Hexavalent Chromium Targets Securin to Drive Numerical Chromosome Instability in Human Lung Cells

Hexavalent chromium [Cr(VI)] is a known human lung carcinogen with widespread exposure in environmental and occupational settings. Despite well-known cancer risks, the molecular mechanisms of Cr(VI)-induced carcinogenesis are not well understood, but a major driver of Cr(VI) carcinogenesis is chromosome instability. Previously, we reported Cr(VI) induced numerical chromosome instability, premature centriole disengagement, centrosome amplification, premature centromere division, and spindle assembly checkpoint bypass. A key regulator of these events is securin, which acts by regulating the cleavage ability of separase. Thus, in this study we investigated securin disruption by Cr(VI) exposure. We exposed human lung cells to a particulate Cr(VI) compound, zinc chromate, for acute (24 h) and prolonged (120 h) time points. We found prolonged Cr(VI) exposure caused marked decrease in securin levels and function. After prolonged exposure at the highest concentration, securin protein levels were decreased to 15.3% of control cells, while securin mRNA quantification was 7.9% relative to control cells. Additionally, loss of securin function led to increased separase activity manifested as enhanced cleavage of separase substrates; separase, kendrin, and SCC1. These data show securin is targeted by prolonged Cr(VI) exposure in human lung cells. Thus, a new mechanistic model for Cr(VI)-induced carcinogenesis emerges with centrosome and centromere disruption as key components of numerical chromosome instability, a key driver in Cr(VI) carcinogenesis.

Correlation Between Serum ESPL1 and Hepatitis B Virus-related Hepatocellular Carcinoma Histological Grade: A Chinese Single-center Case-control Study

BACKGROUND/AIM

Serum markers to determine the histological grade of hepatitis B virus (HBV)-related hepatocellular carcinoma (HCC) are still limited. This study aimed to investigate if serum extra spindle pole bodies-like 1 (ESPL1) protein could reflect the histological grade of HBV-related HCC.

MATERIALS AND METHODS

A total of 154 patients with HBV-related HCC were enrolled in the experimental group and 41 non-HBV-related patients in the control. Enzyme-linked immunosorbent assay was used to detect serum ESPL1 levels. The differences in serological ESPL1, alpha-fetoprotein (AFP), and des-gamma-carboxy prothrombin (DCP) were compared between the two groups. HCC tumor diameter was measured, and pathological examination was performed to compare the relationship between ESPL1, AFP, and DCP and tumor size and histological grade.

RESULTS

Serum AFP and DCP levels showed no significant difference between experimental group and control group, and increased when the tumor diameter increased but were not related to HCC histological grade. Serological ESPL1 levels were higher in the experimental group than those in the control group, and positively correlated with the histological grade. In the experimental group, tumor size and histological grade were almost independent (Kappa=0.000); patients with medium size tumors had the highest serum ESPL1 levels and the highest proportion of poorly differentiated carcinomas, whereas 75.6% of patients with small size tumors had moderately differentiated carcinomas and only 20% well differentiated carcinomas.

CONCLUSION

Serum ESPL1 can reflect the malignant degree of HBV-related HCC and is helpful in identifying small size HCC tumors.

PP2ACdc55 dephosphorylates Pds1 and inhibits spindle elongation in S. cerevisiae

PP2ACdc55 (the form of protein phosphatase 2A containing Cdc55) regulates cell cycle progression by reversing cyclin-dependent kinase (CDK)- and polo-like kinase (Cdc5)-dependent phosphorylation events. In S. cerevisiae, Cdk1 phosphorylates securin (Pds1), which facilitates Pds1 binding and inhibits separase (Esp1). During anaphase, Esp1 cleaves the cohesin subunit Scc1 and promotes spindle elongation. Here, we show that PP2ACdc55 directly dephosphorylates Pds1 both in vivo and in vitro Pds1 hyperphosphorylation in a cdc55 deletion mutant enhanced the Pds1-Esp1 interaction, which played a positive role in Pds1 nuclear accumulation and in spindle elongation. We also show that nuclear PP2ACdc55 plays a role during replication stress to inhibit spindle elongation. This pathway acted independently of the known Mec1, Swe1 or spindle assembly checkpoint (SAC) checkpoint pathways. We propose a model where Pds1 dephosphorylation by PP2ACdc55 disrupts the Pds1-Esp1 protein interaction and inhibits Pds1 nuclear accumulation, which prevents spindle elongation, a process that is elevated during replication stress.

Wapl is an essential regulator of chromatin structure and chromosome segregation

Mammalian genomes contain several billion base pairs of DNA that are packaged in chromatin fibres. At selected gene loci, cohesin complexes have been proposed to arrange these fibres into higher-order structures, but how important this function is for determining overall chromosome architecture and how the process is regulated are not well understood. Using conditional mutagenesis in the mouse, here we show that depletion of the cohesin-associated protein Wapl stably locks cohesin on DNA, leads to clustering of cohesin in axial structures, and causes chromatin condensation in interphase chromosomes. These findings reveal that the stability of cohesin-DNA interactions is an important determinant of chromatin structure, and indicate that cohesin has an architectural role in interphase chromosome territories. Furthermore, we show that regulation of cohesin-DNA interactions by Wapl is important for embryonic development, expression of genes such as c-myc (also known as Myc), and cell cycle progression. In mitosis, Wapl-mediated release of cohesin from DNA is essential for proper chromosome segregation and protects cohesin from cleavage by the protease separase, thus enabling mitotic exit in the presence of functional cohesin complexes.

Meiotic HORMA domain proteins prevent untimely centriole disengagement during Caenorhabditis elegans spermatocyte meiosis

In many species where oocytes lack centrosomes, sperm contribute both genetic material and centriole(s) to the zygote. Correct centriole organization during male meiosis is critical to guarantee a normal bipolar mitotic spindle in the zygote. During Caenorhabditis elegans male meiosis, centrioles normally undergo two rounds of duplication, resulting in haploid sperm each containing a single tightly engaged centriole pair. Here we identify an unanticipated role for C. elegans HORMA (Hop1/Rev7/Mad2) domain proteins HTP-1/2 and HIM-3 in regulating centriole disengagement during spermatocyte meiosis. In him-3 and htp-1 htp-2 mutants, centrioles separate inappropriately during meiosis II, resulting in spermatids with disengaged centrioles. Moreover, extra centrosomes are detected in a subset of zygotes. Together, these data implicate HIM-3 and HTP-1/2 in preventing centriole disengagement during meiosis II. We showed previously that HTP-1/2 prevents premature loss of sister chromatid cohesion during the meiotic divisions by inhibiting removal of meiotic cohesin complexes containing the REC-8 subunit. Worms lacking REC-8, or expressing a mutant separase protein with elevated local concentration at centrosomes and in sperm, likewise exhibit inappropriate centriole separation during spermatocyte meiosis. These observations are consistent with HIM-3 and HTP-1/2 preventing centriole disengagement by inhibiting separase-dependent cohesin removal. Our data suggest that the same specialized meiotic mechanisms that function to prevent premature release of sister chromatid cohesion during meiosis I in C. elegans also function to inhibit centriole separation at meiosis II, thereby ensuring that the zygote inherits the appropriate complement of chromosomes and centrioles.

A single cohesin complex performs mitotic and meiotic functions in the protist tetrahymena

The cohesion of sister chromatids in the interval between chromosome replication and anaphase is important for preventing the precocious separation, and hence nondisjunction, of chromatids. Cohesion is accomplished by a ring-shaped protein complex, cohesin; and its release at anaphase occurs when separase cleaves the complex's α-kleisin subunit. Cohesin has additional roles in facilitating DNA damage repair from the sister chromatid and in regulating gene expression. We tested the universality of the present model of cohesion by studying cohesin in the evolutionarily distant protist Tetrahymena thermophila. Localization of tagged cohesin components Smc1p and Rec8p (the α-kleisin) showed that cohesin is abundant in mitotic and meiotic nuclei. RNAi knockdown experiments demonstrated that cohesin is crucial for normal chromosome segregation and meiotic DSB repair. Unexpectedly, cohesin does not detach from chromosome arms in anaphase, yet chromosome segregation depends on the activity of separase (Esp1p). When Esp1p is depleted by RNAi, chromosomes become polytenic as they undergo multiple rounds of replication, but fail to separate. The cohesion of such bundles of numerous chromatids suggests that chromatids may be connected by factors in addition to topological linkage by cohesin rings. Although cohesin is not detected in transcriptionally active somatic nuclei, its loss causes a slight defect in their amitotic division. Notably, Tetrahymena uses a single version of α-kleisin for both mitosis and meiosis. Therefore, we propose that the differentiation of mitotic and meiotic cohesins found in most other model systems is not due to the need of a specialized meiotic cohesin, but due to additional roles of mitotic cohesin.

Post-replicative repair involves separase-dependent removal of the kleisin subunit of cohesin

DNA double-strand break repair is critical for cell viability and involves highly coordinated pathways to restore DNA integrity at the lesion. An early event during homology-dependent repair is resection of the break to generate progressively longer 3' single-strand tails that are used to identify suitable templates for repair. Sister chromatids provide near-perfect sequence homology and are therefore the preferred templates during homologous recombination. To provide a bias for the use of sisters as donors, cohesin--the complex that tethers sister chromatids together--is recruited to the break to enforce physical proximity. Here we show that DNA breaks promote dissociation of cohesin loaded during the previous S phase in budding yeast, and that damage-induced dissociation of cohesin requires separase, the protease that dissolves cohesion in anaphase. Moreover, a separase-resistant allele of the gene coding for the α-kleisin subunit of cohesin, Mcd1 (also known as Scc1), reduces double-strand break resection and compromises the efficiency of repair even when loaded during DNA damage. We conclude that post-replicative DNA repair involves cohesin dissociation by separase to promote accessibility to repair factors during the coordinated cellular response to restore DNA integrity.

Oscillation of APC/C activity during cell cycle arrest promotes centrosome amplification

Centrosome duplication is licensed by the disengagement, or 'uncoupling', of centrioles during late mitosis. However, arrest of cells in G2 can trigger premature centriole disengagement. Here, we show that premature disengagement results from untimely activation of the anaphase-promoting complex (APC/C), leading to securin degradation and release of active separase. Although APC/C activation during G2 arrest is dependent on polo-like kinase 1 (Plk1)-mediated degradation of the APC/C inhibitor, early mitotic inhibitor 1 (Emi1), Plk1 also has a second APC/C-independent role in promoting disengagement. Importantly, APC/C and Plk1 activity also stimulates centriole disengagement in response to hydroxyurea or DNA damage-induced cell-cycle arrest and this leads to centrosome amplification. However, the reduplication of disengaged centrioles is dependent on cyclin-dependent kinase 2 (Cdk2) activity and Cdk2 activation coincides with a subsequent inactivation of the APC/C and re-accumulation of cyclin A. Although release from these arrests leads to mitotic entry, the presence of disengaged and/or amplified centrosomes results in the formation of abnormal mitotic spindles that lead to chromosome mis-segregation. Thus, oscillation of APC/C activity during cell cycle arrest promotes both centrosome amplification and genome instability.

Cyclin A2 is required for sister chromatid segregation, but not separase control, in mouse oocyte meiosis

In meiosis, two specialized cell divisions allow the separation of paired chromosomes first, then of sister chromatids. Separase removes the cohesin complex holding sister chromatids together in a stepwise manner from chromosome arms in meiosis I, then from the centromere region in meiosis II. Using mouse oocytes, our study reveals that cyclin A2 promotes entry into meiosis, as well as an additional unexpected role; namely, its requirement for separase-dependent sister chromatid separation in meiosis II. Untimely cyclin A2-associated kinase activity in meiosis I leads to precocious sister separation, whereas inhibition of cyclin A2 in meiosis II prevents it. Accordingly, endogenous cyclin A is localized to kinetochores throughout meiosis II, but not in anaphase I. Additionally, we found that cyclin B1, but not cyclin A2, inhibits separase in meiosis I. These findings indicate that separase-dependent cohesin removal is differentially regulated by cyclin B1 and A2 in mammalian meiosis.

The proteolytic activity of separase in BCR-ABL-positive cells is increased by imatinib

Separase, an endopeptidase required for the separation of sister-chromatides in mitotic anaphase, triggers centriole disengagement during centrosome duplication. In cancer, separase is frequently overexpressed, pointing to a functional role as an aneuploidy promoter associated with centrosomal amplification and genomic instability. Recently, we have shown that centrosomal amplification and subsequent chromosomal aberrations are a hallmark of chronic myeloid leukemia (CML), increasing from chronic phase (CP) toward blast crisis (BC). Moreover, a functional linkage of p210BCR-ABL tyrosine kinase activity with centrosomal amplification and clonal evolution has been established in long-term cell culture experiments. Unexpectedly, therapeutic doses of imatinib (IM) did not counteract; instead induced similar centrosomal alterations in vitro. We investigated the influence of IM and p210BCR-ABL on Separase as a potential driver of centrosomal amplification in CML. Short-term cell cultures of p210BCR-ABL-negative (NHDF, UROtsa, HL-60, U937), positive (K562, LAMA-84) and inducible (U937p210BCR-ABL/c6 (Tet-ON)) human cell lines were treated with therapeutic doses of IM and analyzed by qRT-PCR, Western blot analysis and quantitative Separase activity assays. Decreased Separase protein levels were observed in all cells treated with IM in a dose dependent manner. Accordingly, in all p210BCR-ABL-negative cell lines, decreased proteolytic activity of Separase was found. In contrast, p210BCR-ABL-positive cells showed increased Separase proteolytic activity. This activation of Separase was consistent with changes in the expression levels of Separase regulators (Separase phosphorylation at serine residue 1126, Securin, CyclinB1 and PP2A). Our data suggest that regulation of Separase in IM-treated BCR-ABL-positive cells occurs on both the protein expression and the proteolytic activity levels. Activation of Separase proteolytic activity exclusively in p210BCR-ABL-positive cells during IM treatment may act as a driving force for centrosomal amplification, contributing to genomic instability, clonal evolution and resistance in CML.

Separase biosensor reveals that cohesin cleavage timing depends on phosphatase PP2A(Cdc55) regulation

In anaphase, sister chromatids separate abruptly and are then segregated by the mitotic spindle. The protease separase triggers sister separation by cleaving the Scc1/Mcd1 subunit of the cohesin ring that holds sisters together. Polo-kinase phosphorylation of Scc1 promotes its cleavage, but the underlying regulatory circuits are unclear. We developed a separase biosensor in Saccharomyces cerevisiae that provides a quantitative indicator of cohesin cleavage in single cells. Separase is abruptly activated and cleaves most cohesin within 1 min, after which anaphase begins. Cohesin near centromeres and telomeres is cleaved at the same rate and time. Protein phosphatase PP2A(Cdc55) inhibits cohesin cleavage by counteracting polo-kinase phosphorylation of Scc1. In early anaphase, the previously described separase inhibition of PP2A(Cdc55) promotes cohesin cleavage. Thus, separase acts directly on Scc1 and also indirectly, through inhibition of PP2A(Cdc55), to stimulate cohesin cleavage, providing a feedforward loop that may contribute to a robust and timely anaphase.

Separase sensor reveals dual roles for separase coordinating cohesin cleavage and cdk1 inhibition

Complete dissociation of sister chromatid cohesion and subsequent induction of poleward movement of disjoined sisters are two essential events underlying chromosome segregation; however, how cells coordinate these two processes is not well understood. Here, we developed a fluorescence-based sensor for the protease separase that mediates cohesin cleavage. We found that separase undergoes an abrupt activation shortly before anaphase onset in the vicinity of chromosomes. This activation profile of separase depends on the abilities of two of its binding proteins, securin and cyclin B1, to inhibit its protease activity and target it to chromosomes. Subsequent to its proteolytic activation, separase then binds to and inhibits a subset of cyclin B1-cdk1, which antagonizes cdk1-mediated phosphorylation on chromosomes and facilitates poleward movement of sisters in anaphase. Therefore, by consecutively acting as a protease and a cdk1 inhibitor, separase coordinates two key processes to achieve simultaneous and abrupt separation of sister chromatids.

Separases: biochemistry and function

Tight regulation of cell cycle is of critical importance for eukaryotic biology and is achieved through a combined action of a large number of highly specialized proteins. Separases are evolutionarily conserved caspase-like proteases playing a crucial role in cell cycle regulation, as they execute sister chromatid separation at metaphase to anaphase transition. In contrast to extensively studied yeast and metazoan separases, very little is known about the role of separases in plant biology. Here we describe the molecular mechanisms of separase-mediated chromatid segregation in yeast and metazoan models, discuss new emerging but less-understood functions of separases and highlight major gaps in our knowledge about plant separases.

Kendrin is a novel substrate for separase involved in the licensing of centriole duplication

The centrosome, consisting of a pair of centrioles surrounded by pericentriolar material, directs the formation of bipolar spindles during mitosis. Aberrant centrosome number can promote chromosome instability, which is implicated in tumorigenesis. Thus, centrosome duplication needs to be tightly regulated to occur only once per cell cycle. Separase, a cysteine protease that triggers sister chromatid separation, is involved in centriole disengagement, which licenses centrosomes for the next round of duplication. However, at least two questions remain unsolved: what is the substrate relevant to the disengagement, and how does separase, activated at anaphase onset, act on the disengagement that occurs during late mitosis. Here, we show that kendrin, also named pericentrin, is cleaved by activated separase at a consensus site in vivo and in vitro, and this leads to the delayed release of kendrin from the centrosome later in mitosis. Furthermore, we demonstrate that expression of a noncleavable kendrin mutant suppresses centriole disengagement and subsequent centriole duplication. Based on these results, we propose that kendrin is a novel and crucial substrate for separase at the centrosome, protecting the engaged centrioles from premature disengagement and thereby blocking reduplication until the cell passes through mitosis.

The sister bonding of duplicated chromosomes

Sister chromatid cohesion and separation are two fundamental chromosome dynamics that are essential to equal chromosome segregation during cell proliferation. In this review, I will discuss the major steps that regulate these dynamics during mitosis, with an emphasis on vertebrate cells. The implications of these machineries outside of sister chromatid cohesion and separation are also discussed.

Separase loss of function cooperates with the loss of p53 in the initiation and progression of T- and B-cell lymphoma, leukemia and aneuploidy in mice

Background

Cohesin protease Separase plays a key role in faithful segregation of sister chromatids by cleaving the cohesin complex at the metaphase to anaphase transition. Homozygous deletion of ESPL1 gene that encodes Separase protein results in embryonic lethality in mice and Separase overexpression lead to aneuploidy and tumorigenesis. However, the effect of Separase haploinsufficiency has not been thoroughly investigated.

Methodology/Principal Findings

Here we examined the effect of ESPL1 heterozygosity using a hypomorphic mouse model that has reduced germline Separase activity. We report that while ESPL1 mutant (ESPL1 ^+/hyp) mice have a normal phenotype, in the absence of p53, these mice develop spontaneous T- and B-cell lymphomas, and leukemia with a significantly shortened latency as compared to p53 null mice. The ESPL1 hypomorphic, p53 heterozygous transgenic mice (ESPL1^+/hyp, p53^+/−) also show a significantly reduced life span with an altered tumor spectrum of carcinomas and sarcomas compared to p53^+/− mice alone. Furthermore, ESPL1^+/hyp, p53^−/− mice display significantly higher levels of genetic instability and aneuploidy in normal cells, as indicated by the abnormal metaphase counts and SKY analysis of primary splenocytes.

Conclusions/Significance

Our results indicate that reduced levels of Separase act synergistically with loss of p53 in the initiation and progression of B- and T- cell lymphomas, which is aided by increased chromosomal missegregation and accumulation of genomic instability. ESPL1^+/hyp, p53^−/− mice provide a new animal model for mechanistic study of aggressive lymphoma and also for preclinical evaluation of new agents for its therapy.

Cleavage of cohesin rings coordinates the separation of centrioles and chromatids

Cohesin pairs sister chromatids by forming a tripartite Scc1-Smc1-Smc3 ring around them. In mitosis, cohesin is removed from chromosome arms by the phosphorylation-dependent prophase pathway. Centromeric cohesin is protected by shugoshin 1 and protein phosphatase 2A (Sgo1-PP2A) and opened only in anaphase by separase-dependent cleavage of Scc1 (refs 4-6). Following chromosome segregation, centrioles loosen their tight orthogonal arrangement, which licenses later centrosome duplication in S phase. Although a role of separase in centriole disengagement has been reported, the molecular details of this process remain enigmatic. Here, we identify cohesin as a centriole-engagement factor. Both premature sister-chromatid separation and centriole disengagement are induced by ectopic activation of separase or depletion of Sgo1. These unscheduled events are suppressed by expression of non-cleavable Scc1 or inhibition of the prophase pathway. When endogenous Scc1 is replaced by artificially cleavable Scc1, the corresponding site-specific protease triggers centriole disengagement. Separation of centrioles can alternatively be induced by ectopic cleavage of an engineered Smc3. Thus, the chromosome and centrosome cycles exhibit extensive parallels and are coordinated with each other by dual use of the cohesin ring complex.

The radially swollen 4 separase mutation of Arabidopsis thaliana blocks chromosome disjunction and disrupts the radial microtubule system in meiocytes

The caspase-family protease, separase, is required at the onset of anaphase to cleave the cohesin complex that joins replicated sister chromatids. However, in various eukaryotes, separase has acquired additional and distinct functions. A single amino-acid substitution in separase is responsible for phenotypes of the Arabidopsis thaliana mutant, radially swollen 4 (rsw4). This is a conditional mutant, resembling the wild type at the permissive temperature (∼20°C) and expressing mutant phenotypes at the restrictive temperature (∼30°C). Root cells in rsw4 at the restrictive temperature undergo non-disjunction and other indications of the loss of separase function. To determine to what extent separase activity remains at 30°C, we examined the effect of the mutation on meiosis, where the effects of loss of separase activity through RNA interference are known; and in addition, we examined female gametophyte development. Here, we report that, at the restrictive temperature, replicated chromosomes in rsw4 meiocytes typically fail to disjoin and the cohesin complex remains at centromeres after metaphase. Meiotic spindles appear normal in rsw4 male meiocytes; however the mutation disrupts the radial microtubule system, which is replaced by asymmetric arrays. Surprisingly, female gametophyte development was relatively insensitive to loss of separase activity, through either rsw4 or RNAi. These effects confirm that phenotypes in rsw4 result from loss of separase activity and establish a role for separase in regulating cell polarization following male meiosis.

Separase phosphosite mutation leads to genome instability and primordial germ cell depletion during oogenesis

To ensure equal chromosome segregation and the stability of the genome during cell division, Separase is strictly regulated primarily by Securin binding and inhibitory phosphorylation. By generating a mouse model that contained a mutation to the inhibitory phosphosite of Separase, we demonstrated that mice of both sexes are infertile. We showed that Separase deregulation leads to chromosome mis-segregation, genome instability, and eventually apoptosis of primordial germ cells (PGCs) during embryonic oogenesis. Although the PGCs of mutant male mice were completely depleted, a population of PGCs from mutant females survived Separase deregulation. The surviving PGCs completed oogenesis but produced deficient initial follicles. These results indicate a sexual dimorphism effect on PGCs from Separase deregulation, which may be correlated with a gender-specific discrepancy of Securin. Our results reveal that Separase phospho-regulation is critical for genome stability in oogenesis. Furthermore, we provided the first evidence of a pre-zygotic mitotic chromosome segregation error resulting from Separase deregulation, whose sex-specific differences may be a reason for the sexual dimorphism of aneuploidy in gametogenesis.

Splitting the nucleus: what's wrong with the tripartite ring model?

The segregation of sister DNA molecules at mitosis involves their traction to opposite poles by microtubules attached to kinetochores. By creating tension required to stabilize kinetochore microtubules, sister chromatid cohesion has a key role in ensuring that sister kinetochores attach to microtubules with opposing polarity, a process known as biorientation. Cohesion is mediated by a cohesin complex whose Smc1, Smc3, and kleisin subunits form a tripartite ring thought to hold sister DNAs together by entrapping them (the ring model). Sister chromatid disjunction at the onset of anaphase is triggered by a thiol protease called separase whose activation, only when all chromosomes have bioriented, opens the cohesin ring by cleaving its kleisin subunit. Separase is inhibited by the binding of an inhibitory chaperone called securin whose destruction at the hands of a ubiquitin protein ligase called the anaphase-promoting complex/cyclosome (APC/C) is essential for kleisin cleavage and sister chromatid disjunction. We describe microinjection experiments showing that cohesin cleavage and Cdk1 down-regulation are sufficient to drive formation of daughter nuclei in cells arrested in metaphase due to inactivation of the APC/C and describe chemical cross-linking experiments consistent with the ring model. How sister DNAs enter the cohesin ring and are retained inside for long periods of time after the completion of DNA replication remains poorly understood.

Cell division cycle 6, a mitotic substrate of polo-like kinase 1, regulates chromosomal segregation mediated by cyclin-dependent kinase 1 and separase

Defining the links between cell division and DNA replication is essential for understanding normal cell cycle progression and tumorigenesis. In this report we explore the effect of phosphorylation of cell division cycle 6 (Cdc6), a DNA replication initiation factor, by polo-like kinase 1 (Plk1) on the regulation of chromosomal segregation. In mitosis, the phosphorylation of Cdc6 was highly increased, in correlation with the level of Plk1, and conversely, Cdc6 is hypophosphorylated in Plk1-depleted cells, although cyclin A- and cyclin B1-dependent kinases are active. Binding between Cdc6 and Plk1 occurs through the polo-box domain of Plk1, and Cdc6 is phosphorylated by Plk1 on T37. Immunohistochemistry studies reveal that Cdc6 and Plk1 colocalize to the central spindle in anaphase. Expression of T37V mutant of Cdc6 (Cdc6-TV) induces binucleated cells and incompletely separated nuclei. Wild-type Cdc6 but not Cdc6-TV binds cyclin-dependent kinase 1 (Cdk1). Expression of wild-type Plk1 but not kinase-defective mutant promotes the binding of Cdc6 to Cdk1. Cells expressing wild-type Cdc6 display lower Cdk1 activity and higher separase activity than cells expressing Cdc6-TV. These results suggest that Plk1-mediated phosphorylation of Cdc6 promotes the interaction of Cdc6 and Cdk1, leading to the attenuation of Cdk1 activity, release of separase, and subsequent anaphase progression.

[Change of structural maintenance of chromosome (SMC)1, SMC3, Separase and Securin expression in BEAS-2B malignant transformation cell induced by coal tar pitch smoke extracts]

OBJECTIVE

to study the role of structural maintenance of chromosome (SMC)1, SMC3, Separase and Securin in tumorgenesis that contact with coal tar pitch.

METHODS

the BEAS-2B cells was induced by coal tar pitch smoke extracts to form malignant transformation cell model in vitro. The gene expression levels of mRNA were assessed by real-time quantitative RT-PCR, and the protein expression variation were determined by cell culture overslip of immunohistochemical methods.

RESULTS

in malignant transformation cells, the mRNA and the protein expression level of SMC1 gene was not statistically significantly different compared with the BEAS-2B group and DMSO group (P > 0.05); SMC3 and Separase was increased and Securin was decreased (P < 0.05), while the difference between other two control groups was not significant (P > 0.05).

CONCLUSIONS

the up expression level of SMC3 and Separase and the down expression level of Securin are involved in the process that evolves into malignant transformation in bronchial epithelial cells BEAS-2B induced by coal tar pitch smoke extracts.

Determinants of Rad21 localization at the centrosome in human cells

Cohesin proteins help maintain the physical associations between sister chromatids that arise in S-phase and are removed in anaphase. Recent studies found that cohesins also localize to the centrosomes, the organelles that organize the mitotic bipolar spindle. We find that the cohesin protein Rad21 localizes to centrosomes in a manner that is dependent upon known regulators of sister chromatid cohesion as well as regulators of centrosome function. These data suggest that Rad21 functions at the centrosome and that the regulators of Rad21 coordinate the centrosome and chromosomal functions of cohesin.

Rec8 phosphorylation by casein kinase 1 and Cdc7-Dbf4 kinase regulates cohesin cleavage by separase during meiosis

During meiosis, two rounds of chromosome segregation after a single round of DNA replication produce haploid gametes from diploid precursors. At meiosis I, maternal and paternal kinetochores are pulled toward opposite poles, and chiasmata holding bivalent chromosomes together are resolved by cleavage of cohesin's alpha-kleisin subunit (Rec8) along chromosome arms. This creates dyad chromosomes containing a pair of chromatids joined solely by cohesin at centromeres that had resisted cleavage. The discovery that centromeric Rec8 is protected from separase during meiosis I by shugoshin/MEI-S332 proteins that bind PP2A phosphatase suggests that phosphorylation either of separase or cohesin may be necessary for Rec8 cleavage. We show here that multiple phosphorylation sites within Rec8 as well as two different kinases, casein kinase 1delta/epsilon (CK1delta/epsilon) and Dbf4-dependent Cdc7 kinase (DDK), are required for Rec8 cleavage and meiosis I nuclear division. Rec8 with phosphomimetic mutations is no longer protected from separase at centromeres and is cleaved even when the two kinases are inhibited. Our data suggest that PP2A protects centromeric cohesion by opposing CK1delta/epsilon- and DDK-dependent phosphorylation of Rec8.

A conditional mutation in Arabidopsis thaliana separase induces chromosome non-disjunction, aberrant morphogenesis and cyclin B1;1 stability

The caspase family protease, separase, is required at anaphase onset to cleave the cohesin complex, which joins sister chromatids. However, among eukaryotes, separases have acquired novel functions. Here, we show that Arabidopsis thaliana radially swollen 4 (rsw4), a temperature-sensitive mutant isolated previously on the basis of root swelling, harbors a mutation in At4g22970, the A. thaliana separase. Loss of separase function in rsw4 at the restrictive temperature is indicated by the widespread failure of replicated chromosomes to disjoin. Surprisingly, rsw4 has neither pronounced cell cycle arrest nor anomalous spindle formation, which occur in other eukaryotes upon loss of separase activity. However, rsw4 roots have disorganized cortical microtubules and accumulate the mitosis-specific cyclin, cyclin B1;1, excessive levels of which have been associated with altered microtubules and morphology. Cyclin B1;1 also accumulates in certain backgrounds in response to DNA damage, but we find no evidence for aberrant responses to DNA damage in rsw4. Our characterization of rsw4 leads us to hypothesize that plant separase, in addition to cleaving cohesin, regulates cyclin B1;1, with profound ramifications for morphogenesis.

DNA-dependent cohesin cleavage by separase

Eukaryotic genomes are organized into chromosomes. In order to maintain genomic stability during cell proliferation, a series of elaborate processes is employed to ensure that chromosomes are duplicated and segregated equally into daughter cells. Sister chromatid cohesion, a tight association of duplicated sister chromatids, allows their attachment to the opposite centrosomes. Sister chromatid cohesion depends on the cohesin complex, a proteinaceous ring that entraps the chromatids together. At the metaphase-to-anaphase transition, a protease called separase is activated and completely dissolves the cohesion by cleaving SCC1, a subunit of the cohesin complex. As one of the key executors of anaphase, separase is regulated temporally and spatially by often redundant mechanisms. A recent study revealed that chromosomal DNA is required as a cofactor for the cleavage of cohesin to occur. This DNA dependence is the underlying biochemical mechanism that allows separase to selectively cleave only the chromosome-associated cohesin. We propose that the chromosomal DNA dependent cohesin cleavage by separase is a component of a regulatory pathway that cells utilize to protect the bulk of cohesin. This intact cohesin becomes immediately available in G(1) to resume its other function-regulation of gene transcription by means of chromatin insulation.

Polo kinase and separase regulate the mitotic licensing of centriole duplication in human cells

It has been proposed that separase-dependent centriole disengagement at anaphase licenses centrosomes for duplication in the next cell cycle. Here we test whether such a mechanism exists in intact human cells. Loss of separase blocked centriole disengagement during mitotic exit and delayed assembly of new centrioles during the following S phase; however, most engagements were eventually dissolved. We identified Polo-like kinase 1 (Plk1) as a parallel activator of centriole disengagement. Timed inhibition of Plk1 mapped its critical period of action to late G2 or early M phase, i.e., prior to securin destruction and separase activation at anaphase onset. Crucially, when cells exited mitosis after downregulation of both separase and Plk1, centriole disengagement failed completely, and subsequent centriole duplication in interphase was also blocked. Our results indicate that Plk1 and separase act at different times during M phase to license centrosome duplication, reminiscent of their roles in removing cohesin from chromosomes.

Arabidopsis separase functions beyond the removal of sister chromatid cohesion during meiosis

Separase is a capase family protease that is required for the release of sister chromatid cohesion during meiosis and mitosis. Proteolytic cleavage of the alpha-kleisin subunit of the cohesin complex at the metaphase-to-anaphase transition is essential for the proper segregation of chromosomes. In addition to its highly conserved role in cleaving the alpha-kleisin subunit, separase appears to have acquired additional diverse activities in some organisms, including involvement in mitotic and meiotic anaphase spindle assembly and elongation, interphase spindle pole body positioning, and epithelial cell reorganization. Results from the characterization of Arabidopsis (Arabidopsis thaliana) separase (ESP) demonstrated that meiotic expression of ESP RNA interference blocked the proper removal of cohesin from chromosomes and resulted in the presence of a mixture of fragmented chromosomes and intact bivalents. The presence of large numbers of intact bivalents raised the possibility that separase may also have multiple roles in Arabidopsis. In this report, we show that meiotic expression of ESP RNA interference blocks the removal of cohesin during both meiosis I and II, results in alterations in nonhomologous centromere association, disrupts the radial microtubule system after telophase II, and affects the proper establishment of nuclear cytoplasmic domains, resulting in the formation of multinucleate microspores.

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, we provide evidence that PP2A's recruitment to chromosomes may be sufficient to protect cohesin from separase in mammalian oocytes.

Smc5-Smc6-dependent removal of cohesin from mitotic chromosomes

The function of the essential cohesin-related Smc5-Smc6 complex has remained elusive, though hypomorphic mutants have defects late in recombination, in checkpoint maintenance, and in chromosome segregation. Recombination and checkpoints are not essential for viability, and Smc5-Smc6-null mutants die in lethal mitoses. This suggests that the chromosome segregation defects may be the source of lethality in irradiated Smc5-Smc6 hypomorphs. We show that in smc6 mutants, following DNA damage in interphase, chromosome arm segregation fails due to an aberrant persistence of cohesin, which is normally removed by the Separase-independent pathway. This postanaphase persistence of cohesin is not dependent on DNA damage, since the synthetic lethality of smc6 hypomorphs with a topoisomerase II mutant, defective in mitotic chromosome structure, is also due to the retention of cohesin on undamaged chromosome arms. In both cases, Separase overexpression bypasses the defect and restores cell viability, showing that defective cohesin removal is a major determinant of the mitotic lethality of Smc5-Smc6 mutants.

Role of cleavage by separase of the Rec8 kleisin subunit of cohesin during mammalian meiosis I

Proteolytic activity of separase is required for chiasma resolution during meiosis I in mouse oocytes. Rec8, the meiosis-specific alpha-kleisin subunit of cohesin, is a key target of separase in yeast. Is the equivalent protein also a target in mammals? We show here that separase cleaves mouse Rec8 at three positions in vitro but only when the latter is hyper-phosphorylated. Expression of a Rec8 variant (Rec8-N) that cannot be cleaved in vitro at these sites causes sterility in male mice. Their seminiferous tubules lack a normal complement of 2 C secondary spermatocytes and 1 C spermatids and contain instead a high proportion of cells with enlarged nuclei. Chromosome spreads reveal that Rec8-N expression has no effect in primary spermatocytes but produces secondary spermatocytes and spermatids with a 4 C DNA content, suggesting that the first and possibly also the second meiotic division is abolished. Expression of Rec8-N in oocytes causes chromosome segregation to be asynchronous and delays its completion by 2-3 hours during anaphase I, probably due to inefficient proteolysis of Rec8-N by separase. Despite this effect, chromosome segregation must be quite accurate as Rec8-N does not greatly reduce female fertility. Our data is consistent with the notion that Rec8 cleavage is important and probably crucial for the resolution of chiasmata in males and females.

Recruitment of separase to mitotic chromosomes is regulated by Aurora B

Accurate segregation of chromosome, initiated by abrupt and irreversible dissolution of sister-chromatid cohesion at anaphase, is crucial for the faithful inheritance of parental genomes during eukaryotic cell division. The dissolution of sister-chromatid cohesion is catalyzed by separase after the destruction of securin by the anaphase-promoting complex/cyclosome (APC/C). However, separase was localized to the mitotic centrosome, raising the question as how separase hydrolyzes sister-chromatid cohesion of centromere at the anaphase onset. Here we show that separase is associated with mitotic chromosomes and this association is regulated by Aurora B kinase. Using a panel of separase antibodies, we found that separase protein was accumulated in mitosis and degraded at the end of telophase. To study the spatiotemporal distribution of separase in mitosis, we carried out immunofluorescence microscopic analyses. Surprisingly, separase was found to be associated with mitotic chromosomes from prophase to metaphase and dissociated from the chromosomes in anaphase right after sister chromatids separation. Staining of isolated mitotic chromosomes from Nocodazole-arrested cells revealed that separase is concentrated at the centromeric cohesion. To examine if any mitotic kinases are responsible for chromosomal localization of separase in mitosis, we carried out RNAi-mediated knockdown and found that association of separase with mitotic chromosomes was a function of Aurora B. Consistent with the phenotype seen in the Aurora B-repressed cells, inhibition of Aurora B kinase by hersperadin prevents the association of separase with chromosomes. Our results suggest that Aurora B kinase activity helps coordinate the association of separase with chromosome and the initiation of sister-chromatid separation.

Overexpression and mislocalization of the chromosomal segregation protein separase in multiple human cancers

PURPOSE

Separase, an endopeptidase, plays a pivotal role in chromosomal segregation by separating sister chromatids during the metaphase to anaphase transition. Using a mouse mammary tumor model we have recently shown that overexpression of Separase induces aneuploidy and tumorigenesis (Zhang et al., Proc Natl Acad Sci 2008;105:13033). In the present study, we have investigated the expression level of Separase across a wide range of human tumors.

EXPERIMENTAL DESIGN

To examine the expression levels and localization of Separase in human tumors, we have performed immunofluorescence microscopy using human Separase antibody and tumor tissue arrays from osteosarcoma, colorectal, breast, and prostate cancers with appropriate normal controls.

RESULTS

We show that Separase is significantly overexpressed in osteosarcoma, breast, and prostate tumor specimens. There is a strong correlation of tumor status with the localization of Separase into the nucleus throughout all stages of the cell cycle. Unlike the normal control tissues, where Separase localization is exclusively cytoplasmic in nondividing cells, human tumor samples show significantly higher number of resting cells with a strong nuclear Separase staining. Additionally, overexpression of Separase transcript strongly correlates with high incidence of relapse, metastasis, and lower 5-year overall survival rate in breast and prostate cancer patients.

CONCLUSION

These results further strengthen our hypothesis that Separase might be an oncogene, whose overexpression induces tumorigenesis, and indicates that Separase overexpression and aberrant nuclear localization are common in many tumor types and may predict outcome in some human cancers.

Separase is recruited to mitotic chromosomes to dissolve sister chromatid cohesion in a DNA-dependent manner

Sister chromatid separation is triggered by the separase-catalyzed cleavage of cohesin. This process is temporally controlled by cell-cycle-dependent factors, but its biochemical mechanism and spatial regulation remain poorly understood. We report that cohesin cleavage by human separase requires DNA in a sequence-nonspecific manner. Separase binds to DNA in vitro, but its proteolytic activity, measured by its autocleavage, is not stimulated by DNA. Instead, biochemical characterizations suggest that DNA mediates cohesin cleavage by bridging the interaction between separase and cohesin. In human cells, a fraction of separase localizes to the mitotic chromosome. The importance of the chromosomal DNA in cohesin cleavage is further demonstrated by the observation that the cleavage of the chromosome-associated cohesins is sensitive to nuclease treatment. Our observations explain why chromosome-associated cohesins are specifically cleaved by separase and the soluble cohesins are left intact in anaphase.

Shugoshin prevents cohesin cleavage by PP2A(Cdc55)-dependent inhibition of separase

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. We investigated the mechanism by which the protector protein Shugoshin counteracts cohesin cleavage in budding yeast. We show that Shugoshin can prevent separase activation independently of securin. Instead, PP2A(Cdc55) is essential for Shugoshin-mediated inhibition of separase. Loss of both securin and Cdc55 leads to premature sister chromatid separation, resulting in aneuploidy. We propose that Cdc55 is a separase inhibitor that acts downstream from Shugoshin under conditions where sister chromatids are not under tension.

Functional characterization of cohesin SMC3 and separase and their roles in the segregation of large and minichromosomes in Trypanosoma brucei

Minichromosomes in the nuclear genome of Trypanosoma brucei exhibit unusual patterns of mitotic segregation. To address whether differences in their mode of segregation in relation to large chromosomes are reflected at a molecular level, we characterized two different proteins that have highly conserved functions in eukaryotic chromosomes segregation: the SMC3 protein, a component of the chromatid cohesion apparatus, and the protease separase that resolves the cohesin complex at the onset of anaphase and has, in other organisms, additional functions during mitosis. Using in situ hybridization we show that RNA interference-mediated depletion of SMC3 has no visible effect on the segregation of the minichromosomal population but interferes with the faithful mitotic separation of large chromosomes. In contrast, separase depletion causes missegregation of both mini- and large chromosomes. We also show that SMC3 persists as a soluble protein throughout the cell cycle and only associates with chromatin between G1 and metaphase. Separase is present in the cell during the entire cell cycle, but is excluded from the nucleus until the metaphase-anaphase transition, thereby providing a potential control mechanism to prevent the untimely cleavage of the cohesin complex.

Mitotic exit in the absence of separase activity

In budding yeast, three interdigitated pathways regulate mitotic exit (ME): mitotic cyclin-cyclin-dependent kinase (Cdk) inactivation; the Cdc14 early anaphase release (FEAR) network, including a nonproteolytic function of separase (Esp1); and the mitotic exit network (MEN) driven by interaction between the spindle pole body and the bud cortex. Here, we evaluate the contributions of these pathways to ME kinetics. Reducing Cdk activity is critical for ME, and the MEN contributes strongly to ME efficiency. Esp1 contributes to ME kinetics mainly through cohesin cleavage: the Esp1 requirement can be largely bypassed if cells are provided Esp1-independent means of separating sister chromatids. In the absence of Esp1 activity, we observed only a minor ME delay consistent with a FEAR defect. Esp1 overexpression drives ME in Cdc20-depleted cells arrested in metaphase. We have found that this activity of overexpressed Esp1 depended on spindle integrity and the MEN. We defined the first quantitative measure for Cdc14 release based on colocalization with the Net1 nucleolar anchor. This measure indicates efficient Cdc14 release upon MEN activation; release driven by Esp1 in the absence of microtubules was inefficient and incapable of driving ME. We also found a novel role for the MEN: activating Cdc14 nuclear export, even in the absence of Net1.

Studies of meiosis disclose distinct roles of cohesion in the core centromere and pericentromeric regions

During meiosis, a single round of genome duplication is followed by two sequential rounds of chromosome segregation. Through this process, a diploid parent cell generates gametes with a haploid set of chromosomes. A characteristic of meiotic chromosome segregation is a stepwise loss of sister chromatid cohesion along chromosomal arms and at centromeres. Whereas arm cohesion plays an important role in ensuring homologue disjunction at meiosis I, persisting cohesion at pericentromeric regions throughout meiosis I is essential for the faithful equational segregation of sisters in the following meiosis II, similar to mitosis. A widely conserved pericentromeric protein called shugoshin, which associates with protein phosphatase 2A (PP2A), plays a critical role in this protection of cohesin. Another key aspect of meiosis I is the establishment of monopolar attachment of sister kinetochores to spindle microtubules. Cohesion or physical linkage at the core centromeres, where kinetochores assemble, may conjoin sister kinetochores, leading to monopolar attachment. A meiosis-specific kinetochore factor such as fission yeast Moa1 or budding yeast monopolin contributes to this regulation. We propose that cohesion at the core centromere and pericentromeric regions plays distinct roles, especially in defining the orientation of kinetochores.

Effect of separase depletion on ionizing radiation-induced cell cycle checkpoints and survival in human lung cancer cell lines

OBJECTIVES

This study is to evaluate the effect of separase depletion on cell cycle progression of irradiated and non-irradiated cells through the G(2)/M phases and consecutive cell survival.

MATERIALS AND METHODS

Separase was depleted with siRNA in two human non-small cell lung carcinoma (NSCLC) cell lines. Cell cycle progression, mitotic fraction, DNA repair, apoptotic and clonogenic cell death were determined.

RESULTS

By depletion of endogenous separase with siRNA in NSCLCs, we showed that separase affects progression through the G(2) phase. In non-irradiated exponentially growing cells, separase depletion led to an increased G(2) accumulation from 17.2% to 29.1% in H460 and from 15.7% to 30.9% in A549 cells and a decrease in mitotic cells. Depletion of separase significantly (P < 0.01) increased the fraction of radiation-induced G(2) arrested cells 30-56 h after irradiation and led to decrease in the mitotic fraction. This was associated with increased double-strand break repair as measured by gamma-H2AX foci kinetics in H460 cells and to a lesser extent in A549 cells. In addition, a decrease in the expression of mitotic linked cell death after irradiation was found.

CONCLUSIONS

These results indicate that separase has additional targets involved in regulation of G(2) to M progression after DNA damage. Prolonged G(2) phase arrest in the absence of separase has consequences on repair of damaged DNA and cell death.

Cleavage of Mcd1 by caspase-like protease Esp1 promotes apoptosis in budding yeast

Over the last decade, yeast has been used successfully as a model system for studying the molecular mechanism of apoptotic cell death. Here, we report that Mcd1, the yeast homology of human cohesin Rad21, plays an important role in hydrogen peroxide-induced apoptosis in yeast. On induction of cell death, Mcd1 is cleaved and the C-terminal fragment is translocated from nucleus into mitochondria, causing the decrease of mitochondrial membrane potential and the amplification of cell death in a cytochrome c-dependent manner. We further demonstrate that the caspase-like protease Esp1 has dual functions and that it is responsible for the cleavage of Mcd1 during the hydrogen peroxide-induced apoptosis. When apoptosis is induced, Esp1 is released from the anaphase inhibitor Pds1. The activated Esp1 acts as caspase-like protease for the cleavage of Mcd1, which enhances the cell death via its translocation from nucleus to mitochondria.

Inhibitory phosphorylation of separase is essential for genome stability and viability of murine embryonic germ cells

Activity of separase, a cysteine protease that cleaves sister chromatid cohesin at the onset of anaphase, is tightly regulated to ensure faithful chromosome segregation and genome stability. Two mechanisms negatively regulate separase: inhibition by securin and phosphorylation on serine 1121. To gauge the physiological significance of the inhibitory phosphorylation, we created a mouse strain in which Ser1121 was mutated to Ala (S1121A). Here we report that this S1121A point mutation causes infertility in mice. We show that germ cells in the mutants are depleted during development. We further demonstrate that S1121A causes chromosome misalignment during proliferation of the postmigratory primordial germ cells, resulting in mitotic arrest, aneuploidy, and eventual cell death. Our results indicate that inhibitory phosphorylation of separase plays a critical role in the maintenance of sister chromatid cohesion and genome stability in proliferating postmigratory primordial germ cells.

Higher eukaryotes rely on a separate cell lineage, the germline, to pass genetic information from generation to generation. To ensure faithful transmission of genetic information, cell cycle checkpoint mechanisms are engaged during mitotic and meiotic divisions of germ cells. The identity and function of these checkpoints is not well understood. In mammals, the germline is specified early in embryogenesis as primordial germ cells (PGCs) at the epiblast stage (around embryonic day 5.0 in mice). PGCs then migrate out from their birthplace and arrive at the genital ridge several days later. In the genital ridge, PGCs undergo a great expansion in number through mitosis. During this expansion, PGCs critically depend on the inhibitory phosphorylation of separase to prevent premature separation of sister chromatids and hence progeny with abnormal chromosome number. Separase is a protease which cleaves the Scc1 subunit of sister chromatid cohesin complex. Its activity must be suppressed before all sisters are aligned at the metaphase plate. Two mechanisms are known that can inhibit separase: phosphorylation and binding by securin, both of which are activated at the spindle assembly checkpoint. Although these two mechanisms are redundant in somatic cells, our results indicate that the inhibitory phosphorylation of separase is uniquely required in the germline.

A single point mutation of separase that blocks its phosphorylation has a profound and dominant effect on germ cell biology.

The protease activity of yeast separase (esp1) is required for anaphase spindle elongation independently of its role in cleavage of cohesin

Separase is a caspase-family protease required for the metaphase-anaphase transition in eukaryotes. In budding yeast, the separase ortholog, Esp1, has been shown to cleave a subunit of cohesin, Mcd1 (Scc1), thereby releasing sister chromatids from cohesion and allowing anaphase. However, whether Esp1 has other substrates required for anaphase has been controversial. Whereas it has been reported that cleavage of Mcd1 is sufficient to trigger anaphase in the absence of Esp1 activation, another study using a temperature-sensitive esp1 mutant concluded that depletion of Mcd1 was not sufficient for anaphase in the absence of Esp1 function. Here we revisit the issue and demonstrate that neither depletion of Mcd1 nor ectopic cleavage of Mcd1 by Tev1 protease is sufficient to support anaphase in an esp1 temperature-sensitive mutant. Furthermore, we demonstrate that the catalytic activity of the Esp1 protease is required for this Mcd1-independent anaphase function. These data suggest that another protein, possibly a spindle-associated protein, is cleaved by Esp1 to allow anaphase. Such a function is consistent with the previous observation that Esp1 localizes to the mitotic spindle during anaphase.

What makes centromeric cohesion resistant to separase cleavage during meiosis I but not during meiosis II?

Segregation of chromosomes during meiosis I is triggered by separase cleavage of the cohesin's Rec8 subunit along chromosome arms. Centromeric cohesin is protected from separase cleavage during meiosis I by Sgo1/MEI-S332 proteins in complex with protein phosphatase 2A (PP2A). This retention of centromeric sister chromatid cohesion is essential for faithful segregation of chromatids during the second meiotic division. While Sgo1/PP2A complex is required for protecting centromeric sister chromatid cohesion during meiosis I, it is not known what renders the centromeric cohesion sensitive to separase cleavage during meiosis II. Our data suggest that the absence of Sgo1 and PP2A from meiosis II centromeres is not sufficient to render centromeric cohesion sensitive to cleavage by separase and additional factors are required to ensure the removal of centromeric cohesion during meiosis II.

Many faces of separase regulation

In the last decade, yeast genetics has played an instrumental role in identifying conserved components of the machinery that holds sister chromatids together and promotes their separation at anaphase. However, we still lack a molecular understanding which can explain the 'all-or-nothing' nature of sister chromatid separation. A formidable challenge for the future will be to elucidate how multiple layers of control act at both the level of separase and its substrate to regulate the global cleavage of cohesin at anaphase. Such a challenge will benefit from the development of in vivo real-time, high-resolution biomarkers of separase activation and cohesin cleavage. Furthermore, a move towards specifically compromising securin's inhibitory function without affecting the protein's chaperone role is required if we are to understand the true value of other pathways controlling separase activation and cohesin cleavage.

Astrin is required for the maintenance of sister chromatid cohesion and centrosome integrity

Faithful chromosome segregation in mitosis requires the formation of a bipolar mitotic spindle with stably attached chromosomes. Once all of the chromosomes are aligned, the connection between the sister chromatids is severed by the cysteine protease separase. Separase also promotes centriole disengagement at the end of mitosis. Temporal coordination of these two activities with the rest of the cell cycle is required for the successful completion of mitosis. In this study, we report that depletion of the microtubule and kinetochore protein astrin results in checkpoint-arrested cells with multipolar spindles and separated sister chromatids, which is consistent with untimely separase activation. Supporting this idea, astrin-depleted cells contain active separase, and separase depletion suppresses the premature sister chromatid separation and centriole disengagement in these cells. We suggest that astrin contributes to the regulatory network that controls separase activity.

The complete removal of cohesin from chromosome arms depends on separase

Cohesin needs to be removed from chromosomes to allow sister chromatid separation in mitosis. In vertebrates, two pathways contribute to this process. The prophase pathway, which requires phosphorylation of the cohesin subunit SA2 and a cohesin-binding protein, called Wapl, removes the bulk of cohesin from the chromosome arms in early mitosis and allows the resolution of the chromosome arms. At anaphase onset, the protease separase removes centromere-enriched cohesin by proteolytic cleavage of another cohesin subunit, Scc1 (Rad21, Mcd1), which allows the separation of sister chromatids. When anaphase onset is delayed by the spindle-assembly checkpoint, the complete removal of cohesin from chromosome arms but not from centromeres generates typical X- or V-shaped chromosomes. Here, we found that cohesion between chromosome arms is preserved if mitosis is arrested with the proteasome inhibitor MG132. This arm cohesion depends on cohesin complexes that are protected by the shugoshin protein Sgo1, which appears to be distributed on chromosome arms as well as on centromeres in early mitosis. In cells lacking separase or expressing non-cleavable Scc1, arm cohesion was not efficiently removed during nocodazole arrest. Our observations suggest that a fraction of arm cohesin is protected by Sgo1, which prevents cohesin from being removed by the prophase pathway, and that separase is partly activated in nocodazole-arrested cells and removes the arm cohesin protected by Sgo1.