Amino acid transporters yeast

From a genetical point of view, Saccharomyces cerevisiae is an ideal organism which may be considered the Escherichia coli of eukaryotic cells [4,5]. This is true in particular for the study of metabolic regulation and for that of membrane transport [6]. Finally, the astonishing resemblance between many yeast proteins and certain mammalian-cell proteins has seriously broadened the scope of interest. Although a few reports have appeared on amino acid transport in some other yeasts, most investigations in this field have used strains of Saccharomyces cerevisiae. 

Furthermore, the multiplicity and diversity of amino acid transporters allow yeast to accumulate amino acids for both biosynthesis and catabolism under a... 

Feedback inhibition of amino acid transporters by amino acids synthesized by the cells might be responsible for the well known fact that blocking protein synthesis by cycloheximide in Saccharomyces cerevisiae inhibits the uptake of most amino acids [56]. Indeed, under these conditions, endogenous amino acids continue to accumulate. This situation, which precludes studying amino acid transport in yeast in the presence of inhibitors of protein synthesis, is very different from that observed in bacteria, where amino acid uptake is commonly measured in the presence of chloramphenicol in order to isolate the uptake process from further metabolism of accumulated substances. In yeast, when nitrogen starvation rather than cycloheximide is used to block protein synthesis, this leads to very high uptake activity. This fact supports the feedback inhibition interpretation of the observed cycloheximide effect. 

Ethanol strongly limits amino acid transport. It modifies the composition and the properties of the phospholipids of the plasmic membrane. The membrane becomes more permeable. The H+ ions of the medium massively penetrate the interior of the cell by simple diffusion. The membrane ATPase must increase its operation to control the intracellular pH. As soon as this task monopolizes the ATPase, the symport of the amino acids no longer functions. In other words, at the beginning of fermentation, and for as long as the ethanol concentration in the must is low, yeasts can rapidly assimilate amino acids and concentrate them in the vacuoles for later use, according to then-biosynthesis needs. 

Suspended solids also supply yeasts with nutritional elements and adsorb certain metabolic inhibitors. In fact, these two effects are related and significant The lipid fraction of suspended solids provides the principal nutritional supply (Section 13.5.1)—in particular, long chain unsaturated fatty acids (Cig) that the yeast can incorporate into its own membrane phospholipids. Sugar and amino acid transport systems across the yeast membrane are conseqnently improved. Due to their hydrophobic Upid content suspended solids are capable of adsorbing toxic inhibitive fatty acids freed in the jnice during alcoholic fermentation (Cg, Cio, C12). The combination of these two effects (lipidic nutrition and toxic fatty acid adsorption) produces a survival factor effect for yeasts (Section 3.5.2) (Ollivier et al., 1987 Alexandre et al., 1994). 

Abstract Polyamine (PA) transport has been analyzed at the ceU and organ levels in several plant species, but currently the molecular mechanisms of PA transport are not completely understood. Several papers recently identified plant PA transporters. A study of Arabidopsis oxidative stress responses to the herbicide paraquat (PQ) identified a LAT (L-type amino acid transporter) family protein as a transporter of both PQ and PA. Other studies based on yeast complementation analyses revealed that LAT family genes in rice md Arabidopsis function as PA transporters. 

In Saccharomyces cerevisiae, as in most eukaryotic cells, the plasma membrane is not freely permeable to nitrogenous compounds such as amino acids. Therefore, the first step in their utilization is their catalyzed transport across the plasma membrane. Most of the transported amino acids are accumulated inside the yeast cells against a concentration gradient. When amino acids are to be used as a general source of nitrogen, this concentration is crucial because most enzymes which catalyze the first step of catabolic pathways have a low affinity for their substrates. 

A rather satisfactory explanation of the irreversibility of amino acid accumulation in yeast cells is that it might result from specific regulatory mechanisms capable of immobilizing the transporters in a closed position. Uptake of amino acids by a number of permeases does indeed appear to be regulated by specific, and possibly allosteric, feedback inhibition. This idea is based on the fact that a number of transport systems seem to be specifically inhibited by their internally accumulated... 

The data presented in Table 3, which includes the amino acid composition of baker s yeast and Candida krusei cytochrome c for comparison, show that Ustilago and Neurospora cytochrome c contain the same number of total residues. In seven instances, the number of residues of a particular amino acid/mole are identical. Thus, even in the absence of a sequence for the Ustilago cytochrome it can be concluded that this protein, unlike the siderochromes, has suffered little alteration in the progression from the Ascomycetes to the Basidiomycetes. This can be ascribed to the varying function of the two types of molecules. Cytochrome c must fit into a relatively specific slot bounded by a reductase and an oxidase and it has hence evolved much more slowly than the more freely acting transport agents where the specificity constraints are less demanding. 

In bacteria and yeasts, Li+ has strain-dependent, inhibitory, and morphological effects upon growth. The driving force behind the transport of carbohydrates and amino acids in bacteria is the proton gradient, and in both E. coli [228] and Salmonella typhimurium cells [229], Li+ stimulates the movement of proline into cells via a Li+/proline symport and the transport of melibiose via a cotransport pathway [230]. In both cases, Li+ is replacing Na+ and results in the inhibition of growth. 

Administration of inorganic trivalent chromium compounds or extracts of brewers yeast resulted in decreased blood glucose levels and cholesterol levels and regression of atherosclerotic plaques (Pi-Sunyer and Offenbacher 1984). Improved insulin sensitivity also resulted in an increased incorporation of amino acids into proteins and cell transport of amino acid in rats receiving supplemental chromium (Roginski and Mertz 1969). 

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