Serum androgen levels in adolescents with type 1 diabetes: relationship to pubertal stage and metabolic control

Delayed sexual maturation is still frequently seen in adolescents with type 1 diabetes. A close relationship between insulin and androgen metabolism has been found in a number of studies. Our study was designed to investigate whether or not abnormalities in androgen secretion could play a role in the onset of sexual maturation in adolescents with type 1 diabetes. We have asked whether or not there was a correlation between daily insulin dosage, duration of diabetes, metabolic control, age, pubertal stage, and body mass index (BMI) versus serum androgen concentrations. Basal total and free testosterone, dehydroepiandrosterone-sulfate (DHEA-S), dihydrotestosterone (DHT), sex hormone binding globulin (SHBG) and 3alpha-androstanediol glucuronide (3alpha diol-G) plasma concentrations were measured in 36 pubertal boys and 31 pubertal girls with type 1 diabetes and in 59 sex- and pubertal stage-matched control subjects without diabetes. Significantly higher serum total testosterone (p<0.01) and free testosterone (p<0.05) levels were found in females and males with type 1 diabetes than in controls at pubertal stage 5. DHEA-S, SHBG, DHT and 3alpha diol G concentrations in patients with diabetes were not significantly different from those in controls. There was no correlation between daily insulin requirements and serum androgen levels. These data suggest that adolescents with diabetes have similar serum levels of DHEA-S, SHBG, DHT and 3alpha diol G as healthy subjects at all stages of puberty. However, there are significant differences in serum testosterone and free testosterone levels in adolescents with diabetes when compared to healthy, sex- and pubertal stage-matched controls in late puberty. We hypothesize that the increased testosterone levels in patients with diabetes could relate to reduced fertility in females, disorders of sexual maturation and an increased risk for cardiovascular complications later in life.

Solution structure and dynamics of Crh, the Bacillus subtilis catabolite repression HPr

The solution structure and dynamics of the Bacillus subtilis HPr-like protein, Crh, have been investigated using NMR spectroscopy. Crh exhibits high sequence identity (45 %) to the histidine-containing protein (HPr), a phospho-carrier protein of the phosphoenolpyruvate (PEP):carbohydrate phosphotransferase system, but contains no catalytic His15, the site of PEP-dependent phosphorylation in HPr. Crh also forms a mixture of monomers and dimers in solution whereas HPr is known to be monomeric. Complete backbone and side-chain assignments were obtained for the monomeric form, and 60 % of the dimer backbone resonances; allowing the identification of the Crh dimer interface from chemical-shift mapping. The conformation of Crh was determined to a precision of 0.46(+/-0.06) A for the backbone atoms, and 1.01(+/-0.08) A for the heavy atoms. The monomer structure is similar to that of known HPr 2.67(+/-0.22) A (C(alpha) rmsd), but has a few notable differences, including a change in the orientation of one of the helices (B), and a two-residue shift in beta-sheet pairing of the N-terminal strand with the beta4 strand. This shift results in a shortening of the surface loop present in HPr and consequently provides a flatter surface in the region of dimerisation contact, which may be related to the different oligomeric nature of these two proteins. A binding site of phospho-serine(P-Ser)-Crh with catabolite control protein A (CcpA) is proposed on the basis of highly conserved surface side-chains between Crh and HPr. This binding site is consistent with the model of a dimer-dimer interaction between P-Ser-Crh and CcpA. (15)N relaxation measured in the monomeric form also identified differential local mobility in the helix B which is located in the vicinity of this site.

Control of Bacillus subtilis mtl operon expression by complex phosphorylation-dependent regulation of the transcriptional activator MtlR

Many bacteria transport mannitol via the mtlAF-encoded phosphoenolpyruvate (PEP):carbohydrate phosphotransferase system (PTS). In most firmicutes the transcriptional activator MtlR controls expression of the mtl operon. MtlR possesses an N-terminal DNA binding domain, two PTS regulation domains (PRDs), an EIIB(Gat)- and EIIA(Mtl)-like domain. These four regulatory domains contain one or two potential PTS phosphorylation sites. Replacement of His-342 or His-399 in PRD2 with Ala prevented the phosphorylation of Bacillus subtilis MtlR by PEP, EI and HPr. These mutations as well as EI inactivation caused a loss of MtlR function in vivo. In contrast, phosphomimetic replacement of His-342 with Asp rendered MtlR constitutively active. The absence of phosphorylation in PRD2 serves as catabolite repression mechanism. When EIIA(Mtl) and the soluble EIIB(Mtl) domain of the EIICB(Mtl) permease were included in the phosphorylation mixture, His-599 in the EIIA-like domain of MtlR also became phosphorylated. Replacement of His-599 with Asp rendered MtlR inactive, while His599Ala replacement caused slightly constitutive, glucose-repressible MtlR activity. Doubly mutated His342Ala/His599Ala MtlR was still phosphorylated by EI, HPr and EIIA(Mtl) at Cys-419 in the EIIB(Gat)-like domain. Cys419Ala replacement and deletion of EIIA(Mtl) caused strong constitutive glucose-repressible MtlR activity. This is the first report that Cys phosphorylation controls PRD-containing transcriptional activators.

The 1.9 A resolution structure of phospho-serine 46 HPr from Enterococcus faecalis

The histidine-containing phosphocarrier protein HPr is a central component of the phosphoenolpyruvate:sugar phosphotransferase system (PTS), which transfers metabolic carbohydrates across the cell membrane in many bacterial species. In Gram-positive bacteria, phosphorylation of HPr at conserved serine 46 (P-Ser-HPr) plays several regulatory roles within the cell; the major regulatory effect of P-Ser-HPr is its inability to act as a phosphocarrier substrate in the enzyme I reaction of the PTS. In order to investigate the structural nature of HPr regulation by phosphorylation at Ser46, the structure of the P-Ser-HPr from the Gram- positive bacterium Enterococcus faecalis has been determined. X-ray diffraction analysis of P-Ser-HPr crystals provided 10,043 unique reflections, with a 95.1 % completeness of data to 1.9 A resolution. The structure was solved using molecular replacement, with two P-Ser-HPr molecules present in the asymmetric unit. The final R-value and R(Free) are 0.178 and 0.239, respectively. The overall tertiary structure of P-Ser-HPr is that of other HPr structures. However the active site in both P-Ser-HPr molecules was found to be in the "open" conformation. Ala16 of both molecules were observed to be in a state of torsional strain, similar to that seen in the structure of the native HPr from E. faecalis. Regulatory phosphorylation at Ser46 does not induce large structural changes to the HPr molecule. The B-helix was observed to be slightly lengthened as a result of Ser46 phosphorylation. Also, the water mediated Met51-His15 interaction is maintained, again similar to that of the native E. faecalis HPr. The major structural, and thus regulatory, effect of phosphorylation at Ser46 is disruption of the hydrophobic interactions between EI and HPr, in particular the electrostatic repulsion between the phosphoryl group on Ser46 and Glu84 of EI and the prevention of a potential interaction of Met48 with a hydrophobic pocket of EI.

In vitro characterization of the Bacillus subtilis protein tyrosine phosphatase YwqE

Both gram-negative and gram-positive bacteria possess protein tyrosine phosphatases (PTPs) with a catalytic Cys residue. In addition, many gram-positive bacteria have acquired a new family of PTPs, whose first characterized member was CpsB from Streptococcus pneumoniae. Bacillus subtilis contains one such CpsB-like PTP, YwqE, in addition to two class II Cys-based PTPs, YwlE and YfkJ. The substrates for both YwlE and YfkJ are presently unknown, while YwqE was shown to dephosphorylate two phosphotyrosine-containing proteins implicated in UDP-glucuronate biosynthesis, YwqD and YwqF. In this study, we characterize YwqE, compare the activities of the three B. subtilis PTPs (YwqE, YwlE, and YfkJ), and demonstrate that the two B. subtilis class II PTPs do not dephosphorylate the physiological substrates of YwqE.

Analysis of the elements of catabolite repression in Clostridium acetobutylicum ATCC 824

The PTSH gene, encoding the phosphotransferase protein HPr, from Clostridium acetobutylicum ATCC 824 was identified from the genome sequence, cloned and shown to complement a PTSH mutant of Escherichia coli. The deduced protein sequence shares significant homology with HPr proteins from other low-GC gram-positive bacteria, although the highly conserved sequence surrounding the Ser-46 phosphorylation site is not well preserved in the clostridial protein. Nevertheless, the HPr was phosphorylated in an ATP-dependent manner in cell-free extracts of C. Acetobutylicum. Furthermore, purified His-tagged HPr from Bacillus Subtilis was also a substrate for the clostridial HPr kinase/phosphorylase. This phosphorylation reaction is a key step in the mechanism of carbon catabolite repression proposed to operate in B. Subtilis and other low-GC gram-positive bacteria. Putative genes encoding the HPr kinase/phosphorylase and the other element of this model, namely the catabolite control protein CcpA, were identified from the C. Acetobutylicum genome sequence, suggesting that a similar mechanism of carbon catabolite repression may operate in this industrially important organism.

Phosphoenolpyruvate-dependent phosphotransferase system of Staphylococcus aureus: 1H nuclear magnetic resonance studies on phosphorylated and unphosphorylated factor IIIlac and its interaction with the phosphocarrier protein HPr

The trimeric phosphocarrier protein factor III specific for galactosides was investigated by 1H NMR spectroscopy. The protomer contains four histidyl residues with acidic pK values in the range 5.6-6.2. One of the histidyl residues, His-B, carries the phosphoryl group. The pK value of His-B increases from 6.0 to 8.6 upon phosphorylation. To determine the position of the phosphoryl group with respect to the nitrogens required the isolation of a peptide T-2 containing the phosphorylated active-center histidine and one of the other histidines. The pK value and the chemical shift of the phosphopeptide clearly indicated the phosphorus to be bound to the N-3 atom of the imidazole ring. The temperature dependence of the factor III spectrum demonstrates multiple conformations which exchange rapidly on the NMR time scale. Titration of factor III with HPr protein showed an upfield shift of the active-center histidine, indicating complex formation between both proteins. Phosphorylation of both proteins abolished the interaction, which is plausible from mechanistic considerations.

Transcription regulators controlled by interaction with enzyme IIB components of the phosphoenolpyruvate: sugar phosphotransferase system

Numerous bacteria possess transcription activators and antiterminators composed of regulatory domains phosphorylated by components of the phosphoenolpyruvate:sugar phosphotransferase system (PTS). These domains, called PTS regulation domains (PRDs), usually contain two conserved histidines as potential phosphorylation sites. While antiterminators possess two PRDs with four phosphorylation sites, transcription activators contain two PRDs plus two regulatory domains resembling PTS components (EIIA and EIIB). The activity of these transcription regulators is controlled by up to five phosphorylations catalyzed by PTS proteins. Phosphorylation by the general PTS components EI and HPr is usually essential for the activity of PRD-containing transcription regulators, whereas phosphorylation by the sugar-specific components EIIA or EIIB lowers their activity. For a specific regulator, for example the Bacillus subtilis mtl operon activator MtlR, the functional phosphorylation sites can be different in other bacteria and consequently the detailed mode of regulation varies. Some of these transcription regulators are also controlled by an interaction with a sugar-specific EIIB PTS component. The EIIBs are frequently fused to the membrane-spanning EIIC and EIIB-mediated membrane sequestration is sometimes crucial for the control of a transcription regulator. This is also true for the Escherichia coli repressor Mlc, which does not contain a PRD but nevertheless interacts with the EIIB domain of the glucose-specific PTS. In addition, some PRD-containing transcription activators interact with a distinct EIIB protein located in the cytoplasm. The phosphorylation state of the EIIB components, which changes in response to the presence or absence of the corresponding carbon source, affects their interaction with transcription regulators. This article is part of a Special Issue entitled: Inhibitors of Protein Kinases (2012).

Catabolite repression resistance of gnt operon expression in Bacillus subtilis conferred by mutation of His-15, the site of phosphoenolpyruvate-dependent phosphorylation of the phosphocarrier protein HPr

Carbon catabolite repression of the gnt operon of Bacillus subtilis is mediated by the catabolite control protein CcpA and by HPr, a phosphocarrier protein of the phosphotransferase system. ATP-dependent phosphorylation of HPr at Ser-46 is required for carbon catabolite repression as ptsH1 mutants in which Ser-46 of HPr is replaced with an unphosphorylatable alanyl residue are resistant to carbon catabolite repression. We here demonstrate that mutation of His-15 of HPr, the site of phosphoenolpyruvate-dependent phosphorylation, also prevents carbon catabolite repression of the gnt operon. A strain which expressed two mutant HPrs (one in which Ser-46 is replaced by Ala [S46A HPr] and one in which His-15 is replaced by Ala [H15A HPr]) on the chromosome was barely sensitive to carbon catabolite repression, although the H15A mutant HPr can be phosphorylated at Ser-46 by the ATP-dependent HPr kinase in vitro and in vivo. The S46D mutant HPr which structurally resembles seryl-phosphorylated HPr has a repressive effect on gnt expression even in the absence of a repressing sugar. By contrast, the doubly mutated H15E,S46D HPr, which resembles the doubly phosphorylated HPr because of the negative charges introduced by the mutations at both phosphorylation sites, had no such effect. In vitro assays substantiated these findings and demonstrated that in contrast to the wild-type seryl-phosphorylated HPr and the S46D mutant HPr, seryl-phosphorylated H15A mutant HPr and H15E,S46D doubly mutated HPr did not interact with CcpA. These results suggest that His-15 of HPr is important for carbon catabolite repression and that either mutation or phosphorylation at His-15 can prevent carbon catabolite repression.

Regulation of glycerol metabolism in Enterococcus faecalis by phosphoenolpyruvate-dependent phosphorylation of glycerol kinase catalyzed by enzyme I and HPr of the phosphotransferase system

Using a polyclonal antibody against glycerol kinase from Enterococcus faecalis, we could demonstrate that glycerol kinase is inducible by growth on glycerol-containing medium and that during growth on glycerol the enzyme is mainly phosphorylated. Glucose and other sugars metabolized via the Embden-Meyerhof pathway strongly repressed the synthesis of glycerol kinase, while if glycerol was also present during growth, low activity, reflecting partial induction and the presence of mainly unphosphorylated, less active enzyme, was found. With gluconate, which is also a substrate of the phosphotransferase system, repression of glycerol kinase was less severe, but the enzyme was mainly present in the less active, unphosphorylated form. Effects of growth on different carbon sources on glycerol uptake are also reported.

Cloning and characterization of the Bacillus subtilis prkA gene encoding a novel serine protein kinase

We have cloned and sequenced a 3574-bp Bacillus subtilis (Bs) DNA fragment located between the nrdA and citB genes at about 169 degrees on the chromosome. An Escherichia coli strain, LBG1605, carrying a mutated ptsH gene (encoding HPr (His-containing protein) of the bacterial phosphotransferase system (PTS)) and complemented for PTS activity with the ptsH of Staphylococcus carnosus, exhibited reduced mannitol fermentation activity when transformed with a plasmid bearing this 3574-bp Bs fragment. This fragment contained an incomplete and two complete open reading frames (ORFs). The product of the first complete ORF, a protein composed of 235 amino acids (aa) (25038 Da), was found to be responsible for the observed reduced mannitol fermentation. The 3' part of this 705-bp second ORF and the 428-bp incomplete first ORF encode aa sequences exhibiting almost 40% sequence identify. However, the function of these two proteins remains unknown. The third ORF, the 1893-bp prkA gene, encodes a protein (PrkA) of 72889 Da. PrkA possesses the A-motif of nucleotide-binding proteins and exhibits distant homology to eukaryotic protein kinases. Several of the essential aa in the loops known to form the active site of cyclic adenosine 3',5'-monophosphate (cAMP)-dependent protein kinase appeared to be conserved in PrkA. After expression of prkA and purification of PrkA, we could demonstrate that PrkA can indeed phosphorylate a Bs 60-kDa protein at a Ser residue.

Dimerization of Crh by reversible 3D domain swapping induces structural adjustments to its monomeric homologue Hpr

The crystal structure of the regulatory protein Crh from Bacillus subtilis was solved at 1.8A resolution and showed an intertwined dimer formed by N-terminal beta1-strand swapping of two monomers. Comparison with the monomeric NMR structure of Crh revealed a domain swap induced conformational rearrangement of the putative interaction site with the repressor CcpA. The resulting conformation closely resembles that observed for the monomeric Crh homologue HPr, indicating that the Crh dimer is the active form binding to CcpA. An analogous dimer of HPr can be constructed without domain swapping, suggesting that HPr may dimerize upon binding to CcpA. Our data suggest that reversible 3D domain swapping of Crh might be an efficient regulatory mechanism to modulate its activity.

Bacillus subtilis mutant LicT antiterminators exhibiting enzyme I- and HPr-independent antitermination affect catabolite repression of the bglPH operon

The Bacillus subtilis antiterminator LicT regulates the expression of bglPH and bglS, which encode the enzymes for the metabolism of aryl-beta-glucosides and the beta-glucanase BglS. The N-terminal domain of LicT (first 55 amino acids) prevents the formation of rho-independent terminators on the respective transcripts by binding to target sites overlapping these terminators. Proteins of the phosphoenolpyruvate:carbohydrate phosphotransferase system (PTS) regulate the antitermination activity of LicT by phosphorylating histidines in its two PTS regulation domains (PRDs). Phosphorylation at His-100 in PRD-1 requires the PTS proteins enzyme I and HPr and the phosphorylated permease BglP and inactivates LicT. During transport and phosphorylation of aryl-beta-glucosides, BglP is dephosphorylated, which renders LicT active and thus leads to bglPH and bglS induction. In contrast, phosphorylation at His-207 and/or His-269 in PRD-2, which requires only enzyme I and HPr, is absolutely necessary for LicT activity and bglPH and bglS expression. We isolated spontaneous licT mutants expressing bglPH even when enzyme I and HPr were absent (as indicated by the designation "Pia" [PTS-independent antitermination]). Introduced in a ptsHI(+) strain, two classes of licT(Pia) mutations could be distinguished. Mutants synthesizing LicT(Pia) antiterminators altered in PRD-2 still required induction by aryl-beta-glucosides, whereas mutations affecting PRD-1 caused constitutive bglPH expression. One of the two carbon catabolite repression (CCR) mechanisms operative for bglPH requires the rho-independent terminator and is probably prevented when LicT is activated by P approximately His-HPr-dependent phosphorylation in PRD-2 (where the prefix "P approximately " stands for "phospho"). During CCR, the small amount of P approximately His-HPr present in cells growing on repressing PTS sugars probably leads to insufficient phosphorylation at PRD-2 of LicT and therefore to reduced bglPH expression. In agreement with this concept, mutants synthesizing a P approximately His-HPr-independent LicT(Pia) had lost LicT-modulated CCR.

Molecular analysis of the glucose-specific phosphoenolpyruvate : sugar phosphotransferase system from Lactobacillus casei and its links with the control of sugar metabolism

Lactobacillus caseitransports glucose preferentially by a mannose-class phosphoenolpyruvate : sugar phosphotransferase system (PTS). The genomic analysis ofL. caseiallowed the authors to find a gene cluster (manLMNO) encoding the IIAB (manL), IIC (manM) and IID (manN) proteins of a mannose-class PTS, and a putative 121 aa protein of unknown function (encoded bymanO), homologues of which are also present inmanclusters that encode glucose/mannose transporters in other Gram-positive bacteria. TheL. casei manoperon is constitutively expressed into amanLMNOmessenger, but an additionalmanOtranscript was also detected. Upstream of themanoperon, two genes (upsRandupsA) were found which encode proteins resembling a transcriptional regulator and a membrane protein, respectively. Disruption of eitherupsRorupsAdid not affectmanLMNOtranscription, and had no effect on glucose uptake. Cells carrying amanOdeletion transported glucose at a rate similar to that of the wild-type strain. By contrast, amanMdisruption resulted in cells unable to transport glucose by the PTS, thus confirming the functional role of themangenes. In addition, themanMmutant exhibited neither inducer exclusion of maltose nor glucose repression. This result confirms the need for glucose transport through the PTS to trigger these regulatory processes inL. casei.

Body fat mass, leptin and puberty

Leptin, the ob gene product, provides a molecular basis for the lipostatic theory of the regulation of energy balance. Leptin circulates as a monomeric 16 kDa protein in rodent and human plasma and is also bound to leptin binding proteins that may form large high molecular weight complexes. Initial models of leptin action included leptin-deficient ob/ob mice and leptin-insensitive db/db mice. Peripheral or central administration of leptin reduced body weight, adiposity, and food intake in ob/ob mice but not in db/db mice. In ob/ob mice leptin treatment restored fertility. Leptin interacts with many messenger molecules in the brain. For example, leptin suppresses neuropeptide Y (NPY) expression in the arcuate nucleus. Increased NPY activity has an inhibitory effect on the gonadotropin axis and represents a direct mechanism for inhibiting sexual maturation and reproductive function in conditions of food restriction and/or energy expenditure. By modulating the hypothalamo-pituitary-gonadal axis both directly and indirectly, leptin may thus serve as the signal from fat to the brain about the adequacy of fat stores for pubertal development and reproduction. Normal leptin secretion is necessary for normal reproductive function to proceed and leptin may be a signal allowing for the point of initiation of and progression toward puberty.

Leptin as a metabolic regulator during fetal and neonatal life and in childhood and adolescence

Body weight is regulated by a feedback loop in which peripheral signals report nutritional information to an integratory center in the brain. The cloning of the ob gene is consistent with this concept and suggests that body fat content in adult rodents is regulated by a negative feedback loop centered in the hypothalamus/1-8/. In a recent report, two severely obese children with congenital leptin deficiency due to a homozygous frame-shift mutation involving the deletion of a single guanine nucleotide in codon 133 of the ob gene have been described. This discovery provides the first genetic evidence that leptin is an important regulator of energy balance in humans. However, it has become increasingly clear that apart from leptin's function in the central nervous system and in regulation of energy balance, leptin also acts in the periphery and might be important as a hormone modulating processes in regard to reproduction, glucose metabolism and insulin resistance, as well as growth and development of many tissues and organs either directly or indirectly. This report reviews some of the topics of leptin research that are of particular importance and relevance for pediatric and adolescent medicine and for pediatric endocrinology in particular.

The Lactobacillus casei ptsHI47T mutation causes overexpression of a LevR-regulated but RpoN-independent operon encoding a mannose class phosphotransferase system

A proteome analysis of Lactobacillus casei mutants that are affected in carbon catabolite repression revealed that a 15-kDa protein was strongly overproduced in a ptsHI47T mutant. This protein was identified as EIIA of a mannose class phosphotransferase system (PTS). A 7.1-kb DNA fragment containing the EIIA-encoding open reading frame and five other genes was sequenced. The first gene encodes a protein resembling the RpoN (sigma54)-dependent Bacillus subtilis transcription activator LevR. The following pentacistronic operon is oriented in the opposite direction and encodes four proteins with strong similarity to the proteins of the B. subtilis Lev-PTS and one protein of unknown function. The genes present on the 7.1-kb DNA fragment were therefore called levR and levABCDX. The levABCDX operon was induced by fructose and mannose. No "-12, -24" promoter typical of RpoN-dependent genes precedes the L. casei lev operon, and its expression was therefore RpoN independent but required LevR. Phosphorylation of LevR by P approximately His-HPr stimulates its activity, while phosphorylation by P approximately EIIBLev inhibits it. Disruption of the EIIBLev-encoding levB gene therefore led to strong constitutive expression of the lev operon, which was weaker in a strain carrying a ptsI mutation preventing phosphorylation by both P approximately EIIBLev and P approximately His-HPr. Expression of the L. casei lev operon is also subject to P-Ser-HPr-mediated catabolite repression. The observed slow phosphoenolpyruvate- and ATP-dependent phosphorylation of HPrI47T as well as the slow phosphoryl group transfer from the mutant P approximately His-HPr to EIIALev are assumed to be responsible for the elevated expression of the lev operon in the ptsHI47T mutant.

An ectoprotein kinase of group C streptococci binds hyaluronan and regulates capsule formation

A 56-kDa protein had been isolated and cloned from protoplast membranes of group C streptococci that had erroneously been identified as hyaluronan synthase. The function of this protein was reexamined. When streptococcal membranes were separated on an SDS-polyacrylamide gel and renatured, a 56-kDa protein was detected that had kinase activity for a casein substrate. When this recombinant protein was expressed in Escherichia coli and incubated in the presence of [32P]ATP, it was responsible for phosphorylation of two proteins with 30 and 56 kDa that were not present in the control lysate. The 56-kDa protein was specifically phosphorylated in an immunoprecipitate of a detergent extract of the recombinant E. coli lysate with antibodies against the 56-kDa protein, indicating that it was autophosphorylated. The E. coli lysate containing the recombinant protein could bind hyaluronan, and hyaluronan binding was abolished by the addition of ATP. Kinetic analysis of hyaluronan synthesis and release from isolated protoplast membranes indicated that phosphorylation by ATP stimulated hyaluronan release and synthesis. Incubation of membranes with antibodies to the 56-kDa protein increased hyaluronan release. The addition of [32P]ATP to intact streptococci led to rapid phosphorylation of two proteins, 56 and 75 kDa each at threonine residues. This phosphorylation was neither observed with [32P]phosphate nor in the presence of trypsin, indicating that the kinase was localized extracellularly. The addition of ATP to growing group C streptococci led to increased hyaluronan synthesis and release. However marked differences were found between group A and group C streptococci. Antibodies against the 56-kDa protein from group C streptococci did not recognize proteins from group A strains, and a homologous DNA sequence could not be detected by polymerase chain reaction or Southern blotting. In addition, Group A streptococci did not retain a large hyaluronan capsule like group C strains. These results indicated that the 56-kDa protein is an ectoprotein kinase specific for group C streptococci that regulates hyaluronan capsule shedding by phosphorylation.

Enterococcus faecalis utilizes maltose by connecting two incompatible metabolic routes via a novel maltose 6'-phosphate phosphatase (MapP)

Similar to Bacillus subtilis, Enterococcus faecalis transports and phosphorylates maltose via a phosphoenolpyruvate (PEP):maltose phosphotransferase system (PTS). The maltose-specific PTS permease is encoded by the malT gene. However, E. faecalis lacks a malA gene encoding a 6-phospho-α-glucosidase, which in B. subtilis hydrolyses maltose 6'-P into glucose and glucose 6-P. Instead, an operon encoding a maltose phosphorylase (MalP), a phosphoglucomutase and a mutarotase starts upstream from malT. MalP was suggested to split maltose 6-P into glucose 1-P and glucose 6-P. However, purified MalP phosphorolyses maltose but not maltose 6'-P. We discovered that the gene downstream from malT encodes a novel enzyme (MapP) that dephosphorylates maltose 6'-P formed by the PTS. The resulting intracellular maltose is cleaved by MalP into glucose and glucose 1-P. Slow uptake of maltose probably via a maltodextrin ABC transporter allows poor growth for the mapP but not the malP mutant. Synthesis of MapP in a B. subtilis mutant accumulating maltose 6'-P restored growth on maltose. MapP catalyses the dephosphorylation of intracellular maltose 6'-P, and the resulting maltose is converted by the B. subtilis maltose phosphorylase into glucose and glucose 1-P. MapP therefore connects PTS-mediated maltose uptake to maltose phosphorylase-catalysed metabolism. Dephosphorylation assays with a wide variety of phospho-substrates revealed that MapP preferably dephosphorylates disaccharides containing an O-α-glycosyl linkage.

Structural characterizations of glycerol kinase: unraveling phosphorylation-induced long-range activation

Glycerol metabolism provides a central link between sugar and fatty acid catabolism. In most bacteria, glycerol kinase plays a crucial role in regulating channel/facilitator-dependent uptake of glycerol into the cell. In the firmicute Enterococcus casseliflavus, this enzyme's activity is enhanced by phosphorylation of the histidine residue (His232) located in its activation loop, approximately 25 A from its catalytic cleft. We reported earlier that some mutations of His232 altered enzyme activities; we present here the crystal structures of these mutant GlpK enzymes. The structure of a mutant enzyme with enhanced enzymatic activity, His232Arg, reveals that residues at the catalytic cleft are more optimally aligned to bind ATP and mediate phosphoryl transfer. Specifically, the position of Arg18 in His232Arg shifts by approximately 1 A when compared to its position in wild-type (WT), His232Ala, and His232Glu enzymes. This new conformation of Arg18 is more optimally positioned at the presumed gamma-phosphate location of ATP, close to the glycerol substrate. In addition to structural changes exhibited at the active site, the conformational stability of the activation loop is decreased, as reflected by an approximately 35% increase in B factors ("thermal factors") in a mutant enzyme displaying diminished activity, His232Glu. Correlating conformational changes to alteration of enzymatic activities in the mutant enzymes identifies distinct localized regions that can have profound effects on intramolecular signal transduction. Alterations in pairwise interactions across the dimer interface can communicate phosphorylation states over 25 A from the activation loop to the catalytic cleft, positioning Arg18 to form favorable interactions at the beta,gamma-bridging position with ATP. This would offset loss of the hydrogen bonds at the gamma-phosphate of ATP during phosphoryl transfer to glycerol, suggesting that appropriate alignment of the second substrate of glycerol kinase, the ATP molecule, may largely determine the rate of glycerol 3-phosphate production.

Membrane sequestration by the EIIB domain of the mannitol permease MtlA activates the Bacillus subtilis mtl operon regulator MtlR

In most firmicutes expression of the mannitol operon is regulated by MtlR. This transcription activator is controlled via phosphorylation of its regulatory domains by components of the phosphoenolpyruvate : carbohydrate phosphotransferase system (PTS). We found that activation of Bacillus subtilis MtlR also requires an interaction with the EIIB(Mtl) domain of the mannitol permease MtlA (EIICB(Mtl) ). The constitutive expression of the mtlAFD operon in an mtlF mutant was prevented when entire mtlA or only its 3' part (EIIB(Mtl) ) were deleted. Yeast two-hybrid experiments revealed a direct interaction of the EIIB(Mtl) domain with the two C-terminal domains of MtlR. Complementation of the Δ3'-mtlA ΔmtlF or ΔmtlAFD mutants with mtlA restored constitutive MtlR activity, whereas complementation with only 3'-mtlA had no effect. Moreover, synthesis of EIIB(Mtl) in strains producing constitutively active MtlR caused MtlR inactivation. Interestingly, EIIB(Mtl) fused to the trans-membrane protein YwqC restored constitutive MtlR activity in the above mutants. Replacing the phosphorylatable Cys with Asp in MtlA or soluble EIIB(Mtl) lowered MtlR activation, indicating that MtlR does not interact with phosphorylatyed EIIB(Mtl) . Induction of the B. subtilis mtl operon therefore follows a novel regulation mechanism where the transcription activator needs to be sequestered to the membrane by unphosphorylated EIICB(Mtl) in order to be functional.

CcpA represses the expression of the divergent cit operons of Enterococcus faecalis through multiple cre sites

BACKGROUND

In Enterococcus faecalis the genes encoding the enzymes involved in citrate metabolism are organized in two divergent operons, citHO and oadHDB-citCDEFX-oadA-citMG (citCL locus). Expression of both operons is specifically activated by adding citrate to the medium. This activation is mediated by binding of the GntR-like transcriptional regulator (CitO) to the cis-acting sequences located in the cit intergenic region. Early studies indicated that citrate and glucose could not be co-metabolized suggesting some form of catabolite repression, however the molecular mechanism remained unknown.

RESULTS

In this study, we observed that the citHO promoter is repressed in the presence of sugars transported by the Phosphoenolpyruvate:carbohydrate Phosphotranserase System (PTS sugars). This result strongly suggested that Carbon Catabolic Repression (CCR) impedes the expression of the activator CitO and the subsequent induction of the cit pathway. In fact, we demonstrate that CCR is acting on both promoters. It is partially relieved in a ccpA-deficient E. faecalis strain indicating that a CcpA-independent mechanism is also involved in regulation of the two operons. Furthermore, sequence analysis of the citH/oadH intergenic region revealed the presence of three putative catabolite responsive elements (cre). We found that they are all active and able to bind the CcpA/P-Ser-HPr complex, which downregulates the expression of the cit operons. Systematic mutation of the CcpA/P-Ser-HPr binding sites revealed that cre1 and cre2 contribute to citHO repression, while cre3 is involved in CCR of citCL.

CONCLUSION

In conclusion, our study establishes that expression of the cit operons in E. faecalis is controlled by CCR via CcpA-dependent and -independent mechanisms.