Superior robustness of ExEm-spFRET to IIem-spFRET method in live-cell FRET measurement

Detecting Förster resonance energy transfer in living cells by conventional and spectral flow cytometry

Assays based on Förster resonance energy transfer (FRET) can be used to study many processes in cell biology. Although this is most often done with microscopy for fluorescence detection, we report two ways to measure FRET in living cells by flow cytometry. Using a conventional flow cytometer and the "3-cube method" for intensity-based calculation of FRET efficiency, we measured the enzymatic activity of specific kinases in cells expressing a genetically-encoded reporter. For both AKT and protein kinase A, the method measured kinase activity in time-course, dose-response, and kinetic assays. Using the Cytek Aurora spectral flow cytometer, which applies linear unmixing to emission measured in multiple wavelength ranges, FRET from the same reporters was measured with greater single-cell precision, in real time and in the presence of other fluorophores. Results from gene-knockout studies suggested that spectral flow cytometry might enable the sorting of cells on the basis of FRET. The methods we present provide convenient and flexible options for using FRET with flow cytometry in studies of cell biology.

Mechanofluorescent Polymer Brush Surfaces that Spatially Resolve Surface Solvation

Polymer brushes, consisting of densely end-tethered polymers to a surface, can exhibit rapid and sharp conformational transitions due to specific stimuli, which offer intriguing possibilities for surface-based sensing of the stimuli. The key toward unlocking these possibilities is the development of methods to readily transduce signals from polymer conformational changes. Herein, we report on single-fluorophore integrated ultrathin (<40 nm) polymer brush surfaces that exhibit changing fluorescence properties based on polymer conformation. The basis of our methods is the change in occupied volume as the polymer brush undergoes a collapse transition, which enhances the effective concentration and aggregation of the integrated fluorophores, leading to a self-quenching of the fluorophores' fluorescence and thereby reduced fluorescence lifetimes. By using fluorescence lifetime imaging microscopy, we reveal spatial details on polymer brush conformational transitions across complex interfaces, including at the air-water-solid interface and at the interface of immiscible liquids that solvate the surface. Furthermore, our method identifies the swelling of polymer brushes from outside of a direct droplet (i.e., the polymer phase with vapor above), which is controlled by humidity. These solvation-sensitive surfaces offer a strong potential for surface-based sensing of stimuli-induced phase transitions of polymer brushes with spatially resolved output in high resolution.

Probing membrane protein interactions and signaling molecule homeostasis in plants by Förster resonance energy transfer analysis

Membrane proteins have key functions in signal transduction, transport, and metabolism. Therefore, deciphering the interactions between membrane proteins provides crucial information on signal transduction and the spatiotemporal organization of protein complexes. However, detecting the interactions and behaviors of membrane proteins in their native environments remains difficult. Förster resonance energy transfer (FRET) is a powerful tool for quantifying the dynamic interactions and assembly of membrane proteins without disrupting their local environment, supplying nanometer-scale spatial information and nanosecond-scale temporal information. In this review, we briefly introduce the basic principles of FRET and assess the current state of progress in the development of new FRET techniques (such as FRET-FLIM, homo-FRET, and smFRET) for the analysis of plant membrane proteins. We also describe the various FRET-based biosensors used to quantify the homeostasis of signaling molecules and the active state of kinases. Furthermore, we summarize recent applications of these advanced FRET sensors in probing membrane protein interactions, stoichiometry, and protein clustering, which have shed light on the complex biological functions of membrane proteins in living plant cells.

A plant-unique ESCRT component, FYVE4, regulates multivesicular endosome biogenesis and plant growth

During evolution, land plants generated unique proteins that participate in endosomal sorting and multivesicular endosome (MVE) biogenesis, many of them with specific phosphoinositide-binding capabilities. Nonetheless, the function of most plant phosphoinositide-binding proteins in endosomal trafficking remains elusive. Here, we analysed several Arabidopsis mutants lacking predicted phosphoinositide-binding proteins and first identified fyve4-1 as a mutant with a hypersensitive response to high-boron conditions and defects in degradative vacuolar sorting of membrane proteins such as the borate exporter BOR1-GFP. FYVE4 encodes a plant-unique, FYVE domain-containing protein that interacts with SNF7, a core component of ESCRT-III (Endosomal Sorting Complex Required for Transport III). FYVE4 affects the membrane association of the late-acting ESCRT components SNF7 and VPS4, and modulates the formation of intraluminal vesicles (ILVs) inside MVEs. The critical function of FYVE4 in the ESCRT pathway was further demonstrated by the strong genetic interactions with SNF7B and LIP5. Although the fyve4-1, snf7b and lip5 single mutants were viable, the fyve4-1 snf7b and fyve4-1 lip5 double mutants were seedling lethal, with strong defects in MVE biogenesis and vacuolar sorting of ubiquitinated membrane proteins. Taken together, we identified FYVE4 as a novel plant endosomal regulator, which functions in ESCRTing pathway to regulate MVE biogenesis.

Comparing the performance of mScarlet-I, mRuby3, and mCherry as FRET acceptors for mNeonGreen

Förster Resonance Energy Transfer (FRET) has become an immensely powerful tool to profile intra- and inter-molecular interactions. Through fusion of genetically encoded fluorescent proteins (FPs) researchers have been able to detect protein oligomerization, receptor activation, and protein translocation among other biophysical phenomena. Recently, two bright monomeric red fluorescent proteins, mRuby3 and mScarlet-I, have been developed. These proteins offer much improved physical properties compared to previous generations of monomeric red FPs that should help facilitate more general adoption of Green/Red FRET. Here we assess the ability of these two proteins, along with mCherry, to act as a FRET acceptor for the bright, monomeric, green-yellow FP mNeonGreen using intensiometric FRET and 2-photon Fluorescent Lifetime Imaging Microscopy (FLIM) FRET techniques. We first determined that mNeonGreen was a stable donor for 2-photon FLIM experiments under a variety of imaging conditions. We then tested the red FP's ability to act as FRET acceptors using mNeonGreen-Red FP tandem construct. With these constructs we found that mScarlet-I and mCherry are able to efficiently FRET with mNeonGreen in spectroscopic and FLIM FRET. In contrast, mNeonGreen and mRuby3 FRET with a much lower efficiency than predicted in these same assays. We explore possible explanations for this poor performance and determine mRuby3's protein maturation properties are a major contributor. Overall, we find that mNeonGreen is an excellent FRET donor, and both mCherry and mScarlet-I, but not mRuby3, act as practical FRET acceptors, with the brighter mScarlet-I out performing mCherry in intensiometric studies, but mCherry out performing mScarlet-I in instances where consistent efficiency in a population is critical.

ExEm-FRET two-hybrid assay: FRET two-hybrid assay based on linear unmixing of excitation-emission spectra

Simultaneous linear unmixing of excitation-emission spectra (ExEm-unmixing)-based fluorescence resonance energy transfer (FRET) two-hybrid assay method, named as ExEm-FRET two-hybrid assay, was developed for evaluating the stoichiometric ratio of macromolecular complexes in living cells. Linear unmixing of the excitation-emission spectra (SDA) of cells obtains the weight factors of donor (WD), acceptor (WA) and acceptor sensitization (WS), yielding ED and EA (donor- and acceptor-centric FRET efficiency) images. ExEm-FRET two-hybrid assay employs pixel-to-pixel titration curves of ED/EA versus the free acceptor (Ca)/donor (Cd) concentration deduced from the three weight factors to obtain EA,max and ED,max (the maximal EA and ED), thus yielding the stoichiometric ratio (EA,max/ED,max) of donor-tagged protein to acceptor-tagged protein.