Molecular mechanisms of the activation of maturation-promoting factor during goldfish oocyte maturation

Oocyte maturation is triggered by the activation in the oocyte cytoplasm of maturation-promoting factor (MPF), which consists of cdc2 (a catalytic subunit) and cyclin B (a regulatory subunit). Immature goldfish oocytes contain only inactive monomeric 35-kDa cdc2 and do not stockpile cyclin B. In maturing oocytes, activation of cdc2 is associated with its Thr161 phosphorylation and mobility shift on SDS-PAGE from 35 to 34 kDa after binding to cyclin B. Using mutant cdc2, we show that Thr161 phosphorylation is required for both the downward shift and the kinase activation. Since cdc2 Tyr15 is not phosphorylated after binding to cyclin B, it does not require dephosphorylation. This situation is obviously different from that in immature Xenopus oocytes, in which the cdc2-cyclin B complex preexists with cdc2 phosphorylated on both Tyr15 and Thr161, thereby requiring Tyr15 dephosphorylation catalyzed by cdc25 phosphatase for MPF activation. These results indicate that these species employ different mechanisms of MPF activation during oocyte maturation, although the final molecular structure of the active MPF (cdc2 bound to cyclin B and phosphorylated on Thr161) is identical.