Vacuolar protein sorting in fission yeast: cloning, biosynthesis, transport, and processing of carboxypeptidase Y from Schizosaccharomyces pombe

Atg20- and Atg24-family proteins promote organelle autophagy in fission yeast

Autophagy cargos include not only soluble cytosolic materials but also bulky organelles, such as ER and mitochondria. In budding yeast, two proteins that contain the PX domain and the BAR domain, Atg20 and Atg24 (also known as Snx42 and Snx4, respectively) are required for organelle autophagy and contribute to general autophagy in a way that can be masked by compensatory mechanisms. It remains unclear why these proteins are important for organelle autophagy. Here, we show that in a distantly related fungal organism, the fission yeast Schizosaccharomyces pombe, autophagy of ER and mitochondria is induced by nitrogen starvation and is promoted by three Atg20- and Atg24-family proteins - Atg20, Atg24 and SPBC1711.11 (named here as Atg24b). These proteins localize at the pre-autophagosomal structure, or phagophore assembly site (PAS), during starvation. S. pombe Atg24 forms a homo-oligomer and acts redundantly with Atg20 and Atg24b, and the latter two proteins can form a hetero-oligomer. The organelle autophagy defect caused by the loss of these proteins is associated with a reduction of autophagosome size and a decrease in Atg8 accumulation at the PAS. These results provide new insights into the autophagic function of Atg20- and Atg24-family proteins.

Vacuolar proteases from Candida glabrata: Acid aspartic protease PrA, neutral serine protease PrB and serine carboxypeptidase CpY. The nitrogen source influences their level of expression

BACKGROUND

The Saccharomyces cerevisiae vacuole is actively involved in the mechanism of autophagy and is important in homeostasis, degradation, turnover, detoxification and protection under stressful conditions. In contrast, vacuolar proteases have not been fully studied in phylogenetically related Candida glabrata.

AIMS

The present paper is the first report on proteolytic activity in the C. glabrata vacuole.

METHODS

Biochemical studies in C. glabrata have highlighted the presence of different kinds of intracellular proteolytic activity: acid aspartyl proteinase (PrA) acts on substrates such as albumin and denatured acid hemoglobin, neutral serine protease (PrB) on collagen-type hide powder azure, and serine carboxypeptidase (CpY) on N-benzoyl-tyr-pNA.

RESULTS

Our results showed a subcellular fraction with highly specific enzymatic activity for these three proteases, which allowed to confirm its vacuolar location. Expression analyses were performed in the genes CgPEP4 (CgAPR1), CgPRB1 and CgCPY1 (CgPRC), coding for vacuolar aspartic protease A, neutral protease B and carboxypeptidase Y, respectively. The results show a differential regulation of protease expression depending on the nitrogen source.

CONCLUSIONS

The proteases encoded by genes CgPEP4, CgPRB1 and CgCPY1 from C. glabrata could participate in the process of autophagy and survival of this opportunistic pathogen.

Calnexin is essential for survival under nitrogen starvation and stationary phase in Schizosaccharomyces pombe

Cell fate is determined by the balance of conserved molecular mechanisms regulating death (apoptosis) and survival (autophagy). Autophagy is a process by which cells recycle their organelles and macromolecules through degradation within the vacuole in yeast and plants, and lysosome in metazoa. In the yeast Schizosaccharomyces pombe, autophagy is strongly induced under nitrogen starvation and in aging cells. Previously, we demonstrated that calnexin (Cnx1p), a highly conserved transmembrane chaperone of the endoplasmic reticulum (ER), regulates apoptosis under ER stress or inositol starvation. Moreover, we showed that in stationary phase, Cnx1p is cleaved into two moieties, L_Cnx1p and S_Cnx1p. Here, we show that the processing of Cnx1p is regulated by autophagy, induced by nitrogen starvation or cell aging. The cleavage of Cnx1p involves two vacuolar proteases: Isp6, which is essential for autophagy, and its paralogue Psp3. Blocking autophagy through the knockout of autophagy-related genes (atg) results in inhibition of both, the cleavage and the trafficking of Cnx1p from the ER to the vacuole. We demonstrate that Cnx1p is required for cell survival under nitrogen-starvation and in chronological aging cultures. The death of the mini_cnx1 mutant (overlapping S_cnx1p) cells is accompanied by accumulation of high levels of reactive-oxygen species (ROS), a slowdown in endocytosis and severe cell-wall defects. Moreover, mutant cells expressing only S_Cnx1p showed cell wall defects. Co-expressing mutant overlapping the L_Cnx1p and S_Cnx1p cleavage products reverses the death, ROS phenotype and cell wall defect to wild-type levels. As it is involved in both apoptosis and autophagy, Cnx1p could be a nexus for the crosstalk between these pro-death and pro-survival mechanisms. Ours, and observations in mammalian systems, suggest that the multiple roles of calnexin depend on its sub-cellular localization and on its cleavage. The use of S. pombe should assist in further shedding light on the multiple roles of calnexin.

Vsl1p cooperates with Fsv1p for vacuolar protein transport and homotypic fusion in Schizosaccharomyces pombe

Members of the SNARE protein family participate in the docking-fusion step of several intracellular vesicular transport events. Saccharomyces cerevisiae Vam7p was identified as a SNARE protein that acts in vacuolar protein transport and membrane fusion. However, in Schizosaccharomyces pombe, there have been no reports regarding the counterpart of Vam7p. Here, we found that, although the SPCC594.06c gene has low similarity to Vam7p, the product of SPCC594.06c has a PX domain and SNARE motif like Vam7p, and thus we designated the gene Sch. pombe vsl1(+) (Vam7-like protein 1). The vsl1Δ cells showed no obvious defect in vacuolar protein transport. However, cells of the vsl1Δ mutant with a deletion of fsv1(+), which encodes another SNARE protein, displayed extreme defects in vacuolar protein transport and vacuolar morphology. Vsl1p was localized to the vacuolar membrane and prevacuolar compartment, and its PX domain was essential for proper localization. Expression of the fusion protein GFP-Vsl1p was able to suppress ZnCl2 sensitivity and the vacuolar protein sorting defect in the fsv1Δ cells. Moreover, GFP-Vsl1p was mislocalized in a pep12Δ mutant and in cells overexpressing fsv1(+). Importantly, overexpression of Sac. cerevisiae VAM7 could suppress the sensitivity to ZnCl2 of vsl1Δ cells and the vacuolar morphology defect of vsl1Δfsv1Δ cells in Sch. pombe. Taken together, these data suggest that Vsl1p and Fsv1p are required for vacuolar protein transport and membrane fusion, and they function cooperatively with Pep12p in the same membrane-trafficking step.

Mitochondrial ABC Transporter Atm1p Is Required for Protection against Oxidative Stress and Vacuolar Functions inSchizosaccharomyces pombe

A potential correlation between mitochondrial and vacuolar functions is known to exit in yeast. Fission yeast atm1(+), SPAC15A10.01, encodes a putative half-type ABC transporter with an N-terminal mitochondrial-targeting signal. In an attempt to evaluate the possible involvement of mitochondrion in vacuole function, a functional analysis of atm1(+) was performed by gene disruption. Growth of the atm1 mutant was inhibited in the presence of oxidizing agents, and S. cerevisiae Atm1p was found to complement this growth defect. atm1Delta cells exhibited defects in fluid-phase endocytosis and vacuolar fusion under hypotonic stress. GFP-tagged Atm1p was observed to be localized in the mitochondria. These data strongly suggest that fission yeast Atm1p was not only involved in protection against oxidative stress, but also played a role in vacuolar functions.

Global analysis of fission yeast mating genes reveals new autophagy factors

Macroautophagy (autophagy) is crucial for cell survival during starvation and plays important roles in animal development and human diseases. Molecular understanding of autophagy has mainly come from the budding yeast Saccharomyces cerevisiae, and it remains unclear to what extent the mechanisms are the same in other organisms. Here, through screening the mating phenotype of a genome-wide deletion collection of the fission yeast Schizosaccharomyces pombe, we obtained a comprehensive catalog of autophagy genes in this highly tractable organism, including genes encoding three heretofore unidentified core Atg proteins, Atg10, Atg14, and Atg16, and two novel factors, Ctl1 and Fsc1. We systematically examined the subcellular localization of fission yeast autophagy factors for the first time and characterized the phenotypes of their mutants, thereby uncovering both similarities and differences between the two yeasts. Unlike budding yeast, all three Atg18/WIPI proteins in fission yeast are essential for autophagy, and we found that they play different roles, with Atg18a uniquely required for the targeting of the Atg12–Atg5·Atg16 complex. Our investigation of the two novel factors revealed unforeseen autophagy mechanisms. The choline transporter-like protein Ctl1 interacts with Atg9 and is required for autophagosome formation. The fasciclin domain protein Fsc1 localizes to the vacuole membrane and is required for autophagosome-vacuole fusion but not other vacuolar fusion events. Our study sheds new light on the evolutionary diversity of the autophagy machinery and establishes the fission yeast as a useful model for dissecting the mechanisms of autophagy.

Autophagy is a eukaryotic cellular process that transports cytoplasmic contents into lysosomes/vacuoles for degradation. It has been linked to multiple human diseases, including cancer and neurodegenerative disorders. The molecular machinery of autophagy was first identified and has been best characterized in the budding yeast Saccharomyces cerevisiae, but little is known about the autophagy machinery in another important unicellular model organism, the fission yeast Schizosaccharomyces pombe. In this study, we performed an unbiased and comprehensive screening of the fission yeast autophagy genes by profiling the mating phenotypes of nearly 3000 deletion strains. Following up on the screening results, we systematically characterized both previously known and newly identified fission yeast autophagy factors by examining their localization and the phenotype of their mutants. Our analysis increased the number of experimentally defined fission yeast autophagy factors from 14 to 23, including two novel factors that act in ways different from all previously known autophagy proteins. Together, our data reveal unexpected evolutionary divergence of autophagy mechanisms and establish a new model system for unraveling the molecular details of the autophagy process.

N-glycans are not required for the efficient degradation of the mutant Saccharomyces cerevisiae CPY* in Schizosaccharomyces pombe

In eukaryotic cells, aberrant proteins generated in the endoplasmic reticulum (ER) are degraded by the ER-associated degradation (ERAD) pathway. Here, we report on the ERAD pathway of the fission yeast Schizosaccharomyces pombe. We constructed and expressed Saccharomyces cerevisiae wild-type CPY (ScCPY) and CPY-G255R mutant (ScCPY*) in S. pombe. While ScCPY was glycosylated and efficiently transported to the vacuoles in S. pombe, ScCPY* was retained in the ER and was not processed to the matured form in these cells. Cycloheximide chase experiments revealed that ScCPY* was rapidly degraded in S. pombe, and its degradation depended on Hrd1p and Ubc7p homologs. We also found that Mnl1p and Yos9p, proteins that are essential for ERAD in S. cerevisiae, were not required for ScCPY* degradation in S. pombe. Moreover, the null-glycosylation mutant of ScCPY, CPY*0000, was rapidly degraded by the ERAD pathway. These results suggested that N-linked oligosaccharides are not important for the recognition of luminal proteins for ERAD in S. pombe cells.

Schizosaccharomyces pombe Pep12p is required for vacuolar protein transport and vacuolar homotypic fusion

In eukaryotic cells, SNARE proteins are essential for intracellular vesicle trafficking. Several SNARE proteins are required for vacuolar protein transport and vacuolar biogenesis in Saccharomyces cerevisiae. Previously we demonstrated that one of the fission yeast SNARE proteins, Pep12p, is not required for vacuolar fusion process in Schizosaccharomyces pombe. We have re-examined the function of S. pombe Pep12p using the newly created pep12(+) deletion strain. Deletion of the fission yeast pep12(+) gene results in pleiotropic phenotypes consistent with the absence of normal vacuoles, including missorting of vacuolar carboxypeptidase Y-and various ion- and drug-sensitivities. GFP-Pep12 fusion protein is mostly localized at the vacuolar membrane and the prevacuolar compartment. The S. pombe pep12Δ mutation phenocopies that of vps33Δ, suggesting that both Pep12p and Vps33p act at the same membrane fusion step in S. pombe, and both mutations cause vacuolar deficiency.

Enhanced production and secretion of heterologous proteins by the filamentous fungus Aspergillus oryzae via disruption of vacuolar protein sorting receptor gene Aovps10

Filamentous fungi have received attention as hosts for heterologous protein production because of their high secretion capability and eukaryotic posttranslational modifications. However, despite these positive attributes, a bottleneck in posttranscriptional processing limits protein yields. The vacuolar protein sorting gene VPS10 encodes a sorting receptor for the recognition and delivery of several yeast vacuolar proteins. Although it can also target recombinant and aberrant proteins for vacuolar degradation, there is limited knowledge of the effect of its disruption on heterologous protein production. In this study, cDNA encoding AoVps10 from the filamentous fungus Aspergillus oryzae was cloned and sequenced. Microscopic observation of the transformant expressing AoVps10 fused with enhanced green fluorescent protein showed that the fusion protein localized at the Golgi and prevacuolar compartments. Moreover, disruption of the Aovps10 gene resulted in missorting and secretion of vacuolar carboxypeptidase AoCpyA into the medium, indicating that AoVps10 is required for sorting of vacuolar proteins to vacuoles. To investigate the extracellular production levels of heterologous proteins, DeltaAovps10 mutants expressing either bovine chymosin (CHY) or human lysozyme (HLY) were constructed. Interestingly, the DeltaAovps10 mutation increased the maximum extracellular production levels of CHY and HLY by 3- and 2.2-fold, respectively. Western blot analysis of extracellular heterologous proteins also demonstrated an improvement in productivity. These results suggest that AoVps10 plays a role in the regulation of heterologous protein secretion in A. oryzae and may be involved in the vacuolar protein degradation through the Golgi apparatus.

A versatile selection system for folding competent proteins using genetic complementation in a eukaryotic host

Recombinant expression of native or modified eukaryotic proteins is pivotal for structural and functional studies and for industrial and pharmaceutical production of proteins. However, it is often impeded by the lack of proper folding. Here, we present a stringent and broadly applicable eukaryotic in vivo selection system for folded proteins. It is based on genetic complementation of the Schizosaccharomyces pombe growth marker gene invertase fused C-terminally to a protein library. The fusion proteins are directed to the secretion system, utilizing the ability of the eukaryotic protein quality-control systems to retain misfolded proteins in the ER and redirect them for cytosolic degradation, thereby only allowing folded proteins to reach the cell surface. Accordingly, the folding potential of the tested protein determines the ability of autotrophic colony growth. This system was successfully demonstrated using a complex insertion mutant library of TNF-alpha, from which different folding competent mutant proteins were uncovered.

Autophagy in the fission yeast Schizosaccharomyces pombe

Autophagy is a non-selective degradation process in eukaryotic cells. The genome sequence of the fission yeast Schizosaccharomyces pombe has revealed that many of the genes required for autophagy are common between the fission yeast and budding yeast, suggesting that the basic machinery of autophagy is conserved between these species. Autophagy in fission yeast is specifically induced by nitrogen starvation based on monitoring a GFP-Atg8p marker. Upon nitrogen starvation, fission yeast cells exit the vegetative cell cycle and initiate sexual differentiation to produce spores. Most of the nitrogen used for de novo protein synthesis during sporulation derives from the autophagic protein degradation system. This review focuses on the recent advances in the role of autophagy in fission yeast.

The dynamin-related protein Vps1 regulates vacuole fission, fusion and tubulation in the fission yeast, Schizosaccharomyces pombe

Fission yeast cells lacking the dynamin-related protein (DRP) Vps1 had smaller vacuoles with reduced capacity for both fusion and fission in response to hypotonic and hypertonic conditions respectively. vps1Delta cells showed normal vacuolar protein sorting, actin organisation and endocytosis. Over-expression of vps1 transformed vacuoles from spherical to tubular. Tubule formation was enhanced in fission conditions and required the Rab protein Ypt7. Vacuole tubulation by Vps1 was more extensive in the absence of a second DRP, Dnm1. Both dnm1Delta and the double mutant vps1Delta dnm1Delta showed vacuole fission defects similar to that of vps1Delta. Over-expression of vps1 in dnm1Delta, or of dnm1 in vps1Delta failed to rescue this phenotype. Over-expression of dnm1 in wild-type cells, on the other hand, induced vacuole fission. Our results are consistent with a model of vacuole fission in which Vps1 creates a tubule of an appropriate diameter for subsequent scission by Dnm1.

S. pombe btn1, the orthologue of the Batten disease gene CLN3, is required for vacuole protein sorting of Cpy1p and Golgi exit of Vps10p

Batten disease is characterised by lysosomal dysfunction. The most common type of the disease is caused by mutations in the membrane protein CLN3, whose function is unknown. We show that the fission yeast orthologue Btn1p, previously implicated in vacuole function, is required for correct sorting of the vacuole hydrolase carboxypeptidase Y (Cpy1p). This is, in part, due to a defect in trafficking of Vps10p, the sorting receptor for Cpy1p, from the Golgi to the trans-Golgi network in btn1Delta cells. Our data also implicate btn1 in other Vps10-independent Cpy1-sorting pathways. Furthermore, btn1 affects the number, intracellular location and structure of Golgi compartments. We show that the prevacuole location of Btn1p is at the Golgi, because Btn1p colocalises predominantly with the Golgi marker Gms1p in compartments that are sensitive to Brefeldin A. Btn1p function might be linked to that of Vps34p, a phosphatidylinositol 3-kinase, because Btn1p acts as a multicopy suppressor of the severe Cpy1p vacuole protein-sorting defect of vps34Delta cells. Together, these results indicate an important role for Btn1p in the Golgi complex, which affects Golgi homeostasis and vacuole protein sorting. We propose a similar role for CLN3 in mammalian cells.

Multiple functions of ergosterol in the fission yeast Schizosaccharomyces pombe

Sterols are a major class of membrane lipids in eukaryotes. In Schizosaccharomyces pombe, sterol 24-C-methyltransferase (Erg6p), C-8 sterol isomerase (Erg2p), C-5 sterol desaturase (Erg31p, Erg32p), C-22 sterol desaturase (Erg5p) and C-24 (28) sterol reductase (Sts1p/Erg4p) have been predicted, but not yet determined, to catalyse a sequence of reactions from zymosterol to ergosterol. Disruption mutants of these genes were unable to synthesize ergosterol, and most were tolerant to the polyene drugs amphotericin B and nystatin. Disruption of erg31(+) or erg32(+) did not cause ergosterol deficiency or tolerance to polyene drugs, indicating that the two C-5 sterol desaturases have overlapping functions. GFP-tagged DRM (detergent-resistant membrane)-associated protein Pma1p localized to the plasma membrane in ergDelta mutants. DRM fractionation revealed that the association between Pma1-GFP and DRM was weakened in erg6Delta but not in other erg mutants. Several GFP-tagged plasma membrane proteins were tested, and an amino acid permease homologue, SPBC359.03c, was found to mislocalize to intracellular punctate structures in the ergDelta mutants. These results indicate that these proteins are responsible for ergosterol biosynthesis in fission yeast, similar to the situation in Saccharomyces cerevisiae. Furthermore, in fission yeast, ergosterol is important for plasma membrane structure and function and for localization of plasma membrane proteins.

PXA domain-containing protein Pxa1 is required for normal vacuole function and morphology in Schizosaccharomyces pombe

PhoX homology (PX) domain-containing proteins play critical roles in vesicular trafficking, protein sorting, and lipid modification in eukaryotic cells. Several proteins with PX domains contain an associated domain termed PXA (PX-associated). Although PXA domain-containing proteins are required for some important cellular processes, the function of the PXA domain is unknown. We identified three PXA domain-containing proteins in Schizosaccharomyces pombe. S. pombe Pxa1p (SPAC5D6.07c) contained only the PXA domain, not the PX domain. To elucidate the role of the PXA domain in eukaryotic cells, we constructed and characterized a disruption mutant, pxa1. The pxa1 disruptant contained enlarged vacuoles and exhibited mislocalization of vacuolar carboxypeptidase Y (CPY). The conversion rate from pro- to mature-CPY was greatly impaired in pxa1 cells, and fluorescence microscopy indicated that a sorting receptor for CPY, Vps10p, mislocalized to the vacuolar membrane. The mutants were also deficient in vacuolar sorting of a multivesicular body (MVB) marker, a ubiquitin-GFP-carboxypeptidase S (Ub-GFP-CPS) fusion protein. Taken together, these results indicate that Pxa1 protein is required for normal vacuole function and morphology in S. pombe.

Biochemical characterization and structural prediction of a novel cytosolic leucyl aminopeptidase of the M17 family from Schizosaccharomyces pombe

A new leucyl aminopeptidase activity has been identified in the fission yeast Schizosaccharomyces pombe. The enzyme, which has been purified and named leucyl aminopeptidase yspII (LAP yspII), had a molecular mass of 320 and 54 kDa by gel filtration and SDS/PAGE, respectively, suggesting a homohexameric structure. The enzyme cleaved synthetic aminoacyl-4-nitroanilides at an optimum of pH 8.5, and preferred leucine and methionine as N-terminal amino acids. A clear dependence on Mn2+ concentration for activity was found, and an apparent association constant of 0.33 mM was calculated for the metal ion. Bestatin behaved as a competitive inhibitor of LAP yspII (K(i) = 0.14 microM), while chelating agents such as chloroquine, EDTA and 1,10-phenanthroline also reduced enzyme activity. A MALDI-MS analysis, followed by sequencing of two of the resulting peptides, showed that LAP yspII undoubtedly corresponds to the putative aminopeptidase C13A11.05 identified in the S. pombe genome project. The protein exhibited nearly 40% sequence identity to fungal and mammalian aminopeptidases belonging to the M17 family of metallopeptidases. Catalytic residues (Lys292 and Arg366), as well as those involved in coordination with the cocatalytic metal ions (Lys280, Asp285, Asp303, Asp362 and Glu364) and those forming the hydrophobic pocket for substrate binding (Met300, Asn360, Ala363, Thr390, Leu391, Ala483 and Met486), were perfectly conserved among all known aminopeptidases. The S. pombe enzyme is predicted to be formed two clearly distinguished domains with a well conserved C-terminal catalytic domain showing a characteristic topology of eight beta-sheets surrounded by alpha-helical segments in the form of a saddle.

Essential roles of class E Vps proteins for sorting into multivesicular bodies in Schizosaccharomyces pombe

The multivesicular body (MVB) sorting pathway is required for a number of biological processes, including downregulation of cell-surface proteins and protein sorting into the vacuolar lumen. The function of this pathway requires endosomal sorting complexes required for transport (ESCRT) composed of class E vacuolar protein sorting (Vps) proteins in Saccharomyces cerevisiae, many of which are conserved in Schizosaccharomyces pombe. Of these, sst4/vps27 (homologous to VPS27) and sst6 (similar to VPS23) have been identified as suppressors of sterility in ste12Delta (sst), although their functions have not been uncovered to date. In this report, these two sst genes are shown to be required for vacuolar sorting of carboxypeptidase Y (CPY) and an MVB marker, the ubiquitin-GFP-carboxypeptidase S (Ub-GFP-CPS) fusion protein, despite the lack of the ubiquitin E2 variant domain in Sst6p. Disruption mutants of a variety of other class E vps homologues also had defects in sorting of CPY and Ub-GFP-CPS. Sch. pombe has a mammalian AMSH homologue, sst2. Phenotypic analyses suggested that Sst2p is a class E Vps protein. Taken together, these results suggest that sorting into multivesicular bodies is dependent on class E Vps proteins, including Sst2p, in Sch. pombe.

Valproic acid affects membrane trafficking and cell-wall integrity in fission yeast

Valproic acid (VPA) is widely used to treat epilepsy and manic-depressive illness. Although VPA has been reported to exert a variety of biochemical effects, the exact mechanisms underlying its therapeutic effects remain elusive. To gain further insights into the molecular mechanisms of VPA action, a genetic screen for fission yeast mutants that show hypersensitivity to VPA was performed. One of the genes that we identified was vps45+, which encodes a member of the Sec1/Munc18 family that is implicated in membrane trafficking. Notably, several mutations affecting membrane trafficking also resulted in hypersensitivity to VPA. These include ypt3+ and ryh1+, both encoding a Rab family protein, and apm1+, encoding the mu1 subunit of the adaptor protein complex AP-1. More importantly, VPA caused vacuolar fragmentation and inhibited the glycosylation and the secretion of acid phosphatase in wild-type cells, suggesting that VPA affects membrane trafficking. Interestingly, the cell-wall-damaging agents such as micafungin or the inhibition of calcineurin dramatically enhanced the sensitivity of wild-type cells to VPA. Consistently, VPA treatment of wild-type cells enhanced their sensitivity to the cell-wall-digesting enzymes. Altogether, our results suggest that VPA affects membrane trafficking, which leads to the enhanced sensitivity to cell-wall damage in fission yeast.

Synthesis and function of membrane phosphoinositides in budding yeast, Saccharomyces cerevisiae

It is now well appreciated that derivatives of phosphatidylinositol (PtdIns) are key regulators of many cellular processes in eukaryotes. Of particular interest are phosphoinositides (mono- and polyphosphorylated adducts to the inositol ring in PtdIns), which are located at the cytoplasmic face of cellular membranes. Phosphoinositides serve both a structural and a signaling role via their recruitment of proteins that contain phosphoinositide-binding domains. Phosphoinositides also have a role as precursors of several types of second messengers for certain intracellular signaling pathways. Realization of the importance of phosphoinositides has brought increased attention to characterization of the enzymes that regulate their synthesis, interconversion, and turnover. Here we review the current state of our knowledge about the properties and regulation of the ATP-dependent lipid kinases responsible for synthesis of phosphoinositides and also the additional temporal and spatial controls exerted by the phosphatases and a phospholipase that act on phosphoinositides in yeast.

Loss of a GPI-anchored membrane protein Aah3p causes a defect in vacuolar protein sorting in Schizosaccharomyces pombe

Schizosaccharomyces pombe has four alpha-amylase homologs (Aah1p-Aah4p) with a glycosylphosphatidylinositol (GPI) modification site at the C-terminal end. Disruption mutants of aah genes were tested for mislocalization of vacuolar carboxypeptidase Y (CPY), and aah3Delta was found to secrete CPY. The conversion rate from pro- to mature CPY was greatly impaired in aah3Delta, and fluorescence microscopy inidicated that a sorting receptor for CPY, Vps10p, mislocalized to the vacuolar membrane. These results indicate that aah3Delta had a defect in the retrograde transport of Vps10p, and that Aah3p is the first S. pombe specific protein required for vacuolar protein sorting.

Vacuolar protein sorting receptor in Schizosaccharomyces pombe

The mechanism by which soluble proteins, such as carboxypeptidase Y, reach the vacuole inSaccharomyces cerevisiaeis very similar to the mechanism of lysosomal protein sorting in mammalian cells. Vps10p is a receptor for transport of soluble vacuolar proteins inS. cerevisiae.vps10+, a gene encoding a homologue ofS. cerevisiae PEP1/VPS10, has been identified and deleted from the fission yeastSchizosaccharomyces pombe. Deletion of thevps10+gene resulted in missorting and secretion ofSch. pombevacuolar carboxypeptidase Cpy1p, indicating that it is required for targeting Cpy1p to the vacuole.Sch. pombeVps10p (SpVps10p) is a type I transmembrane protein and its C-terminal cytoplasmic tail domain is essential for Cpy1p transport to the vacuole. Cells expressing green fluorescent protein-tagged SpVps10p produced a punctate pattern of fluorescence, indicating that SpVps10p was largely localized in the Golgi compartment. In addition,Sch. pombe vps26+,vps29+andvps35+, encoding homologues of theS. cerevisiaeretromer componentsVPS26,VPS29andVPS35, were identified and deleted. Fluorescence microscopy demonstrated that SpVps10p mislocalized to the vacuolar membrane in these mutants. These results indicate that thevps26+,vps29+andvps35+gene products are required for retrograde transport of SpVps10p from the prevacuolar compartment back to the Golgi inSch. pombecells.

Alternative protein secretion: The Mam1 ABC transporter supports secretion of M-factor linked GFP in fission yeast

To examine whether the fission yeast Mam1 ABC transporter can be used for secretion of heterologous proteins, thereby bypassing the classical secretion pathway, we have analyzed chimeric forms of the M-factor precursor. It was demonstrated that GFP can be exported when fused to both the amino-terminal prosequence from mfm1 and a CaaX motif. This secretion was dependent on the Mam1 transporter and not the classical secretion pathway. The secretion efficiency of GFP, however, was relatively low and most of the reporter protein was trapped in the vacuolar membranes. Our findings suggest that the Mam1 ABC protein is a promiscuous peptide transporter that can accommodate globular proteins of a relatively large size. Furthermore, our results help in defining the sequences required for processing and secretion of natural M-factor.

Comparison of different signal peptides for secretion of heterologous proteins in fission yeast

In the fission yeast Schizosaccharomyces pombe, there are relatively few signal peptides available and most reports of their activity have not been comparative. Using sequence information from the S. pombe genome database we have identified three putative signal peptides, designated Cpy, Amy and Dpp, and compared their ability to support secretion of green fluorescent protein (GFP). In the comparison we also included the two well-described secretion signals derived from the precursors of, respectively, the Saccharomyces cerevisiae alpha-factor and the S. pombe P-factor. The capability of the tested signal peptides to direct secretion of GFP varied greatly. The alpha-factor signal did not confer secretion to GFP and all the produced GFP was trapped intracellular. In contrast, the Cpy signal peptide supported efficient secretion of GFP with yields approximating 10 mg/L. We also found that the use of an attenuated version of the S. cerevisiae URA3 marker substantially increases vector copy number and expression yield in fission yeast.

Development of a genetic transformation system using new selectable markers for fission yeast Schizosaccharomyces pombe

We describe the development of a new transformation system, using multiple auxotrophic marker genes, for the fission yeast Schizosaccharomyces pombe. We developed three new auxotrophic marker genes (arg12(+), tyr1(+) and ade7(+)) and generated a new host strain, YF043, by Cre-loxP-mediated gene disruption. YF043 possessed six mutated biosynthetic genes (leu1-32, ura4-M190T, arg12::loxP, tyr1::loxP, ade7::loxP and his2::loxP). The combination of this host strain and the new selectable markers can be used for gene disruption using the same preexisting transformation systems. In addition, Sz. pombe vectors were constructed, containing selectable marker genes that complement the auxotrophies of YF043. These new vectors are available for gene disruption and heterologous protein expression in strain YF043. The new Sz. pombe host strain will be a useful tool for molecular genetic studies of Sz. pombe where multiple recombinant modifications or multiple mutations are needed.

The serine carboxypeptidase like gene family of rice (Oryza sativa L. ssp. japonica)

Serine carboxypeptidases (SCPs) comprise a large family of protein hydrolyzing enzymes and have roles ranging from protein turnover and C-terminal processing to wound responses and xenobiotic metabolism. The proteins can be classified into three groups, namely carboxypeptidase I, II and III, based on their coding protein sequences and the fact that each family is characterized by a central catalytic domain of unique topology designated as the "alpha/beta hydrolase fold". The available SCP protein sequences have been utilized as datasets to build a HMM (hidden Markov model) profile, which is used to search the rice (Oryza sativa L. ssp. japonica) proteome. A total of 71 SCP and serine carboxypeptidase-like (SCPL) protein-coding genes exist in rice. The intron-exon structure, chromosome localization, expression and characteristics of encoded protein sequences of the 71 putative genes are reviewed.

Sorting nexin homologues are targets of phosphatidylinositol 3-phosphate in sporulation of Schizosaccharomyces pombe

Schizosaccharomyces pombe defective in phosphatidylinositol (PtdIns) 3-kinase shows various defects in forespore membrane formation, including onset, growth orientation, and closure. Downstream factors of PtdIns 3-kinase in this system were explored. Among various phox homology (PX) domain-containing proteins, Vps5p and Vps17p, homologues of sorting nexins, were found to be required for efficient sporulation. Cells defective in these proteins showed a disordered growth orientation of the forespore membrane, as is the case with Deltapik3 cells. Vps5p and Vps17p with mutations in the PX domains failed to suppress the defects of their relevant disruptants. Vps5p and Vps17p migrated toward the the forespore membrane in a pik3+-dependent manner, suggesting that these proteins may interact with PtdIns(3)P. Electron-microscopic analysis revealed that the forespore membrane fails to engulf the nucleus in some of these cells, accumulating vesicle-like bodies similar to those seen in Deltaspo3 cells. These results suggest that Vps5p and Vps17p are the targets of PtdIns(3)P in vesicle transport required for onset of the forespore membrane formation.

Pmr1, a P-type ATPase, and Pdt1, an Nramp homologue, cooperatively regulate cell morphogenesis in fission yeast: the importance of Mn2+ homeostasis

Schizosaccharomyces pombe pmr1+ gene is homologous to Saccharomyces cerevisiae PMR1 gene, which encodes the P-type Ca2+/Mn2+-ATPase. Addition of Mn2+, as well as Ca2+, to the medium induced pmr1+ gene expression in a calcineurin-dependent manner. The pmr1 knockout (Deltapmr1) cells exhibited hypersensitivity to EGTA. A screen for high gene dosage-suppressors of the EGTA-hypersensitive phenotype of Deltapmr1 led to the identification of pdt1+ gene, which encodes an Nramp-related metal transporter. The Deltapmr1 cells showed round cell morphology. Although Deltapdt1 cells appeared normal in the regular medium, it showed round cell morphology similar to that of the Deltapmr1 cells when Mn2+ was removed from the medium. The removal of Mn2+ also exacerbated the round morphology of the Deltapmr1 cells. The Deltapmr1Deltapdt1 double mutants grew very slowly and showed extremely aberrant cell morphology with round, enlarged and depolarized shape. The addition of Mn2+, but not Ca2+, to the medium completely suppressed the morphological defects, while both Mn2+ and Ca2+ markedly improved the slow growth of the double mutants. These results suggest that Pmr1 and Pdt1 cooperatively regulate cell morphogenesis through the control of Mn2+ homeostasis, and that calcineurin functions as a Mn2+ sensor as well as a Mn2+ homeostasis regulator.

Characterization of Schizosaccharomyces pombe mutants defective in vacuolar acidification and protein sorting

The vacuolar H+-ATPases (V-ATPases) are ATP-dependent proton pumps responsible for acidification of intracellular compartments in eukaryotic cells. To investigate the functional roles of the V-ATPase in Schizosaccharomyces pombe, the gene vma1 encoding subunit A or vma3 encoding subunit c was disrupted. Both deletion mutants lost the capacity for vacuolar acidification in vivo, and showed sensitivity to neutral pH or high concentrations of divalent cations including Ca2+. The delivery of FM4-64 to the vacuolar membrane and accumulation of Lucifer Yellow CH were strongly inhibited in the vma1 and vma3 mutants. Moreover, deletion of the S. pombe vma1+ or vma3+ gene resulted in pleiotropic phenotypes consistent with lack of vacuolar acidification, including the missorting of vacuolar carboxypeptidase Y, abnormal vacuole morphology, and mating defects. These findings suggest that V-ATPase is essential for endocytosis, ion and pH homeostasis, and for intracellular targeting of vacuolar proteins and vacuolar biogenesis in S. pombe.

Identification of a SNARE protein required for vacuolar protein transport in Schizosaccharomyces pombe

Intracellular vesicle trafficking is mediated by a set of SNARE proteins in eukaryotic cells. Several SNARE proteins are required for vacuolar protein transport and vacuolar biogenesis in Saccharomyces cerevisiae. A search of the Schizosaccharomyces pombe genome database revealed a total of 17 SNARE-related genes. Although no homologs of Vam3p, Nyv1p, and Vam7p have been found in S. pombe, we identified one SNARE-like protein that is homologous to S. cerevisiae Pep12p. However, the disruptants transport vacuolar hydrolase CPY (SpCPY) to the vacuole normally, suggesting that the Pep12 homolog is not required for vacuolar protein transport in S. pombe cells. To identify the SNARE protein(s) involved in Golgi-to-vacuole protein transport, we have deleted four SNARE homolog genes in S. pombe. SpCPY was significantly missorted to the cell surface on deletion of one of the SNARE proteins, Fsv1p (SPAC6F12.03c), with no apparent S. cerevisiae ortholog. In addition, sporulation, endocytosis, and in vivo vacuolar fusion appear to be normal in fsv1Delta cells. These results showed that Fsv1p is mainly involved in vesicle-mediated protein transport between the Golgi and vacuole in S. pombe cells.

Characterization of vps33+, a gene required for vacuolar biogenesis and protein sorting in Schizosaccharomyces pombe

From the fission yeast Schizosaccharomyces pombe we have identified and deleted vps33, a gene encoding a homologue of VPS33, which is required for vacuolar biogenesis in S. cerevisiae cells. When the vps33(+) gene is disrupted, Sz. pombe strains are temperature-sensitive for growth and contain numerous small vesicular structures stained with FM4-64 in the cells. Deletion of the Sz. pombe vps33(+) gene results in pleiotropic phenotypes consistent with the absence of normal vacuoles, including missorting of vacuolar carboxypeptidase Y, various ion- and drug-sensitivities, and sporulation defects. These results are consistent with Vps33p being necessary for the morphogenesis of vacuoles and subsequent expression of vacuolar functions in Sz. pombe cells.

Heterologous expression and characterization of Schizosaccharomyces pombe vacuolar carboxypeptidase Y in Saccharomyces cerevisiae

To investigate the intracellular transport mechanism of the vacuolar carboxypeptidase of Schizosaccharomyces pombe (SpCPY), SpCPY was expressed in Saccharomyces cerevisiae and its biosynthesis and sorting were examined. When Sac. cerevisiae prc1Delta, devoid of intrinsic (Sc) CPY activity, was transformed with a plasmid carrying the Sch. pombe cpy1(+) gene, CPY activity was restored. Pulse-chase experiments revealed that SpCPY is initially synthesized in a pro-precursor form and then converted to a heterodimer, the mature form, in Sac. cerevisiae cells. SpCPY was not processed into intermediate or mature forms in pep4 mutant cells, indicating that SpCPY was proteolytically cleaved in a PEP4-dependent manner in Sac. cerevisiae. Several vps mutants, which are defective in vacuolar protein-sorting, exhibited a defect in the maturation of SpCPY. Moreover, the maturation of SpCPY was severely inhibited in a vps10 strain, although the pro- segment of SpCPY does not contain a QRPL-like sequence, which is the putative targeting signal of ScCPY. When SpCPY was expressed in a wild-type strain, more than 90% of ScCPY was normally sorted to the vacuole, indicating that SpCPY does not compete with ScCPY for vacuolar sorting. In contrast, expression of SpCPY resulted in a missorting of a ScCPY-invertase fusion protein to the cell surface. These results suggested that there are two different binding sites for SpCPY and ScCPY on Vps10p and that the binding of SpCPY to Vps10p interferes with the binding of a ScCPY-invertase fusion protein.

Application of a Saccharomyces cerevisiae model to study requirements for trafficking of Yersinia pestis YopM in eucaryotic cells

YopM is a leucine-rich repeat (LRR) virulence protein that is delivered into host cells when any of the three human-pathogenic species of Yersinia binds to mammalian cells. It exhibits heterogeneity of size and sequence among the yersiniae, but the functional consequences of this variability are not yet known. Yersinia pestis YopM was previously shown to accumulate in the nuclei of infected HeLa cells by a mechanism that requires vesicular trafficking. In this study, we characterized the trafficking of Y. pestis YopM in a Saccharomyces cerevisiae model previously found to support nuclear localization of YopM from an enteropathogenic Yersinia strain (C. F. Lesser and S. I. Miller, EMBO J. 20:1840-1849, 2001). Y. pestis YopM was N-terminally fused to the yeast enhanced green fluorescent protein (yEGFP) and inducibly expressed in the cytoplasm. yEGFP-YopM localized to the yeast nucleus, showing that this property is conserved for YopMs so far tested and that infection and the presence of other Yops are not required for its trafficking. When expressed in S. cerevisiae that is temperature sensitive for vesicular transport, YopM failed to accumulate in the nucleus at the nonpermissive temperature but did accumulate when the permissive temperature was restored. This shows that vesicular trafficking also is required in yeast for normal localization of YopM. YopM consists of a 71-residue leader sequence, 15 LRRs, and a 32-residue tail. Deletion analysis revealed that the leader sequence or tail is alone insufficient to direct YopM to the nucleus, showing that the LRR structure is required. Both the N-terminal and C-terminal halves of YopM localized to the nucleus, indicating the possible presence of two nuclear localization signals (NLSs) in YopM or domains in YopM where an NLS-containing protein might bind; this fits with the presence of two highly conserved regions among Yersinia YopMs. yEGFP-YopM lacking LRRs 4 to 7 or 7 to 10 accumulated in the nucleus in yeast, and YopM lacking these LRRs concentrated normally in the HeLa cell nucleus after delivery by Yersinia infection, showing that these LRRs are not essential for YopM trafficking in eucaryotic cells. However, because Y. pestis carrying either of these YopMs is strongly compromised in virulence in mice, these findings revealed that LRRs 4 to 10 map a region of YopM or support a conformation of YopM that is necessary for a pathogenic effect.

Vesicle-mediated Protein Transport Pathways to the Vacuole in Schizosaccharomyces pombe

The vacuole of Saccharomyces cerevisiae plays essential roles not only for osmoregulation and ion homeostasis but also down-regulation (degradation) of cell surface proteins and protein and organellar turnover. Genetic selections and genome-wide screens in S. cerevisiae have resulted in the identification of a large number of genes required for delivery of proteins to the vacuole. Although the complete genome sequence of the fission yeast Schizosaccharomyces pombe has been reported, there have been few reports on the proteins required for vacuolar protein transport and vacuolar biogenesis in S. pombe. Recent progress in the S. pombe genome project of has revealed that most of the genes required for vacuolar biogenesis and protein transport are conserved between S. pombe and S. cerevisiae. This suggests that the basic machinery of vesicle-mediated protein delivery to the vacuole is conserved between the two yeasts. Identification and characterization of the fission yeast counterparts of the budding yeast Vps and Vps-related proteins have facilitated our understanding of protein transport pathways to the vacuole in S. pombe. This review focuses on the recent advances in vesicle-mediated protein transport to the vacuole in S. pombe.

Role of the Rab GTP-binding protein Ypt3 in the fission yeast exocytic pathway and its connection to calcineurin function

A genetic screen for mutations synthetically lethal with fission yeast calcineurin deletion led to the identification of Ypt3, a homolog of mammalian Rab11 GTP-binding protein. A mutant with the temperature-sensitive ypt3-i5 allele showed pleiotropic phenotypes such as defects in cytokinesis, cell wall integrity, and vacuole fusion, and these were exacerbated by FK506-treatment, a specific inhibitor of calcineurin. Green fluorescent protein (GFP)-tagged Ypt3 showed cytoplasmic staining that was concentrated at growth sites, and this polarized localization required the actin cytoskeleton. It was also detected as a punctate staining in an actin-independent manner. Electron microscopy revealed that ypt3-i5 mutants accumulated aberrant Golgi-like structures and putative post-Golgi vesicles, which increased remarkably at the restrictive temperature. Consistently, the secretion of GFP fused with the pho1(+) leader peptide (SPL-GFP) was abolished at the restrictive temperature in ypt3-i5 mutants. FK506-treatment accentuated the accumulation of aberrant Golgi-like structures and caused a significant decrease of SPL-GFP secretion at a permissive temperature. These results suggest that Ypt3 is required at multiple steps of the exocytic pathway and its mutation affects diverse cellular processes and that calcineurin is functionally connected to these cellular processes.

Phosphatidylinositol 3-phosphate 5-kinase is required for the cellular response to nutritional starvation and mating pheromone signals in Schizosaccharomyces pombe

BACKGROUND

Phosphatidylinositol (3,5) bisphosphate, which is converted from phosphatidylinositol 3-phosphate by phosphatidylinositol 3-phosphate 5-kinase, is implicated in vacuolar functions and the sorting of cell surface proteins within endosomes in the endocytic pathway of budding yeast. A homologous protein, SpFab1p, has been found in the fission yeast Schizosaccharomyces pombe, but its role is not known.

RESULTS

Here we report that SpFab1p is encoded by ste12+ known as a fertility gene in S. pombe. The ste12 mutant grew normally under stress-free conditions, but was highly vacuolated and swelled at high temperatures and under starvation conditions. In nitrogen-free medium, ste12 cells were arrested in G1 phase, but partially defective in the expression of genes responsible for mating and meiosis. The ste12 mutant was defective both in the production of, and in the response to, mating pheromones. The amount of the pheromone receptor protein Map3p, was substantially decreased in ste12 cells. Map3p was transported to the cell surface, then internalized and eventually transported to the vacuolar lumen, even in the ste12 mutant.

CONCLUSION

The results indicate that phosphatidylinositol(3,5)bisphosphate is essential for cellular responses to various stresses and for the mating pheromone signalling under starvation conditions.

Characterization of a Schizosaccharomyces pombe mutant deficient in UDP-galactose transport activity

In fission yeast, Schizosaccharomyces pombe, the carbohydrate components of the cell wall consist of galactomannan, unlike in Saccharomyces cerevisiae. We previously found that the disruption of gms1+, a gene encoding the UDP-galactose transporter required for the synthesis of galactomannan, led to the complete defect of cell surface galactosylation in Sz. pombe. The Deltagms1 strain is therefore useful for the analysis of physiological properties of galactose residues in Sz. pombe. The deletion strain of gms1+ was viable; however, itshowed an aberrant cell morphology and increased sensitivities to digestion with beta-glucanase and to various drugs, such as hygromycin B, sodium orthovanadate and Calcofluor white. A reduction of galactomannan layers of the cell wall in the Deltagms1 strain was observed by scanning and transmission electron microscopic analyses. The addition of osmotic stabilizer suppressed the morphologic defect of the Deltagms1 cells, while other phenotypes were weakly suppressed. The Deltagms1 (h90) strain was incapable of sexual conjugation during nutritional starvation. These results suggest that the cell surface galactosylation is required not only for non-sexual flocculation but also for sexual conjugation in Sz. pombe.

Functional characterization of Gms1p/UDP-galactose transporter in Schizosaccharomyces pombe

Galactosylation of glycoproteins in the fission yeast Schizosaccharomyces pombe requires the transport of UDP-galactose as substrate for the galactosyltransferase into the lumen of the Golgi apparatus, which is achieved by the UDP-galactose transporter. We isolated a mutant (gms1) that is deficient in galactosylation of cell surface glycoproteins in Sz.pombe, and found that the gms1(+) gene encodes a UDP-galactose transporter. In the prediction of secondary structure of the Gms1 protein, an eight-membrane-spanning structure was obtained. Fluorescent microscopy revealed the functional Gms1-GFP fusion protein to be stably localized at the Golgi membrane. Sequencing analysis of the coding region of Gms1p derived from galactosylation-defective mutants identified a single amino acid mutation (A102T or A258E) located within the putative transmembrane region, helix 2 or helix 7, respectively. The mutagenized Gms1(A102T or A258E)p exhibited loss of UDP-galactose transport activity but no change in the localization to the Golgi membrane. The C-terminal truncated Gms1p mutants demonstrated that the C-terminal hydrophilic region was dispensable for targeting and function as UDP-galactose transporter at the Golgi membrane. We suggest that the putative eighth (the most C-terminus-proximal) transmembrane helix of Gms1p is critical to targeting from ER to the Golgi membrane.

Sxa2 is a serine carboxypeptidase that degrades extracellular P-factor in the fission yeast Schizosaccharomyces pombe

Stimulating the fission yeast Schizosaccharomyces pombe with mating pheromones brings about responses that lead to cell conjugation. Persistent stimulation does not, however, induce a continuous response as the cells become desensitized to the presence of the pheromone. One mechanism that contributes to desensitization in M-cells is the release of a carboxypeptidase that inactivates the extracellular P-factor pheromone. Production of the carboxypeptidase requires a functional sxa2 gene. In this study, we report the first molecular characterization of the Sxa2 protein and provide direct evidence that it is the carboxypeptidase that degrades P-factor. Sxa2 is synthesized as a precursor that undergoes an internal cleavage event catalysed by a protease with specificity for basic residues. This generates a series of catalytically active N-terminal fragments and an inactive C-terminal fragment. Cleavage is essential for activation of the carboxypeptidase and, although the C-terminal fragment is inactive, it is required for the N-terminal fragment to attain activity.

Functional analysis of the human NRAMP family expressed in fission yeast

The Bcg/Ity/Lsh locus in the mouse genome regulates macrophage activation for antimicrobial activity against intracellular pathogens, and the positional cloning of this locus identified the Nramp1 (natural resistance-associated macrophage protein) gene. Nramp2 was initially isolated as a homologue of Nramp1. Recently, the rat divalent metal transporter DMT1 was identified electrophysiologically, and was found to be an isoform of Nramp2, a mutation which was subsequently identified in rats suffering from hereditary iron-deficiency anaemia. Despite the 64% amino acid sequence identity of Nramp1 and Nramp2, no divalent metal transport activity has yet been detected from Nramp1, and the function of Nramp1 on the molecular level is still unclear. To investigate the divalent metal transport activity of NRAMP molecules, we constructed four chimeric NRAMP genes by swapping the domains of human NRAMP1 and NRAMP2 with each other. The functional characteristics of wild-type NRAMP1, NRAMP2 and their chimeras were determined by expression in the divalent metal transporter-disrupted strain of fission yeast, pdt1Delta, and we analysed the divalent metal transport activity by complementation of the EGTA- and pH-sensitive phenotype of pdt1Delta. Replacement of the N-terminal cytoplasmic domain of NRAMP2 with the NRAMP1 counterpart resulted in inactive chimeras, indicating that the functional difference between NRAMP1 and NRAMP2 is located in this region. However, results obtained with the reverse construct and other chimeras indicated that these regions are not solely responsible for the differences in EGTA- and pH-sensitivity of NRAMP1 and NRAMP2. These findings indicate that NRAMP1 itself cannot represent the divalent metal transport activity in S. pombe and the additional protein segments of the molecules located elsewhere in NRAMP1 are also functionally distinct from their NRAMP2 counterparts.

Genetic evidence for the heterodimeric structure of glucosidase II. The effect of disrupting the subunit-encoding genes on glycoprotein folding

It has been proposed that in rat and murine tissues glucosidase II (GII) is formed by two subunits, GIIalpha and GIIbeta, respectively, responsible for the catalytic activity and the retention of the enzyme in the endoplasmic reticulum (ER). To test this proposal we disrupted genes (gls2alpha(+) and gls2beta(+)) encoding GIIalpha and GIIbeta homologs in Schizosaccharomyces pombe. Both mutant cells (gls2alpha and gls2beta) were completely devoid of GII activity in cell-free assays. Nevertheless, N-oligosaccharides formed in intact gls2alpha cells were identified as Glc(2)Man(9)GlcNAc(2) and Glc(2)Man(8)GlcNAc(2), whereas gls2beta cells formed, in addition, small amounts of Glc(1)Man(9)GlcNAc(2). It is suggested that this last compound was formed by GIIalpha transiently present in the ER. Monoglucosylated oligosaccharides facilitated glycoprotein folding in S. pombe as mutants, in which formation of monoglucosylated glycoproteins was completely (gls2alpha) or severely (gls2beta and UDP-Glc:glycoprotein:glucosyltransferase null) diminished, showed ER accumulation of misfolded glycoproteins when grown in the absence of exogenous stress as revealed by (a) induction of binding protein-encoding mRNA and (b) accumulation of glycoproteins bearing ER-specific oligosaccharides. Moreover, the same as in mammalian cell systems, formation of monoglucosylated oligosaccharides decreased the folding rate and increased the folding efficiency of glycoproteins as pulse-chase experiments revealed that carboxypeptidase Y arrived at a higher rate but in decreased amounts to the vacuoles of gls2alpha than to those of wild type cells.

The UDP-Glc:Glycoprotein glucosyltransferase is essential for Schizosaccharomyces pombe viability under conditions of extreme endoplasmic reticulum stress

Interaction of monoglucosylated oligosaccharides with ER lectins (calnexin and/or calreticulin) facilitates glycoprotein folding but this interaction is not essential for cell viability under normal conditions. We obtained two distinct single Schizosaccharomyces pombe mutants deficient in either one of the two pathways leading to the formation of monoglucosylated oligosaccharides. The alg6 mutant does not glucosy- late lipid-linked oligosaccharides and transfers Man9GlcNAc2 to nascent polypeptide chains and the gpt1 mutant lacks UDP-Glc:glycoprotein glucosyltransferase (GT). Both single mutants grew normally at 28 degreesC. On the other hand, gpt1/alg6 double-mutant cells grew very slowly and with a rounded morphology at 28 degreesC and did not grow at 37 degreesC. The wild-type phenotype was restored by transfection of the double mutant with a GT-encoding expression vector or by addition of 1 M sorbitol to the medium, indicating that the double mutant is affected in cell wall formation. It is suggested that facilitation of glycoprotein folding mediated by the interaction of monoglucosylated oligosaccharides with calnexin is essential for cell viability under conditions of extreme ER stress such as underglycosylation of proteins caused by the alg6 mutation and high temperature. In contrast, gls2/alg6 double-mutant cells that transfer Man9GlcNAc2 and that are unable to remove the glucose units added by GT as they lack glucosidase II (GII), grew at 37 degreesC and had, when grown at 28 degreesC, a phenotype of growth and morphology almost identical to that of wild-type cells. These results indicate that facilitation of glycoprotein folding mediated by the interaction of calnexin and monoglucosylated oligosaccharides does not necessarily require cycles of reglucosylation-deglucosylation catalyzed by GT and GII.

A misfolded protein conformation is not a sufficient condition for in vivo glucosylation by the UDP-Glc:glycoprotein glucosyltransferase

A key element in the quality control of glycoprotein folding is the UDP-Glc:glycoprotein glucosyltransferase (GT), which in cell-free assays exclusively glucosylates misfolded glycoproteins. In order to test if such a protein conformation is a sufficient condition for in vivo glucosylation of all N-linked oligosaccharides by GT, a Schizosaccharomyces pombe double mutant (gls2/alg6) was constructed. With this mutant, Man9GlcNAc2 is transferred to proteins and no removal of glucose units added by GT occurs as it lacks glucosidase II. The same proportion of glucosylated (Glc1Man9GlcNAc2) and unglucosylated (Man9GlcNAc2 and Man8GlcNAc2) endoplasmic reticulum (ER)-specific compounds was produced when cells were pre-incubated for 10, 20 or 30 min and further incubated with [14C]glucose for 10 min at 28 degrees C with or without 5 mM dithiothreitol (DTT), thus indicating not only that DTT did not affect protein glucosylation but also that no increased glucosylation of glycoproteins occurred in the presence of the drug. Monitoring Golgi-specific modifications of oligosaccharides after pulse-chase experiments performed in the presence or absence of 5 mM DTT showed that exit of the bulk of glycoproteins synthesized from the ER and thence their proper folding had been prevented by the drug. Cells pulse-chase labeled at 37 degrees C in the absence of DTT also yielded glucosylated and unglucosylated protein-linked oligosaccharides without Golgi-specific modifications. It was concluded that a misfolded protein conformation is not a sufficient condition for in vivo glucosylation of all N-linked oligosaccharides by GT.

A minisatellite sequence within the propeptide region of the vacuolar carboxypeptidase Y gene of Schizosaccharomyces pombe

We describe the presence of a minisatellite sequence that displays length polymorphisms in the fission yeast Schizosaccharomyces pombe. The minisatellite sequence was found to reside within the propeptide region of the vacuolar carboxypeptidase Y gene. The minisatellite sequence, which was found only at a single locus, was mitotically stable and displayed length polymorphisms between the two varieties of S. pombe (S. pombe var. pombe and S. pombe var. malidevorans). The minisatellite sequence, however, appeared to be species specific and was absent in other members of the Schizosaccharomyces genus. This report constitutes the first experimental demonstration of the presence of such sequences in yeasts.

Isolation and characterization of an invertase and its repressor genes from Schizosaccharomyces pombe

PCR was used to isolate an invertase homolog gene from the fission yeast Schizosaccharomyces pombe. The cloned inv1(+) gene encodes a protein of 581 amino acids with 16 potential asparagine-linked glycosylation sites, and has 39% and 38% identity to the Schwanniomyces occidentalis and Saccharomyces cerevisiae SUC2 invertases. When the inv1(+) gene was disrupted, S. pombe strains lacked detectable invertase activity. This result showed that the inv1(+) gene encodes only one active invertase in S. pombe cells. The transcription of inv1(+) is repressed in the presence of glucose. The transcription of inv1(+) was not affected in cyr1Delta strain which lacks adenylate cyclase activity, unlike transcription of S. pombe fbp1(+) gene. We have identified an S. pombe gene (scr1(+)) that encodes a homolog of the Aspergillus nidulans CREA which is required for glucose repression of the glyconeogenic pathway. Although the deletion of scr1(+) did not influence the transcription of fbp1(+) gene, glucose repression of the inv1(+) gene was severely affected. These results showed that glucose repression of inv1(+) gene is dependent on scr1(+) gene, and S. pombe cAMP signalling pathway may not be essential for glucose repression of inv1(+) gene.