Domain swapping is a mechanism for forming protein dimers and oligomers with high specificity. It is distinct from other forms of oligomerization in that the binding interface is formed by reciprocal exchange of polypeptide segments. Swapping plays a physiological role in protein-protein recognition and it can also potentially be exploited as a mechanism for controlled self assembly. Here, we demonstrate that domain-swapped interfaces can be engineered by inserting one protein into a surface loop of another protein. The key to facilitating a domain swap is to destabilize the protein when it is monomeric but not when it is oligomeric. We achieve this condition by employing the ‘mutually exclusive folding’ design to apply conformational stress to the monomeric state. Ubiquitin is inserted into one of six surface loops of barnase. The 38 Å amino-to-carboxy terminal distance of ubiquitin stresses the barnase monomer, causing it to split at the point of insertion. The 2.2 Å X-ray structure of one insertion variant reveals that strain is relieved by intermolecular folding with an identically-unfolded barnase domain, resulting in a domain-swapped polymer. All six constructs oligomerize suggesting that inserting ubiquitin into each surface loop of barnase results in a similar domain-swapping event. Binding affinity can be tuned by varying the length of the peptide linkers used to join the two proteins, which modulates the extent of stress. Engineered, swapped proteins have the potential to be used to fabricate ‘smart’ biomaterials, or as binding modules from which to assemble heterologous, multi-subunit protein complexes.
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