Dynamic Regulation of Emi2 by Emi2-Bound Cdk1/Plk1/CK1 and PP2A-B56 in Meiotic Arrest of Xenopus Eggs

Abstract

In vertebrates, unfertilized eggs are arrested at metaphase of meiosis II by Mos and Emi2, an inhibitor of the APC/C ubiquitin ligase. In Xenopus, Cdk1 phosphorylates Emi2 and both destabilizes and inactivates it, whereas Mos recruits PP2A phosphatase to antagonize the Cdk1 phosphorylation. However, how Cdk1 phosphorylation inhibits Emi2 is largely unknown. Here we show that multiple N-terminal Cdk1 phosphorylation motifs bind cyclin B1-Cdk1 itself, Plk1, and CK1δ/ε to inhibit Emi2. Plk1, after rebinding to other sites by self-priming phosphorylation, partially destabilizes Emi2. Cdk1 and CK1δ/ε sequentially phosphorylate the C-terminal APC/C-docking site, thereby cooperatively inhibiting Emi2 from binding the APC/C. In the presence of Mos, however, PP2A-B56β/ε bind to Emi2 and keep dephosphorylating it, particularly at the APC/C-docking site. Thus, Emi2 stability and activity are dynamically regulated by Emi2-bound multiple kinases and PP2A phosphatase. Our data also suggest a general role for Cdk1 substrate phosphorylation motifs in M phase regulation.

Punctuated cyclin synthesis drives early embryonic cell cycle oscillations

“…This caused a decrease in the level of cyclin synthesized when proteasome activity was permitted (Figure 4C, left, boxes), whereas blocking the proteasome (Figure 4C, left, open circles) caused accumulation of cyclin that matched levels seen in extracts with unaltered CDK1 activities. This confirmed that increasing CDK1 activity can increase APC activity in CSF extracts—which had been reported previously (Hansen et al , 2007; Wu et al , 2007; Isoda et al , 2011)—but did not slow the production of cyclin B. This is the opposite of what we observed prior to and during M phase of mitotic cycles and demonstrates that CDK1 activity does not exert the same inhibition on cyclin synthesis in mature oocytes/eggs (CSF extract) that it does in embryos (cycling extract).…”

“…The control of meiotic cyclin B translation in the unfertilized egg, however, is different from that of mitotic cycles. Polyadenylation of cyclin B1 mRNA has been reported in the unfertilized egg (Sheets et al , 1994; Ballantyne et al , 1997; Kim and Richter, 2007; Belloc et al , 2008), and we confirmed that meiotic CSF extracts are capable of continuous cyclin translation in the presence of elevated CDK1 activity, in agreement with previously published studies (Hansen et al , 2007; Wu et al , 2007; Isoda et al , 2011). Therefore, although translation of cyclin B in meiotic M phase is continuous despite CDK1 activity, it is attenuated during M phase of mitotic cycles.…”

Cyclin B3 Is a Mitotic Cyclin that Promotes the Metaphase-Anaphase Transition

“…However, cyclin:Cdk kinase activity phosphorylates certain APC/C subunits, such as Cdc27, as well as phosphorylating and inactivating an inhibitor of the APC/C, Emi2 [30, 31]. We suggest that Cyclin B3:Cdk1 has unique substrate specificity [32] and spatial-temporal distribution that make it an effective activator of APC/C.…”

OSD1 Promotes Meiotic Progression via APC/C Inhibition and Forms a Regulatory Network with TDM and CYCA1;2/TAM

“…This suggests that TAM could regulate OSD1 to prevent precocious meiotic exit. Alternatively, phosphorylation by TAM could inactivate OSD1 and thus allow exit from meiosis I. Interestingly, the activity and stability of Emi2/Erp1 - the vertebrate meiotic APC/C inhibitor - is regulated by phosphorylation [33], [34]. Further functional analysis of the OSD1 putative phosphorylation sites is required to establish the role of this CYCA1;2/TAM-CDKA;1-mediated phosphorylation in meiotic cell cycle progression.…”