Transducin activation by molecular species of rhodopsin other than metarhodopsin II

Apo-Opsin Exists in Equilibrium Between a Predominant Inactive and a Rare Highly Active State

Bleaching adaptation in rod photoreceptors is mediated by apo-opsin, which activates phototransduction with effective activity 10⁵- to 10⁶-fold lower than that of photoactivated rhodopsin (meta II). However, the mechanism that produces such low opsin activity is unknown. To address this question, we sought to record single opsin responses in mouse rods. We used mutant mice lacking efficient calcium feedback to boosts rod responses and generated a small fraction of opsin by photobleaching ∼1% of rhodopsin. The bleach produced a dramatic increase in the frequency of discrete photoresponse-like events. This activity persisted for hours, was quenched by 11-cis-retinal, and was blocked by uncoupling opsin from phototransduction, all indicating opsin as its source. Opsin-driven discrete activity was also observed in rods containing non-activatable rhodopsin, ruling out transactivation of rhodopsin by opsin. We conclude that bleaching adaptation is mediated by opsin that exists in equilibrium between a predominant inactive and a rare meta II-like state.SIGNIFICANCE STATEMENT Electrophysiological analysis is used to show that the G-protein-coupled receptor opsin exists in equilibrium between a predominant inactive and a rare highly active state that mediates bleaching adaptation in photoreceptors.

Delayed S-cone sensitivity losses following the onset of intense yellow backgrounds linked to the lifetime of a photobleaching product?

Thirty years ago, Mollon, Stockman, & Polden (1987) reported that after the onset of intense yellow 581-nm backgrounds, S-cone threshold rose unexpectedly for several seconds before recovering to the light-adapted steady-state value-an effect they called: "transient-tritanopia of the second kind" (TT2). Given that 581-nm lights have little direct effect on S-cones, TT2 must arise indirectly from the backgrounds' effects on the L- and M-cones. We attribute the phenomenon to the action of an unknown L- and M-cone photobleaching product, X, which acts at their outputs like an "equivalent" background light that then inhibits S-cones at a cone-opponent, second-site. The time-course of TT2 is similar in form to the lifetime of X in a two-stage, first-order biochemical reaction A→X→C with successive best-fitting time-constants of 3.09 ± 0.35 and 7.73 ± 0.70 s. Alternatively, with an additional slowly recovering exponential "restoring-force" with a best-fitting time-constant 23.94 ± 1.42 s, the two-stage best-fitting time-constants become 4.15 ± 0.62 and 6.79 ± 1.00 s. Because the time-constants are roughly independent of the background illumination, and thus the rate of photoisomerization, A→X is likely to be a reaction subsidiary to the retinoid cycle, perhaps acting as a buffer when the bleaching rate is too high. X seems to be logarithmically related to S-cone threshold, which may result from the logarithmic cone-opponent, second-site response compression after multiplicative first-site adaptation. The restoring-force may be the same cone-opponent force that sets the rate of S-cone recovery following the unusual threshold increase following the offset of dimmer yellow backgrounds, an effect known as "transient-tritanopia" (TT1).

Constitutively active rhodopsin mutants causing night blindness are effectively phosphorylated by GRKs but differ in arrestin-1 binding

The effects of activating mutations associated with night blindness on the stoichiometry of rhodopsin interactions with G protein-coupled receptor kinase 1 (GRK1) and arrestin-1 have not been reported. Here we show that the monomeric form of WT rhodopsin and its constitutively active mutants M257Y, G90D, and T94I, reconstituted into HDL particles are effectively phosphorylated by GRK1, as well as two more ubiquitously expressed subtypes, GRK2 and GRK5. All versions of arrestin-1 tested (WT, pre-activated, and constitutively monomeric mutants) bind to monomeric rhodopsin and show the same selectivity for different functional forms of rhodopsin as in native disc membranes. Rhodopsin phosphorylation by GRK1 and GRK2 promotes arrestin-1 binding to a comparable extent, whereas similar phosphorylation by GRK5 is less effective, suggesting that not all phosphorylation sites on rhodopsin are equivalent in promoting arrestin-1 binding. The binding of WT arrestin-1 to phospho-opsin is comparable to the binding to its preferred target, P-Rh*, suggesting that in photoreceptors arrestin-1 only dissociates after opsin regeneration with 11-cis-retinal, which converts phospho-opsin into inactive phospho-rhodopsin that has lower affinity for arrestin-1. Reduced binding of arrestin-1 to the phospho-opsin form of G90D mutant likely contributes to night blindness caused by this mutation in humans.

The 9-methyl group of retinal is essential for rapid Meta II decay and phototransduction quenching in red cones

Cone photoreceptors of the vertebrate retina terminate their response to light much faster than rod photoreceptors. However, the molecular mechanisms underlying this rapid response termination in cones are poorly understood. The experiments presented here tested two related hypotheses: first, that the rapid decay rate of metarhodopsin (Meta) II in red-sensitive cones depends on interactions between the 9-methyl group of retinal and the opsin part of the pigment molecule, and second, that rapid Meta II decay is critical for rapid recovery from saturation of red-sensitive cones after exposure to bright light. Microspectrophotometric measurements of pigment photolysis, microfluorometric measurements of retinol production, and single-cell electrophysiological recordings of flash responses of salamander cones were performed to test these hypotheses. In all cases, cones were bleached and their visual pigment was regenerated with either 11-cis retinal or with 11-cis 9-demethyl retinal, an analogue of retinal lacking the 9-methyl group. Meta II decay was four to five times slower and subsequent retinol production was three to four times slower in red-sensitive cones lacking the 9-methyl group of retinal. This was accompanied by a significant slowing of the recovery from saturation in cones lacking the 9-methyl group after exposure to bright (>0.1% visual pigment photoactivated) but not dim light. A mathematical model of the turn-off process of phototransduction revealed that the slower recovery of photoresponse can be explained by slower Meta decay of 9-demethyl visual pigment. These results demonstrate that the 9-methyl group of retinal is required for steric chromophore-opsin interactions that favor both the rapid decay of Meta II and the rapid response recovery after exposure to bright light in red-sensitive cones.

Turning cones off: the role of the 9-methyl group of retinal in red cones

Our ability to see in bright light depends critically on the rapid rate at which cone photoreceptors detect and adapt to changes in illumination. This is achieved, in part, by their rapid response termination. In this study, we investigate the hypothesis that this rapid termination of the response in red cones is dependent on interactions between the 9-methyl group of retinal and red cone opsin, which are required for timely metarhodopsin (Meta) II decay. We used single-cell electrical recordings of flash responses to assess the kinetics of response termination and to calculate guanylyl cyclase (GC) rates in salamander red cones containing native visual pigment as well as visual pigment regenerated with 11-cis 9-demethyl retinal, an analogue of retinal in which the 9-methyl group is missing. After exposure to bright light that photoactivated more than ∼0.2% of the pigment, red cones containing the analogue pigment had a slower recovery of both flash response amplitudes and GC rates (up to 10 times slower at high bleaches) than red cones containing 11-cis retinal. This finding is consistent with previously published biochemical data demonstrating that red cone opsin regenerated in vitro with 11-cis 9-demethyl retinal exhibited prolonged activation as a result of slowed Meta II decay. Our results suggest that two different mechanisms regulate the recovery of responsiveness in red cones after exposure to light. We propose a model in which the response recovery in red cones can be regulated (particularly at high light intensities) by the Meta II decay rate if that rate has been inhibited. In red cones, the interaction of the 9-methyl group of retinal with opsin promotes efficient Meta II decay and, thus, the rapid rate of recovery.

Breaking the covalent bond--a pigment property that contributes to desensitization in cones

Retinal rod and cone pigments consist of an apoprotein, opsin, covalently linked to a chromophore, 11-cis retinal. Here we demonstrate that the formation of the covalent bond between opsin and 11-cis retinal is reversible in darkness in amphibian red cones, but essentially irreversible in red rods. This dissociation, apparently a general property of cone pigments, results in a surprisingly large amount of free opsin--about 10% of total opsin--in dark-adapted red cones. We attribute this significant level of free opsin to the low concentration of intracellular free 11-cis retinal, estimated to be only a tiny fraction (approximately 0.1 %) of the pigment content in red cones. With its constitutive transducin-stimulating activity, the free cone opsin produces an approximately 2-fold desensitization in red cones, equivalent to that produced by a steady light causing 500 photoisomerizations s-1. Cone pigment dissociation therefore contributes to the sensitivity difference between rods and cones.

Adaptation in vertebrate photoreceptors

When light is absorbed within the outer segment of a vertebrate photoreceptor, the conformation of the photopigment rhodopsin is altered to produce an activated photoproduct called metarhodopsin II or Rh(*). Rh(*) initiates a transduction cascade similar to that for metabotropic synaptic receptors and many hormones; the Rh(*) activates a heterotrimeric G protein, which in turn stimulates an effector enzyme, a cyclic nucleotide phosphodiesterase. The phosphodiesterase then hydrolyzes cGMP, and the decrease in the concentration of free cGMP reduces the probability of opening of channels in the outer segment plasma membrane, producing the electrical response of the cell. Photoreceptor transduction can be modulated by changes in the mean light level. This process, called light adaptation (or background adaptation), maintains the working range of the transduction cascade within a physiologically useful region of light intensities. There is increasing evidence that the second messenger responsible for the modulation of the transduction cascade during background adaptation is primarily, if not exclusively, Ca(2+), whose intracellular free concentration is decreased by illumination. The change in free Ca(2+) is believed to have a variety of effects on the transduction mechanism, including modulation of the rate of the guanylyl cyclase and rhodopsin kinase, alteration of the gain of the transduction cascade, and regulation of the affinity of the outer segment channels for cGMP. The sensitivity of the photoreceptor is also reduced by previous exposure to light bright enough to bleach a substantial fraction of the photopigment in the outer segment. This form of desensitization, called bleaching adaptation (the recovery from which is known as dark adaptation), seems largely to be due to an activation of the transduction cascade by some form of bleached pigment. The bleached pigment appears to activate the G protein transducin directly, although with a gain less than Rh(*). The resulting decrease in intracellular Ca(2+) then modulates the transduction cascade, by a mechanism very similar to the one responsible for altering sensitivity during background adaptation.

Molecular basis of dark adaptation in rod photoreceptors

Following exposure of the eye to an intense light that 'bleaches' a significant fraction of the rhodopsin, one's visual threshold is initially greatly elevated, and takes tens of minutes to recover to normal. The elevation of visual threshold arises from events occurring within the rod photoreceptors, and the underlying molecular basis of these events and of the rod's recovery is now becoming clearer. Results obtained by exposing isolated toad rods to hydroxylamine solution indicate that, following small bleaches, the primary intermediate causing elevation of visual threshold is metarhodopsin II, in its phosphorylated and arrestin-bound form. This product activates transduction with an efficacy about 100 times greater than that of opsin.

Enhancement of opsin activity by all-trans-retinal

The rod cell photoreceptor apoprotein, opsin, activates the G-protein, transducin, although at a much reduced level than light-activated rhodopsin. The ability of all-trans-retinal to enhance opsin apoprotein activity was investigated using a guanyl nucleotide exchange assay on transducin. All-trans-retinal enhanced opsin activity in a concentration-dependent manner. At high concentrations of all-trans-retinal, the activity of the all-trans-retinal-opsin complex was comparable to that from an equimolar amount of metarhodopsin(II). However, in contrast to metarhodopsin(II), the active all-trans-retinalopsin complex did not require a stable Schiff base linkage between opsin and all-trans-retinal. The lack of a stable Schiff base and differences in activity at high pH imply that opsin and all-trans-retinal form a complex that is distinct from metarhodopsin(II). The ability of all-trans-retinal to stimulate the transduction cascade may be a source of post-bleach noise in photoreceptors.

A comparison of the efficiency of G protein activation by ligand-free and light-activated forms of rhodopsin

Activation of the photoreceptor G protein transducin (Gt) by opsin, the ligand-free form of rhodopsin, was measured using rod outer segment membranes with densities of opsin and Gt similar to those found in rod cells. When GTPgammaS was used as the activating nucleotide, opsin catalyzed transducin activation with an exponential time course with a rate constant k(act) on the order of 2 x 10(-3)s(-1). Comparison under these conditions to activation by flash-generated metarhodopsin II (MII) revealed that opsin- and R*-catalyzed activation showed similar kinetics when MII was present at a surface density approximately 10(-6) lower than that of opsin. Thus, in contrast to some previous reports, we find that the catalytic potency of opsin is only approximately 10(-6) that of MII. In the presence of residual retinaldehyde-derived species present in membranes treated with hydroxylamine after bleaching, the apparent k(act) observed was much higher than that for opsin, suggesting a possible explanation for previous reports of more efficient activation by opsin. These results are important for considering the possible role of opsin in the diverse phenomena in which it has been suggested to play a key role, such as bleaching desensitization and retinal degeneration induced by continuous light or vitamin A deprivation.

Modulation of opsin apoprotein activity by retinal. Dark activity of rhodopsin formed at low temperature

The bovine opsin apoprotein activates transducin, although at a much reduced level than light-activated rhodopsin (Surya, A., Foster, K., and Knox, B. (1995) J. Biol. Chem. 270, 5024-5031). The ability of retinal to modulate opsin apoprotein activity was investigated using a guanyl nucleotide exchange assay on transducin. 11-cis-Retinal reacted with opsin at 22 degrees C to (a) reform pigment having maximal absorbance at 500 nm and (b) reduce opsin activity by >80%. Pigment formation also occurred at 0 degrees C with a t1/2 of 260 min. However, unlike rhodopsin formed at 22 degrees C (R22), the rhodopsin formed at 0 degrees C (R0) activated transducin with the same half-saturating concentration as opsin in an exhaustive binding assay. Thus, the formation of a protonated Schiff base associated with 500 nm absorbance does not by itself lead to the inactivation of opsin. The R0 conformation was partially inactivated by incubation at 22 degrees C (t1/2 = 61 +/- 9 min), suggesting that it may be an intermediate conformation in the regeneration of rhodopsin.

Effect of hydroxylamine on photon-like events during dark adaptation in toad rod photoreceptors

1. The suction pipette technique was used to investigate the recovery of toad rod photoreceptors following small bleaches of 0.2-3% of the rhodopsin. 2. The reduction in sensitivity and the increase in noise elicited by bleaches were measured, and from these measurements the underlying rate of occurrence of photon-like events was calculated as a function of time after the bleach. 3. Exposure to hydroxylamine solution was used to hasten the decomposition of the metarhodopsin photoproducts. The outer segment was exposed to 110 mM hydroxylamine in a low-Ca2+ Ringer solution for a period of 10-50 s beginning 10-17 min after the bleaching exposure. 4. By the time of the hydroxylamine exposure, the flash sensitivity and response kinetics had returned almost to normal, and were not significantly altered by the exposure. 5. Following hydroxylamine exposure, the rate of spontaneous photon-like events in the rods declined rapidly to near pre-bleach levels. 6. We conclude that hydroxylamine reduces the rate of occurrence of photon-like events induced by a bleach, and we postulate that this reduction results from the removal of metarhodopsin (most likely metarhodopsin II) from the outer segment. 7. Our results are consistent with a model in which photon-like events result from reversal of the reactions (phosphorylation and capping by arrestin) that lead to inactivation of the activated form of rhodopsin, Rh*.

Transducin activation by the bovine opsin apoprotein

The interaction of the bovine opsin apoprotein with transducin in rod outer segment membranes was investigated using a guanyl nucleotide exchange assay. In exhaustive binding experiments, opsin activates transducin, with half-maximal exchange activity occurring at 0.8 mol of opsin/mol of transducin. The opsin activity was light-insensitive, hydroxylamine-resistant, unaffected by stoichiometric concentrations of retinaloxime, and more heat-labile than rhodopsin. The t1/2 of transducin activation in the presence of excess opsin was 8.5 min, compared with 0.7 min for metarhodopsin (II). The second-order rate constants were determined to be 0.012 pmol of guanosine 5'-(gamma-thio)triphosphate (GTP gamma S) bound per min/nM opsin and 0.35 pmol of GTP gamma S bound per min/nM metarhodopsin (II). Opsin was able to activate more than one transducin, although there appeared to be a turnover-dependent inactivation of the apoprotein. Opsin showed a broad pH range (5.8-7.4) for optimal activity, with no activity in buffers of pH > 9, whereas metarhodopsin (II) exhibited activity at pH > 9. Regulation of opsin activity by stoichiometric amounts of retinal was observed, with inhibition by 11-cis-retinal and stimulation by all-trans-retinal. A model for opsin activity is proposed.

Photoreceptor rod outer segment 48-kDa protein has ATPase activity

The role of 48-kDa protein in visual transduction remains unresolved. Two hypotheses for its role in quenching the light activation of cyclic GMP cascade suggest that the protein binds to either phosphodiesterase or phosphorylated rhodopsin. Since the protein is also reported to bind ATP, we anticipated that the protein may have ATP hydrolyzing activity, and in analogy with the GTP-binding protein of the rod outer segments, such activity may be greatly enhanced by the elements of transduction cyclic GMP cascade, permitting the protein to function cyclically as GTP-binding protein does. We found that purified 48-kDa protein hydrolyzes ATP but at a slow rate of 0.04-0.05 per min. The Km for ATP is about 45-65 microM. The activity is inhibited noncompetitively by ADP with a Ki of about 50 microM. The ATPase activity of 48-kDa protein is not affected by rhodopsin, bleached rhodopsin, phosphorylated rhodopsin, unactivated cyclic GMP phosphodiesterase, or phosphodiesterase (PDE) activated by GMP PNP-bound G-protein. These data show that although 48-kDa protein has ATPase activity, lack of regulation of this activity by the elements of visual transduction makes it unlikely for this activity to have a role in quenching the light activation of cyclic GMP cascade.

Sensitization of bleached rod photoreceptors by 11-cis-locked analogues of retinal

Photoactivation of rhodopsin initiates both excitation and adaptation in vertebrate rod photoreceptors. Bleaching of rhodopsin to free opsin and all-trans-retinal in isolated rods produces a stable desensitization (bleaching adaptation) that is much larger than expected from pigment depletion alone. In our experiments, a 93% bleach produced a 500-fold increase in the light intensity required for saturation of the light response. This component of adaptation was 32-fold larger than the 16-fold increase expected from pigment depletion alone. 11-cis-Retinal, when delivered to isolated rods from liposomes, combines with free opsin to form a bleachable photopigment that fully restores sensitivity. 11-cis-Locked analogues of retinal combine with opsin to form unbleachable pigments in isolated bleached rods from the tiger salamander. They restore sensitivity to a substantial (16- to 25-fold) but incomplete extent. The analogues apparently relieve a stable component of adaptation when they interact with opsin. Because these analogues do not detectably excite rods, the structural requirements of both retinal and opsin for the relief of adaptation are different from those of excitation. The biochemical basis of light adaptation resulting from pigment bleaching and the minimum structural requirements of retinal for its relief remain to be determined.