Phosphatase 2A and Polo kinase, two antagonistic regulators of Cdc25 activation and MPF auto-amplification

The auto-catalytic activation of the cyclin-dependent kinase Cdc2 or MPF (M -phase promoting factor) is an irreversible process responsible for the ent ry into M phase, In Xenopus oocyte, a positive feed-back loop between Cdc2 kinase and its activating phosphatase Cdc25 allows the abrupt activation of MPF and the entry into the first meiotic division, We have studied the Cdc 2/Cdc25 feedback loop using cell-free systems derived from Xenopus prophase -arrested oocyte, Our findings support the following two-step model for MPF amplification: during the first step, Cdc25 acquires a basal catalytic act ivity resulting in a linear activation of Cdc2 kinase, In turn Cdc2 partial ly phosphorylates Cdc25 but no amplification takes place; under this condit ion Plx1 kinase and its activating kinase, Plkk1 are activated, However, th eir activity is not required for the partial phosphorylation of Cdc25, This first step occurs independently of PP2A or Suc1/Cks-dependent Cdc25/Cdc2 a ssociation, On the contrary, the second step involves the full phosphorylat ion and activation of Cdc25 and the initiation of the amplification loop, I t depends both on PP2A inhibition and Plx1 kinase activity, Suc1-dependent Cdc25/Cdc2 interaction is required for this process.