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At least 1,045 records · Page 58Linked to original sources
PubMed2002

Two highly conserved glutamate residues critical for type III sodium-dependent phosphate transport revealed by uncoupling transport function from retroviral receptor function.

Type III sodium-dependent phosphate (NaP(i)) cotransporters, Pit1 and Pit2, have been assigned housekeeping P(i) transport functions and suggested involved in chondroblastic and osteoblastic mineralization and ectopic calcification. Both proteins exhibit dual function, thus, besides being transporters, they also serve as receptors for several gammaretroviruses. We here show that it is possible to uncouple transport and receptor functions of a type III NaP(i) cotransporter and thus exploit the retroviral receptor function as a control for proper processing and folding of mutant proteins. Thus exchanging two putative transmembranic glutamate residues in human Pit2, Glu(55) and Glu(575), with glutamine or with lysine severely impaired or knocked out, respectively, P(i) transport function, but left viral receptor function undisturbed. Both glutamates are conserved in type III NaP(i) cotransporters, in fungal NaP(i) cotransporters PHO-4 and Pho89, and in other known or putative phosphate permeases from a number of species and are the first residues shown to be critical for type III NaP(i) cotransport. Their putative transmembranic positions together with the presented data are consistent with Glu(55) and Glu(575) being parts of a cation liganding site or playing roles in conformational changes associated with substrate transport. Finally, the results also show that Pit2 retroviral receptor function per se is not dependent on Pit2 P(i) transport function.

Amino Acid Substitution↗
PubMed1966

Auxin Transport in Zea mays L. Coleoptiles I. Influence of Gravity on the Transport of Indoleacetic Acid-2-C.

(14)C-methylene labeled IAA was used to determine the influence of reorientation with respect to gravity on auxin transport in Zea mays L. coleoptile segments. It was observed that inversion of the segments leads to a decrease in the capacity to transport (14)C-IAA basipetally, as well as, in certain instances, the linear velocity of that transport. Segments were also reoriented horizontally, and the transport velocity and capacity of the upper and lower tissue halves compared with vertical halves. There was no significant change in the velocity, but the transport capacity of lower halves was higher than that of the vertical halves, which in turn was higher than the capacity of the horizontal upper halves. It is suggested that the geocurvature of horizontally placed coleoptiles may be caused primarily by the effect of reorientation on auxin transport.

Journal Article↗
PubMed1993

p-Aminohippurate transport in rat renal brush-border membranes: a potential-sensitive transport system and an anion exchanger.

Transport mechanisms of p-aminohippurate (PAH) were investigated in rat renal brush-border membrane vesicles. The uptake of PAH was stimulated by an inside-positive membrane potential created by K+ and valinomycin. This potential-stimulated uptake of PAH was inhibited by various anion transport inhibitors and was saturable. In addition, PAH uptake in the presence of valinomycin was linearly increased in proportion to log[K+]out/[K+]in. On one hand, PAH uptake was stimulated by [14C]PAH/PAH or [14C]PAH/Cl- exchange, and the [14C]PAH/PAH exchange was insensitive to the membrane potential. The uptake by the exchanger was also inhibited by anion transport inhibitors, but the potential-stimulated uptake of PAH was more sensitive to furosemide and 4,4'-diisothiocyano-2,2'-disulfonic stilbene. On the contrary, [14C]PAH/PAH exchange was more sensitive to urate than the potential-stimulated uptake of PAH. These findings indicate that PAH is transported by two distinct transport systems in rat renal brush-border membranes, a potential-sensitive transport system and an anion exchanger.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed1979

Divalent cation transport in kidney slices. I. Properties of calcium transport in slices of rat kidney cortex and the effects of diuretics.

Correlative studies on transports of Ca2+ and Na+ and the properties of Ca2+ transport were carried out in rat kidney cortex slices. Ouabain had no effect on the Ca2+ transport but did inhibit the Na+ transport extensively. With addition of 2,4-dinitrophenol to the incubation medium, or under the anaerobic conditions, the effluxes of Ca2+ and Na+ were inhibited while the Ca2+ influx was enhanced significantly. Sulfhydryl inhibitors such as ethacrynic acid, mersalyl and p-chloromercuribenzoic acid suppressed Ca2+ efflux and stimulated Ca2+ influx. When the slices of kidney cortex were treated with these inhibitors, there was a reduction in the content of cellular ATP. The present results suggest that Ca2+ transport may be partly independent of Na+ transport, and that Ca2+ efflux may require energy-yielding processes.

Adenosine Triphosphate↗
PubMed1994

Determination of peritoneal transport characteristics with 24-hour dialysate collections: dialysis adequacy and transport test.

Although the 24-hour collection of dialysate provides a very accurate measure of the adequacy of dialysis, it is not known if it can also determine peritoneal membrane transport characteristics. In this prospective study, 101 24-hour dialysate collections were immediately followed by a standard peritoneal equilibration test (PET). Four- and 24-h dialysate-to-plasma (D/P) ratios were determined for creatinine and urea. The correlation coefficients between these two tests were 0.86 for the adjusted D/P creatinine and 0.71 for the D/P urea, whereas the standard errors of estimate were 0.054 and 0.060, respectively. Patients were classified into one of four transport groups on the basis of the mean and standard deviation of the adjusted D/P creatinine values, and these values were similar to those generated from the PET data. Rates of ultrafiltration were also defined for patients undergoing 2.0- and 2.5-L dwells. A survey of a subset of these patients demonstrated that the 24-h collection was preferred to the PET for determining transport characteristics. Therefore, the 24-h dialysate collection can be used to monitor both peritoneal membrane transport characteristics and adequacy. This technique, which has been named the "dialysis adequacy and transport test," has the potential for significant cost savings when it is used for the routine follow-up of both peritoneal transport and adequacy of dialysis.

Adolescent↗
PubMed1984

Mitochondrial phosphate transport. Large scale isolation and characterization of the phosphate transport protein from beef heart mitochondria.

The phosphate transport protein from beef heart mitochondria has been purified on a large scale by hydroxylapatite chromatography in the presence of sodium dodecyl sulfate and urea. As shown by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (silver stain), the pure phosphate transport protein preparation consists of two protein bands (alpha and beta, ratio 1:1) with similar mobilities (34 kDa) which display identical peptide maps if fragmented with either CNBr or HCl/dimethyl sulfoxide/HBr. The complete amino acid composition of phosphate transport protein is presented. Quantitative determination of N-terminal amino acids underlines the purity of the preparation and shows for alpha and beta the identical amino-terminals H2N-Ala-Val-Glu-Glu-Glx-Tyr-. Qualitative digestion shows that carboxypeptidase A is able to release at least three amino acids from the C termini of the alpha as well as the beta band of phosphate transport protein. The nature of these two protein bands is discussed. The sum of phosphate transport protein (alpha + beta) per total mitochondrial protein amounts to 2.3% or 1.4 nmol of phosphate transport protein (34 kDa) per nmol of cytochrome b.

Amino Acids↗
PubMed1996

Transport of levofloxacin in a kidney epithelial cell line, LLC-PK1: interaction with organic cation transporters in apical and basolateral membranes.

The interactions of levofloxacin, a pyridonecarboxylic acid antibacterial drug, with the organic cation transport systems expressed in a pig kidney epithelial cell line, LLC-PK1, were examined. The transcellular transport of tetraethylammonium was remarkably inhibited by levofloxacin, accompanied by a marked increase in the cellular accumulation of tetraethylammonium in the LLC-PK1, monolayers grown on collagen-coated membrane filters. The results obtained by efflux and uptake of tetraethylammonium revealed that levofloxacin drastically inhibited the apical transport activity rather than the basolateral uptake of tetraethylammonium. Under conditions in which the apical efflux of tetraethylammonium was blocked by pretreatment with p-chloromercuribenzene sulfonate, levofloxacin showed a moderate inhibitory effect against the basolateral uptake of tetraethylammonium. Transepithelial flux of levofloxacin from the basolateral side to the apical side was much greater than the flux in the opposite direction. The flux of levofloxacin was influenced by the apical side pH, resulting in a decreased cellular accumulation by lowering pH. The basal-to-apical transport and cellular accumulation of levofloxacin were not inhibited by either tetraethylammonium or cimetidine. These results suggested that levofloxacin interacts with the apical H+/organic cation antiport system to a greater extent than with the basolateral system. However, transcellular transport of levofloxacin would be mediated by the transport systems which are distinct from the systems for tetraethylammonium in LLC-PK1 cells.

Cell Line↗
PubMed1996

Rabphilin-3A is transported with fast anterograde axonal transport and associated with synaptic vesicles.

Rab3a has been proposed to be involved in the process of exocytosis. It undergoes a membrane association/dissociation cycle which is dependent upon alteration in its GTP-GDP state. Rabphilin-3A is one of the most attractive candidates for a Rab3a effector molecule in synaptic vesicle exocytosis. It binds selectively to the GTP-bound form of Rab3a as well as of Rab3c. Fast axonal transport was studied by crushing spinal roots and sciatic nerves, resulting in accumulations of transported, organelle-bound substances. Rabphilin-3A was transported with fast anterograde transport, but < 10% of the proximal accumulation was detected in the recycling vesicle population. The pattern for accumulation of rabphilin-3A was similar to that of Rab3a accumulation, including the poor recycling. However, synaptophysin and SV2, two transmembrane synaptic vesicle proteins, accumulated in large amounts in many axons distal to the crushes, and the amounts of recycling synaptophysin and SV2, in percentages of anterograde accumulations, were about 70% and 60%, respectively. Double-labelling showed a high degree of colocalization between Rab3a and rabphilin-3A in spinal roots and sciatic nerves. The two proteins essentially colocalized in the nerve terminals of the spinal cord and in motor endplates. Immuno-EM studies demonstrated that rabphilin-3A was present on clear small synaptic vesicles in the proximal segment, but was absent from vesicles in the distal segment. Rabphilin-3A also associated with the synaptic vesicles in the presynaptic compartment. Results indicate that rabphilin-3A is associated with the membrane of synaptic vesicles during anterograde transport. Furthermore, as it colocalized in most, but not all, structures with Rab3a, results support the hypothesis that rabphilin-3A is interacting with Rab3a.

Adaptor Proteins, Signal Transducing↗
PubMed2001

Glial transporters for glutamate, glycine and GABA I. Glutamate transporters.

The termination of chemical neurotransmission in the CNS involves the rapid removal of neurotransmitter from synapses by specific transport systems. Such mechanism operates for the three major amino acid neurotransmitters glutamate, gamma-aminobutyric acid (GABA) and glycine. To date, five different high-affinity Na(+)-dependent glutamate (Glu) transporters have been cloned: GLT1, GLAST, EAAC1, EAAT4 and EAAT5. The first two are expressed mainly by glial cells, and seem to be the predominant Glu transporters in the brain. A major function of Glu uptake in the nervous system is to prevent extracellular Glu concentrations from raising to neurotoxic levels in which glial transporters seem to play a critical role in protecting neurons from glutamate-induced excitotoxicity. Under particular conditions, glial GluTs have been shown to release Glu by reversal of activity, in a Ca(2+)--and energy-independent fashion. Furthermore, an activity of these transporters as ion channels or transducing units coupled to G-proteins has recently been reported. The localization, stoichiometry, and regulation of glial GluTs are outlined, as well as their possible contributions to nervous system diseases as ALS, AD and ischemic damage.

ATP-Binding Cassette Transporters↗
PubMed1997

The rat serotonin transporter: identification of cysteine residues important for substrate transport.

Reduction and alkylation of disulfide bonds are known to affect substrate translocation by and antidepressant binding to the serotonin transporter (SERT). To identify functionally relevant cysteine residues, we substituted 16 cysteins of the rat SERT by alanine or serine residues and analyzed the transport and binding properties of the respective mutant transporters after heterologous expression in a mammalian cell line. Replacement of cysteine 209 by serine resulted in a marked reduction of the maximal transport rate, loss of positive cooperativity, and insensitivity to treatment with disulfide reducing agents, indicating that cysteine 209 participates in a structurally important disulfide bridge. Replacement of cysteine residues 147, 200, 369, and 540 caused a complete loss of both substrate transport and antidepressant binding, a result that is likely to reflect impaired processing and/or cell surface expression of the mutated polypeptides.

Alanine↗
PubMed1999

Human LAT1, a subunit of system L amino acid transporter: molecular cloning and transport function.

We report here on the cloning and functional characterization of human LAT1, a subunit of the amino acid transport system L. The hLAT1 cDNA, obtained from a human placental cDNA library, codes for a protein of 507 amino acids. When functionally expressed in mammalian cells together with the heavy chain of the rat 4F2 antigen (r4F2hc), hLAT1 induces the transport of neutral amino acids. When expressed independently, neither hLAT1 nor r4F2hc was capable of amino acid transport to any significant extent. Thus, the hLAT1-r4F2hc heterodimeric complex is responsible for the observed amino acid transport. The transport process induced by the heterodimer is Na+ independent and is not influenced by pH. It recognizes exclusively neutral amino acids with high affinity. LAT1-specific mRNA is expressed in most human tissues with the notable exception of the intestine.

Amino Acid Sequence↗
PubMed1999

GFP-Human high-affinity carnitine transporter OCTN2 protein: subcellular localization and functional restoration of carnitine uptake in mutant cell lines with the carnitine transporter defect.

Individuals with the plasmalemmal high-affinity carnitine transporter defect present with progressive infantile-onset carnitine-responsive cardiomyopathy, lipid storage myopathy, recurrent hypoglycemic hypoketotic encephalopathy, and failure to thrive. The carnitine uptake defect (CUD) has been documented in their cultured skin fibroblasts, lymphoblasts, and/or myoblasts. The cDNA encoding the high-affinity sodium-dependent human carnitine transporter OCTN2 has recently been cloned. We used the green fluorescent protein (GFP) as a living marker for positively transfected cells in our expression studies of the high-affinity carnitine transporter OCTN2 cDNA in cell lines with the CUD. Transfection of cell lines from 12 unrelated patients (nine fibroblast and three lymphoblastoid) with a GFP construct harboring the wild-type full-length OCTN2 cDNA was done using LipoTAXI. Transient and stable expression of the recombinant GFP-human carnitine transporter OCTN2 cDNA was surveyed, and transient transfection of the fibroblast and stable transfection of the lymphoblastoid cell lines were achieved. There was functional restoration of carnitine uptake in the transfected mutant cell lines, thereby confirming the identity of the transfected cDNA. In addition, we report the first demonstration of the subcellular localization of an in-frame fusion GFP-human high-affinity carnitine transporter OCTN2 protein in the plasma membrane by confocal laser-scanning fluorescence microscopy.

Biological Transport↗
PubMed2000

Molecular identification of a rat novel organic anion transporter moat1, which transports prostaglandin D(2), leukotriene C(4), and taurocholate.

We have isolated a rat novel multispecific organic anion transporter, moat1. The isolated clones were originated by alternative splicing of the moat1 mRNA. The nucleotide sequences predict a protein of 682 amino acids with moderate sequence similarity to LST-1, the oatp family, and the prostaglandin transporter. Northern blot analysis of rat moat1 identified a predominant transcript of 4.4 kilonucleotides in all tissues. Northern blot and in situ hybridization analyses of rat brain further indicated that moat1 mRNA is widely distributed in neuronal cells of the central nervous system, especially in the hippocampus and cerebellum. moat1 transports prostaglandin D(2) (K(m); 35.5 nM), leukotriene C(4) (K(m); 3.2 microM) and taurocholate (K(m); 17.6 microM) in a sodium-independent manner. moat1 also transports prostaglandin E(1), E(2), thromboxane B(2), and iloprost but not dehydroepiandrosterone sulfate and digoxin, of which the substrate specificity is similar, but definitively different from those of any other organic anion transporters.

Alternative Splicing↗
PubMed2006

Structure/function relationships in serotonin transporter: new insights from the structure of a bacterial transporter.

Serotonin transporter (SERT) serves the important function of taking up serotonin (5-HT) released during serotonergic neurotransmission. It is the target for important therapeutic drugs and psychostimulants. SERT catalyzes the influx of 5-HT together with Na+ and Cl- in a 1:1:1 stoichiometry. In the same catalytic cycle, there is coupled efflux of one K+ ion. SERT is one member of a large family of amino acid and amine transporters that is believed to utilize similar mechanisms of transport. A bacterial member of this family was recently crystallized, revealing the structural basis of these transporters. In light of the new structure, previous results with SERT have been re-interpreted, providing new insight into the substrate binding site, the permeation pathway, and the conformational changes that occur during the transport cycle.

Amino Acid Sequence↗
PubMed2006

Taurine transport and transporter localization in peripheral blood lymphocytes of controls and major depression patients.

Taurine concentration is increased in peripheral blood lymphocytes in a group of depressed subjects. After this observation [3H]taurine transport was measured in lymphocytes of 10 major depression patients (19-60 years old), diagnosed by the criteria of the American Psychiatric Association, with moderate severity as determined by Hamilton Scale for Depression (32 +/- 6). The control group comprised 10 subjects (20-56 years old). Taurine transporter and CD4+ (helpers) and CD8+ (cytolytic) T lymphocytes were immunolabeled. No significant differences were observed in immunochemical analyses. CD4+ cells were 53% and CD8+ cells 28% of the total lymphocytes in both controls and patients. Two taurine transport components, high and low affinity, were demonstrated in both groups. Taurine transporter was present in 16% of CD4+ and CD8+ lymphocytes in controls and patients. This preliminary report exhibits the presence of taurine transporter in peripheral blood lymphocytes but no differences were observed between controls and depressed subjects.

Adolescent↗
PubMed2002

Astroglial dopamine transport is mediated by norepinephrine transporter.

The aim of this study was to clarify the characteristics of the dopamine (DA) transport mechanism in cultured rat cortical astrocytes. Reverse transcription-polymerase chain reaction (RT-PCR) with DA transporter (DAT)-, norepinephrine (NE) transporter (NET)- and organic cation transporter 3 (OCT3)-specific primers demonstrated that rat cortical astrocytes and frontal cortex expressed DAT, NET and OCT3 mRNA. Specific [(3)H]DA and [(3)H]NE uptake were each partly inhibited by 1 micro M decynium 22, an extraneuronal monoamine transporter (EMT) and OCT inhibitor. The selective NE uptake inhibitor nisoxetine (0.1 micro M) and the tricyclic antidepressant desipramine (1 micro M) very potently inhibited the specific uptake of both [(3)H]DA and [(3)H]NE in astrocytes. In contrast, 0.1 micro M GBR-12935, a selective and potent DA uptake inhibitor, had no inhibitory activity on the uptake of either compound. These results suggest that cortical astrocytes regulate extracellular DA and NE concentrations through the uptake of DA and NE by the glial NET but not by DAT. Furthermore, an uptake(2) mechanism contributes to DA uptake in cortical astrocytes.

Animals↗
PubMed2006

Organic anion transporting polypeptides of the OATP/SLCO superfamily: identification of new members in nonmammalian species, comparative modeling and a potential transport mode.

Organic anion-transporting polypeptides (human, OATPs; other animals, Oatps; gene symbol, SLCO/Slco) form a transport protein superfamily that mediates the translocation of amphipathic substrates across the plasma membrane of animal cells. So far, OATPs/Oatps have been identified in human, rat and mouse tissues. In this study, we used bioinformatic tools to detect new members of the OATP/SLCO superfamily in nonmammalian species and to build models for the three-dimensional structure of OATPs/Oatps. New OATP/SLCO superfamily members, some of which form distinct novel families, were identified in chicken, zebrafish, frog, fruit fly and worm species. The lack of OATP/SLCO superfamily members in plants, yeast and bacteria suggests the emergence of an ancient Oatp protein in an early ancestor of the animal kingdom. Structural models were generated for the representative members OATP1B3 and OATP2B1 based on the known structures of the major facilitator superfamily of transport proteins. A model was also built for the large extracellular region between transmembrane helices 9 and 10, following the identification of a novel homology with the Kazal-type serine protease inhibitors. Along with the electrostatic potential and the conservation of key amino acid residues, we propose a common transport mechanism for all OATPs/Oatps, whereby substrates are translocated through a central, positively charged pore in a rocker-switch type of mechanism. Several amino acid residues were identified that may play crucial roles in the proposed transport mechanism.

Amino Acid Sequence↗
PubMed2003

Expression of a yeast-derived invertase in companion cells results in long-distance transport of a trisaccharide in an apoplastic loader and influences sucrose transport.

Companion cell-specific expression of a cytosolic invertase from yeast ( Saccharomyces cerevisiae) was used as a tool to synthesise oligosaccharides in the sieve element/companion cell complex and study whether oligosaccharides could be transported in the phloem of an apoplastically loading species. Potato ( Solanum tuberosum L.) plants expressing the invertase under the control of the Agrobacterium tumefaciens rolC promoter produced the trisaccharide 6-kestose in leaves, which was transported via the phloem and accumulated in tubers of transgenic plants. In graft experiments with rolC invertase plants as scion and wild-type rootstocks, 6-kestose accumulated in tubers to levels comparable to sucrose. This shows that long-distance transport of oligosaccharides is possible in apoplastically loading plants, which normally transport only sucrose. The additional transport route for assimilates neither led to elevated photosynthetic activity nor to increased tuber yield. Enhanced sucrose turnover in companion cells caused large amounts of glucose and fructose to be exuded from leaf petioles, and elevated levels of sucrose were detected in phloem exudates. While the latter indicates a higher capacity for sucrose loading into the phloem due to increased metabolic activity of companion cells, the massive release of hexoses catalysed by the invertase seemed to interfere with assimilate delivery to sink organs.

Biological Transport↗