Preventive and therapeutic effects of SkQ1-containing Visomitin eye drops against light-induced retinal degeneration

The human retina is constantly affected by light of varying intensity, this being especially true for photoreceptor cells and retinal pigment epithelium. Traditionally, photoinduced damages of the retina are induced by visible light of high intensity in albino rats using the LIRD (light-induced retinal degeneration) model. This model allows study of pathological processes in the retina and the search for retinoprotectors preventing retinal photodamage. In addition, the etiology and mechanisms of retina damage in the LIRD model have much in common with the mechanisms of the development of age-related retinal disorders, in particular, with age-related macular degeneration (AMD). We have studied preventive and therapeutic effects of Visomitin eye drops (based on the mitochondria-targeted antioxidant SkQ1) on albino rat retinas damaged by bright light. In the first series of experiments, rats receiving Visomitin for two weeks prior to illumination demonstrated significantly less expressed atrophic and degenerative changes in the retina compared to animals receiving similar drops with no SkQ1. In the second series, the illuminated rats were treated for two weeks with Visomitin or similar drops without SkQ1. The damaged retinas of the experimental animals were repaired much more effectively than those of the control animals. Therefore, we conclude that Visomitin SkQ1-containing eye drops have pronounced preventive and therapeutic effects on the photodamaged retina and might be recommended as a photoprotector and a pharmaceutical preparation for the treatment of AMD in combination with conventional medicines.

Cancer-retina antigens -- a new group of tumor antigens

Some photoreceptor proteins normally specific for the eye retina are aberrantly expressed in malignant tumors. These proteins include recoverin, visual rhodopsin, transducin, cGMP-phosphodiesterase 6 (PDE 6), cGMP-dependent cationic channels, guanylyl cyclase 1, rhodopsin kinase, and arrestin. By analogy with cancer-testis antigens, these photoreceptor proteins form the group of cancer-retina antigens. It is shown that an aberrant demethylation of the promoter region of recoverin is involved in the aberrant expression of this protein. The cascade Wnt5a → Frizzled-2 → transducin → PDE 6 is shown to function in skin melanoma cells, and this suggests that these cancer-retina antigens can play a functional role. The events accompanying the signal transduction in this cascade, including those involving calcium ions and cGMP-dependent protein kinase (protein kinase G), are discussed.

Light-induced disulfide dimerization of recoverin under ex vivo and in vivo conditions

Despite vast knowledge of the molecular mechanisms underlying photochemical damage of photoreceptors, linked to progression of age-related macular degeneration, information on specific protein targets of the light-induced oxidative stress is scarce. Here, we demonstrate that prolonged intense illumination (halogen bulb, 1500 lx, 1-5 h) of mammalian eyes under ex vivo (cow) or in vivo (rabbit) conditions induces disulfide dimerization of recoverin, a Ca(2+)-dependent inhibitor of rhodopsin kinase. Western blotting and mass spectrometry analysis of retinal extracts reveals illumination time-dependent accumulation of disulfide homodimers of recoverin and its higher order disulfide cross-linked species, including a minor fraction of mixed disulfides with intracellular proteins (tubulins, etc.). Meanwhile, monomeric bovine recoverin remains mostly reduced. These effects are accompanied by accumulation of disulfide homodimers of visual arrestin. Histological studies demonstrate that the light-induced oxidation of recoverin and arrestin occurs in intact retina (illumination for 2 h), while illumination for 5 h is associated with damage of the photoreceptor layer. A comparison of ex vivo levels of disulfide homodimers of bovine recoverin with redox dependence of its in vitro thiol-disulfide equilibrium (glutathione redox pair) gives the lowest estimate of redox potential in rod outer segments under illumination from -160 to -155 mV. Chemical crosslinking and dynamic light scattering data demonstrate an increased propensity of disulfide dimer of bovine recoverin to multimerization/aggregation. Overall, the oxidative stress caused by the prolonged intense illumination of retina might affect rhodopsin desensitization via concerted disulfide dimerization of recoverin and arrestin. The developed herein models of eye illumination are useful for studies of the light-induced thiol oxidation of visual proteins.

Regulatory function of the C-terminal segment of guanylate cyclase-activating protein 2

Neuronal responses to Ca2+-signals are provided by EF-hand-type neuronal Ca2+-sensor (NCS) proteins, which have similar core domains containing Ca2+-binding and target-recognizing sites. NCS proteins vary in functional specificity, probably depending on the structure and conformation of their non-conserved C-terminal segments. Here, we investigated the role of the C-terminal segment in guanylate cyclase activating protein-2, GCAP2, an NCS protein controlling the Ca2+-dependent regulation of photoreceptor guanylate cyclases. We obtained two chimeric proteins by exchanging C-terminal segments between GCAP2 and its photoreceptor homolog recoverin, a Ca2+-sensor controlling rhodopsin kinase (RK) activity. The exchange affected neither the structural integrity of GCAP2 and recoverin nor the Ca2+-sensitivity of GCAP2. Intrinsic fluorescence, circular dichroism, biochemical studies and hydrophobic dye probing revealed Ca2+-dependent conformational transition of the C-terminal segment of GCAP2 occurring in the molecular environment of both proteins. In Ca2+-GCAP2, the C-terminal segment was constrained and its replacement provided the protein with approximately two-fold inhibitory activity towards RK, suggesting that the segment contributes to specific target recognition by interfering with RK-binding. Upon Ca2+-release, it became less constrained and more available for phosphorylation by cyclic nucleotide-dependent protein kinase. The transition from the Ca2+-bound to the apo-state exposed hydrophobic sites in GCAP2, and was associated with its activating function without affecting its dimerization. The released C-terminal segment participated further in photoreceptor membrane binding making it sensitive to phosphorylation. Thus, the C-terminal segment in GCAP2 confers target selectivity, facilitates membrane binding and provides sensitivity of the membrane localization of the protein to phosphorylation by signaling kinases.

Organotypic culture of neural retina as a research model of neurodegeneration of ganglion cells

Organotypic models deserve special attention among the large variety of methods of vertebrate retina cultivation. The purpose of this study was to make a detailed qualitative and quantitative characterization of a model employing roller organotypic cultivation of the neural retina of rat eye posterior segment, with special attention to morphological and functional characteristics of retinal ganglion cells. The study included morphological analysis of retina histological preparations as well as estimation of RNA synthesis and evaluation of neuron survival by the Brachet and TUNEL methods, respectively. Retina has been shown to display normal morphofunctional characteristics for the first 12 h of cultivation. After 24 h, a substantial number of ganglion cells underwent pyknosis and stopped RNA synthesis. Almost all the cells of the retinal ganglion layer became apoptotic by 3-4 days in vitro. In the course of cultivation, neural retina is detached from the underlying layers of the posterior eye segment and undergoes significant cytoarchitectonic changes. The causes of ganglion cell death during organotypic cultivation of eye posterior segment are discussed. This method can serve as a suitable model for the screening of new retinoprotectors and for research on ganglion cell death resulting from retina degenerative diseases, e.g. glaucoma.

In vivo immunoregulatory properties of the novel mitochondria-targeted antioxidant SkQ1

Reactive oxygen species (ROS) is a group of highly reactive oxygen-containing chemicals. ROS are essential for various biological functions, including cell survival and growth, proliferation and differentiation. At the same time ROS production is connected to a number of disorders, such as chronic inflammation, age-related diseases and cancers. In the immune system, ROS are involved in the defence of the host organism, immune response and immune regulation. One of the main sites of ROS generation in the cell is mitochondrial electron transport. In contrast to a number of traditional antioxidants, the novel mitochondria-targeted antioxidant SkQ1 exerts its antioxidant properties even in nanomolar concentrations. In this work, we investigated immunomodulatory properties of SkQ1 and demonstrated that treatment of mice with SkQ1 led to a decrease in percentage of CD8(+) T cells but not of CD4(+) T cells. We documented a decrease of a relative number of naïve T cells with a simultaneous increase in percentage of effector memory T cells. Central memory T cells had also a trend to be increased after SkQ1 treatment. In fraction of dendritic cells, we found an increase in percentage of plasmacytoid dendritic cells. In the case of myeloid cells, SkQ1 treatment decreased significantly the percentage of granulocytes. No effect of SkQ1 was observed on regulatory T cells, natural killer cells, natural killer T cells, as well as on freshly isolated CD8(+) T or CD4(+) T cells, indicating the indirect influence of SkQ1 on immune cells.

Synergetic effect of recoverin and calmodulin on regulation of rhodopsin kinase

Phosphorylation of photoactivated rhodopsin by rhodopsin kinase (RK or GRK1), a first step of the phototransduction cascade turnoff, is under the control of Ca²⁺/recoverin. Here, we demonstrate that calmodulin, a ubiquitous Ca²⁺-sensor, can inhibit RK, though less effectively than recoverin does. We have utilized the surface plasmon resonance technology to map the calmodulin binding site in the RK molecule. Calmodulin does not interact with the recoverin-binding site within amino acid residues M1-S25 of the enzyme. Instead, the high affinity calmodulin binding site is localized within a stretch of amino acid residues V150-K175 in the N-terminal regulatory region of RK. Moreover, the inhibitory effect of calmodulin and recoverin on RK activity is synergetic, which is in agreement with the existence of separate binding sites for each Ca²⁺-sensing protein. The synergetic inhibition of RK by both Ca²⁺-sensors occurs over a broader range of Ca²⁺-concentration than by recoverin alone, indicating increased Ca²⁺-sensitivity of RK regulation in the presence of both Ca²⁺-sensors. Taken together, our data suggest that RK regulation by calmodulin in photoreceptor cells could complement the well-known inhibitory effect of recoverin on RK.

Autoantibodies against the Ca(2+)-binding protein recoverin in blood sera of patients with various oncological diseases

In cancer, the retinal Ca(2+)-binding protein recoverin is a paraneoplastic antigen, the aberrant expression of which is capable of triggering the appearance of specific autoantibodies in the serum of patients with malignant tumors and the subsequent development of a paraneoplastic syndrome, cancer-associated retinopathy (CAR). The frequency of serum autoantibodies against recoverin (AAR), earlier determined at a rate of 15-20% in lung cancer, is much higher than the frequency of CAR syndrome, which is approximately 1%. In the present study, we estimated for the first time the frequencies of serum AAR in patients with various types of malignancies other than lung cancer. Patient biospecimens were collected to analyze for the presence of AAR. Additionally, various cell lines were cultivated and analyses were performed using Western blotting and RT-PCR. Results showed that in all cases tested, the AAR frequencies did not exceed 10%. Five AAR-positive patients with various types of cancer were available for ophthalmological investigation and only one of these patients had CAR syndrome. This result is consistent with the conclusion made in our previous studies of lung cancer that serum AAR do not necessarily trigger the development of CAR syndrome.

Amino acid sequences of two immune-dominant epitopes of recoverin are involved in Ca2+/recoverin-dependent inhibition of phosphorylation of rhodopsin

Antibodies AB(60-72) and AB(80-92) against two immune-dominant epitopes of photoreceptor Ca(2+)-binding protein recoverin, 60-DPKAYAQHVFRSF-72 and 80-LDFKEYVIALHMT-92, which can be exposed in a Ca(2+)-dependent manner, were obtained. The presence of AB(60-72) or AB(80-92) results in a slight increase in Ca(2+)-affinity of recoverin and does not affect significantly a Ca(2+)-myristoyl switch mechanism of the protein. However in the presence of AB(60-72) or AB(80-92) recoverin loses its ability to interact with rhodopsin kinase and consequently to perform a function of Ca(2+)-sensitive inhibitor of rhodopsin phosphorylation in photoreceptor cells.

Oxidation mimicking substitution of conservative cysteine in recoverin suppresses its membrane association

Recoverin belongs to the family of intracellular Ca(2+)-binding proteins containing EF-hand domains, neuronal calcium sensors (NCS). In photoreceptor outer segments, recoverin is involved into the recovery of visual cycle via Ca(2+)-dependent interaction with disk membranes and inhibition of rhodopsin kinase. The function of a conservative within NCS family Cys residue in the inactive EF-loop 1 remains unclear, but previous study has shown its vulnerability to oxidation under mild oxidizing conditions. To elucidate the influence of oxidation of the conservative Cys39 in recoverin the properties of its C39D mutant, mimicking oxidative conversion of Cys39 into sulfenic, sulfinic or sulfonic acids have been studied using intrinsic fluorescence, circular dichroism, and equilibrium centrifugation methods. The C39D substitution results in essential changes in structural, physico-chemical and physiological properties of the protein: it reduces α-helical content, decreases thermal stability and suppresses protein affinity for photoreceptor membranes. The latter effect precludes proper functioning of the Ca(2+)-myristoyl switch in recoverin. The revealed significance of oxidation state of Cys39 for maintaining the protein functional status shows that it may serve as redox sensor in vision and suggests an explanation of the available data on localization and light-dependent translocation of recoverin in rod photoreceptors.

Aberrant demethylation of the recoverin gene is involved in the aberrant expression of recoverin in cancer cells

The Ca(2+) -binding protein recoverin is normally specific for the retina. Recoverin aberrantly expressed in lung and melanoma tumors can trigger the host immune response followed by the development of a paraneoplastic neurological syndrome represented by cancer- and melanoma-associated retinopathy, respectively. The mechanisms, underlying the aberrant expression of recoverin in tumor cells, have remained unknown. The data obtained in this study suggest that (i) DNA methylation participates in the repression of synthesis of mRNA for recoverin in normal tissues and (ii) aberrant hypomethylation of the recoverin gene region, overlapping the promoter up-stream of the first exon and the first exon itself, is involved in the aberrant expression of recoverin in tumor cells.

cGMP-phosphodiesterase 6, transducin and Wnt5a/Frizzled-2-signaling control cGMP and Ca(2+) homeostasis in melanoma cells

Malignant melanoma is one of the most aggressive human neoplasms which develop from the malignant transformation of normal epithelial melanocytes and share the lineage with retinal cells. cGMP-phosphodiesterase 6 (PDE6) is one of the cancer-retina antigens newly identified in melanoma cells. Normally, PDE6 hydrolyzes the photoreceptor second messenger cGMP allowing the visual signal transduction in photoreceptor cells. cGMP also play an important signaling role in stimulating melanogenesis in human melanocytes. Here, we present evidence that PDE6 is a key enzyme regulating the cGMP metabolism in melanoma cells. Decrease in intracellular cGMP leads to calcium accumulation in melanoma cells. In these cells, cGMP-phosphodiesterase 6 can be activated by another cancer-retina antigen, transducin, through Wnt5a-Frizzled-2 cascade, which leads to a lowering of cGMP and an increase in intracellular calcium mobilization. Thus, the aberrant expression of PDE6 may control cGMP metabolism and calcium homeostasis in melanoma cells.

Mechanism of rhodopsin kinase regulation by recoverin

Recoverin is suggested to inhibit rhodopsin kinase (GRK1) at high [Ca(2+)] in the dark state of the photoreceptor cell. Decreasing [Ca(2+)] terminates inhibition and facilitates phosphorylation of illuminated rhodopsin (Rh*). When recoverin formed a complex with GRK1, it did not interfere with the phosphorylation of a C-terminal peptide of rhodopsin (S338-A348) by GRK1. Furthermore, while GRK1 competed with transducin on interaction with rhodopsin and thereby suppressed GTPase activity of transducin, recoverin in the complex with GRK1 did not influence this competition. Constructs of GRK1 that encompass its N-terminal, catalytic or C-terminal domains were used in pull-down assays and surface plasmon resonance analysis to monitor interaction. Ca(2+)-recoverin bound to the N-terminus of GRK1, but did not bind to the other constructs. GRK1 interacted with rhodopsin also by its N-terminus in a light-dependent manner. No interaction was observed with the C-terminus. We conclude that inhibition of GRK1 by recoverin is not the result of their direct competition for the same docking site on Rh*, although the interaction sites of GRK1/Rh* and GRK1/recoverin partially overlap. The N-terminus of GRK1 is recognized by Rh* leading to a conformational change which moves the C-terminus of Rh* into the catalytic kinase groove. Ca(2+)-recoverin interacting with the N-terminus of GRK1 prevents this conformational change and thus blocks Rh* phosphorylation by GRK1.

Membrane binding of the neuronal calcium sensor recoverin - modulatory role of the charged carboxy-terminus

BACKGROUND

The Ca2+-binding protein recoverin operates as a Ca2+-sensor in vertebrate photoreceptor cells. It undergoes a so-called Ca2+-myristoyl switch when cytoplasmic Ca2+-concentrations fluctuate in the cell. Its covalently attached myristoyl-group is exposed at high Ca2+-concentrations and enables recoverin to associate with lipid bilayers and to inhibit its target rhodopsin kinase. At low Ca2+-concentrations the myristoyl group is inserted into a hydrophobic pocket of recoverin thereby relieving inhibitory constraint on rhodopsin kinase. Hydrophobic and electrostatic interactions of recoverin with membranes have not been clearly determined, in particular the function of the positively charged carboxy-terminus in recoverin 191QKVKEKLKEKKL202 in this context is poorly understood.

RESULTS

Binding of myristoylated recoverin to lipid bilayer depends on the charge distribution in phospholipids. Binding was tested by equilibrium centrifugation and surface plasmon resonance (SPR) assays. It is enhanced to a certain degree by the inclusion of phosphatidylserine (up to 60%) in the lipid mixture. However, a recoverin mutant that lacked the charged carboxy-terminus displayed the same relative binding amplitudes as wildtype (WT) recoverin when bound to neutral or acidic lipids. Instead, the charged carboxy-terminus of recoverin has a significant impact on the biphasic dissociation of recoverin from membranes. On the other hand, the nonmyristoylated WT and truncated mutant form of recoverin did not bind to lipid bilayers to a substantial amount as binding amplitudes observed in SPR measurements are similar to bulk refractive index changes.

CONCLUSION

Our data indicate a small, but evident electrostatic contribution to the overall binding energy of recoverin association with lipid bilayer. Properties of the charged carboxy-terminus are consistent with a role of this region as an internal effector region that prolongs the time recoverin stays on the membrane by influencing its Ca2+-sensitivity.

Surface plasmon resonance study of g protein/receptor coupling in a lipid bilayer-free system

Surface plasmon resonance (SPR) spectroscopy is a technique to study protein-protein interactions in real time; however, application of SPR spectroscopy for investigations of membrane receptors is difficult with respect to functional and uniform immobilization of receptors on a biosensor surface. In the current study, we developed a simple, direct, biosensor-based approach to monitor the molecular interactions between G protein transducin (Gt) and rhodopsin (Rho), a prototypical G protein-coupled receptor (GPCR). Detergent-solubilized dark-adapted Rho was captured onto a biosensor surface via lectin interaction, enabling site-directed immobilization of the receptor that made its cytoplasmic surface accessible to a coupling G protein. The system resembled the natural system with respect to receptor density, binding of Gt following flash or constant light application, fast GTP-dependent dissociation of Gt from Rho, regeneration of Rho, and dependence of Gt binding on light intensity and on concentration of Gt. The apparent KD of the Gt/Rho interaction was 13.6 nM. Our results validate the use of SPR spectroscopy as a tool to study G protein activation in GPCR systems and could be extended for application to other interaction partners of GPCRs.

Ca2+-dependent conformational changes in the neuronal Ca2+-sensor recoverin probed by the fluorescent dye Alexa647

Recoverin belongs to the superfamily of EF-hand Ca2+-binding proteins and operates as a Ca2+-sensor in vertebrate photoreceptor cells, where it regulates the activity of rhodopsin kinase GRK1 in a Ca2+-dependent manner. Ca2+-dependent conformational changes in recoverin are allosterically controlled by the covalently attached myristoyl group. The amino acid sequence of recoverin harbors a unique cysteine at position 38. The cysteine can be modified by the fluorescent dye Alexa647 using a maleimide-thiol coupling step. Introduction of Alexa647 into recoverin did not disturb the biological function of recoverin, as it can regulate rhodopsin kinase activity like unlabeled recoverin. Performance of the Ca2+-myristoyl switch of labeled recoverin was monitored by Ca2+-dependent association with immobilized lipids using surface plasmon resonance spectroscopy. When the Ca2+-concentration was varied, labeled myristoylated recoverin showed a 37%-change in fluorescence emission and a 34%-change in excitation intensity, emission and excitation maxima shifted by 6 and 18 nm, respectively. In contrast, labeled nonmyristoylated recoverin exhibited only minimal changes. Time-resolved fluorescence measurements showed biexponentiell fluorescence decay, in which the slower time constant of 2 ns was specifically influenced by Ca2+-induced conformational changes. A similar influence on the slower time constant was observed with the recoverin mutant RecE85Q that has a disabled EF-hand 2, but no such influence was detected with the mutant RecE121Q (EF-hand 3 is nonfunctional) that contains the myristoyl group in a clamped position. We conclude from our results that Alexa647 bound to cysteine 38 can monitor the conformational transition in recoverin that is under control of the myristoyl group.

Critical role of electrostatic interactions of amino acids at the cytoplasmic region of helices 3 and 6 in rhodopsin conformational properties and activation

The cytoplasmic sides of transmembrane helices 3 and 6 of G-protein-coupled receptors are connected by a network of ionic interactions that play an important role in maintaining its inactive conformation. To investigate the role of such a network in rhodopsin structure and function, we have constructed single mutants at position 134 in helix 3 and at positions 247 and 251 in helix 6, as well as combinations of these to obtain double mutants involving the two helices. These mutants have been expressed in COS-1 cells, immunopurified using the rho-1D4 antibody, and studied by UV-visible spectrophotometry. Most of the single mutations did not affect chromophore formation, but double mutants, especially those involving the T251K mutant, resulted in low yield of protein and impaired 11-cis-retinal binding. Single mutants E134Q, E247Q, and E247A showed the ability to activate transducin in the dark, and E134Q and E247A enhanced activation upon illumination, with regard to wild-type rhodopsin. Mutations E247A and T251A (in E134Q/E247A and E134Q/T251A double mutants) resulted in enhanced activation compared with the single E134Q mutant in the dark. A role for Thr(251) in this network is proposed for the first time in rhodopsin. As a result of these mutations, alterations in the hydrogen bond interactions between the amino acid side chains at the cytoplasmic region of transmembrane helices 3 and 6 have been observed using molecular dynamics simulations. Our combined experimental and modeling results provide new insights into the details of the structural determinants of the conformational change ensuing photoactivation of rhodopsin.

Photoreceptor proteins as cancer-retina antigens

Melanocytes, melanoma and photoreceptor cells are of neuroectodermal origin and have a certain sensitivity to light. In this study, we present evidence for photoreceptor proteins that are responsible for visual transduction and its regulation function as a new class of cancer antigens in melanoma. Visual rhodopsin, transducin, cGMP-phosphodiesterase 6, cGMP-dependent channels, guanylyl cyclase, rhodopsin kinase, recoverin and arrestin are expressed in melanoma and can induce antibody responses in patients. Melanocytes also express mRNA of all photoreceptor genes besides transducin, but were devoid of the corresponding protein, which was tested for rhodopsin, cGMP-phosphodiesterase, guanylyl cyclase and recoverin. Furthermore, we show for the first time that some healthy tissues express mRNA of these genes, but never protein. Expression profiles and autoantibody responses were confirmed in the MT/ret and the HGF(tg)/Ink4a(-/-) transgenic mouse melanoma models. We propose a molecular transition of cancer-retina antigens from mRNA expression in melanocytes to protein expression in melanoma. Our work provides the basis for analyzing regulation of photoreceptor gene expression in normal and malignant cells as well as possible therapeutic tumor targeting using the newly defined class of cancer-retina antigens.

Recoverin as a cancer-retina antigen

In photoreceptor cells the Ca(2+) -binding protein recoverin controls phosphorylation of the visual receptor rhodopsin by inhibiting rhodopsin kinase (GRK-1). It can also serve as a paraneoplastic antigen in the development of retinal degeneration in some patients with cancer. The aberrant expression of recoverin in cancer cells and the presence of autoantibodies against recoverin are essential for the occurrence of cancer-associated retinopathy, which finally results in the apoptosis of photoreceptor cells. Noteworthy in cancer patients, the aberrant recoverin expression and the appearance of autoantibodies against recoverin are more frequent than paraneoplastic syndromes. We suggest the term "cancer-retina antigens" for this kind of proteins like recoverin that are solely expressed in retina and tumor tissues and evoke antibodies and/or T cells in patients with cancer. The rare development of a paraneoplastic syndrome is possibly caused by this immune response and probably depends on further events allowing to overcome the blood-retina barrier and the immune privileged status of the retina. It is still unknown whether aberrantly expressed recoverin could have a specific function in cancer cells, though it is suggested that it can be functionally associated with G-protein-coupled receptor kinases. This paper reviews the present knowledge on paraneoplastic syndromes associated with the aberrant expression of recoverin. A possible application of recoverin as a potential target for immunotherapy of cancer is discussed.

Tuning of a neuronal calcium sensor

Recoverin is a Ca(2+)-regulated signal transduction modulator expressed in the vertebrate retina that has been implicated in visual adaptation. An intriguing feature of recoverin is a cluster of charged residues at its C terminus, the functional significance of which is largely unclear. To elucidate the impact of this segment on recoverin structure and function, we have investigated a mutant lacking the C-terminal 12 amino acids. Whereas in myristoylated recoverin the truncation causes an overall decrease in Ca(2+) sensitivity, results for the non-myristoylated mutant indicate that the truncation primarily affects the high affinity EF-hand 3. The three-dimensional structure of the mutant has been determined by x-ray crystallography. In addition to significant changes in average coordinates compared with wild-type recoverin, the structure provides strong indication of increased conformational flexibility, particularly in the C-terminal domain. Based on these observations, we propose a novel role of the C-terminal segment of recoverin as an internal modulator of Ca(2+) sensitivity.

Ca2+/recoverin dependent regulation of phosphorylation of the rhodopsin mutant R135L associated with retinitis pigmentosa

No single molecular mechanism accounts for the effect of mutations in rhodopsin associated with retinitis pigmentosa. Here we report on the specific effect of a Ca2+/recoverin upon phosphorylation of the autosomal dominant retinitis pigmentosa R135L rhodopsin mutant. This mutant shows specific features like impaired G-protein signaling but enhanced phosphorylation in the shut-off process. We now report that R135L hyperphosphorylation by rhodopsin kinase is less efficiently inhibited by Ca2+/recoverin than wild-type rhodopsin. This suggests an involvement of Ca2+/recoverin into the molecular pathogenic effect of the mutation in retinitis pigmentosa which is the cause of rod photoreceptor cell degeneration. This new proposed role of Ca2+/recoverin may be one of the specific features of the proposed new Type III class or rhodopsin mutations associated with retinitis pigmentosa.

One of the Ca2+ binding sites of recoverin exclusively controls interaction with rhodopsin kinase

Recoverin is a neuronal calcium sensor protein that controls the activity of rhodopsin kinase in a Ca(2+)-dependent manner. Mutations in the EF-hand Ca2+ binding sites are valuable tools for investigating the functional properties of recoverin. In the recoverin mutant E121Q (Rec E121Q ) the high-affinity Ca2+ binding site is disabled. The non-myristoylated form of Rec E121Q binds one Ca2+ via its second Ca(2+)-binding site (EF-hand 2), whereas the myristoylated variant does not bind Ca2+ at all. Binding of Ca2+ to non-myristoylated Rec E121Q apparently triggers exposure of apolar side chains, allowing for association with hydrophobic matrices. Likewise, an interaction surface for the recoverin target rhodopsin kinase is constituted upon Ca2+ binding to the non-acylated mutant. Structural changes resulting from Ca(2+)-occupation of EF-hand 2 in myristoylated and non-myristoylated recoverin variants are discussed in terms of critical conditions required for biological activity.