Phototropin LOV domains exhibit distinct roles in regulating photoreceptor function

Phototropins (phot1 and phot2) are autophosphorylating serine/threonine kinases that function as photoreceptors for phototropism, light-induced chloroplast movement, and stomatal opening in Arabidopsis. The N-terminal region of phot1 and phot2 contains two specialized PAS domains, designated LOV1 and LOV2, which function as binding sites for the chromophore flavin mononucleotide (FMN). Both LOV1 and LOV2 undergo a self-contained photocycle, which involves the formation of a covalent adduct between the FMN chromophore and a conserved active-site cysteine residue (Cys39). Replacement of Cys39 with alanine abolishes the light-induced photochemical reaction of LOV1 and LOV2. Here we have used the Cys39Ala mutation to investigate the role of LOV1 and LOV2 in regulating phototropin function. Photochemical analysis of a bacterially expressed LOV1 + LOV2 fusion protein indicates that LOV2 functions as the predominant light-sensing domain for phot1. LOV2 also plays a major role in mediating light-dependent autophosphorylation of full-length phot1 expressed in insect cells and transgenic Arabidopsis. Moreover, photochemically active LOV2 alone in full-length phot1 is sufficient to elicit hypocotyl phototropism in transgenic Arabidopsis, whereas photochemically active LOV1 alone is not. Further photochemical and biochemical analyses also indicate that the LOV1 and LOV2 domains of phot2 exhibit distinct roles. The significance for the different roles of the phototropin LOV domains is discussed.     

A LOV story: the signaling state of the phot1 LOV2 photocycle involves chromophore-triggered protein structure relaxation, as probed by far-UV time-resolved optical rotatory dispersion spectroscopy

Light-, oxygen-, or voltage-regulated (LOV1 and LOV2) domains bind flavin mononucleotide (FMN) and activate the phototropism photoreceptors phototropin 1 (phot1) and phototropin 2 (phot2) by using energy from absorbed blue light. Upon absorption of blue light, chromophore and protein conformational changes trigger the kinase domain for subsequent autophosphorylation and presumed downstream signal transduction. To date, the light-induced photocycle of the phot1 LOV2 protein is known to involve formation of a triplet flavin mononucleotide (FMN) chromophore followed by the appearance of a FMN adduct within 4 micros [Swartz, T. E., Corchnoy, S. B., Christie, J. M., Lewis, J. W., Szundi, I., Briggs, W. R., and Bogomolni, R. A. (2001) J. Biol. Chem. 276, 36493-36500] before thermal decay back to the dark state. To probe the mechanism by which the blue light information is relayed from the chromophore to the protein, nanosecond time-resolved optical rotatory dispersion (TRORD) spectroscopy, which is a direct probe of global secondary structure, was used to study the phot1 LOV2 protein in the far-UV region. These TRORD experiments reveal a previously unobserved intermediate species (tau approximately 90 micros) that is characterized by a FMN adduct chromophore and partially unfolded secondary structure (LOV390(S2)). This intermediate appears shortly after the formation of the FMN adduct. For LOV2, formation of a long-lived species that is ready to interact with a receptor domain for downstream signaling is much faster by comparison with formation of a similar species in other light-sensing proteins.     

Photochemical properties of the flavin mononucleotide-binding domains of the phototropins from Arabidopsis,rice, and Chlamydomonas reinhardtii

Phototropins (phot1 and phot2, formerly designated nph1 and npl1) are blue-light receptors that mediate phototropism, blue light-induced chloroplast relocation, and blue light-induced stomatal opening in Arabidopsis. Phototropins contain two light, oxygen, or voltage (LOV) domains at their N termini (LOV1 and LOV2), each a binding site for the chromophore flavin mononucleotide (FMN). Their C termini contain a serine/threonine protein kinase domain. Here, we examine the kinetic properties of the LOV domains of Arabidopsis phot1 and phot2, rice (Oryza sativa) phot1 and phot2, and Chlamydomonas reinhardtii phot. When expressed in Escherichia coli, purified LOV domains from all phototropins examined bind FMN tightly and undergo a self-contained photocycle, characterized by fluorescence and absorption changes induced by blue light (T. Sakai, T. Kagawa, M. Kasahara, T.E. Swartz, J.M. Christie, W.R. Briggs, M. Wada, K. Okada [2001] Proc Natl Acad Sci USA 98: 6969-6974; M. Salomon, J.M. Christie, E. Knieb, U. Lempert, W.R. Briggs [2000] Biochemistry 39: 9401-9410). The photocycle involves the light-induced formation of a cysteinyl adduct to the C(4a) carbon of the FMN chromophore, which subsequently breaks down in darkness. In each case, the relative quantum efficiencies for the photoreaction and the rate constants for dark recovery of LOV1, LOV2, and peptides containing both LOV domains are presented. Moreover, the data obtained from full-length Arabidopsis phot1 and phot2 expressed in insect cells closely resemble those obtained for the tandem LOV-domain fusion proteins expressed in E. coli. For both Arabidopsis and rice phototropins, the LOV domains of phot1 differ from those of phot2 in their reaction kinetic properties and relative quantum efficiencies. Thus, in addition to differing in amino acid sequence, the phototropins can be distinguished on the basis of the photochemical cycles of their LOV domains. The LOV domains of C. reinhardtii phot also undergo light-activated spectral changes consistent with cysteinyl adduct formation. Thus, the phototropin family extends over a wide evolutionary range from unicellular algae to higher plants.     

The photocycle of a flavin-binding domain of the blue light photoreceptor phototropin.

The plant blue light receptor, phot1, a member of the phototropin family, is a plasma membrane-associated flavoprotein that contains two ( approximately 110 amino acids) flavin-binding domains, LOV1 and LOV2, within its N terminus and a typical serine-threonine protein kinase domain at its C terminus. The LOV (light, oxygen, and voltage) domains belong to the PAS domain superfamily of sensor proteins. In response to blue light, phototropins undergo autophosphorylation. E. coli-expressed LOV domains bind riboflavin-5'-monophosphate, are photochemically active, and have major absorption peaks at 360 and 450 nm, with the 450 nm peak having vibronic structure at 425 and 475 nm. These spectral features correspond to the action spectrum for phototropism in higher plants. Blue light excitation of the LOV2 domain generates, in less than 30 ns, a transient approximately 660 nm-absorbing species that spectroscopically resembles a flavin triplet state. This putative triplet state subsequently decays with a 4-micros time constant into a 390 nm-absorbing metastable form. The LOV2 domain (450 nm) recovers spontaneously with half-times of approximately 50 s. It has been shown that the metastable species is likely a flavin-cysteine (Cys(39) thiol) adduct at the flavin C(4a) position. A LOV2C39A mutant generates the early photoproduct but not the adduct. Titrations of LOV2 using chromophore fluorescence as an indicator suggest that Cys(39) exists as a thiolate.     

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 ? 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.     

13C Isotopologue editing of FMN bound to phototropin domains

The plant blue light receptor phototropin comprises a protein kinase domain and two FMN-binding LOV domains (LOV1 and LOV2). Blue light irradiation of recombinant LOV domains is conducive to the addition of a cysteinyl thiolate group to carbon 4a of the FMN chromophore, and spontaneous cleavage of that photoadduct completes the photocycle of the receptor. The present study is based on (13)C NMR signal modulation observed after reconstitution of LOV domains of different origins with random libraries of (13)C-labeled FMN isotopologues. Using this approach, all (13)C signals of FMN bound to LOV1 and LOV2 domains of Avena sativa and to the LOV2 domain of the fern, Adiantum capillus-veneris, could be unequivocally assigned under dark and under blue light irradiation conditions. (13)C Chemical shifts of FMN are shown to be differently modulated by complexation with the LOV domains under study, indicating slight differences in the binding interactions of FMN and the apoproteins.     

The photochemistry of the light-, oxygen-, and voltage-sensitive domains in the algal blue light receptor phot

Phot proteins are blue light photoreceptors in plants and algae that mainly regulate photomovement responses. They contain two light-, oxygen-, and voltage-sensitive (LOV) domains and a serine/threonine kinase domain. Both LOV domains noncovalently bind a flavin mononucleotide (FMN) as chromophore. Upon blue light illumination, the LOV domains undergo a photocycle, transiently forming a covalent adduct of the FMN moiety with a nearby cysteine residue. The presence of two light-sensitive domains in the photoreceptor raises the question about the differences in properties and function between LOV1 and LOV2. As a model system, the photocycles of the LOV1 and LOV2 domains from phot of the green alga Chlamydomonas reinhardtii have been studied in detail, both separately and in a tandem construct. Here we give an overview about the results on the individual behavior of the domains and their interaction. Furthermore, the current status in the understanding of the role of LOV1 in phot in general is presented.     

LOV Takes a Pick: Thermodynamic and Structural Aspects of the Flavin-LOV-Interaction of the Blue-Light Sensitive Photoreceptor YtvA from Bacillus subtilis

LOV domains act as versatile photochromic switches servicing multiple effector domains in a variety of blue light sensing photoreceptors abundant in a multitude of organisms from all kingdoms of life. The perception of light is realized by a flavin chromophore that upon illumination reversibly switches from the non-covalently bound dark-state to a covalently linked flavin-LOV adduct. It is usually assumed that most LOV domains preferably bind FMN, but heterologous expression frequently results in the incorporation of all natural occurring flavins, i.e. riboflavin, FMN and FAD. Over recent years, the structures, photochemical properties, activation mechanisms and physiological functions of a multitude of LOV proteins have been studied intensively, but little is known about its affinities to physiologically relevant flavins or the thermodynamics of the flavin-LOV interaction. We have investigated the interaction of the LOV domain of the well characterized bacterial photoreceptor YtvA with riboflavin, FMN and FAD by ITC experiments providing binding constants and thermodynamic profiles of these interactions. For this purpose, we have developed a protocol for the production of the apo forms of YtvA and its isolated LOV domain and we demonstrate that the latter can be used as a molecular probe for free flavins in cell lysates. Furthermore, we show here using NMR spectroscopic techniques and Analytical Ultracentrifugation that the flavin moiety stabilizes the conformation of the LOV domain and that dimerization of YtvA is caused not only by intermolecular LOV-LOV but also by STAS-STAS contacts.     

Phototropins and Their LOV Domains：Versatile Plant Blue-Light Receptors

The phototropins phot1 and phot2 are plant blue-light receptors that mediate phototropism, chloroplast movements, stomatal opening, leaf expansion, the rapid Inhibition of hypocotyl growth in etiolated seedlings, and possibly solar tracking by leaves in those species in which It occurs. The phototroplns are plasma membrane-associated hydrophilic proteins with two chromophore domains （designated LOV1 and LOV2 for their resemblance to domains In other signaling proteins that detect light, oxygen, or voltage） in their Nterminal half and a classic serine/threonlne kinase domain in their C-terminal half. Both chromophore domains bind flavin mononucleotide （FMN） and both undergo light-activated formation of a covalent bond between a nearby cystelne and the C（4a） carbon of the FMN to form the signaling state. LOV2-cystelnyl adduct formation leads to the release downstream of a tightly bound amphlpathlc α-helix, a step required for activation of the klnase function. This cysteinyl adduct then slowly decays over a matter of seconds or minutes to return the photoreceptor chromophore modules to their ground state. Functional LOV2 is required for light-activated phosphorylation and for various blue-light responses mediated by the phototroplns. The function of LOV1 is still unknown, although It may serve to modulate the signal generated by LOV2. The LOV domain Is an ancient chromophore module found In a wide range of otherwise unrelated proteins In fungi and prokaryotes, the latter Including cyanobacterla, eubacterla, and archaea. Further general reviews on the phototropins are those by Celaya and Liscum （2005） and Christie and Briggs （2005）.     

Phototropin receptor kinase activation by blue light

Phototropins (phot1 and phot2) are blue light-activated serine/threonine protein kinases that function to mediate a variety of adaptive processes that serve to optimize the photosynthetic efficiency of plants and thereby promote their growth. Light sensing by the phototropins is mediated by a repeated motif located within the N-terminal region of the protein designated the LOV domain. Although phototropins possess two LOV photosensors (LOV1 and LOV2), recent biophysical and structure-function analyses clearly indicate that the LOV2 domain plays a predominant role in regulating phototropin kinase activity owing to specific protein changes that occur in response to LOV2 photoexcitation. In particular, the central β-sheet scaffold plays a role in propagating the photochemical signal generated from within LOV2 to protein changes at the surface that are necessary for kinase activation.Key words: phototropin, LOV domain, FMN, cysteinyl adduct, amphipathic helix, receptor autophosphoryation     

Mutational analysis of phototropin 1 provides insights into the mechanism underlying LOV2 signal transmission

Phototropins (phot1 and phot2) are blue light-activated serine/threonine protein kinases that elicit a variety of photoresponses in plants. Light sensing by the phototropins is mediated by two flavin mononucleotide (FMN)-binding domains, designated LOV1 and LOV2, located in the N-terminal region of the protein. Exposure to light results in the formation of a covalent adduct between the FMN chromophore and a conserved cysteine residue within the LOV domain. LOV2 photoexcitation is essential for phot1 function in Arabidopsis and is necessary to activate phot1 kinase activity through light-induced structural changes within a conserved alpha-helix situated C-terminal to LOV2. Here we have used site-directed mutagenesis to identify further amino acid residues that are important for phot1 activation by light. Mutagenesis of bacterially expressed LOV2 and full-length phot1 expressed in insect cells indicates that perturbation of the conserved salt bridge on the surface of LOV2 does not play a role in receptor activation. However, mutation of a conserved glutamine residue (Gln(575)) within LOV2, reported previously to be required to propagate structural changes at the LOV2 surface, attenuates light-induced autophosphorylation of phot1 expressed in insect cells without compromising FMN binding. These findings, in combination with double mutant analyses, indicate that Gln(575) plays an important role in coupling light-driven cysteinyl adduct formation from within LOV2 to structural changes at the LOV2 surface that lead to activation of the C-terminal kinase domain.     

A base-catalyzed mechanism for dark state recovery in the Avena sativa phototropin-1 LOV2 domain

Phototropins are autophosphorylating serine/threonine kinases responsible for blue-light perception in plants; their action gives rise to phototropism, chloroplast relocation, and opening of stomatal guard cells. The kinase domain constitutes the C-terminal part of Avena sativa phototropin 1. The N-terminal part contains two light, oxygen, or voltage (LOV) sensing domains, LOV1 and LOV2; each binds a flavin mononucleotide (FMN) chromophore (lambdamax = 447 nm, termed D447) and forms the light-sensitive domains, of which LOV2 is the principal component. Blue-light absorption produces a covalent adduct between a very conserved nearby cysteine residue and the C(4a) atom of the FMN moiety via the triplet state of the flavin. The covalent adduct thermally decays to regenerate the D447 dark state, with a rate that may vary by several orders of magnitude between different species. We report that the imidazole base can act as a very efficient enhancer of the dark recovery of A. sativa phot1 LOV2 (AsLOV2) and some other well-characterized LOV domains. Imidazole accelerates the thermal decay of AsLOV2 by 3 orders of magnitude in the submolar concentration range, via a base-catalyzed mechanism involving base abstraction of the FMN N(5)-H adduct state and subsequent reprotonation of the reactive cysteine. The LOV2 crystal structure suggests that the imidazole molecules may act from a cavity located in the vicinity of the FMN, explaining its high efficiency, populated through a channel connecting the cavity to the protein surface. Use of pH titration and chemical inactivation by diethyl pyrocarbonate (DEPC) suggests that histidines located at the surface of the LOV domain act as base catalysts via an as yet unidentified H-bond network, operating at a rate of (55 s)-1 at pH 8. In addition, molecular processes other than histidine-mediated base catalysis contibute significantly to the total thermal decay rate of the adduct and operate at a rate constant of (65 s)-1, leading to a net adduct decay time constant of 30 s at pH 8.     

Photoexcited structure of a plant photoreceptor domain reveals a light-driven molecular switch

The phototropins are flavoprotein kinases that control phototropic bending, light-induced chloroplast movement, and stomatal opening in plants. Two flavin mononucleotide binding light, oxygen, or voltage (LOV) domains are the sites for initial photochemistry in these blue light photoreceptors. We have determined the steady state, photoexcited crystal structure of a flavin-bound LOV domain. The structure reveals a unique photochemical switch in the flavin binding pocket in which the absorption of light drives the formation of a reversible covalent bond between a highly conserved Cys residue and the flavin cofactor. This provides a molecular picture of a cysteinyl-flavin covalent adduct, the presumed signaling species that leads to phototropin kinase activation and subsequent signal transduction. We identify closely related LOV domains in two eubacterial proteins that suggests the light-induced conformational change evident in this structure is an ancient biomolecular response to light, arising before the appearance of plants.     

Structural tuning of the fluorescent protein iLOV for improved photostability

Fluorescent proteins derived from light, oxygen, or voltage (LOV) domains offer advantages over green fluorescent protein (GFP) from their small size and efficacy under anaerobic conditions. The flavoprotein improved LOV (iLOV) was engineered from the blue light receptor phototropin as a reporter of viral infection. To inform the molecular basis for the improved, photoreversible, fluorescent properties of iLOV, we employed directed evolution and determined five LOV crystallographic structures. Comparative structural analyses between iLOV and its progenitors reveal mutation-induced constraints in the environment of the flavin mononucleotide (FMN) chromophore; in iLOV, the methyl group of Thr-394 "crowds" the FMN isoalloxazine ring, Leu-470 triggers side chain "flipping" of Leu-472, and the terminal FMN phosphate shows increased anchoring. We further engineered iLOV variants that are readily detectable in bacterial and mammalian cells due to order-of-magnitude photostability increases. Structure determination of a resulting representative photostable iLOV (phiLOV) variant reveals additional constraints on the chromophore. Aromatic residues Tyr-401 and Phe-485 in phiLOV sandwich the FMN isoalloxazine ring from both sides, whereas Ser-390 anchors the side chain of FMN-interacting Gln-489 Our combined structural and mutational results reveal that constraining the FMN fluorophore yields improved photochemical properties for iLOV and its new photostable derivative. These findings provide a framework for structural fine-tuning of LOV scaffold proteins to maximize their potential as oxygen-independent fluorescent reporters.     

Heterogeneous environment of the S-H group of Cys966 near the flavin chromophore in the LOV2 domain of Adiantum neochrome1

The primary photochemistry of the blue-light sensor protein, phototropin, is adduct formation between the C4a atom of the flavin mononucleotide (FMN) chromophore and a nearby, reactive cysteine (Cys966), following decay of the triplet excited state of FMN. The distance between the C4a position of FMN and the sulfur atom of Cys966 is 4.2 A in the LOV2 domain of Adiantum neochrome 1 (neo1-LOV2), a fusion protein of phototropin containing the phytochrome chromophoric domain. We previously reported the presence of an unreactive fraction in neo1-LOV2 at low temperatures, which presumably originated from the heterogeneous environment of Cys966 [Iwata, T., Nozaki, D., Tokutomi, S., Kagawa, T., Wada, M., and Kandori, H. (2003) Biochemistry 42, 8183-8191]. The present study showed that (i) 28% forms an adduct at 77 K (state I), (ii) 50% forms an adduct at 150 K but not at 77 K (state II), and (iii) 22% does not form an adduct at 150 K (state III). By Fourier transform infrared (FTIR) spectroscopy, we observed the S-H stretching frequencies at 2570 and 2562 cm-1 for state I and at 2563 cm-1 for state II, suggesting that the microenvironment of the S-H group of Cys966 determines the reactivity at low temperatures. Adduct formation is more efficient for state I than for states II and III. Molecular dynamics simulation strongly suggests that the observed multiple structures originate from the isomeric forms of Cys966. We thus concluded that there are multiple local structures of FMN and cysteine in neo1-LOV2, each of which is thermally converted by protein fluctuation at physiological temperatures.     

Mapping of low- and high-fluence autophosphorylation sites in phototropin 1

Phototropins, originally detected by their blue light-dependent autophosphorylation, are plant photoreceptors involved in several blue light responses such as phototropism, chloroplast relocation, leaf expansion, rapid inhibition of hypocotyl growth, and stomatal opening. Three domains have been identified in phototropin sequences, two chromophore binding domains (LOV1 and LOV2) and a kinase domain. We describe here two additional domains, the N-terminus upstream of LOV1 and the hinge region between LOV1 and LOV2, as the regions for autophosphorylation; the phosphorylation sites were identified by site-directed mutagenesis as S27, S30, S274, S300, S317, S325, S332, and S349 of the PHOT1a sequence of Avena sativa. Investigation of the autophosphorylation in vivo revealed that serines close to the LOV1 domain are phosphorylated at lower fluence of blue light than the serines close to the LOV2 domain. Recovery of phosphorylation in vivo during a dark period after saturating irradiation is caused by dephosphorylation rather than by degradation of the phosphorylated form and new synthesis of nonphosphorylated phototropin. The results were obtained by a combination of autophosphorylation of phototropin with phosphorylation of recombinant domains by protein kinase A, which turned out to have the same site specificity as the phototropin kinase, followed by proteolysis and separation of phosphopeptides. With the knowledge of the phosphorylation sites, the physiological and biochemical consequences of autophosphorylation can now be approached by site-directed mutagenesis of phototropins.     

The phototropin family as photoreceptors for blue light-induced chloroplast relocation

Blue light-induced chloroplast accumulation and avoidance relocation movements are controlled by the blue light photoreceptor phototropin. The Arabidopsis thaliana genome has two phototropin genes encoding phot1 and phot2. Each of these photoreceptors contains two LOV (light oxygen and voltage) domains and a kinase domain. The LOV domains absorb blue light though an associated flavin mononucleotide chromophore, while the kinase domain is thought to be associated with signal transduction. The phototropins control not only chloroplast relocation movement, but also blue light-induced phototropic responses, leaf expansion and stomatal opening. Here I review the role of phototropin as a photoreceptor for chloroplast photorelocation movement. Electronic Publication     

FMN Binding and Photochemical Properties of Plant Putative Photoreceptors Containing Two LOV Domains,LOV/LOV Proteins

LOV domains function as blue light-sensing modules in various photoreceptors in plants, fungi, algae, and bacteria. A LOV/LOV protein (LLP) has been found from Arabidopsis thaliana (AtLLP) as a two LOV domain-containing protein. However, its function remains unknown. We isolated cDNA clones coding for an LLP homolog from tomato (Solanum lycopersicum) and two homologs from the moss Physcomitrella patens. The tomato LLP (SlLLP) contains two LOV domains (LOV1 and LOV2 domains), as in AtLLP. Most of the amino acids required for association with chromophore are conserved in both LOV domains, except that the amino acid at the position equivalent to the cysteine essential for cysteinyl adduct formation is glycine in the LOV1 domain as in AtLLP. When expressed in Escherichia coli, SlLLP binds FMN and undergoes a self-contained photocycle upon irradiation of blue light. Analyses using mutant SlLLPs revealed that SlLLP binds FMN in both LOV domains, although the LOV1 domain does not show spectral changes on irradiation. However, when Gly⁶⁶ in the LOV1 domain, which is located at the position equivalent to the essential cysteine of LOV domains, is replaced by cysteine, the mutated LOV1 domain shows light-induced spectral changes. In addition, all four LOV domains of P. patens LLPs (PpLLP1 and PpLLP2) show the typical features of LOV domains, including the reactive cysteine in each. This study shows that plants have a new LOV domain-containing protein family with the typical biochemical and photochemical properties of other LOV domain-containing proteins such as the phototropins.