Mutation of the SPS1-encoded protein kinase of Saccharomyces cerevisiae leads to defects in transcription and morphology during spore formation

Coexistence of genetically different Rhizophagus irregularis isolates induces genes involved in a putative fungal mating response

Arbuscular mycorrhizal fungi (AMF) are of great ecological importance because of their effects on plant growth. Closely related genotypes of the same AMF species coexist in plant roots. However, almost nothing is known about the molecular interactions occurring during such coexistence. We compared in planta AMF gene transcription in single and coinoculation treatments with two genetically different isolates of Rhizophagus irregularis in symbiosis independently on three genetically different cassava genotypes. Remarkably few genes were specifically upregulated when the two fungi coexisted. Strikingly, almost all of the genes with an identifiable putative function were known to be involved in mating in other fungal species. Several genes were consistent across host plant genotypes but more upregulated genes involved in putative mating were observed in host genotype (COL2215) compared with the two other host genotypes. The AMF genes that we observed to be specifically upregulated during coexistence were either involved in the mating pheromone response, in meiosis, sexual sporulation or were homologs of MAT-locus genes known in other fungal species. We did not observe the upregulation of the expected homeodomain genes contained in a putative AMF MAT-locus, but observed upregulation of HMG-box genes similar to those known to be involved in mating in Mucoromycotina species. Finally, we demonstrated that coexistence between the two fungal genotypes in the coinoculation treatments explained the number of putative mating response genes activated in the different plant host genotypes. This study demonstrates experimentally the activation of genes involved in a putative mating response and represents an important step towards the understanding of coexistence and sexual reproduction in these important plant symbionts.

A Noncanonical Hippo Pathway Regulates Spindle Disassembly and Cytokinesis During Meiosis in Saccharomyces cerevisiae

Meiosis in the budding yeast Saccharomyces cerevisiae is used to create haploid yeast spores from a diploid mother cell. During meiosis II, cytokinesis occurs by closure of the prospore membrane, a membrane that initiates at the spindle pole body and grows to surround each of the haploid meiotic products. Timely prospore membrane closure requires SPS1, which encodes an STE20 family GCKIII kinase. To identify genes that may activate SPS1, we utilized a histone phosphorylation defect of sps1 mutants to screen for genes with a similar phenotype and found that cdc15 shared this phenotype. CDC15 encodes a Hippo-like kinase that is part of the mitotic exit network. We find that Sps1 complexes with Cdc15, that Sps1 phosphorylation requires Cdc15, and that CDC15 is also required for timely prospore membrane closure. We also find that SPS1, like CDC15, is required for meiosis II spindle disassembly and sustained anaphase II release of Cdc14 in meiosis. However, the NDR-kinase complex encoded by DBF2/DBF20MOB1 which functions downstream of CDC15 in mitotic cells, does not appear to play a role in spindle disassembly, timely prospore membrane closure, or sustained anaphase II Cdc14 release. Taken together, our results suggest that the mitotic exit network is rewired for exit from meiosis II, such that SPS1 replaces the NDR-kinase complex downstream of CDC15.

Non-coding RNAs as cell wall regulators in Saccharomyces cerevisiae

The cell wall of Saccharomyces cerevisiae is an extracellular organelle crucial for preserving its cellular integrity and detecting environmental cues. The cell wall is composed of mannoproteins attached to a polysaccharide network and is continuously remodelled as cells undergo cell division, mating, gametogenesis or adapt to stressors. This makes yeast an excellent model to study the regulation of genes important for cell wall formation and maintenance. Given that certain yeast strains are pathogenic, a better understanding of their life cycle is of clinical relevance. This is why transcriptional regulatory mechanisms governing genes involved in cell wall biogenesis or maintenance have been the focus of numerous studies. However, little is known about the roles of long non-coding RNAs (lncRNAs), a class of transcripts that are thought to possess little or no protein coding potential, in controlling the expression of cell wall-related genes. This review outlines currently known mechanisms of lncRNA-mediated regulation of gene expression in S. cerevisiae and describes examples of lncRNA-regulated genes encoding cell wall proteins. We suggest that the association of currently annotated lncRNAs with the coding sequences and/or promoters of cell wall-related genes highlights a potential role for lncRNAs as important regulators of the yeast cell wall structure.

Dermatophyte Resistance to Antifungal Drugs: Mechanisms and Prospectus

Dermatophytes comprise pathogenic fungi that have a high affinity for the keratinized structures present in nails, skin, and hair, causing superficial infections known as dermatophytosis. A reasonable number of antifungal drugs currently exist on the pharmaceutical market to control mycoses; however, their cellular targets are restricted, and fungi may exhibit tolerance or resistance to these agents. For example, the stress caused by antifungal and cytotoxic drugs in sub-inhibitory concentrations promotes compensatory stress responses, with the over-expression of genes involved in cellular detoxification, drug efflux, and signaling pathways being among the various mechanisms that may contribute to drug tolerance. In addition, the ATP-binding cassette transporters in dermatophytes that are responsible for cellular efflux can act synergistically, allowing one to compensate for the absence of the other, revealing the complexity of drug tolerance phenomena. Moreover, mutations in genes coding for target enzymes could lead to substitutions in amino acids involved in the binding of antifungal agents, hindering their performance and leading to treatment failure. The relevance of each one of these mechanisms of resistance to fungal survival is hard to define, mainly because they can act simultaneously in the cell. However, an understanding of the molecular mechanisms involved in the resistance/tolerance processes, the identification of new antifungal targets, as well as the prospective of new antifungal compounds among natural or synthetic products, are expected to bring advances and new insights that facilitate the improvement or development of novel strategies for antifungal therapy.

Timely Closure of the Prospore Membrane Requires SPS1 and SPO77 in Saccharomyces cerevisiae

During sporulation in Saccharomyces cerevisiae, a double lipid bilayer called the prospore membrane is formed de novo, growing around each meiotic nucleus and ultimately closing to create four new cells within the mother cell. Here we show that SPS1, which encodes a kinase belonging to the germinal center kinase III family, is involved in prospore membrane development and is required for prospore membrane closure. We find that SPS1 genetically interacts with SPO77 and see that loss of either gene disrupts prospore membrane closure in a similar fashion. Specifically, cells lacking SPS1 and SPO77 produce hyperelongated prospore membranes from which the leading edge protein complex is not removed from the prospore membrane in a timely fashion. The SPS1/SPO77 pathway is required for the proper phosphorylation and stability of Ssp1, a member of the leading edge protein complex that is removed and degraded when the prospore membrane closes. Genetic dissection of prospore membrane closure finds SPS1 and SPO77 act in parallel to a previously described pathway of prospore membrane closure that involves AMA1, an activator of the meiotic anaphase promoting complex.

Developmental Coordination of Gamete Differentiation with Programmed Cell Death in Sporulating Yeast

The gametogenesis program of the budding yeast Saccharomyces cerevisiae, also known as sporulation, employs unusual internal meiotic divisions, after which all four meiotic products differentiate within the parental cell. We showed previously that sporulation is typically accompanied by the destruction of discarded immature meiotic products through their exposure to proteases released from the mother cell vacuole, which undergoes an apparent programmed rupture. Here we demonstrate that vacuolar rupture contributes to de facto programmed cell death (PCD) of the meiotic mother cell itself. Meiotic mother cell PCD is accompanied by an accumulation of depolarized mitochondria, organelle swelling, altered plasma membrane characteristics, and cytoplasmic clearance. To ensure that the gametes survive the destructive consequences of developing within a cell that is executing PCD, we hypothesized that PCD is restrained from occurring until spores have attained a threshold degree of differentiation. Consistent with this hypothesis, gene deletions that perturb all but the most terminal postmeiotic spore developmental stages are associated with altered PCD. In these mutants, meiotic mother cells exhibit a delay in vacuolar rupture and then appear to undergo an alternative form of PCD associated with catastrophic consequences for the underdeveloped spores. Our findings reveal yeast sporulation as a context of bona fide PCD that is developmentally coordinated with gamete differentiation.

The GCKIII kinase Sps1 and the 14-3-3 isoforms, Bmh1 and Bmh2, cooperate to ensure proper sporulation in Saccharomyces cerevisiae

Sporulation in the budding yeast Saccharomyces cerevisiae is a developmental program initiated in response to nutritional deprivation. Sps1, a serine/threonine kinase, is required for sporulation, but relatively little is known about the molecular mechanisms through which it regulates this process. Here we show that SPS1 encodes a bona-fide member of the GCKIII subfamily of STE20 kinases, both through phylogenetic analysis of the kinase domain and examination of its C-terminal regulatory domain. Within the regulatory domain, we find Sps1 contains an invariant ExxxPG region conserved from plant to human GCKIIIs that we call the EPG motif; we show this EPG motif is important for SPS1 function. We also find that Sps1 is phosphorylated near its N-terminus on Threonine 12, and that this phosphorylation is required for the efficient production of spores. In Sps1, Threonine 12 lies within a 14-3-3 consensus binding sequence, and we show that the S. cerevisiae 14-3-3 proteins Bmh1 and Bmh2 bind Sps1 in a Threonine 12-dependent fashion. This interaction is significant, as BMH1 and BMH2 are required during sporulation and genetically interact with SPS1 in sporulating cells. Finally, we observe that Sps1, Bmh1 and Bmh2 are present in both the nucleus and cytoplasm during sporulation. We identify a nuclear localization sequence in Sps1 at amino acids 411-415, and show that this sequence is necessary and sufficient for nuclear localization. Taken together, these data identify regions within Sps1 critical for its function and indicate that SPS1 and 14-3-3s act together to promote proper sporulation in S. cerevisiae.

Transmembrane signaling in Saccharomyces cerevisiae as a model for signaling in metazoans: state of the art after 25 years

In the very first article that appeared in Cellular Signalling, published in its inaugural issue in October 1989, we reviewed signal transduction pathways in Saccharomyces cerevisiae. Although this yeast was already a powerful model organism for the study of cellular processes, it was not yet a valuable instrument for the investigation of signaling cascades. In 1989, therefore, we discussed only two pathways, the Ras/cAMP and the mating (Fus3) signaling cascades. The pivotal findings concerning those pathways undoubtedly contributed to the realization that yeast is a relevant model for understanding signal transduction in higher eukaryotes. Consequently, the last 25 years have witnessed the discovery of many signal transduction pathways in S. cerevisiae, including the high osmotic glycerol (Hog1), Stl2/Mpk1 and Smk1 mitogen-activated protein (MAP) kinase pathways, the TOR, AMPK/Snf1, SPS, PLC1 and Pkr/Gcn2 cascades, and systems that sense and respond to various types of stress. For many cascades, orthologous pathways were identified in mammals following their discovery in yeast. Here we review advances in the understanding of signaling in S. cerevisiae over the last 25 years. When all pathways are analyzed together, some prominent themes emerge. First, wiring of signaling cascades may not be identical in all S. cerevisiae strains, but is probably specific to each genetic background. This situation complicates attempts to decipher and generalize these webs of reactions. Secondly, the Ras/cAMP and the TOR cascades are pivotal pathways that affect all processes of the life of the yeast cell, whereas the yeast MAP kinase pathways are not essential. Yeast cells deficient in all MAP kinases proliferate normally. Another theme is the existence of central molecular hubs, either as single proteins (e.g., Msn2/4, Flo11) or as multisubunit complexes (e.g., TORC1/2), which are controlled by numerous pathways and in turn determine the fate of the cell. It is also apparent that lipid signaling is less developed in yeast than in higher eukaryotes. Finally, feedback regulatory mechanisms seem to be at least as important and powerful as the pathways themselves. In the final chapter of this essay we dare to imagine the essence of our next review on signaling in yeast, to be published on the 50th anniversary of Cellular Signalling in 2039.

Histone phosphorylation: a chromatin modification involved in diverse nuclear events

Histone posttranslational modifications are key components of diverse processes that modulate chromatin structure. These marks function as signals during various chromatin-based events, and act as platforms for recruitment, assembly or retention of chromatin-associated factors. The best-known function of histone phosphorylation takes place during cellular response to DNA damage, when phosphorylated histone H2A(X) demarcates large chromatin domains around the site of DNA breakage. However, multiple studies have also shown that histone phosphorylation plays crucial roles in chromatin remodeling linked to other nuclear processes. In this review, we summarize the current knowledge of histone phosphorylation and describe the many kinases and phosphatases that regulate it. We discuss the key roles played by this histone mark in DNA repair, transcription and chromatin compaction during cell division and apoptosis. Additionally, we describe the intricate crosstalk that occurs between phosphorylation and other histone modifications and allows for sophisticated control over the chromatin remodeling processes.

Sporulation in the budding yeast Saccharomyces cerevisiae

In response to nitrogen starvation in the presence of a poor carbon source, diploid cells of the yeast Saccharomyces cerevisiae undergo meiosis and package the haploid nuclei produced in meiosis into spores. The formation of spores requires an unusual cell division event in which daughter cells are formed within the cytoplasm of the mother cell. This process involves the de novo generation of two different cellular structures: novel membrane compartments within the cell cytoplasm that give rise to the spore plasma membrane and an extensive spore wall that protects the spore from environmental insults. This article summarizes what is known about the molecular mechanisms controlling spore assembly with particular attention to how constitutive cellular functions are modified to create novel behaviors during this developmental process. Key regulatory points on the sporulation pathway are also discussed as well as the possible role of sporulation in the natural ecology of S. cerevisiae.

High-resolution view of the yeast meiotic program revealed by ribosome profiling

Meiosis is a complex developmental process that generates haploid cells from diploid progenitors. We measured messenger RNA (mRNA) abundance and protein production through the yeast meiotic sporulation program and found strong, stage-specific expression for most genes, achieved through control of both mRNA levels and translational efficiency. Monitoring of protein production timing revealed uncharacterized recombination factors and extensive organellar remodeling. Meiotic translation is also shifted toward noncanonical sites, including short open reading frames (ORFs) on unannnotated transcripts and upstream regions of known transcripts (uORFs). Ribosome occupancy at near-cognate uORFs was associated with more efficient ORF translation; by contrast, some AUG uORFs, often exposed by regulated 5' leader extensions, acted competitively. This work reveals pervasive translational control in meiosis and helps to illuminate the molecular basis of the broad restructuring of meiotic cells.

Ste20-related kinases: effectors of signaling and morphogenesis in fungi

The family of Ste20-related kinases is conserved from yeast to mammals and includes the p21 activated kinases (PAKs) and germinal centre kinases (GCKs). These kinases have been shown to be involved in signaling through mitogen activated protein kinase (MAPK) pathways and in morphogenesis through the regulation of cytokinesis and actin-dependent polarized growth. This review concentrates on the role of Ste20-related kinases in fungi where recent research has revealed roles for both PAKs and GCKs in the regulation of cytokinesis and in previously unidentified roles in promoting hyphal growth and differentiation of asexual development structures. In particular, the importance of PAKs during pathogenesis will be examined.

Expression, stability, and replacement of glucan-remodeling enzymes during developmental transitions in Saccharomyces cerevisiae

Sporulation is a developmental variation of the yeast life cycle whereby four spores are produced within a diploid cell, with proliferation resuming after germination. The GAS family of glycosylphosphatidylinositol-anchored glucan-remodeling enzymes exemplifies functional interplay between paralogous genes during the yeast life cycle. GAS1 and GAS5 are expressed in vegetative cells and repressed during sporulation while GAS2 and GAS4 exhibit a reciprocal pattern. GAS3 is weakly expressed in all the conditions and encodes an inactive protein. Although Gas1p functions in cell wall formation, we show that it persists during sporulation but is relocalized from the plasma membrane to the epiplasm in a process requiring End3p-mediated endocytosis and the Sps1 protein kinase of the p21-activated kinase family. Some Gas1p is also newly synthesized and localized to the spore membrane, but this fraction is dispensable for spore formation. By way of contrast, the Gas2-Gas4 proteins, which are essential for spore wall assembly, are rapidly degraded after spore formation. On germination, Gas1p is actively synthesized and concentrated in the growing part of the spore, which is essential for its elongation. Thus Gas1p is the primary glucan-remodeling enzyme required in vegetative growth and during reentry into the proliferative state. The dynamic interplay among Gas proteins is crucial to couple glucan remodeling with morphogenesis in developmental transitions.

SOK1 translocates from the Golgi to the nucleus upon chemical anoxia and induces apoptotic cell death

SOK1 is a Ste20 protein kinase of the germinal center kinase (GCK) family that has been shown to be activated by oxidant stress and chemical anoxia, a cell culture model of ischemia. More recently, it has been shown to be localized to the Golgi apparatus, where it functions in a signaling pathway required for cell migration and polarization. Herein, we demonstrate that SOK1 regulates cell death after chemical anoxia, as its down-regulation by RNA interference enhances cell survival. Furthermore, expression of SOK1 elicits apoptotic cell death by activating the intrinsic pathway. We also find that a cleaved form of SOK1 translocates from the Golgi to the nucleus after chemical anoxia and that this translocation is dependent on both caspase activity and on amino acids 275-292, located immediately C-terminal to the SOK1 kinase domain. Furthermore, SOK1 entry into the nucleus is important for the cell death response since SOK1 mutants unable to enter the nucleus do not induce cell death. In summary, SOK1 is necessary to induce cell death and can induce death when overexpressed. Furthermore, SOK1 appears to play distinctly different roles in stressed versus non-stressed cells, regulating cell death in the former.

Function and regulation in MAPK signaling pathways: lessons learned from the yeast Saccharomyces cerevisiae

Signaling pathways that activate different mitogen-activated protein kinases (MAPKs) elicit many of the responses that are evoked in cells by changes in certain environmental conditions and upon exposure to a variety of hormonal and other stimuli. These pathways were first elucidated in the unicellular eukaryote Saccharomyces cerevisiae (budding yeast). Studies of MAPK pathways in this organism continue to be especially informative in revealing the molecular mechanisms by which MAPK cascades operate, propagate signals, modulate cellular processes, and are controlled by regulatory factors both internal to and external to the pathways. Here we highlight recent advances and new insights about MAPK-based signaling that have been made through studies in yeast, which provide lessons directly applicable to, and that enhance our understanding of, MAPK-mediated signaling in mammalian cells.

Genetic evidence for a SPO1-dependent signaling pathway controlling meiotic progression in yeast

The yeast spindle pole body (SPB) plays a unique role in meiosis, initiating both spindle assembly and prospore membrane synthesis. SPO1, induced early in development, encodes a meiosis-specific phospholipase B (PLB) homolog required at three stages of SPB morphogenesis: MI, MII, and spore formation. Here we report in-depth analysis of the SPO1 gene including its transcriptional control by regulators of early gene expression, protein localization to the ER lumen and periplasmic space, and molecular genetic studies of its role in meiosis. Evidence is presented that multiple arrest points in spo1Delta occur independently, demonstrating that Spo1 acts at distinct steps. Loss of Spo1 is suppressed by high-copy glycosylphosphatidylinositol (GPI) proteins, dependent on sequence, timing, and strength of induction in meiosis. Since phosphatidylinositol (PI) serves as both an anchor component and a lipase substrate, we hypothesized that GPI-protein expression might substitute for Spo1 by decreasing levels of its potential substrates, PI and phosphatidylinositol phosphates (PIPs). Partial spo1Delta complementation by PLB3 (encoding a unique PLB capable of cleaving PI) and relatively strong Spo1 binding to PI(4)P derivatives (via a novel N-terminal lysine-rich fragment essential for Spo1 function) are consistent with this view. Epistasis of SPO1 mutations to those in SPO14 (encoding a PLD involved in signaling) and physical interaction of Spo1 with Spo23, a protein regulating PI synthesis required for wild-type sporulation, further support this notion. Taken together these findings implicate PI and/or PIPs in Spo1 function and suggest the existence of a novel Spo1-dependent meiosis-specific signaling pathway required for progression of MI, MII, and spore formation via regulation of the SPB.

SSP2 and OSW1, two sporulation-specific genes involved in spore morphogenesis in Saccharomyces cerevisiae

Spore formation in Saccharomyces cerevisiae requires the synthesis of prospore membranes (PSMs) followed by the assembly of spore walls (SWs). We have characterized extensively the phenotypes of mutants in the sporulation-specific genes, SSP2 and OSW1, which are required for spore formation. A striking feature of the osw1 phenotype is asynchrony of spore development, with some spores displaying defects in PSM formation and others spores in the same ascus blocked at various stages in SW development. The Osw1 protein localizes to spindle pole bodies (SPBs) during meiotic nuclear division and subsequently to PSMs/SWs. We propose that Osw1 performs a regulatory function required to coordinate the different stages of spore morphogenesis. In the ssp2 mutant, nuclei are surrounded by PSMs and SWs; however, PSMs and SWs often also encapsulate anucleate bodies both inside and outside of spores. In addition, the SW is not as thick as in wild type. The ssp2 mutant defect is partially suppressed by overproduction of either Spo14 or Sso1, both of which promote the fusion of vesicles at the outer plaque of the SPB early in PSM formation. We propose that Ssp2 plays a role in vesicle fusion during PSM formation.

Phosphorylation of histone H4 Ser1 regulates sporulation in yeast and is conserved in fly and mouse spermatogenesis

Sporulation in Saccharomyces cerevisiae is a highly regulated process wherein a diploid cell gives rise to four haploid gametes. In this study we show that histone H4 Ser1 is phosphorylated (H4 S1ph) during sporulation, starting from mid-sporulation and persisting to germination, and is temporally distinct from earlier meiosis-linked H3 S10ph involved in chromosome condensation. A histone H4 S1A substitution mutant forms aberrant spores and has reduced sporulation efficiency. Deletion of sporulation-specific yeast Sps1, a member of the Ste20 family of kinases, nearly abolishes the sporulation-associated H4 S1ph modification. H4 S1ph may promote chromatin compaction, since deletion of SPS1 increases accessibility to antibody immunoprecipitation; furthermore, either deletion of Sps1 or an H4 S1A substitution results in increased DNA volume in nuclei within spores. We find H4 S1ph present during Drosophila melanogaster and mouse spermatogenesis, and similar to yeast, this modification extends late into sperm differentiation relative to H3 S10ph. Thus, H4 S1ph may be an evolutionarily ancient histone modification to mark the genome for gamete-associated packaging.

Ascospore formation in the yeast Saccharomyces cerevisiae

Sporulation of the baker's yeast Saccharomyces cerevisiae is a response to nutrient depletion that allows a single diploid cell to give rise to four stress-resistant haploid spores. The formation of these spores requires a coordinated reorganization of cellular architecture. The construction of the spores can be broadly divided into two phases. The first is the generation of new membrane compartments within the cell cytoplasm that ultimately give rise to the spore plasma membranes. Proper assembly and growth of these membranes require modification of aspects of the constitutive secretory pathway and cytoskeleton by sporulation-specific functions. In the second phase, each immature spore becomes surrounded by a multilaminar spore wall that provides resistance to environmental stresses. This review focuses on our current understanding of the cellular rearrangements and the genes required in each of these phases to give rise to a wild-type spore.

Saccharomyces cerevisiae Sps1p regulates trafficking of enzymes required for spore wall synthesis

SPS1 encodes a sporulation-specific protein with homology to the Ste20/p21-activated kinase family. Deletion of SPS1 impinges on the formation of the spore wall, which surrounds each of the haploid nuclei generated by the meiotic divisions. Here, we demonstrate that the new internal membranes that surround the meiotic nuclei appear normal in the absence of Sps1p. Analyses of spore wall layers by immunohistochemistry suggest that the inner layers are not efficiently deposited. The defect in spore wall morphogenesis is most likely a consequence of mislocalization of enzymes required for the synthesis of the spore wall layers as both Chs3p, the major chitin synthase in yeast, and Gsc2/Fks2p, a glucan synthase transcriptionally upregulated during sporulation, fail to reach the prospore membrane in the sps1 mutant. Furthermore, localization of Chs3p to the prospore membrane is not dependent on Shc1p, a sporulation-specific homolog of Chs4p, which is required for recruitment of Chs3p to the bud neck in vegetative cells. Sps1p colocalized with Chs3p to peripheral and internal punctate structures and prospore membranes. We propose that Sps1p promotes sporulation, in part, by regulating the intracellular movement of proteins required for spore wall formation.

Morphogenetic pathway of spore wall assembly in Saccharomyces cerevisiae

The Saccharomyces cerevisiae spore is protected from environmental damage by a multilaminar extracellular matrix, the spore wall, which is assembled de novo during spore formation. A set of mutants defective in spore wall assembly were identified in a screen for mutations causing sensitivity of spores to ether vapor. The spore wall defects in 10 of these mutants have been characterized in a variety of cytological and biochemical assays. Many of the individual mutants are defective in the assembly of specific layers within the spore wall, leading to arrests at discrete stages of assembly. The localization of several of these gene products has been determined and distinguishes between proteins that likely are involved directly in spore wall assembly and probable regulatory proteins. The results demonstrate that spore wall construction involves a series of dependent steps and provide the outline of a morphogenetic pathway for assembly of a complex extracellular structure.

End3p-mediated endocytosis is required for spore wall formation in Saccharomyces cerevisiae

During sporulation in Saccharomyces cerevisiae, vesicles transported to the vicinity of spindle pole bodies are fused to each other to generate bilayered prospore membranes (PSMs). PSMs encapsulate the haploid nuclei that arise from the meiotic divisions and serve as platforms for spore wall deposition. Membrane trafficking plays an important role in supplying vesicles for these processes. The endocytosis-deficient mutant, end3Delta, sporulated poorly and the spores produced lost resistance to ether vapor, suggesting that END3-mediated endocytosis is important for sporulation. End3p-GFP localized to cell and spore peripheries in vegetative and sporulating cells and colocalized with actin structures. Correspondingly, the actin cytoskeleton appeared aberrant during sporulation in end3Delta. Analysis of meiosis in end3Delta mutants revealed that the meiotic divisions occurred with wild-type kinetics. Furthermore, PSMs were assembled normally. However, the levels of proteins required for spore wall synthesis and components of the spore wall layers at spores were reduced, indicating that end3Delta mutants are defective in spore wall synthesis. Thus, END3-mediated endocytosis is important for spore wall formation. Additionally, cytological analyses suggest that trafficking between the plasma membrane and PSMs is important earlier during sporulation.

Lessons from the genome sequence of Neurospora crassa: tracing the path from genomic blueprint to multicellular organism

We present an analysis of over 1,100 of the approximately 10,000 predicted proteins encoded by the genome sequence of the filamentous fungus Neurospora crassa. Seven major areas of Neurospora genomics and biology are covered. First, the basic features of the genome, including the automated assembly, gene calls, and global gene analyses are summarized. The second section covers components of the centromere and kinetochore complexes, chromatin assembly and modification, and transcription and translation initiation factors. The third area discusses genome defense mechanisms, including repeat induced point mutation, quelling and meiotic silencing, and DNA repair and recombination. In the fourth section, topics relevant to metabolism and transport include extracellular digestion; membrane transporters; aspects of carbon, sulfur, nitrogen, and lipid metabolism; the mitochondrion and energy metabolism; the proteasome; and protein glycosylation, secretion, and endocytosis. Environmental sensing is the focus of the fifth section with a treatment of two-component systems; GTP-binding proteins; mitogen-activated protein, p21-activated, and germinal center kinases; calcium signaling; protein phosphatases; photobiology; circadian rhythms; and heat shock and stress responses. The sixth area of analysis is growth and development; it encompasses cell wall synthesis, proteins important for hyphal polarity, cytoskeletal components, the cyclin/cyclin-dependent kinase machinery, macroconidiation, meiosis, and the sexual cycle. The seventh section covers topics relevant to animal and plant pathogenesis and human disease. The results demonstrate that a large proportion of Neurospora genes do not have homologues in the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe. The group of unshared genes includes potential new targets for antifungals as well as loci implicated in human and plant physiology and disease.

Cell signaling in yeast sporulation

Gametogenesis is essential for the propagation of all sexually reproducing organisms and consists of halving the chromosome number through meiosis, and the subsequent packaging of the haploid products into gametes. Meiosis and gamete formation must be tightly coupled to ensure the formation of viable progeny; perturbations result in infertility, inviability, and birth defects. In the yeast Saccharomyces cerevisiae, sexual reproduction occurs via sporulation and is similar in many respects to gametogenesis in mammals. An increasing number of conserved signaling molecules have been shown to be essential for yeast sporulation; recent studies reveal molecular insights into how these molecules regulate this intricate differentiation program.

Evidence from comparative genomics for a complete sexual cycle in the 'asexual' pathogenic yeast Candida glabrata

BACKGROUND

Candida glabrata is a pathogenic yeast of increasing medical concern. It has been regarded as asexual since it was first described in 1917, yet phylogenetic analyses have revealed that it is more closely related to sexual yeasts than other Candida species. We show here that the C. glabrata genome contains many genes apparently involved in sexual reproduction.

RESULTS

By genome survey sequencing, we find that genes involved in mating and meiosis are as numerous in C. glabrata as in the sexual species Kluyveromyces delphensis, which is its closest known relative. C. glabrata has a putative mating-type (MAT) locus and a pheromone gene (MFALPHA2), as well as orthologs of at least 31 other Saccharomyces cerevisiae genes that have no known roles apart from mating or meiosis, including FUS3, IME1 and SMK1.

CONCLUSIONS

We infer that C. glabrata is likely to have an undiscovered sexual stage in its life cycle, similar to that recently proposed for C. albicans. The two Candida species represent two distantly related yeast lineages that have independently become both pathogenic and 'asexual'. Parallel evolution in the two lineages as they adopted mammalian hosts resulted in separate but analogous switches from overtly sexual to cryptically sexual life cycles, possibly in response to defense by the host immune system.

Phosphorylation and maximal activity of Saccharomyces cerevisiae meiosis-specific transcription factor Ndt80 is dependent on Ime2

The Saccharomyces cerevisiae meiosis-specific transcription factor Ndt80 is responsible for the induction of a class of genes referred to as middle sporulation genes. Among the members of this family are the B-type cyclins and other genes whose products are required for meiotic chromosome division and spore morphogenesis. Inactivation of NDT80 leads to a failure to induce the middle sporulation genes and a subsequent arrest in pachytene. The expression of NDT80 is itself highly regulated. The initial transcription of NDT80 is dependent upon the protein kinase Ime2; once Ndt80 protein accumulates, it activates its own promoter, thus generating an autoactivation loop. In addition to being transcriptionally regulated, Ndt80 protein is posttranslationally regulated. Phosphorylation of Ndt80 occurs coincident with its activation as a transcription factor. If expressed prematurely in meiosis, Ndt80 accumulates initially in an unmodified form that is subsequently modified by phosphorylation. In contrast, Ndt80 expressed in ime2 mutant strains does not become modified and has a reduced ability to activate transcription of its target genes. Ime2 can also phosphorylate Ndt80 in vitro, further supporting a direct role for Ime2 in the phosphorylation of Ndt80. These data indicate that Ime2 plays a novel and previously unexpected role in promoting chromosome dissemination and progress through meiotic development by activating Ndt80.

Role of Ndt80, Sum1, and Swe1 as targets of the meiotic recombination checkpoint that control exit from pachytene and spore formation in Saccharomyces cerevisiae

The meiotic recombination checkpoint, which is triggered by defects in recombination or chromosome synapsis, arrests sporulating cells of Saccharomyces cerevisiae at pachytene by preventing accumulation of active Clb-Cdc28. We compared the effects of manipulating the three known targets of the meiotic recombination checkpoint, NDT80, SWE1, and SUM1, in dmc1-arrested cells. Ndt80 is an activator of a set of middle sporulation-specific genes (MSGs), which includes CLB genes and genes involved in spore wall formation; Swe1 inhibits Clb-Cdc28 activity; and Sum1 is a repressor of NDT80 and some MSGs. Activation of the checkpoint leads to inhibition of Ndt80 activity and to stabilization of Swe1 and Sum1. Thus, dmc1-arrested cells fail to express MSGs, arrest at pachytene, and do not form spores. Our study shows that dmc1/dmc1 sum1/sum1 cells expressed MSGs prematurely and at high levels, entered the meiotic divisions efficiently, and in some cases formed asci containing mature spores. In contrast, dmc1/dmc1 swe1/swe1 cells expressed MSGs at a very low level, were inefficient and delayed in entry into the meiotic divisions, and never formed mature spores. We found that cells of dmc1/dmc1 sum1/sum1 ndt80/ndt80 and dmc1/dmc1 swe1/swe1 ndt80/ndt80 strains arrested at pachytene and that dmc1/dmc1 or dmc1/dmc1 swe1/swe1 cells overexpressing NDT80 were less efficient in bypassing checkpoint-mediated arrest than dmc1/dmc1 sum1/sum1 cells. Our results are consistent with previous suggestions that increased Clb-Cdc28 activity, caused by mutation of SWE1 or by an NDT80-dependent increase in CLB expression, allows dmc1/dmc1 cells to exit pachytene and that subsequent upregulation of Ndt80 activity by a feedback mechanism promotes entry into the meiotic divisions. Spore morphogenesis, however, requires efficient and timely activation of MSGs, which we speculate was achieved in dmc1/dmc1 sum1/sum1 cells by premature expression of NDT80.

Regulation of the premiddle and middle phases of expression of the NDT80 gene during sporulation of Saccharomyces cerevisiae

The NDT80 gene of Saccharomyces cerevisiae, which encodes a global activator of transcription of middle sporulation-specific genes, is first expressed after the activation of early meiotic genes but prior to activation of middle sporulation-specific genes. Both upstream repression sequence 1 (URS1) and mid-sporulation element (MSE) sites are present in the promoter region of the NDT80 gene; these elements have been shown previously to contribute to the regulation of expression of early and middle sporulation-specific genes, respectively, by mediating repression in growing cells and activation at specific times during sporulation. In this study, we have shown that the overlapping windows of URS1- and MSE-mediated repression and activation are responsible for the distinctive premiddle expression pattern of the NDT80 gene. Our data suggest that a Sum1-associated repression complex bound at the NDT80 MSE sites prevents Ime1 tethered at the NDT80 URS1 sites from activating transcription of the NDT80 gene at the time that Ime1-dependent activation of early URS1-regulated meiotic genes is occurring. We propose that a decrease in the efficiency of Sum1-mediated repression as cells progress through the early events of the sporulation program allows the previously inactive Ime1 tethered at the URS1(NDT80) sites to promote a low level of expression of the NDT80 gene. This initial phase of URS1-dependent NDT80 expression is followed by Ndt80-dependent upregulation of its own expression, which requires the MSE(NDT80) sites and occurs concomitantly with Ndt80-dependent activation of a set of middle MSE-regulated sporulation-specific genes. Mutation of IME2 prevents expression of NDT80 in sporulating cells. We show in this study that NDT80 is expressed and that middle genes are activated in cells of an Deltaime2/Deltaime2 Deltasum1/Deltasum1 strain in sporulation medium. This suggests that Ime2 activates expression of NDT80 by eliminating Sum1-mediated repression.

Regulation of cell separation in the dimorphic fungus Ustilago maydis

During its haploid phase the dimorphic fungus Ustilago maydis grows vegetatively by budding. We have identified two genes, don1 and don3, which control the separation of mother and daughter cells. Mutant cells form tree-like clusters in liquid culture and grow as ring-like (donut-shaped) colonies on solid medium. In wild-type U. maydis cells, two distinct septa are formed during cytokinesis and delimit a fragmentation zone. Cells defective for either don1 or don3 display only a single septum and fail to complete cell separation. don1 encodes a guanine nucleotide exchange factor (GEF) of the Dbl family specific for Rho/Rac GTPases. Don3 belongs to the germinal-centre-kinase (GC) subfamily of Ste20-like protein kinases. We have isolated the U. maydis homologues of the small GTP binding proteins Rho2, Rho3, Rac1 and Cdc42. Out of these, only Cdc42 interacts specifically with Don1 and Don3 in the yeast two-hybrid system. We propose that Don1 and Don3 regulate the initiation of the secondary septum, which is required for proper cell separation.

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.

CAK1 promotes meiosis and spore formation in Saccharomyces cerevisiae in a CDC28-independent fashion

CAK1 encodes a protein kinase in Saccharomyces cerevisiae whose sole essential mitotic role is to activate the Cdc28p cyclin-dependent kinase by phosphorylation of threonine-169 in its activation loop. SMK1 encodes a sporulation-specific mitogen-activated protein (MAP) kinase homolog that is required to regulate the postmeiotic events of spore wall assembly. CAK1 was previously identified as a multicopy suppressor of a weakened smk1 mutant and shown to be required for spore wall assembly. Here we show that Smk1p, like other MAP kinases, is phosphorylated in its activation loop and that Smk1p is not activated in a cak1 missense mutant. Strains harboring a hyperactivated allele of CDC28 that is CAK1 independent and that lacks threonine-169 still require CAK1 to activate Smk1p. The data indicate that Cak1p functions upstream of Smk1p by activating a protein kinase other than Cdc28p. We also found that mutants lacking CAK1 are blocked early in meiotic development, as they show substantial delays in premeiotic DNA synthesis and defects in the expression of sporulation-specific genes, including IME1. The early meiotic role of Cak1p, like the postmeiotic role in the Smk1p pathway, is CDC28 independent. The data indicate that Cak1p activates multiple steps in meiotic development through multiple protein kinase targets.

A Gip1p-Glc7p phosphatase complex regulates septin organization and spore wall formation

Sporulation of Saccharomyces cerevisiae is a developmental process in which a single cell is converted into four haploid spores. GIP1, encoding a developmentally regulated protein phosphatase 1 interacting protein, is required for spore formation. Here we show that GIP1 and the protein phosphatase 1 encoded by GLC7 play essential roles in spore development. The gip1Delta mutant undergoes meiosis and prospore membrane formation normally, but is specifically defective in spore wall synthesis. We demonstrate that in wild-type cells, distinct layers of the spore wall are deposited in a specific temporal order, and that gip1Delta cells display a discrete arrest at the onset of spore wall deposition. Localization studies revealed that Gip1p and Glc7p colocalize with the septins in structures underlying the growing prospore membranes. Interestingly, in the gip1Delta mutant, not only is Glc7p localization altered, but septins are also delocalized. Similar phenotypes were observed in a glc7-136 mutant, which expresses a Glc7p defective in interacting with Gip1p. These results indicate that a Gip1p-Glc7p phosphatase complex is required for proper septin organization and initiation of spore wall formation during sporulation.

Interaction of Knr4 protein, a protein involved in cell wall synthesis, with tyrosine tRNA synthetase encoded by TYS1 in Saccharomyces cerevisiae

The Knr4 protein, known to be involved in the regulation of cell wall assembly in Saccharomyces cerevisiae, strongly interacts with the tyrosine tRNA synthetase protein encoded by TYS1 as demonstrated by the genetic two-hybrid system and a biochemical pull-down experiment using GST--Tys1p fusion. Data reported here raise the possibility that this physical interaction between these proteins is required for dityrosine formation during the sporulation process. In addition, it is shown that the efficiency of spores formation was drastically reduced in diploid cells homozygous for the disruption of KNR4 or for a temperature-sensitive mutation of TYS1, although this effect could be independent of their protein interaction. Altogether, these data provide novel functions of Knr4p and Tys1p to those that were known before.

The Ste20 group kinases as regulators of MAP kinase cascades

Ste20p (sterile 20 protein) is a putative yeast mitogen-activated protein kinase kinase kinase kinase (MAP4K) involved in the mating pathway. Its homologs in mammals, Drosophila, Caenorhabditis elegans and other organisms make up a large emerging group of protein kinases including 28 members in human. The Ste20 group kinases are further divided into the p21-activated kinase (PAK) and germinal center kinase (GCK) families. They are characterized by the presence of a conserved kinase domain and a noncatalytic region of great structural diversity that enables the kinases to interact with various signaling molecules and regulatory proteins of the cytoskeleton. This review describes the phylogenetic relationships of the Ste20 group kinases based on discussions with many researchers in this field. With the newly established phylogenetic relationships, crucial arguments can be advanced regarding the functions of these kinases as upstream activators of the MAPK pathways and possible activity as MAP4Ks. Their involvement in apoptosis, morphogenesis and cytoskeletal rearrangements is also discussed.

Map kinases in fungal pathogens

MAP kinases in eukaryotic cells are well known for transducing a variety of extracellular signals to regulate cell growth and differentiation. Recently, MAP kinases homologous to the yeast Fus3/Kss1 MAP kinases have been identified in several fungal pathogens and found to be important for appressorium formation, invasive hyphal growth, and fungal pathogenesis. This MAP kinase pathway also controls diverse growth or differentiation processes, including conidiation, conidial germination, and female fertility. MAP kinases homologous to yeast Slt2 and Hog1 have also been characterized in Candida albicans and Magnaporthe grisea. Mutants disrupted of the Slt2 homologues have weak cell walls, altered hyphal growth, and reduced virulence. The Hog1 homologues are dispensable for growth but are essential for regulating responses to hyperosmotic stress in C. albicans and M. grisea. Overall, recent studies have indicated that MAP kinase pathways may play important roles in regulating growth, differentiation, survival, and pathogenesis in fungal pathogens.

Mps1p regulates meiotic spindle pole body duplication in addition to having novel roles during sporulation

Sporulation in yeast requires that a modified form of chromosome segregation be coupled to the development of a specialized cell type, a process akin to gametogenesis. Mps1p is a dual-specificity protein kinase essential for spindle pole body (SPB) duplication and required for the spindle assembly checkpoint in mitotically dividing cells. Four conditional mutant alleles of MPS1 disrupt sporulation, producing two distinct phenotypic classes. Class I alleles of mps1 prevent SPB duplication at the restrictive temperature without affecting premeiotic DNA synthesis and recombination. Class II MPS1 alleles progress through both meiotic divisions in 30-50% of the population, but the asci are incapable of forming mature spores. Although mutations in many other genes block spore wall formation, the cells produce viable haploid progeny, whereas mps1 class II spores are unable to germinate. We have used fluorescently marked chromosomes to demonstrate that mps1 mutant cells have a dramatically increased frequency of chromosome missegregation, suggesting that loss of viability is due to a defect in spindle function. Overall, our cytological data suggest that MPS1 is required for meiotic SPB duplication, chromosome segregation, and spore wall formation.

SPAK, a STE20/SPS1-related kinase that activates the p38 pathway

We have cloned a member of the STE20/SPS1 protein kinase family from a transformed rat pancreatic beta cell line. SPAK (STE20/SPS1-related, proline alanine-rich kinase) belongs to the SPS1 subfamily of STE20 kinases and is highly conserved between species. SPAK is expressed ubiquitously, although preferentially in brain and pancreas. Biochemical characterization of SPAK catalytic activity demonstrates that is a serine/threonine kinase that can phosphorylate itself and an exogenous substrate in vitro. SPAK is immunoprecipitated from transfected mammalian cells as a complex with another, as yet uncharacterized, serine/threonine kinase which is capable of phosphorylating catalytically-inactive SPAK and myelin basic protein in an in vitro kinase assay. SPAK specifically activates the p38 pathway in cotransfection assays. Like MST1 and MST2, SPAK contains a putative caspase cleavage site at the junction of the catalytic domain and the C-terminal region. Full-length SPAK is expressed in the cytoplasm in transfected cells, while a mutant corresponding to caspase-cleaved SPAK is expressed predominantly in the nucleus. The similarity of SPAK to other SPS1 family members, its ability to activate the p38 pathway, in addition to its putative caspase cleavage site, provide evidence that SPAK may act as a novel mediator of stress-activated signals. Oncogene (2000) 19, 4290 - 4297

Response of Saccharomyces cerevisiae to severe osmotic stress: evidence for a novel activation mechanism of the HOG MAP kinase pathway

The HOG/p38 MAP kinase route is an important stress-activated signal transduction pathway that is well conserved among eukaryotes. Here, we describe a novel mechanism of activation of the HOG pathway in budding yeast. This mechanism operates upon severe osmostress conditions (1.4 M NaCl) and is independent of the Sln1p and Sho1p osmosensors. The alternative input feeds into the HOG pathway MAPKK Pbs2p and requires activation of Pbs2p by phosphorylation. We show that, upon severe osmotic shock, Hog1p nuclear accumulation and phosphorylation is delayed compared with mild stress. Moreover, both events lost their transient pattern, presumably because of the absence of negative feedback mediated by Ptp2p tyrosine phosphatase, which we found to be localized in the nucleus. Under severe osmotic stress conditions, the delayed nuclear accumulation correlates with a delay in stress-responsive gene expression. Severe osmoshock leads to a situation in which active and nuclear-localized Hog1p is transiently unable to induce transcription of osmotic stress-responsive genes. It also appeared from our studies that the Sho1p osmosensor is less active under severe osmotic stress conditions, whereas the Sln1p/Ypd1p/Ssk1p sensor and signal transducer functions normally under these circumstances.

Transcriptional regulation of meiosis in yeast

The genes required for meiosis and sporulation in yeast are expressed at specific points in a highly regulated temporal pathway. Recent experiments using DNA microarrays to examine gene expression during meiosis and the identification of many regulatory factors have provided important advances in our understanding of how genes are regulated at the different stages of meiosis.

Neutrophil products and alterations in epithelial junctional proteins: prevention of artifactual degradation

Recent reports of disruption of endothelial cell adherens junction proteins during neutrophil adhesion and transmigration have been challenged as being partly due to post-fixation artifactual release of neutrophil-derived proteases. In this study we examined alterations in the epithelial junctional complex during neutrophil adhesion. Using standard fixation protocols, neutrophil addition to epithelial monolayers resulted in gross disruption of apical junction protein immunofluorescence. However, the inclusion of a post fixation incubation step with formic acid resulted in epitope preservation. These observations indicate that neutrophil derived products, likely proteases, remain active despite prolonged exposure to conventional fixatives. This may result in diffuse and artifactual loss of epithelial junctional protein immunofluorescence. Formic acid prevents this loss of epitope staining and may be considered as an agent to preserve protease-sensitive endothelial or epithelial immunoreactivity.

Molecular cloning and characterization of a novel human STE20-like kinase, hSLK

We have cloned a human counterpart to a guinea pig STE20-like kinase cDNA, designated human SLK (hSLK), from a human lung carcinomatous cell line A549 cDNA library. hSLK cDNA encodes a novel 1204 amino acid serine/threonine kinase for which the kinase domain located at the N-terminus shares considerable homology to that of the STE20-like kinase family. The C-terminal domain of hSLK includes both the coiled-coil structure and four Pro/Glu/Ser/Thr-rich (PEST) sequences, but not the GTPase-binding domain (GBD) that is characteristic of the p21-activated kinase (PAK) family, polyproline consensus binding sites, or the Leu-rich domain seen in the group I germinal center kinases (GCKs). Northern blot analysis indicated that hSLK was ubiquitously expressed. hSLK overexpressed in COS-7 cells phosphorylates itself as well as myelin basic protein used as a substrate. On the other hand, hSLK cannot activate any of the three well-characterized mitogen-activated protein kinase MAPK (ERK, JNK/SAPK and p38) pathways. Moreover, hSLK kinase activity is not upregulated by constitutive active forms of GTPases (RasV12, RacV12 and Cdc42V12). These structural and functional properties indicate that hSLK should be considered to be a new member of group II GCKs.

KFC, a Ste20-like kinase with mitogenic potential and capability to activate the SAPK/JNK pathway

The Sterile-20 (Ste20) family of serine-threonine kinases has been implicated in the activation of the stress-activated protein kinase pathways. However, the physiological role has remained ambiguous for most of the investigated mammalian Ste20's. Here we report the cloning of a novel Ste20-like kinase, from chicken embryo fibroblast (CEF) cells, which we have named KFC, for Kinase From Chicken. The 898 amino acid full-length KFC protein contains an amino-terminal kinase domain, an adjacent downstream serine-rich region, and a C-terminal tail containing a coiled-coil domain. Here we show that the coiled-coil domain of KFC negatively regulates the intrinsic kinase activity. We have also identified a splice variant of KFC in which there is a 207 nucleotide in-frame deletion. This deletion of 69 amino acids encompasses the serine-rich region. These two isoforms, called KFCL, for full-length, and KFCS for spliced (or short) form, not only differ in structure, but also in biological properties. Stable CEF cells overexpressing KFCL, but not KFCS, have a significant increase in growth rate when compared to parental cells. This mitogenic effect is the first such reported for this family of kinases. Finally, we found that KFC, when activated by truncation of the regulatory C-terminus, has a specific activation of the stress-activated protein kinase (SAPK/JNK) pathway.

Essential functions of protein tyrosine phosphatases PTP2 and PTP3 and RIM11 tyrosine phosphorylation in Saccharomyces cerevisiae meiosis and sporulation

Tyrosine phosphorylation plays a central role in eukaryotic signal transduction. In yeast, MAP kinase pathways are regulated by tyrosine phosphorylation, and it has been speculated that other biochemical processes may also be regulated by tyrosine phosphorylation. Previous genetic and biochemical studies demonstrate that protein tyrosine phosphatases (PTPases) negatively regulate yeast MAP kinases. Here we report that deletion of PTP2 and PTP3 results in a sporulation defect, suggesting that tyrosine phosphorylation is involved in regulation of meiosis and sporulation. Deletion of PTP2 and PTP3 blocks cells at an early stage of sporulation before premeiotic DNA synthesis and induction of meiotic-specific genes. We observed that tyrosine phosphorylation of several proteins, including 52-, 43-, and 42-kDa proteins, was changed in ptp2Deltaptp3Delta homozygous deletion cells under sporulation conditions. The 42-kDa tyrosine-phosphorylated protein was identified as Mck1, which is a member of the GSK3 family of protein kinases and previously known to be phosphorylated on tyrosine. Mutation of MCK1 decreases sporulation efficiency, whereas mutation of RIM11, another GSK3 member, specifically abolishes sporulation; therefore, we investigated regulation of Rim11 by Tyr phosphorylation during sporulation. We demonstrated that Rim11 is phosphorylated on Tyr-199, and the Tyr phosphorylation is essential for its in vivo function, although Rim11 appears not to be directly regulated by Ptp2 and Ptp3. Biochemical characterizations indicate that tyrosine phosphorylation of Rim11 is essential for the activity of Rim11 to phosphorylate substrates. Our data demonstrate important roles of protein tyrosine phosphorylation in meiosis and sporulation

CLN1 and its repression by Xbp1 are important for efficient sporulation in budding yeast

Xbp1, a transcriptional repressor of Saccharomyces cerevisiae with homology to Swi4 and Mbp1, is induced by stress and starvation during the mitotic cycle. It is also induced late in the meiotic cycle. Using RNA differential display, we find that genes encoding three cyclins (CLN1, CLN3, and CLB2), CYS3, and SMF2 are downregulated when Xbp1 is overexpressed and that Xbp1 can bind to sequences in their promoters. During meiosis, XBP1 is highly induced and its mRNA appears at the same time as DIT1 mRNA, but its expression remains high for up to 24 h. As such, it represents a new class of meiosis-specific genes. Xbp1-deficient cells are capable of forming viable gametes, although ascus formation is delayed by several hours. Furthermore, Xbp1 target genes are normally repressed late in meiosis, and loss of XBP1 results in their derepression. Interestingly, we find that a deletion of CLN1 also reduces the efficiency of sporulation and delays the meiotic program but that sporulation in a Deltacln1 Deltaxbp1 strain is not further delayed. Thus, CLN1 may be Xbp1's primary target in meiotic cells. We hypothesize that CLN1 plays a role early in the meiotic program but must be repressed, by Xbp1, at later stages to promote efficient sporulation.

Human JIK, a novel member of the STE20 kinase family that inhibits JNK and is negatively regulated by epidermal growth factor

Mammalian members related to Saccharomyces cerevisiae serine/threonine kinase STE20 can be divided into two subfamilies based on their structure and function. The PAK subfamily is characterized by an N-terminal p21-binding domain (also known as CRIB domain), a C-terminal kinase domain, and is regulated by the small GTP-binding proteins Rac1 and Cdc42Hs. The second group is represented by the GCK-like members, which contain an N-terminal catalytic domain and lack the p21-binding domain. Some of them have been demonstrated to induce c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) cascade, while others have been shown to be activated by a subset of stress conditions or apoptotic agents, although little is known about their specific function. Here, we have identified a novel human STE20-related serine/threonine kinase, belonging to the GCK-like subfamily. This kinase does not induce the JNK/SAPK pathway, but, instead, inhibits the basal activity of JNK/SAPK, and diminishes its activation in response to human epidermal growth factor (EGF). Therefore, we designated this molecule JIK for JNK/SAPK-inhibitory kinase. The inhibition of JNK/SAPK signaling pathway by JIK was found to occur between the EGF receptor and the small GTP-binding proteins Rac1 and Cdc42Hs. In contrast, JIK does not activate nor does it inhibit ERK2, ERK6, p38, or ERK5. Furthermore, JIK kinase activity is not modulated by any exogenous stimuli, but, interestingly, it is dramatically decreased upon EGF receptor activation. Thus, JIK might represent the first member of the STE20 kinase family whose activity can be negatively regulated by tyrosine kinase receptors, and whose downstream targets inhibit, rather than enhance, JNK/SAPK activation.

Regulation and function of PLDs in yeast

While yeast contain multiple phospholipase D activities, only one, encoded by SPO14, appears to be a member of the phosphatidylcholine-specific phospholipase D gene family. Genetic analyses have revealed a role for this enzyme in regulated membrane trafficking events.

Distinct steps in yeast spore morphogenesis require distinct SMK1 MAP kinase thresholds

The SMK1 mitogen-activated protein kinase is required for spore morphogenesis in Saccharomyces cerevisiae. In contrast to the multiple aberrant spore wall assembly patterns seen even within a single smk1 null ascus, different smk1 missense mutants block in a coordinated fashion at intermediate stages. One smk1 mutant forms asci in which the four spores are surrounded only by prospore wall-like structures, while another smk1 mutant forms asci in which the spores are surrounded by inner but not outer spore wall layers. Stepwise increases in gene dosage of a hypomorphic smk1 allele allow for the completion of progressively later morphological and biochemical events and for the acquisition of distinct spore-resistance phenotypes. Furthermore, smk1 allelic spore phenotypes can be recapitulated by reducing wild-type SMK1 expression. The data demonstrate that SMK1 is required for the execution of multiple steps in spore morphogenesis that require increasing thresholds of SMK1 activity. These results suggest that quantitative changes in mitogen-activated protein kinase signaling play a role in coordinating multiple events of a single cellular differentiation program.

SWM1, a developmentally regulated gene, is required for spore wall assembly in Saccharomyces cerevisiae

Meiosis in Saccharomyces cerevisiae is followed by encapsulation of haploid nuclei within multilayered spore walls. Formation of this spore-specific wall requires the coordinated activity of enzymes involved in the biosynthesis of its components. Completion of late events in the sporulation program, leading to spore wall formation, requires the SWM1 gene. SWM1 is expressed at low levels during vegetative growth but its transcription is strongly induced under sporulating conditions, with kinetics similar to those of middle sporulation-specific genes. Homozygous swm1Delta diploids proceed normally through both meiotic divisions but fail to produce mature asci. Consistent with this finding, swm1Delta mutant asci display enhanced sensitivity to enzymatic digestion and heat shock. Deletion of SWM1 specifically affects the expression of mid-late and late sporulation-specific genes. All of the phenotypes observed are similar to those found for the deletion of SPS1 or SMK1, two putative components of a sporulation-specific MAP kinase cascade. However, epistasis analyses indicate that Swm1p does not form part of the Sps1p-Smk1p-MAP kinase pathway. We propose that Swm1p, a nuclear protein, would participate in a different signal transduction pathway that is also required for the coordination of the biochemical and morphological events occurring during the last phase of the sporulation program.

Cell polarity and morphogenesis in budding yeast

Eukaryotic cells respond to intracellular and extracellular cues to direct asymmetric cell growth and division. The yeast Saccharomyces cerevisiae undergoes polarized growth at several times during budding and mating and is a useful model organism for studying asymmetric growth and division. In recent years, many regulatory and cytoskeletal components important for directing and executing growth have been identified, and molecular mechanisms have been elucidated in yeast. Key signaling pathways that regulate polarization during the cell cycle and mating response have been described. Since many of the components important for polarized cell growth are conserved in other organisms, the basic mechanisms mediating polarized cell growth are likely to be universal among eukaryotes.