The phosphorylation of ARPP19 by Greatwall renders the autoamplification of MPF independent of PKA in Xenopus oocytes

Abstract

Entry into mitosis or meiosis relies on the coordinated action of kinases and phosphatases that ultimately leads to the activation of Cyclin B-Cdk1, also called MPF for M-phase promoting factor. Vertebrate oocytes are blocked in prophase of the first meiotic division, an arrest tightly controlled by a high PKA activity. Reentry into meiosis depends on Cdk1 activation that obeys a two steps mechanism: a catalytic amount of Cdk1 is generated in a PKA and protein synthesis-dependent manner; then a regulatory network called MPF auto-amplification loop is initiated. This second step is independent of PKA and protein synthesis. However, none of the molecular components of the auto-amplification loop identified so far acts independently of PKA. Therefore, the protein rendering this process independent of PKA in oocytes remains unknown. Using a physiological intact cell system, the Xenopus oocyte, we show that the phosphorylation of ARPP19 at S67 by the Greatwall kinase promotes its binding to the PP2A-B55 δ phosphatase, thus inhibiting its activity. This process is controlled by Cdk1 and plays an essential role within the Cdk1 auto-amplification loop for entry into the first meiotic division. Moreover, once phosphorylated by Greatwall, ARPP19 escapes the negative regulation exerted by PKA. It also promotes MPF activation independently of protein synthesis, provided a small amount of Mos is present. Taken together, these findings reveal that PP2A-B55δ, Greatwall and ARPP19 are not only required for entry into meiotic divisions, but are also pivotal effectors within the Cdk1 auto-regulatory loop responsible for its independence toward PKA negative control.