Joakim Norbeck

Purification and Characterization of Two Isoenzymes of DL-Glycerol-3-phosphatase from Saccharomyces cerevisiae IDENTIFICATION OF THE CORRESPONDING GPP1 AND GPP2 GENES AND EVIDENCE FOR OSMOTIC REGULATION OF Gpp2p EXPRESSION BY THE OSMOSENSING MITOGEN-ACTIVATED PROTEIN KINASE SIGNAL TRANSDUCTION PATHWAY

The existence of specific DL-glycerol-3-phosphatase (EC) activity in extracts of Saccharomyces cerevisiae was confirmed by examining strains lacking nonspecific acid and alkaline phosphatase activities. During purification of the glycerol-3-phosphatase, two isozymes having very similar molecular weights were isolated by gel filtration and anion exchange chromatography. By microsequencing of trypsin-generated peptides the corresponding genes were identified as previously sequenced open reading frames of unknown function. The two genes, GPP1 (YIL053W) and GPP2 (YER062C) encode proteins that show 95% amino acid identity and have molecular masses of 30.4 and 27.8 kDa, respectively. The intracellular concentration of Gpp2p increases in cells subjected to osmotic stress, while the production of Gpp1p is unaffected by changes of external osmolarity. Both isoforms have a high specificity for DL-glycerol-3-phosphate, pH optima at 6.5, and KG3Pm in the range of 3-4 mM. The osmotic induction of Gpp2p is blocked in cells that are defective in the HOG-mitogen-activated protein kinase pathway, indicating that GPP2 is a target gene for this osmosensing signal transduction pathway. Together with DOG1 and DOG2, encoding two highly homologous enzymes that dephosphorylate 2-deoxyglucose-6-phosphate, GPP1 and GPP2 constitute a new family of genes for low molecular weight phosphatases.

Metabolic and regulatory changes associated with growth of Saccharomyces cerevisiae in 1.4 M NaCl. Evidence for osmotic induction of glycerol dissimilation via the dihydroxyacetone pathway

The salt-instigated protein expression of Saccharomyces cerevisiae during growth in either 0.7 or 1.4 M NaCl was studied by two-dimensional polyacrylamide gel electrophoresis. The 73 protein spots that were identified as more than 3-fold responsive in 1.4 M NaCl were further grouped by response class (halometric, low-salt, and high-salt regulation). Roughly 40% of these responsive proteins were found to decrease in expression, while at higher magnitudes of change (>8-fold) only induction was recorded. Enolase 1 (Eno1p) was the most increasing protein by absolute numbers per cell, but not by -fold change, and the enzymes involved in glycerol synthesis, Gpd1p and Gpp2p, were also induced to a similar degree as Eno1p. We furthermore present evidence for salt induction of glycerol dissimilation via dihydroxyacetone and also identify genes putatively encoding the two enzymes involved; dihydroxyacetone kinase (DAK1 and DAK2) and glycerol dehydrogenase (YPR1 and GCY1). The GPD1, GPP2, GCY1, DAK1, and ENO1 genes all displayed a halometric increase in the amount of transcript. This increase was closely linked to the salt-induced rate of protein synthesis of the corresponding proteins, indicating mainly transcriptional regulation of expression for these genes. A consensus element with homology to the URS sequence of the ENO1 promoter was found in the promoters of the GPD1, GPP2, GCY1, and DAK1 genes.

Identification and specificities of N‐terminal acetyltransferases from Saccharomyces cerevisiae

N‐terminal acetylation can occur cotranslationally on the initiator methionine residue or on the penultimate residue if the methionine is cleaved. We investigated the three N‐terminal acetyltransferases (NATs), Ard1p/Nat1p, Nat3p and Mak3p. Ard1p and Mak3p are significantly related to each other by amino acid sequence, as is Nat3p, which was uncovered in this study using programming alignment procedures. Mutants deleted in any one of these NAT genes were viable, but some exhibited diminished mating efficiency and reduced growth at 37°C, and on glycerol and NaCl‐containing media. The three NATs had the following substrate specificities as determined in vivo by examining acetylation of 14 altered forms of iso‐1‐cytochrome c and 55 abundant normal proteins in each of the deleted strains: Ard1p/Nat1p, subclasses with Ser‐, Ala‐, Gly‐ and Thr‐termini; Nat3p, Met‐Glu‐ and Met‐Asp‐ and a subclass of Met‐Asn‐termini; and Mak3p subclasses with Met‐Ile‐ and Met‐Leu‐termini. In addition, a special subclass of substrates with Ser‐Glu‐ Phe‐, Ala‐Glu‐Phe‐ and Gly‐Glu‐Phe‐termini required all three NATs for acetylation.

Adhesion of Type 1-Fimbriated Escherichia coli to Abiotic Surfaces Leads to Altered Composition of Outer Membrane Proteins

Phenotypic differences between planktonic bacteria and those attached to abiotic surfaces exist, but the mechanisms involved in the adhesion response of bacteria are not well understood. By the use of two-dimensional (2D) polyacrylamide gel electrophoresis, we have demonstrated that attachment of Escherichia coli to abiotic surfaces leads to alteration in the composition of outer membrane proteins. A major decrease in the abundance of resolved proteins was observed during adhesion of type 1-fimbriated E. coli strains, which was at least partly caused by proteolysis. Moreover, a study of fimbriated and nonfimbriated mutants revealed that these changes were due mainly to type 1 fimbria-mediated surface contact and that only a few changes occurred in the outer membranes of nonfimbriated mutant strains. Protein synthesis and proteolytic degradation were involved to different extents in adhesion of fimbriated and nonfimbriated cells. While protein synthesis appeared to affect adhesion of only the nonfimbriated strain, proteolytic activity mostly seemed to contribute to adhesion of the fimbriated strain. Using matrix-assisted laser desorption ionization-time of flight mass spectrometry, six of the proteins resolved by 2D analysis were identified as BtuB, EF-Tu, OmpA, OmpX, Slp, and TolC. While the first two proteins were unaffected by adhesion, the levels of the last four were moderately to strongly reduced. Based on the present results, it may be suggested that physical interactions between type 1 fimbriae and the surface are part of a surface-sensing mechanism in which protein turnover may contribute to the observed change in composition of outer membrane proteins. This change alters the surface characteristics of the cell envelope and may thus influence adhesion.

Nucleocytoplasmic Distribution of Budding Yeast Protein Kinase A Regulatory Subunit Bcy1 Requires Zds1 and Is Regulated by Yak1-Dependent Phosphorylation of Its Targeting Domain

ABSTRACT In Saccharomyces cerevisiae the subcellular distribution of Bcy1 is carbon source dependent. In glucose-grown cells, Bcy1 is almost exclusively nuclear, while it appears more evenly distributed between nucleus and cytoplasm in carbon source-derepressed cells. Here we show that phosphorylation of its N-terminal domain directs Bcy1 to the cytoplasm. Biochemical fractionation revealed that the cytoplasmic fraction contains mostly phosphorylated Bcy1, whereas unmodified Bcy1 is predominantly present in the nuclear fraction. Site-directed mutagenesis of two clusters (I and II) of serines near the N terminus to alanine resulted in an enhanced nuclear accumulation of Bcy1 in ethanol-grown cells. In contrast, substitutions to Asp led to a dramatic increase of cytoplasmic localization in glucose-grown cells. Bcy1 modification was found to be dependent on Yak1 kinase and, consequently, in ethanol-grown yak1 cells the Bcy1 remained nuclear. A two-hybrid screen aimed to isolate genes encoding proteins that interact with the Bcy1 N-terminal domain identified Zds1. In ethanol-grown zds1 cells, cytoplasmic localization of Bcy1 was largely absent, while overexpression of ZDS1 led to increased cytoplasmic Bcy1 localization. Zds1 does not regulate Bcy1 modification since this was found to be unaffected inzds1 cells. However, in zds1 cells cluster II-mediated, but not cluster I-mediated, cytoplasmic localization of Bcy1 was found to be absent. Altogether, these results suggest that Zds1-mediated cytoplasmic localization of Bcy1 is regulated by carbon source-dependent phosphorylation of cluster II serines, while cluster I acts in a Zds1-independent manner.

The level of cAMP-dependent protein kinase A activity strongly affects osmotolerance andosmo-instigated gene expression changes inSaccharomyces cerevisiae

The influence of cAMP‐dependent protein kinase (PKA) on protein expression during exponential growth under osmotic stress was studied by two‐dimensional polyacrylamide gel electrophoresis (2D–PAGE). The responses of isogenic strains (tpk2Δtpk3Δ) with either constitutively low (tpk1w1), regulated (TPK1) or constitutively high (TPK1bcy1Δ) PKA activity were compared. The activity of cAMP‐dependent protein kinase (PKA) was shown to be a major determinant of osmotic shock tolerance. Proteins with increased expression during growth under sodium chloride stress could be grouped into three classes with respect to PKA activity, with the glycerol metabolic proteins GPD1, GPP2 and DAK1 standing out as independent of PKA. The other osmotically induced proteins displayed a variable dependence on PKA activity; fully PKA‐dependent genes were TPS1 and GCY1, partly PKA‐dependent genes were ENO1, TDH1, ALD3 and CTT1. The proteins repressed by osmotic stress also fell into distinct classes of PKA‐dependency. Ymr116c was PKA‐independent, while Pgi1p, Sam1p, Gdh1p and Vma1p were fully PKA‐dependent. Hxk2p, Pdc1p, Ssb1p, Met6p, Atp2p and Hsp60p displayed a partially PKA‐dependent repression. The promotors of all induced PKA‐dependent genes have STRE sites in their promotors suggestive of a mechanism acting via Msn2/4p. The mechanisms governing the expression of the other classes are unknown. From the protein expression data we conclude that a low PKA activity causes a protein expression resembling that of osmotically stressed cells, and furthermore makes cells tolerant to this type of stress. Copyright

Two-dimensional electrophoretic separation of yeast proteins using a non-linear wide range (pH 3–10) immobilized pH gradient in the first dimension; reproducibility and evidence for isoelectric focusing of alkaline (pI >7) proteins

The proteome of the yeast Saccharomyces cerevisiae was analysed by two‐dimensional (2D) polyacrylamide gel electrophoresis utilizing a non‐linear immobilized pH gradient (3–10) in the first‐dimensional separation. Cells were labelled by [35S]methionine incorporation in the respiro‐fermentative phase during exponential growth on glucose. Gels were run, visualized with phosphoimager technology and all resolved proteins automatically quantified. Proteins were well resolved over the whole pH interval, and evidence for isoelectric focusing on the basic side of the pattern was generated by sequencing of some spots, revealing the 2D positions of Tef1p, Pgk1p, Gpm1p, Tdh1p and Shm2p. Roughly 25% of the spots were resolved at the alkaline side of the pattern (pI>7). The position reproducibility was high and in the range 1–2 mm in the x‐and y‐dimension, respectively. No quantitative variation was linked to a certain size or charge class of resolved proteins, and the average quantitative standard deviation was 17±11%. The obtained immobilized pH gradient based pattern could easily be compared to the old ampholine‐based 2D pattern, and the previously reported identifications could thus be transferred. Our yeast pattern currently contains 43 known proteins, all identified by protein sequencing. Utilizing these identified proteins, relevant pI and Mr scales in the pattern were constructed. Normalization of the expression of identified spots by compensating for the number of methionine residues a protein contains allowed stoichiometric comparisons. The most dominant proteins under these growth conditions were Tdh3p, Fba1p, Eno2p and Tef1p/Tef2p, all being expressed at more than 500 000 copies per cell. The differential carbon source response during exponential growth on either glucose, galactose or ethanol was examined for the alkaline proteins identified by micro‐sequencing in this study.

Dihydroxyacetone kinases in Saccharomyces cerevisiae are involved in detoxification of dihydroxyacetone.

The genes YML070W/DAK1 andYFL053W/DAK2 in the yeast Saccharomyces cerevisiae were characterized by a combined genetic and biochemical approach that firmly functionally classified their encoded proteins as dihydroxyacetone kinases (DAKs), an enzyme present in most organisms. The kinetic properties of the two isoforms were similar, exhibiting K m (DHA) of 22 and 5 μm and K m (ATP) of 0.5 and 0.1 mm for Dak1p and Dak2p, respectively. We furthermore show that their substrate, dihydroxyacetone (DHA), is toxic to yeast cells and that the detoxification is dependent on functional DAK. The importance of DAK was clearly apparent for cells where both isogenes were deleted (dak1Δdak2Δ), since this strain was highly sensitive to DHA. In the opposite case, overexpression of either DAK1 or DAK2 made thedak1Δdak2Δ highly resistant to DHA. In fact, overexpression of either DAK provided cells with the capacity to grow efficiently on DHA as the only carbon and energy source, with a generation time of about 5 h. The DHA toxicity was shown to be strongly dependent on the carbon and energy source utilized, since glucose efficiently suppresses the lethality, whereas galactose or ethanol did so to a much lesser extent. However, this suppression was found not to be explained by differences in DHA uptake, since uptake kinetics revealed a simple diffusion mechanism with similar capacity independent of carbon source. Salt addition strongly aggravated the DHA toxicity, independent of carbon source. Furthermore, the DHA toxicity was not linked to the presence of oxygen or to the known harmful agents methylglyoxal and formaldehyde. It is proposed that detoxification of DHA may be a vital part of the physiological response during diverse stress conditions in many species.

NADH-reductive stress in Saccharomyces cerevisiae induces the expression of the minor isoform of glyceraldehyde-3-phosphate dehydrogenase ( TDH1 )

Abstract A strain of Saccharomyces cerevisiae lacking the GPD2 gene, encoding one of the glycerol-3-phosphate dehydrogenases, grows slowly under anaerobic conditions, due to reductive stress caused by the accumulation of cytoplasmic NADH. We used 2D-PAGE to study the effect on global protein expression of reductive stress in the anaerobically grown gpd2Δ strain. The most striking response was a strongly elevated expression of Tdh1p, the minor isoform of glyceraldehyde-3-phosphate dehydrogenase. This increased expression could be reversed by the addition of acetoin, a NADH-specific redox sink, which furthermore largely restored anaerobic growth of the gpd2Δ strain. Additional deletion of the TDH1 gene (but not of TDH2 or TDH3) improved anaerobic growth of the gpd2Δ strain. We therefore propose that TDH1 has properties not displayed by the other TDH isogenes and that its expression is regulated by reductive stress caused by an excess of cytoplasmic NADH.

A complete inventory of all enzymes in the eukaryotic methionine salvage pathway

The methionine salvage pathway is universally used to regenerate methionine from 5′‐methylthioadenosine, a byproduct of certain reactions involving S‐adenosylmethionine. We identified and verified the genes encoding the enzymes of all steps in this cycle in a commonly used eukaryotic model system: the yeast Saccharomyces cerevisiae. The genes encoding 5′‐methylthioribose‐1‐phosphate isomerase and 5′‐methylthioribulose‐1‐phosphate dehydratase are herein named MRI1 and MDE1, respectively. The 5′‐methylthioadenosine phosphorylase was verified as Meu1p, the 2,3‐dioxomethiopentane‐1‐phosphate enolase/phosphatase as Utr4p and the aci‐reductone dioxygenase as Adi1p. The homologue of the enolase/phosphatase gene, YNL010w, was excluded from its candidate role in the cycle. The methodology used involved auxotrophic growth tests and analysis of intracellular 5′‐methylthioadenosine in deletion mutants. The last step, a transamination of 4‐methylthio‐2‐oxobutyrate to yield methionine, was found to be a highly redundant step. It was catalysed by amino acid transaminases, mainly coupled with aromatic and branched chain amino acids as amino donors, but also with proline, lysine and glutamate/glutamine. The aromatic amino acid transaminases, Aro8p and Aro9p, and the branched chain amino acid transaminases, Bat1p and Bat2p, seemed to be the main enzymes exhibiting 4‐methylthio‐2‐oxobutyrate transaminase activity. Bat2p was found to be less specific and used proline, lysine, tyrosine and glutamate as amino donors in addition to the branched chain amino acids. Thus, for the first time, all enzymes of the methionine salvage pathway were identified in a eukaryote.