Protein fold determined by paramagnetic magic-angle spinning solid-state NMR spectroscopy

Biomacromolecules that are challenging for the usual structural techniques can be studied with atomic resolution by solid-state NMR spectroscopy. However, the paucity of distance restraints >5 angstrom, traditionally derived from measurements of magnetic dipole-dipole couplings between protein nuclei, is a major bottleneck that hampers such structure elucidation efforts. Here, we describe a general approach that enables the rapid determination of global protein fold in the solid phase via measurements of nuclear paramagnetic relaxation enhancements (PREs) in several analogues of the protein of interest containing covalently attached paramagnetic tags, without the use of conventional internuclear distance restraints. The method is demonstrated using six cysteine-EDTA-Cu2+ mutants of the 56-residue B1 immunoglobulin-binding domain of protein G, for which similar to 230 longitudinal backbone N-15 PREs corresponding to distances of similar to 10-20 angstrom were obtained. The mean protein fold determined in this manner agrees with the X-ray structure with a backbone atom root-mean-square deviation of 1.8 angstrom.