Reactivation of the phospho form of the NAD-dependent glutamate dehydrogenase by a yeast protein phosphatase

H2O2 Induces Major Phosphorylation Changes in Critical Regulators of Signal Transduction, Gene Expression, Metabolism and Developmental Networks in Aspergillus nidulans

Reactive oxygen species (ROS) regulate several aspects of cell physiology in filamentous fungi including the antioxidant response and development. However, little is known about the signaling pathways involved in these processes. Here, we report Aspergillus nidulans global phosphoproteome during mycelial growth and show that under these conditions, H2O2 induces major changes in protein phosphorylation. Among the 1964 phosphoproteins we identified, H2O2 induced the phosphorylation of 131 proteins at one or more sites as well as the dephosphorylation of a larger set of proteins. A detailed analysis of these phosphoproteins shows that H2O2 affected the phosphorylation of critical regulatory nodes of phosphoinositide, MAPK, and TOR signaling as well as the phosphorylation of multiple proteins involved in the regulation of gene expression, primary and secondary metabolism, and development. Our results provide a novel and extensive protein phosphorylation landscape in A. nidulans, indicating that H2O2 induces a shift in general metabolism from anabolic to catabolic, and the activation of multiple stress survival pathways. Our results expand the significance of H2O2 in eukaryotic cell signaling.

Multiple Forms of Glutamate Dehydrogenase in Animals: Structural Determinants and Physiological Implications

Glutamate dehydrogenase (GDH) of animal cells is usually considered to be a mitochondrial enzyme. However, this enzyme has recently been reported to be also present in nucleus, endoplasmic reticulum and lysosomes. These extramitochondrial localizations are associated with moonlighting functions of GDH, which include acting as a serine protease or an ATP-dependent tubulin-binding protein. Here, we review the published data on kinetics and localization of multiple forms of animal GDH taking into account the splice variants, post-translational modifications and GDH isoenzymes, found in humans and apes. The kinetic properties of human GLUD1 and GLUD2 isoenzymes are shown to be similar to those published for GDH1 and GDH2 from bovine brain. Increased functional diversity and specific regulation of GDH isoforms due to alternative splicing and post-translational modifications are also considered. In particular, these structural differences may affect the well-known regulation of GDH by nucleotides which is related to recent identification of thiamine derivatives as novel GDH modulators. The thiamine-dependent regulation of GDH is in good agreement with the fact that the non-coenzyme forms of thiamine, i.e., thiamine triphosphate and its adenylated form are generated in response to amino acid and carbon starvation.

Insights into the in vivo regulation of glutamate dehydrogenase from the foot muscle of an estivating land snail

Land snails, Otala lactea, survive in seasonally hot and dry environments by entering a state of aerobic torpor called estivation. During estivation, snails must prevent excessive dehydration and reorganize metabolic fuel use so as to endure prolonged periods without food. Glutamate dehydrogenase (GDH) was hypothesized to play a key role during estivation as it shuttles amino acid carbon skeletons into the Krebs cycle for energy production and is very important to urea biosynthesis (a key molecule used for water retention). Analysis of purified foot muscle GDH from control and estivating conditions revealed that estivated GDH was approximately 3-fold more active in catalyzing glutamate deamination as compared to control. This kinetic difference appears to be regulated by reversible protein phosphorylation, as indicated by ProQ Diamond phosphoprotein staining and incubations that stimulate endogenous protein kinases and phosphatases. The increased activity of the high-phosphate form of GDH seen in the estivating land snail foot muscle correlates well with the increased use of amino acids for energy and increased synthesis of urea for water retention during prolonged estivation.

Regulation of liver glutamate dehydrogenase by reversible phosphorylation in a hibernating mammal

Glutamate dehydrogenase (GDH) is a key enzyme that links amino acid and carbohydrate metabolism in cells. Regulation is likely most important when organisms are confronted with extreme stresses such as the low environmental temperatures and lack of food associated with winter. Many small mammals, such as Richardson's ground squirrels, Spermophilus richardsonii, cope with these conditions by hibernating. Animals enter long periods of profound torpor where metabolic rate is greatly suppressed, body temperature drops to near-ambient and all metabolic needs must be met from fixed internal body stores of fuels. To investigate how GDH is regulated under these conditions, kinetic properties of GDH were analyzed in liver from euthermic and torpid squirrels, revealing significant differences in V(max), K(m) glutamate, K(a) ADP and inhibition by urea between the two forms of GDH. These data suggested an activation of the glutamate-oxidizing activity of GDH in the hypometabolic state. Subsequent experiments suggested that the molecular basis of the kinetic differences was a change in the protein phosphorylation state of GDH between euthermia and torpor. Specifically, liver GDH appears to be dephosphorylated and activated when animals transition into torpor and this may serve to promote amino acid oxidation to contribute to energy production and gluconeogenesis. This is the first study to show that mammalian liver GDH can be regulated by reversible phosphorylation, providing an important new regulatory mechanism for GDH control.

A specific alkaline phosphatase from Saccharomyces cerevisiae with protein phosphatase activity

In this paper, specific PHO13 alkaline phosphatase from Saccharomyces cerevisiae was demonstrated to possess phosphoprotein phosphatase activity on the phosphoseryl proteins histone II-A and casein. The enzyme is a monomeric protein with molecular mass of 60 kDa and hydrolyzes p-nitrophenyl phosphate with maximal activity at pH 8.2 with strong dependence on Mg2+ ions and an apparent Km of 3.6 x 10(-5) M. No other substrates tested except phosphorylated histone II-A and casein were hydrolyzed at any significant rate. These data suggest that the physiological role of the p-nitrophenyl phosphate-specific phosphatase may involve participation in reversible protein phosphorylation. 1988 Federation of European Microbiological Societies.

Substrate specificity of the phosphorylated fructose-1,6-bisphosphatase dephosphorylating protein phosphatase from Saccharomyces cerevisiae

Enzymatic dephosphorylation of the phosphorylated forms of five different yeast enzymes has been studied: fructose-1,6-bisphosphatase, glycogen phosphorylase, neutral trehalase, NAD-glutamate dehydrogenase and 6-phosphofructo-2-kinase. Phosphorylated fructose-1,6-bisphosphatase and phosphorylated 6-phosphofructo-2-kinase were present in extracts of starved yeast cells which had been incubated for 10 min with glucose. Phosphorylated glycogen phosphorylase, neutral trehalase and NAD-glutamate dehydrogenase were obtained by incubation of yeast extract with ATP, cyclic AMP and Mg2+. After incubation with commercially available preparations of alkaline phosphatase, all five phosphorylated enzymes studied showed the changes in catalytic activity that would be expected as a consequence of dephosphorylation. The recently purified yeast enzyme which dephosphorylates phosphorylated fructose-1,6-bisophosphatase (Horn and Holzer (1987) however, was found to be active only with the phosphorylated fructose-1,6-bisphosphatase, but not with the other four phosphorylated enzymes studied. By contrast, a crude extract from yeast showed dephosphorylating activity towards all five substrates. Substrate specificity with the five phosphorylated enzymes studied of different phosphoprotein phosphatases from yeast prepared by others is discussed.

Partial purification and characterization of the interconvertible forms of trehalase from Saccharomyces cerevisiae

Cryptic trehalase from Saccharomyces cerevisiae was purified about 3000-fold. The recovery of 970% of the original "activity" indicated the removal of an inhibitor of the enzyme. Active trehalase, obtained through phosphorylation of cryptic trehalase by cAMP-dependent protein kinase, was isolated by chromatography on DEAE-cellulose. A major phosphorylated protein, with an apparent Mr of 86,000, was detected after SDS-polyacrylamide gel electrophoresis. This protein band correlated exactly with the elution profile of trehalase activity and 32Pi incorporation into the enzyme on DEAE-cellulose chromatography. Partially purified active trehalase showed absolute specificity towards trehalose with an apparent Km of 4.79 X 10(-3) M. Both forms of the enzyme showed an apparent molecular weight of 160,000, by gel filtration. Centrifugation on a glycerol density gradient indicated multiple forms of trehalase-c, with Mr of 320,000, 160,000, and 80,000. After activation of each of these forms by protein kinase, a single form of trehalase-a was observed, with a Mr of 160,000. Trehalase-c appears to be a totally inactive form of the enzyme. The only mechanism of activation seems to be phosphorylation by cAMP-dependent protein kinase. When the protein kinase concentration was varied, at a fixed trehalase-c concentration, a sigmoidal activation plot was obtained. This result suggests the occurrence of multiple forms of cryptic trehalase.

Isolation and characterization of a phosphoprotein phosphatase-deficient mutant in yeast

The ppd1 mutant of yeast, Saccharomyces cerevisiae, was isolated as a suppressor of the cyr2 mutation which caused alteration of the catalytic subunit of cAMP-dependent protein kinase. Three peaks of phosphoprotein phosphatase activity (peak I, II and III) were identified by DEAE-Sephacel chromatography of crude extracts of the wild-type strain. The ppd1 mutant was deficient in peak III phosphoprotein phosphatase activity. The peak III enzyme efficiently utilized the phosphorylated forms of NAD-dependent glutamate dehydrogenase and trehalase as substrate. The ppd1 mutation did not suppress the cyr1, CYR3 or ras1 ras2 mutations. The ppd1 locus was located on chromosome II and had identical characteristics with glc1. The ppd1 mutation suppressed the G1 arrest caused by nutritional limitation, but maintained sensitivity to mating pheromone. In diploids homozygous for the ppd1 mutation, no premeiotic DNA replication and commitment to intragenic recombination occurred and no spores were formed, suggesting that the accumulation of phosphorylated proteins in the absence of one of the phosphoprotein phosphatases is required for mitosis but not for the initiation of meiosis.

The presence of two heat-stable inhibitors of phosphoprotein phosphatases in the dimorphic fungus, Mucor rouxii

Two heat-stable inhibitors (a and b) of phosphoprotein phosphatases I and II from Mucor rouxii were isolated from mycelium of the fungus. They were partially purified from extracts by heating, DEAE-cellulose chromatography, and Sephadex G-75 gel filtration. The molecular weights of inhibitors a and b, estimated by gel filtration, are 5,000 and 20,000 respectively. Inhibitor a acts similarly on both enzymes while inhibitor b is relatively more active on enzyme II. Storage of inhibitor b at -20 degrees C for several weeks resulted in a partial conversion to a lower-molecular-weight form with properties similar to those of inhibitor a.

Characterization of phosphoprotein phosphatases and phosphorylase phosphatase from yeast

Three peaks of protein phosphatase (phosphoprotein phosphohydrolase, EC 3.1.3.16) activity (fractions a, b and c) acting on muscle phosphorylase (1,4-alpha-D-glucan:orthophosphate alpha-D-glucosyltransferase, EC 2.4.1.1) were separated by DEAE-cellulose chromatography of yeast extracts. In contrast to fractions a and b, only fraction c was able to liberate phosphate from 32P-labelled inactivated yeast phosphorylase. The activity of fraction c on both substrates was totally dependent on the presence of bivalent metal ions (Mg2+, Mn2+), and was activated by Mg . ATP. Following freezing in the presence of mercaptoethanol, fractions a and b were also able to dephosphorylate yeast phosphorylase. Rabbit muscle phosphoprotein phosphatase inhibitors 1 and 2 showed that yeast phosphatases acting on muscle phosphorylase were inhibited by inhibitor 2 but not by inhibitor 1. The action of fraction c on yeast phosphorylase was not inhibited by either inhibitor. The native yeast phosphorylase phosphatase (EC 3.1.3.17) was purified 8000-fold by ion-exchange chromatography, casein-Sepharose chromatography and Sephadex G-200 gel filtration. The purified enzyme was unable to dephosphorylate rabbit muscle phosphorylase a, but acted on casein phosphate (Km 3.3 mg/ml). Molecular weight was estimated to be 78 000 and pH optimum 6.5-7.5. Activity of the enzyme was dependent on bivalent metal ions (Mg2+, Mn2+) and was inhibited by fluoride (Ki 20 mM) and succinate (Ki 10 mM).