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

Residue coevolution and mutational landscape for OmpR and NarL response regulator subfamilies

DNA-binding response regulators (DBRRs) are a broad class of proteins that operate in tandem with their partner kinase proteins to form two-component signal transduction systems in bacteria. Typical DBRRs are composed of two domains where the conserved N-terminal domain accepts transduced signals and the evolutionarily diverse C-terminal domain binds to DNA. These domains are assumed to be functionally independent, and hence recombination of the two domains should yield novel DBRRs of arbitrary input/output response, which can be used as biosensors. This idea has been proved to be successful in some cases; yet, the error rate is not trivial. Improvement of the success rate of this technique requires a deeper understanding of the linker-domain and inter-domain residue interactions, which have not yet been thoroughly examined. Here, we studied residue coevolution of DBRRs of the two main subfamilies (OmpR and NarL) using large collections of bacterial amino acid sequences to extensively investigate the evolutionary signatures of linker-domain and inter-domain residue interactions. Coevolutionary analysis uncovered evolutionarily selected linker-domain and inter-domain residue interactions of known experimental structures, as well as previously unknown inter-domain residue interactions. We examined the possibility of these inter-domain residue interactions as contacts that stabilize an inactive conformation of the DBRR where DNA binding is inhibited for both subfamilies. The newly gained insights on linker-domain/inter-domain residue interactions and shared inactivation mechanisms improve the understanding of the functional mechanism of DBRRs, providing clues to efficiently create functional DBRR-based biosensors. Additionally, we show the feasibility of applying coevolutionary landscape models to predict the functionality of domain-swapped DBRR proteins. The presented result demonstrates that sequence information can be used to filter out bioengineered DBRR proteins that are predicted to be nonfunctional due to a high negative predictive value.

General strategies for using amino acid sequence data to guide biochemical investigation of protein function

The rapid increase of '-omics' data warrants the reconsideration of experimental strategies to investigate general protein function. Studying individual members of a protein family is likely insufficient to provide a complete mechanistic understanding of family functions, especially for diverse families with thousands of known members. Strategies that exploit large amounts of available amino acid sequence data can inspire and guide biochemical experiments, generating broadly applicable insights into a given family. Here we review several methods that utilize abundant sequence data to focus experimental efforts and identify features truly representative of a protein family or domain. First, coevolutionary relationships between residues within primary sequences can be successfully exploited to identify structurally and/or functionally important positions for experimental investigation. Second, functionally important variable residue positions typically occupy a limited sequence space, a property useful for guiding biochemical characterization of the effects of the most physiologically and evolutionarily relevant amino acids. Third, amino acid sequence variation within domains shared between different protein families can be used to sort a particular domain into multiple subtypes, inspiring further experimental designs. Although generally applicable to any kind of protein domain because they depend solely on amino acid sequences, the second and third approaches are reviewed in detail because they appear to have been used infrequently and offer immediate opportunities for new advances. Finally, we speculate that future technologies capable of analyzing and manipulating conserved and variable aspects of the three-dimensional structures of a protein family could lead to broad insights not attainable by current methods.

Generalizable strategy to analyze domains in the context of parent protein architecture: A CheW case study

Domains are the three-dimensional building blocks of proteins. An individual domain can occur in a variety of domain architectures that perform unique functions and are subject to different evolutionary selective pressures. We describe an approach to evaluate the variability in amino acid sequences of a single domain across architectural contexts. The ability to distinguish different evolutionary outcomes of one protein domain can help determine whether existing knowledge about a specific domain will apply to an uncharacterized protein, lead to insights and hypotheses about function, and guide experimental priorities. We developed and tested our approach on CheW-like domains (PF01584), which mediate protein/protein interactions and are difficult to compare experimentally. CheW-like domains occur in CheW scaffolding proteins, CheA kinases, and CheV proteins that regulate bacterial chemotaxis. We analyzed 16 domain architectures that included 94% of all CheW-like domains found in nature. We identified six Classes of CheW-like domains with presumed functional differences. CheV and most CheW proteins contained Class 1 domains, whereas some CheW proteins contained Class 6 (~20%) or Class 2 (~1%) domains instead. Most CheA proteins contained Class 3 domains. CheA proteins with multiple Hpt domains contained Class 4 domains. CheA proteins with two CheW-like domains contained one Class 3 and one Class 5. We also created SimpLogo, an innovative method for visualizing amino acid composition across large sets of multiple sequence alignments of arbitrary length. SimpLogo offers substantial advantages over standard sequence logos for comparison and analysis of related protein sequences. The R package for SimpLogo is freely available.

Azorhizobium caulinodans chemotaxis is controlled by an unusual phosphorelay network

Azorhizobium caulinodans is a nitrogen-fixing bacterium that forms root nodules on its host legume, Sesbania rostrata. This agriculturally significant symbiotic relationship is important in lowland rice cultivation, and allows for nitrogen fixation under flood conditions. Chemotaxis plays an important role in bacterial colonization of the rhizosphere. Plant roots release chemical compounds that are sensed by bacteria, triggering chemotaxis along a concentration gradient toward the roots. This gives motile bacteria a significant competitive advantage during root surface colonization. Although plant-associated bacterial genomes often encode multiple chemotaxis systems, A. caulinodans appears to encode only one. The che cluster on the A. caulinodans genome contains cheA, cheW, cheY2, cheB, and cheR. Two other chemotaxis genes, cheY1 and cheZ, are located independently from the che operon. Both CheY1 and CheY2 are involved in chemotaxis, with CheY1 being the predominant signaling protein. A. caulinodans CheA contains an unusual set of C-terminal domains: a CheW-like/Receiver pair (termed W2-Rec), follows the more common single CheW-like domain. W2-Rec impacts both chemotaxis and CheA function. We found a preference for transfer of phosphoryl groups from CheA to CheY2, rather than to W2-Rec or CheY1, which appears to be involved in flagellar motor binding. Furthermore, we observed increased phosphoryl group stabilities on CheY1 compared to CheY2 or W2-Rec. Finally, CheZ enhanced dephosphorylation of CheY2 substantially more than CheY1, but had no effect on the dephosphorylation rate of W2-Rec. This network of phosphotransfer reactions highlights a previously uncharacterized scheme for regulation of chemotactic responses. IMPORTANCE Chemotaxis allows bacteria to move towards nutrients and away from toxins in their environment. Chemotactic movement provides a competitive advantage over non-specific motion. CheY is an essential mediator of the chemotactic response with phosphorylated and unphosphorylated forms of CheY differentially interacting with the flagellar motor to change swimming behavior. Previously established schemes of CheY dephosphorylation include action of a phosphatase and/or transfer of the phosphoryl group to another receiver domain that acts as a sink. Here, we propose A. caulinodans uses a concerted mechanism in which the Hpt domain of CheA, CheY2, and CheZ function together as a dual sink system to rapidly reset chemotactic signaling. To the best of our knowledge, this mechanism is unlike any that have previously been evaluated. Chemotaxis systems that utilize both receiver and Hpt domains as phosphate sinks likely occur in other bacterial species.

Predicted Functional and Structural Diversity of Receiver Domains in Fungal Two-Component Regulatory Systems

Fungal two-component regulatory systems incorporate receiver domains into hybrid histidine kinases (HHKs) and response regulators. We constructed a nonredundant database of 670 fungal receiver domain sequences from 51 species sampled from nine fungal phyla. A much greater proportion (21%) of predicted fungal response regulators did not belong to known groups than previously appreciated. Receiver domains in Rim15 response regulators from Ascomycota and other phyla are very different from one another, as are the duplicate receiver domains in group XII HHKs. Fungal receiver domains from five known types of response regulators and 20 known types of HHKs exhibit distinct patterns of amino acids at conserved and variable positions known to be structurally and functionally important in bacterial receiver domains. We inferred structure/activity relationships from the patterns and propose multiple experimentally testable hypotheses about the mechanisms of signal transduction mediated by fungal receiver domains.

Reconstitution of an intact clock reveals mechanisms of circadian timekeeping

[Figure: see text].

Role of Position K+4 in the Phosphorylation and Dephosphorylation Reaction Kinetics of the CheY Response Regulator

Two-component signaling is a primary method by which microorganisms interact with their environments. A kinase detects stimuli and modulates autophosphorylation activity. The signal propagates by phosphotransfer from the kinase to a response regulator, eliciting a response. Response regulators operate over a range of time scales, corresponding to their related biological processes. Response regulator active site chemistry is highly conserved, but certain variable residues can influence phosphorylation kinetics. An Ala-to-Pro substitution (K+4, residue 113) in the Escherichia coli response regulator CheY triggers a constitutively active phenotype; however, the A113P substitution is too far from the active site to directly affect phosphochemistry. To better understand the activating mechanism(s) of the substitution, we analyzed receiver domain sequences to characterize the evolutionary role of the K+4 position. Although most featured Pro, Leu, Ile, and Val residues, chemotaxis-related proteins exhibited atypical Ala, Gly, Asp, and Glu residues at K+4. Structural and in silico analyses revealed that CheY A113P adopted a partially active configuration. Biochemical data showed that A113P shifted CheY toward a more activated state, enhancing autophosphorylation. By characterizing CheY variants, we determined that this functionality was transmitted through a hydrophobic network bounded by the β5α5 loop and the α1 helix of CheY. This region also interacts with the phosphodonor CheA_P1, suggesting that binding generates an activating perturbation similar to the A113P substitution. Atypical residues like Ala at the K+4 position likely serve two purposes. First, restricting autophosphorylation may minimize background noise generated by intracellular phosphodonors such as acetyl phosphate. Second, optimizing interactions with upstream partners may help prime the receiver domain for phosphorylation.

Phospho-dependent signaling during the general stress response by the atypical response regulator and ClpXP adaptor RssB

In the model organism Escherichia coli and related species, the general stress response relies on tight regulation of the intracellular levels of the promoter specificity subunit RpoS. RpoS turnover is exclusively dependent on RssB, a two-domain response regulator that functions as an adaptor that delivers RpoS to ClpXP for proteolysis. Here, we report crystal structures of the receiver domain of RssB both in its unphosphorylated form and bound to the phosphomimic BeF₃ ^- . Surprisingly, we find only modest differences between these two structures, suggesting that truncating RssB may partially activate the receiver domain to a "meta-active" state. Our structural and sequence analysis points to RssB proteins not conforming to either the Y-T coupling scheme for signaling seen in prototypical response regulators, such as CheY, or to the signaling model of the less understood FATGUY proteins.