Protein Kinase A Acts at Multiple Points To Inhibit Xenopus Oocyte Maturation

Abstract

In Xenopus oocytes, initiation of maturation is dependent on reduction of cyclic AMP-dependent protein kinase (PKA) activity and the synthesis of the mos proto-oncogene product. Mos is required during meiosis I for the activation of both maturation-promoting factor (MPF) and mitogen-activated protein kinase (MAPK). Here we show that injection of the catalytic subunit of PKA (PKA_c) prevented progesterone-induced synthesis of endogenous Mos as well as downstream MPF and MAPK activation. However, PKA_c did not prevent injected soluble Mos product from activating MAPK. While MAPK is activated during Mos-PKA_c coinjection, attendant MPF activation is blocked. Additionally, PKA_c caused a potent block in the electrophoretic mobility shift of cdc25 that is associated with phosphatase activation. This inhibition of cdc25 activity was not reversed by progesterone, Mos, or MPF. We conclude that PKA_c acts as a negative regulator at several points in meiotic maturation by preventing both Mos translation and MPF activation.