Electrostatic and steric interactions determine bacteriorhodopsin single-molecule biomechanics.

Area of Science:

• Structural Biology
• Biophysics
• Membrane Protein Dynamics

Background:

• Bacteriorhodopsin (bR) is a light-driven proton pump in haloarchaea.
• Its function relies on photon energy conversion into structural changes for proton translocation.
• Understanding bR biomechanics at the submolecular level is key to its efficient function.

Purpose of the Study:

• To investigate the role of tryptophan (Trp) residues in bacteriorhodopsin's mechanical properties.
• To elucidate how Trp residues contribute to the protein's scaffold and proton pumping mechanism.
• To explore the influence of salt concentration on bR structure and function.

Main Methods:

• Single molecule force spectroscopy was employed.
• Experiments were conducted at varying salt concentrations.
• Analysis focused on the mechanical contributions of specific amino acid residues.

Main Results:

• Tryptophan residues form a rigid extracellular scaffold, creating major unfolding barriers.
• This Trp-based network encloses the retinal, dictates local mechanical properties, and anchors bR.
• Extracellular Trp residues facilitate ion binding for proton release and transport, while the cytoplasmic side offers flexibility.

Conclusions:

• A stable, extracellular Trp-based network is critical for bacteriorhodopsin's mechanical stability and function.
• This network controls proton translocation by anchoring the protein and facilitating ion binding.
• The identified Trp network is likely conserved in other haloarchaeal rhodopsins, contributing to their high efficiency.