The APC/C Inhibitor XErp1/Emi2 Is Essential for Xenopus Early Embryonic Divisions

Abstract

In many vertebrates, mature eggs await fertilization arrested at metaphase of meiosis II. Recently, XErp1/Emi2 was identified as the cytostatic factor mediating the metaphase-II arrest in mature Xenopus eggs. XErp1 prevents anaphase onset by directly inhibiting the ubiquitin-ligase Anaphase promoting complex/cyclosome (APC/C). Upon fertilization, XErp1 is targeted for degradation resulting in APC/C activation and the consequent exit from meiosis.In canonical cell cycles, Emi1 and the spindle-assembly-checkpoint control mitotic progression by inhibiting the APC/C. On the contrary, early embryonic divisions lack these APC/C-inhibitory components, which raises the question of how these cycles are regulated. In the work presented here it is shown that XErp1 quickly reaccumulates in the early embryonic divisions of Xenopus and that it is essential as mitotic APC/C inhibitor regulating the timely destruction of APC/C substrates.Loss of XErp1 by injection of antisense morpholio oligos (MOs) in one-cell embryos is lethal. Co-injection of XErp1 mRNA not targeted by the MOs completely rescues the phenotype and the injected embryos develop into healthy tadpoles, which demonstrates that the phenotype is specific for XErp1. Furthermore, in contrast to exit from meiosis II, protein levels of XErp1 remain constant during the early embryonic divisions, but XErp1 is phosphorylated in a cell cycle dependent manner. Cyclin dependent kinase 1 (Cdk1/Cyclin B) and the Protein Phosphatase 2A (PP2A) together with cyclic AMP dependent Protein Kinase A (PKA) act as regulators of XErp1. Phosphorylation of XErp1 by Cdk1 inhibits its APC/C inhibitory function resulting in APC/C activation. PP2A antagonizes the Cdk1 mediated inactivation of XErp1 and promotes its binding to the APC/C. PKA phosphorylates XErp1 at sites critical for PP2A recruitment. Thus, Cdk1 and PP2A/PKA are at the core of early mitotic cell cycles by antagonistically controlling XErp1-activity, which results in oscillating APC/C-activity driving the rapid cleavage divisions.