Bright ligand-activatable fluorescent protein for high-quality multicolor live-cell super-resolution microscopy

We introduce UnaG as a green-to-dark photoswitching fluorescent protein capable of high-quality super-resolution imaging with photon numbers equivalent to the brightest photoswitchable red protein. UnaG only fluoresces upon binding of a fluorogenic metabolite, bilirubin, enabling UV-free reversible photoswitching with easily controllable kinetics and low background under Epi illumination. The on- and off-switching rates are controlled by the concentration of the ligand and the excitation light intensity, respectively, where the dissolved oxygen also promotes the off-switching. The photo-oxidation reaction mechanism of bilirubin in UnaG suggests that the lack of ligand-protein covalent bond allows the oxidized ligand to detach from the protein, emptying the binding cavity for rebinding to a fresh ligand molecule. We demonstrate super-resolution single-molecule localization imaging of various subcellular structures genetically encoded with UnaG, which enables facile labeling and simultaneous multicolor imaging of live cells. UnaG has the promise of becoming a default protein for high-performance super-resolution imaging. Photoconvertible proteins occupy two color channels thereby limiting multicolour localisation microscopy applications. Here the authors present UnaG, a new green-to-dark photoswitching fluorescent protein for super-resolution imaging, whose activation is based on a noncovalent binding with bilirubin

Fluorescence Microscopy with Nanometer Resolution

Throughout the twentieth century, it was widely accepted that a light microscope relying on propagating light waves and conventional optical lenses could not discern details that were much finer than about half the wavelength of light, or 200−400nm, due to diffraction. However, in the 1990s, the potential for overcoming the diffraction barrier was realized, and microscopy concepts were defined that now resolve fluorescent features down to molecular dimensions. This chapter discusses the simple yet powerful principles that make it possible to neutralize the resolution-limiting role of diffraction in far-field fluorescence nanoscopy methods such as STED, RESOLFT, PALM/"​"​STORM, or PAINT. In a nutshell, feature molecules residing closer than the diffraction barrier are transferred to different (quantum) states, usually a bright fluorescent state and a dark state, so that they become discernible for a brief period of detection. With nanoscopy, the interior of transparent samples, such as living cells and tissues, can be imaged at the nanoscale. A fresh look at the foundations shows that an in-depth description of the basic principles spawns powerful new concepts. Although they differ in some aspects, these concepts harness a local intensity minimum (of a doughnut-shaped or a standing wave pattern) for determining the coordinate of the fluorophore(s) to be registered. Most strikingly, by using an intensity minimum of the excitation light to establish the fluorophore position, MINFLUX nanoscopy has obtained the ultimate (super)resolution: the size of a molecule (≈1nm)