Activation of p42 MAP kinase and the release of oocytes from cell cycle arrest

Clam oocytes are arrested naturally at the G₂/M border in meiosis and contain an inactive 42 kDa ERK/MAP kinase, p42^MAPK. Following fertilization, p42^MAPK is rapidly phosphorylated on tyrosine residues and concomitantly activated. Both tyrosine phosphorylation and activation of p42^MAPK begin within 2-3 min of fertilization, peak at ∼15 min, then rapidly decline and disappear around the end of meiosis I. Neither the tyrosine phosphorylated form of p42^MAPK nor p42^MAPK activity reappears during meiosis II or the succeeding mitotic cell cycles. High doses of molybdate, a potent PTPase inhibitor, block the phosphorylation of p42^MAPK and entry into the cell cycle. Lower doses of molybdate delay both p42^MAPK phosphorylation and the release from cell cycle arrest, but once cells have re-entered the cell cycle, they continue with near-normal timing. These results argue that the transient activation of p42^MAPK at fertilization is a one-time event linked to release from cell cycle arrest. In trying to reconcile this one-time activation of p42^MAPK in clam embryos with the recurring, M-phase specific activation of MBP/MAP kinases reported in other systems, we show that cdc2 kinase contributes a major portion of the MBP kinase activity in mitotic extracts. Furthermore, a small fraction of p42^MAPK and other related kinases are present in p13^suc1-bound material, cautioning against the use of p13^suc1 beads for experiments where, in addition to cdc2, the unaccounted presence of other kinase activities could be misleading.