Higher resolution in localization microscopy by slower switching of a photochromic protein.

Photoswitchable fluorophores play an essential role in super-resolution fluorescence microscopy, including techniques such as photoactivated localization microscopy (PALM). A determining factor in the precision of the images generated by PALM measurements is the photon numbers that can be detected from the fluorophores. Dronpa is a reversibly photoswitchable fluorescent protein that has been successfully used in PALM experiments. The number of photons per switching cycle that can be acquired for Dronpa depends on its off-switching rate, limiting the number of photons that can be recorded. In this study we report our discovery that the tetrameric ancestor of Dronpa, 22G, shows slower switching, and develop a mutant that displays switching kinetics between those of Dronpa and 22G. We show that the kinetics of the photoswitching are strongly related to self-association of the protein, supporting our view of dynamic flexibility as determining in the photoswitching. Similarly we find that higher-resolution PALM images can be acquired with slower-switching proteins due to their higher number of emitted photons per switching cycle.