PROTEINS 2014 * Iron, piracy and conflict on the bacterial high seas: Ferredoxin containing antimicrobial proteins identify novel iron acquisition system in Pectobacterium (#133)
Colicin-like bacteriocins are narrow spectrum antimicrobial proteins produced widely by gram-negative bacteria for intra-species competition. These proteins consist of a toxin domain of diverse function, which is delivered across the bacterial envelope by additional domains that parasitize outer-membrane nutrient importation machinery. Bacteriocins characterized to date, bind tightly to a specific nutrient receptor and thread an intrinsically unstructured translocation domain (IUTD) into the periplasm. This domain utilizes a specific binding motif to recruit nutrient uptake machinery for cell entry [1] . Our recent work on protein antimicrobials led to the discovery of the pectocins, a novel class of bacteriocins produced by the phyopathogen Pectobacterium. Pectocins have a unique structure consisting of a toxin domain fused to a 2Fe-2S plant-type ferredoxin. We have shown that this ferredoxin domain replaces the conventional bacteriocin cell entry domains and can deliver toxins of diverse function to the periplasm [2].
To explain this highly unusual role of ferredoxin we have utilized genetic and protein interaction techniques to identify the outer-membrane receptor required for pectocin cell entry. This receptor, designated FupA, allows Pectobacterium to utilize plant-ferredoxin as an iron source, which given its lifestyle as a plant pathogen allows it to obtain iron from its host during infection.
Biochemical and biophysical analysis of recombinant FupA has shown it binds to plant-ferredoxin with extremely high affinity but that the strength of this interaction varies depending on the ferredoxin’s plant of origin, suggesting adaption of the system to a specific host. Additionally we have solved the crystal structure of pectocin M2, which shows it possesses dimensions compatible with traversing the lumen of FupA. As the pectocins lack the IUTD required for direct interaction with uptake machinery in the periplasm, it is likely that the pectocin cytotoxic domains hitches a ride on ferredoxin through the lumen of FupA. This in turn suggests that ferredoxin in imported into the periplasm by FupA and subsequently processed to extract its iron.
