III. The regulation of cell cycle progression by ubiquitin ligases
Selective protein destruction by the ubiquitin proteasome system (UPS) is an integral regulatory element ensuring switch like and irreversible cell cycle transitions. Mature Xenopus egg cells like mammalian ones await fertilization arrested at metaphase of the second meiotic division (MII). While the existence of a biochemical activity – termed cytostatic factor (CSF) – responsible for the MII arrest of mature eggs was postulated already in 1971 by Masui and Markert, the molecular nature of CSF has remained a mystery since then. Our studies aimed to solve the mystery succeeded in the identification of XErp1/Emi2, an evolutionary conserved F-box protein. XErp1 mediates the MII-arrest of mature eggs by directly inhibiting the ubiquitin ligase anaphase promoting complex / cyclosome (APC/C). Upon fertilization, calmodulin-dependent kinase II (CaMKII) and Polo-like kinase 1 (Plx1) cooperate to target XErp1 for destruction resulting in APC/C activation, cyclin-B and securin destruction and, hence, in exit from meiosis.
Following fertilization, Xenopus embryos undergo eleven (division 2-12) highly synchronous and fast divisions. Notably, these rapid divisions are devoid of gap phases, known regulators of the APC/C such as the spindle assembly checkpoint (SAC) and the early mitotic inhibitor Emi1 and inhibitory phosphorylations of the master cell cycle regulator cyclin-dependent kinase 1 (Cdk1). Intrigued by the lack of these regulatory mechanisms, we aimed to dissect the mechanism that drives the highly synchronous and rapid early embryonic divisions. By combining biochemical assays with depletion/rescue experiments in intact Xenopus embryos we could demonstrate that the APC/C inhibitory activity of XErp1 is essential for early embryonic divisions. Depletion of XErp1 results in untimely destruction of APC/C substrates resulting in an embryonic lethal phenotype. Further studies revealed a complex regulatory network composed of Cdk1 and protein phosphatase 2A (PP2A) that tightly controls XErp1 function during embryonic divisions. Further studies aim to understand how the different regulatory elements communicate to ensure the synchrony of these specialized divisions. We expect that insights derived from these studies will also contribute to a better understanding of more complex somatic divisions subject to SAC regulation.