Purification and characterization of the Saccharomyces cerevisiae BGL2 gene product, a cell wall endo-beta-1,3-glucanase

Growth temperature exerts differential physiological and transcriptional responses in laboratory and wine strains of Saccharomyces cerevisiae

Laboratory strains of Saccharomyces cerevisiae have been widely used as a model for studying eukaryotic cells and mapping the molecular mechanisms of many different human diseases. Industrial wine yeasts, on the other hand, have been selected on the basis of their adaptation to stringent environmental conditions and the organoleptic properties that they confer to wine. Here, we used a two-factor design to study the responses of a standard laboratory strain, CEN.PK113-7D, and an industrial wine yeast strain, EC1118, to growth temperatures of 15 degrees C and 30 degrees C in nitrogen-limited, anaerobic, steady-state chemostat cultures. Physiological characterization revealed that the growth temperature strongly impacted the biomass yield of both strains. Moreover, we found that the wine yeast was better adapted to mobilizing resources for biomass production and that the laboratory yeast exhibited higher fermentation rates. To elucidate mechanistic differences controlling the growth temperature response and underlying adaptive mechanisms between the strains, DNA microarrays and targeted metabolome analysis were used. We identified 1,007 temperature-dependent genes and 473 strain-dependent genes. The transcriptional response was used to identify highly correlated gene expression subnetworks within yeast metabolism. We showed that temperature differences most strongly affect nitrogen metabolism and the heat shock response. A lack of stress response element-mediated gene induction, coupled with reduced trehalose levels, indicated that there was a decreased general stress response at 15 degrees C compared to that at 30 degrees C. Differential responses among strains were centered on sugar uptake, nitrogen metabolism, and expression of genes related to organoleptic properties. Our study provides global insight into how growth temperature affects differential physiological and transcriptional responses in laboratory and wine strains of S. cerevisiae.

Amyloid-like properties of Saccharomyces cerevisiae cell wall glucantransferase Bgl2p: prediction and experimental evidences

Glucantransferase Bgl2p is a major conserved cell wall constituent described for a wide range of yeast species. In the baker's yeast Saccharomyces cerevisiae it is the only non-covalently bound cell wall protein that cannot be released from cell walls by sequential SDS and trypsin treatment. It contains seven amyloidogenic determinants. Circular dichroism analysis and fluorescence spectroscopy with thioflavin T indicate the presence of beta-sheet structures in Bgl2p isolates. Bgl2p forms fibrils, a process that is enforced in the presence of other cell wall components. Thus the data obtained is the first evidence for amyloid-like properties of yeast cell wall protein-glucantransferase Bgl2p.

Mass spectrometric identification of covalently bound cell wall proteins from the fission yeast Schizosaccharomyces pombe

The cell wall of Schizosaccharomyces pombe is bilayered, consisting of an inner layer of mainly polysaccharides and an outer layer of galactomannoproteins. We present a detailed analysis of the cell wall proteome. Six covalently-bound cell wall proteins (CWPs) were identified using tandem mass spectrometry, including four predicted GPI-dependent CWPs (Gas1p, Gas5p, Ecm33p and Pwp1p) and two alkali-sensitive CWPs (Psu1p and Asl1p). Gas1p and Gas5p belong to glycoside hydrolase family 72, and are believed to be involved in 1,3-beta-glucan elongation. Ecm33p belongs to a ubiquitous fungal protein family with an unknown but crucial function in cell wall integrity. Pwp1p is an abundant protein with an unknown but probably non-enzymatic function. All four CWPs were present in HF-pyridine extracts, indicating that they are linked via a phosphodiester bridge to the glucan network. Psu1p is a homologue of the Saccharomyces cerevisiae Sun family, whereas Asl1p has no homologues in S. cerevisiae but is related to Aspergillus fumigatus and Ustilago maydis proteins. Finally, although the protein content of Sz. pombe cell walls is only slightly less than in S. cerevisiae and Candida albicans, the amount of carbohydrate added to the proteins was found to be two- to three-fold decreased, consistent with earlier reported differences in outer chain N-glycosylation.

Extraction of cell surface-associated proteins from living yeast cells

To extract cell surface-associated proteins from living fungal cells, reducing agents such as beta-mercaptoethanol and dithiothreitol are often used. We show here that both compounds are moderately lipophilic and may perturb the plasma membrane, thus causing the release of cytosolic proteins, especially at high extraction temperatures. To avoid artifacts, we recommend using (a) a low concentration of the reducing agent for only a short period of time, and (b) an extraction temperature of 4 degrees C to protect the integrity of the plasma membrane. Similarly, biotinylation of cell surface proteins should be carried out at low temperatures in the absence of dimethylsulphoxide.

The CRH family coding for cell wall glycosylphosphatidylinositol proteins with a predicted transglycosidase domain affects cell wall organization and virulence of Candida albicans

In Candida albicans UTR2 (CSF4), CRH11, and CRH12 are members of a gene family (the CRH family) that encode glycosylphosphatidylinositol-dependent cell wall proteins with putative transglycosidase activity. Deletion of genes of this family resulted in additive sensitivity to compounds interfering with normal cell wall formation (Congo red, calcofluor white, SDS, and high Ca(2+) concentrations), suggesting that these genes contribute to cell wall organization. A triple mutant lacking UTR2, CRH11, and CRH12 produced a defective cell wall, as inferred from increased sensitivity to cell wall-degrading enzymes, decreased ability of protoplasts to regenerate a new wall, constitutive activation of Mkc1p, the mitogen-activated protein kinase of the cell wall integrity pathway, and an increased chitin content of the cell wall. Importantly, this was accompanied by a decrease in alkali-insoluble 1,3-beta-glucan but not total glucan content, suggesting that formation of the linkage between 1,3-beta-glucan and chitin might be affected. In support of this idea, localization of a Utr2p-GFP fusion protein largely coincided with areas of chitin incorporation in C. albicans. As UTR2 and CRH11 expression is regulated by calcineurin, a serine/threonine protein phosphatase involved in tolerance to antifungal drugs, cell wall morphogenesis, and virulence, this points to a possible relationship between calcineurin and the CRH family. Deletion of UTR2, CRH11, and CRH12 resulted in only a partial overlap with calcineurin-dependent phenotypes, suggesting that calcineurin has additional targets. Interestingly, cells deleted for UTR2, CRH11, and CRH12 were, like a calcineurin mutant, avirulent in a mouse model of systemic infection but retained the capacity to colonize target organs (kidneys) as the wild type. In conclusion, this work establishes the role of UTR2, CRH11, and CRH12 in cell wall organization and integrity.

Cell wall assembly in Saccharomyces cerevisiae

An extracellular matrix composed of a layered meshwork of beta-glucans, chitin, and mannoproteins encapsulates cells of the yeast Saccharomyces cerevisiae. This organelle determines cellular morphology and plays a critical role in maintaining cell integrity during cell growth and division, under stress conditions, upon cell fusion in mating, and in the durable ascospore cell wall. Here we assess recent progress in understanding the molecular biology and biochemistry of cell wall synthesis and its remodeling in S. cerevisiae. We then review the regulatory dynamics of cell wall assembly, an area where functional genomics offers new insights into the integration of cell wall growth and morphogenesis with a polarized secretory system that is under cell cycle and cell type program controls.

Enzymic activity of the K5-type yeast killer toxin and its characterization

K5-type yeast killer toxin secreted by P. anomala NCYC 434 cells has a broad killing spectrum. Competitive inhibiton of killer activity showed that glucans, mainly the beta-1,3 glucan, represent the primary toxin binding site within the cell wall of sensitive cells. Its hydrolytic activity on laminarin in an exo-like fashion revealed that the toxin exerts its killing effect by exo-beta-1,3-glucanase activity. Its specific activity on laminarin was 120 U/mg, and the Michaelis constants K(m) and V(max) for laminarin hydrolysis were 0.25 mg/ml and 370 micromol/min/mg. The toxin exerted its cytocidal effect after 2 h contact with the target cells. Production of the toxin by the cells was induced only when they were grown in culture media rich in beta-glucan sources, and the addition of glucose increased the specific production rate. The enzymic activity of the toxin was fully inhibited by Hg(+2), but increased with some other metal ions, most of all by Pb(+2).

Structure and assembly of epiglucan, the extracellular (1→3;1→6)-β-glucan produced by the fungus Epicoccum nigrum strain F19

In a previous article [Carbohydr. Res.2001, 331, 163-171] two different structures for the possible modular repeating unit of the extracellular beta-glucan, epiglucan produced by the fungus Epicoccum nigrum strain F19 were proposed. Clarifying which was the more likely one was considered essential before attempts were made to understand how epiglucan was assembled by this fungus. Data from Smith degradation analyses of epiglucan were consistent with the repeating unit of structure I, where single glucosyl residues are attached by (1-->6)-beta-linkages to two out of every three glucosyl residues in the (1-->3)-beta-linked glucan backbone. Repeated Smith degradations of 14C-glucose labelled epiglucan showed that chain elongation occurred from its non-reducing end. Side chain insertion into the growing glucan was followed by analysis of real time incorporation of 13C-glucose into epiglucan by 13C NMR, and 14C-glucose by enzymic digestion of the synthesised 14C-epiglucan. All data obtained were consistent with the view that single (1-->6)-beta-linked glucosyl side residues are inserted simultaneously as the glucan backbone elongates.

Isolation and characterization of the Aspergillus nidulans eglC gene encoding a putative β-1,3-endoglucanase

The Aspergillus nidulans eglC gene, which encodes a putative beta-1,3-endoglucanase, was isolated from a chromosome-specific library by using an expressed sequence tag, esd0113. The EglC open reading frame encodes a 465 amino acid polypeptide, of which the amino acid sequence showed 46% similarity to that of Saccharomyces cerevisiae beta-1,3-endoglucanase. The eglC transcript level at the early stages of asexual and sexual developments was dependent on the presence of the nsdD gene that encodes a GATA-type transcription factor, confirming that the nsdD gene is necessary for full accumulation of the eglC transcript. Deletion of the eglC gene did not affect the radial growth rate, the germination rate of conidia, and both of asexual and sexual development. However, deletion of the gene led to hyphae more resistant to a cell wall-lyzing enzyme, implying that the cell wall structure of the eglC-null mutant is altered from a wild type one. Furthermore, deletion of the fadA and sfaD genes, that encode a Galpha and a Gbeta subunits of a heterotrimeric G protein, respectively, did not affect the eglC transcript level at the early developmental stages. In contrast, deletion of the flbA gene, that codes for a regulatory protein having an RGS (regulator of G protein signaling) motif, led to decrease in the eglC transcript level. The eglC transcript level was not higher in a creA mutant than in a wild type, indicating that the eglC gene is not sensitive to carbon-catabolite repression.

Comprehensive Proteomic Analysis of Saccharomyces cerevisiae Cell Walls

The cell wall of yeast contains proteins that are covalently bound to the glycan network. These cell wall proteins (CWPs) mediate cell-cell interactions and may be involved in cell wall biosynthesis. Using tandem mass spectrometry, we have identified 19 covalently bound CWPs of Saccharomyces cerevisiae. Twelve of them are shown for the first time to be covalently incorporated into the cell wall. The identified proteins include 12 predicted glycosylphosphatidylinositol-modified CWPs, all four members of the Pir protein family, and three additional proteins (Scw4p, Scw10p, and Tos1p) that are, like Pir proteins, connected to the cell wall glycan network via an alkali-sensitive linkage. However, Scw4p, Scw10p, and Tos1p do not contain internal repeat sequences shown to be essential for Pir protein incorporation and may represent a separate class of CWPs. Strikingly, seven of the identified proteins (Gas1p, Gas3p, Gas5p, Crh1p, Utr2p, Scw4p, and Scw10p) are classified as glycoside hydrolases. Phenotypic analysis of deletion mutants lacking the corresponding CWP-encoding genes indicated that most of them have altered cell wall properties, which reinforces the importance of the identified proteins for proper cell wall formation. In particular, gas1Delta and ecm33Delta were highly sensitive to Calcofluor White and high temperature, whereas gas1Delta, scw4Delta, and tos1Delta were highly resistant to incubation with beta-1,3-glucanase. The CWP identification method developed here relies on directly generating tryptic peptides from isolated cell walls and is independent of the nature of the covalent linkages between CWPs and cell wall glycans. Therefore, it will probably be equally effective in many other fungi.

GPI7 Involved in Glycosylphosphatidylinositol Biosynthesis Is Essential for Yeast Cell Separation

GPI7 is involved in adding ethanolaminephosphate to the second mannose in the biosynthesis of glycosylphosphatidylinositol (GPI) in Saccharomyces cerevisiae. We isolated gpi7 mutants, which have defects in cell separation and a daughter cell-specific growth defect at the non-permissive temperature. WSC1, RHO2, ROM2, GFA1, and CDC5 genes were isolated as multicopy suppressors of gpi7-2 mutant. Multicopy suppressors could suppress the growth defect of gpi7 mutants but not the cell separation defect. Loss of function mutations of genes involved in the Cbk1p-Ace2p pathway, which activates the expression of daughter-specific genes for cell separation after cytokinesis, bypassed the temperature-sensitive growth defect of gpi7 mutants. Furthermore, deletion of EGT2, one of the genes controlled by Ace2p and encoding a GPI-anchored protein required for cell separation, ameliorated the temperature sensitivity of the gpi7 mutant. In this mutant, Egt2p was displaced from the septal region to the cell cortex, indicating that GPI7 plays an important role in cell separation via the GPI-based modification of daughter-specific proteins in S. cerevisiae.

Kluyveromyces phaffii killer toxin active against wine spoilage yeasts: purification and characterization

The killer toxin secreted by Kluyveromyces phaffii (KpKt) is active against spoilage yeast under winemaking conditions and thus has potential applications in the biocontrol of undesired micro-organisms in the wine industry. Biochemical characterization and N-terminal sequencing of the purified toxin show that KpKt is a glycosylated protein with a molecular mass of 33 kDa. Moreover, it shows 93% and 80% identity to a beta-1,3-glucanase of Saccharomyces cerevisiae and a beta-1,3-glucan transferase of Candida albicans, respectively, and it is active on laminarin and glucan, thus showing a beta-glucanase activity. Competitive inhibition of killer activity by cell-wall polysaccharides suggests that glucan (beta-1,3 and beta-1,6 branched glucans) represents the first receptor site of the toxin on the envelope of the sensitive target. Flow cytometry analysis of the sensitive target after treatment with KpKt and K1 toxin of S. cerevisiae, known to cause loss of cell viability via formation of pores in the cell membrane, suggests a different mode of action for KpKt.

Gyp5p and Gyl1p are involved in the control of polarized exocytosis in budding yeast

We report here elements for functional characterization of two members of the Saccharomyces cerevisiae Ypt/Rab GTPase activating proteins family (GAP): Gyp5p, a potent GAP in vitro for Ypt1p and Sec4p, and the protein Ymr192wp/APP2 that we propose to rename Gyl1p (GYp like protein). Immunofluorescence experiments showed that Gyp5p and Gyl1p partly colocalize at the bud emergence site, at the bud tip and at the bud neck during cytokinesis. Subcellular fractionation and co-immunoprecipitation experiments showed that Gyp5p and Gyl1p co-fractionate with post-Golgi vesicles and plasma membrane, and belong to the same protein complexes in both localizations. We found by co-immunoprecipitation experiments that a fraction of Gyp5p interacts with Sec4p, a small GTPase involved in exocytosis, and that a fraction of Gyl1p associates at the plasma membrane with the Gyp5p/Sec4p complexes. We showed also that GYP5 genetically interacts with SEC2, which encodes the Sec4p exchange factor. Examination of the gyp5Δgyl1Δ mutants grown at 13°C revealed a slight growth defect, a secretion defect and an accumulation of secretory vesicles in the small-budded cells. These data suggest that Gyp5p and Gyl1p are involved in control of polarized exocytosis.

Fungal cell wall chitinases and glucanases

The fungal cell wall is a complex structure composed of chitin, glucans and other polymers, and there is evidence of extensive cross-linking between these components. The wall structure is highly dynamic, changing constantly during cell division, growth and morphogenesis. Hydrolytic enzymes, closely associated with the cell wall, have been implicated in the maintenance of wall plasticity and may have roles during branching and cross-linking of polymers. Most fungal cell wall hydrolases identified to date have chitinase or glucanase activity and this short article reviews the apparent functions of these enzymes in unicellular and filamentous fungi, and the mechanisms that regulate enzyme activity in yeasts.

Sed1p and Srl1p are required to compensate for cell wall instability in Saccharomyces cerevisiae mutants defective in multiple GPI-anchored mannoproteins

The covalently linked cell wall protein Ccw12p of Saccharomyces cerevisiae is a GPI-anchored protein (V. Mrsa et al., 1999, J Bacteriol 181: 3076-3086). Although only 121 amino acids long, the haemagglutinin-tagged protein released by laminarinase from the cell wall possesses an apparent molecular mass of > 300 kDa. A membrane-bound form with an apparent molecular mass of 58 kDa is highly O- and N-glycosylated and contains the GPI anchor. With a half-life of 2 min, the membrane form is transformed to the > 300 kDa form. The deletion mutant ccw12Delta grows slower than the wild type, is highly sensitive to Calcofluor white and contains 2.5 times more chitin. Further, compared with wild-type yeast, significantly more proteins are released from intact cells when treated with dithiothreitol. Interestingly, these defects become less pronounced when further GPI-anchored cell wall proteins are deleted. Mutant DeltaGPI (simultaneous deletion of CCW12, CCW13/DAN1, CCW14, TIP1 and CWP1) is similar in many respects to wild-type yeast. To find out how the cell wall is stabilized in mutant DeltaGPI, a genome-wide transcription analysis was performed. Of 159 significantly regulated genes, 14 encode either known or suspected cell wall-associated proteins. Analysis of genes affected in transcription revealed that SED1 and SRL1 in particular are required to reconstruct cell wall stability in the absence of multiple GPI-anchored mannoproteins.

The Global Transcriptional Response to Transient Cell Wall Damage in Saccharomyces cerevisiae and Its Regulation by the Cell Integrity Signaling Pathway

In the yeast Saccharomyces cerevisiae, environmental stress conditions that damage the cell wall lead to activation of the so-called "compensatory mechanism," aimed at preserving cell integrity through a remodeling of this extracellular matrix. Here we used DNA microarrays to investigate the molecular basis of this response to two agents that induce transient cell wall damage; namely Congo Red and Zymolyase. Treatment of the cells with these two agents elicited the up-regulation of 132 and 101 genes respectively, the main functional groups among them being involved in cell wall construction and metabolism. The main response does not occur until hours after exposure to the cell wall-perturbing agent. In some cases, this response was transient, but more sustained in others, especially in the case of the genes involved in cell wall remodeling. Clustering of these data together with those from the response to constitutive cell wall damage, revealed the existence of a cluster of co-regulated genes that was strongly induced under all conditions assayed. Those genes induced by cell wall damage showed an enrichment in DNA binding motifs for Rlm1p, Crz1p, SBF (Swi4p/Swi6p), Msn2p/Msn4p, Ste12p, and Tec1p transcription factors, suggesting a complex regulation of this response together with the possible involvement of several signaling pathways. With the exception of PHO89 and FKS2, none of the genes induced by Congo Red was up-regulated in a slt2 strain. Moreover, characterization of the transcriptional response to Congo Red in a rlm1 mutant strain revealed that only a few genes (i.e. PHO89, FKS2, YLR042C, and CHA1) were induced at least partially independently of the transcription factor Rlm1p, the rest being totally dependent on this transcription factor for their activation. Our findings consistently demonstrate that the cell integrity signaling pathway regulates the cell wall damage compensatory response, mainly through transcriptional activation mediated by Rlm1p.

Cell wall-associated enzymes in fungi

This review compiles and discusses previous reports on the identity of wall-associated enzymes (WAEs) in fungi and addresses critically the widely different terminologies used in the literature to specify the type of bonding of WAEs to other entities of the cell wall compartment, the extracellular matrix (ECM). A facile and rapid fractionation protocol for catalytically active WAEs is presented, which uses crude cell walls as the experimental material, a variety of test enzymes (including representatives of polysaccharide synthases and hydrolases, phosphatases, gamma-glutamyltransferases, pyridine-nucleotide dehydrogenases and phenol-oxidising enzymes) and a combination of simple hydrophilic and hydrophobic extractants. The protocol provides four fully operationally defined classes of WAEs, with constituent members of each class displaying the same basic type of physicochemical interaction with binding partners in situ. The routine application of the protocol to different species and cell types could yield easily accessible data useful for building-up a general objective information retrieval system of WAEs, suitable as an heuristic basis both for the unravelling of the role and for the biotechnological potentialities of WAEs. A detailed account is given of the function played in the ECM by WAEs in the metabolism of chitin (chitin synthase, chitinase and beta-N-acetylhexosaminidase) and of phenols (tyrosinase).

The GAP activity of Msb3p and Msb4p for the Rab GTPase Sec4p is required for efficient exocytosis and actin organization

Polarized growth in Saccharomyces cerevisiae is thought to occur by the transport of post-Golgi vesicles along actin cables to the daughter cell, and the subsequent fusion of the vesicles with the plasma membrane. Previously, we have shown that Msb3p and Msb4p genetically interact with Cdc42p and display a GTPase-activating protein (GAP) activity toward a number of Rab GTPases in vitro. We show here that Msb3p and Msb4p regulate exocytosis by functioning as GAPs for Sec4p in vivo. Cells lacking the GAP activity of Msb3p and Msb4p displayed secretory defects, including the accumulation of vesicles of 80-100 nm in diameter. Interestingly, the GAP activity of Msb3p and Msb4p was also required for efficient polarization of the actin patches and for the suppression of the actin-organization defects in cdc42 mutants. Using a strain defective in polarized secretion and actin-patch organization, we showed that a change in actin-patch organization could be a consequence of the fusion of mistargeted vesicles with the plasma membrane.

Eng1p, an endo-1,3-beta-glucanase localized at the daughter side of the septum, is involved in cell separation in Saccharomyces cerevisiae

ENG1 (YNR067c), a gene encoding a new endo-1,3-beta-glucanase, was cloned by screening a genomic library with a DNA probe obtained by PCR with synthetic oligonucleotides designed according to conserved regions found between yeast exo-1,3-beta-glucanases (Exglp, Exg2p, and Ssglp). Eng1p shows strong sequence similarity to the product of the Saccharomyces cerevisiae ACF2 gene, involved in actin assembly "in vitro," and to proteins present in other yeast and fungal species. It is also related to plant glucan-binding elicitor proteins, which trigger the onset of a defense response upon fungal infection. Eng1p and Acf2p/Eng2p are glucan-hydrolyzing proteins that specifically act on 1,3-beta linkages, with an endolytic mode of action. Eng1p is an extracellular, heavily glycosylated protein, while Acf2p/Eng2p is an intracellular protein with no carbohydrate linked by N-glycosidic bonds. ENG1 transcription fluctuates periodically during the cell cycle; maximal accumulation occurs during the M/G1 transition and is dependent on the transcription factor Ace2p. Interestingly, eng1 deletion mutants show defects in cell separation, and Eng1p localizes asymmetrically to the daughter side of the septum, suggesting that this protein is involved, together with chitinase, in the dissolution of the mother-daughter septum.

Microbial cellulose utilization: fundamentals and biotechnology

Fundamental features of microbial cellulose utilization are examined at successively higher levels of aggregation encompassing the structure and composition of cellulosic biomass, taxonomic diversity, cellulase enzyme systems, molecular biology of cellulase enzymes, physiology of cellulolytic microorganisms, ecological aspects of cellulase-degrading communities, and rate-limiting factors in nature. The methodological basis for studying microbial cellulose utilization is considered relative to quantification of cells and enzymes in the presence of solid substrates as well as apparatus and analysis for cellulose-grown continuous cultures. Quantitative description of cellulose hydrolysis is addressed with respect to adsorption of cellulase enzymes, rates of enzymatic hydrolysis, bioenergetics of microbial cellulose utilization, kinetics of microbial cellulose utilization, and contrasting features compared to soluble substrate kinetics. A biological perspective on processing cellulosic biomass is presented, including features of pretreated substrates and alternative process configurations. Organism development is considered for "consolidated bioprocessing" (CBP), in which the production of cellulolytic enzymes, hydrolysis of biomass, and fermentation of resulting sugars to desired products occur in one step. Two organism development strategies for CBP are examined: (i) improve product yield and tolerance in microorganisms able to utilize cellulose, or (ii) express a heterologous system for cellulose hydrolysis and utilization in microorganisms that exhibit high product yield and tolerance. A concluding discussion identifies unresolved issues pertaining to microbial cellulose utilization, suggests approaches by which such issues might be resolved, and contrasts a microbially oriented cellulose hydrolysis paradigm to the more conventional enzymatically oriented paradigm in both fundamental and applied contexts.

Microbial cellulose utilization: fundamentals and biotechnology

Fundamental features of microbial cellulose utilization are examined at successively higher levels of aggregation encompassing the structure and composition of cellulosic biomass, taxonomic diversity, cellulase enzyme systems, molecular biology of cellulase enzymes, physiology of cellulolytic microorganisms, ecological aspects of cellulase-degrading communities, and rate-limiting factors in nature. The methodological basis for studying microbial cellulose utilization is considered relative to quantification of cells and enzymes in the presence of solid substrates as well as apparatus and analysis for cellulose-grown continuous cultures. Quantitative description of cellulose hydrolysis is addressed with respect to adsorption of cellulase enzymes, rates of enzymatic hydrolysis, bioenergetics of microbial cellulose utilization, kinetics of microbial cellulose utilization, and contrasting features compared to soluble substrate kinetics. A biological perspective on processing cellulosic biomass is presented, including features of pretreated substrates and alternative process configurations. Organism development is considered for "consolidated bioprocessing" (CBP), in which the production of cellulolytic enzymes, hydrolysis of biomass, and fermentation of resulting sugars to desired products occur in one step. Two organism development strategies for CBP are examined: (i) improve product yield and tolerance in microorganisms able to utilize cellulose, or (ii) express a heterologous system for cellulose hydrolysis and utilization in microorganisms that exhibit high product yield and tolerance. A concluding discussion identifies unresolved issues pertaining to microbial cellulose utilization, suggests approaches by which such issues might be resolved, and contrasts a microbially oriented cellulose hydrolysis paradigm to the more conventional enzymatically oriented paradigm in both fundamental and applied contexts.

Correct GPI-anchor synthesis is required for the incorporation of endoglucanase/glucanosyltransferase Bgl2p into the Saccharomyces cerevisiae cell wall

The SSU21/MCD4 gene encodes an essential component of the glycosylphosphatidylinositol (GPI)-anchor synthesis pathway in Saccharomyces cerevisiae. Here we demonstrate that the ssu21 mutation affected the transport and the incorporation into the cell wall of the major non-GPI yeast cross-linker - endoglucanase/glucanosyltransferase Bgl2p. This mutation also led to a decrease in the levels of both known types of cell wall mannoproteins, those covalently linked with glucan and SDS-extractable proteins. Our results indicate that the precision of the GPI-anchor synthesis is essential for cell wall assembly and suggest the strong interdependence of different groups of cell wall proteins during their incorporation into the cell wall.

Yeast Cdc42 functions at a late step in exocytosis, specifically during polarized growth of the emerging bud

The Rho family GTPase Cdc42 is a key regulator of cell polarity and cytoskeletal organization in eukaryotic cells. In yeast, the role of Cdc42 in polarization of cell growth includes polarization of the actin cytoskeleton, which delivers secretory vesicles to growth sites at the plasma membrane. We now describe a novel temperature-sensitive mutant, cdc42-6, that reveals a role for Cdc42 in docking and fusion of secretory vesicles that is independent of its role in actin polarization. cdc42-6 mutants can polarize actin and deliver secretory vesicles to the bud, but fail to fuse those vesicles with the plasma membrane. This defect is manifested only during the early stages of bud formation when growth is most highly polarized, and appears to reflect a requirement for Cdc42 to maintain maximally active exocytic machinery at sites of high vesicle throughput. Extensive genetic interactions between cdc42-6 and mutations in exocytic components support this hypothesis, and indicate a functional overlap with Rho3, which also regulates both actin organization and exocytosis. Localization data suggest that the defect in cdc42-6 cells is not at the level of the localization of the exocytic apparatus. Rather, we suggest that Cdc42 acts as an allosteric regulator of the vesicle docking and fusion apparatus to provide maximal function at sites of polarized growth.

A novel family of cell wall-related proteins regulated differently during the yeast life cycle

The Saccharomyces cerevisiae Ygr189c, Yel040w, and Ylr213c gene products show significant homologies among themselves and with various bacterial beta-glucanases and eukaryotic endotransglycosidases. Deletion of the corresponding genes, either individually or in combination, did not produce a lethal phenotype. However, the removal of YGR189c and YEL040w, but not YLR213c, caused additive sensitivity to compounds that interfere with cell wall construction, such as Congo red and Calcofluor White, and overexpression of YEL040w led to resistance to these compounds. These genes were renamed CRH1 and CRH2, respectively, for Congo red hypersensitive. By site-directed mutagenesis we found that the putative glycosidase domain of CRH1 was critical for its function in complementing hypersensitivity to the inhibitors. The involvement of CRH1 and CRH2 in the development of cell wall architecture was clearly shown, since the alkali-soluble glucan fraction in the crh1Delta crh2Delta strain was almost twice the level in the wild-type. Interestingly, the three genes were subject to different patterns of transcriptional regulation. CRH1 and YLR213c (renamed CRR1, for CRH related) were found to be cell cycle regulated and also expressed under sporulation conditions, whereas CRH2 expression did not vary during the mitotic cycle. Crh1 and Crh2 are localized at the cell surface, particularly in chitin-rich areas. Consistent with the observed expression patterns, Crh1-green fluorescent protein was found at the incipient bud site, around the septum area in later stages of budding, and in ascospore envelopes. Crh2 was found to localize mainly at the bud neck throughout the whole budding cycle, in mating projections and zygotes, but not in ascospores. These data suggest that the members of this family of putative glycosidases might exert a common role in cell wall organization at different stages of the yeast life cycle.

Genome-wide analysis of gene expression regulated by the yeast cell wall integrity signalling pathway

The cell integrity pathway of Saccharomyces cerevisiae monitors cell wall remodelling during growth and differentiation. Additionally, this pathway responds to environmental stresses that challenge the integrity of the cell wall. We conducted a genome-wide survey of genes whose expression was altered in response to activation of Mpk1/Slt2, the MAP kinase, under the control of cell integrity signalling. We identified 25 genes whose regulation was altered by Mpk1 activity. Among these, 20 were positively regulated by Mpk1, and five were negatively regulated. Most of the genes identified encode either known or suspected cell wall proteins or enzymes involved in cell wall biogenesis. These include glycosyl-phosphatidylinositol (GPI) proteins, members of the Pir family of cell wall proteins, Mpk1 itself and others. All of the regulation detected was mediated by the Rlm1 transcription factor, a MADS-box protein that is phosphorylated and activated by Mpk1, but for which no transcriptional targets had been identified. A similar pattern of regulation was observed when cell integrity signalling was induced by environmental stress (i.e. temperature upshift).

Cloning and characterization of the EXG1 gene from the yeast Yarrowia lipolytica

The YlEXG1 gene of Yarrowia lipolytica, encoding an exo-1, 3-beta-glucanase, was isolated by screening a genomic library with a DNA probe obtained by PCR amplification, using oligonucleotides designed according to conserved regions in the EXG1, EXG2 and SSG1 genes from Saccharomyces cerevisiae. YlEXG1 consists of a 1263 bp open reading frame encoding a protein of 421 amino acids with a calculated molecular weight of 48 209 Da. Northern blot analysis revealed a unique YlEXG1-specific transcript, 1.4 kb long. A putative pre(signal)-peptide of 15 amino acids is proposed at the N-terminal domain of the primary translation product. The deduced amino acid sequence shares a high degree of homology with exo-1, 3-beta-glucanases from other yeast species, including S. cerevisiae, Kluyveromyces lactis, Pichia angusta and Debaryomyces occidentalis. YlExg1p contains the invariant amino acid positions which have been shown to be important in the catalytic function of family 5 glycosyl hydrolases. Chromoblot analysis indicated that YlEXG1 is located on chromosome VI. Disruption of YlEXG1 did not result in a phenotype under laboratory conditions and did not prevent the yeast-hypha transition. The sequence data reported in this paper have been assigned EMBL Accession No. Z46872.

Purification and properties of a basic endo-beta-1,6-glucanase (BGN16.1) from the antagonistic fungus Trichoderma harzianum

The antagonistic fungus Trichoderma harzianum CECT 2413 produces at least two extracellular beta-1,6-glucanases, among other hydrolases acting on polysaccharides from fungal cell walls, when grown in chitin as the sole carbon source. We have previously reported on the purification and biochemical characterization of the major activity, which corresponds to an acidic enzyme named BGN16.2 [de la Cruz, J., Pintor-Toro, J.A., Benítez, T. & Llobell, A. (1995) J. Bacteriol. 177, 1864-1871]. In this paper, we report on the purification to electrophoretical homogeneity of BGN16.1, the second beta-1, 6-glucanase enzyme. BGN16.1 was purified by ammonium sulfate precipitation followed by adsorption and digestion of pustulan (a beta-1,6-glucan), chromatofocusing and gel-filtration chromatography. BGN16.1 is a non-glycosylated protein with an apparent molecular mass of 51 kDa and a basic isoelectric point (pI 7.4-7.7). The enzyme was active toward substrates containing beta-1,6-glycosidic linkages, including yeast cell walls. The Km was 0.8 mg x mL-1 with pustulan as the substrate. Reaction product analysis by HPLC clearly indicated that BGN16.1 has an endo-hydrolytic mode of action. The probable role of this enzyme in the antagonistic action of T. harzianum is also discussed.

Hydrolase and transferase activities of the beta-1,3-exoglucanase of Candida albicans

The exo-beta-1,3-glucanase of Candida albicans (Exg) has a marked specificity for beta-1,3-glucosidic linkages as judged by the kinetic constants for p-nitophenyl beta-glucoside, beta-linked disaccharides of glucose (laminaribiose, gentiobiose, and cellobiose), oligosaccharides of the laminari series, laminarin and pustulan. The kcat/Km ratios for a series of laminari oligosaccharides from -biose to -heptaose showed that Exg has an extended substrate-binding site which contains at least five binding sites for sugar residues. Binding at position +2 (the third sugar residue) increases the kcat twofold while positions +3 and +4 lower the Km value further and thereby increase the catalytic efficiency. Exg catalyses an efficient transglucosylation reaction with high concentrations of laminari-oligosaccharides which specifically form beta-1,3 linkages and with yields up to 50%. The rate of the transglucosylation is concentration-dependent and can be more than 10 times faster than the hydrolytic reaction with excess donor substrates such as laminaritriose and laminarihexaose. The kinetics of Exg and the predicted substrate-binding site for up to five sugar residues are consistent with a recent structural analysis of the enzyme-binding site.

Cloning and characterization of 1,3-beta-glucanase-encoding genes from non-conventional yeasts

The molecular cloning of 1,3-beta-glucanase-encoding genes from different yeast species was achieved by screening genomic libraries with DNA probes obtained by PCR-amplification using oligonucleotides designed according to conserved regions in the EXG1, EXG2 and SSG1 genes from Saccharomyces cerevisiae. The nucleotide sequence of the KlEXG1 (Kluyveromyces lactis), HpEXG1 (Hansenula polymorpha) and SoEXG1 (Schwanniomyces occidentalis) genes was determined. K1EXG1 consists of a 1287 bp open reading frame encoding a protein of 429 amino acids (49,815 Da). HpEXG1 specifies a 435-amino acid polypeptide (49,268 Da) which contains two potential N-glycosylation sites. SoEXG1 encodes a protein of 425 residues (49,132 Da) which contains one potential site for N-linked glycosylation. Expression in S. cerevisiae of KlEXG1, SoEXG1 or HpEXG1 under control of their native promoters resulted in the secretion of active 1,3-beta-glucanases. Disruption of KlEXG1 did not result in a phenotype under laboratory conditions. Comparison of the primary translation products encoded by KlEXG1, HpEXG1 and SoEXG1 with the previously characterized exo-1,3-beta-glucanases from S. cerevisiae and C. albicans reveals that enzymes with this type of specificity constitute a family of highly conserved proteins in yeasts. KlExg1p, HpExg1p and SoExg1p contain the invariant amino acid positions which have been shown to be important in the catalytic function of family 5 glycosyl hydrolases.

The Gas1 glycoprotein, a putative wall polymer cross-linker

The yeast cell wall, which for years has been regarded as a static cellular component, has been revealed to be dynamic in its structure and composition and complex in its enzymatic activity. The S. cerevisiae cell wall is composed of beta-1,3/beta-1,6-glucans, mannoproteins, and chitin, which are assembled into an extracellular matrix essential for maintenance of cell integrity. Gas1p, a glycoprotein anchored to the outer leaflet of the plasma membrane through a glycosylphosphatidylinositol, plays a key role in cell wall assembly. Loss of Gas1p leads to several morphogenetic defects and to a decrease in the amount of cross-links between the cell wall glucans. These defects in turn trigger a compensatory response that guarantees cell viability. Several Gas1p homologs have been isolated from Candida species and S. pombe. The Gas1p family also includes two plant proteins with endo-beta-1,3-glucanase activity. Sequence comparisons reveal that Gas1p family proteins have a modular organization of domains. The genetic and molecular analyses reviewed here suggest that Gas1p could play a role as a polymer cross-linker, presumably by catalyzing a transglycosylation reaction.

Tropomyosin-containing actin cables direct the Myo2p-dependent polarized delivery of secretory vesicles in budding yeast

The actin cytoskeleton in budding yeast consists of cortical patches and cables, both of which polarize toward regions of cell growth. Tropomyosin localizes specifically to actin cables and not cortical patches. Upon shifting cells with conditionally defective tropomyosin to restrictive temperatures, actin cables disappear within 1 min and both the unconventional class V myosin Myo2p and the secretory vesicle-associated Rab GTPase Sec4p depolarize rapidly. Bud growth ceases and the mother cell grows isotropically. When returned to permissive temperatures, tropomyosin-containing cables reform within 1 min in polarized arrays. Cable reassembly permits rapid enrichment of Myo2p at the focus of nascent cables as well as the Myo2p- dependent recruitment of Sec4p and the exocyst protein Sec8p, and the initiation of bud emergence. With the loss of actin cables, cortical patches slowly assume an isotropic distribution within the cell and will repolarize only after restoration of cables. Therefore, actin cables respond to polarity cues independently of the overall distribution of cortical patches and are able to directly target the Myo2p-dependent delivery of secretory vesicles and polarization of growth.

A 1,3-beta-glucanosyltransferase isolated from the cell wall of Aspergillus fumigatus is a homologue of the yeast Bgl2p

A 1,3-beta-glucanosyltransferase which introduces intrachain 1,6-beta linkages into 1,3-beta-glucan was isolated from the cell wall of Aspergillus fumigatus. The biochemical and molecular characterization of the A. fumigatus transferase showed it was homologous to the Saccharomyces cerevisiae and Candida albicans transferase Bgl2p. A null mutant constructed in A. fumigatus by gene replacement did not show a distinct phenotype from the parental strain. The putative function of this major cell-wall-associated protein is discussed.

Cell wall and secreted proteins of Candida albicans: identification, function, and expression

The cell wall is essential to nearly every aspect of the biology and pathogenicity of Candida albicans. Although it was initially considered an almost inert cellular structure that protected the protoplast against osmotic offense, more recent studies have demonstrated that it is a dynamic organelle. The major components of the cell wall are glucan and chitin, which are associated with structural rigidity, and mannoproteins. The protein component, including both mannoprotein and nonmannoproteins, comprises some 40 or more moieties. Wall proteins may differ in their expression, secretion, or topological location within the wall structure. Proteins may be modified by glycosylation (primarily addition of mannose residues), phosphorylation, and ubiquitination. Among the secreted enzymes are those that are postulated to have substrates within the cell wall and those that find substrates in the extracellular environment. Cell wall proteins have been implicated in adhesion to host tissues and ligands. Fibrinogen, complement fragments, and several extracellular matrix components are among the host proteins bound by cell wall proteins. Proteins related to the hsp70 and hsp90 families of conserved stress proteins and some glycolytic enzyme proteins are also found in the cell wall, apparently as bona fide components. In addition, the expression of some proteins is associated with the morphological growth form of the fungus and may play a role in morphogenesis. Finally, surface mannoproteins are strong immunogens that trigger and modulate the host immune response during candidiasis.

Correct targeting of a vacuolar tobacco chitinase in Saccharomyces cerevisiae--post-translational modifications are dependent on the host strain

The chitinase gene FB7-1 of Nicotiana tabacum cv. samsun line 5 was expressed in the two Saccharomyces cerevisiae strains, INVSC2 and H4, under the control of the GAL1 promoter from S. cerevisiae and a multicopy plasmid vector. Both yeast strains express the plant gene as enzymatic active proteins. In transformants of the strain INVSC2, 94% of the total plant chitinase is contained inside the cells, probably within the vacuole which has been confirmed by subcellular fractionation as well as immunohistochemical experiments. This retention inside the cells is due to the C-terminally located 7 amino acids long vacuolar targeting peptide of the prochitinase. When this sequence was removed, chitinase was transported into the culture medium. Pulse-chase experiments revealed that during translation in transformants of both yeast strains one chitinase polypeptide can be immunoadsorbed with specific antibodies. In the case of INVSC2-transformants, newly formed chitinase is modified in a 60 min chase to slightly increase its molecular mass, whereas in H4-transformants the molecular mass constantly remained 32 kDa. By Western blot analysis two chitinase corresponding polypeptides of 32 and 37 kDa were accumulated in the culture medium of both transformants carrying the chitinase gene without the vacuolar targeting sequence. The larger one was very likely O-glycosylated. Whereas, both polypepitdes were also detected in cell extracts of the H4-transformant, only the smaller one was found in the INVSC2-transformant. The plant chitinase passed through the endoplasmic reticulum on its way to the vacuole. The N-terminal signal peptide responsible for the uptake into the endoplasmic reticulum is cleaved correctly. However, cleavage of the vacuolar targeting peptide located at the C-terminus, to give the mature chitinase is obviously influenced by the genetic background of the host strain. In INVSC2-transformants chitinase accumulates in its mature form whereas both the polypeptides of H4-transformants retain their vacuolar targeting peptide. Our results demonstrate that in the case of plant class I chitinase, the plant sorting signal is recognized in yeast cells but post-translational modifications are influenced by the host strain.

Over-expression of the Saccharomyces cerevisiae exo-beta-1,3-glucanase gene together with the Bacillus subtilis endo-beta-1,3-1,4-glucanase gene and the Butyrivibrio fibrisolvens endo-beta-1,4-glucanase gene in yeast

The EXG1 gene encoding the main Saccharomyces cerevisiae exo-beta-1,3-glucanase was cloned and over-expressed in yeast. The Bacillus subtilis endo-1,3-1,4-beta-glucanase gene (beg1) and the Butyrivibrio fibrisolvens endo-beta-1,4-glucanase gene (end1) were fused to the secretion signal sequence of the yeast mating pheromone alpha-factor (MF alpha 1S) and inserted between the yeast alcohol dehydrogenase II gene promoter (ADH2P) and terminator (ADH2T). Constructs ADH2P-MF alpha 1S-beg1-ADH2T and ADH2P-MF alpha 1S-end 1-ADH2T designated BEG1 and END1, respectively, were expressed separately and jointly with EXG1 in S. cerevisiae. The construction of fur 1 ura3 S. cerevisiae strains allowed for the autoselection of these multicopy URA3-based plasmids in rich medium. Enzyme assays confirmed that co-expression of EXG1, BEG1 and END1 enhanced glucan degradation by S. cerevisiae.

Phenotype in Candida albicans of a disruption of the BGL2 gene encoding a 1,3-beta-glucosyltransferase

The BGL2 gene encodes a unique 1,3-beta-glucosyltransferase (Bgl2p) present in the cell wall of Candida albicans and other fungi. Although believed to be involved in cell wall assembly, disruption of the gene in saccharomyces cerevisiae showed no apparent phenotype. We performed sequential disruptions of the BGL2 loci in a homozygous ura3 clinical isolate of C. albicans using the URA3 blaster method, in order to investigate the role of Bgl2p in this dimorphic, pathogenic fungus. Strain CACW-1 contained disruptions of both homologues of the BGL2 gene and lacked Bgl2p, as assessed by protein extraction, SDS-PAGE and Western blot analysis, and enzyme assay; however, residual non-Bgl2p transferase activity was detected. CACW-1 was attenuated in virulence for mice when compared to an isogenic parent strain, and fewer organisms were recovered from the kidneys of infected animals. Additional phenotypic changes included: (1) a dramatic increase in the sensitivity to the chitin synthesis inhibitor nikkomycin Z when CACW-1 cells were incubated at 37 or 42 degrees C; (2) an 8.7 +/- 1.6% slower growth rate at 37 degrees C for CACW-1 when compared to its isogenic parent; and (3) aggregation of CACW-1 cells during stationary phase and/or incubation of stationary phase cells in phosphate buffer. Characterization of SDS-extracted cell walls did not reveal any significant differences in the levels of 1,3-beta- or 1,6-beta-glucan. These data reveal that loss of Bgl2p does have a phenotype in C. albicans, and indicate that (1) loss of Bgl2p function renders cells more dependent on chitin for wall integrity, and attenuates virulence (probably due to subtle changes in wall structure), and (2) that additional 1,3-beta-glucosyltransferases are present in the C. albicans BGL2 disruptant.

A novel beta-(1-3)-glucanosyltransferase from the cell wall of Aspergillus fumigatus

Cell wall transferases utilizing beta-(1-3)-glucan chains as substrates may play important roles in cell wall assembly and rearrangement, as beta-(1-3)-glucan is a major structural component of the cell wall of many fungi. A novel beta-(1-3)-glucanosyltransferase was purified to apparent homogenei ty from an autolysate of the cell wall of Aspergillus fumigatus. The enzyme had a molecular mass of 49 kDa and contained approximately 5 kDa of N-linked carbohydrate. The enzyme catalyzed an initial endo-type splitting of a beta-(1-3)-glucan molecule, followed by linkage of the newly generated reducing end to the nonreducing end of another beta-(1-3)-glucan molecule. Laminarioligosaccharides of size G10 and greater were donor substrates for the transferase. Laminarioligosaccharides of size G5 and greater formed acceptors. The enzyme was able to reuse initial transferase products as donors and acceptors in extended incubations, resulting in the formation of increasingly larger transferase products until they became insoluble. The major initial products from an incubation of the transferase with borohydride-reduced G11 (rG11) were rG6 and rG16. 1H NMR analysis of the rG16 transferase product showed it was a laminarioligosaccharide, indicating that the enzyme forms a beta-(1-3)-linkage during transfer. The enzyme may have a key function in vivo by allowing the integration of newly synthesized glucan into the wall and promoting cell wall expansion during cell growth.

Beta-glucan synthesis in Bradyrhizobium japonicum: characterization of a new locus (ndvC) influencing beta-(1-->6) linkages

Bradyrhizobium japonicum synthesizes periplasmic cyclic beta-(1-->3),beta-(1-->6)-D-glucans during growth in hypoosmotic environments, and evidence is growing that these molecules may have a specific function during plant-microbe interactions in addition to osmoregulation. Site-directed Tn5 mutagenesis of the DNA region upstream of ndvB resulted in identification of a new gene (ndvC) involved in beta-(1--> 3), beta-(1-->6)-glucan synthesis and in nodule development. The predicted translation product was a polypeptide (ca. 62 kDa) with several transmembrane domains. It contained a sequence characteristic of a conserved nucleoside-sugar-binding motif found in many bacterial enzymes and had 51% similarity with a beta-glucanosyltransferase from Candida albicans. B. japonicum carrying a Tn5 insertion in ndvC resulted in synthesis of altered cyclic beta-glucans composed almost entirely of beta-(1--> 3)-glycosyl linkages. The mutant strain was only slightly sensitive to hypoosmotic growth conditions compared with the ndvB mutant, but it was severely impaired in symbiotic interactions with soybean (Glycine max). Nodulation was delayed by 8 to 10 days, and many small nodule-like structures apparently devoid of viable bacteria were formed. This finding suggests that the structure of the beta-glucan molecule is important for a successful symbiotic interaction, and beta-glucans may have a specific function in addition to their role in hypoosmotic adaptation.

A novel endo-beta-1,3-glucanase, BGN13.1, involved in the mycoparasitism of Trichoderma harzianum

The mycoparasitic fungus Trichoderma harzianum CECT 2413 produces at least three extracellular beta-1,3-glucanases. The most basic of these extracellular enzymes, named BGN13.1, was expressed when either fungal cell wall polymers or autoclaved mycelia from different fungi were used as the carbon source. BGN13.1 was purified to electrophoretic homogeneity and was biochemically characterized. The enzyme was specific for beta-1,3 linkages and has an endolytic mode of action. A synthetic oligonucleotide primer based on the sequence of an internal peptide was designed to clone the cDNA corresponding to BGN13.1. The deduced amino acid sequence predicted a molecular mass of 78 kDa for the mature protein. Analysis of the amino acid sequence indicates that the enzyme contains three regions, one N-terminal leader sequence; another, nondefined sequence; and one cysteine-rich C-terminal sequence. Sequence comparison shows that this beta-1,3-glucanase, first described for filamentous fungi, belongs to a family different from that of its previously described bacterial, yeast, and plant counterparts. Enzymatic-activity, protein, and mRNA data indicated that bgn13.1 is repressed by glucose and induced by either fungal cell wall polymers or autoclaved yeast cells and mycelia. Finally, experimental evidence showed that the enzyme hydrolyzes yeast and fungal cell walls.

Parallel secretory pathways to the cell surface in yeast

Saccharomyces cerevisiae mutants that have a post-Golgi block in the exocytic pathway accumulate 100-nm vesicles carrying secretory enzymes as well as plasma membrane and cell-wall components. We have separated the vesicle markers into two groups by equilibrium isodensity centrifugation. The major population of vesicles contains Bg12p, an endoglucanase destined to be a cell-wall component, as well as Pma1p, the major plasma membrane ATPase. In addition, Snc1p, a synaptobrevin homologue, copurifies with these vesicles. Another vesicle population contains the periplasmic enzymes invertase and acid phosphatase. Both vesicle populations also contain exoglucanase activity; the major exoglucanase normally secreted from the cell, encoded by EXG1, is carried in the population containing periplasmic enzymes. Electron microscopy shows that both vesicle groups have an average diameter of 100 nm. The late secretory mutants sec1, sec4, and sec6 accumulate both vesicle populations, while neither is detected in wild-type cells, early sec mutants, or a sec13 sec6 double mutant. Moreover, a block in endocytosis does not prevent the accumulation of either vesicle species in an end4 sec6 double mutant, further indicating that both populations are of exocytic origin. The accumulation of two populations of late secretory vesicles indicates the existence of two parallel routes from the Golgi to the plasma membrane.

Changes in cell wall composition of deformed ras1- cells of Schizosaccharomyces pombe

Disruption of the Schizosaccharomyces pombe ras1 gene results in a morphological transformation to large spheres, in contrast to wild-type cells which grow as rods. Chemical analysis of isolated cell walls showed no significant changes in saccharide content but an increase in protein and phosphate contents in ras1- walls relative to parent walls. Polymers tightly bound to the cell wall were solubilized by SDS treatment. Several compounds with molar mass ranging from 22 to 130 kDa and more were resolved by gel filtration and SDS-PAGE. Among low-molar-mass species, a component moving as a band at 31 kDa was conspicuous in ras1- cell walls. It was solubilized by heating in Tris-HCl buffer and shown to have a beta-1,3-glucanase activity against laminarin. The level of the enzyme was by 30% higher in the ras1- cell wall than in the wild-type cell wall. This enzyme may participate in the remodelling of the rigid glucan network and account (at least partially) for the aberrant cell shape. The ras1- cell wall contained a high level of charged polymers, especially phosphoproteins, raising the appealing possibility that ras1- is involved in a putative kinase cascade required to sense and respond to external stimuli destined for the cell wall. Although the present study shows that ras1 loss of function and altered cell wall composition are closely linked defects, it has still to be shown that the ras1 protein is directly involved in alterations found in the mutant cell walls.

Molecular basis of cell integrity and morphogenesis in Saccharomyces cerevisiae

In fungi and many other organisms, a thick outer cell wall is responsible for determining the shape of the cell and for maintaining its integrity. The budding yeast Saccharomyces cerevisiae has been a useful model organism for the study of cell wall synthesis, and over the past few decades, many aspects of the composition, structure, and enzymology of the cell wall have been elucidated. The cell wall of budding yeasts is a complex and dynamic structure; its arrangement alters as the cell grows, and its composition changes in response to different environmental conditions and at different times during the yeast life cycle. In the past few years, we have witnessed a profilic genetic and molecular characterization of some key aspects of cell wall polymer synthesis and hydrolysis in the budding yeast. Furthermore, this organism has been the target of numerous recent studies on the topic of morphogenesis, which have had an enormous impact on our understanding of the intracellular events that participate in directed cell wall synthesis. A number of components that direct polarized secretion, including those involved in assembly and organization of the actin cytoskeleton, secretory pathways, and a series of novel signal transduction systems and regulatory components have been identified. Analysis of these different components has suggested pathways by which polarized secretion is directed and controlled. Our aim is to offer an overall view of the current understanding of cell wall dynamics and of the complex network that controls polarized growth at particular stages of the budding yeast cell cycle and life cycle.

Regulation of cell wall beta-glucan assembly: PTC1 negatively affects PBS2 action in a pathway that includes modulation of EXG1 transcription

Analysis of genes involved in yeast cell wall beta-glucan assembly has led to the isolation of EXG1, PBS2 and PTC1. EXG1 and PBS2 were isolated as genes that, when expressed from multicopy plasmids, led to a dominant killer toxin-resistant phenotype. The PTC1 gene was cloned by functional complementation of the calcofluor white-hypersensitive mutant cwh47-1. PTC1/CWH47 is the structural gene for a type 2C serine/threonine phosphatase, EXG1 codes for an exo-beta-glucanase, and PBS2 encodes a MAP kinase kinase in the Pbs2p-Hog1p signal transduction pathway. Overexpression of EXG1 on a 2 mu plasmid led to reduction in a cell wall beta 1,6-glucan and caused killer resistance in wild type cells; while the exg1 delta mutant displayed modest increases in killer sensitivity and beta 1,6-glucan levels. Disruption of PTC1/CWH47 and overexpression of PBS2 gave rise to similar beta-glucan related phenotypes, with higher levels of EXG1 transcription, increased exo-beta-glucanase activity, reduced beta 1,6-glucan levels, and resistance to killer toxin. Genetic analysis revealed that loss of function of the PBS2 gene was epistatic to PTC1/CWH47 disruption, indicating a functional role for the Ptc1p/Cwh47p phosphatase in the Pbs2p-Hog1p signal transduction pathway. These results suggest that Ptc1p/Cwh47p and Pbs2p play opposing regulatory roles in cell wall glucan assembly, and that this is effected in part by modulating Exg1p activity.

Kinetics of beta-1,3 glucan interaction at the donor and acceptor sites of the fungal glucosyltransferase encoded by the BGL2 gene

Formation of branched glucan, glucan-glucan cross links, and glucan-chitin cross links most likely involves the action of fungal wall glucanases and transglycosylases. We developed an HPLC assay using radiolabeled substrates in order to study the kinetics of interaction of donor and acceptor molecules with a glucosyltransferase present in the cell walls of both Saccharomyces cerevisiae and Candida albicans. Purified transferase first forms an activated intermediate from a donor beta-1,3 glucan, releasing free disaccharide. The activated intermediate is transferred, in the presence of an appropriate acceptor beta-1,3 glucan, yielding a linear glucan containing a beta-1,6 linkage at the transfer site [Yu, L., Goldman, R., Sullivan, P., Walker, G. & Fesik, S. W. (1993) J. Biomol. NMR 3, 429-441]. An apparent Km of 0.41 mM for the acceptor site was determined using laminaritetraose as the acceptor. An apparent Km of 31 mM for the donor site was determined using increasing concentrations of laminaripentaose, and monitoring formation of laminaribiose. The enzyme functioned as a glucanase at low concentrations of acceptor molecules, with excess H2O competing for reaction at the activated donor site, thus resulting in hydrolysis. However, as the concentration of acceptor increased, the reaction shifted from hydrolysis to glucosyltransfer. The reaction appeared specific for beta-1,3 glucan as acceptor, in as much as no transfer was detected when either hexa-N-acetyl-chitohexaose or maltooligosaccharides were used as acceptors. The roles of such an enzymic activity in cell wall metabolism is discussed in terms of repair, cross linking and incorporation of newly synthesized chains of beta-1,3 glucan into the previously existing cell wall structure.

Another brick in the wall? Recent developments concerning the yeast cell envelope

To a yeast, the cell wall is an important living organelle performing a number of vital functions, including osmotic and physical protection, selective permeability barrier, immobilized enzyme support and cell-cell recognition and adhesion. Our basic model of wall structure involves attachment of secreted mannoproteins to a fibrillar inner layer of beta-glucan. Recent work has emphasised the importance of chitin in lateral walls, examined the mechanisms of attachment of mannoproteins to the various cell wall glucan fractions and elucidated the pathway of beta-glucan synthesis, by means of resistance to glucan-binding killer toxins. The conventional view of wall structure has been challenged by the discovery of a class of GPI-anchored, serine/threonine-rich wall-proteins. It has been suggested, that these proteins are anchored in the plasma membrane, spanning the wall with extended O-glycosylated structures and protruding out into the medium. Examination of these proteins shows a diversity of structures, sizes and behaviour that makes it improbable that these represent a new class of wall proteins. The possible roles of one of these proteins associated with flocculation, Flo1p, are discussed.

Expression of the Butyrivibrio fibrisolvens endo-beta-1,4-glucanase gene together with the Erwinia pectate lyase and polygalacturonase genes in Saccharomyces cerevisiae

Recombinant Saccharomyces cerevisiae strains capable of simultaneous secretion of bacterial glucanase and pectinase enzymes have been developed. The Butyrivibrio fibrrisolvens endo-beta-1,4-glucanase gene (end1), the Erwinia chrysanthemi pectate lyase gene (pelE) and E. carotovora polygalacturonase gene (peh1) were each inserted between a yeast expression-secretion cassette and yeast gene terminator, and cloned into yeast-centromeric shuttle vectors. Transcription initiation signals present in the expression-secretion cassette were derived from the yeast alcohol dehydrogenase gene promoter (ADC1P), whereas the transcription termination signals were derived from the yeast tryptophan synthase gene terminator (TRP5T). Secretion of glucanase and pectinases was directed by the signal sequence of the yeast mating pheromone alpha-factor (MF alpha 1S). These YCplac111-based constructs, designated END1, PEL5, AND PEH1, respectively, were transformed into S. cerevisiae. The END1, PEL5 and PEH1 constructs were co-expressed in laboratory strains of S. cerevisiae as well as in wine and distillers' yeasts. DNA-RNA hybridization analysis showed the presence of END1, PEL5 and PEH1 transcripts. Carboxymethylcellulose and polypectate agarose assays revealed the production of biologically active endo-beta-1,4-glucanase, pectate lyase and polygalacturonase by the S. cerevisiae transformants. Interestingly, although the same expression-secretion cassette was used in all three constructs, time-course assays indicated that the pectinases were secreted before the glucanase. It is tempting to speculate that the bulkiness of the END1-encoded protein and the five alternating repeats of Pro-Asp-Pro-Thr(Gln)-Pro-Val-Asp within the glucanase moiety could be involved in the delayed secretion of the glucanase.