Cells proliferate by means of the mitotic cell cycle, a process underlying growth and development in all living organisms. Entry into mitosis depends on the activation of MPF (M-phase Promoting Factor, or Cdk1-cyclin B complex), the universal mitotic inducer in eukaryotic cells, which induces the phosphorylation of numerous proteins implicated in mitosis progression. Understanding the control of the cell cycle is pivotal to diverse and important biological problems and has thus fascinated biologists for many years. Despite this long-standing preoccupation, there is a surprising lack of knowledge on key points of the cell cycle, notably the control the G2-M transition. The OOCAMP project aims to improve our understanding of the G2-M transition by a physiological approach using oocytes of an amphibian (Xenopus) and a cnidarian (the jellyfish Clytia), simple and powerful experimental systems characterised by natural cell cycle arrest points released by defined stimuli to induce cell cycle progression. During oogenesis, oocytes enter meiosis and stop in prophase of the 1st meiotic division (equivalent to G2). External stimuli release this arrest to promote oocyte maturation by initiating signal transduction pathways, ultimately activating MPF. cAMP is a key immediate regulator, but intriguingly it acts in opposite ways in different species, creating the cAMP paradox. In vertebrates, cytoplasmic cAMP concentrations, and consequently activity of the kinase PKA, is maintained high in the arrested oocyte and must be down regulated to promote meiotic resumption (a situation reminiscent of the cAMP control exerted on the G2 phase of the mitotic cell cycle). In contrast, in many invertebrates including Clytia, elevation of cAMP levels and PKA activity are required to promote release from the G2 block. We plan to gain insight into cAMP-based regulation by comparing and cross-testing molecular regulation of cAMP modulators and targets between powerful experimental model species, Xenopus and Clytia, representing the two opposite modes of cAMP action. We will specifically address two key unsolved questions: the nature of the receptor systems that respond to oocyte maturation stimuli upstream of cAMP, and the identity of the PKA substrate that provokes MPF activation downstream of cAMP. Our approach will involve classical biochemical and molecular biology manipulations of candidate participant proteins combined with microinjection into oocytes of both species and monitoring of the physiological and molecular responses by microscopy and biochemical assays. We will focus on a small protein called ARPP19 (or ENSA), indicated by our preliminary results in Xenopus to be an excellent candidate for the key element maintaining prophase arrest as a PKA substrate. A distinct phosphorylation of ARPP19 triggered by the Greatwall kinase converts ARPP19 into a powerful MPF activator, essential for M-phase entry. We have identified a Clytia ortholog of ARPP19 with conserved Gwl and PKA phosphorylation sites, which will be functionally compared with its Xenopus counterparts at the cellular and molecular levels. In a parallel study we will attempt to identify the receptor of the peptide that stimulates oocyte maturation in Clytia by a novel transcriptomics-approach. The two OOCAMP partners, both with confirmed experience in the study of meiotic maturation, show excellent complementarity both in their favored animal models and their technical approaches and expertise (biochemical/molecular vs cell biology/imaging), which will drive the accomplishment of the project. To conclude, the OOCAMP program aims at understanding the biochemical systems that connect the extracellular maturation-inducing stimuli and the activation of MPF. The study of this key physiological process should continue to provide important contributions to knowledge of the hormonal regulation of reproduction, cell cycle regulation, signal transduction and mechanisms of oncogenesis.