Metal ion induced self-assembly of a designed peptide into a triple- stranded α-helical bundle: A novel metal binding site in the hydrophobic core

Coiled coils, which mediate the associations and regulate the functions of various proteins, have a representative amino acid sequence of (defgabc)(n) heptad repeats and usually have hydrophobic residues at the a and d positions. We have designed a triple-stranded parallel α-helical coiled coil, in which the amino acid sequence is YGG(IEKKIEA)₄. To construct a peptide that undergoes metal ion induced self-assembly into a triple- stranded coiled coil, we engineered a metal binding site in the hydrophobic core of the coiled coil. We replaced two IIe residues of the third heptad with His residues. The peptide had a random structure in aqueous solution. In contrast, in the presence of a transition metal ion, the peptide exhibited an α-helical conformation. The metal-complexed peptide was triple stranded and had a parallel orientation, as determined by sedimentation equilibrium and fluorescence quenching analyses. Metal ion titrations monitored by circular dichroism revealed that the dissociation constants, K(d), were 35 ± 1 μM for Co(II), 5.0 ± 0.3 μM for Ni(II), 17 ± 1 μM for Cu(II), and 23 ± 2 μM for Zn(II). The Ni(II) binds to the His residues, as judged by both pH titration monitored by circular dichroism and metal ion titration monitored by nuclear magnetic resonance. The highest affinity for Ni(II) suggests that the metal binding site has six-coordinated octahedral geometry. Thus, the peptide is a useful tool to control the associations of functional domains attached to the peptide.