Sugar phosphate:sugar transphosphorylation coupled to exchange group translocation catalyzed by the enzyme II complexes of the phosphoenolpyruvate:sugar phosphotransferase system in membrane vesicles of Escherichia coli

Protein:Protein interactions in the cytoplasmic membrane apparently influencing sugar transport and phosphorylation activities of the e. coli phosphotransferase system

The multicomponent phosphoenolpyruvate (PEP)-dependent sugar-transporting phosphotransferase system (PTS) in Escherichia coli takes up sugar substrates from the medium and concomitantly phosphorylates them, releasing sugar phosphates into the cytoplasm. We have recently provided evidence that many of the integral membrane PTS permeases interact with the fructose PTS (FruA/FruB) [1]. However, the biochemical and physiological significance of this finding was not known. We have carried out molecular genetic/biochemical/physiological studies that show that interactions of the fructose PTS often enhance, but sometimes inhibit the activities of other PTS transporters many fold, depending on the target PTS system under study. Thus, the glucose (Glc), mannose (Man), mannitol (Mtl) and N-acetylglucosamine (NAG) permeases exhibit enhanced in vivo sugar transport and sometimes in vitro PEP-dependent sugar phosphorylation activities while the galactitol (Gat) and trehalose (Tre) systems show inhibited activities. This is observed when the fructose system is induced to high levels and prevented when the fruA/fruB genes are deleted. Overexpression of the fruA and/or fruB genes in the absence of fructose induction during growth also enhances the rates of uptake of other hexoses. The β-galactosidase activities of man, mtl, and gat-lacZ transcriptional fusions and the sugar-specific transphosphorylation activities of these enzyme transporters were not affected either by frustose induction or by fruAB overexpression, showing that the rates of synthesis of the target PTS permeases were not altered. We thus suggest that specific protein-protein interactions within the cytoplasmic membrane regulate transport in vivo (and sometimes the PEP-dependent phosphorylation activities in vitro) of PTS permeases in a physiologically meaningful way that may help to provide a hierarchy of preferred PTS sugars. These observations appear to be applicable in principle to other types of transport systems as well.

Lipid dependencies, biogenesis and cytoplasmic micellar forms of integral membrane sugar transport proteins of the bacterial phosphotransferase system

Permeases of the prokaryotic phosphoenolpyruvate-sugar phosphotransferase system (PTS) catalyse sugar transport coupled to sugar phosphorylation. The lipid composition of a membrane determines the activities of these enzyme/transporters as well as the degree of coupling of phosphorylation to transport. We have investigated mechanisms of PTS permease biogenesis and identified cytoplasmic (soluble) forms of these integral membrane proteins. We found that the catalytic activities of the soluble forms differ from those of the membrane-embedded forms. Transport via the latter is much more sensitive to lipid composition than to phosphorylation, and some of these enzymes are much more sensitive to the lipid environment than others. While the membrane-embedded PTS permeases are always dimeric, the cytoplasmic forms are micellar, either monomeric or dimeric. Scattered published evidence suggests that other integral membrane proteins also exist in cytoplasmic micellar forms. The possible functions of cytoplasmic PTS permeases in biogenesis, intracellular sugar phosphorylation and permease storage are discussed.

Biophysical studies of the membrane-embedded and cytoplasmic forms of the glucose-specific Enzyme II of the E. coli phosphotransferase system (PTS)

The glucose Enzyme II transporter complex of the Escherichia coli phosphotransferase system (PTS) exists in at least two physically distinct forms: a membrane-integrated dimeric form, and a cytoplasmic monomeric form, but little is known about the physical states of these enzyme forms. Six approaches were used to evaluate protein-protein and protein-lipid interactions in this system. Fluorescence energy transfer (FRET) using MBP-II(Glc)-YFP and MBP-II(Glc)-CFP revealed that the homodimeric Enzyme II complex in cell membranes is stable (FRET(-)) but can be dissociated and reassociated to the heterodimer only in the presence of Triton X100 (FRET(+)). The monomeric species could form a heterodimeric species (FRET(+)) by incubation and purification without detergent exposure. Formaldehyde cross linking studies, conducted both in vivo and in vitro, revealed that the dimeric MBP-II(Glc) activity decreased dramatically with increasing formaldehyde concentrations due to both aggregation and activity loss, but that the monomeric MBP-II(Glc) retained activity more effectively in response to the same formaldehyde treatments, and little or no aggregation was observed. Electron microscopy of MBP-II(Glc) indicated that the dimeric form is larger than the monomeric form. Dynamic light scattering confirmed this conclusion and provided quantitation. NMR analyses provided strong evidence that the dimeric form is present primarily in a lipid bilayer while the monomeric form is present as micelles. Finally, lipid analyses of the different fractions revealed that the three lipid species (PE, PG and CL) are present in all fractions, but the monomeric micellar structure contains a higher percentage of anionic lipids (PG & CL) while the dimeric bilayer form has a higher percentage of zwitterion lipids (PE). Additionally, evidence for a minor dimeric micellar species, possibly an intermediate between the monomeric micellar and the dimeric bilayer forms, is presented. These results provide convincing evidence for interconvertible physical forms of Enzyme-II(Glc).

In vitro interconversion of the soluble and membrane- integrated forms of the Escherichia coli glucose enzyme II of the phosphoenolpyruvate-dependent sugar-transporting phosphotransferase system

In previous publications, we have shown that integral membrane sugar permeases of the bacterial phosphotransferase system can exist in a 'soluble' (probably micellar) monomeric form (SII) as well as a membrane-integrated dimeric form (MII). We here show that the two forms of the his-tagged glucose permease of Escherichia coli can be interconverted in vitro. Conversion of MII to SII is promoted by (1) low protein concentration, (2) detergent, (3) high pH, and (4) phospholipase A(2) treatment. Conversion of SII to MII is promoted by: (1) high protein concentration, (2) adherence to and elution from an Ni(2+) column, (3) neutral pH, and (4) incorporation into phospholipid liposomes.

Competition between two pathways for sugar uptake by the phosphoenolpyruvate-dependent sugar phosphotransferase system in Salmonella typhimurium

The interaction between the two pathways for glucose entry via the phosphoenolpyruvate:sugar phosphotransferase system, i.e. via enzyme II-A/II-B and enzymes II-BGlc/IIIGlc, was studied in Salmonella typhimurium. Thio-beta-D-glucoside and 5-thio-D-glucose were shown to be substrates of P-pyruvate:sugar phosphotransferase specific for enzyme II-BGlc both in intact cells and in toluene-treated cells of S. typhimurium. The activity of the II-A/II-B pathway was strongly inhibited by the presence of II-BGlc substrates. It is concluded that the two pathways compete for phosphoryl groups provided by P-pyruvate, and that under the conditions tested the flow of phosphoryl groups through enzyme I/HPr is the rate-limiting step in vivo of activity of the pathways studied. The results corroborate the proposed mechanism of the regulatory function of the P-pyruvate:sugar phosphotransferase system which predicts a net dephosphorylation of components of the P-pyruvate:sugar phosphotransferase in the presence of a substrate of P-pyruvate:sugar phosphotransferase.

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.

Dependency of sugar transport and phosphorylation by the phosphoenolpyruvate-dependent phosphotransferase system on membranous phosphatidylethanolamine in Escherichia coli: studies with a pssA mutant lacking phosphatidylserine synthase

An isogenic pair of Escherichia coli strains lacking ( pssA) and possessing (wild-type) the enzyme phosphatidylserine synthase was used to estimate the effects of the total lack of phosphatidylethanolamine (PE), the major phospholipid in E. coli membranes, on the activities of several sugar permeases (enzymes II) of the phosphoenolpyruvate:sugar phosphotransferase system (PTS). The mutant exhibits greatly elevated levels of phosphatidylglycerol (PG), a lipid that has been reported to stimulate the in vitro activities of several PTS permeases. The activities, thermal stabilities, and detergent sensitivities of three PTS permeases, the glucose enzyme II (II(Glc)), the mannose enzyme II (II(Man)) and the mannitol enzyme II (II(Mtl)), were characterized. Western blot analyses revealed that the protein levels of II(Glc) were not appreciably altered by the loss of PE. In the pssA mutant, II(Glc) and II(Man) activities were depressed both in vivo and in vitro, with the in vivo transport activities being depressed much more than the in vitro phosphorylation activities. II(Mtl) also exhibited depressed transport activity in vivo but showed normal phosphorylation activities in vitro. II(Man) and II(Glc) exhibited greater thermal lability in the pssA mutant membranes than in the wild-type membranes, but II(Mtl) showed enhanced thermal stability. All three enzymes were activated by exposure to TritonX100 (0.4%) or deoxycholate (0.2%) and inhibited by SDS (0.1%), but II(Mtl) was the least affected. II(Man) and, to a lesser degree, II(Glc) were more sensitive to detergent treatments in the pssA mutant membranes than in the wild-type membranes while II(Mtl) showed no differential effect. The results suggest that all three PTS permeases exhibit strong phospholipid dependencies for transport activity in vivo but much weaker and differential dependencies for phosphorylation activities in vitro, with II(Man) exhibiting the greatest and II(Mtl) the least dependency. The effects of lipid composition on thermal sensitivities and detergent activation responses paralleled the effects on in vitro phosphorylation activities. These results together with those previously published suggest that, while the in vivo transport activities of all PTS enzymes II require an appropriate anionic to zwitterionic phospholipid balance, the in vitro phosphorylation activities of these same enzymes show much weaker and differential dependencies. Alteration of the phospholipid composition of the membrane thus allows functional dissection of transport from the phosphorylation activities of PTS enzyme complexes.

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.

Studies on the Escherichia coli glucose-specific permease, PtsG, with a point mutation in its N-terminal amphipathic leader sequence

Previous work has resulted in the isolation of several mutant glucose permeases (II(Glc) or PtsG) of the Escherichia coli phosphotransferase system (PTS) with altered N-terminal amphipathic leader sequences. The mutations were reported to (1). broaden permease substrate specificity, (2). promote facilitated diffusion of some sugars and (3). increase ptsG gene transcription. Detailed biochemical analyses were conducted, showing that one such mutant (V12F-II(Glc)) (1). contains dramatically increased amounts of II(Glc), (2). displays correspondingly increased in vitro phosphorylation and in vivo transport activities, (3). shows increased utilization of several metabolizable sugars and (4). shows decreased susceptibility to detergent activation. These results are interpreted as suggesting that the V12F substitution in the N-terminal amphipathic leader sequence of II(Glc) alters the facility with which the permease is integrated into the membrane. Consequent changes in conformation alter its catalytic properties and increase its affinity for the pleiotropic transcriptional repressor, Mlc. These changes together are proposed to promote transcription of the ptsG gene and account for the observed phenotypic changes.

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.

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

Allosteric regulation of several sugar transport systems such as those specific for lactose, maltose and melibiose in Escherichia coli (inducer exclusion) is mediated by the glucose-specific enzyme IIA (IIAGlc) of the phosphoenolpyruvate:sugar phosphotransferase system (PTS). Deletion mutations in the cytoplasmic N and C termini of the lactose permease protein, LacY, and replacement of all cysteine residues in LacY with other residues did not prevent IIAGlc-mediated inhibition of lactose uptake, but several point and insertional mutations in the central cytoplasmic loop of this permease abolished transport regulation and IIAGlc binding. The results substantiate the conclusion that regulation of the lactose permease in E. coli by the PTS is mediated by a primary interaction of IIAGlc with the central cytoplasmic loop of the permease.

The mannitol repressor (MtlR) of Escherichia coli

The mannitol operon of Escherichia coli, encoding the mannitol-specific enzyme II of the phosphotransferase system (Mt1A) and mannitol phosphate dehydrogenase (Mt1D), is here shown to contain a single additional downstream open reading frame which encodes the mannitol repressor (Mt1R). Mt1R contains 195 amino acids and has a calculated molecular weight of 21,990 and a calculated pI of 4.5. It is homologous to the product of an open reading frame (URF2D) upstream of the E. coli gapB gene but represents a novel type of transcriptional regulatory protein.

Presence of protein constituents of the gram-positive bacterial phosphotransferase regulatory system in Acholeplasma laidlawii

Acholeplasma species have been reported to lack a functional phosphoenolpyruvate:sugar phosphotransferase system (PTS). We show here that Acholeplasma laidlawii possesses activities of enzyme I, HPr, HPr(ser) kinase, and HPr(ser-P) phosphatase but lacks detectable activities of enzymes II of the PTS. HPr from this organism was purified, and the regulatory properties of the kinase and phosphatase were characterized and shown to differ from those of previously studied bacteria. The results suggest the presence of an incomplete PTS in A. laidlawii which has the potential to function in a unique regulatory capacity.

Deletion mutants of the Escherichia coli K-12 mannitol permease: dissection of transport-phosphorylation, phospho-exchange, and mannitol-binding activities

We have constructed a series of deletion mutations of the cloned Escherichia coli K-12 mtlA gene, which encodes the mannitol-specific enzyme II of the phosphoenolpyruvate (PEP)-dependent carbohydrate phosphotransferase system. This membrane-bound permease consists of 637 amino acid residues and is responsible for the concomitant transport and phosphorylation of D-mannitol in E. coli. Deletions into the 3' end of mtlA were constructed by exonuclease III digestion. Restriction mapping of the resultant plasmids identified several classes of deletions that lacked approximately 5% to more than 75% of the gene. Immunoblotting experiments revealed that many of these plasmids expressed proteins within the size range predicted by the restriction analyses, and all of these proteins were membrane localized, which demonstrated that none of the C-terminal half of the permease is required for membrane insertion. Functional analyses of the deletion proteins, expressed in an E. coli strain deleted for the chromosomal copy of mtlA, showed that all but one of the strains containing confirmed deletions were inactive in transport and PEP-dependent phosphorylation of mannitol, but deletions removing up to at least 117 amino acid residues from the C terminus of the permease were still active in catalyzing phospho exchange between mannitol 1-phosphate and mannitol. A deletion protein that lacked 240 residues from the C terminus of the permease was inactive in phospho exchange but still bound mannitol with high affinity. These experiments localize sites important for transport and PEP-dependent phosphorylation to the extreme C terminus of the mannitol permease, sites important for phospho exchange to between residues 377 and 519, and sites necessary for mannitol binding to the N-terminal 60% of the molecule. The results are discussed with respect to the fact that the mannitol permease consists of structurally independent N- and C-terminal domains.

Altered transcriptional patterns affecting several metabolic pathways in strains of Salmonella typhimurium which overexpress the fructose regulon

Expression of beta-galactosidase in transcriptional fusions with the pps gene (encoding phosphoenolpyruvate [PEP] synthase), the aceBAK operon (encoding malate synthase, isocitrate lyase, and isocitrate dehydrogenase kinase, respectively), and the phs operon (encoding either thiosulfate reductase or a regulatory protein controlling its expression) was studied in Salmonella typhimurium. beta-Galactosidase synthesis in these strains was repressible either by growth in the presence of glucose or by the presence of a fruR mutation, which resulted in the constitutive expression of the fructose (fru) regulon. Five enzymes of gluconeogenesis (PEP synthase, PEP carboxykinase, isocitrate lyase, malate synthase, and fructose-1,6-diphosphatase) were shown to be repressed either by growth in the presence of glucose or the fruR mutation, while the glycolytic enzymes, enzyme I and enzymes II of the phosphotransferase system as well as phosphofructokinase, were induced either by growth in the presence of glucose or the fruR mutation. Overexpression of the cloned fru regulon genes (not including fruR) resulted in parallel repression of representative gluconeogenic, Krebs cycle, and glyoxylate shunt enzymes. Studies with temperature-sensitive mutants of S. typhimurium which synthesized heat-labile IIIFru proteins provided evidence that this protein plays a role in the regulation of gluconeogenic substrate utilization. Other mutant analyses revealed a complex relationship between fru gene expression and the expression of genes encoding gluconeogenic enzymes. Taken together, the results suggest that a number of genes encoding catabolic, biosynthetic, and amphibolic enzymes in enteric bacteria are transcriptionally regulated by a complex catabolite repression/activation mechanism which may involve enzyme IIIFru of the phosphotransferase system as one component of the regulatory system.

The phosphoenolpyruvate-dependent fructose-specific phosphotransferase system in Rhodopseudomonas sphaeroides. Energetics of the phosphoryl group transfer from phosphoenolpyruvate to fructose

Energy coupling to fructose transport in Rhodopseudomonas sphaeroides is achieved by phosphorylation of the membrane-spanning fructose-specific carrier protein, EFruII. The phosphoryl group of phosphoenolpyruvate is transferred to EFruII via the cytoplasmic component SF (soluble factor). The standard free enthalpy of hydrolysis of the two phosphorylated proteins has been estimated from isotope exchange measurements in chemical equilibrium. The delta G degrees for SF-P is -60.5 kJ/mol. The standard free enthalpy for hydrolysis of EII-P is -37.9 kJ/mol, but -45.2 kJ/mol when SF is still complexed to it, as in the overall reaction. Therefore the standard free enthalpy of hydrolysis of SF X EII-P is 70% of the standard free enthalpy of hydrolysis of P-enolpyruvate. The measurements reveal two regulation sites in the system. First, the phosphorylation of SF is inhibited by pyruvate when the concentration ratio of pyruvate/P-enolpyruvate becomes too high. Second, a low concentration of internal fructose prevents the phosphorylation of the carrier by the internal fructose-1-P pool when the concentration of the latter becomes too high or the phosphorylation rate by P-enolpyruvate too slow. Furthermore comparison of the isotope exchange and the overall phosphotransferase reaction kinetics leads to the conclusion that binding of fructose to the carrier is a slow step relative to the phosphoryl group transfer from EFruII to fructose.

Phosphate/hexose 6-phosphate antiport in Streptococcus lactis

After growth in appropriate media, resting cells of Streptococcus lactis 7962 showed a rapid exchange between external and internal pools of inorganic phosphate. This exchange was not found in other strains of S. lactis (ML3, 133, or K1) or in Streptococcus faecalis. Phosphate exchange in S. lactis 7962 did not require other anions or cations in the assay medium, nor was phosphate influx affected by the membrane potential and pH gradient formed during glycolysis. Thus, the exchange reaction was independent of known ionic drivers (H+, Na+, OH-, etc.). Experiments testing inhibitions of phosphate entry suggested that alternative substrates for exchange included arsenate, as well as the 6-phosphates of glucose, 2-deoxyglucose, fructose, mannose, or glucosamine, and direct studies with 2-deoxyglucose 6-phosphate verified that resting cells could accumulate this sugar phosphate to levels expected for exchange with internal phosphate. Two other observations supported the idea of an exchange between phosphate and sugar phosphate. First, early addition of the heterologous substrate blocked entry of the test compound, whereas later addition caused efflux of preaccumulated material. Second, expression of phosphate exchange and 2-deoxyglucose 6-phosphate transport varied in parallel. Both activities were found at high levels after growth in medium supplemented with rhamnose or arabinose, at intermediate levels with addition of galactose, and at low levels after growth with glucose, fructose, or mannose. We conclude that these findings describe a novel anion antiporter that mediates the exchange of phosphate (arsenate) and sugar 6-phosphates.

Mechanism of inducer expulsion in Streptococcus pyogenes: a two-step process activated by ATP

The mechanism of methyl-beta-D-thiogalactoside-phosphate (TMG-P) expulsion from Streptococcus pyogenes was studied. The expulsion elicited by glucose was not due to exchange vectorial transphosphorylation between the expelled TMG and the incoming glucose since more beta-galactoside was displaced than glucose taken up, and the stoichiometry between TMG and glucose transport was inconstant. Instead, two distinct and sequential reactions, intracellular dephosphorylation of TMG-P followed by efflux of free TMG, mediated the expulsion. This was shown by temporary accumulation of free TMG effected by competitive inhibition of its efflux and by the aid of arsenate, which arrested dephosphorylation of TMG-P but did not affect efflux of free TMG formed intracellularly before arsenate addition. The competitive inhibition of TMG efflux by its structural analogs suggests that a transport protein facilitates the expulsion. Iodoacetate or fluoride prevented TMG-P dephosphorylation and its expulsion. However, provision of ATP via the arginine deiminase pathway restored these activities in the presence of the glycolytic inhibitors and stimulated expulsion in their absence. Other amino acids tested did not promote this restoration, and canavanine or norvaline severely inhibited it. Arginine without glucose neither elicited the dephosphorylation nor evoked the expulsion of TMG-P. Ionophores or ATPase inhibitors did not prevent the expulsion as elicited by glucose or its restoration by arginine. The results suggest that activation of the dephosphorylation-expulsion mechanism occurs independently of a functional glycolytic pathway, requires ATP provision, and is possibly due to protein phosphorylation controlled by a yet unknown metabolite. The in vivo phosphorylation of a protein (approximate molecular weight - 10,000) under the conditions of expulsion was demonstrated.

Regulation of lactose transport by the phosphoenolpyruvate-sugar phosphotransferase system in membrane vesicles of Escherichia coli

Regulation of lactose uptake by the phosphoenolpyruvate-sugar phosphotransferase system (PTS) has been demonstrated in membrane vesicles of Escherichia coli strain ML 308-225. Substrates of the phosphotransferase system inhibited D-lactate energized uptake of lactose but did not inhibit uptake of either L-alanine or L-proline. This inhibition was reversed by intravesicular (but not extravesicular) phosphoenolpyruvate. Lactose uptake was also inhibited by enzyme IIIglc preparations that were shocked into the vesicles, and this inhibition was reversed by phosphoenolpyruvate. Intravesicular HPr and enzyme I stimulated methyl alpha-glycoside uptake but did not inhibit or stimulate lactose accumulation. Vesicles maintained at 0 degree C for several days partially lost 1) the ability to take up lactose, 2) the ability to accumulate PTS substrates, and 3) PTS-mediated regulation. Phosphoenolpyruvate addition restored all of these activities. These results support a mechanism in which the relative proportions of phosphorylated and nonphosphorylated forms of a phosphotransferase constituent regulate the activity of the lactose permease.

Evidence for the functional association of enzyme I and HPr of the phosphoenolpyruvate-sugar phosphotransferase system with the membrane in sealed vesicles of Escherichia coli

Several independent assay procedures were used to estimate the activities of the enzyme constituents of the phosphoenolpyruvate-sugar phosphotransferase system (PTS) in osmotically shocked bacterial membrane vesicles. The soluble enzymes of the system were found to be in association with the membrane by several criteria. Phosphoenolpyruvate-dependent sugar phosphorylation was catalyzed by this membrane-bound enzyme system far more efficiently than by a mixture of the individual enzymes at corresponding concentrations. By contrast, the rates of the phosphoryl exchange reactions catalyzed by enzyme I and the enzyme II complexes were essentially the same for the associated and dissociated forms of the system. Functional association of the PTS-enzyme complex was stabilized by Mg++ and phosphoenolypyruvate and could be destroyed by detergent treatment, sonication, or by passage of the vesicle preparation through a French pressure cell. These results lead to the possibility that in the intact bacterial cell the soluble enzymes of the phosphotransferase system exist, in part, as peripheral membrane constituents associated with the integral membrane enzyme II complexes.

Genetic dissection of catalytic activities of the Salmonella typhimurium mannitol enzyme II

Approximately 60 mutants of Salmonella typhimurium were isolated which exhibited altered levels of the activities of the mannitol enzyme II. The mutants were grouped into six distinct categories based on their mannitol fermentation, transport, chemotaxis, and phosphorylation activities.

Vectorial and nonvectorial transphosphorylation catalyzed by enzymes II of the bacterial phosphotransferase system

Vectorial transphosphorylation of hexitols, catalyzed by enzymes II of the bacterial phosphotransferase system, was studied in intact cells and membrane vesicles of Escherichia coli. In strains depleted of phosphoenolpyruvate and unable to metabolize the internal hexitol phosphate, internal mannitol-1-phosphate stimulated uptake of extracellular [14C]mannitol, whereas external mannitol stimulated release of [14C]mannitol from the intracellular [14C]mannitol-1-phosphate pool. The stoichiometry of mannitol uptake to mannitol release was 1:1. Glucitol did not promote release of [14C]mannitol from the mannitol phosphate pool but stimulated release of [14C]glucitol from internal glucitol phosphate pools when the glucitol enzyme II was induced to high levels. In E coli cells and membrane vesicles, both vectorial and nonvectorial transphosphorylation reactions of hexitols and hexoses were demonstrated. The nonvectorial reactions, but not the vectorial reactions, catalyzed by the mannitol and glucose enzymes II, were inhibited by p-chloromercuriphenyl sulfonate, a membrane-impermeable sulfhydryl reagent which inactivates enzymes II. Similarly, glucose-6-sulfate, an inhibitor of the glucose enzyme II-catalyzed transphosphorylation reaction, specifically inhibited the nonvectorial reaction. This compound was shown to be a noncompetitive inhibitor of methyl alpha-glucoside phosphorylation employing phospho-HPr as the phosphate donor. It apparently exerts its inhibitory effect by exclusive binding to the sugar phosphate binding site on the enzyme II complex. The results are consistent with the conclusion that enzymes II can exist in two distinct dispositions in the membrane, one of which catalyzes vectorial transphosphorylation, and the other catalyzes nonvectorial transphosphorylation.

Physiological function of periplasmic hexose phosphatase in Salmonella typhimurium

Hydrolysis of sugar phosphates by crude and purified preparations of periplasmic hexose phosphatase from Salmonella typhimurium followed Michaelis-Menten kinetics. The enzyme bound glucose 1-phosphate with high affinity (Km = 10 microM) but bound glucose 6-phosphate with low affinity (Km = 2,000 microM). The order of substrate affinities was glucose 1-phosphate greater than mannose 1-phosphate = galactose 1-phosphate greater than fructose 1-phosphate greater than glucose 6-phosphate. These results and others suggest that the physiological function of the enzyme is the periplasmic hydrolysis of hexose 1-phosphates.

Catalytic activities associated with the enzymes II of the bacterial phosphotransferase system

The phosphotransferase system (PTS) in Escherichia coli is a multifunctional, multicomponent enzyme system. Its primary functions deal with carbon source acquisition, while its secondary functions are concerned with the regulation of bacterial physiology. The primary functions of the system include 1) extracellular detection, 2) unidirectional and exchange transmembrane transport, and 3) phosphoenolpyruvate-dependent and sugar phosphate-dependent phosphorylation of the sugar substrates of the system. The secondary functions include 1) regulation of the activities of adenylate cyclase and various non-PTS permeases and 2) regulation of the induced synthesis of several PTS enzymes. Both the primary and secondary functions appear to be elicited by the binding of a sugar substrate to an Enzyme II complex. One of these integral transmembrane enzymes, the mannitol Enzyme II (IImtl), has been solubilized with detergent, purified to homogeneity, and reconstituted in an artificial membrane system. The molecular weight of this protein, IImtl, is 60,000 daltons. It possesses an extracellular sugar binding site and distinct intracellular combining sites for sugar phosphate and phospho-HPr. An essential sulfhydryl group and an antibody combining site are localized to the cytoplasmic surface of the enzyme, while a dextran combining site is localized to the external surface. Preliminary experiments suggest that the different functions of the Enzyme IImtl can be dissected by genetic and biochemical techniques. These studies emphasize the functional complexity of the PTS and its integral membrane protein constituents.

Kinetic study of a phosphoryl exchange reaction between fructose and fructose 1-phosphate catalyzed by the membrane-bound enzyme II of the phosphoenolpyruvate-fructose 1-phosphotransferase system of Bacillus subtilis

A phosphoryl exchange reaction between fructose 1-phosphate and fructose was found to be catalyzed by a membrane preparation isolated from Bacillus subtilis. The regulation of the biosynthesis of the activity in the wild type as well as in the regulation mutants fruB closely correlates with that of the membrane-bound enzyme II of the phosphoenolpyruvate fructose 1-phosphotransferase system which is known to mediate the transmembrane vectorial phosphorylation of fructose. The computed analysis of the kinetic data shows that the mechanism of the enzyme II is ping-pong, i.e. that a phosphoryl-enzyme intermediate occurs in the reaction. The apparent dissociation constants of the enzyme II/fructose 1-phosphate complex and of the phosphoryl enzyme II/fructose complex are estimated. The value of the standard free energy of the hydrolysis of the bond between the phosphoryl moiety and the enzyme suggests a covalent bonding. This intermediate is assumed to occur in the physiological functioning of the enzyme which utilizes the phosphocarrier protein HPr as phosphoryl donor. The exchange reaction is competitively inhibited by high fructose concentrations: this indicates that the same site of the enzyme binds fructose and fructose 1-phosphate, this site being accessible to fructose on the external side of the membrane when the enzyme is phosphorylated.