Topological switching between an alpha-beta parallel protein and a remarkably helical molten globule

Area of Science:

• Protein folding dynamics
• Structural biology
• Biochemistry

Background:

• Partially folded protein species, often molten globules, transiently exist during protein folding.
• These molten globules possess secondary structure but lack tertiary packing, making them prone to aggregation and implicated in pathologies.
• The apoflavodoxin molten globule is an off-pathway intermediate requiring unfolding before native protein formation.

Purpose of the Study:

• To spectroscopically characterize the conformation of an off-pathway molten globule intermediate of apoflavodoxin.
• To investigate the structural plasticity of a single polypeptide sequence under native-like conditions.

Main Methods:

• Site-directed mutagenesis: substituting phenylalanine (F44) with tyrosine (Y44) to trap the molten globule state.
• Spectroscopic characterization to determine the protein's conformation.

Main Results:

• The F44Y mutation allowed trapping of the apoflavodoxin molten globule under native-like conditions.
• The trapped molten globule exhibited a helical topology, lacking the beta-sheet present in the native protein.
• This demonstrates a significant topological switch from an alpha-beta fold to a helical fold.

Conclusions:

• A single apoflavodoxin sequence can adopt distinct, unrelated protein folds.
• Topological switching between different protein structures is a plausible phenomenon in protein structure diversity.
• Understanding these alternative folds is crucial for deciphering protein folding pathways and associated diseases.