Rational design of a hexameric protein assembly stabilized by metal chelation.

Protein-based self-assembled nanostructures hold tremendous promise as smart materials. One strategy to control the assembly of individual protein modules takes advantage of the directionality and high affinity bonding afforded by metal chelation. Here, we describe the use of 2,2'-bipyridine units (Bpy) as side chains to template the assembly of large structures (MW approx. 35 000 Da) in a metal-dependent manner. The structures are trimers of independently folded 3-helix bundles, and are held together by 2 Me(Bpy)3 complexes. The assemblies are stable to thermal denaturation, and are more than 90% helical at 90°C. Circular dichroism spectroscopy shows that one of the 2 possible (Bpy)3 enantiomers is favored over the other. Because of the sequence pliability of the starting peptides, these constructs could find use to organize functional groups at controlled positions within a supramolecular assembly.