The ability to chemically control protein-protein interactions would allow the interrogation of dynamic cellular processes and lead to a better understanding and exploitation of self-assembling protein architectures. Here we introduce a new engineering strategy--reverse metal-templated interface redesign (rMeTIR)--that transforms a natural protein-protein interface into one that only engages in selective response to a metal ion. We have applied rMeTIR to render the self-assembly of the cage-like protein ferritin controllable by divalent copper binding, which has allowed the study of the structure and stability of the isolated ferritin monomer, the demonstration of the primary role of conserved hydrogen-bonding interactions in providing geometric specificity for cage assembly and the uniform chemical modification of the cage interior under physiological conditions. Notably, copper acts as a structural template for ferritin assembly in a manner that is highly reminiscent of RNA sequences that template virus capsid formation.