Rise of intracellular Ca2+ level causes the decrease of cyclin B1 and Mos in the newt eggs at fertilization

Unfertilized eggs of the newt, Cynops pyrrhogaster, are arrested at the sec ond meiotic metaphase, with activity of the M-phase promoting factor (MPF) maintained at a high level. After fertilization, the eggs resume the cell c ycle, and emit the second polar body. When the change in [Ca2+](i) in the f ertilized eggs was monitored by aequorin, an early increase in [Ca2+](i) wa s observed 5-10 min after insemination and continued for about 30 sec. A la te increase in [Ca2+](i) then occurred 10-15 min after fertilization and co ntinued for 30-40 min. The injection of 1,2-Bis (2 aminophenoxy) ethane-N,N ,N',N',-tetraacetic acid (BAPTA) into unfertilized eggs inhibited reinitiat ion of the cell cycle after fertilization. Western blot analysis with antib odies against cyclin B1 or Mos indicated that both cycljn B1 and Mos were p resent in unfertilized eggs, but both disappeared within 30 min after ferti lization. Treatment with Ca2+-ionophore decreased both cyclin B1 and Mos. C hymotryptjc activity in Cynops egg extracts was not significantly increased after fertilization or activation by treatment with the Ca2+-ionophore. No change in [Ca2+](i) was observed following treatment with cycloheximide, b ut the amount of both cyclin B1 and Mos rapidly decreased. These results in dicate that resumption of meiosis in Cynops eggs is induced by an increase in [Ca2+](i) at fertilization, which causes degradation of both cyclin B1 a nd Mos by inhibition of de novo synthesis of those proteins. Mel. Reprod. D ev. 53:341-349, 1999. (C) 1999 Wiley-Liss, Inc.