Substrate selectivity in arginine-dependent acid resistance in enteric bacteria

Area of Science:

• Microbiology
• Biochemistry
• Molecular Biology

Background:

• Enteric bacteria require acid resistance systems to survive the stomach's low pH.
• The arginine transporter AdiC is crucial for the arginine-dependent acid resistance system.
• AdiC imports arginine and exports agmatine, a process thought to involve virtual proton pumping.

Purpose of the Study:

• To experimentally determine the specific arginine/agmatine exchange mechanism employed by AdiC under acidic conditions.
• To clarify whether AdiC mediates Arg(+)/Agm(2+) or Arg(2+)/Agm(2+) exchange.
• To investigate the basis of AdiC's substrate selection.

Main Methods:

• Utilized an oriented liposome system capable of maintaining a three-unit pH gradient.
• Performed experiments to analyze the electrogenic nature of AdiC-mediated Arg/Agm exchange.
• Conducted further investigations into AdiC's substrate selection mechanism.

Main Results:

• Demonstrated that AdiC-mediated Arg/Agm exchange is strongly electrogenic, moving positive charge outward.
• Provided evidence that AdiC predominantly catalyzes Arg(+)/Agm(2+) exchange.
• Revealed that AdiC selects substrates based on overall charge (valence) rather than local protonation states.

Conclusions:

• AdiC primarily mediates Arg(+)/Agm(2+) exchange, supporting effective virtual proton pumping essential for bacterial acid resistance.
• The mechanism of substrate selection by AdiC relies on charge, challenging previous hypotheses based on crystal structures.