Arrestin of bovine photoreceptors reveals strong ATP binding

Nucleotides in ocular secretions: their role in ocular physiology

The eye is the sense organ that permits the detection of light owing to the existence of a sophisticated neuronal array, called the retina, which is responsive to photons. The correct functioning of this complex system requires the coordination of several intraocular structures that ultimately permit the perfect focusing of images on the neural retina. Light has to pass through different media: the tear, the cornea, aqueous humour, lens, and vitreous humour before it reaches the retina. Moreover, the composition and structure of some of these media can change due to several physiological mechanisms. Nucleotides are active components of the humours bathing relevant ocular structures. The tear contains nucleotides and dinucleotides that control the process of tearing, wound healing and protects of superficial infections. In the inner eye, the aqueous humour also presents a collection of mono and dinucleotides that affect pupil contraction, aqueous humour production and accommodation. Behind the lens and between this structure and the retina the vitreous humour can modify the physiology of the retinal cells, mostly the ganglion cells. By investigating the actions of nucleotides and dinucleotide present in the ocular humours we will be able not only to understand the functioning of the ocular structures but also to develop new pharmacological therapies for pathologies such as dry eye, glaucoma or retinal detachment.

Brain-type creatine kinase in photoreceptor cell outer segments: role of a phosphocreatine circuit in outer segment energy metabolism and phototransduction

Different isoforms of creatine kinase, an important enzyme of vertebrate energy metabolism, were localized in bovine photoreceptor cells, with particular emphasis on the identification and quantification of the brain-type isoform within the outer segment compartment. Using immunofluorescence and immunoelectron microscopy, brain-type creatine kinase was shown to be present in bovine photoreceptor cell outer and inner segments. The presence of this isoenzyme in rod outer segments was additionally confirmed by immunoblotting and immunolabeling of isolated rod outer segments. The content of creatine kinase in rod outer segments was quantified by measuring creatine kinase activity after membrane disruption with detergent. The ATP regeneration potential provided by the creatine kinase in isolated, washed bovine rod outer segments was 1.2 +/- (0.4) i.u. mg-1 rhodopsin. This value was calculated to be at least an order of magnitude larger than that necessary to replenish the energy required for cGMP resynthesis in rod outer segments, and high enough to regenerate the entire ATP pool of rod outer segments within the time span of a photic cycle. A mitochondrial creatine kinase isoenzyme was located within the ellipsoid portions of bovine rod and cone inner segments by immunofluorescence microscopy and, using immunogold staining, was specifically localized in the mitochondria clustered within bovine rod and cone inner segments. These results suggest that vertebrate photoreceptor cells contain a functional phosphocreatine circuit. Outer segment creatine kinase may play an important role in phototransduction by providing energy for the visual cycle, maintaining high local ATP/ADP ratios and consuming protons produced by enzymes located in the outer segment.

Arrestin-subtypes in insect antennae

Arrestin is supposed to be involved in uncoupling receptor-mediated second messenger cascades. Clones encoding proteins homologous to arrestin have been isolated from antennal cDNA libraries of Locusta migratoria and Heliothis virescens. Based on the size and several characteristic motifs, the two proteins are considered as members of different arrestin subfamilies. One of the subtypes, which has also been found in Drosophila, lacks the regulatory acidic C-terminal. The putative site of interaction with phosphorylated receptors, a cationic region in the primary structure, is conserved in all identified arrestins from locust to human.

Study of the structure of arrestin (S-antigen) from bovine photoreceptors by FTIR spectroscopy

Fourier transform-infrared spectroscopy has been used for the study of the secondary structure of arrestin from bovine retina rod cells. Spectra have been obtained in H2O and in D2O media. Resolution enhancement of the amide I secondary structure-sensitive overlapped component bands has been achieved by means of Fourier self-deconvolution and Fourier derivation. In order to obtain a quantitative estimation of the proportion of amino acid residues involved in each type of secondary structure, bands at the resolved frequencies have been curve-fitted to the deconvolved amide I contour by means of a least-squares best-fitting iterative program. The analysis of the results suggests that the secondary structure of arrestin comprises 56-63% of extended strands, 12-19% of turns and bends, 15% of alpha-helices and 10% of undefined and irregular segments.

Structural properties of arrestin studied by chemical modification and circular dichroism

A unique conformation of arrestin is crucial for its interaction with phosphorylated photolyzed rhodopsin. Conformational changes in arrestin were investigated using chemical modification and circular dichroism. We studied the kinetics of sulfhydryl modification of bovine arrestin in order to determine whether its conformation is altered by the presence of ligands or salts at different ionic strengths. We found that all three cysteines (stoichiometry was 2.7 +/- 0.06 3-carboxy-4-nitrophenyl sulfide (NbS)/arrestin) are accessible for modification by NbS2. Under pseudo-first-order conditions (30-100-fold excess of NbS2 over arrestin), the modifications of the 3 cysteines are indistinguishable. At higher concentrations of NbS2 (150-300-fold excess), the pseudo-first-order plot is not linear, and the reaction can be resolved into two processes that involve two classes of sulfhydryl groups. Addition of CaCl2, MgCl2, inorganic phosphate, MgATP, or MgGTP had little effect on the rate of modification of the cysteine residues; however, heparin and inositol hexakisphosphate, which have been shown to induce conformational changes in arrestin, block modification of one sulfhydryl group of arrestin and accelerate the modification of the remaining two. Analysis of CD spectra revealed that arrestin has virtually no alpha-helical structure, about 40% beta-structure, about 18% beta-turns, and about 40% other structure. The CD spectrum for arrestin did not change in the presence of heparin. These studies suggest that arrestin exists in equilibrium between two or more conformational states. However, it is proposed that conversion between these conformations occur without altering significantly the secondary structure of arrestin.

Evidence for ATP-ase activity of arrestin from bovine photoreceptors

In vertebrate photoreceptors the soluble protein arrestin (45 kDa) is involved in controlling the light dependent activity of receptor proteins such as transducin or the cGMP-phosphodiesterase. Arrestin has further been identified as the retinal-S-antigen which is assumed to cause the autoimmune disease uveitis. In a first communication a binding of the nucleotide ATP to arrestin was described. In this subsequent study it is shown that arrestin is also able to hydrolyse ATP at a rate of (5.1 +/- 0.3) x 10(-3) U/mg.min with C1/2 = 93 +/- 5 nM and a Hill coefficient n = 1.8 +/- 0.1 at pH 7.2 and 20 degrees C. These findings suggest a new insight into the process of regulating photoreceptor activity.

Immunolocalization of arrestin (S-antigen) in rods of pearl mutant and wild-type mice

The pearl mutant mouse is hypopigmented and exhibits a significantly elevated dark-adapted (DA) threshold in comparison to the congenic wild-type mouse. The primary cause of the elevated DA threshold is not known. The subcellular immunolocalization of arrestin/S-antigen reflects the state of adaptation of rod photoreceptors. In this study, quantitative immunoelectron microscopy was used to examine the subcellular distribution of arrestin in wild-type and pearl rods as a function of light exposure. The goal was to determine whether arrestin distribution within rods of pearl and wild-type mice responds to background luminance in a comparable manner. The level of arrestin immunolabeling in DA (unilluminated) rods of pearl retinas was indistinguishable from that measured in wild-type rods. By contrast, arrestin immunolabeling in light-adapted (LA) pearl rod outer segments (ROS) was significantly greater than in wild-type LA ROS. Relative to DA ROS, arrestin labeling density increased 1.6 fold in wild-type ROS following light adaptation, as compared to a 4 fold increase in pearl ROS. These data suggest that although arrestin levels in DA pearl rods are indistinguishable from that of DA wild-type rods, net changes in arrestin immunolocalization in response to light exposure reflect the effects of the pearl mutation at the level of the rod outer segment. The possible implication of this finding is discussed in view of the proposed role of arrestin in the down-regulation of the enzymatic cascade of phototransduction.

Photoreceptor rod outer segment 48-kDa protein has ATPase activity

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

Differential expression of mRNA and protein encoding retinal and pineal S-antigen during the light/dark cycle

S-Antigen is a soluble cell protein unique to the retina and pineal gland. In the former, it is a well-characterized molecule that participates in light-induced signal transduction in photoreceptor cells. In the latter, the functional role is presently not known. The expression of S-antigen and its mRNA was examined in the rat retina and pineal gland throughout the diurnal cycle and with light interruption of the dark cycle. A cDNA for rat S-antigen was isolated from a pineal gland library to examine the mRNAs. A 1.7-kb mRNA for S-antigen was observed in both the pineal gland and the retina. Retinal S-antigen mRNA was expressed throughout the diurnal cycle and increased with light interruption of the dark cycle. In contrast, pineal gland S-antigen mRNA levels were detectable only during the dark and were absent preceding and during light. The phenotypic expression of immunoreactive S-antigen, identified with two S-antigen monoclonal antibodies (MAbs), MAb A9C6 and MAb C10C10, was analyzed by sodium dodecyl sulfate (SDS)-polyacrylamide gel (PAGE) and isoelectric focusing (IEF) electrophoresis. Immunoblot analysis of gels after SDS-PAGE revealed a single 46-kDa protein in retina. In contrast, two bands of approximately 43 and 46 kDa were identified in the pineal gland. Immunoblots of the retinal extracts separated by IEF electrophoresis revealed five S-antigen isomers, which vary quantitatively throughout the diurnal cycle and when light interrupted the dark cycle. Immunoblots of the pineal gland samples separated by IEF electrophoresis indicated that the pineal gland possesses four pineal gland-specific forms of S-antigen in addition to the five forms present in the retina. The differences observed in the mRNA and protein analyses suggest tissue-specific structural components for S-antigen in the retina and pineal gland that are not regulated in the same manner.