Structure and mechanism of the mammalian fructose transporter GLUT5.

The altered activity of the fructose transporter GLUT5, an isoform of the facilitated-diffusion glucose transporter family, has been linked to disorders such as type 2 diabetes and obesity. GLUT5 is also overexpressed in certain tumour cells, and inhibitors are potential drugs for these conditions. Here we describe the crystal structures of GLUT5 from Rattus norvegicus and Bos taurus in open outward- and open inward-facing conformations, respectively. GLUT5 has a major facilitator superfamily fold like other homologous monosaccharide transporters. On the basis of a comparison of the inward-facing structures of GLUT5 and human GLUT1, a ubiquitous glucose transporter, we show that a single point mutation is enough to switch the substrate-binding preference of GLUT5 from fructose to glucose. A comparison of the substrate-free structures of GLUT5 with occluded substrate-bound structures of Escherichia coli XylE suggests that, in addition to global rocker-switch-like re-orientation of the bundles, local asymmetric rearrangements of carboxy-terminal transmembrane bundle helices TM7 and TM10 underlie a 'gated-pore' transport mechanism in such monosaccharide transporters.

A case study of measles vaccination for university students during the measles outbreak in Tokyo, Japan, 2007.

In April 2007, seven students belonging to the same class at Teikyo University developed measles. To prevent the spread of infection, 27 of 106 students in the same class who had low anti-measles antibody titers as measured by hemagglutination inhibition (HI) assay were vaccinated. After the outbreak had subsided, the HI values were investigated in 103 students, and they answered questionnaires about their health condition during the period of the outbreak and their previous clinical histories of measles, including vaccination records. There was no new case of measles after introduction of the vaccination program. However, the HI titers of 42% of the students who were not vaccinated in this program were significantly elevated. Fever and catarrhal signs occurred in 7 of these students with pre-exposure titers of 8 or less. The post-exposure HI titers of 71% of students who were unaffected by measles and had high HI titers (>8) before the epidemic did not increase. These results suggested that people with low HI titers may become potential carriers of measles and that measurement of pre-exposure HI anti-measles antibody titer is a useful method for selection of candidates to undergo vaccination.

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

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

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

We previously identified Asn(331) in transmembrane segment 7 (TM7) as a key residue determining substrate affinity in Hxt2, a moderately high-affinity facilitative glucose transporter of Saccharomyces cerevisiae. To gain further insight into the structural basis of substrate recognition by yeast glucose transporters, we have now studied Hxt7, whose affinity for glucose is the highest among the major hexose transporters. The functional role of Asp(340) in Hxt7, the residue corresponding to Asn(331) of Hxt2, was examined by replacing it with each of the other 19 amino acids. Such replacement of Asp(340) generated transporters with various affinities for glucose, with the affinity of the Cys(340) mutant surpassing that of the wild-type Hxt7. To examine the structural role of Asp(340) in the substrate translocation pathway, we performed cysteine-scanning mutagenesis of the 21 residues in TM7 of a functional Cys-less Hxt7 mutant in conjunction with exposure to the hydrophilic sulfhydryl reagent p-chloromercuribenzenesulfonate (pCMBS). The transport activity of the D340C mutant of Cys-less Hxt7, in which Asp(340) is replaced with Cys, was completely inhibited by pCMBS, indicating that Asp(340) is located in a water-accessible position. This D340C mutant showed a sensitivity to pCMBS that was approximately 70 times that of the wild-type Hxt7, and it was protected from pCMBS inhibition by the substrates d-glucose and 2-deoxy-d-glucose but not by l-glucose. These results indicate that Asp(340) is situated at or close to a substrate recognition site and is a key residue determining high-affinity glucose transport by Hxt7, supporting the notion that yeast glucose transporters share a common mechanism for substrate recognition.

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

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

Nephrocalcinosis in glucose-galactose malabsorption: nephrocalcinosis and proximal tubular dysfunction in a young infant with a novel mutation of SGLT1.

We report an association of proximal renal tubular dysfunction in a 50-day-old girl with glucose-galactose malabsorption who was found to have nephrocalcinosis, but no sign of nephrolithiasis. A novel homozygous nonsense mutation at 267Arg-->stop (CGA-->TGA) in the Na(+)-dependent glucose transporter (SGLT1) was found in loop 5 connecting transmembrane segments 6 and 7, indicating the complete loss of glucose transport activity. This case indicates that hypercalcaemia, nephrocalcinosis and proximal tubular dysfunction may be seen in association with glucose-galactose malabsorption and that most of these abnormalities improve with a glucose-galactose-free diet.

Microsatellite polymorphism in Japanese mongrel dogs.

The genetic variability of 182 unrelated mongrel dogs living in various areas of Japan (from Hokkaido to Okinawa) was studied by collecting their blood. Ten microsatellite loci were chosen from different autosomal chromosomes. After combining a few rare adjoining alleles to allelic classes, it was confirmed that the Hardy-Weinberg equilibrium was attained in each locus. The polymorphic information contents (PICs) of the loci, Ren37A11, Ren48E01, AHTk253, ZuBeCa30, Ren277K09, Ren42N13, AHT130, PEZ03, PEZ12, and AHT121, were 0.58, 0.63, 0.67, 0.67, 0.68, 0.71, 0.79, 0.80, 0.80, and 0.80, and the power of discriminations (PDs) were 0.80, 0.85, 0.87, 0.88, 0.88, 0.89, 0.94, 0.94, 0.94, and 0.94, respectively. The combined mean exclusion chance (MEC) was 0.9995, indicating that these microsatellite loci are useful for kinship testing of Japanese dogs.

Tricks of the trade used to accelerate high-resolution structure determination of membrane proteins.

The rate at which X-ray structures of membrane proteins are solved is on a par with that of soluble proteins in the late 1970s. There are still many obstacles facing the membrane protein structural community. Recently, there have been several technical achievements in the field that have started to dramatically accelerate structural studies. Here, we summarize these so-called 'tricks-of-the-trade' and include case studies of several mammalian transporters.

Identification by comprehensive chimeric analysis of a key residue responsible for high affinity glucose transport by yeast HXT2.

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

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

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

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

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

Comprehensive chimeric analysis of amino acid residues critical for high affinity glucose transport by Hxt2 of Saccharomyces cerevisiae.

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