Structural Basis and IP6 Requirement for Pds5-Dependent Cohesin Dynamics

Structural Basis of an Asymmetric Condensin ATPase Cycle

The condensin protein complex plays a key role in the structural organization of genomes. How the ATPase activity of its SMC subunits drives large-scale changes in chromosome topology has remained unknown. Here we reconstruct, at near-atomic resolution, the sequence of events that take place during the condensin ATPase cycle. We show that ATP binding induces a conformational switch in the Smc4 head domain that releases its hitherto undescribed interaction with the Ycs4 HEAT-repeat subunit and promotes its engagement with the Smc2 head into an asymmetric heterodimer. SMC head dimerization subsequently enables nucleotide binding at the second active site and disengages the Brn1 kleisin subunit from the Smc2 coiled coil to open the condensin ring. These large-scale transitions in the condensin architecture lay out a mechanistic path for its ability to extrude DNA helices into large loop structures.

PDS5 proteins are required for proper cohesin dynamics and participate in replication fork protection

Cohesin is a chromatin-bound complex that mediates sister chromatid cohesion and facilitates long-range interactions through DNA looping. How the transcription and replication machineries deal with the presence of cohesin on chromatin remains unclear. The dynamic association of cohesin with chromatin depends on WAPL cohesin release factor (WAPL) and on PDS5 cohesin-associated factor (PDS5), which exists in two versions in vertebrate cells, PDS5A and PDS5B. Using genetic deletion in mouse embryo fibroblasts and a combination of CRISPR-mediated gene editing and RNAi-mediated gene silencing in human cells, here we analyzed the consequences of PDS5 depletion for DNA replication. We found that either PDS5A or PDS5B is sufficient for proper cohesin dynamics and that their simultaneous removal increases cohesin's residence time on chromatin and slows down DNA replication. A similar phenotype was observed in WAPL-depleted cells. Cohesin down-regulation restored normal replication fork rates in PDS5-deficient cells, suggesting that chromatin-bound cohesin hinders the advance of the replisome. We further show that PDS5 proteins are required to recruit WRN helicase-interacting protein 1 (WRNIP1), RAD51 recombinase (RAD51), and BRCA2 DNA repair associated (BRCA2) to stalled forks and that in their absence, nascent DNA strands at unprotected forks are degraded by MRE11 homolog double-strand break repair nuclease (MRE11). These findings indicate that PDS5 proteins participate in replication fork protection and also provide insights into how cohesin and its regulators contribute to the response to replication stress, a common feature of cancer cells.

Organization of Chromosomal DNA by SMC Complexes

Structural maintenance of chromosomes (SMC) complexes are key organizers of chromosome architecture in all kingdoms of life. Despite seemingly divergent functions, such as chromosome segregation, chromosome maintenance, sister chromatid cohesion, and mitotic chromosome compaction, it appears that these complexes function via highly conserved mechanisms and that they represent a novel class of DNA translocases.

Towards a Unified Model of SMC Complex Function

Protein complexes built of structural maintenance of chromosomes (SMC) and kleisin subunits, including cohesin, condensin and the Smc5/6 complex, are master organizers of genome architecture in all kingdoms of life. How these large ring-shaped molecular machines use the energy of ATP hydrolysis to change the topology of chromatin fibers has remained a central unresolved question of chromosome biology. A currently emerging concept suggests that the common principle that underlies the essential functions of SMC protein complexes in the control of gene expression, chromosome segregation or DNA damage repair is their ability to expand DNA into large loop structures. Here, we review the current knowledge about the biochemical and structural properties of SMC protein complexes that might enable them to extrude DNA loops and compare their action to other motor proteins and nucleic acid translocases. We evaluate the currently predominant models of active loop extrusion and propose a detailed version of a 'scrunching' model, which reconciles much of the available mechanistic data and provides an elegant explanation for how SMC protein complexes fulfill an array of seemingly diverse tasks during the organization of genomes.

Structural analyses of inositol phosphate second messengers bound to signaling effector proteins

The higher-order inositol phosphate second messengers inositol tetrakisphosphate (IP4), inositol pentakisphosphate (IP5) and inositol hexakisphosphate (IP6) are important signaling molecules that regulate DNA-damage repair, cohesin dynamics, RNA-editing, retroviral assembly, nuclear transport, phosphorylation, acetylation, crotonylation, and ubiquitination. This functional diversity has made understanding how inositol polyphosphates regulate cellular processes challenging to dissect. However, some inositol phosphates have been unexpectedly found in X-ray crystal structures, occasionally revealing structural and mechanistic details of effector protein regulation before functional consequences have been described. This review highlights a sampling of crystal structures describing the interaction between inositol phosphates and protein effectors. This list includes the RNA editing enzyme "adenosine deaminase that acts on RNA 2" (ADAR2), the Pds5B regulator of cohesin dynamics, the class 1 histone deacetylases (HDACs) HDAC1 and HDAC3, and the PH domain of Bruton's tyrosine kinase (Btk). One of the most important enzymes responsible for higher-order inositol phosphate synthesis is inositol polyphosphate multikinase (IPMK), which plays dual roles in both inositol and phosphoinositide signaling. Structures of phosphoinositide lipid binding proteins have also revealed new aspects of protein effector regulation, as mediated by the nuclear receptors Steroidogenic Factor-1 (SF-1, NR5A2) and Liver Receptor Homolog-1 (LRH-1, NR5A2). Together, these studies underscore the structural diversity in binding interactions between effector proteins and inositol phosphate small signaling molecules, and further support that detailed structural studies can lead to new biological discovery.

A synthetic biological approach to reconstitution of inositide signaling pathways in bacteria

Inositide lipid (PIP) and soluble (IP) signaling pathways produce essential cellular codes conserved in eukaryotes. In many cases, deconvoluting metabolic and functional aspects of individual pathways are confounded by promiscuity and multiplicity of PIP and IP kinases and phosphatases. We report a molecular genetic approach that reconstitutes eukaryotic inositide lipid and soluble pathways in a prokaryotic cell which inherently lack inositide kinases and phosphatases in their genome. By expressing synthetic cassettes of eukaryotic genes, we have reconstructed the heterologous formation of a range of inositide lipids, including PI(3)P, PI(4,5)P₂ and PIP₃. In addition, we report the reconstruction of lipid-dependent production of inositol hexakisphosphate (IP₆). Our synthetic system is scalable, reduces confounding metabolic issues, for example it is devoid of inositide phosphatases and orthologous kinases, and enables accurate characterization gene product enzymatic activity and substrate selectivity. This genetically engineered tool is designed to help interpret metabolic pathways and may facilitate in vivo testing of regulators and small molecule inhibitors. In summary, heterologous expression of inositide pathways in bacteria provide a malleable experimental platform for aiding signaling biologists and offers new insights into metabolism of these essential pathways.

The Many Roles of Cohesin in Drosophila Gene Transcription

The cohesin protein complex mediates sister chromatid cohesion to ensure accurate chromosome segregation, and also influences gene transcription in higher eukaryotes. Modest deficits in cohesin function that do not alter chromosome segregation cause significant birth defects. The mechanisms by which cohesin participates in gene regulation have been studied in Drosophila, revealing that it is involved in gene activation by transcriptional enhancers and epigenetic gene silencing mediated by Polycomb group proteins. Recent studies reveal that early DNA replication origins are important for determining which genes associate with cohesin, and suggest that cohesin at replication origins is important for establishing both sister chromatid cohesion and enhancer-promoter communication.

Loop formation by SMC complexes: turning heads, bending elbows, and fixed anchors

From the dynamic interphase genome to compacted mitotic chromosomes, DNA is organized by the conserved SMC complexes cohesin and condensin. The picture is emerging that these complexes structure the genome through a shared basic principle that involves the formation and processive enlargement of chromatin loops. This appears to be an asymmetric process, in which the complex anchors at the base of a loop and then enlarges the loop in a one-sided manner. We discuss the latest insights into how ATPase-driven conformational changes within these complexes may enlarge loops, and consider how asymmetric DNA reeling can bring together genomic elements in a symmetric manner.

Structural basis of HEAT-kleisin interactions in the human condensin I subcomplex

Condensin I is a multi‐protein complex that plays an essential role in mitotic chromosome assembly and segregation in eukaryotes. It is composed of five subunits: two SMC (SMC2 and SMC4), a kleisin (CAP‐H), and two HEAT‐repeat (CAP‐D2 and CAP‐G) subunits. Although balancing acts of the two HEAT‐repeat subunits have been demonstrated to enable this complex to support the dynamic assembly of chromosomal axes in vertebrate cells, its underlying mechanisms remain poorly understood. Here, we report the crystal structure of a human condensin I subcomplex comprising hCAP‐G and hCAP‐H. hCAP‐H binds to the concave surfaces of a harp‐shaped HEAT‐repeat domain of hCAP‐G. Physical interaction between hCAP‐G and hCAP‐H is indeed essential for mitotic chromosome assembly recapitulated in Xenopus egg cell‐free extracts. Furthermore, this study reveals that the human CAP‐G‐H subcomplex has the ability to interact with not only double‐stranded DNA, but also single‐stranded DNA, suggesting functional divergence of the vertebrate condensin I complex in proper mitotic chromosome assembly.

Perturbing cohesin dynamics drives MRE11 nuclease-dependent replication fork slowing

Pds5 is required for sister chromatid cohesion, and somewhat paradoxically, to remove cohesin from chromosomes. We found that Pds5 plays a critical role during DNA replication that is distinct from its previously known functions. Loss of Pds5 hinders replication fork progression in unperturbed human and mouse cells. Inhibition of MRE11 nuclease activity restores fork progression, suggesting that Pds5 protects forks from MRE11-activity. Loss of Pds5 also leads to double-strand breaks, which are again reduced by MRE11 inhibition. The replication function of Pds5 is independent of its previously reported interaction with BRCA2. Unlike Pds5, BRCA2 protects forks from nucleolytic degradation only in the presence of genotoxic stress. Moreover, our iPOND analysis shows that the loading of Pds5 and other cohesion factors on replication forks is not affected by the BRCA2 status. Pds5 role in DNA replication is shared by the other cohesin-removal factor Wapl, but not by the cohesin complex component Rad21. Interestingly, depletion of Rad21 in a Pds5-deficient background rescues the phenotype observed upon Pds5 depletion alone. These findings support a model where loss of either component of the cohesin releasin complex perturbs cohesin dynamics on replication forks, hindering fork progression and promoting MRE11-dependent fork slowing.

Condensins and cohesins - one of these things is not like the other!

Condensins and cohesins are highly conserved complexes that tether together DNA loci within a single DNA molecule to produce DNA loops. Condensin and cohesin structures, however, are different, and the DNA loops produced by each underlie distinct cell processes. Condensin rods compact chromosomes during mitosis, with condensin I and II complexes producing spatially defined and nested looping in metazoan cells. Structurally adaptive cohesin rings produce loops, which organize the genome during interphase. Cohesin-mediated loops, termed topologically associating domains or TADs, antagonize the formation of epigenetically defined but untethered DNA volumes, termed compartments. While condensin complexes formed through cis-interactions must maintain chromatin compaction throughout mitosis, cohesins remain highly dynamic during interphase to allow for transcription-mediated responses to external cues and the execution of developmental programs. Here, I review differences in condensin and cohesin structures, and highlight recent advances regarding the intramolecular or cis-based tetherings through which condensins compact DNA during mitosis and cohesins organize the genome during interphase.

A positive feedback mechanism ensures proper assembly of the functional inner centromere during mitosis in human cells

The inner centromere region of a mitotic chromosome critically regulates sister chromatid cohesion and kinetochore-microtubule attachments. However, the molecular mechanism underlying inner centromere assembly remains elusive. Here, using CRISPR/Cas9-based gene editing in HeLa cells, we disrupted the interaction of Shugoshin 1 (Sgo1) with histone H2A phosphorylated on Thr-120 (H2ApT120) to selectively release Sgo1 from mitotic centromeres. Interestingly, cells expressing the H2ApT120-binding defective mutant of Sgo1 have an elevated rate of chromosome missegregation accompanied by weakened centromeric cohesion and decreased centromere accumulation of the chromosomal passenger complex (CPC), an integral part of the inner centromere and a key player in the correction of erroneous kinetochore-microtubule attachments. When artificially tethered to centromeres, a Sgo1 mutant defective in binding protein phosphatase 2A (PP2A) is not able to support proper centromeric cohesion and CPC accumulation, indicating that the Sgo1-PP2A interaction is essential for the integrity of mitotic centromeres. We further provide evidence indicating that Sgo1 protects centromeric cohesin to create a binding site for the histone H3-associated protein kinase Haspin, which not only inhibits the cohesin release factor Wapl and thereby strengthens centromeric cohesion but also phosphorylates histone H3 at Thr-3 to position CPC at inner centromeres. Taken together, our findings reveal a positive feedback-based mechanism that ensures proper assembly of the functional inner centromere during mitosis. They further suggest a causal link between centromeric cohesion defects and chromosomal instability in cancer cells.

Interaction of the Warsaw breakage syndrome DNA helicase DDX11 with the replication fork-protection factor Timeless promotes sister chromatid cohesion

Establishment of sister chromatid cohesion is coupled to DNA replication, but the underlying molecular mechanisms are incompletely understood. DDX11 (also named ChlR1) is a super-family 2 Fe-S cluster-containing DNA helicase implicated in Warsaw breakage syndrome (WABS). Herein, we examined the role of DDX11 in cohesion establishment in human cells. We demonstrated that DDX11 interacts with Timeless, a component of the replication fork-protection complex, through a conserved peptide motif. The DDX11-Timeless interaction is critical for sister chromatid cohesion in interphase and mitosis. Immunofluorescence studies further revealed that cohesin association with chromatin requires DDX11. Finally, we demonstrated that DDX11 localises at nascent DNA by SIRF analysis. Moreover, we found that DDX11 promotes cohesin binding to the DNA replication forks in concert with Timeless and that recombinant purified cohesin interacts with DDX11 in vitro. Collectively, our results establish a critical role for the DDX11-Timeless interaction in coordinating DNA replication with sister chromatid cohesion, and have important implications for understanding the molecular basis of WABS.

Targeting nonsense-mediated mRNA decay in colorectal cancers with microsatellite instability

Nonsense-mediated mRNA decay (NMD) is responsible for the degradation of mRNAs with a premature termination codon (PTC). The role of this system in cancer is still quite poorly understood. In the present study, we evaluated the functional consequences of NMD activity in a subgroup of colorectal cancers (CRC) characterized by high levels of mRNAs with a PTC due to widespread instability in microsatellite sequences (MSI). In comparison to microsatellite stable (MSS) CRC, MSI CRC expressed increased levels of two critical activators of the NMD system, UPF1/2 and SMG1/6/7. Suppression of NMD activity led to the re-expression of dozens of PTC mRNAs. Amongst these, several encoded mutant proteins with putative deleterious activity against MSI tumorigenesis (e.g., HSP110DE9 chaperone mutant). Inhibition of NMD in vivo using amlexanox reduced MSI tumor growth, but not that of MSS tumors. These results suggest that inhibition of the oncogenic activity of NMD may be an effective strategy for the personalized treatment of MSI CRC.

Structural basis for Scc3-dependent cohesin recruitment to chromatin

The cohesin ring complex is required for numerous chromosomal transactions including sister chromatid cohesion, DNA damage repair and transcriptional regulation. How cohesin engages its chromatin substrate has remained an unresolved question. We show here, by determining a crystal structure of the budding yeast cohesin HEAT-repeat subunit Scc3 bound to a fragment of the Scc1 kleisin subunit and DNA, that Scc3 and Scc1 form a composite DNA interaction module. The Scc3-Scc1 subcomplex engages double-stranded DNA through a conserved, positively charged surface. We demonstrate that this conserved domain is required for DNA binding by Scc3-Scc1 in vitro, as well as for the enrichment of cohesin on chromosomes and for cell viability. These findings suggest that the Scc3-Scc1 DNA-binding interface plays a central role in the recruitment of cohesin complexes to chromosomes and therefore for cohesin to faithfully execute its functions during cell division.

Inositol phosphates are assembly co-factors for HIV-1

A short, 14-amino-acid segment called SP1, located in the Gag structural protein1, has a critical role during the formation of the HIV-1 virus particle. During virus assembly, the SP1 peptide and seven preceding residues fold into a six-helix bundle, which holds together the Gag hexamer and facilitates the formation of a curved immature hexagonal lattice underneath the viral membrane2,3. Upon completion of assembly and budding, proteolytic cleavage of Gag leads to virus maturation, in which the immature lattice is broken down; the liberated CA domain of Gag then re-assembles into the mature conical capsid that encloses the viral genome and associated enzymes. Folding and proteolysis of the six-helix bundle are crucial rate-limiting steps of both Gag assembly and disassembly, and the six-helix bundle is an established target of HIV-1 inhibitors4,5. Here, using a combination of structural and functional analyses, we show that inositol hexakisphosphate (InsP6, also known as IP6) facilitates the formation of the six-helix bundle and assembly of the immature HIV-1 Gag lattice. IP6 makes ionic contacts with two rings of lysine residues at the centre of the Gag hexamer. Proteolytic cleavage then unmasks an alternative binding site, where IP6 interaction promotes the assembly of the mature capsid lattice. These studies identify IP6 as a naturally occurring small molecule that promotes both assembly and maturation of HIV-1.

Dysregulated circRNAs in plasma from active tuberculosis patients

Endogenous circular RNAs (circRNAs) have been reported in various diseases. However, their role in active TB remains unknown. The study was aimed to determine plasma circRNA expression profile to characterize potential biomarker and improve our understanding of active TB pathogenesis. CircRNA expression profiles were screened by circRNA microarrays in active TB plasma samples. Dysregulated circRNAs were then verified by qRT-PCR. CircRNA targets were predicted based on analysis of circRNA-miRNA-mRNA interaction. GO and KEGG pathway analyses were used to predict the function of circRNA. ROC curve was calculated to evaluate diagnostic value for active TB. A total of 75 circRNAs were significantly dysregulated in active TB plasma. By further validation, hsa_circRNA_103571 exhibited significant decrease in active TB patients and showed potential interaction with active TB-related miRNAs such as miR-29a and miR-16. Bioinformatics analysis revealed that hsa_circRNA_103571 was primarily involved in ras signalling pathway, regulation of actin cytoskeleton, T- and B-cell receptor signalling pathway. ROC curve analysis suggested that hsa_circRNA_103571 had significant value for active TB diagnosis. Circulating circRNA dysregulation may play a role in active TB pathogenesis. Hsa_circRNA_103571 may be served as a potential biomarker for active TB diagnosis, and hsa_circRNA_103571-miRNA-mRNA interaction may provide some novel mechanism for active TB.

Establishing and dissolving cohesion during the vertebrate cell cycle

Replicated chromatids are held together from the time they emerge from the replication fork until their separation in anaphase. This process, known as cohesion, promotes faithful DNA repair by homologous recombination in interphase and ensures accurate chromosome segregation in mitosis. Identification of cohesin thirty years ago solved a long-standing question about the nature of the linkage keeping together the sister chromatids. Cohesin is an evolutionarily conserved complex composed of a heterodimer of the Structural Maintenance of Chromosomes (SMC) family of ATPases, Smc1 and Smc3, the kleisin subunit Rad21 and a Huntingtin/EF3/PP2A/Tor1 (HEAT) repeat domain-containing subunit named SA/STAG. In addition to mediating cohesion, cohesin plays a major role in genome organization. Cohesin functions rely on the ability of the complex to entrap DNA topologically and in a dynamic manner. Establishment of cohesion during S phase requires coordination with the DNA replication machinery and restricts the dynamic behaviour of at least a fraction of cohesin. Dissolution of cohesion in subsequent mitosis is regulated by multiple mechanisms that ensure that daughter cells receive the correct number of intact chromosomes. We here review recent progress on our understanding of how these processes are regulated in somatic vertebrate cells.

SMC Complexes: Universal DNA Looping Machines with Distinct Regulators

What drives the formation of chromatin loops has been a long-standing question in chromosome biology. Recent work provides major insight into the basic principles behind loop formation. Structural maintenance of chromosomes (SMC) complexes, that are conserved from bacteria to humans, are key to this process. The SMC family includes condensin and cohesin, which structure chromosomes to enable mitosis and long-range gene regulation. We discuss novel insights into the mechanism of loop formation and the implications for how these complexes ultimately shape chromosomes. A picture is emerging in which these complexes form small loops that they then processively enlarge. It appears that SMC complexes act by family-wide basic principles, with complex-specific levels of control.

MCM2-7-dependent cohesin loading during S phase promotes sister-chromatid cohesion

DNA replication transforms cohesin rings dynamically associated with chromatin into the cohesive form to establish sister-chromatid cohesion. Here, we show that, in human cells, cohesin loading onto chromosomes during early S phase requires the replicative helicase MCM2-7 and the kinase DDK. Cohesin and its loader SCC2/4 (NIPBL/MAU2 in humans) associate with DDK and phosphorylated MCM2-7. This binding does not require MCM2-7 activation by CDC45 and GINS, but its persistence on activated MCM2-7 requires fork-stabilizing replisome components. Inactivation of these replisome components impairs cohesin loading and causes interphase cohesion defects. Interfering with Okazaki fragment processing or nucleosome assembly does not impact cohesion. Therefore, MCM2-7-coupled cohesin loading promotes cohesion establishment, which occurs without Okazaki fragment maturation. We propose that the cohesin-loader complex bound to MCM2-7 is mobilized upon helicase activation, transiently held by the replisome, and deposited behind the replication fork to encircle sister chromatids and establish cohesion.

HP1 links centromeric heterochromatin to centromere cohesion in mammals

Heterochromatin protein-1 (HP1) is a key component of heterochromatin. Reminiscent of the cohesin complex which mediates sister-chromatid cohesion, most HP1 proteins in mammalian cells are displaced from chromosome arms during mitotic entry, whereas a pool remains at the heterochromatic centromere region. The function of HP1 at mitotic centromeres remains largely elusive. Here, we show that double knockout (DKO) of HP1α and HP1γ causes defective mitosis progression and weakened centromeric cohesion. While mutating the chromoshadow domain (CSD) prevents HP1α from protecting sister-chromatid cohesion, centromeric targeting of HP1α CSD alone is sufficient to rescue the cohesion defects in HP1 DKO cells. Interestingly, HP1-dependent cohesion protection requires Haspin, an antagonist of the cohesin-releasing factor Wapl. Moreover, HP1α CSD directly binds the N-terminal region of Haspin and facilitates its centromeric localization. The need for HP1 in cohesion protection can be bypassed by centromeric targeting of Haspin or inhibiting Wapl activity. Taken together, these results reveal a redundant role for HP1α and HP1γ in the protection of centromeric cohesion through promoting Haspin localization at mitotic centromeres in mammalian cells.

A role for the Smc3 hinge domain in the maintenance of sister chromatid cohesion

Cohesin is a conserved protein complex required for sister chromatid cohesion, chromosome condensation, DNA damage repair, and regulation of transcription. Although cohesin functions to tether DNA duplexes, the contribution of its individual domains to this activity remains poorly understood. We interrogated the Smc3p subunit of cohesin by random insertion mutagenesis. Analysis of a mutant in the Smc3p hinge revealed an unexpected role for this domain in cohesion maintenance and condensation. Further investigation revealed that the Smc3p hinge functions at a step following cohesin's stable binding to chromosomes and independently of Smc3p's regulation by the Eco1p acetyltransferase. Hinge mutant phenotypes resemble loss of Pds5p, which binds opposite the hinge near Smc3p's head domain. We propose that a specific conformation of the Smc3p hinge and Pds5p cooperate to promote cohesion maintenance and condensation.

Brca2, Pds5 and Wapl differentially control cohesin chromosome association and function

The cohesin complex topologically encircles chromosomes and mediates sister chromatid cohesion to ensure accurate chromosome segregation upon cell division. Cohesin also participates in DNA repair and gene transcription. The Nipped-B-Mau2 protein complex loads cohesin onto chromosomes and the Pds5-Wapl complex removes cohesin. Pds5 is also essential for sister chromatid cohesion, indicating that it has functions beyond cohesin removal. The Brca2 DNA repair protein interacts with Pds5, but the roles of this complex beyond DNA repair are unknown. Here we show that Brca2 opposes Pds5 function in sister chromatid cohesion by assaying precocious sister chromatid separation in metaphase spreads of cultured cells depleted for these proteins. By genome-wide chromatin immunoprecipitation we find that Pds5 facilitates SA cohesin subunit association with DNA replication origins and that Brca2 inhibits SA binding, mirroring their effects on sister chromatid cohesion. Cohesin binding is maximal at replication origins and extends outward to occupy active genes and regulatory sequences. Pds5 and Wapl, but not Brca2, limit the distance that cohesin extends from origins, thereby determining which active genes, enhancers and silencers bind cohesin. Using RNA-seq we find that Brca2, Pds5 and Wapl influence the expression of most genes sensitive to Nipped-B and cohesin, largely in the same direction. These findings demonstrate that Brca2 regulates sister chromatid cohesion and gene expression in addition to its canonical role in DNA repair and expand the known functions of accessory proteins in cohesin's diverse functions.

DNA entry, exit and second DNA capture by cohesin: insights from biochemical experiments

Cohesin is a ring-shaped, multi-subunit ATPase assembly that is fundamental to the spatiotemporal organization of chromosomes. The ring establishes a variety of chromosomal structures including sister chromatid cohesion and chromatin loops. At the core of the ring is a pair of highly conserved SMC (Structural Maintenance of Chromosomes) proteins, which are closed by the flexible kleisin subunit. In common with other essential SMC complexes including condensin and the SMC5-6 complex, cohesin encircles DNA inside its cavity, with the aid of HEAT (Huntingtin, elongation factor 3, protein phosphatase 2A and TOR) repeat auxiliary proteins. Through this topological embrace, cohesin is thought to establish a series of intra- and interchromosomal interactions by tethering more than one DNA molecule. Recent progress in biochemical reconstitution of cohesin provides molecular insights into how this ring complex topologically binds and mediates DNA-DNA interactions. Here, I review these studies and discuss how cohesin mediates such chromosome interactions.

Cohesin Function in Cohesion, Condensation, and DNA Repair Is Regulated by Wpl1p via a Common Mechanism in Saccharomyces cerevisiae

Cohesin tethers DNA to mediate sister chromatid cohesion, chromosome condensation, and DNA repair. How the cell regulates cohesin to perform these distinct functions remains to be elucidated. One cohesin regulator, Wpl1p, was characterized in Saccharomyces cerevisiae as a promoter of efficient cohesion and an inhibitor of condensation. Wpl1p is also required for resistance to DNA-damaging agents. Here, we provide evidence that Wpl1p promotes the timely repair of DNA damage induced during S-phase. Previous studies have indicated that Wpl1p destabilizes cohesin's binding to DNA by modulating the interface between the cohesin subunits Mcd1p and Smc3p Our results suggest that Wpl1p likely modulates this interface to regulate all of cohesin's biological functions. Furthermore, we show that Wpl1p regulates cohesion and condensation through the formation of a functional complex with another cohesin-associated factor, Pds5p In contrast, Wpl1p regulates DNA repair independently of its interaction with Pds5p Together, these results suggest that Wpl1p regulates distinct biological functions of cohesin by Pds5p-dependent and -independent modulation of the Smc3p/Mcd1p interface.

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.

Topologically associating domains and chromatin loops depend on cohesin and are regulated by CTCF, WAPL, and PDS5 proteins

Mammalian genomes are spatially organized into compartments, topologically associating domains (TADs), and loops to facilitate gene regulation and other chromosomal functions. How compartments, TADs, and loops are generated is unknown. It has been proposed that cohesin forms TADs and loops by extruding chromatin loops until it encounters CTCF, but direct evidence for this hypothesis is missing. Here, we show that cohesin suppresses compartments but is required for TADs and loops, that CTCF defines their boundaries, and that the cohesin unloading factor WAPL and its PDS5 binding partners control the length of loops. In the absence of WAPL and PDS5 proteins, cohesin forms extended loops, presumably by passing CTCF sites, accumulates in axial chromosomal positions (vermicelli), and condenses chromosomes. Unexpectedly, PDS5 proteins are also required for boundary function. These results show that cohesin has an essential genome-wide function in mediating long-range chromatin interactions and support the hypothesis that cohesin creates these by loop extrusion, until it is delayed by CTCF in a manner dependent on PDS5 proteins, or until it is released from DNA by WAPL.

Shaping chromosomes by DNA capture and release: gating the SMC rings

SMC proteins organize chromosomes to coordinate essential nuclear processes such as gene expression and DNA recombination as well as to segregate chromosomes during cell division. SMC mediated DNA bridging keeps sister chromatids aligned for much of the cell cycle, while the active extrusion of DNA loops by SMC presumably compacts chromosomes. Chromosome superstructure is thus given by the number of DNA linkages and the size of chromosomal DNA loops, which in turn depend on the dynamics of SMC loading and unloading. The latter is regulated by the intrinsic SMC ATPase activity, multiple external factors and post-translational modification. Here, I highlight recent advances in our understanding of DNA capture and release by SMC-with a focus on cohesin.

Sororin is enriched at the central region of synapsed meiotic chromosomes

During meiotic prophase, cohesin complexes mediate cohesion between sister chromatids and promote pairing and synapsis of homologous chromosomes. Precisely how the activity of cohesin is controlled to promote these events is not fully understood. In metazoans, cohesion establishment between sister chromatids during mitotic divisions is accompanied by recruitment of the cohesion-stabilizing protein Sororin. During somatic cell division cycles, Sororin is recruited in response to DNA replication-dependent modification of the cohesin complex by ESCO acetyltransferases. How Sororin is recruited and acts in meiosis is less clear. Here, we have surveyed the chromosomal localization of Sororin and its relationship to the meiotic cohesins and other chromatin modifiers with the objective of determining how Sororin contributes to meiotic chromosome dynamics. We show that Sororin localizes to the cores of meiotic chromosomes in a manner that is dependent on synapsis and the synaptonemal complex protein SYCP1. In contrast, cohesin, with which Sororin interacts in mitotic cells, shows axial enrichment on meiotic chromosomes even in the absence of synapsis between homologs. Using high-resolution microscopy, we show that Sororin is localized to the central region of the synaptonemal complex. These results indicate that Sororin regulation during meiosis is distinct from its regulation in mitotic cells and may suggest that it interacts with a distinctly different partner to ensure proper chromosome dynamics in meiosis.

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

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

The crystal structure of mammalian inositol 1,3,4,5,6-pentakisphosphate 2-kinase reveals a new zinc-binding site and key features for protein function

Inositol 1,3,4,5,6-pentakisphosphate 2-kinases (IP₅ 2-Ks) are part of a family of enzymes in charge of synthesizing inositol hexakisphosphate (IP₆) in eukaryotic cells. This protein and its product IP₆ present many roles in cells, participating in mRNA export, embryonic development, and apoptosis. We reported previously that the full-length IP₅ 2-K from Arabidopsis thaliana is a zinc metallo-enzyme, including two separated lobes (the N- and C-lobes). We have also shown conformational changes in IP₅ 2-K and have identified the residues involved in substrate recognition and catalysis. However, the specific features of mammalian IP₅ 2-Ks remain unknown. To this end, we report here the first structure for a murine IP₅ 2-K in complex with ATP/IP₅ or IP₆. Our structural findings indicated that the general folding in N- and C-lobes is conserved with A. thaliana IP₅ 2-K. A helical scaffold in the C-lobe constitutes the inositol phosphate-binding site, which, along with the participation of the N-lobe, endows high specificity to this protein. However, we also noted large structural differences between the orthologues from these two eukaryotic kingdoms. These differences include a novel zinc-binding site and regions unique to the mammalian IP₅ 2-K, as an unexpected basic patch on the protein surface. In conclusion, our findings have uncovered distinct features of a mammalian IP₅ 2-K and set the stage for investigations into protein-protein or protein-RNA interactions important for IP₅ 2-K function and activity.

Releasing the cohesin ring: A rigid scaffold model for opening the DNA exit gate by Pds5 and Wapl

The ring-shaped ATPase machine, cohesin, regulates sister chromatid cohesion, transcription, and DNA repair by topologically entrapping DNA. Here, we propose a rigid scaffold model to explain how the cohesin regulators Pds5 and Wapl release cohesin from chromosomes. Recent studies have established the Smc3-Scc1 interface as the DNA exit gate of cohesin, revealed a requirement for ATP hydrolysis in ring opening, suggested regulation of the cohesin ATPase activity by DNA and Smc3 acetylation, and provided insights into how Pds5 and Wapl open this exit gate. We hypothesize that Pds5, Wapl, and SA1/2 form a rigid scaffold that docks on Scc1 and anchors the N-terminal domain of Scc1 (Scc1N) to the Smc1 ATPase head. Relative movements between the Smc1-3 ATPase heads driven by ATP and Wapl disrupt the Smc3-Scc1 interface. Pds5 binds the dissociated Scc1N and prolongs this open state of cohesin, releasing DNA. We review the evidence supporting this model and suggest experiments that can further test its key principles.

The N-Terminal Non-Kinase-Domain-Mediated Binding of Haspin to Pds5B Protects Centromeric Cohesion in Mitosis

Sister-chromatid cohesion, mediated by the multi-subunit cohesin complex, must be precisely regulated to prevent chromosome mis-segregation. In prophase and prometaphase, whereas the bulk of cohesin on chromosome arms is removed by its antagonist Wapl, cohesin at centromeres is retained to ensure chromosome biorientation until anaphase onset. It remains incompletely understood how centromeric cohesin is protected against Wapl in mitosis. Here we show that the mitotic histone kinase Haspin binds to the cohesin regulatory subunit Pds5B through a conserved YGA/R motif in its non-catalytic N terminus, which is similar to the recently reported YSR-motif-dependent binding of Wapl to Pds5B. Knockout of Haspin or disruption of Haspin-Pds5B interaction causes weakened centromeric cohesion and premature chromatid separation, which can be reverted by centromeric targeting of a N-terminal short fragment of Haspin containing the Pds5B-binding motif or by prevention of Wapl-dependent cohesin removal. Conversely, excessive Haspin capable of binding Pds5B displaces Wapl from Pds5B and suppresses Wapl activity, and it largely bypasses the Wapl antagonist Sgo1 for cohesion protection. Taken together, these data indicate that the Haspin-Pds5B interaction is required to ensure proper sister-chromatid cohesion, most likely through antagonizing Wapl-mediated cohesin release from mitotic centromeres.

Pds5 Regulates Sister-Chromatid Cohesion and Chromosome Bi-orientation through a Conserved Protein Interaction Module

Sister-chromatid cohesion is established by the cohesin complex in S phase and persists until metaphase, when sister chromatids are captured by microtubules emanating from opposite poles [1]. The Aurora-B-containing chromosome passenger complex (CPC) plays a crucial role in achieving chromosome bi-orientation by correcting erroneous microtubule attachment [2]. The centromeric localization of the CPC relies largely on histone H3-T3 phosphorylation (H3-pT3), which is mediated by the mitotic histone kinase Haspin/Hrk1 [3-5]. Hrk1 localization to centromeres depends largely on the cohesin subunit Pds5 in fission yeast [5]; however, it is unknown how Pds5 regulates Hrk1 localization. Here we identify a conserved Hrk1-interacting motif (HIM) in Pds5 and a Pds5-interacting motif (PIM) in Hrk1 in fission yeast. Mutations in either motif result in the displacement of Hrk1 from centromeres. We also show that the mechanism of Pds5-dependent Hrk1 recruitment is conserved in human cells. Notably, the PIM in Haspin/Hrk1 is reminiscent of the YSR motif found in the mammalian cohesin destabilizer Wapl and stabilizer Sororin, both of which bind PDS5 [6-12]. Similarly, and through the same motifs, fission yeast Pds5 binds to Wpl1/Wapl and acetyltransferase Eso1/Eco1, in addition to Hrk1. Thus, we have identified a protein-protein interaction module in Pds5 that serves as a chromatin platform for regulating sister-chromatid cohesion and chromosome bi-orientation.

Familial STAG2 germline mutation defines a new human cohesinopathy

We characterize a novel human cohesinopathy originated from a familial germline mutation of the gene encoding the cohesin subunit STAG2, which we propose to call STAG2-related X-linked Intellectual Deficiency. Five individuals carry a STAG2 p.Ser327Asn (c.980 G > A) variant that perfectly cosegregates with a phenotype of syndromic mental retardation in a characteristic X-linked recessive pattern. Although patient-derived cells did not show overt sister-chromatid cohesion defects, they exhibited altered cell cycle profiles and gene expression patterns that were consistent with cohesin deficiency. The protein level of STAG2 in patient cells was normal. Interestingly, STAG2 S327 is located at a conserved site crucial for binding to SCC1 and cohesin regulators. When expressed in human cells, the STAG2 p.Ser327Asn mutant is defective in binding to SCC1 and other cohesin subunits and regulators. Thus, decreased amount of intact cohesin likely underlies the phenotypes of STAG2-SXLID. Intriguingly, recombinant STAG2 p.Ser327Asn binds normally to SCC1, WAPL, and SGO1 in vitro, suggesting the existence of unknown in vivo mechanisms that regulate the interaction between STAG2 and SCC1.

A newly discovered developmental disease is characterized by mutations in a subunit of the cohesin protein involved in cell division. A team led by Sérgio Pena from GENE—Núcleo de Genética Médica, Brazil, and Hongtao Yu from the University of Texas Southwestern Medical Center, USA, describe a Brazilian family with five male relatives, all with intellectual deficiency, short stature, and other abnormalities. The family tree pointed toward an X-linked pattern of inheritance, so the researchers performed a network analysis of 24 genes on the X chromosome known to contribute to mental retardation. They found that all five individuals had a mutation in a gene called STAG2, which encodes a subunit of cohesin. The mutant STAG2 did not bind properly to other cohesin subunits in human cells, and patient-derived cells exhibited altered cell cycle profiles. The researchers propose calling the disease “STAG2-related X-linked intellectual deficiency”.

Overall Shapes of the SMC-ScpAB Complex Are Determined by Balance between Constraint and Relaxation of Its Structural Parts

The SMC-ScpAB complex plays a crucial role in chromosome organization and segregation in many bacteria. It is composed of a V-shaped SMC dimer and an ScpAB subcomplex that bridges the two Structural Maintenance of Chromosomes (SMC) head domains. Despite its functional significance, the mechanistic details of SMC-ScpAB remain obscure. Here we provide evidence that ATP-dependent head-head engagement induces a lever movement of the SMC neck region, which might help to separate juxtaposed coiled-coil arms. Binding of the ScpA N-terminal domain (NTD) to the SMC neck region is negatively regulated by the ScpB C-terminal domain. Mutations in the ScpA NTD compromise this regulation and profoundly affect the overall shape of the complex. The SMC hinge domain is structurally relaxed when free from coiled-coil juxtaposition. Taken together, we propose that the structural parts of SMC-ScpAB are subjected to the balance between constraint and relaxation, cooperating to modulate dynamic conformational changes of the whole complex.

Twilight reloaded: the peptide experience

The de facto commoditization of biomolecular crystallography as a result of almost disruptive instrumentation automation and continuing improvement of software allows any sensibly trained structural biologist to conduct crystallographic studies of biomolecules with reasonably valid outcomes: that is, models based on properly interpreted electron density. Robust validation has led to major mistakes in the protein part of structure models becoming rare, but some depositions of protein-peptide complex structure models, which generally carry significant interest to the scientific community, still contain erroneous models of the bound peptide ligand. Here, the protein small-molecule ligand validation tool Twilight is updated to include peptide ligands. (i) The primary technical reasons and potential human factors leading to problems in ligand structure models are presented; (ii) a new method used to score peptide-ligand models is presented; (iii) a few instructive and specific examples, including an electron-density-based analysis of peptide-ligand structures that do not contain any ligands, are discussed in detail; (iv) means to avoid such mistakes and the implications for database integrity are discussed and (v) some suggestions as to how journal editors could help to expunge errors from the Protein Data Bank are provided.

Evolution of condensin and cohesin complexes driven by replacement of Kite by Hawk proteins

Mitotic chromosome condensation, sister chromatid cohesion, and higher order folding of interphase chromatin are mediated by condensin and cohesin, eukaryotic members of the SMC (structural maintenance of chromosomes)-kleisin protein family. Other members facilitate chromosome segregation in bacteria [1]. A hallmark of these complexes is the binding of the two ends of a kleisin subunit to the apices of V-shaped Smc dimers, creating a tripartite ring capable of entrapping DNA (Figure 1A). In addition to creating rings, kleisins recruit regulatory subunits. One family of regulators, namely Kite dimers (Kleisin interacting winged-helix tandem elements), interact with Smc-kleisin rings from bacteria, archaea and the eukaryotic Smc5-6 complex, but not with either condensin or cohesin [2]. These instead possess proteins containing HEAT (Huntingtin/EF3/PP2A/Tor1) repeat domains whose origin and distribution have not yet been characterized. Using a combination of profile Hidden Markov Model (HMM)-based homology searches, network analysis and structural alignments, we identify a common origin for these regulators, for which we propose the name Hawks, i.e. HEAT proteins associated with kleisins.

Structure of the cohesin loader Scc2

The functions of cohesin are central to genome integrity, chromosome organization and transcription regulation through its prevention of premature sister-chromatid separation and the formation of DNA loops. The loading of cohesin onto chromatin depends on the Scc2-Scc4 complex; however, little is known about how it stimulates the cohesion-loading activity. Here we determine the large 'hook' structure of Scc2 responsible for catalysing cohesin loading. We identify key Scc2 surfaces that are crucial for cohesin loading in vivo. With the aid of previously determined structures and homology modelling, we derive a pseudo-atomic structure of the full-length Scc2-Scc4 complex. Finally, using recombinantly purified Scc2-Scc4 and cohesin, we performed crosslinking mass spectrometry and interaction assays that suggest Scc2-Scc4 uses its modular structure to make multiple contacts with cohesin.

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.

Cohesin Mutations in Cancer

Cohesin is a large ring-shaped protein complex, conserved from yeast to human, which participates in most DNA transactions that take place in the nucleus. It mediates sister chromatid cohesion, which is essential for chromosome segregation and homologous recombination (HR)-mediated DNA repair. Together with architectural proteins and transcriptional regulators, such as CTCF and Mediator, respectively, it contributes to genome organization at different scales and thereby affects transcription, DNA replication, and locus rearrangement. Although cohesin is essential for cell viability, partial loss of function can affect these processes differently in distinct cell types. Mutations in genes encoding cohesin subunits and regulators of the complex have been identified in several cancers. Understanding the functional significance of these alterations may have relevant implications for patient classification, risk prediction, and choice of treatment. Moreover, identification of vulnerabilities in cancer cells harboring cohesin mutations may provide new therapeutic opportunities and guide the design of personalized treatments.

Crystal structure of the cohesin loader Scc2 and insight into cohesinopathy

The ring-shaped cohesin complex topologically entraps chromosomes and regulates chromosome segregation, transcription, and DNA repair. The cohesin core consists of the structural maintenance of chromosomes 1 and 3 (Smc1-Smc3) heterodimeric ATPase, the kleisin subunit sister chromatid cohesion 1 (Scc1) that links the two ATPase heads, and the Scc1-bound adaptor protein Scc3. The sister chromatid cohesion 2 and 4 (Scc2-Scc4) complex loads cohesin onto chromosomes. Mutations of cohesin and its regulators, including Scc2, cause human developmental diseases termed cohesinopathy. Here, we report the crystal structure of Chaetomium thermophilum (Ct) Scc2 and examine its interaction with cohesin. Similar to Scc3 and another Scc1-interacting cohesin regulator, precocious dissociation of sisters 5 (Pds5), Scc2 consists mostly of helical repeats that fold into a hook-shaped structure. Scc2 binds to Scc1 through an N-terminal region of Scc1 that overlaps with its Pds5-binding region. Many cohesinopathy mutations target conserved residues in Scc2 and diminish Ct Scc2 binding to Ct Scc1. Pds5 binding to Scc1 weakens the Scc2-Scc1 interaction. Our study defines a functionally important interaction between the kleisin subunit of cohesin and the hook of Scc2. Through competing with Scc2 for Scc1 binding, Pds5 might contribute to the release of Scc2 from loaded cohesin, freeing Scc2 for additional rounds of loading.

Roles for APRIN (PDS5B) in homologous recombination and in ovarian cancer prediction

APRIN (PDS5 cohesin associated factor B) interacts with both the cohesin complex and the BRCA2 tumor suppressor. How APRIN influences cohesion and DNA repair processes is not well understood. Here, we show that APRIN is recruited to DNA damage sites. We find that APRIN interacts directly with RAD51, PALB2 and BRCA2. APRIN stimulates RAD51-mediated DNA strand invasion. APRIN also binds DNA with an affinity for D-loop structures and single-strand (ss) DNA. APRIN is a new homologous recombination (HR) mediator as it counteracts the RPA inhibitory effect on RAD51 loading to ssDNA. We show that APRIN strongly improves the annealing of complementary-strand DNA and that it can stimulate this process in synergy with BRCA2. Unlike cohesin constituents, its depletion has no impact on class switch recombination, supporting a specific role for this protein in HR. Furthermore, we show that low APRIN expression levels correlate with a better survival in ovarian cancer patients and that APRIN depletion sensitizes cells to the PARP inhibitor Olaparib in xenografted zebrafish. Our findings establish APRIN as an important and specific actor of HR, with cohesin-independent functions.

Cohesin Removal along the Chromosome Arms during the First Meiotic Division Depends on a NEK1-PP1γ-WAPL Axis in the Mouse

Mammalian NIMA-like kinase-1 (NEK1) is a dual-specificity kinase highly expressed in mouse germ cells during prophase I of meiosis. Loss of NEK1 induces retention of cohesin on chromosomes at meiotic prophase I. Timely deposition and removal of cohesin is essential for accurate chromosome segregation. Two processes regulate cohesin removal: a non-proteolytic mechanism involving WAPL, sororin, and PDS5B and direct cleavage by separase. Here, we demonstrate a role for NEK1 in the regulation of WAPL loading during meiotic prophase I, via an interaction between NEK1 and PDS5B. This regulation of WAPL by NEK1-PDS5B is mediated by protein phosphatase 1 gamma (PP1γ), which both interacts with and is a phosphotarget of NEK1. Taken together, our results reveal that NEK1 phosphorylates PP1γ, leading to the dephosphorylation of WAPL, which, in turn, results in its retention on chromosome cores to promote loss of cohesion at the end of prophase I in mammals.

Opposing Functions of the N-terminal Acetyltransferases Naa50 and NatA in Sister-chromatid Cohesion

During the cell cycle, sister-chromatid cohesion tethers sister chromatids together from S phase to the metaphase-anaphase transition and ensures accurate segregation of chromatids into daughter cells. N-terminal acetylation is one of the most prevalent protein covalent modifications in eukaryotes and is mediated by a family of N-terminal acetyltransferases (NAT). Naa50 (also called San) has previously been shown to play a role in sister-chromatid cohesion in metazoans. The mechanism by which Naa50 contributes to cohesion is not understood however. Here, we show that depletion of Naa50 in HeLa cells weakens the interaction between cohesin and its positive regulator sororin and causes cohesion defects in S phase, consistent with a role of Naa50 in cohesion establishment. Strikingly, co-depletion of NatA, a heterodimeric NAT complex that physically interacts with Naa50, rescues the sister-chromatid cohesion defects and the resulting mitotic arrest caused by Naa50 depletion, indicating that NatA and Naa50 play antagonistic roles in cohesion. Purified recombinant NatA and Naa50 do not affect each other's NAT activity in vitro Because NatA and Naa50 exhibit distinct substrate specificity, we propose that they modify different effectors and regulate sister-chromatid cohesion in opposing ways.

Topology and structure of an engineered human cohesin complex bound to Pds5B

The cohesin subunits Smc1, Smc3 and Scc1 form large tripartite rings which mediate sister chromatid cohesion and chromatin structure. These are thought to entrap DNA with the help of the associated proteins SA1/2 and Pds5A/B. Structural information is available for parts of cohesin, but analyses of entire cohesin complexes are limited by their flexibility. Here we generated a more rigid 'bonsai' cohesin by truncating the coiled coils of Smc1 and Smc3 and used single-particle electron microscopy, chemical crosslinking-mass spectrometry and in silico modelling to generate three-dimensional models of cohesin bound to Pds5B. The HEAT-repeat protein Pds5B forms a curved structure around the nucleotide-binding domains of Smc1 and Smc3 and bridges the Smc3-Scc1 and SA1-Scc1 interfaces. These results indicate that Pds5B forms an integral part of the cohesin ring by contacting all other cohesin subunits, a property that may reflect the complex role of Pds5 proteins in controlling cohesin-DNA interactions.