two proteins) for the outer membrane, and one (with three proteins) for the inner membrane. To account for the uneven distribution of B12 in living forms, we suggest that the B12 synthetic pathway may have evolved to allow anaerobic fermentation of small molecules in the absence of an external electron acceptor. Later, evolution of the pathway produced siroheme, (allowing use of inorganic electron acceptors), chlorophyll (O2 production), and heme (aerobic respiration). As oxygen became a larger part of the atmosphere, many organisms lost fermentative functions and retained dependence on newer, B12 functions that did not involve fermentation. Paradoxically, Salmonella spp. synthesize B12 only anaerobically but can use B12 (for degradation of ethanolamine and propanediol) only with oxygen. Genetic analysis of the operons for these degradative functions indicate that anaerobic degradation is important. Recent results suggest that B12 can be synthesized and used during anaerobic respiration using tetrathionate (but not nitrate or fumarate) as an electron acceptor. The branch of enteric taxa from which Salmonella spp. and E. coli evolved appears to have lost the ability to synthesize B12 and the ability to use it in propanediol and glycerol degradation. Salmonella spp., but not E. coli, have acquired by horizontal transfer the ability to synthesize B12 and degrade propanediol. The acquired ability to degrade propanediol provides the selective force that maintains B12 synthesis in this group.

Structure-function analysis of the vitamin B12 receptor of Escherichia coli by means of informational suppression

We describe a genetic analysis of the vitamin B12 receptor of Escherichia coli. Through the use of informational suppression, we have been able to generate a family of receptor variants, each identical save for a single, known substitution (Ser, Gln, Lys, Tyr, Leu, Cys, Phe) at a known site. We have studied 22 different mutants, 14 in detail, distributed throughout the length of the btuB gene. Most amino acid substitutions have a pleiotropic effect with respect to all ligands tested, the two colicins E1 and E3, the T5-like bacteriophage BF23, and vitamin B12. (The dramatic effect of a single amino acid substitution is also well exemplified by the G142A missense change which renders the receptor completely non-functional.) In some instances, however, we have been able to modify a subset of receptor functions (viz. Q62, Q150 and Q299 and the response to phage BF23). These data are summarized on a two-dimensional folding model for the BtuB protein in the outer membrane (devised using both amphipathic beta-strand analysis and sequence conservation amongst the TonB-dependent receptors). In addition, we report that the extreme C-terminus of BtuB is vital for receptor localization and provide evidence for it being a membrane-spanning beta-sheet with residue L588 situated on its hydrophobic surface. Two of the C-terminal btuB mutations are located within the region of overlap with the recently identified dga (murl) gene.

Purification and properties of chicken egg-white cobalamin-binding protein

A cobalamin-binding protein has been purified from chicken egg-white by using a combination of conventional and high performance ion-exchange chromatography. Following initial purification by DEAE-cellulose, ammonium sulphate precipitation, Sephacryl S-200 CM-cellulose and affinity chromatography, appropriate fractions were further purified using the Pharmacia fast protein liquid chromatography (FPLC) system. Using this method of purification, egg-white CBP has been purified more rapidly and with greater recovery than with conventional column chromatography. The homogeneity of this protein was verified by SDS-PAGE. The Mr was 37,000 by SDS-PAGE and 39,000 by gel filtration, which indicated that it was a glycoprotein. The stokes radius was 4.1 nm and pI was 4.3. The protein bound 57COB12 with a molar ratio of 1/1 and kd of 0.40 microM. The egg-white CBP was composed of 294 amino acid residues. Thiol groups and metal ions were not connected with the Cbl-binding activities.

Interdependence of calcium and cobalamin binding by wild-type and mutant BtuB protein in the outer membrane of Escherichia coli

The binding of calcium and cobalamin to outer membranes from cells of Escherichia coli that contained amplified levels of wild-type or mutant btuB was studied. The mutant (BBam50) had an aspartyl-prolyl dipeptide inserted after the original 50th amino acid residue of the mature BtuB protein, which is within a region that shows extensive homology with the ferric siderophore receptors. This insertion resulted in cleavage of the BtuB in two places. The larger form retained the insertion but had lost 11 amino acid residues from the amino terminus. The smaller form was cut at the insertion site. Both the wild-type protein and the larger form of mutant BtuB showed calcium-dependent cobalamin binding with the same affinity for cobalamin, although the mutant had a much lower affinity for calcium. The smaller form of the mutant BtuB protein had a greatly reduced affinity for cobalamin, which was probably the result of inactivation of the cobalamin-dependent calcium-binding site. Cobalamin-dependent calcium binding was measured in wild-type BtuB preparations and was found to have the same corrinoid specificity and response to various corrinoid concentrations as shown previously for cobalamin binding. The results are consistent with a role for calcium in the cobalamin pump of the outer membrane of E. coli and show that a conserved part of the BtuB protein is required for the cobalamin-dependent binding of calcium.

Characterization of FadL-specific fatty acid binding in Escherichia coli

The product of the fadL gene (FadL) is a central component of the long-chain fatty acid transport system of Escherichia coli. When fatty acid activation is blocked by a mutation in the structural gene for acyl CoA synthetase (fadD) transport is inhibited allowing a FadL-specific fatty acid binding activity to be measured. This binding activity was 4- to 6-fold greater in the fadL+ fadD strain LS6928 when compared to the delta fadLfadD strain LS6929. With long-chain fatty acids, this binding activity was saturable and it was estimated that there were approx. 35,000 FadL-specific oleic acid binding sites per cell in the fadL+ strain LS6928. The FadL-specific fatty acid binding affinity was highest for oleic acid (18:1) and palmitic acid (16:0) giving apparent KD values of 2.3.10(-7) M and 8.8.10(-7) M, respectively. FadL-specific binding affinity of myristic acid (14:0) was nearly an order of magnitude less and no FadL-specific binding of decanoic acid (10:0) could be measured. Two lines of evidence suggest that FadL-fatty acid binding occurs by a hydrophobic interaction: (1) There was a preference for the long-chain substrates oleic acid and palmitic acid; and (2) oleic acid binding activity was not significantly changed over the pH range 5.0 to 8.0. The FadL-specific binding of oleic acid in the fadL+ strain LS6928 could be blocked by preincubation with antisera raised against purified FadL providing a clear correlation between the activity and identity of FadL. The binding activity associated with FadL was measured in vesicles of the outer membrane following passage over the hydrophobic resin Lipidex 1000. The KD of oleic acid binding attributable to FadL in outer membranes vesicles (6.0.10(-7) M) was in close agreement with that determined in whole cells. Overall, these studies demonstrated that FadL binds long-chain fatty acids with a relatively high affinity prior to their transport across the outer membrane.

Purification and partial characterization of a cobalamin-binding protein from chicken egg yolk

Cobalamin-binding protein has been purified from chicken egg yolk by using DEAE-cellulose with a NaCl gradient. The resultant protein fraction was subjected to bioaffinity chromatography. The Mr was 38,000 by SDS-PAGE and 39,000 by gel filtration, and indicated that it was a glycoprotein. The Stokes radius was 4.3 nm and the pI 4.1. The protein bound 57CO.B12 with a molar ratio of 1:1 and a Kd of 0.41 microM. The CBP composed 296 amino acids residues. The protein-ligand interaction was inhibited by Cbl analogues.

Vitamin B12 transport in Escherichia coli K12 does not require the btuE gene of the btuCED operon

Transport of vitamin B12 across the cytoplasmic membrane of Escherichia coli requires the products of btuC and btuD, two genes in the btuCED operon. The role of btuE, the central gene of this operon, was examined. Deletions within btuE were constructed by removal of internal restriction fragments and were crossed onto the chromosome by allelic replacement. In-frame deletions that removed 20% or 82% of the btuE coding region did not affect expression of the distal btuD gene. These nonpolar deletions had little effect on vitamin B12 binding (whole cells or periplasmic fraction) and transport. They did not affect the utilization of vitamin B12 or other cobalamins for methionine biosynthesis, even in strains with decreased outer membrane transport of vitamin B12. The btuE mutations did not impair adenosyl-cobalamin dependent catabolism of ethanolamine or repression of btuB expression. Thus, despite its genetic location in the transport operon, the btuE product plays no essential role in vitamin B12 transport.

Two outer membrane transport systems for vitamin B12 in Salmonella typhimurium

The involvement of an outer membrane transport component for vitamin B12 uptake in Salmonella typhimurium, analogous to the btuB product in Escherichia coli, was investigated. Mutants of S. typhimurium selected for resistance to bacteriophage BF23 carried mutations at the btuB locus (butBS) (formerly called bfe, at the analogous map position as the E. coli homolog) and were defective in high-affinity vitamin B12 uptake. The cloned E. coli btuB gene (btuBE) hybridized to S. typhimurium genomic DNA and restored vitamin B12 transport activity to S. typhimurium btuBS mutants. An Mr-60,000 protein in the S. typhimurium outer membrane was repressed by growth with vitamin B12 and was eliminated in a btuBS mutant. The btuBS product thus appears to play the same role in vitamin B12 transport by S. typhimurium as does the E. coli btuBE product. A second vitamin B12 transport system that is not present in E. coli was found by cloning a fragment of S. typhimurium DNA that complemented btuB mutants for vitamin B12 utilization. In addition to this plasmid with a 6-kilobase insert of S. typhimurium DNA, vitamin B12 utilization by E. coli btuB strains required the btuC and btuD products, necessary for transport across the cytoplasmic membrane, but not the btuE or tonB product. The plasmid conferred low levels of vitamin B12-binding and energy-dependent transport activity but not susceptibility to phage BF23 or utilization of dicyanocobinamide. The cloned S. typhimurium DNA encoding this new transport system did not hybridize to the btuBE gene or to E. coli chromosomal DNA and therefore does not carry the S. typhimurium btuBS locus. Increased production of an Mr -84,000 polypeptide associated with the outer membrane was seen. The new locus appears to be carried on the large plasmid in most S. typhimurium strains. Thus S. typhimurium possesses both high- and low-affinity systems for uptake of cobalamins across the outer membrane.

Separate regulatory systems for the repression of metE and btuB by vitamin B12 in Escherichia coli

Synthesis of the btuB-encoded outer membrane receptor for vitamin B12 and the metE-encoded homocysteine methyltransferase is repressed by growth of Escherichia coli in the presence of vitamin B12. The regulation by vitamin B12 of the production of beta-galactosidase in strains carrying btuB-lac or metE-lac operon fusions indicated that repression of both genes operates at the transcriptional level. Selection for expression of these fusions under repressive conditions allowed isolation of second-site mutations in which repressibility by vitamin B12 had been lost. Mutations in metH and metF prevented vitamin B12-dependent regulation of metE, but not that of btuB. Mutations in btuB and other genes involved in uptake of the vitamin eliminated or reduced repression. Mutations in the newly identified gene, btuR, controlled the repressibility of btuB, but had no effect on metE regulation. The btuR gene resides at 27.9 min on the genetic map in the gene order cysB-topA-btuR-trp; it acts in a trans-dominant manner and appears to encode a repressor of btuB transcription.

Nucleotide sequence of the btuCED genes involved in vitamin B12 transport in Escherichia coli and homology with components of periplasmic-binding-protein-dependent transport systems

The products of the btuCED region of the Escherichia coli chromosome participate in the transport of vitamin B12 across the cytoplasmic membrane. The nucleotide sequence of the 3,410-base-pair HindIII-HincII DNA fragment carrying a portion of the himA gene and the entire btuCED region was determined. Comparison of the location of the open reading frames with the gene boundaries defined by transposon insertions allowed the assignment of polypeptide products to gene sequences. The btuC product is a highly nonpolar integral membrane protein of molecular weight 31,683. The distribution of hydrophobic regions suggests the presence of numerous membrane-spanning domains. The btuD product is a relatively polar but membrane-associated polypeptide of Mr 27,088 and contains segments bearing extensive homology to the ATP-binding peripheral membrane constituents of periplasmic binding protein-dependent transport systems. Other regions of this protein are similar to portions of the outer membrane vitamin B12 receptor. The btuE product (Mr 20,474) appears to have a periplasmic location. It has the mean hydropathy of a soluble protein but lacks an obvious signal sequence. The cellular locations and structural and sequence homologies of the Btu polypeptides point to the similarity of these three proteins to components of binding protein-dependent transport systems. However, the dependence on a periplasmic vitamin B12-binding protein has not yet been demonstrated.

Outer-membrane permeability of bacteria

Gram-negative bacteria evolved to survive under the conditions in which a number of hazardous compounds are abundant. The outer membrane which protects the cell interior acts as a barrier against such hazardous agents, yet the cells must incorporate the chemicals that are essential for the cellular activity. The devices that Gram-negative bacteria developed to incorporate such essence are the transmembrane pores. These pores could be subdivided into three categories: (1) pore made of porins has a weak solute selectivity; (2) pore made of lamB protein and tsx proteins hold intermediate solute specificity. and (3) pores for the diffusion of vitamin B12 and ferric ion-chelator complexes have a tight solute specificity. Porins are identified from a number of Gram-negatives and from the outer membrane of mitochondria of various sources. Studies on the diffusion properties of these outer-membrane proteins provided essential information to understand membrane transports.

Analysis of btuB receptor function by use of nonsense suppression

Informational suppression of btuB nonsense mutants allows the study of the effect of known, single amino acid substitutions on receptor function. We found that ligand uptake is largely unaffected by such amino acid changes. The few instances in which certain substitutions destroyed sensitivity to the two lethal agents (phage BF23, colicin E3) without affecting vitamin B12 uptake suggest a common region on the btuB receptor involved in the binding of these proteinaceous agents.

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

A protein with a relative subunit molecular weight of 81000 (81K) has been isolated in virtually pure form from the outer membrane of low iron grown cells of Escherichia coli K12. The 81K protein, which is part of the receptor complex for translocation of the siderophore ferric enterobactin, displays activity in vitro for binding both ferric enterobactin and colicin B. The dissociation constant for the 81K-ferric enterobactin compound at 4 degrees C in 2% Triton-0.1 M Tris, pH 7, was determined to be 10 nM. The N-terminal amino acid was identified as phenylalanine, and the amino acid composition was shown to be similar to that published for the ferric aerobactin-cloacin receptor of Enterobacter cloacae. A plasmid-bearing strain of E. coli was employed to confirm that degradation of 81K to a slightly smaller, inactive form (81K) is performed by a second outer membrane component, protein a. The endoproteolytic action of protein a was verified by the finding of alanine as the N-terminal residue of 81K. A survey of enteric species suggests that the 81K-protein a interaction is confined to the K12 strain of E. coli.

Interference by methylcobalamin analogues with synthesis of cobalamin coenzymes in human lymphocytes in vitro

1. 72 h uptake of cyano[57Co]cobalamin and formation of 57Co-labelled methylcobalamin, adenosylcobalamin and hydroxocobalamin has been estimated with and without the addition of methylcobalamin analogues in phytohaemagglutinin-stimulated lymphocytes from healthy human subjects. 2. Difluorochloromethylcobalamin reduced cell uptake of cyanocobalamin and caused a disproportionate reduction in synthesis of adenosylcobalamin. 3. Methylcobalamin-palladium trichloride reduced cell uptake of cyanobalamin more effectively than did difluorochloromethylcobalamin and reduced the formation of methylcobalamin, adenosylcobalamin and hydroxocobalamin in proportion. 4. The results suggest that in addition to inhibiting uptake of cyanocobalamin, one or both compounds may have interfered directly with the mechanism of synthesis of the cobalamin coenzymes.

A lipopolysaccharide-binding cell-surface protein from Salmonella minnesota. Isolation, partial characterization and occurrence in different Enterobacteriaceae

1. Protein extracts obtained from Salmonella minnesota Re mutant cells by treatment with EDTA/NaC1 solution contain a protein which exhibits high affinity to bacterial lipopolysaccharides. The isolation and partial characterization of this lipopolysaccharide-binding protein is described. 2. The protein was purified from EDTA extracts by a two-step procedure consisting of ion-exchange chromatography on CM-Sephadex and preparative polyacrylamide gel electrophoresis at pH 9.5. The yield of the total purification procedure was around 16%. 3. The resulting protein preparation was homogeneous on the basis of disc gel electrophoresis, dodecylsulfate gel electrophoresis, isoelectric focusing in polyacrylamide gel and immunoelectrophoresis. 4. The isoelectric point of the protein was found to be 10.3 at 4 degrees C. Its molecular weight determined by dodecylsulfate gel electrophoresis is 15000. Its amino acid composition is characterized by the absence of histidine and proline, a low content in tyrosine and high amounts of alanine, lysine, aspartic and glutamic acid residues, or their respective amides. 5. The lipopolysaccharide-protein association was shown to be mainly due to ionic interactions of the basic protein with negatively charged groups (probably phosphate and pyrophosphate groups) of the lipid A moiety. 6. Purified lipopolysaccharide-binding protein is immunogenic in rabbits, thus enabling the preparation of specific antiserum. 7. The protein is located at the surface of Salmonella minnesota Re mutant cells as revealed by antiserum absorption with total bacteria. Ferritin-labelling studies further demonstrated that it is evenly spread over the entire cell surface. 8. Comparative antiserum absorption studies using smooth and rough strains of Salmonella minnesota, Salmonella typhimurium, Escherichia coli, Klebsiella and Shigella revealed the presence of lipopolysaccharide-binding protein (or a serologically cross-reacting antigen) in most of the strains tested. From these results the protein can be considered as a common antigen of Enterobacteriaceae.

Repression of synthesis of the vitamin B12 receptor in Escherichia coli

Growth of Escherichia coli K-12 strains in the presence of the vitamin cyanocobalamin (B12) resulted in an 80 to 90% reduction in B12 uptake activity of washed cells. Coincident with the decline in uptake activity was the depression of B12-binding activity in energy-poisoned cells, suggesting that growth in B12 resulted in the repression of synthesis of the B12 receptor protein in the outer membrane. Growth in the presence of B12 led to marked reduction in sensitivity to the E colicins, whose adsorption to cells requires the B12 receptor, and to a decrease in the amount of a band on electropherograms of outer membrane proteins. That polypeptide was also missing from mutants altered at btuB, the locus encoding the B12 receptor. Addition of B12 to growing cultures resulted in the exponential decline in specific activity of B12 uptake, as expected for dilution of functional receptors by further growth. Repression of receptor synthesis appears to be regulated by the level of intracellular, rather than extracellular, B12 and is separate from the regulation of the methionine biosynthetic pathway. Mutants altered in btuC, which are defective in accumulation and retention of B12, exhibit a much lower degree of repressibility.

Role of lipopolysaccharide and outer membrane protein of Escherichia coli K-12 in the receptor activity for bacteriophage T4

Lipopolysaccharide isolated from Escherichia coli K-12 did not inactivate phage T4, although the cell envelopes with 1% sodium deoxycholate resulted in the release of cytoplasmic membrane proteins, 70% of the lipopolysaccharide, and almost all of the phospholipid. The reconstitution of phage receptor activity was achieved from deoxycholate-soluble and -insoluble fractions by dialysis against a solution of magnesium chloride. Lipopolysaccharide was the only essential component in the deoxycholate-soluble fraction. PhageT4-resistant mutants YA21-6 and YA21-82, having defects in the deoxycholate-soluble and -insoluble fractions, respectively, were isolated. The deoxycholate-soluble fraction of YA21-6 possessed heptoseless lipopolysaccharide, and this defect was responsible for the phage resistance. The deoxycholate-insoluble fraction of YA21-82 lacked outer membrane protein O-8. The addition of O-8 to this fraction together with the wild-type lipopolysaccharide resulted in the appearance of the receptor activity. Furthermore, the reconstitution was successfully achieved with only O-8 and the wild-type lipopolysaccharide, indicating that O-8 was an essential component in the deoxycholate-insoluble fraction.

Genetic analysis of components involved in vitamin B12 uptake in Escherichia coli

The products of three genes are involved in cyanocobalamin (B(12)) uptake in Escherichia coli. btuB (formerly bfe), located at min 88 on the Escherichia coli linkage map, codes for a protein component of the outer membrane which serves as receptor for B(12), the E colicins, and bacteriophage BF23. Four phenotypic classes of mutants varying in response to these agents were found to carry mutations that, based on complementation and reversion analyses, reside in the single btuB cistron. In one mutant class, ligand binding to the receptor appeared to be normal, but subsequent B(12) uptake was defective. The level of receptor and rate of uptake were responsive to btuB gene dosage. Previous studies showed that the tonB product was necessary for energy-dependent B(12) uptake but not for its binding. Other than those in tonB, no mutations that conferred insensitivity to group B colicins affected B(12) utilization. The requirement for the btuB and tonB products could be bypassed by elevated levels of B(12) (>1 muM) or by mutations compromising the integrity of the outer membrane as a permeability barrier. Utilization of elevated B(12) concentrations in strains lacking the btuB-tonB uptake system was dependent on the function of the btuC product. This gene was located at 37.7 min on the linkage map, with the order pps-btuC-pheS. Strains altered in btuC but with an intact btuB-tonB system were only slightly impaired in B(12) utilization, being defective in its accumulation. This defect was manifested as inability to retain B(12), such that intracellular label was almost completely lost by exchange or efflux. It is proposed that btuC encodes a transport system for B(12) in the periplasm.