(A) Crystal structures of two Vivid monomers in the dark state (PDB: 2PD7), and the dimer in the lit state (PDB: 3RH8). (B) Cartoon representation of LightR design. Two tandemly connected VVD photoreceptors inserted in the catalytic domain disrupt the catalytic activity of the protein in the dark. Dimerization of VVD in response to blue light restores the protein activity. (C) Crystal structure of c-Src catalytic domain (PDB:1Y57) with the insertion site G288 in magenta. The insertion site is connected to the catalytically important G-loop , highlighted in red, by a β-strand. Schematic below shows the amino acid sequence of the wild type Src residues around the insertion site and the resulting construct with LightR insertion. Insertion site G288 in WT Src is shown in magenta, asymmetric flexible GPGGSGG and GSGGPG linkers are depicted in purple, VVD proteins are shown in orange and blue, and 22-residue flexible linker is shown in grey. (D, E) Computational modeling of structural changes in the catalytic domain of LightR-Src. Color scale reflects the degree of deviation from the position in the crystal structure of Src (PDB: 1Y57). (D) Structural models reflecting the average RMSD of each residue for the dark and the lit states. (E) Comparative heat map of RMSD values for each residue over the course of the simulation for Src catalytic domain in lit and dark states. Zoomed-in insert shows changes in the G-loop. RMSD values represent an average from three independent 100 ns simulations for each state, lit and dark.