SUBDOMAIN FOLDING OF THE COILED-COIL LEUCINE-ZIPPER FROM THE BZIP TRANSCRIPTIONAL ACTIVATOR GCN4

One popular model for protein folding, the framework model, postulates initial formation of secondary structure elements, which then assembl e into the native conformation. However, short peptides that correspon d to secondary structure elements in proteins are often only marginall y stable in isolation. A 33-residue peptide (GCN4-p1) corresponding to the GCN4 leucine zipper folds as a parallel, two-stranded coiled coil [O'Shea, E. K., Klemm, J. D., Kim, P. S., and Alber, T. A. (1991) Sci ence 254, 539-544]. Deletion of the first residue (Arg 1) results in l ocal, N-terminal unfolding of the coiled coil, suggesting that a stabl e subdomain of GCN4-p1 can form. N- and C-terminal deletion studies re sult in a 23-residue peptide, corresponding to residues 8-30 of GCN4-p 1, that folds as a parallel, two-stranded coiled coil with substantial stability (the melting temperature of a 1 mM solution is 43 degrees C at pH 7). In contrast, a closely related 23-residue peptide (residues 11-33 of GCN4-p1) is predominantly unfolded, even at 0 degrees C, as observed previously for many isolated peptides of similar length. Thus , specific tertiary packing interactions between two short units of se condary structure can be energetically more important in stabilizing f olded structure than secondary structure propensities. These results p rovide strong support for the notion that stable, cooperatively folded subdomains are the important determinants of protein folding.