CSE1 and CSE2, two new genes required for accurate mitotic chromosome segregation in Saccharomyces cerevisiae

Dual RNA-Seq of H5N1 Avian Influenza Virus and Host Cell Transcriptomes Reveals Novel Insights Into Host-Pathogen Cross Talk

H5N1 avian influenza virus (AIV) is a highly pathogenic influenza virus that poses a substantial threat to poultry production and public health. A comprehensive understanding of host-pathogen interactions for AIV requires knowledge of gene expression changes in both the pathogen and the host upon infection. We report the use of dual RNA sequencing technology to uncover trends in gene expression in H5N1 AIV and chickens (DF1 cells) during the course of infection. The expression of all viral genes increased continuously from 0 to 20 h post infection. We also identified 2,762 differentially expressed host genes during infection. Pathway analysis found that genes related to the signaling pathways of DNA replication, T cell activation, NF-kappa B signaling pathway, and RNA degradation were significantly enriched. We demonstrated that the cis-acting lncRNA MSTRG.14019.1 targeted CSE1L and may affect virus replication. This study provides a more comprehensive and detailed understanding of host-virus interactions at the RNA level during the course of H5N1 AIV infection.

Dissecting the roles of Cse1 and Nup2 in classical NLS-cargo release in vivo

The importin α/β transport machinery mediates the nuclear import of cargo proteins that bear a classical nuclear localization sequence (cNLS). These cargo proteins are linked to the major nuclear protein import factor, importin‐β, by the importin‐α adapter, after which cargo/carrier complexes enter the nucleus through nuclear pores. In the nucleus, cargo is released by the action of RanGTP and the nuclear pore protein Nup2, after which the importins are recycled to the cytoplasm for further transport cycles. The nuclear export of importin‐α is mediated by Cse1/CAS. Here, we exploit structures of functionally important complexes to identify residues that are critical for these interactions and provide insight into how cycles of protein import and recycling of importin‐α occur in vivo using a Saccharomyces cerevisiae model. We examine how these molecular interactions impact protein localization, cargo import, function and complex formation. We show that reversing the charge of key residues in importin‐α (Arg44) or Cse1 (Asp220) results in loss of function of the respective proteins and impairs complex formation both in vitro and in vivo. To extend these results, we show that basic residues in the Nup2 N‐terminus are required for both Nup2 interaction with importin‐α and Nup2 function. These results provide a more comprehensive mechanistic model of how Cse1, RanGTP and Nup2 function in concert to mediate cNLS‐cargo release in the nucleus.

Directional transport of cargoes between the nucleus and cytoplasm is mediated by receptors that bind cargo in one compartment and release cargo into a destination compartment. Cargoes that contain a cNLS are recognized by importin‐α in the cytoplasm. Release factors including the importin‐α export receptor, Cse1, and a nuclear pore complex protein, Nup2, ensure efficient cargo delivery into the nucleus. Interactions defined by previous structural studies are required for productive interactions between importin‐α, Cse1, and Nup2 to occur in vivo.

Septin-associated proteins Aim44 and Nis1 traffic between the bud neck and the nucleus in the yeast Saccharomyces cerevisiae

In budding yeast, a collar of septin filaments at the neck between a mother cell and its bud marks the incipient site for cell division and serves as a scaffold that recruits proteins required for proper spatial and temporal execution of cytokinesis. A set of interacting proteins that localize at or near the bud neck, including Aim44/Gps1, Nba1 and Nis1, also has been implicated in preventing Cdc42‐dependent bud site re‐establishment at the division site. We found that, at their endogenous level, Aim44 and Nis1 robustly localize sequentially at the septin collar. Strikingly, however, when overproduced, both proteins shift their subcellular distribution predominantly to the nucleus. Aim44 localizes with the inner nuclear envelope, as well as at the plasma membrane, whereas Nis1 accumulates within the nucleus, indicating that these proteins normally undergo nucleocytoplasmic shuttling. Of the 14 yeast karyopherins, Kap123/Yrb4 is the primary importin for Aim44, whereas several importins mediate Nis1 nuclear entry. Conversely, Kap124/Xpo1/Crm1 is the primary exportin for Nis1, whereas both Xpo1 and Cse1/Kap109 likely contribute to Aim44 nuclear export. Even when endogenously expressed, Nis1 accumulates in the nucleus when Nba1 is absent. When either Aim44 or Nis1 are overexpressed, Nba1 is displaced from the bud neck, further consistent with the mutual interactions of these proteins. Collectively, our results indicate that a previously unappreciated level at which localization of septin‐associated proteins is controlled is via regulation of their nucleocytoplasmic shuttling, which places constraints on their availability for complex formation with other partners at the bud neck.

Regulation of Budding Yeast CENP-A levels Prevents Misincorporation at Promoter Nucleosomes and Transcriptional Defects

The exclusive localization of the histone H3 variant CENP-A to centromeres is essential for accurate chromosome segregation. Ubiquitin-mediated proteolysis helps to ensure that CENP-A does not mislocalize to euchromatin, which can lead to genomic instability. Consistent with this, overexpression of the budding yeast CENP-A^Cse4 is lethal in cells lacking Psh1, the E3 ubiquitin ligase that targets CENP-A^Cse4 for degradation. To identify additional mechanisms that prevent CENP-A^Cse4 misincorporation and lethality, we analyzed the genome-wide mislocalization pattern of overexpressed CENP-A^Cse4 in the presence and absence of Psh1 by chromatin immunoprecipitation followed by high throughput sequencing. We found that ectopic CENP-A^Cse4 is enriched at promoters that contain histone H2A.Z^Htz1 nucleosomes, but that H2A.Z^Htz1 is not required for CENP-A^Cse4 mislocalization. Instead, the INO80 complex, which removes H2A.Z^Htz1 from nucleosomes, promotes the ectopic deposition of CENP-A^Cse4. Transcriptional profiling revealed gene expression changes in the psh1Δ cells overexpressing CENP-A^Cse4. The down-regulated genes are enriched for CENP-A^Cse4 mislocalization to promoters, while the up-regulated genes correlate with those that are also transcriptionally up-regulated in an htz1Δ strain. Together, these data show that regulating centromeric nucleosome localization is not only critical for maintaining centromere function, but also for ensuring accurate promoter function and transcriptional regulation.

Chromosomes carry the genetic material in cells. When cells divide, each daughter cell must inherit a single copy of each chromosome. The centromere is the locus on each chromosome that ensures the equal distribution of chromosomes during cell division. One essential protein involved in this task is CENP-A^Cse4, which normally localizes exclusively to centromeres. Here, we investigated where CENP-A^Cse4 spreads in the genome when parts of its regulatory machinery are removed. We found that CENP-A^Cse4 becomes mislocalized to promoters, the region upstream of each gene that controls the activity of the gene. Consistent with this, the mislocalization of CENP-A^Cse4 to promoters leads to problems with gene activity. Our work shows that mislocalization of centromeric proteins can have effects beyond chromosome segregation defects, such as interfering with gene expression on chromosome arms.

The Cellular Apoptosis Susceptibility Protein (CAS) Promotes Tumor Necrosis Factor-related Apoptosis-inducing Ligand (TRAIL)-induced Apoptosis and Cell Proliferation

A signature event during the cell intrinsic apoptotic pathway is mitochondrial outer membrane permeabilization, leading to formation of the apoptosome, a caspase activation complex. The cellular apoptosis susceptibility protein (CAS) can facilitate apoptosome assembly by stimulating nucleotide exchange on Apaf-1 following binding of cytochrome c. We report here that CAS expression itself is up-regulated during tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis, and knockdown of CAS renders cells resistant to TRAIL. We find that TRAIL induces up-regulation of CAS in a posttranscriptional, caspase-8-dependent manner through degradation of cIAP1, an E3 ligase that targets CAS for ubiquitin-dependent proteasomal degradation. We identified a novel signaling pathway whereby caspase-8 engages a feedforward cascade that leads to CAS up-regulation and amplifies the apoptotic signal. Furthermore, in silico analysis revealed that expression of CAS is up-regulated at both the mRNA and DNA levels in human breast tumors, consistent with its role in promoting cell proliferation. Overexpression of various oncogenes led to CAS up-regulation in non-transformed cells. Intriguingly, oncogene-induced CAS up-regulation also resulted in greater susceptibility to TRAIL-induced cell death, consistent with its proapoptotic function. These findings suggest that CAS plays contrasting roles in proliferation and apoptosis and that overexpression of CAS in tumors could serve as a potential biomarker to guide therapeutic choices.

Genome rearrangements caused by depletion of essential DNA replication proteins in Saccharomyces cerevisiae

Genetic screens of the collection of ~4500 deletion mutants in Saccharomyces cerevisiae have identified the cohort of nonessential genes that promote maintenance of genome integrity. Here we probe the role of essential genes needed for genome stability. To this end, we screened 217 tetracycline-regulated promoter alleles of essential genes and identified 47 genes whose depletion results in spontaneous DNA damage. We further showed that 92 of these 217 essential genes have a role in suppressing chromosome rearrangements. We identified a core set of 15 genes involved in DNA replication that are critical in preventing both spontaneous DNA damage and genome rearrangements. Mapping, classification, and analysis of rearrangement breakpoints indicated that yeast fragile sites, Ty retrotransposons, tRNA genes, early origins of replication, and replication termination sites are common features at breakpoints when essential replication genes that suppress chromosome rearrangements are downregulated. We propose mechanisms by which depletion of essential replication proteins can lead to double-stranded DNA breaks near these features, which are subsequently repaired by homologous recombination at repeated elements.

Origins and activity of the Mediator complex

The Mediator is a large, multisubunit RNA polymerase II transcriptional regulator that was first identified in Saccharomyces cerevisiae as a factor required for responsiveness of Pol II and the general initiation factors to DNA binding transactivators. Since its discovery in yeast, Mediator has been shown to be an integral and highly evolutionarily conserved component of the Pol II transcriptional machinery with critical roles in multiple stages of transcription, from regulation of assembly of the Pol II initiation complex to regulation of Pol II elongation. Here we provide a brief overview of the evolutionary origins of Mediator, its subunit composition, and its remarkably diverse collection of activities in Pol II transcription.

Cse1l is a negative regulator of CFTR-dependent fluid secretion

Transport of chloride through the cystic fibrosis transmembrane conductance regulator (CFTR) channel is a key step in regulating fluid secretion in vertebrates [1, 2]. Loss of CFTR function leads to cystic fibrosis [1, 3, 4], a disease that affects the lungs, pancreas, liver, intestine, and vas deferens. Conversely, uncontrolled activation of the channel leads to increased fluid secretion and plays a major role in several diseases and conditions including cholera [5, 6] and other secretory diarrheas [7] as well as polycystic kidney disease [8-10]. Understanding how CFTR activity is regulated in vivo has been limited by the lack of a genetic model. Here, we used a forward genetic approach in zebrafish to uncover CFTR regulators. We report the identification, isolation, and characterization of a mutation in the zebrafish cse1l gene that leads to the sudden and dramatic expansion of the gut tube. We show that this phenotype results from a rapid accumulation of fluid due to the uncontrolled activation of the CFTR channel. Analyses in zebrafish larvae and mammalian cells indicate that Cse1l is a negative regulator of CFTR-dependent fluid secretion. This work demonstrates the importance of fluid homeostasis in development and establishes the zebrafish as a much-needed model system to study CFTR regulation in vivo.

Cellular apoptosis susceptibility (CSE1L/CAS) protein in cancer metastasis and chemotherapeutic drug-induced apoptosis

The cellular apoptosis susceptibility (CSE1L/CAS) protein is highly expressed in cancer, and its expression is positively correlated with high cancer stage, high cancer grade, and worse outcomes of patients. CSE1L (or CAS) regulates chemotherapeutic drug-induced cancer cell apoptosis and may play important roles in mediating the cytotoxicities of chemotherapeutic drugs against cancer cells in cancer chemotherapy. CSE1L was originally regarded as a proliferation-associated protein and was thought to regulate the proliferation of cancer cells in cancer progression. However, the results of experimental studies showed that enhanced CSE1L expression is unable to increase proliferation of cancer cells and CSE1L regulates invasion and metastasis but not proliferation of cancer cells. Recent studies revealed that CSE1L is a secretory protein, and there is a higher prevalence of secretory CSE1L in the sera of patients with metastatic cancer. Therefore, CSE1L may be a useful serological marker for screening, diagnosis and prognosis, assessment of therapeutic responses, and monitoring for recurrence of cancer. In this paper, we review the expression of CSE1L in cancer and discuss why CSE1L regulates the invasion and metastasis rather than the proliferation of cancer.

Connecting mutations of the RNA polymerase II C-terminal domain to complex phenotypic changes using combined gene expression and network analyses

The C-terminal domain (CTD) of the largest subunit in DNA-dependent RNA polymerase II (RNAP II) is essential for mRNA synthesis and processing, through coordination of an astounding array of protein-protein interactions. Not surprisingly, CTD mutations can have complex, pleiotropic impacts on phenotype. For example, insertions of five alanine residues between CTD diheptads in yeast, which alter the CTD's overall tandem structure and physically separate core functional units, dramatically reduce growth rate and result in abnormally large cells that accumulate increased DNA content over time. Patterns by which specific CTD-protein interactions are disrupted by changes in CTD structure, as well as how downstream metabolic pathways are impacted, are difficult to target for direct experimental analyses. In an effort to connect an altered CTD to complex but quantifiable phenotypic changes, we applied network analyses of genes that are differentially expressed in our five alanine CTD mutant, combined with established genetic interactions from the Saccharomyces cerevisiae Genome Database (SGD). We were able to identify candidate genetic pathways, and several key genes, that could explain how this change in CTD structure leads to the specific phenotypic changes observed. These hypothetical networks identify links between CTD-associated proteins and mitotic function, control of cell cycle checkpoint mechanisms, and expression of cell wall and membrane components. Such results can help to direct future genetic and biochemical investigations that tie together the complex impacts of the CTD on global cellular metabolism.

Yeast karyopherin Kap95 is required for cell cycle progression at Start

Background

The control of the subcellular localization of cell cycle regulators has emerged as a crucial mechanism in cell division regulation. The active transport of proteins between the nucleus and the cytoplasm is mediated by the transport receptors of the β-karyopherin family. In this work we characterized the terminal phenotype of a mutant strain in β-karyopherin Kap95, a component of the classical nuclear import pathway.

Results

When KAP95 was inactivated, most cells arrested at the G2/M phase of the cell cycle, which is in agreement with the results observed in mutants in the other components of this pathway. However, a number of cells accumulate at G1, suggesting a novel role of Kap95 and the classical import pathway at Start. We investigated the localization of Start transcription factors. It is known that Swi6 contains a classical NLS that interacts with importin α. Here we show that the in vivo nuclear import of Swi6 depends on Kap95. For Swi4, we identified a functional NLS between amino acids 371 and 376 that is sufficient and necessary for Swi4 to enter the nucleus. The nuclear import driven by this NLS is mediated by karyopherins Kap95 and Srp1. Inactivation of Kap95 also produces a dramatic change in the localization of Mbp1 since the protein is mainly detected in the cytoplasm. Two functionally redundant Kap95- and Srp1-dependent NLSs were identified in Mbp1 between amino acids 27-30 and 166-181. Nuclear accumulation was not completely abolished in a kap95 mutant or in the Mbp1 mutated in the two NLSs, suggesting that alternative pathways might contribute to the Mbp1 nuclear import to a lesser extent.

Conclusions

Kap95 plays an essential role at the initiation of the cell cycle by driving the nuclear import of Swi4, Swi6 and Mbp1, the three transcription factors responsible for the gene expression at Start. This transport depends on the specific nuclear localization signals present in cargo proteins.

A systematic screen for transcriptional regulators of the yeast cell cycle

Transcription factors play a key role in the regulation of cell cycle progression, yet many of the specific regulatory interactions that control cell cycle transcription are still unknown. To systematically identify new yeast cell cycle transcription factors, we used a quantitative flow cytometry assay to screen 268 transcription factor deletion strains for defects in cell cycle progression. Our results reveal that 20% of nonessential transcription factors have an impact on cell cycle progression, including several recently identified cyclin-dependent kinase (Cdk) targets, which have not previously been linked to cell cycle transcription. This expanded catalog of cell-cycle-associated transcription factors will be a valuable resource for decoding the transcriptional regulatory interactions that govern progression through the cell cycle. We conducted follow-up studies on Sfg1, a transcription factor with no previously known role in cell cycle progression. Deletion of Sfg1 retards cells in G(1), and overexpression of Sfg1 delays cells in the G(2)/M phase. We find that Sfg1 represses early G(1), Swi5/Ace2-regulated genes involved in mother-daughter cell separation. We also show that Sfg1, a known in vitro cyclin-dependent kinase target, is phosphorylated in vivo on conserved Cdk phosphorylation sites and that phosphorylation of Sfg1 is necessary for its role in promoting cell cycle progression. Overall, our work increases the number of transcription factors associated with cell cycle progression, strongly indicates that there are many more unexplored connections between the Cdk-cyclin oscillator and cell cycle transcription, and suggests a new mechanism for the regulation of cell separation during the M/G(1) phase transition.

A protein interaction map of the mitotic spindle

The mitotic spindle consists of a complex network of proteins that segregates chromosomes in eukaryotes. To strengthen our understanding of the molecular composition, organization, and regulation of the mitotic spindle, we performed a system-wide two-hybrid screen on 94 proteins implicated in spindle function in Saccharomyces cerevisiae. We report 604 predominantly novel interactions that were detected in multiple screens, involving 303 distinct prey proteins. We uncovered a pattern of extensive interactions between spindle proteins reflecting the intricate organization of the spindle. Furthermore, we observed novel connections between kinetochore complexes and chromatin-modifying proteins and used phosphorylation site mutants of NDC80/TID3 to gain insights into possible phospho-regulation mechanisms. We also present analyses of She1p, a novel spindle protein that interacts with the Dam1 kinetochore/spindle complex. The wealth of protein interactions presented here highlights the extent to which mitotic spindle protein functions and regulation are integrated with each other and with other cellular activities.

A survey of essential gene function in the yeast cell division cycle

Mutations impacting specific stages of cell growth and division have provided a foundation for dissecting mechanisms that underlie cell cycle progression. We have undertaken an objective examination of the yeast cell cycle through flow cytometric analysis of DNA content in TetO(7) promoter mutant strains representing 75% of all essential yeast genes. More than 65% of the strains displayed specific alterations in DNA content, suggesting that reduced function of an essential gene in most cases impairs progression through a specific stage of the cell cycle. Because of the large number of essential genes required for protein biosynthesis, G1 accumulation was the most common phenotype observed in our analysis. In contrast, relatively few mutants displayed S-phase delay, and most of these were defective in genes required for DNA replication or nucleotide metabolism. G2 accumulation appeared to arise from a variety of defects. In addition to providing a global view of the diversity of essential cellular processes that influence cell cycle progression, these data also provided predictions regarding the functions of individual genes: we identified four new genes involved in protein trafficking (NUS1, PHS1, PGA2, PGA3), and we found that CSE1 and SMC4 are important for DNA replication.

Importin-beta family members mediate alpharetrovirus gag nuclear entry via interactions with matrix and nucleocapsid

The retroviral Gag polyprotein orchestrates the assembly and release of virus particles from infected cells. We previously reported that nuclear transport of the Rous sarcoma virus (RSV) Gag protein is intrinsic to the virus assembly pathway. To identify cis- and trans-acting factors governing nucleocytoplasmic trafficking, we developed novel vectors to express regions of Gag in Saccharomyces cerevisiae. The localization of Gag proteins was examined in the wild type and in mutant strains deficient in members of the importin-beta family. We confirmed the Crm1p dependence of the previously identified Gag p10 nuclear export signal. The known nuclear localization signal (NLS) in MA (matrix) was also functional in S. cerevisiae, and additionally we discovered a novel NLS within the NC (nucleocapsid) domain of Gag. MA utilizes Kap120p and Mtr10p import receptors while nuclear entry of NC involves the classical importin-alpha/beta (Kap60p/95p) pathway. NC also possesses nuclear targeting activity in avian cells and contains the primary signal for the import of the Gag polyprotein. Thus, the nucleocytoplasmic dynamics of RSV Gag depend upon the counterbalance of Crm1p-mediated export with two independent NLSs, each interacting with distinct nuclear import factors.

Yeast phosphatidylinositol 4-kinase, Pik1, has essential roles at the Golgi and in the nucleus

Phosphatidylinositol 4-kinase, Pik1, is essential for viability. GFP-Pik1 localized to cytoplasmic puncta and the nucleus. The puncta colocalized with Sec7-DsRed, a marker of trans-Golgi cisternae. Kap95 (importin-beta) was necessary for nuclear entry, but not Kap60 (importin-alpha), and exportin Msn5 was required for nuclear exit. Frq1 (frequenin orthologue) also is essential for viability and binds near the NH2 terminus of Pik1. Frq1-GFP localized to Golgi puncta, and Pik1 lacking its Frq1-binding site (or Pik1 overexpressed in frq1Delta cells) did not decorate the Golgi, but nuclear localization was unperturbed. Pik1(Delta10-192), which lacks its nuclear export sequence, displayed prominent nuclear accumulation and did not rescue inviability of pik1Delta cells. A Pik1-CCAAX chimera was excluded from the nucleus and also did not rescue inviability of pik1Delta cells. However, coexpression of Pik1(Delta10-192) and Pik1-CCAAX in pik1Delta cells restored viability. Catalytically inactive derivatives of these compartment-restricted Pik1 constructs indicated that PtdIns4P must be generated both in the nucleus and at the Golgi for normal cell function.

Importin 7 and importin alpha/importin beta are nuclear import receptors for the glucocorticoid receptor

The vertebrate glucocorticoid receptor (GR) is cytoplasmic without hormone and localizes to the nucleus after hormone binding. GR has two nuclear localization signals (NLS): NL1 is similar in sequence to the SV40 NLS; NL2 is poorly defined, residing in the ligand-binding domain. We found that GR displayed similar hormone-regulated compartmentalization in Saccharomyces cerevisiae and required the Sxm1 nuclear import receptor for NL2-mediated import. Two metazoan homologues of Sxm1, importin 7 and importin 8, bound both NL1 and NL2, whereas importin alpha selectively bound NL1. In an in vitro nuclear import assay, both importin 7 and the importin alpha-importin beta heterodimer could import a GR NL1 fragment. Under these conditions, full-length GR localized to nuclei in the presence but not absence of an unidentified component in cell extracts. Interestingly, importin 7, importin 8, and importin alpha bound GR even in the absence of hormone; thus, hormonal control of localization is exerted at a step downstream of import receptor binding.

Regulation of PI4,5P2 synthesis by nuclear-cytoplasmic shuttling of the Mss4 lipid kinase

The essential phospholipid PI4,5P(2) is generated by a well conserved PI4P 5-kinase, Mss4, in yeast. Balanced production and turnover of PI4,5P(2) is important for normal organization of the actin cytoskeleton and cell viability. Previous studies have shown that multiple PI phosphatases can regulate PI4,5P(2) levels. We report a new, unexpected regulatory mechanism for PI4,5P(2) homeostasis, directed by nuclear-cytoplasmic shuttling of the lipid kinase. We show that Mss4 is a phosphoprotein, which contains a functional nuclear localization signal (NLS) and can shuttle between the cytoplasm and the nucleus. Temperature-conditional mss4 cells that accumulate Mss4 protein in the nucleus exhibit reduced levels of PI4,5P(2), depolarization of the actin cytoskeleton and a block in Mss4 phosphorylation, suggesting an essential role for phosphorylated Mss4 at the plasma membrane. Through the isolation of gene dosage-dependent suppressors of mss4 mutants, we identified Bcp1, a protein enriched in the nucleus, which is required for Mss4 nuclear export and is related to the mammalian BRCA2-interacting protein BCCIP. Together, these studies suggest a new mechanism for lipid kinase regulation through regulated nuclear-cytoplasmic shuttling.

A Los1p-independent pathway for nuclear export of intronless tRNAs in Saccharomycescerevisiae

Los1p, the Saccharomyces cerevisiae exportin-t homologue, binds tRNA and functions in pre-tRNA splicing and export of mature tRNA from the nucleus to the cytosol. Because LOS1 is unessential in yeast, other pathways for tRNA nuclear export must exist. We report that Cca1p, which adds nucleotides C, C, and A to the 3' end of tRNAs, is a multicopy suppressor of the defect in tRNA nuclear export caused by los1 null mutations. Mes1p, methionyl-tRNA synthetase, also suppresses the defect in nuclear export of tRNA(Met) in los1 cells. Thus, Cca1p and Mes1p seem to function in a Los1p-independent tRNA nuclear export pathway. Heterokaryon analysis indicates that Cca1p is a nucleus/cytosol-shuttling protein, providing the potential for Cca1p to function as an exporter or an adapter in this tRNA nuclear export pathway. In yeast, most mutations that affect tRNA nuclear export also cause defects in pre-tRNA splicing leading to tight coupling of the splicing and export processes. In contrast, we show that overexpressed Cca1p corrects the nuclear export, but not the pre-tRNA-splicing defects of los1Kan(r) cells, thereby uncoupling pre-tRNA splicing and tRNA nuclear export.

Cse1l is essential for early embryonic growth and development

The CSE1L gene, the human homologue of the yeast chromosome segregation gene CSE1, is a nuclear transport factor that plays a role in proliferation as well as in apoptosis. CSE1 and CSE1L are essential genes in Saccharomyces cerevisiae and mammalian cells, as shown by conditional yeast mutants and mammalian cell culture experiments with antisense-mediated depletion of CSE1L. To analyze whether CSE1L is also essential in vivo and whether its absence can be compensated for by other genes or mechanisms, we have cloned the murine CSE1L gene (Cse1l) and analyzed its tissue- and development-specific expression: Cse1l was detected at embryonic day 7.0 (E7.0), E11.0, E15.0, and E17.0, and in adults, high expression was observed in proliferating tissues. Subsequently, we inactivated the Cse1l gene in embryonic stem cells to generate heterozygous and homozygous knockout mice. Mice heterozygous for Cse1l appear normal and are fertile. However, no homozygous pups were born after interbreeding of heterozygous mice. In 30 heterozygote interbreeding experiments, 50 Cse1l wild-type mice and 100 heterozygotes were born but no animal with both Cse1l alleles deleted was born. Embryo analyses showed that homozygous mutant embryos were already disorganized and degenerated by E5.5. This implicates with high significance (P < 0.0001, Pearson chi-square test) an embryonically lethal phenotype of homozygous murine CSE1 deficiency and suggests that Cse1l plays a critical role in early embryonic development.

Calcineurin-dependent nuclear import of the transcription factor Crz1p requires Nmd5p

Calcineurin is a conserved Ca2+/calmodulin-specific serine-threonine protein phosphatase that mediates many Ca2+-dependent signaling events. In yeast, calcineurin dephosphorylates Crz1p, a transcription factor that binds to the calcineurin-dependent response element, a 24-bp promoter element. Calcineurin-dependent dephosphorylation of Crz1p alters Crz1p nuclear localization. This study examines the mechanism by which calcineurin regulates the nuclear localization of Crz1p in more detail. We describe the identification and characterization of a novel nuclear localization sequence (NLS) in Crz1p, which requires both basic and hydrophobic residues for activity, and show that the karyopherin Nmd5p is required for Crz1p nuclear import. We also demonstrate that the binding of Crz1p to Nmd5p is dependent upon its phosphorylation state, indicating that nuclear import of Crz1p is regulated by calcineurin. Finally, we demonstrate that residues in both the NH2- and COOH-terminal portions of Crz1p are required for regulated Crz1p binding to Nmd5p, supporting a model of NLS masking for regulating Crz1p nuclear import.

The yeast hnRNP-Like proteins Yra1p and Yra2p participate in mRNA export through interaction with Mex67p

Yra1p is an essential nuclear protein which belongs to the evolutionarily conserved REF (RNA and export factor binding proteins) family of hnRNP-like proteins. Yra1p contributes to mRNA export in vivo and directly interacts with RNA and the shuttling mRNP export receptor Mex67p in vitro. Here we describe a second nonessential Saccharomyces cerevisiae family member, called Yra2p, which is able to complement a YRA1 deletion when overexpressed. Like other REF proteins, Yra1p and Yra2p consist of two highly conserved N- and C-terminal boxes and a central RNP-like RNA-binding domain (RBD). These conserved regions are separated by two more variable regions, N-vr and C-vr. Surprisingly, the deletion of a single conserved box or the deletion of the RBD in Yra1p does not affect viability. Consistently, neither the conserved N and C boxes nor the RBD is required for Mex67p and RNA binding in vitro. Instead, the N-vr and C-vr regions both interact with Mex67p and RNA. We further show that Yra1 deletion mutants which poorly interact with Mex67p in vitro affect the association of Mex67p with mRNP complexes in vivo and are paralleled by poly(A)(+) RNA export defects. These observations support the idea that Yra1p promotes mRNA export by facilitating the recruitment of Mex67p to the mRNP.

Factors affecting nuclear export of the 60S ribosomal subunit in vivo

In Saccharomyces cerevisiae, the 60S ribosomal subunit assembles in the nucleolus and then is exported to the cytoplasm, where it joins the 40S subunit for translation. Export of the 60S subunit from the nucleus is known to be an energy-dependent and factor-mediated process, but very little is known about the specifics of its transport. To begin to address this problem, an assay was developed to follow the localization of the 60S ribosomal subunit in S. cerevisiae. Ribosomal protein L11b (Rpl11b), one of the approximately 45 ribosomal proteins of the 60S subunit, was tagged at its carboxyl terminus with the green fluorescent protein (GFP) to enable visualization of the 60S subunit in living cells. A panel of mutant yeast strains was screened for their accumulation of Rpl11b-GFP in the nucleus as an indicator of their involvement in ribosome synthesis and/or transport. This panel included conditional alleles of several rRNA-processing factors, nucleoporins, general transport factors, and karyopherins. As predicted, conditional alleles of rRNA-processing factors that affect 60S ribosomal subunit assembly accumulated Rpl11b-GFP in the nucleus. In addition, several of the nucleoporin mutants as well as a few of the karyopherin and transport factor mutants also mislocalized Rpl11b-GFP. In particular, deletion of the previously uncharacterized karyopherin KAP120 caused accumulation of Rpl11b-GFP in the nucleus, whereas ribosomal protein import was not impaired. Together, these data further define the requirements for ribosomal subunit export and suggest a biological function for KAP120.

Evidence for Gal3p's cytoplasmic location and Gal80p's dual cytoplasmic-nuclear location implicates new mechanisms for controlling Gal4p activity in Saccharomyces cerevisiae

Genetics and in vitro studies have shown that the direct interaction between Gal3p and Gal80p plays a central role in galactose-dependent Gal4p-mediated GAL gene expression in the yeast Saccharomyces cerevisiae. Precisely how Gal3p-Gal80p interaction effects induction is not clear. It has been assumed that Gal3p interacts with Gal80p in the nucleus upon galactose addition to release Gal80p inhibition of Gal4p. Although Gal80p has been shown to possess nuclear localization signal (NLS) peptides, the subcellular distribution of neither Gal80p nor Gal3p was previously determined. Here we report that Gal3p is located in the cytoplasm and apparently excluded from the nucleus. We show that Gal80p is located in both the cytoplasm and the nucleus. Converting Gal80p into a nucleus-localized protein (NLS-Gal80p) by exogenous NLS addition impairs GAL gene induction. The impaired induction can be partially suppressed by targeting Gal3p to the nucleus (NLS-Gal3p). We document a very rapid association between NLS-Gal3p and Gal80p in vivo in response to galactose, illustrating that the nuclear import of Gal80p is very rapid and efficient. We also demonstrate that nucleus-localized NLS-Gal80p can move out of the nucleus and shuttle between nuclei in yeast heterokaryons. These results are the first indication that the subcellular distribution dynamics of the Gal3 and Gal80 proteins play a role in regulating Gal4p-mediated GAL gene expression in vivo.

Yeast Ran-binding protein 1 (Yrb1) shuttles between the nucleus and cytoplasm and is exported from the nucleus via a CRM1 (XPO1)-dependent pathway

The RanGTP-binding protein RanBP1, which is located in the cytoplasm, has been implicated in release of nuclear export complexes from the cytoplasmic side of the nuclear pore complex. Here we show that Yrb1 (the yeast homolog of RanBP1) shuttles between the nucleus and the cytoplasm. Nuclear import of Yrb1 is a facilitated process that requires a short basic sequence within the Ran-binding domain (RBD). By contrast, nuclear export of Yrb1 requires an intact RBD, which forms a ternary complex with the Xpo1 (Crm1) NES receptor in the presence of RanGTP. Nuclear export of Yrb1, however, is insensitive towards leptomycin B, suggesting a novel type of substrate recognition between Yrb1 and Xpo1. Taken together, these data suggest that ongoing nuclear import and export is an important feature of Yrb1 function in vivo.

Nuclear export of the small ribosomal subunit requires the ran-GTPase cycle and certain nucleoporins

After their assembly in the nucleolus, ribosomal subunits are exported from the nucleus to the cytoplasm. After export, the 20S rRNA in the small ribosomal subunit is cleaved to yield 18S rRNA and the small 5' ITS1 fragment. The 5' ITS1 RNA is normally degraded by the cytoplasmic Xrn1 exonuclease, but in strains lacking XRN1, the 5' ITS1 fragment accumulates in the cytoplasm. Using the cytoplasmic localization of the 5' ITS1 fragment as an indicator for the export of the small ribosomal subunit, we have identified genes that are required for ribosome export. Ribosome export is dependent on the Ran-GTPase as mutations in Ran or its regulators caused 5' ITS1 to accumulate in the nucleoplasm. Mutations in the genes encoding the nucleoporin Nup82 and in the NES exporter Xpo1/Crm1 also caused the nucleoplasmic accumulation of 5' ITS1. Mutants in a subset of nucleoporins and in the nuclear transport factors Srp1, Kap95, Pse1, Cse1, and Mtr10 accumulate the 5' ITS1 in the nucleolus and affect ribosome assembly. In contrast, we did not detect nuclear accumulation of 5' ITS1 in 28 yeast strains that have mutations in other genes affecting nuclear trafficking.

New yeast genes important for chromosome integrity and segregation identified by dosage effects on genome stability

Phenotypes produced by gene overexpression may provide important clues to gene function. Here, we have performed a search for genes that affect chromo-some stability when overexpressed in the budding yeast Saccharomyces cerevisiae. We have obtained clones encompassing 30 different genes. Twenty-four of these genes have been previously characterized. Most of them are involved in chromatin dynamics, cell cycle control, DNA replication or mitotic chromosome segregation. Six novel genes obtained in this screen were named CST (chromosome stability). Based on the pattern of genomic instability, inter-action with checkpoint mutations and sensitivity to chromosome replication or segregation inhibitors, we conclude that overexpression of CST4 specifically interferes with mitotic chromosome segregation, and CST6 affects some aspect of DNA metabolism. The other CST genes had complex pleiotropic phenotypes. We have created deletions of five genes obtained in this screen, CST9, CST13, NAT1, SBA1 and FUN30. None of these genes is essential for viability, and deletions of NAT1 and SBA1 cause chromosome instability, a phenotype not previously associated with these genes. This work shows that analysis of dosage effects is complementary to mutational analysis of chromosome transmission fidelity, as it allows the identification of chromosome stability genes that have not been detected in mutational screens.

Nuclear import of the TATA-binding protein: mediation by the karyopherin Kap114p and a possible mechanism for intranuclear targeting

Binding of the TATA-binding protein (TBP) to the promoter is the first and rate limiting step in the formation of transcriptional complexes. We show here that nuclear import of TBP is mediated by a new karyopherin (Kap) (importin) family member, Kap114p. Kap114p is localized to the cytoplasm and nucleus. A complex of Kap114p and TBP was detected in the cytosol and could be reconstituted using recombinant proteins, suggesting that the interaction was direct. Deletion of the KAP114 gene led to specific mislocalization of TBP to the cytoplasm. We also describe two other potential minor import pathways for TBP. Consistent with other Kaps, the dissociation of TBP from Kap114p is dependent on RanGTP. However, we could show that double stranded, TATA-containing DNA stimulates this RanGTP-mediated dissociation of TBP, and is necessary at lower RanGTP concentrations. This suggests a mechanism where, once in the nucleus, TBP is preferentially released from Kap114p at the promoter of genes to be transcribed. In this fashion Kap114p may play a role in the intranuclear targeting of TBP.

A Crm1p-independent nuclear export path for the mRNA-associated protein, Npl3p/Mtr13p

mRNA export involves association of mRNAs with nucleoplasmic proteins, delivery to the nuclear pore complex, translocation to the cytoplasm, and reimport of recycling components. Many yeast mutants inhibit mRNA export, but there is little information concerning the RNA carriers and steps of transport that they affect. The hnRNP/serine-arginine-rich-like protein, Npl3p/Mtr13p, binds poly(A)+ RNA and shuttles between the nucleus and cytoplasm. Its export accelerates on inhibition of RNA synthesis. In vivo tests show that its export requires two proteins with putative leucine-rich nuclear export signals: Gle1p, Mex67p, and several additional nuclear and nuclear pore complex-associated proteins. Surprisingly, a nonnuclear pool of an import factor (the importin alpha homologue, Srp1p) is also required. Changes in the methylation status of Npl3p do not correlate with its nucleocytoplasmic distribution. A crm1 mutant that inhibits export of proteins with leucine-rich nuclear export signals and mRNAs does not inhibit Npl3p export. Moreover, several proteins needed for Npl3p export are not needed for export of a typical Crm1p cargo. Thus, Npl3p export requires only a subset of proteins implicated in mRNA export, suggesting that more than one mRNA export path exists. A distinct group of mutants, including a mutation of a member of the importin beta superfamily, inhibits Npl3p reimport from the cytoplasm.

Activator-specific requirement of yeast mediator proteins for RNA polymerase II transcriptional activation

The multisubunit Mediator complex of Saccharomyces cerevisiae is required for most RNA polymerase II (Pol II) transcription. The Mediator complex is composed of two subcomplexes, the Rgr1 and Srb4 subcomplexes, which appear to function in the reception of activator signals and the subsequent modulation of Pol II activity, respectively. In order to determine the precise composition of the Mediator complex and to explore the specific role of each Mediator protein, our goal was to identify all of the Mediator components. To this end, we cloned three previously unidentified Mediator subunits, Med9/Cse2, Med10/Nut2, and Med11, and isolated mutant forms of each of them to analyze their transcriptional defects. Differential display and Northern analyses of mRNAs from wild-type and Mediator mutant cells demonstrated an activator-specific requirement for each Mediator subunit. Med9/Cse2 and Med10/Nut2 were required, respectively, for Bas1/Bas2- and Gcn4-mediated transcription of amino acid biosynthetic genes. Gal11 was required for Gal4- and Rap1-mediated transcriptional activation. Med11 was also required specifically for MFalpha1 transcription. On the other hand, Med6 was required for all of these transcriptional activation processes. These results suggest that distinct Mediator proteins in the Rgr1 subcomplex are required for activator-specific transcriptional activation and that the activation signals mediated by these Mediator proteins converge on Med6 (or the Srb4 subcomplex) to modulate Pol II activity.

NTF2 mediates nuclear import of Ran

Importin beta family transport receptors shuttle between the nucleus and the cytoplasm and mediate transport of macromolecules through nuclear pore complexes (NPCs). The interactions between these receptors and their cargoes are regulated by binding RanGTP; all receptors probably exit the nucleus complexed with RanGTP, and so should deplete RanGTP continuously from the nucleus. We describe here the development of an in vitro system to study how nuclear Ran is replenished. Nuclear import of Ran does not rely on simple diffusion as Ran's small size would permit, but instead is stimulated by soluble transport factors. This facilitated import is specific for cytoplasmic RanGDP and employs nuclear transport factor 2 (NTF2) as the actual carrier. NTF2 binds RanGDP initially to NPCs and probably also mediates translocation of the NTF2-RanGDP complex to the nuclear side of the NPCs. A direct NTF2-RanGDP interaction is crucial for this process, since point mutations that disturb the RanGDP-NTF2 interaction also interfere with Ran import. The subsequent nuclear accumulation of Ran also requires GTP, but not GTP hydrolysis. The release of Ran from NTF2 into the nucleus, and thus the directionality of Ran import, probably involves nucleotide exchange to generate RanGTP, for which NTF2 has no detectable affinity, followed by binding of the RanGTP to an importin beta family transport receptor.

Cse1p is involved in export of yeast importin alpha from the nucleus

Proteins bearing a nuclear localization signal (NLS) are targeted to the nucleus by the heterodimeric transporter importin. Importin alpha binds to the NLS and to importin beta, which carries it through the nuclear pore complex (NPC). Importin disassembles in the nucleus, evidently by binding of RanGTP to importin beta. The importin subunits are exported separately. We investigated the role of Cse1p, the Saccharomyces cerevisiae homologue of human CAS, in nuclear export of Srp1p (yeast importin alpha). Cse1p is located predominantly in the nucleus but also is present in the cytoplasm and at the NPC. We analyzed the in vivo localization of the importin subunits fused to the green fluorescent protein in wild-type and cse1-1 mutant cells. Srp1p but not importin beta accumulated in nuclei of cse1-1 mutants, which are defective in NLS import but not defective in NLS-independent import pathways. Purified Cse1p binds with high affinity to Srp1p only in the presence of RanGTP. The complex is dissociated by the cytoplasmic RanGTP-binding protein Yrb1p. Combined with the in vivo results, this suggests that a complex containing Srp1p, Cse1p, and RanGTP is exported from the nucleus and is subsequently disassembled in the cytoplasm by Yrb1p. The formation of the trimeric Srp1p-Cse1p-RanGTP complex is inhibited by NLS peptides, indicating that only NLS-free Srp1p will be exported to the cytoplasm.

Phosphorylation regulates association of the transcription factor Pho4 with its import receptor Pse1/Kap121

The transcription factor Pho4 is phosphorylated and localized predominantly to the cytoplasm when budding yeast are grown in phosphate-rich medium and is unphosphorylated and localized to the nucleus upon phosphate starvation. We have investigated the requirements for nuclear import of Pho4 and find that Pho4 enters the nucleus via a nonclassical import pathway that utilizes the importin beta family member Pse1/Kap121. Pse1 binds directly to Pho4 and is required for its import in vivo. We have defined the nuclear localization signal on Pho4 and demonstrate that it is required for Pse1 binding in vitro and is sufficient for PSE1-dependent import in vivo. Phosphorylation of Pho4 inhibits its interaction with Pse1, providing a mechanism by which phosphorylation may regulate import of Pho4 in vivo.

A mutation in NPS1/STH1, an essential gene encoding a component of a novel chromatin-remodeling complex RSC, alters the chromatin structure of Saccharomyces cerevisiae centromeres

The NPS1/STH1 gene encodes a nuclear protein essential for the progression of G2/M phase in Saccharomyces cerevisiae . Nps1p shares homology to Snf2/Swi2p, a subunit of a protein complex known as the SNF/SWI complex. Recently, Nps1p was found to be a component of a protein complex termed RSC (3) essential for mitotic growth, whereas its function is unknown. We isolated a temperature-sensitive mutant allele of NPS1 , nps1-105, and found that the mutation increases the sensitivity to thiabendazole (TBZ). At the restrictive temperature, nps1-105 arrested at the G2/M phase in MAD1-dependent manner and missegregated the mini-chromosome with higher frequency than the wild type cells. The nuclease digestion of the chromatin of the mutant cells revealed that the mutation causes the alteration of the chromatin structure around centromeres at the restrictive temperature. The results suggested that, in the nps1-105 mutant, impaired chromatin structure surrounding centromeres may lead to an impairment of kinetochore function and the cells arrest at G2/M phase through the spindle-assembly checkpoint system.

A member of the Ran-binding protein family, Yrb2p, is involved in nuclear protein export

Yeast cells mutated in YRB2, which encodes a nuclear protein with similarity to other Ran-binding proteins, fail to export nuclear export signal (NES)-containing proteins including HIV Rev out of the nucleus. Unlike Xpo1p/Crm1p/exportin, an NES receptor, Yrb2p does not shuttle between the nucleus and the cytoplasm but instead remains inside the nucleus. However, by both biochemical and genetic criteria, Yrb2p interacts with Xpo1p and not with other members of the importin/karyopherin beta superfamily. Moreover, the Yrb2p region containing nucleoporin-like FG repeats is important for NES-mediated protein export. Taken together, these data suggest that Yrb2p acts inside the nucleus to mediate the action of Xpo1p in at least one of several nuclear export pathways.

A novel yeast screen for mitotic arrest mutants identifies DOC1, a new gene involved in cyclin proteolysis

B-type cyclins are rapidly degraded at the transition between metaphase and anaphase and their ubiquitin-mediated proteolysis is required for cells to exit mitosis. We used a novel enrichment to isolate new budding mutants that arrest the cell cycle in mitosis. Most of these mutants lie in the CDC16, CDC23, and CDC27 genes, which have already been shown to play a role in cyclin proteolysis and encode components of a 20S complex (called the cyclosome or anaphase promoting complex) that ubiquitinates mitotic cyclins. We show that mutations in CDC26 and a novel gene, DOC1, also prevent mitotic cyclin proteolysis. Mutants in either gene arrest as large budded cells with high levels of the major mitotic cyclin (Clb2) protein at 37 degrees C and cannot degrade Clb2 in G1-arrested cells. Cdc26 associates in vivo with Doc1, Cdc16, Cdc23, and Cdc27. In addition, the majority of Doc1 cosediments at 20S with Cdc27 in a sucrose gradient, indicating that Cdc26 and Doc1 are components of the anaphase promoting complex.

Importin/karyopherin protein family members required for mRNA export from the nucleus

The yeast Saccharomyces cerevisiae contains three proteins (Kap104p, Pse1p, and Kap123p) that share similarity to the 95-kDa beta subunit of the nuclear transport factor importin (also termed karyopherin and encoded by KAP95/RSL1 in yeast). Proteins that contain nuclear localization sequences are recognized in the cytoplasm and delivered to the nucleus by the heterodimeric importin complex. A second importin-related protein, transportin, delivers a subset of heterogeneous nuclear ribonucleoproteins (hnRNPs) to the nucleoplasm. We now show that in contrast to loss of importin beta (Kap95p/Rsl1p) and transportin (Kap104p), conditional loss of Pse1p in a strain lacking Kap123p results in a specific block of mRNA export from the nucleus. Overexpression of Sxm1p, a protein related to Cse1p in yeast and to the human cellular apoptosis susceptibility protein, relieves the defects of cells lacking Pse1p and Kap123p. Thus, a major role of Pse1p, Kap123p, and Sxm1p may be nuclear export rather than import, suggesting a symmetrical relationship between these processes.

A novel class of RanGTP binding proteins

The importin-alpha/beta complex and the GTPase Ran mediate nuclear import of proteins with a classical nuclear localization signal. Although Ran has been implicated also in a variety of other processes, such as cell cycle progression, a direct function of Ran has so far only been demonstrated for importin-mediated nuclear import. We have now identified an entire class of approximately 20 potential Ran targets that share a sequence motif related to the Ran-binding site of importin-beta. We have confirmed specific RanGTP binding for some of them, namely for two novel factors, RanBP7 and RanBP8, for CAS, Pse1p, and Msn5p, and for the cell cycle regulator Cse1p from Saccharomyces cerevisiae. We have studied RanBP7 in more detail. Similar to importin-beta, it prevents the activation of Ran's GTPase by RanGAP1 and inhibits nucleotide exchange on RanGTP. RanBP7 binds directly to nuclear pore complexes where it competes for binding sites with importin-beta, transportin, and apparently also with the mediators of mRNA and U snRNA export. Furthermore, we provide evidence for a Ran-dependent transport cycle of RanBP7 and demonstrate that RanBP7 can cross the nuclear envelope rapidly and in both directions. On the basis of these results, we propose that RanBP7 might represent a nuclear transport factor that carries an as yet unknown cargo, which could apply as well for this entire class of related RanGTP-binding proteins.

TPR proteins required for anaphase progression mediate ubiquitination of mitotic B-type cyclins in yeast

The abundance of B-type cyclin-CDK complexes is determined by regulated synthesis and degradation of cyclin subunits. Cyclin proteolysis is required for the final exit from mitosis and for the initiation of a new cell cycle. In extracts from frog or clam eggs, degradation is accompanied by ubiquitination of cyclin. Three genes, CDC16, CDC23, and CSE1 have recently been shown to be required specifically for cyclin B proteolysis in yeast. To test whether these genes are required for cyclin ubiquitination, we prepared extracts from G1-arrested yeast cells capable of conjugating ubiquitin to the B-type cyclin Clb2. The ubiquitination activity was cell cycle regulated, required Clb2's destruction box, and was low if not absent in cdc16, cdc23, cdc27, and cse1 mutants. Furthermore all these mutants were also defective in ubiquitination of another mitotic B-type cyclin, Clb3. The Cdc16, Cdc23, and Cdc27 proteins all contain several copies of the tetratricopeptide repeat and are subunits of a complex that is required for the onset of anaphase. The finding that gene products that are required for ubiquitination of Clb2 and Clb3 are also required for cyclin proteolysis in vivo provides the best evidence so far that cyclin B is degraded via the ubiquitin pathway in living cells. Xenopus homologues of Cdc16 and Cdc27 have meanwhile been shown to be associated with a 20S particle that appears to function as a cell cycle-regulated ubiquitin-protein ligase.

The human CAS protein which is homologous to the CSE1 yeast chromosome segregation gene product is associated with microtubules and mitotic spindle

Human CAS cDNA contains a 971-aa open reading frame that is homologous to the essential yeast gene CSE1. CSE1 is involved in chromosome segregation and is necessary for B-type cyclin degradation in mitosis. Using antibodies to CAS, it was shown that CAS levels are high in proliferating and low in nonproliferating cells. Here we describe the distribution of CAS in cells and tissues analyzed with antibodies against CAS. CAS is an approximately 100-kDa protein present in the cytoplasm of proliferating cells at levels between 2 x 10(5) and 1 x 10(6) molecules per cell. The intracellular distribution of CAS resembles that of tubulin. In interphase cells, anti-CAS antibody shows microtubule-like patterns and in mitotic cells it labels the mitotic spindle. CAS is removed from microtubules by mild detergent treatment (cytoskeleton preparations) and in vincristine- or taxol-treated cells. CAS is diffusely distributed in the cytoplasm with only traces present in tubulin paracrystals or bundles. Thus, CAS appears to be associated with but not to be an integral part of microtubules. Immunohistochemical staining of frozen tissues shows elevated amounts of CAS in proliferating cells such as testicular spermatogonia and cells in the basal layer cells of the colon. CAS was also concentrated in the respiratory epithelium of the trachea and in axons and Purkinje cells in the cerebellum. These cells contain many microtubules. The cellular location of CAS is consistent with an important role in cell division as well as in ciliary movement and vesicular transport.

Cloning and characterization of a cellular apoptosis susceptibility gene, the human homologue to the yeast chromosome segregation gene CSE1

We recently isolated human cDNA fragments that render MCF-7 breast cancer cells resistant to cell death caused by Pseudomonas exotoxin, Pseudomonas exotoxin-derived immunotoxins, diphtheria toxin, and tumor necrosis factor. We report here that one of these fragments is an antisense fragment of a gene homologous to the essential yeast chromosome segregation gene CSE1. Cloning and analysis of the full-length cDNA of the human CSE1 homologue, which we name CAS for cellular apoptosis susceptibility gene, reveals a protein coding region with similar length (971 amino acids for CAS, 960 amino acids for CSE1) and 59% overall protein homology to the yeast CSE1 protein. The conservation of this gene indicates it has an important function in human cells consistent with the essential role of CSE1 in yeast. CAS is highly expressed in human tumor cell lines and in human testis and fetal liver, tissues that contain actively dividing cells. Furthermore, CAS expression increases when resting human fibroblasts are induced to proliferate and decreases when they are growth-arrested. Thus, CAS appears to play an important role in both toxin and tumor necrosis factor-mediated cell death, as well as in cell proliferation.

The yeast nuclear import receptor is required for mitosis

The nuclear import system is highly conserved among eukaryotes. Here we report the effects of a conditional mutation in SRP1, which encodes a Saccharomyces cerevisiae homolog of the vertebrate nuclear import receptor importin. Importin was isolated as a factor required for the initial targeting step of a nuclear import substrate to the nuclear envelope in a mammalian in vitro assay. We show that yeast Srp1 is similarly required for protein import. In addition, Srp1 is also required for the execution of mitosis: we demonstrate that cells containing a conditional mutation of SRP1 arrest with a G2/M phenotype in a manner analogous to classic cdc mutants. This defect may be due to the failure of the mutant to degrade the mitotic cyclin Clb2 and other proteins required for mitosis. The requirement of a nuclear import receptor for cell cycle-regulated proteolysis implies that import of cell cycle regulators into the nucleus is critical for cell cycle progression.

Evidence that the MIF2 gene of Saccharomyces cerevisiae encodes a centromere protein with homology to the mammalian centromere protein CENP-C

The MIF2 gene of Saccharomyces cerevisiae has been implicated in mitosis. Here we provide genetic evidence that MIF2 encodes a centromere protein. Specifically, we found that mutations in MIF2 stabilize dicentric minichromosomes and confer high instability (i.e., a synthetic acentric phenotype) to chromosomes that bear a cis-acting mutation in element I of the yeast centromeric DNA (CDEI). Similarly, we observed synthetic phenotypes between mutations in MIF2 and trans-acting mutations in three known yeast centromere protein genes-CEP1/CBF1/CPF1, NDC10/CBF2, and CEP3/CBF3B. In addition, the mif2 temperature-sensitive phenotype can be partially rescued by increased dosage of CEP1. Synthetic lethal interactions between a cep1 null mutation and mutations in either NDC10 or CEP3 were also detected. Taken together, these data suggest that the Mif2 protein interacts with Cep1p at the centromere and that the yeast centromere indeed exists as a higher order protein-DNA complex. The Mif2 and Cep1 proteins contain motifs of known transcription factors, suggesting that assembly of the yeast centromere is analogous to that of eukaryotic enhancers and origins of replication. We also show that the predicted Mif2 protein shares two short regions of homology with the mammalian centromere Ag CENP-C and that two temperature-sensitive mutations in MIF2 lie within these regions. These results provide evidence for structural conservation between yeast and mammalian centromeres.

CEP3 encodes a centromere protein of Saccharomyces cerevisiae

We have designed a screen to identify mutants specifically affecting kinetochore function in the yeast Saccharomyces cerevisiae. The selection procedure was based on the generation of "synthetic acentric" minichromosomes. "Synthetic acentric" minichromosomes contain a centromere locus, but lack centromere activity due to combination of mutations in centromere DNA and in a chromosomal gene (CEP) encoding a putative centromere protein. Ten conditional lethal cep mutants were isolated, seven were found to be alleles of NDC10 (CEP2) encoding the 110-kD protein of yeast kinetochore. Three mutants defined a novel essential gene CEP3. The CEP3 product (Cep3p) is a 71-kD protein with a potential DNA-binding domain (binuclear Zn-cluster). At nonpermissive temperature the cep3 cells arrest with an undivided nucleus and a short mitotic spindle. At permissive temperature the cep3 cells are unable to support segregation of minichromosomes with mutations in the central part of element III of yeast centromere DNA. These minichromosomes, when isolated from cep3 cultures, fail to bind bovine microtubules in vitro. The sum of genetic, cytological and biochemical data lead us to suggest that the Cep3 protein is a DNA-binding component of yeast centromere. Molecular mass and sequence comparison confirm that Cep3p is the p64 component of centromere DNA binding complex Cbf3 (Lechner, 1994).

DBF8, an essential gene required for efficient chromosome segregation in Saccharomyces cerevisiae

To investigate chromosome segregation in Saccharomyces cerevisiae, we examined a collection of temperature-sensitive mutants that arrest as large-budded cells at restrictive temperatures (L. H. Johnston and A. P. Thomas, Mol. Gen. Genet. 186:439-444, 1982). We characterized dbf8, a mutation that causes cells to arrest with a 2c DNA content and a short spindle. DBF8 maps to chromosome IX near the centromere, and it encodes a 36-kDa protein that is essential for viability at all temperatures. Mutational analysis reveals that three dbf8 alleles are nonsense mutations affecting the carboxy-terminal third of the encoded protein. Since all of these mutations confer temperature sensitivity, it appears that the carboxyl-terminal third of the protein is essential only at a restrictive temperature. In support of this conclusion, an insertion of URA3 at the same position also confers a temperature-sensitive phenotype. Although they show no evidence of DNA damage, dbf8 mutants exhibit increased rates of chromosome loss and nondisjunction even at a permissive temperature. Taken together, our data suggest that Dbf8p plays an essential role in chromosome segregation.

DBF8, an essential gene required for efficient chromosome segregation in Saccharomyces cerevisiae

To investigate chromosome segregation in Saccharomyces cerevisiae, we examined a collection of temperature-sensitive mutants that arrest as large-budded cells at restrictive temperatures (L. H. Johnston and A. P. Thomas, Mol. Gen. Genet. 186:439-444, 1982). We characterized dbf8, a mutation that causes cells to arrest with a 2c DNA content and a short spindle. DBF8 maps to chromosome IX near the centromere, and it encodes a 36-kDa protein that is essential for viability at all temperatures. Mutational analysis reveals that three dbf8 alleles are nonsense mutations affecting the carboxy-terminal third of the encoded protein. Since all of these mutations confer temperature sensitivity, it appears that the carboxyl-terminal third of the protein is essential only at a restrictive temperature. In support of this conclusion, an insertion of URA3 at the same position also confers a temperature-sensitive phenotype. Although they show no evidence of DNA damage, dbf8 mutants exhibit increased rates of chromosome loss and nondisjunction even at a permissive temperature. Taken together, our data suggest that Dbf8p plays an essential role in chromosome segregation.

SCM2, a tryptophan permease in Saccharomyces cerevisiae, is important for cell growth

SCM2, a novel gene encoding a yeast tryptophan permease, was cloned as a high-copy-number suppressor of cse2-1. The cse2-1 mutation causes cold sensitivity, temperature sensitivity and chromosome missegregation. However, only the cold-sensitive phenotype of cse2-1 cells is suppressed by SCM2 at high copy. SCM2 is located on the left arm of yeast chromosome XV, adjacent to SUP3 and encodes a 65 kDa protein that is highly homologous to known amino acid permeases. Four out of five disrupted scm2 alleles (scm2 delta 1-delta 4) cause slow growth, whereas one disrupted allele (scm2 delta 5) is lethal. Cells with both the scm2 delta 1 and trp1-delta 101 mutations exhibit a synthetic cold-sensitive phenotype and grow much more slowly at the permissive temperature than cells with a single scm2 delta 1 or trp1-delta 101 mutation. A region of the predicted SCM2 protein is identical to the partial sequence recently reported for the yeast tryptophan permease TAP2, indicating that SCM2 and TAP2 probably encode the same protein.

Yeast Srp1p has homology to armadillo/plakoglobin/beta-catenin and participates in apparently multiple nuclear functions including the maintenance of the nucleolar structure

SRP1, a suppressor of certain temperature-sensitive mutations in RNA polymerase I in Saccharomyces cerevisiae, encodes a protein that is associated with nuclear pores. By using a system of conditional SRP1 expression and by isolating temperature-sensitive srp1 mutants, we have demonstrated that Srp1p is essential for maintenance of the crescent-shaped nucleolar structure, RNA transcription, and the proper functions of microtubules as inferred from analysis of nuclear division/segregation and immunofluorescence microscopy of microtubules. Different mutant alleles showed significantly different phenotypes in relation to these apparently multiple functional roles of the protein. We have also found that eight imperfect 42-amino-acid tandem repeats present in Srp1p are similar to the 42-amino-acid repeats in armadillo/plakoglobin/beta-catenin proteins present in adhesive junction complexes of higher eukaryotes. We discuss this similarity in connection with the observed pleiotropic effects of srp1 mutations.