Mei332 homologs in plants

During meiosis, sequential release of sister chromatid cohesion (SSC) during two successive nuclear divisions allows the production of haploid gametes from diploid progenitor cells. Release of SSC along chromosome arms allows first a reductional segregation of homologs, and, subsequently, release of centromeric cohesion at anaphase II allows the segregation of chromatids. The Shugoshin (SGO) protein family plays a major role in the protection of centromeric cohesion in Drosophila and yeast. A maize mutant was isolated that displays premature loss of centromeric cohesion at anaphase I. This phenotype is due to the absence of ZmSGO1 protein, a maize shugoshin homolog. ZmSGO1 is localized to the centromeres. The ZmSGO1 protein is not found on mitotic chromosomes and has no obvious mitotic function. On the basis of these results, it is proposed that ZmSGO1 specifically maintains centromeric cohesion during meiosis I and therefore it is suggested that SGO1 core functions during meiosis are conserved across kingdoms and in large-genome species. However, in contrast to other Shugoshins, an early and REC8-dependent recruitment of ZmSGO1 is observed in maize, suggesting that control of SGO1 recruitment to chromosomes is different in plants than in other model organisms (Hamant, 2005).

Mei332 homologs in yeast

Meiosis comprises a pair of specialized nuclear divisions that produce haploid germ cells. To accomplish this, sister chromatids must segregate together during the first meiotic division (meiosis I), which requires that sister chromatid cohesion persists at centromeres. The factors that protect centromeric cohesion during meiosis I have remained elusive. This study identifies Sgo1 (shugoshin), a protector of the centromeric cohesin Rec8 in fission yeast. A homologue of Sgo1 was also identified in budding yeast. Evidence is provided that shugoshin is widely conserved among eukaryotes. Moreover, Sgo2, a paralogue of shugoshin, was identified in fission yeast that is required for faithful mitotic chromosome segregation. Localization of Sgo1 and Sgo2 at centromeres requires the kinase Bub1 (see Drosophila Bub1), identifying shugoshin as a crucial target for the kinetochore function of Bub1. These findings provide insights into the evolution of meiosis and kinetochore regulation during mitosis and meiosis (Kitajima, 2004).

The halving of chromosome number that occurs during meiosis depends on three factors: (1) homologs must pair and recombine; (2) sister centromeres must attach to microtubules that emanate from the same spindle pole, which ensures that homologous maternal and paternal pairs can be pulled in opposite directions (called homolog biorientation); (3) cohesion between sister centromeres must persist after the first meiotic division to enable their biorientation at the second. A screen performed in fission yeast to identify meiotic chromosome missegregation mutants has identified a conserved protein called Sgo1 that is required to maintain sister chromatid cohesion after the first meiotic division. An orthologous protein is described in the budding yeast S. cerevisiae (Sc), which has not only meiotic but also mitotic chromosome segregation functions. Deletion of Sc SGO1 not only causes frequent homolog nondisjunction at meiosis I but also random segregation of sister centromeres at meiosis II. Meiotic cohesion fails to persist at centromeres after the first meiotic division, and sister centromeres frequently separate precociously. Sgo1 is a kinetochore-associated protein whose abundance declines at anaphase I but, nevertheless, persists on chromatin until anaphase II. The finding that Sgo1 is localized to the centromere at the time of the first division suggests that it may play a direct role in preventing the removal of centromeric cohesin. The similarity in sequence composition, chromosomal location, and mutant phenotypes of sgo1 mutants in two distant yeasts with that of MEI-S332 in Drosophila suggests that these proteins define an orthologous family conserved in most eukaryotic lineages (Katis, 2004).

Meiosis produces haploid gametes from diploid progenitor cells. This reduction is achieved by two successive nuclear divisions after one round of DNA replication. Correct chromosome segregation during the first division depends on sister kinetochores being oriented toward the same spindle pole while homologous kinetochores must face opposite poles. Segregation during the second division depends on retention of sister chromatid cohesion between centromeres until the onset of anaphase II, which in Drosophila melanogaster depends on a protein called Mei-S332 that binds to centromeres. Two homologs of Mei-S332 have been identified in fission yeast using a knockout screen. Together with their fly ortholog they define a protein family conserved from fungi to mammals. The two identified genes, sgo1 and sgo2, are required for retention of sister centromere cohesion between meiotic divisions and kinetochore orientation during meiosis I, respectively. The amount of meiotic cohesin's Rec8 subunit retained at centromeres after meiosis I is reduced in Deltasgo1, but not in Deltasgo2, cells, and Sgo1 appears to regulate cleavage of Rec8 by separase. Both Sgo1 and Sgo2 proteins localize to centromere regions. The abundance of Sgo1 protein normally declines after the first meiotic division, but extending its expression by altering its 3'UTR sequences does not greatly affect meiosis II. Its mere presence within the cell might therefore be insufficient to protect centromeric cohesion. In conclusion, a conserved protein family based on Mei-S332 has been identified. The two fission yeast homologs are implicated in meiosis I kinetochore orientation and retention of centromeric sister chromatid cohesion until meiosis II (Rabitsch, 2004).

During meiosis, two chromosome segregation phases follow a single round of DNA replication. Factors that are required to establish this specialized cell cycle have been identified by examining meiotic chromosome segregation in a collection of yeast strains lacking all nonessential genes. This analysis revealed Sgo1, Chl4, and Iml3 to be important for retaining centromeric cohesin until the onset of anaphase II. Consistent with this role, Sgo1 localizes to centromeric regions but dissociates at the onset of anaphase II. The screen described in this study provides a comprehensive analysis of the genes required for the meiotic cell cycle and identifies three factors important for the stepwise loss of sister chromatid cohesion (Marston, 1994).

The stepwise loss of cohesins, the complexes that hold sister chromatids together, is required for faithful meiotic chromosome segregation. Cohesins are removed from chromosome arms during meiosis I but are maintained around centromeres until meiosis II. This study shows that Sgo1, a protein required for protecting centromeric cohesins from removal during meiosis I, localizes to cohesin-associated regions (CARs) at the centromere and the 50-kb region surrounding it. Establishment of this Sgo1-binding domain requires the 120-base-pair (bp) core centromere, the kinetochore component Bub1, and the meiosis-specific factor Spo13. Interestingly, cohesins and the kinetochore proteins Iml3 and Chl4 are necessary for Sgo1 to associate with pericentric regions but less so for Sgo1 to associate with the core centromeric regions. Finally, this study shows that the 50-kb Sgo1-binding domain is the chromosomal region where cohesins are protected from removal during meiosis I. The results identify the portions of chromosomes where cohesins are protected from removal during meiosis I and show that kinetochore components and cohesins themselves are required to establish this cohesin protective domain (Kiburz, 2005).

Chromosome alignment on the mitotic spindle is monitored by the spindle checkpoint. Sgo1, a protein involved in meiotic chromosome cohesion, was identified as a spindle checkpoint component. Budding yeast cells with mutations in SGO1 respond normally to microtubule depolymerization but not to lack of tension at the kinetochore, and they have difficulty attaching sister chromatids to opposite poles of the spindle. Sgo1 is thus required for sensing tension between sister chromatids during mitosis, and its degradation when they separate may prevent cell cycle arrest and chromosome loss in anaphase, a time when sister chromatids are no longer under tension (Indjeian, 2005).

Chromosomes must biorient on the mitotic spindle, with the two sisters attached to opposite spindle poles. The spindle checkpoint detects unattached chromosomes and monitors biorientation by detecting the lack of tension between two sisters attached to the same pole. After the spindle has been depolymerized and allowed to reform, budding yeast sgo1 mutants fail to biorient their sister chromatids and die as cells divide. In sgo1 mutants, chromosomes attach to microtubules normally but cannot reorient if both sisters attach to the same pole. The mutants' fate depends on the position of the spindle poles when the chromosomes attach to microtubules. If the poles have separated, sister chromatids biorient, but if the poles are still close, sister chromatids often attach to the same pole, missegregate, and cause cell death. These observations argue that budding yeast mitotic chromosomes have an intrinsic, geometric bias to biorient on the spindle. When the poles have already separated, attaching one kinetochore to one pole predisposes its sister to attach to the opposite pole, allowing the cells to segregate the chromosomes correctly. When the poles have not separated, the second kinetochore eventually attaches to either of the two poles randomly, causing orientation errors that are corrected in the wild-type but not in sgo1 mutants. In the absence of spindle damage, sgo1 cells divide successfully, suggesting that kinetochores only make stable attachments to microtubules after the cells have entered mitosis and separated their spindle poles (Indjeian, 2007).

Homologue segregation during the first meiotic division requires the proper spatial regulation of sister chromatid cohesion and its dissolution along chromosome arms, but its protection at centromeric regions. This protection requires the conserved MEI-S332/Sgo1 proteins that localize to centromeric regions and also recruit the PP2A phosphatase by binding its regulatory subunit, Rts1. Centromeric Rts1/PP2A then locally prevents cohesion dissolution possibly by dephosphorylating the protein complex cohesin. This study shows that Aurora B kinase in Saccharomyces cerevisiae (Ipl1) is also essential for the protection of meiotic centromeric cohesion. Coupled with a previous study in Drosophila, this meiotic function of Aurora B kinase appears to be conserved among eukaryotes. Furthermore, Sgo1 recruits Ipl1 to centromeric regions. In the absence of Ipl1, Rts1 can initially bind to centromeric regions but disappears from these regions after anaphase I onset. It is suggested that centromeric Ipl1 ensures the continued centromeric presence of active Rts1/PP2A, which in turn locally protects cohesin and cohesion (Yu, 2007).

Fission yeast has two members of the Shugoshin family, Sgo1 and Sgo2. Although Sgo1 has clearly been established as a protector of centromere cohesion in meiosis I, the roles of Sgo2 remain elusive. This study shows that Sgo2 is required to ensure proper chromosome biorientation upon recovery from a prolonged spindle checkpoint arrest. Consistent with this, Sgo2 is essential for maintaining the Passenger proteins on centromeres upon checkpoint activation. Interestingly, lack of Sgo2 has a more penetrant effect on the localization of Survivin than on the two other Passenger proteins INCENP and Aurora B, and the Survivin-INCENP complex but not the INCENP-Aurora B complex is destabilized in the absence of Sgo2. Finally this study shows that the conserved C-terminus of Sgo2 is crucial to maintain Sgo2 and Passenger proteins localization on centromeres upon prolonged checkpoint activation. Taken together, these results demonstrate that Sgo2 is important for chromosome biorientation and that it controls docking of the Passenger proteins on chromosomes in early mitotic cells (Vanoosthuyse, 2007).

Chromosome segregation must be executed accurately during both mitotic and meiotic cell divisions. Sgo1 plays a key role in ensuring faithful chromosome segregation in at least two ways. During meiosis this protein regulates the removal of cohesins, the proteins that hold sister chromatids together, from chromosomes. During mitosis, Sgo1 is required for sensing the absence of tension caused by sister kinetochores not being attached to microtubules emanating from opposite poles. This study describes a differential requirement for Sgo1 in the segregation of homologous chromosomes and sister chromatids. Sgo1 plays only a minor role in segregating homologous chromosomes at meiosis I. In contrast, Sgo1 is important to bias sister kinetochores toward biorientation. It is suggested that Sgo1 acts at sister kinetochores to promote their biorientation (Kiburz, 2008).

Chromosome segregation is triggered by separase, an enzyme that cleaves cohesin, the protein complex that holds sister chromatids together. Separase activation requires the destruction of its inhibitor, securin, which occurs only upon the correct attachment of chromosomes to the spindle. However, other mechanisms restrict separase activity to the appropriate window in the cell cycle because cohesin is cleaved in a timely manner in securin-deficient cells. This study investigated the mechanism by which the protector protein Shugoshin counteracts cohesin cleavage in budding yeast. Shugoshin prevents separase activation independently of securin. Instead, PP2ACdc55 is essential for Shugoshin-mediated inhibition of separase. Loss of both securin and Cdc55 leads to premature sister chromatid separation, resulting in aneuploidy. It is proposed that Cdc55 is a separase inhibitor that acts downstream from Shugoshin under conditions where sister chromatids are not under tension (Clift, 2009).

The separation of chromosomes during cell division is an irreversible event that must be tightly controlled to safeguard against aneuploidy. Sister kinetochores attach to microtubules from opposite spindle pole bodies at metaphase in preparation for their segregation during anaphase of mitosis. The cohesin complex facilitates this process by linking sister chromatids, thereby resisting opposing microtubule forces to generate tension at kinetochores. Once all sister chromatids have made proper bipolar attachments, cohesin is abruptly destroyed, due to cleavage of its Scc1/Mcd1/Rad21 subunit by a protease known as separase, thereby triggering chromosome segregation (Clift, 2009).

Separase must be exquisitely controlled. An inhibitory chaperone, known as securin, plays a key role in preventing separase activation. Securin is destroyed at the onset of anaphase owing to its ubiquitination by the anaphase-promoting complex (APC), coupled to its activator, Cdc20, thereby liberating separase. The spindle assembly checkpoint (SAC) prevents securin ubiquitination in response to defective kinetochore-microtubule attachments by inhibiting APCCdc20 (Clift, 2009).

Despite the importance of securin in preventing separase activation, it is clear that other mechanisms exist to restrict cohesin cleavage to the appropriate window in the cell cycle. Although fission yeast and Drosophila securins are essential for viability, budding yeast cells lacking securin are viable and initiate anaphase in a timely manner. Similarly, securin-deficient mice appear normal and mammalian cells exhibit only mild or transient phenotypes in the absence of securin. An additional level of separase regulation is indeed achieved in vertebrate cells by the inhibitory phosphorylation-dependent binding of Cdk1/cyclin B1 (Clift, 2009).

In addition to this temporal control, cohesin cleavage is additionally subject to spatial regulation during meiosis. Separase-dependent cleavage of the meiosis-specific Scc1 homolog, Rec8, occurs on chromosome arms during meiosis I but is prevented in the vicinity of centromeres until meiosis II owing to the Shugoshin/Mei-S332 family of protector proteins. Shugoshin (Sgo1) achieves this, at least in part, through recruitment of the protein phosphatase 2A, coupled to its B' regulatory subunit (Rts1 in budding yeast) to centromeres. PP2ARts1 is thought to maintain Rec8 around centromeres in its unphosphorylated state, making it refractory to cleavage by separase. However, prevention of Rec8 phosphorylation either by Cdc5 depletion or mutation of its phosphorylation sites blocks cohesin cleavage only in the presence of Sgo1 (Brar, 2006). This suggests that Sgo1 prevents cohesin cleavage in ways other than inhibiting cohesin phosphorylation. Shugoshins also contribute to accurate chromosome segregation during mitosis by monitoring and promoting sister kinetochore biorientation (Indjeian, 2005; Indjeian, 2007; Kawashima, 2007; Vanoosthuyse, 2007; Kiburz, 2008; Clift, 2009 and references therein).

The centromeric cohesin protection and sister kinetochore biorientation functions of Shugoshins all culminate in the restraint of separase activity. This study sought to learn more about the pathways by which Shugoshins accomplish their functions in cell division. Budding yeast has a single Shugoshin, SGO1, that both protects centromeric cohesin during meiosis I and monitors the biorientation of sister chromatids during mitosis. This study shows here that Sgo1 can inhibit separase activity independently of securin. PP2ACdc55 acts downstream from Sgo1 to fully inhibit separase and both monitors sister kinetochore biorientation and prevents cohesin cleavage during meiosis. Cells lacking both securin and Cdc55 prematurely separate sister chromatids, giving rise to aneuploidy. It is proposed that Cdc55 is a separase inhibitor that is activated by Sgo1 when sister chromatids are not under tension (Clift, 2009).

Tension-dependent removal of pericentromeric shugoshin is an indicator of sister chromosome biorientation

During mitosis and meiosis, sister chromatid cohesion resists the pulling forces of microtubules, enabling the generation of tension at kinetochores upon chromosome biorientation. How tension is read to signal the bioriented state remains unclear. Shugoshins form a pericentromeric platform that integrates multiple functions to ensure proper chromosome biorientation. This study shows that budding yeast shugoshin Sgo1 dissociates from the pericentromere reversibly in response to tension. The antagonistic activities of the kinetochore-associated Bub1 kinase (see Drosophila Bub1-related kinase) and the Sgo1-bound phosphatase protein phosphatase 2A (PP2A)-Rts1 (see Drosophila Twins) underlie a tension-dependent circuitry that enables Sgo1 removal upon sister kinetochore biorientation. Sgo1 dissociation from the pericentromere triggers dissociation of condensin and Aurora B (see Drosophila Aurora B) from the centromere, thereby stabilizing the bioriented state. Conversely, forcing sister kinetochores to be under tension during meiosis I leads to premature Sgo1 removal and precocious loss of pericentromeric cohesion. Overall, this study shows that the pivotal role of shugoshin is to build a platform at the pericentromere that attracts activities that respond to the absence of tension between sister kinetochores. Disassembly of this platform in response to intersister kinetochore tension signals the bioriented state. Therefore, tension sensing by shugoshin is a central mechanism by which the bioriented state is read (Nerusheva, 2014).

Mei332 homologs in C. elegans

The Shugoshin/Aurora circuitry that controls the timely release of cohesins from sister chromatids in meiosis and mitosis is widely conserved among eukaryotes, although little is known about its function in organisms whose chromosomes lack a localized centromere. This study shows that C. elegans chromosomes rely on an alternative mechanism to protect meiotic cohesin that is shugoshin-independent and instead involves the activity of a new chromosome-associated protein named LAB-1 (Long Arm of the Bivalent). LAB-1 preserves meiotic sister chromatid cohesion by restricting the localization of the C. elegans Aurora B kinase, AIR-2, to the interface between homologs via the activity of the PP1/Glc7 phosphatase GSP-2. The localization of LAB-1 to chromosomes of dividing embryos and the suppression of mitotic-specific defects in air-2 mutant embryos with reduced LAB-1 activity support a global role of LAB-1 in antagonizing AIR-2 in both meiosis and mitosis. Although the localization of a GFP fusion and the analysis of mutants and RNAi-mediated knockdowns downplay a role for the C. elegans shugoshin protein in cohesin protection, shugoshin nevertheless helps to ensure the high fidelity of chromosome segregation at metaphase I. It is proposed that, in C. elegans, a LAB-1-mediated mechanism evolved to offset the challenges of providing protection against separase activity throughout a larger chromosome area (de Carvalho, 2009).

Mei332 homologs in vertebrates

Cohesion between sister chromatids is essential for their bi-orientation on mitotic spindles. It is mediated by a multisubunit complex called cohesin. In yeast, proteolytic cleavage of cohesin's alpha kleisin subunit at the onset of anaphase removes cohesin from both centromeres and chromosome arms and thus triggers sister chromatid separation. In animal cells, most cohesin is removed from chromosome arms during prophase via a separase-independent pathway involving phosphorylation of its Scc3-SA1/2 subunits. Cohesin at centromeres is refractory to this process and persists until metaphase, whereupon its alpha kleisin subunit is cleaved by separase, which is thought to trigger anaphase. What protects centromeric cohesin from the prophase pathway? Potential candidates are proteins, known as shugoshins, that are homologous to Drosophila Mei-S332 and yeast Sgo1 proteins, which prevent removal of meiotic cohesin complexes from centromeres at the first meiotic division. A vertebrate shugoshin-like protein associates with centromeres during prophase and disappears at the onset of anaphase. Its depletion by RNA interference causes HeLa cells to arrest in mitosis. Most chromosomes bi-orient on a metaphase plate, but precocious loss of centromeric cohesin from chromosomes is accompanied by loss of all sister chromatid cohesion, the departure of individual chromatids from the metaphase plate, and a permanent cell cycle arrest, presumably due to activation of the spindle checkpoint. Remarkably, expression of a version of Scc3-SA2 whose mitotic phosphorylation sites have been mutated to alanine alleviates the precocious loss of sister chromatid cohesion and the mitotic arrest of cells lacking shugoshin. These data suggest that shugoshin, and by inference its Drosophila homolog Mei-S332, prevents phosphorylation of cohesin's Scc3-SA2 subunit at centromeres during mitosis. This ensures that cohesin persists at centromeres until activation of separase causes cleavage of its alpha kleisin subunit. Centromeric cohesion is one of the hallmarks of mitotic chromosomes. These results imply that it is not an intrinsically stable property, because it can easily be destroyed by mitotic kinases, which are kept in check by shugoshin (Marston, 2004).

Drosophila MEI-S332 and fungal Sgo1 genes are essential for sister centromere cohesion in meiosis I. The related vertebrate Sgo localizes to kinetochores and is required to prevent premature sister centromere separation in mitosis, thus providing an explanation for the differential cohesion observed between the arms and the centromeres of mitotic sister chromatids. Sgo is degraded by the anaphase-promoting complex, allowing the separation of sister centromeres in anaphase. Intriguingly, Sgo interacts strongly with microtubules in vitro and it regulates kinetochore microtubule stability in vivo, consistent with a direct microtubule interaction. Sgo is thus critical for mitotic progression and chromosome segregation and provides an unexpected link between sister centromere cohesion and microtubule interactions at kinetochores (Salic, 2004).

Sister chromatids in mammalian cells remain attached mostly at their centromeres at metaphase because of the loss of cohesion along chromosome arms in prophase. Bub1 retains centromeric cohesion in mitosis of human cells. Depletion of Bub1 or Shugoshin (Sgo1) in HeLa cells by RNA interference causes massive missegregation of sister chromatids that originates at centromeres. Surprisingly, loss of chromatid cohesion in Bub1 and Sgo1 RNA-interference cells does not appear to require the full activation of separase but, instead, triggers a mitotic arrest that depends on Mad2 and Aurora B. Bub1 maintains the steady-state levels and centromeric localization of Sgo1. Therefore, Bub1 protects centromeric cohesion through Shugoshin in mitosis (Tang, 2004).

Shugoshin (Sgo) proteins constitute a conserved protein family defined as centromeric protectors of Rec8-containing cohesin complexes in meiosis. In vertebrate mitosis, Scc1/Rad21-containing cohesin complexes (see Drosophila Rad21) are also protected at centromeres because arm cohesin, but not centromeric cohesin, is largely dissociated in prophase and prometaphase. The dissociation process is dependent on the activity of polo-like kinase (Plk1) and partly dependent on Aurora B. Recently, it has been demonstrated that vertebrate shugoshin is required for preserving centromeric cohesion during mitosis; however, whether human shugoshin protects cohesin itself was not addressed. The persistence of human Scc1 at centromeres in mitosis is indeed dependent on human Sgo1. In fission yeast, Sgo localization depends on Bub1, a conserved spindle checkpoint protein, which is enigmatically also required for chromosome congression during prometaphase in vertebrate cells. Human Sgo1 fails to localize at centromeres in Bub1-repressed cells, and centromeric cohesion is significantly loosened. Remarkably, in these cells, Sgo1 relocates to chromosomes all along their length and provokes ectopic protection from dissociation of Scc1 on chromosome arms. These results reveal a hitherto concealed role for human Bub1 in defining the persistent cohesion site of mitotic chromosomes (Kitajima, 2005).

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. This study demonstrates 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. It is proposed 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 (Kawashima, 2007).

This study demonstrates that human shugoshin hSgo1 associates with Survivin and Aurora and requires these components for its centromeric localization. Together with the recent finding in Drosophila that the Aurora kinase complex is required for centromeric localization of Sgo/Mei-S332 (Resnick, 2006), these studies suggest that the linkage between shugoshin and Aurora kinase complex is conserved among eukaryotes. Studies in human cells present the strongest data to date indicating the existence of a complex including shugoshin and Survivin in vivo; hSgo1 could coprecipitate with Survivin better than Aurora in extracts prepared from chromatin fraction. This result fits with the immunoprecipitation using a cross-linker in fission yeast and with genetic results indicating that Sgo2 closely interacts with Bir1/Survivin for the centromeric localization. Although the linkage between shugoshin and the Aurora kinase complex is conserved across species, the precise manner of interaction has apparently diverged. The centromeric localization of Drosophila Mei-S332 reportedly requires phosphorylation by Aurora (Resnick, 2006); however, fission yeast Sgo2 does not require it, albeit Sgo2, like Mei-S332, is a good substrate of Ark1 in vitro. Whereas fission yeast shugoshin (Sgo2) is required for the localization and function of Aurora kinase complex at centromeres, Drosophila Mei-S332 as well as human Sgo1 is not required for the localization of the Aurora kinase complex (Resnick, 2006), albeit the centromeric function of the Aurora kinase complex might nevertheless be regulated by Mei-S332 (Kawashima, 2007).

The sole shugoshin protein in budding yeast seems to play dual roles in protecting centromeric cohesin at meiosis I (but not at mitosis) as well as in establishing tension-generating attachment at mitosis. Drosophila SGO/MEI-S332 mutants show nondisjunction of homologs at meiosis I and a reduced ratio of meta/anaphase (but only slight or little defect in cohesion) in mitosis. Therefore, it is suggested that Mei-S332, the sole shugoshin of Drosophila, is also required for establishing tension-generating attachment, like fission yeast Sgo2. The localization of the Aurora kinase complex does not depend on Mei-S332; however, it is tenable that the activation of centromeric Aurora kinase complex may somewhat depend on Mei-S332 since they physically interact in vitro (Resnick, 2006). Similarly, fission yeast Sgo2 might play an additional role in activating centromeric Aurora rather than merely promoting its localization. Given that hSgo1 associates with Survivin (and Aurora) in HeLa cells, a similar functional link is conceivable also in human cells (Kawashima, 2007).

Studies in fission yeast enabled definition of two distinct shugoshin functions or pathways that are carried out by two diverged shugoshins, Sgo1 and Sgo2; the former interacts with PP2A to protect cohesin, but the latter interacts with the Aurora kinase complex to facilitate centromeric Aurora function. It is speculated that the ancestral shugoshin molecule played dual roles at kinetochores like in budding yeast or Drosophila; fission yeast shugoshin might have divided the labor to Sgo1 and Sgo2. Thus, these findings of a functional link between Sgo2 and the Aurora kinase complex open a new view that shugoshin in general may play a role in facilitating Aurora function at centromeres, thereby ensuring tension-generating kinetochore microtubule attachment. At the centromere, microtubule attachment is ensured by tension across centromeres, which is generated depending on the cohesion between sister chromatids. Therefore, cohesion and tension are two sides of a 'coin' ensuring bipolar attachment of kinetochores. It is suggested that the original role of shugoshin was to guarantee bipolar attachment rather than to protect cohesin, because fission yeast and presumably budding yeast, two primitive eukaryotes, exhibit this role only during mitosis. The protection role, once acquired, might facilitate the generation of tension by counteracting the spindle force, improving the fidelity of chromosome segregation. This function might have been modified to evolve meiosis, in which the requirement for centromeric protection is more essential and therefore has been preserved in all eukaryotes. Whatever the validity of this view, the finding of how Sgo2 acts will contribute to understand the fundamental regulation of eukaryotic chromosome segregation (Kawashima, 2007).

Structure and function of the PP2A-shugoshin interaction

Accurate chromosome segregation during mitosis and meiosis depends on shugoshin proteins that prevent precocious dissociation of cohesin from centromeres. Shugoshins associate with PP2A, which is thought to dephosphorylate cohesin and thereby prevent cleavage by separase during meiosis I. A crystal structure of a complex between a fragment of human Sgo1 and an AB'C PP2A holoenzyme reveals that Sgo1 forms a homodimeric parallel coiled coil that docks simultaneously onto PP2A's C and B' subunits. Sgo1 homodimerization is a prerequisite for PP2A binding. While hSgo1 interacts only with the AB'C holoenzymes, its relative, Sgo2, interacts with all PP2A forms and may thus lead to dephosphorylation of distinct substrates. Mutant shugoshin proteins defective in the binding of PP2A cannot protect centromeric cohesin from separase during meiosis I or support the spindle assembly checkpoint in yeast. Finally, evidence is provided that PP2A's recruitment to chromosomes may be sufficient to protect cohesin from separase in mammalian oocytes (Xu, 2009).

Sgol2 provides a regulatory platform that coordinates essential cell cycle processes during meiosis I in oocytes

Accurate chromosome segregation depends on coordination between cohesion resolution and kinetochore-microtubule interactions (K-fibers), a process regulated by the spindle assembly checkpoint (SAC). How these diverse processes are coordinated remains unclear. This study shows that in mammalian oocytes Shugoshin-like protein 2 (Sgol2; Drosophila homolog Mei-S332) in addition to protecting cohesin, plays an important role in turning off the SAC, in promoting the congression and bi-orientation of bivalents on meiosis I spindles, in facilitating formation of K-fibers and in limiting bivalent stretching. Sgol2's ability to protect cohesin depends on its interaction with PP2A, as is its ability to silence the SAC, with the latter being mediated by direct binding to Mad2. In contrast, its effect on bivalent stretching and K-fiber formation is independent of PP2A and mediated by recruitment of MCAK and inhibition of Aurora C kinase activity respectively. By virtue of its multiple interactions, Sgol2 links many of the processes essential for faithful chromosome segregation (Rattani, 2013).

The production of haploid gametes from diploid germ cells depends on two rounds of chromosome segregation (meiosis I and II) without an intervening round of DNA replication. Defects during the first or second meiotic division in oocytes lead to formation of aneuploid eggs, which in humans occurs with a frequency between 10 and 30% and is a major cause of fetal miscarriage. Understanding the causes of meiotic chromosome missegregation will require clarifying not only the forces and regulatory mechanisms governing meiotic chromosome segregation but also how these are coordinated (Rattani, 2013).

At the heart of this process are two opposing forces. The pulling forces produced by kinetochore-microtubules attachments (K-fibers) and resisting forces generated by sister chromatid cohesion, which counteracts K-fiber forces if and when kinetochores attach to microtubules with different polarities. During meiosis I, cohesion along chromosome arms holds bivalent chromosomes together following the creation of chiasmata produced by reciprocal recombination between homologous non-sister chromatids. This cohesion must persist during the attachment of maternal and paternal kinetochores to microtubules from opposite poles (bi-orientation) and resist the resulting spindle forces. The degree of traction exerted by meiotic spindles must create sufficient tension to facilitate bi-orientation. During the bi-orientation process, the initial attachment of kinetochores to the surface of the microtubule lattice (lateral attachment) is converted to attachments to their plus ends (end on attachment), creating K-fibers. Because this process is intrinsically error prone, inappropriate attachments, for example those that connect maternal and paternal kinetochores to the same pole, must be disrupted through phosphorylation of kinetochore proteins by Aurora B/C kinases. However, because these kinases disrupt K-fibers, they must subsequently be down-regulated once bivalents bi-orient correctly (Rattani, 2013).

The first meiotic division is eventually triggered by activation of a gigantic ubiquitin protein ligase called the anaphase-promoting complex or cyclosome (APC/C) whose destruction of securin and cyclin B activates a thiol protease called separase that cleaves the kleisin subunit of the cohesin complex holding sister chromatids together. This process converts chromosomes from bivalents to dyads. It is delayed until all bivalents have bi-oriented by the production, at kinetochores that have not yet come under tension, of a potent inhibitor of the APC/C called the mitotic checkpoint complex (MCC) whose Mad2 subunit binds tightly to the APC/C's Cdc20 co-activator protein. This regulatory mechanism, called the spindle assembly checkpoint (SAC), must be turned off before APC/CCdc20 can direct destruction of securin and cyclin B and thereby activate separase. Another pre-condition for cleavage, at least in yeast, is phosphorylation of cohesin's kleisin subunit by a pair of protein kinases, namely CK1δ/ε and DDK. During the first meiotic division, cohesin is phosphorylated along chromosome arms but not at centromeres, which ensures that only cohesion along arms is destroyed by separase at the onset of anaphase I. The consequent persistence of cohesion at centromeres promotes bi-orientation of dyads during meiosis II (Rattani, 2013).

Centromeric cohesin avoids phosphorylation and therefore cleavage during the first meiotic division because separase activation is preceded by the recruitment to centromeres of orthologues of the Drosophila MEI-S332 protein, called shugoshins. Members of this family contain a conserved C-terminal basic region and an N-terminal homodimeric parallel coiled coil, which provides a docking site for protein phosphatase 2A's (PP2A) C and B' subunits (Xu, 2009). In budding yeast, mutant proteins defective specifically in PP2A binding fail to confer protection of centromeric cohesion during meiosis I (Rattani, 2013).

Mammals have two members of the shugoshin family: Shugoshin-like protein 1 (Sgol1), and Shugoshin-like protein 2 (Sgol2). The former prevents centromeric cohesin from a process called the 'prophase pathway' that removes cohesin from chromosomes by a non-proteolytic mechanism soon after cells enter mitosis (McGuinness, 2005; Liu, 2013). Sgol2, on the other hand, protects centromeric cohesin from separase at the first meiotic division (Lee, 2008; Llano, 2008). Sgol2's coiled coil domain binds PP2A in vitro (Xu, 2009) but whether this is vital for protecting centromeric cohesion is not known. Sgol2 also interacts with MCAK (Huang, 2007), a microtubule depolymerizing kinesin, implicated in correcting inappropriate kinetochore-microtubule interactions (error correction), and with Mad2 an essential component of the MCC (Orth, 2011). Shugoshins, though not explicitly Sgol2, have also been implicated in recruiting to kinetochores the Aurora B kinase, necessary both for the SAC and for error correction (Tsukahara, 2010; Yamagishi, 2010). Based on its interactions, Sgol2 has been linked to cohesion protection, the spindle assembly checkpoint, and error correction pathways. However, the physiological significance of these multiple interactions remain unclear (Rattani, 2013).

This study shows that Sgol2 defective in PP2A binding fails to protect centromeric cohesin, as found for Sgo1 in yeast (Xu, 2009). However, if this were the sole function of Sgol2, then chromosome behavior during meiosis I should be unaffected. Surprisingly it was found that Sgol2 deficiency caused striking changes in chromosome and microtubule dynamics. It delayed bi-orientation of bivalents on meiosis I spindles, caused increased bivalent stretching, and greatly increased Aurora B/C kinase activity at kinetochores, which was accompanied by an increase in lateral and a decrease in end on kinetochore-microtubules attachments. Lastly, Sgol2 deficiency delayed considerably APC/CCdc20 activation, suggesting that it was required to shut off the SAC. Sgol2 helps to turn off the SAC by binding directly both PP2A and the MCC protein Mad2, it moderates chromosome stretching by recruiting to kinetochores the kinesin MCAK, and likely promotes formation of K-fibers by down-regulating the activity of Aurora B/C kinase specifically at kinetochores. Meiotic chromosome segregation is a highly complex process dependent on an array of different biochemical processes. These findings imply that through its multi-domain structure, Sgol2 has an important if not unique role in coordinating many of the key processes within this array (Rattani, 2013).

APC/CCdh1 enables removal of Shugoshin-2 from the arms of bivalent chromosomes by moderating Cyclin-dependent kinase activity

In mammalian females, germ cells remain arrested as primordial follicles. Resumption of meiosis is heralded by germinal vesicle breakdown, condensation of chromosomes, and their eventual alignment on metaphase plates. At the first meiotic division, anaphase-promoting complex/cyclosome associated with Cdc20 (APC/CCdc20; see Drosophila Cdc20) activates separase (see Drosophila Separase) and thereby destroys cohesion along chromosome arms. Because cohesion around centromeres is protected by shugoshin-2 (see Drosophila mei-S332), sister chromatids remain attached through centromeric/pericentromeric cohesin. This study shows that, by promoting proteolysis of cyclins and Cdc25B (see Drosophila String) at the germinal vesicle (GV) stage, APC/C associated with the Cdh1 protein (APC/CCdh1; see Drosophila Fizzy-related) delays the increase in Cdk1 (see Drosophila Cdk2) activity, leading to germinal vesicle breakdown (GVBD). More surprisingly, by moderating the rate at which Cdk1 is activated following GVBD, APC/CCdh1 creates conditions necessary for the removal of shugoshin-2 from chromosome arms by the Aurora B/C kinase (see Drosophila Aurora B), an event crucial for the efficient resolution of chiasmata (Rattani, 2017).

mei-S332: Biological Overview | Regulation | Developmental Biology | Effects of Mutation | References

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