Non-strict strand orientation of the Ca2+-induced dimerization of a conantokin peptide variant with sequence-shifted γ-carboxyglutamate residues

We have previously found a new mode of metal ion-induced helix–helix assembly for the γ-carboxyglutamate (Gla)-containing, neuroactive conantokin (con) peptides that is independent of the hydrophobic effect. In these unique “metallo-zipper” assemblies of con-G and con-T[K7γ], interhelical Ca2+ coordination induces dimer formation with strictly antiparallel chain orientation in conantokin peptides in which Gla residues are positioned at “i, i + 4, i + 7, i + 11” intervals. In order to probe the property of self-assembly in conantokin peptides with an extended Gla network, a con-T variant (con-T-tri) was synthesized that contains five Gla residues spaced at “i, i + 4, i + 7, i + 11, i + 14” intervals. Sedimentation equilibrium analyses showed that Ca2+, but not Mg2+, was capable of promoting con-T-tri self-assembly. Oxidation and rearrangement assays with Cys-containing con-T-tri variants revealed that the peptide strands in the complex can orient in both parallel and antiparallel forms. Stable parallel and antiparallel dimeric forms of con-T-tri were modeled using disulfide-linked peptides and the biological viability of these species was confirmed by electrophysiology. These findings suggest that small changes within the helix–helix interface of the conantokins can be exploited to achieve desired modes of strand alignment.