Figure 1.

Regulation of oocyte maturation. Immature oocytes are held in G₂ arrest through the activity of PKA, which is stimulated by GPR3-dependent production of cAMP. Progesterone signaling leads to a loss of PKA activity, leading to disinhibition of the CDK1 activator Cdc25 and stimulation of the CDK1 inhibitor Wee1. Additionally, progesterone stimulates translation of both Mos and cyclin B proteins (the former also stimulating translation of the latter) through induction of Eg2, which phosphorylates and activates CPEB to unmask the messages and promote their polyadenylation. In parallel, progesterone stimulates translation of RINGO, which can activate CDK1 independently of cyclin and make it phosphorylate and inhibit the CDK1 inhibitor Myt1. All of these events result in activation of MPF, which drives the oocyte into MI. Maturation continues via the Mos-MEK-ERK pathway as shown. Blockade of APC (anaphase-promoting complex) activity by CSF and Emi2 holds the mature oocyte at a second arrest until the time of fertilization.