SH2-catalytic domain linker heterogeneity influences allosteric coupling across the SFK family

LIMS-Kinase provides sensitive and generalizable label-free in vitro measurement of kinase activity using mass spectrometry

Measurements of kinase activity are important for kinase-directed drug development, analysis of inhibitor structure and function, and understanding mechanisms of drug resistance. Sensitive, accurate, and miniaturized assay methods are crucial for these investigations. Here, we describe a label-free, high-throughput mass spectrometry-based assay for studying individual kinase enzymology and drug discovery in a purified system, with a focus on validated drug targets as benchmarks. We demonstrate that this approach can be adapted to many known kinase substrates and highlight the benefits of using mass spectrometry to measure kinase activity in vitro, including increased sensitivity. We speculate that this approach to measuring kinase activity will be generally applicable across most of the kinome, enabling research on understudied kinases and kinase drug discovery.

Identification of novel Src, Bcl-2 dual inhibitors by the pharmacophore model, molecular docking, and molecular dynamics simulations

Src is a tyrosine kinase that plays a key role in cell proliferation, migration, invasion, and angiogenesis.

Parallel Chemoselective Profiling for Mapping Protein Structure

Solution-based structural techniques complement high-resolution structural data by providing insight into the oft-missed links between protein structure and dynamics. Here, we present Parallel Chemoselective Profiling, a solution-based structural method for characterizing protein structure and dynamics. Our method utilizes deep mutational scanning saturation mutagenesis data to install amino acid residues with specific chemistries at defined positions on the solvent-exposed surface of a protein. Differences in the extent of labeling of installed mutant residues are quantified using targeted mass spectrometry, reporting on each residue's local environment and structural dynamics. Using our method, we studied how conformation-selective, ATP-competitive inhibitors affect the local and global structure and dynamics of full-length Src kinase. Our results highlight how parallel chemoselective profiling can be used to study a dynamic multi-domain protein, and suggest that our method will be a useful addition to the relatively small toolkit of existing protein footprinting techniques.

How ATP-Competitive Inhibitors Allosterically Modulate Tyrosine Kinases That Contain a Src-like Regulatory Architecture

Small molecule kinase inhibitors that stabilize distinct ATP binding site conformations can differentially modulate the global conformation of Src-family kinases (SFKs). However, it is unclear which specific ATP binding site contacts are responsible for modulating the global conformation of SFKs and whether these inhibitor-mediated allosteric effects generalize to other tyrosine kinases. Here, we describe the development of chemical probes that allow us to deconvolute which features in the ATP binding site are responsible for the allosteric modulation of the global conformation of Src. We find that the ability of an inhibitor to modulate the global conformation of Src's regulatory domain-catalytic domain module relies mainly on the influence it has on the conformation of a structural element called helix αC. Furthermore, by developing a set of orthogonal probes that target a drug-sensitized Src variant, we show that stabilizing Src's helix αC in an active conformation is sufficient to promote a Src-mediated, phosphotransferase-independent alteration in cell morphology. Finally, we report that ATP-competitive, conformation-selective inhibitors can influence the global conformation of tyrosine kinases beyond the SFKs, suggesting that the allosteric networks we observe in Src are conserved in kinases that have a similar regulatory architecture. Our study highlights that an ATP-competitive inhibitor's interactions with helix αC can have a major influence on the global conformation of some tyrosine kinases.

Renaissance of Allostery to Disrupt Protein Kinase Interactions

Protein-protein interactions often regulate the activity of protein kinases by allosterically modulating the conformation of the ATP-binding site. Bidirectional allostery implies that reverse modulation (i.e., from the ATP-binding site to the interaction and regulatory sites) must also be possible. Here, we review both the allosteric regulation of protein kinases and recent work describing how compounds binding at the ATP-binding site can promote or inhibit protein kinase interactions at regulatory sites via the reverse mechanism. Notably, the pharmaceutical industry has been developing compounds that bind to the ATP-binding site of protein kinases and potently disrupt protein-protein interactions between target protein kinases and their regulatory interacting partners. Learning to modulate allosteric processes will facilitate the development of protein-protein interaction modulators.

Lipid-targeting pleckstrin homology domain turns its autoinhibitory face toward the TEC kinases

Bruton’s tyrosine kinase (BTK) is targeted in treatment of immune cancers. As patients experience drug resistance, there is a need for alternative approaches to inhibit BTK. Other recently published findings clarify the role of the BTK pleckstrin homology (PH) domain in mediating activation via dimerization and sensing of ligand concentration at the membrane. Work presented here provides insight into the autoinhibitory BTK structure that has so far been elusive via crystallographic methods. In the resting state, the BTK PH domain binds to the activation loop face of the kinase domain and allosterically alters key sites within the kinase domain. The findings define a new regulatory site, the PH/kinase interface, that can be exploited in drug discovery efforts.

The pleckstrin homology (PH) domain is well known for its phospholipid targeting function. The PH-TEC homology (PHTH) domain within the TEC family of tyrosine kinases is also a crucial component of the autoinhibitory apparatus. The autoinhibitory surface on the PHTH domain has been previously defined, and biochemical investigations have shown that PHTH-mediated inhibition is mutually exclusive with phosphatidylinositol binding. Here we use hydrogen/deuterium exchange mass spectrometry, nuclear magnetic resonance (NMR), and evolutionary sequence comparisons to map where and how the PHTH domain affects the Bruton’s tyrosine kinase (BTK) domain. The data map a PHTH-binding site on the activation loop face of the kinase C lobe, suggesting that the PHTH domain masks the activation loop and the substrate-docking site. Moreover, localized NMR spectral changes are observed for non–surface-exposed residues in the active site and on the distal side of the kinase domain. These data suggest that the association of PHTH induces allosteric conformational shifts in regions of the kinase domain that are critical for catalysis. Through statistical comparisons of diverse tyrosine kinase sequences, we identify residues unique to BTK that coincide with the experimentally determined PHTH-binding surface on the kinase domain. Our data provide a more complete picture of the autoinhibitory conformation adopted by full-length TEC kinases, creating opportunities to target the regulatory domains to control the function of these kinases in a biological setting.

Dynamic regulatory features of the protein tyrosine kinases

The SRC, Abelson murine leukemia viral oncogene homolog 1, TEC and C-terminal SRC Kinase families of non-receptor tyrosine kinases (collectively the Src module kinases) mediate an array of cellular signaling processes and are therapeutic targets in many disease states. Crystal structures of Src modules kinases provide valuable insights into the regulatory mechanisms that control activation and generate a framework from which drug discovery can advance. The conformational ensembles visited by these multidomain kinases in solution are also key features of the regulatory machinery controlling catalytic activity. Measurement of dynamic motions within kinases substantially augments information derived from crystal structures. In this review, we focus on a body of work that has transformed our understanding of non-receptor tyrosine kinase regulation from a static view to one that incorporates how fluctuations in conformational ensembles and dynamic motions influence activation status. Regulatory dynamic networks are often shared across and between kinase families while specific dynamic behavior distinguishes unique regulatory mechanisms for select kinases. Moreover, intrinsically dynamic regions of kinases likely play important regulatory roles that have only been partially explored. Since there is clear precedence that kinase inhibitors can exploit specific dynamic features, continued efforts to define conformational ensembles and dynamic allostery will be key to combating drug resistance and devising alternate treatments for kinase-associated diseases.

Targeting Dynamic ATP-Binding Site Features Allows Discrimination between Highly Homologous Protein Kinases

ATP-competitive inhibitors that demonstrate exquisite selectivity for specific members of the human kinome have been developed. Despite this success, the identification of highly selective inhibitors is still very challenging, and it is often not possible to rationally engineer selectivity between the ATP-binding sites of kinases, especially among closely related family members. Src-family kinases (SFKs) are a highly homologous family of eight multidomain, nonreceptor tyrosine kinases that play general and specialized roles in numerous cellular processes. The high sequence and functional similarities between SFK members make it hard to rationalize how selectivity can be gained with inhibitors that target the ATP-binding site. Here, we describe the development of a series of inhibitors that are highly selective for the ATP-binding sites of the SFKs Lyn and Hck over other SFKs. By biochemically characterizing how these selective ATP-competitive inhibitors allosterically influence the global conformation of SFKs, we demonstrate that they most likely interact with a binding pocket created by the movement of the conformationally flexible helix αC in the ATP-binding site. With a series of sequence swap experiments, we show that sensitivity to this class of selective inhibitors is due to the identity of residues that control the conformational flexibility of helix αC rather than any specific ATP-binding site interactions. Thus, the ATP-binding sites of highly homologous kinases can be discriminated by targeting heterogeneity within conformationally flexible regions.

Allosteric Regulation of Protein Kinases Downstream of PI3-Kinase Signalling

Allostery is a basic principle that enables proteins to process and transmit cellular information. Protein kinases evolved allosteric mechanisms to transduce cellular signals to downstream signalling components or effector molecules. Protein kinases catalyse the transfer of the terminal phosphate from ATP to protein substrates upon specific stimuli. Protein kinases are targets for the development of small molecule inhibitors for the treatment of human diseases. Drug development has focussed on ATP-binding site, while there is increase interest in the development of drugs targeting alternative sites, i.e. allosteric sites. Here, we review the mechanism of regulation of protein kinases, which often involve the allosteric modulation of the ATP-binding site, enhancing or inhibiting activity. We exemplify the molecular mechanism of allostery in protein kinases downstream of PI3-kinase signalling with a focus on phosphoinositide-dependent protein kinase 1 (PDK1), a model kinase where small compounds can allosterically modulate the conformation of the kinase bidirectionally.

The Src module: an ancient scaffold in the evolution of cytoplasmic tyrosine kinases

Tyrosine kinases were first discovered as the protein products of viral oncogenes. We now know that this large family of metazoan enzymes includes nearly one hundred structurally diverse members. Tyrosine kinases are broadly classified into two groups: the transmembrane receptor tyrosine kinases, which sense extracellular stimuli, and the cytoplasmic tyrosine kinases, which contain modular ligand-binding domains and propagate intracellular signals. Several families of cytoplasmic tyrosine kinases have in common a core architecture, the "Src module," composed of a Src-homology 3 (SH3) domain, a Src-homology 2 (SH2) domain, and a kinase domain. Each of these families is defined by additional elaborations on this core architecture. Structural, functional, and evolutionary studies have revealed a unifying set of principles underlying the activity and regulation of tyrosine kinases built on the Src module. The discovery of these conserved properties has shaped our knowledge of the workings of protein kinases in general, and it has had important implications for our understanding of kinase dysregulation in disease and the development of effective kinase-targeted therapies.

Allosteric Modulation of JNK Docking Site Interactions with ATP-Competitive Inhibitors

The c-Jun N-terminal kinases (JNKs) play a wide variety of roles in cellular signaling processes, dictating important, and even divergent, cellular fates. These essential kinases possess docking surfaces distal to their active sites that interact with diverse binding partners, including upstream activators, downstream substrates, and protein scaffolds. Prior studies have suggested that the interactions of certain protein-binding partners with one such JNK docking surface, termed the D-recruitment site (DRS), can allosterically influence the conformational state of the ATP-binding pocket of JNKs. To further explore the allosteric relationship between the ATP-binding pockets and DRSs of JNKs, we investigated how the interactions of the scaffolding protein JIP1, as well as the upstream activators MKK4 and MKK7, are allosterically influenced by the ATP-binding site occupancy of the JNKs. We show that the affinity of the JNKs for JIP1 can be divergently modulated with ATP-competitive inhibitors, with a >50-fold difference in dissociation constant observed between the lowest- and highest-affinity JNK1-inhibitor complexes. Furthermore, we found that we could promote or attenuate phosphorylation of JNK1's activation loop by MKK4 and MKK7, by varying the ATP-binding site occupancy. Given that JIP1, MKK4, and MKK7 all interact with JNK DRSs, these results demonstrate that there is functional allostery between the ATP-binding sites and DRSs of these kinases. Furthermore, our studies suggest that ATP-competitive inhibitors can allosterically influence the intracellular binding partners of the JNKs.

Allosteric Modulation of Src Family Kinases with ATP-Competitive Inhibitors

The Src family kinases (SFKs) are an important family of tyrosine kinases that are allosterically regulated by their SH2 and SH3 domains. Engagement of SFK SH2 and SH3 domains with their intramolecular ligands leads to reduced kinase activity by stabilizing an inactive ATP-binding site conformation. Disruption of these intramolecular interactions stabilizes a more active ATP-binding site conformation and restores SFK activity. Interestingly, this allosteric relationship is bidirectional in that ATP-competitive ligands that stabilize distinct active site conformations can divergently modulate the abilities of the regulatory SH2 and SH3 domains to participate in intermolecular interactions. Here, we describe a series of assays that profile the bidirectional relationship between the ATP-binding sites and regulatory domains of SFKs. These methods can be used to discover ATP-competitive inhibitors that are selective for distinct ATP-binding site conformations of SFKs and for characterizing the effects that ATP-competitive inhibitors of SFKs have on domains that are distal to their site of interaction.

Same but not alike: Structure, flexibility and energetics of domains in multi-domain proteins are influenced by the presence of other domains

The majority of the proteins encoded in the genomes of eukaryotes contain more than one domain. Reasons for high prevalence of multi-domain proteins in various organisms have been attributed to higher stability and functional and folding advantages over single-domain proteins. Despite these advantages, many proteins are composed of only one domain while their homologous domains are part of multi-domain proteins. In the study presented here, differences in the properties of protein domains in single-domain and multi-domain systems and their influence on functions are discussed. We studied 20 pairs of identical protein domains, which were crystallized in two forms (a) tethered to other proteins domains and (b) tethered to fewer protein domains than (a) or not tethered to any protein domain. Results suggest that tethering of domains in multi-domain proteins influences the structural, dynamic and energetic properties of the constituent protein domains. 50% of the protein domain pairs show significant structural deviations while 90% of the protein domain pairs show differences in dynamics and 12% of the residues show differences in the energetics. To gain further insights on the influence of tethering on the function of the domains, 4 pairs of homologous protein domains, where one of them is a full-length single-domain protein and the other protein domain is a part of a multi-domain protein, were studied. Analyses showed that identical and structurally equivalent functional residues show differential dynamics in homologous protein domains; though comparable dynamics between in-silico generated chimera protein and multi-domain proteins were observed. From these observations, the differences observed in the functions of homologous proteins could be attributed to the presence of tethered domain. Overall, we conclude that tethered domains in multi-domain proteins not only provide stability or folding advantages but also influence pathways resulting in differences in function or regulatory properties.

High prevalence of multi-domain proteins in proteomes has been attributed to higher stability and functional and folding advantages of the multi-domain proteins. Influence of tethering of domains on the overall properties of proteins has been well studied but its influence on the properties of the constituent domains is largely unaddressed. Here, we investigate the influence of tethering of domains in multi-domain proteins on the structural, dynamics and energetics properties of the constituent domains and its implications on the functions of proteins. To this end, comparative analyses were carried out for identical protein domains crystallized in tethered and untethered forms. Also, comparative analyses of single-domain proteins and their homologous multi-domain proteins were performed. The analyses suggest that tethering influences the structural, dynamic and energetic properties of constituent protein domains. Our observations hint at regulation of protein domains by tethered domains in multi-domain systems, which may manifest at the differential function observed between single-domain and homologous multi-domain proteins.

SNAP-tagged Chikungunya Virus Replicons Improve Visualisation of Non-Structural Protein 3 by Fluorescence Microscopy

Chikungunya virus (CHIKV), a mosquito-borne alphavirus, causes febrile disease, muscle and joint pain, which can become chronic in some individuals. The non-structural protein 3 (nsP3) plays essential roles during infection, but a complete understanding of its function is lacking. Here we used a microscopy-based approach to image CHIKV nsP3 inside human cells. The SNAP system consists of a self-labelling enzyme tag, which catalyses the covalent linking of exogenously supplemented synthetic ligands. Genetic insertion of this tag resulted in viable replicons and specific labelling while preserving the effect of nsP3 on stress granule responses and co-localisation with GTPase Activating Protein (SH3 domain) Binding Proteins (G3BPs). With sub-diffraction, three-dimensional, optical imaging, we visualised nsP3-positive structures with variable density and morphology, including high-density rod-like structures, large spherical granules, and small, low-density structures. Next, we confirmed the utility of the SNAP-tag for studying protein turnover by pulse-chase labelling. We also revealed an association of nsP3 with cellular lipid droplets and examined the spatial relationships between nsP3 and the non-structural protein 1 (nsP1). Together, our study provides a sensitive, specific, and versatile system for fundamental research into the individual functions of a viral non-structural protein during infection with a medically important arthropod-borne virus (arbovirus).

Paralog-Specific Patterns of Structural Disorder and Phosphorylation in the Vertebrate SH3-SH2-Tyrosine Kinase Protein Family

One of the largest multigene families in Metazoa are the tyrosine kinases (TKs). These are important multifunctional proteins that have evolved as dynamic switches that perform tyrosine phosphorylation and other noncatalytic activities regulated by various allosteric mechanisms. TKs interact with each other and with other molecules, ultimately activating and inhibiting different signaling pathways. TKs are implicated in cancer and almost 30 FDA-approved TK inhibitors are available. However, specific binding is a challenge when targeting an active site that has been conserved in multiple protein paralogs for millions of years. A cassette domain (CD) containing SH3-SH2-Tyrosine Kinase domains reoccurs in vertebrate nonreceptor TKs. Although part of the CD function is shared between TKs, it also presents TK specific features. Here, the evolutionary dynamics of sequence, structure, and phosphorylation across the CD in 17 TK paralogs have been investigated in a large-scale study. We establish that TKs often have ortholog-specific structural disorder and phosphorylation patterns, while secondary structure elements, as expected, are highly conserved. Further, domain-specific differences are at play. Notably, we found the catalytic domain to fluctuate more in certain secondary structure elements than the regulatory domains. By elucidating how different properties evolve after gene duplications and which properties are specifically conserved within orthologs, the mechanistic understanding of protein evolution is enriched and regions supposedly critical for functional divergence across paralogs are highlighted.

Structural and Functional Analysis of the Allosteric Inhibition of IRE1α with ATP-Competitive Ligands

The accumulation of unfolded proteins under endoplasmic reticulum (ER) stress leads to the activation of the multidomain protein sensor IRE1α as part of the unfolded protein response (UPR). Clustering of IRE1α lumenal domains in the presence of unfolded proteins promotes kinase trans-autophosphorylation in the cytosol and subsequent RNase domain activation. Interestingly, there is an allosteric relationship between the kinase and RNase domains of IRE1α, which allows ATP-competitive inhibitors to modulate the activity of the RNase domain. Here, we use kinase inhibitors to study how ATP-binding site conformation affects the activity of the RNase domain of IRE1α. We find that diverse ATP-competitive inhibitors of IRE1α promote dimerization and activation of RNase activity despite blocking kinase autophosphorylation. In contrast, a subset of ATP-competitive ligands, which we call KIRAs, allosterically inactivate the RNase domain through the kinase domain by stabilizing monomeric IRE1α. Further insight into how ATP-competitive inhibitors are able to divergently modulate the RNase domain through the kinase domain was gained by obtaining the first structure of apo human IRE1α in the RNase active back-to-back dimer conformation. Comparison of this structure with other existing structures of IRE1α and integration of our extensive structure activity relationship (SAR) data has led us to formulate a model to rationalize how ATP-binding site ligands are able to control the IRE1α oligomeric state and subsequent RNase domain activity.

Bivalent Inhibitors of c-Src Tyrosine Kinase That Bind a Regulatory Domain

We have developed a general methodology to produce bivalent kinase inhibitors for c-Src that interact with the SH2 and ATP binding pockets. Our approach led to a highly selective bivalent inhibitor of c-Src. We demonstrate impressive selectivity for c-Src over homologous kinases. Exploration of the unexpected high level of selectivity yielded insight into the inherent flexibility of homologous kinases. Finally, we demonstrate that our methodology is modular and both the ATP-competitive fragment and conjugation chemistry can be swapped.

Structural properties of the linkers connecting the N- and C- terminal domains in the MocR bacterial transcriptional regulators

Peptide inter-domain linkers are peptide segments covalently linking two adjacent domains within a protein. Linkers play a variety of structural and functional roles in naturally occurring proteins. In this work we analyze the sequence properties of the predicted linker regions of the bacterial transcriptional regulators belonging to the recently discovered MocR subfamily of the GntR regulators. Analyses were carried out on the MocR sequences taken from the phyla Actinobacteria, Firmicutes, Alpha-, Beta- and Gammaproteobacteria. The results suggest that MocR linkers display phylum-specific characteristics and unique features different from those already described for other classes of inter-domain linkers. They show an average length significantly higher: 31.8 ± 14.3 residues reaching a maximum of about 150 residues. Compositional propensities displayed general and phylum-specific trends. Pro is dominating in all linkers. Dyad propensity analysis indicate Pro–Pro as the most frequent amino acid pair in all linkers. Physicochemical properties of the linker regions were assessed using amino acid indices relative to different features: in general, MocR linkers are flexible, hydrophilic and display propensity for β-turn or coil conformations. Linker sequences are hypervariable: only similarities between MocR linkers from organisms related at the level of species or genus could be found with sequence searches. The results shed light on the properties of the linker regions of the new MocR subfamily of bacterial regulators and may provide knowledge-based rules for designing artificial linkers with desired properties.

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An overview of the structural properties of MocR inter-domain linkers is reported.

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Linker length distribution is heterogeneous in different phyla.

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Linkers are flexible, hydrophilic and have coil conformation propensity.

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Pro and Pro–Pro dyads are very frequent in all the linkers.

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MocR linkers display a few properties different from those reported for other linkers.