Tuning Into the Gut: Molecular Insights Into Vibrio cholerae’s CqsR Sensor

Vibrio cholerae, the etiologic agent of cholera, relies on a sophisticated quorum-sensing (QS) network to orchestrate virulence, biofilm formation, and host colonization. While the receptors CqsS and LuxPQ respond to well-characterized autoinducers, the receptor CqsR has remained more enigmatic. Structure-function studies from the Neiditch lab at Rutgers New Jersey Medical School in Newark, NJ now provide a high-resolution look at how CqsR recognizes ethanolamine—a prevalent metabolite in the human gut—to guide the pathogen’s niche recognition.

Central to this discovery is the classification of CqsR’s periplasmic domain as a double Cache domain, the most common extracellular sensory module in prokaryotes. Despite their prevalence, structural comparisons between ligand-bound and ligand-free (apo) states of Cache receptors are rare. By determining X-ray crystal structures of CqsR in complex with ethanolamine and its analogs, serinol and L-alaninol, alongside an apo-like structure using a D198N mutant, researchers have mapped the precise molecular basis of signal recognition. The binding site located in the membrane-distal Cache domain features a "hot spot" residue, Asp198, which acts as a platform to coordinate both the amine and hydroxyl groups of ethanolamine. This coordination enforces a high-energy gauche configuration for the ligand. A hydrophobic lid, composed of residues like Trp151 and Trp138, further secures the ligand’s alkane carbons. The critical nature of Asp198 was confirmed by both LC-MS/MS and genetic assays; the D198N substitution abolished ligand binding and rendered the receptor unresponsive to ethanolamine in vivo.

Thermodynamic assays using microscale thermophoresis (MST) revealed tight binding to ethanolamine (Kd = 1.3 μM), with structural flexibility in Trp138 allowing the pocket to accommodate small α-amino substituents found in L-alaninol and serinol. However, the receptor is highly specific: modifying the amine functionality or replacing the hydroxyl with a carboxylic acid, as seen in glycine or alanine, completely abolishes binding. Interestingly, while amino acids and quaternary amines bind Cache domains in distinct orientations, 2-amino alcohols like ethanolamine pivot around the amine nitrogen to form a unique conserved pose. How does this binding event translate into a cellular response? By aligning the apo and bound structures, the study identified that ethanolamine binding triggers a compaction of the membrane-distal domain. Because the α1 helix remains relatively stationary, this compaction forces the membrane-proximal domain to rotate anticlockwise. This conformational shift likely transmits a signal through the transmembrane region to regulate the cytoplasmic histidine kinase activity. In the V. cholerae QS circuit, this signal inhibits the kinase activity of CqsR, eventually leading to a high-cell-density gene expression pattern.

An important nuance of CqsR signaling is that V. cholerae does not produce enough

ethanolamine on its own to trigger this response, suggesting that ethanolamine acts primarily as a host-derived niche recognition signal. Furthermore, the study suggests that other unidentified ligands—from the host, other microbes, or the environment—might also modulate CqsR, given its ability to bind various 2-amino alcohol analogs. From an antimicrobial perspective, these insights are important. Understanding the "rules" of Cache domain binding allows for the rational design of signaling agonists or antagonists that could potentially disrupt bacterial communication and virulence.

As researchers continue to decode the chemical language of the gut, CqsR stands as a primary example of how pathogens sense their environment to time their attacks. To visualize the mechanical nature of this transition, one might imagine the Cache domain as a spring-loaded latch; the ethanolamine ligand acts as the key that, once inserted, pulls the mechanism tight, causing the entire handle—the membrane-proximal domain—to twist and signal the machinery on the other side of the door.

Guarnaccia AM, Steenhaut AD, Olenic SD, Na J, Perez LJ, Ng WL, Neiditch MB. Structure-function studies of Vibrio cholerae quorum-sensing receptor CqsR signal recognition. PLoS Pathog. 2025 Sep 4;21(9):e1013447. doi: 10.1371/journal.ppat.1013447. PMID: 40906767; PMCID: PMC12410723. https://journals.plos.org/plospathogens/article?id=10.1371/journal.ppat.1013447