Abstract
Recent results indicate that regulating the balance between cyclin-B-Cdc2 kinase, also known as M-phase-promoting factor (MPF), and protein phosphatase 2A (PP2A) is crucial to enable correct mitotic entry and exit. In this work, we studied the regulatory mechanisms controlling the cyclin-B-Cdc2 and PP2A balance by analysing the activity of the Greatwall kinase and PP2A, and the different components of the MPF amplification loop (Myt1, Wee1, Cdc25) during the first embryonic cell cycle. Previous data indicated that the Myt1-Wee1-Cdc25 equilibrium is tightly regulated at the G2-M and M-G1 phase transitions; however, no data exist regarding the regulation of this balance during M phase and interphase. Here, we demonstrate that constant regulation of the cyclin-B-Cdc2 amplification loop is required for correct mitotic division and to promote correct timing of mitotic entry. Our results show that removal of Cdc25 from metaphase-II-arrested oocytes promotes mitotic exit, whereas depletion of either Myt1 or Wee1 in interphase egg extracts induces premature mitotic entry. We also provide evidence that, besides the cyclin-B-Cdc2 amplification loop, the Greatwall-PP2A pathway must also be tightly regulated to promote correct first embryonic cell division. When PP2A is prematurely inhibited in the absence of cyclin-B-Cdc2 activation, endogenous cyclin-A-Cdc2 activity induces irreversible aberrant mitosis in which there is, first, partial transient phosphorylation of mitotic substrates and, second, subsequent rapid and complete degradation of cyclin A and cyclin B, thus promoting premature and rapid exit from mitosis.
Highlights
Entry and exit of mitosis are regulated by the cyclin-B–Cdc2 complex, known as the M-phase-promoting factor (MPF)
We studied the regulatory mechanisms controlling the cyclin-B–Cdc2 and phosphatase 2A (PP2A) balance by analysing the activity of the Greatwall kinase and PP2A, and the different components of the MPF amplification loop (Myt1, Wee1, Cdc25) during the first embryonic cell cycle
Previous data indicated that the Myt1-Wee1-Cdc25 equilibrium is tightly regulated at the G2-M and M-G1 phase transitions; no data exist regarding the regulation of this balance during M phase and interphase
Summary
Entry and exit of mitosis are regulated by the cyclin-B–Cdc complex, known as the M-phase-promoting factor (MPF). At the end of G2, abrupt dephosphorylation of Tyr and Thr residues by Cdc triggers initial activation of cyclin-B–Cdc, which in turn further activates Cdc and inactivates Wee and Myt by phosphorylation, resulting in full activation of the cyclin-B–Cdc complex (Hoffmann et al, 1993; Izumi et al, 1992; Mueller et al, 1995b; Russell and Nurse, 1987b; Smythe and Newport, 1992). This positive-feedback mechanism is called the MPF amplification loop
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