The β subunit of CKII negatively regulates Xenopus oocyte maturation

CKII (formerly known as casein kinase II) is a ubiquitously expressed enzyme that plays an important role in regulating cell growth and differentiation. The β subunit of CKII (CKIIβ) is not catalytic but forms heterotetramers with the catalytic subunit α to generate an α(2)β(2) holoenzyme. In Xenopus oocytes, CKIIβ also associates with another serine/threonine kinase, Mos. As a key regulator of meiosis, Mos is necessary and sufficient to initiate oocyte maturation. We have previously shown that the binding of CKIIβ to Mos represses Mos-mediated mitogen-activated protein kinase (MAPK) activation and that the ectopic expression of CKIIβ inhibits progesterone-induced Xenopus oocyte maturation. We have now used an antisense oligonucleotide technique to reduce the endogenous CKIIβ protein level in Xenopus oocytes, and we find that oocytes with a reduced content of CKIIβ are more sensitive to low doses of progesterone and show accelerated MAPK activation and germinal vesicle breakdown. Furthermore, ectopic expression of a Mos-binding fragment of CKIIβ suppressed the effect of antisense oligonucleotide. These results suggest that the endogenous CKIIβ normally sets a threshold level for Mos protein, which must be exceeded for Mos to activate the MAPK signaling pathway and induce oocyte maturation.