Selective labeling and unlabeling strategies in protein solid-state NMR spectroscopy

Area of Science:

• Biochemistry
• Structural Biology
• Biophysics

Background:

• Selective isotope labeling is crucial for Nuclear Magnetic Resonance (NMR) spectroscopy, enhancing spectral resolution for complex biological systems.
• Solid-state NMR studies are increasingly investigating large protein systems, where signal overlap presents significant challenges.

Purpose of the Study:

• To provide a comprehensive overview of amino acid metabolism in Escherichia coli (E. coli) relevant to isotope labeling strategies.
• To define accessible combinations of amino acids for specific 13C or 15N labeling or unlabeling in E. coli.
• To demonstrate the application of these labeling strategies for large protein systems using the ABC transporter BmrA as an example.

Main Methods:

• Detailed analysis of amino acid metabolic pathways in E. coli.
• Supplementation of minimal media with specific labeled or unlabeled amino acids during bacterial protein expression.
• Application of solid-state NMR techniques, including 2D DARR experiments, for resonance assignment of selectively labeled proteins.

Main Results:

• Identification of amino acid interconversion pathways (scrambling) in E. coli that impact labeling strategies.
• Demonstration of successful selective labeling of the 600-residue membrane protein BmrA by strategic amino acid supplementation.
• Reduced spectral overlap in NMR spectra of selectively labeled BmrA, facilitating resonance assignment and sequential assignment initiation.

Conclusions:

• A thorough understanding of E. coli amino acid metabolism is essential for designing effective selective isotope labeling schemes.
• Selective labeling strategies significantly improve the feasibility of solid-state NMR studies on large and complex proteins.
• The presented approach enables resonance assignment in challenging systems, advancing the structural and functional investigation of large proteins.