Lanthanide Ion-Induced Folding of De Novo Designed Coiled-Coil Polypeptides

To construct a novel isoleucine zipper polypeptide that undergoes a random to coiled coil transition upon lanthanide ion binding, we designed a 37-residue polypeptide (Pep3) with a γ-carboxy glutamic acid (γ-Gla) as metal binding site. Pep3 was designed based on the sequence of the isoleucine zipper polypeptide (Pep1), forming a triple-stranded coiled coil. The coiled-coil structure was destabilized by ionic repulsions among γ-Gla residues at position 21 (hydrophobic 3a position) of one strand. On the other hand, a stable coiled coil formed in the presence of Eu 3+ ions. This structural change in the designed polypeptide was selective toward Eu 3+ ion from the circular dichroism (CD) measurements. Furthermore, we designed a lanthanide ion-induced heterotrimeric coiled-coil-assembled system by using two kinds of polypeptides, Pep4 and Pep5. CD spectroscopy, sedimentation equilibrium centrifugation, gel filtration, and HPLC analyses indicate that Pep4 and Pep5 could noncovalently assemble in a 1:2 ratio in the presence of Eu 3+ ions.