A PDZ tandem repeat folds and unfolds via different pathways

Area of Science:

• Biochemistry
• Structural Biology
• Protein Dynamics

Background:

• Protein folding and unfolding experiments traditionally assume microscopic reversibility, where folding and unfolding are reverse processes.
• This assumption holds for single-domain proteins but often fails for complex proteins exhibiting irreversible unfolding.
• Complex proteins with irreversible unfolding are typically considered unsuitable for folding kinetics studies.

Purpose of the Study:

• To investigate protein folding and unfolding pathways under different experimental conditions.
• To reconcile the discrepancy between theoretical assumptions of reversibility and experimental observations in complex proteins.
• To demonstrate the applicability of folding kinetics studies to proteins with potentially irreversible unfolding.

Main Methods:

• Utilized the X11 PDZ1-PDZ2 tandem repeat as a model system.
• Conducted protein folding and unfolding experiments under varied conditions.
• Analyzed folding and unfolding pathways to identify differences.

Main Results:

• Observed distinct folding and unfolding pathways for the X11 PDZ1-PDZ2 tandem repeat.
• Demonstrated that different experimental conditions lead to different pathways.
• Showcased that complex proteins can exhibit different pathways, challenging the assumption of simple reversibility.

Conclusions:

• The X11 PDZ1-PDZ2 tandem repeat exhibits condition-dependent folding and unfolding pathways.
• Apparent irreversibility in complex protein unfolding can be reconciled by considering different pathways under varied conditions.
• Folding kinetics studies can be applied to complex proteins by accounting for pathway complexity.