Phosphorylation of conserved serine residues does not regulate the ability of mosxe protein kinase to induce oocyte maturation or function as cytostatic factor

Mos in the oocyte: how to use MAPK independently of growth factors and transcription to control meiotic divisions

In many cell types, the mitogen-activated protein kinase (MAPK) also named extracellular signal-regulated kinase (ERK) is activated in response to a variety of extracellular growth factor-receptor interactions and leads to the transcriptional activation of immediate early genes, hereby influencing a number of tissue-specific biological activities, as cell proliferation, survival and differentiation. In one specific cell type however, the female germ cell, MAPK does not follow this canonical scheme. In oocytes, MAPK is activated independently of growth factors and tyrosine kinase receptors, acts independently of transcriptional regulation, plays a crucial role in controlling meiotic divisions, and is under the control of a peculiar upstream regulator, the kinase Mos. Mos was originally identified as the transforming gene of Moloney murine sarcoma virus and its cellular homologue was the first proto-oncogene to be molecularly cloned. What could be the specific roles of Mos that render it necessary for meiosis? Which unique functions could explain the evolutionary cost to have selected one gene to only serve for few hours in one very specific cell type? This review discusses the original features of MAPK activation by Mos and the roles of this module in oocytes.

Kicked by Mos and tuned by MPF-the initiation of the MAPK cascade in Xenopus oocytes

The mitogen-activated protein kinase (MAPK) cascade is a paradigmatic signaling cascade, which plays a crucial role in many aspects of cellular events. The main initiator of the cascade in Xenopus oocytes is the oncoprotein Mos. After activation of the cascade, Mos activity is stabilized by MAPK via a feedback loop. Mos concentration levels are, however, not controlled by MAPK alone. In this paper we show, by imposing either a sustained or a peaked activity of M-phase promoting factor (MPF) (Cdc2-cyclin B), how the latter regulates the dynamics of Mos. Our experiments are supported by a detailed kinetic model for the Mos-MPF-MAPK network, which takes into account the three different phosphorylation states of Mos and, as a consequence, allows us to determine the time evolution of Mos under control of MPF. Our work opens a path toward a more complete and biologically realistic quantitative understanding of the dynamic interdependence of Mos and MPF in Xenopus oocytes.

Mechanistic studies of the mitotic activation of Mos

The protein kinase Mos is responsible for the activation of MEK1 and p42 mitogen-activated protein kinase during Xenopus oocyte maturation and during mitosis in Xenopus egg extracts. Here we show that the activation of Mos depends upon the phosphorylation of Ser 3, a residue previously implicated in the regulation of Mos stability; the dephosphorylation of Ser 105, a previously unidentified phosphorylation site conserved in Mos proteins; and the regulated dissociation of Mos from CK2beta. Mutation of Ser 3 to alanine and/or mutation of Ser 105 to glutamate produces a Mos protein that is defective for M-phase activation, as assessed by in vitro kinase assays, and defective for induction of oocyte maturation and maintenance of the spindle assembly checkpoint in extracts. Interestingly, Ser 105 is situated at the beginning of helix alphaC in the N-terminal lobe of the Mos kinase domain. Changes in the orientation of this helix have been previously implicated in the activation of Cdk2 and Src family tyrosine kinases. Our work suggests that Ser 105 dephosphorylation represents a novel mechanism for reorienting helix alphaC.

Dissection of c-MOS degron

c-MOS, a MAP kinase kinase kinase, is a regulator of oocyte maturation. The concentration of c-MOS is controlled in part through its conditional degradation. Previous studies proposed the "second-codon rule", according to which the N-terminal proline (Pro) of c-MOS is a destabilizing residue that targets c-MOS for degradation. We analyzed the degradation signal (degron) of c-MOS in Xenopus oocytes, found it to be a portable degron, and demonstrated that, contrary to the model above, the N-terminal Pro residue of c-MOS is entirely dispensable for its degradation if Ser-2 (encoded Ser-3) of c-MOS is replaced by a small non-phosphorylatable residue such as Gly. The dependence of c-MOS degradation on N-terminal Pro is shown to be caused by a Pro-mediated downregulation of the net phosphorylation of Ser-2, a modification that halts c-MOS degradation in oocytes. Thus, the N-terminal Pro residue of c-MOS is not a recognition determinant for a ubiquitin ligase, in agreement with earlier evidence that Pro is a stabilizing residue in the N-end rule.

Cyclin B/cdc2 induces c-Mos stability by direct phosphorylation in Xenopus oocytes

The c-Mos proto-oncogene product plays an essential role during meiotic divisions in vertebrate eggs. In Xenopus, it is required for progression of oocyte maturation and meiotic arrest of unfertilized eggs. Its degradation after fertilization is essential to early embryogenesis. In this study we investigated the mechanisms involved in c-Mos degradation. We present in vivo evidence for ubiquitin-dependent degradation of c-Mos in activated eggs. We found that c-Mos degradation is not directly dependent on the anaphase-promoting factor activator Fizzy/cdc20 but requires cyclin degradation. We demonstrate that cyclin B/cdc2 controls in vivo c-Mos phosphorylation and stabilization. Moreover, we show that cyclin B/cdc2 is capable of directly phosphorylating c-Mos in vitro, inducing a similar mobility shift to the one observed in vivo. Tryptic phosphopeptide analysis revealed a practically identical in vivo and in vitro phosphopeptide map and allowed identification of serine-3 as the largely preferential phosphorylation site as previously described (Freeman et al., 1992). Altogether, these results demonstrate that, in vivo, stability of c-Mos is directly regulated by cyclin B/cdc2 kinase activity.

c-Mos proteolysis is independent of the CA(2+) rise induced by 6-DMAP in Xenopus oocytes

In Xenopus oocytes, metaphase II arrest is due to a cytostatic factor (CSF) that involves c-Mos, maintaining a high MPF (cdk1/cyclin B) activity in the cell. At fertilization, a rise in intracellular calcium triggers the proteolysis of both cyclin B and c-Mos. The kinase inhibitor 6-dimethylaminopurine (6-DMAP) is also able to release matured Xenopus oocytes from metaphase II block. This is characterized by c-Mos proteolysis without degradation of cyclin B. We hypothesized that 6-DMAP induced an increase in intracellular calcium. Using the calcium-sensitive fluorescent dye Fura-2, we observed a systematic increase in intracellular calcium following 6-DMAP application. In matured oocytes previously microinjected with the calcium chelator BAPTA, no calcium changes occurred after 6-DMAP addition; however, c-Mos was still proteolysed. In oocytes at the GVBD stage, c-Mos proteolysis occurred in response to 6-DMAP but not to calcium ionophore treatment. We suggest that c-Mos proteolysis is rather controlled by a phosphorylation-dependent process.

A single Glu(62)-to-Lys(62) mutation in the Mos residues of the R7Delta447Gag-tMos protein causes the mutant virus to induce brain lesions

We previously reported that R7Delta447, a 2954-base-pair (bp) laboratory-generated Moloney murine sarcoma virus, induced subcutaneous tumors in about 14% of infected mice but did not induce brain lesions. We now report that R7Delta447K, a spontaneous mutant of R7Delta447, induced brain lesions as well as subcutaneous tumors in all injected mice. The genomes of the two viruses differ in a single base pair: the deduced Glu(62) of the Mos residue of the R7Delta447 Gag-tMos protein is changed to Lys(62). More R7Delta447 than R7Delta447K focus-forming units were detected in both NIH3T3 and mouse cerebral vascular endothelial (MCVE) cells. However, R7Delta447K transformed NIH3T3 and MCVE cells more acutely than did R7Delta447. A distinctive feature that distinguished the morphologic transformation of R7Delta447- and R7Delta447K-infected MCVE cells is the markedly prolonged spindle-shaped phase exhibited by R7Delta447-infected MCVE cells. In addition, R7Delta447K was more efficient in inducing the phosphorylation of ERK1/2 than R7Delta447 in both MCVE and NIH3T3 cells. Moreover morphologic transformation was inhibited, and levels of phosphorylated ERK1/2 were reduced when R7Delta447- or R7Delta447K-infected NIH3T3 or MCVE cells were grown in the presence of the MEK1/2-specific inhibitor PD98095. Thus, we have identified a key residue in the Gag-tMos protein that profoundly affects activation of the Mos/MEK/ERK pathway, virus and cell replication, morphologic transformation in vitro and pathogenicity in vivo.

Hsp90 is required for c-Mos activation and biphasic MAP kinase activation in Xenopus oocytes

During Xenopus oocyte maturation, the Mos protein kinase is synthesized and activates the MAP kinase cascade. In this report, we demonstrate that the synthesis and activation of Mos are two separable processes. We find that Hsp90 function is required for activation and phosphorylation of Mos and full activation of the MAP kinase cascade. Once Mos is activated, Hsp90 function is no longer required. We show that Mos interacts with both Hsp90 and Hsp70, and that there is an inverse relationship between association of Mos with these two chaperones. We propose that Mos protein kinase is activated by a novel mechanism involving sequential association with Hsp70 and Hsp90 as well as phosphorylation. We also present evidence for a two-phase activation of MAP kinase in Xenopus oocytes.

Xenopus oocyte maturation: new lessons from a good egg

Fully grown Xenopus oocytes can remain in their immature state essentially indefinitely, or, in response to the steroid hormone progesterone, can be induced to develop into fertilizable eggs. This process is termed oocyte maturation. Oocyte maturation is initiated by a novel plasma membrane steroid hormone receptor. Progesterone brings about inhibition of adenylate cyclase and activation of the Mos/MEK1/p42 MAP kinase cascade, which ultimately brings about the activation of the universal M phase trigger Cdc2/cyclin B. Oocyte maturation provides an interesting example of how signaling cascades entrain the cell cycle clock to environmental changes.

Evidence of an interaction between Mos and Hsp70: a role of the Mos residue serine 3 in mediating Hsp70 association

c-Mos is a germ cell-specific MAP kinase kinase kinase (MAPKKK) that plays an essential role during meiotic divisions of oocytes. c-Mos is a key component of an activity, cytostatic factor, required for metaphase II arrest of unfertilized eggs in vertebrates. To understand the regulation of c-Mos, we are investigating c-Mos-interacting proteins. We provide evidence that mouse c-Mos binds to Hsp70, a molecular chaperone. Hsp70 was found to associate with Mos ectopically expressed in COS-1 cells. Mos-Hsp70 complexes could be immunoprecipitated with both Mos and Hsp70 antibodies. Despite a low-abundance of Mos, the Hsp70 antibody immunoprecipitated Mos as the major protein. Of importance, the Mos protein present in anti-Hsp70 immunoprecipitates functioned as an active MAPKKK indicating that it is not grossly misfolded. It is known that c-Mos protein kinase activity in cell extracts of transfected COS-1 or NIH3T3 cells is labile. We found that the inclusion of adenosine triphosphate (ATP) in cell extracts protected against the loss of Mos kinase activity. In the absence of ATP from cell extracts, protein kinase activity of Mos was lost within 6 h on ice even though the Mos protein was not degraded and remained bound to Hsp70. Based on our identification of c-Mos-Hsp70 interaction, one of the roles of ATP may be to assist the regulation of c-Mos via ATP involvement in the protein-folding function of Hsp70 and possibly other molecular chaperones. We also detected by coimmunoprecipitation a physical association between endogenous c-Mos and Hsp70 in Xenopus eggs. To provide further evidence for the functional significance of Hsp70 interaction to Mos function, we show that the residue serine 3 in Mos, which is important for the regulation of protein kinase activity of Mos is also important for Hsp70 association.

Evidence of a functional interaction between serine 3 and serine 25 Mos phosphorylation sites. A dominant inhibitory role of serine 25 phosphorylation on Mos protein kinase

Recently, we identified the major in vivo phosphorylation site on v-Mos as Ser-56, which is phosphorylated by cyclic AMP dependent protein kinase (PKA). Others have shown that c-Mos phosphorylation at Ser-3 (equivalent to Ser-34 in v-Mos) is important for the interaction of c-Mos with its substrate MEK and for its stability and cytostatic factor activity in eggs. To investigate the role of Ser-56 phosphorylation, we generated site-directed mutants of v-Mos that would mimic phosphorylation in terms of charge at positions 56 and 34. After mutating serine (S) residues with alanine (A) or glutamic acid (E) in different combinations, various v-Mos mutants were expressed in a rabbit reticulocyte lysate in vitro translation system and in COS-1 or NIH/3T3 cells. The effect of mutations on Mos function was evaluated by in vitro protein kinase assays and by the ability of Mos to cause neoplastic transformation of NIH/3T3 cells. The S56E but not the S56A mutation inhibited v-Mos kinase activity suggesting that Ser-56 phosphorylation has an inhibitory role. As predicted from Xenopus c-Mos studies, S34A but not S34E mutation inhibited v-Mos activity. Studies with the double mutants showed that the S56E mutation but not S56A mutation inhibited v-Mos kinase activity of both S34A and S34E mutants. Interestingly, the S56A mutation blocked the inhibitory effect of the S34A mutation on v-Mos kinase suggesting that in c-Mos the corresponding serine (Ser-25) can influence the regulation of c-Mos by Ser-3. Results showing inhibition of v-Mos kinase activity of the S34E mutant by the S56E mutation is significant as it suggests that doubly phosphorylated Mos at these residues would be inactive. Because residues corresponding to both v-Mos Ser-34 and Ser-56 are evolutionarily conserved in c-Mos, the kinase activity of c-Mos during meiosis may also be regulated in the same manner as v-Mos kinase activity.

Mos and the cell cycle

The mos proto-oncogene-encoded serine/threonine protein kinase plays a key cell cycle-regulatory role during meiosis. The Mos protein is required for the activation and stabilisation of M phase-promoting factor MPF. As a component of a large multiprotein complex known as the cytostatic factor (CSF), Mos is involved in causing metaphase II arrest of eggs in vertebrates. Upon expression in somatic cells, Mos causes cell cycle perturbations resulting in cytotoxicity and neoplastic transformation. All the known biological activities of Mos are mediated through activation of the mitogen activated protein (MAP) kinase pathway. Here we discuss the interrelationship between Mos and other cell cycle regulators.

Identification of the autophosphorylation sites of the Xenopus laevis Pim-1 proto-oncogene-encoded protein kinase

Pim-1 is an oncogene-encoded serine/threonine kinase expressed primarily in cells of the hematopoietic and germ line lineages. Previously identified only in mammals, pim-1 cDNA was cloned and sequenced from the African clawed frog Xenopus laevis. The coding region of Xenopus pim-1 encoded a protein of 324 residues, which exhibited 64% amino acid identity with the full-length human cognate. Xenopus Pim-1 was expressed in bacteria as a glutathione S-transferase (GST) fusion protein and in COS cells. Phosphoamino acid analysis revealed that recombinant Pim-1 autophosphorylated on serine and threonine and to a more limited extent on tyrosine. Electrospray ionization mass spectroscopy was undertaken to locate these phosphorylation sites, and the primary autophosphorylation site of GST-Pim-1 was identified as Ser-190 with Thr-205 and Ser-4 being minor sites. Ser-190, which immediately follows the high conserved Asp-Phe-Gly motif in catalytic subdomain VII, is also featured in more than 20 other protein kinases. To evaluate the importance of the Ser-190 site on the phosphotransferase activity of Pim-1, Ser-190 was mutated to either alanine or glutamic acid, and the constructs were expressed in bacteria as GST fusion proteins and in COS cells. These mutants confirmed that Ser-190 is a major autophosphorylation site of Pim-1 and indicated that phosphorylation of Pim-1 on the Ser-190 residue may serve to activate this kinase.

Synthesis and function of Mos: the control switch of vertebrate oocyte meiosis

One distinguishing feature of vertebrate oocyte meiosis is its discontinuity; oocytes are released from their prophase I arrest, usually by hormonal stimulation, only to again halt at metaphase II, where they await fertilization. The product of the c-mos proto-oncogene, Mos, is a key regulator of this maturation process. Mos is a serine-threonine kinase that activates and/or stabilizes maturation-promoting factor (MPF), the master cell cycle switch, through a pathway that involves the mitogen-activated protein kinase (MAPK) cascade. Oocytes arrested at prophase I lack detectable levels of Mos, which must be synthesized from a pool of maternal mRNAs for proper maturation. While Mos is necessary throughout maturation in Xenopus, it seems to be required only for meiosis II in the mouse. The translational activation of c-mos mRNA at specific times during meiosis requires cytoplasmic polyadenylation. Cis- and trans-acting factors for polyadenylation are, therefore, essential elements of maturation.

Identification of an autoinhibitory region in the activation loop of the Mos protein kinase

The Mos protein is a serine/threonine protein kinase which acts to regulate progression through meiosis in vertebrate oocytes. Although Mos function is dependent on its ability to act as a protein kinase, little is known about the factors which regulate Mos kinase activity. To understand the mechanism by which Mos kinase activity is regulated, we have used molecular modeling to construct a three-dimensional model of Mos based on the crystallographic coordinates of cyclic AMP-dependent kinase (PKA). This model identified a loop in Mos which is positioned near the active site and appears capable of blocking substrate access to the active site. Mutagenesis was used to construct altered forms of the Mos protein with deletions of parts or all of the loop. In vitro kinase assays showed that Mos proteins with the loop removed had up to a fourfold increase in kinase activity compared with the wild-type protein, indicating that the loop acts in an autoinhibitory manner for Mos kinase activity. Point mutations were also made on individual residues of the loop which were determined from the molecular model to be capable of reaching the active site. Determination of the kinase activities of these mutants showed that individual mutations in the loop region are capable of either increasing or decreasing kinase activity with regard to the wild-type protein. These data suggest that the loop identified in Mos acts as an autoinhibitor of kinase activity.

Inhibition of v-Mos kinase activity by protein kinase A

We investigated the effect of cyclic AMP-dependent protein kinase (PKA ) on v-Mos kinase activity. Increase in PKA activity in vivo brought about either by forskolin treatment or by overexpression of PKA catalytic subunit resulted in a significant inhibition of v-Mos kinase activity. The purified PKA catalytic subunit was able to phosphorylate recombinant p37v-mos in vitro, suggesting that the mechanism of in vivo inhibition of v-Mos kinase involves direct phosphorylation by PKA. Combined tryptic phosphopeptide two-dimensional mapping analysis and in vitro mutagenesis studies indicated that Ser-56 is the major in vivo phosphorylation site on v-Mos. In vivo phosphorylation at Ser-56 correlated with slower migration of the v-Mos protein during sodium dodecyl sulfate-polyacrylamide gel electrophoresis. However, even though Ser-56 was phosphorylated by PKA, this phosphorylation was not involved in the inhibition of v-Mos kinase. The alanine-for-serine substitution at residue 56 did not affect the ability of v-Mos to autophosphorylate in vitro or, more importantly, to activate MEK1 in transformed NIH 3T3 cells. We identified Ser-263 phosphorylation, the Ala-263 mutant of v-Mos was not inhibited by forskolin treatment. From our results, we propose that the known inhibitory role of PKA in the initiation of oocyte maturation in mice could be explained at least in part by its inhibition of Mos kinase.

Ser-3 is important for regulating Mos interaction with and stimulation of mitogen-activated protein kinase kinase

Mos is a germ cell-specific serine/threonine protein kinase that activates mitogen-activated protein kinase (MAPK) through MAPK kinase (MKK). In Xenopus oocytes, Mos synthesis is required for progesterone-induced activation of MAPK and maturation promoting factor. Injection of Mos or active MAPK causes mitotic arrest in early embryos, suggesting that Mos also acts via MKK and MAPK to induce the arrest of unfertilized eggs in metaphase of meiosis II. We have investigated whether Mos activity is regulated by phosphorylation. Previous studies have identified Ser-3 as the principal autophosphorylation site. We show that Mos interacts with the catalytic domain of MKK in a Saccharomyces cerevisiae two-hybrid test. Acidic substitutions of the sites phosphorylated by Mos in MKK reduce the interaction, implying that the complex may dissociate after phosphorylation of MKK by Mos. Furthermore, the Mos-MKK interaction requires Mos kinase activity, suggesting that Mos autophosphorylation may be involved in the interaction. Substitution of Ser-3 of Mos with Ala reduces the interaction with MKK and also reduces both the activation of MKK by Mos in vitro and cleavage arrest induced by Mos fusion protein in Xenopus embryos. By contrast, substitution of Ser-3 by Glu, an acidic amino acid that mimics phosphoserine, fosters the Mos interaction with MKK and permits activation of MKK in vitro and Mos-induced cleavage arrest. Moreover, the Glu-3 substitution increases the interaction of a kinase-inactive Mos mutant with MKK. Taken together, these results suggest that an important step in Mos activation involves the phosphorylation at Ser-3, which promotes Mos interaction with and activation of MKK.

On the c-mos proto-oncogene product during meiotic maturation in bovine oocytes cultured in vitro

The present study was carried out using dot-blot Western analysis with pp39mos-specific polyclonal antibodies to examine the quantitative and qualitative changes of c-mos proto-oncogene product, Mos, during bovine oocyte maturation in vitro. Mos is present throughout meiotic maturation, is produced from around the onset of meiotic resumption, and is phosphorylated on germinal vesicle breakdown. These results indicate for the first time that the synthesis and phosphorylation of Mos during maturation culture play a key role in the accomplishment of meiosis in bovine oocytes.

The protein kinase mos activates MAP kinase kinase in vitro and stimulates the MAP kinase pathway in mammalian somatic cells in vivo

The mos protooncogene encodes a serine/threonine protein kinase that is only expressed at significant levels in germ cells. Recombinant malE-mos protein (Xenopus mos protooncogene fused in frame to the maltose binding protein of E. coli) activates MAP kinase in cell-free extracts prepared from Xenopus oocytes and eggs. Here we show that malE-mos immunoprecipitates from Xenopus extracts phosphorylate and activate MAP kinase kinase in vitro, indicating that mos can function as a MAP kinase kinase kinase. Moreover, ectopic expression of mos in mammalian somatic cells, that lack any endogenous mos protein, triggers the activation of MAP kinase in vivo. These results identify the mos protooncogene as a direct activator of the MAP kinase pathway, with the potential to activate this kinase cascade even in cells where normally there is no expression of mos.

Mos induces the in vitro activation of mitogen-activated protein kinases in lysates of frog oocytes and mammalian somatic cells

Mitogen-activated protein kinases (MAPKs) are rapidly and transiently activated when both quiescent Go-arrested cells and G2-arrested oocytes are stimulated to reenter the cell cycle. We previously developed a cell-free system from lysates of quiescent Xenopus oocytes that responds to oncogenic H-ras protein by activating a MAPK, p42MAPK. Here, we show that the oncogenic protein kinase mos is also a potent activator of p42MAPK in these lysates. Mos also induces p42MAPK activation in lysates of activated eggs taken at a time when neither mos nor p42MAPK is normally active, showing that the mos-responsive MAPK activation pathway persists beyond the stage where mos normally functions. Similarly, lysates of somatic cells (rabbit reticulocytes) also retain a mos-inducible MAPK activation pathway. The mos-induced activation of MAPKs in all three lysates leads to phosphorylation of the pp90rsk proteins, downstream targets of the MAPK signaling pathway in vivo. The in vitro activation of MAPKs by mos in cell-free systems derived from oocytes and somatic cells suggests that mos contributes to oncogenic transformation by inappropriately inducing the activation of MAPKs.

Raf-1 protein kinase is important for progesterone-induced Xenopus oocyte maturation and acts downstream of mos

In somatic cells, the Raf-1 serine/threonine protein kinase is activated by several polypeptide growth factors. We investigated the role of Raf-1 in progesterone-induced meiotic maturation of Xenopus laevis oocytes. Raf-1 enzymatic activity and phosphorylation (reflected by a mobility shift on sodium dodecyl sulfate gels) were increased in oocytes following progesterone stimulation. The increase in Raf-1 activity was concurrent with an elevation in the activity of mitogen-activated protein (MAP) kinase. When RNA encoding an oncogenic form of Raf-1 (v-Raf) was injected into immature oocytes, MAP kinase mobility shift, germinal vesicle breakdown, and histone H1 phosphorylation increased markedly. When RNA encoding a dominant-negative version of Raf-1 was injected, progesterone-induced oocyte maturation was blocked. When RNA encoding Xenopus mos (mosxe) was injected into oocytes, Raf-1 and MAP kinase mobility shifts were observed after several hours. Also, when antisense mosxe oligonucleotides were injected into oocytes, progesterone-induced Raf-1 and MAP kinase mobility shifts were blocked. Finally, when antisense mosxe oligonucleotides were coinjected with v-Raf RNA into oocytes, histone H1 kinase activation, germinal vesicle breakdown, and MAP kinase mobility shift occurred. These findings suggest that Raf-1 activity is required for progesterone-induced oocyte maturation and that Raf-1 is downstream of mosxe activity.

Raf-1 protein kinase is important for progesterone-induced Xenopus oocyte maturation and acts downstream of mos

In somatic cells, the Raf-1 serine/threonine protein kinase is activated by several polypeptide growth factors. We investigated the role of Raf-1 in progesterone-induced meiotic maturation of Xenopus laevis oocytes. Raf-1 enzymatic activity and phosphorylation (reflected by a mobility shift on sodium dodecyl sulfate gels) were increased in oocytes following progesterone stimulation. The increase in Raf-1 activity was concurrent with an elevation in the activity of mitogen-activated protein (MAP) kinase. When RNA encoding an oncogenic form of Raf-1 (v-Raf) was injected into immature oocytes, MAP kinase mobility shift, germinal vesicle breakdown, and histone H1 phosphorylation increased markedly. When RNA encoding a dominant-negative version of Raf-1 was injected, progesterone-induced oocyte maturation was blocked. When RNA encoding Xenopus mos (mosxe) was injected into oocytes, Raf-1 and MAP kinase mobility shifts were observed after several hours. Also, when antisense mosxe oligonucleotides were injected into oocytes, progesterone-induced Raf-1 and MAP kinase mobility shifts were blocked. Finally, when antisense mosxe oligonucleotides were coinjected with v-Raf RNA into oocytes, histone H1 kinase activation, germinal vesicle breakdown, and MAP kinase mobility shift occurred. These findings suggest that Raf-1 activity is required for progesterone-induced oocyte maturation and that Raf-1 is downstream of mosxe activity.

Mos stimulates MAP kinase in Xenopus oocytes and activates a MAP kinase kinase in vitro

Several protein kinases, including Mos, maturation-promoting factor (MPF), mitogen-activated protein (MAP) kinase, and MAP kinase kinase (MAPKK), are activated when Xenopus oocytes enter meiosis. De novo synthesis of the Mos protein is required for progesterone-induced meiotic maturation. Recently, bacterially synthesized maltose-binding protein (MBP)-Mos fusion protein was shown to be sufficient to initiate meiosis I and MPF activation in fully grown oocytes in the absence of protein synthesis. Here we show that MAP kinase is rapidly phosphorylated and activated following injection of wild-type, but not kinase-inactive mutant, MBP-Mos into fully grown oocytes. MAP kinase activation by MBP-Mos occurs within 20 min, much more rapidly than in progesterone-treated oocytes. The MBP-Mos fusion protein also activates MPF, but MPF activation does not occur until approximately 2 h after injection. Extracts from oocytes injected with wild-type but not kinase-inactive MBP-Mos contain an activity that can phosphorylate MAP kinase, suggesting that Mos directly or indirectly activates a MAPKK. Furthermore, activated MBP-Mos fusion protein is able to phosphorylate and activate a purified, phosphatase-treated, rabbit muscle MAPKK in vitro. Thus, in oocytes, Mos is an upstream activator of MAP kinase which may function through direct phosphorylation of MAPKK.

Mos stimulates MAP kinase in Xenopus oocytes and activates a MAP kinase kinase in vitro

Several protein kinases, including Mos, maturation-promoting factor (MPF), mitogen-activated protein (MAP) kinase, and MAP kinase kinase (MAPKK), are activated when Xenopus oocytes enter meiosis. De novo synthesis of the Mos protein is required for progesterone-induced meiotic maturation. Recently, bacterially synthesized maltose-binding protein (MBP)-Mos fusion protein was shown to be sufficient to initiate meiosis I and MPF activation in fully grown oocytes in the absence of protein synthesis. Here we show that MAP kinase is rapidly phosphorylated and activated following injection of wild-type, but not kinase-inactive mutant, MBP-Mos into fully grown oocytes. MAP kinase activation by MBP-Mos occurs within 20 min, much more rapidly than in progesterone-treated oocytes. The MBP-Mos fusion protein also activates MPF, but MPF activation does not occur until approximately 2 h after injection. Extracts from oocytes injected with wild-type but not kinase-inactive MBP-Mos contain an activity that can phosphorylate MAP kinase, suggesting that Mos directly or indirectly activates a MAPKK. Furthermore, activated MBP-Mos fusion protein is able to phosphorylate and activate a purified, phosphatase-treated, rabbit muscle MAPKK in vitro. Thus, in oocytes, Mos is an upstream activator of MAP kinase which may function through direct phosphorylation of MAPKK.

Regulatory mechanisms in meiosis

Meiosis can be viewed both as a process of cell differentiation and as a modification of the mitotic cell cycle. Here we describe recent progress in defining a variety of regulatory mechanisms that govern the meiotic divisions. Studies in the yeast Saccharomyces cerevisiae and in higher organisms have led to complementary insights into these controls.

Cyclin synthesis: who needs it?

Studies of the G2 to M transition in amphibian oocytes, in combination with in vitro mitotic systems and yeast genetic analysis, have significantly contributed to our understanding of the mechanisms by which M-phase is regulated. Historically, oocyte maturation has provided a number of valuable initial observations, but the biochemical elucidation of cell cycle control mechanisms has proved more tractable in cell-free extracts of frog eggs which reproduce aspects of early embryogenic mitosis. Recent experiments examining the importance of protein synthesis in the maturing oocyte have highlighted some important differences between mitosis and meiosis. Additional controls found in meiosis but not embryonic mitosis, are similar to controls found in somatic cells. This suggests that understanding the differences, as well as the similarities, between meiosis in the oocyte and mitosis in the early embryo will help us to learn more about the way in which cells enter and leave mitosis.

Current concepts of cell-cycle regulation and its relationship to oocyte maturation, fertilization and embryo development

Genetic and biochemical approaches have contributed to an explosion of literature on cell-cycle control. Regulation of the cell-cycle is controlled by a series of kinases and phosphatases. Key control points are during the G(1)-S and G(2)-M transitions. During both transitions, cyclins interact with a specific kinase to allow a cell to pass through that phase. The meiotic maturation of oocytes, fertilization and embryo development are all events influenced by cell-cycle regulation. Understanding cell-cycle control should provide new ways for gamete and embryo biologists to approach culture and development problems.