NTR1 encodes a high affinity oligopeptide transporter in Arabidopsis

Transport of cytokinins mediated by purine transporters of the PUP family expressed in phloem, hydathodes, and pollen of Arabidopsis

Nucleobases and derivatives like cytokinins and caffeine are translocated in the plant vascular system. Transport studies in cultured Arabidopsis cells indicate that adenine and cytokinin are transported by a common H+-coupled high-affinity purine transport system. Transport properties are similar to that of Arabidopsis purine transporters AtPUP1 and 2. When expressed in yeast, AtPUP1 and 2 mediate energy-dependent high-affinity adenine uptake, whereas AtPUP3 activity was not detectable. Similar to the results from cell cultures, purine permeases (PUP) mediated uptake of adenine can be inhibited by cytokinins, indicating that cytokinins are transport substrates. Direct measurements demonstrate that AtPUP1 is capable of mediating uptake of radiolabeled trans-zeatin. Cytokinin uptake is strongly inhibited by adenine and isopentenyladenine but is poorly inhibited by 6-chloropurine. A number of physiological cytokinins including trans- and cis-zeatin are also efficient competitors for AtPUP2-mediated adenine uptake, suggesting that AtPUP2 is also able to mediate cytokinin transport. Furthermore, AtPUP1 mediates transport of caffeine and ribosylated purine derivatives in yeast. Promoter-reporter gene studies point towards AtPUP1 expression in the epithem of hydathodes and the stigma surface of siliques, suggesting a role in retrieval of cytokinins from xylem sap to prevent loss during guttation. The AtPUP2 promoter drives GUS reporter gene activity in the phloem of Arabidopsis leaves, indicating a role in long-distance transport of adenine and cytokinins. Promoter activity of AtPUP3 was only found in pollen. In summary, three closely related PUPs are differentially expressed in Arabidopsis and at least two PUPs have properties similar to the adenine and cytokinin transport system identified in Arabidopsis cell cultures.

Involvement of CjMDR1, a plant multidrug-resistance-type ATP-binding cassette protein, in alkaloid transport in Coptis japonica

Alkaloids comprise one of the largest groups of plant secondary metabolites. Berberine, a benzylisoquinoline alkaloid, is preferentially accumulated in the rhizome of Coptis japonica, a ranunculaceous plant, whereas gene expression for berberine biosynthetic enzymes has been observed specifically in root tissues, which suggests that berberine synthesized in the root is transported to the rhizome, where there is high accumulation. We recently isolated a cDNA encoding a multidrug-resistance protein (MDR)-type ATP-binding cassette (ABC) transporter (Cjmdr1) from berberine-producing cultured C. japonica cells, which is highly expressed in the rhizome. Functional analysis of Cjmdr1 by using a Xenopus oocyte expression system showed that CjMDR1 transported berberine in an inward direction, resulting in a higher accumulation of berberine in Cjmdr1-injected oocytes than in the control. Typical inhibitors of ABC proteins, such as vanadate, nifedipine, and glibenclamide, as well as ATP depletion, clearly inhibited this CjMDR1-dependent berberine uptake, suggesting that CjMDR1 functioned as an ABC transporter. Conventional membrane separation methods showed that CjMDR1 was localized in the plasma membrane of C. japonica cells. In situ hybridization indicated that Cjmdr1 mRNA was expressed preferentially in xylem tissues of the rhizome. These findings strongly suggest that CjMDR1 is involved in the translocation of berberine from the root to the rhizome.

Identification of an Arabidopsis mitochondrial succinate-fumarate translocator

Complementation of a yeast acr1 mutant carrying a deletion of the succinate/fumarate carrier gene enabled functional identification of a mitochondrial succinate translocator from Arabidopsis thaliana (AtmSFC1). Thus complementation of yeast mutants is applicable also for identification and characterization of organellar transporters. Reverse transcription polymerase chain reaction and promoter-GUS fusion showed expression of AtmSFC1 in 2 day old dark grown seedlings, which declined in cotyledons during further development, consistent with a role in export of fumarate for gluconeogenesis during lipid mobilization at early germination of Arabidopsis seeds. In mature plants, expression was found in developing and germinating pollen, suggesting a role in ethanolic fermentation.

Expression pattern of a nuclear encoded mitochondrial arginine-ornithine translocator gene from Arabidopsis

BACKGROUND

Arginine and citrulline serve as nitrogen storage forms, but are also involved in biosynthetic and catabolic pathways. Metabolism of arginine, citrulline and ornithine is distributed between mitochondria and cytosol. For the shuttle of intermediates between cytosol and mitochondria transporters present on the inner mitochondrial membrane are required. Yeast contains a mitochondrial translocator for ornithine and arginine, Ort1p/Arg11p. Ort1p/Arg11p is a member of the mitochondrial carrier family (MCF) essential for ornithine export from mitochondria. The yeast arg11 mutant, which is deficient in Ort1p/Arg11p grows poorly on media lacking arginine.

RESULTS

High-level expression of a nuclear encoded Arabidopsis thaliana homolog (AtmBAC2) of Ort1p/Arg11p was able to suppress the growth deficiency of arg11. RT-PCR analysis demonstrated expression of AtmBAC2 in all tissues with highest levels in flowers. Promoter-GUS fusions showed preferential expression in flowers, i.e. pollen, in the vasculature of siliques and in aborted seeds. Variable expression was observed in leaf vasculature. Induction of the promoter was not observed during the first two weeks in seedlings grown on media containing NH4NO3, arginine or ornithine as sole nitrogen sources.

CONCLUSION

AtmBAC2 was isolated as a mitochondrial transporter for arginine in Arabidopsis. The absence of expression in developing seeds and in cotyledons of seedlings indicates that other transporters are responsible for storage and mobilization of arginine in seeds.

Molecular cloning of columbamine O-methyltransferase from cultured Coptis japonica cells

To identify all of the O-methyltransferase genes involved in isoquinoline alkaloid biosynthesis in Coptis japonica cells, we sequenced 1014 cDNA clones isolated from high-alkaloid-producing cultured cells of C. japonica. Among them, we found all three reported O-methyltransferases and an O-methyltransferase-like cDNA clone (CJEST64). This cDNA was quite similar to S-adenosyl-l-methionine:coclaurine 6-O-methyltransferase and S-adenosyl-l-methionine:isoflavone 7-O-methyltransferase. As S-adenosyl-l-methionine:columbamine O-methyltransferase, which catalyzes the conversion of columbamine to palmatine, is one of the remaining unelucidated components in isoquinoline alkaloid biosynthesis in C. japonica, we heterologously expressed the protein in Escherichia coli and examined the activity of columbamine O-methyltransferase. The recombinant protein clearly showed O-methylation activity using columbamine, as well as (S)-tetrahydrocolumbamine, (S)-, (R,S)-scoulerine and (R,S)-2,3,9,10-tetrahydroxyprotoberberine as substrates. This result clearly indicated that EST analysis was useful for isolating the candidate gene in a relatively well-characterized biosynthetic pathway. The relationship between the structure and substrate recognition of the O-methyltransferases involved in isoquinoline alkaloid biosynthesis, and a reconsideration of the biosynthetic pathway to palmatine are discussed.

Visualization of maltose uptake in living yeast cells by fluorescent nanosensors

Compartmentation of metabolic reactions and thus transport within and between cells can be understood only if we know subcellular distribution based on nondestructive dynamic monitoring. Currently, methods are not available for in vivo metabolite imaging at cellular or subcellular levels. Limited information derives from methods requiring fixation or fractionation of tissue (1, 2). We thus developed a flexible strategy for designing protein-based nanosensors for a wide spectrum of solutes, allowing analysis of changes in solute concentration in living cells. We made use of bacterial periplasmic binding proteins (PBPs), where we show that, on binding of the substrate, PBPs transform their hinge-bend movement into increased fluorescence resonance energy transfer (FRET) between two coupled green fluorescent proteins. By using the maltose-binding protein as a prototype, nanosensors were constructed allowing in vitro determination of FRET changes in a concentration-dependent fashion. For physiological applications, mutants with different binding affinities were generated, allowing dynamic in vivo imaging of the increase in cytosolic maltose concentration in single yeast cells. Control sensors allow the exclusion of the effect from other cellular or environmental parameters on ratio imaging. Thus the myriad of PBPs recognizing a wide spectrum of different substrates is suitable for FRET-based in vivo detection, providing numerous scientific, medical, and environmental applications.

Reconstitution in yeast of the Arabidopsis SOS signaling pathway for Na+ homeostasis

The Arabidopsis thaliana SOS1 protein is a putative Na+/H+ antiporter that functions in Na+ extrusion and is essential for the NaCl tolerance of plants. sos1 mutant plants share phenotypic similarities with mutants lacking the protein kinase SOS2 and the Ca2+ sensor SOS3. To investigate whether the three SOS proteins function in the same response pathway, we have reconstituted the SOS system in yeast cells. Expression of SOS1 improved the Na+ tolerance of yeast mutants lacking endogenous Na+ transporters. Coexpression of SOS2 and SOS3 dramatically increased SOS1-dependent Na+ tolerance, whereas SOS2 or SOS3 individually had no effect. The SOS2/SOS3 kinase complex promoted the phosphorylation of SOS1. A constitutively active form of SOS2 phosphorylated SOS1 in vitro independently of SOS3, but could not fully substitute for the SOS2/SOS3 kinase complex for activation of SOS1 in vivo. Further, we show that SOS3 recruits SOS2 to the plasma membrane. Although sos1 mutant plants display defective K+ uptake at low external concentrations, neither the unmodified nor the SOS2/SOS3-activated SOS1 protein showed K+ transport capacity in vivo, suggesting that the role of SOS1 on K+ uptake is indirect. Our results provide an example of functional reconstitution of a plant response pathway in a heterologous system and demonstrate that the SOS1 ion transporter, the SOS2 protein kinase, and its associated Ca2+ sensor SOS3 constitute a functional module. We propose a model in which SOS3 activates and directs SOS2 to the plasma membrane for the stimulatory phosphorylation of the Na+ transporter SOS1.

Differential expression and function of Arabidopsis thaliana NHX Na+/H+ antiporters in the salt stress response

The Arabidopsis thaliana vacuolar Na+/H+ antiporter AtNHX1 is a salt tolerance determinant. Predicted amino acid sequence similarity, protein topology and the presence of functional domains conserved in AtNHX1 and prototypical mammalian NHE Na+/H+ exchangers led to the identification of five additional AtNHX genes (AtNHX2-6). The AtNHX1 and AtNHX2 mRNAs are the most prevalent transcripts among this family of genes in seedling shoots and roots. A lower-abundance AtNHX5 mRNA is present in both shoots and roots, whereas AtNHX3 transcript is expressed predominantly in roots. AtNHX4 and AtNHX6 mRNAs were detected only by RT-PCR. AtNHX1, 2 or 5 suppress, with differential efficacy, the Na+/Li+-sensitive phenotype of a yeast mutant that is deficient in the endosomal/vacuolar Na+/H+ antiporter ScNHX1. Ion accumulation data indicate that these AtNHX proteins function to facilitate Na+ ion compartmentalization and maintain intracellular K+ status. Seedling steady-state mRNA levels of AtNHX1 and AtNHX2 increase similarly after treatment with NaCl, an equi-osmolar concentration of sorbitol, or ABA, whereas AtNHX5 transcript abundance increases only in response to salt treatment. Hyper-osmotic up-regulation of AtNHX1, 2 or 5 expression is not dependent on the SOS pathway that controls ion homeostasis. However, steady-state AtNHX1, 2 and 5 transcript abundance is greater in sos1, sos2 and sos3 plants growing in medium that is not supplemented with sorbitol or NaCl, providing evidence that transcription of these genes is negatively affected by the SOS pathway in the absence of stress. AtNHX1 and AtNHX2 transcripts accumulate in response to ABA but not to NaCl in the aba2-1, mutant indicating that the osmotic responsiveness of these genes is ABA-dependent. An as yet undefined stress signal pathway that is ABA- and SOS-independent apparently controls transcriptional up-regulation of AtNHX5 expression by hyper-saline shock. Similar to AtNHX1, AtNHX2 is localized to the tonoplast of plant cells. Together, these results implicate AtNHX2 and 5, together with AtNHX1, as salt tolerance determinants, and indicate that AtNHX2 has a major function in vacuolar compartmentalization of Na+.

Peptide transport in plants

Recent completion of the Arabidopsis genome revealed that this organism has ten times more peptide transporters than any other sequenced organism (prokaryote or eukaryote). These transporters are found in three protein families: the ABC-type transporters; the di- and tripeptide transporters; and the newly described tetra- and pentapeptide oligopetide transporters. The abundance of these transporters suggests that they play diverse and important roles in plant growth and development. Possible substrates for these transporters include glutathione, gamma-glutamyl peptides, hormone-amino acid conjugates, phytosulfokine, peptide-like compounds and peptide phytotoxins. However, the exact role of peptide transport in plants is still undefined.

Intra- and intermolecular interactions in sucrose transporters at the plasma membrane detected by the split-ubiquitin system and functional assays

Interaction of two separately expressed halves of sucrose transporter SUT1 was detected by an optimized split-ubiquitin system. The halves reconstitute sucrose transport activity at the plasma membrane with affinities similar to the intact protein. The halves do not function independently, and an intact central loop is not required for membrane insertion, plasma membrane targeting, and transport. Under native conditions, the halves associate into higher molecular mass complexes. Furthermore, the N-terminal half of the low-affinity SUT2 interacts functionally with the C-terminal half of SUT1. Since the N terminus of SUT2 determines affinity for sucrose, the reconstituted chimera has lower affinity than SUT1. The split-ubiquitin system efficiently detects intramolecular interactions in membrane proteins, and can be used to dissect transporter structure.

Potassium- or sodium-efflux ATPase, a key enzyme in the evolution of fungi

Potassium is the most abundant cation in cells. Therefore, plant-associated fungi and intracellular parasites are permanently or circumstantially exposed to high K(+) and must avoid excessive K(+) accumulation activating K(+) efflux systems. Because high K(+) and high pH are compatible in natural environments, free-living organisms cannot keep a permanent transmembrane DeltapH and cannot rely only on K(+)/H(+) antiporters, as do mitochondria. This study shows that the Schizosaccharomyces pombe CTA3 is a K(+)-efflux ATPase, and that other fungi are furnished with Na(+)-efflux ATPases, which also pump Na(+). All these fungal ATPases, including those pumping only Na(+), form a phylogenetic group, IID or ENA, among P-type ATPases. By searching in databases and partial cloning of ENA genes in species of Zygomycetes and Basidiomycetes, the authors conclude that probably all fungi have these genes. This study indicates that fungal K(+)- or Na(+)-ATPases evolved from an ancestral K(+)-ATPase, through processes of gene duplication. In yeast hemiascomycetes these duplications have occurred recently and produced bifunctional ATPases, whereas in Neurospora, and probably in other euascomycetes, they occurred earlier in evolution and produced specialized ATPases. In Schizosaccharomyces, adaptation to Na(+) did not involve the duplication of the K(+)-ATPase and thus it retains an enzyme which is probably close to the original one. The parasites Leishmania and Trypanosoma have ATPases phylogenetically related to fungal K(+)-ATPases, which are probably functional homologues of the fungal enzymes.

Geranyl diphosphate:4-hydroxybenzoate geranyltransferase from Lithospermum erythrorhizon. Cloning and characterization of a ket enzyme in shikonin biosynthesis

Two cDNAs encoding geranyl diphosphate:4-hy- droxybenzoate 3-geranyltransferase were isolated from Lithospermum erythrorhizon by nested PCR using the conserved amino acid sequences among polyprenyl- transferases for ubiquinone biosynthesis. They were functionally expressed in yeast COQ2 disruptant and showed a strict substrate specificity for geranyl diphosphate as the prenyl donor, in contrast to ubiquinone biosynthetic enzymes, suggesting that they are involved in the biosynthesis of shikonin, a naphthoquinone secondary metabolite. Regulation of their expression by various culture conditions coincided with that of geranyltransferase activity and the secondary metabolites biosynthesized via this enzyme. This is the first established plant prenyltransferase that transfers the prenyl chain to an aromatic substrate.

The arabidopsis Na+/H+ exchanger AtNHX1 catalyzes low affinity Na+ and K+ transport in reconstituted liposomes

In saline environments, plants accumulate Na(+) in vacuoles through the activity of tonoplast Na(+)/H(+) antiporters. The first gene for a putative plant vacuolar Na(+)/H(+) antiporter, AtNHX1, was isolated from Arabidopsis and shown to increase plant tolerance to NaCl. However, AtNHX1 mRNA was up-regulated by Na(+) or K(+) salts in plants and substituted for the homologous protein of yeast to restore tolerance to several toxic cations. To study the ion selectivity of the AtNHX1 protein, we have purified a histidine-tagged version of the protein from yeast microsomes by Ni(2+) affinity chromatography, reconstituted the protein into lipid vesicles, and measured cation-dependent H(+) exchange with the fluorescent pH indicator pyranine. The protein catalyzed Na(+) and K(+) transport with similar affinity in the presence of a pH gradient. Li(+) and Cs(+) ions were also transported with lower affinity. Ion exchange by AtNHX1 was inhibited 70% by the amiloride analog ethylisopropyl-amiloride. Our data indicate a role for intracellular antiporters in organelle pH control and osmoregulation.

Molecular cloning and characterization of coclaurine N-methyltransferase from cultured cells of Coptis japonica

S-adenosyl-L-methionine:coclaurine N-methyltransferase (CNMT) converts coclaurine to N-methylcoclaurine in isoquinoline alkaloid biosynthesis. The N-terminal amino acid sequence of Coptis CNMT was used to amplify the corresponding cDNA fragment and later to isolate full-length cDNA using 5'- and 3'-rapid amplification of cDNA ends (RACE). The nucleotide sequence and predicted amino acid sequence showed that the cDNA encoded 358 amino acids, which contained a putative S-adenosyl-L-methionine binding domain and showed relatively high homology to tomato phosphoethanolamine-N-methyltransferase. A recombinant protein was expressed in Escherichia coli, and its CNMT activity was confirmed. Recombinant CNMT was purified to homogeneity, and enzymological characterization confirmed that Coptis CNMT has quite broad substrate specificity, i.e. not only for 6-O-methylnorlaudanosoline and norreticuline but also for 6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline. The evolution of N-methyltransferases in secondary metabolism is discussed based on sequence similarity.

Characterization of a developmentally regulated amino acid transporter (AAT1p) of the rust fungus Uromyces fabae

summary In the rust fungus Uromyces fabae, invasion of the host plant and haustorium formation are accompanied by the activation of many genes (PIGs =in planta induced genes). In addition to the previously described AAT2 (PIG2), AAT1 (PIG27) was found to encode a protein with a high similarity to fungal amino acid permeases. AAT1 transcripts are present in germinated hyphae and throughout the mycelium later in the infection process, but occur at the highest levels in haustoria. Expression of AAT1p in a histidine uptake-defective yeast mutant revealed energy-dependent transport of (14)C-histidine, with a K(M) value of 25.8 microm. In addition, complementation analysis revealed AAT1-dependent transport for lysine. Using Xenopus oocytes as expression system, AAT1p-dependent symport of protons with a broad spectrum of amino acids was observed, with the highest activities obtained with histidine and lysine. These results confirm that in rust fungi, the expression of amino acid transporters is developmentally regulated and occurs preferentially in the parasitic phase of development.

Molecular characterization of two ammonium transporters from the ectomycorrhizal fungus Hebeloma cylindrosporum

Heterologous expression of the yeast triple Mep mutant has enabled the first molecular characterization of AMT/MEP family members in an ectomycorrhizal fungus. External hyphae, which play a key role in nitrogen nutrition of trees, are considered as the absorbing structure of the ectomycorrhizal symbiosis and therefore molecular studies on ammonium transport in hyphae are urgently needed. The kinetic properties of AMT2 and AMT3 from Hebeloma cylindrosporum were studied in Saccharomyces cerevisiae. Expression of HcAmts in the yeast triple Mep mutant restored ammonium retention within cells. The HcAmts did not complement the ammonium sensing defect phenotype of Mep2Delta cells during pseudohyphal differentiation. Northern blot analysis in H. cylindrosporum showed that the HcAMTs were up-regulated upon nitrogen deprivation and down-regulated by ammonium.

The role of haustoria in sugar supply during infection of broad bean by the rust fungus Uromyces fabae

Biotrophic plant pathogenic fungi differentiate specialized infection structures within the living cells of their host plants. These haustoria have been linked to nutrient uptake ever since their discovery. We have for the first time to our knowledge shown that the flow of sugars from the host Vicia faba to the rust fungus Uromyces fabae seems to occur largely through the haustorial complex. One of the most abundantly expressed genes in rust haustoria, the expression of which is negligible in other fungal structures, codes for a hexose transporter. Functional expression of the gene termed HXT1 in Saccharomyces cerevisiae and Xenopus laevis oocytes assigned a substrate specificity for D-glucose and D-fructose and indicated a proton symport mechanism. Abs against HXT1p exclusively labeled haustoria in immunofluorescence microscopy and the haustorial plasma membrane in electron microscopy. These results suggest that the fungus concentrates this transporter in haustoria to take advantage of a specialized compartment of the haustorial complex. The extrahaustorial matrix, delimited by the plasma membranes of both host and parasite, constitutes a newly formed apoplastic compartment with qualities distinct from those of the bulk apoplast. This organization might facilitate the competition of the parasite with natural sink organs of the host.

TRANSPORTERS RESPONSIBLE FOR THE UPTAKE AND PARTITIONING OF NITROGENOUS SOLUTES

The acquisition and allocation of nitrogenous compounds are essential processes in plant growth and development. The huge economic and environmental costs resulting from the application of nitrogen fertilizers make this topic very important. A diverse array of transporters varying in their expression pattern and also in their affinity, specificity, and capacity for nitrogenous compounds has been identified. Now the future challenge is to define their individual contribution to nitrogen nutrition and signalling processes. Here we have reviewed recent advances in the identification and molecular characterization of these transporters, concentrating on mechanisms existing at the plasma membrane. The review focuses on nitrate, ammonium, and amino acid transporter familes, but we also briefly describe what is known at the molecular level about peptide transporters and a recently identified family implicated in the transport of purines and their derivatives.

Cloning of a lymphatic peptide/histidine transporter

Although peptide transport across the plasma membrane has been characterized well in the kidney and the intestine, the functional relevance of this transport in other organs has not been addressed. Here we report the cloning of a cDNA for a novel peptide/histidine transporter found in the rat (rPHT2), whose mRNA is expressed mainly in the lymphatic system. rPHT2 encodes a protein of 582 amino acids and showed 49% identity with the brain PHT (PHT1) [Yamashita, Shimada, Guo, Sato, Kohmura, Hayakawa, Takagi and Tohyama (1997) J. Biol. Chem. 272, 10205-10211]. rPHT2 mRNA was abundant in lung, spleen and thymus, and detected faintly in brain, liver, adrenal gland and heart by Northern-blot analysis and reverse transcriptase PCR (RT-PCR). Intense signals for the gene were found in immunocytes using in situ hybridization. Ectopic expression of rPHT2 protein in HEK-293T cells and BHK cells was not found on the cell surface, but was found on the lysosomal membrane using light- and electron-microscopic analysis. Recombinant rPHT2 protein reconstituted into liposomes showed proton-dependent transport activity with histidine and histidyl-leucine. These findings suggest that rPHT2 is involved in the protein catabolic pathway in the lymphatic system.

Functional identification of the glycerol permease activity of Arabidopsis thaliana NLM1 and NLM2 proteins by heterologous expression in Saccharomyces cerevisiae

NLM proteins (NOD26-like major intrinsic proteins) from plants contain amino acid sequence signatures which can be found in aquaporins including plant plasma membrane intrinsic proteins and tonoplast intrinsic proteins and glycerol permeases such as the Escherichia coli GlpF and the yeast FPS1 proteins. Heterologous expression of two members of the NLM subgroup from Arabidopsis thaliana (AtNLM1 and AtNLM2) in baker's yeast demonstrated the glycerol permease activity in addition to the previously described aquaporin activity of AtNLM1. The transport was non-saturable up to 100 mM extracellular glycerol concentration. Longer-chain sugar alcohols did not compete with the transport of radiolabelled glycerol and hexoses were also not transported through the pore.

Regulation of sugar, amino acid and peptide plant membrane transporters

During the past few years, various cDNAs encoding the proton cotransporters which mediate the uptake of sucrose, hexoses, amino acids and peptides across the plant plasma membrane have been cloned. This has made possible some preliminary insight into the regulation of the activity of these transporters at various levels. The paper summarises the present status of knowledge and gaps relative to their transcriptional control (organ, tissue and cell specificity, response to the environment) and post-transcriptional control (targeting and turnover, kinetic and thermodynamic control, lipidic environment, phosphorylation). This outline and the description of a few cases (the sink/source transition of the leaf, the pollen grain, the legume seed) serve as a basis for suggesting some directions for future research.

Molecular cloning of the calcium and sodium ATPases in Neurospora crassa

Using PCR, reverse transcription-PCR (RT-PCR) and colony hybridization in a genomic library, we isolated six genes which encode type II P-type ATPases in Neurospora crassa. The six full-length cDNAs were cloned in a yeast expression vector and transformed into Saccharomyces cerevisiae null Ca2+- or Na+-ATPase mutants. Three cDNAs suppressed the defect of the Ca2+ mutant and two of these protected from Mn2+ toxicity. One cDNA suppressed the defect of the Na+ mutant and two cDNAs were not functional in S. cerevisiae. The expression of the transcripts of the six genes in the presence of Ca2+, Na+, high pH or supporting an osmotic shock indicated that, with the exception of one of the Ca2+-ATPases, the main function of the cloned ATPases is the adaptation to stress conditions. The relationship between the cloned fungal Ca2+- and Na+-ATPases and plant type II P-ATPases is discussed.

Cloning and functional characterization of a constitutively expressed nitrate transporter gene, OsNRT1, from rice

Elucidating how rice (Oryza sativa) takes up nitrate at the molecular level could help improve the low recovery rate (<50%) of nitrogen fertilizer in rice paddies. As a first step toward that goal, we have cloned a nitrate transporter gene from rice called OsNRT1. OsNRT1 is a new member of a growing transporter family called PTR, which consists not only of nitrate transporters from higher plants that are homologs of the Arabidopsis CHL1 (AtNRT1) protein, but also peptide transporters from a wide variety of genera including animals, plants, fungi, and bacteria. However, despite the fact that OsNRT1 shares a higher degree of sequence identity with the two peptide transporters from plants (approximately 50%) than with the nitrate transporters (approximately 40%) of the PTR family, no peptide transport activity was observed when OsNRT1 was expressed in either Xenopus oocytes or yeast. Furthermore, contrasting the dual-affinity nitrate transport activity of CHL1, OsNRT1 displayed only low-affinity nitrate transport activity in Xenopus oocytes, with a K(m) value of approximately 9 mM. Northern-blot and in situ hybridization analysis indicated that OsNRT1 is constitutively expressed in the most external layer of the root, epidermis and root hair. These data strongly indicate that OsNRT1 encodes a constitutive component of a low-affinity nitrate uptake system for rice.

Intestinal peptide transport systems and oral drug availability

The intestinal peptide transport system has broad substrate specificities. In addition to its physiological function of absorbing di- and tripeptides resulting from the digestion of dietary proteins, this transport system also absorbs some orally administered peptidomimetic drugs, including beta-lactam antibiotics, angiotensin converting enzyme inhibitors, renin inhibitors, bestatin, thrombin inhibitors, and thyrotropin-releasing hormone and its analogues. There have been several studies on the mechanism and substrate structure-affinity relationship for this transport system. Rapid progress has been made recently in studies on the molecular basis of the intestinal peptide transport system. A protein apparently involved in peptide transport has been isolated from rabbit small intestines, and genes for human intestinal peptide transporters have been cloned, sequenced and functionally expressed. This review summarizes these studies and addresses the pharmaceutical potential of the intestinal peptide transport system.

Functional characterization and expression analysis of the amino acid permease RcAAP3 from castor bean

A polymerase chain reaction-based library screening procedure was used to isolate RcAAP3, an amino acid permease cDNA from castor bean (Ricinus communis). RcAAP3 is 1.7 kb in length, with an open reading frame that encodes a protein with a calculated molecular mass of 51 kD. Hydropathy analysis indicates that the RcAAP3 protein is highly hydrophobic in nature with nine to 11 putative transmembrane domains. RcAAP3-mediated uptake of citrulline in a yeast transport mutant showed saturable kinetics with a K(m) of 0.4 mM. Transport was higher at acidic pH and was inhibited by the protonophore carbonylcyanide-m-chlorophenylhydrazone, suggesting a proton-coupled transport mechanism. Citrulline uptake was strongly inhibited (72%) by the permeable sulfydryl reagent N-ethylmaleimide, but showed lower sensitivity (30% inhibition) to the nonpermeable reagent p-chloromercuribenzenesulfonic acid. Diethylpyrocarbonate, a histidine modifier, inhibited citrulline uptake by 80%. A range of amino acids inhibited citrulline uptake, suggesting that RcAAP3 may be a broad substrate permease that can transport neutral and basic amino acids with a lower affinity for acidic amino acids. Northern analysis indicated that RcAAP3 is widely expressed in source and sink tissues of castor bean, and that the pattern of expression is distinct from RcAAP1 and RcAAP2.

Identification of a pollen-specific sucrose transporter-like protein NtSUT3 from tobacco

Pollen cells are symplasmically isolated during maturation and germination. Pollen therefore needs to take up nutrients via membrane carriers. Physiological measurements on pollen indicate sucrose transport in the pollen tube. A cDNA encoding a pollen-specific sucrose transporter-like protein NtSUT3 was isolated from a tobacco pollen cDNA library. NtSUT3 expression is detected only in pollen and is restricted to late pollen development, pollen germination and pollen tube growth. Altogether these data indicate that pollen is supplied not only with glucose, but also with sucrose through a specific sucrose transporter. The respective contribution of each transport pathway may change during pollen tube growth.

Transporters for ammonium, amino acids and peptides are expressed in pitchers of the carnivorous plant Nepenthes

Insect capture and digestion contribute substantially to the nitrogen budget of carnivorous plants. In Nepenthes, insect-derived nitrogenous compounds are imported from the pitcher fluid and transported throughout the plant via the vascular tissue to support growth. Import and distribution of nutrients may require transmembrane nitrogen transporters. Representatives of three classes of genes encoding transporters for the nitrogenous compounds ammonium, amino acids and peptides were identified in Nepenthes pitchers. The expression at the cellular level of an ammonium transporter gene, three amino acid transporter genes, and one peptide transporter gene were investigated in the insect trapping organs of Nepenthes. Expression of the ammonium transporter gene NaAMT1 was detected in the head cells of digestive glands in the lower part of the pitcher where NaAMT1 may function in ammonium uptake from the pitcher fluid. One amino acid transporter gene, NaAAP1, was expressed in bundle sheath cells surrounding the vascular tissue. To understand the locations where transmembrane transport could be required within the pitcher, symplasmic and apoplasmic continuity was probed using fluorescent dyes. Symplasmic connections were not found between cortical cells and vascular bundles. Therefore, the amino acid transporter encoded by NaAAP1 may be involved in transport of amino acids into the vascular tissue. In contrast, expression of the peptide transporter gene NaNTR1 was detected in phloem cells of the vascular tissue within pitchers. NaNTR1 may function in the export of nitrogen from the pitcher by loading peptides into the phloem.

Cloning of a cDNA coding for an amino acid carrier from Ricinus communis (RcAAP1) by functional complementation in yeast: kinetic analysis, inhibitor sensitivity and substrate specificity

A cDNA for the amino acid permease gene RcAAP1 has been isolated from Ricinus communis by yeast complementation and subjected to a detailed kinetic analysis. RcAAP1 cDNA is 1.5 kb with an open reading frame that codes for a protein with 486 amino acids and a calculated molecular mass of 53.1 kDa. RcAAP1-mediated histidine uptake was pH dependent with highest transport rates at acidic pH; it was sensitive to protonophores and uncouplers and the Km for histidine uptake was 96 microM. The substrate specificity was investigated by measuring the levels of inhibition of histidine uptake by a range of amino acids. The basic amino acids (histidine, lysine and arginine) showed strongest inhibition of uptake whereas acidic amino acids competed less effectively. Alanine was the most efficient competitor of the neutral amino acids. Glutamine, serine, asparagine, methionine and cysteine showed moderate inhibition whereas threonine, isoleucine, leucine, phenylalanine, tyrosine and tryptophan showed only low levels of inhibition. Glycine, proline and citrulline caused slight stimulation. More detailed competition kinetics indicated that both lysine and arginine showed simple competitive inhibition of histidine uptake. When direct uptake measurements were carried out, both lysine and arginine were found to be effective substrates for RcAAP1.

Cloning and functional characterisation of a peptide transporter expressed in the scutellum of barley grain during the early stages of germination

A peptide transport gene (HvPTR1) expressed in the scutella of germinating barley grain has been cloned by an RT-PCR approach. Sequence analysis of the full length cDNA (2260 bp) revealed an open reading frame encoding a 579 amino acid protein of predicted molecular mass 63 kDa, which displayed 58% identity to the Arabidopsis thaliana peptide transporter AtPTR2-B. Expression of HvPTR1 in Xenopus laevis oocytes conferred a 48-fold increase in alanyl-[14C]phenylalanine uptake relative to water injected oocytes, confirming the function of HvPTR1 as a peptide transporter. HvPTR1 expression was detectable only in the scutellum of the germinating barley grain, with no transcript found in roots, shoots or the embryo axis. Transcript levels increased rapidly from 6 to 24 h imbibition, correlating with the development of peptide transport activity in the barley scutellum. Peptide transport provides a significant source of organic nitrogen to the barley embryo for use in germination and growth processes associated with the early stages of seedling development. The temporal and spatial pattern of HvPTR1 expression is consistent with a central role for HvPTR1 in the transport of peptides in the germinating barley grain.

The plasma membrane H(+)-ATPase from the biotrophic rust fungus Uromyces fabae: molecular characterization of the gene (PMA1) and functional expression of the enzyme in yeast

To study the molecular basis of biotrophic nutrient uptake by plant parasitic rust fungi, the gene (Uf-PMA1) encoding the plasma membrane H(+)-ATPase from Uromyces fabae was isolated. Uf-PMA1 exists probably as a single gene. However, two nearly identical sequences were identified; the similarity apparently is due to two Uf-PMA1 alleles in the dikaryotic hyphae. Multiple Uf-PMA1 transcripts were observed during early rust development, and reduced amounts of a single Uf-PMA1 mRNA were observed in haustoria and rust-infected leaves. This is in contrast to elevated enzyme activity in haustoria compared to germinated spores (C. Struck, M. Hahn, and K. Mendgen. Fungal Genet. Biol. 20:30-35, 1996). Unexpectedly, the PMA1-encoded rust protein is more similar to H(+)-ATPases from plants (55% identity) than from ascomycetous fungi (36% identity). When the rust PMA1 cDNA was expressed in Saccharomyces cerevisiae, both the wild-type enzyme and a mutant derivative (delta 76) deleted for the 76 C-terminal amino acids were able to support growth of a yeast strain lacking its own H(+)-ATPases. Compared to the wild-type, the delta 76 mutant enzyme displayed increased affinity to ATP, a higher vanadate sensitivity, and a more alkaline pH optimum. These results indicate that the C-terminal region of the rust enzyme exhibits auto-regulatory properties.

Cloning and functional characterization of a Brassica napus transporter that is able to transport nitrate and histidine

A full-length cDNA for a membrane transporter was isolated from Brassica napus by its sequence homology to a previously cloned Arabidopsis low affinity nitrate transporter. The cDNA encodes a predicted protein of 589 amino acid residues with 12 putative transmembrane domains. The transporter belongs to a multigene family with members that have been identified in bacteria, fungi, plants, and animals and that are able to transport a range of different nitrogen-containing substrates, including amino acids, peptides, and nitrate. To identify the substrates of this plant gene, we have expressed the protein in Xenopus oocytes. The properties of the transporter are consistent with a proton cotransport mechanism for nitrate, and the voltage dependence of the Km for nitrate was determined. The Km for nitrate was shown to increase from 4 to 14 mM as the membrane voltage became more negative from -40 to -180 mV. Oocytes expressing the gene could accumulate internal nitrate to concentrations higher than those measured in water-injected controls. A range of different substrate molecules for the transporter was tested, but of these, histidine gave the largest currents, although the affinity was in the millimolar range. The pH dependence of the activity of the transporter was different for the substrates, with histidine transport favored at alkaline and nitrate at acid external pH. Kinetic analysis of the mechanism of histidine transport suggests a cotransport of protons and the neutral form of the amino acid, with the Km for histidine decreasing at more negative membrane voltages. This gene is the first member of this family of transporters for which the transport of two very different types of substrate, nitrate and histidine, has been demonstrated.

Regulation of a plant plasma membrane sucrose transporter by phosphorylation

The protein phosphatase inhibitor okadaic acid (OA) either provided directly to sugar beet (Beta vulgaris L.) leaf discs or infiltrated in the leaf blade rapidly inhibited sucrose uptake. Methyl okadaic acid, a biologically inactive analogue of OA, had only a marginal effect on uptake. OA inhibited proton-motive force-driven uptake of sucrose into plasma membrane vesicles, without affecting their proton permeability. OA did not significantly affect the amount of sucrose transporters present in the vesicles, as estimated by ELISA with specific antibodies. It is concluded that OA directly inhibits the activity of a H+-sucrose cotransporter of the plant plasma membrane, likely by maintaining it in a phosphorylated form.

Molecular and structural features of the proton-coupled oligopeptide transporter superfamily

Work in the area of molecular biology of transport proteins has unveiled the presence of a distinct peptide transporter superfamily whose members extend from the prokaryotic to the eukaryotic kingdom. There are two subgroups within this superfamily, one subgroup harnessing the energy necessary for active transport from a transmembrane H+ gradient and the other subgroup relying directly on ATP hydrolysis. In addition to the use of different driving forces, the two subgroups are also distinguishable with regard to molecular structure and operational mechanism. This review is intended to analyze critically the molecular nature of the members of the H+ gradient-dependent peptide transporter subgroup, with emphasis on the cloning strategies utilized in the isolation of the individual transporter cDNAs or genes; on the structural patterns, motifs, and conserved amino acid residues common to constituent members of the subgroup; and on the characteristic topological features of the individual members.

NO3--induced pH changes in mammalian cells. Evidence for an NO3--H+ cotransporter

The effect of NO3- on intracellular pH (pHi) was assessed microfluorimetrically in mammalian cells in culture. In cells of human, hamster, and murine origin addition of extracellular NO3- induced an intracellular acidification. This acidification was eliminated when the cytosolic pH was clamped using ionophores or by perfusing the cytosol with highly buffered solutions using patch-pipettes, ruling out spectroscopic artifacts. The NO3-- induced pH change was not due to modulation of Na+/H+ exchange, since it was also observed in Na+/H+ antiport-deficient mutants. Though NO3- is known to inhibit vacuolar-type (V) H+-ATPases, this effect was not responsible for the acidification since it persisted in the presence of the potent V-ATPase inhibitor bafilomycin A1. NO3-/HCO3- exchange as the underlying mechanism was ruled out because acidification occurred despite nominal removal of HCO3-, despite inhibition of the anion exchanger with disulfonic stilbenes and in HEK 293 cells, which seemingly lack anion exchangers (Lee, B. S., R.B. Gunn, and R.R. Kopito. 1991. J. Biol. Chem. 266:11448- 11454). Accumulation of intracellular NO3-, measured by the Greiss method after reduction to NO2-, indicated that the anion is translocated into the cells along with the movement of acid equivalents. The simplest model to explain these observations is the cotransport of NO3- with H+ (or the equivalent counter-transport of NO3- for OH-). The transporter appears to be bi-directional, operating in the forward as well as reverse directions. A rough estimate of the fluxes of NO3- and acid equivalents suggests a one-to-one stoichiometry. Accordingly, the rate of transport was unaffected by sizable changes in transmembrane potential. The cytosolic acidification was a saturable function of the extracellular concentration of NO3- and was accentuated by acidification of the extracellular space. The putative NO3--H+ cotransport was inhibited markedly by ethacrynic acid and by alpha-cyano-4-hydroxycinnamate, but only marginally by 4, 4'-diisothiocyanostilbene-2,2' disulfonate or by p-chloromercuribenzene sulfonate. The transporter responsible for NO3--induced pH changes in mammalian cells may be related, though not identical, to the NO3--H+ cotransporter described in Arabidopsis and Aspergillus. The mammalian cotransporter may be important in eliminating the products of NO metabolism, particularly in cells that generate vast amounts of this messenger. By cotransporting NO3- with H+ the cells would additionally eliminate acid equivalents from activated cells that are metabolizing actively, without added energetic investment and with minimal disruption of the transmembrane potential, inasmuch as the cotransporter is likely electroneutral.

A putative amino acid transporter is specifically expressed in haustoria of the rust fungus Uromyces fabae

A cDNA library constructed from haustoria of the rust fungus Uromyces fabae was screened for clones that are differentially expressed in haustoria. One family of cDNAs (in planta-induced gene 2 [PIG2] was isolated and found to encode a protein with high homologies to fungal amino acid transporters. A cDNA clone containing the complete coding region of PIG2 and the corresponding genomic clone were isolated and sequenced, revealing the presence of 17 introns in the PIG2 gene. Expression of PIG2 mRNA appeared to be restricted to haustoria. With antibodies raised against synthetic peptides, the PIG2-encoded protein was found in membranes fractions of isolated haustoria but not of germinated rust spores. With immunofluorescence microscopy, the putative amino acid transporter was localized to plasma membranes of the haustorial bodies, but not detected in the haustorial neck, haustorial mother cells, or intercellular fungal hyphae growing within infected leaf tissue. These data present for the first time molecular evidence that the rust haustorium plays a special role in the uptake of nutrients from an infected host cell.

Cloning and functional expression of a brain peptide/histidine transporter

Here we report the cloning and functional characterization of a rat novel peptide/histidine transporter (PHT1), which was expressed in the brain and the retina. The cDNA encodes the predicted protein of 572 amino acid residues with 12 putative membrane-spanning domains. The amino acid sequence has moderate homology with a nonspecific peptide transporter found in the plant. When expressed in Xenopus laevis oocytes, PHT1 cRNA induced high affinity proton-dependent histidine transport activity. This transport process was inhibited by dipeptides and tripeptides but not by free amino acids such as glutamate, glycine, leucine, methionine, and aspartate. Dipeptide carnosine transport activity was also confirmed by direct uptake measurement. By in situ hybridization analysis, PHT1 mRNA was widely distributed throughout whole brain. Especially, intense hybridization signals were found in the hippocampus, choroid plexus, cerebellum, and pontine nucleus. Signals were located in both the neuronal and small nonneuronal cells in these areas. PHT1 protein could contribute to uptake of oligopeptides, which function as neuromodulators, and clearance of degraded neuropeptides and be a new member in the growing superfamily of proton-coupled peptide and nitrate transporters, although its structure, localization, and pharmacological characteristics are unique among these members.

Preferential expression of an ammonium transporter and of two putative nitrate transporters in root hairs of tomato

Root hairs as specialized epidermal cells represent part of the outermost interface between a plant and its soil environment. They make up to 70% of the root surface and, therefore, are likely to contribute significantly to nutrient uptake. To study uptake systems for mineral nitrogen, three genes homologous to Arabidopsis nitrate and ammonium transporters (AtNrt1 and AtAmt1) were isolated from a root hair-specific tomato cDNA library. Accumulation of LeNrt1-1, LeNrt1-2, and LeAmt1 transcripts was root-specific, with no detectable transcripts in stems or leaves. Expression was root cell type-specific and regulated by nitrogen availability. LeNrt1-2 mRNA accumulation was restricted to root hairs that had been exposed to nitrate. In contrast, LeNrt1-1 transcripts were detected in root hairs as well as other root tissues under all nitrogen treatments applied. Analogous to LeNrt1-1, the gene LeAmt1 was expressed under all nitrogen conditions tested, and root hair-specific mRNA accumulation was highest following exposure to ammonium. Expression of LeAMT1 in an ammonium uptake-deficient yeast strain restored growth on low ammonium medium, confirming its involvement in ammonium transport. Root hair specificity and characteristics of substrate regulation suggest an important role of the three genes in uptake of mineral nitrogen.

Seed and vascular expression of a high-affinity transporter for cationic amino acids in Arabidopsis

In most plants amino acids represent the major transport form for organic nitrogen. A sensitive selection system in yeast mutants has allowed identification of a previously unidentified amino acid transporter in Arabidopsis. AAT1 encodes a hydrophobic membrane protein with 14 membrane-spanning regions and shares homologies with the ecotropic murine leukemia virus receptor, a bifunctional protein serving also as a cationic amino acid transporter in mammals. When expressed in yeast, AAT1 mediates high-affinity transport of basic amino acids, but to a lower extent also recognizes acidic and neutral amino acids. AAT1-mediated histidine transport is sensitive to protonophores and occurs against a concentration gradient, indicating that AAT1 may function as a proton symporter. AAT1 is specifically expressed in major veins of leaves and roots and in various floral tissues--i.e., and developing seeds.