A requirement for calcium in the transport of cobalamin across the outer membrane of Escherichia coli

Membrane transport of cobalamin

A wide variety of organisms encode cobalamin-dependent enzymes catalyzing essential metabolic reactions, but the cofactor cobalamin (vitamin B12) is only synthesized by a subset of bacteria and archaea. The biosynthesis of cobalamin is complex and energetically costly, making cobalamin variants and precursors metabolically valuable. Auxotrophs for these molecules have evolved uptake mechanisms to compensate for the lack of a synthesis pathway. Bacterial transport of cobalamin involves the passage over one or two lipidic membranes in Gram-positive and -negative bacteria, respectively. In higher eukaryotes, a complex system of carriers, receptors and transporters facilitates the delivery of the essential molecule to the tissues. Biochemical and genetic approaches have identified different transporter families involved in cobalamin transport. The majority of the characterized cobalamin transporters are active transport systems that belong to the ATP-binding cassette (ABC) superfamily of transporters. In this chapter, we describe the different cobalamin transport systems characterized to date that are present in bacteria and humans, as well as yet-to-be-identified transporters.

Structural intermediates observed only in intact Escherichia coli indicate a mechanism for TonB-dependent transport

Outer membrane TonB-dependent transporters facilitate the uptake of trace nutrients and carbohydrates in Gram-negative bacteria and are essential for pathogenic bacteria and the health of the microbiome. Despite this, their mechanism of transport is still unknown. Here, pulse electron paramagnetic resonance (EPR) measurements were made in intact cells on the Escherichia coli vitamin B₁₂ transporter, BtuB. Substrate binding was found to alter the C-terminal region of the core and shift an extracellular substrate binding loop 2 nm toward the periplasm; moreover, this structural transition is regulated by an ionic lock that is broken upon binding of the inner membrane protein TonB. Significantly, this structural transition is not observed when BtuB is reconstituted into phospholipid bilayers. These measurements suggest an alternative to existing models of transport, and they demonstrate the importance of studying outer membrane proteins in their native environment.

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.

Host resistance factor SLC11A1 restrictsSalmonellagrowth through magnesium deprivation

The pleiotropic host resistance factor SLC11A1 (NRAMP1) defends against diverse intracellular pathogens in mammals by yet-unknown mechanisms. We comparedSalmonellainfection of coisogenic mice with differentSLC11A1alleles. SLC11A1 reducedSalmonellareplication and triggered up-regulation of uptake systems for divalent metal cations but no other stress responses. SLC11A1 modestly diminished iron availability and acutely restrictedSalmonellaaccess to magnesium. Growth ofSalmonellacells in the presence of SLC11A1 was highly heterogeneous and inversely correlated with expression of the crucial magnesium transporter genemgtB. We observed superimposable single-cell patterns in mice lacking SLC11A1 when we restrictedSalmonellaaccess to magnesium by impairing its uptake. Together, these findings identify deprivation of the main group metal magnesium as the main resistance mechanism of SLC11A1 againstSalmonella.

Specificity of cobamide remodeling, uptake and utilization in Vibrio cholerae

Cobamides are a group of compounds including vitamin B₁₂ that can vary at the lower base position of the nucleotide loop. They are synthesized de novo by only a subset of prokaryotes, but some organisms encode partial biosynthesis pathways for converting one variant to another (remodeling) or completing biosynthesis from an intermediate (corrinoid salvaging). Here, we explore the cobamide specificity in Vibrio cholerae through examination of three natural variants representing major cobamide groups: commercially available cobalamin, and isolated pseudocobalamin and p-cresolylcobamide. We show that BtuB, the outer membrane corrinoid transporter, mediates the uptake of all three variants and the intermediate cobinamide. Our previous work suggested that V. cholerae could convert pseudocobalamin produced by cyanobacteria into cobalamin. In this work, cobamide specificity in V. cholerae is demonstrated by remodeling of pseudocobalamin and salvaging of cobinamide to produce cobalamin. Cobamide remodeling in V. cholerae is distinct from the canonical pathway requiring amidohydrolase CbiZ, and heterologous expression of V. cholerae CobS was sufficient for remodeling. Furthermore, function of V. cholerae cobamide-dependent methionine synthase MetH was robustly supported by cobalamin and p-cresolylcobamide, but not pseudocobalamin. Notably, the inability of V. cholerae to produce and utilize pseudocobalamin contrasts with enteric bacteria like Salmonella.

Role of the Native Outer-Membrane Environment on the Transporter BtuB

BtuB is a TonB-dependent transporter that permits the high-affinity binding and transport of cobalamin (CBL), or vitamin B₁₂, across the asymmetric outer membrane (OM) of Gram-negative bacteria. It has been shown that Ca²⁺ binding is necessary for high-affinity binding of CBL to BtuB, and earlier simulations suggested that calcium ions serve to stabilize key substrate-binding extracellular loops. However, those simulations did not account for the lipopolysaccharides in the OM. To illuminate the roles of both Ca²⁺ and lipopolysaccharides in protein functionality, we performed simulations of apo and Ca²⁺-loaded BtuB in symmetric and asymmetric bilayers. The simulations reveal that the oligosaccharides of LPS stabilize the extracellular loops to some degree, apparently obviating the need for Ca²⁺. However, it is shown that Ca²⁺ ions stabilize a key substrate-binding loop to an even greater degree, as well as reposition specific CBL-binding residues, bringing them closer to the organization found in the CBL-bound structure. These results indicate the importance of including realistic membrane models when simulating outer-membrane proteins.

Distance Measurement on an Endogenous Membrane Transporter in E. coli Cells and Native Membranes Using EPR Spectroscopy

Membrane proteins may be influenced by the environment, and they may be unstable in detergents or fail to crystallize. As a result, approaches to characterize structures in a native environment are highly desirable. Here, we report a novel general strategy for precise distance measurements on outer membrane proteins in whole Escherichia coli cells and isolated outer membranes. The cobalamin transporter BtuB was overexpressed and spin-labeled in whole cells and outer membranes and interspin distances were measured to a spin-labeled cobalamin using pulse EPR spectroscopy. A comparative analysis of the data reveals a similar interspin distance between whole cells, outer membranes, and synthetic vesicles. This approach provides an elegant way to study conformational changes or protein-protein/ligand interactions at surface-exposed sites of membrane protein complexes in whole cells and native membranes, and provides a method to validate outer membrane protein structures in their native environment.

Bacterial ATP-driven transporters of transition metals: physiological roles, mechanisms of action, and roles in bacterial virulence

Maintaining adequate intracellular levels of transition metals is fundamental to the survival of all organisms. While all transition metals are toxic at elevated intracellular concentrations, metals such as iron, zinc, copper, and manganese are essential to many cellular functions. In prokaryotes, the concerted action of a battery of membrane-embedded transport proteins controls a delicate balance between sufficient acquisition and overload. Representatives from all major families of transporters participate in this task, including ion-gradient driven systems and ATP-utilizing pumps. P-type ATPases and ABC transporters both utilize the free energy of ATP hydrolysis to drive transport. Each of these very different families of transport proteins has a distinct role in maintaining transition metal homeostasis: P-type ATPases prevent intracellular overloading of both essential and toxic metals through efflux while ABC transporters import solely the essential ones. In the present review we discuss how each system is adapted to perform its specific task from mechanistic and structural perspectives. Despite the mechanistic and structural differences between P-type ATPases and ABC transporters, there is one important commonality: in many clinically relevant bacterial pathogens, transporters of transition metals are essential for virulence. Here we present several such examples and discuss how these may be exploited for future antibacterial drug development.

Coupling of calcium and substrate binding through loop alignment in the outer-membrane transporter BtuB

In Gram-negative bacteria, TonB-dependent outer-membrane transporters bind large, scarce organometallic substrates with high affinity preceding active transport. The cobalamin transporter BtuB requires the additional binding of two Ca(2+) ions before substrate binding can occur, but the underlying molecular mechanism is unknown. Using the crystallographic structures available for different bound states of BtuB, we have carried out extended molecular dynamics simulations of multiple functional states of BtuB to address the role of Ca(2+) in substrate recruitment. We find that Ca(2+) binding both stabilizes and repositions key extracellular loops of BtuB, optimizing interactions with the substrate. Interestingly, replacement by Mg(2+) abolishes this effect, in accordance with experiments. Using a set of new force-field parameters developed for cyanocobalamin, we also simulated the substrate-bound form of BtuB, where we observed interactions not seen in the crystal structure between the substrate and loops previously found to be important for binding and transport. Based on our results, we suggest that the large size of cobalamin compared to other TonB-dependent transporter substrates explains the requirement of Ca(2+) binding for high-affinity substrate recruitment in BtuB.

Observations on the Calcium Dependence and Reversibility of Cobalamin Transport across the Outer Membrane of Escherichia coli

The calcium dependence of cobalamin (Cbl) binding to the BtuB protein of Escherichia coli and the reversibility of its function in the transport of Cbl across the outer membrane have been examined. The results show that the two calcium-binding sites in BtuB that were identified previously by others are responsible for the calcium dependence of high affinity Cbl binding. The affinity of the pure BtuB protein for Cbl was approximately 1000-fold higher in the presence of saturating levels of calcium than in its absence. The affinities of BtuB for both Cbl and calcium were decreased by insertion of alanine residues at position 51 of the mature protein and were increased by several mutations and deletions in the TonB box. Experiments on the uptake of Cbl into the periplasmic space showed that this process is reversible and that the exit of Cbl back into the medium does not require the protonmotive force. Our interpretation of these results is that the role of the TonB-ExbB-ExbD complex, potentiated by the protonmotive force, is to reduce the affinity of the Cbl-binding site, thus increasing the rate of Cbl release into the periplasmic space. The evidence also indicates that access of the Cbl-binding site of BtuB to the periplasmic space does not require removal of the hatch domain from the barrel.

In meso structure of the cobalamin transporter, BtuB, at 1.95 A resolution

Crystals of the apo form of the vitamin B12 and colicin receptor, BtuB, that diffract to 1.95 A have been grown by the membrane-based in meso technique. The structure of the protein differs in several details from that of its counterpart grown by the more traditional, detergent-based (in surfo) method. Some of these differences include (i) the five N-terminal residues are resolved in meso, (ii) residues 57-62 in the hatch domain and residues 574-581 in loop 21-22 are disordered in meso and are ordered in surfo, (iii) residues 278-287 in loop 7-8 are resolved in meso, (iv) residues 324-331 in loop 9-10, 396-411 in loop 13-14, 442-458 in loop 15-16 and 526-541 in loop 19-20 have large differences in position between the two crystal forms, as have residues 86-96 in the hatch domain, and (v) the conformation of residues 6 and 7 in the Ton box (considered critical to signal transduction and substrate transport) are entirely different in the two structures. Importantly, the in meso orientation of residues 6 and 7 is similar to that of the vitamin B12-charged state. These data suggest that the "substrate-induced" 180 degrees -rotation of residues 6 and 7 reported in the literature may not be a unique signalling event. The extent to which these findings agree with structural, dynamic and functional insights gleaned from site-directed spin labelling and electron paramagnetic resonance measurements is evaluated. Packing in in meso grown crystals is dense and layered, consistent with the current model for crystallogenesis of membrane proteins in lipidic mesophases. Layered packing has been used to locate the transmembrane hydrophobic surface of the protein. Generally, this is consistent with tryptophan, tyrosine, lipid and CalphaB-factor distributions in the protein, and with predictions based on transfer free energy calculations.

The Escherichia coli outer membrane cobalamin transporter BtuB: structural analysis of calcium and substrate binding, and identification of orthologous transporters by sequence/structure conservation

Gram-negative bacteria possess specialized active transport systems that function to transport organometallic cofactors or carriers, such as cobalamins, siderophores, and porphyrins, across their outer membranes. The primary components of each transport system are an outer membrane transporter and the energy-coupling protein TonB. In Escherichiacoli, the TonB-dependent outer membrane transporter BtuB carries out active transport of cobalamin (Cbl) substrates across its outer membrane. Cobalamins bind to BtuB with nanomolar affinity. Previous studies implicated calcium in high-affinity binding of cyanocobalamin (CN-Cbl) to BtuB. We previously solved four structures of BtuB or BtuB complexes: an apo-structure of a methionine-substitution mutant (used to obtain experimental phases by selenomethionine single-wavelength anomalous diffraction studies); an apo-structure of wild-type BtuB; a binary complex of calcium and wild-type BtuB; and a ternary complex of calcium, CN-Cbl and wild-type BtuB. We present an analysis of the binding of calcium in the binary and ternary complexes, and show that calcium coordination changes upon substrate binding. High-affinity CN-Cbl binding and calcium coordination are coupled. We also analyze the binding mode of CN-Cbl to BtuB, and compare and contrast this binding to that observed in other proteins that bind Cbl. BtuB binds CN-Cbl in a manner very different from Cbl-utilizing enzymes and the periplasmic Cbl binding protein BtuF. Homology searches of bacterial genomes, structural annotation based on the presence of conserved Cbl-binding residues identified by analysis of our BtuB structure, and detection of homologs of the periplasmic Cbl-binding binding protein BtuF enable identification of putative BtuB orthologs in enteric and non-enteric bacterial species.

Spectroscopic Evidence that Osmolytes Used in Crystallization Buffers Inhibit a Conformation Change in a Membrane Protein

BtuB is a bacterial outer-membrane protein that transports vitamin B(12). Spectroscopic studies using site-directed spin labeling (SDSL) indicate that the N-terminus of BtuB undergoes a dramatic structural change from a docked (folded) to an undocked (unfolded) configuration upon substrate binding. However, this dramatic conformational change is not observed in the crystal structures of BtuB. Here, we make an attempt to resolve this discrepancy and find that the effects of solutes can explain the discrepancy between the results obtained using these two methods. Specifically, if SDSL is performed with the buffers used for the crystallization of BtuB, the substrate-induced order-disorder transition of the N-terminal Ton box observed in intact membranes is blocked. Moreover, poly(ethylene glycol) 3350, which is a component of the crystallization and soaking buffers, is shown to inhibit this structural transition. It is likely that the crystal structure of BtuB in its holo form represents an osmotically trapped conformation. Conformational changes involving relatively modest energy differences and significant hydration changes may be sensitive to solutes used during crystallization, and this example demonstrates the value of combining multiple structural methods in the examination of protein structure and function.

Enzymatic E-colicins bind to their target receptor BtuB by presentation of a small binding epitope on a coiled-coil scaffold

Toxins and viruses often initiate their attacks by binding to specific proteins on the surfaces of target cells. Bacterial toxins (e.g. bacteriocins) and viruses (bacteriophages) targeting Gram-negative bacteria typically bind to outer membrane proteins. Bacterial E-colicins target Escherichia coli by binding to the outer membrane cobalamin transporter BtuB. Colicins are tripartite molecules possessing receptor-binding, translocation, and toxin domains connected by long coiled-coil alpha-helices. Surprisingly, the crystal structure of colicin E3 does not possess a recognizable globular fold in its receptor-binding domain. We hypothesized that the binding epitope of enzymatic E-colicins is a short loop connecting the two alpha-helices that comprise the coiled-coil region and that this flanking coiled-coil region serves to present the loop in a binding-capable conformation. To test this hypothesis, we designed and synthesized a 34-residue peptide (E-peptide-1) corresponding to residues Ala366-Arg399 of the helix-loop-helix region of colicin E3. Cysteines placed near the ends of the peptide (I372C and A393C) enabled crosslinking for reduction of conformational entropy and formation of a peptide structure that would present the loop epitope. A fluorescent analog was also made for characterization of binding by measurement of fluorescence polarization. Our analysis shows the following. (i). E-peptide-1 is predominantly random coil in aqueous solution, but disulfide bond formation increases its alpha-helical content in both aqueous buffer and solvents that promote helix formation. (ii). Fluorescein-labeled E-peptide-1 binds to purified BtuB in a calcium-dependent manner with a Kd of 43.6 +/- 4.9 nm or 2370 +/- 670 nm in the presence or absence of calcium, respectively. (iii). In the presence of calcium, cyanocobalamin (CN-Cbl) displaces E-peptide-1 with a nanomolar inhibition constant (Ki = 78.9 +/- 5.6 nm). We conclude that the BtuB binding sites for cobalamins and enzymatic E-colicins are overlapping but inequivalent and that the distal loop and (possibly) the short alpha-helical flanking regions are sufficient for high affinity binding.

Substrate-induced transmembrane signaling in the cobalamin transporter BtuB

The outer membranes of Gram-negative bacteria possess transport proteins essential for uptake of scarce nutrients. In TonB-dependent transporters, a conserved sequence of seven residues, the Ton box, faces the periplasm and interacts with the inner membrane TonB protein to energize an active transport cycle. A critical mechanistic step is the structural change in the Ton box of the transporter upon substrate binding; this essential transmembrane signaling event increases the affinity of the transporter for TonB and enables active transport to proceed. We have solved crystal structures of BtuB, the outer membrane cobalamin transporter from Escherichia coli, in the absence and presence of cyanocobalamin (vitamin B(12)). In these structures, the Ton box is ordered and undergoes a conformational change in the presence of bound substrate. Calcium has been implicated as a necessary factor for the high-affinity binding (K(d) approximately 0.3 nM) of cyanocobalamin to BtuB. We observe two bound calcium ions that order three extracellular loops of BtuB, thus providing a direct (and unusual) structural role for calcium.

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.

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.

Effect of precultivation conditions on colicin susceptibility in Escherichia coli

The effect of 20 colicins and cloacin was studied after various precultivations. Nutrient agar supplemented with subinhibitory concentration of EDTA used for precultivation or elevating the growth-temperature of the inoculum from 37 degrees C to 42 degrees C increased the susceptibility of wild-type (smooth) Escherichia coli strains to the inhibitory action of some colicins. There were great differences among the colicins in respect to these effects. In case of rough mutants, their sensitivities did not change or eventually decrease after EDTA or heat pretreatment. The LPS pattern in SDS-PAGE of smooth cells grown in EDTA-containing nutrient medium changed in some degree towards the rough character. In case of precultivation at 42 degrees C this change was less considerable. It is supposed that both factors applied during precultivation have influence on colicin sensitivity by means of the change of receptor activity caused by LPS modification.

Calcium in bacteria: a solution to which problem?

Calcium and calcium-binding proteins including those resembling calmodulin are implicated in numerous diverse processes in bacteria. These processes include chemotaxis, sporulation, virulence, the transport of sugars and proteins, phosphorylation, heat shock, the initiation of DNA replication, septation, nucleoid structure, nuclease activity and recombination, the stability of the envelope, and phospholipid synthesis and configuration. That such varied processes should have a common factor, calcium, suggests major underlying principles of calcium metabolism which have yet to be discovered.

TonB and the gram-negative dilemma

TonB protein serves as an energy transducer to couple cytoplasmic membrane energy to high-affinity active transport of iron siderophores and vitamin B12 across the outer membranes of Gram-negative bacteria. The biochemical mechanism of the energy transduction remains to be determined, but important details are already known. TonB is targeted to and anchored in the cytoplasmic membrane by a single membrane-spanning domain and spans the periplasm to physically interact with outer-membrane receptors of the transport ligands. TonB-dependent energy transduction is modulated by ExbB protein, which stabilizes TonB, and possibly by several other proteins including ExbC, ExbD, and TolQ. TonB has a relatively short functional half-life that is accelerated when rates of active transport across the outer membrane are increased. A model that incorporates this information, as well as some tempered speculation, is presented.

Vitamin B12 transport in Escherichia coli: energy coupling between membranes

Cells of Escherichia coli possess high-affinity active transport systems of vitamin B12 and iron-siderophore complexes. Specific outer-membrane proteins carry out the energy-dependent transport across the outer membrane, in conjunction with the TonB coupling protein. Mutagenesis experiments have identified a conserved region near the amino-terminus of the outer-membrane transporters that is necessary for energy-coupled transport. The ability of extragenic suppressor mutations in tonB to correct the transport defect indicates that TonB couples the proton-motive force to the outer-membrane proteins by direct contact.

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.

Substitution of Co alpha-(5-hydroxybenzimidazolyl)cobamide (factor III) by vitamin B12 in Methanobacterium thermoautotrophicum

Methanobacterium thermoautotrophicum grown on mineral medium contains 120 nmol of Co alpha-(5-hydroxybenzimidazolyl)cobamides (derivatives of factor III) per g of dry cell mass as the sole cobamide. The bacterium assimilated several corrinoids and benzimidazole bases during autotrophic growth. The corrinoids were converted into factor III; however, after three transfers in 5,6-dimethylbenzimidazole (200 microM)-supplemented mineral medium, derivatives of factor III were completely replaced by derivatives of vitamin B12, which is atypical for methanogens. The total cobamide content of these cells and their growth rate were not affected compared with factor III-containing cells. Therefore, the high cobamide content rather than a particular type of cobamide is required for metabolism of methanogens. Derivatives of factor III are not essential cofactors of cobamide-containing enzymes from methanogenic bacteria, but they are the result of a unique biosynthetic ability of these archaebacteria. The cobamide biosynthesis include unspecific enzymes, which made it possible either to convert non-species-derived corrinoids into derivatives of factor III or to synthesize other types of cobamides than factor III. The cobamide biosynthesis is regulated by its end product. In addition, the uptake of extracellular cobamides is controlled, and the assimilated corrinoids regulate cellular cobamide biosynthesis.