Hitting the target: emerging technologies in the search for kinase substrates

The glycosyltransferase ALG3 is an AKT substrate that regulates protein N-glycosylation

The PI3K/AKT signaling pathway is frequently dysregulated in cancer and controls key cellular processes such as survival, proliferation, metabolism and growth. Protein glycosylation is essential for proper protein folding and is also often deregulated in cancer. Cancer cells depend on increased protein folding to sustain oncogene-driven proliferation rates. The N-glycosyltransferase asparagine-linked glycosylation 3 homolog (ALG3), a rate-limiting enzyme during glycan biosynthesis, catalyzes the addition of the first mannose to glycans in an alpha-1,3 linkage. Here we show that ALG3 is phosphorylated downstream of the PI3K/AKT pathway in both growth factor-stimulated cells and PI3K/AKT hyperactive cancer cells. AKT directly phosphorylates ALG3 in the amino terminal region at Ser11/Ser13. CRISPR/Cas9-mediated depletion of ALG3 leads to improper glycan formation and induction of endoplasmic reticulum stress, the unfolded protein response, and impaired cell proliferation. Phosphorylation of ALG3 at Ser11/Ser13 is required for glycosylation of cell surface receptors EGFR, HER3 and E-cadherin. These findings provide a direct link between PI3K/AKT signaling and protein glycosylation in cancer cells.

Protein kinases on carbon metabolism: potential targets for alternative chemotherapies against toxoplasmosis

The apicomplexan parasite Toxoplasma gondii is the causative agent of toxoplasmosis, a global disease that significantly impacts human health. The clinical manifestations are mainly observed in immunocompromised patients, including ocular damage and neuronal alterations leading to psychiatric disorders. The congenital infection leads to miscarriage or severe alterations in the development of newborns. The conventional treatment is limited to the acute phase of illness, without effects in latent parasites; consequently, a cure is not available yet. Furthermore, considerable toxic effects and long-term therapy contribute to high treatment abandonment rates. The investigation of exclusive parasite pathways would provide new drug targets for more effective therapies, eliminating or reducing the side effects of conventional pharmacological approaches. Protein kinases (PKs) have emerged as promising targets for developing specific inhibitors with high selectivity and efficiency against diseases. Studies in T. gondii have indicated the presence of exclusive PKs without homologs in human cells, which could become important targets for developing new drugs. Knockout of specific kinases linked to energy metabolism have shown to impair the parasite development, reinforcing the essentiality of these enzymes in parasite metabolism. In addition, the specificities found in the PKs that regulate the energy metabolism in this parasite could bring new perspectives for safer and more efficient therapies for treating toxoplasmosis. Therefore, this review provides an overview of the limitations for reaching an efficient treatment and explores the role of PKs in regulating carbon metabolism in Toxoplasma, discussing their potential as targets for more applied and efficient pharmacological approaches.

Nestin Selectively Facilitates the Phosphorylation of the Lissencephaly-Linked Protein Doublecortin (DCX) by cdk5/p35 to Regulate Growth Cone Morphology and Sema3a Sensitivity in Developing Neurons

Nestin, an intermediate filament protein widely used as a marker of neural progenitors, was recently found to be expressed transiently in developing cortical neurons in culture and in developing mouse cortex. In young cortical cultures, nestin regulates axonal growth cone morphology. In addition, nestin, which is known to bind the neuronal cdk5/p35 kinase, affects responses to axon guidance cues upstream of cdk5, specifically, to Sema3a. Changes in growth cone morphology require rearrangements of cytoskeletal networks, and changes in microtubules and actin filaments are well studied. In contrast, the roles of intermediate filament proteins in this process are poorly understood, even in cultured neurons. Here, we investigate the molecular mechanism by which nestin affects growth cone morphology and Sema3a sensitivity. We find that nestin selectively facilitates the phosphorylation of the lissencephaly-linked protein doublecortin (DCX) by cdk5/p35, but the phosphorylation of other cdk5 substrates is not affected by nestin. We uncover that this substrate selectivity is based on the ability of nestin to interact with DCX, but not with other cdk5 substrates. Nestin thus creates a selective scaffold for DCX with activated cdk5/p35. Last, we use cortical cultures derived from Dcx KO mice to show that the effects of nestin on growth cone morphology and on Sema3a sensitivity are DCX-dependent, thus suggesting a functional role for the DCX-nestin complex in neurons. We propose that nestin changes growth cone behavior by regulating the intracellular kinase signaling environment in developing neurons. The sex of animal subjects is unknown.SIGNIFICANCE STATEMENT Nestin, an intermediate filament protein highly expressed in neural progenitors, was recently identified in developing neurons where it regulates growth cone morphology and responsiveness to the guidance cue Sema3a. Changes in growth cone morphology require rearrangements of cytoskeletal networks, but the roles of intermediate filaments in this process are poorly understood. We now report that nestin selectively facilitates phosphorylation of the lissencephaly-linked doublecortin (DCX) by cdk5/p35, but the phosphorylation of other cdk5 substrates is not affected. This substrate selectivity is based on preferential scaffolding of DCX, cdk5, and p35 by nestin. Additionally, we demonstrate a functional role for the DCX-nestin complex in neurons. We propose that nestin changes growth cone behavior by regulating intracellular kinase signaling in developing neurons.

The Tuberin and Cyclin B1 complex functions as a novel G2/M sensor of serum conditions and Akt signaling

A great deal of ground breaking work has determined that the Tuberin and Hamartin Complex function as a negative regulator of protein synthesis and cell cycle progression through G1/S. This is largely attributed to the GTPase activity of Tuberin that indirectly inhibits the mammalian target of rapamycin (mTOR). During times of ample nutrition Tuberin is inhibited by growth factor signaling, including direct phosphorylation by Akt/PKB, allowing for activation of mTOR and subsequent protein synthesis. It is well rationalized that maintaining homeostasis requires communication between cell growth (mTOR signaling) and cell division (cell cycle regulation), however how this occurs mechanistically has not been resolved. This work demonstrates that in the presence of high serum, and/or Akt signaling, direct binding between Tuberin and the G2/M cyclin, Cyclin B1, is stabilized and the rate of mitotic entry is decreased. Importantly, we show that this results in an increase in cell size. We propose that this represents a novel cell cycle checkpoint linking mitotic onset with the nutritional status of the cell to control cell growth.

The Quest for MAP Kinase Substrates: Gaining Momentum

Mitogen-activated protein kinase (MAPK) pathways are versatile signaling mechanisms in all eukaryotes. Their signaling outputs are defined by the protein substrates phosphorylated by MAPKs. An expanding list of substrates has been identified by high-throughput screens and targeted approaches in plants. The majority of these are phosphorylated by MPK3/6, and a few by MPK4, which are the best-characterized plant MAPKs, participating in the regulation of numerous biological processes. The identified substrates clearly represent the functional diversity of MAPKs: they are associated with pathogen defense, abiotic stress responses, ethylene signaling, and various developmental functions. Understanding their outputs is integral to unraveling the complex regulatory mechanisms of MAPK cascades. We review here methodological approaches and provide an overview of known MAPK substrates.

Active site-adjacent phosphorylation at Tyr-397 by c-Abl kinase inactivates caspase-9

Caspase-9 (casp-9) is an initiator caspase and plays a central role in activating apoptotic cell death. Control of all caspases is tightly regulated by a series of phosphorylation events enacted by several different kinases. Caspase-9 is the most heavily phosphorylated of all caspases, with phosphorylation of at least 11 distinct residues in all three caspase-9 domains by nine kinases. Caspase-9 phosphorylation by the non-receptor tyrosine kinase c-Abl at Tyr-153 reportedly leads to caspase-9 activation. All other phosphorylation events on caspases have been shown to block proteolytic function by a number of mechanisms, so we sought to unravel the molecular mechanism of the putative caspase-9 activation by phosphorylation. Surprisingly, we observed no evidence for Tyr-153 phosphorylation of caspase-9 in vitro or in cells, suggesting that Tyr-153 is not phosphorylated by c-Abl. Instead, we identified a new site for c-Abl-mediated phosphorylation, Tyr-397. This residue is adjacent to the caspase-9 active site but, as a member of the second shell, not a residue that directly contacts substrate. Our results further indicate that Tyr-397 is the dominant site of c-Abl phosphorylation both in vitro and upon c-Abl activation in cells. Of note, phosphorylation at this site inhibits caspase-9 activity, and the bulk of the added phosphate moiety appeared to directly block substrate binding. c-Abl plays both proapoptotic and prosurvival roles, and our findings suggest that c-Abl's effects on caspase-9 activity promote the prosurvival mode.

Systems-level identification of PKA-dependent signaling in epithelial cells

G protein stimulatory α-subunit (G_αs)-coupled heptahelical receptors regulate cell processes largely through activation of protein kinase A (PKA). To identify signaling processes downstream of PKA, we deleted both PKA catalytic subunits using CRISPR-Cas9, followed by a "multiomic" analysis in mouse kidney epithelial cells expressing the G_αs-coupled V2 vasopressin receptor. RNA-seq (sequencing)-based transcriptomics and SILAC (stable isotope labeling of amino acids in cell culture)-based quantitative proteomics revealed a complete loss of expression of the water-channel gene Aqp2 in PKA knockout cells. SILAC-based quantitative phosphoproteomics identified 229 PKA phosphorylation sites. Most of these PKA targets are thus far unannotated in public databases. Surprisingly, 1,915 phosphorylation sites with the motif x-(S/T)-P showed increased phosphooccupancy, pointing to increased activity of one or more MAP kinases in PKA knockout cells. Indeed, phosphorylation changes associated with activation of ERK2 were seen in PKA knockout cells. The ERK2 site is downstream of a direct PKA site in the Rap1GAP, Sipa1l1, that indirectly inhibits Raf1. In addition, a direct PKA site that inhibits the MAP kinase kinase kinase Map3k5 (ASK1) is upstream of JNK1 activation. The datasets were integrated to identify a causal network describing PKA signaling that explains vasopressin-mediated regulation of membrane trafficking and gene transcription. The model predicts that, through PKA activation, vasopressin stimulates AQP2 exocytosis by inhibiting MAP kinase signaling. The model also predicts that, through PKA activation, vasopressin stimulates Aqp2 transcription through induction of nuclear translocation of the acetyltransferase EP300, which increases histone H3K27 acetylation of vasopressin-responsive genes (confirmed by ChIP-seq).

K-BILDS: A Kinase Substrate Discovery Tool

Kinases catalyze protein phosphorylation to regulate cell signaling events. However, identifying kinase substrates is challenging due to the often low abundance and dynamic nature of protein phosphorylation. Development of novel techniques to identify kinase substrates is necessary. Here, we report kinase-catalyzed biotinylation with inactivated lysates for discovery of substrates (K-BILDS) as a tool to identify direct substrates of a kinase. As a proof of concept, K-BILDS was applied to cAMP-dependent protein kinase A (PKA) with HeLa cell lysates. Subsequent enrichment and MS/MS analysis identified 279 candidate PKA substrates, including 56 previously known PKA substrates. Of the candidate substrates, nuclear autoantigenic sperm protein (NASP), BCL2-associated athanogene 3 (BAG3), and 14-3-3 protein Tau (YWHAQ) were validated as novel PKA substrates. K-BILDS provides a valuable tool to identify direct substrates of any protein kinase.

Kinase-Associated Phosphoisoform Assay: a novel candidate-based method to detect specific kinase-substrate phosphorylation interactions in vivo

BACKGROUND

Protein kinases are important components of signalling pathways, and kinomes have remarkably expanded in plants. Yet, our knowledge of kinase substrates in plants is scarce, partly because tools to analyse protein phosphorylation dynamically are limited. Here we describe Kinase-Associated Phosphoisoform Assay, a flexible experimental method for directed experiments to study specific kinase-substrate interactions in vivo. The concept is based on the differential phosphoisoform distribution of candidate substrates transiently expressed with or without co-expression of activated kinases. Phosphorylation status of epitope-tagged proteins is subsequently detected by high-resolution capillary isoelectric focusing coupled with nanofluidic immunoassay, which is capable of detecting subtle changes in isoform distribution.

RESULTS

The concept is validated by showing phosphorylation of the known mitogen-activated protein kinase (MAPK) substrate, ACS6, by MPK6. Next, we demonstrate that two transcription factors, WUS and AP2, both of which are shown to be master regulators of plant development by extensive genetic studies, exist in multiple isoforms in plant cells and are phosphorylated by activated MAPKs.

CONCLUSION

As plant development flexibly responds to environmental conditions, phosphorylation of developmental regulators by environmentally-activated kinases may participate in linking external cues to developmental regulation. As a counterpart of advances in unbiased screening methods to identify potential protein kinase substrates, such as phosphoproteomics and computational predictions, our results expand the candidate-based experimental toolkit for kinase research and provide an alternative in vivo approach to existing in vitro methodologies.

Oncogenic Receptor Tyrosine Kinases Directly Phosphorylate Focal Adhesion Kinase (FAK) as a Resistance Mechanism to FAK-Kinase Inhibitors

Focal adhesion kinase (FAK) is a major drug target in cancer and current inhibitors targeted to the ATP-binding pocket of the kinase domain have entered clinical trials. However, preliminary results have shown limited single-agent efficacy in patients. Despite these unfavorable data, the molecular mechanisms that drive intrinsic and acquired resistance to FAK-kinase inhibitors are largely unknown. We have demonstrated that receptor tyrosine kinases (RTK) can directly bypass FAK-kinase inhibition in cancer cells through phosphorylation of FAK's critical tyrosine 397 (Y397). We also showed that HER2 forms a direct protein-protein interaction with the FAK-FERM-F1 lobe, promoting direct phosphorylation of Y397. In addition, FAK-kinase inhibition induced two forms of compensatory RTK reprogramming: (i) the rapid phosphorylation and activation of RTK signaling pathways in RTK^High cells and (ii) the long-term acquisition of RTKs novel to the parental cell line in RTK^Low cells. Finally, HER2 +: cancer cells displayed resistance to FAK-kinase inhibition in 3D growth assays using a HER2 isogenic system and HER2⁺ cancer cell lines. Our data indicate a novel drug resistance mechanism to FAK-kinase inhibitors whereby HER2 and other RTKs can rescue and maintain FAK activation (pY397) even in the presence of FAK-kinase inhibition. These data may have important ramifications for existing clinical trials of FAK inhibitors and suggest that individual tumor stratification by RTK expression would be important to predict patient response to FAK-kinase inhibitors. Mol Cancer Ther; 15(12); 3028-39. ©2016 AACR.

Identifying novel targets of oncogenic EGF receptor signaling in lung cancer through global phosphoproteomics

Mutations in the epidermal growth factor receptor (EGFR) kinase domain occur in 10-30% of lung adenocarcinoma and are associated with tyrosine kinase inhibitor (TKI) sensitivity. We sought to identify the immediate direct and indirect phosphorylation targets of mutant EGFRs in lung adenocarcinoma. We undertook SILAC strategy, phosphopeptide enrichment, and quantitative MS to identify dynamic changes of phosphorylation downstream of mutant EGFRs in lung adenocarcinoma cells harboring EGFR(L858R) and EGFR(L858R/T790M) , the TKI-sensitive, and TKI-resistant mutations, respectively. Top canonical pathways that were inhibited upon erlotinib treatment in sensitive cells, but not in the resistant cells include EGFR, insulin receptor, hepatocyte growth factor, mitogen-activated protein kinase, mechanistic target of rapamycin, ribosomal protein S6 kinase beta 1, and Janus kinase/signal transducer and activator of transcription signaling. We identified phosphosites in proteins of the autophagy network, such as ULK1 (S623) that is constitutively phosphorylated in these lung adenocarcinoma cells; phosphorylation is inhibited upon erlotinib treatment in sensitive cells, but not in resistant cells. Finally, kinase-substrate prediction analysis from our data indicated that substrates of basophilic kinases from, AGC and Calcium and calmodulin-dependent kinase groups, as well as STE group kinases were significantly enriched and those of proline-directed kinases from, CMGC and Casein kinase groups were significantly depleted among substrates that exhibited increased phosphorylation upon EGF stimulation and reduced phosphorylation upon TKI inhibition. This is the first study to date to examine global phosphorylation changes upon erlotinib treatment of lung adenocarcinoma cells and results from this study provide new insights into signaling downstream of mutant EGFRs in lung adenocarcinoma. All MS data have been deposited in the ProteomeXchange with identifier PXD001101 (http://proteomecentral.proteomexchange.org/dataset/PXD001101).

Targeting of nucleotide-binding proteins by HAMLET--a conserved tumor cell death mechanism

HAMLET (Human Alpha-lactalbumin Made LEthal to Tumor cells) kills tumor cells broadly suggesting that conserved survival pathways are perturbed. We now identify nucleotide-binding proteins as HAMLET binding partners, accounting for about 35% of all HAMLET targets in a protein microarray comprising 8000 human proteins. Target kinases were present in all branches of the Kinome tree, including 26 tyrosine kinases, 10 tyrosine kinase-like kinases, 13 homologs of yeast sterile kinases, 4 casein kinase 1 kinases, 15 containing PKA, PKG, PKC family kinases, 15 calcium/calmodulin-dependent protein kinase kinases and 13 kinases from CDK, MAPK, GSK3, CLK families. HAMLET acted as a broad kinase inhibitor in vitro, as defined in a screen of 347 wild-type, 93 mutant, 19 atypical and 17 lipid kinases. Inhibition of phosphorylation was also detected in extracts from HAMLET-treated lung carcinoma cells. In addition, HAMLET recognized 24 Ras family proteins and bound to Ras, RasL11B and Rap1B on the cytoplasmic face of the plasma membrane. Direct cellular interactions between HAMLET and activated Ras family members including Braf were confirmed by co-immunoprecipitation. As a consequence, oncogenic Ras and Braf activity was inhibited and HAMLET and Braf inhibitors synergistically increased tumor cell death in response to HAMLET. Unlike most small molecule kinase inhibitors, HAMLET showed selectivity for tumor cells in vitro and in vivo. The results identify nucleotide-binding proteins as HAMLET targets and suggest that dysregulation of the ATPase/kinase/GTPase machinery contributes to cell death, following the initial, selective recognition of HAMLET by tumor cells. The findings thus provide a molecular basis for the conserved tumoricidal effect of HAMLET, through dysregulation of kinases and oncogenic GTPases, to which tumor cells are addicted.

Analytical challenges translating mass spectrometry-based phosphoproteomics from discovery to clinical applications

Phosphoproteomics is the systematic study of one of the most common protein modifications in high throughput with the aim of providing detailed information of the control, response, and communication of biological systems in health and disease. Advances in analytical technologies and strategies, in particular the contributions of high-resolution mass spectrometers, efficient enrichments of phosphopeptides, and fast data acquisition and annotation, have catalyzed dramatic expansion of signaling landscapes in multiple systems during the past decade. While phosphoproteomics is an essential inquiry to map high-resolution signaling networks and to find relevant events among the apparently ubiquitous and widespread modifications of proteome, it presents tremendous challenges in separation sciences to translate it from discovery to clinical practice. In this mini-review, we summarize the analytical tools currently utilized for phosphoproteomic analysis (with focus on MS), progresses made on deciphering clinically relevant kinase-substrate networks, MS uses for biomarker discovery and validation, and the potential of phosphoproteomics for disease diagnostics and personalized medicine.

Manipulation of endogenous kinase activity in living cells using photoswitchable inhibitory peptides

Optogenetic control of endogenous signaling can be an important tool for probing cell behavior. Using the photoresponse of the LOV2 domain of Avena sativa phototropin 1, we developed analogues of kinase inhibitors whose activity is light dependent. Inhibitory peptides were appended to the Jα helix, where they potently inhibited kinases in the light but were sterically blocked from kinase interaction in the dark. Photoactivatable inhibitors for cyclic-AMP dependent kinase (PKA) and myosin light chain kinase (MLCK) are described, together with studies that shed light on proper positioning of the peptides in the LOV domain. These inhibitors altered endogenous signaling in living cells and produced light-dependent changes in cell morphodynamics.

SchA-p85-FAK complex dictates isoform-specific activation of Akt2 and subsequent PCBP1-mediated post-transcriptional regulation of TGFβ-mediated epithelial to mesenchymal transition in human lung cancer cell line A549

A post-transcriptional pathway by which TGF-β modulates expression of specific proteins, Disabled-2 (Dab2) and Interleukin-like EMT Inducer (ILEI), inherent to epithelial to mesenchymal transition (EMT) in murine epithelial cells through Akt2-mediated phosphorylation of poly r(C) binding protein (PCBP1), has been previously elucidated. The aims of the current study were to determine if the same mechanism is operative in the non-small cell lung cancer (NSCLC) cell line, A549, and to delineate the underlying mechanism. Steady-state transcript and protein expression levels of Dab2 and ILEI were examined in A549 cells treated with TGF-β for up to 48 h. Induction of translational de-repression in this model was quantified by polysomal fractionation followed by qRT-PCR. The underlying mechanism of isoform-specific activation of Akt2 was elucidated through a combination of co-immunoprecipitation studies. TGF-β induced EMT in A549 cells concomitant with translational upregulation of Dab2 and ILEI proteins through isoform-specific activation of Akt2 followed by phosphorylation of PCBP1 at serine-43. Our experiments further elucidated that the adaptor protein SchA is phosphorylated at tyrosine residues following TGF-β treatment, which initiated a signaling cascade resulting in the sequential recruitment of p85 subunit of PI3K and focal adhesion kinase (FAK). The SchA-FAK-p85 complex subsequently selectively recruited and activated Akt2, not Akt1. Inhibition of the p85 subunit through phosphorylated 1257 peptide completely attenuated EMT in these cells. We have defined the underlying mechanism responsible for isoform-specific recruitment and activation of Akt2, not Akt1, during TGF-β-mediated EMT in A549 cells. Inhibition of the formation of this complex thus represents an important and novel therapeutic target in metastatic lung carcinoma.

Signaling specificity in the Akt pathway in biology and disease

Akt/PKB is a key master regulator of a wide range of physiological functions including metabolism, proliferation, survival, growth, angiogenesis and migration and invasion. The Akt protein kinase family comprises three highly related isoforms encoded by different genes. The initial observation that the Akt isoforms share upstream activators as well as several downstream effectors, together with the high sequence homology suggested that their functions were mostly redundant. By contrast, an increasing body of evidence has recently uncovered the concept of Akt isoform signaling specificity, supported by distinct phenotypes displayed by animal strains genetically modified for each of the three genes, as well as by the identification of isoform-specific substrates and association with discrete subcellular locations. Given that Akt is regarded as a promising therapeutic target in a number of pathologies, it is essential to dissect the relative contributions of each isoform, as well as the degree of compensation in pathophysiological function. Here we summarize our view of how Akt selectivity is achieved in the context of subcellular localization, isoform-specific substrate phosphorylation and context-dependent functions in normal and pathophysiological settings.

Quantitative phosphoproteomic profiling of PINK1-deficient cells identifies phosphorylation changes in nuclear proteins

The Parkinson's disease (PD) associated gene PINK1 encodes a protein kinase that mediates the phosphorylation of multiple proteins involved in mitochondrial homeostasis. The broader downstream signaling events mediated by PINK1 kinase activity have not been well documented. We combine quantitative phosphoproteomic strategies with siRNA mediated PINK1 knock down in mammalian cells to identify alterations of phosphorylation events downstream of PINK1. Although down-regulation of PINK1 has no major effect on the proteome expression in these cells, phosphorylation of over one hundred proteins was reduced reflecting basal levels of phosphorylation signaling events downstream of PINK1. Motif analysis of the residues flanking the phosphorylation sites indicates proline-directed kinase specificity. Surprisingly, we found that the downstream signaling nodes included many transcription factors, as well as nuclear proteins involved in DNA and RNA metabolism. Thus, PINK1 dependent phosphorylation signaling may regulate nuclear activities.

Identification of direct tyrosine kinase substrates based on protein kinase assay-linked phosphoproteomics

Protein kinases are implicated in multiple diseases such as cancer, diabetes, cardiovascular diseases, and central nervous system disorders. Identification of kinase substrates is critical to dissecting signaling pathways and to understanding disease pathologies. However, methods and techniques used to identify bona fide kinase substrates have remained elusive. Here we describe a proteomic strategy suitable for identifying kinase specificity and direct substrates in high throughput. This approach includes an in vitro kinase assay-based substrate screening and an endogenous kinase dependent phosphorylation profiling. In the in vitro kinase reaction route, a pool of formerly phosphorylated proteins is directly extracted from whole cell extracts, dephosphorylated by phosphatase treatment, after which the kinase of interest is added. Quantitative proteomics identifies the rephosphorylated proteins as direct substrates in vitro. In parallel, the in vivo quantitative phosphoproteomics is performed in which cells are treated with or without the kinase inhibitor. Together, proteins phosphorylated in vitro overlapping with the kinase-dependent phosphoproteome in vivo represents the physiological direct substrates in high confidence. The protein kinase assay-linked phosphoproteomics was applied to identify 25 candidate substrates of the protein-tyrosine kinase SYK, including a number of known substrates and many novel substrates in human B cells. These shed light on possible new roles for SYK in multiple important signaling pathways. The results demonstrate that this integrated proteomic approach can provide an efficient strategy to screen direct substrates for protein tyrosine kinases.

Using phospho-motif antibodies to determine kinase substrates

Phosphorylation of substrates by protein kinases regulates a myriad of cellular processes, ranging from proliferation and migration to autophagy, senescence, and apoptosis. Kinase substrate selectivity is largely dependent on the amino acid sequence surrounding the phosphorylation site; therefore, substrate-directed, phosphorylation-state-sensitive, motif-specific ("phospho-motif") antibodies represent powerful tools to identify novel kinase substrates and to investigate mechanisms of substrate phosphorylation in many signaling pathways typically associated with human malignancies. Phospho-motif antibodies are engineered to recognize proteins that contain a phosphorylated residue in the context of a specific motif. They are raised against a library of phospho-peptides comprising both the phosphorylated residue and the surrounding residues that determine kinase specificity, with degenerate residues taking up the remaining positions. Currently, several categories of phospho-motif antibody are commercially available, which may be used to specifically detect Ser, Thr, Ser/Thr, or Tyr residues phosphorylated by different protein kinase families. These antibodies are commonly used in immunoprecipitation and/or immunoblotting protocols to determine kinase-induced substrate phosphorylation. This unit describes the use of phospho-motif antibodies to elucidate the kinase(s) responsible for phosphorylating substrate proteins.

Universal quantitative kinase assay based on diagonal SCX chromatography and stable isotope dimethyl labeling provides high-definition kinase consensus motifs for PKA and human Mps1

In order to understand cellular signaling, a clear understanding of kinase-substrate relationships is essential. Some of these relationships are defined by consensus recognition motifs present in substrates making them amendable for phosphorylation by designated kinases. Here, we explore a method that is based on two sequential steps of strong cation exchange chromatography combined with differential stable isotope labeling, to define kinase consensus motifs with high accuracy. We demonstrate the value of our method by evaluating the motifs of two very distinct kinases: cAMP regulated protein kinase A (PKA) and human monopolar spindle 1 (Mps1) kinase, also known as TTK. PKA is a well-studied basophilic kinase with a relatively well-defined motif and numerous known substrates in vitro and in vivo. Mps1, a kinase involved in chromosome segregation, has been less well characterized. Its substrate specificity is unclear and here we show that Mps1 is an acidophilic kinase with a striking tendency for phosphorylation of threonines. The final outcomes of our work are high-definition kinase consensus motifs for PKA and Mps1. Our generic method, which makes use of proteolytic cell lysates as a source for peptide-substrate libraries, can be implemented for any kinase present in the kinome.

Correlation profiling for determining kinase-substrate relationships

Arguably the most important issue in phosphorylation studies is the determination of kinase-substrate relationships. Kinase Activity-Abundance Correlation (KAAC) profiling is a technique that can be used to suggest the kinase responsible for a particular phosphorylation event. The method involves the separation of a lysate at the protein level, after which fractions are analyzed to produce an activity profile for each substrate peptide. This activity profile is correlated with the kinase abundance profiles (obtained using shotgun proteomics) to obtain candidate kinases responsible for phosphorylation of the substrate peptide. We demonstrate this approach by determining potential kinase-substrate pairs for six peptides, selected from the literature, that were shown to be upregulated during mitosis. Finally, for a subset of fractions we evaluated the use of stable isotope labeling to improve quantification.

Current technologies to identify protein kinase substrates in high throughput

Since the discovery of protein phosphorylation as an important modulator of many cellular processes, the involvement of protein kinases in diseases, such as cancer, diabetes, cardiovascular diseases, and central nervous system pathologies, has been extensively documented. Our understanding of many disease pathologies at the molecular level, therefore, requires the comprehensive identification of substrates targeted by protein kinases. In this review, we focus on recent techniques for kinase substrate identification in high throughput, in particular on genetic and proteomic approaches. Each method with its inherent advantages and limitations is discussed.

A highly scalable peptide-based assay system for proteomics

We report a scalable and cost-effective technology for generating and screening high-complexity customizable peptide sets. The peptides are made as peptide-cDNA fusions by in vitro transcription/translation from pools of DNA templates generated by microarray-based synthesis. This approach enables large custom sets of peptides to be designed in silico, manufactured cost-effectively in parallel, and assayed efficiently in a multiplexed fashion. The utility of our peptide-cDNA fusion pools was demonstrated in two activity-based assays designed to discover protease and kinase substrates. In the protease assay, cleaved peptide substrates were separated from uncleaved and identified by digital sequencing of their cognate cDNAs. We screened the 3,011 amino acid HCV proteome for susceptibility to cleavage by the HCV NS3/4A protease and identified all 3 known trans cleavage sites with high specificity. In the kinase assay, peptide substrates phosphorylated by tyrosine kinases were captured and identified by sequencing of their cDNAs. We screened a pool of 3,243 peptides against Abl kinase and showed that phosphorylation events detected were specific and consistent with the known substrate preferences of Abl kinase. Our approach is scalable and adaptable to other protein-based assays.

Sensitive kinase assay linked with phosphoproteomics for identifying direct kinase substrates

Our understanding of the molecular control of many disease pathologies requires the identification of direct substrates targeted by specific protein kinases. Here we describe an integrated proteomic strategy, termed kinase assay linked with phosphoproteomics, which combines a sensitive kinase reaction with endogenous kinase-dependent phosphoproteomics to identify direct substrates of protein kinases. The unique in vitro kinase reaction is carried out in a highly efficient manner using a pool of peptides derived directly from cellular kinase substrates and then dephosphorylated as substrate candidates. The resulting newly phosphorylated peptides are then isolated and identified by mass spectrometry. A further comparison of these in vitro phosphorylated peptides with phosphopeptides derived from endogenous proteins isolated from cells in which the kinase is either active or inhibited reveals new candidate protein substrates. The kinase assay linked with phosphoproteomics strategy was applied to identify unique substrates of spleen tyrosine kinase (Syk), a protein-tyrosine kinase with duel properties of an oncogene and a tumor suppressor in distinctive cell types. We identified 64 and 23 direct substrates of Syk specific to B cells and breast cancer cells, respectively. Both known and unique substrates, including multiple centrosomal substrates for Syk, were identified, supporting a unique mechanism that Syk negatively affects cell division through its centrosomal kinase activity.

A novel whole-cell lysate kinase assay identifies substrates of the p38 MAPK in differentiating myoblasts

Background

The p38α mitogen-activated protein kinase (MAPK) is a critical mediator of myoblast differentiation, and does so in part through the phosphorylation and regulation of several transcription factors and chromatin remodelling proteins. However, whether p38α is involved in processes other than gene regulation during myogenesis is currently unknown, and why other p38 isoforms cannot compensate for its loss is unclear.

Methods

To further characterise the involvement of p38α during myoblast differentiation, we developed and applied a simple technique for identifying relevant in vivo kinase substrates and their phosphorylation sites. In addition to identifying substrates for one kinase, the technique can be used in vitro to compare multiple kinases in the same experiment, and we made use of this to study the substrate specificities of the p38α and β isoforms.

Results

Applying the technique to p38α resulted in the identification of seven in vivo phosphorylation sites on six proteins, four of which are cytoplasmic, in lysate derived from differentiating myoblasts. An in vitro comparison with p38β revealed that substrate specificity does not discriminate these two isoforms, but rather that their distinguishing characteristic appears to be cellular localisation.

Conclusion

Our results suggest p38α has a novel cytoplasmic role during myogenesis and that its unique cellular localisation may be why p38β and other isoforms cannot compensate for its absence. The substrate-finding approach presented here also provides a necessary tool for studying the hundreds of protein kinases that exist and for uncovering the deeper mechanisms of phosphorylation-dependent cell signalling.

Human Protein Reference Database and Human Proteinpedia as resources for phosphoproteome analysis

Human Protein Reference Database (HPRD) is a rich resource of experimentally proven features of human proteins. Protein information in HPRD includes protein-protein interactions, post-translational modifications, enzyme/substrate relationships, disease associations, tissue expression, and subcellular localization of human proteins. Although, protein-protein interaction data from HPRD has been widely used by the scientific community, its phosphoproteome data has not been exploited to its full potential. HPRD is one of the largest documentations of human phosphoproteins in the public domain. Currently, phosphorylation data in HPRD comprises of 95,016 phosphosites mapped on to 13,041 proteins. Additionally, enzyme-substrate reactions responsible for 5930 phosphorylation events were also documented. Significant improvements in technologies and high-throughput platforms in biomedical investigations led to an exponential increase of biological data and phosphoproteomic data in recent years. Human Proteinpedia, a community annotation portal developed by us, has also contributed to the significant increase in phosphoproteomic data in HPRD. A large number of phosphorylation events have been mapped on to reference sequences available in HPRD and Human Proteinpedia along with associated protein features. This will provide a platform for systems biology approaches to determine the role of protein phosphorylation in protein function, cell signaling, biological processes and their implication in human diseases. This review aims to provide a composite view of phosphoproteomic data pertaining to human proteins in HPRD and Human Proteinpedia.

Eukaryote-like serine/threonine kinases and phosphatases in bacteria

Genomic studies have revealed the presence of Ser/Thr kinases and phosphatases in many bacterial species, although their physiological roles have largely been unclear. Here we review bacterial Ser/Thr kinases (eSTKs) that show homology in their catalytic domains to eukaryotic Ser/Thr kinases and their partner phosphatases (eSTPs) that are homologous to eukaryotic phosphatases. We first discuss insights into the enzymatic mechanism of eSTK activation derived from structural studies on both the ligand-binding and catalytic domains. We then turn our attention to the identified substrates of eSTKs and eSTPs for a number of species and to the implications of these findings for understanding their physiological roles in these organisms.

Controlling the activity of the Tec kinase Itk by mutation of the phenylalanine gatekeeper residue

The regulatory spine is a set of conserved residues that are assembled and disassembled upon activation and inactivation of kinases. We recently identified the regulatory spine within the immunologically important Tec family kinases and have shown that in addition to the core spine residues within the kinase domain itself, contributions from the SH2-kinase linker region result in an extended spine structure for this kinase family. Disruption of the regulatory spine, either by mutation or by removal of the amino-terminal SH2-kinase linker region or by mutation of core spine residues, leads to inactivation of the Tec kinases. With a focus on the Tec family members, Itk and Btk, we now show that the gatekeeper residue is also critical for the assembly of the regulatory spine. Mutation of the bulky Itk F434 gatekeeper residue to alanine or glycine inactivates Itk. The activity of the Itk F434A mutant can be recovered by a secondary site mutation within the N-terminal lobe, specifically L432I. The Itk L432I mutation likely rescues the activity of the gatekeeper F434A mutation by promoting the assembly of the regulatory spine. We also show that mutation of the Itk and Btk gatekeeper residues to methionine is sufficient to activate the isolated kinase domains of Tec kinases in the absence of the amino-terminal SH2-kinase linker. Thus, shifting the conformational equilibrium between the assembled and disassembled states of the regulatory spine by changing the nature of the gatekeeper residue is key to regulating the activity of Tec kinases.

Atg1-mediated myosin II activation regulates autophagosome formation during starvation-induced autophagy

Autophagy is a membrane-mediated degradation process of macromolecule recycling. Although the formation of double-membrane degradation vesicles (autophagosomes) is known to have a central role in autophagy, the mechanism underlying this process remains elusive. The serine/threonine kinase Atg1 has a key role in the induction of autophagy. In this study, we show that overexpression of Drosophila Atg1 promotes the phosphorylation-dependent activation of the actin-associated motor protein myosin II. A novel myosin light chain kinase (MLCK)-like protein, Spaghetti-squash activator (Sqa), was identified as a link between Atg1 and actomyosin activation. Sqa interacts with Atg1 through its kinase domain and is a substrate of Atg1. Significantly, myosin II inhibition or depletion of Sqa compromised the formation of autophagosomes under starvation conditions. In mammalian cells, we found that the Sqa mammalian homologue zipper-interacting protein kinase (ZIPK) and myosin II had a critical role in the regulation of starvation-induced autophagy and mammalian Atg9 (mAtg9) trafficking when cells were deprived of nutrients. Our findings provide evidence of a link between Atg1 and the control of Atg9-mediated autophagosome formation through the myosin II motor protein.

Phosphoproteomics by mass spectrometry: insights, implications, applications and limitations

Phosphorylation of proteins is a predominant, reversible post-translational modification. It is central to a wide variety of physiological responses and signaling mechanisms. Recent advances have allowed the global scope of phosphorylation to be addressed by mass spectrometry using phosphoproteomic approaches. In this perspective, we discuss four aspects of phosphoproteomics: the insights and implications from recently published phosphoproteomic studies and the applications and limitations of current phosphoproteomic strategies. Since approximately 50,000 known phosphorylation sites do not yet have any ascribed function, we present our perspectives on a major function of protein phosphorylation that may be of predictive value in hypothesis-based investigations. Finally, we discuss strategies to measure the stoichiometry of phosphorylation in a proteome-wide manner that is not provided by current phosphoproteomic approaches.

Perspectives for the use of structural information and chemical genetics to develop inhibitors of Janus kinases

Gain-of-function mutations in the genes encoding Janus kinases have been discovered in various haematologic diseases. Jaks are composed of a FERM domain, an SH2 domain, a pseudokinase domain and a kinase domain, and a complex interplay of the Jak domains is involved in regulation of catalytic activity and association to cytokine receptors. Most activating mutations are found in the pseudokinase domain. Here we present recently discovered mutations in the context of our structural models of the respective domains. We describe two structural hotspots in the pseudokinase domain of Jak2 that seem to be associated either to myeloproliferation or to lymphoblastic leukaemia, pointing at the involvement of distinct signalling complexes in these disease settings. The different domains of Jaks are discussed as potential drug targets. We present currently available inhibitors targeting Jaks and indicate structural differences in the kinase domains of the different Jaks that may be exploited in the development of specific inhibitors. Moreover, we discuss recent chemical genetic approaches which can be applied to Jaks to better understand the role of these kinases in their biological settings and as drug targets.

Phosphorylation of the mutant K303R estrogen receptor alpha at serine 305 affects aromatase inhibitor sensitivity

We previously identified a lysine to arginine transition at residue 303 (K303R) in ERα in invasive breast cancers, which confers resistance to the aromatase inhibitor (AI) anastrozole (Ana) when expressed in MCF-7 breast cancer cells. Here we show that AI resistance arises through an enhanced cross-talk of the IGF-1R/IRS-1/Akt pathway with ERα, and the serine (S) residue 305 adjacent to the K303R mutation plays a key role in mediating this cross-talk. The ERα S305 residue is an important site that modifies response to tamoxifen; thus, we questioned whether this site could also influence AI response. We generated stable transfectants expressing wild-type (WT), K303R ERα, or a double K303R/S305A mutant receptor, and found that the AI-resistant phenotype associated with expression of the K303R mutation was dependent on activation of S305 within the receptor. Ana significantly reduced growth in K303R/S305A-expressing cells. Preventing S305 phosphorylation with a blocking peptide inhibited IGF-1R/IRS-1/Akt activation, and also restored AI sensitivity. Our data suggest that the K303R mutation and the S305 ERα residue may be a novel determinant of aromatase inhibitor response in breast cancer, and blockade of S305 phosphorylation represents a new therapeutic strategy for treating tumors resistant to hormone therapy.

Quantifying oncogenic phosphotyrosine signaling networks through systems biology

Pathways linking oncogenic mutations to increased proliferative or migratory capacity are poorly characterized, yet provide potential targets for therapeutic intervention. As tyrosine phosphorylation signaling networks are known to mediate proliferation and migration, and frequently go awry in cancers, a comprehensive understanding of these networks in normal and diseased states is warranted. To this end, recent advances in mass spectrometry, protein microarrays, and computational algorithms provide insight into various aspects of the network including phosphotyrosine identification, analysis of kinase/phosphatase substrates, and phosphorylation-mediated protein-protein interactions. Here we detail technological advances underlying these system-level approaches and give examples of their applications. By combining multiple approaches, it is now possible to quantify changes in the phosphotyrosine signaling network with various oncogenic mutations, thereby unveiling novel therapeutic targets.

Understanding protein phosphorylation on a systems level

Protein kinase phosphorylation is central to the regulation and control of protein and cellular function. Over the past decade, the development of many high-throughput approaches has revolutionized the understanding of protein phosphorylation and allowed rapid and unbiased surveys of phosphoproteins and phosphorylation events. In addition to this technological advancement, there have also been computational improvements; recent studies on network models of protein phosphorylation have provided many insights into the cellular processes and pathways regulated by phosphorylation. This article gives an overview of experimental and computational techniques for identifying and analyzing protein phosphorylation on a systems level.

A predictive phosphorylation signature of lung cancer

Background

Aberrant activation of signaling pathways drives many of the fundamental biological processes that accompany tumor initiation and progression. Inappropriate phosphorylation of intermediates in these signaling pathways are a frequently observed molecular lesion that accompanies the undesirable activation or repression of pro- and anti-oncogenic pathways. Therefore, methods which directly query signaling pathway activation via phosphorylation assays in individual cancer biopsies are expected to provide important insights into the molecular “logic” that distinguishes cancer and normal tissue on one hand, and enables personalized intervention strategies on the other.

Results

We first document the largest available set of tyrosine phosphorylation sites that are, individually, differentially phosphorylated in lung cancer, thus providing an immediate set of drug targets. Next, we develop a novel computational methodology to identify pathways whose phosphorylation activity is strongly correlated with the lung cancer phenotype. Finally, we demonstrate the feasibility of classifying lung cancers based on multi-variate phosphorylation signatures.

Conclusions

Highly predictive and biologically transparent phosphorylation signatures of lung cancer provide evidence for the existence of a robust set of phosphorylation mechanisms (captured by the signatures) present in the majority of lung cancers, and that reliably distinguish each lung cancer from normal. This approach should improve our understanding of cancer and help guide its treatment, since the phosphorylation signatures highlight proteins and pathways whose phosphorylation should be inhibited in order to prevent unregulated proliferation.

Sensitive multiplexed analysis of kinase activities and activity-based kinase identification

Constitutive activation of one or more kinase signaling pathways is a hallmark of many cancers. Here we extend the previously described mass spectrometry-based KAYAK approach by monitoring kinase activities from multiple signaling pathways simultaneously. This improved single-reaction strategy, which quantifies the phosphorylation of 90 synthetic peptides in a single mass spectrometry run, is compatible with nanogram to microgram amounts of cell lysate. Furthermore, the approach enhances kinase monospecificity through substrate competition effects, faithfully reporting the signatures of many signaling pathways after mitogen stimulation or of basal pathway activation differences across a panel of well-studied cancer cell lines. Hierarchical clustering of activities from related experiments groups peptides phosphorylated by similar kinases together and, when combined with pathway alteration using pharmacological inhibitors, distinguishes underlying differences in potency, off-target effects and genetic backgrounds. Finally, we introduce a strategy to identify the kinase, and even associated protein complex members, responsible for phosphorylation events of interest.

Quantitative phosphoproteomic analysis of T cell receptor signaling reveals system-wide modulation of protein-protein interactions

Protein phosphorylation events during T cell receptor (TCR) signaling control the formation of complexes among proteins proximal to the TCR, the activation of kinase cascades, and the activation of transcription factors; however, the mode and extent of the influence of phosphorylation in coordinating the diverse phenomena associated with T cell activation are unclear. Therefore, we used the human Jurkat T cell leukemia cell line as a model system and performed large-scale quantitative phosphoproteomic analyses of TCR signaling. We identified 10,665 unique phosphorylation sites, of which 696 showed TCR-responsive changes. In addition, we analyzed broad trends in phosphorylation data sets to uncover underlying mechanisms associated with T cell activation. We found that, upon stimulation of the TCR, phosphorylation events extensively targeted protein modules involved in all of the salient phenomena associated with T cell activation: patterning of surface proteins, endocytosis of the TCR, formation of the F-actin cup, inside-out activation of integrins, polarization of microtubules, production of cytokines, and alternative splicing of messenger RNA. Further, case-by-case analysis of TCR-responsive phosphorylation sites on proteins belonging to relevant functional modules together with network analysis allowed us to deduce that serine-threonine (S-T) phosphorylation modulated protein-protein interactions (PPIs) in a system-wide fashion. We also provide experimental support for this inference by showing that phosphorylation of tubulin on six distinct serine residues abrogated PPIs during the assembly of microtubules. We propose that modulation of PPIs by stimulus-dependent changes in S-T phosphorylation state is a widespread phenomenon applicable to many other signaling systems.

Protein tyrosine kinase characterization based on fully automated synthesis of (phospho) peptide arrays in microplates

In view of the importance of information transfer mediated throughout the cell by recognition, phosphorylation or dephosphorylation of kinases, their adapters, or substrates, this method was developed. The method provides a potent research tool for rapidly generating and testing these substrates as modeled by synthetic peptide arrays. The peptides or phosphorylated peptides are automatically generated on the inner surfaces of microplate wells, covalently linked to a polylysine polymer so that they are in a sterically favorable conformation, immediately available for in situ testing. Products up to 18 amino acids long have shown excellent mass spectral homogeneity. Thus, determinate peptide libraries can be ready for testing in as little as 2 days after the conception of an experiment. The process can be easily automated using robotic liquid handlers and is extremely rapid, sensitive, and economical. Optionally, the method can be upgraded to a higher throughput level using more powerful workstations with greater capacity, such as the Biomek FX, or any similar robotics capable of transfer-from-file logic to guide synthesis cycles.

Distal recognition sites in substrates are required for efficient phosphorylation by the cAMP-dependent protein kinase

Protein kinases are important mediators of signal transduction in eukaryotic cells, and identifying the substrates of these enzymes is essential for a complete understanding of most signaling networks. In this report, novel substrate-binding variants of the cAMP-dependent protein kinase (PKA) were used to identify substrate domains required for efficient phosphorylation in vivo. Most wild-type protein kinases, including PKA, interact only transiently with their substrates. The substrate domains identified were distal to the sites of phosphorylation and were found to interact with a C-terminal region of PKA that was itself removed from the active site. Only a small set of PKA alterations resulted in a stable association with substrates, and the identified residues were clustered together within the hydrophobic core of this enzyme. Interestingly, these residues stretched from the active site of the enzyme to the C-terminal substrate-binding domain identified here. This spatial organization is conserved among the entire eukaryotic protein kinase family, and alteration of these residues in a second, unrelated protein kinase also resulted in a stable association with substrates. In all, this study identified distal sites in PKA substrates that are important for recognition by this enzyme and suggests that the interaction of these domains with PKA might influence specific aspects of substrate binding and/or release.

Akt phosphorylation of La regulates specific mRNA translation in glial progenitors

The Akt signaling pathway activity increases as normal tissue progresses to malignant transformation, and regulates the translation of specific messenger RNAs (mRNAs) through multiple mechanisms. We have identified one such mechanism of Akt-dependent translation control as involving the lupus autoantigen La. La is an RNA-associated protein that contains multiple trafficking elements to support the interaction with RNAs in different subcellular locations. We show here that the La protein is a direct target of the serine/threonine protein kinase Akt on threonine 301, and La nuclear export in mouse glial progenitors, as well as its association with polysomes is modulated by Akt activity. Using a functional approach to determine the network of genes affected by La in the cytoplasm by microarray analysis of polysome-bound mRNAs, we found that La binds 34% of the polysome bound mRNAs and regulates the expression of a specific pool of mRNAs under KRas/Akt activation. Therefore, La appears to be an important contributor to Akt-mediated translational regulation of these transcripts in murine glial cells.

Identification of proteins that interact with catalytically active calcium-dependent protein kinases from Arabidopsis

Calcium-dependent protein kinases (CDPKs) are essential sensor-transducers of calcium signaling pathways in plants. Functional characterization of CDPKs is of great interest because they play important roles during growth, development, and in response to a wide range of environmental stimuli. The Arabidopsis genome encodes 34 CDPKs, but very few substrates of these enzymes have been identified. In this study, we exploited the unique characteristics of CDPKs to develop an efficient approach for the discovery of CDPK-interacting proteins. High-throughput, semi-automated yeast two-hybrid interaction screens with two different cDNA libraries each containing 18 million prey clones were performed using catalytically impaired and constitutively active AtCPK4 and AtCPK11 variants as baits. The use of the constitutively active versions of the CPK baits improved the recovery of positive interacting proteins relative to the wild type kinase. Titration of interaction strength by growth under increasing concentrations of 3-aminotriazole (3-AT), a histidine analog and competitive inhibitor of the His3 gene product, confirmed these results. Possible mechanisms for this observed improvement are discussed. The reproducibility of this approach was assessed by the overlap of several interacting proteins of AtCPK4 and AtCPK11 and the recovery of several putative substrates and indicated that yeast two-hybrid screens using constitutively active and/or catalytically impaired forms of CDPK provides a useful tool to identify potential substrates of the CDPK family and potentially the entire protein kinase superfamily.

Tuning a three-component reaction for trapping kinase substrate complexes

The upstream protein kinases responsible for thousands of phosphorylation events in the phosphoproteome remain to be discovered. We developed a three-component chemical reaction which converts the transient noncovalent substrate-kinase complex into a covalently cross-linked product by utilizing a dialdehyde-based cross-linker, 1. Unfortunately, the reaction of 1 with a lysine in the kinase active site and an engineered cysteine on the substrate to form an isoindole cross-linked product could not be performed in the presence of competing cellular proteins due to nonspecific side reactions. In order to more selectively target the cross-linker to protein kinases in cell lysates, we replaced the weak, kinase-binding adenosine moiety of 1 with a potent protein kinase inhibitor scaffold. In addition, we replaced the o-phthaldialdehyde moiety in 1 with a less-reactive thiophene-2,3-dicarboxaldehyde moiety. The combination of these two structural modifications provides for cross-linking of a cysteine-containing substrate to its corresponding kinase in the presence of competing cellular proteins.

Predicting protein post-translational modifications using meta-analysis of proteome scale data sets

Protein post-translational modifications are an important biological regulatory mechanism, and the rate of their discovery using high throughput techniques is rapidly increasingly. To make use of this wealth of sequence data, we introduce a new general strategy designed to predict a variety of post-translational modifications in several organisms. We used the motif-x program to determine phosphorylation motifs in yeast, fly, mouse, and man and lysine acetylation motifs in man. These motifs were then scanned against proteomic sequence data using a newly developed tool called scan-x to globally predict other potential modification sites within these organisms. 10-fold cross-validation was used to determine the sensitivity and minimum specificity for each set of predictions, all of which showed improvement over other available tools for phosphoprediction. New motif discovery is a byproduct of this approach, and the phosphorylation motif analyses provide strong evidence of evolutionary conservation of both known and novel kinase motifs.

Covalent capture of kinase-specific phosphopeptides reveals Cdk1-cyclin B substrates

We describe a method for rapid identification of protein kinase substrates. Cdk1 was engineered to accept an ATP analog that allows it to uniquely label its substrates with a bio-orthogonal phosphate analog tag. A highly specific, covalent capture-and-release methodology was developed for rapid purification of tagged peptides derived from labeled substrate proteins. Application of this approach to the discovery of Cdk1-cyclin B substrates yielded identification of >70 substrates and phosphorylation sites. Many of these sites are known to be phosphorylated in vivo, but most of the proteins have not been characterized as Cdk1-cyclin B substrates. This approach has the potential to expand our understanding of kinase-substrate connections in signaling networks.

Profiling of UV-induced ATM/ATR signaling pathways

To ensure survival in the face of genomic insult, cells have evolved complex mechanisms to respond to DNA damage, termed the DNA damage checkpoint. The serine/threonine kinases ataxia telangiectasia-mutated (ATM) and ATM and Rad3-related (ATR) activate checkpoint signaling by phosphorylating substrate proteins at SQ/TQ motifs. Although some ATM/ATR substrates (Chk1, p53) have been identified, the lack of a more complete list of substrates limits current understanding of checkpoint pathways. Here, we use immunoaffinity phosphopeptide isolation coupled with mass spectrometry to identify 570 sites phosphorylated in UV-damaged cells, 498 of which are previously undescribed. Semiquantitative analysis yielded 24 known and 192 previously uncharacterized sites differentially phosphorylated upon UV damage, some of which were confirmed by SILAC, Western blotting, and immunoprecipitation/Western blotting. ATR-specific phosphorylation was investigated by using a Seckel syndrome (ATR mutant) cell line. Together, these results provide a rich resource for further deciphering ATM/ATR signaling and the pathways mediating the DNA damage response.

Dissecting kinase signaling pathways

Aberrant protein kinase signaling is a hallmark of many human diseases including cancer, diabetes, and neurological disorders. Kinase inhibitors have shown to be successful at treating some of these diseases, implying that understanding kinase signaling pathways may lead to additional, non-kinase drug targets. However, identifying substrates of protein kinases is difficult due to the universality of the chemical mechanism kinases utilize and the ability of multiple kinases to phosphorylate the same protein substrates. In this review, we explore the advantages and disadvantages of several techniques for identifying kinase substrates. Once putative substrates are identified, their validation as physiological substrates remains a major challenge. We propose three criteria for confirming the physiological relevance of a putative substrate's interaction with a kinase.

Clustering of phosphorylation site recognition motifs can be exploited to predict the targets of cyclin-dependent kinase

Protein kinases are critical to cellular signalling and post-translational gene regulation, but their biological substrates are difficult to identify. We show that cyclin-dependent kinase (CDK) consensus motifs are frequently clustered in CDK substrate proteins. Based on this, we introduce a new computational strategy to predict the targets of CDKs and use it to identify new biologically interesting candidates. Our data suggest that regulatory modules may exist in protein sequence as clusters of short sequence motifs.

Reconstitution of modular PDK1 functions on trans-splicing of the regulatory PH and catalytic kinase domains

The serine-threonine protein kinases PDK1 and PKB each contain a pleckstrin homology (PH) domain that binds the membrane-bound phosphatidylinositol 3,4,5-triphosphate [PI(3,4,5)P3] second messenger and is required for PDK1-catalyzed phosphorylation and activation of PKB. While X-ray structures have been reported for the individual regulatory PH and catalytic kinase domain constructs of both PDK1 and PKB, diffraction quality crystals of full length constructs have yet to be obtained, likely due to conformational heterogeneity. In developing alternative approaches to understanding the potential role of conformational dynamics in regulating PKB phosphorylation by PDK1, an efficient in vitro method for protein trans-splicing was developed, which utilizes the N- and C-terminal split inteins of the gene dnaE from Nostoc punctiforme [(N)NpuDnaE] and Synechocystis sp. strain PCC6803 [(C)SspDnaE], respectively. For conjugating the regulatory PH domain to the catalytic kinase domain of PDK1, the recombinant trans-splicing fusion constructs KINASE(AEY)-(N)NpuDnaE-His6 and GST-His6-(C)SspDnaE-(CMN)PH were designed, PCR assembled, overexpressed, and affinity purified. The cross-reacting (N)NpuDnaE and (C)SspDnaE inteins generated full length spliced-PDK1 with kobs = (2.8 +/- 0.3) x 10(-5) s(-1) and with < or =5% of any competing trans-cleavage reactions. Spliced-PDK1 was efficiently purified to > or =95% homogeneity from the reaction mixture by subsequent His6 affinity and ion exchange chromatography steps. In vitro kinase assays and phosphopeptide mapping studies confirmed that spliced-PDK1 retained the ability to colocalize and selectively phosphorylate Thr-309 of PKBbeta in a PI(3,4,5)P3-dependent manner. The high-level production and reconstitution of functional spliced-PDK1 establishes the feasibility of incorporating domain-specific biophysical probes for spectroscopic studies of regulatory PH domain mediated catalytic specificity.

Differential phosphorylation of NG2 proteoglycan by ERK and PKCalpha helps balance cell proliferation and migration

Two distinct Thr phosphorylation events within the cytoplasmic domain of the NG2 proteoglycan help regulate the cellular balance between proliferation and motility. Protein kinase Calpha mediates the phosphorylation of NG2 at Thr2256, resulting in enhanced cell motility. Extracellular signal-regulated kinase phosphorylates NG2 at Thr2314, stimulating cell proliferation. The effects of NG2 phosphorylation on proliferation and motility are dependent on beta1-integrin activation. Differential cell surface localization of the two distinctly phosphorylated forms of NG2 may be the mechanism by which the NG2-beta1-integrin interaction promotes proliferation in one case and motility in the other. NG2 phosphorylated at Thr2314 colocalizes with beta1-integrin on microprotrusions from the apical cell surface. In contrast, NG2 phosphorylated at Thr2256 colocalizes with beta1-integrin on lamellipodia at the leading edges of cells. Thus, phosphorylation and the resulting site of NG2-integrin localization may determine the specific downstream effects of integrin signaling.