Folding of a designed simple ankyrin repeat protein

Area of Science:

• Protein biophysics
• Molecular biology
• Structural biology

Background:

• Ankyrin repeats (AR) are tandemly arranged 33-residue motifs forming elongated domains crucial for diverse cellular functions.
• Consensus libraries of AR repeats have been recently developed.
• Protein E1_5 is a minimal AR domain composed of a single consensus repeat with flanking capping repeats.

Purpose of the Study:

• To perform a biophysical characterization of the designed minimal ankyrin repeat protein E1_5.
• To investigate the folding and unfolding mechanisms of E1_5.
• To assess the thermodynamic and kinetic properties of this minimal AR domain.

Main Methods:

• Differential scanning calorimetry (DSC) to assess thermal stability and unfolding.
• Spectroscopic techniques to monitor protein unfolding.
• Kinetic experiments (folding/unfolding rates) and chevron-plot analysis.

Main Results:

• E1_5 exhibits compact folding, with thermodynamic parameters indicating a stable native state.
• Unfolding can be modeled as a two-state transition at low temperatures (5°C) based on spectroscopic and calorimetric data.
• Deviations from two-state behavior are observed at elevated temperatures, suggesting intermediate population.
• Kinetic and equilibrium unfolding parameters at 5°C show excellent agreement.
• Chevron-plot analysis reveals a very compact transition state.

Conclusions:

• The designed minimal ankyrin repeat protein E1_5 folds via a simple two-state mechanism at low temperatures.
• Equilibrium intermediates become populated at higher temperatures, indicating a temperature-dependent folding pathway.
• E1_5 possesses thermodynamic and kinetic properties characteristic of compactly folded proteins.
• The study validates the favorable properties of the consensus AR framework for protein design.