ExbB and ExbD do not function independently in TonB-dependent energy transduction.

Area of Science:

• Microbiology
• Molecular Biology
• Biochemistry

Background:

• The TonB-dependent energy transduction system is crucial for nutrient uptake in Gram-negative bacteria like E. coli.
• ExbB and ExbD proteins are key components of this system, encoded by the exb operon.
• Previous models proposed distinct roles for ExbB and ExbD based on their cellular ratios.

Purpose of the Study:

• To investigate the specific roles of ExbB and ExbD in the TonB energy transduction cycle.
• To test the hypothesis that ExbB and ExbD form distinct complexes with different functions.
• To elucidate the mechanism of TonB's interaction with the cytoplasmic membrane.

Main Methods:

• Experimental testing of the distinct complex hypothesis.
• Analysis of TonB's response to proton motive force in the presence and absence of ExbB and ExbD.
• Assessment of TonB's association with the cytoplasmic membrane.

Main Results:

• The hypothesis of distinct ExbB/ExbD complexes was rejected.
• Both ExbB and ExbD are required for TonB to conformationally respond to proton motive force.
• Both ExbB and ExbD are essential for TonB's association with the cytoplasmic membrane.

Conclusions:

• ExbB and ExbD function together, not in separate complexes, for TonB energy transduction.
• A single, large complex of ExbB and ExbD is likely involved in TonB's interaction with the cytoplasmic membrane.
• This complex may reach a molecular weight of up to 520 kDa.