TYROSINE PHOSPHORYLATION OF P34(CDC2) IN METAPHASE II-ARRESTED PIG OOCYTES RESULTS IN PRONUCLEUS FORMATION WITHOUT CHROMOSOME SEGREGATION

At the G2-M boundary, maturation-promoting factor (MPF) activation is usually induced in one or both of two ways; tyrosine dephosphorylation of p34(cdc2) Or synthesis of cyclin B according to cell type and spec ies. At the end of M-phase, however, MPF inactivation is normally trig gered only by cyclin degradation. We investigated whether tyrosine pho sphorylation of p34(cdc2) 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, whe n MII oocytes were treated with vanadate, an inhibitor of tyrosine pho sphatases, they were released from MII arrest, but MPF was inactivated by further tyrosine phosphorylation of p34(cdc2) rather than cyclin B 1 degradation. The vanadate-induced exit from M-phase is distinct from normal M-phase exit, which is accompanied by cyclin B1 degradation; t he former lacks both sister chromatid separation and second polar body emission. Vanadate itself has no inhibitory effect on chromosome segr egation 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. Final ly, we propose that MPF inactivation by tyrosine phosphorylation of p3 4(cdc2) enables MII oocytes to form an interphase nucleus, but not to segregate sister chromatid due to the absence of the mechanisms requir ed to trigger sister chromatid separation such as anaphase-promoting c omplex (APC)-mediated proteolysis, on the signaling pathway from intra cellular Ca2+ increase to MPF inactivation, Mol. Reprod. Dev. 52:107-1 16, 1999. (C) 1999 Wiley-Liss, Inc.