Localization Microscopy using Noncovalent Fluorogen Activation by Genetically Encoded Fluorogen-Activating Proteins

The noncovalent equilibrium activation of a fluorogenic malachite green dye and its cognate fluorogen‐activating protein (FAP) can produce a sparse labeling distribution of densely tagged genetically encoded proteins, enabling single molecule detection and super‐resolution imaging in fixed and living cells. These sparse labeling conditions are achieved by control of the dye concentration in the milieu, and do not require any photoswitching or photoactivation. The labeling is achieved by using physiological buffers and cellular media, in which additives and switching buffers are not required to obtain super‐resolution images. We evaluate the super‐resolution properties and images obtained from a selected FAP clone fused to actin, and show that the photon counts per object are between those typically reported for fluorescent proteins and switching‐dye pairs, resulting in 10–30 nm localization precision per object. This labeling strategy complements existing approaches, and may simplify multicolor labeling of cellular structures. Seeing is believing: Equilibrium control of the activated density of fluorogen‐activating proteins with a low concentration of fluorogenic dye enables genetically targeted binding‐activated localization microscopy in fixed and living cells.