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At least 19 recordsLinked to original sources
PubMed1990

Amino acid and cDNA sequence of bovine phosducin, a soluble phosphoprotein from photoreceptor cells.

Vertebrate photoreceptor cells contain a soluble phosphoprotein, phosducin, which complexes with the beta, gamma subunits of the GTP-binding protein, transducin. Light-induced changes in cyclic nucleotide levels modulate the phosphorylation of phosducin by protein kinase A. The complete amino acid sequence of purified phosducin from bovine retinas was determined by Edman degradation from overlapping polypeptides derived from enzymatic digestion by trypsin and Staphylococcus aureus V8 protease or from chemical degradation by cyanogen bromide. Excluding the unidentified group which blocks the NH2 terminus, phosducin contains 245 amino acids with a calculated molecular weight of 28,185 and isoelectric point of pH 4.5. Phosducin is enriched with acidic and sulfur-containing amino acids, having 32 glutamic acid, 16 aspartic acid, 9 methionine, and 5 cysteine residues. It also contains 24 serine and 8 threonine residues, of which only serine 73 is located within a consensus phosphorylation sequence (-RKMS(P)QV-) for cyclic nucleotide-dependent protein kinase. Secondary structure analysis predicts the presence of 62% alpha-helix, 22% beta-sheet, and 16% random coil, with eight turns. Computer-aided searches of protein data banks revealed no apparent homology to any sequenced protein except that coded by a MEKA cDNA clone (Kuo, C-H., Akiyama, M., and Miki, N. (1989) Mol. Brain Res. 6, 1-10) which deviates from the confirmed phosducin sequence in the last 15 amino acids. Sequence analysis of a cDNA clone for bovine retinal phosducin confirmed that the MEKA clone deviation resulted from an unidentified cDNA guanosine nucleotide, a shifted reading frame and a premature stop codon.

Amino Acid Sequence↗
PubMed2001

Activation, deactivation, and adaptation in vertebrate photoreceptor cells.

Visual transduction captures widespread interest because its G-protein signaling motif recurs throughout nature yet is uniquely accessible for study in the photoreceptor cells. The light-activated currents generated at the photoreceptor outer segment provide an easily observed real-time measure of the output of the signaling cascade, and the ease of obtaining pure samples of outer segments in reasonable quantity facilitates biochemical experiments. A quiet revolution in the study of the mechanism has occurred during the past decade with the advent of gene-targeting techniques. These have made it possible to observe how transduction is perturbed by the deletion, overexpression, or mutation of specific components of the transduction apparatus.

Animals↗
PubMed1998

Vertebrate photoreceptor cell development and disease.

Photoreceptors provide an excellent model for studies of vertebrate neuronal differentiation, and many human diseases resulting in blindness primarily affect photoreceptors. There is therefore great interest in studying the cellular and molecular mechanisms of photoreceptor development. This article discusses our current understanding of this process, including the recent discovery of the homeodomain transcription factor Crx and its potential role in diseases affecting human vision.

Animals↗
PubMed1998

[Signal transduction in photoreceptor cells].

Vertebrate photoreceptors respond to light with a brief hyperpolarization of their membrane potential. In the dark, photoreceptors are depolarized by cation influx through channels in the plasma membrane which are kept open by the second messenger cGMP. Light absorption activates an enzyme cascade that hydrolytically destroys cGMP, resulting in channel closure and hyperpolarization of the membrane. In addition, processes are initiated that allow photoreceptors to adapt their sensitivity to the ambient illumination. Although these adaptational mechanisms are less well understood, it is clear that they are strongly controlled by the intracellular Ca2+ concentration. This review describes our present knowledge about the signal transduction and its fine tuning by a complex network of Ca(2+)-mediated processes in vertebrate photoreceptors.

Animals↗
PubMed1995

Involvement of cGMP and calcium in the photoresponse in vertebrate photoreceptor cells.

The biochemical and physiological events responsible for reaction of the rod photoreceptor cells in the vertebrate eye have provided insight into the mechanisms involved in signal transduction in other cells. The light-mediated changes in the photopigment, rhodopsin, in the photoreceptive region of these photoreceptors initiates a cascade of events that result in changes in conductance through a specific ligand-gated receptor. This process involves activation of a specific heterotrimeric GTP-binding protein, transducin, and is mediated by changes in the intracellular concentration of two intracellular second messengers, cGMP and calcium. The current hypotheses are discussed of the mechanisms involved in various aspects of the photoresponse, particularly photoexcitation, termination, and recovery.

Animals↗
PubMed2002

Centrins, a novel group of Ca2+-binding proteins in vertebrate photoreceptor cells.

Changes in the intracellular Ca2+-concentration affect the visual signal transduction cascade directly or more often indirectly through Ca2+-binding proteins. Here we review recent findings on centrins in photoreceptor cells of the mammalian retina. Centrins are members of a highly conserved subgroup of the EF-hand superfamily of Ca2+-binding proteins commonly associated with centrosome-related structures. In vertebrate photoreceptor cells, centrins are also prominent components in the connecting cilium linking the light sensitive outer segment with the biosynthetically active inner segment compartment. Recent findings demonstrate that Ca2+-activated centrin forms a complex with the visual G-protein transducin in photoreceptor cells. This Ca2+-dependent assembly of G-proteins with centrin is a novel aspect of the supply of signaling proteins in sensory cells, and a potential link between molecular translocations and signal transduction in general.

Amino Acid Motifs↗
PubMed1994

Functional coupling of a Ca2+/calmodulin-dependent nitric oxide synthase and a soluble guanylyl cyclase in vertebrate photoreceptor cells.

Electrophysiological recordings on retinal rod cells, horizontal cells and on-bipolar cells indicate that exogenous nitric oxide (NO) has neuromodulatory effects in the vertebrate retina. We report here endogenous NO formation in mammalian photoreceptor cells. Photoreceptor NO synthase resembled the neuronal NOS type I from mammalian brain. NOS activity utilized the substrate L-arginine (Km = 4 microM) and the cofactors NADPH, FAD, FMN and tetrahydrobiopterin. The activity showed a complete dependence on the free calcium concentration ([Ca2+]) and was mediated by calmodulin. NO synthase activity was sufficient to activate an endogenous soluble guanylyl cyclase that copurified in photoreceptor preparations. This functional coupling was strictly controlled by the free [Ca2+] (EC50 = 0.84 microM). Activation of the soluble guanylyl cyclase by endogenous NO was up to 100% of the maximal activation of this enzyme observed with the exogenous NO donor compound sodium nitroprusside. This NO/cGMP pathway was predominantly localized in inner and not in outer segments of photoreceptors. Immunocytochemically, we localized NO synthase type I mainly in the ellipsoid region of the inner segments and a soluble guanylyl cyclase in cell bodies of cone photoreceptor cells. We conclude that in photoreceptors endogenous NO is functionally coupled to a soluble guanylyl cyclase and suggest that it has a neuromodulatory role in visual transduction and in synaptic transmission in the outer retina.

Amino Acid Oxidoreductases↗
PubMed1990

Molecular cloning, primary structure, and orientation of the vertebrate photoreceptor cell protein peripherin in the rod outer segment disk membrane.

Peripherin, a 39-kDa membrane protein, has been previously localized to the rim region of the vertebrate rod photoreceptor disk membrane by use of monoclonal antibodies and immunocytochemical labeling techniques. As an initial step in determining the structure and function of this protein, we have cloned and sequenced cDNA containing its complete coding sequence. A bovine retinal lambda gt11 expression library was screened with the antibodies, and a 583 base pair clone was initially isolated. The remaining part of the coding sequence was obtained from subsequent rescreenings of the same library and an independent lambda gt10 library. A C-terminal CNBr fragment of peripherin was purified by immunoaffinity chromatography and reverse-phase high-performance liquid chromatography. The amino acid sequence of the isolated C-terminal peptide and the N-terminal sequence analysis of immunoaffinity-purified peripherin are in agreement with the cDNA sequence. The cDNA sequence predicts that there are possibly four transmembrane domains. On the basis of immunocytochemical studies and sequence analysis, the hydrophilic C-terminal segment containing the antigenic sites for the antiperipherin monoclonal antibodies has been localized on the cytoplasmic side of the disk membrane. There are three consensus sequences for asparagine-linked glycosylation. Deglycosylation studies have indicated that at least one of these sites is utilized. The possible function of peripherin in relation to its primary structure is discussed.

Amino Acid Sequence↗
PubMed2005

cGMP signaling in vertebrate retinal photoreceptor cells.

The visual transduction pathway in vertebrate photoreceptors transforms a light stimulus entering the photoreceptor outer segments into an electrical response at the synapses of rod and cone photoreceptor cells. This process is mediated by complex biochemical pathways that precisely regulate cGMP levels, thereby controlling the extent, duration, and adaptation of the photoreceptor to the light stimulus. This review first summarizes the major mechanisms of regulating cytoplasmic cGMP levels (synthesis, degradation, buffering, and efflux) as well as the primary targets of action of cGMP (cyclic nucleotide-gated ion channels, cGMP-dependent protein kinase, and cGMPregulated phosphodiesterases). This information is applied to our current understanding of how these processes operate in the signal-transducing outer segment of rod and cone photoreceptors to carry out visual excitation, recovery, and adaptation in response to light stimulation.

Animals↗
PubMed2002

Photoreceptor specific guanylate cyclases in vertebrate phototransduction.

Two membrane bound guanylate cyclases are expressed in vertebrate photoreceptor cells. They serve a key function in photoreceptor physiology as they synthesize the intracellular transmitter of photoexcitation guanosine 3',5'-cyclic monophosphate (cGMP). Both cyclases named ROS-GC1 and ROS-GC2 form a subclass of membrane bound cyclases and differ in many aspects from hormone peptide receptor guanylate cyclases. One unique feature is their regulation by three small Ca2+-binding proteins called GCAPs. These regulatory proteins sense changes in the cytoplasmic Ca2+-concentration [Ca2+] during illumination and activate ROS-GCs when the [Ca2+] decreases below the value in a dark adapted cell of 500-600 nM. Recent work has identified the target regions of GCAP-1 in ROS-GC1. In addition to GCAPs several other proteins including aktin, tubulin, a glutamic-acid-rich protein and a GTPase accelerating protein (RGS9) were found to interact with ROS-GC1 and probably form a multiprotein complex.

Adenosine Triphosphate↗
PubMed1991

Actin filaments and photoreceptor membrane turnover.

The shape and turnover of photoreceptor membranes appears to depend on associated actin filaments. In dipterans, the photoreceptor membrane is microvillar. It is turned over by the addition of new membrane at the bases of the microvilli and by subsequent shedding, mostly from the distal ends. Each microvillus contains actin filaments as a component of its cytoskeletal core. Two myosin I-like proteins co-localize with the actin filaments. It is suggested that one of the myosin I-like proteins might be linked to the microvillar membrane. By interacting with the actin filaments, this motor should move the membrane of a microvillus in a distal direction, thus providing a possible mechanism for the turnover of the membrane. A vertebrate photoreceptor cell contains a small cluster of actin filaments in its connecting cilium at the site where new transductive disk membranes are formed. Disruption of the actin filaments perturbs disk morphogenesis. The most likely explanation for this perturbation is that the process of initiating a new disk is inhibited. Conventional myosin (myosin II) is found in the connecting cilium with the same distribution as actin. A simple model is proposed to illustrate how the actin-myosin system of the connecting cilium might function to initiate the morphogenesis of a disk membrane.

Actins↗
PubMed1999

Modulation of rhythmic melatonin synthesis in Xenopus retinal photoreceptors by cyclic AMP.

Cyclic AMP regulates melatonin synthesis in vertebrate photoreceptor cells. In the present study, we investigated whether the circadian rhythm of melatonin synthesis in Xenopus retinal photoreceptor layers is driven by rhythmic changes in cyclic AMP. When the photoreceptor layers were continuously treated with 8-(4-chlorophenylthio)-cyclic AMP (8-CPT-cAMP) at a saturating concentration (1 mM), melatonin release was increased at all times of the day, but robust melatonin rhythms were maintained for 2 days in constant darkness (DD). We also measured cyclic AMP efflux and melatonin release simultaneously from photoreceptor layers that were continuously treated with forskolin and/or 3-isobutyl-1-methylxanthine (IBMX) in light-dark (LD) and DD. Circadian rhythmicity was observed in melatonin release, but not in cyclic AMP efflux, suggesting that changes of melatonin levels are not always caused by the changes of the cyclic AMP levels. In addition, the simultaneous treatment of forskolin and IBMX appeared to saturate sensitivity of melatonin synthesis to cyclic AMP, but this treatment did not abolish melatonin rhythms. These results suggest that circadian rhythms of melatonin can be driven without rhythmic changes of cyclic AMP, and that cyclic AMP regulates melatonin in parallel with the output pathways from the circadian oscillator.

1-Methyl-3-isobutylxanthine↗
PubMed2003

Rhodopsin phosphorylation: 30 years later.

Phototransduction in vertebrate photoreceptor cells mediated by rhodopsin is one of the most comprehensively examined G protein-coupled receptor (GPCR) signaling pathways. The signal transduction pathway can be mapped from the initial absorption of light to conformational changes within rhodopsin, through activation of the G protein transducin, and to the ultimate closure of the cation cGMP-gated channels in the plasma membrane. Furthermore, phototransduction has become an intensely studied model system for understanding the desensitizing processes that allow reduced non-linear responses of photoreceptor cells to increasing levels of illumination. Although some general themes appear to occur in GPCR systems, the details of these desensitizing processes are likely to be specific to each of the receptors. These differences are attributed to the fact that each receptor has unique kinetic constraints, amplification levels, tolerance to basal constitutive activity, intracellular internalization and recycling, redundancy of isoforms, and morphologies of the cell of their expression. One of the biochemical processes that are believed to be a common part of this desensitization of the GPCR-mediated cascade is receptor phosphorylation catalyzed by members of a small family of the GPCR kinases. The enzymatic, physiological and genetic aspects of rhodopsin phosphorylation and rhodopsin kinase have been characterized extensively over the last 30 yr. However, new structurally based approaches to examining rhodopsin kinase and rhodopsin phosphorylation are still awaiting further investigations. We present here a summary of the current understanding of rhodopsin phosphorylation and the properties of rhodopsin kinase, along with some expectations of future investigations into these topics.

Animals↗