The yeast cyclins Pc16p and Pc17p are involved in the control of glycogen storage by the cyclin-dependent protein kinase Pho85p

Glycogen Metabolism in Candida albicans Impacts Fitness and Virulence during Vulvovaginal and Invasive Candidiasis

The polymorphic fungus Candida albicans remains a leading cause of both invasive and superficial mycoses, including vulvovaginal candidiasis (VVC). Metabolic plasticity, including carbohydrate catabolism, confers fitness advantages at anatomical site-specific host niches. C. albicans possesses the capacity to accumulate and store carbohydrates as glycogen and can consume intracellular glycogen stores when nutrients become limited. In the vaginal environment, estrogen promotes epithelial glycogen accumulation and C. albicans colonization. However, whether these factors are mechanistically linked is unexplored. Here, we characterized the glycogen metabolism pathways in C. albicans and investigated whether these impact the long-term survival of C. albicans, both in vitro and in vivo during murine VVC, or virulence during systemic infection. SC5314 and 6 clinical isolates demonstrated impaired growth when glycogen was used as the sole carbon source, suggesting that environmental glycogen acquisition is limited. The genetic deletion and complementation of key genes involved in glycogen metabolism in Saccharomyces cerevisiae confirmed that GSY1 and GLC3, as well as GPH1 and GDB1, are essential for glycogen synthesis and catabolism in C. albicans, respectively. Potential compensatory roles for a glucoamylase encoded by SGA1 were also explored. Competitive survival assays revealed that gsy1Δ/Δ, gph1Δ/Δ, and gph1Δ/Δ sga1Δ/Δ mutants exhibited long-term survival defects in vitro under starvation conditions and in vivo during vaginal colonization. A complete inability to catabolize glycogen (gph1Δ/Δ sga1Δ/Δ) also rendered C. albicans significantly less virulent during disseminated infections. This is the first study fully validating the glycogen metabolism pathways in C. albicans, and the results further suggest that intracellular glycogen catabolism positively impacts the long-term fitness of C. albicans in nutrient deficient environments and is important for full virulence. IMPORTANCE Glycogen is a highly branched polymer of glucose and is used across the tree of life as an efficient and compact form of energy storage. Whereas glycogen metabolism pathways have been studied in model yeasts, they have not been extensively explored in pathogenic fungi. Using a combination of microbiologic, molecular genetic, and biochemical approaches, we reveal orthologous functions of glycogen metabolism genes in the fungal pathogen Candida albicans. We also provide evidence that extracellular glycogen poorly supports growth across the Candida species and clinical isolates. Competitive fitness assays reveal that the loss of glycogen synthesis or catabolism significantly impacts survival during both in vitro starvation and the colonization of the mouse vagina. Moreover, a global glycogen catabolism mutant is rendered less virulent during murine invasive candidiasis. Therefore, this work demonstrates that glycogen metabolism in C. albicans contributes to survival and virulence in the mammalian host and may be a novel antifungal target.

The Neurospora crassa PCL-1 cyclin is a PHO85-1 (PGOV) kinase partner that directs the complex to glycogen metabolism and is involved in calcium metabolism regulation

Cyclins are a family of proteins characterized by possessing a cyclin box domain that mediates binding to cyclin dependent kinases (CDKs) partners. In this study, the search for a partner cyclin of the PHO85-1 CDK retrieved PCL-1 an ortholog of yeast Pcls (for Pho85 cyclins) that performs functions common to Pcls belonging to different cyclin families. We show here that PCL-1, as a typical cyclin, is involved in cell cycle control and cell progression. In addition, PCL-1 regulates glycogen metabolism; Δpcl-1 cells accumulate higher glycogen levels than wild-type cells and the glycogen synthase (GSN) enzyme is less phosphorylated and, therefore, more active in the mutant cells. Together with PHO85-1, PCL-1 phosphorylates in vitro GSN at the Ser636 amino acid residue. Modeling studies identified PHO85-1 and PCL-1 as a CDK/cyclin complex, with a conserved intermolecular region stabilized by hydrophobic and polar interactions. PCL-1 is also involved in calcium and NaCl stress response. Δpcl-1 cells are sensitive to high NaCl concentration; on the contrary, they grow better and overexpress calcium responsive genes under high calcium chloride concentration compared to the wild-type strain. The expression of the calcium-responsive CRZ-1 transcription factor is modulated by PCL-1, and this transcription factor seems to be less phosphorylated in Δpcl-1 cells since exhibits nuclear location in these cells in the absence of calcium. Our results show that PCL-1 locates at different cell regions suggesting that it may determine its activity by controlling its intracellular location and reveal an interesting functional divergence between yeast and filamentous fungus cyclins.

Restriction Checkpoint Controls Bradyzoite Development in Toxoplasma gondii

Human toxoplasmosis is a life-threatening disease caused by the apicomplexan parasite Toxoplasma gondii. Rapid replication of the tachyzoite is associated with symptomatic disease, while suppressed division of the bradyzoite is responsible for chronic disease. Here, we identified the T. gondii cell cycle mechanism, the G1 restriction checkpoint (R-point), that operates the switch between parasite growth and differentiation. Apicomplexans lack conventional R-point regulators, suggesting adaptation of alternative factors. We showed that Cdk-related G1 kinase TgCrk2 forms alternative complexes with atypical cyclins (TgCycP1, TgCycP2, and TgCyc5) in the rapidly dividing developmentally incompetent RH and slower dividing developmentally competent ME49 tachyzoites and bradyzoites. Examination of cyclins verified the correlation of cyclin expression with growth dependence and development capacity of RH and ME49 strains. We demonstrated that rapidly dividing RH tachyzoites were dependent on TgCycP1 expression, which interfered with bradyzoite differentiation. Using the conditional knockdown model, we established that TgCycP2 regulated G1 duration in the developmentally competent ME49 tachyzoites but not in the developmentally incompetent RH tachyzoites. We tested the functions of TgCycP2 and TgCyc5 in alkaline induced and spontaneous bradyzoite differentiation (rat embryonic brain cells) models. Based on functional and global gene expression analyses, we determined that TgCycP2 also regulated bradyzoite replication, while signal-induced TgCyc5 was critical for efficient tissue cyst maturation. In conclusion, we identified the central machinery of the T. gondii restriction checkpoint comprised of TgCrk2 kinase and three atypical T. gondii cyclins and demonstrated the independent roles of TgCycP1, TgCycP2, and TgCyc5 in parasite growth and development. IMPORTANCE Toxoplasma gondii is a virulent and abundant human pathogen that puts millions of silently infected people at risk of reactivation of the chronic disease. Encysted bradyzoites formed during the chronic stage are resistant to current therapies. Therefore, insights into the mechanism of tissue cyst formation and reactivation are major areas of investigation. The fact that rapidly dividing parasites differentiate poorly strongly suggests that there is a threshold of replication rate that must be crossed to be considered for differentiation. We discovered a cell cycle mechanism that controls the T. gondii growth-rest switch involved in the conversion of dividing tachyzoites into largely quiescent bradyzoites. This switch operates the T. gondii restriction checkpoint using a set of atypical and parasite-specific regulators. Importantly, the novel T. gondii R-point network was not present in the parasite's human and animal hosts, offering a wealth of new and parasite-specific drug targets to explore in the future.

IP7-SPX Domain Interaction Controls Fungal Virulence by Stabilizing Phosphate Signaling Machinery

In the human-pathogenic fungus Cryptococcus neoformans, the inositol polyphosphate signaling pathway is critical for virulence. We recently demonstrated the key role of the inositol pyrophosphate IP₇ (isomer 5-PP-IP₅) in driving fungal virulence; however, the mechanism of action remains elusive. Using genetic and biochemical approaches, and mouse infection models, we show that IP₇ synthesized by Kcs1 regulates fungal virulence by binding to a conserved lysine surface cluster in the SPX domain of Pho81. Pho81 is the cyclin-dependent kinase (CDK) inhibitor of the phosphate signaling (PHO) pathway. We also provide novel mechanistic insight into the role of IP₇ in PHO pathway regulation by demonstrating that IP₇ functions as an intermolecular "glue" to stabilize Pho81 association with Pho85/Pho80 and, hence, promote PHO pathway activation and phosphate acquisition. Blocking IP₇-Pho81 interaction using site-directed mutagenesis led to a dramatic loss of fungal virulence in a mouse infection model, and the effect was similar to that observed following PHO81 gene deletion, highlighting the key importance of Pho81 in fungal virulence. Furthermore, our findings provide additional evidence of evolutionary divergence in PHO pathway regulation in fungi by demonstrating that IP₇ isomers have evolved different roles in PHO pathway control in C. neoformans and nonpathogenic yeast.IMPORTANCE Invasive fungal diseases pose a serious threat to human health globally with >1.5 million deaths occurring annually, 180,000 of which are attributable to the AIDS-related pathogen, Cryptococcus neoformans Here, we demonstrate that interaction of the inositol pyrophosphate, IP₇, with the CDK inhibitor protein, Pho81, is instrumental in promoting fungal virulence. IP₇-Pho81 interaction stabilizes Pho81 association with other CDK complex components to promote PHO pathway activation and phosphate acquisition. Our data demonstrating that blocking IP₇-Pho81 interaction or preventing Pho81 production leads to a dramatic loss in fungal virulence, coupled with Pho81 having no homologue in humans, highlights Pho81 function as a potential target for the development of urgently needed antifungal drugs.

The RESTRICTION checkpoint: a window of opportunity governing developmental transitions in Toxoplasma gondii

The life cycle of Toxoplasma gondii is characterized by active replication alternating with periods of rest. Encysted dormant sporozoites and bradyzoites initiate active replication as tachyzoites and merozoites. Here we explore the role of the cell cycle with a focus on the canonical G₁ RESTRICTION checkpoint (R-point) as the integrator governing developmental decisions in T. gondii. This surveillance mechanism, which licenses replication, creates a window of opportunity in G₁ for cellular reorganization in the execution of developmental transitions. We also explore the unique status of the bradyzoite, the only life cycle stage executing both a forward (entry into the sexual cycle) and reverse (recrudescence) developmental transitions as a multipotent cell. These opposing decisions are executed through the common machinery of the RESTRICTION checkpoint.

CYCP2;1 integrates genetic and nutritional information to promote meristem cell division in Arabidopsis

In higher plants, cell cycle activation in the meristems at germination is essential for the initiation of post-embryonic development. We previously identified the signaling pathways of homeobox transcription factor STIMPY and metabolic sugars as two interacting branches of the regulatory network that is responsible for activating meristematic tissue proliferation in Arabidopsis. In this study, we found that CYCP2;1 is both a direct target of STIMPY transcriptional activation and an early responder to sugar signals. Genetic and molecular studies show that CYCP2;1 physically interacts with three of the five mitotic CDKs in Arabidopsis, and is required for the G2 to M transition during meristem activation. Taken together, our results suggest that CYCP2;1 acts as a permissive control of cell cycle progression during seedling establishment by directly linking genetic control and nutritional cues with the activity of the core cell cycle machinery.

Acid phosphatases of budding yeast as a model of choice for transcription regulation research

Acid phosphatases of budding yeast have been studied for more than forty years. This paper covers biochemical characteristics of acid phosphatases and different aspects in expression regulation of eukaryotic genes, which were researched using acid phosphatases model. A special focus is devoted to cyclin-dependent kinase Pho85p, a negative transcriptional regulator, and its role in maintaining mitochondrial genome stability and to pleiotropic effects of pho85 mutations.

Regulation of glycogen metabolism in yeast and bacteria

Microorganisms have the capacity to utilize a variety of nutrients and adapt to continuously changing environmental conditions. Many microorganisms, including yeast and bacteria, accumulate carbon and energy reserves to cope with the starvation conditions temporarily present in the environment. Glycogen biosynthesis is a main strategy for such metabolic storage, and a variety of sensing and signaling mechanisms have evolved in evolutionarily distant species to ensure the production of this homopolysaccharide. At the most fundamental level, the processes of glycogen synthesis and degradation in yeast and bacteria share certain broad similarities. However, the regulation of these processes is sometimes quite distinct, indicating that they have evolved separately to respond optimally to the habitat conditions of each species. This review aims to highlight the mechanisms, both at the transcriptional and at the post-transcriptional level, that regulate glycogen metabolism in yeast and bacteria, focusing on selected areas where the greatest increase in knowledge has occurred during the last few years. In the yeast system, we focus particularly on the various signaling pathways that control the activity of the enzymes of glycogen storage. We also discuss our recent understanding of the important role played by the vacuole in glycogen metabolism. In the case of bacterial glycogen, special emphasis is placed on aspects related to the genetic regulation of glycogen metabolism and its connection with other biological processes.

Function of protein phosphatase-1, Glc7, in Saccharomyces cerevisiae

Budding yeast, Saccharomyces cerevisiae, and its close relatives are unique among eukaryotes in having a single gene, GLC7, encoding protein phosphatase-1 (PP1). This enzyme with a highly conserved amino acid sequence controls many processes in all eukaryotic cells. Therefore, the study of Glc7 function offers a unique opportunity to gain a comprehensive understanding of this critical regulatory enzyme. This review summarizes our current knowledge of how Glc7 function modulates processes in the cytoplasm and nucleus. Additionally, global Glc7 regulation is described.

Negative regulation of Vps34 by Cdk mediated phosphorylation

Vacuolar protein sorting 34 (Vps34) complexes, the class III PtdIns3 kinase, specifically phosphorylate the D3 position of PtdIns to produce PtdIns3P. Vps34 is involved in the control of multiple key intracellular membrane trafficking pathways including endocytic sorting and autophagy. In mammalian cells, Vps34 interacts with Beclin 1, an ortholog of Atg6 in yeast, to regulate the production of PtdIns3P and autophagy. We show that Vps34 is phosphorylated on Thr159 by Cdk1, which negatively regulates its interaction with Beclin 1 during mitosis. Cdk5/p25, a neuronal Cdk shown to play a role in Alzheimer's disease, can also phosphorylate Thr159 of Vps34. Phosphorylation of Vps34 on Thr159 inhibits its interaction with Beclin 1. We propose that phosphorylation of Thr159 in Vps34 is a key regulatory mechanism that controls the class III PtdIns3 kinase activity in cell-cycle progression, development, and human diseases including neurodegeneration and cancers.

Positive or negative roles of different cyclin-dependent kinase Pho85-cyclin complexes orchestrate induction of autophagy in Saccharomyces cerevisiae

As a major intracellular degradation pathway, autophagy is tightly regulated to prevent cellular dysfunction in all eukaryotic cells. The rapamycin-sensitive Tor kinase complex 1 is a major regulator of autophagy. Several other nutrient-sensory kinases also play critical roles to precisely modulate autophagy; however, the network of regulatory mechanisms remains largely elusive. We used genetic analyses to elucidate the mechanism by which the stress-responsive, cyclin-dependent kinase Pho85 and its corresponding cyclin complexes antagonistically modulate autophagy in Saccharomyces cerevisiae. When complexed with cyclins Pho80 and Pcl5, Pho85 negatively regulates autophagy through downregulating the protein kinase Rim15 and the transcription factors Pho4 and Gcn4. The cyclins Clg1, Pcl1, and Pho80, in concert with Pho85, positively regulate autophagy through promoting the degradation of Sic1, a negative regulator of autophagy that targets Rim15. Our results suggest a model in which Pho85 and its cyclin complexes have opposing roles in autophagy regulation.

Yeast Gcn4p stabilization is initiated by the dissociation of the nuclear Pho85p/Pcl5p complex

Protein stability of the c-jun-like yeast bZIP transcriptional activator Gcn4p is exclusively controlled in the yeast nucleus. Phosphorylation by the nuclear Pho85p cyclin-dependent protein kinase, a functional homolog of mammalian Cdk5, initiates the Gcn4p degradation pathway in complex with the cyclin Pcl5p. We show that the initial step in Gcn4p stabilization is the dissociation of the Pho85p/Pcl5p complex. Pcl7p, another nuclear and constantly present cyclin, is required for Gcn4p stabilization and is able to associate to Pho85p independently of the activity of the Gcn4p degradation pathway. In addition, the nuclear cyclin-dependent Pho85p kinase inhibitor Pho81p is required for Gcn4p stabilization. Pho81p only interacts with Pcl5p when Gcn4p is rapidly degraded but constitutively interacts with Pcl7p. Our data suggest that Pcl7p and Pho81p are antagonists of the Pho85p/Pcl5p complex formation in a yet unknown way, which are specifically required for Gcn4p stabilization. We suggest that dissociation of the Pho85p/Pcl5p complex as initial step in Gcn4p stabilization is a prerequisite for a shift of equilibrium to an increased amount of the Pho85p/Pcl7p complexes and subsequently results in decreased Gcn4p phosphorylation and therefore increased stability of the transcription factor.

Regulation of yeast glycogen phosphorylase by the cyclin-dependent protein kinase Pho85p

Yeast accumulate glycogen in response to nutrient limitation. The key enzymes of glycogen synthesis and degradation, glycogen synthase, and phosphorylase, are regulated by reversible phosphorylation. Phosphorylation inactivates glycogen synthase but activates phosphorylase. The kinases and phosphatases that control glycogen synthase are well characterized whilst the enzymes modifying phosphorylase are poorly defined. Here, we show that the cyclin-dependent protein kinase, Pho85p, which we have previously found to regulate glycogen synthase also controls the phosphorylation state of phosphorylase.

The identification of Pcl1-interacting proteins that genetically interact with Cla4 may indicate a link between G1 progression and mitotic exit

In budding yeast, Cla4 and Ste20, two p21-activated kinases, contribute to numerous morphogenetic processes. Loss of Ste20 or Cla4 individually confers distinct phenotypes, implying that they regulate different processes. However, loss of both proteins is lethal, suggesting some functional overlap. To explore the role(s) of Cla4, we and others have sought mutations that are lethal in a cla4 Delta strain. These mutations define >60 genes. Recently, both Ste20 and Cla4 have been implicated in mitotic exit. Here, we identify a genetic interaction between PHO85, which encodes a cyclin-dependent kinase, and CLA4. We further show that the Pho85-coupled G(1) cyclins Pcl1 and Pcl2 contribute to this Pho85 role. We performed a two-hybrid screen with Pcl1. Three Pcl1-interacting proteins were identified: Ncp1, Hms1, and a novel ATPase dubbed Epa1. Each of these proteins interacts with Pcl1 in GST pull-down experiments and is specifically phosphorylated by Pcl1.Pho85 complexes. NCP1, HMS1, and EPA1 also genetically interact with CLA4. Like Cla4, the proteins Hms1, Ncp1, and Pho85 appear to affect mitotic exit, a conclusion that follows from the mislocalization of Cdc14, a key mitotic regulator, in strains lacking these proteins. We propose a model in which the G(1) Pcl1.Pho85 complex regulates mitotic exit machinery.

The Pho80-like cyclin of Aspergillus nidulans regulates development independently of its role in phosphate acquisition

In Saccharomyces cerevisiae, phosphate acquisition enzymes are regulated by a cyclin-dependent kinase (Pho85), a cyclin (Pho80), the cyclin-dependent kinase inhibitor Pho81, and the helix-loop-helix transcription factor Pho4 (the PHO system). Previous studies in Aspergillus nidulans indicate that a Pho85-like kinase, PHOA, does not regulate the classic PHO system but regulates development in a phosphate-dependent manner. A Pho80-like cyclin has now been isolated through its interaction with PHOA. Surprisingly, unlike PHOA, An-PHO80 does play a negative role in the PHO system. Similarly, an ortholog of Pho4 previously identified genetically as palcA also regulates the PHO system. However, An-PHO81, a putative cyclin-dependent kinase inhibitor, does not regulate the PHO system. Therefore, there are significant differences between the classic PHO system conserved between S. cerevisiae and Neurospora crassa compared with that which has evolved in A. nidulans. Most interestingly, under low phosphate conditions, the An-PHO80 cyclin also promotes sexual development while having a negative effect on asexual development. These effects are independent of the role An-PHO80 has in the classic PHO system. However, in high phosphate medium, An-PHO80 affects development because of deregulation of the PHO system as loss of palcA(Pho4) function negates the developmental defects caused by lack of An-pho80. Therefore, under low phosphate conditions the An-PHO80 cyclin regulates development independently of the PHO system, whereas in high phosphate it affects development through the PHO system. The data indicate that a single cyclin can control various aspects of growth and development in a multicellular organism.

Regulation of phosphate acquisition in Saccharomyces cerevisiae

Membrane transport systems active in cellular inorganic phosphate (P(i)) acquisition play a key role in maintaining cellular P(i) homeostasis, independent of whether the cell is a unicellular microorganism or is contained in the tissue of a higher eukaryotic organism. Since unicellular eukaryotes such as yeast interact directly with the nutritious environment, regulation of P(i) transport is maintained solely by transduction of nutrient signals across the plasma membrane. The individual yeast cell thus recognizes nutrients that can act as both signals and sustenance. The present review provides an overview of P(i) acquisition via the plasma membrane P(i) transporters of Saccharomyces cerevisiae and the regulation of internal P(i) stores under the prevailing P(i) status.

Pho85 phosphorylates the Glc7 protein phosphatase regulator Glc8 in vivo

The budding yeast Glc7 serine/threonine protein phosphatase-1 is regulated by Glc8, the yeast ortholog of mammalian phosphatase inhibitor-2. In this work, we demonstrated that similarly to inhibitor-2, Glc8 function is regulated by phosphorylation. The cyclin-dependent protein kinase, Pho85, in conjunction with the related cyclins Pcl6 and Pcl7 comprise the major Glc8 kinase in vivo and in vitro. Several glc7 mutations are dependent on the presence of Glc8 for viability. For example, glc7 alleles R121K, R142H, and R198D are lethal in combination with a glc8 deletion. We found that glc7-R121K is lethal in combination with a pho85 deletion. This finding indicates that Pho85 is the sole Glc8 kinase in vivo. Furthermore, glc7-R121K is also lethal when combined with deletions of pcl6, plc7, pcl8, and pcl10, indicating that these related cyclins redundantly activate Pho85 for Glc8 phosphorylation in vivo. In vitro kinase assays and genetic results indicate that Pho85 cyclins Pcl6 and Pcl7 comprise the predominant Glc8 kinase.

Dissection of a complex phenotype by functional genomics reveals roles for the yeast cyclin-dependent protein kinase Pho85 in stress adaptation and cell integrity

Cyclin-dependent kinases (Cdks) are key regulators of the cell division cycle. Pho85 is a multifunctional Cdk in budding yeast involved in aspects of metabolism, the cell cycle, cell polarity, and gene expression. Consistent with a broad spectrum of functions, Pho85 associates with a family of 10 cyclins and deletion of PHO85 causes a pleiotropic phenotype. Discovering the physiological substrates of protein kinases is a major challenge, and we have pursued a number of genomics approaches to reveal the processes regulated by Pho85 and to understand the root cause of reduced cellular fitness in pho85Delta mutant strains. We used a functional-genomics approach called synthetic genetic array (SGA) analysis to systematically identify strain backgrounds in which PHO85 is required for viability. In parallel, we used DNA microarrays to examine the genome-wide transcriptional consequences of deleting PHO85 or members of the Pho85 cyclin family. Using this pairwise approach coupled with phenotypic tests, we uncovered clear roles for Pho85 in cell integrity and the response to adverse growth conditions. Importantly, our combined approach allowed us to ascribe new aspects of the complex pho85 phenotype to particular cyclins; our data highlight a cell integrity function for the Pcl1,2 subgroup of Pho85 Cdks that is independent of a role for the Pho80-Pho85 kinase in the response to stress. Using a modification of the SGA technique to screen for suppressors of pho85Delta strain growth defects, we found that deletion of putative vacuole protein gene VTC4 suppressed the sensitivity of the pho85Delta strain to elevated CaCl(2) and many other stress conditions. Expression of VTC4 is regulated by Pho4, a transcription factor that is inhibited by the Pho80-Pho85 kinase. Genetic tests and electron microscopy experiments suggest that VTC4 is a key target of Pho4 and that Pho80-Pho85-mediated regulation of VTC4 expression is required for proper vacuole function and for yeast cell survival under a variety of suboptimal conditions. The integration of multiple genomics approaches is likely to be a generally useful strategy for extracting functional information from pleiotropic mutant phenotypes.

Protein phosphatase type 1 regulates ion homeostasis in Saccharomyces cerevisiae

Protein phosphatase type 1 (PP1) is encoded by the essential gene GLC7 in Saccharomyces cerevisiae. glc7-109 (K259A, R260A) has a dominant, hyperglycogen defect and a recessive, ion and drug sensitivity. Surprisingly, the hyperglycogen phenotype is partially retained in null mutants of GAC1, GIP2, and PIG1, which encode potential glycogen-targeting subunits of Glc7. The R260A substitution in GLC7 is responsible for the dominant and recessive traits of glc7-109. Another mutation at this residue, glc7-R260P, confers only salt sensitivity, indicating that the glycogen and salt traits of glc7-109 are due to defects in distinct physiological pathways. The glc7-109 mutant is sensitive to cations, aminoglycosides, and alkaline pH and exhibits increased rates of l-leucine and 3,3'-dihexyloxacarbocyanine iodide uptake, but it is resistant to molar concentrations of sorbitol or KCl, indicating that it has normal osmoregulation. KCl suppresses the ion and drug sensitivities of the glc7-109 mutant. The CsCl sensitivity of this mutant is suppressed by recessive mutations in PMA1, which encodes the essential plasma membrane H(+)ATPase. Together, these results indicate that Glc7 regulates ion homeostasis by controlling ion transport and/or plasma membrane potential, a new role for Glc7 in budding yeast.

Analysis of respiratory mutants reveals new aspects of the control of glycogen accumulation by the cyclin-dependent protein kinase Pho85p

The PHO85 gene of Saccharomyces cerevisiae encodes a cyclin-dependent protein kinase that can interact with 10 different cyclins (Pcls). In conjunction with Pcl8p and Pcl10p, Pho85p phosphorylates and regulates glycogen synthase. Respiratory-deficient strains, such as coq3 mutants, have reduced glycogen stores and contain hyperphosphorylated and inactive glycogen synthase. We show here that pho85 coq3 mutants have dephosphorylated and active glycogen synthase yet do not maintain glycogen reserves. In contrast, deletion of PCL8 and PCL10 in the coq3 mutant background partially restores glycogen accumulation. This suggested the existence of inputs from Pho85p into glycogen storage, independent of Pcl8p and Pcl10p, and acting antagonistically.