A putative multicopper protein secreted by an atypical type II secretion system involved in the reduction of insoluble electron acceptors in Geobacter sulfurreducens

• Published in: Microbiology (Society for General Microbiology) 2006-08, Vol.152 (8), p.2257-2264
• Main Authors: Mehta, Teena, Childers, Susan E, Glaven, Richard, Lovley, Derek R, Mester, Tunde
• Format: Article
• Language: English
• Subjects: Bacterial Outer Membrane Proteins - genetics; Bacterial Outer Membrane Proteins - physiology; Bacteriology; Biological and medical sciences; Electron Transport; Ferric Compounds - metabolism; Fimbriae Proteins - genetics; Fimbriae Proteins - physiology; Fundamental and applied biological sciences. Psychology; Genetics; Geobacter - metabolism; Geobacter sulfurreducens; Leptothrix discophora; Manganese Compounds - metabolism; Metabolism. Enzymes; Microbiology; Mutation; Oxidation-Reduction; Oxides - metabolism
• ISSN: 1350-0872; 1465-2080
• DOI: 10.1099/mic.0.28864-0

Department of Microbiology, Morrill IV North, University of Massachusetts, Amherst, MA 01003, USA Correspondence Tünde Mester tmester{at}microbio.umass.edu Extracellular electron transfer onto Fe(III) oxides in Geobacter sulfurreducens is considered to require proteins that must be exported to the outer surface of the cell. In order to investigate this, the putative gene for OxpG, the pseudopilin involved in a type II general secretion pathway of Gram-negative bacteria, was deleted. The mutant was unable to grow with insoluble Fe(III) oxide as the electron acceptor. Growth on soluble Fe(III) was not affected. An analysis of proteins that accumulated in the periplasm of the oxpG mutant, but not in the wild-type, led to the identification of a secreted protein, OmpB. OmpB is predicted to be a multicopper protein, with highest homology to the manganese oxidase, MofA, from Leptothrix discophora . OmpB contains a potential Fe(III)-binding site and a fibronectin type III domain, suggesting a possible role for this protein in accessing Fe(III) oxides. OmpB was localized to the membrane fraction of G. sulfurreducens and in the supernatant of growing cultures, consistent with the type II secretion system exporting OmpB. A mutant in which ompB was deleted had the same phenotype as the oxpG mutant, suggesting that the failure to export OmpB was responsible for the inability of the oxpG -deficient mutant to reduce Fe(III) oxide. This is the first report that proposes a role for a multicopper oxidase-like protein in an anaerobic organism. These results further emphasize the importance of outer-membrane proteins in Fe(III) oxide reduction and suggest that outer-membrane proteins other than c -type cytochromes are required for Fe(III) oxide reduction in Geobacter species. Abbreviations: MALDI-MS, matrix-assisted-laser-desorption/ionization time-of-flight mass spectrometry; NTA, nitrilotriacetic acid; PSD, post-source decay Four supplementary figures are available with the online version of this paper. Present address: Department of Geological Sciences, University of Idaho, Moscow, ID 83844 – 3022, USA.