The SARS-CoV-2 Nsp3 macrodomain reverses PARP9/DTX3L-dependent ADP-ribosylation induced by interferon signaling.

SARS-CoV-2 nonstructural protein 3 (Nsp3) contains a macrodomain that is essential for coronavirus pathogenesis and is thus an attractive target for drug development. This macrodomain is thought to counteract the host interferon (IFN) response, an important antiviral signalling cascade, via the reversal of protein ADP-ribosylation, a posttranslational modification catalyzed by host poly(ADP-ribose) polymerases (PARPs). However, the main cellular targets of the coronavirus macrodomain that mediate this effect are currently unknown. Here, we use a robust immunofluorescence-based assay to show that activation of the IFN response induces ADP-ribosylation of host proteins and that ectopic expression of the SARS-CoV-2 Nsp3 macrodomain reverses this modification in human cells. We further demonstrate that this assay can be used to screen for on-target and cell-active macrodomain inhibitors. This IFN-induced ADP-ribosylation is dependent on PARP9 and its binding partner DTX3L, but surprisingly the expression of the Nsp3 macrodomain or the deletion of either PARP9 or DTX3L does not impair IFN signaling or the induction of IFN-responsive genes. Our results suggest that PARP9/DTX3L-dependent ADP-ribosylation is a downstream effector of the host IFN response and that the cellular function of the SARS-CoV-2 Nsp3 macrodomain is to hydrolyze this end product of IFN signaling, rather than to suppress the IFN response itself.

Viral infection and smell loss: The case of COVID-19.

Olfactory disorders have been increasingly reported in individuals infected with SARS-CoV-2, the virus causing the coronavirus disease 2019 (COVID-19). Losing the sense of smell has a strong impact on the quality of life, since it may lead to malnutrition, weight loss, food poisoning, depression, and exposure to dangerous chemicals. Individuals who suffer from anosmia (inability to smell) also cannot sense the flavor of food, which is a combination of taste and smell. Interestingly, infected individuals have reported sudden loss of smell with no congested nose, as is frequently observed in common colds or other upper respiratory tract infections. These observations suggest that SARS-CoV-2 infection leads to olfactory loss through a distinct mechanism, which is still unclear. This article provides an overview of olfactory loss and the recent findings relating to COVID-19. Possible mechanisms of SARS-CoV-2-induced olfactory loss are also discussed.

Epidermal growth factor (EGF) triggers nuclear calcium signaling through the intranuclear phospholipase Cδ-4 (PLCδ4).

Calcium (Ca2+) signaling within the cell nucleus regulates specific cellular events such as gene transcription and cell proliferation. Nuclear and cytosolic Ca2+ levels can be independently regulated, and nuclear translocation of receptor tyrosine kinases (RTKs) is one way to locally activate signaling cascades within the nucleus. Nuclear RTKs, including the epidermal growth factor receptor (EGFR), are important for processes such as transcriptional regulation, DNA-damage repair, and cancer therapy resistance. RTKs can hydrolyze phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) within the nucleus, leading to Ca2+ release from the nucleoplasmic reticulum by inositol 1,4,5-trisphosphate receptors. PI(4,5)P2 hydrolysis is mediated by phospholipase C (PLC). However, it is unknown which nuclear PLC isoform is triggered by EGFR. Here, using subcellular fractionation, immunoblotting and fluorescence, siRNA-based gene knockdowns, and FRET-based biosensor reporter assays, we investigated the role of PLCδ4 in epidermal growth factor (EGF)-induced nuclear Ca2+ signaling and downstream events. We found that EGF-induced Ca2+ signals are inhibited when translocation of EGFR is impaired. Nuclear Ca2+ signals also were reduced by selectively buffering inositol 1,4,5-trisphosphate (InsP3) within the nucleus. EGF induced hydrolysis of nuclear PI(4,5)P2 by the intranuclear PLCδ4, rather than by PLCγ1. Moreover, protein kinase C, a downstream target of EGF, was active in the nucleus of stimulated cells. Furthermore, PLCδ4 and InsP3 modulated cell cycle progression by regulating the expression of cyclins A and B1. These results provide evidence that EGF-induced nuclear signaling is mediated by nuclear PLCδ4 and suggest new therapeutic targets to modulate the proliferative effects of this growth factor.

FBXO25 regulates MAPK signaling pathway through inhibition of ERK1/2 phosphorylation.

The FBXO25 mediates degradation of ELK-1 and thus inhibits transcriptional activation of immediate early genes (iEG). Here we show that FBXO25 regulates yet another node of this signaling pathway, by decreasing MAPK/ERK activity. We show that induction of FBXO25 reduced ERK1/2 phosphorylation independently of MEK1/2. Accordingly, in HAP1 FBXO25 knockout cells (FBXO25KO), we observed that upon PMA treatment ERK1/2 was more active than in parental cells. An increase in cell proliferation under receptor mediated activation of the ERK signaling pathway in FBXO25KO cells was also observed. Taken together we show that FBXO25 functions as a negative regulator of MAPK signaling though the reduction of ERK1/2 activation.

Structural shifts of aldehyde dehydrogenase enzymes were instrumental for the early evolution of retinoid-dependent axial patterning in metazoans.

Aldehyde dehydrogenases (ALDHs) catabolize toxic aldehydes and process the vitamin A-derived retinaldehyde into retinoic acid (RA), a small diffusible molecule and a pivotal chordate morphogen. In this study, we combine phylogenetic, structural, genomic, and developmental gene expression analyses to examine the evolutionary origins of ALDH substrate preference. Structural modeling reveals that processing of small aldehydes, such as acetaldehyde, by ALDH2, versus large aldehydes, including retinaldehyde, by ALDH1A is associated with small versus large substrate entry channels (SECs), respectively. Moreover, we show that metazoan ALDH1s and ALDH2s are members of a single ALDH1/2 clade and that during evolution, eukaryote ALDH1/2s often switched between large and small SECs after gene duplication, transforming constricted channels into wide opened ones and vice versa. Ancestral sequence reconstructions suggest that during the evolutionary emergence of RA signaling, the ancestral, narrow-channeled metazoan ALDH1/2 gave rise to large ALDH1 channels capable of accommodating bulky aldehydes, such as retinaldehyde, supporting the view that retinoid-dependent signaling arose from ancestral cellular detoxification mechanisms. Our analyses also indicate that, on a more restricted evolutionary scale, ALDH1 duplicates from invertebrate chordates (amphioxus and ascidian tunicates) underwent switches to smaller and narrower SECs. When combined with alterations in gene expression, these switches led to neofunctionalization from ALDH1-like roles in embryonic patterning to systemic, ALDH2-like roles, suggesting functional shifts from signaling to detoxification.

FERM domain interaction with myosin negatively regulates FAK in cardiomyocyte hypertrophy.

Focal adhesion kinase (FAK) regulates cellular processes that affect several aspects of development and disease. The FAK N-terminal FERM (4.1 protein-ezrin-radixin-moesin homology) domain, a compact clover-leaf structure, binds partner proteins and mediates intramolecular regulatory interactions. Combined chemical cross-linking coupled to MS, small-angle X-ray scattering, computational docking and mutational analyses showed that the FAK FERM domain has a molecular cleft (~998 Å(2)) that interacts with sarcomeric myosin, resulting in FAK inhibition. Accordingly, mutations in a unique short amino acid sequence of the FERM myosin cleft, FP-1, impaired the interaction with myosin and enhanced FAK activity in cardiomyocytes. An FP-1 decoy peptide selectively inhibited myosin interaction and increased FAK activity, promoting cardiomyocyte hypertrophy through activation of the AKT-mammalian target of rapamycin pathway. Our findings uncover an inhibitory interaction between the FAK FERM domain and sarcomeric myosin that presents potential opportunities to modulate the cardiac hypertrophic response through changes in FAK activity.

Chromatin Immunoprecipitation on Fixed Tissues and Cell Lines.

Chromatin immunoprecipitation (ChIP) has been used over three decades to characterize protein-DNA interactions in cells and tissues. The samples are initially cross-linked with formaldehyde and subjected to lysis with detergents followed by sonication to generate smaller fragments of DNA covalently bound to nuclear proteins. Antibodies against the protein of interest are then used to immunoprecipitate the protein-DNA complex, allowing for the purification of the genomic locations bound to the protein of interest. The DNA can then be analyzed by several techniques such as PCR, quantitative PCR, DNA microarrays, and next-generation sequencing (NGS). ChIP can be used in cell culture systems and animal tissues to provide insights into how the identity of cells and tissues is established, and how transcriptional programs can be disrupted or reactivated in disease states.

Odorant receptors as potential drug targets.

Odorant receptors (ORs) belong to a large family of G protein-coupled receptors (GPCRs) that are highly expressed by olfactory sensory neurons of the nose. Accumulating evidence indicates that they are also expressed in a variety of nonolfactory tissues, which makes them new potential drug targets. Here we discuss the challenges and strategies to target these receptors.

DNA methylation of the promoter region at the CREB1 binding site is a mechanism for the epigenetic regulation of brain-specific PKMζ.

Protein kinase M zeta, PKMζ, is a brain enriched kinase with a well characterized role in Long-Term Potentiation (LTP), the activity-dependent strengthening of synapses involved in long-term memory formation. However, little is known about the molecular mechanisms that maintain the tissue specificity of this kinase. Here, we characterized the epigenetic factors, mainly DNA methylation, regulating PKMζ expression in the human brain. The PRKCZ gene has an upstream promoter regulating Protein kinase C ζ (PKCζ), and an internal promoter driving PKMζ expression. A demethylated region, including a canonical CREB binding site, situated at the internal promoter was only observed in human CNS tissues. The induction of site-specific hypermethylation of this region resulted in decreased CREB1 binding and downregulation of PKMζ expression. Noteworthy, CREB binding sites were absent in the upstream promoter of PRKCZ locus, suggesting a specific mechanism for regulating PKMζ expression. These observations were validated using a system of human neuronal differentiation from induced pluripotent stem cells (iPSCs). CREB1 binding at the internal promoter was detected only in differentiated neurons, where PKMζ is expressed. The same epigenetic mechanism in the context of CREB binding site was identified in other genes involved in neuronal differentiation and LTP. Additionally, aberrant DNA hypermethylation at the internal promoter was observed in cases of Alzheimer's disease, correlating with decreased expression of PKMζ in patient brains. Altogether, we present a conserved epigenetic mechanism regulating PKMζ expression and other genes enhanced in the CNS with possible implications in neuronal differentiation and Alzheimer's disease.

Retinoic acid and VEGF delay smooth muscle relative to endothelial differentiation to coordinate inner and outer coronary vessel wall morphogenesis.

RATIONALE: Major coronary vessels derive from the proepicardium, the cellular progenitor of the epicardium, coronary endothelium, and coronary smooth muscle cells (CoSMCs). CoSMCs are delayed in their differentiation relative to coronary endothelial cells (CoEs), such that CoSMCs mature only after CoEs have assembled into tubes. The mechanisms underlying this sequential CoE/CoSMC differentiation are unknown. Retinoic acid (RA) is crucial for vascular development and the main RA-synthesizing enzyme is progressively lost from epicardially derived cells as they differentiate into blood vessel types. In parallel, myocardial vascular endothelial growth factor (VEGF) expression also decreases along coronary vessel muscularization. OBJECTIVE: We hypothesized that RA and VEGF act coordinately as physiological brakes to CoSMC differentiation. METHODS AND RESULTS: In vitro assays (proepicardial cultures, cocultures, and RALDH2 [retinaldehyde dehydrogenase-2]/VEGF adenoviral overexpression) and in vivo inhibition of RA synthesis show that RA and VEGF act as repressors of CoSMC differentiation, whereas VEGF biases epicardially derived cell differentiation toward the endothelial phenotype. CONCLUSION: Experiments support a model in which early high levels of RA and VEGF prevent CoSMC differentiation from epicardially derived cells before RA and VEGF levels decline as an extensive endothelial network is established. We suggest this physiological delay guarantees the formation of a complex, hierarchical, tree of coronary vessels.

Identification of εPKC targets during cardiac ischemic injury.

BACKGROUND: Epsilon-protein kinase C (εPKC) protects the heart from ischemic injury. However, the mechanism(s) of εPKC cardioprotection is still unclear. Identification of the εPKC targets may aid in elucidating the εPKC-mediated cardioprotective mechanisms. Previous studies, using εPKC transgenic mice and difference in gel electrophoresis, identified proteins involved in glucose metabolism, the expression of which was modified by εPKC. Those studies were accompanied by metabolomic analysis, suggesting that increased glucose oxidation may be responsible for the cardioprotective effect of εPKC. Whether these εPKC-mediated alterations were because of differences in protein expression or phosphorylation was not determined. METHODS AND RESULTS: In the present study, we used an εPKC -specific activator peptide, ψεRACK, combined with phosphoproteomics, to find εPKC targets, and identified that the proteins whose phosphorylation was altered by selective activation of εPKC were mostly mitochondrial proteins. Analysis of the mitochondrial phosphoproteome led to the identification of 55 spots, corresponding to 37 individual proteins, exclusively phosphorylated, in the presence of ψεRACK. The majority of the proteins identified were involved in glucose and lipid metabolism, components of the respiratory chain as well as mitochondrial heat shock proteins. CONCLUSIONS: The protective effect of εPKC during ischemia involves phosphorylation of several mitochondrial proteins involved in glucose and lipid metabolism and oxidative phosphorylation. Regulation of these metabolic pathways by εPKC phosphorylation may lead to εPKC-mediated cardioprotection induced by ψεRACK.

Structural analysis of TrkA mutations in patients with congenital insensitivity to pain reveals PLCγ as an analgesic drug target.

Chronic pain is a major health issue, and the search for new analgesics has become increasingly important because of the addictive properties and unwanted side effects of opioids. To explore potentially new drug targets, we investigated mutations in the NTRK1 gene found in individuals with congenital insensitivity to pain with anhidrosis (CIPA). NTRK1 encodes tropomyosin receptor kinase A (TrkA), the receptor for nerve growth factor (NGF) and that contributes to nociception. Molecular modeling and biochemical analysis identified mutations that decreased the interaction between TrkA and one of its substrates and signaling effectors, phospholipase Cγ (PLCγ). We developed a cell-permeable phosphopeptide derived from TrkA (TAT-pQYP) that bound the Src homology domain 2 (SH2) of PLCγ. In HEK-293T cells, TAT-pQYP inhibited the binding of heterologously expressed TrkA to PLCγ and decreased NGF-induced, TrkA-mediated PLCγ activation and signaling. In mice, intraplantar administration of TAT-pQYP decreased mechanical sensitivity in an inflammatory pain model, suggesting that targeting this interaction may be analgesic. The findings demonstrate a strategy to identify new targets for pain relief by analyzing the signaling pathways that are perturbed in CIPA.

Immunodominant antibody responses directed to SARS-CoV-2 hotspot mutation sites and risk of immune escape.

Introduction: Considering the likely need for the development of novel effective vaccines adapted to emerging relevant CoV-2 variants, the increasing knowledge of epitope recognition profile among convalescents and afterwards vaccinated with identification of immunodominant regions may provide important information. Methods: We used an RBD peptide microarray to identify IgG and IgA binding regions in serum of 71 COVID-19 convalescents and 18 vaccinated individuals. Results: We found a set of immunodominant RBD antibody epitopes, each recognized by more than 30% of the tested cohort, that differ among the two different groups and are within conserved regions among betacoronavirus. Of those, only one peptide, P44 (S415-429), recognized by 68% of convalescents, presented IgG and IgA antibody reactivity that positively correlated with nAb titers, suggesting that this is a relevant RBD region and a potential target of IgG/IgA neutralizing activity. Discussion: This peptide is localized within the area of contact with ACE-2 and harbors the mutation hotspot site K417 present in gamma (K417T), beta (K417N), and omicron (K417N) variants of concern. The epitope profile of vaccinated individuals differed from convalescents, with a more diverse repertoire of immunodominant peptides, recognized by more than 30% of the cohort. Noteworthy, immunodominant regions of recognition by vaccinated coincide with mutation sites at Omicron BA.1, an important variant emerging after massive vaccination. Together, our data show that immune pressure induced by dominant antibody responses may favor hotspot mutation sites and the selection of variants capable of evading humoral response.

Olfactory Dysfunction in Frontline Health Care Professionals During COVID-19 Pandemic in Brazil.

Upper respiratory viral infections can decrease the sense of smell either by inflammatory restriction of nasal airflow that carries the odorant molecules or through interference in olfactory sensory neuron function. During the coronavirus disease 2019 (COVID-19) pandemic, triggered by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), worldwide reports of severe smell loss (anosmia/hyposmia) revealed a different type of olfactory dysfunction associated with respiratory virus infection. Since self-reported perception of smell is subjective and SARS-CoV-2 exposure is variable in the general population, we aimed to study a population that would be more homogeneously exposed to the virus. Here, we investigated the prevalence of olfactory loss in frontline health professionals diagnosed with COVID-19 in Brazil, one of the major epicenters of the disease. We also analyzed the rate of olfactory function recovery and the particular characteristics of olfactory deficit in this population. A widely disclosed cross-sectional online survey directed to health care workers was developed by a group of researchers to collect data concerning demographic information, general symptoms, otolaryngological symptoms, comorbidities, and COVID-19 test results. Of the 1,376 health professionals who completed the questionnaire, 795 (57.8%) were working directly with COVID-19 patients, either in intensive care units, emergency rooms, wards, outpatient clinics, or other areas. Five-hundred forty-one (39.3%) participants tested positive for SARS-CoV-2, and 509 (37%) were not tested. Prevalence of olfactory dysfunction in COVID-19-positive subjects was 83.9% (454 of 541) compared to 12.9% (42 of 326) of those who tested negative and to 14.9% (76 of 509) of those not tested. Olfactory dysfunction incidence was higher in those working in wards, emergency rooms, and intensive care units compared to professionals in outpatient clinics. In general, remission from olfactory symptoms was frequent by the time of responses. Taste disturbances were present in 74.1% of infected participants and were significantly associated with hyposmia. In conclusion, olfactory dysfunction is highly correlated with exposure to SARS-CoV-2 in health care professionals, and remission rates up to 2 weeks are high.

High-throughput screening and validation of antibodies against synaptic proteins to explore opioid signaling dynamics.

Antibodies represent powerful tools to examine signal transduction pathways. Here, we present a strategy integrating multiple state-of-the-art methods to produce, validate, and utilize antibodies. Focusing on understudied synaptic proteins, we generated 137 recombinant antibodies. We used yeast display antibody libraries from the B cells of immunized rabbits, followed by FACS sorting under stringent conditions to identify high affinity antibodies. The antibodies were validated by high-throughput functional screening, and genome editing. Next, we explored the temporal dynamics of signaling in single cells. A subset of antibodies targeting opioid receptors were used to examine the effect of treatment with opiates that have played central roles in the worsening of the 'opioid epidemic.' We show that morphine and fentanyl exhibit differential temporal dynamics of receptor phosphorylation. In summary, high-throughput approaches can lead to the identification of antibody-based tools required for an in-depth understanding of the temporal dynamics of opioid signaling.

Phosphoproteomics profiling suggests a role for nuclear ßΙPKC in transcription processes of undifferentiated murine embryonic stem cells.

Protein kinase C (PKC) plays a key role in embryonic stem cell (ESC) proliferation, self-renewal, and differentiation. However, the function of specific PKC isoenzymes have yet to be determined. Of the PKCs expressed in undifferentiated ESCs, ßIPKC was the only isoenzyme abundantly expressed in the nuclei. To investigate the role of ßΙPKC in these cells, we employed a phosphoproteomics strategy and used two classical (cPKC) peptide modulators and one ßIPKC-specific inhibitor peptide. We identified 13 nuclear proteins that are direct or indirect ßΙPKC substrates in undifferentiated ESCs. These proteins are known to be involved in regulating transcription, splicing, and chromatin remodeling during proliferation and differentiation. Inhibiting ßΙPKC had no effect on DNA synthesis in undifferentiated ESCs. However, upon differentiation, many cells seized to express ßΙPKC and ßΙPKC was frequently found in the cytoplasm. Taken together, our results suggest that ßIPKC takes part in the processes that maintain ESCs in their undifferentiated state.

Specific modulation of protein kinase activity via small peptides.

Protein kinases are key drug targets for several diseases. However, despite large efforts, the development of drugs that are specific for these kinases has been a great challenge. The discovery that protein-protein interactions can be mediated by small linear sequences has invigorated the bioinformatics field and there are increasing efforts to create computer programs that identify and characterize linear sequences responsible for protein-protein interactions. The development of peptides as modulators of target protein activity is facilitated by in silico approaches. There are numerous applications for peptide modulators of protein-protein interactions and peptide modulators of kinase activity. Peptide modulators may be used either as tools to elucidate specific signaling pathways, as drug leads, or as drugs themselves. In the present review we will discuss the development of such peptide modulators and some of their applications.

Exploring Morphine-Triggered PKC-Targets and Their Interaction with Signaling Pathways Leading to Pain via TrkA.

It is well accepted that treatment of chronic pain with morphine leads to µ opioid receptor (MOR) desensitization and the development of morphine tolerance. MOR activation by the selective peptide agonist, D-Ala2, N-MePhe4, Gly-ol]-enkephalin(DAMGO), leads to robust G protein receptor kinase activation, ß-arrestin recruitment, and subsequent receptor endocytosis, which does not occur in an activation by morphine. However, MOR activation by morphine induces receptor desensitization, in a Protein kinase C (PKC) dependent manner. PKC inhibitors have been reported to decrease receptor desensitization, reduce opiate tolerance, and increase analgesia. However, the exact role of PKC in these processes is not clearly delineated. The difficulties in establishing a particular role for PKC have been, in part, due to the lack of reagents that allow the selective identification of PKC targets. Recently, we generated a conformation state-specific anti-PKC antibody that preferentially recognizes the active state of this kinase. Using this antibody to selectively isolate PKC substrates and a proteomics strategy to establish the identity of the proteins, we examined the effect of morphine treatment on the PKC targets. We found an enhanced interaction of a number of proteins with active PKC, in the presence of morphine. In this article, we discuss the role of these proteins in PKC-mediated MOR desensitization and analgesia. In addition, we posit a role for some of these proteins in mediating pain by TrKA activation, via the activation of transient receptor potential cation channel subfamily V member 1 (TRPV1). Finally, we discuss how these new PKC interacting proteins and pathways could be targeted for the treatment of pain.

Generating Conformation-Specific Polyclonal and Monoclonal Anti-Protein Kinase C Antibodies and Anti-Active State Specific PKC Antibodies.

The protein kinase C (PKC) family of serine/ threonine kinases has been shown to play active roles as either suppressors or promoters of carcinogenesis in different types of tumors. Using antibodies that preferentially recognize the active conformation of classical PKCs (cPKCs), we have previously shown that in breast cancer samples the expression levels of cPKCs were similar in estrogen receptor-positive (ER+ ) as compared to triple-negative tumors; however, the levels of active cPKCs were different. Determining the activation status of PKCs and other kinases in tumors may thus aid therapeutic decisions. Further, in basic science these tools may be used to understand the spatial and temporal dynamics of PKC signaling under different stimuli and for co-immunoprecipitation studies to detect binding partners and substrates of active cPKCs. In this article, we describe how monoclonal and polyclonal anti-active state PKC antibodies can be obtained using rational approaches to select bona fide epitopes through inspection of the crystal structure of classical PKCs coupled to molecular modeling studies. We believe that this methodology can be used for other kinases and multi-domain enzymes that undergo changes in their conformation upon activation. © 2018 by John Wiley & Sons, Inc.

Revisiting protein kinase-substrate interactions: Toward therapeutic development.

Despite the efforts of pharmaceutical companies to develop specific kinase modulators, few drugs targeting kinases have been completely successful in the clinic. This is primarily due to the conserved nature of kinases, especially in the catalytic domains. Consequently, many currently available inhibitors lack sufficient selectivity for effective clinical application. Kinases phosphorylate their substrates to modulate their activity. One of the important steps in the catalytic reaction of protein phosphorylation is the correct positioning of the target residue within the catalytic site. This positioning is mediated by several regions in the substrate binding site, which is typically a shallow crevice that has critical subpockets that anchor and orient the substrate. The structural characterization of this protein-protein interaction can aid in the elucidation of the roles of distinct kinases in different cellular processes, the identification of substrates, and the development of specific inhibitors. Because the region of the substrate that is recognized by the kinase can be part of a linear consensus motif or a nonlinear motif, advances in technology beyond simple linear sequence scanning for consensus motifs were needed. Cost-effective bioinformatics tools are already frequently used to predict kinase-substrate interactions for linear consensus motifs, and new tools based on the structural data of these interactions improve the accuracy of these predictions and enable the identification of phosphorylation sites within nonlinear motifs. In this Review, we revisit kinase-substrate interactions and discuss the various approaches that can be used to identify them and analyze their binding structures for targeted drug development.