CDC46/MCM5, a yeast protein whose subcellular localization is cell cycle-regulated, is involved in DNA replication at autonomously replicating sequences

Saccharomyces cerevisiae cells containing mutations in the cell-division-cycle gene CDC46 arrest with a large bud and a single nucleus with unreplicated DNA at the non-permissive temperature. This G1/S arrest, together with the increased rates of mitotic chromosome loss and recombination phenotype, suggests that these mutants are defective in DNA replication. The subcellular localization of the CDC46 protein changes with the cell cycle; it is nuclear between the end of M phase and the G1/S transition but is cytoplasmic in other phases of the cell cycle. Here we show that CDC46 is identical to MCM5, based on complementation analysis of the mcm5-1 and cdc46-1 alleles, complementation of the minichromosome maintenance defect of mcm5-1 by CDC46, and the genetic linkage of these two genes. Like mcm5-1, cdc46-1 and cdc46-5 also show a minichromosome maintenance defect thought to be associated with DNA replication initiation at autonomously replicating sequences. Taken together, these observations suggest that CDC46/MCM5 acts during a very narrow window at the G1/S transition or the beginning of S phase by virtue of its nuclear localization to effect the initiation of DNA replication at autonomously replicating sequences.

The Saccharomyces cerevisiae NPS1 gene, a novel CDC gene which encodes a 160 kDa nuclear protein involved in G2 phase control

We have cloned the gene NPS1 (nuclear protein of Saccharomyces) which encodes a nuclear protein of mol. wt 156 735 Daltons (1359 amino acids) essential for cell growth. NPS1 contains a 2 kb sequence that is highly homologous to the S. cerevisiae SNF2/GAM1 gene known as a transcriptional regulator for multiple genes. However, the NPS1 gene was found to have a distinct function from SNF2/GAM1. The growth of the cells carrying a nps1 delta :: URA3 deletion allele and galactose-inducible NPS1 on a plasmid was arrested under NPS1-repressed conditions with a cell cycle arrest phenotype, being arrested at the large-bud stage with a single nucleus that had a DNA content of G2/M phase. When the arrested cells were further incubated under NPS1-repressed conditions, re-replication of DNA occurred in some of the arrested cells without passage through mitosis. In the predicted amino acid sequence of NPS1, sequences homologous to the catalytic domain of protein kinases were found. We constructed a mutation which results in the substitution of a highly conserved lysine residue (Lys792) in the presumed ATP-binding site of this kinase-like domain with a glutamic acid codon. The mutant gene failed to rescue the growth defect caused by NPS1 disruption, suggesting that Lys792 is essential for the function of NPS1.

The nuclear-mitotic apparatus protein is important in the establishment and maintenance of the bipolar mitotic spindle apparatus

The formation and maintenance of the bipolar mitotic spindle apparatus require a complex and balanced interplay of several mechanisms, including the stabilization and separation of polar microtubules and the action of various microtubule motors. Nonmicrotubule elements are also present throughout the spindle apparatus and have been proposed to provide a structural support for the spindle. The Nuclear-Mitotic Apparatus protein (NuMA) is an abundant 240 kD protein that is present in the nucleus of interphase cells and concentrates in the polar regions of the spindle apparatus during mitosis. Sequence analysis indicates that NuMA possesses an unusually long alpha-helical central region characteristic of many filament forming proteins. In this report we demonstrate that microinjection of anti-NuMA antibodies into interphase and prophase cells results in a failure to form a mitotic spindle apparatus. Furthermore, injection of metaphase cells results in the collapse of the spindle apparatus into a monopolar microtubule array. These results identify for the first time a nontubulin component important for both the establishment and stabilization of the mitotic spindle apparatus in multicellular organisms. We suggest that nonmicrotubule structural components may be important for these processes.

Differential effects of phorbol esters on proliferation and calcyclin expression in human endometrial carcinoma cells

Calcyclin is a member of the S-100 family of calcium-binding proteins, whose expression is enhanced when quiescent cells are exposed to mitogenic signals. The function of calcyclin is unknown, but it is thought to be involved in modulating the intracellular calcium concentration following mitogenic stimuli. Since activation of protein kinase C (PKC) also occurs following stimulation of quiescent cells by a variety of mitogens, we have investigated the relationship between calcyclin expression and PKC activation in three human endometrial adenocarcinoma cell lines. The addition of 10(-7) M 4 beta-phorbol 12-myristate 13-acetate (PMA) to HEC-50 and HEC-1B cell cultures resulted in a change in cell morphology, an inhibition of proliferation, an increase in calcyclin transcription rate, and an increase in calcyclin mRNA and calcyclin protein levels. In contrast, PMA had no effect on cell morphology or cell proliferation in the Ishikawa adenocarcinoma cell line but enhanced calcyclin expression. Another bioactive phorbol ester had the same effect, whereas the calcium ionophore A23187 and the non-phorbol-ester-type tumor promoter thapsigargin had no effect on calcyclin expression. The effect of PMA on calcyclin expression was blocked by the simultaneous addition of the PKC inhibitor staurosporine and by protein synthesis inhibition with cycloheximide. RNase protection assays and primer extension analysis demonstrated that PMA enhanced transcription from all three of the previously identified transcription start sites in the calcyclin gene. These data clearly demonstrate a dissociation between calcyclin expression and cellular proliferation and suggest that the enhanced calcyclin expression which is seen in quiescent cells following mitogenic stimuli may result from activation of the PKC system.

Cell cycle regulation of CDK2 activity by phosphorylation of Thr160 and Tyr15

We have examined the role of phosphorylation in the regulation of human cyclin-dependent kinase-2 (CDK2), a protein closely related to the cell cycle regulatory kinase CDC2. We find that CDK2 from HeLa cells contains three major tryptic phosphopeptides. Analysis of site-directed mutant proteins, expressed by transient transfection of COS cells, demonstrates that the two major phosphorylation sites are Tyr15 (Y15) and Thr160 (T160). Additional phosphorylation probably occurs on Thr14 (T14). Replacement of T160 with alanine abolishes the kinase activity of CDK2, indicating that phosphorylation at this site (as in CDC2) is required for kinase activity. Mutation of Y15 and T14 stimulates kinase activity, demonstrating that phosphorylation at these sites (as in CDC2) is inhibitory. Similarly, CDK2 is activated in vitro by dephosphorylation of Y15 and T14 by the phosphatase CDC25. Analysis of HeLa cells synchronized at various cell cycle stages indicates that CDK2 phosphorylation on T160 increases during S phase and G2, when CDK2 is most active. Phosphorylation on the inhibitory sites T14 and Y15 is also maximal during S phase and G2. Thus, the activity of a subpopulation of CDK2 molecules is inhibited at a time in the cell cycle when overall CDK2 activity is increased.

Identification of a new set of cell cycle-regulatory genes that regulate S-phase transcription of histone genes in Saccharomyces cerevisiae

Histone mRNA synthesis is tightly regulated to S phase of the yeast Saccharomyces cerevisiae cell cycle as a result of transcriptional and posttranscriptional controls. Moreover, histone gene transcription decreases rapidly if DNA replication is inhibited by hydroxyurea or if cells are arrested in G1 by the mating pheromone alpha-factor. To identify the transcriptional controls responsible for cycle-specific histone mRNA synthesis, we have developed a selection for mutations which disrupt this process. Using this approach, we have isolated five mutants (hpc1, hpc2, hpc3, hpc4, and hpc5) in which cell cycle regulation of histone gene transcription is altered. All of these mutations are recessive and belong to separate complementation groups. Of these, only one (hpc1) falls in one of the three complementation groups identified previously by other means (M. A. Osley and D. Lycan, Mol. Cell. Biol. 7:4204-4210, 1987), indicating that at least seven different genes are involved in the cell cycle-specific regulation of histone gene transcription. hpc4 is unique in that derepression occurs only in the presence of hydroxyurea but not alpha-factor, suggesting that at least one of the regulatory factors is specific to histone gene transcription after DNA replication is blocked. One of the hpc mutations (hpc2) suppresses delta insertion mutations in the HIS4 and LYS2 loci. This effect allowed the cloning and sequence analysis of HPC2, which encodes a 67.5-kDa, highly charged basic protein.

Okadaic acid and p13suc1 modulate the reinitiation of meiosis in mouse oocytes

Short-term exposure to okadaic acid (OA), a specific inhibitor of protein phosphatases 1 and 2A, induced resumption of meiosis, including metaphase spindle formation, in mouse oocytes treated with a phosphodiesterase inhibitor, while long incubations with OA arrested oocyte maturation at a step prior to spindle formation. To explore the basis for this difference, the overall patterns of protein synthesis and phosphorylation and the production of tissue-type plasminogen activator (tPA), the synthesis of which is induced after germinal vesicle breakdown (GVBD), were analyzed under various OA treatments. Short-term exposure to OA led to tPA production and did not greatly affect the maturation-associated changes in protein phosphorylation. By contrast, a long application of OA did not result in tPA production and induced more marked changes in protein phosphorylation. Microinjection into prophase oocytes of the product of the fission yeast gene p13suc1, known to inhibit p34cdc2 kinase activation and/or activity, prevented meiotic reinitiation. This effect was overcome by microinjection of OA, at concentrations higher than those required for induction of maturation in the absence of p13suc1. These observations suggest that inhibition of phosphatase 1 or 2A or both triggers meiotic resumption by acting at the same site or at a site proximal to the p13suc1-sensitive step of cdc2 kinase activation.

Transforming growth factor-beta 1 induces expression of statin during differentiation of human promonocytic leukemia cells

Transforming growth factor-Beta (TGF-beta) is a potent growth inhibitor for several cell types including epithelial cells and hematopoietic progenitor cells. Using a human promonocytic leukemia cell line, THP-1, we have shown that TGF-beta inhibits their proliferation and promotes differentiation into cells exhibiting macrophage-like properties. Therefore, a key question is whether TGF-beta influences the expression of genes associated with proliferation and/or growth inhibition. TGF-beta treatment of THP-1 cells results in downregulation of expression of c-myc. We also observe that TGF-beta 1-treated cells express reduced levels of the cell cycle regulated histone, H2B, but express elevated levels of an RNA splicing variant of this histone that has been observed to be upregulated in growth inhibited and terminally differentiated cells. In addition, a nuclear protein associated with senescence and withdrawal of cells from the cell cycle, statin, is also expressed by THP-1 cells in response to TGF-beta 1 treatment. These results suggest that TGF-beta 1 is capable of inducing expression of specific nuclear proteins associated with differentiation and/or cessation of proliferation that may result in changes in nuclear organization and altered gene expression. Such changes in nuclear organization may be incompatible with continued proliferation of the cells.

S100 alpha, CAPL, and CACY: molecular cloning and expression analysis of three calcium-binding proteins from human heart

Elevated levels of intracellular calcium are a major cause of myocardial dysfunction. To find possible mediators of the deregulated calcium we searched for EF-hand calcium-binding proteins of the S100 family. By PCR technology we identified three members of the S100 protein family (S100 alpha, CACY, and CAPL) in the human heart. We cloned the corresponding cDNAs and examined their expression levels in various human tissues by Northern blot analysis. All three proteins are expressed at high levels in the human heart. Whereas CACY and CAPL mRNAs are expressed ubiquitously, S100 alpha mRNA is restricted to heart, skeletal muscle, and brain. Interestingly, the expression pattern of S100 alpha, CACY, and CAPL in human tissues differs significantly from that in rodent tissues.

Statin, a protein specifically present in nonproliferating cells, is a phosphoprotein and forms a complex with a 45-kilodalton serine/threonine kinase

The protein statin is found in nuclei of nonproliferating cells. Here we report that statin is a phosphoprotein, phosphorylated at serine residues in cultured cells. During immunoprecipitation with anti-statin (S44) antibody, a 45-kDa protein co-precipitates with the 57-kDa statin. In vitro kinase assays demonstrate that the S44 immunoprecipitates can phosphorylate, besides statin, immunoglobulins, enolase, and casein, at either serine or serine/threonine residues. Kinase assays with immunoprecipitated proteins performed on casein- or enolase-impregnated gels show that these substrates are phosphorylated by the 45-kDa (p45) protein. When the S44 immunoprecipitates from human cultured fibroblasts with different in vitro life-spans were compared, the p45 kinase activity was present only in young nongrowing and senescent cells, but not in young growing ones. In other cell cultures, the kinase is detected only in protein complexes precipitated from quiescent 3T3 cells, but not from cycling 3T3 cells or from transformed human glioma (U251-4) cells. Cell fractionation studies, indicating that the phosphorylating activity of S44 immunoprecipitates correlates both qualitatively and quantitatively with the amount of statin present, provide strong evidence that in vivo statin is specifically associated with the p45 kinase. These results suggest that the nonproliferation-specific nature of statin is indeed related to the phosphorylated property of this protein and maybe contributed by the associated kinase.

E2F: a link between the Rb tumor suppressor protein and viral oncoproteins

The cellular transcription factor E2F, previously identified as a component of early adenovirus transcription, has now been shown to be important in cell proliferation control. E2F appears to be a functional target for the action of the tumor suppressor protein Rb that is encoded by the retinoblastoma susceptibility gene. The disruption of this E2F-Rb interaction, as well as a complex involving E2F in association with the cell cycle-regulated cyclin A-cdk2 kinase complex, may be a common mechanism of action for the oncoproteins encoded by the DNA tumor viruses.

Functional analysis of nucleosome assembly protein, NAP-1. The negatively charged COOH-terminal region is not necessary for the intrinsic assembly activity

A nucleosome assembly protein (NAP-1) of Saccharomyces cerevisiae facilitates the association of histones with DNA to form nucleosomes in vitro at physiological ionic conditions. The cloned gene was expressed in Escherichia coli using a T7 expression system, and the protein (417 amino acid residues) was purified by Mono Q column chromatography. Various deletion fragments of NAP-1 protein were also produced, and their nucleosome assembly activity was examined by supercoiling assay. The internal fragment containing the residues 43-365 was necessary and sufficient for the activity, and a long stretch of negatively charged region near the carboxyl terminus was dispensable. This minimal size fragment could form the 12 S NAP-1-histone complex as the whole protein could, whereas deleted fragments on either side could bind with core histones only to form aggregates.

CDC43 and RAM2 encode the polypeptide subunits of a yeast type I protein geranylgeranyltransferase

The question regarding the identity of the alpha and beta subunits of the yeast type I protein geranylgeranyltransferase was explored using prokaryotic expression of candidate genes. The Saccharomyces cerevisiae CDC43 and RAM2 genes were expressed in Escherichia coli and cell extracts examined for the ability to transfer [3H]geranylgeranyl diphosphate to an appropriate CaaX protein substrate. Individual expression of each gene yielded no activity; however, co-expression of the two genes resulted in high levels of [3H] geranylgeranyl incorporation into the substrate protein Ras-Cys-Val-Val-Leu. The activity was partially purified yielding approximately 12,600 units/liter. The partially purified enzyme geranylgeranylated the Ras-Cys-Val-Val-Leu, Ras-Cys-Ala-Ile-Leu, Ras-Cys-Ile-Ile-Leu, and Ras-Cys-Thr-Ile-Leu substrates but not the Ras-Cys-Val-Leu-Ser or Ras-Ser-Val-Leu-Ser substrates. The protein geranylgeranyltransferase was highly specific for geranylgeranyl diphosphate and poorly transferred farnesyl. The recombinant enzyme was indistinguishable from the native type I geranylgeranyltransferase in yeast extracts. As has been reported for the protein farnesyltransferase, the yeast type I protein geranylgeranyltransferase is also a magnesium-requiring, zinc metalloenzyme. Interestingly, the recombinant enzyme functioned with calcium as the only divalent cation, although addition of zinc increased calcium-dependent activity 2-fold.

p13suc1 suppresses the catalytic function of p34cdc2 kinase for intermediate filament proteins, in vitro

The regulation of p34cdc2 kinase activity controls the entry into and exit from mitosis. Although genetic and biochemical evidence suggested close interactions between cyclins, p13suc1 and p34cdc2 kinase, the roles of p13suc1 on p34cdc2 kinase functions remain unclear. To examine the effects of p13suc1 on p34cdc2 kinase function we developed a simple purification procedure for p34cdc2 kinase, unassociated with p13suc1. The key to the purification procedures we used was buffer containing 0.5 M NaCl and 50% ethylene glycol, as a specific elutant of p34cdc2 kinase from p13suc1-Sepharose. This purified p34cdc2 kinase stoichiometrically phosphorylated vimentin and desmin. Exogenous p13suc1 suppressed the phosphorylation of these filament proteins by the kinase and prevented disassembly, although histone H1 phosphorylation was not affected. Peptide mapping analysis showed a similar extent of inhibition by p13suc1 for all five phosphorylation sites by p34cdc2 kinase of vimentin and desmin, hence these p13suc1-induced inhibitions are probably not site-specific. It thus appears that p13suc1 has a selective effect on the catalytic activity of p34cdc2 kinase for these filament proteins.

Identification of a 60-kilodalton Rb-binding protein, RBP60, that allows the Rb-E2F complex to bind DNA

Several reports have indicated that the product of the retinoblastoma gene (Rb) complexes with the transcription factor E2F. We present evidence that the DNA-binding of the Rb-E2F complex involves another cellular factor. Addition of Rb to purified preparations of E2F does not generate an Rb-E2F complex that can bind DNA, and in fact, we see an inhibition of the DNA-binding ability of E2F. On the other hand, addition of Rb to cruder preparations of E2F results in the formation of an Rb-E2F complex (E2Fr) that can bind DNA and produces a distinct complex in gel retardation assays. We have identified and purified a 60-kDa protein that allows the Rb-E2F complex to bind DNA, and we show that this 60-kDa protein exerts its effect by directly interacting with Rb.

Cyclin E/cdk2 and cyclin A/cdk2 kinases associate with p107 and E2F in a temporally distinct manner

Cyclin E is classified as a putative G1 cyclin on the basis of its cyclic pattern of mRNA expression, with maximal levels being detected near the G1/S boundary. We report here that cyclin E is found associated with the transcription factor E2F in a temporally regulated fashion. E2F is known to be a critical transcription factor for the expression of some S phase-specific proteins and is thought to be important for a series of others. Antisera specific for cyclin E were raised and used to demonstrate an association between cyclin E and E2F. This cyclin E/E2F complex was seen in a variety of human cell lines from various tissues, but its appearance was detected primarily during the G1 phase of the cell cycle. The cyclin E/E2F association decreased as cells entered S phase, just as the association of E2F with cyclin A became detectable. We characterized the cyclin E-E2F complex further to show that both the cyclin-dependent kinase-2 (cdk2) and p107 were present. Therefore, the p107/E2F complex is associated with two different cdk2 kinase complexes--one containing cyclin A and the other containing cyclin E--and the appearance of these complexes is temporally regulated during the cell cycle. The presence of cyclin E/E2F complexes in the G1 phase suggests a role for cyclin E in the control of genes required for the G1-to-S transition.

On the multiple simultaneous superposition of molecular structures by rigid body transformations

A method of optimally superimposing n coordinate sets on each other by rigid body transformations, which minimizes the sum of all n (n - 1)/2 pairwise residuals, is presented. In the solution phase the work load is approximately linear on n, is independent of the size of the structures, is independent of their initial orientations, and terminates in one cycle if n = 2 of if the coordinate sets are exactly superposable, and otherwise takes a number of cycles dependent only on genuine shape differences. Enantiomorphism, if present, is detected, in which case the option exists to reverse or not to reverse the chirality of relevant coordinate sets. The method also offers a rational approach to the problem of multiple minima and has successfully identified four distinct minima in such a case. Source code, which is arranged to enable the study of the disposition of domains in multidomain structures, is available from the author.

A mammalian dual specificity protein kinase, Nek1, is related to the NIMA cell cycle regulator and highly expressed in meiotic germ cells

Screening of mouse cDNA expression libraries with antibodies to phosphotyrosine resulted in repeated isolation of cDNAs that encode a novel mammalian protein kinase of 774 amino acids, termed Nek1. Nek1 contains an N-terminal protein kinase domain which is most similar (42% identity) to the catalytic domain of NIMA, a protein kinase which controls initiation of mitosis in Aspergillus nidulans. In addition, both Nek1 and NIMA have a long, basic C-terminal extension, and are therefore similar in overall structure. Despite its identification with anti-phosphotyrosine antibodies, Nek1 contains sequence motifs characteristic of protein serine/threonine kinases. The Nek1 kinase domain, when expressed in bacteria, phosphorylated exogenous substrates primarily on serine/threonine, but also on tyrosine, indicating that Nek1 is a dual specificity kinase with the capacity to phosphorylate all three hydroxyamino acids. Like NIMA, Nek1 preferentially phosphorylated beta-casein in vitro. In situ RNA analysis of nek1 expression in mouse gonads revealed a high level of expression in both male and female germ cells, with a distribution consistent with a role in meiosis. These results suggest that Nek1 is a mammalian relative of the fungal NIMA cell cycle regulator.

E2F mediates dihydrofolate reductase promoter activation and multiprotein complex formation in human cytomegalovirus infection

The adenovirus immediate-early protein E1A activates the adenovirus E2 promoter and several cellular gene promoters through transcription factor E2F. The immediate-early proteins of human cytomegalovirus (HCMV) can complement an E1A-deficient adenovirus mutant and activate the adenovirus E2 promoter. HCMV also has been shown to activate the adenovirus E2 promoter. On the basis of these findings, we have investigated whether HCMV can activate the promoter of the cellular dihydrofolate reductase (DHFR) gene, which requires E2F binding for maximal promoter activity. We show that HCMV activates the DHFR promoter and that products of the HCMV major immediate-early gene region mediate the activation of the promoter specifically through the E2F site. We used gel mobility shift assays to search for potential molecular mechanisms for this activation and found an "infection-specific" multimeric complex that bound to the E2F sites in the DHFR and E2 promoters in extracts from HCMV-infected cells but not in extracts from uninfected cells. Several antibodies against HCMV immediate-early gene products had no effect on this infection-specific complex. Subsequently, the complex was found to contain E2F, cyclin A, p33cdk2, and p107 and to be similar to S-phase-specific complexes that recently have been identified in several cell types. A functional role for the binding of the cyclin A-p33cdk2 complex to cellular gene promoters has yet to be demonstrated; however, HCMV infection causes the induction of both cellular DNA replication and transcription of growth-related genes containing E2F sites in their promoters. The findings described above therefore may relate to both of these effects of HCMV infection. We also provide evidence that some of the molecular events associated with adenovirus infection are different from those associated with HCMV infection.

Regulation of p105wee1 and p34cdc2 during meiosis in Schizosaccharomyces pombe

Temperature-sensitive pat1 mutants of the fission yeast Schizosaccharomyces pombe can be induced to undergo meiosis at the restrictive temperature, irrespective of the mat1 configuration and the nutritional conditions. Using a combination of exit from stationary phase and thermal inactivation of the 52-kilodalton protein kinase that is encoded by the pat1 (also called ran1) gene, highly synchronous meiotic cultures were obtained. Synthesis and tyrosyl phosphorylation of p34cdc2 was evident during meiotic G1 and S phases. During this period there was increased expression of p105wee1, a protein kinase implicated in the tyrosyl phosphorylation of p34cdc2. Following a relatively brief G2 period, during which a reduction in the steady-state level of p105wee1 occurred, there was an approximately 19-fold increase in the histone H1 phosphotransferase activity of p34cdc2. Only a single peak of histone H1 kinase activation was observed, which implies that unlike meiosis in amphibians and echinoderms, p34cdc2 is functional only during one of the meiotic divisions in S. pombe, presumably meiosis II. Stimulation of the kinase activity of p34cdc2 was associated with its tyrosyl dephosphorylation. This is analogous to mitotic M phase and suggests parallels in the mechanism of activation of p34cdc2 during mitosis and one of the meiotic divisions in S. pombe.

Inactivation of the p34cdc2-cyclin B complex by the human WEE1 tyrosine kinase

Entry into mitosis in Schizosaccharomyces pombe is negatively regulated by the wee1+ gene, which encodes a protein kinase with serine-, theonine-, and tyrosine-phosphorylating activities. The wee1+ kinase negatively regulates mitosis by phosphorylating p34cdc2 on tyrosine 15, thereby inactivating the p34cdc2-cyclin B complex. The human homolog of the wee1+ gene (WEE1Hu) was overproduced in bacteria and assayed in an in vitro system. Unlike its fission yeast homolog, the product of the WEE1Hu gene encoded a tyrosine-specific protein kinase. The human WEE1 kinase phosphorylated the p34cdc2-cyclin B complex on tyrosine 15 but not on threonine 14 in vitro and inactivated the p34cdc2-cyclin B kinase. This inhibition was reversed by the human Cdc25C protein, which catalyzed the dephosphorylation of p34cdc2. These results indicate that the product of the WEE1Hu gene directly regulates the p34cdc2-cyclin B complex in human cells and that a kinase other than that encoded by WEE1Hu phosphorylates p34cdc2 on threonine 14.

Efficient isolation and mapping of rad genes of the fungus Coprinus cinereus using chromosome-specific libraries

We have constructed cosmid libraries from electrophoretically separated chromosomes of the basidiomycete Coprinus cinereus. These libraries greatly facilitate the isolation of genes by complementation of mutant phenotypes and are particularly useful for map-based cloning strategies. From a library constructed from two co-migrating C.cinereus chromosomes, we isolated a clone that complements the C.cinereus rad9-1 mutation. Examination of this clone showed that it complements both the repair and meiotic defects of this mutant. Restriction fragment length polymorphism mapping using a portion of this clone showed that it maps to the rad9 locus. In addition, a single copy of transforming DNA is sufficient to complement the rad9-1 defects. Thus, we believe we have cloned the rad9 gene itself. We also used a chromosome-specific library and backcrossed isolates to rapidly identify a cosmid clone which is tightly linked to the rad11 locus and is therefore a suitable starting point for a chromosome walk. These rapid methods of gene mapping and isolation should be applicable to any organism with separable chromosomes.

Expression of the non-proliferation-specific protein, statin, in grey matter neuroglia of the aging rat brain

The monoclonal antibody, S-44, identifies statin, a 57 kDa nuclear protein which appears to be expressed exclusively in non-proliferating cells. We previously demonstrated that in the aging rat corpus callosum approximately one third of neuroglia are statin-negative, suggesting the existence of an unexpectedly large cycling glial compartment. In the present study, double-labeling of individual cultured astroglia with [3H]thymidine and the S-44 antibody provided direct evidence for the non-proliferative status of statin-positive cells. The S-44 antibody was used to immuno-localize statin and thereby determine growth fractions for neuroglia in various grey matter regions of 3-, 18-, and 33-month-old rats. The proportion of statin-negative (cycling) cells for the three ages combined ranged from about 24% in the molecular layer of the dentate gyrus to 38% in the molecular layer of the parietal cortex. In most regions surveyed total glial counts and proportions of statin-positive and -negative cells did not vary significantly as a function of advancing age. These results suggest that (i) as in corpus callosum, pools of cycling neuroglia in various grey matter regions are far in excess of those previously predicted by S-phase labeling with [3H]thymidine or BUdR, and (ii) ratios of proliferating-to-quiescent neuroglia are tightly regulated over much of the animal's adult life span. These conserved ratios may be used as markers of normal CNS senescence, and deviations thereof may indicate the presence and extent of intervening neuropathologic processes.

Mutations in the cdc10 start gene of Schizosaccharomyces pombe implicate the region of homology between cdc10 and SWI6 as important for p85cdc10 function

The cdc10 gene of the fission yeast Schizosaccharomyces pombe is required for traverse of start and commitment to the mitotic cell division cycle rather than other fates. The product of the gene, p85cdc10, is a component of a factor that is thought to be involved in regulating the transcription of genes that are required for DNA synthesis. In order to define regions of the p85cdc10 protein that are important for its function a fine structure genetic map of the cdc10 gene was derived and the sequences of 13 cdc10ts mutants determined. The 13 mutants tested define eight alleles. Eleven of the mutants are located in the region that contains the two copies of the cdc10/SWI6 repeat motif, implicating it as important for p85cdc10 function.

Molecular genetic studies of the Cdc7 protein kinase and induced mutagenesis in yeast

The Saccharomyces cerevisiae CDC7 gene encodes a protein kinase that functions in DNA replication, repair, and meiotic recombination. The sequence of several temperature-sensitive (ts) cdc7 mutations was determined and correlated with protein kinase consensus domain structure. The positions of these ts alleles suggests some general principles for predicting ts protein kinase mutations. Pedigree segregation lag analysis demonstrated that all of the mutant proteins are less active or less stable than wild-type Cdc7p. Two new mutations were constructed, one by site-directed and the other by insertional mutagenesis. All of the cdc7 mutants were assayed for induced mutagenesis in response to mutagenic agents at the permissive temperature. Some cdc7 mutants were found to be hypomutable, while others are hypermutable. The differences in mutability are observed most clearly when log phase cells are used. Both hypo- and hypermutability are recessive to wild type. Cdc7p may participate in DNA repair by phosphorylating repair enzymes or by altering chromatin structure to allow accessibility to DNA lesions.

A delay in the Saccharomyces cerevisiae cell cycle that is induced by a dicentric chromosome and dependent upon mitotic checkpoints

Dicentric chromosomes are genetically unstable and depress the rate of cell division in Saccharomyces cerevisiae. We have characterized the effects of a conditionally dicentric chromosome on the cell division cycle by using microscopy, flow cytometry, and an assay for histone H1 kinase activity. Activating the dicentric chromosome induced a delay in the cell cycle after DNA replication and before anaphase. The delay occurred in the absence of RAD9, a gene required to arrest cell division in response to DNA damage. The rate of dicentric chromosome loss, however, was elevated in the rad9 mutant. A mutation in BUB2, a gene required for arrest of cell division in response to loss of microtubule function, diminished the delay. Both RAD9 and BUB2 appear to be involved in the cellular response to a dicentric chromosome, since the conditionally dicentric rad9 bub2 double mutant was highly inviable. We conclude that a dicentric chromosome results in chromosome breakage and spindle aberrations prior to nuclear division that normally activate mitotic checkpoints, thereby delaying the onset of anaphase.

Characterization and cell cycle-regulated expression of mouse B-myb

We have isolated a full-length mouse B-myb cDNA clone and used this to examine cell cycle-regulated expression of this gene. Mouse B-Myb was predicted to comprise 704 amino acids and to be 84% homologous with human B-Myb. There were three regions of extensive amino acid homology which may indicate functional domains: the first corresponded to the c-Myb DNA-binding domain, while the second had no counterpart in c-Myb but was instead homologous to a short segment of the related A-myb protein. The third region of homology is partially conserved in both c-Myb and A-Myb and may correspond to the c-Myb negative regulatory domain. Stimulation of quiescent 3T3 fibroblasts with serum was found to result in induction of B-myb expression in late G1 and to lead to high levels of gene transcripts that persisted through S phase. Similarly, maximum B-myb mRNA levels were reached in G2/M synchronized cells prior to entry into S phase. These results are consistent with a role in G1/S transition as has been suggested for c-myb.

Functional interaction between p21rap1A and components of the budding pathway in Saccharomyces cerevisiae

The rap1A gene encodes a 21-kDa, ras-related GTP-binding protein (p21rap1A) of unknown function. A close structural homolog of p21rap1A (65% identity in the amino-terminal two-thirds) is the RSR1 gene product (Rsr1p) of Saccharomyces cerevisiae. Although Rsr1p is not essential for growth, its presence is required for nonrandom selection of bud sites. To assess the similarity of these proteins at the functional level, wild-type and mutant forms of p21rap1A were tested for complementation of activities known to be fulfilled by Rsr1p. Expression of p21rap1A, like multicopy expression of RSR1, suppressed the conditional lethality of a temperature-sensitive cdc24 mutation. Point mutations predicted to affect the localization of p21rap1A or its ability to cycle between GDP and GTP-bound states disrupted suppression of cdc24ts, while other mutations in the 61-65 loop region improved suppression. Expression of p21rap1A could not, however, suppress the random budding phenotype of rsr1 cells. p21rap1A also apparently interfered with the normal activity of Rsrlp, causing random budding in diploid wild-type cells, suggesting an inability of p21rap1A to interact appropriately with Rsr1p regulatory proteins. Consistent with this hypothesis, we found an Rsr1p-specific GTPase-activating protein (GAP) activity in yeast membranes which was not active toward p21rap1A, indicating that p21rap1A may be predominantly GTP bound in yeast cells. Coexpression of human Rap1-specific GAP suppressed the random budding due to expression of p21rap1A or its derivatives, including Rap1AVal-12. Although Rap1-specific GAP stimulated the GTPase of Rsr1p in vitro, it did not dominantly interfere with Rsr1p function in vivo. A chimera consisting of Rap1A1-165::Rsr1p166-272 did not exhibit normal Rsr1p function in the budding pathway. These results indicated that p21rap1A and Rsr1p share at least partial functional homology, which may have implications for p21rap1A function in mammalian cells.

RHO gene products, putative small GTP-binding proteins, are important for activation of the CAL1/CDC43 gene product, a protein geranylgeranyltransferase in Saccharomyces cerevisiae

Two multicopy suppressors of the cal1-1 mutation in the yeast Saccharomyces cerevisiae have been isolated and characterized. They are identical to the yeast RHO1 and RHO2 genes, which encode putative small GTP-binding proteins. Multiple copies of either RHO gene suppressed temperature-sensitive growth of the cal1-1 mutant but did not suppress the cal1 null mutant. Genetic analysis suggests that overproduction of either RHO gene product acts for activation of the CAL1 gene product.

Proliferative activity of colonic mucosa at different distances from primary adenocarcinoma as determined by the presence of statin: a nonproliferation-specific nuclear protein

The field change is one hypothesis concerning the development of colorectal carcinoma. Removal of a carcinoma without its entire surrounding altered mucosa may result in the development of a recurrence. S44, a monoclonal antibody directed against statin, a nuclear protein expressed in nonproliferating cells in either a quiescent or senescent state, was used to determine the rate of cell growth in colorectal mucosa at different distances from carcinomas. The specimens of 18 patients undergoing resection of a colorectal carcinoma were immediately opened after operation, and strips of mucosa were taken at distances of 1 cm, 5 cm, and 10 cm from the carcinoma. For each location, 10 longitudinally oriented crypts were evaluated for statin-positive cells identified by the presence of a dark brown peroxidase-conjugated antibody reaction product. The average percentage of statin-positive cells per crypt was significantly lower at a 1-cm distance from the carcinoma compared with the mucosa located 5 and 10 cm from the carcinoma (20.89 +/- 4.33 at 1 cm, 32.41 +/- 5.27 at 5 cm, and 34.23 +/- 6.45 at 10 cm). None of the calculated parameters showed any significant difference between the 5-cm and 10-cm locations. The fact that the proliferation rate of the mucosal cells returns to the normal level at 5 cm from the margin of the carcinoma suggests that cells located within this distance still retain proliferative potential even though they are morphologically indistinguishable from their normal counterparts. We conclude that failure to remove this transitional, potentially proliferative mucosa may result in subsequent development of anastomotic or perianastomotic recurrences.

The retinoblastoma protein region required for interaction with the E2F transcription factor includes the T/E1A binding and carboxy-terminal sequences

Recent experiments in understanding the mechanism of the retinoblastoma protein (RB) function have revealed the existence of several cellular proteins that are complexed with RB. One of these cellular proteins is the E2F transcription factor, which was originally identified due to its inducibility by E1A during an adenovirus infection. The E2F recognition sequence is found in the promoters of several cellular genes involved in growth control, including several oncogenes. In this report, we provide evidence that the interaction of E2F and RB is mediated through a region on RB where viral oncogenes such as SV40 T antigen and adenovirus E1A bind and where tumorigenic mutations also cluster. Additional carboxy-terminal sequences are also required for the interaction with E2F. These observations provide evidence for a direct connection between tumor suppressor function and the gene expression program leading to cellular growth regulation.

Binding of the human E2F transcription factor to the retinoblastoma protein but not to cyclin A is abolished in HPV-16-immortalized cells

The adenovirus E1A, SV40 large T and papillomavirus E7 proteins immortalize primary cells by virtue of their ability to bind the retinoblastoma gene product (pRB) and other cellular proteins, including cyclin A and the prRB-related protein, p107. It has been demonstrated that these viral oncogene products will prevent the inhibition of positive growth regulators by pRB, one of them being the E2F transcription factor. Here we show that the interactions of pRB and cyclin A with E2F are present also in normal keratinocytes and in primary human fibroblasts. In human keratinocytes immortalized by human papillomavirus 16 (HPV-16), expressing high levels of HPV-16 E7 protein, complexes between E2F and pRB are disrupted. In this cell line, as well as in HeLa cells which express HPV-18 E7, complexes containing E2F and cyclin A are maintained, indicating that this interaction is not sensitive to the viral oncoprotein and that cyclin A can associate with E2F independently of pRB. In vitro binding experiments suggest that the E7 gene product is able to preferentially abolish the interaction of pRB with E2F, leaving the cyclin A complexes intact. Our findings suggest that E7-dependent immortalization of human cells is associated with modifications of E2F multiprotein complexes.

The growth arrest-specific gene, gas1, is involved in growth suppression

This report describes the structure of the mRNA, the protein product, and the growth-regulating activity of one of the growth arrest-specific genes, gas1. From the predicted amino acid sequence, in vitro translation of gas1 mRNA, and immunofluorescence of cells in culture, it appears that the gas1 protein is an integral plasma membrane protein whose expression is linked to growth arrest. When gas1 is overexpressed from a constitutive promoter in quiescent cells, the serum-induced transition from the G0 to the S phase of the cell cycle is inhibited without affecting the normal early serum response. Ectopic expression of the gas1 gene by microinjection in normal and transformed NIH 3T3 cell lines with the notable exception of SV40-transformed 3T3 cells leads to inhibition of DNA synthesis. Thus, gas1 appears to be one component of a negative circuit that governs growth suppression. Its effect is, however, abolished in SV40-transformed cells.

Anatomy of a transcription factor important for the start of the cell cycle in Saccharomyces cerevisiae

Entry of yeast cells into the mitotic cell cycle (Start) involves a form of the CDC28 kinase that associates with G1-specific cyclins encoded by CLN1 and CLN2 (ref. 1). The onset of Start may be triggered by the activation of CLN1 and CLN2 transcription in late G1 (ref. 2). SWI4 and SWI6 are components of a factor (SBF) that binds the CACGAAAA (SCB) promoter elements responsible for activation in late G1 of the HO endonuclease, CLN1 and CLN2 genes. A related factor (MBF) containing SWI6 and a 120K protein binds to the ACGCGTNA (MCB) promoter elements responsible for late G1-specific transcription of DNA replication genes. Nothing is known about how these heteromeric proteins bind DNA. We show here that SWI4 contains a novel DNA-binding domain at its N terminus that alone binds specifically to SCBs and a C-terminal domain that binds to SWI6. SWI4's DNA-binding domain is similar to an N-terminal domain of the cdc10 protein that is a component of an MBF-like factor from Schizosaccharomyces pombe and is required for Start. An involvement of this kind of DNA-binding domain in transcriptional controls at Start may therefore be a conserved feature of eukaryotic cells.

Expression of TTK, a novel human protein kinase, is associated with cell proliferation

We have isolated the full-length sequence for a unique human kinase, designated TTK. TTK was initially identified by screening of a T cell expression library with anti-phosphotyrosine antibodies. The kinases most closely related to TTK are the SPK1 serine, threonine and tyrosine kinase, the Pim1, PBS2, and CDC2 serine/threonine kinases, and the TIK kinase which was also identified through screening of an expression library with anti-phosphotyrosine antibodies. However, the relationships are distant with less than 25% identity. Nevertheless, TTK is highly conserved throughout phylogeny with hybridizing sequences being detected in mammals, fish, and yeast. TTK mRNA is present at relatively high levels in testis and thymus, tissues which contain a large number of proliferating cells, but is not detected in most other benign tissues. Freshly isolated cells from most malignant tumors assessed expressed TTK mRNA. As well, all rapidly proliferating cell lines tested expressed TTK mRNA. Escherichia coli expressing the complete kinase domain of TTK contain markedly elevated levels of phosphoserine and phosphothreonine as well as slightly increased levels of phosphotyrosine. Taken together, these findings suggest that expression of TTK, a previously unidentified member of the family of kinases which can phosphorylate serine, threonine, and tyrosine hydroxyamino acids, is associated with cell proliferation.

CDC25-dependent induction of inositol 1,4,5-trisphosphate and diacylglycerol in Saccharomyces cerevisiae by nitrogen

The addition of ammonium sulfate to starved yeast cells leads to a 3- to 4-fold rapid increase of the second messengers inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG), the products of phosphoinositide-specific phospholipase C (PI-PLC). This response is reduced by dissecting the RAS-activating Cdc25 protein, and is completely abolished by the cdc25-1 mutation even at permissive temperature. Starved cdc25-1 mutant cells have a strongly reduced IP3 content, but an at least 10-fold increased DAG level compared to the isogenic wild-type strain. NH4 does not stimulate cAMP synthesis, and glucose does not induce IP3 and DAG. Our data suggest that the Cdc25 protein controls a nitrogen-specific signalling pathway involving the effector PI-PLC, in addition to the glucose-induced activation of adenylyl cyclase (AC).

The Drosophila l(1)zw10 gene product, required for accurate mitotic chromosome segregation, is redistributed at anaphase onset

Mutations in the gene l(1)zw10 disrupt the accuracy of chromosome segregation in a variety of cell types during the course of Drosophila development. Cytological analysis of mutant larval brain neuroblasts shows very high levels of aneuploid cells. Many anaphase figures are aberrant, the most frequent abnormality being the presence of lagging chromosomes that remain in the vicinity of the metaphase plate when the other chromosomes have migrated toward the spindle poles. Finally, the centromeric connection between sister chromatids in mutant neuroblasts treated with colchicine often appears to be broken, in contrast with similarly treated control neuroblasts. The 85-kD protein encoded by the l(1)zw10 locus displays a dynamic pattern of localization in the course of the embryonic cell cycle. It is excluded from the nuclei during interphase, but migrates into the nuclear zone during prometaphase. At metaphase, the zw10 antigen is found in a novel filamentous structure that may be specifically associated with kinetochore microtubules. Upon anaphase onset, there is an extremely rapid redistribution of the zw10 protein to a location at or near the kinetochores of the separating chromosomes.

Identification and characterization of the regulated pattern of expression of a novel mouse gene, meg1, during the meiotic cell cycle

The gene designated meg1 (meiosis expressed gene) is a new mouse gene identified during a search for mammalian genes potentially involved in meiotic processes. Two classes of complementary DNAs were isolated from an adult mouse testis complementary DNA library, which shared the same 3' end including the entire putative coding region but differed in their 5' ends. Only one of these complementary DNA classes appeared to correspond to the very abundant 0.75-kilobase testicular transcript of meg1. Sequence analysis predicts a 10.8-kilodalton protein which is highly charged and lysine rich. It is also relatively rich in potential phosphoacceptor amino acids (approximately 17%), several of which are located in phosphorylation consensus sequences. The pattern of expression of meg1 was studied utilizing a combined Northern blot and in situ hybridization analysis. Of the adult tissues examined, meg1 transcripts were detected exclusively in testis. Analysis of mRNA from testes of two germ cell deficient mutant strains did not reveal significant levels of meg1 transcripts. Analysis of RNA from enriched populations of spermatogenic cells from adult testes and localization by in situ hybridization revealed that meg1 transcripts are most abundant in pachytene spermatocytes. These results suggest a role for meg1 during germ cell differentiation, possibly during meiotic prophase.

Identification of a mammalian gene structurally and functionally related to the CDC25 gene of Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae CDC25 gene encodes a nucleotide-exchange-factor (NEF) that can convert the inactive GDP-bound state of RAS proteins to an active RAS-GTP complex. CDC25 can activate the yeast RAS proteins as well as the human H-ras protein. CDC25 is a member of a family of yeast genes that likely encode NEFs capable of regulating the RAS-related proteins found in yeast. By aligning the amino acid sequence of CDC25-related gene products we found a number of conserved motifs. Using degenerate oligonucleotides that encode these conserved sequences, we have used polymerase chain reactions to amplify fragments of mouse and human cDNAs related to the yeast CDC25 gene. We show that a chimeric molecule, part mouse and part yeast CDC25, can suppress the loss of CDC25 function in the yeast S. cerevisiae.

Identification and analysis of a DNA fragment from Saccharomyces kluyveri that can complement the loss of CDC25 function in Saccharomyces cerevisiae

In the budding yeast, Saccharomyces cerevisiae, the function of wild-type Ras proteins is dependent on the CDC25 protein, which promotes the exchange of guanine nucleotides bound to Ras. To facilitate the identification of proteins which similarly regulate Ras function in higher eukaryotes, we have identified the CDC25 gene from another budding yeast, Saccharomyces kluyveri, by low-stringency hybridization to an S. cerevisiae CDC25 restriction fragment. This protein, SKCDC25, shares significant amino acid homology with CDC25, SCD25, and Ste6 of Schizosaccharomyces pombe in the C-terminal portion of the protein. The expression of SKCDC25 in a temperature-sensitive cdc25 strain of S. cerevisiae complements the loss of endogenous CDC25 activity. The identification of the highly conserved C-terminal sequences, which direct bona fide CDC25 activity within these proteins, will aid in the isolation of CDC25 genes from higher eukaryotes.

Cloning of chromosome I DNA from Saccharomyces cerevisiae: analysis of the FUN52 gene, whose product has homology to protein kinases

A gene whose product has homology to protein kinases and is closely related to the Aspergillus nidulans nimA cell-cycle gene was identified on chromosome I of the yeast, Saccharomyces cerevisiae. This gene has been temporarily designated FUN52, where FUN is the acronym for 'function unknown now'. In A. nidulans, nimA is required to enter mitosis. In addition, overexpression of nimA causes premature onset of mitosis and cell cycle arrest. In contrast, S. cerevisiae cells that were either deleted for FUN52 or were overexpressing it had no detectable growth phenotypes. FUN52 proved to be the same as the previously identified KIN3 gene [Jones and Rosamond, Gene 90 (1990) 87-92] that was reported to map on chromosome VI.

Regulated temporal and spatial expression of the calcium-binding proteins calcyclin and OPN (osteopontin) in mouse tissues during pregnancy

In situ hybridization and northern/slot blot analyses were used to quantify the expression of calcyclin (2A9, 5B10), osteopontin (opn, secreted phosphoprotein, 2ar) and calmodulin mRNAs in mouse tissues that support pregnancy. High-to-moderate levels of the mRNAs of all three genes were detected at discrete locations in the uterus, decidua and placenta as a function of gestation time. Calmodulin expression was constant in these tissues; calcyclin mRNA was high during early pregnancy and declined after day 8-9 of gestation; and opn mRNA was undetectable before day 7, with maximal levels on days 9-12 in each of these tissues. Calcyclin, but not opn, expression was also observed in the chorioamnion after day 12. Calcyclin was expressed throughout the decidua on day 8 but became restricted to the primary (antimesometrial) decidual zone and decidua lateralis on day 9, and the decidua capsularis after day 9. By contrast, opn mRNA was localized on day 9 to the mesometrial triangle, which contains a large population of granulated metrial gland cells, and to the decidua basalis. These two genes may serve as markers for the two types of decidual tissue. We suggest that one function of OPN, which may be an indicator of cells in the decidua that have a bone marrow genealogy, is to mediate the flux of calcium from the maternal circulation to the developing embryo.

RCC1, a regulator of mitosis, is essential for DNA replication

Temperature-sensitive mutants in the RCC1 gene of BHK cells fail to maintain a correct temporal order of the cell cycle and will prematurely condense their chromosomes and enter mitosis at the restrictive temperature without having completed S phase. We have used Xenopus egg extracts to investigate the role that RCC1 plays in interphase nuclear functions and how this role might contribute to the known phenotype of temperature-sensitive RCC1 mutants. By immunodepleting RCC1 protein from egg extracts, we find that it is required for neither chromatin decondensation nor nuclear formation but that it is absolutely required for the replication of added sperm chromatin DNA. Our results further suggest that RCC1 does not participate enzymatically in replication but may be part of a structural complex which is required for the formation or maintenance of the replication machinery. By disrupting the replication complex, the loss of RCC1 might lead directly to disruption of the regulatory system which prevents the initiation of mitosis before the completion of DNA replication.

A cDNA encoding a pRB-binding protein with properties of the transcription factor E2F

The retinoblastoma protein (pRB) plays an important role in the control of cell proliferation, apparently by binding to and regulating cellular transcription factors such as E2F. Here we describe the characterization of a cDNA clone that encodes a protein with properties of E2F. This clone, RBP3, was identified by the ability of its gene product to interact with pRB. RBP3 bound to pRB both in vitro and in vivo, and this binding was competed by viral proteins known to disrupt pRB-E2F association. RBP3 bound to E2F recognition sequences in a sequence-specific manner. Furthermore, transient expression of RBP3 caused a 10-fold transactivation of the adenovirus E2 promoter, and this transactivation was dependent on the E2F recognition sequences. These properties suggest that RBP3 encodes E2F, or an E2F-like protein.

Expression cloning of a cDNA encoding a retinoblastoma-binding protein with E2F-like properties

An expression vector was modified to permit the rapid synthesis of purified, 32P-labeled, glutathione S-transferase (GST)-retinoblastoma (RB) fusion proteins. The products were used to screen lambda gt11 expression libraries, from which we cloned a cDNA encoding a polypeptide (RBAP-1) capable of binding directly to a putative functional domain (the pocket) of the retinoblastoma gene product (RB). The RB "pocket" is known to bind, directly or indirectly, to the cellular transcription factor, E2F, implicated in cell growth control. We have found that RBAP-1 copurifies with E2F, interacts specifically with the adenovirus E4 ORF 6/7 protein, binds specifically and directly to a known E2F DNA recognition sequence, and contains a functional tranasactivation domain. Therefore, RBAP-1 is a species of E2F and can bind specifically to the RB pocket.

In vitro interaction between Saccharomyces cerevisiae CDC25 and RAS2 proteins

In Saccharomyces cerevisiae the CDC25 protein is a positive regulator of RAS/cAMP pathway [1-4], enhancing the GDP-releasing rate of RAS2 protein [5]. In this work we have tried to detect a direct interaction between CDC25 and RAS2 gene products. The results indicate that both the whole RAS2 protein and a truncated version that lacks approximately 25 C-terminal residues interact specifically with the CDC25 protein. On the contrary, a derivative of RAS2 that lacks the 112 C-terminal residues as well as the p21TI-ras is not able to bind the CDC25 protein in our assay conditions. The 310 C-terminal aminoacids of CDC25 bind RAS2 while a C-terminus deletion within this aminoacid stretch abolishes the binding. The possible physiological significance of these findings is discussed.

A rat cerebellar protein containing the cdc10/SWI6 motif

Systematic analysis of soluble proteins in developing rat cerebellum by an automated two-dimensional liquid-chromatography system detected a number of proteins which increased transiently during the initial stage of postnatal development. One of the proteins, V-1, was isolated using a liquid-chromatography system, and its amino acid sequence was determined by analysis of the purified protein. The sequence showed that the V-1 protein consists of 117 amino acids with an acetylated N-terminus, and has 2.5 internal sequence repeats of 33 amino acids. Computer retrieval of the sequence indicated that the repeated sequences have a structural characteristics of the cdc10/SWI6 motif, which is found in a series of proteins, including those involved in cell-cycle control and cell-fate determination in yeast, Drosophila melanogaster and Caenorhabditis elegans. The structure of V-1, coupled with its controlled expression in early postnatal development, implies a potential role for V-1 in cerebellar morphogenesis.