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1.
Mol Microbiol ; 114(1): 151-171, 2020 07.
Article in English | MEDLINE | ID: mdl-32198949

ABSTRACT

Sinorhizobium (Ensifer) meliloti is a model example of a soil alpha-proteobacterium which induces the formation of nitrogen-fixing symbiotic nodules on the legume roots. In contrast to all other rhizobacterial species, S. meliloti contains multiple homologs of nucleobase transporter genes that belong to NAT/NCS2 family (Nucleobase-Ascorbate Transporter/Nucleobase-Cation Symporter-2). We analyzed functionally all (six) relevant homologs of S. meliloti 1,021 using Escherichia coli K-12 as a host and found that five of them are high-affinity transporters for xanthine (SmLL9), uric acid (SmLL8, SmLL9, SmX28), adenine (SmVC3, SmYE1), guanine (SmVC3), or hypoxanthine (SmVC3). Detailed analysis of substrate profiles showed that two of these transporters display enlarged specificity (SmLL9, SmVC3). SmLL9 is closely related in sequence with the xanthine-specific XanQ of E. coli. We subjected SmLL9 to rationally designed site-directed mutagenesis and found that the role of key binding-site residues of XanQ is conserved in SmLL9, whereas a single amino-acid change (S93N) converts the xanthine/uric-acid transporter SmLL9 to a xanthine-preferring variant, due to disruption of an essential hydrogen bond with the C8 oxygen of uric acid. The results highlight the presence of several different purine nucleobase transporters in S. meliloti and imply that the purine transport might be important in the nodule symbiosis involving S. meliloti.


Subject(s)
Biological Transport, Active/genetics , Sinorhizobium meliloti/genetics , Sinorhizobium meliloti/metabolism , Symporters/genetics , Symporters/metabolism , Adenine/metabolism , Escherichia coli K12/genetics , Escherichia coli K12/metabolism , Guanine/metabolism , Hypoxanthine/metabolism , Membrane Transport Proteins/genetics , Membrane Transport Proteins/metabolism , Plant Root Nodulation/physiology , Rhizosphere , Root Nodules, Plant/microbiology , Uric Acid/metabolism , Xanthine/metabolism
2.
J Exp Bot ; 71(10): 3110-3125, 2020 05 30.
Article in English | MEDLINE | ID: mdl-32016431

ABSTRACT

Monosaccharide transporters (MSTs) represent key components of the carbon transport and partitioning mechanisms in plants, mediating the cell-to-cell and long-distance distribution of a wide variety of monosaccharides. In this study, we performed a thorough structural, molecular, and physiological characterization of the monosaccharide transporter gene family in the model legume Medicago truncatula. The complete set of MST family members was identified with a novel bioinformatic approach. Prolonged darkness was used as a test condition to identify the relevant transcriptomic and metabolic responses combining MST transcript profiling and metabolomic analysis. Our results suggest that MSTs play a pivotal role in the efficient partitioning and utilization of sugars, and possibly in the mechanisms of carbon remobilization in nodules upon photosynthate-limiting conditions, as nodules are forced to acquire a new role as a source of both C and N.


Subject(s)
Medicago truncatula , Carbon/metabolism , Medicago truncatula/genetics , Medicago truncatula/metabolism , Membrane Transport Proteins , Monosaccharides , Nitrogen Fixation , Plant Proteins/genetics , Plant Proteins/metabolism , Symbiosis
3.
Biochim Biophys Acta Biomembr ; 1861(9): 1546-1557, 2019 09 01.
Article in English | MEDLINE | ID: mdl-31283918

ABSTRACT

Recombinant expression systems for mammalian membrane transport proteins are often limited by insufficient yields to support structural studies, inadequate post-translational processing and problems related with improper membrane targeting or cytotoxicity. Use of alternative expression systems and optimization of expression/purification protocols are constantly needed. In this work, we explore the applicability of the laboratory strain LEXSY of the ancient eukaryotic microorganism Leishmania tarentolae as a new expression system for mammalian nucleobase permeases of the NAT/NCS2 (Nucleobase-Ascorbate Transporter/Nucleobase-Cation Symporter-2) family. We achieved the heterologous expression of the purine-pyrimidine permease rSNBT1 from Rattus norvegicus (tagged at C-terminus with a red fluorescent protein), as confirmed by confocal microscopy and biochemical analysis of the subcellular fractions enriched in membrane proteins. The cDNA of rSNBT1 has been subcloned in a pLEXSY-sat-mrfp1vector and used to generate transgenic L. tarentolae-rsnbt1-mrfp1 strains carrying the pLEXSY-sat-rsnbt1-mrfp1 plasmid either episomally or integrated in the chromosomal DNA. The chimeric transporter rSNBT1-mRFP1 is targeted to the ER and the plasma membrane of the L. tarentolae promastigotes. The transgenic strains are capable of transporting nucleobases that are substrates of rSNBT1 but also of the endogenous L. tarentolae nucleoside/nucleobase transporters. A dipyridamole-resistant Na+-dependent fraction of uptake is attributed to the exogenously expressed rSNBT1.


Subject(s)
Leishmania/genetics , Nucleobase Transport Proteins/genetics , Protein Engineering/methods , Animals , Animals, Genetically Modified , Biological Transport/genetics , Ion Transport , Leishmania/metabolism , Membrane Transport Proteins/metabolism , Nucleobase Transport Proteins/metabolism , Purines , Pyrimidines , Rats , Sodium/metabolism , Symporters/metabolism
4.
Mol Microbiol ; 108(2): 204-219, 2018 04.
Article in English | MEDLINE | ID: mdl-29437264

ABSTRACT

The uracil permease UraA of Escherichia coli is a structurally known prototype for the ubiquitous Nucleobase-Ascorbate Transporter (NAT) or Nucleobase-Cation Symporter-2 (NCS2) family and represents a well-defined subgroup of bacterial homologs that remain functionally unstudied. Here, we analyze four of these homologs, including RutG of E. coli which shares 35% identity with UraA and is encoded in the catabolic rut (pyrimidine utilization) operon. Using amplified expression in E. coli K-12, we show that RutG is a high-affinity permease for uracil, thymine and, at low efficiency, xanthine and recognizes also 5-fluorouracil and oxypurinol. In contrast, UraA and the homologs from Acinetobacter calcoaceticus and Aeromonas veronii are permeases specific for uracil and 5-fluorouracil. Molecular docking indicates that thymine is hindered from binding to UraA by a highly conserved Phe residue which is absent in RutG. Site-directed replacement of this Phe with Ala in the three uracil-specific homologs allows high-affinity recognition and/or transport of thymine, emulating the RutG profile. Furthermore, all RutG orthologs from enterobacteria retain an Ala at this position, implying that they can use both uracil and thymine and, possibly, xanthine as substrates and provide the bacterial cell with a range of catabolizable nucleobases.


Subject(s)
Escherichia coli Proteins/chemistry , Escherichia coli Proteins/metabolism , Escherichia coli/enzymology , Membrane Transport Proteins/chemistry , Membrane Transport Proteins/metabolism , Pyrimidines/metabolism , Uracil/metabolism , Bacteria/classification , Bacteria/enzymology , Bacteria/genetics , Escherichia coli/chemistry , Escherichia coli/genetics , Escherichia coli/metabolism , Escherichia coli Proteins/genetics , Membrane Transport Proteins/genetics , Molecular Docking Simulation , Multigene Family , Operon , Phylogeny , Pyrimidines/chemistry , Substrate Specificity , Thymine/chemistry , Thymine/metabolism , Uracil/chemistry
5.
J Biol Chem ; 288(52): 36827-40, 2013 Dec 27.
Article in English | MEDLINE | ID: mdl-24214977

ABSTRACT

The evolutionarily broad family nucleobase-cation symporter-2 (NCS2) encompasses transporters that are conserved in binding site architecture but diverse in substrate selectivity. Putative purine transporters of this family fall into one of two homology clusters: COG2233, represented by well studied xanthine and/or uric acid permeases, and COG2252, consisting of transporters for adenine, guanine, and/or hypoxanthine that remain unknown with respect to structure-function relationships. We analyzed the COG2252 genes of Escherichia coli K-12 with homology modeling, functional overexpression, and mutagenesis and showed that they encode high affinity permeases for the uptake of adenine (PurP and YicO) or guanine and hypoxanthine (YjcD and YgfQ). The two pairs of paralogs differ clearly in their substrate and ligand preferences. Of 25 putative inhibitors tested, PurP and YicO recognize with low micromolar affinity N(6)-benzoyladenine, 2,6-diaminopurine, and purine, whereas YjcD and YgfQ recognize 1-methylguanine, 8-azaguanine, 6-thioguanine, and 6-mercaptopurine and do not recognize any of the PurP ligands. Furthermore, the permeases PurP and YjcD were subjected to site-directed mutagenesis at highly conserved sites of transmembrane segments 1, 3, 8, 9, and 10, which have been studied also in COG2233 homologs. Residues irreplaceable for uptake activity or crucial for substrate selectivity were found at positions occupied by similar role amino acids in the Escherichia coli xanthine- and uric acid-transporting homologs (XanQ and UacT, respectively) and predicted to be at or around the binding site. Our results support the contention that the distantly related transporters of COG2233 and COG2252 use topologically similar side chain determinants to dictate their function and the distinct purine selectivity profiles.


Subject(s)
Escherichia coli K12 , Escherichia coli Proteins , Membrane Transport Proteins , Models, Molecular , Nucleoside Transport Proteins , Escherichia coli K12/chemistry , Escherichia coli K12/genetics , Escherichia coli K12/metabolism , Escherichia coli Proteins/antagonists & inhibitors , Escherichia coli Proteins/chemistry , Escherichia coli Proteins/genetics , Escherichia coli Proteins/metabolism , Ligands , Membrane Transport Proteins/chemistry , Membrane Transport Proteins/genetics , Membrane Transport Proteins/metabolism , Mutagenesis, Site-Directed , Nucleoside Transport Proteins/antagonists & inhibitors , Nucleoside Transport Proteins/chemistry , Nucleoside Transport Proteins/genetics , Nucleoside Transport Proteins/metabolism , Protein Structure, Tertiary , Structural Homology, Protein , Structure-Activity Relationship
6.
J Biol Chem ; 285(45): 35011-20, 2010 Nov 05.
Article in English | MEDLINE | ID: mdl-20802252

ABSTRACT

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 (259)FLVVGTIYLLSVLEAVGDITATAMVSRRPIQGEEYQSRLKGGVLADGLVSVIASAV(314) and (342)TIAVMLVILGLFP(354) 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 (IC(50) 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).


Subject(s)
Cell Membrane/metabolism , Escherichia coli K12/metabolism , Escherichia coli Proteins/metabolism , Nucleobase Transport Proteins/metabolism , Amino Acid Motifs , Amino Acid Substitution , Cell Membrane/genetics , Cysteine/genetics , Cysteine/metabolism , Escherichia coli K12/genetics , Escherichia coli Proteins/genetics , Mutation, Missense , Nucleobase Transport Proteins/genetics
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