Tissue-specific alternative splicing of hybrid Shaker/lacZ genes correlates with kinetic differences in Shaker K+ currents in vivo

Alternative splicing of the Shaker (Sh) locus of Drosophila generates several transcripts with divergent 5' and 3' domains that produce kinetically distinct K+ currents in Xenopus oocytes. Although suggestive that alternative splicing may be involved in generating K+ channel diversity, clear tissue-specific differences in the distribution of particular Sh gene products have not been demonstrated. Using lacZ as a reporter gene for accurate splicing of variable Sh3' domains, we observe differences in beta-galactosidase expression patterns in transgenic animals that indicate both temporal and spatial regulation of 3' splice choice. The differences in 3'splice choice can account for variation in recovery kinetics of Sh-encoded K+ currents recorded in adult flies. The results indicate that tissue-specific expression of functionally distinct Sh K+ channels is regulated, in part, at the level of pre-mRNA splicing and implicate sequences in or around the 3' splice sites in regulating the choice of 3' domain.

The Ras signaling pathway in Drosophila

During Drosophila eye development, a Ras cascade mediates the decision between neuronal and non-neuronal differentiation of the R7 photoreceptor precursor. Recent genetic and molecular studies have identified a set of protein kinases as components of the Ras cascade and nuclear targets of the cascade, including Yan, Pointed, Jun, and Phyllopod. The Ras cascade functions in other Drosophila signal transduction pathways, eliciting a distinct response in each case, presumably through phosphorylation of specific transcription factors.

The Drosophila cation channel trpl expressed in insect Sf9 cells is stimulated by agonists of G-protein-coupled receptors

Structures and regulations of vertebrate channels responsible for sustained calcium elevations after hormone stimulation are largely unknown. Therefore, the Drosophila photoreceptor channels, trp and trpl, which are assumed to be involved in calcium influx, serve as model system, trpl expressed in Sf9 cells showed spontaneous activity. Hormonal stimulations of calcium influx (detected by fura-2) and of an outwardly rectifying current were observed in Sf9 cells coinfected with baculoviruses encoding trpl and various heptahelical receptors for histamine, thrombin, and thromboxane A2, all known to cause phospholipase C-beta activation in mammalian cells. Although the identity of the G-proteins and of possible second messengers involved need to be clarified, it is clear that trpl represents a receptor/G-protein regulated cation channel.

Regulation of PLC-mediated signalling in vivo by CDP-diacylglycerol synthase

CDP-diacylglycerol synthase (CDS) is an enzyme required for the regeneration of the signalling molecule phosphatidylinositol-4,5-bisphosphate (PtdlnsP2) from phosphatidic acid. A photo-receptor cell-specific isoform of CDS from Drosophila is a key regulator of phototransduction, a G-protein-coupled signalling cascade mediated by phospholipase C. cds mutants cannot sustain a light-activated current as a result of depletion of PtdlnsP2. Overexpression of CDS increases the amplitude of the light response, demonstrating that availability of PtdlnsP2 is a determinant in the gain of this pathway. cds mutants undergo light-dependent retinal degeneration which can be suppressed by a mutation in phospholipase C. Thus, enzymes involved in PtdlnsP2 metabolism regulate phosphoinositide-mediated signalling cascades in vivo.

Photolysis of caged Ca2+ facilitates and inactivates but does not directly excite light-sensitive channels in Drosophila photoreceptors

Substantial evidence implicates the phosphoinositide cascade in invertebrate phototransduction, but the final pathway of excitation remains obscure. In order to test the hypothesis that Ca2+ is the excitatory messenger rapid concentration jumps of cytosolic Ca2+ were achieved in dissociated Drosophila photoreceptors by flash photolysis of the caged Ca2+ compounds DM-nitrophen and nitr-5. Both compounds were introduced via patch pipettes used to record whole-cell currents. Calibrations using INDO-1 and Mag-INDO-1 indicated that photolysis of DM-nitrophen (5 mM loaded with 4 mM Ca2+), raised Ca, to ca. 20-50 microM, and nitr-5 (same loading) to ca. 1-2 microM. In mutants lacking light responses (ora, lacking rhodopsin; norpA, lacking phospholipase C; trp, which is inactivated by conditioning lights), the only current evoked by photolysis of DM-nitrophen was a small inward current with no detectable latency. This current did not reverse at +80 mV and was blocked by substitution of external Na+ for Li+, suggesting it represents activation of an electrogenic Na+/Ca2+ exchanger. A similar current was also the only current elicited by caged Ca2+ during the 5 msec latent period in wild type (WT) photoreceptors. To investigate possible modulatory effects of caged Ca2+ on the light-activated conductance, cells were first stimulated with a saturating light stimulus, itself incapable of releasing significant Ca2+, and then the photolytic flash was discharged during the response. During the rising phase of the response, photolysis of DM-nitrophen (but not nitr-5) induced a pronounced facilitation in WT photoreceptors. When photolysed during the plateau phase both DM-nitrophen and nitr-5 induced a rapid inactivation of the light-induced current. By contrast, in trp photoreceptors, which lack one class of Ca2+ permeable light-sensitive channel, photolysis of DM-nitrophen induced a significant facilitation during the falling phase of the response, but during the rising phase photolysis significantly depressed the overall response. In conclusion, caged Ca2+ failed to activate any channels in Drosophila photoreceptors but profoundly affected the light-dependent channels once they have been activated.

Measurement of cytosolic Ca2+ concentration in Limulus ventral photoreceptors using fluorescent dyes

Several Ca-sensitive fluorescent dyes (fura-2, mag-fura-2 and Calcium Green-5N) were used to measure intracellular calcium ion concentration, Cai, accompanying light-induced excitation of Limulus ventral nerve photoreceptors. A ratiometric procedure was developed for quantification of Calcium Green-5N fluorescence. A mixture of Calcium Green-5N and a Ca-insensitive dye, ANTS, was injected in the cell and the fluorescence intensities of both dyes were used to calculate the spatial average of Cai within the light-sensitive R lobe of the photoreceptor. In dark-adapted photoreceptors, the initial Cai was 0.40 +/- 0.22 microM (SD, n = 7) as measured with fura-2. Cai peaked in the light-sensitive R lobe at 700-900 ms after the onset of an intense measuring light step, when the spatial average of Cai within the R lobe reached 68 +/- 14 and 62 +/- 37 microM (SD, n = 5) as measured with mag-fura-2 and Calcium Green-5N, respectively. The rate of Cai rise was calculated to be approximately 350 microM/s under the measuring conditions. The resting level of Mg2+ was estimated to be 1.9 +/- 0.9 mM, calculated from mag-fura-2 measurements. To investigate the effect of adapting light on the initial Cai level in the R lobe, a 1-min step of 420 nm background light was applied before each measurement. The first significant (P < 0.05) change in the initial level of Cai occurred even at the lowest adapting light intensity, which delivered approximately 3 x 10(3) effective photons/s. The relative sensitivity of the light-adapted photoreceptors was linearly related to the relative Cai on a double log plot with slope between -4.3 and -5.3. We were unable to detect a Cai rise preceding the light-activated receptor potential. The Cai rise, measured with Calcium Green-5N, lagged 14 +/- 5 ms (SD, n = 32) behind the onset of the receptor potential at room temperature in normal ASW. In the absence of extracellular Ca2+ and at 10 degrees C, this lag increased to 44 +/- 12 ms (SD, n = 17).

Isolation and expression of an arrestin cDNA from the horseshoe crab lateral eye

Electrophysiological studies of photoreceptors from the horseshoe crab Limulus polyphemus continue to provide fundamental new knowledge of the photoresponse in invertebrates. Therefore, it is of particular interest to characterize the molecular components of the photoresponse in this system. Here we describe an arrestin cloned from a cDNA library constructed using poly(A)+ RNA isolated from Limulus lateral eyes. The protein, deduced from the arrestin cDNA, is most similar to arrestin from locust antennae (56% identity) and Drosophila phosrestin I (53% identity). Limulus arrestin was expressed in a heterologous system, and its properties were compared with those of a 46-kDa light-regulated phosphoprotein (pp46A) in Limulus photoreceptors described in previous studies from this laboratory. Arrestin and pp46A (a) have the same apparent molecular weight on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, (b) have an isoelectric point in the basic pH range, (c) require calmodulin and elevated Ca2+ levels for phosphorylation, (d) are immunoreactive with monoclonal antibody C10C10 directed against a sequence in bovine arrestin (S-antigen) that is perfectly conserved in the deduced arrestin protein, and (e) are associated with photoreceptors. We conclude that the arrestin described here and pp46A are the same protein. The results of this and previous studies show that in Limulus photoreceptors, light regulates the phosphorylation of arrestin in complex ways.

Transfer of graded potentials at the photoreceptor-interneuron synapse

To characterize the transfer of graded potentials and the properties of the associated noise in the photoreceptor-interneuron synapse of the blowfly (Calliphora vicina) compound eye, we recorded voltage responses of photoreceptors (R1-6) and large monopolar cells (LMC) evoked by: (a) steps of light presented in the dark; (b) contrast steps; and (c) pseudorandomly modulated contrast stimuli at backgrounds covering 6 log intensity units. Additionally, we made recordings from photoreceptor axon terminals. Increased light adaptation gradually changed the synaptic signal transfer from low-pass to band-pass filtering. This was accompanied by decreased synaptic delay and increased contrast gain, but the overall synaptic gain and the intrinsic noise (i.e., transmission noise) were reduced. Based on these results, we describe a descriptive synaptic model, in which the kinetics of the tonic transmitter (histamine) release from the photoreceptor axon terminals change with mean photoreceptor depolarization. During signal transmission, tonic transmitter release is augmented by voltage-dependent contrast-enhancing mechanisms in the photoreceptor axons that produce fast transients from the rising phases of the photoreceptor responses and add these enhanced voltages to the original photoreceptor responses. The model can predict the experimental findings and it agrees with the recently proposed theory of maximizing sensory information.

In situ assay of light-stimulated G protein activity in Drosophila photoreceptor G protein beta mutants

An in situ 35S-labeled guanosine 5'-3-O-(thio)triphosphate (GTP gamma S) binding procedure was developed to assay light-stimulated G protein activity in Drosophila compound eyes. We found that Drosophila with mutations in G beta e, an abundant photoreceptor-specific G protein beta subunit essential for photoexcitation, are defective in light-stimulated [35S]GTP gamma S binding. We confirmed that G beta e interacts with a GTP-binding protein by demonstrating that immunoprecipitation of G beta e is sensitive to GTP gamma S. These results suggest that G beta e functions as the beta subunit of a heterotrimeric G protein that couples photoactivation of rhodopsin to downstream components in the Drosophila phototransduction cascade.

Genetic dissection of light-induced Ca2+ influx into Drosophila photoreceptors

Invertebrate photoreceptors use the inositol-lipid signaling cascade for phototransduction. A useful approach to dissect this pathway and its regulation has been provided by the isolation of Drosophila visual mutants. We measured extracellular changes of Ca2+ [delta Ca2+]o in Drosophila retina using Ca(2+)-selective microelectrodes in both the transient receptor potential (trp) mutant, in which the calcium permeability of the light-sensitive channels is greatly diminished and in the inactivation-but-no-afterpotential C (inaC) mutant which lacks photoreceptor-specific protein kinase C (PKC). Illumination induced a decrease in extracellular [Ca2+] with kinetics and magnitude that changed with light intensity. Compared to wild-type, the light-induced decrease in [Ca2+]o (the Ca2+ signal) was diminished in trp but significantly enhanced in inaC. The enhanced Ca2+ signal was diminished in the double mutant inaC;trp indicating that the effect of the trp mutation overrides the enhancement observed in the absence of eye-PKC. We suggest that the decrease in [Ca2+]o reflects light-induced Ca2+ influx into the photoreceptors and that the trp mutation blocks a large fraction of this Ca2+ influx, while the absence of eye specific PKC leads to enhancement of light-induced Ca2+ influx. This suggestion was supported by Ca2+ measurements in isolated ommatidia loaded with the fluorescent Ca2+ indicator, Ca Green-5N, which indicated an approximately threefold larger light-induced increase in cellular Ca2+ in inaC relative to WT. Our observations are consistent with the hypothesis that TRP is a light activated Ca2+ channel and that the increased Ca2+ influx observed in the absence of PKC is mediated mainly via the TRP channel.

Activation and regeneration of rhodopsin in the insect visual cycle

Light absorption by rhodopsin generates metarhodopsin, which activates heterotrimeric guanine nucleotide-binding proteins (G proteins) in photoreceptor cells of vertebrates and invertebrates. In contrast to vertebrate metarhodopsins, most invertebrate metarhodopsins are thermally stable and regenerate rhodopsin by absorption of a second photon. In experiments with Rh1 Drosophila rhodopsin, the thermal stability of metarhodopsin was found not to be an intrinsic property of the visual pigment but a consequence of its interaction with arrestin (49 kilodaltons). The stabilization of metarhodopsin resulted in a large decrease in the efficiency of G protein activation. Light absorption by thermally stable metarhodopsin initially regenerated an inactive rhodopsin-like intermediate, which was subsequently converted in the dark to active rhodopsin. The accumulation of inactive rhodopsin at higher light levels may represent a mechanism for gain regulation in the insect visual cycle.

Unambiguous total synthesis of the enantiomers of myo-inositol 1,3,4-trisphosphate: 1L-myo-inositol 1,3,4-trisphosphate mobilizes intracellular Ca2+ in Limulus photoreceptors

Syntheses of the enantiomers of myo-inositol 1,3,4-trisphosphate are described. 1,4-Di-O-allyl-myo-inositol was regioselectively p-methoxybenzylated at the 3-position to give 1,4-di-O-allyl-3-O-(p-methoxybenzyl)-myo-inositol followed by benzylation of the remaining free hydroxyl groups to give the key intermediate 1,4-di-O-allyl-2,5,6-tri-O-benzyl-3-O-(p-methoxybenzyl)-myo-inositol. Removal of the p-methoxybenzyl and allyl groups gave 2,4,5-tri-O-benzyl-myo-inositol which was phosphitylated with bis(benzyloxy)(diisopropylamino)phosphine to give the fully protected trisphosphite triester. Oxidation using tert-butyl hydroperoxide gave 2,5,6-tri-O-benzyl-1,3,4-tris(dibenzylphospho)-myo-inositol, and deprotection using sodium in liquid ammonia gave racemic myo-inositol 1,3,4-trisphosphate. Deprotection of the key intermediate 1,4-di-O-allyl-2,5,6-tri-O-benzyl-3-O-(p-methoxybenzyl)-myo-inositol by isomerization of allyl groups followed by mild acid hydrolysis gave 2,4,5-tri-O-benzyl-1-O-(p-methoxybenzyl)-myo-inositol, which was converted to the diastereoisomeric (bis-(-)-camphanates. The diastereoisomers were separated by column chromatography and the camphanates and the p-methoxybenzyl group removed by saponification and acid hydrolysis, respectively, for each diastereoisomer to give the enantiomers of 2,4,5-tri-O-benzyl-myo-inositol. The absolute configurations of the latter were established by conversion of 1L-2,5,6-tri-O-benzyl-3-O-(p-methoxybenyl)-myo-inositol to the known 1L-1,2,4,5,6-penta-O-benzyl-myo-inositol. Phosphorylation and deblocking gave the D- and L-enantiomers of myo-inositol 1,3,4-trisphosphate. Biological evaluation in Limulus photoreceptors showed that 1L-myo-inositol 1,3,4-trisphosphate was much more active than the D-enantiomer, producing repetitive bursts of depolarization due to mobilization of intracellular calcium.

Cloning of a structural and functional homolog of the circadian clock gene period from the giant silkmoth Antheraea pernyi

The period (per) gene of Drosophila plays an important role in circadian clock function. Interestingly, homologs of per have not been cloned outside of dipteran species. Using a PCR strategy, we now report the cloning of the cDNA of a per homolog from the silkmoth Antheraea pernyi. The cDNA encodes a protein of 849 amino acids, which shows highest identity (39%) with the per protein of Drosophila virilis. Stretches of high identity between moth and fly proteins are in the amino terminus, the PAS region, and the region surrounding the site of the per mutation in Drosophila. Moth per homolog mRNA levels exhibit a prominent circadian variation in adult heads, and per protein antibodies show a pronounced variation of per antigen staining in photoreceptor nuclei. With sequence information derived from moth and flies, per-like cDNA fragments were readily cloned by PCR from other moth species and a third insect order.

Single K+ channels closed by light and opened by cyclic GMP in molluscan extra-ocular photoreceptor cells

We report the first recordings of the light-sensitive channel which is active during dark and is closed by light in the Onchidium extra-ocular photoreceptor cells. This light-sensitive channel was K-selective and was not blocked by extracellular Ca2+ and Mg2+. Application of cyclic GMP to excised inside-out patches activated (opened) a channel that appeared to be the same as the light-sensitive channel recorded from the same membrane in the intact cell.

Mechanism of arrestin 2 function in rhabdomeric photoreceptors

Arrestins have emerged as one family of proteins that mediate the inactivation of G-protein-coupled receptors. We have isolated cDNA clones encoding two arrestin isoforms of the dipteran visual system, Calliphora arrestin 1 (Arr1) and arrestin 2 (Arr2). Microsequencing established that the arr2 gene encodes the Calliphora 49-kDa protein characterized previously as a photoreceptor-specific protein that undergoes reversible binding to light-activated rhodopsin and thereby activates the phosphorylation of metarhodopsin. Ultrastructural localization of Arr2 to the rhabdomeral part of the photoreceptor cell and quantitation of the amount of Arr2 bound suggest that Arr2 directly interacts with light-activated rhodopsin. In a reconstituted system containing affinity purified Arr2 and isolated rhabdomeric membranes, Arr2 binds to non-phosphorylated and phosphorylated metarhodopsin with comparable affinity. Reaction time courses reveal that Arr2 binding precedes phosphorylation of metarhodopsin, contrary to what has been reported so far for vertebrate photoreceptors. The phosphorylation-independent binding of Arr2 to metarhodopsin provides a mechanism for the rapid inactivation of the long-lived activated rhodopsin state which is characteristic for invertebrate photoreceptors. The dephosphorylation of rhodopsin is catalyzed by a Ca(2+)-dependent protein phosphatase which is shown here for the first time to exist in a membrane-associated form. Only metarhodopsin molecules with bound Arr2 are resistant to dephosphorylation. Thus, in fly photoreceptors, Arr2 acts as a regulatory protein that controls the phosphorylation as well as the dephosphorylation of the light-activated visual pigment.

Mutations in calphotin, the gene encoding a Drosophila photoreceptor cell-specific calcium-binding protein, reveal roles in cellular morphogenesis and survival

Calphotin is a Drosophila photoreceptor cell-specific protein expressed very early in eye development, at the time when cell-type decisions are being made. Calphotin is a very hydrophobic and proline-rich protein which lacks obvious transmembrane domains. The cDNA encoding Calphotin was mapped to a region removed by a set of existing chromosomal deletions. Mutations that alter photoreceptor cell structure and development were isolated that fail to complement these deletions. These mutations fall into two classes. Class I mutations alter the structure of the rhabdomere, a photoreceptor cell organelle specialized for phototransduction. Class II mutations have rough eyes, due to misorientation of the rhabdomeres and photoreceptor cell death. Transformation rescue of these phenotypes in transgenic flies bearing calphotin genomic DNA indicates that both classes of mutations are in the calphotin gene. Analysis of these mutations suggest that Calphotin plays important roles in both rhabdomere development and in photoreceptor cell survival.

Blockage of the light-sensitive conductance in hyperpolarizing photoreceptors of the scallop. Effects of tetraethylammonium and 4-aminopyridine

The tight-seal whole-cell recording technique was used to examine the effect of tetraethylammonium (TEA) and 4-aminopyridine (4-AP) on the photocurrent of hyperpolarizing ciliary photoreceptors isolated from the distal retina of the bay scallop (Pecten irradians). In these cells, light causes an increase in a conductance that is highly selective to potassium ions. Extracellular application of TEA at a concentration of 50 mM produced a modest, reversible block (approximately 35% at -20 mV holding potential). The blockage was weakly voltage dependent, increasing by approximately 20% for a 20-mV hyperpolarization, suggestive of a site of interaction superficially located within the electric field of the membrane. Treatment with TEA produced no significant changes either in the light sensitivity of the photocurrent or in its kinetics. The effects of superfusion with 4-AP were more dramatic: the light-evoked current was nearly abolished (> 95%) at submillimolar concentrations, with a half-maximal dose of approximately 0.6 microns. The blockage had a rapid onset and was slowly reversible. No significant use or voltage dependency were observed. A number of control experiments indicated that the phototransduction cascade remained functional during treatment with 4-AP: the early receptor current, the prolonged after current and its suppression, the photoresponse kinetics and the light sensitivity of the cell were little affected by 4-AP, suggesting that the suppression of the photocurrent is due to blockage of the light-sensitive channels, rather than impairment of some of the activation steps. The results are discussed in the light of a possible kinship between the light-activated potassium channels of invertebrate hyperpolarizing photoreceptors and the family of rapidly-inactivating voltage-dependent potassium channels, which typically exhibit high susceptibility to blockage by this drug.

Facilitation of the responses to injections of inositol 1,4,5-trisphosphate analogs in Limulus ventral photoreceptors

Injection of inositol 1,4,5-trisphosphate and its metabolically resistant analogs InsP3S3 and L-chiro-2,3,5-InsP3 into the ventral photoreceptors of Limulus results in the release of calcium from internal stores and in a current flow into the cells. We show here that the dependence of the current response on the amount of analog injected is supralinear. The injections also facilitate the responses to subsequent injections. We analyze the kinetics of the responses either by very slow application of the analogs directly into the lobe that is sensitive to InsP3 and light or by delivering a pulse into the nonsensitive lobe of the cell, in both cases creating a ramp of rising concentration in the sensitive region. Typically, a long latent period was followed by a strong brief inward current. The ratio between the latency and the duration of the response, defined as twice the time from half-amplitude to the peak of the response, reaches values greater than 10. Our analysis shows that this value cannot be attained within realistic models whose only nonlinearity is the cooperative binding of the ligand to its receptor. The observed ratio, however, can be achieved with a positive feedback model. Treatments that lead to partial depletion of calcium stores reversibly increase the latency of the response. We conclude that the mechanisms of the response of Limulus ventral eye to the metabolically resistant analogs of InsP3 probably involves a positive feedback mechanism and that the carrier of the feedback is likely to be Ca2+.

Constitutive overexpression of the Drosophila period protein inhibits period mRNA cycling

The Drosophila period gene (per) is a likely component of a circadian pacemaker. per protein (PER) participates in the regulation of its own expression, at least in part at the transcriptional level. There is at present no direct evidence that the effect of PER on its own transcription is intracellular. Results presented in this paper show that (i) the circadian oscillations of both per mRNA and PER protein are quantitatively similar in eye photoreceptor cells and in brain; (ii) constitutive overexpression of PER only in photoreceptors R1-R6 represses endogenous per RNA cycling in these cells but not in other per-expressing cells; (iii) the overexpression construct has no effect on locomotor activity rhythms. These results indicate that the autoregulation of per expression is a direct, intracellular event and suggest that each per-expressing cell contains an autonomous oscillator of which the per feedback loop is a component.

Immunolocalization of a Drosophila phosphatidylinositol transfer protein (rdgB) in normal and rdgA mutant photoreceptor cells with special reference to the subrhabdomeric cisternae

Distribution of rdgB protein, which was recently shown to be a Drosophila phosphatidylinositol transfer protein, was studied in the photoreceptor cells of compound eyes of normal and rdgAPC47 mutant of Drosophila melanogaster by immunoelectron microscopy using (1) pre-embedding HRP staining, (2) pre-embedding NANOGOLD labeling followed by siliver enhancement, (3) and post-embedding colloidal gold labeling methods. In normal cells, immunoreactivity was localized to the membranes of subrhabdomeric cisternae (SRC) and adjacent plasma membranes at the bases of photoreceptive microvilli. In rdgAPC47 mutant cells, whose SRC gradually degenerate, the immunoreactivity in the plasma membranes at the bases of microvilli disappeared in parallel with the degeneration of SRC. Instead, the abnormally proliferated membranous structures became labeled by the antibody. These results indicate that the rdgB protein is normally localized to the SRC membranes and probably adjacent photoreceptive membranes, suggesting the involvement of the rdgB protein in the phosphatidylinositol transfer between SRC and photoreceptive membranes.

Rotation of photoreceptor clusters in the developing Drosophila eye requires the nemo gene

The Drosophila eye consists of a reiterative hexagonal array of photoreceptor cell clusters, the ommatidia. During normal morphogenesis, the clusters in the dorsal or ventral halves of the disc rotate 90 degrees in opposite directions, forming mirror images across a dorsoventral equator. In the mutant nemo (nmo), there is an initial turning of approximately 45 degrees, but further rotation is blocked. Genetic mosaic analysis indicates that the nmo gene acts upon each cluster as a whole; normal nmo function in one or more photoreceptor cells appears to be sufficient to induce full rotation. The nmo gene sequence encodes a serine/threonine protein kinase homolog, suggesting that the kinase is required to initiate the second step of rotation. In another mutant, roulette, excessive rotation through varying angles occurs in many ommatidia. This defect is suppressed by nmo, indicating that nmo acts upstream in a rotation-regulating pathway.

Induction of photoresponse by the hydrolysis of polyphosphoinositides in the Hermissenda type B photoreceptor

Direct evidence that the photoresponse of the Hermissenda type B photoreceptor cell is triggered directly by the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) was obtained. Neomycin and spermine, which inhibit PIP2 breakdown, suppressed light response, while injection of inositol 1,4,5-trisphosphate (IP3), guanosine 5'-(3-O-thio)triphosphate (GTP gamma S), guanosine 5'-(2-O-thio)diphosphate (GDP beta S), cAMP, cGMP did not alter the light-induced Na+ influx underlying the photoresponse. Suppression of the photoresponse was also observed with decrease of total amount of membraneous PIP2 induced by injection of the phosphoinositides (PI) turnover inhibitors, isobutylmethylxanthine (IBMX), LiCl and R 59022.

An eye-specific G beta subunit essential for termination of the phototransduction cascade

Heterotrimeric G proteins couple various receptors to intracellular effector molecules. Although the role of the G alpha subunit in effector activation, guanine nucleotide exchange and GTP hydrolysis has been well studied, the cellular functions of the G beta subunits are less well understood. G beta gamma dimers bind G alpha subunits and anchor them to the membrane for presentation to the receptor. In specific systems, the G beta subunits have also been implicated in direct coupling to ion channels and to effector molecules. We have isolated Drosophila melanogaster mutants defective in an eye-specific G-protein beta-subunit (G beta e), and show here that the beta-subunit is essential for G-protein-receptor coupling in vivo. Remarkably, G beta mutants are also severely defective in the deactivation of the light response, demonstrating an essential role for the G beta subunit in terminating the active state of this signalling cascade.

Accumulation of calcium in degenerating photoreceptors of several Drosophila mutants

The hypothesis that a large, possibly toxic, increase in cellular calcium accompanies photoreceptor cell degeneration in several different Drosophila mutants was tested. The calcium content of wild type and mutant photoreceptors of Drosophila was measured using rapid freezing of the eyes and energy-dispersive x-ray analysis (e.d.x.) of cryosections and semithin sections of cryosubstituted material. Light- and dark-raised mutants of the following strains were studied: retinal degeneration B (rdgB); retinal degeneration C (rdgC); neither inactivation nor afterpotential C (ninaC), and no receptor potential A (norpA). These are light-dependent retinal degeneration mutants in which the affected gene products had been previously shown as myosin-kinase (ninaC), calcium-dependent phosphoprotein phosphatase (rdgC), phosphoinositide transfer protein (rdgB), and phospholipase C (norpA). In light-raised mutants, ommatidia of variable degrees of degeneration were observed. Mass-dense globular bodies of 200-500 nm diameter in relatively large quantities were found in the degenerating photoreceptor of all the mutants tested. These subcellular globules were found to have a very high calcium content, which was not found in wild type or in nondegenerating photoreceptors of the mutants. Nondegenerating photoreceptors were found not only in dark-raised mutants, but in smaller quantities also in light-raised mutants. Usually these globular structures contained high levels of phosphorus, indicating that at least part of the calcium in the mutant photoreceptors is precipitated as calcium phosphate. The results indicate that a large increase in cellular calcium accompanies light-induced photoreceptor degeneration in degenerating Drosophila mutants even when induced by very different mutations, suggesting that the calcium accumulation is a secondary rather than a primary effect in the degeneration process.