Deficiency of Pkc1 activity affects glycerol metabolism in Saccharomyces cerevisiae

Yeast Gup1(2) Proteins Are Homologues of the Hedgehog Morphogens Acyltransferases HHAT(L): Facts and Implications

In multiple tissues, the Hedgehog secreted morphogen activates in the receiving cells a pathway involved in cell fate, proliferation and differentiation in the receiving cells. This pathway is particularly important during embryogenesis. The protein HHAT (Hedgehog O-acyltransferase) modifies Hh morphogens prior to their secretion, while HHATL (Hh O-acyltransferase-like) negatively regulates the pathway. HHAT and HHATL are homologous to Saccharomyces cerevisiae Gup2 and Gup1, respectively. In yeast, Gup1 is associated with a high number and diversity of biological functions, namely polarity establishment, secretory/endocytic pathway functionality, vacuole morphology and wall and membrane composition, structure and maintenance. Phenotypes underlying death, morphogenesis and differentiation are also included. Paracrine signalling, like the one promoted by the Hh pathway, has not been shown to occur in microbial communities, despite the fact that large aggregates of cells like biofilms or colonies behave as proto-tissues. Instead, these have been suggested to sense the population density through the secretion of quorum-sensing chemicals. This review focuses on Gup1/HHATL and Gup2/HHAT proteins. We review the functions and physiology associated with these proteins in yeasts and higher eukaryotes. We suggest standardisation of the presently chaotic Gup-related nomenclature, which includes KIAA117, c3orf3, RASP, Skinny, Sightless and Central Missing, in order to avoid the disclosure of otherwise unnoticed information.

Signaling through Lrg1, Rho1 and Pkc1 Governs Candida albicans Morphogenesis in Response to Diverse Cues

The capacity to transition between distinct morphological forms is a key virulence trait for diverse fungal pathogens. A poignant example of a leading opportunistic fungal pathogen of humans for which an environmentally responsive developmental program underpins virulence is Candida albicans. C. albicans mutants that are defective in the transition between yeast and filamentous forms typically have reduced virulence. Although many positive regulators of C. albicans filamentation have been defined, there are fewer negative regulators that have been implicated in repression of filamentation in the absence of inducing cues. To discover novel negative regulators of filamentation, we screened a collection of 1,248 C. albicans homozygous transposon insertion mutants to identify those that were filamentous in the absence of inducing cues. We identified the Rho1 GAP Lrg1, which represses filamentous growth by stimulating Rho1 GTPase activity and converting Rho1 to its inactive, GDP-bound form. Deletion of LRG1 or introduction of a RHO1 mutation that locks Rho1 in constitutively active, GTP-bound state, leads to filamentation in the absence of inducing cues. Deletion of the Rho1 downstream effector PKC1 results in defective filamentation in response to diverse host-relevant inducing cues, including serum. We further established that Pkc1 is not required to sense filament-inducing cues, but its kinase activity is critical for the initiation of filamentous growth. Our genetic analyses revealed that Pkc1 regulates filamentation independent of the canonical MAP kinase cascade. Further, although Ras1 activation is not impaired in a pkc1Δ/pkc1Δ mutant, adenylyl cyclase activity is reduced, consistent with a model in which Pkc1 functions in parallel with Ras1 in regulating Cyr1 activation. Thus, our findings delineate a signaling pathway comprised of Lrg1, Rho1 and Pkc1 with a core role in C. albicans morphogenesis, and illuminate functional relationships that govern activation of a central transducer of signals that control environmental response and virulence programs.

A permease encoded by STL1 is required for active glycerol uptake by Candida albicans

Candida albicans accumulates large amounts of the polyols glycerol and d-arabitol when the cells are exposed to physiological conditions relevant to stress and virulence in animals. Intracellular concentrations of glycerol are determined by rates of glycerol production and catabolism and of glycerol uptake and efflux through the plasma membrane. We and others have studied glycerol production in C. albicans, but glycerol uptake by C. albicans has not been studied. In the present study, we found that [(14)C]glycerol uptake by C. albicans SC5314 was (i) accumulative; (ii) dependent on proton-motive force; (iii) unaffected by carbon source; and (iv) unaffected by large molar excesses of d-arabitol or other polyols. The respective K(m) and V(max) values were 2.1 mM and 460 micromol h(-1) (g dry wt)(-1) in glucose medium and 2.6 mM and 268 micromol h(-1) (g dry wt)(-1) in glycerol medium. To identify the C. albicans glycerol uptake protein(s), we cloned the C. albicans homologues of the Saccharomyces cerevisiae genes GUP1 and STL1, both of which are known to be involved in glycerol transport. When multicopy plasmids encoding C. albicans STL1, C. albicans STL2 and C. albicans GUP1 were introduced into the corresponding S. cerevisiae null mutants, the transformants all acquired the ability to grow on minimal glycerol medium; however, only S. cerevisiae stl1 null mutants transformed with C. albicans STL1 actively took up extracellular [(14)C]glycerol. When both chromosomal alleles of C. albicans STL1 were deleted from C. albicans BWP17, the resulting stl1 null mutants grew poorly on minimal glycerol medium, and their ability to transport [(14)C]glycerol into the cell was markedly reduced. In contrast, deletion of both chromosomal alleles of C. albicans STL2 or of C. albicans GUP1 had no significant effects on [(14)C]glycerol uptake or the ability to grow on minimal glycerol medium. Northern blot analysis indicated that C. albicans STL1 was expressed in both glucose and glycerol media, conditions under which we detected wild-type active glycerol uptake. Furthermore, STL1 was highly expressed in salt-stressed cells; however, the stl1 null mutant was no more sensitive to salt stress than wild-type controls. We also detected high levels of STL2 expression in glycerol-grown cells, even though deletion of this gene did not influence glycerol uptake activity in glycerol-grown cells. We conclude from the results above that a plasma-membrane H(+) symporter encoded by C. albicans STL1 actively transports glycerol into C. albicans cells.

The early steps of glucose signalling in yeast

In the presence of glucose, yeast undergoes an important remodelling of its metabolism. There are changes in the concentration of intracellular metabolites and in the stability of proteins and mRNAs; modifications occur in the activity of enzymes as well as in the rate of transcription of a large number of genes, some of the genes being induced while others are repressed. Diverse combinations of input signals are required for glucose regulation of gene expression and of other cellular processes. This review focuses on the early elements in glucose signalling and discusses their relevance for the regulation of specific processes. Glucose sensing involves the plasma membrane proteins Snf3, Rgt2 and Gpr1 and the glucose-phosphorylating enzyme Hxk2, as well as other regulatory elements whose functions are still incompletely understood. The similarities and differences in the way in which yeasts and mammalian cells respond to glucose are also examined. It is shown that in Saccharomyces cerevisiae, sensing systems for other nutrients share some of the characteristics of the glucose-sensing pathways.

Glucose repression over Saccharomyces cerevisiae glycerol/H+ symporter gene STL1 is overcome by high temperature

High temperature promotes an improved activity of the Saccharomyces cerevisiae glycerol/H(+) symporter encoded by STL1, which correlates well with Stl1p levels. This happens in both fermentable and respiratory metabolic growth conditions, though the induction in the latter is much higher. The relief of glucose repression by high temperature at the level of protein expression and activity (Stl1p) is reported for the first time. We reason that the glycerol internal levels fine-tuning, under heat-stress as in other physiological condition, can be achieved with the contribution of the tight regulation of the symporter.

Thermosensitivity of the Saccharomyces cerevisiae gpp1gpp2 double deletion strain can be reduced by overexpression of genes involved in cell wall maintenance

A Saccharomyces cerevisiae strain in which the GPP1 and GPP2 genes, both encoding glycerol-3-phosphate phosphatase isoforms, are deleted, displays both osmo- and thermosensitive (ts) phenotypes. We isolated genes involved in cell wall maintenance as multicopy suppressors of the gpp1gpp2 ts phenotype. We found that the gpp1gpp2 strain is hypersensitive to cell wall stress such as treatment with beta-1,3-glucanase containing cocktail Zymolyase and chitin-binding dye Calcofluor-white (CFW). Sensitivity to Zymolyase was rescued by overexpression of SSD1, while CFW sensitivity was rescued by SSD1, FLO8 and WSC3-genes isolated as multicopy suppressors of the gpp1gpp2 ts phenotype. Some of the isolated suppressor genes (SSD1, FLO8) also rescued the lytic phenotype of slt2 deletion strain. Additionally, the sensitivity to CFW was reduced when the cells were supplied with glycerol. Both growth on glycerol-based medium and overexpression of SSD1, FLO8 or WSC3 had additive suppressing effect on CFW sensitivity of the gpp1gpp2 mutant strain. We also confirmed that the internal glycerol level changed in cells exposed to cell wall perturbation.

The yeast osmosensitive mutantfps1Δ transformed by the cauliflower BobTIP1;1 aquaporin withstand a hypo-osmotic shock

Osmoregulation plays an important role in cellular responses to osmotic stress in plants and in yeast. Aquaporins contribute to osmotic adjustment by facilitating transport of water or solutes across membranes. The tonoplastic water channel BobTIP1;1 (original name BobTIP26-1) genes are upregulated during dessication stress in cauliflower meristematic tissue. To investigate the physiological importance of BobTIP1;1, we expressed it in a Saccharomyces cerevisiae osmosensitive mutant fps1Delta. We showed that the defect in the yeast glycerol plasma membrane transporter is complemented by a plant cDNA encoding the aquaporin BobTIP1;1 which is localized in the vacuolar membrane of the complemented yeast cells. To our knowledge, this is the first example of a plant aquaporin for which localization in the vacuolar membrane of yeast cells is related to an osmoresistant phenotype under hypo-osmotic shock.