The greatwall kinase is dominant over PKA in controlling the antagonistic function of ARPP19 in Xenopus oocytes.

The small protein ARPP19 plays a dual role during oocyte meiosis resumption. In Xenopus, ARPP19 phosphorylation at S109 by PKA is necessary for maintaining oocytes arrested in prophase of the first meiotic division. Progesterone downregulates PKA, leading to the dephosphorylation of ARPP19 at S109. This initiates a transduction pathway ending with the activation of the universal inducer of M-phase, the kinase Cdk1. This last step depends on ARPP19 phosphorylation at S67 by the kinase Greatwall. Hence, phosphorylated by PKA at S109, ARPP19 restrains Cdk1 activation while when phosphorylated by Greatwall at S67, ARPP19 becomes an inducer of Cdk1 activation. Here, we investigate the functional interplay between S109 and S67-phosphorylations of ARPP19. We show that both PKA and Gwl phosphorylate ARPP19 independently of each other and that Cdk1 is not directly involved in regulating the biological activity of ARPP19. We also show that the phosphorylation of ARPP19 at S67 that activates Cdk1, is dominant over the inhibitory S109 phosphorylation. Therefore our results highlight the importance of timely synchronizing ARPP19 phosphorylations at S109 and S67 to fully activate Cdk1.