matrimony: Biological Overview | References
Gene name - matrimony
Cytological map position - 66C11-66C11
Function - signaling
Symbol - mtrm
FlyBase ID: FBgn0010431
Genetic map position - 3L: 8,406,115..8,407,203 [+]
Classification - Sterile alpha motif
Cellular location - cytoplasmic and nuclear
|Recent literature||Bonner, A. M. and Hawley, R. S. (2018). Functional consequences of the evolution of matrimony, a meiosis-specific inhibitor of Polo kinase. Mol Biol Evol. PubMed ID: 30351378
Meiosis is a defining characteristic of eukaryotes, believed to have evolved only once, over one billion years ago. While the general progression of meiotic events is conserved across multiple diverse organisms, the specific pathways and proteins involved can be highly divergent, even within species from the same genus. This study investigate the rapid evolution of Matrimony (Mtrm), a female meiosis-specific regulator of Polo kinase (Polo) in Drosophila. Mtrm physically interacts with Polo and is required to restrict the activity of Polo during meiosis. Despite Mtrm's critical role in meiosis, sequence conservation within the genus Drosophila is poor. To explore the functional significance of this rapid divergence, Mtrm proteins from 12 different Drosophila species were expressed in the D. melanogaster female germline. Distantly related Mtrm homologs are able to both physically interact with D. melanogaster Polo and rescue the meiotic defects seen in mtrm mutants. However, these distant homologs are not properly degraded after the completion of meiosis. Rather, they continue to inhibit Polo function in the early embryo, resulting in dominant maternal-effect lethality. The ability of Mtrm to be properly degraded, and thus release Polo, is partially due to residues or motifs found within Mtrm's least-conserved regions. It is hypothesized that, while Mtrm regions critical for its meiotic function are under strong purifying selection, changes that occurred in its unconserved regions may have been advantageous, potentially by affecting the timing or duration of meiosis and/or the early embryonic divisions.
|Bonner, A. M., Hughes, S. E. and Hawley, R. S. (2020). Regulation of Polo Kinase by Matrimony Is Required for Cohesin Maintenance during Drosophila melanogaster Female Meiosis. Curr Biol. PubMed ID: 32008903
The Drosophila PLK Polo kinase (Polo) is inhibited by the female meiosis-specific protein Matrimony (Mtrm) in a stoichiometric manner. Drosophila Polo localizes strongly to kinetochores and to central spindle microtubules during prometaphase and metaphase I of female meiosis. Mtrm protein levels increase dramatically after nuclear envelope breakdown. This study shows that Mtrm is enriched along the meiotic spindle and that loss of mtrm results in mislocalization of the catalytically active form of Polo. The mtrm gene is haploinsufficient, and heterozygosity for mtrm results in high levels of achiasmate chromosome missegregation. In mtrm/(+) heterozygotes, there is a low level of sister centromere separation, as well as precocious loss of cohesion along the arms of achiasmate chromosomes. However, mtrm-null females are sterile, and sister chromatid cohesion is abolished on all chromosomes, leading to a failure to properly congress or orient chromosomes in metaphase I. These data demonstrate a requirement for the inhibition of Polo, perhaps by sequestering Polo to the microtubules during Drosophila melanogaster female meiosis and suggest that catalytically active Polo is a distinct subset of the total Polo population within the oocyte that requires its own regulation.
Many meiotic systems in female animals include a lengthy arrest in G2 that separates the end of pachytene from nuclear envelope breakdown (NEB). However, the mechanisms by which a meiotic cell can arrest for long periods of time (decades in human females) have remained a mystery. The Drosophila Matrimony (Mtrm) protein is expressed from the end of pachytene until the completion of meiosis I. Loss-of-function mtrm mutants result in precocious NEB. Coimmunoprecipitation experiments reveal that Mtrm physically interacts with Polo kinase (Polo) in vivo, and multidimensional protein identification technology mass spectrometry analysis reveals that Mtrm binds to Polo with an approximate stoichiometry of 1:1. Mutation of a Polo-Box Domain (PBD) binding site in Mtrm ablates the function of Mtrm and the physical interaction of Mtrm with Polo. The meiotic defects observed in mtrm/+ heterozygotes are fully suppressed by reducing the dose of polo+, demonstrating that Mtrm acts as an inhibitor of Polo. Mtrm acts as a negative regulator of Polo during the later stages of G2 arrest. Indeed, both the repression of Polo expression until stage 11 and the inactivation of newly synthesized Polo by Mtrm until stage 13 play critical roles in maintaining and properly terminating G2 arrest. These data suggest a model in which the eventual activation of Cdc25 (Drosophila Twine) by an excess of Polo at stage 13 triggers NEB and entry into prometaphase (Xiang, 2007).
The mechanism of the lengthy arrest in G2 that separates the end of pachytene from nuclear envelope breakdown (NEB) (which is a characterization of many female meiotic systems) has remained a mystery. One can imagine that both the maintenance and the termination of this arrest might involve either or both of two mechanisms: the transcriptional or translational repression of a protein that induces NEB, and thus meiotic entry, or the presence of an inhibitory protein that precludes entry into the first meiotic division. Because Drosophila females exhibit a prolonged G2 arrest and are amenable to both genetic and cytological analyses, they provide an ideal system in which to study this problem (Xiang, 2007).
The ovaries of Drosophila females are composed of a bundle of ovarioles, each of which contains a number of oocytes arranged in order of their developmental stages. For the purposes of this study, the process of oogenesis may be said to consist of three separate sets of divisions: the initial stem cell divisions, which create primary cystoblasts; four incomplete cystoblast divisions, which create a 16-cell cyst that contains the oocyte; and the two meiotic divisions. Although a great deal is known regarding the mechanisms that control cystoblast divisions and oocyte differentiation, relatively little is known about the mechanisms by which the progression of meiosis is controlled (Xiang, 2007).
As is the case in many meiotic systems, female meiosis in Drosophila involves preprogrammed developmental pauses. The two most prominent pauses during Drosophila meiosis are an arrest that separates the end of pachytene at stages 5-6 from NEB at stage 13, and a second pause that begins with metaphase I arrest at stage 14 and continues until the egg passes through the oviduct. It is the release of this second preprogrammed arrest event that initiates anaphase I and allows the completion of meiosis I followed by meiosis II. The end of meiotic prophase by dissolution of the synaptonemal complex (SC) at stages 5-6 is separated from the beginning of the meiotic divisions, which is defined by NEB at stage 13, by approximately 40 h to allow for oocyte growth (Xiang, 2007).
Elucidating the mechanisms that arrest meiotic progression at the end of prophase, but then allow onset of NEB and the initiation of meiotic spindle formation some 40 h later, was of great interest. One intriguing possibility is that during this period of meiotic arrest, the oocyte actively blocks the function of cell cycle regulatory proteins such as cyclin dependent kinase 1 (Cdk1), the phosphatase Cdc25, and Polo kinase (Polo), all of which promote meiotic progression just as they do during mitotic growth. Recently, Polo was shown to be expressed in the germarium and required for the proper entry of Drosophila oocytes into meiotic prophase, as defined by the assembly of the SC (Mirouse, 2006). Decreased levels of Polo resulted in delayed entry into meiotic prophase, whereas overexpression of Polo caused a dramatic increase in the number of cystocyte cells entering meiotic prophase, indicating that Polo is involved both in the initiation of SC formation and in the restriction of meiosis to the oocyte. How then is Polo, which is known to play multiple roles in promoting meiotic and mitotic progression (Lee, 2003a; Lee, 2003b), prevented from compelling the differentiated oocyte to proceed further into meiosis? One component of this regulation may well lie in the fact that Polo is not expressed during much of oogenesis. Polo is clearly visible in the germarium but is then absent until stage 11, when it begins to accumulate to high levels in the oocyte. A second component of Polo regulation is mediated by binding to the protein product of the matrimony (mtrm) gene, which occurs from stage 11 until the onset of NEB at stage 13. This binding serves to inhibit Polo in the early stages of its expression, and thus prevents precocious nuclear envelope breakdown (Xiang, 2007).
The mtrm gene was first identified in a deficiency screen for loci that were required in two doses for faithful meiotic chromosome segregation (Harris, 2003). mtrm/+ heterozygotes display a significant defect in achiasmate segregation (the meiotic process that ensures the segregation of those homologs that, for various reasons, fail to undergo crossingover). As a result of this defect, mtrm/+ heterozygotes exhibit high levels of achiasmate nondisjunction. As homozygotes, mtrm mutants are fully viable but exhibit complete female sterility. This study shows that the Mtrm protein prevents precocious NEB. Indeed, the effects of reducing the dose of mtrm on meiotic progression and on chromosome segregation are easily explained as the consequence of precocious NEB at stages 11 or 12, and can be suppressed by simultaneously reducing the copy number of polo+. In addition, the effects of heterozygosity for loss-of-function alleles of mtrm can be phenocopied by increasing the copy number of polo+. These genetic interactions suggest that Mtrm negatively regulates Polo in vivo (Xiang, 2007).
Interestingly, Mtrm was shown to interact physically with Polo by a global yeast two-hybrid study. This yeast two-hybrid finding reflects a true physical interaction in vivo by both coimmunoprecipitation studies and by multidimensional protein identification technology (MudPIT) mass spectrometry experiments, which indicate that Mtrm binds to Polo with an approximate stoichiometry of 1:1. Moreover, ablating one of the two putative Polo binding sites on Mtrm by mutation prevents the physical interaction between Polo and Mtrm and renders the mutated Mtrm protein functionless. This experiment, along with genetic interaction studies, provides compelling evidence that the function of the binding of Mtrm to Polo is to inhibit Polo, and not vice versa (Xiang, 2007).
The analysis of mtrm mutants allows examination of the effects of premature Polo function during oogenesis. The evidence shows that in the absence of Mtrm, newly synthesized Polo is capable of inducing NEB from stage 11 onward. As a result of this precocious NEB, chromosomes are not properly compacted into a mature karyosome and they are released prematurely onto the meiotic spindle. In many cases, the centromeres of achiasmate bivalents subsequently fail to co-orient (Xiang, 2007).
The mtrm gene was first identified as a dosage-sensitive meiotic locus; heterozygosity for a loss-of-function allele of mtrm specifically induced the failed segregation of achiasmate homologs (Harris, 2003). The mtrm gene encodes a 217-amino acid protein with two Polo-Box Domain (PBD) binding sites (STP and SSP) and a C-terminal SAM/Pointed domain. The studies reported in this paper rely primarily on a null allele of mtrm (mtrm126), which removes 80 bp of upstream sequence and the sequences encoding the first 41 amino acids of the Mtrm protein (Xiang, 2007).
Western blot analysis using an antibody to Mtrm reveals that Mtrm can be detected only in ovaries. This is consistent with a previous report by Arbeitman (2002), which showed that the expression profile of the mtrm gene product was strictly maternal and that its expression was reduced greater than 10-fold over 0-6.5 h of embryonic development. The specificity of this antibody is demonstrated by the fact that no signal was detected by either Western blotting or by immunofluorescence of ovarioles homozygous for the mtrm126 mutant. Immunofluorescence studies using the same antibody reveal that Mtrm is expressed as a diffuse nuclear protein in the oocytes and nurse cells beginning at stage 4-5. The Mtrm signal was not restricted to the karyosome itself; but rather Mtrm seems to fill the space in the entire nucleus. Although Mtrm is restricted to the nucleus until approximately stage 10, it localizes throughout the oocyte in later stages. Mtrm brightly stains both the oocyte nucleus and cytoplasm between stage 11 and stage 12, but staining is greatly reduced at stage 13, the stage at which NEB occurs (Xiang, 2007).
The data presented in this study that Mtrm serves to inactivate newly synthesized Polo during the period of meiotic progression that precedes NEB. An excess of functional (unbound) Polo, produced by reducing the amount of available Mtrm, causes the early onset of NEB. This early entry into prometaphase releases an immature karyosome into the cytoplasm, which then fails to properly align the centromeres of achiasmate chromosomes on the prometaphase spindle. These observations raise a number of questions ranging from the role of Polo in mediating the G2/M transition in oogenesis to the role of the karyosome structure in facilitating the proper segregation of achiasmate chromosomes (Xiang, 2007).
The trigger for the G2/M transition is activation of Cdk1 by Cdc25, and multiple lines of evidence suggest that Polo can activate Cdc25. First, in Caenorhabditis elegans, RNAi experiments demonstrate that ablation of Polo prevents NEB. Second, the Xenopus Polo homolog Plx1 is activated in vivo during oocyte maturation with the same kinetics as Cdc25. Additionally, microinjection of Plx1 accelerates the activation of both Cdc25 and cyclinB-Cdk1. Moreover, microinjection of either an antibody to Plx1 or kinase-dead mutant of Plx1 inhibits the activation of Cdc25 and its target cyclinB-Cdk1. Another demonstrated that injection of a constitutively active form of Plx1 accelerated Cdc25 activation. These studies support "the concept that Plx1 is the 'trigger' kinase for the activation of Cdc25 during the G2/M transition." Finally, a small molecule inhibitor of Polo kinase (BI 2536) also results in extension of prophase (Lenart, 2007). These data are consistent with the view that the presence of functional (unbound) Polo plays a critical role in ending the extended G2 that is characteristic of oogenesis in most animals (Xiang, 2007).
In light of these data, it is tempting to suggest that in wild-type Drosophila oocytes, the large quantity of Mtrm deposited into the oocyte from stage 10 onward inhibits the Polo that is either newly synthesized or transported into the oocyte during stages 11-12. However, at stage 13, an excess of functional Polo is created when the number of Polo proteins exceeds the available amount of inhibitory Mtrm proteins. This unencumbered and thus functional Polo then serves to activate Cdc25, initiating the chain of events that leads to NEB and the initiation of prometaphase. In the absence of a sufficient amount of Mtrm, an excess of Polo causes the precocious activation of Cdc25, and thus an early G2/M transition. A model describing this hypothesis is presented. Based on this model, one can visualize that decreasing the dose of Mtrm or increasing the dose of Polo will hasten NEB, whereas simultaneous reduction in the dose of both proteins should allow for proper timing of NEB (Xiang, 2007).
Mtrm's first PBD binding site (T40) is required for its interaction with Polo. Mtrm T40 has to be first phosphorylated by a priming kinase, such as one of the Cdks or MAPKs, and was indeed detected as phosphorylated in the mass spectrometry dataset. The NetPhosK algorithm predicts T40 to be a Cdk5 site, and the serines immediately distal to T40 (S48 and S52), which were also detected as phosphorylated are sites for proline-directed kinases such as Cdk or MAPK sites as well. The other prominent phosphorylation event occurs at S137, which could be a Polo phosphorylation site since it falls within a Polo consensus (D/E-X-S/T-Ø-X-D/E). Although the combined sequence coverage for Mtrm was 44%, indicating that some phosphorylated sites might have been missed, Mtrm S137 is a suitable binding site for activated Polo, in agreement with the processive phosphorylation model. At this point of these studies, Mtrm T40 priming kinase or the kinase responsible for Polo activating phosphorylation on T182 has not been identified (Xiang, 2007).
The finding that Polo not only is able to bind to Mtrm in vivo in a 1:1 ratio, but also is fully phosphorylated on T182 in its activation loop suggests a method by which Mtrm serves to inhibit Polo. In general, enzymes are usually not recovered from affinity purifications at levels similar to their targets. They do not form stable complexes, but rather form transient interactions with their substrates, which is how efficient catalysis is achieved. In this instance, Mtrm is able to sequester activated Polo away in a stable binary complex over a long period of time. It is only when this equilibrium is disturbed at the onset of stage 13 by the production of an excess of Polo or by degradation of Mtrm that Polo can be released. The molecular determinants of the Mtrm::Polo sequestration event are not clear, but it would be interesting to test whether the serines found phosphorylated in the vicinity of Mtrm PBD binding sites play a role in locking the binary complex into place (Xiang, 2007).
The data demonstrate that a reduction in the levels of Mtrm results in the release of an incompletely compacted karyosome that rapidly dissolves into individual bivalents during the early stages of spindle formation. For chiasmate bivalents, this is apparently not a problem, because they still co-orient correctly. However, the nonexchange bivalents frequently fail to co-orient properly, such that both homologs are oriented toward the same pole (but often occupy two different arcs of the spindle). This initial failure of proper co-orientation leads to high frequencies of nondisjunction as demonstrated by the genetic studies and analysis of metaphase I images (Xiang, 2007).
Although achiasmate homologs are properly co-oriented in wild-type oocytes, it has been noted that such homologs can often vacillate between the poles such that two achiasmate homologs are often found on the same arc of the same half-spindle during mid to late prometaphase (Gilliland, 2007). These chromosomes are often observed to be physically associated. This situation is quite different from the defect observed in mtrm/+ heterozygotes, where the homologs are neither physically associated nor on the same arc of the spindle (Xiang, 2007).
It is tempting to suggest that the chromosome segregation defects observed in mtrm/+ heterozygotes are simply the result of precocious release of an incompletely re-compacted karyosome. According to this explanation, the defects observed in meiotic chromosome segregation are solely the consequence of premature NEB. (Implicit in this model is the assumption that it is the events that occur during karyosome re-compaction, at stages 11 and 12, that serve to initially bi-orient achiasmate chromosomes, and there is no direct evidence to support such a hypothesis) (Xiang, 2007).
Alternatively, Polo plays multiple roles in the meiotic process (Lee, 2003a; Lee, 2003b), and it is possible that the chromosome segregation defects that were seen represent effects of excess Polo that are un-related to the precocious breakdown of the nuclear envelope. Such a view is supported by two observations. First, the bivalent individualization observed after NEB in mtrm/+ oocytes does not disrupt FM7-X pairings. Second, although heterozygosity for twine in mtrm126/+ heterozygotes suppresses the frequency of precocious NEB from 42%, two alleles of twine tested (twe1 and twek08310) failed to suppress the levels of meiotic nondisjunction observed in FM7/X; mtrm126/+ heterozygotes. These data suggest that the effects of excess Polo on nondisjunction may not be regulated via Cdc25/Twine, but rather by the effects of excess Polo on some other as-yet-unidentified Polo target. This suggests that the effects of Mtrm on the level of Polo might affect multiple Polo-related processes (Xiang, 2007).
Support for such an idea that Mtrm can inhibit Polo-regulated proteins that are unrelated to NEB comes from the observation that the ectopic expression of Drosophila Mtrm in Schizosaccharomyces pombe blocks karyokinesis, producing long multi-septate cells with only one or two large nuclei (Edgar, 1994; Edgar, personal communication to Xiang, 2007). This phenotype is similar, if not identical to, that exhibited by mutants in the S. pombe Polo homolog plo1 (Plo1), which fail in later stages of mitosis due to the role of Plo1 in activating the septation initiation network to trigger cytokinesis and cell division. However, Plo1 also plays a role in bipolar spindle assembly that might also be inhibited in the Mtrm expressing cells, but this function of Plo1 is less well understood (Xiang, 2007).
Thus the possibility exists that the effect of mtrm mutants on meiotic chromosome segregation may well not be the direct consequence of early NEB, but rather may be due to the role of Polo in other meiotic activities, such as spindle formation or the combined effects of these defects with precocious NEB. Efforts to identify such processes and their components are underway (Xiang, 2007).
Finally, it should be noted that while Mtrm is the first known protein that is able to inactivate Polo by physical interaction with Polo itself; there is certainly additional mechanisms of Polo regulation. For example, Archambault (2007) has described mutants in the gene that encodes Greatwall/Scant kinase, which have both late meiotic and mitotic defects. Although there is no evidence for a physical interaction between these two kinases, the authors speculate that the function of the Greatwall kinase serves to antagonize that of Polo. The Scant mutations create a hyperactive form of Greatwall, which might be expected to lower the dosage Polo, and thus perhaps partially suppress the defects observed in mtrm/+ heterozygotes. Indeed, exactly such a suppressive effect has been observed in Scant homozygotes (however, this suppression is much weaker than that obtained by heterozygosity for loss of function alleles of polo) (Xiang, 2007).
The data presented in this study demonstrate that Mtrm acts as a negative regulator of Polo during the later stages of G2 arrest during meiosis. Indeed, both the repression of Polo expression until stage 11 and the inactivation of newly synthesized Polo by Mtrm until stage 13 play critical roles in maintaining and properly terminating G2 arrest. These data suggest a model in which the eventual activation of Cdc25 by an excess of Polo at stage 13 triggers NEB and entry into prometaphase. Although the data do shed some light on the mechanism by which Mtrm inhibits Polo, it is not entirely clear whether Polo's ability to phosphorylate targets other than Cdc25 might be blocked by Mtrm::Polo binding. These issues will need to be addressed in the future studies. Finally, it is noted that although small molecule inhibitors of Polo have been identified, Mtrm represents the first case of a protein inhibitor of Polo. It would be most exciting to identify functional orthologs of Mtrm outside of the genus Drosophila. Perhaps that might best be accomplished through a screen for oocyte-specific Polo-interacting proteins (Xiang, 2007).
Oocytes are stockpiled with proteins and mRNA that are required to drive the initial mitotic divisions of embryogenesis. But are there proteins specific to meiosis whose levels must be decreased to begin embryogenesis properly? The Drosophila protein Cortex (Cort) is a female, meiosis-specific activator of the Anaphase Promoting Complex/Cyclosome (APC/C), an E3 ubiquitin ligase. Immunoprecipitation of Cortex followed by mass spectrometry was performed, and the Polo kinase inhibitor Matrimony (Mtrm) was identified as a potential interactor with Cort. In vitro binding assays showed Mtrm and Cort can bind directly. Mtrm protein levels are reduced dramatically during the oocyte-to-embryo transition, and this downregulation does not take place in cort mutant eggs, consistent with Mtrm being a substrate of APCCort. Mtrm was shown to be subject to APCCort-mediated proteasomal degradation, and a putative APC/C recognition motif was identified in Mtrm that when mutated partially stabilized the protein in the embryo. Furthermore, overexpression of Mtrm in the early embryo caused aberrant nuclear divisions and developmental defects, and these were enhanced by decreasing levels of active Polo. These data indicate APC(Cort) ubiquitylates Mtrm at the oocyte-to-embryo transition, thus preventing excessive inhibition of Polo kinase activity due to Mtrm's presence (Whitfield, 2013).
Despite its pivotal role in development, regulation of the oocyte-to-embryo transition is poorly understood. Given the maternal stockpiles in the oocyte, mechanistic differences between meiosis and mitosis, and meiosis-specific forms of the APC/C, it is crucial to determine which proteins need to be degraded to switch correctly from meiosis to mitosis. The meiosis-specific activator Cort is essential for the transition from oocyte to embryo despite Fzy/Cdc20's presence. Cortex's existence raised the possibility that degradation of particular meiosis-specific proteins may be necessary for the onset of embryogenesis. This study shows this to be the case: the Cort form of the APC/C is required for Mtrm's destruction at the oocyte-to-embryo transition. Furthermore, reduced levels of Mtrm heading into embryogenesis are necessary for proper development, indicative of requirements for differential levels of the protein in meiosis and mitosis (Whitfield, 2013).
A requirement for reduction in levels of Mtrm is illustrated by the deleterious effects of overexpression of the protein in the embryo. A crucial role for Mtrm degradation in the transition from oocyte to embryo is supported by the observation that reduction in levels of Mtrm protein can suppress the developmental block caused by low activity of Cort. In the grau mutants, levels of Cort are reduced, and the mutant oocytes arrest in meiosis. By mutating a single copy of the mtrm gene, this arrest was overcome, the eggs progressed, and several nuclear divisions occurred (Whitfield, 2013).
Mtrm provides key insights into how protein degradation can be regulated at the oocyte-to-embryo transition. Mtrm is not completely removed from the embryo, illustrating that its protein levels are important and degradation does not have to be an all-or-none process. In this case, APCCort acts as a rheostat, allowing for high levels of Mtrm in meiosis and low levels in mitosis. Consistent with this, it is interesting that stabilized forms of Mtrm present at lower levels than the overexpressed wild-type form did not exhibit an embryonic phenotype. mCherry-Mtrm also is present at levels lower than endogenous Mtrm in stage 14 oocytes, and therefore may never reach high enough levels to be able to cause the developmental defects seen with the overexpressed form of Mtrm. This offers evidence for a specific threshold of Mtrm that can be tolerated in the early embryo (Whitfield, 2013).
Polo kinase is a critical regulator of both mitosis and meiosis, and is conserved from yeast to humans. polo (and its orthologs) help regulate mitotic/meiotic entry, chromosome segregation, centrosome dynamics, and cytokinesis. With such diverse roles during mitosis and meiosis, Polo function must be carefully regulated. Up-regulation of human Polo-like kinase (Plk1) is prevalent in many human cancers, and identifying potent inhibitors of Plk1 is the focus of much research. In Drosophila, without inhibition by Mtrm during prophase of meiosis I, Polo prematurely triggers nuclear envelope breakdown (through activation of the Cdc25 phosphatase) and eventually leads to chromosome nondisjunction. Mutation of polo has direct consequences on female meiotic progression as well. During Drosophila embryogenesis, expression of Scant, a hyperactive form of the Polo antagonist Greatwall kinase, leads to dissociated centrosomes from prophase nuclei. Embryos homozygous for polo1 show a wide array of defects, including irregular DNA masses with disorganized spindles, reminiscent of the mtrm overexpression phenotype. These data illustrate the importance of Polo kinase in both mitosis and meiosis, and that improper regulation of its activity can have disastrous consequences on cell division (Whitfield, 2013).
Current evidence suggests that Mtrm regulates Polo activity during both meiosis and mitosis. The current results shed light on how the oocyte/embryo might use the same protein to regulate Polo during such drastically different cell divisions. The data indicate meiosis requires high levels of Mtrm protein/Polo inhibition, while low levels of Mtrm are needed for early embryogenesis. This is likely a mechanism to allow for fine tuning of Polo activity during the rapid divisions of the syncytial embryo (Whitfield, 2013).
The results of this study provide an interesting biological counterpoint to a recent study on the S. cerevisiae meiosis-specific APC/C activator Ama1. Previously, Ama1 had been known to act later in meiosis, regulating spore formation and Cdc20 degradation at meiosis II, It has been shown that APCAma1 also acts earlier in meiosis to clear out mitotic regulators (including Polo/Cdc5) during the extended meiotic prophase I. Consequently, cells lacking Ama1 exit prematurely from prophase I. It is interesting that two meiosis-specific APC/C activators have now been tied to regulation of Polo kinase. Ama1 has a direct, inhibitory effect early in meiosis, whereas Cort seemingly activates Polo indirectly through degradation of Mtrm late in meiosis (Whitfield, 2013).
Mtrm is not likely to be the only specific substrate of Cort, and it will be exciting to search for more APCCort substrates in the future. It will also be interesting to examine whether Cort targets continue to follow a graded versus all-or-none pattern of degradation during the oocyte-to-embryo transition. Further study of meiosis-specific APC/C activators will give valuable insight into the distinctions between meiotic and mitotic regulation and the control of the onset of embryogenesis (Whitfield, 2013).
In Drosophila oocytes, euchromatic homolog-homolog associations are released at the end of pachytene, while heterochromatic pairings persist until metaphase I. A screen of 123 autosomal deficiencies for dominant effects on achiasmate chromosome segregation has identified a single gene that is haplo-insufficient for homologous achiasmate segregation and whose product may be required for the maintenance of such heterochromatic pairings. Of the deficiencies tested, only one exhibited a strong dominant effect on achiasmate segregation, inducing both X and fourth chromosome nondisjunction in FM7/X females. Five overlapping deficiencies showed a similar dominant effect on achiasmate chromosome disjunction and mapped the haplo-insufficient meiotic gene to a small interval within 66C7-12. A P-element insertion mutation in this interval exhibits a similar dominant effect on achiasmate segregation, inducing both high levels of X and fourth chromosome nondisjunction in FM7/X females and high levels of fourth chromosome nondisjunction in X/X females. The insertion site for this P element lies immediately upstream of CG18543, and germline expression of a UAS-CG18543 cDNA construct driven by nanos-GAL4 fully rescues the dominant meiotic defect. It is concluded that CG18543 is the haplo-insufficient gene, and this gene has been named matrimony (mtrm). Cytological studies of prometaphase and metaphase I in mtrm hemizygotes demonstrate that achiasmate chromosomes are not properly positioned with respect to their homolog on the meiotic spindle. One possible, albeit speculative, interpretation of these data is that the presence of only a single copy of mtrm disrupts the function of whatever 'glue' holds heterochromatically paired homologs together from the end of pachytene until metaphase I (Harris, 2003; full text of article).
Search PubMed for articles about Drosophila Matrimony
Arbeitman, M. N., et al. (2002). Gene expression during the life cycle of Drosophila melanogaster. Science 297: 2270-2275. PubMed ID: 12351791
Archambault, V., Zhao, X., Carpenter, A. T. and Glover, D. M. (2007). Mutations in Drosophila Greatwall/Scant reveal its roles in mitosis and meiosis and suggest interdependence with Polo kinase. PLoS Genet 3(11): e200. PubMed ID: 17997611
Edgar, B. A., et al. (1994). Distinct molecular mechanism regulate cell cycle timing at successive stages of Drosophila embryogenesis. Genes Dev 8: 440-452. PubMed ID: 7510257
Gilliland, W. D., et al. (2007). The multiple roles of Mps1 in Drosophila female meiosis. PLoS Genet 3(7): e113. PubMed ID: 17630834
Harris, D., et al. (2003). A deficiency screen of the major autosomes identifies a gene (matrimony) that is haplo-insufficient for achiasmate segregation in Drosophila oocytes. Genetics 165(2): 637-52. PubMed ID: 14573476
Lee, B. H. and Amon, A. (2003). Polo kinase-meiotic cell cycle coordinator. Cell Cycle 2: 400-402. PubMed ID: 18482691
Lee, B. H. and Amon, A. (2003). Role of Polo-like kinase CDC5 in programming meiosis I chromosome segregation. Science 300: 482-486. PubMed ID: 12663816
Lenart, P., et al. (2007). The small-molecule inhibitor BI 2536 reveals novel insights into mitotic roles of polo-like kinase 1. Curr Biol 17: 304-315. PubMed ID: 17291761
Mirouse, V., Formstecher, E. and Couderc, J. L. (2006). Interaction between Polo and BicD proteins links oocyte determination and meiosis control in Drosophila. Development 133: 4005-4013. PubMed ID: 16971474
Whitfield, Z. J., Chisholm, J., Hawley, R. S. and Orr-Weaver, T. L. (2013). A Meiosis-Specific Form of the APC/C Promotes the Oocyte-to-Embryo Transition by Decreasing Levels of the Polo Kinase Inhibitor Matrimony. PLoS Biol 11: e1001648. PubMed ID: 24019759
Xiang, Y., et al. (2007). The inhibition of polo kinase by matrimony maintains G2 arrest in the meiotic cell cycle. PLoS Biol. 5(12): e323. PubMed ID: 18052611
date revised: 10 October 2013
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