A superprotein triangle driven by nickel(II) coordination: exploiting non-natural metal ligands in protein self-assembly.

We previously devised a strategy (metal-directed protein self-assembly, MDPSA) that utilizes the simultaneous stability, lability, and directionality of metal-ligand bonds to drive protein-protein interactions. Here we show that both the structural and functional scopes of MDPSA can be broadened by incorporation of non-natural metal-chelating ligands onto protein surfaces. A cytochrome cb(562) variant, MBP-Phen1, which features a covalently attached phenanthroline (Phen) group on its surface, self-assembles into an unusual triangular architecture (Ni(3):MBP-Phen1(3)) upon binding Ni as a result of specific Phen-protein interactions. The crystal structure of Ni(3):MBP-Phen1(3) reveals that the Phen group is buried in a small pocket on the protein surface, which results in an unsaturated Ni coordination environment.