Probing the energetic and kinetic impact of topologically conserved interactions in the SIV gp41 six-helix bundle.

In this study we used an engineered six-helix bundle construct corresponding to the fusogenic core of the SIV gp41 protein as a model system to investigate the folding of a trimeric protein, which acquires a compact structure upon association of largely unstructured monomeric peptides. Thirteen mutants were generated in order to gain information about the thermodynamic and kinetic roles of topologically conserved tertiary interactions to folding and stability. The effect of the mutations was assessed by circular dichroism spectroscopy from urea-induced equilibrium unfolding experiments and in time-resolved mode to follow the kinetics of refolding and unfolding. While individual experiments can be interpreted in terms of a simple monomer-trimer refolding/unfolding reaction mechanism, comparison of equilibrium and kinetic data reveals that some variants clearly deviate from this two-state behavior and that most proteins cannot be classified as two-state folders without some reservations. Nevertheless, following "quasi-φ-value" and "quasi-β(T)-value" analyses, we propose that the highest-energy barrier along the folding pathway is passed in the trimeric state, after the C-terminal half of each monomer chain is "fixed" in anti-parallel orientation to the surface of the central, still nascent N-terminal coiled-coil.