Ultrahigh resolution protein structures using NMR chemical shift tensors

NMR chemical shift tensors (CSTs) in proteins, as well as their orientations, represent an important new restraint class for protein structure refinement and determination. Here, we present the first determination of both CST magnitudes and orientations for C-13 alpha and N-15 (peptide backbone) groups in a protein, the beta 1 IgG binding domain of protein G from Streptococcus spp., GB1. Site-specific C-13 alpha and N-15 CSTs were measured using synchronously evolved recoupling experiments in which C-13 and N-15 tensors were projected onto the H-1-C-13 and H-1-N-15 vectors, respectively, and onto the N-15-C-13 vector in the case of C-13 alpha. The orientations of the C-13 alpha CSTs to the H-1-C-13 and C-13-N-15 vectors agreed well with the results of ab initio calculations, with an rmsd of approximately 8 degrees. In addition, the measured N-15 tensors exhibited larger reduced anisotropies in alpha-helical versus beta-sheet regions, with very limited variation (18 +/- 4 degrees) in the orientation of the z-axis of the N-15 CST with respect to the H-1-N-15 vector. Incorporation of the C-13 alpha CST restraints into structure calculations, in combination with isotropic chemical shifts, transferred echo double resonance C-13-N-15 distances and vector angle restraints, improved the backbone rmsd to 0.16 angstrom (PDB ID code 2LGI) and is consistent with existing X-ray structures (0.51 angstrom agreement with PDB ID code 2QMT). These results demonstrate that chemical shift tensors have considerable utility in protein structure refinement, with the best structures comparable to 1.0-angstrom crystal structures, based upon empirical metrics such as Ramachandran geometries and chi(1)/chi(2) distributions, providing solid-state NMR with a powerful tool for de novo structure determination.