Polo-like kinase confers MPF autoamplification competence to growingXenopusoocytes

Abstract

During oogenesis, the Xenopus oocyte is blocked in prophase of meiosis I. It becomes competent to resume meiosis in response to progesterone at the end of its growing period (stage VI of oogenesis). Stage IV oocytes contain a store of inactive pre-MPF (Tyr15-phosphorylated Cdc2 bound to cyclin B2); the Cdc25 phosphatase that catalyzes Tyr15 dephosphorylation of Cdc2 is also present. However, the positive feedback loop that allows MPF autoamplification is not functional at this stage of oocyte growth. We report that when cyclin B is overexpressed in stage IV oocytes, MPF autoamplification does not occur and the newly formed cyclin B-Cdc2 complexes are inactivated by Tyr15 phosphorylation, indicating that Myt1 kinase remains active and that Cdc25 is prevented to be activated. Plx1 kinase (or polo-like kinase), which is required for Cdc25 activation and MPF autoamplification in full grown oocytes is not expressed at the protein level in small stage IV oocytes. In order to determine if Plx1 could be the missing regulator that prevents MPF autoamplification, polo kinase was overexpressed in stage IV oocytes. Under these conditions, the MPF-positive feedback loop was restored. Moreover, we show that acquisition of autoamplification competence does not require the Mos/MAPK pathway.

Xenopus Polo-like kinase Plx1: a multifunctional mitotic kinase

“…However, in these G 2 oocytes, the inactive form of Cdc2/cyclin B and Cdc25C are both present, but the Cdc2/cyclin B autoamplification positive feedback loop with Cdc25C is not functional due to the lack of Plx1 at the protein level. Overexpression of Plx1 restored the feedback loop (Karaiskou et al, 2004). Similar results were also obtained in studies carried out with Xenopus egg extracts.…”

“…The observation that recombinant Plx1 phosphorylates Cdc25C and stimulates its activity in vitro, and that phosphorylated Cdc25C reacted strongly with mitotic phosphoprotein antibody, MPM-2 (Kumagai and Dunphy, 1996), strongly suggests that Plx1 is a upstream kinase of Cdc25C in vivo. Since the initial work, more studies have been carried out in Xenopus egg extracts and oocytes, and the results show that Plx1 is indeed the initial phosphorylating and activating kinase for Cdc25C in vivo and is also required for the Cdc2/ cyclin B autoamplification loop with Cdc25C (Abrieu et al, 1998;Qian et al, 1998aQian et al, , 1999Qian et al, , 2001Karaiskou et al, 2004).…”

The Centrosome and Its Duplication Cycle

“…Removal of this phosphate from stockpiled Cdk1/cyclin B complexes by Cdc25 phosphatases then unleashes a wave of Cdk1-cyclin B activity that promotes mitotic commitment (Nurse 1990). Work in Xenopus laevis oocyte/egg extracts established that an initial impact of newly activated Cdk1-cyclin B is to use Polo kinase to enhance Cdc25 and repress Wee1 activities (Hoffmann et al 1993;Maller 1993, 1995;Strausfeld et al 1994;Kovelman and Russell 1996;Kumagai and Dunphy 1996;Abrieu et al 1998;Karaiskou et al 1999Karaiskou et al , 2004Lindqvist et al 2009). These feedback loops convert mitotic commitment into a bistable, all or nothing, switch (Ferrell 2008).…”

The Plk3-Cdc25 circuit

“…Genetic and biochemical approaches with several organisms indicate that Plks regulate diverse cellular events at various cell-cycle stages. The Xenopus Plx1 is a critical trigger that initiates the onset of the G 2 /M transition Kumagai and Dunphy (1996), and appears to be part of the Cdk1/cyclin B amplification loop in meiotic maturation of Xenopus oocytes (Karaiskou et al, 2004). Plx1 also participates in the destruction of targets of the anaphase-promoting complex (APC) to mediate mitotic exit in the early embryonic cell cycle (Brassac et al, 2000).…”

“…The Xenopus Plk, Plx, is required for small oocytes to re-enter meiosis in response to progesterone (Karaiskou et al, 2004). Plx was the first Plk shown to be required for the G2/M transition.…”