Folding of a three-stranded coiled coil

Abstract

Coiled coils consist of two or more amphipathic α-helices wrapped around each other to form a superhelical structure stabilized at the interhelical interface by hydrophobic residues spaced in a repeating 3–4 sequence pattern. Dimeric coiled coils have been shown to often form in a single step reaction in which association and folding of peptide chains are tightly coupled. Here, we ask whether such a simple folding mechanism may also apply to the formation of a three-stranded coiled coil. The designed 29-residue peptide LZ16A was shown previously to be in a concentration-dependent equilibrium between unfolded monomer (M), folded dimer (D), and folded trimer (T ). We show by time-resolved fluorescence change experiments that folding of LZ16A to D and T can be described by [equation] and [equation]. The following rate constants were determined (25 °C, pH 7): k₁ = 7.8 × 10⁴ M⁻¹ s⁻¹, k₋₁ = 0.015 s⁻¹, k₂ = 6.5 × 10⁵ M⁻¹ s⁻¹, and k₋₂ = 1.1 s⁻¹. In a separate experiment, equilibrium binding constants were determined from the change with concentration of the far-ultraviolet circular dichroism spectrum of LZ16A and were in good agreement with the kinetic rate constants according to K_D = k₁/2k₋₁ and K_T = k₂/k₋₂. Furthermore, pulsed hydrogen-exchange experiments indicated that only unfolded M and folded D and T were significantly populated during folding. The results are compatible with a two-step reaction in which a subpopulation of association competent (e.g., partly helical) monomers associate to dimeric and trimeric coiled coils.