Crosslinking snapshots of bacterial chemoreceptor squads

Area of Science:

• Microbiology
• Molecular Biology
• Biochemistry

Background:

• Bacterial chemoreceptors form signaling complexes, typically trimers of dimers, to detect environmental stimuli.
• The team signaling model suggests mixed receptor squads are essential for signal transduction.
• The precise structural organization of these squads, especially in the absence of core signaling proteins, remains unclear.

Purpose of the Study:

• To investigate the in vivo formation and structural geometry of mixed bacterial chemoreceptor squads.
• To determine if mixed receptor squads can assemble independently of CheA and CheW proteins.
• To confirm if the trimer-of-dimers geometry underlies these pre-signaling receptor assemblies.

Main Methods:

• Cysteine-directed in vivo crosslinking using specifically engineered serine (Tsr) and aspartate (Tar) receptors.
• Utilized a trigonal geometry reporter and a trifunctional maleimide reagent to capture trimer-of-dimers structure.
• Analyzed crosslinking products in bacterial cells lacking CheA and CheW.

Main Results:

• Detected mixed receptor-crosslinking products, confirming the formation of Tsr-Tar dimers within squads.
• Demonstrated that mixed receptor squads form in the absence of CheA and CheW.
• Crosslinking efficiency was independent of receptor methylation and ligand-binding state.
• Evidence suggests most receptors exist in higher-order assemblies, supporting the trimer-of-dimers model.
• Crosslinking in mutant strains correlated with signaling behavior, validating the structural model.

Conclusions:

• Bacterial chemoreceptors can self-assemble into mixed trimers of dimers (squads) independent of cytoplasmic signaling proteins.
• The trimer-of-dimers arrangement is a fundamental structural unit preceding functional chemoreceptor signaling teams.
• These findings provide crucial insights into the initial stages of bacterial chemotaxis signal processing.