The Physiopathological Role of the Exchangers Belonging to the SLC37 Family (original) (raw)
Related papers
SLC37A1 and SLC37A2 Are Phosphate-Linked, Glucose-6-Phosphate Antiporters
PLoS ONE, 2011
Blood glucose homeostasis between meals depends upon production of glucose within the endoplasmic reticulum (ER) of the liver and kidney by hydrolysis of glucose-6-phosphate (G6P) into glucose and phosphate (P i). This reaction depends on coupling the G6P transporter (G6PT) with glucose-6-phosphatase-a (G6Pase-a). Only one G6PT, also known as SLC37A4, has been characterized, and it acts as a P i-linked G6P antiporter. The other three SLC37 family members, predicted to be sugarphosphate:P i exchangers, have not been characterized functionally. Using reconstituted proteoliposomes, we examine the antiporter activity of the other SLC37 members along with their ability to couple with G6Pase-a. G6PT-and mockproteoliposomes are used as positive and negative controls, respectively. We show that SLC37A1 and SLC37A2 are ERassociated, P i-linked antiporters, that can transport G6P. Unlike G6PT, neither is sensitive to chlorogenic acid, a competitive inhibitor of physiological ER G6P transport, and neither couples to G6Pase-a. We conclude that three of the four SLC37 family members are functional sugar-phosphate antiporters. However, only G6PT/SLC37A4 matches the characteristics of the physiological ER G6P transporter, suggesting the other SLC37 proteins have roles independent of blood glucose homeostasis.
The FASEB Journal, 2008
Glycogen storage disease type Ib (GSD-Ib) is caused by deficiencies in the glucose-6-phosphate (G6P) transporter (G6PT) that have been well characterized. Interestingly, deleterious mutations in the G6PT gene were identified in clinical cases of GSD type Ic (GSD-Ic) proposed to be deficient in an inorganic phosphate (P i ) transporter. We hypothesized that G6PT is both the G6P and P i transporter. Using reconstituted proteoliposomes we show that both G6P and P i are efficiently taken up into P i -loaded G6PTproteoliposomes. The G6P uptake activity decreases as the internal:external P i ratio decreases and the P i uptake activity decreases in the presence of external G6P. Moreover, G6P or P i uptake activity is not detectable in P i -loaded proteoliposomes containing the p.R28H G6PT null mutant. The G6PT-proteoliposomemediated G6P or P i uptake is inhibited by cholorgenic acid and vanadate, both specific G6PT inhibitors. Glucose-6-phosphatase-␣ (G6Pase-␣), which facilitates microsomal G6P uptake by G6PT, fails to stimulate G6P uptake in P i -loaded G6PT-proteoliposomes, suggesting that the G6Pase-␣-mediated stimulation is caused by decreasing G6P and increasing P i concentrations in microsomes. Taken together, our results suggest that G6PT has a dual role as a G6P and a P i transporter and that GSD-Ib and GSD-Ic are deficient in the same G6PT gene.-Chen, S.-Y., Pan, C.-.J, Nandigama, K., Mansfield, B., Ambudkar, S., Chou, J. The glucose-6-phosphate transporter is a phosphate-linked antiporter deficient in glycogen storage disease type Ib and Ic. FASEB J. 22, 2206 -2213 (2008)
The SLC37 family of sugar-phosphate/phosphate exchangers
Current topics in membranes, 2014
The SLC37 family members are endoplasmic reticulum (ER)-associated sugar-phosphate/phosphate (P(i)) exchangers. Three of the four members, SLC37A1, SLC37A2, and SLC37A4, function as Pi-linked glucose-6-phosphate (G6P) antiporters catalyzing G6P:P(i) and P(i):P(i) exchanges. The activity of SLC37A3 is unknown. SLC37A4, better known as the G6P transporter (G6PT), has been extensively characterized, functionally and structurally, and is the best characterized family member. G6PT contains 10 transmembrane helices with both N and C termini facing the cytoplasm. The primary in vivo function of the G6PT protein is to translocate G6P from the cytoplasm into the ER lumen where it couples with either the liver/kidney/intestine-restricted glucose-6-phosphatase-α (G6Pase-α or G6PC) or the ubiquitously expressed G6Pase-β (or G6PC3) to hydrolyze G6P to glucose and P(i). The G6PT/G6Pase-α complex maintains interprandial glucose homeostasis, and the G6PT/G6Pase-β complex maintains neutrophil energy...
Human Molecular Genetics, 2002
Glycogen storage disease type Ib (GSD-Ib) is caused by a deficiency in the glucose-6-phosphate transporter (G6PT), a 10 transmembrane domain endoplasmic reticulum protein. To date, 69 G6PT mutations, including 28 missenses and 2 codon deletions, have been identified in GSD-Ib patients. We previously characterized 15 of the missense and one codon deletion mutations using a pSVL-based expression assay. A lack of sensitivity in this assay limited the discrimination between mutations that lead to loss of function and mutations that leave a low residual activity. We now report an improved G6PT assay, based on an adenoviral vector-mediated expression system and its use in the functional characterization of all 30 codon mutations found in GSD-Ib patients. Twenty of the naturally occurring mutations completely abolish microsomal G6P uptake activity while the other 10 mutations, including 5 previously characterized ones, partially inactivate the transporter. This information should greatly facilitate genotype-phenotype correlation. We also report a structurefunction analysis of G6PT. In addition to the 3 destabilizing mutations reported previously, we now show that the G50R, C176R, V235del, G339C and G339D mutations also compromise the G6PT stability. Mutation analysis of the amino-terminal domain of G6PT shows that it is required for optimal G6P uptake activity. Finally, we show that degradation of both wild-type and mutant G6PT is inhibited by a potent proteasome inhibitor, lactacystin, demonstrating that G6PT is a substrate for proteasome-mediated degradation.
Human variant glucose-6-phosphate transporter is active in microsomal transport
Human Genetics, 2000
Glycogen storage disease type 1b (GSD-1b) is caused by deficiencies in the glucose-6-phosphate transporter (G6PT), which works together with glucose-6phosphatase to maintain glucose homeostasis. In humans, there are two alternatively spliced transcripts, G6PT and variant G6PT (vG6PT), differing by the inclusion of a 66bp exon-7 sequence in vG6PT. We have previously shown that the G6PT protein functions as a microsomal glucose-6-phosphate (G6P) transporter, which is anchored to the endoplasmic reticulum by ten transmembrane helices. Here, we demonstrate that vG6PT is also active in microsomal G6P transport. The additional 22 amino acids in vG6PT is predicted to constitute a part of the luminal loop 4. Our data indicate that this loop plays no vital role in microsomal G6P transport. Further, we show that G6PT mRNA is expressed in all organs and tissues examined, but that the vG6PT transcript is expressed exclusively in the brain, heart, and skeletal muscle. These results raise the possibility that mutations in exon-7 of the G6PT gene, which would not perturb glucose homeostasis, might have other deleterious effects.
The human sugar-phosphate/phosphate exchanger family SLC37
Pfl�gers Archiv European Journal of Physiology, 2004
The SLC37 family of four predicted proteins is an almost unexplored group of transmembrane sugar transporters. Of the four proteins/genes assigned to date to this family, only one is well known, the SLC37A4 gene (also known as the glucose-6-phosphate transporter 1, G6PT1) mutated in the glycogen storage disease non-1A type. Data on SLC3A1 gene expression are available for humans, while data on SLC37A2 are available for mice. The last SLC37 family member, SLC37A3, is only a putative gene/protein identified by in silico analyses. The four genes are not clustered in a single chromosome as regions and the identity of their predicted polypeptides is between 60 and 20%. Here we propose a new nomenclature for the SLC37 proteins (SPX: sugar-phosphate exchangers) numbered according to the gene numbering.
Human Genetics, 1999
Glycogen storage disease type 1 (GSD-1) is a group of autosomal recessive disorders caused by deficiencies in glucose-6-phosphatase (G6Pase) and the associated substrate/product transporters. Molecular genetic studies have demonstrated that GSD-1a and GSD-1b are caused by mutations in the G6Pase enzyme and a glucose-6-phosphate transporter (G6PT), respectively. While kinetic studies of G6Pase catalysis predict that the index GSD-1c patient is deficient in a pyrophosphate/phosphate transporter, the existence of a separate locus for GSD-1c remains unclear. We have previously shown that the G6Pase gene of the index GSD-1c patient is intact; we now show that the G6PT gene of this patient is normal, strongly suggesting the existence of a distinct GSD-1c locus.
FEBS Letters, 1999
Glycogen storage diseases type 1 (GSD 1) are a group of autosomal recessive disorders characterized by impairment of terminal steps of glycogenolysis and gluconeogenesis. Mutations of the glucose-6-phosphatase gene are responsible for the most frequent form of GSD 1, the subtype 1a, while mutations of the glucose-6-phosphate transporter gene (G6PT) have recently been shown to cause the non 1a forms of GSD, namely the 1b and 1c subtypes. Here, we report on the analysis by single-stranded conformation polymorphism (SSCP) and/or DNA sequencing of the exons of the G6PT in 14 patients diagnosed either as affected by the GSD 1b or 1c subtypes. Mutations in the G6PT gene were found in all patients. Four of the detected mutations were novel mutations, while the others were previously described. Our results confirm that the GSD 1b and 1c forms are due to mutations in the same gene, i.e. the G6PT gene. We also show that the same kind of mutation can be associated or not with evident clinical complications such as neutrophil impairment. Since no correlation between the type and position of the mutation and the severity of the disease was found, other unknown factors may cause the expression of symptoms, such as neutropenia, which dramatically influence the severity of the disease.
The American Journal of Human Genetics, 1998
Glycogen-storage diseases type I (GSD type I) are due to a deficiency in glucose-6-phosphatase, an enzymatic system present in the endoplasmic reticulum that plays a crucial role in blood glucose homeostasis. Unlike GSD type Ia, types Ib and Ic are not due to mutations in the phosphohydrolase gene and are clinically characterized by the presence of associated neutropenia and neutrophil dysfunction. Biochemical evidence indicates the presence of a defect in glucose-6-phosphate (GSD type Ib) or inorganic phosphate (Pi) (GSD type Ic) transport in the microsomes. We have recently cloned a cDNA encoding a putative glucose-6-phosphate translocase. We have now localized the corresponding gene on chromosome 11q23, the region where GSD types Ib and Ic have been mapped. Using SSCP analysis and sequencing, we have screened this gene, for mutations in genomic DNA, from patients from 22 different families who have GSD types Ib and Ic. Of 20 mutations found, 11 result in truncated proteins that are probably nonfunctional. Most other mutations result in substitutions of conserved or semiconserved residues. The two most common mutations (Gly339Cys and 1211-1212 delCT) together constitute ∼40% of the disease alleles. The fact that the same mutations are found in GSD types Ib and Ic could indicate either that Pi and glucose-6-phosphate are transported in microsomes by the same transporter or that the biochemical assays used to differentiate Pi and glucose-6phosphate transport defects are not reliable.