Vacuoles: The sole compartments of digestive enzymes in yeast (Saccharomyces cerevisiae)?

Intracellular sphingolipid sorting drives membrane phase separation in the yeast vacuole

The yeast vacuole membrane can phase separate into ordered and disordered domains, a phenomenon that is required for micro-lipophagy under nutrient limitation. Despite its importance as a biophysical model and physiological significance, it is not yet resolved if specific lipidome changes drive vacuole phase separation. Here we report that the metabolism of sphingolipids (SLs) and their sorting into the vacuole membrane can control this process. We first developed a vacuole isolation method to identify lipidome changes during the onset of phase separation in early stationary stage cells. We found that early stationary stage vacuoles are defined by an increased abundance of putative raft components, including 40% higher ergosterol content and a nearly 3-fold enrichment in complex SLs (CSLs). These changes were not found in the corresponding whole cell lipidomes, indicating that lipid sorting is associated with domain formation. Several facets of SL composition-headgroup stoichiometry, longer chain lengths, and increased hydroxylations-were also markers of phase-separated vacuole lipidomes. To test SL function in vacuole phase separation, we carried out a systematic genetic dissection of their biosynthetic pathway. The abundance of CSLs controlled the extent of domain formation and associated micro-lipophagy processes, while their headgroup composition altered domain morphology. These results suggest that lipid trafficking can drive membrane phase separation in vivo and identify SLs as key mediators of this process in yeast.

How and why intralumenal membrane fragments form during vacuolar lysosome fusion

Lysosomal membrane fusion mediates the last step of the autophagy and endocytosis pathways and supports organelle remodeling and biogenesis. Because fusogenic proteins and lipids concentrate in a ring at the vertex between apposing organelle membranes, the encircled area of membrane can be severed and internalized within the lumen as a fragment upon lipid bilayer fusion. How or why this intralumenal fragment forms during fusion, however, is not entirely clear. To better understand this process, we studied fragment formation during homotypic vacuolar lysosome membrane fusion in Saccharomyces cerevisiae Using cell-free fusion assays and light microscopy, we find that GTPase activation and trans-SNARE complex zippering have opposing effects on fragment formation and verify that this affects the morphology of the fusion product and regulates transporter protein degradation. We show that fragment formwation is limited by stalk expansion, a key intermediate of the lipid bilayer fusion reaction. Using electron microscopy, we present images of hemifusion diaphragms that form as stalks expand and propose a model describing how the fusion machinery regulates fragment formation during lysosome fusion to control morphology and protein lifetimes.

The three-dimensional morphology of Candida albicans as seen by high-resolution scanning electron microscopy

The fine structure of Candida albicans has been repeatedly described by transmission electron microscopy, whereas studies by high-resolution scanning electron microscopy (HRSEM) are rare and devoted solely to the study of its external morphology. This report describes the results of an HRSEM study on C. albicans carried out by an osmium maceration protocol modified to better retain the structural characteristics of this yeast. Thus, we visualized various intracellular structures including invaginations of cell membrane (plasmalemmasomes), nuclear envelope, mitochondria, the vacuolar system, and two additional structures that might represent a form of endoplasmic reticulum and the Golgi apparatus. The present investigation, which for the first time shows the organelles of C. albicans at the 3D level, may lead to a better understanding of its cell physiology.

The yeast vacuolar membrane proteome

Transport of solutes between the cytosol and the vacuolar lumen is of crucial importance for various functions of vacuoles, including ion homeostasis; detoxification; storage of different molecules such as amino acids, phosphate, and calcium ions; and proteolysis. To identify proteins that catalyze solute transport across the vacuolar membrane, the membrane proteome of purified Saccharomyces cerevisiae vacuoles was analyzed. Subtractive proteomics was used to distinguish contaminants from true vacuolar proteins by comparing the relative abundances of proteins in pure and crude preparations. A robust statistical analysis combining enrichment ranking with the double boundary iterative group analysis revealed that 148 proteins were significantly enriched in the pure vacuolar preparations. Among these proteins were well characterized vacuolar proteins, such as the subunits of the vacuolar H(+)-ATPase, but also proteins that had not previously been assigned to a cellular location, many of which are likely novel vacuolar membrane transporters, e.g. for nucleosides and oligopeptides. Although the majority of contaminating proteins from other organelles were depleted from the pure vacuolar membranes, some proteins annotated to reside in other cellular locations were enriched along with the vacuolar proteins. In many cases the enrichment of these proteins is biologically relevant, and we discuss that a large group is involved in membrane fusion and protein trafficking to vacuoles and may have multiple localizations. Other proteins are degraded in vacuoles, and in some cases database annotations are likely to be incomplete or incorrect. Our work provides a wealth of information on vacuolar biology and a solid basis for further characterization of vacuolar functions.

Analysis of the vacuolar luminal proteome of Saccharomyces cerevisiae

Despite its large size and the numerous processes in which it is implicated, neither the identity nor the functions of the proteins targeted to the yeast vacuole have been defined comprehensively. In order to establish a methodological platform and protein inventory to address this shortfall, we refined techniques for the purification of 'proteomics-grade' intact vacuoles. As confirmed by retention of the preloaded fluorescent conjugate glutathione-bimane throughout the fractionation procedure, the resistance of soluble proteins that copurify with this fraction to digestion by exogenous extravacuolar proteinase K, and the results of flow cytometric, western and marker enzyme activity analyses, vacuoles prepared in this way retain most of their protein content and are of high purity and integrity. Using this material, 360 polypeptides species associated with the soluble fraction of the vacuolar isolates were resolved reproducibly by 2D gel electrophoresis. Of these, 260 were identified by peptide mass fingerprinting and peptide sequencing by MALDI-MS and liquid chromatography coupled to ion trap or quadrupole TOF tandem MS, respectively. The polypeptides identified in this way, many of which correspond to alternate size and charge states of the same parent translation product, can be assigned to 117 unique ORFs. Most of the proteins identified are canonical vacuolar proteases, glycosidases, phosphohydrolases, lipid-binding proteins or established vacuolar proteins of unknown function, or other proteases, glycosidases, lipid-binding proteins, regulatory proteins or proteins involved in intermediary metabolism, protein synthesis, folding or targeting, or the alleviation of oxidative stress. On the basis of the high purity of the vacuolar preparations, the electrophoretic properties of the proteins identified and the results of quantitative proteinase K protection measurements, many of the noncanonical vacuolar proteins identified are concluded to have entered this compartment for breakdown, processing and/or salvage purposes.

Measurement of fast dynamic intracellular pH in Saccharomyces cerevisiae using benzoic acid pulse

pH affects many processes on cell metabolism, such as enzyme kinetics. To enhance the understanding of the living cells, it is therefore indispensable to have a method to monitor the pH in living cells. To accomplish this, a dynamic intracellular pH measurement method applying low concentration benzoic acid pulse was developed. The method was thoroughly validated and successfully implemented for measuring fast dynamic intracellular pH of Saccharomyces cerevisiae in response to a glucose pulse perturbation performed in the BioSCOPE set-up. Fast drop in intracellular pH followed by partial alkalinization was observed following the pulse. The low concentration benzoic acid pulse which was implemented in the method avoids the undesirable effects that may be introduced by benzoic acid to cell metabolism.

Synergistic Fungicidal Activities of Amphotericin B and N-Methyl-N″-dodecylguanidine: A Constituent of Polyol Macrolide Antibiotic Niphimycin

The synergy between the alkylguanidinium chain of niphimycin (NM), a polyol macrolide antibiotic, and polyene macrolide amphotericin B (AmB) without such an alkyl side chain was examined using N-methyl-N"-alkylguanidines as its synthetic analogs. Among the analogs, N-methyl-N"-dodecylguanidine (MC12) most strongly inhibited the growth of Saccharomyces cerevisiae cells and those of other fungal strains in synergy with AmB. MC12 itself was not lethal but the analog could be a cause of a rapid cell death progression of yeast cells in the presence of AmB at a nonlethal concentration. Their combined actions resulted in the generation of NM-like fungicidal activity that depended on plasma membrane disability and cellular reactive oxygen species production. We also found an aberrant vacuolar morphogenesis and an associated vacuolar membrane disability in cells treated simultaneously with MC12 and AmB, as in the case of NM-treated cells. These findings support the idea that the alkylguanidinium chain plays a major role in the fungicidal activity of NM in cooperation with the polyol lactone ring as its enhancer.

Inorganic polyphosphates and exopolyphosphatases in cell compartments of the yeast Saccharomyces cerevisiae under inactivation of PPX1 and PPN1 genes

Purified fractions of cytosol, vacuoles, nuclei, and mitochondria of Saccharomyces cerevisiae possessed inorganic polyphosphates with chain lengths characteristic of each individual compartment. The most part (80-90%) of the total polyphosphate level was found in the cytosol fractions. Inactivation of a PPX1 gene encoding ~40-kDa exopolyphosphatase substantially decreased exopolyphosphatase activities only in the cytosol and soluble mitochondrial fraction, the compartments where PPX1 activity was localized. This inactivation slightly increased the levels of polyphosphates in the cytosol and vacuoles and had no effect on polyphosphate chain lengths in all compartments. Exopolyphosphatase activities in all yeast compartments under study critically depended on the PPN1 gene encoding an endopolyphosphatase. In the single PPN1 mutant, a considerable decrease of exopolyphosphatase activity was observed in all the compartments under study. Inactivation of PPN1 decreased the polyphosphate level in the cytosol 1.4-fold and increased it 2- and 2.5-fold in mitochondria and vacuoles, respectively. This inactivation was accompanied by polyphosphate chain elongation. In nuclei, this mutation had no effect on polyphosphate level and chain length as compared with the parent strain CRY. In the double mutant of PPX1 and PPN1, no exopolyphosphatase activity was detected in the cytosol, nuclei, and mitochondria and further elongation of polyphosphates was observed in all compartments.

Functional morphology of the secretory pathway organelles in yeast

The glycoprotein secretory pathway of yeast serves mainly for cell surface growth and cell division. It involves a centrifugal transport of transit macromolecules among organelles, whose membranes contain resident proteins needed for driving the transport. These resident membrane proteins return by retrograde vesicular transport. Apart from this, the pathway involves endocytosis. The model yeast Saccharomyces cerevisiae and vertebrate cells were found to contain very similar gene products regulating the molecular mechanism of glycoprotein transport, and the cellular mechanism of their secretion pathways was therefore also presumed to be identical. Biochemists have postulated that, in S. cerevisiae, the translocation of peptides through the endoplasmic reticulum membranes into the lumen of ER cisternae and the core glycosylation is followed by a vector-mediated transport into the functional cascade of the Golgi system cisternae and between them. This is the site of maturation and sorting of glycoproteins, before the ultimate transport by other vectors involving either secretion from the cells (exocytosis across the plasmalemma into the cell wall) or transport into the lysosome-like vacuole via a prevacuolar compartment, which serves at the same time as a primary endosome. The established cellular model of secretion deals with budding yeast; interphase yeast cells, in which the secretion is limited and which predominate in exponential cultures, have not been taken into consideration. The quality of organelle imaging in S. cerevisiae ultra-thin sections depends on the fixation technique used and on specimen contrasting by metals. The results achieved by combinations of different techniques differ mostly in the imaging of bilayers of membrane interfaces and the transparence of the matrix phase. Fixation procedures are decisive for the results of topochemical localisations of cellular antigenic components or enzyme activities, which form the basis of the following survey of functional morphology of organelles involved in the yeast secretory pathway. The existing results of these studies do not confirm all aspects of the vertebrate model of the Golgi apparatus proposed by molecular geneticists to hold for S. cerevisiae, and alternative models of the cellular mechanism of secretion in this yeast are, therefore, also discussed.

X-ray structures of five renin inhibitors bound to saccharopepsin: exploration of active-site specificity

Saccharopepsin is a vacuolar aspartic proteinase involved in activation of a number of hydrolases. The enzyme has great structural homology to mammalian aspartic proteinases including human renin and we have used it as a model system to study the binding of renin inhibitors by X-ray crystallography. Five medium-to-high resolution structures of saccharopepsin complexed with transition-state analogue renin inhibitors were determined. The structure of a cyclic peptide inhibitor (PD-129,541) complexed with the proteinase was solved to 2.5 A resolution. This inhibitor has low affinity for human renin yet binds very tightly to the yeast proteinase (K(i)=4 nM). The high affinity of this inhibitor can be attributed to its bulky cyclic moiety spanning P(2)-P(3)' and other residues that appear to optimally fit the binding sub-sites of the enzyme. Superposition of the saccharopepsin structure on that of renin showed that a movement of the loop 286-301 relative to renin facilitates tighter binding of this inhibitor to saccharopepsin. Our 2.8 A resolution structure of the complex with CP-108,420 shows that its benzimidazole P(3 )replacement retains one of the standard hydrogen bonds that normally involve the inhibitor's main-chain. This suggests a non-peptide lead in overcoming the problem of susceptible peptide bonds in the design of aspartic proteinase inhibitors. CP-72,647 which possesses a basic histidine residue at P(2), has a high affinity for renin (K(i)=5 nM) but proves to be a poor inhibitor for saccharopepsin (K(i)=3.7 microM). This may stem from the fact that the histidine residue would not bind favourably with the predominantly hydrophobic S(2) sub-site of saccharopepsin.

Alcohol acetyltransferases and the significance of ester synthesis in yeast

This paper reviews our current knowledge of yeast alcohol acyltransferases. Much of this information has been gathered over the past 10 years through the application of powerful yeast molecular biology techniques. Evidence from gene disruption and expression analysis of members of the alcohol acyltransferase (ATF) gene family indicates that different ester synthases are involved in the synthesis of esters during alcoholic fermentation. The natural physiological rationale behind these enzyme activities remains unclear. However, it is believed that these enzymes may be involved in very different functions, including cellular fatty acid homeostasis and detoxification mechanisms. Insights into the regulation of yeast ester synthesis by oxygen and unsaturated fatty acids have contributed to our understanding of the general mechanisms of gene regulation. In particular, control mechanisms that underpin the oxygen-mediated regulation of ATF1 gene transcription appear to be closely linked to those involved in the regulation of fatty acid metabolism. Data pertaining to the regulation of ATF1 gene transcription have been integrated into a working model for future research.

New aspects of inorganic polyphosphate metabolism and function

The review analyzes the results of recent studies on the biochemistry of high-molecular inorganic poly-phosphates (PolyPs). The data obtained lead to the following main conclusions. PolyPs are polyfunctional compounds. The main role of PolyPs is their participation in the regulation of metabolism both at the genetic and metabolic levels. Among the functions of PolyPs known at present, the most important are the following: phosphate and energy storage; regulation of the levels of ATP and other nucleotide and nucleoside-containing coenzymes; participation in the regulation of homeostasis and storage of inorganic cations and other positively charged solutes in an osmotically inert form; participation in membrane transport processes mediated by poly-beta-Ca2+-hydroxybutyrate complexes; participation in the formation and functions of cell surface structures; control of gene activity; and regulation of activities of the enzymes and enzyme assemblies involved in the metabolism of nucleic acids and other acid biopolymers. However, the functions of PolyPs vary among organisms of different evolutionary levels. The metabolism and functions of PolyPs in each cellular compartment of procaryotes (cell wall, plasma membrane, cytosol) and eucaryotes (nuclei, vacuoles, mitochondria, plasma membrane, cell wall, mitochondria, cytosol) are unique. The synthesis and degradation of PolyPs in the organelles of eucaryotic cells are possibly mediated by different sets of enzymes. This is consistent with of the endosymbiotic hypothesis of eucaryotic cell origin. Some aspects of the biochemistry of high-molecular PolyPs are considered to be of great significance to the approach to biotechnological, ecological and medical problems.

Pth1/Vam3p is the syntaxin homolog at the vacuolar membrane of Saccharomyces cerevisiae required for the delivery of vacuolar hydrolases

The PEP12 homolog Pth1p (Pep twelve homolog 1) is predicted to be similar in size to Pep12p, the endosomal syntaxin homolog that mediates docking of Golgi-derived transport vesicles and, like other members of the syntaxin family, is predicted to be a cytoplasmically oriented, integral membrane protein with a C-terminal transmembrane domain. Kinetic analyses indicate that deltapth1/vam3 mutants fail to process the soluble vacuolar hydrolase precursors and that PrA, PrB and most of CpY accumulate within the cell in their Golgi-modified P2 precursor forms. This is in contrast to a pep12 mutant in which P2CpY is secreted from the cell. Furthermore, pep12 is epistatic to pth1/vam3 with respect to the CpY secretion phenotype. Alkaline phosphatase, a vacuolar membrane hydrolase, accumulates in its precursor form in the deltapth1/vam3 mutant. Maturation of pro-aminopeptidase I, a hydrolase precursor delivered directly to the vacuole from the cytoplasm, is also blocked in the deltapth1/vam3 mutant. Subcellular fractionation localizes Pth1/Vam3p to vacuolar membranes. Based on these data, we propose that Pth1/Vam3p is the vacuolar syntaxin/t-SNARE homolog that participates in docking of transport vesicles at the vacuolar membrane and that the function of Pth1/Vam3p impinges on at least three routes of protein delivery to the yeast vacuole.

Genetic interaction with vps8-200 allows partial suppression of the vestigial vacuole phenotype caused by a pep5 mutation in Saccharomyces cerevisiae

pep5 mutants of Saccharomyces cerevisiae accumulate inactive precursors to the vacuolar hydrolases. In addition, they show a vestigial vacuole morphology and a sensitivity to growth on media containing excess divalent cations. This pleiotropic phenotype observed for pep5::TRP1 mutants is partially suppressed by the vps8-200 allele. pep5::TRP1 vps8-200 mutants show near wild-type levels of mature-sized soluble vacuolar hydrolases, growth on zinc-containing medium, and a more "wild-type" vacuolar morphology; however, aminopeptidase I and alkaline phosphatase accumulate as precursors. These data suggest that Pep5p is a bifunctional protein and that the TRP1 insertion does not eliminate function, but results in a shorter peptide that can interact with Vps8-200p, allowing for partial function. vps8 deletion/disruption mutants contain a single enlarged vacuole. This genetic interaction was unexpected, since Pep5p was thought to interact more directly with the vacuole, and Vps8p is thought to play a role in transport between the Golgi complex and the prevacuolar compartment. The data are consistent with Pep5p functioning both at the site of Vps8p function and more closely proximal to the vacuole. They also provide evidence that the three transport pathways to the vacuole either converge or share gene products at late step(s) in the pathway(s).

Comparison of exopolyphosphatases of different yeast cell compartments

Purified cell-envelope polyphosphatase as well as polyphoshatase activities of cytosol and isolated vacuoles, of nuclei and mitochondria of the yeast Saccharomyces cerevisiae were compared. The polyphosphatases of cell envelope and cytosol are similar, the polyphosphatases of nuclei, vacuoles and mitochondria differ in their kinetic properties, substrate specificity, requirements in divalent cations and in some effector actions both from these and from each other.

Phosphate transport in yeast vacuoles

The vacuole of the yeast Saccharomyces cerevisiae is a major storage compartment for phosphate. We have measured phosphate transport across the vacuolar membrane. Isolated intact vacuoles take up large amounts of added [32P]phosphate by counterflow exchange with phosphate present in the vacuoles at the time of their isolation. The bidirectional phosphate transporter has an intrinsic dissociation constant for phosphate of 0.4 mM. Exchange mediated by this carrier is faster than unidirectional efflux of phosphate from the vacuoles. The transporter is highly selective for phosphate; of other anions tested, only arsenate is also a substrate. Transport is strongly pH-dependent with increasing activity at lower pH. Similar phosphate transport behavior was observed in right-side-out vacuolar membrane vesicles.

The glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase of Candida albicans is a surface antigen

A lambda gt11 cDNA library from Candida albicans ATCC 26555 was screened by using pooled sera from two patients with systemic candidiasis and five neutropenic patients with high levels of anti-C. albicans immunoglobulin M antibodies. Seven clones were isolated from 60,000 recombinant phages. The most reactive one contained a 0.9-kb cDNA encoding a polypeptide immunoreactive only with sera from patients with systemic candidiasis. The whole gene was isolated from a genomic library by using the cDNA as a probe. The nucleotide sequence of the coding region showed homology (78 to 79%) to the Saccharomyces cerevisiae TDH1 to TDH3 genes coding for glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and their amino acid sequences showed 76% identity; thus, this gene has been named C. albicans TDH1. A rabbit polyclonal antiserum against the purified cytosolic C. albicans GAPDH (polyclonal antibody [PAb] anti-CA-GAPDH) was used to identify the GAPDH in the beta-mercaptoethanol extracts containing cell wall moieties. Indirect immunofluorescence demonstrated the presence of GAPDH at the C. albicans cell surface, particularly on the blastoconidia. Semiquantitative flow cytometry analysis showed the sensitivity of this GAPDH form to trypsin and its resistance to be removed with 2 M NaCl or 2% sodium dodecyl sulfate. The decrease in fluorescence in the presence of soluble GAPDH indicates the specificity of the labelling. In addition, a dose-dependent GAPDH enzymatic activity was detected in intact blastoconidia and germ tube cells. This activity was reduced by pretreatment of the cells with trypsin, formaldehyde, and PAb anti-CA-GAPDH. These observations indicate that an immunogenic, enzymatically active cell wall-associated form of the glycolytic enzyme GAPDH is found at the cell surface of C. albicans cells.

Purification and characterization of two isoenzymes of DL-glycerol-3-phosphatase from Saccharomyces cerevisiae. Identification of the corresponding GPP1 and GPP2 genes and evidence for osmotic regulation of Gpp2p expression by the osmosensing mitogen-activated protein kinase signal transduction pathway

The existence of specific dl-glycerol-3-phosphatase (EC 3.1.3.21) activity in extracts of Saccharomyces cerevisiae was confirmed by examining strains lacking nonspecific acid and alkaline phosphatase activities. During purification of the glycerol-3-phosphatase, two isozymes having very similar molecular weights were isolated by gel filtration and anion exchange chromatography. By microsequencing of trypsin-generated peptides the corresponding genes were identified as previously sequenced open reading frames of unknown function. The two genes, GPP1 (YIL053W) and GPP2 (YER062C) encode proteins that show 95% amino acid identity and have molecular masses of 30.4 and 27.8 kDa, respectively. The intracellular concentration of Gpp2p increases in cells subjected to osmotic stress, while the production of Gpp1p is unaffected by changes of external osmolarity. Both isoforms have a high specificity for dl-glycerol-3-phosphate, pH optima at 6.5, and KmG3P in the range of 3-4 mM. The osmotic induction of Gpp2p is blocked in cells that are defective in the HOG-mitogen-activated protein kinase pathway, indicating that GPP2 is a target gene for this osmosensing signal transduction pathway. Together with DOG1 and DOG2, encoding two highly homologous enzymes that dephosphorylate 2-deoxyglucose-6-phosphate, GPP1 and GPP2 constitute a new family of genes for low molecular weight phosphatases.

Isolation and characterization of a novel yeast gene, ATH1, that is required for vacuolar acid trehalase activity

We have isolated a plasmid containing a gene, ATH1, that results in eight- to ten-fold higher acid trehalase activity in yeast cells when present in high copy. The screening procedure was based on overproduction-induced mislocalization of acid trehalase activity; overproduction of vacuolar enzymes that transit through the secretory pathway leads to secretion to the cell surface. A DNA fragment that confers cell surface expression of acid trehalase activity was cloned and sequenced. The deduced amino acid sequence displayed no homology to known proteins, indicating that we have identified a novel gene. A deletion in the genomic copy of the ATH1 gene eliminates vacuolar acid trehalase activity. These results suggest that ATH1 may be the structural gene encoding vacuolar acid trehalase or that the gene product may be essential regulatory component involved in control of trehalase activity.

The gene for a major exopolyphosphatase of Saccharomyces cerevisiae

The gene encoding a major exopolyphosphatase (scPPX1) in Saccharomyces cerevisiae (H. Wurst and A. Kornberg, J. Biol. Chem. 269:10996-11001, 1994) has been isolated from a genomic library. The gene, located at 57 kbp from the end of the right arm of chromosome VIII, encodes a protein of 396 amino acids. Overexpression in Escherichia coli allowed the ready purification of a recombinant form of the enzyme. Disruption of the gene did not affect the growth rate of S. cerevisiae. Lysates from the mutants displayed considerably lower exopolyphosphatase activity than the wild type. The enzyme is located in the cytosol, whereas the vast accumulation of polyphosphate (polyP) of the yeast is in the vacuole. Disruption of PPX1 in strains with and without deficiencies in vacuolar proteases allowed the identification of exopolyphosphatase activity in the vacuole. This residual activity was strongly reduced in the absence of vacuolar proteases, indicating a dependence on proteolytic activation. A 50-fold-lower protease-independent activity could be distinguished from this protease-dependent activity by different patterns of expression during growth and activation by arginine. With regard to the levels of polyP in various mutants, those deficient in vacuolar ATPase retain less than 1% of the cellular polyP, a loss that is not offset by additional mutations that eliminate the cytosolic exopolyphosphatase and the vacuolar polyphosphatases dependent on vacuolar protease processing.

Ultracytochemical evidence of Golgi functions in microvesicles at all phases of cell cycle in Saccharomyces cerevisiae

The topical question of Golgi compartment identity in the ascomycetous yeast Saccharomyces cerevisiae is illustrated by a multiple ultracytochemical approach. For this eucaryotic single-cell organism the established scheme of secretory transport via a cascade of cisternae housing different functions of Golgi apparatus has been deduced principally of genetic and molecular analyses ex situ and confirms the mammalian secretion scheme. Nevertheless, ultracytochemical in situ localizations of enzyme activities engaged in secretion represented evidence for localization of important steps of secretory glycoprotein maturation in two morphologically distinct populations of transport microvesicles formed from endoplasmic reticulum and Golgi cisternae. Both types of microvesicles function in exocytosis or transport into lysosomal vacuoles and have identical charge. However, their presence differs in interphase and in budding cells of S. cerevisiae. Smooth, larger membrane bound microvesicles are conspicuous at the onset of budding and at construction of scars, while the coated, smaller microvesicles of globular ultrastructure are present constitutively, throughout the cell cycle. Because the established model of the yeast secretory path considers only the part of the budding phase preceding the onset of mitosis, an alternative scheme for the cellular mechanism of glycoprotein secretion in S. cerevisiae that distinguishes interphase and budding yeast, has been established. The lumen of microvesicles contains proteases catalysing maturation of the mating pheromone alpha-factor (yscIV, yscF), vacuolar protease yscY, alkaline phosphohydrolase, polyphosphorylated components of the bud scar and glycoproteins. The in situ approach also reveals a minimum level of alpha-factor precursor processing proteolytic activity at the budding phase of cells, a transient presence of polyphosphorylated compounds in the bud scars and their transport by microvesicles. Ultracytochemical reactions suggest that the nuclear envelope lumen houses certain functions attributed to endoplasmic reticulum and that some steps of outer-chain glycosylation may occur in microvesicles. Microvesicles which contain proteases and polyphosphorylated intermediates also appear in juvenile vacuoles (lysosomes). Ultracytochemical findings show the Golgi compartment of S. cerevisiae to consist not only of discrete endoplasmic cisternae, immunodetected by others as sites of outer chain alpha-1,6-mannosylation and of the Golgi membrane marker proteins Sec7p and Ypt1p, but also of microvesicles moving either to the cell plasma membrane or to vacuoles. The previously hypothesized hierarchy of segregated yeast Golgi cisternae was not revealed by ultracytochemical findings.(ABSTRACT TRUNCATED AT 400 WORDS)

Chemiosmotic coupling of ion transport in the yeast vacuole: its role in acidification inside organelles

Acidification inside the vacuo-lysosome systems is ubiquitous in eukaryotic organisms and essential for organelle functions. The acidification of these organelles is accomplished by proton-translocating ATPase belonging to the V-type H(+)-ATPase superfamily. However, in terms of chemiosmotic energy transduction, electrogenic proton pumping alone is not sufficient to establish and maintain those compartments inside acidic. Current studies have shown that the in situ acidification depends upon the activity of V-ATPase and vacuolar anion conductance; the latter is required for shunting a membrane potential (interior positive) generated by the positively charged proton translocation. Yeast vacuoles possess two distinct Cl- transport systems both participating in the acidification inside the vacuole, a large acidic compartment with digestive and storage functions. These two transport systems have distinct characteristics for their kinetics of Cl- uptake or sensitivity to a stilbene derivative. One shows linear dependence on a Cl- concentration and is inhibited by 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS). The other shows saturable kinetics with an apparent Km for Cl- of approximately 20 mM. Molecular mechanisms of the chemiosmotic coupling in the vacuolar ion transport and acidification inside are discussed in detail.

Analysis of two mutated vacuolar proteins reveals a degradation pathway in the endoplasmic reticulum or a related compartment of yeast

The fate of a mutant form of each of the two yeast vacuolar enzymes proteinase yscA (PrA) and carboxypeptidase yscY (CPY) has been investigated. Both mutant proteins are rapidly degraded after entering the secretory pathway. Mutant PrA is deleted in 37 amino acids spanning the processing site region of the PrA pro-peptide. The mutant enzyme shows no activity towards maturation of itself or other vacuolar hydrolases, a function of wild-type PrA. Mutant CPY carries an Arg instead of a Gly residue in a highly conserved region, two positions distant from the active-site Ser. In contrast to wild-type CPY, the mutant form was quickly degraded by trypsin in vitro, indicating an altered structure. Using antisera specific for alpha-1-->6 and alpha-1-->3 outer-chain mannose linkages, no Golgi-specific carbohydrate modification could be detected on either mutant protein. Subcellular fractionation studies located both mutant enzymes in the endoplasmic reticulum. Degradation kinetics of both proteins show the same characteristics, indicating similar degradation pathways. The degradation process was shown to be independent of a functional sec18 gene product and takes place before Golgi-specific carbohydrate modifications occur. The proteasome, the major proteolytic activity of the cytoplasm, is not involved in this degradation event. All degradation characteristics of the two mutant proteins are consistent with a degradation process within the endoplasmic reticulum ('ER degradation').

Serine carboxypeptidases: a new and versatile family of enzymes

Several structural and biochemical studies in the past year have revealed the potential application of a new family of serine proteinase, the serine carboxypeptidases, in peptide synthesis, carboxy-terminal peptide sequencing and the production of biologically active carboxy-terminal peptide amides. The recent determination of the high-resolution crystal structures for two members of the family, CPDW-II and CPDY, should permit protein engineering to further increase their utility.

Sterol composition of yeast organelle membranes and subcellular distribution of enzymes involved in sterol metabolism

Organelles of the yeast Saccharomyces cerevisiae were isolated and analyzed for sterol composition and the activity of three enzymes involved in sterol metabolism. The plasma membrane and secretory vesicles, the fractions with the highest sterol contents, contain ergosterol as the major sterol. In other subcellular membranes, which exhibit lower sterol contents, intermediates of the sterol biosynthetic pathway were found at higher percentages. Lipid particles contain, in addition to ergosterol, large amounts of zymosterol, fecosterol, and episterol. These sterols are present esterified with long-chain fatty acids in this subcellular compartment, which also harbors practically all of the triacylglycerols present in the cell but very little phospholipids and proteins. Sterol delta 24-methyltransferase, an enzyme that catalyzes one of the late steps in sterol biosynthesis, was localized almost exclusively in lipid particles. Steryl ester formation is a microsomal process, whereas steryl ester hydrolysis occurs in the plasma membrane and in secretory vesicles. The fact that synthesis, storage, and hydrolysis of steryl esters occur in different subcellular compartments gives rise to the view that ergosteryl esters of lipid particles might serve as intermediates for the supply of ergosterol from internal membranes to the plasma membrane.

Biogenesis of the yeast vacuole (lysosome). Proteinase yscB contributes molecularly and kinetically to vacuolar hydrolase-precursor maturation

The vacuolar proteinase yscB (PrB) has been implicated in the final maturation of procarboxypeptidase yscY (pro-CpY) to the mature wild-type form CpYb of 61 kDa. In PrB-deficient mutants, only the proteinase yscA processed form CpYa of 62 kDa is found [Mechler, B., Müller, H. & Wolf, D. H. (1987) EMBO J. 6, 2157-2163]. We report now that, akin to CpY, two forms of mature proteinase yscA (PrA) can be distinguished. In PrB-deficient mutant cells, PrAa, migrating at about 43 kDa in SDS/PAGE, is found, whereas PrAb, found in wild-type cells, had the known molecular mass of 42 kDa. In the PrB-deficient strain, pro-PrA and pro-CpY matured only to the higher-molecular-mass forms, PrAa and CpYa, and the maturation of both precursors was slower than in the isogenic wild-type strain. Pulse-labeling experiments indicated that the mature forms, PrAb or CpYb, are generated directly in the PrB-containing wild-type strain in vivo. In vitro experiments showed that PrB is able to trigger maturation of its 42-kDa pro-PrB precursor to mature PrB in the absence of PrA. Mature PrB and its proteolytic activity, however, shows a higher stability in the presence of mature PrA. The data indicate a molecular and kinetic participation of proteinase yscB in vacuolar hydrolase precursor maturation.

Ubiquitin, a central component of selective cytoplasmic proteolysis, is linked to proteins residing at the locus of non-selective proteolysis, the vacuole

Ubiquitin, an evolutionary highly conserved protein, is known to be involved in selective proteolysis in the cytoplasm. Here we show that ubiquitin-protein conjugates are also found in the yeast vacuole. Mutants defective in the major vacuolar endopeptidases, proteinase yscA and yscB, lead to accumulation of ubiquitin-protein conjugates in this cellular organelle.