THE ROLE OF HELIX FORMATION IN THE FOLDING OF A FULLY ALPHA-HELICAL COILED-COIL

To determine when secondary structure forms as two chains coalesce to form an alpha-helical dimer, the folding rates of variants of the coil ed coil region of GCN4 were compared, Residues at non-perturbing posit ions along the exterior length of the helices were substituted one at a time with alanine and glycine to vary helix propensity and therefore dimer stability. For all variants, the bimolecular folding rate remai ns largely unchanged; the unfolding rate changes to largely account fo r the change in stability. Thus, contrary to most folding models, wide spread helix is not yet formed at the rate-limiting step in the foldin g pathway. The high-energy transition state is a collapsed form that c ontains little if any secondary structure, as suggested for the globul ar protein cytochrome c (Sosnick et al., Proteins 24:413-426, 1996). ( C) 1996 Wiley-Liss, Inc.