Phosphoenolpyruvate-dependent phosphorylation site in enzyme IIIglc of the Escherichia coli phosphotransferase system

Characterization of the transcriptionally active form of dephosphorylated DctD complexed with dephospho-IIAGlc

Bacterial enhancer-binding proteins (bEBPs) acquire a transcriptionally active state via phosphorylation. However, transcriptional activation by the dephosphorylated form of bEBP has been observed in DctD, which belongs to Group I bEBP. The formation of a complex between dephosphorylated DctD (d-DctD) and dephosphorylated IIAGlc (d-IIAGlc) is a prerequisite for the transcriptional activity of d-DctD. In the present study, characteristics of the transcriptionally active complex composed of d-IIAGlc and phosphorylation-deficient DctD (DctDD57Q) of Vibrio vulnificus were investigated in its multimeric conformation and DNA-binding ability. DctDD57Q formed a homodimer that could not bind to the DNA. In contrast, when DctDD57Q formed a complex with d-IIAGlc in a 1:1 molar ratio, it produced two conformations: dimer and dodecamer of the complex. Only the dodecameric complex exhibited ATP-hydrolyzing activity and DNA-binding affinity. For successful DNA-binding and transcriptional activation by the dodecameric d-IIAGlc/DctDD57Q complex, extended upstream activator sequences were required, which encompass the nucleotide sequences homologous to the known DctD-binding site and additional nucleotides downstream. This is the first report to demonstrate the molecular characteristics of a dephosphorylated bEBP complexed with another protein to form a transcriptionally active dodecameric complex, which has an affinity for a specific DNA-binding sequence.IMPORTANCEResponse regulators belonging to the bacterial two-component regulatory system activate the transcription initiation of their regulons when they are phosphorylated by cognate sensor kinases and oligomerized to the appropriate multimeric states. Recently, it has been shown that a dephosphorylated response regulator, DctD, could activate transcription in a phosphorylation-independent manner in Vibrio vulnificus. The dephosphorylated DctD activated transcription as efficiently as phosphorylated DctD when it formed a complex with dephosphorylated form of IIAGlc, a component of the glucose-phosphotransferase system. Functional mimicry of this complex with the typical form of transcriptionally active phosphorylated DctD led us to study the molecular characteristics of this heterodimeric complex. Through systematic analyses, it was surprisingly determined that a multimer constituted with 12 complexes gained the ability to hydrolyze ATP and recognize specific upstream activator sequences containing a typical inverted-repeat sequence flanked by distinct nucleotides.

How Bacterial Pathogens Coordinate Appetite with Virulence

Cells adjust growth and metabolism to nutrient availability. Having access to a variety of carbon sources during infection of their animal hosts, facultative intracellular pathogens must efficiently prioritize carbon utilization. Here, we discuss how carbon source controls bacterial virulence, with an emphasis on Salmonella enterica serovar Typhimurium, which causes gastroenteritis in immunocompetent humans and a typhoid-like disease in mice, and propose that virulence factors can regulate carbon source prioritization by modifying cellular physiology. On the one hand, bacterial regulators of carbon metabolism control virulence programs, indicating that pathogenic traits appear in response to carbon source availability. On the other hand, signals controlling virulence regulators may impact carbon source utilization, suggesting that stimuli that bacterial pathogens experience within the host can directly impinge on carbon source prioritization. In addition, pathogen-triggered intestinal inflammation can disrupt the gut microbiota and thus the availability of carbon sources. By coordinating virulence factors with carbon utilization determinants, pathogens adopt metabolic pathways that may not be the most energy efficient because such pathways promote resistance to antimicrobial agents and also because host-imposed deprivation of specific nutrients may hinder the operation of certain pathways. We propose that metabolic prioritization by bacteria underlies the pathogenic outcome of an infection.

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.

Functional dissection of the phosphotransferase system provides insight into the prevalence of Faecalibacterium prausnitzii in the host intestinal environment

Faecalibacterium prausnitzii is a dominant member of healthy human colon microbiota, regarded as a beneficial gut bacterium due to its ability to produce anti-inflammatory substances. However, little is known about how F. prausnitzii utilizes the nutrients present in the human gut, influencing its prevalence in the host intestinal environment. The phosphoenolpyruvate (PEP):carbohydrate phosphotransferase system (PTS) is a widely distributed and highly efficient carbohydrate transport system found in most bacterial species that catalyses the simultaneous phosphorylation and import of cognate carbohydrates; its components play physiological roles through interaction with other regulatory proteins. Here, we performed a systematic analysis of the 16 genes encoding putative PTS components (2 enzyme I, 2 HPr, and 12 enzyme II components) in F. prausnitzii A2-165. We identified the general PTS components responsible for the PEP-dependent phosphotransfer reaction and the sugar-specific PTS components involved in the transport of two carbohydrates, N-acetylglucosamine and fructose, among five enzyme II complexes. We suggest that the dissection of the functional PTS in F. prausnitzii may help to understand how this species outcompetes other bacterial species in the human intestine.

Glucose-Specific Enzyme IIA of the Phosphoenolpyruvate:Carbohydrate Phosphotransferase System Modulates Chitin Signaling Pathways in Vibrio cholerae

In Vibrio cholerae, the genes required for chitin utilization and natural competence are governed by the chitin-responsive two-component system (TCS) sensor kinase ChiS. In the classical TCS paradigm, a sensor kinase specifically phosphorylates a cognate response regulator to activate gene expression. However, our previous genetic study suggested that ChiS stimulates the non-TCS transcriptional regulator TfoS by using mechanisms distinct from classical phosphorylation reactions (S. Yamamoto, J. Mitobe, T. Ishikawa, S. N. Wai, M. Ohnishi, H. Watanabe, and H. Izumiya, Mol Microbiol 91:326-347, 2014, https://doi.org/10.1111/mmi.12462). TfoS specifically activates the transcription of tfoR, encoding a small regulatory RNA essential for competence gene expression. Whether ChiS and TfoS interact directly remains unknown. To determine if other factors mediate the communication between ChiS and TfoS, we isolated transposon mutants that turned off tfoR::lacZ expression but possessed intact chiS and tfoS genes. We demonstrated an unexpected association of chitin-induced signaling pathways with the glucose-specific enzyme IIA (EIIA^glc) of the phosphoenolpyruvate:carbohydrate phosphotransferase system (PTS) for carbohydrate uptake and catabolite control of gene expression. Genetic and physiological analyses revealed that dephosphorylated EIIA^glc inactivated natural competence and tfoR transcription. Chitin-induced expression of the chb operon, which is required for chitin transport and catabolism, was also repressed by dephosphorylated EIIA^glc Furthermore, the regulation of tfoR and chb expression by EIIA^glc was dependent on ChiS and intracellular levels of ChiS were not affected by disruption of the gene encoding EIIA^glc These results define a previously unknown connection between the PTS and chitin signaling pathways in V. cholerae and suggest a strategy whereby this bacterium can physiologically adapt to the existing nutrient status.IMPORTANCE The EIIA^glc protein of the PTS coordinates a wide variety of physiological functions with carbon availability. In this report, we describe an unexpected association of chitin-activated signaling pathways in V. cholerae with EIIA^glc The signaling pathways are governed by the chitin-responsive TCS sensor kinase ChiS and lead to the induction of chitin utilization and natural competence. We show that dephosphorylated EIIA^glc inhibits both signaling pathways in a ChiS-dependent manner. This inhibition is different from classical catabolite repression that is caused by lowered levels of cyclic AMP. This work represents a newly identified connection between the PTS and chitin signaling pathways in V. cholerae and suggests a strategy whereby this bacterium can physiologically adapt to the existing nutrient status.

Enterococcus faecalis Uses a Phosphotransferase System Permease and a Host Colonization-Related ABC Transporter for Maltodextrin Uptake

Maltodextrin is a mixture of maltooligosaccharides, which are produced by the degradation of starch or glycogen. They are mostly composed of α-1,4- and some α-1,6-linked glucose residues. Genes presumed to code for the Enterococcus faecalis maltodextrin transporter were induced during enterococcal infection. We therefore carried out a detailed study of maltodextrin transport in this organism. Depending on their length (3 to 7 glucose residues), E. faecalis takes up maltodextrins either via MalT, a maltose-specific permease of the phosphoenolpyruvate (PEP):carbohydrate phosphotransferase system (PTS), or the ATP binding cassette (ABC) transporter MdxEFG-MsmX. Maltotriose, the smallest maltodextrin, is primarily transported by the PTS permease. A malT mutant therefore exhibits significantly reduced growth on maltose and maltotriose. The residual uptake of the trisaccharide is catalyzed by the ABC transporter, because a malT mdxF double mutant no longer grows on maltotriose. The trisaccharide arrives as maltotriose-6″-P in the cell. MapP, which dephosphorylates maltose-6'-P, also releases P_i from maltotriose-6″-P. Maltotetraose and longer maltodextrins are mainly (or exclusively) taken up via the ABC transporter, because inactivation of the membrane protein MdxF prevents growth on maltotetraose and longer maltodextrins up to at least maltoheptaose. E. faecalis also utilizes panose and isopanose, and we show for the first time, to our knowledge, that in contrast to maltotriose, its two isomers are primarily transported via the ABC transporter. We confirm that maltodextrin utilization via MdxEFG-MsmX affects the colonization capacity of E. faecalis, because inactivation of mdxF significantly reduced enterococcal colonization and/or survival in kidneys and liver of mice after intraperitoneal infection.IMPORTANCE Infections by enterococci, which are major health care-associated pathogens, are difficult to treat due to their increasing resistance to clinically relevant antibiotics, and new strategies are urgently needed. A largely unexplored aspect is how these pathogens proliferate and which substrates they use in order to grow inside infected hosts. The use of maltodextrins as a source of carbon and energy was studied in Enterococcus faecalis and linked to its virulence. Our results demonstrate that E. faecalis can efficiently use glycogen degradation products. We show here that depending on the length of the maltodextrins, one of two different transporters is used: the maltose-PTS transporter MalT, or the MdxEFG-MsmX ABC transporter. MdxEFG-MsmX takes up longer maltodextrins as well as complex molecules, such as panose and isopanose.

The phosphocarrier protein HPr of Neisseria meningitidis interacts with the transcription regulator CrgA and its deletion affects capsule production, cell adhesion, and virulence

The bacterial phosphotransferase system (PTS) transports and phosphorylates sugars, but also carries out numerous regulatory functions. The β-proteobacterium Neisseria meningitidis possesses an incomplete PTS unable to transport carbon sources because it lacks a membrane component. Nevertheless, the residual phosphorylation cascade is functional and the meningococcal PTS was therefore expected to carry out regulatory roles. Interestingly, a ΔptsH mutant (lacks the PTS protein HPr) exhibited reduced virulence in mice and after intraperitoneal challenge it was rapidly cleared from the bloodstream of BALB/c mice. The rapid clearance correlates with lower capsular polysaccharide production by the ΔptsH mutant, which is probably also responsible for its increased adhesion to Hec-1-B epithelial cells. In addition, compared to the wild-type strain more apoptotic cells were detected when Hec-1-B cells were infected with the ΔptsH strain. Coimmunoprecipitation revealed an interaction of HPr and P-Ser-HPr with the LysR type transcription regulator CrgA, which among others controls its own expression. Moreover, ptsH deletion caused increased expression of a ΦcrgA-lacZ fusion. Finally, the presence of HPr or phospho-HPr's during electrophoretic mobility shift assays enhanced the affinity of CrgA for its target sites preceding crgA and pilE, but HPr did not promote CrgA binding to the sia and pilC1 promoter regions.

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.

The central metabolism regulator EIIAGlc switches Salmonella from growth arrest to acute virulence through activation of virulence factor secretion

The ability of Salmonella to cause disease depends on metabolic activities and virulence factors. Here, we show that a key metabolic protein, EIIAGlc, is absolutely essential for acute infection, but not for Salmonella survival, in a mouse typhoid fever model. Surprisingly, phosphorylation-dependent EIIAGlc functions, including carbohydrate transport and activation of adenylate cyclase for global regulation, do not explain this virulence phenotype. Instead, biochemical studies, in vitro secretion and translocation assays, and in vivo genetic epistasis experiments suggest that EIIAGlc binds to the type three secretion system 2 (TTSS-2) involved in systemic virulence, stabilizes its cytoplasmic part including the crucial TTSS-2 ATPase, and activates virulence factor secretion. This unexpected role of EIIAGlc reveals a striking direct link between central Salmonella metabolism and a crucial virulence mechanism.

Carbon catabolite repression of the maltose transporter revealed by X-ray crystallography

Efficient carbon utilization is critical to the survival of microorganisms in competitive environments. To optimize energy usage, bacteria have developed an integrated control system to preferentially uptake carbohydrates that support rapid growth. The availability of a preferred carbon source such as glucose represses the synthesis and activities of proteins necessary for the transport and metabolism of secondary carbon sources. This regulatory phenomenon is defined as carbon catabolite repression (CCR)¹. In enteric bacteria, the key player of CCR is a component of the glucose-specific phosphotransferase system, enzyme IIA (EIIA^Glc)^1,2. It is known that unphosphorylated EIIA^Glc binds and inhibits a variety of transporters when glucose is available^1,2. However, understanding the underlying molecular mechanism has been hindered by the complete absence of structures for any EIIA^Glc-transporter complexes. Here, we present the 3.9 Å crystal structure of EIIA^Glc in complex with the maltose transporter, an ATP-binding cassette (ABC) transporter. The structure shows that two EIIA^Glc molecules bind to the cytoplasmic ATPase subunits, stabilizing the transporter in an inward-facing conformation and preventing the structural rearrangements necessary for ATP hydrolysis. We also show that the half maximal inhibitory concentrations of the full-length EIIA^Glc and an N-terminal truncation mutant differ by 60 fold, consistent with the hypothesis that the N-terminal region, disordered in the crystal structure, functions as a membrane-anchor to increase the effective EIIA^Glc concentration at the membrane^3,4. Together these data suggest a model of how the central regulatory protein EIIA^Glc allosterically inhibits maltose uptake in E. coli.

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.

The Cold Shock Response of Lactobacillus casei: Relation between HPr Phosphorylation and Resistance to Freeze/Thaw Cycles

When carrying out a proteome analysis with a ptsH3 mutant of Lactobacillus casei, we found that the cold shock protein CspA was significantly overproduced compared to the wild-type strain. We also noticed that CspA and CspB of L. casei and CSPs from other organisms exhibit significant sequence similarity to the C-terminal part of EIIA(Glc), a glucose-specific component of the phosphoenolpyruvate:sugar phosphotransferase system. This similarity suggested a direct interaction of HPr with CSPs, as histidyl-phosphorylated HPr has been shown to phosphorylate EIIA(Glc) in its C-terminal part. We therefore compared the cold shock response of several carbon catabolite repression mutants to that of the wild-type strain. Following a shift from 37 degrees C to lower temperatures (20, 15 or 10 degrees C), all mutants showed significantly reduced growth rates. Moreover, glucose-grown mutants unable to form P-Ser-HPr (ptsH1, hprK) exhibited drastically increased sensitivity to freeze/thaw cycles. However, when the same mutants were grown on ribose or maltose, they were similarly resistant to freezing and thawing as the wild-type strain. Although subsequent biochemical and genetic studies did not allow to identify the form of HPr implicated in the resistance to cold and freezing conditions, they strongly suggested a direct interaction of HPr or one of its phospho-derivatives with CspA and/or another, hitherto undetected cold shock protein in L. casei.

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.

Effects of Mutations and Truncations on the Kinetic Behavior of IIAGlc, a Phosphocarrier and Regulatory Protein of the Phosphoenolpyruvate Phosphotransferase System of Escherichia coli

IIAGlc, a component of the glucose-specific phosphoenolpyruvate:phosphotransferase system (PTS) of Escherichia coli, is important in regulating carbohydrate metabolism. In Glc uptake, the phosphotransfer sequence is: phosphoenolpyruvate --> Enzyme I --> HPr --> IIAGlc --> IICBGlc --> Glc. (HPr is the first phosphocarrier protein of the PTS.) We previously reported two classes of IIAGlc mutations that substantially decrease the P-transfer rate constants to/from IIAGlc. A mutant of His75 which adjoins the active site (His90), (H75Q), was 0.5% as active as wild-type IIAGlc in the reversible P-transfer to HPr. Two possible explanations were offered for this result: (a) the imidazole ring of His75 is required for charge delocalization and (b) H75Q disrupts the hydrogen bond network: Thr73, His75, phospho-His90. The present studies directly test the H-bond network hypothesis. Thr73 was replaced by Ser, Ala, or Val to eliminate the network. Because the rate constants for phosphotransfer to/from HPr were largely unaffected, we conclude that the H-bond network hypothesis is not correct. In the second class of mutants, proteolytic truncation of seven residues of the IIAGlc N terminus caused a 20-fold reduction in phosphotransfer to membrane-bound IICBGlc from Salmonella typhimurium. Here, we report the phosphotransfer rates between two genetically constructed N-terminal truncations of IIAGlc (Delta7 and Delta16) and the proteins IICBGlc and IIBGlc (the soluble cytoplasmic domain of IICBGlc). The truncations did not significantly affect reversible P-transfer to IIBGlc but substantially decreased the rate constants to IICBGlc in E. coli and S. typhimurium membranes. The results support the hypothesis (Wang, G., Peterkofsky, A., and Clore, G. M. (2000) J. Biol. Chem. 275, 39811-39814) that the N-terminal 18-residue domain "docks" IIAGlc to the lipid bilayer of membranes containing IICBGlc.

The phosphotransferase system of Streptomyces coelicolor

We have investigated the crr gene of Streptomyces coelicolor that encodes a homologue of enzyme IIAGlucose of Escherichia coli, which, as a component of the phosphoenolpyruvate-dependent sugar phosphotransferase system (PTS) plays a key role in carbon regulation by triggering glucose transport, carbon catabolite repression, and inducer exclusion. As in E. coli, the crr gene of S. coelicolor is genetically associated with the ptsI gene that encodes the general phosphotransferase enzyme I. The gene product IIACrr was overproduced, purified, and polyclonal antibodies were obtained. Western blot analysis revealed that IIACrr is expressed in vivo. The functionality of IIACrr was demonstrated by phosphoenolpyruvate-dependent phosphorylation via enzyme I and the histidine-containing phosphoryl carrier protein HPr. Phosphorylation was abolished when His72, which corresponds to the catalytic histidine of E. coli IIAGlucose, was mutated. The capacity of IIACrr to operate in sugar transport was shown by complementation of the E. coli glucose-PTS. The striking functional resemblance between IIACrr and IIAGlucose was further demonstrated by its ability to confer inducer exclusion of maltose to E. coli. A specific interaction of IIACrr with the maltose permease subunit MalK from Salmonella typhimurium was uncovered by surface plasmon resonance. These data suggest that this IIAGlucose-like protein may be involved in carbon metabolism in S. coelicolor.

Negative regulation of the pts operon by Mlc: mechanism underlying glucose induction in Escherichia coli

BACKGROUND

The pts operon of Escherichia coli consists of three genes ptsH, ptsI and crr, each encoding for central components of the phosphoenolpyruvate: carbohydrate phosphotransferase system, HPr, enzyme I and IIAGlc, respectively. Transcription of the pts operon is stimulated when glucose is present in the culture medium. One of the two major promoters, P0, is responsible for this glucose induction. However, no regulatory protein responsible for the glucose induction of the pts operon has been identified yet and molecular mechanism by which glucose stimulates the pts transcription is not known.

RESULTS

We found by Northern blotting that the pts mRNA levels in cells lacking Mlc, a new global repressor of carbohydrate metabolism, were increased without external glucose and that the addition of glucose had no effect on the pts mRNA levels in the mutant cells. Western blotting revealed that the enzyme I level in the mlc- cells was also elevated without glucose and no further increase in the enzyme I level was observed in the presence of glucose. S1 analysis revealed that transcription of the glucose-sensitive promoter, P0, occurs constitutively in the mlc- cells independently from the external glucose. In vitro transcription studies indicated that Mlc strongly inhibited P0 transcription. DNase I footprinting experiment revealed that Mlc bound to P0 promoter region to prevent RNA polymerase binding at P0.

CONCLUSION

We conclude that Mlc is a repressor for the pts transcription acting as a major regulatory protein involved in the glucose induction of pts operon. We propose that glucose induces the pts transcription by modulating the Mlc activity. The mechanism by which glucose modulates the Mlc action remains to be studied.

A mutated PtsG, the glucose transporter, allows uptake of D-ribose

Mutations arose from an Escherichia coli strain defective in the high (Rbs/ribose) and low (Als/allose and Xyl/xylose) affinity D-ribose transporters, which allow cells to grow on D-ribose. Genetic tagging and mapping of the mutations revealed that two loci in the E. coli linkage map are involved in creating a novel ribose transport mechanism. One mutation was found in ptsG, the glucose-specific transporter of phosphoenolpyruvate:carbohydrate phosphotransferase system and the other in mlc, recently reported to be involved in the regulation of ptsG. Five different mutations in ptsG were characterized, whose growth on D-ribose medium was about 80% that of the high affinity system (Rbs+). Two of them were found in the predicted periplasmic loops, whereas three others are in the transmembrane region. Ribose uptakes in the mutants, competitively inhibited by D-glucose, D-xylose, or D-allose, were much lower than that of the high affinity transporter but higher than those of the Als and Xyl systems. Further analyses of the mutants revealed that the rbsK (ribokinase) and rbsD (function unknown) genes are involved in the ribose transport through PtsG, indicating that the phosphorylation of ribose is not mediated by PtsG and that some unknown metabolic function mediated by RbsD is required. It was also found that D-xylose, another sugar not involved in phosphorylation, was efficiently transported through the wild-type or mutant PtsG in mlc-negative background. The efficiencies of xylose and glucose transports are variable in the PtsG mutants, depending on their locations, either in the periplasm or in the membrane. In an extreme case of the transmembrane change (I283T), xylose transport is virtually abolished, indicating that the residue is directly involved in determining sugar specificity. We propose that there are at least two domains for substrate specificity in PtsG with slightly altered recognition properties.

Phosphorylation and functional properties of the IIA domain of the lactose transport protein of Streptococcus thermophilus

The lactose-H+ symport protein (LacS) of Streptococcus thermophilus has a carboxyl-terminal regulatory domain (IIALacS) that is homologous to a family of proteins and protein domains of the phosphoenolpyruvate:carbohydrate phosphotransferase system (PTS) in various organisms, of which IIAGlc of Escherichia coli is the best-characterized member. On the basis of these similarities, it was anticipated that IIALacS would be able to perform one or more functions associated with IIAGlc, i.e., carry out phosphoryl transfer and/or affect other catabolic functions. The gene fragment encoding IIALacS was overexpressed in Escherichia coli, and the protein was purified in two steps by metal affinity and anion-exchange chromatography. IIALacS was unable to restore glucose uptake in a IIAGlc-deficient strain, which is consistent with a very low rate of phosphorylation of IIALacS by phosphorylated HPr (HPr approximately P) from E. coli. With HPr approximately P from S. thermophilus, the rate was more than 10-fold higher, but the rate constants for the phosphorylation of IIALacS (k1 = 4.3 x 10(2) M-1 s-1) and dephosphorylation of IIALacS approximately P by HPr (k-1 = 1.1 x 10(3) M-1 s-1) are still at least 4 orders of magnitude lower than for the phosphoryltransfer between IIAGlc and HPr from E. coli. This finding suggests that IIALacS has evolved into a protein domain whose main function is not to transfer phosphoryl groups rapidly. On the basis of sequence alignment of IIA proteins with and without putative phosphoryl transfer functions and the known structure of IIAGlc, we constructed a double mutant [IIALacS(I548E/G556D)] that was predicted to have increased phosphoryl transfer activity. Indeed, the phosphorylation rate of IIALacS(I548E/G556D) by HPr approximately P increased (k1 = 4.0 x 10(3) M-1 s-1) and became nearly independent of the source of HPr approximately P (S. thermophilus, Bacillus subtilis, or E. coli). The increased phosphoryl transfer rate of IIALacS(I548E/G556D) was insufficient to complement IIAGlc in PTS-mediated glucose transport in E. coli. Both IIALacS and IIALacS(I548E/G556D) could replace IIAGlc, but in another function: they inhibited glycerol kinase (inducer exclusion) when present in the unphosphorylated form.

A promiscuous binding surface: crystal structure of the IIA domain of the glucose-specific permease from Mycoplasma capricolum

BACKGROUND

The phosphoenolpyruvate:sugar phosphotransferase system (PTS) is a bacterial and mycoplasma system responsible for the uptake of some sugars, concomitant with their phosphorylation. The sugar-specific component of the system, enzyme II (EII),consists of three domains, EIIA, EIIB and EIIC. EIIA and ELLB are cytoplasmic and EIIC is an integral membrane protein that contains the sugar-binding site. Phosphoenolpyruvate (PEP) provides the source of the phosphoryl group, which is transferred via several phosphoprotein intermediates, eventually being transferred to the internalized sugar. Along the pathway, EIIA accepts a phosphoryl group from the phosphocarrier protein HPr and transfers it to EIIB. The structure of the glucose-specific EIIA (EIIAglc) from Mycoplasma capricolum reported here facilitates understanding of the nature of the interactions between this protein and its partners.

RESULTS

The crystal structure of EIIAglc from M. capricolum has been determined at 2.5 A resolution. two neighboring EIIAglc molecules associate with one another in a front-to-back fashion, such that Glu149 of one molecule forms electrostatic interactions with the active-site histidine residues, His90 and His75, of the other. Glu149 is therefore considered to mimic the interaction that a phosphorylated histidine of a partner protein makes with EIIA. Another interaction, an ion pair between the active-site Asp94 and Lys168 of a neighboring molecule, may be analogous to the interaction between Asp94 of EIIAglc and Arg17 of HPr. Analysis of molecular packing in this crystal, and in the crystals of two other homologous proteins from Escherichia coli and Bacillus subtilis, reveals that in all cases active-site hydrophobic residues are involved in crystal contacts, but in each case a different region of the neighboring molecule is involved. The transition-state complexes of M. capricolum EIIAglc with HPr and EIIBglc have been modeled; in each case, different structural units are shown to interact with EIIAglc. Many of the interactions are hydrophobic with no sequence specificity. The only specific interaction, other than that formed by the phosphoryl group, involves ion pairs between two invariant aspartate residues of EIIAglc and arginine/lysine residues of HPr or EIIBglc.

CONCLUSIONS

The non-discriminating nature of the hydrophobic interactions that EIIAglc forms with a variety of partners may be a consequence of the requirement for interaction with a variety of proteins that show no sequence or structural similarity. Nevertheless, specificity is provided by an ion-pair interaction that is enhanced by the apolar nature of the interface.

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

The glucose transporter of the bacterial phosphotransferase system (PTS) consists of a hydrophilic (IIAGlc) and a transmembrane subunit (IICBGlc). IICBGlc has two domains (C and B), which are linked by a highly invariant sequence. Transport of glucose by IIC and phosphorylation by IIB are tightly coupled processes. Three motifs that are strongly conserved in 12 homologous PTS transporters, namely two invariant arginines (Arg-424 and Arg-426) adjacent to the phosphorylation site (Cys-421), the invariant interdomain sequence KTPGRED, and two conserved histidines (His-211 and His-212) in the IIC domain were mutated and the mutant proteins characterized in vivo and in vitro for transport and phosphorylation activity. Replacement of the strongly beta-turn favoring residues Thr and Gly of the linker by alpha-helix favoring Ala results in strong reduction of activity, whereas the substitutions of the other residues have only minor effects. The R424K and R426K mutants can be phosphorylated by IIAGlc but can no longer donate the phosphoryl group to glucose. The H211Q and H212Q mutants continue to phosphorylate glucose at a reduced rate but H212Q can no longer transport glucose. Mixtures of purified R424K/H212Q and R426K/H212Q have 10% of wild-type phosphorylation activity and when coexpressed in Escherichia coli support glucose transport.

Modulation of Escherichia coli adenylyl cyclase activity by catalytic-site mutants of protein IIA(Glc) of the phosphoenolpyruvate:sugar phosphotransferase system

It is demonstrated here that in Escherichia coli, the phosphorylated form of the glucose-specific phosphocarrier protein IIA(Glc) of the phosphoenolpyruvate:sugar phosphotransferase system is an activator of adenylyl cyclase and that unphosphorylated IIA(Glc) has no effect on the basal activity of adenylyl cyclase. To elucidate the specific role of IIA(Glc) phosphorylation in the regulation of adenylyl cyclase activity, both the phosphorylatable histidine (H90) and the interactive histidine (H75) of IIA(Glc) were mutated by site-directed mutagenesis to glutamine and glutamate. Wild-type IIA(Glc) and the H75Q mutant, in which the histidine in position 75 has been replaced by glutamine, were phosphorylated by the phosphohistidine-containing phosphocarrier protein (HPr-P) and were equally potent activators of adenylyl cyclase. Neither the H90Q nor the H90E mutant of IIA(Glc) was phosphorylated by HPr-P, and both failed to activate adenylyl cyclase. Furthermore, replacement of H75 by glutamate inhibited the appearance of a steady-state level of phosphorylation of H90 of this mutant protein by HPr-P, yet the H75E mutant of IIA(Glc) was a partial activator of adenylyl cyclase. The H75E H90A double mutant, which cannot be phosphorylated, did not activate adenylyl cyclase. This suggests that the H75E mutant was transiently phosphorylated by HPr-P but the steady-state level of the phosphorylated form of the mutant protein was decreased due to the repulsive forces of the negatively charged glutamate at position 75 in the catalytic pocket. These results are discussed in the context of the proximity of H75 and H90 in the IIA(Glc) structure and the disposition of the negative charge in the modeled glutamate mutants.

Structural studies of the Escherichia coli signal transducing protein IIAGlc: implications for target recognition

In Escherichia coli, the glucose-specific phosphocarrier protein of the phosphotransferase system (PTS), IIAGlc (IIIGlc in older literature), is also the central regulatory protein of the PTS. Depending upon its state of phosphorylation, IIAGlc binds to a number of different proteins that display no apparent sequence homology. Previous structural studies suggested that nonspecific hydrophobic interactions, specific salt bridges, and an intermolecular Zn(II) binding site contribute to the wide latitude in IIAGlc binding sites. Two new crystal forms of IIAGlc have been solved at high resolution, and the models were compared to those previously studied. The major intermolecular contacts in the crystals differ in detail, but all involve the hydrophobic active site of IIAGlc interacting with a hydrophobic patch on a neighbor and all are shown to be surprisingly similar to the physiologically relevant regulatory interaction of IIAGlc with glycerol kinase. In two crystal forms, a helix on one molecule interacts with the face of another, while in the other crystal form, the primary crystal contact consists of a strand of beta-sheet that contributes to an intermolecular Zn(II) binding site with tetrahedral ligation identical to that of the zinc peptidase thermolysin. Thus, relatively nonspecific hydrophobic interactions combined with specific salt bridges and an intermolecular cation binding site (cation-promoted association) permit a regulatory protein to bind to target proteins that have little or no sequence or structural homology with one another. It is suggested that signal transduction by IIAGlc is a binary switch in which phosphorylation at the active site directly controls binding to target molecules.

Cloning and sequencing of two enterococcal glpK genes and regulation of the encoded glycerol kinases by phosphoenolpyruvate-dependent, phosphotransferase system-catalyzed phosphorylation of a single histidyl residue

The glpK genes of Enterococcus casseliflavus and Enterococcus faecalis, encoding glycerol kinase, the key enzyme of glycerol uptake and metabolism in bacteria, have been cloned and sequenced. The translated amino acid sequences exhibit strong homology to the amino acid sequences of other bacterial glycerol kinases. After expression of the enterococcal glpK genes in Escherichia coli, both glycerol kinases were purified and were found to be phosphorylated by enzyme I and the histidine-containing protein of the phosphoenolpyruvate:glycose phosphotransferase system. Phosphoenolpyruvate-dependent phosphorylation caused a 9-fold increase in enzyme activity. The site of phosphorylation in glycerol kinase of E. casseliflavus was determined as His-232. Site-specific mutagenesis was used to replace His-232 in glycerol kinase of E. casseliflavus with an alanyl, glutamate, or arginyl residue. The mutant proteins could no longer be phosphorylated confirming that His-232 of E. casseliflavus glycerol kinase represents the site of phosphorylation. The His232 --> Arg glycerol kinase exhibited an about 3-fold elevated activity compared with wild-type glycerol kinase. Fructose 1,6-bisphosphate was found to inhibit E. casseliflavus glycerol kinase activity. However, neither EIIAGlc from E. coli nor the EIIAGlc domain of Bacillus subtilis had an inhibitory effect on glycerol kinase of E. casseliflavus.

Purification by Ni2+ affinity chromatography, and functional reconstitution of the transporter for N-acetylglucosamine of Escherichia coli

The N-acetyl-D-glucosamine transporter (IIGlcNAc) of the bacterial phosphotransferase system couples vectorial translocation to phosphorylation of the transported GlcNAc. IIGlcNAc of Escherichia coli containing a carboxyl-terminal affinity tag of six histidines was purified by Ni2+ chelate affinity chromatography. 4 mg of purified protein was obtained from 10 g (wet weight) of cells. Purified IIGlcNAc was reconstituted into phospholipid vesicles by detergent dialysis and freeze/thaw sonication. IIGlcNAc was oriented randomly in the vesicles as inferred from protein phosphorylation studies. Import and subsequent phosphorylation of GlcNAc were measured with proteoliposomes preloaded with enzyme I, histidine-containing phosphocarrier protein, and phosphoenolpyruvate. Uptake and phosphorylation occurred in a 1:1 ratio. Active extrusion of GlcNAc entrapped in vesicles was also measured by the addition of enzyme I, histidine-containing phosphocarrier protein, and phosphoenolpyruvate to the outside of the vesicles. The Km for vectorial phosphorylation and non-vectorial phosphorylation were 66. 6 +/- 8.2 microM and 750 +/- 19.6 microM, respectively. Non-vectorial phosphorylation was faster than vectorial phosphorylation with kcat 15.8 +/- 0.9 s-1 and 6.2 +/- 0.7 s-1, respectively. Using exactly the same conditions, the purified transporters for mannose (IIABMan, IICMan, IIDMan) and glucose (IICBGlc, IIAGlc) were also reconstituted for comparison. Although the vectorial transport activities of IICBAGlcNAc and IICBGlc. IIAGlc are inhibited by non-vectorial phosphorylation, no such effect was observed with the IIABMan.IICMan.IIDMan complex. This suggests that the molecular mechanisms underlying solute transport and phosphorylation are different for different transporters of the phosphotransferase system.

Regulation of bacterial sugar-H+ symport by phosphoenolpyruvate-dependent enzyme I/HPr-mediated phosphorylation

The lactose-H+ symport protein (LacS) of Streptococcus thermophilus has a C-terminal hydrophilic domain that is homologous to IIA protein(s) domains of the phosphoenolpyruvate:sugar phosphotransferase system (PTS). C-terminal truncation mutants were constructed and expressed in Escherichia coli and their properties were analyzed. Remarkably, the entire IIA domain (160 amino acids) could be deleted without significant effect on lactose-H+ symport and galactoside equilibrium exchange. Analysis of the LacS mutants in S. thermophilus cells suggested that transport is affected by PTS-mediated phosphorylation of the IIA domain. For further studies, membrane vesicles of S. thermophilus were fused with cytochrome c oxidase-containing liposomes, and, when appropriate, phosphoenolpyruvate (PEP) plus purified enzyme I and heat-stable protein HPr were incorporated into the hybrid membranes. Generation of a protonmotive force (delta p) in the hybrid membranes resulted in accumulation of lactose, whereas uptake of the PTS sugar sucrose was not observed. With PEP and the energy-coupling proteins enzyme I and HPr of the PTS on the inside, high rates of sucrose uptake were observed, whereas delta p-driven lactose uptake by wild-type LacS was inhibited. This inhibition was not observed with LacS(delta 160) and LacS(H552R), indicating that PEP-dependent enzyme I/HPr-mediated phosphorylation of the IIA domain (possibly the conserved His-552 residue) modulates lactose-H+ symport activity.

Unraveling a bacterial hexose transport pathway

Structural information about proteins involved in bacterial hexose transport mediated by the phosphoenolpyruvate:sugar phosphotransferase system is rapidly accumulating. Within the past year, two crystal structures and two solution NMR structures of the histidine-containing phosphocarrier protein have been reported, adding structural details to previous NMR and crystallographic work on this protein and on enzyme IIA. The crystal structure of the regulatory complex between the glucose enzyme IIA and glycerol kinase has been determined, and the association of the histidine-containing phosphocarrier protein and either the glucose enzyme IIA or the mannitol enzyme IIA have been studied by NMR. Proposals concerning the mechanism of phosphoryl transfer and the protein-protein interactions involved may now be tested more rigorously using these data.

The mannose transporter of Escherichia coli K12: oligomeric structure, and function of two conserved cysteines

The mannose transporter of E. coli is a member of the phosphotransferase system. It consists of two membrane spanning subunits, IICMan (27.64 kDa) and IIDMan (31.02 kDa) and a peripheral subunit IIABMan (35.02 kDa). It acts by a mechanism that couples vectorial translocation to phosphorylation of the substrate. The subunit ratio determined from densitometric scans of polyacrylamide gels is close to IIABMan2 IICMan1 IIDMan2. A molecular mass of 100 +/- 20 kDa was calculated from electronmicrographs of freeze fractured proteoliposomes containing particles of the IICMan/IIDMan subcomplex with a mean diameter of 6.3 +/- 1.1 nm. This is most compatible with IICMan:IIDMan subunit compositions of 1:2 (89.7 kDa). Fusion proteins between IICMan and IIDMan were generated, with the subunits connected either by a two-residue linker or a 20 residue Ala Pro rich hinge. The fusion proteins had 5%-15% of control phosphotransferase activity. The one with the Ala Pro rich linker could be cleaved with trypsin resulting in a 7 fold increase of activity while the fusion with the two residue linker was resistant to limited trypsinolysis. Taking into account the inside-out orientation of the membrane vesicles the C-terminus of IICMan and the N-terminus of IIDMan are both predicted to be on the cytoplasmic side of the membrane. Two cysteines in IICMan and IIDMan which are conserved in the homologous subunits of the fructose transporter of Bacillus subtilis and of sorbose transporter of Klebsiella pneumoniae are not necessary for phosphotransferase function.

Role of a conserved histidine residue, His-195, in the activities of the Escherichia coli mannitol permease

The mannitol permease, an enzyme II of the phosphoenolpyruvate-dependent carbohydrate phosphotransferase system (PTS) of Escherichia coli, carries out the transport and phosphorylation of D-mannitol in this organism. Previous studies have shown that His-554 and Cys-384 in the mannitol permease are sequentially phosphorylated in reactions necessary for the transport and phosphorylation of the substrate. These residues are located in a large cytoplasmic domain of the protein. Interaction of the permease with mannitol, and its membrane translocation, however, must involve the N-terminal, transmembrane domain (EIIC domain) of the protein. In this report, we use site-directed mutagenesis and mutant complementation to investigate the role of His-195 in the EIIC domain of the mannitol permease, a residue that is conserved in many PTS permeases. In a previous report [Weng, Q.-P., Elder, J., & Jacobson, G. R. (1992) J. Biol. Chem. 267, 19529-19535], we inferred a role for His-195 that involves its hydrogen-bonding ability. Here we show that His-195 plays a role in high-affinity mannitol binding. Moreover, mutant complementation studies show that a functional His-195 must be on the same subunit as a functional Cys-384 in a permease dimer for phosphotransfer to mannitol to occur. These results and kinetic studies of His-195 mutant proteins imply that His-195 also may play an important role in this phosphotransfer reaction. His-195 is predicted to be in a cytoplasmic "loop" in the EIIC domain of the mannitol permease, in which several other residues have been shown to have roles in mannitol permease activity.

Phosphoenolpyruvate:carbohydrate phosphotransferase systems of bacteria

Numerous gram-negative and gram-positive bacteria take up carbohydrates through the phosphoenolpyruvate (PEP):carbohydrate phosphotransferase system (PTS). This system transports and phosphorylates carbohydrates at the expense of PEP and is the subject of this review. The PTS consists of two general proteins, enzyme I and HPr, and a number of carbohydrate-specific enzymes, the enzymes II. PTS proteins are phosphoproteins in which the phospho group is attached to either a histidine residue or, in a number of cases, a cysteine residue. After phosphorylation of enzyme I by PEP, the phospho group is transferred to HPr. The enzymes II are required for the transport of the carbohydrates across the membrane and the transfer of the phospho group from phospho-HPr to the carbohydrates. Biochemical, structural, and molecular genetic studies have shown that the various enzymes II have the same basic structure. Each enzyme II consists of domains for specific functions, e.g., binding of the carbohydrate or phosphorylation. Each enzyme II complex can consist of one to four different polypeptides. The enzymes II can be placed into at least four classes on the basis of sequence similarity. The genetics of the PTS is complex, and the expression of PTS proteins is intricately regulated because of the central roles of these proteins in nutrient acquisition. In addition to classical induction-repression mechanisms involving repressor and activator proteins, other types of regulation, such as antitermination, have been observed in some PTSs. Apart from their role in carbohydrate transport, PTS proteins are involved in chemotaxis toward PTS carbohydrates. Furthermore, the IIAGlc protein, part of the glucose-specific PTS, is a central regulatory protein which in its nonphosphorylated form can bind to and inhibit several non-PTS uptake systems and thus prevent entry of inducers. In its phosphorylated form, P-IIAGlc is involved in the activation of adenylate cyclase and thus in the regulation of gene expression. By sensing the presence of PTS carbohydrates in the medium and adjusting the phosphorylation state of IIAGlc, cells can adapt quickly to changing conditions in the environment. In gram-positive bacteria, it has been demonstrated that HPr can be phosphorylated by ATP on a serine residue and this modification may perform a regulatory function.

Tautomeric states of the active-site histidines of phosphorylated and unphosphorylated IIIGlc, a signal-transducing protein from Escherichia coli, using two-dimensional heteronuclear NMR techniques

IIIGlc is an 18.1-kDa signal-transducing phosphocarrier protein of the phosphoenolpyruvate:glycose phosphotransferase system from Escherichia coli. The 1H, 15N, and 13C histidine ring NMR signals of both the phosphorylated and unphosphorylated forms of IIIGlc have been assigned using two-dimensional 1H-15N and 1H-13C heteronuclear multiple-quantum coherence (HMQC) experiments and a two-dimensional 13C-13C-1H correlation spectroscopy via JCC coupling experiment. The data were acquired on uniformly 15N-labeled and uniformly 15N/13C-labeled protein samples. The experiments rely on one-bond and two-bond J couplings that allowed for assignment of the signals without the need for the analysis of through-space (nuclear Overhauser effect spectroscopy) correlations. The 15N and 13C chemical shifts were used to determine that His-75 exists predominantly in the N epsilon 2-H tautomeric state in both the phosphorylated and unphosphorylated forms of IIIGlc, and that His-90 exists primarily in the N delta 1-H state in the unphosphorylated protein. Upon phosphorylation of the N epsilon 2 nitrogen of His-90, the N delta 1 nitrogen remains protonated, resulting in the formation of a charged phospho-His-90 moiety. The 1H, 15N, and 13C signals of the phosphorylated and unphosphorylated proteins showed only minor shifts in the pH range from 6.0 to 9.0. These data indicate that the pK alpha values for both His-75 and His-90 in IIIGlc and His-75 in phospho-IIIGlc are less than 5.0, and that the pK alpha value for phospho-His-90 is greater than 10. The results are presented in relation to previously obtained structural data on IIIGlc, and implications for proposed mechanisms of phosphoryl transfer are discussed.

Three-dimensional structures of the central regulatory proteins of the bacterial phosphotransferase system, HPr and enzyme IIAglc

Enzyme IIA and HPr are central regulatory proteins of the bacterial phosphoenolpyruvate:sugar phosphotransferase (PTS) system. Three-dimensional structures of the glucose enzyme IIA domain (IIAglc) and HPr of Bacillus subtilis and Escherichia coli have been studied by both X-ray crystallography and Nuclear Magnetic Resonance (NMR) Spectroscopy. Phosphorylation of HPr of B. subtilis and IIAglc of E. coli have also been characterized by NMR spectroscopy. In addition, the binding interfaces of B. subtilis HPr and IIAglc have been identified from backbone chemical shift changes. This paper reviews these recent advances in the understanding of the three-dimensional structures of HPr and IIAglc and their interaction with each other.

Structural comparison of phosphorylated and unphosphorylated forms of IIIGlc, a signal-transducing protein from Escherichia coli, using three-dimensional NMR techniques

The 18.1-kDa protein IIIGlc from Escherichia coli acts as both a phosphocarrier protein in the phosphoenolpyruvate:glycose phosphotransferase system (PTS) and as a signal-transducing protein with respect to the uptake of non-PTS sugars. Phosphorylation of IIIGlc at the N epsilon (N3) position of His-90 was effected through a regeneration system that included MgCl2, DTT, excess PEP, and catalytic amounts of Enzyme I and HPr. NH, 15N, and 13C alpha signal assignments for P-IIIGlc were made through comparison of 15N-1H correlation spectra (HSQC) of uniformly 15N-labeled preparations of phosphorylated and unphosphorylated protein and through analysis of three-dimensional triple-resonance HNCA spectra of P-IIIGlc uniformly labeled with both 15N and 13C. Backbone and side-chain 1H and 13C beta signals were assigned using 3D heteronuclear HCCH-COSY and HCCH-TOCSY spectra of P-IIIGlc. Using this approach, the assignments were made without reference to nuclear Overhauser effect data or assumptions regarding protein structure. The majority of NH, 15N, H alpha, and 13C alpha chemical shifts measured for P-IIIGlc were identical to those obtained for the unphosphorylated protein [Pelton, J. G., Torchia, D. A., Meadow, N. D., Wong, C.-Y., & Roseman, S. (1991) Biochemistry 30, 10043]. Those signals that exhibited shifts corresponded to residues within four segments (1) Leu-87-Gly-100, (2) Val-36-Val-46, (3) His-75-Ser-78, and (4) Ala-131-Val-138. These four segments are in close proximity to the active site residues His-75 and His-90 in the unphosphorylated protein [Worthylake, D., Meadow, N. D., Roseman, S., Liao, D., Hertzberg, O., & Remington, S.J. (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 10382], and the chemical shift data provide strong evidence that if any structural changes accompany phosphorylation, they are confined to residues in these four segments. This conclusion is confirmed by comparing NOEs observed in 3D 15N/13C NOESY-HMQC spectra of the two forms of the protein. No NOE differences are seen for residues having the same chemical shifts in IIIGlc and P-IIIGlc. Furthermore, with the exception of residues Ala-76, Asp-94, and Val-96, the NOEs of residues (in the four segments) which exhibited chemical shift differences also had the same NOEs in IIIGlc and P-IIIGlc. In the case of residues Ala-76, Asp-94, and Val-96, minor differences in NOEs, corresponding to interproton distances changes of less than 1.5 A, were observed.(ABSTRACT TRUNCATED AT 400 WORDS)

Mannitol-specific enzyme II of the phosphoenolpyruvate-dependent phosphotransferase system of Staphylococcus carnosus. Sequence and expression in Escherichia coli and structural comparison with the enzyme IImannitol of Escherichia coli

The enzyme IImannitol (EIImtl) of the phosphoenolpyruvate-dependent phosphotransferase system (PTS) catalyses the uptake and concomitant phosphorylation of mannitol by bacteria; it is specified by the gene mtlA. MtlA is located near the genes mtlF and mtlD in the staphylococcal genome, encoding the enzyme IIImtl and the mannitol-1-phosphate dehydrogenase, respectively. We present the cloning of the whole operon by a novel complementation system which is generally suitable for cloning Gram-positive PTS genes. The nucleotide sequence of a 2.5-kbp subclone spanning mtlA has been determined. From the deduced amino acid sequence, it is predicted that the membrane-protein EIImtl consists of 505 amino acid residues (54112 Da). The protein has the expected hydropathy profile of an integral-membrane protein. The NH2-terminal part of the enzyme resides within the membrane, whereas the COOH-terminus of the enzyme has the properties of a soluble protein. Comparison with the known amino acid sequence of EIImtl of Escherichia coli [Lee, C. A. & Saier, M. H. (1983) J. Biol. Chem. 258, 10761-10767] showed significant similarity. The motif containing the cysteine, which is the putative second phosphorylation site in EIImtl of E. coli [Pas, H. H. & Robillard, G. T. (1988) Biochemistry 27, 5835-5839], is well conserved in EIImtl of Staphylococcus carnosus. Chemical modification of the single active site cysteine residue by Ellman's reagent leads to total inactivation, which can be reversed by treatment with 2-mercaptoethanol.

A functional protein hybrid between the glucose transporter and the N-acetylglucosamine transporter of Escherichia coli

The glucose and N-acetylglucosamine-specific transporters (IIGlc/IIIGlc and IIGlcNAc) of the bacterial phosphotransferase system mediate carbohydrate uptake across the cytoplasmic membrane concomitant with substrate phosphorylation. The two transporters have 40% amino acid sequence identity. Eight chimeric proteins between the two transporters were made by gene reconstruction. All hybrid proteins could be expressed, some inhibited cell growth, and one was active. The active hybrid transporter consists of the transmembrane domain (residues 1-386) of the IIGlc subunit and the two hydrophilic domains (residues 370-648) of IIGlcNAc. The N-terminal hydrophilic domain of IIGlcNAc contains the transiently phosphorylated cysteine-412. The hybrid protein is specific for glucose, which indicates that the sugar specificity determinant is in the transmembrane domain and that the cysteine from which the phosphoryl group is transferred to the substrate is not part of the binding site. The protein sequence (LKTPGRED) at which the successful fusion occurred has the characteristic properties of an interdomain oligopeptide linker (Argos, P., 1990, J. Mol. Biol. 211, 943-958).

Crystallization of the IIA domain of the glucose permease of Bacillus subtilis

The IIA domain of the glucose permease of the phosphoenolpyruvate: sugar phosphotransferase system (PTS) from Bacillus subtilis has been crystallized. Crystals are obtained from ammonium sulfate solution. They diffract to at least 2.2 A resolution, and belong to space group C222(1), with unit cell dimensions: a = 74.2 A; b = 54.9 A; c = 67.0 A.

1H, 15N, and 13C NMR signal assignments of IIIGlc, a signal-transducing protein of Escherichia coli, using three-dimensional triple-resonance techniques

IIIGlc is an 18.1-kDa signal-transducing phosphocarrier protein of the phosphoenolpyruvate:glycose phosphotransferase system (PTS) of Escherichia coli. Virtually complete (98%) backbone 1H, 15N, and 13C nuclear magnetic resonance (NMR) signal assignments were determined by using a battery of triple-resonance three-dimensional (3D) NMR pulse sequences. In addition, nearly complete (1H, 95%; 13C, 85%) side-chain 1H and 13C signal assignments were obtained from an analysis of 3D 13C HCCH-COSY and HCCH-TOCSY spectra. These experiments rely almost exclusively upon one- and two-bond J couplings to transfer magnetization and to correlate proton and heteronuclear NMR signals. Hence, essentially complete signal assignments of this 168-residue protein were made without any assumptions regarding secondary structure and without the aid of a crystal structure, which is not yet available. Moreover, only three samples, one uniformly 15N-enriched, one uniformly 15N/13C-enriched, and one containing a few types of amino acids labeled with 15N and/or 13C, were needed to make the assignments. The backbone assignments together with the 3D 15N NOESY-HMQC and 13C NOESY-HMQC data have provided extensive information about the secondary structure of this protein [Pelton, J.G., Torchia, D.A., Meadow, N.D., Wong, C.-Y., & Roseman, S (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 3479-3488]. The nearly complete set of backbone and side-chain atom assignments reported herein provide a basis for studies of the three-dimensional structure and dynamics of IIIGlc as well as its interactions with a variety of membrane and cytoplasmic proteins.

Structure of the IIA domain of the glucose permease of Bacillus subtilis at 2.2-A resolution

The crystal structure of the IIA domain of the glucose permease of the phosphoenolpyruvate:sugar phosphotransferase system (PTS) from Bacillus subtilis has been determined at 2.2-A resolution. Refinement of the structure is in progress, and the current R-factor is 0.201 (R = sigma h parallel Fo magnitude of - Fc parallel/sigma h magnitude of Fo, where magnitude of Fo and magnitude of Fc are the observed and calculated structure factor amplitudes, respectively) for data between 6.0- and 2.2-A resolution for which F greater than or equal to 2 sigma (F). This is an antiparallel beta-barrel structure that incorporates "Greek key" and "jellyroll" topological motifs. A shallow depression is formed at the active site by part of the beta-sheet and an omega-loop flanking one side of the sheet. His83, the histidyl residue which is the phosphorylation target of HPr and which transfers the phosphoryl group to the IIB domain of the permease, is located at the C-terminus of a beta-strand. The N epsilon atom is partially solvated and also interacts with the N epsilon atom of a second histidyl residue, His68, located at the N-terminus of an adjacent beta-strand, suggesting they share a proton. The geometry of the hydrogen bond is imperfect, though. Electrostatic interactions with other polar groups and van der Waals contacts with the side chains of two flanking phenylalanine residues assure the precise orientation of the imidazole rings. The hydrophobic nature of the surface around the His83-His68 pair may be required for protein-protein recognition by HPr or/and by the IIB domain of the permease. The side chains of two aspartyl residues, Asp31 and Asp87, are oriented toward each other across a narrow groove, about 7 A from the active-site His83, suggesting they may play a role in protein-protein interaction. A model of the phosphorylated form of the molecule is proposed, in which oxygen atoms of the phosphoryl group interact with the side chain of His68 and with the main-chain nitrogen atom of a neighboring residue, Val89. The model, in conjunction with previously reported site-directed mutagenesis experiments, suggests that the phosphorylation of His83 may be accompanied by the protonation of His68. This may be important for the interaction with the IIB domain of the permease and/or play a catalytic role in the phosphoryl transfer from IIA to IIB.

Evolutionary relationships among the permease proteins of the bacterial phosphoenolpyruvate: sugar phosphotransferase system. Construction of phylogenetic trees and possible relatedness to proteins of eukaryotic mitochondria

The amino acid sequences of 15 sugar permeases of the bacterial phosphoenolpyruvate-dependent phosphotransferase system (PTS) were divided into four homologous segments, and these segments were analyzed to give phylogenetic trees. The permease segments fell into four clusters: the lactose-cellobiose cluster, the fructose-mannitol cluster, the glucose-N-acetylglucosamine cluster, and the sucrose-beta-glucoside cluster. Sequences of the glucitol and mannose permeases (clusters 5 and 6, respectively) were too dissimilar to establish homology with the other permeases, but short regions of statistically significant sequence similarities were noted. The functional and structural relationships of these permease segments are discussed. Some of the homologous PTS permeases were found to exhibit sufficient sequence similarity to subunits 4 and 5 of the eukaryotic mitochondrial NADH dehydrogenase complex to suggest homology. Moreover, subunits 4 and 5 of this complex appeared to be homologous to each other, suggesting that these PTS and mitochondrial proteins comprise a superfamily. The integral membrane subunits of the evolutionarily divergent mannose PTS permease, the P and M subunits, exhibited limited sequence similarity to subunit 6 of the mitochondrial F1F0-ATPase and subunit 5b of cytochrome oxidase, respectively. These results suggest that PTS sugar permeases and mitochondrial proton-translocating proteins may be related, although the possibility of convergent evolution cannot be ruled out.

Polypeptide backbone resonance assignments and secondary structure of Bacillus subtilis enzyme IIIglc determined by two-dimensional and three-dimensional heteronuclear NMR spectroscopy

The enzyme IIIglc-like domain of Bacillus subtilis IIglc (IIIglc, 162 residues, 17.4 kDa) has been cloned and overexpressed in Escherichia coli. Sequence-specific assignment of the backbone 1H and 15N resonances has been carried out with a combination of homonuclear and heteronuclear two-dimensional and heteronuclear three-dimensional (3D) NMR spectroscopy. Amide proton solvent exchange rate constants have been determined from a series of 1H-15N heteronuclear single-quantum coherence (HSQC) spectra acquired following dissolution of the protein in D2O. Major structural features of IIIglc have been inferred from the pattern of short-, medium- and long-range NOEs in 3D heteronuclear 1H nuclear Overhauser effect 1H-15N multiple-quantum coherence (3D NOESY-HMQC) spectra, together with the exchange rate constants. IIIglc contains three antiparallel beta-sheets comprised of eight, three, and two beta-strands. In addition, five beta-bulges were identified. No evidence of regular helical structure was found. The N-terminal 15 residues of the protein appear disordered, which is consistent with their being part of the Q-linker that connects the C-terminal enzyme IIIglc-like domain to the membrane-bound IIglc domain. Significantly, two histidine residues, His 68 and His 83, which are important for phosphotransferase function, are found from NOE measurements to be in close proximity at the ends of adjacent strands in the major beta-sheet.