Fission yeast p13 blocks mitotic activation and tyrosine dephosphorylation of the Xenopus cdc2 protein kinase

Intrinsic fluorescence properties and structural analysis of p13(suc1) from Schizosaccharomyces pombe

p13(suc1) acts in the fission yeast cell division cycle as a component of p34(cdc2). In the present work, structural information contained in the intrinsic fluorescence of p13(suc1) has been extracted by steady-state and time-resolved fluorescence techniques. In its native form, the steady-state emission spectrum of p13(suc1) is centered at 336 nm. Upon denaturation by guanidine HCl (4.0 M), the emission spectrum is shifted to 355-360 nm and the fluorescence intensity decreases 70%. The same changes are not obtained with p13(suc1) at 56 degrees C or after incubation at 100 degrees C, and the protein appears to be substantially temperature-stable. The fluorescence decay of p13(suc1) is best described by three discrete lifetimes of 0.6 ns (tau1), 2.9 ns (tau2), and 6.1 ns (tau3), with amplitudes that are dependent on the native or unfolded state of the protein. Under native conditions, the two predominant decay-associated spectra, DAS-tau2 (lambdamax = 332 nm) and DAS-tau3 (lambdamax = 340 nm), derive from two different excitation DAS. Moreover distinct quenching mechanisms and collisional accessibilities (kq(tau2)>>kq(tau3)) are resolved for each lifetime. An interpretation in terms of specific tryptophan residue (or protein conformer)-lifetime assignments is presented. The decay of the fluorescence anisotropy of native p13(suc1) is best described by a double exponential decay. The longer correlation time recovered (9 ns </= phi2 </= 15ns) can be associated with the rotational motion of the protein as a whole and a Stokes radius of 21.2 A has been calculated for p13(suc1). Anisotropy measurements obtained as a function of temperature indicate that, in solution, the protein exists exclusively as a prolate monomer. In 1 mM zinc, changes of the anisotropy decay parameters are compatible with subunits oligomerization.

Regulation of the acquisition of meiotic competence in the mouse: changes in the subcellular localization of cdc2, cyclin B1, cdc25C and wee1, and in the concentration of these proteins and their transcripts

During their development, mammalian oocytes acquire the ability to resume meiosis. We demonstrate that the concentration of p34cdc2 increases during the acquisition of meiotic competence, as determined by immunoblotting, whereas the concentration of cyclin B1 decreases. Laser-scanning confocal microscopy corroborated these changes and furthermore indicate that an increase occurs in the nuclear concentration of each protein. Results of immunoblotting experiments demonstrate that associated with the acquisition of meiotic competence is an increase in the concentration of cdc25C, an activator of p34cdc2/cyclin B kinase, and a decrease in wee1, an inhibitor of cdc2/cyclin B kinase. These changes were again corroborated by laser-scanning confocal microscopy, which also indicates that an increase in the nuclear concentration of wee1 occurs. The concentration of the transcripts encoding these proteins, however, is essentially similar in meiotically incompetent and competent oocytes. Thus, these changes in protein concentration that occur during oocyte development likely reflect changes in the translational efficiency of their mRNAs. Consistent with this is that the relative rate of synthesis of p34cdc2 in meiotically competent oocytes is approximately 3 times greater than that in meiotically incompetent oocytes, whereas the stability of newly synthesized p34cdc2 is essentially the same in each cell type.

Localization and function of tyrosine-phosphorylated protein in pig oocytes

Several proteins with phosphorylated tyrosine residues have been shown to be closely involved in the control meiotic nuclear division. We identified a 42-kD protein in pig oocytes, using a polyclonal antibody to a synthetic phosphotyrosine construct that increases significantly in amount after 12 hr of maturation culture, and is discretely localized to condensing and condensed chromosomes. However, since microinjection of the antibody into oocytes blocks spindle formation, the role of this protein appears to be at that stage rather than directly in chromosome condensation. Specificity of action of the 42-kD protein indicates that it may be a phosphorylation-dependent component necessary for successful spindle assembly.

Xe-p9, a Xenopus Suc1/Cks homolog, has multiple essential roles in cell cycle control

The small Suc1/Cks protein is a ubiquitous subunit of Cdk/cyclin complexes, but its precise function has remained unclear. We have isolated a Xenopus homolog, Xe-p9, of the Suc1/Cks protein by virtue of its ability to rescue a fission yeast mutant that enters mitosis prematurely. To assess its functional role in cell cycle control, we have both overexpressed p9 in Xenopus egg extracts and immunodepleted the protein from these extracts. We found that addition of recombinant His6-p9 to egg extracts results in a pronounced delay of mitosis that can be attributed to an inhibition of the tyrosine dephosphorylation of the inactive Cdc2/cyclin B complex. In immunodepletion studies, we observed that the consequences of removing p9 from egg extracts depend on the stage of the cell cycle. Specifically, in the case of interphase extracts, the removal of p9 abolishes the entry into mitosis as a result of a failure in the activation of the Cdc2/cyclin B complex by tyrosine dephosphorylation. Furthermore, mitotic extracts lacking p9 fail to exit mitosis because of a defect in the destruction of cyclin B. Collectively, these results indicate that p9 has multiple essential roles in the cell cycle by governing the interaction of the Cdc2/cyclin B complex with both positive and negative regulators.

Leishmania mexicana p12cks1, a homologue of fission yeast p13suc1, associates with a stage-regulated histone H1 kinase

We have isolated a Leishmania mexicana homologue of the fission yeast suc1 gene using PCR with oligonucleotides designed to conserved regions of cdc2 kinase subunits (cks). The product of cks1 is a 12 kDa polypeptide, which has 70% identity with human p9cks1 and 44% identity with fission yeast p13suc1.p12cks1 was detected in the three life-cycle stages of L. mexicana by immunoblotting. Recombinant p12cks1 (p12cks1his) bound to agarose beads was used as a matrix to affinity-select histone H1 kinase complexes from Leishmania, yeast and bovine extracts. Immunoblotting showed that yeast and bovine cdc2 kinase bound to p12cks1his, thus demonstrating functional homology between L. mexicana p12cks1 and yeast p13suc1. Histone H1 kinase activity was found at a high level in the proliferative promastigote and amastigote forms of L. mexicana, but at a low level in the non-dividing metacyclic form. These activities are likely to be the same as the leishmanial p13suc1 binding kinase (SBCRK) described previously [Mottram, Kinnaird, Shiels, Tait and Barry (1993) J. Biol. Chem. 268, 21044-21051]. A distinct cdc2-related kinase, L. mexicana CRK1, was also found to associate with p12cks1his but affinity-depletion experiments showed that CRK1 was not responsible for the histone H1 kinase activity associating with p12cks1his in promastigote cell extracts. The finding that p12cks1 associates with at least two cdc2-related kinases, SBCRK and CRK1, is consistent with the presence of a large gene family of cdc2-related kinases in trypanosomatids, a situation thought to be more similar to higher eukaryotes than yeast.

A novel MAP kinase phosphatase is localised in the branchial arch region and tail tip of Xenopus embryos and is inducible by retinoic acid

Using a differential display strategy, we have isolated a cDNA corresponding to a mRNA which is induced by retinoic acid treatment of late gastrula Xenopus embryos, and much more strongly induced by retinoic acid and cycloheximide. The cDNA, designated X17C, encodes a novel mitogen-activated protein (MAP) kinase phosphatase of 378 amino acid residues which is only distantly related to other known MAP kinase phosphatases. In normal embryogenesis, the X17C mRNA is expressed after the midblastula transition and accumulates during gastrulation. In neurula and tailbud stage embryos the mRNA is localised in two domains, one in the anterior region of the embryo, and one at the tail tip. When expressed from synthetic mRNA injected into oocytes, the X17C protein is found within the cytosolic fraction and not in the nucleus. The X17C protein dephosphorylates and inactivates Xenopus MAP kinase in oocytes stimulated to undergo maturation by progesterone. We indicate the application of X17C as a tool for interfering with MAP kinase signaling in somatic cells of embryos, using FGF receptor-mediated MAP kinase activation in animal cap explants.

Crystal structure and mutational analysis of the human CDK2 kinase complex with cell cycle-regulatory protein CksHs1

The 2.6 Angstrom crystal structure for human cyclin-dependent kinase 2(CDK2) in complex with CksHs1, a human homolog of essential yeast cell cycle-regulatory proteins suc1 and Cks1, reveals that CksHs1 binds via all four beta strands to the kinase C-terminal lobe. This interface is biologically critical, based upon mutational analysis, but far from the CDK2 N-terminal lobe, cyclin, and regulatory phosphorylation sites. CDK2 binds the Cks single domain conformation and interacts with conserved hydrophobic residues plus His-60 and Glu-63 in their closed beta-hinge motif conformation. The beta hinge opening to form the Cks beta-interchanged dimer sterically precludes CDK2 binding, providing a possible mechanism regulating CDK2-Cks interactions. One face of the complex exposes the sequence-conserved phosphate-binding region on Cks and the ATP-binding site on CDK2, suggesting that CKs may target CDK2 to other phosphoproteins during the cell cycle.

Crystal structure of the yeast cell-cycle control protein, p13suc1, in a strand-exchanged dimer

BACKGROUND

p13(suc1) from fission yeast is a member of the CDC28 kinase specific (CKS) class of cell-cycle control proteins, that includes CKS1 from budding yeast and the human homologues CksHs1 and CksHs2. p13(suc1) participates in the regulation of p34(cdc2), a cyclin-dependent kinase controlling the G1-S and the G2-M transitions of the cell cycle. The CKS proteins are believed to exert their regulatory activity by binding to the kinase, in which case their function may be governed by their conformation or oligomerization state. Previously determined X-ray structures of p13(suc1), CksHs1 and CksHs2 show that these proteins share a common fold but adopt different oligomeric states. Monomeric forms of p13(suc1) and CksHs1 have been solved. In addition, CksHs2 and p13(suc1) have been observed by X-ray crystallography in assemblies of strand-exchanged dimers. Analysis of various assemblies of the CKS proteins, as found in different crystal forms, should help to clarify their role in cell-cycle control.

RESULTS

We report the X-ray crystal structure of p13(suc1) to 1.95 A resolution in space group C2221. It is present in the crystals as a strand-exchanged dimer. The overall monomeric fold is preserved in each lobe of the dimer but a single beta-strand (Ile94-Asp102) is exchanged between the central beta-sheets of each molecule.

CONCLUSIONS

Strand exchange, which has been observed for p13(suc1) in two different space groups, and for CksHs2, is now confirmed to be an intrinsic feature of the CKS family. A switch between levels of assembly may serve to coordinate the function of the CKS proteins in cell-cycle control.

A developmental timer regulates degradation of cyclin E1 at the midblastula transition during Xenopus embryogenesis

We have analyzed cyclin E1, a protein that is essential for the G1/S transition, during early development in Xenopus embryos. Cyclin E1 was found to be abundant in eggs, and after fertilization, until the midblastula transition (MBT) when levels of cyclin E1 protein, and associated kinase activity, were found to decline precipitously. Our results suggest that the reduced level of the cyclin E1 protein detected after the MBT does not occur indirectly as a result of degradation of the maternally encoded cyclin E1 mRNA. Instead, the stability of cyclin E1 protein appears to play a major role in reduction of cyclin E1 levels at this time. Cyclin E1 protein was found to be stable during the cleavage divisions but degraded with a much shorter half-life after the MBT. Activation of cyclin E1 protein turnover occurs independent of cell cycle progression, does not require ongoing protein synthesis, and is not triggered as a result of the ratio of nuclei to cytoplasm in embryonic cells that initiates the MBT. We therefore propose that a developmental timing mechanism measures an approximately 5-hr time period, from the time of fertilization, and then allows activation of a protein degradative pathway that regulates cyclin E1. Characterization of the timer suggests that it might be held inactive in eggs by a mitogen-activated protein kinase signal transduction pathway.

Cell cycle control by Xenopus p28Kix1, a developmentally regulated inhibitor of cyclin-dependent kinases

We have isolated Xenopus p28Kix1, a member of the p21CIP1/p27KIP1/p57KIP2 family of cyclin-dependent kinase (Cdk) inhibitors. Members of this family negatively regulate cell cycle progression in mammalian cells by inhibiting the activities of Cdks. p28 shows significant sequence homology with p21, p27, and p57 in its N-terminal region, where the Cdk inhibition domain is known to reside. In contrast, the C-terminal domain of p28 is distinct from that of p21, p27, and p57. In co-immunoprecipitation experiments, p28 was found to be associated with Cdk2, cyclin E, and cyclin A, but not the Cdc2/cyclin B complex in Xenopus egg extracts. Xenopus p28 associates with the proliferating cell nuclear antigen, but with a substantially lower affinity than human p21. In kinase assays with recombinant Cdks, p28 inhibits pre-activated Cdk2/cyclin E and Cdk2/cyclin A, but not Cdc2/cyclin B. However, at high concentrations, p28 does prevent the activation of Cdc2/cyclin B by the Cdk-activating kinase. Consistent with the role of p28 as a Cdk inhibitor, recombinant p28 elicits an inhibition of both DNA replication and mitosis upon addition to egg extracts, indicating that it can regulate multiple cell cycle transitions. The level of p28 protein shows a dramatic developmental profile: it is low in Xenopus oocytes, eggs, and embryos up to stage 11, but increases approximately 100-fold between stages 12 and 13, and remains high thereafter. The induction of p28 expression temporally coincides with late gastrulation. Thus, although p28 may play only a limited role during the early embryonic cleavages, it may function later in development to establish a somatic type of cell cycle. Taken together, our results indicate that Xenopus p28 is a new member of the p21/p27/p57 class of Cdk inhibitors, and that it may play a role in developmental processes.

An inhibitor of p34cdc2/cyclin B that regulates the G2/M transition in Xenopus extracts

The activity of maturation-promoting factor (MPF), a protein kinase complex composed of p34cdc2 and cyclin B, is undetectable during interphase but rises abruptly at the G2/M transition to induce mitosis. After the synthesis of cyclin B, the suppression of MPF activity before mitosis has been attributed to the phosphorylation of p34cdc2 on sites (threonine-14 and tyrosine-15) that inhibit its catalytic activity. We previously showed that the activity of the mitotic p34cdc2/cyclin B complex is rapidly suppressed when added to interphase Xenopus extracts that lack endogenous cyclin B. Here we show that a mutant of p34cdc2 that cannot be inhibited by phosphorylation (threonine-14-->alanine, tyrosine-15-->phenylalanine) is also susceptible to inactivation, demonstrating that inhibitory mechanisms independent of threonine-14 and tyrosine-15 phosphorylation must exist. We have partially characterized this inhibitory pathway as one involving a reversible binding inhibitor of p34cdc2/cyclin B that is tightly associated with cell membranes. Kinetic analysis suggests that this inhibitor, in conjunction with the kinases that mediate the inhibitory phosphorylations on p34cdc2, maintains the interphase state in Xenopus; it may play an important role in the exact timing of the G2/M transition.

A model for a bistable biochemical trigger of mitosis

The activation of maturation promoting factor (MPF, cyclin B/Cdc2), which starts mitosis, is modeled as a bistable biochemical switch or trigger. A small, slow parameter change can cause an abrupt transition by a saddle-node bifurcation from a stable steady state of low activity to one of high activity. The switch is not reversed if the parameter change is reversed (hysteresis). The dynamical features necessary for this triggering action are the presence of two stable steady states (low-activity and high-activity), and one unstable steady state. The key biochemical kinetic features of the model are (1) mutual activation by MPF and Cdc25, which makes the activation of MPF effectively autocatalytic, and (2) binding of MPF by Suc1, which inhibits MPF autocatalysis and stabilizes the low-activity steady state until the amount of MPF begins to approach or exceed stoichiometrically the amount of Suc1, then allows strong autocatalysis and full activation. The special virtues of bistable triggering, and the general types of biochemical mechanism which can produce it, are discussed.

Crystal structure of the cell cycle-regulatory protein suc1 reveals a beta-hinge conformational switch

The Schizosaccharomyces pombe cell cycle-regulatory protein suc1, named as the suppressor of cdc2 temperature-sensitive mutations, is essential for cell cycle progression. To understand suc1 structure-function relationships and to help resolve conflicting interpretations of suc1 function based on genetic studies of suc1 and its functional homologs in both lower and higher eukaryotes, we have determined the crystal structure of the beta-interchanged suc1 dimer. Each domain consists of three alpha-helices and a four-stranded beta-sheet, completed by the interchange of terminal beta-strands between the two subunits. This beta-interchanged suc1 dimer, when compared with the beta-hairpin single-domain folds of suc1, reveals a beta-hinge motif formed by the conserved amino acid sequence HVPEPH. This beta-hinge mediates the subunit conformation and assembly of suc1: closing produces the intrasubunit beta-hairpin and single-domain fold, whereas opening leads to the intersubunit beta-strand interchange and interlocked dimer assembly reported here. This conformational switch markedly changes the surface accessibility of sequence-conserved residues available for recognition of cyclin-dependent kinase, suggesting a structural mechanism for beta-hinge-mediated regulation of suc1 biological function. Thus, suc1 belongs to the family of domain-swapping proteins, consisting of intertwined and dimeric protein structures in which the dual assembly modes regulate their function.

The protein kinase from mitotic human cells that phosphorylates Ser-209 on the casein kinase II beta-subunit is p34cdc2

Casein kinase II is a highly conserved enzyme that is essential for viability. In cells, the casein kinase II beta-subunit is phosphorylated at an autophosphorylation site and at a site (Ser-209) that is maximally phosphorylated in mitotic cells. To identify protein kinase activities that phosphorylate Ser-209, we fractionated extracts from mitosis-arrested human Burkitt lymphoma MANCA cells. A single Ser-209 kinase activity was detected following each fractionation step. The Ser-209 kinase was purified to a specific activity of approx. 250 nmol/min per mg and efficiently phosphorylated histone H1, a synthetic peptide containing Ser-209 (Ser-209 peptide), myelin basic protein and casein. Immunoblot analysis demonstrated that all fractions containing Ser-209 kinase activity contained p34cdc2. Furthermore, depletion of the Ser-209 kinase activity with p13suc1-Sepharose and anti-p34cdc2 antiserum demonstrated conclusively that the isolated Ser-209 kinase is p34cdc2. These studies provide strong biochemical evidence that p34cdc2 is the enzyme that phosphorylates Ser-209 on the beta-subunit of CKII in mitotic cells. In addition, these results indicate that the Ser-209 peptide can be utilized as a specific reagent for the assay of p34cdc2 activity in mitotic extracts, since no other Ser-209 peptide kinase activities were detected.

Myt1: a membrane-associated inhibitory kinase that phosphorylates Cdc2 on both threonine-14 and tyrosine-15

Cdc2 is the cyclin-dependent kinase that controls entry of cells into mitosis. Phosphorylation of Cdc2 on threonine-14 and tyrosine-15 inhibits the activity of the enzyme and prevents premature initiation of mitosis. Although Wee1 has been identified as the kinase that phosphorylates tyrosine-15 in various organisms, the threonine-14-specific kinase has not been isolated. A complementary DNA was cloned from Xenopus that encodes Myt1, a member of the Wee1 family that was discovered to phosphorylate Cdc2 efficiently on both threonine-14 and tyrosine-15. Myt1 is a membrane-associated protein that contains a putative transmembrane segment. Immunodepletion studies suggested that Myt1 is the predominant threonine-14-specific kinase in Xenopus egg extracts. Myt1 activity is highly regulated during the cell cycle, suggesting that this relative of Wee1 plays a role in mitotic control.

Drosophila Wee1 kinase rescues fission yeast from mitotic catastrophe and phosphorylates Drosophila Cdc2 in vitro

Cdc2 kinase activity is required for triggering entry into mitosis in all known eukaryotes. Elaborate mechanisms have evolved for regulating Cdc2 activity so that mitosis occurs in a timely manner, when preparations for its execution are complete. In Schizosaccharomyces pombe, Wee1 and a related Mik1 kinase are Cdc2-inhibitory kinases that are required for preventing premature activation of the mitotic program. To identify Cdc2-inhibitory kinases in Drosophila, we screened for cDNA clones that rescue S. pombe wee1- mik1- mutants from lethal mitotic catastrophe. One of the genes identified in this screen, Drosophila wee1 (Dwee1), encodes a new Wee1 homologue. Dwee1 kinase is closely related to human and Xenopus Wee1 homologues, and can inhibit Cdc2 activity by phosphorylating a critical tyrosine residue. Dwee1 mRNA is maternally provided to embryos, and is zygotically expressed during the postblastoderm divisions of embryogenesis. Expression remains high in the proliferating cells of the central nervous system well after cells in the rest of the embryo have ceased dividing. The loss of zygotically expressed Dwee1 does not lead to mitotic catastrophe during postblastoderm cycles 14 to 16. This result may indicate that maternally provided Dwee1 is sufficient for regulating Cdc2 during embryogenesis, or it may reflect the presence of a redundant Cdc2 inhibitory kinase, as in fission yeast.

Effects of different kinases and phosphatases on nuclear and cytoplasmic maturation of bovine oocytes

Phosphorylation is considered as a common post-translational modification implicated in the control of various key enzymes. In somatic and germinal cells, important regulators of the cell cycle are controlled by their phosphorylation status, and some act as kinases or phosphatases themselves. Bovine oocytes are blocked in the germinal vesicle (GV) stage until either an LH surge occurs or until oocytes are released from the inhibitory influence of the follicle. Meiotic resumption in vitro is therefore an excellent model for the study of phosphorylation events that occur in the G2/M transition, a control point of the cellular cycle. To better understand this transition, we have modulated, either directly or indirectly, kinases using known effectors (epidermal growth factor, EGF; isobutylmethylxanthine-forskolin, Bx-Fk; 6-dimethylaminopurine, 6-DMAP) or phosphatases (okadaic acid, OA) or cycloheximide, which is known to inhibit maturation through protein synthesis suppression. With this procedure, influence on meiotic resumption and phosphoprotein patterns was verified. Both EGF and OA accelerated nuclear maturation after 9 hr of culture. Only 23% (n = 140) and 9% (n = 111) of oocytes were still at GV stage with EGF and OA, respectively, compared to 41% (n = 105) of control oocytes. The different treatments changed the protein patterns in oocytes. In cumulus cells, the patterns were especially modified by the OA treatment. Characteristic changes that occur in germ cells were also identified. Nuclear maturation was inhibited by modulators of kinase (6-DMAP, GV = 74%, n = 126; cAMP dependent protein kinase (PKA) stimulators, Bx-Fk, GV = 71%, n = 129) likewise, phosphoprotein patterns were affected, especially in oocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

Assembly/disassembly of the nuclear envelope membrane. Characterization of the membrane-chromatin interaction using partially purified regulatory enzymes

Assembly and disassembly of the nucleus at mitosis in eukaryotes involves the reversible interaction of chromatin with the nuclear membrane. Previously we have shown that this interaction is regulated by the antagonistic activities of a kinase and a phosphatase. The kinase promotes membrane release while the phosphatase stimulates binding. In this report we describe four steps in the purification of the kinase needed for release of membranes from chromatin. We also show that the release kinase and the mitotic initiation kinase, cdc2, are distinct and are separated from each other during the second purification step. Reconstitution experiments using these two kinases demonstrate that the release kinase and cdc2 kinase work in concert to cause membrane release from chromatin. In phosphorylation experiments, protein targets that are substrates for the regulatory release kinase are identified on the membranes. These phosphorylated proteins ae candidates for regulated proteins mediating membrane-chromatin interaction. Finally, we find that membrane release activity can also be extracted from membranes by high salt treatment, indicating a possible dual localization of this activity.

Active cyclin B-cdc2 kinase does not inhibit DNA replication and cannot drive prematurely fertilized sea urchin eggs into mitosis

Feedback mechanisms preventing M phase occurrence before S phase completion are assumed to depend on inhibition of cyclin B-cdc2 kinase activation by unreplicated DNA. In sea urchin, fertilization stimulates protein synthesis and releases eggs from G1 arrest. We found that in the one-cell sea urchin embryo cyclin B-cdc2 kinase undergoes partial activation before S phase, reaching in S phase a level that is sufficient for G2-M phase transition. S phase entry is not inhibited by this level of cyclin B-dependent kinase activity. Inhibition of DNA replication by aphidicolin suppresses nuclear envelope breakdown, yet it does not prevent the microtubule array from being converted from its interphasic to its mitotic state. Moreover, mitotic cytoplasmic events occur at the same time in control and aphidicolin-treated embryos. Thus unreplicated DNA only prevents mitotic nuclear, not cytoplasmic, events from occurring prematurely. These results together show that the inhibition of cyclin B-cdc2 kinase activation is probably not the only mechanism that prevents mitotic nuclear events from occurring as long as DNA replication has not been completed. In contrast, cytoplasmic mitotic events seem to be controlled by a timing mechanism independent of DNA replication, set up at fertilization, that prevents premature opening of a window for mitotic events.

Cyclins and cyclin-dependent kinases: a biochemical view

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Identification of a developmental timer regulating the stability of embryonic cyclin A and a new somatic A-type cyclin at gastrulation

We have identified a second Xenopus cyclin A, called cyclin A2. Cyclin A2 is a 46.6-kD protein that shows a greater homology to human cyclin A than to the previously identified Xenopus cyclin A1. It is present throughout embryonic development (up to stage 46 at least) and is found in adult tissues as well as in Xenopus tissue culture cell lines. In contrast, cyclin A1 is present in eggs and early embryos but cannot be detected in late embryos or in tissue culture cells. We have found that the maternally stored pools of mRNAs encoding both of these cyclin A proteins are stable until the onset of gastrulation and then are degraded abruptly. At this time, new transcription replaces cyclin A2 mRNA. Interestingly, we have also observed a dramatic change in the stability of the cyclin A proteins at this time. Prior to the onset of gastrulation, cyclin A1 protein is stable during interphase of the cell cycle. At gastrulation, however, both A1 and A2 proteins turn over rapidly during interphase of the cell cycle. Together, these results indicate that developmental programs controlling cyclin A protein and mRNA stability are activated at gastrulation. We have shown that this program is independent of new transcription beginning at the mid-blastula transition. Furthermore, treatment of early stage embryos with cycloheximide demonstrates that activation of this degradative program is independent of cell division and translation. Collectively, our observations suggest that a previously uncharacterized timing mechanism activates new degradative pathways at the onset of gastrulation, which could play an essential role in releasing cells from maternal programming.

Raf1 interaction with Cdc25 phosphatase ties mitogenic signal transduction to cell cycle activation

The Ras and Raf1 proto-oncogenes transduce extracellular signals that promote cell growth. Cdc25 phosphatases activate the cell division cycle by dephosphorylation of critical threonine and tyrosine residues within the cyclin-dependent kinases. We show here that Cdc25 phosphatase associates with raf1 in somatic mammalian cells and in meiotic frog oocytes. Furthermore, Cdc25 phosphatase can be activated in vitro in a Raf1-dependent manner. We suggest that activation of the cell cycle by the Ras/Raf1 pathways might be mediated in part by Cdc25.

Oligomerization state in solution of the cell cycle regulators p13suc1 from the fission yeast and p9cksphy from the myxomycete Physarum, two members of the cks family

The cks proteins (for cdc2 kinase subunit) are essential cell cycle regulators. They interact strongly with the mitotic cdc2 kinase, but the mechanism and the biological function of this association still await understanding. The oligomerization state in solution of two members of this ubiquitous protein family, the suc1 gene product from the fission yeast and the newly cloned cksphy gene product from the myxomycete Physarum, was investigated by small-angle X-ray scattering (SAXS) and biochemical methods. We found that the major molecular species are monodispersed monomeric proteins. Minor amounts of dimeric suc1 proteins were also found, but no equilibrium between the two forms was observed and surprisingly, the hexameric assemblies observed in the crystal structure of the human ckshs2 homolog were not detected. These apparent discrepancies between proteins that display cross-complementation address the question of the control of the cks oligomerization process and its link to the biological function.

The cell cycle and suc1: from structure to function?

Structures have recently been determined for the yeast Schizosaccharomyces pombe cell cycle regulatory protein, CKS/suc1, and its human equivalent. The structures provide some long-awaited clues about the role of CKS/suc1 in cell cycle control.

p13suc1 of Schizosaccharomyces pombe regulates two distinct forms of the mitotic cdc2 kinase

suc1 is an essential gene initially identified for its ability to rescue certain temperature-sensitive alleles of cdc2 in Schizosaccharomyces pombe. The role of suc1 in the regulation of the cdc2 kinase is not well understood. In our study, we have characterized the biochemical effect of loss of suc1 function on specific cdc2-cyclin complexes. We show that the cig1 cyclin is associated with cdc2 and that the cdc2-cig1 kinase is activated at mitosis, with kinetics similar to those of the cdc2-cdc13 kinase. We provide evidence that loss of suc1 function affects the kinase activity of the two distinct mitotic forms of the cdc2 kinase. We also show that a dramatic increase in the level of the cdc13 protein is associated with loss of suc1. These results suggest that mitosis cannot be properly completed in the absence of suc1, possibly because of an increase in the level of cdc2-cdc13 complex, and support the idea of a role for suc1 in the regulation of multiple forms of the cdc2 kinase.

Morphological effects of caffeine, okadaic acid and genistein in one-cell mouse embryos blocked in G2 by X-irradiation

One-cell mouse embryos of the Balb/c strain normally divide at 18.5 h p.c. (post conception), but they suffer an extremely long G2 arrest when irradiated with 2 Gy X-rays 8 h p.c. at the early pronuclear stage. This could be an indirect effect of radiation on tyrosine dephosphorylation of the p34cdc2 subunit of a maturation or mitosis promoting factor (MPF), which normally occurs at the end of G2. This, in turn, would maintain MPF in an inactivated form and block entry into mitosis. Preliminary studies were undertaken at the morphological level to assess indirectly the validity of this hypothesis. For this purpose, irradiated and control embryos were exposed to different compounds, which are known to interfere, directly or indirectly, with the state of phosphorylation/dephosphorylation of p34cdc2. Caffeine (CAF; 2 mM) did not affect the time of first division of control embryos, but it completely suppressed the radiation-induced G2 arrest of embryos exposed to this compound from 17 h p.c., i.e. 1.5 h before the normal time of first cleavage. Under the same conditions, okadaic acid (OA; 3 microM), a specific inhibitor of phosphatases I and IIA, induced a rapid pronuclear membrane breakdown and a block of all control and irradiated embryos at metaphase. Genistein (GEN; 92 or 185 microM). A potent inhibitor of tyrosine kinases, increased the radiation-induced G2 arrest and even induced a dose-dependent G2 arrest in the control embryos. Embryos were exposed at different times following irradiation to a mixture of either CAF (2 or 5 mM) or OA (3 or 10 microM), and cycloheximide (CH; 5 micrograms/ml), a potent protein synthesis inhibitor. Reversion of G2-arrest by CAF was still seen in embryos exposed to CAF+CH from 17 h p.c. However, the proportion of irradiated embryos eventually able to cleave was lower than that obtained under the conditions of exposure to CAF alone. Embryos exposed to CAF+CH before 17 h p.c. were not able to cleave, regardless of the concentration of CAF used. Nuclear envelope breakdown still occurred in 100% control and irradiated embryos, following exposure to 3 microM OA+CH from 10 h p.c., or to 10 microM OA+CH from 8.5 p.c.(ABSTRACT TRUNCATED AT 400 WORDS)

Control of the Cdc2/cyclin B complex in Xenopus egg extracts arrested at a G2/M checkpoint with DNA synthesis inhibitors

Proliferating eukaryotic cells possess checkpoint mechanisms that block cell division in the presence of unreplicated or damaged DNA. Using cell-free extracts from Xenopus eggs, we have investigated the mechanisms underlying the inability of a recombinant Cdc2/cyclin B complex to induce mitosis in the presence of incompletely replicated DNA. We found that the activities of the kinases and phosphatases that regulate the major phosphorylation sites on Cdc2 (e.g., tyrosine 15, threonine 14, and threonine 161) are not altered significantly under conditions where Xenopus extracts remain stably arrested in interphase due to the presence of the replication inhibitor aphidicolin. However, at threshold concentrations, a Cdc2/cyclin B complex containing a mutant Cdc2 subunit that cannot be phosphorylated on either tyrosine 15 or threonine 14 displays a markedly reduced capacity to induce mitosis in the presence of aphidicolin. This observation indicates that the replication checkpoint in Xenopus egg extracts functions without the inhibitory tyrosine and threonine phosphorylation of Cdc2. We provide evidence that the checkpoint-dependent suppression of the Cdc2/cyclin B complex involves a titratable inhibitor that is regulated by the presence of unreplicated DNA.

Cell cycle regulation of a Xenopus Wee1-like kinase

Using a polymerase chain reaction-based strategy, we have isolated a gene encoding a Wee1-like kinase from Xenopus eggs. The recombinant Xenopus Wee1 protein efficiently phosphorylates Cdc2 exclusively on Tyr-15 in a cyclin-dependent manner. The addition of exogenous Wee1 protein to Xenopus cell cycle extracts results in a dose-dependent delay of mitotic initiation that is accompanied by enhanced tyrosine phosphorylation of Cdc2. The activity of the Wee1 protein is highly regulated during the cell cycle: the interphase, underphosphorylated form of Wee1 (68 kDa) phosphorylates Cdc2 very efficiently, whereas the mitotic, hyperphosphorylated version (75 kDa) is weakly active as a Cdc2-specific tyrosine kinase. The down-modulation of Wee1 at mitosis is directly attributable to phosphorylation, since dephosphorylation with protein phosphatase 2A restores its kinase activity. During interphase, the activity of this Wee1 homolog does not vary in response to the presence of unreplicated DNA. The mitosis-specific phosphorylation of Wee1 is due to at least two distinct kinases: the Cdc2 protein and another activity (kinase X) that may correspond to an MPM-2 epitope kinase. These studies indicate that the down-regulation of Wee1-like kinase activity at mitosis is a multistep process that occurs after other biochemical reactions have signaled the successful completion of S phase.

Cell cycle regulation of the p34cdc2 inhibitory kinases

In cells of higher eukaryotic organisms the activity of the p34cdc2/cyclin B complex is inhibited by phosphorylation of p34cdc2 at two sites within its amino-terminus (threonine 14 and tyrosine 15). In this study, the cell cycle regulation of the kinases responsible for phosphorylating p34cdc2 on Thr14 and Tyr15 was examined in extracts prepared from both HeLa cells and Xenopus eggs. Both Thr14- and Tyr15- specific kinase activities were regulated in a cell cycle-dependent manner. The kinase activities were high throughout interphase and diminished coincident with entry of cells into mitosis. In HeLa cells delayed in G2 by the DNA-binding dye Hoechst 33342, Thr14- and Tyr15-specific kinase activities remained high, suggesting that a decrease in Thr14- and Tyr15- kinase activities may be required for entry of cells into mitosis. Similar cell cycle regulation was observed for the Thr14/Tyr15 kinase(s) in Xenopus egg extracts. These results indicate that activation of CDC2 and entry of cells into mitosis is not triggered solely by activation of the Cdc25 phosphatase but by the balance between Thr14/Tyr15 kinase and phosphatase activities. Finally, we have detected two activities capable of phosphorylating p34cdc2 on Thr14 and/or Tyr15 in interphase extracts prepared from Xenopus eggs. An activity capable of phosphorylating Tyr15 remained soluble after ultracentrifugation of interphase extracts whereas a second activity capable of phosphorylating both Thr14 and Tyr15 pelleted. The pelleted fraction contained activities that were detergent extractable and that phosphorylated p34cdc2 on both Thr14 and Tyr15. The Thr14- and Tyr15-specific kinase activities co-purified through three successive chromatographic steps indicating the presence of a dual-specificity protein kinase capable of acting on p34cdc2.

Isolation and characterization of phosphoprotein phosphatase 1 from alfalfa

Protein phosphatases are central regulatory components of diverse processes in eukaryotes and are among the most highly conserved proteins known. In this paper, we report the cloning and sequencing of a type 1 protein phosphatase (pp1Ms) cDNA from alfalfa. Southern analysis indicates the presence of a gene family of PP1 proteins in alfalfa. The pp1Ms open reading frame is very similar to one of five predicted Arabidopsis type 1 protein phosphatases, indicating that different subtypes are individually conserved. Expression of the alfalfa pp1Ms in a temperature-sensitive Schizosaccharomyces pombe PP1 mutant, dis2-11, revealed no complementation, suggesting that PP1Ms is not involved in mitotic regulation. In different plant organs, different pp1Ms transcript levels were observed; in contrast, mRNA levels remained constant in all phases of the cell cycle and in logarithmically growing cells. However, when cells entered stationary phase pp1Ms transcript levels decreased considerably.

A single p34cdc2 protein kinase (encoded by nimXcdc2) is required at G1 and G2 in Aspergillus nidulans

We have cloned and sequenced a homolog of cdc2 from Aspergillus nidulans that can complement the Schizosaccharomyces pombe cdc2-33 mutation. The gene was deleted and is required for continued nuclear DNA replication but not for mitochondrial DNA replication. Three different temperature-sensitive alleles were generated by reverse genetics. All of the mutations generate the nim phenotype of A. nidulans. The new gene was designated nimXcdc2 as it is not allelic to any of the other nim genes (nimA to nimW) of A. nidulans. Reciprocal shift experiments place an essential function for nimXcdc2 in G1 and G2. Antipeptide antibodies were generated that detect NIMXcdc2, and antisera were also generated to detect NIMEcyclinB. The two p34cdc2 protein species previously detected in A. nidulans, p34 and p37, both precipitate using NIMXcdc2 C-terminus-specific antibodies but only p34 co-precipitates with NIMEcyclinB. Dephosphorylation of denatured p34 converts it to the p37 form, showing p37 to be the non-phosphorylated form of NIMXcdc2. The phosphorylation of p34 is therefore associated with its interaction with NIMEcyclinB.

Evidence for a dual role for TC4 protein in regulating nuclear structure and cell cycle progression

TC4, a ras-like G protein, has been implicated in the feedback pathway linking the onset of mitosis to the completion of DNA replication. In this report we find distinct roles for TC4 in both nuclear assembly and cell cycle progression. Mutant and wild-type forms of TC4 were added to Xenopus egg extracts capable of assembling nuclei around chromatin templates in vitro. We found that a mutant TC4 protein defective in GTP binding (GDP-bound form) suppressed nuclear growth and prevented DNA replication. Nuclear transport under these conditions approximated normal levels. In a separate set of experiments using a cell-free extract of Xenopus eggs that cycles between S and M phases, the GDP-bound form of TC4 had dramatic effects, blocking entry into mitosis even in the complete absence of nuclei. The effect of this mutant TC4 protein on cell cycle progression is mediated by phosphorylation of p34cdc2 on tyrosine and threonine residues, negatively regulating cdc2 kinase activity. Therefore, we provide direct biochemical evidence for a role of TC4 in both maintaining nuclear structure and in the signaling pathways that regulate entry into mitosis.

Distribution of the cdc2 gene product in normal tissues: an immunocytochemical study using four new monoclonal antibodies

The reproductive cycle in eukaryotic cells is partly controlled by the p34 protein kinase, the product of the cdc2 gene. We report here the tissue reactivity of four new anti-cdc2 monoclonal antibodies in relation to the known proliferation markers Ki-67 and JC1. In tissues where proliferation occurs, germinal centres in the tonsil, basal layers of tonsular epithelium and skin, cortex of the thymus, seminiferous tubules of the testis and epithelium of the colon, the anti-cdc2 antibodies gave positive nuclear staining as did the proliferation markers. The percentage of positive cells was, however, lower with the cdc2 antibodies. Given the role of the cdc2 gene at specific points of the cell cycle, these antibodies are potentially useful as markers of different phases of the cell cycle and may help to detect abnormalities in cell cycle control in disease.

Identification of two steps during Xenopus ribosomal gene transcription that are sensitive to protein phosphorylation

Protein kinase(s) and protein phosphatase(s) present in a Xenopus S-100 transcription extract strongly influence promoter-dependent transcription by RNA polymerase I. The protein kinase inhibitor 6-dimethyl-aminopurine causes transcription to increase, while the protein phosphatase inhibitor okadaic acid causes transcription to decrease. Repression is also observed with inhibitor 2, and the addition of extra protein phosphatase 1 stimulates transcription, indicating that the endogenous phosphatase is a type 1 enzyme. Partial fractionation of the system, single-round transcription reactions, and kinetic experiments show that two different steps during ribosomal gene transcription are sensitive to protein phosphorylation: okadaic acid affects a step before or during transcription initiation, while 6-dimethylaminopurine stimulates a process "late" in the reaction, possibly reinitiation. The present results are a clear demonstration that transcription by RNA polymerase I can be regulated by protein phosphorylation.

Nuclear concentration and mitotic dispersion of the essential cell cycle protein, p13suc1, examined in living cells

Stamen hair cells of Tradescantia virginiana have been microinjected with p13suc1 labeled with carboxyfluorescein (CF) and studied throughout the division cycle in living cells by using the confocal laser scanning microscope. The protein, p13suc1, is essential for the rapid inactivation of the key mitotic catalyst, p34cdc2 kinase, at anaphase and for completion of nuclear division. During interphase or prophase, CF-p13suc1 concentrates quickly (< 2 min) in nuclei, reaching levels that are approximately 2-fold greater than those in the cytoplasm. At nuclear envelope breakdown, CF-p13suc1 permeates throughout the entire spindle and nonspindle cytoplasm. The protein is excluded from the tightly condensed chromosomes but otherwise no regions accumulate or exclude the protein. It remains evenly distributed throughout metaphase, anaphase, and well into cytokinesis; however, during telophase CF-p13suc1 reconcentrates in the daughter nuclei.

Identification of two steps during Xenopus ribosomal gene transcription that are sensitive to protein phosphorylation

Protein kinase(s) and protein phosphatase(s) present in a Xenopus S-100 transcription extract strongly influence promoter-dependent transcription by RNA polymerase I. The protein kinase inhibitor 6-dimethyl-aminopurine causes transcription to increase, while the protein phosphatase inhibitor okadaic acid causes transcription to decrease. Repression is also observed with inhibitor 2, and the addition of extra protein phosphatase 1 stimulates transcription, indicating that the endogenous phosphatase is a type 1 enzyme. Partial fractionation of the system, single-round transcription reactions, and kinetic experiments show that two different steps during ribosomal gene transcription are sensitive to protein phosphorylation: okadaic acid affects a step before or during transcription initiation, while 6-dimethylaminopurine stimulates a process "late" in the reaction, possibly reinitiation. The present results are a clear demonstration that transcription by RNA polymerase I can be regulated by protein phosphorylation.

Inhibition of cdc2 activation by INH/PP2A

INH, a type 2A protein phosphatase (PP2A), negatively regulates entry into M phase and the cyclin B-dependent activation of cdc2 in Xenopus extracts. INH appears to be central to the mechanism of the trigger for mitotic initiation, as it prevents the premature activation of cdc2. We first show that INH is a conventional form of PP2A with a B alpha regulatory subunit. We next explore the mechanism by which it inhibits cdc2 activation by examining the effect of purified PP2A on the reaction pathways controlling cdc2 activity. Our results suggest that although PP2A inhibits the switch in tyrosine kinase and tyrosine phosphatase activities accompanying mitosis, this switch is a consequence of the inhibition of some other rate-limiting event. In the preactivation phase, PP2A inhibits the pathway leading to T161 phosphorylation, suggesting that this activity may be one of the rate-limiting events for transition. However, our results also suggest that the accumulation of active cdc2/cyclin complexes during the lag is only one of the events required for triggering entry into mitosis.

Theoretical dynamics of the cyclin B-MPF system: a possible role for p13suc1

In dividing cells, entry into mitosis is caused by maturation promoting factor (MPF), which is formed autocatalytically by activation of a complex of p34cdc2 and cyclin B. This biochemical system may oscillate, causing repeated mitosis. It is shown mathematically that the oscillatory tendency would be enhanced by a cofactor which binds to MPF and inhibits its autocatalytic action. A candidate for such a cofactor is the suc1 gene product p13, which binds to p34cdc2/cyclin B complex and inhibits MPF-induced MPF activation. At a steady rate of cyclin biosynthesis, with small amounts converted to MPF, p13suc1 would have to be titrated by MPF before autocatalysis could begin. This would have three possibly important effects: (1) it would determine the 'threshold' cyclin accumulation (and hence the corresponding time-delay) for MPF activation; (2) it would cause the accumulation of a backlog of MPF precursor (tyrosine-phosphorylated p34cdc2/cyclin B) sufficient to produce a substantial MPF pulse when MPF autocatalysis begins; (3) it would give the autocatalysis a high reaction order, which tends to destabilize the steady state, promote autonomous oscillations, and enhance the triggering property (excitability) of the system. The MPF pulse generated by this system may be essential for the proper triggering of the events of M phase, including the cyclin degradation which inactivates MPF at the end of M phase. This model offers explanations for several puzzling effects of p13suc1, including the fact that p13suc1, though an inhibitor of MPF activation, is nevertheless necessary for mitosis.

Histone H1 kinase activity, germinal vesicle breakdown and M phase entry in mouse oocytes

Meiotic reinitiation of the mouse oocyte is characterized by a slow entry into metaphase I, beginning with germinal vesicle breakdown and ending with spindle formation. It is accompanied by a cascade of protein kinases and phosphatases increasing protein phosphorylation. The activation of histone H1 kinase and that of the mitogen-activated protein kinase p42 have been compared during spontaneous or okadaic acid-induced meiotic reinitiation. In spontaneously maturing oocytes, histone H1 kinase activity increases before germinal vesicle breakdown (2-fold), in a protein synthesis-independent manner. It is associated with the disappearance of the upper migrating form of p34cdc2, which, in our system, seems to represent the tyrosine phosphorylated form. Following germinal vesicle breakdown, histone H1 kinase activity culminates (8-fold) in metaphase I and requires protein synthesis. Activation by phosphorylation of p42MAPK is observed as a permanent shift upward-migrating form and by its myelin basic protein kinase activity. It occurs after germinal vesicle breakdown and depends on protein synthesis. In contrast, no increase of histone H1 kinase is detectable in oocytes induced to reinitiate meiosis by a transient inhibition of okadaic acid-sensitive phosphatase(s), either before germinal vesicle breakdown or during the following 7 hours of culture. A slight increase is nevertheless evident after 17 hours, when oocytes are arrested with an abnormal metaphase I spindle. The upper migrating form of p34cdc2 is present for 8 hours. The activation of p42MAPK begins before germinal vesicle breakdown.(ABSTRACT TRUNCATED AT 250 WORDS)

p27Kip1, a cyclin-Cdk inhibitor, links transforming growth factor-beta and contact inhibition to cell cycle arrest

Cell-cell contact and TGF-beta can arrest the cell cycle in G1. Mv1Lu mink epithelial cells arrested by either mechanism are incapable of assembling active complexes containing the G1 cyclin, cyclin E, and its catalytic subunit, Cdk2. These growth inhibitory signals block Cdk2 activation by raising the threshold level of cyclin E necessary to activate Cdk2. In arrested cells the threshold is set higher than physiological cyclin E levels and is determined by an inhibitor that binds to cyclin E-Cdk2 complexes. A 27-kD protein that binds to and prevents the activation of cyclin E-Cdk2 complexes can be purified from arrested cells but not from proliferating cells, using cyclin E-Cdk2 affinity chromatography. p27 is present in proliferating cells, but it is sequestered and unavailable to interact with cyclin E-Cdk2 complexes. Cyclin D2-Cdk4 complexes bind competitively to and down-regulate the activity of p27 and may thereby act in a pathway that reverses Cdk2 inhibition and enables G1 progression.

Phosphatidylinositol 3-kinase activity is important for progesterone-induced Xenopus oocyte maturation

In somatic cells, phosphatidylinositol 3-kinase (PI3 kinase) is a critical intermediary in growth factor-induced mitogenesis. We have examined the role of this enzyme in meiotic maturation of Xenopus laevis oocytes. PI3 kinase activity was present in immunoprecipitates of the p85 subunit of PI3 kinase from immature oocytes and markedly increased following progesterone stimulation. Injection of bacterially expressed protein corresponding to the C-terminal SH2 domain of p85 (SH2-C) inhibited progesterone-induced PI3 kinase activation and meiotic maturation. Injection of protein corresponding to the N-terminal SH2 domain or the SH3 domain of p85 did not inhibit PI3 kinase activation or maturation. SH2-C did not inhibit oocyte maturation induced by c-mos RNA injection. In addition, radiolabelled SH2-C was used to probe oocyte lysates, revealing that a novel 200-kDa protein bound to SH2-C. This protein may be an important mediator of progesterone-induced lipid metabolism in oocytes.

Phosphatidylinositol 3-kinase activity is important for progesterone-induced Xenopus oocyte maturation

In somatic cells, phosphatidylinositol 3-kinase (PI3 kinase) is a critical intermediary in growth factor-induced mitogenesis. We have examined the role of this enzyme in meiotic maturation of Xenopus laevis oocytes. PI3 kinase activity was present in immunoprecipitates of the p85 subunit of PI3 kinase from immature oocytes and markedly increased following progesterone stimulation. Injection of bacterially expressed protein corresponding to the C-terminal SH2 domain of p85 (SH2-C) inhibited progesterone-induced PI3 kinase activation and meiotic maturation. Injection of protein corresponding to the N-terminal SH2 domain or the SH3 domain of p85 did not inhibit PI3 kinase activation or maturation. SH2-C did not inhibit oocyte maturation induced by c-mos RNA injection. In addition, radiolabelled SH2-C was used to probe oocyte lysates, revealing that a novel 200-kDa protein bound to SH2-C. This protein may be an important mediator of progesterone-induced lipid metabolism in oocytes.