Progesterone (Pg) binds to both intracellular iPR and plasma membrane- bound mPR. (Right Top) After binding to Pg, iPR is recruited to the membrane associated protein tyrosine kinase p60c- src, which induces activation of the MAPK signaling pathway. This results in activation of p90Rsk and the subsequent phosphorylation and inactivation of Myt1, which favors formation of the activated cell cycle complex cyclin B-Cdc2. Activation of cyclin B-Cdc2 causes breakdown of the germinal vesicle and the initiation of oocyte maturation. (Left). In contrast, binding of Pg to mPR leads to inhibition of adenylyl cyclase (AC) through activation of Gi or inhibition of Gs. This leads to a decrease in the cAMPdependent kinase PKA, which relieves inhibition of Cdc25C (the phosphatase that dephosphorylates and activates cyclin B-Cdc2) and also indirectly promotes the activation of MAPK signaling. PKA also regulates the initiation of oocyte maturation through other effects that are independent of PKA activity.
