LeProT1, a transporter for proline, glycine betaine, and gamma-amino butyric acid in tomato pollen.
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During maturation, pollen undergoes a period of dehydration accompanied by the accumulation of compatible solutes. Solute import across the pollen plasma membrane, which occurs via proteinaceous transporters, is required to support pollen development and also for subsequent germination and pollen tube growth. Analysis of the free amino acid composition of various tissues in tomato revealed that the proline content in flowers was 60 times higher than in any other organ analyzed. Within the floral organs, proline was confined predominantly to pollen, where it represented >70% of total free amino acids. Uptake experiments demonstrated that mature as well as germinated pollen rapidly take up proline. To identify proline transporters in tomato pollen, we isolated genes homologous to Arabidopsis proline transporters. LeProT1 was specifically expressed both in mature and germinating pollen, as demonstrated by RNA in situ hybridization. Expression in a yeast mutant demonstrated that LeProT1 transports proline and gamma-amino butyric acid with low affinity and glycine betaine with high affinity. Direct uptake and competition studies demonstrate that LeProT1 constitutes a general transporter for compatible solutes. (+info)
Cloning and functional characterization of a new multispecific organic anion transporter, OAT-K2, in rat kidney.
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We have isolated a cDNA coding a new organic anion transporter, OAT-K2, expressed specifically in rat kidney. The OAT-K2 cDNA had an open reading frame encoding a 498-amino acid protein (calculated molecular mass of 55 kDa) that shows 91% identity with the rat kidney-specific organic anion transporter, OAT-K1. Reverse transcription-coupled polymerase chain reaction analyses revealed that the OAT-K2 mRNA was expressed predominantly in the proximal convoluted tubules, proximal straight tubules, and cortical collecting ducts. When expressed in Xenopus oocytes, OAT-K2 stimulated the uptake of hydrophobic organic anions, such as taurocholate, methotrexate, folate, and prostaglandin E2, although its homolog OAT-K1 transported methotrexate and folate, but not taurocholate and prostaglandin E2. In MDCK cells stably transfected with the OAT-K1 and OAT-K2 cDNAs, each transporter was localized functionally to the apical membranes and showed transport activity similar to that in the oocyte. Moreover, the efflux of preloaded taurocholate was also enhanced across the apical membrane in OAT-K2 transfectant. The taurocholate transport by OAT-K2-expressing cells showed saturability (Km = 10.3 microM). Several organic anions, bile acids, cardiac glycosides, and steroids had potent inhibitory effects on the OAT-K2-mediated taurocholate transport in the transfectant. These findings suggest that the OAT-K2 participates in epithelial transport of hydrophobic anionic compounds in the kidney. (+info)
Muscle contractile activity increases fatty acid metabolism and transport and FAT/CD36.
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We have examined whether 1) fatty acid (FA) uptake, 2) FA transporter expression, and 3) FA metabolism are increased when the oxidative capacity of skeletal muscle is increased. The oxidative capacities of red and white tibialis anterior and extensor digitorum longus muscles were increased via chronic stimulation (10 Hz, 24 h/day for 7 days). The contralateral muscles served as controls. After 7 days of increased muscle activity 1) palmitate uptake by giant sarcolemmal vesicles was increased twofold (P < 0.05), 2) the expression of FA translocase (FAT)/CD36 was increased at both the mRNA (3.2- to 10-fold) and protein (3.4-fold) levels, and 3) palmitate oxidation and esterification into triacylglycerols and phospholipids were increased 1.5-, 2.7-, and 1.7-fold, respectively (P < 0.05). These data show that when the oxidative capacity of muscle is increased, there is a parallel increase in the rate of FA transport and FA transporters at the sarcolemmal membrane, which is associated with the enhanced expression of the membrane transporter FAT/CD36. (+info)
Identification of the amine-polyamine-choline transporter superfamily 'consensus amphipathic region' as the target for inactivation of the Escherichia coli GABA transporter GabP by thiol modification reagents. Role of Cys-300 in restoring thiol sensitivity to Gabp lacking Cys.
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The Escherichia coli gamma-aminobutyric acid transporter GabP (gab permease) contains a functionally significant cysteine residue (Cys-300) within its consensus amphipathic region (CAR), a putative channel-forming structure that extends out of transmembrane helix 8 and into the adjoining cytoplasmic loop 8-9 of transporters from the amine-polyamine-choline (APC) superfamily. Here we show that of the five cysteine residues (positions 158, 251, 291, 300 and 443) in the E. coli GabP, Cys-300 is the one that renders the transport activity sensitive to inhibition by thiol modification reagents: whereas substituting Ala for Cys-300 mimics the inhibitory effect of thiol modification, substituting Ala at position 158, 251, 291 or 443 preserves robust transport activity and confers no resistance to thiol inactivation; and whereas the robustly active Cys-300 single-Cys mutant is fully sensitive to thiol modification, other single-Cys mutants (Cys at 158, 251, 291 or 443) exhibit kinetically compromised transport activities that resist further chemical inactivation by thiol reagents. The present study reveals additionally that Cys-300 exhibits (1) sensitivity to hydrophobic thiol reagents, (2) general resistance to bulky (fluorescein 5-maleimide) and/or charged {2-sulphonatoethyl methanethiosulphonate or [2-(trimethylammonium)ethyl] methanethiosulphonate} thiol reagents and (3) a peculiar sensitivity to p-chloromercuribenzenesulphonate (PCMBS). The accessibility of PCMBS to Cys-300 (located midway through the lipid bilayer) might be related to the structural similarity that it shares with guvacine (1, 2,3,6-tetrahydro-3-pyridinecarboxylic acid), a transported GabP substrate. These structural requirements for thiol sensitivity provide the first chemical evidence consistent with channel-like access to the polar surface of the CAR, a physical configuration that might provide a basis for understanding how this region impacts the function of APC transporters generally [Closs, Lyons, Kelly and Cunningham (1993) J. Biol. Chem. 268, 20796-20800] and the gab permease particularly [Hu and King (1998) Biochem. J. 300, 771-776]. (+info)
PDZ-mediated interactions retain the epithelial GABA transporter on the basolateral surface of polarized epithelial cells.
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The PDZ target motifs located in the C-terminal end of many receptors and ion channels mediate protein-protein interactions by binding to specific PDZ-containing proteins. These interactions are involved in the localization of surface proteins on specialized membrane domains of neuronal and epithelial cells. However, the molecular mechanism responsible for this PDZ protein-dependent polarized localization is still unclear. This study first demonstrated that the epithelial gamma-aminobutyric acid (GABA) transporter (BGT-1) contains a PDZ target motif that mediates the interaction with the PDZ protein LIN-7 in Madin-Darby canine kidney (MDCK) cells, and then investigated the role of this interaction in the basolateral localization of the transporter. It was found that although the transporters from which the PDZ target motif was deleted were still targeted to the basolateral surface, they were not retained but internalized in an endosomal recycling compartment. Furthermore, an interfering BGT peptide determined the intracellular relocation of the native transporter. These data indicate that interactions with PDZ proteins determine the polarized surface localization of target proteins by means of retention and not targeting mechanisms. PDZ proteins may, therefore, act as a sort of membrane protein sorting machinery which, by recognizing retention signals (the PDZ target sequences), prevents protein internalization. (+info)
Multiple G protein-coupled receptors initiate protein kinase C redistribution of GABA transporters in hippocampal neurons.
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Neurotransmitter transporters function in synaptic signaling in part through the sequestration and removal of neurotransmitter from the synaptic cleft. A recurring theme of transporters is that many can be functionally regulated by protein kinase C (PKC); some of this regulation occurs via a redistribution of the transporter protein between the plasma membrane and the cytoplasm. The endogenous triggers that lead to PKC-mediated transporter redistribution have not been elucidated. G-protein-coupled receptors that activate PKC are likely candidates to initiate transporter redistribution. We tested this hypothesis by examining the rat brain GABA transporter GAT1 endogenously expressed in hippocampal neurons. Specific agonists of G-protein-coupled acetylcholine, glutamate, and serotonin receptors downregulate GAT1 function. This functional inhibition is dose-dependent, mimicked by PKC activators, and prevented by specific receptor antagonists and PKC inhibitors. Surface biotinylation experiments show that the receptor-mediated functional inhibition correlates with a redistribution of GAT1 from the plasma membrane to intracellular locations. These data demonstrate (1) that endogenous GAT1 function can be regulated by PKC via subcellular redistribution, and (2) that signaling via several different G-protein-coupled receptors can mediate this effect. These results raise the possibility that some effects of G-protein-mediated alterations in synaptic signaling might occur through changes in the number of transporters expressed on the plasma membrane and subsequent effects on synaptic neurotransmitter levels. (+info)
Protein-mediated palmitate uptake and expression of fatty acid transport proteins in heart giant vesicles.
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Giant sarcolemmal vesicles were isolated from rat heart and hindlimb muscles for a) characterization of long-chain fatty acid transport in the absence of metabolism and b) comparison of fatty acid transport protein expression with fatty acid transport. Giant vesicles contained cytosolic fatty acid binding protein. Palmitate uptake was completely divorced from its metabolism. All palmitate taken up was recovered in the intravesicular cytosol as unesterified FA. Palmitate uptake by heart vesicles exhibited a K m of 9.7 nm, similar to that of muscle (K m = 9.7 nm). Vmax (2.7 pmol/mg protein/s) in heart was 8-fold higher than in muscle (0.34 pmol/mg protein/s). Palmitate uptake was inhibited in heart (55-80%) and muscle (31-50%) by trypsin, phloretin, sulfo-N-succinimidyloleate (SSO), or a polyclonal antiserum against the 40 kDa plasma membrane fatty acid binding protein (FABPpm). Palmitate uptake by heart and by red and white muscle vesicles correlated well with the expression of fatty acid translocase (FAT/CD36) and fatty acid binding protein FABPpm, which may act in concert. The expression of fatty acid transport protein (FATP), was 10-fold lower in heart vesicles than in white muscle vesicles. It is concluded that long-chain fatty acid uptake by heart and muscle vesicles is largely protein-mediated, involving FAT/CD36 and FABPpm. The role of FATP in muscle and heart remains uncertain. (+info)
Passive water and ion transport by cotransporters.
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1. The rabbit Na+-glucose (SGLT1) and the human Na+-Cl--GABA (GAT1) cotransporters were expressed in Xenopus laevis oocytes, and passive Na+ and water transport were studied using electrical and optical techniques. Passive water permeabilities (Lp) of the cotransporters were determined from the changes in oocyte volume in response to osmotic gradients. The specific SGLT1 and GAT1 Lp values were obtained by measuring Lp in the presence and absence of blockers (phlorizin and SKF89976A). In the presence of the blockers, the Lp values of oocytes expressing SGLT1 and GAT1 were indistinguishable from the Lp of control oocytes. Passive Na+ transport (Na+ leak) was obtained from the blocker-sensitive Na+ currents in the absence of substrates (glucose and GABA). 2. Passive Na+ and water transport through SGLT1 were blocked by phlorizin with the same sensitivity (inhibitory constant (Ki), 3-5 microM). When Na+ was replaced with Li+, phlorizin also inhibited Li+ and water transport, but with a lower affinity (Ki, 100 microM). When Na+ was replaced by choline, which is not transported, the SGLT1 Lp was indistinguishable from that in Na+ or Li+, but in this case water transport was less sensitive to phlorizin. 3. The activation energies (Ea) for passive Na+ and water transport through SGLT1 were 21 and 5 kcal mol-1, respectively. The high Ea for Na+ transport is comparable to that of Na+-glucose cotransport and indicates that the process is dependent on conformational changes of the protein, while the low Ea for water transport is similar to that of water channels (aquaporins). 4. GAT1 also behaved as an SKF89976A-sensitive water channel. We did not observe passive Na+ transport through GAT1. 5. We conclude that passive water and Na+ transport through cotransporters depend on different mechanisms: Na+ transport occurs by a saturable uniport mechanism, and water permeation is through a low conductance water channel. In the case of SGLT1, we suggest that both the water channel and water cotransport could contribute to isotonic fluid transport across the intestinal brush border membrane. (+info)