Hexoses like glucose, galactose and fructose serve as basic fuel molecules for eukaryotic cells. Indeed, glucose is the main energy source for mammalian cells. These sugars are unable to diffuse across cellular membranes, and require transporter proteins for entry into and exit out of cells. Four gene families encode hexose transporter proteins (He et al, 2009). SLC2 family contains 14 genes and encode facilitative glucose transporters (GLUTs) (Uldry M and Thorens B, 2004). SLC5 family contains 12 genes and encode Na+/glucose symporters (Wright EM and Turk E, 2004). SLC37 family contains 4 members and encode sugar-phosphate/phosphate exchangers (Bartoloni L and Antonarakis SE, 2004). SLC45 family has 4 members and encode putative sugar/H+ symporters.
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This CandidateSet contains sequences identified by William Pearson's analysis of Reactome catalyst entities. Catalyst entity sequences were used to identify analagous sequences that shared overall homology and active site homology. Sequences in this Candidate set were identified in an April 24, 2012 analysis.
This CandidateSet contains sequences identified by William Pearson's analysis of Reactome catalyst entities. Catalyst entity sequences were used to identify analagous sequences that shared overall homology and active site homology. Sequences in this Candidate set were identified in an April 24, 2012 analysis.
This CandidateSet contains sequences identified by William Pearson's analysis of Reactome catalyst entities. Catalyst entity sequences were used to identify analagous sequences that shared overall homology and active site homology. Sequences in this Candidate set were identified in an April 24, 2012 analysis.
This CandidateSet contains sequences identified by William Pearson's analysis of Reactome catalyst entities. Catalyst entity sequences were used to identify analagous sequences that shared overall homology and active site homology. Sequences in this Candidate set were identified in an April 24, 2012 analysis.
This CandidateSet contains sequences identified by William Pearson's analysis of Reactome catalyst entities. Catalyst entity sequences were used to identify analagous sequences that shared overall homology and active site homology. Sequences in this Candidate set were identified in an April 24, 2012 analysis.
This CandidateSet contains sequences identified by William Pearson's analysis of Reactome catalyst entities. Catalyst entity sequences were used to identify analagous sequences that shared overall homology and active site homology. Sequences in this Candidate set were identified in an April 24, 2012 analysis.
This CandidateSet contains sequences identified by William Pearson's analysis of Reactome catalyst entities. Catalyst entity sequences were used to identify analagous sequences that shared overall homology and active site homology. Sequences in this Candidate set were identified in an April 24, 2012 analysis.
The transport of extracellular glucose and galactose into the cytosol, coupled to the uptake of two sodium ions for each hexose transported is mediated by SGLT1. In the small intestine, SGLT1 is localized on the lumenal surfaces of enterocytes and thus mediates the uptake of dietary glucose and galactose, which can be released into the circulation in a separate transport step mediated by basolaterally localized GLUT2 (Wright et al. 2004). The specificity of SGLT1 has been worked out by studying sugar transport in plasma membrane vesicles containing recombinant human SGLT1 protein (Quick et al. 2003). Consistent with these in vitro results, children lacking functional SGLT1 protein fail to absord dietary glucose and galactose (Martin et al. 1996).
The reversible transport of extracellular fructose into the cytosol is mediated by GLUT5. In the small intestine, GLUT5 is localized on the lumenal surfaces of enterocytes (Davidson et al. 1992) and thus mediates the uptake of dietary fructose, which can be released into the circulation in a separate transport step mediated by basolaterally localized GLUT2. The specificity of GLUT5 has been worked out by studying sugar transport in Xenopus oocytes expressing recombinant human GLUT5 protein (Burant et al. 1992).
The reversible facilitated diffusion of fructose, galactose, and glucose from the cytosol to the extracellular space is mediated by the GLUT2 transporter in the plasma membrane. In the epithelial cells of the small intestine, the basolateral localization of GLUT2 (Thorens et al. 1990) enables hexose sugars derived from the diet and taken up by the action of the SGLT1 and GLUT5 transporters to be released into the circulation. The specificity of the GLUT2 transporter has been established directly through biochemical assays of purified recombinant proteins (Colville et al. 1993; Wu et al. 1998) and indirectly through studies of patients deficient in GLUT2 transporter protein (Santer et al. 1997).
The protein SLC6A18 was first identified as an amino acid transporter based on sequence similarity to other members of the SLC6 protein family (Hoglund et al. 2005). It is annotated here as mediating glycine uptake based on the phenotype of mice homozygous for a null mutation in the homologous gene (Quan et al. 2004).
The plasma membrane transport protein SLC6A6 mediates the uptake of taurine and beta-alanine. Together with each amino acid molecule, 2 sodium ions and 1 chloride ion are taken up. SLC6A6 is widely expressed in the body (Ramamoorthy et al. 1994).
The plasma membrane transport protein SLC6A12 (BGT-1) mediates the uptake of GABA (gamma-aminobutyrate) and betaine and, less efficiently, of diminobutyrate (DABA) and beta-alanine. Together with each amino acid molecule, 3 sodium ions and 2 chloride ions are taken up. In the body, SLC6A12 is expressed in the proximal tubules of the kidney and cells of the central nervous system (Rasola et al. 1995; Matskevitch et al. 1999).
SLC6A20, associated with the plasma membrane, mediates the uptake of proline plus a sodium ion. The human protein is expressed in the intestine and kidney (Takanaga et al. 2005).
SLC6A15, associated with the plasma membrane, mediates the uptake of a broad range of amino acids plus a sodium ion, transporting branched-chain amiono acids and methionine most efficiently. The human protein is expressed in the brain (Takanaga et al. 2005).
SLC6A19 mediates the uptake of neutral amino acids across the plasma membrane. Uptake of an amino acid molecule is accompanied by uptake of a sodium ion. The protein is abundant in cells in the small intestine and kidney. Its deficiency is associated with Hartnup disorder, the failure to take up neutral amino acids efficiently from the gut lumen and to reabsorb them in the proximal kidney tubule (Kleta et al. 2004, Seow et al. 2004).
SLC6A14, associated with the plasma membrane, mediates the uptake of multiple basic and nonpolar amino acids as well as beta-alanine. Uptake of one amino acid molecule is accompanied by uptake of two sodium ions and a chloride ion. As assessed by Northern blotting, SLC6A14 is expressed at high levels in lung but only at low levels, if at all, in intestine or kidney (Sloan & Mager 1999, Anderson et al. 2008).
The human gene SLC6A3 encodes the sodium-dependent dopamine transporter, DAT which mediates the re-uptake of dopamine from the synaptic cleft (Vandenbergh DJ et al, 2000). Dopamine can then be degraded by either COMT or monoamine oxidase.
The human gene SLC6A4 encodes the sodium-dependent serotonin transporter 5HTT which mediates the re-uptake of serotonin from the synaptic cleft thus terminating the action of serotonin (Canli T and Lesch KP, 2007). The serotonin taken up in the cytosol from the synaptic cleft may be recycled into synaptic vesicles or metabolized.
SLC2A7 encodes GLUT7, a class II facilitative glucose transporter which was cloned from a human intestinal cDNA library (Li Q et al, 2004). It has a high affinity for glucose and fructose uptake. GLUT7 is found predominantly in the small intestine, colon, testis and prostate.
SLC2A11 encodes GLUT11 (Doege H et al, 2001), another member of the class II facilitative glucose transporters. It has the highest similarity with GLUT5 and in humans, three isoforms are expressed (GLUT11A-C) (Sasaki T et al, 2001). Human GLUT11 has been shown to transport glucose and fructose but not galactose when expressed in Xenopus oocytes ( Scheepers A et al, 2005).
The class I facilitative glucose transporters contain GLUT1-4. As well as glucose, these proteins can transport other hexoses such as fructose, galactose and glucosamine.
GLUT2 is expressed by SLC2A2 and is a low affinity glucose transporter (Fukumoto H et al, 1988). It is expressed mainly in the kidney, liver and pancreatic beta-cells. In beta-cells, it functions as a glucose-sensor for insulin secretion and in the liver, it allows for bi-directional glucose transport. In this reaction, it is shown to mediate the influx of glucose. In the next reaction, it is shown to be mediating efflux of glucose. Defects in SLC2A2 are the cause of Fanconi-Bickel syndrome (FBS). It is characterized by hepatorenal glycogen accumulation, proximal renal tubular dysfunction, and impaired utilization of glucose and galactose (Burwinkel B et al, 1999).
SLC2A3 encodes GLUT3 which is mainly expressed in the brain but also in a wide range of tissues. If has a high affinity for glucose and can also transport other sugars (Kayano T et al, 1988). GLUT4, encoded by SLC2A4, is an insulin-responsive glucose transporter found in heart, skeletal muscle, brain and adipose tissue. Due to its sensitivity to insulin, it may play a role in diabetes. In a non-insulin condition, GLUT4 is localized in intracellular GLUT4-containing vesicles. On insulin stimulation, GLUT4 translocates to the plasma membrane where it can increase glucose transport 10-20-fold (Fukumoto H et al, 1989). Defects in SLC2A4 may be a cause of non-insulin-dependent diabetes mellitus (NIDDM) (Kusari J et al, 1991; Choi WH et al, 1991). GLUT1 is curated in its own reaction in this section.
The human SLC2A9 gene encodes two isoforms of class II facilitative glucose transporter 9; GLUT9 (Phay et al. 2000) and GLUT9DeltaN (Augustin et al. 2004). GLUT9 is expressed mainly in kidney (proximal tubules of epithelial cells) and liver while GLUT9DeltaN is expressed mainly in kidney and placenta. SLC2A9 mediates the transport of urate (uric acid), the end product of purine metabolism in humans and great apes. In addition it mediates the uptake of fructose (Fru) and glucose (Glc) at a low rate (Vitart et al. 2008). Mutations in SLC2A9 influence serum urate concentrations with excess serum accumulation of urate leading to the development of gout (Vitart et al. 2008).
Class III facilitative transporters consist of five members; GLUT6, 8, 10, 12 and HMIT (a H+/myo-inositol transporter). They possess a characteristic glycosylation site on loop 9 (found in loop 1 of classes I and II transporters).
Four class III facilitative transporters can transport glucose. SLC2A6 encodes GLUT6, expressed mainly in brain, spleen and leucocytes (Doege H et al, 2000a). In literature, this protein is incorrectly described as GLUT9. SLC2A8 encodes GLUT8 and is expressed in brain, testis and adipose tissue (Doege H et al, 2000b). SLC2A10 (located in the Type 2 diabetes-linked region of human chromosome 20q12-13.1) encodes GLUT10, a transporter with high affinity for glucose (McVie-Wylie AJ et al, 2001) . GLUT10 is highly expressed in liver and pancreas but is present in most tissues in lower levels. Defects in SLC2A10 are the cause of arterial tortuosity syndrome (ATS), an autosomal recessive disorder characterized by tortuosity and elongation of major arteries, often resulting in death at a young age (Coucke PJ et al, 2006). SLC2A12 encodes GLUT12, which is highly expressed in skeletal muscle, heart and prostate, with lower levels in brain, placenta and kidney. It was originally cloned from the human breast cancer cell line MCF-7 (Rogers S et al, 2002).
SLC2A13 encodes a H+/myo-inositol co-transporter, HMIT (Uldry M et al, 2001) which is abundantly expressed in the brain. A proton is co-transported with myo-inositol uptake into cells. No glucose transport function has been detected to date.
The human gene SLC5A9 encodes a low affinity transporter for glucose and mannose (SGLT4). Of the tissues tested, SGLT4 appears to be highly expressed in the kidney and intestine, with lower levels detected in the liver. Human SGLT4 expressed in african green monkey cells exhibited glucose and mannose co-transport with Na+ ions (Tazawa S et al, 2005).
The human SLC5A11 gene encodes a high affinity myo-inositol transporter (SMIT2, SGLT6) (Ostlund RE et al, 1996; Roll P et al, 2002). It can transport myo-inositol and D-chiro-inositol, together with two Na+ ions. It was thought SGLT6 could transport glucose but there is no evidence for this so far (Lin X et al, 2009).
The human SLC5A7 gene encodes a sodium- and chloride-dependent, high affinity choline transporter, CHT (Apparsundaram et al. 2000). CHT transports choline (Cho) from the extracellular space into neuronal cells and is dependent on Na+ and Cl- ions for transport (Okuda & Haga 2000). Choline uptake is the rate-limiting step in acetylcholine synthesis.
The human gene SLC5A2 encodes a sodium-dependent glucose transporter, SGLT2 (Wells et al. 1992). SLC5A2 is expressed in many tissues but primarily in the kidney, specifically the renal proximal tubules (S1 and S2 segments). It is a low affinity, high capacity transporter of glucose across the apical membrane, with co-transport of Na+ ions in a 1:1 ratio. Unlike SGLT1, it doesn't transport galactose. SLC5A2 is the main transporter of glucose in the kidney, responsible for approximately 98% of glucose reabsorption (remainder by SGLT1). Defects in SLC5A2 are the cause of renal glucosuria (GLYS1), an autosomal recessive renal tubular disorder (Calado et al. 2004). A separate sodium dependent glucose transporter NAGLT1, was identified in the multifacilitator superfamily (MFS) and could be a transporter of glucose in kidney proximal tubules. Its rat orthologue, Naglt1, has been shown to mediate tubular reabsorption of glucose (Horiba et al. 2003). By similarity, SLC5A1, 4 and 9 are predicted proteins that transport glucose in a Na+-dependent manner.
The human SLC5A3 gene encodes a Na+/myo-inositol transporter, SMIT (Berry GT et al, 1995). SMIT functions in cellular osmoregulation and is expressed in many human tissues including skeletal muscle, brain, kidney, and placenta. It transports myo-inositol together with two Na+ ions. SMIT is also thought to act as a glucose sensor that generates an intracellular glucose signal.
The human SLC10A6 gene encodes a sodium-dependant organic anion transporter, SOAT. Highest expressions of the gene are in testis, placenta and pancreas. Unlike the other SLC10A gene products, SOAT shows no affinity for binding bile acids. However, SOAT is able to transport sulpho-conjugated bile acids such as taurolithocholate 3-sulphate (Geyer J et al, 2007). It can also transport the structurally similar sulphated steroids (not shown here), thus SOAT may play a role in delivery of these prohormones to testis, pancreas and placenta.
The human gene SLC13A4 encodes a sodium/sulphate co-transporter NaS2 (Girard JP et al, 1999). NaS2 is highly expressed in placenta and testis, mainly in high endothelial venules. Kinetic experiments suggest two sodium ions co-transported for every sulphate ion (Markovich D et al, 2005).
Mammalian sodium/dicarboxylate cotransporters (transport succinate and other Krebs cycle intermediates) fall into two categories based on their substrate affinity. The human gene SLC13A3 encodes a high-affinity sodium/dicarboxlate co-transporter, NaDC3. NaDC3 is expressed in the basolateral membrane of renal proximal tubular epithelial cells, sinusoidal membrane of hepatocytes, and brain synaptosomes. Kinetic studies in human retinal pigment epithelial (HRPE) cells suggest three sodium ions co-transported with every divalent succinate (Wang H et al, 2000).
The human SLC13A5 gene encodes a sodium-coupled citrate transporter, NACT. This gene is expressed mainly in the liver, with lower levels in brain and testis. NACT has a preference for trivalent citrate (Inoue K et al, 2002).
The human gene SLC13A1 encodes a sodium/sulphate co-transporter NaS1 (Lee A et al, 2000). NaS1 is almost exclusively expressed in the kidney and mediates sulphate reabsorption there.
Mammalian sodium/dicarboxylate cotransporters (transport succinate and other Krebs cycle intermediates) fall into two categories based on their substrate affinity. The human gene SLC13A2 encodes a low-affinity sodium/dicarboxlate co-transporter, NaDC1. NaDC1 is highly expressed in the brush-border membranes of kidney and intestinal cells and reabsorbs Krebs cycle intermediates, such as succinate and citrate, from the glomerular filtrate (Pajor AM, 1996).
Four members of the SLC16A gene family encode classical monocarboxylate transporters, MCT1-4. They all function as proton-dependent transporters of monocarboxylic acids such as lactate and pyruvate and ketone bodies such as acetacetate and beta-hydroxybutyrate. These processes are crucial in the regulation of energy metabolism and acid-base homeostasis.
SLC16A1 encodes MCT1, a ubiquitiously expressed protein (Garcia et al. 1994). Defects in SLC16A1 are the cause of symptomatic deficiency in lactate transport (SDLT), resulting in an acidic intracellular environment and muscle degeneration (Merezhinskaya et al. 2000). Activating promotor mutations in SLC16A1 are associated with exercise-induced hyperinsulinism (EIHI), a dominantly inherited hypoglycemic disorder characterized by inappropriate insulin secretion during anaerobic exercise or on pyruvate load (Otonkoski et al. 2000). SLC16A7 encodes MCT2, a high affinity pyruvate transporter highly expressed in testis (Lin et al. 1998). SLC16A8 encodes MCT3 (Yoon et al. 1999).
Human RPE (retinal pigment epithelial) cells express two proton-coupled monocarboxylate transporters: MCT1 in the apical membrane and MCT3 in the basolateral membrane. This suggests that the coordinated activities of these two transporters could facilitate the transepithelial transport of lactate from the retina to the choroid. (Philip et al. 2003).
The human gene family SLC47 encodes 2 multidrug and toxin extrusion (MATE) proteins. Mammalian MATE-type transporters are responsible for the final step in the excretion of metabolic waste and xenobiotic organic cations in the kidney and liver through electroneutral exchange of H(+). MATE1 is primarily expressed in the kidney and liver, where it is localized to the luminal membranes of the urinary tubules and bile canaliculi. When expressed in HEK293 cells, MATE1 mediates H(+)-coupled electroneutral exchange of various drugs (Otsuka M et al, 2005). MATE2 is a human kidney-specific H+/organic cation antiporter that is responsible for the tubular secretion of cationic drugs across the brush border membranes (Masuda S et al, 2006). Substrates for both MATEs include tetraethylammonium, 1-methyl-4-phenylpyridinium, cimetidine, metformin, creatinine, guanidine and procainamide (Tanihara Y et al, 2007).
Natural resistance-associated macrophage proteins (NRAMPs) regulate macrophage activation for antimicrobial activity against intracellular pathogens. They do this by mediating metal ion transport across macrophage membranes and the subsequent use of these ions in the control of free radical formation. The human gene SLC11A1 encodes NRAMP1 (Kishi F, 2004; Kishi F and Nobumoto M, 1995) which can utilize the protonmotive force to mediate divalent iron (Fe2+), zinc (Zn2+) and manganese (Mn2+) influx to or efflux from phagosomes.
The primary site for absorption of dietary iron is the duodenum. Ferrous iron (Fe2+) is taken up from the gut lumen across the apical membranes of enterocytes and released into the portal vein circulation across basolateral membranes. The human gene SLC11A2 encodes the divalent cation transporter DCT1 (NRAMP2, Natural resistance-associated macrophage protein 2). DCT1 resides on the apical membrane of enterocytes and mediates the uptake of many metal ions, particularly ferrous iron, into these cells (Tandy et al. 2000).
The human gene SLC30A1 encodes the zinc transporter ZnT1. It is widely expressed throughout the body and it's expression is regulated by zinc. ZnT1 is the only member of the SLC30 gene family that is located on the plasma membrane and mediates the transport of zinc out of the cell (Devergnas S et al, 2004).
The human gene SLC30A3 encodes zinc transporter ZnT3. Through experiments on the mouse homologue Znt3, it is thought this transporter is expressed mainly in brain and testis and mediated the accumulation of zinc into synaptic vesicles (Palmiter et al, 1996).
The human gene SLC30A5 encodes the zinc transporter ZnT5. This protein is widely expressed but is most abundant in pancreatic beta cells (Kambe T et al, 2002). In these cells, ZnT5 mediates the transport of zinc into secretory granules that contain insulin.
The human gene SLC30A7 encodes the zinc transporter ZnT7. It is thought to be present in the small intestine and lung in humans (Kirschke CP and Huang L, 2003). Functional properties assigned to ZnT7 are based on studies conducted with mouse experiments.
The human SLC30A8 gene encodes the zinc transporter ZnT8 which is specifically expressed in pancreatic beta cells (Chimienti et al. 2005). Zinc is required for zinc-insulin crystallization within secretory vesicles of these cells. After glucose stimulation, large amounts of zinc are secreted locally in the extracellular matrix together with insulin. It has been suggested that this co-secreted zinc plays a role in islet cell paracrine and/or autocrine communication (Chimienti F et al, 2006). Loss of function mutations in SLC30A8 are strongly protective against type 2 diabetes, suggesting SLC20A8 inhibition as a therapeutic target in T2D prevention. (Flannick et al. 2014).
Two human genes mediate the transport of zinc into the TGN and they are both localized to the TGN. The human gene SLC30A6 encodes the zinc transporter ZnT6. By Western blot studies, ZnT6 is only found in the brain and lung in human (Huang L et al, 2002).
Copper (Cu2+) is essential for many important biological processes such as mitochondrial oxidative phosphorylation, detoxification of free radicals, iron metabolism and neurotransmiter synthesis. Too much influx results in cell poisoning. In humans, there are two member of the SLC31 gene family that are implicated in copper transport. The human gene SLC31A1 encodes human copper transporter 1, hCTR1 and is ubiquitiously expressed, with highest levels seen in the liver.. It was first identified by functional complementation in ctr1-deficient yeast (Zhou B and Gitschier J, 1997). hCTR1 exists as a homotrimer at the plasma membrane of cells (De Feo CJ et al, 2007) and is responsible for high-affinity copper uptake (Lee J et al, 2002). The second gene product, hCTR2, has not be characterized yet.
The human gene SLC39A6 encodes the zinc transporter ZIP6 (LIV-1). The gene is oestrogen-regulated and has been implicated in metastatic breast cancer. ZIP6 mediates the transport of zinc into cells, is localized to the plasma membrane and is expressed mainly in hormonal tissues such as breast, prostate and brain (Taylor KM et al, 2003).
The human gene SLC39A14 encodes the zinc transporter ZIP14. This protein is ubiquitously expressed with higher expression seen in heart, liver and pancreas. ZIP14 is localized to the plasma membrane and mediates zinc influx into cells (Taylor KM et al, 2005).
The human gene SLC39A10 encodes the zinc transporter hZIP10. It is thought to be involved in the invasive behaviour of breast cancer cells where depletion of hZIP10 and intracellular zinc levels inhibit the migratory effects these cells (Kagara N et al, 2007). Functional characterization of this transporter was elucidated in the rat orthologue rZip10.
The primary site for absorption of dietary iron is the duodenum. Ferrous iron (Fe2+) is taken up from the gut lumen across the apical membranes of enterocytes and released into the portal vein circulation across basolateral membranes. The human gene SLC40A1 encodes the metal transporter protein MTP1 (aka ferroportin or IREG1). This protein resides on the basolateral membrane of enterocytes and mediates ferrous iron efflux into the portal vein (Schimanski et al. 2005). MTP1 colocalizes with hephaestin (HEPH) which stablizes MTP1 and is necessary for the efflux reaction to occur (Han & Kim 2007, Chen et al. 2009). As well as the dudenum, MTP1 is also highly expressed on macrophages (where it mediates iron efflux from the breakdown of haem) and the placenta (where it may mediate the transport of iron from maternal to foetal circulation). It is also expressed in muscle and spleen.
The human gene SLC39A8 encodes the zinc transporter ZIP8 (BIGM103, BCG-induced integral membrane protein in monocyte clone 103 protein). It is highly expressed in the pancreas and localized to the plasma membrane where it mediates the influx of zinc into the cell (Besecker B et al, 2008). The highly homologous mouse ZIP8 have been shown to play important roles in regulating zinc homeostasis and determining sensitivity to cadmium toxicity in mouse testicular endothelium and kidney tissue. It is expected the human transporter plays a similar role (He L et al, 2009). ZIP8 belongs to the LZT subfamily of ZIP transporters.
The human gene SLC39A7 encodes the zinc transporter ZIP7 (HKE4, Histidine-rich membrane protein Ke4). It is expressed in many tissues but especially in liver, kidney and the hormonal tissues apart from brain. Unlike the other members of the LZT subfamily, ZIP7 is localized to intracellular membranes of the ER rather than the plasma membrane and mediates zinc efflux into the cytoplasm (Taylor KM et al, 2004).
The human gene SLC39A5 encodes the zinc transporter hZIP5 (Wang F et al, 2004). Highest expressions are seen in liver, kidney, pancreas and throughout the small intestine and colon with little expression detected in other tissues. hZIP5 is localized to the basolateral membrane of cells in these tissues. The functionality of ZIP5 as a zinc transporter was determined using mouse protein (mZIP5) (Wang F et al, 2004).
The human gene SLC39A1 encodes the human zinc transporter hZIP1. It is ubiquitously expressed (Lioumi M et al, 1999) and mediates the influx of zinc into cells (Gaither LA and Eide DJ, 2001). The human gene SLC39A2 encodes the zinc transporter hZIP2. It is expressed exclusively in prostate and uterine epithelial cells and mediates zinc transport into cells (Gaither LA and Eide DJ, 2000).
Normal prostate cells have the ability to accumulate high levels of zinc. In prostate cancer, hZIP1-3 transporters are down-regulated and the cells lose the ability to accumulate zinc. Zinc plays a role as a tumour-suppressing agent thus prostate cells can become cancerous. Silencing of the genes that express hZIP1-3 transporters is a required event for malignancy (Costello LC et al, 1999; Desouki MM et al, 2007).
The human gene SLC39A4 encodes the zinc transporter hZIP4 (Kury S et al, 2002). The role of zinc in tumour progression is complicated and, subsequently, so are the role of ZIP transporters. For example, ZIP4 can actually enhance cancer progression (Li M et al, 2007; Li M et al, 2009). Defects in SLC39A4 result in the inherited condition acrodermatitis enteropathica (AE) results from defective absorption of dietary zinc from the duodenum and jejunum. Clinical features include growth retardation, immune system dysfunction, severe dermatitis and mental disorders (Wang K et al, 2002).
Magnesium (Mg2+) is an abundant cation that is important for many intracellular biochemical functions, especially as a cofactor for ATP. Intracellular Mg2+ concentrations must be finely regulated and recently, transporters for this cation have been elucidated. The human genes SLC41A1 and SLC41A2 encode magnesium transport proteins 1 and 2 respectively (MagT1 and MagT2). They both mediate the uptake of Mg2+ into cells (Goytain A and Quamme GA, 2005; Sahni J et al, 2007). A third human gene, SLC41A3, is also thought to encode a MagT protein but has not been characterized yet.
Noradrenaline (NAd, norepinephrine) is a neurotransmitter whose action is mediated by the noradrenaline transporter NAT1. NAT1 is a monoamine transporter that transports noradrenaline from the synapse back to its vesicles for storage until later use. NAT1 is encoded by the human gene SLC6A2 and is expressed in the CNS and adrenal gland (Pacholczyk et al. 1991). Defects in SLC6A2 results in orthostatic intolerance (OI), which is a syndrome characterized by lightheadedness, fatigue and development of symptoms during upright standing, relieved by sitting back down again (Shannon et al. 2000).
Four transporters mediate GABA uptake in the brain, GAT1-3 and BGT1. They terminates the action of GABA by high affinity sodium-dependent reuptake into presynaptic terminals. Transport of GABA by GAT1-3 is proposed to be accompanied by 2Na+ ions and 1 Cl- ion (Loo DD et al, 2000).
SLC6A1 encodes a sodium- and chloride-dependent GABA transporter 1, GAT1, which is the predominant GABA transporter in brain. It is widely distributed in the brain and co-localized to GABAergic neurons (Nelson H et al, 1990). SLC6A13 encodes a sodium- and chloride-dependent GABA transporter 2, GAT2, which is localized to GABAergic neurons in the brain. It is also found in retina, liver and kidney (Christiansen B et al, 2007). SLC6A11 encodes a sodium- and chloride-dependent GABA transporter 3, GAT3. It is expressed in the brain and localizes to GABAergic neurons (Borden LA et al, 1994).
The amino acid L-proline can act as a neurotransmitter. Its actions are terminated by its re-uptake from the synaptic cleft into the pre-synaptic terminal in the brain. This re-uptake is mediated by a sodium-dependent proline transporter, PROT (Shafqat S et al, 1995).
The amino acid glycine plays an important role in neurotransmission. Its action is terminated by rapid re-uptake into the pre-synaptic terminal or surrounding glial cells. This re-uptake is mediated by the sodium- and chloride-dependent glycine transporters 1 and 2 (GLYT1 and GLYT2 respectively). GLYT1 is encoded by the human gene SLC6A9 and is expressed in the brain, liver, kidney, pancreas, lung and placenta (Kim KM et al, 1994). GLYT2 is encoded by the human gene SLC6A5 and is predominantly expressed in the medulla (Morrow JA et al, 1998). Defects in SLC6A5 cause startle disease (STHE or hyperekplexia). STHE is is a human neurological disorder characterized by an excessive startle response (Rees MI et al, 2006).
Carrier-mediated urea transport allows rapid urea movement across the cell membrane, which is particularly important in the process of urinary concentration and for rapid urea equilibrium in non-renal tissues. Two carriers exist in humans, HUT2 which is renal-specific (Olives B et al, 1996) and HUT11, which is erythrocyte-specific (Olives B et al, 1994).
The human gene SLC18A1 encodes the vesicular monoamine transporter 1 (VMAT1) (Erickson JD et al, 1996). VMAT1 is mainly expressed in neuroendocrine cells. The human gene SLC18A2 encodes VMAT2 (Erickson JD and Eiden LE, 1993). Both transporters can mediate the transport of biogenic amines into secretory vesicles, which can then discharge their contents into the extracellular space by exocytosis. Predominant biogenic amines these proteins can transport are serotonin, dopamine, adrenaline, noradrenaline and histamine.
The human gene RhCG encodes the Rhesus blood group family type C glycoprotein which is mainly expressed in kidney collecting duct but also found in testis. RhCG is located on the apical membrane and mediates the bi-directional transport of ammonium into and out of renal collecting duct cells in an electroneutral manner, with H+ transported the other way (Liu Z et al, 2000; Marini AM et al, 2000).
The human gene RHAG encodes a Rhesus blood group family type A glycoprotein which is expressed specifically in erythroid cells. It is thought to mediate ammonium export from these cells (Marini et al. 2000, Westhoff et al. 2002). Defects in RHAG are the cause of regulator type Rh-null hemolytic anemia (RHN, Rh-deficiency syndrome). RHN is a form of chronic hemolytic anemia (Hyland et al. 1998).
The human gene RhBG encodes a Rhesus blood group family type B glycoprotein which is expressed mainly in the kidney but is also found in the liver. The liver and kidney are important tissues for ammonium metabolism and excretion. RhBG is located on the basolateral membrane and mediates the reversible transport of ammonium in and out of renal collecting duct cells in an electroneutral manner, with H+ transported the other way (Ludewig U, 2004; Liu Z et al, 2001).
Choline (Cho) transports from the extracellular space through the plasma membrane via the choline transporter-like proteins (SLC44A1-5 also known as CTL1-5) to the cytosol (Okuda & Haga 2000, Traiffort et al. 2005, O'Regan et al. 2000).
CTL1 is broadly expressed on leukocytes and endothelial cells (Wille et al. 2001). CTL2 is highly expressed in human inner ear and is the target of antibody-induced hearing loss (Nair et al. 2004).
The human gene RhCG encodes the Rhesus blood group family type C glycoprotein which is mainly expressed in kidney collecting duct but also found in testis. RhCG is located on the apical membrane and mediates the bi-directional transport of ammonium into and out of renal collecting duct cells in an electroneutral manner, with H+ transported the other way (Liu Z et al, 2000; Marini AM et al, 2000).
The human gene RhBG encodes a Rhesus blood group family type B glycoprotein which is expressed mainly in the kidney but is also found in the liver. The liver and kidney are important tissues for ammonium metabolism and excretion. RhBG is located on the basolateral membrane and mediates the reversible transport of ammonium in and out of renal collecting duct cells in an electroneutral manner, with H+ transported the other way (Ludewig U, 2004; Liu Z et al, 2001).
GLUT (glucose transporter) homotetramers associated with the plasma membrane mediate the facilitated diffusion of glucose between the extracellular space and the cytosol. Four human GLUT isoforms have been identified, members of a larger family of transporter proteins (Joost et al. 2002), encoded by the SCLC2A1, 2, 3, and 4 genes. Conserved sequence motifs in the GLUT proteins suggest the existence of shared structural features confirmed by in situ labeling and mutagenesis studies. Each GLUT protein has twelve membrane spanning domains organized to form an aqueous channel. While monomeric protein can form such a channel and transport glucose, kinetic studies suggest that the functional form of the protein is a homotetramer.
Different GLUT proteins are expressed in different tissues. GLUT1 is expressed by many cell types, notably endothelial cells, red blood cells and cells of the brain. Its low Km for glucose (~1 mM) relative to normal blood glucose concentration (~5 mM) allows these cells to take up glucose independent of changes in blood glucose levels. Its abundance in red blood cells has allowed it to be purified and biochemically characterized (Hruz & Mueckler 2001, Liu et al. 2001). Cytosolic ATP associates with GLUT1 and inhibits its glucose transporter activity.
Fibroblast growth factor 21 (FGF21), is a potent regulator of glucose uptake in mouse 3T3-L1 and primary human adipocytes. FGF21 activity is likely to be mediated through a significant increase in gene expression of GLUT1 (Kharitonenkov et al. 2005).
The human gene SLC22A1 expresses the organic cation transporter 1 (OCT1) mainly in the liver. It can mediate the reversible transport of a broad array of organic cations with various structures and molecular weights including the model compounds 1-methyl-4-phenylpyridinium (MPP), tetraethylammonium (TEA), N-1-methylnicotinamide (NMN) and 4-(4-(dimethylamino)styryl)-N-methylpyridinium (Zhang L et al, 1998; Gorboulev V et al, 1997).
The human gene SLC22A4 encodes the ergothioneine transporter (ETT). It was originally discovered as an organic cation/carnitine transporter (OCTN1) (Tamai et al. 1997) but its main substrate is not carnitine. It is widely expressed and transports ergothioneine more than 100 times more efficiently than tetraethylammonium and carnitine (Grundemann et al. 2005), leading to the name change from OCTN1 to ETT. Defects in SLC22A4 could be implicated in rheumatoid arthritis (RA; MIM:180300), an inflammatory disease with autoimmune features and a complex genetic component (Tokuhiro et al. 2003, Yamada et al. 2004). An intronic SNP (slc2F2) in intron 1, consists of a susceptible T allele and a nonsusceptible C allele. The runt-related transcription factor 1 (RUNX1) has a suppressive effect on SLC22A4 expression and this suppression appears to be stronger with the susceptible T allele of SLC22A4 than with the nonsusceptible C allele, leading to a lower expression of SLC22A4 (Tokuhiro et al. 2003).
The human gene SLC22A2 encodes the organic cation transporter OCT2. It is expressed in a variety of tissues, especially the kidney and placenta. OCT2 can mediate the reversible transport of a broad array of organic cations with various structures and molecular weights including the model compounds 1-methyl-4-phenylpyridinium (MPP), tetraethylammonium (TEA), N-1-methylnicotinamide (NMN) and 4-(4-(dimethylamino)styryl)-N-methylpyridinium (Gorboulev V et al, 1997). Pharmaceuticals that up-regulate OCT2 in the kidney can increase the renal excretion of cationic drugs.
The human gene SLC22A5 encodes the organic cation/carnitine transporter 2 (OCTN2). OCTN2 is strongly expressed in the kidney, skeletal muscle, heart and placenta (Tamai et al. 1998). Defects in SLC22A5 are the cause of systemic primary carnitine deficiency (CDSP) (Tang et al. 1999) and susceptibility to Crohn disease (CD) (Peltekova et al. 2004). The human gene SLC22A15 encodes the fly-like putative transporter1 (FLIPT1). FLIPT1 is a novel transporter highly expressed in kidney and brain is shown to be homologous to other carnitine transporters (Eraly & Nigan 2002). The human gene SLC22A16 encodes the organic cation/carnitine transporter 6 (also called the fly-like putative transporter 2, FLIPT2) (Enomoto et al. 2002). FLIPT2 is strongly expressed in the testis and epididymas as well as generally in other tissues and in leukaemia cells ( Enomoto et al. 2002, Gong et al. 2002). All of these transporters are sodium-dependent, high affinity carnitine cotransporters.
The human gene SLC22A3 encodes organic cation transporter OCT3. It is mainly expressed in skeletal muscle, liver, placenta, kidney and heart, and to a lesser extent in brain. OCT3 is involved in the biliary excretion of cationic drugs. In CNS, ganglia and heart, OCT3 regulates the interstitial concentrations of monoamine neurotransmitters and cationic drugs. In placenta, OCT3 is responsible for the release of acetylcholine during nonneuronal cholinergic regulation (Grundemann D et al,1998; Wu X et al, 2000).
The human gene SLC22A1 expresses the organic cation transporter 1 (OCT1) mainly in the liver. It can mediate the reversible transport of a broad array of organic cations with various structures and molecular weights including the model compounds 1-methyl-4-phenylpyridinium (MPP), tetraethylammonium (TEA), N-1-methylnicotinamide (NMN) and 4-(4-(dimethylamino)styryl)-N-methylpyridinium (Zhang L et al, 1998; Gorboulev V et al, 1997).
The human gene SLC22A6 encodes organic anion transporter1 (OAT1). It was originally characterized in mouse as Novel Kidney Transcript (NKT). OAT1 is located on the basolateral membrane of the proximal tubule in human kidney as well as in the brain (Reid G et al, 1998; Lu R et al, 1999; Hosoyamada M et al, 1999). The human gene SLC22A7 encodes organic anion transporter 2 (OAT2) and is highly expressed in the liver and kidney (Sun W et al, 2001; Kobayashi Y et al, 2005). The human gene SLC22A8 encodes organic anion transporter 3 (OAT3) which is expressed mainly in the brain and kidney (Race JE et al, 1999; Bakhiya A et al, 2003). OAT1-3 transport organic anions such as p-aminohippurate and drugs such as cimetidine and acyclovir. This transport is is coupled with an efflux of one molecule of endogenous dicarboxylic acid such as alpha-ketoglutarate (2-oxoglutarate). OAT2 is classified as both a transporter of organic anions and sulphate conjugates.
The human gene SLC22A2 encodes the organic cation transporter OCT2. It is expressed in a variety of tissues, especially the kidney and placenta. OCT2 can mediate the reversible transport of a broad array of organic cations with various structures and molecular weights including the model compounds 1-methyl-4-phenylpyridinium (MPP), tetraethylammonium (TEA), N-1-methylnicotinamide (NMN) and 4-(4-(dimethylamino)styryl)-N-methylpyridinium (Gorboulev V et al, 1997). Pharmaceuticals that up-regulate OCT2 in the kidney can increase the renal excretion of cationic drugs.
The human gene SLC22A7 encodes organic anion transporter 2 (OAT2) and is highly expressed in the liver and kidney (Sun W et al, 2001; Kobayashi Y et al, 2005). The human gene SLC22A11 encodes organic anion transporter 4 (OAT4) which is highly expressed in the placenta and kidney (Cha SH et al, 2000; Ekaratanawong S et al, 2004). Both of these transporters mediate the influx of sulfate conjugates with antiport of dicarboxylic acid. OAT2 is classified as both a transporter of organic anions and sulphate conjugates.
The human gene SLC22A3 encodes organic cation transporter OCT3. It is mainly expressed in skeletal muscle, liver, placenta, kidney and heart, and to a lesser extent in brain. OCT3 is involved in the biliary excretion of cationic drugs. In CNS, ganglia and heart, OCT3 regulates the interstitial concentrations of monoamine neurotransmitters and cationic drugs. In placenta, OCT3 is responsible for the release of acetylcholine during nonneuronal cholinergic regulation (Grundemann D et al,1998; Wu X et al, 2000).
Urate is a naturally occurring product of purine metabolism and is a scavenger of biological oxidants. Uric acid readily precipitates out of aqueous solutions causing gout and kidney stones. Due to this ability, changes in urate levels are implicated in numerous disease processes. The human gene SLC22A12 encodes urate transporter 1 (URAT1), predominantly expressed in the kidney and is involved in the regulation of blood urate levels. This transport can be trans-stimulated by organic anions such as L-lactate (LACT) (Enomoto et al. 2002). Defects in SLC22A12 result in idiopathic renal hypouricaemia (lack of blood urate) (Wakida et al. 2005).
The human gene SLC22A18 encodes organic cation transporter-like protein 2 (ORCTL2). It is expressed at high levels in kidney, liver and colon and at lower levels in heart, brain and lung. ORCTL2 can transport chloroquine and quinidine with the antiport of protons (Reece et al. 1998). Defects in SLC22A18 may play a role in tumorigenesis (Schwienbacher et al. 1998). How SLC22A18 might be involved in growth regulation is poorly understood. There is speculation that it may be involved in resistance to chemotherapy drugs and/or in the export of genotoxic substances whose retention may increase the risk of tumor formation.
SLC40A1 (MTP1 aka ferroportin or IREG1) is highly expressed on macrophages where it mediates iron efflux from the breakdown of haem (Schimanski et al. 2005). SLC40A1 colocalizes with ceruloplasmin (CP) which stablizes SLC40A1 and is necessary for the efflux reaction to occur (Texel et al. 2008). Six copper ions are required as cofactor. Ceruloplasmin (CP) also catalyses the conversion of iron from ferrous (Fe2+) to ferric form (Fe3+), thereby assisting in its transport in the plasma in association with transferrin, which can only carry iron in the ferric state. As well as being expressed on macrophages, SLC40A1 is also highly expressed in the duodenum, placenta (where it may mediate the transport of iron from maternal to foetal circulation), muscle and spleen.
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transported by
VMAT1/2transported by
VMAT1/2transported by
NaDC1transported by
NaDC1transported by
NRAMP1transported by
NRAMP1transported by
SMIT2transported by
SMIT2conjugate
substratesconjugate
substratesSerotonin
transporterAnnotated Interactions
transported by
VMAT1/2transported by
VMAT1/2transported by
NaDC1transported by
NaDC1transported by
NRAMP1transported by
NRAMP1transported by
SMIT2transported by
SMIT2conjugate
substratesconjugate
substratesSLC2A11 encodes GLUT11 (Doege H et al, 2001), another member of the class II facilitative glucose transporters. It has the highest similarity with GLUT5 and in humans, three isoforms are expressed (GLUT11A-C) (Sasaki T et al, 2001). Human GLUT11 has been shown to transport glucose and fructose but not galactose when expressed in Xenopus oocytes ( Scheepers A et al, 2005).
GLUT2 is expressed by SLC2A2 and is a low affinity glucose transporter (Fukumoto H et al, 1988). It is expressed mainly in the kidney, liver and pancreatic beta-cells. In beta-cells, it functions as a glucose-sensor for insulin secretion and in the liver, it allows for bi-directional glucose transport. In this reaction, it is shown to mediate the influx of glucose. In the next reaction, it is shown to be mediating efflux of glucose. Defects in SLC2A2 are the cause of Fanconi-Bickel syndrome (FBS). It is characterized by hepatorenal glycogen accumulation, proximal renal tubular dysfunction, and impaired utilization of glucose and galactose (Burwinkel B et al, 1999).
SLC2A3 encodes GLUT3 which is mainly expressed in the brain but also in a wide range of tissues. If has a high affinity for glucose and can also transport other sugars (Kayano T et al, 1988). GLUT4, encoded by SLC2A4, is an insulin-responsive glucose transporter found in heart, skeletal muscle, brain and adipose tissue. Due to its sensitivity to insulin, it may play a role in diabetes. In a non-insulin condition, GLUT4 is localized in intracellular GLUT4-containing vesicles. On insulin stimulation, GLUT4 translocates to the plasma membrane where it can increase glucose transport 10-20-fold (Fukumoto H et al, 1989). Defects in SLC2A4 may be a cause of non-insulin-dependent diabetes mellitus (NIDDM) (Kusari J et al, 1991; Choi WH et al, 1991). GLUT1 is curated in its own reaction in this section.
Four class III facilitative transporters can transport glucose. SLC2A6 encodes GLUT6, expressed mainly in brain, spleen and leucocytes (Doege H et al, 2000a). In literature, this protein is incorrectly described as GLUT9. SLC2A8 encodes GLUT8 and is expressed in brain, testis and adipose tissue (Doege H et al, 2000b). SLC2A10 (located in the Type 2 diabetes-linked region of human chromosome 20q12-13.1) encodes GLUT10, a transporter with high affinity for glucose (McVie-Wylie AJ et al, 2001) . GLUT10 is highly expressed in liver and pancreas but is present in most tissues in lower levels. Defects in SLC2A10 are the cause of arterial tortuosity syndrome (ATS), an autosomal recessive disorder characterized by tortuosity and elongation of major arteries, often resulting in death at a young age (Coucke PJ et al, 2006). SLC2A12 encodes GLUT12, which is highly expressed in skeletal muscle, heart and prostate, with lower levels in brain, placenta and kidney. It was originally cloned from the human breast cancer cell line MCF-7 (Rogers S et al, 2002).
SLC16A1 encodes MCT1, a ubiquitiously expressed protein (Garcia et al. 1994). Defects in SLC16A1 are the cause of symptomatic deficiency in lactate transport (SDLT), resulting in an acidic intracellular environment and muscle degeneration (Merezhinskaya et al. 2000). Activating promotor mutations in SLC16A1 are associated with exercise-induced hyperinsulinism (EIHI), a dominantly inherited hypoglycemic disorder characterized by inappropriate insulin secretion during anaerobic exercise or on pyruvate load (Otonkoski et al. 2000). SLC16A7 encodes MCT2, a high affinity pyruvate transporter highly expressed in testis (Lin et al. 1998). SLC16A8 encodes MCT3 (Yoon et al. 1999).
Human RPE (retinal pigment epithelial) cells express two proton-coupled monocarboxylate transporters: MCT1 in the apical membrane and MCT3 in the basolateral membrane. This suggests that the coordinated activities of these two transporters could facilitate the transepithelial transport of lactate from the retina to the choroid. (Philip et al. 2003).
MATE1 is primarily expressed in the kidney and liver, where it is localized to the luminal membranes of the urinary tubules and bile canaliculi. When expressed in HEK293 cells, MATE1 mediates H(+)-coupled electroneutral exchange of various drugs (Otsuka M et al, 2005). MATE2 is a human kidney-specific H+/organic cation antiporter that is responsible for the tubular secretion of cationic drugs across the brush border membranes (Masuda S et al, 2006). Substrates for both MATEs include tetraethylammonium, 1-methyl-4-phenylpyridinium, cimetidine, metformin, creatinine, guanidine and procainamide (Tanihara Y et al, 2007).
The human gene SLC11A1 encodes NRAMP1 (Kishi F, 2004; Kishi F and Nobumoto M, 1995) which can utilize the protonmotive force to mediate divalent iron (Fe2+), zinc (Zn2+) and manganese (Mn2+) influx to or efflux from phagosomes.
The human gene SLC39A14 encodes the zinc transporter ZIP14. This protein is ubiquitously expressed with higher expression seen in heart, liver and pancreas. ZIP14 is localized to the plasma membrane and mediates zinc influx into cells (Taylor KM et al, 2005).
The human gene SLC40A1 encodes the metal transporter protein MTP1 (aka ferroportin or IREG1). This protein resides on the basolateral membrane of enterocytes and mediates ferrous iron efflux into the portal vein (Schimanski et al. 2005). MTP1 colocalizes with hephaestin (HEPH) which stablizes MTP1 and is necessary for the efflux reaction to occur (Han & Kim 2007, Chen et al. 2009). As well as the dudenum, MTP1 is also highly expressed on macrophages (where it mediates iron efflux from the breakdown of haem) and the placenta (where it may mediate the transport of iron from maternal to foetal circulation). It is also expressed in muscle and spleen.
The human gene SLC39A2 encodes the zinc transporter hZIP2. It is expressed exclusively in prostate and uterine epithelial cells and mediates zinc transport into cells (Gaither LA and Eide DJ, 2000).
Normal prostate cells have the ability to accumulate high levels of zinc. In prostate cancer, hZIP1-3 transporters are down-regulated and the cells lose the ability to accumulate zinc. Zinc plays a role as a tumour-suppressing agent thus prostate cells can become cancerous. Silencing of the genes that express hZIP1-3 transporters is a required event for malignancy (Costello LC et al, 1999; Desouki MM et al, 2007).
The human gene SLC39A4 encodes the zinc transporter hZIP4 (Kury S et al, 2002). The role of zinc in tumour progression is complicated and, subsequently, so are the role of ZIP transporters. For example, ZIP4 can actually enhance cancer progression (Li M et al, 2007; Li M et al, 2009). Defects in SLC39A4 result in the inherited condition acrodermatitis enteropathica (AE) results from defective absorption of dietary zinc from the duodenum and jejunum. Clinical features include growth retardation, immune system dysfunction, severe dermatitis and mental disorders (Wang K et al, 2002).
SLC6A1 encodes a sodium- and chloride-dependent GABA transporter 1, GAT1, which is the predominant GABA transporter in brain. It is widely distributed in the brain and co-localized to GABAergic neurons (Nelson H et al, 1990). SLC6A13 encodes a sodium- and chloride-dependent GABA transporter 2, GAT2, which is localized to GABAergic neurons in the brain. It is also found in retina, liver and kidney (Christiansen B et al, 2007). SLC6A11 encodes a sodium- and chloride-dependent GABA transporter 3, GAT3. It is expressed in the brain and localizes to GABAergic neurons (Borden LA et al, 1994).
CTL1 is broadly expressed on leukocytes and endothelial cells (Wille et al. 2001). CTL2 is highly expressed in human inner ear and is the target of antibody-induced hearing loss (Nair et al. 2004).
Different GLUT proteins are expressed in different tissues. GLUT1 is expressed by many cell types, notably endothelial cells, red blood cells and cells of the brain. Its low Km for glucose (~1 mM) relative to normal blood glucose concentration (~5 mM) allows these cells to take up glucose independent of changes in blood glucose levels. Its abundance in red blood cells has allowed it to be purified and biochemically characterized (Hruz & Mueckler 2001, Liu et al. 2001). Cytosolic ATP associates with GLUT1 and inhibits its glucose transporter activity.
Fibroblast growth factor 21 (FGF21), is a potent regulator of glucose uptake in mouse 3T3-L1 and primary human adipocytes. FGF21 activity is likely to be mediated through a significant increase in gene expression of GLUT1 (Kharitonenkov et al. 2005).
OAT1-3 transport organic anions such as p-aminohippurate and drugs such as cimetidine and acyclovir. This transport is is coupled with an efflux of one molecule of endogenous dicarboxylic acid such as alpha-ketoglutarate (2-oxoglutarate). OAT2 is classified as both a transporter of organic anions and sulphate conjugates.
Serotonin
transporter