Phosphorylation of ovine rhodopsin. Identification of the phosphorylated sites

The Role of Reversible Phosphorylation of Drosophila Rhodopsin

Vertebrate and fly rhodopsins are prototypical GPCRs that have served for a long time as model systems for understanding GPCR signaling. Although all rhodopsins seem to become phosphorylated at their C-terminal region following activation by light, the role of this phosphorylation is not uniform. Two major functions of rhodopsin phosphorylation have been described: (1) inactivation of the activated rhodopsin either directly or by facilitating binding of arrestins in order to shut down the visual signaling cascade and thus eventually enabling a high-temporal resolution of the visual system. (2) Facilitating endocytosis of activated receptors via arrestin binding that in turn recruits clathrin to the membrane for clathrin-mediated endocytosis. In vertebrate rhodopsins the shutdown of the signaling cascade may be the main function of rhodopsin phosphorylation, as phosphorylation alone already quenches transducin activation and, in addition, strongly enhances arrestin binding. In the Drosophila visual system rhodopsin phosphorylation is not needed for receptor inactivation. Its role here may rather lie in the recruitment of arrestin 1 and subsequent endocytosis of the activated receptor. In this review, we summarize investigations of fly rhodopsin phosphorylation spanning four decades and contextualize them with regard to the most recent insights from vertebrate phosphorylation barcode theory.

The physiological roles of arrestin-1 in rod photoreceptor cells

Arrestin-1 is the second most abundant protein in rod photoreceptors and is nearly equimolar to rhodopsin. Its well-recognized role is to "arrest" signaling from light-activated, phosphorylated rhodopsin, a prototypical G protein-coupled receptor. In doing so, arrestin-1 plays a key role in the rapid recovery of the light response. Arrestin-1 exists in a basal conformation that is stabilized by two independent sets of intramolecular interactions. The intramolecular constraints are disrupted by encountering (1) active conformation of the receptor (R*) and (2) receptor-attached phosphates. Requirement for these two events ensures its highly specific high-affinity binding to phosphorylated, light-activated rhodopsin (P-R*). In the dark-adapted state, the basal form is further organized into dimers and tetramers. Emerging data suggest pleiotropic roles of arrestin-1 beyond the functional range of rod cells. These include light-induced arrestin-1 translocation from the inner segment to the outer segment, a process that may be protective against cellular damage incurred by constitutive signaling. Its expanding list of binding partners also hints at additional, yet to be characterized functions. Uncovering these novel roles of arrestin-1 is a subject of future studies.

Arrestin interactions with G protein-coupled receptors

G-protein-coupled receptors (GPCRs) are the primary interaction partners for arrestins. The visual arrestins, arrestin1 and arrestin4, physiologically bind to only very few receptors, i.e., rhodopsin and the color opsins, respectively. In contrast, the ubiquitously expressed nonvisual variants β-arrestin1 and 2 bind to a large number of receptors in a fairly nonspecific manner. This binding requires two triggers, agonist activation and receptor phosphorylation by a G-protein-coupled receptor kinase (GRK). These two triggers are mediated by two different regions of the arrestins, the "phosphorylation sensor" in the core of the protein and a less well-defined "activation sensor." Binding appears to occur mostly in a 1:1 stoichiometry, involving the N-terminal domain of GPCRs, but in addition a second GPCR may loosely bind to the C-terminal domain when active receptors are abundant.Arrestin binding initially uncouples GPCRs from their G-proteins. It stabilizes receptors in an active conformation and also induces a conformational change in the arrestins that involves a rotation of the two domains relative to each other plus changes in the polar core. This conformational change appears to permit the interaction with further downstream proteins. The latter interaction, demonstrated mostly for β-arrestins, triggers receptor internalization as well as a number of nonclassical signaling pathways.Open questions concern the exact stoichiometry of the interaction, possible specificity with regard to the type of agonist and of GRK involved, selective regulation of downstream signaling (=biased signaling), and the options to use these mechanisms as therapeutic targets.

Constitutively active rhodopsin and retinal disease

Rhodopsin is the light receptor in rod photoreceptor cells of the retina that initiates scotopic vision. In the dark, rhodopsin is bound to the chromophore 11-cis retinal, which locks the receptor in an inactive state. The maintenance of an inactive rhodopsin in the dark is critical for rod photoreceptor cells to remain highly sensitive. Perturbations by mutation or the absence of 11-cis retinal can cause rhodopsin to become constitutively active, which leads to the desensitization of photoreceptor cells and, in some instances, retinal degeneration. Constitutive activity can arise in rhodopsin by various mechanisms and can cause a variety of inherited retinal diseases including Leber congenital amaurosis, congenital night blindness, and retinitis pigmentosa. In this review, the molecular and structural properties of different constitutively active forms of rhodopsin are overviewed, and the possibility that constitutive activity can arise from different active-state conformations is discussed.

Lessons from photoreceptors: turning off g-protein signaling in living cells

Phototransduction in retinal rods is one of the most extensively studied G-protein signaling systems. In recent years, our understanding of the biochemical steps that regulate the deactivation of the rod's response to light has greatly improved. Here, we summarize recent advances and highlight some of the remaining puzzles in this model signaling system.

Lipid second messengers and related enzymes in vertebrate rod outer segments

Rod outer segments (ROSs) are specialized light-sensitive organelles in vertebrate photoreceptor cells. Lipids in ROS are of considerable importance, not only in providing an adequate environment for efficient phototransduction, but also in originating the second messengers involved in signal transduction. ROSs have the ability to adapt the sensitivity and speed of their responses to ever-changing conditions of ambient illumination. A major contributor to this adaptation is the light-driven translocation of key signaling proteins into and out of ROS. The present review shows how generation of the second lipid messengers from phosphatidylcholine, phosphatidic acid, and diacylglycerol is modulated by the different illumination states in the vertebrate retina. Findings suggest that the light-induced translocation of phototransduction proteins influences the enzymatic activities of phospholipase D, lipid phosphate phosphatase, diacylglyceride lipase, and diacylglyceride kinase, all of which are responsible for the generation of the second messenger molecules.

Phospholipase D from photoreceptor rod outer segments is a downstream effector of RhoA: evidence of a light-dependent mechanism

Photoreceptor cells contain rod outer segments (ROS) which are specialized light-sensitive organelles. The biological function of ROS is to generate a photoresponse, which occurs via the classic transducin-mediated pathway. Moreover, ROS undergo light-regulated membrane turnover and protein translocation whose mechanisms have not been fully elucidated to date. Phospholipase D (PLD) is a key enzyme involved in lipid signal transduction and membrane trafficking. We have previously reported that PLD activity is present in purified ROS (Salvador, G.A., Giusto, N.M., 1998. Characterization of phospholipase D activity in bovine photoreceptor membranes. Lipids 33, 853-860). We now demonstrate that ROS PLD activity is enhanced by phosphatidylinositol bisphosphate (PIP2) and cytosolic factors in a GTP dependent-manner. Western blot analysis demonstrates the presence of PLD1 isoform in purified ROS. In ROS obtained from dark-adapted retinas (DROS), PIP2-dependent PLD activity was higher than that observed in ROS obtained from light-adapted retinas (LROS). In addition, experiments carried out in the presence of C3 toxin inhibited PLD activity from DROS whereas pertussis toxin did not affect the enzyme activity. Western blot analysis demonstrates the presence of RhoA, a PLD upstream-regulator. Moreover, RhoA levels were higher in DROS with respect to those in LROS. The present study reports evidence of the involvement of the small G-protein, RhoA, in ROS PLD regulation. Our data strongly suggest that RhoA regulates ROS PLD activity under a light-dependent mechanism.

Toward a unified model of vertebrate rod phototransduction

Recently, we introduced a phototransduction model that was able to account for the reproducibility of vertebrate rod single-photon responses (SPRs) (Hamer et al., 2003). The model was able to reproduce SPR statistics by means of stochastic activation and inactivation of rhodopsin (R*), transducin (G alpha ), and phosphodiesterase (PDE). The features needed to capture the SPR statistics were (1) multiple steps of R* inactivation by means of multiple phosphorylations (followed by arrestin capping) and (2) phosphorylation dependence of the affinity between R* and the three molecules competing to bind with R* (G alpha, arrestin, and rhodopsin kinase). The model was also able to account for several other rod response features in the dim-flash regime, including SPRs obtained from rods in which various elements of the cascade have been genetically disabled or disrupted. However, the model was not tested under high light-level conditions. We sought to evaluate the extent to which the multiple phosphorylation model could simultaneously account for single-photon response behavior, as well as responses to high light levels causing complete response saturation and/or significant light adaptation (LA). To date no single model, with one set of parameters, has been able to do this. Dim-flash responses and statistics were simulated using a hybrid stochastic/deterministic model and Monte-Carlo methods as in Hamer et al. (2003). A dark-adapted flash series, and stimulus paradigms from the literature eliciting various degrees of light adaptation (LA), were simulated using a full differential equation version of the model that included the addition of Ca2+-feedback onto rhodopsin kinase via recoverin. With this model, using a single set of parameters, we attempted to account for (1) SPR waveforms and statistics (as in Hamer et al., 2003); (2) a full dark-adapted flash-response series, from dim flash to saturating, bright flash levels, from a toad rod; (3) steady-state LA responses, including LA circulating current (as in Koutalos et al., 1995) and LA flash sensitivity measured in rods from four species; (4) step responses from newt rods ( Forti et al., 1989) over a large dynamic range; (5) dynamic LA responses, such as the step-flash paradigm of Fain et al. (1989), and the two-flash paradigm of Murnick and Lamb (1996); and (6) the salient response features from four knockout rod preparations. The model was able to meet this stringent test, accounting for almost all the salient qualitative, and many quantitative features, of the responses across this broad array of stimulus conditions, including SPR reproducibility. The model promises to be useful in testing hypotheses regarding both normal and abnormal photoreceptor function, and is a good starting point for development of a full-range model of cone phototransduction. Informative limitations of the model are also discussed.

Functional Diversity of Endothelin Pathways in Human Lung Fibroblasts May Be Based on Structural Diversity of the Endothelin Receptors

Posttranslational modifications of the endothelin receptors A and B from human lung fibroblasts were investigated before and after stimulation of the cells with (dA)(30)-5'-S-EMC-endothelin-1. The patterns of phosphorylation and palmitoylation of both receptors were much more complicated than expected. In both the stimulated and the unstimulated states, multiple isoforms differing in the number and location of posttranslational modifications were present. MS analyses suggested rapid changes in these isoforms following stimulation. Overall, the ETA receptor was modified at 20 sites (15 phosphorylation, five palmitoylation sites) and ETB at 17 sites (13 phosphorylation, four palmitoylation sites). Part of the structural diversity involved hypermodification of short sequence regions, and it is suggested that this could represent a mechanism for incremental modulation of receptor activity. It is postulated that the observed structural diversity over disparate parts of the receptor sequences forms the basis for parallel stimulation of different signaling pathways at spatially and functionally distinct ET receptors differing in posttranslational modifications.

Multiple steps of phosphorylation of activated rhodopsin can account for the reproducibility of vertebrate rod single-photon responses

Single-photon responses (SPRs) in vertebrate rods are considerably less variable than expected if isomerized rhodopsin (R*) inactivated in a single, memoryless step, and no other variability-reducing mechanisms were available. We present a new stochastic model, the core of which is the successive ratcheting down of R* activity, and a concomitant increase in the probability of quenching of R* by arrestin (Arr), with each phosphorylation of R* (Gibson, S.K., J.H. Parkes, and P.A. Liebman. 2000. Biochemistry. 39:5738-5749.). We evaluated the model by means of Monte-Carlo simulations of dim-flash responses, and compared the response statistics derived from them with those obtained from empirical dim-flash data (Whitlock, G.G., and T.D. Lamb. 1999. Neuron. 23:337-351.). The model accounts for four quantitative measures of SPR reproducibility. It also reproduces qualitative features of rod responses obtained with altered nucleotide levels, and thus contradicts the conclusion that such responses imply that phosphorylation cannot dominate R* inactivation (Rieke, F., and D.A. Baylor. 1998a. Biophys. J. 75:1836-1857; Field, G.D., and F. Rieke. 2002. Neuron. 35:733-747.). Moreover, the model is able to reproduce the salient qualitative features of SPRs obtained from mouse rods that had been genetically modified with specific pathways of R* inactivation or Ca2+ feedback disabled. We present a theoretical analysis showing that the variability of the area under the SPR estimates the variability of integrated R* activity, and can provide a valid gauge of the number of R* inactivation steps. We show that there is a heretofore unappreciated tradeoff between variability of SPR amplitude and SPR duration that depends critically on the kinetics of inactivation of R* relative to the net kinetics of the downstream reactions in the cascade. Because of this dependence, neither the variability of SPR amplitude nor duration provides a reliable estimate of the underlying variability of integrated R* activity, and cannot be used to estimate the minimum number of R* inactivation steps. We conclude that multiple phosphorylation-dependent decrements in R* activity (with Arr-quench) can confer the observed reproducibility of rod SPRs; there is no compelling need to invoke a long series of non-phosphorylation dependent state changes in R* (as in Rieke, F., and D.A. Baylor. 1998a. Biophys. J. 75:1836-1857; Field, G.D., and F. Rieke. 2002. Neuron. 35:733-747.). Our analyses, plus data and modeling of others (Rieke, F., and D.A. Baylor. 1998a. Biophys. J. 75:1836-1857; Field, G.D., and F. Rieke. 2002. Neuron. 35:733-747.), also argue strongly against either feedback (including Ca2+-feedback) or depletion of any molecular species downstream to R* as the dominant cause of SPR reproducibility.

Ca2+-dependent control of rhodopsin phosphorylation: recoverin and rhodopsin kinase

Over many years until the middle of the 1980s, the main problem in vision research had been the mechanism of transducing the visual signal from photobleached rhodopsin to the cationic channels in the plasma membrane of a photoreceptor to trigger the electrophysiological response of the cell. After cGMP was proven to be the secondary messenger, the main intriguing question has become the mechanisms of negative feedback in photoreceptors to modulate their response to varying conditions of illumination. Although the mechanisms of light-adaptation are not completely understood, it is obvious that Ca2+ plays a crucial role in these mechanisms and that the effects of Ca2+ can be mediated by several Ca2+-binding proteins. One of them is recoverin. The leading candidate for the role of an intracellular target for recoverin is believed to be rhodopsin kinase, a member of a family of G-protein-coupled receptor kinases. The present review considers recoverin, rhodopsin kinase and their interrelationships in the in vitro as well as in vivo contexts.

Effect of light and protein phosphorylation on photoreceptor rod outer segment acyltransferase activity

Rod outer segments (ROS) exhibit high acyltransferase (AT) activity, the preferred substrate of which being lysophosphatidylcholine. To study factors possibly regulating ROS AT activity purified ROS membranes were assayed under conditions under which protein kinase C (PKC), cAMP-dependent protein kinase (PKA), and phosphatases were stimulated or inhibited. PKC activation produced a significant increase in the acylation of phosphatidylethanolamine (PE) and phosphatidylinositol (PI) with oleate, it inhibited phosphatidylcholine (PC) acylation, and phosphatidylserine (PS) and phosphatidic acid (PA) acylation remained unchanged. ROS PKA activation resulted in increased oleate incorporation into PS and PI while the acylation of PC, PE, and PA remained unchanged. Inhibition of ROS PKC or PKA produced, as a general trait, inverse effects with respect to those observed under kinase-stimulatory conditions. ROS phosphatase 2A was inhibited by using okadaic acid, and the changes observed in AT activity are described. These findings suggest that changes in ROS protein phosphorylation produce specific changes in AT activity depending on the phospholipid substrate. The effect of light on AT activity in ROS membranes was also studied and it is reported that acylation in these membranes remains unchanged independent of the illumination condition used.

Mass spectrometric analysis of porcine rhodopsin

Rhodopsin is the dim light photosensitive pigment of animals. In this work, we undertook to study the structure of rhodopsin from swine and compare it with bovine and rat rhodopsin. Porcine rhodopsin was analyzed using methodology developed previously for mass spectrometric analysis of integral membrane proteins. Combining efficient protein cleavage and high performance liquid chromatography separation with the sensitivity of mass spectrometry (MS), this technique allows the observation of the full protein map and the posttranslational modifications of the protein in a single experiment. The rhodopsin protein from a single porcine eye was sequenced completely, with the exception of two single-amino acid fragments and one two-amino acid fragment, and the gene sequence reported previously was confirmed. The posttranslational modifications, similar to the ones reported previously for bovine and rat rhodopsin, were also identified. Although porcine rhodopsin has a high degree of homology to bovine and rat rhodopsins and most of their posttranslational modifications are identical, the glycosylation and phosphorylation patterns observed were different. These results show that rhodopsin from a single porcine eye can be characterized completely by MS. This technology opens the possibility of rhodopsin structural and functional studies aided by powerful mass spectrometric analysis, using the fellow eye as an internal control.

Rapid and reproducible deactivation of rhodopsin requires multiple phosphorylation sites

Efficient single-photon detection by retinal rod photoreceptors requires timely and reproducible deactivation of rhodopsin. Like other G protein-coupled receptors, rhodopsin contains multiple sites for phosphorylation at its COOH-terminal domain. Transgenic and electrophysiological methods were used to functionally dissect the role of the multiple phosphorylation sites during deactivation of rhodopsin in intact mouse rods. Mutant rhodopsins bearing zero, one (S338), or two (S334/S338) phosphorylation sites generated single-photon responses with greatly prolonged, exponentially distributed durations. Responses from rods expressing mutant rhodopsins bearing more than two phosphorylation sites declined along smooth, reproducible time courses; the rate of recovery increased with increasing numbers of phosphorylation sites. We conclude that multiple phosphorylation of rhodopsin is necessary for rapid and reproducible deactivation.

Evolution of visual pigments and related molecules

The molecular phylogenetic tree of vertebrate visual pigments, constructed on the basis of amino acid sequence identity, suggests that the visual pigments can be classified into five groups (L, ML, MS, S and Rh) and that their genes have evolved along these five gene lines. Goldfish has a UV-sensitive visual pigment (S group) localized in miniature single cone cells. Medaka has one type of rod cell containing rhodopsin (Rh group) and four types of cone cells, each of which contains a specific visual pigment with an absorption maximum that differs from those of the others. Frogs have a violet-sensitive visual pigment (S group) in small single cone cells and a blue-sensitive visual pigment (MS group) in green rod cells. Although nocturnal and diurnal geckos have rod- and cone-based retinas, respectively, they have phylogenetically closely related visual pigments. The pigments in each line may have restricted absorption maxima. We have cloned cDNAs encoding molecules involved in the phototransduction system of visual cells, such as phosphodiesterase, opsin kinase and arrestin. We then constructed phylogenetic trees of these molecules with the deduced amino acid sequences. The resulting phylogenetic trees show that these molecules are classified into two groups; one is expressed in cones and another in rods, suggesting that rods and cones contain homologous molecules with different amino acid sequences. These differences may result in the different light responses of rods and cones.

Spectral tuning of a circadian photopigment in a subterranean 'blind' mammal (Spalax ehrenbergi)

The atrophied subcutaneous eyes of Spalax ehrenbergi (the blind mole rat) express a long wavelength sensitive (LWS) cone opsin. Our data provide strong evidence that this photopigment is spectrally tuned to enhance photon capture in the red light environment of the eye. Furthermore, novel mechanisms appear partially responsible for this sensory fine-tuning. These data support the hypothesis that the LWS opsin of Spalax acts as a functional photopigment and that it is not a 'residue' of the pre-subterranean visual system. As the eye of Spalax has only one known function, the entrainment of circadian rhythms to environmental light, the LWS photopigment is implicated in this task. These results, together with our recent findings that rod and cone photopigments are not required for murine photoentrainment, suggest that multiple photopigments (classical and novel) mediate the effects of light on the mammalian circadian system.

Structure-activity relationships of G protein-coupled receptors

The primary function of cell-surface receptors is to discriminate the specific signaling molecule or ligand from a large array of chemically diverse extracellular substances and to activate an effector signaling cascade that triggers an intracellular response and eventually a biological effect. G protein-coupled cell-surface receptors (GPCRs) mediate their intracellular actions through the activation of guanine nucleotide-binding signal-transducing proteins (G proteins), which form a diverse family of regulatory GTPases that, in the GTP-bound state, bind and activate downstream membrane-localized effectors. Hundreds of GPCRs signal through one or more of these G proteins in response to a large variety of stimuli including photons, neurotransmitters, and hormones of variable molecular structure. The mechanisms by which these ligands provoke activation of the receptor/G-protein system are highly complex and multifactorial. Knowledge and mapping of the structural determinants and requirements for optimal GPCR function are of paramount importance, not only for a better and more detailed understanding of the molecular basis of ligand action and receptor function in normal and abnormal conditions, but also for a rational design of early diagnostic and therapeutic tools that may allow exogenous regulation of receptor and G protein function in disease processes.

Correlations in palmitoylation and multiple phosphorylation of rat bradykinin B2 receptor in Chinese hamster ovary cells

Rat bradykinin B2 receptor from unstimulated Chinese hamster ovary cells transfected with the corresponding cDNA has been isolated, and subsequent mass spectrometric analysis of multiple phosphorylated species and of the palmitoylation attachment site is described. Bradykinin B2 receptor was isolated on oligo(dT)-cellulose using N-(epsilon-maleimidocaproyloxy)succinimide-Met-Lys-bradykinin coupled to a protected (dA)30-mer. This allowed a one-step isolation of the receptor on an oligo(dT)-cellulose column via variation solely of salt concentration. After enzymatic in-gel digestion, matrix-assisted laser desorption ionization and electrospray ion trap mass spectrometric analysis of the isolated rat bradykinin B2 receptor showed phosphorylation at Ser365, Ser371, Ser378, Ser380, and Thr374. Further phosphorylation at Tyr352 and Tyr161 was observed. Rat bradykinin receptor B2 receptor is also palmitoylated at Cys356. All of the phosphorylation sites except for Tyr161 cluster at the carboxyl-terminal domain of the receptor located on the cytoplasmic face of the cell membrane. Surprisingly, many of the post-translational modifications were shown by MSn mass spectroscopic analysis to be correlated pairwise, e.g. diphosphorylation at Ser365 and Ser371, at Ser378 and Ser380, and at Thr374 and Ser380 as well as mutually exclusive phosphorylation at Tyr352 and palmitoylation at Cys356. The last correlation may be involved in a receptor internalization motif. Pairwise correlations and mutual exclusion of phosphorylation and palmitoylation suggest critical roles of multiple post-translational modifications for the regulation of activity, coupling to intracelluar signaling pathways, and/or sequestration of the bradykinin receptor.

Phosphorylation of rod outer segment proteins modulates phosphatidylethanolamine N-methyltransferase and phospholipase A2 activities in photoreceptor membranes

The activities of enzymes involved in lipid metabolism--phospholipase A2 (PLA2) and phosphatidylethanolamine N-methyltransferase (PE N-MTase)--were found to be differently affected by pre-incubation of rod outer segments (ROS) under protein phosphorylating or dephosphorylating conditions. Exposure to cAMP-dependent protein kinase (PKA), under dark or light conditions, produced a significant increase in PE N-MTase activity, whereas PLA2 activity decreased. Under standard protein kinase C (PKC) phosphorylating conditions in light, PE N-MTase activity was stimulated and PLA2 activity was not affected. When the assays were performed in the dark, both enzymatic activities were unaffected when compared to the corresponding controls. Incubation of ROS membranes in light in the presence of PKC activators phorbol 12,13-dibutyrate (PDBu) and dioctanoylglycerol (DOG) resulted in the same pattern of changes in enzyme activities as described for standard PKC phosphorylating condition. Pre-incubation of membranes with the PKC inhibitor H-7 reduced the stimulation of PDBu on PE N-MTase activity, and had no effect on PLA2 activity in ROS membranes incubated with the phorbol ester. Pre-treatment of isolated ROS with alkaline phosphatase resulted in decreased PE N-MTase activity and produced a significant stimulation of PLA2 activity under dark as well as under light conditions when compared to the corresponding controls. These findings suggest that ROS protein phosphorylation and dephosphorylation modulates PE N-MTase and PLA2 activities in isolated ROS, and that these activities are independently and specifically modulated by particular kinases. Furthermore, dephosphorylation of ROS proteins has the opposite effect to that produced by protein phosphorylation on the enzymes studied.

The role of receptor kinases and arrestins in G protein-coupled receptor regulation

G protein-coupled receptors (GPRs) play a key role in controlling hormonal regulation of numerous second-messenger pathways. However, following agonist activation, most GPRs rapidly lose their ability to respond to hormone. For many GPRs, this process, commonly referred to as desensitization, appears to be primarily mediated by two protein families: G protein-coupled receptor kinases (GRKs) and arrestins. GRKs specifically bind to the agonist-occupied receptor, thereby promoting receptor phosphorylation, which in turn leads to arrestin binding. Arrestin binding precludes receptor/G protein interaction leading to functional desensitization. Many GPRs are then removed from the plasma membrane via clathrin-mediated endocytosis. Recent studies have implicated endocytosis in the resensitization of GPRs and have linked both GRKs and arrestins to this process. In this review, we discuss the role of GRKs and arrestins in regulating agonist-specific signaling and trafficking of GPRs.

Rhodopsin phosphorylation and its role in photoreceptor function

Light-stimulated phosphorylation of rhodopsin was first described 25 years ago. This paper reviews the progress that has been made towards (i) understanding the nature of the enzymes that phosphorylate and dephosphorylate rhodopsin (ii) identifying the sites of phosphorylation on rhodopsin and (iii) understanding the physiological importance of rhodopsin phosphorylation. Many important questions related to rhodopsin phosphorylation remain unanswered and new strategies and methods are needed to address issues such as the roles of Ca2+ and recoverin. We present one such method that uses mass spectrometry to quantitate rhodopsin phosphorylation in intact mouse retinas.

A novel subtype of G-protein-coupled receptor kinase, GRK7, in teleost cone photoreceptors

Two kinds of retinal cDNA fragments (OIGRK-R and -C) encoding the putative G-protein-coupled receptor kinases (GRKs) were isolated from medaka, Oryzias latipes. OIGRK-R appears to be closely related to the rhodopsin kinase (RK) found in the outer segments of mammalian photoreceptors, but the deduced amino acid sequence of OIGRK-C shows less than 50% identity to those of GRKs known to date, suggesting that OIGRK-C is a novel GRK subtype (GRK7). The mRNA of OIGRK-R is detectable in rods, and that of OIGRK-C is found in all four types of cone photoreceptor. The C-terminal of OIGRK-R has a consensus sequence for farnesylation, whereas, surprisingly, OIGRK-C has a consensus sequence for geranylgeranylation. Our result are consistent with the concept that lower vertebrates have rod- and cone-specific opsin kinases.

Post-translational modifications of endothelin receptor B from bovine lungs analyzed by mass spectrometry

A new mild experimental approach for isolation of peptide membrane receptors and subsequent analysis of post-translational modifications is described. Endothelin receptors A and B were isolated on oligo(dT)-cellulose using N-(epsilon-maleimidocaproyloxy)succinimide endothelin coupled to a protected (dA)-30-mer. This allowed a one-step isolation of the receptor from oligo(dT)-cellulose via variation solely of salt concentration. The identity of the receptor was confirmed by direct amino acid sequencing of electroblotted samples or by using antibodies against ETA and ETB receptors. The method used here is very fast, requires only very mild elution conditions and, for the first time, gave both ETA and ETB receptors concurrently in very good yield. Following enzymatic in-gel digestion, MALDI, and electrospray ion trap mass spectrometric analysis of the isolated endothelin B receptor showed phosphorylation at Ser-304, -418, -438, -439, -440, and -441. Further phosphorylation at either Ser-434 or -435 was observed. The endothelin B receptor is also palmitoylated at Cys residues 402 and 404. Phosphorylation of Ser304 may play a role in Hirschsprung's disease.

The C-terminal domain of the human EP4 receptor confers agonist-induced receptor desensitization in a receptor hybrid with the rat EP3beta receptor

Prostaglandin E2 receptors (EPR), which belong to the family of heterotrimeric G protein-coupled ectoreceptors with seven transmembrane domains, can be classified into four subtypes according to their ligand binding and G protein coupling specificity. Of these, EP3betaR is coupled to Gi, whereas EP4R is coupled to Gs. EP4R, in contrast to EP3betaR, shows agonist-induced desensitization. The C-terminal domain and the third intracellular loop of these receptors have been implicated in G protein coupling specificity and desensitization. Here, receptor hybrids consisting of the main portion of rat EP3betaR and either the C-terminal domain or the third intracellular loop of human EP4R were used to study the contribution of the respective receptor domains to G protein coupling and desensitization. Neither the EP4R C-terminal domain nor the EP4R third intracellular loop alone was sufficient to change the coupling specificity of the rEP3hEP4 receptor hybrids from Gi to Gs or to confer additional Gs coupling. However, the EP4R C-terminal domain but not the third intracellular loop was necessary and sufficient to mediate rapid agonist-induced, second messenger-independent desensitization in the Gi-coupled hybrid receptors.

Phosphorylation of phospholipase C-coupled receptors

Early work on G-protein-coupled receptor (GPCR) phosphorylation focused on the adenylyl cyclase-linked beta-adrenoceptor, where phosphorylation at sites on the C-terminal tail and within the third intracellular loop results in receptor desensitisation. In recent years, intense research activity has revealed that a large number of GPCR subtypes exist as phosphoproteins, where the level of phosphorylation is dramatically increased subsequent to receptor stimulation. Among these receptor subtypes are those receptors coupled to phospholipase C (PLC). It appears, therefore, that regulation via receptor phosphorylation is a mechanism employed by all but a few GPCRs, including those coupled to PLC. Because the majority of GPCRs are coupled to the phosphoinositide signalling pathway, receptor phosphorylation of PLC-coupled receptors is a regulatory process with profound physiological significance for a huge array of biological responses. This review discusses the properties of homologous and heterologous phosphorylation of PLC-coupled receptors, together with the receptor kinases involved and the functional significance of receptor phosphorylation.

Arrestins expressed in killifish photoreceptor cells

Two kinds of cDNA fragments (KfhArr-R and KfhArr-C) encoding the putative arrestins of killifish, Oryzias latipes, were isolated. The distributions of these transcripts were investigated by in situ hybridization, and it was demonstrated that KfhArr-R and KfhArr-C are expressed in, respectively, rod and all four types of cone cells. The deduced amino acid sequences of KfhArr-R and KfhArr-C are closely related to human S-antigen (rod arrestin) and X-arrestin (cone arrestin), respectively. Phylogenetic analysis of arrestin sequences suggests that vertebrate visual arrestins form a single cluster distinct from other arrestins and diverged to form rod and cone subtypes before the divergence between teleosts and tetrapods. It is speculated that the divergence pattern of vertebrate visual arrestins may prove to be reflected in the divergence of the proteins participating in the respective phototransduction cascades.

A novel and ancient vertebrate opsin

We describe the identification of a novel opsin gene isolated from the eyes of Atlantic salmon. The cDNA sequence predicts a protein that has the key features of an opsin, but shows only 32-42% amino acid identity to the known opsin families. Phylogenetic analysis suggests that this opsin is a member of a hitherto unrecognised opsin family that diverged early in the evolution of vertebrate photopigments. We have tentatively called this opsin family the vertebrate ancient (VA) opsins. The identification of VA opsin may ultimately help to resolve some of the uncharacterised photoreceptor functions of the eye, which include the regulation of circadian rhythms, pupil size and corneal pigmentation.

Novel mechanism for the activation of rhodopsin kinase: implications for other G protein-coupled receptor kinases (GRK's)

ATP, its nonhydrolyzable analogue, AMP-PNP, and albumin were found to promote the dissociation of rhodopsin kinase from rod outer segments (ROS) containing photoactivated-rhodopsin (Rho*). These features were embodied in a protocol for the recovery of rhodopsin kinase from incubations containing ROS which had been subjected to a wide range of treatments. It was found that the supernatants recovered from mixtures containing ATP, rhodopsin kinase, and photolyzed ROS membranes catalyzed a Rho*-independent peptide phosphorylation as well as dark-phosphorylation of rhodopsin. The activities of this activated kinase in the two aforementioned assays were 7-8% of the maximum intrinsic activity found in appropriate standard assays (i.e., light-stimulated phosphorylation of rhodopsin and Rho*-dependent peptide phosphorylation). The activated kinase reverted to its inactive resting-state in a time dependent fashion, giving a tau 1/2 of decay of approximately 2 min. The intrinsic activity of kinase as measured by the standard assay, however, remained constant during this decay period. No positive evidence was found to suggest that the interconversion activated kinase <--> inactive kinase occurred by a phosphorylation event. Cumulatively, the results show that the interaction of rhodopsin kinase.ATP complex with Rho* leads to the formation, presumably due to the reorganization of the protein structure, of a soluble active kinase species which reverts to the inactive resting state in a time-dependent fashion.

Mechanism of rhodopsin phosphorylation

A key reaction in the inactivation of rhodopsin is its phosphorylation by rhodopsin kinase. In recent years, extensive studies related to rhodopsin kinase function and enzymatic properties were carried out. Rhodopsin kinase is a Ser/Thr protein kinase and a member of the G protein-coupled receptor kinases sub-family (GRKs) which consists of six recently identified members. Photolyzed rhodopsin is phosphorylated by rhodopsin kinase sequentially, with the first phosphate transferred preferentially to Ser-338, and subsequent phosphates transferred to Ser-343 and Thr-336. The binding of arrestin to the receptor, and reduction of the photolyzed chromophore all-trans-retinal to all-trans-retinol limits physiologically significant phosphorylation at no more than three sites (H. Ohguro, R.S. Johnson, L.H. Ericsson, K.A. Walsh and K. Palczewski, Biochemistry, 33 (1994) 1023). A similar phosphorylation reaction is implicated in most, if not all, G protein-coupled receptors during their desensitization.

Arrestin binding determines the rate of inactivation of the G protein-coupled receptor rhodopsin in vivo

G protein-coupled receptor inactivation is a crucial feature of cellular signaling systems; this process determines the catalytic lifetime of the activated receptor and is necessary for response termination. Although previous work has indicated a class of models in which several sequential steps are required for receptor inactivation, the rate-limiting event is still unclear. In this paper, we develop a theory that describes the kinetics of inactivation of the G protein-coupled receptor rhodopsin based on the rate of arrestin binding and test the theory using a combination of genetic and electrophysiological techniques in Drosophila photoreceptors. The theory quantitatively describes the inactivation kinetics of activated rhodopsin in vivo and can be independently tested with molecular and spectroscopic data. The results demonstrate that the rate of arrestin binding determines the kinetics of receptor inactivation in vivo and thus is the event that controls signal amplification at the first step of this G protein-coupled transduction cascade.

Rhodopsin kinase: studies on the sequence of and the recognition motif for multiphosphorylations

Peptides of 10-12 amino acids in length, which overlapped with the sequence of the last 20 amino acids in the C-terminal tail of rhodopsin, were synthesized and used as substrates for rhodopsin kinase. In all cases the phosphorylation of the peptides was found to be greatly stimulated (> 20-fold) by the presence of light-activated rhodopsin (Rho*). The incorporation of 32P at seven Ser/Thr residues that are the potential sites of phosphorylation was quantified, and the results were analyzed in terms of two parameters. First, a global comparison of phosphorylation at each site was made when the propensity for the modification was found to be in the order: Ser 343 > Ser 338 > Thr 336 > Ser 334, Thr 342 > Thr 335, Thr 340. Second, the peptides were aligned on a hypothetical template with the residue to be phosphorylated occupying the P-position, and the manner in which the nature of the surrounding residues effected the phosphorylation was assessed. It was found that the optimal phosphorylation of the P-site Ser/Thr occurs if it has at least one residue on the amino side and five on the acyl side and also contains a neutral residue, preferably small (A, P, S, T) at the P+4 position.(ABSTRACT TRUNCATED AT 250 WORDS)

A segment corresponding to amino acids Val170-Arg182 of bovine arrestin is capable of binding to phosphorylated rhodopsin

In retinal rods, photoexcited rhodopsin (R*) is inactivated upon phosphorylation by rhodopsin kinase and the subsequent binding of arrestin. We have studied the structural role of a cationic region of bovine arrestin (Val170-Arg182) using anti-peptide IgGs specifically recognizing this segment and the corresponding oligopeptide. Our results clearly indicate that amino acids Val170-Arg182 are shielded within the arrestin-rhodopsin-complex and very likely belong to a binding domain of arrestin for phosphorylated R*. The purified anti-peptide IgGs strongly reacted with isolated arrestin but did not recognize arrestin when bound to phosphorylated R*. In agreement with these experiments, the oligopeptide Val170-Arg182 was found to compete with arrestin for binding to phosphorylated R*. Increasing concentrations of this peptide caused an oligomerization of phosphorylated rhodopsin when illuminated by white light as well as in the dark. Unphosphorylated rhodopsin did not oligomerize up to a 400-fold molar ratio of peptide/rhodopsin. Limited proteolysis of the phosphorylated carboxy-terminus of rhodopsin with endoproteinase Asp-N caused a significant decrease in the peptide-induced formation of oligomers. Therefore, Val170-Arg182 of bovine arrestin probably interacts with the phosphorylated carboxy-terminus of rhodopsin. The data presented support the proposal of Palczewski et al. (1991c) considering the region Lys163-Arg182 in bovine arrestin to be a possible binding domain for phosphorylated R*.

The phospho-opsin phosphatase from bovine rod outer segments. An insight into the mechanism of stimulation of type-2A protein phosphatase activity by protamine

The vertebrate visual transduction system involves a cycle of phosphorylation and dephosphorylation of a transmembranous photoreceptor (rhodopsin). Upon illumination, the activated photoreceptor (metarhodopsin-II) is phosphorylated by a specific kinase on up to seven serine and threonine residues. A dephosphorylation process must then be undertaken to return the photoreceptor to its ground state. Initial work, along with studies using the rabbit skeletal muscle catalytic subunit of protein phosphatase 2A, indicated that the phosphatase responsible was a member of the type-2 family. The work has been further extended and using 1000 bovine retinae, the catalytic subunit and a holoenzyme form of phospho-opsin phosphatase were purified 2100-fold and 550-fold respectively. The stimulation of the activities of both these fractions with protamine sulphate and the inhibition by okadaic acid are consistent with the fact that these phosphatases belong to the type-2A family. Western blotting using a variety of specific antibodies established that the catalytic subunit (36 kDa, C subunit) was indeed of type 2A, while the holoenzyme was a heterotrimer comprising the preceding catalytic subunit complexed to two other polypeptides of 55 kDa (B subunit) and 65 kDa (A subunit), both of which were of alpha subtype; phospho-opsin phosphatase may thus be described as a trimeric enzyme containing the ABC subunits of type-2A protein phosphatase, i.e. PP2A1. The dephosphorylation of phospho-opsin by both fractions was found to be stimulated (4-8-fold) by the presence of protamine sulphate (250 micrograms/ml; 50 microM). However, when phospho-peptides corresponding to the C-terminal region of opsin were used, these were maximally dephosphorylated without requiring the presence of protamine; at equivalent concentrations of substrates the phospho-peptides were dephosphorylated (in the absence of protamine) at rates which were approximately equal to those obtained with phospho-opsin (in the presence of protamine). It was shown that type-1 phosphatases had little activity against these phospho-peptides. Furthermore, if phospho-opsin was treated with protamine, the activity of the phosphatase assumed an elevated level and was not significantly stimulated by the addition of exogenous protamine. This effect could be reversed by washing the protamine-treated substrate with 1 M NaCl, whence the protamine-dependent stimulation returned to normal levels. To this end, studies revealed that protamine was binding to the particulate substrate in a ratio of protamine/opsin of 0.7:1. The cumulative finding may be rationalised by suggesting that the effect of protamine is a substrate-directed phenomenon and a hypothetical mechanism for this effect is considered.

Location of agonist-dependent-phosphorylation sites in the third intracellular loop of muscarinic acetylcholine receptors (m2 subtype)

Muscarinic acetylcholine receptors (mAChR, human m2 subtype) expressed in Sf9 (Spodoptera frugiperda) cells using the baculovirus system were purified and subjected to phosphorylation by a mAChR kinase, which was partially purified from porcine cerebrum. Two bands with apparent molecular masses of 59 kDa and 39 kDa as determined by SDS/PAGE were found to be phosphorylated in an agonist-dependent manner. Both bands were labeled by the irreversible muscarinic ligand [3H]propylbenzilylcholine mustard. Molecular masses of the [32P]phosphorylated or [3H]propylbenzilylcholine-mustard-labeled bands decreased following treatment with N-glycanase. The 59-kDa and 39-kDa bands were converted to 52-kDa and 32-kDa bands, respectively, indicating that both the 59-kDa and 39-kDa bands contain the amino-terminal region where glycosylation sites are present. The ratio of incorporated [32P]phosphate and bound [3H]propylbenzilylcholine mustard was essentially the same for the 59-kDa and 39-kDa bands, indicating that all the phosphorylation sites reside in the sequence of 39 kDa from the amino-terminal region. The amounts of incorporated [32P]phosphate were estimated to be 10-11/receptor, with 7-8 serine and 3-4 threonine, but no phosphorylated tyrosine residues. Further treatment of [32P]phosphorylated or [3H]propylbenzilylcholine-mustard-labeled receptors with V8 protease indicated that the phosphorylation sites were not present in 30-kDa amino-terminal segment. These results indicate that the phosphorylation sites are localized in the range 30-39 kDa from the amino terminus, which consists of primarily the central part of the third intracellular loop. Consistent with this conclusion, a fusion protein containing glutathione S-transferase linked to a peptide corresponding to residues 227-324 of the central part of the third intracellular loop was found to be phosphorylated by the mAChR kinase in a heparin-sensitive manner.

Analysis of protein modifications: recent advances in detection, characterization and mapping

The past year has seen several contributions, both in methods for determining and characterizing chemical modifications of proteins and in related technologies used to map peptides. These contributions mainly involve improvements in capillary zone electrophoresis and in various applications of mass spectrometry.

Phosphorylation of bovine rod photoreceptor cyclic GMP phosphodiesterase

The cyclic GMP phosphodiesterase (PDE) of retinal rods plays a key role in phototransduction and consists of two catalytic subunits (PDE alpha and PDE beta) and two identical inhibitory subunits (PDE gamma). Here we report that PDE alpha and PDE gamma are phosphorylated by protein kinase(s) C (PKC) from brain and rod outer segments (ROS). These same two types of PKC also phosphorylate PDE alpha in trypsin-activated PDE (without PDE gamma). In contrast, cyclic-AMP-dependent protein kinase catalytic subunit phosphorylates both PDE alpha and PDE beta, but not PDE gamma. This kinase does not phosphorylate trypsin-activated PDE. The synthetic peptides AKVISNLLGPREAAV (PDE alpha 30-44) and KQRQTRQFKSKPPKK (PDE gamma 31-45) inhibited phosphorylation of PDE by PKC from ROS. These data suggest that sites (at least one for each subunit) for phosphorylation of PDE by PKC are localized in these corresponding regions of PDE alpha and PDE gamma. Isoenzyme-specific PKC antibodies against peptides unique to the alpha, beta, gamma, delta, epsilon and zeta isoforms of protein kinase C were used to show that a major form of PKC in ROS is PKC alpha. However, other minor forms were also present.

Phosphorylation sites in bovine rhodopsin

Bovine rhodopsin has been phosphorylated in rod outer segments by ATP and endogenous rhodopsin kinase. Mono-, di-, and triphosphorylated rhodopsins have been prepared by chromatofocusing. Nearly all of the phosphate is found in peptide 330-348, formed by digestion of phosphorhodopsins with endoproteinase Asp-N. Sequence analysis of the phosphopeptides shows that monophosphorylated rhodopsin consists of a mixture containing rhodopsins phosphorylated at 338Ser and 343Ser. Diphosphorylated rhodopsin is phosphorylated at both 338Ser and 343Ser. When rhodopsin becomes triphosphorylated it does not become phosphorylated on 334Ser but appears to become phosphorylated on one or more of the four threonine residues: 335Thr, 336Thr, 340Thr, and 342Thr.

Cooperativity during multiple phosphorylations catalyzed by rhodopsin kinase: supporting evidence using synthetic phosphopeptides

Rhodopsin kinase is a key component in the shutdown of visual transduction. The phosphorylation of rhodopsin's C-terminus was evaluated using synthetic peptides derived from the last 12 amino acids (337-348) as substrates and their phosphorylated counterparts as inhibitors. It was found that synthetic peptides were phosphorylated at the serine residue corresponding to Ser-343 in the primary sequence of bovine rhodopsin. The phosphopeptides were prepared by incorporating into the peptide chain a trityl-protected serine derivative at the site destined to contain the phosphoryl group. The trityl group was selectively released with 20% (v/v) dichloroacetic acid; the free hydroxyl group was then phosphitylated with di-tert-butyl N,N-diethylphosphoramidite, and the resulting phosphite derivative was oxidized with m-chloroperoxybenzoic acid. The phosphopeptides were found to have a greater affinity for the kinase compared with their nonphosphorylated counterparts; for the peptides corresponding to residues 337-348 of rhodopsin the affinity increased in the order VSKTETSQVAPA < VSKTETS[PO3H2]QVAPA < VS[PO3H2]KTETS[PO3H2]QVAPA. The results are interpreted to support the cooperativity hypotheses proposed previously [Wilden, U., & Kühn, H. (1982) Biochemistry 21, 3014-3022; Aton, B. R., Litman, B. J., & Jackson, M. L. (1984) Biochemistry 23, 1737-1741].

Phosphorylation of solubilised dark-adapted rhodopsin. Insights into the activation of rhodopsin kinase

A protocol for the separation of phosphorhodopsin from phospho-opsin has been developed. The method takes advantage of the finding that, while 0.5% N,N-dimethyldodecylamine-N-oxide completely solubilises membrane-embedded phosphorhodopsin, at this concentration of detergent, phospho-opsin is only sparingly soluble. Phosphorhodopsin solubilised in this manner may be freed from contaminant phospho-opsin by chromatography on hydroxyapatite. Using this method, the rhodopsin-kinase-catalysed phosphorylation of photoexcited rhodopsin and native rhodopsin was studied in rod outer-segment membranes at different levels of bleaching. Prior to analysis of the phosphorylation mixture, the phosphorylated form of photoexcited rhodopsin was converted into phospho-opsin by treatment with NH2OH. It was found that, while at a 5% bleach level the amount of phosphorhodopsin produced was 15% that of phospho-opsin, at 60% bleaching the phosphorhodopsin was less than 1% of phospho-opsin. The phosphorylation reaction under different bleaching conditions was also studied in a completely soluble system (using 2% dodecyl maltoside) and the pattern of phosphate incorporation into rhodopsin versus opsin was identical to that in the membrane system. We have previously proposed that rhodopsin kinase normally exists in an inactive form and is only activated following interaction with photoexcited rhodopsin. The present work strengthens this conclusion and also shows that, following activation, the kinase preferentially phosphorylates photoexcited rhodopsin but can also act upon unbleached rhodopsin. Two possible mechanisms for the activation of the kinase are considered. From the distribution of phosphorhodopsin and phospho-opsin at different bleaching levels, the relative rates of the phosphorylation of photoexcited rhodopsin (kR*) and rhodopsin (kR) were calculated. kR*/kR values for the membrane system of 71 +/- 20 and, for the solubilised system, of 80 +/- 19 were obtained. The algebraic equation used to obtain these values highlights the fact that the ratio of the concentrations of the two substrates, photoexcited rhodopsin and rhodopsin, in a sample, determines the final distribution of phosphate between bleached and unbleached rhodopsin. This conclusion may contribute to the understanding of 'high-gain' phosphorylation observed previously.