A Brownian ratchet model for ferredoxin import into Pectobacterium.

The TBDT FusA is a β-barrel protein in the outer membrane (labeled “OM”), with the M16 peptidase FusC on the trans side of the outer membrane, in the periplasm. Initial interactions of plant ferredoxin with the cis face of FusA have been modeled, and complementary surfaces would provide for a docking reaction [9]. As discussed in the text, some degree of unfolding of ferredoxin is required in order to allow its entry into the translocation channel of the FusA β-barrel. Engagement of an unfolded ferredoxin segment to FusC would establish the conditions for a Brownian ratchet to drive vectorial movement and thereby power protein import through the channel of the FusA β-barrel domain. Given the size of ferredoxin, a complete unfolding of the protein would not be required. Liberation of iron would require proteolysis of ferredoxin by either FusC or another protease in the periplasm. TBDT, TonB-dependent transporter.