An alpha-helical burst in the src SH3 folding pathway

Area of Science:

• Protein folding dynamics
• Biophysical characterization of protein structure

Background:

• Src SH3 is a small, single-domain, all-beta-sheet protein.
• Understanding protein folding pathways is crucial for molecular biology and disease research.

Purpose of the Study:

• To investigate the folding behavior of src SH3 under various conditions.
• To characterize the nature of folding intermediates in src SH3.

Main Methods:

• Circular dichroism (CD) spectroscopy for secondary structure analysis.
• Fluorescence spectroscopy for monitoring conformational changes.
• X-ray solution scattering to determine the compactness of folding intermediates.

Main Results:

• An alpha-helix-rich intermediate was observed during src SH3 folding, present at both subzero and above 0°C temperatures.
• Kinetic CD experiments indicated approximately 26% alpha-helix content in the intermediate.
• X-ray scattering showed the intermediate to be compact, though not fully packed.
• CD analysis correlated the burst phase amplitude with helical fraction, supporting a model of initial alpha-helical collapse.

Conclusions:

• The src SH3 folding pathway involves a distinct alpha-helix-rich intermediate.
• This intermediate's properties suggest a folding mechanism initiated by the collapse of transient alpha-helical structures.
• The findings provide insights into the early stages of protein folding for small, all-beta-sheet proteins.