During the last 2 years, several novel genes that encode glucose transporter-like proteins have been identified and characterized. Because of their sequence similarity with GLUT1, these genes appear to belong to the family of solute carriers 2A ( SLC2A, protein symbol GLUT). Sequence comparisons of all 13 family members allow the definition of characteristic sugar/polyol transporter signatures: (1) the presence of 12 membrane-spanning helices, (2) seven conserved glycine residues in the helices, (3) several basic and acidic residues at the intracellular surface of the proteins, (4) two conserved tryptophan residues, and (5) two conserved tyrosine residues. On the basis of sequence similarities and characteristic elements, the extended GLUTfamily can be divided intothree subfamilies, namely class I (the previously known glucose transporters GLUT1-4), class II (the previously known fructose transporter GLUT5, the GLUT7, GLUT9 and GLUT11), and class III (GLUT6, 8, 10, 12, and the myoinositol transporter HMIT...

The extended GLUT-family of sugar/polyol transport facilitators: nomenclature, sequence characteristics, and potential function of its novel members (review).

The glucose transporter GLUT1 catalyses facilitative diffusion of glucose into erythrocytes and is responsible for glucose supply to the brain and other organs. Dysfunctional mutations may lead to GLUT1 deficiency syndrome, whereas overexpression of GLUT1 is a prognostic indicator for cancer. Despite decades of investigation, the structure of GLUT1 remains unknown. Here we report the crystal structure of human GLUT1 at 3.2 A resolution. The full-length protein, which has a canonical major facilitator superfamily fold, is captured in an inward-open conformation. This structure allows accurate mapping and potential mechanistic interpretation of disease-associated mutations in GLUT1. Structure-based analysis of these mutations provides an insight into the alternating access mechanism of GLUT1 and other members of the sugar porter subfamily. Structural comparison of the uniporter GLUT1 with its bacterial homologue XylE, a proton-coupled xylose symporter, allows examination of the transport mechanisms of both passive facilitators and active transporters. The structure of human GLUT1 in an inward-open conformation is reported; access to the structure of the human protein, instead of just a bacterial homologue, made it possible to map (inactivating) mutations associated with GLUT1 deficiency syndrome onto the structure. GLUT1 is a membrane protein that is responsible for the uptake of glucose into erythrocytes and other cells. The structure of a proton-coupled xylose symporter that is a bacterial homologue of GLUT1 has been reported previously and here Nieng Yan and colleagues report the structure of human GLUT1 in an inward-open conformation. Having access to the structure of the human protein, the authors were able to map inactivating mutations associated with GLUT1 deficiency syndrome — also known as De Vivo syndrome — onto their structure. Because elevated expression levels of GLUT1 have been observed in several cancer types, access to this structure may facilitate the development of new anticancer agents.

Crystal structure of the human glucose transporter GLUT1

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.

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Functional properties and genomics of glucose transporters.

The determination of the crystal structures of small-molecule transporters has shed light on the conformational changes that take place during structural isomerization from outward- to inward-facing states. Rather than using a simple rocking movement of two bundles around a central substrate-binding site, it has become clear that even the most simplistic transporters utilize rearrangements of nonrigid bodies. In the most dramatic cases, one bundle is fixed while the other, structurally divergent, bundle carries the substrate some 18 A across the membrane, which in this review is termed an elevator alternating-access mechanism. Here, we compare and contrast rocker-switch, rocking-bundle, and elevator alternating-access mechanisms to highlight shared features and novel refinements to the basic alternating-access model.

Shared Molecular Mechanisms of Membrane Transporters

The protein family of facilitative glucose transporters comprises 14 isoforms that share common structural features such as 12 transmembrane domains, N- and C-termini facing the cytoplasm of the cell, and a N-glycosylation side either within the first or fifth extracellular loop. Based on their sequence homology, three classes can be distinguished: class I includes GLUT1-4 and GLUT14, class II the "odd transporters" GLUT5, 7, 9, 11, and class III the "even transporters" GLUT6, 8, 10, 12 and the proton driven myoinositol transporter HMIT (or GLUT13). With the cloning and characterization of the more recent class II and III isoforms, it became apparent that despite their structural similarities, the different isoforms not only show a distinct tissue-specific expression pattern but also show distinct characteristics such as alternative splicing, specific (sub)cellular localization, and affinities for a spectrum of substrates. This review summarizes the current understanding of the physiological role for the various transport facilitators based on human genetically inherited disorders or single-nucleotide polymorphisms and knockout mice models. The emphasis of the review will be on the potential functional role of the more recent isoforms.

https://iubmb.onlinelibrary.wiley.com/doi/pdf/10.1002/iub.315

The protein family of glucose transport facilitators: It's not only about glucose after all.

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 acti...

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

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.

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.

Volker Monser

Papers

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

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.