Light-induced modification of Drosophila retinal polypeptides in vivo

Unstructural Biology of TRP Ion Channels: The Role of Intrinsically Disordered Regions in Channel Function and Regulation

The first genuine high-resolution single particle cryo-electron microscopy structure of a membrane protein determined was a transient receptor potential (TRP) ion channel, TRPV1, in 2013. This methodical breakthrough opened up a whole new world for structural biology and ion channel aficionados alike. TRP channels capture the imagination due to the sheer endless number of tasks they carry out in all aspects of animal physiology. To date, structures of at least one representative member of each of the six mammalian TRP channel subfamilies as well as of a few non-mammalian families have been determined. These structures were instrumental for a better understanding of TRP channel function and regulation. However, all of the TRP channel structures solved so far are incomplete since they miss important information about highly flexible regions found mostly in the channel N- and C-termini. These intrinsically disordered regions (IDRs) can represent between a quarter to almost half of the entire protein sequence and act as important recruitment hubs for lipids and regulatory proteins. Here, we analyze the currently available TRP channel structures with regard to the extent of these "missing" regions and compare these findings to disorder predictions. We discuss select examples of intra- and intermolecular crosstalk of TRP channel IDRs with proteins and lipids as well as the effect of splicing and post-translational modifications, to illuminate their importance for channel function and to complement the prevalently discussed structural biology of these versatile and fascinating proteins with their equally relevant 'unstructural' biology.

Genetic dissection of the phosphoinositide cycle in Drosophila photoreceptors

Phototransduction in Drosophila is mediated by phospholipase C-dependent hydrolysis of PIP_2-, and is an important model for phosphoinositide signalling. Although generally assumed to operate by generic machinery conserved from yeast to mammals, some key elements of the phosphoinositide cycle have yet to be identified in Drosophila photoreceptors. Here, we used transgenic flies expressing fluorescently tagged probes (P4M and Tb^R332H), which allow in vivo quantitative measurements of PI4P and PIP₂ dynamics in photoreceptors of intact living flies. Using mutants and RNA interference for candidate genes potentially involved in phosphoinositide turnover, we identified Drosophila PI4KIIIα (CG10260) as the PI4-kinase responsible for PI4P synthesis in the photoreceptor membrane. Our results also indicate that PI4KIIIα activity requires rbo (the Drosophila orthologue of Efr3) and CG8325 (orthologue of YPP1), both of which are implicated as scaffolding proteins necessary for PI4KIIIα activity in yeast and mammals. However, our evidence indicates that the recently reported central role of dPIP5K59B (CG3682) in PIP₂ synthesis in the rhabdomeres should be re-evaluated; although PIP₂ resynthesis was suppressed by RNAi directed against dPIP5K59B, little or no defect was detected in a reportedly null mutant (dPIP5K¹⁸ ).

Phototransduction in Drosophila Is Compromised by Gal4 Expression but not by InsP3 Receptor Knockdown or Mutation

Drosophila phototransduction is mediated by phospholipase C, leading to activation of transient receptor potential (TRP) and TRP-like (TRPL) channels by mechanisms that are unresolved. A role for InsP₃ receptors (IP₃Rs) had been excluded because IP₃R mutants (itpr) appeared to have normal light responses; however, this was recently challenged by Kohn et al. (“Functional cooperation between the IP3 receptor and phospholipase C secures the high sensitivity to light of Drosophila photoreceptors in vivo,” Journal of Neuroscience 35:2530), who reported defects in phototransduction after IP₃R-RNAi knockdown. They concluded that InsP₃-induced Ca²⁺ release plays a critical role in facilitating channel activation, and that previous failure to detect IP₃R phenotypes resulted from trace Ca²⁺ in electrodes substituting for InsP₃-induced Ca²⁺ release. In an attempt to confirm this, we performed electroretinograms, whole-cell recordings, and GCaMP6f Ca²⁺ imaging from both IP₃R-RNAi flies and itpr-null mutants. Like Kohn et al., we used GMRGal4 to drive expression of UAS-IP₃R-RNAi, but we also used controls expressing GMRGal4 alone. We describe several GMRGal4 phenotypes suggestive of compromised development, including reductions in sensitivity, dark noise, potassium currents, and cell size and capacitance, as well as extreme variations in sensitivity between cells. However, we found no effect of IP₃R RNAi or mutation on photoreceptor responses or Ca²⁺ signals, indicating that the IP₃R plays little or no role in Drosophila phototransduction.

Speed and sensitivity of phototransduction in Drosophila depend on degree of saturation of membrane phospholipids

Drosophila phototransduction is mediated via a G-protein-coupled PLC cascade. Recent evidence, including the demonstration that light evokes rapid contractions of the photoreceptors, suggested that the light-sensitive channels (TRP and TRPL) may be mechanically gated, together with protons released by PLC-mediated PIP2 hydrolysis. If mechanical gating is involved we predicted that the response to light should be influenced by altering the physical properties of the membrane. To achieve this, we used diet to manipulate the degree of saturation of membrane phospholipids. In flies reared on a yeast diet, lacking polyunsaturated fatty acids (PUFAs), mass spectrometry showed that the proportion of polyunsaturated phospholipids was sevenfold reduced (from 38 to ∼5%) but rescued by adding a single species of PUFA (linolenic or linoleic acid) to the diet. Photoreceptors from yeast-reared flies showed a 2- to 3-fold increase in latency and time to peak of the light response, without affecting quantum bump waveform. In the absence of Ca(2+) influx or in trp mutants expressing only TRPL channels, sensitivity to light was reduced up to ∼10-fold by the yeast diet, and essentially abolished in hypomorphic G-protein mutants (Gαq). PLC activity appeared little affected by the yeast diet; however, light-induced contractions measured by atomic force microscopy or the activation of ectopic mechanosensitive gramicidin channels were also slowed ∼2-fold. The results are consistent with mechanosensitive gating and provide a striking example of how dietary fatty acids can profoundly influence sensory performance in a classical G-protein-coupled signaling cascade.

Light-dependent phosphorylation of the Drosophila inactivation no afterpotential D (INAD) scaffolding protein at Thr170 and Ser174 by eye-specific protein kinase C

Drosophila inactivation no afterpotential D (INAD) is a PDZ domain-containing scaffolding protein that tethers components of the phototransduction cascade to form a supramolecular signaling complex. Here, we report the identification of eight INAD phosphorylation sites using a mass spectrometry approach. PDZ1, PDZ2, and PDZ4 each harbor one phosphorylation site, three phosphorylation sites are located in the linker region between PDZ1 and 2, one site is located between PDZ2 and PDZ3, and one site is located in the N-terminal region. Using a phosphospecific antibody, we found that INAD phosphorylated at Thr170/Ser174 was located within the rhabdomeres of the photoreceptor cells, suggesting that INAD becomes phosphorylated in this cellular compartment. INAD phosphorylation at Thr170/Ser174 depends on light, the phototransduction cascade, and on eye-Protein kinase C that is attached to INAD via one of its PDZ domains.

Proteomics of Drosophila compound eyes: early studies, now, and the future--light-induced protein phosphorylation as an example

In the past three decades, efforts to understand the molecular mechanisms underlying photoreceptor transduction of the fruit fly Drosophila melanogaster experienced drastic waves of technological development that involve multiple areas of scientific disciplines; the multidisciplinary approach includes a classical genetic manipulation in which random mutations are created and phenotypes are screened, a modern genetics maneuver in which a specific gene relevant to a hypothesis is molecularly cloned and manipulated, and, more recently, direct studies of proteins by proteomics technologies in combination with modern molecular biology and electrophysiology. This paper will review efforts that originated three decades ago in Professor William L. Pak's laboratory at Purdue University to study proteins involved in the Drosophila photoreceptor transduction process and show the power of such multidisciplinary approach that involves collaboration between molecular genetics, electrophysiology, and proteomics.

The history of the prolonged depolarizing afterpotential (PDA) and its role in genetic dissection of Drosophila phototransduction

In invertebrate photoreceptors, the photopigment exhibits a long-lived and physiologically active photoproduct, called metarhodopsin (M). The long life of invertebrate M implies that under physiological conditions, M and the original pigment state rhodopsin, R, are in photoequilibrium. In many invertebrates, the absorption spectra of R and M states are different, allowing large photopigment conversion between R and M states. These net pigment molecules conversions between R and M are the basis of the prolonged depolarizing afterpotential (PDA) phenomenology, which is the main subject of this review. A large net conversion of R to M disrupts phototransduction termination at the photopigment level, which in turn results in sustained excitation long after the light is turned off. Throughout this period, the photoreceptors are partially desensitized and are insensitive (or less sensitive) to subsequent test lights. In Drosophila, the PDA tests the maximal capacity of the photoreceptor cell to maintain excitation for an extended period and is strictly dependent on the presence of high concentrations of rhodopsin and the transient receptor potential (TRP) channels. Therefore, it detects even minor defects in rhodopsin or TRP biogenesis and easily scores deficient replenishment of phototransduction components, which results in temporary desensitization of the phototransduction process. Indeed, the introduction and use of PDA to screen for phototransduction-defective Drosophila mutants by Pak and colleagues yielded a plethora of new and most interesting visual mutants. Remarkably, to this day, the PDA mutants that Pak and his colleagues isolated are the main source of mutants for analysis of the Drosophila visual system.

Retinophilin is a light-regulated phosphoprotein required to suppress photoreceptor dark noise in Drosophila

Photoreceptor cells achieve high sensitivity, reliably detecting single photons, while limiting the spontaneous activation events responsible for dark noise. We used proteomic, genetic, and electrophysiological approaches to characterize Retinophilin (RTP) (CG10233) in Drosophila photoreceptors and establish its involvement in dark-noise suppression. RTP possesses membrane occupation and recognition nexus (MORN) motifs, a structure shared with mammalian junctophilins and other membrane-associated proteins found within excitable cells. We show the MORN repeats, and both the N- and C-terminal domains, are required for RTP localization in the microvillar light-gathering organelle, the rhabdomere. RTP exists in multiple phosphorylated isoforms under dark conditions and is dephosphorylated by light exposure. An RTP deletion mutant exhibits a high rate of spontaneous membrane depolarization events in dark conditions but retains the normal kinetics of the light response. Photoreceptors lacking neither inactivation nor afterpotential C (NINAC) myosin III, a motor protein/kinase, also display a similar dark-noise phenotype as the RTP deletion. We show that NINAC mutants are depleted for RTP. These results suggest the increase in dark noise in NINAC mutants is attributable to lack of RTP and, furthermore, defines a novel role for NINAC in the rhabdomere. We propose that RTP is a light-regulated phosphoprotein that organizes rhabdomeric components to suppress random activation of the phototransduction cascade and thus increases the signaling fidelity of dark-adapted photoreceptors.

Novel eye-specific calmodulin methylation characterized by protein mapping in Drosophila melanogaster

Post-translational methylation of the epsilon-amino group of lysine residues regulates a number of protein functions. Calmodulin, a key modulator of intracellular calcium signaling, is methylated on lysine 115 in many species. Although the amino acid sequence of calmodulin is highly conserved in eukaryotes, it has been shown that lysine 115 is not methylated in Drosophila calmodulin and no other methylation site has been reported. In this study, we characterized in vivo modification states of Drosophila calmodulin using proteomic methodology involving the protein mapping of microdissected Drosophila tissues on 2-D gels. We found that Drosophila calmodulin was highly expressed in methylated forms in the compound eye, whereas its methylation was hardly detected in other tissues. We identified that lysine 94 located in an EF-hand III is the methylation site in Drosophila calmodulin. The predominance of methylated calmodulin in the compound eye may imply the involvement of calmodulin in photoreceptor-specific functions through methylation.

Application of fluorescence difference gel electrophoresis technology in searching for protein biomarkers in chick myopia

The lens-induced myopia (LIM) in response to concave lens (negative lens) is a well established animal model for studying myopia development. However, the exact visual and neurochemical signaling mechanisms involving myopic eye growth are yet to be elucidated. The feasibility of applying a novel two-dimensional fluorescence difference gel electrophoresis technique for global protein profilings and a search for differential protein expressions in LIM were explored in the present study. Two-dimensional polyacrylamide gel electrophoresis was performed employing a "minimal Lysine labeling" approach and a reverse CyeDye experimental protocol using retinal tissue from chicks. The retinal protein profiles between myopic and control eyes were found to be very similar. More than a thousand protein spots could be detected on a 2D gel. Sixteen and ten protein spots were found to be up-regulated and down-regulated respectively in the myopic eyes according to our preset criteria with the inclusion of an internal pool standard. About 65% of those filtered spots could be successfully identified by peptide mass fingerprinting by matrix-assisted laser desorption ionization-time-of-flight mass spectrometry . Most of the differentially expressed proteins were found to be related to cytoskeletal or oxidative functions. According to the prediction of subcellular locations, most of them (about 84%) were classified as cytoplasmic proteins. The cellular functions for those differentially expressed proteins were reported and their possible involvements in the compensated eye growth were discussed. We have optimized a workable protocol for the study of the differential retinal protein expressions in the LIM using 2D-DIGE approach which was shown to have a number of advantages over the traditional 2D electrophoresis technique.

lazaro encodes a lipid phosphate phosphohydrolase that regulates phosphatidylinositol turnover during Drosophila phototransduction

An essential step in Drosophila phototransduction is the hydrolysis of phosphatidylinositol 4,5 bisphosphate PI(4,5)P2 by phospholipase Cbeta (PLCbeta) to generate a second messenger that opens the light-activated channels TRP and TRPL. Although the identity of this messenger remains unknown, recent evidence has implicated diacylglycerol kinase (DGK), encoded by rdgA, as a key enzyme that regulates its levels, mediating both amplification and response termination. In this study, we demonstrate that lazaro (laza) encodes a lipid phosphate phosphohydrolase (LPP) that functions during phototransduction. We demonstrate that the synergistic activity of laza and rdgA regulates response termination during phototransduction. Analysis of retinal phospholipids revealed a reduction in phosphatidic acid (PA) levels and an associated reduction in phosphatidylinositol (PI) levels. Together our results demonstrate the contribution of PI depletion to the rdgA phenotype and provide evidence that depletion of PI and its metabolites might be a key signal for TRP channel activation in vivo.

Highly sensitive multistage mass spectrometry enables small-scale analysis of protein glycosylation from two-dimensional polyacrylamide gels

Structural characterization of glycoproteins remains among the most challenging areas of glycomics due to the requirement of large quantities of samples and laborious biochemical steps involved in the analytical procedure. Here we report the structural characterization of glycoproteins separated on a 2-D gel by using a MALDI-QIT-TOF MS where QIT is quadrupole IT. The combination of MALDI-ion source and QIT appears to generate a unique tendency to cause fragmentation of glycopeptides without collision-induced dissociation. The majority of such fragmentations observed in our study result from the cleavage of sugar linkages, but not of peptide-peptide or peptide-sugar linkages. This unique feature allows us to perform pseudo-MS3 analysis of a fragmented glycopeptide. A small gel spot of a glycoprotein in the abundance range of low picomoles was enough for the mass spectrometer to analyze fragmentation pathway of the sugar linkage and peptide backbone. In this study, we demonstrate direct determination of glycosylation sites and N-linked glycan-sequences of the tryptic glycopeptides of Drosophila glycoproteins. Glycopeptides with various MWs up to approximately 4000 Da were suitable for structural analysis, including its attachment site and the amino acid sequence, of the glycopeptide through multistage mass spectrometric analysis.

Calcium homeostasis in fly photoreceptor cells

In fly photoreceptor cells, two processes dominate the Ca2+ homeostasis: light-induced Ca2+ influx through members of the TRP family of ion channels, and Ca2+ extrusion by Na+/Ca2+ exchange. Ca2+ release from intracellular stores is quantitatively insignificant. Both, the light-activated channels and the Ca2+-extruding exchangers are located in or close to the rhabdomeric microvilli, small protrusions of the plasma membrane. The microvilli also contain the molecular machinery necessary for generating quantum bumps, short electrical responses caused by the absorption of a single photon. Due to this anatomical arrangement, the light-induced Ca2+ influx results in two separate Ca2+ signals that have different functions: a global, homogeneous increase of the Ca2+ concentration in the cell body, and rapid but large amplitude Ca2+ transients in the microvilli. The global rise of the Ca2+ concentration mediates light adaptation, via regulatory actions on the phototransduction cascade, the voltage-gated K+ channels and small pigment granules controlling the light intensity. The local Ca2+ transients in the microvilli are responsible for shaping the quantum bumps into fast, all-or-nothing events. They achieve this by facilitating strongly the phototransduction cascade at early stages ofthe light response and subsequently inhibiting it. Many molecular targets of these feedback mechanisms have been identified and characterized due to the availability of numerous Drosophila mutant showing defects in the phototransduction.

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.

The formation of stable rhodopsin-arrestin complexes induces apoptosis and photoreceptor cell degeneration

Although many different mutations in humans and Drosophila cause retinal degeneration, in most cases, a molecular mechanism for the degeneration has not been found. We now demonstrate the existence of stable, persistent complexes between rhodopsin and its regulatory protein arrestin in several different retinal degeneration mutants. Elimination of these rhodopsin-arrestin complexes by removing either rhodopsin or arrestin rescues the degeneration phenotype. Furthermore, we show that the accumulation of these complexes triggers apoptotic cell death and that the observed retinal degeneration requires the endocytic machinery. This suggests that the endocytosis of rhodopsin-arrestin complexes is a molecular mechanism for the initiation of retinal degeneration. We propose that an identical mechanism may be responsible for the pathology found in a subset of human retinal degenerative disorders.

Normal phototransduction in Drosophila photoreceptors lacking an InsP(3) receptor gene

The Drosophila light-sensitive channels TRP and TRPL are prototypical members of an ion channel family responsible for a variety of receptor-mediated Ca(2+) influx phenomena, including store-operated calcium influx. While phospholipase Cbeta is essential, downstream events leading to TRP and TRPL activation remain unclear. We investigated the role of the InsP(3) receptor (InsP(3)R) by generating mosaic eyes homozygous for a deficiency of the only known InsP(3)R gene in Drosophila. Absence of gene product was confirmed by RT-PCR, Western analysis, and immunocytochemistry. Mutant photoreceptors underwent late onset retinal degeneration; however, whole-cell recordings from young flies demonstrated that phototransduction was unaffected, quantum bumps, macroscopic responses in the presence and absence of external Ca(2+), light adaptation, and Ca(2+) release from internal stores all being normal. Using the specific TRP channel blocker La(3+) we demonstrated that both TRP and TRPL channel functions were unaffected. These results indicate that InsP(3)R-mediated store depletion does not underlie TRP and TRPL activation in Drosophila photoreceptors.

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.

A role for the light-dependent phosphorylation of visual arrestin

Arrestins are regulatory proteins that participate in the termination of G protein-mediated signal transduction. The major arrestin in the Drosophila visual system, Arrestin 2 (Arr2), is phosphorylated in a light-dependent manner by a Ca2+/calmodulin-dependent protein kinase and has been shown to be essential for the termination of the visual signaling cascade in vivo. Here, we report the isolation of nine alleles of the Drosophila photoreceptor cell-specific arr2 gene. Flies carrying each of these alleles underwent light-dependent retinal degeneration and displayed electrophysiological defects typical of previously identified arrestin mutants, including an allele encoding a protein that lacks the major Ca2+/calmodulin-dependent protein kinase site. The phosphorylation mutant had very low levels of phosphorylation and lacked the light-dependent phosphorylation observed with wild-type Arr2. Interestingly, we found that the Arr2 phosphorylation mutant was still capable of binding to rhodopsin; however, it was unable to release from membranes once rhodopsin had converted back to its inactive form. This finding suggests that phosphorylation of arrestin is necessary for the release of arrestin from rhodopsin. We propose that the sequestering of arrestin to membranes is a possible mechanism for retinal disease associated with previously identified rhodopsin alleles in humans.

Requirement for the NINAC kinase/myosin for stable termination of the visual cascade

Activation of the Drosophila photoresponse is a rapid process that results in plasma membrane Ca2+ and Na+ conductances. Ca2+ functions in negative feedback regulation of Drosophila vision including deactivation. Protein kinase C (PKC) binds directly to Ca2+ and is required for deactivation. However, the consequences of disrupting phosphorylation of any individual PKC substrate in the Drosophila retina have not been addressed. In the current work, we show that NINAC p174, which consists of a protein kinase domain joined to the head region of myosin heavy chain, is a phosphoprotein and is phosphorylated in vitro by PKC. Mutation of either of two PKC sites in the p174 tail resulted in an unusual defect in deactivation that had not been detected previously for other ninaC alleles or other loci. After cessation of the light stimulus, there appeared to be a transient reactivation of the visual cascade. This phenotype suggests that a mechanism exists to prevent reactivation of the visual cascade and that p174 participates in this process.

Phosphorylation of the InaD gene product, a photoreceptor membrane protein required for recovery of visual excitation

In an approach directed to isolate and characterize key proteins of the transduction cascade in photoreceptors using the phosphoinositide signaling pathway, we have isolated the Calliphora homolog of the Drosophila InaD gene product, which in Drosophila InaD mutants causes slow deactivation of the light response. By screening a retinal cDNA library with antibodies directed against photoreceptor membrane proteins, we have isolated a cDNA coding for an amino acid sequence of 665 residues (Mr = 73,349). The sequence displays 65.3% identity (77.3% similarity) with the Drosophila InaD gene product. Probing Western blots with monospecific antibodies directed against peptides comprising amino acids 272-542 (anti-InaD-(272-542)) or amino acids 643-655 (anti-InaD-(643-655)) of the InaD gene product revealed that the Calliphora InaD protein is specifically associated with the signal-transducing rhabdomeral photoreceptor membrane from which it can be extracted by high salt buffer containing 1.5 M NaCl. As five out of eight consensus sequences for protein kinase C phosphorylation reside within stretches of 10-16 amino acids that are identical in the Drosophila and Calliphora InaD protein, the InaD gene product is likely to be a target of protein kinase C. Phosphorylation studies with isolated rhabdomeral photoreceptor membranes followed by InaD immunoprecipitation revealed that the InaD protein is a phosphoprotein. In vitro phosphorylation is, at least to some extent, Ca 2+ dependent and activated by phorbol 12-myristate 13-acetate. The inaC-encoded eye-specific form of a protein kinase C (eye-PKC) is co-precipitated by antibodies specific for the InaD protein from detergent extracts of rhabdomeral photoreceptor membranes, suggesting that the InaD protein and eye-PKC are interacting in these membranes. Co-precipitating with the InaD protein and eye-PKC are two other key components of the transduction pathway, namely the trp protein, which is proposed to form a Ca2+ channel, and the norpA-encoded phospholipase C, the primary target enzyme of the transduction pathway. It is proposed that the rise of the intracellular Ca2+ concentration upon visual excitation initiates the phosphorylation of the InaD protein by eye-PKC and thereby modulates its function in the control of the light response.

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.

Light-dependent in vivo phosphorylation of an inhibitory subunit of cGMP-phosphodiesterase in frog rod photoreceptor outer segments

In vivo phosphorylation of P gamma, an inhibitory subunit of cGMP-phosphodiesterase of frog (Rana catesbeiana) photoreceptor rod outer segments, was investigated using a quick-freezing technique and a newly developed method for the preparation of rod outer segments. Light-dependent phosphorylation of P gamma was observed. Okadaic acid, a potent inhibitor of protein phosphatases 1 and 2A, enhanced the apparent incorporation of 32P into P gamma, suggesting that P gamma is in equilibrium between phosphorylation and dephosphorylation. Neither phorbol ester, a potent activator of protein kinase C, nor changes in the extracellular Ca2+ concentration affected the in vivo phosphorylation of P gamma.

Light-induced binding of proteins to rhabdomeric membranes in the retina of crayfish (Procambarus clarkii)

Light-induced protein interaction as part of the process of visual transduction in arthropods with rhabdomeric photoreceptors was investigated biochemically by using crayfish retina. Two kinds of retinal buffer soluble proteins (one of 40 kDa and the other of 46 kDa) were found to bind to the irradiated rhabdomeric membranes both in vitro and in vivo. The proteins bound to the membranes in the presence of metarhodopsin. An antibody against mouse arrestin (S-antigen) cross-reacted with the 40 kDa protein. These results suggest that the binding of the proteins to the membranes is caused by the formation of metarhodopsin, and that the 40 kDa protein has a similar structure to arrestin.

Dependence of calmodulin localization in the retina on the NINAC unconventional myosin

Calmodulin is a highly conserved regulatory protein found in all eukaryotic organisms which mediates a variety of calcium ion-dependent signalling pathways. In the Drosophila retina, calmodulin was concentrated in the photoreceptor cell microvillar structure, the rhabdomere, and was found in lower amounts in the sub-rhabdomeral cytoplasm. This calmodulin localization was dependent on the NINAC (neither inactivation nor afterpotential C) unconventional myosins. Mutant flies lacking the rhabdomere-specific p174 NINAC protein did not concentrate calmodulin in the rhabdomere, whereas flies lacking the sub-rhabdomeral p132 isoform had no detectable cytoplasmic calmodulin. Furthermore, a defect in vision resulted when calmodulin was not concentrated in the rhabdomeres, suggesting a role for calmodulin in the regulation of fly phototransduction. A general function of unconventional myosins may be to control the subcellular distribution of calmodulin.

Drosophila homologs of two mammalian intracellular Ca(2+)-release channels: identification and expression patterns of the inositol 1,4,5-triphosphate and the ryanodine receptor genes

We have identified and cloned portions of two Drosophila genes homologous to two classes of mammalian intracellular Ca(2+)-release channels, the ryanodine receptor and the inositol 1,4,5-triphosphate (IP3) receptor. The Drosophila ryanodine receptor gene (dry) encodes an approx. 15 kb mRNA. It is expressed in the mesoderm of early stage-9 embryos and subsequently in somatic muscles and their precursor cells. In adults, dry mRNA was detected in tubular muscles and at a lower level in neuronal tissues. Embryonic expression of the Drosophila IP3 receptor gene (dip) appears more dynamic and is associated with developing anterior sense organs. In adults, dip expression occurs in several tissues, and relatively high levels of dip mRNA in adult antennae suggest a role for this gene product during olfactory transduction.

Phosrestins I and II: arrestin homologs which undergo differential light-induced phosphorylation in the Drosophila photoreceptor in vivo

The 49-kDa phosphoprotein gene and Dmarrestin gene encode two distinct arrestin homologs in the Drosophila photoreceptor. We find that two DNA fragments representing the Dmarrestin gene hybrid-selected a mRNA the in vitro translation of which produced a protein corresponding to the 39-kDa phosphoprotein previously reported by us. We propose to name these phosphorylated homologs of arrestin phosrestin I (49-kDa protein) and phosrestin II (39-kDa protein or the Dmarrestin gene product). We find that phosrestins I and II follow different time courses of phosphorylation in vivo; in the time period (approximate seconds) during which 43% of phosrestin I became phosphorylated, the phosphorylated state of phosrestin II remained unchanged from that of the nonilluminated flies. These results indicate that phosrestins I and II probably occupy different functional roles in the Drosophila photoreceptor.

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.

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.

The Drosophila ninaC locus encodes two photoreceptor cell specific proteins with domains homologous to protein kinases and the myosin heavy chain head

The fruit fly Drosophila melanogaster has been extensively used to identify genes required for photoreceptor cell function. We show that the ninaC gene, originally isolated as a Drosophila visual mutation with an electrophysiological phenotype, encodes two novel cytoskeletal proteins. We identified the DNA sequences encoding the ninaC gene by rescuing the electrophysiological phenotype using P-element-mediated germ line transformation. The ninaC locus is expressed as two extensively overlapping mRNAs encoding proteins of 1135 and 1501 amino acids. Both proteins contain a putative protein kinase domain joined to a domain homologous to the head region of the myosin heavy chain and are spatially restricted to photoreceptor cells.

Quantitative tissue isolation from Drosophila freeze-dried in acetone

Freeze-drying procedures were developed to enable collection of tissues from Drosophila flies. The flies were frozen in acetone at -86 or -94 degrees C, and dehydrated therein. After drying, many tissues could be easily taken in entirety and free of neighbouring tissues without action of degradative enzymes. Seven polypeptide species specific to retina, and nine specific to cornea, were identified on two-dimensional electrophoretograms. Phospholipids of the dried tissues could be studied by t.l.c., and phosphatidic acid of the fly head was found to occur predominantly in the retina. Activity of three enzymes in the dried tissues could be assayed. The results of protein, phospholipid and enzyme analyses were corroborated by analyses by 'genetic dissection' using an eyeless mutant line.

Electrophoretic analysis of proteins from night-blind mutants of Phycomyces

Using two-dimensional gel electrophoresis, we have analyzed proteins from a plasma membrane-enriched fraction from Phycomyces sporangiophores. Specifically, we have compared gels for night-blind mutants and a wild-type strain to find proteins involved in the early steps of the sensory transduction chain for phototropism. In the gels for a mutant affected in the gene madA, a protein spot [51 kilodaltons (kdal) and pI 6.35] appears that is absent from the wild-type and the other mad mutants. Mutants affect in either of two madB alleles lack a protein spot (57 kdal and pI 6.6) that is present in the wild-type and all other mad strains; this spot probably represents the madB gene product. In some madC mutants, two spots (59 kdal, pI 6.5, with a covalently linked flavin; and 50 kdal, pI 6.4) are absent; however, in other madC strains, one or both of these spots are present. These four protein spots that are altered in madA, madB, and madC mutants may represent component of the photoreceptor complex responsible for phototropism in Phycomyces.

Light-induced phosphorylation of retina-specific polypeptides of Drosophila in vivo

A moderate light stimulus induced isoelectric point (pI) changes in three classes of retina-specific polypeptides (80, 49, and 39 kilodaltons) of Drosophila in vivo. When inorganic phosphate labeled with phosphorus-32 was fed to flies, the radioactive label was incorporated into these polypeptides during the pI changes, indicating light-induced phosphorylation of the polypeptides. A 1-millisecond flash induced a detectable amount of phosphorylation in the 80- and 49-kilodalton polypeptides within 3 seconds. These results, and our previous results with norpA mutants, suggest that phosphorylation of these two polypeptides may be involved in some early stages of photoreceptor excitation or its modulation.

Defective phospholipid metabolism in the retinular cell membrane of norpA (no receptor potential) visual transduction mutants of Drosophila

The phosphorylation of photoreceptor phospholipids in the three alleles of Drosophila visual mutants (norpA: no receptor potential A gene) was studied. In the normal strain, the gamma-32P of ATP was transferred mainly to phosphatidic acid (PA) and diphosphoinositide (DPI), while, in the mutants, we found that the phosphorylation of PA was drastically reduced, but that of DPI was not. The radioactivity incorporation into PA closely parallels with the degree of the mutant genes' expressivity among the three alleles of norpA tested. Therefore, the abnormality found in the phosphorylation of diglycerol to PA may be closely related to the primary mutant defect in the phototransduction mechanism.