Glucose transport in the extremely thermoacidophilic Sulfolobus solfataricus involves a high-affinity membrane-integrated binding protein

Posttranslational protein modification in Archaea

One of the first hurdles to be negotiated in the postgenomic era involves the description of the entire protein content of the cell, the proteome. Such efforts are presently complicated by the various posttranslational modifications that proteins can experience, including glycosylation, lipid attachment, phosphorylation, methylation, disulfide bond formation, and proteolytic cleavage. Whereas these and other posttranslational protein modifications have been well characterized in Eucarya and Bacteria, posttranslational modification in Archaea has received far less attention. Although archaeal proteins can undergo posttranslational modifications reminiscent of what their eucaryal and bacterial counterparts experience, examination of archaeal posttranslational modification often reveals aspects not previously observed in the other two domains of life. In some cases, posttranslational modification allows a protein to survive the extreme conditions often encountered by Archaea. The various posttranslational modifications experienced by archaeal proteins, the molecular steps leading to these modifications, and the role played by posttranslational modification in Archaea form the focus of this review.

Analysis of ATPases of putative secretion operons in the thermoacidophilic archaeon Sulfolobus solfataricus

Gram-negative bacteria use a wide variety of complex mechanisms to secrete proteins across their membranes or to assemble secreted proteins into surface structures. As most archaea only possess a cytoplasmic membrane surrounded by a membrane-anchored S-layer, the organization of such complexes might be significantly different from that in Gram-negative bacteria. Five proteins of Sulfolobus solfataricus, SSO0120, SSO0572, SSO2316, SSO2387 and SSO2680, which are homologous to secretion ATPases of bacterial type II, type IV secretion systems and the type IV pili assembly machinery, were identified. The operon structures of these putative secretion systems encoding gene clusters and the expression patterns of the ATPases under different growth conditions were determined, and it was established that all five putative ATPases do show a divalent cation-dependent ATPase activity at high temperature. These results show that the archaeal secretion systems are related to the bacterial secretion systems and might be powered in a similar way.

The high-affinity maltose/trehalose ABC transporter in the extremely thermophilic bacterium Thermus thermophilus HB27 also recognizes sucrose and palatinose

We have studied the transport of trehalose and maltose in the thernophilic bacterium Thermus thermophilus HB27, which grows optimally in the range of 70 to 75 degrees C. The K(m) values at 70 degrees C were 109 nM for trehalose and 114 nM for maltose; also, a high K(m) (424 nM) was found for the uptake of sucrose. Competition studies showed that a single transporter recognizes trehalose, maltose, and sucrose, while d-galactose, d-fucose, l-rhamnose, l-arabinose, and d-mannose were not competitive inhibitors. In the recently published genome of T. thermophilus HB27, two gene clusters designated malEFG1 (TTC1627 to -1629) and malEFG2 (TTC1288 to -1286) and two monocistronic genes designated malK1 (TTC0211) and malK2 (TTC0611) are annotated as trehalose/maltose and maltose/maltodextrin transport systems, respectively. To find out whether any of these systems is responsible for the transport of trehalose, the malE1 and malE2 genes, lacking the sequence encoding the signal peptides, were expressed in Escherichia coli. The binding activity of pure recombinant proteins was analyzed by equilibrium dialysis. MalE1 was able to bind maltose, trehalose, and sucrose but not glucose or maltotetraose (K(d) values of 103, 67, and 401 nM, respectively). Mutants with disruptions in either malF1 or malK1 were unable to grow on maltose, trehalose, sucrose, or palatinose, whereas mutants with disruption in malK2 or malF2 showed no growth defect on any of these sugars. Therefore, malEFG1 encodes the binding protein and the two transmembrane subunits of the trehalose/maltose/sucrose/palatinose ABC transporter, and malK1 encodes the ATP-binding subunit of this transporter. Despite the presence of an efficient transporter for trehalose, this compound was not used by HB27 for osmoprotection. MalE1 and MalE2 exhibited extremely high thermal stability: melting temperatures of 90 degrees C for MalE1 and 105 degrees C for MalE2 in the presence of 2.3 M guanidinium chloride. The latter protein did not bind any of the sugars examined and is not implicated in a maltose/maltodextrin transport system. This work demonstrates that malEFG1 and malK1 constitute the high-affinity ABC transport system of T. thermophilus HB27 for trehalose, maltose, sucrose, and palatinose.

Functional analysis of detergent-solubilized and membrane-reconstituted ATP-binding cassette transporters

ATP-binding cassette (ABC) transporters are vital to any living system and are involved in the translocation of a wide variety of substances, from ions and nutrients to high molecular weight proteins. This chapter describes methods used to purify and membrane reconstitute ABC transporters in a fully functional state. The procedures are largely based on our experience with substrate-binding protein-dependent ABC uptake systems from bacteria, but the approaches should be applicable to multisubunit membrane complexes in general. Also, we present simple methods, based on substrate binding or translocation, to follow the activity of the protein complexes in detergent-solubilized and/or membrane-reconstituted state(s).

Preliminary crystallographic studies of glucose dehydrogenase from the promiscuous Entner-Doudoroff pathway in the hyperthermophilic archaeon Sulfolobus solfataricus

The hyperthermophilic archaeon Sulfolobus solfataricus grows optimally above 353 K and can metabolize glucose and its C4 epimer galactose via a non-phosphorylative variant of the Entner-Doudoroff pathway involving catalytically promiscuous enzymes that can operate with both sugars. The initial oxidation step is catalysed by glucose dehydrogenase (SsGDH), which can utilize both NAD and NADP as cofactors. The enzyme operates with glucose and galactose at similar catalytic efficiency, while its substrate profile also includes a range of other five- and six-carbon sugars. Crystals of the 164 kDa SsGDH homotetramer have been grown under a variety of conditions. The best crystals to date diffract to 1.8 A on a synchrotron source, have orthorhombic symmetry and belong to space group P2(1)2(1)2. Attempts are being made to solve the structure by MAD and MR.

Gluconate dehydratase from the promiscuous Entner-Doudoroff pathway in Sulfolobus solfataricus

An investigation has been carried out into gluconate dehydratase from the hyperthermophilic Archaeon Sulfolobus solfataricus. The enzyme has been purified from cell extracts of the organism and found to be responsible for both gluconate and galactonate dehydratase activities. It was shown to be a 45 kDa monomer with a half-life of 41 min at 95 degrees C and it exhibited similar catalytic efficiency with both substrates. Taken alongside the recent work on glucose dehydrogenase and 2-keto-3-deoxygluconate aldolase, this report clearly demonstrates that the entire non-phosphorylative Entner-Doudoroff pathway of S. solfataricus is promiscuous for the metabolism of both glucose and galactose.

X-ray structures of the maltose-maltodextrin-binding protein of the thermoacidophilic bacterium Alicyclobacillus acidocaldarius provide insight into acid stability of proteins

Maltose-binding proteins act as primary receptors in bacterial transport and chemotaxis systems. We report here crystal structures of the thermoacidostable maltose-binding protein from Alicyclobacillus acidocaldarius, and explore its modes of binding to maltose and maltotriose. Further, comparison with the structures of related proteins from Escherichia coli (a mesophile), and two hyperthermophiles (Pyrococcus furiosus and Thermococcus litoralis) allows an investigation of the basis of thermo- and acidostability in this family of proteins.The thermoacidophilic protein has fewer charged residues than the other three structures, which is compensated by an increase in the number of polar residues. Although the content of acidic and basic residues is approximately equal, more basic residues are exposed on its surface whereas most acidic residues are buried in the interior. As a consequence, this protein has a highly positive surface charge. Fewer salt bridges are buried than in the other MBP structures, but the number exposed on its surface does not appear to be unusual. These features appear to be correlated with the acidostability of the A. acidocaldarius protein rather than its thermostability. An analysis of cavities within the proteins shows that the extremophile proteins are more closely packed than the mesophilic one. Proline content is slightly higher in the hyperthermophiles and thermoacidophiles than in mesophiles, and this amino acid is more common at the second position of beta-turns, properties that are also probably related to thermostability. Secondary structural content does not vary greatly in the different structures, and so is not a contributing factor.

On the role of the two extracytoplasmic substrate-binding domains in the ABC transporter OpuA

Members of two transporter families of the ATP-binding cassette (ABC) superfamily use two or even four extracytoplasmic substrate-binding domains (SBDs) for transport. We report on the role of the two SBDs in the translocation cycle of the ABC transporter OpuA from Lactococcus lactis. Heterooligomeric OpuA complexes with only one SBD or one functional and one non-functional SBD (inactivated by covalent linkage of a substrate mimic) have been constructed, and the substrate binding and transport kinetics of the purified transporters, reconstituted in liposomes, have been determined. The data indicate that the two SBDs of OpuA interact in a cooperative manner in the translocation process by stimulating either the docking of the SBDs onto the translocator or the delivery of glycine betaine to the translocator. It appears that one of these initial steps, but not the later steps in translocation or resetting of the system to the initial state, is rate determining for transport. These new insights on the functional role of the extracytoplasmic SBDs are discussed in the light of the current knowledge of substrate-binding-protein-dependent ABC transporters.

Formation of the productive ATP-Mg2+-bound dimer of GlcV, an ABC-ATPase from Sulfolobus solfataricus

The ABC-ATPase GlcV from Sulfolobus solfataricus energizes an ABC transporter mediating glucose uptake. In ABC transporters, two ABC-ATPases are believed to form a head-to-tail dimer, with both monomers contributing conserved residues to each of the two productive active sites. In contrast, isolated GlcV, although active, behaves apparently as a monomer in the presence of ATP-Mg(2+), AMPPNP-Mg(2+) or ATP alone. To resolve the oligomeric state of the active form of GlcV, we analysed the effects of changing the putative catalytic base, residue E166, into glutamine or alanine. Both mutants are, to different extents, defective in ATP hydrolysis, and gel-filtration experiments revealed their dimerization in the presence of ATP-Mg(2+). Mutant E166Q forms dimers also in the presence of ATP alone, without Mg(2+), whereas dimerization of mutant E166A requires both ATP and Mg(2+). These results confirm earlier reports for other ABC-ATPases, but for the first time suggest the occurrence of a fast equilibrium between ATP-bound monomers and ATP-bound dimers. We further mutated two highly conserved residues of the ABC signature motif, S142 and G144, into alanine. The G144A mutant is completely inactive and fails to dimerize, indicating an essential role of this residue in stabilizing the productive dimeric state. Mutant S142A retained considerable activity, and was able to dimerize, thus implying that the interaction of the serine with ATP is not essential for dimerization and catalysis. Furthermore, although the E166A and G144A mutants each alone are inactive, they produce an active heterodimer, showing that disruption of one active site can be tolerated. Our data suggest that ABC-ATPases with partially degenerated catalytic machineries, as they occur in vivo, can still form productive dimers to drive transport.

The unique features of glycolytic pathways in Archaea

An early divergence in evolution has resulted in two prokaryotic domains, the Bacteria and the Archaea. Whereas the central metabolic routes of bacteria and eukaryotes are generally well-conserved, variant pathways have developed in Archaea involving several novel enzymes with a distinct control. A spectacular example of convergent evolution concerns the glucose-degrading pathways of saccharolytic archaea. The identification, characterization and comparison of the glycolytic enzymes of a variety of phylogenetic lineages have revealed a mosaic of canonical and novel enzymes in the archaeal variants of the Embden-Meyerhof and the Entner-Doudoroff pathways. By means of integrating results from biochemical and genetic studies with recently obtained comparative and functional genomics data, the structure and function of the archaeal glycolytic routes, the participating enzymes and their regulation are re-evaluated.

Metabolic pathway promiscuity in the archaeon Sulfolobus solfataricus revealed by studies on glucose dehydrogenase and 2-keto-3-deoxygluconate aldolase

The hyperthermophilic Archaeon Sulfolobus solfataricus metabolizes glucose by a non-phosphorylative variant of the Entner-Doudoroff pathway. In this pathway glucose dehydrogenase and gluconate dehydratase catalyze the oxidation of glucose to gluconate and the subsequent dehydration of gluconate to 2-keto-3-deoxygluconate. 2-Keto-3-deoxygluconate (KDG) aldolase then catalyzes the cleavage of 2-keto-3-deoxygluconate to glyceraldehyde and pyruvate. The gene encoding glucose dehydrogenase has been cloned and expressed in Escherichia coli to give a fully active enzyme, with properties indistinguishable from the enzyme purified from S. solfataricus cells. Kinetic analysis revealed the enzyme to have a high catalytic efficiency for both glucose and galactose. KDG aldolase from S. solfataricus has previously been cloned and expressed in E. coli. In the current work its stereoselectivity was investigated by aldol condensation reactions between D-glyceraldehyde and pyruvate; this revealed the enzyme to have an unexpected lack of facial selectivity, yielding approximately equal quantities of 2-keto-3-deoxygluconate and 2-keto-3-deoxygalactonate. The KDG aldolase-catalyzed cleavage reaction was also investigated, and a comparable catalytic efficiency was observed with both compounds. Our evidence suggests that the same enzymes are responsible for the catabolism of both glucose and galactose in this Archaeon. The physiological and evolutionary implications of this observation are discussed in terms of catalytic and metabolic promiscuity.

Crystal structures of the ATPase subunit of the glucose ABC transporter from Sulfolobus solfataricus: nucleotide-free and nucleotide-bound conformations

The ABC-ATPase GlcV energizes a binding protein-dependent ABC transporter that mediates glucose uptake in Sulfolobus solfataricus. Here, we report high-resolution crystal structures of GlcV in different states along its catalytic cycle: distinct monomeric nucleotide-free states and monomeric complexes with ADP-Mg(2+) as a product-bound state, and with AMPPNP-Mg(2+) as an ATP-like bound state. The structure of GlcV consists of a typical ABC-ATPase domain, comprising two subdomains, connected by a linker region to a C-terminal domain of unknown function. Comparisons of the nucleotide-free and nucleotide-bound structures of GlcV reveal re-orientations of the ABCalpha subdomain and the C-terminal domain relative to the ABCalpha/beta subdomain, and switch-like rearrangements in the P-loop and Q-loop regions. Additionally, large conformational differences are observed between the GlcV structures and those of other ABC-ATPases, further emphasizing the inherent flexibility of these proteins. Notably, a comparison of the monomeric AMPPNP-Mg(2+)-bound GlcV structure with that of the dimeric ATP-Na(+)-bound LolD-E171Q mutant reveals a +/-20 degrees rigid body re-orientation of the ABCalpha subdomain relative to the ABCalpha/beta subdomain, accompanied by a local conformational difference in the Q-loop. We propose that these differences represent conformational changes that may have a role in the mechanism of energy-transduction and/or allosteric control of the ABC-ATPase activity in bacterial importers.

Archaeal homolog of bacterial type IV prepilin signal peptidases with broad substrate specificity

A large number of secretory proteins in the thermoacidophile Sulfolobus solfataricus are synthesized as a precursor with an unusual leader peptide that resembles bacterial type IV prepilin signal sequences. This set of proteins includes the flagellin subunit but also various solute binding proteins. Here we describe the identification of the S. solfataricus homolog of bacterial type IV prepilin peptidases, termed PibD. PibD is an integral membrane protein that is phylogenetically related to the bacterial enzymes. When heterologously expressed in Escherichia coli, PibD is capable of processing both the flagellin and glucose-binding protein (GlcS) precursors. Site-directed mutagenesis of the GlcS signal peptide shows that the substrate specificity of PibD is consistent with the variations found in proteins with type IV prepilin-like signal sequences of S. solfataricus. We conclude that PibD is responsible for the processing of these secretory proteins in S. solfataricus.

Effect of amino acid starvation on glucose transport in two archaeal organisms

When protein synthesis is arrested by amino acid starvation, Escherichia coli wild-type strains show stringent control (SC) over stable RNA (sRNA) accumulation as well as a large number of other growth-related processes. One of the events under SC is transport of metabolites. Thus, under amino acid starvation, E. coli fails to accumulate the non-metabolizable glucose analog alpha-methyl-D-glucoside, whereas isogenic relaxed strains continue to take up this glucose analog. Unlike the Bacteria, most wild-type archaeal strains show relaxed control of sRNA accumulation, although a number of stringent strains have been identified. In order to determine whether stringency in the Archaea affects physiological events different from sRNA accumulation, transport of glucose analogs was examined under amino acid starvation in two stringent archaeal strains, Haloferax volcanii and Sulfolobus acidocaldarius. The experiments were performed with 2-deoxy-D-glucose, which was shown to be transported, but metabolized very limitedly. Unlike E. coli, H. volcanii and S. acidocaldarius continued to transport 2-deoxy-D-glucose under amino acid starvation. Thus, in both Archaea glucose analog transport is not under SC, as it is in E. coli.

ABC transporters: one, two or four extracytoplasmic substrate-binding sites?

Two families of ATP-binding cassette (ABC) transporters in which one or two extracytoplasmic substrate-binding domains are fused to either the N- or C-terminus of the translocator protein have been detected. This suggests that two, or even four, substrate-binding sites may function in the ABC transporter complex. This domain organization in ABC transporters, widely represented among microorganisms, raises new possibilities for how the substrate-binding protein(s) (SBPs) might interact with the translocator. One appealing hypothesis is that multiple substrate-binding sites in proximity to the entry site of the translocation pore enhance the transport capacity. We also discuss the implications of multiple substrate-binding sites in close proximity to the translocator in terms of broadened substrate specificity and possible cooperative interactions between SBPs and the translocator.

S-decyl-glutathione nonspecifically stimulates the ATPase activity of the nucleotide-binding domains of the human multidrug resistance-associated protein, MRP1 (ABCC1)

The human multidrug resistance-associated protein(MRP1) is an ATP-dependent efflux pump that transports anionic conjugates, and hydrophobic compounds in a glutathione dependent manner. Similar to the other, well-characterized multidrug transporter P-gp, MRP1 comprises two nucleotide-binding domains (NBDs) in addition to transmembrane domains. However, whereas the NBDs of P-gp have been shown to be functionally equivalent, those of MRP1 differ significantly. The isolated NBDs of MRP1 have been characterized in Escherichia coli as fusions with either the glutathione-S-transferase (GST) or the maltose-binding domain (MBP). The nonfused NBD1 was obtained by cleavage of the fusion protein with thrombin. The GST-fused forms of NBD1 and NBD2 hydrolyzed ATP with an apparent K(m) of 340 microm and a V(max) of 6.0 nmol P(I) x mg-1 x min-1, and a K(m) of 910 microm ATP and a V(max) of 7.5 nmol P(I) x mg-1 x min-1, respectively. Remarkably, S-decyl-glutathione, a conjugate specifically transported by MRP1 and MRP2, was able to stimulate the ATPase activities of the isolated NBDs more than 2-fold in a concentration-dependent manner. However,the stimulation of the ATPase activity was found to coincide with the formation of micelles by S-decyl-glutathione. Equivalent stimulation of ATPase activity could be obtained by surfactants with similar critical micelle concentrations.

Sugar transport in (hyper)thermophilic archaea

Hyperthermophilic archaea show important metabolic adaptations for growth on carbohydrates under hostile conditions. For carbohydrate uptake so far only ABC-type transporters have been described that are equipped with a uniquely high affinity as compared to mesophilic bacterial systems. This allows these organisms to efficiently scavenge all available carbohydrates from the extreme environment.

Cellobiose uptake in the hyperthermophilic archaeon Pyrococcus furiosus is mediated by an inducible, high-affinity ABC transporter

The hyperthermophilic archaeon Pyrococcus furiosus can utilize different beta-glucosides, like cellobiose and laminarin. Cellobiose uptake occurs with high affinity (K(m) = 175 nM) and involves an inducible binding protein-dependent transport system. The cellobiose binding protein (CbtA) was purified from P. furiosus membranes to homogeneity as a 70-kDa glycoprotein. CbtA not only binds cellobiose but also cellotriose, cellotetraose, cellopentaose, laminaribiose, laminaritriose, and sophorose. The cbtA gene was cloned and functionally expressed in Escherichia coli. cbtA belongs to a gene cluster that encodes a transporter that belongs to the Opp family of ABC transporters.

Purification and characterization of the heterologously expressed trehalose/maltose ABC transporter complex of the hyperthermophilic archaeon Thermococcus litoralis

We report the purification of the maltose/trehalose transporter complex MalFGK of the hyperthermophilic archaeon Thermococcus litoralis. The complex was expressed in Escherichia coli, solubilized in dodecyl maltoside and purified with the aid of a histidine tag on one of the membrane proteins. One hundred grams of cells yielded 3 mg of pure complex. The final product showed ATPase activity at 70 degrees C and was soluble at low detergent concentration. ATPase activity was not due to dissociation of the MalK subunit from the integral membrane proteins MalF and MalG but could not be further stimulated by trehalose/maltose binding protein (TMBP), be it the native protein as isolated from T. litoralis or the soluble engineered protein. The purified native TMBP was identified as a glycoprotein.

Peptides and ATP binding cassette peptide transporters

In this review our knowledge of ATP binding cassette (ABC) transporters specific for peptides is discussed. Besides serving a role in nutrition of the cell, the systems participate in various signaling processes that allow (micro)organisms to monitor the local environment. In bacteria, these include regulation of gene expression, competence development, sporulation, DNA transfer by conjugation, chemotaxis, and virulence development, and the role of ABC transporters in each of these processes is discussed. Particular attention is paid to the specificity determinants of peptide receptors and transporters in relation to their structure and to the mechanisms of peptide binding.

ABC transporters catalyzing carbohydrate uptake

ATP binding cassette (ABC) transporters mediating the uptake of carbohydrates comprise two subfamilies (CUT1, CUT2) that differ with respect to the chemical nature of their substrates, subunit composition, and conserved sequence motifs. In this article, current knowledge of members of each family is summarized with special emphasis on the well-characterized transport systems for maltose/maltodextrin and ribose, respectively, of enterobacteria.

Sugar transport in Sulfolobus solfataricus is mediated by two families of binding protein-dependent ABC transporters

The extreme thermoacidophilic archaeon Sulfolobus solfataricus grows optimally at 80 degrees C and pH 3 and uses a variety of sugars as sole carbon and energy source. Glucose transport in this organism is mediated by a high-affinity binding protein-dependent ATP-binding cassette (ABC) transporter. Sugar-binding studies revealed the presence of four additional membrane-bound binding proteins for arabinose, cellobiose, maltose and trehalose. These glycosylated binding proteins are subunits of ABC transporters that fall into two distinct groups: (i) monosaccharide transporters that are homologous to the sugar transport family containing a single ATPase and a periplasmic-binding protein that is processed at an unusual site at its amino-terminus; (ii) di- and oligosaccharide transporters, which are homologous to the family of oligo/dipeptide transporters that contain two different ATPases, and a binding protein that is synthesized with a typical bacterial signal sequence. The latter family has not been implicated in sugar transport before. These data indicate that binding protein-dependent transport is the predominant mechanism of transport for sugars in S. solfataricus.

The crystal structure of a liganded trehalose/maltose-binding protein from the hyperthermophilic Archaeon Thermococcus litoralis at 1.85 A

We report the crystallization and structure determination at 1.85 A of the extracellular, membrane-anchored trehalose/maltose-binding protein (TMBP) in complex with its substrate trehalose. TMBP is the substrate recognition site of the high-affinity trehalose/maltose ABC transporter of the hyperthermophilic Archaeon Thermococcus litoralis. In vivo, this protein is anchored to the membrane, presumably via an N-terminal cysteine lipid modification. The crystallized protein was N-terminally truncated, resulting in a soluble protein exhibiting the same binding characteristics as the wild-type protein. The protein shows the characteristic features of a transport-related, substrate-binding protein and is structurally related to the maltose-binding protein (MBP) of Escherichia coli. It consists of two similar lobes, each formed by a parallel beta-sheet flanked by alpha-helices on both sides. Both are connected by a hinge region consisting of two antiparallel beta-strands and an alpha-helix. As in MBP, the substrate is bound in the cleft between the lobes by hydrogen bonds and hydrophobic interactions. However, compared to maltose binding in MBP, direct hydrogen bonding between the substrate and the protein prevails while apolar contacts are reduced. To elucidate factors contributing to thermostability, we compared TMBP with its mesophilic counterpart MBP and found differences known from similar investigations. Specifically, we find helices that are longer than their structurally equivalent counterparts, and fewer internal cavities.

Maltose and maltodextrin transport in the thermoacidophilic gram-positive bacterium Alicyclobacillus acidocaldarius is mediated by a high-affinity transport system that includes a maltose binding protein tolerant to low pH

We have studied the uptake of maltose in the thermoacidophilic gram-positive bacterium Alicyclobacillus acidocaldarius, which grows best at 57 degrees C and pH 3.5. Under these conditions, accumulation of [(14)C]maltose was observed in cells grown with maltose but not in those grown with glucose. At lower temperatures or higher pH values, the transport rates substantially decreased. Uptake of radiolabeled maltose was inhibited by maltotetraose, acarbose, and cyclodextrins but not by lactose, sucrose, or trehalose. The kinetic parameters (K(m) of 0.91 +/- 0.06 microM and V(max) ranging from 0.6 to 3.7 nmol/min/mg of protein) are consistent with a binding protein-dependent ATP binding cassette (ABC) transporter. A corresponding binding protein (MalE) that interacts with maltose with high affinity (K(d) of 1.5 microM) was purified from the culture supernatant of maltose-grown cells. Immunoelectron microscopy revealed distribution of the protein throughout the cell wall. The malE gene was cloned and sequenced. Five additional open reading frames, encoding components of a maltose transport system (MalF and MalG), a putative transcriptional regulator (MalR), a cyclodextrinase (CdaA), and an alpha-glucosidase (GlcA), were identified downstream of malE. The malE gene lacking the DNA sequence that encodes the signal sequence was expressed in Escherichia coli. The purified wild-type and recombinant proteins bind maltose with high affinity over a wide pH range (2.5 to 7) and up to 80 degrees C. Recombinant MalE cross-reacted with an antiserum raised against the wild-type protein, thereby indicating that the latter is the product of the malE gene. The MalE protein might be well suited as a model to study tolerance of proteins to low pH.

Diversity of transport mechanisms: common structural principles

Traditionally, prokaryotic solute transport systems are classified into major groups based on the energetic requirement of the transport process. These include the secondary transporters that are driven by a proton or sodium motive force, and the ATP-binding cassette (ABC) primary transporters, which use the hydrolysis of ATP to fuel transport. These transporters are specified by entirely different architectures of polypeptides. Recently, transport systems have been discovered that are composed of combinations of distinct functional modules of both secondary and ABC transporters. These findings indicate that during evolution the combination of integral membrane transport proteins with either a periplasmic solute-binding protein or a cytosolic ATPase, or both, have resulted in distinct classes of transporters with unique architectures and properties.

On the binding mechanism of the peptide receptor of the oligopeptide transport system of Lactococcus lactis

Lactococcus lactis degrades exogenous proteins such as beta-casein to peptides of 4-30 amino acids, and uses these as nitrogen sources. The binding protein or receptor (OppA(Ll)) of the oligopeptide transport system (Opp) of L.LACTIS: has the unique capacity to bind peptides from five up to at least 20 residues. To study the binding mechanism of OppA(Ll), nonameric peptides were used in which the cysteine at position 1, 3, 4, 5, 6, 7 or 9 was selectively labeled with either bulky and non-fluorescent or bulky and fluorescent groups. Also, nonameric peptides with a non-natural residue, azatryptophan, at positions 3 or 7 were used. The fluorescence of azatryptophan reports on the polarity of the environment. The studies indicate that the binding protein encloses the first six amino acids of the peptide, whereas the remaining residues stick out and interact with the surface of the binding protein. The peptide binding mechanism of OppA(Ll) is discussed in relation to known three-dimensional structures of members of this class of proteins, and an adaptation of the general binding mechanism is proposed.