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Emi2 at the Crossroads: Where CSF Meets MPF

David V. Hansen, Joseph R. Pomerening, Matthew K. Summers, Julie J. Miller, James E. Ferrell, Jr and Peter K. Jackson

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Vertebrate eggs arrest at metaphase of meiosis II due to an activity known as cytostatic factor (CSF). CSF antagonizes the ubiquitin ligase activity of the anaphase-promoting complex/cyclosome (APC/C), preventing cyclin B destruction and meiotic exit until fertilization occurs. A puzzling feature of CSF arrest is that APC/C inhibition is leaky. Ongoing cyclin B synthesis is counterbalanced by a limited amount of APC/C-mediated cyclin B destruction; thus, cyclin B/Cdc2 activity remains at steady state. How the APC/C can be slightly active toward cyclin B, and yet restrained from ubiquitinating cyclin B altogether, is unknown. Emi2/XErp1 is the critical CSF component directly responsible for APC/C inhibition during CSF arrest. Fertilization triggers the Ca2+-dependent destruction of Emi2, releasing the APC/C to ubiquitinate the full pool of cyclin B and initiate completion of meiosis. Previously, we showed that a phosphatase maintains Emi2’s APC/C-inhibitory activity in CSF-arrested Xenopus egg extracts. Here, we demonstrate that phosphatase inhibition permits Emi2 phosphorylation at thr-545 and -551, which inactivates Emi2. Furthermore, we provide evidence that adding excess cyclin B to CSF extracts stimulates Cdc2 phosphorylation of these same residues, antagonizing Emi2-APC/C association. Our findings suggest a model wherein the pool of Emi2 acts analogously to a rheostat by integrating Cdc2 and phosphatase activities to prevent cyclin B overaccumulation and Cdc2 hyperactivity during the indefinite period of time between arrival at metaphase II and eventual fertilization. Finally, we propose that inactivation of Emi2 by Cdc2 permits mitotic progression during early embryonic cleavage cycles.

Authors

David V. Hansen

Genentech, Inc., San Francisco, California

Joseph R. Pomerening

Stanford University School of Medicine, Stanford, California

Matthew K. Summers

Genentech, Inc., San Francisco, California

Julie J. Miller

Stanford University School of Medicine, Stanford, California

James E. Ferrell, Jr

Stanford University School of Medicine, Stanford, California

Peter K. Jackson

Genentech, Inc., San Francisco, California

This is an open-access article

 Download PDF

If the document does not open, please right-click on the link (control-click on a Macintosh) and select the option to save the file to disk.