Genome-wide identification and expression analysis of the StSWEET family genes in potato (Solanum tuberosum L.).

BACKGROUND: The sugar will eventually be exported transporter (SWEET) family is a novel type of membrane-embedded sugar transporter that contains seven transmembrane helices with two MtN3/saliva domains. The SWEET family plays crucial roles in multiple processes, including carbohydrate transportation, development, environmental adaptability and host-pathogen interactions. Although SWEET genes, especially those involved in response to biotic stresses, have been extensively characterized in many plants, they have not yet been studied in potato. OBJECTIVE: The identification of StSWEET genes provides important candidates for further functional analysis and lays the foundation for the production of good quality and high yield potatoes through molecular breeding. METHODS: In this study, StSWEET genes were identified using a genome-wide search method. A comprehensive analysis of StSWEET family through bioinformatics methods, such as phylogenetic tree, gene structure and promoter prediction analysis. The expression profiles of StSWEET genes in different potato tissues and under P. infestans attack and sugar stress were studied using quantitative real-time polymerase chain reaction (qRT-PCR). RESULTS: Phylogenetic analysis classified 33 StSWEET genes into four groups containing 12, 5, 12 and 4 genes. Furthermore, the gene structures and conserved motifs found that the StSWEET genes are very conservative during evolution. The chromosomal localization pattern showed that the distribution and density of the StSWEETs on 10 potato chromosomes were uneven and basically clustered. Predictive promoter analysis indicated that StSWEET proteins are associated with cell growth, development, secondary metabolism, and response to biotic and abiotic stresses. Finally, the expression patterns of the StSWEET genes in different tissues and the induction of P. infestans and the process of the sugar stress were investigated to obtain the tissue-specific and stress-responsive candidates. CONCLUSION: This study systematically identifies the SWEET gene family in potato at the genome-wide level, providing important candidates for further functional analysis and contributing to a better understanding of the molecular basis of development and tolerance in potato.

Sugar transporters in Fabaceae, featuring SUT MST and SWEET families of the model plant Medicago truncatula and the agricultural crop Pisum sativum.

Sugar transporters play a crucial role for plant productivity, as they coordinate sugar fluxes from source leaf towards sink organs (seed, fruit, root) and regulate the supply of carbon resources towards the microorganisms of the rhizosphere (bacteria and fungi). Thus, sugar fluxes mediated by SUT (sucrose transporters), MST (monosaccharide transporters) and SWEET (sugar will eventually be exported transporters) families are key determinants of crop yield and shape the microbial communities living in the soil. In this work, we performed a systematic search for sugar transporters in Fabaceae genomes, focusing on model and agronomical plants. Here, we update the inventory of sugar transporter families mining the latest version of the Medicago truncatula genome and identify for the first time SUT MST and SWEET families of the agricultural crop Pisum sativum. The sugar transporter families of these Fabaceae species comprise respectively 7 MtSUT 7 PsSUT, 72 MtMST 59 PsMST and 26 MtSWEET 22 PsSWEET. Our comprehensive phylogenetic analysis sets a milestone for the scientific community, as we propose a new and simple nomenclature to correctly name SUT MST and SWEET families. Then, we searched for transcriptomic data available for our gene repertoire. We show that several clusters of homologous genes are co-expressed in different organs, suggesting that orthologous sugar transporters may have a conserved function. We focused our analysis on gene candidates that may be involved in remobilizing resources during flowering, grain filling and in allocating carbon towards roots colonized by arbuscular mycorrhizal fungi and Rhizobia. Our findings open new perspectives for agroecological applications in legume crops, as for instance improving the yield and quality of seed productions and promoting the use of symbiotic microorganisms.

Phylogenomic analysis of UDP-dependent glycosyltransferases provides insights into the evolutionary landscape of glycosylation in plant metabolism.

Glycosylated metabolites generated by UDP-dependent glycosyltransferases (UGTs) play critical roles in plant interactions with the environment as well as human and animal nutrition. The evolution of plant UGTs has previously been explored, but with a limited taxon sampling. In this study, 65 fully sequenced plant genomes were analyzed, and stringent criteria for selection of candidate UGTs were applied to ensure a more comprehensive taxon sampling and reliable sequence inclusion. In addition to revealing the overall evolutionary landscape of plant UGTs, the phylogenomic analysis also resolved the phylogenetic association of UGTs from free-sporing plants and gymnosperms, and identified an additional UGT group (group R) in seed plants. Furthermore, lineage-specific expansions and contractions of UGT groups were detected in angiosperms, with the total number of UGTs per genome remaining constant generally. The loss of group Q UGTs in Poales and Brassicales, rather than functional convergence in the group Q containing species, was supported by a gene tree of group Q UGTs sampled from many species, and further corroborated by the absence of group Q homologs on the syntenic chromosomal regions in Arabidopsis thaliana (Brassicales). Branch-site analyses of the group Q UGT gene tree allowed for identification of branches and amino acid sites that experienced episodic positive selection. The positively selected sites are located on the surface of a representative group Q UGT (PgUGT95B2), away from the active site, suggesting their role in protein folding/stability or protein-protein interactions.

Sugar Transporter Proteins (STPs) in Gramineae Crops: Comparative Analysis, Phylogeny, Evolution, and Expression Profiling.

Sugar transporter proteins (STPs), such as H+/sugar symporters, play essential roles in plants' sugar transport, growth, and development, and possess an important potential to enhance plants' performance of multiple agronomic traits, especially crop yield and stress tolerance. However, the evolutionary dynamics of this important gene family in Gramineae crops are still not well-documented and functional differentiation of rice STP genes remain unclear. To address this gap, we conducted a comparative genomic study of STP genes in seven representative Gramineae crops, which are Brachypodium distachyon (Bd), Hordeum vulgare (Hv), Setaria italica (Si), Sorghum bicolor (Sb), Zea mays (Zm), Oryza rufipogon (Or), and Oryza sativa ssp. japonica (Os). In this case, a total of 177 STP genes were identified and grouped into four clades. Of four clades, the Clade I, Clade III, and Clade IV showed an observable number expansion compared to Clade II. Our results of identified duplication events and divergence time of duplicate gene pairs indicated that tandem, Whole genome duplication (WGD)/segmental duplication events play crucial roles in the STP gene family expansion of some Gramineae crops (expect for Hv) during a long-term evolutionary process. However, expansion mechanisms of the STP gene family among the tested species were different. Further selective force studies revealed that the STP gene family in Gramineae crops was under purifying selective forces and different clades and orthologous groups with different selective forces. Furthermore, expression analysis showed that rice STP genes play important roles not only in flower organs development but also under various abiotic stresses (cold, high-temperature, and submergence stresses), blast infection, and wounding. The current study highlighted the expansion and evolutionary patterns of the STP gene family in Gramineae genomes and provided some important messages for the future functional analysis of Gramineae crop STP genes.

Transcriptome profiling reveals differential gene expression of detoxification enzymes in a hemimetabolous tobacco pest after feeding on jasmonate-silenced Nicotiana attenuata plants.

BACKGROUND: The evolutionary arms race between plants and insects has driven the co-evolution of sophisticated defense mechanisms used by plants to deter herbivores and equally sophisticated strategies that enable phytophagous insects to rapidly detoxify the plant's defense metabolites. In this study, we identify the genetic determinants that enable the mirid, Tupiocoris notatus, to feed on its well-defended host plant, Nicotiana attenuata, an outstanding model for plant-insect interaction studies. RESULTS: We used an RNAseq approach to evaluate the global gene expression of T. notatus after feeding on a transgenic N. attenuata line which does not accumulate jasmonic acid (JA) after herbivory, and consequently accumulates very low levels of defense metabolites. Using Illumina sequencing, we generated a de novo assembled transcriptome which resulted in 63,062 contigs (putative transcript isoforms) contained in 42,610 isotigs (putative identified genes). Differential expression analysis based on RSEM-estimated transcript abundances identified 82 differentially expressed (DE) transcripts between T. notatus fed on wild-type and the defenseless plants. The same analysis conducted with Corset-estimated transcript abundances identified 59 DE clusters containing 85 transcripts. In both analyses, a larger number of DE transcripts were found down-regulated in mirids feeding on JA-silenced plants (around 70%). Among these down-regulated transcripts we identified seven transcripts possibly involved in the detoxification of N. attenuata defense metabolite, specifically, one glutathione-S-transferase (GST), one UDP-glucosyltransferase (UGT), five cytochrome P450 (P450s), and six serine proteases. Real-time quantitative PCR confirmed the down-regulation for six transcripts (encoding GST, UGT and four P450s) and revealed that their expression was only slightly decreased in mirids feeding on another N. attenuata transgenic line specifically silenced in the accumulation of diterpene glycosides, one of the many classes of JA-mediated defenses in N. attenuata. CONCLUSIONS: The results provide a transcriptional overview of the changes in a specialist hemimetabolous insect associated with feeding on host plants depleted in chemical defenses. Overall, the analysis reveals that T. notatus responses to host plant defenses are narrow and engages P450 detoxification pathways. It further identifies candidate genes which can be tested in future experiments to understand their role in shaping the T. notatus-N. attenuata interaction.

A knowledge-based molecular screen uncovers a broad-spectrum OsSWEET14 resistance allele to bacterial blight from wild rice.

Transcription activator-like (TAL) effectors are type III-delivered transcription factors that enhance the virulence of plant pathogenic Xanthomonas species through the activation of host susceptibility (S) genes. TAL effectors recognize their DNA target(s) via a partially degenerate code, whereby modular repeats in the TAL effector bind to nucleotide sequences in the host promoter. Although this knowledge has greatly facilitated our power to identify new S genes, it can also be easily used to screen plant genomes for variations in TAL effector target sequences and to predict for loss-of-function gene candidates in silico. In a proof-of-principle experiment, we screened a germplasm of 169 rice accessions for polymorphism in the promoter of the major bacterial blight susceptibility S gene OsSWEET14, which encodes a sugar transporter targeted by numerous strains of Xanthomonas oryzae pv. oryzae. We identified a single allele with a deletion of 18 bp overlapping with the binding sites targeted by several TAL effectors known to activate the gene. We show that this allele, which we call xa41(t), confers resistance against half of the tested Xoo strains, representative of various geographic origins and genetic lineages, highlighting the selective pressure on the pathogen to accommodate OsSWEET14 polymorphism, and reciprocally the apparent limited possibilities for the host to create variability at this particular S gene. Analysis of xa41(t) conservation across the Oryza genus enabled us to hypothesize scenarios as to its evolutionary history, prior to and during domestication. Our findings demonstrate that resistance through TAL effector-dependent loss of S-gene expression can be greatly fostered upon knowledge-based molecular screening of a large collection of host plants.

Sporadic and familial glut1ds Italian patients: A wide clinical variability.

PURPOSE: GLUT1 deficiency syndrome is a treatable neurological disorder characterized by developmental delay, movement disorders and epilepsy. It is caused by mutations in the SLC2A1 gene inherited as an autosomal dominant trait with complete penetrance, even if most detected SCL2A1 mutations are de novo. Our aim is to present a wide series of Italian patients to highlight the differences among subjects with de novo mutations and those with familial transmission. METHODS: We present clinical and genetic features in a series of 22 GLUT1DS Italian patients. Our patients were classified in two different groups: familial cases including GLUT1DS patients with genetically confirmed affected relatives and sporadic cases with detection of SLC2A1 de novo mutation. RESULTS: We found remarkable differences in the severity of the clinical picture regarding the type of genetic inheritance (sporadic versus familial): sporadic patients were characterized by an earlier epilepsy-onset and higher degree of intellectual disability. No significant differences were found in terms of type of movement disorder, whilst Paroxysmal Exertion-induced Dyskinesia (PED) is confirmed to be the most characteristic movement disorder type in GLUT1DS. In familial cases the clinical manifestation of the disease was particularly variable and heterogeneous, also including asymptomatic patients or those with minimal-symptoms. CONCLUSION: The finding of a "mild" phenotype in familial GLUT1DS gives rise to several questions: the real incidence of the disease, treatment option with ketogenic diet in adult patients and genetic counseling.

On the evolution of hexose transporters in kinetoplastid Protozoans [corrected].

Glucose, an almost universally used energy and carbon source, is processed through several well-known metabolic pathways, primarily glycolysis. Glucose uptake is considered to be the first step in glycolysis. In kinetoplastids, a protozoan group that includes relevant human pathogens, the importance of glucose uptake in different phases of the life cycles is well established, and hexose transporters have been proposed as targets for therapeutic drugs. However, little is known about the evolutionary history of these hexose transporters. Hexose transporters contain an intracellular N- and C- termini, and 12 transmembrane spans connected by alternate intracellular and extracellular loops. In the present work we tested the hypothesis that the evolutionary rate of the transmembrane span is different from that of the whole sequence and that it is possible to define evolutionary units inside the sequence. The phylogeny of whole molecules was compared to that of their transmembrane spans and the loops connecting the transmembrane spans. We show that the evolutionary units in these proteins primarily consist of clustered rather than individual transmembrane spans. These analyses demonstrate that there are evolutionary constraints on the organization of these proteins; more specifically, the order of the transmembrane spans along the protein is highly conserved. Finally, we defined a signature sequence for the identification of kinetoplastid hexose transporters.

Expansion of hexose transporter genes was associated with the evolution of aerobic fermentation in yeasts.

The genetic basis of organisms' adaptation to different environments is a central issue of molecular evolution. The budding yeast Saccharomyces cerevisiae and its relatives predominantly ferment glucose into ethanol even in the presence of oxygen. This was suggested to be an adaptation to glucose-rich habitats, but the underlying genetic basis of the evolution of aerobic fermentation remains unclear. In S. cerevisiae, the first step of glucose metabolism is transporting glucose across the plasma membrane, which is carried out by hexose transporter proteins. Although several studies have recognized that the rate of glucose uptake can affect how glucose is metabolized, the role of HXT genes in the evolution of aerobic fermentation has not been fully explored. In this study, we identified all members of the HXT gene family in 23 fully sequenced fungal genomes, reconstructed their evolutionary history to pinpoint gene gain and loss events, and evaluated their adaptive significance in the evolution of aerobic fermentation. We found that the HXT genes have been extensively amplified in the two fungal lineages that have independently evolved aerobic fermentation. In contrast, reduction of the number of HXT genes has occurred in aerobic respiratory species. Our study reveals a strong positive correlation between the copy number of HXT genes and the strength of aerobic fermentation, suggesting that HXT gene expansion has facilitated the evolution of aerobic fermentation.

The Vitis vinifera sugar transporter gene family: phylogenetic overview and macroarray expression profiling.

BACKGROUND: In higher plants, sugars are not only nutrients but also important signal molecules. They are distributed through the plant via sugar transporters, which are involved not only in sugar long-distance transport via the loading and the unloading of the conducting complex, but also in sugar allocation into source and sink cells. The availability of the recently released grapevine genome sequence offers the opportunity to identify sucrose and monosaccharide transporter gene families in a woody species and to compare them with those of the herbaceous Arabidopsis thaliana using a phylogenetic analysis. RESULTS: In grapevine, one of the most economically important fruit crop in the world, it appeared that sucrose and monosaccharide transporter genes are present in 4 and 59 loci, respectively and that the monosaccharide transporter family can be divided into 7 subfamilies. Phylogenetic analysis of protein sequences has indicated that orthologs exist between Vitis and Arabidospis. A search for cis-regulatory elements in the promoter sequences of the most characterized transporter gene families (sucrose, hexoses and polyols transporters), has revealed that some of them might probably be regulated by sugars. To profile several genes simultaneously, we created a macroarray bearing cDNA fragments specific to 20 sugar transporter genes. This macroarray analysis has revealed that two hexose (VvHT1, VvHT3), one polyol (VvPMT5) and one sucrose (VvSUC27) transporter genes, are highly expressed in most vegetative organs. The expression of one hexose transporter (VvHT2) and two tonoplastic monosaccharide transporter (VvTMT1, VvTMT2) genes are regulated during berry development. Finally, three putative hexose transporter genes show a preferential organ specificity being highly expressed in seeds (VvHT3, VvHT5), in roots (VvHT2) or in mature leaves (VvHT5). CONCLUSIONS: This study provides an exhaustive survey of sugar transporter genes in Vitis vinifera and revealed that sugar transporter gene families in this woody plant are strongly comparable to those of herbaceous species. Dedicated macroarrays have provided a Vitis sugar transporter genes expression profiling, which will likely contribute to understand their physiological functions in plant and berry development. The present results might also have a significant impact on our knowledge on plant sugar transporters.

Correlated changes between regulatory cis elements and condition-specific expression in paralogous gene families.

Gene duplication is integral to evolution, providing novel opportunities for organisms to diversify in function. One fundamental pathway of functional diversification among initially redundant gene copies, or paralogs, is via alterations in their expression patterns. Although the mechanisms underlying expression divergence are not completely understood, transcription factor binding sites and nucleosome occupancy are known to play a significant role in the process. Previous attempts to detect genomic variations mediating expression divergence in orthologs have had limited success for two primary reasons. First, it is inherently challenging to compare expressions among orthologs due to variable trans-acting effects and second, previous studies have quantified expression divergence in terms of an overall similarity of expression profiles across multiple samples, thereby obscuring condition-specific expression changes. Moreover, the inherently inter-correlated expressions among homologs present statistical challenges, not adequately addressed in many previous studies. Using rigorous statistical tests, here we characterize the relationship between cis element divergence and condition-specific expression divergence among paralogous genes in Saccharomyces cerevisiae. In particular, among all combinations of gene family and TFs analyzed, we found a significant correlation between TF binding and the condition-specific expression patterns in over 20% of the cases. In addition, incorporating nucleosome occupancy reveals several additional correlations. For instance, our results suggest that GAL4 binding plays a major role in the expression divergence of the genes in the sugar transporter family. Our work presents a novel means of investigating the cis regulatory changes potentially mediating expression divergence in paralogous gene families under specific conditions.

Structurally reduced monosaccharide transporters in an evolutionarily conserved red alga.

The unicellular red alga Galdieria sulphuraria is a facultative heterotrophic member of the Cyanidiaceae, a group of evolutionary highly conserved extremophilic red algae. Uptake of various sugars and polyols is accomplished by a large number of distinct plasma membrane transporters. We have cloned three transporters [GsSPT1 (G. sulphuraria sugar and polyol transporter 1), GsSPT2 and GsSPT4], followed their transcriptional regulation and assayed their transport capacities in the heterologous yeast system. SPT1 is a conserved type of sugar/H(+) symporter with 12 predicted transmembrane-spanning domains, whereas SPT2 and SPT4 represent monosaccharide transporters, characterized by only nine hydrophobic domains. Surprisingly, all three proteins are functional plasma membrane transporters, as demonstrated by genetic complementation of a sugar uptake-deficient yeast mutant. Substrate specificities were broad and largely redundant, except for glucose, which was only taken up by SPT1. Comparison of SPT1 and truncated SPT1(Delta1-3) indicated that the N-terminus of the protein is not required for sugar transport or substrate recognition. However, its deletion affected substrate affinity as well as maximal transport velocity and released the pH dependency of sugar uptake. In line with these results, uptake by SPT2 and SPT4 was active but not pH-dependent, making a H(+) symport mechanism unlikely for the truncated proteins. We postulate SPT2 and SPT4 as functional plasma membrane transporters in G. sulphuraria. Most likely, they originated from genes encoding active monosaccharide/H(+) symporters with 12 transmembrane-spanning domains.

Cloning and characterization of a glucose 6-phosphate/phosphate translocator from Oryza sativa.

Plastids of nongreen tissues import carbon as a source of biosynthetic pathways and energy, and glucose 6-phosphate is the preferred hexose phosphate taken up by nongreen plastids. A cDNA clone encoding glucose 6-phosphate/phosphate translocator (GPT) was isolated from a cDNA library of immature seeds of rice and named as OsGPT. The cDNA has one uninterrupted open reading frame encoding a 42 kDa polypeptide possessing transit peptide consisting of 70 amino acid residues. The OsGPT gene maps on chromosome 8 of rice and is linked to the quantitative trait locus for 1000-grain weight. The expression of OsCPT is mainly restricted to heterotrophic tissues. These results suggest that glucose 6-phosphate imported via GPT can be used for starch biosynthesis in rice nongreen plastids.

The nucleotide-sugar transporter family: a phylogenetic approach.

Nucleotide sugar transporters (NST) establish the functional link of membrane transport between the nucleotide sugars synthesized in the cytoplasm and nucleus, and the glycosylation processes that take place in the endoplasmic reticulum (ER) and Golgi apparatus. The aim of the present work was to perform a phylogenetic analysis of 87 bank annotated protein sequences comprising all the NST so far characterized and their homologues retrieved by BLAST searches, as well as the closely related triose-phosphate translocator (TPT) plant family. NST were classified in three comprehensive families by linking them to the available experimental data. This enabled us to point out both the possible ER subcellular targeting of these transporters mediated by the dy-lysine motif and the substrate recognition mechanisms specific to each family as well as an important acceptor site motif, establishing the role of evolution in the functional properties of each NST family.

Investigating the conformational states of the rabbit Na+/glucose cotransporter.

The Na(+) and voltage-dependence of transient rabbit Na(+)/glucose cotransporter (rSGLT1) kinetics was studied with the two-electrode voltage-clamp technique and Xenopus laevis oocytes. Using step changes in membrane potential, in the absence of glucose but with 100 or 10 mM Na(+), transient currents were measured corresponding to binding/debinding of Na(+) and conformational changes of the protein. Previously, only a single time constant has been published for rSGLT1. We, however, observed three decay components; a fast (tau(f), 0.5-1 ms) voltage- and Na(+)-independent decay, and medium (tau(m), 0.5-4 ms) and slow (tau(s), 8-50 ms) voltage- and Na(+)-dependent decays. Transient currents were simulated and fit using a four-state model to obtain kinetic parameters for the system. The four-state model was able to reconstitute an assortment of experimental data.

Fluorodeoxyglucose uptake and glucose transporter expression in experimental inflammatory lesions and malignant tumours: effects of insulin and glucose loading.

The expression of glucose transporters (GLUTs) and its relationship to fluorodeoxyglucose accumulation in malignant tumours have been well investigated, while such a relation has not been studied in inflammatory lesions. The aim of the present study was to investigate the effects of insulin and glucose loading on the expression of GLUTs in inflammatory lesions and compare them with those in malignant tumours in relation to fluorodeoxyglucose accumulation. All tissue specimens used in this study were obtained in our previous study, in which rats were inoculated with allogenic hepatoma cells (KDH-8), Staphylococcus aureus, or turpentine oil into the left calf muscle and divided into three subgroups: insulin loaded, glucose loaded, and control groups. The expression of glucose transporters (GLUT-1 to GLUT-5) was investigated by immunostaining the lesions (n=5-6, for each group). In all control groups, the expression levels of GLUT-1 and GLUT-3 were significantly higher than those of GLUT-2, GLUT-4 and GLUT-5. Insulin loading did not significantly affect the expression levels of GLUT-1 and GLUT-3 in these lesions except for a significant but slight decrease in the GLUT-1 expression level in the inflammatory lesion of non-infectious origin (89% of the control value). Glucose loading significantly decreased the expression level of GLUT-1 in the inflammatory lesion of non-infectious origin (70% of the control value, P<0.01), and that of GLUT-3 in the inflammatory lesion of infectious origin (70% of the control value, P<0.05), while the expression levels of GLUT-1 and GLUT-3 in the tumour were not significantly affected. These results demonstrate the effects of insulin and glucose loading on the expression level of a molecule (GLUT proteins). The decreased GLUT-1 and GLUT-3 expression levels induced by glucose loading may partly explain the impaired FDG uptake observed in our previous study.

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.

Genetic multiplicity of the human UDP-glucuronosyltransferases and regulation in the gastrointestinal tract.

The metabolism of ingested foods and orally administered drugs occurs in the hepato-gastrointestinal tract. This process is facilitated by several supergene families that catalyze oxidative metabolism as well as conjugation of the small molecular weight substances that enter the systemic circulation through resorption in the gastrointestinal tract. The catalytic action carried out by one of several conjugation reactions leads to the eventual elimination of the resultant metabolites from the cell. As early as 1959 (R. T. Williams, Detoxification Mechanisms) it was suggested that the detoxification of most agents is efficiently performed by the phase II conjugation reactions, because the addition of bulky, water-soluble groups to the target substrates facilitates the partitioning of these metabolites from the lipid into the aqueous compartments of the cell. The combined efforts of the phase II reactions provides remarkable redundancy in a biological system that seems to be designed to assure that many endogenously generated catabolic products as well as exogenous agents introduced through the surface tissues of the digestive tracts are efficiently removed through excretion to the bile or urine. In this review, we focus on recent findings that highlight the genetic multiplicity and regulatory patterns of the phase II superfamily UDP-glucuronosyltransferases (UGTs). Although much is known regarding the number of UGTs that make up the UGT1 and UGT2 gene families, as demonstrated after the characterization of expressed cDNAs, examples are also presented in which information obtained from the human genome project will aid in the final characterization of the genetic multiplicity. In addition, tools have now been developed and examples presented to identify the expression patterns of the UGTs in human tissues, paying particular attention to expression patterns of these genes in the hepato-gastrointestinal tract.

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

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 GLUT family can be divided into three 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 myo-inositol transporter HMIT1). Functional characteristics have been reported for some of the novel GLUTs. Like GLUT1-4, they exhibit a tissue/cell-specific expression (GLUT6, leukocytes, brain; GLUT8, testis, blastocysts, brain, muscle, adipocytes; GLUT9, liver, kidney; GLUT10, liver, pancreas; GLUT11, heart, skeletal muscle). GLUT6 and GLUT8 appear to be regulated by sub-cellular redistribution, because they are targeted to intra-cellular compartments by dileucine motifs in a dynamin dependent manner. Sugar transport has been reported for GLUT6, 8, and 11; HMIT1 has been shown to be a H+/myo-inositol co-transporter. Thus, the members of the extended GLUT family exhibit a surprisingly diverse substrate specificity, and the definition of sequence elements determining this substrate specificity will require a full functional characterization of all members.