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

Abstract

Mastering Early Divisions The regulation of canonical mitotic cell cycles is well understood, but the basic principles of the rapid, synchronized early mitotic divisions in embryos remains a mystery. Early embryos lack key mitotic regulators such as checkpoints, the anaphase-promoting complex/cyclosome (APC/C)-inhibitory protein Emi1, and the inhibitory phosphorylations of cyclin-dependent kinase 1 (Cdk1). Working in Xenopus embryos, Tischer et al. (p. 520, published online 27 September) identified XErp1/Emi2 as a mitotic APC/C-inhibitor essential for early mitotic divisions. The mitotic APC/C-inhibitory activity of XErp1 is positively regulated by protein kinase A (PKA) and protein phosphatase IIA (PP2A), which antagonizes Cdk1's inhibitory effect on XErp1. Thus, Cdk1 and PP2A/PKA appear to act antagonistically to control XErp1 activity, which results in the oscillatory activation and inactivation of the APC/C required for fast and synchronous mitotic divisions.