Glucose transport activity and photolabelling with 3-[125I]iodo-4-azidophenethylamido-7-O-succinyldeacetyl (IAPS)-forskolin of two mutants at tryptophan-388 and -412 of the glucose transporter GLUT1: dissociation of the binding domains of forskolin and glucose

Camelus dromedarius glucose transporter 4: in silico analysis, cloning, expression, purification and characterisation in E. coli

Camels have exceptional carbohydrate metabolism as their plasma glucose level is high and have low whole body insulin sensitivity, similar to that observed in type 2 diabetes patients. We aimed at studing an important component of insulin signalling pathway, the GLUT4, in camel. Camelus dromedarius GLUT4 (CdGLUT4) CDS is 1530 nucleotide in length that encodes for a 55KDa protein. CdGLUT4 has 23 amino acid substitutions and 3N-glycosylation sites, compared to 2 in Human GLUT4. 3 D structures of CdGLUT4 and HsGLUT4 generated by homology modelling revealed conservation of characteristic signature motifs. CdGLUT4 was cloned and expressed optimally in C43(DE3)pLysS strain and maximum detergent solubility was observed in n-Dodecyl-β-d-maltopyranoside. These preliminary data provide information on residual differences between CdGLUT4 and HsGLUT4 that may be responsible for camel's unique glucose metabolism. These differences are postulated to assist in designing and development of efficacious GLUT4 that might help in management of diabetic patients.

The glucose transporter 1 -GLUT1- from the white shrimp Litopenaeus vannamei is up-regulated during hypoxia

During hypoxia the shrimp Litopenaeus vannamei accelerates anaerobic glycolysis to obtain energy; therefore, a correct supply of glucose to the cells is needed. Facilitated glucose transport across the cells is mediated by a group of membrane embedded integral proteins called GLUT; being GLUT1 the most ubiquitous form. In this work, we report the first cDNA nucleotide and deduced amino acid sequences of a glucose transporter 1 from L. vannamei. A 1619 bp sequence was obtained by RT-PCR and RACE approaches. The 5´ UTR is 161 bp and the poly A tail is exactly after the stop codon in the mRNA. The ORF is 1485 bp and codes for 485 amino acids. The deduced protein sequence has high identity to GLUT1 proteins from several species and contains all the main features of glucose transporter proteins, including twelve transmembrane domains, the conserved motives and amino acids involved in transport activity, ligands binding and membrane anchor. Therefore, we decided to name this sequence, glucose transporter 1 of L. vannamei (LvGLUT1). A partial gene sequence of 8.87 Kbp was also obtained; it contains the complete coding sequence divided in 10 exons. LvGlut1 expression was detected in hemocytes, hepatopancreas, intestine gills, muscle and pleopods. The higher relative expression was found in gills and the lower in hemocytes. This indicates that LvGlut1 is ubiquitously expressed but its levels are tissue-specific and upon short-term hypoxia, the GLUT1 transcripts increase 3.7-fold in hepatopancreas and gills. To our knowledge, this is the first evidence of expression of GLUT1 in crustaceans.

Putative sugar transporters of the mustard leaf beetle Phaedon cochleariae: their phylogeny and role for nutrient supply in larval defensive glands

Background

Phytophagous insects have emerged successfully on the planet also because of the development of diverse and often astonishing defensive strategies against their enemies. The larvae of the mustard leaf beetle Phaedon cochleariae, for example, secrete deterrents from specialized defensive glands on their back. The secretion process involves ATP-binding cassette transporters. Therefore, sugar as one of the major energy sources to fuel the ATP synthesis for the cellular metabolism and transport processes, has to be present in the defensive glands. However, the role of sugar transporters for the production of defensive secretions was not addressed until now.

Results

To identify sugar transporters in P. cochleariae, a transcript catalogue was created by Illumina sequencing of cDNA libraries. A total of 68,667 transcripts were identified and 68 proteins were annotated as either members of the solute carrier 2 (SLC2) family or trehalose transporters. Phylogenetic analyses revealed an extension of the mammalian GLUT6/8 class in insects as well as one group of transporters exhibiting distinctive conserved motifs only present in the insect order Coleoptera. RNA-seq data of samples derived from the defensive glands revealed six transcripts encoding sugar transporters with more than 3,000 counts. Two of them are exclusively expressed in the glandular tissue. Reduction in secretions production was accomplished by silencing two of four selected transporters. RNA-seq experiments of transporter-silenced larvae showed the down-regulation of the silenced transporter but concurrently the up-regulation of other SLC2 transporters suggesting an adaptive system to maintain sugar homeostasis in the defensive glands.

Conclusion

We provide the first comprehensive phylogenetic study of the SLC2 family in a phytophagous beetle species. RNAi and RNA-seq experiments underline the importance of SLC2 transporters in defensive glands to achieve a chemical defense for successful competitive interaction in natural ecosystems.

Functional properties and genomics of glucose transporters

Glucose is the major energy source for mammalian cells as well as an important substrate for protein and lipid synthesis. Mammalian cells take up glucose from extracellular fluid into the cell through two families of structurallyrelated glucose transporters. The facilitative glucose transporter family (solute carriers SLC2A, protein symbol GLUT) mediates a bidirectional and energy-independent process of glucose transport in most tissues and cells, while the NaM(+)/glucose cotransporter family (solute carriers SLC5A, protein symbol SGLT) mediates an active, Na(+)-linked transport process against an electrochemical gradient. The GLUT family consists of thirteen members (GLUT1-12 and HMIT). Phylogenetically, the members of the GLUT family are split into three classes based on protein similarities. Up to now, at least six members of the SGLT family have been cloned (SGLT1-6). In this review, we report both the genomic structure and function of each transporter as well as intra-species comparative genomic analysis of some of these transporters. The affinity for glucose and transport kinetics of each transporter differs and ranges from 0.2 to 17mM. The ability of each protein to transport alternative substrates also differs and includes substrates such as fructose and galactose. In addition, the tissue distribution pattern varies between species. There are different regulation mechanisms of these transporters. Characterization of transcriptional control of some of the gene promoters has been investigated and alternative promoter usage to generate different protein isoforms has been demonstrated. We also introduce some pathophysiological roles of these transporters in human.

Functional studies of the T295M mutation causing Glut1 deficiency: glucose efflux preferentially affected by T295M

Glucose transporter type 1 (Glut1) deficiency syndrome (Glut1 DS, OMIM: #606777) is characterized by infantile seizures, acquired microcephaly, developmental delay, hypoglycorrhachia (CSF glucose <40 mg/dL), and decreased erythrocyte glucose uptake (56.1 +/- 17% of control). Previously, we reported two patients with a mild Glut1 deficiency phenotype associated with a heterozygous GLUT1 T295M mutation and normal erythrocyte glucose uptake. We assessed the pathogenicity of T295M in the Xenopus laevis oocyte expression system. Under zero-trans influx conditions, the T295M Vmax (590 pmol/min/oocyte) was 79% of the WT value and the Km (14.3 mM) was increased compared with WT (9.6 mM). Under zero-trans efflux conditions, both the Vmax (1216 pmol/min/oocyte) and Km (8.8 mM) in T295M mutant Glut1 were markedly decreased in comparison to the WT values (7443 pmol/min/oocyte and 90.8 mM). Western blot analysis and confocal studies confirmed incorporation of the T295M mutant protein into the plasma membrane. The side chain of M295 is predicted to block the extracellular "gate" for glucose efflux in our Glut-1 molecular model. We conclude that the T295M mutation specifically alters Glut1 conformation and asymmetrically affects glucose flux across the cell by perturbing efflux more than influx. These findings explain the seemingly paradoxical findings of Glut1 DS with hypoglycorrhachia and "normal" erythrocyte glucose uptake.

Glucose transporter 8 (GLUT8) from the red imported fire ant, Solenopsis invicta Buren (Hymenoptera: Formicidae)

We have cloned the fire ant glucose transporter 8 (GLUT8) cDNA providing the first molecular characterization of a GLUT8 in insects. Glucose is a poly-alcohol and, due to its high hydrophilicity, cannot move across cell membranes. GLUT8 is a putative facilitative transporter for the cellular import and export of glucose. The complete 2,974-bp cDNA encodes a 501-residue protein with a predicted molecular mass of 54.8 kDa. Transcripts were detected in the brain, midgut, hindgut, Malpighian tubule, fat body, ovary, and testis. The highest transcriptional expression was found in fat body. Northern blot analysis revealed different transcript sizes in mated queen brains, alate female ovaries, and male testes. We propose that four other sequences obtained from insect genome projects from the honey bee Apis mellifera (ENSAPMP00000006624), the malaria mosquito Anopheles gambiae (EAA11842), and the fruit fly Drosophila melanogaster (AAQ23604 and AAM52591) are likely the orthologues of the fire ant GLUT8. Phylogenetic relationships in insect glucose transporters are presented.

Cloning and characterization of glucose transporter 11, a novel sugar transporter that is alternatively spliced in various tissues

We have cloned and characterized a novel glucose transporter (GLUT11) that is alternatively spliced. The GLUT11 gene maps to chromosome 22q11.2 and consists of 13 exons. The long form (GLUT11-L) cDNA uses 13 exons to produce a protein containing 503 amino acids. The short form of GLUT11 (GLUT-11) cDNA is missing exon 2 and produces a protein of 496 amino acids with a 14 amino acid N-terminal difference compared to the long form. GLUT11 has significant similarity to known GLUTs and contains 12 putative membrane-spanning helices along with sugar transporter signature motifs that have previously been shown to be essential for transport activity. The putative glycosylation site of GLUT11 is present in loop 1. Northern blot analysis showed that GLUT11 mRNA is expressed in a number of tissues and most abundantly in the skeletal muscle and heart. RT-PCR assay showed that GLUT11 is alternatively spliced and the two isoforms are distributed differently in various tissues. Immunofluorescence microscopy demonstrated that GLUT11-L resides on the plasma membrane when overexpressed in HEK293T cells. Western blot analysis revealed that GLUT11-L runs as a broad band of approximately 42 kDa that was converted to a 38 kDa polypeptide by PNGase F digestion. Furthermore, a liposome reconstitution functional assay showed that GLUT11-L has glucose transport activity.

Immunolocalization of GLUTX1 in the testis and to specific brain areas and vasopressin-containing neurons

GLUTX1 or GLUT8 is a newly characterized glucose transporter isoform that is expressed at high levels in the testis and brain and at lower levels in several other tissues. Its expression was mapped in the testis and brain by using specific antibodies. In the testis, immunoreactivity was expressed in differentiating spermatocytes of type 1 stage but undetectable in mature spermatozoa. In the brain, GLUTX1 distribution was selective and localized to a variety of structures, mainly archi- and paleocortex. It was found in hippocampal and dentate gyrus neurons as well as amygdala and primary olfactory cortex. In these neurons, its location was close to the plasma membrane of cell bodies and sometimes in proximal dendrites. High GLUTX1 levels were detected in the hypothalamus, supraoptic nucleus, median eminence, and the posterior pituitary. Neurons of these areas synthesize and secrete vasopressin and oxytocin. As shown by double immunofluorescence microscopy and immunogold labeling, GLUTX1 was expressed only in vasopressin neurons. By immunogold labeling of ultrathin cryosections microscopy, GLUTX1 was identified in dense core vesicles of synaptic nerve endings of the supraoptic nucleus and secretory granules of the vasopressin positive neurons. This localization suggests an involvement of GLUTX1 both in specific neuron function and endocrine mechanisms.

Molecular cloning of a member of the facilitative glucose transporter gene family GLUT11 (SLC2A11) and identification of transcription variants

We isolated a member of the facilitative glucose transporter (GLUT) gene family (GLUT11; SLC2A11 as a HGMW-approved symbol) based on the analysis of a human genomic BAC clone KB1125A3 located on band q11.2 of human chromosome 22. The gene GLUT11/SLC2A11 consists of 12 exons spanning over 29 kb in size and is located between two genes, SMARCB1 and MIF. The deduced amino acid sequence indicated the topological features of transmembrane helices and sequence motifs which are common to the GLUT protein family. The cDNA cloning revealed the presence of three types of variation in its transcripts. The first variation is caused by the existence of three distinct first exons (SLC2A11-a, -b, and -c). PCR analysis of multi-tissue-derived cDNA panels indicated the differential expression of these transcript variants. The second variation is caused by skipping over one exon (exon 6). The third variation is caused by the premature transcription termination at a site between exon 8 and exon 9. Both exon skipping and premature termination caused frameshift, resulting in the production of truncated GLUT11/SLC2A11 transcripts. These results suggested that transcription of GLUT11/SCL2A11 gene is controlled in a complex manner.

Characterization of human glucose transporter (GLUT) 11 (encoded by SLC2A11), a novel sugar-transport facilitator specifically expressed in heart and skeletal muscle

Human GLUT11 (encoded by the solute carrier 2A11 gene, SLC2A11) is a novel sugar transporter which exhibits significant sequence similarity with the members of the GLUT family. The amino acid sequence deduced from its cDNAs predicts 12 putative membrane-spanning helices and all the motifs (sugar-transporter signatures) that have previously been shown to be essential for sugar-transport activity. The closest relative of GLUT11 is the fructose transporter GLUT5 (sharing 41.7% amino acid identity with GLUT11). The human GLUT11 gene (SLC2A11) consists of 12 exons and is located on chromosome 22q11.2. In human tissues, a 7.2 kb transcript of GLUT11 was detected exclusively in heart and skeletal muscle. Transfection of COS-7 cells with GLUT11 cDNA significantly increased the glucose-transport activity reconstituted from membrane extracts as well as the specific binding of the sugar-transporter ligand cytochalasin B. In contrast to that of GLUT4, the glucose-transport activity of GLUT11 was markedly inhibited by fructose. It is concluded that GLUT11 is a novel, muscle-specific transport facilitator that is a member of the extended GLUT family of sugar/polyol-transport facilitators.

Structural analysis of the GLUT1 facilitative glucose transporter (review)

The structure of the human erythrocyte facilitative glucose transporter (GLUT1) has been intensively investigated using a wide array of chemical and biophysical approaches. Despite the lack of a crystal structure for any of the facilitative monosaccharide transport proteins, detailed information regarding primary and secondary structure, membrane topology, transport kinetics, and functionally important residues has allowed the construction of a sophisticated working model for GLUT1 tertiary structure. The existing data support the formation of a central aqueous channel formed by the juxtaposition of several amphipathic transmembrane-spanning alpha-helices. The results of extensive mutational analysis of GLUT1 have elucidated many of the structural determinants of the glucose permeation pathway. Continued application of currently available technologies will allow further refinement of this working model. In addition to providing insights into the molecular basis of both normal and disordered glucose homeostasis, this detailed understanding of structure/function relationships within GLUT1 can provide a basis for understanding transport carried out by other members of the major facilitator superfamily.

Activity and genomic organization of human glucose transporter 9 (GLUT9), a novel member of the family of sugar-transport facilitators predominantly expressed in brain and leucocytes

The GLUT9 gene encodes a cDNA which exhibits significant sequence similarity with members of the glucose transporter (GLUT) family. The gene is located on chromosome 9q34 and consists of 10 exons separated by short introns. The amino acid sequence deduced from its cDNA predicts 12 putative membrane-spanning helices and all the motifs (sugar-transporter signatures) that have previously been shown to be essential for transport activity. A striking characteristic of GLUT9 is the presence of two arginines in the putative helices 7 and 8 at positions where the organic anion transporters harbour basic residues. The next relative of GLUT9 is the glucose transporter GLUT8/GLUTX1 (44.8% amino acid identity with GLUT9). A 2.6-kb transcript of GLUT9 was detected in spleen, peripheral leucocytes and brain. Transfection of COS-7 cells with GLUT9 produced expression of a 46-kDa membrane protein which exhibited reconstitutable glucose-transport activity and low-affinity cytochalasin-B binding. It is concluded that GLUT9 is a novel member of the family of sugar-transport facilitators with a tissue-specific function.

Activity and genomic organization of human glucose transporter 9 (GLUT9), a novel member of the family of sugar-transport facilitators predominantly expressed in brain and leucocytes

The GLUT9 gene encodes a cDNA which exhibits significant sequence similarity with members of the glucose transporter (GLUT) family. The gene is located on chromosome 9q34 and consists of 10 exons separated by short introns. The amino acid sequence deduced from its cDNA predicts 12 putative membrane-spanning helices and all the motifs (sugar-transporter signatures) that have previously been shown to be essential for transport activity. A striking characteristic of GLUT9 is the presence of two arginines in the putative helices 7 and 8 at positions where the organic anion transporters harbour basic residues. The next relative of GLUT9 is the glucose transporter GLUT8/GLUTX1 (44.8% amino acid identity with GLUT9). A 2.6-kb transcript of GLUT9 was detected in spleen, peripheral leucocytes and brain. Transfection of COS-7 cells with GLUT9 produced expression of a 46-kDa membrane protein which exhibited reconstitutable glucose-transport activity and low-affinity cytochalasin-B binding. It is concluded that GLUT9 is a novel member of the family of sugar-transport facilitators with a tissue-specific function.

Activity and genomic organization of human glucose transporter 9 (GLUT9), a novel member of the family of sugar-transport facilitators predominantly expressed in brain and leucocytes

The GLUT9 gene encodes a cDNA which exhibits significant sequence similarity with members of the glucose transporter (GLUT) family. The gene is located on chromosome 9q34 and consists of 10 exons separated by short introns. The amino acid sequence deduced from its cDNA predicts 12 putative membrane-spanning helices and all the motifs (sugar-transporter signatures) that have previously been shown to be essential for transport activity. A striking characteristic of GLUT9 is the presence of two arginines in the putative helices 7 and 8 at positions where the organic anion transporters harbour basic residues. The next relative of GLUT9 is the glucose transporter GLUT8/GLUTX1 (44.8% amino acid identity with GLUT9). A 2.6-kb transcript of GLUT9 was detected in spleen, peripheral leucocytes and brain. Transfection of COS-7 cells with GLUT9 produced expression of a 46-kDa membrane protein which exhibited reconstitutable glucose-transport activity and low-affinity cytochalasin-B binding. It is concluded that GLUT9 is a novel member of the family of sugar-transport facilitators with a tissue-specific function.

GLUT8, a novel member of the sugar transport facilitator family with glucose transport activity

GLUT8 is a novel glucose transporter-like protein that exhibits significant sequence similarity with the members of the sugar transport facilitator family (29.4% of amino acids identical with GLUT1). Human and mouse sequence (86.2% identical amino acids) comprise 12 putative membrane-spanning helices and several conserved motifs (sugar transporter signatures), which have previously been shown to be essential for transport activity, e.g. GRK in loop 2, PETPR in loop 6, QQLSGVN in helix 7, DRAGRR in loop 8, GWGPIPW in helix 10, and PETKG in the C-terminal tail. An expressed sequence tag (STS A005N15) corresponding with the 3'-untranslated region of GLUT8 has previously been mapped to human chromosome 9. COS-7 cells transfected with GLUT8 cDNA expressed a 42-kDa protein exhibiting specific, glucose-inhibitable cytochalasin B binding (K(D) = 56.6 +/- 18 nm) and reconstitutable glucose transport activity (8.1 +/- 1. 4 nmol/(mg protein x 10 s) versus 1.1 +/- 0.1 in control transfections). In human tissues, a 2.4-kilobase pair transcript was predominantly found in testis, but not in testicular carcinoma. Lower amounts of the mRNA were detected in most other tissues including skeletal muscle, heart, small intestine, and brain. GLUT8 mRNA was found in testis from adult, but not from prepubertal rats; its expression in human testis was suppressed by estrogen treatment. It is concluded that GLUT8 is a sugar transport facilitator with glucose transport activity and a hormonally regulated testicular function.

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)).

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.

Serine-294 and threonine-295 in the exofacial loop domain between helices 7 and 8 of glucose transporters (GLUT) are involved in the conformational alterations during the transport process

The role of a conserved polar motif (STS) in the exofacial loop between helices 7 and 8 of GLUT4 for transporter function was investigated by site-directed mutagenesis and expression of the constructs in COS-7 cells. Reconstituted glucose-transport activity, cytochalasin B binding and photolabelling with the exofacial label 2-N4-(1-azi-2,2,2-trifluoroethyl)benzoyl-1, 3-bis-(d-mannosyloxy)-2-propylamine (ATB-BMPA) were assayed in membranes from transfected cells and corrected for immunoreactivity of expressed transporters. Replacement of Ser-294 with Ala or Thr suppressed transport activity and cytochalasin B binding. ATB-BMPA photolabelling was normal in S294A mutants, and even increased in S294T mutants. Replacement of Thr-295 with Ala suppressed transport activity and cytochalasin B binding, whereas ATB-BMPA photolabelling was normal; substitution of Ser failed to alter the investigated parameters. Similarly, exchanging Ser-296 for Ala generated a normally functioning protein. The data suggest that Ser-294 and Thr-295 are involved in the conformational change in GLUT during the transport process, and that their substitution may arrest the transporter in an outward-facing conformation.

Role of conserved arginine and glutamate residues on the cytosolic surface of glucose transporters for transporter function

The role of conserved arginine and glutamic acid residues at the cytoplasmic surface of the GLUT4 for transporter function was investigated by site-directed mutagenesis and expression of the constructs in COS-7 cells. Reconstituted glucose transport activity, cytochalasin B binding, and photolabeling with the exofacial label 2-N4-(1-azi-2,2,2-trifluoroethyl)benzoyl-1, 3-bis(d-mannosyloxy)-2-propylamine (ATB-BMPA) was assayed in membranes from transfected cells and corrected for immunoreactivity of expressed transporters. Exchange of Arg 92 (R92L amino acid residues are numbered according to the corresponding residues in the GLUT1) or Arg 333/334 (RR333/4LA) reduced or suppressed transport activity with no or very little effect on photolabeling with ATB-BMPA and cytochalasin B binding. It is suggested that the lack of these residues selectively disturbes the substrate-induced conformational change of the carrier during transport. Exchange of Glu 146 (E146D) or Arg 153 (R153L) markedly reduced transport activity, ATB-BMPA photolabeling, and cytochalasin B binding. Transport activity and ATB-BMPA labeling were abolished in the mutants E329Q, E393D, and R400L, whereas binding of cytochalasin B was normal. Thus, exchange of Glu 329, Glu 393, and Arg 400 appears to arrest the transporter in an inward facing conformation. It is concluded that the conserved arginine and glutamate residues at the cytoplasmic surface of the glucose transporter GLUT4 are essential for its appropriate conformation, and that it is the interaction of charged residues which mediates the oscillation between outward and inward facing states.

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.

Glucose transport activity and ligand binding (cytochalasin B, IAPS-forskolin) of chimeric constructs of GLUT2 and GLUT4 expressed in COS-7-cells

Chimeric constructs of glucose transporters GLUT2 and GLUT4 were transiently expressed in COS-7 cells in order to determine regions of the proteins responsible for their differences in activity and ligand binding. Exchange of the C-terminal tail (aa 479-509) of GLUT4 failed to affect glucose transport activity assayed at 1 mM glucose or ligand binding (cytochalasin B, IAPS-forskolin). In contrast, exchange of the C-terminal half of GLUT4 (aa 222-509) for that of GLUT2 markedly reduced ligand binding (Kd of cytochalasin B binding 1.88 +/- 0.2 microM vs. 0.21 +/- 0.06 in the wild-type GLUT4), and moderately (25%) reduced glucose transport activity. These data support the conclusion that the domains determining differences in ligand binding between GLUT4 and GLUT2 are located in the C-terminal half of the glucose transporters.

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.

The role of tryptophans 371 and 395 in the binding of antibiotics and the transport of sugars by the D-galactose-H+ symport protein (GalP) from Escherichia coli

The interactions between the D-galactose-H+ symporter (GalP) from Escherichia coli and the inhibitory antibiotics, cytochalasin B and forskolin, and the substrates, D-galactose and H+, have been investigated for the wild-type protein and the mutants Trp-371-->Phe and Trp-395-->Phe, so that the roles of these residues in the structure-activity relationship could be assessed. Neither mutation prevented photolabeling by either [4-3H]cytochalasin B or by 3-[125I]iodo-4-azidophenethyl-amido-7-O-succinyldesacetylforskolin ([125I]APS-forskolin). However, measurements of protein fluorescence show that both residues are in structural domains, the conformations of which are perturbed by the binding of cytochalasin B or forskolin. Moreover, both mutations cause a substantial decrease in the affinity of the inward-facing site of the GalP protein for cytochalasin B, 10- and 43-fold, respectively, but have little effect upon the affinity of this site for forskolin, 0.8- and 2.6-fold reductions, respectively. Both these mutations change the equilibrium between the putative outward- (T1) and inward-facing (T2) conformations, so that the inward-facing form is more favored. They also stabilize a different conformational state, "T3-antibiotic," in which the initial interactions between the protein and antibiotics are tightened. Overall, this has the effect of compensating for the reduction in affinity for cytochalasin B, so that the respective overall Kd values are 0.74- and 3.5-fold that of the wild type, while causing a slight increase, 1.5- and 3.2-fold, respectively, in affinity of the mutants for forskolin. The Trp-371-->Phe mutation causes a 15-fold reduction in the affinity of the inward-facing site for D-galactose, suggesting that this residue forms part of the sugar binding site. In contrast, the Trp-395-->Phe mutation has no effect upon the affinity of the inward-facing site for D-galactose. These effects may be related to the reduction in galactose-H+ symport activity only in the Trp-371-->Phe mutant, although it still effects active transport to the same extent as the Trp395-->Phe mutant. However, there is a 10-20-fold increase in the Km values for energized transport of D-galactose for both mutants.

Mutation of two conserved arginine residues in the glucose transporter GLUT4 supresses transport activity, but not glucose-inhibitable binding of inhibitory ligands

Two arginine residues (RR333/334) in the conserved GRR motif located in the endofacial loop between helix 8 and 9 of the glucose transporter GLUT4 were substituted for leucine and alanine, respectively. Reconstituted glucose transport activity of the construct (GLUT4-RR333/4LA) expressed in COS-7 or LM(TK-) cells was less than 10% of that of the wild-type GLUT4. In contrast, binding of the inhibitory ligand cytochalasin B and glucose-inhibitable photolabeling with IAPS-forskolin were not significantly affected. Exchange of a histidine residue (H337Q) previously believed to be involved in the binding of inhibitory ligands failed to affect any of the investigated parameters. These data suggest that positive charges in the GRR motif at the cytoplasmic surface of the transporter participate in the conformational changes of the carrier protein during the process of facilitated diffusion.

Analysis of the structural features of the C-terminus of GLUT1 that are required for transport catalytic activity

C-terminally truncated and mutated forms of GLUT1 have been constructed to determine the minimum structure at the C-terminus required for glucose transport activity and ligand binding at the outer and inner binding sites. Four truncated mutants have been constructed (CTD24 to CTD27) in which 24 to 27 amino acids are deleted. In addition, point substitutions of R468-->L, F467-->L and G466-->E have been produced. Chinese hamster ovary clones which were transfected with these mutant GLUT1s were shown, by Western blotting and cell-surface carbohydrate labelling, to have expression levels which were comparable with the wild-type clone. Wild-type levels of 2-deoxy-D-glucose transport activity were retained only in the clone transfected with the construct in which 24 amino acids were deleted (CTD24). The CTD25, CTD26 and CTD27 clones showed markedly reduced transport activity. From a kinetic comparison of the CTD24 and CTD26 clones it was found that the reduced transport was mainly associated with a reduced Vmax. value for 2-deoxy-D-glucose uptake but with a slight lowering of the Km. These data establish that the 24 amino acids at the C-terminus of GLUT1 are not required for the transport catalysis. However, the point mutations of F467L and G466E (26 and 27 residues from the C-terminus) did not significantly perturb the kinetics of 2-deoxy-D-glucose transport. The substitution of R468L produced a slight, but significant, lowering of the Km. The ability of the truncated GLUt1s to bind the exofacial ligand, 2-N-4-(1-zai-2,2,2-trifluoroethyl)benzoyl-1,3-bis-(D-mannos- 4-yl-oxy) -2-propylamine (ATB-BMPA), and the endofacial ligand, cytochalasin B, were assessed by photolabelling procedures. The ability to bind ATB-BMPA was retained only in the CTD24 truncated mutant and was reduced to levels comparable with those of the non-transfected clone in the other mutant clones. Cytochalasin B labelling was unimpaired in all four mutated GLUT1s. These data establish that a minimum structure at the C-terminus of GLUT1, which is required for the conformational change to expose the exofacial site, includes amino acids at positions Phe-467 and Arg-468; however, these amino acids are not individually essential.

Substitution of conserved tyrosine residues in helix 4 (Y143) and 7 (Y293) affects the activity, but not IAPS-forskolin binding, of the glucose transporter GLUT4

Six tyrosine residues (Y28, Y143, Y292, Y293, Y308, Y432(1)) which are conserved in all mammalian glucose transporters were substituted for phenylalanine by site-directed mutagenesis, and mutant glucose transporters were transiently expressed in COS-7 cells. Glucose transport activity as assessed by reconstitution of the solubilized transporters into lecithin liposomes was reduced by 70% in the mutant Y143F and appeared to be abolished in Y293F, but was not affected by substitution of Y28, Y292, Y308 and Y432. In contrast, covalent binding of the photolabel 125IAPS-forskolin was normal in all mutants. Stable expression of the mutants Y143F, Y293F, and Y292F in LTK cells yielded identical results. These data indicate that only two of the 6 conserved helical tyrosine residues, located in helices 4 and 7, are essential for full activity, but not for IAPS-forskolin binding of the GLUT4.