The cell cycle and suc1: from structure to function?

Auxin is required for the assembly of A-type cyclin-dependent kinase complexes in tobacco cell suspension culture

Although activation of A-type cyclin-dependent kinase (CDKA) is required for plant cell division, little is known about how CDKA is activated before commitment to cell division. Here, we show that auxin is required for the formation of active CDKA-associated complexes, promoting assembly of the complex in tobacco suspension culture Bright Yellow-2 (BY-2) cells. Protein gel blot analysis revealed that CDKA levels increased greatly after stationary-phase BY-2 cells were subcultured into fresh medium to re-enter the cell cycle. However, these increasing levels subsided when cells were subcultured into auxin-deprived medium, and a subtle increase was observed after subculturing into sucrose-deprived medium. Additionally, p13(suc1)-associated kinase activity did not increase significantly after subculturing into either auxin- or sucrose-deprived medium, but increased strongly after subculturing into medium containing both auxin and sucrose. Using gel filtration, we found that p13(suc1)-associated kinase activity against tobacco retinoblastoma-related protein was maximal in fractions corresponding to the molecular mass of the cyclin/CDKA complex. Interestingly, this peak distribution of high molecular-mass fractions of CDKA disappeared after cells were subcultured into auxin-deprived medium. These findings suggest an important role for auxin in the assembly of active CDKA-associated complexes.

Genome-wide annotation and expression profiling of cell cycle regulatory genes in Chlamydomonas reinhardtii

Eukaryotic cell cycles are driven by a set of regulators that have undergone lineage-specific gene loss, duplication, or divergence in different taxa. It is not known to what extent these genomic processes contribute to differences in cell cycle regulatory programs and cell division mechanisms among different taxonomic groups. We have undertaken a genome-wide characterization of the cell cycle genes encoded by Chlamydomonas reinhardtii, a unicellular eukaryote that is part of the green algal/land plant clade. Although Chlamydomonas cells divide by a noncanonical mechanism termed multiple fission, the cell cycle regulatory proteins from Chlamydomonas are remarkably similar to those found in higher plants and metazoans, including the proteins of the RB-E2F pathway that are absent in the fungal kingdom. Unlike in higher plants and vertebrates where cell cycle regulatory genes have undergone extensive duplication, most of the cell cycle regulators in Chlamydomonas have not. The relatively small number of cell cycle genes and growing molecular genetic toolkit position Chlamydomonas to become an important model for higher plant and metazoan cell cycles.

Annotation in three dimensions. PINTS: Patterns in Non-homologous Tertiary Structures

The detection of local structural patterns in proteins (e.g. active sites) can provide insights into protein function in the absence of sequence or fold similarity. Methods to detect such similarities are key during structural annotation, for example with results from Structural Genomics initiatives. PINTS (Patterns in Non-homologous Tertiary Structures, http://pints.embl.de) performs database searches for such patterns and most importantly provides a measure of statistical significance for any similarity uncovered. To aid functional annotation of proteins, we allow comparisons of pre-defined patterns against databases of complete structures and of entire structures to databases of particular residues likely to be functionally important.

Cks1-dependent proteasome recruitment and activation of CDC20 transcription in budding yeast

Cks proteins are small evolutionarily conserved proteins that interact genetically and physically with cyclin-dependent kinases. However, in spite of a large body of genetic, biochemical and structural research, no compelling unifying model of their functions has emerged. Here we show, by investigating the essential role of Cks1 in Saccharomyces cerevisiae, that the protein is primarily involved in promoting mitosis by modulating the transcriptional activation of the APC/C protein-ubiquitin ligase activator Cdc20. Cks1 is required for both the periodic dissociation of Cdc28 kinase from the CDC20 promoter and the periodic association of the proteasome with the promoter. We propose that the essential role of Cks1 is to recruit the proteasome to, and/or dissociate the Cdc28 kinase from, the CDC20 promoter, thus facilitating transcription by remodelling transcriptional complexes or chromatin associated with the CDC20 gene.

Solution NMR study of the monomeric form of p13suc1 protein sheds light on the hinge region determining the affinity for a phosphorylated substrate

Cyclin-dependent kinase subunit (CKS) proteins bind to cyclin-dependent kinases and target various proteins to phosphorylation and proteolysis during cell division. Crystal structures showed that CKS can exist both in a closed monomeric conformation when bound to the kinase and in an inactive C-terminal beta-strand-exchanged conformation. With the exception of the hinge loop, however, both crystal structures are identical, and no new protein interface is formed in the dimer. Protein engineering studies have pinpointed the crucial role of the proline 90 residue of the p13(suc1) CKS protein from Schizosaccharomyces pombe in the monomer-dimer equilibrium and have led to the concept of a loaded molecular spring of the beta-hinge motif. Mutation of this hinge proline into an alanine stabilizes the protein and prevents the occurrence of swapping. However, other mutations further away from the hinge as well as ligand binding can equally shift the equilibrium between monomer and dimer. To address the question of differential affinity through relief of the strain, here we compare the ligand binding of the monomeric form of wild-type S. pombe p13(suc1) and its hinge mutant P90A in solution by NMR spectroscopy. We indeed observed a 5-fold difference in affinity with the wild-type protein being the most strongly binding. Our structural study further indicates that both wild-type and the P90A mutant proteins adopt in solution the closed conformation but display different dynamic properties in the C-terminal beta-sheet involved in domain swapping and protein interactions.

Crystal structure and mutational analysis of the Saccharomyces cerevisiae cell cycle regulatory protein Cks1: implications for domain swapping, anion binding and protein interactions

BACKGROUND

The Saccharomyces cerevisiae protein Cks1 (cyclin-dependent kinase subunit 1) is essential for cell-cycle progression. The biological function of Cks1 can be modulated by a switch between two distinct molecular assemblies: the single domain fold, which results from the closing of a beta-hinge motif, and the intersubunit beta-strand interchanged dimer, which arises from the opening of the beta-hinge motif. The crystal structure of a cyclin-dependent kinase (Cdk) in complex with the human Cks homolog CksHs1 single-domain fold revealed the importance of conserved hydrophobic residues and charged residues within the beta-hinge motif.

RESULTS

The 3.0 A resolution Cks1 structure reveals the strict structural conservation of the Cks alpha/beta-core fold and the beta-hinge motif. The beta hinge identified in the Cks1 structure includes a novel pivot and exposes a cluster of conserved tyrosine residues that are involved in Cdk binding but are sequestered in the beta-interchanged Cks homolog suc1 dimer structure. This Cks1 structure confirms the conservation of the Cks anion-binding site, which interacts with sidechain residues from the C-terminal alpha helix of another subunit in the crystal.

CONCLUSIONS

The Cks1 structure exemplifies the conservation of the beta-interchanged dimer and the anion-binding site in evolutionarily distant yeast and human Cks homologs. Mutational analyses including in vivo rescue of CKS1 disruption support the dual functional roles of the beta-hinge residue Glu94, which participates in Cdk binding, and of the anion-binding pocket that is located 22 A away and on an opposite face to Glu94. The Cks1 structure suggests a biological role for the beta-interchanged dimer and the anion-binding site in targeting Cdks to specific phosphoproteins during cell-cycle progression.

Role of Suc1 in the activation of the cyclosome by protein kinase Cdk1/cyclin B

A large complex, called the cyclosome or anaphase-promoting complex, has specific and regulated protein-ubiquitin ligase activity that targets mitotic regulators (such as cyclin B) for degradation at the end of mitosis. In early embryonic cell cycles the cyclosome is inactive in the interphase, but is subsequently converted by protein kinase Cdk1/cyclin B to an active, phosphorylated form, in a process that includes an initial lag period. This time lag may be important to prevent premature self-inactivation of Cdk1/cyclin B before the end of mitosis. We have previously observed that the phosphorylated form of the cyclosome binds to Suc1, a protein that associates with Cdk1 and with phosphate-containing compounds. We now report that low, physiological concentrations of Suc1 stimulate the activation of the interphase form of the cyclosome by the protein kinase. When Suc1 was present from the beginning of the incubation together with protein kinase Cdk1/cyclin B, activation of the cyclosome took place with the normal lag kinetics. However, when interphase cyclosome was first incubated with protein kinase Cdk1/cyclin B without Suc1, the subsequent addition of Suc1 caused a rapid burst of cyclosome activation and the lag was completely abolished. These findings are consistent with the interpretation that following initial slow phosphorylations of the cyclosome by the protein kinase, Suc1 accelerates multiple phosphorylations that culminate in the full activation of the cyclosome. In support of this interpretation, we find that Suc1 stimulates the phosphorylation of several proteins in the preparation of interphase cyclosome and that the effect of Suc1 on phosphorylation was augmented by prior incubation of interphase cyclosome with protein kinase Cdk1/cyclin B.

Stability and folding of the cell cycle regulatory protein, p13(suc1)

p13(suc1) (suc1) is a member of the CDC28 kinase specific family of cell cycle regulatory proteins that bind to the cyclin-dependent kinase CDK2 and regulate its activity. suc1 has two distinct conformational and assembly states, a compact globular monomer and a beta strand-exchanged dimer. The dimerisation is an example of domain-swapping, and is mediated by a molecular hinge mechanism that is conserved across the entire CKS family. It has been proposed that the function of suc1 may be modulated by the dimerisation process with monomer-dimer switching occurring in response to a change in the cell environment. We have investigated the stability and folding of suc1 as a first step in determining the mechanism and functional role of the strand exchange. Suc1 unfolds reversibly at equilibrium in a two-state manner with a free energy of unfolding of 7.2 kcal mol-1. The kinetics of folding and unfolding are complex, and double-jump stopped-flow methods revealed that there are at least three parallel folding pathways arising from distinct unfolded and partly folded, intermediate states. The major population of unfolded species fold rapidly according to a three-state mechanism, D1->I1->N, with a rate constant for the formation of native species, N, from the intermediate, I1, of 65 s-1 in water. Two minor populations of unfolded molecules fold more slowly. Folding of one population is limited by proline isomerisation in a partly folded state, and some expansion of the protein is required for isomerisation to occur. The other population could be assigned to rate-limiting isomerisation of the peptidyl-proline bond of residue 90, which is located in the molecular hinge. A minor, fast phase was detected in the unfolding kinetics that corresponds to unfolding of a small population of a distinct native-like form. Heterogeneity was removed upon mutation of Pro90 to Ala. The unfolding kinetics of the strand-exchanged dimer were also investigated and showed that the dimer unfolds at the same rate as the monomer.

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.

Cyclin-stimulated binding of Cks proteins to cyclin-dependent kinases

Although Cks proteins were the first identified binding partners of cyclin-dependent protein kinases (cdks), their cell cycle functions have remained unclear. To help elucidate the function of Cks proteins, we examined whether their binding to p34(cdc2) (the mitotic cdk) varies during the cell cycle in Xenopus egg extracts. We observed that binding of human CksHs2 to p34(cdc2) was stimulated by cyclin B. This stimulation was dependent on the activating phosphorylation of p34(cdc2) on Thr-161, which follows cyclin binding and is mediated by the cdk-activating kinase. Neither the inhibitory phosphorylations of p34(cdc2) nor the catalytic activity of p34(cdc2) was required for this stimulation. Stimulated binding of CksHs2 to another cdk, p33(cdk2), required both cyclin A and activating phosphorylation. Our findings support recent models that suggest that Cks proteins target active forms of p34(cdc2) to substrates.

Detection of protein three-dimensional side-chain patterns: new examples of convergent evolution

Detection of recurring three-dimensional side-chain patterns is a potential means of inferring protein function. This paper presents a new method for detecting such patterns and discusses various implications. The method allows detection of side-chain patterns without any prior knowledge of function, requiring only protein structure data and associated multiple sequence alignments. A recursive, depth-first search algorithm finds all possible groups of identical amino acids common to two protein structures independent of sequence order. The search is highly constrained by distance constraints, and by ignoring amino acids unlikely to be involved in protein function. A weighted root-mean-square deviation (RMSD) between equivalenced groups of amino acids is used as a measure of similarity. The statistical significance of any RMSD is assigned by reference to a distribution fitted to simulated data. Searches with the Ser/His/Asp catalytic triad, a His/His porphyrin binding pattern, and the zinc-finger Cys/Cys/His/His pattern are performed to test the method on known examples. An all-against-all comparison of representatives from the structural classification of proteins (SCOP) is performed, revealing several new examples of evolutionary convergence to common patterns of side-chains within different tertiary folds and in different orders along the sequence. These include a di-zinc binding Asp/Asp/His/His/Ser pattern common to alkaline phosphatase/bacterial aminopeptidase, and an Asp/Glu/His/His/Asn/Asn pattern common to the active sites of DNase I and endocellulase E1. Implications for protein evolution, function prediction and the rational design of functional regulators are discussed.

Effect of water stress on cell division and cell-division-cycle 2-like cell-cycle kinase activity in wheat leaves

In wheat (Triticum aestivum) seedlings subjected to a mild water stress (water potential of -0.3 MPa), the leaf-elongation rate was reduced by one-half and the mitotic activity of mesophyll cells was reduced to 42% of well-watered controls within 1 d. There was also a reduction in the length of the zone of mesophyll cell division to within 4 mm from the base compared with 8 mm in control leaves. However, the period of division continued longer in the stressed than in the control leaves, and the final cell number in the stressed leaves reached 85% of controls. Cyclin-dependent protein kinase enzymes that are required in vivo for DNA replication and mitosis were recovered from the meristematic zone of leaves by affinity for p13(suc1). Water stress caused a reduction in H1 histone kinase activity to one-half of the control level, although amounts of the enzyme were unaffected. Reduced activity was correlated with an increased proportion of the 34-kD Cdc2-like kinase (an enzyme sharing with the Cdc2 protein of other eukaryotes the same size, antigenic sites, affinity for p13(suc1), and H1 histone kinase catalytic activity) deactivated by tyrosine phosphorylation. Deactivation to 50% occurred within 3 h of stress imposition in cells at the base of the meristematic zone and was therefore too fast to be explained by a reduction in the rate at which cells reached mitosis because of slowing of growth; rather, stress must have acted more immediately on the enzyme. The operation of controls slowing the exit from the G1 and G2 phases is discussed. We suggest that a water-stress signal acts on Cdc2 kinase by increasing phosphorylation of tyrosine, causing a shift to the inhibited form and slowing cell production.

Campylobacter jejuni cytolethal distending toxin causes a G2-phase cell cycle block

Cytolethal distending toxin (CDT) from the diarrheagenic bacterium Campylobacter jejuni was shown to cause a rapid and specific cell cycle arrest in HeLa and Caco-2 cells. Within 24 h of treatment, CDT caused HeLa cells to arrest with a 4N DNA content, indicative of cells in G2 or early M phase. Immunofluorescence studies indicated that the arrested cells had not entered M phase, since no evidence of tubulin reorganization or chromatin condensation was visible. CDT treatment was also shown to cause HeLa cells to accumulate the inactive, tyrosine-phosphorylated form of CDC2. These results indicated that CDT treatment results in a failure to activate CDC2, which leads to cell cycle arrest in G2. This mechanism of action is novel for a bacterial toxin and provides a model for the generation of diarrheal disease by C. jejuni and other diarrheagenic bacteria that produce CDT.

Suc1: cdc2 affinity reagent or essential cdk adaptor protein?

CKS proteins, for which the original member, p13suc1, was identified as a suppressor of cdc2 alleles in S. Pombe, have long served as a reagent for the purification of p34cdc2, whereas their biological function has remained elusive. Apparently conflicting data derived from different model systems may indicate a diversity of function for these proteins. Several new observations in yeast and Xenopus egg extracts together with new structural information tends to enhance the hypothesis that CKS proteins function to alter the activity of cdc2 at several important points in the cell cycle. Here we review previous observations and recent data that suggest CKS proteins serve as adaptor proteins that modify the functions of cdc2 throughout the cell cycle.

Cyclin-dependent kinases: engines, clocks, and microprocessors

Cyclin-dependent kinases (Cdks) play a well-established role in the regulation of the eukaryotic cell division cycle and have also been implicated in the control of gene transcription and other processes. Cdk activity is governed by a complex network of regulatory subunits and phosphorylation events whose precise effects on Cdk conformation have been revealed by recent crystallographic studies. In the cell, these regulatory mechanisms generate an interlinked series of Cdk oscillators that trigger the events of cell division.

Binding of activated cyclosome to p13(suc1). Use for affinity purification

Previous studies have indicated that a approximately 1,500-kDa complex, designated the cyclosome or anaphase-promoting complex, has a regulated cyclin-ubiquitin ligase activity that targets cyclin B for degradation at the end of mitosis. The cyclosome is inactive in the interphase of the embryonic cell cycle and is converted to the active form in late mitosis in a phosphorylation-dependent process initiated by protein kinase Cdc2-cyclin B. We show here that the active, phosphorylated form of the cyclosome from clam oocytes binds to p13(suc1), a protein known to associate with Cdc2. The following evidence indicates that the binding of the cyclosome to p13(suc1) is not mediated via the Cdc2-cyclin B complex: (a) activated cyclosome binds to p13(suc1)-Sepharose following its separation from Cdc2-cyclin B by gel filtration chromatography; (b) cyclosome from interphase extracts, activated by a kinase in which cyclin B has been replaced by an N-terminally truncated derivative fused to glutathione S-transferase, binds well to p13(suc1)-Sepharose but not to glutathione-agarose. An alternative possibility, that the phosphorylated cyclosome binds directly to a phosphate-binding site of p13(suc1), is supported by the observation that the cyclosome is efficiently eluted from p13(suc1)-Sepharose by phosphate-containing compounds. This information was utilized to develop a procedure for the affinity purification of the cyclosome. A factor abundant in the fraction not adsorbed to p13(suc1)-Sepharose stimulates the activity of purified cyclosome. It is suggested that binding of Suc1 may have a role in the regulation of cyclosome activity.

The dynamics of cyclin dependent kinase structure

In the past year, several new crystal structures have provided exciting insights into the conformational changes underlying the regulation of cyclin-dependent kinases. We now understand the structural basis of many of the mechanisms by which cyclin-dependent kinases are regulated, including activation by cyclin binding and phosphorylation, inhibition by the inhibitor p27, and binding by the CKS proteins.

Xe-p9, a Xenopus Suc1/Cks homolog, has multiple essential roles in cell cycle control

The small Suc1/Cks protein is a ubiquitous subunit of Cdk/cyclin complexes, but its precise function has remained unclear. We have isolated a Xenopus homolog, Xe-p9, of the Suc1/Cks protein by virtue of its ability to rescue a fission yeast mutant that enters mitosis prematurely. To assess its functional role in cell cycle control, we have both overexpressed p9 in Xenopus egg extracts and immunodepleted the protein from these extracts. We found that addition of recombinant His6-p9 to egg extracts results in a pronounced delay of mitosis that can be attributed to an inhibition of the tyrosine dephosphorylation of the inactive Cdc2/cyclin B complex. In immunodepletion studies, we observed that the consequences of removing p9 from egg extracts depend on the stage of the cell cycle. Specifically, in the case of interphase extracts, the removal of p9 abolishes the entry into mitosis as a result of a failure in the activation of the Cdc2/cyclin B complex by tyrosine dephosphorylation. Furthermore, mitotic extracts lacking p9 fail to exit mitosis because of a defect in the destruction of cyclin B. Collectively, these results indicate that p9 has multiple essential roles in the cell cycle by governing the interaction of the Cdc2/cyclin B complex with both positive and negative regulators.

Leishmania mexicana p12cks1, a homologue of fission yeast p13suc1, associates with a stage-regulated histone H1 kinase

We have isolated a Leishmania mexicana homologue of the fission yeast suc1 gene using PCR with oligonucleotides designed to conserved regions of cdc2 kinase subunits (cks). The product of cks1 is a 12 kDa polypeptide, which has 70% identity with human p9cks1 and 44% identity with fission yeast p13suc1.p12cks1 was detected in the three life-cycle stages of L. mexicana by immunoblotting. Recombinant p12cks1 (p12cks1his) bound to agarose beads was used as a matrix to affinity-select histone H1 kinase complexes from Leishmania, yeast and bovine extracts. Immunoblotting showed that yeast and bovine cdc2 kinase bound to p12cks1his, thus demonstrating functional homology between L. mexicana p12cks1 and yeast p13suc1. Histone H1 kinase activity was found at a high level in the proliferative promastigote and amastigote forms of L. mexicana, but at a low level in the non-dividing metacyclic form. These activities are likely to be the same as the leishmanial p13suc1 binding kinase (SBCRK) described previously [Mottram, Kinnaird, Shiels, Tait and Barry (1993) J. Biol. Chem. 268, 21044-21051]. A distinct cdc2-related kinase, L. mexicana CRK1, was also found to associate with p12cks1his but affinity-depletion experiments showed that CRK1 was not responsible for the histone H1 kinase activity associating with p12cks1his in promastigote cell extracts. The finding that p12cks1 associates with at least two cdc2-related kinases, SBCRK and CRK1, is consistent with the presence of a large gene family of cdc2-related kinases in trypanosomatids, a situation thought to be more similar to higher eukaryotes than yeast.

Crystal structure and mutational analysis of the human CDK2 kinase complex with cell cycle-regulatory protein CksHs1

The 2.6 Angstrom crystal structure for human cyclin-dependent kinase 2(CDK2) in complex with CksHs1, a human homolog of essential yeast cell cycle-regulatory proteins suc1 and Cks1, reveals that CksHs1 binds via all four beta strands to the kinase C-terminal lobe. This interface is biologically critical, based upon mutational analysis, but far from the CDK2 N-terminal lobe, cyclin, and regulatory phosphorylation sites. CDK2 binds the Cks single domain conformation and interacts with conserved hydrophobic residues plus His-60 and Glu-63 in their closed beta-hinge motif conformation. The beta hinge opening to form the Cks beta-interchanged dimer sterically precludes CDK2 binding, providing a possible mechanism regulating CDK2-Cks interactions. One face of the complex exposes the sequence-conserved phosphate-binding region on Cks and the ATP-binding site on CDK2, suggesting that CKs may target CDK2 to other phosphoproteins during the cell cycle.

Crystal structure of the yeast cell-cycle control protein, p13suc1, in a strand-exchanged dimer

BACKGROUND

p13(suc1) from fission yeast is a member of the CDC28 kinase specific (CKS) class of cell-cycle control proteins, that includes CKS1 from budding yeast and the human homologues CksHs1 and CksHs2. p13(suc1) participates in the regulation of p34(cdc2), a cyclin-dependent kinase controlling the G1-S and the G2-M transitions of the cell cycle. The CKS proteins are believed to exert their regulatory activity by binding to the kinase, in which case their function may be governed by their conformation or oligomerization state. Previously determined X-ray structures of p13(suc1), CksHs1 and CksHs2 show that these proteins share a common fold but adopt different oligomeric states. Monomeric forms of p13(suc1) and CksHs1 have been solved. In addition, CksHs2 and p13(suc1) have been observed by X-ray crystallography in assemblies of strand-exchanged dimers. Analysis of various assemblies of the CKS proteins, as found in different crystal forms, should help to clarify their role in cell-cycle control.

RESULTS

We report the X-ray crystal structure of p13(suc1) to 1.95 A resolution in space group C2221. It is present in the crystals as a strand-exchanged dimer. The overall monomeric fold is preserved in each lobe of the dimer but a single beta-strand (Ile94-Asp102) is exchanged between the central beta-sheets of each molecule.

CONCLUSIONS

Strand exchange, which has been observed for p13(suc1) in two different space groups, and for CksHs2, is now confirmed to be an intrinsic feature of the CKS family. A switch between levels of assembly may serve to coordinate the function of the CKS proteins in cell-cycle control.

Cyclin dependent kinase regulation

Cyclin-dependent kinases (CDKs) are key regulators of the cell cycle and their activities are consequently tightly regulated. Recent developments in the field of CDK regulation have included the discovery and characterization of CDK inhibitors. These developments have had an impact on our understanding of how other signalling pathways may be linked to the cell cycle machinery.