Saccharomyces cerevisiae Aqr1 is an internal-membrane transporter involved in excretion of amino acids

A gene repertoire for nitrogen transporters in Laccaria bicolor

Ectomycorrhizal interactions established between the root systems of terrestrial plants and hyphae from soil-borne fungi are the most ecologically widespread plant symbioses. The efficient uptake of a broad range of nitrogen (N) compounds by the fungal symbiont and their further transfer to the host plant is a major feature of this symbiosis. Nevertheless, we far from understand which N form is preferentially transferred and what are the key molecular determinants required for this transfer. Exhaustive in silico analysis of N-compound transporter families were performed within the genome of the ectomycorrhizal model fungus Laccaria bicolor. A broad phylogenetic approach was undertaken for all families and gene regulation was investigated using whole-genome expression arrays. A repertoire of proteins involved in the transport of N compounds in L. bicolor was established that revealed the presence of at least 128 gene models in the genome of L. bicolor. Phylogenetic comparisons with other basidiomycete genomes highlighted the remarkable expansion of some families. Whole-genome expression arrays indicate that 92% of these gene models showed detectable transcript levels. This work represents a major advance in the establishment of a transportome blueprint at a symbiotic interface, which will guide future experiments.

Generic and specific transcriptional responses to different weak organic acids in anaerobic chemostat cultures ofSaccharomyces cerevisiae

Transcriptional responses to four weak organic acids (benzoate, sorbate, acetate and propionate) were investigated in anaerobic, glucose-limited chemostat cultures of Saccharomyces cerevisiae. To enable quantitative comparison of the responses to the acids, their concentrations were chosen such that they caused a 50% decrease of the biomass yield on glucose. The concentration of each acid required to achieve this yield was negatively correlated with membrane affinity. Microarray analysis revealed that each acid caused hundreds of transcripts to change by over twofold relative to reference cultures without added organic acids. However, only 14 genes were consistently upregulated in response to all acids. The moderately lipophilic compounds benzoate and sorbate and, to a lesser extent, the less lipophilic acids acetate and propionate showed overlapping transcriptional responses. Statistical analysis for overrepresented functional categories and upstream regulatory elements indicated that responses to the strongly lipophilic acids were focused on genes related to the cell wall, while acetate and propionate had a stronger impact on membrane-associated transport processes. The fact that S. cerevisiae exhibits a minimal generic transcriptional response to weak organic acids along with extensive specific responses is relevant for interpreting and controlling weak acid toxicity in food products and in industrial fermentation processes.

The yeast ammonium transport protein Mep2 and its positive regulator, the Npr1 kinase, play an important role in normal and pseudohyphal growth on various nitrogen media through retrieval of excreted ammonium

Three ammonium transport systems of the Mep/Amt/Rh superfamily contribute to ammonium uptake for use as a nitrogen source in Saccharomyces cerevisiae. A specific sensor role has further been proposed for Mep2 in the stimulation of pseudohyphal development during ammonium limitation. Optimal ammonium transport by the Mep proteins requires the Npr1 kinase, a potential target of the target-of-rapamycin signalling pathway. We show here that the growth impairment of cells lacking Npr1 on many nitrogen sources is shared by cells deprived of the three Mep proteins and is a consequence of deficient ammonium retrieval. Expression of a newly isolated Npr1-independent and hyperactive Mep2 in cells lacking Npr1 and/or the Mep proteins restores growth on low ammonium but also on other nitrogen sources. This hyperactive Mep2 variant efficiently counteracts ammonium excretion. Hence, ammonium uptake activity plays an important role in compensating for leakage of catabolic ammonium. Our data also reveal that the requirement of Npr1 for ammonium-induced pseudohyphal growth is an indirect consequence of its necessity for Mep2-mediated ammonium transport. Finally, we show that Mep2 participates, through ammonium leakage compensation, in pseudohyphal growth induced by amino acid starvation. This argues further in favour of tight coupling of Mep2 transport and sensor functions.

Transporters for uptake and allocation of organic nitrogen compounds in plants

Nitrogen is an essential macronutrient for plant growth. Following uptake from the soil or assimilation within the plant, organic nitrogen compounds are transported between organelles, from cell to cell and over long distances in support of plant metabolism and development. These translocation processes require the function of integral membrane transporters. The review summarizes our current understanding of the molecular mechanisms of organic nitrogen transport processes, with a focus on amino acid, ureide and peptide transporters.

Ammonium toxicity and potassium limitation in yeast

DNA microarray analysis of gene expression in steady-state chemostat cultures limited for potassium revealed a surprising connection between potassium and ammonium: potassium limits growth only when ammonium is the nitrogen source. Under potassium limitation, ammonium appears to be toxic for Saccharomyces cerevisiae. This ammonium toxicity, which appears to occur by leakage of ammonium through potassium channels, is recapitulated under high-potassium conditions by over-expression of ammonium transporters. Although ammonium toxicity is well established in metazoans, it has never been reported for yeast. To characterize the response to ammonium toxicity, we examined the filtrates of these cultures for compounds whose excretion might serve to detoxify the ammonium (such as urea in mammals). Using liquid chromatography-tandem mass spectrometry to assay for a wide array of metabolites, we detected excreted amino acids. The amounts of amino acids excreted increased in relation to the severity of growth impairment by ammonium, suggesting that amino acid excretion is used by yeast for ammonium detoxification.

Saccharomyces cerevisiae multidrug resistance transporter Qdr2 is implicated in potassium uptake, providing a physiological advantage to quinidine-stressed cells

The QDR2 gene of Saccharomyces cerevisiae encodes a putative plasma membrane drug:H(+) antiporter that confers resistance against quinidine, barban, bleomycin, and cisplatin. This work provides experimental evidence of defective K(+) (Rb(+)) uptake in the absence of QDR2. The direct involvement of Qdr2p in K(+) uptake is reinforced by the fact that increased K(+) (Rb(+)) uptake due to QDR2 expression is independent of the Trk1p/Trk2p system. QDR2 expression confers a physiological advantage for the yeast cell during the onset of K(+) limited growth, due either to a limiting level of K(+) in the growth medium or to the presence of quinidine. This drug decreases the K(+) uptake rate and K(+) accumulation in the yeast cell, especially in the Deltaqdr2 mutant. Qdr2p also helps to sustain the decrease of intracellular pH in quinidine-stressed cells in growth medium at pH 5.5 by indirectly promoting H(+) extrusion affected by the drug. The results are consistent with the hypothesis that Qdr2p may also couple K(+) movement with substrate export, presumably with quinidine. Other clues to the biological role of QDR2 in the yeast cell come from two additional lines of experimental evidence. First, QDR2 transcription is activated under nitrogen (NH(4)(+)) limitation or when the auxotrophic strain examined enters stationary phase due to leucine limitation, this regulation being dependent on general amino acid control by Gcn4p. Second, the amino acid pool is higher in Deltaqdr2 cells than in wild-type cells, indicating that QDR2 expression is, directly or indirectly, involved in amino acid homeostasis.

Ammonia: a candidate for nitrogen transfer at the mycorrhizal interface

In mycorrhizal associations, the fungal partner assists its plant host with nitrogen and phosphorus uptake while obtaining photosynthetically fixed carbon. Recent studies in mycorrhiza have highlighted the potential for direct transfer of ammonia from fungal to plant cells. This presents a new perspective on nitrogen transfer at the mycorrhizal interface, which is discussed here in light of recent progress made in characterizing a large array of membrane proteins that could fulfil the function of transporting ammonia.

The yeast multidrug transporter Qdr3 (Ybr043c): localization and role as a determinant of resistance to quinidine, barban, cisplatin, and bleomycin

Saccharomyces cerevisiae ORF YBR043c, predicted to code for a transporter of the major facilitator superfamily required for multiple drug resistance, encodes a plasma membrane protein that confers resistance to quinidine and barban, as observed before for its close homologues QDR1 and QDR2. This ORF was, thus, named the QDR3 gene. The increased expression of QDR3, or QDR2, also leads to increased resistance to the anticancer agents cisplatin and bleomycin. However, no evidence for increased QDR3 expression in yeast cells exposed to all these inhibitory compounds was found. Transport assays support the concept that Qdr3 is involved, even if opportunistically, in the active export of quinidine out of yeast cell. A correlation was established between the efficiency of quinidine active export mediated by Qdr3p, Qdr2p or Qdr1p, and the efficacy of the expression of the encoding genes in alleviating the deleterious action of quinidine, as well as of the other compounds (QDR2>QDR3>>>QDR1).