An M-phase-specific protein kinase of Xenopus oocytes: partial purification and possible mechanism of its periodic activation

Involvement of protein kinase A and casein kinase II in the in vivo protein kinase activities in prophase arrested Xenopus oocytes

In vivo beta casein phosphorylation was analysed in Xenopus full-grown oocytes arrested in the prophase of the meiotic cell division. The phosphorylation was inhibited by the protein kinase inhibitor (PKI) and also by heparin (3 micrograms/ml; final concentration). beta casein phosphorylation was increased by spermine (2 mM). Therefore, protein kinase A and casein kinase II are both active in vivo in full-grown oocytes and may be involved in the prophase arrest of meiotic cell division.

Identification of the ATP + Mg-dependent and polycation-stimulated protein phosphatases in the germinal vesicle of the Xenopus oocyte

Two protein phosphatase activities were characterized in the germinal vesicle of Xenopus laevis oocytes after manual dissection of the nucleus. One enzyme can be classified as an active form of the ATP + Mg-dependent (AMD) phosphatase, the other as a polycation-stimulated (PCS) phosphatase. The activity of the PCS phosphatase is localized exclusively in the soluble compartment of the nucleus (nucleoplasm). The catalytic subunit of the AMD phosphatase activity is associated either with the nuclear particulate fraction or with an inhibitory subunit in the nucleoplasm.

Purification of MPF from starfish: identification as the H1 histone kinase p34cdc2 and a possible mechanism for its periodic activation

MPF extracted from starfish oocytes copurifies with an M phase-specific H1 histone kinase encoded by a homolog of the fission yeast cell cycle control gene cdc2+. The most purified preparations contain p34cdc2 as the only major protein. Activation of the p34cdc2 kinase is correlated with appearance of the MPF activity both in vivo and in vitro. The increase in protein kinase activity is associated with p34cdc2 dephosphorylation and the decrease in protein kinase activity on leaving M phase with rephosphorylation. Microinjection of a peptide perfectly conserved in p34cdc2 from yeast to humans induces meiotic maturation, suggesting that an inhibitory component in G2 arrested oocytes interacts with this region of the p34cdc2 kinase. We propose that initiation of M phase is brought about by the dephosphorylation of p34cdc2, leading to increase in its protein kinase activity.

Translation of cyclin mRNA is necessary for extracts of activated xenopus eggs to enter mitosis

The cyclins are a family of proteins encoded by maternal mRNA. Cyclin polypeptides accumulate during interphase and are destroyed during mitosis at about the time of entry into anaphase. We show here that Xenopus oocytes contain mRNAs encoding two cyclins that are major translation products in a cell-free extract from activated eggs. Cutting these mRNAs with antisense oligonucleotides and endogenous RNAase H blocks entry into mitosis in a cell-free egg extract. The extracts can enter mitosis if either of the cyclin mRNAs is left intact. We conclude that the synthesis of these cyclins is necessary for mitotic cell cycles in cleaving Xenopus embryos.

Okadaic acid, a specific protein phosphatase inhibitor, induces maturation and MPF formation in Xenopus laevis oocytes

Micro-injection of, or incubation with okadaic acid (OA), a specific phosphatase inhibitor, can induce formation of maturation-promoting factor (MPF) and germinal vesicle breakdown (GVBD) in Xenopus laevis oocytes. Comparison of the dose-response curves of OA on maturation, isolated enzymes and phosphatase activities in crude oocyte preparations suggests that inhibition of both polycation-stimulated (PCS) and ATP,Mg-dependent (AMD) phosphatases is sufficient but requires that a critical phosphorylation level is attained of one or several of their substrates, resulting in the formation of active MPF and meiotic maturation.

cdc2 is a component of the M phase-specific histone H1 kinase: evidence for identity with MPF

A so-called "growth-associated" or "M phase-specific" histone H1 kinase (H1K) has been described in a wide variety of eukaryotic cell types. In starfish oocytes, the hormone 1-methyladenine triggers synchronous meiotic divisions that are accompanied by a rapid 30-fold stimulation of H1K activity. We have substantially purified this activated enzyme and find that it is enriched for a protein of 34 kd. Quantitative immunoblotting of the column fractions with antibodies raised against p34, the product of the fission yeast cdc2 gene, revealed complete coelution of the H1K activity and a 34 kd anti-cdc2 cross-reactive protein. Starfish H1K also displayed the same apparent molecular weight, on a molecular sizing column, as the mitotically activated p13/p34/p62 protein kinase complex of HeLa cells. p13, the product of the fission yeast suc1+ gene, interacts tightly with p34 in yeast, Xenopus, and HeLa cells. H1K from starfish binds strongly to p13-Sepharose and the time course of 1-methyladenine-induced H1K activation, whether assayed in crude extract or on p13-Sepharose beads, is identical. These results indicate that a cdc2 homolog is a subunit of the M phase-specific H1K of starfish meiotic oocytes. Since this protein is also a subunit of the M-phase promoting factor (MPF) of Xenopus oocytes, we suggest that H1K and MPF are the same entity, and that histone H1 is likely to be one substrate of the pleiotropic MPF.