Exploring kinase cosubstrate promiscuity: monitoring kinase activity through dansylation

Kinase-catalyzed crosslinking: A comparison of ATP-crosslinker analogs

Protein phosphorylation is catalyzed by kinases to regulate cellular events and disease states. Identifying kinase-substrate relationships represents a powerful strategy to understand cell biology and disease yet remains challenging due to the rapid dynamics of phosphorylation. Over the last decade, several γ-phosphoryl modified ATP analogs containing crosslinkers were developed to covalently conjugate kinases, their substrates, and their associated proteins for subsequent characterization. Here, kinetics and crosslinking experiments demonstrated that the UV-activated analogs, ATP-aryl azide and ATP-benzophenone, offered the most robust crosslinking, whereas electrophilic ATP-aryl fluorosulfate promoted the most effective proximity-enabled crosslinking. The data will guide future applications of kinase-catalyzed crosslinking to study normal and disease biology.

Kinase-catalyzed biotinylation for discovery and validation of substrates to multispecificity kinases NME1 and NME2

Protein phosphorylation by kinases regulates mammalian cell functions, such as growth, division, and signal transduction. Among human kinases, NME1 and NME2 are associated with metastatic tumor suppression but remain understudied due to the lack of tools to monitor their cellular substrates. In particular, NME1 and NME2 are multispecificity kinases phosphorylating serine, threonine, histidine, and aspartic acid residues of substrate proteins, and the heat and acid sensitivity of phosphohistidine and phosphoaspartate complicates substrate discovery and validation. To provide new substrate monitoring tools, we established the γ-phosphate-modified ATP analog, ATP-biotin, as a cosubstrate for phosphorylbiotinylation of NME1 and NME2 cellular substrates. Building upon this ATP-biotin compatibility, the Kinase-catalyzed Biotinylation with Inactivated Lysates for Discovery of Substrates method enabled validation of a known substrate and the discovery of seven NME1 and three NME2 substrates. Given the paucity of methods to study kinase substrates, ATP-biotin and the Kinase-catalyzed Biotinylation with Inactivated Lysates for Discovery of Substrates method are valuable tools to characterize the roles of NME1 and NME2 in human cell biology.

Kinase-catalyzed Biotinylation with Inactivated Lysates for Discovery of Substrates (K-BILDS)

Protein phosphorylation is catalyzed by kinases to regulate a large variety of cellular activities, including growth and signal transduction. Methods to identify kinase substrates are crucial to fully understand phosphorylation-mediated cellular events and disease states. Here, we report a set of protocols to identify substrates of a target kinase using Kinase-catalyzed Biotinylation with Inactivated Lysates for Discovery of Substrates (K-BILDS). As described in these protocols, K-BILDS involves inactivation of endogenous kinases in lysates, followed by addition of an active exogenous kinase and the γ-phosphate-modified ATP analog ATP-biotin for kinase-catalyzed biotinylation of cellular substrates. Avidin enrichment isolates biotinylated substrates of the active kinase, which can be monitored by western blot. Substrates of the target kinase can also be discovered using mass spectrometry analysis. Key advantages of K-BILDS include compatibility with any lysate, tissue homogenate, or complex mixture of biological relevance and any active kinase of interest. K-BILDS is a versatile method for studying or discovering substrates of a kinase of interest to characterize biological pathways thoroughly. © 2023 Wiley Periodicals LLC. Basic Protocol 1: FSBA treatment of lysates to inactivate kinases Basic Protocol 2: Kinase-catalyzed Biotinylation with Inactivated Lysates for Discovery of Substrates (K-BILDS).

Affinity-Based Kinase-Catalyzed Crosslinking to Study Kinase-Substrate Pairs

Phosphorylation of proteins by kinase enzymes is a post-translational modification involved in a myriad of biological events, including cell signaling and disease development. Identifying the interactions between a kinase and its phosphorylated substrate(s) is necessary to characterize phosphorylation-mediated cellular events and encourage development of kinase-targeting drugs. One method for substrate-kinase identification utilizes photocrosslinking γ-phosphate-modified ATP analogues to covalently link kinases to their substrates for subsequent monitoring. Because photocrosslinking ATP analogues require UV light, which could influence cell biology, we report here two ATP analogues, ATP-aryl fluorosulfate (ATP-AFS) and ATP-hexanoyl bromide (ATP-HexBr), that crosslink kinase-substrate pairs via proximity-mediated reactions without the need for UV irradiation. Both ATP-AFS and ATP-HexBr acted as cosubstrates with a variety of kinases for affinity-based crosslinking, with ATP-AFS showing more robust complexes. Importantly, ATP-AFS promoted crosslinking in lysates, which demonstrates compatibility with complex cellular mixtures for future application to kinase-substrate identification.

Analysis of protein phosphorylation combining capillary electrophoresis with ATP analog labeling technique

Protein phosphorylation is one of the most basic mechanisms for regulating and controlling protein biological activity and function, and it is also a very important posttranslational modification process. Protein phosphorylation participates in and regulates many life activities such as signal transduction, gene expression, cell cycle, and so on. In this paper, we propose a method for the determination of the protein phosphorylation combining capillary electrophoresis (CE) with ATP analog labeling technique. We synthesized two new ATP analogs (ATP-NB and ATP-TATD-NB) functionalized by norbornene. Using Abl kinase as a model, we established a method for the determination of the kinase activity in solution and lysate by CE with laser-induced fluorescence detection (CE-LIF). This method was used to evaluate the efficiencies of kinase inhibitors. The IC₅₀ values obtained are basically consistent with the reports. By D-A reaction (inverse electron demand Diels-Alder reaction) to label TZ-BODIPY fluorescence, we also realized the phosphorylation fluorescence detection of substrate peptide. Then, we used fluorescence confocal microscopy imaging technology to study the phosphorylation of proteins in vivo by the D-A reaction of ATP-NB and TZ-BODIPY. Our preliminary results documented that the combination of CE-LIF with analog ATP-NB labeling technique is an effective strategy for the determination of the protein phosphorylation and the kinase activity and for screening of kinase inhibitors. The D-A reaction of ATP-NB and TZ-BODIPY also laid the foundation for the subsequent in situ study of protein phosphorylation.

Analysis of protein phosphorylation in solution and in cells by using an ATP analogue in combination with fluorescence techniques

Protein phosphorylation is a very important mechanism for regulating and controlling the activity and function of proteins, and is closely associated with signal transduction, gene expression, cell cycle and other life activities in organisms. In this paper, we proposed a new strategy for studying protein phosphorylation in living cells by combining fluorescence resonance energy transfer (FRET) with a small molecule adenosine 5'-triphosphate (ATP) analogue. We synthesized a new ATP analogue functionalized by norbornene (ATP-NB), and a tetrazine modified fluorescent probe Cyanine3 (TZ-Cy3). Based on the inverse electron demand Diels-Alder (D-A) reaction, ATP-NB phosphorylated proteins in solution and in living cells were in situ labelled with TZ-Cy3. By combining FRET with fluorescence correlation spectroscopy (FRET-FCS) and imaging technology, we established an efficient method for studying the phosphorylation of proteins in solution and in living cells using an ATP analogue instead of natural ATP. We studied the effects of phosphatase inhibitors on the phosphorylation of proteins in living cells. Our results documented that ATP-NB is a small molecule ATP analogue with hydrophobicity, which can penetrate cells and efficiently phosphorylate proteins in living cells. This strategy is well suitable for in situ study of protein phosphorylation in living cells.

A Panel of Protein Kinase Chemosensors Distinguishes Different Types of Fatty Liver Disease

The worldwide incidence of fatty liver disease continues to rise, which may account for concurrent increases in the frequencies of more aggressive liver ailments. Given the existence of histologically identical fatty liver disease subtypes, there is a critical need for the identification of methods that can classify disease and potentially predict progression. Herein, we show that a panel of protein kinase chemosensors can distinguish fatty liver disease subtypes. These direct activity measurements highlight distinct differences between histologically identical fatty liver diseases arising from diets rich in fat versus alcohol and identify a previously unreported decrease in p38α activity associated with a high-fat diet. In addition, we have profiled kinase activities in both benign (diet-induced) and progressive (STAM) disease models. These experiments provide temporal insights into kinase activity during disease development and progression. Altogether, this work provides the basis for the future development of clinical diagnostics and potential treatment strategies.

Chitosan-assisted permeabilization of ATP-biotin for live cell kinase-catalyzed biotinylation

Kinases are essential cell signaling enzymes that phosphorylate protein substrates using ATP as the universal cosubstrate. A wide variety of ATP analogs have been used in kinase research, although the studies are limited by the cell impermeability of ATP. Here we describe the use of the cationic polymer deacetylated chitosan to permeabilize ATP analogs for live cell applications, including kinase-catalyzed biotinylation.

Phosphate-Modified Nucleotides for Monitoring Enzyme Activity

Nucleotides modified at the terminal phosphate position have been proven to be interesting entities to study the activity of a variety of different protein classes. In this chapter, we present various types of modifications that were attached as reporter molecules to the phosphate chain of nucleotides and briefly describe the chemical reactions that are frequently used to synthesize them. Furthermore, we discuss a variety of applications of these molecules. Kinase activity, for instance, was studied by transfer of a phosphate modified with a reporter group to the target proteins. This allows not only studying the activity of kinases, but also identifying their target proteins. Moreover, kinases can also be directly labeled with a reporter at a conserved lysine using acyl-phosphate probes. Another important application for phosphate-modified nucleotides is the study of RNA and DNA polymerases. In this context, single-molecule sequencing is made possible using detection in zero-mode waveguides, nanopores or by a Förster resonance energy transfer (FRET)-based mechanism between the polymerase and a fluorophore-labeled nucleotide. Additionally, fluorogenic nucleotides that utilize an intramolecular interaction between a fluorophore and the nucleobase or an intramolecular FRET effect have been successfully developed to study a variety of different enzymes. Finally, also some novel techniques applying electron paramagnetic resonance (EPR)-based detection of nucleotide cleavage or the detection of the cleavage of fluorophosphates are discussed. Taken together, nucleotides modified at the terminal phosphate position have been applied to study the activity of a large diversity of proteins and are valuable tools to enhance the knowledge of biological systems.

Different Enzymatic Processing of γ-Phosphoramidate and γ-Phosphoester-Modified ATP Analogues

Monitoring the activity of ATP-consuming enzymes provides the basis for elucidating their modes of action and regulation. Although a number of ATP analogues have been developed for this, their scope is restricted because of the limited acceptance by respective enzymes. In order to clarify which kind of phosphate-modified ATP analogues are accepted by the α-β-phosphoanhydride-cleaving ubiquitin-activating enzyme 1 (UBA1) and the β-γ-phosphoanhydride-cleaving focal adhesion kinase (FAK), we tested phosphoramidate- and phosphoester-modified ATP analogues. UBA1 and FAK were able to convert phosphoramidate-modified ATP analogues, even with a bulky modification like biotin. In contrast, a phosphoester-modified analogue was poorly accepted. These results demonstrate that minor variations in the design of ATP analogues for monitoring ATP utilization have a significant impact on enzymatic acceptance.

Directed Chemical Evolution with an Outsized Genetic Code

The first demonstration that macromolecules could be evolved in a test tube was reported twenty-five years ago. That breakthrough meant that billions of years of chance discovery and refinement could be compressed into a few weeks, and provided a powerful tool that now dominates all aspects of protein engineering. A challenge has been to extend this scientific advance into synthetic chemical space: to enable the directed evolution of abiotic molecules. The problem has been tackled in many ways. These include expanding the natural genetic code to include unnatural amino acids, engineering polyketide and polypeptide synthases to produce novel products, and tagging combinatorial chemistry libraries with DNA. Importantly, there is still no small-molecule analog of directed protein evolution, i.e. a substantiated approach for optimizing complex (≥ 10^9 diversity) populations of synthetic small molecules over successive generations. We present a key advance towards this goal: a tool for genetically-programmed synthesis of small-molecule libraries from large chemical alphabets. The approach accommodates alphabets that are one to two orders of magnitude larger than any in Nature, and facilitates evolution within the chemical spaces they create. This is critical for small molecules, which are built up from numerous and highly varied chemical fragments. We report a proof-of-concept chemical evolution experiment utilizing an outsized genetic code, and demonstrate that fitness traits can be passed from an initial small-molecule population through to the great-grandchildren of that population. The results establish the practical feasibility of engineering synthetic small molecules through accelerated evolution.

The generality of kinase-catalyzed biotinylation

Kinase-catalyzed protein phosphorylation is involved in a wide variety of cellular events. Development of methods to monitor phosphoproteins in normal and diseased states is critical to fully characterize cell signaling. Towards phosphoprotein analysis tools, our lab reported kinase-catalyzed labeling where γ-phosphate modified ATP analogs are utilized by kinases to label peptides or protein substrates with a functional tag. In particular, the ATP-biotin analog was developed for kinase-catalyzed biotinylation. However, kinase-catalyzed labeling has been tested rigorously with only a few kinases, preventing use of ATP-biotin as a general tool. Here, biotinylation experiments, gel or HPLC-based quantification, and kinetic measurements indicated that twenty-five kinases throughout the kinome tree accepted ATP-biotin as a cosubstrate. With this rigorous characterization of ATP-biotin compatibility, kinase-catalyzed labeling is now immediately useful for studying phosphoproteins and characterizing the role of phosphorylation in various biological events.

Design and evaluation of a real-time activity probe for focal adhesion kinase

Focal adhesion kinase (FAK) has been identified as a potential therapeutic target for the treatment of metastatic cancers. Herein we describe the design, synthesis and optimization of a direct activity sensor for FAK and its application to screening FAK inhibitors. We find that the position of the sensing moiety, a phosphorylation-sensitive sulfonamido-oxine fluorophore, can dramatically influence the performance of peptide sensors for FAK. Real-time fluorescence activity assays using an optimized sensor construct, termed FAKtide-S2, are highly reproducible (Z' = 0.91) and are capable of detecting as little as 1 nM recombinant FAK. Utilizing this robust assay format, we define conditions for the screening of FAK inhibitors and demonstrate the utility of this platform using a set of well-characterized small molecule kinase inhibitors. Additionally, we provide the selectivity profile of FAKtide-S2 among a panel of closely related enzymes, identifying conditions for selectively monitoring FAK activity in the presence of off-target enzymes. In the long term, the chemosensor platform described in this work can be used to identify novel FAK inhibitor scaffolds and potentially assess the efficacy of FAK inhibitors in disease models.

A real-time, fluorescence-based assay for Rho-associated protein kinase activity

Inhibitors of Rho-associated protein kinase (ROCK) enzymatic activity have been shown to reduce the invasive phenotype observed in metastatic hepatocellular carcinoma (HCC). We describe the design, synthesis, and evaluation of a direct probe for ROCK activity utilizing a phosphorylation-sensitive sulfonamido-oxine fluorophore, termed Sox. The Sox fluorophore undergoes an increase in fluorescence upon phosphorylation of a proximal amino acid via chelation-enhanced fluorescence (CHEF, ex. = 360 nm and em. = 485 nm), allowing for the direct visualization of the rate of phosphate addition to a peptide substrate over time. Our optimal probe design, ROCK-S1, is capable of sensitively reporting ROCK activity with a limit of detection of 10 pM and a high degree of reproducibility (Z'-factor = 0.6 at 100 pM ROCK2). As a proof-of-principle for high-throughput screening (HTS) we demonstrate the ability to rapidly assess the efficacy of a 78 member, small molecule library against ROCK2 using a robotics platform. We identify two previously unreported ROCK2 inhibitor scaffolds, PHA665752 and IKK16, with IC50 values of 3.6 μM and 247 nM respectively. Lastly, we define conditions for selectively monitoring ROCK activity in the presence of potential off-target enzymes (PKCα, PKA, and PAK) with similar substrate specificities.

A Cell-Permeable ATP Analogue for Kinase-Catalyzed Biotinylation

ATP analogues have been powerful compounds for the study of kinase-catalyzed phosphorylation. However, the cell impermeability of ATP analogues has largely limited their use to in vitro lysate-based experiments. Herein, we report the first cell-permeable ATP analogue, ATP-polyamine-biotin (APB). APB is shown to promote biotin labeling of kinase substrates in live cells and has future applications in phosphoprotein purification and analysis. More generally, these studies provide a foundation for the development of additional cell-permeable ATP analogues for cell-signaling research.

Synthesis of γ-Phosphate-Labeled and Doubly Labeled Adenosine Triphosphate Analogs

This unit describes the synthesis of γ-phosphate-labeled and doubly labeled adenosine triphosphate (ATP) analogs and their characterization using the phosphodiesterase I from Crotalus adamanteus (snake venom phosphodiesterase; SVPD). In the key step of the synthesis, ATP or an ATP analog, bearing a linker containing a trifluoroacetamide group attached to the nucleoside, are modified with an azide-containing linker at the terminal phosphate using an alkylation reaction. Subsequently, different labels are introduced to the linkers by transformation of one functional group to an amine and coupling to an N-hydroxysuccinimide ester. Specifically, the Staudinger reaction of the azide is employed as a straightforward means to obtain an amine in the presence of various labels. Furthermore, the fluorescence characteristics of a fluorogenic, doubly labeled ATP analog are investigated following enzymatic cleavage by SVPD.

Biotinylated phosphoproteins from kinase-catalyzed biotinylation are stable to phosphatases: implications for phosphoproteomics

Kinase-catalyzed protein phosphorylation is involved in a wide variety of cellular events. Development of methods to monitor phosphorylation is critical to understand cell biology. Our lab recently discovered kinase-catalyzed biotinylation, where ATP-biotin is utilized by kinases to label phosphopeptides or phosphoproteins with a biotin tag. To exploit kinase-catalyzed biotinylation for phosphoprotein purification and identification in a cellular context, the susceptibility of the biotin tag to phosphatases was characterized. We found that the phosphorylbiotin group on peptide and protein substrates was relatively insensitive to protein phosphatases. To understand how phosphatase stability would impact phosphoproteomics research applications, kinase-catalyzed biotinylation of cell lysates was performed in the presence of kinase or phosphatase inhibitors. We found that biotinylation with ATP-biotin was sensitive to inhibitors, although with variable effects compared to ATP phosphorylation. The results suggest that kinase-catalyzed biotinylation is well suited for phosphoproteomics studies, with particular utility towards monitoring low-abundance phosphoproteins or characterizing the influence of inhibitor drugs on protein phosphorylation.

Unexpected biotinylation using ATP-γ-Biotin-LC-PEO-amine as a kinase substrate

Protein phosphorylation is the most widely studied post-translational modification. Reversible protein phosphorylation is implicated in the regulation of a broad range of cellular processes. As such, there is extensive interest in simple and sensitive procedures for the isolation and detection of phosphorylated proteins. Synthetic analogues of ATP, with a biotin linked to the gamma-phosphate of ATP, have been reported to biotinylate kinase substrates in a kinase-catalyzed reaction. This could be an extremely attractive and versatile method for affinity enrichment of phosphorylated proteins. However, as we report here, the commercially available biotin-ATP analogue, ATP-γ-Biotin-LC-PEO-amine, is capable of biotinylating proteins independent of kinase activity. In fact, we demonstrate that this reagent is capable of non-specifically biotinylating any protein. Although the mechanism of biotinylation is not known, this report uncovers a flaw in a commercially available reagent and also highlights the importance of control experiments when developing new biochemical tools to study enzyme activity.

Structural analysis of ATP analogues compatible with kinase-catalyzed labeling

Kinase-catalyzed protein phosphorylation is an important biochemical process involved in cellular functions. We recently discovered that kinases promiscuously accept γ-modified ATP analogues as cosubstrates and used several ATP analogues as tools for studying protein phosphorylation. Herein, we explore the structural requirements of γ-modified ATP analogues for kinase compatibility. To understand the influence of linker length and composition, a series of ATP analogues was synthesized, and the efficiency of kinase-catalyzed labeling was determined by quantitative mass spectrometry. This study on factors influencing kinase cosubstrate promiscuity will enable design of ATP analogues for a variety of kinase-catalyzed labeling reactions.

Synthesis and stability of phosphate modified ATP analogues

Nucleotides modified at the phosphate have numerous applications. Nevertheless, the number of attachment modes is limited and little is known about their stability. Here, we present results on the elaboration of the synthesis of five classes of ATP analogues and studies concerning their stability. We show that the nitrogen-linked ATP analogue is less stable, whereas the oxygen- and novel carbon-linked adenosine tri- and tetraphosphate analogues are stable from pH 3 to 12 rendering them interesting for further applications and designs.

Kinase-Catalyzed Biotinylation

Kinase-catalyzed protein phosphorylation plays an essential role in a variety of biological processes. Methods to detect phosphoproteins and phosphopeptides in cellular mixtures will aid in cell biological and signaling research. Our laboratory recently discovered the utility of γ-modified ATP analogues as tools for studying phosphorylation. Specifically, ATP-biotin can be used for labeling and visualizing phosphoproteins from cell lysates. Because the biotin tag is suitable for protein detection, the biotinylation reaction can be applied to multiple phosphoproteomics applications. Herein we report a general protocol for labeling phosphopeptides and phosphoproteins in biological samples using kinase-catalyzed biotinylation.

Use of 5'-γ-ferrocenyl adenosine triphosphate (Fc-ATP) bioconjugates having poly(ethylene glycol) spacers in kinase-catalyzed phosphorylations

The 5'-γ-ferrocenyl adenosine triphosphate (Fc-ATP) bioconjugates (3 and 4), containing the poly(ethylene glycol) spacers, were synthesized and compared to a hydrophobic analogue as co-substrates for the following protein kinases: sarcoma related kinase (Src), cyclin-dependent kinase (CDK), casein kinase II (CK2α), and protein kinase A (PKA). Electrochemical kinase assays indicate that the hydrophobic Fc-ATP analogue was an optimal co-substrate for which K(M) values were determined to be in the 30-200 μM range, depending on the particular protein kinase. The luminescence kinase assay demonstrated the kinase utility for all Fc-ATP conjugates, which is in line with the electrochemical data. Moreover, Fc-ATP bioconjugates exhibit competitive behavior with respect to ATP. Relatively poor performance of the polar Fc-ATP bioconjugates as co-substrates for protein kinases was presumably due to the additional H-bonding and electrostatic interactions of the poly(ethylene glycol) linkers of Fc-ATP with the kinase catalytic site and the target peptides. Phosphorylation of the full-length protein, His-tagged pro-caspase-3, was demonstrated through Fc-phosphoamide transfer to the Ser residues of the surface-bound protein by electrochemical means. These results suggest that electrochemical detection of the peptide and protein Fc-phosphorylation via tailored Fc-ATP co-substrates may be useful for probing protein-protein interactions.

Chemical approaches towards unravelling kinase-mediated signalling pathways

Protein kinases control the function of about one third of cellular proteins by catalysing the transfer of the γ-phosphate group of ATP onto their substrate proteins. Protein phosphatases counter this action and also control the activation status of many kinases. Cellular responses to environmental changes, or signalling events, temporarily tilt the balance of protein phosphorylation and dephosphorylation to one side or the other. The identification of protein-kinase-substrate pairs and substrate-phosphatase pairs is critical to understanding cell function and how cells respond to environmental changes. Identification of these substrate-enzyme pairs is non-trivial, because of the structural and mechanistic conservation of the catalytic cores of protein kinases. In this tutorial review we review recent progress towards identifying protein-kinase-substrate pairs by emphasising the use of chemical genetics and purpose-designed ATP analogues that target one particular protein kinase. In addition, we discuss activity-based chemical profiling approaches, based on ATP analogues, for the detection of active kinases.

Design of an encodable tyrosine kinase-inducible domain: detection of tyrosine kinase activity by terbium luminescence

Tyrosine kinases are critical mediators of intracellular signaling and of intracellular responses to extracellular signaling. Changes in tyrosine kinase activity are implicated in numerous human diseases, including cancers, diabetes, and pathogen infectivity. To address questions in tyrosine phosphorylation, we have designed a protein tyrosine kinase-inducible domain, a small, genetically encodable protein motif whose structure is dependent on its tyrosine phosphorylation state. Tyrosine kinase-inducible domain peptides are based on EF-hand loops in which a structurally critical Glu12 residue is replaced by tyrosine at residue 11 or at residue 15 of the protein. Tyrosine kinase-inducible domain peptides bind terbium(III) in a phosphorylation-dependent manner, showing strong terbium luminescence when phosphorylated but weak terbium luminescence when not phosphorylated. Lanthanide binding was confirmed by NMR. A tyrosine kinase-inducible domain peptide, pKID-Abl, was designed to incorporate a recognition sequence of the Abl kinase. Incubation of pKID-Abl with Abl kinase resulted in a large increase in terbium luminescence. This increase in luminescence was abolished when pKID-Abl and Abl kinase were incubated with the Abl kinase inhibitor Gleevec. In addition, incubation of phosphorylated pKID-Abl with the tyrosine phosphatase YOP resulted in a large reduction in terbium luminescence. pKID-Abl was employed as a fluorescent sensor of Abl tyrosine kinase activity in HeLa cell extracts, exhibiting low luminescence with extracts from serum-starved cells and increased luminescence using extracts from EGF-treated cells. These results indicate that tyrosine kinase-inducible domains may be used as sensors of tyrosine kinase and tyrosine phosphatase activity and in the detection of tyrosine kinase inhibitors.