Evolutionary engineering a larger porin using a loop-to-hairpin mechanism

Area of Science:

• Protein evolution
• Membrane protein structure
• Biochemistry

Background:

• Protein diversification is typically driven by gene duplication, with β-hairpins as repeating units in outer membrane β-barrels.
• A computational study proposed an alternative mechanism: loop-to-hairpin transitions increasing strand numbers in β-barrels.

Approach:

• Created a chimeric protein by replacing loop L3 of a 16-stranded β-barrel with the β-hairpin region of an 18-stranded homolog.
• Tested the stability and structural characteristics of the resulting chimeric protein.

Key Points:

• The chimeric protein demonstrated stability within the cell membrane.
• The chimera exhibited increased β-structure and a larger pore size.
• These characteristics are consistent with the proposed loop-to-hairpin transition model.

Conclusions:

• Provides the first experimental evidence supporting the loop-to-hairpin transition as a mechanism for outer membrane β-barrel diversification.
• Suggests alternative evolutionary pathways beyond simple hairpin duplications for protein structural evolution.