The ligand-binding domain of a chemoreceptor from Comamonas testosteroni has a previously unknown homotrimeric structure

The ligand binding domain of a type IV pilus chemoreceptor PilJ has a different fold from that of another PilJ-type receptor McpN

Bacterial chemoreceptors sense the extracellular signals and regulate bacterial motilities, biofilm formation, etc. The periplasmic ligand binding domains of chemoreceptors occur as different structural folds and recognize a diversity of chemical molecules. In Pseudomonas aeruginosa (PAO1), two bacterial chemoreceptors, McpN (PA2788) and PilJ (PA0411), are proposed to both contain a PilJ-like ligand-binding domain (LBD) (Pfam motif PF13675) and involved in nitrate chemotaxis and type IV pilus-mediated motility, respectively. The LBDs of McpN and PilJ consist of 135 and 263 residues, respectively, and share very low sequence identity, suggesting they might occur as different structures. Here, we found that PilJ-LBD folded into an HBM module, the same as the sensor domains of McpS-LBD and TorS-LBD, but it differed from that of McpN-LBD. We also observed a trimer in SEC and AUC and proposed a trimeric model based on the crystal structure. Based on the sequence, we classified the Pfam containing McpN-LBD and PilJ-LBD into three classes: sPilJ (single PilJ) represented by McpN-LBD with only one PilJ domain, dPilJ (dual PilJ) that contained dual PilJ domains, and hPilJ (hybrid PilJ) that comprises of a PilJ domain and another non-PilJ domain. Our work indicates a significant structural difference between the ligand binding domains of PilJ and McpN and will help our further study on both kinds of chemoreceptors.

The volatile organic compound acetoin enhances the colonization of Azorhizobium caulinodans ORS571 on Sesbania rostrata

Chemoreceptors play a crucial role in assisting bacterial sensing and response to environmental stimuli. Genome analysis of Azorhizobium caulinodans ORS571 revealed the presence of 43 putative chemoreceptors, but their biological functions remain largely unknown. In this study, we identified the chemoreceptor AmaP (methyl-accepting protein of A. caulinodans), characterized by the presence of the CHASE3 domain and exhibited a notable response to acetoin. Thus, we investigated the effect of acetoin sensing on its symbiotic association with the host. Our findings uncovered a compelling role for acetoin as a key player in enhancing various facets of A. caulinodans ORS571's performance including biofilm formation, colonization, and nodulation abilities. Notably, acetoin bolstered A. caulinodans ORS571's efficacy in promoting the growth of S. rostrata, even under moderate salt stress conditions. This study not only broadens our understanding of the AmaP protein with its distinctive CHASE3 domain but also highlights the promising potential of acetoin in fortifying the symbiotic relationship between A. caulinodans and Sesbania rostrata.

Attractant and repellent induce opposing changes in the four-helix bundle ligand-binding domain of a bacterial chemoreceptor

Motile bacteria navigate toward favorable conditions and away from unfavorable environments using chemotaxis. Mechanisms of sensing attractants are well understood; however, molecular aspects of how bacteria sense repellents have not been established. Here, we identified malate as a repellent recognized by the MCP2201 chemoreceptor in a bacterium Comamonas testosteroni and showed that it binds to the same site as an attractant citrate. Binding determinants for a repellent and an attractant had only minor differences, and a single amino acid substitution in the binding site inverted the response to malate from a repellent to an attractant. We found that malate and citrate affect the oligomerization state of the ligand-binding domain in opposing way. We also observed opposing effects of repellent and attractant binding on the orientation of an alpha helix connecting the sensory domain to the transmembrane helix. We propose a model to illustrate how positive and negative signals might be generated.

Accessing nutrients as the primary benefit arising from chemotaxis

About half of the known bacterial species perform chemotaxis that gains them access to sites that are optimal for growth and survival. The motility apparatus and chemotaxis signaling pathway impose a large energetic and metabolic burden on the cell. There is almost no limit to the type of chemoeffectors that are recognized by bacterial chemoreceptors. For example, they include hormones, neurotransmitters, quorum-sensing molecules, and inorganic ions. However, the vast majority of chemoeffectors appear to be of metabolic value. We review here the experimental evidence indicating that accessing nutrients is the main selective force that led to the evolution of chemotaxis.

The structural analysis of the periplasmic domain of Sinorhizobium meliloti chemoreceptor McpZ reveals a novel fold and suggests a complex mechanism of transmembrane signaling

Chemotaxis is a fundamental process whereby bacteria seek out nutrient sources and avoid harmful chemicals. For the symbiotic soil bacterium Sinorhizobium meliloti, the chemotaxis system also plays an essential role in the interaction with its legume host. The chemotactic signaling cascade is initiated through interactions of an attractant or repellent compound with chemoreceptors or methyl-accepting chemotaxis proteins (MCPs). S. meliloti possesses eight chemoreceptors to mediate chemotaxis. Six of these receptors are transmembrane proteins with periplasmic ligand-binding domains (LBDs). The specific functions of McpW and McpZ are still unknown. Here, we report the crystal structure of the periplasmic domain of McpZ (McpZPD) at 2.7 Å resolution. McpZPD assumes a novel fold consisting of three concatenated four-helix bundle modules. Through phylogenetic analyses, we discovered that this helical tri-modular domain fold arose within the Rhizobiaceae family and is still evolving rapidly. The structure, offering a rare view of a ligand-free dimeric MCP-LBD, reveals a novel dimerization interface. Molecular dynamics calculations suggest ligand binding will induce conformational changes that result in large horizontal helix movements within the membrane-proximal domains of the McpZPD dimer that are accompanied by a 5 Å vertical shift of the terminal helix toward the inner cell membrane. These results suggest a mechanism of transmembrane signaling for this family of MCPs that entails both piston-type and scissoring movements. The predicted movements terminate in a conformation that closely mirrors those observed in related ligand-bound MCP-LBDs.

D-amino acids signal a stress-dependent run-away response in Vibrio cholerae

To explore favourable niches while avoiding threats, many bacteria use a chemotaxis navigation system. Despite decades of studies on chemotaxis, most signals and sensory proteins are still unknown. Many bacterial species release D-amino acids to the environment; however, their function remains largely unrecognized. Here we reveal that D-arginine and D-lysine are chemotactic repellent signals for the cholera pathogen Vibrio cholerae. These D-amino acids are sensed by a single chemoreceptor MCPDRK co-transcribed with the racemase enzyme that synthesizes them under the control of the stress-response sigma factor RpoS. Structural characterization of this chemoreceptor bound to either D-arginine or D-lysine allowed us to pinpoint the residues defining its specificity. Interestingly, the specificity for these D-amino acids appears to be restricted to those MCPDRK orthologues transcriptionally linked to the racemase. Our results suggest that D-amino acids can shape the biodiversity and structure of complex microbial communities under adverse conditions.

Gas and light: triggers of c-di-GMP-mediated regulation

The widespread bacterial second messenger c-di-GMP is responsible for regulating many important physiological functions such as biofilm formation, motility, cell differentiation, and virulence. The synthesis and degradation of c-di-GMP in bacterial cells depend, respectively, on diguanylate cyclases and c-di-GMP-specific phosphodiesterases. Since c-di-GMP metabolic enzymes (CMEs) are often fused to sensory domains, their activities are likely controlled by environmental signals, thereby altering cellular c-di-GMP levels and regulating bacterial adaptive behaviors. Previous studies on c-di-GMP-mediated regulation mainly focused on downstream signaling pathways, including the identification of CMEs, cellular c-di-GMP receptors, and c-di-GMP-regulated processes. The mechanisms of CME regulation by upstream signaling modules received less attention, resulting in a limited understanding of the c-di-GMP regulatory networks. We review here the diversity of sensory domains related to bacterial CME regulation. We specifically discuss those domains that are capable of sensing gaseous or light signals and the mechanisms they use for regulating cellular c-di-GMP levels. It is hoped that this review would help refine the complete c-di-GMP regulatory networks and improve our understanding of bacterial behaviors in changing environments. In practical terms, this may eventually provide a way to control c-di-GMP-mediated bacterial biofilm formation and pathogenesis in general.

Comparative Genomics of Cyclic di-GMP Metabolism and Chemosensory Pathways in Shewanella algae Strains: Novel Bacterial Sensory Domains and Functional Insights into Lifestyle Regulation

Shewanella spp. play important ecological and biogeochemical roles, due in part to their versatile metabolism and swift integration of stimuli. While Shewanella spp. are primarily considered environmental microbes, Shewanella algae is increasingly recognized as an occasional human pathogen. S. algae shares the broad metabolic and respiratory repertoire of Shewanella spp. and thrives in similar ecological niches. In S. algae, nitrate and dimethyl sulfoxide (DMSO) respiration promote biofilm formation strain specifically, with potential implication of taxis and cyclic diguanosine monophosphate (c-di-GMP) signaling. Signal transduction systems in S. algae have not been investigated. To fill these knowledge gaps, we provide here an inventory of the c-di-GMP turnover proteome and chemosensory networks of the type strain S. algae CECT 5071 and compare them with those of 41 whole-genome-sequenced clinical and environmental S. algae isolates. Besides comparative analysis of genetic content and identification of laterally transferred genes, the occurrence and topology of c-di-GMP turnover proteins and chemoreceptors were analyzed. We found S. algae strains to encode 61 to 67 c-di-GMP turnover proteins and 28 to 31 chemoreceptors, placing S. algae near the top in terms of these signaling capacities per Mbp of genome. Most c-di-GMP turnover proteins were predicted to be catalytically active; we describe in them six novel N-terminal sensory domains that appear to control their catalytic activity. Overall, our work defines the c-di-GMP and chemosensory signal transduction pathways in S. algae, contributing to a better understanding of its ecophysiology and establishing S. algae as an auspicious model for the analysis of metabolic and signaling pathways within the genus Shewanella.

IMPORTANCEShewanella spp. are widespread aquatic bacteria that include the well-studied freshwater model strain Shewanella oneidensis MR-1. In contrast, the physiology of the marine and occasionally pathogenic species Shewanella algae is poorly understood. Chemosensory and c-di-GMP signal transduction systems integrate environmental stimuli to modulate gene expression, including the switch from a planktonic to sessile lifestyle and pathogenicity. Here, we systematically dissect the c-di-GMP proteome and chemosensory pathways of the type strain S. algae CECT 5071 and 41 additional S. algae isolates. We provide insights into the activity and function of these proteins, including a description of six novel sensory domains. Our work will enable future analyses of the complex, intertwined c-di-GMP metabolism and chemotaxis networks of S. algae and their ecophysiological role.

A catalogue of signal molecules that interact with sensor kinases, chemoreceptors and transcriptional regulators

Bacteria have evolved many different signal transduction systems that sense signals and generate a variety of responses. Generally, most abundant are transcriptional regulators, sensor histidine kinases and chemoreceptors. Typically, these systems recognize their signal molecules with dedicated ligand-binding domains (LBDs), which, in turn, generate a molecular stimulus that modulates the activity of the output module. There are an enormous number of different LBDs that recognize a similarly diverse set of signals. To give a global perspective of the signals that interact with transcriptional regulators, sensor kinases and chemoreceptors, we manually retrieved information on the protein-ligand interaction from about 1,200 publications and 3D structures. The resulting 811 proteins were classified according to the Pfam family into 127 groups. These data permit a delineation of the signal profiles of individual LBD families as well as distinguishing between families that recognize signals in a promiscuous manner and those that possess a well-defined ligand range. A major bottleneck in the field is the fact that the signal input of many signaling systems is unknown. The signal repertoire reported here will help the scientific community design experimental strategies to identify the signaling molecules for uncharacterised sensor proteins.

The structural basis for signal promiscuity in a bacterial chemoreceptor

Signalling through chemosensory pathways is typically initiated by the binding of signal molecules to the chemoreceptor ligand binding domain (LBD). The PcaY_PP chemoreceptor from Pseudomonas putida KT2440 is characterized by an unusually broad signal range, and minimal requisites for signal binding are the presence of a C6-membered ring and that of a carboxyl group. Previous studies have shown that only some of the multiple signals recognized by this chemoreceptor are of apparent metabolic value. We report here high-resolution structures of PcaY_PP-LBD in the absence and presence of four cognate chemoeffectors and glycerol. The domain formed a four-helix bundle (4HB), and both ligand binding sites of the dimer were occupied with the high-affinity ligands protocatechuate and quinate, whereas the lower-affinity ligands benzoate and salicylate were present in only one site. Ligand binding was verified by microcalorimetric titration of site-directed mutants revealing important roles of an arginine and number of polar residues that establish an extensive hydrogen bonding network with bound ligands. The comparison of the apo and holo structures did not provide evidence for this receptor employing a transmembrane signalling mechanism that involves piston-like shifts of the final helix. Instead, ligand binding caused rigid-body scissoring movements of both monomers of the dimer. Comparisons with the 4HB domains of the Tar and Tsr chemoreceptors revealed significant structural differences. Importantly, the ligand binding site in PcaY_PP-LBD is approximately 8 Å removed from that of the Tar and Tsr receptors. Data indicate a significant amount of structural and functional diversity among 4HB domains. DATABASES: The coordinates and structure factors have been deposited in the protein data band with the following IDs: 6S1A (apo form), 6S18 (bound glycerol), 6S33 (bound protocatechuate), 6S38 (bound quinate), 6S3B (bound benzoate) and 6S37 (bound salicylate).

A defined structural unit enables de novo design of small-molecule-binding proteins

The de novo design of proteins that bind highly functionalized small molecules represents a great challenge. To enable computational design of binders, we developed a unit of protein structure-a van der Mer (vdM)-that maps the backbone of each amino acid to statistically preferred positions of interacting chemical groups. Using vdMs, we designed six de novo proteins to bind the drug apixaban; two bound with low and submicromolar affinity. X-ray crystallography and mutagenesis confirmed a structure with a precisely designed cavity that forms favorable interactions in the drug-protein complex. vdMs may enable design of functional proteins for applications in sensing, medicine, and catalysis.