CDK7 and CCNH Are Components of CDK-Activating Kinase and Are Required for Meiotic Progression of Pig Oocytes. Fujii W et al. CDK-activating kinase (CAK) phosphorylates threonine 161 (T161) of CDC2, a catalytic subunit of maturation/M-phase promoting factor (MPF), and is essential for MPF activation in mitosis. CAK has been thought to consist of a catalytic subunit, a regulatory subunit and an assembly factor: CDK7, CCNH (also known as Cyclin H) and MNAT1 (also known as MAT1), respectively. Although it is known that the meiotic progression of oocytes is regulated by MPF activity, the role of CAK in meiosis is still unclear. In the present study, we attempted to confirm the involvement of CAK in the meiotic progression of porcine immature oocytes. The T161 phosphorylation of CDC2 was found around germinal vesicle breakdown (GVBD) and thereafter, from 18 h to 48 h of culture. The GVBD rate at 18 h was increased by the overexpression of CDC2, but not mutated CDC2 (T161 replaced by alanine). Transcripts of CDK7, CCNH and MNAT1 were detectable throughout the culture period and their protein distribution patterns during oocyte maturation were the same as those reported in mitotic somatic cells. Overexpression of CDK7 or CCNH accelerated the meiotic events, such as meiotic resumption, T161 phosphorylation of CDC2, CCNB (also known as Cyclin B) synthesis and MPF activation. On the contrary, knockdown of CDK7 or CCNH caused the inhibition of these meiotic events. In contrast, overexpression and antisense RNA injection of MNAT1 had no influence on meiotic resumption, the status of T161 phosphorylation of CDC2, or MPF activity. These results suggest that CDK7 and CCNH activate CDC2 by T161 phosphorylation, and comprise CAK, which is required for normal meiotic progression during porcine oocyte maturation.