The cytoskeleton in mRNA localization and cell differentiation

Asymmetric distribution of cytoplasmic proteins and messenger RNAs has been implicated in several instances of cell differentiation. Microtubules have been suggested to direct mRNA localization in Drosophila and Xenopus oocytes but motor proteins that might transport mRNAs have not yet been identified. Recent data imply that in Drosophila, Caenorhabditis elegans and budding yeast, proteins of the actin cytoskeleton, including unconventional myosins, play active roles in the segregation of differentiation factors and mRNAs.

Long-range interactions at the HO promoter

The SWI5 gene encodes a zinc finger DNA-binding protein required for the transcriptional activation of the yeast HO gene. There are two Swi5p binding sites in the HO promoter, site A at -1800 and site B at -1300. Swi5p binding at site B has been investigated in some detail, and we have shown that Swi5p binds site B in a mutually cooperative fashion with Pho2p, a homeodomain protein. In this report, we demonstrate that Swi5p and Pho2p bind cooperatively to both sites A and B but that there are differences in binding to these two promoter sites. It has been shown previously that point mutations in either Swi5p binding site only modestly reduce HO expression in a PHO2 strain. We show that these mutant promoters are completely inactive in a pho2 mutant. We have created stronger point mutations at the two Swi5p binding sites within the HO promoter, and we show that the two binding sites, separated by 500 bp, are both absolutely required for HO expression, independent of PHO2. These results create an apparent dilemma, as the strong mutations at the Swi5p binding sites show that both binding sites are required for HO expression, but the earlier binding site mutations allow Swi5p to activate HO, but only in the presence of Pho2p. To explain these results, a model is proposed in which physical interaction between Swi5p proteins bound to these two sites separated by 500 bp is required for activation of the HO promoter. Experimental evidence is presented that supports the model. In addition, through deletion analysis we have identified a region near the amino terminus of Swi5p that is required for PHO2-independent activation of HO, suggesting that this region mediates the long-range interactions between Swi5p molecules bound at the distant sites.

Crystal structure of the DNA-binding domain of Mbp1, a transcription factor important in cell-cycle control of DNA synthesis

BACKGROUND

During the cell cycle, cells progress through four distinct phases, G1, S, G2 and M; transcriptional controls play an important role at the transition between these phases. MCB-binding factor (MBF), a transcription factor from budding yeast, binds to the so-called MCB (MluI cell-cycle box) elements found in the promoters of many DNA synthesis genes, and activates the transcription of those at the G1-->S phase transition. MBF is comprised of two proteins, Mbp1 and Swi6.

RESULTS

The three-dimensional structure of the N-terminal DNA-binding domain of Mbp1 has been determined by multiwavelength anomalous diffraction from crystals of the selenomethionyl variant of the protein. The structure is composed of a six-stranded beta sheet interspersed with two pairs of alpha helices. The most conserved core region among Mbp1-related transcription factors folds into a central helix-turn-helix motif with a short N-terminal beta strand and a C-terminal beta hairpin.

CONCLUSIONS

Despite little sequence similarity, the structure within the core region of the Mbp1 N-terminal domain exhibits a similar fold to that of the DNA-binding domains of other proteins, such as hepatocyte nuclear factor-3gamma and histone H5 from eukaryotes, and the prokaryotic catabolite gene activator. However, the structure outside the core region defines Mbp1 as a larger entity with substructures that stabilize and display the helix-turn-helix motif.

Identification of subunits of the anaphase-promoting complex of Saccharomyces cerevisiae

Entry into anaphase and proteolysis of B-type cyclins depend on a complex containing the tetratricopeptide repeat proteins Cdc16p, Cdc23p, and Cdc27p. This particle, called the anaphase-promoting complex (APC) or cyclosome, functions as a cell cycle-regulated ubiquitin-protein ligase. Two additional subunits of the budding yeast APC were identified: The largest subunit, encoded by the APC1 gene, is conserved between fungi and vertebrates and shows similarity to BIMEp from Aspergillus nidulans. A small heat-inducible subunit is encoded by the CDC26 gene. The yeast APC is a 36S particle that contains at least seven different proteins.

At the heart of the budding yeast cell cycle

It might now seem obvious that the mechanisms regulating cell division would be found to be a highly conserved feature of eukaryotic cells. This was less clear 20 years ago when the pioneering genetic studies of the cell cycle were initiated. This article presents one view as to what lies at the heart of the budding yeast cell cycle. It is written on the premise that most of the key players, such as cyclin-dependent kinases, the anaphase-promoting complex, the origin recognition complex, Cdc6p and Mcm proteins, were performing similar functions in the common ancestor of yeast and man. Ideas about the budding yeast cell cycle might, therefore, have universal significance for other eukaryotic cells.

The transcription factor Swi5 regulates expression of the cyclin kinase inhibitor p40SIC1

DNA replication in budding yeast cells depends on the activation of the Cdc28 kinase (Cdk1 of Saccharomyces cerevisiae) associated with B-type cyclins Clb1 to Clb6. Activation of the kinase depends on proteolysis of the Cdk inhibitor p40SIC1 in late G1, which is mediated by the ubiquitin-conjugating enzyme Cdc34 and two other proteins, Cdc4 and Cdc53. Inactivation of any one of these three proteins prevents p40SIC1 degradation and causes cells to arrest in G1 with active Cln kinases but no Clb-associated Cdc28 kinase activity. Deletion of SIC1 allows these mutants to replicate. p40SIC1 disappears at the G1/S transition and reappears only after nuclear division. Cell cycle-regulated proteolysis seems largely responsible for this pattern, but transcriptional control could also contribute; SIC1 RNA accumulates to high levels as cells exit M phase. To identify additional factors necessary for the inhibition of the Cdk1/Cdc28 kinase in G1, we isolated mutants that can replicate DNA in the absence of Cdc4 function. Mutations in three loci (SIC1, SWI5, and RIC3) were identified. We have shown that high SIC1 transcript levels at late M phase depend on Swi5. Swi5 accumulates in the cytoplasm during S, G2, and M phases of the cell cycle but enters the nuclei at late anaphase. Our data suggest that cell cycle-regulated nuclear accumulation of Swi5 is responsible for the burst of SIC1 transcription at the end of anaphase. This transcriptional control may be important for inactivation of the Clb/Cdk1 kinase in G2/M transition and during the subsequent G1 period.

EGT2 gene transcription is induced predominantly by Swi5 in early G1

In a screen for cell cycle-regulated genes in the yeast Saccharomyces cerevisiae, we have identified a gene, EGT2, which is involved in cell separation in the G1 stage of the cell cycle. Transcription of EGT2 is tightly regulated in a cell cycle-dependent manner. Transcriptional levels peak at the boundary of mitosis and early G1 The transcription factors responsible for EGT2 expression in early G1 are Swi5 and, to a lesser extent, Ace2. Swi5 is involved in the transcriptional activation of the HO gene during late G1 and early S phase, and Ace2 induces CTS1 transcription during early and late G1 We show that Swi5 activates EGT2 transcription as soon as it enters the nucleus at the end of mitosis in a concentration-dependent manner. Since Swi5 is unstable in the nucleus, its level drops rapidly, causing termination of EGT2 transcription before cells are committed to the next cell cycle. However, Swi5 is still able to activate transcription of HO in late G1 in conjunction with additional activators such as Swi4 and Swi6.

Activation of S-phase-promoting CDKs in late G1 defines a "point of no return" after which Cdc6 synthesis cannot promote DNA replication in yeast

In eukaryotic cells, DNA replication is confined to a discrete period of the cell cycle and does not usually recur until after anaphase. In the budding yeast Saccharomyces cerevisiae, assembly of pre-replication complexes (pre-RCs) at future origins as cells exit mitosis (or later during G1 is necessary for subsequent initiation of DNA replication triggered by activation in late G1 of Cdc28/Cdk1 kinases associated with B-type cyclins Clb1-Clb6. The absence of pre-RCs during G2 and M phases could explain why origins of DNA replication fire only once during the cell cycle, even though S-phase-promoting Cdks remain active from the beginning of S phase through the end of M phase. Formation of pre-RCs and their maintenance during G1 depend on the synthesis and activity of an unstable protein encoded by CDC6. We find that Cdc6 synthesis can only promote DNA replication in a restricted window of the cell cycle: between destruction of Clbs after anaphase and activation of Clb5/ and Clb6/Cdk1 in late G1. The latter corresponds to a "point of no return," after which Cdc6 synthesis can no longer promote DNA replication. Cdc6 protein can be made throughout the cell cycle and, in certain circumstances, can accumulate within the nuclei of G2 and M phase cells without inducing re-replication. Thus, control over Cdc6 degradation and/or nuclear localization is not crucial for preventing origin re-firing. Our data are consistent with the notion that cells can no longer incorporate de novo synthesized Cdc6 into pre-RCs once C1b/Cdk1 kinases have been activated. We show that Cdc6p associates with Clb/Cdk1 kinases from late G1 until late anaphase, which might be important for inhibiting pre-RC assembly during S, G2, and M phases. Inhibition of pre-RC assembly by the same kinases that trigger initiation explains how origins are prevented from re-firing until Clb kinases are destroyed after anaphase.

TPR proteins required for anaphase progression mediate ubiquitination of mitotic B-type cyclins in yeast

The abundance of B-type cyclin-CDK complexes is determined by regulated synthesis and degradation of cyclin subunits. Cyclin proteolysis is required for the final exit from mitosis and for the initiation of a new cell cycle. In extracts from frog or clam eggs, degradation is accompanied by ubiquitination of cyclin. Three genes, CDC16, CDC23, and CSE1 have recently been shown to be required specifically for cyclin B proteolysis in yeast. To test whether these genes are required for cyclin ubiquitination, we prepared extracts from G1-arrested yeast cells capable of conjugating ubiquitin to the B-type cyclin Clb2. The ubiquitination activity was cell cycle regulated, required Clb2's destruction box, and was low if not absent in cdc16, cdc23, cdc27, and cse1 mutants. Furthermore all these mutants were also defective in ubiquitination of another mitotic B-type cyclin, Clb3. The Cdc16, Cdc23, and Cdc27 proteins all contain several copies of the tetratricopeptide repeat and are subunits of a complex that is required for the onset of anaphase. The finding that gene products that are required for ubiquitination of Clb2 and Clb3 are also required for cyclin proteolysis in vivo provides the best evidence so far that cyclin B is degraded via the ubiquitin pathway in living cells. Xenopus homologues of Cdc16 and Cdc27 have meanwhile been shown to be associated with a 20S particle that appears to function as a cell cycle-regulated ubiquitin-protein ligase.

Asymmetric accumulation of Ash1p in postanaphase nuclei depends on a myosin and restricts yeast mating-type switching to mother cells

Cell division in haploid yeast gives rise to a "mother" cell capable of mating-type switching and a "daughter" cell that is not. Switching is initiated by the HO endonuclease, whose gene is only transcribed in cells that have previously given birth to a bud (mother cells). HO expression depends on a minimyosin, She1p/Myo4p, which accumulates preferentially in growing buds. We describe a gene, ASH1, that is necessary to repress HO in daughters. ASH1 encodes a zinc finger protein whose preferential accumulation in daughter cell nuclei at the end of anaphase depends on She1p/Myo4p. The greater abundance of Ash1p in daughter cells is responsible for restricting HO expression to mother cells.

Mother cell-specific HO expression in budding yeast depends on the unconventional myosin myo4p and other cytoplasmic proteins

Certain cell types give rise to progeny that adopt different patterns of gene expression in the absence of any differences in their environment. Cells of budding yeast give birth to mother and daughter cells that differ in that only mother cells express the HO endonuclease gene and thereby switch mating types. We describe the identification of five genes, called SHE1-SHE5, that encode cytoplasmic proteins required for mother-specific HO expression. She1p, which is identical to the minimyosin Myo4p, and She3p are not, however, mother-specific proteins. On the contrary, they accumulate in growing buds. She proteins might be required for the transport of factors that promote HO repression from the mother cell into its bud. In an accompanying paper, we show that SHE genes are needed for the accumulation in daughter nuclei of Ash1p, a repressor of HO.

Switching transcription on and off during the yeast cell cycle: Cln/Cdc28 kinases activate bound transcription factor SBF (Swi4/Swi6) at start, whereas Clb/Cdc28 kinases displace it from the promoter in G2

When yeast cells reach a critical size in late G1 they simultaneously start budding, initiate DNA synthesis, and activate transcription of a set of genes that includes G1 cyclins CLN1, CLN2, and many DNA synthesis genes. Cell cycle-regulated expression of CLN1, CLN2 genes is attributable to the heteromeric transcription factor complex SBF. SBF is composed of Swi4 and Swi6 and binds to the promoters of CLN1 and CLN2. Different cyclin-Cdc28 complexes have different effects on late G1-specific transcription. Activation of transcription at the G1/S boundary requires Cdc28 and one of the G1 cyclins Cln1-Cln3, whereas repression of SBF-regulated genes in G2 requires the association of Cdc28 with G2-specific cyclins Clb1-Clb4. Using in vivo genomic footprinting, we show that SBF (Swi4/Swi6) binding to SCB elements (Swi4/Swi6 cell cycle box) in the CLN2 promoter is cell cycle regulated. SBF binds to the promoter prior to the activation of transcription in late G1, suggesting that Cln/Cdc28 kinase regulates the ability of previously bound SBF to activate transcription. In contrast, SBF dissociates from the CLN2 promoter when transcription is repressed during G2 and M phases, suggesting that Clb1-Clb4 repress SBF activity by inhibiting its DNA-binding activity. Switching transcription on and off by different mechanisms could be important to ensure that Clns are activated only once per cell cycle and could be a conserved feature of cell cycle-regulated transcription.

An essential role for the Cdc6 protein in forming the pre-replicative complexes of budding yeast

Origins of DNA replication in Saccharomyces cerevisiae are bound by two protein complexes during the cell cycle. Post-replicative complexes closely resemble those generated in vitro by purified origin recognition complex (ORC), which is essential for DNA replication in vivo. Pre-replicative complexes (pre-RCs) are characterized by an extended region of nuclease protection overlapping the ORC footprint. We show here that the Cdc6 protein (Cdc6p), which is necessary for origin firing in vivo, is essential for the establishment and maintenance of pre-RCs, suggesting that it is a component of these complexes. Without Cdc6p, G1 origins closely resemble post-replicative origins, providing evidence that ORC is also a component of pre-RCs. These results suggest that pre-RCs play an essential role in initiating DNA replication and support a two-step mechanism for the assembly of functional initiation complexes.

Terminal differentiation of normal chicken erythroid progenitors: shortening of G1 correlates with loss of D-cyclin/cdk4 expression and altered cell size control

Detailed knowledge is available about the molecular makeup of the cell cycle clock in dividing cells. However, comparatively little is known about cell cycle regulation during terminal differentiation. Here we describe a primary cell system in which this question can be addressed. Normal avian erythroid progenitors undergo continuous self-renewal in suspension culture in the presence of growth factors and hormones, allowing us to obtain large cell numbers (10(10)-10(11)). By replacing these "self-renewal factors" with erythropoietin and insulin, the cells can be induced to synchronous, terminal differentiation. During the first 72 h, the cells undergo five cell divisions. Thereafter, they arrest in G1 and complete their maturation into RBC without further divisions. Sixteen to 24 h after induction of differentiation, the cell cycle length decreased from about 20 to 12 h. This shortened doubling time was due to a drastic reduction of G1 (from 12 to 5 h), while S- and G2-phase lengths were not affected. At the same time, the differentiating cells underwent an extensive and concerted switch in their gene expression pattern. During the subsequent four cell divisions, the cell volume decreased from about 300 to less than 70 femtoliters, but the rate of protein synthesis normalized to cell volume remained constant. Interestingly, the shortening of G1 was accompanied by a rapid down-regulation of D-type cyclins and their partner, cyclin-dependent kinase type 4 (cdk4), while expression of S- and G2-M-associated cell cycle regulators (cyclin A and cdk1/cdc2) remained high until the cells arrested in G1 72-96 h after differentiation induction. We conclude that concerted reprogramming of progenitor gene expression during erythroid differentiation is accompanied by profoundly altered cell cycle progression involving the loss or alteration of cell size control at the restriction point.

Roles and regulation of Cln-Cdc28 kinases at the start of the cell cycle of Saccharomyces cerevisiae

In budding yeast G1 cells increase in cell mass until they reach a critical cell size, at which point (called Start) they enter S phase, bud and duplicate their spindle pole bodies. Activation of the Cdc28 protein kinase by G1-specific cyclins Cln1, Cln2 or Cln3 is necessary for all three Start events. Transcriptional activation of CLN1 and CLN2 by SBF and MBF transcription factors also requires an active Cln-Cdc28 kinase and it has therefore been proposed that the sudden accumulation of CLN1 and CLN2 transcripts during late G1 occurs via a positive feedback loop. We report that whereas Cln1 and Cln2 are required for the punctual execution of most, if not all, other Start-related events, they are not required for the punctual activation of SBF- or MBF-driven transcription. Cln3, on the other hand, is essential. By turning off cyclin B proteolysis and turning on proteolysis of the cyclin B-Cdc28 inhibitor p40SIC1, Cln1 and Cln2 kinases activate cyclin B-Cdc28 kinases and thereby trigger S phase. Thus the accumulation of Cln1 and Cln2 kinases which starts the yeast cell cycle is set in motion by prior activation of SBF- and MBF-mediated transcription by Cln3-Cdc28 kinase. This dissection of regulatory events during late G1 demands a rethinking of Start as a single process that causes cells to be committed to the mitotic cell cycle.

Evolution of the cell cycle

Cell proliferation involves duplication of all cell constituents and their more-or-less equal segregation to daughter cells. It seems probable that the performance of primitive cell-like structures would have been dogged by poor duplication and segregation fidelity, and by parasitism. This favoured evolution of the genome and with it the distinction between 'genomic' components like chromosomes whose synthesis is periodic and most other 'functional' components whose synthesis is continuous. Eukaryotic cells evolved from bacterial ancestors whose fused genome was replicated from a single origin and whose means of segregating sister chromatids depended on fixing their identity at replication. Evolution of an endo- or cytoskeleton, initially as means of consuming other bacteria, eventually enabled evolution of the mitotic spindle and a new means of segregating sister chromatids whose replication could be initiated from multiple origins. In this primitive eukaryotic cell, S and M phases might have been triggered by activation of a single cyclin-dependent kinase whose destruction along with that of other proteins would have triggered anaphase. Mitotic non-disjunction would have greatly facilitated genomic expansion, now possible due to multiple origins, and thereby accelerated the tempo of evolution when permitted by environmental conditions.

Ste20-like protein kinases are required for normal localization of cell growth and for cytokinesis in budding yeast

The yeast Ste20 protein kinase is involved in pheromone response. Mammalian homologs of Ste20 exist, but their function remains unknown. We identified a novel yeast STE20 homolog, CLA4, in a screen for mutations lethal in the absence of the G1 cyclins Cln1 and Cln2. Cla4 is involved in budding and cytokinesis and interacts with Cdc42, a GTPase required for polarized cell growth. Despite a cytokinesis defect, cla4 mutants are viable. However, double cla4 ste20 mutants cannot maintain septin rings at the bud neck and cannot undergo cytokinesis. Mutations in CDC12, which encodes one of the septins, were found in the same screen. Cla4 and Ste20 kinases apparently share a function in localizing cell growth with respect to the septin ring.

Cdc6 is an unstable protein whose de novo synthesis in G1 is important for the onset of S phase and for preventing a ‘reductional’ anaphase in the budding yeast Saccharomyces cerevisiae

S phase entry depends on cyclin-dependent kinases whose activation during late G1 due partly to the synthesis of unstable cyclin subunits. We identify here a second type of unstable protein, Cdc6, whose synthesis during G1 is important for initiation of DNA replication. The CDC6 gene is normally transcribed at the end of mitosis, but in cells with a prolonged G1 phase there is a second burst of transcription in late G1. The former is due to Swi5, while the latter is due to MBF or SBF transcription factors. Small G1 cells that cannot synthesize Cdc6 in late G1 progress through S phase very slowly. Cells that transcribe CDC6 neither at the end of mitosis nor in late G1 fail to replicate DNA but, despite this, undergo mitosis and produce daughter cells with fractional DNA contents. This 'reductional' anaphase occurs with almost wild-type kinetics and depends on the activity of G2 cyclins. Thus, cells that fail to duplicate chromosomes due to a cdc6 defect cannot prevent the onset of mitosis, unlike other mutants with replication defects. We show, by fluorescence in situ hybridization, that chromosomes which remain unduplicated due to a lack of Cdc6 synthesis are segregated intact to spindle poles during the 'reductional' anaphase.

Cyclin-dependent kinase site-regulated signal-dependent nuclear localization of the SW15 yeast transcription factor in mammalian cells

Control over the nuclear transport of transcription factors (TFs) represents a level of gene regulation integral to cellular processes such as differentiation, transformation and signal transduction. The Saccharomyces cerevisiae TF SWI5 is excluded from the nucleus in a cell cycle-dependent fashion, mediated by phosphorylation by the cyclin-dependent kinase (cdk) CDC28. Nuclear entry occurs in G1. beta-galactosidase fusion proteins carrying SWI5 amino acids 633-682, including the nuclear localization sequence (NLS: Lys-Lys-Tyr-Glu-Asn-Val-Val-Ile-Lys-Arg-Ser-Pro-Arg-Lys-Arg-Gly-Arg-Pro- Arg-Lys655) were analyzed for subcellular localization in appropriate temperature-sensitive yeast strains blocked in G1 or G2/M using indirect immunofluorescence, and for nuclear import kinetics in living rat hepatoma or Vero African green monkey kidney cells microinjected with fluorescently labeled bacterially expressed protein and quantitative confocal laser microscopy. Cell cycle-dependent nuclear localization in yeast was both NLS and cdk site-dependent, whereby mutation of the cdk site serines (Ser646 and Ser664) to alanine resulted in constitutive nuclear localization. In mammalian cells, the SWI5 fusion proteins were similarly transported to the nucleus in an NLS-dependent fashion, while the mutation to Ala of the cdk site serines increased the maximal level of nuclear accumulation from about 1- to over 8-fold. We suggest that phosphorylation at the cdk sites inhibits nuclear transport of SWI5, consistent with our previous observations for the inhibition of SV40 large tumor antigen nuclear transport by phosphorylation by the cdk cdc2. The results indicate for the first time that a yeast NLS and, fascinatingly, its regulatory mechanisms are functional in higher eukaryotes, implying the universal nature of regulatory signals for protein transport to the nucleus.

Cell cycle-regulated transcription of the CLB2 gene is dependent on Mcm1 and a ternary complex factor

Clb2 is the major B-type mitotic cyclin required for entry into mitosis in the budding yeast Saccharomyces cerevisiae. We showed that accumulation of CLB2 transcripts in G2 cells is controlled at the transcriptional level and identified a 55-bp upstream activating sequence (UAS) containing an Mcm1 binding site as being necessary and sufficient for cell cycle regulation. Sequences within the cell cycle-regulated UAS were shown to bind Mcm1 in vitro, and mutation which abolished Mcm1-dependent DNA binding activity eliminated cell cycle-regulated transcription in vivo. A second protein with no autonomous DNA binding activity was also recruited into Mcm1-UAS complexes, generating a ternary complex. A point mutation in the CLB2 UAS which blocked ternary complex formation, but still allowed Mcm1 to bind, resulted in loss of cell cycle regulation in vivo, suggesting that the ternary complex factor is also important in control of CLB2 transcription. We discuss the possibility that the CLB2 gene is coregulated with other genes known to be regulated with the same periodicity and suggest that Mcm1 and the ternary complex factor may coordinately regulate several other G2-regulated transcripts.

Genes involved in sister chromatid separation are needed for B-type cyclin proteolysis in budding yeast

B-type cyclin destruction is necessary for exit from mitosis and the initiation of a new cell cycle. Through the isolation of mutants, we have identified three essential yeast genes, CDC16, CDC23, and CSE1, which are required for proteolysis of the B-type cyclin CLB2 but not of other unstable proteins. cdc23-1 mutants are defective in both entering and exiting anaphase. Their failure to exit anaphase can be explained by defective cyclin proteolysis. CDC23 is required at the metaphase/anaphase transition to separate sister chromatids, and we speculate that it might promote proteolysis of proteins that hold sister chromatids together. Proteolysis of CLB2 is initiated in early anaphase, but a fraction of CLB2 remains stable until anaphase is complete.

The B-type cyclin kinase inhibitor p40SIC1 controls the G1 to S transition in S. cerevisiae

When yeast cells reach a critical size, they initiate bud formation, spindle pole body duplication, and DNA replication almost simultaneously. All three events depend on activation of Cdc28 protein kinase by the G1 cyclins Cln1, -2, and -3. We show that DNA replication also requires activation of Cdc28 by B-type (Clb) cyclins. A sextuple clb1-6 mutant arrests as multibudded G1 cells that resemble cells lacking the Cdc34 ubiquitin-conjugating enzyme. cdc34 mutants cannot enter S phase because they fail to destroy p40SIC1, which is a potent inhibitor of Clb but not Cln forms of the Cdc28 kinase. In wild-type cells, p40SIC1 protein appears at the end of mitosis and disappears shortly before S phase. Proteolysis of a cyclin-specific inhibitor of Cdc28 is therefore an essential aspect of the G1 to S phase transition.

Closing the cell cycle circle in yeast: G2 cyclin proteolysis initiated at mitosis persists until the activation of G1 cyclins in the next cycle

It is thought that DNA replication and mitosis in yeast are triggered by oscillations in the level of G1-specific (CLN1 and CLN2) and G2-specific (CLB1-CLB4) cyclins, which determine the substrate specificity of the CDC28 protein kinase. It is not understood how the time and order of appearance of different cyclin types are determined. We show here that CLB2 proteolysis, which is important for transition from mitosis to G1, is not confined to a narrow window at the end of mitosis as previously thought but continues until reactivation of CDC28 by CLN cyclins toward the end of the subsequent G1 period. Thus, cell cycle-regulated proteolysis prevents accumulation of G2-specific CLB cyclins during G1 and thereby ensures that the CLN-associated forms of the CDC28 kinase are activated without interference from CLB cyclins. Accumulation of CLN cyclins leads to inactivation of CLB cyclin proteolysis, which is a precondition for subsequent activation of G2-specific B-type cyclins.

Cell cycle regulated transcription in yeast

At least four different classes of cell cycle regulated gene exist in yeast: G1 cyclins and DNA synthesis genes are expressed in late G1; histone genes in S phase; genes for transcription factors, cell cycle regulators and replication initiation proteins in G2; and genes needed for cell separation as cells enter G1. Early and late G1-specific transcription is mediated by the Swi5/Ace2 and Swi4/Swi6 classes of factor, respectively. Changes in cyclin/Cdc28 kinases may be involved in all classes of regulation. Transcriptional control of cyclin genes has an important role in regulating cell cycle progression.

pct1+, which encodes a new DNA-binding partner of p85cdc10, is required for meiosis in the fission yeast Schizosaccharomyces pombe

The transcriptional activation of genes at late G1 is an important regulatory step in the commitment to a new cell division cycle. In Schizosaccharomyces pombe, this regulation is mediated by MCB elements that serve as binding sites for the MBF/DSC-1 complex. The cdc10(+)-encoded protein is a component of this complex. We report the cloning of a new gene, pct1+, encoding a 73-kD protein that interacts with p85cdc10 to form an MCB-binding heteromer. Pct1+ is related to, but distinct from, the res1+/sct1+ gene that also encodes a p85cdc10 partner. p73pct1 has centrally located ankyrin repeats and a putative amino-terminal DNA-binding domain that has extensive sequence similarity to the DNA-binding domains of the Saccharomyces cerevisiae SWI4 and MBP1 proteins. The p73pct1/p85cdc10 complex binds both in vitro and in vivo to MCB but not SCB or E2F sites. Overexpression of pct1+ is sufficient to rescue the growth of the cdc10-129 temperature-sensitive mutant at the restrictive temperature, although it is unable to rescue a cdc10 null mutation. A deletion of pct1+ is not lethal but does result in a severe meiotic defect. Our results indicate that there are two cdc10-containing heteromeric complexes that bind to MCB elements and play differential roles in mitotic division and meiosis.

Yeast G1 cyclins CLN1 and CLN2 and a GAP-like protein have a role in bud formation

Cyclin-dependent protein kinases have a central role in cell cycle regulation. In Saccharomyces cerevisiae, Cdc28 kinase and the G1 cyclins Cln1, 2 and 3 are required for DNA replication, duplication of the spindle pole body and bud emergence. These three independent processes occur simultaneously in late G1 when the cells reach a critical size, an event known as Start. At least one of the three Clns is necessary for Start. Cln3 is believed to activate Cln1 and Cln2, which can then stimulate their own accumulation by means of a positive feedback loop. They (or Cln3) also activate another pair of cyclins, Clb5 and 6, involved in initiating S phase. Little is known about the role of Clns in spindle pole body duplication and budding. We report here the isolation of a gene (CLA2/BUD2/ERC25) that codes for a homologue of mammalian Ras-associated GTPase-activating proteins (GAPs) and is necessary for budding only in cln1 cln2 cells. This suggests that Cln1 and Cln2 may have a direct role in bud formation.

Yeast origin recognition complex is involved in DNA replication and transcriptional silencing

The HMR E silencer represses transcription of silent mating-type genes in the budding yeast Saccharomyces cerevisiae and contains three redundant regulatory elements A, E and B (ref. 1). The A element contains the 11 base pair consensus sequence that is essential for the firing of DNA replication origins. A multisubunit protein called the origin recognition complex (ORC) binds specifically to this consensus sequence within yeast origins in vitro and in vivo. We isolated mutants in A element-mediated silencing and report here that one of the genes we identified, RRR1, encodes ORC2, the 72K subunit of ORC. RRR1/ORC2 is an essential gene, but the rrr1-316 allele, which is viable, is defective in the replication of nuclear DNA and the maintenance of the 2-microns episomal DNA. This is, to our knowledge, the first genetic evidence that ORC is involved in DNA replication and silencing.

Mechanisms that help the yeast cell cycle clock tick: G2 cyclins transcriptionally activate G2 cyclins and repress G1 cyclins

In budding yeast, G1 cyclins such as CLN1 and CLN2 are expressed in G1 and S phases, while mitotic cyclins such as CLB1 and CLB2 are expressed in G2 and M phases. We find that the CLBs play a central role in the transition from CLNs to CLBs: the CLBs stimulate their own expression while repressing that of CLNs. This negative regulation of CLNs may occur via the transcription factor SWI4, because CLBs are necessary for G2 repression of SCB-regulated genes like CLN1 and CLN2 but not for repression of MCB-regulated genes like DNA polymerase and CLB5. Furthermore, SW14 associates with CLB2 protein and is a substrate for the CLB2-associated CDC28 kinase in vitro.

A role for the transcription factors Mbp1 and Swi4 in progression from G1 to S phase

In budding yeast genes that encode G1 cyclins and proteins involved in DNA synthesis are transcriptionally activated in late G1. A transcription factor, called SBF, is composed of Swi4 and Swi6 proteins and activates transcription of G1 cyclin genes. A different, but related, complex called MBF binds to MCB elements (Mlu I cell cycle box) found in the promoter of most DNA synthesis genes. MBF contains Swi6 and a 120-kilodalton protein (p120). MBF was purified and the gene encoding p120 (termed MBP1) was cloned. A deletion of MBP1 was not lethal but led to deregulated expression of DNA synthesis genes, indicating a direct regulatory role for MBF in MCB-driven transcription. Mbp1 is related to Swi4. Strains deleted for both MBP1 and SWI4 were inviable, demonstrating that transcriptional activation by MBF and SBF has an important role in the transition from G1 to S phase.

CLB5 and CLB6, a new pair of B cyclins involved in DNA replication in Saccharomyces cerevisiae

The functions of the Cdc28 protein kinase in DNA replication and mitosis in Saccharomyces cerevisiae are thought to be determined by the type of cyclin subunit with which it is associated. G1-specific cyclins encoded by CLN1, CLN2, and CLN3 are required for entry into the cell cycle (Start) and thereby for S phase, whereas G2-specific B-type cyclins encoded by CLB1, CLB2, CLB3, and CLB4 are required for mitosis. We describe a new family of B-type cyclin genes, CLB5 and CLB6, whose transcripts appear in late G1 along with those of CLN1, CLN2, and many genes required for DNA replication. Deletion of CLB6 has little or no effect, but deletion of CLB5 greatly extends S phase, and deleting both genes prevents the timely initiation of DNA replication. Transcription of CLB5 and CLB6 is normally dependent on Cln activity, but ectopic CLB5 expression allows cells to proliferate in the absence of Cln cyclins. Thus, the kinase activity associated with Clb5/6 and not with Cln cyclins may be responsible for S-phase entry. Clb5 also has a function, along with Clb3 and Clb4, in the formation of mitotic spindles. Our observation that CLB5 is involved in the initiation of both S phase and mitosis suggests that a single primordial B-type cyclin might have been sufficient for regulating the cell cycle of the common ancestor of many, if not all, eukaryotes.

Transcription factors important for starting the cell cycle in yeast

Unlike early embryonic cleavage divisions in certain animals, cell-cycle progression in yeast and probably also in all metazoan somatic cells requires the periodic transcriptional activation of certain key genes. Thus far, the only clear examples are genes that encode a class of unstable 'cyclin' proteins, which bind and activate the cdc2/Cdc28 protein kinase: the G1-specific cyclins encoded by CLN1 and CLN2, a B-type cyclin implicated in DNA replication encoded by CLB5; and four B-type cyclins involved in mitosis encoded by CLB1, 2, 3, 4. CLN1, CLN2, and CLB5 are transcribed in late G1, as cells undergo Start. A transcription factor composed of Swi4 and Swi6 proteins (called SBF) activates CLN1 and CLN2 transcription via a positive feedback loop in which Cln proteins activate their own transcription. A different but related transcription factor called MBF seems responsible for the late G1-specific transcription of most DNA replication genes including CLB5. We have purified MBF and shown that it contains Swi6 and a 110-120 kDa protein distinct from Swi4 (p120) that contacts DNA. Thus, we propose that SBF and MBF share a common regulatory subunit (Swi6) but recognize their promoter elements via distinct DNA binding subunits.

Destruction of the CDC28/CLB mitotic kinase is not required for the metaphase to anaphase transition in budding yeast

It is widely assumed that degradation of mitotic cyclins causes a decrease in mitotic cdc2/CDC28 kinase activity and thereby triggers the metaphase to anaphase transition. Two observations made on the budding yeast Saccharomyces cerevisiae are inconsistent with this scenario: (i) anaphase occurs in the presence of high levels of kinase in cdc15 mutants and (ii) overproduction of a B-type mitotic cyclin causes arrest not in metaphase as previously reported but in telophase. Kinase destruction is therefore implicated in the exit from mitosis rather than the entry into anaphase. The behaviour of esp1 mutants shows in addition that kinase destruction can occur in the absence of anaphase completion. The execution of anaphase and the destruction of CDC28 kinase activity therefore appear to take place independently of one another.

Control of the yeast cell cycle by the Cdc28 protein kinase

It is becoming increasingly apparent that the diverse functions of Cdc28 during the yeast cell cycle are performed by forms of the kinase that are distinguished by their cyclin subunits. Entry into the cell cycle at START involves the Cln cyclins. S phase needs Clb5 or Clb6 B-type cyclins. Bipolar mitotic spindle formation involves Clb1-4 B-type cyclins. Much of the order and timing of the cell cycle events may involve the progressive activation of Cdc28 kinase activities associated with different cyclins, whose periodicity during the cycle is determined by both transcriptional and post-transcriptional controls.

Regulating the HO endonuclease in yeast

The pedigree of mating-type switching in yeast is determined by the transcription pattern of the HO endonuclease gene, which is expressed during late G1 in mother cells but not at all in daughter cells. The late-G1 specificity of HO transcription depends on a heteromeric factor, SBF, which is composed of the Swi4 and Swi6 proteins. Mother-cell specificity involves a second site-specific DNA-binding factor, Swi5, which is synthesized in the G2 and M phases and only enters the nucleus at the end of mitosis. Swi5 enters mother and daughter nuclei in equal amounts and most is then rapidly degraded. It has been suggested that in mothers but not in daughters some Swi5 protein escapes degradation and persists until SBF is activated in late G1. This subset of Swi5 molecules may constitute a mother cell's memory.

MAP kinase-related FUS3 from S. cerevisiae is activated by STE7 in vitro

Pheromone-stimulated haploid yeast cells undergo a differentiation process that allows them to mate. Transmission of the intracellular signal involves threonine and tyrosine phosphorylation of the redundant FUS3 and KSS1 kinases, which are members of the MAP kinase family. FUS3/KSS1 phosphorylation depends on two additional kinases, STE11 and STE7 (refs 2, 5, 6). Genetic analyses predict an ordered pathway where STE11 acts before STE7 and FUS3/KSS1 (refs 2, 7). Here we report that STE7 is a dual-specificity kinase that modifies FUS3 at the appropriate sites and stimulates its catalytic activity in vitro. From these data and previous genetic results, we argue that STE7 is the physiological activator of FUS3. Recent indications that MAP kinase activators are related to STE7 suggest that signal transduction pathways in many, if not all, eukaryotic organisms use homologous kinase cascades.

SWI5 instability may be necessary but is not sufficient for asymmetric HO expression in yeast

Homothallic haploid yeast cells divide to produce a mother cell that switches mating type and a daughter cell that does not. This pattern is the result of HO endonuclease transcription exclusively in mother cells, and there only transiently in late G1 as cells undergo Start. SWI5 encodes an HO transcription factor that is expressed during the S, G2, and M phases of the cell cycle. The lack of synthesis of SWI5 during G1 is essential to prevent HO transcription in daughter cells. Thus, HO must be activated by SWI5 protein synthesized in the previous cell cycle if it is to be properly regulated. SWI5 is inherited by both mother and daughter cells, and we show here that most of it is rapidly degraded during early G1. More stable mutant SWI5 proteins cause daughter cells to switch mating type, suggesting that SWI5 destruction is necessary to prevent HO expression in daughters. We show further that mother cells can still express HO when stimulated to undergo Start after arrest in early G1 for several hours. We propose that a small fraction of the SWI5 protein inherited by mother cells is extremely stable and that the crucial difference between mothers and daughters with regard to HO transcription is their differential ability to sequester SWI5 in a stable form, possibly as a component of transcription complexes on the HO promoter.

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.

Characterization of four B-type cyclin genes of the budding yeast Saccharomyces cerevisiae

The previously described CLB1 and CLB2 genes encode a closely related pair of B-type cyclins. Here we present the sequences of another related pair of B-type cyclin genes, which we term CLB3 and CLB4. Although CLB1 and CLB2 mRNAs rise in abundance at the time of nuclear division, CLB3 and CLB4 are turned on earlier, rising early in S phase and declining near the end of nuclear division. When all possible single and multiple deletion mutants were constructed, some multiple mutations were lethal, whereas all single mutants were viable. All lethal combinations included the clb2 deletion, whereas the clb1 clb3 clb4 triple mutant was viable, suggesting a key role for CLB2. The inviable multiple clb mutants appeared to have a defect in mitosis. Conditional clb mutants arrested as large budded cells with a G2 DNA content but without any mitotic spindle. Electron microscopy showed that the spindle pole bodies had duplicated but not separated, and no spindle had formed. This suggests that the Clb/Cdc28 kinase may have a relatively direct role in spindle formation. The two groups of Clbs may have distinct roles in spindle formation and elongation.

Signal transduction in Saccharomyces cerevisiae requires tyrosine and threonine phosphorylation of FUS3 and KSS1

The FUS3 and KSS1 kinases are components of the pheromone-dependent signal transduction pathway in yeast. We show that FUS3 and KSS1 become rapidly phosphorylated after pheromone treatment. Similar to mammalian MAP kinases, this modification occurs at two amino acids of FUS3, threonine-180 and tyrosine-182. A mutation introduced at either position results in complete loss of function in vivo. Amino acid substitutions that destroy catalytic activity of the kinase do not prevent phosphorylation of the mutant products, a result that excludes an autocatalytic activation pathway. The modification of FUS3 is dependent on kinases encoded by the STE11 and STE7 genes. Furthermore, a hyperactive allele of STE11 causes increased phosphorylation of FUS3 in the absence of pheromone stimulation. Thus, either STE7 or STE11 could be the kinase responsible for the phosphorylation of FUS3.

A central role for SWI6 in modulating cell cycle Start-specific transcription in yeast

Most genes involved in DNA replication in the yeast Saccharomyces cerevisiae are transcribed transiently during late G1 as cells become committed to a new cell cycle at Start. Their promoters all contain one or more versions of an 8-base-pair motif (ACGCGTNA) containing an MluI restriction enzyme site and called the MluI cell-cycle box (MCB). MCBs are both necessary and sufficient for the late G1-specific transcription of the TMP1 thymidylate synthase and POL1 DNA polymerase genes. A different late G1-specific 8-base-pair transcription element called the SCB (CACGAAAA; ref. 5) is bound by a factor containing the Swi4 and Swi6 proteins. We describe here the formation in vitro of complexes on TMP1 MCBs that contain the Swi6 protein and, we suggest, a protein of relative molecular mass 120,000 (p120) that is distinct from Swi4. Transcription due to SCBs and MCBs occurs in the absence of Swi6 but it is no longer correctly regulated in the cell cycle. We suggest that Swi6 is an essential regulatory subunit of two different Start-dependent transcription factors. One factor (SBF) contains Swi4 and binds to SCBs, whereas the other (MBF) contains the protein p120 and binds MCBs.

Regulation of p34CDC28 tyrosine phosphorylation is not required for entry into mitosis in S. cerevisiae

Progression from G2 to M phase in eukaryotes requires activation of a protein kinase composed of p34cdc2/CDC28 associated with G1-specific cyclins. In some organisms the activation of the kinase at the G2/M boundary is due to dephosphorylation of a highly conserved tyrosine residue at position 15 (Y15) of the cdc2 protein. Here we report that in the budding yeast Saccharomyces cerevisiae, p34CDC28 also undergoes cell-cycle regulated dephosphorylation on an equivalent tyrosine residue (Y19). However, in contrast to previous observations in S. pombe, Xenopus and mammalian cells, dephosphorylation of Y19 is not required for the activation of the CDC28/cyclin kinase. Furthermore, mutation of this tyrosine residue does not affect dependence of mitosis on DNA synthesis nor does it abolish G2 arrest induced by DNA damage. Our data imply that regulated phosphorylation of this tyrosine residue is not the 'universal' means by which the onset of mitosis is determined. We propose that there are other unidentified controls that regulate entry into mitosis.

A new method for the isolation of recombinant baculovirus

An improved method for the isolation of baculovirus recombinants is described. The method involves the replication and maintenance of the baculovirus genome in the yeast Saccharomyces cerevisiae which was accomplished by the isolation of a baculovirus recombinant containing yeast ARS and CEN sequences ensuring stable replication in yeast and a URA3 selectable marker. The viral DNA maintained its ability to replicate in insect cells. An efficient and rapid selection system was set up, to isolate viral recombinants in yeast; DNA from selected yeast colonies was transfected into insect cells to obtain recombinant virus. We demonstrate the utility of this system by isolating recombinant viruses that express two different members of the CREB/ATF family of transcription factors.

SWI6 is a regulatory subunit of two different cell cycle START-dependent transcription factors in Saccharomyces cerevisiae

Most genes involved in DNA replication in the yeast Saccharomyces cerevisiae are transcribed transiently during late G1 as cells undergo START. Their promoters all contain one or more versions of an 8-base pair motif (ACGCGTNA) called the MluI cell cycle box (MCB). MCBs have been shown to be both necessary and sufficient for the late G1-specific transcription of the TMP1 thymidylate synthase and POLI DNA polymerase genes. A different late G1-specific transcription element called the SCB (CACGAAAA) is bound by a factor containing the SWI4 and SWI6 proteins. We describe here the formation in vitro of complexes on TMP1 MCBs that contain the SWI6 protein and, we suggest, a 120 kDa protein that is distinct from SWI4. Transcription due to SCBs and MCBs occurs in the absence of SWI6 but it is no longer correctly cell cycle regulated. We suggest that SWI6 is an essential regulatory subunit of two different START-dependent transcription factors. One factor (SBF) contains SWI4 and binds to SCBs whereas the other (MBF) contains p120 and binds MCBs.