ABSTRACT
Cell polarity networks are defined by quantitative features of their constituent feedback circuits, which must be tuned to enable robust and stable polarization, while also ensuring that networks remain responsive to dynamically changing cellular states and/or spatial cues during development. Using the PAR polarity network as a model, we demonstrate that these features are enabled by the dimerization of the polarity protein PAR-2 via its N-terminal RING domain. Combining theory and experiment, we show that dimer affinity is optimized to achieve dynamic, selective, and cooperative binding of PAR-2 to the plasma membrane during polarization. Reducing dimerization compromises positive feedback and robustness of polarization. Conversely, enhanced dimerization renders the network less responsive due to kinetic trapping of PAR-2 on internal membranes and reduced sensitivity of PAR-2 to the anterior polarity kinase, aPKC/PKC-3. Thus, our data reveal a key role for a dynamically oligomeric RING domain in optimizing interaction affinities to support a robust and responsive cell polarity network, and highlight how optimization of oligomerization kinetics can serve as a strategy for dynamic and cooperative intracellular targeting.
Subject(s)
Cell Membrane , Cell Polarity , Protein Kinase C , Protein Multimerization , Cell Membrane/metabolism , Protein Kinase C/metabolism , Animals , Protein BindingABSTRACT
Mutations in FBXO7 have been discovered to be associated with an atypical parkinsonism. We report here a new homozygous missense mutation in a paediatric patient that causes an L250P substitution in the dimerisation domain of Fbxo7. This alteration selectively ablates the Fbxo7-PI31 interaction and causes a significant reduction in Fbxo7 and PI31 levels in patient cells. Consistent with their association with proteasomes, patient fibroblasts have reduced proteasome activity and proteasome subunits. We also show PI31 interacts with the MiD49/51 fission adaptor proteins, and unexpectedly, PI31 acts to facilitate SCFFbxo7-mediated ubiquitination of MiD49. The L250P mutation reduces the SCFFbxo7 ligase-mediated ubiquitination of a subset of its known substrates. Although MiD49/51 expression was reduced in patient cells, there was no effect on the mitochondrial network. However, patient cells show reduced levels of mitochondrial function and mitophagy, higher levels of ROS and are less viable under stress. Our study demonstrates that Fbxo7 and PI31 regulate proteasomes and mitochondria and reveals a new function for PI31 in enhancing the SCFFbxo7 E3 ubiquitin ligase activity.
Subject(s)
F-Box Proteins , Mitochondria , Proteasome Endopeptidase Complex , Ubiquitination , Humans , F-Box Proteins/genetics , F-Box Proteins/metabolism , Proteasome Endopeptidase Complex/metabolism , Proteasome Endopeptidase Complex/genetics , Mitochondria/metabolism , Mitochondria/genetics , Mutation, Missense , Mitophagy/genetics , Fibroblasts/metabolism , Male , HEK293 Cells , FemaleABSTRACT
Atypical protein kinase Cs (aPKCs) are part of the PKC family of protein kinases and are atypical because they don't respond to the canonical PKC activators diacylglycerol (DAG) and Ca2+. They are central to the organization of polarized cells and are deregulated in several cancers. aPKC recruitment to the plasma membrane compartment is crucial to their encounter with substrates associated with polarizing functions. However, in contrast with other PKCs, the mechanism by which atypical PKCs are recruited there has remained elusive until recently. Here, we bring aPKC into the fold, summarizing recent reports on the direct recruitment of aPKC to membranes, providing insight into seemingly discrepant findings and integrating them with existing literature.
Subject(s)
Protein Kinase C , Protein Kinase C/metabolism , Cell Membrane/metabolismABSTRACT
Atypical PKCs are cell polarity kinases that operate at the plasma membrane where they function within multiple molecular complexes to contribute to the establishment and maintenance of polarity. In contrast to the classical and novel PKCs, atypical PKCs do not respond to diacylglycerol cues to bind the membrane compartment. Until recently, it was not clear how aPKCs are recruited; whether aPKCs can directly interact with membranes or whether they are dependent on other protein interactors to do so. Two recent studies identified the pseudosubstrate region and the C1 domain as direct membrane interaction modules; however, their relative importance and coupling are unknown. We combined molecular modeling and functional assays to show that the regulatory module of aPKCι, comprising the PB1 pseudosubstrate and C1 domains, forms a cooperative and spatially continuous invariant membrane interaction platform. Furthermore, we show the coordinated orientation of membrane-binding elements within the regulatory module requires a key PB1-C1 interfacial ß-strand (beta-strand linker). We show this element contains a highly conserved Tyr residue that can be phosphorylated and that negatively regulates the integrity of the regulatory module, leading to membrane release. We thus expose a hitherto unknown regulatory mechanism of aPKCι membrane binding and release during cell polarization.
Subject(s)
Cell Membrane , Protein Kinase C , Protein Processing, Post-Translational , Cell Membrane/metabolism , Phosphorylation , Protein Kinase C/metabolism , Tyrosine/metabolism , Humans , HEK293 Cells , Protein Binding , Mutation , Cell Polarity/physiologyABSTRACT
Topoisomerases are essential enzymes that recognize and modify the topology of DNA to allow DNA replication and transcription to take place. Topoisomerases are divided into type I topoisomerases, that cleave one DNA strand to modify DNA topology, and type II, that cleave both DNA strands. Topoisomerases normally rapidly religate cleaved-DNA once the topology has been modified. Topoisomerases do not recognize specific DNA sequences, but actively cleave positively supercoiled DNA ahead of transcription bubbles or replication forks, and negative supercoils (or precatenanes) behind, thus allowing the unwinding of the DNA-helix to proceed (during both transcription and replication). Drugs that stabilize DNA-cleavage complexes with topoisomerases produce cytotoxic DNA damage and kill fast-dividing cells; they are widely used in cancer chemotherapy. Oligonucleotide-recognizing topoisomerase inhibitors (OTIs) have given drugs that stabilize DNA-cleavage complexes specificity by linking them to either: (i) DNA duplex recognizing triplex forming oligonucleotide (TFO-OTIs) or DNA duplex recognizing pyrrole-imidazole-polyamides (PIP-OTIs) (ii) or by conventional Watson-Crick base pairing (WC-OTIs). This converts compounds from indiscriminate DNA-damaging drugs to highly specific targeted DNA-cleaving OTIs. Herein we propose simple strategies to enable DNA-duplex strand invasion of WC-OTIs giving strand-invading SI-OTIs. This will make SI-OTIs similar to the guide RNAs of CRISPR/Cas9 nuclease bacterial immune systems. However, an important difference between OTIs and CRISPR/Cas9, is that OTIs do not require the introduction of foreign proteins into cells. Recent successful oligonucleotide therapeutics for neurodegenerative diseases suggest that OTIs can be developed to be highly specific gene editing agents for DNA lesions that cause neurodegenerative diseases.
Subject(s)
Neurodegenerative Diseases , Oligonucleotides , DNA/metabolism , DNA Topoisomerases, Type I/genetics , DNA Topoisomerases, Type I/metabolism , DNA Topoisomerases, Type II/metabolism , DNA, Superhelical , Humans , Imidazoles , Neurodegenerative Diseases/drug therapy , Neurodegenerative Diseases/genetics , Nylons , Oligonucleotides/chemistry , Pyrroles , Topoisomerase I Inhibitors/pharmacology , Topoisomerase II Inhibitors , Topoisomerase Inhibitors/pharmacology , Topoisomerase Inhibitors/therapeutic useABSTRACT
Deficits in axonal transport are one of the earliest pathological outcomes in several models of amyotrophic lateral sclerosis (ALS), including SOD1G93A mice. Evidence suggests that rescuing these deficits prevents disease progression, stops denervation, and extends survival. Kinase inhibitors have been previously identified as transport enhancers, and are being investigated as potential therapies for ALS. For example, inhibitors of p38 mitogen-activated protein kinase and insulin growth factor receptor 1 have been shown to rescue axonal transport deficits in vivo in symptomatic SOD1G93A mice. In this work, we investigated the impact of RET, the tyrosine kinase receptor for glial cell line-derived neurotrophic factor (GDNF), as a modifier of axonal transport. We identified the fundamental interplay between RET signalling and axonal transport in both wild-type and SOD1G93A motor neurons in vitro. We demonstrated that blockade of RET signalling using pharmacological inhibitors and genetic knockdown enhances signalling endosome transport in wild-type motor neurons and uncovered a divergence in the response of primary motor neurons to GDNF compared with cell lines. Finally, we showed that inhibition of the GDNF-RET signalling axis rescues in vivo transport deficits in early symptomatic SOD1G93A mice, promoting RET as a potential therapeutic target in the treatment of ALS.
Subject(s)
Amyotrophic Lateral Sclerosis , Axonal Transport , Glial Cell Line-Derived Neurotrophic Factor , Proto-Oncogene Proteins c-ret , Amyotrophic Lateral Sclerosis/metabolism , Animals , Axonal Transport/physiology , Disease Models, Animal , Glial Cell Line-Derived Neurotrophic Factor/metabolism , Mice , Mice, Transgenic , Motor Neurons/metabolism , Proto-Oncogene Proteins c-ret/metabolism , Superoxide Dismutase-1/genetics , Superoxide Dismutase-1/metabolismABSTRACT
Mutations of the rearranged during transfection (RET) kinase are frequently reported in cancer, which make it as an attractive therapeutic target. Herein, we discovered a series of N-trisubstituted pyrimidine derivatives as potent inhibitors for both wild-type (wt) RET and RETV804M, which is a resistant mutant for several FDA-approved inhibitors. The X-ray structure of a representative inhibitor with RET revealed that the compound binds in a unique pose that bifurcates beneath the P-loop and confirmed the compound as a type I inhibitor. Through the structure-activity relationship (SAR) study, compound 20 was identified as a lead compound, showing potent inhibition of both RET and RETV804M. Additionally, compound 20 displayed potent antiproliferative activity of CCDC6-RET-driven LC-2/ad cells. Analysis of RET phosphorylation indicated that biological activity was mediated by RET inhibition. Collectively, N-trisubstituted pyrimidine derivatives could serve as scaffolds for the discovery and development of potent inhibitors of type I RET and its gatekeeper mutant for the treatment of RET-driven cancers.
Subject(s)
Adenocarcinoma of Lung/drug therapy , Lung Neoplasms/drug therapy , Mutation , Protein Kinase Inhibitors/pharmacology , Proto-Oncogene Proteins c-ret/antagonists & inhibitors , Pyrimidines/chemistry , Adenocarcinoma of Lung/pathology , Apoptosis , Cell Proliferation , Humans , Lung Neoplasms/pathology , Protein Kinase Inhibitors/chemistry , Proto-Oncogene Proteins c-ret/genetics , Structure-Activity Relationship , Tumor Cells, Cultured , Wound HealingABSTRACT
The PKCε-regulated genome protective pathway provides transformed cells a failsafe to successfully complete mitosis. Despite the necessary role for Aurora B in this programme, it is unclear whether its requirement is sufficient or if other PKCε cell cycle targets are involved. To address this, we developed a trapping strategy using UV-photocrosslinkable amino acids encoded in the PKCε kinase domain. The validation of the mRNA binding protein SERBP1 as a PKCε substrate revealed a series of mitotic events controlled by the catalytic form of PKCε. PKCε represses protein translation, altering SERBP1 binding to the 40 S ribosomal subunit and promoting the assembly of ribonucleoprotein granules containing SERBP1, termed M-bodies. Independent of Aurora B, SERBP1 is shown to be necessary for chromosome segregation and successful cell division, correlating with M-body formation. This requirement for SERBP1 demonstrates that Aurora B acts in concert with translational regulation in the PKCε-controlled pathway exerting genome protection.
Subject(s)
Chromosome Segregation , Mitosis , Protein Biosynthesis , Protein Kinase C-epsilon/metabolism , RNA-Binding Proteins/metabolism , Aurora Kinase B/metabolism , HEK293 Cells , HeLa Cells , HumansABSTRACT
A requirement for PKCε in exiting from the Aurora B dependent abscission checkpoint is associated with events at the midbody, however, the recruitment, retention and action of PKCε in this compartment are poorly understood. Here, the prerequisite for 14-3-3 complex assembly in this pathway is directly linked to the phosphorylation of Aurora B S227 at the midbody. However, while essential for PKCε control of Aurora B, 14-3-3 association is shown to be unnecessary for the activity-dependent enrichment of PKCε at the midbody. This localisation is demonstrated to be an autonomous property of the inactive PKCε D532N mutant, consistent with activity-dependent dissociation. The C1A and C1B domains are necessary for this localisation, while the C2 domain and inter-C1 domain (IC1D) are necessary for retention at the midbody. Furthermore, it is shown that while the IC1D mutant retains 14-3-3 complex proficiency, it does not support Aurora B phosphorylation, nor rescues division failure observed with knockdown of endogenous PKCε. It is concluded that the concerted action of multiple independent events facilitates PKCε phosphorylation of Aurora B at the midbody to control exit from the abscission checkpoint.
Subject(s)
14-3-3 Proteins/metabolism , Aurora Kinase B/metabolism , Cytokinesis , Protein Kinase C-epsilon/metabolism , 14-3-3 Proteins/genetics , Aurora Kinase B/genetics , HEK293 Cells , Humans , Phosphorylation , Protein Kinase C-epsilon/genetics , Signal Transduction , Spindle ApparatusABSTRACT
RET receptor tyrosine kinase plays vital developmental and neuroprotective roles in metazoans. GDNF family ligands (GFLs) when bound to cognate GFRα co-receptors recognize and activate RET stimulating its cytoplasmic kinase function. The principles for RET ligand-co-receptor recognition are incompletely understood. Here, we report a crystal structure of the cadherin-like module (CLD1-4) from zebrafish RET revealing interdomain flexibility between CLD2 and CLD3. Comparison with a cryo-electron microscopy structure of a ligand-engaged zebrafish RETECD-GDNF-GFRα1a complex indicates conformational changes within a clade-specific CLD3 loop adjacent to the co-receptor. Our observations indicate that RET is a molecular clamp with a flexible calcium-dependent arm that adapts to different GFRα co-receptors, while its rigid arm recognizes a GFL dimer to align both membrane-proximal cysteine-rich domains. We also visualize linear arrays of RETECD-GDNF-GFRα1a suggesting that a conserved contact stabilizes higher-order species. Our study reveals that ligand-co-receptor recognition by RET involves both receptor plasticity and strict spacing of receptor dimers by GFL ligands.
Subject(s)
Glial Cell Line-Derived Neurotrophic Factor Receptors/metabolism , Glial Cell Line-Derived Neurotrophic Factor/metabolism , Proto-Oncogene Proteins c-ret/metabolism , Zebrafish Proteins/metabolism , Zebrafish/metabolism , Animals , Cadherins/metabolism , Cryoelectron Microscopy , Crystallography, X-Ray , Models, Molecular , Multiprotein Complexes/chemistry , Protein Binding , Protein Conformation , Protein Domains , Proto-Oncogene Proteins c-ret/chemistry , Zebrafish Proteins/chemistryABSTRACT
The maturing mutational landscape of cancer genomes, the development and application of clinical interventions and evolving insights into tumour-associated functions reveal unexpected features of the protein kinase C (PKC) family of serine/threonine protein kinases. These advances include recent work showing gain or loss-of-function mutations relating to driver or bystander roles, how conformational constraints and plasticity impact this class of proteins and how emergent cancer-associated properties may offer opportunities for intervention. The profound impact of the tumour microenvironment, reflected in the efficacy of immune checkpoint interventions, further prompts to incorporate PKC family actions and interventions in this ecosystem, informed by insights into the control of stromal and immune cell functions. Drugging PKC isoforms has offered much promise, but when and how is not obvious.
Subject(s)
Neoplasms/enzymology , Protein Kinase C/physiology , Animals , Humans , Isoenzymes/physiology , Mutation , Phosphorylation , Promoter Regions, Genetic , Protein Kinase C/antagonists & inhibitors , Protein Kinase C/genetics , Tumor MicroenvironmentABSTRACT
RET receptor tyrosine kinase is a driver oncogene in human cancer. We recently identified the clinical drug candidate Pz-1, which targets RET and VEGFR2. A key in vivo metabolite of Pz-1 is its less active demethylated pyrazole analogue. Using bioisosteric substitution methods, here, we report the identification of NPA101.3, lacking the structural liability for demethylation. NPA101.3 showed a selective inhibitory profile and an inhibitory concentration 50 (IC50) of <0.003 µM for both RET and VEGFR2. NPA101.3 inhibited phosphorylation of all tested RET oncoproteins as well as VEGFR2 and proliferation of cells transformed by RET. Oral administration of NPA101.3 (10 mg/kg/day) completely prevented formation of tumors induced by RET/C634Y-transformed cells, while it weakened, but did not abrogate, formation of tumors induced by a control oncogene (HRAS/G12V). The balanced synchronous inhibition of both RET and VEGFR2, as well the resistance to demethylation, renders NPA101.3 a potential clinical candidate for RET-driven cancers.
Subject(s)
Antineoplastic Agents/therapeutic use , Neoplasms/drug therapy , Protein Kinase Inhibitors/therapeutic use , Proto-Oncogene Proteins c-ret/antagonists & inhibitors , Vascular Endothelial Growth Factor Receptor-2/antagonists & inhibitors , Animals , Antineoplastic Agents/metabolism , Antineoplastic Agents/pharmacology , Cell Line, Tumor , Drug Discovery , Female , Humans , Mice , Mice, Inbred BALB C , Models, Molecular , Mutation , NIH 3T3 Cells , Polypharmacology , Protein Binding , Protein Kinase Inhibitors/metabolism , Protein Kinase Inhibitors/pharmacology , Proto-Oncogene Proteins c-ret/genetics , Proto-Oncogene Proteins c-ret/metabolism , Signal Transduction/drug effects , Vascular Endothelial Growth Factor Receptor-2/metabolismABSTRACT
The structure-specific endonuclease XPF-ERCC1 participates in multiple DNA damage repair pathways including nucleotide excision repair (NER) and inter-strand crosslink repair (ICLR). How XPF-ERCC1 is catalytically activated by DNA junction substrates is not currently understood. Here we report cryo-electron microscopy structures of both DNA-free and DNA-bound human XPF-ERCC1. DNA-free XPF-ERCC1 adopts an auto-inhibited conformation in which the XPF helical domain masks the ERCC1 (HhH)2 domain and restricts access to the XPF catalytic site. DNA junction engagement releases the ERCC1 (HhH)2 domain to couple with the XPF-ERCC1 nuclease/nuclease-like domains. Structure-function data indicate xeroderma pigmentosum patient mutations frequently compromise the structural integrity of XPF-ERCC1. Fanconi anaemia patient mutations in XPF often display substantial in-vitro activity but are resistant to activation by ICLR recruitment factor SLX4. Our data provide insights into XPF-ERCC1 architecture and catalytic activation.
Subject(s)
DNA-Binding Proteins/chemistry , DNA-Binding Proteins/metabolism , DNA/metabolism , Endonucleases/chemistry , Endonucleases/metabolism , Binding Sites , Cryoelectron Microscopy , DNA-Binding Proteins/genetics , Endonucleases/genetics , Fanconi Anemia/enzymology , Fanconi Anemia/genetics , Humans , Models, Molecular , Mutation , Protein Conformation , Protein Domains , Protein Multimerization , Structure-Activity Relationship , Xeroderma Pigmentosum/enzymology , Xeroderma Pigmentosum/geneticsABSTRACT
We report the design and optimisation of novel oligonucleotide substrates for a sensitive fluorescence assay for high-throughput screening and functional studies of the DNA repair enzyme, XPF-ERCC1, with a view to accelerating inhibitor and drug discovery.
Subject(s)
DNA Repair , DNA-Binding Proteins/metabolism , Endonucleases/metabolism , DNA-Binding Proteins/chemistry , DNA-Binding Proteins/genetics , Dimerization , Endonucleases/chemistry , Endonucleases/genetics , Humans , Oligonucleotides/chemistry , Oligonucleotides/metabolism , Substrate Specificity , TemperatureABSTRACT
RPEL proteins, which contain the G-actin-binding RPEL motif, coordinate cytoskeletal processes with actin dynamics. We show that the ArhGAP12- and ArhGAP32-family GTPase-activating proteins (GAPs) are RPEL proteins. We determine the structure of the ArhGAP12/G-actin complex, and show that G-actin contacts the RPEL motif and GAP domain sequences. G-actin inhibits ArhGAP12 GAP activity, and this requires the G-actin contacts identified in the structure. In B16 melanoma cells, ArhGAP12 suppresses basal Rac and Cdc42 activity, F-actin assembly, invadopodia formation and experimental metastasis. In this setting, ArhGAP12 mutants defective for G-actin binding exhibit more effective downregulation of Rac GTP loading following HGF stimulation and enhanced inhibition of Rac-dependent processes, including invadopodia formation. Potentiation or disruption of the G-actin/ArhGAP12 interaction, by treatment with the actin-binding drugs latrunculin B or cytochalasin D, has corresponding effects on Rac GTP loading. The interaction of G-actin with RPEL-family rhoGAPs thus provides a negative feedback loop that couples Rac activity to actin dynamics.
Subject(s)
Actins/metabolism , GTPase-Activating Proteins/metabolism , cdc42 GTP-Binding Protein/metabolism , Actins/drug effects , Animals , Bridged Bicyclo Compounds, Heterocyclic/pharmacology , Cytochalasin D/pharmacology , GTPase-Activating Proteins/drug effects , GTPase-Activating Proteins/genetics , Guanosine Triphosphate/metabolism , Humans , Mice , Protein Binding/drug effects , Protein Binding/genetics , Rabbits , Thiazolidines/pharmacology , cdc42 GTP-Binding Protein/drug effects , rac GTP-Binding Proteins/metabolismABSTRACT
The elaboration of polarity is central to organismal development and to the maintenance of functional epithelia. Among the controls determining polarity are the PAR proteins, PAR6, aPKCι and PAR3, regulating both known and unknown effectors. Here, we identify FARP2 as a 'RIPR' motif-dependent partner and substrate of aPKCι that is required for efficient polarisation and junction formation. Binding is conferred by a FERM/FA domain-kinase domain interaction and detachment promoted by aPKCι-dependent phosphorylation. FARP2 is shown to promote GTP loading of Cdc42, which is consistent with it being involved in upstream regulation of the polarising PAR6-aPKCι complex. However, we show that aPKCι acts to promote the localised activity of FARP2 through phosphorylation. We conclude that this aPKCι-FARP2 complex formation acts as a positive feedback control to drive polarisation through aPKCι and other Cdc42 effectors.This article has an associated First Person interview with the first author of the paper.
Subject(s)
Epithelial Cells/cytology , Guanine Nucleotide Exchange Factors/metabolism , Protein Kinase C/metabolism , Tight Junctions/metabolism , cdc42 GTP-Binding Protein/metabolism , Caco-2 Cells , Cell Polarity , Guanine Nucleotide Exchange Factors/genetics , HCT116 Cells , Humans , PhosphorylationABSTRACT
The use of kinase-directed precision medicine has been heavily pursued since the discovery and development of imatinib. Annually, it is estimated that around â¼20â¯000 new cases of tropomyosin receptor kinase (TRK) cancers are diagnosed, with the majority of cases exhibiting a TRK genomic rearrangement. In this Perspective, we discuss current development and clinical applications for TRK precision medicine by providing the following: (1) the biological background and significance of the TRK kinase family, (2) a compilation of known TRK inhibitors and analysis of their cocrystal structures, (3) an overview of TRK clinical trials, and (4) future perspectives for drug discovery and development of TRK inhibitors.
Subject(s)
Antineoplastic Agents/therapeutic use , Neoplasms/drug therapy , Protein Kinase Inhibitors/therapeutic use , Receptor, trkA/antagonists & inhibitors , Receptor, trkB/antagonists & inhibitors , Receptor, trkC/antagonists & inhibitors , Animals , Antineoplastic Agents/metabolism , Antineoplastic Agents/pharmacology , Catalytic Domain , Cell Line, Tumor , Drug Discovery , Humans , Mice, Inbred BALB C , Precision Medicine/methods , Protein Kinase Inhibitors/metabolism , Protein Kinase Inhibitors/pharmacology , Rats, Sprague-Dawley , Receptor, trkA/chemistry , Receptor, trkA/metabolism , Receptor, trkB/chemistry , Receptor, trkB/metabolism , Receptor, trkC/chemistry , Receptor, trkC/metabolismABSTRACT
Resistance to vandetanib, a type I RET kinase inhibitor, developed in a patient with metastatic lung adenocarcinoma harboring a CCDC6-RET fusion that initially exhibited a response to treatment. The resistant tumor acquired a secondary mutation resulting in a serine-to-phenylalanine substitution at codon 904 in the activation loop of the RET kinase domain. The S904F mutation confers resistance to vandetanib by increasing the ATP affinity and autophosphorylation activity of RET kinase. A reduced interaction with the drug is also observed in vitro for the S904F mutant by thermal shift assay. A crystal structure of the S904F mutant reveals a small hydrophobic core around F904 likely to enhance basal kinase activity by stabilizing an active conformer. Our findings indicate that missense mutations in the activation loop of the kinase domain are able to increase kinase activity and confer drug resistance through allosteric effects.