Smart fluorescent proteins: innovation for barrier-free superresolution imaging in living cells.

During the past decade, several novel fluorescence microscopy techniques have emerged that achieve incredible spatial and temporal resolution beyond the diffraction limit. These microscopy techniques depend on altered optical setups, unique fluorescent probes, or post-imaging analysis. Many of these techniques also depend strictly on the use of unique fluorescent proteins (FPs) with special photoswitching properties. These photoswitchable FPs are capable of switching between two states in response to light. All localization precision and patterned illumination techniques-such as photo-activation localization microscopy, stochastic optical reconstruction microscopy, reversible saturable optically linear transitions, and saturated structured illumination microscopy-take advantage of these inherent switching properties to achieve superior spatial resolution. This review provides extensive analysis of the positive and negative aspects of photoswitchable FPs, highlighting their application in diffraction-unlimited imaging and suggesting the most suitable fluorescent proteins for superresolution imaging.