Structural examination of phi-value analysis in protein folding

Area of Science:

• Protein folding dynamics
• Structural biology
• Biophysics

Background:

• Phi-value analysis is a common protein engineering technique to characterize protein folding intermediates and transition states.
• This method relies on assumptions about the native-like conformations of intermediates and the minimal structural perturbation by mutations, which have not been experimentally verified.
• High-resolution structures of folding intermediates and transition states are crucial for validating these assumptions.

Purpose of the Study:

• To experimentally test the validity of assumptions underlying Phi-value analysis.
• To investigate the structural characteristics of a protein folding intermediate, specifically the presence of non-native interactions.
• To assess the impact of mutations on the structure of folding intermediates.

Main Methods:

• Nuclear Magnetic Resonance (NMR) spectroscopy was used to determine the high-resolution structure of a folding intermediate of the four-helix bundle protein Rd-apocytochrome b(562).
• Phi values were determined for 14 hydrophobic core residues within the intermediate.
• The structure of a mutant (F65A) of the intermediate was also determined to assess the effect of mutation.

Main Results:

• The determined structure of the folding intermediate revealed extensive non-native hydrophobic interactions within the three folded helices.
• All measured Phi values fell within the typical 0 to 1 range, indicating that these non-native interactions were not detectable by standard Phi-value analysis.
• The mutation F65A did not significantly perturb the structure of the folding intermediate, supporting the assumption that such mutations do not alter intermediate structures.

Conclusions:

• The findings challenge the assumption that protein folding intermediates possess native-like conformations, as significant non-native interactions can exist and remain undetected by Phi-value analysis.
• While Phi-value analysis may be useful for assessing the energetics of specific residue interactions, it is not reliable for determining the detailed structures of folding intermediates and transition states.
• The study highlights the limitations of current methods in fully characterizing the complex landscape of protein folding pathways.