The Effect of Fluorescent Protein Tags on Phosphoglycerate Kinase Stability Is Nonadditive

Fluorescence lifetime multiplexing with fluorogen activating protein FAST variants

In this paper, we propose a fluorescence-lifetime imaging microscopy (FLIM) multiplexing system based on the fluorogen-activating protein FAST. This genetically encoded fluorescent labeling platform employs FAST mutants that activate the same fluorogen but provide different fluorescence lifetimes for each specific protein-dye pair. All the proposed probes with varying lifetimes possess nearly identical and the smallest-in-class size, along with quite similar steady-state optical properties. In live mammalian cells, we target these chemogenetic tags to two intracellular structures simultaneously, where their fluorescence signals are clearly distinguished by FLIM. Due to the unique structure of certain fluorogens under study, their complexes with FAST mutants display a monophasic fluorescence decay, which may facilitate enhanced multiplexing efficiency by reducing signal cross-talks and providing optimal prerequisites for signal separation upon co-localized and/or spatially overlapped labeling.

HYPK: A marginally disordered protein sensitive to charge decoration

Intrinsically disordered proteins (IDPs) that lie close to the empirical boundary separating IDPs and folded proteins in Uversky's charge-hydropathy plot may behave as "marginal IDPs" and sensitively switch conformation upon changes in environment (temperature, crowding, and charge screening), sequence, or both. In our search for such a marginal IDP, we selected Huntingtin-interacting protein K (HYPK) near that boundary as a candidate; PKIα, also near that boundary, has lower secondary structure propensity; and Crk1, just across the boundary on the folded side, has higher secondary structure propensity. We used a qualitative Förster resonance energy transfer-based assay together with circular dichroism to simultaneously probe global and local conformation. HYPK shows several unique features indicating marginality: a cooperative transition in end-to-end distance with temperature, like Crk1 and folded proteins, but unlike PKIα; enhanced secondary structure upon crowding, in contrast to Crk1 and PKIα; and a cross-over from salt-induced expansion to compaction at high temperature, likely due to a structure-to-disorder transition not seen in Crk1 and PKIα. We then tested HYPK's sensitivity to charge patterning by designing charge-flipped variants including two specific sequences with identical amino acid composition that markedly differ in their predicted size and response to salt. The experimentally observed trends, also including mutants of PKIα, verify the predictions from sequence charge decoration metrics. Marginal proteins like HYPK show features of both folded and disordered proteins that make them sensitive to physicochemical perturbations and structural control by charge patterning.

A Combination of Library Screening and Rational Mutagenesis Expands the Available Color Palette of the Smallest Fluorogen-Activating Protein Tag nanoFAST

NanoFAST is the smallest fluorogen-activating protein, consisting of only 98 amino acids, used as a genetically encoded fluorescent tag. Previously, only a single fluorogen with an orange color was revealed for this protein. In the present paper, using rational mutagenesis and in vitro screening of fluorogens libraries, we expanded the color palette of this tag. We discovered that E46Q is one of the key substitutions enabling the range of possible fluorogens to be expanded. The introduction of this and several other substitutions has made it possible to use not only orange but also red and green fluorogens with the modified protein.

Identification of structural motifs critical for human G6PC2 function informed by sequence analysis and an AlphaFold2-predicted model

G6PC2 encodes a glucose-6-phosphatase (G6Pase) catalytic subunit, primarily expressed in pancreatic islet β cells, which modulates the sensitivity of insulin secretion to glucose and thereby regulates fasting blood glucose (FBG). Mutational analyses were conducted to validate an AlphaFold2 (AF2)-predicted structure of human G6PC2 in conjunction with a novel method to solubilize and purify human G6PC2 from a heterologous expression system. These analyses show that residues forming a predicted intramolecular disulfide bond are essential for G6PC2 expression and that residues forming part of a type 2 phosphatidic acid phosphatase (PAP2) motif are critical for enzyme activity. Additional mutagenesis shows that residues forming a predicted substrate cavity modulate enzyme activity and substrate specificity and residues forming a putative cholesterol recognition amino acid consensus (CRAC) motif influence protein expression or enzyme activity. This CRAC motif begins at residue 219, the site of a common G6PC2 non-synonymous single-nucleotide polymorphism (SNP), rs492594 (Val219Leu), though the functional impact of this SNP is disputed. In microsomal membrane preparations, the L219 variant has greater activity than the V219 variant, but this difference disappears when G6PC2 is purified in detergent micelles. We hypothesize that this was due to a differential association of the two variants with cholesterol. This concept was supported by the observation that the addition of cholesteryl hemi-succinate to the purified enzymes decreased the Vmax of the V219 and L219 variants ∼8-fold and ∼3 fold, respectively. We anticipate that these observations should support the rational development of G6PC2 inhibitors designed to lower FBG.

Protein Folding Stability and Kinetics in Alginate Hydrogels

Proteins are commonly encapsulated in alginate gels for drug delivery and tissue-engineering applications. However, there is limited knowledge of how encapsulation impacts intrinsic protein properties such as folding stability or unfolding kinetics. Here, we use fast relaxation imaging (FReI) to image protein unfolding in situ in alginate hydrogels after applying a temperature jump. Based on changes in the Förster resonance energy transfer (FRET) response of FRET-labeled phosphoglycerate kinase (PGK), we report the quantitative impact of multiple alginate hydrogel concentrations on protein stability and folding dynamics. The gels stabilize PGK by increasing its melting temperature up to 18.4 °C, and the stabilization follows a nonmonotonic dependence on the alginate density. In situ kinetic measurements also reveal that PGK deviates more from two-state folding behavior in denser gels and that the gel decreases the unfolding rate and accelerates the folding rate of PGK, compared to buffer. Phi-value analysis suggests that the folding transition state of an encapsulated protein is structurally similar to that of folded protein. This work reveals both beneficial and negative impacts of gel encapsulation on protein folding, as well as potential mechanisms contributing to altered stability.

Chemical interactions modulate λ6-85 stability in cells

Because steric crowding is most effective when the crowding agent is similar in size to the molecule that it acts upon and the average macromolecule inside cells is much larger than a small protein or peptide, steric crowding is not predicted to affect their folding inside cells. On the other hand, chemical interactions should perturb in-cell structure and stability because they arise from interactions between the surface of the small protein or peptide and its environment. Indeed, previous in vitro measurements of the λ-repressor fragment, λ6-85 , in crowding matrices comprised of Ficoll or protein crowders support these predictions. Here, we directly quantify the in-cell stability of λ6-85 and distinguish the contribution of steric crowding and chemical interactions to its stability. Using a FRET-labeled λ6-85 construct, we find that the fragment is stabilized by 5°C in-cells compared to in vitro. We demonstrate that this stabilization cannot be explained by steric crowding because, as anticipated, Ficoll has no effect on λ6-85 stability. We find that the in-cell stabilization arises from chemical interactions, mimicked in vitro by mammalian protein extraction reagent (M-PER™). Comparison between FRET values in-cell and in Ficoll confirms that U-2 OS cytosolic crowding is reproduced at macromolecule concentrations of 15% w/v. Our measurements validate the cytomimetic of 15% Ficoll and 20% M-PER™ that we previously developed for protein and RNA folding studies. However, because the in-cell stability of λ6-85 is reproduced by 20% v/v M-PER™ alone, we predict that this simplified mixture could be a useful tool to predict the in-cell behaviors of other small proteins and peptides.

Glutamate Permeability of Chicken Best1

Bestrophin-1 (Best1) is a calcium (Ca²⁺)-activated chloride (Cl^-) channel which has a phylogenetically conserved channel structure with an aperture and neck in the ion-conducting pathway. Mammalian mouse Best1 (mBest1) has been known to have a permeability for large organic anions including gluconate, glutamate, and D-serine, in addition to several small monovalent anions, such as Cl^‑, bromine (Br^-), iodine (I^-), and thiocyanate (SCN^-). However, it is still unclear whether non-mammalian Best1 has a glutamate permeability through the ion-conducting pathway. Here, we report that chicken Best1 (cBest1) is permeable to glutamate in a Ca²⁺-dependent manner. The molecular docking and molecular dynamics simulation showed a glutamate binding at the aperture and neck of cBest1 and a glutamate permeation through the ion-conducting pore, respectively. Moreover, through electrophysiological recordings, we calculated the permeability ratio of glutamate to Cl^- (P_Glutamate/P_Cl) as 0.28 based on the reversal potential shift by ion substitution from Cl^- to glutamate in the internal solution. Finally, we directly detected the Ca²⁺-dependent glutamate release through cBest1 using the ultrasensitive two-cell sniffer patch technique. Our results propose that Best1 homologs from non-mammalian (cBest1) to mammalian (mBest1) have a conserved permeability for glutamate.

Co-Translational Folding of Multi-Domain Proteins

The majority of proteins in nature are composed of multiple domains connected in a single polypeptide. How these long sequences fold into functional structures without forming toxic misfolds or aggregates is poorly understood. Their folding is inextricably linked to protein synthesis and interactions with cellular machinery, making mechanistic studies challenging. Recent progress has revealed critical features of multi-domain protein folding in isolation and in the context of translation by the ribosome. In this review, we discuss challenges and progress in understanding multi-domain protein folding, and highlight how molecular interactions shape folding and misfolding pathways. With the development of new approaches and model systems, the stage is now set for mechanistically exploring the folding of large multi-domain proteins.

A Patched-Like Protein PsPTL Is Not Essential for the Growth and Response to Various Stresses in Phytophthora sojae

Patched (Ptc) and Patched-related (Ptr) proteins containing sterol-sensing domains (SSD) and Patched domains are highly conserved in eukaryotes for lipid transport and metabolism. Four proteins containing predicted SSD and Patched domains were simultaneously found by searching the Phytophthora sojae genome database, and one of them was identified as a Patched-like (PTL) protein. Here, we investigated the biological function of PsPTL. The expression level of PsPTL was higher during mycelial and sporulation stages, compared to zoospore (ZO), cyst, and germinated-cyst stages, without significant change during infection. However, deletion of PsPTL using CRISPR/Cas9 had no significant effect on the growth, development, or virulence of P. sojae. Further investigations showed that PsPTL is not essential for P. sojae to cope with external stresses such as temperature, pH, oxidative and osmotic pressure. In addition, this gene did not appear to play an essential role in P. sojae’s response to exogenous sterols. The transcript levels of the other three proteins containing predicted SSD and Patched domains were also not significantly upregulated in PsPTL deletion transformants. Our studies demonstrated that PsPTL is not an essential protein for P. sojae under the tested conditions, and more in-depth research is required for revealing the potential functions of PsPTL under special conditions or in other signaling pathways.

Protein folding and surface interaction phase diagrams in vitro and in cells

Protein stability is subject to environmental perturbations such as pressure and crowding, as well as sticking to other macromolecules and quinary structure. Thus, the environment inside and outside the cell plays a key role in how proteins fold, interact, and function on the scale from a few molecules to macroscopic ensembles. This review discusses three aspects of protein phase diagrams: first, the relevance of phase diagrams to protein folding and function in vitro and in cells; next, how the evolution of protein surfaces impacts on interaction phase diagrams; and finally, how phase separation plays a role on much larger length-scales than individual proteins or oligomers, when liquid phase-separated regions form to assist protein function and cell homeostasis.

GFP fluorescence tagging alters dynamin-related protein 1 oligomerization dynamics and creates disassembly-refractory puncta to mediate mitochondrial fission

Green fluorescent protein (GFP)-tagging is the prevalent strategy to monitor protein dynamics in living cells. However, the consequences of appending the bulky GFP moiety to the protein of interest are rarely investigated. Here, using a powerful combination of quantitative fluorescence spectroscopic and imaging techniques, we have examined the oligomerization dynamics of the GFP-tagged mitochondrial fission GTPase dynamin-related protein 1 (Drp1) both in vitro and in vivo. We find that GFP-tagged Drp1 exhibits impaired oligomerization equilibria in solution that corresponds to a greatly diminished cooperative GTPase activity in comparison to native Drp1. Consequently, GFP-tagged Drp1 constitutes aberrantly stable, GTP-resistant supramolecular assemblies both in vitro and in vivo, neither of which reflects a more dynamic native Drp1 oligomerization state. Indeed, GFP-tagged Drp1 is detected more frequently per unit length over mitochondria in Drp1-null mouse embryonic fibroblasts (MEFs) compared to wild-type (wt) MEFs, indicating that the drastically reduced GTP turnover restricts oligomer disassembly from the mitochondrial surface relative to mixed oligomers comprising native and GFP-tagged Drp1. Yet, GFP-tagged Drp1 retains the capacity to mediate membrane constriction in vitro and mitochondrial division in vivo. These findings suggest that instead of robust assembly-disassembly dynamics, persistent Drp1 higher-order oligomerization over membranes is sufficient for mitochondrial fission.

Measuring protein stability in the GroEL chaperonin cage reveals massive destabilization

The thermodynamics of protein folding in bulk solution have been thoroughly investigated for decades. By contrast, measurements of protein substrate stability inside the GroEL/ES chaperonin cage have not been reported. Such measurements require stable encapsulation, that is no escape of the substrate into bulk solution during experiments, and a way to perturb protein stability without affecting the chaperonin system itself. Here, by establishing such conditions, we show that protein stability in the chaperonin cage is reduced dramatically by more than 5 kcal mol^-1 compared to that in bulk solution. Given that steric confinement alone is stabilizing, our results indicate that hydrophobic and/or electrostatic effects in the cavity are strongly destabilizing. Our findings are consistent with the iterative annealing mechanism of action proposed for the chaperonin GroEL.

Non-Invasive Plasmonic-Based Real-Time Characterization of Cardiac Drugs on Cardiomyocytes Functional Behavior

In the fabrication of cardiac tissue, an important factor is continuous measurement of its contraction features. A module that allows for a dynamic system capable of noninvasive and label-free monitoring of the contraction profile under administering chemicals and drugs is highly valuable for understanding accurate tissue mechanobiology. In this research, we have successfully demonstrated the use of surface plasmon resonance (SPR) technology for the first time to characterize the contractility of cardiac cells in response to Blebbistatin and ATP drug exposure in real tme. An optimal flow rate of 10 μL/min was selected for a continuous flow of warm media,and 10 μM drug administration effect was detected with high spatiotemporal sensitivity on contracting cardiomyocytes. Our drug screening has identified the source of the SPR periodic signal to be direct cell contraction rather than action potentials or calcium signaling. Per our results, SPR has high potential in applications in least-interference real-time and label-free tissue characterizations and cellular properties analysis from a functional and structural point of view.

Competitive Binding Study Revealing the Influence of Fluorophore Labels on Biomolecular Interactions

Fluorescence methods are important tools in modern biology. Direct labeling of biomolecules with a fluorophore might, however, change interaction surfaces. Here, we introduce a competitive binding assay in combination with fluorescence correlation spectroscopy that reports binding affinities of both labeled and unlabeled biomolecules to their binding target. We investigated how fluorophore labels at different positions of a DNA oligonucleotide affect hybridization to a complementary oligonucleotide and found dissociation constants varying within 2 orders of magnitude. We next demonstrated that placing a fluorophore label at position Leu280 in the protein ligand internalin B does not alter the binding affinity to the MET receptor tyrosine kinase, compared to unlabeled internalin B. Our approach is simple to implement and can be applied to investigate the influence of fluorophore labels in a large variety of biomolecular interactions.

Heat shock-induced chaperoning by Hsp70 is enabled in-cell

Recent work has shown that weak protein-protein interactions are susceptible to the cellular milieu. One case in point is the binding of heat shock proteins (Hsps) to substrate proteins in cells under stress. Upregulation of the Hsp70 chaperone machinery at elevated temperature was discovered in the 1960s, and more recent studies have shown that ATPase activity in one Hsp70 domain is essential for control of substrate binding by the other Hsp70 domain. Although there are several denaturant-based assays of Hsp70 activity, reports of ATP-dependent binding of Hsp70 to a globular protein substrate under heat shock are scarce. Here we show that binding of heat-inducible Hsp70 to phosphoglycerate kinase (PGK) is remarkably different in vitro compared to in-cell. We use fluorescent-labeled mHsp70 and ePGK, and begin by showing that mHsp70 passes the standard β-galactosidase assay, and that it does not self-aggregate until 50°C in presence of ATP. Yet during denaturant refolding or during in vitro heat shock, mHsp70 shows only ATP-independent non-specific sticking to ePGK, as evidenced by nearly identical results with an ATPase activity-deficient K71M mutant of Hsp70 as a control. Addition of Hsp40 (co-factor) or Ficoll (crowder) does not reduce non-specific sticking, but cell lysate does. Therefore, Hsp70 does not act as an ATP-dependent chaperone on its substrate PGK in vitro. In contrast, we observe only specific ATP-dependent binding of mHsp70 to ePGK in mammalian cells, when compared to the inactive Hsp70 K71M mutant. We hypothesize that enhanced in-cell activity is not due to an unknown co-factor, but simply to a favorable shift in binding equilibrium caused by the combination of crowding and osmolyte/macromolecular interactions present in the cell. One candidate mechanism for such a favorable shift in binding equilibrium is the proven ability of Hsp70 to bind near-native states of substrate proteins in vitro. We show evidence for early onset of binding in-cell. Our results suggest that Hsp70 binds PGK preemptively, prior to its full unfolding transition, thus stabilizing it against further unfolding. We propose a "preemptive holdase" mechanism for Hsp70-substrate binding. Given our result for PGK, more proteins than one might think based on in vitro assays may be chaperoned by Hsp70 in vivo. The cellular environment thus plays an important role in maintaining proper Hsp70 function.

Weak Chemical Interactions That Drive Protein Evolution: Crowding, Sticking, and Quinary Structure in Folding and Function

In recent years, better instrumentation and greater computing power have enabled the imaging of elusive biomolecule dynamics in cells, driving many advances in understanding the chemical organization of biological systems. The focus of this Review is on interactions in the cell that affect both biomolecular stability and function and modulate them. The same protein or nucleic acid can behave differently depending on the time in the cell cycle, the location in a specific compartment, or the stresses acting on the cell. We describe in detail the crowding, sticking, and quinary structure in the cell and the current methods to quantify them both in vitro and in vivo. Finally, we discuss protein evolution in the cell in light of current biophysical evidence. We describe the factors that drive protein evolution and shape protein interaction networks. These interactions can significantly affect the free energy, ΔG, of marginally stable and low-population proteins and, due to epistasis, direct the evolutionary pathways in an organism. We finally conclude by providing an outlook on experiments to come and the possibility of collaborative evolutionary biology and biophysical efforts.

Assessment of GFP Tag Position on Protein Localization and Growth Fitness in Yeast

While protein tags are ubiquitously utilized in molecular biology, they harbor the potential to interfere with functional traits of their fusion counterparts. Systematic evaluation of the effect of protein tags on function would promote accurate use of tags in experimental setups. Here we examine the effect of green fluorescent protein tagging at either the N or C terminus of budding yeast proteins on subcellular localization and functionality. We use a competition-based approach to decipher the relative fitness of two strains tagged on the same protein but on opposite termini and from that infer the correct, physiological localization for each protein and the optimal position for tagging. Our study provides a first of a kind systematic assessment of the effect of tags on the functionality of proteins and provides a step toward broad investigation of protein fusion libraries.

Labeling for Quantitative Comparison of Imaging Measurements in Vitro and in Cells

Qualitative imaging of biomolecular localization and distribution inside cells has revolutionized cell biology. Most of these powerful techniques require modifications to the target biomolecule. Over the past 10 years, these techniques have been extended to quantitative measurements, from in-cell protein folding rates to complex dissociation constants to RNA lifetimes. Such measurements can be affected even when a target molecule is just mildly perturbed by its labels. Here, the impact of labeling on protein (and RNA) structure, stability, and function in cells is discussed via practical examples from the recent literature. General guidelines for selecting and validating modification sites are provided to bring the best from cell biology and imaging to quantitative biophysical experiments inside cells.

Protein Assembly and Building Blocks: Beyond the Limits of the LEGO Brick Metaphor

Proteins, like other biomolecules, have a modular and hierarchical structure. Various building blocks are used to construct proteins of high structural complexity and diverse functionality. In multidomain proteins, for example, domains are fused to each other in different combinations to achieve different functions. Although the LEGO brick metaphor is justified as a means of simplifying the complexity of three-dimensional protein structures, several fundamental properties (such as allostery or the induced-fit mechanism) make deviation from it necessary to respect the plasticity, softness, and cross-talk that are essential to protein function. In this work, we illustrate recently reported protein behavior in multidomain proteins that deviates from the LEGO brick analogy. While earlier studies showed that a protein domain is often unaffected by being fused to another domain or becomes more stable following the formation of a new interface between the tethered domains, destabilization due to tethering has been reported for several systems. We illustrate that tethering may sometimes result in a multidomain protein behaving as "less than the sum of its parts". We survey these cases for which structure additivity does not guarantee thermodynamic additivity. Protein destabilization due to fusion to other domains may be linked in some cases to biological function and should be taken into account when designing large assemblies.

Design and Properties of Genetically Encoded Probes for Sensing Macromolecular Crowding

Cells are highly crowded with proteins and polynucleotides. Any reaction that depends on the available volume can be affected by macromolecular crowding, but the effects of crowding in cells are complex and difficult to track. Here, we present a set of Förster resonance energy transfer (FRET)-based crowding-sensitive probes and investigate the role of the linker design. We investigate the sensors in vitro and in vivo and by molecular dynamics simulations. We find that in vitro all the probes can be compressed by crowding, with a magnitude that increases with the probe size, the crowder concentration, and the crowder size. We capture the role of the linker in a heuristic scaling model, and we find that compression is a function of size of the probe and volume fraction of the crowder. The FRET changes observed in Escherichia coli are more complicated, where FRET-increases and scaling behavior are observed solely with probes that contain the helices in the linker. The probe with the highest sensitivity to crowding in vivo yields the same macromolecular volume fractions as previously obtained from cell dry weight. The collection of new probes provides more detailed readouts on the macromolecular crowding than a single sensor.

Direct Imaging of Protein Stability and Folding Kinetics in Hydrogels

We apply fast relaxation imaging (FReI) as a novel technique for investigating the folding stability and dynamics of proteins within polyacrylamide hydrogels, which have diverse and widespread uses in biotechnology. FReI detects protein unfolding in situ by imaging changes in fluorescence resonance energy transfer (FRET) after temperature jump perturbations. Unlike bulk measurements, diffraction-limited epifluorescence imaging combined with fast temperature perturbations reveals the impact of local environment effects on protein-biomaterial compatibility. Our experiments investigated a crowding sensor protein (CrH2) and phosphoglycerate kinase (PGK), which undergoes cooperative unfolding. The crowding sensor quantifies the confinement effect of the cross-linked hydrogel: the 4% polyacrylamide hydrogel is similar to aqueous solution (no confinement), while the 10% hydrogel is strongly confining. FRAP measurements and protein concentration gradients in the 4% and 10% hydrogels further support this observation. PGK reveals that noncovalent interactions of the protein with the polymer surface are more important than confinement for determining protein properties in the gel: the mere presence of hydrogel increases protein stability, speeds up folding relaxation, and promotes irreversible binding to the polymer even at the solution-gel interface, whereas the difference between the 4% and the 10% hydrogels is negligible despite their large difference in confinement. The imaging capabilities of FReI, demonstrated to be diffraction limited, further revealed spatially homogeneous protein unfolding across the hydrogels at 500 nm length scales and revealed differences in protein properties at the gel-solution boundary.

Macromolecular Crowding In Vitro, In Vivo, and In Between

Biochemical processes take place in heterogeneous and highly volume-occupied or crowded environments that can considerably influence the reactivity and distribution of participating macromolecules. We summarize here the thermodynamic consequences of excluded-volume and long-range nonspecific intermolecular interactions for macromolecular reactions in volume-occupied media. In addition, we summarize and compare the information content of studies of crowding in vitro and in vivo. We emphasize the importance of characterizing the behavior not only of labeled tracer macromolecules but also the composition and behavior of unlabeled macromolecules in the immediate vicinity of the tracer. Finally, we propose strategies for extending quantitative analyses of crowding in simple model systems to increasingly complex media up to and including intact cells.

ReAsH as a Quantitative Probe of In-Cell Protein Dynamics

The tetracysteine (tc) tag/biarsenical dye system (FlAsH or ReAsH) promises to combine the flexibility of fluorescent protein tags with the small size of dye labels, allowing in-cell study of target proteins that are perturbed by large protein tags. Quantitative thermodynamic and kinetic studies in-cell using FlAsH and ReAsH have been hampered by methodological complexities presented by the fluorescence properties of the tag-dye complex probed by either Förster resonance energy transfer (FRET) or direct excitation. We label the model protein phosphoglycerate kinase (PGK) with AcGFP1 and ReAsH for direct comparison with AcGFP1/mCherry-labeled PGK. We find that fast relaxation imaging (FReI), combining millisecond temperature jump kinetics with fluorescence microscopy detection, circumvents many of the difficulties encountered working with the ReAsH system, allowing us to obtain quantitative FRET measurements of protein stability and kinetics both in vitro and in cells. We also demonstrate the to us surprising result that fluorescence from directly excited, unburied ReAsH at the C-terminus of the model protein also reports on folding in vitro and in cells. Comparing the ReAsH-labeled protein to a construct labeled with two fluorescent protein tags allows us to evaluate how a bulkier protein tag affects protein dynamics in cells and in vitro. We find that the average folding rate in the cell is closer to the in vitro rate with the smaller tag, highlighting the effect of tags on quantitative in-cell measurements.