EXPRESSION AND ACTIVITY OF P40(MO15), THE CATALYTIC SUBUNIT OF CDK-ACTIVATING KINASE, DURING XENOPUS OOGENESIS AND EMBRYOGENESIS

Threonine 161 phosphorylation of p34(cdc2) and its equivalent threonine 160 in p33(cdk2) by cdk-activating kinase (CAK) is essential for the activation of these cyclin-dependent kinases. We have studied the expression and associated kinase activity of p40(MO15), the catalytic subunit of CAK, during Xenopus oogenesis, meiotic maturation, and early development to understand in more detail how cdk kinases are regulated during these events. We find that p40(MO15) is a stable protein with a half-life >16 h that is accumulated during oogenesis. p40(MO15) protein and its associated CAK activity are localized predominantly to the germinal vesicle; however, a small but significant proportion is found in the cytoplasm. The amount of p40(MO15) detected in stage VI oocytes remains unchanged through meiotic maturation, fertilization, and early embryogenesis. Significantly, p40 was found to be constitutively active during oogenesis, meiotic maturation, and the rapid mitotic cycles of early development. This suggests that regulation of p34(cdc2) and p33(cdk2) activity during cell cycle progression does not involve changes in the level or activity of p40(MO15)/CAK.