A high-affinity uptake system for branched-chain amino acids in Saccharomyces cerevisiae

The Candida albicans GAP gene family encodes permeases involved in general and specific amino acid uptake and sensing

The Saccharomyces cerevisiae general amino acid permease Gap1 (ScGap1) not only mediates the uptake of most amino acids but also functions as a receptor for the activation of protein kinase A (PKA). Fungal pathogens can colonize different niches in the host, each containing various levels of different amino acids and sugars. The Candida albicans genome contains six genes homologous to the S. cerevisiae GAP1. The expression of these six genes in S. cerevisiae showed that the products of all six C. albicans genes differ in their transport capacities. C. albicans Gap2 (CaGap2) is the true orthologue of ScGap1 as it transports all tested amino acids. The other CaGap proteins have narrower substrate specificities though CaGap1 and CaGap6 transport several structurally unrelated amino acids. CaGap1, CaGap2, and CaGap6 also function as sensors. Upon detecting some amino acids, e.g., methionine, they are involved in a rapid activation of trehalase, a downstream target of PKA. Our data show that CaGAP genes can be functionally expressed in S. cerevisiae and that CaGap permeases communicate to the intracellular signal transduction pathway similarly to ScGap1.

Deletion of RTS1, encoding a regulatory subunit of protein phosphatase 2A, results in constitutive amino acid signaling via increased Stp1p processing

In Saccharomyces cerevisiae, extracellular amino acids are sensed at the plasma membrane by the SPS sensor, consisting of the transporter homologue Ssy1p, Ptr3p, and the endoprotease Ssy5p. Amino acid sensing results in proteolytic truncation of the transcription factors Stp1p and Stp2p, followed by their relocation from the cytoplasm to the nucleus, where they activate transcription of amino acid permease genes. We screened a transposon mutant library for constitutively signaling mutants, with the aim of identifying down-regulating components of the SPS-mediated pathway. Three isolated mutants were carrying a transposon in the RTS1 gene, which encodes a regulatory subunit of protein phosphatase 2A. We investigated the basal activity of the AGP1 and BAP2 promoters in rts1delta cells and found increased transcription from these promoters, as well as increased Stp1p processing, even in the absence of amino acids. Based on our findings we propose that the phosphatase complex containing Rts1p keeps the SPS-mediated pathway down-regulated in the absence of extracellular amino acids by dephosphorylating a component of the pathway.

Transcriptional profiling of extracellular amino acid sensing in Saccharomyces cerevisiae and the role of Stp1p and Stp2p

S. cerevisiae responds to the presence of amino acids in the environment through the membrane-bound complex SPS, by altering transcription of several genes. Global transcription analysis shows that 46 genes are induced by L-citrulline. Under the given conditions there appears to be only one pathway for induction with L-citrulline, and this pathway is completely dependent on the SPS component, Ssy1p, and either of the transcription factors, Stp1p and Stp2p. Besides the effects on amino acid permease genes, an ssy1 and an stp1 stp2 mutant exhibit a number of other transcriptional phenotypes, such as increased expression of genes subject to nitrogen catabolite repression and genes involved in stress response. A group of genes involved in the upper part of the glycolysis, including those encoding hexose transporters Hxt4p, Hxt5p, Hxt6p, Hxt7p, hexokinase Hxk1p, glyceraldehyde 3-phosphate dehydrogenase Tdh1p and glucokinase (Glk1p), shows increased transcription levels in either or both of the mutants. Also, most of the structural genes involved in trehalose and glycogen synthesis and a few genes in the glyoxylate cycle and the pentose phosphate pathway are derepressed in the ssy1 and stp1 stp2 strains.

Regulation of expression of the amino acid transporter gene BAP3 in Saccharomyces cerevisiae

The BAP3 gene of Saccharomyces cerevisiae encodes a protein with a high similarity to the BAP2 gene product, a high-affinity permease for branched-chain amino acids. In this paper, we show that, like BAP2, the expression of the BAP3 gene in S. cerevisiae is induced by the addition of branched-chain amino acids to the medium. Unexpectedly, most other naturally occurring L-amino acids found in proteins (with the exception of proline, lysine, arginine and histidine) have the same effect on the expression of BAP3. The induction of BAP3 expression appears to be dependent on Stp1p, a nuclear protein, previously shown to be involved in pre-tRNA maturation and also required for the expression of BAP2, as induction is no longer observed in an stp1 - mutant. The transcriptional regulator Leu3p is not involved in the induction of BAP3 expression, but may act as a repressor of BAP3 expression in the absence of leucine, as can be inferred from a transcriptional analysis in a Deltaleu3 mutant. By extensive deletion analysis of the BAP3 promoter fused to a GUS reporter, as well as by fusions of different parts of the BAP3 promoter to a LacZ reporter, we have found that a portion of the BAP3 promoter from - 418 to - 392 relative to the ATG start codon is both necessary and sufficient for the Stp1p-dependent induction of BAP3 expression by (most) amino acids. We have therefore named this sequence UASaa (amino acid-dependent upstream activator sequence). Neither Stp1p nor Leu3p appear to bind to the UASaa, at least in vitro, as judged from gel retardation assays. Sequences similar to the UASaa can be found in the promoters of BAP2, PTR2 and TAT1; genes that, like BAP3, encode permeases inducible by amino acids, suggesting that amino acid induction of all these genes is exerted via a common mechanism.

Mutations in five loci affecting GAP1-independent uptake of neutral amino acids in yeast

In order to identify genes involved in uptake of isoleucine, leucine and valine in Saccharomyces cerevisiae we isolated mutants that, on a complex medium, were sensitive to an inhibitor of the biosynthesis of the branched-chain amino acids. Mutants that in a secondary screen showed reduced uptake of isoleucine, leucine and valine when growing in synthetic complete medium were further characterized. Genetic analysis identified five loci, named ssy1 through ssy5. ssy2 corresponds to the previously characterized bap1 mutation, which we recently have found to be allelic to stp1. ssy1, ssy3 and ssy5 exhibit a reduced uptake of phenylalanine, methionine and threonine, as well. Furthermore, they are resistant to several neutral amino acid analogs. ssy4 only affects uptake of few neutral amino acids and is as sensitive as the wild type to the amino acid analogs tested. It was previously found that a C-terminal truncation of 29 codons of BAP2, which encodes a branched-chain amino acid permease, results in increased uptake of the branched-chain amino acids. We find epistasis of the C-terminally truncated BAP2 gene over the ssy4 mutation, while the other ssy mutations are epistatic over the truncated BAP2 gene. SSY1, SSY3 and SSY5 were cloned from a low-copy genomic library by complementation of the mutants. The SSY3 gene and the SSY5 gene show no significant homology to any sequence in the databases. SSY1 is a member of the major family of genes encoding amino acid permeases in yeast. We discuss possible roles of Ssy1p in amino acid uptake.

Cha4p of Saccharomyces cerevisiae activates transcription via serine/threonine response elements

The CHA1 gene of Saccharomyces cerevisiae encodes the catabolic L-serine (L-threonine) deaminase responsible for the utilization of serine/threonine as nitrogen sources. Previously, we identified two serine/threonine response elements in the CHA1 promoter, UASCHA. We report isolation of a mutation, cha4-1, that impairs serine/threonine induction of CHA1 transcription. The cha4-1 allele causes noninducibility of a CHA1 p-lacZ translational gene fusion, indicating that Cha4p exerts its action through the CHA1 promoter. Molecular and genetic mapping positioned the cha4 locus 17 cM centromere proximal to put1 on chromosome XII. The coding region of CHA4 predicts a 648-amino acid protein with a DNA-binding motif (residues 43-70) belonging to the Cys6 zinc cluster class. Gel retardation employing a recombinant peptide, Cha4p1-174, demonstrated that the peptide in vitro specifically binds UASCHA. Binding is abolished by a G-C to T-A mutation in the middle bases of the two CEZ-elements in UASCHA. The transcriptional activating ability of UASCHA derivatives in vivo correlates with their ability to bind Cha4p1-174 in vitro. We conclude that Cha4p is a positive regulator of CHA1 transcription and that Cha4p alone, or as part of a complex, is binding UASCHA.

Isolation and characterization of a cDNA encoding Arabidopsis thaliana 3-hydroxy-3-methylglutaryl-coenzyme A synthase

An 1.7-kb Arabidopsis thaliana (At) cDNA was isolated by complementation of a bap1 mutation affecting the transport of branched-chain amino acids (aa) in the yeast Saccharomyces cerevisiae. The determination of the nucleotide (nt) sequence revealed an open reading frame of 1383 nt which may encode a protein of 461 aa with a predicted molecular mass of 51,038 Da. The deduced aa sequence exhibited strong similarities with mammalian 3-hydroxy-3-methylglutaryl-coenzyme A synthase (HMGS) sequences. Although former biochemical studies have suggested that acetoacetyl-coenzyme A thiolase (AACT) and HMGS activities were carried by a single protein in plants, complementation studies and measurements of enzymatic activities clearly showed that the At HMGS is devoid of AACT activity.

BAP2, a gene encoding a permease for branched-chain amino acids in Saccharomyces cerevisiae

To select the gene coding for an isoleucine permease, an isoleucine dependent strain (ilv1 cha1) was transformed with a yeast genomic multicopy library, and colonies growing at a low isoleucine concentration were selected. Partial sequencing of the responsible plasmid insert revealed the presence of a previously sequenced 609 codon open reading frame of chromosome II with homology to known permeases. Deletion, extra dosage and C-terminal truncation of this gene were constructed in a strain lacking the general amino acid permease, and amino acid uptake was measured during growth in synthetic complete medium. The following observations prompted us to name the gene BAP2 (branched-chain amino acid permease). Deletion of BAP2 reduced uptake of leucine, isoleucine and valine by 25-50%, while the uptake of 8 other L-alpha-amino acids was unaltered or slightly increased. Introduction of BAP2 on a centromere-based vector, leading to a gene dosage of two or slightly more, caused a 50% increase in leucine uptake and a smaller increase for isoleucine and valine. However, when the 29 C-terminal codons of the plasmid-borne copy of BAP2 were substituted, the cells more than doubled the uptake of leucine, isoleucine and valine, while no or little increase in uptake was observed for the other 8 amino acids.

The immunosuppressant FK506 inhibits amino acid import in Saccharomyces cerevisiae

The immunosuppressants cyclosporin A, FK506, and rapamycin inhibit growth of unicellular eukaryotic microorganisms and also block activation of T lymphocytes from multicellular eukaryotes. In vitro, these compounds bind and inhibit two different types of peptidyl-prolyl cis-trans isomerases. Cyclosporin A binds cyclophilins, whereas FK506 and rapamycin bind FK506-binding proteins (FKBPs). Cyclophilins and FKBPs are ubiquitous, abundant, and targeted to multiple cellular compartments, and they may fold proteins in vivo. Previously, a 12-kDa cytoplasmic FKBP was shown to be only one of at least two FK506-sensitive targets in the yeast Saccharomyces cerevisiae. We find that a second FK506-sensitive target is required for amino acid import. Amino acid-auxotrophic yeast strains (trp1 his4 leu2) are FK506 sensitive, whereas prototrophic strains (TRP1 his4 leu2, trp1 HIS4 leu2, and trp1 his4 LEU2) are FK506 resistant. Amino acids added exogenously to the growth medium mitigate FK506 toxicity. FK506 induces GCN4 expression, which is normally induced by amino acid starvation. FK506 inhibits transport of tryptophan, histidine, and leucine into yeast cells. Lastly, several genes encoding proteins involved in amino acid import or biosynthesis confer FK506 resistance. These findings demonstrate that FK506 inhibits amino acid import in yeast cells, most likely by inhibiting amino acid transporters. Amino acid transporters are integral membrane proteins which import extracellular amino acids and constitute a protein family sharing 30 to 35% identity, including eight invariant prolines. Thus, the second FK506-sensitive target in yeast cells may be a proline isomerase that plays a role in folding amino acid transporters during transit through the secretory pathway.

The immunosuppressant FK506 inhibits amino acid import in Saccharomyces cerevisiae

The immunosuppressants cyclosporin A, FK506, and rapamycin inhibit growth of unicellular eukaryotic microorganisms and also block activation of T lymphocytes from multicellular eukaryotes. In vitro, these compounds bind and inhibit two different types of peptidyl-prolyl cis-trans isomerases. Cyclosporin A binds cyclophilins, whereas FK506 and rapamycin bind FK506-binding proteins (FKBPs). Cyclophilins and FKBPs are ubiquitous, abundant, and targeted to multiple cellular compartments, and they may fold proteins in vivo. Previously, a 12-kDa cytoplasmic FKBP was shown to be only one of at least two FK506-sensitive targets in the yeast Saccharomyces cerevisiae. We find that a second FK506-sensitive target is required for amino acid import. Amino acid-auxotrophic yeast strains (trp1 his4 leu2) are FK506 sensitive, whereas prototrophic strains (TRP1 his4 leu2, trp1 HIS4 leu2, and trp1 his4 LEU2) are FK506 resistant. Amino acids added exogenously to the growth medium mitigate FK506 toxicity. FK506 induces GCN4 expression, which is normally induced by amino acid starvation. FK506 inhibits transport of tryptophan, histidine, and leucine into yeast cells. Lastly, several genes encoding proteins involved in amino acid import or biosynthesis confer FK506 resistance. These findings demonstrate that FK506 inhibits amino acid import in yeast cells, most likely by inhibiting amino acid transporters. Amino acid transporters are integral membrane proteins which import extracellular amino acids and constitute a protein family sharing 30 to 35% identity, including eight invariant prolines. Thus, the second FK506-sensitive target in yeast cells may be a proline isomerase that plays a role in folding amino acid transporters during transit through the secretory pathway.

A study of branched-chain amino acid aminotransferase and isolation of mutations affecting the catabolism of branched-chain amino acids in Saccharomyces cerevisiae

The specific activity of branched-chain amino acid aminotransferase was highest when S. cerevisiae was grown in minimal medium containing a branched-chain amino acid as nitrogen source. Growth in complex media with glycerol or ethanol gave moderately high levels, whereas with glucose and fructose the specific activity was very low. Mutagenesis defined three genes (BAA1 to BAA3) required for branched-chain amino acid catabolism. The baa1 mutation reduced the specific activity of the aminotransferase, the stationary phase density in YEPD and caused gross morphological disturbance. Branched-chain amino acid aminotransferase is essential for sporulation.