Structural features underlying selective inhibition of protein kinase CK2 by ATP site-directed tetrabromo-2-benzotriazole

Two novel crystal structures of Zea mays protein kinase CK2alpha catalytic subunit, one in complex with the specific inhibitor 4,5,6,7-tetrabromobenzotriazole (TBB) and another in the apo-form, were solved at 2.2 A resolution. These structures were compared with those of the enzyme in presence of ATP and GTP (the natural cosubstrates) and the inhibitor emodin. Interaction of TBB with the active site of CK2alpha is mainly due to van der Waals contacts, with the ligand fitting almost perfectly the cavity. One nitrogen of the five-membered ring interacts with two charged residues, Glu 81 and Lys 68, in the depth of the cavity, through two water molecules. These are buried in the active site and are also generally found in the structures of CK2alpha enzyme analyzed so far, with the exception of the complex with emodin. In the N-terminal lobe, the position of helix alphaC is particularly well preserved in all the structures examined; the Gly-rich loop is displaced from the intermediate position it has in the apo-form and in the presence of the natural cosubstrates (ATP/GTP) to either an upper (with TBB) or a lower position (with emodin). The selectivity of TBB for CK2 appears to be mainly dictated by the reduced size of the active site which in most other protein kinases is too large for making stable interactions with this inhibitor.     

Structural analysis of the lymphocyte-specific kinase Lck in complex with non-selective and Src family selective kinase inhibitors.

BACKGROUND: The lymphocyte-specific kinase Lck is a member of the Src family of non-receptor tyrosine kinases. Lck catalyzes the initial phosphorylation of T-cell receptor components that is necessary for signal transduction and T-cell activation. On the basis of both biochemical and genetic studies, Lck is considered an attractive cell-specific target for the design of novel T-cell immunosuppressants. To date, the lack of detailed structural information on the mode of inhibitor binding to Lck has limited the discovery of novel Lck inhibitors. RESULTS: We report here the high-resolution crystal structures of an activated Lck kinase domain in complex with three structurally distinct ATP-competitive inhibitors: AMP-PNP (a non-selective, non-hydrolyzable ATP analog); staurosporine (a potent but non-selective protein kinase inhibitor); and PP2 (a potent Src family selective protein tyrosine kinase inhibitor). Comparison of these structures reveals subtle but important structural changes at the ATP-binding site. Furthermore, PP2 is found to access a deep, hydrophobic pocket near the ATP-binding cleft of the enzyme; this binding pocket is not occupied by either AMP-PNP or staurosporine. CONCLUSIONS: The potency of staurosporine against Lck derives in part from an induced movement of the glycine-rich loop of the enzyme upon binding of this ligand, which maximizes the van der Waals interactions present in the complex. In contrast, PP2 binds tightly and selectively to Lck and other Src family kinases by making additional contacts in a deep, hydrophobic pocket adjacent to the ATP-binding site; the amino acid composition of this pocket is unique to Src family kinases. The structures of these Lck complexes offer useful structural insights as they demonstrate that kinase selectivity can be achieved with small-molecule inhibitors that exploit subtle topological differences among protein kinases.     

Selectivity of 4,5,6,7-tetrabromobenzotriazole, an ATP site-directed inhibitor of protein kinase CK2 ('casein kinase-2')

The specificity of 4,5,6,7-tetrabromo-2-azabenzimidazole (TBB), an ATP/GTP competitive inhibitor of protein kinase casein kinase-2 (CK2), has been examined against a panel of 33 protein kinases, either Ser/Thr- or Tyr-specific. In the presence of 10 microM TBB (and 100 microM ATP) only CK2 was drastically inhibited (>85%) whereas three kinases (phosphorylase kinase, glycogen synthase kinase 3 beta and cyclin-dependent kinase 2/cyclin A) underwent moderate inhibition, with IC(50) values one--two orders of magnitude higher than CK2 (IC(50)=0.9 microM). TBB also inhibits endogenous CK2 in cultured Jurkat cells. A CK2 mutant in which Val66 has been replaced by alanine is much less susceptible to inhibition by TBB as well as by another ATP competitive inhibitor, emodin. These data show that TBB is a quite selective inhibitor of CK2, that can be used in cell-based assays.     

Pim-1 ligand-bound structures reveal the mechanism of serine/threonine kinase inhibition by LY294002

Pim-1 is an oncogene-encoded serine/threonine kinase primarily expressed in hematopoietic and germ cell lines. Pim-1 kinase was originally identified in Maloney murine leukemia virus-induced T-cell lymphomas and is associated with multiple cellular functions such as proliferation, survival, differentiation, apoptosis, and tumorigenesis (Wang, Z., Bhattacharya, N., Weaver, M., Petersen, K., Meyer, M., Gapter, L., and Magnuson, N. S. (2001) J. Vet. Sci. 2, 167-179). The crystal structures of Pim-1 complexed with staurosporine and adenosine were determined. Although a typical two-domain serine/threonine protein kinase fold is observed, the inter-domain hinge region is unusual in both sequence and conformation; a two-residue insertion causes the hinge to bulge away from the ATP-binding pocket, and a proline residue in the hinge removes a conserved main chain hydrogen bond donor. Without this hydrogen bond, van der Waals interactions with the hinge serve to position the ligand. The hinge region of Pim-1 resembles that of phosphatidylinositol 3-kinase more closely than it does other protein kinases. Although the phosphatidylinositol 3-kinase inhibitor LY294002 also inhibits Pim-1, the structure of the LY294002.Pim-1 complex reveals a new binding mode that may be general for Ser/Thr kinases.     

Features and potentials of ATP-site directed CK2 inhibitors

A panel of quite specific, fairly potent and cell-permeable inhibitors of protein kinase CK2 belonging to the classes of condensed polyphenolic compounds, tetrabromobenzimidazole/triazole derivatives and indoloquinazolines have been developed, with K(i) values in the submicromolar range. Nine structures have been solved to date of complexes between the catalytic alpha subunit of CK2 and a number of these compounds, many of which display a remarkable specificity toward CK2 as compared to a panel of >30 kinases tested. The structural basis for such selectivity appears to reside in the shape and size of a hydrophobic pocket adjacent to the ATP binding site where these ATP competitive ligands are entrapped mainly by van der Waals interactions and by an energy contribution derived from the hydrophobic effect. In CK2, this cavity is smaller than in the majority of other protein kinases due to a number of unique bulky apolar residues. Consequently, the replacement of two of these residues (V66 and I174) in human CK2 alpha with alanines gives rise to mutants, which are markedly less susceptible than wild type to these classes of inhibitors. Cell-permeable CK2 inhibitors have been successfully employed, either alone or in combination with CK2 mutants refractory to inhibition, to dissect signalling pathways affected by CK2 and/or to validate the identification of in vivo targets of this pleiotropic kinase. Moreover, the remarkable pro-apoptotic efficacy of these compounds toward cell lines derived from a wide spectrum of tumors, disclose the possibility that in perspective CK2 inhibitors might become leads for the development of anti-cancer drugs.     

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-(δ2-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. © 1995 Wiley-Liss, Inc.     

Enzymatic activity with an incomplete catalytic spine: insights from a comparative structural analysis of human CK2?? and its paralogous isoform CK2????

Eukaryotic protein kinases are fundamental factors for cellular regulation and therefore subject of strict control mechanisms. For full activity a kinase molecule must be penetrated by two stacks of hydrophobic residues, the regulatory and the catalytic spine that are normally well conserved among active protein kinases. We apply this novel spine concept here on CK2α, the catalytic subunit of protein kinase CK2. Homo sapiens disposes of two paralog isoforms of CK2α (hsCK2α and hsCK2α'). We describe two new structures of hsCK2α constructs one of which in complex with the ATP-analog adenylyl imidodiphosphate and the other with the ATP-competitive inhibitor 3-(4,5,6,7-tetrabromo-1H-benzotriazol-1-yl)propan-1-ol. The former is the first hsCK2α structure with a well defined cosubstrate/magnesium complex and the second with an open β4/β5-loop. Comparisons of these structures with existing CK2α/CK2α' and cAMP-dependent protein kinase (PKA) structures reveal: in hsCK2α' an open conformation of the interdomain hinge/helix αD region that is critical for ATP-binding is found corresponding to an incomplete catalytic spine. In contrast hsCK2α often adopts the canonical, PKA-like version of the catalytic spine which correlates with a closed conformation of the hinge region. HsCK2α can switch to the incomplete, non-canonical, hsCK2α'-like state of the catalytic spine, but this transition apparently depends on binding of either ATP or of the regulatory subunit CK2β. Thus, ATP looks like an activator of hsCK2α rather than a pure cosubstrate.     

Targeting Conformational Activation of CDK2 Kinase

Cyclin‐dependent kinases constitute attractive pharmacological targets for cancer therapeutics, yet inhibitors in clinical trials target the ATP‐binding pocket of the CDK and therefore suffer from limited selectivity and emergence of resistance. The more recent development of allosteric inhibitors targeting conformational plasticity of protein kinases offers promising perspectives for therapeutics. In particular tampering with T‐loop dynamics of CDK2 kinase would provide a selective means of inhibiting this kinase, by preventing its conformational activation. To this aim we engineered a fluorescent biosensor that specifically reports on conformational changes of CDK2 activation loop and is insensitive to ATP or ATP‐competitive inhibitors, which constitutes a highly sensitive probe for identification of selective T‐loop modulators. This biosensor was successfully applied to screen a library of small chemical compounds leading to discovery of a family of quinacridine analogs, which potently inhibit cancer cell proliferation, and promote accumulation of cells in S phase and G2. These compounds bind CDK2/ Cyclin A, inhibit its kinase activity, compete with substrate binding, but not with ATP, and dock onto the T‐loop of CDK2. The best compound also binds CDK4 and CDK4/Cyclin D1, but not CDK1. The strategy we describe opens new doors for the discovery of a new class of allosteric CDK inhibitors for cancer therapeutics.     

Kinase selectivity profiling by inhibitor affinity chromatography

As new drugs rapidly advance into clinical trials, comprehensive identification of their intracellular targets becomes fundamental for the full understanding of the molecular basis of their efficacy and toxicity. This is particularly important when the targets belong to a large family and the inhibitors recognize a conserved site among different members of the class. A typical example is the kinase family, where efforts are aimed at the development of inhibitors of distinct kinases for therapeutic applications in oncology, inflammation and other disease areas. In this case, inhibitors targeting the ATP pocket may cross react with different kinases, as well as with other proteins that bind ATP. This review critically discusses the available approaches for kinase selectivity profiling. It also reviews some examples of inhibitor affinity chromatography applied to inhibitors of kinases and other protein families as a tool to identify and characterize their intracellular targets.     

Kinase selectivity profiling by inhibitor affinity chromatography

As new drugs rapidly advance into clinical trials, comprehensive identification of their intracellular targets becomes fundamental for the full understanding of the molecular basis of their efficacy and toxicity. This is particularly important when the targets belong to a large family and the inhibitors recognize a conserved site among different members of the class. A typical example is the kinase family, where efforts are aimed at the development of inhibitors of distinct kinases for therapeutic applications in oncology, inflammation and other disease areas. In this case, inhibitors targeting the ATP pocket may cross react with different kinases, as well as with other proteins that bind ATP. This review critically discusses the available approaches for kinase selectivity profiling. It also reviews some examples of inhibitor affinity chromatography applied to inhibitors of kinases and other protein families as a tool to identify and characterize their intracellular targets.     

Discovery of cyclin-dependent kinase inhibitor, CR229, using structurebased drug screening

To generate new scaffold candidates as highly selective and potent cyclin-dependent kinase (CDK) inhibitors, structure-based drug screening was performed utilizing 3D pharmacophore conformations of known potent inhibitors. As a result, CR229 (6-bromo-2,3,4,9-tetrahydro-carbolin-1-one) was generated as the hit-compound. A computational docking study using the X-ray crystallographic structure of CDK2 in complex with CR229 was evaluated. This predicted binding mode study of CR229 with CDK2 demonstrated that CR229 interacted effectively with the Leu83 and Glu81 residues in the ATP-binding pocket of CDK2 for the possible hydrogen bond formation. Furthermore, biochemical studies on inhibitory effects of CR229 on various kinases in the human cervical cancer HeLa cells demonstrated that CR229 was a potent inhibitor of CDK2 (IC50: 3 microM), CDK1 (IC50: 4.9 microM), and CDK4 (IC50: 3 microM), yet had much less inhibitory effect (IC50: >20 microM) on other kinases, such as casein kinase 2-1 (CK2- alpha1), protein kinase A (PKA), and protein kinase C (PKC). Accordingly, these data demonstrate that CR229 is a potent CDK inhibitor with anticancer efficacy.     

The crystal structure of choline kinase reveals a eukaryotic protein kinase fold

Choline kinase catalyzes the ATP-dependent phosphorylation of choline, the first committed step in the CDP-choline pathway for the biosynthesis of phosphatidylcholine. The 2.0 A crystal structure of a choline kinase from C. elegans (CKA-2) reveals that the enzyme is a homodimeric protein with each monomer organized into a two-domain fold. The structure is remarkably similar to those of protein kinases and aminoglycoside phosphotransferases, despite no significant similarity in amino acid sequence. Comparisons to the structures of other kinases suggest that ATP binds to CKA-2 in a pocket formed by highly conserved and catalytically important residues. In addition, a choline binding site is proposed to be near the ATP binding pocket and formed by several structurally flexible loops.     

Inhibition of protein kinase CK2 by flavonoids and tyrphostins. A structural insight

Sixteen flavonoids and related compounds have been tested for their ability to inhibit three acidophilic Ser/Thr protein kinases: the Golgi apparatus casein kinase (G-CK) recently identified with protein FAM20C, protein kinase CK1, and protein kinase CK2. While G-CK is entirely insensitive to all compounds up to 40 μM concentration, consistent with the view that it is not a member of the kinome, and CK1 is variably inhibited in an isoform-dependent manner by fisetin and luteolin, and to a lesser extent by myricetin and quercetin, CK2 is susceptible to drastic inhibition by many flavonoids, displaying with six of them IC(50) values < 1 μM. A common denominator of these compounds (myricetin, quercetin, fisetin, kaempferol, luteolin, and apigenin) is a flavone scaffold with at least two hydroxyl groups at positions 7 and 4'. Inhibition is competitive with respect to the phospho-donor substrate ATP. The crystal structure of apigenin and luteolin in complex with the catalytic subunit of Zea mays CK2 has been solved, revealing their ability to interact with both the hinge region (Val116) and the positive area near Lys68 and the conserved water W1, the two main polar ligand anchoring points in the CK2 active site. Modeling experiments account for the observation that luteolin but not apigenin inhibits also CK1. The observation that luteolin shares its pyrocatechol moiety with tyrphostin AG99 prompted us to solve also the structure of this compound in complex with CK2. AG99 was found inside the ATP pocket, consistent with its mode of inhibition competitive with respect to ATP. As in the case of luteolin, the pyrocatechol group of AG99 is critical for binding, interacting with the positive area in the deepest part of the CK2 active site.     

c-Src Binds to the Cancer Drug Ruxolitinib with an Active Conformation

The cancer drug Ruxolitinib is a potent janus kinase inhibitor approved for the treatment of the myeloproliferative neoplasms. In addition, Ruxolitinib has weak inhibitory activity against a panel of other kinases, including Src kinase. There is no structural information of Ruxolitinib binding to any kinase. In this paper, we determined the crystal structure of c-Src kinase domain in complex of Ruxolitinib at a resolution of 2.26 Å. C-Src kinase domain adopts the DFG-in active conformation upon Ruxolitinib binding, indicating Ruxolitinib is a type I inhibitor for c-Src. Ruxolitinib forms two hydrogen bonds with Met341, a water-mediated hydrogen bond with Thr338, and a number of van der Waals contacts with c-Src. Ruxolitinib was then docked into the ligand-binding pocket of a previously solved JAK1 structure. From the docking result, Ruxolitinib also binds JAK1 as a type I inhibitor, with more interactions and a higher shape complementarity with the ligand-binding pocket of JAK1 compared to that of c-Src. Since Ruxolitinib is a relatively small inhibitor and there is sizeable cavity between Ruxolitinib and c-Src ligand-binding pocket, we propose to modify Ruxolitinib to develop more potent inhibitors to c-Src.     

Structure-guided discovery of cyclin-dependent kinase inhibitors

CDK2 inhibitors containing the related bicyclic heterocycles pyrazolopyrimidines and imidazopyrazines were discovered through high-throughput screening. Crystal structures of inhibitors with these bicyclic cores and two more related ones show that all but one have a common binding mode featuring two hydrogen bonds (H-bonds) to the backbone of the kinase hinge region. Even though ab initio computations indicated that the imidazopyrazine core would bind more tightly to the hinge, pyrazolopyrimidines gain an advantage in potency through participation of N4 in an H-bond network involving two catalytic residues and bridging water molecules. Further insight into inhibitor/CDK2 interactions was gained from analysis of additional crystal structures. Significant gains in potency were obtained by optimizing the fit of hydrophobic substituents to the gatekeeper region of the ATP binding site. The most potent inhibitors have good selectivity.     

The structure of CYP101D2 unveils a potential path for substrate entry into the active site

We describe in the present paper mutations of the catalytic subunit α of PKA (protein kinase A) that introduce amino acid side chains into the ATP-binding site and progressively transform the pocket to mimic that of Aurora protein kinases. The resultant PKA variants are enzymatically active and exhibit high affinity for ATP site inhibitors that are specific for Aurora kinases. These features make the Aurora-chimaeric PKA a valuable tool for structure-based drug discovery tasks. Analysis of crystal structures of the chimaera reveal the roles for individual amino acid residues in the binding of a variety of inhibitors, offering key insights into selectivity mechanisms. Furthermore, the high affinity for Aurora kinase-specific inhibitors, combined with the favourable crystallizability properties of PKA, allow rapid determination of inhibitor complex structures at an atomic resolution. We demonstrate the utility of the Aurora-chimaeric PKA by measuring binding kinetics for three Aurora kinase-specific inhibitors, and present the X-ray structures of the chimaeric enzyme in complex with VX-680 (MK-0457) and JNJ-7706621 [Aurora kinase/CDK (cyclin-dependent kinase) inhibitor].     

Crystal structures of active SRC kinase domain complexes

c-Src was the first proto-oncoprotein to be identified, and has become the focus of many drug discovery programs. Src structures of a major inactive form have shown how the protein kinase is rigidified by several interdomain interactions; active configurations of Src are generated by release from this "assembled" or "bundled" form. Despite the importance of Src as a drug target, there is relatively little structural information available regarding the presumably more flexible active forms. Here we report three crystal structures of a dimeric active c-Src kinase domain, in an apo and two ligand complexed forms, with resolutions ranging from 2.9A to 1.95A. The structures show how the kinase domain, in the absence of the rigidifying interdomain interactions of the inactivation state, adopts a more open and flexible conformation. The ATP site inhibitor CGP77675 binds to the protein kinase with canonical hinge hydrogen bonds and also to the c-Src specific threonine 340. In contrast to purvalanol B binding in CDK2, purvalanol A binds in c-Src with a conformational change in a more open ATP pocket.     

Synthesis and evaluation of heteroaryl substituted diazaspirocycles as scaffolds to probe the ATP-binding site of protein kinases

With the success of protein kinase inhibitors as drugs to target cancer, there is a continued need for new kinase inhibitor scaffolds. We have investigated the synthesis and kinase inhibition of new heteroaryl-substituted diazaspirocyclic compounds that mimic ATP. Versatile syntheses of substituted diazaspirocycles through ring-closing metathesis were demonstrated. Diazaspirocycles directly linked to heteroaromatic hinge binder groups provided ligand efficient inhibitors of multiple kinases, suitable as starting points for further optimization. The binding modes of representative diazaspirocyclic motifs were confirmed by protein crystallography. Selectivity profiles were influenced by the hinge binder group and the interactions of basic nitrogen atoms in the scaffold with acidic side-chains of residues in the ATP pocket. The introduction of more complex substitution to the diazaspirocycles increased potency and varied the selectivity profiles of these initial hits through engagement of the P-loop and changes to the spirocycle conformation, demonstrating the potential of these core scaffolds for future application to kinase inhibitor discovery.     

Validation of an Allosteric Binding Site of Src Kinase Identified by Unbiased Ligand Binding Simulations

Allostery plays a primary role in regulating protein activity, making it an important mechanism in human disease and drug discovery. Identifying allosteric regulatory sites to explore their biological significance and therapeutic potential is invaluable to drug discovery; however, identification remains a challenge. Allosteric sites are often “cryptic” without clear geometric or chemical features. Since allosteric regulatory sites are often less conserved in protein kinases than the orthosteric ATP binding site, allosteric ligands are commonly more specific than ATP competitive inhibitors. We present a generalizable computational protocol to predict allosteric ligand binding sites based on unbiased ligand binding simulation trajectories. We demonstrate the feasibility of this protocol by revisiting our previously published ligand binding simulations using the first identified viral proto-oncogene, Src kinase, as a model system. The binding paths for kinase inhibitor PP1 uncovered three metastable intermediate states before binding the high-affinity ATP-binding pocket, revealing two previously known allosteric sites and one novel site. Herein, we validate the novel site using a combination of virtual screening and experimental assays to identify a V-type allosteric small-molecule inhibitor that targets this novel site with specificity for Src over closely related kinases. This study provides a proof-of-concept for employing unbiased ligand binding simulations to identify cryptic allosteric binding sites and is widely applicable to other protein–ligand systems.     

Activation mechanism of CDK2: role of cyclin binding versus phosphorylation

Activation of the cyclin-dependent kinases is a two-step process involving cyclin binding followed by phosphorylation at a conserved threonine residue within the kinase activation loop. In this study, we describe the separate roles of cyclin A binding versus phosphorylation in the overall activation mechanism of CDK2. Interaction of CDK2 with cyclin A results in a partially active complex that is moderately defective in the binding of the protein substrate, but not ATP, and severely defective in both phosphoryl group transfer and turnover. Alternatively, phosphorylation of the CDK2 monomer also results in a partially activated species, but one that is severely (> or = 480-fold) defective in substrate binding exclusively. Catalytic turnover in the phosphorylated CDK2 monomer is largely unimpaired (approximately 8-fold lower). Our data support a model for the activation of CDK2 in vivo, in which interaction of unphosphorylated CDK2 with cyclin A serves to configure the active site for ground-state binding of both ATP and the protein substrate, and further aligns ATP in the transition state for phosphoryl transfer. Optimizing the alignment of protein substrates in the phosphoryl transfer reaction is the principal role of phosphorylation at Thr(160).