The role of the Escherichia coli lambda receptor in the transport of maltose and maltodextrins

Maltose-Derivatized Fluorescence Turn-On Imaging Probe for Bacteria Detection

We report a maltose-derivatized fluorescence turn-on imaging probe, Mal-Cz, to detect E. coli and Staphylococci. The fluorescence turn-on is achieved through an intramolecular C-H insertion reaction of the perfluoroaryl azide-functionalized carbazole to give a fluorescent product. Confocal fluorescence microscopy confirmed the successful uptake of Mal-Cz by E. coli and Staphylococci upon photoactivation. The Mal-Cz probe could selectively detect E. coli and S. epidermidis in the presence of P. aeruginosa and M. smegmatis without interference from these bacteria. Both the photoactivation and bacteria detection can be accomplished using a hand-held UV lamp at 365 nm, with the limit of detection of 103 CFU/mL by the naked eye. Mal-Cz could also be used to detect E. coli and S. epidermidis spiked in milk by the naked eye under a hand-held UV lamp. The uptake of Mal-Cz requires metabolically active bacteria: the uptake was reduced in stationary phase bacteria and was diminished in bacteria that were killed by heating or treating with antibiotics or sodium azide. The uptake decreased with increasing concentration of added free maltose, indicating that Mal-Cz hijacked the maltose uptake pathways. In E. coli, the maltose transport systems, including maltoporin LamB, maltose binding protein MBP, and the maltose ATP binding cassette (ABC) transporter MalFGK2, are all critical for the transport of Mal-Cz. The uptake was diminished in the deletion mutants ΔLamB, ΔMalE, ΔMalF, and ΔMalK.

Genetic Analysis of Protein Translocation

Cells in all domains of life must translocate newly synthesized proteins both across membranes and into membranes. In eukaryotes, proteins are translocated into the lumen of the ER or the ER membrane. In prokaryotes, proteins are translocated into the cytoplasmic membrane or through the membrane into the periplasm for Gram-negative bacteria or the extracellular space for Gram-positive bacteria. Much of what we know about protein translocation was learned through genetic selections and screens utilizing lacZ gene fusions in Escherichia coli. This review covers the basic principles of protein translocation and how they were discovered and developed. In particular, we discuss how lacZ gene fusions and the phenotypes conferred were exploited to identify the genes involved in protein translocation and provide insights into their mechanisms of action. These approaches, which allowed the elucidation of processes that are conserved throughout the domains of life, illustrate the power of seemingly simple experiments.

β-CD-Functionalized Microdevice for Rapid Capture and Release of Bacteria

Most procedures for detecting pathogens in liquid media require an initial concentration step. In this regard, carbohydrates have proven to be attractive affinity ligands for the solid-phase capture of bacteria that use lectins for adhesion to host cell membranes. However, the use of cyclodextrin-immobilized substrates to selectively trap bacteria has not been explored before. Here, using quartz-crystal microbalance with dissipation monitoring experiments, we demonstrate that functionalization of surfaces by β-cyclodextrin (β-CD) can not only allow for rapid and efficient capture of bacterial cells in liquid but also their facile elution with an aqueous solution of a selectively methylated β-CD derivative as a competitive molecule. This capture/elution strategy, which is based on host-guest interactions between membrane components of the bacterial cell and the CD cavities, is performed in physiological conditions and can be integrated in a microchip. Indeed, proof-of-concept studies showed the potential of β-CD-modified micropillar-integrated microfluidic devices for concentration of bacteria. The results obtained with Escherichia coli suggest that this approach could be broadly applicable among Gram-negative bacteria, which share common cell membrane structures.

Tuning the affinity of anion binding sites in porin channels with negatively charged residues: molecular details for OprP

The cell envelope of the Gram negative opportunistic pathogen Pseudomonas aeruginosa is poorly permeable to many classes of hydrophilic molecules including antibiotics due to the presence of the narrow and selective porins. Here we focused on one of the narrow-channel porins, that is, OprP, which is responsible for the high-affinity uptake of phosphate ions. Its two central binding sites for phosphate contain a number of positively charged amino acids together with a single negatively charged residue (D94). The presence of this negatively charged residue in a binding site for negatively charged phosphate ions is highly surprising due to the potentially reduced binding affinity. The goal of this study was to better understand the role of D94 in phosphate binding, selectivity, and transport using a combination of mutagenesis, electrophysiology, and free-energy calculations. The presence of a negatively charged residue in the binding site is critical for this specific porin OprP as emphasized by the evolutionary conservation of such negatively charged residue in the binding site of several anion-selective porins. Mutations of D94 in OprP to any positively charged or neutral residue increased the binding affinity of phosphate for OprP. Detailed analysis indicated that this anionic residue in the phosphate binding site of OprP, despite its negative charge, maintained energetically favorable phosphate binding sites in the central region of the channel and at the same time decreased residence time thus preventing excessively strong binding of phosphate that would oppose phosphate flux through the channel. Intriguingly mutations of D94 to positively charged residues, lysine and arginine, resulted in very different binding affinities and free energy profiles, indicating the importance of side chain conformations of these positively charged residues in phosphate binding to OprP.

Structural basis for substrate specificity in the Escherichia coli maltose transport system

ATP-binding cassette (ABC) transporters are molecular pumps that harness the chemical energy of ATP hydrolysis to translocate solutes across the membrane. The substrates transported by different ABC transporters are diverse, ranging from small ions to large proteins. Although crystal structures of several ABC transporters are available, a structural basis for substrate recognition is still lacking. For the Escherichia coli maltose transport system, the selectivity of sugar binding to maltose-binding protein (MBP), the periplasmic binding protein, does not fully account for the selectivity of sugar transport. To obtain a molecular understanding of this observation, we determined the crystal structures of the transporter complex MBP-MalFGK2 bound with large malto-oligosaccharide in two different conformational states. In the pretranslocation structure, we found that the transmembrane subunit MalG forms two hydrogen bonds with malto-oligosaccharide at the reducing end. In the outward-facing conformation, the transmembrane subunit MalF binds three glucosyl units from the nonreducing end of the sugar. These structural features explain why modified malto-oligosaccharides are not transported by MalFGK2 despite their high binding affinity to MBP. They also show that in the transport cycle, substrate is channeled from MBP into the transmembrane pathway with a polarity such that both MBP and MalFGK2 contribute to the overall substrate selectivity of the system.

Gene tandem-mediated selection of coliphage lambda-receptive Agrobacterium, Pseudomonas, and Rhizobium strains

Agrobacterium, Pseudomonas, and Rhizobium spp. have been made receptive to coliphage lambda. To achieve this, recombinant (pTROY) plasmids carrying a constitutive Escherichia coli lamB gene encoding the lambda receptor and expressed from an insertion sequence 3 (IS3) promoter were introduced into various bacteria. Because the wild-type lambda receptor was not expressed in these bacteria, a procedure called the lamB gene tandem protocol was used to select lamB alleles that expressed the lambda receptor. This gene tandem protocol may have general use in adapting genes with nonselectable traits to different organisms. Agrobacterium, Pseudomonas, and Rhizobium strains carrying pTROY41613, which encoded a gene tandem-selected lambda receptor, could be quantitatively transduced with lambda-packaged cosmids. With the ability to confer lambda receptivity on organisms, phage lambda now becomes a general DNA delivery agent.

Identification of Lactococcus lactis genes required for bacteriophage adsorption

The aim of this work was to identify genes in Lactococcus lactis subsp. lactis IL1403 and Lactococcus lactis subsp. cremoris Wg2 important for adsorption of the 936-species phages bIL170 and phi 645, respectively. Random insertional mutagenesis of the two L. lactis strains was carried out with the vector pGh9:ISS1, and integrants that were resistant to phage infection and showed reduced phage adsorption were selected. In L. lactis IL1403 integration was obtained in the ycaG and rgpE genes, whereas in L. lactis Wg2 integration was obtained in two genes homologous to ycbC and ycbB of L. lactis IL1403. rgpE and ycbB encode putative glycosyltransferases, whereas ycaG and ycbC encode putative membrane-spanning proteins with unknown functions. Interestingly, ycaG, rgpE, ycbC, and ycbB are all part of the same operon in L. lactis IL1403. This operon is probably involved in biosynthesis and transport of cell wall polysaccharides (WPS). Binding and infection studies showed that phi645 binds to and infects L. lactis Wg2, L. lactis IL1403, and L. lactis IL1403 strains with pGh9:ISS1 integration in ycaG and rgpE, whereas bIL170 binds to and infects only L. lactis IL1403 and cannot infect Wg2. These results indicate that phi 645 binds to a WPS structure present in both L. lactis IL1403 and L. lactis Wg2, whereas bIL170 binds to another WPS structure not present in L. lactis Wg2. Binding of bIL170 and phi 645 to different WPS structures was supported by alignment of the receptor-binding proteins of bIL170 and phi 645 that showed no homology in the C-terminal part.

Display of proteins on bacteria

Display of heterologous proteins on the surface of microorganisms, enabled by means of recombinant DNA technology, has become an increasingly used strategy in various applications in microbiology, biotechnology and vaccinology. Gram-negative, Gram-positive bacteria, viruses and phages are all being investigated in such applications. This review will focus on the bacterial display systems and applications. Live bacterial vaccine delivery vehicles are being developed through the surface display of foreign antigens on the bacterial surfaces. In this field, 'second generation' vaccine delivery vehicles are at present being generated by the addition of mucosal targeting signals, through co-display of adhesins, in order to achieve targeting of the live bacteria to immunoreactive sites to thereby increase immune responses. Engineered bacteria are further being evaluated as novel microbial biocatalysts with heterologous enzymes immobilized as surface exposed on the bacterial cell surface. A discussion has started whether bacteria can find use as new types of whole-cell diagnostic devices since single-chain antibodies and other type of tailor-made binding proteins can be displayed on bacteria. Bacteria with increased binding capacity for certain metal ions can be created and potential environmental or biosensor applications for such recombinant bacteria as biosorbents are being discussed. Certain bacteria have also been employed for display of various poly-peptide libraries for use as devices in in vitro selection applications. Through various selection principles, individual clones with desired properties can be selected from such libraries. This article explains the basic principles of the different bacterial display systems, and discusses current uses and possible future trends of these emerging technologies.

Bacillus subtilis contains a cyclodextrin-binding protein which is part of a putative ABC-transporter

Bacillus subtilis is able to grow on alpha-, beta- and gamma-cyclodextrins as a carbon source via a yet unknown metabolizing system. Sequence analysis of the B. subtilis genome reveals that the putative yvfK-yvfO operon seems to be involved in cyclodextrin utilization, containing the open reading frame yvfK, now termed cycB. The amino acid sequence derived from the DNA sequence bears high similarities to solute-binding proteins from B. subtilis, as well as to cymE from Klebsiella oxytoca and malE from Escherichia coli, both encoding solute-binding proteins able to interact with cyclodextrins. A [His](6)-tagged variant of CycB from B. subtilis was constructed, overproduced in E. coli and purified. The modified protein has been used to study its substrate specificity by surface plasmon resonance and fluorescence spectroscopy. From these data, CycB can be classified as a cyclodextrin-binding protein which interacts with all three natural cyclodextrins: alpha, beta and gamma, thereby showing the highest affinity to gamma-cyclodextrin.

Ligand-induced structural changes to maltodextrin-binding protein as studied by solution NMR spectroscopy

Solution NMR studies on the physiologically relevant ligand-free and maltotriose-bound states of maltodextrin-binding protein (MBP) are presented. Together with existing data on MBP in complex with beta-cyclodextrin (non-physiological, inactive ligand), these new results provide valuable information on changes in local structure, dynamics and global fold that occur upon ligand binding to this two-domain protein. By measuring a large number of different one-bond residual dipolar couplings, the domain conformations, critical for biological function, were investigated for all three states of MBP. Structural models of the solution conformation of MBP in a number of different forms were generated from the experimental dipolar coupling data and X-ray crystal structures using a quasi-rigid-body domain orientation algorithm implemented in the structure calculation program CNS. Excellent agreement between relative domain orientations in ligand-free and maltotriose-bound solution conformations and the corresponding crystal structures is observed. These results are in contrast to those obtained for the MBP/beta-cyclodextrin complex where the solution state is found to be approximately 10 degrees more closed than the crystalline state. The present study highlights the utility of residual dipolar couplings for orienting protein domains or macromolecules with respect to each other.

In vivo and in vitro studies of transmembrane beta-strand deletion, insertion or substitution mutants of the Escherichia coli K-12 maltoporin

LamB of Escherichia coli K12, also called maltoporin, is an outer membrane protein, which specifically facilitates the diffusion of maltose and maltodextrin through the bacterial outer membrane. Each monomer is composed of an 18-stranded antiparallel beta-barrel. In the present work, on the basis of the known X-ray structure of LamB, the effects of modifications of the beta-barrel domain of maltoporin were studied in vivo and in vitro. We show that: (i) the substitution of the pair of strands beta13-beta14 of the E. coli maltoporin with the corresponding pair of strands from the functionally related maltoporin of Salmonella typhimurium yielded a protein active in vivo and in vitro; and (ii) the removal of one pair of beta-strands (deletion beta13-beta14) from the E. coli maltoporin, or its replacement by a pair of strands from the general porin OmpF of E. coli, leads to recombinant proteins that lost in vivo maltoporin activities but still kept channel formation and carbohydrate binding in vitro. We also inserted into deletion beta13-beta14 the portion of the E. coli LamB protein comprising strands beta13 to beta16. This resulted in a protein expected to have 20 beta-strands and which completely lost all LamB-specific activities in vivo and in vitro.

Ferric enterobactin binding and utilization by Neisseria gonorrhoeae

FetA, formerly designated FrpB, an iron-regulated, 76-kDa neisserial outer membrane protein, shows sequence homology to the TonB-dependent family of receptors that transport iron into gram-negative bacteria. Although FetA is commonly expressed by most neisserial strains and is a potential vaccine candidate for both Neisseria gonorrhoeae and Neisseria meningitidis, its function in cell physiology was previously undefined. We now report that FetA functions as an enterobactin receptor. N. gonorrhoeae FA1090 utilized ferric enterobactin as the sole iron source when supplied with ferric enterobactin at approximately 10 microM, but growth stimulation was abolished when an omega (Omega) cassette was inserted within fetA or when tonB was insertionally interrupted. FA1090 FetA specifically bound 59Fe-enterobactin, with a Kd of approximately 5 microM. Monoclonal antibodies raised against the Escherichia coli enterobactin receptor, FepA, recognized FetA in Western blots, and amino acid sequence comparisons revealed that residues previously implicated in ferric enterobactin binding by FepA were partially conserved in FetA. An open reading frame downstream of fetA, designated fetB, predicted a protein with sequence similarity to the family of periplasmic binding proteins necessary for transporting siderophores through the periplasmic space of gram-negative bacteria. An Omega insertion within fetB abolished ferric enterobactin utilization without causing a loss of ferric enterobactin binding. These data show that FetA is a functional homolog of FepA that binds ferric enterobactin and may be part of a system responsible for transporting the siderophore into the cell.

Maltose/maltodextrin system of Escherichia coli: transport, metabolism, and regulation

The maltose system of Escherichia coli offers an unusually rich set of enzymes, transporters, and regulators as objects of study. This system is responsible for the uptake and metabolism of glucose polymers (maltodextrins), which must be a preferred class of nutrients for E. coli in both mammalian hosts and in the environment. Because the metabolism of glucose polymers must be coordinated with both the anabolic and catabolic uses of glucose and glycogen, an intricate set of regulatory mechanisms controls the expression of mal genes, the activity of the maltose transporter, and the activities of the maltose/maltodextrin catabolic enzymes. The ease of isolating many of the mal gene products has contributed greatly to the understanding of the structures and functions of several classes of proteins. Not only was the outer membrane maltoporin, LamB, or the phage lambda receptor, the first virus receptor to be isolated, but also its three-dimensional structure, together with extensive knowledge of functional sites for ligand binding as well as for phage lambda binding, has led to a relatively complete description of this sugar-specific aqueous channel. The periplasmic maltose binding protein (MBP) has been studied with respect to its role in both maltose transport and maltose taxis. Again, the combination of structural and functional information has led to a significant understanding of how this soluble receptor participates in signaling the presence of sugar to the chemosensory apparatus as well as how it participates in sugar transport. The maltose transporter belongs to the ATP binding cassette family, and although its structure is not yet known at atomic resolution, there is some insight into the structures of several functional sites, including those that are involved in interactions with MBP and recognition of substrates and ATP. A particularly astonishing discovery is the direct participation of the transporter in transcriptional control of the mal regulon. The MalT protein activates transcription at all mal promoters. A subset also requires the cyclic AMP receptor protein for transcription. The MalT protein requires maltotriose and ATP as ligands for binding to a dodecanucleotide MalT box that appears in multiple copies upstream of all mal promoters. Recent data indicate that the ATP binding cassette transporter subunit MalK can directly inhibit MalT when the transporter is inactive due to the absence of substrate. Despite this wealth of knowledge, there are still basic issues that require clarification concerning the mechanism of MalT-mediated activation, repression by the transporter, biosynthesis and assembly of the outer membrane and inner membrane transporter proteins, and interrelationships between the mal enzymes and those of glucose and glycogen metabolism.

Extensive features of tight oligosaccharide binding revealed in high-resolution structures of the maltodextrin transport/chemosensory receptor

BACKGROUND

Active-transport processes perform a vital function in the life of a cell, maintaining cell homeostasis and allowing access of nutrients. Maltodextrin/maltose-binding protein (MBP; M(r) = 40k) is a receptor protein which serves as an initial high-affinity binding component of the active-transport system of maltooligosaccharides in bacteria. MBP also participates in chemotaxis towards maltooligosaccharides. The interaction between MBP and specific cytoplasmic membrane proteins initiates either active transport or chemotaxis. In order to gain new understanding of the function of MBP, especially its versatility in binding different linear and cyclic oligosaccharides with similar affinities, we have undertaken high-resolution X-ray analysis of three oligosaccharide-bound structures.

RESULTS

The structures of MBP complexed with maltose, maltotriose and maltotetraose have been refined to high resolutions (1.67 to 1.8 A). These structures provide details at the atomic level of many features of oligosaccharide binding. The structures reveal differences between buried and surface binding sites and show the importance of hydrogen bonds and van der Waals interactions, especially those resulting from aromatic residue stacking. Insights are provided into the structural plasticity of the protein, the binding affinity and the binding specificity with respect to alpha/beta anomeric preference and oligosaccharide length. In addition, the structures demonstrate the different conformations that can be adopted by the oligosaccharide within the complex.

CONCLUSIONS

MBP has a two-domain structure joined by a hinge-bending region which contains the substrate-binding groove. The bound maltooligosaccharides have a ribbon-like structure: the edges of the ribbon are occupied by polar hydroxyl groups and the flat surfaces are composed of nonpolar patches of the sugar ring faces. The polar groups and nonpolar patches are heavily involved in forming hydrogen bonds and van der Waals contacts, respectively, with complimentary residues in the groove. Hinge-bending between the two domains enables the participation of both domains in the binding and sequestering of the oligosaccharides. Changes in the subtle contours of the binding site allow binding of maltodextrins of varying length with similarly high affinities. The fact that the three bound structures are essentially identical ensures productive interaction with the oligomeric membrane proteins, which are distinct for transport and chemotaxis.

Chaperone properties of the bacterial periplasmic substrate-binding proteins

Bacterial periplasmic substrate-binding proteins are initial receptors in the process of active transport across cell membranes and/or chemotaxis. Each of them binds a specific substrate (e.g. sugar, amino acid, or ion) with high affinity. For transport, each binding protein interacts with a cognate membrane complex consisting of two hydrophobic proteins and two subunits of a hydrophilic ATPase. For chemotaxis, binding proteins interact with specific membrane chemotaxis receptors. We report, herewith, that the oligopeptide-binding protein OppA of Escherichia coli, the maltose-binding protein MalE of E. coli, and the galactose-binding protein MglB of Salmonella typhimurium interact with unfolded and denatured proteins, such as the molecular chaperones that are involved in protein folding and protein renaturation after stress. These periplasmic substrate-binding proteins promote the functional folding of citrate synthase and alpha-glucosidase after urea denaturation. They prevent the aggregation of citrate synthase under heat shock conditions, and they form stable complexes with several unfolded proteins, such as reduced carboxymethyl alpha-lactalbumin and unfolded bovine pancreatic trypsin inhibitor. These chaperone-like functions are displayed by both the liganded and ligand-free forms of binding proteins, and they occur at binding protein concentrations that are 10-100-fold lower than their periplasmic concentration. These results suggest that bacterial periplasmic substrate-binding proteins, in addition to their function in transport and chemotaxis, might be implicated in protein folding and protection from stress in the periplasm.

Further genetic analysis of the C-terminal external loop region in Escherichia coli maltoporin

LamB specifically facilitates the diffusion of maltose and maltodextrins through the bacterial outer membrane, and acts as a general (i.e. non-specific) porin for small hydrophilic molecules (< 600 daltons). We reported previously that deletion of the last predicted external domain near the C-terminus of the Eschirichia coli LamB protein (residues 376 to 405), affected in vivo the binding and transport of maltodextrins (specific pore functions), and also increased bacterial sensitivity to large antibiotics. The residues covered by this deletion correspond almost exactly to the major cell surface loop of LamB on the structural model based on X-ray crystallography (loop L9, residues 375 to 405). The L9 loop comprises a large central portion, which varies in size and sequence between the LamB proteins from different species. This variable region is flanked by two highly charged and conserved portions, which overlap with the adjacent beta strands. To identify subregions in L9 that influence the pore properties of LamB, we constructed and analysed nine mutants in loop L9 and its flanking sequences. Deletion of the 23-amino-acids central variable portion of the loop (residues 379 to 401), and deletion of the downstream conserved region (residues 402 to 409), only moderately affected specific maltoporin function. In contrast, deletion of the conserved region (residues 372 to 378) upstream of the variable portion strongly decreased specific maltoporin function and also increased sensitivity to large antibiotics, accounting for most, if not all, of the effects of the complete deletion of L9.

Topology of the membrane protein LamB by epitope tagging and a comparison with the X-ray model

We previously developed a genetic approach to study, with a single antibody, the topology of the outer membrane protein LamB, an Escherichia coli porin with specificity towards maltodextrins and a receptor for bacteriophage lambda. Our initial procedure consisted of inserting at random the same reporter epitope (the C3 neutralization epitope from poliovirus) into permissive sites of LamB (i.e., sites which tolerate insertions without deleterious effects on the protein activities or the cell). A specific monoclonal antibody was then used to examine the position of the inserted epitope with respect to the protein and the membrane. In the present work, we set up a site-directed procedure to insert the C3 epitope at new sites in order to distinguish between two-dimensional folding models. This allowed us to identify two new surface loops of LamB and to predict another periplasmic exposed region. The results obtained by random and directed epitope tagging are analyzed in light of the recently published X-ray structure of the LamB protein. Study of 23 hybrid LamB-C3 proteins led to the direct identification of five of the nine external loops (L4, L5, L6, L7, and L9) and led to the prediction of four periplasmic loops (I1, I4, I5, and I8) of LamB. Nine of the hybrid proteins did not lead to topological conclusions, and none led to the wrong predictions or conclusions. The comparison indicates that parts of models based on secondary structure predictions alone are not reliable and points to the importance of experimental data in the establishment of outer membrane protein topological models. The advantages and limitations of genetic foreign epitope insertion for the study of integral membrane proteins are discussed.

A role for residue 151 of LamB in bacteriophage lambda adsorption: possible steric effect of amino acid substitutions

LamB is the cell surface receptor for bacteriophage lambda. LamB missense mutations yielding resistance to lambda have been previously grouped in two classes. Class I mutants block growth of lambda with wild-type host range (lambda h+) but support growth of one-step extended-host-range mutants (lambda h). Class II mutants block lambda h but support growth of two-step extended host range mutants (lambda hh*). While Class I mutations occur at 11 different amino acid sites, in five distinct portions of LamB, all the Class II mutations analyzed previously correspond to the same G-to-D change at amino acid 151. We generated by in vitro mutagenesis four different new substitutions at site 151 (to S, V, R, and C). Two of the mutants (G-151-->V [G151V] and G151R) were of Class II, while the two others (G151S and G151C) were of Class I, demonstrating that not only the site but also the nature of the substitutions at residue 151 was critical for the phage sensitivity phenotypes. The introduction of a negatively charged, a positively charged, or an aliphatic nonpolar residue at site 151 of LamB prevented both lambda h+ and lambda h adsorption, indicating that the block is not due to a charge effect. In contrast to G151D, which was sensitive to all the lambda hh* phages, G151V and G151R conferred sensitivity to only four of the five lambda hh* phages. Thus, G151V and G151R represent a new subclass of Class II LamB mutations that is more restrictive with respect to the growth of lambda hh*. Our results agree with the hypothesis that residue 151 belongs to an accessibility gate controlling the access to the phage tight-binding site and that substitutions at this residue affect the access of the phage to the binding site in relation to the size of the substitute side chain (surface area): the most restrictive changes are G151V and G151R, followed to a lesser extent by G151D and they by G151S and G151C.

Adsorption of bacteriophage lambda on the LamB protein of Escherichia coli K-12: point mutations in gene J of lambda responsible for extended host range

LamB is the cell surface receptor for bacteriophage lambda. LamB missense mutations yielding resistance to lambda group in two classes. Class I mutants block the growth of lambda with the wild-type host range (lambda h+) but support the growth of one-step host range mutants (lambda h). Class II mutants block lambda h but support the growth of two-step host range mutant (lambda hh*) phages. To identify amino acid residues in the J protein (the tail fiber of phage lambda) responsible for the extended host range phenotype of mutants of phage (lambda h+), we selected a series of one-step (lambda h) and two-step (lambda hh*) host range mutants and analyzed their corresponding J genes. Three different class I LamB missense mutants (mutations at sites 247, 245, and 148) were used to select 11 independent, new, one-step host range mutants (lambda h phages). DNA sequence analysis revealed a single-amino-acid change in each case. The 11 alterations affected only three residues in the distal part of J, corresponding to a Val-->Ala change at site 1077 in five cases, a Thr-->Met change at site 1040 in three cases, and a Leu-->Pro change at site 1127 in three cases. Recombination experiments confirmed that in the cases tested, the mutations identified were indeed responsible for the extended host range phenotype. The class II LamB mutant (Gly-->Asp at site 151) was used to select two-step extended host range mutants (lambda hh* phages) from three new lambda h phages, corresponding to different amino acid modifications in the J protein (at sites 1040, 1077, and 1127). The new lambda hh* phages analyzed corresponded to either double or triple point mutations located at the distal end of the J protein. In all, seven residues involved in the extended host range properties of lambda mutants were identified in the distal part of the J protein, suggesting that the last C-terminal portion of the J protein participates directly in the adsorption of the phage onto LamB. In agreement with the fact that the lambda h mutants (and the lambda hh* mutants) could grow on all of the lamB class I mutations tested, we found tha the nature of the J mutations did not depend on the LamB class I mutant used to select them. This is interpreted as meaning that the mutated residues in the J protein and in the LamB mutants are not involved in allele-specific protein-protein interactions. Rather, the LamB mutations would block a step in phage adsorption, and this block would be overcome by the mutations in the J protein.

The ompS gene of Vibrio cholerae encodes a growth-phase-dependent maltoporin

The outer membrane of Vibrio cholerae contains a maltose-inducible major protein, OmpS (43 kDa), that is common to different isolates. Nucleotide sequence analysis of the corresponding structural gene, ompS, revealed an open reading frame encoding a 412-amino-acid polypeptide. The amino acid sequence of OmpS is similar to that of LamB, the Escherichia coli maltoporin, and to ScrY or Klebsiella pneumoniae, although the antigenic determinants of these proteins are different. The cloned ompS gene complemented an ompS mutation of V. cholerae and the corresponding polypeptide could function as a maltoporin in a LamB- mutant of E. coli. The promoter region of ompS is highly homologous to the malK-lamB promoter of E. coli and the ompS gene is controlled by MalT in E. coli. This indicates that the same kind of regulatory mechanism is used to activate the ompS expression in V. cholerae and malK-lamB expression in E. coli. An ompS-lacZ transcriptional fusion was used to demonstrate a dual control in ompS expression; the ompS gene is responsive to the inducers maltose and trehalose but in their absence it is also expressed in response to growth-phase. These different modes of induction might be of importance during different stages of V. cholerae infection.

The importance of the binding-protein-dependent Mgl system to the transport of glucose in Escherichia coli growing on low sugar concentrations

Glucose limitation in chemostats derepressed the binding-protein-dependent Mgl transport system, which is strongly repressed during growth in batch culture with high glucose levels. The limitation-induced Mgl activity was higher than that of batch cultures "fully induced" for the Mgl system after growth on glycerol plus fucose. Mgl- strains were impaired compared to Mgl+ bacteria in removing glucose from sugar-limited chemostats and were outcompeted in mixed continuous culture on limiting glucose. The influence of Mgl was not observed on growth with limiting maltose or non-carbohydrates, and thus was specific for glucose, a known substrate of the Mgl system. In the absence of the two glucose-specific membrane components of the phosphoenolpyruvate:sugar phosphotransferase system, non-PTS-dependent growth on glucose was observed in continuous culture, but only under sugar-limited conditions derepressing the Mgl system and not in glucose-rich batches or continuous culture. Hence growth of Escherichia coli on glucose at micromolar concentrations involves a significant contribution of a binding-protein-dependent transport system. The participation of multiple transporters in glucose transport can account for the complex non-hyperbolic dependence of growth-rate on glucose concentration and for discrepancies in studies attempting to describe growth on glucose purely in terms of phosphotransferase kinetics.

Two-dimensional crystals of Escherichia coli maltoporin and their interaction with the maltose-binding protein

We have reconstituted Escherichia coli maltoporin into phospholipid membranes at low lipid-to-protein ratios to produce two-dimensional crystals of this membrane protein. Electron microscopy of negatively stained membranes showed three different types of arrays, two of them hexagonal and the third rectangular, all diffracting to approximately (2 nm)-1. Furthermore, we have core-constituted maltoporin with the maltose-binding protein from E. coli, a soluble periplasmic protein that has been proposed to interact with maltoporin. One of the hexagonal arrays was found to bind maltose-binding protein molecules in a regular way, while the maltose-binding protein binding sites were not accessible in the other crystal forms. Difference maps from averaged decorated arrays and undecorated controls showed three symmetry-related maltose-binding protein binding sites per maltoporin trimer, of which not more than one is likely to be occupied at a given time. Using multivariate statistical analysis to select similar unit cells of the decorated maltoporin array, we have obtained a map showing the rough outline of a maltose-binding protein molecule interacting with the pore formed by a maltoporin trimer.

In vitro synthesized bacterial outer membrane protein is integrated into bacterial inner membranes but translocated across microsomal membranes

The LamB protein is an integral membrane protein of the outer membrane of Escherichia coli. We have now found that, when synthesized in an E. coli cell-free translation system supplemented with inverted vesicles derived from the E. coli inner membrane, LamB protein is integrated into the vesicle membrane as assayed by its resistance to extraction at alkaline pH. These data suggest that the inner membrane is the primary site for integration of LamB protein prior to subsequent sorting to the outer membrane. When synthesized in a wheat germ cell-free translation system supplemented with canine microsomal membranes, LamB protein is glycosylated at one or two cryptic sites, and surprisingly, it is translocated across instead of being integrated into the vesicle membrane. We suggest that the translocation machinery of the microsomal membrane, although able to recognize the signal sequence(s) of LamB, is unable to recognize its stop-transfer sequence(s), thereby yielding translocation instead of integration.

Substrate specificity of the Escherichia coli maltodextrin transport system and its component proteins

Maltooligosaccharides up to maltoheptaose are transported by the maltodextrin transport system of Escherichia coli. The overall substrate specificity of the transport system was investigated by using 15 maltodextrin analogues with various modifications at the reducing end of the oligosaccharides as competing substrates. The binding interaction of the analogues with maltoporin in the outer membrane and the periplasmic maltose-binding protein, the two protein components of the transport system with known specificity for maltodextrins, was also investigated. All analogues containing several alpha, 1----4-glucosyl linkages were bound with high affinity by maltoporin and maltose-binding protein, regardless of O-methyl, O-nitrophenyl, beta-glucosyl or beta-fructosyl substitutions at the reducing end of the dextrins. Introduction of a negative charge or lack of a ring structure at the reducing end were also ineffective in abolishing binding by these two proteins. These results suggest that the structure of the reducing glucose is not important in the binding specificity of maltoporin or maltose-binding protein. However, the high affinity of these proteins for analogues was not in itself sufficient for recognition by the transport system overall. Maltohexaitol, 4-nitrophenyl alpha-maltotetraoside and 4-beta-D-maltopentaosyl-D-glucopyranose were bound with the same affinity as comparable maltodextrins by both maltoporin and maltose-binding protein but were poorly recognized by the transport system. These results suggest that another, yet uninvestigated component of the transport system has a more restricted specificity towards changes at the reducing end of the maltodextrin molecule.

Maltose-binding protein does not modulate the activity of maltoporin as a general porin in Escherichia coli

Maltoporin (lambda receptor) is part of the maltose transport system in Escherichia coli and is necessary for the facilitated diffusion of maltose and maltodextrins across the outer membrane. Maltoporin also allows the diffusion of nonmaltodextrin substrates, albeit with less efficiency. The preference of maltoporin for maltodextrins in vivo is thought to be the result of an interaction of maltoporin with the maltose-binding protein, the malE gene product. In a recent report Heuzenroeder and Reeves (J. Bacteriol. 144:431-435, 1980) suggested that this interaction establishes a gating mechanism which inhibits the diffusion of nonmaltodextrin substrates, such as lactose. To reinvestigate this important conclusion, we constructed ompR malTc strains carrying either the malE+ gene, the nonpolar malE444 deletion, or the malE254 allele, which specifies an interaction-deficient maltose-binding protein. Lactose uptake was measured at different concentrations below the Km of this transport system and under conditions where transport was limited by the diffusion through maltoporin. We found no difference in the kinetics of lactose uptake irrespective of the malE allele. We conclude that the maltose-binding protein does not modulate the activity of maltoporin as a general outer membrane porin.

Phosphate transport in Pseudomonas aeruginosa. Involvement of a periplasmic phosphate-binding protein

A binding protein for inorganic phosphate was purified to apparent homogeneity from the shock fluids of phosphate-limited Pseudomonas aeruginosa. The purified protein bound one molecule of phosphate per molecule of binding protein with an average Kd of 0.34 microM. Arsenate, pyrophosphate and polyphosphates up to 15 units long could inhibit the binding of phosphate to the binding protein, although organic phosphates, such as glucose 6-phosphate, glycerol 3-phosphate and adenosine 5'-monophosphate could not. Mutants lacking the phosphate-binding protein were isolated and shown to be deficient in phosphate transport compared with wild-type cells. Two kinetically distinct systems for phosphate uptake could be observed in wild-type cells, with apparent Km values of 0.46 +/- 0.10 microM (high affinity) and 12.0 +/- 1.6 microM (low affinity). In contrast, only a single low-affinity transport system was observable in mutants lacking the binding protein (Km apparent = 19.3 +/- 1.4 microM Pi), suggesting the involvement of the binding protein in the inducible high-affinity phosphate-uptake system of P. aeruginosa.

Extension of bacteriophage lambda host range: selection, cloning, and characterization of a constitutive lambda receptor gene

A set of plasmids has been constructed that carry a constitutive lamB gene (LamBc phenotype) from Escherichia coli and that confer functional phage lambda receptors to bacteria other than E. coli. This E. coli LamBc strain has been selected to escape both maltose-inducible and glucose-repressible control. Constitutivity results from an IS-3 insertion, carrying a mobile promoter, proximal to lamB. The LamBc DNA has been cloned into both broad and narrow host-range plasmids, and the resulting pTROY plasmids have been transferred to diverse bacteria. Both Salmonella typhimurium/pTROY and Klebsiella pneumoniae/pTROY strains efficiently adsorb phage lambda; Pseudomonas aeruginosa/pTROY strains do not. Introduction of a functional E. coli LamB protein into foreign bacterial will allow these bacteria carrying pTROY plasmids to be infected by phage lambda recombinant DNA libraries, phage lambda::Tn insertion mutagenesis vectors, and in vivo lambda-packaged cosmids.

Affinity engineering of maltoporin: variants with enhanced affinity for particular ligands

Affinity-chromatographic selection on immobilized starch was used to selectively enhance the affinity of the maltodextrin-specific pore protein ( maltoporin , LamB protein, or lambda receptor protein) in the outer membrane of E. coli. Selection strategies were established for rare bacteria in large populations producing maltoporin variants with enhanced affinities for both starch and maltose, for starch but not maltose and for maltose but not starch. Three classes of lamB mutants with up to eight-fold increase in affinity for particular ligands were isolated. These mutants provide a unique range of modifications in the specificity of a transport protein.

Outer membrane porin proteins F, P, and D1 of Pseudomonas aeruginosa and PhoE of Escherichia coli: chemical cross-linking to reveal native oligomers

Native oligomers of three Pseudomonas aeruginosa outer membrane porin proteins and one Escherichia coli porin were demonstrated by using a chemical cross-linking technique. P. aeruginosa protein F, the major constitutive outer membrane porin, was cross-linked to dimers in outer membrane and whole-cell cross-linking experiments. Purified preparations of P. aeruginosa proteins F, D1 (glucose induced), and P (phosphate starvation induced) and E. coli protein PhoE (Ic) were also cross-linked to reveal dimers and trimers upon two-dimensional sodium dodecyl sulfate-polyacrylamide electrophoretic analysis. Cross-linking of protein F was abolished by pretreatment of the protein with sodium dodecyl sulfate, indicating that the cross-linked products were due to native associations in the outer membrane.

A mutant form of maltose-binding protein of Escherichia coli deficient in its interaction with the bacteriophage lambda receptor protein

In one malE mutant known to be deficient in the transport of maltose and maltodextrins across the outer membrane, the altered MalE protein was shown to be defective in its interaction with the phage lambda receptor, or LamB protein, of the outer membrane.

Reconstitution of maltose transport in Escherichia coli: conditions affecting import of maltose-binding protein into the periplasm of calcium-treated cells

The reconstitution of active transport by the Ca2+ -induced import of exogenous binding protein was studied in detail in whole cells of a malE deletion mutant lacking the periplasmic maltose-binding protein. A linear increase in reconstitution efficiency was observed by increasing the Ca2+ - concentration in the reconstitution mixture up to 400 mM. A sharp pH optimum around pH 7.5 was measured for reconstitution. Reconstitution efficiency was highest at 0 degree C and decreased sharply with increasing temperature. The time necessary for optimal reconstitution at 0 degree C and 250 mM Ca2+ was about 1 min. The competence for reconstitution was highest in exponentially growing cultures with cell densities up to 1 X 10(9)/ml and declined when the cells entered the stationary-growth phase. The apparent Km for maltose uptake was the same as that of wild-type cells (1 to 2 microM). Vmax at saturating maltose-binding protein concentration was 125 pmol per min per 7.5 X 10(7) cells (30% of the wild-type activity). The concentration of maltose-binding protein required for half-maximal reconstitution was about 1 mM. The reconstitution procedure appears to be generally applicable. Thus, galactose transport in Escherichia coli could also be reconstituted by its respective binding protein. Maltose transport in E. coli was restored by maltose-binding protein isolated from Salmonella typhimurium. Finally, in S. typhimurium, histidine transport was reconstituted by the addition of shock fluid containing histidine-binding protein to a hisJ deletion mutant lacking histidine-binding protein. The method is fast and general enough to be used as a screening procedure to distinguish between transport mutants in which only the binding protein is affected and those in which additional transport components are affected.

On the insertion of proteins into membranes

Recent data concerning the primary structure and the interactions of proteins with membranes suggest the existence of two classes of integral membrane proteins. In the first class, the polypeptide chain crosses the membrane only once. The membrane penetrating fragment is markedly hydrophobic and contains several positive charges on its C-terminal border. In the second class, the protein is folded in a complex fashion within the membrane and the knowledge of its amino acid sequence is not sufficient to predict the manner in which the protein interacts with the membrane.

Isolation, by affinity chromatography, of mutant escherichia coli cells with novel regulation of lamB expression

Affinity chromatography was used as a positive genetic selection technique for the isolation of cells exhibiting high levels of surface receptor expression. Starting from a large population of Escherichia coli with no maltodextrin receptor due to a deletion of malT, the positive regulator gene required for receptor synthesis, cells were chromatographically enriched that could bind to starch-Sepharose, an immobilized ligand of the receptor. One such isolate showed over 25% of wild-type-induced levels of receptor in the absence of malT and levels higher than that of the wild type in a malT+ background. In contrast to wild-type cells, receptor expression in the isolate was insensitive to control by cAMP. The maltodextrin receptor synthesized by the mutant was identical to wild-type protein in terms of ligand affinity and electrophoretic mobility and was dependent on lamB, the structural gene for the receptor. The directed evolution of this novel form of lamB expression was dependent on at least two mutations in the isolate.

Bacteriophage lambda receptor protein in Escherichia coli K-12: lowered affinity of some mutant proteins for maltose-binding protein in vitro

Mutant and wild-type LamB proteins (phage lambda receptor proteins) were purified by affinity chromatography with immobilized maltose-binding protein, and their transport functions were tested in reconstituted liposomes. Two mutant proteins exhibited a marked decrease in affinity for immobilized maltose-binding protein, as well as altered transport rates.

Study of binding protein-ligand interaction by ammonium sulfate-assisted adsorption on cellulose esters filters

A technique for the study of neutral carbohydrate binding protein-ligand interaction is described in this report. It is based on filtration on cellulose esters filters of a mixture of the binding protein and the radioactive ligand, following a treatment of this mixture with ammonium sulfate; the technique is described for the galactose binding protein and for the maltose binding protein of Escherichia coli. For the galactose binding protein, an ammonium sulfate concentration far below that required for precipitation of the protein is sufficient to promote an almost complete retention of the protein on the filters. Furthermore, the addition of ammonium sulfate does not modify the amount of preexisting binding protein-ligand complex, and, in much less than one second, leads to a conformation of the protein-ligand complex which does not allow further ligand binding or dissociation. Hence, the technique is not only very useful for the detection of binding proteins in crude extracts and during purification procedures, it is also of value in the determination of the kinetic parameters of protein-ligand interactions.

The periplasmic maltose-binding protein modifies the channel-forming characteristics of maltoporin

Maltoporin, a protein spanning Escherichia coli outer membranes, modifies electrical conductance of membranes due to its channel-forming properties. This observation was made by conductance measurements across planar bilayers which were derived from unextracted, isolated outer membrane vesicles using a porin-deficient E. coli strain. Alternatively, proteoliposomes reconstituted with detergent-solubilized homogeneous maltoporin and phospholipids were used. With either membrane preparation, channel conductance was observed, although no discrete conductance levels were detected. The presence of lipopolysaccharide, a bacterial glycolipid, was not required, nor did it affect channel activity. In the presence of the water-soluble periplasmic maltose-binding protein, conductance fluctuations occurred in discrete steps, demonstrating opening and closing events of channels. Multiple step sizes (1/3, 2/3 and 1 ns in 1 M KCl) in single channel traces suggest cooperative opening and closing of up to three channels. The action of maltose-binding protein is highly asymmetrical, and its affinity to maltoporin is very high (KD = 1.5 X 10(-7) M). Association of maltose-binding protein to maltoporin shifts, for a given polarity, the equilibrium between open and closed states in favour of closed states. This result matches earlier in vivo studies, and supports the physiological significance of the observations made.

cir, a gene conferring resistance to colicin I maps between mgl and fpk on the Escherichia coli chromosome

With the help of the tetracycline resistance transposon Tn10 in and around the mgl genes the gene cir was mapped. cir is 80% cotransducible with mgl by P1 transduction. The sequence of the surrounding markers in clockwise order was established as: cdd fpk cir mgl gyrA. The direction of transcription in cir was determined as clockwise on the Escherichia coli chromosome. The gene product of cir, an outer membrane receptor for colicin I, is not part of the mgl operon. It is not regulated by D-fucose, the inducer of the mgl system and mutants defective in cir are unimpaired in the uptake of substrates of the mgl-dependent transport system.

Sequence of the malK gene in E.coli K12

We present the sequence of gene malK which encodes a component of the system for maltose transport in E.coli K12. We also determined the position of deletion (S50) which fuses malK to the following gene lamB; the malK-lamB protein hybrid contains all of the malK protein. The mRNA corresponding to the last two thirds of gene malK could form stable stem and loop structures. The malK protein, as deduced from the gene sequence, would include 370 residues and correspond to a molecular weight of 40700. The sequence as well as sequence comparisons with the ndh protein of E.coli are discussed in terms of the location and function of the malK protein.

Outer membrane protein P of Pseudomonas aeruginosa: regulation by phosphate deficiency and formation of small anion-specific channels in lipid bilayer membranes

A new major outer membrane protein, P, was induced in Pseudomonas aeruginosa PAO1 upon growth in medium containing 0.2 mM or less inorganic phosphate. Studies with media containing different levels of phosphate and with mutants of PAO1 suggested that protein P was coregulated with alkaline phosphatase and phospholipase C. Protein P was substantially purified and shown to form sodium dodecyl sulfate-resistant oligomers on polyacrylamide gels. The incorporation of purified protein P into artificial lipid bilayers resulted in an increase of the membrane conductance by many orders of magnitude. Single-channel experiments demonstrated that protein P channels were substantially smaller than all previously studied porins from P. aeruginosa and enteric bacteria, with an average single-channel conductance in 1 M NaCl of 0.25 nS. The protein P channel was apparently not voltage induced or regulated. The results of single-channel conductance experiments, using a variety of different salts, allowed a minimum channel diameter estimate of 0.7 nm. Furthermore, from these results it was concluded that the protein P channel was highly specific for anions. Zero-current potential measurements confirmed that protein P was at least 30-fold more permeable for Cl- than for K+ ions. The possible biological role of the small, anion-specific protein P channels in phosphate uptake from the medium is discussed.