Dynamic switching mechanisms in LOV1 and LOV2 domains of plant phototropins

Structural basis of respiratory complex adaptation to cold temperatures

In response to cold, mammals activate brown fat for respiratory-dependent thermogenesis reliant on the electron transport chain. Yet, the structural basis of respiratory complex adaptation upon cold exposure remains elusive. Herein, we combined thermoregulatory physiology and cryoelectron microscopy (cryo-EM) to study endogenous respiratory supercomplexes from mice exposed to different temperatures. A cold-induced conformation of CI:III2 (termed type 2) supercomplex was identified with a ∼25° rotation of CIII2 around its inter-dimer axis, shortening inter-complex Q exchange space, and exhibiting catalytic states that favor electron transfer. Large-scale supercomplex simulations in mitochondrial membranes reveal how lipid-protein arrangements stabilize type 2 complexes to enhance catalytic activity. Together, our cryo-EM studies, multiscale simulations, and biochemical analyses unveil the thermoregulatory mechanisms and dynamics of increased respiratory capacity in brown fat at the structural and energetic level.

Unusual photodynamic characteristics of the light-oxygen-voltage domain of phototropin linked to terrestrial adaptation of Klebsormidium nitens

Phototropin (Phot), a blue light-sensing LOV domain protein, mediates blue light responses and is evolutionarily conserved across the green lineage. Klebsormidium nitens, a green terrestrial alga, presents a valuable opportunity to study adaptive responses from aquatic to land habitat transitions. We determined the crystal structure of Klebsormidium nitens Phot LOV1 domain (KnLOV1) in the dark and engineered different mutations (R60K, Q122N, and D33N) to modulate the lifetime of the photorecovery cycle. We observed unusual, slow recovery kinetics in the wild-type KnLOV1 domain (τ = 41 ± 3 min) compared to different mutants (R60K: τ = 2.0 ± 0.1 min, Q122N: τ = 1.7 ± 0.1 min, D33N: τ = 9.6 ± 0.1 min). Crystal structures of wild-type KnLOV1 and mutants revealed subtle but critical changes near the protein chromophore that is responsible for modulating protein dark recovery time. Our findings shed light on the unique structural and biochemical characteristics of the newly studied KnLOV1 and its evolutionary importance for phototropin-mediated physiology.

Structural dynamics of protein-protein association involved in the light-induced transition of Avena sativa LOV2 protein

The Light-oxygen-voltage-sensing domain (LOV) superfamily, found in enzymes and signal transduction proteins, plays a crucial role in converting light signals into structural signals, mediating various biological mechanisms. While time-resolved spectroscopic studies have revealed the dynamics of the LOV-domain chromophore's electronic structures, understanding the structural changes in the protein moiety, particularly regarding light-induced dimerization, remains challenging. Here, we utilize time-resolved X-ray liquidography to capture the light-induced dimerization of Avena sativa LOV2. Our analysis unveils that dimerization occurs within milliseconds after the unfolding of the A'α and Jα helices in the microsecond time range. Notably, our findings suggest that protein-protein interactions (PPIs) among the β-scaffolds, mediated by helix unfolding, play a key role in dimerization. In this work, we offer structural insights into the dimerization of LOV2 proteins following structural changes in the A'α and Jα helices, as well as mechanistic insights into the protein-protein association process driven by PPIs.

Structural basis of respiratory complexes adaptation to cold temperatures

In response to cold, mammals activate brown fat for respiratory-dependent thermogenesis reliant on the electron transport chain (1, 2). Yet, the structural basis of respiratory complex adaptation to cold remains elusive. Herein we combined thermoregulatory physiology and cryo-EM to study endogenous respiratory supercomplexes exposed to different temperatures. A cold-induced conformation of CI:III 2 (termed type 2) was identified with a ∼25° rotation of CIII 2 around its inter-dimer axis, shortening inter-complex Q exchange space, and exhibiting different catalytic states which favor electron transfer. Large-scale supercomplex simulations in lipid membrane reveal how unique lipid-protein arrangements stabilize type 2 complexes to enhance catalytic activity. Together, our cryo-EM studies, multiscale simulations and biochemical analyses unveil the mechanisms and dynamics of respiratory adaptation at the structural and energetic level.

Disruption of the FMN-A524 interaction cascade and Glu513-induced collapse of the hydrophobic barrier promotes light-induced Jα-helix unfolding in AsLOV2

The C-terminal Jα-helix of the Avena sativa's Light Oxygen and Voltage (AsLOV2) protein, unfolds on exposure to blue light. This characteristic seeks relevance in applications related to engineering novel biological photoswitches. Using molecular dynamics simulations and the Markov state modeling (MSM) approach we provide the mechanism that explains the stepwise unfolding of the Jα-helix. The unfolding was resolved into seven steps represented by the structurally distinguishable states distributed over the initiation and the post initiation phases. Whereas, the initiation phase occurs due to the collapse of the interaction cascade FMN-Q513-N492-L480-W491-Q479-V520-A524, the onset of the post initiation phase is marked by breaking of the hydrophobic interactions between the Jα-helix and the Iβ-strand. This study indicates that the displacement of N492 out of the FMN binding pocket, not necessarily requiring Q513, is essential for the initiation of the Jα-helix unfolding. Rather, the structural reorientation of Q513 activates the protein to cross the hydrophobic barrier and enter the post initiation phase. Similarly, the structural deviations in N482, rather than its integral role in unfolding, could enhance the unfolding rates. Furthermore, the MSM studies on the wild-type and the Q513 mutant, provide the spatiotemporal roadmap that lay out the possible pathways of structural transition between the dark and the light states of the protein. Overall, the study provides insights useful to enhance the performance of AsLOV2-based photoswitches.

Network analysis of chromophore binding site in LOV domain

Photoreceptor proteins are versatile toolbox for developing biosensors for optogenetic applications. These molecular tools get activated upon illumination of blue light, which in turn offers a non-invasive method for gaining high spatiotemporal resolution and precise control of cellular signal transduction. The Light-Oxygen-Voltage (LOV) domain family of proteins is a well-recognized system for constructing optogenetic devices. Translation of these proteins into efficient cellular sensors is possible by tuning their photochemistry lifetime. However, the bottleneck is the need for more understanding of the relationship between the protein environment and photocycle kinetics. Significantly, the effect of the local environment also modulates the electronic structure of chromophore, which perturbs the electrostatic and hydrophobic interaction within the binding site. This work highlights the critical factors hidden in the protein networks, linking with their experimental photocycle kinetics. It presents an opportunity to quantitatively examine the alternation in chromophore's equilibrium geometry and identify details which have substantial implications in designing synthetic LOV constructs with desirable photocycle efficiency.

Engineering of optogenetic devices for biomedical applications in mammalian synthetic biology

Gene- and cell-based therapies are the next frontiers in the field of medicine. Both are transformative and innovative therapies; however, a lack of safety data limits the translation of such promising technologies to the clinic. Improving the safety and promoting the clinical translation of these therapies can be achieved by tightly regulating the release and delivery of therapeutic outputs. In recent years, the rapid development of optogenetic technology has provided opportunities to develop precision-controlled gene- and cell-based therapies, in which light is introduced to precisely and spatiotemporally manipulate the behaviour of genes and cells. This review focuses on the development of optogenetic tools and their applications in biomedicine, including photoactivated genome engineering and phototherapy for diabetes and tumours. The prospects and challenges of optogenetic tools for future clinical applications are also discussed.

Signal transduction in light-oxygen-voltage receptors lacking the active-site glutamine

In nature as in biotechnology, light-oxygen-voltage photoreceptors perceive blue light to elicit spatiotemporally defined cellular responses. Photon absorption drives thioadduct formation between a conserved cysteine and the flavin chromophore. An equally conserved, proximal glutamine processes the resultant flavin protonation into downstream hydrogen-bond rearrangements. Here, we report that this glutamine, long deemed essential, is generally dispensable. In its absence, several light-oxygen-voltage receptors invariably retained productive, if often attenuated, signaling responses. Structures of a light-oxygen-voltage paradigm at around 1 Å resolution revealed highly similar light-induced conformational changes, irrespective of whether the glutamine is present. Naturally occurring, glutamine-deficient light-oxygen-voltage receptors likely serve as bona fide photoreceptors, as we showcase for a diguanylate cyclase. We propose that without the glutamine, water molecules transiently approach the chromophore and thus propagate flavin protonation downstream. Signaling without glutamine appears intrinsic to light-oxygen-voltage receptors, which pertains to biotechnological applications and suggests evolutionary descendance from redox-active flavoproteins.

Light-oxygen-voltage (LOV) photoreceptors perceive blue light to elicit spatio-temporally defined cellular responses, and their signalling process has been extensively characterized. Here the authors report that the light signal is still transduced in the absence of a conserved Gln residue, thought to be key.

Toward Multiplexed Optogenetic Circuits

Owing to its ubiquity and easy availability in nature, light has been widely employed to control complex cellular behaviors. Light-sensitive proteins are the foundation to such diverse and multilevel adaptive regulations in a large range of organisms. Due to their remarkable properties and potential applications in engineered systems, exploration and engineering of natural light-sensitive proteins have significantly contributed to expand optogenetic toolboxes with tailor-made performances in synthetic genetic circuits. Progressively, more complex systems have been designed in which multiple photoreceptors, each sensing its dedicated wavelength, are combined to simultaneously coordinate cellular responses in a single cell. In this review, we highlight recent works and challenges on multiplexed optogenetic circuits in natural and engineered systems for a dynamic regulation breakthrough in biotechnological applications.

High-resolution structure and dynamics of mitochondrial complex I-Insights into the proton pumping mechanism

Mitochondrial NADH:ubiquinone oxidoreductase (complex I) is a 1-MDa membrane protein complex with a central role in energy metabolism. Redox-driven proton translocation by complex I contributes substantially to the proton motive force that drives ATP synthase. Several structures of complex I from bacteria and mitochondria have been determined, but its catalytic mechanism has remained controversial. We here present the cryo-EM structure of complex I from Yarrowia lipolytica at 2.1-Å resolution, which reveals the positions of more than 1600 protein-bound water molecules, of which ~100 are located in putative proton translocation pathways. Another structure of the same complex under steady-state activity conditions at 3.4-Å resolution indicates conformational transitions that we associate with proton injection into the central hydrophilic axis. By combining high-resolution structural data with site-directed mutagenesis and large-scale molecular dynamic simulations, we define details of the proton translocation pathways and offer insights into the redox-coupled proton pumping mechanism of complex I.

Real-Time Tracking of Proton Transfer from the Reactive Cysteine to the Flavin Chromophore of a Photosensing Light Oxygen Voltage Protein

LOV (light oxygen voltage) proteins are photosensors ubiquitous to all domains of life. A variant of the short LOV protein from Dinoroseobacter shibae (DsLOV) exhibits an exceptionally fast photocycle. We performed time-resolved molecular spectroscopy on DsLOV-M49S and characterized the formation of the thio-adduct state with a covalent bond between the reactive cysteine (C72) and C_4a of the FMN. By use of a tunable quantum cascade laser, the weak absorption change of the vibrational band of S-H stretching vibration of C57 was resolved with a time resolution of 10 ns. Deprotonation of C72 proceeded with a time constant of 12 μs which tallies the rise of the thio-adduct state. These results provide valuable information for the mechanistic interpretation of light-induced structural changes in LOV domains, which involves the choreographed sequence of proton transfers, changes in electron density distributions, spin alterations of the latter, and transient bond formation and breakage. Such molecular insight will help develop new optogenetic tools based on flavin photoreceptors.

QM calculations predict the energetics and infrared spectra of transient glutamine isomers in LOV photoreceptors

Photosensory receptors containing the flavin-binding light-oxygen-voltage (LOV) domain are modular proteins that fulfil a variety of biological functions ranging from gene expression to phototropism. The LOV photocycle is initiated by blue-light and involves a cascade of intermediate species, including an electronically excited triplet state, that leads to covalent bond formation between the flavin mononucleotide (FMN) chromophore and a nearby cysteine residue. Subsequent conformational changes in the polypeptide chain arise due to the remodelling of the hydrogen bond network in the cofactor binding pocket, whereby a conserved glutamine residue plays a key role in coupling FMN photochemistry with LOV photobiology. Although the dark-to-light transition of LOV photosensors has been previously addressed by spectroscopy and computational approaches, the mechanistic basis of the underlying reactions is still not well understood. Here we present a detailed computational study of three distinct LOV domains: EL222 from Erythrobacter litoralis, AsLOV2 from the second LOV domain of Avena sativa phototropin 1, and RsLOV from Rhodobacter sphaeroides LOV protein. Extended protein-chromophore models containing all known crucial residues involved in the initial steps (femtosecond-to-microsecond) of the photocycle were employed. Energies and rotational barriers were calculated for possible rotamers and tautomers of the critical glutamine side chain, which allowed us to postulate the most energetically favoured glutamine orientation for each LOV domain along the assumed reaction path. In turn, for each evolving species, infrared difference spectra were constructed and compared to experimental EL222 and AsLOV2 transient infrared spectra, the former from original work presented here and the latter from the literature. The good agreement between theory and experiment permitted the assignment of the majority of observed bands, notably the ∼1635 cm-1 transient of the adduct state to the carbonyl of the glutamine side chain after rotation. Moreover, both the energetic and spectroscopic approaches converge in suggesting a facile glutamine flip at the adduct intermediate for EL222 and more so for AsLOV2, while for RsLOV the glutamine keeps its initial configuration. Additionally, the computed infrared shifts of the glutamine and interacting residues could guide experimental research addressing early events of signal transduction in LOV proteins.

Computational Investigation of Structural and Spectroscopic Properties of LOV-Based Proteins with Improved Fluorescence

Flavin-based fluorescent proteins are a class of fluorescent reporters derived from light, oxygen, and voltage (LOV) sensing proteins. Through mutagenesis, natural LOV proteins have been engineered to obtain improved fluorescence properties. In this study, we combined extended classical Molecular Dynamics simulations and multiscale Quantum Mechanics/Molecular Mechanics methods to clarify the relationship between structural and dynamic changes induced by specific mutations and the spectroscopic response. To reach this goal we compared two LOV variants, one obtained by the single mutation needed to photochemically inactivate the natural system, and the other (iLOV) obtained through additional mutations and characterized by a significantly improved fluorescence. Our simulations confirmed the "flipping and crowding" effect induced in iLOV by the additional mutations and revealed its mechanism of action. We also showed that these mutations, and the resulting differences in the composition and flexibility of the binding pockets, are not reflected in significant shifts of the excitation and emission energies, in agreement with the similarity of the spectra measured for the two systems. However, a small but consistent reduction was found in the Stokes shift of iLOV, suggesting a reduction of the intermolecular reorganization experienced by the chromophore after excitation, which could slow down its internal conversion to the ground state and improve the fluorescence.

A theoretical study on the signal transduction process of bacterial photoreceptor PpSB1 based on the Markov state model

Light-oxygen-voltage (LOV) domains are blue light sensors and play an important role in signal transduction in many organisms. Generally, LOV domains use chromophores to absorb photons, and then photochemical reactions will occur to convert light energy into chemical energy and transduce it to the main chain of proteins. These reactions can cause conformational rearrangement of proteins, and thus leading to signal transduction. Therefore, it is important to study the signal transduction process of LOV domains for understanding the control mechanism of cellular functions. However, how small photochemical changes in the active sites of the LOV domains lead to large conformational rearrangements of proteins, which in turn lead to signal transduction, has been puzzling us for a long time. Currently, the LOV domains are mainly studied in plants. The signal transduction mechanism of LOV domains in bacteria is still unclear. In this work, the Markov state model (MSM) combined with molecular dynamics (MD) simulations was applied to investigate the signal transduction process of the LOV protein from pseudomonas putida (PpSB1-LOV). The present work will play an important role in understanding the signal transduction mechanism of PpSB1-LOV domains, which may provide theoretical basis for the design and improvement of LOV-based optogenetic tools.

Multiple Reaction Pathways in the Morphinone Reductase-Catalyzed Hydride Transfer Reaction

Morphinone reductase (MR) is an important model system for studying the contribution of protein motions to H-transfer reactions. In this research, we used quantum mechanical/molecular mechanics (QM/MM) simulation together with transition path sampling (TPS) simulation to study two important topics of current research on MR: the existence of multiple catalytic reaction pathways and the involvement of fast protein motions in the catalytic process. We have discovered two reaction pathways for the wild type and three reaction pathways for the N189A mutant. With the committor distribution analysis method, we found reaction coordinates for all five reaction pathways. Only one wild-type reaction pathway has a rate-promoting vibration from His186, while all of the other four pathways do not involve any protein motions in their catalytic process through the transition state. The rate-promoting vibration in the wild-type MR, which comes from a direction perpendicular to the donor-acceptor axis, functions to decrease the donor-acceptor distance by causing a subtle "out-of-plane" motion of a donor atom. By comparing reaction pathways between the two enzymes, we concluded that the major effect of the N189A point mutation is to increase the active site volume by altering the active site backbone and eliminating the Asn189 side chain. This effect causes a different NADH geometry at the reactant state, which very well explains the different reaction mechanisms between the two enzymes, as well as the disappearance of the His186 rate-promoting vibrations in the N189A mutant. The unfavorable geometry of the NADH pyridine ring induced by the N189A point mutation is the potential cause of multiple reaction pathways in N189A mutants.

New Light on the Mechanism of Phototransduction in Phototropin

Phototropins are photoreceptor proteins that regulate blue light-dependent biological processes for efficient photosynthesis in plants and algae. The proteins consist of a photosensory domain that responds to the ambient light and an output module that triggers cellular responses. The photosensory domain of phototropin from Chlamydomonas reinhardtii contains two conserved LOV (light-oxygen-voltage) domains with flavin chromophores. Blue light triggers the formation of a covalent cysteine-flavin adduct and upregulates the phototropin kinase activity. Little is known about the structural mechanism that leads to kinase activation and how the two LOV domains contribute to this. Here, we investigate the role of the LOV1 domain from C. reinhardtii phototropin by characterizing the structural changes occurring after blue light illumination with nano- to millisecond time-resolved X-ray solution scattering. By structurally fitting the data with atomic models generated by molecular dynamics simulations, we find that adduct formation induces a rearrangement of the hydrogen bond network from the buried chromophore to the protein surface. In particular, the change in conformation and the associated hydrogen bonding of the conserved glutamine 120 induce a global movement of the β-sheet, ultimately driving a change in the electrostatic potential on the protein surface. On the basis of the change in the electrostatics, we propose a structural model of how LOV1 and LOV2 domains interact and regulate the full-length phototropin from C. reinhardtii. This provides a rationale for how LOV photosensor proteins function and contributes to the optimal design of optogenetic tools based on LOV domains.

A Noncanonical Chromophore Reveals Structural Rearrangements of the Light-Oxygen-Voltage Domain upon Photoactivation

Light-oxygen-voltage (LOV) domains are increasingly used to engineer photoresponsive biological systems. While the photochemical cycle is well documented, the allosteric mechanism by which formation of a cysteinyl-flavin adduct leads to activation is unclear. Via replacement of flavin mononucleotide (FMN) with 5-deazaflavin mononucleotide (5dFMN) in the Aureochrome1a (Au1a) transcription factor from Ochromonas danica, a thermally stable cysteinyl-5dFMN adduct was generated. High-resolution crystal structures (<2 Å) under different illumination conditions with either FMN or 5dFMN chromophores reveal three conformations of the highly conserved glutamine 293. An allosteric hydrogen bond network linking the chromophore via Gln293 to the auxiliary A′α helix is observed. With FMN, a “flip” of the Gln293 side chain occurs between dark and lit states. 5dFMN cannot hydrogen bond through the C5 position and proved to be unable to support Au1a domain dimerization. Under blue light, the Gln293 side chain instead “swings” away in a conformation distal to the chromophore and not previously observed in existing LOV domain structures. Together, the multiple side chain conformations of Gln293 and functional analysis of 5dFMN provide new insight into the structural requirements for LOV domain activation.

Anomalous pressure effects on the photoreaction of a light-sensor protein from Synechocystis, PixD (Slr1694), and the compressibility change of its intermediates

SyPixD (Slr1694) is a blue-light receptor that contains a BLUF (blue-light sensor using a flavin chromophore) domain for the function of phototaxis. The key reaction of this protein is a light-induced conformational change and subsequent dissociation reaction from the decamer to the dimer. In this study, anomalous effects of pressure on this reaction were discovered, and changes in the compressibility of its short-lived intermediates were investigated. While the absorption spectra of the dark and light states are not sensitive to pressure, the formation yield of the first intermediate decreases with pressure to about 40% at 150 MPa. Upon blue-light illumination with a sufficiently strong intensity, the transient grating signal, which represents the dissociation of the SyPixD decamer, was observed at 0.1 MPa, and the signal intensity significantly decreased with increasing pressure. This behavior shows that the dissociation of the decamer from the second intermediate state is suppressed by pressure. However, while the decamer undergoes no dissociation upon excitation of one monomer unit at 0.1 MPa, dissociation is gradually induced with increasing pressure. For solving this strange behavior, the compressibility changes of the intermediates were measured as a function of pressure at weak light intensity. Interestingly, the compressibility change was negative at low pressure, but became positive with increasing pressure. Because the compressibility is related to the volume fluctuation, this observation suggests that the driving force for this reaction is fluctuation of the protein. The relationship between the cavities at the interfaces of the monomer units and the reactivity was also discussed.

Sequential conformational transitions and α-helical supercoiling regulate a sensor histidine kinase

Sensor histidine kinases are central to sensing in bacteria and in plants. They usually contain sensor, linker, and kinase modules and the structure of many of these components is known. However, it is unclear how the kinase module is structurally regulated. Here, we use nano- to millisecond time-resolved X-ray scattering to visualize the solution structural changes that occur when the light-sensitive model histidine kinase YF1 is activated by blue light. We find that the coiled coil linker and the attached histidine kinase domains undergo a left handed rotation within microseconds. In a much slower second step, the kinase domains rearrange internally. This structural mechanism presents a template for signal transduction in sensor histidine kinases.

Sensor histidine kinases (SHK) consist of sensor, linker and kinase modules and different models for SHK signal transduction have been proposed. Here the authors present nano- to millisecond time-resolved X-ray scattering measurements, which reveal a structural mechanism for kinase domain activation in SHK.

Role of water and protein dynamics in proton pumping by respiratory complex I

Membrane bound respiratory complex I is the key enzyme in the respiratory chains of bacteria and mitochondria, and couples the reduction of quinone to the pumping of protons across the membrane. Recently solved crystal or electron microscopy structures of bacterial and mitochondrial complexes have provided significant insights into the electron and proton transfer pathways. However, due to large spatial separation between the electron and proton transfer routes, the molecular mechanism of coupling remains unclear. Here, based on atomistic molecular dynamics simulations performed on the entire structure of complex I from Thermus thermophilus, we studied the hydration of the quinone-binding site and the membrane-bound subunits. The data from simulations show rapid diffusion of water molecules in the protein interior, and formation of hydrated regions in the three antiporter-type subunits. An unexpected water-protein based connectivity between the middle of the Q-tunnel and the fourth proton channel is also observed. The protonation-state dependent dynamics of key acidic residues in the Nqo8 subunit suggest that the latter may be linked to redox-coupled proton pumping in complex I. We propose that in complex I the proton and electron transfer paths are not entirely separate, instead the nature of coupling may in part be ‘direct’.

Time-resolved fluctuation during the photochemical reaction of a photoreceptor protein: phototropin1LOV2-linker

Although the relationship between structural fluctuations and reactions is important for elucidating reaction mechanisms, experimental data describing such fluctuations of reaction intermediates are sparse. In order to investigate structural fluctuations during a protein reaction, the compressibilities of intermediate species after photoexcitation of a phot1LOV2-linker, which is a typical LOV domain protein with the C-terminal linker including the J-α helix and used recently for optogenetics, were measured in the time-domain by the transient grating and transient lens methods with a high pressure optical cell. The yield of covalent bond formation between the chromophore and a Cys residue (S state formation) relative to that at 0.1 MPa decreased very slightly with increasing pressure. The fraction of the reactive species that yields the T state (linker-unfolded state) decreased almost proportionally with pressure (0.1-200 MPa) to about 65%. Interestingly, the volume change associated with the reaction was much more pressure sensitive. By combining these data, the compressibility changes for the short lived intermediate (S state) and the final product (T state) formation were determined. The compressibility of the S state was found to increase compared with the dark (D) state, and the compressibility decreased during the transition from the S state to the T state. The compressibility change is discussed in terms of cavities inside the protein. By comparing the crystal structures of the phot1LOV2-linker at dark and light states, we concluded that the cavity volumes between the LOV domain and the linker domain increase in the S state, which explains the enhanced compressibility.

Regulation of neural gene transcription by optogenetic inhibition of the RE1-silencing transcription factor

Optogenetics provides new ways to activate gene transcription; however, no attempts have been made as yet to modulate mammalian transcription factors. We report the light-mediated regulation of the repressor element 1 (RE1)-silencing transcription factor (REST), a master regulator of neural genes. To tune REST activity, we selected two protein domains that impair REST-DNA binding or recruitment of the cofactor mSin3a. Computational modeling guided the fusion of the inhibitory domains to the light-sensitive Avena sativa light-oxygen-voltage-sensing (LOV) 2-phototrophin 1 (AsLOV2). By expressing AsLOV2 chimeras in Neuro2a cells, we achieved light-dependent modulation of REST target genes that was associated with an improved neural differentiation. In primary neurons, light-mediated REST inhibition increased Na(+)-channel 1.2 and brain-derived neurotrophic factor transcription and boosted Na(+) currents and neuronal firing. This optogenetic approach allows the coordinated expression of a cluster of genes impinging on neuronal activity, providing a tool for studying neuronal physiology and correcting gene expression changes taking place in brain diseases.

Light-induced structural changes in a short light, oxygen, voltage (LOV) protein revealed by molecular dynamics simulations-implications for the understanding of LOV photoactivation

The modularity of light, oxygen, voltage (LOV) blue-light photoreceptors has recently been exploited for the design of LOV-based optogenetic tools, which allow the light-dependent control of biological functions. For the understanding of LOV sensory function and hence the optimal design of LOV-based optogentic tools it is essential to gain an in depth atomic-level understanding of the underlying photoactivation and intramolecular signal-relay mechanisms. To address this question we performed molecular dynamics simulations on both the dark- and light-adapted state of PpSB1-LOV, a short dimeric bacterial LOV-photoreceptor protein, recently crystallized under constant illumination. While LOV dimers remained globally stable during the light-state simulation with regard to the Jα coiled-coil, distinct conformational changes for a glutamine in the vicinity of the FMN chromophore are observed. In contrast, multiple Jα-helix conformations are sampled in the dark-state. These changes coincide with a displacement of the Iβ and Hβ strands relative to the light-state structure and result in a correlated rotation of both LOV core domains in the dimer. These global changes are most likely initiated by the reorientation of the conserved glutamine Q116, whose side chain flips between the Aβ (dark state) and Hβ strand (light state), while maintaining two potential hydrogen bonds to FMN-N5 and FMN-O4, respectively. This local Q116-FMN reorientation impacts on an inter-subunit salt-bridge (K117-E96), which is stabilized in the light state, hence accounting for the observed decreased mobility. Based on these findings we propose an alternative mechanism for dimeric LOV photoactivation and intramolecular signal-relay, assigning a distinct structural role for the conserved “flipping” glutamine. The proposed mechanism is discussed in light of universal applicability and its implications for the understanding of LOV-based optogenetic tools.

Redox-induced activation of the proton pump in the respiratory complex I

Complex I functions as a redox-linked proton pump in the respiratory chains of mitochondria and bacteria, driven by the reduction of quinone (Q) by NADH. Remarkably, the distance between the Q reduction site and the most distant proton channels extends nearly 200 Å. To elucidate the molecular origin of this long-range coupling, we apply a combination of large-scale molecular simulations and a site-directed mutagenesis experiment of a key residue. In hybrid quantum mechanics/molecular mechanics simulations, we observe that reduction of Q is coupled to its local protonation by the His-38/Asp-139 ion pair and Tyr-87 of subunit Nqo4. Atomistic classical molecular dynamics simulations further suggest that formation of quinol (QH2) triggers rapid dissociation of the anionic Asp-139 toward the membrane domain that couples to conformational changes in a network of conserved charged residues. Site-directed mutagenesis data confirm the importance of Asp-139; upon mutation to asparagine the Q reductase activity is inhibited by 75%. The current results, together with earlier biochemical data, suggest that the proton pumping in complex I is activated by a unique combination of electrostatic and conformational transitions.

Insight into structural rearrangements and interdomain interactions related to electron transfer between flavin mononucleotide and heme in nitric oxide synthase: A molecular dynamics study

Calmodulin (CaM) binding to nitric oxide synthase (NOS) enables a conformational change, in which the FMN domain shuttles between the FAD and heme domains to deliver electrons to the active site heme center. A clear understanding of this large conformational change is critical, since this step is the rate-limiting in NOS catalysis. Herein molecular dynamics simulations were conducted on a model of an oxygenase/FMN (oxyFMN) construct of human inducible NOS (iNOS). This is to investigate the structural rearrangements and the domain interactions related to the FMN-heme interdomain electron transfer (IET). We carried out simulations on the iNOS oxyFMN·CaM complex models in [Fe(III)][FMNH(-)] and [Fe(II)][FMNH] oxidation states, the pre- and post-IET states. The comparison of the dynamics and conformations of the iNOS construct at the two oxidation states has allowed us to identify key factors related to facilitating the FMN-heme IET process. The computational results demonstrated, for the first time, that the conformational change is redox-dependent. Predictions of the key interacting sites in optimal interdomain FMN/heme docking are well supported by experimental data in the literature. An intra-subunit pivot region is predicted to modulate the FMN domain motion and correlate with existence of a bottleneck in the conformational sampling that leads to the electron transfer-competent state. Interactions of the residues identified in this work are proposed to ensure that the FMN domain moves with appropriate degrees of freedom and docks to proper positions at the heme domain, resulting in efficient IET and nitric oxide production.

LOV-based optogenetic devices: light-driven modules to impart photoregulated control of cellular signaling

The Light-Oxygen-Voltage domain family of proteins is widespread in biology where they impart sensory responses to signal transduction domains. The small, light responsive LOV modules offer a novel platform for the construction of optogenetic tools. Currently, the design and implementation of these devices is partially hindered by a lack of understanding of how light drives allosteric changes in protein conformation to activate diverse signal transduction domains. Further, divergent photocycle properties amongst LOV family members complicate construction of highly sensitive devices with fast on/off kinetics. In the present review we discuss the history of LOV domain research with primary emphasis on tuning LOV domain chemistry and signal transduction to allow for improved optogenetic tools.

Transient conformational fluctuation of TePixD during a reaction

Knowledge of the dynamical behavior of proteins, and in particular their conformational fluctuations, is essential to understanding the mechanisms underlying their reactions. Here, transient enhancement of the isothermal partial molar compressibility, which is directly related to the conformational fluctuation, during a chemical reaction of a blue light sensor protein from the thermophilic cyanobacterium Thermosynechococcus elongatus BP-1 (TePixD, Tll0078) was investigated in a time-resolved manner. The UV-Vis absorption spectrum of TePixD did not change with the application of high pressure. Conversely, the transient grating signal intensities representing the volume change depended significantly on the pressure. This result implies that the compressibility changes during the reaction. From the pressure dependence of the amplitude, the compressibility change of two short-lived intermediate (I1 and I2) states were determined to be +(5.6 ± 0.6) × 10(-2) cm(3) ⋅ mol(-1) ⋅ MPa(-1) for I1 and +(6.6 ± 0.7) × 10(-2) cm(3) ⋅ mol(-1) ⋅ MPa(-1) for I2. This result showed that the structural fluctuation of intermediates was enhanced during the reaction. To clarify the relationship between the fluctuation and the reaction, the compressibility of multiply excited TePixD was investigated. The isothermal compressibility of I1 and I2 intermediates of TePixD showed a monotonic decrease with increasing excitation laser power, and this tendency correlated with the reactivity of the protein. This result indicates that the TePixD decamer cannot react when its structural fluctuation is small. We concluded that the enhanced compressibility is an important factor for triggering the reaction of TePixD. To our knowledge, this is the first report showing enhanced fluctuations of intermediate species during a protein reaction, supporting the importance of fluctuations.

The dark recovery rate in the photocycle of the bacterial photoreceptor YtvA is affected by the cellular environment and by hydration

We report thermal recovery kinetics of the lit state into the parental dark state, measured for the blue light-sensing photoreceptor YtvA inside overexpressing E. coli and B. subtilis bacterial cells, performed for the wild type and several mutated proteins. Recovery was followed as a recovery of the fluorescence, as this property is only found for the parental but not for the photochemically generated lit state. When cells were deposited onto a microscope glass plate, the observed thermal recovery rate in the photocycle was found ca. ten times faster in comparison to purified YtvA in solution. When the E. coli or B. subtilis colonies were soaked in an isotonic buffer, the dark relaxation became again much slower and was very similar to that observed for YtvA in solution. The observed effects show that rate constants can be tuned by the cellular environment through factors such as hydration.

Exploring the multiscale signaling behavior of phototropin1 from Chlamydomonas reinhardtii using a full-residue space kinetic Monte Carlo molecular dynamics technique

Devising analysis tools for elucidating the regulatory mechanism of complex enzymes has been a challenging task for many decades. It generally requires the determination of the structural-dynamical information of protein solvent systems far from equilibrium over multiple length and time scales, which is still difficult both theoretically and experimentally. To cope with the problem, we introduce a full-residue space multiscale simulation method based on a combination of the kinetic Monte Carlo and molecular dynamics techniques, in which the rates of the rate-determining processes are evaluated from a biomolecular forcefield on the fly during the simulation run by taking into account the full space of residues. To demonstrate its reliability and efficiency, we explore the light-induced functional behavior of the full-length phototropin1 from Chlamydomonas reinhardtii (Cr-phot1) and its various subdomains. Our results demonstrate that in the dark state the light oxygen voltage-2-Jα (LOV2-Jα) photoswitch inhibits the enzymatic activity of the kinase, whereas the LOV1-Jα photoswitch controls the dimerization with the LOV2 domain. This leads to the repulsion of the LOV1-LOV2 linker out of the interface region between both LOV domains, which results in a positively charged surface suitable for cell-membrane interaction. By contrast, in the light state, we observe that the distance between both LOV domains is increased and the LOV1-LOV2 linker forms a helix-turn-helix (HTH) motif, which enables gene control through nucleotide binding. Finally, we find that the kinase is activated through the disruption of the Jα-helix from the LOV2 domain, which is followed by a stretching of the activation loop (A-loop) and broadening of the catalytic cleft of the kinase.

Signaling mechanisms of LOV domains: new insights from molecular dynamics studies

Phototropins are one of several classes of photoreceptors used by plants and algae to respond to light. These proteins contain flavin-binding LOV (Light-Oxygen-Voltage) domains that form covalent cysteine-flavin adducts upon exposure to blue light, leading to the enhancement of phototropin kinase activity. Several lines of evidence suggest that adduct formation in the phototropin LOV2 domains leads to the dissociation of an alpha helix (Jα) from these domains as part of the light-induced activation process. However, crystal structures of LOV domains both in the presence and absence of the Jα helix show very few differences between dark and illuminated states, and thus the precise mechanism through which adduct formation triggers helical dissociation remains poorly understood. Using Avena sativa phototropin 1 LOV2 as a model system, we have studied the interactions of the LOV domain core with the Jα helix through a series of equilibrium molecular dynamics simulations. Here we show that conformational transitions of a conserved glutamine residue in the flavin binding pocket are coupled to altered dynamics of the Jα helix both through a shift in dynamics of the main β-sheet of the LOV domain core and through a secondary pathway involving the N-terminal A'α helix.

Photochemistry of Arabidopsis phototropin 1 LOV1: transient tetramerization

The photochemical reaction of the LOV1 (light-oxygen-voltage 1) domain of phototropin 1 from Arabidopsis thaliana was investigated by the time-resolved transient grating method. As with other LOV domains, an absorption spectral change associated with an adduct formation between its chromophore (flavin mononucleotide) and a cysteine residue was observed with a time constant of 1.1 μs. After this reaction, a significant diffusion coefficient (D) change (D of the reactant = 8.2 × 10(-11) m(2) s(-1), and D of the photoproduct = 6.4 × 10(-11) m(2) s(-1)) was observed with a time constant of 14 ms at a protein concentration of 270 μM. From the D value of the ground state and the peak position in size exclusion chromatography, we have confirmed that the phot1LOV1 domain exists as a dimer in the dark. The D-value and the concentration dependence of the rate indicated that the phot1LOV1 domain associates to form a tetramer (dimerization of the dimer) upon photoexcitation. We also found that the chromophore is released from the binding pocket of the LOV domain when it absorbs two photons within a pulse duration, which occurs in addition to the normal photocycle reaction. On the basis of these results, we discuss the molecular mechanism of the light dependent role of the phot1LOV1 domain.

Computational spectroscopy, dynamics, and photochemistry of photosensory flavoproteins

Extensive interest in photosensory proteins stimulated computational studies of flavins and flavoproteins in the past decade. This review is dedicated to the three central topics of these studies: calculations of flavin UV-visible and IR spectra, simulated dynamics of photoreceptor proteins, and flavin photochemistry. Accordingly, this chapter is divided into three parts; each part describes corresponding computational protocols, summarizes computational results, and discusses the emerging mechanistic picture.

Dynamics of the amino-terminal and carboxyl-terminal helices of Arabidopsis phototropin 1 LOV2 studied by the transient grating

Recently, conformational changes of the amino-terminal helix (A'α helix), in addition to the reported conformational changes of the carboxyl-terminal helix (Jα helix), have been proposed to be important for the regulatory function of the light-oxygen-voltage 2 domain (LOV2) of phototropin 1 from Arabidopsis. However, the reaction dynamics of the A'α helix have not been examined. Here, the unfolding reactions of the A'α and Jα helices of the LOV2 domain of phototropin 1 from Arabidopsis thaliana were investigated by the time-resolved transient grating (TG) method. A mutant (T469I mutant) that renders the A'α helix unfolded in the dark state showed unfolding of the Jα helix with a time constant of 1 ms, which is very similar to the time constant reported for the wild-type LOV2-linker sample. Furthermore, a mutant (I608E mutant) that renders the Jα helix unfolded in the dark state exhibited an unfolding process of the A'α helix with a time constant of 12 ms. On the basis of these experimental results, it is suggested that the unfolding reactions of these helices occurs independently.

Light-induced subunit dissociation by a light-oxygen-voltage domain photoreceptor from Rhodobacter sphaeroides

Light-oxygen-voltage (LOV) domains bind a flavin chromophore to serve as blue light sensors in a wide range of eukaryotic and prokaryotic proteins. LOV domains are associated with a variable effector domain or a separate protein signaling partner to execute a wide variety of functions that include regulation of kinases, generation of anti-sigma factor antagonists, and regulation of circadian clocks. Here we present the crystal structure, photocycle kinetics, association properties, and spectroscopic features of a full-length LOV domain protein from Rhodobacter sphaeroides (RsLOV). RsLOV exhibits N- and C-terminal helical extensions that form an unusual helical bundle at its dimer interface with some resemblance to the helical transducer of sensory rhodopsin II. The blue light-induced conformational changes of RsLOV revealed from a comparison of light- and dark-state crystal structures support a shared signaling mechanism of LOV domain proteins that originates with the light-induced formation of a flavin-cysteinyl photoadduct. Adduct formation disrupts hydrogen bonding in the active site and propagates structural changes through the LOV domain core to the N- and C-terminal extensions. Single-residue variants in the active site and dimer interface of RsLOV alter photoadduct lifetimes and induce structural changes that perturb the oligomeric state. Size exclusion chromatography, multiangle light scattering, small-angle X-ray scattering, and cross-linking studies indicate that RsLOV dimerizes in the dark but, upon light excitation, dissociates into monomers. This light-induced switch in oligomeric state may prove to be useful for engineering molecular associations in controlled cellular settings.

Modeling the interfacial interactions between CrtS and CrtR from Xanthophyllomyces dendrorhous , a P450 system involved in astaxanthin production

Xanthophyllomyces dendrorhous is a natural source of astaxanthin, a carotenoid widely used in the food industry. In this yeast, astaxanthin is synthesized from β-carotene by a cytochrome P450, CrtS, which depends on CrtR, the four-domain cytochrome P450 reductase (CPR). Although Saccharomyces cerevisiae has an endogenous CPR (ScCPR), expression of CrtS does not result in astaxanthin production unless it is coexpressed with CrtR. Assuming that CrtS could interact with the FMN-binding domain of either CrtR or ScCPR (XdFMNbd and ScFMNbd, respectively), the aim of this work was to identify possible interaction differences between these alternative complexes by protein modeling and short molecular dynamics simulations. Considering the recently proposed membrane orientation of a mammalian P450, our CrtS-CrtR model predicts that both N-terminal ends stand adjacent to the membrane plane, allowing their anchoring. Compared with the possible interface between CrtS and both FMNbd, the Xanthophyllomyces system appears to be stabilized by more saline bridges.

The amino-terminal helix modulates light-activated conformational changes in AsLOV2

The mechanism of light-triggered conformational change and signaling in light-oxygen-voltage (LOV) domains remains elusive in spite of extensive investigation and their use in optogenetic studies. The LOV2 domain of Avenasativa phototropin 1 (AsLOV2), a member of the Per-Arnt-Sim (PAS) family, contains a flavin mononucleotide chromophore that forms a covalent bond with a cysteine upon illumination. This event leads to the release of the carboxy-terminal Jα helix, the biological output signal. Using mutational analysis, circular dichroism, and NMR, we find that the largely ignored amino-terminal helix is a control element in AsLOV2's light-activated conformational change. We further identify a direct amino-to-carboxy-terminal "input-output" signaling pathway. These findings provide a framework to rationalize the LOV domain architecture, as well as the signaling mechanisms in both isolated and tandem arrangements of PAS domains. This knowledge can be applied in engineering LOV-based photoswitches, opening up new design strategies and improving existing ones.

Signals of LOV1: a computer simulation study on the wildtype LOV1-domain of Chlamydomonas reinhardtii and its mutants

Phototropins are photoreceptors regulating the blue-light response in plants and bacteria. They consist of two LOV (light oxygen voltage sensitive) domains each containing a non-covalently bound flavin-mononucleotide (FMN) chromophore, which are connected to a serine/threonine-kinase. Upon illumination, the LOV-domains undergo conformational changes, triggering a signal cascade in the organism through kinase activation. Here, we present results from molecular dynamics simulations in which we investigate the signal transduction pathway of the wildtype LOV1-domain of Chlamydomonas reinhardtii and a methyl-mercaptan (MM) adduct of its Cys57Gly-mutant at the molecular level. In particular, we analyzed the effect of covalent-bond formation between the reactive cysteine Cys57 and the FMN-reaction center, as well as the subsequent charge redistribution, on the spatio-dynamical behavior of the LOV1-domain. We compare the calculation results with experimental data and demonstrate that these adduct state characteristics have an important influence on the response of this photosensor. The light-induced changes implicate primarily an alteration of the surface charge distribution through rearrangement of the highly flexible Cα-, Dα- and Eα-helices including the Glu51-Lys91-salt bridge on the hydrophilic side of the protein domain and a β-sheet tightening process via coupling of the Aβ- and Bβ-strands. Our findings confirm the aptitude of the LOV1-domain to function as a dimerization partner, allowing the green alga to adapt its reproduction and growth speed to the environmental conditions.

Modulation of the photocycle of a LOV domain photoreceptor by the hydrogen-bonding network

An extended hydrogen-bonding (HB) network stabilizes the isoalloxazine ring of the flavin mononucleotide (FMN) chromophore within the photosensing LOV domain of blue-light protein receptors, via interactions between the C(2)═O, N(3)H, C(4)═O, and N(5) groups and conserved glutamine and asparagine residues. In this work we studied the influence of the HB network on the efficiency, kinetics, and energetics of a LOV protein photocycle, involving the reversible formation of a FMN-cysteine covalent adduct. The following results were found for mutations of the conserved amino acids N94, N104, and Q123 in the Bacillus subtilis LOV protein YtvA: (i) Increased (N104D, N94D) or strongly reduced (N94A) rate of adduct formation; this latter mutation extends the lifetime of the flavin triplet state, i.e., adduct formation, more than 60-fold, from 2 μs for the wild-type (WT) protein to 129 μs. (ii) Acceleration of the overall photocycle for N94S, N94A, and Q123N, with recovery lifetimes 20, 45, and 85 times faster than for YtvA-WT, respectively. (iii) Slight modifications of FMN spectral features, correlated with the polarization of low-energy transitions. (iv) Strongly reduced (N94S) or suppressed (Q123N) structural volume changes accompanying adduct formation, as determined by optoacoustic spectroscopy. (v) Minor effects on the quantum yield, with the exception of a considerable reduction for Q123N, i.e., 0.22 vs 0.49 for YtvA-WT. The data stress the importance of the HB network in modulating the photocycle of LOV domains, while at the same time establishing a link with functional responses.

Effect of computational methodology on the conformational dynamics of the protein photosensor LOV1 from Chlamydomonas reinhardtii

LOV domains are the light-sensitive protein domains of plant phototropins and bacteria. They photochemically form a covalent bond between a flavin mononucleotide (FMN) chromophore and a cysteine, attached to the apo-protein, upon irradiation with blue light, which triggers a signal in the adjacent kinase. Although their signaling state has been well characterized through experimental means, their signal transduction pathway as well as dark-state activity are generally only poorly understood. Here we show results from molecular dynamics simulations where we investigated the effect of thermostating and long-range electrostatics on the solution structure and dynamical behavior of the wild-type LOV1 domain from the green algae Chlamydomonas reinhardtii in the dark. We demonstrate that these computational issues can dramatically affect the conformational fluctuations of such protein domains by suppressing configurations far from equilibrium or destabilizing local configurations, leading to artificial changes of the protein secondary structure as well as the H-bond network formed by the amino acids and the FMN. By comparing our calculation results with recent experimental data, we show that the non-invasive thermostating strategy, where the protein solute is only indirectly coupled to the thermostat via the solvent, in conjunction with the particle-mesh Ewald technique, provides dark-state conformers, which are in consistency with experimental observations. Moreover, our calculations indicate that the LOV1 domains can alter the intersystem crossing rate and rate of adduct formation by adjusting the population distribution of these dark-state conformers. This might permit them to function as a modulator of the signal intensity under low light conditions.

ELECTRONIC SUPPLEMENTARY MATERIAL

The online version of this article (doi:10.1007/s12154-011-0060-z) contains supplementary material, which is available to authorized users.

Kinetics of conformational changes of the FKF1-LOV domain upon photoexcitation

The photochemical reaction dynamics of a light-oxygen-voltage (LOV) domain from the blue light sensor protein, FKF1 (flavin-binding Kelch repeat F-box) was studied by means of the pulsed laser-induced transient grating method. The observed absorption spectral changes upon photoexcitation were similar to the spectral changes observed for typical LOV domain proteins (e.g., phototropins). The adduct formation took place with a time constant of 6 μs. After this reaction, a significant conformational change with a time constant of 6 ms was observed as a change in the diffusion coefficient. An FKF1-LOV mutant without the conserved loop connecting helices E and F, which is present only in the FKF1/LOV Kelch protein 2/ZEITLUPE family, did not show these slow phase dynamics. This result indicates that the conformational change in the loop region represents a major change in the FKF1-LOV photoreaction.