The glucose transporter of the Escherichia coli phosphotransferase system. Mutant analysis of the invariant arginines, histidines, and domain linker

Structure and mechanism of a phosphotransferase system glucose transporter

Glucose is the primary source of energy for many organisms and is efficiently taken up by bacteria through a dedicated transport system that exhibits high specificity. In Escherichia coli, the glucose-specific transporter IICBGlc serves as the major glucose transporter and functions as a component of the phosphoenolpyruvate-dependent phosphotransferase system. Here, we report cryo-electron microscopy (cryo-EM) structures of the glucose-bound IICBGlc protein. The dimeric transporter embedded in lipid nanodiscs was captured in the occluded, inward- and occluded, outward-facing conformations. Together with biochemical and biophysical analyses, and molecular dynamics (MD) simulations, we provide insights into the molecular basis and dynamics for substrate recognition and binding, including the gates regulating the binding sites and their accessibility. By combination of these findings, we present a mechanism for glucose transport across the plasma membrane. Overall, this work provides molecular insights into the structure, dynamics, and mechanism of the IICBGlc transporter in a native-like lipid environment.

Removing carbon catabolite repression in Parageobacillus thermoglucosidasius DSM 2542

Parageobacillus thermoglucosidasius is a thermophilic bacterium of interest for lignocellulosic biomass fermentation. However, carbon catabolite repression (CCR) hinders co-utilization of pentoses and hexoses in the biomass substrate. Hence, to optimize the fermentation process, it is critical to remove CCR in the fermentation strains with minimal fitness cost. In this study, we investigated whether CCR could be removed from P. thermoglucosidasius DSM 2542 by mutating the Ser46 regulatory sites on HPr and Crh to a non-reactive alanine residue. It was found that neither the ptsH1 (HPr-S46A) nor the crh1 (Crh-S46A) mutation individually eliminated CCR in P. thermoglucosidasius DSM 2542. However, it was not possible to generate a ptsH1 crh1 double mutant. While the Crh-S46A mutation had no obvious fitness effect in DSM 2542, the ptsH1 mutation had a negative impact on cell growth and sugar utilization under fermentative conditions. Under these conditions, the ptsH1 mutation was associated with the production of a brown pigment, believed to arise from methylglyoxal production, which is harmful to cells. Subsequently, a less directed adaptive evolution approach was employed, in which DSM 2542 was grown in a mixture of 2-deoxy-D-glucose(2-DG) and xylose. This successfully removed CCR from P. thermoglucosidasius DSM 2542. Two selection strategies were applied to optimize the phenotypes of evolved strains. Genome sequencing identified key mutations affecting the PTS components PtsI and PtsG, the ribose operon repressor RbsR and adenine phosphoribosyltransferase APRT. Genetic complementation and bioinformatics analysis revealed that the presence of wild type rbsR and apt inhibited xylose uptake or utilization, while ptsI and ptsG might play a role in the regulation of CCR in P. thermoglucosidasius DSM 2542.

Transporters of glucose and other carbohydrates in bacteria

Glucose arguably is the most important energy carrier, carbon source for metabolites and building block for biopolymers in all kingdoms of life. The proper function of animal organs and tissues depends on the continuous supply of glucose from the bloodstream. Most animals can resorb only a small number of monosaccharides, mostly glucose, galactose and fructose, while all other sugars oligosaccharides and dietary fibers are degraded and metabolized by the microbiota of the lower intestine. Bacteria, in contrast, are omnivorous. They can import and metabolize structurally different sugars and, as a consortium of different species, utilize almost any sugar, sugar derivative and oligosaccharide occurring in nature. Bacteria have membrane transport systems for the uptake of sugars against steep concentration gradients energized by ATP, the proton motive force and the high energy glycolytic intermediate phosphoenolpyruvate (PEP). Different uptake mechanisms and the broad range of overlapping substrate specificities allow bacteria to quickly adapt to and colonize changing environments. Here, we review the structures and mechanisms of bacterial representatives of (i) ATP-dependent cassette (ABC) transporters, (ii) major facilitator (MFS) superfamily proton symporters, (iii) sodium solute symporters (SSS) and (iv) enzyme II integral membrane subunits of the bacterial PEP-dependent phosphotransferase system (PTS). We give a short overview on the distribution of transporter genes and their phylogenetic relationship in different bacterial species. Some sugar transporters are hijacked for import of bacteriophage DNA and antibacterial toxins (bacteriocins) and they facilitate the penetration of polar antibiotics. Finally, we describe how the expression and activity of certain sugar transporters are controlled in response to the availability of sugars and how the presence and uptake of sugars may affect pathogenicity and host-microbiota interactions.

Carbohydrate Transport by Group Translocation: The Bacterial Phosphoenolpyruvate: Sugar Phosphotransferase System

The Bacterial Phosphoenolpyruvate (PEP) : Sugar Phosphotransferase System (PTS) mediates the uptake and phosphorylation of carbohydrates, and controls the carbon- and nitrogen metabolism in response to the availability of sugars. PTS occur in eubacteria and in a few archaebacteria but not in animals and plants. All PTS comprise two cytoplasmic phosphotransferase proteins (EI and HPr) and a species-dependent, variable number of sugar-specific enzyme II complexes (IIA, IIB, IIC, IID). EI and HPr transfer phosphorylgroups from PEP to the IIA units. Cytoplasmic IIA and IIB units sequentially transfer phosphates to the sugar, which is transported by the IIC and IICIID integral membrane protein complexes. Phosphorylation by IIB and translocation by IIC(IID) are tightly coupled. The IIC(IID) sugar transporters of the PTS are in the focus of this review. There are four structurally different PTS transporter superfamilies (glucose, glucitol, ascorbate, mannose) . Crystal structures are available for transporters of two superfamilies: bcIIC^mal (MalT, 5IWS, 6BVG) and bcIIC^chb (ChbC, 3QNQ) of B. subtilis from the glucose family, and IIC^asc (UlaA, 4RP9, 5ZOV) of E. coli from the ascorbate superfamily . They are homodimers and each protomer has an independent transport pathway which functions by an elevator-type alternating-access mechanism. bcIIC^mal and bcIIC^chb have the same fold, IIC^asc has a completely different fold. Biochemical and biophysical data accumulated in the past with the transporters for mannitol (IICBA^mtl) and glucose (IICB^glc) are reviewed and discussed in the context of the bcIIC^mal crystal structures. The transporters of the mannose superfamily are dimers of protomers consisting of a IIC and a IID protein chain. The crystal structure is not known and the topology difficult to predict. Biochemical data indicate that the IICIID complex employs a different transport mechanism . Species specific IICIID serve as a gateway for the penetration of bacteriophage lambda DNA across, and insertion of class IIa bacteriocins into the inner membrane. PTS transporters are inserted into the membrane by SecYEG translocon and have specific lipid requirements. Immunoelectron- and fluorescence microscopy indicate a non-random distribution and supramolecular complexes of PTS proteins.

The Structure of a Sugar Transporter of the Glucose EIIC Superfamily Provides Insight into the Elevator Mechanism of Membrane Transport

The phosphoenolpyruvate:carbohydrate phosphotransferase systems are found in bacteria, where they play central roles in sugar uptake and regulation of cellular uptake processes. Little is known about how the membrane-embedded components (EIICs) selectively mediate the passage of carbohydrates across the membrane. Here we report the functional characterization and 2.55-Å resolution structure of a maltose transporter, bcMalT, belonging to the glucose superfamily of EIIC transporters. bcMalT crystallized in an outward-facing occluded conformation, in contrast to the structure of another glucose superfamily EIIC, bcChbC, which crystallized in an inward-facing occluded conformation. The structures differ in the position of a structurally conserved substrate-binding domain that is suggested to play a central role in sugar transport. In addition, molecular dynamics simulations suggest a potential pathway for substrate entry from the periplasm into the bcMalT substrate-binding site. These results provide a mechanistic framework for understanding substrate recognition and translocation for the glucose superfamily EIIC transporters.

2D and 3D crystallization of the wild-type IIC domain of the glucose PTS transporter from Escherichia coli

The bacterial phosphoenolpyruvate: sugar phosphotransferase system serves the combined uptake and phosphorylation of carbohydrates. This structurally and functionally complex system is composed of several conserved functional units that, through a cascade of phosphorylated intermediates, catalyze the transfer of the phosphate moiety from phosphoenolpyruvate to the substrate, which is bound to the integral membrane domain IIC. The wild-type glucose-specific IIC domain (wt-IIC(glc)) of Escherichia coli was cloned, overexpressed and purified for biochemical and functional characterization. Size-exclusion chromatography and scintillation-proximity binding assays showed that purified wt-IIC(glc) was homogenous and able to bind glucose. Crystallization was pursued following two different approaches: (i) reconstitution of wt-IIC(glc) into a lipid bilayer by detergent removal through dialysis, which yielded tubular 2D crystals, and (ii) vapor-diffusion crystallization of detergent-solubilized wt-IIC(glc), which yielded rhombohedral 3D crystals. Analysis of the 2D crystals by cryo-electron microscopy and the 3D crystals by X-ray diffraction indicated resolutions of better than 6Å and 4Å, respectively. Furthermore, a complete X-ray diffraction data set could be collected and processed to 3.93Å resolution. These 2D and 3D crystals of wt-IIC(glc) lay the foundation for the determination of the first structure of a bacterial glucose-specific IIC domain.

Structural insight into the PTS sugar transporter EIIC

BACKGROUND

The enzyme IIC (EIIC) component of the phosphotransferase system (PTS) is responsible for selectively transporting sugar molecules across the inner bacterial membrane. This is accomplished in parallel with phosphorylation of the sugar, which prevents efflux of the sugar back across the membrane. This process is a key part of an extensive signaling network that allows bacteria to efficiently utilize preferred carbohydrate sources.

SCOPE OF REVIEW

The goal of this review is to examine the current understanding of the structural features of the EIIC and how it mediates concentrative, selective sugar transport. The crystal structure of an N,N'-diacetylchitobiose transporter is used as a structural template for the glucose superfamily of PTS transporters.

MAJOR CONCLUSIONS

Comparison of protein sequences in context with the known EIIC structure suggests that members of the glucose superfamily of PTS transporters may exhibit variations in topology. Despite these differences, a conserved histidine and glutamate appear to have roles shared across the superfamily in sugar binding and phosphorylation. In the proposed transport model, a rigid body motion between two structural domains and movement of an intracellular loop provide the substrate binding site with alternating access, and reveal a surface required for interaction with the phosphotransfer protein responsible for catalysis.

GENERAL SIGNIFICANCE

The structural and functional data discussed here give a preliminary understanding of how transport in EIIC is achieved. However, given the great sequence diversity between varying glucose-superfamily PTS transporters and lack of data on conformational changes needed for transport, additional structures of other members and conformations are still required. This article is part of a Special Issue entitled: Structural biochemistry and biophysics of membrane proteins.

The phosphoenolpyruvate-dependent glucose-phosphotransferase system from Escherichia coli K-12 as the center of a network regulating carbohydrate flux in the cell

The phosphoenolpyruvate-(PEP)-dependent-carbohydrate:phosphotransferase systems (PTSs) of enteric bacteria constitute a complex transport and sensory system. Such a PTS usually consists of two cytoplasmic energy-coupling proteins, Enzyme I (EI) and HPr, and one of more than 20 different carbohydrate-specific membrane proteins named Enzyme II (EII), which catalyze the uptake and concomitant phosphorylation of numerous carbohydrates. The most prominent representative is the glucose-PTS, which uses a PTS-typical phosphorylation cascade to transport and phosphorylate glucose. All components of the glucose-PTS interact with a large number of non-PTS proteins to regulate the carbohydrate flux in the bacterial cell. Several aspects of the glucose-PTS have been intensively investigated in various research projects of many groups. In this article we will review our recent findings on a Glc-PTS-dependent metalloprotease, on the interaction of EIICB(Glc) with the regulatory peptide SgrT, on the structure of the membrane spanning C-domain of the glucose transporter and on the modeling approaches of ptsG regulation, respectively, and discuss them in context of general PTS research.

Expression, purification, crystallization and preliminary X-ray analysis of the EIICGlc domain of the Escherichia coli glucose transporter

The glucose-import system of Escherichia coli consists of a hydrophilic EIIA(Glc) subunit and a transmembrane EIICB(Glc) subunit. EIICB(Glc) (UniProt P69786) contains two domains: the transmembrane EIIC(Glc) domain (40.6 kDa) and the cytoplasmic EIIB(Glc) domain (8.0 kDa), which are fused by a linker that is strongly conserved among its orthologues. The EIICB(Glc) subunit can be split within this motif by trypsin. Here, the crystallization of the tryptic EIIC(Glc) domain is described. A complete data set was collected to 4.5 A resolution at 100 K.

Domain complementation studies reveal residues critical for the activity of the mannitol permease from Escherichia coli

This paper presents domain complementation studies in the mannitol transporter, EIImtl, from Escherichia coli. EIImtl is responsible for the transport and concomitant phosphorylation of mannitol over the cytoplasmic membrane. By using tryptophan-less EIImtl as a basis, each of the four phenylalanines located in the cytoplasmic loop between putative transmembrane helices II and III in the membrane-embedded C domain were replaced by tryptophan, yielding the mutants W97, W114, W126, and W133. Except for W97, these single-tryptophan mutants exhibited a high, wild-type-like, binding affinity for mannitol. Of the four mutants, only W114 showed a high mannitol phosphorylation activity. EIImtl is functional as a dimer and the effect of these mutations on the oligomeric activity was investigated via heterodimer formation (C/C domain complementation studies). The low phosphorylation activities of W126 and W133 could be increased 7-28 fold by forming heterodimers with either the C domain of W97 (IICmtlW97) or the inactive EIImtl mutant G196D. W126 and W133, on the other hand, did not complement each other. This study points towards a role of positions 97, 126 and 133 in the oligomeric activation of EIImtl. The involvement of specific residue positions in the oligomeric functioning of a sugar-translocating EII protein has not been presented before.

Ins and outs of glucose transport systems in eubacteria

Glucose is the classical carbon source that is used to investigate the transport, metabolism, and regulation of nutrients in bacteria. Many physiological phenomena like nutrient limitation, stress responses, production of antibiotics, and differentiation are inextricably linked to nutrition. Over the years glucose transport systems have been characterized at the molecular level in more than 20 bacterial species. This review aims to provide an overview of glucose uptake systems found in the eubacterial kingdom. In addition, it will highlight the diverse and sophisticated regulatory features of glucose transport systems.

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.

Hexose/Pentose and Hexitol/Pentitol Metabolism

Escherichia coli and Salmonella enterica serovar Typhimurium exhibit a remarkable versatility in the usage of different sugars as the sole source of carbon and energy, reflecting their ability to make use of the digested meals of mammalia and of the ample offerings in the wild. Degradation of sugars starts with their energy-dependent uptake through the cytoplasmic membrane and is carried on further by specific enzymes in the cytoplasm, destined finally for degradation in central metabolic pathways. As variant as the different sugars are, the biochemical strategies to act on them are few. They include phosphorylation, keto-enol isomerization, oxido/reductions, and aldol cleavage. The catabolic repertoire for using carbohydrate sources is largely the same in E. coli and in serovar Typhimurium. Nonetheless, significant differences are found, even among the strains and substrains of each species. We have grouped the sugars to be discussed according to their first step in metabolism, which is their active transport, and follow their path to glycolysis, catalyzed by the sugar-specific enzymes. We will first discuss the phosphotransferase system (PTS) sugars, then the sugars transported by ATP-binding cassette (ABC) transporters, followed by those that are taken up via proton motive force (PMF)-dependent transporters. We have focused on the catabolism and pathway regulation of hexose and pentose monosaccharides as well as the corresponding sugar alcohols but have also included disaccharides and simple glycosides while excluding polysaccharide catabolism, except for maltodextrins.

Effect of modification of the length and flexibility of the acyl carrier protein-thioesterase interdomain linker on functionality of the animal fatty acid synthase

A natural linker of approximately 20 residues connects the acyl carrier protein with the carboxy-terminal thioesterase domain of the animal fatty acid synthase. This study examines the effects of changes in the length and amino acid composition of this linker on catalytic activity, product composition, and segmental motion of the thioesterase domain. Deletion of 10 residues, almost half of the interdomain linker, had no effect on either mobility of the thioesterase domain, estimated from fluorescence polarization of a pyrenebutyl methylphosphono moiety bound covalently to the active site serine residue, or functionality of the fatty acid synthase; further shortening of the linker limited mobility of the thioesterase domain and resulted in reduced fatty acid synthase activity and an increase in product chain length from 16 to 18 and 20 carbon atoms. Surprisingly, however, even when the entire linker region was deleted, the fatty acid synthase retained 28% activity. Lengthening of the linker, by insertion of an unusually long acyl carrier protein-thioesterase linker from a modular polyketide synthase, increased mobility of the thioesterase domain without having any significant effect on catalytic properties of the complex. Interdomain linkers could also be used to tether, to the acyl carrier protein domain of the fatty acid synthase, a thioesterase active toward shorter chain length acyl thioesters generating novel short-chain fatty acid synthases. These studies reveal that although truncation of the interdomain linker partially impacts the ability of the thioesterase domain to terminate growth of the acyl chain, the overall integrity of the fatty acid synthase is quite tolerant to moderate changes in linker length and flexibility. The retention of fatty acid synthesizing activity on deletion of the entire linker region implies that the inherent flexibility of the phosphopantetheine "swinging arm" also contributes significantly to the successful docking of the long-chain acyl moiety in the thioesterase active site.

Characterization of soluble enzyme II complexes of the Escherichia coli phosphotransferase system

Plasmid-encoded His-tagged glucose permease of Escherichia coli, the enzyme IIBCGlc (IIGlc), exists in two physical forms, a membrane-integrated oligomeric form and a soluble monomeric form, which separate from each other on a gel filtration column (peaks 1 and 2, respectively). Western blot analyses using anti-His tag monoclonal antibodies revealed that although IIGlc from the two fractions migrated similarly in sodium dodecyl sulfate gels, the two fractions migrated differently on native gels both before and after Triton X-100 treatment. Peak 1 IIGlc migrated much more slowly than peak 2 IIGlc. Both preparations exhibited both phosphoenolpyruvate-dependent sugar phosphorylation activity and sugar phosphate-dependent sugar transphosphorylation activity. The kinetics of the transphosphorylation reaction catalyzed by the two IIGlc fractions were different: peak 1 activity was subject to substrate inhibition, while peak 2 activity was not. Moreover, the pH optima for the phosphoenolpyruvate-dependent activities differed for the two fractions. The results provide direct evidence that the two forms of IIGlc differ with respect to their physical states and their catalytic activities. These general conclusions appear to be applicable to the His-tagged mannose permease of E. coli. Thus, both phosphoenolpyruvate-dependent phosphotransferase system enzymes exist in soluble and membrane-integrated forms that exhibit dissimilar physical and kinetic properties.

Crystal structure of the Citrobacter freundii dihydroxyacetone kinase reveals an eight-stranded alpha-helical barrel ATP-binding domain

Dihydroxyacetone kinases are a sequence-conserved family of enzymes, which utilize two different phosphoryldonors, ATP in animals, plants and some bacteria, and a multiphosphoprotein of the phosphoenolpyruvate carbohydrate phosphotransferase system in bacteria. Here we report the 2.5-A crystal structure of the homodimeric Citrobacter freundii dihydroxyacetone kinase complex with an ATP analogue and dihydroxyacetone. The N-terminal domain consists of two alpha/beta-folds with a molecule of dihydroxyacetone covalently bound in hemiaminal linkage to the N epsilon 2 of His-220. The C-terminal domain consists of a regular eight-helix alpha-barrel. The eight helices form a deep pocket, which includes a tightly bound phospholipid. Only the lipid headgroup protrudes from the surface. The nucleotide is bound on the top of the barrel across from the entrance to the lipid pocket. The phosphate groups are coordinated by two Mg2+ ions to gamma-carboxyl groups of aspartyl residues. The ATP binding site does not contain positively charged or aromatic groups. Paralogues of dihydroxyacetone kinase also occur in association with transcription regulators and proteins of unknown function pointing to biological roles beyond triose metabolism.

Glucose and trehalose PTS permeases of Spiroplasma citri probably share a single IIA domain, enabling the spiroplasma to adapt quickly to carbohydrate changes in its environment

Spiroplasma citri is a plant-pathogenic mollicute phylogenetically related to Gram-positive bacteria. Spiroplasma cells are restricted to the phloem sieve tubes and are transmitted from plant to plant by the leafhopper vector Circulifer haematoceps. In the plant sieve tubes, S. citri grows on glucose and fructose, whereas in the leafhopper haemolymph the spiroplasma must grow on trehalose, the major sugar in insects. Previous studies in this laboratory have shown that fructose utilization was a key factor of spiroplasmal pathogenicity. To further study the implication of sugar metabolism in the interactions of S. citri with its plant host and its leafhopper vector, genes encoding permease enzymes II (EII(Glc) and EII(Tre)) of the S. citri phosphoenolpyruvate : glucose and phosphoenolpyruvate : trehalose phosphotransferase systems (PTS) were characterized. Mapping studies revealed that the EII(Glc) complex was split into two distinct polypeptides, IIA(Glc) and IICB(Glc), encoded by two separate genes, crr and ptsG, respectively. As expected, S. citri polypeptides IIA(Glc) and IICB(Glc) were more phylogenetically related to their counterparts from Gram-positive than to those from Gram-negative bacteria. The trehalose operon consisted of three genes treR, treP and treA, encoding a transcriptional regulator, the PTS permease (EII(Tre)) and the amylase, respectively. However, in contrast to the fructose-PTS permease, which is encoded as a single polypeptide (IIABC(Fru)) containing the three domains A, B and C, the trehalose-PTS permease (IIBC(Tre)) lacks its own IIA domain. No trehalose-specific IIA could be identified in the spiroplasmal genome, suggesting that the IIBC(Tre) permease probably functions with the IIA(Glc) domain. In agreement with this statement, yeast two-hybrid system experiments revealed that the IIA(Glc) domain interacted not only with IIB(Glc) but also with the IIB(Tre) domain. The results are discussed with respect to the ability of the spiroplasma to adapt from the phloem sap of the host plant to the haemolymph and salivary gland cells of the insect vector.

Solution structure of the phosphoryl transfer complex between the signal-transducing protein IIAGlucose and the cytoplasmic domain of the glucose transporter IICBGlucose of the Escherichia coli glucose phosphotransferase system

The solution structure of the final phosphoryl transfer complex in the glucose-specific arm of the Escherichia coli phosphotransferase system, between enzyme IIAGlucose (IIAGlc) and the cytoplasmic B domain (IIBGlc) of the glucose transporter IICBGlc, has been solved by NMR. The interface (approximately 1200-A2 buried surface) is formed by the interaction of a concave depression on IIAGlc with a convex protrusion on IIBGlc. The phosphoryl donor and acceptor residues, His-90 of IIAGlc and Cys-35 of IIBGlc (residues of IIBGlc are denoted in italics) are in close proximity and buried at the center of the interface. Cys-35 is primed for nucleophilic attack on the phosphorus atom by stabilization of the thiolate anion (pKa approximately 6.5) through intramolecular hydrogen bonding interactions with several adjacent backbone amide groups. Hydrophobic intermolecular contacts are supplemented by peripheral electrostatic interactions involving an alternating distribution of positively and negatively charged residues on the interaction surfaces of both proteins. Salt bridges between the Asp-38/Asp-94 pair of IIAGlc and the Arg-38/Arg-40 pair of IIBGlc neutralize the accumulation of negative charge in the vicinity of both the Sgamma atom of Cys-35 and the phosphoryl group in the complex. A pentacoordinate phosphoryl transition state is readily accommodated without any change in backbone conformation, and the structure of the complex accounts for the preferred directionality of phosphoryl transfer between IIAGlc and IIBGlc. The structures of IIAGlc.IIBGlc and the two upstream complexes of the glucose phosphotransferase system (EI.HPr and IIAGlc.HPr) reveal a cascade in which highly overlapping binding sites on HPr and IIAGlc recognize structurally diverse proteins.

Mutations which uncouple transport and phosphorylation in the D-mannitol phosphotransferase system of Escherichia coli K-12 and Klebsiella pneumoniae 1033-5P14

Mutants of Escherichia coli K-12 were isolated which lack the normal phosphotransferase system-dependent catabolic pathway for D-mannitol (Mtl). In some mutants the pts genes for the general proteins enzyme I and histidine protein of the phosphoenolpyruvate-dependent carbohydrate phosphotransferase systems were deleted. Other mutants expressed truncated mannitol-specific enzymes II (II(Mtl)) which lacked the IIA(Mtl) or IIBA(Mtl) domain(s), and the mtlA genes originated either from E. coli K-12 or from Klebsiella pneumoniae 1033-5P14. The dalD gene from Klebsiella oxytoca M5a1 was cloned on single-copy plasmids and transformed into the strains described above. This gene encodes an NAD-dependent D-arabinitol dehydrogenase (DalD) which converts D-arabinitol into D-xylulose and also converts D-mannitol into D-fructose. The different strains were used to isolate mutations which allow efficient transport of mannitol through the nonphosphorylated II(Mtl) complexes by selecting for growth on this polyhydric alcohol. More than 40 different mutants were analyzed to determine their ability to grow on mannitol, as well as their ability to bind and transport free mannitol and, after restoration of the missing domain(s), their ability to phosphorylate mannitol. Four mutations were identified (E218A, E218V, H256P, and H256Y); all of these mutations are located in the highly conserved loop 5 of the IIC membrane-bound transporter, and two are located in its GIHE motif. These mutations were found to affect the various functions in different ways. Interestingly, in the presence of all II(Mtl) variants, whether they were in the truncated form or in the complete form, in the phosphorylated form or in the nonphosphorylated form, and in the wild-type form or in the mutated form, growth occurred on the low-affinity analogue D-arabinitol with good efficiency, while only the uncoupled mutated forms transported mannitol at a high rate.

Soluble sugar permeases of the phosphotransferase system in Escherichia coli: evidence for two physically distinct forms of the proteins in vivo

The bacterial phosphoenolpyruvate-dependent phosphotransferase system (PTS) consists of a set of cytoplasmic energy-coupling proteins and various integral membrane permeases/sugar phosphotransferases, each specific for a different sugar. We have conducted biochemical analyses of three PTS permeases (enzymes II), the glucose permease (IIGlc), the mannitol permease (IIMtl) and the mannose permease (IIMan). These enzymes each catalyse two vectorial/chemical reactions, sugar phosphorylation using phosphoenolpyruvate (PEP) as the phosphoryl donor, dependent on enzyme I, HPr and IIA as well as IIBC (the PEP reaction), and transphosphorylation using a sugar phosphate (glucose-6-P for IIGlc and IIMan; mannitol-1-P for IIMtl) as the phosphoryl donor, dependent only on IIBC (the TP reaction). When crude extracts of French-pressed or osmotically shocked Escherichia coli cells are centrifuged in an ultracentrifuge at high speed, 5-20% of the enzyme II activity remains in the high-speed supernatant, and passage through a gel filtration column gives two activity peaks, one in the void volume exhibiting high PEP-dependent and TP activities, and a second included peak with high PEP-dependent activity and high (IIMan), moderate (IIGlc) or negligible (IIMtl) TP activities. Both log and stationary phase cells exhibit comparable relative amounts of pelletable and soluble enzyme II activities, but long-term exposure of cells to chloramphenicol results in selective loss of the soluble fraction with retention of much of the pelleted activity concomitant with extensive protein degradation. Short-term exposure of cells to chloramphenicol results in increased activities in both fractions, possibly because of increased lipid association, with more activation in the soluble fraction than in the pelleted fraction. Western blot analyses show that the soluble IIGlc exhibits a subunit size of about 45 kDa, and all three soluble enzymes II elute from the gel filtration column with apparent molecular weights of 40-50 kDa. We propose that enzymes II of the PTS exist in two physically distinct forms in the E. coli cell, one tightly integrated into the membrane and one either soluble or loosely associated with the membrane. We also propose that the membrane-integrated enzymes II are largely dimeric, whereas the soluble enzymes II, retarded during passage through a gel filtration column, are largely monomeric.

Analysis of the interaction between the global regulator Mlc and EIIBGlc of the glucose-specific phosphotransferase system in Escherichia coli

Mlc is a global regulator acting as a transcriptional repressor for several genes and operons of Escherichia coli encoding sugar-metabolizing enzymes and uptake systems. The repressing activity of Mlc is inactivated by binding to the dephosphorylated form of EIICB(Glc) (PtsG), which is formed during the transport of glucose. Here, we demonstrate that EIIB(Glc), the cytoplasmic domain of PtsG, alone is sufficient to inactivate Mlc but only when EIIB(Glc) is attached to the membrane by a protein anchor, which can be unrelated to PtsG. Several EIIB(Glc) mutants, which were altered in and around the phosphorylation site (Cys-421) of EIIB(Glc), were tested for their ability to bind Mlc and to affect transcriptional repression by Mlc. The exchange of Cys-421 with serine or aspartate still allowed binding to Mlc, and in addition, derepression became constitutive, i.e. independent of phosphoenolpyruvate-dependent phosphotransferase system (PTS) phosphorylation. Mutations were made in the surface-exposed residues in the vicinity of Cys-421 and identified Arg-424 as essential for binding to Mlc. Binding of Mlc to the EIIB(Glc) constructs in membrane preparations paralleled their ability to derepress Mlc-dependent transcription in vivo. These observations demonstrate that it is not the charge change at Cys-421, produced by PTS phosphorylation, that allows Mlc binding but rather the structural change in the environment surrounding Cys-421 that the phosphorylation provokes. Native Mlc exists as a tetramer. Deleting 18 amino acids from the C-terminal removes a putative amphipathic helix and results in dimeric Mlc that is no longer able to repress.

Dependency of sugar transport and phosphorylation by the phosphoenolpyruvate-dependent phosphotransferase system on membranous phosphatidyl glycerol in Escherichia coli: studies with a pgsA mutant lacking phosphatidyl glycerophosphate synthase

It has been reported that phosphatidyl glycerol (PG) is specifically required for the in vitro activities of the hexose-phosphorylating Enzymes II of the Escherichia coli phosphoenolpyruvate-dependent sugar transporting phosphotransferase system (PTS). We have examined this possibility by measuring the properties of a null pgsA mutant that lacks detectable PG. The mutant showed lower in vitro phosphorylation activities towards several sugars when both PEP-dependent and sugar-phosphate-dependent [14C]sugar phosphorylation reactions were measured. The order of dependency on PG for the different enzymes II was: IIMannose > IIGlucose > IIFructose > IIMannitol. Nonsedimentable (40000 rpm for 2 h) Enzymes II exhibited a greater dependency on PG than pelletable Enzymes II. Western blot analyses showed that the glucose Enzyme II is present in normal amounts. Transport and fermentation measurements revealed diminished activities for all Enzymes II. Thermal stability of all of these enzymes except the mannitol-specific Enzyme II was significantly decreased by the pgsA mutation, and sensitivity to detergent treatments was enhanced. Sugar transport proved to be the most sensitive indicator of proper Enzyme II-phospholipid association. Our results show that PG stimulates but is not required for Enzyme II function in E. coli.

The glucose-specific carrier of the Escherichia coli phosphotransferase system

Thirteen glucose analogues bearing electrophilic groups were synthesized (five of them for the first time) and screened as inhibitors of the glucose transporter (EIIGlc) of the Escherichia coli phosphoenolpyruvate-sugar phosphotransferase system (PTS). 2',3'-Epoxypropyl beta-d-glucopyranoside (3a) is an inhibitor and also a pseudosubstrate. Five analogues are inhibitors of nonvectorial Glc phosphorylation by EIIGlc but not pseudosubstrates. They are selective for EIIGlc as demonstrated by comparison with EIIMan, another Glc-specific but structurally different transporter. 3a is the only analogue that inhibits EIIGlc by binding to the high-affinity cytoplasmic binding site and also strongly inhibits sugar uptake mediated by this transporter. The most potent inhibitor in vitro, methyl 6,7-anhydro-d,l-glycero-alpha-d-gluco-heptopyranoside (1d), preferentially interacts with the low-affinity cytoplasmic site but only weakly inhibits Glc uptake. Binding and/or phosphorylation from the cytoplasmic side of EIIGlc is more permissive than sugar binding and/or translocation of substrates via the periplasmic site. EIIGlc is rapidly inactivated by the 6-O-bromoacetyl esters of methyl alpha-d-glucopyranoside (1a) and methyl alpha-d-mannopyranoside (1c), methyl 6-deoxy-6-isothiocyanato-alpha-d-glucopyranoside (1e), beta-d-glucopyranosyl isothiocyanate (3c) and beta-d-glucopyranosyl phenyl isothiocyanate (3d). Phosphorylation of EIIGlc protects, indicating that inactivation occurs by alkylation of Cys421. Glc does not protect, but sensitizes EIIGlc for inactivation by 1e and 3d, which is interpreted as the effect of glucose-induced conformational changes in the dimeric transporter. Glc also sensitizes EIIGlc for inactivation by 1a and 1c of uptake by starved cells. This indicates that Cys421 which is located on the cytoplasmic domain of EIIGlc becomes transiently accessible to substrate analogues on the periplasmic side of the transporter.

Sugar recognition by the glucose and mannose permeases of Escherichia coli. Steady-state kinetics and inhibition studies

The glucose (EII(Glc)) and mannose (EII(Man)) permeases of the phosphoenolpyruvate:sugar phosphotransferase system (PTS) of Escherichia coli belong to structurally different families of PTS transporters. The sugar recognition mechanism of the two transporters is compared using as inhibitors and pseudosubstrates all possible monodeoxy analogues, monodeoxyfluoro analogues, and epimers of D-glucose. The analogues were tested as phosphoryl acceptors in vitro and as uptake inhibitors with intact cells. Both EII have a high K(m) of phosphorylation for glucose modified at C-4 and C-6, and these analogues also are weak inhibitors of uptake. Conversely, modifications at C-1 (and also at C-2 with EII(Man)) were well tolerated. OH-3 is proposed to interact with hydrogen bond donors on EII(Glc) and EII(Man), since only substitution by fluorine was tolerated. Glucose-6-aldehydes, which exist as gem-diols in aqueous solution, are potent and highly selective inhibitors of "nonvectorial" phosphorylation by EII(Glc) (K(I) 3-250 microM). These aldehydes are comparatively weak inhibitors of transport by EII(Glc) and of phosphorylation and transport by EII(Man). Both transporters display biphasic kinetics (with glucose and some analogues) but simple Michaelis-Menten kinetics with 3-fluoroglucose (and other analogues). Kinetic simulations of the phosphorylation activities measured with different substrates and inhibitors indicate that two independent activities are present at the cytoplasmic side of the transporter. A working model that accounts for the kinetic data is presented.

Intein-mediated cyclization of a soluble and a membrane protein in vivo: function and stability

Cyclized subunits of the E. coli glucose transporter were produced in vivo by intein mediated trans-splicing. IIA(Glc) is a beta-sandwich protein, IICB(Glc) spans the membrane eight times. Genes encoding the circularly permuted precursors U(Cdelta)-IIA(Glc)-U(Ndelta) and U(Cdelta)-IICB(Glc)-U(Ndelta) were assembled from DNA fragments encoding the 3' and 5' segments of the recA intein of M. tuberculosis and crr and ptsG of E. coli, respectively. A 20-residues long, Ala-Pro rich linker peptide and/or a histidine tag were used to join the native N- and C-termini in the cyclized proteins. The cyclized proteins complemented growth of glucose auxotrophic strains. Purified, cyclized IIA(Glc) and IICB(Glc) had 100 and 25%, respectively, of wild-type glucose phosphotransferase activity. They had an increased electrophoretic mobility, which decreased upon linearization of the proteins with chymotrypsin. Cyclized IIA(Glc) displayed increased stability against temperature and GuHCl-induced unfolding (75 vs. 70 degrees C; 1.52 vs. 1.05 M).

Metabolism of sucrose and its five isomers by Fusobacterium mortiferum

Fusobacterium mortiferum utilizes sucrose [glucose-fructose in alpha(1-->2) linkage] and its five isomeric alpha-D-glucosyl-D-fructoses as energy sources for growth. Sucrose-grown cells are induced for both sucrose-6-phosphate hydrolase (S6PH) and fructokinase (FK), but the two enzymes are not expressed above constitutive levels during growth on the isomeric compounds. Extracts of cells grown previously on the sucrose isomers trehalulose alpha(1-->1), turanose alpha(1-->3), maltulose alpha(1-->4), leucrose alpha(1-->5) and palatinose alpha(1-->6) contained high levels of an NAD+ plus metal-dependent phospho-alpha-glucosidase (MalH). The latter enzyme was not induced during growth on sucrose. MalH catalysed the hydrolysis of the 6'-phosphorylated derivatives of the five isomers to yield glucose 6-phosphate and fructose, but sucrose 6-phosphate itself was not a substrate. Unexpectedly, MalH hydrolysed both alpha- and beta-linked stereomers of the chromogenic analogue p-nitrophenyl glucoside 6-phosphate. The gene malH is adjacent to malB and malR, which encode an EII(CB) component of the phosphoenolpyruvate-dependent sugar:phosphotransferase system and a putative regulatory protein, respectively. The authors suggest that for F. mortiferum, the products of malB and malH catalyse the phosphorylative translocation and intracellular hydrolysis of the five isomers of sucrose and of related alpha-linked glucosides. Genes homologous to malB and malH are present in both Klebsiella pneumoniae and the enterohaemorrhagic strain Escherichia coli O157:H7. Both these organisms grew well on sucrose, but only K. pneumoniae exhibited growth on the isomeric compounds.

Carbohydrate transporters of the bacterial phosphoenolpyruvate: sugar phosphotransferase system (PTS)

The glucose transporter of Escherichia coli couples translocation with phosphorylation of glucose. The IICB(Glc) subunit spans the membrane eight times. Split, circularly permuted and cyclized forms of IICB(Glc) are described. The split variant was 30 times more active when the two proteins were encoded by a dicistronic mRNA than by two genes. The stability and activity of circularly permuted forms was improved when they were expressed as fusion proteins with alkaline phosphatase. Cyclized IICB(Glc) and IIA(Glc) were produced in vivo by RecA intein-mediated trans-splicing. Purified, cyclized IIA(Glc) and IICB(Glc) had 100% and 30% of wild-type glucose phosphotransferase activity, respectively. Cyclized IIA(Glc) displayed increased stability against temperature and GuHCl-induced unfolding.

Expression of Semliki Forest virus E1 protein in Escherichia coli. Low pH-induced pore formation

Exposure of Semliki Forest virus 1 to mildly acidic conditions results in conformational changes of the viral spike proteins, which in turn leads to a pore formation across its membrane. The ability to form a pore has been ascribed to the ectodomain of the Semliki Forest virus (SFV) E1 spike protein. To elucidate whether the E1 protein per se is sufficient for low pH-dependent pore formation, we expressed E1 in Escherichia coli in an inducible manner using the pET11c expression system. The data obtained clearly showed that the E1 protein was expressed in the bacterial cell membrane and that exposure of E. coli expressing the SFV E1 protein to low pH (<6.2) resulted in a permeability change of the membrane. Thus, we conclude that the E1 protein of SFV per se is sufficient to promote pore formation under mildly acidic conditions.

Mutational analysis of the functional role of conserved arginine and lysine residues in transmembrane domains of the murine reduced folate carrier

The reduced folate carrier (RFC1) plays a major role in the delivery of folates into mammalian cells. RFC1 is an anion exchanger with seven conserved positively charged amino acid residues within 12 predicted transmembrane domains. This article explores the role of these residues in transport function by the development of cell lines in which arginines and lysines in RFC1 were replaced with leucine by site-directed mutagenesis. Three cell lines transfected with R131L, R155L, or R366L all lacked activity, despite high levels of protein expression in the plasma membrane, suggesting the crucial role of these amino acid residues in RFC1 function. In several mutant carriers, R26L, R42L, and K332L, there was little or no change in the influx K(t) value for MTX or influx K(i) value for folic acid. However, the R26L, R42L, and K332L carriers had decreased affinity for reduced folates. This was most prominent for K404L, which had 11- and 4-fold increases in influx K(i) for 5-methyl-THF and 5-formyl-THF, respectively, compared with L1210 cells. The marked influx stimulation observed with wild-type carrier when extracellular chloride was decreased was significantly diminished when influx was mediated by the K404L carrier, but was only slightly decreased with the R26L, R42L, and K332L mutants. This suggested that the K404 residue may be a major site of inhibition by chloride in the wild-type carrier. These studies indicate the important role that some positively charged residues within transmembrane domains of RFC1 play in RFC1 function.

Glucose transporter mutants of Escherichia coli K-12 with changes in substrate recognition of IICB(Glc) and induction behavior of the ptsG gene

In Escherichia coli K-12, the major glucose transporter with a central role in carbon catabolite repression and in inducer exclusion is the phosphoenolpyruvate-dependent glucose:phosphotransferase system (PTS). Its membrane-bound subunit, IICB(Glc), is encoded by the gene ptsG; its soluble domain, IIA(Glc), is encoded by crr, which is a member of the pts operon. The system is inducible by D-glucose and, to a lesser degree, by L-sorbose. The regulation of ptsG transcription was analyzed by testing the induction of IICB(Glc) transporter activity and of a single-copy Phi(ptsGop-lacZ) fusion. Among mutations found to affect directly ptsG expression were those altering the activity of adenylate cyclase (cyaA), the repressor DgsA (dgsA; also called Mlc), the general PTS proteins enzyme I (ptsI) and histidine carrier protein HPr (ptsH), and the IIA(Glc) and IIB(Glc) domains, as well as several authentic and newly isolated UmgC mutations. The latter, originally thought to map in the repressor gene umgC outside the ptsG locus, were found to represent ptsG alleles. These affected invariably the substrate specificity of the IICB(Glc) domain, thus allowing efficient transport and phosphorylation of substrates normally transported very poorly or not at all by this PTS. Simultaneously, all of these substrates became inducers for ptsG. From the analysis of the mutants, from cis-trans dominance tests, and from the identification of the amino acid residues mutated in the UmgC mutants, a new regulatory mechanism involved in ptsG induction is postulated. According to this model, the phosphorylation state of IIB(Glc) modulates IIC(Glc) which, directly or indirectly, controls the repressor DgsA and hence ptsG expression. By the same mechanism, glucose uptake and phosphorylation also control the expression of the pts operon and probably of all operons controlled by the repressor DgsA.

The glucose transporter of the Escherichia coli phosphotransferase system: linker insertion mutants and split variants

The IICB(Glc) subunit of the glucose transporter acts by a mechanism which couples vectorial translocation with phosphorylation of the substrate. It contains 8 transmembrane segments connected by 4 periplasmic, 2 short, 1 long (80 residues), cytoplasmic loops and an independently folding cytoplasmic domain at the C-terminus. Random DNase I cleavage, EcoRI linker insertion, and screening for transport-active mutants afforded 12 variants with between 46% and 116% of wild-type sugar phosphorylation activity. They carried inserts of up to 29 residues and short deletions in periplasmic loops 1, 2, and 3, in the long cytoplasmic loop 3, and in the linker region between the membrane spanning IIC(Glc) and the cytoplasmic IIB(Glc) domains. Disruption of the gene at the sites of linker insertion decreased the expression level and diminished phosphotransferase activity to between 7% and 32%. IICB(Glc) with a discontinuity in the cytoplasmic loop was purified to homogeneity as a stable complex. It was active only if encoded by a dicistronic operon but not if encoded by two genes on two different replicons, suggesting that spatial proximity of the nascent polypeptide chains is important for folding and membrane assembly.

Folding and activity of circularly permuted forms of a polytopic membrane protein

The transmembrane subunit of the Glc transporter (IICB(Glc)), which mediates uptake and concomitant phosphorylation of glucose, spans the membrane eight times. Variants of IICB(Glc) with the native N and C termini joined and new N and C termini in the periplasmic and cytoplasmic surface loops were expressed in Escherichia coli. In vivo transport/in vitro phosphotransferase activities of the circularly permuted variants with the termini in the periplasmic loops 1 to 4 were 35/58, 32/37, 0/3, and 0/0% of wild type, respectively. The activities of the variants with the termini in the cytoplasmic loops 1 to 3 were 0/25, 0/4 and 24/70, respectively. Fusion of alkaline phosphatase to the periplasmic C termini stabilized membrane integration and increased uptake and/or phosphorylation activities. These results suggest that internal signal anchor and stop transfer sequences can function as N-terminal signal sequences in a circularly permuted alpha-helical bundle protein and that the orientation of transmembrane segments is determined by the amino acid sequence and not by the sequential appearance during translation. Of the four IICB(Glc) variants with new termini in periplasmic loops, only the one with the discontinuity in loop 4 is inactive. The sequences of loop 4 and of the adjacent TM7 and TM8 are conserved in all phosphoenolpyruvate-dependent carbohydrate:phosphotransferase system transporters of the glucose family.

Purification and electron microscopic characterization of the membrane subunit (IICB(Glc)) of the Escherichia coli glucose transporter

The glucose transporter of the bacterial phosphotransferase system mediates sugar transport across the cytoplasmic membrane concomitant with sugar phosphorylation. It consists of a cytoplasmic subunit IIA(Glc) and the transmembrane subunit IICB(Glc). IICB(Glc) was purified to homogeneity by urea/alkali washing of membranes and nickel-chelate affinity chromatography. About 1.5 mg highly pure IICB(Glc) representing 77% of the total activity present in the membranes was obtained from 8g (wet weight) of cells. IICB(Glc) was reconstituted into lipid bilayers by temperature-controlled dialysis to yield small 2D crystals and by a rapid detergent-dilution procedure to yield densely packed vesicles. Electron microscopy and digital image processing of the negatively stained 2D crystals revealed a trigonal lattice with a unit cell size of a = b = 14.5 nm. The unit cell morphology exhibited three dimers of IICB(Glc) surrounding the threefold symmetry center. Single particle analysis of IICB(Glc) in proteoliposomes obtained by detergent dialysis also showed predominantly dimeric structures.

Mutational adaptation of Escherichia coli to glucose limitation involves distinct evolutionary pathways in aerobic and oxygen-limited environments

Mutational adaptations leading to improved glucose transport were followed with Escherichia coli K-12 growing in glucose-limited continuous cultures. When populations were oxygen limited as well as glucose limited, all bacteria within 280 generations contained mutations in a single codon of the ptsG gene. V12F and V12G replacements in the enzyme IIBC(Glc) component of the glucose phosphotransferase system were responsible for improved transport. In stark contrast, ptsG mutations were uncommon in fully aerobic glucose-limited cultures, in which polygenic mutations in mgl, mlc, and malT (regulating an alternate high-affinity Mgl/LamB uptake pathway) spread through the adapted population. Hence the same organism adapted to the same selection (glucose limitation) by different evolutionary pathways depending on a secondary environmental factor. The clonal diversity in the adapted populations was also significantly different. The PtsG V12F substitution under O(2) limitation contributed to a universal "winner clone" whereas polygenic, multiallelic changes led to considerable polymorphism in aerobic cultures. Why the difference in adaptive outcomes? E. coli physiology prevented scavenging by the LamB/Mgl system under O(2) limitation; hence, ptsG mutations provided the only adaptive pathway. But ptsG mutations in aerobic cultures are overtaken by mgl, mlc, and malT adaptations with better glucose-scavenging ability. Indeed, when an mglA::Tn10 mutant with an inactivated Mgl/LamB pathway was introduced into two independent aerobic chemostats, adaptation of the Mgl(-) strain involved the identical ptsG mutation found under O(2)-limited conditions with wild-type or Mgl(-) bacteria.

Mechanism of phosphoryl transfer in the dimeric IIABMan subunit of the Escherichia coli mannose transporter

The mannose transporter of bacterial phosphoenolpyruvate:sugar phosphotransferase system (PTS) mediates uptake of mannose, glucose, and related hexoses by a mechanism that couples translocation with phosphorylation of the substrate. It consists of the transmembrane IICMan.IIDMan complex and the cytoplasmic IIABMan subunit. IIABMan has two domains (IIA and IIB) that are linked by a 60-A long alanine-proline-rich linker. IIABMan transfers phosphoryl groups from the phospho-histidine-containing phospho-carrier protein of the PTS to His-10 on IIA, hence to His-175 on IIB, and finally to the 6'-OH of the transported hexose. IIABMan occurs as a stable homodimer. The subunit contact is mediated by a swap of beta-strands and an extensive contact area between the IIA domains. The H10C and H175C single and the H10C/H175C double mutants were used to characterize the phosphoryl transfer between IIA to IIB. Subunits do not exchange between dimers under physiological conditions, but slow phosphoryl transfer can take place between subunits from different dimers. Heterodimers of different subunits were produced in vitro by GuHCl-induced unfolding and refolding of mixtures of two different homodimers. With respect to wild-type homodimers, the heterodimers have the following activities: wild-type.H10C, 50%; wild-type.H175C 45%; H10C.H175C, 37%; and wild-type.H10C/H175C (double mutant), 29%. Taken together, this indicates that both cis and trans pathways contribute to the maximal phosphotransferase activity of IIABMan. A phosphoryl group on a IIA domain can be transferred either to the IIB domain on the same or on the second subunit in the dimer, and interruption of one of the two pathways results in a reduction of the activity to 70-80% of the control.

The glucose transporter of Escherichia coli with circularly permuted domains is active in vivo and in vitro

The bacterial phosphotransferase system (PTS) consists of two energy-coupling soluble proteins (enzyme I and HPr) and a large number of inner membrane transporters (enzymes II) that mediate concomitant phosphorylation and translocation of sugars and hexitols. The transporters consist of three functional units (IIA, IIB, IIC), which occur either as protein subunits or domains of a multidomain polypeptide. The membrane-spanning IIC domain contains the substrate binding site; IIA and IIB are phosphorylation domains that transfer phosphate from HPr to the transported sugar. The transporter complexes of the PTS are good examples for variation of design by modular assembly of domains and subunits. The domain order is IIC-IIB in the membrane subunit of the Escherichia coli glucose transporter (IICBGlc) and IIB-IIC in Salmonella typhimurium sucrose transporter (IIBCScr). The phosphorylation domain of IICBGlc was translocated from the carboxyl-terminal to the amino-terminal end of the IIC domain, and the activity of the circularly permuted form was optimized by variation of the length and the composition of the interdomain linker. IIBapCGlc with an alanine-proline-rich interdomain linker has 70% of the control specific activity after purification and reconstitution into proteoliposomes. These results indicate that the amino-terminal end of IICBGlc must be on the cytoplasmic side of the inner membrane, that membrane insertion of the IIC domain is insensitive to the modification of its amino-terminal end, and that a domain swap as it could occur by a single DNA translocation event can rapidly lead to a functional protein. However, IIB could not be substituted for by glucokinase. Fusion proteins between the IIC domain and glucokinase do not transport and phosphorylate glucose in an ATP-dependent mechanism, although the IIC moiety displays transport activity upon complementation with soluble subclonal IIB, and the glucokinase moiety retains ATP-dependent nonvectorial kinase activity. This indicates that IIC and IIB are two cooperative units and not only sequentially acting upon a common substrate, and that translocation of glucose must be conformationally coupled to the phosphorylation/dephosphorylation cycle of IIB.

Mutational analysis of invariant arginines in the IIAB(Man) subunit of the Escherichia coli phosphotransferase system

The mannose transporter of bacterial phosphotransferase system mediates uptake of mannose, glucose, and related hexoses by a mechanism that couples translocation with phosphorylation of the substrate. It consists of the transmembrane IIC(Man)-IID(Man) complex and the cytoplasmic IIAB(Man) subunit. IIAB(Man) has two flexibly linked domains, IIA(Man) and IIB(Man), each containing a phosphorylation site (His-10 and His-175). Phosphoryl groups are transferred from the phosphoryl carrier protein phospho-HPr to His-10, hence to His-175 and finally to the 6' OH of the transported hexose. Phosphate-binding sites and phosphate-catalytic sites frequently contain arginines, which by their guanidino group can stabilize phosphate through hydrogen bonding and electrostatic interactions. IIB(Man) contains five arginines which are invariant in the homologous IIB subunits of Escherichia coli, Klebsiella pneumoniae and Bacillus subtilis. The IIA domains have no conserved arginines. The five arginines were replaced by Lys or Gln one at a time, and the mutants were analyzed for transport and phosphorylation activity. All five IIB mutants can still be phosphorylated at His-175 by the IIA domain. R172Q is completely inactive with respect to glucose phosphotransferase (phosphoryltransfer from His-175 to the 6' OH of Glc) and hexose transport activity. R168Q has no hexose transport and strongly reduced phosphotransferase activity. R204K has no transport but almost normal phosphotransferase activity. R304Q has only slightly reduced transport activity. R190K behaves like wild-type IIAB(Man). Arg-168, Arg-172, and Arg-304 are part of the hydrogen bonding network on the surface of IIB, which contains the active site His-175 and the interface with the IIA domain (Schauder, S., Nunn, R.S., Lanz, R., Erni, B. and Schirmer, T. (1998) J. Mol. Biol. 276, 591-602) (Protein Data Bank accession code 1BLE). Arg-204 is at the putative interface between IIB(Man) and the IIC(Man)-IID(Man) complex.