Regulation of centromere localization of the Drosophila Shugoshin MEI-S332 and sister-chromatid cohesion in meiosis

ATP synthase F1 subunits recruited to centromeres by CENP-A are required for male meiosis

The histone H3 variant CENP-A epigenetically defines the centromere and is critical for chromosome segregation. Here we report an interaction between CENP-A and subunits of the mitochondrial ATP synthase complex in the germline of male Drosophila. Furthermore, we report that knockdown of CENP-A, as well as subunits ATPsyn-α, -βlike (a testis-specific paralogue of ATPsyn-β) and -γ disrupts sister centromere cohesion in meiotic prophase I. We find that this disruption is likely independent of reduced ATP levels. We identify that ATPsyn-α and -βlike localise to meiotic centromeres and that this localisation is dependent on the presence of CENP-A. We show that ATPsyn-α directly interacts with the N-terminus of CENP-A in vitro and that truncation of its N terminus perturbs sister centromere cohesion in prophase I. We propose that the CENP-A N-terminus recruits ATPsyn-α and -βlike to centromeres to promote sister centromere cohesion in a nuclear function that is independent of oxidative phosphorylation.

The histone H3 CENP-A is known to play a role during meiosis but its role in the testes in the fly is unknown. Here, the authors identify the mitochondrial metabolic protein complex ATP synthase F₁ as interacting with CENP-A, promoting centromere cohesion during meiosis and affecting fly fertility.

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.

Germline-specific labeling of the somatic chromosomes by protein phosphatase 2A and histone H3S28 phosphorylation in Acricotopus lucidus

Additional chromosomes limited to the germline (=Ks) were established as a special form of germline-soma differentiation in the Orthocladiinae, a subfamily of the Chironomidae (Bauer and Beermann in Z Naturforsch 7b: 557-563, 1952). The Ks together with the somatic chromosomes (=Ss) pass through a complex chromosome cycle with elimination at mitosis and a monopolar migration of all Ks. The dissimilar behavior of Ks and Ss in these exceptional mitoses initiated the search for differential chromosome marks in the orthocladiid Acricotopus lucidus. The search, using immunofluorescence, revealed that in metaphases of male gonial mitoses, and both meiotic divisions, the Ss are fully labeled by protein phosphatase 2A (PP2A) and histone H3S28ph, while in metaphases of somatic cells both marks were detected only at the centromeres of the Ss. In another orthocladiid, Psectrocladius obvius, the same labeling pattern of the Ss as in A. lucidus was established for H3S28ph, but not for PP2A, which was localised solely at the centromeres. In Chironomus nuditaris, a species possessing no Ks, PP2A and H3S28ph signals were always restricted to the centromeres. High levels of H3K4me3, a marker of transcriptionally competent chromatin, were detected on the Ss in metaphases I of C. nuditaris, while in both orthocladiids, the Ss in metaphases I were devoid of H3K4me3 signals. This strongly supports an earlier idea of a silencing of the Ss in male meiosis of A. lucidus suggesting the possibility of extending this concept to the Orthocladiinae. The germline-soma differentiation in A. lucidus is not only made apparent by the occurrence of Ks but also by a germline-specific labeling of the Ss by PP2A and H3S28ph.

Playing polo during mitosis: PLK1 takes the lead

Polo-like kinase 1 (PLK1), the prototypical member of the polo-like family of serine/threonine kinases, is a pivotal regulator of mitosis and cytokinesis in eukaryotes. Many layers of regulation have evolved to target PLK1 to different subcellular structures and to its various mitotic substrates in line with its numerous functions during mitosis. Collective work is starting to illuminate an important set of substrates for PLK1: the mitotic kinases that together ensure the fidelity of the cell division process. Amongst these, recent developments argue that PLK1 regulates the activity of the histone kinases Aurora B and Haspin to define centromere identity, of MPS1 to initiate spindle checkpoint signaling, and of BUB1 and its pseudokinase paralog BUBR1 to coordinate spindle checkpoint activation and inactivation. Here, we review the recent work describing the regulation of these kinases by PLK1. We highlight common themes throughout and argue that a major mitotic function of PLK1 is as a master regulator of these key kinases.

Bioinformatical analysis of eukaryotic shugoshins reveals meiosis-specific features of vertebrate shugoshins

Background

Shugoshins (SGOs) are proteins that protect cohesins located at the centromeres of sister chromatids from their early cleavage during mitosis and meiosis in plants, fungi, and animals. Their function is to prevent premature sister-chromatid disjunction and segregation. The study focused on the structural differences among SGOs acting during mitosis and meiosis that cause differences in chromosome behavior in these two types of cell division in different organisms.

Methods

A bioinformatical analysis of protein domains, conserved amino acid motifs, and physicochemical properties of 32 proteins from 25 species of plants, fungi, and animals was performed.

Results

We identified a C-terminal amino acid motif that is highly evolutionarily conserved among the SGOs protecting centromere cohesion of sister chromatids in meiotic anaphase I, but not among mitotic SGOs. This meiotic motif is arginine-rich in vertebrates. SGOs differ in different eukaryotic kingdoms by the sets and locations of amino acid motifs and the number of α-helical regions in the protein molecule.

Discussion

These structural differences between meiotic and mitotic SGOs probably could be responsible for the prolonged SGOs resistance to degradation during meiotic metaphase I and anaphase I. We suggest that the “arginine comb” in C-end meiotic motifs is capable of interaction by hydrogen bonds with guanine bases in the minor groove of DNA helix, thus protecting SGOs from hydrolysis. Our findings support independent evolution of meiosis in different lineages of multicellular organisms.

Nucleolar activity and CENP-C regulate CENP-A and CAL1 availability for centromere assembly in meiosis

The centromere-specific histone CENP-A is the key epigenetic determinant of centromere identity. Whereas most histones are removed from mature sperm, CENP-A is retained to mark paternal centromeres. In Drosophila males we show that the centromere assembly factors CAL1 and CENP-C are required for meiotic chromosome segregation, CENP-A assembly and maintenance on sperm, as well as fertility. In meiosis, CENP-A accumulates with CAL1 in nucleoli. Furthermore, we show that CENP-C normally limits the release of CAL1 and CENP-A from nucleoli for proper centromere assembly in meiotic prophase I. Finally, we show that RNA polymerase I transcription is required for efficient CENP-A assembly in meiosis, as well as centromere tethering to nucleoli.

Summary: Novel roles are uncovered for centromere assembly factors CENP-C and CAL1 in meiotic chromosome segregation, CENP-A assembly and maintenance of sperm, as well as fertility in Drosophila males.

Shugoshins: tension-sensitive pericentromeric adaptors safeguarding chromosome segregation

The shugoshin/Mei-S332 family are proteins that associate with the chromosomal region surrounding the centromere (the pericentromere) and that play multiple and distinct roles in ensuring the accuracy of chromosome segregation during both mitosis and meiosis. The underlying role of shugoshins appears to be to serve as pericentromeric adaptor proteins that recruit several different effectors to this region of the chromosome to regulate processes critical for chromosome segregation. Crucially, shugoshins undergo changes in their localization in response to the tension that is exerted on sister chromosomes by the forces of the spindle that will pull them apart. This has led to the idea that shugoshins provide a platform for activities required at the pericentromere only when sister chromosomes lack tension. Conversely, disassembly of the shugoshin pericentromeric platform may provide a signal that sister chromosomes are under tension. Here the functions and regulation of these important tension-sensitive pericentromeric proteins are discussed.