Endoplasmic reticulum degradation impedes olfactory G-protein coupled receptor functional expression

BACKGROUND

Research on olfactory G-protein coupled receptors (GPCRs) has been severely impeded by poor functional expression in heterologous systems. Previously, we demonstrated that inefficient olfactory receptor (OR) expression at the plasma membrane is attributable, in part, to degradation of endoplasmic reticulum (ER)-retained ORs by the ubiquitin-proteasome system and sequestration of ORs in ER aggregates that are degraded by autophagy. Thus, experiments were performed to test the hypothesis that attenuation of ER degradation improves OR functional expression in heterologous cells.

RESULTS

To develop means to increase the functional expression of ORs, we devised an approach to measure activation of the mOREG OR (Unigene # Mm.196680; Olfr73) through coupling to an olfactory cyclic nucleotide-gated cation channel (CNG). This system, which utilizes signal transduction machinery coupled to OR activation in native olfactory sensory neurons, was used to demonstrate that degradation, both by the ubiquitin-proteasome system and autophagy, limits mOREG functional expression. The stimulatory effects of proteasome and autophagy inhibitors on mOREG function required export from the ER and trafficking through the biosynthetic pathway.

CONCLUSIONS

These findings demonstrate that poor functional expression of mOREG in heterologous cells is improved by blocking proteolysis. Inhibition of ER degradation may improve the function of other ORs and assist future efforts to elucidate the molecular basis of odor discrimination.

The interplay o light and heat in bleaching rhodopsin

Rhodopsin, the pigment of the retinal rods, can be bleached either by light or by high temperature. Earlier work had shown that when white light is used the bleaching rate does not depend on temperature, and so must be independent of the internal energy of the molecule. On the other hand thermal bleaching in the dark has a high temperature dependence from which one can calculate that the reaction has an apparent activation energy of 44 kg. cal. per mole. It has now been shown that the bleaching rate of rhodopsin becomes temperature-dependent in red light, indicating that light and heat cooperate in activating the molecule. Apparently thermal energy is needed for bleaching at long wave lengths where the quanta are not sufficiently energy-rich to bring about bleaching by themselves. The temperature dependence appears at 590 mµ. This is the longest wave length at which bleaching by light proceeds without thermal activation, and corresponds to a quantum energy of 48.5 kg. cal. per mole. This value of the minimum energy to bleach rhodopsin by light alone is in agreement with the activation energy of thermal bleaching in the dark. At wave lengths between 590 and 750 mµ, the longest wave length at which the bleaching rate was fast enough to study, the sum of the quantum energy and of the activation energy calculated from the temperature coefficients remains between 44 and 48.5 kg. cal. This result shows that in red light the energy deficit of the quanta can be made up by a contribution of thermal energy from the internal degrees of freedom of the rhodopsin molecule. The absorption spectrum of rhodopsin, which is not markedly temperature-dependent at shorter wave lengths, also becomes temperature-dependent in red light of wave lengths longer than about 570 to 590 mµ. The temperature dependence of the bleaching rate is at least partly accounted for by the temperature coefficient of absorption. There is some evidence that the temperature coefficient of bleaching is somewhat greater than the temperature coefficient of absorption at wave lengths longer than 590 mmicro;. This means that the thermal energy of the molecule is a more critical factor in bleaching than in absorption. It shows that some of the molecules which absorb energy-deficient quanta of red light are unable to supply the thermal component of the activation energy needed for bleaching, so bringing about a fall in the quantum efficiency. The experiments show that there is a gradual transition between the activation of rhodopsin by light and the activation by internal energy. It is suggested that energy can move freely between the prosthetic group and the protein moiety of the molecule. In this way a part of the large amount of energy in the internal degrees of freedom of rhodopsin could become available to assist in thermal activation. Assuming that the minimum energy required for bleaching is 48.5 kg. cal., an equation familiar in the study of unimolecular reaction has been used to estimate the number of internal degrees of freedom, n, involved in supplying the thermal component of the activation energy when rhodopsin is bleached in red light. It was found that n increases from 2 at 590 mµ to a minimum value of 15 at 750 mµ. One wonders what value n has at 1050 mµ, where vision still persists, and where rhodopsin molecules may supply some 16 kg. cal. of thermal energy per mole in order to make up for the energy deficit of the quanta.

The role of sulfhydryl groups in the bleaching and synthesis of rhodopsin

The condensation of retinene₁ with opsin to form rhodopsin is optimal at pH about 6, a pH which favors the condensation of retinene₁ with sulfhydryl rather than with amino groups. The synthesis of rhodopsin, though unaffected by the less powerful sulfhydryl reagents, monoiodoacetic acid and its amide, is inhibited completely by p-chloromercuribenzoate (PCMB). This inhibition is reversed in part by the addition of glutathione. PCMB does not attack rhodopsin itself, nor does it react with retinene₁. Its action in this system is confined to the —SH groups of opsin. Under some conditions the synthesis of rhodopsin is aided by the presence of such a sulfhydryl compound as glutathione, which helps to keep the —SH groups of opsin free and reduced. By means of the amperometric silver titration of Kolthoff and Harris, it is shown that sulfhydryl groups are liberated in the bleaching of rhodopsin, two such groups for each retinene₁ molecule that appears. This is true equally of rhodopsin from the retinas of cattle, frogs) and squid. The exposure of new sulfhydryl groups adds an important element to the growing evidence that relates the bleaching of rhodopsin to protein denaturation. The place of sulfhydryl groups in the structure of rhodopsin is still uncertain. They may be concerned directly in binding the chromophore to opsin; or alternatively they may furnish hydrogen atoms for some reductive change by which the chromophore is formed from retinene₁. In the amperometric silver titration, the bleaching of rhodopsin yields directly an electrical variation. This phenomenon may have some fundamental connection with the role of rhodopsin in visual excitation, and may provide a model of the excitation process in general.

Iodopsin

The iodopsin system found in the cones of the chicken retina is identical with the rhodopsin system in its carotenoids. It differs only in the protein-the opsin -with which carotenoid combines. The cone protein may be called photopsin to distinguish it from the scotopsins of the rods. Iodopsin bleaches in the light to a mixture of photopsin and all-trans retinene. The latter is reduced by alcohol dehydrogenase and cozymase to all-trans vitamin A(1). Iodopsin is resynthesized from photopsin and a cis isomer of vitamin A, neovitamin Ab or the corresponding neoretinene b, the same isomer that forms rhodopsin. The synthesis of iodopsin from photopsin and neoretinene b is a spontaneous reaction. A second cis retinene, isoretinene a, forms iso-iodopsin (lambda(max) 510 mmicro). The bleaching of iodopsin in moderate light is a first-order reaction (Bliss). The synthesis of iodopsin from neoretinene b and opsin is second-order, like that of rhodopsin, but is very much more rapid. At 10 degrees C. the velocity constant for iodopsin synthesis is 527 times that for rhodopsin synthesis. Whereas rhodopsin is reasonably stable in solution from pH 4-9, iodopsin is stable only at pH 5-7, and decays rapidly at more acid or alkaline reactions. The sulfhydryl poison, p-chloromercuribenzoate, blocks the synthesis of iodopsin, as of rhodopsin. It also bleaches iodopsin in concentrations which do not attack rhodopsin. Hydroxylamine also bleaches iodopsin, yet does not poison its synthesis. Hydroxylamine acts by competing with the opsins for retinene. It competes successfully with chicken, cattle, or frog scotopsin, and hence blocks rhodopsin synthesis; but it is less efficient than photopsin in trapping retinene, and hence does not block iodopsin synthesis. Though iodopsin has not yet been prepared in pure form, its absorption spectrum has been computed by two independent procedures. This exhibits an alpha-band with lambda(max) 562 mmicro, a minimum at about 435 mmicro, and a small beta-band in the near ultraviolet at about 370 mmicro. The low concentration of iodopsin in the cones explains to a first approximation their high threshold, and hence their status as organs of daylight vision. The relatively rapid synthesis of iodopsin compared with rhodopsin parallels the relatively rapid dark adaptation of cones compared with rods. A theoretical relation is derived which links the logarithm of the visual sensitivity with the concentration of visual pigment in the rods and cones. Plotted in these terms, the course of rod and cone dark adaptation resembles closely the synthesis of rhodopsin and iodopsin in solution. The spectral sensitivities of rod and cone vision, and hence the Purkinje phenomenon, have their source in the absorption spectra of rhodopsin and iodopsin. In the chicken, for which only rough spectral sensitivity measurements are available, this relation can be demonstrated only approximately. In the pigeon the scotopic sensitivity matches the spectrum of rhodopsin; but the photopic sensitivity is displaced toward the red, largely or wholly through the filtering action of the colored oil globules in the pigeon cones. In cats, guinea pigs, snakes, and frogs, in which no such colored ocular structures intervene, the scotopic and photopic sensitivities match quantitatively the absorption spectra of rhodopsin and iodopsin. In man the scotopic sensitivity matches the absorption spectrum of rhodopsin; but the photopic sensitivity, when not distorted by the yellow pigmentations of the lens and macula lutea, lies at shorter wave lengths than iodopsin. This discrepancy is expected, for the human photopic sensitivity represents a composite of at least three classes of cone concerned with color vision.

In silico characterisation and chromosomal localisation of human RRH (peropsin)--implications for opsin evolution

BACKGROUND

The vertebrate opsins are proteins which utilise a retinaldehyde chromophore in their photosensory or photoisomerase roles in the visual/irradiance detection cycle. The majority of the opsins, such as rod and cone opsins, have a very highly conserved gene structure suggesting a common lineage. Exceptions to this are RGR-opsin and melanopsin, whose genes have very different intron insertion positions. The gene structure of another opsin, peropsin (retinal pigment epithelium-derived rhodopsin homologue, RRH) is unknown.

RESULTS

By in silico analysis of the GenBank database we have determined that the human RRH comprises 7 exons spanning approximately 16.5 kb and is localised to chromosome 4q25 in the following gene sequence: cen-EGF-RRH-IF-qter - a position that excludes this gene as a candidate for the RP29 autosomal recessive retinitis pigmentosa locus. A comparison of opsin gene structures reveals that RRH and RGR share two common intron (introns 1 and 4) insertion positions which may reflect a shared ancestral gene.

CONCLUSION

The opsins comprise a diverse group of genes which appear to have arisen from three different lineages. These lineages comprise the "classical opsin superfamily" which includes the rod and cone opsins, pinopsin, VA-opsin, parapinopsin and encephalopsin; the RRH and RGR group; and the melanopsin line. A common lineage for RRH and RGR, together with their sites of expression in the RPE, indicates that peropsin may act as a retinal isomerase.

The photosensitive retinal pigments of fishes from relatively turbid coastal waters

Digitonin extracts have been prepared from the retinae of a dozen species of marine and euryhaline teleost fishes from turbid water habitats. Spectrophotometric analysis of the extracts shows that the photosensitive retinal pigments of these species have maximum absorption above 500 mµ. In nine species there are retinene₁ pigments with λ_max between 504 and 512 mµ. In the marine but euryhaline mullet, Mugil cephalus, there is a porphyropsin with λ_max 520 mµ. A mixture of rhodopsin and porphyropsin in an extract of a marine puffer, Sphoeroides annulatus, was disclosed by partial bleaching with colored light. In addition, one other species has a 508 mµ pigment, of which the nature of the chromophore was not determined. The habitats in which these fishes live are relatively turbid, with the water greenish or yellowish in color. The spectral transmission of such waters is probably maximal between 520 and 570 mµ. It is suggested that the fishes have become adapted to these conditions by small but significant shifts in spectral absorption of their retinal pigments. These pigments are decidedly more effective than rhodopsin in absorption of wavelengths above 500 mµ. This offers a possible interpretation of the confusing array of retinal pigments described from marine and euryhaline fishes.

The thermal stability of rhodopsin and opsin

Rhodopsin, the red photosensitive pigment of rod vision, is composed of a specific cis isomer of retinene, neo-b (11-cis), joined as chromophore to a colorless protein, opsin. We have investigated the thermal denaturation of cattle rhodopsin and opsin in aqueous digitonin solution, and in isolated rod outer limbs. Both rhodopsin and opsin are more stable in rods than in solution. In solution as well as in rods, moreover, rhodopsin is considerably more stable than opsin. The chromophore therefore protects opsin against denaturation. This is true whether rhodopsin is extracted from dark-adapted retinas, or synthesized in vitro from neo-b retinene and opsin. Excess neo-b retinene does not protect rhodopsin against denaturation. The protection involves the specific relationship between the chromophore and opsin. Similar, though somewhat less, protection is afforded opsin by the stereoisomeric iso-a (9-cis) chromophore in isorhodopsin. The Arrhenius activation energies (E(a)) and entropies of activation (DeltaSdouble dagger) are much greater for thermal denaturation of rhodopsin and isorhodopsin than of opsin. Furthermore, these values differ considerably for rhodopsins from different species -frog, squid, cattle-presumably due to species differences in the opsins. Heat or light bleaches rhodopsin by different mechanisms, yielding different products. Light stereoisomerizes the retinene chromophore; heat denatures the opsin. Photochemical bleaching therefore yields all-trans retinene and native opsin; thermal bleaching, neo-b retinene and denatured opsin.

The action of enzymes on rhodopsin

The effects have been examined of chymotrypsin, pepsin, trypsin, and pancreatic lipase on cattle rhodopsin in digitonin solution. The digestion of rhodopsin by chymotrypsin was measured by the hydrolysis of peptide bonds (formol titration), changes in pH, and bleaching. The digestion proceeds in two stages: an initial rapid hydrolysis which exposes about 30 amino groups per molecule, without bleaching; superimposed on a slower hydrolysis which exposes about 50 additional amino groups, with proportionate bleaching. The chymotryptic action begins at pH about 6.0 and increases logarithmically in rate to pH 9.2. Trypsin and pepsin also bleach rhodopsin in solution. A preparation of pancreatic lipase bleached it slightly, but no more than could be explained by contamination with proteases. In digitonin solution each rhodopsin molecule is associated in a micelle with about 200 molecules of digitonin; yet the latter do not appear to hinder enzyme action. It is suggested that the digitonin sheath is sufficiently fluid to be penetrated on collision with an enzyme molecule; and that once together the enzyme and substrate are held together by intermolecular attractive forces, and by the "cage effect" of bombardment by surrounding solvent molecules. The two stages of chymotryptic digestion of rhodopsin may correspond to an initial rapid fragmentation, such as has been observed with many proteinases and substrates; superimposed upon a slower digestion of the fragments. Since the first phase involves no bleaching, this may mean that rhodopsin can be broken into considerably smaller fragments without loss of optical properties.

Retinoic acid delays transcription of human retinal pigment neuroepithelium marker genes in ARPE-19 cells

The effect of retinoic acid on the differentiation of a human retinal pigment epithelium-derived cell line ARPE-19 was studied. Differentiation of ARPE-19 cells is delayed by retinoic acid. The minimum all-trans-retinoic acid concentration needed for delay of ARPE-19 differentiation is 1 microM. A delay of differentiation was also observed using 1 microM 9-cis or 13-cis-retinoic acid. Differentiation at the molecular level was studied by analyzing transcription of two RPE-marker genes, RPE65 and peropsin. In the presence of 1 microM retinoic acid the onset of transcription of both genes was delayed by 2-3 weeks. We conclude that all-trans-, 9-cis-, and 13-cis-retinoic acid delay differentiation of ARPE-19 cells into cells that phenotypically resemble cells from the human retinal pigment epithelium.

Inherited retinal dystrophy in the rat

Retinal dystrophies, known in man, dog, mouse, and rat, involve progressive loss of photoreceptor cells with onset during or soon after the developmental period. Functional (electroretinogram), chemical (rhodopsin analyses) and morphological (light and electron microscopy) data obtained in the rat indicated two main processes: (a) overproduction of rhodopsin and an associated abnormal lamellar tissue component, (b) progressive loss of photoreceptor cells. The first abnormality recognized was the appearance of swirling sheets or bundles of extracellular lamellae between normally developing retinal rods and pigment epithelium; membrane thickness and spacing resembled that in normal outer segments. Rhodopsin content reached twice normal values, was present in both rods and extracellular lamellae, and was qualitatively normal, judged by absorption maximum and products of bleaching. Photoreceptors attained virtually adult form and ERG function. Then rod inner segments and nuclei began degenerating; the ERG lost sensitivity and showed selective depression of the a-wave at high luminances. Outer segments and lamellae gradually degenerated and rhodopsin content decreased. No phagocytosis was seen, though pigment cells partially dedifferentiated and many migrated through the outer segment-debris zone toward the retina. Eventually photoreceptor cells and the b-wave of the ERG entirely disappeared. Rats kept in darkness retained electrical activity, rhodopsin content, rod structure, and extracellular lamellae longer than litter mates in light.

The spectral sensitivity of crayfish and lobster vision

(1) The spectral sensitivity function for the compound eye of the crayfish has been determined by recording the retinal action potentials elicited by monochromatic stimuli. Its peak lies at approximately 570 mµ. (2) Similar measurements made on lobster eyes yield functions with maxima in the region of 520 to 525 mµ, which agree well with the absorption spectrum of lobster rhodopsin if minor allowances are made for distortion by known screening pigments. (3) The crayfish sensitivity function, since it is unaffected by selective monochromatic light adaptation, must be determined by a single photosensitive pigment. The absorption maximum of this pigment may be inferred with reasonable accuracy from the sensitivity data. (4) The visual pigment of the crayfish thus has its maximum absorption displaced by 50 to 60 mµ towards the red end of the spectrum from that of the lobster and other marine crustacea. This shift parallels that found in both rod and cone pigments between fresh water and marine vertebrates. In the crayfish, however, an altered protein is responsible for the shift and not a new carotenoid chromophore as in the vertebrates. (5) The existence of this situation in a new group of animals (with photoreceptors which have been evolved independently from those of vertebrates) strengthens the view that there may be strong selection for long wavelength visual sensitivity in fresh water.

Direct action of light in naturally pigmented muscle fibers. I. Action spectrum for contraction in eel iris sphincter

Contraction due to light in excised eel irises appears to follow a simple first order law. The action spectrum for contraction has a maximum which agrees with the eel rhodopsin absorption maximum. Inasmuch as rhodopsin is the rod pigment-opsin complex and the iris sphincter pupillae evolves from the pigment epithelium of the retina in the region of the iris, the muscle pigment might be the same as the visual pigment. In the human eye the contraction of the iris sphincter is activated only by light incident on the retina and the pupil diameter varies inversely with the square root of the light intensity. The inverse first power relation observed in the present experiments suggests a more primitive origin for the light reaction in eel irises. Relaxation is a much slower process and can be approximated as the sum of two first order processes.

In situ microspectrophotometric studies on the pigments of single retinal rods

Three spectral entities have been observed in single intact frog rod outer segments at 506 mmu, 480 mmu and 380 mmu. It is likely that the peak of 506 mmu was somewhat altered by bleaching reactions and originated at about 510 mmu. This is identified with the 502 mmu frog rhodopsin of digitonin extracts. Spectra in polarized light have the same maximum, identifying the dichroism of rods with rhodopsin. The dichroic ratio is around 6, giving the outer segment an axial density of 0.09/5mu or 0.9 OD total, with a pigment concentration of 2 to 3 mM. The dichroism data are used to compute the angle separating the rhodopsin molecular absorption vectors in rods from perfect restriction to a plane. This angle is 16 degrees or 23 degrees depending on which of two assumptions one chooses for the type of molecular ordering. The spectral peaks at 480 mmu and 380 mmu are thought to correspond respectively to metarhodopsin and retinene. Disappearance of the former is accompanied by accumulation of the latter. This reaction seems to occur more slowly in the intact outer segment than the corresponding reaction in solution. Spread of bleaching spectra from illuminated to dark areas of the same rod did not occur over distances of 2 mu or greater. Spectra were similar from rod to rod and from point to point on the same rod showing that frog rods are spectrally homogeneous both individually and collectively.

Response of the timeless protein to light correlates with behavioral entrainment and suggests a nonvisual pathway for circadian photoreception

The period (per) and timeless (tim) genes are required for circadian behavioral rhythms in Drosophila. The current model for how these rhythms entrain to light is based upon the light induced decrease in timeless protein (TIM) levels. We show here that the TIM response to light correlates with the effect of light on the behavioral rhythm. To identify components of the entrainment pathway, we also assayed the TIM response in flies with mutant visual systems. Flies that lacked eyes displayed a normal response in lateral neurons. The TIM response to a light pulse was attenuated in flies that were mutant for the transient receptor potential (trp) and trp-like (trpl) genes, which are required for calcium conductance in the visual transduction cascade. The reduced TIM response was accompanied by a reduced phase shift in the behavioral rhythm, but neither response was completely eliminated, and the trpl;trp flies entrain to light-dark cycles, suggesting that these genes perturb some aspect of circadian entrainment when mutated but are not essential for it. The TIM response was also unaffected in ninaE flies that lack the rhodopsin protein (rh1). These results support the hypothesis that circadian entrainment does not rely on the visual system and likely involves a dedicated pathway for photoreception.

Anti-apoptosis therapy: a way of treating neural degeneration?

Many degenerative diseases involve apoptotic cell death--can they be treated with apoptosis inhibitors, while protecting the normal physiological function of the rescued cells? Reason for optimism comes from a recent study of mutant flies with an analogue of the human degenerative disease retinitis pigmentosa.

Peropsin, a novel visual pigment-like protein located in the apical microvilli of the retinal pigment epithelium

A visual pigment-like protein, referred to as peropsin, has been identified by large-scale sequencing of cDNAs derived from human ocular tissues. The corresponding mRNA was found only in the eye, where it is localized to the retinal pigment epithelium (RPE). Peropsin immunoreactivity, visualized by light and electron microscopy, localizes the protein to the apical face of the RPE, and most prominently to the microvilli that surround the photoreceptor outer segments. These observations suggest that peropsin may play a role in RPE physiology either by detecting light directly or by monitoring the concentration of retinoids or other photoreceptor-derived compounds.

Steric hindrance between chromophore substituents as the driving force of rhodopsin photoisomerization: 10-methyl-13-demethyl retinal containing rhodopsin

A visual chromophore analogue, 10-methyl-13-demethyl (dm) retinal, was synthesized and reconstituted with bleached bovine rhodopsin to form a visual pigment derivative with absorbance maximum at 505 nm. The investigations with this new compound were stimulated from recent results using 13-dm retinal as a chromophore that revealed a remarkable loss in quantum efficiency (phi of 13-dm retinal-containing rhodopsin: 0.30, Ternieden and Gärtner, J. Photochem. Photobiol. B Biol, 33, 83-86, 1996). The quantum efficiency of the new pigment was determined as 0.59 by quantitative bleaching using reconstituted rhodopsin as a reference. The very similar quantum efficiencies of rhodopsin and the new pigment give experimental support for the recently presented hypothesis that a steric hindrance between the substituents at positions 10 and 13 in 11-cis-retinal is elevated during the photoisomerization and thus facilitates the rapid photoisomerization of the visual chromophore (Peteanu et al., Proc. Natl. Acad. Sci. USA 90, 11762-11766, 1993). Such steric hindrance is removed from the molecule by the elimination of the methyl group from position 13 and can be re-established via a rearrangement of the substitution pattern by introducing a methyl group at position 10 of 13-dm retinal.

CHANGES IN ELECTRICAL CONDUCTANCE OF RHODOPSIN ON PHOTOLYSIS

The change in electrical conductance of rhodopsin solutions was studied with flash-photolysis techniques. The whole pattern of the conductance change on illumination consists of three different processes: (I) the initial decrease, (II) the increase, and (III) the slow decrease, which are in decreasing order of reaction rate. The processes I, II, and III can be most distinctly recognized on flash illumination of acid, slightly acid, and alkaline rhodopsins, respectively. On the other hand, the bleaching of rhodopsin also shows at least three successive phases of different rates, but none of them corresponds in reaction rate to any of the processes of the conductance change. The conductance change may be related to conformational changes of opsin following photoisomerization of retinene, being due to hydrogen or hydroxyl ions and some other inorganic electrolytes. The amount of the change, especially the initial decrease, is proportional to the amount of rhodopsin bleached, even when the photochemical back reaction towards rhodopsin and isorhodopsin occurs in the chromophore depending on the intensity of illumination. Of the three processes, the slow decrease is most severely affected by aging, but the initial decrease and increase are slightly affected. These two processes promptly caused by illumination are connected closely to the conformational changes during the conversion of rhodopsin to metarhodopsin, and perhaps to the initial stage of excitation of rod cells.

VISUAL PIGMENTS IN SINGLE RODS AND CONES OF THE HUMAN RETINA. DIRECT MEASUREMENTS REVEAL MECHANISMS OF HUMAN NIGHT AND COLOR VISION

Difference spectra of the visual pigments have been measured in single rods and cones of a parafoveal region of the human retina. Rods display an absorption maximum (lambdamax) at about 505 mmicro associated with rhodopsin. Three kinds of cones were measured: a blue-sensitive cone with Amaxe about 450 mpf; two green-sensitive cones with Xmaa about 525 mumicro; and a red-sensitive cone with lambdamax about 555 mmicro These are presumably samples of the three types of cone responsible for human color vision.

TAUTOMERIC FORMS OF METARHODOPSIN

Light isomerizes the chromophore of rhodopsin, 11-cis retinal (formerly retinene), to the all-trans configuration. This introduces a succession of unstable intermediates—pre-lumirhodopsin, lumirhodopsin, metarhodopsin —in which all-trans retinal is still attached to the chromophoric site on opsin. Finally, retinal is hydrolyzed from opsin. The present experiments show that metarhodopsin exists in two tautomeric forms, metarhodopsins I and II, with λ_max 478 and 380 mµ. Metarhodopsin I appears first, then enters into equilibrium with metarhodopsin II. In this equilibrium, the proportion of metarhodopsin II is favored by higher temperature or pH, neutral salts, and glycerol. The change from metarhodopsin I to II involves the binding of a proton by a group with pK 6.4 (imidazole?), and a large increase of entropy. Metarhodopsin II has been confused earlier with the final mixture of all-trans retinal and opsin (λ_max 387 mµ), which it resembles in spectrum. These two products are, however, readily distinguished experimentally.

NEURAL AND PHOTOCHEMICAL MECHANISMS OF VISUAL ADAPTATION IN THE RAT

The effects of light adaptation on the increment threshold, rhodopsin content, and dark adaptation have been studied in the rat eye over a wide range of intensities. The electroretinogram threshold was used as a measure of eye sensitivity. With adapting intensities greater than 1.5 log units above the absolute ERG threshold, the increment threshold rises linearly with increasing adapting intensity. With 5 minutes of light adaptation, the rhodopsin content of the eye is not measurably reduced until the adapting intensity is greater than 5 log units above the ERG threshold. Dark adaptation is rapid (i.e., completed in 5 to 10 minutes) until the eye is adapted to lights strong enough to bleach a measurable fraction of the rhodopsin. After brighter light adaptations, dark adaptation consists of two parts, an initial rapid phase followed by a slow component. The extent of slow adaptation depends on the fraction of rhodopsin bleached. If all the rhodopsin in the eye is bleached, the slow fall of threshold extends over 5 log units and takes 2 to 3 hours to complete. The fall of ERG threshold during the slow phase of adaptation occurs in parallel with the regeneration of rhodopsin. The slow component of dark adaptation is related to the bleaching and resynthesis of rhodopsin; the fast component of adaptation is considered to be neural adaptation.

PHOTOREVERSAL OF RHODOPSIN BLEACHING

A mechanistic scheme, showing certain steps of rhodopsin bleaching, provides two ways of viewing the bleaching process: (a) The rate of bleaching depends upon the net rate of accumulation of labile species; and (b) the number of labile molecules which accumulates in a certain period is the number which has absorbed an odd number of quanta by the end of that period. Both views, based on the photoreversibility of bleaching, lend themselves to concise mathematical formulation. The expected amounts of bleaching at various intensities, calculated according to these formulae, give very close fits to the experimental data. The relevance of these results to other experiments is pointed out and emphasis is placed on the explanation of observed quantum efficiencies which obtain at both low and high intensities.

Langmuir-Blodgett films of photosensitive proteins

The striking properties of monolayers and multilayers of photosensitive proteins obtained by using the Langmuir-Blodgett technique are described. The close packing of the protein molecules, which preserve most of the properties found in solution, seems to be the main cause for their thermal stability, which in some cases reached a temperature of 200 degrees C without the loss of the protein secondary structure. The review is focused on three of the most intensively studied photosensitive proteins, namely photosynthetic reaction centres, bacteriorhodopsin and bovine rhodopsin, and on their possible applications as molecular optical devices.

Constitutive overexpression of the Drosophila period protein inhibits period mRNA cycling

The Drosophila period gene (per) is a likely component of a circadian pacemaker. per protein (PER) participates in the regulation of its own expression, at least in part at the transcriptional level. There is at present no direct evidence that the effect of PER on its own transcription is intracellular. Results presented in this paper show that (i) the circadian oscillations of both per mRNA and PER protein are quantitatively similar in eye photoreceptor cells and in brain; (ii) constitutive overexpression of PER only in photoreceptors R1-R6 represses endogenous per RNA cycling in these cells but not in other per-expressing cells; (iii) the overexpression construct has no effect on locomotor activity rhythms. These results indicate that the autoregulation of per expression is a direct, intracellular event and suggest that each per-expressing cell contains an autonomous oscillator of which the per feedback loop is a component.

Cytoplasmic retinal localization of an evolutionary homolog of the visual pigments

A rhodopsin-related protein is preferentially expressed at high levels in retinal pigment epithelium (RPE) and in Müller cells. The putative RPE-retinal G protein-coupled receptor (RGR) was localized in light-adapted bovine retina by means of electron microscopic immunocytochemistry. In the RPE, the protein was localized to a widespread intracellular compartment. Except for the region adjacent to the basal surface, the RPE cytoplasm was labeled throughout the cell including the apical surface. In Müller cells also RGR was found in the intracellular compartment, especially in the cytoplasm in the region of the Müller cell endfeet and proximal cell processes. Subcellular fractionation studies of bovine RPE and neural retina indicated that RGR is a membrane-bound protein. The intracellular localization of RGR is a unique variation in the subcellular distribution of seven-transmembrane-domain receptors and suggests an unconventional role for RGR in the signal transduction process.

Conditional cell ablation in Drosophila

Targeting of cell ablation agents under the control of tissue-specific promoters promises to be an important tool for studies of development and function in higher organisms. Temperature-sensitive cell ablation agents, recently developed for Drosophila, extend control to temporal as well as spatial aspects of toxin expression. Here we discuss achievements to date, together with a novel form of enhancer trap technology with the potential for driving toxin expression in a large range of cell types.