1
|
Iyalomhe O, Herrick DZ, Cafiso DS, Maloney PC. Closure of the cytoplasmic gate formed by TM5 and TM11 during transport in the oxalate/formate exchanger from Oxalobacter formigenes. Biochemistry 2014; 53:7735-44. [PMID: 25409483 PMCID: PMC4270380 DOI: 10.1021/bi5012173] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/04/2022]
Abstract
![]()
OxlT, the oxalate/formate exchanger
of Oxalobacter
formigenes, is a member of the major facilitator superfamily
of transporters. In the present work, substrate (oxalate) was found
to enhance the reactivity of the cysteine mutant S336C on the cytoplasmic
end of helix 11 to methanethiosulfonate ethyl carboxylate. In addition,
S336C is found to spontaneously cross-link to S143C in TM5 in either
native or reconstituted membranes under conditions that support transport.
Continuous wave EPR measurements are consistent with this result and
indicate that positions 143 and 336 are in close proximity in the
presence of substrate. These two residues are localized within helix
interacting GxxxG-like motifs (G140LASG144 and
S336DIFG340) at the cytoplasmic poles of TM5
and TM11. Pulse EPR measurements were used to determine distances
and distance distributions across the cytoplasmic or periplasmic ends
of OxlT and were compared with the predictions of an inside-open homology
model. The data indicate that a significant population of transporter
is in an outside-open configuration in the presence of substrate;
however, each end of the transporter exhibits significant conformational
heterogeneity, where both inside-open and outside-open configurations
are present. These data indicate that TM5 and TM11, which form part
of the transport pathway, transiently close during transport and that
there is a conformational equilibrium between inside-open and outside-open
states of OxlT in the presence of substrate.
Collapse
Affiliation(s)
- Osigbemhe Iyalomhe
- Department of Physiology, The Johns Hopkins University, School of Medicine , 725 North Wolfe Street, Baltimore, Maryland 21205, United States
| | | | | | | |
Collapse
|
2
|
The Structure and Function of OxlT, the Oxalate Transporter of Oxalobacter formigenes. J Membr Biol 2014; 248:641-50. [DOI: 10.1007/s00232-014-9728-y] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/02/2014] [Accepted: 09/05/2014] [Indexed: 01/01/2023]
|
3
|
Yadav K, Kumar C, Archana G, Naresh Kumar G. Pseudomonas fluorescens ATCC 13525 containing an artificial oxalate operon and Vitreoscilla hemoglobin secretes oxalic acid and solubilizes rock phosphate in acidic alfisols. PLoS One 2014; 9:e92400. [PMID: 24705024 PMCID: PMC3976251 DOI: 10.1371/journal.pone.0092400] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/08/2013] [Accepted: 02/21/2014] [Indexed: 11/18/2022] Open
Abstract
Oxalate secretion was achieved in Pseudomonas fluorescens ATCC 13525 by incorporation of genes encoding Aspergillus niger oxaloacetate acetyl hydrolase (oah), Fomitopsis plaustris oxalate transporter (FpOAR) and Vitreoscilla hemoglobin (vgb) in various combinations. Pf (pKCN2) transformant containing oah alone accumulated 19 mM oxalic acid intracellularly but secreted 1.2 mM. However, in the presence of an artificial oxalate operon containing oah and FpOAR genes in plasmid pKCN4, Pf (pKCN4) secreted 13.6 mM oxalate in the medium while 3.6 mM remained inside. This transformant solubilized 509 μM of phosphorus from rock phosphate in alfisol which is 4.5 fold higher than the Pf (pKCN2) transformant. Genomic integrants of P. fluorescens (Pf int1 and Pf int2) containing artificial oxalate operon (plac-FpOAR-oah) and artificial oxalate gene cluster (plac-FpOAR-oah, vgb, egfp) secreted 4.8 mM and 5.4 mM oxalic acid, released 329 μM and 351 μM P, respectively, in alfisol. The integrants showed enhanced root colonization, improved growth and increased P content of Vigna radiata plants. This study demonstrates oxalic acid secretion in P. fluorescens by incorporation of an artificial operon constituted of genes for oxalate synthesis and transport, which imparts mineral phosphate solubilizing ability to the organism leading to enhanced growth and P content of V. radiata in alfisol soil.
Collapse
Affiliation(s)
- Kavita Yadav
- Molecular Microbial Biochemistry Laboratory, Department of Biochemistry, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara, India
| | - Chanchal Kumar
- Molecular Microbial Biochemistry Laboratory, Department of Biochemistry, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara, India
| | - G. Archana
- Department of Microbiology and Biotechnology Centre, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara, India
| | - G. Naresh Kumar
- Molecular Microbial Biochemistry Laboratory, Department of Biochemistry, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara, India
- * E-mail:
| |
Collapse
|
4
|
Kang DC, Venkataraman PA, Dumont ME, Maloney PC. Oligomeric state of the oxalate transporter, OxlT. Biochemistry 2011; 50:8445-53. [PMID: 21866906 DOI: 10.1021/bi201175y] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/09/2023]
Abstract
OxlT, the oxalate transporter of Oxalobacter formigenes, was studied to determine its oligomeric state in solution and in the membrane. Three independent approaches were used. First, we used triple-detector (SEC-LS) size exclusion chromatography to analyze purified OxlT in detergent/lipid micelles. These measurements evaluate protein mass in a manner independent of contributions from detergent and lipid; such work shows an average OxlT mass near 47 kDa for detergent-solubilized material, consistent with that expected for monomeric OxlT (46 kDa). A disulfide-linked OxlT mutant was used to verify that it was possible detect dimers under these conditions. A second approach used amino-reactive cross-linkers of varying spacer lengths to study OxlT in detergent/lipid micelles and in natural or artificial membranes, followed by analysis via sodium dodecyl sulfate-polyacrylamide gel electrophoresis. These tests, performed under conditions where the presence of dimers can be documented for either of two known dimeric transporters (AdiC or TetL), indicate that OxlT exists as a monomer in the membrane and retains this status upon detergent solubilization. In a final test, we showed that reconstitution of OxlT into lipid vesicles at variable protein/lipid ratios has no effect on the specific activity of subsequent oxalate transport, as the OxlT content varies between 0.027 and 5.4 OxlT monomers/proteoliposome. We conclude that OxlT is a functional monomer in the membrane and in detergent/lipid micelles.
Collapse
Affiliation(s)
- Di-Cody Kang
- Department of Physiology, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, United States
| | | | | | | |
Collapse
|
5
|
Bauer J, Fritsch MJ, Palmer T, Unden G. Topology and Accessibility of the Transmembrane Helices and the Sensory Site in the Bifunctional Transporter DcuB of Escherichia coli. Biochemistry 2011; 50:5925-38. [DOI: 10.1021/bi1019995] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Julia Bauer
- Institute for Microbiology and Wine Research, Johannes Gutenberg-University, Becherweg 15, 55099 Mainz, Germany
| | - Max J. Fritsch
- College of Life Sciences, Division of Molecular Microbiology, University of Dundee, Scotland
| | - Tracy Palmer
- College of Life Sciences, Division of Molecular Microbiology, University of Dundee, Scotland
| | - Gottfried Unden
- Institute for Microbiology and Wine Research, Johannes Gutenberg-University, Becherweg 15, 55099 Mainz, Germany
| |
Collapse
|
6
|
Tran AX, Dong C, Whitfield C. Structure and functional analysis of LptC, a conserved membrane protein involved in the lipopolysaccharide export pathway in Escherichia coli. J Biol Chem 2010; 285:33529-33539. [PMID: 20720015 PMCID: PMC2963376 DOI: 10.1074/jbc.m110.144709] [Citation(s) in RCA: 104] [Impact Index Per Article: 7.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022] Open
Abstract
LptC is a conserved bitopic inner membrane protein from Escherichia coli involved in the export of lipopolysaccharide from its site of synthesis in the cytoplasmic membrane to the outer membrane. LptC forms a complex with the ATP-binding cassette transporter, LptBFG, which is thought to facilitate the extraction of lipopolysaccharide from the inner membrane and release it into a translocation pathway that includes the putative periplasmic chaperone LptA. Cysteine modification experiments established that the catalytic domain of LptC is oriented toward the periplasm. The structure of the periplasmic domain is described at a resolution of 2.2-Å from x-ray crystallographic data. The periplasmic domain of LptC consists of a twisted boat structure with two β-sheets in apposition to each other. The β-sheets contain seven and eight antiparallel β-strands, respectively. This structure bears a high degree of resemblance to the crystal structure of LptA. Like LptA, LptC binds lipopolysaccharide in vitro. In vitro, LptA can displace lipopolysaccharide from LptC (but not vice versa), consistent with their locations and their proposed placement in a unidirectional export pathway.
Collapse
Affiliation(s)
- An X Tran
- From the Department of Molecular and Cellular Biology, University of Guelph, Guelph, Ontario N1G 2W1, Canada
| | - Changjiang Dong
- Biomedical Sciences Research Complex, School of Chemistry, University of St. Andrews, Fife KY16 9ST, Scotland, United Kingdom
| | - Chris Whitfield
- From the Department of Molecular and Cellular Biology, University of Guelph, Guelph, Ontario N1G 2W1, Canada.
| |
Collapse
|
7
|
Structural and functional importance of transmembrane domain 3 (TM3) in the aspartate:alanine antiporter AspT: topology and function of the residues of TM3 and oligomerization of AspT. J Bacteriol 2009; 191:2122-32. [PMID: 19181816 DOI: 10.1128/jb.00830-08] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
AspT, the aspartate:alanine antiporter of Tetragenococcus halophilus, a membrane protein of 543 amino acids with 10 putative transmembrane (TM) helices, is the prototype of the aspartate:alanine exchanger (AAE) family of transporters. Because TM3 (isoleucine 64 to methionine 85) has many amino acid residues that are conserved among members of the AAE family and because TM3 contains two charged residues and four polar residues, it is thought to be located near (or to form part of) the substrate translocation pathway that includes the binding site for the substrates. To elucidate the role of TM3 in the transport process, we carried out cysteine-scanning mutagenesis. The substitutions of tyrosine 75 and serine 84 had the strongest inhibitory effects on transport (initial rates of l-aspartate transport were below 15% of the rate for cysteine-less AspT). Considerable but less-marked effects were observed upon the replacement of methionine 70, phenylalanine 71, glycine 74, arginine 76, serine 83, and methionine 85 (initial rates between 15% and 30% of the rate for cysteine-less AspT). Introduced cysteine residues at the cytoplasmic half of TM3 could be labeled with Oregon green maleimide (OGM), whereas cysteines close to the periplasmic half (residues 64 to 75) were not labeled. These results suggest that TM3 has a hydrophobic core on the periplasmic half and that hydrophilic residues on the cytoplasmic half of TM3 participate in the formation of an aqueous cavity in membranes. Furthermore, the presence of l-aspartate protected the cysteine introduced at glycine 62 against a reaction with OGM. In contrast, l-aspartate stimulated the reactivity of the cysteine introduced at proline 79 with OGM. These results demonstrate that TM3 undergoes l-aspartate-induced conformational alterations. In addition, nonreducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis analyses and a glutaraldehyde cross-linking assay suggest that functional AspT forms homo-oligomers as a functional unit.
Collapse
|
8
|
Culham DE, Vernikovska Y, Tschowri N, Keates RAB, Wood JM, Boggs JM. Periplasmic loops of osmosensory transporter ProP in Escherichia coli are sensitive to osmolality. Biochemistry 2009; 47:13584-93. [PMID: 19049385 DOI: 10.1021/bi801576x] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
Abstract
ProP is an osmosensory transporter. The activities of ProP and ProP*, a cysteine-less, His(6)-tagged ProP variant, increase with osmotic pressure in cells and proteoliposomes. In proteoliposomes, ProP activity is osmolality-dependent only if the magnitude of the membrane potential (DeltaPsi) exceeds 100 mV. Some amino acid replacements rendered ProP activity osmolality-insensitive [e.g., Y44M in transmembrane segment 1 (TMI); S62C in periplasmic loop 1 (loop P1)], whereas others elevated the osmolality at which ProP activates (e.g., A59C). This suggested that the environments and/or conformations of TMI and loop P1 might be osmolality-dependent. This report correlates structural dynamics of ProP with osmoregulation of its transport activity. Residues in periplasmic loops were replaced with Cys, and changes in their environments were detected by monitoring their reactivities with N-ethylmaleimide (NEM). Increasing osmolality markedly increased the NEM reactivity of some Cys residues (e.g., C59, loop P1; C415-C418, loop P6) but not others (e.g., C293, loop P4; C348, loop P5). The NEM reactivity of C62 was insensitive to osmolality, as expected. Substitution Y44M rendered the transport activities of ProP*-A59C and ProP*-Q415C, and the NEM reactivities of the introduced Cys, osmolality-insensitive. Furthermore, osmolality did not affect the reactivity of C59 in cells lacking a protonmotive force, consistent with evidence that DeltaPsi is required for osmosensing by ProP. These results indicate that the osmotically induced increases in NEM reactivity of C59 and C415 in energized bacteria are due to a conformational change of ProP in response to osmolality. They therefore constitute the first direct evidence of an osmotically induced conformational change associated with osmosensing by a transporter.
Collapse
Affiliation(s)
- Doreen E Culham
- Department of Molecular and Cellular Biology, University of Guelph, Guelph, ON, Canada N1G 2W1
| | | | | | | | | | | |
Collapse
|
9
|
Wang X, Ye L, McKinney CC, Feng M, Maloney PC. Cysteine scanning mutagenesis of TM5 reveals conformational changes in OxlT, the oxalate transporter of Oxalobacter formigenes. Biochemistry 2008; 47:5709-17. [PMID: 18452311 DOI: 10.1021/bi8001314] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
We constructed a single-cysteine panel encompassing TM5 of the oxalate transporter, OxlT. The 25 positions encompassed by TM5 were largely tolerant of mutagenesis, and functional product was recovered for 21 of the derived variants. For these derivatives, thiol-directed MTS-linked agents (MTSEA, MTSCE, and MTSES) were used as probes of transporter function, yielding 11 mutants that responded to probe treatment, as indicated by effects on oxalate transport. Further study identified three biochemical phenotypes among these responders. Group 1 included seven mutants, exemplified by G151C, displaying substrate protection against probe inhibition. Group 2 was comprised of a single mutant, P156C, which had unexpected behavior. In this case, we observed increased activity if weak acid/base or neutral probes were used, while exposure to probes introducing a fixed charge led to decreased function. In both instances, the presence of substrate prevented the observed response. Group 3 contained three mutants (e.g., S143C) in which probe sensitivity was increased by the presence of substrate. The finding of substrate-protectable probe modification in groups 1 and 2 suggests that TM5 lies on the permeation pathway, as do its structural counterparts, TM2, TM8, and TM11. In addition, we speculate that substrate binding facilitates TM5 conformational changes that allow new regions to become accessible to MTS-linked probes (group 3). These biochemical data are consistent with the recently developed OxlT homology model.
Collapse
Affiliation(s)
- Xicheng Wang
- Department of Physiology, Johns Hopkins Medical School, Baltimore, Maryland 21205, USA
| | | | | | | | | |
Collapse
|
10
|
Nie Y, Ermolova N, Kaback HR. Site-directed alkylation of LacY: effect of the proton electrochemical gradient. J Mol Biol 2007; 374:356-64. [PMID: 17920075 DOI: 10.1016/j.jmb.2007.09.006] [Citation(s) in RCA: 40] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/23/2007] [Revised: 08/28/2007] [Accepted: 09/03/2007] [Indexed: 11/19/2022]
Abstract
Previous N-ethylmaleimide-labeling studies show that ligand binding increases the reactivity of single-Cys mutants located predominantly on the periplasmic side of LacY and decreases reactivity of mutants located for the most part of the cytoplasmic side. Thus, sugar binding appears to induce opening of a periplasmic pathway with closing of the cytoplasmic cavity resulting in alternative access of the sugar-binding site to either side of the membrane. Here we describe the use of a fluorescent alkylating reagent that reproduces the previous observations with respect to sugar binding. We then show that generation of an H(+) electrochemical gradient (Delta(mu (H)+), interior negative) increases the reactivity of single-Cys mutants on the periplasmic side of the sugar-binding site and in the putative hydrophilic pathway. The results suggest that Delta(mu (H)+), like sugar, acts to increase the probability of opening on the periplasmic side of LacY.
Collapse
Affiliation(s)
- Yiling Nie
- Department of Physiology, University of California Los Angeles, Los Angeles, CA 90095-1662, USA
| | | | | |
Collapse
|
11
|
Foley DA, Sharpe HJ, Otte S. Membrane topology of the endoplasmic reticulum to Golgi transport factor Erv29p. Mol Membr Biol 2007; 24:259-68. [PMID: 17520482 DOI: 10.1080/09687860601178518] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/23/2022]
Abstract
Secretory proteins are transported from the endoplasmic reticulum to the Golgi apparatus via COPII-coated intermediates. Yeast Erv29p is a transmembrane protein cycling between these compartments. It is conserved across species, with one ortholog found in each genome studied, including the surf-4 protein in mammals. Yeast Erv29p acts as a receptor, loading a specific subset of soluble cargo, including glycosylated alpha factor pheromone precursor and carboxypeptidase Y, into vesicles. As the eukaryotic secretory pathway is highly conserved, mammalian surf-4 may perform a similar role in the transport of unknown substrates. Here we report the membrane topology of yeast Erv29p, which we solved by minimally invasive cysteine accessibility scanning using thiol-specific biotinylation and fluorescent labeling methods. Erv29p contains four transmembrane domains with both termini exposed to the cytosol. Two luminal loops may contain a recognition site for hydrophobic export signals on soluble cargo.
Collapse
Affiliation(s)
- Deirdre A Foley
- Department of Biochemistry and Molecular Genetics, University of Illinois at Chicago, Illinois, USA
| | | | | |
Collapse
|
12
|
Nanatani K, Fujiki T, Kanou K, Takeda-Shitaka M, Umeyama H, Ye L, Wang X, Nakajima T, Uchida T, Maloney PC, Abe K. Topology of AspT, the aspartate:alanine antiporter of Tetragenococcus halophilus, determined by site-directed fluorescence labeling. J Bacteriol 2007; 189:7089-97. [PMID: 17660287 PMCID: PMC2045216 DOI: 10.1128/jb.00088-07] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The gram-positive lactic acid bacterium Tetragenococcus halophilus catalyzes the decarboxylation of L-aspartate (Asp) with release of L-alanine (Ala) and CO(2). The decarboxylation reaction consists of two steps: electrogenic exchange of Asp for Ala catalyzed by an aspartate:alanine antiporter (AspT) and intracellular decarboxylation of the transported Asp catalyzed by an L-aspartate-beta-decarboxylase (AspD). AspT belongs to the newly classified aspartate:alanine exchanger family (transporter classification no. 2.A.81) of transporters. In this study, we were interested in the relationship between the structure and function of AspT and thus analyzed the topology by means of the substituted-cysteine accessibility method using the impermeant, fluorescent, thiol-specific probe Oregon Green 488 maleimide (OGM) and the impermeant, nonfluorescent, thiol-specific probe [2-(trimethylammonium)ethyl]methanethiosulfonate bromide. We generated 23 single-cysteine variants from a six-histidine-tagged cysteineless AspT template. A cysteine position was assigned an external location if the corresponding single-cysteine variant reacted with OGM added to intact cells, and a position was assigned an internal location if OGM labeling required cell lysis. The topology analyses revealed that AspT has a unique topology; the protein has 10 transmembrane helices (TMs), a large hydrophilic cytoplasmic loop (about 180 amino acids) between TM5 and TM6, N and C termini that face the periplasm, and a positively charged residue (arginine 76) within TM3. Moreover, the three-dimensional structure constructed by means of the full automatic modeling system indicates that the large hydrophilic cytoplasmic loop of AspT possesses a TrkA_C domain and a TrkA_C-like domain and that the three-dimensional structures of these domains are similar to each other even though their amino acid sequences show low similarity.
Collapse
Affiliation(s)
- Kei Nanatani
- Department of Molecular and Cell Biology, Graduate School of Agricultural Science, Tohoku University, Sendai, 981-8555 Japan
| | | | | | | | | | | | | | | | | | | | | |
Collapse
|
13
|
Wang X, Sarker RI, Maloney PC. Analysis of substrate-binding elements in OxlT, the oxalate:formate antiporter of Oxalobacter formigenes. Biochemistry 2006; 45:10344-50. [PMID: 16922510 PMCID: PMC2443686 DOI: 10.1021/bi060746v] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
An OxlT homology model suggests R272 and K355 in transmembrane helices 8 and 11, respectively, are critical to OxlT-mediated transport. We offer positive evidence supporting this idea by studying OxlT function after cysteine residues were separately introduced at these positions. Without further treatment, both mutant proteins had a null phenotype when they were reconstituted into proteoliposomes. By contrast, significant recovery of function occurred when proteoliposomes were treated with MTSEA (methanethiosulfonate ethylamine), a thiol-specific reagent that implants a positively charged amino group. In each case, there was a 2-fold increase in the Michaelis constant (K(M)) for oxalate self-exchange (from 80 to 160 microM), along with a 5-fold (K355C) or 100-fold (R272C) reduction in V(max) compared to that of the cysteine-less parental protein. Analysis by MALDI-TOF confirmed that MTSEA introduced the desired modification. We also examined substrate selectivity for the treated derivatives. While oxalate remained the preferred substrate, there was a shift in preference among other substrates so that the normal rank order (oxalate > malonate > formate) was altered to favor smaller substrates (oxalate > formate > malonate). This shift is consistent with the idea that the substrate-binding site is reduced in size via introduction of the SCH(2)CH(2)NH(3)(+) adduct, which generates a side chain that is approximately 1.85 A longer than that of lysine or arginine. These findings lead us to conclude that R272 and K355 are essential components of the OxlT substrate-binding site.
Collapse
Affiliation(s)
- Xicheng Wang
- Department of Physiology, Johns Hopkins Medical School, Baltimore, MD 21205
| | - Rafiquel I. Sarker
- Department of Physiology, Johns Hopkins Medical School, Baltimore, MD 21205
| | - Peter C. Maloney
- Department of Physiology, Johns Hopkins Medical School, Baltimore, MD 21205
| |
Collapse
|
14
|
Lesoine JF, Holmberg B, Maloney P, Wang X, Novotny L, Knauf PA. Development of an spFRET method to measure structure changes in ion exchange proteins. Acta Physiol (Oxf) 2006; 187:141-7. [PMID: 16734750 DOI: 10.1111/j.1748-1716.2006.01533.x] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
AIM Understanding the mechanism of tightly coupled ion exchange proteins, important effectors of cell volume regulation and other physiologically important transport processes requires means to observe dynamic changes in structure during the transport cycle. As a step towards this goal, we have applied single-pair fluorescence resonance energy transfer to a monomeric bacterial oxalate-formate exchanger (OxlT). METHODS A His-9 tagged OxlT mutant containing two cysteines at positions 17 and 224 was labelled with cyanine dye maleimides (Cy3 donor and Cy5 acceptor) and attached to glass coverslips for measurements of donor and acceptor emission from single molecules, as described (P. Pal et al. Biophys J89, L11, 2005). RESULTS Time-series data from 20 spots containing donor and acceptor provided evidence for single-pair energy transfer. From the efficiency of energy transfer, the mean donor-acceptor distance was determined to be 44.2 A. Considering the size of the probes, this is in good agreement with the Calpha distance of 39.6 A for the corresponding sites found in the OxlT structural (homology) model (Q. Yang et al. Proc Natl Acad Sci102, 8513, 2005). CONCLUSION These results demonstrate the feasibility of single-pair fluorescence resonance energy transfer to measure distances between known sites in single OxlT molecules. This technique provides a potential means to test models for transport-related conformational changes, as well as to detect real-time structure alterations during the catalytic transport cycle.
Collapse
Affiliation(s)
- J F Lesoine
- Institute of Optics, University of Rochester, Rochester, NY 14627, USA.
| | | | | | | | | | | |
Collapse
|
15
|
Yang Q, Wang X, Ye L, Mentrikoski M, Mohammadi E, Kim YM, Maloney PC. Experimental tests of a homology model for OxlT, the oxalate transporter of Oxalobacter formigenes. Proc Natl Acad Sci U S A 2005; 102:8513-8. [PMID: 15932938 PMCID: PMC1150865 DOI: 10.1073/pnas.0503533102] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/07/2005] [Indexed: 11/18/2022] Open
Abstract
Using the x-ray structure of the glycerol 3-phosphate transporter (GlpT), we devised a model for the distantly related oxalate transporter, OxlT. The model accommodates all earlier biochemical information on OxlT, including the idea that Lys-355 lies on the permeation pathway, and predicts that Lys-355 and a second positive center, Arg-272, comprise the binding site for divalent oxalate. Study of R272K, R272A, and R272Q derivatives verifies that Arg-272 is essential, and comparisons with GlpT show that both anion transporters bind substrates within equivalent domains. In 22 single-cysteine variants in TM7 and TM8, topology as marked by accessibility to Oregon green maleimide is predicted by the model, with similar concordance for 52 positions probed earlier. The model also reconciles cross-linking of a cysteine pair placed near the periplasmic ends of TM2 and TM7, and retrospective study of TM2 and TM11 confirms that positions supporting disulfide trapping lie at a helical interface. Our work describes a pathway to the modeling of OxlT and other transporters in the major facilitator superfamily and outlines simple experimental tests to evaluate such proposals.
Collapse
Affiliation(s)
- Qiang Yang
- Department of Physiology, Johns Hopkins Medical School, 725 North Wolfe Street, Baltimore, MD 21205-2185, USA
| | | | | | | | | | | | | |
Collapse
|
16
|
Bogdanov M, Zhang W, Xie J, Dowhan W. Transmembrane protein topology mapping by the substituted cysteine accessibility method (SCAM(TM)): application to lipid-specific membrane protein topogenesis. Methods 2005; 36:148-71. [PMID: 15894490 PMCID: PMC4104023 DOI: 10.1016/j.ymeth.2004.11.002] [Citation(s) in RCA: 126] [Impact Index Per Article: 6.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/01/2004] [Revised: 11/15/2004] [Accepted: 11/15/2004] [Indexed: 01/03/2023] Open
Abstract
We provide an overview of lipid-dependent polytopic membrane protein topogenesis, with particular emphasis on Escherichia coli strains genetically altered in their lipid composition and strategies for experimentally determining the transmembrane organization of proteins. A variety of reagents and experimental strategies are described including the use of lipid mutants and thiol-specific chemical reagents to study lipid-dependent and host-specific membrane protein topogenesis by substituted cysteine site-directed chemical labeling. Employing strains in which lipid composition can be controlled temporally during membrane protein synthesis and assembly provides a means to observe dynamic changes in protein topology as a function of membrane lipid composition.
Collapse
Affiliation(s)
- Mikhail Bogdanov
- Department of Biochemistry and Molecular Biology, University of Texas-Houston, Medical School, Houston, TX 77030, USA
| | - Wei Zhang
- Department of Biochemistry and Molecular Biology, University of Texas-Houston, Medical School, Houston, TX 77030, USA
| | - Jun Xie
- Department of Biochemistry and Molecular Biology, University of Texas-Houston, Medical School, Houston, TX 77030, USA
| | - William Dowhan
- Department of Biochemistry and Molecular Biology, University of Texas-Houston, Medical School, Houston, TX 77030, USA
| |
Collapse
|
17
|
Howitt SM. The role of cysteine residues in the sulphate transporter, SHST1: construction of a functional cysteine-less transporter. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2005; 1669:95-100. [PMID: 15893511 DOI: 10.1016/j.bbamem.2005.01.002] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/13/2004] [Revised: 01/06/2005] [Accepted: 01/07/2005] [Indexed: 10/25/2022]
Abstract
We investigated the role of cysteine residues in the sulphate transporter, SHST1, with the aim of generating a functional cysteine-less variant. SHST1 contains five cysteine residues and none was essential for function. However, replacement of C421 resulted in a reduction in transport activity. Sulphate transport by C205 mutants was dependent on the size of the residue at this position. Alanine at position 205 resulted in a complete loss of function whereas leucine resulted in a 3-fold increase in sulphate transport relative to wild type SHST1. C205 is located in a putative intracellular loop and our results suggest that this loop may be important for sulphate transport. By replacing C205 with leucine and the other four cysteine residues with alanine, we constructed a cysteine-less variant of SHST1 that has transport characteristics indistinguishable from wild type. This construct will be useful for further structure and function studies of SHST1.
Collapse
Affiliation(s)
- Susan M Howitt
- School of Biochemistry and Molecular Biology, The Faculties, Australian National University, Canberra, ACT 0200, Australia.
| |
Collapse
|
18
|
Henderson NS, So SSK, Martin C, Kulkarni R, Thanassi DG. Topology of the outer membrane usher PapC determined by site-directed fluorescence labeling. J Biol Chem 2004; 279:53747-54. [PMID: 15485883 DOI: 10.1074/jbc.m409192200] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
In contrast to typical membrane proteins that span the lipid bilayer via transmembrane alpha-helices, bacterial outer membrane proteins adopt a beta-barrel architecture composed of antiparallel transmembrane beta-strands. The topology of outer membrane proteins is difficult to predict accurately using computer algorithms, and topology mapping protocols commonly used for alpha-helical membrane proteins do not work for beta-barrel proteins. We present here the topology of the PapC usher, an outer membrane protein required for assembly and secretion of P pili by the chaperone/usher pathway in uropathogenic Escherichia coli. An initial attempt to map PapC topology by insertion of protease cleavage sites was largely unsuccessful due to lack of cleavage at most sites and the requirement to disrupt the outer membrane to identify periplasmic sites. We therefore adapted a site-directed fluorescence labeling technique to permit topology mapping of outer membrane proteins using small molecule probes in intact bacteria. Using this method, we demonstrated that PapC has the potential to encode up to 32 transmembrane beta-strands. Based on experimental evidence, we propose that the usher consists of an N-terminal beta-barrel domain comprised of 26 beta-strands and that a distinct C-terminal domain is not inserted into the membrane but is located instead within the lumen of the N-terminal beta-barrel similar to the plug domains encoded by the outer membrane iron-siderophore uptake proteins.
Collapse
Affiliation(s)
- Nadine S Henderson
- Department of Molecular Genetics and Microbiology, Center for Infectious Diseases, Stony Brook University, Stony Brook, NY 11794-5120, USA
| | | | | | | | | |
Collapse
|
19
|
Heymann JAW, Hirai T, Shi D, Subramaniam S. Projection structure of the bacterial oxalate transporter OxlT at 3.4A resolution. J Struct Biol 2004; 144:320-6. [PMID: 14643200 DOI: 10.1016/j.jsb.2003.09.002] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022]
Abstract
OxlT is a bacterial transporter protein with 12 transmembrane segments that belongs to the Major Facilitator Superfamily of transporters. It facilitates the exchange of oxalate and formate across the membrane of the Gram-negative bacterium Oxalobacter formigenes. From an electron crystallographic analysis of two-dimensional, tube-like crystals of OxlT, we have previously determined the three-dimensional structure of this transporter at 6.5 A resolution. Here, we report conditions to obtain crystalline, two-dimensional sheets of OxlT with diameters exceeding 2 microm. Images of the crystalline sheets were recorded at liquid nitrogen temperatures on a transmission electron microscope equipped with a field-emission gun, operated at 300 kV. Computed optical diffraction patterns from the best images display measurable reflections to about 3.4A, and electron diffraction patterns show spots to about 3.2 A resolution in the best cases. As in the case of the tube-like crystals, the new crystalline sheets also belong to the p22(1)2(1) symmetry group. However, the unit cell dimensions of 102.7A x 67.3 A are significantly smaller in one direction than those previously observed with the tube-like crystals that display unit cell dimensions of 100.3A x 79.0 A. Different regions of OxlT are involved in intermolecular contacts in the two types of crystals, and the improved resolution of the sheet crystals appears to be mainly attributable to this tighter packing of the monomers within the unit cell.
Collapse
Affiliation(s)
- Jürgen A W Heymann
- Laboratory of Biochemistry, National Cancer Institute, National Institutes of Health, Bethesda, MD 20892, USA
| | | | | | | |
Collapse
|
20
|
Root P, Mutus B. O-Aminobenzoyl-S-nitrosohomocysteine, a fluorogenic probe for cell-surface thiol determinations via a microtiter plate assay. Anal Biochem 2003; 320:299-302. [PMID: 12927837 DOI: 10.1016/s0003-2697(03)00393-2] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Affiliation(s)
- Paul Root
- Department of Chemistry and Biochemistry, University of Windsor, Windsor, Ontario N9B 3P4, Canada
| | | |
Collapse
|
21
|
Ye L, Maloney PC. Structure/function relationships in OxlT, the oxalate/formate antiporter of Oxalobacter formigenes: assignment of transmembrane helix 2 to the translocation pathway. J Biol Chem 2002; 277:20372-8. [PMID: 11919184 DOI: 10.1074/jbc.m111140200] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
We constructed a single cysteine panel encompassing transmembrane helix two (TM2) of OxlT, the oxalate/formate antiporter of Oxalobacter formigenes. Among the 21 positions targeted, cysteine substitution identified one (phenylalanine 59) as essential to OxlT expression and three (glutamine 56, glutamine 66, and serine 69) as potentially critical to OxlT function. By probing membranes with a bulky hydrophilic probe (Oregon Green maleimide) we also located a central inaccessible core of at least eight residues in length, extending from leucine 61 to glycine 68. Functional assays based on reconstitution of crude detergent extracts showed that of single cysteine mutants within the TM2 core only the Q63C variant was substantially (> or =95%) inhibited by thiol-specific agents (carboxyethyl methanethiosulfonate and ethylsulfonate methanethiosulfonate). Subsequent analytical work using the purified Q63C protein showed that inhibition by ethylsulfonate methanethiosulfonate was blocked by substrate and that the concentration dependence of such substrate protection occurred with a binding constant of 0.16 mm oxalate, comparable with the Michaelis constant observed for oxalate transport (0.23 mm). These findings lead us to conclude that position 63 lies on the OxlT translocation pathway. Our conclusion is strengthened by the finding that position 63, along with most other positions relevant to TM2 function, is found on a helical face that can be cross-linked to the pathway-facing surface of TM11 (Fu, D., Sarker, R. I., Bolton, E., and Maloney, P. C. (2001) J. Biol. Chem. 276, 8753-8760).
Collapse
Affiliation(s)
- Liwen Ye
- Department of Physiology, Johns Hopkins Medical School, Baltimore, Maryland 21205, USA
| | | |
Collapse
|
22
|
Kim YM, Ye L, Maloney PC. Helix proximity in OxlT, the oalate:formate antiporter of oxalobacter formigenes. Cross-linking between TM2 and TM11. J Biol Chem 2001; 276:36681-6. [PMID: 11457863 DOI: 10.1074/jbc.m106079200] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Experiments were designed to evaluate the proximity of transmembrane helices two (TM2) and eleven (TM11) in the tertiary structure of OxlT, the oxalate:formate exchange transporter of Oxalobacter formigenes. A tandem duplication of the Factor Xa protease cleavage site (IEGRIEGR) was inserted into the central cytoplasmic loop of an OxlT cysteine-less derivative in which an endogenous cleavage site had been eliminated by mutagenesis (R248Q). Using this host, double cysteine derivatives were constructed so as to pair one of seventeen positions in TM2 with one of four positions in TM11. Following treatment of membrane vesicles with Cu(II)(1,10-phenanthroline)(3), molecular iodine, or N,N'-o-phenylenedimaleimide, samples were exposed to Factor Xa, and disulfide bond formation was assessed after SDS-polyacrylamide gel electrophoresis by staining with antibody directed against the OxlT C terminus. In the absence of disulfide bond formation, exposure to Factor Xa revealed the expected C-terminal 22-kDa fragment, a result unaffected by the presence of reductant. By contrast, after disulfide formation, OxlT mobility remained at 35 kDa, and appearance of the 22-kDa fragment required addition of 200 mm dithiothreitol prior to electrophoresis. The four TM11 positions chosen for cysteine substitution lie on a helical face known to interact with substrate. Similarly, TM2 positions supporting disulfide trapping were also confined to a single helical face. We conclude that TM2 and TM11 are in close juxtaposition to one another in the tertiary structure of OxlT.
Collapse
Affiliation(s)
- Y M Kim
- Department of Physiology, Johns Hopkins Medical School, Baltimore, Maryland 21205, USA
| | | | | |
Collapse
|
23
|
Heymann JA, Sarker R, Hirai T, Shi D, Milne JL, Maloney PC, Subramaniam S. Projection structure and molecular architecture of OxlT, a bacterial membrane transporter. EMBO J 2001; 20:4408-13. [PMID: 11500368 PMCID: PMC125264 DOI: 10.1093/emboj/20.16.4408] [Citation(s) in RCA: 57] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
The major facilitator superfamily (MFS) represents the largest collection of evolutionarily related members within the class of membrane 'carrier' proteins. OxlT, a representative example of the MFS, is an oxalate-transporting membrane protein in Oxalobacter formigenes. From an electron crystallographic analysis of two-dimensional crystals of OxlT, we have determined the projection structure of this membrane transporter. The projection map at 6 A resolution indicates the presence of 12 transmembrane helices in each monomer of OxlT, with one set of six helices related to the other set by an approximate internal two-fold axis. The projection map reveals the existence of a central cavity, which we propose to be part of the pathway of oxalate transport. By combining information from the projection map with related biochemical data, we present probable models for the architectural arrangement of transmembrane helices in this protein superfamily.
Collapse
Affiliation(s)
| | - Rafiquel Sarker
- Laboratories of Biochemistry and
Cell Biology, National Cancer Institute, National Institutes of Health, Bethesda, MD 20892, and Department of Physiology, The Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA Corresponding author e-mail:
J.A.W.Heymann, R.Sarker and T.Hirai contributed equally to this work
| | | | | | - Jacqueline L.S. Milne
- Laboratories of Biochemistry and
Cell Biology, National Cancer Institute, National Institutes of Health, Bethesda, MD 20892, and Department of Physiology, The Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA Corresponding author e-mail:
J.A.W.Heymann, R.Sarker and T.Hirai contributed equally to this work
| | - Peter C. Maloney
- Laboratories of Biochemistry and
Cell Biology, National Cancer Institute, National Institutes of Health, Bethesda, MD 20892, and Department of Physiology, The Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA Corresponding author e-mail:
J.A.W.Heymann, R.Sarker and T.Hirai contributed equally to this work
| | - Sriram Subramaniam
- Laboratories of Biochemistry and
Cell Biology, National Cancer Institute, National Institutes of Health, Bethesda, MD 20892, and Department of Physiology, The Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA Corresponding author e-mail:
J.A.W.Heymann, R.Sarker and T.Hirai contributed equally to this work
| |
Collapse
|
24
|
Hall JA, Maloney PC. Transmembrane segment 11 of UhpT, the sugar phosphate carrier of Escherichia coli, is an alpha-helix that carries determinants of substrate selectivity. J Biol Chem 2001; 276:25107-13. [PMID: 11349129 DOI: 10.1074/jbc.m102017200] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
In Escherichia coli, transport of hexose 6-phosphates is mediated by the P(i)-linked antiport carrier, UhpT, a member of the major facilitator superfamily. We showed earlier that Lys(391), a member of an intrahelical salt bridge (Asp(388)/Lys(391)) in the eleventh transmembrane segment (TM11) of this transporter, can function as a determinant of substrate selectivity (Hall, J. A., Fann, M.-C., and Maloney, P. C. (1999) J. Biol. Chem. 274, 6148-6153). Here, we examine in detail the role of TM11 in setting substrate preference. Derivatives having an uncompensated cationic charge at either position 388 or 391 (the D388C, D388V, or D388K/K391C variants) are gain-of-function mutants in which phosphoenolpyruvate, not sugar 6-phosphate, is the preferred organic substrate. By contrast, when an uncompensated anionic charge is placed at position 388 (K391C), we observed behavior consistent with an increased preference for monovalent rather than divalent sugar 6-phosphate. Because positions 388 and 391 lie deep within the UhpT hydrophobic sector, these findings suggested that an extended length of TM11 may be accessible to external substrates and probes. To explore this issue, we used a panel of TM11 single cysteine variants to examine the transport of glucose 6-phosphate in the presence and absence of the membrane-impermeant, thiol-reactive agent p-chloromercuribenzosulfonate (PCMBS). Accessibility to PCMBS, together with the pattern of substrate protection against PCMBS inhibition, leads us to conclude that TM11 spans the membrane as an alpha-helix, with approximately two-thirds of its surface lining a substrate translocation pathway. We suggest that this feature is a general property of carrier proteins in the major facilitator superfamily and that for this reason residues in TM11 will serve to carry determinants of substrate selectivity.
Collapse
Affiliation(s)
- J A Hall
- Department of Physiology, Johns Hopkins University Medical School, 725 N. Wolfe St., Baltimore, MD 21205, USA
| | | |
Collapse
|
25
|
Ye L, Jia Z, Jung T, Maloney PC. Topology of OxlT, the oxalate transporter of Oxalobacter formigenes, determined by site-directed fluorescence labeling. J Bacteriol 2001; 183:2490-6. [PMID: 11274108 PMCID: PMC95165 DOI: 10.1128/jb.183.8.2490-2496.2001] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The topology of OxlT, the oxalate:formate exchange protein of Oxalobacter formigenes, was established by site-directed fluorescence labeling, a simple strategy that generates topological information in the context of the intact protein. Accessibility of cysteine to the fluorescent thiol-directed probe Oregon green maleimide (OGM) was examined for a panel of 34 single-cysteine variants, each generated in a His(9)-tagged cysteine-less host. The reaction with OGM was readily scored by examining the fluorescence profile after sodium dodecyl sulfate-polyacrylamide gel electrophoresis of material purified by Ni2+ linked affinity chromatography. A position was assigned an external location if its single-cysteine derivative reacted with OGM added to intact cells; a position was designated internal if OGM labeling required cell lysis. We also showed that labeling of external, but not internal, positions was blocked by prior exposure of cells to the impermeable and nonfluorescent thiol-specific agent ethyltrimethylammonium methanethiosulfonate. Of the 34 positions examined in this way, 29 were assigned unambiguously to either an internal or external location; 5 positions could not be assigned, since the target cysteine failed to react with OGM. There was no evidence of false-positive assignment. Our findings document a simple and rapid method for establishing the topology of a membrane protein and show that OxlT has 12 transmembrane segments, confirming inferences from hydropathy analysis.
Collapse
Affiliation(s)
- L Ye
- Department of Physiology, Johns Hopkins Medical School, Baltimore, Maryland 21205, USA
| | | | | | | |
Collapse
|
26
|
Fu D, Sarker RI, Abe K, Bolton E, Maloney PC. Structure/function relationships in OxlT, the oxalate-formate transporter of oxalobacter formigenes. Assignment of transmembrane helix 11 to the translocation pathway. J Biol Chem 2001; 276:8753-60. [PMID: 11113128 DOI: 10.1074/jbc.m008417200] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
OxlT, the oxalate:formate antiporter of Oxalobacter formigenes, has a lone charged residue, lysine 355 (Lys-355), at the center of transmembrane helix 11 (TM11). Because Lys-355 is the only charged residue in the hydrophobic sector, we tested the hypothesis that lysine 355 contributes to the binding site for the anionic substrate, oxalate. This idea was supported by mutational analysis, which showed that of five variants studied (Lys-355 --> Cys, Gly, Gln, Arg, or Thr), residual function was found for only the K355R derivative, in which catalytic efficiency had fallen 2,600-fold. Further insight came from a study of TM11 single-cysteine mutants, using the impermeant, thiol-specific reagents, carboxyethyl methanethiosulfonate and ethyltrimethylammonium methanethiosulfonate. Of the five reactive positions identified in TM11, four were at the cytoplasmic or periplasmic ends of TM11 (S344C and A345C, and G366C and A370C, respectively), whereas the fifth was at the center of the helix (S359C). Added study with carboxyethyl methanethiosulfonate and ethylsulfonate methylthiosulfonate showed that the attack on S359C could be blocked by the presence of the substrate, oxalate, and that protection could be predicted quantitatively by a kinetic model in which S359C is accessible only in the unliganded form of OxlT. Parallel study showed that the proteoliposomes used in such work contained OxlT of right side-out and inside-out orientations in about equal amounts. Accordingly, full inhibition of S359C by the impermeable methanethiosulfonate-linked probes must reflect an approach from both the cytosolic and periplasmic surfaces of the protein. This, coupled with the finding of substrate protection, leads us to conclude that S359C lies on the translocation pathway through OxlT. Since position 359 and 355 lie on the same helical face, we suggest that Lys-355 also lies on the translocation pathway, consistent with the idea that the essential nature of Lys-355 reflects its role in binding the anionic substrate, oxalate.
Collapse
Affiliation(s)
- D Fu
- Department of Physiology, The Johns Hopkins University Medical School, Baltimore, Maryland 21205, USA
| | | | | | | | | |
Collapse
|
27
|
van Geest M, Lolkema JS. Membrane topology and insertion of membrane proteins: search for topogenic signals. Microbiol Mol Biol Rev 2000; 64:13-33. [PMID: 10704472 PMCID: PMC98984 DOI: 10.1128/mmbr.64.1.13-33.2000] [Citation(s) in RCA: 153] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Integral membrane proteins are found in all cellular membranes and carry out many of the functions that are essential to life. The membrane-embedded domains of integral membrane proteins are structurally quite simple, allowing the use of various prediction methods and biochemical methods to obtain structural information about membrane proteins. A critical step in the biosynthetic pathway leading to the folded protein in the membrane is its insertion into the lipid bilayer. Understanding of the fundamentals of the insertion and folding processes will significantly improve the methods used to predict the three-dimensional membrane protein structure from the amino acid sequence. In the first part of this review, biochemical approaches to elucidate membrane protein topology are reviewed and evaluated, and in the second part, the use of similar techniques to study membrane protein insertion is discussed. The latter studies search for signals in the polypeptide chain that direct the insertion process. Knowledge of the topogenic signals in the nascent chain of a membrane protein is essential for the evaluation of membrane topology studies.
Collapse
Affiliation(s)
- M van Geest
- Department of Microbiology, Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, 9751 NN Haren, The Netherlands
| | | |
Collapse
|
28
|
Gründling A, Bläsi U, Young R. Biochemical and genetic evidence for three transmembrane domains in the class I holin, lambda S. J Biol Chem 2000; 275:769-76. [PMID: 10625606 DOI: 10.1074/jbc.275.2.769] [Citation(s) in RCA: 67] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
lambda S, the prototype class I holin gene, encodes three potential transmembrane domains in its 107 codons, whereas 21 S, the class II prototype spans only 71 codons and encodes two transmembrane domains. Many holin genes, including lambda S and 21 S, have the "dual-start" regulatory motif at the N terminus, suggesting that class I and II holins have the same topology. The primary structure of 21 S strongly suggests a bitopic "helical-hairpin" topology, with N and C termini on the cytoplasmic side of the membrane. However, lambda S chimeras with an N-terminal signal sequence show Lep-dependent function, indicating that the N-terminal domain of S requires export. Here the signal sequence chimera is shown to be sensitive to the missense change A52V, which blocks normal S function. Moreover, cysteine-modification studies in isolated membranes using a collection of S variants with single-cysteine substitutions show that the positions in the core of the 3 putative transmembrane domains of lambda S are protected. Also, S proteins with single-cysteine substitutions in the predicted cytoplasmic and periplasmic loops are more efficiently labeled in inverted membrane vesicles and whole cells, respectively. These data constitute direct evidence that the holin S(lambda) has three transmembrane domains and indicate that class I and class II holins have different topologies, despite regulatory and functional homology.
Collapse
Affiliation(s)
- A Gründling
- Department of Biochemistry and Biophysics Texas A&M University, College Station, Texas 77843-2128, USA
| | | | | |
Collapse
|
29
|
Fann MC, Maloney PC. Functional symmetry of UhpT, the sugar phosphate transporter of Escherichia coli. J Biol Chem 1998; 273:33735-40. [PMID: 9837961 DOI: 10.1074/jbc.273.50.33735] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
UhpT, the sugar phosphate transporter of Escherichia coli, acts to exchange internal inorganic phosphate for external hexose 6-phosphate. Because of this operational asymmetry, we studied variants in which right-side-out (RSO) or inside-out (ISO) orientations could be analyzed independently to ask whether the inward- and outward-facing UhpT surfaces have different substrate specificities. To study the RSO orientation, we constructed a histidine-tagged derivative, His10K291C/K294N, in which the sole external tryptic cleavage site (Lys294) had been removed. Functional assay as well as immunoblot analysis showed that trypsin treatment of proteoliposomes containing His10K291C/K294N led to loss of about 50% of the original population, reflecting retention of only the RSO population. To study the ISO orientation, we used a His10V284C derivative, in which a newly inserted external cysteine (Cys284) conferred sensitivity to the thiol-reactive agent, 3-(N-maleimidylpropionyl)biocytin. In this case, 3-(N-maleimidylpropionyl)biocytin treatment of proteoliposomes containing His10V284C gave about a 60% loss of activity, and immunodetection of biotin showed parallel modification of an equivalent fraction of the original population. Together, such findings indicate that the UhpT RSO and ISO orientations are in about equal proportion in proteoliposomes and that a single population can be generated by exposure of these derivatives to the appropriate agent. This allowed us to study proteoliposomes with UhpT functioning in RSO orientation (His10K291C/K294N) or ISO orientation (His10V284C) with respect to the kinetics of glucose 6-phosphate transport by phosphate-loaded proteoliposomes and also the inhibitions found with 2-deoxy-glucose 6-phosphate, mannose 6-phosphate, galactose 6-phosphate, fructose 6-phosphate, and inorganic phosphate. We found no significant differences in the behavior of UhpT in its different orientations, indicating that the transporter possesses an overall functional symmetry.
Collapse
Affiliation(s)
- M C Fann
- Department of Physiology, Johns Hopkins Medical School, Baltimore, Maryland 21205, USA
| | | |
Collapse
|