Suppression of scant identifies Endos as a substrate of greatwall kinase and a negative regulator of protein phosphatase 2A in mitosis

Protein phosphatase 2A (PP2A) plays a major role in dephosphorylating the targets of the major mitotic kinase Cdk1 at mitotic exit, yet how it is regulated in mitotic progression is poorly understood. Here we show that mutations in either the catalytic or regulatory twins/B55 subunit of PP2A act as enhancers of gwl(Scant), a gain-of-function allele of the Greatwall kinase gene that leads to embryonic lethality in Drosophila when the maternal dosage of the mitotic kinase Polo is reduced. We also show that heterozygous mutant endos alleles suppress heterozygous gwl(Scant); many more embryos survive. Furthermore, heterozygous PP2A mutations make females heterozygous for the strong mutation polo(11) partially sterile, even in the absence of gwl(Scant). Heterozygosity for an endos mutation suppresses this PP2A/polo(11) sterility. Homozygous mutation or knockdown of endos leads to phenotypes suggestive of defects in maintaining the mitotic state. In accord with the genetic interactions shown by the gwl(Scant) dominant mutant, the mitotic defects of Endos knockdown in cultured cells can be suppressed by knockdown of either the catalytic or the Twins/B55 regulatory subunits of PP2A but not by the other three regulatory B subunits of Drosophila PP2A. Greatwall phosphorylates Endos at a single site, Ser68, and this is essential for Endos function. Together these interactions suggest that Greatwall and Endos act to promote the inactivation of PP2A-Twins/B55 in Drosophila. We discuss the involvement of Polo kinase in such a regulatory loop.     

PP2A-twins is antagonized by greatwall and collaborates with polo for cell cycle progression and centrosome attachment to nuclei in drosophila embryos

Cell division and development are regulated by networks of kinases and phosphatases. In early Drosophila embryogenesis, 13 rapid nuclear divisions take place in a syncytium, requiring fine coordination between cell cycle regulators. The Polo kinase is a conserved, crucial regulator of M-phase. We have recently reported an antagonism between Polo and Greatwall (Gwl), another mitotic kinase, in Drosophila embryos. However, the nature of the pathways linking them remained elusive. We have conducted a comprehensive screen for additional genes functioning with polo and gwl. We uncovered a strong interdependence between Polo and Protein Phosphatase 2A (PP2A) with its B-type subunit Twins (Tws). Reducing the maternal contribution of Polo and PP2A-Tws together is embryonic lethal. We found that Polo and PP2A-Tws collaborate to ensure centrosome attachment to nuclei. While a reduction in Polo activity leads to centrosome detachments observable mostly around prophase, a reduction in PP2A-Tws activity leads to centrosome detachments at mitotic exit, and a reduction in both Polo and PP2A-Tws enhances the frequency of detachments at all stages. Moreover, we show that Gwl antagonizes PP2A-Tws function in both meiosis and mitosis. Our study highlights how proper coordination of mitotic entry and exit is required during embryonic cell cycles and defines important roles for Polo and the Gwl-PP2A-Tws pathway in this process.     

Mutations in Drosophila Greatwall/Scant reveal its roles in mitosis and meiosis and interdependence with Polo kinase

Polo is a conserved kinase that coordinates many events of mitosis and meiosis, but how it is regulated remains unclear. Drosophila females having only one wild-type allele of the polo kinase gene and the dominant Scant mutation produce embryos in which one of the centrosomes detaches from the nuclear envelope in late prophase. We show that Scant creates a hyperactive form of Greatwall (Gwl) with altered specificity in vitro, another protein kinase recently implicated in mitotic entry in Drosophila and Xenopus. Excess Gwl activity in embryos causes developmental failure that can be rescued by increasing maternal Polo dosage, indicating that coordination between the two mitotic kinases is crucial for mitotic progression. Revertant alleles of Scant that restore fertility to polo–Scant heterozygous females are recessive alleles or deficiencies of gwl; they show chromatin condensation defects and anaphase bridges in larval neuroblasts. One recessive mutant allele specifically disrupts a Gwl isoform strongly expressed during vitellogenesis. Females hemizygous for this allele are sterile, and their oocytes fail to arrest in metaphase I of meiosis; both homologues and sister chromatids separate on elongated meiotic spindles with little or no segregation. This allelic series of gwl mutants highlights the multiple roles of Gwl in both mitotic and meiotic progression. Our results indicate that Gwl activity antagonizes Polo and thus identify an important regulatory interaction of the cell cycle.     

alpha-Endosulfine is a conserved protein required for oocyte meiotic maturation in Drosophila

Meiosis is coupled to gamete development and must be well regulated to prevent aneuploidy. During meiotic maturation, Drosophila oocytes progress from prophase I to metaphase I. The molecular factors controlling meiotic maturation timing, however, are poorly understood. We show that Drosophila alpha-endosulfine (endos) plays a key role in this process. endos mutant oocytes have a prolonged prophase I and fail to progress to metaphase I. This phenotype is similar to that of mutants of cdc2 (synonymous with cdk1) and of twine, the meiotic homolog of cdc25, which is required for Cdk1 activation. We found that Twine and Polo kinase levels are reduced in endos mutants, and identified Early girl (Elgi), a predicted E3 ubiquitin ligase, as a strong Endos-binding protein. In elgi mutant oocytes, the transition into metaphase I occurs prematurely, but Polo and Twine levels are unaffected. These results suggest that Endos controls meiotic maturation by regulating Twine and Polo levels, and, independently, by antagonizing Elgi. Finally, germline-specific expression of the human alpha-endosulfine ENSA rescues the endos mutant meiotic defects and infertility, and alpha-endosulfine is expressed in mouse oocytes, suggesting potential conservation of its meiotic function.     

Cell cycle regulation of Greatwall kinase nuclear localization facilitates mitotic progression

Cell division requires the coordination of critical protein kinases and phosphatases. Greatwall (Gwl) kinase activity inactivates PP2A-B55 at mitotic entry to promote the phosphorylation of cyclin B–Cdk1 substrates, but how Gwl is regulated is poorly understood. We found that the subcellular localization of Gwl changed dramatically during the cell cycle in Drosophila. Gwl translocated from the nucleus to the cytoplasm in prophase. We identified two critical nuclear localization signals in the central, poorly characterized region of Gwl, which are required for its function. The Polo kinase associated with and phosphorylated Gwl in this region, promoting its binding to 14-3-3ε and its localization to the cytoplasm in prophase. Our results suggest that cyclin B–Cdk1 phosphorylation of Gwl is also required for its nuclear exclusion by a distinct mechanism. We show that the nucleo-cytoplasmic regulation of Gwl is essential for its functions in vivo and propose that the spatial regulation of Gwl at mitotic entry contributes to the mitotic switch.     

The M Phase Kinase Greatwall (Gwl) Promotes Inactivation of PP2A/B55δ, a Phosphatase Directed Against CDK Phosphosites

We have previously shown that Greatwall kinase (Gwl) is required for M phase entry and maintenance in Xenopus egg extracts. Here, we demonstrate that Gwl plays a crucial role in a novel biochemical pathway that inactivates, specifically during M phase, “antimitotic” phosphatases directed against phosphorylations catalyzed by cyclin-dependent kinases (CDKs). A major component of this phosphatase activity is heterotrimeric PP2A containing the B55δ regulatory subunit. Gwl is activated during M phase by Cdk1/cyclin B (MPF), but once activated, Gwl promotes PP2A/B55δ inhibition with no further requirement for MPF. In the absence of Gwl, PP2A/B55δ remains active even when MPF levels are high. The removal of PP2A/B55δ corrects the inability of Gwl-depleted extracts to enter M phase. These findings support the hypothesis that M phase requires not only high levels of MPF function, but also the suppression, through a Gwl-dependent mechanism, of phosphatase(s) that would otherwise remove MPF-driven phosphorylations.     

Spatial regulation of greatwall by Cdk1 and PP2A-Tws in the cell cycle

Entry into mitosis requires the phosphorylation of multiple substrates by cyclin B-Cdk1, while exit from mitosis requires their dephosphorylation, which depends largely on the phosphatase PP2A in complex with its B55 regulatory subunit (Tws in Drosophila). At mitotic entry, cyclin B-Cdk1 activates the Greatwall kinase, which phosphorylates Endosulfine proteins, thereby activating their ability to inhibit PP2A-B55 competitively. The inhibition of PP2A-B55 at mitotic entry facilitates the accumulation of phosphorylated Cdk1 substrates. The coordination of these enzymes involves major changes in their localization. In interphase, Gwl is nuclear while PP2A-B55 is cytoplasmic. We recently showed that Gwl suddenly relocalizes from the nucleus to the cytoplasm in prophase, before nuclear envelope breakdown and that this controlled localization of Gwl is required for its function. We and others have shown that phosphorylation of Gwl by cyclin B-Cdk1 at multiple sites is required for its nuclear exclusion, but the precise mechanisms remained unclear. In addition, how Gwl returns to its nuclear localization was not explored. Here we show that cyclin B-Cdk1 directly inactivates a Nuclear Localization Signal in the central region of Gwl. This phosphorylation facilitates the cytoplasmic retention of Gwl, which is exported to the cytoplasm in a Crm1-dependent manner. In addition, we show that PP2A-Tws promotes the return of Gwl to its nuclear localization during cytokinesis. Our results indicate that the cyclic changes in Gwl localization at mitotic entry and exit are directly regulated by the antagonistic cyclin B-Cdk1 and PP2A-Tws enzymes.     

Protein phosphatase 1 is essential for Greatwall inactivation at mitotic exit

Entry into mitosis is mediated by the phosphorylation of key cell cycle regulators by cyclin-dependent kinase 1 (Cdk1). In Xenopus embryos, the M-phase-promoting activity of Cdk1 is antagonized by protein phosphatase PP2A-B55. Hence, to ensure robust cell cycle transitions, Cdk1 and PP2A-B55 must be regulated so that their activities are mutually exclusive. The mechanism underlying PP2A-B55 inactivation at mitotic entry is well understood: Cdk1-activated Greatwall (Gwl) kinase phosphorylates Ensa/Arpp19, thereby enabling them to bind to and inhibit PP2A-B55. However, the re-activation of PP2A-B55 during mitotic exit, which is essential for cell cycle progression, is less well understood. Here, we identify protein phosphatase PP1 as an essential component of the PP2A-B55 re-activation pathway in Xenopus embryo extracts. PP1 initiates the re-activation of PP2A-B55 by dephosphorylating Gwl. We provide evidence that PP1 targets the auto-phosphorylation site of Gwl, resulting in efficient Gwl inactivation. This step is necessary to facilitate subsequent complete dephosphorylation of Gwl by PP2A-B55. Thus, by identifying PP1 as the phosphatase initiating Gwl inactivation, our study provides the molecular explanation for how Cdk1 inactivation is coupled to PP2A-B55 re-activation at mitotic exit.     

Determinants for activation of the atypical AGC kinase Greatwall during M phase entry

The atypical AGC kinase Greatwall (Gwl) mediates a pathway that prevents the precocious removal of phosphorylations added to target proteins by M phase-promoting factor (MPF); Gwl is thus essential for M phase entry and maintenance. Gwl itself is activated by M phase-specific phosphorylations that are investigated here. Many phosphorylations are nonessential, being located within a long nonconserved region, any part of which can be deleted without effect. Using mass spectrometry and mutagenesis, we have identified 3 phosphorylation sites (phosphosites) critical to Gwl activation (pT193, pT206, and pS883 in Xenopus laevis) located in evolutionarily conserved domains that differentiate Gwl from related kinases. We propose a model in which the initiating event for Gwl activation is phosphorylation by MPF of the proline-directed sites T193 and T206 in the presumptive activation loop. After this priming step, Gwl can intramolecularly phosphorylate its C-terminal tail at pS883; this site probably plays a role similar to that of the tail/Z motif of other AGC kinases. These events largely (but not completely) explain the full activation of Gwl at M phase.     

subito encodes a kinesin-like protein required for meiotic spindle pole formation in Drosophila melanogaster

The female meiotic spindle lacks a centrosome or microtubule-organizing center in many organisms. During cell division, these spindles are organized by the chromosomes and microtubule-associated proteins. Previous studies in Drosophila melanogaster implicated at least one kinesin motor protein, NCD, in tapering the microtubules into a bipolar spindle. We have identified a second Drosophila kinesin-like protein, SUB, that is required for meiotic spindle function. At meiosis I in males and females, sub mutations affect only the segregation of homologous chromosomes. In female meiosis, sub mutations have a similar phenotype to ncd; even though chromosomes are joined by chiasmata they fail to segregate at meiosis I. Cytological analyses have revealed that sub is required for bipolar spindle formation. In sub mutations, we observed spindles that were unipolar, multipolar, or frayed with no defined poles. On the basis of these phenotypes and the observation that sub mutations genetically interact with ncd, we propose that SUB is one member of a group of microtubule-associated proteins required for bipolar spindle assembly in the absence of the centrosomes. sub is also required for the early embryonic divisions but is otherwise dispensable for most mitotic divisions.     

The spindle-associated transmembrane protein Axs identifies a new family of transmembrane proteins in eukaryotes

Axs mutations disrupt both the progression of the meiotic cell cycle and meiotic chromosome segregation in Drosophila. Axs protein co-localizes with endoplasmic reticulum components and is present within a novel structure ensheathing the meiotic spindle. We show that Axs encodes the founding member of a eukaryotic family of trans-membrane proteins.     

Australin: a chromosomal passenger protein required specifically for Drosophila melanogaster male meiosis

The chromosomal passenger complex (CPC), which is composed of conserved proteins aurora B, inner centromere protein (INCENP), survivin, and Borealin/DASRA, localizes to chromatin, kinetochores, microtubules, and the cell cortex in a cell cycle-dependent manner. The CPC is required for multiple aspects of cell division. Here we find that Drosophila melanogaster encodes two Borealin paralogues, Borealin-related (Borr) and Australin (Aust). Although Borr is a passenger in all mitotic tissues studied, it is specifically replaced by Aust for the two male meiotic divisions. We analyzed aust mutant spermatocytes to assess the effects of fully inactivating the Aust-dependent functions of the CPC. Our results indicate that Aust is required for sister chromatid cohesion, recruitment of the CPC to kinetochores, and chromosome alignment and segregation but not for meiotic histone phosphorylation or spindle formation. Furthermore, we show that the CPC is required earlier in cytokinesis than previously thought; cells lacking Aust do not initiate central spindle formation, accumulate anillin or actin at the cell equator, or undergo equatorial constriction.     

Separation of meiotic and mitotic effects of claret non-disjunctional on chromosome segregation in Drosophila.

The claret (ca) locus in Drosophila encodes a kinesin-related motor molecule that is required for proper distribution of chromosomes in meiosis in females and in the early mitotic divisions of the embryo. Here we demonstrate that a mutant allele of claret non-disjunctional (ca(nd)), non-claret disjunctional Dominant (ncdD), causes abnormalities in meiotic chromosome segregation, but is near wild-type with respect to early mitotic chromosome segregation. DNA sequence analysis of this mutant allele reveals two missense mutations compared with the predicted wild-type protein. One mutation lies in a proposed microtubule binding region of the motor domain and affects an amino acid residue that is conserved in all kinesin-related proteins reported to date. This region of the motor domain can be used to distinguish meiotic and mitotic motor function, defining an amino acid sequence criterion for classifying motors according to function. ncdD's mutant meiotic effect, but near wild-type mitotic effect, suggests that interactions of the ca motor protein with spindle microtubules differ in meiosis and mitosis.     

Spatiotemporal coordination of Greatwall-Endos-PP2A promotes mitotic progression

Mitotic entry involves inhibition of protein phosphatase 2A bound to its B55/Tws regulatory subunit (PP2A-B55/Tws), which dephosphorylates substrates of mitotic kinases. This inhibition is induced when Greatwall phosphorylates Endos, turning it into an inhibitor of PP2A-Tws. How this mechanism operates spatiotemporally in the cell is incompletely understood. We previously reported that the nuclear export of Greatwall in prophase promotes mitotic progression. Here, we examine the importance of the localized activities of PP2A-Tws and Endos for mitotic regulation. We find that Tws shuttles through the nucleus via a conserved nuclear localization signal (NLS), but expression of Tws in the cytoplasm and not in the nucleus rescues the development of tws mutants. Moreover, we show that Endos must be in the cytoplasm before nuclear envelope breakdown (NEBD) to be efficiently phosphorylated by Greatwall and to bind and inhibit PP2A-Tws. Disrupting the cytoplasmic function of Endos before NEBD results in subsequent mitotic defects. Evidence suggests that this spatiotemporal regulation is conserved in humans.     

The Spindle-Associated Transmembrane Protein Axs Identifies a New Family of Transmembrane Proteins in Eukaryotes

Axs mutations disrupt both the progression of the meiotic cell cycle and meiotic chromosome segregation in Drosophila. Axs protein co-localizes with endoplasmic reticulum components and is present within a novel structure ensheathing the meiotic spindle. We show that Axs encodes the founding member of a eukaryotic family of trans-membrane proteins.     

Phylogenetic conservation of cytostatic factor related genes in the ascidian Ciona intestinalis

In all vertebrates, mature oocytes arrest at the metaphase of the II meiotic division, while some invertebrates arrest at metaphase-I, others at prophase-I. Fertilization induces completion of meiosis and entry into the first mitotic division. Several experimental models have been considered from both vertebrates and invertebrates in order to shed light on the peculiar aspects of meiotic division, such as the regulation of the cytostatic factor (CSF) and the maturation promoting factor (MPF) in metaphase I or II. Recently, we proposed the oocytes of ascidian Ciona intestinalis as a new model to study the meiotic division. Here, taking advantage of the recent publication of the C. intestinalis genome, we presented a phylogenetic analysis of key molecular components of the CSF-related machinery. We showed that the Mos/MAP kinase pathway is perfectly conserved in ascidians. We demonstrated the presence of a CSF-like activity in metaphase-I arrested C. intestinalis oocytes able to block cell division in two-cell embryos. We further investigated the regulation of CSF by demonstrating that both CSF and MPF inactivation, at the exit of metaphase-I, are independent from protein synthesis, indicating the absence of short-lived factors that regulate metaphase stability, as in other invertebrate species. The results obtained suggest that meiotic regulation in C. intestinalis resembles that of vertebrates, such as Xenopus accordingly to the position of this organism in the evolutionary tree.     

A Drosophila orthologue of larp protein family is required for multiple processes in male meiosis

It is important for the proper execution of cell division in both mitosis and meiosis that the chromosome segregation, cytokinesis, and partition of cell organelles progress in smooth coordination. We show here that the mitochondria inheritance is closely linked with microtubules during meiotic divisions in Drosophila males. They are first clustered in a cell equator at metaphase associated with astral microtubules and then distributed along central spindle microtubules after anaphase. The molecular mechanism for the microtubule-dependent inheritance of mitochondria in male meiosis has not been demonstrated yet. We first isolated mutations for a larp gene that is highly conserved among eukaryotes and showed that these mutant males exhibited multiple meiotic phenotypes such as a failure of chromosome segregation, cytokinesis, and mitochondrial partition. Our cytological examination revealed that the mutants showed defects in spindle pole organization and spindle formation. The larp encodes a Drosophila orthologue of a La-related protein containing a domain exhibiting an outstanding homology with a La type RNA-binding protein. Surprisingly, the dLarp protein is localized in the cytoplasm of the male germ line cells, as observed by its distinct co-localization with mitochondria in early spermatocytes and during meiotic divisions. We discuss here the essential role that dLarp plays in multiple processes in Drosophila male meiosis.     

fumble encodes a pantothenate kinase homolog required for proper mitosis and meiosis in Drosophila melanogaster

A number of fundamental processes comprise the cell division cycle, including spindle formation, chromosome segregation, and cytokinesis. Our current understanding of these processes has benefited from the isolation and analysis of mutants, with the meiotic divisions in the male germline of Drosophila being particularly well suited to the identification of the required genes. We show here that the fumble (fbl) gene is required for cell division in Drosophila. We find that dividing cells in fbl-deficient testes exhibit abnormalities in bipolar spindle organization, chromosome segregation, and contractile ring formation. Cytological analysis of larval neuroblasts from null mutants reveals a reduced mitotic index and the presence of polyploid cells. Molecular analysis demonstrates that fbl encodes three protein isoforms, all of which contain a domain with high similarity to the pantothenate kinases of A. nidulans and mouse. The largest Fumble isoform is dispersed in the cytoplasm during interphase, concentrates around the spindle at metaphase, and localizes to the spindle midbody at telophase. During early embryonic development, the protein localizes to areas of membrane deposition and/or rearrangement, such as the metaphase and cellularization furrows. Given the role of pantothenate kinase in production of Coenzyme A and in phospholipid biosynthesis, this pattern of localization is suggestive of a role for fbl in membrane synthesis. We propose that abnormalities in synthesis and redistribution of membranous structures during the cell division cycle underlie the cell division defects in fbl mutant cells.     

Pachytene Arrest and Other Meiotic Effects of the Start Mutations in Saccharomyes Cerevisiae

Mutations in the Start class of cell division cycle genes (CDC28, CDC36 and CDC39) define the point in the G1 phase of the vegetative cycle at which the cell becomes committed to completing another round of cell division. Genetic, cytological and biochemical data demonstrate that these mutations cause meiotic cells to become arrested at pachytene following completion of both chromosomal DNA replication and spindle pole body (SPB) duplication. In contrast these mutations have previously been found to cause arrest of the mitotic cell cycle prior to either of these landmark events, so the role of the Start genes in these events during vegetative growth must be indirect. Our observations are consistent with the hypothesis that CDC28, CDC36 and CDC39 are required for irreversible commitment to nuclear division in both the mitotic and meiotic pathways. CDC28 was additionally found to be required for the SPB separation that precedes spindle formation in preparation for the second meiotic division. Cytological and genetic analyses of this requirement revealed both that such separation may fail independently at either SPB and that ascospore formation can proceed independently of SPB separation.     

mei-38 is required for chromosome segregation during meiosis in Drosophila females

Meiotic chromosome segregation occurs in Drosophila oocytes on an acentrosomal spindle, which raises interesting questions regarding spindle assembly and function. One is how to organize a bipolar spindle without microtubule organizing centers at the poles. Another question is how to orient the chromosomes without kinetochore capture of microtubules that grow from the poles. We have characterized the mei-38 gene in Drosophila and found it may be required for chromosome organization within the karyosome. Nondisjunction of homologous chromosomes occurs in mei-38 mutants primarily at the first meiotic division in females but not in males where centrosomes are present. Most meiotic spindles in mei-38 oocytes are bipolar but poorly organized, and the chromosomes appear disorganized at metaphase. mei-38 encodes a novel protein that is conserved in the Diptera and may be a member of a multigene family. Mei-38 was previously identified (as ssp1) due to a role in mitotic spindle assembly in a Drosophila cell line. MEI-38 protein localizes to a specific population of spindle microtubules, appearing to be excluded from the overlap of interpolar microtubules in the central spindle. We suggest MEI-38 is required for the stability of parallel microtubules, including the kinetochore microtubules.