Pds5B is required for cohesion establishment and Aurora B accumulation at centromeres

Cohesin mediates sister chromatid cohesion and contributes to the organization of interphase chromatin through DNA looping. In vertebrate somatic cells, cohesin consists of Smc1, Smc3, Rad21, and either SA1 or SA2. Three additional factors Pds5, Wapl, and Sororin bind to cohesin and modulate its dynamic association with chromatin. There are two Pds5 proteins in vertebrates, Pds5A and Pds5B, but their functional specificity remains unclear. Here, we demonstrate that Pds5 proteins are essential for cohesion establishment by allowing Smc3 acetylation by the cohesin acetyl transferases (CoATs) Esco1/2 and binding of Sororin. While both proteins contribute to telomere and arm cohesion, Pds5B is specifically required for centromeric cohesion. Furthermore, reduced accumulation of Aurora B at the inner centromere region in cells lacking Pds5B impairs its error correction function, promoting chromosome mis‐segregation and aneuploidy. Our work supports a model in which the composition and function of cohesin complexes differs between different chromosomal regions.     

Cohesin acetylation and Wapl‐Pds5 oppositely regulate translocation of cohesin along DNA

Cohesin is a ring‐shaped protein complex that plays a crucial role in sister chromatid cohesion and gene expression. The dynamic association of cohesin with chromatin is essential for these functions. However, the exact nature of cohesin dynamics, particularly cohesin translocation, remains unclear. We evaluated the dynamics of individual cohesin molecules on DNA and found that the cohesin core complex possesses an intrinsic ability to traverse DNA in an adenosine triphosphatase (ATPase)‐dependent manner. Translocation ability is suppressed in the presence of Wapl‐Pds5 and Sororin; this suppression is alleviated by the acetylation of cohesin and the action of mitotic kinases. In Xenopus laevis egg extracts, cohesin is translocated on unreplicated DNA in an ATPase‐ and Smc3 acetylation‐dependent manner. Cohesin movement changes from bidirectional to unidirectional when cohesin faces DNA replication; otherwise, it is incorporated into replicating DNA without being translocated or is dissociated from replicating DNA. This study provides insight into the nature of individual cohesin dynamics and the mechanisms by which cohesin achieves cohesion in different chromatin contexts.     

C-terminus of Sororin interacts with SA2 and regulates sister chromatid cohesion

Sororin is a conserved protein required for accurate separation of sister chromatids in each cell cycle. Sororin is recruited to chromatin during DNA replication, protects sister chromatid cohesion in S and G2 phase, and regulates the resolution of sister chromatid cohesion in mitosis. Sororin binds to cohesin complex, but how Sororin and cohesin subunits interact remains unclear. Here we report that the C-terminus of Sororin, especially the last 12 amino acid (aa) residues, is important for Sororin to bind cohesin core subunit SA2. Deletion of the last 12aa residues not only inhibits the interactions between Sororin and SA2 but also causes precocious chromosome separation. Our data suggest that the C-terminus of Sororin functions as an anchor binding to SA2, which facilitates other conserved motifs on Sororin to interact with other proteins to regulate sister chromatid cohesion and separation.     

Sororin is a master regulator of sister chromatid cohesion and separation

The maintenance of sister chromatid cohesion from S phase to the onset of anaphase relies on a small but evolutionarily conserved protein called Sororin. Sororin is a phosphoprotein and its dynamic localization and function are regulated by protein kinases, such as Cdk1/cyclin B and Erk2. The association of Sororin with chromatin requires cohesin to be preloaded to chromatin and modification of Smc3 during DNA replication. Sororin antagonizes the function of Wapl in cohesin releasing from S to G₂ phase and promotes cohesin release from sister chromatid arms in prophase via interaction with Plk1. This review focuses on progress of the identification and regulation of Sororin during cell cycle; role of post-translational modification on Sororin function; role of Sororin in the maintenance and resolution of sister chromatid cohesion; and finally discusses Sororin’s emerging role in cancer and the potential issues that need be addressed in the future.     

Sororin is a master regulator of sister chromatid cohesion and separation

The maintenance of sister chromatid cohesion from S phase to the onset of anaphase relies on a small but evolutionarily conserved protein called Sororin. Sororin is a phosphoprotein and its dynamic localization and function are regulated by protein kinases, such as Cdk1/cyclin B and Erk2. The association of Sororin with chromatin requires cohesin to be preloaded to chromatin and modification of Smc3 during DNA replication. Sororin antagonizes the function of Wapl in cohesin releasing from S to G₂ phase and promotes cohesin release from sister chromatid arms in prophase via interaction with Plk1. This review focuses on progress of the identification and regulation of Sororin during cell cycle; role of post-translational modification on Sororin function; role of Sororin in the maintenance and resolution of sister chromatid cohesion; and finally discusses Sororin’s emerging role in cancer and the potential issues that need be addressed in the future.     

A kinase‐dependent role for Haspin in antagonizing Wapl and protecting mitotic centromere cohesion

Sister‐chromatid cohesion mediated by the cohesin complex is fundamental for precise chromosome segregation in mitosis. Through binding the cohesin subunit Pds5, Wapl releases the bulk of cohesin from chromosome arms in prophase, whereas centromeric cohesin is protected from Wapl until anaphase onset. Strong centromere cohesion requires centromeric localization of the mitotic histone kinase Haspin, which is dependent on the interaction of its non‐catalytic N‐terminus with Pds5B. It remains unclear how Haspin fully blocks the Wapl–Pds5B interaction at centromeres. Here, we show that the C‐terminal kinase domain of Haspin (Haspin‐KD) binds and phosphorylates the YSR motif of Wapl (Wapl‐YSR), thereby directly inhibiting the YSR motif‐dependent interaction of Wapl with Pds5B. Cells expressing a Wapl‐binding‐deficient mutant of Haspin or treated with Haspin inhibitors show centromeric cohesion defects. Phospho‐mimetic mutation in Wapl‐YSR prevents Wapl from binding Pds5B and releasing cohesin. Forced targeting Haspin‐KD to centromeres partly bypasses the need for Haspin–Pds5B interaction in cohesion protection. Taken together, these results indicate a kinase‐dependent role for Haspin in antagonizing Wapl and protecting centromeric cohesion in mitosis.     

A conserved motif at the C terminus of sororin is required for sister chromatid cohesion

Sororin is a positive regulator of sister chromatid cohesion that interacts with the cohesin complex. Sororin is required for the increased stability of the cohesin complex on chromatin following DNA replication and sister chromatid cohesion during G(2). The mechanism by which sororin ensures cohesion is currently unknown. Because the primary sequence of sororin does not contain any previously characterized structural or functional motifs, we have undertaken a structure-function analysis of the sororin protein. Using a series of mutant derivatives of sororin, we show that the ability of sororin to bind to chromatin is separable from both its role in sister chromatid cohesion and its interaction with the cohesin complex. We also show that derivatives of sororin with deletions or mutations in the conserved C terminus fail to rescue the loss-of-cohesion phenotype caused by sororin RNAi and that these mutations also abrogate the association of sororin with the cohesin complex. Our data suggest that the interaction of the highly conserved motif at the C terminus of sororin with the cohesin complex is critical to its ability to mediate sister chromatid cohesion.     

Pds5 promotes cohesin acetylation and stable cohesin-chromosome interaction

Pds5 and Wpl1 act as anti-establishment factors preventing sister-chromatid cohesion until counteracted in S-phase by the cohesin acetyl-transferase Eso1. However, Pds5 is also required to maintain sister-chromatid cohesion in G2. Here, we show that Pds5 is essential for cohesin acetylation by Eso1 and ensures the maintenance of cohesion by promoting a stable cohesin interaction with replicated chromosomes. The latter requires Eso1 only in the presence of Wapl, indicating that cohesin stabilization relies on Eso1 only to neutralize the anti-establishment activity. We suggest that Eso1 requires Pds5 to counteract anti-establishment. This allows both cohesion establishment and Pds5-dependent stable cohesin binding to chromosomes.     

Cohesin acetyltransferase Esco2 is a cell viability factor and is required for cohesion in pericentric heterochromatin

Sister chromatid cohesion, mediated by cohesin and regulated by Sororin, is essential for chromosome segregation. In mammalian cells, cohesion establishment and Sororin recruitment to chromatin-bound cohesin depends on the acetyltransferases Esco1 and Esco2. Mutations in Esco2 cause Roberts syndrome, a developmental disease in which mitotic chromosomes have a 'railroad' track morphology. Here, we show that Esco2 deficiency leads to termination of mouse development at pre- and post-implantation stages, indicating that Esco2 functions non-redundantly with Esco1. Esco2 is transiently expressed during S-phase when it localizes to pericentric heterochromatin (PCH). In interphase, Esco2 depletion leads to a reduction in cohesin acetylation and Sororin recruitment to chromatin. In early mitosis, Esco2 deficiency causes changes in the chromosomal localization of cohesin and its protector Sgo1. Our results suggest that Esco2 is needed for cohesin acetylation in PCH and that this modification is required for the proper distribution of cohesin on mitotic chromosomes and for centromeric cohesion.     

An Eco1-independent sister chromatid cohesion establishment pathway in S. cerevisiae

Cohesion between sister chromatids, mediated by the chromosomal cohesin complex, is a prerequisite for their alignment on the spindle apparatus and segregation in mitosis. Budding yeast cohesin first associates with chromosomes in G1. Then, during DNA replication in S-phase, the replication fork-associated acetyltransferase Eco1 acetylates the cohesin subunit Smc3 to make cohesin’s DNA binding resistant to destabilization by the Wapl protein. Whether stabilization of cohesin molecules that happen to link sister chromatids is sufficient to build sister chromatid cohesion, or whether additional reactions are required to establish these links, is not known. In addition to Eco1, several other factors contribute to cohesion establishment, including Ctf4, Ctf18, Tof1, Csm3, Chl1 and Mrc1, but little is known about their roles. Here, we show that each of these factors facilitates cohesin acetylation. Moreover, the absence of Ctf4 and Chl1, but not of the other factors, causes a synthetic growth defect in cells lacking Eco1. Distinct from acetylation defects, sister chromatid cohesion in ctf4Δ and chl1Δ cells is not improved by removing Wapl. Unlike previously thought, we do not find evidence for a role of Ctf4 and Chl1 in Okazaki fragment processing, or of Okazaki fragment processing in sister chromatid cohesion. Thus, Ctf4 and Chl1 delineate an additional acetylation-independent pathway that might hold important clues as to the mechanism of sister chromatid cohesion establishment.     

Sororin actively maintains sister chromatid cohesion

Cohesion between sister chromatids is established during DNA replication but needs to be maintained to enable proper chromosome–spindle attachments in mitosis or meiosis. Cohesion is mediated by cohesin, but also depends on cohesin acetylation and sororin. Sororin contributes to cohesion by stabilizing cohesin on DNA. Sororin achieves this by inhibiting WAPL, which otherwise releases cohesin from DNA and destroys cohesion. Here we describe mouse models which enable the controlled depletion of sororin by gene deletion or auxin‐induced degradation. We show that sororin is essential for embryonic development, cohesion maintenance, and proper chromosome segregation. We further show that the acetyltransferases ESCO1 and ESCO2 are essential for stabilizing cohesin on chromatin, that their only function in this process is to acetylate cohesin''s SMC3 subunit, and that DNA replication is also required for stable cohesin–chromatin interactions. Unexpectedly, we find that sororin interacts dynamically with the cohesin complexes it stabilizes. This implies that sororin recruitment to cohesin does not depend on the DNA replication machinery or process itself, but on a property that cohesin acquires during cohesion establishment.     

Sororin pre‐mRNA splicing is required for proper sister chromatid cohesion in human cells

Sister chromatid cohesion, which depends on cohesin, is essential for the faithful segregation of replicated chromosomes. Here, we report that splicing complex Prp19 is essential for cohesion in both G2 and mitosis, and consequently for the proper progression of the cell through mitosis. Inactivation of splicing factors SF3a120 and U2AF65 induces similar cohesion defects to Prp19 complex inactivation. Our data indicate that these splicing factors are all required for the accumulation of cohesion factor Sororin, by facilitating the proper splicing of its pre‐mRNA. Finally, we show that ectopic expression of Sororin corrects defective cohesion caused by Prp19 complex inactivation. We propose that the Prp19 complex and the splicing machinery contribute to the establishment of cohesion by promoting Sororin accumulation during S phase, and are, therefore, essential to the maintenance of genome stability.     

Wapl controls the dynamic association of cohesin with chromatin

Cohesin establishes sister-chromatid cohesion from S phase until mitosis or meiosis. To allow chromosome segregation, cohesion has to be dissolved. In vertebrate cells, this process is mediated in part by the protease separase, which destroys a small amount of cohesin, but most cohesin is removed from chromosomes without proteolysis. How this is achieved is poorly understood. Here, we show that the interaction between cohesin and chromatin is controlled by Wapl, a protein implicated in heterochromatin formation and tumorigenesis. Wapl is associated with cohesin throughout the cell cycle, and its depletion blocks cohesin dissociation from chromosomes during the early stages of mitosis and prevents the resolution of sister chromatids until anaphase, which occurs after a delay. Wapl depletion also increases the residence time of cohesin on chromatin in interphase. Our data indicate that Wapl is required to unlock cohesin from a particular state in which it is stably bound to chromatin.     

Cell-cycle regulation of cohesin stability along fission yeast chromosomes

Sister chromatid cohesion is mediated by cohesin, but the process of cohesion establishment during S-phase is still enigmatic. In mammalian cells, cohesin binding to chromatin is dynamic in G1, but becomes stabilized during S-phase. Whether the regulation of cohesin stability is integral to the process of cohesion establishment is unknown. Here, we provide evidence that fission yeast cohesin also displays dynamic behavior. Cohesin association with G1 chromosomes requires continued activity of the cohesin loader Mis4/Ssl3, suggesting that repeated loading cycles maintain cohesin binding. Cohesin instability in G1 depends on wpl1, the fission yeast ortholog of mammalian Wapl, suggestive of a conserved mechanism that controls cohesin stability on chromosomes. wpl1 is nonessential, indicating that a change in wpl1-dependent cohesin dynamics is dispensable for cohesion establishment. Instead, we find that cohesin stability increases at the time of S-phase in a reaction that can be uncoupled from DNA replication. Hence, cohesin stabilization might be a pre-requisite for cohesion establishment rather than its consequence.     

The cohesion stabilizer sororin favors DNA repair and chromosome segregation during mouse oocyte meiosis

Maintenance and timely termination of cohesion on chromosomes ensures accurate chromosome segregation to guard against aneuploidy in mammalian oocytes and subsequent chromosomally abnormal pregnancies. Sororin, a cohesion stabilizer whose relevance in antagonizing the anti-cohesive property of Wings-apart like protein (Wapl), has been characterized in mitosis; however, the role of Sororin remains unclear during mammalian oocyte meiosis. Here, we show that Sororin is required for DNA damage repair and cohesion maintenance on chromosomes, and consequently, for mouse oocyte meiotic program. Sororin is constantly expressed throughout meiosis and accumulates on chromatins at germinal vesicle (GV) stage/G2 phase. It localizes onto centromeres from germinal vesicle breakdown (GVBD) to metaphase II stage. Inactivation of Sororin compromises the GVBD and first polar body extrusion (PBE). Furthermore, Sororin inactivation induces DNA damage indicated by positive γH2AX foci in GV oocytes and precocious chromatin segregation in MII oocytes. Finally, our data indicate that PlK1 and MPF dissociate Sororin from chromosome arms without affecting its centromeric localization. Our results define Sororin as a determinant during mouse oocyte meiotic maturation by favoring DNA damage repair and chromosome separation, and thereby, maintaining the genome stability and generating haploid gametes.     

RBMX: a regulator for maintenance and centromeric protection of sister chromatid cohesion

Cohesion is essential for the identification of sister chromatids and for the biorientation of chromosomes until their segregation. Here, we have demonstrated that an RNA-binding motif protein encoded on the X chromosome (RBMX) plays an essential role in chromosome morphogenesis through its association with chromatin, but not with RNA. Depletion of RBMX by RNA interference (RNAi) causes the loss of cohesin from the centromeric regions before anaphase, resulting in premature chromatid separation accompanied by delocalization of the shugoshin complex and outer kinetochore proteins. Cohesion defects caused by RBMX depletion can be detected as early as the G2 phase. Moreover, RBMX associates with the cohesin subunits, Scc1 and Smc3, and with the cohesion regulator, Wapl. RBMX is required for cohesion only in the presence of Wapl, suggesting that RBMX is an inhibitor of Wapl. We propose that RBMX is a cohesion regulator that maintains the proper cohesion of sister chromatids.     

Interaction of Sororin protein with polo-like kinase 1 mediates resolution of chromosomal arm cohesion

Unlike in budding yeast, sister chromatid cohesion in vertebrate cells is resolved in two steps: cohesin complexes are removed from sister chromatid arms during prophase via phosphorylation, whereas centromeric cohesins are removed at anaphase by Separase. Phosphorylation of cohesin subunit SA2 by polo-like kinase 1 (Plk1) is required for the removal of cohesins at prophase, but how Plk1 is recruited to phosphorylate SA2 during prophase is currently not known. Here we report that Sororin, a cohesin-interacting protein essential for sister chromatid cohesion, plays a novel role in the resolution of sister chromatid arms by direct interaction with Plk1. We identified an evolutionarily conserved motif (ST(159)P) on Sororin, which was phosphorylated by Cdk1/cyclin B and bound to the polo box domain of Plk1. Mutating Thr(159) into alanine prevented the interaction of Plk1 and Sororin and inhibited the resolution of chromosomal arm cohesion. We propose that Sororin is phosphorylated by Cdk1/cyclin B at prophase and acts as a docking protein to bring Plk1 into proximity with SA2, resulting in the phosphorylation of SA2 and the removal of cohesin complexes from chromosomal arms.     

Sister Chromatid Cohesion

During S phase, not only does DNA have to be replicated, but also newly synthesized DNA molecules have to be connected with each other. This sister chromatid cohesion is essential for the biorientation of chromosomes on the mitotic or meiotic spindle, and is thus an essential prerequisite for chromosome segregation. Cohesion is mediated by cohesin complexes that are thought to embrace sister chromatids as large rings. Cohesin binds to DNA dynamically before DNA replication and is converted into a stably DNA-bound form during replication. This conversion requires acetylation of cohesin, which in vertebrates leads to recruitment of sororin. Sororin antagonizes Wapl, a protein that is able to release cohesin from DNA, presumably by opening the cohesin ring. Inhibition of Wapl by sororin therefore “locks” cohesin rings on DNA and allows them to maintain cohesion for long periods of time in mammalian oocytes, possibly for months or even years.DNA replication during the synthesis (S) phase generates identical DNA molecules, which, in their chromatinized form, are called sister chromatids. The pairs of sister chromatids remain united as part of one chromosome during the subsequent gap (G₂) phase and during early mitosis, in prophase, prometaphase, and metaphase. During these stages of mitosis chromosomes condense, in most eukaryotes the nuclear envelope breaks down, and in all species chromosomes are ultimately attached to both poles of the mitotic spindle. Only once this biorientation has been achieved for all chromosomes, the sister chromatids are separated from each other in anaphase and transported toward opposite spindle poles of the mother cell, enabling its subsequent division into two genetically identical daughter cells.This series of events critically depends on the fact that sister chromatids remain physically connected with each other from S phase until metaphase. This physical connection, called sister chromatid cohesion, opposes the pulling forces that are generated by microtubules that attach to kinetochores and thereby enables the biorientation of chromosomes on the mitotic spindle (Tanaka et al. 2000b). Without cohesion, sister chromatids could therefore not be segregated symmetrically between the forming daughter cells, resulting in aneuploidy. For the same reasons, cohesion is essential for chromosome segregation in meiosis I and meiosis II. Cohesion defects in human oocytes can lead to aneuploidy, which is thought to be the major cause of spontaneous abortion, because only a few types of aneuploidy are compatible with viability, such as trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), and trisomy 13 (Patau syndrome) (Hunt and Hassold 2010). Studying the mechanisms of cohesion is therefore essential for understanding how the genome is passed properly from one cell generation to the next.In addition, sister chromatid cohesion facilitates the repair of DNA double-strand breaks in cells that have replicated their DNA, where such breaks can be repaired by a homologous recombination mechanism that uses the undamaged sister chromatid as a template (for review, see Watrin et al. 2006). Furthermore, mutations in the proteins that are required for sister chromatid cohesion can cause defects in chromatin structure and gene regulation, and can in rare cases lead to congenital developmental disorders, called Cornelia de Lange syndrome, Roberts/SC Phocomelia syndrome, and Warsaw Breakage syndrome (for review, see Mannini et al. 2010).     

Sororin loads to the synaptonemal complex central region independently of meiotic cohesin complexes

The distribution and regulation of the cohesin complexes have been extensively studied during mitosis. However, the dynamics of their different regulators in vertebrate meiosis is largely unknown. In this work, we have analyzed the distribution of the regulatory factor Sororin during male mouse meiosis. Sororin is detected at the central region of the synaptonemal complex during prophase I, in contrast with the previously reported localization of other cohesin components in the lateral elements. This localization of Sororin depends on the transverse filaments protein SYCP1, but not on meiosis‐specific cohesin subunits REC8 and SMC1β. By late prophase I, Sororin accumulates at centromeres and remains there up to anaphase II. The phosphatase activity of PP2A seems to be required for this accumulation. We hypothesize that Sororin function at the central region of the synaptonemal complex could be independent on meiotic cohesin complexes. In addition, we suggest that Sororin participates in the regulation of centromeric cohesion during meiosis in collaboration with SGO2‐PP2A.     

Cohesin's DNA Exit Gate Is Distinct from Its Entrance Gate and Is Regulated by Acetylation

Sister chromatid cohesion is mediated by entrapment of sister DNAs by a tripartite ring composed of cohesin's Smc1, Smc3, and α-kleisin subunits. Cohesion requires acetylation of Smc3 by Eco1, whose role is to counteract an inhibitory (antiestablishment) activity associated with cohesin's Wapl subunit. We show that mutations abrogating antiestablishment activity also reduce turnover of cohesin on pericentric chromatin. Our results reveal?a "releasing" activity inherent to cohesin complexes transiently associated with Wapl that catalyzes their dissociation from chromosomes. Fusion of Smc3's nucleotide binding domain to α-kleisin's N-terminal domain also reduces cohesin turnover within pericentric chromatin and permits establishment of Wapl-resistant cohesion in the absence of Eco1. We suggest that releasing activity opens the Smc3/α-kleisin interface, creating a DNA exit gate distinct from its proposed entry gate at the Smc1/3 interface. According to this notion, the function of Smc3 acetylation is to block its dissociation from α-kleisin. The functional implications of regulated ring opening are discussed.