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.     

Dynamic analysis of the KlGAL regulatory system in Kluyveromyces lactis: a comparative study with Saccharomyces cerevisiae

The GAL regulatory system is highly conserved in yeast species of Saccharomyces cerevisiae and Kluyveromyces lactis. While the GAL system is a well studied system in S. cerevisiae, the dynamic behavior of the KlGAL system in K. lactis has not been characterized. Here, we have characterized the GAL system in yeast K. lactis by developing a dynamic model and comparing its performance to its not-so-distant cousin S. cerevisiae. The present analysis demonstrates the significance of the autoregulatory feedbacks due to KlGal4p, KlGal80p, KlGal1p and Lac12p on the dynamic performance of the KlGAL switch. The model predicts the experimentally observed absence of bistability in the wild type strain of K. lactis, unlike the short term memory of preculturing conditions observed in S. cerevisiae. The performance of the GAL switch is distinct for the two yeast species although they share similarities in the molecular components. The analysis suggests that the whole genome duplication of S. cerevisiae, which resulted in a dedicated inducer protein, Gal3p, may be responsible for the high sensitivity of the system to galactose concentrations. On the other hand, K. lactis uses a bifunctional protein as an inducer in addition to its galactokinase activity, which restricts its regulatory role and hence higher galactose levels in the medium are needed to trigger the GAL system. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s11693-011-9082-7) contains supplementary material, which is available to authorized users.     

Transformation of Kluyveromyces fragilis

For the transformation of the yeast species Kluyveromyces fragilis, we have constructed a vector containing a bacterial kanamycin resistance (Kmr) gene, the TRP1 gene of Saccharomyces cerevisiae, and an autonomously replicating sequence of Kluyveromyces lactis called KARS2 . By utilizing the method based on treatment by alkali cations and with the Kmr gene as the selective marker, a wild-type strain of K. fragilis was transformed to resistance against the antibiotic G418 . In the transformed cell the plasmid replicates autonomously. The same plasmid could also be used to transform S. cerevisiae trp1 mutant to Trp+. Thus, KARS2 of K. lactis enables the vector to replicate in K. fragilis, K. lactis, and S. cerevisiae, whereas ARS1 of S. cerevisiae allows autonomous replication only in S. cerevisiae.     

Phenotypic expression of Kluyveromyces lactis killer toxin against Saccharomyces spp.

The secretion of killer toxins by some strains of yeasts is a phenomenon of significant industrial importance. The activity of a recently discovered Kluyveromyces lactis killer strain against a sensitive Saccharomyces cerevisiae strain was determined on peptone-yeast extract-nutrient agar plates containing as the carbon source glucose, fructose, galactose, maltose, or glycerol at pH 4.5 or 6.5. Enhanced activity (50 to 90% increase) was found at pH 6.5, particularly on the plates containing galactose, maltose, or glycerol, although production of the toxin in liquid medium was not significantly different with either glucose or galactose as the carbon source. Results indicated that the action of the K. lactis toxin was not mediated by catabolite repression in the sensitive strain. Sensitivities of different haploid and polyploid Saccharomyces yeasts to the two different killer yeasts S. cerevisiae (RNA-plasmid-coded toxin) and K. lactis (DNA-plasmid-coded toxin) were tested. Three industrial polyploid yeasts sensitive to the S. cerevisiae killer yeast were resistant to the K. lactis killer yeast. The S. cerevisiae killer strain itself, however, was sensitive to the K. lactis killer yeast.     

Phenotypic expression of Kluyveromyces lactis killer toxin against Saccharomyces spp

The secretion of killer toxins by some strains of yeasts is a phenomenon of significant industrial importance. The activity of a recently discovered Kluyveromyces lactis killer strain against a sensitive Saccharomyces cerevisiae strain was determined on peptone-yeast extract-nutrient agar plates containing as the carbon source glucose, fructose, galactose, maltose, or glycerol at pH 4.5 or 6.5. Enhanced activity (50 to 90% increase) was found at pH 6.5, particularly on the plates containing galactose, maltose, or glycerol, although production of the toxin in liquid medium was not significantly different with either glucose or galactose as the carbon source. Results indicated that the action of the K. lactis toxin was not mediated by catabolite repression in the sensitive strain. Sensitivities of different haploid and polyploid Saccharomyces yeasts to the two different killer yeasts S. cerevisiae (RNA-plasmid-coded toxin) and K. lactis (DNA-plasmid-coded toxin) were tested. Three industrial polyploid yeasts sensitive to the S. cerevisiae killer yeast were resistant to the K. lactis killer yeast. The S. cerevisiae killer strain itself, however, was sensitive to the K. lactis killer yeast.     

Why does Kluyveromyces lactis not grow under anaerobic conditions? Comparison of essential anaerobic genes of Saccharomyces cerevisiae with the Kluyveromyces lactis genome

Although some yeast species, e.g. Saccharomyces cerevisiae, can grow under anaerobic conditions, Kluyveromyces lactis cannot. In a systematic study, we have determined which S. cerevisiae genes are required for growth without oxygen. This has been done by using the yeast deletion library. Both aerobically essential and nonessential genes have been tested for their necessity for anaerobic growth. Upon comparison of the K. lactis genome with the genes found to be anaerobically important in S. cerevisiae, which yielded 20 genes that are missing in K. lactis, we hypothesize that lack of import of sterols might be one of the more important reasons that K. lactis cannot grow in the absence of oxygen.     

The UDPase activity of the Kluyveromyces lactis Golgi GDPase has a role in uridine nucleotide sugar transport into Golgi vesicles.

In Saccharomyces cerevisiae a Golgi lumenal GDPase (ScGda1p) generates GMP, the antiporter required for entry of GDP-mannose, from the cytosol, into the Golgi lumen. Scgda1 deletion strains have severe defects in N- and O-mannosylation of proteins and glycosphingolipids. ScGda1p has also significant UDPase activity even though S. cerevisiae does not utilize uridine nucleotide sugars in its Golgi lumen. Kluyveromyces lactis, a species closely related to S. cerevisiae, transports UDP-N-acetylglucosamine into its Golgi lumen, where it is the sugar donor for terminal N-acetylglucosamine of the mannan chains. We have identified and cloned a K. lactis orthologue of ScGda1p. KlGda1p is 65% identical to ScGda1p and shares four apyrase conserved regions with other nucleoside diphosphatases. KlGda1p has UDPase activity as ScGda1p. Transport of both GDP-mannose, and UDP-GlcNAc was decreased into Golgi vesicles from Klgda1 null mutants, demonstrating that KlGda1p generates both GMP and UMP required as antiporters for guanosine and uridine nucleotide sugar transport into the Golgi lumen. Membranes from Klgda1 null mutants showed inhibition of glycosyltransferases utilizing uridine- and guanosine-nucleotide sugars, presumably due to accumulation of nucleoside diphosphates because the inhibition could be relieved by addition of apyrase to the incubations. KlGDA1 and ScGDA1 restore the wild-type phenotype of the other yeast gda1 deletion mutant. Surprisingly, KlGDA1 has only a role in O-glycosylation in K. lactis but also complements N-glycosylation defects in S. cerevisiae. Deletion mutants of both genes have altered cell wall stability and composition, demonstrating a broader role for the above enzymes.     

Activation of the yeast cell wall integrity MAPK pathway by zymolyase depends on protease and glucanase activities and requires the mucin-like protein Hkr1 but not Msb2

Yeast adaptation to conditions in which cell wall integrity is compromised mainly relies on the cell wall integrity (CWI) mitogen-activated protein kinase (MAPK) pathway. Zymolyase, a mixture of cell wall-digesting enzymes, triggers a peculiar signaling mechanism in which activation of the CWI pathway is dependent on the high-osmolarity glycerol MAPK pathway. We have identified inhibitors of the principal enzyme activities present in zymolyase and tested their effect on the activation of the MAPK of the CWI pathway, Slt2/Mpk1. Eventually, only β-1,3-glucanase and protease activities were essential to elicit Slt2 activation and confer lytic power to zymolyase. Moreover, we show that the osmosensor Hkr1 is required for signaling, being the most upstream element identified to date.     

An approach to the hypoxic and oxidative stress responses in Kluyveromyces lactis by analysis of mRNA levels

Genome duplication, after the divergence of Saccharomyces cerevisiae from Kluyveromyces lactis along evolution, has been proposed as a mechanism of yeast evolution from strict aerobics, such as Candida albicans, to facultatives/fermentatives, such as S. cerevisiae. This feature, together with the preponderance of respiration and the use of the pentose phosphate pathway in glucose utilization, makes K. lactis a model yeast for studies related to carbon and oxygen metabolism. In this work, and based on the knowledge of the sequence of the genome of K. lactis, obtained by the Génolevures project, we have constructed DNA arrays from K. lactis including a limited amount of selected probes. They are related to the aerobiosis-hypoxia adaptation and to the oxidative stress response, and have been used to test changes in mRNA levels in response to hypoxia and oxidative stress generated by H(2)O(2). The study was carried out in both wild-type and rag2 mutant K. lactis strains in which glycolysis is blocked at the phosphoglucose isomerase step. This approach is the first analysis carried out in K. lactis for the majority of the genes selected.     

Evidence for a lectin in Kluyveromyces sp. that is involved in co-flocculation with Schizosaccharomyces pombe

Co-flocculation is the aggregation of yeasts belonging to different genera or species. Kluyveromyces bulgaricus and Kluyveromyces lactis 5c are self-flocculent, but they can also co-flocculate with the non-flocculent yeast Schizosaccharomyces pombe 972 h(-). This co-flocculation is inhibited by D-galactose and galactose derivatives and involves the binding of a galactose-specific proteinic receptor (or lectin) of Kluyveromyces sp. to the cell wall galactomannans of S. pombe. The proteinic receptor is strongly anchored in the cell wall, it was partially purified by affinity chromatography using immobilized S. pombe galactomannans. This galactose-specific proteinic receptor does not appear to interfere in K. bulgaricus or K. lactis self-flocculation, which is mediated by another galactose-specific lectin weakly linked at the cell wall.     

Secretion of hen egg white lysozyme from Kluyveromyces lactis

Hen egg white (HEW) lysozyme was correctly processed and efficiently secreted from an alternative yeast, Kluyveromyces lactis. We constructed secretion vectors using PHO5, PGK, and LAC4 promoters, and found that the highest secretion was obtained under the direction of the PGK promoter in non-selective rich medium. K. lactis secreted HEW lysozyme with two-fold higher efficiency than S. cerevisiae, estimated by using a K. lactis-S. cerevisiae shuttle vector.     

Intergeneric transfer of deoxyribonucleic acid killer plasmids, pGKl1 and pGKl2, from Kluyveromyces lactis into Saccharomyces cerevisiae by cell fusion

Two novel linear deoxyribonucleic acid plasmids, pGKl1 and pGKl2, were isolated from the yeast Kluyveromyces lactis. K. lactis strains harboring the pGK1 plasmids killed a certain group of yeasts, including Saccharomyces cerevisiae, Saccharomyces italicus, Saccharomyces rouxii, K. lactis, Kluyveromyces thermotolerans, Kluyvermyces vanudenii, Torulopsis glabrata, Candida utilis, and Candida intermedia. In this experiment, the pGKl1 and pGKl2 plasmids were intergenerically transferred from a K. lactis killer strain into a non-killer (killer-sensitive) strain of S. cerevisiae by the use of a protoplast fusion technique. Both of the pGKl plasmids replicated autonomously and stably in the new host cells of S. cerevisiae and could coexist with the resident 2-micrometers deoxyribonucleic acid plasmid. The S. cerevisiae cells which accepted the pGKl plasmids expressed the same killer phenotype as that of the donor K. lactis killer and became resistant to the K. lactis killer. The pGKl plasmids existing in the S. cerevisiae cells were cured by treatment with ethidium bromide, and the killer and resistance characters were simultaneously lost. From there results, it was concluded that both the killer and the resistance genes are located on the pGKl plasmids.     

How do I begin? Sensing extracellular stress to maintain yeast cell wall integrity

The cell wall integrity (CWI) signalling pathway is necessary to remodel the yeast cell wall during normal morphogenesis and in response to cell surface stress. In the Baker's yeast Saccharomyces cerevisiae, a set of five membrane-spanning sensors, namely Wsc1, Wsc2, Wsc3, Mid2 and Mtl1, detect perturbations in the cell wall and/or the plasma membrane and activate a downstream signal transduction pathway with a central MAP kinase module. As a consequence, the expression of genes whose products are involved in cell wall structure and remodelling is induced. This review summarises our recent results on sensor structure and function, as well as the advances made regarding sensor mechanics.     

Cohesin dysfunction results in cell wall defects in budding yeast

Cohesin is a conserved chromatin-binding multisubunit protein complex involved in diverse chromosomal transactions such as sister-chromatid cohesion, chromosome condensation, regulation of gene expression, DNA replication, and repair. While working with a budding yeast temperature-sensitive mutant, mcd1-1, defective in a cohesin subunit, we observed that it was resistant to zymolyase, indicating an altered cell wall organization. The budding yeast cell wall is a strong but elastic structure essential for maintenance of cell shape and protection from extreme environmental challenges. Here, we show that the cohesin complex plays an important role in cell wall maintenance. Cohesin mutants showed high chitin content in the cell wall and sensitivity to multiple cell wall stress-inducing agents. Interestingly, temperature-dependent lethality of cohesin mutants was osmoremedial, in a HOG1-MAPK pathway-dependent manner, suggesting that the temperature sensitivity of these mutants may arise partially from cell wall defects. Moreover, Mpk1 hyper-phosphorylation indicated activation of the cell wall integrity (CWI) signaling pathway in cohesin mutants. Genetic interaction analysis revealed that the CWI pathway is essential for survival of mcd1-1 upon additional cell wall stress. The cell wall defect was independent of the cohesion function and accompanied by misregulation of expression of several genes having cell wall-related functions. Our findings reveal a requirement of cohesin in maintenance of CWI that is independent of the CWI pathway, and that may arise from cohesin’s role in regulating the expression of multiple genes encoding proteins involved in cell wall organization and biosynthesis.