Involvement of the central loop of the lactose permease of Escherichia coli in its allosteric regulation by the glucose-specific enzyme IIA of the phosphoenolpyruvate-dependent phosphotransferase system

Transition of Dephospho-DctD to the Transcriptionally Active State via Interaction with Dephospho-IIA Glc

Exopolysaccharides (EPSs), biofilm-maturing components of Vibrio vulnificus, are abundantly produced when the expression of two major EPS gene clusters is activated by an enhancer-binding transcription factor, DctD₂, whose expression and phosphorylation are induced by dicarboxylic acids. Surprisingly, when glucose was supplied to V. vulnificus, similar levels of expression of these clusters occurred, even in the absence of dicarboxylic acids. This glucose-dependent activation was also mediated by DctD₂, whose expression was sequentially activated by the transcription regulator NtrC. Most DctD₂ in cells grown without dicarboxylic acids was present in a dephosphorylated state, known as the transcriptionally inactive form. However, in the presence of glucose, a dephosphorylated component of the glucose-specific phosphotransferase system, d-IIA^Glc, interacted with dephosphorylated DctD₂ (d-DctD₂). While d-DctD₂ did not show any affinity to a DNA fragment containing the DctD-binding sequences, the complex of d-DctD₂ and d-IIA^Glc exhibited specific and efficient DNA binding, similar to the phosphorylated DctD₂. The d-DctD₂-mediated activation of the EPS gene clusters’ expression was not fully achieved in cells grown with mannose. Furthermore, the degrees of expression of the clusters under glycerol were less than those under mannose. This was caused by an antagonistic and competitive effect of GlpK, whose expression was increased by glycerol, in forming a complex with d-DctD₂ by d-IIA^Glc. The data demonstrate a novel regulatory pathway for V. vulnificus EPS biosynthesis and biofilm maturation in the presence of glucose, which is mediated by d-DctD₂ through its transition to the transcriptionally active state by interacting with available d-IIA^Glc.

Interaction of mammalian and plant H+/sucrose transporters with 14-3-3 proteins

The solute carrier 45 family (SLC45) was defined in the course of the Human Genome Project and consists of four members, A1-A4, which show only 20-30% identity of amino acid sequences among each other. All these members exhibit an identity of ∼20% to plant H⁺/sucrose cotransporters. Recently, we expressed members of the murine SLC45 family in yeast cells and demonstrated that they are, like their plant counterparts, H⁺/sucrose cotransporters. In contrast with the plant proteins, SLC45 transporters recognise also the monosaccharides glucose and fructose as physiological substrates and seem to be involved in alternative sugar supply as well as in osmoregulation of several mammalian tissues. In the present study, we provide novel insights into the regulation of SLC45 transporters. By screening for interaction partners, we found a 14-3-3 protein as a promising candidate for control of transport activity. Indeed, co-expression of the gamma isoform of murine 14-3-3 protein in yeast and Xenopus oocytes led to a significant decrease in transport rates of the murine SLC45 transporters as well as of the plant H⁺/sucrose transporter Sut1.

Metabolic engineering of Escherichia coli to produce succinate from soybean hydrolysate under anaerobic conditions

It is of great economic interest to produce succinate from low-grade carbon sources, which can enhance the competitiveness of the biological route. In this study, succinate producer Escherichia coli CT550/pHL413KF1 was further engineered to efficiently use the mixed sugars from non-food based soybean hydrolysate to produce succinate under anaerobic conditions. Since many common E. coli strains fail to use galactose anaerobically even if they can use it aerobically, the glucose, and galactose related sugar transporters were deactivated individually and evaluated. The PTS system was found to be important for utilization of mixed sugars, and galactose uptake was activated by deactivating ptsG. In the ptsG- strain, glucose, and galactose were used simultaneously. Glucose was assimilated mainly through the mannose PTS system while galactose was transferred mainly through GalP in a ptsG- strain. A new succinate producing strain, FZ591C which can efficiently produce succinate from the mixed sugars present in soybean hydrolysate was constructed by integration of the high succinate yield producing module and the galactose utilization module into the chromosome of the CT550 ptsG- strain. The succinate yield reached 1.64 mol/mol hexose consumed (95% of maximum theoretical yield) when a mixed sugars feedstock was used as a carbon source. Based on the three monitored sugars, a nominal succinate yield of 1.95 mol/mol was observed as the strain can apparently also use some other minor sugars in the hydrolysate. In this study, we demonstrate that FZ591C can use soybean hydrolysate as an inexpensive carbon source for high yield succinate production under anaerobic conditions, giving it the potential for industrial application.

Sugar Influx Sensing by the Phosphotransferase System of Escherichia coli

The phosphotransferase system (PTS) plays a pivotal role in the uptake of multiple sugars in Escherichia coli and many other bacteria. In the cell, individual sugar-specific PTS branches are interconnected through a series of phosphotransfer reactions, thus creating a global network that not only phosphorylates incoming sugars but also regulates a number of cellular processes. Despite the apparent importance of the PTS network in bacterial physiology, the holistic function of the network in the cell remains unclear. Here we used Förster resonance energy transfer (FRET) to investigate the PTS network in E. coli, including the dynamics of protein interactions and the processing of different stimuli and their transmission to the chemotaxis pathway. Our results demonstrate that despite the seeming complexity of the cellular PTS network, its core part operates in a strikingly simple way, sensing the overall influx of PTS sugars irrespective of the sugar identity and distributing this information equally through all studied branches of the network. Moreover, it also integrates several other specific metabolic inputs. The integrated output of the PTS network is then transmitted linearly to the chemotaxis pathway, in stark contrast to the amplification of conventional chemotactic stimuli. Finally, we observe that default uptake through the uninduced PTS network correlates well with the quality of the carbon source, apparently representing an optimal regulatory strategy.

The bacterial phosphotransferase system (PTS) mediates uptake of multiple sugars from the environment and also controls cell physiology and swimming behavior in sugar gradients. In Escherichia coli and other bacteria, the PTS consists of a number of sugar-specific branches, interconnected via shared components through a series of phosphotransfer reactions. Whereas most previous studies have focused on understanding individual PTS branches, the holistic function of the entire PTS network in the cell remained elusive. In this study we address this question by investigating the dynamics of multiple protein interactions within the cellular PTS network upon stimulation with sugars and other metabolites. We demonstrate that despite its seeming complexity, the core part of the PTS network operates in a strikingly simple way, sensing the overall influx of PTS sugars and key metabolites into the cell and utilizing this information to control bacterial behavior. We further show that the default influx of the carbon source correlates with its quality, and we use computer simulations to demonstrate that this correlation apparently represents an optimal regulatory strategy.

Control of Transposon-Mediated Directed Mutation by the Escherichia coli Phosphoenolpyruvate:Sugar Phosphotransferase System

The phosphoenolpyruvate:sugar phosphotransferase system (PTS) has been shown to control transport, cell metabolism and gene expression. We here present results supporting the novel suggestion that in certain instances it also regulates the mutation rate. Directed mutations are defined as mutations that occur at higher frequencies when beneficial than when neutral or detrimental. To date, the occurrence of directed point mutations has not been documented and confirmed, but a few examples of transposon-mediated directed mutations have been reported. Here we focus on the first and best-studied example of directed mutation, which involves the regulation of insertion sequence-5 hopping into a specific site upstream of the glpFK glycerol utilization operon in Escherichia coli. This insertional event specifically activates expression of the glpFK operon, allowing the growth of wild-type cells with glycerol as a carbon source in the presence of nonmetabolizable glucose analogues which normally block glycerol utilization. The sugar-transporting PTS controls this process by regulating levels of cytoplasmic glycerol-3-phosphate and cyclic (c)AMP as established in previous publications. Direct involvement of the glycerol repressor, GlpR, and the cAMP receptor protein, Crp, in the regulation of transposon-mediated directed mutation has been demonstrated.

The bacterial phosphoenolpyruvate:carbohydrate phosphotransferase system: regulation by protein phosphorylation and phosphorylation-dependent protein-protein interactions

The bacterial phosphoenolpyruvate (PEP):carbohydrate phosphotransferase system (PTS) carries out both catalytic and regulatory functions. It catalyzes the transport and phosphorylation of a variety of sugars and sugar derivatives but also carries out numerous regulatory functions related to carbon, nitrogen, and phosphate metabolism, to chemotaxis, to potassium transport, and to the virulence of certain pathogens. For these different regulatory processes, the signal is provided by the phosphorylation state of the PTS components, which varies according to the availability of PTS substrates and the metabolic state of the cell. PEP acts as phosphoryl donor for enzyme I (EI), which, together with HPr and one of several EIIA and EIIB pairs, forms a phosphorylation cascade which allows phosphorylation of the cognate carbohydrate bound to the membrane-spanning EIIC. HPr of firmicutes and numerous proteobacteria is also phosphorylated in an ATP-dependent reaction catalyzed by the bifunctional HPr kinase/phosphorylase. PTS-mediated regulatory mechanisms are based either on direct phosphorylation of the target protein or on phosphorylation-dependent interactions. For regulation by PTS-mediated phosphorylation, the target proteins either acquired a PTS domain by fusing it to their N or C termini or integrated a specific, conserved PTS regulation domain (PRD) or, alternatively, developed their own specific sites for PTS-mediated phosphorylation. Protein-protein interactions can occur with either phosphorylated or unphosphorylated PTS components and can either stimulate or inhibit the function of the target proteins. This large variety of signal transduction mechanisms allows the PTS to regulate numerous proteins and to form a vast regulatory network responding to the phosphorylation state of various PTS components.

Evidencing the role of lactose permease in IPTG uptake by Escherichia coli in fed-batch high cell density cultures

The lac-operon and its components have been studied for decades and it is widely used as one of the common systems for recombinant protein production in Escherichia coli. However, the role of the lactose permease, encoded by the lacY gene, when using the gratuitous inducer IPTG for the overexpression of heterologous proteins, is still a matter of discussion. A lactose permease deficient strain was successfully constructed. Growing profiles and acetate production were compared with its parent strain at shake flask scale. Our results show that the lac-permease deficient strain grows slower than the parent in defined medium at shake flask scale, probably due to a downregulation of the phosphotransferase system (PTS). The distributions of IPTG in the medium and inside the cells, as well as recombinant protein production were measured by HPLC-MS and compared in substrate limiting fed-batch fermentations at different inducer concentrations. For the mutant strain, IPTG concentration in the medium depletes slower, reaching at the end of the culture higher concentration values compared with the parent strain. Final intracellular and medium concentrations of IPTG were similar for the mutant strain, while higher intracellular concentrations than in medium were found for the parent strain. Comparison of the distribution profiles of IPTG of both strains in fed-batch fermentations showed that lac-permease is crucially involved in IPTG uptake. In the absence of the transporter, apparently IPTG only diffuses, while in the presence of lac-permease, the inducer accumulates in the cytoplasm at higher rates emphasizing the significant contribution of the permease-mediated transport.

Maltose transport in Lactobacillus casei and its regulation by inducer exclusion

Transport of maltose in Lactobacillus casei BL23 is subject to regulation by inducer exclusion. The presence of glucose or other rapidly metabolized carbon sources blocks maltose transport by a control mechanism that depends on the phosphorylation of the HPr protein at serine residue 46. We have identified the L. casei gene cluster for maltose/maltodextrin utilization by sequence analysis and mutagenesis. It is composed of genes coding for a transcriptional regulator, oligosaccharide hydrolytic enzymes, an ABC transporter (MalEFGK2) and the enzymes for the metabolism of maltose or the degradation products of maltodextrins: maltose phosphorylase and beta-phospho-glucomutase. These genes are induced by maltose and repressed by the presence of glucose via the catabolite control protein A (CcpA). A mutant strain was constructed which expressed the hprKV267F allele and therefore formed large amounts of P-Ser-HPr even in the absence of a repressive carbon source. In this mutant, transport of maltose was severely impaired, whereas transport of sugars not subject to inducer exclusion was not changed. These results strengthen the idea that P-Ser-HPr controls inducer exclusion and make the maltose system of L. casei a suitable model for studying this process in Firmicutes.

How phosphotransferase system-related protein phosphorylation regulates carbohydrate metabolism in bacteria

The phosphoenolpyruvate(PEP):carbohydrate phosphotransferase system (PTS) is found only in bacteria, where it catalyzes the transport and phosphorylation of numerous monosaccharides, disaccharides, amino sugars, polyols, and other sugar derivatives. To carry out its catalytic function in sugar transport and phosphorylation, the PTS uses PEP as an energy source and phosphoryl donor. The phosphoryl group of PEP is usually transferred via four distinct proteins (domains) to the transported sugar bound to the respective membrane component(s) (EIIC and EIID) of the PTS. The organization of the PTS as a four-step phosphoryl transfer system, in which all P derivatives exhibit similar energy (phosphorylation occurs at histidyl or cysteyl residues), is surprising, as a single protein (or domain) coupling energy transfer and sugar phosphorylation would be sufficient for PTS function. A possible explanation for the complexity of the PTS was provided by the discovery that the PTS also carries out numerous regulatory functions. Depending on their phosphorylation state, the four proteins (domains) forming the PTS phosphorylation cascade (EI, HPr, EIIA, and EIIB) can phosphorylate or interact with numerous non-PTS proteins and thereby regulate their activity. In addition, in certain bacteria, one of the PTS components (HPr) is phosphorylated by ATP at a seryl residue, which increases the complexity of PTS-mediated regulation. In this review, we try to summarize the known protein phosphorylation-related regulatory functions of the PTS. As we shall see, the PTS regulation network not only controls carbohydrate uptake and metabolism but also interferes with the utilization of nitrogen and phosphorus and the virulence of certain pathogens.

Efficient production of truncated thermostable xylanases from Rhodothermus marinus in Escherichia coli fed-batch cultures

A cultivation strategy for the production of two truncated thermostable recombinant xylanases (Xyn1deltaN and Xyn1deltaNC) was developed. Fed-batch cultivations of Escherichia coli strain BL21(DE3) with a controlled exponential glucose feed led to high specific production of the recombinant proteins. Addition of complex nutrients (e.g. Tryptone Soya Broth (TSB)) to the media were shown to increase both the specific growth rate during the production phase and the production per cell. The final cell-mass concentration depended on the time of induction in relation to both the feed-start and the expected time at which the cultivation had to be terminated due to oxygen transfer limitations or cell lysis. The gene used for the genetic constructions (encoding Xyn1deltaN and Xyn1deltaNC) was originally isolated from Rhodothermus marinus. Recombinant protein expression was controlled by the T7 lac-promoter and induced in the fed-batch phase at low glucose concentrations by the single addition of either lactose or isopropyl-thio-beta-d-galactoside (IPTG). In lactose-induced cells, the production of recombinant xylanase was delayed for approximately 30 min in comparison with those induced with IPTG, but the specific product levels were comparable at 3 h after induction. At this time, approximately 35% of the intracellular protein content was constituted by recombinant xylanase. Under the cultivation conditions used, production of the shorter deletion derivative (Xyn1deltaNC) led to nonspecific leakage and cell lysis, starting 1.5 or 2 h after induction with IPTG or lactose, respectively. At 3 h after induction, 50% of the produced protein (Xyn1deltaNC) was found in the culture medium. This was not the case for the longer protein (Xyn1deltaN), where only 10% of the xylanase leaked into the medium.

Allosteric enzymes as biosensors for molecular diagnosis

Biosensors are hybrid analytical devices that amplify signals generated from the specific interaction between a receptor and the analyte, through a biochemical mechanism. Biosensors use tissues, whole cells, artificial membranes or cell components like proteins or nucleic acids as receptors, coupled to a physicochemical signal transducer. Allosteric enzymes exhibit a catalytic activity that is modulated by specific effectors, through binding to receptor sites that are distinct from the active site. Several enzymes, catalyzing easily measurable reactions, have been engineered to allosterically respond to specific ligands, being themselves the main constituent of new-generation biosensors. The molecular basis, robustness and application of allosteric enzymatic biosensing are revised here.

Size comparisons among integral membrane transport protein homologues in bacteria, Archaea, and Eucarya

Integral membrane proteins from over 20 ubiquitous families of channels, secondary carriers, and primary active transporters were analyzed for average size differences between homologues from the three domains of life: Bacteria, Archaea, and Eucarya. The results showed that while eucaryotic homologues are consistently larger than their bacterial counterparts, archaeal homologues are significantly smaller. These size differences proved to be due primarily to variations in the sizes of hydrophilic domains localized to the N termini, the C termini, or specific loops between transmembrane alpha-helical spanners, depending on the family. Within the Eucarya domain, plant homologues proved to be substantially smaller than their animal and fungal counterparts. By contrast, extracytoplasmic receptors of ABC-type uptake systems in Archaea proved to be larger on average than those of their bacterial homologues, while cytoplasmic enzymes from different organisms exhibited little or no significant size differences. These observations presumably reflect evolutionary pressure and molecular mechanisms that must have been operative since these groups of organisms diverged from each other.

Mutation of the ptsG gene results in increased production of succinate in fermentation of glucose by Escherichia coli

Escherichia coli NZN111 is blocked in the ability to grow fermentatively on glucose but gave rise spontaneously to a mutant that had this ability. The mutant carries out a balanced fermentation of glucose to give approximately 1 mol of succinate, 0. 5 mol of acetate, and 0.5 mol of ethanol per mol of glucose. The causative mutation was mapped to the ptsG gene, which encodes the membrane-bound, glucose-specific permease of the phosphotransferase system, protein EIICB(glc). Replacement of the chromosomal ptsG gene with an insertionally inactivated form also restored growth on glucose and resulted in the same distribution of fermentation products. The physiological characteristics of the spontaneous and null mutants were consistent with loss of function of the ptsG gene product; the mutants possessed greatly reduced glucose phosphotransferase activity and lacked normal glucose repression. Introduction of the null mutant into strains not blocked in the ability to ferment glucose also increased succinate production in those strains. This phenomenon was widespread, occurring in different lineages of E. coli, including E. coli B.

HPr(His approximately P)-mediated phosphorylation differently affects counterflow and proton motive force-driven uptake via the lactose transport protein of Streptococcus thermophilus

The lactose transport protein (LacS) of Streptococcus thermophilus has a C-terminal hydrophilic domain that is homologous to IIA protein and protein domains of the phosphoenolpyruvate:carbohydrate phosphotransferase system (PTS). The IIA domain of LacS is phosphorylated on His-552 by the general energy coupling proteins of the PTS, which are Enzyme I and HPr. To study the effect of phosphorylation on transport, the LacS protein was purified and incorporated into liposomes with the IIA domain facing outwards. This allowed the phosphorylation of the membrane-reconstituted protein by purified HPr(His approximately P) of S. thermophilus. Phosphorylation of LacS increased the V(max) of counterflow transport, whereas the V(max) of the proton motive force (delta p)-driven lactose uptake was not affected. In line with a range of kinetic studies, we propose that phosphorylation affects the rate constants for the reorientation of the ternary complex (LacS with bound lactose plus proton), which is rate-determining for counterflow but not for delta p-driven transport.

The central cytoplasmic loop of the major facilitator superfamily of transport proteins governs efficient membrane insertion

Deletion of 5 residues (Delta5) from the central cytoplasmic loop of the lactose permease of Escherichia coli has no significant effect on expression or activity, whereas Delta12 leads to increased rates of permease turnover after membrane insertion and decreased transport activity, and Delta20 abolishes insertion and activity. By expressing Delta12 or Delta20 in two halves, both expression and activity are restored to levels approximating wild type. Replacing deleted residues with random hydrophilic amino acids also leads to full recovery. However, introduction of hydrophobic residues decreases expression and activity in a context-dependent manner. Thus, a minimum length of the central cytoplasmic loop is vital for proper insertion, stability, and efficient transport activity, because of constraints at the cytoplasmic ends of helices VI and VII. Furthermore, the results are consistent with the idea that the middle cytoplasmic loop provides a temporal delay between insertion of the first six helices into the membrane before insertion of the second six helices.

Topography of the surface of the Escherichia coli phosphotransferase system protein enzyme IIAglc that interacts with lactose permease

The unphosphorylated form of enzyme IIAglc of the Escherichia coli phosphoenolpyruvate:sugar phosphotransferase system inhibits transport catalyzed by lactose permease. We (Seok et al. (1997) Proc. Natl. Acad. Sci. U.S.A. 94, 13515-13519) previously characterized the area on the cytoplasmic face of lactose permease that interacts with enzyme IIAglc, using radioactive enzyme IIAglc. Subsequent studies (Sondej et al. (1999) Proc. Natl. Acad. Sci. U.S.A. 96, 3525-3530) suggested consensus binding sequences on proteins that interact with enzyme IIAglc. The present study characterizes a region on the surface of enzyme IIAglc that interfaces with lactose permease. Acetylation of lysine residues by sulfosuccinimidyl acetate treatment of enzyme IIAglc, but not lactose permease, reduced the degree of interaction between the two proteins. To localize the lysine residue(s) on enzyme IIAglc that is(are) involved in the regulatory interaction, selected lysine residues were mutagenized. Conversion of nine separate lysines to glutamic acid resulted in proteins that were still capable of phosphoryl acceptance from HPr. Except for Lys69, all the modified proteins were as effective as the wild-type enzyme IIAglc in a test for binding to lactose permease. The Lys69 mutant was also defective in phosphoryl transfer to glucose permease. To derive further information concerning the contact surface, additional selected residues in the vicinity of Lys69 were mutagenized and tested for binding to lactose permease. On the basis of these studies, a model for the region of the surface of enzyme IIAglc that interacts with lactose permease is proposed.

Deduction of consensus binding sequences on proteins that bind IIAGlc of the phosphoenolpyruvate:sugar phosphotransferase system by cysteine scanning mutagenesis of Escherichia coli lactose permease

Mediated by the protein IIAGlc, the phosphoenolpyruvate:sugar phosphotransferase system plays a role in the regulation of activity of other sugar transport systems in Escherichia coli. By using a direct binding assay, a collection of single-Cys replacement mutants in cytoplasmic loops of lactose permease were evaluated for their capacity to bind IIAGlc. Selected Cys replacements in loops IV/V or VI/VII result in loss of binding activity. Analysis of the mutagenesis results together with multiple sequence alignments of a family of proteins that interacts with IIAGlc provides the basis for developing two regions of consensus sequence in those partner proteins necessary for binding to IIAGlc. The requirement for two interaction regions is interpreted in the regulatory framework of a substrate-dependent conformational change that brings those two regions into an orientation optimal for binding IIAGlc.

Sequencing and characterization of the xyl operon of a gram-positive bacterium, Tetragenococcus halophila

The xyl operon of a gram-positive bacterium, Tetragenococcus halophila (previously called Pediococcus halophilus), was cloned and sequenced. The DNA was about 7.7 kb long and contained genes for a ribose binding protein and part of a ribose transporter, xylR (a putative regulatory gene), and the xyl operon, along with its regulatory region and transcription termination signal, in this order. The DNA was AT rich, the GC content being 35.8%, consistent with the GC content of this gram-positive bacterium. The xyl operon consisted of three genes, xylA, encoding a xylose isomerase, xylB, encoding a xylulose kinase, and xylE, encoding a xylose transporter, with predicted molecular weights of 49,400, 56,400, and 51,600, respectively. The deduced amino acid sequences of the XylR, XylA, XylB, and XylE proteins were similar to those of the corresponding proteins in other gram-positive and -negative bacteria, the similarities being 37 to 64%. Each polypeptide of XylB and XylE was expressed functionally in Escherichia coli. XylE transported D-xylose in a sodium ion-dependent manner, suggesting that it is the first described xylose/Na+ symporter. The XylR protein contained a consensus sequence for binding catabolites of glucose, such as glucose-6-phosphate, which has been discovered in glucose and fructose kinases in bacteria. Correspondingly, the regulatory region of this operon contained a putative binding site of XylR with a palindromic structure. Furthermore, it contained a consensus sequence, CRE (catabolite-responsive element), for binding CcpA (catabolite control protein A). We speculate that the transcriptional regulation of this operon resembles the regulation of catabolite-repressible operons such as the amy, lev, xyl, and gnt operons in various gram-positive bacteria. We discuss the significance of the regulation of gene expression of this operon in T. halophila.

Topology of allosteric regulation of lactose permease

Sugar transport by some permeases in Escherichia coli is allosterically regulated by the phosphorylation state of the intracellular regulatory protein, enzyme IIAglc of the phosphoenolpyruvate:sugar phosphotransferase system. A sensitive radiochemical assay for the interaction of enzyme IIAglc with membrane-associated lactose permease was used to characterize the binding reaction. The binding is stimulated by transportable substrates such as lactose, melibiose, and raffinose, but not by sugars that are not transported (maltose and sucrose). Treatment of lactose permease with N-ethylmaleimide, which blocks ligand binding and transport by alkylating Cys-148, also blocks enzyme IIAglc binding. Preincubation with the substrate analog beta-D-galactopyranosyl 1-thio-beta-D-galactopyranoside protects both lactose transport and enzyme IIAglc binding against inhibition by N-ethylmaleimide. A collection of lactose permease replacement mutants at Cys-148 showed, with the exception of C148V, a good correlation of relative transport activity and enzyme IIAglc binding. The nature of the interaction of enzyme IIAglc with the cytoplasmic face of lactose permease was explored. The N- and C-termini, as well as five hydrophilic loops in the permease, are exposed on the cytoplasmic surface of the membrane and it has been proposed that the central cytoplasmic loop of lactose permease is the major determinant for interaction with enzyme IIAglc. Lactose permease mutants with polyhistidine insertions in cytoplasmic loops IV/V and VI/VII and periplasmic loop VII/VIII retain transport activity and therefore substrate binding, but do not bind enzyme IIAglc, indicating that these regions of lactose permease may be involved in recognition of enzyme IIAglc. Taken together, these results suggest that interaction of lactose permease with substrate promotes a conformational change that brings several cytoplasmic loops into an arrangement optimal for interaction with the regulatory protein, enzyme IIAglc. A topological map of the proposed interaction is presented.

Computer-based analyses of the protein constituents of transport systems catalysing export of complex carbohydrates in bacteria

Bacteria synthesize and secrete an array of complex carbohydrates including exopolysaccharides (EPSs), capsular polysaccharides (CPSs), lipopolysaccharides (LPSs), lipo-oligosaccharides (LOSs) and teichoic acids (TCAs). We have analysed the families of homologous proteins that appear to mediate excretion of complex carbohydrates into or across the bacterial cell envelope. Two principal families of cytoplasmic-membrane transport systems appear to drive polysaccharide export: polysaccharide-specific transport (PST) systems and ATP-binding cassette-2 (ABC-2) systems. We present evidence that the secretion of CPSs and EPSs, but not of LPSs, LOSs or TCAs via a PST or ABC-2 system requires the presence of a cytoplasmic-membrane-periplasmic auxiliary protein (MPA1 or MPA2, respectively) in both Gram-negative and Gram-positive bacteria as well as an outer-membrane auxiliary (OMA) protein in Gram-negative bacteria. While all OMA proteins are included within a single family, MPA1 and MPA2 family proteins are not demonstrably homologous to each other, even though they share common topological features. Moreover, MPA1 family proteins (which function with PST systems), but not MPA2 family proteins (which function with ABC-2 systems), possess cytoplasmic ATP-binding domains that may either exist as separate polypeptide chains (for those from Gram-positive bacteria) or constitute the C-terminal domain of the MPA1 polypeptide chain (for those from Gram-negative bacteria). The sizes, substrate specificities and regions of relative conservation and hydrophobicity are defined allowing functional and structural predictions as well as delineation of family-specific sequence motifs. Each family is characterized phylogenetically.