The Phe-Ile Zipper: A Specific Interaction Motif Drives Antiparallel Coiled-Coil Hexamer Formation

Coiled
coils are a robust motif for exploring amino acid interactions,
generating unique supramolecular structures, and expanding the functional
properties of biological materials. A recently discovered antiparallel
coiled-coil hexamer (ACC-Hex, peptide 1) exhibits a unique
interaction in which Phe and Ile residues from adjacent α-helices
interact to form a Phe-Ile zipper within the hydrophobic core. Analysis
of the X-ray crystallographic structure of ACC-Hex suggests that the
stability of the six-helix bundle relies on specific interactions
between the Phe and Ile residues. The Phe-Ile zipper is unprecedented
and represents a powerful tool for utilizing the Phe-Ile interactions
to direct supramolecular assembly. To further probe and understand
the limits of the Phe-Ile zipper, we designed peptide sequences with
natural and unnatural amino acids placed at the Phe and Ile residue
positions. Using size exclusion chromatography and small-angle X-ray
scattering, we found that the proper assembly of ACC-Hex from monomers
is sensitive to subtle changes in side chain steric bulk and hydrophobicity
introduced by mutations at the Phe and Ile residue positions. Of the
sequence variants that formed ACC-Hex, mutations in the hydrophobic
core significantly affected the stability of the hexamer, from a ΔGuw of 2–8 kcal mol–1. Additional sequences were designed to further probe and enhance
the stability of the ACC-Hex system by maximizing salt bridging between
the solvent-exposed residues. Finally, we expanded on the generality
of the Phe-Ile zipper, creating a unique sequence that forms an antiparallel
hexamer that is topologically similar to ACC-Hex but atomistically
unique.