The single helix in protein L is largely disrupted at the rate-limiting step in folding

To investigate the role of helix formation in the folding of protein L, a 6 2 residue alpha/beta protein, we studied the consequences of both single an d multiple mutations in the helix on the kinetics of folding. A triple muta nt with 11 additional carbon atoms in core residues in the amino-terminal p ortion of the helix folded substantially faster than wild type, suggesting that hydrophobic association with residues elsewhere in the protein occurs at the rate-limiting step in folding. However, helix-destabilizing mutation s had little effect on the rate of folding; in particular, a triple glycine substitution on the solvent-exposed side of the helix increased the unfold ing rate 56-fold while reducing the folding rate less than threefold. Thus, in contrast to the predictions of models of folding involving the coalesce nce of well-formed secondary structure elements, die single helix in protei n L arrears to be largely disrupted at the rate-limiting step in folding an d unfolding. (C) 1998 Academic Press.