title: Characterization of the surfaceome of the metal-reducing bacterium Desulfotomaculum reducens.
abstract: Desulfotomaculum reducens strain MI-1 is a Gram-positive, sulfate-reducing bacterium also capable of reducing Fe(III). Metal reduction in Gram-positive bacteria is poorly understood. Here, we investigated Fe(III) reduction with lactate, a non-fermentable substrate, as the electron donor. Lactate consumption is concomitant to Fe(III) reduction, but does not support significant growth, suggesting that little energy can be conserved from this process and that it may occur fortuitously. D. reducens can reduce both soluble [Fe(III)-citrate] and insoluble (hydrous ferric oxide, HFO) Fe(III). Because physically inaccessible HFO was not reduced, we concluded that reduction requires direct contact under these experimental conditions. This implies the presence of a surface exposed reductase capable of transferring electrons from the cell to the extracellular electron acceptor. With the goal of characterizing the role of surface proteins in D. reducens and of identifying candidate Fe(III) reductases, we carried out an investigation of the surface proteome (surfaceome) of D. reducens. Cell surface exposed proteins were extracted by trypsin cell shaving or by lysozyme treatment, and analyzed by liquid chromatography-tandem mass spectrometry. This investigation revealed that the surfaceome fulfills many functions, including solute transport, protein export, maturation and hydrolysis, peptidoglycan synthesis and modification, and chemotaxis. Furthermore, a few redox-active proteins were identified. Among these, three are putatively involved in Fe(III) reduction, i.e., a membrane-bound hydrogenase 4Fe-4S cluster subunit (Dred_0462), a heterodisulfide reductase subunit A (Dred_0143) and a protein annotated as alkyl hydroperoxide reductase but likely functioning as a thiol-disulfide oxidoreductase (Dred_1533).
authors.affiliation: Environmental Microbiology Laboratory, Environmental Engineering Institute, École Polytechnique Fédérale de Lausanne Lausanne, Switzerland.
authors.forename: Elena
authors.initials: E
authors.lastname: Dalla Vecchia
authors.affiliation: Environmental Microbiology Laboratory, Environmental Engineering Institute, École Polytechnique Fédérale de Lausanne Lausanne, Switzerland.
authors.forename: Paul P
authors.initials: PP
authors.lastname: Shao
authors.affiliation: Environmental Microbiology Laboratory, Environmental Engineering Institute, École Polytechnique Fédérale de Lausanne Lausanne, Switzerland.
authors.forename: Elena
authors.initials: E
authors.lastname: Suvorova
authors.affiliation: Proteomics Core Facility, Core Facility PTECH, École Polytechnique Fédérale de Lausanne Lausanne, Switzerland.
authors.forename: Diego
authors.initials: D
authors.lastname: Chiappe
authors.affiliation: Proteomics Core Facility, Core Facility PTECH, École Polytechnique Fédérale de Lausanne Lausanne, Switzerland.
authors.forename: Romain
authors.initials: R
authors.lastname: Hamelin
authors.affiliation: Environmental Microbiology Laboratory, Environmental Engineering Institute, École Polytechnique Fédérale de Lausanne Lausanne, Switzerland.
authors.forename: Rizlan
authors.initials: R
authors.lastname: Bernier-Latmani
keywords: Desulfotomaculum reducens; Fe(III) reduction; Gram-positive bacteria; cell-wall protein; extracellular electron transfer; membrane protein; surfaceome