Phosphorylation sites in bovine rhodopsin

A broadly conserved g-protein-coupled receptor kinase phosphorylation mechanism controls Drosophila smoothened activity

Hedgehog (Hh) signaling is essential for normal growth, patterning, and homeostasis of many tissues in diverse organisms, and is misregulated in a variety of diseases including cancer. Cytoplasmic Hedgehog signaling is activated by multisite phosphorylation of the seven-pass transmembrane protein Smoothened (Smo) in its cytoplasmic C-terminus. Aside from a short membrane-proximal stretch, the sequence of the C-terminus is highly divergent in different phyla, and the evidence suggests that the precise mechanism of Smo activation and transduction of the signal to downstream effectors also differs. To clarify the conserved role of G-protein-coupled receptor kinases (GRKs) in Smo regulation, we mapped four clusters of phosphorylation sites in the membrane-proximal C-terminus of Drosophila Smo that are phosphorylated by Gprk2, one of the two fly GRKs. Phosphorylation at these sites enhances Smo dimerization and increases but is not essential for Smo activity. Three of these clusters overlap with regulatory phosphorylation sites in mouse Smo and are highly conserved throughout the bilaterian lineages, suggesting that they serve a common function. Consistent with this, we find that a C-terminally truncated form of Drosophila Smo consisting of just the highly conserved core, including Gprk2 regulatory sites, can recruit the downstream effector Costal-2 and activate target gene expression, in a Gprk2-dependent manner. These results indicate that GRK phosphorylation in the membrane proximal C-terminus is an evolutionarily ancient mechanism of Smo regulation, and point to a higher degree of similarity in the regulation and signaling mechanisms of bilaterian Smo proteins than has previously been recognized.

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.

The role of arrestins in visual and disease processes of the eye

Visual arrestins are well known for their function in quenching the phototransduction process in rods and cones. Perhaps not as well known is their participation in multiple other processes in the normal and disease states of the eye. This chapter covers the range of the known functions of the visual arrestins, beginning with their classical role in quenching light-activated visual pigments. The role of visual arrestins is also reviewed from the perspective of their dynamic mobility whereby they redistribute significantly between the compartments of highly polarized photoreceptor cells. Additional roles of the visual arrestins are also reviewed based on new interacting partners that have been discovered over the past decade. Finally, the contribution of the visual arrestins to diseases of the visual system is explored.

The visual pigments of a deep-sea teleost, the pearl eye Scopelarchus analis

The eyes of deep-sea fish have evolved to function under vastly reduced light conditions compared to those that inhabit surface waters. This has led to a bathochromatic shift in the spectral location of maximum absorbance (lambda(max)) of their rod (RH1) pigments and the loss of cone photoreceptors. There are exceptions to this, however, as demonstrated by the deep-sea pearl eye Scopelarchus analis. Here we show the presence of two RH1 pigments (termed RH1A and RH1B) and a cone RH2 pigment. This is therefore the first time that the presence of a cone pigment in a deep-sea fish has been confirmed by molecular analysis. The lambda(max) values of the RH1A and RH1B pigments at 486 and 479 nm, respectively, have been determined by in vitro expression of the recombinant opsins and show the typical short-wave shifts of fish that live in deep water compared to surface dwellers. RH1B, however, is expressed only in more adult fish and lacks key residues for phosphorylation, indicating that it may not be involved in image formation. In contrast, the RH2 pigment has additional residues near the C terminus that may be involved in phosphorylation and does not show temporal changes in expression. The distribution of these pigments within the multiple retinae of S. analis is discussed.

Current methods for phosphoprotein isolation and enrichment

The phosphorylation of proteins is a central paradigm of signal transduction. The substitution of neutral hydroxyl groups of serine, threonine and tyrosine with a negatively charged phosphate group alters the physicochemical and immunogenic properties of the protein, which then can be used to isolate these isoforms. In the last decades several different techniques were applied, attempting to selectively enrich protein populations with this post-translational modification. This review aims to give an overview on the arsenal of available methods to extract phosphoproteins focusing on chromatographic approaches.

Structure of bovine rhodopsin in a trigonal crystal form

We have determined the structure of bovine rhodopsin at 2.65 A resolution using untwinned native crystals in the space group P3(1), by molecular replacement from the 2.8 A model (1F88) solved in space group P4(1). The new structure reveals mechanistically important details unresolved previously, which are considered in the membrane context by docking the structure into a cryo-electron microscopy map of 2D crystals. Kinks in the transmembrane helices facilitate inter-helical polar interactions. Ordered water molecules extend the hydrogen bonding networks, linking Trp265 in the retinal binding pocket to the NPxxY motif near the cytoplasmic boundary, and the Glu113 counterion for the protonated Schiff base to the extracellular surface. Glu113 forms a complex with a water molecule hydrogen bonded between its main chain and side-chain oxygen atoms. This can be expected to stabilise the salt-bridge with the protonated Schiff base linking the 11-cis-retinal to Lys296. The cytoplasmic ends of helices H5 and H6 have been extended by one turn. The G-protein interaction sites mapped to the cytoplasmic ends of H5 and H6 and a spiral extension of H5 are elevated above the bilayer. There is a surface cavity next to the conserved Glu134-Arg135 ion pair. The cytoplasmic loops have the highest temperature factors in the structure, indicative of their flexibility when not interacting with G protein or regulatory proteins. An ordered detergent molecule is seen wrapped around the kink in H6, stabilising the structure around the potential hinge in H6. These findings provide further explanation for the stability of the dark state structure. They support a mechanism for the activation, initiated by photo-isomerisation of the chromophore to its all-trans form, that involves pivoting movements of kinked helices, which, while maintaining hydrophobic contacts in the membrane interior, can be coupled to amplified translation of the helix ends near the membrane surfaces.

Crystals of Native and Modified Bovine Rhodopsins and Their Heavy Atom Derivatives

Rhodopsin, the pigment protein responsible for dim-light vision, is a G protein-coupled receptor that converts light absorption into the activation of a G protein, transducin, to initiate the visual response. We have crystallised detergent-solubilised bovine rhodopsin in the native form and after chemical modifications as needles 10-40 microm in cross-section. The crystals belong to the trigonal space group P3(1), with two molecules of rhodopsin per asymmetric unit, related by a non-crystallographic 2-fold axis parallel with the crystallographic screw axis along c (needle axis). The unit cell dimensions are a=103.8 A, c=76.6 A for native rhodopsin, but vary over a wide range after heavy atom derivatisation, with a between 101.5 A and 113.9 A, and c between 76.6 A and 79.2 A. Rhodopsin molecules are packed with the bundle of transmembrane helices tilted from the c-axis by about 100 degrees . The two molecules in the asymmetric unit form contacts along the entire length of their transmembrane helices 5 in an antiparallel orientation, and they are stacked along the needle axis according to the 3-fold screw symmetry. Hence hydrophobic contacts are prominent at protein interfaces both along and normal to the needle axis. The best crystals of native rhodopsin in this crystal form diffracted X-rays from a microfocused synchrotron source to 2.55 A maximum resolution. We describe steps taken to extend the diffraction limit from about 10 A to 2.6 A.

Conformational changes in the phosphorylated C-terminal domain of rhodopsin during rhodopsin arrestin interactions

Phosphorylation of activated G-protein-coupled receptors and the subsequent binding of arrestin mark major molecular events of homologous desensitization. In the visual system, interactions between arrestin and the phosphorylated rhodopsin are pivotal for proper termination of visual signals. By using high resolution proton nuclear magnetic resonance spectroscopy of the phosphorylated C terminus of rhodopsin, represented by a synthetic 7-phosphopolypeptide, we show that the arrestin-bound conformation is a well ordered helix-loop structure connected to rhodopsin via a flexible linker. In a model of the rhodopsin-arrestin complex, the phosphates point in the direction of arrestin and form a continuous negatively charged surface, which is stabilized by a number of positively charged lysine and arginine residues of arrestin. Opposite to the mostly extended structure of the unphosphorylated C-terminal domain of rhodopsin, the arrestin-bound C-terminal helix is a compact domain that occupies a central position between the cytoplasmic loops and occludes the key binding sites of transducin. In conjunction with other binding sites, the helix-loop structure provides a mechanism of shielding phosphates in the center of the rhodopsin-arrestin complex and appears critical in guiding arrestin for high affinity binding with rhodopsin.

Visual pigment phosphorylation but not transducin translocation can contribute to light adaptation in zebrafish cones

The ability of cone photoreceptors to adapt to light is extraordinary. In this study we evaluated two biochemical processes, visual pigment phosphorylation and transducin translocation, for their ability to contribute to light adaptation in zebrafish cones. Since cytoplasmic Ca2+ regulates light adaptation, the sensitivities of these processes to both light and Ca2+ were examined. Cytoplasmic Ca2+ regulates the sites of light-stimulated phosphorylation. Unexpectedly, we found that Ca2+ also regulates the extent of phosphorylation of unbleached cone pigments. Immunocytochemical analyses revealed that neither light nor cytoplasmic Ca2+ influences the localization of transducin in zebrafish cones.

The rod opsin pigments from two marsupial species, the South American bare-tailed woolly opossum and the Australian fat-tailed dunnart

Rod visual pigment genes have been studied in a wide range of vertebrates including a number of mammalian species. However, no marsupials have yet been examined. To correct this omission, we have studied the rod pigments in two marsupial species, the nocturnal and frugivorous bare-tailed woolly opossum, Caluromys philander, from Central and South America, and the arhythmic and insectivorous fat-tailed dunnart, Sminthopsis crassicaudata, from Australia. Phylogenetic analysis establishes that the cloned opsin sequences are orthologues of rod opsin genes from other vertebrate species. The deduced amino acid sequences show that both possess glutamate at residue 122, a feature of rod opsins, and the corresponding gene follows the typical vertebrate rod opsin pattern of five exons separated by four introns. Compared to other vertebrates, a stretch of five residues near the C-terminus is deleted in the rod opsin of both marsupials and all eutherian mammals. From microspectrophotometric measurements, the pigments in the two species show an 8 nm difference in peak absorbance; the molecular basis for this spectral shift is discussed and two candidate substitutions are identified.

Differential spatial and temporal phosphorylation of the visual receptor, rhodopsin, at two primary phosphorylation sites in mice exposed to light

Phosphorylation of rhodopsin critically controls the visual transduction cascade by uncoupling it from the G-protein transducin. The kinase primarily responsible for this phosphorylation is rhodopsin kinase, a substrate-regulated kinase that phosphorylates light-activated rhodopsin. Protein kinase C has been implicated in controlling the phosphorylation of both light-activated and dark-adapted rhodopsin. Two of the major rhodopsin phosphorylation sites in vivo, Ser(334) and Ser(338), are effective protein kinase C phosphorylation sites in vitro, while the latter is preferentially phosphorylated by rhodopsin kinase in vitro. Using phosphospecific antibodies against each of these two sites, we show that both sites are under differential spatial and temporal regulation. Exposure of mice to light results in rapid phosphorylation of Ser(338) that is evenly distributed along the rod outer segment. Phosphorylation of Ser(334) is considerably slower, begins at the base of the rod outer segment, and spreads to the top of the photoreceptor over time. In addition, we show that phosphorylation of both sites is abolished in rhodopsin kinase(-/-) mice, revealing an absolute requirement for rhodopsin kinase to phosphorylate rhodopsin. This requirement may reflect the need for priming phosphorylations at rhodopsin kinase sites allowing for subsequent phosphorylation by protein kinase C at Ser(334). In this regard, treatment of mouse retinas with phorbol esters results in a 4-fold increase in phosphorylation on Ser(334), with no significant effect on the phosphorylation of Ser(338). Our results are consistent with light triggering rapid priming phosphorylations of rhodopsin by rhodopsin kinase, followed by a slower phosphorylation on Ser(334), which is regulated by protein kinase C.

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.

Insertional mutagenesis and immunochemical analysis of visual arrestin interaction with rhodopsin

Visual arrestin inactivates the phototransduction cascade by specifically binding to light-activated phosphorylated rhodopsin. This study describes the combined use of insertional mutagenesis and immunochemical approaches to probe the structural determinants of arrestin function. Recombinant arrestins with insertions of a 10-amino acid c-Myc tag (EQKLISEEDL) were expressed in yeast and characterized. When the tag was placed on the C terminus after amino acid 399, between amino acids 99 and 100 or between residues 162 and 163, binding to rhodopsin was found to be very similar to that of wild-type arrestin. Two stable mutants with Myc insertions in the 68-78 loop were also generated. Binding to rhodopsin was markedly decreased for one (72myc73) and completely abolished for the other (77myc78). Limited proteolysis assays using trypsin in the absence or presence of heparin were performed on all mutants and confirmed their overall conformational integrity. Rhodopsin binding to either 162myc163 or 72myc73 arrestins in solution was completely inhibited in the presence of less than a 2-fold molar excess of anti-Myc antibody relative to arrestin. In contrast, the antibody did not block the interaction of the 399myc or 99myc100 arrestins with rhodopsin. These results indicate that an interactive surface for rhodopsin is located on or near the concave region of the N-domain of arrestin.

Agonist-induced signaling, desensitization, and internalization of a phosphorylation-deficient AT1A angiotensin receptor

An analysis of the functional role of a diacidic motif (Asp236-Asp237) in the third intracellular loop of the AT1A angiotensin II (Ang II) receptor (AT1-R) revealed that substitution of both amino acids with alanine (DD-AA) or asparagine (DD-NN) residues diminished Ang II-induced receptor phosphorylation in COS-7 cells. However, Ang II-stimulated inositol phosphate production, mitogen-activated protein kinase, and AT1 receptor desensitization and internalization were not significantly impaired. Overexpression of dominant negative G protein-coupled receptor kinase 2 (GRK2)K220M decreased agonist-induced receptor phosphorylation by approximately 40%, but did not further reduce the impaired phosphorylation of DD-AA and DD-NN receptors. Inhibition of protein kinase C by bisindolylmaleimide reduced the phosphorylation of both the wild-type and the DD mutant receptors by approximately 30%. The inhibitory effects of GRK2K220M expression and protein kinase C inhibition by bisindolylmaleimide on agonist-induced phosphorylation were additive for the wild-type AT1-R, but not for the DD mutant receptor. Agonist-induced internalization of the wild-type and DD mutant receptors was similar and was unaltered by coexpression of GRK2K220M. These findings demonstrate that an acidic motif at position 236/237 in the third intracellular loop of the AT1-R is required for optimal Ang II-induced phosphorylation of its carboxyl-terminal tail by GRKs. Furthermore, the properties of the DD mutant receptor suggest that not only Ang II-induced signaling, but also receptor desensitization and internalization, are independent of agonist-induced GRK-mediated phosphorylation of the AT1 receptor.

Rhodopsin: structural basis of molecular physiology

The crystal structure of rod cell visual pigment rhodopsin was recently solved at 2.8-A resolution. A critical evaluation of a decade of structure-function studies is now possible. It is also possible to begin to explain the structural basis for several unique physiological properties of the vertebrate visual system, including extremely low dark noise levels as well as high gain and color detection. The ligand-binding pocket of rhodopsin is remarkably compact, and several apparent chromophore-protein interactions were not predicted from extensive mutagenesis or spectroscopic studies. The transmembrane helices are interrupted or kinked at multiple sites. An extensive network of interhelical interactions stabilizes the ground state of the receptor. The helix movement model of receptor activation, which might apply to all G protein-coupled receptors (GPCRs) of the rhodopsin family, is supported by several structural elements that suggest how light-induced conformational changes in the ligand-binding pocket are transmitted to the cytoplasmic surface. The cytoplasmic domain of the receptor is remarkable for a carboxy-terminal helical domain extending from the seventh transmembrane segment parallel to the bilayer surface. Thus the cytoplasmic surface appears to be approximately the right size to bind to the transducin heterotrimer in a one-to-one complex. Future high-resolution structural studies of rhodopsin and other GPCRs will form a basis to elucidate the detailed molecular mechanism of GPCR-mediated signal transduction.

Multiple phosphorylation of rhodopsin and the in vivo chemistry underlying rod photoreceptor dark adaptation

Dark adaptation requires timely deactivation of phototransduction and efficient regeneration of visual pigment. No previous study has directly compared the kinetics of dark adaptation with rates of the various chemical reactions that influence it. To accomplish this, we developed a novel rapid-quench/mass spectrometry-based method to establish the initial kinetics and site specificity of light-stimulated rhodopsin phosphorylation in mouse retinas. We also measured phosphorylation and dephosphorylation, regeneration of rhodopsin, and reduction of all-trans retinal all under identical in vivo conditions. Dark adaptation was monitored by electroretinography. We found that rhodopsin is multiply phosphorylated and then dephosphorylated in an ordered fashion following exposure to light. Initially during dark adaptation, transduction activity wanes as multiple phosphates accumulate. Thereafter, full recovery of photosensitivity coincides with regeneration and dephosphorylation of rhodopsin.

Confronting complexity: the interlink of phototransduction and retinoid metabolism in the vertebrate retina

Absorption of light by rhodopsin or cone pigments in photoreceptors triggers photoisomerization of their universal chromophore, 11-cis-retinal, to all-trans-retinal. This photoreaction is the initial step in phototransduction that ultimately leads to the sensation of vision. Currently, a great deal of effort is directed toward elucidating mechanisms that return photoreceptors to the dark-adapted state, and processes that restore rhodopsin and counterbalance the bleaching of rhodopsin. Most notably, enzymatic isomerization of all-trans-retinal to 11-cis-retinal, called the visual cycle (or more properly the retinoid cycle), is required for regeneration of these visual pigments. Regeneration begins in rods and cones when all-trans-retinal is reduced to all-trans-retinol. The process continues in adjacent retinal pigment epithelial cells (RPE), where a complex set of reactions converts all-trans-retinol to 11-cis-retinal. Although remarkable progress has been made over the past decade in understanding the phototransduction cascade, our understanding of the retinoid cycle remains rudimentary. The aim of this review is to summarize recent developments in our current understanding of the retinoid cycle at the molecular level, and to examine the relevance of these reactions to phototransduction.

Vertebrate photoreceptors

The basis of the duplex theory of vision is examined in view of the dazzling array of data on visual pigment sequences and the pigments they form, on the microspectrophotometry measurements of single photoreceptor cells, on the kinds of photoreceptor cascade enzymes, and on the electrophysiological properties of photoreceptors. The implications of the existence of five distinct visual pigment families are explored, especially with regard to what pigments are in what types of photoreceptors, if there are different phototransduction enzymes associated with different types of photoreceptors, and if there are electrophysiological differences between different types of cones.

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.

The 2.8 A crystal structure of visual arrestin: a model for arrestin's regulation

G protein-coupled signaling is utilized by a wide variety of eukaryotes for communicating information from the extracellular environment. Signal termination is achieved by the action of the arrestins, which bind to activated, phosphorylated G protein-coupled receptors. We describe here crystallographic studies of visual arrestin in its basal conformation. The salient features of the structure are a bipartite molecule with an unusual polar core. This core is stabilized in part by an extended carboxy-terminal tail that locks the molecule into an inactive state. In addition, arrestin is found to be a dimer of two asymmetric molecules, suggesting an intrinsic conformational plasticity. In conjunction with biochemical and mutagenesis data, we propose a molecular mechanism by which arrestin is activated for receptor binding.

Effects of phosphorylation on the structure of the G-protein receptor rhodopsin

Upon activation by light, rhodopsin is subject to phosphorylation by rhodopsin kinase at serine and threonine residues in the carboxyl terminal region of the protein. A 19 amino acid peptide that corresponds to the carboxyl terminal end of rhodopsin (residues 330-348) and contains these phosphorylation sites was synthesized. The structure of this peptide was determined using two-dimensional proton NMR. The structure of this peptide was similar to that determined for this region in peptides corresponding to the carboxyl 33 and 43 amino acids of rhodopsin. The effect of phosphorylation on the structure of the carboxyl terminal domain of rhodopsin was determined by solving the solution structures of the peptide containing residues 330-348 with phosphorylation at one (residue 343), three (residues 343, 338, and 334) and seven residues (residues 334, 335, 336, 338, 340, 342, 343). These data indicate that the major structural change occurs upon phosphorylation of the first residue, and that an additional structural change occurs with seven phosphates.

Rhodopsin and phototransduction

Recent studies on rhodopsin structure and function are reviewed and the properties of vertebrate as well as invertebrate rhodopsin described. Open issues such as the 'red shift' of the absorbance spectra are emphasized in the light of the present model of the retinal-binding pocket. The processes that restore the rhodopsin content in photoreceptors are also presented with a comparison between vertebrate and invertebrate visual systems. The central role of rhodopsin in the phototransduction cascade becomes evident by examining the main reports on light-activated conformational changes of rhodopsin and its interaction with transducin. Shut-off mechanisms are considered by reporting the studies on the sites of rhodopsin phosphorylation and arrestin binding. Furthermore, recent findings on the energetics of phototransduction point out that the ATP needed for photoreception in vertebrates is synthesized in the outer segments where phototransduction events take place.

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.

Rhodopsin: a prototypical G protein-coupled receptor

A variety of spectroscopic and biochemical studies of recombinant site-directed mutants of rhodopsin and related visual pigments have been reported over the past 9 years. These studies have elucidated key structural elements common to visual pigments. In addition, systematic analysis of the chromophore-binding pocket in rhodopsin and cone pigments has led to an improved understanding of the mechanism of the opsin shift, and of particular molecular determinants underlying color vision in humans. Identification of the conformational changes that occur on rhodopsin photoactivation has been of particular recent concern. Assignments of light-dependent molecular alterations to specific regions of the chromophore have also been attempted by studying native opsins regenerated with synthetic retinal analogs. Site-directed mutagenesis of rhodopsin has also provided useful information about the retinal-binding pocket and the molecular mechanism of rhodopsin photoactivation. Individual molecular groups have been identified to undergo structural alterations or environmental changes during photoactivation. Analysis of particular mutant pigments in which specific groups are locked into their respective "off" or "on" states has provided a framework to identify determinants of the active conformation, as well as the minimal number of intramolecular transitions required to switch between inactive and active conformations. A simple model for the active state of rhodopsin can be compared to structural models of its ground state to localize chromophore-protein interactions that may be important in the photoactivation mechanism. This review focuses on the recent functional characterization of site-directed mutants of bovine rhodopsin and some cone pigments. In addition, an attempt is made to reconcile previous key findings and existing structural models with information gained from the analysis of site-directed mutant pigments.

The N-formyl peptide receptor: a model for the study of chemoattractant receptor structure and function

N-formyl peptides, such as fMet-Leu-Phe, are one of the most potent chemoattractants for phagocytic leukocytes. The interaction of N-formyl peptides with their specific cell surface receptors has been studied extensively and used as a model system for the characterization of G-protein-coupled signal transduction in phagocytes. The cloning of the N-formyl peptide receptor cDNA from several species and the identification of homologous genes have allowed detailed studies of structural and functional aspects of the receptor. Recent findings that the receptor is expressed in nonhematopoietic cells and that nonformylated peptides can activate the receptor suggest potentially novel functions and the existence of additional ligands for this receptor.

High levels of rhodopsin phosphorylation in missense mutations of C-terminal region of rhodopsin

Rhodopsin phosphorylation was investigated using synthetic C-terminal peptides from rhodopsin. The peptides were phosphorylated by expressed rhodopsin kinase (RK) in the presence of a photolyzed truncated rhodopsin at the C-terminus. No peptide phosphorylation was detected under dark or in conditions in which RK was inactive. However, the phosphorylation rate was significantly higher in the following three peptides: (345M Rho, 330DDEASTTVSKTETSQMAPA; 347S Rho, 330DDEASTTVSKTETSQVASA; and 347L Rho: 330DDEASTTVSKTETSQVALA) taken from missense mutations of rhodopsin found in patients with autosomal retinitis pigmentosa (ADRP) as compared with that from wild-type rhodopsin (330DDEASTTVSKTETSQVAPA). Distribution of the phosphorylation showed a similar ratio among three serines (334, 338 and 343) in 347L Rho mutation to wild type. However, 345M Rho and 347S Rho peptides showed higher phosphorylation at Ser334. The data obtained suggests that an abnormally high rate of phosphorylation in missense mutations around the rhodopsin C-terminus may change the position of phosphorylation and inactivation process of the visual transduction.

A distance measurement between specific sites on the cytoplasmic surface of bovine rhodopsin in rod outer segment disk membranes

Structural information on mammalian integral membrane proteins is scarce. As part of work on an alternative approach to the structure of bovine rhodopsin, a method was devised to obtain an intramolecular distance between two specific sites on rhodopsin while in the rod outer segment disk membrane. In this report, the distance between the rhodopsin kinase phosphorylation site(s) on the carboxyl terminal and the top of the third transmembrane helix was measured on native rhodopsin. Rhodopsin was labeled with a nuclear spin label (31P) by limited phosphorylation with rhodopsin kinase. Major phosphorylation occurs at serines 343 and 338 on the carboxyl terminal. The phosphorylated rhodopsin was then specifically labeled on cysteine 140 with an electron spin label. Magic angle spinning 31P-nuclear magnetic resonance revealed the resonance arising from the phosphorylated protein. The enhancement of the transverse relaxation of this resonance by the paramagnetic spin label was observed. The strength of this perturbation was used to determine the through-space distance between the phosphorylation site(s) and the spin label position. A distance of 18 +/- 3 A was obtained.

Rhodopsin phosphorylation sites and their role in arrestin binding

Rhodopsin, the rod cell photoreceptor, undergoes rapid desensitization upon exposure to light, resulting in uncoupling of the receptor from its G protein, transducin (Gt). Phosphorylation of serine and threonine residues located in the COOH terminus of rhodopsin is the first step in this process, followed by the binding of arrestin. In this study, a series of mutants was generated in which these COOH-terminal phosphorylation substrate sites were substituted with alanines. These mutants were expressed in HEK-293 cells and analyzed for their ability to be phosphorylated by rhodopsin kinase and to bind arrestin. The results demonstrate that rhodopsin kinase can efficiently phosphorylate other serine and threonine residues in the absence of the sites reported to be the preferred substrates for rhodopsin kinase. A correlation was observed between the level of rhodopsin phosphorylation and the amount of arrestin binding to these mutants. However, mutants T340A and S343A demonstrated a significant reduction in arrestin binding even though the level of phosphorylation was similar to that of wild-type rhodopsin. Substitution of Thr-340 and Ser-343 with glutamic acid residues (T340E and S343E, respectively) was not sufficient to promote the binding of arrestin in the absence of phosphorylation by rhodopsin kinase. When S343E was phosphorylated, its ability to bind arrestin was similar to that of wild-type rhodopsin. Surprisingly, arrestin binding to phosphorylated T340E did not increase to the level observed for wild-type rhodopsin. These results suggest that 2 amino acids, Thr-340 and Ser-343, play important but distinct roles in promoting the binding of arrestin to rhodopsin.

Two homologous phosphorylation domains differentially contribute to desensitization and internalization of the m2 muscarinic acetylcholine receptor

Short term exposure of m2 muscarinic acetylcholine receptors (m2 mAChRs) to agonist causes a rapid phosphorylation of the activated receptors, followed by a profound loss in the ability of the m2 mAChR to activate its signaling pathways. We have used site-directed mutagenesis to identify two clusters of Ser/Thr residues in the third intracellular loop of the m2 mAChR that can serve as redundant targets for agonist-dependent phosphorylation. Mutation of both clusters of Ser/Thr residues to alanines abolished agonist-dependent phosphorylation, while wild-type levels of m2 mAChR phosphorylation were observed in mutant receptors with only one or the other cluster mutated. However, the functional effects of phosphorylation of these two "redundant" clusters were not equivalent. No receptor desensitization was observed in an m2 mAChR with residues Thr307-Ser311 mutated to alanine residues. In contrast, mutation of the other Ser/Thr cluster, residues Ser286-Ser290, to alanines produced a receptor that continued to desensitize. Internalization of the m2 mAChR was promoted by phosphorylation of either cluster, suggesting that distinct mechanisms with unique structural requirements act downstream of m2 mAChR phosphorylation to mediate receptor desensitization and receptor internalization.

Identification of protein kinase C phosphorylation sites on bovine rhodopsin

The protein kinase C phosphorylation sites on bovine rhodopsin were identified using proteolytic, phosphoamino acid, mass spectrometric, and peptide sequencing analyses. Tryptic removal of the 9 carboxyl-terminal residues of rhodopsin revealed that a major fraction of the phosphates incorporated by protein kinase C are in a region containing Ser334, Thr335, and Thr336. Phosphoamino acid analysis of the tryptic product established that Ser334 accounts for approximately 65% of the phosphorylation in this region. Analysis of the endoproteinase Asp-N-generated carboxyl terminus of rhodopsin by mass spectrometry and peptide sequencing revealed that Ser338 is also a primary phosphorylation site, with minor phosphorylation of Ser343. Quantitation of high pressure liquid chromatography-separated phosphopeptides, taken together with phosphoamino acid analysis of the tryptic product, revealed that Ser334 and Ser338 were phosphorylated equally and each accounted for approximately 35% of the total phosphorylation; Thr335/336 accounted for just under 20% of the phosphorylation, and Ser343 accounted for 10%. Thus, the primary protein kinase C sites are Ser334 and Ser338, with minor phosphorylation of Thr335/336 and Ser343. Ser334 and Ser338 have recently been identified as the primary sites of phosphorylation of rhodopsin in vivo (Ohguro, H., Van Hooser, J. P., Milam, A. H., and Palczewski, K. (1995) J. Biol. Chem. 270, 14259-14262). Of these sites, only Ser338 is a significant substrate for rhodopsin kinase in vitro. Identification of Ser334 as a primary protein kinase C target in vitro is consistent with protein kinase C modulating the phosphorylation of this site in vivo.

Contribution of protein kinase C to the phosphorylation of rhodopsin in intact retinas

Similar to other G protein-coupled receptors, the visual receptor, rhodopsin, is phosphorylated by both a substrate-regulated kinase, rhodopsin kinase, and a second messenger-regulated kinase, protein kinase C. In the present study, the extent of involvement of protein kinase C in the light-dependent phosphorylation of rhodopsin in intact retinas was assessed using a specific activator (phorbol ester) and specific inhibitor (calphostin C) of protein kinase C. Kinetic analysis of rhodopsin phosphorylation following different illumination conditions revealed that hyperactivation of protein kinase C with phorbol ester resulted in a relative increase in rhodopsin phosphorylation that peaked 10-15 min after the onset of illumination. Following this period, the rate of rhodopsin dephosphorylation was increased in the phorbol ester-treated retinas, so that by about 30 min the amount of phosphorylation was similar to that in control retinas. Treatment of retinas with calphostin C, a potent regulatory domain-directed inhibitor of protein kinase C, resulted in an approximately 50% reduction in the light-dependent phosphorylation of rhodopsin. This inhibitor had no effect on the activity of rhodopsin kinase in vitro. Last, we show that frog rhodopsin is phosphorylated in vitro by protein kinase C from frog rod outer segments, indicating that this kinase could directly modulate rhodopsin in vivo. In conclusion, the present results reveal that the kinetics of rhodopsin phosphorylation/dephosphorylation differ markedly, depending on whether protein kinase C or rhodopsin kinase activity dominates, and that, under the conditions studied, protein kinase C contributes to approximately half of the phosphorylation of rhodopsin in intact frog retinas.

Regulatory mechanisms that modulate signalling by G-protein-coupled receptors

The large and functionally diverse group of G-protein-coupled receptors includes receptors for many different signalling molecules, including peptide and non-peptide hormones and neuro-transmitters, chemokines, prostanoids and proteinases. Their principal function is to transmit information about the extracellular environment to the interior of the cell by interacting with the heterotrimeric G-proteins, and they thereby participate in many aspects of regulation. Cellular responses to agonists of these receptors are usually rapidly attenuated. Mechanisms of signal attenuation include removal of agonists from the extracellular fluid, receptor desensitization, endocytosis and down-regulation. Agonists are removed by dilution, uptake by transporters and enzymic degradation. Receptor desensitization is mediated by receptor phosphorylation by G-protein receptor kinases and second-messenger kinases, interaction of phosphorylated receptors with arrestins and receptor uncoupling from G-proteins. Agonist-induced receptor endocytosis also contributes to desensitization by depleting the cell surface of high-affinity receptors, and recycling of internalized receptors contributes to resensitization of cellular responses. Receptor down-regulation is a form of desensitization that occurs during continuous, long-term exposure of cells to receptor agonists. Down-regulation, which may occur during the development of drug tolerance, is characterized by depletion of the cellular receptor content, and is probably mediated by alterations in the rates of receptor degradation and synthesis. These regulatory mechanisms are important, as they govern the ability of cells to respond to agonists. A greater understanding of the mechanisms that modulate signalling may lead to the development of new therapies and may help to explain the mechanism of drug tolerance.