Using Genetically Encodable Self-Assembling GdIII Spin Labels To Make In-Cell Nanometric Distance Measurements

Double electron–electron resonance (DEER) can be used to study the structure of a protein in its native cellular environment. Until now, this has required isolation, in vitro labeling, and reintroduction of the protein back into the cells. We describe a completely biosynthetic approach that avoids these steps. It exploits genetically encodable lanthanide‐binding tags (LBT) to form self‐assembling GdIII metal‐based spin labels and enables direct in‐cell measurements. This approach is demonstrated using a pair of LBTs encoded one at each end of a 3‐helix bundle expressed in E. coli grown on GdIII‐supplemented medium. DEER measurements directly on these cells produced readily detectable time traces from which the distance between the GdIII labels could be determined. This work is the first to use biosynthetically produced self‐assembling metal‐containing spin labels for non‐disruptive in‐cell structural measurements. In a spin: A 3‐helix bundle flanked by two lanthanide‐binding tags was expressed in E. coli. GdIII added to the medium is able to enter the cell and bind to the protein tags to give a fully biosynthetic spin‐labeled protein. DEER measurements on intact cells showed that this is a feasible approach for in‐cell structural studies of proteins under native cell conditions.