Chemical Probes of Bacterial Signal Transduction Reveal That Repellents Stabilize and Attractants Destabilize the Chemoreceptor Array

Abstract

The signal transduction cascade responsible for bacterial chemotaxis serves as a model for understanding how cells perceive and respond to their environments. Bacteria react to chemotactic signals by migrating toward attractants and away from repellents. Recent data suggest that the amplification of attractant stimuli depends on receptor collaboration: occupied and unoccupied chemoreceptors act together to relay attractant signals. Attractant signal transmission, therefore, depends on the organization of the chemoreceptors into a lattice of signaling proteins. The importance of this lattice for transducing repellent signals was unexplored. Here, we investigate the role of inter-receptor communication on repellent responses in Escherichia coli. Previously, we found that multivalent displays of attractants are more potent than their monovalent counterparts. To examine the importance of the chemoreceptor lattice in repellent signaling, we synthesized ligands displaying multiple copies of the repellent leucine. Monomeric leucine and low-valency leucine-displaying polymers were sensed as repellents. In contrast, multivalent displays of leucine capable of binding multiple chemoreceptors function not as potent repellents but as attractants. Intriguingly, chemical cross-linking studies indicate that these multivalent ligands, like monovalent attractants, disrupt the cellular chemoreceptor lattice. Thus, repellents stabilize the intrinsic chemoreceptor lattice, and attractants destabilize it. These results indicate that signals can be transmitted with high sensitivity via the disruption of protein-protein interactions. Moreover, our data demonstrate that repellents can be transformed into attractants merely by their multivalent display. These results have implications for designing agonists and antagonists for other signaling systems.

Salt-driven Equilibrium between Two Conformations in the HAMP Domain from Natronomonas pharaonis

“…Affecting the dynamics of the HAMP domain could be an effective tool to modulate the whole transducer signaling domain dynamics. This would easily lead to changes in the interaction between dimers within the chemoreceptors trimers (52) and also in the cluster super-structure suggested for the chemo-and photoreceptors, an assumption for which now experimental evidence also exists (49). Future NMR and sitedirected spin labeling EPR experiments will unravel the functional and molecular details of the transition between different dynamic states of HAMP domains.…”

“…In fact, the notion that a two-state equilibrium between populations with different degrees of dynamics can be the language of the activation/deactivation of the cytoplasmic domain was again put forward by Hazelbauer et al (48). Recently, evidence for a stabilization/ destabilization mechanism for the bacterial signal transduction were also provided in vivo (49).…”

Attractant Binding Induces Distinct Structural Changes to the Polar and Lateral Signaling Clusters in Bacillus subtilis Chemotaxis

“…Consistent with this model, the activity of the CheA kinase in E. coli was found to increase as the density of receptors increases (13,14). In further support of this model for clustering and kinase regulation, multivalent ligands that destabilized the clusters in E. coli behaved as attractants, whereas ones that stabilized the clusters behaved as repellents (15,16).…”

“…Studies where attractant binding was found to reduce polar receptor clustering employed immunofluorescence (12,15). Those studies that failed to observe any changes in receptor clustering employed live cell microscopy and fluorescent protein fusions (17,18,21).…”

How the “Melting” and “Freezing” of Protein Molecules May Be Used in Cell Signaling

“…There is resonance here with a series of studies of the cluster of receptors responsible for chemotaxis in bacteria, the most recent of which by Borrock et al appears on page 101 of this issue (6). Many bacteria move in response to their chemical environment by the mechanism of chemotaxis, which involves a two-component signal transduction system.…”