SUMMARY We review the mechanisms responsible for amino acid homeostasis in Saccharomyces cerevisiae and other fungi. Amino acid homeostasis is essential for cell growth and survival. Hence, the de novo synthesis reactions, metabolic conversions, and transport of amino acids are tightly regulated. Regulation varies from nitrogen pool sensing to control by individual amino acids and takes place at the gene (transcription), protein (posttranslational modification and allostery), and vesicle (trafficking and endocytosis) levels. The pools of amino acids are controlled via import, export, and compartmentalization. In yeast, the majority of the amino acid transporters belong to the APC (amino acid-polyamine-organocation) superfamily, and the proteins couple the uphill transport of amino acids to the electrochemical proton gradient. Although high-resolution structures of yeast amino acid transporters are not available, homology models have been successfully exploited to determine and engineer the catalytic and regulatory functions of the proteins. This has led to a further understanding of the underlying mechanisms of amino acid sensing and subsequent downregulation of transport. Advances in optical microscopy have revealed a new level of regulation of yeast amino acid transporters, which involves membrane domain partitioning. The significance and the interrelationships of the latest discoveries on amino acid homeostasis are put in context.

The import of basic amino acids in Saccharomyces cerevisiae has been reported to be unidirectional, which is not typical of how secondary transporters work. Since studies of energy coupling and transport kinetics are complicated in vivo, we purified the major lysine transporter (Lyp1) of yeast and reconstituted the protein into lipid vesicles. We show that the Michaelis constant (KM) of transport from out-to-in is well in the millimolar range and at least 3 to 4-orders of magnitude higher than that of transport in the opposite direction, disfavoring the efflux of solute via Lyp1. We also find that at low values of the proton motive force, the transport by Lyp1 is comparatively slow. We benchmarked the properties of eukaryotic Lyp1 to that of the prokaryotic homologue LysP and find that LysP has a similar KM for transport from in-to-out and out-to-in, consistent with rapid influx and efflux. We thus explain the previously described unidirectional nature of lysine transport in S. cerevisiae by the extraordinary kinetics of Lyp1 and provide a mechanism and rationale for previous observations. The high asymmetry in transport together with secondary storage in the vacuole allow the cell to accumulate basic amino acids to very high levels.

Amino acid permeases (AAPs) in the plasma membrane (PM) of Saccharomyces cerevisiae are responsible for the uptake of amino acids and involved in regulation of their cellular levels. Here, we report on a strong and complex module for PM association found in the C-terminal tail of AAPs. Using in silico analyses and mutational studies we found that the C-terminal sequences of Gap1, Bap2, Hip1, Tat1, Tat2, Mmp1, Sam3, Agp1, and Gnp1 are about 50 residues long, associate with the PM, and have features that discriminate them from the termini of organellar amino acid transporters. We show that this sequence (named PM asseq ) contains an amphipathic ␣-helix and the FWC signature, which is palmitoylated by palmitoyltransferase Pfa4. Variations of PM asseq , found in different AAPs, lead to different mobilities and localization patterns, whereas the disruption of the sequence has an adverse effect on cell viability. We propose that PM asseq modulates the function and localization of AAPs along the PM. PM asseq is one of the most complex protein signals for plasma membrane association across species and can be used as a delivery vehicle for the PM.Yeast transport amino acids across the plasma membrane (PM), 4 the vacuolar membrane (VM), and to a lesser extent the mitochondrial membrane (1). In the plasma and vacuolar membranes, there are 22 and 11 secondary amino acid transporters, respectively. These transporters are polytopic membrane proteins with 10 -14 transmembrane segments. They belong to three superfamilies: amino acid/polyamine/organocation (APC), major facilitator superfamily (MFS), and amino acid/ polyamine transporter II (AAPTII) (2). The amino acid transporters belonging to the APC superfamily are often referred to as amino acid permeases (AAPs). They are mainly localized in the PM and can be highly specific, e.g. transport only one (enantiomer) amino acid or have a broad range of substrates, like the general amino acid permease Gap1.Alongside the transporter function, additional roles have been proposed for some AAPs. The most prominent example is Gap1, which has a receptor function whereby it signals the protein kinase A (PKA) pathway (3). This so-called transceptor function has also been described for the phosphate transporter Pho84 and the ammonium transporter Mep2 but not for any other AAPs (4, 5). On the other end of the spectrum is Ssy1, an endoplasmic reticulum (ER)-resident AAP member, that plays a role in amino acid sensing but has no transport function (6 -8). The levels of AAPs at the PM are modulated by several transcriptional and post-translational control mechanisms, e.g. nitrogen catabolite repression, general amino acid control, and substrate/stress-induced endocytosis (9 -12). Although for several AAPs many details of these control mechanisms have been elucidated, quite a few questions concerning the spatiotemporal regulation of AAPs remain unanswered. Which mechanism ensures that the function is performed in the right organelle and what determines the positioning of transporters within diff...

Amino acids are essential metabolites but can also be toxic when present at high levels intracellularly. Substrate-induced downregulation of amino acid transporters in Saccharomyces cerevisiae is thought to be a mechanism to avoid this toxicity. It has been shown that unregulated uptake by the general amino acid permease Gap1 causes cells to become sensitive to amino acids. Here, we show that overexpression of eight other amino acid transporters (Agp1, Bap2, Can1, Dip5, Gnp1, Lyp1, Put4, or Tat2) also induces a growth defect when specific single amino acids are present at concentrations of 0.5–5 mM. We can now state that all proteinogenic amino acids, as well as the important metabolite ornithine, are growth inhibitory to S. cerevisiae when transported into the cell at high enough levels. Measurements of initial transport rates and cytosolic pH show that toxicity is due to amino acid accumulation and not to the influx of co-transported protons. The amino acid sensitivity phenotype is a useful tool that reports on the in vivo activity of transporters and has allowed us to identify new transporter-specific substrates.

BilE has been reported as a bile resistance determinant that plays an important role in colonization of the gastrointestinal tract by Listeria monocytogenes, the causative agent of listeriosis. The mechanism(s) by which BilE mediates bile resistance are unknown. BilE shares significant sequence similarity with ATP-binding cassette (ABC) importers that contribute to virulence and stress responses by importing quaternary ammonium compounds that act as compatible solutes. Assays using related compounds have failed to demonstrate transport mediated by BilE. The putative substrate-binding domain (SBD) of BilE was expressed in isolation and the crystal structure solved at 1.5 Å. Although the overall fold is characteristic of SBDs, the binding site varies considerably relative to the well-characterized homologs ProX from Archaeoglobus fulgidus and OpuBC and OpuCC from Bacillus subtilis. This suggests that BilE may bind an as-yet unknown ligand. Elucidation of the natural substrate of BilE could reveal a novel bile resistance mechanism.