Nanocage

Variant ferritin polypeptides comprising a modified amino acid sequence of a wild-type ferritin (e.g. bacterioferritin), the modified sequence being in a dimeric subunit interface or the N-terminus of the polypeptide, wherein the variant may assemble into a ferritin nanocage when contacted with a nucleating agent (e.g. gold, iron, copper). Also claimed are fusion proteins comprising wild-type ferritin and a peptide selected from an antibody, antibody binding fragment, a fluorophore, a His tag and a nucleating agent binding peptide; a ferritin nanocage comprising the variant ferritin polypeptide or fusion protein.