Microbiology (2005), 151, 835–840 DOI 10.1099/mic.0.27591-0 Identification of a gene cluster encoding an arginine ATP-binding-cassette transporter in the genome of the thermophilic Gram-positive bacterium Geobacillus stearothermophilus strain DSMZ 13240 Rebecca Fleischer, Antje Wengner,3 Frank Scheffel, Heidi Landmesser and Erwin Schneider Humboldt Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät I, Institut für Biologie, Bakterienphysiologie, Chausseestr. 117, D-10115 Berlin, Germany Correspondence Erwin Schneider erwin.schneider@rz.hu-berlin.de Received 26 August 2004 Revised 17 November 2004 Accepted 22 November 2004 A single gene cluster encoding components of a putative ATP-binding cassette (ABC) transporter for basic amino acids was identified in the incomplete genome sequence of the thermophilic Gram-positive bacterium Geobacillus stearothermophilus by BLAST searches. The cluster comprises three genes, and these were amplified from chromosomal DNA of G. stearothermophilus, ligated into plasmid vectors and expressed in Escherichia coli. The purified solute-binding protein (designated ArtJ) was demonstrated to bind L-arginine with high affinity (Kd=0?39±0?06 mM). Competition experiments revealed only partial inhibition by excess L-lysine (38 %) and L-ornithine (46 %), while no inhibition was observed with L-histidine or other amino acids tested. The membrane-associated transport complex, composed of a permease (designated ArtM) and an ATPase component (designated ArtP), was solubilized from E. coli membranes by decanoylsucrose and purified by metal-affinity chromatography. The ArtMP complex, when incorporated into liposomes formed from a crude extract of G. stearothermophilus lipids, displayed ATPase activity in the presence of ArtJ only. Addition of L-arginine further stimulated the activity twofold. ATP hydrolysis was optimal at 60 6C and sensitive to the specific inhibitor vanadate. Analysis of kinetic parameters revealed a maximal velocity of ATP hydrolysis of 0?71 mmol Pi min”1 (mg protein)”1 and a Km (ATP) of 1?59 mM. Together, these results identify the ArtJMP complex as a high-affinity arginine ABC transporter. INTRODUCTION Amino acids are key intermediates in both carbon and nitrogen metabolism in bacteria. Some are also implicated in osmoregulation and pH homeostasis (Meng & Bennett, 1992; Sleator & Hill, 2001). Although bacteria are generally capable of synthesizing amino acids from metabolic intermediates, the uptake of amino acids from the environment is energetically favoured. Thus, it is hardly surprising that due to the chemical diversity of the side chains, a diverse number of transport systems have evolved. These belong to either the amino acid/polyamine/organocation (APC) superfamily or to the ATP-binding cassette (ABC) superfamily (Saier, 2000). ABC transporters share a common architectural organization comprising two hydrophobic transmembrane domains that form the translocation 3Present address: Max-Delbrück-Center Berlin-Buch, Robert-RössleStr. 10, 13092 Berlin, Germany. Abbreviation: ABC, ATP-binding cassette. 0002-7591 G 2005 SGM pathway and two hydrophilic nucleotide-binding (ABC) domains that hydrolyse ATP (Higgins, 1992). Members of the subclass of ABC import systems that is confined to prokaryotes require an additional extracellular substratebinding protein (or solute receptor) for function (Van der Heide & Poolman, 2002). ABC transporters mediating the uptake of amino acids have been classified in three separate subfamilies designated PAAT (polar amino acid transporters), HAAT (hydrophobic amino acid transporters) and MUT (methionine uptake transporters) (Hosie & Poole, 2001; Saier, 2000; Zhang et al., 2003). The PAAT family comprises most of the known transporters, including the well-studied histidine permease of Salmonella enterica serovar Typhimurium (S. typhimurium) (reviewed by Schneider, 2003). However, very few of the other family members are functionally characterized. In particular, this holds true for transport systems from thermophilic micro-organisms. For example, in the Gram-positive bacterium Bacillus (now Geobacillus) Downloaded from www.microbiologyresearch.org by IP: 78.47.19.138 On: Sun, 02 Oct 2016 10:43:49 Printed in Great Britain 835 R. Fleischer and others stearothermophilus NUB36, which grows best at 55–60 uC, only two genes have been characterized so far, the products of which are involved in glutamine uptake (Wu & Welker, 1991). The proteins displayed significant sequence identities to the ABC subunits and solute-binding proteins, respectively, of glutamine, histidine or arginine transport systems in Escherichia coli/S. typhimurium. This observation is consistent with the result of computer-aided analyses of complete genome sequences that group ABC transporters for glutamine and basic amino acids in one subfamily (Dassa & Bouige, 2001). Thus, in order to obtain a more complete overview of the repertoire of ABC transporters for polar amino acids in G. stearothermophilus, we searched the incomplete genomic sequence of strain DSMZ 13240 (see http://www.genome.ou.edu/bstearo.html) by BLAST (at http://www.ncbi.nlm.nih.gov/) using the ATPase subunit HisP of the histidine/lysine-arginine-ornithine transporter (LAO/HisJ-QMP) of S. typhimurium (Schneider, 2003) as the seed sequence. The data revealed highest identity (52 % compared to 37 % for the next best hit) to a putative ABC protein (27 kDa) encoded within contig 450 (the numbering is still subject to change). The corresponding gene is part of a cluster that additionally includes ORFs for a putative solute-binding protein (29 kDa) and a membraneintegral (permease) subunit (24 kDa). All three gene products also displayed similar identities to the corresponding components of the arginine (ArtPIQMJ) (Wissenbach et al., 1995) and glutamine (GlnHPQ) (Nohno et al., 1986) transporters of E. coli and to an arginine transporter (YqiXYZ) of Bacillus subtilis (Sekowska et al., 2001). However, attempts to more precisely predict the substrate by PSIBLAST searches gave contradicting results. While the ATPase and membrane-integral subunits displayed highest similarities to glutamine transport components, the solutebinding protein was predicted to be most similar to the arginine-binding protein (ArtJ) of E. coli (Wissenbach et al., 1995). Thus, in order to clearly identify the physiological substrate(s) we have cloned and overexpressed the genes in E. coli and characterized the functions of their purified products by binding assays and in a reconstituted system. The results show that the gene cluster encodes a highaffinity arginine ABC transporter with low affinities for lysine and ornithine. In accordance with the E. coli nomenclature and supported by sequence alignments the genes are designated artJMP. METHODS Bacterial strains, plasmids and growth conditions. All bac- terial strains and plasmids used in this study are listed in Table 1. G. stearothermophilus was grown in peptone broth (medium 220, DSMZ, Braunschweig, Germany) at 55 uC. E. coli strains were grown in LB medium (Miller, 1972) at 37 uC. When required, ampicillin was added at 100 mg ml21. Growth of all bacterial strains was followed by measuring the OD650. Standard DNA techniques. Isolation of genomic DNA of G. stearo- thermophilus, preparation of plasmids, digestion by endonucleases, ligation reactions and PCR were performed as described previously (Hülsmann et al., 2000). Cloning of artJ. The artJ gene was amplified by PCR using the Taq platinum polymerase (Invitrogen), and G. stearothermophilus chromosomal DNA as the template. The primers (59-CGC TCC TCG AGG CGG GCG GCA AGT CAA CGG AAA CAA G-39; 59-GAT CCA GAA TTC TCA CCT TTC TCT ACG AAA AAT GCG TAA CTT GC-39) were designed in such a way that the amplified fragment lacked the 59 sequence encoding the signal peptide and that in the translated mature polypeptide, alanine substituted for the N-terminal cysteine residue. The amplified fragment was ligated with pGEM-T, resulting in plasmid pAW10. To overexpress artJ in E. coli the gene was further subcloned (as an XhoI–EcoRI fragment) into expression vector pBAD/HisA. The resulting plasmid was designated pAW11. Cloning of artMP. The artMP genes were co-amplified by PCR as one fragment. The primers (59-AGA ACG GGA TCC GAT TTT Table 1. Bacterial strains and plasmids Strain or plasmid Strains G. stearothermophilus DSM13240 E. coli JM109 TOP10 Plasmids pBAD/HisA pGEM-T pQE60 pAW10 pAW11 pRF2 Relevant genotype or description Source or reference Wild-type DSMZ* recA1 endA1 gyrA96 thi-1 hsdR17 supE44 relA1 D(lac–proAB) [F9 traD36 proAB lacIq lacZDM15] F9 mrcA D(mrr–hsdRMS–mcrBC) w80lacZDM15 DlacX74 deoR recA1 araD139 D(araA–leu)7697 galU galK rpsL endA1 nupG Stratagene ParaBAD, araC, histidine fusion vector, Apr PT7, lacZ, Apr PT5, histidine fusion vector, Apr artJ (Cys1RAla) as PCR fragment on pGEM-T artJ (Cys1RAla) as XhoI–EcoRI fragment on pBAD/HisA artMP as BamHI–BglII fragment on pQE60 Invitrogen Invitrogen Promega Qiagen This study This study This study *DSMZ, Deutsche Sammlung von Mikroorganismen und Zellkulturen, Braunschweig, Germany. 836 Downloaded from www.microbiologyresearch.org by IP: 78.47.19.138 On: Sun, 02 Oct 2016 10:43:49 Microbiology 151 Arginine-ABC transporter of G. stearothermophilus CGT TTT GAT ATT ATT GTC-39; 59-GAG TGA GAT CTA AAT ACT TTT GAC AAA AAC GCT TTT G-39) were designed in such a way that the start codon of artM was replaced by a BamHI site and that a BglII site substituted for the stop codon of artP. The resulting 1?4 kb fragment was cloned into expression vector pQE60, yielding plasmid pRF2. Purification of ArtJ. E. coli strain TOP10(pAW11) was grown in LB, containing ampicillin, to an OD650 of 0?5 and supplemented with 0?02 % arabinose to induce artJ expression, and growth was continued for 4 h. Cells were harvested, resuspended in 50 mM MOPS/KOH, pH 7, 100 mM NaCl, 0?1 mM PMSF and disintegrated by one passage through a French press at 14 000 p.s.i. (96?6 MPa). After ultracentrifugation (45 min at 200 000 g), ArtJ was recovered in the supernatant and subsequently purified by immobilized metal-affinity chromatography using a Co2+-loaded resin (TALON, Clontech). Purification of ArtMP. E. coli strain JM109(pRF2) was grown under the same conditions as TOP10(pAW11) but adding 0?5 mM IPTG for induction of gene expression. For purification of ArtMP, the membrane fraction obtained after cell disruption and ultracentrifugation (1 h at 200 000 g) was resuspended in 50 mM Tris/HCl, pH 8, 5 % (v/v) glycerol, 0?1 mM PMSF and solubilized with 1?2 % decanoylsucrose. After ultracentrifugation (30 min at 200 000 g) the supernatant was adjusted to 10 mM ATP and 20 mM 2-mercaptoethanol and purified on a Co2+-loaded affinity matrix in the presence of 0?2 % decanoylsucrose. Isolation of total lipids from G. stearothermophilus. Lipids were extracted from cells of G. stearothermophilus by a published method (Folch et al., 1957). Briefly, cells (10 g wet weight) were washed once with 50 mM Tris/HCl, pH 8, homogenized in 20 vols chloroform/methanol (2 : 1, v/v) and stirred for 18 h at room temperature. Undissolved material was removed by filtration and the filtrate was extracted with 0?2 vols double-distilled water. After separation into two phases, the lower (organic) phase was evaporated to dryness and washed with acetone for 1 h at room temperature. After removing acetone with a pipette, the sediment was dried under a stream of nitrogen, dissolved to 20 mg ml21 in chloroform and stored at 220 uC. Preparation of proteoliposomes. The ArtMP complex was incor- porated into liposomes essentially as described previously (Scheffel et al., 2004). Lipids (20 mg) were dried under a stream of nitrogen, and slowly redissolved in 50 mM MOPS/KOH, pH 7?5, containing 1 % octyl b-D-glucopyranoside. The mixture was then sonicated for 15 min, and then ArtMP (50 mg), ArtJ (120 mg) and L-arginine (1 mM, final concentration) were added to the lipid/detergent mixture. These values correspond to a 7?6-fold molar excess of binding protein over transport complex, which is in the range of what has been proven optimal for the maltose and histidine ABC transporters (Landmesser et al., 2002; Hall et al., 1998; Liu et al., 1997). Proteoliposomes were formed by removal of detergent by adsorption to Biobeads (100 mg; Bio-Rad) at 4 uC overnight. After replacing the beads with a new batch incubation was continued for 1 h. The mixture was subsequently centrifuged for 1 min at 300 g to pellet the beads and the supernatant was assayed for ATPase activity. As controls proteoliposomes were prepared by the same procedure but omitting ArtJ/L-arginine or L-arginine. Binding assay. Binding of L-[14C]arginine to purified ArtJ was performed by the method of Richarme & Kepes (1983). Standard assay mixtures (100 ml) containing 50 mM MOPS/KOH, pH 7?5, 150 mM NaCl, 5 % (v/v) glycerol and purified ArtJ (3 mg) were preheated at 60 uC for 1 min prior to the addition of L-[U-14C]arginine (Hartmann Analytic; 264 mCi mmol21, 9?77 GBq; 5 mM final concentration). After 1 min, the reaction was terminated by adding http://mic.sgmjournals.org 2 ml of an ice-cold saturated (NH4)2SO4 solution and immediately filtered through a nitrocellulose filter (0?45 mm). The filter was washed once with the same solution, followed by distilled water, and retained radioactivity was determined in a liquid scintillation counter. In competition experiments, unlabelled L-amino acids (0?5 mM) were added 1 min prior to the addition of L-[14C]arginine. To determine the binding constant, ArtJ was first subjected to a denaturation/renaturation procedure using guanidine hydrochloride (6 M) to remove excess bound unlabelled substrate (Hall et al., 1998). Analytical methods. Hydrolysis of ATP was assayed in microtitre plates essentially as described by Landmesser et al., (2002). Protein was assayed by using the BCA kit from Bio-Rad. SDS-PAGE and immunoblot analyses were performed as described by Hülsmann et al. (2000). RESULTS AND DISCUSSION Identification of the substrate specificity of ArtJ Substrate specificity of an ABC importer is primarily determined by the extracellular solute-binding protein. In Gram-positive bacteria, binding proteins are anchored to the cytoplasmic membrane by fatty acids that are covalently linked to the N-terminal cysteine residue (Sutcliffe & Russell, 1995). No evidence exists that the lipid modification is crucial for substrate binding or transport (Hülsmann et al., 2000; Horlacher et al., 1998). However, when genes encoding solute-binding proteins are expressed in E. coli, overproduction is often poor (Horlacher et al., 1998) or can have deleterious effects on the host cells (Sutcliffe & Russell, 1995). Low protein yields were also observed when genes encoding lipo-binding proteins were expressed with their natural signal sequence, which is required for translocation across the cytoplasmic membrane via the Sec apparatus (Horlacher et al., 1998; Kempf et al., 1997). Thus, to avoid any of these potential problems and to assure the availability of sufficient amounts of purified protein for the intended binding and reconstitution studies, we changed the N-terminal cysteine residue to alanine and deleted the signal sequence (see Methods for details). As a consequence, when overproduced as an N-terminal fusion with six consecutive histidine residues using strain TOP10(pAW11), ArtJ was recovered with the cytosolic fraction. Purification was achieved by using a Co2+-loaded affinity matrix and was verified by SDS-PAGE (Fig. 1a). In order to identify the substrate(s) of ArtJ, binding assays using radiolabelled amino acids were performed at 60 uC. Initial experiments identified L-arginine as the most likely ligand. To further explore the arginine-binding activity the protein was first subjected to a denaturation–renaturation procedure using 6 M guanidine hydrochloride in order to remove bound unlabelled amino acids. Subsequent binding experiments at increasing concentrations of L-[14C]arginine and analysis of the data by a Scatchard plot revealed a dissociation constant (Kd) of 0?39±0?06 mM. This value is in good agreement with the dissociation constant reported for the arginine-binding protein ArtJ of E. coli (0?4 mM; Wissenbach et al., 1995) but about 20 times higher than Downloaded from www.microbiologyresearch.org by IP: 78.47.19.138 On: Sun, 02 Oct 2016 10:43:49 837 R. Fleischer and others (a) kDa (b) kDa 45 His6-ArtJ 31 31 ArtP-His6 ArtM 21 1 2 3 4 1 2 3 that of the lysine-arginine-ornithine-binding protein (LAO) of S. typhimurium (Nikaido & Ames, 1992). When binding assays were performed in the presence of 100-fold excess of unlabelled amino acids, only L-lysine (38 %) and L-ornithine (46 %) partially inhibited binding of L-[14C]arginine (5 mM) to ArtJ. In contrast, histidine or glutamine had no effect (Table 2). These data clearly identified ArtJ as a high-affinity arginine-binding protein with low affinities for lysine and ornithine, thereby suggesting that the complete gene cluster encodes a transporter with preference for arginine. In order to demonstrate this in vitro we also purified the ArtMP proteins. Functional characterization of the ArtJMP transport system in proteoliposomes The ArtMP proteins were overproduced from E. coli strain JM109(pRF2). After solubilization of the membrane Table 2. Arginine-binding activity of ArtJ in the presence of competing substrates The assays were performed with 5 mM L-[14C]arginine and competing amino acids were added in their L-forms at 500 mM. The data represent the mean of two experiments. Addition None Arginine Asparagine Aspartate Cysteine Glutamate Glutamine Histidine Lysine Ornithine Binding of L-[14C]arginine (% of control) 100* 1?6 99?6 97?9 88?2 93?9 93?1 94?5 62 54?1 *The control value was 12?1 pmol arginine (mg ArtJ)21. 838 4 Fig. 1. Summary of purification of arginine transport components. Protein samples were run on an SDS gel and subsequently stained with Coomassie blue R250. (a) Purification of His6-ArtJ. Lanes: 1, standard proteins; 2, cytosolic fraction of strain TOP10(pAW11); 3, pooled fractions eluted from TALON matrix with 100 mM imidazole (4?5 mg); 4, denatured and renatured ArtJ (3?7 mg). (b) Purification of ArtMP-His6. Lanes: 1, standard proteins; 2, cytoplasmicmembrane fraction of strain JM109(pRF2); 3, proteins solubilized with 1?2 % decanoylsucrose; 4, pooled fractions eluted from TALON matrix with 100 mM imidazole (9 mg). The upper band was identified as ArtP by immunoblotting using an anti-His-tag antibody. fraction with decanoylsucrose, purification was achieved by affinity chromatography via a His6 extension at the carboxy terminus of ArtP. SDS-PAGE of peak fractions revealed two protein bands of the expected apparent molecular mass (Fig. 1b). The upper band was identified as ArtP by immunoblotting using an anti-His-tag antibody (not shown). These results also demonstrate that ArtM and ArtP form a stable complex in detergent solution. ABC transporter functions of ArtJMP were subsequently studied in proteoliposomes by assaying the following wellestablished criteria: (i) stimulation of ATPase activity of the complex by the cognate binding protein (Davidson et al., 1992; Liu et al., 1997; Landmesser et al., 2002; Scheffel et al., 2004), and (ii) sensitivity of this activity to the specific inhibitor orthovanadate (Landmesser et al., 2002; Sharma & Davidson, 2000). To this end, proteoliposomes were prepared from purified ArtMP and a crude total lipid extract of G. stearothermophilus (mainly composed of phosphatidylethanolamine, diphosphatidylglycerol and phosphatidylglycerol; Jurado et al., 1991) in the presence of L-arginine (1 mM) and/or ArtJ. By this procedure, substrate and binding protein were enclosed in the lumen of the proteoliposomes. Assuming that incorporation of complex molecules occurs randomly with respect to orientation, as has been demonstrated for example for the maltose and histidine ABC transporters of S. typhimurium (Landmesser et al., 2002; Liu & Ames, 1997), only proteoliposomes with the ArtP subunits facing the medium would contribute to ATPase activity. As shown in Fig. 2, proteoliposomes prepared in the absence of ArtJ and arginine exhibited only marginal ATPase activity; however, the activity was markedly stimulated by ArtJ. Moreover, loading the proteoliposomes with ArtJ and L-arginine further increased the initial rate of ATP hydrolysis by a factor of 1?6. Similar rates of substrate-dependent stimulation of ATPase activity were reported for the maltose (Landmesser et al., 2002) and histidine (Liu et al., 1997) transporters of S. typhimurium. Consistent with the growth conditions of Downloaded from www.microbiologyresearch.org by IP: 78.47.19.138 On: Sun, 02 Oct 2016 10:43:49 Microbiology 151 Arginine-ABC transporter of G. stearothermophilus REFERENCES Davidson, A. L., Shuman, H. A. & Nikaido, H. (1992). Mechanism of maltose transport in Escherichia coli: transmembrane signaling by periplasmic binding proteins. Proc Natl Acad Sci U S A 89, 2360–2364. Dassa, E. & Bouige, P. (2001). The ABC of ABCs: a phylogenetic and functional classification of ABC systems in living organisms. Res Microbiol 152, 211–229. Folch, J., Lees, M. & Sloane Stanley, G.-H. (1957). 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