The Cks1/Cks2 axis fine-tunes Mll1 expression and is crucial for MLL-rearranged leukaemia cell viability

The Cdc28 protein kinase subunits, Cks1 and Cks2, play dual roles in Cdk-substrate specificity and Cdk-independent protein degradation, in concert with the E3 ubiquitin ligase complexes SCFSkp2 and APCCdc20. Notable targets controlled by Cks include p27 and Cyclin A. Here, we demonstrate that Cks1 and Cks2 proteins interact with both the MllN and MllC subunits of Mll1 (Mixed-lineage leukaemia 1), and together, the Cks proteins define Mll1 levels throughout the cell cycle. Overexpression of CKS1B and CKS2 is observed in multiple human cancers, including various MLL-rearranged (MLLr) AML subtypes. To explore the importance of MLL-Fusion Protein regulation by CKS1/2, we used small molecule inhibitors (MLN4924 and C1) to modulate their protein degradation functions. These inhibitors specifically reduced the proliferation of MLLr cell lines compared to primary controls. Altogether, this study uncovers a novel regulatory pathway for MLL1, which may open a new therapeutic approach to MLLr leukaemia.

Bub1-mediated adaptation of the spindle checkpoint

During cell division, the spindle checkpoint ensures accurate chromosome segregation by monitoring the kinetochore–microtubule interaction and delaying the onset of anaphase until each pair of sister chromosomes is properly attached to microtubules. The spindle checkpoint is deactivated as chromosomes start moving toward the spindles in anaphase, but the mechanisms by which this deactivation and adaptation to prolonged mitotic arrest occur remain obscure. Our results strongly suggest that Cdc28-mediated phosphorylation of Bub1 at T566 plays an important role for the degradation of Bub1 in anaphase, and the phosphorylation is required for adaptation of the spindle checkpoint to prolonged mitotic arrest.

The spindle checkpoint protects cells from aneuploidy by monitoring the status of the kinetochore-microtubule attachment. Defects in this checkpoint pathway and in kinetochore-microtubule attachment can cause substantial aneuploidy in cells. The duration of the mitotic arrest induced by the spindle checkpoint is not indefinite: cells eventually exit from mitosis and re-enter interphase. Because continued activation of the spindle checkpoint is lethal, adaptation to the spindle checkpoint arrest is essential so that cells have a chance for survival as opposed to certain death. However, adaptation of the spindle checkpoint has a flip side—adapted cells could have an increased chance of aneuploidy due to premature mitotic exit. Thus, it is essential that this mechanism be regulated appropriately. Despite the importance of understanding the adaptation of the spindle checkpoint, little is known to date about this mechanism. We found that Cdc28-mediated phosphorylation of Bub1 at T566 plays an important role for adaptation of the spindle checkpoint, a finding providing the molecular insight on how adaptation to prolonged mitotic arrest induced by the spindle checkpoint occurs.

DNA degradation at unprotected telomeres in yeast is regulated by the CDK1 (Cdc28/Clb) cell-cycle kinase

In the absence of functional telomeric cap protection, the ends of eukaryotic chromosomes are subject to DNA damage responses that lead to cell-cycle arrest and, eventually, genomic instability. However, the controlling activities responsible for the initiation of genome instability on unprotected telomeres remained unclear. Here we show that in budding yeast, unprotected telomeres undergo a tightly cell-cycle-regulated DNA degradation. Ablation of the function of essential capping proteins Cdc13p or Stn1p only caused telomere degradation in G2/M, but not in G1 of the cell cycle. Accordingly, G1-arrested cells with unprotected telomeres remained viable, while G2/M-arrested cells failed to recover. The data also show that completion of S phase and the activity of the S-Cdk1 kinase were required for telomere degradation. These results strongly suggest that after a loss of the telomere capping function, telomere-led genome instability is caused by tightly regulated cellular DNA repair attempts.

The Ama1-directed anaphase-promoting complex regulates the Smk1 mitogen-activated protein kinase during meiosis in yeast

Smk1 is a meiosis-specific MAPK homolog in Saccharomyces cerevisiae that regulates the postmeiotic program of spore formation. Similar to other MAPKs, it is activated via phosphorylation of the T-X-Y motif in its regulatory loop, but the signals controlling Smk1 activation have not been defined. Here we show that Ama1, a meiosis-specific activator of the anaphase-promoting complex/cyclosome (APC/C), promotes Smk1 activation during meiosis. A weakened allele of CDC28 suppresses the sporulation defect of an ama1 null strain and increases the activation state of Smk1. The function of Ama1 in regulating Smk1 is independent of the FEAR network, which promotes exit from mitosis and exit from meiosis I through the Cdc14 phosphatase. The data indicate that Cdc28 and Ama1 function in a pathway to trigger Smk1-dependent steps in spore morphogenesis. We propose that this novel mechanism for controlling MAPK activation plays a role in coupling the completion of meiosis II to gamete formation.

Ime2p and Cdc28p: co-pilots driving meiotic development

Meiosis can be considered an elaboration of the cell division cycle in the sense that meiosis combines cell-cycle processes with programs specific to meiosis. Each phase of the cell division cycle is driven forward by cell-cycle kinases (Cdk) and coordinated with other phases of the cycle through checkpoint functions. Meiotic differentiation is also controlled by these two types of regulation; however, recent study in the budding yeast S. cerevisiae indicates that progression of meiosis is also controlled by a master regulator specific to meiosis, namely the Ime2p kinase. Below, I describe the overlapping roles of Ime2p and Cdk during meiosis in yeast and speculate on how these two kinases cooperate to drive the progression of meiosis.

Loss of meiotic rereplication block in Saccharomyces cerevisiae cells defective in Cdc28p regulation

Cdc28p is the major cyclin-dependent kinase in Saccharomyces cerevisiae. Its activity is required for blocking the reinitiation of DNA replication during mitosis. Here, we show that under conditions where Cdc28p activity is improperly regulated--either through the loss of function of the Schizosaccharomyces pombe wee1 ortholog Swe1p or through the expression of a dominant CDC28 allele, CDC28AF--diploid yeast cells are able to complete several rounds of premeiotic DNA replication within a single meiotic cell cycle. Moreover, a percentage of mutant cells exhibit a "multispore" phenotype, possessing the ability to package more than four spores within a single ascus. These multispored asci contain both even and odd numbers of viable spores. In order for meiotic rereplication and multispore formation to occur, cells must initiate homologous recombination and maintain proper chromosome cohesion during meiosis I. Rad9p- or Rad17p-dependent checkpoint mechanisms are not required for multispore formation and neither are the B-type cyclin Clb6p and the cyclin-dependent kinase inhibitor Sic1p. Finally, we present evidence of a possible role for a Cdc55p-dependent protein phosphatase 2A in initiating meiotic replication.

Cdc28/Cdk1 Regulates Spindle Pole Body Duplication through Phosphorylation of Spc42 and Mps1

Duplication of the Saccharomyces cerevisiae spindle pole body (SPB) once per cell cycle is essential for bipolar spindle formation and accurate chromosome segregation during mitosis. We have investigated the role that the major yeast cyclin-dependent kinase Cdc28/Cdk1 plays in assembly of a core SPB component, Spc42, to better understand how SPB duplication is coordinated with cell cycle progression. Cdc28 is required for SPB duplication and Spc42 assembly, and we found that Cdc28 directly phosphorylates Spc42 to promote its assembly into the SPB. The Mps1 kinase, previously shown to regulate Spc42 phosphorylation and assembly, is also a Cdc28 substrate, and Cdc28 phosphorylation of Mps1 is needed to maintain wild-type levels of Mps1 in cells. Analysis of nonphosphorylatable mutants in SPC42 and MPS1 indicates that direct Spc42 phosphorylation and indirect regulation of Spc42 through Mps1 are two overlapping pathways by which Cdc28 regulates Spc42 assembly and SPB duplication during the cell cycle.

Assembly interdependence among the S. cerevisiae bud neck ring proteins Elm1p, Hsl1p and Cdc12p

In Saccharomyces cerevisiae, a complex comprising more than 20 different polypeptides assembles in a ring at the neck between the mother cell and the bud. This complex functions to coordinate cell morphology with cell division. Relatively little is known about this control system, including the physical relationships between the components of the neck ring. This study addressed the assembly interactions of three components of the ring, specifically the protein kinases Elm1p and Hsl1p and the septin Cdc12p. Specific amino acid substitutions in each of these three proteins were identified that either cause or suppress a characteristic phenotype of abnormally elongated cells and delay in the G(2)-M transition. Each protein was fused to green fluorescent protein, and its ability to localize at the neck was monitored in vivo in cells of various genotypes. Localization of Hsl1p to the neck requires Elm1p function. Elm1p localized normally in the absence of Hsl1p, although a specific point mutation in Hsl1p clearly affected Elm1p localization. The cdc12-122 mutation prevented assembly of Elm1p or Hsl1p into the neck ring. Normal assembly of Cdc12p at the neck was dependent upon Elm1p and also, to a smaller extent, on Hsl1p. Ectopic localization of Cdc12p at the bud tip was observed frequently in elm1 mutants and also, to a lesser extent, in hsl1 mutants. Thus, Elm1p is a key factor in the assembly and/or maintenance of Hsl1p, as well as at least one septin, into the bud neck ring.

Requirement of Cks2 for the first metaphase/anaphase transition of mammalian meiosis

We generated mice lacking Cks2, one of two mammalian homologs of the yeast Cdk1-binding proteins, Suc1 and Cks1, and found them to be viable but sterile in both sexes. Sterility is due to failure of both male and female germ cells to progress past the first meiotic metaphase. The chromosomal events up through the end of prophase I are normal in both CKS2-/- males and females, suggesting that the phenotype is due directly to failure to enter anaphase and not a consequence of a checkpoint-mediated metaphase I arrest.

Evolution of eukaryotic cell cycle regulation: stepwise addition of regulatory kinases and late advent of the CDKs

Protein kinases regulate a number of critical events in mitosis and meiosis. A study of the evolution of kinases involved in cell cycle control (CCC) might shed light on the evolution of the eukaryotic cell cycle. In particular, applying quantitative phylogenetic methods to key CCC kinases could provide information on the relative timing of gene duplication events. To investigate the evolution of CCC kinases, we constructed phylogenetic trees for the CDC28 family and performed statistical tests of the tree topology. This family includes the cyclin-dependent kinases (CDKs), which are key regulators of the eukaryotic cell cycle, as well as other CCC kinases. We found that CDKs and, in particular, the principal cell cycle regulator Cdc28p, branch off the phylogenetic tree at a late stage, after several other kinases involved in either mitosis or meiosis regulation. On the basis of this tree topology, it is proposed that, at early stages of evolution, the eukaryotic cell cycle was not controlled by CDKs and that only a subset of extant kinases, notably the DNA damage checkpoint kinase Chk1p, were in place. During subsequent evolution, a series of duplications of kinase genes occurred, gradually adding more kinases to the CCC system, the CDKs being among the last major additions.

The Rho-GAP Bem2p plays a GAP-independent role in the morphogenesis checkpoint

The Saccharomyces cerevisiae morphogenesis checkpoint delays mitosis in response to insults that impair actin organization and/or bud formation. The delay is due to accumulation of the inhibitory kinase Swe1p, which phosphorylates the cyclin-dependent kinase Cdc28p. Having screened through a panel of yeast mutants with defects in cell morphogenesis, we report here that the polarity establishment protein Bem2p is required for the checkpoint response. Bem2p is a Rho-GTPase activating protein (GAP) previously shown to act on Rho1p, and we now show that it also acts on Cdc42p, the GTPase primarily responsible for establishment of cell polarity in yeast. Whereas the morphogenesis role of Bem2p required GAP activity, the checkpoint role of Bem2p did not. Instead, this function required an N-terminal Bem2p domain. Thus, this single protein has a GAP-dependent role in promoting cell polarity and a GAP-independent role in responding to defects in cell polarity by enacting the checkpoint. Surprisingly, Swe1p accumulation occurred normally in bem2 cells, but they were nevertheless unable to promote Cdc28p phosphorylation. Therefore, Bem2p defines a novel pathway in the morphogenesis checkpoint.

Cdc28 and Ime2 possess redundant functions in promoting entry into premeiotic DNA replication in Saccharomyces cerevisiae

In the budding yeast Saccharomyces cerevisiae initiation and progression through the mitotic cell cycle are determined by the sequential activity of the cyclin-dependent kinase Cdc28. The role of this kinase in entry and progression through the meiotic cycle is unclear, since all cdc28 temperature-sensitive alleles are leaky for meiosis. We used a "heat-inducible Degron system" to construct a diploid strain homozygous for a temperature-degradable cdc28-deg allele. We show that this allele is nonleaky, giving no asci at the nonpermissive temperature. We also show, using this allele, that Cdc28 is not required for premeiotic DNA replication and commitment to meiotic recombination. IME2 encodes a meiosis-specific hCDK2 homolog that is required for the correct timing of premeiotic DNA replication, nuclear divisions, and asci formation. Moreover, in ime2Delta diploids additional rounds of DNA replication and nuclear divisions are observed. We show that the delayed premeiotic DNA replication observed in ime2Delta diploids depends on a functional Cdc28. Ime2Delta cdc28-4 diploids arrest prior to initiation of premeiotic DNA replication and meiotic recombination. Ectopic overexpression of Clb1 at early meiotic times advances premeiotic DNA replication, meiotic recombination, and nuclear division, but the coupling between these events is lost. The role of Ime2 and Cdc28 in initiating the meiotic pathway is discussed.

A Cdc28 mutant uncouples G1 cyclin phosphorylation and ubiquitination from G1 cyclin proteolysis

Proteolysis of the yeast G(1) cyclins is triggered by their Cdc28-dependent phosphorylation. Phosphorylated Cln1 and Cln2 are ubiquitinated by the SCF-Grr1 complex and then degraded by the 26 S proteasome. In this study, we identified a cak1 allele in a genetic screen for mutants that stabilize the yeast G(1) cyclins. Further characterization showed that Cln2HA was hypophosphorylated, unable to bind Cdc28, and stabilized in cak1 mutants at the restrictive temperature. Hypophosphorylation of Cln2HA could thus explain its stabilization. To test this possibility, we expressed a Cak1-independent mutant of Cdc28 (Cdc28-43244) in cak1 mutants and found that Cln2HA phosphorylation was restored, but surprisingly, the phospho-Cln2HA was stabilized. When bound to Cdc28-43244, Cln2HA was recognized and polyubiquitinated by SCF-Grr1. The Cdc28-43244 mutant thus reveals an unexpected complexity in the degradation of polyubiquitinated Cln2HA by the proteasome.

The cyclin-dependent kinase Cdc28p regulates distinct modes of Cdc6p proteolysis during the budding yeast cell cycle

BACKGROUND

Cdc28p, the major cyclin-dependent kinase in budding yeast, prevents re-replication within each cell cycle by preventing the reassembly of Cdc6p-dependent pre-replicative complexes (pre-RCs) once origins have fired. Cdc6p is a rapidly degraded protein that must be synthesised in each cell cycle and is present only during the G1 phase.

RESULTS

We found that, at different times in the cell cycle, there are distinct modes of Cdc6p proteolysis. Before Start, Cdc6p proteolysis did not require either the anaphase-promoting complex (APC/C) or the SCF complex, which mediate the major cell cycle regulated ubiquitination pathways, nor did it require Cdc28p activity or any of the potential Cdc28p phosphorylation sites in Cdc6p. In fact, the activation of B cyclin (Clb)-Cdc28p kinase inactivated this pathway of Cdc6p degradation later in the cell cycle. Activation of the G1 cyclins (Clns) caused Cdc6p degradation to become extremely rapid. This degradation required the SCF(CDC4) and Cdc28p consensus sites in Cdc6p, but did not require Clb5 and Clb6. Later in the cell cycle, SCF(CDC4)-dependent Cdc6p proteolysis remained active but became less rapid.

CONCLUSIONS

Levels of Cdc6p are regulated in several ways by the Cdc28p cyclin-dependent kinase. The Cln-dependent elimination of Cdc6p, which does not require the S-phase-promoting cyclins Clb5 and Clb6, suggests that the ability to assemble pre-RCs is lost before, not concomitant with, origin firing.

Analysis of genetic interactions between DHH1, SSD1 and ELM1 indicates their involvement in cellular morphology determination in Saccharomyces cerevisiae

The DHH1 gene of Saccharomyces cerevisiae belongs to a family of genes that encode highly conserved DEAD-box proteins commonly present in various eukaryotic organisms. Its precise function in yeast has not yet been well documented. To investigate its role in vivo, we constructed a DHH1 disruptant, characterized it genetically and searched for genes the mutations in which would cause synthetic lethality in combination with the DHH1 disruption. CDC28, ELM1 and SSD1 were thus found to be such candidates and we subsequently analysed their interactions. Mutations in ELM1 were previously reported to result in the elongation of cells. We confirmed this phenotype and observed in addition elongated bud formation in an Elm1p overproducing strain. Also, Elm1p fused with the green fluorescent protein (GFP) was found to be localized at the bud neck. These and other observations seem to suggest that Elm1p plays a role during cytokinesis in S. cerevisiae. The phenotypes of strains harbouring either delta dhh1 delta elm1 or ssd1-d delta elm1 were very similar to each other, showing abnormal cellular morphology and defects in cytokinesis and mitosis. Furthermore, DHH1 and SSD1 could functionally complement each other in the ade2 red colour pigment formation, hypersensitivity to SDS, growth on synthetic media and at high temperature. A triple mutant, delta dhh1 ssd1-d delta elm1, apparently had very fragile cell walls and could grow only in a medium supplemented with 1 M sorbitol.

Ectopic expression of cdc2/cdc28 kinase subunit Homo sapiens 1 uncouples cyclin B metabolism from the mitotic spindle cell cycle checkpoint

Primary human fibroblasts arrest growth in response to the inhibition of mitosis by mitotic spindle-depolymerizing drugs. We show that the mechanism of mitotic arrest is transient and implicates a decrease in the expression of cdc2/cdc28 kinase subunit Homo sapiens 1 (CKsHs1) and a delay in the metabolism of cyclin B. Primary human fibroblasts infected with a retroviral vector that drives the expression of a mutant p53 protein failed to downregulate CKsHs1 expression, degraded cyclin B despite the absence of chromosomal segregation, and underwent DNA endoreduplication. In addition, ectopic expression of CKsHs1 interfered with the control of cyclin B metabolism by the mitotic spindle cell cycle checkpoint and resulted in a higher tendency to undergo DNA endoreduplication. These results demonstrate that an altered regulation of CKsHs1 and cyclin B in cells that carry mutant p53 undermines the mitotic spindle cell cycle checkpoint and facilitates the development of aneuploidy. These data may contribute to the understanding of the origin of heteroploidy in mutant p53 cells.

Ubiquitination and degradation of the substrate recognition subunits of SCF ubiquitin-protein ligases

The S. cerevisiae SCFCdc4p ubiquitin-protein ligase complex promotes cell cycle transitions through degradation of cell cycle regulators. To investigate SCFCdc4p regulation in vivo, we examined the stability of individual SCFCdc4p components. Whereas Cdc53p and Skp1p were stable, Cdc4p, the F box-containing component responsible for substrate recognition, was short lived and subject to SCF-mediated ubiquitination. Grr1p, another F box component of SCF complexes, was also ubiquitinated. A stable truncated Cdc4pF-beta-gal hybrid protein capable of binding Skp1p and entering into an SCF complex interfered with proteolysis of SCF targets and inhibited cell proliferation. The finding that the F box-containing SCF components are unstable suggests a mechanism of regulating SCF function through ubiquitination and proteolysis of F box components.

The Polo-related kinase Cdc5 activates and is destroyed by the mitotic cyclin destruction machinery in S. cerevisiae

BACKGROUND

Following chromosome segregation in anaphase, ubiquitin-dependent degradation of mitotic cyclins contributes to the exit from mitosis. A key step in this process is catalyzed by a ubiquitin-protein ligase known as the anaphase-promoting complex (APC), the regulation of which is poorly understood. The Polo-related protein kinase Cdc5 in Saccharomyces cerevisiae might encode a regulator of the APC, because cdc5 mutant cells arrest with a late mitotic phenotype similar to that observed in cells with defective cyclin destruction.

RESULTS

We investigated the role of Cdc5 in the regulation of mitotic cyclin degradation. In cdc5-1 mutant cells, we observed a defect in the destruction of cyclins and a reduction in the cyclin-ubiquitin ligase activity of the APC. Overexpression of CDC5 resulted in increased APC activity and mitotic cyclin destruction in asynchronous cells or in cells arrested in metaphase. CDC5 mutation or overexpression did not affect the degradation of the APC substrate Pds 1, which is normally degraded at the metaphase-to-anaphase transition. Cyclin-specific APC activity in cells overexpressing CDC5 was reduced in the absence of the APC regulatory proteins Hct 1 and Cdc20. In G1, Cdc5 itself was degraded by an APC-dependent and Hct1-dependent mechanism.

CONCLUSIONS

We conclude that Cdc5 is a positive regulator of cyclin-specific APC activity in late mitosis. Degradation of Cdc5 in G1 might provide a feedback mechanism by which the APC destroys its activator at the onset of the next cell cycle.

Nik1: a Nim1-like protein kinase of S. cerevisiae interacts with the Cdc28 complex and regulates cell cycle progression

BACKGROUND

The eukaryotic cell cycle is driven by CDK-cyclin complexes. A number of proteins interact either with CDK or the CDK complex to regulate CDK activity. A search for novel cell cycle regulators in the budding yeast Saccharomyces cerevisiae yielded multicopy suppressors of the cdc2-L7 mutation of the fission yeast, Schizosaccharomyces pombe.

RESULTS

One of the isolated genes was found to encode a putative protein kinase similar to Nim1 of S. pombe and was termed NIK1 (Nim1-like kinase 1). Transcription of NIK1 was periodic and peaked at the G1/S boundary. Although NIK1 is not essential, delta nik1 cells showed G2 delay and hydroxyurea (HU) sensitivity. Anomalously elongated buds were observed in the stationary phase or in the presence of HU. Moreover, DNA was aberrantly distributed in the delta nik1 cdc28 double mutant. Genetical and biochemical evidence suggests that Nik1 interacts with the Cdc28 complex.

CONCLUSIONS

Nik is a structural and functional homologue of Nim1. Nik1 interacts with the Cdc28 complex and functions not only at the G2/M transition but also at other points of the cell cycle.

Distinct roles of yeast MEC and RAD checkpoint genes in transcriptional induction after DNA damage and implications for function

In eukaryotic cells, checkpoint genes cause arrest of cell division when DNA is damaged or when DNA replication is blocked. In this study of budding yeast checkpoint genes, we identify and characterize another role for these checkpoint genes after DNA damage-transcriptional induction of genes. We found that three checkpoint genes (of six genes tested) have strong and distinct roles in transcriptional induction in four distinct pathways of regulation (each defined by induction of specific genes). MEC1 mediates the response in three transcriptional pathways, RAD53 mediates two of these pathways, and RAD17 mediates but a single pathway. The three other checkpoint genes (including RAD9) have small (twofold) but significant roles in transcriptional induction in all pathways. One of the pathways that we identify here leads to induction of MEC1 and RAD53 checkpoint genes themselves. This suggests a positive feedback circuit that may increase the cell's ability to respond to DNA damage. We make two primary conclusions from these studies. First, MEC1 appears to be the key regulator because it is required for all responses (both transcriptional and cell cycle arrest), while other genes serve only a subset of these responses. Second, the two types of responses, transcriptional induction and cell cycle arrest, appear distinct because both require MEC1 yet only cell cycle arrest requires RAD9. These and other results were used to formulate a working model of checkpoint gene function that accounts for roles of different checkpoint genes in different responses and after different types of damage. The conclusion that the yeast MEC1 gene is a key regulator also has implications for the role of a putative human homologue, the ATM gene.

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.

Cell cycle control of morphogenesis in budding yeast

A detailed description of the cytoskeletal rearrangements that orchestrate bud formation is beginning to emerge from studies on yeast morphogenesis. In this review, we focus on recent advances in our understanding of how the timing of these rearrangements is controlled. Dramatic changes in cell polarity that occur in G1 (polarization to the bud site), G2 (depolarization within the bud), and mitosis (repolarization to the mother/bud neck) are triggered by changes in the kinase activity of Cdc28, the universal regulator of cell cycle progression. The hunt for Cdc28 morphogenesis substrates is on.

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.

Collaboration of G1 cyclins in the functional inactivation of the retinoblastoma protein

The retinoblastoma gene product (pRB) constrains cell proliferation by preventing cell-cycle progression from the G1 to S phase. Its growth-inhibitory effects appear to be reversed by hyperphosphorylation occurring during G1. This process is thought to involve G1 cyclins and cyclin-dependent kinases (cdks). Here we report that the cell cycle-dependent phosphorylation of mammalian pRB is faithfully reproduced when it is expressed in Saccharomyces cerevisiae. As is the case in mammalian cells, this phosphorylation requires an intact oncoprotein-binding domain and is inhibited by a negative growth factor, in this case a mating pheromone. Expression of pRB in cln (-) mutants indicates that specific combinations of endogenous G1 cyclins, Cln3 and either Cln1 or Cln2 are required for pRB hyperphosphorylation in yeast. Moreover, expression of mammalian G1 cyclins in cln (-) yeast cells indicates that the functions of Cln2 and Cln3 in pRB hyperphosphorylation can be complemented by human cyclin E and cyclin D1, respectively. These observations suggest a functional heterogeneity among G1 cyclin-cdk complexes and indicate a need for the involvement of multiple G1 cyclins in promoting pRB hyperphosphorylation and resulting cell-cycle progression.

Genetic analysis of Cln/Cdc28 regulation of cell morphogenesis in budding yeast

The CLN1, CLN2 and CLN3 gene family of G1-acting cyclin homologs of Saccharomyces cerevisiae is functionally redundant: any one of the three Cln proteins is sufficient for activation of Cdc28p protein kinase activity for cell cycle START. The START event leads to multiple processes (including DNA replication and bud emergence); how Cln/Cdc28 activity activates these processes remains unclear. CLN3 is substantially different in structure and regulation from CLN1 and CLN2, so its functional redundancy with CLN1 and CLN2 is also poorly understood. We have isolated mutations that alter this redundancy, making CLN3 insufficient for cell viability in the absence of CLN1 and CLN2 expression. Mutations causing phenotypes specific for the cell division cycle were analyzed in detail. Mutations in one gene result in complete failure of bud formation, leading to depolarized cell growth. This gene was identified as BUD2, previously described as a non-essential gene required for proper bud site selection but not required for budding and viability. Bud2p is probably the GTPase-activating protein for Rsr1p/Bud1p [Park, H., Chant, I. and Herskowitz, I. (1993) Nature, 365, 269-274]; we find that Rsr1p is required for the bud2 lethal phenotype. Mutations in two other genes (ERC10 and ERC19) result in a different morphogenetic defect: failure of cytokinesis resulting in the formation of long multinucleate tubes. These results suggest direct regulation of diverse aspects of bud morphogenesis by Cln/Cdc28p activity.

Starting to cycle: G1 controls regulating cell division in budding yeast

In Saccharomyces cerevisiae, START has been shown to comprise a series of tightly regulated reactions by which the cellular environment is assessed and under appropriate conditions, cells are commited to a further round of mitotic division. The key effector of START is the product of the CDC28 gene and the mechanisms by which the protein kinase activity of this gene product is regulated at START are well characterized. This is in contrast to the events which follow p34CDC28 activation and the way in which progress to S phase is achieved, which are less clear. We suggest two possible models to describe the regulation of these events. Firstly, it is conceivable that the only post-START targets of the p34CDC28/G1 cyclin kinase complex are components of the SBF and DSC1 transcription factors. This would require that either SBF or DSC1 regulates CDC4 function either directly by activating the transcription of CDC4 itself or else indirectly by activating the transcription of a mediator of CDC4 function in a manner analogous to the way in which the control of CDC7 function may be mediated by transcriptional regulation of DBF4 (Jackson et al., 1993). Potential regulatory effectors of CDC4 function include SCM4, which suppresses cdc4 mutations in an allele-specific manner (Smith et al., 1992) or its homologue HFS1 (J. Hartley & J. Rosamond, unpublished). This possibility is supported by the finding that CDC4 has no upstream SCB or MCB elements, whereas SCM4 and HFS1 have either an exact or close match to the SCB. This model would further require that genes needed for bud emergence and spindle pole body duplication are also subject to transcriptional regulation by DSC1 or SBF. An alternative model is that the p34CDC28/G1 cyclin complexes have several targets post-START, one being DSC1 and the others being as yet unidentified components of the pathways leading to CDC4 function, spindle pole body duplication and bud emergence. This model could account for the functional redundancy observed amongst the G1 cyclins with the various cyclins providing substrate specificity for the kinase complex. We suggest that a complex containing Cln3 protein is primarily responsible for, and acts most efficiently on, the targets containing Swi6 protein (SBF and DSC1), with complexes containing other G1 cyclins (Cln1 and/or Cln2 proteins) principally involved in activating the other pathways. However, there must be overlap in the function of these complexes with each cyclin able to substitute for some or all of the functions when necessary, albeit with differing efficiencies. This hypothesis is supported by several observations.(ABSTRACT TRUNCATED AT 400 WORDS)

Negative regulation of FAR1 at the Start of the yeast cell cycle

In budding yeast, a switch between the mutually exclusive pathways of cell cycle progression and conjugation is controlled at Start in late G1 phase. Mating pheromones promote conjugation by arresting cells in G1 phase before Start. Pheromone-induced cell cycle arrest requires a functional FAR1 gene. We have found that FAR1 transcription and protein accumulation are regulated independently during the cell cycle. FAR1 RNA and protein are highly expressed in early G1, but decline sharply at Start. Far1 is phosphorylated just before it disappears at Start, suggesting that modification may target Far1 for degradation. Although FAR1 mRNA levels rise again during late S or G2 phase, reaccumulation of Far1 protein to functional levels is restricted until after nuclear division.

The Cdk-associated protein Cks1 functions both in G1 and G2 in Saccharomyces cerevisiae

The CKS1 gene of Saccharomyces cerevisiae encodes a small essential protein shown to interact genetically and physically with the Cdc28 protein kinase. To investigate the specific functions of the CKS1 gene product, conditional temperature-sensitive mutant alleles were generated. The mutations were found to impair the ability of cells to undergo both the G1/S-phase and G2/M-phase transitions of the cell cycle, as well as the ability to bud. Mutants were not defective, however, in their ability to activate Cdc28 kinase as assayed in vitro on the substrate histone H1. It is likely, therefore, that Cks1 mediates a more specialized function of the Cdc28 kinase such as its ability to form specific multimeric complexes or to localize properly in cellular compartments.

Saccharomyces cerevisiae cell cycle: cdc28 and the G1 cyclins

Two families of cyclin-like proteins have been found in S. cerevisiae. The clb proteins are the mitotic cyclins. The cln proteins provide an essential function, are required for the G1/S transition, and appear to be rate-limiting for START, but have no obvious role elsewhere in the cycle. The cln proteins are unstable; they form complexes with cdc28; the complexes have protein kinase activity; and at least one of the clns oscillates in abundance through the cell cycle. The action of the cln cyclins at START suggests that they may be 'G1 cyclins'.