Melatonin induces alterations in protein expression in the Xenopus laevis retina

The hormone melatonin is synthesized by pinealocytes and retinal photoreceptors with a diurnal rhythm. Melatonin produced in the retina at night is thought to exert local modulatory effects by binding to specific receptors in several different retinal cell types. The mechanisms by which melatonin influences circadian activity in retinal cells is poorly understood. Suppression of cyclic AMP synthesis appears to be a major signaling pathway in response to melatonin receptor binding in many tissues. A potential downstream consequence of melatonin-induced changes in cyclic AMP concentrations and protein phosphorylation is the up- or down-regulation of expression of specific genes. In this report, we examined the changes in expression levels of specific proteins in the neural retina and retinal pigment epithelium (RPE) in response to melatonin treatment, because both of these tissues express melatonin receptors. Neural retina and RPE isolated from the eyes of Xenopus laevis were treated with or without 1 microM melatonin for 6 hr, then the rapidly synthesized tissue proteins were radiolabeled by a 15 min incubation with 35S-methionine, and the proteins were subsequently analyzed by two-dimensional gel electrophoresis and autoradiography. In both the neural retina and RPE, the densities of some specific proteins were altered in response to melatonin treatment, and the few protein spots that were altered were distinct between the two tissues. These results support the concept that one function of melatonin may be to regulate the expression of specific genes and the consequent protein levels, and that the target genes may differ according to the cell or tissue type.

Transgenic Bcl-2 expressed in photoreceptor cells confers both death-sparing and death-inducing effects

To examine its potential role within the retina as a modulator of cell death and photoreceptor degeneration, bcl-2 expression was targeted to the photoreceptors of transgenic mice by the human IRBP promoter. Three transgenic families were established, with levels of transgene expression between 0.2 and two-fold relative to that of endogenous bcl-2. The effect of bcl-2 expression on genetically programmed photoreceptor degeneration was evaluated by crossing these transgenic mice with mice that develop a rapid degeneration of rod photoreceptors due to expression of a distinct transgene, SV40 T antigen (Tag). Transgenic Bcl-2 was localized to photoreceptor inner segments and was capable of abrogating the activation of caspase activity and the resulting cell death associated with ectopic expression of Tag. However, Bcl-2 itself ultimately caused photoreceptor cell death and retinal degeneration. Several proteins not expressed normally in Tag or other transgenic retinas undergoing photoreceptor degeneration were induced in the Bcl-2 transgenic retinas. Analysis by mass spectroscopy identified one of these proteins as alphaA-crystallin, a member of a protein family that associates with cellular stress. Since Bcl-2 can promote as well as spare cell death in the same photoreceptor population, its potential utility in ameliorating photoreceptor death in human hereditary blinding disorders is compromised.

The emerging role of mass spectrometry in molecular biosciences: studies of protein phosphorylation in fly eyes as an example

Modern mass spectrometry (MS) streamlined with two-dimensional gel electrophoresis, in-gel digestion and HPLC-interfaced electrospray ionization quadrupole MS or matrix-assisted laser desorption ionization time-of-flight MS enables us to analyse proteins at a minuscule scale. We present here two examples of MS applications in which (1) we identified the in vivo phosphorylation site of Drosophila arrestin, phosrestin I (PRI), and (2) we revealed the identity of an 80 kDa phosphoprotein (80K) in Drosophila eyes to be the InaD gene product, a member of the PDZ domain proteins. Available evidence suggests that PRI quenches the activation of rhodopsin and that the InaD protein adjusts photoreceptor responsiveness by assembling/disassembling components involved in photoreceptor transduction in flies. PRI undergoes a reversible phosphorylation at a single site, and 80K at multiple sites. The phosphorylation states of PRI and 80K depend on the intensity and/or duration of light stimuli. From these results we postulate that these proteins function as a molecular switch adjusting the signalling cascade through phosphorylation. The combination of two-dimensional gel electrophoresis with MS will be a powerful tool for detailed investigation of such complex switching processes. The techniques described here can be applied also to other complex signalling systems.

Ultrastructural localization of beta-arrestin-1 and -2 in rat lumbar spinal cord

beta-arrestins play significant roles in agonist-mediated desensitization of G protein-coupled receptors. Although the presence of beta-arrestin subtypes, beta-arrestin-1 and(- 2) in rat brain has been studied extensively, their existence in the spinal cord has not been described. In the current study, we performed immunohistochemical analyses of beta-arrestins at both light and electron microscopic levels using rat lumbar 1-2 spinal cord segments. Intense immunoreactivity for beta-arrestin-1 was found in the motoneurons in lamina IX of the ventral horn and elongated cells in the dorsal nucleus of Clarke. Modest immunoreactivity was detected among the neurons of laminae V and VII/VIII, and weaker immunoreactivity in laminae III, IV, and X. beta-arrestin-2 immunoreactivity was also distributed through laminae III-X in the order of IX > dorsal nucleus of Clarke > V > VII/VIII > IV > III > X. Laminae I and II did not show immunoreactivity. At the electron microscopic level, both beta-arrestin-immunoreactive and nonimmunoreactive dendrites were observed, whereas axons and terminal boutons were devoid of immunoreactivity. In immunoreactive dendrites most beta-arrestin immunoreactivity was distributed throughout the cytoplasm, demonstrating their association with microtubules. In addition, strong immunoreactivity was often found at postsynaptic densities. Our results thus suggest beta-arrestins' possible involvement in both motor and sensory mechanisms at the postsynaptic level in rat lumbar spinal cord.

Predominant presence of beta-arrestin-1 in small sensory neurons of rat dorsal root ganglia

Reverse transcription-polymerase chain reaction and western immunoblot analyses were performed to demonstrate the presence of beta-arrestin-1 in rat dorsal root ganglion. beta-Arrestin-1 existed as two alternatively spliced variants, although predominantly in its untruncated form. Several factors affected the visualization of the truncated version on a sodium dodecyl sulfate-polyacrylamide gel; however, the isoform was clearly detected on a two-dimensional gel. We further localized beta-arrestin-1 immunoreactivity in the sensory neurons of the 5th lumbar dorsal root ganglia. Beta-arrestin-1-immunoreactive neurons accounted for approximately 60% of the sensory neurons, and approximately 88% of the beta-Arrestin-1 immunoreactive neurons fell into a category of small neurons having a diameter of 10-30 microm. Members of the arrestin superfamily play crucial roles in the desensitization of G protein-coupled receptors. Our data demonstrating the presence of beta-arrestin-1 in the rat dorsal root ganglion at both messenger RNA and protein levels support the idea that beta-arrestin- participates in receptor desensitization in the sensory neurons. Furthermore, because small-size neurons of dorsal root ganglion are often implicated in nociception, the predominant presence of beta-arrestin-1 immunoreactivity in small-size sensory neurons suggests that beta-arrestin-1 may have a role modulating nociceptive signals.

Initiating ocular proteomics for cataloging bovine retinal proteins: microanalytical techniques permit the identification of proteins derived from a novel photoreceptor preparation

Though some mechanisms of photoreception have been well characterized, others remain obscure. Presumably, most, if not all, of the major players in photoreceptor-specific functions are present in large amounts in the photoreceptor layer, and a catalog of these proteins will prove a useful tool for vision researchers. As a first step toward a complete catalog of photoreceptor cells, we have developed a novel method for isolating the photoreceptor cell monolayer from bovine retina. Electron microscopic studies of both the photoreceptor layer and the residual retina from which the photoreceptor layer had been removed, indicate that the preparation contains the photoreceptor outer segments and the majority of the inner segments. Proteins were extracted from the isolated photoreceptor cell layer as well as the rest of the retina with isoelectric focusing lysis buffer, and the protein components were separated by two-dimensional gel electrophoresis. The obtained protein maps reveal several classes of proteins that appear to be expressed more abundantly or specifically in the photoreceptor layer than in the rest of the retina. Four of these protein spots were excised and in-gel digested with trypsin, and the digests were extracted with solvent. The mixture of peptides digested from each protein was analyzed by high performance liquid chromatography interfaced with electrospray ionization tandem quadrupole mass spectrometry or by matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Some of the peptides were isolated and their sequences were determined by gas phase Edman degradation. RNA transcripts extracted from the photoreceptor layer or the whole retina were subjected to Northern blot analysis as well as to reverse transcriptase-polymerase chain reaction amplification of probes for the successful selection of cDNA clones. These data permit both the identification of virtually any protein detectable on a two-dimensional gel, and also enable the corresponding cDNA clone to be selected. We have validated this approach by identifying aspartate aminotransferase and creatine kinase from the populations of abundant photoreceptor layer proteins. Both aspartate aminotransferase and creatine kinase are of mitochondrial origin and are thought to play crucial roles in photoreceptor functions by producing glutamate and ATP, respectively. We also identified two photoreceptor layer specific proteins: an acidic and high molecular weight protein, interphotoreceptor retinoid-binding protein, and an acidic and small molecular weight protein, recoverin.The technique presented here will allow vision researchers to discover and identify the proteins that are expressed specifically or abundantly in the photoreceptor cell as well as the proteins that undergo post-translational modification or modulation in expression under a defined biological condition. With the use of this technology, we anticipate that a researcher who knows only the 2-D gel position of a protein of interest can identify the protein, isolate a cDNA clone, and move into molecular genetic studies. Moreover, this streamlined technology will enable one to assemble a catalog of photoreceptor proteins using a minute amount of materials in a short period of time. We believe that such a catalog will serve as a valuable resource for vision investigators and will accelerate the rate of research progress.

Differential expression of alternative splice variants of beta-arrestin-1 and -2 in rat central nervous system and peripheral tissues

Members of arrestin/beta-arrestin protein family are thought to participate in agonist-mediated desensitization of G-protein-coupled receptors, including rhodopsin and beta2-adrenergic receptor. Unlike in human and cow, splice variants of this protein family in rat have not been studied extensively, and there has been no report on their existence at protein level. Hence, a previous report by others on the localization of both beta-arrestin-1 and -2 in a wide range of innervated rat tissues could imply their broad receptor specificity. In this report we show the presence of two alternatively spliced forms of beta-arrestin-1 in several rat tissues using both reverse transcription-polymerase chain reaction and Western immunoblot. Splicing of beta-arrestin-1 pre-mRNA appears to be subject to differential regulation between the rat CNS and peripheral tissues. In contrast, we detected no splice variants of beta-arrestin-2 in rat. A comparison of the genomic DNA sequences of bovine and rat beta-arrestin-2, where the splicing of bovine beta-arrestin-2 mRNA has been reported, revealed a high degree of homology in their organization of exons and introns as well as certain differences that might be responsible for the different processing of beta-arrestin-2 mRNA in the two species. Our two-dimensional isoelectric focusing gels using rat spinal cord and heart tissues demonstrate isoelectric heterogeneity of rat beta-arrestin-1, suggesting that beta-arrestin-1 is subject to post-translational modification unlike beta-arrestin-2.

Nonradioactive phosphopeptide assay by matrix-assisted laser desorption ionization time-of-flight mass spectrometry: application to calcium/calmodulin-dependent protein kinase II

Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF) was used to quantify the phosphopeptide produced by calcium/calmodulin-dependent protein kinase II (CaMK II). MALDI-TOF measurements were performed in a linear and positive ion mode with delayed extraction excited at various laser powers and at different sampling positions, i.e., different loci of laser illumination. We find that the ratio of the peak area of the substrate (S) to that of its monophosphorylated form (SP) for a given mixture is constant, independent of the laser powers and/or of the sample loci illuminated by the laser. We also find that the fraction of phosphorylation determined by MALDI-TOF, or fMALDI-TOF, is proportionally smaller than that determined by HPLC, or fHPLC; the ratio fMALDI-TOF/fHPLC was 0.797 +/- 0.0229 (99% confidence limit, n = 7) for a 30-mer peptide substrate used in this study. A low mass gate, which turns off the detector temporarily, improved the ratio fMALDI-TOF/fHPLC to 0.917 +/- 0.0184 (99% confidence limit, n = 7). Our interpretation of this result is that the reduction of the phosphopeptide peak in the MALDI-TOF measurement is likely to be caused by a temporal loss of detector function rather than by a lower efficiency of ionization for the phosphopeptide compared with its parent species. In these measurements the experimental errors, up to the 50% phosphorylation state, were less than 5%. After an adjustment made based on the fMALDI-TOF/fHPLC ratio of 0.917, MALDI-TOF gave an accurate measurement for the kinetics of the CaMK II phosphorylation reaction. Since only a small volume of the reaction mixture, typically containing 3 to 50 pmol of substrate, is required for the MALDI-TOF measurement, this method can be adapted to a nonradioactive microscale assay for CaMK II and also for other protein kinases.

Reversible ischemia increases levels of Alzheimer amyloid protein precursor without increasing levels of mRNA in the rabbit spinal cord

In a rabbit spinal cord ischemia model (RSCIM), the time courses of neuropathological damage of the spinal cord and neurological impairment of the motor functions are well established, demonstrating that the extent of neuropathological damage and the severity of neurological impairment are closely correlated. We used the RSCIM to elucidate the effects of reversible (15 min) and irreversible (60 min) ischemia on the endogenous levels of amyloid protein precursors (APPs) at both the mRNA and protein levels in the caudolumbar/sacral region of the spinal cord. We speculate that endogenous APPs are induced by ischemia as either trophic factors or stress-induced proteins in the RSCIM. A 15-min occlusion transiently increased the APP protein levels in neurons, which returned to the original levels by the end of 60 min occlusion. The increase in APP protein levels during 15-min ischemic insult does not appear to involve regulation at the mRNA level. The increased level of APPs, particularly of the soluble form, could support the possibility that APPs play a neuroprotective role in the RSCIM as stress-induced proteins. In contrast, failure to maintain the increased APP protein levels or to increase the mRNA, as seen in the 60-min ischemia samples, may be one of the causal factors that induce necrosis and neuronal cell death leading to irreversible neurological impairment.

Residues within the polycationic region of cGMP phosphodiesterase gamma subunit crucial for the interaction with transducin alpha subunit. Identification by endogenous ADP-ribosylation and site-directed mutagenesis

Interaction between the gamma subunit (Pgamma) of cGMP phosphodiesterase and the alpha subunit (Talpha) of transducin is a key step for the regulation of cGMP phosphodiesterase in retinal rod outer segments. Here we have utilized a combination of specific modification by an endogenous enzyme and site-directed mutagenesis of the Pgamma polycationic region to identify residues required for the interaction with Talpha. Pgamma, free or complexed with the alphabeta subunit (Palphabeta) of cGMP phosphodiesterase, was specifically radiolabeled by prewashed rod membranes in the presence of [adenylate-32P]NAD. Identification of ADP-ribose in the radiolabeled Pgamma and radiolabeling of arginine-replaced mutant forms of Pgamma indicate that both arginine 33 and arginine 36 are similarly ADP-ribosylated by endogenous ADP-ribosyltransferase, but only one arginine is modified at a time. Pgamma complexed with Talpha (both GTP- and GDP-bound forms) was not ADP-ribosylated; however, agmatine, which cannot interact with Talpha, was ADP-ribosylated in the presence of Talpha, suggesting that a Pgamma domain containing these arginines is masked by Talpha. A Pgamma mutant (R33,36K), as well as wild type Pgamma, inhibited both GTP hydrolysis of Talpha and GTP binding to Talpha. Moreover, GTP-bound Talpha activated Palphabeta that had been inhibited by R33,36K. However, another Pgamma mutant (R33,36L) could not inhibit these Talpha functions. In addition, GTP-bound Talpha could not activate Palphabeta inhibited by R33,36L. These results indicate that a Pgamma domain containing these arginines is required for its interaction with Talpha, but not with Palphabeta, and that positive charges in these arginines are crucial for the interaction.

Vitamin B6 modulates expression of albumin gene by inactivating tissue-specific DNA-binding protein in rat liver

The level of albumin mRNA in the liver of vitamin B6-deficient rats was found to be 7-fold higher than that of control rats. Since the transcriptional activity of the albumin gene, as measured by a nuclear run-on assay, was increased 5-fold in vitamin B6 deficiency, the higher concentration of albumin mRNA in the liver of vitamin-deficient rats could be attributed to the enhanced rate of transcription. The promoter proximal sequences of the albumin gene interact with a number of tissue-specific transcription factors including HNF-1 and C/EBP. We determined the binding activities of liver nuclear extracts to the HNF-1- and C/EBP-binding sites by gel mobility-shift assay and found that the activities of the extract prepared from liver of vitamin B6-deficient rats were greater than those of controls. As the concentrations of C/EBP in nuclear extracts from control and vitamin-deficient rats, estimated by Western-blot analysis, were essentially the same, the lower binding activity of the extract from control liver is probably due to inactivation of tissue-specific factors by pyridoxal phosphate and/or its analogues. We therefore examined the effect of pyridoxal phosphate and its analogues on the binding activity of nuclear extract in vitro and found that only pyridoxal phosphate effectively inhibited the binding. These observations indicate that vitamin B6 modulates albumin gene expression through a novel mechanism that involves inactivation of tissue-specific transcription factors by direct interaction with pyridoxal phosphate.

Pyridoxal 5'-phosphate modulates expression of cytosolic aspartate aminotransferase gene by inactivation of glucocorticoid receptor

The level of mRNA for cytosolic aspartate aminotransferase (cAST) in the liver of vitamin B6-deficient rats was found to be 7-fold higher than that of the control rats. The administration of hydrocortisone to adrenalectomized vitamin B6-deficient rats induced expression of hepatic cAST mRNA and the induction was suppressed by the simultaneous administration of pyridoxine. Since the 5' regulatory region of the rat cAST gene contains several sequences showing homology to glucocorticoid-responsive elements, we synthesized an oligonucleotide probe of glucocorticoid-responsive element sequence and assayed the binding activity of liver nuclear extract to the oligonucleotide by gel mobility shift analysis. We found that the binding activity of nuclear extract prepared from the liver of vitamin B6-deficient rats was far greater than that of the control rats, indicating that the DNA-binding activity of glucocorticoid receptor was enhanced by vitamin B6 deficiency. We further found that preincubation of the nuclear extract from the vitamin-deficient liver with pyridoxal 5'-phosphate brought about a rapid and extensive decrease in the binding of the extract to the glucocorticoid-responsive element. Congeners of pyridoxal phosphate, such as pyridoxamine 5'-phosphate, pyridoxal, pyridoxamine and pyridoxine, did not show an inhibitory effect. These observations suggest that pyridoxal 5'-phosphate modulates cAST gene expression by inactivating the binding activity of glucocorticoid receptor to glucocorticoid-responsive elements.

Phosrestin I, an arrestin homolog that undergoes light-induced phosphorylation in dipteran photoreceptors

Two classes of phosphorylated homologs of vertebrate arrestins, designated phosrestins I (PRI) and phosrestin II (PRII), are expressed in the photoreceptors of a fruit fly, Drosophila melanogaster. This study presents evidence that the housefly, Musca domestica, also has a protein similar to Drosophila PRI. Our conclusion is based on the following evidence. (1) We identified a Musca photoreceptor protein exhibiting a molecular mass (51 kDa) and an isoelectric point (pI = 8.6) similar to those of Drosophila PRI. This Musca protein, designated Musca PRI, changes its pI upon illumination in vivo. Drosophila PRI. This Musca protein, designated Musca PRI, changes its pI upon illumination in vivo. (2) Rabbit antibodies raised against Musca PRI, against bovine arrestin, and against a synthetic peptide based on the Drosophila PRI sequence stained the Drosophila and Musca PRIs specifically on 1 and 2-dimensional Western immunoblots. (3) Both Drosophila and Musca PRIs incorporated 32P-radioactivity from gamma-32P-ATP in cell-free homogenates of retinas. Partial peptide digestions of Drosophila and Musca PRIs revealed similarity between these proteins. We observed that Drosophila PRI exists in the random preparation, but it also exists in other subcellular fractions. Immunocytochemistry at the EM level revealed a distribution of both Drosophila and Musca PRI epitopes in membranous vesicular structures in the cytosol as well as in the rhabdomeric microvillar membranes where the visual pigment, rhodopsin, exists. Such distribution of PRI epitopes suggests that PRI and its light-dependent phosphorylation may function in a space remote from the rhabdomere as well as the immediate milieu of photoreception.

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.

Effect of vitamin B6 deficiency on the expression of glycogen phosphorylase mRNA in rat liver and skeletal muscle

The effect of vitamin B6 deficiency on the expression of glycogen phosphorylase mRNA in rat liver and skeletal muscle was investigated. The level of phosphorylase mRNA in the muscle of vitamin B6-deficient rats was reduced to 40% of that in the control rats. By contrast, the phosphorylase mRNA level was increased 5-fold in the liver of the deficient animals. It was also found that the expression of the beta-actin gene, generally regarded as a 'housekeeping' gene, was unaffected by B6 deficiency in the muscle but was enhanced in the liver of the deficient animals. These observations suggest that vitamin B6 may modulate the transcriptional activation of the phosphorylase gene in a tissue-specific manner.

Vitamin B6 deficiency causes activation of RNA polymerase and general enhancement of gene expression in rat liver

The effect of vitamin B6 deficiency on the activity of RNA polymerase and expression of several mRNAs in rat liver was investigated. The activities of RNA polymerase I and II in the liver of vitamin B6-deficient rats were found to be higher than the control rats by 30%. The expression of several mRNAs, including mRNAs for beta-actin and glyceraldehyde-3-phosphate dehydrogenase, and the content of poly(A)+ RNA were also increased in vitamin deficiency. These observations suggest that vitamin B6 influences gene expression in the liver, at least in part, by modulating the activity of RNA polymerase.

Developmental changes in the expression of HMG 2a protein

The levels of HMG 2a chromosomal protein and its mRNA change during the post-hatched development of chicks were investigated. The contents of both HMG 2a and 2b proteins of liver, heart, brain, muscle and gizzard were abundant in the newly hatched chicks but their contents decreased significantly in those tissues of the 70-day-old chicks. The HMG 2a mRNA levels of liver, heart and brain in 70-day-old chick decreased to about 40% of those mRNA in the newly hatched chicks while the HMG 2a mRNA levels of muscle and gizzard in the 70-day-old chicks increased 5- and 3-fold, respectively. These results suggest that the decrease in the HMG 2a protein contents of the muscle and gizzard in the 70-day-old chicks may be largely due to the stimulation of HMG 2a protein degradation or the reduction of HMG 2a mRNA translation.

Drosocrystallin, a major 52 kDa glycoprotein of the Drosophila melanogaster corneal lens. Purification, biochemical characterization, and subcellular localization

We have identified a 52 kDa protein, which is a potent substrate for cholera toxin-dependent ADP-ribosylation, in the compound eye preparation of the fruit fly, Drosophila melanogaster. We find that the 52 kDa protein is a glycoprotein and a Ca2+ binder bearing a high content of leucine, serine and glycine. By microsequencing we determined its 13 N-terminal sequence, AYL*PIDLNQLAK, with the asterisk representing an ambiguous signal. In order to study further the 52 kDa protein we have raised a polyclonal antibody against a synthetic oligopeptide representing the N-terminal 13 residues of the 52 kDa protein. By immunogold labelling with the antibody, the epitopes were localized at the EM level to the laminated corneal lens. The number of the gold particles per microns2 in the electron-dense layer of the corneal lens was 2.5 times higher than that of the electron-lucent layer. The pattern of the 52 kDa protein distribution in the corneal lens suggests that the 52 kDa protein is the major protein component that participates in the pattern formation of the alternate refractive indices of the D. melanogaster corneal lens. An X-ray dispersion analysis in situ revealed that the laminated corneal lens contained a higher concentration of Ca2+, supporting the hypothesis that the 52 kDa protein binds Ca2+ in vivo. To the best of our knowledge, this is the first report that identifies the protein entity of an arthropod corneal lens. We propose to designate this 52 kDa protein drosocrystallin.

A 49-kilodalton phosphoprotein in the Drosophila photoreceptor is an arrestin homolog

The gene encoding the 49-kilodalton protein that undergoes light-induced phosphorylation in the Drosophila photoreceptor has been isolated and characterized. The encoded protein has 401 amino acid residues and a molecular mass of 44,972 daltons, and it shares approximately 42 percent amino acid sequence identity with arrestin (S-antigen), which has been proposed to quench the light-induced cascade of guanosine 3',5'-monophosphate hydrolysis in vertebrate photoreceptors. Unlike the 49-kilodalton protein, however, arrestin, which appears to bind to phosphorylated rhodopsin, has not itself been reported to undergo phosphorylation. In vitro, Ca2+ was the only agent found that would stimulate the phosphorylation of the 49-kilodalton protein. The phosphorylation of this arrestin-like protein in vivo may therefore be triggered by a Ca2+ signal that is likely to be regulated by light-activated phosphoinositide-specific phospholipase C.