Control of H<Superscript>+</Superscript>/Lactose Coupling by Ionic Interactions in the Lactose Permease of<Emphasis Type="Italic">Escherichia coli</Emphasis>

A combinatorial approach was used to study putative interactions among six ionizable residues (Asp-240, Glu-269, Arg-302, Lys-319, His-322, and Glu-325) in the lactose permease. Neutral mutations were made involving five ion pairs that had not been previously studied. Double mutants, R302L/E325Q and D240N/H322Q, had moderate levels of downhill [¹⁴C]-lactose transport. Mutants in which only one of these six residues was left unchanged (pentuple mutants) were also made. A Pent269⁻ mutant (in which only Glu-269 remains) catalyzed a moderate level of downhill lactose transport. Pent240⁻ and Pent 322⁺ also showed low levels of downhill lactose transport. Additionally, a Pent240⁻ mutant exhibited proton transport upon addition of melibiose, but not lactose. This striking result demonstrates that neutralization of up to five residues of the lactose permease does not abolish proton transport. A mutant with neutral replacements at six ionic residues (hextuple mutant) had low levels of downhill lactose transport, but no uphill accumulation or proton transport. Since none of the mutants in this study catalyzes active accumulation of lactose, this is consistent with other reports that have shown that each residue is essential for proper coupling. Nevertheless, none of the six ionizable residues is individually required for substrate-induced proton cotransport. These results suggest that the H⁺ binding domain may be elsewhere in the permease or that cation binding may involve a flexible network of charged residues.This revised version was published online in August 2005 with a corrected cover date.     

The Conserved Motif in Hydrophilic Loop 2/3 and Loop 8/9 of the Lactose Permease of Escherichia coli. Analysis of Suppressor Mutations

The major facilitator superfamily (MFS) of transport proteins, which includes the lactose permease of Escherichia coli, contains a conserved motif G-X-X-X-D/E-R/K-X-G-R/K-R/K in the loops that connect transmembrane segments 2 and 3, and transmembrane segments 8 and 9. In three previous studies (Jessen-Marshall, A.E., & Brooker, R.J. 1996. J. Biol. Chem. 271:1400–1404; Jessen-Marshall, A.E., Parker, N., & Brooker, R.J. 1997. J. Bacteriol. 179:2616–2622; and Pazdernik, N., Cain, S.M., & Brooker, R.J. 1997. J. Biol. Chem. 272:26110–26116), suppressor mutations at twenty different sites were identified which restore function to mutant permeases that have deleterious mutations in the conserved loop 2/3 or loop 8/9 motif. In the current study, several of these second-site suppressor mutations have been separated from the original mutation in the conserved motif. The loop 2/3 suppressors were then coupled to a loop 8/9 mutation (P280L) and the loop 8/9 suppressors were coupled to a loop 2/3 mutation (i.e., G64S) to determine if the suppressors could restore function only to a loop 2/3 mutation, a loop 8/9 mutation, or both. The single parent mutations changing the first position in loop 2/3 (i.e., G64S) and loop 8/9 (i.e., P280L) had less than 4% lactose transport activity. Interestingly, most of the suppressors were very inhibitory when separated from the parent mutation. Two suppressors, A50T and G370V, restored substantial transport activity when individually coupled to the mutation in loop 2/3 and also when coupled to the corresponding mutation in loop 8/9. In other words, these suppressors could alleviate a defect imposed by mutations in either half of the permease. From a kinetic analysis, these suppressors were shown to exert their effects by increasing the V _max values for lactose transport compared with the single G64S and P280L strains. These results are discussed within the context of our model in which the two halves of the lactose permease interact at a rotationally symmetrical interface, and that lactose transport is mediated by conformational changes at the interface. Received: 18 November 1999/Revised: 11 April 2000     

Proper Fatty Acid Composition Rather than an Ionizable Lipid Amine Is Required for Full Transport Function of Lactose Permease from Escherichia coli

Energy-dependent uphill transport but not energy-independent downhill transport by lactose permease (LacY) is impaired when expressed in Escherichia coli cells or reconstituted in liposomes lacking phosphatidylethanolamine (PE) and containing only anionic phospholipids. The absence of PE results in inversion of the N-terminal half and misfolding of periplasmic domain P7, which are required for uphill transport of substrates. Replacement of PE in vitro by lipids with no net charge (phosphatidylcholine (PC), monoglucosyl diacylglycerol (GlcDAG), or diglucosyl diacylglycerol (GlcGlcDAG)) supported wild type transmembrane topology of the N-terminal half of LacY. The restoration of uphill transport in vitro was dependent on LacY native topology and proper folding of P7. Support of uphill transport by net neutral lipids in vitro (PE > PC ≫ GlcDAG ≠ GlcGlcDAG provided that PE or PC contained one saturated fatty acid) paralleled the results observed previously in vivo (PE = PC > GlcDAG ≠ GlcGlcDAG). Therefore, a free amino group is not required for uphill transport as previously concluded based on the lack of in vitro uphill transport when fully unsaturated PC replaced E. coli-derived PE. A close correlation was observed in vivo and in vitro between the ability of LacY to carry out uphill transport, the native conformation of P7, and the lipid headgroup and fatty acid composition. Therefore, the headgroup and the fatty acid composition of lipids are important for defining LacY topological organization and catalytically important structural features, further illustrating the direct role of lipids, independent of other cellular factors, in defining membrane protein structure/function.     

Identification of Functional Amino Acid Residues Involved in Polyamine and Agmatine Transport by Human Organic Cation Transporter 2

Polyamine (putrescine, spermidine and spermine) and agmatine uptake by the human organic cation transporter 2 (hOCT2) was studied using HEK293 cells transfected with pCMV6-XL4/hOCT2. The Km values for putrescine and spermidine were 7.50 and 6.76 mM, and the Vmax values were 4.71 and 2.34 nmol/min/mg protein, respectively. Spermine uptake by hOCT2 was not observed at pH 7.4, although it inhibited both putrescine and spermidine uptake. Agmatine was also taken up by hOCT2, with Km value: 3.27 mM and a Vmax value of 3.14 nmol/min/mg protein. Amino acid residues involved in putrescine, agmatine and spermidine uptake by hOCT2 were Asp⁴²⁷, Glu⁴⁴⁸, Glu⁴⁵⁶, Asp⁴⁷⁵, and Glu⁵¹⁶. In addition, Glu⁵²⁴ and Glu⁵³⁰ were involved in putrescine and spermidine uptake activity, and Glu⁵²⁸ and Glu⁵⁴⁰ were weakly involved in putrescine uptake activity. Furthermore, Asp⁵⁵¹ was also involved in the recognition of spermidine. These results indicate that the recognition sites for putrescine, agmatine and spermidine on hOCT2 strongly overlap, consistent with the observation that the three amines are transported with similar affinity and velocity. A model of spermidine binding to hOCT2 was constructed based on the functional amino acid residues.     

Probing the Periplasmic-Open State of Lactose Permease in Response to Sugar Binding and Proton Translocation

Based on the crystal structure of lactose permease (LacY) open to the cytoplasm, a hybrid molecular simulation approach with self-guided Langevin dynamics is used to describe conformational changes that lead to a periplasmic-open state. This hybrid approach consists of implicit (IM) and explicit (EX) membrane simulations and requires self-guided Langevin dynamics to enhance protein motions during the IM simulations. The pore radius of the lumen increases by 3.5 Å on the periplasmic side and decreases by 2.5 Å on the cytoplasmic side (relative to the crystal structure), suggesting a lumen that is fully open to the periplasm to allow for extracellular sugar transport and closed to the cytoplasm. Based on our simulations, the mechanism that triggers this conformational change to the periplasmic-open state is the protonation of Glu269 and binding of the disaccharide. Then, helix packing is destabilized by breaking of several side chains involved in hydrogen bonding (Asn245, Ser41, Glu374, Lys42, and Gln242). For the periplasmic-open conformations obtained from our simulations, helix-helix distances agree well with experimental measurements using double electron-electron resonance, fluorescence resonance energy transfer, and varying sized cross-linkers. The periplasmic-open conformations are also in compliance with various substrate accessibility/reactivity measurements that indicate an opening of the protein lumen on the periplasmic side on sugar binding. The comparison with these measurements suggests a possible incomplete closure of the cytoplasmic half in our simulations. However, the closure is sufficient to prevent the disaccharide from transporting to the cytoplasm, which is in accordance with the well-established alternating access model. Ser53, Gln60, and Phe354 are determined to be important in sugar transport during the periplasmic-open stage of the sugar transport cycle and the sugar is found to undergo an orientational change in order to escape the protein lumen.     

A Triple Mutant, K319N/H322Q/E325Q, of the Lactose Permease Cotransports H+ with Thiodigalactoside

In a previous study, we characterized a lactose permease mutant (K319N/E325Q) that can transport H⁺ ions with sugar. This result was surprising because other studies had suggested that Glu-325 plays an essential role in H⁺ binding. To determine if the lactose permease contains one or more auxiliary H⁺ binding sites, we began with the K319N/E325Q strain, which catalyzes a sugar-dependent H⁺ leak, and isolated third site suppressor mutations that blocked the H⁺ leak. Three types of suppressors were obtained: H322Y, H322R, and M299I. These mutations blocked the H⁺ leak and elevated the apparent K _m value for lactose. The M299I and H322Y suppressors could still transport H⁺ with β-d-thiodigalactoside (TDG), but the H322R strain appeared uncoupled for H⁺/sugar cotransport. Four mutant strains containing a nonionizable substitution at codon 322 (H322Q) were analyzed. None of these were able to catalyze uphill accumulation of lactose, however, all showed some level of substrate-induced proton accumulation. The level seemed to vary based on the substrate being analyzed (lactose or TDG). Most interestingly, a triple mutant, K319N/H322Q/E325Q, catalyzed robust H⁺ transport with TDG. These novel results suggest an alternative mechanism of lactose permease cation binding and transport, possibly involving hydronium ion (H₃O⁺). Received: 6 November 2000/Revised: 23 March 2001     

Identification of Transport-critical Residues in a Folate Transporter from the Folate-Biopterin Transporter (FBT) Family

The Synechocystis Slr0642 protein and its plastidial Arabidopsis (Arabidopsis thaliana) ortholog At2g32040 belong to the folate-biopterin transporter (FBT) family within the major facilitator superfamily. Both proteins transport folates when expressed in Escherichia coli. Because the structural requirements for transport activity are not known for any FBT protein, we applied mutational analysis to identify residues that are critical to transport and interpreted the results using a comparative structural model based on E. coli lactose permease. Folate transport was assessed via the growth of an E. coli pabA abgT strain, which cannot synthesize or take up folates or p-aminobenzoylglutamate. In total, 47 residues were replaced with Cys or Ala. Mutations at 22 positions abolished folate uptake without affecting Slr0642 expression in membranes, whereas other mutations had no effect. Residues important for function mostly line the predicted central cavity and are concentrated in the core α-helices H1, H4, H7, and H10. The essential residue locations are consistent with a folate-binding site lying roughly equidistant from both faces of the transporter. Arabidopsis has eight FBT proteins besides At2g32040, often lacking conserved critical residues. When six of these proteins were expressed in E. coli or in Leishmania folate or pterin transporter mutants, none showed evidence of folate or pterin transport activity, and only At2g32040 was isolated by functional screening of Arabidopsis cDNA libraries in E. coli. Such negative data could reflect roles in transport of other substrates. These studies provide the first insights into the native structure and catalytic mechanism of FBT family carriers.     

Identification of Conformationally Sensitive Residues Essential for Inhibition of Vesicular Monoamine Transport by the Noncompetitive Inhibitor Tetrabenazine

Vesicular monoamine transporter 2 (VMAT2) transports monoamines into storage vesicles in a process that involves exchange of the charged monoamine with two protons. VMAT2 is a member of the DHA12 family of multidrug transporters that belongs to the major facilitator superfamily of secondary transporters. Tetrabenazine (TBZ) is a non-competitive inhibitor of VMAT2 that is used in the treatment of hyperkinetic disorders associated with Huntington disease and Tourette syndrome. Previous biochemical studies suggested that the recognition site for TBZ and monoamines is different. However, the precise mechanism of TBZ interaction with VMAT2 remains unknown. Here we used a random mutagenesis approach and selected TBZ-resistant mutants. The mutations clustered around the lumenal opening of the transporter and mapped to either conserved proline or glycine, or to residues immediately adjacent to conserved proline and glycine. Directed mutagenesis provides further support for the essential role of the latter residues. Our data strongly suggest that the conserved α-helix breaking residues identified in this work play an important role in conformational rearrangements required for TBZ binding and substrate transport. Our results provide a novel insight into the mechanism of transport and TBZ binding by VMAT2.     

Limits on the Tightness of Coupling in Active Transport

Control of the coupled reaction sequence in active transport depends on systematic changes in the properties of the carrier protein as the reaction proceeds. These changes would have to be brought about by specific interactions with the substrate, the binding forces being used to stabilize either (i) a carrier state with altered properties or (ii) the transition state in a carrier transformation. In the first case the tightness of coupling (the ratio of the coupled rate to slippage) will at first rise with the increment in binding energy in the altered state but will approach an upper limit when overly strong binding forces retard substrate dissociation in a subsequent step in the coupled reaction sequence. Primary and secondary active transport are subject to this limitation because the coupling mechanism necessarily involves intermediates in which the substrate is strongly bound. Exchange-only transport is not necessarily subject to the same limitation because the mechanism can involve only a substrate-catalyzed change in carrier state. The available data, although scant, agree with these conclusions. Received: 3 June 1998/Revised: 22 September 1998     

Cysteine-scanning Analysis of Helices TM8, TM9a,and TM9b and Intervening Loops in the YgfO Xanthine Permease: A CARBOXYL GROUP IS ESSENTIAL AT ASP-276

Bacterial and fungal members of the ubiquitous nucleobase-ascorbate transporter (NAT/NCS2) family use the NAT signature motif, a conserved 11-amino acid sequence between amphipathic helices TM9a and TM9b, to define function and selectivity of the purine binding site. To examine the role of flanking helices TM9a, TM9b, and TM8, we employed Cys-scanning analysis of the xanthine-specific homolog YgfO from Escherichia coli. Using a functional mutant devoid of Cys residues (C-less), each amino acid residue in sequences ²⁵⁹FLVVGTIYLLSVLEAVGDITATAMVSRRPIQGEEYQSRLKGGVLADGLVSVIASAV³¹⁴ and ³⁴²TIAVMLVILGLFP³⁵⁴ including these TMs (underlined) was replaced individually with Cys, except the irreplaceable Glu-272 and Asp-304, which had been studied previously. Of 67 single Cys mutants, 55 accumulate xanthine to 35–140% of the steady state observed with C-less, five (I265C, D276C, I277C, G299C, L350C) accumulate to low levels (10–20%) and seven (T278C, A279C, T280C, A281C, G305C, G351C, P354C) show negligible expression in the membrane. Extensive mutagenesis reveals that a carboxyl group is needed at Asp-276 for high activity and that D276E differs from wild type as it recognizes 8-methylxanthine (K_i 79 μm) but fails to recognize 2-thioxanthine, 3-methylxanthine or 6-thioxanthine; bulky replacements of Ala-279 or Thr-280 and replacements of Gly-305, Gly-351, or Pro-354 impair activity or expression. Single Cys mutants V261C, A273C, G275C, and S284C are sensitive to inactivation by N-ethylmaleimide and sensitivity of G275C (IC₅₀ 15 μm) is enhanced in the presence of substrate. The data suggest that residues crucial for the transport mechanism cluster in two conserved motifs, at the cytoplasmic end of TM8 (EXXGDXXAT) and in TM9a (GXXXDG).     

Phospholipid Flippases Lem3p-Dnf1p and Lem3p-Dnf2p Are Involved in the Sorting of the Tryptophan Permease Tat2p in Yeast

The type 4 P-type ATPases are flippases that generate phospholipid asymmetry in membranes. In budding yeast, heteromeric flippases, including Lem3p-Dnf1p and Lem3p-Dnf2p, translocate phospholipids to the cytoplasmic leaflet of membranes. Here, we report that Lem3p-Dnf1/2p are involved in transport of the tryptophan permease Tat2p to the plasma membrane. The lem3Δ mutant exhibited a tryptophan requirement due to the mislocalization of Tat2p to intracellular membranes. Tat2p was relocalized to the plasma membrane when trans-Golgi network (TGN)-to-endosome transport was inhibited. Inhibition of ubiquitination by mutations in ubiquitination machinery also rerouted Tat2p to the plasma membrane. Lem3p-Dnf1/2p are localized to endosomal/TGN membranes in addition to the plasma membrane. Endocytosis mutants, in which Lem3p-Dnf1/2p are sequestered to the plasma membrane, also exhibited the ubiquitination-dependent missorting of Tat2p. These results suggest that Tat2p is ubiquitinated at the TGN and missorted to the vacuolar pathway in the lem3Δ mutant. The NH₂-terminal cytoplasmic region of Tat2p containing ubiquitination acceptor lysines interacted with liposomes containing acidic phospholipids, including phosphatidylserine. This interaction was abrogated by alanine substitution mutations in the basic amino acids downstream of the ubiquitination sites. Interestingly, a mutant Tat2p containing these substitutions was missorted in a ubiquitination-dependent manner. We propose the following model based on these results; Tat2p is not ubiquitinated when the NH₂-terminal region is bound to membrane phospholipids, but if it dissociates from the membrane due to a low level of phosphatidylserine caused by perturbation of phospholipid asymmetry in the lem3Δ mutant, Tat2p is ubiquitinated and then transported from the TGN to the vacuole.     

Determinants of Substrate and Cation Transport in the Human Na+/Dicarboxylate Cotransporter NaDC3

Metabolic intermediates, such as succinate and citrate, regulate important processes ranging from energy metabolism to fatty acid synthesis. Cytosolic concentrations of these metabolites are controlled, in part, by members of the SLC13 gene family. The molecular mechanism underlying Na⁺-coupled di- and tricarboxylate transport by this family is understood poorly. The human Na⁺/dicarboxylate cotransporter NaDC3 (SLC13A3) is found in various tissues, including the kidney, liver, and brain. In addition to citric acid cycle intermediates such as α-ketoglutarate and succinate, NaDC3 transports other compounds into cells, including N-acetyl aspartate, mercaptosuccinate, and glutathione, in keeping with its dual roles in cell nutrition and detoxification. In this study, we construct a homology structural model of NaDC3 on the basis of the structure of the Vibrio cholerae homolog vcINDY. Our computations are followed by experimental testing of the predicted NaDC3 structure and mode of interaction with various substrates. The results of this study show that the substrate and cation binding domains of NaDC3 are composed of residues in the opposing hairpin loops and unwound portions of adjacent helices. Furthermore, these results provide a possible explanation for the differential substrate specificity among dicarboxylate transporters that underpin their diverse biological roles in metabolism and detoxification. The structural model of NaDC3 provides a framework for understanding substrate selectivity and the Na⁺-coupled anion transport mechanism by the human SLC13 family and other key solute carrier transporters.     

Identification of a Key Residue Determining Substrate Affinity in the Yeast Glucose Transporter Hxt7: A TWO-DIMENSIONAL COMPREHENSIVE STUDY*

We previously identified Asn³³¹ in transmembrane segment 7 (TM7) as a key residue determining substrate affinity in Hxt2, a moderately high-affinity facilitative glucose transporter of Saccharomyces cerevisiae. To gain further insight into the structural basis of substrate recognition by yeast glucose transporters, we have now studied Hxt7, whose affinity for glucose is the highest among the major hexose transporters. The functional role of Asp³⁴⁰ in Hxt7, the residue corresponding to Asn³³¹ of Hxt2, was examined by replacing it with each of the other 19 amino acids. Such replacement of Asp³⁴⁰ generated transporters with various affinities for glucose, with the affinity of the Cys³⁴⁰ mutant surpassing that of the wild-type Hxt7. To examine the structural role of Asp³⁴⁰ in the substrate translocation pathway, we performed cysteine-scanning mutagenesis of the 21 residues in TM7 of a functional Cys-less Hxt7 mutant in conjunction with exposure to the hydrophilic sulfhydryl reagent p-chloromercuribenzenesulfonate (pCMBS). The transport activity of the D340C mutant of Cys-less Hxt7, in which Asp³⁴⁰ is replaced with Cys, was completely inhibited by pCMBS, indicating that Asp³⁴⁰ is located in a water-accessible position. This D340C mutant showed a sensitivity to pCMBS that was ∼70 times that of the wild-type Hxt7, and it was protected from pCMBS inhibition by the substrates d-glucose and 2-deoxy-d-glucose but not by l-glucose. These results indicate that Asp³⁴⁰ is situated at or close to a substrate recognition site and is a key residue determining high-affinity glucose transport by Hxt7, supporting the notion that yeast glucose transporters share a common mechanism for substrate recognition.     

Aspartic Acid 397 in Subunit B of the Na+-pumping NADH:Quinone Oxidoreductase from Vibrio cholerae Forms Part of a Sodium-binding Site,Is Involved in Cation Selectivity,and Affects Cation-binding Site Cooperativity

The Na⁺-pumping NADH:quinone complex is found in Vibrio cholerae and other marine and pathogenic bacteria. NADH:ubiquinone oxidoreductase oxidizes NADH and reduces ubiquinone, using the free energy released by this reaction to pump sodium ions across the cell membrane. In a previous report, a conserved aspartic acid residue in the NqrB subunit at position 397, located in the cytosolic face of this protein, was proposed to be involved in the capture of sodium. Here, we studied the role of this residue through the characterization of mutant enzymes in which this aspartic acid was substituted by other residues that change charge and size, such as arginine, serine, lysine, glutamic acid, and cysteine. Our results indicate that NqrB-Asp-397 forms part of one of the at least two sodium-binding sites and that both size and charge at this position are critical for the function of the enzyme. Moreover, we demonstrate that this residue is involved in cation selectivity, has a critical role in the communication between sodium-binding sites, by promoting cooperativity, and controls the electron transfer step involved in sodium uptake (2Fe-2S → FMN_C).     

Identification of a Region Critically Involved in the Interaction of Phlorizin with the Rabbit Sodium-D-Glucose Cotransporter SGLT1

In order to define potential interaction sites of SGLT1 with the transport inhibitor phlorizin, mutagenesis studies were performed in a hydrophobic region of loop 13 (aa 604–610), located extracellularly, close to the C-terminus. COS 7 cells were transiently transfected with the mutants and the kinetic parameters of α-methyl-d-glucopyranoside (AMG) uptake into the cells were investigated. Replacement of the respective amino acids with lysine reduced the maximal uptake rate: Y604K showed 2.2%, L606K 48.4%, F607K 15.1%, C608K 13.1%, G609K 14.1%, and L610K 17.2% of control. In all mutants the apparent K _i for phlorizin increased at least by a factor of 5 compared to the wild-type K _i of 4.6 ± 0.7 μmol/l; most striking changes were observed for Y604K (K _i = 75.3 ± 19.0 μmol/l) and C608K (K _i = 83.6 ± 13.9 μmol/l). Replacement of these amino acids with a nonpolar amino acid instead of lysine such as in Y604F, Y604G and C608A showed markedly higher affinities for phlorizin. In cells expressing the mutants the apparent affinity of AMG uptake for the sugar was not statistically different from that of the wild type (K _m = 0.8 ± 0.2 mmol/l). These studies suggest that the region between amino acids 604 and 610 is involved in the interaction between SGLT1 and phlorizin, probably by providing a hydrophobic pocket for one of the aromatic rings of the aglucone moiety of the glycoside. Received: 29 March 2001/Revised: 15 June 2001     

Organic Anion Transporting Polypeptides of the OATP/SLCO Superfamily: Identification of New Members in Nonmammalian Species, Comparative Modeling and a Potential Transport Mode

Organic anion-transporting polypeptides (human, OATPs; other animals, Oatps; gene symbol, SLCO/Slco) form a transport protein superfamily that mediates the translocation of amphipathic substrates across the plasma membrane of animal cells. So far, OATPs/Oatps have been identified in human, rat and mouse tissues. In this study, we used bioinformatic tools to detect new members of the OATP/SLCO superfamily in nonmammalian species and to build models for the three-dimensional structure of OATPs/Oatps. New OATP/SLCO superfamily members, some of which form distinct novel families, were identified in chicken, zebrafish, frog, fruit fly and worm species. The lack of OATP/SLCO superfamily members in plants, yeast and bacteria suggests the emergence of an ancient Oatp protein in an early ancestor of the animal kingdom. Structural models were generated for the representative members OATP1B3 and OATP2B1 based on the known structures of the major facilitator superfamily of transport proteins. A model was also built for the large extracellular region between transmembrane helices 9 and 10, following the identification of a novel homology with the Kazal-type serine protease inhibitors. Along with the electrostatic potential and the conservation of key amino acid residues, we propose a common transport mechanism for all OATPs/Oatps, whereby substrates are translocated through a central, positively charged pore in a rocker-switch type of mechanism. Several amino acid residues were identified that may play crucial roles in the proposed transport mechanism. Electronic Supplementary Material Electronic Supplementary material is available for this article at and accessible for authorised users.     

Mutants of the Lactose Carrier of Escherichia coli Which Show Altered Sugar Recognition Plus a Severe Defect in Sugar Accumulation

Lactose and melibiose are actively accumulated by the wild-type Escherichia coli lactose carrier, which is an integral membrane protein energized by the proton motive force. Mutants of the E. coli lactose carrier were isolated by their ability to grow on minimal plates with succinate plus IPTG in the presence of the toxic lactose analog β-thio-o-nitrophenylgalactoside (TONPG). TONPG-resistant mutants were streaked on melibiose MacConkey indicator plates, and red clones were picked. These melibiose positive mutants were then streaked on lactose MacConkey plates, and white clones were picked. Transport assays indicated that the mutants had altered sugar recognition and a defect in sugar accumulation. The mutants had a poor apparent K _m for both lactose and melibiose in transport. One mutant had almost no ability to take up lactose, but melibiose downhill transport was 58% (V _max ) of normal. All of the mutants accumulated methyl-α-d-galactopyranoside (TMG) to only 8% or less of normal, and two failed to accumulate. Immunoblot analysis of the mutant lactose carrier proteins indicated that loss of sugar transport activity was not due to loss of expression in the membrane. Nucleotide sequencing of the lacY gene from the mutants revealed changes in the following amino acids of the lactose carrier: M23I, W151L, G257D, A295D and G377V. Two of the mutants (G257D and G377V) are novel in that they represent the first amino acids in periplasmic loops to be implicated with changes in sugar recognition. We conclude that the amino acids M23, W151, G257, A295 and G377 of the E. coli lactose carrier play either a direct or an indirect role in sugar recognition and accumulation. Received: 12 October 1999/Revised: 21 December 1999     

CtpA,a Copper-translocating P-type ATPase Involved in the Biogenesis of Multiple Copper-requiring Enzymes

The ctpA (ccoI) gene product, a putative inner membrane copper-translocating P1B-type ATPase present in many bacteria, has been shown to be involved only in the cbb₃ assembly in Rhodobacter capsulatus and Bradyrhizobium japonicum. ctpA was disrupted in Rubrivivax gelatinosus, and the mutants showed a drastic decrease in both cbb₃ and caa₃ oxidase activities. Inactivation of ctpA results also in a decrease in the amount of the nitrous oxide reductase, NosZ. This pleiotropic phenotype could be partially rescued by excess copper in the medium, indicating that CtpA is likely a copper transporter that supplies copper-requiring proteins in the membrane with this metal. Although CtpA shares significant sequence homologies with the homeostasis copper efflux P1B-type ATPases including the bacterial CopA and the human ATP7A and ATP7B, disruption of ctpA did not result in any sensitivity to excess copper. This indicates that the CtpA is not crucial for copper tolerance but is involved in the assembly of membrane and periplasmic copper enzymes in this bacterium. The potential roles of CtpA in bacteria in comparison with CopA are discussed.     

Residues in the Polar Loop of Subunit c in Escherichia coli ATP Synthase Function in Gating Proton Transport to the Cytoplasm

Rotary catalysis in F₁F₀ ATP synthase is powered by proton translocation through the membrane-embedded F₀ sector. Proton binding and release occur in the middle of the membrane at Asp-61 on the second transmembrane helix (TMH) of subunit c, which folds in a hairpin-like structure with two TMHs. Previously, the aqueous accessibility of Cys substitutions in the transmembrane regions of subunit c was probed by testing the inhibitory effects of Ag⁺ or Cd²⁺ on function, which revealed extensive aqueous access in the region around Asp-61 and on the half of TMH2 extending to the cytoplasm. In the current study, we surveyed the Ag⁺ and Cd²⁺ sensitivity of Cys substitutions in the loop of the helical hairpin and used a variety of assays to categorize the mechanisms by which Ag⁺ or Cd²⁺ chelation with the Cys thiolates caused inhibition. We identified two distinct metal-sensitive regions in the cytoplasmic loop where function was inhibited by different mechanisms. Metal binding to Cys substitutions in the N-terminal half of the loop resulted in an uncoupling of F₁ from F₀ with release of F₁ from the membrane. In contrast, substitutions in the C-terminal half of the loop retained membrane-bound F₁ after metal treatment. In several of these cases, inhibition was shown to be due to blockage of passive H⁺ translocation through F₀ as assayed with F₀ reconstituted into liposomes. The results suggest that the C-terminal domain of the cytoplasmic loop may function in gating H⁺ translocation to the cytoplasm.