Roles and regulation of the Drosophila centromere cohesion protein MEI-S332 family

Translesion synthesis polymerases contribute to meiotic chromosome segregation and cohesin dynamics in Schizosaccharomycespombe

Translesion synthesis polymerases (TLSPs) are non-essential error-prone enzymes that ensure cell survival by facilitating DNA replication in the presence of DNA damage. In addition to their role in bypassing lesions, TLSPs have been implicated in meiotic double-strand break repair in several systems. Here, we examine the joint contribution of four TLSPs to meiotic progression in the fission yeast Schizosaccharomyces pombe. We observed a dramatic loss of spore viability in fission yeast lacking all four TLSPs, which is accompanied by disruptions in chromosome segregation during meiosis I and II. Rec8 cohesin dynamics are altered in the absence of the TLSPs. These data suggest that the TLSPs contribute to multiple aspects of meiotic chromosome dynamics.

Cloning and characterization of the homoeologous genes for the Rec8-like meiotic cohesin in polyploid wheat

BACKGROUND

Meiosis is a specialized cell division critical for gamete production in the sexual reproduction of eukaryotes. It ensures genome integrity and generates genetic variability as well. The Rec8-like cohesin is a cohesion protein essential for orderly chromosome segregation in meiotic cell division. The Rec8-like genes and cohesins have been cloned and characterized in diploid models, but not in polyploids. The present study aimed to clone the homoeologous genes (homoeoalleles) for Rec8-like cohesin in polyploid wheat, an important food crop for humans, and to characterize their structure and function under a polyploid condition.

RESULTS

We cloned two Rec8-like homoeoalleles from tetraploid wheat (TtRec8-A1 and TtRec8-B1) and one from hexaploid wheat (TaRec8-D1), and performed expression and functional analyses of the homoeoalleles. Also, we identified other two Rec8 homoeoalleles in hexaploid wheat (TaRec8-A1 and TaRec8-B1) and the one in Aegilops tauschii (AetRec8-D1) by referencing the DNA sequences of the Rec8 homoeoalleles cloned in this study. The coding DNA sequences (CDS) of these six Rec8 homoeoalleles are all 1,827 bp in length, encoding 608 amino acids. They differed from each other primarily in introns although single nucleotide polymorphisms were detected in CDS. Substantial difference was observed between the homoeoalleles from the subgenome B (TtRec8-B1 and TaRec8-B1) and those from the subgenomes A and D (TtRec8-A1, TaRec8-A1, and TaRec8-D1). TtRec8-A1 expressed dominantly over TtRec8-B1, but comparably to TaRec8-D1, in polyploid wheat. In addition, we developed the antibody against wheat Rec8 and used the antibody to detect Rec8 cohesin in the Western blotting and subcellular localization analyses.

CONCLUSIONS

The Rec8 homoeoalleles from the subgenomes A and D are transcriptionally more active than the one from the subgenome B in polyploid wheat. The structural variation and differential expression of the Rec8 homoeoalleles indicate a unique cross-genome coordination of the homoeologous genes in polyploid wheat, and imply the distinction of the wheat subgenome B from the subgenomes A and D in the origin and evolution.

Drosophila Dalmatian combines sororin and shugoshin roles in establishment and protection of cohesion

Sister chromatid cohesion is crucial to ensure chromosome bi-orientation and equal chromosome segregation. Cohesin removal via mitotic kinases and Wapl has to be prevented in pericentromeric regions in order to protect cohesion until metaphase, but the mechanisms of mitotic cohesion protection remain elusive in Drosophila Here, we show that dalmatian (Dmt), an ortholog of the vertebrate cohesin-associated protein sororin, is required for protection of mitotic cohesion in flies. Dmt is essential for cohesion establishment during interphase and is enriched on pericentromeric heterochromatin. Dmt is recruited through direct association with heterochromatin protein-1 (HP1), and this interaction is required for cohesion. During mitosis, Dmt interdependently recruits protein phosphatase 2A (PP2A) to pericentromeric regions, and PP2A binding is required for Dmt to protect cohesion. Intriguingly, Dmt is sufficient to protect cohesion upon heterologous expression in human cells. Our findings of a hybrid system, in which Dmt exerts both sororin-like establishment functions and shugoshin-like heterochromatin-based protection roles, provide clues to the evolutionary modulation of eukaryotic cohesion regulation systems.

Meiosis in male Drosophila

Meiosis entails sorting and separating both homologous and sister chromatids. The mechanisms for connecting sister chromatids and homologs during meiosis are highly conserved and include specialized forms of the cohesin complex and a tightly regulated homolog synapsis/recombination pathway designed to yield regular crossovers between homologous chromatids. Drosophila male meiosis is of special interest because it dispenses with large segments of the standard meiotic script, particularly recombination, synapsis and the associated structures. Instead, Drosophila relies on a unique protein complex composed of at least two novel proteins, SNM and MNM, to provide stable connections between homologs during meiosis I. Sister chromatid cohesion in Drosophila is mediated by cohesins, ring-shaped complexes that entrap sister chromatids. However, unlike other eukaryotes Drosophila does not rely on the highly conserved Rec8 cohesin in meiosis, but instead utilizes two novel cohesion proteins, ORD and SOLO, which interact with the SMC1/3 cohesin components in providing meiotic cohesion.

Regulation of centromere localization of the Drosophila Shugoshin MEI-S332 and sister-chromatid cohesion in meiosis

The Shugoshin (Sgo) protein family helps to ensure proper chromosome segregation by protecting cohesion at the centromere by preventing cleavage of the cohesin complex. Some Sgo proteins also influence other aspects of kinetochore-microtubule attachments. Although many Sgo members require Aurora B kinase to localize to the centromere, factors controlling delocalization are poorly understood and diverse. Moreover, it is not clear how Sgo function is inactivated and whether this is distinct from delocalization. We investigated these questions in Drosophila melanogaster, an organism with superb chromosome cytology to monitor Sgo localization and quantitative assays to test its function in sister-chromatid segregation in meiosis. Previous research showed that in mitosis in cell culture, phosphorylation of the Drosophila Sgo, MEI-S332, by Aurora B promotes centromere localization, whereas Polo phosphorylation promotes delocalization. These studies also suggested that MEI-S332 can be inactivated independently of delocalization, a conclusion supported here by localization and function studies in meiosis. Phosphoresistant and phosphomimetic mutants for the Aurora B and Polo phosphorylation sites were examined for effects on MEI-S332 localization and chromosome segregation in meiosis. Strikingly, MEI-S332 with a phosphomimetic mutation in the Aurora B phosphorylation site prematurely dissociates from the centromeres in meiosis I. Despite the absence of MEI-S332 on meiosis II centromeres in male meiosis, sister chromatids segregate normally, demonstrating that detectable levels of this Sgo are not essential for chromosome congression, kinetochore biorientation, or spindle assembly.

Shugoshins function as a guardian for chromosomal stability in nuclear division

Accurate chromosome segregation during mitosis and meiosis is regulated and secured by several distinctly different yet intricately connected regulatory mechanisms. As chromosomal instability is a hallmark of a majority of tumors as well as a cause of infertility for germ cells, extensive research in the past has focused on the identification and characterization of molecular components that are crucial for faithful chromosome segregation during cell division. Shugoshins, including Sgo1 and Sgo2, are evolutionarily conserved proteins that function to protect sister chromatid cohesion, thus ensuring chromosomal stability during mitosis and meiosis in eukaryotes. Recent studies reveal that Shugoshins in higher animals play an essential role not only in protecting centromeric cohesion of sister chromatids and assisting bi-orientation attachment at the kinetochores, but also in safeguarding centriole cohesion/engagement during early mitosis. Many molecular components have been identified that play essential roles in modulating/mediating Sgo functions. This review primarily summarizes recent advances on the mechanisms of action of Shugoshins in suppressing chromosomal instability during nuclear division in eukaryotic organisms.

Phosphorylation of mammalian Sgo2 by Aurora B recruits PP2A and MCAK to centromeres

Shugoshin (Sgo) is a conserved centromeric protein. Mammalian Sgo1 collaborates with protein phosphatase 2A (PP2A) to protect mitotic cohesin from the prophase dissociation pathway. Although another shugoshin-like protein, Sgo2, is required for the centromeric protection of cohesion in germ cells, its precise molecular function remains largely elusive. We demonstrate that hSgo2 plays a dual role in chromosome congression and centromeric protection of cohesion in HeLa cells, while the latter function is exposed only in perturbed mitosis. These functions partly overlap with those of Aurora B, a kinase setting faithful chromosome segregation. Accordingly, we identified the phosphorylation of hSgo2 by Aurora B at the N-terminal coiled-coil region and the middle region, and showed that these phosphorylations separately promote binding of hSgo2 to PP2A and MCAK, factors required for centromeric protection and chromosome congression, respectively. Furthermore, these phosphorylations are essential for localizing PP2A and MCAK to centromeres. This mechanism seems applicable to germ cells as well. Thus, our study identifies Sgo2 as a hitherto unknown crucial cellular substrate of Aurora B in mammalian cells.

Coupling meiotic chromosome axis integrity to recombination

During meiosis, DNA events of recombination occur in direct physical association with underlying chromosome axes. Meiotic cohesin Rec8 and cohesin-associated Spo76/Pds5 are prominent axis components. Two observations indicate that recombination complexes can direct the local destabilization of underlying chromosome axes. First, in the absence of Rec8, Spo76/Pds5 is lost locally at sites of late-persisting Msh4 foci, with a concomitant tendency for loosening of intersister and interhomolog connectedness at the affected sites. This loss is dependent on initiation of recombination. Second, in wild-type prophase, local separation of sister axes is seen at sites of synaptonemal complex-associated recombination nodules. Additional findings reveal that Rec8 localizes to both axis and bulk chromatin and is required for chromatin compactness. Further, Rec8 is essential for maintenance of sister cohesion, along arms and centromeres, during the pachytene-to-diplotene transition, revealing an intrinsic tendency for destabilization of sister cohesion during this period. This finding shows how the loss of sister connectedness, in arm and/or centric regions, could lead to the segregation defects that are seen in the human "maternal age effect" and how Rec8 could be a target of that effect. Finally, Rec8 plays related, but synergistic roles with Spo76/Pds5, indicating auxiliary roles for meiotic and mitotic cohesion-associated components.

Australin: a chromosomal passenger protein required specifically for Drosophila melanogaster male meiosis

The chromosomal passenger complex (CPC), which is composed of conserved proteins aurora B, inner centromere protein (INCENP), survivin, and Borealin/DASRA, localizes to chromatin, kinetochores, microtubules, and the cell cortex in a cell cycle–dependent manner. The CPC is required for multiple aspects of cell division. Here we find that Drosophila melanogaster encodes two Borealin paralogues, Borealin-related (Borr) and Australin (Aust). Although Borr is a passenger in all mitotic tissues studied, it is specifically replaced by Aust for the two male meiotic divisions. We analyzed aust mutant spermatocytes to assess the effects of fully inactivating the Aust-dependent functions of the CPC. Our results indicate that Aust is required for sister chromatid cohesion, recruitment of the CPC to kinetochores, and chromosome alignment and segregation but not for meiotic histone phosphorylation or spindle formation. Furthermore, we show that the CPC is required earlier in cytokinesis than previously thought; cells lacking Aust do not initiate central spindle formation, accumulate anillin or actin at the cell equator, or undergo equatorial constriction.

sSgo1, a major splice variant of Sgo1, functions in centriole cohesion where it is regulated by Plk1

Shugoshin 1 (Sgo1) functions as a protector of centromeric cohesion of sister chromatids in higher eukaryotes. Here, we provide evidence for a previously unrecognized role for Sgo1 in centriole cohesion. Sgo1 depletion via RNA interference induces the formation of multiple centrosome-like structures in mitotic cells that result from the separation of paired centrioles. Sgo1+/- mitotic murine embryonic fibroblasts display split centrosomes. Localization study of two major endogenous splice variants of Sgo1 indicates that the smaller variant, sSgo1, is found at the centrosome in interphase and at spindle poles in mitosis. sSgo1 interacts with Plk1 and its spindle pole localization is Plk1 dependent. Centriole splitting induced by Sgo1 depletion or expression of a dominant negative mutant is suppressed by ectopic expression of sSgo1 or by Plk1 knockdown. Our studies strongly suggest that sSgo1 plays an essential role in protecting centriole cohesion, which is partly regulated by Plk1.

Shugoshin enables tension-generating attachment of kinetochores by loading Aurora to centromeres

Fission yeast shugoshin Sgo1 is meiosis specific and cooperates with protein phosphatase 2A to protect centromeric cohesin at meiosis I. The other shugoshin-like protein Sgo2, which requires the heterochromatin protein Swi6/HP1 for full viability, plays a crucial role for proper chromosome segregation at both mitosis and meiosis; however, the underlying mechanisms are totally elusive. We here demonstrate that, unlike Sgo1, Sgo2 is dispensable for centromeric protection of cohesin. Instead, Sgo2 interacts with Bir1/Survivin and promotes Aurora kinase complex localization to the pericentromeric region, to correct erroneous attachment of kinetochores and thereby enable tension-generating attachment. Forced localization of Bir1 to centromeres partly restored the defects of sgo2Delta. This newly identified interaction of shugoshin with Survivin is conserved between mitosis and meiosis and presumably across eukaryotes. We propose that ensuring bipolar attachment of kinetochores is the primary role of shugoshin and the role of cohesion protection might have codeveloped to facilitate this process.

INCENP and Aurora B promote meiotic sister chromatid cohesion through localization of the Shugoshin MEI-S332 in Drosophila

The chromosomal passenger complex protein INCENP is required in mitosis for chromosome condensation, spindle attachment and function, and cytokinesis. Here, we show that INCENP has an essential function in the specialized behavior of centromeres in meiosis. Mutations affecting Drosophila incenp profoundly affect chromosome segregation in both meiosis I and II, due, at least in part, to premature sister chromatid separation in meiosis I. INCENP binds to the cohesion protector protein MEI-S332, which is also an excellent in vitro substrate for Aurora B kinase. A MEI-S332 mutant that is only poorly phosphorylated by Aurora B is defective in localization to centromeres. These results implicate the chromosomal passenger complex in directly regulating MEI-S332 localization and, therefore, the control of sister chromatid cohesion in meiosis.

Shugoshin: guardian spirit at the centromere

A recently emerging protein family, shugoshin, plays a crucial role in the centromeric protection of cohesin, which is responsible for sister chromatid cohesion. This is especially important at the first meiotic division, where cohesin is cleaved by separase only along chromosome arms while the centromeric cohesin must be preserved. In vertebrate cells, arm cohesion is largely lost during prophase and prometaphase in order to facilitate sister chromatid resolution, whereas centromeric cohesion is preserved until the bipolar attachment of sister chromatids is established. Vertebrate shugoshin plays an essential role in protecting centromeric cohesin from prophase dissociation. In yeast, shugoshin also has a crucial role in sensing the loss of tension at kinetochores and in generating the spindle checkpoint signal.

Basic mechanism of eukaryotic chromosome segregation

We now have firm evidence that the basic mechanism of chromosome segregation is similar among diverse eukaryotes as the same genes are employed. Even in prokaryotes, the very basic feature of chromosome segregation has similarities to that of eukaryotes. Many aspects of chromosome segregation are closely related to a cell cycle control that includes stage-specific protein modification and proteolysis. Destruction of mitotic cyclin and securin leads to mitotic exit and separase activation, respectively. Key players in chromosome segregation are SMC-containing cohesin and condensin, DNA topoisomerase II, APC/C ubiquitin ligase, securin-separase complex, aurora passengers, and kinetochore microtubule destabilizers or regulators. In addition, the formation of mitotic kinetochore and spindle apparatus is absolutely essential. The roles of principal players in basic chromosome segregation are discussed: most players have interphase as well as mitotic functions. A view on how the centromere/kinetochore is formed is described.