Three-dimensional model of the cyclin-dependent kinase 1 (CDK1): Ab initio active site parameters for molecular dynamics studies of CDKS

Cyclin-dependent kinase 1 (CDK1) is an interesting target for potential anticancer drugs, and its three-dimensional (3D) structure is presently unknown. The purpose of this work was to build a 3D model of CDK1, which could be used in drug design studies. The protein 3D structure was homology modeled, based on the known crystal structure of CDK2, and new nonbonded parameters for the Mg(2+) coordination complex were developed by means of ab initio quantum chemical calculations. These parameters were both obtained and validated using the CDK2 structure as reference, and then they were used for the refinement of the CDK1 model. The resulting CDK1 structure was satisfactory and stable at room temperature, as shown by the molecular dynamics simulations carried out over a 1-ns time interval on the entire protein. A number of representative kinases in the active and inactive form, including the inactive CDK1 modeled in this work, were compared. The results illustrate the conformational variability of the activation loop of the inactive form of the kinases and suggest a way for selectively targeting the single CDKs.

Phosphorylation by cdc2-CyclinB1 kinase releases cytoplasmic dynein from membranes

Movement of various cargoes toward microtubule minus ends is driven by the microtubule motor cytoplasmic dynein (CD). Many cargoes are motile only during certain cell cycle phases, suggesting that CD function may be under cell cycle control. Phosphorylation of the CD light intermediate chain (DLIC) has been suggested to play a crucial role in modulating CD function during the Xenopus embryonic cell cycle, where CD-driven organelle movement is active in interphase but greatly reduced in metaphase. This down-regulation correlates with hyperphosphorylation of DLIC and release of CD from the membrane. Here we investigate the role of the key mitotic kinase, cdc2-cyclinB1, in this process. We show that DLIC within the native Xenopus CD complex is an excellent substrate for purified Xenopus cdc2-glutathione S-transferase (GST) cyclinB1 (cdc2-GSTcyclinB1) kinase. Mass spectrometry of native DLIC revealed that a conserved cdc2 site (Ser-197) previously implicated in the metaphase modulation of CD remains phosphorylated in interphase and so is unlikely to be the key regulatory site. We also demonstrate that incubating interphase membranes with cdc2-GSTcyclinB1 kinase results in substantial release of CD from the membrane. These data suggest that phosphorylation of DLIC by cdc2 kinase leads directly to the loss of membrane-associated CD and an inhibition of organelle movement.

Identification of cylin-dependent kinase 1 inhibitors of a new chemical type by structure-based design and database searching

We have selected cyclin-dependent kinase 1 (CDK1), an enzyme participating in the regulation of the cell cycle, as a target in our efforts to discover new antitumor agents. By exploiting available structural information, we designed an ATP-site directed ligand scaffold that allowed us to identify 4-(3-methyl-1,4-dioxo-1,4-dihydro-naphthalen-2-ylamino)-benzenesulfonamide as a new potent inhibitor of CDK1 in a subsequent database search. The synthesis and testing of some analogues confirmed the interest of this class of compounds as novel CDK1 inhibitors.

The Arabidopsis D-type cyclins CycD2 and CycD3 both interact in vivo with the PSTAIRE cyclin-dependent kinase Cdc2a but are differentially controlled

D-type cyclins (CycD) play key roles in linking the Arabidopsis cell cycle to extracellular and developmental signals, but little is known of their regulation at the post-transcriptional level or of their cyclin-dependent kinase (CDK) partners. Using new antisera to CycD2 and CycD3, we demonstrate that the CDK partner of these Arabidopsis cyclins is the PSTAIRE-containing CDK Cdc2a. Previous analysis has shown that transcript levels of CycD2 and CycD3 are regulated in response to sucrose levels and that both their mRNA levels and kinase activity are induced with different kinetics during the G(1) phase of cells reentering the division cycle from quiescence. Here we analyze the protein levels and kinase activity of CycD2 and CycD3. We show that CycD3 protein and kinase activity parallel the abundance of its mRNA and that CycD3 protein is rapidly lost from cells in stationary phase or following sucrose removal. In contrast to both CycD3 and the regulation of its own mRNA levels, CycD2 protein is present at constant levels. CycD2 kinase activity is regulated by sequestration of CycD2 protein in a form inaccessible to immunoprecipitation and probably not complexed to Cdc2a.

Does a cdc2 kinase-like recognition motif on the core protein of hepadnaviruses regulate assembly and disintegration of capsids?

Hepadnaviruses are enveloped viruses, each with a DNA genome packaged in an icosahedral nucleocapsid, which is the site of viral DNA synthesis. In the presence of envelope proteins, DNA-containing nucleocapsids are assembled into virions and secreted, but in the absence of these proteins, nucleocapsids deliver viral DNA into the cell nucleus. Presumably, this step is identical to the delivery of viral DNA during the initiation of an infection. Unfortunately, the mechanisms triggering the disintegration of subviral core particles and delivery of viral DNA into the nucleus are not yet understood. We now report the identification of a sequence motif resembling a serine- or threonine-proline kinase recognition site in the core protein at a location that is required for the assembly of core polypeptides into capsids. Using duck hepatitis B virus, we demonstrated that mutations at this sequence motif can have profound consequences for RNA packaging, DNA replication, and core protein stability. Furthermore, we found a mutant with a conditional phenotype that depended on the cell type used for virus replication. Our results support the hypothesis predicting that this motif plays a role in assembly and disassembly of viral capsids.

Imidazo[2,1 -b]thiazolylmethylene- and indolylmethylene-2-indolinones: a new class of cyclin-dependent kinase inhibitors. Design, synthesis, and CDK1/cyclin B inhibition

Compounds containing a 2-indolinone moiety linked to imidazothiazole and indole fragments were studied as cyclin-dependent kinase inhibitors. The activity of all the new derivatives was tested in vitro against CDK1/cyclinB and the selectivity towards two other kinases was determined for the most promising compounds. The binding mode of one representative compound was investigated by means of a three-dimensional model of the inhibitor-CDK1 complex. The work allowed us to identify (2-chloroindolyl)methylene-2-indolinone as a new lead of a class of CDK1/cyclinB inhibitors, whose potency can be improved by the introduction of suitable variations on the basic molecular skeleton.

Isolation and characterization of recombinant antibody fragments against CDC2a from Arabidopsis thaliana

In order to obtain recombinant antibody fragments that bind the cell-cycle protein CDC2a from Arabidopsis thaliana (CDC2aAt), two phage display libraries of single-chain variable (scFv) fragments were constructed. One library was derived from mice immunized with recombinant CDC2aAt N-terminally fused to a His6-tag (His-CDC2aAt) and the other was made out of an anti-PSTAIRE hybridoma cell line. Six specific His-CDC2aAt-binding phage clones (3D1, 3D2, 3D10, 3D25, 4D21 and 4D47) were isolated by panning. The isolated monoclonal phage clones, as well as the soluble scFv fragments produced in the periplasm of Escherichia coli, bind His-CDC2aAt in ELISA and on Western blots. Moreover, four clones (3D1, 3D2, 3D10 and 4D21) detect specifically CDC2aAt from Arabidopsis cell suspensions on Western blots. Clone 4D21 binds the PSTAIRE epitope, whereas the 3D1, 3D2 and 3D10 clones bind, as yet unidentified, epitopes of CDC2aAt. Furthermore, the accumulation and antigen-binding activity of these scFv fragments in a reducing environment were assessed. No interaction could be shown between the scFv fragments and CDC2aAt in a yeast two-hybrid assay. However, after transient expression of the scFv fragments in the cytosol of tobacco leaves, three of six scFv fragments (3D1, 3D2 and 3D10) accumulated in the plant cytosol and ELISA results indicate that these scFv fragments retained antigen-binding activity.

3D-QSAR CoMFA on cyclin-dependent kinase inhibitors

Several series of cyclin-dependent kinase inhibitors previously prepared in our laboratory were compared using 3D-QSAR (CDK1) and docking (CDK2) techniques. Evaluation of our own library of 93 purine derivatives served to establish the model which was validated by evaluation of an external library of 71 compounds. The best predictions were obtained with the CoMFA standard model (q(2) = 0.68, r(2) = 0.90) and with the CoMSIA combined steric, electrostatic, and lipophilic fields (q(2) = 0.74, r(2) = 0.90). The CDK1 3D-QSAR model was then superimposed to the ATP/CDK2 binding site, giving direct contour maps of the different fields. Although too few compounds were evaluated on CDK5 to derive a 3D-QSAR model, some interesting SARs have been deduced. Comparison of the results obtained from both methods helped with understanding the specific activity of some compounds and designing new specific CDK inhibitors.

Identification of novel purine and pyrimidine cyclin-dependent kinase inhibitors with distinct molecular interactions and tumor cell growth inhibition profiles

Substituted guanines and pyrimidines were tested as inhibitors of cyclin B1/CDK1 and cyclin A3/CDK2 and soaked into crystals of monomeric CDK2. O6-Cyclohexylmethylguanine (NU2058) was a competitive inhibitor of CDK1 and CDK2 with respect to ATP (Ki values: CDK1, 5 +/- 1 microM; CDK2, 12 +/- 3 microM) and formed a triplet of hydrogen bonds (i.e., NH-9 to Glu 81, N-3 to Leu 83, and 2-NH2 to Leu 83). The triplet of hydrogen bonding and CDK inhibition was reproduced by 2,6-diamino-4-cyclohexylmethyloxy-5-nitrosopyrimidine (NU6027, Ki values: CDK1, 2.5 +/- 0.4 microM; CDK2, 1.3 +/- 0.2 microM). Against human tumor cells, NU2058 and NU6027 were growth inhibitory in vitro (mean GI50 values of 13 +/- 7 microM and 10 +/- 6 microM, respectively), with a pattern of sensitivity distinct from flavopiridol and olomoucine. These CDK inhibition and chemosensitivity data indicate that the distinct mode of binding of NU2058 and NU6027 has direct consequences for enzyme and cell growth inhibition.

Structure-based design of potent CDK1 inhibitors derived from olomoucine

Cyclin-dependent kinase 1 (CDK1), an enzyme participating in the regulation of the cell cycle, constitutes a possible target in the search for new antitumor agents. Starting from the purine derivative olomoucine and following a structure-based approach, potent inhibitors of this enzyme were rapidly identified. The molecular modeling aspects of this work are described.

Docking-based development of purine-like inhibitors of cyclin-dependent kinase-2

The cell division cycle is controlled by cyclin-dependent kinases (cdk), which consist of a catalytic subunit (cdk1-cdk8) and a regulatory subunit (cyclin A-H). Purine-like inhibitors of cyclin-dependent kinases have recently been found to be of potential use as anticancer drugs. Rigid and flexible docking techniques were used for analysis of binding mode and design of new inhibitors. X-ray structures of three (ATP, olomoucine, roscovitine) cdk2 complexes were available at the beginning of the study and were used to optimize the docking parameters. The new potential inhibitors were then docked into the cdk2 enzyme, and the enzyme/inhibitor interaction energies were calculated and tested against the assayed activities of cdk1 (37 compounds) and cdk2 (9 compounds). A significant rank correlation between the activity and the rigid docking interaction energy has been found. This implies that (i) the rigid docking can be used as a tool for qualitative prediction of activity and (ii) values obtained by the rigid docking technique into the cdk2 active site can also be used for the prediction of cdk1 activity. While the resulting geometries obtained by the rigid docking are in good agreement with the X-ray data, the flexible docking did not always produce the same inhibitor conformation as that found in the crystal.

Architecture of high mobility group protein I-C.DNA complex and its perturbation upon phosphorylation by Cdc2 kinase

The high mobility group I-C (HMGI-C) protein is an abundant component of rapidly proliferating undifferentiated cells. High level expression of this protein is characteristic for early embryonic tissue and diverse tumors. HMGI-C can function as an architectural factor enhancing the activity of transcription factor NF-kappaB on the beta-interferon promoter. The protein has three minor groove DNA-binding domains (AT-hooks). Here, we describe the complex of HMGI-C with a fragment of the beta-interferon promoter. We show that the protein binds to NRDI and PRDII elements of the promoter with its first and second AT-hook, respectively. Phosphorylation by Cdc2 kinase leads to a partial derailing of the AT-hooks from the minor groove, affecting mainly the second binding domain. In contrast, binding to long AT stretches of DNA involves contacts with all three AT-hooks and is marginally sensitive to phosphorylation. Our data stress the importance of conformation of the DNA binding site and protein phosphorylation for its function.

An overview of Plasmodium protein kinases

Protein kinases are key regulators of many biochemical processes in eukaryotic cells. Malaria parasites, in spite of all their peculiarities, are not likely to represent an exception in this respect. Over the past few years, several genes encoding Plasmodium protein kinases have been cloned and characterized; these molecular studies extend previous data on kinase activities in parasite extracts. Here, Barbara Kappes, Christian Doerig and Ralph Graeser present available data on this topic, with an emphasis on cloned protein kinase genes, and discuss the potential outcome of such research in the context of drug development.

The C-terminal domain of the Cdc2 inhibitory kinase Myt1 interacts with Cdc2 complexes and is required for inhibition of G(2)/M progression

Activation of Cdc2, is the universal event controlling the onset of mitosis. In higher eukaryotes, Cdc2 activity is in part regulated by inhibitory phosphorylation of Thr14 and Tyr15, catalyzed by Wee1 and Myt1, which prevents catastrophic premature entry into mitosis. In this study we defined the function of Myt1 by overexpression studies in both S. pombe and a human osteosarcoma cell line. Similar to Wee1, overexpression of human Myt1 prevented entry into mitosis in both cell types; however, Myt1 catalytic activity was not essential for the cell cycle delay observed with human cells. Myt1 expression was restricted to proliferating cells. Furthermore, we detected no major decline in Myt1 protein abundance prior to the entry into mitosis, which coincides with the loss of Myt1 activity. We localized mitotic phosphoepitopes, recognized by the monoclonal antibody MPM-2, to the C-terminal domain of Myt1. The mitotic peptidyl-prolyl isomerase, Pin1, was able to associate with this domain in a phosphorylation-dependent manner. Truncation of the C-terminal domain of Myt1 prevented its ability to induce G(2)/M phase arrest in overexpression studies in human cells and dramatically reduced its ability to phosphorylate Cdc2 in vitro. We demonstrate that the C-terminal domain of Myt1 was required for recruitment of Cdc2, and we infer that this domain lies in the cytoplasm because it can interact with and is phosphorylated by Cdc2. In conclusion, we propose that Myt1 can negatively regulate Cdc2/cyclin B1 and inhibit G(2)/M progression by two means, both of which require the C-terminal domain; first, Myt1 can bind and sequester Cdc2/cyclin B1 in the cytoplasm preventing entry into the nucleus, and, second, it can phosphorylate associated Cdc2/cyclin B1 at Thr14 and Tyr15 thus inhibiting its catalytic activity.

Synthesis and application of functionally diverse 2,6,9-trisubstituted purine libraries as CDK inhibitors

BACKGROUND

Purines constitute a structural class of protein ligands involved in mediating an astonishing array of metabolic processes and signal pathways in all living organisms. Synthesis of purine derivatives targeting specific purine-binding proteins in vivo could lead to versatile lead compounds for use as biological probes or drug candidates.

RESULTS

We synthesized several libraries of 2,6, 9-trisubstituted purines using both solution- and solid-phase chemistry, and screened the compounds for inhibition of cyclin-dependent kinase (CDK) activity and human leukemic cell growth. Lead compounds were optimized by iterative synthesis based on structure-activity relationships (SARs), as well as analysis of several CDK-inhibitor cocrystal structures, to afford several interesting compounds including one of the most potent CDK inhibitors known to date. Unexpectedly, some compounds with similar CDK inhibitory activity arrested cellular proliferation at distinctly different phases of the cell cycle and another inhibitor directly induced apoptosis, bypassing cell-cycle arrest. Some of these compounds selectively inhibited growth of cells derived from specific tumors.

CONCLUSIONS

2,6,9-Trisubstituted purines have various and potent biological activities, despite high concentrations of competing endogenous purine ligands in living cells. Purine libraries constitute a versatile source of small molecules that affect distinct biochemical pathways mediating different cellular functions.

Intra-M phase-promoting factor phosphorylation of cyclin B at the prophase/metaphase transition

Activation of Cdc2-cyclin B (or M phase-promoting factor (MPF)) at the prophase/metaphase transition proceeds in two steps: dephosphorylation of Cdc2 and phosphorylation of cyclin B. We here investigated the regulation of cyclin B phosphorylation using the starfish oocyte model. Cyclin B phosphorylation is not required for Cdc2 kinase activity; both the prophase complex dephosphorylated on Cdc2 with Cdc25 and the metaphase complex dephosphorylated on cyclin B with protein phosphatase 2A display high kinase activities. An in vitro assay of cyclin B kinase activity closely mimics in vivo phosphorylation as shown by phosphopeptide maps of in vivo and in vitro phosphorylated cyclin B. We demonstrate that Cdc2 itself is the cyclin B kinase; cyclin B phosphorylation requires Cdc2 activity both in vivo (sensitivity to vitamin K3, a Cdc25 inhibitor) and in vitro (copurification with Cdc2-cyclin B, requirement of Cdc2 dephosphorylation, and sensitivity to chemical inhibitors of cyclin-dependent kinases). Furthermore, cyclin B phosphorylation occurs as an intra-M phase-promoting factor reaction as shown by the following: 1) active Cdc2 is unable to phosphorylate cyclin B associated to phosphorylated Cdc2, and 2) cyclin B phosphorylation is insensitive to enzyme/substrate dilution. We conclude that, at the prophase/metaphase transition, cyclin B is mostly phosphorylated by its own associated Cdc2 subunit.

cdc2 kinase-mediated phosphorylation of splicing factor SF2/ASF

SR proteins are a family of splicing factors which are important components of spliceosomes. Recent studies suggested that phosphorylation of SR protein might be a key event for the regulation of pre-mRNA splicing and is prevalent in metaphase cells. To investigate the role of cdc2 kinase in cell cycle-dependent phosphorylation of SR protein, we examined its phosphorylation of SF2/ASF, a representative SR protein. SF2/ASF was phosphorylated both by recombinant cdc2 kinase, a cdc2-cyclin B complex, and by cdc2 kinase immunoprecipitated from G2/M phase HeLa cells. In vitro phosphorylation and phosphopeptide mapping of several mutant proteins revealed that cdc2 kinase specifically phosphorylates the RS domain of SF2/ASF with serines 227, 238 and presumably 199 as major phosphorylation sites. These findings suggest the possibility that cdc2 kinase takes part in the cell cycle-dependent phosphorylation of SR protein which regulates the function of spliceosomes.

Cdk2 activity is dispensable for the onset of DNA replication during the first mitotic cycles of the sea urchin early embryo

Earlier work reported the important role of Cdk2 as a regulator of DNA replication in somatic cells and in Xenopus extracts. In the present report we analyze in vivo the involvement of Cdk2 in DNA replication during early embryogenesis using the first mitotic cycles of sea urchin embryos. Unfertilized Sphaerechinus granularis eggs are arrested after the second meiotic cytokinesis. Fertilization resumes the block and induces DNA replication after a short lag period, making sea urchin early embryo a good model for studying in vivo the onset of DNA replication. We show that Cdk2 as well as its potential partner cyclin A are present in the nucleus in G1 and S phase and therefore available for DNA replication. In accordance with data obtained in Xenopus egg extracts we observed that Cdk2 kinase activity is low and stable during the entire cycle. However, in contrast with this in vitro system in which Cdk2 activity is required for the onset of DNA replication, the specific inhibition of Cdk2 kinase by microinjection of the catalytically inactive Cdk2-K33R or the inhibitor p21(Cip1) does not prevent DNA replication. Because olomoucine, DMAP, and emetine treatments did not preclude DNA synthesis, neither cyclin A/Cdk1 nor cyclin B/Cdk1 kinase activities are necessary to replace the absence of Cdk2 kinase in promoting DNA replication. These data suggest that during early embryogenesis Cdks activities, in particular Cdk2, are dispensable in vivo for the initiation step of DNA replication. However, the specific localization of Cdk2 in the nucleus from the beginning of M phase to the end of S phase suggests its involvement in other mechanisms regulating DNA replication such as inhibition of DNA re-replication and/or that its regulating role is achieved through a pathway independent of the kinase activity. We further demonstrate that even after inhibition of Cdk activities, the permeabilization of the nuclear membrane is required to allow a second round of DNA replication. However, in contrast to Xenopus egg extracts, re-replication can take place in the absence of DMAP-sensitive kinase.

Identification and characterization of a novel serine-threonine kinase gene from the Xp22 region

Eukaryotic protein kinases are part of a large and expanding family of proteins. Through our transcriptional mapping effort in the Xp22 region, we have isolated and sequenced the full-length transcript of STK9, a novel cDNA highly homologous to serine-threonine kinases. A number of human genetic disorders have been mapped to the region where STK9 has been localized including Nance-Horan (NH) syndrome, oral-facial-digital syndrome type 1 (OFD1), and a novel locus for nonsyndromic sensorineural deafness (DFN6). To evaluate the possible involvement of STK9 in any of the above-mentioned disorders, a 2416-bp full-length cDNA was assembled. The entire genomic structure of the gene, which is composed of 20 coding exons, was determined. Northern analysis revealed a transcript larger than 9.5 kb in several tissues including brain, lung, and kidney. The mouse homologue (Stk9) was identified and mapped in the mouse in the region syntenic to human Xp. This location is compatible with the location of the Xcat mutant, which shows congenital cataracts very similar to those observed in NH patients. Sequence homologies, expression pattern, and mapping information in both human and mouse make STK9 a candidate gene for the above-mentioned disorders.

Design of new inhibitors for cdc2 kinase based on a multiple pseudosubstrate structure

New inhibitors have been designed for cdc2 kinase based on a multiple pseudosubstrate structure. The new inhibitors have three different structural components: 3,4-bis(indol-3-yl)maleimide, Ac-Cys-(Ser)-Pro-Lys-Lys-NHMe, and ethyloxy group between the two components. Inhibitory activities toward cdc2 and other protein kinases were investigated, and the compound (21) with Ac-Cys-Pro-Lys-Lys-NHMe connected with the triethylene glycol spacer exhibited the most potent inhibition with relatively high selectivity.

Regulation of Cdc2 activity by phosphorylation at T14/Y15

The highly conserved Cdc2 serine/threonine kinase plays a central role in cell cycle progression. Although Cdc2 levels remain constant throughout the cell cycle, Cdc2 kinase activity peaks at the G2/M boundary, in order to drive entry into mitosis. In the model organism Schizosaccharomysces pombe, potentially active Cdc2/Cdc13 kinase complex accumulates throughout the S and G2 phases of the cell cycle. This complex, however, is maintained in an active state by Wee1/Mik1-mediated phosphorylation at Y15 (and, possibly, T14). At the G2/M boundary, the Cdc25 protein phosphatase is activated to dephosphorylate the Cdc2/Cdc13 complex, resulting in abrupt activation of Cdc2 kinase activity and entry into mitosis.

T-loop deletion of CDC2 from breast cancer tissues eliminates binding to cyclin B1 and cyclin-dependent kinase inhibitor p21

The eukaryotic cell cycle is regulated by a highly conserved family of protein kinases, the cyclin-dependent kinases (CDKs). Monomeric free CDKs do not possess enzymatic activity, largely due to the steric hindrance caused by the T-loop at the entrance of the catalytic cleft, making ATP inaccessible to the substrate. Binding of a cyclin, primarily to the NH2-terminal lobe of the CDK that surrounds the PSTAIRE helix, induces a large conformational change in the PSTAIRE helix of the CDK and also causes the T-loop to move out of the way of the catalytic cleft. We identified from breast cancer tissues a novel variant of human CDC2, termed CDC2deltaT, that lacks 171 nucleotides corresponding to 57 amino acids, which compose most of the T-loop. CDC2deltaT was detected in 10 of 14 breast cancer tissues analyzed, whereas it was not detectable in diploid human fibroblast cell lines or in interleukin 2-stimulated normal human lymphocytes. CDC2deltaT protein is unable to complex with cyclin B1 and lacks histone H1 kinase activity. CDC2deltaT also fails to bind to the CDK inhibitor p21. These results indicate that the T-loop not only plays a key role in keeping a free CDK in its inactive state but also in facilitating CDK activation by promoting cyclin binding.

Cloning of a cdc2-related protein kinase from Trypanosoma cruzi that interacts with mammalian cyclins

Two cdc2-related protein kinases (crk), tzcrk3 and tzcrk1, from the protozoan parasite Trypanosoma cruzi were cloned. tzcrk3 encodes a 35 kDa protein sharing 51.5% amino acid identity with human cdc2 and 82% identity with Trypanosoma brucei CRK3. tzcrk1 encodes a 33 kDa protein sharing 52.7% identity with human cdc2 and a high degree of identity (> 78%) with T. brucei CRK1, Leishmania mexicana CRK1 and Trypanosoma congolense CRK1. A recombinant TzCRK1 protein was able to phosphorylate histone HI and retinoblastoma protein. Western blotting using a polyclonal antibody raised against the recombinant TzCRK1 protein showed that the kinase is present in all life cycle stages of the parasite. A PSTAIRE antiserum detected proteins of 32, 33 and 35 kDa, with differential expression in the life cycle of the parasite. Transfection of COS-7 cells with tzcrk1 demonstrated for the first time that a CRK protein can bind mammalian cyclins; TzCRK1 co-immunoprecipitated with cyclins E, D3 and A suggesting a role for this kinase in cell cycle control. These results indicate that T. cruzi might have cyclin homologues that control the activity of the CRK proteins and that a complex mechanism would exist in order to regulate the kinases involved in the cell cycle and the differentiation processes of the parasite.