Moe1, a conserved protein in Schizosaccharomyces pombe, interacts with a Ras effector, Scd1, to affect proper spindle formation

eIF3d: A driver of noncanonical cap-dependent translation of specific mRNAs and a trigger of biological/pathological processes

Eukaryotic initiation factor 3d (eIF3d), a known RNA-binding subunit of the eIF3 complex, is a 66 to 68-kDa protein with an RNA-binding motif and a cap-binding domain. Compared with other eIF3 subunits, eIF3d is relatively understudied. However, recent progress in studying eIF3d has revealed a number of intriguing findings on its role in maintaining eIF3 complex integrity, global protein synthesis, and in biological and pathological processes. It has also been reported that eIF3d has noncanonical functions in regulating translation of a subset of mRNAs by binding to 5'-UTRs or interacting with other proteins independent of the eIF3 complex and additional functions in regulating protein stability. The noncanonical regulation of mRNA translation or protein stability may contribute to the role of eIF3d in biological processes such as metabolic stress adaptation and in disease onset and progression including severe acute respiratory syndrome coronavirus 2 infection, tumorigenesis, and acquired immune deficiency syndrome. In this review, we critically evaluate the recent studies on these aspects of eIF3d and assess prospects in understanding the function of eIF3d in regulating protein synthesis and in biological and pathological processes.

The Multiple Functions of Rho GTPases in Fission Yeasts

The Rho family of GTPases represents highly conserved molecular switches involved in a plethora of physiological processes. Fission yeast Schizosaccharomyces pombe has become a fundamental model organism to study the functions of Rho GTPases over the past few decades. In recent years, another fission yeast species, Schizosaccharomyces japonicus, has come into focus offering insight into evolutionary changes within the genus. Both fission yeasts contain only six Rho-type GTPases that are spatiotemporally controlled by multiple guanine-nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs), and whose intricate regulation in response to external cues is starting to be uncovered. In the present review, we will outline and discuss the current knowledge and recent advances on how the fission yeasts Rho family GTPases regulate essential physiological processes such as morphogenesis and polarity, cellular integrity, cytokinesis and cellular differentiation.

Molecular characterization and functional analysis of subunit 7 of eukaryotic initiation factor 3 from Eimeria tenella

The initiation of translation in eukaryotic cells is stimulated by proteins known as initiation factors (eIFs). A structurally complex eIF composed of multiple subunits, eIF3 has been shown to have various functions in translation in a variety of eukaryotes. Until now, little is known about eIF3 in Eimeria tenella. Based on a previously identified expressed sequence tag(EST), we cloned the eIF3 subunit 7 gene (EteIF3s7) from E. tenella by rapid amplification of the cDNA ends(RACE). The 2278-bp full-length complementary DNA of EteIF3s7 contained a 1716-bp open reading frame (ORF) that encoded a 571-amino acid (aa) polypeptide. The EteIF3s7 protein contained the subunit 7 domain that is characteristic of members of the eIF3 zeta superfamily. The levels of EteIF3s7 messenger RNA and protein were higher in second generation merozoites than in sporulated oocysts, unsporulated oocysts, or sporozoites, and the EteIF3s7 protein was barely detectable in unsporulated oocysts. Our immunofluorescence analysis showed that the EteIF3s7 protein was uniformly distributed throughout the cytoplasm of sporozoites. After sporozoites were incubated in complete medium, the EteIF3s7 protein localized to the anterior region of the parasite. Following the first schizogenous division, the protein was uniformly dispersed in trophozoites, immature schizonts, and mature schizonts, and the EteIF3s7 protein was observed to be closely associated with the parasitophorous vacuole membrane. An anti-rEteIF3s7 polyclonal antibody inhibited the ability of E. tenella to invade DF-1 cells, which suggested that EteIF3s7 might be involved in host cell invasion and required for the growth of the parasite in the host.

Yeast as a model for Ras signalling

For centuries yeast species have been popular hosts for classical biotechnology processes, such as baking, brewing, and wine making, and more recently for recombinant proteins production, thanks to the advantages of unicellular organisms (i.e., ease of genetic manipulation and rapid growth) together with the ability to perform eukaryotic posttranslational modifications. Moreover, yeast cells have been used for few decades as a tool for identifying the genes and pathways involved in basic cellular processes such as the cell cycle, aging, and stress response. In the budding yeast S. cerevisiae the Ras/cAMP/PKA pathway is directly involved in the regulation of metabolism, cell growth, stress resistance, and proliferation in response to the availability of nutrients and in the adaptation to glucose, controlling cytosolic cAMP levels and consequently the cAMP-dependent protein kinase (PKA) activity. Moreover, Ras signalling has been identified in several pathogenic yeasts as a key controller for virulence, due to its involvement in yeast morphogenesis. Nowadays, yeasts are still useful for Ras-like proteins investigation, both as model organisms and as a test tube to study variants of heterologous Ras-like proteins.

Ras-Mediated Signal Transduction and Virulence in Human Pathogenic Fungi

Signal transduction pathways regulating growth and stress responses are areas of significant study in the effort to delineate pathogenic mechanisms of fungi. In-depth knowledge of signal transduction events deepens our understanding of how a fungal pathogen is able to sense changes in the environment and respond accordingly by modulation of gene expression and re-organization of cellular activities to optimize fitness. Members of the Ras protein family are important regulators of growth and differentiation in eukaryotic organisms, and have been the focus of numerous studies exploring fungal pathogenesis. Here, the current data regarding Ras signal transduction are reviewed for three major pathogenic fungi: Cryptococcus neoformans, Candida albicans and Aspergillus fumigatus. Particular emphasis is placed on Ras-protein interactions during control of morphogenesis, stress response and virulence.

Shaping microtubules into diverse patterns: molecular connections for setting up both ends

Microtubules serve as rails for intracellular trafficking and their appropriate organization is critical for the generation of cell polarity, which is a foundation of cell differentiation, tissue morphogenesis, ontogenesis and the maintenance of homeostasis. The microtubule array is not just a static railway network; it undergoes repeated collapse and reassembly in diverse patterns during cell morphogenesis. In the last decade much progress has been made toward understanding the molecular mechanisms governing complex microtubule patterning. This review first revisits the basic principle of microtubule dynamics, and then provides an overview of how microtubules are arranged in highly shaped and functional patterns in cells changing their morphology by factors controlling the fate of microtubule ends.

Schizosaccharomyces pombe Arc3 is a conserved subunit of the Arp2/3 complex required for polarity, actin organization, and endocytosis

We characterized the Schizosaccharomyces pombe arc3 gene, whose product shares sequence homology with that of the budding yeast ARC18 and human ARPC3/p21 subunits of the Arp2/3 complex. Our data showed that Arc3p co-localizes with F-actin patches at the cell ends, but not with F-actin cables or the equatorial actin ring, and binds other subunits of the Arp2/3 complex. Gene deletion analysis showed that arc3 is essential for viability. When arc3 expression was repressed, F-actin patches became dispersed throughout the cell with greatly reduced mobility. Furthermore, in arc3-repressed cells, endocytosis was also inhibited. Human ARPC3 rescued the viability of the Sz. pombe arc3 null mutant; in addition, ARPC3 also localized to F-actin patches in human cells. These data suggest that Arc3p is an evolutionarily conserved subunit of the Arp2/3 complex required for proper F-actin organization and efficient endocytosis.

Shaping fission yeast with microtubules

For cell morphogenesis, the cell must establish distinct spatial domains at specified locations at the cell surface. Here, we review the molecular mechanisms of cell polarity in the fission yeast Schizosaccharomyces pombe. These are simple rod-shaped cells that form cortical domains at cell tips for cell growth and at the cell middle for cytokinesis. In both cases, microtubule-based systems help to shape the cell by breaking symmetry, providing endogenous spatial cues to position these sites. The plus ends of dynamic microtubules deliver polarity factors to the cell tips, leading to local activation of the GTPase cdc42p and the actin assembly machinery. Microtubule bundles contribute to positioning the division plane through the nucleus and the cytokinesis factor mid1p. Recent advances illustrate how the spatial and temporal regulation of cell polarization integrates many elements, including historical landmarks, positive and negative controls, and competition between pathways.

Proteasome nuclear import mediated by Arc3 can influence efficient DNA damage repair and mitosis in Schizosaccharomyces pombe

Proteasomes must efficiently remove their substrates throughout the cells in a timely manner as many of these proteins can be toxic. This study shows that proteasomes can do so efficiently because they are highly mobile. Furthermore this study uncovers that proteasome mobility requires functional Arc3, a subunit of the Arp2/3 complex.

Proteasomes must remove regulatory molecules and abnormal proteins throughout the cell, but how proteasomes can do so efficiently remains unclear. We have isolated a subunit of the Arp2/3 complex, Arc3, which binds proteasomes. When overexpressed, Arc3 rescues phenotypes associated with proteasome deficiencies; when its expression is repressed, proteasome deficiencies intensify. Arp2/3 is best known for regulating membrane dynamics and vesicular transport; thus, we performed photobleaching experiments and showed that proteasomes are readily imported into the nucleus but exit the nucleus slowly. Proteasome nuclear import is reduced when Arc3 is inactivated, leading to hypersensitivity to DNA damage and inefficient cyclin-B degradation, two events occurring in the nucleus. These data suggest that proteasomes display Arc3-dependent mobility in the cell, and mobile proteasomes can efficiently access substrates throughout the cell, allowing them to effectively regulate cell-compartment–specific activities.

Int6 and Moe1 interact with Cdc48 to regulate ERAD and proper chromosome segregation

Int6/eIF3e is implicated in tumorigenesis, but its molecular functions remain unclear. We have studied its fission yeast homolog Yin6, reporting that it regulates proteolysis by controlling the assembly/localization of proteasomes, and binds directly to another conserved protein, Moe1. In the present study, we isolated Cdc48 as a Moe1-binding protein from a yeast two-hybrid screen, and confirmed biochemically that they form a stable complex in fission yeast. Overexpressing Moe1 or Yin6 partially rescued phenotypes of cdc48 mutants; conversely, overexpressing Cdc48 partially rescued phenotypes of moe1 or yin6 mutants. Mutants defective in both Cdc48 and the Yin6-Moe1 complex showed growth defects that were far more severe than either alone. These double mutants were severely deficient in endoplasmic reticulum associated degradation (ERAD), as they were hypersensitive to accumulation of misfolded proteins. In addition, their chromosomes showed frequent defects in spindle attachment and segregation--these mitotic defects correlated with Ase1 and Bir1/survivin mislocalization. These results suggest that Cdc48, Yin6 and Moe1 act in the same protein complex to concertedly control ERAD and chromosome segregation. Many of these properties are evolutionarily conserved in humans, since human Cdc48 rescued the lethality of the yeast cdc48Delta mutant, and Int6 and Moe1/eIF3d bind Cdc48 in human cells.

The eIF3 interactome reveals the translasome, a supercomplex linking protein synthesis and degradation machineries

eIF3 promotes translation initiation, but relatively little is known about its full range of activities in the cell. Here, we employed affinity purification and highly sensitive LC-MS/MS to decipher the fission yeast eIF3 interactome, which was found to contain 230 proteins. eIF3 assembles into a large supercomplex, the translasome, which contains elongation factors, tRNA synthetases, 40S and 60S ribosomal proteins, chaperones, and the proteasome. eIF3 also associates with ribosome biogenesis factors and the importins-beta Kap123p and Sal3p. Our genetic data indicated that the binding to both importins-beta is essential for cell growth, and photobleaching experiments revealed a critical role for Sal3p in the nuclear import of one of the translasome constituents, the proteasome. Our data reveal the breadth of the eIF3 interactome and suggest that factors involved in translation initiation, ribosome biogenesis, translation elongation, quality control, and transport are physically linked to facilitate efficient protein synthesis.

Microtubule-dependent cell morphogenesis in the fission yeast

In many systems, microtubules contribute spatial information to cell morphogenesis, for instance in cell migration and division. In rod-shaped fission yeast cells, microtubules control cell morphogenesis by transporting polarity factors, namely the Tea1-Tea4 complex, to cell tips. This complex then recruits the DYRK kinase Pom1 to cell ends. Interestingly, recent work has shown that these proteins also provide long-range spatial cues to position the division site in the middle of the cell and temporal signals to coordinate cell length with the cell cycle. Here I review how these microtubule-associated proteins form polar morphogenesis centers that control and integrate both spatial and temporal aspects of cell morphogenesis.

Establishing new sites of polarization by microtubules

BACKGROUND

Microtubules (MTs) participate in the spatial regulation of actin-based processes such as cytokinesis and cell polarization. The fission yeast Schizosaccharomyces pombe is a rod-shaped cell that exhibits polarized cell growth at cell tips. MT plus ends contact and shrink from the cell tips and contribute to polarity regulation.

RESULTS

Here, we investigate the effects of changing cell shape on MTs and cell-polarization machinery. We physically bend fission yeast cells by forcing them into microfabricated femtoliter chambers. In these bent cells, MTs maintain a straight axis and contact and shrink from cortical sites at the sides of cells. At these ectopic sites, polarity factors such as bud6p, for3p (formin), and cdc42p are recruited and assemble actin cables in a MT-dependent manner. MT contact at the cortex induces the appearance of a bud6p dot within seconds. The accumulation of polarity factors leads to cell growth at these sites, when the MT-associated polarity factor tea1p is absent. This process is dependent on MTs, mal3p (EB1), moe1p (an EB1-binding protein), and for3p but, surprisingly, is independent of the tea1p-tea4p pathway.

CONCLUSIONS

These studies provide a direct demonstration for how MTs induce actin assembly at specific locations on the cell cortex and begin to identify a new pathway involved in this process. MT interactions with the cortex may be regulated by cortical-attachment sites. These findings highlight the crosstalk between cell shape, polarity mechanisms, and MTs responsible for cell morphogenesis.

Fission yeast translation initiation factor 3 subunit eIF3h is not essential for global translation initiation, but deletion of eif3h+ affects spore formation

The fission yeast Schizosaccharomyces pombe homologue of the p40/eIF3h subunit of mammalian translation initiation factor eIF3 has been characterized in this study. We show that this protein physically associates with the 40S ribosomal particles as a constituent of the multimeric eIF3 protein complex, which consists of all five known eIF3 core subunits (eIF3a, eIF3b, eIF3c, eIF3g and eIF3i) as well as the five non-core subunits (eIF3d, eIF3e, eIF3f, eIF3h and eIF3m) that constitute an eIF3 holocomplex in fission yeast. However, affinity purification of eIF3 from fission yeast cells expressing TAP-tagged eIF3h suggests the presence of distinct forms of eIF3 that differ in their composition of the non-core subunits. Further characterization of eIF3h shows that strains lacking eif3h(+) (eif3hDelta) are viable and show no gross defects, either in vegetative growth or in the rate of in vivo protein synthesis. Polysome profile analysis shows no apparent defects in translation initiation. Furthermore, deletion of eif3h(+) does not affect the ability of the other eIF3 subunits to remain associated with one another in a tight protein complex similar to the situation in wild-type cells. Additionally, we show that human eIF3h can functionally substitute fission yeast eIF3h in complementing in vivo a genetic deletion of eif3h(+). Interestingly, mutant eif3hDelta cells show several prominent phenotypic properties. They are hypersensitive to caffeine and highly defective in meiosis, producing either no spores or incomplete tetrads with a very high frequency. The implications of these results in relation to the functions of eIF3h in Sz. pombe are discussed.

Mechanisms for maintaining microtubule bundles

The dynamics of microtubules (MTs) are crucial to many of their functions. Certain MT structures, such as the mitotic spindle apparatus, exhibit high MT turnover yet maintain their mass stably through long periods of time. Here, we highlight what are emerging as two important mechanisms for maintaining MT bundles: the first, MT nucleation from pre-existing MTs by means of gamma-tubulin-containing complexes; and the second, MT 'rescue' by the stabilizing protein CLASP. As examples, we describe recent advances in understanding the assembly and maintenance of simple MT bundles in fission yeast and plant cells, which have implications for the bundles of the animal mitotic spindle.

Role of Rho GTPases and Rho-GEFs in the regulation of cell shape and integrity in fission yeast

The Rho family of GTPases are highly conserved molecular switches that control some of the most fundamental processes of cell biology, including morphogenesis, vesicular transport, cell division and motility. Guanine nucleotide-exchange factors (GEFs) are directly responsible for the activation of Rho-family GTPases in response to extracellular stimuli. In fission yeast, there are seven Dbl-related GEFs and they activate six Rho-type GTPases within a particular spatio-temporal context. The failure to do so might have consequences reflected in aberrant phenotypes and in some cases lead to cell death. In this review, we briefly summarize the role of Rho GTPases and Rho-GEFs in the establishment and maintenance of cell polarity and cell integrity in Schizosaccharomyces pombe.

Noncore components of the fission yeast gamma-tubulin complex

Relatively little is known about the in vivo function of individual components of the eukaryotic gamma-tubulin complex (gamma-TuC). We identified three genes, gfh1+, mod21+, and mod22+, in a screen for fission yeast mutants affecting microtubule organization. gfh1+ is a previously characterized gamma-TuC protein weakly similar to human gamma-TuC subunit GCP4, whereas mod21+ is novel and shows weak similarity to human gamma-TuC subunit GCP5. We show that mod21p is a bona fide gamma-TuC protein and that, like gfh1Delta mutants, mod21Delta mutants are viable. We find that gfh1Delta and mod21Delta mutants have qualitatively normal microtubule nucleation from all types of microtubule-organizing centers (MTOCs) in vivo but quantitatively reduced nucleation from interphase MTOCs, and this is exacerbated by mutations in mod22+. Simultaneous deletion of gfh1p, mod21p, and alp16p, a third nonessential gamma-TuC protein, does not lead to additive defects, suggesting that all three proteins contribute to a single function. Coimmunoprecipitation experiments suggest that gfh1p and alp16p are codependent for association with a small "core" gamma-TuC, whereas mod21p is more peripherally associated, and that gfh1p and mod21p may form a subcomplex independently of the small gamma-TuC. Interestingly, sucrose gradient analysis suggests that the major form of the gamma-TuC in fission yeast may be a small complex. We propose that gfh1p, mod21p, and alp16 act as facultative "noncore" components of the fission yeast gamma-TuC and enhance its microtubule-nucleating ability.

eIF3: a versatile scaffold for translation initiation complexes

Translation initiation in eukaryotes depends on many eukaryotic initiation factors (eIFs) that stimulate both recruitment of the initiator tRNA, Met-tRNA(i)(Met), and mRNA to the 40S ribosomal subunit and subsequent scanning of the mRNA for the AUG start codon. The largest of these initiation factors, the eIF3 complex, organizes a web of interactions among several eIFs that assemble on the 40S subunit and participate in the different reactions involved in translation. Structural analysis suggests that eIF3 performs this scaffolding function by binding to the 40S subunit on its solvent-exposed surface rather than on its interface with the 60S subunit, where the decoding sites exist. This location of eIF3 seems ideally suited for its other proposed regulatory functions, including reinitiating translation on polycistronic mRNAs and acting as a receptor for protein kinases that control protein synthesis.

Compartmentalized signaling of Ras in fission yeast

Compartment-specific Ras signaling is an emerging paradigm that may explain the multiplex outputs from a single GTPase. The fission yeast, Schizosaccharomyces pombe, affords a simple system in which to study Ras signaling because it has a single Ras protein, Ras1, that regulates two distinct pathways: one that controls mating through a Byr2-mitogen-activated protein kinase cascade and one that signals through Scd1-Cdc42 to maintain elongated cell morphology. We generated Ras1 mutants that are restricted to either the endomembrane or the plasma membrane. Protein binding studies showed that each could interact with the effectors of both pathways. However, when examined in ras1 null cells, endomembrane-restricted Ras1 supported morphology but not mating, and, conversely, plasma membrane-restricted Ras1 supported mating but did not signal to Scd1-Cdc42. These observations provide a striking demonstration of compartment-specific Ras signaling and indicate that spatial specificity in the Ras pathway is evolutionarily conserved.

Oncogenic RAS induces accelerated transition through G2/M and promotes defects in the G2 DNA damage and mitotic spindle checkpoints

Activating mutations of RAS are prevalent in thyroid follicular neoplasms, which commonly have chromosomal losses and gains. In thyroid cells, acute expression of HRAS(V12) increases the frequency of chromosomal abnormalities within one or two cell cycles, suggesting that RAS oncoproteins may interfere with cell cycle checkpoints required for maintenance of a stable genome. To explore this, PCCL3 thyroid cells with conditional expression of HRAS(V12) or HRAS(V12) effector mutants were presynchronized at the G(1)/S boundary, followed by activation of expression of RAS mutants and release from the cell cycle block. Expression of HRAS(V12) accelerated the G(2)/M phase by approximately 4 h and promoted bypass of the G(2) DNA damage and mitotic spindle checkpoints. Accelerated passage through G(2)/M and bypass of the G(2) DNA damage checkpoint, but not bypass of the mitotic spindle checkpoint, required activation of mitogen-activated protein kinase (MAPK). However, selective activation of the MAPK pathway was not sufficient to disrupt the G(2) DNA damage checkpoint, because cells arrested appropriately in G(2) despite conditional expression of HRAS(V12,S35) or BRAF(V600E). By contrast to the MAPK requirement for radiation-induced G(2) arrest, RAS-induced bypass of the mitotic spindle checkpoint was not prevented by pretreatment with MEK inhibitors. These data support a direct role for the MAPK pathway in control of G(2) progression and regulation of the G(2) DNA damage checkpoint. We propose that oncogenic RAS activation may predispose cells to genomic instability through both MAPK-dependent and independent pathways that affect critical checkpoints in G(2)/M.

Mutant KRAS, chromosomal instability and prognosis in colorectal cancer

The RAS gene family provides a global effect on gene expression by encoding small GTP-binding proteins which act as molecular switches connecting extracellular signals with nuclear transcription factors. While wild type RAS proteins are switched off shortly after activation, mutant RAS proteins remain constitutively activated leading to complex interactions among their downstream effectors. For some human tumor types, these interactions were shown to contribute to cancer genesis and progression by inducing changes in cell survival, apoptosis, angiogenesis, invasion and metastasis. This review addresses the controversial link of KRAS mutations in colorectal cancer with chromosomal instability and patient prognosis.

PCI proteins eIF3e and eIF3m define distinct translation initiation factor 3 complexes

BACKGROUND

PCI/MPN domain protein complexes comprise the 19S proteasome lid, the COP9 signalosome (CSN), and eukaryotic translation initiation factor 3 (eIF3). The eIF3 complex is thought to be composed of essential core subunits required for global protein synthesis and non-essential subunits that may modulate mRNA specificity. Interactions of unclear significance were reported between eIF3 subunits and PCI proteins contained in the CSN.

RESULTS

Here, we report the unexpected finding that fission yeast has two distinct eIF3 complexes sharing common core subunits, but distinguished by the PCI proteins eIF3e and the novel eIF3m, which was previously annotated as a putative CSN subunit. Whereas neither eIF3e nor eIF3m contribute to the non-essential activities of CSN in cullin-RING ubiquitin ligase control, eif3m, unlike eif3e, is an essential gene required for global cellular protein synthesis and polysome formation. Using a ribonomic approach, this phenotypic distinction was correlated with a different set of mRNAs associated with the eIF3e and eIF3m complexes. Whereas the eIF3m complex appears to associate with the bulk of cellular mRNAs, the eIF3e complex associates with a far more restricted set. The microarray findings were independently corroborated for a random set of 14 mRNAs by RT-PCR analysis.

CONCLUSION

We propose that the PCI proteins eIF3e and eIF3m define distinct eIF3 complexes that may assist in the translation of different sets of mRNAs.

Effects of {gamma}-tubulin complex proteins on microtubule nucleation and catastrophe in fission yeast

Although gamma-tubulin complexes (gamma-TuCs) are known as microtubule (MT) nucleators, their function in vivo is still poorly defined. Mto1p (also known as mbo1p or mod20p) is a gamma-TuC-associated protein that recruits gamma-TuCs specifically to cytoplasmic MT organizing centers (MTOCs) and interphase MTs. Here, we investigated gamma-TuC function by analyzing MT behavior in mto1Delta and alp4 (GCP2 homologue) mutants. These cells have free, extra-long interphase MTs that exhibit abnormal behaviors such as cycles of growth and breakage, MT sliding, treadmilling, and hyperstability. The plus ends of interphase and spindle MTs grow continuously, exhibiting catastrophe defects that are dependent on the CLIP170 tip1p. The minus ends of interphase MTs exhibit shrinkage and pauses. As mto1Delta mutants lack cytoplasmic MTOCs, cytoplasmic MTs arise from spindle or other intranuclear MTs that exit the nucleus. Our findings show that mto1p and gamma-TuCs affect multiple properties of MTs including nucleation, nuclear attachment, plus-end catastrophe, and minus-end shrinkage.

Microtubule nucleation at non-spindle pole body microtubule-organizing centers requires fission yeast centrosomin-related protein mod20p

BACKGROUND

Many types of differentiated eukaryotic cells display microtubule distributions consistent with nucleation from noncentrosomal intracellular microtubule organizing centers (MTOCs), although such structures remain poorly characterized. In fission yeast, two types of MTOCs exist in addition to the spindle pole body, the yeast centrosome equivalent. These are the equatorial MTOC, which nucleates microtubules from the cell division site at the end of mitosis, and interphase MTOCs, which nucleate microtubules from multiple sites near the cell nucleus during interphase.

RESULTS

From an insertional mutagenesis screen we identified a novel gene, mod20+, which is required for microtubule nucleation from non-spindle pole body MTOCs in fission yeast. Mod20p is not required for intranuclear mitotic spindle assembly, although it is required for cytoplasmic astral microtubule growth during mitosis. Mod20p localizes to MTOCs throughout the cell cycle and is also dynamically distributed along microtubules themselves. We find that mod20p is required for the localization of components of the gamma-tubulin complex to non-spindle pole body MTOCs and physically interacts with the gamma-tubulin complex in vivo. Database searches reveal a family of eukaryotic proteins distantly related to mod20p; these are found in organisms ranging from fungi to mammals and include Drosophila centrosomin.

CONCLUSIONS

Mod20p appears to act by recruiting components of the gamma-tubulin complex to non-spindle pole body MTOCs. The identification of mod20p-related proteins in higher eukaryotes suggests that this may represent a general mechanism for the organization of noncentrosomal MTOCs in eukaryotic cells.

Schizosaccharomyces pombe Ras1 effector, Scd1, interacts with Klp5 and Klp6 kinesins to mediate cytokinesis

Fission yeast Scd1 is an exchange factor for Cdc42 and an effector of Ras1. In a screen for scd1 interacting genes, we isolated klp5 and klp6, which encode presumptive kinesins. Klp5 and Klp6 form a complex to control the same processes, which so far include microtubule dynamics and chromosome segregation. We showed that klp5 or klp6 inactivation in combination with the scd1 deletion (scd1delta) created a synthetic temperature-dependent growth defect. Further genetic analysis demonstrated that Klp5 and Klp6 interacted specifically with the Ras1-Scd1 pathway, but not with the Ras1-Byr2 pathway. In addition, Klp5 and Klp6 can stably associate with Scd1 and Cdc42. A deletion in the Scd1 C terminus, which contains the PB1 domain, prevented Scd1 binding to Klp5/6 and caused a growth defect in Klp5/6 mutant cells that is indistinguishable from that induced by scd1delta. Analysis of the double-mutant phenotype indicated that at the nonpermissive temperature, cells failed to undergo cytokinesis efficiently. These cells contained abnormal contractile rings in which F-actin and Mid1, a key regulator of F-actin ring formation and positioning, are mispositioned and fragmented. These data suggest that Klp5/6 cooperate with the Ras1-Scd1 pathway to influence proper formation of the contractile ring for cytokinesis.

Protein homeostasis: a degrading role for Int6/eIF3e

Similarities between the three related "PCI" complexes--eIF3, the COP9 signalosome and the proteasome lid--have hinted at novel pathways controlling protein homeostasis. Recent experiments with fission yeast have begun to weigh in with genetic evidence.

Schizosaccharomyces pombe Int6 and Ras homologs regulate cell division and mitotic fidelity via the proteasome

Yin6 is a yeast homolog of Int6, which is implicated in tumorigenesis. We show that Yin6 binds to and regulates proteasome activity. Overexpression of Yin6 strengthens proteasome function while inactivation weakens and causes the accumulation of polyubiquitinated proteins including securin/Cut2 and cyclin/Cdc13. Yin6 regulates the proteasome by preferentially interacting with Rpn5, a conserved proteasome subunit, and affecting its localization/assembly. We showed previously that Yin6 cooperates with Ras1 to mediate chromosome segregation; here, we demonstrate that Ras1 similarly regulates the proteasome via Rpn5. In yeast, human Int6 binds Rpn5 and regulates its localization. We propose that human Int6, either alone or cooperatively with Ras, influences proteasome activities via Rpn5. Inactivating Int6 can lead to accumulation of mitotic regulators affecting cell division and mitotic fidelity.

Gef1p, a new guanine nucleotide exchange factor for Cdc42p, regulates polarity in Schizosaccharomyces pombe

Schizosaccharomyces pombe cdc42(+) regulates cell morphology and polarization of the actin cytoskeleton. Scd1p/Ral1p is the only described guanine nucleotide exchange factor (GEF) for Cdc42p in S. pombe. We have identified a new GEF, named Gef1p, specifically regulating Cdc42p. Gef1p binds to inactive Cdc42p but not to other Rho GTPases in two-hybrid assays. Overexpression of gef1(+) increases specifically the GTP-bound Cdc42p, and Gef1p is capable of stimulating guanine nucleotide exchange of Cdc42p in vitro. Overexpression of gef1(+) causes changes in cell morphology similar to those caused by overexpression of the constitutively active cdc42G12V allele. Gef1p localizes to the septum. gef1(+) deletion is viable but causes a mild cell elongation and defects in bipolar growth and septum formation, suggesting a role for Gef1p in the control of cell polarity and cytokinesis. The double mutant gef1delta scd1delta is not viable, indicating that they share an essential function as Cdc42p activators. However, both deletion and overexpression of either gef1(+) or scd1(+) causes different morphological phenotypes, which suggest different functions. Genetic evidence revealed a link between Gef1p and the signaling pathway of Shk1/Orb2p and Orb6p. In contrast, no genetic interaction between Gef1p and Shk2p-Mkh1p pathway was observed.

Two ras pathways in fission yeast are differentially regulated by two ras guanine nucleotide exchange factors

How a given Ras prreotein coordinates multiple signaling inputs and outputs is a fundamental issue of signaling specificity. Schizosaccharomyces pombe contains one Ras, Ras1, that has two distinct outputs. Ras1 activates Scd1, a presumptive guanine nucleotide exchange factor (GEF) for Cdc42, to control morphogenesis and chromosome segregation, and Byr2, a component of a mitogen-activated protein kinase cascade, to control mating. So far there is only one established Ras1 GEF, Ste6. Paradoxically, ste6 null (ste6 Delta) mutants are sterile but normal in cell morphology. This suggests that Ste6 specifically activates the Ras1-Byr2 pathway and that there is another GEF capable of activating the Scd1 pathway. We thereby characterized a potential GEF, Efc25. Genetic data place Efc25 upstream of the Ras1-Scd1, but not the Ras1-Byr2, pathway. Like ras1 Delta and scd1 Delta, efc25 Delta is synthetically lethal with a deletion in tea1, a critical element for cell polarity control. Using truncated proteins, we showed that the C-terminal GEF domain of Efc25 is essential for function and regulated by the N terminus. We conclude that Efc25 acts as a Ras1 GEF specific for the Scd1 pathway. While ste6 expression is induced during mating, efc25 expression is constitutive. Moreover, Efc25 overexpression renders cells hyperelongated and sterile; the latter can be rescued by activated Ras1. This suggests that Efc25 can recruit Ras1 to selectively activate Scd1 at the expense of Byr2. Reciprocally, Ste6 overexpression can block Scd1 activation. We propose that external signals can partly segregate two Ras1 pathways by modulating GEF expression and that GEFs can influence how Ras is coupled to specific effectors.

Minireview: branded from the start-distinct oncogenic initiating events may determine tumor fate in the thyroid

Thyroid follicular neoplasms commonly have aneuploidy, presumably due to chromosomal instability. This property is associated with a greater malignant potential and worse prognosis. Recently, there has been considerable progress in our understanding of mechanisms that may account for chromosomal instability in cancer cells. Many tumors with chromosomal instability have abnormalities in the cell cycle checkpoint that monitors the fidelity of mitosis. Mutations of Bub1 or BubR1, genes coding for kinases involved in mitotic spindle assembly checkpoint signaling, are found in a small subset of aneuploid tumors. Other components of protein complexes responsible for attachment of kinetochores to microtubules, or for cohesion between sister chromatids, may also be subject to alterations during tumor progression. Here, we also discuss the evidence that certain oncogenic events, such as Ras mutations, may predispose cells to chromosomal instability by favoring inappropriate posttranslational changes in mitotic checkpoint components through activation of upstream kinases during tumor initiation or progression.

Sum1, a component of the fission yeast eIF3 translation initiation complex, is rapidly relocalized during environmental stress and interacts with components of the 26S proteasome

Eukaryotic translation initiation factor 3 (eIF3) is a multisubunit complex that plays a central role in translation initiation. We show that fission yeast Sum1, which is structurally related to known eIF3 subunits in other species, is essential for translation initiation, whereas its overexpression results in reduced global translation. Sum1 is associated with the 40S ribosome and interacts stably with Int6, an eIF3 component, in vivo, suggesting that Sum1 is a component of the eIF3 complex. Sum1 is cytoplasmic under normal growth conditions. Surprisingly, Sum1 is rapidly relocalized to cytoplasmic foci after osmotic and thermal stress. Int6 and p116, another putative eIF3 subunit, behave similarly, suggesting that eIF3 is a dynamic complex. These cytoplasmic foci, which additionally comprise eIF4E and RNA components, may function as translation centers during environmental stress. After heat shock, Sum1 additionally colocalizes stably with the 26S proteasome at the nuclear periphery. The relationship between Sum1 and the 26S proteasome was further investigated, and we find cytoplasmic Sum1 localization to be dependent on the 26S proteasome. Furthermore, Sum1 interacts with the Mts2 and Mts4 components of the 26S proteasome. These data indicate a functional link between components of the structurally related eIF3 translation initiation and 26S proteasome complexes.

The p21-activated kinase, Shk1, is required for proper regulation of microtubule dynamics in the fission yeast, Schizosaccharomyces pombe

The p21-activated kinase, Shk1, is required for the proper establishment of cell polarity in the fission yeast, Schizosaccharomyces pombe. We showed recently that loss of the essential Shk1 inhibitor, Skb15, causes significant spindle defects in fission yeast, thus implicating Shk1 as a potential regulator of microtubule dynamics. Here, we show that cells deficient in Shk1 function have malformed interphase microtubules and mitotic microtubule spindles, are hypersensitive to the microtubule-destabilizing drug thiabendazole (TBZ) and cold sensitive for growth. TBZ treatment causes a downregulation of Shk1 kinase activity, which increases rapidly after release of cells from the drug, thus providing a correlation between Shk1 kinase function and active microtubule polymerization. Consistent with a role for Shk1 as a regulator of microtubule dynamics, green fluorescent protein (GFP)-Shk1 fusion proteins localize to interphase microtubules and mitotic microtubule spindles, as well as to cell ends and septum-forming regions of fission yeast cells. We show that loss of Tea1, a cell end- and microtubule-localized protein previously implicated as a regulator of microtubule dynamics in fission yeast, exacerbates the growth and microtubule defects resulting from partial loss of Shk1 and that Shk1 localizes to illicit growth tips produced by tea1 mutant cells. Our results demonstrate that Shk1 is required for the proper regulation of microtubule dynamics in fission yeast and implicate Tea1 as a potential Shk1 regulator.

Moe1 and spInt6, the fission yeast homologues of mammalian translation initiation factor 3 subunits p66 (eIF3d) and p48 (eIF3e), respectively, are required for stable association of eIF3 subunits

The protein encoded by the fission yeast gene, moe1(+) is the homologue of the p66/eIF3d subunit of mammalian translation initiation factor eIF3. In this study, we show that in fission yeast, Moe1 physically associates with eIF3 core subunits as well as with 40 S ribosomal particles as a constituent of the eIF3 protein complex that is similar in size to multisubunit mammalian eIF3. However, strains lacking moe1(+) (Deltamoe1) are viable and show no gross defects in translation initiation, although the rate of translation in the Deltamoe1 cells is about 30-40% slower than wild-type cells. Mutant Deltamoe1 cells are hypersensitive to caffeine and defective in spore formation. These phenotypes of Deltamoe1 cells are similar to those reported previously for deletion of the fission yeast int6(+) gene that encodes the fission yeast homologue of the p48/Int6/eIF3e subunit of mammalian eIF3. Further analysis of eIF3 subunits in Deltamoe1 or Deltaint6 cells shows that in these deletion strains, while all the eIF3 subunits are bound to 40 S particles, dissociation of ribosome-bound eIF3 results in the loss of stable association between the eIF3 subunits. In contrast, eIF3 isolated from ribosomes of wild-type cells are associated with one another in a protein complex. These observations suggest that Moe1 and spInt6 are each required for stable association of eIF3 subunits in fission yeast.

Two related kinesins, klp5+ and klp6+, foster microtubule disassembly and are required for meiosis in fission yeast

The kinesin superfamily of microtubule motor proteins is important in many cellular processes, including mitosis and meiosis, vesicle transport, and the establishment and maintenance of cell polarity. We have characterized two related kinesins in fission yeast, klp5+ and klp6+,, that are amino-terminal motors of the KIP3 subfamily. Analysis of null mutants demonstrates that neither klp5+ nor klp6+, individually or together, is essential for vegetative growth, although these mutants have altered microtubule behavior. klp5Delta and klp6Delta are resistant to high concentrations of the microtubule poison thiabendazole and have abnormally long cytoplasmic microtubules that can curl around the ends of the cell. This phenotype is greatly enhanced in the cell cycle mutant cdc25-22, leading to a bent, asymmetric cell morphology as cells elongate during cell cycle arrest. Klp5p-GFP and Klp6p-GFP both localize to cytoplasmic microtubules throughout the cell cycle and to spindles in mitosis, but their localizations are not interdependent. During the meiotic phase of the life cycle, both of these kinesins are essential. Spore viability is low in homozygous crosses of either null mutant. Heterozygous crosses of klp5Delta with klp6Delta have an intermediate viability, suggesting cooperation between these proteins in meiosis.

Saccharomyces cerevisiae protein Pci8p and human protein eIF3e/Int-6 interact with the eIF3 core complex by binding to cognate eIF3b subunits

Mammalian, plant, and Schizosaccharomyces pombe eukaryotic initiation factor-3 (eIF3) contains a protein homologous to the product of int-6 (eIF3e), a frequent integration site of mouse mammary tumor viruses. By contrast, Saccharomyces cerevisiae does not encode a protein closely related to eIF3e/Int-6. Here, we characterize a novel S. cerevisiae protein (Pci8p, Yil071cp) that contains a PCI (proteasome-COP9 signalosome-eIF3) domain conserved in eIF3e/Int-6. We show that both Pci8p and human eIF3e/Int-6 expressed in budding yeast interact with the yeast eIF3 complex in vivo and in vitro by binding to a discrete segment of its eIF3b subunit Prt1p and that human eIF3e/Int-6 interacts with the human eIF3b segment homologous to the Pci8p-binding site of yeast Prt1p. These results refine our understanding of subunit interactions in the eIF3 complex and suggest structural similarity between human eIF3e/Int-6 and yeast Pci8p. However, deletion of PCI8 had no discernible effect on cell growth or translation initiation as judged by polysome analysis, suggesting that Pci8p is not required for the essential function of eIF3 in translation initiation. Motivated by the involvement of Int-6 in transcriptional control, we investigated the effects of deleting PCI8 on the total mRNA expression profile by oligonucleotide microarray analysis and found reduced mRNA levels for a subset of heat shock proteins in the pci8Delta mutant. We discuss possible dual functions of Pci8p and Int-6 in transcriptional and translational control.

Characterization of a fission yeast mutant which displays defects in cell wall integrity and cytokinesis

The fission yeast cps6-153 mutant was originally isolated based on its hypersensitivity to the spindle poison isopropyl N-3-chlorophenyl carbamate (CIPC). The mutant also shows defects in both cell wall integrity and cytokinesis, resulting in the accumulation of unseparated cells with weakened cell walls. The arrested cells display a disoriented alignment of cytoplasmic microtubules. When the mutant cells are cultivated at high temperature (35 degrees C), both cell walls and septa become very thick. Electron microscopy revealed the disorganized structure of the thickened cell walls and septa, in which fibrillar components were not completely masked with an amorphous matrix. rad25+ was cloned from a genomic library by complementation of the mutant phenotypes, suggesting the involvement of Rad25p, one of two 14-3-3 proteins in S. pombe, in the pathway of cell wall integrity and cytokinesis.

PCI complexes: pretty complex interactions in diverse signaling pathways

Three protein complexes (the proteasome regulatory lid, the COP9 signalosome and eukaryotic translation initiation factor 3) contain protein subunits with a well defined protein domain, the PCI domain. At least two (the COP9 signalosome and the lid) appear to share a common evolutionary origin. Recent advances in our understanding of the structure and function of the three complexes point to intriguing and unanticipated connections between the cellular functions performed by these three protein assemblies, especially between translation initiation and proteolytic protein degradation.

Bob1, a Gim5/MM-1/Pfd5 homolog, interacts with the MAP kinase kinase Byr1 to regulate sexual differentiation in the fission yeast, Schizosaccharomyces pombe

The MAPKK Byr1 is an essential component of a Ras-dependent MAPK module required for sexual differentiation in the fission yeast, Schizosaccharomyces pombe. Here we describe the genetic and molecular characterization of a highly conserved protein, Bob1, which was identified from a two-hybrid screen for Byr1-interacting proteins. Byrl and Bobl proteins coprecipitate from S. pombe cell lysates, and both proteins localize to the tips and septa of S. pombe cells. S. pombe bob1 null (bob1delta) mutants lack obvious growth defects but exhibit a significant mating deficiency, which can be suppressed by overexpression of Byrl. Overexpression of Bob1 also leads to inhibition of mating in S. pombe, and this defect is likewise suppressed by Byrl overexpression. Bob1 is highly homologous in structure to the mammalian MM-1/Pfd5 and budding yeast Gim5/Pfd5-Sc proteins, which have been implicated as regulators of actin and tubulins. Similar to budding yeast gim5/pfd5-Sc mutants, S. pombe bob1delta cells have cytoskeletal defects, as judged by hypersensitivity to cytoskeletal disrupting drugs. byr1delta mutants do not share this characteristic with bob1delta mutants, and byr1delta bob1delta mutants are not significantly more sensitive to cytoskeletal disrupting drugs than cells carrying only the bob1delta mutation. Taken together, our results suggest that Bob1 has Byr1-related function(s) required for proper mating response of S. pombe cells and Byrl-independent function(s) required for normal cytoskeletal control. We show that the human MM-1/Pfd5 protein can substitute for its counterpart in fission yeast, providing evidence that the functions of Bob1-related proteins have been highly conserved through evolution. Our results lead us to propose that Bob1-related proteins may play diverse roles in eukaryotic organisms.

Establishment of a cellular axis in fission yeast

Recent studies in fission yeast Schizosaccharomyces pombe reveal how cells establish a cellular axis that specifies domains as the functional 'ends' and 'middle' of the cell. During interphase, dynamic microtubules position the nucleus at the middle of the cell and orientate microtubule 'plus' ends towards the ends of the cell. At the cell ends, the microtubule plus ends might establish a zone of polarized cell growth and actin assembly by depositing factors such as Tea1p. At the cell middle, the nucleus might specify the position of the actin contractile ring and the future cell division site by positioning cytokinesis factors such as Mid1p.

A mechanism for nuclear positioning in fission yeast based on microtubule pushing

The correct positioning of the nucleus is often important in defining the spatial organization of the cell, for example, in determining the cell division plane. In interphase Schizosaccharomyces pombe cells, the nucleus is positioned in the middle of the cylindrical cell in an active microtubule (MT)-dependent process. Here, we used green fluorescent protein markers to examine the dynamics of MTs, spindle pole body, and the nuclear envelope in living cells. We find that interphase MTs are organized in three to four antiparallel MT bundles arranged along the long axis of the cell, with MT plus ends facing both the cell tips and minus ends near the middle of the cell. The MT bundles are organized from medial MT-organizing centers that may function as nuclear attachment sites. When MTs grow to the cell tips, they exert transient forces produced by plus end MT polymerization that push the nucleus. After an average of 1.5 min of growth at the cell tip, MT plus ends exhibit catastrophe and shrink back to the nuclear region before growing back to the cell tip. Computer modeling suggests that a balance of these pushing MT forces can provide a mechanism to position the nucleus at the middle of the cell.

Fission yeast homolog of murine Int-6 protein, encoded by mouse mammary tumor virus integration site, is associated with the conserved core subunits of eukaryotic translation initiation factor 3

The murine int-6 locus, identified as a frequent integration site of mouse mammary tumor viruses, encodes the 48-kDa eIF3e subunit of translation initiation factor eIF3. Previous studies indicated that the catalytically active core of budding yeast eIF3 consists of five subunits, all conserved in eukaryotes, but does not contain a protein closely related to eIF3e/Int-6. Whereas the budding yeast genome does not encode a protein closely related to murine Int-6, fission yeast does encode an Int-6 ortholog, designated here Int6. We found that fission yeast Int6/eIF3e is a cytoplasmic protein associated with 40 S ribosomes. FLAG epitope-tagged Tif35, a putative core eIF3g subunit, copurified with Int6 and all five orthologs of core eIF3 subunits. An int6 deletion (int6Delta) mutant was viable but grew slowly in minimal medium. This slow growth phenotype was accompanied by a reduction in the amount of polyribosomes engaged in translation and was complemented by expression of human Int-6 protein. These findings support the idea that human and Schizosaccharomyces pombe Int-6 homologs are involved in translation. Interestingly, haploid int6Delta cells showed unequal nuclear partitioning, possibly because of a defect in tubulin function, and diploid int6Delta cells formed abnormal spores. We propose that Int6 is not an essential subunit of eIF3 but might be involved in regulating the activity of eIF3 for translation of specific mRNAs in S. pombe.

Yin6, a fission yeast Int6 homolog, complexes with Moe1 and plays a role in chromosome segregation

The INT6 gene has been implicated in human breast cancer formation, but its function is unknown. We isolated an Int6 homolog from fission yeast, Yin6, by its binding to a conserved protein in the Ras pathway, Moe1. Yin6 and Moe1 converge on the same protein complex to promote microtubule instability/disassembly. Yin6 and Moe1 interact cooperatively: when either protein is absent, the other becomes mislocalized with decreased protein levels. Furthermore, whereas full-length human Int6 rescues the phenotypes of the yin6-null (yin6Delta) mutant cells and binds human Moe1, truncated Int6 proteins found in tumors do not. Importantly, yin6Delta alone impairs chromosome segregation weakly, but yin6Delta together with ras1Delta causes severe chromosome missegregation. These data support a model in which INT6 mutations in humans either alone or together with additional mutations, such as a RAS mutation, may contribute to tumorigenesis by altering genome stability.

A conserved interaction between Moe1 and Mal3 is important for proper spindle formation in Schizosaccharomyces pombe

Moe1 is a conserved fission yeast protein that negatively affects microtubule stability/assembly. We conducted a two-hybrid screen to search for Moe1-binding proteins and isolated Mal3, a homologue of human EB1. We show that Moe1 and Mal3 expressed in bacteria form a complex and that Moe1 and Mal3 expressed in fission yeast cosediment with microtubules. Deletion of either moe1 or mal3 does not result in lethality; however, deletion of both moe1 and mal3 leads to cell death in the cold. The resulting cells appear to die of chromosome missegregation, which correlates with the presence of abnormal spindles. We investigated the cause for the formation of monopolar spindles and found that only one of the two spindle pole bodies (SPBs) contains gamma-tubulin, although both SPBs appear to be equal in size and properly inserted in the nuclear membrane. Moreover, the moe1 mal3 double null mutant in the cold contains abnormally short and abundant interphase microtubule bundles. These data suggest that Moe1 and Mal3 play a role in maintaining proper microtubule dynamics/integrity and distribution of gamma-tubulin to the SPBs during mitosis. Finally, we show that human Moe1 and EB1 can each rescue the phenotype of the moe1 mal3 double null mutant and form a complex, suggesting that these proteins are part of a well-conserved mechanism for regulating spindle functioning.

Fission yeast Ras1 effector Scd1 interacts with the spindle and affects its proper formation

Ras1 GTPase is the Schizosaccharomyces pombe homolog of the mammalian Ha-Ras proto-oncoprotein. Ras1 interacts with Scd1 (aka Ral1), a presumptive guanine nucleotide exchange factor for Cdc42sp, to control organization of the cytoskeleton. In this study, we demonstrated that the scd1 deletion (scd1Delta) induced hypersensitivity to microtubule destabilizing drugs and instability of the minichromosome. Overexpression of scd1 induced formation of abnormal spindles and chromosome missegregation. The scd1 deletion worsened the defects of spindle formation in tubulin mutants; by contrast, it did not induce lethality in mutants defective in the spindle pole bodies. These genetic data suggest that Scd1 can interact with tubulin with substantial specificity to affect proper spindle formation and chromosome segregation. Subcellular localization data further illustrated that a GFP-Scd1 fusion protein can associate with the spindle. Finally, we showed that unlike ras1Delta and scd1Delta, byr2Delta (affecting the Ras1 effector for mating) is not synthetically lethal with the tubulin mutations. These data collectively suggest that the Ras1 pathway can impinge upon microtubules through Scd1, but not Byr2, to affect proper spindle formation and chromosome segregation.

Nucleocytoplasmic shuttling of the Cdc42p exchange factor Cdc24p

Cdc24p, the GDP/GTP exchange factor for the regulator of actin cytoskeleton Cdc42p, localizes to sites of polarized growth. Here we show that Cdc24p shuttles in and out of the yeast nucleus during vegetative growth. Far1p is necessary and sufficient for nuclear accumulation of Cdc24p, suggesting that its nuclear import occurs via an association with Far1p. Nuclear export is triggered either by entry into the cell cycle or by mating pheromone. As Far1p is degraded upon entry into the cell cycle, cell cycle-dependent export of Cdc24p occurs in the absence of Far1p, whereas during mating similar export kinetics indicate that a Cdc24p-Far1p complex is exported. Our results suggest that the nucleus serves as a store of preformed Cdc24p-Far1p complex which is required for chemotropism.

Nuclear sequestration of the exchange factor Cdc24 by Far1 regulates cell polarity during yeast mating

Cytoskeletal rearrangements during the cell cycle and in response to signals are regulated by small Rho-type GTPases, but it is not known how these GTPases are activated in a spatial and temporal manner. Here we show that Cdc24, the guanine-nucleotide exchange factor for the yeast GTPase Cdc42, is sequestered in the cell nucleus by Far1. Export of Cdc24 to a site of cell polarization is mediated by two mechanisms. At bud emergence, activation of the G1 cyclin-dependent kinase Cdc28-Cln triggers degradation of Far1 and, as a result, relocation of Cdc24 to the cytoplasm. Cells overexpressing a non-degradable Far1 were unable to polarize their actin cytoskeleton because they failed to relocate Cdc24 to the incipient bud site. In contrast, in response to mating pheromones, the Far1-Cdc24 complex is exported from the nucleus by Msn5. This mechanism ensures that Cdc24 is targeted to the site of receptor-associated heterotrimeric G-protein activation at the plasma membrane, thereby allowing polarization of the actin cytoskeleton along the morphogenetic gradient of pheromone. Either degradation of Far1 or its nuclear export by Msn5 was sufficient for cell growth, suggesting that the two mechanisms are redundant for cell viability. Taken together, our results indicate that Far1 functions as a nuclear anchor for Cdc24. This sequestration regulates cell polarity in response to pheromones by restricting activation of Cdc42 to the site of pheromone receptor activation.

Direct binding and In vivo regulation of the fission yeast p21-activated kinase shk1 by the SH3 domain protein scd2

The Ste20/p21-activated kinase homolog Shk1 is essential for viability and required for normal morphology, mating, and cell cycle control in the fission yeast Schizosaccharomyces pombe. Shk1 is regulated by the p21 G protein Cdc42, which has been shown to form a complex with the SH3 domain protein Scd2 (also called Ral3). In this study, we investigated whether Scd2 plays a role in regulating Shk1 function. We found that recombinant Scd2 and Shk1 interact directly in vitro and that they interact in vivo, as determined by the two-hybrid assay and genetic analyses in fission yeast. The second of two N-terminal SH3 domains of Scd2 is both necessary and sufficient for interaction with Shk1. While full-length Scd2 interacted with only the R1 N-terminal regulatory subdomain of Shk1, a C-terminal deletion mutant of Scd2 interacted with both the R1 and R3 subdomains of Shk1, suggesting that the non-SH3 C-terminal domain of Scd2 may be involved in defining specificity in SH3 binding domain recognition. Overexpression of Scd2 stimulated the autophosphorylation activity of wild-type Shk1 in fission yeast but, consistent with results of genetic analyses, did not stimulate the activity of a Shk1 protein lacking the R1 subdomain. Results of additional two-hybrid experiments suggest that Scd2 may stimulate Shk1 catalytic function, at least in part, by positively modulating protein-protein interaction between Cdc42 and Shk1. We propose that Scd2 functions as an organizing center, or scaffold, for the Cdc42 complex in fission yeast and that it acts in concert with Cdc42 to positively regulate Shk1 function.

The guanine-nucleotide-exchange factor Cdc24p is targeted to the nucleus and polarized growth sites

Generation of cellular asymmetry or cell polarity plays a critical role in cell-cycle-regulated morphogenetic processes involving the actin cytoskeleton. The GTPase Cdc42 regulates actin rearrangements and signal transduction pathways in all eukaryotic cells [1], and the temporal and spatial regulation of Cdc42p depends on the activity and targeting of its guanine-nucleotide exchange factor (GEF). Cdc24p, the Saccharomyces cerevisiae GEF for Cdc42p, is found in a particulate fraction and localizes to the plasma membrane [2] [3] at sites of polarized growth [4]. We show that Cdc24p labeled with green fluorescent protein (GFP-Cdc24p) was targeted to pre-bud sites, the tips and sides of enlarging buds, and mating projections in pheromone-treated cells. Unexpectedly, GFP-Cdc24p also localized to the nucleus and GFP-Cdc24p levels diminished before nuclear division followed by its reappearance in divided nuclei and mother-bud necks during cytokinesis. The Cdc24p amino-terminal 283 amino acids were necessary and sufficient for nuclear localization, which depended on the cyclin-dependent-kinase inhibitor Far1p. The Cdc24p carboxy-terminal 289 amino acids were necessary and sufficient for targeting to the pre-bud site, bud, mother-bud neck, and mating projection. Targeting was independent of the Cdc24p-binding proteins Far1p, the GTPase Rsr1p/Bud1p, the scaffold protein Bem1p, and the G(beta) subunit Ste4p. These data are consistent with a temporal and spatial regulation of Cdc24p-dependent activation of Cdc42p during the cell cycle.