M. Yamashita et al., MOLECULAR MECHANISMS OF THE ACTIVATION OF MATURATION-PROMOTING FACTORDURING GOLDFISH OOCYTE MATURATION, Developmental biology, 168(1), 1995, pp. 62-75
Oocyte maturation is triggered by the activation in the oocyte cytopla
sm of maturation-promoting factor (MPF), which consists of cdc2 (a cat
alytic subunit) and cyclin B (a regulatory subunit). Immature goldfish
oocytes contain only inactive monomeric 35-kDa cdc2 and do not stockp
ile cyclin B. In maturing oocytes, activation of cdc2 is associated wi
th its Thr161 phosphorylation and mobility shift on SDS-PAGE from 35 t
o 34 kDa after binding to cyclin B. Using mutant cdc2, we show that Th
r161 phosphorylation is required for both the downward shift and the k
inase 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 t
he cdc2-cyclin B complex preexists with cdc2 phosphorylated on both Ty
r15 and Thr161, thereby requiring Tyr15 dephosphorylation catalyzed by
cdc25 phosphatase for MPF activation. These results indicate that the
se 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. (C
) 1995 Academic Press, Inc.