Distribution of chitin in the yeast cell wall. An ultrastructural and chemical study.

The distribution of chitin in Saccharomyces cervisiae primary septa and cell walls was studied with three methods: electron microscopy of colloidal gold particles coated either with wheat germ agglutinin or with one of two different chitinases, fluorescence microscopy with fluorescein isothiocyanate derivatives of the same markers, and enzymatic treatments of [14C]glucosamine-labeled cells. The septa were uniformly and heavily labeled with the gold-attached markers, an indication that chitin was evenly distributed throughout. To study the localization of chitin in lateral walls, alkali-extracted cell ghosts were used. Observations by electron and fluorescence microscopy suggest that lectin-binding material is uniformly distributed over the whole cell ghost wall. This material also appears to be chitin, on the basis of the analysis of the products obtained after treatment of 14C-labeled cell ghosts with lytic enzymes. The chitin of lateral walls can be specifically removed by treatment with beta-(1 leads to 6)-glucanase containing a slight amount of chitinase. During this incubation approximately 7% of the total radioactivity is solubilized, about the same amount liberated when lateral walls of cell ghosts are completely digested with snail glucanase yield primary septa. It is concluded that the remaining chitin, i.e., greater than 90% of the total, is in the septa. The facilitation of chitin removal from the cell wall by beta-(1 leads to 6)-glucanase indicates a strong association between chitin and beta-(1 leads to 6)-glucan. Covalent linkages between the two polysaccharides were not detected but cannot be excluded.

Chitin synthase 1, an auxiliary enzyme for chitin synthesis in Saccharomyces cerevisiae.

Previously, we showed that chitin synthase 2 (Chs2) is required for septum formation in Saccharomyces cerevisiae, whereas chitin synthase 1 (Chs1) does not appear to be an essential enzyme. However, in strains carrying a disrupted CHS1 gene, frequent lysis of buds is observed. Lysis occurs after nuclear separation and appears to result from damage to the cell wall, as indicated by osmotic stabilization and by a approximately 50-nm orifice at the center of the birth scar. Lysis occurs at a low pH and is prevented by buffering the medium above pH 5. A likely candidate for the lytic system is a previously described chitinase that is probably involved in cell separation. The chitinase has a very acidic pH optimum and a location in the periplasmic space that exposes it to external pH. Accordingly, allosamidin, a specific chitinase inhibitor, substantially reduced the number of lysed cells. Because the presence of Chs1 in the cell abolishes lysis, it is concluded that damage to the cell wall is caused by excessive chitinase activity at acidic pH, which can normally be repaired through chitin synthesis by Chs1. The latter emerges as an auxiliary or emergency enzyme. Other experiments suggest that both Chs1 and Chs2 collaborate in the repair synthesis of chitin, whereas Chs1 cannot substitute for Chs2 in septum formation.

The GTP-binding protein Rho1p is required for cell cycle progression and polarization of the yeast cell.

Previous work showed that the GTP-binding protein Rho1p is required in the yeast, Saccharomyces cerevisiae, for activation of protein kinase C (Pkc1p) and for activity and regulation of beta(1-->3)glucan synthase. Here we demonstrate a hitherto unknown function of Rho1p required for cell cycle progression and cell polarization. Cells of mutant rho1(E45I) in the G1 stage of the cell cycle did not bud at 37 degrees C. In those cells actin reorganization and recruitment to the presumptive budding site did not take place at the nonpermissive temperature. Two mutants in adjacent amino acids, rho1(V43T) and rho1(F44Y), showed a similar behavior, although some budding and actin polarization occurred at the nonpermissive temperature. This was also the case for rho1(E45I) when placed in a different genetic background. Cdc42p and Spa2p, two proteins that normally also move to the bud site in a process independent from actin organization, failed to localize properly in rho1(E45I). Nuclear division did not occur in the mutant at 37 degrees C, although replication of DNA proceeded slowly. The rho1 mutants were also defective in the formation of mating projections and in congregation of actin at the projections in the presence of mating pheromone. The in vitro activity of beta(1-->3)glucan synthase in rho1 (E45I), although diminished at 37 degrees C, appeared sufficient for normal in vivo function and the budding defect was not suppressed by expression of a constitutively active allele of PKC1. Reciprocally, when Pkc1p function was eliminated by the use of a temperature-sensitive mutation and beta(1-->3)glucan synthesis abolished by an echinocandin-like inhibitor, a strain carrying a wild-type RHO1 allele was able to produce incipient buds. Taken together, these results reveal a novel function of Rho1p that must be executed in order for the yeast cell to polarize.

CAL1, a gene required for activity of chitin synthase 3 in Saccharomyces cerevisiae.

The CAL1 gene was cloned by complementation of the defect in Calcofluor-resistant calR1 mutants of Saccharomyces cerevisiae. Transformation of the mutants with a plasmid carrying the appropriate insert restored Calcofluor sensitivity, wild-type chitin levels and normal spore maturation. Southern blots using the DNA fragment as a probe showed hybridization to a single locus. Allelic tests indicated that the cloned gene corresponded to the calR1 locus. The DNA insert contains a single open-reading frame encoding a protein of 1,099 amino acids with a molecular mass of 124 kD. The predicted amino acid sequence shows several regions of homology with those of chitin synthases 1 and 2 from S. cerevisiae and chitin synthase 1 from Candida albicans. calR1 mutants have been found to be defective in chitin synthase 3, a trypsin-independent synthase. Transformation of the mutants with a plasmid carrying CAL1 restored chitin synthase 3 activity; however, overexpression of the enzyme was not achieved even with a high copy number plasmid. Since Calcofluor-resistance mutations different from calR1 also result in reduced levels of chitin synthase 3, it is postulated that the products of some of these CAL genes may be limiting for expression of the enzymatic activity. Disruption of the CAL1 gene was not lethal, indicating that chitin synthase 3 is not an essential enzyme for S. cerevisiae.

The function of chitin synthases 2 and 3 in the Saccharomyces cerevisiae cell cycle.

The morphology of three Saccharomyces cerevisiae strains, all lacking chitin synthase 1 (Chs1) and two of them deficient in either Chs3 (calR1 mutation) or Chs2 was observed by light and electron microscopy. Cells deficient in Chs2 showed clumpy growth and aberrant shape and size. Their septa were very thick; the primary septum was absent. Staining with WGA-gold complexes revealed a diffuse distribution of chitin in the septum, whereas chitin was normally located at the neck between mother cell and bud and in the wall of mother cells. Strains deficient in Chs3 exhibited minor abnormalities in budding pattern and shape. Their septa were thin and trilaminar. Staining for chitin revealed a thin line of the polysaccharide along the primary septum; no chitin was present elsewhere in the wall. Therefore, Chs2 is specific for primary septum formation, whereas Chs3 is responsible for chitin in the ring at bud emergence and in the cell wall. Chs3 is also required for chitin synthesized in the presence of alpha-pheromone or deposited in the cell wall of cdc mutants at nonpermissive temperature, and for chitosan in spore walls. Genetic evidence indicated that a mutant lacking all three chitin synthases was inviable; this was confirmed by constructing a triple mutant rescued by a plasmid carrying a CHS2 gene under control of a GAL1 promoter. Transfer of the mutant from galactose to glucose resulted in cell division arrest followed by cell death. We conclude that some chitin synthesis is essential for viability of yeast cells.

Chitin synthetase distribution on the yeast plasma membrane.

Purified, intact yeast plasma membranes were allowed to synthesize chitin, and the nascent chains of polysaccharide were observed either by the fluorescence produced with a brightener or by autoradiography. By both methods, it was concluded that the newly formed chitin emerged at many sites on each membrane. Thus, the synthetase that catalyzes chitin formation has a similar distribution. Since chitin synthetase is found mainly in a zymogen form, these results confirm the hypothesis that initiation of the chitinous primary septum of Saccharomyces occurs by localized activation of the uniformly distributed zymogen.

Rho1p, a yeast protein at the interface between cell polarization and morphogenesis.

The enzyme that catalyzes the synthesis of the major structural component of the yeast cell wall, beta(1-->3)-D-glucan synthase (also known as 1,3-beta-glucan synthase), requires a guanosine triphosphate (GTP) binding protein for activity. The GTP binding protein was identified as Rho1p. The rho1 mutants were defective in GTP stimulation of glucan synthase, and the defect was corrected by addition of purified or recombinant Rho1p. A protein missing in purified preparations from a rho1 strain was identified as Rho1p. Rho1p also regulates protein kinase C, which controls a mitogen-activated protein kinase cascade. Experiments with a dominant positive PKC1 gene showed that the two effects of Rho1p are independent of each other. The colocalization of Rho1p with actin patches at the site of bud emergence and the role of Rho1p in cell wall synthesis emphasize the importance of Rho1p in polarized growth and morphogenesis.

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.

Chitin synthesis and localization in cell division cycle mutants of Saccharomyces cerevisiae.

Growth of Saccharomyces cerevisiae cell cycle mutants cdc3, cdc4, cdc7, cdc24, and cdc28 at a nonpermissive temperature (37 degrees C) resulted in increased accumulation of chitin relative to other cell wall components, as compared with that observed at a permissive temperature (25 degrees C). Wild-type cells showed the same chitin/carbohydrate ratio at both temperatures, whereas mutants cdc13 and cdc21 yielded only a small increase in the ratio at 37 degrees C. These results confirm and extend those reported by B. F. Sloat and J. R. Pringle (Science 200:1171-1173, 1978) for mutant cdc24. The distribution of chitin in the cell wall was studied by electron microscopy, by specific staining with wheat germ agglutinin-colloidal gold complexes. At the permissive temperature, chitin was restricted to the septal region in all strains, whereas at 37 degrees C a generalized distribution of chitin in the cell wall was observed in all mutants. These results do not support a unique interdependence between the product of the cdc24 gene and localization of chitin deposition; they suggest that unbalanced conditions created in the cell by arresting the cycle at different stages result in generalized activation of the chitin synthetase zymogen. Thus, blockage of an event in the cell cycle may lead to consequences that are not functionally related to that event under normal conditions.

Biosynthesis of cell wall and septum during yeast growth.

The primary septum that forms in yeast cells at cytokinesis consists of the polysaccharide chitin. Three chitin synthetases (Chs1, Chs2 and Chs3) have been identified in Saccharomyces cerevisiae. Cloning and disruption of the respective genes showed that Chs3 is responsible for the formation of a chitin ring at the base of an emerging bud and of chitin dispersed in the cell wall, whereas Chs2 catalyzes the synthesis of a chitin disc that completes the primary septum. Chs1 acts as a repair enzyme, replenishing chitin lost through excessive action of a chitinase that facilitates cell separation by degrading part of the septum. The major structural polysaccharide of the yeast cell wall is beta(1->3)glucan. The glucan synthetase complex is bound to the plasma membrane. By differential extraction of the membranes with salt and detergents two solubilized fractions have been obtained which are required, in addition to GTP, for glucan synthesis. Further purification of one of these fractions led to results that indicate a role for other proteins in the modulation of GTP stimulation. A G-protein system appears to function in the regulation of beta(1->3) glucan and cell wall formation in vivo.

The effect of papulacandin B on (1----3)-beta-D-glucan synthetases. A possible relationship between inhibition and enzyme conformation.

The antibiotic, papulacandin B, inhibited growth or (1----3)-beta-D-glucan synthetase (or both) in the fungi Saccharomyces cerevisiae, Hansenula anomala, Neurospora crassa, Cryptococcus laurentii, Schizophyllum commune and Wangiella dermatitidis. No effect was observed on Achlya ambisexualis. There was no apparent correlation between the inhibition of growth and that of the synthetase. With most of the fungal extracts, the inhibition of glucan synthetase by papulacandin B became less pronounced as the substrate (UDP-glucose) concentration was decreased. At very low levels of UDP-glucose, with the enzymes from S. cerevisiae and W. dermatitidis, the antibiotic stimulated the activity of glucan synthetase. As further studied with the W. dermatitidis enzyme, those low concentrations of UDP-glucose corresponded to a sigmoidal portion of the rate vs. substrate curve. The sigmoid segment of the curve extended to higher concentrations of UDP-glucose as the temperature was increased. Concomitantly, the range of substrate concentrations at which papulacandin B stimulated the reaction or was noninhibitory was broadened. It is tentatively concluded that glucan synthetase may exist in more than one interconvertible form. The stimulatory effect of papulacandin B is possibly due to preferential binding to the active form of the enzyme. The equilibrium between these forms could be shifted by structural changes in the membrane in which the enzyme is embedded. The lack of correlation between the effects of papulacandin B in whole cells and in extracts is discussed in terms of the variations in membrane structure in the two situations.

Differential inhibition of chitin synthetases 1 and 2 from Saccharomyces cerevisiae by polyoxin D and nikkomycins.

Polyoxin D, nikkomycin X, and nikkomycin Z are all competitive inhibitors of chitin synthetase 2 (Chs2), the essential enzyme for primary septum formation in Saccharomyces cerevisiae, and of Chs1, a repair enzyme. However, Chs2 is more resistant to these antibiotics than Chs1. When Co2+, the best stimulator of Chs2, was used in the assay for this enzyme, the differences in the Ki values for nikkomycins between the two isozymes reached 3 orders of magnitude. These results point to differences in the active sites of the two isozymes. Polyoxin D was much more effective than nikkomycin Z in inhibiting cell growth. This underlines the importance of the choice of enzyme and of assay conditions when cell wall-synthesizing enzymes are used in screens for possible antifungal agents.

Solubilization and partial purification of yeast chitin synthetase. Confirmation of the zymogenic nature of the enzyme.

Chitin synthetase was solubilized with digitonin from a particulate yeast fraction. The solubilized enzyme, which did not sediment at 200,000 X g and emerged after the void volume in a Sepharose 6B column, was active only after treatment with a protease. This confirms that chitin synthetase exists in the plasma membrane as a zymogen and that initiation of the chitin septum occurs by localized activation of the enzyme. By differential extraction with sodium cholate and digitonin, followed by chromatography on Sepharose 6B, a 20-fold purification of the enzyme was achieved with respect to the crude particles. The purified enzyme showed a requirement for a phospholipid; phosphatidylserine and lysophosphatidylserine were the best activators. Unsaturated fatty acids strongly inhibited synthetase activity, whereas their saturated counterparts were inert. The solubilized enzyme catalyzed the formation of insoluble chitin in the absence of added primer. The synthetic polysaccharide was examined by electron microscopy and found to consist of lozenge-shaped particles about 60 nm long and 10 nm wide.