A D Nagi1, L Regan. 1. Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, CT 06520, USA.
Abstract
BACKGROUND: The loops in proteins are less well characterized than the secondary structural elements that they connect. We have used the four-helix-bundle protein Rop as a model system in which to explore the role of loop length in protein folding and stability. RESULTS: A natural two-residue loop was replaced with a series of glycine linkers up to 10 residues in length. All 10 mutants are highly helical dimers that retain wild-type RNA-binding activity. As loop length is increased, the stability of Rop toward thermal and chemical denaturation is progressively decreased. CONCLUSIONS: All the mutants assume a wild-type-like structure, which suggests that the natural loop does not actively dictate the final protein fold. The strong inverse correlation observed between loop length and stability is well described by a simple polymer model in which the entropy of loop closure is the dominant energetic term. Our results emphasize the importance of optimization of loop length to successful protein design.
BACKGROUND: The loops in proteins are less well characterized than the secondary structural elements that they connect. We have used the four-helix-bundle protein Rop as a model system in which to explore the role of loop length in protein folding and stability. RESULTS: A natural two-residue loop was replaced with a series of glycine linkers up to 10 residues in length. All 10 mutants are highly helical dimers that retain wild-type RNA-binding activity. As loop length is increased, the stability of Rop toward thermal and chemical denaturation is progressively decreased. CONCLUSIONS: All the mutants assume a wild-type-like structure, which suggests that the natural loop does not actively dictate the final protein fold. The strong inverse correlation observed between loop length and stability is well described by a simple polymer model in which the entropy of loop closure is the dominant energetic term. Our results emphasize the importance of optimization of loop length to successful protein design.