p25rum1 promotes proteolysis of the mitotic B-cyclin p56cdc13 during G1 of the fission yeast cell cycle.

The fission yeast Schizosaccharomyces pombe CDK inhibitor p25rum1 plays a major role in regulating cell cycle progression during G1. Here we show that p25rum1 associates with the CDK p34cdc2/p56cdc13 during G1 in normally cycling cells and is required for the rapid proteolysis of p56cdc13. In vitro binding data indicate that p25rum1 has specificity for the B-cyclin p56cdc13 component of the CDK and can bind the cyclin even in the absence of the cyclin destruction box. At the G1-S-phase transition, p25rum1 levels decrease and p56cd13 levels increase. We also show that on release from a G1 block, the rapid disappearance of p25rum1 requires the activity of the CDK p34cdc2/cig1p and that this same CDK phosphorylates p25rum1 in vitro. We propose that the binding of p25rum1 to p56cdc13 promotes cyclin proteolysis during G1, with p25rum1 possibly acting as an adaptor protein, promoting transfer of p56cdc13 to the proteolytic machinery. At the G1-S-phase transition, p25rum1 becomes targeted for proteolysis by a mechanism which may involve p34cdc2/cig1p phosphorylation. As a consequence, at this point in the cell cycle p56cdc13 proteolysis is inhibited, leading to a rise of p56cdc13 levels in preparation for mitosis.     

The Bfa1/Bub2 GAP complex comprises a universal checkpoint required to prevent mitotic exit

At the end of the cell cycle, cyclin-dependent kinase (CDK) activity is inactivated to allow mitotic exit [1]. A protein phosphatase, Cdc14, plays a key role during mitotic exit in budding yeast by activating the Cdh1 component of the anaphase-promoting complex to degrade cyclin B (Clb) and inducing the CDK inhibitor Sic1 to inactivate Cdk1 [2]. To prevent mitotic exit when the cell cycle is arrested at G2/M, cells must prevent CDK inactivation. In the spindle checkpoint pathway, this is accomplished through Bfa1/Bub2, a heteromeric GTPase-activating protein (GAP) that inhibits Clb degradation by keeping the G protein Tem1 inactive [3-5]. Tem1 is required for Cdc14 activation. Here we show that in budding yeast, BUB2 and BFA1 are also required for the maintenance of G2/M arrest in response to DNA damage and to spindle misorientation. cdc13-1 bub2 and cdc13-1 bfa1 but not cdc13-1 mad2 double mutants rebud and reduplicate their DNA at the restrictive temperature. We also found that the delay in mitotic exit in mutants with misoriented spindles depended on BUB2 and BFA1, but not on MAD2. We propose that Bfa1/Bub2 checkpoint pathway functions as a universal checkpoint in G2/M that prevents CDK inactivation in response to cell-cycle delay in G2/M.     

A single fission yeast mitotic cyclin B p34cdc2 kinase promotes both S-phase and mitosis in the absence of G1 cyclins.

Deletion of the fission yeast mitotic B-type cyclin gene cdc13 causes cells to undergo successive rounds of DNA replication. We have used a strain which expresses cdc13 conditionally to investigate re-replication. Activity of Start genes cdc2 and cdc10 is necessary and p34cdc2 kinase is active in re-replicating cells. We tested to see whether other cyclins were required for re-replication using cdc13delta. Further deletion of cig1 and puc1 had no effect, but deletion of cig2/cyc17 caused a severe delay in re-replication. Deletion of cig1 and cig2/cyc17 together abolished re-replication completely and cells arrested in G1. This, and analysis of the temperature sensitive cdc13-117 mutant, suggests that cdc13 can effectively substitute for the G1 cyclin activity of cig2/cyc17. We have characterized p56cdc13 activity and find evidence that in the absence of G1 cyclins, S-phase is delayed until the mitotic p34cdc2-p56cdc13 kinase is sufficiently active. These data suggest that a single oscillation of p34cdc2 kinase activity provided by a single B-type cyclin can promote ordered progression into both DNA replication and mitosis, and that the level of cyclin-dependent kinase activity may act as a master regulator dictating whether cells undergo S-phase or mitosis.     

Cold-sensitive mutants of p34cdc2 that suppress a mitotic catastrophe phenotype in fission yeast.

Summary The p34^cdc2 protein kinase plays a central role in the regulation of the eukaryotic cell cycle, being required both in late G1 for the commitment to S-phase and in late G2 for the initiation of mitosis. p34^cdc2 also determines the precise timing of entry into mitosis in fission yeast, where a number of gene produts that regulate p34^cdc2 activity have been identified and characterised. To investigate further the mitotic role of p34^cdc2 in this organism we have isolated new cold-sensitive p34^cdc2 mutants. These are defective only in their G2 function and are extragenic suppressors of the lethal premature entry into mitosis brought about by mutating the mitotic inhibitor p107^wee1 and overproducing the mitotic activator p80^cdc25. One of the mutant proteins p34^cdc2-E8 is only functional in the absence of p107^wee1, and all the mutant strains have reduced histone H1 kinase activity in vitro. Each mutant allele has been cloned and sequenced, and the lesions responsible for the cold-sensitive phenotypes identified. All the mutations were found to map to regions that are conserved between the fission yeast p34^cdc2 and functional homologues from higher eukaryotes.     

APC(ste9/srw1) promotes degradation of mitotic cyclins in G(1) and is inhibited by cdc2 phosphorylation

Fission yeast ste9/srw1 is a WD-repeat protein highly homologous to budding yeast Hct1/Cdh1 and DROSOPHILA: Fizzy-related that are involved in activating APC/C (anaphase-promoting complex/cyclosome). We show that APC(ste9/srw1) specifically promotes the degradation of mitotic cyclins cdc13 and cig1 but not the S-phase cyclin cig2. APC(ste9/srw1) is not necessary for the proteolysis of cdc13 and cig1 that occurs at the metaphase-anaphase transition but it is absolutely required for their degradation in G(1). Therefore, we propose that the main role of APC(ste9/srw1) is to promote degradation of mitotic cyclins when cells need to delay or arrest the cell cycle in G(1). We also show that ste9/srw1 is negatively regulated by cdc2-dependent protein phosphorylation. In G(1), when cdc2-cyclin kinase activity is low, unphosphorylated ste9/srw1 interacts with APC/C. In the rest of the cell cycle, phosphorylation of ste9/srw1 by cdc2-cyclin complexes both triggers proteolysis of ste9/srw1 and causes its dissociation from the APC/C. This mechanism provides a molecular switch to prevent inactivation of cdc2 in G(2) and early mitosis and to allow its inactivation in G(1).     

Felix Hoppe-Seyler Lecture 1999. Cyclin dependent kinases and regulation of the fission yeast cell cycle.

The cyclin dependent kinases (CDKs), formed by complexes between Cdc2p and the B-cyclins Cig2p and Cdc13p, have a central role in regulating the fission yeast cell cycle and maintaining genomic stability. The CDK Cig2p/Cdc2p controls the onset of S-phase and the CDK Cdc13p/Cdc2p controls the onset of mitosis and ensures that there is only one S-phase in each cell. Cdc13p/Cdc2p can replace Cig2p/Cdc2p forthe onset of S-phase, suggesting that the increasing activity of a single CDK during the cell cycle is sufficient to drive a cell in an orderly fashion into S-phase and into mitosis. If S-phase is incomplete, then inhibition of Cdc13p/ Cdc2p prevents cells with unreplicated DNA from undergoing a catastrophic entry into mitosis. Control of CDK activity is also important to allow cells to exit the cell cycle and accumulate in G1 in response to nutritional deprivation and the presence of pheromone.     

B-type cyclins regulate G1 progression in fission yeast in opposition to the p25rum1 cdk inhibitor.

The onset of S phase in fission yeast is regulated at Start, the point of commitment to the mitotic cell cycle. The p34cdc2 kinase is essential for G1 progression past Start, but until now its regulation has been poorly understood. Here we show that the cig2/cyc17 B-type cyclin has an important role in G1 progression, and demonstrate that p34cdc2 kinase activity is periodically associated with cig2 in G1. Cells lacking cig2 are defective in G1 progression, and this is particularly clear in small cells that must regulate Start with respect to cell size. We also find that the cig1 B-type cyclin can promote G1 progression. Whilst p25rum1 can inhibit cig2/cdc2 activity in vitro, and may transiently inhibit this complex in vivo, cig1 is regulated independently of p25rum1. Since cig1/cdc2 kinase activity peaks in mitotic cells, and decreases after mitosis with similar kinetics to cdc13-associated kinase activity, we suggest that cig2 is likely to be the principal fission yeast G1 cyclin. cig2 protein levels accumulate in G1 cells, and we propose that p25rum1 may transiently inhibit cig2-associated p34cdc2 activity until the critical cell size required for Start is reached.     

p34cdc2: the S and M kinase?

In the yeast cell cycle, the induction of two very different processes, DNA synthesis (S-phase) and mitosis (M-phase), requires the same serine/threonine-specific protein kinase p34cdc2, which has been highly conserved through evolution. On the basis of work conducted largely in multicellular eukaryotes, it has recently been suggested that p34cdc2 is able to perform these two mutually exclusive roles by phosphorylating different sets of substrates through a cell cycle-dependent association with other proteins that dictate the substrate specificity of the protein kinase. To recognize its mitotic substrates, p34cdc2 associates with one of the cyclins--a family of proteins of two distinct but related types (A and B) characterized by their periodic destruction at each mitosis. In interphase, the formation of a complex between p34cdc2 and another protein (or proteins) would allow the phosphorylation of a different set of proteins involved in the G1 to S transition. This review focuses on the evidence for this appealing simple model and the nature of the putative substrates proposed.     

Control over the onset of DNA synthesis in fission yeast

The fission yeast Schizosaccharomyces pombe has been used to identify gene functions required for the cell to become committed to the mitotic cell cycle and to initiate the processes leading to chromosome replication in S-phase. Two gene functions cdc2 and cdc10 must be executed for the cell to traverse 'start' and proceed from G1 into S-phase. Before the completion of these two functions the cell is in an uncommitted state and can undergo alternative developmental fates such as conjugation. A third gene, suc1, has also been identified whose product may interact directly with that of cdc2 at 'start'. The molecular functions of the genes involved in the completion of 'start' have been investigated. The cdc2 gene has been shown to be a protein kinase, suggesting that phosphorylation may be involved in the control over the transition from G1 into S-phase. The biochemical functions of the cdc10 and suc1 gene products have not yet been elucidated. A control at 'start' has also been shown to exist in the budding yeast Saccharomyces cerevisiae. Traverse of 'start' requires the execution of the CDC28 gene function. The cdc2 and CDC28 gene products (lower-case letters represent genes of Schizosaccharomyces pombe, and capital letters genes of Saccharomyces cerevisiae) are functionally homologous, suggesting that the processes involved in traverse of 'start' are highly conserved. An analogous control may also exist in the G1 period of mammalian cells, suggesting that the 'start' control step, after which cells become committed to the mitotic cell cycle, may have been conserved through evolution.     

The cdc2Ms Kinase Is Differently Regulated in the Cytoplasm and in the Nucleus

To study a cyclin-dependent kinase (CDK) from alfalfa (Medicago sativa L.), an antibody was raised against the C-terminal 16 amino acids of the protein cdc2aMs. The cdc2Ms protein was immunopurified with this antibody and its histone kinase activity was measured. The cdc2Ms kinase is activated at the G1/S transition when phosphate-starved cells from the G0 phase re-enter the cell cycle and remain active as cells transit the S, G2, and M phases, indicating that the same CDK regulates all of these phases in alfalfa. In contrast, when cdc2Ms kinase was purified by binding to p13suc1, it was active only in the G2 and M phases. In immunoblots the C-terminal antibody detected an equal amount of the cdc2Ms protein in the cytoplasm and in the nucleus. By indirect immunofluorescence, however, the cytoplasmic form of cdc2Ms could not be found in the S phase of the cells, indicating that the epitope for the cdc2 antibody is not accessible. Binding of putative inhibitor proteins to cdc2 was shown by inactivation of purified plant CDK when cell extracts were added. Furthermore, purified CDK inhibitors, such as the mouse p27kip1 and the yeast p40sic1, blocked the purified plant CDK activity.     

Fission yeast Fizzy-related protein srw1p is a G(1)-specific promoter of mitotic cyclin B degradation

Downregulation of cyclin-dependent kinase (Cdk)-mitotic cyclin complexes is important during cell cycle progression and in G(1) arrested cells undergoing differentiation. srw1p, a member of the Fizzy-related protein family in fission yeast, is required for the degradation of cdc13p mitotic cyclin B during G(1) arrest. Here we show that srw1p is not required for the degradation of cdc13p during mitotic exit demonstrating that there are two systems operative at different stages of the cell cycle for cdc13p degradation, and that srw1p is phosphorylated by Cdk-cdc13p only becoming dephosphorylated during G(1) arrest. We propose that this phosphorylation targets srw1p for proteolysis and inhibits its activity to promote cdc13p turnover.     

Inactivation of the cyclin-dependent kinase Cdc28 abrogates cell cycle arrest induced by DNA damage and disassembly of mitotic spindles in Saccharomyces cerevisiae.

Eukaryotic cells may halt cell cycle progression following exposure to certain exogenous agents that damage cellular structures such as DNA or microtubules. This phenomenon has been attributed to functions of cellular control mechanisms termed checkpoints. Studies with the fission yeast Schizosaccharomyces pombe and mammalian cells have led to the conclusion that cell cycle arrest in response to inhibition of DNA replication or DNA damage is a result of down-regulation of the cyclin-dependent kinases (CDKs). Based on these studies, it has been proposed that inhibition of the CDK activity may constitute a general mechanism for checkpoint controls. Observations made with the budding yeast Saccharomyces cerevisiae, however, appear to disagree with this model. It has been shown that high levels of mitotic CDK activity are present in the budding yeast cells arrested in G2/mitosis as the result of DNA damage or replication inhibition. In this report, we show that a novel mutant allele of the CDC28 gene, encoding the budding yeast CDK, allowed cell cycle passage through mitosis and nuclear division in the presence of DNA damage and the microtubule toxin nocodazole at a restrictive temperature. Unlike the checkpoint-defective mutations in CDKs of fission yeast and mammalian cells, the cdc28 mutation that we identified was recessive and resulted in a loss of the CDK activity, including the Clb2-, Clb5-, and Clb6-associated, but not the Clb3-associated, CDK activities. Examination of several known alleles of cdc28 revealed that they were also, albeit partially, defective in cell cycle arrest in response to UV-generated DNA damage. These findings suggest that Cdc28 kinase in budding yeast may be required for cell cycle arrest resulting from DNA damage and disassembly of mitotic spindles.