Genetic Encoding of -(((Trimethylsilyl)methoxy)carbonyl)-l-lysine for NMR Studies of Protein-Protein and Protein-Ligand Interactions.

Trimethylsilyl (TMS) groups present outstanding NMR probes of biological macromolecules as they produce intense singlets in H NMR spectra near 0 ppm, where few other proton resonances occur. We report a system for genetic encoding of -(((trimethylsilyl)methoxy)carbonyl)-l-lysine (TMSK) for site-specific incorporation into proteins. The system is based on pyrrolysyl-tRNA synthetase mutants, which deliver proteins with high yield and purity and in cell-free protein synthesis. As the TMS signal can readily be identified in 1D H NMR spectra of high-molecular weight systems without the need of isotopic labeling, TMSK delivers an excellent site-specific NMR probe for the study of protein structure and function, which is both inexpensive and convenient. We demonstrate the utility of TMSK to detect ligand binding, measure the rate of conformational change, and assess protein dimerization by paramagnetic relaxation enhancement. In addition, we present a system for dual incorporation of two different unnatural amino acids (TMSK and --butyl-tyrosine) in the same protein in quantities sufficient for NMR spectroscopy. Close proximity of the TMS and -butyl groups was readily detected by nuclear Overhauser effects.