Protocols for the sequential solid-state NMR spectroscopic assignment of a uniformly labeled 25 kDa protein: HET-s(1-227).

The sequence-specific resonance assignment of a protein forms the basis for studies of molecular structure and dynamics, as well as to functional assay studies by NMR spectroscopy. Here we present a protocol for the sequential 13C and 15N resonance assignment of uniformly [15N,13C]-labeled proteins, based on a suite of complementary three-dimensional solid-state NMR spectroscopy experiments. It is directed towards the application to proteins with more than about 100 amino acid residues. The assignments rely on a walk along the backbone by using a combination of three experiments that correlate nitrogen and carbon spins, including the well-dispersed Cbeta resonances. Supplementary spectra that correlate further side-chain resonances can be important for identifying the amino acid type, and greatly assist the assignment process. We demonstrate the application of this assignment protocol for a crystalline preparation of the N-terminal globular domain of the HET-s prion, a 227-residue protein.