Substrate recognition domain of the Gal2 galactose transporter in yeast Saccharomyces cerevisiae as revealed by chimeric galactose-glucose transporters

DCT4-A New Member of the Dicarboxylate Transporter Family in C4 Grasses

Malate transport shuttles atmospheric carbon into the Calvin–Benson cycle during NADP-ME C₄ photosynthesis. Previous characterizations of several plant dicarboxylate transporters (DCT) showed that they efficiently exchange malate across membranes. Here, we identify and characterize a previously unknown member of the DCT family, DCT4, in Sorghum bicolor. We show that SbDCT4 exchanges malate across membranes and its expression pattern is consistent with a role in malate transport during C₄ photosynthesis. SbDCT4 is not syntenic to the characterized photosynthetic gene ZmDCT2, and an ortholog is not detectable in the maize reference genome. We found that the expression patterns of DCT family genes in the leaves of Zea mays, and S. bicolor varied by cell type. Our results suggest that subfunctionalization, of members of the DCT family, for the transport of malate into the bundle sheath plastids, occurred during the process of independent recurrent evolution of C₄ photosynthesis in grasses of the PACMAD clade. We also show that this subfunctionalization is lineage independent. Our results challenge the dogma that key C₄ genes must be orthologues of one another among C₄ species, and shed new light on the evolution of C₄ photosynthesis.

The conservation of polyol transporter proteins and their involvement in lichenized Ascomycota

In lichen symbiosis, polyol transfer from green algae is important for acquiring the fungal carbon source. However, the existence of polyol transporter genes and their correlation with lichenization remain unclear. Here, we report candidate polyol transporter genes selected from the genome of the lichen-forming fungus (LFF) Ramalina conduplicans. A phylogenetic analysis using characterized polyol and monosaccharide transporter proteins and hypothetical polyol transporter proteins of R. conduplicans and various ascomycetous fungi suggested that the characterized yeast' polyol transporters form multiple clades with the polyol transporter-like proteins selected from the diverse ascomycetous taxa. Thus, polyol transporter genes are widely conserved among Ascomycota, regardless of lichen-forming status. In addition, the phylogenetic clusters suggested that LFFs belonging to Lecanoromycetes have duplicated proteins in each cluster. Consequently, the number of sequences similar to characterized yeast' polyol transporters were evaluated using the genomes of 472 species or strains of Ascomycota. Among these, LFFs belonging to Lecanoromycetes had greater numbers of deduced polyol transporter proteins. Thus, various polyol transporters are conserved in Ascomycota and polyol transporter genes appear to have expanded during the evolution of Lecanoromycetes.

Crucial effects of amino acid side chain length in transmembrane segment 5 on substrate affinity in yeast glucose transporter Hxt7

We previously identified Asp(340) in transmembrane segment 7 (TM7) as a key determinant of substrate affinity in Hxt7, a high-affinity facilitative glucose transporter of Saccharomyces cerevisiae. To gain further insight into the structural basis of substrate recognition by Hxt7, we performed cysteine-scanning mutagenesis of 21 residues in TM5 of a Cys-less form of Hxt7. Four residues were sensitive to Cys replacement, among which Gln(209) was found to be essential for high-affinity glucose transport activity. The 17 remaining sites were examined further for the accessibility of cysteine to the hydrophilic sulfhydryl reagent p-chloromercuribenzenesulfonate (pCMBS). Among the Cys mutants, T213C was the only one whose transport activity was completely inhibited by 0.5 mM pCMBS. Moreover, this mutant was protected from pCMBS inhibition by the substrate d-glucose and by 2-deoxy-D-glucose but not by L-glucose, indicating that Thr(213) is situated at or close to a substrate recognition site. The functional role of Thr(213) was further examined with its replacement with each of the other 19 amino acids in wild-type Hxt7. Such replacement generated seven functional transporters with various affinities for glucose. Only three mutants, those with Val, Cys, and Ser at position 213, exhibited high-affinity glucose transport activity. All of these residues possess a side chain length similar to that of Thr, indicating that side chain length at this position is a key determinant of substrate affinity. A working homology model of Hxt7 indicated that Gln(209) and Thr(213) face the central cavity and that Thr(213) is located within van der Waals distance of Asp(340) (TM7).

Bioconversion of lignocellulose-derived sugars to ethanol by engineered Saccharomyces cerevisiae

Lignocellulosic biomass from agricultural and agro-industrial residues represents one of the most important renewable resources that can be utilized for the biological production of ethanol. The yeast Saccharomyces cerevisiae is widely used for the commercial production of bioethanol from sucrose or starch-derived glucose. While glucose and other hexose sugars like galactose and mannose can be fermented to ethanol by S. cerevisiae, the major pentose sugars D-xylose and L-arabinose remain unutilized. Nevertheless, D-xylulose, the keto isomer of xylose, can be fermented slowly by the yeast and thus, the incorporation of functional routes for the conversion of xylose and arabinose to xylulose or xylulose-5-phosphate in Saccharomyces cerevisiae can help to improve the ethanol productivity and make the fermentation process more cost-effective. Other crucial bottlenecks in pentose fermentation include low activity of the pentose phosphate pathway enzymes and competitive inhibition of xylose and arabinose transport into the cell cytoplasm by glucose and other hexose sugars. Along with a brief introduction of the pretreatment of lignocellulose and detoxification of the hydrolysate, this review provides an updated overview of (a) the key steps involved in the uptake and metabolism of the hexose sugars: glucose, galactose, and mannose, together with the pentose sugars: xylose and arabinose, (b) various factors that play a major role in the efficient fermentation of pentose sugars along with hexose sugars, and (c) the approaches used to overcome the metabolic constraints in the production of bioethanol from lignocellulose-derived sugars by developing recombinant S. cerevisiae strains.

Identification of a key residue determining substrate affinity in the yeast glucose transporter Hxt7: a two-dimensional comprehensive study

We previously identified Asn(331) 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 Asp(340) in Hxt7, the residue corresponding to Asn(331) of Hxt2, was examined by replacing it with each of the other 19 amino acids. Such replacement of Asp(340) generated transporters with various affinities for glucose, with the affinity of the Cys(340) mutant surpassing that of the wild-type Hxt7. To examine the structural role of Asp(340) 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 Asp(340) is replaced with Cys, was completely inhibited by pCMBS, indicating that Asp(340) is located in a water-accessible position. This D340C mutant showed a sensitivity to pCMBS that was approximately 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 Asp(340) 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.

Amino acid residues involved in ligand preference of the Snf3 transporter-like sensor in Saccharomyces cerevisiae

Snf3 is a plasma membrane protein in Saccharomyces cerevisiae able to sense the presence of glucose. Although the Snf3 protein does not transport sugars, it shares sequence similarity with various glucose transporters from other organisms. We investigated the sugar specificity/preferences of Snf3. The ability of cells to sense sugars in vivo was monitored by following the degradation of the Mth1 protein, an early event in the signal pathway. Our study reveals that Snf3, in addition to glucose, also senses fructose and mannose, as well as the glucose analogues 2-deoxyglucose, 3-O-methylglucoside and 6-deoxyglucose. The signalling proficiency of a non-phosphorylatable analogue strongly supports the notion that sensing through Snf3 does not require sugar phosphorylation. Sequence comparisons of Snf3 to glucose transporters indicated amino acid residues possibly involved in sensing of sugars other than glucose. By site-specific mutagenesis of the structural gene, roles of specific residues in Snf3 could be established. Change of isoleucine-374 to valine in transmembrane segment 7 of Snf3 partially abolished sensing of fructose and mannose, while mutagenesis causing a change of phenylalanine-462 to tyrosine in transmembrane segment 10 of Snf3 abolished sensing of fructose. Neither of these amino acid changes affected the ability of Snf3 to sense glucose, nor did they permit Snf3 to sense galactose. These data indicate a similarity between a ligand binding site of the sensor Snf3 and binding sites used for facilitated hexose transport in the GLUT proteins.

Identification of a key residue determining substrate affinity in the human glucose transporter GLUT1

Asn(331) in transmembrane segment 7 of the yeast Saccharomyces cerevisiae transporter Hxt2 has been identified as a single key residue for high-affinity glucose transport by comprehensive chimera approach. The glucose transporter GLUT1 of mammals belongs to the same major facilitator superfamily as Hxt2 and may therefore show a similar mechanism of substrate recognition. The functional role of Ile(287) in human GLUT1, which corresponds to Asn(331) in Hxt2, was studied by its replacement with each of the other 19 amino acids. The mutant transporters were individually expressed in a recently developed yeast expression system for GLUT1. Replacement of Ile(287) generated transporters with various affinities for glucose that correlated well with those of the corresponding mutants of the yeast transporter. Residues exhibiting high affinity for glucose were medium-sized, non-aromatic, uncharged and irrelevant to hydrogen-bond capability, suggesting an important role of van der Waals interaction. Sensitivity to phloretin, a specific inhibitor for the presumed exofacial glucose binding site, was decreased in two mutants, whereas that to cytochalasin B, a specific inhibitor for the presumed endofacial glucose binding site, was unchanged in the mutants. These results suggest that Ile(287) is a key residue for maintaining high glucose affinity in GLUT1 as revealed in Hxt2 and is located at or near the exofacial glucose binding site.

Identification by Comprehensive Chimeric Analysis of a Key Residue Responsible for High Affinity Glucose Transport by Yeast HXT2

Hxt2 and Hxt1 are, respectively, high affinity and low affinity facilitative glucose transporter paralogs of Saccharomyces cerevisiae. We have previously investigated which amino acid residues of Hxt2 are important for high affinity transport activity. Studies with all the possible combinations of 12 transmembrane segments (TMs) of Hxt2 and Hxt1 revealed that TMs 1, 5, 7, and 8 of Hxt2 are necessary for high affinity transport. Systematic shuffling of the 20 amino acid residues that differ between Hxt2 and Hxt1 in these TMs subsequently identified 5 residues as important for such activity: Leu(59) and Leu(61) (TM1), Leu(201) (TM5), Asn(331) (TM7), and Phe(366) (TM8). We have now studied the relative importance of these 5 residues by individually replacing them with each of the other 19 residues. Replacement of Asn(331) yielded transporters with various affinities, with those of the Ile(331), Val(331), and Cys(331) mutants being higher than that of the wild type. Replacement of the Hxt2 residues at the other four sites yielded transporters with affinities similar to that of the wild type but with various capacities. A working homology model of the chimeric transporters containing Asn(331) or its 19 replacement residues indicated that those residues at this site that yield high affinity transporters (Ile(331), Val(331), Cys(331)) face the central cavity and are within van der Waals distances of Phe(208) (TM5), Leu(357) (TM8), and Tyr(427) (TM10). Interactions via these residues of the four TMs, which compose a half of the central pore, may thus play a pivotal role in formation of a core structure for high affinity transport.

Identification of essential loops and residues of glucosyltransferase V (GtrV) of Shigella flexneri

Lipopolysaccharide (LPS), particularly the O-antigen component, is one of many virulence determinants necessary for Shigella flexneri pathogenesis. O-antigen modification is mediated by glucosyltransferase (gtr) genes encoded by temperate serotype-converting bacteriophages. The gtrV and gtrX genes encode the GtrV and GtrX glucosyltransferases, respectively. These are integral membrane proteins, which catalyze the transfer of a glucosyl residue via an alpha1,3 linkage to rhamnose II and rhamnose I of the O-antigen unit. This mediates conversion of S. flexneri serotype Y to serotype 5a and X, respectively. Essential regions in the topology of GtrV protein were identified by in vivo recombination and a PCR-mediated approach. A series of GtrX-GtrV and GtrV-GtrX chimeric proteins were constructed based on the fact that GtrV and GtrX share sequence similarity. Analysis of their respective serotype conversion abilities led to the identification of two important periplasmic loops: loops No 2 and No 10 located in the N- and C-termini, respectively. Within these two loops, three conserved motifs were identified; two in loop No 2 and one in loop No 10. These conserved motifs contain acidic residues which were shown to be critical for GtrV function.

Eight amino acid residues in transmembrane segments of yeast glucose transporter Hxt2 are required for high affinity transport

Hxt2 and Hxt1 are high affinity and low affinity facilitative glucose transporter paralogs of Saccharomyces cerevisiae, respectively, that differ at 75 amino acid positions in their 12 transmembrane segments (TMs). Comprehensive analysis of chimeras of these two proteins has previously revealed that TMs 1, 5, 7, and 8 of Hxt2 are required for high affinity glucose transport activity and that leucine 201 in TM5 is the most important in this regard of the 20 amino acid residues in these regions that differ between Hxt2 and Hxt1. To evaluate the importance of the remaining residues, we systematically shuffled the amino acids at these positions and screened the resulting proteins for high affinity and high capacity glucose transport activity. In addition to leucine 201 (TM5), four residues of Hxt2 (leucine 59 and leucine 61 in TM1, asparagine 331 in TM7, and phenylalanine 366 in TM8) were found to be important for such activity. Furthermore, phenylalanine 198 (TM5), alanine 363 (TM8), and either valine 316 (TM7) or alanine 368 (TM8) were found to be supportive of maximal activity. Construction of a homology model suggested that asparagine 331 interacts directly with the substrate and that the other identified residues may contribute to maintenance of protein conformation.

Directed evolution of extradiol dioxygenase by a novel in vivo DNA shuffling

RecA-dependent homologous recombination in Escherichia coli is a very effective way to construct chimeras between two homologous genes. The disadvantage of in vivo method is a small library size of chimeric genes in comparison with in vitro DNA shuffling. In order to overcome the disadvantage, we have developed novel in vivo DNA shuffling methods with successive homologous recombinations. Linearized DNA molecules with two homologous genes were made with ligation rather than the conventional restriction enzyme cleavage between two genes. The three-way ligation of a vector and two homologous bphC genes encoding 2,3-dihydroxybiphenyl 1,2-dioxygenases or the two-way ligation of the donor bphC gene and an acceptor plasmid carrying the homologous bphC gene generated a variety of linearized DNA molecules. The homologous recombination between the genes on the linearized DNA molecules created the large chimeric bphC gene libraries in a recBC sbcA E. coli strain. After three rounds of recombinations, chimeric bphC genes with four-part gene fragments by triple-crossover were easily obtained. By employing a 96-well microtiter plate high-throughput screening, thermally stable chimeric 2,3-dihydroxybiphenyl 1,2-dioxygenases were selected from chimeric bphC gene libraries. This opens up a new way for directed evolution of proteins in vivo.

Identification of Sorbitol Transporters Expressed in the Phloem of Apple Source Leaves

Sorbitol is a major photosynthetic product and a major phloem-translocated component in Rosaceae (e.g. apple, pear, peach, and cherry). We isolated the three cDNAs, MdSOT3, MdSOT4, and MdSOT5 from apple (Malus domestica) source leaves, which are homologous to plant polyol transporters. Yeasts transformed with the MdSOTs took up sorbitol significantly. MdSOT3- and MdSOT5-dependent sorbitol uptake was strongly inhibited by xylitol and myo-inositol, but not or only weakly by mannitol and dulcitol. Apparent K(m) values of MdSOT3 and MdSOT5 for sorbitol were estimated to be 0.71 mM and 3.2 mM, respectively. The protonophore, carbonyl cyanide m-chlorophenylhydrazone (CCCP), strongly inhibited the sorbitol transport. MdSOT3 was expressed specifically in source leaves, whereas MdSOT4 and MdSOT5 were expressed in source leaves and also in some sink organs. MdSOT4 and MdSOT5 expressions were highest in flowers. Fruits showed no or only weak MdSOT expression. Although MdSOT4 and MdSOT5 were also expressed in immature leaves, MdSOT expressions increased with leaf maturation. In addition, in situ hybridization revealed that all MdSOTs were expressed to high levels in phloem of minor veins in source leaves. These results suggest that these MdSOTs are involved in sorbitol loading in Rosaceae.

Comprehensive Chimeric Analysis of Amino Acid Residues Critical for High Affinity Glucose Transport by Hxt2 of Saccharomyces cerevisiae

Chimeras of Hxt2 and Hxt1, high affinity and low affinity glucose transporters, respectively, of Saccharomyces cerevisiae, were previously constructed by random replacement of each of the 12 transmembrane segments (TMs) of Hxt2 with the corresponding region of Hxt1. Characterization of these chimeras revealed that at least TMs 1, 5, 7, and 8 of Hxt2 are required for high affinity transport activity. To determine which amino acid residues in these TMs are important for high affinity glucose transport, we systematically shuffled all of the 20 residues in these regions that differ between Hxt2 and Hxt1. Analysis of 60 independent mutant strains identified as expressing high affinity and high capacity glucose transport activity by selection on glucose-limited agar plates revealed that Leu-201 in TM5 of Hxt2 is most important for such activity and that either Cys-195 or Phe-198 is also required for maximal activity.

Differentiation of dicarboxylate transporters in mesophyll and bundle sheath chloroplasts of maize

In NADP-malic enzyme-type C(4) plants such as maize (Zea mays L.), efficient transport of oxaloacetate and malate across the inner envelope membranes of chloroplasts is indispensable. We isolated four maize cDNAs, ZmpOMT1 and ZmpDCT1 to 3, encoding orthologs of plastidic 2-oxoglutarate/malate and general dicarboxylate transporters, respectively. Their transcript levels were upregulated by light in a cell-specific manner; ZmpOMT1 and ZmpDCT1 were expressed in the mesophyll cell (MC) and ZmpDCT2 and 3 were expressed in the bundle sheath cell (BSC). The recombinant ZmpOMT1 protein expressed in yeast could transport malate and 2-oxoglutarate but not glutamate. By contrast, the recombinant ZmpDCT1 and 2 proteins transported 2-oxoglutarate and glutamate at similar affinities in exchange for malate. The recombinant proteins could also transport oxaloacetate at the same binding sites as those for the dicarboxylates. In particular, ZmpOMT1 transported oxaloacetate at a higher efficiency than malate or 2-oxoglutarate. We also compared the activities of oxaloacetate transport between MC and BSC chloroplasts from maize leaves. The K(m) value for oxaloacetate in MC chloroplasts was one order of magnitude lower than that in BSC chloroplasts, and was close to that determined with the recombinant ZmpOMT1 protein. Southern analysis revealed that maize has a single OMT gene. These findings suggest that ZmpOMT1 participates in the import of oxaloacetate into MC chloroplasts in exchange for stromal malate. In BSC chloroplasts, ZmpDCT2 and/or ZmpDCT3 were expected to import malate that is transported from MC.

Transmembrane segments 1, 5, 7 and 8 are required for high-affinity glucose transport by Saccharomyces cerevisiae Hxt2 transporter

Hxt2 is a high-affinity facilitative glucose transporter of Saccharomyces cerevisiae and belongs to the major facilitator superfamily. Hxt1 shares approximately 70% amino acid identity with Hxt2 in its transmembrane segments (TMs) and inter-TM loops, but transports D-glucose with an affinity about one-tenth of that of Hxt2. To determine which TMs of Hxt2 are important for high-affinity glucose transport, we constructed chimaeras of Hxt2 and Hxt1 by randomly replacing each of the 12 TMs of Hxt2 with the corresponding segment of Hxt1, for a total of 4096 different transporters. Among > 20000 yeast transformants screened, 39 different clones were selected by plate assays of high-affinity glucose-transport activity and sequenced. With only two exceptions, the selected chimaeras contained Hxt2 TMs 1, 5, 7 and 8. We then constructed chimaeras corresponding to all 16 possible combinations of Hxt2 TMs 1, 5, 7 and 8. Only one chimaera, namely that containing all four Hxt2 TMs, exhibited transport activity comparable with that of Hxt2. The K (m) and V (max) values for D-glucose transport, and the substrate specificity of this chimaera were almost identical with those of Hxt2. These results indicate that TMs 1, 5, 7 and 8 are necessary for exhibiting high-affinity glucose-transport activity of Hxt2.

Substrate specificity of nickel/cobalt permeases: insights from mutants altered in transmembrane domains I and II

HoxN, a high-affinity, nickel-specific permease of Ralstonia eutropha H16, and NhlF, a nickel/cobalt permease of Rhodococcus rhodochrous J1, are structurally related members of the nickel/cobalt transporter (NiCoT) family. These transporters have an eight-helix structure and are characterized by highly conserved segments with polar or charged amino acid residues in transmembrane domains (TMDs) II, III, V, and VI. Two histidine residues in a Ni2+ binding motif, the signature sequence of NiCoTs, in TMD II of HoxN have been shown to be crucial for activity. Replacement of the corresponding His residues in NhlF affected both Co2+ and Ni2+ uptake, demonstrating that NhlF employs a HoxN-like mechanism for transport of the two cations. Multiple alignments of bacterial NiCoT sequences identified a striking correlation between a hydrophobic residue (Val or Phe) in TMD II and a position in the center of TMD I occupied by either an Asn (as in HoxN) or a His (as in NhlF). Introducing an isoleucine residue at the latter position strongly reduced HoxN activity and abolished NhlF activity, suggesting that a Lewis base N-donor moiety is important. The Asn-to-His exchange had no effect on HoxN, whereas the converse replacement reduced NhlF-mediated Ni2+ uptake significantly. Replacement of the entire TMD I of HoxN by the respective NhlF segment resulted in a chimera that transported Ni2+ and Co2+ with low capacity. The Val-to-Phe exchange in TMD II of HoxN led to a considerable rise in Ni2+ uptake capacity and conferred to the variant the ability to transport Co2+. NhlF activity dropped in response to the converse mutation. Our data predict that TMDs I and II in NiCoTs spatially interact to form a critical part of the selectivity filter. As seen for the V64F variant of HoxN, modification of this site can increase the velocity of transport and concomitantly reduce the specificity.

Sensing of cytoplasmic pH by bacterial chemoreceptors involves the linker region that connects the membrane-spanning and the signal-modulating helices

The two major chemoreceptors of Escherichia coli, Tsr and Tar, mediate opposite responses to the same changes in cytoplasmic pH (pH(i)). We set out to identify residues involved in pH(i) sensing to gain insight into the general mechanisms of signaling employed by the chemoreceptors. Characterization of various chimeras of Tsr and Tar localized the pH(i)-sensing region to Arg(259)-His(267) of Tar and Gly(261)-Asp(269) of Tsr. This region of Tar contains three charged residues (Arg(259)-Ser(261), Asp(263), and His(267)) that have counterparts of opposite charge in Tsr (Gly(261)-Glu(262), Arg(265), and Asp(269)). The replacement of all of the three charged residues in Tar or Arg(259)-Ser(260) alone by the corresponding residues of Tsr reversed the polarity of pH(i) response, whereas the replacement of Asp(263) or His(267) did not change the polarity but altered the time course of pH(i) response. These results suggest that the electrostatic properties of a short cytoplasmic region within the linker region that connects the second transmembrane helix to the first methylation helix is critical for switching the signaling state of the chemoreceptors during pH sensing. Similar conformational changes of this region in response to external ligands may be critical components of transmembrane signaling.

The Chlorella hexose/H(+)-symporters

The physiology, molecular biology, and biochemistry of the inducible hexose uptake protein of Chlorella kessleri is reviewed. The protein encoded by the HUP1 gene is the most intensively studied membrane transporter of plants. Responsible for substrate accumulation up to 1500-fold, it translocates one proton together with one hexose, and the cell invests 1 ATP per sugar transported. Kinetics suggest that substrate accumulation is mainly brought about by a large delta Km (Kminside >> Kmoutside). The HUP1 protein (534aa) consists of 12 transmembrane helices of which at least helices I, V, VII, and XI interact with the sugar during translocation and participate in lining the transport path through the membrane. The helix packing might very well be identical to the one suggested for the E. coli lac permease, although the mechanism for transport and proton coupling that has been suggested for lac permease (Kaback, 1997) certainly does not hold for the Chlorella symporter; both are distantly related members, however, of the MFS-family of transporters. HUP1 has been functionally expressed in Schizosaccharomyces pombe, Saccharomyces cerevisiae, Escherichia coli, Volvox carteri, and in Xenopus oocytes.

Characterization and expression of monosaccharide transporters (osMSTs) in rice

This study deals with the cloning and characterization of monosaccharide transporter cDNAs in rice. OsMST1-3 (Oryza sativa monosaccharide transporters 1-3) have two sets of putative six transmembrane domains separated by a central long hydrophilic region. Heterologous expression of OsMST3 in the yeast Saccharomyces cerevisiae indicated that OsMST3 has transport activity for some monosaccharides in an energy-dependent H+ co-transport manner. Northern blot and in situ hybridization analyses showed that OsMST3 mRNA is detectable in leaf blades, leaf sheaths, calli and roots, especially the xylem as well as in sclerenchyma cells in the root. These results suggested that OsMST3 is involved in the accumulation of monosaccharides required for cell wall synthesis at the stage of cell thickening.

Monosaccharide transporters in plants: structure, function and physiology

Monosaccharide transport across the plant plasma membrane plays an important role both in lower and higher plants. Algae can switch between phototrophic and heterotrophic growth and utilize organic compounds, such as monosaccharides as additional or sole carbon sources. Higher plants represent complex mosaics of phototrophic and heterotrophic cells and tissues and depend on the activity of numerous transporters for the correct partitioning of assimilated carbon between their different organs. The cloning of monosaccharide transporter genes and cDNAs identified closely related integral membrane proteins with 12 transmembrane helices exhibiting significant homology to monosaccharide transporters from yeast, bacteria and mammals. Structural analyses performed with several members of this transporter superfamily identified protein domains or even specific amino acid residues putatively involved in substrate binding and specificity. Expression of plant monosaccharide transporter cDNAs in yeast cells and frog oocytes allowed the characterization of substrate specificities and kinetic parameters. Immunohistochemical studies, in situ hybridization analyses and studies performed with transgenic plants expressing reporter genes under the control of promoters from specific monosaccharide transporter genes allowed the localization of the transport proteins or revealed the sites of gene expression. Higher plants possess large families of monosaccharide transporter genes and each of the encoded proteins seems to have a specific function often confined to a limited number of cells and regulated both developmentally and by environmental stimuli.

Interaction between the critical aromatic amino acid residues Tyr(352) and Phe(504) in the yeast Gal2 transporter

Three critical aromatic sites have been identified in the yeast galactose transporter Gal2: Tyr(352) at the extracellular boundary of putative transmembrane segment (TM) 7, Tyr(446) in the middle of TM10 and Phe(504) in the middle of TM12. The relationship between these sites was investigated by random mutagenesis of each combination of two of the three residues. Galactose transport-positive clones selected by plate assays encoded Tyr(446) and specific combinations of aromatic residues at sites 352 and 504. Double-site mutants containing aromatic residues at these latter two positions showed either essentially full galactose transport activity (Phe(352)Trp(504) and Trp(352)Trp(504)) or no significant activity (Phe(352)Tyr(504) and Trp(352)Tyr(504)), whereas single-site mutants showed markedly reduced activity. These results are indicative of a specific interaction between sites 352 and 504 of Gal2.

A study of AroP-PheP chimeric proteins and identification of a residue involved in tryptophan transport

In vivo recombination has been used to make a series of AroP-PheP chimeric proteins. Analysis of their respective substrate profiles and activities has identified a small region within span III of AroP which can confer on a predominantly PheP protein the ability to transport tryptophan. Site-directed mutagenesis of the AroP-PheP chimera, PheP, and AroP has established that a key residue involved in tryptophan transport is tyrosine at position 103 in AroP. Phenylalanine is the residue at the corresponding position in PheP. The use of PheP-specific antisera has shown that the inability of certain chimeras to transport any of the aromatic amino acids is not a result of instability or a failure to be inserted into the membrane. Site-directed mutagenesis has identified two significant AroP-specific residues, alanine 107 and valine 114, which are the direct cause of loss of transport activity in chimeras such as A152P. These residues replace a glycine and an alanine in PheP and flank a highly conserved glutamate at position 110. Some suggestions are made as to the possible functions of these residues in the tertiary structure of the proteins.

Three aromatic amino acid residues critical for galactose transport in yeast Gal2 transporter

Tyr(446) in putative transmembrane segment 10 (TM10) of the yeast galactose transporter Gal2 has previously been identified as essential for galactose recognition. In the present study, alignment of the amino acid sequences of 63 sugar transporters or related proteins revealed 14 aromatic sites, including Tyr(446) of Gal2, that are conserved in >75% of these proteins. The importance of the remaining 13 conserved aromatic amino acids was examined individually by random mutagenesis using degenerate primers. Galactose transport-positive clones were identified by plate selection and subjected to DNA sequencing. For those transport-positive clones corresponding to Tyr(352), and Phe(504) mutants, all the amino acid substitutions comprised aromatic residues. The importance of the aromatic residues at these sites was further investigated by replacing them individually with each of the other 19 amino acids and measuring the galactose transport activity of the resulting mutants. Among both Tyr(352) and Phe(504) mutants, the other aromatic amino acids supported galactose transport; no other amino acids conferred high affinity transport activity. Thus, at least three aromatic sites are critical for galactose transport: one at the extracellular boundary of putative TM7 (Tyr(352)), one in the middle of putative TM10 (Tyr(446)), and one in the middle of putative TM12 (Phe(504)).

Metabolic signals trigger glucose-induced inactivation of maltose permease in Saccharomyces

Organisms such as Saccharomyces capable of utilizing several different sugars selectively ferment glucose when less desirable carbon sources are also available. This is achieved by several mechanisms. Glucose down-regulates the transcription of genes involved in utilization of these alternate carbon sources. Additionally, it causes posttranslational modifications of enzymes and transporters, leading to their inactivation and/or degradation. Two glucose sensing and signaling pathways stimulate glucose-induced inactivation of maltose permease. Pathway 1 uses Rgt2p as a sensor of extracellular glucose and causes degradation of maltose permease protein. Pathway 2 is dependent on glucose transport and stimulates degradation of permease protein and very rapid inactivation of maltose transport activity, more rapid than can be explained by loss of protein alone. In this report, we characterize signal generation through pathway 2 using the rapid inactivation of maltose transport activity as an assay of signaling activity. We find that pathway 2 is dependent on HXK2 and to a lesser extent HXK1. The correlation between pathway 2 signaling and glucose repression suggests that these pathways share common upstream components. We demonstrate that glucose transport via galactose permease is able to stimulate pathway 2. Moreover, rapid transport and fermentation of a number of fermentable sugars (including galactose and maltose, not just glucose) are sufficient to generate a pathway 2 signal. These results indicate that pathway 2 responds to a high rate of sugar fermentation and monitors an intracellular metabolic signal. Production of this signal is not specific to glucose, glucose catabolism, glucose transport by the Hxt transporters, or glucose phosphorylation by hexokinase 1 or 2. Similarities between this yeast glucose sensing pathway and glucose sensing mechanisms in mammalian cells are discussed.

Function and regulation of yeast hexose transporters

Glucose, the most abundant monosaccharide in nature, is the principal carbon and energy source for nearly all cells. The first, and rate-limiting, step of glucose metabolism is its transport across the plasma membrane. In cells of many organisms glucose ensures its own efficient metabolism by serving as an environmental stimulus that regulates the quantity, types, and activity of glucose transporters, both at the transcriptional and posttranslational levels. This is most apparent in the baker's yeast Saccharomyces cerevisiae, which has 20 genes encoding known or likely glucose transporters, each of which is known or likely to have a different affinity for glucose. The expression and function of most of these HXT genes is regulated by different levels of glucose. This review focuses on the mechanisms S. cerevisiae and a few other fungal species utilize for sensing the level of glucose and transmitting this information to the nucleus to alter HXT gene expression. One mechanism represses transcription of some HXT genes when glucose levels are high and works through the Mig1 transcriptional repressor, whose function is regulated by the Snf1-Snf4 protein kinase and Reg1-Glc7 protein phosphatase. Another pathway induces HXT expression in response to glucose and employs the Rgt1 transcriptional repressor, a ubiquitin ligase protein complex (SCF(Grr1)) that regulates Rgt1 function, and two glucose sensors in the membrane (Snf3 and Rgt2) that bind glucose and generate the intracellular signal to which Rgt1 responds. These two regulatory pathways collaborate with other, less well-understood, pathways to ensure that yeast cells express the glucose transporters best suited for the amount of glucose available.

Structure and function of facilitative sugar transporters

Sugar transporters from one group of the major facilitator superfamily of membrane transporters. A conserved common central pore structure lies at the heart of these transporters and diverse functionality is brought about by alterations to this pore or regions associated with it. Recent mutagenesis studies of sugar transporters within the framework of tenable models for the distantly related lactose permease argue that this model is a good paradigm for other members of the major facilitator superfamily.

Contribution to substrate recognition of two aromatic amino acid residues in putative transmembrane segment 10 of the yeast sugar transporters Gal2 and Hxt2

The comprehensive study of chimeras between the Gal2 galactose transporter and the Hxt2 glucose transporter of Saccharomyces cerevisiae has shown that Tyr446 is essential and Trp455 is important for galactose recognition by Gal2. Consistent with this finding, replacement of the corresponding Phe431 and Tyr440 residues of Hxt2 with Tyr and Trp, respectively, allowed Hxt2 to transport galactose, suggesting that the two amino acid residues in putative transmembrane segment 10 play a definite role in galactose recognition (Kasahara, M., Shimoda, E., and Maeda, M. (1997) J. Biol. Chem. 272, 16721-16724). Replacement of Trp455 of Gal2 with any of the other 19 amino acids was shown to reduce galactose transport activity to between 0 and <20% of that of wild-type Gal2. The role of Phe431 in Hxt2 was similarly studied. Other than Phe, only Tyr at position 431 was able to support glucose transport activity, at the reduced level of <20%. In contrast, replacement of Tyr440 of Hxt2 with other amino acids revealed that most replacements, with the exception of Pro and charged amino acids, supported glucose transport activity. The importance of residue 431 in sugar recognition was more pronounced in a modified Hxt2 in which Tyr440 was replaced with Trp. Glucose transport was supported only by the aromatic amino acids Phe, Tyr, and Trp at position 431, and galactose transport was supported only by Tyr. These results suggest that an aromatic amino acid located in the middle of transmembrane segment 10 (Tyr446 in Gal2 and Phe431 in Hxt2) plays a critical role in substrate recognition in the yeast sugar transporter family to which Gal2 and Hxt2 belong.

Tryptophan 388 in putative transmembrane segment 10 of the rat glucose transporter Glut1 is essential for glucose transport

The molecular mechanism of substrate recognition in membrane transport is not well understood. Two amino acid residues, Tyr446 and Trp455 in transmembrane segment 10 (TM10), have been shown to be important for galactose recognition by the yeast Gal2 transporter; Tyr446 was found to be essential in that its replacement by any of the other 19 amino acids abolished transport activity (Kasahara, M., Shimoda, E., and Maeda, M. (1997) J. Biol. Chem. 272, 16721-16724). The Glut1 glucose transporter of animal cells belongs to the same Glut transporter family as does Gal2 and thus might be expected to show a similar mechanism of substrate recognition. The role of the two amino acids, Phe379 and Trp388, in rat Glut1 corresponding to Tyr446 and Trp455 of Gal2 was therefore studied. Phe379 and Trp388 were individually replaced with each of the other 19 amino acids, and the mutant Glut1 transporters were expressed in yeast. The expression level of most mutants was similar to that of the wild-type Glut1, as revealed by immunoblot analysis. Glucose transport activity was assessed by reconstituting a crude membrane fraction of the yeast cells in liposomes. No significant glucose transport activity was observed with any of Trp388 mutants, whereas the Phe379 mutants showed reduced or no activity. These results indicate that the two aromatic amino acids in TM10 of Glut1 are important for glucose transport. However, unlike Gal2, the residue at the cytoplasmic end of TM10 (Trp388, corresponding to Trp455 of Gal2), rather than that in the middle of TM10 (Phe379, corresponding to Tyr446 of Gal2), is essential for transport activity.

Chimeric purine transporters of Aspergillus nidulans define a domain critical for function and specificity conserved in bacterial, plant and metazoan homologues

In Aspergillus nidulans, purine uptake is mediated by three transporter proteins: UapA, UapC and AzgA. UapA and UapC have partially overlapping functions, are 62% identical and have nearly identical predicted topologies. Their structural similarity is associated with overlapping substrate specificities; UapA is a high-affinity, high-capacity specific xanthine/uric acid transporter. UapC is a low/moderate-capacity general purine transporter. We constructed and characterized UapA/UapC, UapC/UapA and UapA/UapC/UapA chimeric proteins and UapA point mutations. The region including residues 378-446 in UapA (336-404 in UapC) has been shown to be critical for purine recognition and transport. Within this region, we identified: (i) one amino acid residue (A404) important for transporter function but probably not for specificity and two residues (E412 and R414) important for UapA function and specificity; and (ii) a sequence, (F/Y/S)X(Q/E/P) NXGXXXXT(K/R/G), which is highly conserved in all homologues of nucleobase transporters from bacteria to man. The UapC/UapA series of chimeras behaves in a linear pattern and leads to an univocal assignment of functional domains while the analysis of the reciprocal and 'sandwich' chimeras revealed unexpected inter-domain interactions. cDNAs coding for transporters including the specificity region defined by these studies have been identified for the first time in the human and Caenorhabditis elegans databases.

Alteration of substrate affinities and specificities of the Chlorella Hexose/H+ symporters by mutations and construction of chimeras

The cDNAs HUP1 and HUP2 of Chlorella kessleri code for monosaccharide/H+ symporters that can be functionally expressed in Schizosaccharomyces pombe. By random mutagenesis three HUP1 mutants with an increased Km value for D-glucose were isolated. The 40-fold increase in Km of the first mutant is due to the amino acid exchange N436I in putative transmembrane helix XI. Two substitutions were found in a second (G97C/I303N) and third mutant (G120D/F292L), which show a 270-fold and 50-fold increase in Km for D-glucose, respectively. An investigation of the individual mutations revealed that the substitutions I303N and F292L (both in helix VII) cause the Km shifts seen in the corresponding double mutants. These mutations together with those previously found support the hypothesis that helices V, VII, and XI participate in the transmembrane sugar pathway. Whereas for most mutants obtained so far the Km change for D-glucose is paralleled by a corresponding change for other hexoses tested, the exchange D44E exclusively alters the Km for D-glucose. Moreover the pH profile of this mutant is shifted by more than 2 pH units to alkaline values, indicating that the activity of the transporter may require deprotonation of the corresponding carboxyl group. Chimeric transporters were constructed to study the 100-fold lower affinity for D-galactose of the HUP1 symporter as compared with that of the HUP2 protein. A crucial determinant for the differential D-galactose recognition was shown to be associated with the first external loop. The effect could be pinpointed to a single amino acid change: replacement of Asn-45 of HUP1 with isoleucine, the corresponding amino acid of HUP2, yields a transporter with a 20 times higher affinity for D-galactose. The reverse substitution (I47N) decreases the affinity of HUP2 for D-galactose 20-fold.

Substrate recognition domains as revealed by active hybrids between the D-arabinitol and ribitol transporters from Klebsiella pneumoniae

Two new genes, dalT and rbtT, have been cloned from the dal operon for D-arabinitol and the rbt operon for ribitol uptake and degradation, respectively, in Klebsiella pneumoniae 1033-5P14, derivative KAY2026. Each gene codes for a specific transporter which, based on sequence data, belongs to a large family of carbohydrate transporters which constitutes 12 transmembrane helices. DalT and RbtT show an unusually high similarity (86.2% identical residues for totals of 425 and 427 amino acids, respectively). This allowed the construction of DalT'-Rbt"T and RbtT'-Dal'T crossover hybrids by using a natural restriction site overlapping Met202. This site is located within the large cytoplasmic loop which connects the putative helices 6 and 7 and in particular the amino- and the carboxy-terminal halves of the transporters. Both hybrids have close to normal transport activities but essentially the substrate specificities and kinetic properties of the amino-terminal half. This result localizes essential substrate binding and recognition sites to the amino-terminal halves of the proteins in this important class of carbohydrate transporters.

The molecular genetics of hexose transport in yeasts

Transport across the plasma membrane is the first, obligatory step of hexose utilization. In yeast cells the uptake of hexoses is mediated by a large family of related transporter proteins. In baker's yeast Saccharomyces cerevisiae the genes of 20 different hexose transporter-related proteins have been identified. Six of these transmembrane proteins mediate the metabolically relevant uptake of glucose, fructose and mannose for growth, two others catalyze the transport of only small amounts of these sugars, one protein is a galactose transporter but also able to transport glucose, two transporters act as glucose sensors, two others are involved in the pleiotropic drug resistance process, and the functions of the remaining hexose transporter-related proteins are not yet known. The catabolic hexose transporters exhibit different affinities for their substrates, and expression of their corresponding genes is controlled by the glucose sensors according to the availability of carbon sources. In contrast, milk yeast Kluyveromyces lactis contains only a few different hexose transporters. Genes of other monosaccharide transporter-related proteins have been found in fission yeast Schizosaccharomyces pombe and in the xylose-fermenting yeast Pichia stipitis. However, the molecular genetics of hexose transport in many other yeasts remains to be established. The further characterization of this multigene family of hexose transporters should help to elucidate the role of transport in yeast sugar metabolism.

Amino acid residues responsible for galactose recognition in yeast Gal2 transporter

A novel, systematic approach was used to identify amino acid residues responsible for substrate recognition in the transmembrane 10 region of the Gal2 galactose transporter of Saccharomyces cerevisiae. A mixture of approximately 25,000 distinct plasmids that encode all the combinations of 12 amino acids in transmembrane 10 that are different in Gal2 and the homologous glucose transporter Hxt2 was synthesized. Selection of galactose transport-positive clones on galactose limited agar plates yielded 19 clones, all of which contained the Tyr446 residue found in Gal2. 14 of the 19 clones contained Trp455 found in Gal2, whereas the other 5 contained Cys455, a residue not found in either Gal2 or Hxt2. When Tyr446 of Gal2 was replaced with any of the other 19 amino acids, no galactose transport activity was observed in the resulting transporters, indicating that Tyr446 plays an essential role in the transport of this sugar. Replacement of 2 amino acids of Hxt2 with the corresponding Tyr446 and Trp455 of Gal2 allowed the modified Hxt2 to transport galactose. The Km of galactose transport for the modified transporter was 8-fold higher than that of Gal2. These results and other evidence unequivocally show that Tyr446 is essential and Trp455 is important for the discrimination of galactose versus glucose.

Catabolite inactivation of the galactose transporter in the yeast Saccharomyces cerevisiae: ubiquitination, endocytosis, and degradation in the vacuole

When Saccharomyces cerevisiae cells growing on galactose are transferred onto glucose medium containing cycloheximide, an inhibitor of protein synthesis, a rapid reduction of Gal2p-mediated galactose uptake is observed. We show that glucose-induced inactivation of Gal2p is due to its degradation. Stabilization of Gal2p in pra1 mutant cells devoid of vacuolar proteinase activity is observed. Subcellular fractionation and indirect immunofluorescence showed that the Gal2 transporter accumulates in the vacuole of the mutant cells, directly demonstrating that its degradation requires vacuolar proteolysis. In contrast, Gal2p degradation is proteasome independent since its half-life is unaffected in pre1-1 pre2-2, cim3-1, and cim5-1 mutants defective in several subunits of the protease complex. In addition, vacuolar delivery of Gal2p was shown to be blocked in conditional end3 and end4 mutants at the nonpermissive temperature, indicating that delivery of Gal2p to the vacuole occurs via the endocytic pathway. Taken together, the results presented here demonstrate that glucose-induced proteolysis of Gal2p is dependent on endocytosis and vacuolar proteolysis and is independent of the functional proteasome. Moreover, we show that Gal2p is ubiquitinated under conditions of glucose-induced inactivation.

Characterization of rat Glut4 glucose transporter expressed in the yeast Saccharomyces cerevisiae: comparison with Glut1 glucose transporter

Rat Glut4 glucose transporter was expressed in the yeast Saccharomyces cerevisiae, but was retained in an intracellular membranous compartment and did not contribute to glucose uptake by intact cells. A crude membrane fraction was prepared and reconstituted in liposome with the use of the freeze-thaw/sonication method. D-glucose-specific, cytochalasin B inhibitable glucose transport activity was observed. Kinetic analysis of D-glucose transport was performed by an integrated rate equation approach. The K(m) under zero-trans influx condition was 12 +/- 1 mM (mean +/- S.E., n = 3) and that under equilibrium exchange condition was 22 +/- 3 mM (n = 4). D-glucose transport was inhibited by 2-deoxy-D-glucose or 3-O-methyl-D-glucose, but not by D-allose, D-fructose or L-glucose. Cytochalasin B, phloretin and phlorizin inhibited D-glucose transport, but neither p-chloromercuribenzoic acid (pCMB) (0-0.1 mM) nor p-chloromercuribenzene sulfonic acid (pCMBS) (0-1.0 mM) inhibited this activity. High concentrations of HgCl2 were required to inhibit D-glucose transport (IC50, 370 microM). Comparing these properties to those of rat Glut1 we found two notable differences; (1) in Glut1, K(m) under zero-trans influx was significantly smaller than that under equilibrium exchange but in Glut4 less than two-fold difference was seen between these two K(m) values; and (2) Glut1 was inhibited with pCMB, pCMBS and low concentrations of HgCl2 (IC50, 3.5 microM), whereas Glut4 was almost insensitive to SH reagents. To examine the role of the exofacial cysteine, we replaced Met-455 of Glut4 (corresponding to Cys-429 of Glut1) with cysteine. The mutated Glut4 was inhibited by pCMB or pCMBS and the IC50 of HgCl2 decreased to 47 microM, whereas K(m), substrate specificity and the sensitivity to cytochalasin B were not significantly changed, indicating that the existence of exofacial cysteine contributed only to increase SH sensitivity in Glut4.

The C-terminal domain of Snf3p is sufficient to complement the growth defect of snf3 null mutations in Saccharomyces cerevisiae: SNF3 functions in glucose recognition

The SNF3 protein, Snf3p, of Saccharomyces cerevisiae was initially thought to be a high affinity glucose transporter required for efficient catabolism of low glucose concentrations. We now report evidence suggesting that Snf3p is a regulatory protein and not a catabolic transporter. The C-terminal domain of Snf3p is able to complement the growth defect on solid media of snf3 null mutants independent of attachment to the membrane-spanning domains. However, the C-terminal domain is unable to fully restore high affinity glucose transport to a snf3 null strain. Examination of deletions of the C-terminal domain of intact SNF3 demonstrates that this region is required for both the growth and transport functions of Snf3p. Loss of the SNF3 gene leads to a long-term adaptation phenotype for cells grown in liquid medium at low substrate concentrations in the presence of the respiratory inhibitor, antimycin A. The presence of the C-terminal domain shortens the time required for adaptation in a snf3 null strain. Thus, Snf3p appears to affect ability to adapt to low substrate conditions, but does not confer an absolute defect in uptake of substrate. Taken together, these data suggest that Snf3p is a regulatory protein likely functioning in the detection of glucose.

A novel signal transduction pathway in Saccharomyces cerevisiae defined by Snf3-regulated expression of HXT6

We show that cells deleted for SNF3, HXT1, HXT2, HXT3, HXT4, HXT6, and HXT7 do not take up glucose and cannot grow on media containing glucose as a sole carbon source. The expression of Hxt1, Hxt2, Hxt3, Hxt6, or Gal2 in these cells resulted in glucose transport and allowed growth on glucose media. In contrast, the expression of Snf3 failed to confer glucose uptake or growth on glucose. HXT6 is highly expressed on raffinose, low glucose, or nonfermentable carbon sources but is repressed in the presence of high concentrations of glucose. The maintenance of HXT6 glucose repression is strictly dependent on Snf3 and not on intracellular glucose. In snf3 delta cells expression of HXT6 is constitutive even when the entire repertoire of HXT genes is present and glucose uptake is abundant. In addition, glucose repression of HXT6 does not require glucose uptake by HXT1, HXT2, HXT3 or HXT4. We show that a signal transduction pathway defined by the Snf3-dependent hexose regulation of HXT6 is distinct from but also overlaps with general glucose regulation pathways in Saccharomyces cerevisiae. Finally, glucose repression of ADH2 and SUC2 is intact in snf3 delta hxt1 delta hxt2 delta hxt3 delta hxt4 delta hxt6 delta hxt7 delta gal2 cells, suggesting that the sensing and signaling mechanism for general glucose repression is independent from glucose uptake.

The dopamine transporter carboxyl-terminal tail. Truncation/substitution mutants selectively confer high affinity dopamine uptake while attenuating recognition of the ligand binding domain

In order to delineate structural motifs regulating substrate affinity and recognition for the human dopamine transporter (DAT), we assessed [3H]dopamine uptake kinetics and [3H]CFT binding characteristics of COS-7 cells transiently expressing mutant DATs in which the COOH terminus was truncated or substituted. Complete truncation of the carboxyl tail from Ser582 allowed for the expression of biphasic [3H]dopamine uptake kinetics displaying both a low capacity (Vmax approximately 0.4 pmol/10(5) cells/min) high affinity (Km approximately 300 nM) component and one exhibiting low affinity (Km approximately 15 microM] and high capacity (Vmax approximately 5 pmol/10(5)cells/min) with a concomitant 40% decrease in overall apparent Vmax relative to wild type (WT) DAT. Truncation of the last 22 amino acids or substitution of the DAT-COOH tail with sequences encoding the intracellular carboxyl-terminal of either dopamine D1 or D5 receptors produced results that were identical to those with the fully truncated DAT, suggesting that the induction of biphasic dopamine uptake kinetics is likely conferred by removal of DAT-specific sequence motifs distal to Pro597. The attenuation of WT transport activity, either by lowering levels of DAT expression or by pretreatment of cells with phorbol 12-myristate 13-acetate (1 microM), did not affect the kinetics of [3H]dopamine transport. The estimated affinity of dopamine (Ki approximately 180 nM) for all truncated/substituted DAT mutants was 10-fold lower than that of WT DAT (approximately 2000 nM) and appears selective for the endogenous substrate, since the estimated inhibitory constants for numerous putative substrates or uptake inhibitors were virtually identical to those obtained for WT DATs. In marked contrast, DAT truncation/substitution mutants displayed significantly reduced high affinity [3H]CFT binding interactions with estimated Ki values for dopamine and numerous other substrates and inhibitors tested from 10-100-fold lower than that observed for WT DAT. Moreover, co-expression of truncated and/or substituted DATs with WT transporter failed to reconstitute functional or pharmacological activities associated with both transporters. Instead, complete restoration of uniphasic low affinity [3H]dopamine uptake kinetics (Km approximately 2000 nM) and high affinity substrate and inhibitor [3H]CFT binding interactions attributable to WT DATs were evident. These data clearly suggest the functional independence and differential regulation of the dopamine translocation process from the characteristics exhibited by its ligand binding domain. The lack of functional phenotypic expression of mutant DAT activities in cells co-expressing WT transporter is consistent with the contention that native DATs may exist as multisubunit complexes, the formation and maintenance of which is dependent upon sequences encoded within the carboxyl tail.

Transmembrane segment 10 is important for substrate recognition in Ga12 and Hxt2 sugar transporters in the yeast Saccharomyces cerevisiae

A systematic series of chimeras between Ga12 galactose transporter and Hxt2 glucose transporter in yeast was produced to delineate the essential domain for substrate recognition. A domain of 101 amino acids close to the COOH-terminus that has been previously identified as the critical substrate recognition region was further divided into four subdomains, by introducing five restriction enzyme sites at exactly corresponding locations of both genes without changing coding amino acids. When each of all possible 16 modified genes was expressed, all the galactose transport-active chimeras were found to possess Ga12-derived transmembrane segment (TM) 10. Of the 35 amino acids in the TM1O region, only 12 differ between Ga12 and Hxt2, indicating that these 12 amino acids include the critical residue(s) responsible for the differential recognition of galactose and glucose in these transporters.

MEMBRANE TRANSPORT CARRIERS

▪ Abstract  Plant and fungal membrane proteins catalyzing the transmembrane translocation of small molecules without directly using ATP or acting as channels are discussed in this review. Facilitators, ion-cotransporters, and exchange translocators mainly for sugars, amino acids, and ions that have been cloned and characterized from Saccharomyces cerevisiae and from various plant sources have been tabulated. The membrane topology and structure of the most extensively studied carriers (lac permease of Escherichia coli, Glut1 of man, HUP1 of Chlorella) are discussed in detail as well as the kinetic analysis of specific Na+ and H+ cotransporters. Finally, the knowledge concerning regulatory phenomena of carriers—mainly of S. cerevisiae—is summarized.

Sugar binding to Na+/glucose cotransporters is determined by the carboxyl-terminal half of the protein

d-Glucose is absorbed across the proximal tubule of the kidney by two Na+/glucose cotransporters (SGLT1 and SGLT2). The low affinity SGLT2 is expressed in the S1 and S2 segments, has a Na+:glucose coupling ratio of 1, a K0.5 for sugar of approximately 2 mM, and a K0.5 for Na+ of approximately 1 mM. The high affinity SGLT1, found in the S3 segment, has a coupling ratio of 2, and K0.5 for sugar and Na+ of approximately 0.2 and 5 mM, respectively. We have constructed a chimeric protein consisting of amino acids 1-380 of porcine SGLT2 and amino acids 381-662 of porcine SGLT1. The chimera was expressed in Xenopus oocytes, and steady-state kinetics were characterized by a two-electrode voltage-clamp. The K0.5 for alpha-methyl-d-glucopyranoside (0.2 mM) was similar to that for SGLT1, and like SGLT1 the chimera transported D-galactose and 3-O-methylglucose. In contrast, SGLT2 transports poorly D-galactose and excludes 3-O-methylglucose. The apparent K0.5Na was 3.5 mM (at -150 mV), and the Hill coefficient ranged between 0.8 and 1.5. We conclude that recognition/transport of organic substrate is mediated by interactions distal to amino acid 380, while cation binding is determined by interactions arising from the amino- and carboxyl-terminal halves of the transporters. Surprisingly, the chimera transported alpha-phenyl derivatives of D-glucose as well as the inhibitors of sugar transport: phlorizin, deoxyphlorizin, and beta-D-glucopyranosylphenyl isothiocyanate are transported with high affinity (K0.5 for phlorizin was 5 microM). Thus, the pocket for organic substrate binding is increased from 10 x 5 x 5 (A) for SGLT1 to 11 x 18 x 5 (A) for the chimera.

Expression of the rat GLUT1 glucose transporter in the yeast Saccharomyces cerevisiae

We expressed the rat GLUT1 facilitative glucose transporter in the yeast Saccharomyces cerevisiae with the use of a galactose-inducible expression system. Confocal immunofluorescence microscopy indicated that a majority of this protein is retained in an intracellular structure that probably corresponds to endoplasmic reticulum. Yeast cells expressing GLUT1 exhibited little increase in glucose-transport activity. We prepared a crude membrane fraction from these cells and made liposomes with this fraction using the freeze-thaw/sonication method. In this reconstituted system, D-glucose-transport activity was observed with a Km for D-glucose of 3.4 +/- 0.2 mM (mean +/- S.E.M.) and was inhibited by cytochalasin B (IC50= 0.44 +/- 0.03 microM), HgCl2 (IC50)= 3.5 +/- 0.5 microM), phloretin (IC50= 49 +/- 12 microM) and phloridzin (IC50= 355 +/- 67 microM). To compare these properties with native GLUT1 we made reconstituted liposomes with a membrane fraction prepared from human erythrocytes, in which the Km of D-glucose transport and ICs of these inhibitors were approximately equal to those obtained with GLUT1 made by yeast. When the relative amounts of GLUT1 in the crude membrane fractions were measured by quantitative immunoblotting, the specific activity of the yeast-made GLUT1 was 110% of erythrocyte GLUT1, indicating that GLUT1 expressed in yeast is fully active in glucose transport.

Importance of the first external loop for substrate recognition as revealed by chimeric Chlorella monosaccharide/H+ symporters

It had been shown previously by heterologous expression in Schizosaccharomyces pombe, that the two monosaccharide/H+ symporters HUP1 and HUP2 of Chlorella kessleri differ significantly concerning their substrate specificity: HUP1 transports predominantly D-glucose while HUP2 prefers D-galactose. Several chimeric transporters were constructed and their substrate specificities determined. Surprisingly, it is sufficient to replace the first part of the external loop 1 of the HUP1 symporter by the corresponding portion of HUP2 to improve transport and also to decrease the Km value for D-galactose. Additional data indicating the importance of the first loop for substrate recognition and binding are discussed.

Computer-assisted nonlinear regression analysis of the multicomponent glucose uptake kinetics of Saccharomyces cerevisiae

The kinetics of glucose uptake in Saccharomyces cerevisiae are complex. An Eadie-Hofstee (rate of uptake versus rate of uptake over substrate concentration) plot of glucose uptake shows a nonlinear form typical of a multicomponent system. The nature of the constituent components is a subject of debate. It has recently been suggested that this nonlinearity is due to either a single saturable component together with free diffusion of glucose or a single constitutive component with a variable Km, rather than the action of multiple hexose transporters. Genetic data support the existence of a family of differentially regulated glucose transporters, encoded by the HXT genes. In this work, kinetic expressions and nonlinear regression analysis, based on an improved zero trans-influx assay, were used to address the nature of the components of the transport system. The results indicate that neither one component with free diffusion nor a single permease with a variable Km can explain the observed uptake rates. Results of uptake experiments, including the use of putative alternative substrates as inhibitory compounds, support the model derived from genetic analyses of a multicomponent system with at least two components, one a high-affinity carrier and the other a low-affinity carrier. This approach was extended to characterize the activity of the SNF3 protein and identify its role in the depression of high-affinity uptake. The kinetic data support a role of SNF3 as a regulatory protein that may not itself be a transporter.