stf1: a new suppressor of the mitotic control gene, cdc25, in Schizosaccharomyces pombe

A novel element in the mitotic control, stf1, has been identified genetically by its ability to rescue cdc25-22 as well as a gene disruption of cdc25. This is the first phenotypically non-wee mutation shown to do so. stf1-1 functions additively with cdc2-1w, cdc2-3w, or wee1-6 to rescue cdc25. The available data are consistent with the wild-type gene product operating either on the same pathway as cdc25 or to stimulate cdc2 by a pathway independent of cdc25 or wee1. The stf1 gene has been cloned and sequenced and encodes a putative protein of 50-65 kD, depending on whether a potential intron is present. It is a novel protein with no homology detected in the current data bases. When challenged with hydroxyurea, stf1-1 acts additively with cdc2-3w in rescuing cdc25 mutants and in allowing mitosis to occur without DNA synthesis. It does not appear to play a role in the nutritional sensing pathway nor in the pathway mediating radiation-induced G2 delay.

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.

CDC6 mRNA fluctuates periodically in the yeast cell cycle

Using cultures synchronized by two independent procedures, alpha-factor arrest and centrifugal elutriation, we have investigated the expression of the Saccharomyces cerevisiae CDC6 gene through the cell cycle. Our results show that the CDC6 gene is periodically expressed in the yeast cell cycle. The level of CDC6 transcripts increases in late G1, reaching a peak (approximately 10-20-fold over the initial level) at about the G1/S phase boundary. The peak of CDC6 mRNA was observed to overlap or slightly precede that of the CDC8 message, and to obviously precede that of the histone H2A message by some 25 min. Unlike histone H2A mRNA, the CDC6 mRNA as well as CDC8 mRNA were not affected by hydroxyurea treatment. These results suggest that regulation of H2A mRNA is different from that of CDC6 or CDC8. We have studied the 5'-flanking regions of CDC6 and other cell cycle-regulated genes. DNA sequence analysis of the CDC6 promoter revealed two sequences, 5'-C/GACGCGNC/G-3' and 5'-PuGNAGAAA-3' (where Pu is a purine, and N is any nucleotide), which are repeated three times each. Similar sequence elements have also been found among several cell cycle-regulated genes, including the CDC8 gene, but are not found upstream of histone genes. The possible significance of these elements is discussed.

Purification and biochemical characterization of statin, a nonproliferation specific protein from rat liver

The nuclear protein statin, detectable with specific monoclonal antibodies, is found mostly in nonproliferating cells (Wang, E. (1985) J. Cell Biol. 100, 545-551). In the rat liver a 57-kDa protein designated as rat liver protein 57 (RLp57) was recently identified to carry the epitope for the anti-statin-specific monoclonal antibody, S-44 (Sester, U., Moutsatsos, I. K., and Wang, E. (1989) Exp. Cell Res. 182, 550-558). To characterize further the RLp57 protein, in the present study a polyclonal antibody was raised to the RLp57 protein eluted from polyacrylamide gel. Similar to the anti-statin monoclonal antibody, this polyclonal antibody recognizes a nuclear antigen in nonproliferating fibroblasts and reacts with a 57-kDa protein in rat liver and nonproliferating cells strongly suggesting that RLp57 is a statin protein from rat liver. Two isoforms of RLp57 (isoelectric points between 6.5 and 7.0) were detected after two-dimensional gel electrophoresis and immunoblotting. RLp57 was purified using multiple chromatographic steps, including ion-exchange and affinity chromatography followed by chromatofocusing. These results show that RLp57, a statin protein found in liver, has two isoelectric variants and can be purified to apparent homogeneity by sequential steps of chromatographic procedures.

The C-terminal part of the CDC25 gene product plays a key role in signal transduction in the glucose-induced modulation of cAMP level in Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, addition of glucose to cells grown under glucose-derepressed conditions induces a transient rise in the intracellular level of cAMP. This modulation requires functional elements of the cAMP-producing pathway, adenylate cyclase, ras proteins and the product of CDC25 gene. To determine whether or not the CDC25 gene product is a transducing element in the signal-transmission pathway leading from glucose to ras adenylate cyclase we have made use of the mutated allele RAS2Ile152 whose gene product uncouples the product of CDC25 from adenylate cyclase, but does not promotes other secondary phenotypes. The transient increase in cAMP is lost in cells lacking a functional CDC25 gene product, although they produce a normal amount of cAMP with the RAS2Ile152 gene. This result demonstrates the requirement of CDC25 for mediation of glucose signal transmission. The fact that cells grow normally on glucose in the absence of glucose-induced cAMP signaling confirms that this signaling pathway is not essential for growth on glucose. To further analyze the role of the CDC25 gene product we have made use of truncated versions of the gene. The results show that the C-terminal part of the gene alone is able to mediate glucose-induced activation of the RAS adenylate cyclase pathway.

The glucose-induced polyphosphoinositides turnover in Saccharomyces cerevisiae is not dependent on the CDC25-RAS mediated signal transduction pathway

Recently the polyphosphoinositides (PI) turnover has been related to the control of growth and cell cycle also in Saccharomyces cerevisiae, and the RAS2 and RAS1 gene products have been shown to be involved in the stimulation of PI turnover in G0/G1 arrested yeast cells. Here we show that addition of glucose to previously glucose-starved cells, stimulates, the PI turnover with fast kinetics also in yeast cells that were not arrested in the G0/G1 phase of the cell cycle. In addition PI turnover is equally stimulated in temperature sensitive cdc25-1 and cdc25-5 strains at restrictive temperature, as well as in ras1, ras2-ts strain, suggesting that PI turnover stimulation is not dependent on the CDC25-RAS mediated signal transduction pathway.

Complementation of fission yeast cdc2ts and cdc25ts mutants identifies two cell cycle genes from Drosophila: a cdc2 homologue and string

We have exploited the universality of the molecular mechanisms that control entry into mitosis to clone the Drosophila melanogaster homologues of fission yeast Schizosaccharomyces pombe cell division control (cdc) genes by the complementation of temperature sensitive mutations. The Drosophila genes were expressed in S.pombe as cDNAs from the SP6 promoter. Successful recovery of complementing plasmids required that we first 'adapt' pooled plasmids from a Drosophila embryonic cDNA library for propagation in fission yeast by introducing an ars1-LEU2 DNA fragment into the vector. This library was introduced into S.pombe cdc2 and cdc25 mutants, and plasmids isolated carrying cDNAs that complement these mutations. The gene that encodes the Drosophila cdc2 homologue maps to a single locus in the Drosophila genome at 31E on chromosome 2. It is expressed maternally to provide mRNA in syncytial embryos, and appears to be zygotically expressed in mitotically active regions of the cellularized embryo. We have isolated two different cDNAs that complement cdc25-22. One corresponds to a transcript of string, previously described as the Drosophila homologue of cdc25, and the other to a gene that has not been previously characterized.

The phenotype of the minichromosome maintenance mutant mcm3 is characteristic of mutants defective in DNA replication

MCM3 is an essential gene involved in the maintenance of minichromosomes in yeast cells. It encodes a protein of 971 amino acids that shows striking homology to the Mcm2 protein. We have mapped the mcm3-1 mutation of the left arm of chromosome V approximately 3 kb centromere proximal of anp1. The mcm3-1 mutant was found to be thermosensitive for growth. Under permissive growth conditions, it was defective in minichromosome maintenance in an autonomously replicating sequence-specific manner and showed an increase in chromosome loss and recombination. Under nonpermissive conditions, mcm3-1 exhibited a cell cycle arrest phenotype, arresting at the large-bud stage with an undivided nucleus that had a DNA content of nearly 2n. These phenotypes are consistent with incomplete replication of the genome of the mcm3-1 mutant, possibly as a result of limited replication initiation at selective autonomously replicating sequences leading to cell cycle arrest before mitosis. The phenotype exhibited by the mcm3 mutant is very similar to that of mcm2, suggesting that the Mcm2 and Mcm3 protein may play interacting roles in DNA replication.

Related calcium-binding proteins map to the same subregion of chromosome 1q and to an extended region of synteny on mouse chromosome 3

The serum protein cystic fibrosis-associated antigen (CFAG), present at elevated levels in CF homozygotes and heterozygotes, is now known to consist of two distinct but related subunits (calgranulins A (CAGA) and B (CAGB)). Both show similarity to the S100-related calcium-binding proteins. We have previously assigned CAGA to human chromosome 1q12-q21 and demonstrate here that the cDNA probe for CAGB cosegregates with it in our somatic cell hybrid panel. cDNA probes for the related genes calcyclin (CACY) and a mouse placental protein (18A2, suggested name Capl) enabled us to confirm and refine the in situ hybridization result assigning CACY to chromosome 1q21-25 and to demonstrate that both genes cosegregate with CAGA and CAGB. Capl was mapped to a region of chromosome 3 in the mouse using the BXD recombinant inbred strain mice where the p11 protein (calpactin light chain Cal1l), another S100 family member, has been localized. Cacy is shown to be within 8 kb of Capl in the mouse genome.

Overexpression of the CDC25 gene, an upstream element of the RAS/adenylyl cyclase pathway in Saccharomyces cerevisiae, allows immunological identification and characterization of its gene product

The product of the START gene CDC25, an upstream element of the RAS/adenylyl cyclase pathway in Saccharomyces cerevisiae, was identified using specific antibodies raised against a chimeric beta-galactosidase/CDC25 protein. The CDC25 protein is poorly expressed and can be detected only when the CDC25 gene is overexpressed under the control of the galactose-inducible GAL1-10 strong promoter elements. It has a molecular weight of 180,000, is not glycosylated and is strongly associated with the particulate fraction. After deletion of residues 1255-1550 the protein is found in the soluble fraction.

Expression of statin, a non-proliferation-dependent nuclear protein, in the postnatal rat brain: evidence for substantial retention of neuroglial proliferative capacity with aging

Statin is a 57 kDa protein expressed in nuclei of reversibly and irreversibly growth-arrested (Go-phase) cells. In this report, immunohistochemical localization of statin in the developing and aging rat brain was achieved using the monoclonal antibody, S-44. On postnatal day 2, post-migratory neurons in the developing cerebral cortex were statin-positive. Many statin-negative cells were observed in the lateral subependymal zone of the lateral ventricle. By postnatal day 10, most neuronal nuclei were statin-positive although small numbers of statin-negative neurons were still encountered in the lateral subependymal zone and hippocampal dentate gyrus. At 3, 18 and 33 months, all neuronal nuclei surveyed were statin-positive. These results support the contention that, save for the postnatal persistence of 'germinal zones' such as the subependymal region and dentate gyrus, neuronal proliferation in the rat is largely completed by the time of birth. In striking contrast to neuronal statin expression, a significant fraction of neuroglia in both grey and white matter remains statin-negative at all ages examined. In the corpus callosum, 33.2%, 34.0% and 34.7% of glial nuclei were statin-negative at 3, 18 and 33 months, respectively. These findings indicate that: (i) even in senescent brain, the cycling (statin-negative) glial pool is substantially larger than previously surmised from S-phase labeling experiments; and (ii) during aging, the ratio of noncycling-to-cycling neuroglia remains very tightly regulated. Examination of other non-neuronal cell types revealed that most, if not all, ependymal and choroid plexus epithelial cells were statin-positive in the neonatal and adult brains in keeping with the predominantly prenatal proliferation of these tissues. Our results indicate that statin immunolabeling using the S-44 antibody is a powerful technique for the in situ identification of non-proliferating cells in the developing and aging nervous system.

stf1: non-wee mutations epistatic to cdc25 in the fission yeast Schizosaccharomyces pombe

In Schizosaccharomyces pombe, cdc25 is a cell cycle regulated inducer of mitosis. wee1 and phenotypically wee alleles of cdc2 are epistatic to cdc25. Mutant alleles of a new locus, stf1 (suppressor of twenty-five), identified in a reversion analysis of conditionally lethal cdr1-76 cdc25-22 and cdr2-96 cdc25-22 double mutant strains, also suppress both temperature-sensitive and gene disruption alleles of cdc25. These mutants, by themselves, are phenotypically indistinguishable from wild type strains; hence they represent the first known mutations that are epistatic to cdc25 and do not display a wee phenotype. stf1 genetically interacts with other elements of mitotic control in S. pombe. stf1-1 is additive with wee1-50, cdc2-1w and cdc2-3w for suppression of cdc25-22. Also, like wee1- and cdc2-w, stf1- suppression of cdc25 is reversed by overexpression of the putative type 1 protein phosphatase bws1+/dis2+. Interaction with various mutants and plasmid overexpression experiments suggest that stf1 does not operate either upstream or downstream of wee1. Similarly, it does not operate through cdc25 since it rescues the disruption. stf1 appears to encode an important new element of mitotic control.

Characterization of a sperm-specific nuclear autoantigenic protein. I. Complete sequence and homology with the Xenopus protein, N1/N2

In our studies on specific sperm proteins that function in fertilization, an autoantigenic, postacrosomal sperm protein has been found to originate in the testis as a nuclear-associated protein. This nuclear autoantigenic sperm protein (NASP) contains a C-terminal nuclear translocation signal and has structural similarities to the lamins and other nuclear proteins; and its 2.5 kb mRNA is apparently tissue-, but not species-, specific. DNA clones from a rabbit testis cDNA library and a rabbit genomic library were sequenced in order to characterize NASP. The polyadenylated mRNA has 39 bases of 5' untranslated sequence, an open reading frame of 2043 bases encoding 680 amino acids, and a 104 base 3' untranslated region (2,186). The encoded polypeptide has a calculated molecular weight of 73,533 and a pI = 4.06, containing 25% acidic residues. One clone (R1.2) expressing the C-terminal 446 amino acids was used to express a fusion protein. The expressed R1.2/beta-galactosidase fusion protein was found to be autoantigenic. Secondary structure predictions for NASP showed that 69% of the molecule had a high probability of forming alpha-helices and that several alpha-helical regions had a characteristic repeating heptad pattern that in the intermediate filaments and nuclear lamins is involved in coiled-coil interactions with other molecules. In addition to the nuclear translocation signal common to many nuclear proteins, NASP also showed homology with the Xenopus histone-binding protein, N1/N2.

A yeast mutant, PRP20, altered in mRNA metabolism and maintenance of the nuclear structure, is defective in a gene homologous to the human gene RCC1 which is involved in the control of chromosome condensation

We report on the characterization of the yeast prp20-1 mutant. In this temperature-sensitive mutant, multiple steps of mRNA metabolism are affected. The prp20-1 mutant strain showed alterations in mRNA steady-state levels, defective mRNA splicing and changes in transcription initiation or termination when shifted from the permissive to the non-permissive temperature. In addition, a change in the structure of the nucleus in these cells became apparent. Electron microscopy revealed an altered structure of the nucleoplasm of prp20-1 mutant cells when grown at the non-permissive temperature that was not observed in cells grown at the permissive temperature or in wild-type cells. The wild-type PRP20 gene was isolated and sequenced. The putative PRP20 protein has a molecular weight of 52 kDa. We found that the PRP20 gene is identical to the yeast SRM1 gene (Clark and Sprague 1989). In addition, the PRP20 protein sequence shows significant sequence similarity to the human RCC1 protein (Ohtsubo et al. 1987). This protein has been implicated in the control of chromosome condensation. Based on the phenotype of the prp20-1 mutant and the observed sequence similarity to the human RCC1 protein, we postulate that the yeast PRP20 protein is involved in the control of nuclear organization.

A negative regulator of mitosis in Aspergillus is a putative membrane-spanning protein

The temperature-sensitive cell cycle mutation bimE7 of Aspergillus nidulans causes cells to become blocked in mitosis at a restrictive temperature. Previous work has shown that this mitotic block is induced even when cells are arrested in the S or G2 phase. The mitotic block is also observed in cells carrying a null mutation in bimE, obtained by molecular disruption of the gene (Osmani, S.A., Engle, D.B., Doonan, J.H., and Morris, N.R. (1988) Cell 52, 241-251), indicating that a lack of bimE function is responsible for the phenotype. We have cloned the bimE gene by complementation of the mutant phenotype and have isolated and sequenced its corresponding cDNA. The gene product is encoded by a 6.5-7-kilobase mRNA. The deduced amino acid sequence suggests a protein with three transmembrane domains. The sequence contains numerous potential N-glycosylation sites and several putative cAMP-dependent phosphorylation sites. No homologous protein sequences were found in the common data bases. The bimE gene product is a novel component in the regulation of mitosis.

Site-directed mutagenesis of the Saccharomyces cerevisiae CDC25 gene: effects on mitotic growth and cAMP signalling

A potential membrane-interacting site within the essential growth-controlling carboxy-terminal region of the CDC25 protein was interrupted by a lethal mutation (1461 Tyr----Asp and 1462 Leu----Arg). The elimination of two potential phosphorylation sites found in the same region (1489 Thr----Pro and 1584 Ser----Pro) does not affect growth but completely prevents glucose-induced cAMP signalling in the double mutant, whereas the single mutants produce normal or slightly retarded cAMP signals. A cluster of five potential targets for cAMP-dependent phosphorylation at the amino-terminal region could be deleted without affecting phenotypic properties. It is concluded that the carboxy-terminal 137 residues of the CDC25 protein are involved in three different functions: control of mitotic growth, glucose-induced hyperactivation of adenylate cyclase, and feed-back inhibition of cAMP synthesis.

Statin expression in the untreated and SarCNU-exposed human glioma cell line, SK-MG-1

Cytokinetic analyses of gliomas and other neoplasms rely exclusively on the use of proliferation-dependent markers such as [3H]-thymidine and BuDR incorporation and the detection of growth-dependent proteins such as proliferating cell nuclear antigen (PCNA) and Ki-67. In normal tissues, the monoclonal antibody S-44 recognizes statin, a nuclear protein expressed only in nonproliferating cells. In the present study, indirect immunofluorescence microscopy using S-44 identified nuclear statin in 5.9% of a population of untreated human SK-MG-1 glioma cells in vitro. Incremental doses of the alkylating agent sarcosinamide chloroethylnitrosourea (SarCNU) induced a linear increase in the fraction of statin-positive SK-MG-1 cells. Labeling of nuclear statin with the monoclonal antibody S-44 may be a potentially useful marker of the cytotoxic effects of anticancer drugs in gliomas and other neoplastic tissues.

Cell cycle regulation of p34cdc2 kinase activity in Physarum polycephalum

The regulation of the mitotic histone H1 kinase activity has been analyzed during the naturally synchronous cell cycle of Physarum polycephalum plasmodia. The universal binding property of the p13suc1 Schizosaccharomyces pombe gene product was used to precipitate and assay the cdc2 histone H1 kinase activity. The kinase activity peaks at the beginning of metaphase and its decline, which requires protein synthesis, appears to be an early event during the metaphase process. Microtubular poisons, temperature shifts and DNA synthesis inhibitors were used to perturb cell cycle regulatory pathways and characterize their effects on cdc2 kinase activation. Our results suggest that the full activation of the mitotic kinase requires at least two successive triggering signals involving microtubular components and DNA synthesis.

Identification of a serum-inducible messenger RNA (5B10) as the mouse homologue of calcyclin: tissue distribution and expression in metastatic, ras-transformed NIH 3T3 cells

A mouse mRNA, provisionally designated 5B10, has been cloned based on its inducibility by serum in quiescent murine fibroblasts. Here we report the full-length complementary DNA sequence and a partial characterization. There are about five copies of the gene in the mouse genome. Sequence analysis of the 5B10 coding region reveals 94 and 97% amino acid identity to human and rat calcyclin, respectively. Although the coding region has been highly conserved during evolution of the rodent and human genomes, the untranslated flanking sequences differ significantly. A protein of Mr about 8000 was produced by in vitro translation of the mRNA transcribed in vitro from 5B10 complementary DNA in a riboprobe vector. An antiserum raised against a portion of the predicted human calcyclin protein cross-reacted with this mouse protein. 5B10 mRNA was found in greatest amount in organs containing proliferating cells, e.g., epidermis, skin, stomach, uterus of pregnant mouse, placenta, and decidua. Brain, liver, mature thymus, and skeletal muscle had little or no detectable 5B10 mRNA. 5B10 mRNA levels were higher in cells treated with 7,12-dimethylbenzanthracene and 12-O-tetradecanoylphorbol-13-acetate than in their normal counterparts, suggesting a role in tumorigenesis. In addition, high 5B10 mRNA levels were associated with metastatic ability in a series of ras-transformed cells, in proportion to levels of ras p21 expressed by the cells, implicating 5B10 even more deeply in carcinogenesis.

DNA binding activity and transcriptional activator function of the human B-myb protein compared with c-MYB

Three members of the myb gene family have been identified in human cDNA libraries c-myb, A-myb, and B-myb. We compared the DNA binding properties of the B-myb and c-myb proteins (B-MYB and c-MYB) using bacterially synthesized B-MYB and c-MYB in DNase I footprinting. B-MYB bound to most of the c-MYB binding sites examined, including the c-MYB binding site, MBS-I, in the simian virus (SV) 40 enhancer, in which the most frequent sequence was CCTAACTG. The MBS-I site was an enhancer element dependent on B-MYB and c-MYB in a co-transfection assay that used the B-myb or c-myb expression plasmid. Some sites in the SV40 genome, including the MBS-BI site, had high affinity with B-MYB but little or no affinity with c-MYB, in which the most frequent sequence was AGAAANPyrG. The MBS-BI site was an enhancer element dependent on B-MYB and a very weakly dependent on c-MYB. Our results showed that B-MYB is a transcriptional activator, like c-MYB, and that although B-MYB and c-MYB have similar sequence specificity for DNA binding some sequences were recognized by B-MYB preferentially.

Isolation and sequence analysis of CDC43, a gene involved in the control of cell polarity in Saccharomyces cerevisiae

The Saccharomyces cerevisiae CDC43 gene product is involved in establishing cell polarity during the cell-division cycle. When grown at restrictive temperatures, temperature-sensitive cdc43 mutants are unable to form buds and display delocalized cell-surface deposition [Adams et al., J. Cell Biol. (1990) in press]. We have isolated a cdc43-complementing plasmid from a yeast genomic-DNA library and localized the CDC43 gene, by subcloning and transposon-mutagenesis experiments, to a 1.2-kb region of DNA that contained only one significant ATG-initiated open reading frame of 213 codons. The putative CDC43 gene product contains a possible nuclear-localization signal sequence, a cysteine-rich domain and a histidine-rich domain, and a region that is similar in structure to alpha-helix-turn-alpha-helix structural domains present in some prokaryotic and eukaryotic DNA-binding proteins.

Calcyclin is a calcium and zinc binding protein

Calcyclin, a cell cycle regulated protein, was recently purified from Ehrlich ascites tumour (EAT) cells and shown to be a calcium binding protein. Here we show that calcyclin monomer and dimer also bind zinc ions. Zinc binding sites seem to be different from calcium binding sites since: preincubation with Ca2+ lacks effect on the binding of Zn2+, and Ca2+ (but not Zn2+) increases tyrosine fluorescence intensity. Binding of Zn2+ reduces the extent of the conformational changes induced by Ca2+, and seems to affect Ca2(+)-binding. The data suggest that Ca2+ and Zn2+ might trigger the biological activity of calcyclin.

Enhancement of the GDP-GTP exchange of RAS proteins by the carboxyl-terminal domain of SCD25

In Saccharomyces cerevisiae, the product of the CDC25 gene controls the RAS-mediated production of adenosine 3',5'-monophosphate (cAMP). In vivo the carboxyl-terminal third of the CDC25 gene product is sufficient for the activation of adenylate cyclase. The 3'-terminal part of SCD25, a gene of S. cerevisiae structurally related to CDC25, can suppress the requirement for CDC25. Partially purified preparations of the carboxy-terminal domain of the SCD25 gene product enhanced the exchange rate of guanosine diphosphate (GDP) to guanosine triphosphate (GTP) of pure RAS2 protein by stimulating the release of GDP. This protein fragment had a similar effect on the human c-H-ras-encoded p21 protein. Thus, the SCD25 carboxyl-terminal domain can enhance the regeneration of the active form of RAS proteins.

A versatile microtiter assay for the universal cdc2 cell cycle regulator

A microassay for p34cdc2 based on the high affinity association between cdc2 and Schizosaccharomyces pombe p13suc1 has been developed. p13 purified from Escherichia coli was immobilized on microtiter plates and cellular lysate was incubated in the wells to allow the binding of cdc2 and its associated proteins. p34cdc2 was assayed either as a histone kinase or by immunological methods. The method was optimized for S. pombe cell extracts but can also be applied to other organisms such as Xenopus oocytes or HeLa cells. This rapid assay allows the specific determination of p34cdc2 histone H1 kinase activity in a very large number of samples.

Intramitotic controls in the fission yeast Schizosaccharomyces pombe: the effect of cell size on spindle length and the timing of mitotic events

We have used a new cinemicroscopy technique in combination with antitubulin immunofluorescence microscopy to investigate the timing of mitotic events in cells of the fission yeast Schizosaccharomyces pombe having lengths at division between 7 and 60 microns. Wild-type fission yeast cells divide at a length of 14 microns. Separation of daughter nuclei (anaphase B) proceeds at a rate of 1.6 +/- 0.2 microns min-1, until the spindle extends the length of the cell. Coincident with spindle depolymerization, the nuclei reverse direction and take up positions that will become the center of the two daughter cells. This post-mitotic nuclear migration occurs at a rate of 1.4 +/- 0.5 microns-1. In cells in which the weel+ gene is overexpressed fivefold and that have an average length at mitosis of 28 microns, the rate of nuclear separation was only slightly reduced but, as spindles in these cells measure 20-22 microns, the duration of anaphase B was extended by approximately 40%. By contrast, in the mutant weel.50, which divides at 7 microns, both the rate and duration of anaphase B were indistinguishable from wild type. Nuclei reach the ends of these cells earlier but remain there until a point corresponding to the time of postmitotic nuclear migration in wild type. Thus, the events of mitosis can be extended but not abbreviated. These results are discussed in terms of a mitotic termination control that monitors many different events, one of which is spindle elongation.

The relationship of expression of statin, the nuclear protein of nonproliferating cells, to the differentiation and cell cycle of astroglia in cultures and in situ

Cells in the quiescent, nonproliferative state express a protein, statin, in their nuclei. When the cells reenter the cell cycle, statin disappears and another protein, cyclin, appears. We have examined mouse astroglia at various stages of differentiation in cultures and astroglia in adult mouse brains for the presence of statin. In cultures initiated from the neopallium of newborn mice, the glial fibrillary acidic protein (GFAP)+ stellate astrocytes were statin-negative (statin-) but cyclin-positive (cyclin+). In the same cultures, large flat cells (senescent cells) were statin+ but cyclin-. In frozen sections of the brains of adult mice and in brain smears, GFAP+ astrocytes were statin-. Neither stellate astrocytes grown in cultures for 30 or more days nor astrocytes in adult mouse brain were labeled when pulsed with bromodeoxyuridine (BudR). When astroglia were treated with dibutyrl cyclic adenosine monophosphate (dBcAMP), large stellate cells that closely resemble reactive astrocytes in situ formed. These cells were all statin+ from 11-62 days in vitro; however, reactive astrocytes in mouse neopallium, 4-50 days after a stab wound, were statin-. In colony cultures, senescent cells became statin+, whereas stellate astrocytes and their precursor cells remained statin-. These observations indicate that normal astrocytes both in cultures and in situ retain the potential to divide and probably progress through the cell cycle at a very slow rate.

Regulation of mitosis by cyclic accumulation of p80cdc25 mitotic inducer in fission yeast

The coordination of somatic cell division with cell size must be accomplished by the accumulation of mitotic inducers or the dilution, in the course of cell growth, of mitotic inhibitors. In fission yeast (Schizosaccharomyces pombe), cell size at mitosis is determined by expression of the cdc25+ and nim1+ inducer genes and of the inhibitor gene wee1+, which between them regulate the M-phase protein kinase p34cdc2. We now report that both the phosphoprotein product of cdc25+ (p80cdc25, with apparent relative molecular mass 80,000) and the corresponding messenger RNA increase in concentration as cells proceed through interphase, peaking at mitosis. We propose that the cell-cycle timing of mitosis in somatic cells is regulated by the cyclic accumulation of the cdc25 mitotic inducer, which on reaching a critical level results in activation of p34cdc2 protein kinase. Accumulation of this inducer could play a part in coordinating cell division with growth.

Initiation of meiosis and sporulation in Saccharomyces cerevisiae requires a novel protein kinase homologue

SME1 was cloned due to its high copy number effect: it enabled MATa/MAT alpha diploid cells to undergo meiosis and sporulation in a vegetative medium. Disruption of SME1 resulted in a recessive Spo- phenotype. These results suggest that SME1 is a positive regulator for meiosis. DNA sequencing analysis revealed an open reading frame of 645 amino acids. An amino terminal peptide of ca 400 amino acids in the deduced protein was similar to known protein kinases. Transcription of SME1 was regulated negatively by nitrogen and glucose and positively by MATa/MAT alpha and IME1, another positive regulator gene of meiosis. By complementation analysis, SME1 was found to be identical to IME2, which had been shown to be important in meiosis. These results suggest that IME1 product stimulates meiosis by activating transcription of SME1 (IME2) and that protein phosphorylation is required for initiation of meiosis.

Comparison of thermosensitive alleles of the CDC25 gene involved in the cAMP metabolism of Saccharomyces cerevisiae

The CDC25 gene from Saccharomyces cerevisiae is an essential component of the RAS-adenylate cyclase pathway. Genetic and biochemical evidence has led to the proposal that the gene product may act upstream of RAS, possibly as a guanine nucleotide exchange factor. We report here the cloning, sequencing and characterization of four mutations in the CDC25 gene. All four are missense mutations which reside within the carboxy-terminal quarter of the single open reading frame found within the gene. Three of the four are missense mutations in the same amino acid codon. A search of protein data bases reveals that the carboxy terminus of the putative CDC25 gene product is similar to that of LTE1, a gene required for growth at low temperature and SCD25, a suppressor of cdc25. Taken together these data indicate that the carboxy terminus of CDC25 plays a critical role in the function of the CDC25 gene product and that other proteins, such as LTE1 or SCD25, may have related activities.

In vitro reconstitution of cdc25 regulated S. cerevisiae adenylyl cyclase and its kinetic properties

The attenuated GTP regulation adenylyl cyclase (CDC35) lysates or membranes prepared from cells of a cdc25ts strain is enhanced 2.5- to 6-fold by mixing these lysates or membranes with lysates or membranes from a cdc35ts strain harboring wild-type CDC25. The kinetics of activation of the Saccharomyces cerevisiae adenylyl cyclase in vitro is first order, as is the activation of mammalian adenylyl cyclase. The rate of enzyme activation in the presence of non-hydrolysable analogs of GTP increases with the number of CDC25 gene copies present in the cell. When GppNHp was used the rate of activation of the cyclase in a strain harboring a multicopy plasmid of CDC25 was 7.0-fold higher than the rate in an isogenic strain with the cdc25-2 mutation. The rate of adenylyl cyclase activation from a strain with a disrupted CDC25 gene is 14.7-fold lower than the rate in an isogenic strain containing the CDC25 gene on a multicopy plasmid. The reconstitution experiments described provide direct biochemical evidence for the role of the CDC25 protein in regulating the RAS dependent adenylyl cyclase in S.cerevisiae. The reconstitution experiments and the kinetic experiments may also provide a biochemical assay for the CDC25 protein and can form the basis for its characterization. In this study we also show that adenylyl cyclase activity in ras1ras2byc1 cells is found in the soluble fraction, whereas in wild-type strain it is found in the membrane fraction. Overexpression of the gene CDC25 in the ras1ras2bcy1 strain relocalizes adenylyl cyclase activity to the membrane fraction. This finding suggests a biochemical link between CDC25 and CDC35 in the absence of RAS, in addition to its role in regulating RAS dependent adenylyl cyclase.

Fission yeast cdc25 is a cell-cycle regulated protein

Fission yeast cell division is initiated by the cdc2/cdc13-cyclin protein kinase which in its catalytically active state comprises the mitotic inducer. During interphase the cdc2/cyclin complex is assembled in an inactive state that requires cdc25+ gene function for M-phase activation. The cdc25+ product, a 76 kd phosphoprotein, is shown to oscillate in abundance during the cell cycle, reaching a peak at G2/M, and to be sensitive to nitrogen starvation. The level of cdc25 is subject to feedback regulation involving both cdc25 and cdc2.

Characterization of two populations of statin and the relationship of their syntheses to the state of cell proliferation

Statin has previously been identified to be a 57-kD protein present in the nuclei of quiescent and senescent human fibroblasts, but not in their replicating counterparts (Wang, E. 1985. J. Cell Biol. 100: 545-551). In the present report we demonstrate by immunoprecipitation analysis of fractionated cellular extracts the existence of two populations of statin. The Triton X-100-soluble statin is found in replicating sparse cultures as well as in quiescent confluent cultures and quiescent serum-starved cultures of young human fibroblasts, but the Triton X-100-insoluble, nuclear envelope-localized statin is present only in the quiescent cultures. Two-dimensional gel analysis of the immunoprecipitated cellular fractions reveals that both populations of statin have an isoelectric point of 5.3. Pulse-chase experiments show that statin is synthesized as a 57-kD polypeptide and is not processed from a precursor of different molecular mass. Experiments on serum stimulation of quiescent cells show that synthesis of the Triton X-100-insoluble statin decreases rapidly during the transition from the G0 to S phase, and that this decrease is accompanied by a slower reduction in synthesis of the Triton X-100-soluble statin. These results suggest that the cellular expression of the two populations of statin may be associated with the mechanisms controlling the transition between the growing state and the quiescent state and confirm the previous finding that the Triton X-100-insoluble, nuclear envelope-localized statin could be used as a marker for cells arrested at the G0 phase of the cell cycle.

Premature chromosome condensation is induced by a point mutation in the hamster RCC1 gene

At the nonpermissive temperature, premature chromosome condensation (PCC) occurs in tsBN2 cells derived from the BHK cell line, which can be converted to the Ts+ phenotype by the human RCC1 gene. To prove that the RCC1 gene is the mutant gene in tsBN2 cells, which have RCC1 mRNA and protein of the same sizes as those of BHK cells, RCC1 cDNAs were isolated from BHK and tsBN2 cells and sequenced to search for mutations. The hamster (BHK) RCC1 cDNA encodes a protein of 421 amino acids homologous to the human RCC1 protein. In a comparison of the base sequences of BHK and BN2 RCC1 cDNAs, a single base change, cytosine to thymine (serine to phenylalanine), was found in the 256th codon of BN2 RCC1 cDNA. The same transition was verified in the RCC1 genomic DNA by the polymerase chain reaction method. BHK RCC1 cDNA, but not tsBN2 RCC1 cDNA, complemented the tsBN2 mutation, although both have the same amino acid sequence except for one amino acid at the 256th codon. This amino acid change, serine to phenylalanine, was estimated to cause a profound structural change in the RCC1 protein.

Cloning of Xenopus RCC1 cDNA, a homolog of the human RCC1 gene: complementation of tsBN2 mutation and identification of the product

We isolated a cDNA clone from Xenopus cDNA libraries for the homolog of the human RCC1 gene, which is considered to be a regulator for the onset of chromosome condensation. The cloned Xenopus RCC1 cDNA encoded a protein of 424 amino acids which had the same seven homologous repeated domains of about 60 amino acids as found in human RCC1 cDNA. The overall identity of amino acid sequence between the human and Xenopus RCC1 protein was 76%. Specifically, in the repeated domain the amino acid sequence was highly conserved between both species. The identity of amino acids in this region was 82%. In the N-terminal region, albeit the overall identity was low (35%), some positively charged amino acids were conserved. The transcript for Xenopus RCC1 gene with the length of 2.2-kb was detected in both Xenopus oocytes and cultured somatic cells, A6. The antibody prepared against Xenopus RCC1 protein produced in E. coli recognized 45 and 46 kDa proteins not only in Xenopus oocytes and A6 cells, but also in human and hamster cells. Xenopus RCC1 cDNA complemented the tsBN2 mutation, depending on the amount of its product in transformants. Thus, the RCC1 protein was suggested to regulate the onset of chromosome condensation in the eukaryote from amphibian to mammalian cells.

The differentiation process of intestinal epithelial cells is associated with the appearance of statin, a non-proliferation-specific nuclear protein

The expression of statin, a 57,000 Mr nuclear protein specifically present in non-proliferating cultured fibroblasts, was studied in vivo in the differentiating epithelial cells of the rat intestine. Using immunofluorescence microscopy we found that undifferentiated, proliferating crypt enterocytes are statin negative, whereas the differentiated non-proliferating villus enterocytes are statin positive. The epithelial cells of the intestine were isolated according to different stages of differentiation and the expression of statin was studied biochemically by immunoblotting assays. The prominent band (57,000 Mr) was present in abundance in villus cell fractions but undetectable in crypt cell fractions. These findings were also confirmed by immunofluorescence microscopy on individual intestinal epithelial cells of the different isolated fractions. The results presented here, which are similar to observations made in cell cultures, suggest that statin is a unique protein associated with the non-proliferative state of differentiated cells in tissue.

Different kinetic properties of the two mutants, RAS2Ile152 and RAS2Val19, that suppress the CDC25 requirement in RAS/adenylate cyclase pathway in Saccharomyces cerevisiae

The properties of RAS2Gly19----Val and RAS2Thr152----Ile, two mutants suppressing the CDC25 requirement for the activation of adenylate cyclase in Saccharomyces cerevisiae, were compared with the properties of wild-type RAS2. We examined (a) the guanine nucleotide interaction, (b) the intrinsic GTPase (EC 3.6.1-) activity, and (c) the ability to activate adenylate cyclase in vitro. The low GTPase of RAS2Val19 is associated with an increased stability of the GTP complex. By contrast, RAS2Ile152 shows a strong destabilization of the GDP complex (the dissociation rate constants of the RAS2Ile152.GDP complex is enhanced almost 50 times) and an increased GTPase activity. Remarkably, all the parameters of the interaction with GDP and GTP as well as the catalytic activity are modified by the two mutations in an opposite manner. Our kinetic results show that the functional modifications of RAS2 compensating for the CDC25 inactivation can not only be associated with the presence of a long-lived RAS2.GTP complex, but also with a rapid GDP to GTP exchange reaction. As a striking result, the functional modifications induced by Thr152----Ile activate the adenylate cyclase in vitro much more efficiently than those induced by Gly19----Val. This stresses the importance of a rapid regeneration of the RAS2.GTP complex for the activation of the adenylate cyclase pathway.

Calcium-binding protein from mouse Ehrlich ascites-tumour cells is homologous to human calcyclin

A Ca2+-binding protein was purified from mouse Ehrlich ascites-tumour cells. The protein forms monomers and disulphide-linked dimers, which can be separated by reverse-phase h.p.l.c. A partial amino acid sequence analysis demonstrated that the protein has an EF-hand structure. A striking homology was found to rat and human calcyclin (a member of the S-100 protein family), which is possibly involved in cell-cycle regulation.

Cloning and characterization of NSP1, a locus encoding a component of a CDC25-dependent, nutrient-responsive pathway in Saccharomyces cerevisiae

The NSP1 gene in Saccharomyces cerevisiae has been identified by its ability, when expressed at high levels, to bypass the CDC25 requirement for growth. Sequence analysis of the cloned NSP1 locus suggests that the NSP1 product contains 269 amino acids and has a membrane-spanning domain at its carboxyl terminus. The NSP1 protein does not have sequence similarity to other known proteins, and is not related to the CDC25 protein, or to any of the previously described suppressors of CDC25 mutants. Phosphoprotein analysis of NSP1-suppressed cells indicates that the NSP1 product controls the phosphorylation of two 31 kD proteins whose phosphorylation and dephosphorylation are strongly correlated with cell-cycle arrest and proliferation, respectively, and suggests that the NSP1 product is an important downstream element of a CDC25-dependent, nutrient-responsive, phosphorylation pathway.

The RCC1 protein, a regulator for the onset of chromosome condensation locates in the nucleus and binds to DNA

The RCC1 gene, a regulator for the onset of chromosome condensation was found to encode a protein with a molecular mass of 45 kD, determined using the antibody against the synthetic peptides prepared according to the amino acid sequence of the putative RCC1 protein. The p45 located in the nuclei was released from the isolated nuclei, either by DNase I digestion or by treatment with 0.3 M NaCl. Consistently, p45 bound to the DNA-cellulose column was eluted with 0.3 M NaCl. After sequential treatment with DNase I and 2 M NaCl, almost all of the RCC1 protein were released from the nuclei. Thus, RCC1 protein locates on the chromatin and is not a component of the nuclear matrix. In mitotic cells, p45 is dispersed into the cytoplasm. Presumably, RCC1 protein plays a role in regulating the onset of chromosome condensation, at the level of transcription or of mRNA maturation.

Statin, a nonproliferation-specific protein, is associated with the nuclear envelope and is heterogeneously distributed in cells leaving quiescent state

Statin, a protein of 57,000 daltons, is present primarily in the nuclei of nonproliferating cells of terminally differentiated tissues or of in vitro aged fibroblast cultures. In young growing cells, the protein can be induced to appear in the nuclei once the cell-cycle traverse is blocked by various tissue culture manipulations, such as serum starvation; this expression, however, can be rapidly removed by addition of serum. The disappearance of statin in cells leaving the quiescent state is not uniform along the periphery of the nucleus; it can be distributed in various patterns, such as caps, nodules, patches, or irregular granules. This unusual distribution seems to suggest that preferential sites exist at the region of the nuclear envelope where statin presence may residually remain. The concentration of statin at the nuclear envelope region in cells at G0-quiescent phase is confirmed by the intense staining of fluorescent antibody at the periphery of isolated rat liver nuclei. Further examination of the isolated nuclei reveals that the protein is associated with the lamina compartment of the nuclear envelope; this is evidenced by the results of immunoblotting experiments showing statin presence in the fraction enriched for lamins A-C. Immunogold labelling studies show that the protein is located in the general area of the nuclear envelope. These results suggest that statin in G0-quiescent cells is located predominantly at the nuclear envelope region and that in this vicinity there may exist geometrically sites of statin concentration as evidenced by the heterogeneous distribution in those cells experiencing the departure from the quiescent state.

Identification of a cell cycle-dependent gene product as a sialic acid-binding protein

A Ca2+-dependent sialic acid-binding protein was purified on fetuin-Sepharose from various types of human tissue. The molecular mass was determined to be 10,315 Da by laser desorption mass spectrometry. Partial sequence analysis after cyanogen bromide cleavage that yielded one N-terminus accessible for Edman degradation revealed an identity to an internal stretch following the only methionine residue within a putative amino acid sequence (Mr 10,048), deduced from the cDNA of a cell cycle-specific gene. The reported biochemical identification is a prerequisite to infer the biological role of the so far undetected gene product. Initial glycohistochemical studies with sialic acid-(BSA-biotin) raised evidence for nuclear localization of sialic acid-binding sites that might reflect, at least in part, detection of this protein.

Tissue specific distribution of calcyclin--10.5 kDa Ca2+-binding protein

Expression of calcyclin in different cell lines and mouse tissues was determined with polyclonal antibodies raised against calcyclin from Ehrlich ascites tumour (EAT) cells. The protein was detected in mouse skeletal and cardiac muscle, in lung, kidney and spleen, and was especially enriched in mouse smooth muscle as well as in rat fibroblasts. No positive immunological reaction was detected in mouse brain, liver and intestine and some tumourigenic cell lines. The level of calcyclin mRNA found in different cells and tissues corresponded well to the calcyclin level estimated by immunoblotting. The calcyclin-like protein was purified from mouse stomach and appeared to be very similar to the EAT protein.

Regulation of p34cdc2 protein kinase during mitosis

The cell-cycle timing of mitosis in fission yeast is determined by the cdc25+ gene product activating the p34cdc2 protein kinase leading to mitotic initiation. Protein kinase activity remains high in metaphase and then declines during anaphase. Activation of the protein kinase also requires the cyclin homolog p56cdc13, which also functions post activation at a later stage of mitosis. The continuing function of p56cdc13 during mitosis is consistent with its high level until the metaphase/anaphase transition. At anaphase the p56cdc13 level falls dramatically just before the decline in p34cdc2 protein kinase activity. The behavior of p56cdc13 is similar to that observed for cyclins in oocytes. p13suc1 interacts closely with p34cdc2; it is required during the process of mitosis and may play a role in the inactivation of the p34cdc2 protein kinase. Therefore, the cdc25+, cdc13+, and suc1+ gene products are important for regulating p34cdc2 protein kinase activity during entry into, progress through, and exit from mitosis.

cAMP- and RAS-independent nutritional regulation of plasma-membrane H+-ATPase activity in Saccharomyces cerevisiae

The plasma-membrane ATPase of Saccharomyces cerevisiae is a proton pump whose activity, essential fro proliferation, is subject to regulation by nutritional signals. The previous finding that the CDC25 gene product is required for the glucose-induced H+-ATPase activation suggested that H+-ATPase activity is regulated by cAMP. Analysis of starvation-induced inactivation and glucose-induced activation of the H+-ATPase in mutants affected in activity of the RAS proteins, adenylyl cyclase or cAMP-dependent protein kinase showed that nutritional regulation of H+-ATPase activity does not depend directly on any of these factors. We conclude that adenlyl cyclase does not mediate all nutritional responses. This also indicates that the specific CDC25 requirement for the glucose-induced activation of the H+-ATPase identifies a new function for the CDC25 gene product, a function that appears to be independent of CDC25-mediated modulation of the RAS/adenylyl cyclase/cAMP pathway.

A rat liver 57-kDa protein is identified to share antigenic determinants with statin, a marker for nonproliferating cells

The nuclear protein statin, found uniquely in nonproliferating cells, has been previously identified by mouse monoclonal antibodies designated as S-30 and S-44. We report here the screening of various rat tissues for proteins cross-reacting with these antibodies. As revealed by immunoblotting, two polypeptides (mol wt 80 and 57 kDa) and a group of lower-molecular-weight proteins migrating between 34 and 38 kDa were found to react with the anti-statin antibodies. The most prominent immunoreactivity was observed with a 57-kDa protein present in rat liver. Upon further fractionation of the liver protein extract with ammonium sulfate [(NH4)2SO4] the 57-kDa protein, designated as rat liver protein 57 or RLp57, was detected independently with both anti-statin antibodies in the 30 to 60% (NH4)2SO4 fraction. In order to determine whether rat liver protein 57 is indeed specifically recognized by anti-statin antibody S-44, we used RLp57 transferred onto nitrocellulose paper as a specific substrate for the adsorption of the S-44 antibody from ascites fluid. As shown by indirect immunofluorescence microscopy the solution remaining after adsorption failed to stain human fibroblasts. The adsorbed immunoglobulin, however, upon elution revealed statin-specific nuclear staining activity on senescent fibroblasts. These findings suggest that rat liver protein 57 and the human fibroblast statin share similar antigenic determinants recognized by the statin-specific S-44 antibody. Our results indicate furthermore that the statin previously identified in fibroblasts may represent one member of a group of several antigenically related proteins detectable with specific anti-statin antibodies.

Molecular cloning of Saccharomyces cerevisiae CDC6 gene. Isolation, identification, and sequence analysis

The CDC6 gene product is required for entering the S phase of the cell cycle in Saccharomyces cerevisiae. It has been isolated on recombinant plasmids by selection for complementation of temperature-sensitive alleles with a yeast genomic library. The entire complementing activity is carried on a 1.8-kilobase chromosomal DNA fragment, as revealed by deletion mapping. Northern blotting shows that the size of the CDC6 mRNA is about 1.7 kilobases. A Southern blot of yeast chromosomes which were separated by the field inversion gel electrophoresis method indicates that the isolated DNA fragment is derived from chromosome X. The locus from which the clone was derived was marked by integration with a nutritional marker and found by meiotic mapping to cosegregate with CDC6. Thus, we conclude that we have isolated the authentic CDC6 gene. Nucleotide sequence analysis of the CDC6 gene has revealed an open reading frame that encodes a protein with Mr = 57,969. There are five potential Asn-X-(Ser/Thr) glycosylation sites and a highly conserved nucleotide-binding site in the CDC6 sequence. Although computer surveys indicate overall sequence homology between S. cerevisiae CDC6 protein and Saccharomyces pombe CDC10 START protein, they may not be functionally equivalent as evaluated by the complementation assay.

The cdc2 kinase is a nuclear protein that is essential for mitosis in mammalian cells

A homolog of the fission yeast cdc2-encoded protein kinase (p34) is a component of M phase promoting factor in Xenopus oocytes. The homologous kinase in human HeLa cells is maximally active during mitosis, suggesting a mitotic role in mammalian somatic cells. This has been directly investigated by microinjection of anti-p34 antibodies into serum-stimulated rat fibroblasts. DNA synthesis was unaffected but cell division was quantitatively blocked in injected cells. Injection of antibodies against p13suc1, a component of the p34 kinase complex, did not block mitosis but caused mitotic abnormalities resulting in cells containing multiple micronuclei in the subsequent interphase. p34 localized in the nucleus during interphase. During mitosis, a fraction tightly associated with centrosomes. p13 was more evenly distributed between the nucleus and cytoplasm. These observations demonstrate that cdc2 is a nuclear and centrosomal protein that is required for mitosis in mammalian cells.

Statin expression associated with terminally differentiating and postreplicative lens epithelial cells

The expression of a nuclear (57 kDa) protein statin has been previously characterized as a specific marker of quiescent or senescent aging human fibroblasts in vitro. In these studies we have shown that the expression of statin is associated specifically with the postreplicative and terminally differentiating lens epithelial cell. By monitoring the synthesis of specific lens crystallin proteins, and the morphological and cellular changes associated with this differentiated system, we have demonstrated a close correlation between statin expression and cell commitment to the G0 nonreplicative cell cycle state.

The C-terminal part of a gene partially homologous to CDC 25 gene suppresses the cdc25-5 mutation in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, the product of the CDC25 gene is required for progression in the cell division cycle. It is necessary for cAMP production. It has been suggested that the CDC25 gene product acts through Ras proteins. We report the cloning of a DNA fragment from a new gene able to suppress the thermosensitive phenotype of the cdc25-5 mutation. It is unable to suppress the defect of a mutant of the adenylate cyclase gene or of the ras1, ras2ts double mutant. This DNA fragment prevents the drop in cAMP level in cdc25-5 mutant cells shifted to restrictive temperature. The complementing part of this fragment contains a truncated open reading frame (ORF) corresponding to the 3' end of a gene we named SCD25. The 584-amino acid sequence deduced from this ORF shares 45% identity with the 592-aa C-terminal part of the CDC25 ORF which is sufficient for complementation of cdc25 mutations. Some of the common sequences between these two genes are also partially homologous with the amino acid sequence of LTE1, another gene of S. cerevisiae. The capacity of the SCD25 fragment to suppress a cdc25 mutation and its homology to the C-terminal part of the CDC25 led us to propose that the CDC25 and the SCD25 C-terminal fragments each encode a protein domain which is capable in itself to support a similar biochemical function.