CK2β, which inhibits Mos function, binds to a discrete domain in the N-terminus of Mos

Progesterone stimulates G2-arrested Xenopus oocytes to synthesize Mos, a MAPK kinase kinase required for the coordinated activation of cdc2 and the G2/Meiosis I (MI) transition. Mos leads to activation of MAPK, Rsk, and the inhibition of the cdc2 inhibitor Myt1. Previous work identified CK2β as a Mos-interacting protein, and suggested that CK2β acts as a negative regulator by setting a threshold above which newly made Mos must accumulate to activate MAPK. However, it had not been demonstrated that CK2β directly inhibits Mos. We report here that Mos (52–115) is required for CK2β binding and can serve as a portable binding domain. To test whether CK2β acts at the level of Mos or on a downstream component, we took advantage of previous work that showed injection of Mos arrests rapidly dividing embryonic cells. We find that coinjection of CK2β and Mos into embryonic cells inhibits the ability of Mos to arrest cell division. In contrast, CK2β does not inhibit the mitotic arrest induced by injection of active Rsk. These results argue that CK2β directly binds and inhibits Mos rather than a downstream component, and support that CK2β functions as a molecular buffer that prevents premature MAPK activation and oocyte maturation.