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 42kDa 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.