Inhibition of cyclin-dependent kinases by purine analogues

Levels of endogenous cytokinins, indole-3-acetic acid and abscisic acid during the cell cycle of synchronized tobacco BY-2 cells

Correlation between cell cycle progression and endogenous levels of plant hormones was studied in synchronized tobacco BY-2 cell suspension cultures. Sixteen different cytokinins, indole-3-acetic acid (IAA) and abscisic acid (ABA) were extracted using solid-phase anion exchange chromatography in combination with immunoaffinity purification, and quantified by mass spectrometry. No significant correlation could be identified for IAA and ABA. In contrast, there were sharp peaks in the levels of specific cytokinins (zeatin- and dihydrozeatin-type) at the end of the S phase and during mitosis. The levels of other cytokinins analyzed, including zeatins N- and O-glucosides, remained low, suggesting that the increased amounts of their corresponding non-glucosylated form resulted from de novo synthesis. These findings suggest that zeatin- and dihydrozeatin-type cytokinins might play a specific regulatory role in the progression of the plant cell cycle. One hypothesis to explain cytokinin action is based on a specific interaction with kinases that regulate cell cycle progression, as has been recently shown for the cytokinin analogue olomoucine.

Pho85p, a cyclin-dependent protein kinase, and the Snf1p protein kinase act antagonistically to control glycogen accumulation in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, nutrient levels control multiple cellular processes. Cells lacking the SNF1 gene cannot express glucose-repressible genes and do not accumulate the storage polysaccharide glycogen. The impaired glycogen synthesis is due to maintenance of glycogen synthase in a hyperphosphorylated, inactive state. In a screen for second site suppressors of the glycogen storage defect of snf1 cells, we identified a mutant gene that restored glycogen accumulation and which was allelic with PHO85, which encodes a member of the cyclin-dependent kinase family. In cells with disrupted PHO85 genes, we observed hyperaccumulation of glycogen, activation of glycogen synthase, and impaired glycogen synthase kinase activity. In snf1 cells, glycogen synthase kinase activity was elevated. Partial purification of glycogen synthase kinase activity from yeast extracts resulted in the separation of two fractions by phenyl-Sepharose chromatography, both of which phosphorylated and inactivated glycogen synthase. The activity of one of these, GPK2, was inhibited by olomoucine, which potently inhibits cyclin-dependent protein kinases, and contained an approximately 36-kDa species that reacted with antibodies to Pho85p. Analysis of Ser-to-Ala mutations at the three potential Gsy2p phosphorylation sites in pho85 cells implicated Ser-654 and/or Thr-667 in PHO85 control of glycogen synthase. We propose that Pho85p is a physiological glycogen synthase kinase, possibly acting downstream of Snf1p.

Understanding and controlling the cell cycle with natural products

Small molecule natural products have aided in the discovery and characterization of many proteins critical to the progression and maintenance of the cell cycle. Identification of the direct target of a natural product gives scientists a tool to control a specific aspect of the cell cycle, thus facilitating the study of the cell-cycle machinery.

Binding of the vesicle docking protein p115 to Golgi membranes is inhibited under mitotic conditions

The vesicle docking protein p115 showed saturable, high affinity binding to interphase Golgi membranes. The affinity of binding was up to 20-fold lower using membranes preincubated with mitotic cytosol. In contrast, binding was not affected by mitotic pretreatment of p115. The reduction in p115 binding was mediated by phosphorylation, could be induced by a cyclin-dependent kinase, and was fully reversible. A shift of p115 from membranes to cytosol was also found after fractionating mitotic cells. The functional significance of the decreased binding was addressed by in vitro mitotic incubations which disassemble Golgi cisternae, predominantly producing transport vesicles. The addition of excess p115 decreased loss of membrane from cisternae, indicating that p115's action is limiting while transport vesicles accumulate. The cessation of intra-Golgi traffic in mitosis has been hypothesized to result from an inhibition of membrane fusion while budding of transport vesicles continues. This process also contributes to mitotic Golgi disassembly. Our results imply that there is a mitotic modification to Golgi membranes leading to a reduction in the affinity of the p115 receptor. Reduced p115 binding may play a part in the inhibition of membrane fusion by preventing prior vesicle docking.

betaII protein kinase C is required for the G2/M phase transition of cell cycle

Entry into mitosis requires the coordinated action of multiple mitotic protein kinases. In this report, we investigate the involvement of protein kinase C in the control of mitosis in human cells. Treatment of synchronized HL60 cells with the highly selective protein kinase C (PKC) inhibitor chelerythrine chloride leads to profound cell cycle arrest in G2 phase. The cellular effects of chelerythrine are not due to either direct or indirect inhibition of the known mitotic regulator p34(cdc2)/cyclin B kinase. Rather, several lines of evidence demonstrate that chelerythrine-mediated G2 phase arrest results from selective inhibition and degradation of betaII protein kinase C. First, chelerythrine causes dose-dependent inhibition of betaII PKC in vitro with an IC50 identical to that for G2 phase blockade in whole cells. Second, chelerythrine specifically inhibits betaII PKC-mediated lamin B phosphorylation and mitotic nuclear lamina disassembly. Third, chelerythrine leads to selective loss of betaII PKC during G2 phase in synchronized cells. Fourth, chelerythrine mediates activation-dependent degradation of PKC, indicating that betaII PKC is selectively activated during G2 phase of cell cycle. Taken together, these data demonstrate that betaII PKC activation at G2 phase is required for mitotic nuclear lamina disassembly and entry into mitosis and that betaII PKC-mediated phosphorylation of nuclear lamin B is important in these events.

Evidence that the endogenous histone H1 phosphatase in HeLa mitotic chromosomes is protein phosphatase 1, not protein phosphatase 2A

Histone H1 is highly phosphorylated in mitotic HeLa cells, but is quickly dephosphorylated in vivo at the end of mitosis and in vitro following cell lysis. We show here that okadaic acid and microcystin-LR block the in vitro dephosphorylation of H1 and that they do so directly by inhibiting the histone H1 phosphatase rather than by some indirect mechanism. The concentrations of microcystin and okadaic acid required for inhibition strongly suggest that the histone H1 phosphatase is either PP1 or an unknown protein phosphatase with okadaic acid-sensitivity similar to PP1. The histone H1 phosphatase is predominantly located in chromosomes with at most one copy for every 86 nucleosomes. This tends to support its identification as PP1, since localization in mitotic chromosomes is a characteristic of PP1 but not of the other known okadaic acid-sensitive protein phosphatases. We also show that treatment of metaphase-arrested HeLa cells with staurosporine and olomoucine, inhibitors of p34cdc2 and other protein kinases, rapidly induces reassembly of interphase nuclei and dephosphorylation of histone H1 without chromosome segregation. This result indicates that protein kinase activity must remain elevated to maintain a mitotic block. Using this as a model system for the M- to G1-phase transition, we present evidence from inhibitor studies suggesting that the in vivo histone H1 phosphatase may be either PP1 or another phosphatase with similar okadaic acid-sensitivity, but not PP2A.

Both cyclin A and cyclin E have S-phase promoting (SPF) activity in Xenopus egg extracts

Extracts of activated Xenopus eggs in which protein synthesis has been inhibited support a single round of chromosomal DNA replication. Affinity-depletion of cyclin dependent kinases (Cdks) from these extracts blocks the initiation of DNA replication. We define ‘S-phase promoting factor’ (SPF) as the Cdk activity required for DNA replication in these Cdk-depleted extracts. Recombinant cyclins A and E, but not cyclin B, showed significant SPF activity. High concentrations of cyclin A promoted entry into mitosis, which inhibited DNA replication. In contrast, high concentrations of cyclin E1 promoted neither nuclear envelope disassembly nor full chromosome condensation. In the early embryo cyclin E1 complexes exclusively with Cdk2 and cyclin A is complexed predominantly with Cdc2; only later in development does cyclin A associate with Cdk2. We show that baculovirus-produced complexes of cyclin A-Cd2, cyclin A-Cdk2 and cyclin E-Cdk2 could each provide SPF activity. These results suggest that although in the early Xenopus embryo cyclin E1-Cdk2 is sufficient to support entry into S-phase, cyclin A-Cdc2 provides a significant additional quantity of SPF as its levels rise during S phase.

A cdc2-like kinase distinct from cdk5 is associated with neurofilaments

An immunoprecipitation assay was used to identify protein kinases which are physically associated with neurofilaments (NF) in mouse brain extracts. Using this approach, we show that a cdc2-related kinase is associated with NF. The cdc2-related kinase was found to be distinct from cdk5 and the authentic cdc2 by a number of criteria. Firstly, it has a molecular mass on SDS-PAGE gels of 34 kDa, similar to that of cdc2, but differing from cdk5 (31 kDa). Secondly, it is not recognized by an antibody specific for cdk5. Thirdly, it is recognized by an antibody raised against the C-terminal region of authentic cdc2, but not by an antibody specific for the PSTAIRE motif. Using immunoblotting, we further show that the cdc2-related kinase copurifies with NF isolated from rat tissues. In vitro kinase assays further demonstrated that immunoprecipitated cdc2-related kinase phosphorylates recombinant NF-H protein. Phosphorylation of NF-H by the cdc2-like activity was not affected by 3 microM olomoucine but was inhibited by 10 microM of this kinase inhibitor. Phosphoamino acid analysis of in vitro phosphorylated NF-H indicates that the immunoprecipitated cdc2-related kinase phosphorylates serine residues.

Inhibition of neuronal cyclin-dependent kinase-5 by staurosporine and purine analogs is independent of activation by Munc-18

Neuronal cdk5 can phosphorylate certain lys-ser-pro (KSP) motifs of neurofilaments and tau protein in the nervous system. We have immunoprecipitated the cdk5 from rat brain using a polyclonal antibody raised against the C-terminus of cdk5. The immunoprecipitate has phosphorylated a KSPXK peptide analog of NF-H, as well as histone H1 and a bacterially expressed rat NF-H protein. The kinase activity was inhibited by staurosporine, isopentanyladenine and olomoucine in a dose dependent manner. Kinetic studies indicated Ki values of 39 nM, 38 microM and 8 microM, respectively for staurosporine, isopentanyladenine and olomoucine. The inhibition by staurosporine was non-competitive with respect to phosphoryl acceptor acceptor substrates. Western blot analysis of the immunoprecipitate showed both cdk5 and p67 (Munc-18), a putative regulator molecule of the kinase. Addition of p67 fusion protein enhanced the kinase activity of the immunoprecipitate by 60% above the basal activity. P67 elevated Ki values for both staurosporine and olomoucine. The degree of inhibition at high concentrations of these inhibitors was unaltered by exogenous p67 indicating a lack of competitive interactions with p67. The high affinity of staurosporine for cdk5 suggests that cdk5 may be one of the targets for the neurotropic effect of staurosporine.

Evidence for phosphorylation of CTP:phosphocholine cytidylyltransferase by multiple proline-directed protein kinases

Reversible phosphorylation of CTP:phosphocholine cytidylyltransferase, the rate-limiting enzyme of phosphatidylcholine biosynthesis, is thought to play a role in regulating its activity. In the present study, the hypothesis that proline-directed kinases play a major role in phosphorylating cytidylyltransferase is substantiated using a c-Ha-ras-transfected clone of the human keratinocyte cell line HaCaT. Cellular extracts from epidermal growth factor-stimulated HaCaT cells and from ras-transfected HaCaT cells phosphorylated cytidylyltransferase much stronger as compared with extracts from quiescent HaCaT cells. The tryptic phosphopeptide pattern of cytidylyltransferase phosphorylated by cell-free extracts from ras-transfected HaCaT cells was similar compared with the patterns of cytidylyltransferase phosphorylated by p44mpkmitogen-activated protein kinase and p34cdc2 kinase in vitro, whereas in the case of casein kinase II the pattern was different. Furthermore, in c-Ha-ras-transfected HaCaT cells the in vivo phosphorylation state of cytidylyltransferase was 2-fold higher as compared with untransfected HaCaT cells. This higher phosphorylation of cytidylyltransferase in the ras-transfected clone was reduced to a level below the phosphorylation of cytidylyltransferase in untransfected cells, using olomoucine, a specific inhibitor of proline-directed kinases. The reduced phosphorylation of cytidylyltransferase in olomoucine-treated cells correlated with an enhanced stimulation of enzyme activity by oleic acid.

Identification of plectin as a substrate of p34cdc2 kinase and mapping of a single phosphorylation site

Plectin is an in vitro substrate for various kinases present in cell lysates from mitotic and interphase Chinese hamster ovary cells. Sensitivity of plectin kinase activity to the inhibitor olomoucine, and two-dimensional tryptic peptide mapping of plectin phosphorylated by various kinase preparations suggested that the major plectin kinase activity in mitotic extracts is related to the cell cycle regulator kinase p34cdc2. Bacterial expression of various truncated plectin mutant proteins comprising different domains of the molecule and their phosphorylation by purified p34cdc2kinase revealed that the target site of this kinase resided within plectin's C-terminal globular domain. Among the subdomains of the C-terminal region (six repeats and a short tail sequence), only repeat 6 and the tail were phosphorylated by p34cdc2 kinase. As shown by two-dimensional phosphopeptide mapping, repeat 6, but not the tail, contained a mitosis-specific phosphorylation site targeted by p34cdc2 kinase in intact plectin molecules. By performing site-directed mutagenesis of a potential p34cdc2 recognition sequence motif within the repeat 6 domain, threonine 4542 was identified as the major target for the kinase. Protein kinase A, phosphorylating plectin also within repeat 6, targeted sites that were clearly different from those of p34cdc2 kinase.

Inhibitors of cyclin-dependent kinases promote survival of post-mitotic neuronally differentiated PC12 cells and sympathetic neurons

Previous studies have demonstrated that multiple agents that promote survival of PC12 cells and sympathetic neurons deprived of trophic support also block cell cycle progression. Presently, we address whether inhibition of cell cycle-related cyclin-dependent kinases (CDKs) prevents neuronal cell death. We show that two distinct CDK inhibitors, flavopiridol and olomoucine, suppress the death of neuronal PC12 cells and sympathetic neurons. In addition, we demonstrate that inhibitor concentrations required to promote survival correlate with their ability to inhibit proliferation. Promotion of survival, however, does not correlate with inhibition of extracellular signal-regulated kinase or c-Jun kinase activities or with interference with the activation of c-Jun kinase that accompanies serum/nerve growth factor deprivation. In contrast to their actions on nerve growth factor-differentiated PC12 cells, the CDK inhibitors do not prevent the death of proliferation-competent PC12 cells and, in fact, promote their cell death. These findings support the hypothesis that post-mitotic neuronal cells die after removal of trophic support due to an attempt to re-enter the cell cycle in an uncoordinated and inappropriate manner. We speculate that cycling PC12 cells are not saved by these agents due to a signaling conflict between an inherent oncogenic signal and the inhibition of CDK activity.

Targeting signal transduction for disease therapy

With the advance in the molecular understanding of disease processes, it has been appreciated that many diseases result from the malfunctions of signaling pathways. This recognition has led to intensive research and the development of therapies based on the interception of cellular signaling in diseased cells. In the past two years, success has been achieved using a blocker of the farnesylation of Ras as a tumor inhibitor, a JAK-2 blocker as an efficient inhibitor of recurrent pre-B cell acute lymphoblastic leukemia, and a platelet-derived growth factor receptor kinase as a blocker of restenosis.

M-phase-specific phosphorylation and structural rearrangement of the cytoplasmic cross-linking protein plectin involve p34cdc2 kinase

Plectin, a widespread and abundant cytoskeletal cross-linking protein, serves as a target for protein kinases throughout the cell cycle, without any significant variation in overall phosphorylation level. One of the various phosphorylation sites of the molecule was found to be phosphorylated preferentially during mitosis. By in vivo phosphorylation of ectopically expressed plectin domains in stably transfected Chinese hamster ovary cells, this site was mapped to the C-terminal repeat 6 domain of the polypeptide. The same site has been identified as an in vitro target for p34cdc2 kinase. Mitosis-specific phosphorylation of plectin was accompanied by a rearrangement of plectin structures, changing from a filamentous, largely vimentin-associated state in interphase to a diffuse vimentin-independent distribution in mitosis as visualized by immunofluorescence microscopy. Subcellular fractionation studies showed that in interphase cells up to 80% of cellular plectin was found associated with an insoluble cell fraction mostly consisting of intermediate filaments, while during mitosis the majority of plectin (> 75%) became soluble. Furthermore, phosphorylation of purified plectin by p34cdc2 kinase decreased plectin's ability to interact with preassembled vimentin filaments in vitro. Together, our data suggest that a mitosis-specific phosphorylation involving p34cdc2 kinase regulates plectin's cross-linking activities and association with intermediate filaments during the cell cycle.

Cyclin-dependent kinase 5 (Cdk5) and neuron-specific Cdk5 activators

While cyclin-dependent kinase 5 (Cdk5) is widely distributed in mammalian tissues and in cultured cell lines, Cdk5-associated kinase activity has been demonstrated only in mammalian brains. An active form of Cdk5, called neuronal cdc2-like kinase (Nclk) has been purified from mammalian brain and shown to be a heterodimer of Cdk5 and a 25 kDa protein, which is derived proteolytically from a 35 kDa brain and neuron-specific protein. The protein is essential for the kinase activity of Cdk5 and is therefore designated neuronal Cdk5 activator, p25/35Nck5a. Nclk appears to have important neuronal functions. The changes in Cdk5 and Nck5a expression appear to correlate with the terminal differentiation of neurons of the mouse embryonic brain. Transfection of cultured cortical neurons with dominant negative cdk5 mutants or Nck5a antisense DNA may reduce neurite growth, suggesting that Nclk plays an active role in neuron differentiation. A number of cytoskeletal proteins including neurofilament proteins, the neuron-specific microtubule associated protein tau, and the actin binding protein caldesmon are in vitro substrates of Nclk. Although Nck5a has cyclin-like activity, it shows minimal amino acid sequence identity to members of cyclin family proteins. The mechanism of activation of Cdk5 by Nck5a differs from that of cyclin activation of Cdks in that full Cdk5 kinase activity can be achieved in the absence of phosphorylation of Cdk5. An isoform of Nck5a, a 39 kDa protein has been cloned and shown to share extensive amino acid identity and the mechanism of Cdk5 activation with Nck5a. These proteins may represent a subfamily of Cdk activators distinct from cyclins.

Inhibitors of serine/threonine kinases

Several serine/threonine kinase inhibitors have been described recently that are sufficiently selective, and therefore useful as biochemical probes, for studying the role of kinases in signaling pathways. In addition, these newer classes of kinase inhibitor may well provide an impetus for the development of drugs to attenuate certain cellular responses in the treatment of diseases. Importantly, within the past year, specific and potent inhibitiors have been reported for both the new mitogen-activated protein (MAP) kinase homolog CSBP and MAP kinase kinase-1.

The phosphodiesterase inhibitor SQ 20006 selectively blocks mitogen activation of p70S6k and transition to S phase of the cell division cycle without affecting the steady state phosphorylation of eIF-4E

In quiescent cells high levels of protein synthesis are required in order to re-enter the cell cycle upon stimulation. Initiation of polypeptide synthesis is the step most often subject to regulation, controlled in part by phosphorylation of 40 S ribosomal protein S6 and a number of initiation factors. The kinase responsible for S6 phosphorylation is p70S6k. We now show that the p70S6k pathway can be selectively blocked by the aminopurine analogue, SQ 20006. This agent is known to raise cAMP levels, resulting in activation of protein kinase A. We present evidence that the increase in cAMP is not responsible for the inhibitory effect observed. We also show that SQ 20006 can prevent the activation of p70S6k in a rapid and reversible manner. The compound does not exert its inhibitory activity on p70S6k but can inhibit in vitro two protein kinase C isozymes (alpha and gamma). In a B lymphoblastoid cell line, treatment with SQ 20006 results in inhibition of protein synthesis at the initiation stage. In contrast, when tested directly upon the translational machinery in the reticulocyte lysate, inhibition is manifest at both the level of initiation and elongation. The role of protein kinase A in the modulation of p70S6k and the rate of translation is discussed.

Activation of p70 S6 kinase and erk-encoded mitogen-activated protein kinases is resistant to high cyclic nucleotide levels in Swiss 3T3 fibroblasts

Treatment of Swiss mouse 3T3 fibroblasts with certain cyclic nucleotide phosphodiesterase inhibitors (theophylline, SQ 20,006, and MY-5445) prevents the activation of the M(r) 70,000 S6 kinase (p70S6k) induced by a variety of external stimuli. Concentrations giving half-maximal inhibition were 800, 50, and 25 microM, respectively. Western blot analysis and immunocomplex kinase assays showed that these compounds inhibit the phosphorylation and activation of p70S6k without affecting the erk-encoded mitogen-activated protein (MAP) kinases or the rsk-encoded S6 kinase (p90rsk). A distinct collection of cAMP and cGMP agonists and analogues did not suppress p70S6k activation, indicating that 1) high intracellular cyclic nucleotide concentrations do not antagonize the p70S6k pathway and 2) phosphodiesterase inhibitors block p70S6k activation by a mechanism that is independent of cAMP or cGMP production. The effect of theophylline and SQ 20,006, but not MY-5445, on p70S6k signaling may be due in part to the inhibition of a phosphatidylinositol 3-kinase that acts upstream of p70S6k. Finally, in contrast to many other cell types, cAMP and cGMP were also found to have no inhibitory effect on the MAP kinase/p90rsk signaling pathway in Swiss 3T3 fibroblasts.

Rapamycin, wortmannin, and the methylxanthine SQ20006 inactivate p70s6k by inducing dephosphorylation of the same subset of sites

Activation of p70s6k in cells stimulated with serum correlates with the phosphorylation of seven sites. Pretreatment of Swiss 3T3 cells with the immunosuppressant rapamycin blocks phosphorylation of four of these sites (Thr229, Thr389, Ser404, and Ser411), whereas phosphorylation proceeds in the remaining three sites (Ser418, Thr421, and Ser424). If rapamycin is added postserum stimulation, the pattern of phosphorylation is qualitatively similar except that Ser411 is still highly phosphorylated. The inhibitory effect of rapamycin on serum-induced p70s6k activation and the phosphorylation of Thr229, Thr389, Ser404, and Ser411 is rescued by FK506, providing further evidence that the inhibitory effect is exerted through a complex of rapamycin-FKBP12. Wortmannin treatment pre- or post-serum stimulation inhibits phosphorylation of the same set of sites as rapamycin, supporting the argument that both agents act on the same pathway. Likewise, methylxanthine phosphodiesterase inhibitors block p70s6k activation and phosphorylation of the same set of sites as wortmannin and rapamycin. However, other agents that raise intracellular cAMP levels have no inhibitory effect, leading to the hypothesis that the inhibitory actions of methylxanthines on p70s6k activity are not through activating protein kinase A but through inhibition of an upstream kinase. Together the results indicate that there are two kinase signaling pathways that must converge to activate p70s6k and that only one of these pathways is sensitive to rapamycin, wortmannin, and methylxanthine inhibition.

A role for tubular networks and a COP I-independent pathway in the mitotic fragmentation of Golgi stacks in a cell-free system

Golgi stacks were previously shown to be converted into tubular networks when incubated in mitotic cytosol depleted of the coatomer subunit of COP I coats (Misteli and Warren, 1994). Similar, though smaller, networks are now shown to be an early intermediate on the Golgi fragmentation pathway both in vitro and in vivo. Their appearance mirrors the disappearance of Golgi cisternae and at their peak they constitute 35% of total Golgi membrane. They are consumed by two pathways, the first involving the budding of COP I-coated vesicles described previously (Misteli and Warren, 1994). The second involves a COP I-independent mechanism that leads eventually to a vesicle fraction that is larger in size and more heterogeneous than that produced by the COP I-mechanism. We suggest that both pathways operate concurrently at the onset of mitotic fragmentation. The COP I-independent pathway converts cisternae into tubular networks that then fragment. The COP I-dependent pathway partially consumes first the cisternae at the beginning of the incubation and then the tubular networks that form from them.

Multiple modes of ligand recognition: crystal structures of cyclin-dependent protein kinase 2 in complex with ATP and two inhibitors, olomoucine and isopentenyladenine

Cyclin-dependent kinases (CDKs) are conserved regulators of the eukaryotic cell cycle with different isoforms controlling specific phases of the cell cycle. Mitogenic or growth inhibitory signals are mediated, respectively, by activation or inhibition of CDKs which phosphorylate proteins associated with the cell cycle. The central role of CDKs in cell cycle regulation makes them a potential new target for inhibitory molecules with anti-proliferative and/or anti-neoplastic effects. We describe the crystal structures of the complexes of CDK2 with a weakly specific CDK inhibitor, N6-(delta 2-isopentenyl)adenine, and a strongly specific inhibitor, olomoucine. Both inhibitors are adenine derivatives and bind in the adenine binding pocket of CDK2, but in an unexpected and different orientation from the adenine of the authentic ligand ATP. The N6-benzyl substituent in olomoucine binds outside the conserved binding pocket and is most likely responsible for its specificity. The structural information from the CDK2-olomoucine complex will be useful in directing the search for the next generation inhibitors with improved properties.

Mitotic disassembly of the Golgi apparatus in vivo

Populations enriched in prophase cells were obtained either by using a cell line with a temperature-sensitive mutation in the mitotic kinase, p34cdc2, or by treating cells with olomoucine, an inhibitor of this kinase. Both methods resulted in efficient and reversible block of the cells at the G2/M boundary. After cells were released from the cell cycle block, the morphological changes to the Golgi apparatus were characterised using both quantitative conventional electron microscopy and immuno-gold microscopy. The early mitotic phases were divided into six stages (G2 to pro-metaphase) based on the morphology of the nucleus. During prophase the cross-sectional length of Golgi stacks decreased prior to unstacking. At the same time, small vesicular profiles, typically 50–70 nm in diameter, accumulated in the vicinity of the stacks. The disappearance of Golgi stacks was accompanied by the transient appearance of tubular networks. By the time cells entered prometaphase, the stacks had completely disassembled and only clusters consisting of Golgi vesicles and short tubular elements were left. When cells were released from the G2/M boundary and pulsed briefly with [AlF4]- to prevent uncoating of transport vesicles, vesicular profiles with a morphology reminiscent of COP-coated vesicles appeared. These vesicular profiles were either associated with Golgi stacks or, at later stages, with clusters, but were formed at all stages of disassembly. Together these results provide further support for our model that continued budding of vesicles from the rims of Golgi cisternae is at least partly responsible for the disassembly of the Golgi apparatus.

Purification of an MCM-containing complex as a component of the DNA replication licensing system

Replication licensing factor (RLF) ensures that eukaryotic chromosomal DNA is replicated exactly once in each cell cycle. On exit from metaphase, RLF is activated and binds to or modifies chromatin. This modification (the 'licence') is required for subsequent DNA replication; the licence is also inactivated in the process of replication. Active RLF is not imported into the nucleus, so further DNA replication cannot occur until the DNA is relicensed by passage throught mitosis. We have developed an assay to purify RLF from Xenopus eggs. Activity resolves into two components, RLF-M and RLF-B, both of which are required for licensing. RLF-M has been purified to apparent homogeneity: it consists of three polypeptides, one of which is a Xenopus homologue of the yeast MCM3 protein. Xenopus Mcm3 associates with chomatin in G1 and is removed during replication, consistent with its being a component of the RLF system.

Inhibitors of protein kinases and phosphatases

Inhibitors of eukaryotic protein kinases and phosphatases are a chemically diverse array of natural and synthetic compounds, including medicines, potions and poisons. These substances are valuable pharmacological probes and affinity ligands for the kinases and phosphatases of signalling pathways, enhancing our knowledge of the cellular effects of the pathway in question. More broadly, this basic research is also leading to the development of drugs to control specific cellular responses, and enzyme-based assays to detect toxins in food and water.