title: An inverse correlation between loop length and stability in a four-helix-bundle protein.
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.
authors.affiliation: Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, CT 06520, USA.
authors.forename: A D
authors.initials: AD
authors.lastname: Nagi
authors.forename: L
authors.initials: L
authors.lastname: Regan
mesh_terms: D003001:Cloning, Molecular; D004224:Diterpenes / Q000737:chemistry; D008956:Models, Chemical; D008961:Models, Structural; D016133:Polymerase Chain Reaction; D011487:Protein Conformation; D015202:Protein Engineering; D017510:Protein Folding; D013223:Statistics as Topic; D013816:Thermodynamics; D014461:Ultracentrifugation
chemical_list: D004224:Diterpenes; C011309:mezerein