Regulation of signaling directionality revealed by 3D snapshots of a kinase:regulator complex in action

Leveraging the histidine kinase-phosphatase duality to sculpt two-component signaling

Bacteria must constantly probe their environment for rapid adaptation, a crucial need most frequently served by two-component systems (TCS). As one component, sensor histidine kinases (SHK) control the phosphorylation of the second component, the response regulator (RR). Downstream responses hinge on RR phosphorylation and can be highly stringent, acute, and sensitive because SHKs commonly exert both kinase and phosphatase activity. With a bacteriophytochrome TCS as a paradigm, we here interrogate how this catalytic duality underlies signal responses. Derivative systems exhibit tenfold higher red-light sensitivity, owing to an altered kinase-phosphatase balance. Modifications of the linker intervening the SHK sensor and catalytic entities likewise tilt this balance and provide TCSs with inverted output that increases under red light. These TCSs expand synthetic biology and showcase how deliberate perturbations of the kinase-phosphatase duality unlock altered signal-response regimes. Arguably, these aspects equally pertain to the engineering and the natural evolution of TCSs.

Comprehensive virulence profiling and evolutionary analysis of specificity determinants in Staphylococcus aureus two-component systems

In the Staphylococcus aureus genome, a set of highly conserved two-component systems (TCSs) composed of histidine kinases (HKs) and their cognate response regulators (RRs) sense and respond to environmental stimuli, which drive the adaptation of the bacteria. This study investigates the complex interplay between TCSs in S. aureus USA300, a predominant methicillin-resistant S. aureus strain, revealing shared and unique virulence regulatory pathways and genetic variations mediating signal specificity within TCSs. Using TCS-related mutants from the Nebraska Transposon Mutant Library, we analyzed the effects of inactivated TCS HKs and RRs on the production of various virulence factors, in vitro infection abilities, and adhesion assays. We found that the TCSs' influence on virulence determinants was not associated with their phylogenetic relationship, indicating divergent functional evolution. Using the co-crystallized structure of the DesK-DesR from Bacillus subtilis and the modeled structures of the four NarL TCSs in S. aureus, we identified interacting residues, revealing specificity determinants and conservation within the same TCS, even from different strain backgrounds. The interacting residues were highly conserved within strains but varied between species due to selection pressures and the coevolution of cognate pairs. This study unveils the complex interplay and divergent functional evolution of TCSs, highlighting their potential for future experimental exploration of phosphotransfer between cognate and non-cognate recombinant HK and RRs.IMPORTANCEGiven the widespread conservation of two-component systems (TCSs) in bacteria and their pivotal role in regulating metabolic and virulence pathways, they present a compelling target for anti-microbial agents, especially in the face of rising multi-drug-resistant infections. Harnessing TCSs therapeutically necessitates a profound understanding of their evolutionary trajectory in signal transduction, as this underlies their unique or shared virulence regulatory pathways. Such insights are critical for effectively targeting TCS components, ensuring an optimized impact on bacterial virulence, and mitigating the risk of resistance emergence via the evolution of alternative pathways. Our research offers an in-depth exploration of virulence determinants controlled by TCSs in S. aureus, shedding light on the evolving specificity determinants that orchestrate interactions between their cognate pairs.

A Model of the Full-Length Cytokinin Receptor: New Insights and Prospects

Cytokinins (CK) are one of the most important classes of phytohormones that regulate a wide range of processes in plants. A CK receptor, a sensor hybrid histidine kinase, was discovered more than 20 years ago, but the structural basis for its signaling is still a challenge for plant biologists. To date, only two fragments of the CK receptor structure, the sensory module and the receiver domain, were experimentally resolved. Some other regions were built up by molecular modeling based on structures of proteins homologous to CK receptors. However, in the long term, these data have proven insufficient for solving the structure of the full-sized CK receptor. The functional unit of CK receptor is the receptor dimer. In this article, a molecular structure of the dimeric form of the full-length CK receptor based on AlphaFold Multimer and ColabFold modeling is presented for the first time. Structural changes of the receptor upon interacting with phosphotransfer protein are visualized. According to mathematical simulation and available data, both types of dimeric receptor complexes with hormones, either half- or fully liganded, appear to be active in triggering signals. In addition, the prospects of using this and similar models to address remaining fundamental problems of CK signaling were outlined.

Metal Messengers: Communication in the Bacterial World through Transition-Metal-Sensing Two-Component Systems

Bacteria survive in highly dynamic and complex environments due, in part, to the presence of systems that allow the rapid control of gene expression in the presence of changing environmental stimuli. The crosstalk between intra- and extracellular bacterial environments is often facilitated by two-component signal transduction systems that are typically composed of a transmembrane histidine kinase and a cytosolic response regulator. Sensor histidine kinases and response regulators work in tandem with their modular domains containing highly conserved structural features to control a diverse array of genes that respond to changing environments. Bacterial two-component systems are widespread and play crucial roles in many important processes, such as motility, virulence, chemotaxis, and even transition metal homeostasis. Transition metals are essential for normal prokaryotic physiological processes, and the presence of these metal ions may also influence pathogenic virulence if their levels are appropriately controlled. To do so, bacteria use transition-metal-sensing two-component systems that bind and respond to rapid fluctuations in extracytosolic concentrations of transition metals. This perspective summarizes the structural and metal-binding features of bacterial transition-metal-sensing two-component systems and places a special emphasis on understanding how these systems are used by pathogens to establish infection in host cells and how these systems may be targeted for future therapeutic developments.

Mode of autophosphorylation in bacteriophytochromes RpBphP2 and RpBphP3

Phytochromes are red-light photoreceptors that regulate a wide range of physiological processes in plants, fungi and bacteria. Canonical bacteriophytochromes are photosensory histidine kinases that undergo light-dependent autophosphorylation, thereby regulating cellular responses to red light via two-component signaling pathways. However, the molecular mechanism of kinase activation remains elusive for bacteriophytochromes. In particular, the directionality of autophosphorylation is still an open question in these dimeric photoreceptor kinases. In this work, we perform histidine kinase assays on two tandem bacteriophytochromes RpBphP2 and RpBphP3 from the photosynthetic bacterium Rhodopseudomonas palustris. By examining the kinase activities of full-length bacteriophytochromes and two loss-of-function mutants under different light conditions, we demonstrate that RpBphP2 and RpBphP3 undergo light-dependent trans-phosphorylation between protomers in both homodimeric and heterodimeric forms. We have further determined the crystal structure of the histidine kinase domains of RpBphP2 at 3.19 Å resolution. Based on structural comparisons and homology modeling, we also present a model to account for the actions of trans-autophosphorylation in bacteriophytochromes.

Marginal specificity in protein interactions constrains evolution of a paralogous family

The evolution of novel functions in biology relies heavily on gene duplication and divergence, creating large paralogous protein families. Selective pressure to avoid detrimental cross-talk often results in paralogs that exhibit exquisite specificity for their interaction partners. But how robust or sensitive is this specificity to mutation? Here, using deep mutational scanning, we demonstrate that a paralogous family of bacterial signaling proteins exhibits marginal specificity, such that many individual substitutions give rise to substantial cross-talk between normally insulated pathways. Our results indicate that sequence space is locally crowded despite overall sparseness, and we provide evidence that this crowding has constrained the evolution of bacterial signaling proteins. These findings underscore how evolution selects for "good enough" rather than optimized phenotypes, leading to restrictions on the subsequent evolution of paralogs.

Structural features of sensory two component systems: a synthetic biology perspective

All living organisms include a set of signaling devices that confer the ability to dynamically perceive and adapt to the fluctuating environment. Two-component systems are part of this sensory machinery that regulates the execution of different genetic and/or biochemical programs in response to specific physical or chemical signals. In the last two decades, there has been tremendous progress in our molecular understanding on how signals are detected, the allosteric mechanisms that control intramolecular information transmission and the specificity determinants that guarantee correct wiring. All this information is starting to be exploited in the development of new synthetic networks. Connecting multiple molecular players, analogous to programming lines of code, can provide the resources to build new sophisticated biocomputing systems. The Synthetic Biology field is starting to revolutionize several scientific fields, such as biomedicine and agriculture, propelling the development of new solutions. Expanding the spectrum of available nanodevices in the toolbox is key to unleash its full potential. This review aims to discuss, from a structural perspective, how to take advantage of the vast array of sensor and effector protein modules involved in two-component systems for the construction of new synthetic circuits.

An allosteric switch ensures efficient unidirectional information transmission by the histidine kinase DesK from Bacillus subtilis

Phosphorylation carries chemical information in biological systems. In two-component systems (TCSs), the sensor histidine kinase and the response regulator are connected through phosphoryl transfer reactions that may be uni- or bidirectional. Directionality enables the construction of complex regulatory networks that optimize signal propagation and ensure the forward flow of information. We combined x-ray crystallography, hybrid quantum mechanics/molecular mechanics (QM/MM) simulations, and systems-integrative kinetic modeling approaches to study phosphoryl flow through the Bacillus subtilis thermosensing TCS DesK-DesR. The allosteric regulation of the histidine kinase DesK was critical to avoid back transfer of phosphoryl groups and futile phosphorylation-dephosphorylation cycles by isolating phosphatase, autokinase, and phosphotransferase activities. Interactions between the kinase's ATP-binding domain and the regulator's receiver domain placed the regulator in two distinct positions in the phosphotransferase and phosphatase complexes, thereby determining whether a key glutamine residue in DesK was properly situated to assist in the dephosphorylation reaction. Moreover, an energetically unfavorable phosphotransferase conformation when DesK was not phosphorylated minimized reverse phosphoryl transfer. DesR dimerization and a dissociative phosphoryl transfer reaction also enforced the direction of phosphoryl flow. Shorter or longer distances between the phosphoryl acceptor and donor residues shifted the phosphoryl transfer equilibrium by modulating the stabilizing effect of the Mg²⁺ cofactor. These mechanisms control the directionality of signal transmission and show how structure-encoded allostery stores and transmits information in signaling systems.

Light-regulated gene expression in Bacteria: Fundamentals, advances, and perspectives

Numerous photoreceptors and genetic circuits emerged over the past two decades and now enable the light-dependent i.e., optogenetic, regulation of gene expression in bacteria. Prompted by light cues in the near-ultraviolet to near-infrared region of the electromagnetic spectrum, gene expression can be up- or downregulated stringently, reversibly, non-invasively, and with precision in space and time. Here, we survey the underlying principles, available options, and prominent examples of optogenetically regulated gene expression in bacteria. While transcription initiation and elongation remain most important for optogenetic intervention, other processes e.g., translation and downstream events, were also rendered light-dependent. The optogenetic control of bacterial expression predominantly employs but three fundamental strategies: light-sensitive two-component systems, oligomerization reactions, and second-messenger signaling. Certain optogenetic circuits moved beyond the proof-of-principle and stood the test of practice. They enable unprecedented applications in three major areas. First, light-dependent expression underpins novel concepts and strategies for enhanced yields in microbial production processes. Second, light-responsive bacteria can be optogenetically stimulated while residing within the bodies of animals, thus prompting the secretion of compounds that grant health benefits to the animal host. Third, optogenetics allows the generation of precisely structured, novel biomaterials. These applications jointly testify to the maturity of the optogenetic approach and serve as blueprints bound to inspire and template innovative use cases of light-regulated gene expression in bacteria. Researchers pursuing these lines can choose from an ever-growing, versatile, and efficient toolkit of optogenetic circuits.

The W-Acidic Motif of Histidine Kinase WalK Is Required for Signaling and Transcriptional Regulation in Streptococcus mutans

In Streptococcus mutans, we find that the histidine kinase WalK possesses the longest C-terminal tail (CTT) among all 14 TCSs, and this tail plays a key role in the interaction of WalK with its response regulator WalR. We demonstrate that the intrinsically disordered CTT is characterized by a conserved tryptophan residue surrounded by acidic amino acids. Mutation in the tryptophan not only disrupts the stable interaction, but also impairs the efficient phosphotransferase and phosphatase activities of WalRK. In addition, the tryptophan is important for WalK to compete with DNA containing a WalR binding motif for the WalR interaction. We further show that the tryptophan is important for in vivo transcriptional regulation and bacterial biofilm formation by S. mutans. Moreover, Staphylococcus aureus WalK also has a characteristic CTT, albeit relatively shorter, with a conserved W-acidic motif, that is required for the WalRK interaction in vitro. Together, these data reveal that the W-acidic motif of WalK is indispensable for its interaction with WalR, thereby playing a key role in the WalRK-dependent signal transduction, transcriptional regulation and biofilm formation.

Allostery and protein plasticity: the keystones for bacterial signaling and regulation

Bacteria sense intracellular and environmental signals using an array of proteins as antennas. The information is transmitted from such sensory modules to other protein domains that act as output effectors. Sensor and effector can be part of the same polypeptide or instead be separate diffusible proteins that interact specifically. The output effector modules regulate physiologic responses, allowing the cells to adapt to the varying conditions. These biological machineries are known as signal transduction systems (STSs). Despite the captivating architectural diversity exhibited by STS proteins, a universal feature is their allosteric regulation: signal binding at one site modifies the activity at a physically distant site. Allostery requires protein plasticity, precisely encoded within their 3D structures, and implicating programmed molecular motions. This review summarizes how STS proteins connect stimuli to specific responses by exploiting allostery and protein plasticity. Illustrative examples spanning a wide variety of protein folds will focus on one- and two-component systems (TCSs). The former encompass the entire transmission route within a single polypeptide, whereas TCSs have evolved as separate diffusible proteins that interact specifically, sometimes including additional intermediary proteins in the pathway. Irrespective of their structural diversity, STS proteins are able to modulate their own molecular motions, which can be relatively slow, rigid-body movements, all the way to fast fluctuations in the form of macromolecular flexibility, thus spanning a continuous protein dynamics spectrum. In sum, STSs rely on allostery to steer information transmission, going from simple two-state switching to rich multi-state conformational order/disorder transitions.

The many ways that nature has exploited the unusual structural and chemical properties of phosphohistidine for use in proteins

Histidine phosphorylation is an important and ubiquitous post-translational modification. Histidine undergoes phosphorylation on either of the nitrogens in its imidazole side chain, giving rise to 1- and 3- phosphohistidine (pHis) isomers, each having a phosphoramidate linkage that is labile at high temperatures and low pH, in contrast with stable phosphomonoester protein modifications. While all organisms routinely use pHis as an enzyme intermediate, prokaryotes, lower eukaryotes and plants also use it for signal transduction. However, research to uncover additional roles for pHis in higher eukaryotes is still at a nascent stage. Since the discovery of pHis in 1962, progress in this field has been relatively slow, in part due to a lack of the tools and techniques necessary to study this labile modification. However, in the past ten years the development of phosphoproteomic techniques to detect phosphohistidine (pHis), and methods to synthesize stable pHis analogues, which enabled the development of anti-phosphohistidine (pHis) antibodies, have accelerated our understanding. Recent studies that employed anti-pHis antibodies and other advanced techniques have contributed to a rapid expansion in our knowledge of histidine phosphorylation. In this review, we examine the varied roles of pHis-containing proteins from a chemical and structural perspective, and present an overview of recent developments in pHis proteomics and antibody development.

Comparative analysis of two paradigm bacteriophytochromes reveals opposite functionalities in two-component signaling

Bacterial phytochrome photoreceptors usually belong to two-component signaling systems which transmit environmental stimuli to a response regulator through a histidine kinase domain. Phytochromes switch between red light-absorbing and far-red light-absorbing states. Despite exhibiting extensive structural responses during this transition, the model bacteriophytochrome from Deinococcus radiodurans (DrBphP) lacks detectable kinase activity. Here, we resolve this long-standing conundrum by comparatively analyzing the interactions and output activities of DrBphP and a bacteriophytochrome from Agrobacterium fabrum (Agp1). Whereas Agp1 acts as a conventional histidine kinase, we identify DrBphP as a light-sensitive phosphatase. While Agp1 binds its cognate response regulator only transiently, DrBphP does so strongly, which is rationalized at the structural level. Our data pinpoint two key residues affecting the balance between kinase and phosphatase activities, which immediately bears on photoreception and two-component signaling. The opposing output activities in two highly similar bacteriophytochromes suggest the use of light-controllable histidine kinases and phosphatases for optogenetics.

Interhelical H-Bonds Modulate the Activity of a Polytopic Transmembrane Kinase

DesK is a Histidine Kinase that allows Bacillus subtilis to maintain lipid homeostasis in response to changes in the environment. It is located in the membrane, and has five transmembrane helices and a cytoplasmic catalytic domain. The transmembrane region triggers the phosphorylation of the catalytic domain as soon as the membrane lipids rigidify. In this research, we study how transmembrane inter-helical interactions contribute to signal transmission; we designed a co-expression system that allows studying in vivo interactions between transmembrane helices. By Alanine-replacements, we identified a group of polar uncharged residues, whose side chains contain hydrogen-bond donors or acceptors, which are required for the interaction with other DesK transmembrane helices; a particular array of H-bond- residues plays a key role in signaling, transmitting information detected at the membrane level into the cell to finally trigger an adaptive response.

Nitrate- and Nitrite-Sensing Histidine Kinases: Function, Structure, and Natural Diversity

Under anaerobic conditions, bacteria may utilize nitrates and nitrites as electron acceptors. Sensitivity to nitrous compounds is achieved via several mechanisms, some of which rely on sensor histidine kinases (HKs). The best studied nitrate- and nitrite-sensing HKs (NSHKs) are NarQ and NarX from Escherichia coli. Here, we review the function of NSHKs, analyze their natural diversity, and describe the available structural information. In particular, we show that around 6000 different NSHK sequences forming several distinct clusters may now be found in genomic databases, comprising mostly the genes from Beta- and Gammaproteobacteria as well as from Bacteroidetes and Chloroflexi, including those from anaerobic ammonia oxidation (annamox) communities. We show that the architecture of NSHKs is mostly conserved, although proteins from Bacteroidetes lack the HAMP and GAF-like domains yet sometimes have PAS. We reconcile the variation of NSHK sequences with atomistic models and pinpoint the structural elements important for signal transduction from the sensor domain to the catalytic module over the transmembrane and cytoplasmic regions spanning more than 200 Å.

Cryo-Electron Microscopy of Arabidopsis thaliana Phytochrome A in Its Pr State Reveals Head-to-Head Homodimeric Architecture

Phytochrome photoreceptors regulate vital adaptations of plant development, growth, and physiology depending on the ratio of red and far-red light. The light-triggered Z/E isomerization of a covalently bound bilin chromophore underlies phytochrome photoconversion between the red-absorbing Pr and far-red-absorbing Pfr states. Compared to bacterial phytochromes, the molecular mechanisms of signal propagation to the C-terminal module and its regulation are little understood in plant phytochromes, not least owing to a dearth of structural information. To address this deficit, we studied the Arabidopsis thaliana phytochrome A (AtphyA) at full length by cryo-electron microscopy (cryo-EM). Following heterologous expression in Escherichia coli, we optimized the solvent conditions to overcome protein aggregation and thus obtained photochemically active, near-homogenous AtphyA. We prepared grids for cryo-EM analysis of AtphyA in its Pr state and conducted single-particle analysis. The resulting two-dimensional class averages and the three-dimensional electron density map at 17 Å showed a homodimeric head-to-head assembly of AtphyA. Docking of domain structures into the electron density revealed a separation of the AtphyA homodimer at the junction of its photosensor and effector modules, as reflected in a large void in the middle of map. The overall architecture of AtphyA resembled that of bacterial phytochromes, thus hinting at commonalities in signal transduction and mechanism between these receptors. Our work paves the way toward future studies of the structure, light response, and interactions of full-length phytochromes by cryo-EM.

Bionoi: A Voronoi Diagram-Based Representation of Ligand-Binding Sites in Proteins for Machine Learning Applications

Bionoi is a new software to generate Voronoi representations of ligand-binding sites in proteins for machine learning applications. Unlike many other deep learning models in biomedicine, Bionoi utilizes off-the-shelf convolutional neural network architectures, reducing the development work without sacrificing the performance. When initially generating images of binding sites, users have the option to color the Voronoi cells based on either one of six structural, physicochemical, and evolutionary properties, or a blend of all six individual properties. Encouragingly, after inputting images generated by Bionoi into the convolutional autoencoder, the network was able to effectively learn the most salient features of binding pockets. The accuracy of the generated model is evaluated both visually and numerically through the reconstruction of binding site images from the latent feature space. The generated feature vectors capture well various properties of binding sites and thus can be applied in a multitude of machine learning projects. As a demonstration, we trained the ResNet-18 architecture from Microsoft on Bionoi images to show that it is capable to effectively classify nucleotide- and heme-binding pockets against a large dataset of control pockets binding a variety of small molecules. Bionoi is freely available to the research community at https://github.com/CSBG-LSU/BionoiNet.

Identification of Novel Thermosensors in Gram-Positive Pathogens

Temperature is a crucial variable that every living organism, from bacteria to humans, need to sense and respond to in order to adapt and survive. In particular, pathogenic bacteria exploit host-temperature sensing as a cue for triggering virulence gene expression. Here, we have identified and characterized two integral membrane thermosensor histidine kinases (HKs) from Gram-positive pathogens that exhibit high similarity to DesK, the extensively characterized cold sensor histidine kinase from Bacillus subtilis. Through in vivo experiments, we demonstrate that SA1313 from Staphylococcus aureus and BA5598 from Bacillus anthracis, which likely control the expression of putative ATP binding cassette (ABC) transporters, are regulated by environmental temperature. We show here that these HKs can phosphorylate the non-cognate response regulator DesR, partner of DesK, both in vitro and in vivo, inducing in B. subtilis the expression of the des gene upon a cold shock. In addition, we report the characterization of another DesK homolog from B. subtilis, YvfT, also closely associated to an ABC transporter. Although YvfT phosphorylates DesR in vitro, this sensor kinase can only induce des expression in B. subtilis when overexpressed together with its cognate response regulator YvfU. This finding evidences a physiological mechanism to avoid cross talk with DesK after a temperature downshift. Finally, we present data suggesting that the HKs studied in this work appear to monitor different ranges of membrane lipid properties variations to mount adaptive responses upon cooling. Overall, our findings point out that bacteria have evolved sophisticated mechanisms to assure specificity in the response to environmental stimuli. These findings pave the way to understand thermosensing mediated by membrane proteins that could have important roles upon host invasion by bacterial pathogens.

Structural insights into the signal transduction mechanism of the K+-sensing two-component system KdpDE

Two-component systems (TCSs), which consist of a histidine kinase (HK) sensor and a response regulator (RR), are important for bacteria to quickly sense and respond to various environmental signals. HKs and RRs typically function as a cognate pair, interacting only with one another to transduce signaling. Precise signal transduction in a TCS depends on the specific interactions between the receiver domain (RD) of the RR and the dimerization and histidine phosphorylation domain (DHp) of the HK. Here, we determined the complex structure of KdpDE, a TCS consisting of the HK KdpD and the RR KdpE, which is responsible for K⁺ homeostasis. Both the RD and the DNA binding domain (DBD) of KdpE interacted with KdpD. Although the RD of KdpE and the DHp of KdpD contributed to binding specificity, the DBD mediated a distinct interaction with the catalytic ATP-binding (CA) domain of KdpD that was indispensable for KdpDE-mediated signal transduction. Moreover, the DBD-CA interface largely overlapped with that of the DBD-DNA complex, leading to competition between KdpD and its target promoter in a KdpE phosphorylation-dependent manner. In addition, the extended C-terminal tail of the CA domain was critical for stabilizing the interaction with KdpDE and for signal transduction. Together, these data provide a molecular basis for specific KdpD and KdpE interactions that play key roles in efficient signal transduction and transcriptional regulation by this TCS.

Modulation of Response Regulator CheY Reaction Kinetics by Two Variable Residues That Affect Conformation

Microorganisms and plants utilize two-component systems to regulate adaptive responses to changing environmental conditions. Sensor kinases detect stimuli and alter their autophosphorylation activity accordingly. Signal propagation occurs via the transfer of phosphoryl groups from upstream kinases to downstream response regulator proteins. Removal of phosphoryl groups from the response regulator typically resets the system. Members of the same protein family may catalyze phosphorylation and dephosphorylation reactions with different efficiencies, exhibiting rate constants spanning many orders of magnitude to accommodate response time scales from milliseconds to days. We previously found that variable positions one or two residues to the C-terminal side of the conserved Asp phosphorylation site (D+2) or Thr/Ser (T+1/T+2) in response regulators alter reaction kinetics by direct interaction with phosphodonor or phosphoacceptor molecules. Here, we explore the kinetic effects of amino acid substitutions at the two positions immediately C-terminal to the conserved Lys (K+1/K+2) in the model Escherichia coli response regulator CheY. We measured CheY autophosphorylation and autodephosphorylation rate constants for 27 pairs of K+1/K+2 residues that represent 84% of naturally occurring response regulators. Effects on autodephosphorylation were modest, but autophosphorylation rate constants varied by 2 orders of magnitude, suggesting that the K+1/K+2 positions influence reaction kinetics by altering the conformational spectrum sampled by CheY at equilibrium. Additional evidence supporting this indirect mechanism includes the following: the effect on autophosphorylation rate constants is independent of the phosphodonor, the autophosphorylation rate constants and dissociation constants for the phosphoryl group analog BeF₃ ^- are inversely correlated, and the K+1/K+2 positions are distant from the phosphorylation site.IMPORTANCE We have identified five variable positions in response regulators that allow the rate constants of autophosphorylation and autodephosporylation reactions each to be altered over 3 orders of magnitude in CheY. The distributions of variable residue combinations across response regulator subfamilies suggest that distinct mechanisms associated with different variable positions allow reaction rates to be tuned independently during evolution for diverse biological purposes. This knowledge could be used in synthetic-biology applications to adjust the properties (e.g., background noise and response duration) of biosensors and may allow prediction of response regulator reaction kinetics from the primary amino acid sequence.

Design of a Histidine Kinase FRET Sensor to Detect Complex Signal Integration within Living Bacteria

Histidine kinases (HK) switch between conformational states that promote kinase and phosphatase activities to regulate diverse cellular processes. Past studies have shown that these functional states can display heterogeneity between cells in microbial communities and can vary at the subcellular level. Methods to track and correlate the kinase conformational state with the phenotypic response of living bacteria cells will offer new opportunities to interrogate bacterial signaling mechanisms. As a proof of principle, we incorporated both mClover3 (donor) and mRuby3 (acceptor) fluorescent proteins into the Caulobacter crescentus cell-cycle HK CckA as an in vivo fluorescence resonance energy transfer (FRET) sensor to detect these structural changes. Our engineered FRET sensor was responsive to CckA-specific input signals and detected subcellular changes in CckA signal integration that occurs as cells develop. We demonstrated the potential of using the CckA FRET sensor as an in vivo screening tool for HK inhibitors. In summary, we have developed a new HK FRET sensor design strategy that can be adopted to monitor in vivo changes for interrogation of a broad range of signaling mechanisms in living bacteria.

Structural asymmetry does not indicate hemiphosphorylation in the bacterial histidine kinase CpxA

Histidine protein kinases (HKs) are prevalent prokaryotic sensor kinases that are central to phosphotransfer in two-component signal transduction systems, regulating phosphorylation of response regulator proteins that determine the output responses. HKs typically exist as dimers and can potentially autophosphorylate at each conserved histidine residue in the individual protomers, leading to diphosphorylation. However, analyses of HK phosphorylation in biochemical assays in vitro suggest negative cooperativity, whereby phosphorylation in one protomer of the dimer inhibits phosphorylation in the second protomer, leading to ∼50% phosphorylation of the available sites in dimers. This negative cooperativity is often correlated with an asymmetric domain arrangement, a common structural characteristic of autophosphorylation states in many HK structures. In this study, we engineered covalent dimers of the cytoplasmic domains of Escherichia coli CpxA, enabling us to quantify individual species: unphosphorylated, monophosphorylated, and diphosphorylated dimers. Together with mathematical modeling, we unambiguously demonstrate no cooperativity in autophosphorylation of CpxA despite its asymmetric structures, indicating that these asymmetric domain arrangements are not linked to negative cooperativity and hemiphosphorylation. Furthermore, the modeling indicated that many parameters, most notably minor amounts of ADP generated during autophosphorylation reactions or present in ATP preparations, can produce ∼50% total phosphorylation that may be mistakenly attributed to negative cooperativity. This study also establishes that the engineered covalent heterodimer provides a robust experimental system for investigating cooperativity in HK autophosphorylation and offers a useful tool for testing how symmetric or asymmetric structural features influence HK functions.

Revisiting the pH-gated conformational switch on the activities of HisKA-family histidine kinases

Histidine is a versatile residue playing key roles in enzyme catalysis thanks to the chemistry of its imidazole group that can serve as nucleophile, general acid or base depending on its protonation state. In bacteria, signal transduction relies on two-component systems (TCS) which comprise a sensor histidine kinase (HK) containing a phosphorylatable catalytic His with phosphotransfer and phosphatase activities over an effector response regulator. Recently, a pH-gated model has been postulated to regulate the phosphatase activity of HisKA HKs based on the pH-dependent rotamer switch of the phosphorylatable His. Here, we have revisited this model from a structural and functional perspective on HK853-RR468 and EnvZ-OmpR TCS, the prototypical HisKA HKs. We have found that the rotamer of His is not influenced by the environmental pH, ruling out a pH-gated model and confirming that the chemistry of the His is responsible for the decrease in the phosphatase activity at acidic pH.

Protein Dynamics in Phosphoryl-Transfer Signaling Mediated by Two-Component Systems

The ability to perceive the environment, an essential attribute in living organisms, is linked to the evolution of signaling proteins that recognize specific signals and execute predetermined responses. Such proteins constitute concerted systems that can be as simple as a unique protein, able to recognize a ligand and exert a phenotypic change, or extremely complex pathways engaging dozens of different proteins which act in coordination with feedback loops and signal modulation. To understand how cells sense their surroundings and mount specific adaptive responses, we need to decipher the molecular workings of signal recognition, internalization, transfer, and conversion into chemical changes inside the cell. Protein allostery and dynamics play a central role. Here, we review recent progress on the study of two-component systems, important signaling machineries of prokaryotes and lower eukaryotes. Such systems implicate a sensory histidine kinase and a separate response regulator protein. Both components exploit protein flexibility to effect specific conformational rearrangements, modulating protein-protein interactions, and ultimately transmitting information accurately. Recent work has revealed how histidine kinases switch between discrete functional states according to the presence or absence of the signal, shifting key amino acid positions that define their catalytic activity. In concert with the cognate response regulator's allosteric changes, the phosphoryl-transfer flow during the signaling process is exquisitely fine-tuned for proper specificity, efficiency and directionality.

Hybrid histidine kinase activation by cyclic di-GMP-mediated domain liberation

Expression of bacterial genes in response to various cues is predominantly regulated by 2- or multicomponent systems with autophosphorylation of a histidine kinase (HK), the first component, being controlled by an N-terminal sensor domain. This is followed by phosphotransfer to the receiver domain (Rec) of a cognate transcription factor. In about 20% of all cases, HK core and Rec are fused to form a hybrid HK (HHK). Here, we show the first full-length structure of an HHK and reveal how it gets activated by the second-messenger c-di-GMP that binds to a dedicated pseudo-Rec domain. The mechanism is fundamentally distinct from the canonical mechanism of HK regulation, but may be operational in many HHKs with a predicted pseudo-Rec domain.

Cytosolic hybrid histidine kinases (HHKs) constitute major signaling nodes that control various biological processes, but their input signals and how these are processed are largely unknown. In Caulobacter crescentus, the HHK ShkA is essential for accurate timing of the G1-S cell cycle transition and is regulated by the corresponding increase in the level of the second messenger c-di-GMP. Here, we use a combination of X-ray crystallography, NMR spectroscopy, functional analyses, and kinetic modeling to reveal the regulatory mechanism of ShkA. In the absence of c-di-GMP, ShkA predominantly adopts a compact domain arrangement that is catalytically inactive. C-di-GMP binds to the dedicated pseudoreceiver domain Rec1, thereby liberating the canonical Rec2 domain from its central position where it obstructs the large-scale motions required for catalysis. Thus, c-di-GMP cannot only stabilize domain interactions, but also engage in domain dissociation to allosterically invoke a downstream effect. Enzyme kinetics data are consistent with conformational selection of the ensemble of active domain constellations by the ligand and show that autophosphorylation is a reversible process.

Insights revealed by the co-crystal structure of the Saccharomyces cerevisiae histidine phosphotransfer protein Ypd1 and the receiver domain of its downstream response regulator Ssk1

Two‐component signaling systems are the primary means by which bacteria, archaea, and certain plants and fungi react to their environments. The model yeast, Saccharomyces cerevisiae, uses the Sln1 signaling pathway to respond to hyperosmotic stress. This pathway contains a hybrid histidine kinase (Sln1) that autophosphorylates and transfers a phosphoryl group to its own receiver domain (R1). The phosphoryl group is then transferred to a histidine phosphotransfer protein (Ypd1) that finally passes it to the receiver domain (R2) of a downstream response regulator (Ssk1). Under normal conditions, Ssk1 is constitutively and preferentially phosphorylated in the phosphorelay. Upon detecting hyperosmotic stress, Ssk1 rapidly dephosphorylates and activates the high‐osmolarity glycerol (HOG) pathway, initiating a response. Despite their distinct physiological roles, both Sln1 and Ssk1 bind to Ypd1 at a common docking site. Co‐crystal structures of response regulators in complex with their phosphorelay partners are scarce, leaving many mechanistic and structural details uncharacterized for systems like the Sln1 pathway. In this work, we present the co‐crystal structure of Ypd1 and a near wild‐type variant of the receiver domain of Ssk1 (Ssk1‐R2‐W638A) at a resolution of 2.80 Å. Our structural analyses of Ypd1‐receiver domain complexes, biochemical determination of binding affinities for Ssk1‐R2 variants, in silico free energy estimates, and sequence comparisons reveal distinctive electrostatic properties of the Ypd1/Ssk1‐R2‐W638A complex that may provide insight into the regulation of the Sln1 pathway as a function of dynamic osmolyte concentration.

Signal transduction in photoreceptor histidine kinases

Two-component systems (TCS) constitute the predominant means by which prokaryotes read out and adapt to their environment. Canonical TCSs comprise a sensor histidine kinase (SHK), usually a transmembrane receptor, and a response regulator (RR). In signal-dependent manner, the SHK autophosphorylates and in turn transfers the phosphoryl group to the RR which then elicits downstream responses, often in form of altered gene expression. SHKs also catalyze the hydrolysis of the phospho-RR, hence, tightly adjusting the overall degree of RR phosphorylation. Photoreceptor histidine kinases are a subset of mostly soluble, cytosolic SHKs that sense light in the near-ultraviolet to near-infrared spectral range. Owing to their experimental tractability, photoreceptor histidine kinases serve as paradigms and provide unusually detailed molecular insight into signal detection, decoding, and regulation of SHK activity. The synthesis of recent results on receptors with light-oxygen-voltage, bacteriophytochrome and microbial rhodopsin sensor units identifies recurring, joint signaling strategies. Light signals are initially absorbed by the sensor module and converted into subtle rearrangements of α helices, mostly through pivoting and rotation. These conformational transitions propagate through parallel coiled-coil linkers to the effector unit as changes in left-handed superhelical winding. Within the effector, subtle conformations are triggered that modulate the solvent accessibility of residues engaged in the kinase and phosphatase activities. Taken together, a consistent view of the entire trajectory from signal detection to regulation of output emerges. The underlying allosteric mechanisms could widely apply to TCS signaling in general.

Structural insights into the signalling mechanisms of two-component systems

Two-component systems reprogramme diverse aspects of microbial physiology in response to environmental cues. Canonical systems are composed of a transmembrane sensor histidine kinase and its cognate response regulator. They catalyse three reactions: autophosphorylation of the histidine kinase, transfer of the phosphoryl group to the regulator and dephosphorylation of the phosphoregulator. Elucidating signal transduction between sensor and output domains is highly challenging given the size, flexibility and dynamics of histidine kinases. However, recent structural work has provided snapshots of the catalytic mechanisms of the three enzymatic reactions and described the conformation and dynamics of the enzymatic moiety in the kinase-competent and phosphatase-competent states. Insight into signalling mechanisms across the membrane is also starting to emerge from new crystal structures encompassing both sensor and transducer domains of sensor histidine kinases. In this Progress article, we highlight such important advances towards understanding at the molecular level the signal transduction mechanisms mediated by these fascinating molecular machines.

Two-Component Sensing and Regulation: How Do Histidine Kinases Talk with Response Regulators at the Molecular Level?

Perceiving environmental and internal information and reacting in adaptive ways are essential attributes of living organisms. Two-component systems are relevant protein machineries from prokaryotes and lower eukaryotes that enable cells to sense and process signals. Implicating sensory histidine kinases and response regulator proteins, both components take advantage of protein phosphorylation and flexibility to switch conformations in a signal-dependent way. Dozens of two-component systems act simultaneously in any given cell, challenging our understanding about the means that ensure proper connectivity. This review dives into the molecular level, attempting to summarize an emerging picture of how histidine kinases and cognate response regulators achieve required efficiency, specificity, and directionality of signaling pathways, properties that rely on protein:protein interactions. α helices that carry information through long distances, the fine combination of loose and specific kinase/regulator interactions, and malleable reaction centers built when the two components meet emerge as relevant universal principles.

Structural Basis of Response Regulator Function

Response regulators function as the output components of two-component systems, which couple the sensing of environmental stimuli to adaptive responses. Response regulators typically contain conserved receiver (REC) domains that function as phosphorylation-regulated switches to control the activities of effector domains that elicit output responses. This modular design is extremely versatile, enabling different regulatory strategies tuned to the needs of individual signaling systems. This review summarizes structural features that underlie response regulator function. An abundance of atomic resolution structures and complementary biochemical data have defined the mechanisms for response regulator enzymatic activities, revealed trends in regulatory strategies utilized by response regulators of different subfamilies, and provided insights into interactions of response regulators with their cognate histidine kinases. Among the hundreds of thousands of response regulators identified, variations abound. This article provides a framework for understanding structural features that enable function of canonical response regulators and a basis for distinguishing noncanonical configurations.

Structural basis for inhibition of a response regulator of σS stability by a ClpXP antiadaptor

Dorich et al. present the first crystal structure of RssB bound to an antiadaptor, the DNA damage-inducible IraD. The structural data, together with mechanistic studies, suggest that RssB plasticity is critical for regulation of σ^S degradation.

The stationary phase promoter specificity subunit σ^S (RpoS) is delivered to the ClpXP machinery for degradation dependent on the adaptor RssB. This adaptor-specific degradation of σ^S provides a major point for regulation and transcriptional reprogramming during the general stress response. RssB is an atypical response regulator and the only known ClpXP adaptor that is inhibited by multiple but dissimilar antiadaptors (IraD, IraP, and IraM). These are induced by distinct stress signals and bind to RssB in poorly understood manners to achieve stress-specific inhibition of σ^S turnover. Here we present the first crystal structure of RssB bound to an antiadaptor, the DNA damage-inducible IraD. The structure reveals that RssB adopts a compact closed architecture with extensive interactions between its N-terminal and C-terminal domains. The structural data, together with mechanistic studies, suggest that RssB plasticity, conferred by an interdomain glutamate-rich flexible linker, is critical for regulation of σ^S degradation. Structural modulation of interdomain linkers may thus constitute a general strategy for tuning response regulators.

Helix Cracking Regulates the Critical Interaction between RetS and GacS in Pseudomonas aeruginosa

Recent paradigm shifting discoveries have demonstrated that bacterial signaling kinases engage in unexpected regulatory crosstalk, yet the underlying molecular mechanisms remain largely uncharacterized. The Pseudomonas aeruginosa RetS/GacS system constitutes an ideal model for studying these mechanisms. The in-depth analysis of the kinase region of RetS and RetS/GacS interactions presented here refutes a longstanding model, which posited the formation of a catalytically inactive RetS/GacS heterodimer. Crystallographic studies uncovered structurally dynamic features within the RetS kinase region, suggesting that RetS uses the reversible unfolding of a helix, or helix cracking, to control interactions with GacS. The pivotal importance of this helical region for regulating GacS and, by extension, Pseudomonas aeruginosa virulence, was corroborated via in vivo assays. The implications of this work extend beyond the RetS/GacS system because the helix cracking occurs right next to a highly conserved catalytic residue histidine-424, suggesting this model could represent an emergent archetype for histidine kinase regulation.

Insights into histidine kinase activation mechanisms from the monomeric blue light sensor EL346

Translation of environmental cues into cellular behavior is a necessary process in all forms of life. In bacteria, this process frequently involves two-component systems in which a sensor histidine kinase (HK) autophosphorylates in response to a stimulus before subsequently transferring the phosphoryl group to a response regulator that controls downstream effectors. Many details of the molecular mechanisms of HK activation are still unclear due to complications associated with the multiple signaling states of these large, multidomain proteins. To address these challenges, we combined complementary solution biophysical approaches to examine the conformational changes upon activation of a minimal, blue-light-sensing histidine kinase from Erythrobacter litoralis HTCC2594, EL346. Our data show that multiple conformations coexist in the dark state of EL346 in solution, which may explain the enzyme's residual dark-state activity. We also observe that activation involves destabilization of the helices in the dimerization and histidine phosphotransfer-like domain, where the phosphoacceptor histidine resides, and their interactions with the catalytic domain. Similar light-induced changes occur to some extent even in constitutively active or inactive mutants, showing that light sensing can be decoupled from activation of kinase activity. These structural changes mirror those inferred by comparing X-ray crystal structures of inactive and active HK fragments, suggesting that they are at the core of conformational changes leading to HK activation. More broadly, our findings uncover surprising complexity in this simple system and allow us to outline a mechanism of the multiple steps of HK activation.

Two Mutations Commonly Associated with Daptomycin Resistance in Enterococcus faecium LiaST120A and LiaRW73C Appear To Function Epistatically in LiaFSR Signaling

The cyclic antimicrobial lipopeptide daptomycin is now frequently used as a first-line therapy in serious infections caused by multidrug-resistant Enterococcus faecium. Resistance to daptomycin in E. faecium is mediated by activation of the LiaFSR membrane stress response pathway. Deletion of liaR, encoding the response regulator of the system, restores susceptibility to daptomycin, suggesting that the LiaFSR pathway is a potential target for the development of drugs that would induce hypersusceptibility to daptomycin and make it more difficult for enterococci to become daptomycin-resistant. In clinical isolates of E. faecium, substitutions in the membrane-bound histidine kinase LiaS (T120A) and its response regulator LiaR (W73C) are found together, suggesting a potential epistatic relationship in daptomycin resistance. Using in vitro phosphorylation studies, we show that while the phosphotransfer rate of wild-type LiaS and LiaS^T120A to either wild-type LiaR or LiaR^W73C remains rapid and comparable, the LiaS-dependent dephosphorylation rate of phosphorylated LiaR^W73C is markedly higher. When the two adaptive mutants LiaR^W73C and LiaS^T210A are paired, however, LiaS-mediated LiaR dephosphorylation is restored back to wild-type levels. Taken together with earlier work showing that LiaR^W73C leads to an increased level of oligomerization and subsequently favors an increased level of transcription of the LiaFSR regulon, the net effect of the two commonly found LiaS^T120A and LiaR^W73C alleles would be to coordinately increase the strength and persistence of LiaFSR signaling and decrease daptomycin susceptibility. The in vitro approaches developed in this work also provide the basis for screens for identifying drug candidates that inhibit the LiaFSR pathway.

Measuring the Activities of Two-Component Regulatory System Phosphatases

Two-component regulatory systems (TCSs) are used for signal transduction by organisms from all three phylogenetic domains of the living world. TCSs use transient protein phosphorylation and dephosphorylation reactions to convert stimuli into appropriate responses to changing environmental conditions. Phosphoryl groups flow from ATP to sensor kinases (which detect stimuli) to response regulators (which implement responses) to inorganic phosphate (P_i). The phosphorylation state of response regulators controls their output activity. The rate at which phosphoryl groups are removed from response regulators correlates with the timescale of the corresponding biological function. Dephosphorylation reactions are fastest in chemotaxis TCS and slower in other TCS. Response regulators catalyze their own dephosphorylation, but at least five types of phosphatases are known to enhance dephosphorylation of response regulators. In each case, the phosphatases are believed to stimulate the intrinsic autodephosphorylation reaction. We discuss in depth the properties of TCS (particularly the differences between chemotaxis and nonchemotaxis TCS) relevant to designing in vitro assays for TCS phosphatases. We describe detailed assay methods for chemotaxis TCS phosphatases using loss of ³²P, change in intrinsic fluorescence as a result of dephosphorylation, or release of P_i. The phosphatase activities of nonchemotaxis TCS phosphatases are less well characterized. We consider how the properties of nonchemotaxis TCS affect assay design and suggest suitable modifications for phosphatases from nonchemotaxis TCS, with an emphasis on the P_i release method.

Architecture of the complete oxygen-sensing FixL-FixJ two-component signal transduction system

The symbiotic nitrogen-fixing bacterium Bradyrhizobium japonicum is critical to the agro-industrial production of soybean because it enables the production of high yields of soybeans with little use of nitrogenous fertilizers. The FixL and FixJ two-component system (TCS) of this bacterium ensures that nitrogen fixation is only stimulated under conditions of low oxygen. When it is not bound to oxygen, the histidine kinase FixL undergoes autophosphorylation and transfers phosphate from adenosine triphosphate (ATP) to the response regulator FixJ, which, in turn, stimulates the expression of genes required for nitrogen fixation. We purified full-length B. japonicum FixL and FixJ proteins and defined their structures individually and in complex using small-angle x-ray scattering, crystallographic, and in silico modeling techniques. Comparison of active and inactive forms of FixL suggests that intramolecular signal transduction is driven by local changes in the sensor domain and in the coiled-coil region connecting the sensor and histidine kinase domains. We also found that FixJ exhibits conformational plasticity not only in the monomeric state but also in tetrameric complexes with FixL during phosphotransfer. This structural characterization of a complete TCS contributes both a mechanistic and evolutionary understanding to TCS signal relay, specifically in the context of the control of nitrogen fixation in root nodules.

Transmembrane Signal Transduction in Two-Component Systems: Piston, Scissoring, or Helical Rotation?

Allosteric and transmembrane (TM) signaling are among the major questions of structural biology. Here, we review and discuss signal transduction in four-helical TM bundles, focusing on histidine kinases and chemoreceptors found in two-component systems. Previously, piston, scissors, and helical rotation have been proposed as the mechanisms of TM signaling. We discuss theoretically possible conformational changes and examine the available experimental data, including the recent crystallographic structures of nitrate/nitrite sensor histidine kinase NarQ and phototaxis system NpSRII:NpHtrII. We show that TM helices can flex at multiple points and argue that the various conformational changes are not mutually exclusive, and often are observed concomitantly, throughout the TM domain or in its part. The piston and scissoring motions are the most prominent motions in the structures, but more research is needed for definitive conclusions.

Conformational dynamics of the essential sensor histidine kinase WalK

Two-component systems (TCSs) are key elements in bacterial signal transduction in response to environmental stresses. TCSs generally consist of sensor histidine kinases (SKs) and their cognate response regulators (RRs). Many SKs exhibit autokinase, phosphoryltransferase and phosphatase activities, which regulate RR activity through a phosphorylation and dephosphorylation cycle. However, how SKs perform different enzymatic activities is poorly understood. Here, several crystal structures of the minimal catalytic region of WalK, an essential SK from Lactobacillus plantarum that shares 60% sequence identity with its homologue VicK from Streptococcus mutans, are presented. WalK adopts an asymmetrical closed structure in the presence of ATP or ADP, in which one of the CA domains is positioned close to the DHp domain, thus leading both the β- and γ-phosphates of ATP/ADP to form hydrogen bonds to the ℇ- but not the δ-nitrogen of the phosphorylatable histidine in the DHp domain. In addition, the DHp domain in the ATP/ADP-bound state has a 25.7° asymmetrical helical bending coordinated with the repositioning of the CA domain; these processes are mutually exclusive and alternate in response to helicity changes that are possibly regulated by upstream signals. In the absence of ATP or ADP, however, WalK adopts a completely symmetric open structure with its DHp domain centred between two outward-reaching CA domains. In summary, these structures of WalK reveal the intrinsic dynamic properties of an SK structure as a molecular basis for multifunctionality.

Conformational Changes of an Interdomain Linker Mediate Mechanical Signal Transmission in Sensor Kinase BvgS

The whooping cough agent, Bordetella pertussis, controls the expression of its large virulence regulon in a coordinated manner through the two-component system BvgAS. BvgS is a dimeric, multidomain sensor kinase. Each monomer comprises, in succession, tandem periplasmic Venus flytrap (VFT) domains, a transmembrane segment, a cytoplasmic Per-Arnt-Sim (PAS) domain, a kinase module, and additional phosphorelay domains. BvgS shifts between kinase and phosphatase modes of activity in response to chemical modulators that modify the clamshell motions of the VFT domains. We have shown previously that this regulation involves a shift between distinct states of conformation and dynamics of the two-helix coiled-coil linker preceding the enzymatic module. In this work, we determined the mechanism of signal transduction across the membrane via a first linker, which connects the VFT and PAS domains of BvgS, using extensive cysteine cross-linking analyses and other approaches. Modulator perception by the periplasmic domains appears to trigger a small, symmetrical motion of the transmembrane segments toward the periplasm, causing rearrangements of the noncanonical cytoplasmic coiled coil that follows. As a consequence, the interface of the PAS domains is modified, which affects the second linker and eventually causes the shift of enzymatic activity. The major features of this first linker are well conserved among BvgS homologs, indicating that the mechanism of signal transduction unveiled here is likely to be generally relevant for this family of sensor kinases.IMPORTANCEBordetella pertussis produces virulence factors coordinately regulated by the two-component system BvgAS. BvgS is a sensor kinase, and BvgA is a response regulator that activates gene transcription when phosphorylated by BvgS. Sensor kinases homologous to BvgS are also found in other pathogens. Our goal is to decipher the mechanisms of BvgS signaling, since these sensor kinases may represent new targets for antibacterial agents. Signal perception by the sensor domains of BvgS triggers small motions of the helical linker region underneath. The protein domain that follows this linker undergoes a large conformational change that amplifies the initial signal, causing a shift of activity from kinase to phosphatase. Because BvgS homologs harbor similar regions, these signaling mechanisms are likely to apply generally to that family of sensor kinases.

Multiscale approach to the activation and phosphotransfer mechanism of CpxA histidine kinase reveals a tight coupling between conformational and chemical steps

Sensor histidine kinases (SHKs) are an integral component of the molecular machinery that permits bacteria to adapt to widely changing environmental conditions. CpxA, an extensively studied SHK, is a multidomain homodimeric protein with each subunit consisting of a periplasmic sensor domain, a transmembrane domain, a signal-transducing HAMP domain, a dimerization and histidine phospho-acceptor sub-domain (DHp) and a catalytic and ATP-binding subdomain (CA). The key activation event involves the rearrangement of the HAMP-DHp helical core and translation of the CA towards the acceptor histidine, which presumably results in an autokinase-competent complex. In the present work we integrate coarse-grained, all-atom, and hybrid QM-MM computer simulations to probe the large-scale conformational reorganization that takes place from the inactive to the autokinase-competent state (conformational step), and evaluate its relation to the autokinase reaction itself (chemical step). Our results highlight a tight coupling between conformational and chemical steps, underscoring the advantage of CA walking along the DHp core, to favor a reactive tautomeric state of the phospho-acceptor histidine. The results not only represent an example of multiscale modelling, but also show how protein dynamics can promote catalysis.

Snapshots of the Signaling Complex DesK:DesR in Different Functional States Using Rational Mutagenesis and X-ray Crystallography

We have developed protocols to generate site-specific variants of the histidine-kinase DesK and its cognate response regulator DesR, conducive to trapping different signaling states of the proteins. Co-expression of both partners in E. coli, ensuring an excess of the regulator, was essential for soluble production of the DesK:DesR complexes and further purification. The 3D structures of the complex trapped in the phosphotransferase and in the phosphatase reaction steps, were solved by X-ray crystallography using molecular replacement. The solution was not trivial, and we found that in silico-generated models used as search probes, were instrumental to succeeding in placing a large portion of the complex in the asymmetric unit. Electron density maps were then clear enough to allow for manual model building attaining complete atomic models. These methods contribute to tackling a major challenge in the bacterial signaling field, namely obtaining stable kinase:regulator complexes, in distinct conformational states, amenable for high-resolution crystallographic studies.

Sequential conformational transitions and α-helical supercoiling regulate a sensor histidine kinase

Sensor histidine kinases are central to sensing in bacteria and in plants. They usually contain sensor, linker, and kinase modules and the structure of many of these components is known. However, it is unclear how the kinase module is structurally regulated. Here, we use nano- to millisecond time-resolved X-ray scattering to visualize the solution structural changes that occur when the light-sensitive model histidine kinase YF1 is activated by blue light. We find that the coiled coil linker and the attached histidine kinase domains undergo a left handed rotation within microseconds. In a much slower second step, the kinase domains rearrange internally. This structural mechanism presents a template for signal transduction in sensor histidine kinases.

Sensor histidine kinases (SHK) consist of sensor, linker and kinase modules and different models for SHK signal transduction have been proposed. Here the authors present nano- to millisecond time-resolved X-ray scattering measurements, which reveal a structural mechanism for kinase domain activation in SHK.

Non-canonical activation of histidine kinase KdpD by phosphotransferase protein PtsN through interaction with the transmitter domain

The two-component system KdpD/KdpE governs K⁺ homeostasis by controlling synthesis of the high affinity K⁺ transporter KdpFABC. When sensing low environmental K⁺ concentrations, the dimeric kinase KdpD autophosphorylates in trans and transfers the phosphoryl-group to the response regulator KdpE, which subsequently activates kdpFABC transcription. In Escherichia coli, KdpD can also be activated by interaction with the non-phosphorylated form of the accessory protein PtsN. PtsN stimulates KdpD kinase activity thereby increasing phospho-KdpE levels. Here, we analyzed the interplay between KdpD/KdpE and PtsN. PtsN binds specifically to the catalytic DHp domain of KdpD, which is also contacted by KdpE. Accordingly, PtsN and KdpE compete for binding, providing a paradox. Low levels of non-phosphorylated PtsN stimulate, whereas high amounts reduce kdpFABC expression by blocking access of KdpE to KdpD. Ligand fishing experiments provided insight as they revealed ternary complex formation of PtsN/KdpD₂ /KdpE in vivo demonstrating that PtsN and KdpE bind different protomers in the KdpD dimer. PtsN may bind one protomer to stimulate phosphorylation of the second KdpD protomer, which then phosphorylates bound KdpE. Phosphorylation of PtsN prevents its incorporation in ternary complexes. Interaction with the conserved DHp domain enables PtsN to regulate additional kinases such as PhoR.

Structural Coupling between Autokinase and Phosphotransferase Reactions in a Bacterial Histidine Kinase

Bacterial two-component systems consist of a sensor histidine kinase (HK) and a response regulator (RR). HKs are homodimers that catalyze the autophosphorylation of a histidine residue and the subsequent phosphoryl transfer to its RR partner, triggering an adaptive response. How the HK autokinase and phosphotransferase activities are coordinated remains unclear. Here, we report X-ray structures of the prototypical HK CpxA trapped as a hemi-phosphorylated dimer, and of the receiver domain from the RR partner, CpxR. Our results reveal that the two catalytic reactions can occur simultaneously, one in each protomer of the asymmetric CpxA dimer. Furthermore, the increase of autokinase activity in the presence of phosphotransfer-impaired CpxR put forward the idea of an allosteric switching mechanism, according to which CpxR binding to one CpxA protomer triggers autophosphorylation in the second protomer. The ensuing dynamical model provides a mechanistic explanation of how HKs can efficiently orchestrate two catalytic reactions involving large-scale protein motions.

Blue-light reception through quaternary transitions

Sensory photoreceptors absorb light via their photosensor modules and trigger downstream physiological adaptations via their effector modules. Light reception accordingly depends on precisely orchestrated interactions between these modules, the molecular details of which often remain elusive. Using electron-electron double resonance (ELDOR) spectroscopy and site-directed spin labelling, we chart the structural transitions facilitating blue-light reception in the engineered light-oxygen-voltage (LOV) histidine kinase YF1 which represents a paradigm for numerous natural signal receptors. Structural modelling based on pair-wise distance constraints derived from ELDOR pinpoint light-induced rotation and splaying apart of the two LOV photosensors in the dimeric photoreceptor. Resultant molecular strain likely relaxes as left-handed supercoiling of the coiled-coil linker connecting sensor and effector units. ELDOR data on a photoreceptor variant with an inverted signal response indicate a drastically altered dimer interface but light-induced structural transitions in the linker that are similar to those in YF1. Taken together, we provide mechanistic insight into the signal trajectories of LOV photoreceptors and histidine kinases that inform molecular simulations and the engineering of novel receptors.