Calcineurin promotes APC/C activation at meiotic exit by acting on both XErp1 and Cdc20

Abstract

Vertebrate oocytes await fertilization arrested at metaphase of the second meiotic division. Fertilization triggers a transient calcium wave, which induces the activation of the anaphase‐promoting complex/cyclosome (APC/C) and its co‐activator Cdc20 resulting in the destruction of cyclin B and hence meiotic exit. Two calcium‐dependent enzymes are implicated in fertilization‐induced APC/CCdc20 activation: calcium‐/calmodulin‐dependent kinase type II (CaMKII) and calcineurin (CaN). While the role of CaMKII in targeting the APC/C inhibitor XErp1/Emi2 for destruction is well‐established, it remained elusive how CaN affects APC/CCdc20 activation. Here, we discover that CaN contributes to APC/CCdc20 activation in Xenopus laevis oocytes by two independent but interrelated mechanisms. First, it facilitates the degradation of XErp1 by dephosphorylating it at a site that is part of a phosphorylation‐dependent recruiting motif for PP2A‐B′56, which antagonizes inhibitory phosphorylation of XErp1. Second, it dephosphorylates Cdc20 at an inhibitory site, thereby supporting its APC/C‐activating function. Thus, our comprehensive analysis reveals that CaN contributes to timely APC/C activation at fertilization by both negatively regulating the APC/C inhibitory activity of XErp1 and positively regulating the APC/C‐activating function of Cdc20.