The partial primary structure of bovine rhodopsin and its topography in the retinal rod cell disc membrane

The amino-terminal 39 amino acids of bovine rhodopsin have the sequence where both carbohydrate attachment sites (CHO) contain GlcNAc(3)Man(3). This region of rhodopsin's sequence is exposed at the internal membrane surface of the rod cell disc membrane. Rhodopsin's carboxyl-terminal 40 amino acids have the sequence where amino acid 1? is the carboxyl-terminal amino acid of rhodopsin. Serines and threonines in the sequence 6? ? 15? are phosphorylated by rhodopsin kinase in a light-dependent reaction. Trypsin can digest native rhodopsin, in the disc membrane at and thermolysin can hydrolyze bonds , and . Limited proteolysis by thermolysin at a site internal in the molecule has been exploited in order to prepare rhodopsin as two large fragments, F1 and F2. Cysteine(33)?, is highly reactive in the dark and is modified by N-ethylmaleimide and several alkylating agents. The carboxyl-terminal region 1?-39? reacts with the membrane-impermeable nitrene from N-(4-azido-2-nitrophenyl)-2-aminoethyl sulfonate and is therefore exposed at the external (cytoplasmic) surface of the disc membrane. 1-azldopyrene, a hydrophobic nitrene precursor, is being used to map those regions of the rhodopsin sequence which are located in a hydrophobic environment.

Preparation of carboxyl-terminal tryptic peptides from proteins by cleavage at arginine

A method has been developed for selectively preparing the carboxyl-terminal tryptic peptide of proteins by cleavage at arginyl residues. The succinylated protein is digested with trypsin and the peptides produced are maleylated. Maleylated peptides are then submitted to cation-exchange chromatography in urea at low pH and ionic strength. Arginine-containing peptides are retained by the resin. The carboxyl-terminal peptide emerges unretarded and in pure form. This method has been applied to four proteins of known sequence. Yields as high as 88% have been obtained.

Activation of arrestin: requirement of phosphorylation as the negative charge on residues in synthetic peptides from the carboxyl-terminal region of rhodopsin

PURPOSE

To determine whether substitution of the potential phosphorylation sites of bovine rhodopsin's carboxyl-terminal region with the acidic residues aspartic acid, glutamic acid, or cysteic acid promotes the activation of arrestin.

METHODS

Three peptide analogues of the 19-residue carboxyl-terminal region of rhodopsin (330-348) were synthesized: the fully phosphorylated peptide (7P-peptide), the peptide with all potential phosphorylation sites substituted with glutamic acid (7E-peptide), and the peptide with the phosphorylation sites substituted with cysteic acid (7Cya-peptide). The peptides were tested in assays in which the 7P-peptide had previously been shown to have an effect. Rhodopsin with glutamic acid (Etail) or aspartic acid (Dtail) substituted for the phosphorylation sites in rhodopsin were constructed and expressed in COS-7 cells and tested in an in vitro assay.

RESULTS

Earlier work has demonstrated that the 7P-peptide activates arrestin, showing induction of arrestin binding to light-activated unphosphorylated rhodopsin, inhibition of the light-induced phosphodiesterase (PDE) activity in rod outer segments (ROS) with excess arrestin, increase in the initial rapid proteolysis of arrestin by trypsin, and enhanced reactivity of one of arrestin's sulfhydryl groups with inhibition of the reactivity of another. None of these effects was observed in the presence of 7E-peptide or 7Cya-peptide. The 7Cya-peptide inhibited the PDE activity in ROS, but the same effect was observed both in the presence and the absence of excess arrestin. Because none of the other effects was observed with the 7Cya-peptide, the authors conclude that the 7Cya-peptide does not activate arrestin, but acts, probably nonspecifically, through some other part of the transduction system. Considerable arrestin-mediated rhodopsin inactivation was observed with both the Etail and the Dtail mutant, although these substitutions did not yield rhodopsins that were equivalent to phosphorylated rhodopsin.

CONCLUSIONS

These results, taken together, suggest that the negative charge due to phosphates in the carboxyl-terminal region of rhodopsin are required for the full activation of arrestin and that acidic amino acids (carboxyl and sulfonic) do not mimic the negative charge of phosphorylated residues.

Importance of cryptic myelin basic protein epitopes in the pathogenicity of acute and recurrent anterior uveitis associated with EAE

Lewis rats immunized with myelin basic protein (MBP) develop experimental autoimmune encephalomyelitis (EAE) and associated anterior uveitis (AU), which can relapse without recurring of EAE. In this study, we analyzed the repertoire of MBP epitopes that play a role in acute and recurrent AU by injection of MBP synthetic peptides. In addition to the encephalitogenic epitopes 69-89 and 87-99, several cryptic epitopes were found to be strongly uveitogenic in Lewis rats upon immunization with synthetic peptides, including 100-120, 121-140 and 142-167. However, the peptide corresponding to the MBP residues 1-20 was uniquely capable of inducing AU without EAE. Immunization with intact MBP was not essential for the induction of the recurrence of AU. The responses of T cells from lymph nodes and spleens showed a dominant response to the original disease-induced epitope with responses to secondary epitopes. In conclusion, the analysis of pathogenic determinants important for the induction of uveitis provides further evidence that MBP-specific T cells also contribute to the pathogenesis of anterior uveitis. Moreover, this also suggests that a distinct immunoregulatory mechanism exists in the eye and spinal cord because of the uniqueness of the epitope 1-20 in AU but not EAE, and the capability of MBP-specific T cells of inducing AU without EAE.

Epitope recognition and T cell receptors in recurrent autoimmune anterior uveitis in Lewis rats immunized with myelin basic protein

Lewis rats immunized with myelin basic protein (MBP) develop experimental autoimmune encephalomyelitis (EAE) and associated anterior uveitis (AU). Rats recover and become resistant to further reinduction of EAE. We investigated whether the resistance to reinduction of EAE was associated with the resistance to AU in LEW rats reinjected with MBP. We demonstrated that while rats remained resistant to EAE, they become susceptible to uveitis after recovery, and suffered a second episode of disease. The susceptibility to reinduced disease was associated with the recognition of new MBP epitopes. In contrast to the initial episode of AU, TCR Vbeta8.2 predominance was not observed in the iris/ciliary body. Our results suggest that T cells specific for MBP, which are rapidly reactivated when re-exposed to antigen, are sufficient to induce clinical uveitis in LEW rats. This process may involve a shifting of T cell specificity from the major encephalitogenic peptide utilizing the Vbeta8.2 receptor to a more diverse cell repertoire.

Sulfhydryl reactivity demonstrates different conformational states for arrestin, arrestin activated by a synthetic phosphopeptide, and constitutively active arrestin

The sulfhydryl groups of the three cysteines in bovine arrestin react with DTNB very slowly (over a period of several hours). In the presence of the synthetic phosphopeptide comprising the fully phosphorylated carboxyl-terminal 19 amino acids of bovine rhodopsin, the reactivity of one of the sulfhydryls was enhanced while that of another was greatly reduced. Since this synthetic peptide was shown to activate arrestin with respect to its binding to unphosphorylated, light-activated rhodopsin, the reactivity of the sulfhydryl groups of a constitutively active R175Q arrestin mutant was examined. All three of the sulfhydryl groups of the mutant arrestin R175Q reacted rapidly with DTNB, but not as rapidly as with SDS-denatured arrestin. The arrestin mutant R175Q bound to light-activated, unphosphorylated rhodopsin in ROS disk membranes. The arrestin mutant R175Q also inhibited the light-activated PDE activity with an IC50 of 1.3 microM under the experimental conditions that were used. These data indicate that each of these forms of arrestin is a different conformation. The activated conformation of arrestin that binds to phosphorylated rhodopsin in vivo may be yet another conformation. We conclude that arrestin is a flexible molecule that is able to attain several different conformations, all of which are able to attain the activated functional state of arrestin.

Epitope mapping of anti-rhodopsin antibodies from patients with normal pressure glaucoma

PURPOSE

The presence of anti-rhodopsin antibodies in patients with normal pressure glaucoma (NPG) has been previously demonstrated with western blot analysis and enzyme-linked immunosorbent assay. To learn more about the characteristics, origin, and possible significance of these antibodies, the epitopic specificity of the anti-rhodopsin antibodies was examined in four NPG patients.

METHODS

Antibodies in patient sera were assayed by western blot analysis against purified bovine rhodopsin. Peptides derived from particular segments of the rhodopsin sequence were tested for activity in competing for rhodopsin-antibody binding.

RESULTS

Of a series of nine peptides that constitute most of the hydrophilic regions of rhodopsin, only one, consisting of the C-terminal 25 amino acids, prevented binding of the patient antibodies to rhodopsin. Higher resolution mapping using a set of dodecamers of overlapping sequences from the C-terminal region demonstrated that antibody binding is completely dependent on the last two amino acids. Removing the C-terminal alanine alone, or amidating the C terminus carboxyl group, also eliminated antibody binding.

CONCLUSIONS

Because four of four patient antibodies examined exhibited the identical epitopic specificity, it is likely that a common mechanism underlies their generation. This may indicate that molecular mimicry has occurred, because several pathogens contain similar C-terminal sequences. Although they may serve as diagnostic markers, and provide evidence that there is an autoimmune component in some patients with glaucoma, the role, if any, that these antibodies play in the pathogenesis of the disease remains unknown.

Identification of regions of arrestin that bind to rhodopsin

Arrestin facilitates phototransduction inactivation through binding to photoactivated and phosphorylated rhodopsin (RP). However, the specific portions of arrestin that bind to RP are not known. In this study, two different approaches were used to determine the regions of arrestin that bind to rhodopsin: panning of phage-displayed arrestin fragments against RP and cGMP phosphodiesterase (PDE) activity inhibition using synthetic arrestin peptides spanning the entire arrestin protein. Phage display indicated the predominant region of binding was contained within amino acids 90-140. A portion of this region (residues 95-140) expressed as a fusion protein with glutathione S-transferase is capable of binding to rhodopsin regardless of the activation or phosphorylation state of the receptor. Within this region, the synthetic peptide of residues 109-130 was shown to completely inhibit the binding of arrestin to rhodopsin with an IC50 of 1.1 mM. The relatively high IC50 of this competition suggests that this portion of the molecule may be only one of several regions of binding between arrestin and RP. A survey of synthetic arrestin peptides in the PDE assay indicated that the two most effective inhibitors of PDE activity were peptides of residues 111-130 and 101-120. These results indicate that at least one of the principal regions of binding between arrestin and RP is contained within the region of residues 109-130.

Identification of a guanylyl cyclase-activating protein-binding site within the catalytic domain of retinal guanylyl cyclase 1

Regulation of cAMP and cGMP production is a fundamental step in a broad range of signal transduction systems, including phototransduction. To identify regions within photoreceptor guanylyl cyclase 1 (GC1) that interact with GC-activating proteins (GCAPs), we synthesized the intracellular fragment of GC1, residues 491-1110, as a set of 15 amino acid long, partially overlapping peptides on the surface of individual pins arranged in a microtiter plate format. This pin assay identified 8 peptides derived from different regions of the GC1 intracellular domain that bind GCAPs. Peptide variants containing these sequences were synthesized as free peptides and tested for their ability to inhibit GC1 stimulation by GCAPs. A free peptide,968GTFRMRHMPEVPVRIRIG, from the catalytic domain of GC1 was the strongest inhibitor of GCAP1/GCAP2-mediated activation. In native GC1, this polypeptide fragment is likely to form a loop between alpha-helix 3 and beta-strand 4. When this region in GC1 was replaced by the corresponding sequence of GCAP-insensitive GC type A, GCAPs did not stimulate the GC1 mutant. The corresponding loops in related adenylyl cyclase (AC) are involved in the activating and inhibiting interactions with Gs alpha and Gi alpha, respectively. Thus, despite interacting with different activating proteins, both AC and GC activity may be modulated through their respective regions within catalytic domains.

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.

Regulation of sorting and post-Golgi trafficking of rhodopsin by its C-terminal sequence QVS(A)PA

Several mutations that cause severe forms of the human disease autosomal dominant retinitis pigmentosa cluster in the C-terminal region of rhodopsin. Recent studies have implicated the C-terminal domain of rhodopsin in its trafficking on specialized post-Golgi membranes to the rod outer segment of the photoreceptor cell. Here we used synthetic peptides as competitive inhibitors of rhodopsin trafficking in the frog retinal cell-free system to delineate the potential regulatory sequence within the C terminus of rhodopsin and model the effects of severe retinitis pigmentosa alleles on rhodopsin sorting. The rhodopsin C-terminal sequence QVS(A)PA is highly conserved among different species. Peptides that correspond to the C terminus of bovine (amino acids 324-348) and frog (amino acids 330-354) rhodopsin inhibited post-Golgi trafficking by 50% and 60%, respectively, and arrested newly synthesized rhodopsin in the trans-Golgi network. Peptides corresponding to the cytoplasmic loops of rhodopsin and other control peptides had no effect. When three naturally occurring mutations: Q344ter (lacking the last five amino acids QVAPA), V345M, and P347S were introduced into the frog C-terminal peptide, the inhibitory activity of the peptides was no longer detectable. These observations suggest that the amino acids QVS(A)PA comprise a signal that is recognized by specific factors in the trans-Golgi network. A lack of recognition of this sequence, because of mutations in the last five amino acids causing autosomal dominant retinitis pigmentosa, most likely results in abnormal post-Golgi membrane formation and in an aberrant subcellular localization of rhodopsin.

Rhodopsins from three frog and toad species: sequences and functional comparisons

The frequency of thermal 'dark events' in the membrane current of rhodopsin rods of the bullfrog, Rana catesbeiana, is considerably lower than observed in rods of two toad species, even though all three rhodopsins have approximately the same absorbance characteristics. In order to map amino acid substitutions possibly associated with thermal stability in the genus Rana, the cDNA's coding for the rhodopsins of Bufo bufo, B. marinus and R. temporaria were sequenced and the conceptually translated protein sequences aligned to the previously sequenced rhodopsins of R. catesbeiana, R. pipiens and Xenopus laevis. Across the six anuran species studied, there are sixteen non-conserved substitutions and six changes that include gain or loss of a hydroxyl group. Serine or threonine at position 220 is unique to the three Rana species, phenylalanine at position 270 is unique to all three Ranas and to X. laevis, and phenylalanine at position 274 is unique to both species of the genus Bufo. This investigation produces a list of substitutions that are candidates for future studies of thermal stability. In addition, a number of amino acids are identified that apparently do not influence absorbance characteristics, at least not cumulatively.

Arrangement of rhodopsin transmembrane alpha-helices

Rhodopsins, the photoreceptors in rod cells, are G-protein-coupled receptors with seven hydrophobic segments containing characteristic conserved sequence patterns that define a large family. Members of the family are expected to share a conserved transmembrane structure. Direct evidence for the arrangement of seven alpha-helices was obtained from a 9A projection map of bovine rhodopsin. Structural constraints inferred from a comparison of G-protein-coupled receptor sequences were used to assign the seven hydrophobic stretches in the sequence to features in the projection map. A low-resolution three-dimensional structure of bovine rhodopsin and two projection structures of frog rhodopsin confirmed the position of the three least tilted helices, 4, 6 and 7. A more elongated peak of density for helix 5 indicated that it is tilted or bent, but helices 1, 2 and 3 were not resolved. Here we have used electron micrographs of frozen-hydrated two-dimensional frog rhodopsin crystals to determine the structure of frog rhodopsin. Seven rods of density in the map are used to estimate tilt angles for the seven helices. Density visible on the extracellular side of the membrane suggests a folded domain. Density extends from helix 6 on the intracellular side, and a short connection between helices 1 and 2, and possibly a part of the carboxy terminus, are visible.

Rhodopsins with Similar Absorbance Characteristics but Different Rates of Thermal Activation: Amino Acid Sequences

Thermal activation of the visual pigment is thought to be an important factor that in many cases limits the absolute sensitivity of vision in darkness. It has been suggested that pigments with high \lambdamax (ie with good absorbance at long wavelengths, allowing ‘red-sensitive’ vision) are associated with a cost in terms of high thermal activation rates, degrading signal/noise and hence visual sensitivity [Barlow, 1957 Nature (London)179 255 – 256]. While rhodopsins in different species do show a general correlation between red-sensitivity and high thermal activation rates as measured electrophysiologically in whole rods (Firsov and Govardovskii, 1990 Sensornye Sistemy4 25 – 34), a comparison of the toads Bufo marinus and B. bufo with the bull-frog Rana catesbeiana has suggested that the two properties are not tightly coupled. Toad and bull-frog rhodopsins have almost the same \lambdamax, yet the thermal activation rate in bull-frog rods is lower by almost one order of magnitude [Donner et al, 1990 Journal of Physiology (London)428 673 – 692]. We have sequenced the cDNA coding for the rhodopsins of the two toad species and that of the common frog R. temporaria and aligned them to previously sequenced rhodopsins of R. catesbeiana, R. pipiens and Xenopus laevis in an attempt to identify substitutions that could underlie the greater thermal stability of bull-frog rhodopsin compared with toad rhodopsins.

The open reading frame predicted proteins of 354 amino acids. There was 96% identity between species of the same genus and 90% identity between genera. Across the six species studied, there is a total of 22 non-conserved substitutions and changes that include gain or loss of hydroxyl groups. Our study produced a list of substitutions that apparently do not significantly affect absorbance characteristics. We could not, however, unequivocally identify substitutions important for thermal stability.

Short wavelength-sensitive opsins from the Saharan silver and carpenter ants

We have previously cloned the opsins coding for the long-wavelength visual pigments from the Saharan silver ant and carpenter ant. Here we report two new cDNA clones isolated from cDNA libraries which also code for opsin proteins. These cDNAs code for deduced proteins with 369 amino acids which are 91% identical to each other, but only 38% identical to the previously cloned opsins. Phyletic comparisons suggest that these opsins are likely the ultraviolet sensitive visual pigments, a conclusion that is supported by the presence of a phenylalanine at the counterion position in the third transmembrane segment.

Functional reconstitution of photoreceptor guanylate cyclase with native and mutant forms of guanylate cyclase-activating protein 1

In rod and cone photoreceptor cells, activation of particulate guanylate cyclase (retGC1) is mediated by a Ca2+-binding protein termed GCAP1, that detects changes in [Ca2+]free. In this study, we show that N-acylated GCAP1 restored Ca2+ sensitivity of native and recombinant photoreceptor retGC1. ATP increased the affinity of retGC1 for GCAP1 and accelerated catalysis. Using peptides derived from the GCAP1 sequence, we found that at least three regions, encompassing the N-terminus, the EF-1 motif, and the EF-3 motif, were likely involved in the interaction with retGC1. Mutation of 2Gly to Ala (GCAP1-G2A), which abolished myristoylation and a 25 amino acid truncation at the N-terminus (delta25-GCAP1) reduced retGC1-stimulating activity dramatically, while deletion of 10 amino acids (delta10-GCAP1) reduced the specific activity by only approximately 60% and modified the Ca2+ sensitivity. At 10(-6) M [Ca2+]free, in conditions that inactivated native GCAP1, retGC1 showed significant activity in the presence of delta10-GCAP1. Native and all three mutant forms of GCAP1 had similar affinities for Ca2+ as demonstrated by gel filtration and the changes in tryptophan fluorescence. All mutants bound to ROS membranes in a Ca2+-independent manner, except delta25-GCAP1, which was mostly soluble. These findings suggest that the N-terminal region is important in tethering of GCAP1 to the ROS membranes.

Interval mapping of quantitative trait loci controlling humoral immunity to exogenous antigens: evidence that non-MHC immune response genes may also influence susceptibility to autoimmunity

IgG Ab titers elicited to bovine rhodopsin in CFA differ 8- to 10-fold between H2s identical inbred strains A.SW/snJ (high responder) and SJL/snJ (low responder). This variation in IgG Ab titer resulted from a dramatic difference in the rise in Ab titer occurring during the maturation of the T-dependent humoral immune response. To determine the positions of non-MHC genes controlling this quantitative variation in T-dependent humoral immune responsiveness, 206 reciprocal (A.SW/snJ x SJL/snJ)F2 female progeny were immunized and assayed for anti-rhodopsin responsiveness. The genomes of these progeny were screened with 115 polymorphic simple sequence repeat markers covering >90% of the mouse genome. interval mapping analysis localized the positions of these non-MHC immune response genes to genomic intervals on chromosomes 1, 5, and 13. Interestingly, these three intervals coincide exactly with three intervals recently shown to contain genes contributing to susceptibility to systemic lupus erythematosus and/or the production of autoimmune anti-dsDNA Abs. These results suggest that some genes affecting levels of humoral immune responsiveness to exogenous Ag may also play a role in genetic susceptibility to humoral autoimmune diseases. Analyses of the modes of inheritance demonstrated that high responder alleles were inherited from both parental genomes, indicative of epistatic interactions among genes influencing humoral immune responsiveness.

Interval mapping of quantitative trait loci controlling humoral immunity to exogenous antigens: evidence that non-MHC immune response genes may also influence susceptibility to autoimmunity

IgG Ab titers elicited to bovine rhodopsin in CFA differ 8- to 10-fold between H2s identical inbred strains A.SW/snJ (high responder) and SJL/snJ (low responder). This variation in IgG Ab titer resulted from a dramatic difference in the rise in Ab titer occurring during the maturation of the T-dependent humoral immune response. To determine the positions of non-MHC genes controlling this quantitative variation in T-dependent humoral immune responsiveness, 206 reciprocal (A.SW/snJ x SJL/snJ)F2 female progeny were immunized and assayed for anti-rhodopsin responsiveness. The genomes of these progeny were screened with 115 polymorphic simple sequence repeat markers covering >90% of the mouse genome. interval mapping analysis localized the positions of these non-MHC immune response genes to genomic intervals on chromosomes 1, 5, and 13. Interestingly, these three intervals coincide exactly with three intervals recently shown to contain genes contributing to susceptibility to systemic lupus erythematosus and/or the production of autoimmune anti-dsDNA Abs. These results suggest that some genes affecting levels of humoral immune responsiveness to exogenous Ag may also play a role in genetic susceptibility to humoral autoimmune diseases. Analyses of the modes of inheritance demonstrated that high responder alleles were inherited from both parental genomes, indicative of epistatic interactions among genes influencing humoral immune responsiveness.

Ant opsins: sequences from the Saharan silver ant and the carpenter ant

cDNA clones encoding opsins from compound eyes of carpenter ant, Camponotus abdominalis, and Saharan silver ant, Cataglyphis bombycina, were isolated from cDNA libraries. The opsin cDNAs from each species code for deduced proteins with 378 amino acids which are 92% identical. Of the 30 amino acid differences between the two proteins, 13 are non-conservative. Eight of these non-conservative substitutions are within the membrane spanning domain. The presence of a potential Schiff-base counterion in helix III in both species suggests that these opsins are the protein moiety of the visible range pigments. When compared to all known opsins, these opsins are most similar to the opsin from preying mantis (76% identity at the amino acid level). Phyletic comparisons group the two ant opsins with the other arthropod long wavelength opsins.

The occurrence of serum autoantibodies against enolase in cancer-associated retinopathy

Cancer-associated retinopathy (CAR) is an uncommon paraneoplastic disease in which degeneration of the retina occurs as a remote effect of cancer in a distant part of the body. Immunoreactivity of sera from CAR patients and controls have been analyzed. Immunostaining of human retinal proteins showed that a soluble protein of Mr approximately 46 kDa (p46) is labeled by antibodies from several CAR patients with various types of cancer (lung, breast, bladder, prostate, salivary gland, and gastrointestinal tract cancer and chronic lymphocytic leukemia). These sera did not show reactivity with the 23-kDa protein previously associated with CAR. To identify and further characterize p46, the retinal protein was purified to homogeneity by anion-exchange chromatography and preparative gel electrophoresis. Protein sequence analysis of the peptides from p46 revealed a high homology with human enolase, an important glycolytic enzyme. Although enolase has been previously identified as a product of several types of tumors, and enolase activity has been detected in the sera of some cancer patients, the existence of autoantibodies directed to enolase has not been described. This is the first report of the presence of serum antibodies to retinal enolase in the patients with cancer and the CAR syndrome. When antibodies of specific isotypes (IgG, IgM, and IgA) were measured, IgG1 isotype was dominant. The significance of these antibodies for the disease process is under investigation.

Investigations of antiretinal antibodies in pigmentary retinopathy and other retinal degenerations

Images

Projection structure of frog rhodopsin in two crystal forms

Rhodopsin is the G protein-coupled receptor that upon light activation triggers the visual transduction cascade. Rod cell outer segment disc membranes were isolated from dark-adapted frog retinas and were extracted with Tween detergents to obtain two-dimensional rhodopsin crystals for electron crystallography. When Tween 80 was used, tubular structures with a p2 lattice (a = 32 A, b = 83 A, gamma = 91 degrees) were formed. The use of a Tween 80/Tween 20 mixture favored the formation of larger p22(1)2(1) lattices (a = 40 A, b = 146 A, gamma = 90 degrees). Micrographs from frozen hydrated frog rhodopsin crystals were processed, and projection structures to 7-A resolution for the p22(1)2(1) form and to 6-A resolution for the p2 form were calculated. The maps of frog rhodopsin in both crystal forms are very similar to the 9-A map obtained previously for bovine rhodopsin and show that the arrangement of the helices is the same. In a tentative topographic model, helices 4, 6, and 7 are nearly perpendicular to the plane of the membrane. In the higher-resolution projection maps of frog rhodopsin, helix 5 looks more tilted than it appeared previously. The quality of the two frog rhodopsin crystals suggests that they would be suitable to obtain a three-dimensional structure in which all helices would be resolved.

The sequence of arrestins from rod and cone photoreceptors in the frogs Rana catesbeiana and Rana pipiens. Localization of gene transcripts by reverse-transcription polymerase chain reaction on isolated photoreceptors

Members of the arrestin protein family are known to participate in the inactivation of rhodopsin and other heptahelical receptors. Arrestins bind to the activated and phosphorylated state of these receptors, consequently blocking the ability of the receptors to activate the guanine-nucleotide-binding protein (G protein). We have determined the sequences of four retinal arrestins from two species of frog, Rana catesbeiana and Rana pipiens. Using polymerase chain reaction on reverse-transcribed mRNA isolated from single photoreceptor cells, we show that two of these arrestins are from rod photoreceptors and two rod photoreceptors and two are from cone photoreceptors. Comparison of these arrestins with the twenty known arrestin sequences identifies three regions of the protein that are well conserved across all phylogenetic groups. These regions may function in the binding of the arrestin to the heptahelical receptors. In addition, the Rana arrestins contain a uniquely acidic C-terminal sequence.

Alligator rhodopsin: sequence and biochemical properties

We sequenced selected peptides of alligator rhodopsin that accounted for about half of the total protein. These sequences were confirmed when the total primary structure of alligator rhodopsin was deduced from the cDNA sequence. Differences in the amino-terminal region, compared to that of bovine rhodopsin, account for failure of cross-reactivity of several anti-bovine rhodopsin monoclonal antibodies. Differences in the carboxyl-terminal region give rise to limited antibody cross-reactivity and may also account for a slightly reduced ability of alligator rhodopsin to be phosphorylated by bovine rhodopsin kinase. Alligator rhodopsin regenerates much faster than bovine rhodopsin. The pseudo-first-order rate constant for alligator rhodopsin regeneration is approximately 25 times that of bovine. Phylogenetic analysis of 17 rhodopsin sequences indicates that the alligator is more closely related to the chicken than to the other species examined.

The deduced amino-acid sequence of opsin from rabbit rod photoreceptors

The amino acid (aa) sequence of rabbit opsin from rod photoreceptor cells was determined by direct aa sequencing and conceptual translation from the cDNA. The cDNA (1198 bp) containing the complete coding region encodes a 348-aa opsin protein. Of the 16 rod cell opsins that are known, rabbit opsin is most similar to human opsin (96.3% identity at the aa level).

Synthetic phosphopeptide from rhodopsin sequence induces retinal arrestin binding to photoactivated unphosphorylated rhodopsin

A synthetic heptaphosphopeptide comprising the fully phosphorylated carboxyl terminal phosphorylation region of bovine rhodopsin, residues 330-348, was found to induce a conformational change in bovine arrestin. This caused an alteration of the pattern of limited proteolysis of arrestin similar to that induced by binding phosphorylated rhodopsin or heparin. Unlike heparin, the phosphopeptide also induced light-activated binding of arrestin to both unphosphorylated rhodopsin in disk membranes as well as to endoproteinase Asp-N-treated rhodopsin (des 330-348). These findings suggest that one function of phosphorylation of rhodopsin is to activate arrestin which can then bind to other regions of the surface of the photoactivated rhodopsin.

A splice variant of arrestin. Molecular cloning and localization in bovine retina

Inactivation of photolyzed rhodopsin requires phosphorylation of the receptor and binding of the 48-kDa regulatory protein arrestin. We recently isolated a novel form of arrestin, termed p44, that is truncated at the COOH terminus (Palczewski, K., Buczylko, J., Ohguro, H., Annan, R. S., Carr, S. A., Crabb, J. W., Kaplan, M. W., Johnson, R. S., and Walsh, K. A. (1994) Protein Sci. 3, 319-329) and strongly inhibits Gt activation by non-phosphorylated rhodopsin. p44 is identical to arrestin except at the COOH terminus, where the 35 amino acids of arrestin are replaced by a single alanine residue. p44 is identified as a splice variant of arrestin based on the identical cDNA sequence of p44 with arrestin (except the 3' non-coding regions), the presence of an exon/intron junction at the Ser369 codon, and identical Southern hybridization patterns generated by the 3' non-coding portion of arrestin and p44. Immunocytochemistry reveals that p44 is localized in the photoreceptor outer segment, whereas arrestin is present throughout the cell. This specificity of localization to the outer segment is consistent with a role of p44 in the phototransduction cascade.

Optimization of peptide synthesis on polyethylene rods

Multipin solid-phase peptide synthesis is widely used for epitope mapping of monoclonal and polyclonal antibodies. However, neither the chemical yield nor the homogeneity of products currently match those of solid-phase synthesis of peptides on resins. In order to improve synthesis parameters, we have repeated the standard procedure and introduced modifications during synthesis of model heptapeptides and peptides from the sequence of rhodopsin and other proteins. Good incorporation of amino acids using the multipin peptide synthesis system can now be obtained in less synthesis time and with less costly reagents.

Role of anti-recoverin autoantibodies in cancer-associated retinopathy

PURPOSE

To examine the retina and test the serum of a patient with cancer-associated retinopathy syndrome who was diagnosed with small cell carcinoma of the lung and experienced unexpected visual loss.

METHODS

Proteins from normal human retina were extracted, separated by one- and two-dimensional gel electrophoresis, transferred to PVDF membrane, and used for immunostaining. Antibody specificity was determined by use of solid-phase peptides in a solid-phase immunoassay.

RESULTS

Histologic examination of the retina showed loss of the photoreceptor cell layer. This finding correlated with the results of clinical (loss of vision) and electrophysiologic (abnormal electroretinograph [ERG]) tests. The patient's serum antibodies specifically recognized recoverin, a protein predominantly found in retinal photoreceptor cells. The patient's serum also labeled some higher molecular weight proteins present in normal lung and other normal tissues, as well as in lung cell carcinoma cell lines. The only other tissue in which immunoreactivity against p23 could be found was the optic nerve. Our data revealed a lack of cross-reactivity between specific anti-recoverin antibodies and lung proteins. The results indicate that the patient serum contains more than one type of antibody activity. The autoantibodies were tested for fine immunospecificity by use of solid-phase peptides in a solid-phase immunoassay. Patient's antibodies reacted with a major determinant located in the recoverin sequence 62-68 (PKAYAQH) and with several minor ones.

CONCLUSION

Based on the fact that the recoverin appears to be distributed in several different cell types, we suggest that this protein may be present in cancer cells and may play a role in the pathogenesis of some cancer-associated retinopathies.

The kinetics of multiphosphorylation of rhodopsin

Rhodopsin kinase catalyzes the incorporation of up to seven phosphates into the carboxyl terminal region of freshly bleached rhodopsin. In order to study the mechanism of this reaction, we have separated different phosphorylated species of rhodopsin using Mono P FPLC chromatofocusing chromatography. The purity of the isolated species of rhodopsin was determined by isoelectric focusing. Separation yielded two forms of monophosphorylated and two diphosphorylated species of rhodopsin. Other species, containing up to five phosphates, were not fully separated. The phosphorylated forms of rhodopsin were characterized by competition enzyme-linked immunosorbent assay and immunoblotting using anti-rhodopsin site-specific monoclonal antibodies. A combination of the above methods allowed quantitative determination of the formation of different phosphorylated species of rhodopsin. A computer model for the consecutive time course of rhodopsin phosphorylation was developed and employed to characterize this reaction. Our data suggest that the rate of incorporation of the first phosphates into rhodopsin is slower than the rate of formation of more highly phosphorylated species. These data are supported by results showing that some monophosphorylated synthetic peptides are phosphorylated significantly faster than control unphosphorylated peptides.

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.

Interaction of rhodopsin with the G-protein, transducin

Rhodopsin, upon activation by light, transduces the photon signal by activation of the G-protein, transducin. The well-studied rhodopsin/transducin system serves as a model for the understanding of signal transduction by the large class of G-protein-coupled receptors. The interactive form of rhodopsin, R*, is conformationally similar or identical to rhodopsin's photolysis intermediate Metarhodopsin II (MII). Formation of MII requires deprotonation of rhodopsin's protonated Schiff base which appears to facilitate some opening of the rhodopsin structure. This allows a change in conformation at rhodopsin's cytoplasmic surface that provides binding sites for transducin. Rhodopsin's 2nd, 3rd and putative 4th cytoplasmic loops bind transducin at sites including transducin's 5 kDa carboxyl-terminal region. Site-specific mutagenesis of rhodopsin is being used to distinguish sites on rhodopsin's surface that are important in binding transducin from those that function in activating transducin. These observations are consistent with and extend studies on the action of other G-protein-coupled receptors and their interactions with their respective G proteins.

Induction of experimental autoimmune uveitis with rhodopsin synthetic peptides in Lewis rats

Rhodopsin, a membrane protein of rod photoreceptor cells, induces an experimental autoimmune uveitis (EAU) in Lewis rats. Synthetic peptides derived from rhodopsin sequences that cover hydrophilic, exposed regions of the protein were tested for their capacity of eliciting in vitro T cell proliferation and their ability for inducing EAU in Lewis rats. Rats were injected with rhodopsin's peptides mixed in complete Freund's adjuvant containing M. tuberculosis H37Ra (5 mg/ml) three days after pretreatment with cyclophosphamide (20 mg/kg). ELISA results indicate that all peptides induce antibody responses; however antibody titers differ among sera tested. Immunization with four peptides--the amino-terminus (2-32), loop I-II (61-75), loop V-VI (230-251), and the carboxyl-terminus (324-348 and 331-342) induced both antibody and T cell responses. In all cases, the proliferative responses of cells derived from peptide-injected rats were stronger against the immunizing peptide than against native protein. Three distinct uveitogenic epitopes were identified on rhodopsin's cytoplasmic surface--within the rhodopsin carboxyl-terminus (324-348), loop I-II (61-75), and loop V-VI (230-250). Histopathologically, at the immunized doses, total destruction of the photoreceptor cell layer was observed as compared to the control group. Loop V-VI caused severe inflammation of the retina while the other pathogenic peptides produced less severe destruction with few inflammatory cells present. Our study indicates that the major immunodominant T cell epitope (331-342) is also involved in EAU induction but is not the primary uveitogenic site.

Rhodopsin and phototransduction: a model system for G protein-linked receptors

Rhodopsin is the photoreceptor protein in rod cells of the vertebrate retina and the first member of the class of G protein-coupled receptors for which the amino acid sequence was determined. Rhodopsin is available in greater quantities than any other receptor of its class and therefore has been studied biochemically and biophysically by methods difficult or impossible to apply to its fellow receptors. Such studies support a model in which rhodopsin consists of seven transmembrane helices that form a binding pocket for its ligand, 11-cis retinal. Insights into the structure and function of rhodopsin serve as a model for understanding the structure and function of other members of the receptor class. Rhodopsin undergoes a change in conformation upon photoexcitation and activates a G protein, transducin, and is phosphorylated by a receptor-specific kinase, rhodopsin kinase. The phosphorylated photoactivated rhodopsin is bound by arrestin, thereby terminating activity of the receptor in the signal transduction process. These auxiliary proteins that function with rhodopsin on rod cells serve as models for understanding how other members of the receptor family may function in conjunction with other G proteins, kinases, and arrestin-like proteins.

Genetic control of antibody response to bovine rhodopsin in mice: epitope mapping of rhodopsin structure

Inbred strains of mice of independent haplotype were immunized with bovine rhodopsin. All mice tested except SJL developed significant titers of specific antibodies 21 days after a single immunization. Anti-rhodopsin antibody level differed among conventional inbred strains. Comparison of the immune response to rhodopsin of congenic mice on two different genetic backgrounds showed that animals with an A background typically produced higher levels of specific antibody than mice with a B10 background. Titer of specific antibodies in antisera of mice of the same H-2 haplotype but different Igh haplotype differed; e.g. for H-2d haplotype, NZB (Ighn) generated the highest level of antibody with BALB/c (Igha), DBA/2 (Ighc), and B10.D2 (Ighb) strains giving successively lower responses. The location of immunodominant regions of bovine rhodopsin was investigated in primary sera among strains of mice. Sera were tested for their binding of anti-rhodopsin antibodies to synthetic peptides covering the entire primary structure of rhodopsin. From direct binding studies with hydrophilic rhodopsin peptides, the majority of the antigenic binding sites were localized in the sequence of the amino terminus, the II-III loop and the carboxyl terminus. Binding to these antigenic peptides was not strain restricted. Application of the overlapping synthetic peptide strategy of Geysen enabled refinement of these epitopes and determination of an additional major epitope in the hydrophobic sequence 304-310.

Photoreceptor cell specific proteins of snake pineal

The pineal body of lower vertebrates is saccular and directly photoreceptive. The pineal gland of mammals is parenchymal and not directly photoreceptive. The parenchymal morphology of snake pineal raises questions of direct photoreceptivity of snake pineal and of correspondence of molecular homology with morphological homology. S-antigen and rhodopsin are highly conserved photoreceptor cell specific proteins. We used site-specific monoclonal antibodies (MAb) to study S-antigen and rhodopsin of snake pineal. Immunohistochemical reactivity of snake retina and pineal was compared to that of trout, guinea pig, and rat. MAb's to S-antigen reacted with each pineal and retina tested, but reactivity of individual MAb's with snake tissue was more similar to that with trout than with rat or guinea pig tissue. MAb's to rhodopsin did not react with snake pineal, although they did react with the photoreceptive trout pineal body. MAb's to rhodopsin reacted with retina of each species. These results suggest that although snake pineal is morphologically similar to mammalian pineal, and like mammalian pineal is probably not directly photoreceptive, it does have S-antigen homology with lower vertebrates such as trout.

Differential scanning calorimetry of bovine rhodopsin in rod-outer-segment disk membranes

Rhodopsin-containing retinal rod disk membranes from cattle have been examined by differential scanning calorimetry. Under conditions of 67 mM phosphate pH 7.0, unbleached rod outer segment disk membranes gave a single major endotherm with a temperature of denaturation (Tm) of 71.9 +/- 0.4 degrees C and a thermal unfolding calorimetric enthalpy change (delta Hcal) of 700 +/- 17 kJ/mol rhodopsin. Bleached rod outer segment disk membranes (membranes that had lost their absorbance at 498 nm after exposure to orange light) gave a single major endotherm with a Tm of 55.9 +/- 0.3 degrees C and a delta Hcal of 520 +/- 17 kJ/mol opsin. Neither bleached nor unbleached rod outer segment disk membranes gave endotherms upon thermal rescans. When thermal stability is examined over the pH range of 4-9, the major endotherms of both bleached and unbleached rod outer segment disk membranes were found to show maximum stability at pH 6.1. The observed delta Hcal values for bleached and unbleached rod outer segment disk membranes exhibit membrane concentration dependences which plateau at protein concentrations beyond 1.5 mg/mL. For partially bleached samples of rod outer segment disk membranes, the calorimetric enthalpy change for opsin appears to be somewhat dependent on the degree of bleaching, indicating intramembrane nearest neighbor interactions which affect the unfolding of opsin. Delta Hcal and Tm are particularly useful for assessing stability and testing for completeness of regeneration of rhodopsin from opsin. Other factors such as sample preparation and the presence of low concentrations of ethanol also affect the delta Hcal values while the Tm values remain fairly constant. This shows that the delta Hcal is a sensitive parameter for monitoring environmental changes of rhodopsin and opsin.

Role of the carboxyl-terminal region of arrestin in binding to phosphorylated rhodopsin

The structural and functional properties of arrestin were studied by subjecting the protein to limited proteolysis. Limited proteolysis by trypsin cleaves arrestin (48 kDa), producing 20-25-kDa fragments. Prior to this stage of proteolysis, trypsin produced 46.6-, 45.4-, and 42-kDa fragments. Structural analysis of the proteolytic fragments demonstrated major cleavage at the carboxyl terminus, indicating that the carboxyl terminus is highly exposed. We found that forms of arrestin truncated at their carboxyl terminus maintained their functional properties and bound to phosphorylated rhodopsin. Native arrestin binds only to photoexcited phosphorylated rhodopsin, whereas the truncated arrestin binds to phosphorylated rhodopsin independent of its exposure to light. The truncated forms of arrestin were separated from native arrestin by a chromatographic procedure and subsequently characterized in functional studies. The binding of the truncated forms of arrestin to phosphorylated photoexcited rhodopsin is more tight than the binding of native arrestin as determined by a direct binding assay and the phosphodiesterase assay. We suggest that the acidic carboxyl-terminal region of arrestin may act as a regulator for light-dependent binding to phosphorylated rhodopsin.

Elevated level of protein phosphatase 2A activity in retinas of rd mice

Phosphorylation of rhodopsin is not detectable in vitro in the retina of the rd mouse. We investigated the enzymatic system responsible for this abnormality by measuring the levels of rhodopsin kinase and protein phosphatase 2A in normal (rd/+) and diseased (rd/rd) mouse retinas of several ages. For each enzyme, we developed micro assays that were suitable for measuring enzyme activity in one-half mouse retina. Our results indicate that rhodopsin kinase activity is identical in rd/+ and rd/rd retinas until post-natal day 11, and it decreases thereafter in the rd/rd retina, correlating with the loss of rod photoreceptors that occurs in this tissue. Protein phosphatase 2A has a constant level of activity in rd/+ retinas from ages 5 to 32 days but it is higher than normal in rd/rd retinas from post-natal days 5 to 10. It then decreases to levels that are comparable to those in rd/+ retina. Although the rd/rd extract contains the elevated protein phosphatase 2A activity, when rd/rd and rd/+ retinal extracts are each subjected to gel filtration, the elution profiles of protein phosphatase 2A activity appear to be quantitatively identical. This apparent loss of rd/rd phosphatase activity suggests a difference in the regulatory behavior of the enzyme in the normal and degenerative retinas. Thus, the failure to detect in vitro phosphorylation of rhodopsin in the rd/rd retina seems to result from the elevated level of protein phosphatase 2A activity which could more rapidly remove the phosphate from phosphorylated rhodopsin. Since protein phosphatase 2A is a ubiquitous enzyme with broad specificity, an elevation in its activity also could affect other protein phosphorylations.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies of ligand binding to arrestin

A striking homology is observed between the regions 70-83 and 361-374 of the sequence of bovine arrestin and the calcium-binding loops of calmodulin and troponin C. However, the predicted alpha-helices flanking the calcium-binding site in calmodulin and troponin C are not present in arrestin. Direct measurements therefore were made in order to assess whether arrestin can bind calcium. We found that arrestin does not bind Ca2+ at physiological ionic strength, as determined by equilibrium dialysis, gel filtration, and fluorescence spectroscopy. Rapid and quantitative precipitation of arrestin occurs with Tb3+. The precipitation is reversed by EDTA and blocked by Mg2+ but not by Ca2+. Prompted by several reports, we also investigated whether nucleotides bind to arrestin. Neither ATP nor GTP binds under the conditions tested. Binding of arrestin to photolyzed, phosphorylated rhodopsin also does not influence the binding of calcium or nucleotides.

Anti-rhodopsin monoclonal antibodies of defined specificity: characterization and application

A panel of anti-bovine rhodopsin monoclonal antibodies (MAbs) of defined site-specificity has been prepared and used for functional and topographic studies of rhodopsins. In order to select these antibodies, hybridoma supernatants that contained anti-rhodopsin antibodies have been screened by enzyme-linked immunosorbent assay (ELISA) in the presence of synthetic peptides from rhodopsin's cytoplasmic regions. We selected for antibodies against predominantly linear determinants (as distinct from complex assembled determinants) and have isolated antibodies that recognize rhodopsin's amino terminus, its carboxyl terminus, as well as the hydrophilic helix-connecting regions 61-75, 96-115, 118-203, 230-252 and 310-321. Detailed specificities have been further determined by using a series of overlapping peptides and chemically modified rhodopsins as competitors. A group of seven antibodies with epitopes clustered within the amino terminal region of rhodopsin and a group of 15 antibodies with epitopes within the carboxyl terminal region are described. These MAbs have high affinities for rhodopsin with Kas in the range of 10(8)-10(10) M-1. Some MAbs specific for the carboxyl and amino terminal regions were used to compare these bovine rhodopsin sequences to those of different vertebrates. The MAbs cross-reacted with the different species tested to different extents indicating that there is some similarity in the sequences of these regions. However, some differences in the sequences were indicated by a reduced or absent cross-reactivity with some MAbs. In membrane topographic studies the MAbs showed both the presence and the accessibility of rhodopsin sequences 330-348, 310-321 and 230-252 on the cytoplasmic surface of the disk membrane. Similarly, sequences 1-20 and 188-203 were shown to reside on the lumenal surface of the disk and to be accessible to a macromolecular (antibody) probe. Antibodies directed against rhodopsin's carboxyl terminal sequence did not bind well to highly phosphorylated rhodopsin. Similarly, these antibodies as well as those against the V-VI loop inhibited phosphorylation of rhodopsin. Antibody A11-82P, specific for phosphorylated rhodopsin, recognized rhodopsin containing two or more phosphates and inhibited its further phosphorylation.

Phosphorylation of iodopsin, chicken red-sensitive cone visual pigment

The amino acid sequence has been determined for the carboxyl-terminal 41 amino acids of chicken red-sensitive cone pigment, iodopsin. This sequence is distinct from but structurally homologous to that of other visual pigments. It contains a region rich in the hydroxy amino acids serine and threonine. In the related rod cell visual pigment, rhodopsin, such serines and threonines have previously been identified as sites for phosphorylation by rhodopsin kinase. Phosphorylation of photolyzed rhodopsin serves to terminate its ability to function in visual transduction as an activator of G-protein. We have purified and reconstituted both chicken rhodopsin and chicken iodopsin and shown them to be phosphorylated by bovine rhodopsin kinase. Chicken iodopsin has a Km and Vmax similar to but distinguishably different from that for bovine rhodopsin. These results, in conjunction with other data, suggest that visual pigments in cone cells, upon absorption of light, undergo functional processes similar to those of the visual pigments in rod cells.