Putative mineral-specific proteins synthesized by a metal reducing bacterium

Biological force microscopy (BFM) was combined with two-dimensional gel electrophoresis and mass spectrometry to search for evidence of putative mineral-specific outer membrane proteins (OM) synthesized by Shewanella oneidensis for Fe-oxide binding and/or anaerobic Fe(III) reduction. BFM shows that S. oneidensis possess an affinity towards goethite (FeOOH) but not diaspore (AlOOH) under anaerobic conditions, despite the fact that diaspore is isostructural with goethite and has essentially the same surface charge. The worm-like chain model was used to identify force-signatures indicative of putative OM polypeptides that bind to goethite. Two-dimensional protein expression patterns show that over 100 OM proteins are differentially expressed under aerobic versus anaerobic Fe(III) reducing conditions. Peptide mass fingerprinting and tandem mass spectrometry were used to identify several of the protein spots predominately detected when Fe(III) was the terminal electron acceptor. Among those identified were proteins involved in metal reduction, protein transport and secretion, polysaccharide biosynthesis and export, and hypothetical proteins with unknown functions. A comparison of the BFM and proteomic data suggest that a few specific OM proteins are synthesized by S. oneidensis under anaerobic conditions to function in iron oxide binding and/or Fe(III) reduction. This suggests the intriguing possibility that metal reducing bacteria contain the genetic repertoire to make proteins directed at specific inorganic phases.