The cell cycle control gene cdc2+ of fission yeast encodes a protein kinase potentially regulated by phosphorylation

Activation of p34cdc2 kinase by cyclin A

Functional clam cyclin A and B proteins have been produced using a baculovirus expression system. Both cyclin A and B can induce meiosis I and meiosis II in Xenopus in the absence of protein synthesis. Half-maximal induction occurs at 50 nM for cyclin A and 250 nM for cyclin B. Addition of 25 nM cyclin A to activated Xenopus egg extracts arrested in the cell cycle by treatment with RNase or emetine activates cdc2 kinase to the normal metaphase level and stimulates one oscillatory cell cycle. High levels of cyclin A cause marked hyperactivation of cdc2 kinase and a stable arrest at the metaphase point in the cell cycle. Kinetic studies demonstrate the concentration of cyclin A added does not affect the 10 min lag period required for kinase activation or the timing of maximal activity, but does control the rate of deactivation of cdc2 kinase during exit from mitosis. In addition, exogenous clam cyclin A inhibits the degradation of both A- and B-type endogenous Xenopus cyclins. These results define a system for investigating the biochemistry and regulation of cdc2 kinase activation by cyclin A.

Identification of a 70-base-pair cell cycle regulatory unit within the promoter of the human thymidine kinase gene and its interaction with cellular factors

The promoter of the human thymidine kinase gene contains cis-regulatory elements responsible for its cell-cycle-regulated expression. We report here that a 70-bp region between -133 and -64 is sufficient to confer cell cycle regulation on a heterologous promoter. The 20-bp region between -64 and -83, which contains an inverted CCAAT motif, is important for transcriptional stimulation of this functional unit. The sequence of this CCAAT motif is nearly identical to the consensus sequence for the transcriptional factor CP1. We also examined the specificity and binding activities of cellular factors interacting with the 70-bp fragment. We showed that the cellular factors binding to the 70-bp region are similar during the G1, S, and G2 phases, suggesting that the cell cycle regulatory activity observed must involve processes other than factor binding to the DNA.

The role of CDC28 and cyclins during mitosis in the budding yeast S. cerevisiae

cdc28-1N is a conditional allele that has normal G1 (Start) function but confers a mitotic defect. We have isolated seven genes that in high dosage suppress the growth defect of cdc28-1N cells but not of Start-defective cdc28-4 cells. Three of these (CLB1, CLB2, and CLB4) encode proteins strongly homologous to G2-specific B-type cyclins. Another gene, CLB3, was cloned using PCR, CLB1 and CLB2 encode a pair of closely related proteins; CLB3 and CLB4 encode a second pair. Neither CLB1 nor CLB2 is essential; however, disruption of both is lethal and causes a mitotic defect. Furthermore, the double mutant cdc28-1N clb2::LEU2 is nonviable, whereas cdc28-4 clb2::LEU2 is viable, suggesting that the cdc28-1N protein may be defective in its interaction with B-type cyclins. Our results are consistent with CDC28 function being required in both G1 and mitosis. Its mitotic role, we believe, involves interaction with a family of at least four G2-specific cyclins.

A cyclin B homolog in S. cerevisiae: chronic activation of the Cdc28 protein kinase by cyclin prevents exit from mitosis

A cyclin B homolog was identified in Saccharomyces cerevisiae using degenerate oligonucleotides and the polymerase chain reaction. The protein, designated Scb1, has a high degree of similarity with B-type cyclins from organisms ranging from fission yeast to human. Levels of SCB1 mRNA and protein were found to be periodic through the cell cycle, with maximum accumulation late, most likely in the G2 interval. Deletion of the gene was found not to be lethal, and subsequently other B-type cyclins have been found in yeast functionally redundant with Scb1. A mutant allele of SCB1 that removes an amino-terminal fragment of the encoded protein thought to be required for efficient degradation during mitosis confers a mitotic arrest phenotype. This arrest can be reversed by inactivation of the Cdc28 protein kinase, suggesting that cyclin-mediated arrest results from persistent protein kinase activation.

Phosphorylation differences among proteins of bloodstream developmental stages of Trypanosoma brucei brucei

Early in an infection the bloodstream forms of the African trypanosome Trypanosoma brucei brucei are long, slender and rapidly dividing. Later, non-dividing, short, stumpy forms may be found. In this report we described biochemical differences between the two parasite populations in the phosphorylation of their proteins in vitro. Compared with the slender populations, the non-dividing stumpy forms of the parasites exhibit decreased phosphorylation of an 80 kDa protein and enhanced phosphorylation of 37 kDa and 42 kDa proteins (pp37 and pp42). These changes occurred regardless of whether the stumpy trypanosomes were generated naturally during the course of the infection or induced by difluoromethylornithine treatment. The phosphorylation of pp37 and pp42 occurs on serine and threonine residues and is totally dependent upon the presence of Mn2+ or Mg2+. However, excess Mn2+ or Mg2+ inhibits phosphorylation. Maximal phosphorylation of pp42 occurs with 1 mm-Mn2+ or 10 mm-Mg2+, whereas that of pp37 occurs with 50 mM-Mn2+ or greater than 100 mm-Mg2+. The phosphorylation of pp37 is greatly enhanced by KCl, whereas that of pp42 is only slightly increased by this salt. Ca2+, calmodulin, phospholipids and cyclic AMP have no discernible effect upon the phosphorylation of pp42 or pp37 in vitro, whereas heparin, suramin, polylysine, polyarginine and polyamines all inhibit phosphorylation. Thus the enzymes that phosphorylate pp42 and pp37 have properties similar to, but distinct from, those of mammalian casein kinase II. Since the casein-kinase-like activity is higher in the slender than in the stumpy forms, the enhanced phosphorylation of pp42 and pp37 in the non-dividing parasites is probably a result of the enhanced synthesis of these acidic proteins.

The product of the KIN1 locus in Saccharomyces cerevisiae is a serine/threonine-specific protein kinase

The catalytic domain (30 kDa) of all protein kinases can be aligned for maximum homology, thereby revealing both invariant and highly conserved residues. The KIN1 locus from Saccharomyces cerevisiae was isolated by hybridization to a degenerate oligonucleotide encoding the conserved protein kinase domain, DVWSFG. The predicted amino acid sequence revealed significant homology to the catalytic domain of protein kinases. Using antibodies raised against a bacterial LacZ/KIN1 fusion protein, we have identified by immunoprecipitation the yeast KIN1 gene product as a 145,000 dalton protein (p145KIN1). In exponentially growing yeast cells, the KIN1 protein is phosphorylated primarily on serine residues. The gene product of KIN1 was shown to be a serine/threonine-specific protein kinase in immune complexes, as determined by the transfer of label from [gamma-32P]ATP to either pp145KIN1 or to an exogenously added substrate, alpha-casein. The optimal metal ion concentration in this assay was 20 mM-MnCl2. Subsequent phosphoamino acid analysis of the radiolabelled product, pp145KIN1, demonstrated that this autophosphorylation was specific for serine/threonine residues. There is no apparent difference between wild-type cells and cells containing a disrupted KIN1 gene. The biochemical characterization of protein kinases in simple eukaryotes such as yeast will aid us in determining the role of phosphorylation in cellular growth and physiology.

Identification of a 70-base-pair cell cycle regulatory unit within the promoter of the human thymidine kinase gene and its interaction with cellular factors

The promoter of the human thymidine kinase gene contains cis-regulatory elements responsible for its cell-cycle-regulated expression. We report here that a 70-bp region between -133 and -64 is sufficient to confer cell cycle regulation on a heterologous promoter. The 20-bp region between -64 and -83, which contains an inverted CCAAT motif, is important for transcriptional stimulation of this functional unit. The sequence of this CCAAT motif is nearly identical to the consensus sequence for the transcriptional factor CP1. We also examined the specificity and binding activities of cellular factors interacting with the 70-bp fragment. We showed that the cellular factors binding to the 70-bp region are similar during the G1, S, and G2 phases, suggesting that the cell cycle regulatory activity observed must involve processes other than factor binding to the DNA.

Transforming growth factor beta 1 inhibition of p34cdc2 phosphorylation and histone H1 kinase activity is associated with G1/S-phase growth arrest

Transforming growth factor beta 1 (TGF beta 1) is a potent inhibitor of epithelial cell proliferation. We present data which indicate that epithelial cell proliferation is inhibited when TGF beta 1 is added throughout the prereplicative G1 phase. Cultures become reversibly blocked in late G1 at the G1/S-phase boundary. The inhibitory effects of TGF beta 1 on cell growth occur in the presence of the RNA synthesis inhibitor 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole. Associated with this inhibitory effect is a decrease in the phosphorylation and histone H1 kinase activity of the p34cdc2 protein kinase. These data suggest that TGF beta 1 growth inhibition in epithelial cells involves the regulation of p34cdc2 activity at the G1/S transition.

mik1 and wee1 cooperate in the inhibitory tyrosine phosphorylation of cdc2

wee1 acts antagonistically to cdc25 in the tyrosine dephosphorylation and activation of cdc2, yet biochemical evidence suggests that wee1 is not required for tyrosine phosphorylation and its role is obscure. We show here that a related 66 kd kinase, called mik1, acts redundantly with wee1 in the negative regulation of cdc2 in S. pombe. A null allele of mik1 has no discernible phenotype, but a mik1 wee1 double mutant is hypermitotically lethal: all normal M phase checkpoints are bypassed, including the requirement for initiation of cell cycle "start," completion of S phase, and function of the cdc25+ mitotic activator. In the absence of mik1 and wee1 activity, cdc2 rapidly loses phosphate on tyrosine, both in strains undergoing mitotic lethality and in those that are viable owing to a compensating mutation within cdc2. The data suggest that mik1 and wee1 act cooperatively on cdc2, either directly as the inhibitory tyrosine kinase or as essential activators of that kinase.

Characterization of a cAMP-independent Ca2(+)-inhibited protamine kinase from Candida lipolytica

A cAMP-independent protamine kinase has been purified from extracts of the yeast Candida lipolytica by ion-exchange and affinity chromatography. Two subunits with apparent Mr's of 52,000 and 36,000 were resolved by SDS-PAGE. The purified kinase exhibited about 20% activity with casein and histone Type VII-S as substrates relative to protamine. The enzyme was inactive against other protein substrates tested, and was essentially insensitive to AMP, cAMP, cGMP up to 0.2 mM, the polyamines spermine and spermidine up to 1 mM, N-ethylmaleimide (5 mM), 2-mercaptoethanol (20 mM), or dithiothreitol (2 mM), and several cations like Zn2+, N1+, or Co2+ at 0.1 mM each. Ca2+ at 3 mM inhibited protamine kinase activity by 50%, which was reversed by EGTA.

A novel M phase-specific H1 kinase recognized by the mitosis-specific monoclonal antibody MPM-2

At the onset of mitosis, eukaryotic cells display an abrupt increase in a Ca2(+)- and cyclic nucleotide-independent histone H1 kinase activity, referred to as growth-associated or M phase-specific H1 kinase. The molecular basis for this activity is generally attributed to a kinase complex that consists of the p34cdc2 protein and cyclin, and exhibits maturation-promoting factor (MPF) activity. In the present study, we show that more than one kinase contributes to M phase-specific H1 kinase activity. When mature Xenopus oocyte extract prepared with ATP gamma S and NaF was fractionated by gel filtration, two prominent peaks of H1 kinase activity were detected, with apparent molecular masses of 600 and 150 kDa. The 150-kDa kinase copurified with the p34cdc2 protein and was immobilized by the suc 1 gene product p13 and anti-cyclin B2, which are specific for the cdc2 kinase complex. However, the 600-kDa kinase did not satisfy any of these criteria, thus identifying it as a novel M phase-specific H1 kinase. Only the 600-kDa kinase was recognized by the mitosis-specific monoclonal antibody, MPM-2, which inhibits Xenopus oocyte maturation and immunodepletes MPF activity. Furthermore, not only did the full activation of this kinase (MPM-2 kinase) coincide with the activation of MPF during the cell cycle, but also MPM-2 kinase-positive fractions obtained by gel filtration accelerated progesterone-induced oocyte maturation. It is, therefore, likely that MPM-2 kinase is a positive regulator in the M phase induction pathway.

Transforming growth factor beta 1 inhibition of p34cdc2 phosphorylation and histone H1 kinase activity is associated with G1/S-phase growth arrest

Transforming growth factor beta 1 (TGF beta 1) is a potent inhibitor of epithelial cell proliferation. We present data which indicate that epithelial cell proliferation is inhibited when TGF beta 1 is added throughout the prereplicative G1 phase. Cultures become reversibly blocked in late G1 at the G1/S-phase boundary. The inhibitory effects of TGF beta 1 on cell growth occur in the presence of the RNA synthesis inhibitor 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole. Associated with this inhibitory effect is a decrease in the phosphorylation and histone H1 kinase activity of the p34cdc2 protein kinase. These data suggest that TGF beta 1 growth inhibition in epithelial cells involves the regulation of p34cdc2 activity at the G1/S transition.

Complementation of a yeast cell cycle mutant by an alfalfa cDNA encoding a protein kinase homologous to p34cdc2

The cdc2 protein kinase plays a central role in control of the eukaryotic cell cycle of animals and yeasts. We have isolated a cDNA clone (cdc2Ms) from alfalfa (Medicago sativa L.) that is homologous to the yeast cdc2/CDC28 genes. The encoded protein is 64% identical to the yeast and mammalian counterparts and shows all the prominent structural features known from these organisms. Antibody raised against a 16-amino acid synthetic peptide with crossreactivity against p34 proteins recognized a 34-kilodalton protein in extracts of alfalfa cells. When transferred into a fission yeast, the plant cdc2 homolog can complement a temperature-sensitive cdc2 mutant. Northern analysis revealed higher transcript levels in shoots and suspension cultures than in roots. In addition to the dominant transcript of 1.4 kilobases detected in the poly(A)+fraction, 2.5- and 1.2-kilobase transcripts were detected in total RNA preparations from shoots or somatic embryos. Suspension cultures that were induced to form somatic embryos by an auxin (2,4-dichlorophenoxyacetic acid) showed fluctuations in transcription pattern during the induction period and embryogenesis.

Involvement of p34cdc2 in establishing the dependency of S phase on mitosis

Mutants of cdc2+ can disrupt the dependency of S phase on completion of the previous mitosis. By changing the state of p34cdc2 it is possible to reprogramme a cell from entering mitosis to undergoing S phase. This leads to the proposal that the cell cycle can be considered a p34cdc2 cycle, and has implications for the evolution of life cycles.

Complementation of the mitotic activator, p80cdc25, by a human protein-tyrosine phosphatase

The onset of M phase requires the activation of the pp34 protein kinase in all eukaryotes thus far examined. In Schizosaccharomyces pombe, pp34 is phosphorylated on Tyr15, and dephosphorylation of this residue regulates the initiation of mitosis. In this study, it is shown that dephosphorylation of Tyr15 triggered activation of the pp34-cyclin complex from fission yeast, that a human protein-tyrosine phosphatase can catalyze this event both in vitro and in vivo, and that activation of fission yeast pp34 does not require threonine dephosphorylation. The complementary DNA that encoded the tyrosine phosphatase replaced the mitotic activator p80cdc25, closely associating the cdc25(+)-activating pathway with tyrosine dephosphorylation of pp34.

Identification of mitosis-specific p65 dimer as a component of human M phase-promoting factor

Antisera raised against two mitosis-specific protein kinases from human cells recognized a single 65-kDa polypeptide (p65) that is present in similar amounts in interphase and mitotic cell extracts. Immunoblot analysis of reduced and unreduced extracts revealed that p65 exists as a 65-kDa monomer during interphase but forms a 130-kDa disulfide-linked homodimer during mitosis. Several different antibodies recognizing the p34cdc2 protein kinase and cyclin B components of M phase-promoting factor (MPF) coprecipitated p65 from mitotic but not from interphase extracts. In addition, an anti-p65 immunoaffinity column substantially depleted mitotic extracts of histon H1 kinase activity assayed under conditions diagnostic for MPF. These results suggest that active human MPF may be a complex of p34cdc2, cyclin B, and dimeric p65. A sulfhydryl cycle, proposed in the earlier literature on the biochemistry of mitosis, might underlie the dimerization of p65 and formation of active MPF.

Cell cycle control of DNA replication by a homologue from human cells of the p34cdc2 protein kinase

The regulation of DNA replication during the eukaryotic cell cycle was studied in a system where cell free replication of simian virus 40 (SV40) DNA was used as a model for chromosome replication. A factor, RF-S, was partially purified from human S phase cells based on its ability to activate DNA replication in extracts from G1 cells. RF-S contained a human homologue of the Schizosaccharomyces pombe p34cdc2 kinase, and this kinase was necessary for RF-S activity. The limiting step in activation of the p34 kinase at the G1 to S transition may be its association with a cyclin since addition of cyclin A to a G1 extract was sufficient to start DNA replication. These observations suggest that the role of p34cdc2 in controlling the start of DNA synthesis has been conserved in evolution.

cdc2 gene expression at the G1 to S transition in human T lymphocytes

The product of the cdc2 gene, designated p34cdc2, is a serine-threonine protein kinase that controls entry of eukaryotic cells into mitosis. Freshly isolated human T lymphocytes (G0 phase) were found to have very low amounts of p34cdc2 and cdc2 messenger RNA. Expression of cdc2 increased 18 to 24 hours after exposure of T cells to phytohemagglutinin, coincident with the G1 to S transition. Antisense oligodeoxynucleotides could reduce the increase in cdc2 expression and inhibited DNA synthesis, but had no effect on several early and mid-G1 events, including blastogenesis and expression of interleukin-2 receptors, transferrin receptors, c-myb, and c-myc. Induction of cdc2 required prior induction of c-myb and c-myc. These results suggest that cdc2 induction is part of an orderly sequence of events that occurs at the G1 to S transition in T cells.

The difference in murine CDC2 kinase activity between cytoplasmic and nuclear fractions during the cell cycle

The mouse analog of yeast CDC2+ kinase was detected in the cytoplasmic and nuclear fractions of cultured mouse FM3A cells. Its activity in the nuclear fraction increased in the G2/M phase became seven times higher than that in the G1/S phase, while the activity in the cytoplasmic fraction remained was almost constant from the G1/S to G1 phases. The activity in the cytoplasmic fraction was similar to that in the nuclear fraction in the G2/M phase. The amount of the enzyme remained almost constant during the cell cycle in both the nuclear and cytoplasmic fractions. These findings suggest that the cytoplasmic enzyme might play an independent role in the cell cycle.

Heat-shock and related stress enhance RNA polymerase II C-terminal-domain kinase activity in HeLa cell extracts

Changes in protein kinase activities are thought to contribute to the alteration of gene expression after heat shock and related stresses. In an attempt to identify enzymes which might be involved in both chromatin structure modification and transcriptional switch in heat-shocked cells, we have studied protein kinase activities in heat-shocked cell lysates with two exogenous substrates: a tetramer of a heptapeptide (heptapeptide 4) corresponding to the RNA polymerase II C-terminal domain (CTD), and the histone H1. Heat-shock and arsenite stress were found to stimulate strongly CTD kinase activity. H1 kinase activity was also stimulated but more weakly. Stimulation of CTD and H1 kinases occurs mainly at the early phase of recovery and by a process which is independent of protein synthesis. The stress-induced H1 kinase is shown to contain a molecule related to the mitotic-promoting factor (MPF) Cdc2 component. On the other hand, though Cdc2-related protein has also been reported to be part of a CTD kinase complex, we show that the stress-induced CTD kinase activity corresponds to a distinct entity. It is proposed that stress activation of CTD kinase might be involved in changing the specificity of RNA polymerase II.

Yeast as a model system for understanding the control of DNA replication in Eukaryotes

In the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe, the initiation of DNA replication is controlled at a point called START. At this point, the cellular environment is assessed; only if conditions are appropriate do cells traverse START, thus becoming committed to initiate DNA replication and complete the remainder of the cell cycle. The cdc2+/CDC28+ gene, encoding the protein kinase p34, is a key element in this complex control. The identification of structural and functional homologues of p34 suggests that it has a role in the control of DNA replication in all eukaryotes. The WHI1+, CLN1+ and CLN2+ gene products, identified in S. cerevisiae, are positive regulators that function at START and may interact with p34. Determining how passing the START control point leads to the initiation of DNA replication is a major outstanding challenge in cell cycle studies.

Cell cycle control in eukaryotes: molecular mechanisms of cdc2 activation

cdc2 kinase regulates the progression of eukaryotic cells through the division cycle. Events such as cell growth, DNA replication and mitosis are coordinated through the activation of specific forms of this kinase. Here I discuss our present knowledge of the mechanisms that regulate the activity of cdc2 kinase.

The testis-specific transcript (ferT) of the tyrosine kinase FER is expressed during spermatogenesis in a stage-specific manner

ferT is a testis-specific transcript of FER encoding a truncated version of the potential tyrosine kinase. Using in situ hybridization analysis, we found that ferT was transiently expressed during spermatogenesis and that expression was restricted to spermatocytes at the pachytene stage of meiotic prophase. This pattern of expression is unprecedented by other tyrosine kinases and suggests a role for ferT in a particular stage of spermatogenesis.

Synthesis, phosphorylation, and nuclear localization of human papillomavirus E7 protein in Schizosaccharomyces pombe

The complete E7 protein-encoding open reading frame of human papillomavirus type 16 (HPV-16) was expressed in the fission yeast Schizosaccharomyces pombe, under the control of a cloned yeast promoter. The HPV-16 E7 protein synthesized in S. pombe is a 17-kDa phosphoprotein which is recognized by anti-E7 antibodies (raised in rabbits against E7 fusion protein produced in Escherichia coli). The mobility during sodium dodecyl sulfate-polyacrylamide-gel electrophoresis of native E7 phosphoprotein synthesized in S. pombe is identical to that of the E7 phosphoprotein immunoprecipitated from human CaSki cells. Immunofluorescence staining showed that HPV-16 E7 phosphoprotein is localized in the nuclei of transformed S. pombe. These results indicate that E7 protein synthesized by S. pombe is apparently indistinguishable from HPV-16 E7 protein synthesized in higher eukaryotic cells expressing genes of HPV-16, and also that the phosphorylated, nuclear HPV-16 E7 protein is synthesized in S. pombe in a form compatible with its biological activity.

The testis-specific transcript (ferT) of the tyrosine kinase FER is expressed during spermatogenesis in a stage-specific manner

ferT is a testis-specific transcript of FER encoding a truncated version of the potential tyrosine kinase. Using in situ hybridization analysis, we found that ferT was transiently expressed during spermatogenesis and that expression was restricted to spermatocytes at the pachytene stage of meiotic prophase. This pattern of expression is unprecedented by other tyrosine kinases and suggests a role for ferT in a particular stage of spermatogenesis.

Protein phosphatases are involved in the in vivo activation of histone H1 kinase in mouse oocyte

Histone H1 kinase and protein phosphorylation have been studied in mouse oocyte. Histone H1 kinase activity increases when the oocyte enters M-phase at the time of GVBD and is paralleled with a burst of protein phosphorylation. This activity dramatically drops after parthenogenetic activation induced by puromycin. Okadic acid (OA), a potent inhibitor of protein phosphatases, induces GVBD when oocytes are arrested in the first meiotic prophase by dbc-AMP; the continuous presence of the phosphatase inhibitor, however, inhibits the polymerization of metaphase microtubules. Following activation of metaphase II-arrested mouse eggs by puromycin, OA can induce the breakdown of the nuclear envelope and the activation of histone H1 kinase. This indicates that in the absence of protein synthesis, and therefore of cyclin synthesis, inhibition of protein phosphatases may be sufficient to induce the entry into M-phase during the first cell cycle of the mouse parthenogenetic activated oocyte.

The starfish egg mRNA responsible for meiosis reinitiation encodes cyclin

During meiotic maturation and early embryonic cycles, the activity of maturation-promoting factor (MPF) cycles in exact correspondence with the mitotic cycles. For the appearance of MPF activity in starfish, protein synthesis is required except in the first meiotic cycle. In order to identify newly synthesized proteins involved in the regulation of MPF activity, we extracted poly(A)+ RNA from starfish eggs, and found that the egg poly(A)+ RNA induced germinal vesicle breakdown (GVBD) upon injection into immature oocytes of starfish and Xenopus. The molecular size of the poly(A)+ RNA responsible for GVBD was estimated to be approximately 22S by sucrose density gradient centrifugation. Since these characteristics of the starfish egg poly(A)+ RNA are similar to those of cyclin mRNAs from sea urchin and surf clam eggs, we synthesized a 50-mer antisense-cyclin oligonucleotide probe coding for a part of the sea urchin cyclin cDNA and used this to screen starfish RNA. The Northern blot analysis showed that the starfish egg RNA contained cyclin homologous transcripts. Incubation of the starfish egg poly(A)+ RNA and the antisense-cyclin oligonucleotide with RNase H completely destroyed its GVBD-inducing activity. These results indicated that starfish cyclin mRNA was the only poly(A)+ RNA responsible for GVBD. We constructed a starfish egg cDNA library to clone starfish cyclin cDNA. The longest cDNA clone containing 2190 base pairs was sequenced. The longest open reading frame consisted of 395 amino acid residues, and the predicted molecular size was 48 kDa. Comparison of the deduced amino acid sequences of starfish cyclin with known cyclins indicated that the starfish cyclin belongs to the B-type. Injection of synthetic mRNA of starfish cyclin caused GVBD in immature oocytes of starfish and Xenopus, while injection of synthetic mRNA of human CDC2 had no effect. The Northern blot analysis of starfish RNA extracted at various stages of the meiotic cycles suggested that the starfish cyclin transcript was stored in its polyadenylated form even in immature oocytes and was further polyadenylated at maturation.

Human cDNAs encoding homologs of the small p34Cdc28/Cdc2-associated protein of Saccharomyces cerevisiae and Schizosaccharomyces pombe

The Cks1 protein is a component of the Cdc28 protein kinase in the budding yeast Saccharomyces cerevisiae. This paper reports the cloning of two homologs of the S. cerevisiae CKS1 gene from human cells. These homologs, CKShs1 and CKShs2, both encode proteins of 79 amino acids that share considerable homology at the amino acid level with the products of CKS1 from S. cerevisiae and suc1+ from the fission yeast Schizosaccharomyces pombe. Both human homologs are capable of rescuing a null mutation of the S. cerevisiae CKS1 gene when expressed from the S. cerevisiae GAL1 promoter. S. pombe suc1+ expressed from the GAL1 promoter is also capable of rescuing a S. cerevisiae cks1 null mutation. Ckshs1 or Ckshs2 protein linked to Sepharose beads can bind the Cdc28/Cdc2 protein kinase from both S. cerevisiae and human cells. The CKShs1 and CKShs2 mRNAs are expressed in different patterns through the cell cycle in HeLa cells, which may reflect specialized roles for the encoded proteins.

Periodic biosynthesis of the human M-phase promoting factor catalytic subunit p34 during the cell cycle

The product of the CDC2Hs gene is the protein kinase subunit of the M-phase promoting factor, which is required for entry into mitosis. The activity of this kinase is regulated in a cell cycle-dependent manner by reversible phosphorylation and through association with other proteins. We report here that in HeLa cells, the abundance of the CDC2Hs mRNA and the rate of synthesis of the encoded protein, p34, vary in a cell cycle-dependent manner.

Periodic biosynthesis of the human M-phase promoting factor catalytic subunit p34 during the cell cycle

The product of the CDC2Hs gene is the protein kinase subunit of the M-phase promoting factor, which is required for entry into mitosis. The activity of this kinase is regulated in a cell cycle-dependent manner by reversible phosphorylation and through association with other proteins. We report here that in HeLa cells, the abundance of the CDC2Hs mRNA and the rate of synthesis of the encoded protein, p34, vary in a cell cycle-dependent manner.

Functions of maternal mRNA in early development

In this review, the types of mRNAs found in oocytes and eggs of several animal species, particularly Drosophila, marine invertebrates, frogs, and mice, are described. The roles that proteins derived from these mRNAs play in early development are discussed, and connections between maternally inherited information and embryonic pattern are sought. Comparisons between genetically identified maternally expressed genes in Drosophila and maternal mRNAs biochemically characterized in other species are made when possible. Regulation of the meiotic and early embryonic cell cycles is reviewed, and translational control of maternal mRNA following maturation and/or fertilization is discussed with regard to specific mRNAs.

Identification of a 42-kilodalton phosphotyrosyl protein as a serine(threonine) protein kinase by renaturation

We have surveyed fibroblast lysates for protein kinases that might be involved in mitogenesis. The assay we have used exploits the ability of blotted, sodium dodecyl sulfate-denatured proteins to regain enzymatic activity after guanidine treatment. About 20 electrophoretically distinct protein kinases could be detected by this method in lysates from NIH 3T3 cells. One of the kinases, a 42-kilodalton serine(threonine) kinase (PK42), was found to possess two- to fourfold-higher in vitro activity when isolated from serum-stimulated cells than when isolated from serum-starved cells. This kinase comigrated on sodium dodecyl sulfate-gels with a protein (p42) whose phosphotyrosine content increased in response to serum stimulation. The time courses of p42 tyrosine phosphorylation and PK42 activation were similar, reaching maximal levels within 10 min and returning to basal levels within 5 h. Both p42 tyrosine phosphorylation and PK42 activation were stimulated by low concentrations of phorbol esters, and the responses of p42 and PK42 to TPA were abolished by chronic 12-O-tetradecanoylphorbol-13-acetate (TPA) treatment. Chronic TPA treatment had less effect on serum-induced p42 tyrosine phosphorylation and PK42 activation. PK42 and p42 bound to DEAE-cellulose, and both eluted at a salt concentration of 250 mM. Thus, PK42 and p42 comigrate and cochromatograph, and the kinase activity of PK42 correlates with the tyrosine phosphorylation of p42. These findings suggest that PK42 and p42 are related or identical, that PK42 is activated by tyrosine phosphorylation, and that this tyrosine phosphorylation can be regulated by protein kinase C.

Human p53 is phosphorylated by p60-cdc2 and cyclin B-cdc2

The human anti-oncoprotein p53 is shown to be a substrate of cdc2. The primary site of phosphorylation is serine-315. Serine-315 is phosphorylated by both p60-cdc2 and cyclin B-cdc2 enzymes. The phosphorylation of p53 is cell cycle-dependent. The abundance of p53 also oscillates during the cell cycle. The protein is largely absent from cells that have just completed division but accumulates in cells during G1 phase. Phosphorylation by cdc2 might regulate the antiproliferative activity of p53.

Identification of a 42-kilodalton phosphotyrosyl protein as a serine(threonine) protein kinase by renaturation

We have surveyed fibroblast lysates for protein kinases that might be involved in mitogenesis. The assay we have used exploits the ability of blotted, sodium dodecyl sulfate-denatured proteins to regain enzymatic activity after guanidine treatment. About 20 electrophoretically distinct protein kinases could be detected by this method in lysates from NIH 3T3 cells. One of the kinases, a 42-kilodalton serine(threonine) kinase (PK42), was found to possess two- to fourfold-higher in vitro activity when isolated from serum-stimulated cells than when isolated from serum-starved cells. This kinase comigrated on sodium dodecyl sulfate-gels with a protein (p42) whose phosphotyrosine content increased in response to serum stimulation. The time courses of p42 tyrosine phosphorylation and PK42 activation were similar, reaching maximal levels within 10 min and returning to basal levels within 5 h. Both p42 tyrosine phosphorylation and PK42 activation were stimulated by low concentrations of phorbol esters, and the responses of p42 and PK42 to TPA were abolished by chronic 12-O-tetradecanoylphorbol-13-acetate (TPA) treatment. Chronic TPA treatment had less effect on serum-induced p42 tyrosine phosphorylation and PK42 activation. PK42 and p42 bound to DEAE-cellulose, and both eluted at a salt concentration of 250 mM. Thus, PK42 and p42 comigrate and cochromatograph, and the kinase activity of PK42 correlates with the tyrosine phosphorylation of p42. These findings suggest that PK42 and p42 are related or identical, that PK42 is activated by tyrosine phosphorylation, and that this tyrosine phosphorylation can be regulated by protein kinase C.

Completion of DNA replication is monitored by a feedback system that controls the initiation of mitosis in vitro: studies in Xenopus

During cell division complete DNA replication must occur before mitosis is initiated. Using a cell-free extract derived from Xenopus eggs that oscillates between S phase and mitosis, we have investigated how completion of DNA synthesis is coupled to the initiation of mitosis. We find that Xenopus eggs contain a feedback pathway which suppresses mitosis until replication is completed and that activation of this inhibitory system is dependent on the presence of a threshold concentration of unreplicated DNA. We demonstrate that in the presence of unreplicated DNA the active feedback system inhibits initiation of mitosis by blocking the activation of MPF, a regulator of mitosis found in all eukaryotic cells. Our results demonstrate that the feedback system does not inhibit MPF activation by blocking the synthesis or accumulation of cyclin protein, a subunit of MPF, or by blocking association of cyclin with the cdc2 subunit of MPF. We propose that the feedback system blocks mitosis by maintaining MPF in an inactive state by modulating posttranslational modifications critical for MPF activation.

A novel mammalian protein kinase gene (mak) is highly expressed in testicular germ cells at and after meiosis

We isolated a novel gene designated mak (male germ cell-associated kinase) by using weak cross-hybridization with a tyrosine kinase gene (v-ros). Sequence analysis of the cDNA corresponding to the 2.6-kilobase transcript revealed that the predicted product of rat mak consisted of 622 amino acids and contained protein kinase consensus motifs in its amino-terminal region. Comparison of the deduced amino acid sequence of mak in the kinase domain with those of other protein kinase genes demonstrated that mak was approximately 40% identical to the cdc2-CDC28 gene family in Schizosaccharomyces pombe, Saccharomyces cerevisiae, and humans but less identical to most other protein kinase gene products. Expression of mak was highly tissue specific, and its transcripts were detected almost exclusively in testicular cells entering and after meiosis but hardly detectable in ovarian cells including oocytes, after the dictyotene stage. These results suggest that the mak gene plays an important role in spermatogenesis.

Identification of cell cycle-regulated phosphorylation sites on nuclear lamin C

The mechanism by which MPF induces nuclear lamin disassembly and nuclear envelope breakdown during mitosis was studied in a frog egg extract in which the transition from interphase to mitosis can be induced by the addition of MPF. Bacterially expressed human nuclear lamin C, assembled in vitro into filaments, showed increased phosphorylation on specific sites in the extract in response to MPF. Phosphorylation was accompanied by disassembly of the lamin filaments. We determined the sequences of the sites phosphorylated both in the presence and absence of MPF. The sequence data suggest that multiple protein kinases act on the lamins, and S6 kinase II was identified as one potentially important lamin kinase.

The roles of Drosophila cyclins A and B in mitotic control

We have cloned, sequenced, and characterized the expression of a Drosophila cyclin B gene. The independent evolutionary conservation of A- and B-type cyclins implies that they have distinct roles. Indeed, in mutant embryos deficient in cyclin A, cells that accumulate only cyclin B do not enter mitosis. Thus, in vivo, cyclin B is not sufficient for mitosis. Furthermore, we find that the two cyclins are coexpressed in all proliferating cells throughout development. Though lacking a formal demonstration that cyclin B is essential as it is in other organisms, we propose that each of these proteins fulfills a distinct and essential role in the cell cycle.

The start gene CDC28 and the genetic stability of yeast

The cdc28-srm mutation in Saccharomyces cerevisiae decreases spontaneous and induced mitochondrial rho- mutability and the mitotic stability of native chromosomes and recombinant circular minichromosomes. The effects of cdc28-srm on the genetic stability of cells support the hypothesis that links cell cycle regulation in yeast to changes in chromatin organization dependent on the start gene CDC28 (Hayles and Nurse, 1986).

A novel mammalian protein kinase gene (mak) is highly expressed in testicular germ cells at and after meiosis

We isolated a novel gene designated mak (male germ cell-associated kinase) by using weak cross-hybridization with a tyrosine kinase gene (v-ros). Sequence analysis of the cDNA corresponding to the 2.6-kilobase transcript revealed that the predicted product of rat mak consisted of 622 amino acids and contained protein kinase consensus motifs in its amino-terminal region. Comparison of the deduced amino acid sequence of mak in the kinase domain with those of other protein kinase genes demonstrated that mak was approximately 40% identical to the cdc2-CDC28 gene family in Schizosaccharomyces pombe, Saccharomyces cerevisiae, and humans but less identical to most other protein kinase gene products. Expression of mak was highly tissue specific, and its transcripts were detected almost exclusively in testicular cells entering and after meiosis but hardly detectable in ovarian cells including oocytes, after the dictyotene stage. These results suggest that the mak gene plays an important role in spermatogenesis.

Identification of an activator required for elevation of maturation-promoting factor (MPF) activity by gamma-S-ATP

Maturation-promoting factor (MPF) is a cell cycle control element able to cause cells to enter M-phase upon microinjection and will induce metaphase in nuclei incubated in cell extracts. Previous work has shown that MPF is composed of a complex between p34cdc 2 protein kinase and a B-type cyclin. In the present work gamma-S-ATP was found to cause activation of MPF activity in partially purified preparations, but this activation was lost upon chromatography on Matrex Green gel A. Readdition of other Matrex Green fractions to purified MPF restored the ability of gamma-S-ATP to activate MPF for nuclear breakdown as well as phosphorylation of histone H1. Use of the system described here will facilitate study of p34cdc 2 kinase activation and identification of elements involved in MPF regulation.

Differential phosphorylation of c-Abl in cell cycle determined by cdc2 kinase and phosphatase activity

The product of the c-abl proto-oncogene (c-Abl) is phosphorylated on three sites during interphase and seven additional sites during mitosis. Two interphase and all mitotic c-Abl sites are phosphorylated by cdc2 kinase isolated from either interphase or mitotic cells, with the mitotic cdc2 having an 11-fold higher activity. Inhibition of phosphatases with okadaic acid in interphase cells leads to the phosphorylation of c-Abl mitotic sites, indicating that those sites are preferentially dephosphorylated during interphase. The differential phosphorylation of c-Abl in the cell cycle is therefore determined by an equilibrium between cdc2 kinase and protein phosphatase activities. Treatment of interphase cells with okadaic acid leads to a rounded morphology similar to that observed during mitosis.

Universal control mechanism regulating onset of M-phase

The onset of M-phase is regulated by a mechanism common to all eukaryotic cells. Entry into M-phase is determined by activation of the p34cdc2 protein kinase which requires p34cdc2 dephosphorylation and association with cyclin.

Light-induced dephosphorylation of a 33 kDa protein in the wild-type strain of Neurospora crassa: the regulatory mutants wc-1 and wc-2 are abnormal

Light induces the dephosphorylation of a 33 kdalton protein within 8 min in the wild-type strain of Neurospora crassa. The regulatory mutants, wc-1 and wc-2, have an altered pattern of phosphoproteins in darkness and also after irradiation. Because the wc genes have previously been implicated in photodifferentiation (F. Degli Innocenti and V. E. A. Russo, Genetic analysis of blue light-induced responses in Neurospora crassa, in H. Senger (ed.), Blue Light Effects in Biological Systems, Springer-Verlag, Berlin, Heidelberg, 1984, pp. 213-219), we suggest that protein dephosphorylation may constitute a necessary step in the light-transduction chain of Neurospora crassa.