Understanding the role and the regulation of Arpp19, a central player of cell division – KiARP

Cells proliferate by means of the mitotic cell cycle that supports growth, development and maintenance of all living organisms. Understanding the control of cell cycle has fascinated biologists for many years. Cell division is orchestrated by the phosphorylation of mitotic proteins, under the control of two master enzymes: the kinase Cdk1 and its antagonizing phosphatase, PP2A-B55delta. While the regulation of these enzymes has been widely studied since their discovery, there is still a surprising lack of knowledge on cell division. In particular, the switch-like behavior of M-phase transitions that depends on an antagonistic interplay between kinases and phosphatases remains largely unknown.
The KiPARP project aims at elucidating the regulatory circuits that underlie the cellular decision to divide or not, the conservation of these molecular networks among species and the specific adaptations of the mitotic molecular controls ensuring a meiotic division. To get new insights into these questions, our project is based on a newly identified player of cell division, the small protein Arpp19, which plays a dual function in controlling the switch between Cdk1 and PP2A-B55delta-depending on its phosphorylation. When phosphorylated at S109 by the cAMP-dependent protein kinase, PKA, Arpp19 strongly impairs Cdk1 activation during female meiosis. Conversely, when phosphorylated by the kinase Greatwall at S67, Arpp19 becomes a potent activator of both meiosis and mitosis by inactivating PP2A-B55delta. Hence, depending on its phosphorylation by PKA or Greatwall, Arpp19 authorizes or not entry into cell division.
To decipher the molecular decision to divide, the KiPARP project will investigate how Arpp19 is converted from a negative to a positive regulator of M-phase by using distinct and powerful experimental models, oocytes of various species, cell-free extracts and an engineered mouse cell line. We will elucidate the mechanisms controlled by PKA-phosphorylated Arpp19 that block M-phase entry and further investigate how Arpp19 dephosphorylation at S109 fires the Cdk1-activatory mechanisms inducing M-phase in Xenopus oocytes and cell-free extracts. We will also study the function of a double-phosphorylated form of ARPP19 at both S67 and S109, which is generated during M-phase. We will determine whether S109 phosphorylation of ARPP19 is a conserved mechanism regulating both meiotic and mitotic division, by using physiological models as oocytes from vertebrates (mouse and human) and invertebrates (jellyfish).
The two KiPARP partners have confirmed experiences in studying meiotic and mitotic divisions and show excellent complementarity in their favored experimental models and technical approaches (biochemical/molecular and cell biology/imaging), which will drive the accomplishment of the project. By focusing on the biochemical networks that trigger the flip-flop switch in M-phase, the KiPARP project will help understanding key biological processes involved in reproduction, cell cycle regulation, signal transduction and oncogenesis.