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PubMed2001

Light-induced structural changes occur in the transmembrane helices of the Natronobacterium pharaonis HtrII transducer.

The Natronobacterium pharaonis HtrII (NpHtrII) transducer interacts with its cognate photoactive sensory rhodopsin receptor, NpSRII, to mediate phototaxis responses. NpHtrII is predicted to have two transmembrane helices and a large cytoplasmic domain and to form a homodimer. Single cysteines were substituted into an engineered cysteine-less NpHtrII at 38 positions in its transmembrane domain. Oxidative disulfide cross-linking efficiencies of the monocysteine mutants were measured with or without photoactivation of NpSRII. The rapid cross-linking rates at several positions support that NpHtrII is a dimer when functionally expressed in the Halobacterium salinarum membrane. Thirteen positions in the second transmembrane segment (TM2) exhibited significant light-induced increases in cross-linking efficiency, and they define a single face traversing the length of the segment when modeled as an alpha-helix. Four positions in this helix showing light-induced decreases in efficiency are clustered on the cytoplasmic side of the protein. One of the monocysteine mutants, G83C, showed loss of phototaxis responses, and analysis of double mutants showed that the G83C mutation alters the dark structure of the TM2-TM2' region of NpHtrII. In summary, the results reveal conformationally active regions in the second transmembrane segment of NpHtrII and a face along the length of TM2 that becomes more available for TM2-TM2' cross-linking upon receptor photoactivation. The data also establish that one residue in TM2, Gly83, is critical for maintaining the proper conformation of NpHtrII for signal relay from the photoactivated receptor to the kinase-binding region of the transducer.

Amino Acid Sequence↗
PubMed1999

Transducer-binding and transducer-mutations modulate photoactive-site-deprotonation in sensory rhodopsin I.

Sensory rhodopsin I (SRI) is a seven-transmembrane helix retinylidene protein that mediates color-sensitive phototaxis responses through its bound transducer HtrI in the archaeon Halobacterium salinarum. Deprotonation of the Schiff base attachment site of the chromophore accompanies formation of the SRI signaling state, S(373). We measured the rate of laser flash-induced S(373) formation in the presence and absence of HtrI, and the effects of mutations in SRI or HtrI on the kinetics of this process. In the absence of HtrI, deprotonation occurs rapidly (halftime 10 micros) if the proton acceptor Asp76 is ionized (pK(a) = approximately 7), and only very slowly (halftime > 10 ms) when Asp76 is protonated. Transducer-binding, although it increases the pK(a) of Asp76 so that it is protonated throughout the range of pH studied, results in a first order, pH-independent rate of S(373) formation of approximately 300 micros. Therefore, the complexation of HtrI facilitates the proton-transfer reaction, increasing the rate approximately 50-fold at pH6. Arrhenius analysis shows that HtrI-binding accelerates the reaction primarily by an entropic effect, suggesting HtrI constrains the SRI molecule in the complex. Function-perturbing mutations in SRI and HtrI also alter the rate of S(373) formation and the lambda(max) of the parent state as assessed by laser flash-induced kinetic difference spectroscopy, and shifts to longer wavelength are correlated with slower deprotonation. The data indicate that HtrI affects electrostatic interactions of the protonated Schiff base and not only receives the signal from SRI but also optimizes the photochemical reaction process for SRI signaling.

Archaeal Proteins↗
PubMed2002

Circadian phototransduction and the regulation of biological rhythms.

The vertebrate circadian system that controls most biological rhythms is composed of multiple oscillators with varied hierarchies and complex levels of organization and interaction. The retina plays a key role in the regulation of daily rhythms and light is the main synchronizer of the circadian system. To date, the identity of photoreceptors/photopigments responsible for the entrainment of biological rhythms is still uncertain; however, it is known that phototransduction must occur in the eye because light entrainment is lost with eye removal. The retina is also rhythmic in physiological and metabolic activities as well as in gene expression. Retinal oscillators may act like clocks to induce changes in the visual system according to the phase of the day by predicting environmental changes. These oscillatory and photoreceptive capacities are likely to converge all together on selected retinal cells. The aim of this overview is to present the current knowledge of retinal physiology in relation to the circadian timing system.

Animals↗
PubMed1998

Structure at 0.85 A resolution of an early protein photocycle intermediate.

Protein photosensors from all kingdoms of life use bound organic molecules, known as chromophores, to detect light. A specific double bond within each chromophore is isomerized by light, triggering slower changes in the protein as a whole. The initial movements of the chromophore, which can occur in femtoseconds, are tightly constrained by the surrounding protein, making it difficult to see how isomerization can occur, be recognized, and be appropriately converted into a protein-wide structural change and biological signal. Here we report how this dilemma is resolved in the photoactive yellow protein (PYP). We trapped a key early intermediate in the light cycle of PYP at temperatures below -100 degrees C, and determined its structure at better than 1 A resolution. The 4-hydroxycinnamoyl chromophore isomerizes by flipping its thioester linkage with the protein, thus avoiding collisions resulting from large-scale movement of its aromatic ring during the initial light reaction. A protein-to-chromophore hydrogen bond that is present in both the preceding dark state and the subsequent signalling state of the photosensor breaks, forcing one of the hydrogen-bonding partners into a hydrophobic pocket. The isomerized bond is distorted into a conformation resembling that in the transition state. The resultant stored energy is used to drive the PYP light cycle. These results suggest a model for phototransduction, with implications for bacteriorhodopsin, photoactive proteins, PAS domains, and signalling proteins.

Bacteria↗
PubMed2005

Addition of human melanopsin renders mammalian cells photoresponsive.

A small number of mammalian retinal ganglion cells act as photoreceptors for regulating certain non-image forming photoresponses. These intrinsically photosensitive retinal ganglion cells express the putative photopigment melanopsin. Ablation of the melanopsin gene renders these cells insensitive to light; however, the precise role of melanopsin in supporting cellular photosensitivity is unconfirmed. Here we show that heterologous expression of human melanopsin in a mouse paraneuronal cell line (Neuro-2a) is sufficient to render these cells photoreceptive. Under such conditions, melanopsin acts as a sensory photopigment, coupled to a native ion channel via a G-protein signalling cascade, to drive physiological light detection. The melanopsin photoresponse relies on the presence of cis-isoforms of retinaldehyde and is selectively sensitive to short-wavelength light. We also present evidence to show that melanopsin functions as a bistable pigment in this system, having an intrinsic photoisomerase regeneration function that is chromatically shifted to longer wavelengths.

Animals↗
PubMed2005

Induction of photosensitivity by heterologous expression of melanopsin.

Melanopsin has been proposed to be the photopigment of the intrinsically photosensitive retinal ganglion cells (ipRGCs); these photoreceptors of the mammalian eye drive circadian and pupillary adjustments through direct projections to the brain. Their action spectrum (lambda(max) approximately 480 nm) implicates an opsin and melanopsin is the only opsin known to exist in these cells. Melanopsin is required for ipRGC photosensitivity and for behavioural photoresponses that survive disrupted rod and cone function. Heterologously expressed melanopsin apparently binds retinaldehyde and mediates photic activation of G proteins. However, its amino-acid sequence differs from vertebrate photosensory opsins and some have suggested that melanopsin may be a photoisomerase, providing retinoid chromophore to an unidentified opsin. To determine whether melanopsin is a functional sensory photopigment, here we transiently expressed it in HEK293 cells that stably expressed TRPC3 channels. Light triggered a membrane depolarization in these cells and increased intracellular calcium. The light response resembled that of ipRGCs, with almost identical spectral sensitivity (lambda(max) approximately 479 nm). The phototransduction pathway included Gq or a related G protein, phospholipase C and TRPC3 channels. We conclude that mammalian melanopsin is a functional sensory photopigment, that it is the photopigment of ganglion-cell photoreceptors, and that these photoreceptors may use an invertebrate-like phototransduction cascade.

Animals↗
PubMed2005

Melanopsin-expressing ganglion cells in primate retina signal colour and irradiance and project to the LGN.

Human vision starts with the activation of rod photoreceptors in dim light and short (S)-, medium (M)-, and long (L)- wavelength-sensitive cone photoreceptors in daylight. Recently a parallel, non-rod, non-cone photoreceptive pathway, arising from a population of retinal ganglion cells, was discovered in nocturnal rodents. These ganglion cells express the putative photopigment melanopsin and by signalling gross changes in light intensity serve the subconscious, 'non-image-forming' functions of circadian photoentrainment and pupil constriction. Here we show an anatomically distinct population of 'giant', melanopsin-expressing ganglion cells in the primate retina that, in addition to being intrinsically photosensitive, are strongly activated by rods and cones, and display a rare, S-Off, (L + M)-On type of colour-opponent receptive field. The intrinsic, rod and (L + M) cone-derived light responses combine in these giant cells to signal irradiance over the full dynamic range of human vision. In accordance with cone-based colour opponency, the giant cells project to the lateral geniculate nucleus, the thalamic relay to primary visual cortex. Thus, in the diurnal trichromatic primate, 'non-image-forming' and conventional 'image-forming' retinal pathways are merged, and the melanopsin-based signal might contribute to conscious visual perception.

Animals↗
PubMed2005

A light-sensing knot revealed by the structure of the chromophore-binding domain of phytochrome.

Phytochromes are red/far-red light photoreceptors that direct photosensory responses across the bacterial, fungal and plant kingdoms. These include photosynthetic potential and pigmentation in bacteria as well as chloroplast development and photomorphogenesis in plants. Phytochromes consist of an amino-terminal region that covalently binds a single bilin chromophore, followed by a carboxy-terminal dimerization domain that often transmits the light signal through a histidine kinase relay. Here we describe the three-dimensional structure of the chromophore-binding domain of Deinococcus radiodurans phytochrome assembled with its chromophore biliverdin in the Pr ground state. Our model, refined to 2.5 A resolution, reaffirms Cys 24 as the chromophore attachment site, locates key amino acids that form a solvent-shielded bilin-binding pocket, and reveals an unusually formed deep trefoil knot that stabilizes this region. The structure provides the first three-dimensional glimpse into the photochromic behaviour of these photoreceptors and helps to explain the evolution of higher plant phytochromes from prokaryotic precursors.

Bile Pigments↗
PubMed2001

Melanopsin in cells of origin of the retinohypothalamic tract.

All known eukaryotic organisms exhibit physiological and behavioral rhythms termed circadian rhythms that cycle with a near-24-hour period; in mammals, light is the most potent stimulus for entraining endogenous rhythms to the daily light cycle. Photic information is transmitted via the retinohypothalamic tract (RHT) to the suprachiasmatic nucleus (SCN) in the hypothalamus, where circadian rhythms are generated, but the retinal photopigment that mediates circadian entrainment has remained elusive. Here we show that most retinal ganglion cells (RGCs) that project to the SCN express the photopigment melanopsin.

Animals↗
PubMed1998

Sensing photons.

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Bacterial Proteins↗
PubMed2003

Genetic, temporal and developmental differences between melatonin rhythm generating systems in the teleost fish pineal organ and retina.

Complete melatonin rhythm generating systems, including photodetector, circadian clock and melatonin synthesis machinery, are located within individual photoreceptor cells in two sites in Teleost fish: the pineal organ and retina. In both, light regulates daily variations in melatonin secretion by controlling the activity of arylalkylamine N-acetyltransferase (AANAT). However, in each species examined to date, marked differences exist between the two organs which may involve the genes encoding the photopigments, genes encoding AANAT, the times of day at which AANAT activity and melatonin production peak and the developmental schedule. We review the fish pineal and retinal melatonin rhythm generating systems and consider the evolutional pressures and other factors which led to these differences.

Animals↗
PubMed2003

Encoding time of day and time of year by the avian circadian system.

The most important zeitgeber for seasonal rhythmicity of physiology and behaviour in birds is the annual cycle of photoperiod. Regulatory mechanisms are less well understood in birds than in mammals since photic information can be perceived by photoreceptors in the retina and the pineal gland, as well as in the brain, and photoperiodic time measurement might be performed with reference to at least three autonomous circadian systems, the retina, the pineal gland and a hypothalamic oscillator. In many bird species, the pineal melatonin rhythm plays a central role in circadian organization. Durations of elevated melatonin in the blood reflect night length when animals are kept under natural photoperiodic conditions, as well as under different light/dark schedules in the laboratory. In the house sparrow, time of year is encoded in a particular melatonin signal, being short in duration and high in amplitude in long photoperiods and being long in duration and low in amplitude in short photoperiods, independent of whether the light zeitgeber is natural or artificial or varies in strength. Specific features of the melatonin signal are retained in vivo as well as in vitro when birds or isolated pineal glands are transferred to constant conditions. To regulate daily and seasonal changes of behaviour and physiology, melatonin may act at various target sites, including a complex hypothalamic oscillator that, unlike that in mammals, is not confined to a single cell group in the house sparrow. There is increasing evidence that interactions between two or more components of the songbird circadian pacemaking system are essential to encode and store biologically meaningful information about time, and thus provide the basis for photoperiodic time measurements and after effects in birds.

Animals↗
PubMed2001

Unexpected roles for cryptochrome 2 and phototropin revealed by high-resolution analysis of blue light-mediated hypocotyl growth inhibition.

Blue light (BL) rapidly and strongly inhibits hypocotyl elongation during the photomorphogenic response known as de-etiolation, the transformation of a dark-grown seedling into a pigmented, photoautotrophic organism. In Arabidopsis thaliana, high-resolution studies of hypocotyl growth accomplished by computer-assisted electronic image capture and analysis revealed that inhibition occurs in two genetically independent phases, the first beginning within 30 sec of illumination. The present work demonstrates that phototropin (nph1), the photoreceptor responsible for phototropism, is largely responsible for the initial, rapid inhibition. Signaling from phototropin during the curvature response is dependent upon interaction with NPH3, but the results presented here demonstrate that NPH3 is not necessary for phototropin-dependent growth inhibition. Activation of anion channels, which transiently depolarizes the plasma membrane within seconds of BL, is an early event in the cryptochrome signaling pathway leading to a phase of growth inhibition that replaces the transient phototropin-dependent phase after approximately 30 min of BL. Surprisingly, cry1 and cry2 were found to contribute equally and non-redundantly to anion-channel activation and to growth inhibition between 30 and 120 min of BL. Inspection of the inhibition kinetics displayed by nph1 and nph1cry1 mutants revealed that the cryptochrome phase of inhibition is delayed in seedlings lacking phototropin. This result indicates that BL-activation of phototropin influences cryptochrome signaling leading to growth inhibition. Mutations in the NPQ1 gene, which inhibit BL-induced stomatal opening, do not affect any aspect of the growth inhibition within the first 120 min examined here, and NPQ1 does not affect the activation of anion channels.

Arabidopsis↗
PubMed2002

Gradients in the circadian expression of Per1 and Per2 genes in the rat suprachiasmatic nucleus.

The suprachiasmatic nucleus (SCN) is the mammalian circadian pacemaker, which consists of thousands of oscillator cells. It is believed that the circadian oscillation in each cell is generated by the transcription/(post)-translation feedback loop of a set of clock genes. However, little is known about how these oscillator cells are organized to produce the robust circadian rhythms in the SCN. In the present study, we examined the expression of the clock genes Per1 and Per2 paying particular attention to the topographic compartmentalization of the SCN. In the rat SCN, the dorsomedial (SCNDM) and ventrolateral (SCNVL) compartments are clearly delineated by chemical characteristics of neurons and neuronal afferents. In the SCNDM, Per1 mRNA was initially expressed at the most dorsomedial region along the third ventricle (SCNDMPV, periventricular part of the dorsomedial compartment of the SCN) at CT0, and then spread laterally to the central dorsomedial region (SCNDMCe, central part of the dorsomedial compartment of the SCN), reaching peaks at subjective day and troughs at subjective night. In contrast, in the SCNVL, Per1 expression showed a weak, two-peak pattern in one circadian cycle. Per2 expression was also robust in the SCN, showing very similar circadian profiles among these three subdivisions with a slightly earlier phase in SCNDMPV than that in SCNDMCe. We also investigated the Per1 and Per2 expression in response to a light exposure at early subjective night. The light pulse induced both Per1 and Per2 expression, which was restricted in the SCNVL neurons. The present findings suggest that the phase and amplitude of the circadian expression of clock genes are not uniform, and there are topographic neuron groups that have different properties in the SCN.

Animals↗