Transporters for nitrogenous compounds in plants

Vectorization of agrochemicals: amino acid carriers are more efficient than sugar carriers to translocate phenylpyrrole conjugates in the Ricinus system

Producing quality food in sufficient quantity while using less agrochemical inputs will be one of the great challenges of the twenty-first century. One way of achieving this goal is to greatly reduce the doses of plant protection compounds by improving the targeting of pests to eradicate. Therefore, we developed a vectorization strategy to confer phloem mobility to fenpiclonil, a contact fungicide from the phenylpyrrole family used as a model molecule. It consists in coupling the antifungal compound to an amino acid or a sugar, so that the resulting conjugates are handled by active nutrient transport systems. The method of click chemistry was used to synthesize three conjugates combining fenpiclonil to glucose or glutamic acid with a spacer containing a triazole ring. Systemicity tests with the Ricinus model have shown that the amino acid promoiety was clearly more favorable to phloem mobility than that of glucose. In addition, the transport of the amino acid conjugate is carrier mediated since the derivative of the L series was about five times more concentrated in the phloem sap than its counterpart of the D series. The systemicity of the L-derivative is pH dependent and almost completely inhibited by the protonophore carbonyl cyanide 3-chlorophenylhydrazone (CCCP). These data suggest that the phloem transport of the L-derivative is governed by a stereospecific amino acid carrier system energized by the proton motive force.

The diatom molecular toolkit to handle nitrogen uptake

Nutrient concentrations in the oceans display significant temporal and spatial variability, which strongly affects growth, distribution and survival of phytoplankton. Nitrogen (N) in particular is often considered a limiting resource for prominent marine microalgae, such as diatoms. Diatoms possess a suite of N-related transporters and enzymes and utilize a variety of inorganic (e.g., nitrate, NO3(-); ammonium, NH4(+)) and organic (e.g., urea; amino acids) N sources for growth. However, the molecular mechanisms allowing diatoms to cope efficiently with N oscillations by controlling uptake capacities and signaling pathways involved in the perception of external and internal clues remain largely unknown. Data reported in the literature suggest that the regulation and the characteristic of the genes, and their products, involved in N metabolism are often diatom-specific, which correlates with the peculiar physiology of these organisms for what N utilization concerns. Our study reveals that diatoms host a larger suite of N transporters than one would expected for a unicellular organism, which may warrant flexible responses to variable conditions, possibly also correlated to the phases of life cycle of the cells. All this makes N transporters a crucial key to reveal the balance between proximate and ultimate factors in diatom life.

ZmPTR1, a maize peptide transporter expressed in the epithelial cells of the scutellum during germination

In plants, peptide transporter/nitrate transporter 1 (PTR/NRT1) family proteins transport a variety of substrates such as nitrate, di- and tripepetides, auxin and carboxylates across membranes. We isolated and characterized ZmPTR1, a maize member of this family. ZmPTR1 protein sequence is highly homologous to the previously characterized di- and tripeptide Arabidopsis transporters AtPTR2, AtPTR4 and AtPTR6. ZmPTR1 gene is expressed in the cells of the scutellar epithelium during germination and, to a less extent, in the radicle and the hypocotyl. Arabidopsis thaliana lines overexpressing ZmPTR1 performed better than control plants when grown on a medium with Ala-Ala dipeptide as the unique N source. Our results suggest that ZmPTR1 plays a role in the transport into the embryo of the small peptides produced during enzymatic hydrolysis of the storage proteins in the endosperm.

Spider-fed bromeliads: seasonal and interspecific variation in plant performance

BACKGROUND AND AIMS

Several animals that live on bromeliads can contribute to plant nutrition through nitrogen provisioning (digestive mutualism). The bromeliad-living spider Psecas chapoda (Salticidae) inhabits and breeds on Bromelia balansae in regions of South America, but in specific regions can also appear on Ananas comosus (pineapple) plantations and Aechmea distichantha.

METHODS

Using isotopic and physiological methods in greenhouse experiments, the role of labelled ((15)N) spider faeces and Drosophila melanogaster flies in the nutrition and growth of each host plant was evaluated, as well as seasonal variation in the importance of this digestive mutualism.

KEY RESULTS

Spiders contributed 0·6 ± 0·2 % (mean ± s.e.; dry season) to 2·7 ± 1 % (wet season) to the total nitrogen in B. balansae, 2·4 ± 0·4 % (dry) to 4·1 ± 0·3 % (wet) in An. comosus and 3·8 ± 0·4 % (dry) to 5 ± 1 % (wet) in Ae. distichantha. In contrast, flies did not contribute to the nutrition of these bromeliads. Chlorophylls and carotenoid concentrations did not differ among treatments. Plants that received faeces had higher soluble protein concentrations and leaf growth (RGR) only during the wet season.

CONCLUSIONS

These results indicate that the mutualism between spiders and bromeliads is seasonally restricted, generating a conditional outcome. There was interspecific variation in nutrient uptake, probably related to each species' performance and photosynthetic pathways. Whereas B. balansae seems to use nitrogen for growth, Ae. distichantha apparently stores nitrogen for stressful nutritional conditions. Bromeliads absorbed more nitrogen coming from spider faeces than from flies, reinforcing the beneficial role played by predators in these digestive mutualisms.

A nitrite transporter associated with nitrite uptake by higher plant chloroplasts

Chloroplasts take up cytosolic nitrite during nitrate assimilation. In this study we identified a nitrite transporter located in the chloroplasts of higher plants. The transporter, CsNitr1-L, a member of the proton-dependent oligopeptide transporter (POT) family, was detected during light-induced chloroplast development in de-etiolating cucumber seedlings. We detected a CsNitr1-L-green fluorescent protein (GFP) fusion protein in the chloroplasts of leaf cells and found that an immunoreactive 51 kDa protein was present in the isolated inner envelope membrane of chloroplasts. CsNitr1-L has an isoform, CsNitr1-S, with an identical 484 amino acid core sequence; however, in CsNitr1-S the 120 amino acid N-terminal extension is missing. Saccharomyces cerevisiae cells expressing CsNitr1-S absorbed nitrite from an acidic medium at a slower rate than mock-transformed control cells, and accumulated nitrite to only one-sixth the concentration of the control cells, suggesting that CsNitr1-S enhances the efflux of nitrite from the cell. Insertion of T-DNA in a single CsNitr1-L homolog (At1g68570) in Arabidopsis resulted in nitrite accumulation in leaves to more than five times the concentration found in the wild type. These results show that it is possible that both CsNitr1-L and CsNitr1-S encode efflux-type nitrite transporters, but with different subcellular localizations. CsNitr1-L may possibly load cytosolic nitrite into chloroplast stroma in the chloroplast envelope during nitrate assimilation. The presence of genes homologous to CsNitr1-L in the genomes of Arabidopsis and rice indicates that facilitated nitrite transport is of general physiological importance in plant nutrition.

Peptide and amino acid transporters are differentially regulated during seed development and germination in faba bean

Two peptide transporter (PTR) homologs have been isolated from developing seeds of faba bean (Vicia faba). VfPTR1 was shown to be a functional peptide transporter through complementation of a yeast mutant. Expression patterns of VfPTR1 and VfPTR2 as well as of the amino acid permease VfAAP1 (Miranda et al., 2001) were compared throughout seed development and germination. In developing seeds, the highest levels of VfPTR1 transcripts were reached during midcotyledon development, whereas VfAAP1 transcripts were most abundant during early cotyledon development, before the appearance of storage protein gene transcripts, and were detectable until late cotyledon development. During early germination, VfPTR1 mRNA appeared first in cotyledons and later, during seedling growth, also in axes and roots. Expression of VfPTR2 and VfAAP1 was delayed compared with VfPTR1, and was restricted to the nascent organs of the seedlings. Localization of VfPTR1 transcripts showed that this PTR is temporally and spatially regulated during cotyledon development. In germinating seeds, VfPTR1 mRNA was localized in root hairs and root epidermal cells, suggesting a role in nutrient uptake from the soil. In seedling roots, VfPTR1 was repressed by a dipeptide and by an amino acid, whereas nitrate was without influence.

The export of amino acid in the phloem is altered in wheat plants lacking the short arm of chromosome 7B

Grain protein content is one of the major determinants of the baking and nutritional quality of wheat. It has previously been reported that the ditelosomic line of wheat (Triticum aestivum L.) CSDT7BL, where the short arm of chromosome 7B is missing, shows a lower grain protein concentration than the normal line, but a similar grain yield. In the present paper the growth and nitrogen (N) metabolism of wheat plants cv. Chinese Spring (CS) and its ditelosomic line CSDT7BL were compared. When plants were grown to maturity in pots with different N supplements, the wild-type line showed a higher grain protein concentration and a lower straw N concentration than the ditelosomic line at every N level analysed, suggesting a deficiency in the N remobilization capacity. When 15-d-old plants were grown in a growth cabinet in pots with sand, and supplied with nutrient solutions of different nitrate concentrations, the ditelosomic line showed no differences in N uptake per unit of root dry weight, nitrate reductase activity, nitrate, total N concentration or free amino acid concentration. However, the ditelosomic line showed a decreased capacity to export amino acids in the phloem under high N, independently of the N source. This deficiency was also observed under dark-induced senescence. The diminished export of amino acids to the phloem was principally caused by a decrease in the export of Glu, Asp, and Gln. It is suggested that the decrease in grain protein concentration in the ditelosomic line is a consequence of defective export in the phloem of these amino acids.

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.

Structure, function and regulation of ammonium transporters in plants

Ammonium is an important source of nitrogen for plants. It is taken up by plant cells via ammonium transporters in the plasma membrane and distributed to intracellular compartments such as chloroplasts, mitochondria and vacuoles probably via different transporters in each case. Ammonium is generally not used for long-distance transport of nitrogen within the plant. Instead, most of the ammonium transported into plant cells is assimilated locally via glutamine synthetases in the cytoplasm and plastids. Ammonium is also produced by plant cells during normal metabolism, and ammonium transporters enable it to be moved from intracellular sites of production to sites of consumption. Ammonium can be generated de novo from molecular nitrogen (N(2)) by nitrogen-fixing bacteria in some plant cells, such as rhizobia in legume root nodule cells, and at least one ammonium transporter is implicated in the transfer of ammonium from the bacteria to the plant cytoplasm. Plant physiologists have described many of these ammonium transport processes over the last few decades. However, the genes and proteins that underlie these processes have been isolated and studied only recently. In this review, we consider in detail the molecular structure, function and regulation of plant ammonium transporters. We also attempt to reconcile recent discoveries at the molecular level with our knowledge of ammonium transport at the whole plant level.

Ammonium and methylamine transport by the ectomycorrhizal fungus Paxillus involutus and ectomycorrhizas

Using [(14)C]methylamine as an analogue of ammonium, the kinetics and the energetics of NH(4)(+) transport were studied in the ectomycorrhizal fungus, Paxillus involutus (Batsch) Fr. The apparent half-saturation constant (K(m)) and the maximum uptake rate (V(max)) for the carrier-mediated transport derived from the Eadie-Hofstee transformation were 180 µM and 380 nmol (mg dry wt)(-1) min(-1,) respectively. Both pH dependence and inhibition by protonophores indicate that methylamine transport in P. involutus was dependent on the electrochemical H(+) gradient. Both long-term and short-term uptake experiments were consistent with regulation of ammonium/methylamine transport processes by the presence of an organic nitrogen source. Analysis of methylamine uptake by different P. involutus isolates revealed no obvious trend in the uptake capacities in relation to N deposition at the collection site. Kinetic parameters were determined in P. involutus/Betula pendula (Roth.) axenic association and in detached mycorrhizal roots isolated from forest sites. Enhanced methylamine uptake in the presence of the fungal symbiont was demonstrated. Homogeneous V(max) values were found for axenic and detached mycorrhizas, whereas K(m) values showed greater variations.

Identification and characterization of GABA, proline and quaternary ammonium compound transporters from Arabidopsis thaliana

Arabidopsis thaliana grows efficiently on GABA as the sole nitrogen source, thereby providing evidence for the existence of GABA transporters in plants. Heterologous complementation of a GABA uptake-deficient yeast mutant identified two previously known plant amino acid transporters, AAP3 and ProT2, as GABA transporters with Michaelis constants of 12.9 +/- 1.7 and 1.7 +/- 0.3 mM at pH 4, respectively. The simultaneous transport of [1-14C]GABA and [2,3-3H]proline by ProT2 as a function of pH, provided evidence that the zwitterionic state of GABA is an important parameter in substrate recognition. ProT2-mediated [1-14C]GABA transport was inhibited by proline and quaternary ammonium compounds.

The amino acid/auxin:proton symport permease family

Amino acids and their derivatives are transported into and out of cells by a variety of permease types which comprise several distinct protein families. We here present a systematic analysis of a group of homologous transport proteins which together comprise the eukaryotic-specific amino acid/auxin permease (AAAP) family (TC #2. 18). In characterizing this family, we have (1) identified all sequenced members of the family, (2) aligned their sequences, (3) identified regions of striking conservation, (4) derived a family-specific signature sequence, and (5) proposed a topological model that appears to be applicable to all members of the family. We have also constructed AAAP family phylogenetic trees and dendrograms using six different programs that allow us to trace the evolutionary history of the family, estimate the relatedness of proteins from dissimilar organismal phyla, and evaluate the reliability of the different programs available for phylogenetic studies. The TREE and neighbor-joining programs gave fully consistent results while CLUSTAL W gave similar but non-identical results. Other programs gave less consistent results. The phylogenetic analyses reveal (1) that many plant AAAP family proteins arose recently by multiple gene duplication events that occurred within a single organism, (2) that some plant members of the family with strikingly different specificities diverged early in evolutionary history, and (3) that AAAP family proteins from fungi and animals diverged from the plant proteins long ago, possibly when animals, plants and fungi diverged from each other. The Neurospora protein nevertheless exhibits overlapping specificity with those found in plants. Preliminary evidence is presented suggesting that proteins of the AAAP family are distantly related to proteins of the large ubiquitous amino acid/polyamine/choline family (TC #2.3) as well as to those of two small bacterial amino acid transporter families, the ArAAP family (TC #2.42) and the STP family (TC #2.43).

Chimeric purine transporters of Aspergillus nidulans define a domain critical for function and specificity conserved in bacterial, plant and metazoan homologues

In Aspergillus nidulans, purine uptake is mediated by three transporter proteins: UapA, UapC and AzgA. UapA and UapC have partially overlapping functions, are 62% identical and have nearly identical predicted topologies. Their structural similarity is associated with overlapping substrate specificities; UapA is a high-affinity, high-capacity specific xanthine/uric acid transporter. UapC is a low/moderate-capacity general purine transporter. We constructed and characterized UapA/UapC, UapC/UapA and UapA/UapC/UapA chimeric proteins and UapA point mutations. The region including residues 378-446 in UapA (336-404 in UapC) has been shown to be critical for purine recognition and transport. Within this region, we identified: (i) one amino acid residue (A404) important for transporter function but probably not for specificity and two residues (E412 and R414) important for UapA function and specificity; and (ii) a sequence, (F/Y/S)X(Q/E/P) NXGXXXXT(K/R/G), which is highly conserved in all homologues of nucleobase transporters from bacteria to man. The UapC/UapA series of chimeras behaves in a linear pattern and leads to an univocal assignment of functional domains while the analysis of the reciprocal and 'sandwich' chimeras revealed unexpected inter-domain interactions. cDNAs coding for transporters including the specificity region defined by these studies have been identified for the first time in the human and Caenorhabditis elegans databases.

Physiology of organic nitrogen acquisition by ectomycorrhizal fungi and ectomycorrhizas

Ectomycorrhizal fungi are symbiotically associated microorganisms which ecological importance has been repeatedly demonstrated. There has been a considerable amount of research aimed at assessing the ability of ectomycorrhizal fungi and ectomycorrhizas to utilize organic nitrogen sources. The fate of soil proteins, peptides and amino acids has been studied from a number of perspectives. Exocellular hydrolytic enzymes have been detected and characterized in a number of ectomycorrhizal and ericoid fungi. Studies on amino acid transport through the plasma membrane have demonstrated the ability of ectomycorrhizal fungi to take up the products of proteolytic activities. Investigations on intracellular metabolism of amino acids have allowed the identification of the metabolic pathways involved. Possible intracellular compartmentation of amino acids will be examined by immunocytochemistry. Further translocation of amino acids in symbiotic tissues has been established by experiments using isotopic tracers, although the exact nature of the nitrogenous compounds transferred at the symbiotic interface remained unclear. One of the main future challenges in the physiology of organic nitrogen acquisition is to determine the nature, the regulation and the location of N-compound transporters at the soil-fungus and fungus-plant interfaces. The molecular approach which is just emerging in this particular research area will greatly improve our knowledge. Future research should also address the extent of competition between different ectomycorrhizal species and between different microbial populations for organic nitrogen.

Transport, Compartmentation, and Metabolism of Homoserine in Higher Plant Cells. Carbon-13- and phosphorus-31-nuclear magnetic resonance studies Carbon-13- and Phosphorus-31-Nuclear Magnetic Resonance Studies

The transport, compartmentation, and metabolism of homoserine was characterized in two strains of meristematic higher plant cells, the dicotyledonous sycamore (Acer pseudoplatanus) and the monocotyledonous weed Echinochloa colonum. Homoserine is an intermediate in the synthesis of the aspartate-derived amino acids methionine, threonine (Thr), and isoleucine. Using 13C-nuclear magnetic resonance, we showed that homoserine actively entered the cells via a high-affinity proton-symport carrier (Km approximately 50-60 mum) at the maximum rate of 8 +/- 0.5 mumol h-1 g-1 cell wet weight, and in competition with serine or Thr. We could visualize the compartmentation of homoserine, and observed that it accumulated at a concentration 4 to 5 times higher in the cytoplasm than in the large vacuolar compartment. 31P-nuclear magnetic resonance permitted us to analyze the phosphorylation of homoserine. When sycamore cells were incubated with 100 mum homoserine, phosphohomoserine steadily accumulated in the cytoplasmic compartment over 24 h at the constant rate of 0.7 mumol h-1 g-1 cell wet weight, indicating that homoserine kinase was not inhibited in vivo by its product, phosphohomoserine. The rate of metabolism of phosphohomoserine was much lower (0.06 mumol h-1 g-1 cell wet weight) and essentially sustained Thr accumulation. Similarly, homoserine was actively incorporated by E. colonum cells. However, in contrast to what was seen in sycamore cells, large accumulations of Thr were observed, whereas the intracellular concentration of homoserine remained low, and phosphohomoserine did not accumulate. These differences with sycamore cells were attributed to the presence of a higher Thr synthase activity in this strain of monocot cells.

Specificity and stoichiometry of the Arabidopsis H+/amino acid transporter AAP5

The H+-dependent AAP5 amino acid transporter from Arabidopsis thaliana was expressed in Xenopus oocytes, and we used radiotracer flux and electrophysiology methods to investigate its substrate specificity and stoichiometry. Inward currents of up to 9 microA were induced by a broad spectrum of amino acids, including anionic, cationic, and neutral amino acids. The apparent affinity of AAP5 for amino acids was influenced by the position of side chain branches, bulky ring structures, and charged groups. The maximal current was dependent on amino acid charge, but was relatively independent of amino acid structure. A detailed kinetic analysis of AAP5 using lysine, alanine, glutamate, and histidine revealed H+-dependent differences in the apparent affinity constants for each substrate. The differences were correlated to the effect of H+ concentration on the net charge of each amino acid and suggested that AAP5 transports only the neutral species of histidine and glutamate. Stoichiometry experiments, whereby the uptake of 3H-labeled amino acid and net inward charge were simultaneously measured in voltage-clamped oocytes, showed that the charge:amino acid stoichiometry was 2:1 for lysine and 1:1 for alanine, glutamate, and histidine. The results confirm that histidine is transported in its neutral form and show that the positive charge on lysine contributes to the magnitude of its inward current. Thus, the transport stoichiometry of AAP5 is 1 H+:1 amino acid irrespective of the net charge on the transported substrate. Structural features of amino acid molecules that are involved in substrate recognition by AAP5 are discussed.

MEMBRANE TRANSPORT CARRIERS

▪ Abstract  Plant and fungal membrane proteins catalyzing the transmembrane translocation of small molecules without directly using ATP or acting as channels are discussed in this review. Facilitators, ion-cotransporters, and exchange translocators mainly for sugars, amino acids, and ions that have been cloned and characterized from Saccharomyces cerevisiae and from various plant sources have been tabulated. The membrane topology and structure of the most extensively studied carriers (lac permease of Escherichia coli, Glut1 of man, HUP1 of Chlorella) are discussed in detail as well as the kinetic analysis of specific Na+ and H+ cotransporters. Finally, the knowledge concerning regulatory phenomena of carriers—mainly of S. cerevisiae—is summarized.

Kinetics and specificity of a H+/amino acid transporter from Arabidopsis thaliana

The amino acid transporter AAP1/NAT2 recently cloned from Arabidopsis thaliana was expressed in Xenopus oocytes, and we used electrophysiological, radiotracer flux, and electron microscopic methods to characterize the biophysical properties, kinetics, and specificity of the transporter. Uptake of alanine was H(+)-dependent increasing from 14 pmol/oocyte/h at 0.032 microM H+ to 370 pmol/oocyte/h at 10 microM H+. AAP1 was electrogenic; there was an amino acid-induced depolarization of the oocyte plasma membrane and net inward currents through the transporter due to the transport of amino acids favoring neutral amino acids with shortside chains. The maximal current (imax) for alanine, proline, glutamine, histidine, and glutamate was voltage and [H+]o-dependent. Similarly, the imaxH was voltage and [amino acid]o-dependent. The imax for both H+ and amino acid were dependent on the concentrations of their respective cosubstrates, suggesting that both ligands bind randomly to the transporter. The K0.5 of the transporter for amino acids decreased as [H+]o increased and was lower at negative membrane potentials. The K0.5 for H+ was relatively voltage-independent and decreased as [amino acid]o increased. This positive cooperativity suggests that the transporter operates via a simultaneous mechanism. The Hill coefficients n for amino acids and H+ were > 1, suggesting that the transporter has more than one binding site for both H+ and amino acid. Freeze-fracture electron microscopy was used to estimate the number of transporters expressed in the plasma membrane of oocytes. The density of particles on the protoplasmic face of the plasma membrane of oocytes expressing AAP1 increased approximately 5-fold above water-injected controls and corresponded to a turnover number 350 to 800 s-1.

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.

An overview of membrane transport proteins in Saccharomyces cerevisiae

All eukaryotic cells contain a wide variety of proteins embedded in the plasma and internal membranes, which ensure transmembrane solute transport. It is now established that a large proportion of these transport proteins can be grouped into families apparently conserved throughout organisms. This article presents the data of an in silicio analysis aimed at establishing a preliminary classification of membrane transport proteins in Saccharomyces cerevisiae. This analysis was conducted at a time when about 65% of all yeast genes were available in public databases. In addition to approximately 60 transport proteins whose function was at least partially known, approximately 100 deduced protein sequences of unknown function display significant sequence similarity to membrane transport proteins characterized in yeast and/or other organisms. While some protein families have been well characterized by classical genetic experimental approaches, others have largely if not totally escaped characterization. The proteins revealed by this in silicio analysis also include a putative K+ channel, proteins similar to aquaporins of plant and animal origin, proteins similar to Na+-solute symporters, a protein very similar to electroneural cation-chloride cotransporters, and a putative Na+-H+ antiporter. A new research area is anticipated: the functional analysis of many transport proteins whose existence was revealed by genome sequencing.

NTR1 encodes a high affinity oligopeptide transporter in Arabidopsis

Hterologous complementation of yeast mutants has enabled the isolation of genes encoding several families of amino acid transporters. Among them, NTR1 codes for a membrane protein with weak histidine transport activity. However, at the sequence level, NTR1 is related to rather non-specific oligopeptide transporters from a variety of species including Arabidopsis and to the Arabidopsis nitrate transporter CHL1. A yeast mutant deficient in oligopeptide transport was constructed allowing to show that NTR1 functions as a high affinity, low specificity peptide transporter. In siliques NTR1-expression is restricted to the embryo, implicating a role in the nourishment of the developing seed.

Substrate specificity and expression profile of amino acid transporters (AAPs) in Arabidopsis

Three amino acid transporter genes (AAP3-5) were isolated from Arabidopsis by complementation of a yeast mutant defective in histidine uptake. Transport is driven against a concentration gradient and sensitive to protonophores. Analysis of the substrate specificity demonstrates that the carriers have a broad substrate specificity covering the major transport forms of reduced nitrogen, i.e. glutamine and glutamate. The transporters have similar affinities for glutamate, glutamine, and alanine but differ with respect to valine, phenylalanine, histidine, arginine, and lysine. AAP3 and AAP5 efficiently transport arginine and lysine and are involved in basic amino acid transport. The predicted polypeptides of 53 kDa are highly hydrophobic with 12 putative membrane-spanning regions and show significant homologies to Arabidopsis amino acid transporters AAP1 and AAP2. Each of the genes has a different organ-specific expression in the plant. AAP3 is exclusively expressed in roots and AAP4 mainly in source leaves, stems, and flowers, whereas AAP5 is found in all tissues. The specific distribution in the plant and the different substrate specificities of AAP transporters may indicate that tissues differ both qualitatively and quantitatively regarding import or export of amino acids.