Recoverin inhibits the phosphorylation of dark-adapted rhodopsin more than it does that of bleached rhodopsin: a possible mechanism through which rhodopsin kinase is prevented from participation in a side reaction

Regulation of the methylation status of G protein-coupled receptor kinase 1 (rhodopsin kinase)

Rhodopsin kinase (GRK1) is a member of G protein-coupled receptor kinase family and a key enzyme in the quenching of photolysed rhodopsin activity and desensitisation of the rod photoreceptor neurons. Like some other rod proteins involved in phototransduction, GRK1 is posttranslationally modified at the C terminus by isoprenylation (farnesylation), endoproteolysis and α-carboxymethylation. In this study, we examined the potential mechanisms of regulation of GRK1 methylation status, which have remained unexplored so far. We found that considerable fraction of GRK1 is endogenously methylated. In isolated rod outer segments, its methylation is inhibited and demethylation stimulated by low-affinity nucleotide binding. This effect is not specific for ATP and was observed in the presence of a non-hydrolysable ATP analogue AMP-PNP, GTP and other nucleotides, and thus may involve a site distinct from the active site of the kinase. GRK1 demethylation is inhibited in the presence of Ca(2+) by recoverin. This inhibition requires recoverin myristoylation and the presence of the membranes, and may be due to changes in GRK1 availability for processing enzymes upon its redistribution to the membranes induced by recoverin/Ca(2+). We hypothesise that increased GRK1 methylation in dark-adapted rods due to elevated cytoplasmic Ca(2+) levels would further increase its association with the membranes and recoverin, providing a positive feedback to efficiently suppress spurious phosphorylation of non-activated rhodopsin molecules and thus maximise senstivity of the photoreceptor. This study provides the first evidence for dynamic regulation of GRK1 α-carboxymethylation, which might play a role in the regulation of light sensitivity and adaptation in the rod photoreceptors.

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.

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.

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.

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.

The neuronal calcium-sensor proteins

Changes in intracellular free Ca(2+) concentration ([Ca(2+)](i)) affect many different aspects of neuronal function ranging from millisecond regulation of ion channels to long term changes in gene expression. These effects of Ca(2+) are transduced by Ca(2+)-binding proteins that act as Ca(2+) sensors by binding Ca(2+), undergoing a conformational change and then modifying the function of additional target proteins. Mammalian species express 14 members of the neuronal calcium sensor (NCS) family of EF hand-containing Ca(2+)-binding proteins which are expressed mainly in photoreceptor cells or neurons. Many of the NCS proteins are membrane targeted through their N-terminal myristoylation either constitutively or following exposure of the myristoyl group after Ca(2+) binding (the Ca(2+)/myristoyl switch). The NCS proteins have been implicated in a wide range of functional roles in neuronal regulation, several of which have been confirmed though molecular genetic analyses.

Preparation and characteristics of Ca2+-dependent monoclonal antibodies to recoverin

Thirty-four primary hybridoma clones were prepared which expressed monoclonal antibodies to the Ca(2+)-binding protein recoverin. Among the resulting monoclonal antibodies, two Ca(2+)-dependent clones (mAb3 and mAb19) recognizing recoverin were detected by solid-phase immunoenzyme assay. In the presence of Ca(2+), antibodies of the mAb3 and mAb19 clones bound to recoverin several times better than in the absence of Ca(2+). The mAb3 and mAb19 antibodies recognized epitopes located inside the sequences Pro61-Met91 and Pro57-Tyr64 of the recoverin molecule, respectively. The possible mechanism of the Ca(2+)-dependent recognition of recoverin by the prepared monoclonal antibodies is discussed.

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

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

Ca2+-dependent control of rhodopsin phosphorylation: recoverin and rhodopsin kinase

Over many years until the middle of the 1980s, the main problem in vision research had been the mechanism of transducing the visual signal from photobleached rhodopsin to the cationic channels in the plasma membrane of a photoreceptor to trigger the electrophysiological response of the cell. After cGMP was proven to be the secondary messenger, the main intriguing question has become the mechanisms of negative feedback in photoreceptors to modulate their response to varying conditions of illumination. Although the mechanisms of light-adaptation are not completely understood, it is obvious that Ca2+ plays a crucial role in these mechanisms and that the effects of Ca2+ can be mediated by several Ca2+-binding proteins. One of them is recoverin. The leading candidate for the role of an intracellular target for recoverin is believed to be rhodopsin kinase, a member of a family of G-protein-coupled receptor kinases. The present review considers recoverin, rhodopsin kinase and their interrelationships in the in vitro as well as in vivo contexts.

The neuronal calcium sensor family of Ca2+-binding proteins

Ca(2+) plays a central role in the function of neurons as the trigger for neurotransmitter release, and many aspects of neuronal activity, from rapid modulation to changes in gene expression, are controlled by Ca(2+). These actions of Ca(2+) must be mediated by Ca(2+)-binding proteins, including calmodulin, which is involved in Ca(2+) regulation, not only in neurons, but in most other cell types. A large number of other EF-hand-containing Ca(2+)-binding proteins are known. One family of these, the neuronal calcium sensor (NCS) proteins, has a restricted expression in retinal photoreceptors or neurons and neuroendocrine cells, suggesting that they have specialized roles in these cell types. Two members of the family (recoverin and guanylate cyclase-activating protein) have established roles in the regulation of phototransduction. Despite close sequence similarities, the NCS proteins have distinct neuronal distributions, suggesting that they have different functions. Recent work has begun to demonstrate the physiological roles of members of this protein family. These include roles in the modulation of neurotransmitter release, control of cyclic nucleotide metabolism, biosynthesis of polyphosphoinositides, regulation of gene expression and in the direct regulation of ion channels. In the present review we describe the known sequences and structures of the NCS proteins, information on their interactions with target proteins and current knowledge about their cellular and physiological functions.

Regulation of G protein-coupled receptor kinase subtypes by calcium sensor proteins

G protein-coupled receptor homologous desensitization is intrinsically related to the function of a class of S/T kinases named G protein-coupled receptor kinases (GRK). The GRK family is composed of six cloned members, named GRK1 to 6. Studies from different laboratories have demonstrated that different calcium sensor proteins (CSP) can selectively regulate the activity of GRK subtypes. In the presence of calcium, rhodopsin kinase (GRK1) is inhibited by the photoreceptor-specific CSP recoverin through direct binding. Several other recoverin homologues (including NCS 1, VILIP 1 and hippocalcin) are also able to inhibit GRK1. The ubiquitous calcium-binding protein calmodulin (CaM) can inhibit GRK5 with a high affinity (IC(50)=40-50 nM). A direct interaction between GRK5 and Ca(2+)/CaM was documented and this binding does not influence the catalytic activity of the kinase, but rather reduced GRK5 binding to the membrane. These studies suggest that CSP act as functional analogues in mediating the regulation of different GRK subtypes by Ca(2+). This mechanism is, however, highly selective with respect to the GRK subtypes: while GRK1, but not GRK2 and GRK5, is regulated by recoverin and other NCS, GRK4, 5 and 6, that belong to the GRK4 subfamily, are potently inhibited by CaM, which had little or no effect on members of other GRK subfamilies.

Regulation of G-protein-coupled receptor kinase subtypes by calcium sensor proteins

The process of G-protein-coupled receptor (GPCR) homologous desensitization is intrinsically related to the function of a class of S/T kinases named G-protein-coupled receptor kinases (GRK). GRK family is so far composed of six cloned members, named GRK1 to 6, which are classified into three subfamilies: GRK1 is alone in the first (rhodopsin kinase subfamily), GRK2 and 3 form the second [beta-adrenergic receptor kinase (betaARK) subfamily], and GRK4, 5, and 6 constitute the third (GRK4 subfamily). Recent studies from different laboratories have demonstrated that different calcium sensor proteins (CSP) can selectively regulate the activity of GRK subtypes. In the presence of calcium, rhodopsin kinase (GRK1) is inhibited by the photoreceptor-specific CSP recoverin through direct binding. Several other recoverin homologues (including NCS 1, VILIP 1, and hippocalcin) are also able to inhibit GRK1 in a calcium-dependent manner. The ubiquitous calcium binding protein calmodulin (CaM) can inhibit GRK5 with a high affinity (IC50=40-50 nM). A direct interaction between GRK5 and Ca2+/CaM was documented and this binding did not influence the catalytic activity of the kinase, but rather reduced GRK5 binding to the membrane. These studies suggest that CSP act as functional analogs in mediating the regulation of different GRK subtypes by Ca2+. This mechanism, however, is highly selective with respect to the GRK subtypes: GRK1, but not GRK2 and GRK5, is regulated by recoverin and other NCS, but GRK4, 5, and 6, which belong to the GRK4 subfamily are potently inhibited by CaM, which has little or no effect on members of other GRK subfamilies. Calcium-dependent inhibition of rhodopsin kinase by recoverin represents one of the mechanisms that control adaptation to light. For the other GPCR, CSP-GRK interaction provides a feedback mechanism that can modulate homologous desensitization of these receptors.

Phosphorylation of photolyzed rhodopsin is calcium-insensitive in retina permeabilized by alpha-toxin

Light triggers the phototransduction cascade by activating the visual pigment rhodopsin (Rho --> Rho*). Phosphorylation of Rho* by rhodopsin kinase (RK) is necessary for the fast recovery of sensitivity after intense illumination. Ca2+ ions, acting through Ca2+-binding proteins, have been implicated in the desensitization of phototransduction. One such protein, recoverin, has been proposed to regulate RK activity contributing to adaptation to background illumination in retinal photoreceptor cells. In this report, we describe an in vitro assay system using isolated retinas that is well suited for a variety of biochemical assays, including assessing Ca2+ effects on Rho* phosphorylation. Pieces of bovine retina with intact rod outer segments were treated with pore-forming staphylococcal alpha-toxin, including an alpha-toxin mutant that forms pores whose permeability is modulated by Zn2+. The pores formed through the plasma membranes of rod cells permit the diffusion of small molecules <2 kDa but prevent the loss of proteins, including recoverin (25 kDa). The selective permeability of these pores was confirmed by using the small intracellular tracer N-(2-aminoethyl) biotinamide hydrochloride. Application of [gamma-32P]ATP to alpha-toxin-treated, isolated retina allowed us to monitor and quantify phosphorylation of Rho*. Under various experimental conditions, including low and high [Ca2+]free, the same level of Rho* phosphorylation was measured. No differences were observed between low and high [Ca2+]free conditions, even when rods were loaded with ATP and the pores were closed by Zn2+. These results suggest that under physiological conditions, Rho* phosphorylation is insensitive to regulation by Ca2+ and Ca2+-binding proteins, including recoverin.

Obtaining and characterization of EF-hand mutants of recoverin

Several EF-hand recoverin mutants were obtained and their abilities to bind to photoreceptor membranes and to inhibit rhodopsin kinase were determined. The mutants with the 'spoiled' 2nd, 3rd or (2nd+3rd) EF-hand structures did not act upon the kinase activity in the microM range of Ca2+ concentrations. Mutations of the 4th EF hand, which 'repaired' its Ca2+-binding activity, resulted in recoverin with three 'working' Ca2+-binding sites. The latter mutant inhibited rhodopsin kinase even more effectively than the wild-type recoverin, containing two working Ca2+-binding structures.

Autophosphorylation and ADP regulate the Ca2+-dependent interaction of recoverin with rhodopsin kinase

Recoverin is a 23 kDa myristoylated Ca2+-binding protein that inhibits rhodopsin kinase. We have used surface plasmon resonance to investigate the influences of Ca2+, myristoylation, and adenine nucleotides on the recoverin-rhodopsin kinase interaction. Our analyses confirmed that Ca2+ is required for recoverin to bind RK. Myristoylation had little effect on the affinity of recoverin for the kinase, but it raised the K0.5 for Ca2+ from 150 nM for nonacylated recoverin to 400 nM for myristoylated recoverin. Finally, our studies also revealed two separate and previously unreported effects of adenine nucleotides on the recoverin-rhodopsin kinase binding. The interaction is weakened by autophosphorylation of the kinase, and it is strengthened by the presence of ADP.

Recoverin and hippocalcin distribution in the lamprey (Lampreta fluviatilis) retina

Recoverin is a calcium-sensing protein which is involved in the transduction of light in vertebrate photoreceptors. It is also detected in other retina cell types in which its function is not yet elucidated, and is an autoantigen in a cancer-associated degenerative disease of the retina. Recently, hippocalcin, an homologous protein of recoverin, belonging to the same family of fatty acylated EF-hand calcium binding proteins was described in mammals. The immunohistochemical studies presented in this paper demonstrate, that, in the retina of the lamprey, an Agnathan considered the living ancestor of actual jawed vertebrates, recoverin was present in all photoreceptors and, to a lesser extent in subpopulations of amacrine and ganglion cells whereas hippocalcin was detected in numerous amacrine and ganglion cells and in the inner segments of long photoreceptors. The existence of these calcium-binding proteins shows that they have a high degree of conservation during evolution. Their presence in the same cells that in jawed vertebrates (photoreceptors and ganglion cells for recoverin; amacrine and ganglion cells for hippocalcin) suggests that some retinal functions are well conserved but because they were also found in different cell types than in other species (amacrine for recoverin; photoreceptors for hippocalcin), they may have functions more specific to the lamprey retina.

Rhodopsin phosphorylation and its role in photoreceptor function

Light-stimulated phosphorylation of rhodopsin was first described 25 years ago. This paper reviews the progress that has been made towards (i) understanding the nature of the enzymes that phosphorylate and dephosphorylate rhodopsin (ii) identifying the sites of phosphorylation on rhodopsin and (iii) understanding the physiological importance of rhodopsin phosphorylation. Many important questions related to rhodopsin phosphorylation remain unanswered and new strategies and methods are needed to address issues such as the roles of Ca2+ and recoverin. We present one such method that uses mass spectrometry to quantitate rhodopsin phosphorylation in intact mouse retinas.

GTP-binding-protein-coupled receptor kinases--two mechanistic models

Six vertebrate protein kinases (G-protein-coupled receptor kinases; GRKs) that regulate the function of G-protein-coupled receptors (GPCRs) were recently cloned; several distinct properties set them apart from conventional second-messenger regulated protein kinases. It appears that GRKs bind GPCR* through two separate sites: a high-affinity site, which involves intracellular loops of the activated receptor, and the lower-affinity site, encompassing the phosphorylation region. The high-affinity interaction may involve complementary structural elements of GRKs and GPCRs* rather than precise amino acid alignment, thus allowing broad and overlapping specificities of these kinases, in spite of differences in the sequences of GPCRs. In addition, GRK structures are modified by several posttranslational modifications, including phosphorylation, autophosphorylation, prenylation, carboxymethylation, and palmitoylation, probably affecting properties of these enzymes. While GRKs phosphorylate and inactivate receptor molecules which are engaged in G-protein activation, controversy surrounds whether GRKs might be activated and phosphorylate unstimulated GPCRs, leading to a desensitization of a larger population of the receptors. In this review, mechanistic aspects of GPCR* phosphorylation related to the distinct properties, regulation and modes of action of GRKs are described.

Rhodopsin phosphorylation in bovine rod outer segments is more sensitive to the inhibitory action of recoverin at the low rhodopsin bleaching than it is at the high bleaching

Recoverin, a calcium-binding protein, is supposed to have rhodopsin kinase as a target in the retinal rod cell. In the present work, we show that efficiency of recoverin as an inhibitor of rhodopsin phosphorylation in bovine rod outer segments is inversely proportional to the level of rhodopsin bleaching. These results, together with the data obtained previously in a reconstituted system (Senin et al. (1997) Biochem. J. 321, 551-555), allow us to hypothesize that recoverin might be responsible for a Ca2(+)-dependent regulation of the kinase in vivo, preventing it from participating in the phosphorylation of unbleached rhodopsin.