X-Ray Crystal Structure and Properties of Phanta, a Weakly Fluorescent Photochromic GFP-Like Protein

Engineering a Yellow Thermostable Fluorescent Protein by Rational Design

Thermal green protein (TGP) is an extremely stable, highly soluble synthetic green fluorescent protein. The quantum yield of TGP is lower than the closest related natural fluorescent protein, monomeric Azami-Green. We improved the thermal recovery of TGP through the introduction of a chromophore mutation, Q66E. Furthermore, we developed a yellow thermal protein (YTP) via mutation of histidine 193 to tyrosine. Incorporation of Q66E into YTP (YTP-E) improved chemostability and pH stability. Both YTP and YTP-E have superior thermostability compared to TGP or TGP-E. These proteins offer a new option for green or yellow fluorescence under harsh chemical or thermal conditions.

Arginine 66 Controls Dark-State Formation in Green-to-Red Photoconvertible Fluorescent Proteins

Photoactivated localization microscopy (PALM) is a powerful technique to investigate cellular nanostructures quantitatively and dynamically. However, the use of PALM for molecular counting or single-particle tracking remains limited by the propensity of photoconvertible fluorescent protein markers (PCFPs) to repeatedly enter dark states. By designing the single mutants mEos2-A69T and Dendra2-T69A, we completely swapped the blinking behaviors of mEos2 and Dendra2, two popular PCFPs. We combined X-ray crystallography and single-molecule microscopy to show that blinking in mEos2 and Dendra2 is largely controlled by the orientation of arginine 66, a highly conserved residue in Anthozoan PCFPs. The Arg66 side-chain conformation affects the bleaching and the on-to-off transition quantum yields, as well as the fraction of molecules entering long-lived dark states, resulting in widely different apparent blinking behaviors that largely modulate the efficiency of current blinking correction procedures. The present work provides mechanistic insight into the complex photophysics of Anthozoan PCFPs and will facilitate future engineering of bright and low-blinking variants suitable for PALM.