Trypanosoma brucei eflornithine transporter AAT6 is a low-affinity low-selective transporter for neutral amino acids.

Amino acid transporters are crucial for parasite survival since the cellular metabolism of parasitic protozoa depends on the up-take of exogenous amino acids. Amino acid transporters are also of high pharmacological relevance because they may mediate uptake of toxic amino acid analogues. In the present study we show that the eflornithine transporter AAT6 from Trypanosoma brucei (TbAAT6) mediates growth on neutral amino acids when expressed in Saccharomyces cerevisiae mutants. The transport was electrogenic and further analysed in Xenopus laevis oocytes. Neutral amino acids, proline analogues, eflornithine and acivicin induced inward currents. For proline, glycine and tryptophan the apparent affinities and maximal transport rates increased with more negative membrane potentials. Proline-induced currents were dependent on pH, but not on sodium. Although proline represents the primary energy source of T. brucei in the tsetse fly, down-regulation of TbAAT6-expression by RNAi showed that in culture TbAAT6 is not essential for growth of procyclic form trypanosomes in the presence of glucose or proline as energy source. TbAAT6-RNAi lines of both bloodstream and procyclic form trypanosomes showed reduced susceptibility to eflornithine, whereas the sensitivity to acivicin remained unchanged, indicating that acivicin enters the cell by more than one transporter.

Altered expression of the PTR/NRT1 homologue OsPTR9 affects nitrogen utilization efficiency, growth and grain yield in rice.

The plant PTR/NRT1 (peptide transporter/nitrate transporter 1) gene family comprises di/tripeptide and low-affinity nitrate transporters; some members also recognize other substrates such as carboxylates, phytohormones (auxin and abscisic acid), or defence compounds (glucosinolates). Little is known about the members of this gene family in rice (Oryza sativa L.). Here, we report the influence of altered OsPTR9 expression on nitrogen utilization efficiency, growth, and grain yield. OsPTR9 expression is regulated by exogenous nitrogen and by the day-night cycle. Elevated expression of OsPTR9 in transgenic rice plants resulted in enhanced ammonium uptake, promotion of lateral root formation and increased grain yield. On the other hand, down-regulation of OsPTR9 in a T-DNA insertion line (osptr9) and in OsPTR9-RNAi rice plants had the opposite effect. These results suggest that OsPTR9 might hold potential for improving nitrogen utilization efficiency and grain yield in rice breeding.

Nonredundant regulation of rice arbuscular mycorrhizal symbiosis by two members of the phosphate transporter1 gene family.

Pi acquisition of crops via arbuscular mycorrhizal (AM) symbiosis is becoming increasingly important due to limited high-grade rock Pi reserves and a demand for environmentally sustainable agriculture. Here, we show that 70% of the overall Pi acquired by rice (Oryza sativa) is delivered via the symbiotic route. To better understand this pathway, we combined genetic, molecular, and physiological approaches to determine the specific functions of two symbiosis-specific members of the PHOSPHATE TRANSPORTER1 (PHT1) gene family from rice, ORYsa;PHT1;11 (PT11) and ORYsa;PHT1;13 (PT13). The PT11 lineage of proteins from mono- and dicotyledons is most closely related to homologs from the ancient moss, indicating an early evolutionary origin. By contrast, PT13 arose in the Poaceae, suggesting that grasses acquired a particular strategy for the acquisition of symbiotic Pi. Surprisingly, mutations in either PT11 or PT13 affected the development of the symbiosis, demonstrating that both genes are important for AM symbiosis. For symbiotic Pi uptake, however, only PT11 is necessary and sufficient. Consequently, our results demonstrate that mycorrhizal rice depends on the AM symbiosis to satisfy its Pi demands, which is mediated by a single functional Pi transporter, PT11.

Determinants for Arabidopsis peptide transporter targeting to the tonoplast or plasma membrane.

Di- and tripeptide transporters of the PTR/NRT1 (peptide transporter/nitrate transporter1)-family are localized either at the tonoplast (TP) or plasma membrane (PM). As limited information is available on structural determinants required for targeting of plant membrane proteins, we performed gene shuffling and domain swapping experiments of Arabidopsis PTRs. A 7 amino acid fragment of the hydrophilic N-terminal region of PTR2, PTR4 and PTR6 was required for TP localization and sufficient to redirect not only PM-localized PTR1 or PTR5, but also sucrose transporter SUC2 to the TP. Alanine scanning mutagenesis identified L(11) and I(12) of PTR2 to be essential for TP targeting, while only one acidic amino acid at position 5, 6 or 7 was required, revealing a dileucine (LL or LI) motif with at least one upstream acidic residue. Similar dileucine motifs could be identified in other plant TP transporters, indicating a broader role of this targeting motif in plants. Targeting to the PM required the loop between transmembrane domain 6 and 7 of PTR1 or PTR5. Deletion of either PM or TP targeting signals resulted in retention in internal membranes, indicating that PTR trafficking to these destination membranes requires distinct signals and is in both cases not by default.

Characterization of a transport activity for long-chain peptides in barley mesophyll vacuoles.

The plant vacuole is the largest compartment in a fully expanded plant cell. While only very limited metabolic activity can be observed within the vacuole, the majority of the hydrolytic activities, including proteolytic activities reside in this organelle. Since it is assumed that protein degradation by the proteasome results in the production of peptides with a size of 3-30 amino acids, we were interested to show whether the tonoplast exhibits a transport activity, which could deliver these peptides into the vacuole for final degradation. It is shown here that isolated barley mesophyll vacuoles take up peptides of 9-27 amino acids in a strictly ATP-dependent manner. Uptake is inhibited by vanadate, but not by NH(+)(4), while GTP could partially substitute for ATP. The apparent affinity for the 9 amino acid peptide was 15 µM, suggesting that peptides are efficiently transferred to the vacuole in vivo. Inhibition experiments showed that peptides with a chain length below 10 amino acids did not compete as efficiently as longer peptides for the uptake of the 9 amino acid peptide. Our results suggest that vacuoles contain at least one peptide transporter that belongs to the ABC-type transporters, which efficiently exports long-chain peptides from the cytosol into the vacuole for final degradation.