The Casein Kinase II β Subunit Binds to Mos and Inhibits Mos Activity

Abstract

Mos is a germ cell-specific serine/threonine kinase and is required for Xenopus oocyte maturation. Active Mos stimulates a mitogen-activated protein kinase (MAPK) by directly phosphorylating and activating MAPK kinase (MKK). We report here that the Xenopus homolog of the ␤ subunit of casein kinase II (CKII␤) binds to and regulates Mos. The Mos-interacting region of CKII␤ was mapped to the C terminus. Mos bound to CKII␤ in somatic cells ectopically expressing Mos and CKII␤ as well as in unfertilized Xenopus eggs. CKII␤ inhibited Mos-mediated MAPK activation in rabbit reticulocyte lysates and repressed MKK activation by v-Mos in a coupled kinase assay. In addition, microinjection of CKII␤ mRNA into Xenopus oocytes inhibited progesterone-induced meiotic maturation and MAPK activation, presumably by binding of CKII␤ to Mos and thereby inhibiting MAPK activation. Moreover, this inhibitory phenotype could be rescued by another protein that binds to CKII␤, CKII␣. The ability of ectopic CKII␤ to inhibit meiotic maturation and the detection of a complex between endogenous Mos and CKII␤ suggest that CKII␤ may act as an inhibitor of Mos during oocyte maturation, perhaps setting a threshold beyond which Mos protein must accumulate before it can activate the MAPK pathway.Mos is a germ cell-specific serine/threonine kinase and is present at a very low level in fully grown (stage VI) Xenopus oocytes (46,53,55,65). During oocyte maturation, mos translation is activated by progesterone secreted from the follicle cells (64,65). Oocytes are then released from the prophase arrest of meiosis I and undergo a sequence of events including nuclear envelope breakdown (commonly known as germinal vesicle breakdown [GVBD]), completion of meiosis I, extrusion of the first polar body, meiosis II entry, and subsequent metaphase II arrest as unfertilized eggs (reviewed in reference 45). Upon fertilization, Mos protein is degraded and does not reappear during embryonic development (39,80).Mos is essential for progesterone-induced oocyte maturation. Microinjection of antisense mos oligonucleotides into Xenopus oocytes blocks progesterone-induced GVBD and the activation of maturation-promoting factor (MPF) (31, 65). MPF is a universal M-phase regulator that drives meiotic progression and consists of the p34 cdc2 protein kinase and the cyclin B protein (reviewed in references 32 and 49). In the absence of progesterone, injected Mos protein or mos mRNA can activate MPF and induce GVBD (64,65,82). These results suggest that Mos is necessary and sufficient to initiate meiosis. Mos is also needed for the later stage of oocyte maturation. Extracts of unfertilized eggs in meiosis II contain an activity, cytostatic factor (CSF), that causes metaphase II arrest and can be assayed by the induction of cleavage arrest when injected into blastomeres of dividing embryos (45). Mos has CSF activity, and immunodepletion of Mos from unfertilized Xenopus egg extracts abolishes CSF (15,66,82). Mos is also important for the meiosis II arrest in mice. Female mice lacki...

Modulation of human checkpoint kinase Chk1 by the regulatory β-subunit of protein kinase CK2

“…It is possible that the C-terminus of CK2b recognizes a common structural motif of serine/threonine kinases. However, the fact that CK2b interacts specifically with A-Raf (Boldyreff and Issinger, 1997) but not c-Raf (Chen et al, 1997;Hagemann et al, 1997), suggests that kinase interaction with CK2b may require higher specificity. The structure determination of the human Chk1 kinase (Chen et al, 2000) has indicated that the activity of the kinase domain (residues 1-265) of human Chk1 is over 20-fold more active than the full-length Chk1 kinase toward Cdc25C substrate.…”

“…While CK2 holoenzyme appears to promote cell cycle progression, the regulatory b-subunit is implicated to participate in other pathways. It has been shown that CK2b can inhibit the activity of c-Mos preventing Xenopus oocytes maturation (Chen et al, 1997). Moreover, the transient or the stable ectopic expression of Myc-tagged CK2b inhibits cell proliferation and reduces mitotic cyclin-dependent kinases activity (Li et al, 1999).…”

Identification and Characterization of CKIP-1, a Novel Pleckstrin Homology Domain-containing Protein That Interacts with Protein Kinase CK2

“…Reactions were initiated by the addition of immune complex and were terminated by the addition of EDTA to a concentration of 25 mM. Reactions were centrifuged briefly to pellet immune complexes, and half of the supernatant was spotted on P81 paper to measure incorporation of 32 P into synthetic peptide as described previously (47). All samples were assayed in the presence or absence of synthetic peptide to control for background phosphorylation of proteins in the immune complexes.…”

“…In this regard, it may be notable that there is mounting evidence demonstrating that CK2 interacts with a variety of cellular proteins. For example, CK2␤ has been reported to interact with FGF-2 (24), A-Raf (25,26), Nopp140 (27), p53 (28 -30), p21 WAF1/CIP1 (31), c-MOS (32), and CD5 (33). CK2␣ and CK2␣Ј have also been observed to interact with specific protein partners.…”

An inactive mutant of the α subunit of protein kinase CK2 that traps the regulatory CK2β subunit

“…In this regard it is relevant to emphasize that CK2B has recently been shown to have a role that transcends its regulation of CK2oc activity. The CK2B activation of A-Raf [13,14] and the inhibition of mos [15] by this subunit have been reported to be antagonized by CK2oc and presumably would be also blocked by the aA 156 mutant.…”