Mapping of the dimer interface of the Escherichia coli mannitol permease by cysteine cross-linking

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

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

Structural insight into the PTS sugar transporter EIIC

BACKGROUND

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

SCOPE OF REVIEW

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

MAJOR CONCLUSIONS

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

GENERAL SIGNIFICANCE

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

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

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

Development of a new wheat microarray from a durum wheat totipotent cDNA library used for a powdery mildew resistance study

Totipotent cDNA libraries representative of all the potentially expressed sequences in a genome would be of great benefit to gene expression studies. Here, we report on an innovative method for creating such a library for durum wheat (Triticum turgidum L. var. durum) and its application for gene discovery. The use of suitable quantities of 5-azacytidine during the germination phase induced the demethylation of total DNA, and the resulting seedlings potentially express all of the genes present in the genome. A new wheat microarray consisting of 4925 unigenes was developed from the totipotent cDNA library and used to screen for genes that may contribute to differences in the disease resistance of two near-isogenic lines, the durum wheat cultivar Latino and the line 5BIL-42, which are respectively susceptible and resistant to powdery mildew. Fluorescently labeled cDNA was prepared from the RNA of seedlings of the two near-isogenic wheat lines after infection with a single powdery mildew isolate under controlled conditions in the greenhouse. Hybridization to the microarray identified six genes that were differently expressed in the two lines. Four of the sequences could be assigned putative functions based on their similarity to known genes in public databases. Physical mapping of the six genes localized them to two regions of the genome: the centromeric region of chromosome 5B, where the Pm36 resistance gene was previously localized, and chromosome 6B.

A putative transmembrane leucine zipper of agrobacterium VirB10 is essential for t-pilus biogenesis but not type IV secretion

The Agrobacterium tumefaciens VirB/VirD4 type IV secretion system is composed of a translocation channel and an extracellular T pilus. Bitopic VirB10, the VirB7 lipoprotein, and VirB9 interact to form a cell envelope-spanning structural scaffold termed the "core complex" that is required for the assembly of both structures. The related pKM101-encoded core complex is composed of 14 copies each of these VirB homologs, and the transmembrane (TM) α helices of VirB10-like TraF form a 55-Å-diameter ring at the inner membrane. Here, we report that the VirB10 TM helix possesses two types of putative dimerization motifs, a GxxxA (GA4) motif and two leucine (Leu1, Leu2) zippers. Mutations in the Leu1 motif disrupted T-pilus biogenesis, but these or other mutations in the GA4 or Leu2 motif did not abolish substrate transfer. Replacement of the VirB10 TM domain with a nondimerizing poly-Leu/Ala TM domain sequence also blocked pilus production but not substrate transfer or formation of immunoprecipitable complexes with the core subunits VirB7 and VirB9 and the substrate receptor VirD4. The VirB10 TM helix formed weak homodimers in Escherichia coli, as determined with the TOXCAT assay, whereas replacement of the VirB10 TM helix with the strongly dimerizing TM helix from glycophorin A blocked T-pilus biogenesis in A. tumefaciens. Our findings support a model in which VirB10's TM helix contributes to the assembly or activity of the translocation channel as a weakly self-interacting membrane anchor but establishes a heteromeric TM-TM helix interaction via its Leu1 motif that is critical for T-pilus biogenesis.

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

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

Crystal structure of a phosphorylation-coupled saccharide transporter

Saccharides have a central role in the nutrition of all living organisms. Whereas several saccharide uptake systems are shared between the different phylogenetic kingdoms, the phosphoenolpyruvate-dependent phosphotransferase system exists almost exclusively in bacteria. This multi-component system includes an integral membrane protein EIIC that transports saccharides and assists in their phosphorylation. Here we present the crystal structure of an EIIC from Bacillus cereus that transports diacetylchitobiose. The EIIC is a homodimer, with an expansive interface formed between the amino-terminal halves of the two protomers. The carboxy-terminal half of each protomer has a large binding pocket that contains a diacetylchitobiose, which is occluded from both sides of the membrane with its site of phosphorylation near the conserved His250 and Glu334 residues. The structure shows the architecture of this important class of transporters, identifies the determinants of substrate binding and phosphorylation, and provides a framework for understanding the mechanism of sugar translocation.

Localization of the substrate-binding site in the homodimeric mannitol transporter, EIImtl, of Escherichia coli

The mannitol transporter from Escherichia coli, EII(mtl), belongs to a class of membrane proteins coupling the transport of substrates with their chemical modification. EII(mtl) is functional as a homodimer, and it harbors one high affinity mannitol-binding site in the membrane-embedded C domain (IIC(mtl)). To localize this binding site, 19 single Trp-containing mutants of EII(mtl) were biosynthetically labeled with 5-fluorotryptophan (5-FTrp) and mixed with azi-mannitol, a substrate analog acting as a Förster resonance energy transfer (FRET) acceptor. Typically, for mutants showing FRET, only one 5-FTrp was involved, whereas the 5-FTrp from the other monomer was too distant. This proves that the mannitol-binding site is asymmetrically positioned in dimeric IIC(mtl). Combined with the available two-dimensional projection maps of IIC(mtl), it is concluded that a second resting binding site is present in this transporter. Active transport of mannitol only takes place when EII(mtl) becomes phosphorylated at Cys(384) in the cytoplasmic B domain. Stably phosphorylated EII(mtl) mutants were constructed, and FRET experiments showed that the position of mannitol in IIC(mtl) remains the same. We conclude that during the transport cycle, the phosphorylated B domain has to move to the mannitol-binding site, located in the middle of the membrane, to phosphorylate mannitol.

Spatial arrangement of the beta-glucoside transporter from Escherichia coli

The Escherichia coli BglF protein, a sugar permease of the phosphoenolpyruvate-dependent phosphotransferase system (PTS), catalyzes concomitant transport and phosphorylation of beta-glucosides across the cytoplasmic membrane. Despite intensive studies of PTS permeases, the mechanism that couples sugar translocation to phosphorylation and the nature of the translocation apparatus are poorly understood. Like many PTS permeases, BglF consists of a transmembrane domain, which in addition to transmembrane helices (TMs) contains a big cytoplasmic loop and two hydrophilic domains, one containing a conserved cysteine that phosphorylates the incoming sugar. We previously reported that the big hydrophilic loop, which connects TM VI to TM VII, contains regions that alternate between facing-in and facing-out states and speculated that it is involved in creating the sugar translocation channel. In the current study we used [2-(trimethylammonium)ethyl]methanethiosulfonate bromide (MTSET), a membrane-impermeative thiol-specific reagent, to identify sites that are involved in sugar transport. These sites map to the regions that border the big loop. Using cross-linking reagents that penetrate the cell, we could demonstrate spatial proximity between positions at the center of the big loop and the phosphorylation site, suggesting that the two regions come together to execute sugar phosphotransfer. Additionally, positions on opposite ends of the big loop were found to be spatially close. Cys accessibility analyses suggested that the sugar induces a change in this region. Taken together, our results demonstrate that the big loop participates in creating the sugar pathway and explain the observed coupling between translocation of PTS sugars from the periplasm to the cytoplasm and their phosphorylation.

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

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

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.

In vivo phosphorylation of CFTR promotes formation of a nucleotide-binding domain heterodimer

The human ATP-binding cassette (ABC) protein CFTR (cystic fibrosis transmembrane conductance regulator) is a chloride channel, whose dysfunction causes cystic fibrosis. To gain structural insight into the dynamic interaction between CFTR's nucleotide-binding domains (NBDs) proposed to underlie channel gating, we introduced target cysteines into the NBDs, expressed the channels in Xenopus oocytes, and used in vivo sulfhydryl-specific crosslinking to directly examine the cysteines' proximity. We tested five cysteine pairs, each comprising one introduced cysteine in the NH(2)-terminal NBD1 and another in the COOH-terminal NBD2. Identification of crosslinked product was facilitated by co-expression of NH(2)-terminal and COOH-terminal CFTR half channels each containing one NBD. The COOH-terminal half channel lacked all native cysteines. None of CFTR's 18 native cysteines was found essential for wild type-like, phosphorylation- and ATP-dependent, channel gating. The observed crosslinks demonstrate that NBD1 and NBD2 interact in a head-to-tail configuration analogous to that in homodimeric crystal structures of nucleotide-bound prokaryotic NBDs. CFTR phosphorylation by PKA strongly promoted both crosslinking and opening of the split channels, firmly linking head-to-tail NBD1-NBD2 association to channel opening.

The oligomeric state and stability of the mannitol transporter, EnzymeII(mtl), from Escherichia coli: a fluorescence correlation spectroscopy study

Numerous membrane proteins function as oligomers both at the structural and functional levels. The mannitol transporter from Escherichia coli, EnzymeII(mtl), is a member of the phosphoenolpyruvate-dependent phosphotransferase system. During the transport cycle, mannitol is phosphorylated and released into the cytoplasm as mannitol-1-phosphate. Several studies have shown that EII(mtl) functions as an oligomeric species. However, the oligomerization number and stability of the oligomeric complex during different steps of the catalytic cycle, e.g., substrate binding and/or phosphorylation of the carrier, is still under discussion. In this paper, we have addressed the oligomeric state and stability of EII(mtl) using fluorescence correlation spectroscopy. A functional double-cysteine mutant was site-specifically labeled with either Alexa Fluor 488 or Alexa Fluor 633. The subunit exchange of these two batches of proteins was followed in time during different steps of the catalytic cycle. The most important conclusions are that (1) in a detergent-solubilized state, EII(mtl) is functional as a very stable dimer; (2) the stability of the complex can be manipulated by changing the intermicellar attractive forces between PEG-based detergent micelles; (3) substrate binding destabilizes the complex whereas phosphorylation increases the stability; and (4) substrate binding to the phosphorylated species partly antagonizes the stabilizing effect.

Solution structure of a post-transition state analog of the phosphotransfer reaction between the A and B cytoplasmic domains of the mannitol transporter IIMannitol of the Escherichia coli phosphotransferase system

The solution structure of the post-transition state complex between the isolated cytoplasmic A (IIAMtl) and phosphorylated B (phospho-IIBMtl) domains of the mannitol transporter of the Escherichia coli phosphotransferase system has been solved by NMR. The active site His-554 of IIAMtl was mutated to glutamine to block phosphoryl transfer activity, and the active site Cys-384 of IIBMtl (residues of IIBMtl are denoted in italic type) was substituted by serine to permit the formation of a stable phosphorylated form of IIBMtl. The two complementary interaction surfaces are predominantly hydrophobic, and two methionines on IIBMtl, Met-388 and Met-393, serve as anchors by interacting with two deep pockets on the surface of IIAMtl. With the exception of a salt bridge between the conserved Arg-538 of IIAMtl and the phosphoryl group of phospho-IIBMtl, electrostatic interactions between the two proteins are limited to the outer edges of the interface, are few in number, and appear to be weak. This accounts for the low affinity of the complex (Kd approximately 3.7 mm), which is optimally tuned to the intact biological system in which the A and B domains are expressed as a single polypeptide connected by a flexible 21-residue linker. The phosphoryl transition state can readily be modeled with no change in protein-protein orientation and minimal perturbations in both the backbone immediately adjacent to His-554 and Cys-384 and the side chains in close proximity to the phosphoryl group. Comparison with the previously solved structure of the IIAMtl-HPr complex reveals how IIAMtl uses the same interaction surface to recognize two structurally unrelated proteins and explains the much higher affinity of IIAMtl for HPr than IIBMtl.

Oligomerization of H(+)-pyrophosphatase and its structural and functional consequences

The H(+)-pyrophosphatase (H(+)-PPase) consists of a single polypeptide, containing 16 or 17 transmembrane domains. To determine the higher order oligomeric state of Streptomyces coelicolor H(+)-PPase, we constructed a series of cysteine substitution mutants and expressed them in Escherichia coli. Firstly, we analyzed the formation of disulfide bonds, promoted by copper, in mutants with single cysteine substitutions. 28 of 39 mutants formed disulfide bonds, including S545C, a substitution at the periplasmic side. The formation of intermolecular disulfide bonds suppressed the enzyme activity of several, where the substituted residues were located in the cytosol. Creating disulfide links in the cytosol may interfere with the enzyme's catalytic function. Secondly, we prepared double mutants by introducing second cysteine substitutions into the S545C mutant. These double-cysteine mutants produced cross-linked complexes, estimated to be at least tetramers and possibly hexamers. Thirdly, we co-expressed epitope-tagged, wild type, and inactive mutant H(+)-PPases in E. coli and confirmed the formation of oligomers by co-purifying one subunit using the epitope tag used to label the other. The enzyme activity of these oligomers was markedly suppressed. We propose that H(+)-PPase is present as an oligomer made up of at least two or three sets of dimers.

Modulation of monomer conformation of the BglG transcriptional antiterminator from Escherichia coli

The BglG protein positively regulates expression of the bgl operon in Escherichia coli by binding as a dimer to the bgl transcript and preventing premature termination of transcription in the presence of beta-glucosides. BglG activity is negatively controlled by BglF, the beta-glucoside phosphotransferase, which reversibly phosphorylates BglG according to beta-glucoside availability, thus modulating its dimeric state. BglG consists of an RNA-binding domain and two homologous domains, PRD1 and PRD2. Based on structural studies of a BglG homologue, the two PRDs fold similarly, and the interactions within the dimer are PRD1-PRD1 and PRD2-PRD2. We have recently shown that the affinity between PRD1 and PRD2 of BglG is high, and a fraction of the BglG monomers folds in the cell into a compact conformation, in which PRD1 and PRD2 are in close proximity. We show here that both BglG forms, the compact and noncompact, bind to the active site-containing domain of BglF, IIB(bgl), in vitro. The interaction of BglG with IIB(bgl) or BglF is mediated by PRD2. Both BglG forms are detected as phosphorylated proteins after in vitro phosphorylation with IIB(bgl) and are dephosphorylated by BglF in vitro in the presence of beta-glucosides. Nevertheless, genetic evidence indicates that the interaction of IIB(bgl) and BglF with the compact form is seemingly less favorable. Using in vivo cross-linking, we show that BglF enhances folding of BglG into a compact conformation, whereas the addition of beta-glucosides reduces the amount of this form. Based on these results we suggest a model for the modulation of BglG conformation and activity by BglF.

Unraveling Functional and Structural Interactions between Transmembrane Domains IV and XI of NhaA Na+/H+ Antiporter of Escherichia coli

A functionally important, interface domain between transmembrane segments (TMSs) IV and XI of the NhaA Na+/H+ antiporter of Escherichia coli has been unraveled. Scanning by single Cys replacements identified new mutations (F136C, G125C, and A137C) that cluster in one face of TMS IV and increase dramatically the Km of the antiporter. Whereas G125C, in addition, causes a drastic alkaline shift to the pH dependence of the antiporter, G338C alleviates the pH control of NhaA. Scanning by double Cys replacements (21 pairs of one replacement per TMS) identified genetically eight pairs of residues that showed very strong negative complementation. Cross-linking of the double mutants identified six double mutants (T132C/G338C, D133C/G338C, F136C/S342C, T132C/S342C, A137C/S342C, and A137C/G338C) of which pronounced intramolecular cross-linking defined an interface domain between the two TMSs. Remarkably, cross-linking by a short and rigid reagent (N,N'-o-phenylenedimaleimide) revived the Li+/H+ antiport activity, whereas a shorter reagent (1,2-ethanediyl bismethanethiosulfonate) revived both Na+/H+ and Li+/H+ antiporter activities and even the pH response of the dead mutant T132C/G338C. Hence, cross-linking at this position restores an active conformation of NhaA.

Identification of the dimer interface of the lactose transport protein from Streptococcus thermophilus

The lactose transporter from Streptococcus thermophilus catalyses the symport of galactosides and protons. The carrier domain of the protein harbours the contact sites for dimerization, and the individual subunits in the dimer interact functionally during the transport reaction. As a first step towards the elucidation of the mechanism behind the cooperation between the subunits, regions involved in the dimer interface were determined by oxidative and chemical cross-linking of 12 cysteine substitution mutants. Four positions in the protein were found to be susceptible to intermolecular cross-linking. To ensure that the observed cross-links were not the result of randomly colliding particles, the cross-linking was studied in samples in which either the concentration of LacS in the membrane was varied or the oligomeric state was manipulated. These experiments showed that the cross-links were formed specifically within the dimer. The four regions of the protein located at the dimer interface are close to the extracellular ends of transmembrane segments V and VIII and the intracellular ends of transmembrane segments VI and VII.

Spatial and dynamic interactions between phospholamban and the canine cardiac Ca2+ pump revealed with use of heterobifunctional cross-linking agents

Heterobifunctional thiol to amine cross-linking agents were used to gain new insights on the dynamics and conformational factors governing the interaction between the cardiac Ca2+ pump (SERCA2a) and phospholamban (PLB). PLB is a small protein inhibitor of SERCA2a that reduces enzyme affinity for Ca2+ and thereby regulates cardiac contractility. We found that the PLB monomer with Asn27 or Asn30 changed to Cys (N27C-PLB or N30C-PLB) cross-linked to lysine of SERCA2a within seconds with > or =80% efficiency. Optimal cross-linking occurred at spacer chain lengths of 10 and 15 A for N27C and N30C, respectively. The rapid time course of cross-linking indicated that neither dissociation of PLB pentamers nor binding of PLB monomers to SERCA2a was rate-limiting. Cross-linking occurred only to the E2 (Ca2+-free) conformation of SERCA2a, was strongly favored by nucleotide binding to this state, and was completely inhibited by thapsigargin. Protein sequencing in combination with mutagenesis identified of Lys328 of SERCA2a as the target of cross-linking. A three-dimensional map of interacting residues indicated that the cross-linking distances were entirely compatible with the 10-A distance recently determined between N30C of PLB and Cys318 of SERCA2a. In contrast, Lys3 of PLB did not cross-link to any Lys (or Cys) of SERCA2a, suggesting that previous three-dimensional models that constrain Lys3 near residues 397-400 of thapsigargin-inhibited SERCA2a should be viewed with caution. Furthermore, although earlier models of PLB.SERCA2a are based on thapsigargin-bound SERCA, our results suggest that the nucleotide-bound, E2 conformation is substantially different and represents the key conformational state for interacting with PLB.

A fraction of the BglG transcriptional antiterminator from Escherichia coli exists as a compact monomer

Expression of the bgl operon in Escherichia coli, induced by beta-glucosides, is positively regulated by BglG, a transcriptional antiterminator. In the presence of inducer, BglG dimerizes and binds to the bgl transcript to prevent premature termination of transcription. The dimeric state of BglG is determined by BglF, a membrane-bound enzyme II of the phosphoenolpyruvate-dependent phosphotransferase system (PTS), which reversibly phosphorylates BglG according to beta-glucoside availability. BglG is composed of an RNA-binding domain followed by two homologous PTS regulation domains (PRD1 and PRD2). The predicted structure of dimeric LicT, a BglG homologue from Bacillus subtilis, suggests that the two PRDs adopt a similar structure and that the interactions within the dimer are PRD1-PRD1 and PRD2-PRD2. We have shown recently that the PRD1 and PRD2 domains of BglG can form a stable heterodimer. We report here, based on in vitro and in vivo cross-linking experiments, that a fraction of BglG is present in the cell in a compact form in which PRD1 and PRD2 are in close proximity. The compact form is present mainly in the BglG monomers. Our results imply that the monomer-dimer transition involves a conformational change. The possible role of the compact form in preventing untimely induction of the bgl operon is discussed.

Crosslinking of membrane-embedded cysteines reveals contact points in the EmrE oligomer

EmrE is a small multidrug transporter that extrudes various drugs in exchange with protons, thereby rendering Escherichia coli cells resistant to these compounds. In this study, relative helix packing in the EmrE oligomer solubilized in detergent was probed by intermonomer crosslinking analysis. Unique cysteine replacements in transmembrane domains were shown to react with organic mercurials but not with sulfhydryl reagents, such as maleimides and methanethiosulfonates. A new protocol was developed based on the use of HgCl(2), a compound known to react rapidly and selectively with sulfhydryl groups. The reaction can bridge vicinal pairs of cysteines and form an intermolecular mercury-linked dimer. To circumvent problems inherent to mercury chemistry, a second crosslinker, hexamethylene diisocyanate, was used. After the HgCl(2) treatment, excess reagent was removed and the oligomers were dissociated with a strong denaturant. Only those previously crosslinked reacted with hexamethylene diisocyanate. Thus, vicinal cysteine-substituted residues in the EmrE oligomer were identified. It was shown that transmembrane domain (TM)-1 and TM4 in one subunit are in contact with the corresponding TM1 and TM4, respectively, in the other subunit. In addition, TM1 is also in close proximity to TM4 of the neighboring subunit, suggesting possible arrangements in the binding and translocation domain of the EmrE oligomer. This method should be useful for other proteins with cysteine residues in a low-dielectric environment.