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Photoswitchable proteins offer exciting prospects for remote control of biochemical processes. We propose a general approach to the design of photoswitchable proteins based on the introduction of a photoswitchable intramolecular cross-linker. We chose, as a model, a FynSH3 domain for which the free energy of folding is less than the energy available from photoisomerization of the cross-linker. Taking the experimentally determined structure of the folded protein as a starting point, mutations were made to introduce pairs of Cys residues so that the distance between Cys sulfur atoms matches the ideal length of the cis form, but not the trans form, of the cross-linker. When the trans cross-linker was introduced into this L3C-L29C-T47AFynSH3 mutant, the protein was destabilized so that folded and unfolded forms coexisted. Irradiation of the cross-linker to produce the cis isomer recovered the folded, active state of the …