Light-regulated proteins in Limulus ventral photoreceptor cells

Arrestin in ciliary invertebrate photoreceptors: molecular identification and functional analysis in vivo

Arrestin was identified in ciliary photoreceptors of Pecten irradians, and its role in terminating the light response was established electrophysiologically. Downstream effectors in these unusual visual cells diverge from both microvillar photoreceptors and rods and cones; the finding that key regulatory mechanisms of the early steps of visual excitation are conserved across such distant lineages of photoreceptors underscores that a common blueprint for phototransduction exists across metazoa. Arrestin was detected by Western blot analysis of retinal lysates, and localized in ciliary photoreceptors by immunostaining of whole-eye cryosections and dissociated cells. Two arrestin isoforms were molecularly identified by PCR; these present the canonical N- and C-arrestin domains, and are identical at the nucleotide level over much of their sequence. A high degree of homology to various β-arrestins (up to 70% amino acid identity) was found. In situ hybridization localized the two transcripts within the retina, but failed to reveal finer spatial segregation, possibly because of insufficient differences between the riboprobes. Intracellular dialysis of anti arrestin antibodies into voltage-clamped ciliary photoreceptors produced a gradual slow-down of the photocurrent falling phase, leaving a tail that decayed over many seconds after light termination. The antibodies also caused spectrally neutral flashes to elicit prolonged aftercurrents in the absence of large metarhodopsin accumulation; such aftercurrents could be quenched by chromatic illumination that photoconverts metarhodopsin back to rhodopsin. These observations indicate that the antibodies depleted functionally available arrestin, and implicate this molecule in the deactivation of the photoresponse at the rhodopsin level.

The actin-binding interface of a myosin III is phosphorylated in vivo in response to signals from a circadian clock

Class III unconventional myosins are critical for the normal function of auditory hair cells and the function and maintenance of photoreceptors; however, the roles of class III myosins in these sensory cells are unknown. Class III myosins are unique in that they have a kinase domain at their N-terminus; thus, they may have both signaling and motor functions. In the horseshoe crab Limulus polyphemus, enhanced phosphorylation of an abundant, photoreceptor specific class III myosin at night correlates with well-characterized circadian changes in photoreceptor structure and function. Thus, the Limulus visual system may be particularly useful for investigating the properties, modulation, and functions of a class III myosin. Previously, we showed that two sites within the actin interface of full-length Limulus myosin III expressed in baculovirus are substrates for both cyclic AMP-dependent protein kinase and autophosphorylation. In the current study, mass spectrometry was used to show that these same sites are phosphorylated in the endogenous protein extracted from Limulus lateral eye, and that enhanced phosphorylation at these sites occurs in vivo in response to natural circadian clock input to these eyes. These findings demonstrate in vivo changes in myosin III phosphorylation in response to a natural stimulus. This phosphorylation may modulate myosin III-actin interactions.

Loop 2 of limulus myosin III is phosphorylated by protein kinase A and autophosphorylation

Little is known about the functions of class III unconventional myosins although, with an N-terminal kinase domain, they are potentially both signaling and motor proteins. Limulus myosin III is particularly interesting because it is a phosphoprotein abundant in photoreceptors that becomes more heavily phosphorylated at night by protein kinase A. This enhanced nighttime phosphorylation occurs in response to signals from an endogenous circadian clock and correlates with dramatic changes in photoreceptor structure and function. We seek to understand the role of Limulus myosin III and its phosphorylation in photoreceptors. Here we determined the sites that become phosphorylated in Limulus myosin III and investigated its kinase, actin binding, and myosin ATPase activities. We show that Limulus myosin III exhibits kinase activity and that a major site for both protein kinase A and autophosphorylation is located within loop 2 of the myosin domain, an important actin binding region. We also identify the phosphorylation of an additional protein kinase A and autophosphorylation site near loop 2, and a predicted phosphorylation site within loop 2. We show that the kinase domain of Limulus myosin III shares some pharmacological properties with protein kinase A, and that it is a potential opsin kinase. Finally, we demonstrate that Limulus myosin III binds actin but lacks ATPase activity. We conclude that Limulus myosin III is an actin-binding and signaling protein and speculate that interactions between actin and Limulus myosin III are regulated by both second messenger mediated phosphorylation and autophosphorylation of its myosin domain within and near loop 2.

Sequential phosphorylation of visual arrestin in intactLimulusphotoreceptors: Identification of a highly light-regulated site

The visual arrestins in rhabdomeral photoreceptors are multifunctional phosphoproteins. They are rapidly phosphorylated in response to light, but the functional relevance of this phosphorylation is not yet fully understood. The phosphorylation ofLimulusvisual arrestin is particularly complex in that it becomes phosphorylated on three sites, and one or more of these site are phosphorylated even in the dark. The purpose of this study was to examine in detail the light-stimulated phosphorylation of each of the three sites inLimulusvisual arrestin in intact photoreceptors. We found that light increased the phosphorylation of all three sites (S377, S381, and S396), that S381is a preferred phosphorylation site, and that S377and S381are highly phosphorylated in the dark. The major effect of light was to increase the phosphorylation of S396, the site located closest to the C-terminal and very close to the adaptin binding motif. We speculate that the phosphorylation of this site may be particularly important for regulating the light-driven endocytosis of rhabdomeral membrane.

Circadian efferent input to Limulus eyes: anatomy, circuitry, and impact

Much is known about the anatomy of Limulus retinal efferent neurons and the structural and functional consequences of their activation. Retinal efferent axons arise from cell bodies located in the cheliceral ganglia of the brain, and they project out all of the optic nerves. Their unique neurosecretory-like terminals contact all cell types in lateral eye ommatidia, the retinular cells of the median eye, and the internal rhabdom of ventral photoreceptors. Lateral and median rudimentary photoreceptors are also innervated. The activity of the efferents is circadian. They are active during the subjective night and inactive during the subjective day. Activation of the efferents drives dramatic and diverse changes in the structure and function of Limulus eyes and causes the sensitivity and responsiveness of the eyes to light to increase at night. Relatively little is known about the molecular mechanisms that produce these structural and functional changes, but one efferent-activated biochemical cascade has been identified. The biogenic amine octopamine is released from efferent terminals, and an octopamine-stimulated rise in cAMP in photoreceptors, with a subsequent activation of cAMP-dependent protein kinase, mediates many of the known effects of efferent input. A photoreceptor-specific protein, myosin III, is phosphorylated in response to efferent input; this protein may play a role in the efferent stimulated changes in photoreceptor structure and function. Anatomical, biophysical, biochemical, and molecular approaches are now being effectively combined in studies of Limulus eyes; thus, this preparation should be particularly useful for further detailed investigations of mechanisms underlying the modulation of primary sensory cells by efferent input.

Immunocytochemical localization of opsin, visual arrestin, myosin III, and calmodulin in Limulus lateral eye retinular cells and ventral photoreceptors

The photoreceptors of the horseshoe crab Limulus polyphemus are classical preparations for studies of the photoresponse and its modulation by circadian clocks. An extensive literature details their physiology and ultrastructure, but relatively little is known about their biochemical organization largely because of a lack of antibodies specific for Limulus photoreceptor proteins. We developed antibodies directed against Limulus opsin, visual arrestin, and myosin III, and we have used them to examine the distributions of these proteins in the Limulus visual system. We also used a commercial antibody to examine the distribution of calmodulin in Limulus photoreceptors. Fixed frozen sections of lateral eye were examined with conventional fluorescence microscopy; ventral photoreceptors were studied with confocal microscopy. Opsin, visual arrestin, myosin III, and calmodulin are all concentrated at the photosensitive rhabdomeral membrane, which is consistent with their participation in the photoresponse. Opsin and visual arrestin, but not myosin III or calmodulin, are also concentrated in extra-rhabdomeral vesicles thought to contain internalized rhabdomeral membrane. In addition, visual arrestin and myosin III were found widely distributed in the cytosol of photoreceptors, suggesting that they have functions in addition to their roles in phototransduction. Our results both clarify and raise new questions about the functions of opsin, visual arrestin, myosin III, and calmodulin in photoreceptors and set the stage for future studies of the impact of light and clock signals on the structure and function of photoreceptors.

A myosin III from Limulus eyes is a clock-regulated phosphoprotein

The lateral eyes of the horseshoe crab Limulus polyphemus undergo dramatic daily changes in structure and function that lead to enhanced retinal sensitivity and responsiveness to light at night. These changes are controlled by a circadian neural input that alters photoreceptor and pigment cell shape, pigment migration, and phototransduction. Clock input to the eyes also regulates photomechanical movements within photoreceptors, including membrane shedding. The biochemical mechanisms underlying these diverse effects of the clock on the retina are unknown, but a major biochemical consequence of activating clock input to the eyes is a rise in the concentration of cAMP in photoreceptors and the phosphorylation of a 122 kDa visual system-specific protein. We have cloned and sequenced cDNA encoding the clock-regulated 122 kDa phosphoprotein and show here that it is a new member of the myosin III family. We report that Limulus myosin III is similar to other unconventional myosins in that it binds to calmodulin in the absence of Ca2+; it is novel in that it is phosphorylated within its myosin globular head, probably by cAMP-dependent protein kinase. The protein is present throughout the photoreceptor, including the region occupied by the photosensitive rhabdom. We propose that the phosphorylation of Limulus myosin III is involved in one or more of the structural and functional changes that occur in Limulus eyes in response to clock input.

The Limulus ventral photoreceptor: light response and the role of calcium in a classic preparation

The ventral nerve photoreceptor of the horseshoe crab Limulus polyphemus has been used for many years to investigate basic mechanisms of invertebrate phototransduction. The activation of rhodopsin leads in visual cells of invertebrates to an enzyme cascade at the end of which ion channels in the plasma membrane are transiently opened. This allows an influx of cations resulting in a depolarization of the photoreceptor cell. The receptor current of the Limulus ventral photoreceptor consists of three components which differ in several aspects, such as the time course of activation, the time course of recovery from light adaptation, and the reversal potential. Each component is influenced in a different, characteristic way by various pharmacological manipulations. In addition, at least two types of single photon-evoked events (bumps) and three elementary channel conductances are observed in this photoreceptor cell. These findings suggest that the receptor current components are controlled by three different light-activated enzymatic pathways using three different ligands to increase membrane conductance. Probably one of these ligands is cyclic GMP, another one is activated via the IP3-cascade and calcium, the third one might be cyclic AMP. Calcium ions are very important for the excitation and adaptation of visual cells in invertebrates. The extracellular and intracellular calcium concentrations determine the functional state of the visual cell. A rise in the cytosolic calcium concentration appears to be an essential step in the excitatory transduction cascade. Cytosolic calcium is the major intracellular mediator of adaptation. If the cytosolic calcium level exceeds a certain threshold value after exposure to light it causes the desensitization of the visual cell. On the other hand, from a slight rise in cytosolic calcium facilitation results, i.e. increased sensitivity of the photoreceptor.

Calcium/calmodulin-dependent protein kinase II and arrestin phosphorylation in Limulus eyes

In rhabdomeral photoreceptors, light stimulates the phosphorylation of arrestin, a protein critical for quenching the photoresponse, by activating a calcium/calmodulin-dependent protein kinase (CaM PK). Here we present biochemical evidence that a CaM PK that phosphorylates arrestin in Limulus eyes is structurally similar to mammalian CaM PK II. In addition, cDNAs encoding proteins homologous to mammalian and Drosophila CaM PK II in the catalytic and regulatory domains were cloned and sequenced from a Limulus lateral eye cDNA library. The Limulus sequences are unique, however, in that they lack most of the association domain. The proteins encoded by these sequences may phosphorylate arrestin.

Characterization of protein phosphatases type 1 and type 2A in Limulus nervous tissue: their light regulation in the lateral eye and evidence of involvement in the photoresponse

The activities of both protein phosphatases and protein kinases are responsible for the transient changes in the levels of phosphorylation and probably the functions of protein intermediates involved in the biochemical and physiological mechanisms underlying the photoresponse in photoreceptor cells from both vertebrate and invertebrate organisms. Of the known protein serine/threonine phosphatases, various forms of type 1 (PP 1) and type 2A (PP 2A) protein phosphatases are responsible for dephosphorylating many of the known phosphoproteins including those involved in photoreceptor cell function. In this report, we provide biochemical evidence for both PP 1- and PP 2A-like activities in the visual and nonvisual tissue of the horseshoe crab, Limulus polyphemus, that membrane and soluble forms of both enzymes are present, and that the activities of both enzymes are greater in light- than in dark-adapted lateral eyes. These activities were characterized using glycogen phosphorylase a, a substrate for both PP 1 and PP 2A, and various protein phosphatase inhibitors, including okadaic acid. We also report that okadaic acid, at concentrations required to inhibit PP 1, inhibited physiological functions of photoreceptor cells from the ventral eye, causing a delayed reduction of the resting membrane, and slowing and reducing light responses.

Distinguishing between roles for calcium in Limulus photoreceptor excitation

The role for Ca2+ in the excitation process by which light opens membrane channels in Limulus photoreceptors is discussed. Light initiates a phospholipase C/IP3 pathway that results in a rapid elevation of intracellular Ca2+, but whether this elevation is causal in triggering the light response or merely synergistic to some other second messenger pathway has been unclear. We have developed a procedure using progressive injection of Ca2+ buffers that distinguishes between mediation and synergy models [Shin J-H. Richard EA. Lisman JE. (1992) Ca2+ is an obligatory intermediate in the excitation cascade of Limulus photoreceptors. Neuron, 11, 845-855]. Our conclusion is that Ca2+ mediates all phases of the light-response. Models of this kind had previously been rejected because intracellular injection of Ca2+ buffer can lead to an increase of the late component (> 200 ms) of the response to bright, sustained light. We have used computer simulations of IP3 mediated Ca2+ release to show that the positive and negative regulation of this process by Ca2+ itself together with other feedback loops can explain counterintuitive effects of Ca2+ buffers.

Characterization of a calcium/calmodulin-dependent protein phosphatase in the Limulus nervous tissue and its light regulation in the lateral eye

Calcium (Ca2+) plays an integral role in the light response of the photoreceptors in both vertebrate and invertebrate organisms. In the ventral eye of the horseshoe crab, Limulus polyphemus, a flash of light delivered to a dark-adapted photoreceptor stimulates a rapid rise in intracellular free calcium concentration ([Ca2+]i), which in turn mediates light adaptation. It has previously been demonstrated that in Limulus photoreceptors light, via Ca2+, activates a calcium/calmodulin (Ca2+/CaM)-dependent protein kinase which increases the phosphorylation of arrestin. We now have identified biochemically, a calcium/calmodulin-dependent protein phosphatase (Ca2+/CaM PP) in homogenates of the Limulus lateral and ventral eye, brain, and lateral optic nerve using as a substrate, a 32P-labeled peptide fragment of the regulatory subunit of cAMP-dependent protein kinase (RII). This protein phosphatase shares biochemical properties with calcineurin, a Ca2+/CaM-dependent protein phosphatase (type-2B). Its activity is enhanced by Ca2+, calmodulin and Mn2+; and is inhibited by mastoparan, a calmodulin antagonist, and a synthetic peptide corresponding to the autoinhibitory domain of mammalian calcineurin. Most importantly, light regulates the Ca2+/CaM PP activity in the lateral eye. While there is no difference in basal activity in long-term dark- or light-adapted preparations, Ca2+ enhances Ca2+/CaM PP activity only in long-term light-adapted eyes.

Phosrestin I undergoes the earliest light-induced phosphorylation by a calcium/calmodulin-dependent protein kinase in Drosophila photoreceptors

Activation of PI-PLC initiates two independent branches of protein phosphorylation cascades catalyzed by either PKC or Ca2+/calmodulin-dependent protein kinase (CaMK). We find that phosrestin I (PRI), a Drosophila homolog of vertebrate photoreceptor arrestin, undergoes light-induced phosphorylation on a subsecond time scale which is faster than that of any other protein in vivo. We determine that a CaMK activity is responsible for in vitro PRI phosphorylation at Ser366 in the C-terminal tryptic segment, MetLysSer(P)IleGluGlnHisArg, in which Ser(P) represents phosphoserine366. We also demonstrate that Ser366 is the phosphorylation site of PRI in vivo by identifying the molecular species resulting from in-gel tryptic digestion of purified phospho-PRI using HPLC-electrospray ionization tandem quadrupole mass spectroscopy. From these data, we conclude that the CaMK pathway, not the PKC pathway, is responsible for the earliest protein phosphorylation event following activation of PI-PLC in living Drosophila photoreceptors.

Ca2+ is an obligatory intermediate in the excitation cascade of limulus photoreceptors

We have investigated the role of Ca2+ in the excitation of Limulus photoreceptors by intracellular injection of the Ca2+ buffer, 5,5'-dibromo-BAPTA. Buffer with free Ca2+ of 0.5 or 5 microM slowed the rising edge of the light response over 100-fold and greatly reduced both the transient and plateau phases of the light response, as expected if Ca2+ elevation is necessary for all phases of excitation. Injection of buffers with free Ca2+ of 5 or 45 microM, levels normally reached during light, evoked sustained inward current as expected if Ca2+ is sufficient for excitation. The transduction cascade appears due to a single pathway that sequentially involves 1,4,5-trisphosphate inositol, Ca2+, and cyclic GMP.

Isolation of a novel visual-system-specific arrestin: an in vivo substrate for light-dependent phosphorylation

Absorption of a photon of light by rhodopsin triggers mechanisms responsible for excitation as well as regulation of the phototransduction cascade. Arrestins are a family of proteins that appear to be responsible for terminating the active state of G-protein-coupled receptors. One of the major substrates of light-dependent phosphorylation in the visual cascade of Drosophila was purified and partially sequenced. The complete primary structure of the protein was determined by isolating the corresponding gene, which revealed it to be a new isoform of arrestin, Arr2. Arr2 is 401 residues in length, and shares 47% sequence identity with the Drosophila Arr1 protein and 42% with human arrestin. We show that the two Drosophila arrestin genes are differentially regulated, and that Arr2 is a specific substrate for a calcium-dependent protein kinase. This is the first demonstration of in vivo regulation of arrestins in a transduction cascade, and provides a new level of modulation in the function of G-protein-coupled receptors.

Calcium/calmodulin-stimulated phosphorylation of photoreceptor proteins in Limulus

Calcium (Ca2+) is thought to play a major role in the photoresponse of both vertebrates and invertebrates, but the mechanisms through which Ca2+ exerts its effects are unclear. In many systems, some effects of Ca2+ on cellular processes are thought to be mediated via activation of calcium/calmodulin protein kinase (CaCAM-PK) and the phosphorylation of specific proteins. Thus, protein substrates for CaCAM-PK in photoreceptor cells may be important in mediating the effects of Ca2+ on the photoresponse. In this study, we identify eight substrates for CaCAM-PK found in both the ventral and lateral eyes of Limulus. We focus on a characterization of one of these, a 46-kD substrate. We show that its subcellular distribution in ventral photoreceptors and its isoelectric forms are identical to the 46-kD light-stimulated phosphoprotein (46A) described by Edwards et al. (1989). Furthermore, we present evidence that 46A is unique to photoreceptor cells, and that it is present throughout the cell. Based on the results of this study, and the previous study by Edwards et al. (1989), we propose that 46A is involved in mediating the effects of Ca2+ on Limulus photoreceptor cell function, and that it may be involved in dark adaptation.