The crystal structure of ferritin from Chlorobium tepidum reveals a new conformation of the 4-fold channel for this protein family

Ferritins are ubiquitous iron-storage proteins found in all kingdoms of life. They share a common architecture made of 24 subunits of five α-helices. The recombinant Chlorobium tepidum ferritin (rCtFtn) is a structurally interesting protein since sequence alignments with other ferritins show that this protein has a significantly extended C-terminus, which possesses 12 histidine residues as well as several aspartate and glutamic acid residues that are potential metal ion binding residues. We show that the macromolecular assembly of rCtFtn exhibits a cage-like hollow shell consisting of 24 monomers that are related by 4-3-2 symmetry; similar to the assembly of other ferritins. In all ferritins of known structure the short fifth α-helix adopts an acute angle with respect to the four-helix bundle. However, the crystal structure of the rCtFtn presented here shows that this helix adopts a new conformation defining a new assembly of the 4-fold channel of rCtFtn. This conformation allows the arrangement of the C-terminal region into the inner cavity of the protein shell. Furthermore, two Fe(III) ions were found in each ferroxidase center of rCtFtn, with an average FeA–FeB distance of 3 Å; corresponding to a diferric μ-oxo/hydroxo species. This is the first ferritin crystal structure with an isolated di-iron center in an iron-storage ferritin. The crystal structure of rCtFtn and the biochemical results presented here, suggests that rCtFtn presents similar biochemical properties reported for other members of this protein family albeit with distinct structural plasticity. In all ferritns of known structure the short fifth E α-helix acquire an acute angle to the bundle near the C-terminus tail. However, the crystal structure of the recombinant Chlorobium tepidum ferritin (rCtFtn) shows that the presence of Arg150 in the helix E and the fact that the DE loop is formed by two amino acids produced that the helix E acquire an angle of 90° with respect the four-helical bundle. This conformation is not found in the rest of this family of proteins and defines the conformation of the 4-fold channel of rCtFtn. Despite this, the rCtFtn exhibits a cage-like hollow shell constituting of 24 monomers that are related by 4-3-2 symmetry similar to the assembly of other ferritins from eukaryotes and prokaryotes. [Display omitted] •The recombinant Chlorobium tepidum ferritin is a cage-like multimer consisting of 24 subunits.•Crystal structure of recombinant Chlorobium tepidum ferritin