13C Spin Dilution for Simplified and Complete Solid-State NMR Resonance Assignment of Insoluble Biological Assemblies

A strategy for simplified and complete resonance assignment of insoluble and noncrystalline proteins by solid-state NMR (ssNMR) spectroscopy is presented. Proteins produced with [1-C-13]- or [2-C-13]glucose are very sparsely labeled, and the resulting 2D ssNMR spectra exhibit smaller line widths (by a factor of similar to 2 relative to uniformly labeled proteins) and contain a reduced number of cross-peaks. This allows for an accelerated and straightforward resonance assignment without the necessity of time-consuming 3D spectroscopy or sophisticated pulse sequences. The strategy aims at complete backbone and side-chain resonance assignments based on bidirectional sequential walks. The approach was successfully demonstrated with the de novo assignment of the Type Three Secretion System PrgI needle protein. Using a limited set of simple 2D experiments, we report a 97% complete resonance assignment of the backbone and side-chain C-13 atoms.