Isolation of chitin synthetase from Saccharomyces cerevisiae. Purification of an enzyme by entrapment in the reaction product

Novel Insights into the Wattle and Daub Model of Entamoeba Cyst Wall Formation and the Importance of Actin Cytoskeleton

The "Wattle and Daub" model of cyst wall formation in Entamoeba invadens has been used to explain encystment in Entamoeba histolytica, the causal agent of amoebiasis, and this process could be a potential target for new antiamoebic drugs. In this study, we studied the morphological stages of chitin wall formation in E. invadens in more detail using fluorescent chitin-binding dyes and the immunolocalization of cyst wall proteins. It was found that chitin deposition was mainly initiated on the cell surface at a specific point or at different points at the same time. The cystic wall grew outward and gradually covered the entire surface of the cyst over time, following the model of Wattle and Daub. The onset of chitin deposition was guided by the localization of chitin synthase 1 to the plasma membrane, occurring on the basis of the Jacob lectin in the cell membrane. During encystation, F-actin was reorganized into the cortical region within the early stages of encystation and remained intact until the completion of the chitin wall. The disruption of actin polymerization in the cortical region inhibited proper wall formation, producing wall-less cysts or cysts with defective chitin walls, indicating the importance of the cortical actin cytoskeleton for proper cyst wall formation.

Recent insights into glucans biosynthesis and engineering strategies in edible fungi

Fungal α/β-glucans have significant importance in cellular functions including cell wall structure, host-pathogen interactions and energy storage, and wide application in high-profile fields, including food, nutrition, and pharmaceuticals. Fungal species and their growth/developmental stages result in a diversity of glucan contents, structures and bioactivities. Substantial progresses have been made to elucidate the fine structures and functions, and reveal the potential molecular synthesis pathway of fungal α/β-glucans. Herein, we review the current knowledge about the biosynthetic machineries, including: precursor UDP-glucose synthesis, initiation, elongation/termination and remodeling of α/β-glucan chains, and molecular regulation to maximally produce glucans in edible fungi. This review would provide future perspectives to biosynthesize the targeted glucans and reveal the catalytic mechanism of enzymes associated with glucan synthesis, including: UDP-glucose pyrophosphate phosphorylases (UGP), glucan synthases, and glucanosyltransferases in edible fungi.

Plant cellulose synthase membrane protein isolation directly from Pichia pastoris protoplasts, liposome reconstitution, and its enzymatic characterization

Cellulose is synthesized by a plant cell membrane-integrated processive glycosyltransferase (GT) called cellulose synthase (CesA). Since only a few of these plant CesAs have been purified and characterized to date, there are huge gaps in our mechanistic understanding of these enzymes. The biochemistry and structural biology studies of CesAs are currently hampered by challenges associated with their expression and extraction at high yields. To aid in understanding CesA reaction mechanisms and to provide a more efficient CesA extraction method, two putative plant CesAs - PpCesA5 from Physcomitrella patens and PttCesA8 from Populus tremula x tremuloides that are involved in primary and secondary cell wall formation in plants were expressed using Pichia pastoris as an expression host. We developed a protoplast-based membrane protein extraction approach to directly isolate these membrane-bound enzymes, as confirmed by immunoblotting and mass spectrometry-based analyses. Our method gives 3-4-fold higher purified protein yield than the standard cell homogenization protocol. Our method resulted in liposome reconstituted CesA5 and CesA8 enzymes with similar Michaelis-Menten kinetic constants, K_m = 167 μM, 108 μM and V_max = 7.88 × 10^-5 μmol/min, 4.31 × 10^-5 μmol/min, respectively, in concurrence with the previous studies for enzymes isolated using the standard protocol. Taken together, these results suggest that CesAs involved in primary and secondary cell wall formation can be expressed and purified using a simple and more efficient extraction method. This protocol could help isolate enzymes that unravel the mechanism of native and engineered cellulose synthase complexes involved in plant cell wall biosynthesis.

Recent Advances in Chitin Biosynthesis Associated with the Morphology and Secondary Metabolite Synthesis of Filamentous Fungi in Submerged Fermentation

Metabolites produced by filamentous fungi are used extensively in the food and drug industries. With the development of the morphological engineering of filamentous fungi, numerous biotechnologies have been applied to alter the morphology of fungal mycelia and enhance the yields and productivity of target metabolites during submerged fermentation. Disruption of chitin biosynthesis can modify the cell growth and mycelial morphology of filamentous fungi and regulate the biosynthesis of metabolites during submerged fermentation. In this review, we present a comprehensive coverage of the categories and structures of the enzyme chitin synthase, chitin biosynthetic pathways, and the association between chitin biosynthesis and cell growth and metabolism in filamentous fungi. Through this review, we hope to increase awareness of the metabolic engineering of filamentous fungal morphology, provide insights into the molecular mechanisms of morphological control via chitin biosynthesis, and describe strategies for the application of morphological engineering to enhance the production of target metabolites in filamentous fungi during submerged fermentation.

Structural basis for inhibition and regulation of a chitin synthase from Candida albicans

Chitin is an essential component of the fungal cell wall. Chitin synthases (Chss) catalyze chitin formation and translocation across the membrane and are targets of antifungal agents, including nikkomycin Z and polyoxin D. Lack of structural insights into the action of these inhibitors on Chs has hampered their further development to the clinic. We present the cryo-EM structures of Chs2 from Candida albicans (CaChs2) in the apo, substrate-bound, nikkomycin Z-bound, and polyoxin D-bound states. CaChs2 adopts a unique domain-swapped dimer configuration where a conserved motif in the domain-swapped region controls enzyme activity. CaChs2 has a dual regulation mechanism where the chitin translocation tunnel is closed by the extracellular gate and plugged by a lipid molecule in the apo state to prevent non-specific leak. Analyses of substrate and inhibitor binding provide insights into the chemical logic of Chs inhibition, which can guide Chs-targeted antifungal development.

Functional characteristics of Svl3 and Pam1 that are required for proper cell wall formation in yeast cells

In the budding yeast Saccharomyces cerevisiae, Svl3 and Pam1 proteins work as functional homologues. Loss of their function causes increased levels of chitin deposition in the cell wall and temperature sensitivity, suggesting their involvement in cell wall formation. We found that the N- and C-termini of these proteins have distinctive and critical functions. They contain an N-terminal part that has a probable 2-dehydropantoate 2-reductase domain. In Svl3, this part can be replaced with the yeast 2-dehydropantoate 2-reductase, Pan5, suggesting that Svl3 and its homologues may be able to mediate 2-dehydropantoate 2-reductase function. On the other hand, Svl3 is recruited to the bud tip and bud neck via multiple localization signals in the C-terminal part. One of such signals is the lysine-rich region located in the C-terminal end. The function and localization of Svl3 are significantly disrupted by the loss of this lysine-rich region; however, its localization is not completely abolished by the mutation because another localization signal enables appropriate transport. Svl3 and Pam1 orthologues are found in cells across fungal species. The Svl3 orthologues of Candida glabrata can complement the loss of Svl3 and Pam1 in S. cerevisiae. C. glabrata cells lacking the SVL3 and PAM1 orthologue genes exhibit phenotypes similar to those observed in svl3∆pam1∆ S. cerevisiae cells. Thus, Svl3 homologues may be generally required for the assembly of the cell wall in fungal cells.

Phosphoric Metabolites Link Phosphate Import and Polysaccharide Biosynthesis for Candida albicans Cell Wall Maintenance

Candida species cause hundreds of thousands of invasive infections with high mortality each year. Developing novel antifungal agents is challenging due to the many similarities between fungal and human cells. Maintaining phosphate balance is essential for all organisms but is achieved completely differently by fungi and humans. A protein that imports phosphate into fungal cells, Pho84, is not present in humans and is required for normal cell wall stress resistance and cell wall integrity signaling in C. albicans. Nucleotide sugars, which are phosphate-containing building block molecules for construction of the cell wall, are diminished in cells lacking Pho84. Cell wall-constructing enzymes may be slowed by lack of these building blocks, in addition to being inhibited by drugs. Combined targeting of Pho84 and cell wall-constructing enzymes may provide a strategy for antifungal therapy by which two sequential steps of cell wall maintenance are blocked for greater potency.

The Candida albicans high-affinity phosphate transporter Pho84 is required for normal Target of Rapamycin (TOR) signaling, oxidative stress resistance, and virulence of this fungal pathogen. It also contributes to C. albicans’ tolerance of two antifungal drug classes, polyenes and echinocandins. Echinocandins inhibit biosynthesis of a major cell wall component, beta-1,3-glucan. Cells lacking Pho84 were hypersensitive to other forms of cell wall stress beyond echinocandin exposure, while their cell wall integrity signaling response was weak. Metabolomics experiments showed that levels of phosphoric intermediates, including nucleotides like ATP and nucleotide sugars, were low in pho84 mutant compared to wild-type cells recovering from phosphate starvation. Nonphosphoric precursors like nucleobases and nucleosides were elevated. Outer cell wall phosphomannan biosynthesis requires a nucleotide sugar, GDP-mannose. The nucleotide sugar UDP-glucose is the substrate of enzymes that synthesize two major structural cell wall polysaccharides, beta-1,3- and beta-1,6-glucan. Another nucleotide sugar, UDP-N-acetylglucosamine, is the substrate of chitin synthases which produce a stabilizing component of the intercellular septum and of lateral cell walls. Lack of Pho84 activity, and phosphate starvation, potentiated pharmacological or genetic perturbation of these enzymes. We posit that low substrate concentrations of beta-d-glucan- and chitin synthases, together with pharmacologic inhibition of their activity, diminish enzymatic reaction rates as well as the yield of their cell wall-stabilizing products. Phosphate import is not conserved between fungal and human cells, and humans do not synthesize beta-d-glucans or chitin. Hence, inhibiting these processes simultaneously could yield potent antifungal effects with low toxicity to humans.

Revisiting Old Questions and New Approaches to Investigate the Fungal Cell Wall Construction

The beginning of our understanding of the cell wall construction came from the work of talented biochemists in the 70-80's. Then came the era of sequencing. Paradoxically, the accumulation of fungal genomes complicated rather than solved the mystery of cell wall construction, by revealing the involvement of a much higher number of proteins than originally thought. The situation has become even more complicated since it is now recognized that the cell wall is an organelle whose composition continuously evolves with the changes in the environment or with the age of the fungal cell. The use of new and sophisticated technologies to observe cell wall construction at an almost atomic scale should improve our knowledge of the cell wall construction. This essay will present some of the major and still unresolved questions to understand the fungal cell wall biosynthesis and some of these exciting futurist approaches.

Rab GDP-dissociation inhibitor gdiA is an essential gene required for cell wall chitin deposition in Aspergillus niger

The cell wall is a distinctive feature of filamentous fungi, providing them with structural integrity and protection from both biotic and abiotic factors. Unlike plant cell walls, fungi rely on structurally strong hydrophobic chitin core for mechanical strength together with alpha- and beta-glucans, galactomannans and glycoproteins. Cell wall stress conditions are known to alter the cell wall through the signaling cascade of the cell wall integrity (CWI) pathway and can result in increased cell wall chitin deposition. A previously isolated set of Aspergillus niger cell wall mutants was screened for increased cell wall chitin deposition. UV-mutant RD15.8#16 was found to contain approximately 60% more cell wall chitin than the wild type. In addition to the chitin phenotype, RD15.8#16 exhibits a compact colony morphology and increased sensitivity towards SDS. RD15.8#16 was subjected to classical genetic approach for identification of the underlying causative mutation, using co-segregation analysis and SNP genotyping. Genome sequencing of RD15.8#16 revealed eight SNPs in open reading frames (ORF) which were individually checked for co-segregation with the associated phenotypes, and showed the potential relevance of two genes located on chromosome IV. In situ re-creation of these ORF-located SNPs in a wild type background, using CRISPR/Cas9 genome editing, showed the importance Rab GTPase dissociation inhibitor A (gdiA) for the phenotypes of RD15.8#16. An alteration in the 5' donor splice site of gdiA reduced pre-mRNA splicing efficiency, causing aberrant cell wall assembly and increased chitin levels, whereas gene disruption attempts showed that a full gene deletion of gdiA is lethal.

Priming and elongation of chitin chains: Implications for chitin synthase mechanism

Most fungi have multiple chitin synthases (CSs) that may make chitin at different sites on the cell surface, at different times during growth, and in response to cell wall stress. The structure-based model for CS function is for transfer of GlcNAc from UDP-GlcNAc at the cytoplasmic face of the plasma membrane to the non-reducing end of a growing chitin chain, which is concomitantly translocated through a transmembrane channel formed by the synthase. Two aspects of CS mechanism are investigated: how chains might be initiated, and what governs how long they can get. First, it is shown that CSs incorporate free GlcNAc into di-N-acetylchitobiose and into insoluble chitin in a UDP-GlcNAc-dependent manner, and therefore that GlcNAc primes chitin synthesis. Second, average lengths of insoluble chitin chains were measured by determining the molar ratio of priming GlcNAc to chain-extending, UDP-GlcNAc-derived GlcNAc, and showed dependence on UDP-GlcNAc concentration, approaching a maximum at higher concentrations of substrate. These results, together with previous findings that 2-acylamido GlcN analogues prime formation of chitin oligosaccharides and stimulate chitin synthesis are discussed in the context of the structure-based model, and lead to the following proposals. 1) CSs may "self-prime" by hydrolyzing UDP-GlcNAc to yield GlcNAc. 2) A CS's active site is not continuously occupied by a nascent chitin chain, rather, CSs can release chitin chains, then re-initiate, and therefore synthesize chitin chains in bursts. 3) The length of chitin chains made by a given CS will impact that CS's contribution to construction of the fungal cell wall.

Folding, Assembly, and Persistence: The Essential Nature and Origins of Biopolymers

Life as we know it requires three basic types of polymers: polypeptide, polynucleotide, and polysaccharide. Here we evaluate both universal and idiosyncratic characteristics of these biopolymers. We incorporate this information into a model that explains much about their origins, selection, and early evolution. We observe that all three biopolymer types are pre-organized, conditionally self-complementary, chemically unstable in aqueous media yet persistent because of kinetic trapping, with chiral monomers and directional chains. All three biopolymers are synthesized by dehydration reactions that are catalyzed by molecular motors driven by hydrolysis of phosphorylated nucleosides. All three biopolymers can access specific states that protect against hydrolysis. These protected states are folded, using self-complementary interactions among recurrent folding elements within a given biopolymer, or assembled, in associations between the same or different biopolymer types. Self-association in a hydrolytic environment achieves self-preservation. Heterogeneous association achieves partner-preservation. These universal properties support a model in which life's polymers emerged simultaneously and co-evolved in a common hydrolytic milieu where molecular persistence depended on folding and assembly. We believe that an understanding of the structure, function, and origins of any given type of biopolymer requires the context of other biopolymers.

Heterologous expression of an active chitin synthase from Rhizopus oryzae

Chitin synthases are highly important enzymes in nature, where they synthesize structural components in species belonging to different eukaryotic kingdoms, including kingdom Fungi. Unfortunately, their structure and the molecular mechanism of synthesis of their microfibrilar product remain largely unknown, probably because no fungal active chitin synthases have been isolated, possibly due to their extreme hydrophobicity. In this study we have turned to the heterologous expression of the transcript from a small chitin synthase of Rhizopus oryzae (RO3G_00942, Chs1) in Escherichia coli. The enzyme was active, but accumulated mostly in inclusion bodies. High concentrations of arginine or urea solubilized the enzyme, but their dilution led to its denaturation and precipitation. Nevertheless, use of urea permitted the purification of small amounts of the enzyme. The properties of Chs1 (Km, optimum temperature and pH, effect of GlcNAc) were abnormal, probably because it lacks the hydrophobic transmembrane regions characteristic of chitin synthases. The product of the enzyme showed that, contrasting with chitin made by membrane-bound Chs's and chitosomes, was only partially in the form of short microfibrils of low crystallinity. This approach may lead to future developments to obtain active chitin synthases that permit understanding their molecular mechanism of activity, and microfibril assembly.

A single heterologously expressed plant cellulose synthase isoform is sufficient for cellulose microfibril formation in vitro

Plant cell walls are a composite material of polysaccharides, proteins, and other noncarbohydrate polymers. In the majority of plant tissues, the most abundant polysaccharide is cellulose, a linear polymer of glucose molecules. As the load-bearing component of the cell wall, individual cellulose chains are frequently bundled into micro and macrofibrils and are wrapped around the cell. Cellulose is synthesized by membrane-integrated and processive glycosyltransferases that polymerize UDP-activated glucose and secrete the nascent polymer through a channel formed by their own transmembrane regions. Plants express several different cellulose synthase isoforms during primary and secondary cell wall formation; however, so far, none has been functionally reconstituted in vitro for detailed biochemical analyses. Here we report the heterologous expression, purification, and functional reconstitution of Populus tremula x tremuloides CesA8 (PttCesA8), implicated in secondary cell wall formation. The recombinant enzyme polymerizes UDP-activated glucose to cellulose, as determined by enzyme degradation, permethylation glycosyl linkage analysis, electron microscopy, and mutagenesis studies. Catalytic activity is dependent on the presence of a lipid bilayer environment and divalent manganese cations. Further, electron microscopy analyses reveal that PttCesA8 produces cellulose fibers several micrometers long that occasionally are capped by globular particles, likely representing PttCesA8 complexes. Deletion of the enzyme's N-terminal RING-finger domain almost completely abolishes fiber formation but not cellulose biosynthetic activity. Our results demonstrate that reconstituted PttCesA8 is not only sufficient for cellulose biosynthesis in vitro but also suffices to bundle individual glucan chains into cellulose microfibrils.

AcsA-AcsB: The core of the cellulose synthase complex from Gluconacetobacter hansenii ATCC23769

The gram-negative bacterium, Gluconacetobacter hansenii, produces cellulose of exceptionally high crystallinity in comparison to the cellulose of higher plants. This bacterial cellulose is synthesized and extruded into the extracellular medium by the cellulose synthase complex (CSC). The catalytic component of this complex is encoded by the gene AcsAB. However, several other genes are known to encode proteins critical to cellulose synthesis and are likely components of the bacterial CSC. We have purified an active heterodimer AcsA-AcsB from G. hansenii ATCC23769 to homogeneity by two different methods. With the purified protein, we have determined how it is post-translationally processed, forming the active heterodimer AcsA-AcsB. Additionally, we have performed steady-state kinetic studies on the AcsA-AcsB complex. Finally through mutagenesis studies, we have explored the roles of the postulated CSC proteins AcsC, AcsD, and CcpAx.

A molecular description of cellulose biosynthesis

Cellulose is the most abundant biopolymer on Earth, and certain organisms from bacteria to plants and animals synthesize cellulose as an extracellular polymer for various biological functions. Humans have used cellulose for millennia as a material and an energy source, and the advent of a lignocellulosic fuel industry will elevate it to the primary carbon source for the burgeoning renewable energy sector. Despite the biological and societal importance of cellulose, the molecular mechanism by which it is synthesized is now only beginning to emerge. On the basis of recent advances in structural and molecular biology on bacterial cellulose synthases, we review emerging concepts of how the enzymes polymerize glucose molecules, how the nascent polymer is transported across the plasma membrane, and how bacterial cellulose biosynthesis is regulated during biofilm formation. Additionally, we review evolutionary commonalities and differences between cellulose synthases that modulate the nature of the cellulose product formed.

2-Acylamido analogues of N-acetylglucosamine prime formation of chitin oligosaccharides by yeast chitin synthase 2

Chitin, a homopolymer of β1,4-linked N-acetylglucosamine (GlcNAc) residues, is a key component of the cell walls of fungi and the exoskeletons of arthropods. Chitin synthases transfer GlcNAc from UDP-GlcNAc to preexisting chitin chains in reactions that are typically stimulated by free GlcNAc. The effect of GlcNAc was probed by using a yeast strain expressing a single chitin synthase, Chs2, by examining formation of chitin oligosaccharides (COs) and insoluble chitin, and by replacing GlcNAc with 2-acylamido analogues of GlcNAc. Synthesis of COs was strongly dependent on inclusion of GlcNAc in chitin synthase incubations, and N,N'-diacetylchitobiose (GlcNAc2) was the major reaction product. Formation of both COs and insoluble chitin was also stimulated by GlcNAc2 and by N-propanoyl-, N-butanoyl-, and N-glycolylglucosamine. MALDI analyses of the COs made in the presence of 2-acylamido analogues of GlcNAc showed they that contained a single GlcNAc analogue and one or more additional GlcNAc residues. These results indicate that Chs2 can use certain 2-acylamido analogues of GlcNAc, and likely free GlcNAc and GlcNAc2 as well, as GlcNAc acceptors in a UDP-GlcNAc-dependent glycosyltransfer reaction. Further, formation of modified disaccharides indicates that CSs can transfer single GlcNAc residues.

Biochemical characterization of chitin synthase activity and inhibition in the African malaria mosquito, Anopheles gambiae

Chitin synthase (CHS) is an important enzyme catalyzing the formation of chitin polymers in all chitin containing organisms and a potential target site for insect pest control. However, our understanding of biochemical properties of insect CHSs has been very limited. We here report enzymatic and inhibitory properties of CHS prepared from the African malaria mosquito, Anopheles gambiae. Our study, which represents the first time to use a nonradioactive method to assay CHS activity in an insect species, determined the optimal conditions for measuring the enzyme activity, including pH, temperature, and concentrations of the substrate uridine diphosphate N-acetyl-d-glucosamine (UDP-GlcNAc) and Mg(++) . The optimal pH was about 6.5-7.0, and the highest activity was detected at temperatures between 37°C and 44°C. Dithithreitol is required to prevent melanization of the enzyme extract. CHS activity was enhanced at low concentration of GlcNAc, but inhibited at high concentrations. Proteolytic activation of the activity is significant both in the 500 ×g supernatant and the 40 000 ×g pellet. Our study revealed only slight in vitro inhibition of A. gambiae CHS activity by diflubenzuron and nikkomycin Z at the highest concentration (2.5 μmol/L) examined. There was no in vitro inhibition by polyoxin D at any concentration examined. Furthermore, we did not observe any in vivo inhibition of CHS activity by any of these chemicals at any concentration examined. Our results suggest that the inhibition of chitin synthesis by these chemicals is not due to direct inhibition of CHS in A. gambiae.

Architecture and biosynthesis of the Saccharomyces cerevisiae cell wall

The wall gives a Saccharomyces cerevisiae cell its osmotic integrity; defines cell shape during budding growth, mating, sporulation, and pseudohypha formation; and presents adhesive glycoproteins to other yeast cells. The wall consists of β1,3- and β1,6-glucans, a small amount of chitin, and many different proteins that may bear N- and O-linked glycans and a glycolipid anchor. These components become cross-linked in various ways to form higher-order complexes. Wall composition and degree of cross-linking vary during growth and development and change in response to cell wall stress. This article reviews wall biogenesis in vegetative cells, covering the structure of wall components and how they are cross-linked; the biosynthesis of N- and O-linked glycans, glycosylphosphatidylinositol membrane anchors, β1,3- and β1,6-linked glucans, and chitin; the reactions that cross-link wall components; and the possible functions of enzymatic and nonenzymatic cell wall proteins.

Biosynthesis of non-cellulosic polysaccharides of plant cell walls

Enzymes that make the polymer backbones of plant cell wall polysaccharides have proven to be recalcitrant to biochemical purification. Availability of mutational genetics and genomic tools paved the way for rapid progress in identifying genes encoding various cell wall glycan synthases. Mutational genetics, the primary tool used in unraveling cellulose biosynthesis, was ineffective in assigning function to any of the hemicellulosic, polymerizing glycan synthases. A combination of comparative genomics and functional expression in a heterologous system allowed identification of various cellulose synthase-like (Csl) sequences as being involved in the formation of β-1,4-mannan, β-1,4-glucan, and mixed-linked glucan. A number of xylose-deficient mutants have led to a variety of genes, none of which thus far possesses the motifs known to be conserved among polymerizing β-glycan synthases. Except for xylan synthase, which appears to be an agglomerate of proteins just like cellulose synthase, Golgi glycan synthases already identified suggest that the catalytic polypeptide by itself is sufficient for enzyme activity, most likely as a homodimer. Several of the Csl genes remain to be assigned a function. The possibility of the involvement of various Csl genes in making more than one product remains.

Saccharomyces cerevisiae chitin biosynthesis activation by N-acetylchitooses depends on size and structure of chito-oligosaccharides

Background

To explore chitin synthesis initiation, the effect of addition of exogenous oligosaccharides on in vitro chitin synthesis was studied. Oligosaccharides of various natures and lengths were added to a chitin synthase assay performed on a Saccharomyces cerevisiae membrane fraction.

Findings

N-acetylchito-tetra, -penta and -octaoses resulted in 11 to 25% [¹⁴C]-GlcNAc incorporation into [¹⁴C]-chitin, corresponding to an increase in the initial velocity. The activation appeared specific to N-acetylchitooses as it was not observed with oligosaccharides in other series, such as beta-(1,4), beta-(1,3) or alpha-(1,6) glucooligosaccharides.

Conclusions

The effect induced by the N-acetylchitooses was a saturable phenomenon and did not interfere with free GlcNAc and trypsin which are two known activators of yeast chitin synthase activity in vitro. The magnitude of the activation was dependent on both oligosaccharide concentration and oligosaccharide size.

Enzymatic synthesis of an α-chitin-like substance via lysozyme-mediated transglycosylation

The enzymatic synthesis of an α-chitin-like substance via a non-biosynthetic pathway has been achieved by transglycosylation in an aqueous system of the corresponding substrate, tri-N-acetylchitotriose [(GlcNAc)(3)] for lysozyme. A significant amount of water-insoluble product precipitated out from the reaction system. MALDI-TOFMS analysis showed that the resulting precipitate had a degree of polymerization (DP) of up to 15 from (GlcNAc)(3). Solid-state (13)C NMR analysis revealed that the resulting water-insoluble product is a chitin-like substance consisting of N-acetylglucosamine (GlcNAc) residues joined exclusively in a β-(1→4)-linked chain with stringent regio-/stereoselection. X-ray diffraction (XRD) measurement as well as (13)C NMR analysis showed that the crystal structure of synthetic product corresponds to α-chitin with a high degree of crystallinity. We propose that the multiple oligomers form an α-chitin-like substance as a result of self-assembly via oligomer-oligomer interaction when they precipitate.

The cellular basis of chitin synthesis in fungi and insects: common principles and differences

Chitin is a polymer of N-acetylglucosamine, which assembles into microfibrils of about 20 sugar chains. These microfibrils serve as a structural component of natural biocomposites found in cell walls and specialized extracellular matrices such as cuticles and peritrophic membranes. Chitin synthesis is performed by a wide range of organisms including fungi and insects. The underlying biosynthetic machinery is highly conserved and involves several enzymes, of which the chitin synthase is the key enzyme. This membrane integral glycosyltransferase catalyzes the polymerization reaction. Most of what we know about chitin synthesis derives from studies of fungal and insect systems. In this review, common principles and differences will be worked out at the levels of gene organization, enzymatic properties, cellular localization and regulation.

Differential cell wall remodeling of two chitin synthase deletants Δchs3A and Δchs3B in the pathogenic yeast Candida glabrata

It is known that cell wall remodeling and the salvaging pathway act to compensate for an impaired or a damaged cell wall. Lately, it has been indicated that this mechanism is partly required for resistance to the glucan synthesis inhibitor echinocandin. While cell wall remodeling has been described in mutants of glucan or mannan synthesis, it has not yet been reported in a chitin synthesis mutant. Here, we describe a novel cell wall remodeling and salvaging pathway in chitin synthesis mutants, Δchs3A and Δchs3B, of the pathogenic yeast Candida glabrata. Electron microscopic analysis revealed a thickened mannoprotein layer in Δchs3A cells and a thickened chitin-glucan layer of Δchs3B cells, and it indicated the hypothesis that mannan synthase and chitin-glucan synthase indemnify Δchs3A and Δchs3B cells, respectively. The double-mutant CHS3A and MNN10, encoding α-1,6-mannosyltransferase, showed synergistic stress sensitization, and the Δchs3B strain showed supersensitivity to echinocandins. Hence, these findings support the above hypothesis of remodeling. Furthermore, unlike Δchs3A cells, Δchs3B cells showed supersensitivity to calcineurin inhibitor FK506 and Tor1p kinase inhibitor rapamycin, indicating that the Δchs3B strain uses the calcineurin pathway and a Tor1p kinase for cell wall remodeling.

Expression in E. coli and characterization of the catalytic domain of Botrytis cinerea chitin synthase

BACKGROUND

Chitin synthase 3a (CHS3a) from Botrytis cinerea (Bc) catalyses the multiple transfer of N-acetylglucosamine (GlcNAc) residues to the growing chitin chain. Chitin, a β-1,4 linked GlcNAc homopolymer, is an essential cell wall component of filamentous fungi. Chitin synthase, processive membranous protein, has been recognized as a promising target for new antifungicides. Enzymatic characterizations of chitin synthases have been limited, mainly because purity and amounts of integral enzyme obtained after purification procedures have not been sufficient.

FINDINGS

We undertook the preparation of two BcCHS3a fragment proteins, containing only the central domain and devoid of the N-terminal and transmembrane C-terminal regions. The central domain of CHS3a, named SGC (Spsa GntI Core), is conserved in all UDP-glycosyltransferases and it is believed to contain the active site of the enzyme. CHS3a-SGC protein was totally expressed as inclusion bodies in Escherichia coli. We performed recombinant CHS3a-SGC purification in denaturing conditions, followed by a refolding step. Although circular dichroism spectra clearly exhibited secondary structures of renatured CHS3a-SGC, no chitin synthase activity was detected. Nevertheless CHS3a-SGC proteins show specific binding for the substrate UDP-GlcNAc with a dissociation constant similar to the Michaelis constant and a major contribution of the uracil moiety for recognition was confirmed.

CONCLUSIONS

Milligram-scale quantities of CHS3a-SGC protein with native-like properties such as specific substrate UDP-GlcNAc binding could be easily obtained. These results are encouraging for subsequent heterologous expression of full-length CHS3a.

Purification of an active, oligomeric chitin synthase complex from the midgut of the tobacco hornworm

Chitin formation depends on the activity of a family II glycosyltransferase known as chitin synthase, whose biochemical and structural properties are largely unknown. Previously, we have demonstrated that the chitin portion of the peritrophic matrix in the midgut of the tobacco hornworm, Manduca sexta, is produced by chitin synthase 2 (CHS-2), one of two isoenzymes encoded by the Chs-1 and Chs-2 genes (also named Chs-A and Chs-B), and that CHS-2 is located at the apical tips of the brush border microvilli. Here we report the purification of the chitin synthase from the Manduca midgut as monitored by its activity and immuno-reactivity with antibodies to the chitin synthase. After gel permeation chromatography, the final step of the developed purification protocol, the active enzyme eluted in a fraction corresponding to a molecular mass between 440 and 670 kDa. Native PAGE revealed a single, immuno-reactive band of about 520 kDa, thrice the molecular mass of the chitin synthase monomer. SDS-PAGE and immunoblotting indicated finally that an active, oligomeric complex of the chitin synthase was purified. In summary, the chitin synthase from the midgut of Manduca may prove to be a good model for investigating the enzymes' mode of action.

Phylogenetic diversity of stress signalling pathways in fungi

Background

Microbes must sense environmental stresses, transduce these signals and mount protective responses to survive in hostile environments. In this study we have tested the hypothesis that fungal stress signalling pathways have evolved rapidly in a niche-specific fashion that is independent of phylogeny. To test this hypothesis we have compared the conservation of stress signalling molecules in diverse fungal species with their stress resistance. These fungi, which include ascomycetes, basidiomycetes and microsporidia, occupy highly divergent niches from saline environments to plant or mammalian hosts.

Results

The fungi displayed significant variation in their resistance to osmotic (NaCl and sorbitol), oxidative (H₂O₂ and menadione) and cell wall stresses (Calcofluor White and Congo Red). There was no strict correlation between fungal phylogeny and stress resistance. Rather, the human pathogens tended to be more resistant to all three types of stress, an exception being the sensitivity of Candida albicans to the cell wall stress, Calcofluor White. In contrast, the plant pathogens were relatively sensitive to oxidative stress. The degree of conservation of osmotic, oxidative and cell wall stress signalling pathways amongst the eighteen fungal species was examined. Putative orthologues of functionally defined signalling components in Saccharomyces cerevisiae were identified by performing reciprocal BLASTP searches, and the percent amino acid identities of these orthologues recorded. This revealed that in general, central components of the osmotic, oxidative and cell wall stress signalling pathways are relatively well conserved, whereas the sensors lying upstream and transcriptional regulators lying downstream of these modules have diverged significantly. There was no obvious correlation between the degree of conservation of stress signalling pathways and the resistance of a particular fungus to the corresponding stress.

Conclusion

Our data are consistent with the hypothesis that fungal stress signalling components have undergone rapid recent evolution to tune the stress responses in a niche-specific fashion.

Prenylation of Saccharomyces cerevisiae Chs4p Affects Chitin Synthase III activity and chitin chain length

Chs4p (Cal2/Csd4/Skt5) was identified as a protein factor physically interacting with Chs3p, the catalytic subunit of chitin synthase III (CSIII), and is indispensable for its enzymatic activity in vivo. Chs4p contains a putative farnesyl attachment site at the C-terminal end (CVIM motif) conserved in Chs4p of Saccharomyces cerevisiae and other fungi. Several previous reports questioned the role of Chs4p prenylation in chitin biosynthesis. In this study we reinvestigated the function of Chs4p prenylation. We provide evidence that Chs4p is farnesylated by showing that purified Chs4p is recognized by anti-farnesyl antibody and is a substrate for farnesyl transferase (FTase) in vitro and that inactivation of FTase increases the amount of unmodified Chs4p in yeast cells. We demonstrate that abolition of Chs4p prenylation causes a approximately 60% decrease in CSIII activity, which is correlated with a approximately 30% decrease in chitin content and with increased resistance to the chitin binding compound calcofluor white. Furthermore, we show that lack of Chs4p prenylation decreases the average chain length of the chitin polymer. Prenylation of Chs4p, however, is not a factor that mediates plasma membrane association of the protein. Our results provide evidence that the prenyl moiety attached to Chs4p is a factor modulating the activity of CSIII both in vivo and in vitro.

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.

Regulation of chitin synthesis in the larval midgut of Manduca sexta

In insects, chitin is not only synthesized by ectodermal cells that form chitinous cuticles, but also by endodermal cells of the midgut that secrete a chitinous peritrophic matrix. Using anti-chitin synthase (CHS) antibodies, we previously demonstrated that in the midgut of Manduca sexta, CHS is expressed by two cell types, tracheal cells forming a basal tracheal network and columnar cells forming the apical brush border [Zimoch and Merzendorfer, 2002, Cell Tissue Res. 308, 287-297]. Now, we show that two different genes, MsCHS1 and MsCHS2, encode CHSs of midgut tracheae and columnar cells, respectively. To investigate MsCHS2 expression and activity in the course of the larval development, we monitored chitin synthesis, enzyme levels as well as mRNA amounts. All of the tested parameters were significantly reduced during molting and in the wandering stage when compared to the values obtained from intermolt feeding larvae. By contrast, MsCHS1 appeared to be inversely regulated because its mRNA was detectable only during the molt at the time when tracheal growth occurs at the basal site of the midgut. To further examine midgut chitin synthesis, we measured enzyme activity in crude midgut extracts and different membrane fractions. When we analysed trypsin-mediated proteolytic activation, a phenomenon previously reported for insect and fungal systems, we recognized that midgut chitin synthesis was only activated in crude extracts, but not in the 12,000 g membrane fraction. However, proteolytic activation by trypsin in the 12,000 g membrane fraction could be reconstituted by re-adding a soluble fraction, indicating that limited proteolysis affects an unknown soluble factor, a process that in turn activates chitin synthesis.

Synthase III-dependent Chitin Is Bound to Different Acceptors Depending on Location on the Cell Wall of Budding Yeast

In yeast, chitin is laid down at three locations: a ring at the mother-bud neck, the primary septum and, after cytokinesis, the cell wall of the daughter cell. Some of the chitin is free and the remainder attached to beta(1-3)glucan or beta(1-6)glucan. We recently reported that the chitin ring contributes to the prevention of growth at the mother-bud neck and hypothesized that this inhibition is achieved by a preferential binding of chitin to beta(1-3)glucan at that site. Here, we devised a novel strategy for the analysis of chitin cross-links in [14C]glucosamine-labeled cell walls, involving solubilization in water of alkali-treated walls by carboxymethylation. Intact cell walls or their digestion products with beta(1-3)glucanase or beta(1-6)glucanase were carboxymethylated and fractionated on size columns, and the percentage of chitin bound to different polysaccharides was calculated. Chitin dispersed in the wall was labeled in maturing unbudded cells and that of the ring in early budding cells. The former was mostly attached to beta(1-6)glucan and the latter to beta(1-3)glucan. This confirmed our hypothesis and indicated that the cell has mechanisms to attach chitin, a water-insoluble substance, synthesized here through chitin synthase III, to different acceptors, depending on location. In contrast, most of the chitin synthase II-dependent chitin of the primary septum was free, with the remainder linked to beta(1-3)glucan.

Guar seed beta-mannan synthase is a member of the cellulose synthase super gene family

Genes for the enzymes that make plant cell wall hemicellulosic polysaccharides remain to be identified. We report here the isolation of a complementary DNA (cDNA) clone encoding one such enzyme, mannan synthase (ManS), that makes the beta-1, 4-mannan backbone of galactomannan, a hemicellulosic storage polysaccharide in guar seed endosperm walls. The soybean somatic embryos expressing ManS cDNA contained high levels of ManS activities that localized to Golgi. Phylogenetically, ManS is closest to group A of the cellulose synthase-like (Csl) sequences from Arabidopsis and rice. Our results provide the biochemical proof for the involvement of the Csl genes in beta-glycan formation in plants.

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).

Structure and biosynthesis of fungal cell walls: Methodological approaches

Fungal cell walls possess a characteristic chemical composition differentiating fungal cells from other cell types. For this reason, the mechanisms involved in cell-wall formation represent a potential target for selective antifungal drugs. Understanding the structure and biosynthesis of fungal cell walls opens the ways for design of effective drugs for treating fungal diseases. This article reviews the history methods employed in chemical and structural analysis of fungal cell walls and in studies concerning their formation.

Fungal beta(1,3)-D-glucan synthesis

The polysaccharide beta(1,3)-D-glucan is a component of the cell wall of many fungi. Synthesis of the linear polymer is catalysed by UDP-glucose beta(1,3)-D-glucan beta(3)-D-glucosyltransferase. Because this enzyme has a key role in fungal cell-wall synthesis, and because many organisms that are responsible for human mycoses, including Candida albicans, Aspergillus fumigatus and Cryptococcus neoformans, produce walls that are rich in beta(1,3)-glucan, it has been and remains the focus of intensive study. From early characterization of the enzymatic activity in Saccharomyces cerevisiae, advances have been made in purification of the enzyme, identification of essential subunits and description of regulatory circuitry that controls expression and localization of different components of the multisubunit enzyme complex. Progress in each of these areas has been enhanced dramatically by the availability of specific inhibitors of the enzymatic reaction that produces beta(1,3)-glucan. These natural product inhibitors have utility both as tools to dissect the biology of beta(1,3)-glucan synthase and as sources for development of semisynthetic derivatives with clinical utility in treatment of human fungal disease. This review will focus on the biochemistry, genetics and regulation of the enzyme.

Biochemistry of chitin synthase

This article compiles the papers dealing with the biochemistry of chitin synthase (CS) published during the last decade, provides up-to-date information on the state of knowledge and understanding of chitin synthesis in vitro, and points out some firmly entrenched ideas and tenets of CS biochemistry that have become of age without hardly ever having been critically re-evaluated. The subject is dealt with under the headings "Components of the CS reaction" (educt, cation requirement and intermediates; product), "Regulation of CS" (cooperativity and allostery; non-allosteric activation or priming of CS; latency), "Concerted action of CS and enzymes of chitinolysis", "Inhibition of CS", "Multiplicity of CSs", and "Structure of CS" (the putative UDPGlcNAc-binding domain of CS; identification of CS polypeptides; glycoconjugation). The prospects are outlined of obtaining a refined three-dimensional (3D) model of the catalytic site of CS for biotechnological applications.

CELLULOSE BIOSYNTHESIS: Exciting Times for A Difficult Field of Study

The past few decades have witnessed exciting progress in studies on the biosynthesis of cellulose. In the bacterium Acetobacter xylinum, discovery of the activator of the cellulose synthase, cyclic diguanylic acid, opened the way for obtaining high rates of in vitro synthesis of cellulose. This, in turn, led to purification of the cellulose synthase and for the cloning of genes that encode the catalytic subunit and other proteins that bind the activator and regulate its synthesis and degradation, or that control secretion and crystallization of the microfibrils. In higher plants, a family of genes has been discovered that show interesting similarities and differences from the gene in bacteria that encodes the catalytic subunit of the synthase. Genetic evidence now supports the concept that members of this family encode the catalytic subunit in these organisms, with various members showing tissue-specific expression. Although the cellulose synthase has not yet been purified to homogeneity from plants, recent progress in this area suggests that this will soon be accomplished.

The properties and localization of Saprolegnia monoica chitin synthase differ from those of other fungi

The presence of non-fibrillar alpha-chitin in cellulosic fungi (class Oomycetes) poses intriguing questions as to its role, subcellular localization and evolutionary significance. Previous studies reported on the similarity of chitin synthase from Saprolegnia monoica with that of other fungi. The present work describes important dissimilarities. There was no evidence that the chitin synthase of S. monoica was present in small low-density vesicles (chitosomes). Chitin synthase sedimented with membranous components of high specific gravity (sp. gr. 1.177) that could be partially but distinctly separated from membranes harbouring most of the 1,3-beta-glucan synthase in the cell (sp. gr. 1.158). In contrast to other fungi, the chitin synthase from S. monoica was greatly stimulated by digitonin: both membrane-bound and dissociated chitin synthase showed little activity in the absence of digitonin. As in other fungi, the chitin synthase from S. monoica was solubilized by digitonin and remained zymogenic after dissociation. However, unlike the enzyme from other fungi, the solubilized chitin synthase of S. monoica had a lower sedimentation coefficient, was not stimulated by phospholipids and was not inhibited by high concentrations of digitonin. Unlike the enzyme from Mucor rouxii, the solubilized chitin synthase from S. monoica did not bind to a cation exchanger. The enzyme was partially purified by four-step scheme that included sucrose density-gradient centrifugation, a single passage through a strong anion exchanger and two consecutive passages through a weak anion exchanger. The final preparation contained five to seven polypeptide bands that cochromatographed with the chitin synthase activity, some of which may be part of a presumed chitin synthase macromolecular complex.

Stereochemical requirements of chitin synthase for ligand binding at the allosteric site for N-acetylglucosamine

The substrate kinetics of chitin synthase (CS) were non-Michaelian, irrespective of the type of enzyme preparation studied (105-S chitosomal CS, and 16-S ex walls), even in the presence of saturating GlcNAc. An unexplained idiosyncrasy of this enzyme, which is likely to be responsible for this phenomenon, is evident from the striking non-linearity of product deposition with time at low substrate or low enzyme concentrations, particularly in the absence of GlcNac. The possibility can be excluded that this non-linearity is due to the formation of soluble by-products or intermediates in the form of chito-oligomers, as shown by HPLC/pulsed amperometric detection analysis. Additional evidence was sought for the tenet that CS is homotropically-heterotropically regulated, at least under steady-state reaction conditions. Substrate kinetic curves established from rate data for the linear reaction phase only were used for modelling. These could be reasonably well parameterised on the basis of the Monod mathematical model for co-operative ligand binding. Within a series of test compounds used to assess the stereochemical conditions of the allosteric site of CS for effector binding, N-acetylglucosaminono-1,5-lactone oxime excelled. Requirements for effector binding are as follows: (a) an aminoglucopyranose skeleton with the amino function acetylated, and (b) a single-bonded oxo-function present at C(1), which is preferentially a hydrogen bond donor, that may be equatorially spaced off, but must not be alpha-anomeric. The implications of these findings for chitin synthesis in vivo are discussed in terms of a mechanistically based fitness of CS to operate efficiently under vastly different combinations of substrate could be coordinately linked to the catabolism of chitin.

Characterization and gene cloning of 1,3-beta-D-glucan synthase from Saccharomyces cerevisiae

1,3-beta-D-Glucan synthase of Saccharomyces cerevisiae was solubilized and purified up to 700-fold by product entrapment. The specific activity of the partially purified enzyme was around 4 mumol glucose incorporated.min-1.mg protein-1. In SDS/PAGE, enrichment of a 200-kDa protein was clearly observed in parallel with the increase in specific activity. mAbs that could immunoprecipitate the 1,3-beta-D-glucan synthase activity were isolated, and some of them also recognized this 200-kDa protein in the Western blot. Internal amino acid sequences of this 200-kDa protein were determined after lysyl endopeptidase digestion. With the information of these amino acid sequences, we cloned two genes, GSC1 and GSC2 (glucan synthase of S. cerevisiae 1 and 2), which are very similar to each other (88% at the amino acid level); hydropathy profiles of both proteins suggest that these genes encode integral membrane proteins which can be assumed to have approximately 16 transmembrane domains. Disruption of each gene was not lethal, but disruption of both genes was lethal. The 1,3-beta-D-glucan synthase activities of membrane and partially purified enzyme of gsc1::URA3 cells were significantly lower than those of the wild-type and gsc2::LEU2 cells.

Characterization of chitin synthase 2 of Saccharomyces cerevisiae. Implication of two highly conserved domains as possible catalytic sites

Chitin synthase 2 of Saccharomyces cerevisiae was characterized by means of site-directed mutagenesis and subsequent expression of the mutant enzymes in yeast cells. Chitin synthase 2 shares a region whose sequence is highly conserved in all chitin synthases. Substitutions of conserved amino acids in this region with alanine (alanine scanning) identified two domains in which any conserved amino acid could not be replaced by alanine to retain enzyme activity. These two domains contained unique sequences, Glu561-Asp562-Arg563 and Gln601-Arg602-Arg603-Arg604-Trp605, that were conserved in all types of chitin synthases. Glu561 or arginine at 563, 602, and 603 could be substituted by glutamic acid and lysine, respectively, without significant loss of enzyme activity. However, even conservative substitutions of Asp562 with glutamic acid, Gln601 with asparagine, Arg604 with lysine, or Trp605 with tyrosine drastically decreased the activity, but did not affect apparent Km values for the substrate significantly. In addition to these amino acids, Asp441 was also found in all chitin synthase. The mutant harboring a glutamic acid substitution for Asp441 severely lost activity, but it showed a similar apparent Km value for the substrate. Amounts of the mutant enzymes in total membranes were more or less the same as found in the wild type. Furthermore, Asp441, Asp562, Gln601, Arg604, and Trp605 are completely conserved in other proteins possessing N-acetylglucosaminyltransferase activity such as NodC proteins of Rhizobium bacterias. These results suggest that Asp441, Asp562, Gln601, Arg604, and Trp605 are located in the active pocket and that they function as the catalytic residues of the enzyme.

Cloning and characterization of GNS1: a Saccharomyces cerevisiae gene involved in synthesis of 1,3-beta-glucan in vitro

The GNS1 gene product is required for the synthesis of 1,3-beta-glucan in vitro, since mutations in this gene result in exhibit an 80 to 90% reduction in 1,3-beta-glucan synthase specific activity. gns1 mutant strains display a pleiotropic phenotype including resistance to a pneumocandin B0 analog (L-733,560), slow growth, and mating and sporulation defects. The gns1-1 mutation was genetically mapped to within 1.35 centimorgans from the MAT locus on chromosome III. The wild-type GNS1 gene was isolated by complementing the pneumocandin resistance phenotype of the gns1-1 mutation and by hybridization with a chromosome III-derived sequence being used as a probe. The nucleotide sequence of GNS1 was determined and compared with the homologous region of the chromosome. The genetic and nucleotide sequence analyzes revealed that GNS1 and the open reading frame, YCR34 [S. Oliver, Q. van der Aart, M. Agostoni-Carbone, and the Chromosome III Sequencing Group, Nature (London) 357:38-46, 1992], represent identical loci in the genome. Cells deleted for GNS1 are viable but exhibit slow growth as well as the pleiotropic phenotype of the gns1 mutants. The putative protein product is predicted to be an integral membrane protein with five transmembrane helices displaying an exoplasmic orientation for the N terminus and a cytoplasmic orientation for the C terminus. This protein may be a subunit of 1,3-beta-glucan synthase.

Architecture of the yeast cell wall. The linkage between chitin and beta(1-->3)-glucan

To isolate the putative linkage region between chitin and beta(1-->3)-glucan, Saccharomyces cerevisiae cell walls were digested with beta(1-->3)-endoglucanase and the reducing ends of the enzyme-resistant glucose chain stubs were labeled by reduction with borotritide. The radioactive material was further digested with exochitinase to remove the bulk of the chitin, and the liberated oligosaccharides were fractionated on a sizing column. A single peak (compound I) was found to consist of N-acetylglucosamine, glucose, and glucitol residues in the ratio 1:2:1. By digestion with beta-N-acetylglucosaminidase and by NMR spectroscopy, N-acetylglucosamine was identified as the nonreducing terminus, linked to laminaritriitol by a beta(1-->4) bond. Five additional oligosaccharides were recovered, two being analogs of compound I, with 1 or 3 glucose units, respectively; the remaining three were shown to be the reduced analogs of laminaribiose, laminaritriose, and laminaritetraose. The presence of N-acetylglucosamine-containing oligosaccharides arises from the activity of chitinase in cleaving 2 sugar units sequentially in those chains containing an odd number of N-acetylglucosamine residues; correspondingly, oligosaccharides containing only glucose and sorbitol derived from even-numbered chitin chains, a result implying that chitinase can hydrolyze the linkage between N-acetylglucosamine and glucose. It is concluded that the terminal reducing residue of a chitin chain is attached to the nonreducing end of a beta(1-->3)-glucan chain by a beta(1-->4) linkage. Experiments with appropriate mutants showed that synthesis of the chitin combined with glucan is catalyzed by chitin synthetase 3. The timing and possible mechanism of formation of the chitin-glucan linkage is discussed.

Are yeast chitin synthases regulated at the transcriptional or the posttranslational level?

The three chitin synthases of Saccharomyces cerevisiae, Chs1, Chs2, and Chs3, participate in septum and cell wall formation of vegetative cells and in wall morphogenesis of conjugating cells and spores. Because of the differences in the nature and in the time of execution of their functions, the synthases must be specifically and individually regulated. The nature of that regulation has been investigated by measuring changes in the levels of the three synthases and of the messages of the three corresponding genes, CHS1, CHS2, and CAL1/CSD2/DIT101/KTI2 (referred to below as CAL1/CSD2), during the budding and sexual cycles. By transferring cells carrying CHS2 under the control of a GAL1 promoter from galactose-containing medium to glucose-containing medium, transcription of CHS2 was shut off. This resulted in a rapid disappearance of Chs2, whereas the mRNA decayed much more slowly. Furthermore, Chs2 levels experienced pronounced oscillations during the budding cycle and were decreased in the sexual cycle, indicating that this enzyme is largely regulated by a process of synthesis and degradation. For CHS1 and CAL1/CSD2, however, a stop in transcription was followed by a slow decrease in the level of zymogen (Chs1) or an increase in the level of activity (Chs3), despite a rapid drop in message level in both cases. In synchronized cultures, Chs1 levels were constant during the cell cycle. Thus, for Chs1 and Chs3, posttranslational regulation, probably by activation of latent forms, appears to be predominant. Since Chs2, like Chs1, is found in the cell in the zymogenic form, a posttranslational activation step appears to be necessary for this synthase also.