Shewanella oneidensis, a bacterium admired by biologists for its metal-reducing ability, just gave up one of its structural secrets, thanks to researchers at the University of East Anglia, in England, and Pacific Northwest National Laboratory. The scientists report the first X-ray crystallography structure of a protein in the complex that is responsible for shuttling electrons from the microbe’s innards to solution (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.1017200108). The protein, called MtrF, sits on the outermost membrane of S. oneidensis, and knowledge of its configuration should enable more efficient use of the bacterium in microbial fuel cells and future industrial redox reactions. Together, two of MtrF’s four domains contain 10 heme groups, which are arranged in a staggered cross structure that conducts electrons. “The brilliant finding” in this work “is that there appear to be three active sites where electrons can exit the protein,” says Jeffrey A. Gralnick, a microbiologist at the University of Minnesota, Twin Cities. Electrons can leave directly from the cross tip, the research team shows, and they can potentially leave via shuttle molecules, such as flavins, from each of the cross’s arms.