Proteins of the GFP (green fluorescent protein) family have revolutionized life sciences because they allow the tagging of biological samples in a non-invasive genetically encoded way. 'Phototransformable' fluorescent proteins, in particular, have recently attracted widespread interest, as their fluorescence state can be finely tuned by actinic light, a property central to the development of super-resolution microscopy. Beyond microscopy applications, phototransformable fluorescent proteins are also exquisite tools to investigate fundamental protein dynamics. Using light to trigger processes such as photoactivation, photoconversion, photoswitching, blinking and photobleaching allows the exploration of the conformational landscape in multiple directions. In the present paper, we review how structural dynamics of phototransformable fluorescent proteins can be monitored by combining X-ray crystallography, in crystallo optical spectroscopy and simulation tools such as quantum chemistry/molecular mechanics hybrid approaches. Besides their usefulness to rationally engineer better performing fluorescent proteins for nanoscopy and other biotechnological applications, these investigations provide fundamental insights into protein dynamics.
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