Immunochemical studies on catabolite inactivation of phosphoenolpyruvate carboxykinase in Saccharomyces cerevisiae

Proteolytic regulation of metabolic enzymes by E3 ubiquitin ligase complexes: lessons from yeast

Eukaryotic organisms use diverse mechanisms to control metabolic rates in response to changes in the internal and/or external environment. Fine metabolic control is a highly responsive, energy-saving process that is mediated by allosteric inhibition/activation and/or reversible modification of preexisting metabolic enzymes. In contrast, coarse metabolic control is a relatively long-term and expensive process that involves modulating the level of metabolic enzymes. Coarse metabolic control can be achieved through the degradation of metabolic enzymes by the ubiquitin-proteasome system (UPS), in which substrates are specifically ubiquitinated by an E3 ubiquitin ligase and targeted for proteasomal degradation. Here, we review select multi-protein E3 ligase complexes that directly regulate metabolic enzymes in Saccharomyces cerevisiae. The first part of the review focuses on the endoplasmic reticulum (ER) membrane-associated Hrd1 and Doa10 E3 ligase complexes. In addition to their primary roles in the ER-associated degradation pathway that eliminates misfolded proteins, recent quantitative proteomic analyses identified native substrates of Hrd1 and Doa10 in the sterol synthesis pathway. The second part focuses on the SCF (Skp1-Cul1-F-box protein) complex, an abundant prototypical multi-protein E3 ligase complex. While the best-known roles of the SCF complex are in the regulation of the cell cycle and transcription, accumulating evidence indicates that the SCF complex also modulates carbon metabolism pathways. The increasing number of metabolic enzymes whose stability is directly regulated by the UPS underscores the importance of the proteolytic regulation of metabolic processes for the acclimation of cells to environmental changes.

Intracellular protein degradation: from a vague idea through the lysosome and the ubiquitin-proteasome system and onto human diseases and drug targeting

Between the 1950s and 1980s, scientists were focusing mostly on how the genetic code is transcribed to RNA and translated to proteins, but how proteins are degraded has remained a neglected research area. With the discovery of the lysosome by Christian de Duve it was assumed that cellular proteins are degraded within this organelle. Yet, several independent lines of experimental evidence strongly suggested that intracellular proteolysis is largely non-lysosomal, but the mechanisms involved remained obscure. The discovery of the ubiquitin-proteasome system resolved the enigma. We now recognize that degradation of intracellular proteins is involved in regulation of a broad array of cellular processes, such as cell cycle and division, regulation of transcription factors, and assurance of the cellular quality control. Not surprisingly, aberrations in the system have been implicated in the pathogenesis of human disease, such as malignancies and neurodegenerative disorders, which led subsequently to an increasing effort to develop mechanism-based drugs.

Exploring the topology of the Gid complex, the E3 ubiquitin ligase involved in catabolite-induced degradation of gluconeogenic enzymes

In the yeast Saccharomyces cerevisiae, key regulatory enzymes of gluconeogenesis such as fructose-1,6-bisphosphatase are degraded via the ubiquitin proteasome system when cells are replenished with glucose. Polyubiquitination is carried out by the Gid complex, a multisubunit ubiquitin ligase that consists of seven different Gid (glucose-induced degradation-deficient) proteins. Under gluconeogenic conditions the E3 ligase is composed of six subunits (Gid1/Vid30, Gid2/Rmd5, Gid5/Vid28, Gid7, Gid8, and Gid9/Fyv10). Upon the addition of glucose the regulatory subunit Gid4/Vid24 appears, binds to the Gid complex, and triggers ubiquitination of fructose-1,6-bisphosphatase. All seven proteins are essential for this process; however, nothing is known about the arrangement of the subunits in the complex. Interestingly, each Gid protein possesses several remarkable motifs (e.g. SPRY, LisH, CTLH domains) that may play a role in protein-protein interaction. We, therefore, generated altered versions of individual Gid proteins by deleting or mutating these domains and performed co-immunoprecipitation experiments to analyze the interaction between distinct subunits. Thus, we were able to create an initial model of the topology of this unusual E3 ubiquitin ligase.

Intracellular protein degradation: from a vague idea thru the lysosome and the ubiquitin-proteasome system and onto human diseases and drug targeting

Between the 1950s and 1980s, scientists were focusing mostly on how the genetic code was transcribed to RNA and translated to proteins, but how proteins were degraded had remained a neglected research area. With the discovery of the lysosome by Christian de Duve it was assumed that cellular proteins are degraded within this organelle. Yet, several independent lines of experimental evidence strongly suggested that intracellular proteolysis was largely non-lysosomal, but the mechanisms involved have remained obscure. The discovery of the ubiquitin-proteasome system resolved the enigma. We now recognize that degradation of intracellular proteins is involved in regulation of a broad array of cellular processes, such as cell cycle and division, regulation of transcription factors, and assurance of the cellular quality control. Not surprisingly, aberrations in the system have been implicated in the pathogenesis of human disease, such as malignancies and neurodegenerative disorders, which led subsequently to an increasing effort to develop mechanism-based drugs. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.

Saccharomyces cerevisiae phosphoenolpyruvate carboxykinase: the relevance of Glu299 and Leu460 for nucleotide binding

A homology model of Saccharomyces cerevisiae phosphoenolpyruvate (PEP) carboxykinase (ATP + oxaloacetate right arrow over left arrow ADP + PEP + CO(2)) in complex with its substrates shows that the isobutyl group of Leu460 is in close proximity to the adenine ring of the nucleotide, while the carboxyl group of Glu299 is within hydrogen-bonding distance of the ribose 2'OH. The Leu460Ala mutation caused three-fold and seven-fold increases in the K (m) for ADPMn(-) and ATPMn(2-), respectively, while the Glu299Ala mutation had no effect. Binding studies showed losses of approximately 2 kcal mol(-1) in the nucleotide binding affinity due to the Leu460Ala mutation and no effect for the Glu299Ala mutation. PEP carboxykinase utilized 2'deoxyADP and 2'deoxyATP as substrates with kinetic and equilibrium dissociation constants very similar to those of ADP and ATP, respectively. These results show that the hydrophobic interaction between Leu460 and the adenine ring of the nucleotide significantly contributed to the nucleotide affinity of the enzyme. The 2'deoxy nucleotide studies and the lack of an effect of the Glu299Ala mutation in nucleotide binding suggest that the possible hydrogen bond contributed by Glu299 and the ribose 2'OH group may not be relevant for nucleotide binding.

The Cdc48-Ufd1-Npl4 complex is central in ubiquitin-proteasome triggered catabolite degradation of fructose-1,6-bisphosphatase

The switch from gluconeogenesis to glycolysis in yeast has been shown to require ubiquitin-proteasome dependent elimination of the key enzyme fructose-1,6-bisphosphatase (FBPase). Prior to proteasomal degradation, polyubiquitination of the enzyme occurs via the ubiquitin-conjugating enzymes Ubc1, Ubc4, Ubc5 and Ubc8 in conjunction with a novel multi-subunit ubiquitin ligase, the Gid complex. As an additional machinery required for the catabolite degradation process, we identified the trimeric Cdc48(Ufd1-Npl4) complex and the ubiquitin receptors Dsk2 and Rad23. We show that this machinery acts between polyubiquitination of FBPase and its degradation by the proteasome.

Functional evaluation of serine 252 of Saccharomyces cerevisiae phosphoenolpyruvate carboxykinase

Saccharomyces cerevisiae phosphoenolpyruvate (PEP) carboxykinase mutant Ser252Ala, affecting the conserved Walker A serine residue, was characterized to elucidate the role of this serine residue. The substitution did not result in changes in the protein structure, as indicated by circular dichroism, intrinsic fluorescence spectroscopy, and gel-exclusion chromatography. Kinetic analysis of the mutated enzyme in both directions of the main reaction and in the two secondary reactions showed an approximately 50-fold increase in apparent K(m) for oxaloacetate with minor alterations in the other kinetic parameters. These results show that the hydroxyl group of serine 252 is required for proper oxaloacetate interaction.

The gluconeogenic enzyme fructose-1,6-bisphosphatase is dispensable for growth of the yeast Yarrowia lipolytica in gluconeogenic substrates

The genes encoding gluconeogenic enzymes in the nonconventional yeast Yarrowia lipolytica were found to be differentially regulated. The expression of Y. lipolytica FBP1 (YlFBP1) encoding the key enzyme fructose-1,6-bisphosphatase was not repressed by glucose in contrast with the situation in other yeasts; however, this sugar markedly repressed the expression of YlPCK1, encoding phosphoenolpyruvate carboxykinase, and YlICL1, encoding isocitrate lyase. We constructed Y. lipolytica strains with two different disrupted versions of YlFBP1 and found that they grew much slower than the wild type in gluconeogenic carbon sources but that growth was not abolished as happens in most microorganisms. We attribute this growth to the existence of an alternative phosphatase with a high K(m) (2.3 mM) for fructose-1,6-bisphosphate. The gene YlFBP1 restored fructose-1,6-bisphosphatase activity and growth in gluconeogenic carbon sources to a Saccharomyces cerevisiae fbp1 mutant, but the introduction of the FBP1 gene from S. cerevisiae in the Ylfbp1 mutant did not produce fructose-1,6-bisphosphatase activity or growth complementation. Subcellular fractionation revealed the presence of fructose-1,6-bisphosphatase both in the cytoplasm and in the nucleus.

The early steps of glucose signalling in yeast

In the presence of glucose, yeast undergoes an important remodelling of its metabolism. There are changes in the concentration of intracellular metabolites and in the stability of proteins and mRNAs; modifications occur in the activity of enzymes as well as in the rate of transcription of a large number of genes, some of the genes being induced while others are repressed. Diverse combinations of input signals are required for glucose regulation of gene expression and of other cellular processes. This review focuses on the early elements in glucose signalling and discusses their relevance for the regulation of specific processes. Glucose sensing involves the plasma membrane proteins Snf3, Rgt2 and Gpr1 and the glucose-phosphorylating enzyme Hxk2, as well as other regulatory elements whose functions are still incompletely understood. The similarities and differences in the way in which yeasts and mammalian cells respond to glucose are also examined. It is shown that in Saccharomyces cerevisiae, sensing systems for other nutrients share some of the characteristics of the glucose-sensing pathways.

The yeast GID complex, a novel ubiquitin ligase (E3) involved in the regulation of carbohydrate metabolism

Glucose-dependent regulation of carbon metabolism is a subject of intensive studies. We have previously shown that the switch from gluconeogenesis to glycolysis is associated with ubiquitin-proteasome linked elimination of the key enzyme fructose-1,6-bisphosphatase. Seven glucose induced degradation deficient (Gid)-proteins found previously in a genomic screen were shown to form a complex that binds FBPase. One of the subunits, Gid2/Rmd5, contains a degenerated RING finger domain. In an in vitro assay, heterologous expression of GST-Gid2 leads to polyubiquitination of proteins. In addition, we show that a mutation in the degenerated RING domain of Gid2/Rmd5 abolishes fructose-1,6-bisphosphatase polyubiquitination and elimination in vivo. Six Gid proteins are present in gluconeogenic cells. A seventh protein, Gid4/Vid24, occurs upon glucose addition to gluconeogenic cells and is afterwards eliminated. Forcing abnormal expression of Gid4/Vid24 in gluconeogenic cells leads to fructose-1,6-bisphosphatase degradation. This suggests that Gid4/Vid24 initiates fructose-1,6-bisphosphatase polyubiquitination by the Gid complex and its subsequent elimination by the proteasome. We also show that an additional gluconeogenic enzyme, phosphoenolpyruvate carboxykinase, is subject to Gid complex-dependent degradation. Our study uncovers a new type of ubiquitin ligase complex composed of novel subunits involved in carbohydrate metabolism and identifies Gid4/Vid24 as a major regulator of this E3.

Relevance of Arg457 for the nucleotide affinity of Saccharomyces cerevisiae phosphoenolpyruvate carboxykinase

Phosphoenolpyruvate carboxykinases catalyze one of the first steps in the biosynthesis of glucose and depending on the enzyme origin, preferentially use adenine or guanine nucleotides as substrates. The Saccharomyces cerevisiae enzyme has a marked preference for ADP (or ATP) over other nucleotides. Homology models of the enzyme in complex with ADP or ATP show that the guanidinium group of Arg457 is close to the adenine base, suggesting that this group might be involved in the stabilization of the nucleotide substrate. To evaluate this we have performed the mutation Arg457Met, replacing the positively charged guanidinium group by a neutral residue. The mutated enzyme retained the structural characteristics of the wild-type protein. Fluorescence titration experiments showed that mutation causes a loss of 1.7 kcal mol(-1) in the binding affinity of the enzyme for ADPMn. Similarly, kinetic analyses of the mutated enzyme showed 50-fold increase in K(m) for ADPMn, with minor alterations in the other kinetic parameters. These results show that Arg457 is an important factor for nucleotide binding by S. cerevisiae PEP carboxykinase.

Relevance of phenylalanine 216 in the affinity of Saccharomyces cerevisiae phosphoenolpyruvate carboxykinase for Mn(II)

Saccharomyces cerevisiae phosphoenolpyruvate (PEP) carboxykinase catalyzes the reversible formation of oxaloacetate and adenosine triphosphate from PEP, adenosine diphosphate and carbon dioxide, and uses Mn(2+) as the activating metal ion. Comparison with the crystalline structure of homologous Escherichia coli PEP carboxykinase [Tari et al. (1997) Nature Struct. Biol. 4, 990-994] shows that Lys(213) is one of the ligands to Mn(2+) at the enzyme active site. Coordination of Mn(2+) to a lysyl residue is not common and suggests a low pK (a) value for the epsilon-NH(2) group of Lys(213). In this work, we evaluate the role of neighboring Phe(216) in contributing to provide a low polarity microenvironment suitable to keep the epsilon-NH(2) of Lys(213) in the unprotonated form. Mutation Phe216Tyr shows that the introduction of a hydroxyl group in the lateral chain of the residue produces a substantial loss in the enzyme affinity for Mn(2+), suggesting an increase of the pK (a) of Lys(213). In agreement with this interpretation, theoretical calculations indicate an alkaline shift of 2.8 pH units in the pK (a) of the epsilon-amino group of Lys(213) upon Phe216Tyr mutation.

Intracellular protein degradation: from a vague idea thru the lysosome and the ubiquitin-proteasome system and onto human diseases and drug targeting

Between the 1950s and 1980s, scientists were focusing mostly on how the genetic code is transcribed to RNA and translated to proteins, but how proteins are degraded has remained a neglected research area. With the discovery of the lysosome by Christian de Duve it was assumed that cellular proteins are degraded within this organelle. Yet, several independent lines of experimental evidence strongly suggested that intracellular proteolysis is largely non-lysosomal, but the mechanisms involved remained obscure. The discovery of the ubiquitinproteasome system resolved the enigma. We now recognize that degradation of intracellular proteins is involved in regulation of a broad array of cellular processes, such as cell cycle and division, regulation of transcription factors, and assurance of the cellular quality control. Not surprisingly, aberrations in the system have been implicated in the pathogenesis of human disease, such as malignancies and neurodegenerative disorders, which led subsequently to an increasing effort to develop mechanism-based drugs.

Site-directed mutagenesis study of the microenvironment characteristics of Lys213 of Saccharomyces cerevisiae phosphoenolpyruvate carboxykinase

Saccharomyces cerevisiae phosphoenolpyruvate (PEP) carboxykinase catalyzes the reversible formation of oxaloacetate and adenosine triphosphate from PEP, adenosine diphosphate and carbon dioxide, and uses Mn(2+) as the activating metal ion. Comparison with the crystalline structure of homologous Escherichia coli PEP carboxykinase [Tari et al. Nature Struct. Biol. 4 (1997) 990-994] shows that Lys(213) is one of the ligands to Mn(2+) at the enzyme active site. Coordination of Mn(2+) to a lysyl residue is infrequent and suggests a low pK(a) value for the epsilon-NH(2) group of Lys(213). In this work, we evaluate the role of neighboring Phe(416) in contributing to provide a low polarity microenvironment suitable to keep the epsilon-NH(2) of Lys(213) in the unprotonated form. Mutation Phe416Tyr shows that the introduction of a hydroxyl group in the lateral chain of the residue produces a substantial loss in the enzyme affinity for Mn(2+), suggesting an increase of the pK(a) of Lys(213). A study of the effect of pH on K(m) for Mn(2+) indicate that the affinity of recombinant wild type enzyme for the metal ion is dependent on deprotonation of a group with pK(a) of 7.1+/-0.2, compatible with the low pK(a) expected for Lys(213). This pK(a) value increases at least 1.5 pH units upon Phe416Tyr mutation, in agreement with the expected effect of an increase in the polarity of Lys(213) microenvironment. Theoretical calculations of the pK(a) of Lys(213) indicate a value of 6.5+/-0.9, and it increases to 8.2+/-1.6 upon Phe416Tyr mutation. Additionally, mutation Phe416Tyr causes a loss of 1.3 kcal mol(-1) in the affinity of the enzyme for PEP, an effect perhaps related to the close proximity of Phe(416) to Arg(70), a residue previously shown to be important for PEP binding.

Intracellular protein degradation: from a vague idea thru the lysosome and the ubiquitin-proteasome system and onto human diseases and drug targeting

Between the 1950s and 1980s, scientists were focusing mostly on how the genetic code is transcribed to RNA and translated to proteins, but how proteins are degraded has remained a neglected research area. With the discovery of the lysosome by Christian de Duve it was assumed that cellular proteins are degraded within this organelle. Yet, several independent lines of experimental evidence strongly suggested that intracellular proteolysis is largely non-lysosomal, but the mechanisms involved remained obscure. The discovery of the ubiquitin-proteasome system resolved the enigma. We now recognize that degradation of intracellular proteins is involved in regulation of a broad array of cellular processes, such as cell cycle and division, regulation of transcription factors, and assurance of the cellular quality control. Not surprisingly, aberrations in the system have been implicated in the pathogenesis of human disease, such as malignancies and neurodegenerative disorders, which led subsequently to an increasing effort to develop mechanism-based drugs.

Intracellular protein degradation: from a vague idea thru the lysosome and the ubiquitin-proteasome system and onto human diseases and drug targeting

Between the 1950s and 1980s, scientists were focusing mostly on how the genetic code is transcribed to RNA and translated to proteins, but how proteins are degraded has remained a neglected research area. With the discovery of the lysosome by Christian de Duve, it was assumed that cellular proteins are degraded within this organelle. Yet, several independent lines of experimental evidence strongly suggested that intracellular proteolysis is largely nonlysosomal, but the mechanisms involved remained obscure. The discovery of the ubiquitin-proteasome system resolved the enigma. We now recognize that degradation of intracellular proteins is involved in regulation of a broad array of cellular processes, such as cell cycle and division, regulation of transcription factors, and assurance of the cellular quality control. Not surprisingly, aberrations in the system have been implicated in the pathogenesis of human disease, such as malignancies and neurodegenerative disorders, which led subsequently to an increasing effort to develop mechanism-based drugs.

Intracellular Protein Degradation: From a Vague Idea, through the Lysosome and the Ubiquitin-Proteasome System, and onto Human Diseases and Drug Targeting (Nobel Lecture)

Between the 1950s and 1980s, scientists were focusing mostly on how the genetic code is transcribed to RNA and translated to proteins, but how proteins are degraded has remained a neglected research area. With the discovery of the lysosome by Christian de Duve it was assumed that cellular proteins are degraded within this organelle. Yet, several independent lines of experimental evidence strongly suggested that intracellular proteolysis is largely non-lysosomal, but the mechanisms involved remained obscure. The discovery of the ubiquitin-proteasome system resolved the enigma. We now recognize that degradation of intracellular proteins is involved in regulation of a broad array of cellular processes, such as the cell cycle and division, regulation of transcription factors, and assurance of the cellular quality control. Not surprisingly, aberrations in the system have been implicated in the pathogenesis of human disease, such as malignancies and neurodegenerative disorders, which led subsequently to an increasing effort to develop mechanism-based drugs.

Proteins of newly isolated mutants and the amino-terminal proline are essential for ubiquitin-proteasome-catalyzed catabolite degradation of fructose-1,6-bisphosphatase of Saccharomyces cerevisiae

Addition of glucose to cells of the yeast Saccharomyces cerevisiae growing on a non-fermentable carbon source leads to selective and rapid degradation of fructose-1,6-bisphosphatase. This so called catabolite inactivation of the enzyme is brought about by the ubiquitin-proteasome system. To identify additional components of the catabolite inactivation machinery, we isolated three mutant strains, gid1, gid2, and gid3, defective in glucose-induced degradation of fructose-1,6-bisphospha-tase. All mutant strains show in addition a defect in catabolite inactivation of three other gluconeogenic enzymes: cytosolic malate dehydrogenase, isocitrate lyase, and phosphoenolpyruvate carboxykinase. These findings indicate a common mechanism for the inactivation of all four enzymes. The mutants were also impaired in degradation of short-lived N-end rule substrates, which are degraded via the ubiquitin-proteasome system. Site-directed mutagenesis of the amino-terminal proline residue yielded fructose-1,6-bisphosphatase forms that were no longer degraded via the ubiquitin-proteasome pathway. All amino termini other than proline made fructose-1,6-bisphosphatase inaccessible to degradation. However, the exchange of the amino-terminal proline had no effect on the phosphorylation of the mutated enzyme. Our findings suggest an essential function of the amino-terminal proline residue for the degradation process of fructose-1,6-bisphosphatase. Phosphorylation of the enzyme was not necessary for degradation to occur.

Cloning, sequencing, and overexpression of the Anaerobiospirillum succiniciproducens phosphoenolpyruvate carboxykinase (pckA) gene

The phosphoenolpyruvate (PEP) carboxykinase-encoding gene from the anaerobic, CO2-fixing, succinate-producing bacterium Anaerobiospirillum succiniciproducens was cloned, sequenced, and expressed in Escherichia coli. The gene encoded a 532-residue polypeptide with a calculated molecular mass of 58.7 kDa. The sequence of the A. succiniciproducens PEP carboxykinase was similar to those of all known ATP/ADP-dependent PEP carboxykinases. In particular, the A. succiniciproducens enzyme was 67.3% identical and 79.2% similar to the E. coli enzyme. The A. succiniciproducens pckA transcription start site was determined, and putative promoter regions were identified. The recombinant enzyme was overexpressed in E. coli. The purified enzyme was indiscernible from the native enzyme by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and had the same activity as the native enzyme.

Selective uptake of cytosolic, peroxisomal, and plasma membrane proteins into the yeast lysosome for degradation

When glucose-starved cells are replenished with glucose, the key gluconeogenic enzyme, fructose-1,6-bisphosphatase (FBPase), is selectively targeted from the cytosol to the yeast lysosome (vacuole) for degradation. The glucose-induced targeting of FBPase to the vacuole for degradation occurs in cells grown under a variety of metabolic conditions. Immunoelectron microscopic studies demonstrate that the uptake of FBPase by the vacuole is mediated in part by an autophagic process. FBPase can be found on the vacuolar membrane and also at the sites of membrane invaginations. Furthermore, FBPase is associated with different forms of vesicles, which are induced to accumulate inside the vacuole. We have identified peroxisomes as the organelles that are delivered to the vacuole for degradation when cells are replenished with glucose. Ultrastructural studies indicate that peroxisomes are engulfed by the vacuole by an autophagic process, leading to the destruction of whole organelles in the vacuole. Furthermore, the galactose transporter (Gal2p) is also delivered from the plasma membrane to the vacuole for degradation in response to glucose. Gal2p is delivered to the vacuole through the endocytic pathway, as mutants defective in receptor-mediated endocytosis fail to degrade Gal2p in response to glucose.

Isolation and sequence analysis of cDNAs encoding phosphoenolpyruvate carboxykinase from the PCK-type C4 grass Urochloa panicoides

A rabbit antiserum was raised against phosphoenolpyruvate carboxykinase (PCK) purified from Urochloa panicoides, a PCK-type C4 monocot. The antiserum was used to screen a cDNA expression library constructed from U. panicoides leaf poly(A)+RNA. Inserts from immunoreactive clones were used to rescreen the library and obtain three overlapping cDNAs comprising a 2220 bp composite sequence. The single complete open reading frame of 1872 bp encodes PCK1, a 624 amino acid polypeptide with a predicted molecular mass of 68,474 Da. Comparison of PCK1 with other ATP-dependent PCKs indicates that PCK1 is significantly larger, mainly due to an N-terminal extension of greater than 65 residues, and reveals high sequence identity across the central portion of the protein, especially over seven sub-sequences. One of these sub-sequences spans motifs common to several ATP-utilising enzymes for phosphate and divalent cation binding. The anti-PCK antiserum recognises a 69 kDa polypeptide on immunoblots of either purified PCK or U. panicoides leaf extracts. However, polypeptides of 63, 62, 61 and 60 kDa are also immunoreactive. Amino terminal sequencing of polypeptides from preparations of purified PCK demonstrates that these smaller polypeptides are related to PCK1, and time course experiments show that these polypeptides arise from the breakdown of PCK during isolation. Northern blot analysis indicates that the 2.7 kb PCK mRNA is abundant in green leaves but not in roots or etiolated shoots. Moreover, PCK mRNA levels increase gradually during greening, reaching maximum levels after about 84 h.

Identification of reactive vicinal cysteines in Saccharomyces cerevisiae (ATP) and cytosolic rat liver (GTP) phospho enol pyruvate carboxykinases

Saccharomyces cerevisiae (ATP) and cytosolic rat liver (GTP) phospho enol pyruvate carboxykinases (EC 4.1.1.49/32) have been labeled with N-(1-pyrenyl)-iodoacetamide. Reagent incorporation was completely prevented by the presence of the respective nucleoside diphosphate plus MnCl2. Under appropriate conditions, 2 mol of reagent per mol of enzyme subunit were incorporated. The fluorescence spectra of the labeled proteins showed the pyrene excimer emission band. The pyrenyl-derivatized enzymes were digested with trypsin after carboxymethylation, and two labeled peptides were isolated for each carboxykinase upon reverse-phase high-performance liquid chromatography. Automated Edman degradation of the labeled peptides indicated that cysteines 364 and 457 (yeast enzyme), and cysteines 288 and 413 (rat enzyme) were labeled with the fluorescence SH-specific reagent. The relative reactivity of these residues was characterized. Labeling experiments utilizing the 5,5'-dithiobis(2-nitrobenzoate)-oxidized enzymes suggested that the reactive SH-groups occupy a vicinal position in the tertiary structure of the proteins, probably in the nucleotide-binding region.

Comparative steady-state fluorescence studies of cytosolic rat liver (GTP), Saccharomyces cerevisiae (ATP) and Escherichia coli (ATP) phospho enol pyruvate carboxykinases

Two members of the ATP-dependent class of phospho enol pyruvate (PEP) carboxykinases (Saccharomyces cerevisiae and Escherichia coli PEP carboxykinase), and one member of the GTP-dependent class (the cytosolic rat liver enzyme) have been comparatively analyzed by taking advantage of their intrinsic fluorescence. The S. cerevisiae and the rat liver enzymes show intrinsic fluorescence with a maximum emission characteristic of moderately buried tryptophan residues, while the E. coli carboxykinase shows somewhat more average exposure for these fluorophores. The fluorescence of the three proteins was similarly quenched by the polar compound acrylamide, but differences were observed for the ionic quencher iodide. For the ATP-dependent enzymes, these last experiments indicate more exposure to the aqueous media of the tryptophan population of the E. coli than of the S. cerevisiae enzyme. The effect of nucleotides on the emission intensities and quenching efficiencies revealed substrate-induced conformational changes in the E. coli and cytosolic rat liver PEP carboxykinases. The addition of Mn2+ or of the adenosine nucleotides in the presence of Mg2+ induced an enhancement in the fluorescence of the E. coli enzyme. The addition of guanosine or inosine nucleotides to the rat liver enzyme quenched its fluorescence. From the ligand-induced fluorescence changes, dissociation constants of 40 +/- 6 microM, 10 +/- 0.8 microM, and 15 +/- 1 microM were obtained for Mn2+, MgATP and MgADP binding to the E. coli enzyme, respectively. For the cytosolic rat liver PEP carboxykinase, the respective values for GDP, IDP and ITP binding are 6 +/- 0.5 microM, 6.7 +/- 0.4 microM and 10.1 +/- 1.7 microM. A comparison of the dissociation constants obtained in this work with those reported for other PEP carboxykinases is presented.

Metabolic effects of benzoate and sorbate in the yeast Saccharomyces cerevisiae at neutral pH

Preincubation of yeast cells in the presence of benzoate or sorbate at an extracellular pH value of 6.8 elicited a set of metabolic effects on sugar metabolism, which became apparent after the subsequent glucose addition. They can be summarized as follows: a) reduced glucose consumption; b) inhibition of glucose- and fructose-phosphorylating activities; c) suppression of glucose-triggered peak of hexoses monophosphates; d) substantial reduction of glucose-triggered peak of fructose 2,6-bisphosphate; e) block of catabolite inactivation of fructose-1,6-bisphosphatase and phosphoenolpyruvate carboxy-kinase, but not of cytoplasmic malate dehydrogenase. On the whole this pattern resulted in prevention of glucose-induced switch of metabolism from a gluconeogenetic to a glycolytic state. Our data also show that, unlike former assumptions, intracellular acidification is not likely to mediate the bulk of metabolic effects of benzoate and sorbate, since under our working conditions intracellular pH kept close to neutrality.

Fast high-performance liquid chromatographic purification of Saccharomyces cerevisiae phosphoenolpyruvate carboxykinase

A procedure was established for the rapid isolation of Saccharomyces cerevisiae phosphoenolpyruvate carboxykinase (PEPCK) from an overproducing strain. Overexpression was achieved by the transformation of yeast cells with the multicopy plasmid YEp352 harbouring the PEPCK structural gene. The enzyme was purified to homogeneity using first anion-exchange chromatography on Q-Sepharose followed by hydrophobic interaction chromatography on phenyl-Sepharose and gel filtration on Sephacryl S200. The purified phosphoenolpyruvate carboxykinase was further characterized with respect to the molecular mass, displaying an apparent molecular mass corresponding to a tetrameric form.

The control of trehalose biosynthesis in Saccharomyces cerevisiae: evidence for a catabolite inactivation and repression of trehalose-6-phosphate synthase and trehalose-6-phosphate phosphatase

During diauxic growth of yeast in glucose-rich medium, the accumulation of trehalose started well after complete exhaustion of glucose from the medium. The accumulation of the disaccharide was concomitant with a resumption of cell growth on the ethanol accumulated in the medium, but not with a degradation of glycogen which occurred as soon as glucose had been consumed. In contrast, in a mutant deficient in phosphoenolpyruvate carboxykinase, the synthesis of trehalose coincided exactly with the degradation of glycogen. Upon inoculation of stationary phase wild-type cells into a glucose medium, the activities of trehalose-6-phosphate (Tre6P) synthase and Tre6P phosphatase dropped in parallel to reach only 15% of their initial values after 3 h, and only recovered their original values as cells re-entered stationary phase. In the presence of cycloheximide, the decrease in Tre6P synthase and Tre6P phosphatase activities was restricted to 50-60%, the remaining decrease being inhibited by the drug. Furthermore, the reappearance of the enzyme activities following transfer of cells to an acetate medium was blocked by cycloheximide. It was also shown that loss of activity of these two enzymes required a combination of metabolizable sugars together with a nitrogen source. Low activities of Tre6P synthase and Tre6P phosphatase were measured in mutants with increased adenylate cyclase activity (RAS2ala18val19 mutants). Moreover, derepression of these enzymes at the approach of stationary phase was prevented in a pde2 mutant when it was cultivated in the presence of exogenous cyclic nucleotide. The mechanism of this effect is not clear, but may involve a transcriptional regulation by cAMP of the genes encoding these proteins.

Functions of Intracellular Protein Degradation in Yeast

Diverse vacuolar and nonvacuolar pathways of protein degradation have been described in yeast. In several cases, much is known about the proteases involved, but most of these studies utilized nonphysiological model substrates. On the other hand, many regulatory proteins, such as those involved in cell cycle control, cell type determination, and the regulation of metabolite fluxes through biosynthetic pathways, have been shown to be rapidly and selectively destroyed in vivo, either constitutively or in response to specific regulatory signals. Precisely what molecular features of this class of proteins target them for degradation is largely unknown; this question is an area of intense current interest. A connection has been made between a particular proteolytic mechanism and a specific naturally short-lived protein in only a handful of examples. It is in this regard that the powerful molecular and genetic techniques available in yeast will probably have their greatest impact in the near future. The promise of this type of approach is already becoming apparent with the molecular genetic analysis of the yeast ubiquitin system. Although this work began less than ten years ago, the genes encoding at least 22 proteins involved in ubiquitin-dependent processes have already been isolated, and questions of their physiological and mechanistic function are being answered at an ever quickening pace.

Reactive sulfhydryl groups in Saccharomyces cerevisiae phosphoenolpyruvate carboxykinase

Saccharomyces cerevisiae phosphoenolpyruvate carboxykinase (ATP:oxaloacetate carboxy-lyase (transphosphorylating), EC 4.1.1.49) is inactivated by several thiol- and vicinal dithiol-specific reagents. Titration experiments of the enzyme with 5,5'-dithiobis(2-nitrobenzoate) (DTNB) show the presence of reactive monothiol and vicinal dithiol groups, whose modifications lead to enzyme inactivation. The enzyme is also inactivated by N-(1-pyrenyl)iodoacetamide (PyrIAM), with a binding stoichiometry of approx. 2 mol per mol of enzyme subunit. A high level of pyrene excimer fluorescence is detected on the labeled enzyme, thus implying the reaction of the reagent with two spatially close sulfhydryl groups in the protein. The carboxykinase is not completely inactivated by different vicinal dithiol-specific reagents, thus implying a catalytically non-essential character for these groups. From substrate protection experiments of the enzyme inactivation by DTNB, PyrIAM and vicinal dithiol-specific reagents, it is concluded that the loss of enzyme activity is caused by the modification of both thiol and vicinal dithiol groups in the substrate binding region.

Studies on the degradative mechanism of phosphoenolpyruvate carboxykinase from yeast Saccharomyces cerevisiae

Previous work carried out in our laboratory (Burlini, N., Lamponi S., Radrizzani, M., Monti, E. and Tortora P. (1987) Biochim. Biophys. Acta 930, 220-229) led to the immunological identification of a yeast 65-kDa phosphoprotein as a modified form of phosphoenolpyruvate carboxykinase; moreover the appearance of this phospho form was proven to be independent of cAMP, whereas the glucose-induced inactivation of the native enzyme is cAMP-dependent. Here, we report further investigations on the mechanism of the glucose-triggered degradation of the enzyme which led to the following results: (a) the aforementioned phospho form displayed a binding pattern to 5 AMP-Sepharose 4B quite similar to that of native enzyme, although it did not retain its oligomeric structure, nor was it catalytically active; (b) its phosphate content was of about two residues per monomer; (c) its isoelectric point was slightly higher than that of native enzyme, this shows that the enzyme undergoes additional modifications besides phosphorylation; (d) it represented about 4% of the native enzyme in glucose-depressed cells; (e) other forms immunologically cross-reactive with the native enzyme were also isolated, whose molecular mass was in the range of 60-62 kDa, and they are probable candidates as degradation products of the phospho form; (f) time courses of the native and phospho forms in the presence and the absence of glucose provided data consistent with a kinetic model involving a strong stimulation of the decay of both forms effected by the sugar; (g) in the mutant ABYS1 (Achstetter, T., Emter, O., Ehmann, C. and Wolf, D.H. (1984) J. Biol. Chem. 259, 13334-13343) which is devoid of the four major vacuolar proteinases, the decay pattern was essentially the same as in wild-type; (h) effectors lowering intracellular ATP also retarded the first step of enzyme degradation; this points to an ATP-dependence of this step. Based on these results we propose a degradation mechanism consisting of an initial cAMP- and ATP-dependent modification of the enzyme, followed by a cAMP-independent phosphorylation, which leads to the appearance of the aforementioned monomeric phospho form; this in turn seems to undergo limited proteolysis. These data strongly suggest the occurrence of an intermediate form arising from the native one and whose phosphorylation gives rise to the 65-kDa phosphoprotein described here.

Regulation of yeast fructose-1,6-bisphosphatase in strains containing multicopy plasmids coding for this enzyme

The structural gene for yeast fructose-1,6-bisphosphatase (FbPase) has been cloned by complementation of a strain carrying the fbp1-1 mutation and lacking FbPase activity. By Northern analysis it could be shown that the fbp1-1 mutation does not interfere with the transcription of the gene although no normal active enzyme is produced. Strains transformed with the cloned gene on a multicopy plasmid overproduce FbPase but are still subject to strong catabolite repression. Catabolite inactivation, however, is slowed down in such strains.

Irreversible inactivation of Saccharomyces cerevisiae fructose-1,6-bisphosphatase independent of protein phosphorylation at Ser11

The fructose-1,6-bisphosphatase gene was used with multicopy plasmids to study rapid reversible and irreversible inactivation after addition of glucose to derepressed Saccharomyces cerevisiae cells. Both inactivation systems could inactivate the enzyme, even if 20-fold over-expressed. The putative serine residue, at which fructose-1,6-bisphosphatase is phosphorylated, was changed to an alanine residue without notably affecting the catalytic activity. No rapid reversible inactivation was observed with the mutated enzyme. Nonetheless, the modified enzyme was still irreversibly inactivated, clearly demonstrating that phosphorylation is an independent regulatory circuit that reduces fructose-1,6-bisphosphatase activity within seconds. Furthermore, irreversible glucose inactivation was not triggered by phosphorylation of the enzyme.

Isolation and primary structure of the gene encoding fructose-1,6-bisphosphatase from Saccharomyces cerevisiae

The gene encoding Saccharomyces cerevisiae fructose-1,6-bisphosphatase (FBP1) was isolated. Constructed fbp1::HIS3 null mutants were unable to grow with ethanol, and growth was restored after transformation with the cloned fbp gene. The gene codes for a protein of 347 amino acid residues with an Mr of 38131. Homology with the pig kidney cortex and the sheep liver enzyme is 47.7% and 46.6%, respectively, within a central core of 328 amino acid residues. The cloned promoter size was 318 bp and allowed only low level expression of the gene. This indicates a positive activation site (UAS) upstream of the cloned DNA fragment.

Identification of a phosphorylated form of phosphoenolpyruvate carboxykinase from the yeast Saccharomyces cerevisiae

A phosphoprotein of 65 kDa, as determined by SDS-gel electrophoresis, has been isolated from yeast crude extracts. This phospho form copurifies with phosphoenolpyruvate carboxykinase in the enzyme purification procedure worked out in our laboratory (Tortora, P., Hanozet, G.M. and Guerritore, A. (1985) Anal. Biochem. 144, 179-185). Moreover, both proteins bind strongly to 5'AMP-Sepharose 4B in the presence of Mn2+, whereas a substantially lower binding occurs if Mn2+ is replaced by Mg2+. This binding pattern is consistent with the well-known Mn2+-dependence of yeast phosphoenolpyruvate carboxykinase. These data suggest that the 65-kDa protein might be a phosphorylation product of the native enzyme. Furthermore, although the phospho form is not immunoprecipitated by anti-phosphoenolpyruvate carboxykinase antibodies, addition of Protein A-Sepharose CL-4B to crude extracts preincubated with the antibodies results in the binding to the resin of the phospho form, thus providing immunological evidence for its identification as a modified form of native enzyme. The same 65-kDa phosphoprotein is detectable in extracts from cells grown in the presence of [32P]Pi, as well as in cell extracts incubated with [gamma-32P]ATP. Moreover, digestion of the phosphoprotein with BrCN or with Staphylococcus aureus V8 proteinase, yields two and three fragments, respectively, which appear parallel to digestion products of phosphoenolpyruvate carboxykinase, again supporting the proposed identification. Finally, analysis of the phosphorylated amino acids in the 65-kDa protein shows that phosphoserine is the only labelled phosphoamino acid.

Purification procedure and N-terminal amino acid sequence of yeast malate dehydrogenase isoenzymes

A method has been devised for the rapid isolation of malate dehydrogenase isoenzymes. First, anionic proteins were precipitated with polyethyleneimine, whilst hydrophobic malate dehydrogenase remained in the supernatant fluid. Secondly, the supernatant was 30% saturated with ammonium sulfate and the two isoenzymes were separated by hydrophobic phenyl-Sepharose CL-4B chromatography. For further purification the enzymes were chromatofocused, and polybuffer was removed by hydrophobic chromatography. Affinity chromatography with blue Sepharose CL-6B [1] was used as final purification step. The purified isoenzymes were homogeneous as shown by isoelectric focusing and could be used for N-terminal sequencing. 34 amino acid residues could be identified for the cytoplasmic isoenzyme and 56 amino acid residues for the mitochondrial isoenzyme. Although there are regions of strong homology between both isoenzymes, the sequence differences clearly showed support that both isoenzymes are coded by different genes. Sequence comparison clearly indicated that the N-terminus of the cytoplasmic enzyme extended that of the mitochondrial enzyme by 12 amino acid residues. The amino acid sequence of the extending sequence resembled that of leading sequences known for enzymes which are transported into the mitochondria. The assumed leading sequence is discussed with respect to its possible role in glucose inactivation.

Proteases of Melilotus alba mesophyll protoplasts : II. General properties and effectiveness in degradation of cytosolic and vacuolar enzymes

Proteases from mesophyll protoplasts of Melilotus alba were identified by standard proteolytic assays and separated using different chromatographic techniques. Their characterization also included their subcellular location. Besides the evidence for the multiplicity of the proteolytic enzymes, two protease sets were distinguished endopeptidases, which are exclusively vacuolar, and aminopeptidases, which are widely distributed throughout the cell. Cytosol-located enzymes were tested as substrates of the two sets of proteases, by studying comparatively the time-course changes of enzyme activities during incubation in total protoplast extracts, or in cytosol fractions devoid of vacuolar proteases. The degradation of phosphoenolpyruvate-carboxylase protein, a typical cytosolic enzyme, in the presence of purified amino-and endopeptidases, was also estimated by immunoprecipitation studies. Only the vacuolar endopeptidases are effective in the degradation of cytosolic enzymes. Hydrolytic enzyme activities mostly of vacuolar origin were very stable during incubation in total protoplast extracts. These proteins therefore appear to be particularly resistant to proteolytic attack. The results indicate that, in plants, the effective proteolytic system acting on cytosolic enzymes seems to be vacuole-located, and that the selectivity in protein degradation may be imposed by the susceptibility of the protein being degraded and by its transfer into the vacuoles.

Purification of phosphoenolpyruvate carboxykinase from Saccharomyces cerevisiae and its use for bicarbonate assay

Electrophoretically homogeneous phosphoenolpyruvate carboxykinase (EC 4.1.1.49) from Saccharomyces cerevisiae was obtained in high yields by means of a two-step purification procedure consisting of ion-exchange chromatography and affinity chromatography on adenosine 5'-monophosphate-Sepharose 4B. In the latter step the binding of the enzyme to the resin specifically required the presence of Mn2+. The enzyme was eluted when Mn2+ was removed by addition of ethylenediaminetetraacetate to the elution buffer. Homogeneity, molecular weight, and subunit composition of phosphoenolpyruvate carboxykinase were checked by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and gel filtration. A factor which caused an underestimation of the enzyme activity in crude extracts was identified as adenylate kinase. Finally, a method is proposed for the enzymatic assay of bicarbonate using a purified phosphoenolpyruvate carboxykinase preparation.

Studies on glucose-induced inactivation of gluconeogenetic enzymes in adenylate cyclase and cAMP-dependent protein kinase yeast mutants

Glucose-induced inactivation of the gluconeogenetic enzymes fructose-1,6-biphosphatase, cytoplasmic malate dehydrogenase and phosphoenolpyruvate carboxykinase was tested in yeast mutants defective in adenylate cyclase (cyr1 mutation) and in the cAMP-binding subunit of cAMP-dependent protein kinase (bcy 1 mutation). In the mutant AM7-11D (cyr1 mutation), glucose-induced cAMP overshoot was absent, and no significant inactivation of the gluconeogenetic enzymes was detected, thus supporting the role of cAMP in the process. Moreover, in the mutant AM9-8B (bcy1 mutation), no cAMP-dependent protein kinase activity was evidenced, and, in addition, a normal inactivation pattern was observed, thus indicating that other mechanisms evoked by glucose might be required in the process. In the double mutant AM7-11DR-4 (cyr1 bcy1 mutations), no inactivating effect was triggered by the sugar: this suggests that cAMP exerts some additional effect on the process, besides the activation of cAMP-dependent protein kinase. Furthermore, in AM7-11D, extracellular cAMP triggered about 50% of inactivation of fructose-1,6-bisphosphatase; this effect was largely reversed in acetate medium plus cycloheximide even after 150 min of incubation. However, an extensive and essentially irreversible inactivation was evidenced in the presence of glucose plus cAMP, whereas glucose alone was only slightly effective. Therefore, the reversible effect of cAMP, which probably corresponds to enzyme phosphorylation, seems to be required for the irreversible, probably proteolytic, glucose-stimulated inactivation of this enzyme. Cytoplasmic malate dehydrogenase and phosphoenolpyruvate carboxykinase in AM7-11D were also inactivated by cAMP, and much more by glucose plus cAMP, whereas glucose was practically ineffective. However, reversibility of the effect was not detected, and, in addition, no phosphorylation of phosphoenolpyruvate carboxykinase could be evidenced. Therefore, the sugar quite probably stimulates proteolysis of these enzymes, but the mechanism of cAMP in their degradation has still to be defined.

Studies on rapid reversible and non-reversible inactivation of fructose-1,6-bisphosphatase and malate dehydrogenase in wild-type and glycolytic block mutants of Saccharomyces cerevisiae

Experimental conditions have been elaborated to test for reversibility of the malate dehydrogenase inactivation (E.C.1.1.1.37) after addition of glucose to derepressed yeast cells. Malate dehydrogenase inactivation was shown to be irreversible at all stages of inactivation. In contrast fructose-1,6-bisphosphatase inactivation (E.C.3.1.11) remained reversible for at least 30 min after addition of glucose. Rapid reversible inactivation of fructose-1,6-bisphosphatase and irreversible inactivation of malate dehydrogenase were additionally investigated in glycolytic block mutants. Normal inactivation kinetics were observed in mutants without catalytic activity of phosphoglucose isomerase (E.C.5.3.1.9), phosphofructokinase (E.C.2.7.1.11), triosephosphate isomerase (E.C.5.3.1.1) and phosphoglycerate kinase (E.C.2.7.2.3). Hence, neither type of inactivation depended on the accumulation of any glucose metabolite beyond glucose-6-phosphate. Under anaerobic conditions irreversible inactivation was completely abolished in glycolytic block mutants. In contrast rapid reversible inactivation was independent of energy provided by respiration or fermentation. Reversibility of fructose-1,6-bisphosphatase inactivation was tested under conditions which prevented irreversible malate dehydrogenase inactivation. In these experiments, fructose-1,6-bisphosphatase inactivation remained reversible for at least 120 min, whereas reversibility was normally restricted to about 30 min. This indicated a common mechanism between the irreversible part of fructose-1,6-bisphosphatase inactivation and irreversible malate dehydrogenase inactivation.

Allantoinase from Pseudomonas aeruginosa. Purification, properties and immunochemical characterization of its in vivo inactivation

The catabolic enzyme allantoinase is rapidly inactivated in cells of Pseudomonas aeruginosa when the stationary phase of growth is reached. This process is irreversible since the protein synthesis inhibitor chloramphenicol completely blocked the reappearance of allantoinase activity that is observed when allantoin is added to stationary cells. Purified alloantoinase appeared to be a protein composed of four identical subunits with a molecular weight of 38,000. With antibodies raised against purified allantoinase it was found that allantoinase inactivation is accompanied by a parallel decrease in immunologically reactive material. This suggests that allantoinase inactivation is caused or followed by rapid proteolysis.

Effect of caffeine on glucose-induced inactivation of gluconeogenetic enzymes in Saccharomyces cerevisiae. A possible role of cyclic AMP

The mechanism of catabolite inactivation of three gluconeogenetic enzymes, fructose-1,6-bisphosphatase, cytoplasmic malate dehydrogenase and phosphoenolpyruvate carboxykinase, has been studied in the yeast Saccharomyces cerevisiae. The glucose-induced inactivation of the three enzymes is remarkably retarded by preincubation of the cells with different caffeine concentrations; however, a full conservation of activity has never been obtained, even at the highest drug concentration. Caffeine modifies the metabolic effects produced in the yeast cell by exposure to glucose. It reduces the consumption rate of glucose; changes the glycolytic intermediate pattern, giving rise to a crossover point at the level of the phosphofructokinase/fructose-bisphosphatase cycle; and increases the ATP level and the energy charge. Moreover, it substantially reduces the peak of intracellular cAMP content that immediately follows glucose entry; the magnitude of this effect is dependent on the drug concentration. The effect on the change of intracellular cAMP level appears, among all metabolic effects determined by caffeine, the only plausible one to explain the interference with catabolite inactivation of enzymes. Actually a strong negative correlation between residual activity of each of the three investigated enzymes and intracellular cAMP level has been demonstrated. The existence of a common mechanism of action of cAMP, as the mediating factor for catabolite inactivation of all three enzymes, is proposed.

Characterization of the proteolytic activity firmly attached to yeast phoshoenolpyruvate carboxykinase

Incubation of partially purified yeast phosphoenolpyruvate carboxykinase (ATP:oxaloacetate carboxy-lyase (transphosphorylating), EC 4.1.1.49) with 5% mercaptoethanol and 0.01% sodium dodecyl sulfate at 37 degrees C results in degradation of the enzyme. The degradation can be partially prevented by addition of proteinase B inhibitor 2 or phenylmethylsulfonyl fluoride, an inhibitor of proteinase B and carboxypeptidase Y. The degradation can be completely inhibited by addition of proteinase B inhibitor 2 together with pepstatin, and inhibitor of proteinase A. Thus it appears that proteolytic activities are firmly attached to phosphoenolpyruvate carboxykinase and are identical with the yeast proteinases A and B. The latter conclusion was supported by experiments using the pure yeast proteinases.