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. Biochem J. 2000;350 Pt 3:771-776. [PMID: 10970791 DOI: 10.1042/0264-6021:3500771]

Current understanding of glucose transporter 4 expression and functional mechanisms

Glucose is used aerobically and anaerobically to generate energy for cells. Glucose transporters (GLUTs) are transmembrane proteins that transport glucose across the cell membrane. Insulin promotes glucose utilization in part through promoting glucose entry into the skeletal and adipose tissues. This has been thought to be achieved through insulin-induced GLUT4 translocation from intracellular compartments to the cell membrane, which increases the overall rate of glucose flux into a cell. The insulin-induced GLUT4 translocation has been investigated extensively. Recently, significant progress has been made in our understanding of GLUT4 expression and translocation. Here, we summarized the methods and reagents used to determine the expression levels of Slc2a4 mRNA and GLUT4 protein, and GLUT4 translocation in the skeletal muscle, adipose tissues, heart and brain. Overall, a variety of methods such real-time polymerase chain reaction, immunohistochemistry, fluorescence microscopy, fusion proteins, stable cell line and transgenic animals have been used to answer particular questions related to GLUT4 system and insulin action. It seems that insulin-induced GLUT4 translocation can be observed in the heart and brain in addition to the skeletal muscle and adipocytes. Hormones other than insulin can induce GLUT4 translocation. Clearly, more studies of GLUT4 are warranted in the future to advance of our understanding of glucose homeostasis.

Diagnosis of hepatic glycogenosis in poorly controlled type 1 diabetes mellitus

Hepatic glycogenosis (HG) in type 1 diabetes is a underrecognized complication. Mauriac firstly described the syndrome characterized by hepatomegaly with altered liver enzymes, growth impairment, delay puberty and Cushingoid features, during childhood. HG in adulthood is characterized by the liver disorder (with circulating aminotransferase increase) in the presence of poor glycemic control (elevation of glycated hemoglobin, HbA1c levels). The advances in the comprehension of the metabolic pathways driving to the hepatic glycogen deposition point out the role of glucose transporters and insulin mediated activations of glucokinase and glycogen synthase, with inhibition of glucose-6-phosphatase. The differential diagnosis of HG consists in the exclusion of causes of liver damage (infectious, metabolic, obstructive and autoimmune disease). The imaging study (ultrasonography and/or radiological examinations) gives information about the liver alterations (hepatomegaly), but the diagnosis needs to be confirmed by the liver biopsy. The main treatment of HG is the amelioration of glycemic control that is usually accompanied by the reversal of the liver disorder. In selected cases, more aggressive treatment options (transplantation) have been successfully reported.

Elevated dietary sugar and the heart: experimental models and myocardial remodeling

A dramatic rise in the prevalence of insulin resistance has been paralleled by increasing dietary consumption of sugar. The use of added sweeteners containing fructose (sucrose and high-fructose corn syrup) has increased by 25% over the past 3 decades. High fructose intake has the potential to adversely influence systemic and cellular metabolism via insulin resistance and glycolytic dysregulation. As a tissue that is both insulin sensitive and glycolysis dependent, the heart may be especially vulnerable to fructose over-consumption. In this review, experimental studies of elevated dietary sugar intake are evaluated, including sucrose and fructose dietary manipulation models. The possible role of the GLUT5 transporter as a mediator of cardiomyocyte fructose uptake is considered. The impact of dietary sucrose and fructose on cardiac insulin-dependent signaling in the context of perturbed systemic metabolic response is detailed. Myocardial dysfunction, modified growth, and oxidative stress responses associated with high dietary sugar intake are discussed. Finally, the involvement of the renin-angiotensin system in mediating fructose cardiopathology is considered. This review highlights the importance of obtaining new mechanistic data that can contribute to a more developed understanding of how high sugar intake directly contributes to structural and functional cardiomyopathy.

Glial-neuronal interactions underlying fructose utilization in rat hippocampal slices

Although fructose is commonly used as a sweetener, its effects on brain function are unclear. Using rat hippocampal slices, we found that fructose and mannose, like pyruvate, preserve ATP levels during 3-h of glucose deprivation. Similarly, fructose and mannose restored synaptic potentials (excitatory postsynaptic potential, EPSPs) depressed during glucose deprivation. However, restoration of synaptic responses was slow and only partial with fructose. EPSPs supported by mannose were inhibited by cytochalasin B (CCB), a glucose transport inhibitor, but were not inhibited by alpha-cyano-4-hydroxycinnamate (4-CIN), a monocarboxylate transport inhibitor, indicating that neurons use mannose via glucose transporters. In contrast, both CCB and 4-CIN depressed EPSPs supported by fructose, suggesting that fructose may be taken up by non-neuronal cells through CCB sensitive hexose transporters and metabolized to a monocarboxylate for subsequent use during neuronal respiration. Supporting this possibility, 20 minutes of oxygen deprivation in the presence of fructose resulted in functional and morphological deterioration whereas oxygen deprivation in the presence of glucose or mannose had minimal toxic effects. These results indicate that neuronal fructose utilization differs from glucose and mannose and likely involves release of monocarboxylates from glia.

Channels for water efflux and influx involved in volume regulation of murine spermatozoa

The nature of the membrane channels mediating water transport in murine spermatozoa adjusting to anisotonic conditions was investigated. The volume of spermatozoa subjected to physiologically relevant hypotonic conditions either simultaneously, or after isotonic pre-incubation, with putative water transport inhibitors was monitored. Experiments in which quinine prevented osmolyte efflux, and thus regulatory volume decrease (RVD), revealed whether water influx or efflux was being inhibited. There was no evidence that sodium-dependent solute transporters or facilitative glucose transporters were involved in water transport during RVD of murine spermatozoa since phloretin, cytochalasin B and phloridzin had no effect on volume regulation. However, there was evidence that Hg(2+)- and Ag(+)-sensitive channels were involved in water transport and the possibility that they include aquaporin 8 is discussed. Toxic effects of these heavy metals were ruled out by evidence that mitochondrial poisons had no such effect on volume regulation.

The activity of calpains in lymphocytes is glucose-dependent and is decreased in diabetic patients

Calpains are nonlysosomal calcium-dependent cysteine proteases that participate in insulin secretion and action. Polymorphisms in the calpain-10 gene have been shown to increase the risk for type 2 diabetes. Since white blood cells have been used to study glucose homeostasis, the present study was carried to find out if calpains have different activity and/or expression in accessible cells such as lymphocytes of individuals with or without type 2 diabetes. Fasting blood glucose concentration was significantly higher in diabetic subjects, whereas the difference in the activity of calpains evaluated in basal and stimulating extracellular glucose concentration was significantly higher in the lymphocytes from the control group. The mRNA expression of calpain-10 was similar in the lymphocytes of both patients and controls. The protein blots showed four bands that ranged between 75 and 50 kDa; however, no statistical differences were observed in the expression of the calpain-10 isoforms between controls and patients. Data obtained showed that human lymphocytes express calpain-10 mRNA and protein, showing a similar expression between diabetic and control subjects, nevertheless in the diabetic group calpain activity was less glucose-sensitive.

Characterization of taurine as inhibitor of sodium glucose transporter

The most characterized roles of taurine include osmoregulator and membrane-stabilizing activities. However, much remains to be understood about its role in human physiology concerning its anti-hyperglycemic effect. Studies indicate that taurine-supplemented diet helps alleviate hyperglycemia or insulin resistance. This hypoglycemic effect has been postulated as taurine helping to increase the excretion of cholesterol. Alternatively, this study investigated the effect of taurine on glucose transporter using heterologous expression of sodium-glucose transporter-1 (SGLT-1). SGLT-1 was expressed in Xenopus oocytes and the effect of taurine on the expressed SGLT-1 was analyzed utilizing 2-deoxy-D-glucose (2-DOG) uptake and voltage clamp studies. In the oocytes expressing SGLT-1, taurine was shown to inhibit SGLT-1 activity compared to the non-treated controls in a dose-dependent manner. In the presence of taurine, the glucose uptake was greatly inhibited and the glucose-generated current was significantly inhibited. Synthetic taurine analogs were also shown to be effective in inhibiting SGLT-1 activity in a manner comparable to taurine. These effects might offer a promising opportunity in designing functional foods with anti-hyperglycemic potential by supplementing taurine and its analogs to the diet.

Inhibition of sodium glucose cotransporter-1 expressed inXenopus laevisoocytes by 4-acetoxyscirpendiol from Cordyceps takaomantana (anamorph = Paecilomyces tenuipes)

Cordyceps contains many health-promoting constituents. Recent studies revealed that the fruiting body of cordyceps significantly alleviates hyperglycemia which usually accompanies diabetes mellitus. The mechanism of the anti-hyperglycemic effect by cordyceps, however, is not fully understood. In this study, methanolic extracts were prepared from fruiting bodies of Paecilomyces tenuipes, and 4-beta acetoxyscirpendiol (ASD) was eventually purified from the extracts. The Na+/ glucose transporter-1 (SGLT-1) was expressed in Xenopus oocytes, and the effect of ASD on it was analyzed using voltage clamp and 2-deoxy-D-glucose (2-DOG) uptake studies. Fluorescence microscopy was performed to monitor the effect of ASD on glucose uptake using HEK293 cells expressing recombinant SGLT-1. ASD inhibited SGLT-1 activity, and its two derivatives (2-acetoxyscirpenol and 15-acetoxyscirpendiol), were also effective; 15-acetoxyscirepenol was as inhibitory as ASD while diacetoxyscirpenol had less effect. Thus, the ASD in P. tenuipes may play an important role in lowering blood sugar in the circulatory system along with its derivatives as specific inhibitors of SGLT-1.

Expression of GLUT12 in the fetal membranes of the human placenta

The aim of this study was to characterize the expression of the novel glucose transporter GLUT12 in the fetal membranes of the human placenta. RT-PCR and Western blotting of extracts of amnion and choriodecidua from four normal term placentas identified GLUT12 mRNA and protein expression. In all four samples the signals for GLUT12 were markedly stronger in the choriodecidua than in the amnion, whereas the signals for GLUT1, a glucose transporter know to be expressed in fetal membranes, were similar for the two tissues. In further studies, paraffin sections of fetal membranes were analyzed by immunohistochemistry with GLUT12 and GLUT1-specific polyclonal antibodies. GLUT12 immunoreactivity was localized predominantly to the trophoblast cells in the chorion and to a lesser extent to decidual cells and to epithelial and fibroblast cells of the amnion. GLUT1 was localized to chorionic trophoblast cells and amniotic epithelial and fibroblast cells. GLUT12 expression was predominantly cytoplasmic, whereas GLUT1 was associated with the membrane of the cells. These results show that GLUT12 is expressed in cells of human fetal membranes and suggest that GLUT12 may play a role in the facilitation of glucose transport into these cells.

4-Acetoxyscirpendiol of Paecilomyces tenuipes Inhibits Na+/D-Glucose Cotransporter Expressed in Xenopus laevis Oocytes

Cordyceps, an entomopathogenic fungus, contains many health-promoting ingredients. Recent reports indicate that the consumption of cordyceps helps reduce blood-sugar content in diabetics. However, the mechanism underlying this reduction in circulatory sugar content is not fully understood. Methanolic extracts were prepared from the fruiting bodies of Paecilomyces tenuipes, and 4-beta acetoxyscirpendiol (4-ASD) was eventually isolated and purified. Na(+)/Glucose transporter-1 (SGLT-1) was expressed in Xenopus oocytes, and the effect of 4-ASD on SGLT-1 was analyzed utilizing a voltage clamp and by performing 2-deoxy-D-glucose (2-DOG) uptake studies. 4-ASD was shown to significantly inhibit SGLT-1 activity compared to the non-treated control in a dose-dependent manner. In the presence of the derivatives of 4-ASD (diacetoxyscirpenol or 15-acetoxyscirpendiol), SGLT-1 activity was greatly inhibited in an 4-ASD-like manner. Of these derivatives, 15-acetoxyscirepenol inhibited SGLT-1 as well as 4-ASD, whereas diacetoxyscirpenol was slightly less effective. Taken together, these results strongly indicate that 4-ASD in P. tenuipes may lower blood sugar levels in the circulatory system. We conclude that 4-ASD and its derivatives are effective SGLT-1 inhibitors.

Glucose transporter expression in the central nervous system: relationship to synaptic function

The family of facilitative glucose transporter (GLUT) proteins is responsible for the entry of glucose into cells throughout the periphery and the brain. The expression, regulation and activity of GLUTs play an essential role in neuronal homeostasis, since glucose represents the primary energy source for the brain. Brain GLUTs exhibit both cell type and region specific localizations suggesting that the transport of glucose across the blood-brain barrier is tightly regulated and compartmentalized. As seen in the periphery, insulin-sensitive GLUTs are expressed in the brain and therefore may participate in the central actions of insulin. The aim of this review will be to discuss the localization of GLUTs expressed in the central nervous system (CNS), with a special emphasis upon the recently identified GLUT isoforms. In addition, we will discuss the regulation, activity and insulin-stimulated trafficking of GLUTs in the CNS, especially in relation to the centrally mediated actions of insulin and glucose.

Distinct regulation of glucose transport and GLUT1/GLUT3 transporters by glucose deprivation and IGF-I in chromaffin cells

Effects of prolonged metabolic (glucose deprivation) and hormonal [insulin-like growth factor I (IGF-I)] challenge on regulation of glucose transporter (GLUT) expression, glucose transport rate and possible signaling pathways involved were studied in the neuroendocrine chromaffin cell. The results show that bovine chromaffin cells express both GLUT1 and GLUT3. Glucose deprivation and IGF-I activation led to an elevation of GLUT1 and GLUT3 mRNA, the strongest effect being that of IGF-I on GLUT3 mRNA. Both types of stimulus increased the GLUT1 protein content in a cycloheximide (CHX)-sensitive manner, and the glucose transport rate was elevated by 3- to 4-fold after 48 h under both experimental conditions. IGF-I-induced glucose uptake was totally suppressed by CHX. In contrast, only approximately 50% of transport activation in glucose-deprived cells was sensitive to the protein synthesis inhibitor. Specific inhibitors of mTOR/FRAP and p38 MAPK each partially blocked IGF-I-stimulated glucose transport, but had no effect on transport rate in glucose-deprived cells. The results are consistent with IGF-I-activated transport being completely dependent on new GLUT protein synthesis while the enhanced transport in glucose-deprived cells was partially achieved independent of new synthesis of proteins, suggesting a mechanism relying on preexisting transporters.

GLUT14, a duplicon of GLUT3, is specifically expressed in testis as alternative splice forms

We have identified and cloned GLUT14, a novel member of the glucose transporter family. GLUT14 (SLC2A14) maps to chromosome 12p13.3 (17.1M), about 10 Mb upstream of GLUT3, with which it shares remarkable identity. Until now GLUT14 was thought to be a pseudogene. It consists of 11 exons with a genomic organization similar to that of GLUT3 and likely resulted from a duplication of GLUT3. GLUT14 has two alternatively spliced forms; the shorter form of GLUT14 (GLUT14-S) consists of 10 exons and produces a 497-amino-acid protein that is 94.5% identical to GLUT3. The long form (GLUT14-L) has an additional exon and codes for a protein with 520 amino acids that differs from GLUT14-S only at the N-terminus. GLUT14-S/L contain 12 putative membrane-spanning helices along with sugar-transporter signature motifs that have previously been shown to be essential for sugar transport activity. The putative glycosylation sites of GLUT14-S/L are present in loop 1. In contrast to the expression of GLUT3 in many tissues, both isoforms of GLUT14 are specifically expressed in testis. The mRNA level of GLUT14 in testis is about four times higher than that of GLUT3. Interestingly, the ortholog of GLUT14 is not found in mice. The multiple duplications of GLUT genes suggest that the GLUT family probably emerged by gene duplications and mutations during evolution in different lineages.

Expression of insulin-responsive glucose transporter GLUT4 mRNA in the rat brain and spinal cord: an in situ hybridization study

Following a previous immunocytochemical study of GLUT4 in the rat brain and spinal cord (J. Comp. Neurol. 399 (1998) 492), we now report the distribution and cellular expression of GLUT4 mRNA in the CNS using reverse transcription-polymerase chain reaction and non-radioactive in situ hybridization (ISH). The former technique demonstrated the expression of GLUT4 in the different regions examined while ISH with a specific riboprobe allowed the anatomical localization of GLUT4 mRNA. A strong hybridization signal was detected in the piriform and entorhinal cortices and in the pyramidal cell layer of the hippocampal CA1-CA3 areas. Numerous moderately labeled cells were additionally observed in the dentate gyrus granular layer, subiculum and most neocortical areas, as well as in different nuclei of the limbic and motor systems. In contrast, positive cell groups were scarce in the hypothalamus. In the hindbrain, a strong expression of GLUT4 mRNA was observed in the large cell bodies of the red nucleus and cerebellar Purkinje cell layer. Moreover, different groups of moderately labeled cells were found in the deep cerebellar and medullary motor nuclei, in various reticular fields and in the ventral horn of the spinal cord. The present results of ISH mostly agree with the immunocytochemical data reported by our group, although the immunoreactive cells were generally less numerous. However, the fact that a high expression of GLUT4 mRNA was observed in cell bodies of the piriform lobe, hippocampus and substantia nigra, whereas the immunoreactivity for GLUT4 was low in these regions, suggests the existence of post-transcriptional regulation of GLUT4 expression which may depend on the physiological conditions of the animals.

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.

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.

Mouse GLUT8: genomic organization and regulation of expression in 3T3-L1 adipocytes by glucose

Glucose transporter 8 (GLUT8) is a class III sugar transport facilitator predominantly expressed in testis and insulin-regulated tissues. Here we describe its genomic organization, the identification of its promoter region, and the regulation of its expression in 3T3-L1 cells. The mouse Glut8 gene spans approximately 9 kb, consists of 10 exons, and is highly similar to the human GLUT6 gene. Its 5'-flanking region exhibits promoter activity when fused with a luciferase reporter construct and expressed in HEK-293T cells. A deletion analysis indicated that the critical promoter elements are located in a region between -381 and the transcription start. This region comprises a CAAT box and consensus binding sites for the transcription factors SRY and NF1 that were highly conserved in the mouse and in the human sequence. In 3T3-L1 cells, GLUT8 mRNA levels increased markedly during the differentiation of cells. In contrast to GLUT1, expression of GLUT8 mRNA was significantly reduced by glucose deprivation and by prolonged hypoxia. The present data suggest that the function of GLUT8 is related to the adipocyte-like phenotype of 3T3-L1 cells, and that its expression is controlled by the metabolism of the adipocyte.

Acute regulation of glucose transport in a human megakaryocytic cell line: difference between growth factors and H(2)O(2)

The present study was undertaken to: (i) compare the effect of some hematopoietic growth factors, like interleukine-3, thrombopoietin, granulocyte-megakaryocyte colony-stimulating factor, stem cell factor, and reactive oxygen species such as H(2)O(2) on glucose uptake in a human leukemic megakaryocytic cell line, M07; (ii) investigate the changes in kinetic parameters of the transport activity induced by these stimuli; and (iii) evaluate the effect of genistein, a tyrosine kinase inhibitor, on the glucose uptake activation by the cited agents. The results are as follows: (i) exposure of M07 cells to thrombopoietin, granulocyte-megakaryocyte colony-stimulating factor, and stem cell factor resulted in a rapid stimulation of glucose transport; interleukine-3-treated cells exhibited no increase in the rate of glucose uptake, although M07 proliferation is interleukine-3 dependent; a rapid glucose transport enhancement was also observed when M07 cells were exposed to low doses of H(2)O(2); (ii) the transport kinetic parameters point out that an important difference exists between the effect of cytokines and that of H(2)O(2): cytokines increased predominantly the affinity for glucose, while H(2)O(2) raised both the V(max) and K(m) values; (iii) the isoflavone genistein, at a very low concentration, inhibited the stem cell factor- or H(2)O(2)-induced stimulation of hexose transport, reversing the variations of K(m) and V(max), but it did not affect the transport activity of granulocyte-megakaryocyte colony-stimulating factor-treated cells; and (iv) catalase completely abolished the stimulatory action of H(2)O(2) on glucose transport and slightly prevented the effect of stem cell factor, while caffeic acid phenethyl ester was only able to affect the activation due to stem cell factor.

Amplification of blood-brain barrier GLUT1 glucose transporter gene expression by brain-derived peptides

Glucose is a critical nutrient for the brain, and the transport of this hexose from blood to brain is mediated by the blood-brain barrier (BBB) GLUT1 glucose transporter. The expression of the BBB-GLUT1 gene is compromised in different pathological conditions and it is modulated by brain trophic factors. The brain-derived peptide preparation Cerebrolysin (Cl, EBEWE, Austria) increases the expression of the BBB-GLUT1 via mRNA stabilization. In order to gain more insights into the mechanism of BBB-GLUT1 gene regulation, the present investigation studied the effect of Cl on the expression of both the GLUT1 protein and GLUT1 reporter genes in brain endothelial cultured cells (ECL). Cl markedly increased the expression of reporter genes containing GLUT1 translational control elements and cis-acting elements involved in the stabilization of the GLUT1 mRNA transcript in a dose dependent manner. Cl produced only marginal effects on the reporter gene control lacking the GLUT1 regulatory elements. In parallel experiments, Cl markedly increased the uptake of 3H-2-deoxy-D-glucose and the levels of the GLUT1 protein measured by ELISA. Data presented here demonstrate: (i) that Cl increases the expression of BBB-GLUT1 reporter genes containing regulatory cis-elements involved in the stabilization and translation of the GLUT1 transcript; (ii) that the effect on both regulatory elements cooperates to increase gene expression; and (iii) that the increased levels of the BBB-GLUT1 reporter genes in Cl-treated ECL cells are associated with an increase in the glucose uptake and in the expression of the GLUT1 protein.

Purification and characterization of human erythrocyte glucose transporter in decylmaltoside detergent solution

The facilitative glucose transporter from human erythrocyte membrane, Glut1, was purified by a novel method. The nonionic detergent decylmaltoside was selected for solubilization on the basis of its efficiency to extract Glut1 from the erythrocyte membrane and its ability to maintain the protein in a monodisperse state. A positive, anion-exchange chromatography protocol produced a Glut1 preparation of 95% purity with little copurified lipid. This protein preparation exhibited cytochalasin B binding in detergent solution, as measured by tryptophan fluorescence quenching. The transporter existed as a monomer in decylmaltoside, with a Stokes radius of 50 A and a molecular mass of 147 kDa for the protein-detergent complex. We screened detergent, pH, additive, and lipid and have found conditions to maintain Glut1 monodispersity for 8 days at 25 degrees C or over 5 weeks at 4 degrees C. This Glut1 preparation represents the best available material for two- and three-dimensional crystallization trials of the human glucose transporter protein.

Moving the insulin-regulated glucose transporter GLUT4 into and out of storage

The glucose transporter isoform GLUT4 is unique among the glucose transporter family of proteins in that, in resting cells, it is sequestered very efficiently in a storage compartment. In insulin-sensitive cells, such as fat and muscle, insulin stimulation leads to release of GLUT4 from this reservoir and its translocation to the plasma membrane. This process is crucial for the control of blood and tissue glucose levels. Investigations of the composition and structure of the GLUT4 storage compartment, together with the targeting motifs that direct GLUT4 to this compartment, have been extensive but have been controversial. Recent findings have now provided a clearer consensus of opinion on the mechanisms involved in the formation of this storage compartment. However, another controversy has now emerged, which is unresolved. This concerns the issue of whether the insulin-regulated step occurs at the level of release of GLUT4 from the storage compartment or at the level at which released vesicles fuse with the plasma membrane.