Cryo-EM and Solid State NMR Together Provide a More Comprehensive Structural Investigation of Protein Fibrils

The Tropomyosin 1 isoform I/C C-terminal domain (Tm1-LC) fibril structure is studied jointly with cryogenic electron microscopy (cryo-EM) and solid state nuclear magnetic resonance (NMR). This study demonstrates the complementary nature of these two structural biology techniques. Chemical shift assignments from solid state NMR are used to determine the secondary structure at the level of individual amino acids, which is faithfully seen in cryo-EM reconstructions. Additionally, solid state NMR demonstrates that the region not observed in the reconstructed cryo-EM density is primarily in a highly mobile random coil conformation rather than adopting multiple rigid conformations. Overall, this study illustrates the benefit of investigations combining cryo-EM and solid state NMR to investigate protein fibril structure. Significance The use of multiple techniques to structurally characterize proteins provides models that accurately describe molecular conformations better than a technique used in isolation. Combination approaches allow for the study of proteins not only as rigid objects, but rather dynamic molecules that “breathe” over time. Cryogenic electron microscopy and solid state nuclear magnetic resonance are used jointly to provide a more detailed model of the same protein fibrils, and each technique provides novel insights.