FusB Energizes Import across the Outer Membrane through Direct Interaction with Its Ferredoxin Substrate

Area of Science:

• Microbiology
• Bacterial Physiology
• Iron Acquisition Mechanisms

Background:

• Phytopathogenic *Pectobacterium* spp. require iron for infection.
• Iron acquisition is facilitated by the ferredoxin uptake system, involving ferredoxin import into the periplasm.
• The ferredoxin receptor FusA and protease FusC are known, but the import mechanism remains unclear.

Purpose of the Study:

• To elucidate the mechanism of ferredoxin import across the outer membrane in *Pectobacterium* spp.
• To identify the roles of FusA and FusB in ferredoxin transport.
• To understand the energy transduction process involved in ferredoxin uptake.

Main Methods:

• Genetic analysis involving gene knockouts (FusB, FusA, FusC).
• Analysis of ferredoxin transport to the periplasm.
• Investigation of protein-protein interactions between FusB, FusA, and ferredoxin.

Main Results:

• Protein translocation across the outer membrane is dependent on the TonB-like protein FusB.
• Loss of FusB or FusA, but not FusC, abolishes ferredoxin transport.
• FusB interacts with the TonB box of FusA and the ferredoxin substrate, which is essential for transport.

Conclusions:

• FusA and FusB cooperate to transport ferredoxin across the outer membrane.
• FusB energizes ferredoxin transport by facilitating FusA plug removal and substrate translocation.
• This study reveals an unprecedented direct interaction between a TonB-like protein and its substrate, suggesting a common mechanism for atypical substrate uptake in Gram-negative bacteria.