Differences in the pharmacological activation of visual opsins

Opsins, like many other G-protein-coupled receptors, sustain constitutive activity in the absence of ligand. In partially bleached rods and cones, opsin's activity closes cGMP-gated channels and produces a state of "pigment adaptation" with reduced sensitivity to light and accelerated flash response kinetics. The truncated retinal analogue, beta-ionone, further desensitizes partially bleached green-sensitive salamander rods, but enables partially bleached red-sensitive cones to recover dark-adapted physiology. Structural differences between rod and cone opsins were proposed to explain the effect. Rods and cones, however, also contain different transducins, raising the possibility that G-protein type determines the photoreceptor-specific effects of beta-ionone. To test the two hypotheses, we applied beta-ionone to partially bleached blue-sensitive rods and cones of salamander, two cells that couple the same cone-like opsin to either rod or cone transducin, respectively. Immunocytochemistry confirmed that all salamander rods contain one form of transducin, whereas all cones contain another. beta-Ionone enhanced pigment adaptation in blue-sensitive rods, but it also did so in blue- and UV-sensitive cones. Furthermore, all recombinant salamander rod and cone opsins, with the exception of the red-sensitive cone opsin, activated rod transducin upon the addition of beta-ionone. Thus opsin structure determines the identity of beta-ionone as an agonist or an inverse agonist and in that respect distinguishes the red-sensitive cone opsin from all others.

Visual cycle and its metabolic support in gecko photoreceptors

Photoreceptors of nocturnal geckos are transmuted cones that acquired rod morphological and physiological properties but retained cone-type phototransduction proteins. We have used microspectrophotometry and microfluorometry of solitary isolated green-sensitive photoreceptors of Tokay gecko to study the initial stages of the visual cycle within these cells. These stages are the photolysis of the visual pigment, the reduction of all-trans retinal to all-trans retinol, and the clearance of all-trans retinol from the outer segment (OS) into the interphotoreceptor space. We show that the rates of decay of metaproducts (all-trans retinal release) and retinal-to-retinol reduction are intermediate between those of typical rods and cones. Clearance of retinol from the OS proceeds at a rate that is typical of rods and is greatly accelerated by exposure to interphotoreceptor retinoid-binding protein, IRBP. The rate of retinal release from metaproducts is independent of the position within the OS, while its conversion to retinol is strongly spatially non-uniform, being the fastest at the OS base and slowest at the tip. This spatial gradient of retinol production is abolished by dialysis of saponin-permeabilized OSs with exogenous NADPH or substrates for its production by the hexose monophosphate pathway (NADP+glucose-6-phosphate or 6-phosphogluconate, glucose-6-phosphate alone). Following dialysis by these agents, retinol production is accelerated by several-fold compared to the fastest rates observed in intact cells in standard Ringer solution. We propose that the speed of retinol production is set by the availability of NADPH which in turn depends on ATP supply within the outer segment. We also suggest that principal source of this ATP is from mitochondria located within the ellipsoid region of the inner segment.

Circular dichroism and cross-linking studies of bacteriorhodopsin mutants

Circular dichroism (CD) spectroscopy was employed for native (wild type, WT) bacteriorhodopsin (bR) and several mutant derivatives: R134K, R134H, R82Q, S35C, L66C, and R134C/E194C. Comparative analysis of the CD spectra in visible range shows that only R134C/E194C exhibits biphasic CD, typical for native bR, the other mutants demonstrate CD spectra with significantly smaller or absent negative band. Since the biphasic CD is a feature of hexagonal lattice structure composed by bR trimers in the purple membrane, these mutants and WT were examined by cross-linking studies, which confirmed the same trend towards trimeric organization. Therefore, a single amino acid substitution may lead to drastically different CD spectra without disruption of bR trimeric organization. Thus, although disruption of bR trimeric crystalline lattice structure (e.g., solubilization with detergents) directly results in the disappearance of characteristic bilobe in visible CD, the lack of the bilobe in the CD alone does not predict the absence of trimers.

Rhodopsin phosphorylation in rats exposed to intense light

The damaging effects of intense light on the rat retina are known to vary depending on the time of day of exposure. The purpose of this study was to determine if rhodopsin phosphorylation patterns, a measure of the activity of the pigment, varied in a similar manner. After 10 min in strong light (1400 lux), all six threonine and serine sites in the rat rhodopsin C-terminus were phosphorylated, with mono- to tetraphosphorylation being substantially more prominent than penta- to hexaphosphorylation. The level and multiplicity of rhodopsin phosphorylations were reduced both with the duration of light exposure and the duration of subsequent darkness. Although showing vast differences in susceptibility to light damage, rats exposed at 5 P.M. or 1 A.M. showed similar rhodopsin phosphorylation levels and patterns. These data indicate that a process controlled by circadian rhythm other than rhodopsin phosphorylation is involved either in damaging or mediating the damage evoked by intense light exposure.

A visual pigment expressed in both rod and cone photoreceptors

Rods and cones contain closely related but distinct G protein-coupled receptors, opsins, which have diverged to meet the differing requirements of night and day vision. Here, we provide evidence for an exception to that rule. Results from immunohistochemistry, spectrophotometry, and single-cell RT-PCR demonstrate that, in the tiger salamander, the green rods and blue-sensitive cones contain the same opsin. In contrast, the two cells express distinct G protein transducin alpha subunits: rod alpha transducin in green rods and cone alpha transducin in blue-sensitive cones. The different transducins do not appear to markedly affect photon sensitivity or response kinetics in the green rod and blue-sensitive cone. This suggests that neither the cell topology or the transducin is sufficient to differentiate the rod and the cone response.

Exploring the function of Tyr83 in bacteriorhodopsin: features of the Y83F and Y83N mutants

Tyrosine-83, a residue which is conserved in all halobacterial retinal proteins, is located at the extracellular side in helix C of bacteriorhodopsin. Structural studies indicate that its hydroxyl group is hydrogen bonded to Trp189 and possibly to Glu194, a residue which is part of the proton release complex (PRC) in bacteriorhodopsin. To elucidate the role of Tyr83 in proton transport, we studied the Y83F and Y83N mutants. The Y83F mutation causes an 11 nm blue shift of the absorption spectrum and decreases the size of the absorption changes seen upon dark adaptation. The light-induced fast proton release, which accompanies formation of the M intermediate, is observed only at pH above 7 in Y83F. The pK(a) of the PRC in M is elevated in Y83F to about 7.3 (compared to 5.8 in WT). The rate of the recovery of the initial state (the rate of the O --> BR transition) and light-induced proton release at pH below 7 is very slow in Y83F (ca. 30 ms at pH 6). The amount of the O intermediate is decreased in Y83F despite the longer lifetime of O. The Y83N mutant shows a similar phenotype in respect to proton release. As in Y83F, the recovery of the initial state is slowed several fold in Y83N. The O intermediate is not seen in this mutant. The data indicate that the PRC is functional in Y83F and Y83N but its pK(a) in M is increased by about 1.5 pK units compared to the WT. This suggests that Tyr83 is not the main source for the proton released upon M formation in the WT; however, Tyr83 is involved in the proton release affecting the pK(a) of the PRC in M and the rate of proton transport from Asp85 to PRC during the O --> bR transition. Both the Y83F and the Y83N mutations lead to a greatly decreased functionality of the pigment at high pH because most of the pigment is converted into the inactive P480 species, with a pK(a) 8-9.

Mass spectrometric analysis of integral membrane proteins at the subnanomolar level: application to recombinant photopigments

Integral membrane proteins produced by eukaryotic expression systems are a subject of much current interest in biomedical investigation. Due to the low efficiency of their expression and the limited quantity of the expressed to the total amount of the membrane proteins, they have evaded mass spectrometric analysis. The methodology previously developed for mass spectrometric analysis of integral membrane proteins required proteins that were obtained relatively pure from their native membranes. The previously developed methodology has been modified and applied to the analysis of subnanomolar samples of rhodopsin. Bovine rhodopsin purified by affinity chromatography, from native membranes and from a eukaryotic expression system, was successfully analyzed, obtaining complete sequence coverage for the detection and localization of posttranslational modifications. The methodology presented here will enable mass spectrometric analysis of subnanomolar levels of photopigments or other integral membrane proteins either from their native membranes or as products of expression systems.

Cloning and characterization of three salamander retinal G-protein beta subunits

PURPOSE

Recent evidence has shown that the beta-gamma dimers (beta gamma) of activated heterotrimeric G-proteins are important in many cellular signaling pathways. Since two distinct transducin alpha subunits have been cloned from the salamander retina, we aimed to identify and characterize the G-protein beta (Gbeta) subunits that are involved in visual signal transduction in the salamander.

METHODS

A salamander retina cDNA library was screened using degenerate oligonucleotide primers designed from a compilation of known Gbeta sequences. Tissue specific expression was determined by reverse transcriptase PCR (RT-PCR).

RESULTS

The library screening resulted in the cloning of three full-length sequences, two of which encode proteins of 340 residues and the third being an iniation variant of 353 and 395 residues. No identical matches were found in GenBank but each shows highest homology to G-beta-1 (beta1), G-beta-3 (beta3), and G-beta-5 (beta5 and beta5L) subunits of other species, respectively. The beta1 and beta3 subunits are 84.7% identical to each other but both show only 52% identity to beta5 at the protein level. RT-PCR analysis showed that all the subunits are expressed in multiple tissues, including the retina. However, the beta5L splice variant was found only in the retina.

CONCLUSIONS

Three distinct Gbeta subunit transcripts are expressed in the salamander retina. These subunits have proven to be important in the visual system of mammalian models.

Expression, purification, and MALDI analysis of RPE65

PURPOSE

RPE65 is preferentially expressed in the retinal pigment epithelium (RPE) and is essential for retinal function. The purpose of the study was to develop methods for the expression of the protein, determine the accurate molecular weight of this expressed protein, and quantitate the amount of RPE65 in the bovine RPE.

METHODS

Human RPE65 was expressed in Sf9 cells using the baculovirus system. The subcellular localization was determined by Western blot analysis and immunocytochemistry. An ELISA was developed for RPE65 and used to measure levels in bovine RPE. Recombinant and native RPE65 were purified by affinity chromatography. Molecular mass was determined by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry.

RESULTS

Recombinant human (rH)RPE65 was expressed as a major protein associated with cell membrane in Sf9 cells. The recombinant protein was purified to apparent homogeneity from both the membrane and nonmembrane fractions. The identity of the purified protein was confirmed by Western blot analysis and by partial peptide sequencing. rHRPE65 from the nonmembrane fraction has a mass of 64,867 +/- 80 which is close to the calculated molecular weight from the amino acid sequence including the His-tag (64,663), whereas the membrane-associated rHRPE65 has a molecular mass of 65,380 +/- 150, which is significantly higher than that of the non-membrane-associated form and the calculated molecular weight, suggesting posttranslational modifications. Similarly, native RPE65 was detected in the cytosolic and microsomal fractions of the bovine RPE, with an average level of 3.8 +/- 1.3 and 7.2 +/- 0.4 microg RPE65 per eye, respectively. The cytosolic form had a molecular mass of 61,161 +/- 60, which is close to the calculated value (60,944), whereas that of the microsomal form was 61,961 +/- 170.

CONCLUSIONS

RPE65 is expressed in two forms, one of which is membrane associated and contains significant posttranslational modifications, similar to the native membrane-associated form.

Salamander UV cone pigment: sequence, expression, and spectral properties

The visual pigment from the ultraviolet (UV) cone photoreceptor of the tiger salamander has been cloned, expressed, and characterized. The cDNA contains a full-length open reading frame encoding 347 amino acids. The phylogenetic analysis indicates that the highest sequence homology is to the visual pigments in the S group. The UV opsin was tagged at the carboxy-terminus with the sequence for the 1D4 epitope. This fusion opsin was expressed in COS-1 cells, regenerated with 11-cis retinal (A1) and immuno-purified, yielding a pigment with an absorbance maximum (lambdamax) of 356 nm which is blue shifted from the absorption of retinal itself. The transducin activation assay demonstrated that this pigment is able to activate rod transducin in a light-dependent manner. Regeneration with 11-cis 3,4-dehydroretinal (A2) yielded a pigment with a lambdamax of 360 nm, only 4 nm red shifted from that of the A1 pigment, while bovine rhodopsin generated with A2 showed a 16-nm red shift from the corresponding A1 pigment. These results demonstrate that the trend for a shorter wavelength pigment to have a smaller shift of lambdamax between the A1 and A2 pigments also fits UV pigments. We hypothesize that the small red shift with A2 could be due to a twist in the chromophore that essentially isolates the ring double bond(s) from conjugation with the rest of the polyene chain.

Role of noncovalent binding of 11-cis-retinal to opsin in dark adaptation of rod and cone photoreceptors

Regeneration of visual pigments of vertebrate rod and cone photoreceptors occurs by the initial noncovalent binding of 11-cis-retinal to opsin, followed by the formation of a covalent bond between the ligand and the protein. Here, we show that the noncovalent interaction between 11-cis-retinal and opsin affects the rate of dark adaptation. In rods, 11-cis-retinal produces a transient activation of the phototransduction cascade that precedes sensitivity recovery, thus slowing dark adaptation. In cones, 11-cis-retinal immediately deactivates phototransduction. Thus, the initial binding of the same ligand to two very similar G protein receptors, the rod and cone opsins, activates one and deactivates the other, contributing to the remarkable difference in the rates of rod and cone dark adaptation.

Salamander rods and cones contain distinct transducin alpha subunits

The mammalian retina is known to contain two distinct transducins that interact with their respective rod and cone pigments. However, there are no reports of a nonmammalian species having two distinct transducins. In the present study, we report the cloning and cellular localization of two transducin a subunits (G alpha t) from the tiger salamander. Through degenerate polymerase chain reaction (PCR) and subsequent screening of a salamander retina cDNA library, we have identified two forms of G alpha t. When compared to existing sequences in GenBank, the cloned subunits showed high similarity to rod and cone transducins. The salamander G alpha t-1 has 91.2-93.7% amino acid sequence identity to mammalian rod G alpha t subunits and 79.7-80.9% to mammalian cone Gats. The salamander G alpha t-2 has 86.2-87.9% sequence identity to mammalian cone G alpha ts and 78.9-80.9% to mammalian rod G alpha ts at the amino acid level. The G alpha t-1 cDNA encodes 350 amino acids while the G alpha t-2 cDNA encodes 354 residues, which is typical for rod and cone G alpha ts, respectively, and we thus identified the G alpha t- 1 as rod and G alpha t-2 as cone G alpha t. Sequences identified as effector binding sites and GTPase activity regions are highly conserved between the two subunits. Genomic Southern blot analysis showed that rod and cone G alpha t subunits are both encoded by single-copy genes. Northern blot analysis identified retina-specific transcripts of 3.0 kb for rod G alpha t and 2.6 kb for cone G alpha t. Immunohistochemistry in the flat-mounted salamander retina demonstrated that rod G alpha t is localized to rods, predominantly in the outer segments; similarly, cone G alpha t is localized to cone outer segments. The results confirm that the two sequences encode rod and cone transducins and demonstrate that this lower vertebrate contains two distinct transducins that are localized specifically to rod and cone photoreceptors.

Fluorescence properties of pyrylretinol

A fluorescent analog of retinol, 3,7-dimethyl-9-(1-pyryl)-2E,4E,6E,8E-nonatetr aene-1-ol (referred to as pyrylretinol, or 1) has been synthesized. The fluorescence properties (e.g. quantum yield, lifetime, steady-state anisotropy, and excitation/emission spectra) of this compound in various organic solvents and in dimyristoylphosphatidylcholine (DMPC) liposomes have been studied, and the results are compared with those obtained from 3-methyl-5-(1-pyryl)-2E,4E-pentadiene-1-ol (2), which has the same fused aromatic ring system but a much shorter acyclic chain. 1 and 2 form excimer in aqueous media and fluorescence anisotropies of both 1 and 2 in DMPC liposomes exhibit an abrupt decrease at approximately 21-23 degrees C, which coincides with the main phase transition temperature of DMPC liposomes, indicating that both compounds may be a useful membrane probe. In addition, the binding and quenching capability of pyrylretinol (1) to bovine serum albumin has been investigated. Pyrylretinol (1) binds with BSA with a binding constant of 3.6 x 10(4) M-1, although the value is somewhat lower than that obtained for retinol (3.06 x 10(5) M-1). Pyrylretinol (1) also quenches the BSA intrinsic fluorescence with the quenching rate constant of 1.67 x 10(13) M-1 s-1 and the value is lower than that obtained for retinol (4.06 x 10(13) M-1 s-1). The binding and quenching studies suggest that pyrylretinol (1) may serve as a useful fluorescence probe for structure/function studies of different retinoid binding proteins.

Mass spectrometric analysis of rhodopsin from light damaged rats

PURPOSE

It is well established that the retina is damaged by intense visible light. Rhodopsin has been proposed to be involved in this process. We therefore undertook to examine whether rhodopsin isolated from light damaged animals is structurally altered at the molecular level.

METHODS

Dark reared and dim cyclic light reared 8 week old Sprague-Dawley rats were exposed to intense visible light and sacrificed immediately or 24 h after exposure together with unexposed control animals reared under the same conditions. Rod outer segments were isolated by sucrose gradient ultracentrifugation, their membranes treated with urea, then washed with Tris buffer. The rhodopsin preparations were then reduced, pyridylethylated, delipidated, and cleaved with CNBr. Reversed phase HPLC was used to separate the fragments, and the effluent was analyzed online with a Finnigan LCQ ion trap mass spectrometer. C-terminal phosphorylation was investigated following Asp-N cleavage. MALDI-TOF mass spectrometry was used for the identification of glycosylation.

RESULTS

The rat rhodopsin protein was mapped with the exception of two single amino acid fragments. The reported sequence was confirmed with the exception of the controversial T/S320 residue, which was found to be a threonine. Mono-, di-, tri-, and tetraphosphorylated forms of rhodopsin were found in the light damaged animals. Three sites of phosphorylation were confirmed with MS/MS (tandem mass spectral) data. Single or double phosphorylations were found among these three sites, in various combinations. Dark adaptation completely reversed the phosphorylation in all light damaged animals. Other posttranslational modifications were as previously reported.

CONCLUSIONS

Our results indicate that intense visible light exposure of rats does not lead to oxidative or other primary structural alterations in the rhodopsin protein of rod outer segments. We also report that the mutated rhodopsin (P23H) is present in rat rod outer segments from heterozygous animals and that residue 320 in both normal and mutated rhodopsins is threonine, not serine.

Intrahelical arrangement in the integral membrane protein rhodopsin investigated by site-specific chemical cleavage and mass spectrometry

Site-specific cleavage on the interhelical loop I on the cytoplasmic face of rhodopsin has been observed after activation of a Cu-phenanthroline tethered cleavage reagent attached on the cytoplasmic loop IV. The characterization of the reaction products by mass spectrometry, both of the membrane-bound protein and of the CNBr-cleaved peptides, allows the site of cleavage to be determined precisely. The specific cleavage of the peptide bond between Q64 and H65 on loop I leaves the N-terminal peptide (M1-Q64) intact, confirmed by MALDI-MS detection of the two N-linked glycosyl groups near the N-terminus of rhodopsin. The limited extension of the tether side chain requires a interresidue distance between the cleavage site, Q64, and the site of ligand attachment, C316, of less than 12 A. Upon photoactivation of the receptor, no change in the cleavage pattern is observed; however, a simulated Meta II intermediate activation state indicates a much more complex cleavage pattern. The development of this cleavage method, previously used primarily as a "chemical nuclease", in combination with mass spectrometry, may provide a powerful method on membrane protein conformation studies that can be used to complement other biophysical characterizations.

Evidence for the rate of the final step in the bacteriorhodopsin photocycle being controlled by the proton release group: R134H mutant

Light absorbed by bacteriorhodopsin (bR) leads to a proton being released at the extracellular surface of the purple membrane. Structural studies as well as studies of mutants of bR indicate that several groups form a pathway for proton transfer from the Schiff base to the extracellular surface. These groups include D85, R82, E204, E194, and water molecules. Other residues may be important in tuning the initial state pK(a) values of these groups and in mediating light-induced changes of the pK(a) values. A potentially important residue is R134: it is located close to E194 and might interact electrostatically to affect the pK(a) of E194 and light-induced proton release. In this study we investigated effects of the substitution of R134 with a histidine on light-induced proton release and on the photocycle transitions associated with proton transfer. By measuring the light-induced absorption changes versus pH, we found that the R134H mutation results in an increase in the pK(a) of the proton release group in both the M (0.6 pK unit) and O (0.7 pK unit) intermediate states. This indicates the importance of R134 in tuning the pK(a) of the group that, at neutral and high pH, releases the proton upon M formation (fast proton release) and that, at low pH, releases the proton simultaneously with O decay (slow proton release). The higher pK(a) of the proton release group found in R134H correlates with the slowing of the rate of the O --> bR transition at low pH and probably is the cause of this slowing. The pH dependence of the fraction of the O intermediate is altered in R134H compared to the WT but is similar to that in the E194D mutant: a very small amount of O is present at neutral pH, but the fraction of O increases greatly upon decreasing the pH. These results provide further support for the hypothesis that the O --> bR transition is controlled by the rate of deprotonation of the proton release group. These data also provide further evidence for the importance of the R134-E194 interaction in modulating proton release from D85 after light has led to its being protonated.

Characterization of human lens major intrinsic protein structure

PURPOSE

To determine the primary covalent structure of human lens major intrinsic protein (MIP) in lenses of varying age.

METHODS

MIP was isolated from single human lenses of various ages (7- 86 years) by homogenization of the lenses, followed by centrifugation and urea washes of the membranes. Proteins present in the membrane preparation were reduced, alkylated, and cleaved by CNBr. Peptide fragments were fractionated by reverse-phase high-performance liquid chromatography, and the primary structures of the peptides were determined by tandem mass spectrometry and Edman sequencing.

RESULTS

Complete coverage of the human MIP sequence was observed in the form of CNBr fragments. In addition, peptide structures resulting from in vivo heterogeneous N- and C-terminal cleavage were characterized. The amount of intact MIP decreased with lens age; however, the pattern of truncation did not change from 7 to 86 years. The major site of phosphorylation was identified as serine 235. Asparagine residues 246 and 259 were completely deamidated by age 7 years.

CONCLUSIONS

The major structural modifications of human lens MIP have been determined. Human MIP is heterogeneously modified in lenses ranging in age from 7 to 86 years of age by N- and C-terminal truncation, phosphorylation, and deamidation, resulting in decreased levels of native intact MIP with age.

Studies on pyrylretinal analogues of bacteriorhodopsin

The retinal analogues 3-methyl-5-(1-pyryl)-2E,4E-pentadienal (1) and 3,7-dimethyl-9-(1-pyryl)-2E,4E,6E,8E-nonatetr aenal (2), which contain the tetra aromatic pyryl system, have been synthesized and characterized in order to examine the effect of the extended ring system on the binding capabilities and the function of bacteriorhodopsin (bR). The two bR mutants, E194Q and E204Q, known to have distinct proton-pumping patterns, were also examined so that the effect of the bulky ring system on the proton-pumping mechanism could be studied. Both retinals formed pigments with all three bacterioopsins, and these pigments were found to have absorption maxima in the range 498-516 nm. All the analogue pigments showed activity as proton pumps. The pigment formed from wild-type apoprotein bR with 1 (with the shortened polyene side chain) showed an M intermediate at 400 nm and exhibited fast proton release followed by proton uptake. Extending the polyene side chain to the length identical with retinal, analogue 2 with wild-type apoprotein gave a pigment that shows M and O intermediates at 435 nm and 650 nm, respectively. This pigment shows both fast and slow proton release at pH 7, suggesting that the pKa of the proton release group (in the M-state) is higher in this pigment compared to native bR. Hydrogen azide ions were found to accelerate the rise and decay of the O intermediate at neutral pH in pyryl 2 pigment. The pigments formed between 2 and E194Q and E204Q showed proton-pumping behavior similar to pigments formed with the native retinal, suggesting that the size of the chromophore ring does not alter the protein conformation at these sites.

Two groups control light-induced Schiff base deprotonation and the proton affinity of Asp85 in the Arg82 his mutant of bacteriorhodopsin

Arg(82) is one of the four buried charged residues in the retinal binding pocket of bacteriorhodopsin (bR). Previous studies show that Arg(82) controls the pK(a)s of Asp(85) and the proton release group and is essential for fast light-induced proton release. To further investigate the role of Arg(82) in light-induced proton pumping, we replaced Arg(82) with histidine and studied the resulting pigment and its photochemical properties. The main pK(a) of the purple-to-blue transition (pK(a) of Asp(85)) is unusually low in R82H: 1.0 versus 2.6 in wild type (WT). At pH 3, the pigment is purple and shows light and dark adaptation, but almost no light-induced Schiff base deprotonation (formation of the M intermediate) is observed. As the pH is increased from 3 to 7 the M yield increases with pK(a) 4.5 to a value approximately 40% of that in the WT. A transition with a similar pK(a) is observed in the pH dependence of the rate constant of dark adaptation, k(da). These data can be explained, assuming that some group deprotonates with pK(a) 4.5, causing an increase in the pK(a) of Asp(85) and thus affecting k(da) and the yield of M. As the pH is increased from 7 to 10.5 there is a further 2.5-fold increase in the yield of M and a decrease in its rise time from 200 micros to 75 micros with pK(a) 9. 4. The chromophore absorption band undergoes a 4-nm red shift with a similar pK(a). We assume that at high pH, the proton release group deprotonates in the unphotolyzed pigment, causing a transformation of the pigment into a red-shifted "alkaline" form which has a faster rate of light-induced Schiff base deprotonation. The pH dependence of proton release shows that coupling between Asp(85) and the proton release group is weakened in R82H. The pK(a) of the proton release group in M is 7.2 (versus 5.8 in the WT). At pH < 7, most of the proton release occurs during O --> bR transition with tau approximately 45 ms. This transition is slowed in R82H, indicating that Arg(82) is important for the proton transfer from Asp(85) to the proton release group. A model describing the interaction of Asp(85) with two ionizable residues is proposed to describe the pH dependence of light-induced Schiff base deprotonation and proton release.

Isomerization of all-trans-9- and 13-desmethylretinol by retinal pigment epithelial cells

Photoisomerization of 11-cis-retinal to all-trans-retinal triggers phototransduction in the retinal photoreceptor cells and causes ultimately the sensation of vision. 11-cis-Retinal is enzymatically regenerated through a complex set of reactions in adjacent retinal pigment epithelial cells (RPE). In this study using all-trans-9-desmethylretinol (lacking the C(19) methyl group) and all-trans-13-desmethylretinol (lacking the C(20) methyl group), we explored the effects of C(19) and C(20) methyl group removals on isomerization of these retinols in RPE microsomes. The C(19) methyl group may be involved in the substrate activation, whereas the C(20) methyl group causes steric hindrance with a proton in position C(10) of 11-cis-retinol; thus, removal of this group could accelerate isomerization. We found that all-trans-9-desmethylretinol and all-trans-13-desmethylretinol are isomerized to their corresponding 11-cis-alcohols, although with lower efficiencies than isomerization of all-trans-retinol to 11-cis-retinol. These findings make the mechanism of isomerization through the C(19) methyl group unlikely, because in the case of 9-desmethylretinol, the isomerization would have to progress by proton abstraction from electron-rich olefinic C(9). The differences between all-trans-retinol, all-trans-9-desmethylretinol, and all-trans-13-desmethylretinol appear to be a consequence of the enzymatic properties, and binding affinities of the isomerization system, rather than differences in the chemical or thermodynamic properties of these compounds. This observation is also supported by quantum chemical calculations. It appears that both methyl groups are not essential for the isomerization reaction and are not likely involved in formation of a transition stage during the isomerization process.

Identification of RPE65 in transformed kidney cells

The protein RPE65 has an important role in retinoid processing and/or retinoid transport in the eye. Retinoids are involved in cell differentiation, embryogenesis and carcinogenesis. Since the kidney is known as an important site for retinoid metabolism, the expression of RPE65 in normal kidney and transformed kidney cells has been examined. The RPE65 mRNA was detected in transformed kidney cell lines including the human embryonic kidney cell line HEK293 and the African green monkey kidney cell lines COS-1 and COS-7 by reverse transcription PCR. In contrast, it was not detected in human primary kidney cells or monkey kidney tissues under the same PCR conditions. The RPE65 protein was also identified in COS-7 and HEK293 cells by Western blot analysis using a monoclonal antibody to RPE65, but not in the primary kidney cells or kidney tissues. The RPE65 cDNA containing the full-length encoding region was amplified from HEK293 and COS-7 cells. DNA sequencing showed that the RPE65 cDNA from HEK293 cells is identical to the RPE65 cDNA from the human retinal pigment epithelium. The RPE65 from COS-7 cells shares 98 and 99% sequence identity with human RPE65 at the nucleotide and amino acid levels, respectively. Moreover, the RPE65 mRNA was detected in three out of four renal tumor cultures analyzed including congenital mesoblastic nephroma and clear cell sarcoma of the kidney. These results demonstrated that transformed kidney cells express this retinoid processing protein, suggesting that these transformed cells may have an alternative retinoid metabolism not present in normal kidney cells.

Categorizing reactivity of bacteriorhodopsin cysteine mutants crosslinking to 4-bromoretinal

The structure of bacteriorhodopsin (bR) has been probed by a large number of experimental methods. In earlier work distance constraints measured from the 1BRD Brookhaven structure (1, 2) were used to guide site-directed mutagenesis/affinity labeling experiments (3-5). In the present study we report on the use of limited molecular dynamics (MD) investigations of the same bR/affinity label system. We show here that the chiral center introduced when 4-bromo-all-trans retinal is synthesized produces variable impact on potential crosslinking. Our MD analysis suggests the following ranking of binding site mutants in order of reactivity: R118C > S118C >> S121C > R141C >> S141C >>> R121C, R138C, S138C. Chirality appears to have limited effect for the M118C mutants but shows more dramatic impact for the T121C and S141C mutants. These results are in excellent agreement with the experimental observations and offer encouragement that MD can be a useful component of experimental design with considerable predictive power.

Occupancy of the chromophore binding site of opsin activates visual transduction in rod photoreceptors

The retinal analogue beta-ionone was used to investigate possible physiological effects of the noncovalent interaction between rod opsin and its chromophore 11-cis retinal. Isolated salamander rod photoreceptors were exposed to bright light that bleached a significant fraction of their pigment, were allowed to recover to a steady state, and then were exposed to beta-ionone. Our experiments show that in bleach-adapted rods beta-ionone causes a decrease in light sensitivity and dark current and an acceleration of the dim flash photoresponse and the rate constants of guanylyl cyclase and cGMP phosphodiesterase. Together, these observations indicate that in bleach-adapted rods beta-ionone activates phototransduction in the dark. Control experiments showed no effect of beta-ionone in either fully dark-adapted or background light-adapted cells, indicating direct interaction of beta-ionone with the free opsin produced by bleaching. We speculate that beta-ionone binds specifically in the chromophore pocket of opsin to produce a complex that is more catalytically potent than free opsin alone. We hypothesize that a similar reaction may occur in the intact retina during pigment regeneration. We propose a model of rod pigment regeneration in which binding of 11-cis retinal to opsin leads to activation of the complex accompanied by a decrease in light sensitivity. The subsequent covalent attachment of retinal to opsin completely inactivates opsin and leads to the recovery of sensitivity. Our findings resolve the conflict between biochemical and physiological data concerning the effect of the occupancy of the chromophore binding site on the catalytic potency of opsin. We show that binding of beta-ionone to rod opsin produces effects opposite to its previously described effects on cone opsin. We propose that this distinction is due to a fundamental difference in the interaction of rod and cone opsins with retinal, which may have implications for the different physiology of the two types of photoreceptors.

The proton release group of bacteriorhodopsin controls the rate of the final step of its photocycle at low pH

The factors determining the pH dependence of the formation and decay of the O photointermediate of the bacteriorhodopsin (bR) photocycle were investigated in the wild-type (WT) pigment and in the mutants of Glu-194 and Glu-204, key residues of the proton release group (PRG) in bR. We have found that in the WT the rate constant of O --> bR transition decreases 30-fold upon decreasing the pH from 6 to 3 with a pKa of about 4.3. D2O slows the rise and decay of the O intermediate in the WT at pH 3.5 by a factor of 5.5. We suggest that the rate of the O --> bR transition (which reflects the rate of deprotonation of the primary proton acceptor Asp-85) at low pH is controlled by the deprotonation of the PRG. To test this hypothesis, we studied the E194D mutant. We show that the pKa of the PRG in the ground state of the E194D mutant, when Asp-85 is protonated, is increased by 1.2 pK units compared to that of the WT. We found a similar increase in the pKa of the rate constant of the O --> bR transition in E194D. This provides further evidence that the rate of the O --> bR transition is controlled by the PRG. In a further test, the E194Q mutation, which disables the PRG and slows proton release, almost completely eliminates the pH dependence of O decay at pHs below 6. A second phenomenon we investigated was that in the WT at neutral and alkaline pH the fraction of the O intermediate decreases with pKa 7.5. A similar pH dependence is observed in the mutants in which the PRG is disabled, E194Q and E204Q, suggesting that the decrease in the fraction of the O intermediate with pKa ca. 7.5 is not controlled by the PRG. We propose that the group with pKa 7.5 is Asp-96. The slowing of the reprotonation of Asp-96 at high pH is the cause of the decrease in the rate of the N --> O transition, leading to the decrease in the fraction of O.

Rpe65 is necessary for production of 11-cis-vitamin A in the retinal visual cycle

Mutation of RPE65 can cause severe blindness from birth or early childhood, and RPE65 protein is associated with retinal pigment epithelium (RPE) vitamin A metabolism. Here, we show that Rpe65-deficient mice exhibit changes in retinal physiology and biochemistry. Outer segment discs of rod photoreceptors in Rpe65-/- mice are disorganized compared with those of Rpe65+/+ and Rpe65+/- mice. Rod function, as measured by electroretinography, is abolished in Rpe65-/- mice, although cone function remains. Rpe65-/- mice lack rhodopsin, but not opsin apoprotein. Furthermore, all-trans-retinyl esters over-accumulate in the RPE of Rpe65-/- mice, whereas 11-cis-retinyl esters are absent. Disruption of the RPE-based metabolism of all-trans-retinyl esters to 11-cis-retinal thus appears to underlie the Rpe65-/- phenotype, although cone pigment regeneration may be dependent on a separate pathway.

Cloning and localization of RPE65 mRNA in salamander cone photoreceptor cells1

RPE65 is a potential retinoid-processing protein expressed in the retinal pigment epithelium. Mutations in the RPE65 gene have been shown to cause certain inherited retinal dystrophies. Previous studies have shown that salamander cone photoreceptor cells have a unique retinoid processing mechanism which is distinct from that of rods. To determine whether RPE65 is expressed in photoreceptors, the RPE65 cDNA was cloned from a salamander retinal cDNA library. The deduced protein consists of 533 amino acids and is 85% identical to human and bovine RPE65. The RPE65 mRNA was detected in all of the single cone cells isolated from the salamander retina, as well as in the retinal pigment epithelium by RT-PCR, but not in the isolated rods. The RT-PCR products have been confirmed to be RPE65 by DNA sequencing. The results indicate that this potential retinoid processing protein is expressed in the cone photoreceptor cells but not in rods. Therefore, this protein may contribute to the unique retinoid processing capabilities in salamander cones.

Identification of tryptophan oxidation products in bovine alpha-crystallin

Oxidation is known to affect the structure, activity, and rate of degradation of proteins, and is believed to contribute to a variety of pathological conditions. Metal-catalyzed oxidation (MCO) is a primary oxidizing system in many cell types. In this study, the oxidative effects of a MCO system (the Fenton reaction) on the structure of the tryptophan residues of alpha-crystallin were determined. Tandem mass spectrometry (MS/MS) was utilized to identify specific tryptophan and methionine oxidation products in the bovine alpha-crystallin sequence. After oxidative exposure, alpha-crystallin was digested with trypsin, and the resulting peptides were fractionated by reverse-phase HPLC. Structural analysis by mass spectrometry revealed that tryptophan 9 of alphaA- and tryptophan 60 of alphaB-crystallin were each converted into hydroxytryptophans (HTRP), N-formylkynurenine (NFK), and kynurenine (KYN). However, only HTRP and KYN formation were detected at residue 9 of alphaB-crystallin. Oxidation of methionine 1 of alphaA- and methionine 1 and 68 of alphaB-crystallin was also detected. The products NFK and KYN are of particular importance in the lens, as they themselves are photosensitizers that can generate reactive oxygen species (ROS) upon UV light absorption. The unambiguous identification of HTRP, NFK, and KYN in intact alpha-crystallin represents the first structural proof of the formation of these products in an intact protein, and provides a basis for detailed structural analysis of oxidized proteins generated in numerous pathological conditions.

Molecular cloning of the salamander red and blue cone visual pigments

PURPOSE

Salamander retinas are known to contain at least three cone pigments and two rod pigments. The purpose of this study was to clone and characterize the visual pigments from salamander cones.

METHODS

cDNA fragments of cone pigments were amplified from a salamander retina cDNA library by PCR using a pair of primers with consensus for visual pigments. These fragments were cloned and used as probes for library-screening. The full-length cDNAs were isolated from the retinal library using the cloned PCR products as probes. DNA sequences were determined by the dideoxynucleotide chain termination method.

RESULTS

Two pigment cDNAs were cloned and sequenced from the salamander library. The global GenBank search showed that they do not match any existing sequences but have significant sequence similarity to visual pigments. One of the pigment cDNAs showed a high sequence homology with red cone pigments from other species and thus, was designated as a red cone opsin. The other pigment was designated as a blue cone opsin as it is most homologous to the chicken and goldfish blue cone pigments. Both cDNAs contain a full-length coding region encoding 365 amino acids in the red and 363 amino acids in the blue cone pigment. Hydropathy analysis predicted that both pigments could form seven hydrophobic transmembrane helices. Both pigments retain the key amino acid residues critical for maintaining the structure and function of opsins and have similar G-protein interaction sequences which differ from that of rod opsin. Phylogenetic analysis indicates that the red opsin belongs to the L group and the blue opsin belongs to the M1 group of visual pigments.

CONCLUSIONS

The salamander red and blue cone pigments share high sequence homology with the cone pigments of other species.

Radiolysis-induced oxidation of bovine alpha-crystallin

Radiolysis of water by ionizing radiation results in the production of pure hydroxyl radicals. This technique, combined with analysis by tandem mass spectrometry (MS/MS), has been used to study the effect of hydroxyl radicals on the intact bovine alpha-crystallin protein. After exposure to gamma-irradiation, the oxidized alpha-crystallin was digested with trypsin and the resulting peptides were fractionated by reverse-phase HPLC. The isolated fractions were analyzed by matrix-assisted laser desorption ionization and by MS/MS to determine the locations and identities of the modifications. Structural analysis revealed that methionine 1 of alpha A- and alpha B-crystallin and methionine 68 of alpha B-crystallin were oxidized to methionine sulfoxide. Hydroxytryptophan was formed from each tryptophan residue in alpha-crystallin, although only tryptophan 9 of alpha A-crystallin was converted into N-formylkynurenine. This study has, for the first time, identified the sites of modification and the structures produced in the intact alpha-crystallin protein by exposure to hydroxyl radicals. By determining the consequences of in vitro exposure of alpha-crystallin to pure hydroxyl radicals, the in vivo contribution of this reactive oxygen species to the overall oxidative stress of the lens will be achieved from the identification of the modifications to alpha-crystallin purified from intact human lenses.

Mass spectrometric analysis of integral membrane proteins: application to complete mapping of bacteriorhodopsins and rhodopsin

Integral membrane proteins have not been readily amenable to the general methods developed for mass spectrometric (or internal Edman degradation) analysis of soluble proteins. We present here a sample preparation method and high performance liquid chromatography (HPLC) separation system which permits online HPLC-electrospray ionization mass spectrometry (ESI-MS) and -tandem mass spectrometry (MS/MS) analysis of cyanogen bromide cleavage fragments of integral membrane proteins. This method has been applied to wild type (WT) bacteriorhodopsin (bR), cysteine containing mutants of bR, and the prototypical G-protein coupled receptor, rhodopsin (Rh). In the described method, the protein is reduced and the cysteine residues pyridylethylated prior to separating the protein from the membrane. Following delipidation, the pyridylethylated protein is cleaved with cyanogen bromide. The cleavage fragments are separated by reversed phase HPLC using an isopropanol/acetonitrile/aqueous TFA solvent system and the effluent peptides analyzed online with a Finnigan LCQ Ion Trap Mass Spectrometer. With the exception of single amino acid fragments and the glycosylated fragment of Rh, which is observable by matrix assisted laser desorption ionization (MALDI)-MS, this system permits analysis of the entire protein in a single HPLC run. This methodology will enable pursuit of chemical modification and crosslinking studies designed to probe the three dimensional structures and functional conformational changes in these proteins. The approach should also be generally applicable to analysis of other integral membrane proteins.

Mutation of arginine 134 to lysine alters the pK(a)s of key groups involved in proton pumping by bacteriorhodopsin

Arginine 134 is located near the extracellular surface of bacteriorhodopsin (bR) and may interact with one or more nearby glutamate residues. In the bR mutant R134K, light-induced Schiff-base deprotonation (formation of the M intermediate) exhibits several kinetic components and has a complex pH dependence. The kinetics and pH dependence of M formation were analyzed using the following general guidelines for interpreting M formation: (1) The fastest component of M formation reflects the redistribution of the Schiff-base proton to D85, the usual proton acceptor, in response to the change in the proton affinities of the Schiff base and D85 early in the photocycle; (2) Two additional components of M formation reflect transitions between spectroscopically similar substates of M. By applying these guidelines, supplemented by information about the pK(a)s of D85 and the proton release group from acid (purple-to-blue) and alkaline titrations of the absorption spectra of the unphotolyzed R134K pigment, we explain the pH dependence of M formation as being due to titration of the counterion, D85, and of the proton release group. We calculate, in R134K, that the pKa of D85 is 4.6 in the unphotolyzed state, while the pKa of the proton release group is 8.0 in the unphotolyzed state but drops to approximately 5.8 in the M intermediate. The same value for the pKa of the proton release group in the M intermediate is obtained when we use photocurrent measurements to monitor proton release. The altered values of these pK(a)s relative to the corresponding values in wild-type bR suggest that D85 and the proton release group are coupled more weakly in R134K than in the wild type.

Identification of photooxidation sites in bovine alpha-crystallin

Because UV irradiation of proteins can produce reactive oxygen species and exposure to UV light has been implicated in cataractogenesis, the sites of photooxidation of bovine alpha-crystallin, a major lens protein with molecular chaperone activity, were identified using tandem mass spectrometry (MS/MS). Bovine alpha-crystallin was irradiated with UV light (> 293 nm) for 1, 4 and 8 h, digested with trypsin and analyzed by matrix-assisted laser desorption ionization, time-of-flight mass spectrometry (MALDI) to identify the oxidized sequences. Tryptic peptides were purified by reverse-phase HPLC and oxidized peptides were sequenced by MS/MS to determine the sites of oxidation. Tryptophan fluorescence decreased exponentially with increasing time of UV exposure and peptides containing residues 1-11 of alpha A-crystallin and 1-11, 12-22 and 57-69 of alpha B-crystallin were determined to be oxidized by shifts of 16 D or multiples of 16 Da above the mass of the unmodified peptide. The MALDI analysis revealed single oxidation of all four sequences, which increased with increasing time of UV exposure and possible double oxidation of alpha B 12-22. The specific sites of photooxidation indicate that the N-terminal regions of alpha A- and alpha B-crystallin are exposed to an aqueous environment and are in the vicinity of tryptophan residues from neighboring subunits.

Complete map and identification of the phosphorylation site of bovine lens major intrinsic protein

PURPOSE

To determine the complete primary structure, including posttranslational modifications, of bovine lens major intrinsic protein (MIP) using a recently developed combination of liquid chromatography and mass spectrometry.

METHODS

The MIP was isolated from bovine lenses by sucrose gradient centrifugation and was cleaved with cyanogen bromide (CNBr). A high-performance liquid chromatographic system, developed for hydrophobic protein analysis, was used to separate the cleavage fragments. Matrix-assisted laser desorption ionization and electrospray tandem mass spectrometry were employed to obtain molecular weight and sequence data from bovine MIP CNBr fragments, directly or after subsequent digestion with trypsin.

RESULTS

The complete sequence of bovine MIP was mapped by molecular weight measurements of CNBr fragments, confirming the reported DNA sequence. The C-terminal peptide (177 to 263) was fully sequenced and the major site of phosphorylation was determined to be at serine 235 rather than at the previously reported serine 243. The level of phosphorylation in the native protein was determined to be 25%. No other posttranslational derivatizations were observed with the exception of the previously detected deamidation of asparagine 246.

CONCLUSIONS

These results represent the first complete MIP sequence map at the amino acid level and identify the single major phosphorylation site at serine 235.

Glutamate-194 to cysteine mutation inhibits fast light-induced proton release in bacteriorhodopsin

Substitution of glutamic acid-194, a residue on the extracellular surface of bacteriorhodopsin, with a cysteine inhibits the fast light-induced proton release that normally is coupled with the deprotonation of the Schiff base during the L to M transition. Proton release in this mutant occurs at the very end of the photocycle and coincides with deprotonation of the primary proton acceptor, Asp-85, during the O to bR transition. the E194C mutation also results in a slowing down of the photocycle by about 1 order of magnitude as compared to the wild type and produces a strong effect on the pH dependence of dark adaptation that is interpreted as a drastic reduction or elimination of the coupling between the primary proton acceptor Asp-85 and the proton release group. These data indicate that Glu-194 is a critical component of the proton release complex in bacteriorhodopsin.

Charge movements in the 13-cis photocycles of the bacteriorhodopsin mutants R82K and R82Q

We have examined light-induced currents in oriented membranes of the bacteriorhodopsin mutants R82K and R82Q. Our results suggest that two photocurrent components found in R82K, with 30 and 300 microseconds lifetimes, are due to the photocycle of the 13-cis rather than the all-trans form of the pigment. We investigated the pH dependence of these components and their correspondence to absorbance changes at 660 nm characteristic of photointermediates of the 13-cis cycle. The presence of a D2O effect suggests that the charge motions producing these photocurrents are related to proton or protonated amino acid movement within the molecule. The current amplitudes depend on the protonation states of at least two residues, D85 and (probably) E204. In R82Q, a 10 microseconds photocurrent is observed that also depends on the protonation state of D85 and is similar to the 30 microseconds current in R82K. We attempt to explain these currents in terms of a model for interacting residues in the extracellular half of the bacteriorhodopsin channel.

Proton uptake and release are rate-limiting steps in the photocycle of the bacteriorhodopsin mutant E204Q

In the absence of the putative proton release group, E204, the second half of the photocycle of the E204Q mutant of bacteriorhodopsin is slowed down more than 10-fold compared to the wild type. The effects of pH and D2O on the M decay and O formation rates in E204Q suggest that proton uptake occurs concurrently with the N <--> O transition, possibly coupled with the thermal reisomerization of the retinal. Hence, one of the rate-limiting steps in the slow E204Q photocycle is proton uptake from the outside medium, coincident with the decay of the slow component of M (the N <--> O transition). The second rate-limiting step is the long lifetime of decay of the O state, due to a high activation barrier for the deprotonation of D85 in the O --> bR step of the E204Q photocycle. Addition of the weakly acidic anions azide, cyanate, or formate accelerates the decay of the O intermediate, and restores the total photocycling time to that observed in the wild-type pigment, by accelerating the deprotonation of D85. We also find that azide similarly accelerates the decay of O in the wild type under conditions in which E204 does not deprotonate during the photocycle (pH < 6). It has previously been shown that azide and other weak acids can influence proton transfers in the cytoplasmic half of the protein [Tittor, J., Soell, C., Oesterhelt, D., Butt, H.-J., & Bamberg, E. (1989) EMBO J. 8, 3477-3482]; we suggest that these weak acids can affect proton transfers in the extracellular half of the protein as well.

Mutation of a surface residue, lysine-129, reverses the order of proton release and uptake in bacteriorhodopsin; guanidine hydrochloride restores it

K129 is a residue located in the extracellular loop connecting transmembrane helices D and E of bacteriorhodopsin. Replacement of K129 with a histidine alters the pKa's of two key residues in the proton transport pathway, D85, and the proton release group (probably E204); the resulting pigment has properties that differ markedly from the wild type. 1) In the unphotolyzed state of the K129H mutant, the pKa of D85 is 5.1 +/- 0.1 in 150 mM KCl (compared to approximately 2.6 in the wild-type bacteriorhodopsin), whereas the unphotolyzed-state pKa of E204 decreases to 8.1 +/- 0.1 (from approximately 9.5 in the wild-type pigment). 2) The pKa of E204 in the M state is 7.0 +/- 0.1 in K129H, compared to approximately 5.8 in the wild-type pigment. 3) As a result of the change in the pKa of E204 in M, the order of light-induced proton release and uptake exhibits a dependence on pH in K129H differing from that of the wild type: at neutral pH and moderate salt concentrations (150 mM KCl), light-induced proton uptake precedes proton release, whereas it follows proton release at higher pH. This pumping behavior is similar to that seen in a related bacterial rhodopsin, archaerhodopsin-1, which has a histidine in the position analogous to K129. 4) At alkaline pH, a substantial fraction of all-trans K129H pigment (approximately 30%) undergoes a conversion into a shorter wavelength species, P480, with pKa approximately 8.1, close to the pKa of E204. 5) Guanidine hydrochloride lowers the pKa's of D85 and E204 in the ground state and the pKa of E204 in the M intermediate, and restores the normal order of proton release before uptake at neutral pH. 6) In the K129H mutant the coupling between D85 and E204 is weaker than in wild-type bacteriorhodopsin. In the unphotolyzed pigment, the change in the pKa's of either residue when the other changes its protonation state is only 1.5 units compared to 4.9 units in wild-type bacteriorhodopsin. In the M state of photolyzed K129H pigment, the corresponding change is 1 unit, compared to 3.7 units in the wild-type pigment. We suggest that K129 may be involved in stabilizing the hydrogen bonding network that couples E204 and D85. Substitution of K129 with a histidine residue causes structural changes that alter this coupling and affect the pKa's of E204 and D85.

Formation of free radicals and protein mixed disulfides in rat red cells exposed to dapsone hydroxylamine

The hemolytic activity of dapsone is well known to reside in its N-hydroxylamine metabolites. Addition of dapsone hydroxylamine (DDS-NOH) to red cell suspensions causes damage such that when reintroduced into the circulation of isologous rats, the injured cells are rapidly removed by the spleen. Hemolytic activity is associated with the extensive formation of disulfide-linked hemoglobin adducts on red cell membrane skeletal proteins. To determine if free radicals could be involved in this process, rat red cells were incubated with DDS-NOH in the presence of the spin trap, 5,5'-dimethyl-1-pyrroline-N-oxide (DMPO) and subjected to EPR analysis. Addition of DDS-NOH (25-50 microM) to a red cell suspension gave rise to a four-line (1:2:2:1) EPR spectrum with coupling constants identical to those of a DMPO-hydroxyl radical adduct (DMPO-OH) standard. No other radicals were detected; however, preincubation of red cells with cysteamine caused the DDS-NOH-generated DMPO-OH signal to be replaced by a cysteamine thiyl radical adduct signal. DDS-NOH-treated red cells were also found to contain ferrylhemoglobin, indicating the presence of hydrogen peroxide. Furthermore, DDS-NOH was found to stimulate salicylate hydroxylation in red cell suspensions, confirming the presence of oxygen radicals. These data support the hypothesis that oxygen radicals are involved in the mechanism underlying dapsone-induced hemolytic anemia.

Kallistatin in human ocular tissues: reduced levels in vitreous fluids from patients with diabetic retinopathy

PURPOSE

Kallistatin is a serine proteinase inhibitor, which binds to tissue kallikrein and inhibits its proteolytic activity. This study is to determine the expression, cellular localization and the potential function of kallistatin in the eye.

METHODS

Tissue kallikrein-kallistatin complex formation was performed to detect the kallikrein-binding activity in ocular tissues. Immunoreactive kallistatin was detected and quantified by an enzyme-linked immunosorbent assay using polyclonal antibody specific to human kallistatin. In situ hybridization histochemistry was employed to localize the kallistatin mRNA in human eyes using an antisense riboprobe of kallistatin.

RESULTS

We have identified active kallistatin in the cornea, ciliary body, sclera, choroid, optic nerve, retina, vitreous and aqeous fluids. Kallistatin binds to tissue kallikrein and forms an SDS-stable complex. Immunoreactive kallistatin was identified in these tissues. Linear dose-dependent curves of the tissue extracts of the retina and choroid are parallel to that of purified human kallistatin, suggesting their immunological identity. The kallistatin mRNA was identified in the ciliary muscle, lens epithelial cells, all the layers of retina cells, optic nerve, choroid and vascular endothelial cells. These cells were not stained by the sense riboprobe under the same conditions, indicating the specificity of the hybridization. We also compared immunoreactive kallistatin levels in vitreous fluids from 18 patients with diabetic retinopathy and 17 non-diabetic subjects. The results show that diabetic subjects have significantly lower kallistatin levels (233.0 +/- 14.6 ng/mg protein) compared to non-diabetic subjects (334.1 +/- 26.9 ng/mg protein).

CONCLUSIONS

Kallistatin is produced endogenously in the eye and the decrease in the vitreous kallistatin levels may be involved in diabetic retinopathy.

Mechanisms of opsin activation

Rhodopsin is constrained in an inactive conformation by interactions with 11-cis-retinal including formation of a protonated Schiff base with Lys296. Upon photoisomerization, major structural rearrangements that involve protonation of the active site Glu113 and cytoplasmic acidic residues, including Glu134, lead to the formation of the active form of the receptor, metarhodopsin II b, which decays to opsin. However, an activated receptor may be generated without illumination by addition of all-trans-retinal or its analogues to opsin, as measured in this study by the increased phosphorylation of opsin by rhodopsin kinase. The potency of stimulation depended on the chemical and isomeric nature of the analogues and the length of the polyene chain with all-trans-C17 aldehyde and all-trans-retinal being the most active and trans-C12 aldehyde being the least active. Certain cis-isomers, 11-cis-13-demethyl-retinal and 9-cis-C17 aldehyde, were also active. Most of the retinal analogues tested did not regenerate a spectrally identifiable pigment, and many were incapable of Schiff base formation (ketone, stable oximes, and Schiff base-derivatives of retinal). Thus, receptor activation resulted from formation of non-covalent complexes with opsin. pH titrations suggested that an equilibrium exists between partially active (protonated) and inactive (deprotonated) forms of opsin. These findings are consistent with a model in which protonation of one or more cytoplasmic carboxyl groups of opsin is essential for activity. Upon addition of retinoids, the partially active conformation of opsin is converted to a more active intermediate similar to metarhodopsin II b. The model provides an understanding of the structural requirements for opsin activation and an interpretation of the observed activities of natural and experimental opsin mutants.

Arginine-82 regulates the pKa of the group responsible for the light-driven proton release in bacteriorhodopsin

In wild-type bacteriorhodopsin light-induced proton release occurs before uptake at neutral pH. In contrast, in mutants in which R82 is replaced by a neutral residue (as in R82A and R82Q), only a small fraction of the protons is released before proton uptake at neutral pH; the major fraction is released after uptake. In R82Q the relative amounts of the two types of proton release, "early" (preceding proton uptake) and "late" (following proton uptake), are pH dependent. The main conclusions are that 1) R82 is not the normal light-driven proton release group; early proton release can be observed in the R82Q mutant at higher pH values, suggesting that the proton release group has not been eliminated. 2) R82 affects the pKa of the proton release group both in the unphotolyzed state of the pigment and during the photocycle. In the wild type (in 150 mM salt) the pKa of this group decreases from approximately 9.5 in the unphotolyzed pigment to approximately 5.8 in the M intermediate, leading to early proton release at neutral pH. In the R82 mutants the respective values of pKa of the proton release group in the unphotolyzed pigment and in M are approximately 8 and 7.5 in R82Q (in 1 M salt) and approximately 8 and 6.5 in R82K (in 150 mM KCl). Thus in R82Q the pKa of the proton release group does not decrease enough in the photocycle to allow early proton release from this group at neutral pH. 3) Early proton release in R82Q can be detected as a photocurrent signal that is kinetically distinct from those photocurrents that are due to proton movements from the Schiff base to D85 during M formation and from D96 to the Schiff base during the M-->N transition. 4) In R82Q, at neutral pH, proton uptake from the medium occurs during the formation of O. The proton is released during the O-->bacteriorhodopsin transition, probably from D85 because the normal proton release group cannot deprotonate at this pH. 5) The time constant of early proton release is increased from 85 microseconds in the wild type to 1 ms in R82Q (in 150 mM salt). This can be directly attributed to the increase in the pKa of the proton release group and also explains the uncoupling of proton release from M formation. 6) In the E204Q mutant only late proton release is observed at both neutral and alkaline pH, consistent with the idea that E204 is the proton release group. The proton release is concurrent with the O-->bacteriorhodopsin transition, as in R82Q at neutral pH.

Molecular cloning of a rhodopsin gene from salamander rods

PURPOSE

Salamander photoreceptor cells have been used widely as models in vision research. However, the salamander opsin genes had not been cloned. The purpose of this study was to clone a salamander rhodopsin and to determine its primary structure and cell type-specific expression.

METHODS

Using salamander retina RNA as a template and Xenopus rhodopsin-specific oligonucleotides as primers, reverse transcription and polymerase chain reaction (RT-PCR) were used to amplify and clone a rhodopsin cDNA fragment. This fragment was used as a probe to isolate a full-length cDNA of the rhodopsin from a cDNA library of salamander retina. The dideoxynucleotide chain termination method was used to determine the nucleotide sequence. Single rod and cone cells were isolated by micromanipulation, and the absorbance spectra of the rod outer segments were measured with a photon-counting microspectrophotometer. Individual rod and cone cells were lysed for RT-PCR and Southern blot analysis to detect cell-specific expression of this gene.

RESULTS

A 1.2 kb rhodopsin cDNA containing the full-length coding region of rhodopsin has been cloned and sequenced from the larval tiger salamander, Ambystoma tigrinum. This cDNA encodes 354 amino acids that, by hydropathy profile, could form seven transmembrane domains characteristic of other rhodopsins. Sequence identity was found with other amphibian rhodopsins at the nucleic acid (82% to 83%) and the amino acid (88% to 89%) levels. Key amino acids critical for structure and function of rhodopsin have been retained. The mRNA of this rhodopsin was identified in red rod cells (lambda max 506 nm). No expression of the gene was detected in cone cells.

CONCLUSIONS

The cloned rhodopsin is a newly isolated member of the G protein-coupled receptor superfamily. This protein is expressed in rods but not in cones.

Expression and cellular localization of the kallikrein-kinin system in human ocular tissues

Tissue kallikrein is a serine proteinase which processes kininogens to release bioactive kinins. Kinins mediate a variety of biological processes through the interaction with kinin receptors. Kinins are involved in the regulation of blood pressure and local blood flow, vasodilation, smooth muscle contraction and relaxation, production of pain and inflammation, and stimulation of cell proliferation. The tissue kallikrein-kinin system has been implicated in a number of pathophysiological processes such as hypertension, allergy and diabetes mellitus. In the present study, we have identified the expression and localization of components of the kallikrein-kinin system in the human eye by reverse transcription-polymerase chain reaction (RT-PCR) and Southern blot analyses, and in situ hybridization histochemistry. RT-PCR and Southern blot analyses have detected mRNAs of the key components of the system including tissue kallikrein, low molecular weight kininogen, and bradykinin B1 and B2 receptors at high levels in human retina, choroid and ciliary body, and relatively low levels in the optic nerve. In situ hybridization has identified cellular localization of these four mRNAs in ocular tissues. They are expressed in retinal neuronal cells including the outer nuclear layer, inner nuclear layer and ganglion cell layer. These mRNAs were also identified in endothelial cells of ocular blood vessels, ciliary muscle and lens epithelial cells. The sense riboprobes showed negative staining, which indicates the specificity of the antisense riboprobes. These results suggest that the tissue kallikrein-kinin system is produced endogenously in human ocular tissues. Similar expression patterns of kallikrein, kininogen and kinin receptors indicate that the kallikrein-kinin system may function in an autocrine or paracrine fashion in the eye.