Asparagine-linked carbohydrate does not determine the cellular location of yeast vacuolar nonspecific alkaline phosphatase

Overexpression of polyphosphate polymerases and deletion of polyphosphate phosphatases shorten the replicative lifespan in yeast

We previously found that overexpression of phosphate starvation-responsive genes by disrupting PHO80 led to a shortened replicative lifespan in yeast. To identify lifespan-related genes, we screened upregulated genes in the pho80Δ mutant and focused on the VTC genes, which encode the vacuolar polyphosphate (polyP) polymerase complex. VTC1/VTC2/VTC4 deletion restored the lifespan and intracellular polyP levels in pho80Δ. In the wild type, overexpression of VTC5 or a combination of the other VTCs caused high polyP accumulation and shortened lifespan. Similar phenotypes were caused by the deletion of polyP phosphatase genes-vacuolar PPN1 and cytosolic PPX1. The polyP-accumulating strains exhibited stress sensitivities. Thus, we demonstrated that polyP metabolic enzymes participate in replicative lifespan, and extreme polyP accumulation shortens the lifespan.

Phosphate-responsive signaling pathway is a novel component of NAD+ metabolism in Saccharomyces cerevisiae

Nicotinamide adenine dinucleotide (NAD(+)) is an essential cofactor involved in various cellular biochemical reactions. To date the signaling pathways that regulate NAD(+) metabolism remain unclear due to the dynamic nature and complexity of the NAD(+) metabolic pathways and the difficulty of determining the levels of the interconvertible pyridine nucleotides. Nicotinamide riboside (NmR) is a key pyridine metabolite that is excreted and re-assimilated by yeast and plays important roles in the maintenance of NAD(+) pool. In this study we establish a NmR-specific reporter system and use it to identify yeast mutants with altered NmR/NAD(+) metabolism. We show that the phosphate-responsive signaling (PHO) pathway contributes to control NAD(+) metabolism. Yeast strains with activated PHO pathway show increases in both the release rate and internal concentration of NmR. We further identify Pho8, a PHO-regulated vacuolar phosphatase, as a potential NmR production factor. We also demonstrate that Fun26, a homolog of human ENT (equilibrative nucleoside transporter), localizes to the vacuolar membrane and establishes the size of the vacuolar and cytosolic NmR pools. In addition, the PHO pathway responds to depletion of cellular nicotinic acid mononucleotide (NaMN) and mediates nicotinamide mononucleotide (NMN) catabolism, thereby contributing to both NmR salvage and phosphate acquisition. Therefore, NaMN is a putative molecular link connecting the PHO signaling and NAD(+) metabolic pathways. Our findings may contribute to the understanding of the molecular basis and regulation of NAD(+) metabolism in higher eukaryotes.

Molecular dissection of Erv26p identifies separable cargo binding and coat protein sorting activities

Efficient export of secretory alkaline phosphatase (ALP) from the endoplasmic reticulum depends on the conserved transmembrane sorting adaptor Erv26p/Svp26p. In the present study we investigated the mechanism by which Erv26p couples pro-ALP to the coat protein complex II (COPII) export machinery. Site-specific mutations were introduced into Erv26p, and mutant proteins were assessed in cell-free assays that monitor interactions with pro-ALP cargo and packaging into COPII vesicles. Mutations in the second and third loop domains of Erv26p inhibited interaction with pro-ALP, whereas mutations in the C-terminal tail sequence influenced incorporation into COPII vesicles and subcellular distribution. Interestingly mutations in the second loop domain also influenced Erv26p homodimer associations. Finally we demonstrated that Ktr3p, a cis-Golgi-localized mannosyltransferase, also relies on Erv26p for efficient COPII-dependent export from the endoplasmic reticulum. These findings demonstrate that Erv26p acts as a protein sorting adaptor for a variety of Type II transmembrane cargo proteins and requires domain-specific interactions with both cargo and coat subunits to promote efficient secretory protein transport.

Acclimation of Chlamydomonas reinhardtii to its nutrient environment

To cope with low nutrient availability in nature, organisms have evolved inducible systems that enable them to scavenge and efficiently utilize the limiting nutrient. Furthermore, organisms must have the capacity to adjust their rate of metabolism and make specific alterations in metabolic pathways that favor survival when the potential for cell growth and division is reduced. In this article I will focus on the acclimation of Chlamydomonas reinhardtii, a unicellular, eukaryotic green alga to conditions of nitrogen, sulfur and phosphorus deprivation. This organism has a distinguished history as a model for classical genetic analyses, but it has recently been developed for exploitation using an array of molecular and genomic tools. The application of these tools to the analyses of nutrient limitation responses (and other biological processes) is revealing mechanisms that enable Chlamydomonas to survive harsh environmental conditions and establishing relationships between the responses of this morphologically simple, photosynthetic eukaryote and those of both nonphotosynthetic organisms and vascular plants.

Specific dephosphorylation of phosphopeptides by the yeast alkaline phosphatase encoded by PHO8 gene

Partially purified nonspecific phosphate-repressible alkaline phosphatase from Saccharomyces cerevisiae encoded by PHO8 gene (rALPase), efficiently dephosphorylates phosphohistones and a variety of phosphopeptides. The pho8 mutant, constructed by disruption of the chromosomal counterpart of the PHO8 gene, is lacking in phosphatase activity toward phosphopeptides, confirming that this activity is actually due to rALPase. rALPase activity tested on phosphopeptides is maximum in the pH range 6.5-7.5 and the Km values for these substrates are in the micromolar range, suggesting a possible physiological relevance of this enzyme as a protein phosphatase. rALPase dephosphorylates phosphotyrosyl more efficiently than phosphoseryl peptides, but is poorly active on phosphothreonyl peptides. Its specificity towards synthetic peptides and insensitivity to specific inhibitors and activators of authentic protein phosphatases indicate that rALPase differs from both Ser/Thr- and Tyr-specific protein phosphatases. This conclusion is consistent with the lack of homology with any class of known protein phosphatases.

N-linked glycosylation of proteinase B precursors of the yeast Saccharomyces cerevisiae is not required for proper targeting or processing of the enzyme

Proteinase B precursors are modified by an N-linked carbohydrate side chain at Asn 314. Glycosylation at this position is not required for proper localization, processing, or activation of the enzyme.

Alternative pathways for the sorting of soluble vacuolar proteins in yeast: a vps35 null mutant missorts and secretes only a subset of vacuolar hydrolases

vps35 mutants of Saccharomyces cerevisiae exhibit severe defects in the localization of carboxypeptidase Y, a soluble vacuolar hydrolase. We have cloned the wild-type VPS35 gene by complementation of the vacuolar protein sorting defect exhibited by the vps35-17 mutant. Sequence analysis revealed an open reading frame predicted to encode a protein of 937 amino acids that lacks any obvious hydrophobic domains. Subcellular fractionation studies indicated that 80% of Vps35p peripherally associates with a membranous particulate cell fraction. The association of Vps35p with this fraction appears to be saturable; when overproduced, the vast majority of Vps35p remains in a soluble fraction. Disruption of the VPS35 gene demonstrated that it is not essential for yeast cell growth. However, the null allele of VPS35 results in a differential defect in the sorting of vacuolar carboxypeptidase Y (CPY), proteinase A (PrA), proteinase B (PrB), and alkaline phosphatase (ALP). proCPY was quantitatively missorted and secreted by delta vps35 cells, whereas almost all of proPrA, proPrB, and proALP were retained within the cell and converted to their mature forms, indicating delivery to the vacuole. Based on these observations, we propose that alternative pathways exist for the sorting and/or delivery of proteins to the vacuole.

Yeast carboxypeptidase Y requires glycosylation for efficient intracellular transport, but not for vacuolar sorting, in vivo stability, or activity

Functions of the carbohydrate side chains of the yeast vacuolar enzyme carboxypeptidase Y (CPY) were investigated by removal, through site-directed mutagenesis, of the sequences which act as signals for N-linked glycosylation. The mutant forms of the enzyme were analysed with respect to activity and intracellular sorting, and the stabilities in vivo and in vitro were studied. It was found that carbohydrate was not important for accurate vacuolar targeting of CPY, but that the rate of transport of the unglycosylated CPY through the secretory pathway to the vacuole was reduced. Tunicamycin, which inhibits the formation of asparagine-linked glycosylation, had a similar effect on the transport of CPY at 23 degrees C. However, the absence of N-linked carbohydrate in general had the more dramatic result of blocking the transport of CPY altogether at an increased temperature (37 degrees C). The unglycosylated mutant CPY was not temperature sensitive for transport in the absence of tunicamycin. Analysis of mutant enzymes containing a single glycosyl residue at each of the four positions showed that the residue at position 87 was particularly important for transport. There was no decrease in the intracellular stability of the completely unglycosylated enzyme, and in vitro the rate of heat inactivation of this species was not increased.

Specific cis-acting sequence for PHO8 expression interacts with PHO4 protein, a positive regulatory factor, in Saccharomyces cerevisiae

The PHO8 gene of Saccharomyces cerevisiae encodes repressible alkaline phosphatase (rALPase; EC 3.1.3.1). The rALPase activity of the cells is two to three times higher in medium containing a low concentration of Pi than in high-Pi medium due to transcription of PHO8. The Pi signals are conveyed to PHO8 by binding of PHO4 protein, a positive regulatory factor, to a promoter region of PHO8 (PHO8p) under the influence of the PHO regulatory circuit. Deletion analysis of PHO8p DNA revealed two separate regulatory regions required for derepression of rALPase located at nucleotide positions -704 to -661 (distal region) and -548 to -502 (proximal region) and an inhibitory region located at -421 to -289 relative to the translation initiation codon. Gel retardation experiments showed that a beta-galactosidase-PHO4 fusion protein binds to a 132-bp PHO8p fragment bearing the proximal region but not to a 226-bp PHO8 DNA bearing the distal region. The fusion protein also binds to a synthetic oligonucleotide having the same 12-bp nucleotide sequence as the PHO8p DNA from positions -536 to -525. The 132-bp PHO8p fragment, connected at position -281 of the 5' upstream region of a HIS5'-'lacZ fused gene, could sense Pi signals in vivo, but a 20-bp synthetic oligonucleotide having the same sequence from -544 to -525 of the PHO8p DNA could not. Linker insertions in the PHO8p DNA indicated that the 5-bp sequence 5'-CACGT-3' from positions -535 to -531 is essential for binding the beta-galactosidase-PHO4 fusion protein and for derepression of rALPase.

Protein transport and compartmentation in yeast

Many newly synthesized proteins must be translocated across one or more membranes to reach their destination in the individual organelles or membrane systems. Translocation, mostly requiring an energy source, a signal on the protein itself, loose conformation of the protein and the presence of cytosolic and/or membrane receptor-like proteins, is often accompanied by covalent modifications of transported proteins. In this review I discuss these aspects of protein transport via the classical secretory pathway and/or special translocation mechanisms in the unicellular eukaryotic organism Saccharomyces cerevisiae.

Specific cis-acting sequence for PHO8 expression interacts with PHO4 protein, a positive regulatory factor, in Saccharomyces cerevisiae

The PHO8 gene of Saccharomyces cerevisiae encodes repressible alkaline phosphatase (rALPase; EC 3.1.3.1). The rALPase activity of the cells is two to three times higher in medium containing a low concentration of Pi than in high-Pi medium due to transcription of PHO8. The Pi signals are conveyed to PHO8 by binding of PHO4 protein, a positive regulatory factor, to a promoter region of PHO8 (PHO8p) under the influence of the PHO regulatory circuit. Deletion analysis of PHO8p DNA revealed two separate regulatory regions required for derepression of rALPase located at nucleotide positions -704 to -661 (distal region) and -548 to -502 (proximal region) and an inhibitory region located at -421 to -289 relative to the translation initiation codon. Gel retardation experiments showed that a beta-galactosidase-PHO4 fusion protein binds to a 132-bp PHO8p fragment bearing the proximal region but not to a 226-bp PHO8 DNA bearing the distal region. The fusion protein also binds to a synthetic oligonucleotide having the same 12-bp nucleotide sequence as the PHO8p DNA from positions -536 to -525. The 132-bp PHO8p fragment, connected at position -281 of the 5' upstream region of a HIS5'-'lacZ fused gene, could sense Pi signals in vivo, but a 20-bp synthetic oligonucleotide having the same sequence from -544 to -525 of the PHO8p DNA could not. Linker insertions in the PHO8p DNA indicated that the 5-bp sequence 5'-CACGT-3' from positions -535 to -531 is essential for binding the beta-galactosidase-PHO4 fusion protein and for derepression of rALPase.

The vacuolar protein-targeting signal of yeast carboxypeptidase is functional in oocytes from Xenopus laevis

Carboxypeptidase Y, a yeast vacuolar glycoprotein was expressed in oocytes from Xenopus laevis and its biosynthesis and sorting were examined. In yeast, targeting to the vacuole, the functional equivalent of the lysosome, is not mannose-6-phosphate-receptor dependent. It was found that carboxypeptidase enters the secretory pathway of the oocyte and is there glycosylated, phosphorylated in the carbohydrate part and delivered to the lysosome. Deletion of an amino acid sequence, previously shown to determine intracellular targeting of this enzyme in yeast, caused a loss of phosphorylation and mislocalization of carboxypeptidase Y into the oocyte medium. Inhibition of glycosylation of carboxypeptidase by tunicamycin did not lead to its secretion. In-frame fusion of the targeting domain to a secretory yeast glycoprotein, invertase, did not prevent its secretion. However, a hybrid containing 80% carboxypeptidase abolished invertase secretion. The results indicate that the vacuolar protein-targeting signal from yeast carboxypeptidase can, in principal, function in a higher eukaryote.

The fungal vacuole: composition, function, and biogenesis

The fungal vacuole is an extremely complex organelle that is involved in a wide variety of functions. The vacuole not only carries out degradative processes, the role most often ascribed to it, but also is the primary storage site for certain small molecules and biosynthetic precursors such as basic amino acids and polyphosphate, plays a role in osmoregulation, and is involved in the precise homeostatic regulation of cytosolic ion and basic amino acid concentration and intracellular pH. These many functions necessitate an intricate interaction between the vacuole and the rest of the cell; the vacuole is part of both the secretory and endocytic pathways and is also directly accessible from the cytosol. Because of the various roles and properties of the vacuole, it has been possible to isolate mutants which are defective in various vacuolar functions including the storage and uptake of metabolites, regulation of pH, sorting and processing of vacuolar proteins, and vacuole biogenesis. These mutants show a remarkable degree of genetic overlap, suggesting that these functions are not individual, discrete properties of the vacuole but, rather, are closely interrelated.

Molecular characterization of a specific p-nitrophenylphosphatase gene, PHO13, and its mapping by chromosome fragmentation in Saccharomyces cerevisiae

The structural gene, PHO13, for the specific p-nitrophenyl phosphatase of Saccharomyces cerevisiae was cloned and its nucleotide sequence determined. The deduced PHO13 protein consists of 312 amino acids and its molecular weight is 34635. The disruption of the PHO13 gene produced no effect on cell growth, sporulation, or viability of ascospores. The PHO13 locus was mapped at 1.9 centimorgans from the HO locus on the left arm of chromosome IV. By chromosome fragmentation, the PHO13 locus was found to be located about 72 kb from the left-hand telomere of chromosome IV and distal to the HO locus.

Ultracytochemical localization of the vacuolar marker enzymes alkaline phosphatase, adenosine triphosphatase, carboxypeptidase Y and aminopeptidase reveal new concept of vacuole biogenesis in Saccharomyces cerevisiae

Logarithmic cultures of Saccharomyces cerevisiae strains LBG H 1022, FL-100, X 2180 1A and 1B were studied together with the mutants pep4-3, sec18-1 and sec7-1. The necessary ultrastructural observations showed that, as a rule, juvenile vacuoles were formed de novo from perinuclear endoplasmic reticulum cisternae (ER) packed and inflated with electron-dense (polyanionic) matrix material. This process was disturbed solely in the sec18-1 mutant under non-permissive conditions. The vacuolar marker enzymes adenosine triphosphatase (ATPase) and alkaline phosphohydrolase (ALPase) were assayed by the ultracytochemical cerium precipitation technique. The neutral ATPase was active in vacuolar membranes and in the previously shown (coated) microglobules nearby. ALPase activity was detected in microglobules inside juvenile vacuoles, inside nucleus and in the cytoplasm as well as in the membrane vesicles and in the periplasm. The sites of vacuolar protease carboxypeptidase Y (CPY) activity were assayed using N-CBZ-L-tyrosine-4-methoxy-2-naphthyl-amide (CBZ-Tyr-MNA) as substrate and sites of the amino-peptidase M activity using Leu-MNA as substrate. Hexazotized p-rosaniline served as a coupler for the primary reaction product of both the above proteases (MNA) and the resulting azo-dye was osmicated during postfixation. The CPY reaction product was found in both polar layers of vacuolar membranes (homologous to ER) and in ER membranes enclosing condensed lipoprotein bodies which were taken up by the vacuoles of late logarithmic yeast. Both before and after the uptake into the vacuoles the bodies contained the CPY reaction product in concentric layers or in cavities. Microglobules with CPY activity were also observed. Aminopeptidase was localized in microglobules inside the juvenile vacuoles. These findings combined with the previous cytochemical localizations of polyphosphates and X-prolyl-dipeptidyl (amino)peptidase in S. cerevisiae suggest the following cytologic mechanism for the biosynthetic protein transport: coated microglobules convey metabolites and enzymes either to the cell surface for secretion or enter the vacuoles in all phases of the cell cycle. The membrane vesicles represent an alternative secretory mechanism present in yeast cells only during budding. The homology of the ER with the vacuolar membranes and with the surface membranes of the lipoprotein condensates (bodies) indicates a cotranslational entry of the CPY into these membranes. The secondary transfer of a portion of CPY into vacuoles is probably mediated by the lipoprotein uptake process.

Intracellular sorting and processing of a yeast vacuolar hydrolase: proteinase A propeptide contains vacuolar targeting information

An inactive precursor form of proteinase A (PrA) transits through the early secretory pathway before final vacuolar delivery. We used gene fusions between the gene coding for PrA (PEP4) and the gene coding for the secretory enzyme invertase (SUC2) to identify vacuolar protein-sorting information in the PrA precursor. We found that the 76-amino-acid preprosegment of PrA contains at least two sorting signals: an amino-terminal signal peptide that is cleaved from the protein at the level of the endoplasmic reticulum followed by the prosegment which functions as a vacuolar protein-sorting signal. PrA-invertase hybrid proteins that carried this sequence information were accurately sorted to the yeast vacuole as determined by cell fractionation and immunolocalization studies. Hybrid proteins lacking all or a portion of the PrA prosegment were secreted from the cell. Our gene fusion data together with an analysis of the wild-type PrA protein indicated that N-linked carbohydrate modifications are not required for vacuolar sorting of this protein. Furthermore, results obtained with a set of deletion mutations constructed in the PrA prosegment indicated that this sequence also contributes to proper folding of this polypeptide into a stable transit-competent molecule.

Intracellular sorting and processing of a yeast vacuolar hydrolase: proteinase A propeptide contains vacuolar targeting information

An inactive precursor form of proteinase A (PrA) transits through the early secretory pathway before final vacuolar delivery. We used gene fusions between the gene coding for PrA (PEP4) and the gene coding for the secretory enzyme invertase (SUC2) to identify vacuolar protein-sorting information in the PrA precursor. We found that the 76-amino-acid preprosegment of PrA contains at least two sorting signals: an amino-terminal signal peptide that is cleaved from the protein at the level of the endoplasmic reticulum followed by the prosegment which functions as a vacuolar protein-sorting signal. PrA-invertase hybrid proteins that carried this sequence information were accurately sorted to the yeast vacuole as determined by cell fractionation and immunolocalization studies. Hybrid proteins lacking all or a portion of the PrA prosegment were secreted from the cell. Our gene fusion data together with an analysis of the wild-type PrA protein indicated that N-linked carbohydrate modifications are not required for vacuolar sorting of this protein. Furthermore, results obtained with a set of deletion mutations constructed in the PrA prosegment indicated that this sequence also contributes to proper folding of this polypeptide into a stable transit-competent molecule.

Protein glycosylation in yeast

S. cerevisiae contains many mannose-rich glycoproteins that possess N- and O-linked carbohydrate chains, and both types may even occur on one and the same protein. The steps in the synthesis of asparagine-linked chains begin with assembly and transfer of the lipid-linked precursor to protein in a way common to all eucaryotes. Subsequent modifications lead to mannosyl extensions of various lengths, but complex type carbohydrate structures are not formed. Oligosaccharides O-linked to serine/threonine consist exclusively of mannose in S. cerevisiae. The mannose residue attached directly to the protein is transferred from Dol-P-Man in a unique way, which has been observed so far for fungal cells only. The cellular localization of the glycosylation reactions is summarized and the problem of transmembrane translocation of the sugar precursors at the ER and the Golgi is discussed. Some aspects of secretory (sec) and asparagine linked glycosylation (alg) mutants have been covered, and the various hypotheses related to the possible functions of this costly protein modification process are discussed. The article may also be helpful for those, who want to exploit the yeast's protein synthesizing machinery by genetically manipulating the cells.

Structural characteristics of the PHO8 gene encoding repressible alkaline phosphatase in Saccharomyces cerevisiae

The nucleotide sequence of a 3694-bp DNA fragment bearing the PHO8 gene encoding nonspecific repressible alkaline phosphatase (rALPase; EC 3.1.3.1) of Saccharomyces cerevisiae was determined. The sequence contains a 1698 bp open reading frame (ORF), and the major PHO8 transcription start point at 32 bp upstream from the ATG codon; several minor transcription start points are located between the major start point and ATG. The major start point is most responsive to the phosphate signals. The amino acid (aa) sequence deduced from the ORF contains several homologous regions in common with alkaline phosphatases of Escherichia coli and human placenta. A PHO8 DNA fragment previously isolated [Kaneko et al., Mol. Cell. Biol. 5 (1985) 248-252] was found to be truncated for the region encoding the 22 aa residues at the C terminus of the enzyme, which were replaced with 17 aa encoded by a pBR322 DNA. The modified gene could produce significant rALPase activity without the function of proteinase A which is required for the maturation of rALPase from its precursor.

Transcriptional and post-transcriptional control of PHO8 expression by PHO regulatory genes in Saccharomyces cerevisiae

A DNA fragment bearing the PHO8 gene, which encodes repressible alkaline phosphatase of Saccharomyces cerevisiae, was cloned. Northern hybridizations with the PHO8 DNA as probe indicated that the PHO8 transcript is 1.8 kilobases in length and is more abundant in cells grown in low-phosphate medium than in high-phosphate medium. The pho9 mutant, whose phenotype is defective in the activity of repressible alkaline phosphatase, produced as much of the PHO8 transcript as did the PHO9+ cells. Hence, the PHO9 product should act at the post-transcriptional level. The pho4 mutant could not derepress the PHO8 transcript, whereas the pho80 mutant could, irrespective of the amount of Pi in the medium, as has been suggested by genetic study.

Transcriptional and post-transcriptional control of PHO8 expression by PHO regulatory genes in Saccharomyces cerevisiae

A DNA fragment bearing the PHO8 gene, which encodes repressible alkaline phosphatase of Saccharomyces cerevisiae, was cloned. Northern hybridizations with the PHO8 DNA as probe indicated that the PHO8 transcript is 1.8 kilobases in length and is more abundant in cells grown in low-phosphate medium than in high-phosphate medium. The pho9 mutant, whose phenotype is defective in the activity of repressible alkaline phosphatase, produced as much of the PHO8 transcript as did the PHO9+ cells. Hence, the PHO9 product should act at the post-transcriptional level. The pho4 mutant could not derepress the PHO8 transcript, whereas the pho80 mutant could, irrespective of the amount of Pi in the medium, as has been suggested by genetic study.