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

The N-terminal disordered region of ChsB regulates its efficient transport to the hyphal apical surface in Aspergillus nidulans

In fungi, the cell wall plays a crucial role in morphogenesis and response to stress from the external environment. Chitin is one of the main cell wall components in many filamentous fungi. In Aspergillus nidulans, a class III chitin synthase ChsB plays a pivotal role in hyphal extension and morphogenesis. However, little is known about post-translational modifications of ChsB and their functional impacts. In this study, we showed that ChsB is phosphorylated in vivo. We characterized strains that produce ChsB using stepwise truncations of its N-terminal disordered region or deletions of some residues in that region and demonstrated its involvement in ChsB abundance on the hyphal apical surface and in hyphal tip localization. Furthermore, we showed that some deletions in this region affected the phosphorylation states of ChsB, raising the possibility that these states are important for the localization of ChsB to the hyphal surface and the growth of A. nidulans. Our findings indicate that ChsB transport is regulated by its N-terminal disordered region.

Chitin Synthesis in Yeast: A Matter of Trafficking

Chitin synthesis has attracted scientific interest for decades as an essential part of fungal biology and for its potential as a target for antifungal therapies. While this interest remains, three decades ago, pioneering molecular studies on chitin synthesis regulation identified the major chitin synthase in yeast, Chs3, as an authentic paradigm in the field of the intracellular trafficking of integral membrane proteins. Over the years, researchers have shown how the intracellular trafficking of Chs3 recapitulates all the steps in the intracellular trafficking of integral membrane proteins, from their synthesis in the endoplasmic reticulum to their degradation in the vacuole. This trafficking includes specific mechanisms for sorting in the trans-Golgi network, regulated endocytosis, and endosomal recycling at different levels. This review summarizes the work carried out on chitin synthesis regulation, mostly focusing on Chs3 as a molecular model to study the mechanisms involved in the control of the intracellular trafficking of proteins.

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.

Regulation of cell wall biogenesis in Saccharomyces cerevisiae: the cell wall integrity signaling pathway

The yeast cell wall is a strong, but elastic, structure that is essential not only for the maintenance of cell shape and integrity, but also for progression through the cell cycle. During growth and morphogenesis, and in response to environmental challenges, the cell wall is remodeled in a highly regulated and polarized manner, a process that is principally under the control of the cell wall integrity (CWI) signaling pathway. This pathway transmits wall stress signals from the cell surface to the Rho1 GTPase, which mobilizes a physiologic response through a variety of effectors. Activation of CWI signaling regulates the production of various carbohydrate polymers of the cell wall, as well as their polarized delivery to the site of cell wall remodeling. This review article centers on CWI signaling in Saccharomyces cerevisiae through the cell cycle and in response to cell wall stress. The interface of this signaling pathway with other pathways that contribute to the maintenance of cell wall integrity is also discussed.

Regulation of expression, activity and localization of fungal chitin synthases

The fungal cell wall represents an attractive target for pharmacologic inhibition, as many of the components are fungal-specific. Though targeted inhibition of β-glucan synthesis is effective treatment for certain fungal infections, the ability of the cell wall to dynamically compensate via the cell wall integrity pathway may limit overall efficacy. To date, chitin synthesis inhibitors have not been successfully deployed in the clinical setting. Fungal chitin synthesis is a complex and highly regulated process. Regulation of chitin synthesis occurs on multiple levels, thus targeting of these regulatory pathways may represent an exciting alternative approach. A variety of signaling pathways have been implicated in chitin synthase regulation, at both transcriptional and post-transcriptional levels. Recent research suggests that localization of chitin synthases likely represents a major regulatory mechanism. However, much of the regulatory machinery is not necessarily shared among different chitin synthases. Thus, an in-depth understanding of the precise roles of each protein in cell wall maintenance and repair will be essential to identifying the most likely therapeutic targets.

Dependence of Chs2 ER export on dephosphorylation by cytoplasmic Cdc14 ensures that septum formation follows mitosis

Sequestration of Cdc14 from the cytoplasm ensures Chs2 ER retention after MEN activation. The interdependence of chromosome segregation, MEN activation, decrease in mitotic CDK activity, and Cdc14 dispersal provides an effective mechanism for cells to order late mitotic events.

Cytokinesis, which leads to the physical separation of two dividing cells, is normally restrained until after nuclear division. In Saccharomyces cerevisiae, chitin synthase 2 (Chs2), which lays down the primary septum at the mother–daughter neck, also ensures proper actomyosin ring constriction during cytokinesis. During the metaphase-to-anaphase transition, phosphorylation of Chs2 by the mitotic cyclin-dependent kinase (Cdk1) retains Chs2 at the endoplasmic reticulum (ER), thereby preventing its translocation to the neck. Upon Cdk1 inactivation at the end of mitosis, Chs2 is exported from the ER and targeted to the neck. The mechanism for triggering Chs2 ER export thus far is unknown. We show here that Chs2 ER export requires the direct reversal of the inhibitory Cdk1 phosphorylation sites by Cdc14 phosphatase, the ultimate effector of the mitotic exit network (MEN). We further show that only Cdc14 liberated by the MEN after completion of chromosome segregation, and not Cdc14 released in early anaphase by the Cdc fourteen early anaphase release pathway, triggers Chs2 ER exit. Presumably, the reduced Cdk1 activity in late mitosis further favors dephosphorylation of Chs2 by Cdc14. Thus, by requiring declining Cdk1 activity and Cdc14 nuclear release for Chs2 ER export, cells ensure that septum formation is contingent upon chromosome separation and exit from mitosis.

Expression, stability, and replacement of glucan-remodeling enzymes during developmental transitions in Saccharomyces cerevisiae

Sporulation is a developmental variation of the yeast life cycle whereby four spores are produced within a diploid cell, with proliferation resuming after germination. The GAS family of glycosylphosphatidylinositol-anchored glucan-remodeling enzymes exemplifies functional interplay between paralogous genes during the yeast life cycle. GAS1 and GAS5 are expressed in vegetative cells and repressed during sporulation while GAS2 and GAS4 exhibit a reciprocal pattern. GAS3 is weakly expressed in all the conditions and encodes an inactive protein. Although Gas1p functions in cell wall formation, we show that it persists during sporulation but is relocalized from the plasma membrane to the epiplasm in a process requiring End3p-mediated endocytosis and the Sps1 protein kinase of the p21-activated kinase family. Some Gas1p is also newly synthesized and localized to the spore membrane, but this fraction is dispensable for spore formation. By way of contrast, the Gas2-Gas4 proteins, which are essential for spore wall assembly, are rapidly degraded after spore formation. On germination, Gas1p is actively synthesized and concentrated in the growing part of the spore, which is essential for its elongation. Thus Gas1p is the primary glucan-remodeling enzyme required in vegetative growth and during reentry into the proliferative state. The dynamic interplay among Gas proteins is crucial to couple glucan remodeling with morphogenesis in developmental transitions.

Yeast chitin synthase 2 activity is modulated by proteolysis and phosphorylation

Saccharomyces cerevisiae Chs2 (chitin synthase 2) synthesizes the primary septum after mitosis is completed. It is essential for proper cell separation and is expected to be highly regulated. We have expressed Chs2 and a mutant lacking the N-terminal region in Pichia pastoris in an active form at high levels. Both constructs show a pH and cation dependence similar to the wild-type enzyme, as well as increased activity after trypsin treatment. Using further biochemical analysis, we have identified two mechanisms of chitin synthase regulation. First, it is hyperactivated by a soluble yeast protease. This protease is expressed during exponential growth phase, when budding cells require Chs2 activity. Secondly, LC-MS/MS (liquid chromatography tandem MS) experiments on purified Chs2 identify 12 phosphorylation sites, all in the N-terminal domain. Four of them show the perfect sequence motif for phosphorylation by the cyclin-dependent kinase Cdk1. As we also show that phosphorylation of the N-terminal domain is important for Chs2 stability, these sites might play an important role in the cell cycle-dependent degradation of the enzyme, and thus in cell division.

Function and regulation in MAPK signaling pathways: lessons learned from the yeast Saccharomyces cerevisiae

Signaling pathways that activate different mitogen-activated protein kinases (MAPKs) elicit many of the responses that are evoked in cells by changes in certain environmental conditions and upon exposure to a variety of hormonal and other stimuli. These pathways were first elucidated in the unicellular eukaryote Saccharomyces cerevisiae (budding yeast). Studies of MAPK pathways in this organism continue to be especially informative in revealing the molecular mechanisms by which MAPK cascades operate, propagate signals, modulate cellular processes, and are controlled by regulatory factors both internal to and external to the pathways. Here we highlight recent advances and new insights about MAPK-based signaling that have been made through studies in yeast, which provide lessons directly applicable to, and that enhance our understanding of, MAPK-mediated signaling in mammalian cells.

Exit from mitosis triggers Chs2p transport from the endoplasmic reticulum to mother-daughter neck via the secretory pathway in budding yeast

Budding yeast chitin synthase 2 (Chs2p), which lays down the primary septum, localizes to the mother–daughter neck in telophase. However, the mechanism underlying the timely neck localization of Chs2p is not known. Recently, it was found that a component of the exocyst complex, Sec3p–green fluorescent protein, arrives at the neck upon mitotic exit. It is not clear whether the neck localization of Chs2p, which is a cargo of the exocyst complex, was similarly regulated by mitotic exit. We report that Chs2p was restrained in the endoplasmic reticulum (ER) during metaphase. Furthermore, mitotic exit was sufficient to cause Chs2p neck localization specifically by triggering the Sec12p-dependent transport of Chs2p out of the ER. Chs2p was “forced” prematurely to the neck by mitotic kinase inactivation at metaphase, with chitin deposition occurring between mother and daughter cells. The dependence of Chs2p exit from the ER followed by its transport to the neck upon mitotic exit ensures that septum formation occurs only after the completion of mitotic events.

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.

Cell wall integrity signaling in Saccharomyces cerevisiae

The yeast cell wall is a highly dynamic structure that is responsible for protecting the cell from rapid changes in external osmotic potential. The wall is also critical for cell expansion during growth and morphogenesis. This review discusses recent advances in understanding the various signal transduction pathways that allow cells to monitor the state of the cell wall and respond to environmental challenges to this structure. The cell wall integrity signaling pathway controlled by the small G-protein Rho1 is principally responsible for orchestrating changes to the cell wall periodically through the cell cycle and in response to various forms of cell wall stress. This signaling pathway acts through direct control of wall biosynthetic enzymes, transcriptional regulation of cell wall-related genes, and polarization of the actin cytoskeleton. However, additional signaling pathways interface both with the cell wall integrity signaling pathway and with the actin cytoskeleton to coordinate polarized secretion with cell wall expansion. These include Ca(2+) signaling, phosphatidylinositide signaling at the plasma membrane, sphingoid base signaling through the Pkh1 and -2 protein kinases, Tor kinase signaling, and pathways controlled by the Rho3, Rho4, and Cdc42 G-proteins.

Cell cycle-regulated trafficking of Chs2 controls actomyosin ring stability during cytokinesis

Cytokinesis requires the coordination of many cellular complexes, particularly those involved in the constriction and reconstruction of the plasma membrane in the cleavage furrow. We have investigated the regulation and function of vesicle transport and fusion during cytokinesis in budding yeast. By using time-lapse confocal microscopy, we show that post-Golgi vesicles, as well as the exocyst, a complex required for the tethering and fusion of these vesicles, localize to the bud neck at a precise time just before spindle disassembly and actomyosin ring contraction. Using mutants affecting cyclin degradation and the mitotic exit network, we found that targeted secretion, in contrast to contractile ring activation, requires cyclin degradation but not the mitotic exit network. Analysis of cells in late anaphase bearing exocyst and myosin V mutations show that both vesicle transport and fusion machineries are required for the completion of cytokinesis, but this is not due to a delay in mitotic exit or assembly of the contractile ring. Further investigation of the dynamics of contractile rings in exocyst mutants shows these cells may be able to initiate contraction but often fail to complete the contraction due to premature disassembly during the contraction phase. This phenotype led us to identify Chs2, a transmembrane protein targeted to the bud neck through the exocytic pathway, as necessary for actomyosin ring stability during contraction. Chs2, as the chitin synthase that produces the primary septum, thus couples the assembly of the extracellular matrix with the dynamics of the contractile ring during cytokinesis.

An interactional network of genes involved in chitin synthesis in Saccharomyces cerevisiae

BACKGROUND

In S. cerevisiae the beta-1,4-linked N-acetylglucosamine polymer, chitin, is synthesized by a family of 3 specialized but interacting chitin synthases encoded by CHS1, CHS2 and CHS3. Chs2p makes chitin in the primary septum, while Chs3p makes chitin in the lateral cell wall and in the bud neck, and can partially compensate for the lack of Chs2p. Chs3p requires a pathway of Bni4p, Chs4p, Chs5p, Chs6p and Chs7p for its localization and activity. Chs1p is thought to have a septum repair function after cell separation. To further explore interactions in the chitin synthase family and to find processes buffering chitin synthesis, we compiled a genetic interaction network of genes showing synthetic interactions with CHS1, CHS3 and genes involved in Chs3p localization and function and made a phenotypic analysis of their mutants.

RESULTS

Using deletion mutants in CHS1, CHS3, CHS4, CHS5, CHS6, CHS7 and BNI4 in a synthetic genetic array analysis we assembled a network of 316 interactions among 163 genes. The interaction network with CHS3, CHS4, CHS5, CHS6, CHS7 or BNI4 forms a dense neighborhood, with many genes functioning in cell wall assembly or polarized secretion. Chitin levels were altered in 54 of the mutants in individually deleted genes, indicating a functional relationship between them and chitin synthesis. 32 of these mutants triggered the chitin stress response, with elevated chitin levels and a dependence on CHS3. A large fraction of the CHS1-interaction set was distinct from that of the CHS3 network, indicating broad roles for Chs1p in buffering both Chs2p function and more global cell wall robustness.

CONCLUSION

Based on their interaction patterns and chitin levels we group interacting mutants into functional categories. Genes interacting with CHS3 are involved in the amelioration of cell wall defects and in septum or bud neck chitin synthesis, and we newly assign a number of genes to these functions. Our genetic analysis of genes not interacting with CHS3 indicate expanded roles for Chs4p, Chs5p and Chs6p in secretory protein trafficking and of Bni4p in bud neck organization.

Chitin synthesis in Saccharomyces cerevisiae in response to supplementation of growth medium with glucosamine and cell wall stress

In Saccharomyces cerevisiae most chitin is synthesized by Chs3p, which deposits chitin in the lateral cell wall and in the bud-neck region during cell division. We have recently found that addition of glucosamine (GlcN) to the growth medium leads to a three- to fourfold increase in cell wall chitin levels. We compared this result to the increases in cellular chitin levels associated with cell wall stress and with treatment of yeast with mating pheromone. Since all three phenomena lead to increases in precursors of chitin, we hypothesized that chitin synthesis is at least in part directly regulated by the size of this pool. This hypothesis was strengthened by our finding that addition of GlcN to the growth medium causes a rapid increase in chitin synthesis without any pronounced change in the expression of more than 6,000 genes monitored with Affymetrix gene expression chips. In other studies we found that the specific activity of Chs3p is higher in the total membrane fractions from cells grown in GlcN and from mutants with weakened cell walls. Sucrose gradient analysis shows that Chs3p is present in an inactive form in what may be Golgi compartments but as an active enzyme in other intracellular membrane-bound vesicles, as well as in the plasma membrane. We conclude that Chs3p-dependent chitin synthesis in S. cerevisiae is regulated both by the levels of intermediates of the UDP-GlcNAc biosynthetic pathway and by an increase in the activity of the enzyme in the plasma membrane.

Septins, under Cla4p regulation, and the chitin ring are required for neck integrity in budding yeast

CLA4, encoding a protein kinase of the PAK type, and CDC11, encoding a septin, were isolated in a screen for synthetic lethality with CHS3, which encodes the chitin synthase III catalytic moiety. Although Ste20p shares some essential function with Cla4p, it did not show synthetic lethality with Chs3p. cla4 and cdc11 mutants exhibited similar morphological and septin localization defects, including aberrant and ectopic septa. Myo1p, which requires septins for localization, formed abnormally wide rings in cla4 mutants. In cultures started with unbudded cells, an inhibitor of Chs3p activity, nikkomycin Z, aggravated the abnormalities of cla4 and cdc11 mutants and gave rise to enlarged necks at the mother-bud junction, leading to cell death. It is concluded that Cla4p is required for the correct localization and/or assembly of the septin ring and that both the septin ring and the Chs3p-requiring chitin ring at the mother-bud neck cooperate in maintaining the neck constricted throughout the cell cycle, a vital function in budding yeast.

A Bni4-Glc7 phosphatase complex that recruits chitin synthase to the site of bud emergence

Bni4 is a scaffold protein in the yeast Saccharomyces cerevisiae that tethers chitin synthase III to the bud neck by interacting with septin neck filaments and with Chs4, a regulatory subunit of chitin synthase III. We show herein that Bni4 is also a limiting determinant for the targeting of the type 1 serine/threonine phosphatase (Glc7) to the bud neck. Yeast cells containing a Bni4 variant that fails to associate with Glc7 fail to tether Chs4 to the neck, due in part to the failure of Bni4(V831A/F833A) to localize properly. Conversely, the Glc7-129 mutant protein fails to bind Bni4 properly and glc7-129 mutants exhibit reduced levels of Bni4 at the bud neck. Bni4 is phosphorylated in a cell cycle-dependent manner and Bni4(V831A/F833A) is both hyperphosphorylated and mislocalized in vivo. Yeast cells lacking the protein kinase Hsl1 exhibit increased levels of Bni4-GFP at the bud neck. GFP-Chs4 does not accumulate at the incipient bud site in either a bni4::TRP1 or a bni4(V831A/F833A) mutant but does mobilize to the neck at cytokinesis. Together, these results indicate that the formation of the Bni4-Glc7 complex is required for localization to the site of bud emergence and for subsequent targeting of chitin synthase.

The septation apparatus, an autonomous system in budding yeast

Actomyosin ring contraction and chitin primary septum deposition are interdependent processes in cell division of budding yeast. By fusing Myo1p, as representative of the contractile ring, and Chs2p for the primary septum, to different fluorescent proteins we show herein that the two processes proceed essentially at the same location and simultaneously. Chs2p differs from Myo1p in that it reflects the changes in shape of the plasma membrane to which it is attached and in that it is packed after its action into visible endocytic vesicles for its disposal. To ascertain whether this highly coordinated system could function independently of other cell cycle events, we reexamined the septum-like structures made by the septin mutant cdc3 at various sites on the cell cortex at the nonpermissive temperature. With the fluorescent fusion proteins mentioned above, we observed that in cdc3 at 37 degrees C both Myo1p and Chs2p colocalize at different spots of the cell cortex. A contraction of the Myo1p patch could also be detected, as well as that of a Chs2p patch, with subsequent appearance of vesicles. Furthermore, the septin Cdc12p, fused with yellow or cyan fluorescent protein, also colocalized with Myo1p and Chs2p at the aberrant locations. The formation of delocalized septa did not require nuclear division. We conclude that the septation apparatus, composed of septins, contractile ring, and the chitin synthase II system, can function at ectopic locations autonomously and independently of cell division, and that it can recruit the other elements necessary for the formation of secondary septa.

Chitin synthesis in a gas1 mutant of Saccharomyces cerevisiae

The existence of a compensatory mechanism in response to cell wall damage has been proposed in yeast cells. The increase of chitin accumulation is part of this response. In order to study the mechanism of the stress-related chitin synthesis, we tested chitin synthase I (CSI), CSII, and CSIII in vitro activities in the cell-wall-defective mutant gas1 delta. CSI activity increased twofold with respect to the control, a finding in agreement with an increase in the expression of the CHS1 gene. However, deletion of the CHS1 gene did not affect the phenotype of the gas1 delta mutant and only slightly reduced the chitin content. Interestingly, in chs1 gas1 double mutants the lysed-bud phenotype, typical of chs1 null mutant, was suppressed, although in gas1 cells there was no reduction in chitinase activity. CHS3 expression was not affected in the gas1 mutant. Deletion of the CHS3 gene severely compromised the phenotype of gas1 cells, despite the fact that CSIII activity, assayed in membrane fractions, did not change. Furthermore, in chs3 gas1 cells the chitin level was about 10% that of gas1 cells. Thus, CSIII is the enzyme responsible for the hyperaccumulation of chitin in response to cell wall stress. However, the level of enzyme or the in vitro CSIII activity does not change. This result suggests that an interaction with a regulatory molecule or a posttranslational modification, which is not preserved during membrane fractionation, could be essential in vivo for the stress-induced synthesis of chitin.

Characterization of Yeast Yea4p, a uridine diphosphate-N-acetylglucosamine transporter localized in the endoplasmic reticulum and required for chitin synthesis

Chitin is an essential cell wall component, synthesis of which is regulated throughout the cell cycle in the yeast Saccharomyces cerevisiae. We cloned an S. cerevisiae gene, YEA4, whose product is homologous to the Kluyveromyces lactis uridine diphosphate-N-acetylglucosamine (UDP-GlcNAc) transporter. An epitope-tagged Yea4p localized mainly in the 10,000 x g pellet (P2), suggesting endoplasmic reticulum (ER) localization. Membrane vesicles from the P2 fraction showed an 8-fold higher UDP-GlcNAc transport activity in cells harboring a multicopy YEA4 plasmid than in cells harboring vector alone. The activity distribution is identical with the protein distribution in P2, whether the gene is overexpressed or not, suggesting its native localization in P2. Immunolocalization of epitope-tagged Yea4p further revealed ER localization. The increase in transport activity due to the YEA4 overexpression is specific for UDP-GlcNAc, but not for UDP-galactose and GDP-mannose. Deltayea4-disrupted cells showed a reduced rate of UDP-GlcNAc transport, contained less chitin, and were larger and rounder in shape than the wild type cells. Our results indicate that YEA4 encodes an ER-localized UDP-GlcNAc transporter that is required for cell wall chitin synthesis in S. cerevisiae.

WdChs4p, a homolog of chitin synthase 3 in Saccharomyces cerevisiae, alone cannot support growth of Wangiella (Exophiala) dermatitidis at the temperature of infection

By using improved transformation methods for Wangiella dermatitidis, and a cloned fragment of its chitin synthase 4 structural gene (WdCHS4) as a marking sequence, the full-length gene was rescued from the genome of this human pathogenic fungus. The encoded chitin synthase product (WdChs4p) showed high homology with Chs3p of Saccharomyces cerevisiae and other class IV chitin synthases, and Northern blotting showed that WdCHS4 was expressed at constitutive levels under all conditions tested. Reduced chitin content, abnormal yeast clumpiness and budding kinetics, and increased melanin secretion resulted from the disruption of WdCHS4 suggesting that WdChs4p influences cell wall structure, cellular reproduction, and melanin deposition, respectively. However, no significant loss of virulence was detected when the wdchs4Delta strain was tested in an acute mouse model. Using a wdchs1Delta wdchs2Delta wdchs3Delta triple mutant of W. dermatitidis, which grew poorly but adequately at 25 degrees C, we assayed WdChs4p activity in the absence of activities contributed by its three other WdChs proteins. Maximal activity required trypsin activation, suggesting a zymogenic nature. The activity also had a pH optimum of 7.5, was most stimulated by Mg(2+), and was more inhibited by polyoxin D than by nikkomycin Z. Although the WdChs4p activity had a broad temperature optimum between 30 to 45 degrees C in vitro, this activity alone did not support the growth of the wdchs1Delta wdchs2Delta wdchs3Delta triple mutant at 37 degrees C, a temperature commensurate with infection.

Chs7p, a new protein involved in the control of protein export from the endoplasmic reticulum that is specifically engaged in the regulation of chitin synthesis in Saccharomyces cerevisiae

The Saccharomyces cerevisiae CHS7 gene encodes an integral membrane protein located in the ER which is directly involved in chitin synthesis through the regulation of chitin synthase III (CSIII) activity. In the absence of CHS7 product, Chs3p, but not other secreted proteins, is retained in the ER, leading to a severe defect in CSIII activity and consequently, to a reduced rate of chitin synthesis. In addition, chs7 null mutants show the yeast phenotypes associated with a lack of chitin: reduced mating efficiency and lack of the chitosan ascospore layer, clear indications of Chs7p function throughout the S. cerevisiae biological cycle. CHS3 overexpression does not lead to increased levels of CSIII because the Chs3p excess is retained in the ER. However, joint overexpression of CHS3 and CHS7 increases the export of Chs3p from the ER and this is accompanied by a concomitant increase in CSIII activity, indicating that the amount of Chs7p is a limiting factor for CSIII activity. Accordingly, CHS7 transcription is increased when elevated amounts of chitin synthesis are detected. These results show that Chs7p forms part of a new mechanism specifically involved in Chs3p export from the ER and consequently, in the regulation of CSIII activity.

Chs6p-dependent anterograde transport of Chs3p from the chitosome to the plasma membrane in Saccharomyces cerevisiae

Chitin synthase III (CSIII), an enzyme required to form a chitin ring in the nascent division septum of Saccharomyces cerevisiae, may be transported to the cell surface in a regulated manner. Chs3p, the catalytic subunit of CSIII, requires the product of CHS6 to be transported to or activated at the cell surface. We find that chs6Delta strains have morphological abnormalities similar to those of chs3 mutants. Subcellular fractionation and indirect immunofluorescence indicate that Chs3p distribution is altered in chs6 mutant cells. Order-of-function experiments using end4-1 (endocytosis-defective) and chs6 mutants indicate that Chs6p is required for anterograde transport of Chs3p from an internal endosome-like membrane compartment, the chitosome, to the plasma membrane. As a result, chs6 strains accumulate Chs3p in chitosomes. Chs1p, a distinct chitin synthase that acts during or after cell separation, is transported normally in chs6 mutants, suggesting that Chs1p and Chs3p are independently packaged during protein transport through the late secretory pathway.

Defects in assembly of the extracellular matrix are responsible for altered morphogenesis of a Candida albicans phr1 mutant

Analysis of Candida albicans cells using antibodies directed against Gas1p/Ggp1p, Saccharomyces cerevisiae homolog of Phr1p, revealed that Phr1p is a glycoprotein of about 88 kDa whose accumulation increases with the rise of external pH. This polypeptide is present both in the yeast form and during germ tube induction. In the Phr1- cells at pH 8 the solubility of glucans in alkali is greatly affected. In the parental strain the alkali-soluble/-insoluble glucan ratio shows a 50% decrease at pH 8 with respect to pH 4.5, whereas in the null mutant it is unchanged, indicating the lack of a polymer cross-linker activity induced by the rise of pH. The mutant has a sixfold increase in chitin level and is hypersensitive to calcofluor. Consistently with a role of chitin in strengthening the cell wall, Phr1- cells are more sensitive to nikkomycin Z than the parental strain.

CHS5, a gene involved in chitin synthesis and mating in Saccharomyces cerevisiae

The CHS5 locus of Saccharomyces cerevisiae is important for wild-type levels of chitin synthase III activity. chs5 cells have reduced levels of this activity. To further understand the role of CHS5 in yeast, the CHS5 gene was cloned by complementation of the Calcofluor resistance phenotype of a chs5 mutant. Transformation of the mutant with a plasmid carrying CHS5 restored Calcofluor sensitivity, wild-type cell wall chitin levels, and chitin synthase III activity levels. DNA sequence analysis reveals that CHS5 encodes a unique polypeptide of 671 amino acids with a molecular mass of 73,642 Da. The predicted sequence shows a heptapeptide repeated 10 times, a carboxy-terminal lysine-rich tail, and some similarity to neurofilament proteins. The effects of deletion of CHS5 indicate that it is not essential for yeast cell growth; however, it is important for mating. Deletion of CHS3, the presumptive structural gene for chitin synthase III activity, results in a modest decrease in mating efficiency, whereas chs5delta cells exhibit a much stronger mating defect. However, chs5 cells produce more chitin than chs3 mutants, indicating that CHS5 plays a role in other processes besides chitin synthesis. Analysis of mating mixtures of chs5 cells reveals that cells agglutinate and make contact but fail to undergo cell fusion. The chs5 mating defect can be partially rescued by FUS1 and/or FUS2, two genes which have been implicated previously in cell fusion, but not by FUS3. In addition, mating efficiency is much lower in fus1 fus2 x chs5 than in fus1 fus2 x wild type crosses. Our results indicate that Chs5p plays an important role in the cell fusion step of mating.

Targeting of chitin synthase 3 to polarized growth sites in yeast requires Chs5p and Myo2p

Chitin is an essential structural component of the yeast cell wall whose deposition is regulated throughout the yeast life cycle. The temporal and spatial regulation of chitin synthesis was investigated during vegetative growth and mating of Saccharomyces cerevisiae by localization of the putative catalytic subunit of chitin synthase III, Chs3p, and its regulator, Chs5p. Immunolocalization of epitope-tagged Chs3p revealed a novel localization pattern that is cell cycle-dependent. Chs3p is polarized as a diffuse ring at the incipient bud site and at the neck between the mother and bud in small-budded cells; it is not found at the neck in large-budded cells containing a single nucleus. In large-budded cells undergoing cytokinesis, it reappears as a ring at the neck. In cells responding to mating pheromone, Chs3p is found throughout the projection. The appearance of Chs3p at cortical sites correlates with times that chitin synthesis is expected to occur. In addition to its localization at the incipient bud site and neck, Chs3p is also found in cytoplasmic patches in cells at different stages of the cell cycle. Epitope-tagged Chs5p also localizes to cytoplasmic patches; these patches contain Kex2p, a late Golgi-associated enzyme. Unlike Chs3p, Chs5p does not accumulate at the incipient bud site or neck. Nearly all Chs3p patches contain Chs5p, whereas some Chs5p patches lack detectable Chs3p. In the absence of Chs5p, Chs3p localizes in cytoplasmic patches, but it is no longer found at the neck or the incipient bud site, indicating that Chs5p is required for the polarization of Chs3p. Furthermore, Chs5p localization is not affected either by temperature shift or by the myo2-66 mutation, however, Chs3p polarization is affected by temperature shift and myo2-66. We suggest a model in which Chs3p polarization to cortical sites in yeast is dependent on both Chs5p and the actin cytoskeleton/Myo2p.

Increase in chitin as an essential response to defects in assembly of cell wall polymers in the ggp1delta mutant of Saccharomyces cerevisiae

The GGP1/GAS1 gene codes for a glycosylphosphatidylinositol-anchored plasma membrane glycoprotein of Saccharomyces cerevisiae. The ggp1delta mutant shows morphogenetic defects which suggest changes in the cell wall matrix. In this work, we have investigated cell wall glucan levels and the increase of chitin in ggp1delta mutant cells. In these cells, the level of alkali-insoluble 1,6-beta-D-glucan was found to be 50% of that of wild-type cells and was responsible for the observed decrease in the total alkali-insoluble glucan. Moreover, the ratio of alkali-soluble to alkali-insoluble glucan almost doubled, suggesting a change in glucan solubility. The increase of chitin in ggp1delta cells was found to be essential since the chs3delta ggp1delta mutations determined a severe reduction in the growth rate and in cell viability. Electron microscopy analysis showed the loss of the typical structure of yeast cell walls. Furthermore, in the chs3delta ggp1delta cells, the level of alkali-insoluble glucan was 57% of that of wild-type cells and the alkali-soluble/alkali-insoluble glucan ratio was doubled. We tested the effect of inhibition of chitin synthesis also by a different approach. The ggp1delta cells were treated with nikkomycin Z, a well-known inhibitor of chitin synthesis, and showed a hypersensitivity to this drug. In addition, studies of genetic interactions with genes related to the construction of the cell wall indicate a synthetic lethal effect of the ggp1delta kre6delta and the ggp1delta pkc1delta combined mutations. Our data point to an involvement of the GGP1 gene product in the cross-links between cell wall glucans (1,3-beta-D-glucans with 1,6-beta-D-glucans and with chitin). Chitin is essential to compensate for the defects due to the lack of Ggp1p. Moreover, the activities of Ggp1p and Chs3p are essential to the formation of the organized structure of the cell wall in vegetative cells.

Two sporulation-specific chitin deacetylase-encoding genes are required for the ascospore wall rigidity of Saccharomyces cerevisiae

The formation of the ascospore wall of Saccharomyces cerevisiae requires the coordinate activity of enzymes involved in the biosynthesis of its components such as chitosan, the deacetylated form of chitin. We have cloned the CDA1 and CDA2 genes which together account for the total chitin deacetylase activity of the organism. We have shown that expression of these genes is restricted to a distinct time period during sporulation. The two genes are functionally redundant, each contributing equally to the total chitin deacetylase activity. Diploids disrupted for both genes sporulate as efficiently as wild type cells, and the resulting mutant spores are viable under standard laboratory conditions. However, they fail to emit the natural fluorescence of yeast spores imparted by the dityrosine residues of the outermost ascospore wall layer. Moreover, mutant spores are relatively sensitive to hydrolytic enzymes, ether, and heat shock, a fact that underscores the importance of the CDA genes for the proper formation of the ascospore wall.

Chs1p and Chs3p, two proteins involved in chitin synthesis, populate a compartment of the Saccharomyces cerevisiae endocytic pathway

In Saccharomyces cerevisiae, the synthesis of chitin, a cell-wall polysaccharide, is temporally and spatially regulated with respect to the cell cycle and morphogenesis. Using immunological reagents, we found that steady-state levels of Chs1p and Chs3p, two chitin synthase enzymes, did not fluctuate during the cell cycle, indicating that they are not simply regulated by synthesis and degradation. Previous cell fractionation studies demonstrated that chitin synthase I activity (CSI) exists in a plasma membrane form and in intracellular membrane-bound particles called chitosomes. Chitosomes were proposed to act as a reservoir for regulated transport of chitin synthase enzymes to the division septum. We found that Chs1p and Chs3p resided partly in chitosomes and that this distribution was not cell cycle regulated. Pulse-chase cell fractionation experiments showed that chitosome production was blocked in an endocytosis mutant (end4-1), indicating that endocytosis is required for the formation or maintenance of chitosomes. Additionally, Ste2p, internalized by ligand-induced endocytosis, cofractionated with chitosomes, suggesting that these membrane proteins populate the same endosomal compartment. However, in contrast to Ste2p, Chs1p and Chs3p were not rapidly degraded, thus raising the possibility that the temporal and spatial regulation of chitin synthesis is mediated by the mobilization of an endosomal pool of chitin synthase enzymes.

Differential trafficking and timed localization of two chitin synthase proteins, Chs2p and Chs3p

The deposition of the polysaccharide chitin in the Saccharomyces cerevisiae cell wall is temporally and spatially regulated. Chitin synthase III (Chs3p) synthesizes a ring of chitin at the onset of bud emergence, marking the base of the incipient bud. At the end of mitosis, chitin synthase II (Chs2p) deposits a disk of chitin in the mother-bud neck, forming the primary division septum. Using indirect immunofluorescence microscopy, we have found that these two integral membrane proteins localize to the mother-bud neck at distinct times during the cell cycle. Chs2p is found at the neck at the end of mitosis, whereas Chs3p localizes to a ring on the surface of cells about to undergo bud emergence and in the mother-bud neck of small-budded cells. Cell synchronization and pulse-chase experiments suggest that the timing of Chs2p localization results from cell cycle-specific synthesis coupled to rapid degradation. Chs2p degradation depends on the vacuolar protease encoded by PEP4, indicating that Chs2p is destroyed in the vacuole. Temperature-sensitive mutations that block either the late secretory pathway (sec1-1) or the internalization step of endocytosis (end4-1) also prevent Chs2p degradation. In contrast, Chs3p is synthesized constitutively and is metabolically stable, indicating that Chs2p and Chs3p are subject to different modes of regulation. Differential centrifugation experiments show that a significant proportion of Chs3p resides in an internal compartment that may correspond to a vesicular species called the chitosome (Leal-Morales, C.A., C.E. Bracker, and S. Bartnicki-Garcia. 1988, Proc. Natl. Acad. Sci. USA. 85:8516-8520; Flores Martinez, A., and J. Schwencke. 1988. Biochim. Biophys. Acta. 946:328-336). Fractionation of membranes prepared from mutants defective in internalization (end3-1 and end4-1) indicate that the Chs3p-containing vesicles are endocytically derived. Collectively, these data suggest that the trafficking of Chs2p and Chs3p diverges after endocytosis; Chs3p is not delivered to the vacuole, but instead may be recycled.

Yeast chitin synthases 1 and 2 consist of a non-homologous and dispensable N-terminal region and of a homologous moiety essential for function

Predicted protein sequences of fungal chitin synthases can be divided into a non-homologous N-terminal region and a C-terminal region that shows significant homology among the various synthases. We have explored the function of these domains by constructing a series of nested deletions, extending from either end, in the CHS1 and CHS2 genes of Saccharomyces cerevisiae. In both cases, most or all of the sequences encoding the non-homologous N-terminal region (one-third of the protein for Chs1p and about one-fourth for Chs2p) could be excised, with little effect on the enzymatic activity in vitro of the corresponding synthase or on its function in vivo. However, further small deletions (20-25 amino acids) into the homologous region were deleterious to enzymatic activity and function, and often led to changes in the zymogenic character of the enzymes. Similarly, relatively small (about 75 amino acids) deletions from the C-terminus resulted in loss of enzymatic activity and function of both synthases. Thus, it appears that all the information necessary for membrane localization, enzymatic activity and function resides in the homologous regions of Chs1p and Chs2p, a situation that may also apply to other chitin synthases.

chs-4, a class IV chitin synthase gene from Neurospora crassa

In Saccharomyces cerevisiae, most of the cellular chitin is produced by chitin synthase III, which requires the product encoded by the CSD2/CAL1/DIT101/KT12 gene. We have identified, isolated and structurally characterized as CSD2/CAL1/DIT101/KT12 homologue in the filamentous ascomycete Neurospora crassa and have used a "reverse genetics" approach to determine its role in vivo. The yeast gene was used as a heterologous probe for the isolation of a N. crassa gene(designated chs-4) encoding a polypeptide belonging to a class of chitin synthases which we have designated class IV. The predicted polypeptide encoded by this gene is highly similar to those of S. cerevisiae and Candida albicans. N. crassa strains in which chs-4 had been inactivated by the Repeat-Induced point mutation (RIP) process grew and developed in a normal manner under standard growth conditions. However, when grown in the presence of sorbose (a carbon source which induces morphological changes accompanied by elevated chitin content), chitin levels in the chs-4RIP strain were significantly lower than those observed in the wild type. We suggest that CHS4 may serve as an auxiliary enzyme in N. crassa and that, in contrast to yeasts, it is possible that filamentous fungi may have more than one class IV chitin synthase.