Rafael Daga

Although the function of microtubules (MTs) in chromosome segregation during mitosis is well characterized, much less is known about the role of MTs on chromosomal functions during interphase. In the fission yeast Schizosaccharomyces pombe, dynamic cytoplasmic MT bundles move chromosomes in an oscillatory manner during interphase via linkages through the nuclear envelope (NE) at the spindle pole body (SPB) and other sites. Mto1 is a cytoplasmic factor that mediates the nucleation and attachment of cytoplasmic MTs to the nucleus. Here, we test the function of these cytoplasmic MTs and Mto1 on DNA repair and recombination during interphase. We find that mto1Δ cells exhibit defects in DNA repair and homologous recombination (HR) and abnormal DNA repair factory dynamics. In these cells, sister chromatids are not properly paired, and Rad21 binding along chromosome arms is reduced. Our findings suggest a model in which cytoplasmic MTs and Mto1 facilitate efficient DNA repair and HR by pro...

In eukaryotic cells, the organization of the genome within the nucleus requires the nuclear envelope (NE) and its associated proteins. The nucleus is subjected to mechanical forces produced by the cytoskeleton. The physical properties of the NE and the linkage of chromatin in compacted conformation at sites of cytoskeleton contacts seem to be key for withstanding nuclear mechanical stress. Mechanical perturbations of the nucleus normally occur during nuclear positioning and migration. In addition, cell contraction or expansion occurring for instance during cell migration or upon changes in osmotic conditions also result innuclear mechanical stress. Recent studies in Schizosaccharomyces pombe (fission yeast) have revealed unexpected functions of cytoplasmic microtubules in nuclear architecture and chromosome behavior, and have pointed to NE-chromatin tethers as protective elements during nuclear mechanics. Here, we review and discuss how fission yeast cells can be used to understand ...

In metazoans, the nuclear envelope (NE) breakdown (NEBD) occurs during "open" mitosis and meiosis. In the fission yeast Schizosaccharomyces pombe, the mitosis and the first meiotic division (MI) are "closed," during which the NE is maintained. Intriguingly, during the second meiotic division (MII), the NE is also maintained, but nuclear and cytoplasmic molecules are mixed similarly to open mitosis, a phenomenon of unknown biological significance called "virtual" NEBD (vNEBD). Here, we show that importin-α-dependent nucleocytoplasmic transport regulates spindle disassembly late in anaphase B at MI, as previously reported for mitosis. At MII, however, spindle dissolution is triggered by vNEBD early in anaphase B, a mechanism that short-circuits the nucleocytoplasmic transport system. We demonstrate that the sequential action of these two spindle disassembly systems regulates the spatiotemporal order and ploidy of the meiotic products.

Our view of the nuclear pore complexes (NPCs) as gateways between the nuclear and cytoplasmic compartments has been largely expanded in recent years. NPCs have now demonstrated roles in genome regulation and maintenance from single cells to multicellular organisms. Both NPC proteins as well as components of the NPC basket act as dynamic scaffolds for silencing factors, and chromatin and cell cycle regulators. Components of the NPC basket also couple mRNA production and export, and prevent the exit of unprocessed mRNAs from the nucleus. Our recent work describes a novel function of the fission yeast nuclear basket component - the translocated promoter region (TPR) nucleoporin Alm1 - in proper localization of the proteasome to the nuclear envelope. Here we discuss how regulation of proteasome localization to the nuclear envelope by Alm1 is key to maintain kinetochores homeostasis and proper chromosome segregation.

One important challenge for cell biology in the post-genomic era is to understand how genomes are organized within the nuclear space and how this organization contributes to genome stability, gene expression and DNA metabolism. In yeasts, chromosomes are anchored to the nuclear envelope via specific DNA loci such as centromeres, telomeres and TFIIIC binding sites. This remarkable topological organization requires the presence of distinct DNA sequences and chromatin domains and relies on a large number of proteins that mediate chromatin interactions with the nuclear envelope. Therefore, the nuclear envelope serves as a dynamic scaffold to anchor and organize distinct chromosomal domains, thus generating a higher-order nuclear architecture. This review summarizes the current knowledge on the spatial organization of the fission yeast genome and its implications for nuclear function.

Kinetochores (KTs) are large multiprotein complexes that constitute the interface between centromeric chromatin and the mitotic spindle during chromosome segregation. In spite of their essential role, little is known about how centromeres and KTs are assembled and how their precise stoichiometry is regulated. In this study, we show that the nuclear pore basket component Alm1 is required to maintain both the proteasome and its anchor, Cut8, at the nuclear envelope, which in turn regulates proteostasis of certain inner KT components. Consistently,-deleted cells show increased levels of KT proteins, including CENP-C, spindle assembly checkpoint activation, and chromosome segregation defects. Our data demonstrate a novel function of the nucleoporin Alm1 in proteasome localization required for KT homeostasis.

ABSTRACT One important challenge for cell biology in the post-genomic era is to understand how genomes are organized within the nuclear space and how this organization contributes to genome stability, gene expression and DNA metabolism. In yeasts, chromosomes are anchored to the nuclear envelope via specific DNA loci such as centromeres, telomeres and TFIIIC binding sites. This remarkable topological organization requires the presence of distinct DNA sequences and chromatin domains and relies on a large number of proteins that mediate chromatin interactions with the nuclear envelope. Therefore, the nuclear envelope serves as a dynamic scaffold to anchor and organize distinct chromosomal domains, thus generating a higher-order nuclear architecture. This review summarizes the current knowledge on the spatial organization of the fission yeast genome and its implications for nuclear function.

During each cell cycle, the mitotic spindle is efficiently assembled to achieve chromosome segregation and then rapidly disassembled as cells enter cytokinesis. Although much has been learned about assembly, how spindles disassemble at the end of mitosis remains unclear. Here we demonstrate that nucleocytoplasmic transport at the membrane domain surrounding the mitotic spindle midzone, here named the midzone membrane domain (MMD), is essential for spindle disassembly in Schizosaccharomyces pombe cells. We show that, during anaphase B, Imp1-mediated transport of the AAA-ATPase Cdc48 protein at the MMD allows this disassembly factor to localize at the spindle midzone, thereby promoting spindle midzone dissolution. Our findings illustrate how a separate membrane compartment supports spindle disassembly in the closed mitosis of fission yeast.

Cells of the fission yeast Schizosaccharomyces pombe have a checkpoint mechanism that reportedly monitors the orientation of the mitotic spindle. Astral microtubules in pre-anaphase spindles are thought to contact the contractile actin ring at the plasma membrane in order to rotate the spindle and to sense spindle orientation. Here, we show that these microtubules are actually inside the nuclear envelope.

Regulation of microtubule organizing centers (MTOCs) orchestrates the reorganization of the microtubule (MT) cytoskeleton. In the fission yeast Schizosaccharomyces pombe, an equatorial MTOC (eMTOC) at the cell division site disassembles after cytokinesis, and multiple interphase MTOCs (iMTOCs) appear on the nucleus. Here, we show that, upon eMTOC disassembly, small satellites carrying MTOC components such as the gamma-tubulin complex travel in both directions along interphase MTs. We identify rsp1p, an MTOC protein required for eMTOC disassembly. In rsp1 loss-of-function mutants, the eMTOC persists and organizes an abnormal microtubule aster, while iMTOCs and satellites are greatly reduced. Conversely, rsp1p overexpression inhibits eMTOC formation. Rsp1p is a J domain protein that interacts with an hsp70. Thus, our findings suggest a model in which rsp1p is part of a chaperone-based mechanism that disassembles the eMTOC into satellites, contributing to the dynamic redistribution of MTOC components for organization of interphase microtubules.

During tumour progression, cells accumulate secondary mutations and/or chromosomal aberrations that generate genetic diversity within the tumour cell population. This may result in the acquisition of new properties that increase tumour malignancy, such as invasiveness or resistance to chemotherapy. One of the important mechanisms of chemotherapy resistance is overexpression or biochemical activation of ABC family transporters. ABC transporters remove anti tumour drugs from the cell, reducing their intracellular concentration and producing resistance against a wide range of chemically unrelated drugs, known as multidrug resistant phenotype (MDR). During recent decades, much effort has been devoted to the isolation of compounds able to inhibit the activity of these transporters. However, few such compounds have reached clinical practice and MDR remains a serious complication in cancer therapy. In an innovative approach to finding new ABC inhibitors, we propose using fission yeast Schizosaccharomyces pombe as a biosensor of detoxification that would enable cost-efficient screening of natural compounds and chemical libraries for molecules that revert the MDR phenotype. Existing fission yeast tools provide genetic, biochemical and cell biological analysis, thereby facilitating identification of drug targets. Putative inhibitors and modulators of ABC transporters could be used in combination with chemotherapeutic drugs for the treatment of multidrug resistant tumours.