Non-Covalent Reversibly Photoconvertible Fluorescent Tags for Wash-Free Protein Labeling.

Reversibly photoswitchable fluorophores are widely used in advanced bioimaging, but their design remains demanding. Here, we introduce a new series spanning the whole visible range, which results from combining a large set of fluorogens with the FAST protein scaffold. We first demonstrate that these well-established fluorescent protein labeling tags turn into negative reversible photoswitchers upon decreasing the fluorogen concentration and increasing light intensity. We then show that using not one but two fluorogens adds new responses to illumination. Thus, we obtain positive reversible photoswitchers that increase their brightness under illumination. We also generate a palette of non-covalent reversibly photoconvertible fluorescent proteins changing their fluorescence color upon illumination, a reversible behavior that still remains absent in regular fluorescent proteins. This light-induced color change opens the possibility to discriminate six spectrally similar FAST variants in live cells upon demonstrating the superiority of using multiple spectral channels for exploiting the time dependence of the fluorescence response to illumination.