Molecular basis for the structural stability of an enclosed β-barrel loop

Area of Science:

• Structural biology
• Computational biophysics
• Microbial pathogenesis

Background:

• Autotransporter proteins, like EspP from Escherichia coli O157:H7, are crucial for bacterial outer membrane function.
• Enclosed loops within β-barrel domains play roles in membrane permeability and channel regulation.
• Understanding the stability of these loops is key to deciphering their functional mechanisms.

Purpose of the Study:

• To investigate the structural stability of an enclosed loop (loop 5, L5) in the β domain of the Escherichia coli O157:H7 autotransporter EspP.
• To determine the factors contributing to the thermal and mechanical resilience of EspP L5.
• To compare the stability mechanisms of EspP L5 with those of globular proteins.

Main Methods:

• Molecular dynamics (MD) simulations were employed to study the structural dynamics of EspP L5.
• Simulations assessed stability against thermal perturbations and mechanical pulling forces along the membrane normal.
• Analysis of B-factors from existing high-resolution structures of membrane-embedded β-barrels was performed.

Main Results:

• EspP L5 demonstrated remarkable resistance to both thermal stress and mechanical forces.
• The loop's stability is primarily attributed to geometric constraints and side-chain interactions, particularly hydrogen bonds.
• This contrasts with globular proteins, where backbone hydrogen bonds and hydrophobic interactions are dominant stability factors.

Conclusions:

• EspP L5's robust stability supports its role in maintaining the outer membrane permeability barrier.
• Geometric constraints and side-chain interactions represent a generalizable mechanism for stabilizing enclosed loops in β-barrels.
• This stabilization strategy may be widely utilized in β-barrel channels for diverse functional purposes.