Resumption of meiosis is accompanied by the activation of maturation-promoting factor (MPF) - a complex consisting of a regulatory protein, cyclin B2, and a catalytic protein, Cdc2. The
activity of MPF is controlled by the association of these proteins with each other, and by two
competing enzymes, the Myt1 kinase and Cdc25, that either add or remove phosphate groups
from Cdc2. Active MPF is required for dissolution of the envelope surrounding the nucleus and
formation of the 'spindle' apparatus that will segregate homologous chromosomes. Chromosome
segregation is then triggered by the anaphase-promoting complex (APC), which mediates the
degradation of cyclin B2 and, hence, inactivation of MPF.
Emi1 protein, identified previously as an APC inhibitor, is needed to maintain metaphase II arrest.
Emi1 is a conserved F-box protein that contains a zinc-binding region essential for
APC inhibition and is 35% identical to the human FBXO5 protein, which contains 447 amino acids and has a molecular
mass of approximately 49 kD . Emi1 accumulates before mitosis and is
ubiquitylated and destroyed in mitosis, independent of the APC.
Reimann and Jackson 2002 found that
Emi1 is required for cytostatic factor arrest in vertebrate
eggs.
Adding extra Emi1 to the CSF-containing extracts prevented the
destruction of cyclin B2 that is triggered by the addition of calcium - a process that mimics
fertilization. By contrast, depletion of Emi1 led to cyclin B2 degradation. Furthermore, as shown
previously, adding Emi1 to mitotic blastomeres arrests their division. Their results suggest that
Emi1 arrests the second meiotic division by preventing the destruction of cyclin B2, in a
MAPK-independent way, and that fertilization would lead to the inhibition of Emi1 and activation
of the APC, releasing the egg from metaphase II arrest.
NCBI Summary:
This gene encodes a member of the F-box protein family which is characterized by an approximately 40 amino acid motif, the F-box. The F-box proteins constitute one of the four subunits of the ubiquitin protein ligase complex called SCFs (SKP1-cullin-F-box), which function in phosphorylation-dependent ubiquitination. The F-box proteins are divided into 3 classes: Fbws containing WD-40 domains, Fbls containing leucine-rich repeats, and Fbxs containing either different protein-protein interaction modules or no recognizable motifs. The protein encoded by this gene belongs to the Fbxs class. This protein is similar to xenopus early mitotic inhibitor-1 (Emi1), which is a mitotic regulator that interacts with Cdc20 and inhibits the anaphase promoting complex. Alternatively spliced transcript variants encoding different isoforms have been identified. [provided by RefSeq, Dec 2008]
General function
Comment
Cellular localization
Cytoplasmic, Nuclear
Comment
Ovarian function
Oogenesis, Oocyte maturation
Comment
Inhibition of neddylation causes meiotic arrest in mouse oocyte. Yang M et al. (2019) Mammalian oocyte meiosis is a special form of cell division that provides haploid gametes for fertilization. Unlike in mitosis, post-translational modifications (PTMs) are more crucial during meiosis because of the absence of de novo mRNA transcription. As a classic PTM, protein neddylation is a biological process that mediates protein degradation by modifying cullin proteins and activating the Cullin-Ring E3 ligases. This process plays important roles in various biological processes such as autophagy and tumorigenesis. However, the function of neddylation in germ cells is unknown. In this study, we observed that the inhibition of neddylation by its specific inhibitor MLN4924 significantly arrests mouse oocyte at the stage of metaphase during meiosis. The arrested oocytes display impaired spindles with over-activation of spindle assembly checkpoint (SAC). Accordingly, we identified early mitosis inhibitor 1 (Emi1), a key inhibitor of anaphase-promoting complex/cyclosome (APC/CFzr1), as a substrate of neddylation-mediated protein degradation. Thus, our study uncovered an unknown role of neddylation in female germ cells and suggests that proper neddylation is essential for oocyte maturation.//////////////////
Structural basis for recognition of Emi2 by Polo-like kinase 1 and development of peptidomimetics blocking oocyte maturation and fertilization. Jia JL et al. (2015) In a mammalian oocyte, completion of meiosis is suspended until fertilization by a sperm, and the cell cycle is arrested by a biochemical activity called cytostatic factor (CSF). Emi2 is one of the CSFs, and it maintains the protein level of maturation promoting factor (MPF) by inhibiting ubiquitin ligase anaphase promoting complex/cyclosome (APC/C). Degradation of Emi2 via ubiquitin-mediated proteolysis after fertilization requires phosphorylation by Polo-like kinase 1 (Plk1). Therefore, recognition and phosphorylation of Emi2 by Plk1 are crucial steps for cell cycle resumption, but the binding mode of Emi2 and Plk1 is poorly understood. Using biochemical assays and X-ray crystallography, we found that two phosphorylated threonines (Thr(152) and Thr(176)) in Emi2 are each responsible for the recruitment of one Plk1 molecule by binding to its C-terminal polo box domain (PBD). We also found that meiotic maturation and meiosis resumption via parthenogenetic activation were impaired when Emi2 interaction with Plk1-PBD was blocked by a peptidomimetic called 103-8. Because of the inherent promiscuity of kinase inhibitors, our results suggest that targeting PBD of Plk1 may be an effective strategy for the development of novel and specific contraceptive agents that block oocyte maturation and/or fertilization.//////////////////
Vertebrate eggs are arrested at metaphase of meiosis II with stable cyclin B and high cyclin
B/Cdc2 kinase activity. The ability of the anaphase-promoting complex/cyclosome (APC), an E3
ubiquitin ligase, to trigger cyclin B destruction and metaphase exit is blocked in eggs by the
activity of cytostatic factor (CSF) . CSF was defined as an activity in mature
oocytes that caused mitotic arrest when injected into dividing embryos. Fertilization causes a
transient increase in cytoplasmic calcium concentration leading to CSF inactivation, APC
activation, cyclin B destruction and mitotic exit. The APC activator Cdc20 is required for APC
activation after fertilization. Reimann and Jackson 2002 show that the APCcdc20 inhibitor Emi1 is necessary
and sufficient to inhibit the APC and to prevent mitotic exit in CSF-arrested eggs. CSF extracts
immunodepleted of Emi1 degrade cyclin B, and exit from mitosis prematurely in the absence of
calcium. Addition of Emi1 to these Emi1-depleted extracts blocks premature inactivation of the
CSF-arrested state. Emi1 is required to arrest unfertilized eggs at metaphase of meiosis II and
seems to be the long-sought mediator of CSF activity.
Prophase I arrest and progression to metaphase I in mouse oocytes are controlled by Emi1-dependent regulation of APCCdh1. Marangos P et al. Mammalian oocytes are arrested in prophase of the first meiotic division. Progression into the first meiotic division is driven by an increase in the activity of maturation-promoting factor (MPF). In mouse oocytes, we find that early mitotic inhibitor 1 (Emi1), an inhibitor of the anaphase-promoting complex (APC) that is responsible for cyclin B destruction and inactivation of MPF, is present at prophase I and undergoes Skp1-Cul1-F-box/betaTrCP-mediated destruction immediately after germinal vesicle breakdown (GVBD). Exogenous Emi1 or the inhibition of Emi1 destruction in prophase-arrested oocytes leads to a stabilization of cyclin B1-GFP that is sufficient to trigger GVBD. In contrast, the depletion of Emi1 using morpholino oligonucleotides increases cyclin B1-GFP destruction, resulting in an attenuation of MPF activation and a delay of entry into the first meiotic division. Finally, we show that Emi1-dependent effects on meiosis I require the presence of Cdh1. These observations reveal a novel mechanism for the control of entry into the first meiotic division: an Emi1-dependent inhibition of APC(Cdh1).
Expression regulated by
Comment
Ovarian localization
Oocyte
Comment
Functional interaction between p90(Rsk2) and Emi1 contributes to the metaphase arrest of mouse oocytes.
Paronetto MP, et al .
Vertebrate eggs arrest at metaphase of the second meiotic division before fertilization under the effect of a cytostatic factor (CSF). This arrest is established during oocyte maturation by the MAPK kinase module, comprised of Mos, MEK, MAPKs and p90(Rsk). Maintenance of CSF arrest at metaphase requires inhibitors of the anaphase-promoting complex (APC) like Emi1, which sequesters the APC activator Cdc20. Although it was proposed that the Mos pathway and Emi1 act independently, neither one alone is sufficient to entirely reproduce CSF arrest. Herein we demonstrate that p90(Rsk2) associates with and phosphorylates Emi1 upstream of the binding region for Cdc20, thus stabilizing their interaction. Experiments in transfected cells and two-cell embryos indicate that Emi1 and p90(Rsk2) cooperate to induce the metaphase arrest. Moreover, oocyte maturation was impaired by interfering with the interaction between p90(Rsk2) and Emi1 or by RNA interference of Emi1. Our results indicate that p90(Rsk2) and Emi1 functionally interact during oocyte maturation and that the Mos pathway establishes CSF activity through stabilization of an APC-inhibitory complex composed by Emi1 and Cdc20 before fertilization.