Npp106p, a Schizosaccharomyces pombe nucleoporin similar to Saccharomyces cerevisiae Nic96p, functionally interacts with Rae1p in mRNA export

The Putative RNA-Binding Protein Dri1 Promotes the Loading of Kinesin-14/Klp2 to the Mitotic Spindle and Is Sequestered into Heat-Induced Protein Aggregates in Fission Yeast

Cells form a bipolar spindle during mitosis to ensure accurate chromosome segregation. Proper spindle architecture is established by a set of kinesin motors and microtubule-associated proteins. In most eukaryotes, kinesin-5 motors are essential for this process, and genetic or chemical inhibition of their activity leads to the emergence of monopolar spindles and cell death. However, these deficiencies can be rescued by simultaneous inactivation of kinesin-14 motors, as they counteract kinesin-5. We conducted detailed genetic analyses in fission yeast to understand the mechanisms driving spindle assembly in the absence of kinesin-5. Here, we show that deletion of the dri1 gene, which encodes a putative RNA-binding protein, can rescue temperature sensitivity caused by cut7-22, a fission yeast kinesin-5 mutant. Interestingly, kinesin-14/Klp2 levels on the spindles in the cut7 mutants were significantly reduced by the dri1 deletion, although the total levels of Klp2 and the stability of spindle microtubules remained unaffected. Moreover, RNA-binding motifs of Dri1 are essential for its cytoplasmic localization and function. We have also found that a portion of Dri1 is spatially and functionally sequestered by chaperone-based protein aggregates upon mild heat stress and limits cell division at high temperatures. We propose that Dri1 might be involved in post-transcriptional regulation through its RNA-binding ability to promote the loading of Klp2 on the spindle microtubules.

YTH-RNA-binding protein prevents deleterious expression of meiotic proteins by tethering their mRNAs to nuclear foci

Accurate and extensive regulation of meiotic gene expression is crucial to distinguish germ cells from somatic cells. In the fission yeast Schizosaccharomyces pombe, a YTH family RNA-binding protein, Mmi1, directs the nuclear exosome-mediated elimination of meiotic transcripts during vegetative proliferation. Mmi1 also induces the formation of facultative heterochromatin at a subset of its target genes. Here, we show that Mmi1 prevents the mistimed expression of meiotic proteins by tethering their mRNAs to the nuclear foci. Mmi1 interacts with itself with the assistance of a homolog of Enhancer of Rudimentary, Erh1. Mmi1 self-interaction is required for foci formation, target transcript elimination, their nuclear retention, and protein expression inhibition. We propose that nuclear foci formed by Mmi1 are not only the site of RNA degradation, but also of sequestration of meiotic transcripts from the translation machinery.

Spatial regulation of the KH domain RNA-binding protein Rnc1 mediated by a Crm1-independent nuclear export system in Schizosaccharomyces pombe

RNA-binding proteins (RBPs) play important roles in the posttranscriptional regulation of gene expression, including mRNA stability, transport and translation. Fission yeast rnc1+ encodes a K Homology (KH)-type RBP, which binds and stabilizes the Pmp1 MAPK phosphatase mRNA thereby suppressing the Cl- hypersensitivity of calcineurin deletion and MAPK signaling mutants. Here, we analyzed the spatial regulation of Rnc1 and discovered a putative nuclear export signal (NES)Rnc1 , which dictates the cytoplasmic localization of Rnc1 in a Crm1-independent manner. Notably, mutations in the NESRnc1 altered nucleocytoplasmic distribution of Rnc1 and abolished its function to suppress calcineurin deletion, although the Rnc1 NES mutant maintains the ability to bind Pmp1 mRNA. Intriguingly, the Rnc1 NES mutant destabilized Pmp1 mRNA, suggesting the functional importance of the Rnc1 cytoplasmic localization. Mutation in Rae1, but not Mex67 deletion or overproduction, induced Rnc1 accumulation in the nucleus, suggesting that Rnc1 is exported from the nucleus to the cytoplasm via the mRNA export pathway involving Rae1. Importantly, mutations in the Rnc1 KH-domains abolished the mRNA-binding ability and induced nuclear localization, suggesting that Rnc1 may be exported from the nucleus together with its target mRNAs. Collectively, the functional Rae1-dependent mRNA export system may influence the cytoplasmic localization and function of Rnc1.

Characterization of nuclear pore complex components in fission yeast Schizosaccharomyces pombe

The nuclear pore complex (NPC) is an enormous proteinaceous complex composed of multiple copies of about 30 different proteins called nucleoporins. In this study, we analyzed the composition of the NPC in the model organism Schizosaccharomyces pombe using strains in which individual nucleoporins were tagged with GFP. We identified 31 proteins as nucleoporins by their localization to the nuclear periphery. Gene disruption analysis in previous studies coupled with gene disruption analysis in the present study indicates that 15 of these nucleoporins are essential for vegetative cell growth and the other 16 nucleoporins are non-essential. Among the 16 non-essential nucleoporins, 11 are required for normal progression through meiosis and their disruption caused abnormal spore formation or poor spore viability. Based on fluorescence measurements of GFP-fused nucleoporins, we estimated the composition of the NPC in S. pombe and found that the organization of the S. pombe NPC is largely similar to that of other organisms; a single NPC was estimated as being 45.8–47.8 MDa in size. We also used fluorescence measurements of single NPCs and quantitative western blotting to analyze the composition of the Nup107-Nup160 subcomplex, which plays an indispensable role in NPC organization and function. Our analysis revealed low amounts of Nup107 and Nup131 and high amounts of Nup132 in the Nup107-Nup160 subcomplex, suggesting that the composition of this complex in S. pombe may differ from that in S. cerevisiae and humans. Comparative analysis of NPCs in various organisms will lead to a comprehensive understanding of the functional architecture of the NPC.

Red5 and three nuclear pore components are essential for efficient suppression of specific mRNAs during vegetative growth of fission yeast

Zinc-finger domains are found in many nucleic acid-binding proteins in both prokaryotes and eukaryotes. Proteins carrying zinc-finger domains have important roles in various nuclear transactions, including transcription, mRNA processing and mRNA export; however, for many individual zinc-finger proteins in eukaryotes, the exact function of the protein is not fully understood. Here, we report that Red5 is involved in efficient suppression of specific mRNAs during vegetative growth of Schizosaccharomyces pombe. Red5, which contains five C3H1-type zinc-finger domains, localizes to the nucleus where it forms discrete dots. A red5 point mutation, red5-2, results in the upregulation of specific meiotic mRNAs in vegetative mutant red5-2 cells; northern blot data indicated that these meiotic mRNAs in red5-2 cells have elongated poly(A) tails. RNA-fluorescence in situ hybridization results demonstrate that poly(A)⁺ RNA species accumulate in the nucleolar regions of red5-deficient cells. Moreover, Red5 genetically interacts with several mRNA export factors. Unexpectedly, three components of the nuclear pore complex also suppress a specific set of meiotic mRNAs. These results indicate that Red5 function is important to meiotic mRNA degradation; they also suggest possible connections among selective mRNA decay, mRNA export and the nuclear pore complex in vegetative fission yeast.

The C-terminal domain of Nup93 is essential for assembly of the structural backbone of nuclear pore complexes

ETOC: Nup93 is essential for nuclear pore complex (NPC) assembly, but its interaction partners Nup188 and Nup205 are not. The C-terminus of Nup93 is necessary and sufficient for forming a minimal structural NPC backbone. The remaining part of the protein recruits the Nup62 complex to establish transport-competent NPCs and the diffusion barrier.

Nuclear pore complexes (NPCs) are large macromolecular assemblies that control all transport across the nuclear envelope. They are formed by about 30 nucleoporins (Nups), which can be roughly categorized into those forming the structural skeleton of the pore and those creating the central channel and thus providing the transport and gating properties of the NPC. Here we show that the conserved nucleoporin Nup93 is essential for NPC assembly and connects both portions of the NPC. Although the C-terminal domain of the protein is necessary and sufficient for the assembly of a minimal structural backbone, full-length Nup93 is required for the additional recruitment of the Nup62 complex and the establishment of transport-competent NPCs.

Isolation of synthetic lethal mutations in the rsm1-null mutant of fission yeast

To identify mutations in genes that are genetically linked to rsm1, we performed a synthetic lethal genetic screen in the fission yeast, Schizosaccharomyces pombe. Four mutations that showed synthetic lethality in combination with the rsm1null allele were isolated from approximately 320,000 colonies and defined in three complementation groups. One mutant (SLrsm1) exhibited a significant accumulation of poly(A)(+) RNA in the nucleus under synthetic lethal conditions, while the rest had no mRNA export defects. In addition, some genes (spmex67, rae1, or mlo3) required for mRNA export complemented the growth defects of the identified mutants. These results suggest that the isolated mutants contain mutations in genes that are involved in mRNA export and/or pre-mRNA retention.

Cooperation between the septins and the actomyosin ring and role of a cell-integrity pathway during cell division in fission yeast

A major question about cytokinesis concerns the role of the septin proteins, which localize to the division site in all animal and fungal cells but are essential for cytokinesis only in some cell types. For example, in Schizosaccharomyces pombe, four septins localize to the division site, but deletion of the four genes produces only a modest delay in cell separation. To ask if the S. pombe septins function redundantly in cytokinesis, we conducted a synthetic-lethal screen in a septin-deficient strain and identified seven mutations. One mutation affects Cdc4, a myosin light chain that is an essential component of the cytokinetic actomyosin ring. Five others cause frequent cell lysis during cell separation and map to two loci. These mutations and their dosage suppressors define a signaling pathway (including Rho1 and a novel arrestin) for repairing cell-wall damage. The seventh mutation affects the poorly understood RNA-binding protein Scw1 and severely delays cell separation when combined either with a septin mutation or with a mutation affecting the septin-interacting, anillin-like protein Mid2, suggesting that Scw1 functions in a pathway parallel to that of the septins. Taken together, our results suggest that the S. pombe septins participate redundantly in one or more pathways that cooperate with the actomyosin ring during cytokinesis and that a septin defect causes septum defects that can be repaired effectively only when the cell-integrity pathway is intact.

Nup211, the fission yeast homolog of Mlp1/Tpr, is involved in mRNA export

Synthetic lethal mutants have been previously isolated in fission yeast Schizosaccharomyces pombe, which genetically interact with spmex67, in order to identify the genes involved in mRNA export. The nup211 gene was isolated by complementation of the growth defect in one of the synthetic lethal mutants, SLMex2, under synthetic lethal condition. We showed that Nup211, fission yeast homolog of Mlpl/Mlp2/Tpr, is essential for vegetative growth and Nup211-GFP proteins expressed at endogenous level are localized mainly in nuclear periphery. The accumulation of poly(A)(+) RNA in the nucleus is exhibited when expression of nup211 is repressed or over-expressed. These results suggest that the Nup211 protein plays a pivotal role of mRNA export in fission yeast.

The fission yeast homologue of Glel is essential for growth and involved in mRNA export

We have isolated Glel homologue (named as spglel) as a partial multicopy suppressor of the synthetic lethality of rael-167 elfl-21 in fission yeast Schizosaccharomyces pombe. The spglel is also able to complement partially temperature-sensitive phenotype of rael-167 only at a lower restrictive temperature. The spglel gene contains one intron and encodes a 480 amino-acid protein with predicted molecular weight of 56.2 kDa. We showed that spglel gene is essential for vegetative growth and functional Glel-GFP protein is localized mainly in NPC. The accumulation of poly(A)(+) RNA in the nucleus is exhibited when expression of spglel is repressed or over-expressed. These results suggest that the spGle1 protein is also involved in mRNA export in fission yeast.

Schizosaccharomyces pombe Bub3 is dispensable for mitotic arrest following perturbed spindle formation

The core proteins of the spindle assembly checkpoint (SAC), Mads, Bubs, and Mps1, first identified in the budding yeast, are thought to be functionally and structurally conserved through evolution. We found that fission yeast Bub3 is dispensable for SAC, as bub3 null mutants blocked mitotic progression when spindle formation was disrupted. Consistently, the bub3 mutation only weakly affected the stability of minichromosome Ch16 compared with other SAC mutants. Fission yeast Rae1 has sequence homology with Bub3. The bub3 rae1 double mutant and rae1 single mutant did not have defective SAC, suggesting that these genes do not have overlapping roles for SAC. Observations of living cells revealed that the duration of the mitotic prometaphase/metaphase was longer in the bub3 mutant and was Mad2 dependent. Further, the bub3 mutant was defective in sister centromere association during metaphase. Together, these findings suggest that fission yeast Bub3 is required for normal spindle dynamics, but not for SAC.

The fission yeast Nup107-120 complex functionally interacts with the small GTPase Ran/Spi1 and is required for mRNA export, nuclear pore distribution, and proper cell division

We have characterized Schizosaccharomyces pombe open reading frames encoding potential orthologues of constituents of the evolutionarily conserved Saccharomyces cerevisiae Nup84 vertebrate Nup107-160 nuclear pore subcomplex, namely Nup133a, Nup133b, Nup120, Nup107, Nup85, and Seh1. In spite of rather weak sequence conservation, in vivo analyses demonstrated that these S. pombe proteins are localized at the nuclear envelope. Biochemical data confirmed the organization of these nucleoporins within conserved complexes. Although examination of the S. cerevisiae and S. pombe deletion mutants revealed different viability phenotypes, functional studies indicated that the involvement of this complex in nuclear pore distribution and mRNA export has been conserved between these highly divergent yeasts. Unexpectedly, microscopic analyses of some of the S. pombe mutants revealed cell division defects at the restrictive temperature (abnormal septa and mitotic spindles and chromosome missegregation) that were reminiscent of defects occurring in several S. pombe GTPase Ran (Ran(Sp))/Spi1 cycle mutants. Furthermore, deletion of nup120 moderately altered the nuclear location of Ran(Sp)/Spi1, whereas overexpression of a nonfunctional Ran(Sp)/Spi1-GFP allele was specifically toxic in the Deltanup120 and Deltanup133b mutant strains, indicating a functional and genetic link between constituents of the S. pombe Nup107-120 complex and of the Ran(Sp)/Spi1 pathway.

Elf1p, a member of the ABC class of ATPases, functions as a mRNA export factor in Schizosacchromyces pombe

Rae1p and Mex67p/Tap are conserved mRNA export factors. We have used synthetic lethal genetic screens in Schizosaccharomyces pombe to identify mutations in genes that are functionally linked to rae1 and mex67 in mRNA export. From these screens, we have isolated mutations in a putative S. pombe homologue of the Candida albicans elf1 gene. The elf1 of S. pombe is not an essential gene. When elf1 mutations are combined with rae1-167 mutation, growth and mRNA export is inhibited in the double mutants. This inhibition can be suppressed by the multicopy expression of mex67 suggesting that Mex67p can substitute for the loss of Elf1p function. Elf1p is a non-membrane member of the ATP-binding cassette (ABC) class of ATPase and the GFP-Elf1p fusion localizes to the cytoplasm. Elf1p, expressed and purified from Escherichia coli, binds and hydrolyzes ATP. A mutant Elf1p that carries a glycine to aspartic acid (G731D) mutation within the Walker A domain of the second ATP site retains the ATP binding but loses its ATPase activity in vitro. This mutant protein no longer functions in mRNA export. Taken together, our results show that Elf1p functions as a mRNA export factor along with Rae1p and Mex67p in S. pombe.

Crp79p, like Mex67p, is an auxiliary mRNA export factor in Schizosaccharomyces pombe

The export of mRNA from the nucleus to the cytoplasm involves interactions of proteins with mRNA and the nuclear pore complex. We isolated Crp79p, a novel mRNA export factor from the same synthetic lethal screen that led to the identification of spMex67p in Schizosaccharomyces pombe. Crp79p is a 710-amino-acid-long protein that contains three RNA recognition motif domains in tandem and a distinct C-terminus. Fused to green fluorescent protein (GFP), Crp79p localizes to the cytoplasm. Like Mex67p, Crp79-GFP binds poly(A)(+) RNA in vivo, shuttles between the nucleus and the cytoplasm, and contains a nuclear export activity at the C-terminus that is Crm1p-independent. All of these properties are essential for Crp79p to promote mRNA export. Crp79p import into the nucleus depends on the Ran system. A domain of spMex67p previously identified as having a nuclear export activity can functionally substitute for the nuclear export activity at the C-terminus of Crp79p. Although both Crp79p and spMex67p function to export mRNA, Crp79p does not substitute for all of spMex67p functions and probably is not a functional homologue of spMex67p. We propose that Crp79p is a nonessential mRNA export carrier in S. pombe.

Mrnp41 (Rae 1p) associates with microtubules in HeLa cells and in neurons

Mrnp41 (hRae1p) is an evolutionarily highly conserved protein, which is a potential component of mRNP particles and plays a role in nuclear mRNA export. The protein is mainly localized at the nuclear pore complex, but is also associated with distinct nuclear domains and with a meshwork of numerous small particles in the cytoplasm (Kraemer and Blobel (1997): Proc. Natl. Acad. Sci. USA 91, 1519-1523). We show that the cytoplasmic pattern of mrnp41 is sensitive to treatment with the microtubule (MT)-depolymerizing drug nocodazole which causes disappearance of mrnp41 from the cell periphery and concentration around the nucleus. By immunofluorescence we demonstrate that mrnp41 colocalizes with MT in HeLa cells and displays an MT-like distribution in cultured neurons. Association of mrnp41 with MT is further demonstrated by copurification with MT from pig brain throughout several steps of polymerization and depolymerization. Separation of MT-associated proteins (MAPs) by phosphocellulose (PC) chromatography showed copurification of mrnp41 with MAPs. These data show an association of mrnp41 with MT and, moreover, demonstrate that an intact MT system is necessary for dispersion of mrnp41-containing particles to the cellular periphery. The essential role of mrnp41 in spindle pole separation and cell cycle progression may also be related to its ability to bind to MTs.

Mex67p of Schizosaccharomyces pombe interacts with Rae1p in mediating mRNA export

We identified the Schizosaccharomyces pombe mex67 gene (spmex67) as a multicopy suppressor of rae1-167 nup184-1 synthetic lethality and the rae1-167 ts mutation. spMex67p, a 596-amino-acid-long protein, has considerable sequence similarity to the Saccharomyces cerevisiae Mex67p (scMex67p) and human Tap. In contrast to scMEX67, spmex67 is essential for neither growth nor nuclear export of mRNA. However, an spmex67 null mutation (Deltamex67) is synthetically lethal with the rae1-167 mutation and accumulates poly(A)(+) RNA in the nucleus. We identified a central region (149 to 505 amino acids) within spMex67p that associates with a complex containing Rae1p that complements growth and mRNA export defects of the rae1-167 Deltamex67 synthetic lethality. This region is devoid of RNA-binding, N-terminal nuclear localization, and the C-terminal nuclear pore complex-targeting regions. The (149-505)-green fluorescent protein (GFP) fusion is found diffused throughout the cell. Overexpression of spMex67p inhibits growth and mRNA export and results in the redistribution of the diffused localization of the (149-505)-GFP fusion to the nucleus and the nuclear periphery. These results suggest that spMex67p competes for essential mRNA export factor(s). Finally, we propose that the 149-505 region of spMex67p could act as an accessory factor in Rae1p-dependent transport and that spMex67p participates at various common steps with Rae1p export complexes in promoting the export of mRNA.

The nuclear pore complex: a protein machine bridging the nucleus and cytoplasm

Compositional analysis of nuclear pore complexes (NPCs) is nearing completion, and efforts are now focused on understanding how these protein machines work. Recent analysis of soluble transport factor interactions with NPC proteins reveals distinct and overlapping pathways for movement between the nucleus and cytoplasm. New fluorescence- and microscopy-based strategies have been used to monitor the pathway of NPC assembly and to reveal the dynamics of the NPC during transport.

Nuclear localization of Schizosaccharomyces pombe Mcm2/Cdc19p requires MCM complex assembly

The minichromosome maintenance (MCM) proteins MCM2-MCM7 are conserved eukaryotic replication factors that assemble in a heterohexameric complex. In fission yeast, these proteins are nuclear throughout the cell cycle. In studying the mechanism that regulates assembly of the MCM complex, we analyzed the cis and trans elements required for nuclear localization of a single subunit, Mcm2p. Mutation of any single mcm gene leads to redistribution of wild-type MCM subunits to the cytoplasm, and this redistribution depends on an active nuclear export system. We identified the nuclear localization signal sequences of Mcm2p and showed that these are required for nuclear targeting of other MCM subunits. In turn, Mcm2p must associate with other MCM proteins for its proper localization; nuclear localization of MCM proteins thus requires assembly of MCM proteins in a complex. We suggest that coupling complex assembly to nuclear targeting and retention ensures that only intact heterohexameric MCM complexes remain nuclear.

Regulation of mRNA export by nutritional status in fission yeast

We have isolated a mutation in nup184(nup184-1) that is synthetically lethal with the mRNA export defective rae1-167 mutation in Schizosaccharomyces pombe. The consequence of the synthetic lethality is a defect in mRNA export. The predicted Nup184p is similar to Nup188p of Saccharomyces cerevisiae, and a Nup184p-GFP fusion localizes to the nuclear periphery in a punctate pattern. The Deltanup184 null mutant is viable and also is synthetically lethal with rae1-167. In a rae1(+) background, both the nup184-1 and Deltanup184 mutations confer sensitivity to growth in nutrient-rich medium (YES) that is accompanied by nuclear poly(A)+ RNA accumulation. Removal of the cAMP-dependent protein kinase, Pka1p, relieved the growth and mRNA export defects of nup184 mutants when grown in nutrient-rich medium. The activation of Pka1p is necessary, but not sufficient, to cause the severe poly(A)+ RNA export defects when nup184 mutant cells are incubated in YES, suggesting nutritional status can also regulate poly(A)+ RNA export. Our results suggest that the regulation of poly(A)+ RNA export by Pka1p kinase appears to be indirect, via a translation-dependent step, but post-translationally in response to YES.

The human homologue of Bub3 is required for kinetochore localization of Bub1 and a Mad3/Bub1-related protein kinase

A feedback control mechanism, or cell cycle checkpoint, delays the onset of anaphase until all the chromosomes are correctly aligned on the mitotic spindle. Previously, we showed that the murine homologue of Bub1 is not only required for checkpoint response to spindle damage, but also restrains progression through a normal mitosis (Taylor, S.S., and F. McKeon. 1997. Cell. 89:727-735). Here, we describe the identification of a human homologue of Bub3, a 37-kD protein with four WD repeats. Like Bub1, Bub3 localizes to kinetochores before chromosome alignment. In addition, Bub3 and Bub1 interact in mammalian cells. Deletion mapping was used to identify the domain of Bub1 required for binding Bub3. Significantly, this same domain is required for kinetochore localization of Bub1, suggesting that the role of Bub3 is to localize Bub1 to the kinetochore, thereby activating the checkpoint in response to unattached kinetochores. The identification of a human Mad3/Bub1-related protein kinase, hBubR1, which can also bind Bub3 in mammalian cells, is described. Ectopically expressed hBubR1 also localizes to kinetochores during prometaphase, but only when hBub3 is overexpressed. We discuss the implications of the common interaction between Bub1 and hBubR1 with hBub3 for checkpoint control.