Gene name - fizzy-related
Synonyms - retina aberrant in pattern
Cytological map position -
Function - signaling
Symbol Symbol - fzr
FlyBase ID: FBgn0262699
Genetic map position - 1-
Classification - Trp-Asp repeat (WD-repeat) protein
Cellular location - presumably cytoplasmic
|Recent literature||Meghini, F., Martins, T., Tait, X., Fujimitsu, K., Yamano, H., Glover, D. M. and Kimata, Y. (2016). Targeting of Fzr/Cdh1 for timely activation of the APC/C at the centrosome during mitotic exit. Nat Commun 7: 12607. PubMed ID: 27558644
A multi-subunit ubiquitin ligase, the anaphase-promoting complex/cyclosome (APC/C), regulates critical cellular processes including the cell cycle. To accomplish its diverse functions, APC/C activity must be precisely regulated in time and space. The interphase APC/C activator Fizzy-related (Fzr or Cdh1) is localized at centrosomes in animal cells. However, neither the mechanism of its localization nor its importance is clear. This study identified the centrosome component Spd2 as a major partner of Fzr in Drosophila. The localization of Fzr to the centriole during interphase depends on direct interaction with Spd2. By generating Spd2 mutants unable to bind Fzr, it was shown that centrosomal localization of Fzr is essential for optimal APC/C activation towards its centrosomal substrate Aurora A. Finally, it was shown that Spd2 is also a novel APC/C(Fzr) substrate. This study is the first to demonstrate the critical importance of distinct subcellular pools of APC/C activators in the spatiotemporal control of APC/C activity.
|Neuert, H., Yuva-Aydemir, Y., Silies, M. and Klambt, C. (2017). Different modes of APC/C activation control growth and neuron-glia interaction in the developing Drosophila eye. Development 144(24):4673-4683. PubMed ID: 29084807
The development of the nervous system requires tight control of cell division, fate specification and migration. The anaphase promoting complex/cyclosome (APC/C) is an E3 ubiquitin ligase that affects different steps of cell cycle progression, as well as having postmitotic functions in nervous system development. It can therefore link different developmental stages in one tissue. The two adaptor proteins Fizzy/Cdc20 and Fizzy-Related/Cdh1 confer APC/C substrate specificity. This study shows that two distinct modes of APC/C function act during Drosophila eye development. Fizzy/Cdc20 controls the early growth of the eye disc anlage and the concomitant entry of glial cells onto the disc. In contrast, fzr/cdh1 acts during neuronal patterning and photoreceptor axon growth, and subsequently affects neuron-glia interaction. To further address the postmitotic role of Fzr/Cdh1 in controlling neuron-glia interaction, a series of novel APC/C candidate substrates were identified. Four of the candidate genes are required for fzr/cdh1 dependent neuron-glia interaction, including the dynein light chain Dlc90F. Taken together, these data show how different modes of APC/C activation can couple early growth and neuron-glia interaction during eye disc development.
|Cohen, E., Allen, S. R., Sawyer, J. K. and Fox, D. T. (2018). Fizzy-related dictates a cell cycle switch during organ repair and tissue growth responses in the Drosophila hindgut. Elife 7. PubMed ID: 30117808
Ploidy-increasing cell cycles drive tissue growth in many developing organs. Such cycles, including endocycles, are increasingly appreciated to drive tissue growth following injury or activated growth signaling in mature organs. In these organs, the regulation and distinct roles of different cell cycles remains unclear. This study uncovered a programmed switch between cell cycles in the Drosophila hindgut pylorus. Using an acute injury model, mitosis was identified as the response in larval pyloric cells, whereas endocycles occur in adult pyloric cells. By developing a novel genetic method, DEMISE (Dual-Expression-Method-for-Induced-Site-specific-Eradication), it was shown that the cell cycle regulator Fizzy-related dictates the decision between mitosis and endocycles. After injury, both cycles accurately restore tissue mass and genome content. However, in response to sustained growth signaling, only endocycles preserve epithelial architecture. These data reveal distinct cell cycle programming in response to similar stimuli in mature vs. developmental states and reveal a tissue-protective role of endocycles.
|Moreno-Moreno, O., Torras-Llort, M. and Azorin, F. (2019). The E3-ligases SCFPpa and APC/CCdh1 co-operate to regulate CENP-ACID expression across the cell cycle. Nucleic Acids Res. PubMed ID: 30753559
Centromere identity is determined by the specific deposition of CENP-A, a histone H3 variant localizing exclusively at centromeres. Increased CENP-A expression, which is a frequent event in cancer, causes mislocalization, ectopic kinetochore assembly and genomic instability. Proteolysis regulates CENP-A expression and prevents its misincorporation across chromatin. How proteolysis restricts CENP-A localization to centromeres is not well understood. ^hie e6ury reports that, in Drosophila, CENP-ACID expression levels are regulated throughout the cell cycle by the combined action of SCFPpa and APC/CCdh1. SCFPpa regulates CENP-ACID expression in G1 and, importantly, in S-phase preventing its promiscuous incorporation across chromatin during replication. In G1, CENP-ACID expression is also regulated by APC/CCdh1. This study also shows that Cal1, the specific chaperone that deposits CENP-ACID at centromeres, protects CENP-ACID from SCFPpa-mediated degradation but not from APC/CCdh1-mediated degradation. These results suggest that, whereas SCFPpa targets the fraction of CENP-ACID that is not in complex with Cal1, APC/CCdh1 mediates also degradation of the Cal1-CENP-ACID complex and, thus, likely contributes to the regulation of centromeric CENP-Asup>CID deposition.
The anaphase-promoting complex/cyclosome (APC) is a multisubunit ubiquitin ligase that targets several mitotic regulators for degradation and thereby triggers an exit from mitosis. APC activity is restricted to mitotic stages M and G1. This restriction is achieved by the cell cycle-dependent association of proteins encoded by fizzy and fizzy-related genes functioning during M and G1 respectively. fzr bears the accepted FlyBase designation of retina aberrant in pattern, because of a study showing that mutants show a disruption in eye patterning (Karpilow, 1989). Nevertheless, the alternative name fizzy-related is widely used and will be used in this essay. fzr, a conserved eukaryotic gene, negatively regulates the levels of cyclins A, B, and B3. These mitotic cyclins that bind and activate cdk1(cdc2) are rapidly degraded during exit from M and during G1. fzr is required for cyclin removal during G1 when the embryonic epidermal cell proliferation stops and during G2 during the cell cycle preceding salivary gland endoreduplication. Loss of fzr causes progression through an extra division cycle in the epidermis and inhibition of endoreduplication in the salivary gland, in addition to failure of cyclin removal. Conversely, premature fzr overexpression down-regulates mitotic cyclins, inhibits mitosis, and transforms mitotic cycles into endoreduplication cycles (Sigrist, 1997).
Binding of Cdc20 (the homolog of Fizzy in yeast and vertebrates) and Cdh1 (the homolog of Fizzy-related in yeast and vertebrates) to the APC is differentially regulated. APC-Cdc20 activity is present during mitosis and initiates the metaphase-anaphase transition. The association of Cdc20 with the APC requires phosphorylation of at least one subunit of the APC. Several mitotic kinases have been implicated in this phosphorylation. The dependency of APC phosphorylation on Cdc20 binding ensures that APC-Cdc20 is only active during mitosis. During prophase and prometaphase, APC-Cdc20 activity is furthermore restrained by the spindle checkpoint. This system monitors the presence of unattached kinetochores. Until kinetochores are bound by spindles, they serve as an assembly point for active Mad2 protein. Mad2 binds to Cdc20 and inhibits APC activity. Once all kinetochores are attached and chromosomes are aligned on the metaphase plate, Mad2 inhibition of APC-Cdc20 activity is released (Grosskortenhaus, 2002 and references therein).
Cdh1 is found in association with the APC during later stages of mitosis and G1. This interaction depends on the phosphorylation status of Cdh1 (Kramer, 2000; Zachariae, 1998). Only unphosphorylated Cdh1 is able to bind to and activate the APC (Kotani, 1999; Kramer, 2000). Cdk1 and Cdk2 mediate Cdh1 phosphorylation. Thus, only during stages of low Cdk kinase activity will Cdh1 activate the APC. These requirements are fulfilled during later stages of mitosis, when APC-Cdc20 has induced the degradation of mitotic cyclins, and during G1, when Cdk kinase activity is low. However, the G2 stage is also characterized by low Cdk kinase activity. How Cdh1-dependent APC activity is prevented in these situations has not been addressed so far (Grosskortenhaus, 2002 and references therein).
The mitotic cyclins in Drosophila (Cyclin A [CycA], Cyclin B [CycB], and Cyclin B3) are stable in interphase, degraded during mitosis, and continue to be unstable throughout G1. Cdc20Fzy is required for mitotic cyclin destruction at the metaphase-anaphase transition and is thought to mediate the bulk of cyclin degradation in the first 16 cell cycles in Drosophila. Mutants in fzy arrest in metaphase of cell cycle 16 when the maternal supply of Cdc20Fzy is exhausted. Overexpression of fzy does not cause abnormal cyclin destruction. Thus, Cdc20Fzy is not able to activate the APC at other cell cycle stages. This likely reflects the inability of Cdc20 to interact with unphosphorylated APC (Grosskortenhaus, 2002 and references therein).
Analysis in Drosophila demonstrates that fzr is expressed and of crucial importance when cells terminate cell proliferation during embryogenesis. Loss of fzr results in progression through an extra cell cycle in epidermal cells and in inhibition of endoreduplication in salivary glands. These deviations from the normal developmental cell cycle program are accompanied by a failure to down-regulate mitotic cyclins (cyclins A, B, and B3) that bind and activate cdk1(cdc2) kinase. Premature fzr overexpression, when epidermal cells still proliferate, down-regulates mitotic cyclins followed by inhibition of mitosis. All of these findings are consistent with the idea that Fzr activates degradation of mitotic cyclins and thereby prevents ectopic cdk1(cdc2) activity when cells become postmitotic (Sigrist, 1997).
The consequences of loss of fzr were examined with a deficiency deleting other genes in addition to fzr. However, in these deficient embryos, cell cycle defects occurred exclusively at stages and in tissues corresponding to the dynamic developmental program of fzr expression. Moreover, the various cell cycle defects were all corrected by expressing Hs-fzr in the deficient embryos at levels below the endogenous fzr expression as judged by in situ hybridization. Finally, except for correcting the cell cycle defects in tissues that normally express fzr, this moderate Hs-fzr expression has no effects on cell cycle progression in deficient embryos (Sigrist, 1997).
Ectopic cyclin E expression as well as failure to express the cyclin E/cdk2 inhibitor p27DAP in the embryonic epidermis results in ectopic accumulation of mitotic cyclins and in progression through an ectopic division cycle instead of a G1 arrest. Therefore, ectopic cyclin E/cdk2 activity in the postmitotic epidermal cells has the same phenotypic consequences as loss of fzr, suggesting that Fzr might act primarily as a negative regulator of cyclin E/cdk2 and indirectly on mitotic cyclins. All of the following findings, however, argue strongly against this interpretation. By immunolabeling with anti-cyclin E and anti-DAP antibodies, altered expression in either fzr-deficient embryos or after prd-Gal4-directed UAS-fzr expression was not detected (Sigrist, 1997).
In addition, while ectopic UAS-dap expression in the salivary gland during the larval stages inhibits endoreduplication effectively, UAS-fzr expression has no effect. Finally, when UAS-fzr expression is directed to imaginal disc cells during the larval stages, endoreduplication instead of the normal mitotic proliferation results, as also observed in CycA and Cdc2 mutants, but not in CycE mutants (Sigrist, 1997).
The onset of the developmentally programmed physiological endoreduplication in the salivary gland, therefore, might depend on the inactivation of cdk1 complexes. This inactivation might be achieved by the down-regulation of mitotic cyclins caused by the especially high levels of fzr expression that are observed transiently in the salivary gland before the onset of the first endoreduplication S phase. In the fzr-deficient embryos, mitotic cyclins continue to accumulate and cdk1 activity is expected to be maintained, resulting in the inhibition of endoreduplication (Sigrist, 1997).
While the inactivation of cdk1 is thought to establish the competence to initiate another round of DNA replication, the actual onset of DNA replication is dependent on the activation of cdk1 in yeast and cdk2 in higher eukaryotes. Particular cyclins, Clb5 and Clb6 in yeast and cyclins E and A in higher eukaryotes, are normally involved in this activation under physiological conditions. However, DNA replication can be activated in yeast and higher eukaryotes by cyclin/cdk1 complexes that play little or no role during the G1/S transition under physiological conditions. Moreover, ectopic expression of Drosophila cyclin A, which appears to bind only to cdk1 and not to cdk2, is definitively capable of driving cells from G1 into S. Premature activation of cdk1 must presumably be prevented when cells have to be maintained in the G1 phase. The entry into an additional S phase that is observed in the epidermis of fzr-deficient embryos, therefore, might also result from the failure to down-regulate the mitotic cyclins (Sigrist, 1997).
fzr was identified because of its similarity to fzy, which is required for progression beyond metaphase and mitotic cyclin degradation. fzr and fzy encode highly similar proteins with seven WD repeats in the C-terminal region. WD repeats are found in many proteins with diverse biological function. They are also found in budding yeast Cdc4p, which is required for the ubiquitin-dependent proteolysis of important cell cycle regulators. CDC4 acts in a pathway with CDC53, and interestingly, mutations in cul-1, a C. elegans homolog of CDC53, have recently been shown to result in a similar inability to arrest cell proliferation at the appropriate developmental stage as also apparent in the fzr-deficient embryos (Sigrist, 1997).
The closest yeast relative of fzr, however, is not CDC4 but HCT1, which is required for proteolysis of Clb2p, a budding yeast B-type cyclin with a characteristic destruction box as also present in A- and B-type cyclins of higher eukaryotes. Drosophila fzr appears to be unable to provide HCT1 function in yeast. It remains to be demonstrated that Fzr activates the proteolytic degradation of mitotic cyclins. However, the abnormal cyclin accumulation in both fzr and fzy mutants is not associated with an apparent increase in transcript levels, indicating that Fzr and Fzy down-regulate mitotic cyclins at a posttranscriptional level. If Fzy and Fzr trigger cyclin proteolysis as suspected, it will be important to clarify their functional relationship with proteins known to be required for the ubiquitin-dependent degradation of mitotic cyclins, with Cse1p and the anaphase-promoting complex (APC), a complex composed of several proteins. In addition, it will be interesting to evaluate the relationship of fzr and roughex, an unrelated Drosophila gene with a similar function required at other developmental stages (Sigrist, 1997).
Double mutant analyses demonstrate that Fzy and Fzr are specialized for the down-regulation of mitotic cyclins during either M phase or interphase, respectively. fzy is expressed in proliferating cells and is required for progression beyond metaphase and mitotic cyclin degradation; fzr transcripts accumulate when cells become postmitotic and are required for cyclin down-regulation in G1 during cell cycle exit and in G2 before endoreduplication but not during mitosis. It will be interesting to learn whether fzr is required in proliferating cells progressing through cell cycles with G1 phases (which do not occur during Drosophila embryogenesis). Since Fzy and Fzr promote down-regulation of mitotic cyclins in different cell cycle phases, they might have evolved to respond to different regulatory inputs. A fraction of Fzy is modified precisely during the metaphase/anaphase transition when mitotic cyclin degradation starts. Cyclin degradation during mitosis, and perhaps Fzy activity as well, is controlled by checkpoint mechanisms that monitor spindle integrity and chromosome attachment (Sigrist, 1997)
In contrast, degradation of mitotic cyclins during interphase appears to be important for cell cycle exit and entry into endoreduplication. fzr transcription is regulated by developmental cues that stop embryonic cell proliferation. In addition, Fzr appears to be regulated as well. Observations suggest that Fzr is negatively regulated by cyclin E/cdk2 activity, which in turn is also controlled by the developmental cues that stop embryonic cell proliferation. These developmental cues result in CycE down-regulation and dacapo up-regulation at the stage where cell proliferation is to be arrested. Therefore, the resulting inhibition of cyclin E/cdk2 activity might activate Fzr and thereby prevent mitotic cyclin accumulation and ectopic cdk1 activity when cells become postmitotic (Sigrist, 1997).
It is likely that FZY and Fzr are involved in the degradation of proteins other than the known A- and B-type cyclins. It is thought that the degradation of proteins like the fission yeast cut2 protein is required for sister chromatid separation in mitosis, and fzy is clearly required not only for cyclin degradation but also for sister chromatid separation in mitosis. It is possible, therefore, that Fzy and Fzr trigger the proteolytic degradation of different subsets of proteins (Sigrist, 1997).
In summary, the conserved fzr gene might be generally important in higher eukaryotes for transitions in the developmental cell cycle program (Sigrist, 1997).
The endocycle is a commonly observed variant cell cycle in which cells undergo repeated rounds of DNA replication with no intervening mitosis. How the cell cycle machinery is modified to transform a mitotic cycle into endocycle has long been a matter of interest. In both plants and animals, the transition from the mitotic cycle to the endocycle requires Fzr/Cdh1, a positive regulator of the Anaphase-Promoting Complex/Cyclosome (APC/C). However, because many of its targets are transcriptionally downregulated upon entry into the endocycle, it remains unclear whether the APC/C functions beyond the mitotic/endocycle boundary. This study reports that APC/CFzr/Cdh1 activity is required to promote the G/S oscillation of the Drosophila endocycle. Compromising APC/C activity, after cells have entered the endocycle, inhibits DNA replication and results in the accumulation of multiple APC/C targets, including the mitotic cyclins and Geminin. Notably, the data suggest that the activity of APC/CFzr/Cdh1 during the endocycle is not continuous but is cyclic, as demonstrated by the APC/C-dependent oscillation of the pre-replication complex component Orc1. Taken together, these data suggest a model in which the cyclic activity of APC/CFzr/Cdh1 during the Drosophila endocycle is driven by the periodic inhibition of Fzr/Cdh1 by Cyclin E/Cdk2. It is proposed that, as is observed in mitotic cycles, during endocycles, APC/CFzr/Cdh1 functions to reduce the levels of the mitotic cyclins and Geminin in order to facilitate the relicensing of DNA replication origins and cell cycle progression (Narbonne-Reveau, 2008).
The endocycle provides a useful model for determining the minimum cell cycle inputs required to achieve a G/S oscillation and the once-per-cell-cycle replication of the genome. This study demonstrates that APC/C activity is required for endocycle progression. During the endocycle, mitotic activities are repressed. This is accomplished, at least in part, by preventing the accumulation of the mitotic activators Cyclin A, Cyclin B and Cdc25, which function to activate the mitotic kinase Cdk1. During the mitotic cycle, the mitotic cyclins are periodically targeted for regulated proteolysis by the E3-Ubiquintin ligase the APC/C. Yet the transcriptional downregulation of several APC/C targets at the mitotic/endocycle boundary, including the mitotic cyclins and String/Cdc25, suggested that the proteolytic activity of the APC/C might not be necessary during endocycles. However, this study found that compromising APC/C activity, after cells have entered the endocycle, results in the accumulation of Geminin and the mitotic cyclins, and in a block of DNA replication. Thus, the transcriptional downregulation of APC/C targets observed at the mitotic/endocycle transition is either downstream of APC/C activity and/or not sufficient to maintain low levels of these proteins. Taken together, these data suggest a model in which APC/C promotes the G/S oscillation of the endocycle by preventing the unscheduled accumulation of Geminin and the mitotic cyclins (Narbonne-Reveau, 2008).
During endocycles, APC/C activity prevents the inappropriate accumulation of Geminin, an inhibitor of the DNA replication-licensing factor Cdt1/Dup. When directly expressed in endocycling cells, Geminin efficiently inhibits DNA replication. These results strongly suggest that an essential function of the APC/C during the endocycle is to prevent the unregulated accumulation of Geminin. A similar role has been proposed for the APC/C during endoreplicative cycles of mouse trophoblasts (Gonzalez, 2006). However, the current data indicate that Geminin is not the only essential target of the APC/C during endocycles. A candidate for a second important target of the APC/C during endocycles is Cyclin A. Previous studies have shown that the overexpression of Cyclin A in the salivary gland, between the first and second endocycle, results in variable inhibitory effects on endoreplication. Although the majority of salivary gland cells that overexpress Cyclin A appear to be unaffected, a small percentage show a marked decrease in ploidy values. The reason for this variability is not clear. However, if the inhibitory influence of Cyclin A is mediated through binding and activation of Cdk1, this effect may be greatly amplified in the presence of high levels of String/Cdc25, which removes an inhibitory phosphate from Cdk1. Recent studies indicate that String/Cdc25, which contains both a consensus Ken box and D-box, is a target of the APC/C (Barbara Thomas, personal communication to Narbonne-Reveau, 2008). Therefore, an essential function of the APC/C during endocycles may involve restricting the activity of the mitotic kinase Cdk1, by preventing the accumulation of both Cyclin A and String/Cdc25. Finally, it is noted that the APC/C may have additional essential targets during the endocycle, which have yet to be identified (Narbonne-Reveau, 2008).
The periodic accumulation of the Orc1 protein during endocycles strongly suggests that the activity of the APC/CFzy/Cdh1 may not be continuous but cyclical. Previous work indicates that in Drosophila Cyclin E/Cdk2 inhibits the activity of APC/CFzy/Cdh1. These data are consistent with the observation that phosphorylation of Fzr/Cdh1 by Cdks inhibits the ability of Fzr/Cdh1 to bind and activate the APC/C in yeast, Xenopus and mammals. During the endocycle, the levels of Cyclin E oscillate. Taken together, these observations suggest a model in which APC/CFzy/Cdh1 is regulated by the periodicity of Cyclin E/Cdk2 activity, with high levels of Cyclin E resulting in the inhibition of APC/CFzy/Cdh1 activity and low levels of Cyclin E permitting full APC/CFzy/Cdh1 activity. The current data support this hypothesis. First, it was found that the periodicity of Orc1 levels during the endocycle requires a functional O-box, consistent with the cyclic destruction of Orc1 by APC/CFzy/Cdh1. Second, the levels of Orc1 are sensitive to Cyclin E. Specifically, overexpressing Cyclin E after cells have entered the endocycle results in the accumulation of APC/CFzy/Cdh1 targets, including Orc1, Cyclin A, Cyclin B and Geminin. Thus, the regulatory relationship observed between Cyclin E/Cdk2 and Fzr/Cdh1 that has been reported during mitotic cycles is conserved during endocycles. Finally, in endocycling cells the accumulation of Orc1 occurs during periods of high Cyclin E/Cdk2 activity, when APC/CFzy/Cdh1 dependent proteolysis would be predicted to be low. These data support the idea that the oscillations of Cyclin E/Cdk2 activity drive the periodicity of APC/CFzy/Cdh1 activity during the endocycle (Narbonne-Reveau, 2008).
Although a requirement for the oscillation of APC/CFzy/Cdh1 activity during the Drosophila endocycle has not been formally demonstrated, it is interesting to speculate on how the cyclic, rather than the continuous, activity of the APC/C might serve to facilitate endocycle progression. The data indicate that a period of high APC/CFzy/Cdh1 activity is required during the G phase of the endocycle in order to degrade the mitotic cyclins and Geminin, which can function to inhibit the formation of pre-RCs. However, a period of low APC/C activity may also be important. The continuous activation of APC/CCdh1 significantly slows DNA replication in mouse tissue culture cells. This inhibition may reflect the inability of a cell to accumulate adequate levels of proteins required for DNA replication, such as the APC/CCdh1 target and pre-replication complex component CDC6, in the presence of a constitutively active APC/CCdh1. In Drosophila, continuous APC/CFzy/Cdh1 activity might prevent the accumulation of two pre-RC components, CDC6 and Orc1. Intriguingly, APC/C activity also appears to oscillate during mammalian endocycles. In endocycling mouse trophoblasts, the levels of Cyclin A oscillate, consistent with the regulated destruction of the Cyclin A protein by the APC/C. Additionally, the inhibition of APC/C activity in endocycling trophoblasts results in the accumulation of the APC/C targets Cyclin A and Geminin. Taken together, these observations support a model in which the oscillation of APC/CFzy/Cdh1 activity, which is driven by the regulatory influences of Cdks, promotes efficient cell cycle progression during the endocycle (Narbonne-Reveau, 2008).
The data raises important questions. Why do levels of some APC/CFzy/Cdh1 targets, such as Cyclin A, Cyclin B and Geminin, remain below the level of detection while the levels of Orc1 protein oscillate? What might account for these different modes of regulation? Currently, there is no definitive explanation. However, at least three possibilities, which are not mutually exclusive, are envisaged, that may contribute to this differential behavior. First, it was found that relative to the Cyclin A and geminin, the levels of Orc1 transcript are only minimally downregulated upon entry into the endocycle. Transcriptional downregulation, or changes in transcript stability, may help contribute to the low levels of Geminin and Cyclin A proteins observed during the endocycle. Second, the translational efficiency of a subset of transcripts may be reduced upon entry into the endocycle. Finally, it is possible that the Orc1 protein is not as efficiently targeted by the APC/CFzy/Cdh1 as the mitotic cyclins or Geminin. Indeed the cis-acting sequences that target these proteins for destruction show considerable variability. Orc1 is targeted for APC/CFzy/Cdh1 destruction via a novel motif called the O-box (Araki, 2005). By contrast, Cyclin B and Geminin are targeted by a similar but unique sequence called the destruction-box (D-box), while Drosophila Cyclin A is targeted for destruction by a large complex N-terminal degradation sequence. There is precedence for post-translational regulation of APC/CFzy/Cdh1 targets, resulting in differential expression. In mammalian cells the pre-RC component CDC6, which is structurally related to Orc1, is protected from APC/CFzr/Cdh1 degradation by phosphorylation by Cyclin E/Cdk2. One or all of these potential mechanisms may contribute to the differential expression of various APC/CFzr/Cdh1 targets during the endocycle (Narbonne-Reveau, 2008).
Recent evidence from mice indicates that the depletion of the APC/C inhibitor Emi1/Rca1, results in both a strong decrease in E2F target mRNAs, such as geminin and Cyclin A, as well as APC/C activation. This study suggested that the regulation of APC/C activity, by the inhibitor Emi1/Rca1, drives a positive feedback circuit that controls both protein stability and mRNA expression. Thus, the observed decrease in the levels of at least some APC/C targets that occurs upon depletion of Emi1/Rca1, including Geminin and Cyclin A, are controlled at the levels of transcription and protein stability. Developmentally programmed endocycles may provide a natural example where cell cycle progression occurs in the context of increased APC/CFzr/Cdh1 activity. Thus, a similar positive-feedback circuit may be operating during Drosophila endocycles to downregulate the transcription of E2F target genes. Determining the precise regulatory relationships between the upregulation of APC/CFzr/Cdh1 activity and the transcriptional downregulation of genes such as Cyclin A and geminin, during the Drosophila endocycle represents an exciting area for future research (Narbonne-Reveau, 2008).
The requirement for APC/C activity to promote endocycle progression may help answer several longstanding questions concerning the regulation of the Drosophila endocycle. For example, why does the continuous expression of Cyclin E inhibit cell cycle progression during the endocycle but not the mitotic cycle? Several models have been proposed to explain this difference. First, the breakdown of the nuclear envelope that occurs during the mitotic cycle, but not the endocycle, may allow for a transient decrease in local Cyclin E/Cdk2 activity, thus allowing for the relicensing of DNA replication origins. Alternatively, there may be differences in the machinery required to produce a functional pre-RC in mitotic versus endocycling cells. The current results suggest an alternative model for why endocycles are unusually sensitive to continuous Cyclin E expression. This model is based on the demonstration that endocycle progression requires APC/C activity. Both Fzy/Cdc20 and Fzr/Cdh1 function as activators of the APC/C. However, the regulation of these APC/C activators is very distinct. During the mitotic cycle, the binding of Fzy/Cdc20 to the APC/C is dependent on the phosphorylation of several APC/C subunits by the mitotic kinase Cdk1. By contrast, a Cdk-dependent inhibitory phosphorylation on Fzr/Cdh1 relegates APC/CFzr/Cdh1 activity to late M phase and G1. Because of its requirement for Cdk1 activity, APC/CFzy/Cdc20 is unlikely to be active during most endocycles. Indeed, Drosophila endocycles proceed normally in fzy mutants. Thus, the only available activator of the APC/C during the endocycle is Fzr/Cdh1. As previously discussed, Fzr/Cdh1 is inhibited by Cyclin E/Cdk2 activity. Therefore, it is proposed that during the endocycle, continuous Cyclin E/Cdk2 activity results in the permanent inhibition of the only available activator of the APC/C, Fzr/Cdh1. This leads to the accumulation of Geminin, Cyclin A and other potential targets, which act to block cell cycle progression. Thus, the ability of continuous Cyclin E to inhibit DNA replication during the endocycle may reflect differences in the available activators of the APC/C present in mitotic versus endocycling cells (Narbonne-Reveau, 2008).
Drosophila fizzy is required for cell cycle progression beyond metaphase and for mitotic degradation of A- and B-type cyclins (Dawson, 1993; Dawson, 1995; Sigrist, 1995). The Xenopus fzr gene was identified in an attempt to identify a fzy homolog in Xenopus, an ideal organism for biochemical analyses of cyclin proteolysis. Subsequent analyses have revealed the presence of both, fzy and fzr, in Drosophila and Xenopus. Sequencing projects have identified apparent fzy and fzr homologs in S. pombe, C. elegans, and vertebrates (Sigrist, 1997).
The protein products encoded by the fzy and fzr cDNAs are highly similar in their C-terminal domains, which are composed of seven WD repeats. These repeats, which are found in many functionally diverse proteins, were first identified in ß-transducin. A crystallographic analysis has demonstrated that these WD- or ß-transducin repeats form a seven propeller structure that provides protein-protein interaction faces in trimeric G-protein complexes. In the N-terminal domain, similarities between fzy and fzr are restricted to a few motifs (Sigrist, 1997).
date revised: 15 February 2002
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