Photoswitching-free FRAP analysis with a genetically encoded fluorescent tag

Fluorescence recovery after photobleaching (FRAP) is a widely used imaging technique for measuring protein dynamics in live cells that has provided many important biological insights. Although FRAP presumes that the conversion of a fluorophore from a bright to a dark state is irreversible, GFP as well as other genetically encoded fluorescent proteins now in common use can also exhibit a reversible conversion known as photoswitching. Various studies have shown how photoswitching can cause at least four different artifacts in FRAP, leading to false conclusions about various biological phenomena, including the erroneous identification of anomalous diffusion or the overestimation of the freely diffusible fraction of a cellular protein. Unfortunately, identifying and then correcting these artifacts is difficult. Here we report a new characteristic of an organic fluorophore tetramethylrhodamine bound to the HaloTag protein (TMR-HaloTag), which like GFP can be genetically encoded, but which directly and simply overcomes the artifacts caused by photoswitching in FRAP. We show that TMR exhibits virtually no photoswitching in live cells under typical imaging conditions for FRAP. We also demonstrate that TMR eliminates all of the four reported photoswitching artifacts in FRAP. Finally, we apply this photoswitching-free FRAP with TMR to show that the chromatin decondensation following UV irradiation does not involve loss of nucleosomes from the damaged DNA. In sum, we demonstrate that the TMR Halo label provides a genetically encoded fluorescent tag very well suited for accurate FRAP experiments.