Smaller 145th residue makes fluorescent protein non-fluorescent: Fluorescence lifetimes of enhanced yellow fluorescent protein (eYFP) Y145 mutants and H148 mutants

Unified Role of the 145th Residue on the Fluorescence Lifetime of Fluorescent Proteins from the Jellyfish Aequorea victoria

Finding a unified fluorescence mechanism is essential to develop and utilize fluorescent proteins appropriately. Here, we report the unified role of the 145th residue on the fluorescence efficiency of fluorescent proteins developed from the jellyfish Aequorea victoria by demonstrating the difference and similarity between two representative fluorescent proteins, enhanced green fluorescent protein (eGFP), and enhanced yellow fluorescent protein (eYFP). We determined the fluorescence lifetimes of the 19 different Y145 mutants of eGFP and eYFP by picosecond time-resolved fluorescence spectroscopy. We found that the effect of the 145th mutation on the fluorescence lifetime is significant for eYFP but moderate for eGFP. We compared known crystal structures to clarify the observed difference between eGFP and eYFP. As a result, we conclude that the efficiency of the steric restriction of the chromophore motion by the 145th side chain is essentially the same for both eGFP and eYFP. Meanwhile, the restriction of the chromophore motion by hydrogen bonds is more pronounced for eGFP than for YFP. Balance of the steric effect and hydrogen bonding controls the lifetime of the Y145 mutants for eGFP and eYFP. Furthermore, the steric restriction is induced by the electrostatic effect; the different 145th residue induces a different electrostatic environment around the chromophore. The finding in this study reasonably explains the reported lifetimes of other fluorescent proteins and allows the prediction of the lifetime of unknown fluorescent proteins from jellyfish.

Three Simultaneous Fluorescence Resonance Energy Transfer Processes and Structural Relaxation of Enhanced Yellow Fluorescent Protein Observed by Picosecond Time-Resolved Fluorescence Anisotropy

Fluorescent proteins (FPs) have been widely used to visualize biological processes in living cells. It is essential to understand the underlying fluorescence mechanism to develop novel FPs and to interpret imaging data appropriately. Enhanced yellow fluorescent protein (eYFP) is one of the most typical FPs; however, several reports to date have been limited to individual discussion, which is insufficient to understand the full picture of the dynamics involved. In this study, we focused on the fluorescence resonance energy transfer (FRET) and dimerization behavior and performed picosecond time-resolved fluorescence measurements of eYFP and its A206K mutant, which does not form a dimer. The combination of the dissociation constant and the acid dissociation constant rationally explains the mechanism of ultrafast homo-FRET and ultrafast hetero-FRET. It is also shown that structural relaxation occurs in the dimer after excited-state proton transfer. The formation efficiencies and quaternary structures of dimers consisting of different protonation states are shown to be different. Furthermore, under high-concentration conditions, "slow" homo-FRET with tens of nanoseconds timescale occurs between monomers and dimers. The findings from this study will be applied to other fluorescent proteins such as Aequorea victoria green FP and its mutants and various red FPs with longer conjugation lengths.

Quantification of FRET-induced angular displacement by monitoring sensitized acceptor anisotropy using a dim fluorescent donor

Förster resonance energy transfer (FRET) between fluorescent proteins has become a common platform for designing genetically encoded biosensors. For live cell imaging, the acceptor-to-donor intensity ratio is most commonly used to readout FRET efficiency, which largely depends on the proximity between donor and acceptor. Here, we introduce an anisotropy-based mode of FRET detection (FADED: FRET-induced Angular Displacement Evaluation via Dim donor), which probes for relative orientation rather than proximity alteration. A key element in this technique is suppression of donor bleed-through, which allows measuring purer sensitized acceptor anisotropy. This is achieved by developing Geuda Sapphire, a low-quantum-yield FRET-competent fluorescent protein donor. As a proof of principle, Ca2+ sensors were designed using calmodulin as a sensing domain, showing sigmoidal dose response to Ca2+. By monitoring the anisotropy, a Ca2+ rise in living HeLa cells is observed upon histamine challenging. We conclude that FADED provides a method for quantifying the angular displacement via FRET.

Seeing the long tail: A novel green fluorescent protein, SiriusGFP, for ultra long timelapse imaging

BACKGROUND

Fluorescent proteins (FPs) have widespread uses in cell biology. However, the practical applications of FPs are significantly limited due to their rapid photobleaching and misfolding when fused to target proteins.

NEW METHOD

Using a combination of novel and known mutations to eGFP, we developed a well folded and very photostable variant, SiriusGFP.

RESULTS

The fluorescence spectrum indicated that the excitation and emission peaks of SiriusGFP were red-shifted by 16 and 8 nm, respectively. Co- operative effects of two key mutations, S147R and S205 V, contribute to its photostability. SiriusGFP tagged to the mitochondrial outer membrane protein Omp25 showed sustained fluorescence during continuous 3D-scanning confocal imaging (4D confocal) compared to eGFP-tagged Omp25. Furthermore, with super-resolution structured illumination microscopy (SIM) we demonstrate marked improvements in image quality and resolution (130 nm in XY axis, and 310 nm in Z axis), as well as, decreased artifacts due to photobleaching.

COMPARISON WITH EXISTING METHOD(S)

Compared to eGFP. SiriusGFP shows a 2-fold increase in photostability in vitro, and folds well when fused to the N- and C- termini of cytoplasmic and membrane proteins. While its quantum yield is ˜3 fold lower than eGFP, its decreased brightness was more than compensated by its increasedphotostability in different experimental paradigms allowing practical experimentation without dynamic adjustment of light intensity or fluorescence sampling times.

CONCLUSIONS

We have developed a variant of eGFP, SiriusGFP, that shows over a two fold increase in photostability with utility in methods requiring sustained or high intensity excitation as in 4D confocal or SIM imaging.

A new twist in the photophysics of the GFP chromophore: a volume-conserving molecular torsion couple

Dynamics of a nonplanar GFP chromophore are studied experimentally and theoretically. Coupled torsional motion is responsible for the ultrafast decay.

The simple structure of the chromophore of the green fluorescent protein (GFP), a phenol and an imidazolone ring linked by a methyne bridge, supports an exceptionally diverse range of excited state phenomena. Here we describe experimentally and theoretically the photochemistry of a novel sterically crowded nonplanar derivative of the GFP chromophore. It undergoes an excited state isomerization reaction accompanied by an exceptionally fast (sub 100 fs) excited state decay. The decay dynamics are essentially independent of solvent polarity and viscosity. Excited state structural dynamics are probed by high level quantum chemical calculations revealing that the fast decay is due to a conical intersection characterized by a twist of the rings and pyramidalization of the methyne bridge carbon. The intersection can be accessed without a barrier from the pre-twisted Franck–Condon structure, and the lack of viscosity dependence is due to the fact that the rings twist in the same direction, giving rise to a volume-conserving decay coordinate. Moreover, the rotation of the phenyl, methyl and imidazolone groups is coupled in the sterically crowded structure, with the methyl group translating the rotation of one ring to the next. As a consequence, the excited state dynamics can be viewed as a torsional couple, where the absorbed photon energy leads to conversion of the out-of-plane orientation from one ring to the other in a volume conserving fashion. A similar modification of the range of methyne dyes may provide a new family of devices for molecular machines, specifically torsional couples.