Perturbation of the ground-state electronic structure of FMN by the conserved cysteine in phototropin LOV2 domains

Steric and Electronic Interactions at Gln154 in ZEITLUPE Induce Reorganization of the LOV Domain Dimer Interface

Plants measure light quality, intensity, and duration to coordinate growth and development with daily and seasonal changes in environmental conditions; however, the molecular details linking photochemistry to signal transduction remain incomplete. Two closely related light, oxygen, or voltage (LOV) domain-containing photoreceptor proteins, ZEITLUPE (ZTL) and FLAVIN-BINDING, KELCH REPEAT, F-BOX 1 (FKF1), divergently regulate the protein stability of circadian clock and photoperiodic flowering components to mediate daily and seasonal development. Using structural approaches, we identified that mutations at the Gly46 position led to global rearrangements of the ZTL dimer interface in the isolated ZTL-LOV domain. Specifically, G46S and G46A variants induce a 180° rotation about the ZTL-LOV dimer interface that is coupled to ordering of N- and C-terminal signaling elements. These conformational changes hinge upon rotation of a C-terminal Gln residue (Gln154) analogous to that present in light-state structures of ZTL. In contrast to other LOV proteins, a Q154L variant retains light-state interactions with GIGANTEA (GI), thereby indicating N5 protonation is not required for ZTL signaling. The results presented herein confirm a divergent signaling mechanism within ZTL, whereby steric and electronic effects following adduct formation can be sufficient for signal propagation in LOV proteins containing a Gly residue at position 46. Examination of bacterial LOV structures with Gly residues at the equivalent position suggests that mechanisms of signal transduction in LOV proteins may be fluid across the LOV protein family.

Femtosecond-to-nanosecond dynamics of flavin mononucleotide monitored by stimulated Raman spectroscopy and simulations

Flavin mononucleotide (FMN) belongs to the large family of flavins, ubiquitous yellow-coloured biological chromophores that contain an isoalloxazine ring system. As a cofactor in flavoproteins, it is found in various enzymes and photosensory receptors, like those featuring the light-oxygen-voltage (LOV) domain. The photocycle of FMN is triggered by blue light and proceeds via a cascade of intermediate states. In this work, we have studied isolated FMN in an aqueous solution in order to elucidate the intrinsic electronic and vibrational changes of the chromophore upon excitation. The ultrafast transitions of excited FMN were monitored through the joint use of femtosecond stimulated Raman spectroscopy (FSRS) and transient absorption spectroscopy encompassing a time window between 0 ps and 6 ns with 50 fs time resolution. Global analysis of the obtained transient visible absorption and transient Raman spectra in combination with extensive quantum chemistry calculations identified unambiguously the singlet and triplet FMN populations and addressed solvent dynamics effects. The good agreement between the experimental and theoretical spectra facilitated the assignment of electronic transitions and vibrations. Our results represent the first steps towards more complex experiments aimed at tracking structural changes of FMN embedded in light-inducible proteins upon photoexcitation.

Helical Contributions Mediate Light-Activated Conformational Change in the LOV2 Domain of Avena sativa Phototropin 1

Algae, plants, bacteria, and fungi contain flavin-binding light-oxygen-voltage (LOV) domains that function as blue light sensors to control cellular responses to light. In the second LOV domain of phototropins, called LOV2 domains, blue light illumination leads to covalent bond formation between protein and flavin that induces the dissociation and unfolding of a C-terminally attached α helix (Jα) and the N-terminal helix (A'α). To date, the majority of studies on these domains have focused on versions that contain truncations in the termini, which creates difficulties when extrapolating to the much larger proteins that contain these domains. Here, we study the influence of deletions and extensions of the A'α helix of the LOV2 domain of Avena sativa phototropin 1 (AsLOV2) on the light-triggered structural response of the protein by Fourier-transform infrared difference spectroscopy. Deletion of the A'α helix abolishes the light-induced unfolding of Jα, whereas extensions of the A'α helix lead to an attenuated structural change of Jα. These results are different from shorter constructs, indicating that the conformational changes in full-length phototropin LOV domains might not be as large as previously assumed, and that the well-characterized full unfolding of the Jα helix in AsLOV2 with short A'α helices may be considered a truncation artifact. It also suggests that the N- and C-terminal helices of phot-LOV2 domains are necessary for allosteric regulation of the phototropin kinase domain and may provide a basis for signal integration of LOV1 and LOV2 domains in phototropins.

Directly monitoring the active sites of charge transfer in heterocycles in situ and in real time

Coherent degenerate infrared-infrared-visible sum frequency generation vibrational spectroscopy provides a powerful label-free sensitive probe for charge transfer active sites in heterocyclic molecules in situ and in real time.

Dual Photochemical Reaction Pathway in Flavin-Based Photoreceptor LOV Domain: A Combined Quantum-Mechanics/Molecular-Mechanics Investigation

The primary photochemical reaction of the light, oxygen, and voltage (LOV) domain of the blue-light photosensor YtvA of Bacillus subtilis were investigated using high-level QM(DFT/MRCI)/MM methods. After blue-light excitation, the S_γ atom of the reactive cysteine forms a covalent bond with the C_4a of the flavin mononucleotide (FMN) ring. Two conformations for the side chain of reactive cysteine with occupancies of 70% (conf A) and 30% (conf B) are observed in the X-ray crystallographic structures of the YtvA-LOV ( Möglich , A. ; Moffat , K. J. Mol. Biol. 2007 , 373 , 112 - 126 ). In conf A, the thiol group is directed toward the dimethylbenzene moiety of the FMN ring whereas it is placed directly above the N₅ atom of the FMN ring in conf B. Starting from both conformations, the singlet and triplet excited pathways were evaluated. The singlet states excited from conf A decay nonradiatively to the triplet states by intersystem crossing (ISC). After the formation of a neutral biradical, the triplet states cross over to the electronic ground state by a second ISC and the adducts are efficiently formed. The singlet states excited from conf B are located near the S₁/S₀ conical intersection (CIn). A major fraction returns to the initial states through the CIn. The rest may directly reach the adduct state. Thus, the photoexcitation has a dual reaction pathway. In YtvA-LOV, it is inferred that the efficient triplet excitation from conf A was chosen by bypassing the less efficient singlet excitation from conf B.

Modulation of the flavin-protein interactions in NADH peroxidase and mercuric ion reductase: a resonance Raman study

NADH peroxidase (Npx) and mercuric ion reductase (MerA) are flavoproteins belonging to the pyridine nucleotide:disulfide oxidoreductases (PNDO) and catalyzing the reduction of toxic substrates, i.e., hydrogen peroxide and mercuric ion, respectively. To determine the role of the flavin adenine dinucleotide (FAD) in the detoxification mechanism, the resonance Raman (RR) spectra of these enzymes under various redox and ligation states have been investigated using blue and/or near-UV excitation(s). These data were compared to those previously obtained for glutathione reductase (GR), another enzyme of the PNDO family, but catalyzing the reduction of oxidized glutathione. Spectral differences have been detected for the marker bands of the isoalloxazine ring of Npx, MerA, and GR. They provide evidence for different catalytic mechanisms in these flavoproteins. The RR modes of the oxidized and two-electron reduced (EH₂) forms of Npx are related to very tight flavin-protein interactions maintaining a nearly planar conformation of the isoalloxazine tricycle, a low level of H-bonding at the N₁/N₅ and O₂/O₄ sites, and a strong H-bond at N₃H. They also indicate minimal changes in FAD structure and environment upon either NAD(H) binding or reduction of the sulfinic redox center. All these spectroscopic data support an enzyme functioning centered on the Cys-SO^-/Cys-S^- redox moiety and a neighbouring His residue. On the contrary, the RR data on various functional forms of MerA are indicative of a modulation of both ring II distortion and H-bonding states of the N₅ site and ring III. The Cd(II) binding to the EH₂-NADP(H) complexes, biomimetic intermediates in the reaction of Hg(II) reduction, provokes important spectral changes. They are interpreted in terms of flattening of the isoalloxazine ring and large decreases in H-bonding at the N₅ site and ring III. The large flexibility of the FAD structure and environment in MerA is in agreement with proposed mechanisms involving C_4a(flavin) adducts.

Unfolding of the C-Terminal Jα Helix in the LOV2 Photoreceptor Domain Observed by Time-Resolved Vibrational Spectroscopy

Light-triggered reactions of biological photoreceptors have gained immense attention for their role as molecular switches in their native organisms and for optogenetic application. The light, oxygen, and voltage 2 (LOV2) sensing domain of plant phototropin binds a C-terminal Jα helix that is docked on a β-sheet and unfolds upon light absorption by the flavin mononucleotide (FMN) chromophore. In this work, the signal transduction pathway of LOV2 from Avena sativa was investigated using time-resolved infrared spectroscopy from picoseconds to microseconds. In D2O buffer, FMN singlet-to-triplet conversion occurs in 2 ns and formation of the covalent cysteinyl-FMN adduct in 10 μs. We observe a two-step unfolding of the Jα helix: The first phase occurs concomitantly with Cys-FMN covalent adduct formation in 10 μs, along with hydrogen-bond rupture of the FMN C4═O with Gln-513, motion of the β-sheet, and an additional helical element. The second phase occurs in approximately 240 μs. The final spectrum at 500 μs is essentially identical to the steady-state light-minus-dark Fourier transform infrared spectrum, indicating that Jα helix unfolding is complete on that time scale.

Post-translational control of protein function with light using a LOV-intein fusion protein

Methods for the post-translational control of protein function with light hold much value as tools in cell biology. To this end, we report a fusion protein that consists of DnaE split-inteins, flanking the light sensitive LOV2 domain of Avena sativa. The resulting chimera combines the activities of these two unrelated proteins to enable controlled formation of a functional protein via upregulation of intein splicing with blue light in bacterial and human cells.

Resonance Raman spectroscopy

Flavin is a general name given to molecules having the heteroaromatic ring system of 7,8-dimethylisoalloxazine but practically means riboflavin (Rfl), flavin adenine dinucleotide (FAD), and flavin mononucleotide (FMN) in biological systems, whose structures are illustrated in Fig. 1, together with the atomic numbering scheme and ring numbering of the isoalloxazine moiety. As the isoalloxazine skeleton cannot be synthesized in human cells, it is obtained from diet as Rfl (vitamin B2). FAD and FMN can act as cofactors in flavoenzymes but Rfl does not. Most flavoenzymes catalyze redox reactions of substrates (Miura, Chem Rec 1:183-194, 2001). When O2 serves as the oxidant in the oxidation half cycle of an enzymic reaction, the enzyme is called "flavo-oxidase" but when others do, the enzyme is called "flavo-dehydrogenase." The difference between the two types of oxidative catalysis arises from delicate differences in the π-electron distributions in the isoalloxazine ring, which can be revealed by Raman spectroscopy (Miura, Chem Rec 1:183-194, 2001). Since a flavin is an extremely versatile molecule, the scientific field including chemistry, biochemistry, and enzymology is collectively called "flavonology." It was found recently, however, that the flavin also acts as a chromophore to initiate light-induced DNA repair and signal transductions (Sancar, Chem Rev 103:2203-2237, 2003).

Factors that control the chemistry of the LOV domain photocycle

Algae, plants, bacteria and fungi contain Light-Oxygen-Voltage (LOV) domains that function as blue light sensors to control cellular responses to light. All LOV domains contain a bound flavin chromophore that is reduced upon photon absorption and forms a reversible, metastable covalent bond with a nearby cysteine residue. In Avena sativa LOV2 (AsLOV2), the photocycle is accompanied by an allosteric conformational change that activates the attached phototropin kinase in the full-length protein. Both the conformational change and formation of the cysteinyl-flavin adduct are stabilized by the reduction of the N5 atom in the flavin's isoalloxazine ring. In this study, we perform a mutational analysis to investigate the requirements for LOV2 to photocycle. We mutated all the residues that interact with the chromophore isoalloxazine ring to inert functional groups but none could fully inhibit the photocycle except those to the active-site cysteine. However, electronegative side chains in the vicinity of the chromophore accelerate the N5 deprotonation and the return to the dark state. Mutations to the N414 and Q513 residues identify a potential water gate and H₂O coordination sites. These residues affect the electronic nature of the chromophore and photocycle time by helping catalyze the N5 reduction leading to the completion of the photocycle. In addition, we demonstrate that dehydration leads to drastically slower photocycle times. Finally, to investigate the requirements of an active-site cysteine for photocycling, we moved the nearby cysteine to alternative locations and found that some variants can still photocycle. We propose a new model of the LOV domain photocycle that involves all of these components.

Photoactivation mechanisms of flavin-binding photoreceptors revealed through ultrafast spectroscopy and global analysis methods

Flavin-binding photoreceptor proteins use the isoalloxazine moiety of flavin cofactors to absorb light in the blue/UV-A wavelength region and subsequently translate it into biological information. The underlying photochemical reactions and protein structural dynamics are delicately tuned by the protein environment and represent fundamental reactions in biology and chemistry. Due to their photo-switchable nature, these proteins can be studied efficiently with laser-flash induced transient absorption and emission spectroscopy with temporal precision down to the femtosecond time domain. Here, we describe the application of both visible and mid-IR ultrafast transient absorption and time-resolved fluorescence methods in combination with sophisticated global analysis procedures to elucidate the photochemistry and signal transduction of BLUF (Blue light receptors using FAD) and LOV (Light oxygen voltage) photoreceptor domains.

Illuminating the early signaling pathway of a fungal light-oxygen-voltage photoreceptor

Circadian clocks are molecular timekeepers encountered in a wide variety of organisms, which allow to adapt the cell's metabolism and behavior to the daily and seasonal periods. Their function is regulated by light-sensing proteins, among which Vivid, a light-oxygen-voltage (LOV) sensitive domain of the fungus Neurospora crassa, constitutes one of the most prominent examples. Although the major photochemical and structural changes during the photocycle of this photosensor have been elucidated through experimental means, its signal transduction pathway is still poorly resolved at the molecular level. In this article, we show through molecular dynamics simulation that the primary steps after adduct formation involve a switch of Gln182 in vicinity of the chromophore FAD (flavin-adenine-dinucleotide), followed by a coupling between the Iβ- and Hβ-strands through H-bond formation between Gln182 and Asn161 as well as subsequent weakening of the H-bonding interaction between the Iβ- and Aβ-strands. These processes then induce a reorientation of the Aβ-Bβ-loop with respect to the protein core as well as a simultaneous contraction of the partially unfolded α-helix onto the α-Aβ-linker at the Ncap. Finally, we demonstrate through additional dimer simulations that the light-induced conformational changes, observed in the monomeric case, play a decisive role in controlling the dimerization tendency of Vivid with its partner domains and that the light-state homodimer shows a much larger affinity for aggregation than the dark state.

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.

Modulating LOV domain photodynamics with a residue alteration outside the chromophore binding site

Phototropins, a class of light-activated protein kinases, are essential for several blue light responses in plants and algae, including phototropism. These proteins contain two internal light, oxygen, and voltage sensitive (LOV) domains, which bind flavin chromophores and undergo a reversible photochemical formation of a cysteinyl-flavin adduct as part of the light sensing process. While the photodynamic properties of such photosensory domains are dictated by interactions between the chromophore and surrounding protein, more distant residues can play a significant role as well. Here we explore the role of the Phe434 residue in the photosensory response of the second LOV domain of Avena sativa phototropin 1 (AsLOV2), a model photochemical system for these LOV domains. Phe434 is more than 6 Å from the FMN chromophore in AsLOV2; nevertheless, an F434Y point mutation is likely to change several structural features of the chromophore binding site, as we demonstrate using molecular dynamics simulations. Transient absorption signals spanning 15 decades in time were compared for wild-type AsLOV2 and the F434Y mutant, showing that the latter has significantly altered photodynamics, including (i) a faster intersystem crossing leading to triplet formation on a nanosecond time scale, (ii) biphasic formation of adduct-state kinetics on the microsecond time scale, and (iii) greatly accelerated ground-state recovery kinetics on a second time scale. We present mechanistic models that link these spectroscopic differences to changes in the configuration of the critical cysteine residue and in the chromophore's accessibility to solvent and oxygen according to MD trajectories and purging experiments. Taken together, these results demonstrate the importance of residues outside the chromophore-binding pocket in modulating LOV domain photodynamics.

The primary photophysics of the Avena sativa phototropin 1 LOV2 domain observed with time-resolved emission spectroscopy

The phototropins are blue-light receptors that base their light-dependent action on the reversible formation of a covalent bond between a flavin mononucleotide (FMN) cofactor and a conserved cysteine in light, oxygen or voltage (LOV) domains. The primary reactions of the Avena sativa phototropin 1 LOV2 domain were investigated by means of time-resolved and low-temperature fluorescence spectroscopy. Synchroscan streak camera experiments revealed a fluorescence lifetime of 2.2 ns in LOV2. A weak long-lived component with emission intensity from 600 to 650 nm was assigned to phosphorescence from the reactive FMN triplet state. This observation allowed determination of the LOV2 triplet state energy level at physiological temperature at 16600 cm(-1). FMN dissolved in aqueous solution showed pH-dependent fluorescence lifetimes of 2.7 ns at pH 2 and 3.9-4.1 ns at pH 3-8. Here, too, a weak phosphorescence band was observed. The fluorescence quantum yield of LOV2 increased from 0.13 to 0.41 upon cooling the sample from 293 to 77 K. A pronounced phosphorescence emission around 600 nm was observed in the LOV2 domain between 77 and 120 K in the steady-state emission.

Photoreaction of plant and DASH cryptochromes probed by infrared spectroscopy: the neutral radical state of flavoproteins

Flavoprotein radicals are important intermediates in many biochemical processes. In the blue light sensor plant cryptochrome, the radical state acts as a signaling state. An isolation and assignment of infrared bands of flavin radicals in the most relevant spectral region of carbonyl stretches is missing because of their overlap with absorption of water and the protein moiety. In this study, the neutral radical state of flavoproteins was investigated by Fourier transform infrared difference spectroscopy. The light-induced conversion of oxidized to neutral radical state was monitored in a plant cryptochrome and that of radical to fully reduced state in a DASH cryptochrome. A pure difference spectrum of flavin radical minus oxidized state was obtained from a point mutant of a phototropin LOV (light-, oxygen-, or voltage-sensitive) domain. The analysis of the spectra revealed a correlation between the frequencies of carbonyl vibrations of the flavin radical state and those of its visible absorption. Plant cryptochrome shows a very low frequency of the carbonyl stretch in the radical state. It is postulated that the downshift is caused by the charge of an adjacent aspartate, which donated its proton to flavin N(5). Contributions from the protein moiety to the spectra were isolated for DASH and plant cryptochromes. As a conclusion, the photosensitive domain of plant cryptochromes shows changes in secondary structure upon illumination, which might be related to signaling.

Photophysical characterisation and photo-cycle dynamics of LOV1-His domain of phototropin from Chlamydomonas reinhardtii with roseoflavin monophosphate cofactor

The wild-type phototropin protein phot from the green alga Chlamydomonas reinhardtii with the blue-light photoreceptor domains LOV1 and LOV2 has flavin mononucleotide (FMN) as cofactor. For the LOV1-His domain from phot of C. reinhardtii studied here, the FMN chromophore was replaced by roseoflavin monophosphate (8-dimethylamino-8-demethyl-FMN, RoFMN) during heterologous expression in a riboflavin auxotropic Escherichia coli strain. An absorption and emission spectroscopic characterisation of the cofactor exchanged-LOV1-His (RoLOV1) domain was carried out in aqueous pH 8 phosphate buffer. The fluorescence of RoLOV1 is quenched by photo-induced charge transfer at room temperature. The photo-cyclic dynamics of RoLOV1 was observed by blue-light induced hypochromic and bathochromic absorption changes which recover on a minute timescale in the dark. Photo-excited RoFMN is thought to cause reversible protein and cofactor structural changes. Prolonged intense blue-light exposure caused photo-degradation of RoFMN in RoLOV1 to fully reduced flavin and lumichrome derivatives. Photo-cycle schemes of RoLOV1 and LOV1 are presented, and the photo-degradation dynamics of RoLOV1 is discussed.

Electronic and protein structural dynamics of a photosensory histidine kinase

The bacterium Caulobacter crescentus encodes a two-component signaling protein, LovK, that contains an N-terminal photosensory LOV domain coupled to a C-terminal histidine kinase. LovK binds a flavin cofactor, undergoes a reversible photocycle, and displays regulated ATPase and autophosphorylation activity in response to visible light. Femtosecond to nanosecond visible absorption spectroscopy demonstrates congruence between full-length LovK and isolated LOV domains in the mechanism and kinetics of light-dependent cysteinyl-C4(a) adduct formation and rupture, while steady-state absorption and fluorescence line narrowing (FLN) spectroscopies reveal unique features in the electronic structure of the LovK flavin cofactor. In agreement with other sensor histidine kinases, ATP binds specifically to LovK with micromolar affinity. However, ATP binding to the histidine kinase domain of LovK has no apparent effect on global protein structure as assessed by differential Fourier transform infrared (FTIR) spectroscopy. Cysteinyl adduct formation results in only minor changes in the structure of LovK as determined by differential FTIR. This study provides insight into the structural underpinnings of LOV-mediated signal transduction in the context of a full-length histidine kinase. In particular, the data provide evidence for a model in which small changes in the tertiary/quaternary structure of LovK, as triggered by photon detection in the N-terminal LOV sensory domain, are sufficient to regulate histidine kinase activity.

Influence of the LOV domain on low-lying excited states of flavin: a combined quantum-mechanics/molecular-mechanics investigation

The ground and low-lying excited states of flavin mononucleotide (FMN) in the light, oxygen, and voltage sensitive (LOV) domain of the blue-light photosensor YtvA of Bacillus subtilis were studied by means of combined quantum-mechanical/molecular-mechanical (QM/MM) methods. The FMN cofactor (without the side chain) was treated with density functional theory (DFT) for the geometry optimizations and a combination of DFT and multireference configuration interaction (MRCI) for the determination of the excitation energies, while the protein environment was represented by the CHARMM force field. In addition, several important amino acid side chains, including the reactive cysteine residue, were included in the QM region in order to probe their influence on the spectral properties of the cofactor in two protein conformations. Spin-orbit coupling was taken into account employing an efficient, nonempirical spin-orbit mean-field Hamiltonian. Our results reveal that the protein environment of YtvA-LOV induces spectral shifts for the (pi pi*) states that are similar to those in aqueous solution. In contrast, the blue shifts of the (n pi*) states are smaller in the protein environment, enabling a participation of these states in the decay processes of the optically bright S(1) state. Increased spin-orbit coupling between the initially populated S(1) state and the T(1) and T(2) states is found in YtvA-LOV as compared to free lumiflavine in water. The enhanced singlet-triplet coupling is brought about partially by configuration interaction with (n pi*) states at the slightly out-of-plane distorted minimum geometry. In addition, an external heavy-atom effect is observed when the sulfur atom of the nearby cysteine residue is included in the QM region, in line with experimental findings.

Mechanism-based tuning of a LOV domain photoreceptor

Phototropin-like LOV domains form a cysteinyl-flavin adduct in response to blue light but show considerable variation in output signal and the lifetime of the photo-adduct signaling state. Mechanistic studies of the slow-cycling fungal LOV photoreceptor Vivid (VVD) reveal the importance of reactive cysteine conformation, flavin electronic environment and solvent accessibility for adduct scission and thermal reversion. Proton inventory, pH effects, base catalysis and structural studies implicate flavin N(5) deprotonation as rate-determining for recovery. Substitutions of active site residues Ile74, Ile85, Met135 and Met165 alter photoadduct lifetimes by over four orders of magnitude in VVD, and similar changes in other LOV proteins show analogous effects. Adduct state decay rates also correlate with changes in conformational and oligomeric properties of the protein necessary for signaling. These findings link natural sequence variation of LOV domains to function and provide a means to design broadly reactive light-sensitive probes.

Conformational heterogeneity and propagation of structural changes in the LOV2/Jalpha domain from Avena sativa phototropin 1 as recorded by temperature-dependent FTIR spectroscopy

Phototropins control phototropism, chloroplast movement, stomatal opening, and leaf expansion in plants. Phototropin 1 (phot1) is composed of a kinase domain linked to two blue light-sensing domains, LOV2 and LOV1, which bind flavin mononucleotide. Disruption of the interaction between the LOV2 domain and a helical segment named Jalpha, joining LOV to the kinase domain, induces the subsequent kinase activity of phototropin 1 and further-downstream signal transduction. Here we study the effects of temperature and hydration on the light-triggered signal propagation in the phot1 LOV2 domain of Avena sativa (AsLOV2/Jalpha), using Fourier transform infrared spectroscopy to unravel part of the molecular mechanism of phototropin 1. We report that AsLOV2/Jalpha shows an intense signal in the amide I and II regions, arising mainly from beta-sheet changes and the unbinding of the Jalpha helix from the Per-ARNT-Sim core and its subsequent partial unfolding. Importantly, these structural changes only occur under conditions of full hydration and at temperatures above 280 K. We characterized a newly isolated low-hydration intermediate that shows a downshift of high-frequency amide I signals and that possibly corresponds to loop tightening, without large beta-sheet or Jalpha structural changes. In addition, we report a heterogeneity in AsLOV2/Jalpha involving two different C(4)=O conformer populations, coexisting in the dark state and characterized by C(4)=O carbonyl frequencies at 1712 cm(-1) and 1694 cm(-1) that are attributable to a single H-bond and two H-bonds at this site, respectively. Such conformers display slightly shifted absorption spectra and cause a splitting of the 475-nm band in the ultraviolet/visible spectra of LOV domains at low temperature.

Primary reactions of the LOV2 domain of phototropin studied with ultrafast mid-infrared spectroscopy and quantum chemistry

Phototropins, major blue-light receptors in plants, are sensitive to blue light through a pair of flavin mononucleotide (FMN)-binding light oxygen and voltage (LOV) domains, LOV1 and LOV2. LOV2 undergoes a photocycle involving light-driven covalent adduct formation between a conserved cysteine and the FMN C(4a) atom. Here, the primary reactions of Avena sativa phototropin 1 LOV2 (AsLOV2) were studied using ultrafast mid-infrared spectroscopy and quantum chemistry. The singlet excited state (S1) evolves into the triplet state (T1) with a lifetime of 1.5 ns at a yield of approximately 50%. The infrared signature of S1 is characterized by absorption bands at 1657 cm(-1), 1495-1415 cm(-1), and 1375 cm(-1). The T1 state shows infrared bands at 1657 cm(-1), 1645 cm(-1), 1491-1438 cm(-1), and 1390 cm(-1). For both electronic states, these bands are assigned principally to C=O, C=N, C-C, and C-N stretch modes. The overall downshifting of C=O and C=N bond stretch modes is consistent with an overall bond-order decrease of the conjugated isoalloxazine system upon a pi-pi* transition. The configuration interaction singles (CIS) method was used to calculate the vibrational spectra of the S1 and T1 excited pipi* states, as well as respective electronic energies, structural parameters, electronic dipole moments, and intrinsic force constants. The harmonic frequencies of S1 and T1, as calculated by the CIS method, are in satisfactory agreement with the evident band positions and intensities. On the other hand, CIS calculations of a T1 cation that was protonated at the N(5) site did not reproduce the experimental FMN T1 spectrum. We conclude that the FMN T1 state remains nonprotonated on a nanosecond timescale, which rules out an ionic mechanism for covalent adduct formation involving cysteine-N(5) proton transfer on this timescale. Finally, we observed a heterogeneous population of singly and doubly H-bonded FMN C(4)=O conformers in the dark state, with stretch frequencies at 1714 cm(-1) and 1694 cm(-1), respectively.