Labeling Strategy and Signal Broadening Mechanism of Protein NMR Spectroscopy in Xenopus laevis Oocytes

We used Xenopus laevis oocytes, a paradigm for a variety of biological studies, as a eukaryotic model system for in‐cell protein NMR spectroscopy. The small globular protein GB1 was one of the first studied in Xenopus oocytes, but there have been few reports since then of high‐resolution spectra in oocytes. The scarcity of data is at least partly due to the lack of good labeling strategies and the paucity of information on resonance broadening mechanisms. Here, we systematically evaluate isotope enrichment and labeling methods in oocytes injected with five different proteins with molecular masses of 6 to 54 kDa. ¹⁹F labeling is more promising than ¹⁵N, ¹³C, and ²H enrichment. We also used ¹⁹F NMR spectroscopy to quantify the contribution of viscosity, weak interactions, and sample inhomogeneity to resonance broadening in cells. We found that the viscosity in oocytes is only about 1.2 times that of water, and that inhomogeneous broadening is a major factor in determining line width in these cells.