Location‐Dependent Lanthanide Selectivity Engineered into Structurally Characterized Designed Coiled Coils

Herein we report unprecedented location‐dependent, size‐selective binding to designed lanthanide (Ln3+) sites within miniature protein coiled coil scaffolds. Not only do these engineered sites display unusual Ln3+ selectivity for moderately large Ln3+ ions (Nd to Tb), for the first time we demonstrate that selectivity can be location‐dependent and can be programmed into the sequence. A 1 nm linear translation of the binding site towards the N‐terminus can convert a selective site into a highly promiscuous one. An X‐ray crystal structure, the first of a lanthanide binding site within a coiled coil to be reported, coupled with CD studies, reveal the existence of an optimal radius that likely stems from the structural constraints of the coiled coil scaffold. To the best of our knowledge this is the first report of location‐dependent metal selectivity within a coiled coil scaffold, as well as the first report of location‐dependent Ln3+ selectivity within a protein. Location‐dependent size selectivity can be programmed into designed metallo coiled coils, allowing for the generation of near‐identical selective and promiscuous metal‐ion sites. The first crystal structure of a lanthanide bound within a coiled coil reveals that the existence of an optimal radius stems from the structural constraints of the peptide scaffold. Therefore, a selective site can in principle be designed de novo for any given Ln3+ ion.