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

Expanding the HP1a-binding consensus and molecular grammar for heterochromatin assembly

The recruitment of Heterochromatin Protein 1 (HP1) partners is essential for heterochromatin assembly and function, yet our knowledge regarding their organization in heterochromatin remains limited. Here we show that interactors engage the Drosophila HP1 (HP1a) dimer through a degenerate and expanded form of the previously identified PxVxL motif, which we now term HP1a Access Codes (HACs). These HACs reside in disordered regions, possess high conservation among Drosophila homologs, and contain alternating hydrophobic residues nested in a cluster of positively charged amino acids. These findings and molecular dynamics simulations identify key electrostatic interactions that modulate HP1a-binding strength and provide a dramatically improved HP1a-binding consensus motif that can reveal protein partners and the molecular grammar involved in heterochromatin assembly. We propose HP1a acts as a scaffold for other heterochromatin components containing HAC motifs, which in turn may regulate the function and higher order structure of the heterochromatin compartment.

Protein phosphatase 2A regulates blood cell proliferation and differentiation in Drosophila larval lymph glands

Protein phosphatase 2A (PP2A), one of the most abundant protein phosphatases, has divergent functions in multiple types of cells. Its inactivation has been closely associated with leukemia diseases. However, the physiological function of PP2A for hematopoiesis has been poorly understood in organisms. Drosophila hematopoiesis parallels the vertebrate counterpart in developmental and functional features but involves a much simpler hematopoietic system. Here, utilizing the Drosophila major larval hematopoietic organ lymph gland, we studied the function of PP2A for hematopoiesis in vivo. By knocking down the expression of Pp2A-29B that encodes the scaffold subunit of the PP2A holoenzyme complex, we found that PP2A silencing in the differentiating hemocytes resulted in their excessive proliferation. Furthermore, this PP2A inhibition downregulated the expression of Smoothened (Smo), a crucial component in the Hedgehog pathway, and smo overexpression was able to rescue the phenotypes of PP2A depletion, indicating that Smo functions as a downstream effector of PP2A to restrict the hemocyte proliferation. PDGF/VEGF-receptor (Pvr) overexpression also restored the Smo expression and lymph gland morphology of PP2A silencing, suggesting a PP2A-Pvr-Smo axis to regulate lymph gland growth and hemocyte proliferation. Moreover, inhibiting PP2A activity in the blood progenitor cells promoted their differentiation, but which was independent with Smo. Together, our data suggested that PP2A plays a dual role in the Drosophila lymph gland by preserving the progenitor population and restraining the hemocyte proliferation, to properly regulate the hematopoietic process.

Meiosis-specific functions of kinetochore protein SPC105R required for chromosome segregation in Drosophila oocytes

The reductional division of meiosis I requires the separation of chromosome pairs towards opposite poles. We have previously implicated the outer kinetochore protein SPC105R/KNL1 in driving meiosis I chromosome segregation through lateral attachments to microtubules and coorientation of sister centromeres. To identify the domains of SPC105R that are critical for meiotic chromosome segregation, an RNAi-resistant gene expression system was developed. We found that the SPC105R C-terminal domain (aa 1284-1960) is necessary and sufficient for recruiting NDC80 to the kinetochore and building the outer kinetochore. Furthermore, the C-terminal domain recruits BUBR1, which in turn recruits the cohesion protection proteins MEI-S332 and PP2A. Of the remaining 1283 amino acids, we found the first 473 are most important for meiosis. The first 123 amino acids of the N-terminal half of SPC105R contain the conserved SLRK and RISF motifs that are targets of PP1 and Aurora B kinase and are most important for regulating the stability of microtubule attachments and maintaining metaphase I arrest. The region between amino acids 124 and 473 are required for lateral microtubule attachments and biorientation of homologues, which are critical for accurate chromosome segregation in meiosis I.

Sororin is an evolutionary conserved antagonist of WAPL

Cohesin mediates sister chromatid cohesion to enable chromosome segregation and DNA damage repair. To perform these functions, cohesin needs to be protected from WAPL, which otherwise releases cohesin from DNA. It has been proposed that cohesin is protected from WAPL by SORORIN. However, in vivo evidence for this antagonism is missing and SORORIN is only known to exist in vertebrates and insects. It is therefore unknown how important and widespread SORORIN's functions are. Here we report the identification of SORORIN orthologs in Schizosaccharomyces pombe (Sor1) and Arabidopsis thaliana (AtSORORIN). sor1Δ mutants display cohesion defects, which are partially alleviated by wpl1Δ. Atsororin mutant plants display dwarfism, tissue specific cohesion defects and chromosome mis-segregation. Furthermore, Atsororin mutant plants are sterile and separate sister chromatids prematurely at anaphase I. The somatic, but not the meiotic deficiencies can be alleviated by loss of WAPL. These results provide in vivo evidence for SORORIN antagonizing WAPL, reveal that SORORIN is present in organisms beyond the animal kingdom and indicate that it has acquired tissue specific functions in plants.

Meiosis-specific functions of kinetochore protein SPC105R required for chromosome segregation in Drosophila oocytes

The reductional division of meiosis I requires the separation of chromosome pairs towards opposite poles. We have previously implicated the outer kinetochore protein SPC105R/KNL1 in driving meiosis I chromosome segregation through lateral attachments to microtubules and co-orientation of sister centromeres. To identify the domains of SPC105R that are critical for meiotic chromosome segregation, an RNAi-resistant gene expression system was developed. We found that SPC105R's C-terminal domain (aa 1284-1960) is necessary and sufficient for recruiting NDC80 to the kinetochore and building the outer kinetochore. Furthermore, the C-terminal domain recruits BUBR1, which in turn recruits the cohesion protection proteins MEI-S332 and PP2A. Of the remaining 1283 amino acids, we found the first 473 are most important for meiosis. The first 123 amino acids of the N-terminal half of SPC105R contain the conserved SLRK and RISF motifs that are targets of PP1 and Aurora B kinase and are most important for regulating the stability of microtubule attachments and maintaining metaphase I arrest. The region between amino acids 124 and 473 are required for two activities that are critical for accurate chromosome segregation in meiosis I, lateral microtubule attachments and bi-orientation of homologs.

What AlphaFold tells us about cohesin's retention on and release from chromosomes

Cohesin is a trimeric complex containing a pair of SMC proteins (Smc1 and Smc3) whose ATPase domains at the end of long coiled coils (CC) are interconnected by Scc1. During interphase, it organizes chromosomal DNA topology by extruding loops in a manner dependent on Scc1's association with two large hook-shaped proteins called SA (yeast: Scc3) and Nipbl (Scc2). The latter's replacement by Pds5 recruits Wapl, which induces release from chromatin via a process requiring dissociation of Scc1's N-terminal domain (NTD) from Smc3. If blocked by Esco (Eco)-mediated Smc3 acetylation, cohesin containing Pds5 merely maintains pre-existing loops, but a third fate occurs during DNA replication, when Pds5-containing cohesin associates with Sororin and forms structures that hold sister DNAs together. How Wapl induces and Sororin blocks release has hitherto remained mysterious. In the 20 years since their discovery, not a single testable hypothesis has been proposed as to their role. Here, AlphaFold 2 (AF) three-dimensional protein structure predictions lead us to propose formation of a quarternary complex between Wapl, SA, Pds5, and Scc1's NTD, in which the latter is juxtaposed with (and subsequently sequestered by) a highly conserved cleft within Wapl's C-terminal domain. AF also reveals how Scc1's dissociation from Smc3 arises from a distortion of Smc3's CC induced by engagement of SMC ATPase domains, how Esco acetyl transferases are recruited to Smc3 by Pds5, and how Sororin prevents release by binding to the Smc3/Scc1 interface. Our hypotheses explain the phenotypes of numerous existing mutations and are highly testable.

Transgenerational epigenetic control of constitutive heterochromatin, transposons, and centromeres

Epigenetic mechanisms are important not only for development but also for genome stability and chromosome dynamics. The latter types of epigenetic controls can often be transgenerational. Here, we review recent progress in two examples of transgenerational epigenetic control: i) the control of constitutive heterochromatin and transposable elements and ii) epigenetic mechanisms that regulate centromere specification and functions. We also discuss the biological significance of enigmatic associations among centromeres, transposons, and constitutive heterochromatin.

Construction of gene co-expression networks in cultured silkworm cells and identification of previously uncharacterized lepidopteran-specific genes required for chromosome dynamics

Advances in sequencing technology and bioinformatics have accelerated gene discovery and homology-based functional annotation in many species, and numerous targeted gene studies have greatly expanded the understanding of gene functions. Nevertheless, there are still many genes that lack homology with genes in other evolutionary lineages and are left as genes with unknown functions. We constructed a gene co-expression network from the Bombyx mori ovary-derived cell line, BmN4, and attempted to infer the biological roles of uncharacterized genes based on the correlation between the function-known and unknown genes. Within this network, we focused on the co-expression modules involved in chromosome architecture, dynamics, and integrity, and selected the uncharacterized genes for subsequent RNAi-based phenotypic screening. This approach enabled the identification of 5 genes whose knockdown led to abnormalities in chromosome dynamics and spindle morphology in mitosis. One of them was a recently characterized gene, BmCenp-T, which plays a central role in building the kinetochore protein complex on the silkworm holocentric chromosomes. In this study, we suggest a method for constructing the gene co-expression network and selecting candidate genes for small-scale RNAi screening. This approach is complementary to homology-based annotation and may be useful for the analysis of lineage-specific uncharacterized genes such as orphan genes.

Multiple pools of PP2A regulate spindle assembly, kinetochore attachments and cohesion in Drosophila oocytes

Meiosis in female oocytes lacks centrosomes, the microtubule-organizing centers. In Drosophila oocytes, meiotic spindle assembly depends on the chromosomal passenger complex (CPC). To investigate the mechanisms that regulate Aurora B activity, we examined the role of protein phosphatase 2A (PP2A) in Drosophila oocyte meiosis. We found that both forms of PP2A, B55 and B56, antagonize the Aurora B spindle assembly function, suggesting that a balance between Aurora B and PP2A activity maintains the oocyte spindle during meiosis I. PP2A-B56, which has a B subunit encoded by two partially redundant paralogs, wdb and wrd, is also required for maintenance of sister chromatid cohesion, establishment of end-on microtubule attachments, and metaphase I arrest in oocytes. WDB recruitment to the centromeres depends on BUBR1, MEI-S332 and kinetochore protein SPC105R. Although BUBR1 stabilizes microtubule attachments in Drosophila oocytes, it is not required for cohesion maintenance during meiosis I. We propose at least three populations of PP2A-B56 regulate meiosis, two of which depend on SPC105R and a third that is associated with the spindle.

Functioning mechanisms of Shugoshin-1 in centromeric cohesion during mitosis

Proper regulation of centromeric cohesion is required for faithful chromosome segregation that prevents chromosomal instability. Extensive studies have identified and established the conserved protein Shugoshin (Sgo1/2) as an essential protector for centromeric cohesion. In this review, we summarize the current understanding of how Shugoshin-1 (Sgo1) protects centromeric cohesion at the molecular level. Targeting of Sgo1 to inner centromeres is required for its proper function of cohesion protection. We therefore discuss about the molecular mechanisms that install Sgo1 onto inner centromeres. At metaphase-to-anaphase transition, Sgo1 at inner centromeres needs to be disabled for the subsequent sister-chromatid segregation. A few recent studies suggest interesting models to explain how it is achieved. These models are discussed as well.

Insights into HP1a-Chromatin Interactions

Understanding the packaging of DNA into chromatin has become a crucial aspect in the study of gene regulatory mechanisms. Heterochromatin establishment and maintenance dynamics have emerged as some of the main features involved in genome stability, cellular development, and diseases. The most extensively studied heterochromatin protein is HP1a. This protein has two main domains, namely the chromoshadow and the chromodomain, separated by a hinge region. Over the years, several works have taken on the task of identifying HP1a partners using different strategies. In this review, we focus on describing these interactions and the possible complexes and subcomplexes associated with this critical protein. Characterization of these complexes will help us to clearly understand the implications of the interactions of HP1a in heterochromatin maintenance, heterochromatin dynamics, and heterochromatin's direct relationship to gene regulation and chromatin organization.

Spo13 prevents premature cohesin cleavage during meiosis

Background: Meiosis produces gametes through two successive nuclear divisions, meiosis I and meiosis II. In contrast to mitosis and meiosis II, where sister chromatids are segregated, during meiosis I, homologous chromosomes are segregated. This requires the monopolar attachment of sister kinetochores and the loss of cohesion from chromosome arms, but not centromeres, during meiosis I. The establishment of both sister kinetochore mono-orientation and cohesion protection rely on the budding yeast meiosis I-specific Spo13 protein, the functional homolog of fission yeast Moa1 and mouse MEIKIN. Methods: Here we investigate the effects of loss of SPO13 on cohesion during meiosis I using a live-cell imaging approach. Results: Unlike wild type, cells lacking SPO13 fail to maintain the meiosis-specific cohesin subunit, Rec8, at centromeres and segregate sister chromatids to opposite poles during anaphase I. We show that the cohesin-destabilizing factor, Wpl1, is not primarily responsible for the loss of cohesion during meiosis I. Instead, premature loss of centromeric cohesin during anaphase I in spo13 Δ cells relies on separase-dependent cohesin cleavage. Further, cohesin loss in spo13 Δ anaphase I cells is blocked by forcibly tethering the regulatory subunit of protein phosphatase 2A, Rts1, to Rec8. Conclusions: Our findings indicate that separase-dependent cleavage of phosphorylated Rec8 causes premature cohesin loss in spo13 Δ cells.

Spo13 prevents premature cohesin cleavage during meiosis

Background: Meiosis produces gametes through two successive nuclear divisions, meiosis I and meiosis II. In contrast to mitosis and meiosis II, where sister chromatids are segregated, during meiosis I, homologous chromosomes are segregated. This requires the monopolar attachment of sister kinetochores and the loss of cohesion from chromosome arms, but not centromeres, during meiosis I. The establishment of both sister kinetochore mono-orientation and cohesion protection rely on the budding yeast meiosis I-specific Spo13 protein, the functional homolog of fission yeast Moa1 and mouse MEIKIN.

Methods: Here we investigate the effects of loss of SPO13 on cohesion during meiosis I using a live-cell imaging approach.

Results: Unlike wild type, cells lacking SPO13 fail to maintain the meiosis-specific cohesin subunit, Rec8, at centromeres and segregate sister chromatids to opposite poles during anaphase I. We show that the cohesin-destabilizing factor, Wpl1, is not primarily responsible for the loss of cohesion during meiosis I. Instead, premature loss of centromeric cohesin during anaphase I in spo13Δ cells relies on separase-dependent cohesin cleavage. Further, cohesin loss in spo13Δ anaphase I cells is blocked by forcibly tethering the regulatory subunit of protein phosphatase 2A, Rts1, to Rec8.

Conclusions: Our findings indicate that separase-dependent cleavage of phosphorylated Rec8 causes premature cohesin loss in spo13Δ cells.

Meiotic prophase-like pathway for cleavage-independent removal of cohesin for chromosome morphogenesis

Sister chromatid cohesion is essential for chromosome segregation both in mitosis and meiosis. Cohesion between two chromatids is mediated by a protein complex called cohesin. The loading and unloading of the cohesin are tightly regulated during the cell cycle. In vertebrate cells, cohesin is released from chromosomes by two distinct pathways. The best characterized pathway occurs at the onset of anaphase, when the kleisin component of the cohesin is destroyed by a protease, separase. The cleavage of the cohesin by separase releases entrapped sister chromatids allowing anaphase to commence. In addition, prior to the metaphase-anaphase transition, most of cohesin is removed from chromosomes in a cleavage-independent manner. This cohesin release is referred to as the prophase pathway. In meiotic cells, sister chromatid cohesion is essential for the segregation of homologous chromosomes during meiosis I. Thus, it was assumed that the prophase pathway for cohesin removal from chromosome arms would be suppressed during meiosis to avoid errors in chromosome segregation. However, recent studies revealed the presence of a meiosis-specific prophase-like pathway for cleavage-independent removal of cohesin during late prophase I in different organisms. In budding yeast, the cleavage-independent removal of cohesin is mediated through meiosis-specific phosphorylation of cohesin subunits, Rec8, the meiosis-specific kleisin, and the yeast Wapl ortholog, Rad61/Wpl1. This pathway plays a role in chromosome morphogenesis during late prophase I, promoting chromosome compaction. In this review, we give an overview of the prophase pathway for cohesin dynamics during meiosis, which has a complex regulation leading to differentially localized populations of cohesin along meiotic chromosomes.

Cohesion and cohesin-dependent chromatin organization

Cohesin, one of structural maintenance of chromosomes (SMC) complexes, forms a ring-shaped protein complex, and mediates sister chromatid cohesion for accurate chromosome segregation and precise genome inheritance. The cohesin ring entraps one or two DNA molecules to achieve cohesion, which is further regulated by cohesin-binding proteins and modification enzymes in a cell cycle-dependent manner. Recent significant advancements in Hi-C technologies have revealed numerous cohesin-dependent higher-order chromatin structures. Simultaneously, single-molecule imaging has also unveiled the detailed dynamics of cohesin on DNA and/or chromatin. Thus, those studies are providing novel visions for the authentic chromatin structure regulated by cohesin.

Protein phosphatase 2A interacts with Verthandi/Rad21 to regulate mitosis and organ development in Drosophila

Rad21/Scc1 is a subunit of the cohesin complex implicated in gene regulation as well as sister chromatid cohesion. The level of Rad21/Scc1 must be controlled for proper mitosis and gene expression during development. Here, we identify the PP2A catalytic subunit encoded by microtubule star (mts) as a regulator of Drosophila Rad21/Verthandi (Vtd). Mutations in mts and vtd cause synergistic mitotic defects, including abnormal spindles and loss of nuclei during nuclear division in early embryo. Depletion of Mts and Vtd in developing wing synergistically reduces the Cut protein level, causing severe defects in wing growth. Mts and PP2A subunit Twins (Tws) interact with Vtd protein. Loss of Mts or Tws reduces Vtd protein level. Reduced proteasome function suppresses mitotic defects caused by mutations in mts and vtd. Taken together, this work provides evidence that PP2A is required for mitosis and wing growth by regulating the Vtd level through the proteasomal pathway.

Sister centromere fusion during meiosis I depends on maintaining cohesins and destabilizing microtubule attachments

Sister centromere fusion is a process unique to meiosis that promotes co-orientation of the sister kinetochores, ensuring they attach to microtubules from the same pole during metaphase I. We have found that the kinetochore protein SPC105R/KNL1 and Protein Phosphatase 1 (PP1-87B) regulate sister centromere fusion in Drosophila oocytes. The analysis of these two proteins, however, has shown that two independent mechanisms maintain sister centromere fusion. Maintenance of sister centromere fusion by SPC105R depends on Separase, suggesting cohesin proteins must be maintained at the core centromeres. In contrast, maintenance of sister centromere fusion by PP1-87B does not depend on either Separase or WAPL. Instead, PP1-87B maintains sister centromeres fusion by regulating microtubule dynamics. We demonstrate that this regulation is through antagonizing Polo kinase and BubR1, two proteins known to promote stability of kinetochore-microtubule (KT-MT) attachments, suggesting that PP1-87B maintains sister centromere fusion by inhibiting stable KT-MT attachments. Surprisingly, C(3)G, the transverse element of the synaptonemal complex (SC), is also required for centromere separation in Pp1-87B RNAi oocytes. This is evidence for a functional role of centromeric SC in the meiotic divisions, that might involve regulating microtubule dynamics. Together, we propose two mechanisms maintain co-orientation in Drosophila oocytes: one involves SPC105R to protect cohesins at sister centromeres and another involves PP1-87B to regulate spindle forces at end-on attachments.

SWITCH 1/DYAD is a WINGS APART-LIKE antagonist that maintains sister chromatid cohesion in meiosis

Mitosis and meiosis both rely on cohesin, which embraces the sister chromatids and plays a crucial role for the faithful distribution of chromosomes to daughter cells. Prior to the cleavage by Separase at anaphase onset, cohesin is largely removed from chromosomes by the non-proteolytic action of WINGS APART-LIKE (WAPL), a mechanism referred to as the prophase pathway. To prevent the premature loss of sister chromatid cohesion, WAPL is inhibited in early mitosis by Sororin. However, Sororin homologs have only been found to function as WAPL inhibitors during mitosis in vertebrates and Drosophila. Here we show that SWITCH 1/DYAD defines a WAPL antagonist that acts in meiosis of Arabidopsis. Crucially, SWI1 becomes dispensable for sister chromatid cohesion in the absence of WAPL. Despite the lack of any sequence similarities, we found that SWI1 is regulated and functions in a similar manner as Sororin hence likely representing a case of convergent molecular evolution across the eukaryotic kingdom.

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.

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.

Drosophila protein phosphatases 2A B' Wdb and Wrd regulate meiotic centromere localization and function of the MEI-S332 Shugoshin

Proper segregation of chromosomes in meiosis is essential to prevent miscarriages and birth defects. This requires that sister chromatids maintain cohesion at the centromere as cohesion is released on the chromatid arms when the homologs segregate at anaphase I. The Shugoshin proteins preserve centromere cohesion by protecting the cohesin complex from cleavage, and this has been shown in yeasts to be mediated by recruitment of the protein phosphatase 2A B' (PP2A B'). In metazoans, delineation of the role of PP2A B' in meiosis has been hindered by its myriad of other essential roles. The Drosophila Shugoshin MEI-S332 can bind directly to both of the B' regulatory subunits of PP2A, Wdb and Wrd, in yeast two-hybrid experiments. Exploiting experimental advantages of Drosophila spermatogenesis, we found that the Wdb subunit localizes first along chromosomes in meiosis I, becoming restricted to the centromere region as MEI-S332 binds. Wdb and MEI-S332 show colocalization at the centromere region until release of sister-chromatid cohesion at the metaphase II/anaphase II transition. MEI-S332 is necessary for Wdb localization, but, additionally, both Wdb and Wrd are required for MEI-S332 localization. Thus, rather than MEI-S332 being hierarchical to PP2A B', these proteins reciprocally ensure centromere localization of the complex. We analyzed functional relationships between MEI-S332 and the two forms of PP2A by quantifying meiotic chromosome segregation defects in double or triple mutants. These studies revealed that both Wdb and Wrd contribute to MEI-S332's ability to ensure accurate segregation of sister chromatids, but, as in centromere localization, they do not act solely downstream of MEI-S332.

Dalmatian: spotting the difference in cohesin protectors

The cohesin complex prevents separation of chromosomes following their duplication until the appropriate time during cell division. In vertebrates, establishment and maintenance of cohesin-dependent linkages depend on two distinct proteins, sororin and shugoshin. New findings published in The EMBO Journal show that in Drosophila, the function of both of these cohesin regulators is carried out by a single hybrid protein, Dalmatian.

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.