Metal-dependent assembly of a protein nano-cage

Short, alpha-helical coiled coils provide a simple, modular method to direct the assembly of proteins into higher order structures. We previously demonstrated that by genetically fusing de novo-designed coiled coils of the appropriate oligomerization state to a natural trimeric protein, we could direct the assembly of this protein into various geometrical cages. Here, we have extended this approach by appending a coiled coil designed to trimerize in response to binding divalent transition metal ions and thereby achieve metal ion-dependent assembly of a tetrahedral protein cage. Ni²⁺ , Co²⁺ , Cu²⁺ , and Zn²⁺ ions were evaluated, with Ni²⁺ proving the most effective at mediating protein assembly. Characterization of the assembled protein indicated that the metal ion-protein complex formed discrete globular structures of the diameter expected for a complex containing 12 copies of the protein monomer. Protein assembly could be reversed by removing metal ions with ethylenediaminetetraacetic acid or under mildly acidic conditions.