Preformed secondary structure drives the association reaction of GCN4-p1, a model coiled-coil system

The structure of the transition state for the rate-limiting step in the fol ding and association of the homodimeric coiled-coil peptide GCN4-p1, was pr obed by mutational analysis. A series of quadruple amino acid replacements that spanned the helix propensity scale were made at the four external f po sitions in the heptad repeat. Equilibrium and kinetic circular dichroism st udies demonstrate that both the stability and the unfolding and refolding r ate constants vary with helix propensity but also reflect interactions of t he altered side-chains with their local environments. Pairwise replacements and fragment studies show that the two C-terminal heptads are the likely s ource of the nucleating helices. Helix-helix recognition between preformed elements of secondary structure plays an important role in this fundamental folding reaction. (C) 2000 Academic Press.