Mutational analysis of chitin synthase 2 of Saccharomyces cerevisiae. Identification of additional amino acid residues involved in its catalytic activity

Structure, catalysis, chitin transport, and selective inhibition of chitin synthase

Chitin is one of the most abundant natural biopolymers and serves as a critical structural component of extracellular matrices, including fungal cell walls and insect exoskeletons. As a linear polymer of β-(1,4)-linked N-acetylglucosamine, chitin is synthesized by chitin synthases, which are recognized as targets for antifungal and anti-insect drugs. In this study, we determine seven different cryo-electron microscopy structures of a Saccharomyces cerevisiae chitin synthase in the absence and presence of glycosyl donor, acceptor, product, or peptidyl nucleoside inhibitors. Combined with functional analyses, these structures show how the donor and acceptor substrates bind in the active site, how substrate hydrolysis drives self-priming, how a chitin-conducting transmembrane channel opens, and how peptidyl nucleoside inhibitors inhibit chitin synthase. Our work provides a structural basis for understanding the function and inhibition of chitin synthase.

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.

Deciphering the Chitin Code in Plant Symbiosis, Defense, and Microbial Networks

Chitin is a structural polymer in many eukaryotes. Many organisms can degrade chitin to defend against chitinous pathogens or use chitin oligomers as food. Beneficial microorganisms like nitrogen-fixing symbiotic rhizobia and mycorrhizal fungi produce chitin-based signal molecules called lipo-chitooligosaccharides (LCOs) and short chitin oligomers to initiate a symbiotic relationship with their compatible hosts and exchange nutrients. A recent study revealed that a broad range of fungi produce LCOs and chitooligosaccharides (COs), suggesting that these signaling molecules are not limited to beneficial microbes. The fungal LCOs also affect fungal growth and development, indicating that the roles of LCOs beyond symbiosis and LCO production may predate mycorrhizal symbiosis. This review describes the diverse structures of chitin; their perception by eukaryotes and prokaryotes; and their roles in symbiotic interactions, defense, and microbe-microbe interactions. We also discuss potential strategies of fungi to synthesize LCOs and their roles in fungi with different lifestyles.

The kinetic landscape and interplay of protein networks in cytokinesis

Cytokinesis is executed by protein networks organized into functional modules. Individual proteins within each module have been characterized to various degrees. However, the collective behavior and interplay of the modules remain poorly understood. In this study, we conducted quantitative time-lapse imaging to analyze the accumulation kinetics of more than 20 proteins from different modules of cytokinesis in budding yeast. This analysis has led to a comprehensive picture of the kinetic landscape of cytokinesis, from actomyosin ring (AMR) assembly to cell separation. It revealed that the AMR undergoes biphasic constriction and that the switch between the constriction phases is likely triggered by AMR maturation and primary septum formation. This analysis also provided further insights into the functions of actin filaments and the transglutaminase-like protein Cyk3 in cytokinesis and, in addition, defined Kre6 as the likely enzyme that catalyzes β-1,6-glucan synthesis to drive cell wall maturation during cell growth and division.

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Cytokinesis is executed by protein modules each with a unique kinetic signature

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Actomyosin ring constricts in a biphasic manner that is elaborately regulated

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The transglutaminase-like domain in Cyk3 plays a dual role in cytokinesis

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Kre6 catalyzes β-1,6-glucan synthesis at the cell surface during growth and division

Diversity and evolution of chitin synthases in oomycetes (Straminipila: Oomycota)

The oomycetes are filamentous eukaryotic microorganisms, distinct from true fungi, many of which act as crop or fish pathogens that cause devastating losses in agriculture and aquaculture. Chitin is present in all true fungi, but it occurs in only small amounts in some Saprolegniomycetes and it is absent in Peronosporomycetes. However, the growth of several oomycetes is severely impacted by competitive chitin synthase (CHS) inhibitors. Here, we shed light on the diversity, evolution and function of oomycete CHS proteins. We show by phylogenetic analysis of 93 putative CHSs from 48 highly diverse oomycetes, including the early diverging Eurychasma dicksonii, that all available oomycete genomes contain at least one putative CHS gene. All gene products contain conserved CHS motifs essential for enzymatic activity and form two Peronosporomycete-specific and six Saprolegniale-specific clades. Proteins of all clades, except one, contain an N-terminal microtubule interacting and trafficking (MIT) domain as predicted by protein domain databases or manual analysis, which is supported by homology modelling and comparison of conserved structural features from sequence logos. We identified at least three groups of CHSs conserved among all oomycete lineages and used phylogenetic reconciliation analysis to infer the dynamic evolution of CHSs in oomycetes. The evolutionary aspects of CHS diversity in modern-day oomycetes are discussed. In addition, we observed hyphal tip rupture in Phytophthora infestans upon treatment with the CHS inhibitor nikkomycin Z. Combining data on phylogeny, gene expression, and response to CHS inhibitors, we propose the association of different CHS clades with certain developmental stages.

In Vitro and In Vivo Studies on the Structural Organization of Chs3 from Saccharomyces cerevisiae

Chitin biosynthesis in yeast is accomplished by three chitin synthases (Chs) termed Chs1, Chs2 and Chs3, of which the latter accounts for most of the chitin deposited within the cell wall. While the overall structures of Chs1 and Chs2 are similar to those of other chitin synthases from fungi and arthropods, Chs3 lacks some of the C-terminal transmembrane helices raising questions regarding its structure and topology. To fill this gap of knowledge, we performed bioinformatic analyses and protease protection assays that revealed significant information about the catalytic domain, the chitin-translocating channel and the interfacial helices in between. In particular, we identified an amphipathic, crescent-shaped α-helix attached to the inner side of the membrane that presumably controls the channel entrance and a finger helix pushing the polymer into the channel. Evidence has accumulated in the past years that chitin synthases form oligomeric complexes, which may be necessary for the formation of chitin nanofibrils. However, the functional significance for living yeast cells has remained elusive. To test Chs3 oligomerization in vivo, we used bimolecular fluorescence complementation. We detected oligomeric complexes at the bud neck, the lateral plasma membrane, and in membranes of Golgi vesicles, and analyzed their transport route using various trafficking mutants.

Transportation of Aspergillus nidulans Class III and V Chitin Synthases to the Hyphal Tips Depends on Conventional Kinesin

Cell wall formation and maintenance are crucial for hyphal morphogenesis. In many filamentous fungi, chitin is one of the main structural components of the cell wall. Aspergillus nidulans ChsB, a chitin synthase, and CsmA, a chitin synthase with a myosin motor-like domain (MMD) at its N-terminus, both localize predominantly at the hyphal tip regions and at forming septa. ChsB and CsmA play crucial roles in polarized hyphal growth in A. nidulans. In this study, we investigated the mechanism by which CsmA and ChsB accumulate at the hyphal tip in living hyphae. Deletion of kinA, a gene encoding conventional kinesin (kinesin-1), impaired the localization of GFP-CsmA and GFP-ChsB at the hyphal tips. The transport frequency of GFP-CsmA and GFP-ChsB in both anterograde and retrograde direction appeared lower in the kinA-deletion strain compared to wild type, although the velocities of the movements were comparable. Co-localization of GFP-ChsB and GFP-CsmA with mRFP1-KinA^rigor, a KinA mutant that binds to microtubules but does not move along them, was observed in the posterior of the hyphal tip regions. KinA co-immunoprecipitated with ChsB and CsmA. Co-localization and association of CsmA with KinA did not depend on the MMD. These findings indicate that ChsB and CsmA are transported along microtubules to the subapical region by KinA.

A structural and biochemical model of processive chitin synthesis

Chitin synthases (CHS) produce chitin, an essential component of the fungal cell wall. The molecular mechanism of processive chitin synthesis is not understood, limiting the discovery of new inhibitors of this enzyme class. We identified the bacterial glycosyltransferase NodC as an appropriate model system to study the general structure and reaction mechanism of CHS. A high throughput screening-compatible novel assay demonstrates that a known inhibitor of fungal CHS also inhibit NodC. A structural model of NodC, on the basis of the recently published BcsA cellulose synthase structure, enabled probing of the catalytic mechanism by mutagenesis, demonstrating the essential roles of the DD and QXXRW catalytic motifs. The NodC membrane topology was mapped, validating the structural model. Together, these approaches give insight into the CHS structure and mechanism and provide a platform for the discovery of inhibitors for this antifungal target.

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.

On the function of chitin synthase extracellular domains in biomineralization

Molluscs with various shell architectures evolved around 542-525 million years ago, as part of a larger phenomenon related to the diversification of metazoan phyla. Molluscs deposit minerals in a chitin matrix. The mollusc chitin is synthesized by transmembrane enzymes that contain several unique extracellular domains. Here we investigate the assembly mechanism of the chitin synthase Ar-CS1 via its extracellular domain ArCS1_E22. The corresponding transmembrane protein ArCS1_E22TM accumulates in membrane fractions of the expression host Dictyostelium discoideum. Soluble recombinant ArCS1_E22 proteins can be purified as monomers only at basic pH. According to confocal fluorescence microscopy experiments, immunolabeled ArCS1_E22 proteins adsorb preferably to aragonitic nacre platelets at pH 7.75. At pH 8.2 or pH 9.0 the fluorescence signal is less intense, indicating that protein-mineral interaction is reduced with increasing pH. Furthermore, ArCS1_E22 forms regular nanostructures on cationic substrates as revealed by atomic force microscopy (AFM) experiments on modified mica cleavage planes. These experiments suggest that the extracellular domain ArCS1_E22 is involved in regulating the multiple enzyme activities of Ar-CS1 such as chitin synthesis and myosin movements by interaction with mineral surfaces and eventually by protein assembly. The protein complexes could locally probe the status of mineralization according to pH unless ions and pCO2 are balanced with suitable buffer substances. Taking into account that the intact enzyme could act as a force sensor, the results presented here provide further evidence that shell formation is coordinated physiologically with precise adjustment of cellular activities to the structure, topography and stiffness at the mineralizing interface.

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.

The Chlorella variabilis NC64A genome reveals adaptation to photosymbiosis, coevolution with viruses, and cryptic sex

Chlorella variabilis NC64A, a unicellular photosynthetic green alga (Trebouxiophyceae), is an intracellular photobiont of Paramecium bursaria and a model system for studying virus/algal interactions. We sequenced its 46-Mb nuclear genome, revealing an expansion of protein families that could have participated in adaptation to symbiosis. NC64A exhibits variations in GC content across its genome that correlate with global expression level, average intron size, and codon usage bias. Although Chlorella species have been assumed to be asexual and nonmotile, the NC64A genome encodes all the known meiosis-specific proteins and a subset of proteins found in flagella. We hypothesize that Chlorella might have retained a flagella-derived structure that could be involved in sexual reproduction. Furthermore, a survey of phytohormone pathways in chlorophyte algae identified algal orthologs of Arabidopsis thaliana genes involved in hormone biosynthesis and signaling, suggesting that these functions were established prior to the evolution of land plants. We show that the ability of Chlorella to produce chitinous cell walls likely resulted from the capture of metabolic genes by horizontal gene transfer from algal viruses, prokaryotes, or fungi. Analysis of the NC64A genome substantially advances our understanding of the green lineage evolution, including the genomic interplay with viruses and symbiosis between eukaryotes.

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.

A chitin synthase and its regulator protein are critical for chitosan production and growth of the fungal pathogen Cryptococcus neoformans

Chitin is an essential component of the cell wall of many fungi. Chitin also can be enzymatically deacetylated to chitosan, a more flexible and soluble polymer. Cryptococcus neoformans is a fungal pathogen that causes cryptococcal meningoencephalitis, particularly in immunocompromised patients. In this work, we show that both chitin and chitosan are present in the cell wall of vegetatively growing C. neoformans yeast cells and that the levels of both rise dramatically as cells grow to higher density in liquid culture. C. neoformans has eight putative chitin synthases, and strains with any one chitin synthase deleted are viable at 30 degrees C. In addition, C. neoformans genes encode three putative regulator proteins, which are homologs of Saccharomyces cerevisiae Skt5p. None of these three is essential for viability. However, one of the chitin synthases (Chs3) and one of the regulators (Csr2) are important for growth. Cells with deletions in either CHS3 or CSR2 have several shared phenotypes, including sensitivity to growth at 37 degrees C. The similarity of their phenotypes also suggests that Csr2 specifically regulates chitin synthesis by Chs3. Lastly, both chs3Delta and the csr2Delta mutants are defective in chitosan production, predicting that Chs3-Csr2 complex with chitin deacetylases for conversion of chitin to chitosan. These data suggest that chitin synthesis could be an excellent antifungal target.

Insect chitin synthases: a review

Chitin is the most widespread amino polysaccharide in nature. The annual global amount of chitin is believed to be only one order of magnitude less than that of cellulose. It is a linear polymer composed of N-acetylglucosamines that are joined in a reaction catalyzed by the membrane-integral enzyme chitin synthase, a member of the family 2 of glycosyltransferases. The polymerization requires UDP-N-acetylglucosamines as a substrate and divalent cations as co-factors. Chitin formation can be divided into three distinct steps. In the first step, the enzymes' catalytic domain facing the cytoplasmic site forms the polymer. The second step involves the translocation of the nascent polymer across the membrane and its release into the extracellular space. The third step completes the process as single polymers spontaneously assemble to form crystalline microfibrils. In subsequent reactions the microfibrils combine with other sugars, proteins, glycoproteins and proteoglycans to form fungal septa and cell walls as well as arthropod cuticles and peritrophic matrices, notably in crustaceans and insects. In spite of the good effort by a hardy few, our present knowledge of the structure, topology and catalytic mechanism of chitin synthases is rather limited. Gaps remain in understanding chitin synthase biosynthesis, enzyme trafficking, regulation of enzyme activity, translocation of chitin chains across cell membranes, fibrillogenesis and the interaction of microfibrils with other components of the extracellular matrix. However, cumulating genomic data on chitin synthase genes and new experimental approaches allow increasingly clearer views of chitin synthase function and its regulation, and consequently chitin biosynthesis. In the present review, I will summarize recent advances in elucidating the structure, regulation and function of insect chitin synthases as they relate to what is known about fungal chitin synthases and other glycosyltransferases.

Characterization of Chitin Synthases from Entamoeba

A major component of the Entamoeba cyst wall is chitin, a homopolymer of beta-(1,4)-linked N-acetyl-D-glucosamine. Polymerization of chitin requires the presence of active chitin synthases (CHS), a group of enzymes belonging to the family of beta-glycosyl transferases. CHS have been described for fungi, insects, and nematodes; however, information is lacking about the structure and expression of this class of enzymes in protozoons such as Entamoeba. In this study, the primary structures of two putative E. histolytica CHS (EhCHS-1 and EhCHS-2) were determined by gene cloning and homologous proteins were identified in databases from E. dispar and the reptilian parasite E. invadens. The latter constitutes the widely used model organism for the study of Entamoeba cyst development. The two ameba enzymes revealed between 23% and 33% sequence similarity to CHS from other organisms with full conservation of all residues critically important for CHS activity. Interestingly, EhCHS-1 and EhCHS-2 differed substantially in their predicted molecular weights (73 kD vs. 114 kD) as well as in their isoelectric points (5.04 vs. 8.05), and homology was restricted to a central stretch of about 400 amino acid residues containing the catalytic domain. Outside the catalytic domain, EhCHS-1 was predicted to have seven transmembrane helices (TMH) of which the majority is located within the C-terminal part, resembling the situation found in yeast; whereas, EhCHS-2 is structurally related to nematode or insect chitin synthases, as it contained 17 predicted TMHs of which the majority is located within the N-terminal part of the molecule. Northern blot analysis revealed that genes corresponding to CHS-1 and CHS-2 are not expressed in Entamoeba trophozoites, but substantial amounts of CHS-1 and CHS-2 RNA were present 4 to 8 hours after induction of cyst formation by glucose deprivation of E. invadens. The time-courses of expression differed slightly between the two ameba CHS genes, as in contrast to CHS-1 RNA, expression of CHS-2 RNA was more transient and no plateau was observed between 8 and 16 hours of encystation. However, both CHS RNAs were no longer detectable after 48 hours when most of the cells had been transformed into mature cysts.

Survey of the Botrytis cinerea chitin synthase multigenic family through the analysis of six euascomycetes genomes

We describe a strategy for systematic amplification of chitin synthase genes (chs) in the filamentous ascomycetes plant-pathogen Botrytis cinerea using PCR with multiple degenerate primers designed on specific and conserved sequence motifs. Eight distinct chs genes were isolated, named Bcchs I, II, IIIa, IIIb, IV, V, VI and VII. They probably constitute the entire chs multigenic family of this fungus, as revealed by careful analysis of six euascomycetes genomes. Bcchs I, IIIa, IIIb, IV and VI genes were subjected to DNA walking and their deduced amino acid sequences were compared by hydrophobic cluster analysis (HCA) to localize putative residues critical for CHS activity. HCA also enabled us to highlight three different transmembrane topologies of the CHS membranous isoenzymes. We found that the N-terminal region of the BcCHSI isoenzyme, and its orthologues in other euascomycetes, probably contain folded peptide motifs with conserved tyrosine residues. Their putative role is discussed. The BcCHSVII isoenzyme appeared to belong to a new class of CHS orthologues that was demonstrated by phylogenetic study to branch apart from division 1 and 2 of CHS.

Sequence of a cDNA and expression of the gene encoding a putative epidermal chitin synthase of Manduca sexta

Glycosyltransferases are enzymes that synthesize oligosaccharides, polysaccharides and glycoconjugates. One type of glycosyltransferase is chitin synthase, a very important enzyme in biology, which is utilized by insects, fungi, and other invertebrates to produce chitin, a polysaccharide of beta-1,4-linked N-acetylglucosamine. Chitin is an important component of the insect's exoskeletal cuticle and gut lining. To identify and characterize a chitin synthase gene of the tobacco hornworm, Manduca sexta, degenerate primers were designed from two highly conserved regions in fungal and nematode chitin synthase protein sequences and then used to amplify a similar region from Manduca cDNA. A full-length cDNA of 5152 nucleotides was assembled for the putative Manduca chitin synthase gene, MsCHS1, and sequencing of genomic DNA verified the contiguity of the sequence. The MsCHS1 cDNA has an ORF of 4692 nucleotides that encodes a transmembrane protein of 1564 amino acid residues with a mass of approximately 179 kDa (GenBank no. AY062175). It is most similar, over its entire length of protein sequence, to putative chitin synthases from other insects and nematodes, with 68% identity to enzymes from both the blow fly, Lucilia cuprina, and the fruit fly, Drosophila melanogaster. The similarity with fungal chitin synthases is restricted to the putative catalytic domain, and the MsCHS1 protein has, at equivalent positions, several amino acids that are essential for activity as revealed by mutagenesis of the fungal enzymes. A 5.3-kb transcript of MsCHS1 was identified by northern blot hybridization of RNA from larval epidermis, suggesting that the enzyme functions to make chitin deposited in the cuticle. Further examination by RT-PCR showed that MsCHS1 expression is regulated in the epidermis, with the amount of transcript increasing during phases of cuticle deposition.

Different functions of the class I and class II chitin synthase genes, chsC and chsA, are revealed by repression of chsB expression in Aspergillus nidulans

The filamentous fungus, Aspergillus nidulans, genome contains at least five chitin synthase-encoding genes. chsB is essential for normal hyphal growth. chsA and chsC are likely to be cooperatively required for hyphal wall integrity. In this study, we constructed chsA chsB and chsC chsB double mutants, in which chsB expression was under a repressible promoter [ alcA(p)]. While chsA or chsC single mutants did not show obvious growth defects, the chsA chsB and chsC chsB double mutants showed different phenotypes from the chsB single mutant and from each other under alcA(p)-repressing conditions. The chsA chsB double mutant produced fewer aerial hyphae and the chsC chsB double mutant showed reduced cell mass. These observations support the idea that chsA and chsC each play a different role in hyphal morphogenesis. In addition, the chitin contents of these double mutants were higher than those of the chsB single mutant. When chsA was expressed ectopically under the chsB promoter in the chsB mutant, the growth defects caused by chsB repression were not remedied at all, although an increased level of chsA mRNA was observed. Thus, it is suggested that the gene products of chsA and chsB themselves have different functions in hyphal morphogenesis.

Genetic control of cuticle formation during embryonic development of Drosophila melanogaster

The embryonic cuticle of Drosophila melanogaster is deposited by the epidermal epithelium during stage 16 of development. This tough, waterproof layer is essential for maintaining the structural integrity of the larval body. We have characterized mutations in a set of genes required for proper deposition and/or morphogenesis of the cuticle. Zygotic disruption of any one of these genes results in embryonic lethality. Mutant embryos are hyperactive within the eggshell, resulting in a high proportion reversed within the eggshell (the "retroactive" phenotype), and all show poor cuticle integrity when embryos are mechanically devitellinized. This last property results in embryonic cuticle preparations that appear grossly inflated compared to wild-type cuticles (the "blimp" phenotype). We find that one of these genes, krotzkopf verkehrt (kkv), encodes the Drosophila chitin synthase enzyme and that a closely linked gene, knickkopf (knk), encodes a novel protein that shows genetic interaction with the Drosophila E-cadherin, shotgun. We also demonstrate that two other known mutants, grainy head (grh) and retroactive (rtv), show the blimp phenotype when devitellinized, and we describe a new mutation, called zeppelin (zep), that shows the blimp phenotype but does not produce defects in the head cuticle as the other mutations do.

Evolution and phylogenetic relationships of chitin synthases from yeasts and fungi

Chitin, the structural component that provides rigidity to the cell wall of fungi is the product of chitin synthases (Chs). These enzymes are not restricted to fungi, but are amply distributed in four of the five eukaryotic 'crown kingdoms'. Dendrograms obtained by multiple alignment of Chs revealed that fungal enzymes can be classified into two divisions that branch into at least five classes, independent of fungal divergence. In contrast, oomycetes and animals each possess a single family of Chs. These results suggest that Chs originated as a branch of beta-glycosyl-transferases, once the kingdom Plantae split from the evolutionary line of eukaryotes. The existence of a single class of Chs in animals and Stramenopiles, against the multiple families in fungi, reveals that Chs diversification occurred after fungi departed from these kingdoms, but before separation of fungal groups. Accordingly, each fungal taxon contains members with enzymes belonging to different divisions and classes. Multiple alignment revealed the conservation of specific motifs characteristic of class, division and kingdom, but the strict conservation of only three motifs QXXEY, EDRXL and QXRRW, and seven isolated amino acids in the core region of all Chs. Determination of different structural features in this region of Chs brought to light a noticeable conservation of secondary structure in the proteins.

Chitin synthesis and inhibition: a revisit

Chitin is an abundant biologically important aminopolysaccharide composed of N-acetyl-D-glucosamine units. Individual polymers, which are synthesized intracellularly by chitin synthase (CS), a membrane-bound glycosyl transferase, are translocated across the plasma membrane and coalesce to form rigid crystallites. These crystallites, inter alia, are integral parts of septa and cell walls in yeast and filamentous fungi, respectively, and of cuticles in invertebrates, notably crustaceans and insects. Despite decades of intensive research, many events associated with the complexity of chitin formation and deposition are still obscure, or only partially understood. The list includes the hormonal control of CS at the transcriptional and translational levels as well as the post-translational CS packaging; trafficking and guidance of CS clusters to proper sites in the cells and their intricate insertion into the plasma membranes; activation of the catalytic step and its control or modulation; and translocation of chitin chains across cell membranes, their orientation, fibrillogenesis and association with other extracellular structural components such as polysaccharides (fungi) and cuticular proteins (insects). Also the precise biochemical lesions inflicted by CS inhibitors, such as the acylurea insect growth regulators, are largely unclear. The recent isolation and sequencing of insect CS genes should help in elucidating various aspects of chitin biochemistry and inhibition. In particular, the large number of transmembrane segments, characteristic of the insect CS, are speculated to be involved in chitin translocation and are expected to shed light on the mode of action of acylurea insecticides.

Insect chitin synthase cDNA sequence, gene organization and expression

Chitin is a major component of the cuticle of arthropods. However, the synthesis of chitin is poorly understood. Feeding larvae of the insect Lucilia cuprina on the fungal chitin synthase competitive inhibitor, nikkomycin Z resulted in strong concentration-dependent mortality of the larvae (LD50 = 280 nM). This result demonstrates that chitin is an essential component of this insect. The complete cDNA and deduced amino-acid sequences of the first arthropod chitin synthase-like protein, LcCS-1, from the larvae of the insect L. cuprina have been determined. The cDNA sequence is 5757 bp in length and codes for a large complex protein containing 1592 amino acids (Mr = 180 717). Analysis of the whole protein sequence reveals low, but significant, similarity to yeast chitin synthases with stronger areas of conservation centred on local regions implicated in the active sites of the yeast enzymes. Strikingly, LcCS-1 contains 15-18 potential transmembrane segments, indicating that the protein is an integral membrane protein. Two alternative topographical models of LcCS-1 are described, which involve its association with either the plasma membrane or the membrane of intracellular vesicles. LcCS-1 mRNA is produced in all life stages of the insect with expression in the larval stage limited to the integument and trachea. In a third instar larva the mRNA was localized to a single layer of epidermal cells immediately underlying the procuticle region of the integument. cDNA or genomic sequences that are highly related to fragments of LcCS-1 were demonstrated in three insect orders, one arachnid and Caenorhabditis elegans, thereby attesting to the importance of this enzyme in these chitin-producing organisms. Bioinformatics has been used to deduce the gene sequence and organization of the highly homologous Drosophila melanogaster orthologue of LcCS-1, DmCS-1.

Differential expression of two Blumeria graminis chitin synthase genes

Abstract Two Blumeria graminis chitin synthase genes, designated BgChs1 and BgChs2 were cloned and characterized following the synthesis and use of degenerate PCR primers designed to the conserved regions of fungal chitin synthase (Chs) genes. Their sequences revealed high similarity with the Chs genes previously cloned from other fungi and placed BgChs1 and BgChs2 with the classes I and V, respectively. Each gene was present as a single copy within the barley powdery mildew genome. Semi-quantitative RT-PCR assays revealed BgChs1 to be up-regulated at both the primary germ tube (PGT) and appressorial germ tube (AGT) stages of differentiation whilst the BgChs2 transcript was up-regulated at the PGT stage. The B. graminisbeta-tubulin gene was used as a control for all RT-PCR reactions. The BgChs1 transcript was some 30 fold less abundant than the beta-tubulin transcript and BgChs2 was some 30 fold rarer than the BgChs1 transcript. The effects of the chitin substrate analogues nikkomycin Z and polyoxin D on conidial morphogenesis were assessed. These nucleoside peptide inhibitors did not affect germination but both polyoxin D and nikkomycin Z treatment led to a large population of abnormally swollen 'balloon-shaped' AGTs, whilst by 12 h after inoculation polyoxin treatment caused the swollen germ tubes to burst.

The yeast Chs4 protein stimulates the trypsin-sensitive activity of chitin synthase 3 through an apparent protein-protein interaction

Inducible overexpression of the CHS4 gene under the control of the GAL1 promoter increased Chs3p (chitin synthase 3) activity in Saccharomyces cerevisiae several fold. Approximately half of the Chs3p activity in the membranes of cells overexpressing Chs4p was extracted using CHAPS and cholesteryl hemisuccinate. The detergent-extractable Chs3p activity appeared to be non-zymogenic because incubation with trypsin decreased enzyme activity in both the presence and absence of the substrate, UDP-N-acetylglucosamine. Western blotting confirmed that Chs3p was extracted from membranes by CHAPS and cholesteryl hemisuccinate and revealed that Chs4p was also solubilized using these detergents. Yeast two-hybrid analysis with truncated Chs4p demonstrated that the region of Chs4p between amino acids 269 and 563 is indispensable not only for eliciting the non-zymogenic activity of Chs3p but also for binding of Chs4p to Chs3p. Neither the EF-hand motif nor a possible prenylation site in Chs4p was required for these activities. Thus, it was demonstrated that stimulation of non-zymogenic Chs3p activity by Chs4p requires the amino acid region from 269 to 563 of Chs4p, and it seems that Chs4p activates Chs3p through protein-protein interaction.