Protein ¹⁹F NMR in Escherichia coli

Although overexpression and ¹⁵N enrichment facilitate the observation of resonances from disordered proteins in Escherichia coli, ¹⁵N enrichment alone is insufficient for detecting most globular proteins. Here, we explain this dichotomy and overcome the problem while extending the capability of in-cell NMR by using ¹⁹F-labeled proteins. Resonances from small (∼10 kDa) globular proteins containing the amino acid analogue 3-fluoro-tyrosine can be observed in cells, but for larger proteins the ¹⁹F resonances are broadened beyond detection. Incorporating the amino acid analogue trifluoromethyl-l-phenylalanine allows larger proteins (up to 100 kDa) to be observed in cells. We also show that site-specific structural and dynamic information about both globular and disordered proteins can be obtained inside cells by using ¹⁹F NMR.