Gd³⁺-Trityl-Nitroxide Triple Labeling and Distance Measurements in the Heterooligomeric Cobalamin Transport Complex...

Area of Science:

• Biophysics
• Structural Biology
• Membrane Protein Dynamics

Background:

• Observing proteins in native environments is crucial for understanding their function.
• Pulsed electron-electron double resonance (PELDOR) spectroscopy is a key technique for this.
• Conventional PELDOR with identical spin pairs limits distance measurements.

Purpose of the Study:

• To develop a versatile PELDOR approach for studying heterooligomeric membrane protein complexes.
• To enable independent determination of multiple distances within a single sample.
• To analyze protein-protein interactions and dynamics in native membrane environments.

Main Methods:

• Utilized a Gd³⁺-trityl-nitroxide (NO) three-spin system for PELDOR measurements.
• Applied the method to the four-component cobalamin transport system (cobalamin, BtuB, TonB, BtuF).
• Observed sequential ligand binding and complex formation dynamics in native asymmetric bilayers.

Main Results:

• Successfully determined four independent distances (Gd³⁺-trityl, Gd³⁺-NO, trityl-NO, Gd³⁺-Gd³⁺) in a single sample.
• Demonstrated sequential ligand binding and complex formation in the cobalamin transport system.
• Showed that TonB binding alone releases cobalamin from BtuB in native bilayers.

Conclusions:

• The Gd³⁺-trityl-nitroxide three-spin system is a versatile tool for PELDOR studies.
• This approach facilitates structural and quantitative analysis of dynamic protein interactions.
• Enables investigation of oligomeric complexes within their native biological surroundings.