The dodecameric ferritin from Listeria innocua contains a novel intersubunit iron-binding site

Ferritin is characterized by a highly conserved architecture that comprises 24 subunits assembled into a spherical cage with 432 symmetry. The only kn own exception is the dodecameric ferritin from Listeria innocua. The struct ure of Listeria ferritin has been determined to a resolution of 2.35 Angstr om by molecular replacement, using as a search model the structure of Dps f rom Escherichia coli, The Listeria 12-mer is endowed with 23 symmetry and d isplays the functionally relevant structural features of the ferritin 24-me r, namely the negatively charged channels along the three-fold symmetry axe s that serve for iron entry into the cavity and a negatively charged intern al cavity for iron deposition. The electron density map shows 12 iron ions on the inner surface of the hollow core, at the interface between monomers related by two-fold axes. Analysis of the nature and stereochemistry of the iron-binding ligands reveals strong similarities with known ferroxidase si tes. The L. innocua ferritin site, however, is the first described so far t hat has ligands belonging to two different subunits and is not contained wi thin a four-helix bundle.