Physiologically relevant divalent cations modulate citrate recognition by the McpS chemoreceptor

Area of Science:

• Microbiology
• Biochemistry
• Molecular Biology

Background:

• The McpS chemoreceptor in Pseudomonas putida KT2440 detects tricarboxylic acid (TCA) cycle intermediates, with varying chemotactic responses.
• Citrate, abundant in plant environments, can be a sole carbon source and forms complexes with divalent cations.
• Understanding McpS interactions with citrate and its metal complexes is crucial for bacterial chemotaxis in natural habitats.

Purpose of the Study:

• To investigate the binding affinities of citrate-metal ion complexes using isothermal titration calorimetry.
• To determine the binding preferences of the McpS ligand binding domain (McpS-LBD) for free citrate versus citrate-metal complexes.
• To elucidate the physiological impact of citrate-metal complexation on bacterial chemotaxis.

Main Methods:

• Isothermal titration calorimetry (ITC) to quantify citrate-metal ion complex formation and binding affinities.
• Ligand binding assays using purified McpS-LBD to assess binding to free citrate and various citrate-metal complexes.
• Chemotaxis assays with Pseudomonas putida to evaluate the effect of free citrate and citrate-Mg(2+) complexes on malate-induced taxis.

Main Results:

• Citrate-metal ion complex formation was entropy-driven, with affinities varying from K(D)=157 µM (Mg(2+)) to 3 µM (Ni(2+)).
• McpS-LBD binds free citrate but not complexes with physiologically common Mg(2+) and Ca(2+).
• McpS-LBD recognizes complexes with trace elements like Co(2+), Cd(2+), and Ni(2+), differing from other citrate sensors.
• Free citrate significantly reduces malate-induced chemotaxis in P. putida, while citrate-Mg(2+) complexes have a lesser effect.

Conclusions:

• McpS exhibits selective binding towards free citrate and specific citrate-metal complexes, discriminating based on the metal ion.
• The differential recognition of citrate forms by McpS influences chemotactic responses, impacting bacterial navigation in complex environments.
• These findings provide insights into the molecular mechanisms of chemoreception and bacterial adaptation to nutrient availability and environmental conditions.