Negative regulators of the PHO system in Saccharomyces cerevisiae: isolation and structural characterization of PHO85

The CDK Pho85 inhibits Whi7 Start repressor to promote cell cycle entry in budding yeast

Pho85 is a multifunctional CDK that signals to the cell when environmental conditions are favorable. It has been connected to cell cycle control, mainly in Start where it promotes the G1/S transition. Here we describe that the Start repressor Whi7 is a key target of Pho85 in the regulation of cell cycle entry. The phosphorylation of Whi7 by Pho85 inhibits the repressor and explains most of the contribution of the CDK in the activation of Start. Mechanistically, Pho85 downregulates Whi7 protein levels through the control of Whi7 protein stability and WHI7 gene transcription. Whi7 phosphorylation by Pho85 also restrains the intrinsic ability of Whi7 to associate with promoters. Furthermore, although Whi5 is the main Start repressor in normal cycling cells, in the absence of Pho85, Whi7 becomes the major repressor leading to G1 arrest. Overall, our results reveal a novel mechanism by which Pho85 promotes Start through the regulation of the Whi7 repressor at multiple levels, which may confer to Whi7 a functional specialization to connect the response to adverse conditions with the cell cycle control.

The nutrient-responsive CDK Pho85 primes the Sch9 kinase for its activation by TORC1

Yeast cells maintain an intricate network of nutrient signaling pathways enabling them to integrate information on the availability of different nutrients and adjust their metabolism and growth accordingly. Cells that are no longer capable of integrating this information, or that are unable to make the necessary adaptations, will cease growth and eventually die. Here, we studied the molecular basis underlying the synthetic lethality caused by loss of the protein kinase Sch9, a key player in amino acid signaling and proximal effector of the conserved growth-regulatory TORC1 complex, when combined with either loss of the cyclin-dependent kinase (CDK) Pho85 or loss of its inhibitor Pho81, which both have pivotal roles in phosphate sensing and cell cycle regulation. We demonstrate that it is specifically the CDK-cyclin pair Pho85-Pho80 or the partially redundant CDK-cyclin pairs Pho85-Pcl6/Pcl7 that become essential for growth when Sch9 is absent. Interestingly, the respective three CDK-cyclin pairs regulate the activity and distribution of the phosphatidylinositol-3 phosphate 5-kinase Fab1 on endosomes and vacuoles, where it generates phosphatidylinositol-3,5 bisphosphate that serves to recruit both TORC1 and its substrate Sch9. In addition, Pho85-Pho80 directly phosphorylates Sch9 at Ser726, and to a lesser extent at Thr723, thereby priming Sch9 for its subsequent phosphorylation and activation by TORC1. The TORC1-Sch9 signaling branch therefore integrates Pho85-mediated information at different levels. In this context, we also discovered that loss of the transcription factor Pho4 rescued the synthetic lethality caused by loss of Pho85 and Sch9, indicating that both signaling pathways also converge on Pho4, which appears to be wired to a feedback loop involving the high-affinity phosphate transporter Pho84 that fine-tunes Sch9-mediated responses.

Redundancy or specificity? The role of the CDK Pho85 in cell cycle control

It is generally accepted that progression through the eukaryotic cell cycle is driven by cyclin-dependent kinases (CDKs), which are regulated by interaction with oscillatory expressed proteins called cyclins. CDKs may be separated into 2 categories: essential and non-essential. Understandably, more attention has been focused on essential CDKs because they are shown to control cell cycle progression to a greater degree. After clearly determining the basic and "core" mechanisms of essential CDKs, several questions arise. What role do non-essential CDKs play? Are these CDKs functionally redundant and do they serve as a mere backup? Or might they be responsible for some accessory tasks in cell cycle progression or control? In the present review we will try to answer these questions based on recent findings on the involvement of non-essential CDKs in cell cycle progression. We will analyse the most recent information with regard to these questions in the yeast Saccharomyces cerevisiae, a well-established eukaryotic model, and in its unique non-essential CDK involved in the cell cycle, Pho85. We will also briefly extend our discussion to higher eukaryotic systems.

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.

Simple method to accurately differentiate Candida albicans isolates concurrently using polymorphic patterns of PCR-amplified, species-specific nuclear and mitochondrial targets

We describe a simple method to accurately differentiate Candida albicans isolates by concurrent use of the restriction enzyme digestion patterns for PCR products, targeting two species-specific DNA regions originating from genetically different sources, the nuclear and mitochondrial genomes. The target sequence we used as the nuclear gene was derived from the PHO85 gene, a negative regulator of the PHO system, in which we found a restriction size polymorphism within the two alleles of PHO85 in the diploid genome of this fungus. The mitochondrial target was derived from EO3, a species-specific DNA fragment possessing a small size polymorphism among various clinical isolates. Our results should provide a new tool for molecular epidemiological surveys of patients suffering from candidiasis caused by C. albicans.

Pho85, a multifunctional cyclin-dependent protein kinase in budding yeast

Pho85 is a multifunctional cyclin-dependent kinase (Cdk) in Saccharomyces cerevisiae that has emerged as an important model for the role of Cdks in both cell cycle control and other processes. Pho85 is targeted to its substrates by 10 different cyclins or Pcls. Three of these Pcls have specific roles in G1 phase of the cell cycle, both in regulating G1-specific gene expression and in controlling polarized growth. Many known substrates of the G1 forms of Pho85 are also phosphorylated by the homologous Cdk Cln-Cdc28, suggesting parallel or overlapping roles. Most of the remaining Pcls function in signalling: Pho85 is generally active when environmental conditions are satisfactory, phosphorylating proteins involved in transcription and other regulatory events to keep the stress response and inappropriate activities turned off. Recently, genetic screens for synthetic lethality and synthetic dosage lethality, and proteomic screens for in vitro Pho85 substrates, have revealed more details about how Pho85 functions to regulate a variety of cellular processes.

Identifying novel substrates for mouse Cdk5 kinase using the yeast Saccharomyces cerevisiae

Among the mammalian Cdk family members, Cdk5, activated by the binding of p35, plays an important role in the control of neurogenesis, and its deregulation is thought to be one of the causes of neurodegenerative diseases. Overproduction of Cdk5 and p35 in yeast cells causes growth arrest, probably because of hyperphosphorylation of yeast proteins. We screened mouse brain cDNA that could recover the growth of yeast cells overproducing Cdk5 and p35, hoping that such cDNA encodes a substrate or inhibitor of Cdk5. Mouse brain cDNA library was introduced into a yeast strain in which Cdk5, p35 and mouse cDNA were over-expressed under the control of the GAL promoter, and cDNA plasmids were isolated from the transformants that recovered growth on galactose medium. The analysis of those plasmids revealed that they harbored cDNA that encodes neuronal proteins including SCLIP and CRMP-1, and those with unknown function. We found that Cdk5 could phosphorylate SCLIP and CRMP-1 in vitro and the two proteins in cultured cells showed a mobility shift depending on Cdk5 activity and the presence of specific Ser or Thr residues, indicating that SCLIP and CRMP-1 are likely substrates for Cdk5 in vitro and in cultured cells. Further screening with these systems will enable us to identify more novel substrates and regulators of Cdk5/p35, which will lead to the exploration of Cdk5 function in diverse cellular systems.

A systematic high-throughput screen of a yeast deletion collection for mutants defective in PHO5 regulation

In response to phosphate limitation, Saccharomyces cerevisiae induces transcription of a set of genes important for survival. One of these genes is PHO5, which encodes a secreted acid phosphatase. A phosphate-responsive signal transduction pathway (the PHO pathway) mediates this response through three central components: a cyclin-dependent kinase (CDK), Pho85; a cyclin, Pho80; and a CDK inhibitor (CKI), Pho81. While signaling downstream of the Pho81/Pho80/Pho85 complex to PHO5 expression has been well characterized, little is known about factors acting upstream of these components. To identify missing factors involved in the PHO pathway, we carried out a high-throughput, quantitative enzymatic screen of a yeast deletion collection, searching for novel mutants defective in expression of PHO5. As a result of this study, we have identified at least nine genes that were previously not known to regulate PHO5 expression. The functional diversity of these genes suggests that the PHO pathway is networked with other important cellular signaling pathways. Among these genes, ADK1 and ADO1, encoding an adenylate kinase and an adenosine kinase, respectively, negatively regulate PHO5 expression and appear to function upstream of PHO81.

Cyclin-dependent kinases: a new cell cycle motif?

Increasing evidence suggests that the eukaryotic cell cycle is controlled at several checkpoints by different members of a novel class of protein kinase, the cyclin-dependent kinases. To phosphorylate their substrates, these enzymes bind to proteins of the cyclin family--proteins that are synthesized and degraded at specific points in each cell cycle. The most well known of these kinases is the 34 kDa product of the cdc2 gene in fission yeast, p34cdc2; however, several putative cyclin-dependent kinases have now been cloned or identified. Some of these closely resemble p34cdc2. Here we review these new proteins, their potential roles in the cell cycle and the cyclins with which they may interact.

The PHOA and PHOB Cyclin-Dependent Kinases Perform an Essential Function in Aspergillus nidulans

Unlike Pho85 of Saccharomyces cerevisiae, the highly related PHOA cyclin-dependent kinase (CDK) of Aspergillus nidulans plays no role in regulation of enzymes involved in phosphorous acquisition but instead modulates differentiation in response to environmental conditions, including limited phosphorous. Like PHO85, Aspergillus phoA is a nonessential gene. However, we find that expression of dominant-negative PHOA inhibits growth, suggesting it may have an essential but redundant function. Supporting this we have identified another cyclin-dependent kinase, PHOB, which is 77% identical to PHOA. Deletion of phoB causes no phenotype, even under phosphorous-limited growth conditions. To investigate the function of phoA/phoB, double mutants were selected from a cross of strains containing null alleles and by generating a temperature-sensitive allele of phoA in a ΔphoB background. Double-deleted ascospores were able to germinate but had a limited capacity for nuclear division, suggesting a cell cycle defect. Longer germination revealed morphological defects. The temperature-sensitive phoA allele caused both nuclear division and polarity defects at restrictive temperature, which could be complemented by expression of mammalian CDK5. Therefore, an essential function exists in A. nidulans for the Pho85-like kinase pair PHOA and PHOB, which may involve cell cycle control and morphogenesis.

Controlling transcription by destruction: the regulation of yeast Gcn4p stability

The Gcn4 protein, a member of the AP-1 family of transcription factors, is involved in the expression of more than 500 genes in the budding yeast Saccharomyces cerevisiae. A key role of Gcn4p is the increased expression of many amino acid biosynthesis genes in response to amino acid starvation. The accumulation of this transcription activator is mainly induced by efficient translation of the GCN4 ORF and by stabilisation of the Gcn4 protein. Under normal growth conditions, Gcn4p is a highly unstable protein, thereby resembling many eukaryotic transcription factors, including mammalian Jun and Myc proteins. Gcn4p is degraded by ubiquitin-dependent proteolysis mediated by the Skp1/cullin/F-box (SCF) ubiquitin ligase, which recognises specifically phosphorylated substrates. Two cyclin-dependent protein kinases, Pho85p and Srb10p, have crucial functions in regulating Gcn4p phosphorylation and degradation. The past few years have revealed many novel insights into these regulatory processes. Here, we summarise current knowledge about the factors and mechanisms regulating Gcn4p stability.

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.

Expression of a Pho85 cyclin-dependent kinase is repressed during the dimorphic transition in Sporothrix schenckii

Sporothrix schenckii is a pathogenic fungus that undergoes a dimorphic transition from yeast to mycelium in response to environmental conditions such as cell density, temperature, and calcium. We identified a homolog of the Pho85 cyclin-dependent kinase (Cdk) that mediates cellular responses to environmental conditions in other organisms. By Western blot, three proteins containing the PSTAIRE motif, which characterize the cyclin-dependent protein kinases, were identified in S. schenckii. The gene encoding a Pho85 homolog, PhoSs, was identified and sequenced. The phoSs gene consists of 990bp, contains one intron, and encodes a protein of 306 amino acids. The S. schenckii Pho85 homolog shares features with Cdks, including the PSTAIRE motif, an ATP binding domain, and a serine-threonine kinase domain. By quantitative competitive RT-PCR, expression of the phoSs gene was found to decrease 30-fold during the yeast to mycelium transition. The addition of extracellular calcium accelerated the dimorphic transition and restored phoSs expression. These findings suggest PhoSs may participate in the control of the yeast to mycelium transition in S. schenckii.

Chemical inhibition of the Pho85 cyclin-dependent kinase reveals a role in the environmental stress response

In addition to its well-established role in responding to phosphate starvation, the cyclin-dependent kinase Pho85 has been implicated in a number of other physiological responses of the budding yeast Saccharomyces cerevisiae, including synthesis of glycogen. To comprehensively characterize the range of Pho85-dependent gene expression, we used a chemical genetic approach that enabled us to control Pho85 kinase activity with a cell-permeable inhibitor and whole genome transcript profiling. We found significant phenotypic differences between the rapid loss of activity caused by inhibition and the deletion of the genomic copy of PHO85. We demonstrate that Pho85 controls the expression of not only previously identified glycogen synthetic genes, but also a significant regulon of genes involved in the cellular response to environmental stress. In addition, we show that the effects of this inhibitor are both rapid and reversible, making it well suited to the study of the behavior of dynamic signaling pathways.

Functional analysis of the cyclin-dependent kinase inhibitor Pho81 identifies a novel inhibitory domain

In response to phosphate limitation, Saccharomyces cerevisiae induces transcription of a set of genes important for survival. A phosphate-responsive signal transduction pathway mediates this response by controlling the activity of the transcription factor Pho4. Three components of this signal transduction pathway resemble those used to regulate the eukaryotic cell cycle: a cyclin-dependent kinase (CDK), Pho85; a cyclin, Pho80; and a CDK inhibitor (CKI), Pho81. Pho81 forms a stable complex with Pho80-Pho85 under both high- and low-phosphate conditions, but it only inhibits the kinase when cells are starved for phosphate. Pho81 contains six tandem repeats of the ankyrin consensus domain homologous to the INK4 family of mammalian CKIs. INK4 proteins inhibit kinase activity through an interaction of the ankyrin repeats and the CDK subunits. Surprisingly, we find that a region of Pho81 containing 80 amino acids C terminal to the ankyrin repeats is necessary and sufficient for Pho81's CKI function. The ankyrin repeats of Pho81 appear to have no significant role in Pho81 inhibition. Our results suggest that Pho81 inhibits Pho80-Pho85 with a novel motif.

Structure and function of cyclin-dependent Pho85 kinase of Saccharomyces cerevisiae

Yeast Saccharomyces cerevisiae has five cyclin-dependent protein kinases (Cdks), Cdc28, Srb10, Kin28, Ctk1, and Pho85. Any of these Cdks requires a cyclin partner for its kinase activity and a Cdk/cyclin complex, thus produced, phosphorylates a set of specific substrate proteins to exert its function. The cyclin partners of Srb10, Kin28, and Ctk1 are Srb11, Ccl1, and Ctk2, respectively. In contrast to the fact that each of Srb10, Kin28, and Ctk1 has a single cyclin partner, Cdc28 and Pho85 are polygamous; Cdc28 has 9 cyclins and Pho85 has 10 cyclins. Among these Cdks, Kin28 and Cdc28 are essential Cdks and it is well known that Cdc28 kinase plays a major role in regulating cell cycle progression. Pho85 is a non-essential Cdk but its absence causes a broad spectrum of phenotypes such as constitutive expression of PHO5, inability to utilize non-fermentable carbon sources, defects in cell cycle progression, and so on. Pho85 homologues are expanding to higher eukaryotes. Pho85 is most closely related with Cdk5 in terms of the amino acid sequence. The functional analysis of the domains of Pho85 also supports the close relationship between Pho85 and Cdk5, in which it was shown that the method of regulation of these two kinases is similar. Furthermore, forced expression of the mammalian CDK5 gene in a pho85Delta strain canceled a part of the pho85 defects. In this review, we summarize the functions of both Pho85/cyclin kinase and emphasize yeast Pho85 as valuable model systems to elucidate the functions of their homologues in other organisms.

Pho85 kinase, a yeast cyclin-dependent kinase, regulates the expression of UGP1 encoding UDP-glucose pyrophosphorylase

The PHO85 gene is a negative regulator of the PHO system in the yeast Saccharomyces cerevisiae and encodes a protein kinase (Pho85) highly homologous to the Cdc28 kinase (Cdc28). Ten cyclin-like proteins are known to interact with Pho85, and combination with different cyclins is believed to be responsible for distinct Pho85 functions, including phosphate metabolism, carbon source utilization and cell cycle regulation. However, only a limited number of substrates of Pho85 kinase, including Pho4, Gsy2 and Sicl, have so far been identified. To search for more targets of Pho85 and to clarify the genetic control mechanisms by Pho85 kinase in these cellular functions, we carried out a genome-wide analysis of the effect of a pho85Delta mutation on gene expression. We found that expression of various genes involved in carbon metabolism are affected by the mutation and that among them, UGP1 promoter activity was increased in the absence of Pho85 kinase. This increase in the promoter activity was not observed in a pho4Delta mutant or with a mutant UGP1 promoter that is devoid of putative Pho4 and Bas2 binding sites, suggesting that UGP1 expression is modulated by Pho85 through Pho4. We also found that expression of several Pho85-cyclin genes were altered by the carbon source, the growth phase and Pho85 kinase itself.

Regulation of the Pcl7-Pho85 cyclin-cdk complex by Pho81

Saccharomyces cerevisiae strains lacking a functional Pho85 cyclin-dependent kinase (cdk) exhibit a complex phenotype, including deregulation of phosphatase genes controlled by the transcription factor Pho4, slow growth on rich media, failure to grow using galactose, lactate or glycerol as a carbon source and hyperaccumulation of glycogen. The ability of Pho85 to regulate the transcription factor Pho4 is mediated by its association the Pho80 cyclin. Some other regulatory functions of the Pho85 cdk have been shown to be mediated via its interaction with a recently identified family of Pho80-related cyclins (Pcls). Here, we show that the poorly characterized Pho80-like protein Pcl7 forms a functional kinase complex with the Pho85 cdk, and that the activity of this complex is inhibited in response to phosphate starvation. Additionally, we show that Pcl7 interacts with the phosphate-regulated cyclin-cdk inhibitor Pho81, and that the regulation of the Pcl7-Pho85 complex in response to changes in phosphate levels is dependent on Pho81. Thus, we demonstrate for the first time that the Pho81 regulator is not dedicated to regulating Pho80, but may act to co-ordinate the activity of both the Pho80-Pho85 and Pcl7-Pho85 cyclin-cdk complexes in response to phosphate levels. We also demonstrate that expression of Pcl7 is cell cycle regulated, with maximal activity occurring in mid to late S-phase, perhaps suggesting a role for Pcl7 in cell cycle progression. Finally, we describe the phenotype of pcl7Delta and pcl6Delta yeast strains that have defects in carbon source utilization.

Acclimation of Chlamydomonas reinhardtii to its nutrient environment

To cope with low nutrient availability in nature, organisms have evolved inducible systems that enable them to scavenge and efficiently utilize the limiting nutrient. Furthermore, organisms must have the capacity to adjust their rate of metabolism and make specific alterations in metabolic pathways that favor survival when the potential for cell growth and division is reduced. In this article I will focus on the acclimation of Chlamydomonas reinhardtii, a unicellular, eukaryotic green alga to conditions of nitrogen, sulfur and phosphorus deprivation. This organism has a distinguished history as a model for classical genetic analyses, but it has recently been developed for exploitation using an array of molecular and genomic tools. The application of these tools to the analyses of nutrient limitation responses (and other biological processes) is revealing mechanisms that enable Chlamydomonas to survive harsh environmental conditions and establishing relationships between the responses of this morphologically simple, photosynthetic eukaryote and those of both nonphotosynthetic organisms and vascular plants.

A pcl-like cyclin activates the Res2p-Cdc10p cell cycle "start" transcriptional factor complex in fission yeast

In the fission yeast Schizosaccharomyces pombe, the "start" of the cell cycle is controlled by the two functionally redundant transcriptional regulator complexes, Res1p-Cdc10p and Res2p-Cdc10p, that activate genes essential for the onset and progression of S phase. The activity of the Res2p-Cdc10p complex is regulated at least by the availability of the Rep2 trans-activator subunit in the mitotic cell cycle. We have recently isolated the pas1(+) gene as a multicopy suppressor of the res1 null mutant. This gene encodes a novel cyclin that shares homology with the Pho85 kinase-associated cyclins of the budding yeast Saccharomyces cerevisiae. Genetic analysis reveals that Pas1 cyclin is unrelated to phosphate metabolism and stimulates the G(1)-S transition by specifically activating the Res2p-Cdc10p complex independently of Rep2p. Pas1 cyclin also controls mating pheromone signaling. Cells lacking pas1(+) are highly sensitive to mating pheromone, responding with facilitated G(1) arrest and premature commitment to conjugation. Pas1 cyclin associates in vivo with both Cdc2 and Pef1 kinases, the latter of which is a fission yeast counterpart of the budding yeast Pho85 kinase, but genetic analysis indicates that the Pef1p-associated Pas1p is responsible for the activation of Res2p-Cdc10p during the G(1)-S transition.

Identification of a Candida albicans homologue of the PHO85 gene, a negative regulator of the PHO system in Saccharomyces cerevisiae

In a screen for the protein kinase genes of the human pathogenic yeast Candida albicans, a putative homologue (CaPHO85) of PHO85, a negative regulator of the PHO system of Saccharomyces cerevisiae, which is one of the cyclin-dependent protein kinases (CDKs), was isolated. An open reading frame (ORF) of this gene was identified encoding a predicted protein of 326 amino acids with a calculated molecular weight of 37.6 kDa. The amino acid sequence is highly homologous to S. cerevisiae Pho85 (62% identity) and its Aspergillus nidulans homologue (70% identity), but less homologous to Cdc28 (50% identity) of S. cerevisiae and to its C. albicans homologue CaCdc28 (49% identity), both of which are also CDK. The coding region for the C. albicans gene was interrupted by an intron of 81 nucleotides near the sequence encoding the N-terminal region, similarly to the case of the S. cerevisiae PHO85 gene. Alignment of CaPho85 with various yeast CDKs revealed that most of the domains for ATP-binding and protein kinase activity are conserved among fungal species. Southern blot analysis indicated that CaPHO85 is most likely present as a single copy gene. This gene complemented the pho85 mutation of S. cerevisiae by transformation.

Interactions between Pho85 cyclin-dependent kinase complexes and the Swi5 transcription factor in budding yeast

Pho85 is a cyclin-dependent protein kinase (Cdk) in budding yeast with roles in cell metabolism and cell cycle progression. Activation of Pho85 occurs through association with Pho85 cyclins (Pcls), of which 10 are known. When complexed with the G1 cyclins, Pcl1 and Pcl2, Pho85 is required for cell cycle progression in the absence of the Cdc28-dependent cyclins, Cln1 and Cln2. To identify potential targets of Pcl2-Pho85, we performed a two-hybrid screen using the Pcl2 cyclin as bait and recovered the transcription factor Swi5 as a Pcl2-interacting protein. We performed both biochemical and genetic tests to discover the biological significance of the interaction between Pcl2 and Swi5 seen in the two-hybrid assay. We found that Swi5 interacts in vitro with Pho85 cyclins and is phosphorylated in vitro by the Pho80-Pho85 kinase. We discovered that a subset of genes that are controlled by Swi5 and a homologous transcription factor, Ace2, was misregulated in a pho85 deletion strain; expression of the ASH1 and CTS1 genes was reduced in an ace2 deletion strain, whereas expression of both genes was increased in an ace2Delta pho85Delta double mutant. We also found that overexpression of SWI5 caused cell lethality in a pho85 deletion strain. Our results are consistent with misregulation of Swi5 activity in vivo in the absence of Pho85 and implicate Swi5 as a potential substrate of Pho85 cyclin-dependent kinase complexes.

Mouse cyclin-dependent kinase (Cdk) 5 is a functional homologue of a yeast Cdk, pho85 kinase

Mouse cyclin-dependent kinase (Cdk) 5 and yeast Pho85 kinase share similarities in structure as well as in the regulation of their activity. We found that mouse Cdk5 kinase produced in pho85Delta mutant cells could suppress some of pho85Delta mutant phenotypes including failure to grow on nonfermentable carbon sources, morphological defects, and growth defect caused by Pho4 or Clb2 overproduction. We also demonstrated that Cdk5 coimmunoprecipitated with Pho85-cyclins including Pcl1, Pcl2, Pcl6, Pcl9, and Pho80, and that the immunocomplex could phosphorylate Pho4, a native substrate of Pho85 kinase. Thus mouse Cdk5 is a functional homologue of yeast Pho85 kinase.

The Pho85 kinase, a member of the yeast cyclin-dependent kinase (Cdk) family, has a regulation mechanism different from Cdks functioning throughout the cell cycle

BACKGROUND

The PHO85 gene is a negative regulator of the PHO system in the yeast Saccharomyces cerevisiae and encodes a protein kinase (Pho85p) which is highly homologous to the Cdc28 kinase (Cdc28p). Although the two kinases share a 51% identity and their functional domains are well conserved, PHO85 fails to replace CDC28. Pho85p forms complexes with G1-cyclin homologues, including Pcl1p, Pcl2p and Pcl9p, and is thought to be involved in the cell-cycle regulation at G1 and the end of M. By analysing the genetic and biochemical properties of Pho85p, we studied whether the regulation of Pho85p activity is similar to other cyclin-dependent kinases (Cdks) directly involved in cell cycle regulation.

RESULTS

A functional analysis of various Pho85 mutants revealed that E53 in the PSTAIRE sequence was important for Pho85p function. On the other hand, residues in the T-loop including S166, S167 and E168, appeared dispensable for Pho85p function, suggesting that the phosphorylation of S166, corresponding to T161 of Cdc2p and T169 of Cdc28p, was not required for the kinase activity of Pho85p. Instead, we found that phosphorylation of Y18, corresponding to Y15 of Cdc2p and Y19 of Cdc28p, may be important for the binding of Pho80p but not of Pcl1p, suggesting that tyrosine phosphorylation may function as a signal which discriminates various Pho85-cyclins.

CONCLUSION

In Cdks functioning throughout the cell cycle, tyrosine phosphorylation is inhibitory to the activation of kinase, whereas the phosphorylation of threonine in the T-loop is essential for activation. Our finding indicates that the regulation mechanism of Pho85p activation appears to be distinct from these Cdks.

Regulation of Cdc28 cyclin-dependent protein kinase activity during the cell cycle of the yeast Saccharomyces cerevisiae

The cyclin-dependent protein kinase (CDK) encoded by CDC28 is the master regulator of cell division in the budding yeast Saccharomyces cerevisiae. By mechanisms that, for the most part, remain to be delineated, Cdc28 activity controls the timing of mitotic commitment, bud initiation, DNA replication, spindle formation, and chromosome separation. Environmental stimuli and progress through the cell cycle are monitored through checkpoint mechanisms that influence Cdc28 activity at key cell cycle stages. A vast body of information concerning how Cdc28 activity is timed and coordinated with various mitotic events has accrued. This article reviews that literature. Following an introduction to the properties of CDKs common to many eukaryotic species, the key influences on Cdc28 activity-cyclin-CKI binding and phosphorylation-dephosphorylation events-are examined. The processes controlling the abundance and activity of key Cdc28 regulators, especially transcriptional and proteolytic mechanisms, are then discussed in detail. Finally, the mechanisms by which environmental stimuli influence Cdc28 activity are summarized.

Phosphorylation of sic1, a cyclin-dependent kinase (Cdk) inhibitor, by Cdk including Pho85 kinase is required for its prompt degradation

In the yeast Saccharomyces cerevisiae, Sic1, an inhibitor of Clb-Cdc28 kinases, must be phosphorylated and degraded in G1 for cells to initiate DNA replication, and Cln-Cdc28 kinase appears to be primarily responsible for phosphorylation of Sic1. The Pho85 kinase is a yeast cyclin-dependent kinase (Cdk), which is not essential for cell growth unless both CLN1 and CLN2 are absent. We demonstrate that Pho85, when complexed with Pcl1, a G1 cyclin homologue, can phosphorylate Sic1 in vitro, and that Sic1 appears to be more stable in pho85Delta cells. Three consensus Cdk phosphorylation sites present in Sic1 are phosphorylated in vivo, and two of them are required for prompt degradation of the inhibitor. Pho85 and other G1 Cdks appear to phosphorylate Sic1 at different sites in vivo. Thus at least two distinct Cdks can participate in phosphorylation of Sic1 and may therefore regulate progression through G1.

A cyclin-dependent kinase family member (PHOA) is required to link developmental fate to environmental conditions in Aspergillus nidulans

We addressed the question of whether Aspergillus nidulans has more than one cyclin-dependent kinase gene and identified such a gene, phoA, encoding two PSTAIRE-containing kinases (PHOAM1 and PHOAM47) that probably result from alternative pre-mRNA splicing. PHOAM47 is 66% identical to Saccharomyces cerevisiae Pho85. The function of this gene was studied using phoA null mutants. It functions in a developmental response to phosphorus-limited growth but has no effect on the regulation of enzymes involved in phosphorus acquisition. Aspergillus nidulans shows both asexual and sexual reproduction involving temporal elaboration of different specific cell types. We demonstrate that developmental decisions in confluent cultures depend upon both the initial phosphorus concentration and the inoculation density and that these factors influence development through phoA. In the most impressive cases, absence of phoA resulted in a switch from asexual to sexual development (at pH 8), or the absence of development altogether (at pH 6). The phenotype of phoA deletion strains appears to be specific for phosphorus limitation. We propose that PHOA functions to help integrate environmental signals with developmental decisions to allow ordered differentiation of specific cell types in A.nidulans under varying growth conditions. The results implicate a putative cyclin-dependent kinase in the control of development.

Cyclin partners determine Pho85 protein kinase substrate specificity in vitro and in vivo: control of glycogen biosynthesis by Pcl8 and Pcl10

In Saccharomyces cerevisiae, PHO85 encodes a cyclin-dependent protein kinase (Cdk) with multiple roles in cell cycle and metabolic controls. In association with the cyclin Pho80, Pho85 controls acid phosphatase gene expression through phosphorylation of the transcription factor Pho4. Pho85 has also been implicated as a kinase that phosphorylates and negatively regulates glycogen synthase (Gsy2), and deletion of PHO85 causes glycogen overaccumulation. We report that the Pcl8/Pcl10 subgroup of cyclins directs Pho85 to phosphorylate glycogen synthase both in vivo and in vitro. Disruption of PCL8 and PCL10 caused hyperaccumulation of glycogen, activation of glycogen synthase, and a reduction in glycogen synthase kinase activity in vivo. However, unlike pho85 mutants, pcl8 pcl10 cells had normal morphologies, grew on glycerol, and showed proper regulation of acid phosphatase gene expression. In vitro, Pho80-Pho85 complexes effectively phosphorylated Pho4 but had much lower activity toward Gsy2. In contrast, Pcl10-Pho85 complexes phosphorylated Gsy2 at Ser-654 and Thr-667, two physiologically relevant sites, but only poorly phosphorylated Pho4. Thus, both the in vitro and in vivo substrate specificity of Pho85 is determined by the cyclin partner. Mutation of PHO85 suppressed the glycogen storage deficiency of snf1 or glc7-1 mutants in which glycogen synthase is locked in an inactive state. Deletion of PCL8 and PCL10 corrected the deficit in glycogen synthase activity in both the snf1 and glc7-1 mutants, but glycogen synthesis was restored only in the glc7-1 mutant strain. This genetic result suggests an additional role for Pho85 in the negative regulation of glycogen accumulation that is independent of Pcl8 and Pcl10.

The phosphatase system in Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae has at least six species of acid and alkaline phosphatases with different cellular localizations, as well as inorganic phosphate (Pi) transporters. Most of the genes encoding these enzymes are coordinately repressed and derepressed depending on the Pi concentration in the growth medium. The Pi signals are conveyed to these genes through a regulatory circuit consisting of a set of positive and negative regulatory proteins. This phosphatase system is interested as one of the best systems for studying gene regulation in S. cerevisiae due to the simplicity of phenotype determination in genetic analysis. With this methodological advantage, considerable amounts of genetic and molecular evidence in phosphatase regulation have been accumulated in the past twenty-five years. This article summarizes the current progress of research into this subject.

Regulation of phosphatase synthesis in Saccharomyces cerevisiae--a review

Transcription of the genes encoding acid and alkaline phosphatases and the inorganic phosphate (Pi) transporter of Saccharomyces cerevisiae are coordinately repressed and derepressed depending on the Pi concentration in the culture medium. This phosphatase system is particularly suited for the study of regulatory mechanisms, because the acid phosphatase activity of each colony on a plate is easily detected by specific staining methods and there is a 500-fold difference between the repressed and derepressed levels of acid phosphatase activity. With these advantages, considerable amounts of genetic and molecular evidence have been accumulated in the past two decades. This article summarizes our current knowledge on this subject.

NUC-2, a component of the phosphate-regulated signal transduction pathway in Neurospora crassa, is an ankyrin repeat protein

In response to phosphorus limitation, the fungus Neurospora crassa synthesizes a number of enzymes that function to bring more phosphate into the cell. The NUC-2 protein appears to sense the availability of phosphate and transmits the signal downstream to the regulatory pathway. The nuc-2+ gene has been cloned by its ability to restore growth of a nuc-2 mutant under restrictive conditions of high pH and low phosphate concentration. We mapped the cloned gene to the right arm of linkage group II, consistent with the chromosomal position of the nuc-2 mutation as determined by classical genetic mapping. The nuc-2' open reading frame is interrupted by five introns and codes for a protein of 1066 amino acid residues. Its predicted amino acid sequence has high similarity to that of its homolog in Saccharomyces cerevisiae, PHO81. Both proteins contain six ankyrin repeats, which have been implicated in the cyclin-dependent kinase inhibitory activity of PHO81. The phenotypes of a nuc-2 mutant generated by repeat-induced point mutation and of a strain harboring a UV-induced nuc-2 allele are indistinguishable. Both are unable to grow under the restrictive conditions, a phenotype which is to some degree temperature dependent. The nuc-2+ gene is transcriptionally regulated. A 15-fold increase in the level of the nuc-2+ transcript occurs in response to a decrease in exogenous phosphate concentration.

Deletion of the gene encoding the cyclin-dependent protein kinase Pho85 alters glycogen metabolism in Saccharomyces cerevisiae

Pho85, a protein kinase with significant homology to the cyclin-dependent kinase, Cdc28, has been shown to function in repression of transcription of acid phosphatase (APase, encoded by PHO5) in high phosphate (Pi) medium, as well as in regulation of the cell cycle at G1/S. We described several unique phenotypes associated with the deletion of the PHO85 gene including growth defects on a variety of carbon sources and hyperaccumulation of glycogen in rich medium high in Pi. Hyperaccumulation of glycogen in the pho85 strains is independent of other APase regulatory molecules and is not signaled through Snfl kinase. However, constitutive activation of cAPK suppresses the hyperaccumulation of glycogen in a pho85 mutant. Mutation of the type-1 protein phosphatase encoded by GLC7 only partially suppresses the glycogen phenotype of the pho85 mutant. Additionally, strains containing a deletion of the PHO85 gene show an increase in expression of GSY2. This work provides evidence that Pho85 has functions in addition to transcriptional regulation of APase and cell-cycle progression including the regulation of glycogen levels in the cell and may provide a link between the nutritional state of the cell and these growth related responses.

Structure-function relationships of the yeast cyclin-dependent kinase Pho85

The PHO85 gene of Saccharomyces cerevisiae encodes a cyclin-dependent kinase involved in both transcriptional regulation and cell cycle progression. Although a great deal is known concerning the structure, function, and regulation of the highly homologous Cdc28 protein kinase, little is known concerning these relationships in regard to Pho85. In this study, we constructed a series of Pho85-Cdc28 chimeras to map the region(s) of the Pho85 molecule that is critical for function of Pho85 in repression of acid phosphatase (PHO5) expression. Using a combination of site-directed and ethyl methanesulfonate-induced mutagenesis, we have identified numerous residues critical for either activation of the Pho85 kinase, interaction of Pho85 with the cyclin-like molecule Pho80, or substrate recognition. Finally, analysis of mutations analogous to those previously identified in either Cdc28 or cdc2 of Schizosaccharomyces pombe suggested that the inhibition of Pho85-Pho80 activity in mechanistically different from that seen in the other cyclin-dependent kinases.

A truncated form of the Pho80 cyclin redirects the Pho85 kinase to disrupt vacuole inheritance in S. cerevisiae

Partitioning of the vacuole during cell division in Saccharomyces cerevisiae begins during early S phase and ends in late G2 phase before the yeast nucleus migrates into the bud neck. We have isolated and characterized a new mutant, vac5-1, which is defective in vacuole segregation. Cells with the vac5-1 mutation can form large buds without vacuoles. The VAC5 gene was cloned and is identical to PHO80. PHO80 encodes a cyclin which acts in a complex with a cdc-like kinase, PHO85, as a negative regulator of two transcription factors (PHO2 and PHO4) that govern the expression of metabolic phosphatases. The vacuole inheritance defect in vac5-1 cells is dependent on the presence of the Pho85 kinase and its targets Pho4p and Pho2p. As with other alleles of PHO80, phosphatase levels are elevated in vac5-1 mutants. A suppressor, the COOH-terminal half of the Gal11 transcription factor, rescues the vac5-1 phenotype of defective vacuole inheritance without altering the vac5-1 phenotype of elevated phosphatase levels. In addition, neither maximal nor minimal levels of expression of the inducible "PHO" system phosphatases causes a vacuole inheritance defect. Though vac5-1 is recessive, pho80 delta or pho85 delta strains do not show a defect in vacuole inheritance, suggesting that vac5-1 is not a complete loss-of-function allele. Sequence analysis shows that the vac5-1 allele encodes a truncated form of the Pho80 cyclin and overexpression of vac5-1 in pho80 delta cells causes a vacuole inheritance defect. We conclude that the vac5-1 allele directs the Pho85 kinase to regulate, via transcription factors Pho4 and Pho2, genes that affect vacuole inheritance but which are not known to be under normal PHO pathway control.

A kinase-cyclin pair in the RNA polymerase II holoenzyme

The RNA polymerase II holoenzyme consists of RNA polymerase II, a subset of general transcription factors, and regulatory proteins known as SRB proteins. The genes encoding SRB proteins were isolated as suppressors of mutations in the RNA polymerase II carboxy-terminal domain (CTD). The CTD and SRB proteins have been implicated in the response to transcriptional regulators. We report here the isolation of two new SRB genes, SRB10 and SRB11, which encode kinase- and cyclin-like proteins, respectively. Genetic and biochemical evidence indicates that the SRB10 and SRB11 proteins form a kinase-cyclin pair in the holoenzyme. The SRB10/11 kinase is essential for a normal transcriptional response to galactose induction in vivo. Holoenzymes lacking SRB10/11 kinase function are strikingly deficient in CTD phosphorylation. Although defects in the kinase substantially affect transcription in vivo, purified holoenzymes lacking SRB10/11 kinase function do not show defects in defined in vitro transcription systems, suggesting that the factors necessary to elicit the regulatory role of the SRB10/11 kinase are missing in these systems. These results indicate that the SRB10/11 kinase is involved in CTD phosphorylation and suggest that this modification has a role in the response to transcriptional regulators in vivo.

Functional domains of Pho81p, an inhibitor of Pho85p protein kinase, in the transduction pathway of Pi signals in Saccharomyces cerevisiae

The PHO81 gene is thought to encode an inhibitor of the negative regulators (Pho80p and Pho85p) in the phosphatase (PHO) regulon. Transcription of PHO81 is regulated by Pi signals through the same PHO regulatory system. Elimination of the PHO81 promoter or its substitution by the GAL1 promoter revealed that stimulation of the PHO regulatory system requires both increased transcription of PHO81 and a Pi starvation signal. The predicted Pho81p protein contains 1,179 amino acids (aa) and has six repeats of an ankyrin-like sequence in its central region. The minimum amino acid sequence required for Pho81p function was narrowed down to a 141-aa segment (aa 584 to 724), which contains the fifth and sixth repeats of the ankyrin-like motif. The third to sixth repeats of the ankyrin-like motif of Pho81p have significant similarities to that of p16INK4, which inhibits activity of the human cyclin D-CDK4 kinase complex. Deletion analyses revealed that the N- and C-terminal regions of Pho81p behave as negative and positive regulatory domains, respectively, for the minimal 141-aa region. The negative regulatory activity of the N-terminal domain was antagonized by a C-terminal segment of Pho81p supplied in trans. All four known classes of PHO81c mutations that show repressible acid phosphatase activity in high-Pi medium affect the N-terminal half of Pho81p. An in vitro assay showed that a glutathione S-transferase-Pho81p fusion protein inhibits the Pho85p protein kinase. Association of Pho81p with Pho85p or with the Pho80p-Pho85p complex was demonstrated by the two-hybrid system.

The PCL2 (ORFD)-PHO85 cyclin-dependent kinase complex: a cell cycle regulator in yeast

Cyclin-dependent kinase (cdk) complexes are essential activators of cell cycle progression in all eukaryotes. In contrast to mammalian cells, in which multiple cdk's contribute to cell cycle regulation, the yeast cell cycle is largely controlled by the activity of a single cdk, CDC28. Analysis of the putative G1 cyclin PCL2 (ORFD) identified a second cyclin-cdk complex that contributes to cell cycle progression in yeast. PCL2 interacted with the cdk PHO85 in vivo and in vitro and formed a kinase complex that had G1-periodic activity. Under genetic conditions in which the Start transition was compromised, PHO85 and its associated cyclin subunits were essential for cell cycle commitment. Because PHO85 and another cyclin-like molecule, PHO80, also take part in inorganic phosphate metabolism, this cdk enzyme may integrate responses to nutritional conditions with the cell cycle.

Cell cycle control by a complex of the cyclin HCS26 (PCL1) and the kinase PHO85

The events of the eukaryotic cell cycle are governed by cyclin-dependent kinases (cdk's), whose activation requires association with cyclin regulatory subunits expressed at specific cell cycle stages. In the budding yeast Saccharomyces cerevisiae, the cell cycle is thought to be controlled by a single cdk, CDC28. Passage through the G1 phase of the cell cycle is regulated by complexes of CDC28 and G1 cyclins (CLN1, CLN2, and CLN3). A putative G1 cyclin, HCS26, has recently been identified. In a/alpha diploid cells lacking CLN1 and CLN2, HCS26 is required for passage through G1. HCS26 does not associate with CDC28, but instead associates with PHO85, a closely related protein kinase. Thus, budding yeast, like higher eukaryotes, use multiple cdk's in the regulation of cell cycle progression.

Phosphate-regulated inactivation of the kinase PHO80-PHO85 by the CDK inhibitor PHO81

A complex consisting of the cyclin-dependent kinase (CDK) PHO85 and the cyclin PHO80 phosphorylates and is thought to inactivate the transcription factor PHO4 when yeast cells are grown in medium containing high concentrations of phosphate. The CDK inhibitor PHO81 inhibits the kinase activity of the PHO80-PHO85 complex when Saccharomyces cerevisiae cells are grown in medium depleted of phosphate. A region of PHO81 with similarity to the mammalian CDK inhibitor p16INK4 is sufficient for inhibition in vitro. These studies demonstrate that CDK inhibitors are used to regulate kinases involved in processes other than cell cycle control and suggest that the ankyrin repeat motif may be commonly used for interaction with cyclin-CDK complexes.

Phosphorylation of the transcription factor PHO4 by a cyclin-CDK complex, PHO80-PHO85

Induction of the yeast gene PHO5 is mediated by the transcription factors PHO2 and PHO4. PHO5 transcription is not detectable in high phosphate; it is thought that the negative regulators PHO80 and PHO85 inactivate PHO2 and PHO4. Here it is reported that PHO80 has homology to yeast cyclins and interacts with PHO85, a p34cdc2/CDC28-related protein kinase. The PHO80-PHO85 complex phosphorylates PHO4; this phosphorylation is correlated with negative regulation of PHO5. These results demonstrate the existence of a cyclin-cdk complex that is used for a regulatory process other than cell-cycle control and identify a physiologically relevant substrate for this complex.

MSI3, a multicopy suppressor of mutants hyperactivated in the RAS-cAMP pathway, encodes a novel HSP70 protein of Saccharomyces cerevisiae

The MSI3 gene was isolated as a multicopy suppressor of the heat shock-sensitive phenotype of the ira1 mutation, which causes hyperactivation of the RAS-cAMP pathway. Overexpression of MSI3 also suppresses the heat shock-sensitive phenotype of the bcy1 mutant. Determination of the DNA sequence of MSI3 revealed that MSI3 can encode a 77.4 kDa protein related to the HSP70 family. The amino acid sequence of Msi3p is about 30% identical to that of the Ssa1p of Saccharomyces cerevisiae. This contrasts with the finding that members of the HSP70 family generally show at least 50% amino acid identity. The consensus nucleotide sequence of the heat shock element (HSE) was found in the upstream region of MSI3. Moreover, the steady-state levels of the MSI3 mRNA and protein were increased upon heat shock. These results indicate that the MSI3 gene encodes a novel HSP70-like heat shock protein. Disruption of the MSI3 gene was associated with a temperature sensitive growth phenotype but unexpectedly, thermotolerance was enhanced in the disruptant.

Molecular analysis of the PHO81 gene of Saccharomyces cerevisiae

The PHO81 gene product is a positive regulatory factor required for the synthesis of the phosphate repressible acid phosphatase (encoded by the PHO5 gene) in Saccharomyces cerevisiae. Genetic analysis has suggested that PHO81 may be the signal acceptor molecule; however, the biochemical function of the PHO81 gene product is not known. We have cloned the PHO81 gene and sequenced the promoter. A PHO81-LacZ fusion was shown to be a valid reporter since its expression is regulated by the level of inorganic phosphate and is controlled by the same regulatory factors that regulate PHO5 expression. To elucidate the mechanism by which PHO81 functions, we have isolated and cloned dominant mutations in the PHO81 gene which confer constitutive synthesis of acid phosphatase. We have demonstrated that overexpression of the negative regulatory factor, PHO80, but not the negative regulatory factor PHO85, partially blocks the constitutive acid phosphatase synthesis in a strain containing a dominant constitutive allele of PHO81. This suggests that PHO81 may function by interacting with PHO80 or that these molecules compete for the same target.

Isolation and characterization of a cdc 2 cDNA from Dictyostelium discoideum

A cdc2 homologous sequence was amplified from Dictyostelium discoideum by the polymerase chain reaction and used to isolate several cDNA clones. The amino acid sequence encoded by these cDNAs exhibited approx. 60% identity to the Cdc2 proteins of other species. A cDNA containing the entire coding sequence complemented the temperature sensitive cdc28 mutant of Saccharomyces cerevisiae, although growth of the transformants was slow and limited. Southern blot analysis of restriction digests under high stringency conditions provided evidence that Dictyostelium contains a single cdc2 gene, although at lower stringency multiple fragments were detected, suggesting the existence of a cdc2 gene family. Northern blot analysis of RNA from different stages of Dictyostelium development showed that cdc2 mRNA levels increased during aggregation and then decreased to low levels by the pseudoplasmodial stage of development. By contrast, cdc2 mRNA levels remained relatively constant as cells passed from exponential growth to the stationary phase.

Isolation and characterization of cDNA clones encoding cdc2 homologues from Oryza sativa: a functional homologue and cognate variants

Using probes obtained by PCR amplification, we have isolated two cognate rice cDNAs (cdc2Os-1 and cdc2Os-2) encoding structural homologues of the cdc2+/CDC28 (cdc2) protein kinase from a cDNA library prepared from cultured rice cells. Comparison of the deduced amino acid sequences of cdc2Os-1 and cdc2Os-2 showed that they are 83% identical. They are 62% identical to CDC28 of Saccharomyces cerevisiae and much more similar to the yeast and mammalian p34cdc2 kinases than to rice R2, a cdc2-related kinase isolated previously by screening the same rice cDNA library with a different oligonucleotide probe. Southern blot analysis indicated that the three rice clones (cdc2Os-1, cdc2Os-2 and R2) are derived from distinct genes and are each found in a single copy per rice haploid genome. RNA blot analysis revealed that these genes are expressed in proliferating rice cells and in young rice seedlings. cdc2Os-1 could complement a temperature-sensitive yeast mutant of cdc28. However, despite the similarity in structure, both cdc2Os-2 and R2 were unable to complement the same mutant. Thus, the present results demonstrate the presence of structurally related, but functionally distinct cognates of the cdc2 cell cycle kinase in rice.

Negative regulators of the PHO system of Saccharomyces cerevisiae: characterization of PHO80 and PHO85

Both PHO80 and PHO85 genes are required to establish the repressed state of the PHO system of Saccharomyces cerevisiae. S1 nuclease protection analysis of the PHO85 transcript revealed that the PHO85 gene contains an intron at the 6th codon of the gene. Each of the fusion proteins, LacZ-Pho80 and LacZ-Pho85, was produced into Escherichia coli and used as an antigen to raise antibodies in a rabbit. Using the affinity-purified antibodies in Western blotting experiments, the PHO85 protein was detected as a 36 kDa and the PHO80 protein as a 34 kDa protein. The PHO80 protein was detected only in extracts prepared from an overproducing strain. The immunoprecipitate containing the PHO85 protein showed protein kinase activity suggesting that PHO85 is a protein kinase gene, which is consistent with the observation that the deduced amino acid sequence of the PHO85 protein resembles that of some protein kinases. The PHO80 protein was found to be phosphorylated in the presence of PHO85 protein.

A novel mutation occurring in the PHO80 gene suppresses the PHO4c mutations of Saccharomyces cerevisiae

We have isolated suppressors of a PHO4c (a positive regulator) mutant which normally confers weak constitutivity for acid phosphatase production on the Saccharomyces cell. One dominant suppressor (PHO80-2) was found to be an allele of PHO80 (a negative regulator) that changes G to A, resulting in substitution of isoleucine for methionine 42 of the Pho80 protein. Substitution of valine (PHO80-3) or leucine (PHO80-4) for the same methionine by site-directed mutagenesis also suppressed PHOc. Suppression by PHO80-2) did show some allele specificity. From these results we were able to delimit the region of PHo80 which may interact with the Pho4 protein.

The yeast MCK1 gene encodes a protein kinase homolog that activates early meiotic gene expression

We have identified a yeast gene, MCK1, that encodes a positive regulator of meiosis and spore formation. Sequence analysis revealed that MCK1 encodes a protein kinase homolog identical to YPK1, a phosphotyrosyl protein with demonstrated protein kinase activity. Increased MCK1 gene dosage accelerates the sporulation program; mck1 mutations cause delayed and decreased levels of sporulation. MCK1 is required during sporulation for maximal transcript accumulation from IME1, which encodes a meiotic activator. MCK1 is required in vegetative cells for basal IME1 expression, as evidenced by functional assays of an ime1-HIS3 fusion gene. MCK1 is also required for efficient ascus maturation. Although expression of IME1 from the GAL1 promoter restored high-level sporulation to mck1 mutants, it did not correct the ascus-maturation defect. This observation indicates that MCK1 is required, independently, for both the activation of IME1 and subsequent ascus maturation. Expression of an mck1-lacZ fusion gene was not regulated by the signals that govern meiosis. This observation is consistent with evidence that MCK1 plays a role in governing centromere function during vegetative growth as well as sporulation.