Tuning the electronic absorption of protein-embedded all-trans-retinal

Dual-color optical activation and suppression of neurons with high temporal precision

A well-known phenomenon in the optogenetic toolbox is that all light-gated ion channels, including red-shifted channelrhodopsins (ChRs), are activated by blue light, whereas blue-shifted ChRs are minimally responsive to longer wavelengths. Here, we took advantage of this feature to create a system which allows high-frequency activation of neurons with pulses of red light, while permitting the suppression of action potentials (APs) with millisecond precision by blue light. We achieved this by pairing an ultrafast red-shifted ChR with a blue light-sensitive anion channel of appropriately matching kinetics. This required screening several anion-selective ChRs, followed by a model-based mutagenesis strategy to optimize their kinetics and light spectra. Slice electrophysiology in the hippocampus as well as behavioral inspection of vibrissa movement demonstrate a minimal excitation from blue light. Of significant potential value, in contrast to existing tools, the system we introduce here allows high-frequency optogenetic excitation of neurons with red light, while blue light suppression of APs is confined within the duration of the light pulse.

Engineering of soluble bacteriorhodopsin

Studies and applications of membrane proteins remain challenging due to the requirement of maintaining them in a lipid membrane or a membrane mimic. Modern machine learning-based protein engineering methods offer a possibility of generating soluble analogs of membrane proteins that retain the active site structure and ligand-binding properties; however, clear examples are currently missing. Here, we report successful engineering of proteins dubbed NeuroBRs that mimic the active site (retinal-binding pocket) of bacteriorhodopsin, a light-driven proton pump and well-studied model membrane protein. NeuroBRs are soluble and stable, bind retinal and exhibit photocycles under illumination. The crystallographic structure of NeuroBR_A, determined at anisotropic resolution reaching 1.76 Å, reveals an excellently conserved chromophore binding pocket and tertiary structure. Thus, NeuroBRs are promising microbial rhodopsin mimics for studying retinal photochemistry and potential soluble effector modules for optogenetic tools. Overall, our results highlight the power of modern protein engineering approaches and pave the way towards wider development of molecular tools derived from membrane proteins.

Fluorescent Artificial Receptor for Dopamine based on Molecular Recognition-driven Dynamic Covalent Chemistry in a Lipid Nanoreactor

Molecular recognition and detection of small bioactive molecules, like neurotransmitters, remain a challenge for chemists, whereas nature found an elegant solution in the form of protein receptors. Here, we introduce a concept of a dynamic artificial receptor that synergically combines molecular recognition with dynamic imine bond formation inside a lipid nanoreactor, inducing a fluorescence response. The designed supramolecular system combines a lipophilic recognition ligand derived from a boronic acid, a fluorescent aldehyde based on push-pull styryl pyridine and a phenol-based catalyst. The recognition ligand specifically captures dopamine inside lipid nanodroplets and thus triggers imine bond formation with the aldehyde, producing the emission color change. The rational design of the fluorescent aldehyde, the catalyst, and the recognition ligand allows dramatic acceleration of the imine bond formation required for rapid sensing of dopamine. The nanoprobe enables dopamine detection with micromolar sensitivity and singe-nanoprobe imaging of dopamine gradients through its robust two-color ratiometric response. It displays remarkable selectivity without the interference of competing biogenic primary amines and biological media: blood serum, plasma, urine and cell lysate. The proposed concept of a dynamic artificial receptor offers a solution to the long-standing problem of molecular recognition and sensing of small molecules in complex biological media.

Regulation of Absorption and Emission in a Protein/Fluorophore Complex

Human cellular retinol binding protein II (hCRBPII) was used as a protein engineering platform to rationally regulate absorptive and emissive properties of a covalently bound fluorogenic dye. We demonstrate the binding of a thio-dapoxyl analog via formation of a protonated imine between an active site lysine residue and the chromophore's aldehyde. Rational manipulation of the electrostatics of the binding pocket results in a 204 nm shift in absorption and a 131 nm shift in emission. The protein is readily expressed in mammalian systems and binds with exogenously delivered fluorophore as demonstrated by live-cell imaging experiments.

Excited-State Dynamics of a CRABPII-Based Microbial Rhodopsin Mimic

Microbial rhodopsin, a pivotal photoreceptor protein, has garnered widespread application in diverse fields such as optogenetics, biotechnology, biodevices, etc. However, current microbial rhodopsins are all transmembrane proteins, which both complicates the investigation on the photoreaction mechanism and limits their further applications. Therefore, a specific mimic for microbial rhodopsin can not only provide a better model for understanding the mechanism but also can extend the applications. The human protein CRABPII turns out to be a good template for design mimics on rhodopsin due to the convenience in synthesis and the stability after mutations. Recently, Geiger et al. designed a new CRABPII-based mimic M1-L121E on microbial rhodopsin with the 13-cis, syn (13C) isomerization after irradiation. However, it still remains a question as to how similar it is compared with the natural microbial rhodopsin, in particular, in the aspect of the photoreaction dynamics. In this article, we investigate the excited-state dynamics of this mimic by measuring its transient absorption spectra. Our results reveal that there are two components in the solution of mimic M1-L121E at pH 8, known as protonated Schiff base (PSB) and unprotonated Schiff base (USB) states. In both states, the photoreaction process from 13-cis, syn(13C) to all-trans,anti (AT) is faster than that from the inverse direction. In addition, the photoreaction process in the PSB state is faster than that in the USB state. We compared the isomerization time of the PSB state to that of microbial rhodopsin. Our findings indicate that M1-L121E exhibits behaviors similar to those of microbial rhodopsins in the general pattern of PSB isomerization, where the isomerization from 13C to AT is much faster than its inverse direction. However, our results also reveal significant differences in the excited-state dynamics of the mimic relative to the native microbial rhodopsin, including the slower PSB isomerization rates as well as the unusual USB photoreaction dynamics at pH = 8. By elucidating the distinctive characteristics of mimics M1-L121E, this study enhances our understanding of microbial rhodopsin mimics and their potential applications.

Color Tuning in Bovine Rhodopsin through Polarizable Embedding

Bovine rhodopsin is among the most studied proteins in the rhodopsin family. Its primary activation mechanism is the photoisomerization of 11-cis retinal, triggered by the absorption of a UV-visible photon. Different mutants of the same rhodopsin show different absorption wavelengths due to the influence of the specific amino acid residues forming the cavity in which the retinal chromophore is embedded, and rhodopsins activated at different wavelengths are, for example, exploited in the field of optogenetics. In this letter, we present a procedure for systematically investigating color tuning in models of bovine rhodopsin and a set of its mutants embedded in a membrane bilayer. Vertical excitation energy calculations were carried out with the polarizable embedding potential for describing the environment surrounding the chromophore. We show that polarizable embedding outperformed regular electrostatic embedding in determining both the vertical excitation energies and associated oscillator strengths of the systems studied.

Chromophore-Protein Interactions Affecting the Polyene Twist and π-π* Energy Gap of the Retinal Chromophore in Schizorhodopsins

The properties of a prosthetic group are broadened by interactions with its neighboring residues in proteins. The retinal chromophore in rhodopsins absorbs light, undergoes structural changes, and drives functionally important structural changes in proteins during the photocycle. It is therefore crucial to understand how chromophore-protein interactions regulate the molecular structure and electronic state of chromophores in rhodopsins. Schizorhodopsin is a newly discovered subfamily of rhodopsins found in the genomes of Asgard archaea, which are extant prokaryotes closest to the last common ancestor of eukaryotes and of other microbial species. Here, we report the effects of a hydrogen bond between a retinal Schiff base and its counterion on the twist of the polyene chain and the color of the retinal chromophore. Correlations between spectral features revealed the unexpected fact that the twist of the polyene chain is reduced as the hydrogen bond becomes stronger, suggesting that the twist is caused by tight atomic contacts between the chromophore and nearby residues. In addition, the strength of the hydrogen bond is the primary factor affecting the color-tuning of the retinal chromophore in schizorhodopsins. The findings of this study are valuable for manipulating the molecular structure and electronic state of the chromophore by controlling chromophore-protein interactions.

Automatic Rhodopsin Modeling with Multiple Protonation Microstates

Automatic Rhodopsin Modeling (ARM) is a simulation protocol providing QM/MM models of rhodopsins capable of reproducing experimental electronic absorption and emission trends. Currently, ARM is restricted to a single protonation microstate for each rhodopsin model. Herein, we incorporate an extension of the minimal electrostatic model (MEM) into the ARM protocol to account for all relevant protonation microstates at a given pH. The new ARM+MEM protocol determines the most important microstates contributing to the description of the absorption spectrum. As a test case, we have applied this methodology to simulate the pH-dependent absorption spectrum of a toy model, showing that the single-microstate picture breaks down at certain pH values. Subsequently, we applied ARM+MEM toAnabaenasensory rhodopsin, confirming an improved description of its absorption spectrum when the titration of several key residues is considered.

MicroLED/LED electro-optical integration techniques for non-display applications

MicroLEDs offer an extraordinary combination of high luminance, high energy efficiency, low cost, and long lifetime. These characteristics are highly desirable in various applications, but their usage has, to date, been primarily focused toward next-generation display technologies. Applications of microLEDs in other technologies, such as projector systems, computational imaging, communication systems, or neural stimulation, have been limited. In non-display applications which use microLEDs as light sources, modifications in key electrical and optical characteristics such as external efficiency, output beam shape, modulation bandwidth, light output power, and emission wavelengths are often needed for optimum performance. A number of advanced fabrication and processing techniques have been used to achieve these electro-optical characteristics in microLEDs. In this article, we review the non-display application areas of the microLEDs, the distinct opto-electrical characteristics required for these applications, and techniques that integrate the optical and electrical components on the microLEDs to improve system-level efficacy and performance.

Induction effects on the absorption maxima of photoreceptor proteins

Multiscale simulations have been established as a powerful tool to calculate and predict excitation energies in complex systems such as photoreceptor proteins. In these simulations the chromophore is typically treated using quantum mechanical (QM) methods while the protein and surrounding environment are described by a classical molecular mechanics (MM) force field. The electrostatic interactions between these regions are often treated using electrostatic embedding where the point charges in the MM region polarize the QM region. A more sophisticated treatment accounts also for the polarization of the MM region. In this work, the effect of such a polarizable embedding on excitation energies was benchmarked and compared to electrostatic embedding. This was done for two different proteins, the lipid membrane-embedded jumping spider rhodopsin and the soluble cyanobacteriochrome Slr1393g3. It was found that the polarizable embedding scheme produces absorption maxima closer to experimental values. The polarizable embedding scheme was also benchmarked against expanded QM regions and found to be in qualitative agreement. Treating individual residues as polarizable recovered between 50% and 71% of the QM improvement in the excitation energies, depending on the system. A detailed analysis of each amino acid residue in the chromophore binding pocket revealed that aromatic residues result in the largest change in excitation energy compared to the electrostatic embedding. Furthermore, the computational efficiency of polarizable embedding allowed it to go beyond the binding pocket and describe a larger portion of the environment, further improving the results.

Light controlled reversible Michael addition of cysteine: a new tool for dynamic site-specific labeling of proteins

Cysteine-based Michael addition is a widely employed strategy for covalent conjugation of proteins, peptides, and drugs. The covalent reaction is irreversible in most cases, leading to a lack of control over the process. Utilizing spectroscopic analyses along with X-ray crystallographic studies, we demonstrate Michael addition of an engineered cysteine residue in human Cellular Retinol Binding Protein II (hCRBPII) with a coumarin analog that creates a non-fluorescent complex. UV-illumination reverses the conjugation, yielding a fluorescent species, presumably through a retro-Michael process. This series of events can be repeated between a bound and non-bound form of the cysteine reversibly, resulting in the ON-OFF control of fluorescence. The details of the mechanism of photoswitching was illuminated by recapitulation of the process in light irradiated single crystals, confirming the mechanism at atomic resolution.

Fine spectral tuning of a flavin-binding fluorescent protein for multicolor imaging

Flavin-binding fluorescent proteins are promising genetically encoded tags for microscopy. However, spectral properties of their chromophores (riboflavin, flavin mononucleotide, and flavin adenine dinucleotide) are notoriously similar even between different protein families, which limits applications of flavoproteins in multicolor imaging. Here, we present a palette of 22 finely tuned fluorescent tags based on the thermostable LOV domain from Chloroflexus aggregans. We performed site saturation mutagenesis of three amino acid positions in the flavin-binding pocket, including the photoactive cysteine, to obtain variants with fluorescence emission maxima uniformly covering the wavelength range from 486 to 512 nm. We demonstrate three-color imaging based on spectral separation and two-color fluorescence lifetime imaging of bacteria, as well as two-color imaging of mammalian cells (HEK293T), using the proteins from the palette. These results highlight the possibility of fine spectral tuning of flavoproteins and pave the way for further applications of flavin-binding fluorescent proteins in fluorescence microscopy.

Excited-state dynamics of all-trans protonated retinal Schiff base in CRABPII-based rhodopsin mimics

The rhodopsin mimic is a chemically synthetized complex with retinyl Schiff base (RSB) formed between protein and the retinal chromophore that can mimic the natural rhodopsin-like protein. The artificial rhodopsin mimic is more stable and designable than the natural protein and hence has wider uses in photon detection devices. The mimic structure RSB, like the case in the actual rhodopsin-like protein, undergoes isomerization and protonation throughout the photoreaction process. As a result, understanding the dynamics of the RSB in the photoreaction process is critical. In this study, the ultrafast transient absorption spectra of three mutants of the cellular retinoic acid-binding protein II-based rhodopsin mimic at acidic environment were recorded, from which the related excited-state dynamics of the all-trans protonated RSB (AT-PRSB) were investigated. The transient fluorescence spectra measurements are used to validate some of the dynamic features. We find that the excited-state dynamics of AT-PRSB in three mutants share a similar pattern that differs significantly from the dynamics of 15-cis PRSB of the rhodopsin mimic in neutral solution. By comparing the dynamics across the three mutants, we discovered that the aromatic residues near the β-ionone ring structure of the retinal may help stabilize the AT-PRSB and hence slow down its isomerization rate. The experimental results provide implications on designing a rhodopsin-like protein with significant infrared fluorescence, which can be particularly useful in the applications in biosensing or bioimaging in deeper tissues.

Microsolvation Effects in the Spectral Tuning of Heliorhodopsin

Heliorhodopsins (HeR) are a new category of heptahelical transmembrane photoactive proteins with a covalently linked all-trans retinal. The protonated Schiff base (PSB) nitrogen in the retinal is stabilized by a negatively charged counterion. It is well-known that stronger or weaker electrostatic interactions with the counterion cause a significant spectral blue- or red-shift, respectively, in both microbial and animal rhodopsins. In HeR, however, while Glu107 acts as the counterion, mutations of this residue are not directly correlated with a spectral shift. A molecular dynamics analysis revealed that a water cluster pocket produces a microsolvation effect on the Schiff base, compensating to various extents the replacement of the native counterion. Using a combination of molecular dynamics and quantum mechanical/molecular mechanics (QM/MM), we study this microsolvation effect on the electronic absorption of the retinylidene Schiff base chromophore of HeR.

Far-Red Absorbing Rhodopsins, Insights From Heterodimeric Rhodopsin-Cyclases

The recently discovered Rhodopsin-cyclases from Chytridiomycota fungi show completely unexpected properties for microbial rhodopsins. These photoreceptors function exclusively as heterodimers, with the two subunits that have very different retinal chromophores. Among them is the bimodal photoswitchable Neorhodopsin (NeoR), which exhibits a near-infrared absorbing, highly fluorescent state. These are features that have never been described for any retinal photoreceptor. Here these properties are discussed in the context of color-tuning approaches of retinal chromophores, which have been extensively studied since the discovery of the first microbial rhodopsin, bacteriorhodopsin, in 1971 (Oesterhelt et al., Nature New Biology, 1971, 233 (39), 149-152). Further a brief review about the concept of heterodimerization is given, which is widely present in class III cyclases but is unknown for rhodopsins. NIR-sensitive retinal chromophores have greatly expanded our understanding of the spectral range of natural retinal photoreceptors and provide a novel perspective for the development of optogenetic tools.

Towards translational optogenetics

Optogenetics is widely used to interrogate the neural circuits underlying disease and has most recently been harnessed for therapeutic applications. The optogenetic toolkit consists of light-responsive proteins that modulate specific cellular functions, vectors for the delivery of the transgenes that encode the light-responsive proteins to targeted cellular populations, and devices for the delivery of light of suitable wavelengths at effective fluence rates. A refined toolkit with a focus towards translational uses would include efficient and safer viral and non-viral gene-delivery vectors, increasingly red-shifted photoresponsive proteins, nanomaterials that efficiently transduce near-infrared light deep into tissue, and wireless implantable light-delivery devices that allow for spatiotemporally precise interventions at clinically relevant tissue depths. In this Review, we examine the current optogenetics toolkit and the most notable preclinical and translational uses of optogenetics, and discuss future methodological and translational developments and bottlenecks.

Control of Protonated Schiff Base Excited State Decay within Visual Protein Mimics: A Unified Model for Retinal Chromophores

Artificial biomimetic chromophore-protein complexes inspired by natural visual pigments can feature color tunability across the full visible spectrum. However, control of excited state dynamics of the retinal chromophore, which is of paramount importance for technological applications, is lacking due to its complex and subtle photophysics/photochemistry. Here, ultrafast transient absorption spectroscopy and quantum mechanics/molecular mechanics simulations are combined for the study of highly tunable rhodopsin mimics, as compared to retinal chromophores in solution. Conical intersections and transient fluorescent intermediates are identified with atomistic resolution, providing unambiguous assignment of their ultrafast excited state absorption features. The results point out that the electrostatic environment of the chromophore, modified by protein point mutations, affects its excited state properties allowing control of its photophysics with same power of chemical modifications of the chromophore. The complex nature of such fine control is a fundamental knowledge for the design of bio-mimetic opto-electronic and photonic devices.

Design of Large Stokes Shift Fluorescent Proteins Based on Excited State Proton Transfer of an Engineered Photobase

The incredible potential for fluorescent proteins to revolutionize biology has inspired the development of a variety of design strategies to address an equally broad range of photophysical characteristics, depending on potential applications. Of these, fluorescent proteins that simultaneously exhibit high quantum yield, red-shifted emission, and wide separation between excitation and emission wavelengths (Large Stokes Shift, LSS) are rare. The pursuit of LSS systems has led to the formation of a complex, obtained from the marriage of a rationally engineered protein (human cellular retinol binding protein II, hCRBPII) and different fluorogenic molecules, capable of supporting photobase activity. The large increase in basicity upon photoexcitation leads to protonation of the fluorophore in the excited state, dramatically red-shifting its emission, leading to an LSS protein/fluorophore complex. Essential for selective photobase activity is the intimate involvement of the target protein structure and sequence that enables Excited State Proton Transfer (ESPT). The potential power and usefulness of the strategy was demonstrated in live cell imaging of human cell lines.

Accurate Prediction of Absorption Spectral Shifts of Proteorhodopsin Using a Fragment-Based Quantum Mechanical Method

Many experiments have been carried out to display different colors of Proteorhodopsin (PR) and its mutants, but the mechanism of color tuning of PR was not fully elucidated. In this study, we applied the Electrostatically Embedded Generalized Molecular Fractionation with Conjugate Caps (EE-GMFCC) method to the prediction of excitation energies of PRs. Excitation energies of 10 variants of Blue Proteorhodopsin (BPR-PR105Q) in residue 105GLN were calculated with the EE-GMFCC method at the TD-B3LYP/6-31G* level. The calculated results show good correlation with the experimental values of absorption wavelengths, although the experimental wavelength range among these systems is less than 50 nm. The ensemble-averaged electric fields along the polyene chain of retinal correlated well with EE-GMFCC calculated excitation energies for these 10 PRs, suggesting that electrostatic interactions from nearby residues are responsible for the color tuning. We also utilized the GMFCC method to decompose the excitation energy contribution per residue surrounding the chromophore. Our results show that residues ASP97 and ASP227 have the largest contribution to the absorption spectral shift of PR among the nearby residues of retinal. This work demonstrates that the EE-GMFCC method can be applied to accurately predict the absorption spectral shifts for biomacromolecules.

Color-tuning of natural variants of heliorhodopsin

Microbial rhodopsins are distributed through many microorganisms. Heliorhodopsins are newly discovered but have an unclear function. They have seven transmembrane helices similar to type-I and type-II rhodopsins, but they are different in that the N-terminal region of heliorhodopsin is cytoplasmic. We chose 13 representative heliorhodopsins from various microorganisms, expressed and purified with an N-terminal His tag, and measured the absorption spectra. The 13 natural variants had an absorption maximum (λmax) in the range 530–556 nm similar to proteorhodopsin (λmax = 490–525 nm). We selected several candidate residues that influence rhodopsin color-tuning based on sequence alignment and constructed mutants via site-directed mutagenesis to confirm the spectral changes. We found two important residues located near retinal chromophore that influence λmax. We also predict the 3D structure via homology-modeling of Thermoplasmatales heliorhodopsin. The results indicate that the color-tuning mechanism of type-I rhodopsin can be applied to understand the color-tuning of heliorhodopsin.

Human Cellular Retinol Binding Protein II Forms a Domain-Swapped Trimer Representing a Novel Fold and a New Template for Protein Engineering

Domain-swapping is a mechanism for evolving new protein structure from extant scaffolds, and has been an efficient protein-engineering strategy for tailoring functional diversity. However, domain swapping can only be exploited if it can be controlled, especially in cases where various folds can coexist. Herein, we describe the structure of a domain-swapped trimer of the iLBP family member hCRBPII, and suggest a mechanism for domain-swapped trimerization. It is further shown that domain-swapped trimerization can be favored by strategic installation of a disulfide bond, thus demonstrating a strategy for fold control. We further show the domain-swapped trimer to be a useful protein design template by installing a high-affinity metal binding site through the introduction of a single mutation, taking advantage of its threefold symmetry. Together, these studies show how nature can promote oligomerization, stabilize a specific oligomer, and generate new function with minimal changes to the protein sequence.

NeoR, a near-infrared absorbing rhodopsin

The Rhizoclosmatium globosum genome encodes three rhodopsin-guanylyl cyclases (RGCs), which are predicted to facilitate visual orientation of the fungal zoospores. Here, we show that RGC1 and RGC2 function as light-activated cyclases only upon heterodimerization with RGC3 (NeoR). RGC1/2 utilize conventional green or blue-light-sensitive rhodopsins (λmax = 550 and 480 nm, respectively), with short-lived signaling states, responsible for light-activation of the enzyme. The bistable NeoR is photoswitchable between a near-infrared-sensitive (NIR, λmax = 690 nm) highly fluorescent state (QF = 0.2) and a UV-sensitive non-fluorescent state, thereby modulating the activity by NIR pre-illumination. No other rhodopsin has been reported so far to be functional as a heterooligomer, or as having such a long wavelength absorption or high fluorescence yield. Site-specific mutagenesis and hybrid quantum mechanics/molecular mechanics simulations support the idea that the unusual photochemical properties result from the rigidity of the retinal chromophore and a unique counterion triad composed of two glutamic and one aspartic acids. These findings substantially expand our understanding of the natural potential and limitations of spectral tuning in rhodopsin photoreceptors.

Neuronal octopamine signaling regulates mating-induced germline stem cell increase in female Drosophila melanogaster

Stem cells fuel the development and maintenance of tissues. Many studies have addressed how local signals from neighboring niche cells regulate stem cell identity and their proliferative potential. However, the regulation of stem cells by tissue-extrinsic signals in response to environmental cues remains poorly understood. Here we report that efferent octopaminergic neurons projecting to the ovary are essential for germline stem cell (GSC) increase in response to mating in female Drosophila. The neuronal activity of the octopaminergic neurons is required for mating-induced GSC increase as they relay the mating signal from sex peptide receptor-positive cholinergic neurons. Octopamine and its receptor Oamb are also required for mating-induced GSC increase via intracellular Ca²⁺ signaling. Moreover, we identified Matrix metalloproteinase-2 as a downstream component of the octopamine-Ca²⁺ signaling to induce GSC increase. Our study provides a mechanism describing how neuronal system couples stem cell behavior to environmental cues through stem cell niche signaling.

Illuminating Cellular Biochemistry: Fluorogenic Chemogenetic Biosensors for Biological Imaging

Cellular activity is defined by the precise spatiotemporal regulation of various components, such as ions, small molecules, or proteins. Studying cell physiology consequently requires the optical recording of these processes, notably by using fluorescent biosensors. The recent development of various fluorogenic systems greatly expanded the palette of reporters to be included in these sensors design. Fluorogenic reporters consist of a protein or RNA tag that can complex either an endogenous or a synthetic fluorogenic dye (so-called fluorogen). The intrinsic nature of these tags, along with the high tunability of their cognate chromophore provide interesting features such as far-red to near-infrared emission, oxygen independence, or unprecedented color versatility. These engineered photoreceptors, self-labelling proteins, or noncovalent aptamers and protein tags were rapidly identified as promising reporters to observe biological events. This Minireview focuses on the new perspectives they offer to design unique and innovative biosensors, thus pushing the boundaries of cellular imaging.

Retinol-binding protein 2 (RBP2): biology and pathobiology

Retinol-binding protein 2 (RBP2; originally cellular retinol-binding protein, type II (CRBPII)) is a 16 kDa cytosolic protein that in the adult is localized predominantly to absorptive cells of the proximal small intestine. It is well established that RBP2 plays a central role in facilitating uptake of dietary retinoid, retinoid metabolism in enterocytes, and retinoid actions locally within the intestine. Studies of mice lacking Rbp2 establish that Rbp2 is not required in times of dietary retinoid-sufficiency. However, in times of dietary retinoid-insufficiency, the complete lack of Rbp2 gives rise to perinatal lethality owing to RBP2 absence in both placental (maternal) and neonatal tissues. Moreover, when maintained on a high-fat diet, Rbp2-knockout mice develop obesity, glucose intolerance and a fatty liver. Unexpectedly, recent investigations have demonstrated that RBP2 binds long-chain 2-monoacylglycerols (2-MAGs), including the canonical endocannabinoid 2-arachidonoylglycerol, with very high affinity, equivalent to that of retinol binding. Crystallographic studies establish that 2-MAGs bind to a site within RBP2 that fully overlaps with the retinol binding site. When challenged orally with fat, mucosal levels of 2-MAGs in Rbp2 null mice are significantly greater than those of matched controls establishing that RBP2 is a physiologically relevant MAG-binding protein. The rise in MAG levels is accompanied by elevations in circulating levels of the hormone glucose-dependent insulinotropic polypeptide (GIP). It is not understood how retinoid and/or MAG binding to RBP2 affects the functions of this protein, nor is it presently understood how these contribute to the metabolic and hormonal phenotypes observed for Rbp2-deficient mice.

Engineering of a Red Fluorogenic Protein/Merocyanine Complex for Live-Cell Imaging

A reengineered human cellular retinol binding protein II (hCRBPII), a 15-kDa protein belonging to the intracellular lipid binding protein (iLBP) family, generates a highly fluorescent red pigment through the covalent linkage of a merocyanine aldehyde to an active site lysine residue. The complex exhibits "turn-on" fluorescence, due to a weakly fluorescent aldehyde that "lights up" with subsequent formation of a strongly fluorescent merocyanine dye within the binding pocket of the protein. Cellular penetration of merocyanine is rapid, and fluorophore maturation is nearly instantaneous. The hCRBPII/merocyanine complex displays high quantum yield, low cytotoxicity, specificity in labeling organelles, and compatibility in both cancer cell lines and yeast cells. The hCRBPII/merocyanine tag is brighter than most common red fluorescent proteins.

Selective Colorimetric "Turn-On" Probe for Efficient Engineering of Iminium Biocatalysis

The efficient engineering of iminium biocatalysis has drawn considerable attention, with many applications in pharmaceutical synthesis. Here, we report a tailor-made iminium-activated colorimetric "turn-on" probe, specifically designed as a prescreening tool to facilitate engineering of iminium biocatalysis. Upon complexation of the probe with the catalytic Pro-1 residue of the model enzyme 4-oxalocrotonate tautomerase (4-OT), a brightly colored merocyanine-dye-type structure is formed. 4-OT mutants that formed this brightly colored species upon incubation with the probe proved to have a substantial activity for the iminium-based Michael-type addition of nitromethane to cinnamaldehyde, whereas mutants that showed no staining by the probe exhibited no or very low-level "Michaelase" activity. This system was exploited in a solid-phase prescreening assay termed as activated iminium colony staining (AICS) to enrich libraries for active mutants. AICS prescreening reduced the screening effort up to 20-fold. After two rounds of directed evolution, two artificial Michaelases were identified with up to 39-fold improvement in the activity for the addition of nitromethane to cinnamaldehyde, yielding the target γ-nitroaldehyde product with excellent isolated yield (up to 95%) and enantiopurity (up to >99% ee). The colorimetric activation of the turn-on probe could be extended to the class I aldolase 2-deoxy-d-ribose 5-phosphate aldolase, implicating a broader application of AICS in engineering iminium biocatalysis.

Benchmarking the Performance of Time-Dependent Density Functional Theory Methods on Biochromophores

Quantum chemical calculations are important for elucidating light-capturing mechanisms in photobiological systems. The time-dependent density functional theory (TDDFT) has become a popular methodology because of its balance between accuracy and computational scaling, despite its problems in describing, for example, charge transfer states. As a step toward systematically understanding the performance of TDDFT calculations on biomolecular systems, we study here 17 commonly used density functionals, including seven long-range separated functionals, and compare the obtained results with excitation energies calculated at the approximate second order coupled-cluster theory level (CC2). The benchmarking set includes the first five singlet excited states of 11 chemical analogues of biochromophores from the green fluorescent protein, rhodopsin/bacteriorhodopsin (Rh/bR), and the photoactive yellow protein. We find that commonly used pure density functionals such as BP86, PBE, M11-L, and hybrid functionals with 20-25% of Hartree-Fock (HF) exchange (B3LYP, PBE0) have a tendency to consistently underestimate vertical excitation energies (VEEs) relative to the CC2 values, whereas hybrid density functionals with around 50% HF exchange such as BHLYP, PBE50, and M06-2X and long-range corrected functionals such as CAM-B3LYP, ωPBE, ωPBEh, ωB97X, ωB97XD, BNL, and M11 overestimate the VEEs. We observe that calculations using the CAM-B3LYP and ωPBEh functionals with 65% and 100% long-range HF exchange, respectively, lead to an overestimation of the VEEs by 0.2-0.3 eV for the benchmarking set. To reduce the systematic error, we introduce here two new empirical functionals, CAMh-B3LYP and ωhPBE0, for which we adjusted the long-range HF exchange to 50%. The introduced parameterization reduces the mean signed average (MSA) deviation to 0.07 eV and the root mean square (rms) deviation to 0.17 eV as compared to the CC2 values. In the present study, TDDFT calculations using the aug-def2-TZVP basis sets, the best performing functionals relative to CC2 are ωhPBE0 (rms = 0.17, MSA = 0.06 eV); CAMh-B3LYP (rms = 0.16, MSA = 0.07 eV); and PBE0 (rms = 0.23, MSA = -0.14 eV). For the popular range-separated CAM-B3LYP functional, we obtain an rms value of 0.31 eV and an MSA value of 0.25 eV, which can be compared with the rms and MSA values of 0.37 and -0.31 eV, respectively, as obtained at the B3LYP level.

Engineering the hCRBPII Domain-Swapped Dimer into a New Class of Protein Switches

Protein conformational switches or allosteric proteins play a key role in the regulation of many essential biological pathways. Nonetheless, the implementation of protein conformational switches in protein design applications has proven challenging, with only a few known examples that are not derivatives of naturally occurring allosteric systems. We have discovered that the domain-swapped (DS) dimer of hCRBPII undergoes a large and robust conformational change upon retinal binding, making it a potentially powerful template for the design of protein conformational switches. Atomic resolution structures of the apo- and holo-forms illuminate a simple, mechanical movement involving sterically driven torsion angle flipping of two residues that drive the motion. We further demonstrate that the conformational "readout" can be altered by addition of cross-domain disulfide bonds, also visualized at atomic resolution. Finally, as a proof of principle, we have created an allosteric metal binding site in the DS dimer, where ligand binding results in a reversible 5-fold loss of metal binding affinity. The high resolution structure of the metal-bound variant illustrates a well-formed metal binding site at the interface of the two domains of the DS dimer and confirms the design strategy for allosteric regulation.

Unified Model for Photophysical and Electro-Optical Properties of Green Fluorescent Proteins

Green fluorescent proteins (GFPs) have become indispensable imaging and optogenetic tools. Their absorption and emission properties can be optimized for specific applications. Currently, no unified framework exists to comprehensively describe these photophysical properties, namely the absorption maxima, emission maxima, Stokes shifts, vibronic progressions, extinction coefficients, Stark tuning rates, and spontaneous emission rates, especially one that includes the effects of the protein environment. In this work, we study the correlations among these properties from systematically tuned GFP environmental mutants and chromophore variants. Correlation plots reveal monotonic trends, suggesting that all these properties are governed by one underlying factor dependent on the chromophore's environment. By treating the anionic GFP chromophore as a mixed-valence compound existing as a superposition of two resonance forms, we argue that this underlying factor is defined as the difference in energy between the two forms, or the driving force, which is tuned by the environment. We then introduce a Marcus-Hush model with the bond length alternation vibrational mode, treating the GFP absorption band as an intervalence charge transfer band. This model explains all of the observed strong correlations among photophysical properties; related subtopics are extensively discussed in the Supporting Information. Finally, we demonstrate the model's predictive power by utilizing the additivity of the driving force. The model described here elucidates the role of the protein environment in modulating the photophysical properties of the chromophore, providing insights and limitations for designing new GFPs with desired phenotypes. We argue that this model should also be generally applicable to both biological and nonbiological polymethine dyes.

The origin of the dark S1 state in carotenoids: a comprehensive model

In carotenoids, by analogy to polyenes, the symmetry of the π-electron system is often invoked to explain their peculiar electronic features, in particular the inactivity of the S₀ → S₁ transition in one-photon excitation. In this review, we verify whether the molecular symmetry of carotenoids and symmetry of their π-electron system are supported in experimental and computational studies. We focus on spectroscopic techniques which are sensitive to the electron density distribution, including the X-ray crystallography, electronic absorption, two-photon techniques, circular dichroism, nuclear magnetic resonance, Stark and vibrational spectroscopies, and on this basis we seek for the origin of inactivity of the S₁ state. We come across no experimental and computational evidence for the symmetry effects and the existence of symmetry restrictions on the electronic states of carotenoids. They do not possess an inversion centre and the C_2h symmetry approximation of carotenoid structure is by no means justified. In effect, the application of symmetry rules (and notification) to the electronic states of carotenoids in this symmetry group may lead to a wrong interpretation of experimental data. This conclusion together with the results summarized in the review allows us to advance a consistent model that explains the inactivity of the S₀ → S₁ transition. Within this model, S₁ is never accessible from S₀ due to the negative synergy of (i) the contributions of double excitations of very low probability, which elevate S₁ energy, and (ii) a non-verticality of the S₀ → S₁ transition, due to the breaking of Born-Oppenheimer approximation. Certainly, our simple model requires a further experimental and theoretical verification.

Bile Acid Binding Protein Functionalization Leads to a Fully Synthetic Rhodopsin Mimic

Rhodopsins are photoreceptive proteins using light to drive a plethora of biological functions such as vision, proton and ion pumping, cation and anion channeling, and gene and enzyme regulation. Here we combine organic synthesis, NMR structural studies, and photochemical characterization to show that it is possible to prepare a fully synthetic mimic of rhodopsin photoreceptors. More specifically, we conjugate a bile acid binding protein with a synthetic mimic of the rhodopsin protonated Schiff base chromophore to achieve a covalent complex featuring an unnatural protein host, photoswitch, and photoswitch-protein linkage with a reverse orientation. We show that, in spite of its molecular-level diversity, light irradiation of the prepared mimic fuels a photochromic cycle driven by sequential photochemical and thermal Z/E isomerizations reminiscent of the photocycles of microbial rhodopsins.

Mimicking Microbial Rhodopsin Isomerization in a Single Crystal

Bacteriorhodopsin represents the simplest, and possibly most abundant, phototropic system requiring only a retinal-bound transmembrane protein to convert photons of light to an energy-generating proton gradient. The creation and interrogation of a microbial rhodopsin mimic, based on an orthogonal protein system, would illuminate the design elements required to generate new photoactive proteins with novel function. We describe a microbial rhodopsin mimic, created using a small soluble protein as a template, that specifically photoisomerizes all- trans to 13- cis retinal followed by thermal relaxation to the all- trans isomer, mimicking the bacteriorhodopsin photocycle, in a single crystal. The key element for selective isomerization is a tuned steric interaction between the chromophore and protein, similar to that seen in the microbial rhodopsins. It is further demonstrated that a single mutation converts the system to a protein photoswitch without chromophore photoisomerization or conformational change.

Raman spectroscopy of a near infrared absorbing proteorhodopsin: Similarities to the bacteriorhodopsin O photointermediate

Microbial rhodopsins have become an important tool in the field of optogenetics. However, effective in vivo optogenetics is in many cases severely limited due to the strong absorption and scattering of visible light by biological tissues. Recently, a combination of opsin site-directed mutagenesis and analog retinal substitution has produced variants of proteorhodopsin which absorb maximally in the near-infrared (NIR). In this study, UV-Visible-NIR absorption and resonance Raman spectroscopy were used to study the double mutant, D212N/F234S, of green absorbing proteorhodopsin (GPR) regenerated with MMAR, a retinal analog containing a methylamino modified β-ionone ring. Four distinct subcomponent absorption bands with peak maxima near 560, 620, 710 and 780 nm are detected with the NIR bands dominant at pH <7.3, and the visible bands dominant at pH 9.5. FT-Raman using 1064-nm excitation reveal two strong ethylenic bands at 1482 and 1498 cm-1 corresponding to the NIR subcomponent absorption bands based on an extended linear correlation between λmax and γC = C. This spectrum exhibits two intense bands in the fingerprint and HOOP mode regions that are highly characteristic of the O640 photointermediate from the light-adapted bacteriorhodopsin photocycle. In contrast, 532-nm excitation enhances the 560-nm component, which exhibits bands very similar to light-adapted bacteriorhodopsin and/or the acid-purple form of bacteriorhodopsin. Native GPR and its mutant D97N when regenerated with MMAR also exhibit similar absorption and Raman bands but with weaker contributions from the NIR absorbing components. Based on these results it is proposed that the NIR absorption in GPR-D212N/F234S with MMAR arises from an O-like chromophore, where the Schiff base counterion D97 is protonated and the MMAR adopts an all-trans configuration with a non-planar geometry due to twists in the conjugated polyene segment. This configuration is characterized by extensive charge delocalization, most likely involving nitrogens atoms in the MMAR chromophore.

A Near-Infrared Photoswitchable Protein-Fluorophore Tag for No-Wash Live Cell Imaging

FR-1V, a fluorene-based aldehydic chromophore, binds its target protein as an imine to yield a highly bathochromic pigment, CF-2, a prototypic protein-dye tagging system whose NIR emission can be spatiotemporally switched ON by rapid UV-light activation. This is achieved through photoisomerization of the imine and its subsequent protonation. We demonstrate a no-wash protocol for live cell imaging of subcellular compartments in a variety of mammalian cell lines with minimal fluorescence background.

Relationship between Excited State Lifetime and Isomerization Quantum Yield in Animal Rhodopsins: Beyond the One-Dimensional Landau-Zener Model

We show that the speed of the chromophore photoisomerization of animal rhodopsins is not a relevant control knob for their light sensitivity. This result is at odds with the momentum-driven tunnelling rationale (i.e., assuming a one-dimensional Landau-Zener model for the decay: Zener, C. Non-Adiabatic Crossing of Energy Levels. Proc. R. Soc. London, Ser. A 1932, 137 (833), 696-702) holding that a faster nuclear motion through the conical intersection translates into a higher quantum yield and, thus, light sensitivity. Instead, a model based on the phase-matching of specific excited state vibrational modes should be considered. Using extensive semiclassical hybrid quantum mechanics/molecular mechanics trajectory computations to simulate the photoisomerization of three animal rhodopsin models (visual rhodopsin, squid rhodopsin and human melanopsin), we also demonstrate that phase-matching between three different modes (the reactive carbon and hydrogen twisting coordinates and the bond length alternation mode) is required to achieve high quantum yields. In fact, such "phase-matching" mechanism explains the computational results and provides a tool for the prediction of the photoisomerization outcome in retinal proteins.

A Genetically Encoded Ratiometric pH Probe: Wavelength Regulation-Inspired Design of pH Indicators

Mutants of human cellular retinol-binding protein II (hCRBPII) were engineered to bind a julolidine retinal analogue for the purpose of developing a ratiometric pH sensor. The design relied on the electrostatic influence of a titratable amino acid side chain, which affects the absorption and, thus, the emission of the protein/fluorophore complex. The ratio of emissions obtained at two excitation wavelengths that correspond to the absorption of the two forms of the protein/fluorophore complex, leads to a concentration-independent measure of pH.

Shapes, Dynamics, and Stability of β-Ionone and Its Two Mutants Evidenced by High-Resolution Spectroscopy in the Gas Phase

The conformational landscapes of β-ionone and two mutants (α-ionone and β-damascone) have been analyzed by means of state-of-the-art rotational spectroscopy and quantum-chemical calculations. The experiments performed at high resolution and sensitivity have provided a deep insight into their conformational spaces, assigning more than 8000 transitions corresponding to the rotational structures of 54 different species (3 isomers, 14 conformers, and 40 isotopologues). Methyl internal rotation dynamics were also observed and analyzed. The work proved the great flexibility of β-ionone due to its flatter potential energy surface. This feature confers on β-ionone a wider ability to interconvert between conformers with rather similar energies with respect to its mutants, allowing the retinal ligand to better adapt inside the binding pocket.

The form and function of channelrhodopsin

Channelrhodopsins are light-gated ion channels that, via regulation of flagellar function, enable single-celled motile algae to seek ambient light conditions suitable for photosynthesis and survival. These plant behavioral responses were initially investigated more than 150 years ago. Recently, major principles of function for light-gated ion channels have been elucidated by creating channelrhodopsins with kinetics that are accelerated or slowed over orders of magnitude, by discovering and designing channelrhodopsins with altered spectral properties, by solving the high-resolution channelrhodopsin crystal structure, and by structural model-guided redesign of channelrhodopsins for altered ion selectivity. Each of these discoveries not only revealed basic principles governing the operation of light-gated ion channels, but also enabled the creation of new proteins for illuminating, via optogenetics, the fundamentals of brain function.