Eliminating a Protein Folding Intermediate by Tuning a Local Hydrophobic Contact

Intermediate states in protein folding may slow folding, and sometimes can provide a starting point for aggregation. Recently, the FBP28 WW domain of the formin-binding protein was used as a model for a computational study of the origin and prevention of intermediate-state formation, and local hydrophobic interactions of Leu26 were implicated. Here, we combine new simulations over a broad temperature range with experimental temperature-jump data to study this site in more detail. We replace Leu26 by Asp26 or Trp26 to alter the folding scenario from three-state folding toward two-state or downhill folding at temperatures below the melting point, whereas the wild type shows two-state behavior only near its melting temperature. We offer an explanation of this behavior mainly in terms of principles of hydrophobic interactions.