Abstract
Iron is essential for life. Accessing iron from the environment can be a limiting factor that determines success in a given environmental niche. For bacteria, access of chelated iron from the environment is often mediated by TonB-dependent transporters (TBDTs), which are β-barrel proteins that form sophisticated channels in the outer membrane. Reports of iron-bearing proteins being used as a source of iron indicate specific protein import reactions across the bacterial outer membrane. The molecular mechanism by which a folded protein can be imported in this way had remained mysterious, as did the evolutionary process that could lead to such a protein import pathway. How does the bacterium evolve the specificity factors that would be required to select and import a protein encoded on another organism’s genome? We describe here a model whereby the plant iron–bearing protein ferredoxin can be imported across the outer membrane of the plant pathogen Pectobacterium by means of a Brownian ratchet mechanism, thereby liberating iron into the bacterium to enable its growth in plant tissues. This import pathway is facilitated by FusC, a member of the same protein family as the mitochondrial processing peptidase (MPP). The Brownian ratchet depends on binding sites discovered in crystal structures of FusC that engage a linear segment of the plant protein ferredoxin. Sequence relationships suggest that the bacterial gene encoding FusC has previously unappreciated homologues in plants and that the protein import mechanism employed by the bacterium is an evolutionary echo of the protein import pathway in plant mitochondria and plastids.
Highlights
TonB-dependent transporters (TBDTs) are 22-stranded β-barrel proteins integrated in the bacterial outer membrane
We show how a plant-pathogenic bacteria has reevolved a mechanism, analogous to the protein import pathways that evolved in plant plastids and mitochondria, to import the plant iron–bearing protein ferredoxin from plant tissue
Genome sequence analysis using the hidden Markov model (HMM) built to detect TBDTs revealed that the Australian isolate RMIT1 encodes 23 TBDTs, many of them typical of iron acquisition systems (Table 1)
Summary
TonB-dependent transporters (TBDTs) are 22-stranded β-barrel proteins integrated in the bacterial outer membrane. They contain a plug domain inserted inside the β-barrel that can be actively removed by the inner membrane protein TonB to provide access for the import of vitamins, chelated metals, and other cofactors essential to bacterial viability [1] These various cofactors need to be small enough to pass through the internal channel of the TBDT, a channel whose internal diameter is approximately 20 Å [1,2,3]. Under iron limitation, pathogens like Pectobacterium can remain viable and virulent using proteins such as ferredoxin, pirated from their plant host to supply iron [9] How this important protein transport pathway is mediated by the bacterium had remained unknown and difficult to interpret from evolutionary considerations
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