Iron-sulfur clusters are ubiquitous in biology and function in electron transfer and catalysis. They are assembled from iron and cysteine sulfur on protein scaffolds. Iron is typically stored as iron oxyhydroxide, ferrihydrite, encapsulated in 12 nm shells of ferritin, which buffers cellular iron availability. Here we have characterized IssA, a protein that stores iron and sulfur as thioferrate, an inorganic anionic polymer previously unknown in biology. IssA forms nanoparticles reaching 300 nm in diameter and is the largest natural metalloprotein complex known. It is a member of a widely distributed protein family that includes nitrogenase maturation factors, NifB and NifX. IssA nanoparticles are visible by electron microscopy as electron-dense bodies in the cytoplasm. Purified nanoparticles appear to be generated from 20 nm units containing ∼6,400 Fe atoms and ∼170 IssA monomers. In support of roles in both iron-sulfur storage and cluster biosynthesis, IssA reconstitutes the [4Fe-4S] cluster in ferredoxin in vitro.
Bibliographical noteFunding Information:
This research was supported by a grant from the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy (DOE; DE-FG05-95ER20175 to M.W.W.A.) We thank Robert M. Glaeser for providing negative stain electron micrographs and John P. Shields for assistance with whole-cell electron microscopy. Bioinformatic analysis was conducted using the Georgia Advanced Computing Resource Center, a partnership between the University of Georgia's Office of the Vice President for Research and Office of the Vice President for Information Technology. Work at the University of Saskatchewan was supported by the CRC (G.N.G.), NSERC Canada and CIHR (J.J.H.C is a CIHR-THRUST Associate, M.J.H. held a CIHR PDF award and is a CIHR-THRUST fellow). SSRL is supported by the U.S. DOE and NIH. EPR and IssA-mediated apo-Fd reconstitution studies were supported by a grant from the National Institutes of Health (GM62524 to M.K.J.)
All Science Journal Classification (ASJC) codes
- Biochemistry, Genetics and Molecular Biology(all)
- Physics and Astronomy(all)