Site-directed mutagenesis of the phosphocarrier protein. IIIGlc, a major signal-transducing protein in Escherichia coli

Strain-specific quorum-sensing responses determine virulence properties in Vibrio anguillarum

Bacterial populations communicate using quorum-sensing (QS) molecules and switch on QS regulation to engage in coordinated behaviour such as biofilm formation or virulence. The marine fish pathogen Vibrio anguillarum harbours several QS systems, and our understanding of its QS regulation is still fragmentary. Here, we identify the VanT-QS regulon and explore the diversity and trajectory of traits under QS regulation in Vibrio anguillarum through comparative transcriptomics of two wildtype strains and their corresponding mutants artificially locked in QS-on (ΔvanO) or QS-off (ΔvanT) states. Intriguingly, the two wildtype populations showed different QS responses to cell density changes and operated primarily in the QS-on and QS-off spectrum, respectively. Examining 27 V. anguillarum strains revealed that ~11% were QS-negative, and GFP-reporter measurements of nine QS-positive strains revealed a highly strain-specific nature of the QS responses. We showed that QS controls a plethora of genes involved in processes such as central metabolism, biofilm formation, competence, T6SS, and virulence properties in V. anguillarum, with large strain-specific differences. Moreover, we demonstrated that the QS state is an important driver of virulence towards fish larvae in one of two V. anguillarum strains. We speculate that infections by mixed-strain communities spanning diverse QS strategies optimize the infection efficiency of the pathogen.

The molecular mechanism for carbon catabolite repression of the chitin response in Vibrio cholerae

Vibrio cholerae is a facultative pathogen that primarily occupies marine environments. In this niche, V. cholerae commonly interacts with the chitinous shells of crustacean zooplankton. As a chitinolytic microbe, V. cholerae degrades insoluble chitin into soluble oligosaccharides. Chitin oligosaccharides serve as both a nutrient source and an environmental cue that induces a strong transcriptional response in V. cholerae. Namely, these oligosaccharides induce the chitin sensor, ChiS, to activate the genes required for chitin utilization and horizontal gene transfer by natural transformation. Thus, interactions with chitin impact the survival of V. cholerae in marine environments. Chitin is a complex carbon source for V. cholerae to degrade and consume, and the presence of more energetically favorable carbon sources can inhibit chitin utilization. This phenomenon, known as carbon catabolite repression (CCR), is mediated by the glucose-specific Enzyme IIA (EIIAGlc) of the phosphoenolpyruvate-dependent phosphotransferase system (PTS). In the presence of glucose, EIIAGlc becomes dephosphorylated, which inhibits ChiS transcriptional activity by an unknown mechanism. Here, we show that dephosphorylated EIIAGlc interacts with ChiS. We also isolate ChiS suppressor mutants that evade EIIAGlc-dependent repression and demonstrate that these alleles no longer interact with EIIAGlc. These findings suggest that EIIAGlc must interact with ChiS to exert its repressive effect. Importantly, the ChiS suppressor mutations we isolated also relieve repression of chitin utilization and natural transformation by EIIAGlc, suggesting that CCR of these behaviors is primarily regulated through ChiS. Together, our results reveal how nutrient conditions impact the fitness of an important human pathogen in its environmental reservoir.

Chemoproteomic Profiling of Phosphoaspartate Modifications in Prokaryotes

Phosphorylation at aspartic acid residues represents an abundant and critical post-translational modification (PTM) in prokaryotes. In contrast to most characterized PTMs, such as phosphorylation at serine or threonine, the phosphoaspartate moiety is intrinsically labile, and therefore incompatible with common proteomic profiling methods. Herein, we report a nucleophilic, desthiobiotin-containing hydroxylamine (DBHA) chemical probe that covalently labels modified aspartic acid residues in native proteomes. DBHA treatment coupled with LC-MS/MS analysis enabled detection of known phosphoaspartate modifications, as well as novel aspartic acid sites in the E. coli proteome. Coupled with isotopic labelling, DBHA-dependent proteomic profiling also permitted global quantification of changes in endogenous protein modification status, as demonstrated with the detection of increased E. coli OmpR phosphorylation, but not abundance, in response to changes in osmolarity.

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.

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.

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.

Control of the Bacillus subtilis Antiterminator Protein GlcT by Phosphorylation

Bacillus subtilis transports glucose by the phosphotransferase system (PTS). The genes for this system are encoded in the ptsGHI operon, which is induced by glucose and depends on a termination/antitermination mechanism involving a riboswitch and the RNA-binding antitermination protein GlcT. In the absence of glucose, GlcT is inactive, and a terminator is formed in the leader region of the ptsG mRNA. If glucose is present, GlcT can bind to its RNA target and prevent transcription termination. The GlcT protein is composed of three domains, an N-terminal RNA binding domain and two PTS regulation domains, PTS regulation domain (PRD) I and PRD-II. In this work, we demonstrate that GlcT can be phosphorylated by two PTS proteins, HPr and the glucose-specific enzyme II (EIIGlc). HPr-dependent phosphorylation occurs on PRD-II and has a slight stimulatory effect on GlcT activity. In contrast, EIIGlc phosphorylates the PRD-I of GlcT, and this phosphorylation inactivates GlcT. This latter phosphorylation event links the availability of glucose to the expression of the ptsGHI operon via the phosphorylation state of EIIGlc and GlcT. This is the first in vitro demonstration of a direct phosphorylation of an antiterminator of the BglG family by the corresponding PTS permease.

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

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

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.

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 three-dimensional structure of the nitrogen regulatory protein IIANtr from Escherichia coli

The bacterial rpoN operon codes for sigma 54, which is the key sigma factor that, under nitrogen starvation conditions, activates the transcription of genes needed to assimilate ammonia and glutamate. The rpoN operon contains several other open reading frames that are cotranscribed with sigma 54. The product of one of these, the 17.9 kDa protein IIANtr, is homologous to IIA proteins of the phosphoenolpyruvate:sugar phosphotransferase (PTS) system. IIANtr influences the transcription of sigma 54-dependent genes through an unknown mechanism and may thereby provide a regulatory link between carbon and nitrogen metabolism. Here we describe the 2.35 A X-ray structure of Escherichia coli IIANtr. It is the first structure of a IIA enzyme from the fructose-mannitol family of the PTS. The enzyme displays a novel fold characterized by a central mixed parallel/anti-parallel beta-sheet surrounded by six alpha-helices. The active site His73 is situated in a shallow depression on the protein surface.

The structure of the Escherichia coli phosphotransferase IIAmannitol reveals a novel fold with two conformations of the active site

BACKGROUND

The bacterial phosphoenolpyruvate-dependent phosphotransferase system (PTS) catalyses the cellular uptake and subsequent phosphorylation of carbohydrates. Moreover, the PTS plays a crucial role in the global regulation of various metabolic pathways. The PTS consists of two general proteins, enzyme I and the histidine-containing protein (HPr), and the carbohydrate-specific enzyme II (EII). EIIs are usually composed of two cytoplasmic domains, IIA and IIB, and a transmembrane domain, IIC. The IIA domains catalyse the transfer of a phosphoryl group from HPr to IIB, which phosphorylates the transported carbohydrate. Knowledge of the structures of the IIA proteins may provide insight into the mechanisms by which the PTS couples phosphorylation reactions with carbohydrate specificity.

RESULTS

We have determined the crystal structure of the Escherichia coli mannitol-specific IIA domain, IIAmtl (M(r) 16.3 kDa), by multiple anomalous dispersion analysis of a selenomethionine variant of IIAmtl. The structure was refined at 1.8 A resolution to an R factor of 19.0% (Rfree 24.2%). The enzyme consists of a single five-stranded mixed beta sheet, flanked by helices on both sides. The phosphorylation site (His65) is located at the end of the third beta strand, in a shallow crevice lined with hydrophobic residues. The sidechains of two conserved active-site residues, Arg49 and His111, adopt two different conformations in the four independent IIAmtl molecules. Using a solution structure of phosphorylated HPr, and a combination of molecular modelling and NMR binding experiments, structural models of the HPr-IIAmtl complex were generated.

CONCLUSIONS

The fold of IIAmtl is completely different from the structures of other IIA proteins determined so far. The two conformations of Arg49 and His111 might represent different states of the active site, required for the different phosphoryl transfer reactions in which IIAmtl is involved. A comparison of the HPr-IIAmtl model with models of HPr in complex with other IIA enzymes shows that the overall interaction mode between the two proteins is similar. Differences in the stabilisation of the invariant residue Arg17 of HPr by the different IIA proteins might be part of a subtle mechanism to control the hierarchy of carbohydrate utilisation by the bacterium.

Anion binding to the ubiquitin molecule

Effects of different salts (NaCl, MgCl2, CaCl2, GdmCl, NaBr, NaClO4, NaH2PO4, Na2SO4) on the stability of the ubiquitin molecule at pH 2.0 have been studied by differential scanning calorimetry, circular dichroism, and Tyr fluorescence spectroscopies. It is shown that all of the salts studied significantly increase the thermostability of the ubiquitin molecule, and that this stabilization can be interpreted in terms of anion binding. Estimated thermodynamic parameters of binding for Cl- show that this binding is relatively weak (Kd = 0.15 M) and is characterized by a negative enthalpy of -15 kJ/mol per site. Particularly surprising was the observed stabilizing effect of GdmCl through the entire concentration range studied (0.01-2 M), however, to a lesser extent than stabilization by NaCl. This stabilizing effect of GdmCl appears to arise from the binding of Cl- ions. Analysis of the observed changes in the stability of the ubiquitin molecule in the presence of GdmCl can be adequately described by combining the thermodynamic model of denaturant binding with Cl- binding effects.

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.

Cloning, overexpression, purification, and spectroscopic characterization of human S100P

The calcium-binding protein S100P has been found to be associated with human prostate cancer. We have overexpressed S100P in Escherichia coli using a T7 expression system. A rapid two-step procedure for the isolation of overexpressed S100P leads to a preparation of >95% pure protein with a yield of approximately 150 mg per liter of culture. The structural integrity of recombinant S100P was analyzed using CD and fluorescence spectroscopic techniques. The far-UV CD shows that secondary structure of recombinant S100P consists predominantly of a-helical structure. Both near-UV CD and tyrosine fluorescence spectra show that aromatic residues are involved in the formation of a specific, well packed structure, indicating that the recombinant S100P protein adopts a compact folded conformation. Ca2+ has a profound effect on S100P structure. Near-UV CD and fluorescence intensity of both internal (tyrosine) and external (ANS) probes suggest significant structural rearrangements in the tertiary structure of the molecule. The similarity of far-UV CD spectrum of S100P in the presence and in the absence of Ca2+ suggests that Ca2+ binding has only minor effects on secondary structure.

Structures of active site histidine mutants of IIIGlc, a major signal-transducing protein in Escherichia coli. Effects on the mechanisms of regulation and phosphoryl transfer

IIIGlc (also called IIAGlc), a major signal-transducing protein in Escherichia coli, is also a phosphorylcarrier in glucose uptake. The crystal and NMR structures of IIIGlc show that His90, the phosphoryl acceptor, adjoins His75 in the active site. Glutamine was substituted for His-, giving H75QIIIGlc and H90QIIIGlc, respectively (Presper, K. A., Wong, C.-Y., Liu, L., Meadow, N. D., and Roseman, S. (1989) Proc. Natl. Acad. Sci. U. S. A. 86, 4052-4055), but the mutants showed unexpected properties. H90QIIIGlc loses regulatory functions of IIIGlc, and the phosphoryltransfer rates between HPr/H75QIIIGlc are 200-fold less than HPr/IIIGlc (Meadow, N. D., and Roseman, S. (1996) J. Biol. Chem. 271, 33440-33445). X-ray crystallography, differential scanning calorimetry, and NMR have now been used to determine the structures of the mutants (phospho-H75QIIIGlc was studied by NMR). The three methods gave completely consistent results. Except for the His to Gln substitutions, the only significant structural changes were in a few hydrogen bonds. H90QIIIGlc contains two structured water molecules (to Gln90), which could explain its inability to regulate glycerol kinase. Phospho-IIIGlc contains a chymotrypsin-like, hydrogen bond network (Thr73-His75-O--phosphoryl), whereas phospho-H75QIIIGlc contains only one bond (Gln75-O--phosphoryl). Hydrogen bonds play an essential role in a proposed mechanism for the phosphoryltransfer reaction.

Rate and equilibrium constants for phosphoryltransfer between active site histidines of Escherichia coli HPr and the signal transducing protein IIIGlc

The bacterial phosphoenolpyruvate:glycose phosphotransferase system (PTS) plays a central role in catabolizing many sugars; regulation is effected by phosphorylation of PTS proteins. In Escherichia coli, the phosphoryltransfer sequence for glucose uptake is: PEP --> Enzyme I(His191) --> HPr(His15) --> IIIGlc(His90) --> IIGlc(Cys421) --> glucose. A rapid quench method has now been developed for determining the rate and equilibrium constants of these reactions. The method was validated by control experiments, and gave the following results for phosphoryltransfer between the following protein pairs. For phospho-HPr/IIIGlc (and HPr/phospho-IIIGlc), k1 = 6.1 x 10(7) M-1 s-1, k-1 = 4.7 x 10(7); for the mutant H75QIIIGlc in place of IIIGlc, k1 = 2.8 x 10(5) M-1 s-1, k-1 = 2.3 x 10(5). The derived Keq values agreed with the Keq obtained without use of the rapid quench apparatus. Keq for both reactions is 1-1.5. The rate of phosphoryltransfer between HPr and wild type IIIGlc is close to a diffusion-controlled process, while the reactions involving the mutant H75QIIIGlc are 200-fold slower. These rate differences are explained by an hypothesis for the mechanism of phosphoryltransfer between HPr and IIIGlc based on the structures of mutant and wild type proteins (see Pelton et al. (Pelton, J. G., Torchia, D. A., Remington, S. J., Murphy, K. P., Meadow, N. D., and Roseman, S. (1996) J. Biol. Chem. 271, 33446-33456)).

Regulation of glycerol and maltose uptake by the IIAGlc-like domain of IINag of the phosphotransferase system in Salmonella typhimurium LT2

In Enterobacteriaceae the nonphosphorylated form of IIAGlc of the phosphoenolpyruvate-dependent phosphotransferase system (PTS) can inhibit the uptake and subsequent metabolism of glycerol and maltose by binding to, and inhibiting, glycerol kinase and the MalK protein of the maltose transport system, respectively. In this report we show that the IIAGlc-like domain of the membrane-bound IIN-acetylglucosamine (IINag) of the PTS can replace IIAGlc in a Salmonella typhimurium crr mutant strain that lacks all soluble IIAGlc. The inhibition was most severe in cells which were partially induced for the glycerol or maltose uptake systems. The Streptococcus thermophilus lactose transporter LacS, which also contains a IIAGlc-like domain, could not replace IIAGlc. Neither IINag nor LacS could replace IIAGlc in activation of adenylate cyclase.

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.

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.

Structure of the regulatory complex of Escherichia coli IIIGlc with glycerol kinase

The phosphocarrier protein IIIGlc is an integral component of the bacterial phosphotransferase (PTS) system. Unphosphorylated IIIGlc inhibits non-PTS carbohydrate transport systems by binding to diverse target proteins. The crystal structure at 2.6 A resolution of one of the targets, glycerol kinase (GK), in complex with unphosphorylated IIIGlc, glycerol, and adenosine diphosphate was determined. GK contains a region that is topologically identical to the adenosine triphosphate binding domains of hexokinase, the 70-kD heat shock cognate, and actin. IIIGlc binds far from the catalytic site of GK, indicating that long-range conformational changes mediate the inhibition of GK by IIIGlc. GK and IIIGlc are bound by hydrophobic and electrostatic interactions, with only one hydrogen bond involving an uncharged group. The phosphorylation site of IIIGlc, His90, is buried in a hydrophobic environment formed by the active site region of IIIGlc and a 3(10) helix of GK, suggesting that phosphorylation prevents IIIGlc binding to GK by directly disrupting protein-protein interactions.

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)

A proposed link between nitrogen and carbon metabolism involving protein phosphorylation in bacteria

We demonstrate that certain phosphoryl transfer proteins of the bacterial phosphotransferase system (PTS), the fructose- and mannitol-specific IIA proteins or domains, are homologous to a class of proteins, one of which is known to affect transcription of some of the nitrogen-regulatory sigma 54-dependent operons in Klebsiella pneumoniae. The phosphorylatable histidyl residue in the homologous PTS proteins and the consensus sequence in the vicinity of the active-site histidine are fully conserved in all members that comprise this family of proteins. A phylogenetic tree of the eight protein members of this family was constructed, and a "signature" sequence that can serve for the identification of new protein members of this family is proposed. These observations suggest that PTS-catalyzed protein phosphorylation may provide a regulatory link between carbon and nitrogen assimilation in bacteria.

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.

Low resolution solution structure of the Bacillus subtilis glucose permease IIA domain derived from heteronuclear three-dimensional NMR spectroscopy

A low resolution solution structure of the IIA domain of the Bacillus subtilis phosphoenolpyruvate-sugar phosphotransferase system (PTS) glucose permease has been determined using 945 inter-residue and 724 intra-residue distance constraints derived from three-dimensional 15N and 13C edited NOESY spectra. A total of 15 structures was generated using distance geometry calculations. The protein is comprised of 13 beta-strands forming an antiparallel beta-barrel. The average backbone atomic RMS deviation about the average distance geometry structure for the beta-sheet residues is 1.1 A. The conformations of the loop regions between the beta-strands are less well determined. Backbone distance constraints obtained during the process of sequential assignment were insufficient to correctly calculate the polypeptide fold.

Three-dimensional structure of the Escherichia coli phosphocarrier protein IIIglc

The crystal structure of a proteolytically modified form of the Escherichia coli phosphocarrier and signal transducing protein IIIglc has been determined by multiple isomorphous and molecular replacement. The model has been refined to an R-factor of 0.166 for data between 6- and 2.1-A resolution with an rms deviation of 0.020 A from ideal bond lengths and 3.2 degrees from ideal bond angles. The molecule is a beta-sheet sandwich, with six antiparallel strands on either side. Several short distorted helices line the periphery of the active site, which is a shallow extremely hydrophobic depression approximately 18 A in diameter near the center of one face. The side chains of the active site histidine residues 75 and 90 face each other at the center of the depression, with the N3 positions exposed to solvent, separated by 3.3 A in an excellent position to form adducts with phosphate. Chloroplatinate forms a divalent adduct with both histidyl side chains, suggesting that the phosphodonor reaction might proceed through a similar transition state. The hydrophobic patch forms the primary crystal contact, suggesting a mode of association of IIIglc with other components of the phosphoenolpyruvate-dependent phosphotransferase system.

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.

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.

Secondary structure of the phosphocarrier protein IIIGlc, a signal-transducing protein from Escherichia coli, determined by heteronuclear three-dimensional NMR spectroscopy

IIIGlc is a signal-transducing phosphocarrier protein of the phosphoenolpyruvate:glycose phosphotransferase system of Escherichia coli. The secondary structure of IIIGlc is determined by heteronuclear (15N, 13C) three-dimensional NMR spectroscopy. Sequential, medium-range, and long-range nuclear Overhauser effects seen in NMR spectra are used to elucidate 11 antiparallel beta-strands and four helical segments. The medium-range nuclear Overhauser effect patterns suggest that the helices are either distorted alpha-helices or are of the 3(10) class. The amino acids separating the active-site histidine residues (His75 and His90) form two strands (Ala76-Ser81 and Val85-Phe91) of a six-stranded antiparallel beta-sheet that brings His90 and His75 in close proximity. Sequence similarities in IIIGlc and several other sugar-transport proteins suggest that the histidine residues within these proteins may be arranged in a similar manner. The 18-residue N-terminal peptide that precedes beta-strand Thr19-Ile22 in native IIIGlc is disordered and does not interact with the rest of the protein. Furthermore, removal of the N-terminal heptapeptide by a specific endopeptidase does not affect the structure of the remaining protein, thus explaining the phospho-acceptor activity of modified IIIGlc with the phospho-histidine-containing phosphocarrier protein of this system.