Tyrosine phosphorylation of p34cdc2 in metaphase II-arrested pig oocytes results in pronucleus formation without chromosome segregation

Abstract

At the G2-M boundary, maturation-promoting factor (MPF) activation is usually induced in one or both of two ways; tyrosine dephosphorylation of p34cdc2 or synthesis of cyclin B according to cell type and species. At the end of M-phase, however, MPF inactivation is normally triggered only by cyclin degradation. We investigated whether tyrosine phosphorylation of p34cdc2 can inactivate MPF and what kinds of events are induced in pig metaphase II (MII)-arrested oocytes. First, cyclin B1 in MII-arrested oocytes is degraded upon fertilization. Second, when MII oocytes were treated with vanadate, an inhibitor of tyrosine phosphatases, they were released from MII arrest, but MPF was inactivated by further tyrosine phosphorylation of p34cdc2 rather than cyclin B1 degradation. The vanadate-induced exit from M-phase is distinct from normal M-phase exit, which is accompanied by cyclin B1 degradation; the former lacks both sister chromatid separation and second polar body emission. Vanadate itself has no inhibitory effect on chromosome segregation since calcium ionophore induced chromosome segregation in the presence of vanadate. Furthermore, when MII oocytes were treated with olomoucine, an inhibitor of cyclin-dependent kinases, they exited from MII arrest in a manner similar to vanadate-treated MII oocytes. Finally, we propose that MPF inactivation by tyrosine phosphorylation of p34cdc2 enables MII oocytes to form an interphase nucleus, but not to segregate sister chromatid due to the absence of the mechanisms required to trigger sister chromatid separation such as anaphase-promoting complex (APC)-mediated proteolysis, on the signaling pathway from intracellular Ca2+ increase to MPF inactivation. Mol. Reprod. Dev. 52:107–116, 1999. © 1999 Wiley-Liss, Inc.