A high affinity glutathione transporter has been identified, cloned, and characterized from the yeast Saccharomyces cerevisiae. This transporter, Hgt1p, represents the first high affinity glutathione transporter to be described from any system so far. The strategy for the identification involved investigating candidate glutathione transporters from the yeast genome sequence project followed by genetic and physiological investigations. This approach revealed HGT1 (open reading frame YJL212c) as encoding a high affinity glutathione transporter. Yeast strains deleted in HGT1 did not show any detectable plasma membrane glutathione transport, and hgt1⌬ disruptants were non-viable in a glutathione biosynthetic mutant (gsh1⌬) background. The glutathione repressible transport activity observed in wild type cells was also absent in the hgt1⌬ strains. The transporter was cloned and kinetic studies indicated that Hgt1p had a high affinity for glutathione (K m ‫؍‬ 54 M)) and was not sensitive to competition by amino acids, dipeptides, or other tripeptides. Significant inhibition was observed, however, with oxidized glutathione and glutathione conjugates. The transporter reveals a novel class of transporters that has homologues in other yeasts and plants but with no apparent homologues in either Escherichia coli or in higher eukaryotes other than plants.

The enzyme Psd2 catalyzes endosomal synthesis of the phospholipid PE. While this pool of PE represents a minority of total cellular PE, function of Psd2 is required for normal activity of the vacuolar ABC transporter Ycf1. Psd2 controls vacuolar PE levels by acting at the level of the endosome.

Multidrug resistance in the yeast

Med12 is a transcriptional Mediator subunit most typically associated with negative control of gene expression. Here Med12 is demonstrated to serve as a positive regulator required for activation of multidrug resistance gene expression in yeast cells lacking their mitochondrial genome.

Summary Pathogenic fungi present a special problem in the clinic as the range of drugs that can be used to treat these types of infections is limited. This situation is further complicated by the presence of robust inducible gene networks encoding different proteins that confer tolerance to many available antifungal drugs. The transcriptional control of these multidrug resistance systems in several key fungi will be discussed. Experiments in the non-pathogenic Saccharomyces cerevisiae have provided much of our current understanding of the molecular framework on which fungal multidrug resistance is built. More recent studies on the important pathogenic Candida species, Candida albicans and Candida glabrata, have provided new insights into the organization of the multidrug resistance systems in these organisms. We will compare the circuitry of multidrug resistance networks in these three organisms and suggest that, in addition to the well-accepted drug efflux activities, the regulation of membrane composition by multidrug resistance proteins provides an important contribution to the resistant phenotypes observed.

One of the most common origins of multidrug resistance occurs via the overproduction of ATP-binding cassette (ABC) transporter proteins. These ABC transporters then act as broad specificity drug pumps and efflux a wide range of toxic agents out of the cell. The yeast Saccharomyces cerevisiae exhibits multiple or pleiotropic drug resistance (Pdr) often through the overproduction of a plasma membrane-localized ABC transporter protein called Pdr5p. Expression of the PDR5 gene is controlled by two zinc cluster-containing transcription factors called Pdr1p and Pdr3p. Cells that lack their mitochondrial genome ( 0 cells) strongly induce PDR5 transcription in a Pdr3p-dependent fashion. To identify proteins associated with Pdr3p that might act to regulate this factor, a tandem affinity purification (TAP) moiety was fused to Pdr3p, and this recombinant protein was purified from yeast cells. The same similarities to mammalian cells that have made yeast a powerful model eukaryotic organism restrict the ability to design antifungal drugs without also impacting the health of the animal host. This has led to a limited repertoire of antifungal drugs and makes the development of drug-resistant fungi a special burden on chemotherapy of infections by these organisms. Even more serious is the acquisition of fungi with a multidrug resistance phenotype that can permit tolerance to many antifungal agents with a single genetic change (reviewed in Ref. 1). Saccharomyces cerevisiae mutant strains with a single nuclear mutation have been isolated which enable these mutants to become tolerant of a wide range of toxic compounds. These mutant cells are referred to as having a pleiotropic drug resistant (Pdr) 2 phenotype (reviewed in Ref.2). Mutations either within genes encoding transcriptional regulators of PDR genes or in their regulatory inputs led to overexpression of downstream transporter proteins with associated multidrug resistance. The most extensively studied gene contributing to this pathway in S. cerevisiae is PDR5, which encodes an ATPbinding cassette (ABC) transporter that exhibits broad spectrum drug efflux activity (3-5). Transcription of PDR5 and other Pdr pathway genes are controlled in large measure by the Zn(II) 2 Cys 6 zinc finger regulators Pdr1p and Pdr3p (reviewed in Refs. 6 and 7).Substitution mutant forms of Pdr1p and Pdr3p transcription factors have been identified that lead to high level overexpression of Pdr5p with an associated increase in multidrug resistance (8 -10). Genetic experiments indicate that these mutant transcription factors behave as dominant, hyperactive forms of the regulatory proteins. Pdr1p and Pdr3p share partially overlapping function and control expression of their target genes by binding to a sequence element referred to as the Pdr1p/Pdr3p response element (PDRE) (11,12). The PDR3 gene itself is also controlled by two PDREs in its promoter and involves an autoregulatory loop (13). The relative ease of isolation of these hyperactive transcription factors led to the suggestion that both Pdr1p...

Mature amyloid fibrils are believed to be formed by the lateral association of discrete structural units designated as protofibrils, but this lateral association of protofibrils has never been directly observed. We have recently characterized a thioesterase from Alcaligenes faecalis, which was shown to exist as homomeric oligomers with an average diameter of 21.6 nm consisting of 22 kDa subunits in predominantly beta-sheet structure. In this study, we have shown that upon incubation in a 75% ethanol solution, the oligomeric particles of protein were transformed into amyloid-like fibrils. TEM pictures obtained at various stages during fibril growth helped us to understand to a certain extent the early events in the fibrillization process. When incubated in 75% ethanol, oligomeric particles of protein grew to approximately 35-40 nm in diameter before fusion. Fusion of two oligomers of 35-40 nm resulted in the formation of a fibril. Fibril formation was accompanied by a reduction in the diameter of the particle to approximately 20-25 nm along with concomitant elongation to approximately 110 nm, indicating reorganization and strengthening of the structure. The elongation process continued by sequential addition of oligomeric units to give fibers 500-1000 nm in length with a further reduction in diameter to 17-20 nm. Further elongation resulted in the formation of fibers that were more than 4000 nm in length; the diameter, however, remained constant at 17-20 nm. These data clearly show that the mature fibrils have assembled via longitudinal growth of oligomers and not via lateral association of protofibrils.

Long-chain acyl-CoA thioesterases (EC 3.1.2.2) hydrolyze acyl-CoA esters to nonesterified fatty acids and coenzyme A (CoASH) [1]. These are ubiquitously expressed in bacteria, yeast, plants and mammals, and in most cell compartments, such as endoplasmic reticulum, cytosol, mitochondria and peroxisomes. Several unrelated thioesterases have been purified to homogeneity from plants, animals, and bacteria, and the cDNAs encoding several of them have been cloned and sequenced [2][3][4][5][6][7]. Although the physiological functions of these enzymes remain largely unknown, it is speculated that they regulate lipid metabolism by maintaining appropriate concentrations of acyl-CoA, CoASH, and nonesterified fatty acids. The only established function for acyl-CoA thioesterases is in the termination of fatty acid synthesis in eukaryotes [8].Two thioesterases, I and II, that cleave acyl-CoA molecules in vitro have been characterized from A novel long-chain acyl-CoA thioesterase from Alcaligenes faecalis has been isolated and characterized. The protein was extracted from the cells with 1 m NaCl, which required 1.5-fold, single-step purification to yield near-homogeneous preparations. In solution, the protein exists as homomeric aggregates, of mean diameter 21.6 nm, consisting of 22-kDa subunits. MS ⁄ MS data for peptides obtained by trypsin digestion of the thiosterase did not match any peptide from Escherichia coli thioesterases or any other thioesterases in the database. The thioesterase was associated exclusively with the surface of cells as revealed by ultrastructural studies using electron microscopy and immunogold labeling. It hydrolyzed saturated and unsaturated fatty acyl-CoAs of C 12 to C 18 chain length with V max and K m of 3.58-9.73 lmolAEmin )1 AE(mg protein) )1 and 2.66-4.11 lm, respectively. A catalytically important histidine residue is implicated in the active site of the enzyme. The thioesterase was active and stable over a wide range of temperature and pH. Maximum activity was observed at 65°C and pH 10.5, and varied between 60% and 80% at temperatures of 25-70°C and pH 6.5-10. The thioesterase also hydrolyzed p-nitrophenyl esters of C 2 to C 12 chain length, but substrate competition experiments demonstrated that the long-chain acyl-CoAs are better substrates for thioesterase than p-nitrophenyl esters. When assayed at 37 and 20°C, the affinity and catalytic efficiency of the thioesterase for palmitoleoyl-CoA and cis-vaccenoyl-CoA were reduced approximately twofold at the lower temperature, but remained largely unaltered for palmitoyl-CoA.Abbreviations DTNB, 5,5¢-dithiobis(2-nitrobenzoic acid); TEM, transmission electron microscopy.