Calphotin: a Drosophila photoreceptor cell calcium-binding protein

Contamination of aquatic environment with anticancer reagents influences Daphnia magna - Ecotoxicogenomics approach

Pharmaceuticals used in human medicine contaminate freshwater ecosystems. Chemotherapeutics applied in cancer treatment are found in freshwaters at low concentrations (in the range of ng L^-1) which, however, can be toxic or mutagenic to aquatic organisms. The aim of this study was to determine the impact of the alkylating/crosslinking anticancer agents, cyclophosphamide (CP) and cisplatin (CDDP), at the concentration detected in water, on Daphnia magna life history, transcriptome, and proteome. This filter feeding cladoceran is an important member of the aquatic food webs controlling algal biomass and forming basic food for planktivorous fish. Here, observations of the D. magna growth rate, age at first reproduction, and the number of eggs produced were performed in the presence of CP or CDDP. The D. magna proteins and RNA were isolated and analysed by mass spectrometry and the mRNA-seq method, respectively. Five generations of contact with the pharmaceuticals in question significantly influenced the D. magna life history parameters with the growth rate and number of laid eggs decreased, whereas age at first reproduction was increased. A decrease in survivorship was observed when daphnids were exposed to CP. These changes are the result of modifications in the gene/transcript expression followed by differences in the proteome profile in comparison to the untreated control. The proteome changes were generally in accordance with the modified transcriptome. The ecotoxicogenomics approach makes it possible to get closer to a complete picture of the influence of CP and CDDP on Daphnia. We have gathered evidence that animals in the presence of anticancer pharmaceuticals attempt to cope with permanent stress by changing their proteome and transcriptome profile. Additionally, our analyses indicate that CDDP showed a stronger effect on tested organisms than CP.

Groundnut Bud Necrosis Virus Modulates the Expression of Innate Immune, Endocytosis, and Cuticle Development-Associated Genes to Circulate and Propagate in Its Vector, Thrips palmi

Thrips palmi (Thysanoptera: Thripidae) is the predominant tospovirus vector in Asia-Pacific region. It transmits economically damaging groundnut bud necrosis virus (GBNV, family Tospoviridae) in a persistent propagative manner. Thrips serve as the alternate host, and virus reservoirs making tospovirus management very challenging. Insecticides and host plant resistance remain ineffective in managing thrips–tospoviruses. Recent genomic approaches have led to understanding the molecular interactions of thrips–tospoviruses and identifying novel genetic targets. However, most of the studies are limited to Frankliniella species and tomato spotted wilt virus (TSWV). Amidst the limited information available on T. palmi–tospovirus relationships, the present study is the first report of the transcriptome-wide response of T. palmi associated with GBNV infection. The differential expression analyses of the triplicate transcriptome of viruliferous vs. nonviruliferous adult T. palmi identified a total of 2,363 (1,383 upregulated and 980 downregulated) significant transcripts. The Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses showed the abundance of differentially expressed genes (DEGs) involved in innate immune response, endocytosis, cuticle development, and receptor binding and signaling that mediate the virus invasion and multiplication in the vector system. Also, the gene regulatory network (GRN) of most significant DEGs showed the genes like ABC transporter, cytochrome P450, endocuticle structural glycoprotein, gamma-aminobutyric acid (GABA) receptor, heat shock protein 70, larval and pupal cuticle proteins, nephrin, proline-rich protein, sperm-associated antigen, UHRF1-binding protein, serpin, tyrosine–protein kinase receptor, etc., were enriched with higher degrees of interactions. Further, the expression of the candidate genes in response to GBNV infection was validated in reverse transcriptase-quantitative real-time PCR (RT-qPCR). This study leads to an understanding of molecular interactions between T. palmi and GBNV and suggests potential genetic targets for generic pest control.

Vitamin A deficiency affects gene expression in the Drosophila melanogaster head

Insufficient dietary intake of vitamin A causes various human diseases. For instance, chronic vitamin A deprivation causes blindness, slow growth, impaired immunity, and an increased risk of mortality in children. In contrast to these diverse effects of vitamin A deficiency (VAD) in mammals, chronic VAD in flies neither causes obvious developmental defects nor lethality. As in mammals, VAD in flies severely affects the visual system: it impairs the synthesis of the retinal chromophore, disrupts the formation of the visual pigments (Rhodopsins), and damages the photoreceptors. However, the molecular mechanisms that respond to VAD remain poorly understood. To identify genes and signaling pathways that are affected by VAD, we performed RNA-sequencing and differential gene expression analysis in Drosophila melanogaster. We found an upregulation of genes that are essential for the synthesis of the retinal chromophore, specific aminoacyl-tRNA synthetases, and major nutrient reservoir proteins. We also discovered that VAD affects several genes that are required for the termination of the light response: for instance, we found a downregulation of both arrestin genes that are essential for the inactivation of Rhodopsin. A comparison of the VAD-responsive genes with previously identified blue light stress-responsive genes revealed that the two types of environmental stress trigger largely nonoverlapping transcriptome responses. Yet, both stresses increase the expression of seven genes with poorly understood functions. Taken together, our transcriptome analysis offers insights into the molecular mechanisms that respond to environmental stresses.

Differential Gene Expression in Red Imported Fire Ant (Solenopsis invicta) (Hymenoptera: Formicidae) Larval and Pupal Stages

Solenopsis invicta Buren is an invasive ant species that has been introduced to multiple continents. One such area, the southern United States, has a history of multiple control projects using chemical pesticides over varying ranges, often resulting in non-target effects across trophic levels. With the advent of next generation sequencing and RNAi technology, novel investigations and new control methods are possible. A robust genome-guided transcriptome assembly was used to investigate gene expression differences between S. invicta larvae and pupae. These life stages differ in many physiological processes; of special importance is the vital role of S. invicta larvae as the colonies' "communal gut". Differentially expressed transcripts were identified related to many important physiological processes, including digestion, development, cell regulation and hormone signaling. This dataset provides essential developmental knowledge that reveals the dramatic changes in gene expression associated with social insect life stage roles, and can be leveraged using RNAi to develop effective control methods.

Transcriptome profiling of aging Drosophila photoreceptors reveals gene expression trends that correlate with visual senescence

BACKGROUND

Aging is associated with functional decline of neurons and increased incidence of both neurodegenerative and ocular disease. Photoreceptor neurons in Drosophila melanogaster provide a powerful model for studying the molecular changes involved in functional senescence of neurons since decreased visual behavior precedes retinal degeneration. Here, we sought to identify gene expression changes and the genomic features of differentially regulated genes in photoreceptors that contribute to visual senescence.

RESULTS

To identify gene expression changes that could lead to visual senescence, we characterized the aging transcriptome of Drosophila sensory neurons highly enriched for photoreceptors. We profiled the nuclear transcriptome of genetically-labeled photoreceptors over a 40 day time course and identified increased expression of genes involved in stress and DNA damage response, and decreased expression of genes required for neuronal function. We further show that combinations of promoter motifs robustly identify age-regulated genes, suggesting that transcription factors are important in driving expression changes in aging photoreceptors. However, long, highly expressed and heavily spliced genes are also more likely to be downregulated with age, indicating that other mechanisms could contribute to expression changes at these genes. Lastly, we identify that circular RNAs (circRNAs) strongly increase during aging in photoreceptors.

CONCLUSIONS

Overall, we identified changes in gene expression in aging Drosophila photoreceptors that could account for visual senescence. Further, we show that genomic features predict these age-related changes, suggesting potential mechanisms that could be targeted to slow the rate of age-associated visual decline.

Electrophysiological Method for Whole-cell Voltage Clamp Recordings from Drosophila Photoreceptors

Whole-cell voltage clamp recordings from Drosophila melanogaster photoreceptors have revolutionized the field of invertebrate visual transduction, enabling the use of D. melanogaster molecular genetics to study inositol-lipid signaling and Transient Receptor Potential (TRP) channels at the single-molecule level. A handful of labs have mastered this powerful technique, which enables the analysis of the physiological responses to light under highly controlled conditions. This technique allows control over the intracellular and extracellular media; the membrane voltage; and the fast application of pharmacological compounds, such as a variety of ionic or pH indicators, to the intra- and extracellular media. With an exceptionally high signal-to-noise ratio, this method enables the measurement of dark spontaneous and light-induced unitary currents (i.e. spontaneous and quantum bumps) and macroscopic Light-induced Currents (LIC) from single D. melanogaster photoreceptors. This protocol outlines, in great detail, all the key steps necessary to perform this technique, which includes both electrophysiological and optical recordings. The fly retina dissection procedure for the attainment of intact and viable ex vivo isolated ommatidia in the bath chamber is described. The equipment needed to perform whole-cell and fluorescence imaging measurements are also detailed. Finally, the pitfalls in using this delicate preparation during extended experiments are explained.

Fractional Ca(2+) currents through TRP and TRPL channels in Drosophila photoreceptors

Light responses in Drosophila photoreceptors are mediated by two Ca(2+) permeable cation channels, transient receptor potential (TRP) and TRP-like (TRPL). Although Ca(2+) influx via these channels is critical for amplification, inactivation, and light adaptation, the fractional contribution of Ca(2+) to the currents (Pf) has not been measured. We describe a slow (τ ∼ 350 ms) tail current in voltage-clamped light responses and show that it is mediated by electrogenic Na(+)/Ca(2+) exchange. Assuming a 3Na:1Ca stoichiometry, we derive empirical estimates of Pf by comparing the charge integrals of the exchanger and light-induced currents. For TRPL channels, Pf was ∼17% as predicted by Goldman-Hodgkin-Katz (GHK) theory. Pf for TRP (29%) and wild-type flies (26%) was higher, but lower than the GHK prediction (45% and 42%). As predicted by GHK theory, Pf for both channels increased with extracellular [Ca(2+)], and was largely independent of voltage between -100 and -30 mV. A model incorporating intra- and extracellular geometry, ion permeation, diffusion, extrusion, and buffering suggested that the deviation from GHK predictions was largely accounted for by extracellular ionic depletion during the light-induced currents, and the time course of the Na(+)/Ca(2+) exchange current could be used to obtain estimates of cellular Ca(2+) buffering capacities.

Compartmentalization and Ca2+ buffering are essential for prevention of light-induced retinal degeneration

Fly photoreceptors are polarized cells, each of which has an extended interface between its cell body and the light-signaling compartment, the rhabdomere. Upon intense illumination, rhabdomeric calcium concentration reaches millimolar levels that would be toxic if Ca(2+) diffusion between the rhabdomere and cell body was not robustly attenuated. Yet, it is not clear how such effective attenuation is obtained. Here we show that Ca(2+) homeostasis in the photoreceptor cell relies on the protein calphotin. This unique protein functions as an immobile Ca(2+) buffer localized along the base of the rhabdomere, separating the signaling compartment from the cell body. Generation and analyses of transgenic Drosophila strains, in which calphotin-expression levels were reduced in a graded manner, showed that moderately reduced calphotin expression impaired Ca(2+) homeostasis while calphotin elimination resulted in severe light-dependent photoreceptor degeneration. Electron microscopy, electrophysiology, and optical methods revealed that the degeneration was rescued by prevention of Ca(2+) overload via overexpression of CalX, the Na(+)-Ca(2+) exchanger. In addition, Ca(2+)-imaging experiments showed that reduced calphotin levels resulted in abnormally fast kinetics of Ca(2+) elevation in photoreceptor cells. Together, the data suggest that calphotin functions as a Ca(2+) buffer; a possibility that we directly demonstrate by expressing calphotin in a heterologous expression system. We propose that calphotin-mediated compartmentalization and Ca(2+) buffering constitute an effective strategy to protect cells from Ca(2+) overload and light-induced degeneration.

Genomic and functional studies of Drosophila sex hierarchy regulated gene expression in adult head and nervous system tissues

The Drosophila sex determination hierarchy controls all aspects of somatic sexual differentiation, including sex-specific differences in adult morphology and behavior. To gain insight into the molecular-genetic specification of reproductive behaviors and physiology, we identified genes expressed in the adult head and central nervous system that are regulated downstream of sex-specific transcription factors encoded by doublesex (dsx) and fruitless (fru). We used a microarray approach and identified 54 genes regulated downstream of dsx. Furthermore, based on these expression studies we identified new modes of DSX-regulated gene expression. We also identified 90 and 26 genes regulated in the adult head and central nervous system tissues, respectively, downstream of the sex-specific transcription factors encoded by fru. In addition, we present molecular-genetic analyses of two genes identified in our studies, calphotin (cpn) and defective proboscis extension response (dpr), and begin to describe their functional roles in male behaviors. We show that dpr and dpr-expressing cells are required for the proper timing of male courtship behaviors.

The fruit fly Drosophila is an excellent model system to use to understand the molecular-genetic basis of male courtship behavior, as the potential for this behavior is specified by a well-understood genetic regulatory hierarchy, called the sex determination hierarchy. The sex hierarchy consists of a pre-mRNA splicing cascade that culminates in the production of sex-specific transcription factors, encoded by doublesex (dsx) and fruitless (fru). dsx specifies all the anatomical differences between the sexes, and fru is required for all aspects of male courtship behavior. In this study, we measure gene expression differences between males and females, and between sex hierarchy mutants and wild-type animals, to identify genes that underlie the differences between males and females. We have performed these studies on adult head and nervous system tissues, as these tissues are important for establishing the potential for behaviors. We have identified several genes regulated downstream of dsx and fru and more extensively characterized two genes that are more highly expressed in males. One gene regulated downstream of dsx is expressed in the retina and is known to have a function in visual transduction. The other gene, regulated downstream of fru, plays a role in the timing of male courtship behavior.

RNA editing in Drosophila melanogaster: New targets and functional consequences

Adenosine deaminases that act on RNA [adenosine deaminase, RNA specific (ADAR)] catalyze the site-specific conversion of adenosine to inosine in primary mRNA transcripts. These re-coding events affect coding potential, splice sites, and stability of mature mRNAs. ADAR is an essential gene, and studies in mouse, Caenorhabditis elegans, and Drosophila suggest that its primary function is to modify adult behavior by altering signaling components in the nervous system. By comparing the sequence of isogenic cDNAs to genomic DNA, we have identified and experimentally verified 27 new targets of Drosophila ADAR. Our analyses led us to identify new classes of genes whose transcripts are targets of ADAR, including components of the actin cytoskeleton and genes involved in ion homeostasis and signal transduction. Our results indicate that editing in Drosophila increases the diversity of the proteome, and does so in a manner that has direct functional consequences on protein function.

Microscopy of the Drosophila facet eye: vademecum for standardized fixation, embedding, and sectioning

We describe here a standardized method for histological processing of the Drosophila compound eye. Primary fixation with 2.5% glutaraldehyde, obligatorily supplemented with 0.1% household detergent regularly yielded the best structural preservation, as compared with that of other, more complicated fixation protocols tested. Notably, it proved indispensable not only to cut off the fly's head to facilitate the penetration of the reagents but also to open the chitinous head capsule. For this, we locally pierced the cuticle between the eyes, leaving the head structurally almost intact, a prerequisite for precisely aligning the head for microtomy. We developed a two-step re-embedding procedure allowing for exact and reproducible orientation of the fly heads. Thus, highly comparable series of cross sections through a representative number of ommatidia were obtained. The feasibility of our embedding and sectioning approach is finally demonstrated by three-dimensional reconstructions of the middle segments of the R1, R7, and R8 photoreceptor cells. We present reconstructions from structurally modified ommatidia, as seen after RNAi-mediated depletion of the endosomal adaptor protein p14, and from normal ommatidia corresponding to the wildtype.

Preliminary characterisation of repeat families in the genome of EhV-86, a giant algal virus that infects the marine microalga Emiliania huxleyi

EhV-86 is a large double stranded DNA virus with a 407,339 base pair circular genome that infects the globally important microalga Emiliania huxleyi. It belongs to a new genus of viruses termed the Coccolithoviridae within the algal virus family Phycodnaviridae. By plotting the EhV-86 genome against itself in a dot-plot analysis we revealed three families of distinctly different repeat sequences throughout its genome, designated Family A, B and C. Family A repeats are non-coding, found immediately upstream of 86 predicted coding sequences (CDSs) and are likely to play a crucial role in controlling the expression of the associated CDSs. Family B repeats are GC rich, coding and correspond to possible calcium binding sites in 22 proline-rich domains found in the protein products of eight predicted EhV-86 CDSs. Family C repeats are AT-rich, non-coding and are likely to form part of the origin of replication. We suggest that these repeat regions are of fundamental importance during virus propagation being involved with transcriptional control (Family A), virus adsorption/release (Family B) and DNA replication (Family C).

A crustacean Ca2+-binding protein with a glutamate-rich sequence promotes CaCO3 crystallization

The DD4 mRNA of the penaeid prawn Penaeus japonicus was shown previously to be expressed in the epidermis adjacent to the exoskeleton specifically during the post-moult period, when calcification of the exoskeleton took place. The encoded protein possessed a Ca2+-binding site, suggesting its involvement in the calcification of the exoskeleton. In the present study, an additional ORF (open reading frame) of 289 amino acids was identified at the 5' end of the previous ORF. The newly identified part of the encoded protein included a region of approx. 120 amino acids that was highly rich in glutamate residues, and contained one or more Ca2+-binding sites. In an immunohistochemical study, signals were detected within calcified regions in the endocuticular layer of the exoskeleton. Bacterially expressed partial segments of the protein induced CaCO3 crystallization in vitro. Finally, a reverse transcription-PCR study showed that the expression was limited to an early part of the post-moult period, preceding significant calcification of the exoskeleton. These observations argue for the possibility that the encoded protein, renamed crustocalcin (CCN), promotes formation of CaCO3 crystals in the exoskeleton by inducing nucleation.

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.

Calcium binding to an elastic portion of connectin/titin filaments

Alpha-connectin/titin-1 exists as an elastic filament that links a thick filament with the Z-disk, keeping thick filaments centered within the sarcomere during force generation. We have shown that the connectin filament has an affinity for calcium ions and its binding site(s) is restricted to the beta-connectin/titin-2 portion. We now report the localization and the characterization of calcium-binding sites on beta-connectin. Purified beta-connectin was digested by trypsin into 1700- and 400-kDa fragments. which were then subjected to fluorescence calcium-binding assays. The 400-kDa fragment possesses calcium-binding activity; the binding constant was 1.0 x 10(7) M(-1) and the molar ratio of bound calcium ions to the 400-kDa fragment reached a maximum of 12 at a free calcium ion concentration of approximately 1.0 microM. Antibodies against the 400-kDa fragment formed a sharp dense stripe at the boundary of the A and the I bands, indicating that the calcium-binding domain constitutes the N-terminal region of beta-connectin, that is, the elastic portion of connectin filaments. Furthermore, we estimated the N-terminal location of beta-connectin of various origins (n = 26). Myofibrils were treated with a solution containing 0.1 mM CaCl2 and 70 microM leupeptin to split connectin filaments into beta-connectin and a subfragment, and chain weights of these polypeptides were estimated according to their mobility in 2% polyacrylamide slab gels. The subfragment exhibited a similar chain weight of 1200+/-33 kDa (mean+/-SD), while alpha- and beta-connectins were variable in size according to their origin. These results suggest that the apparent length of the 1200-kDa subfragment portion is almost constant in all instances, about 0.34 microm at the slack condition, therefore that the C-terminus of the 1200-kDa subfragment, that is, the N-terminus of the calcium-binding domain, is at the N2 line region of parent filaments in situ. Because the secondary structure of the 400-kDa fragment was changed by the binding of calcium ions, connectin filaments could be expected to alter their elasticity during the contraction-relaxation cycle of skeletal muscle.

Modular motif, structural folds and affinity profiles of the PEVK segment of human fetal skeletal muscle titin

The extension of the PEVK segment of the giant elastic protein titin is a key event in the elastic response of striated muscle to passive stretch. PEVK behaves mechanically as an entropic spring and is thought to be a random coil. cDNA sequencing of human fetal skeletal PEVK reveals a modular motif with tandem repeats of modules averaging 28 residues and with superrepeats of seven modules. Conformational studies of bacterially expressed 53-kDa fragment (TP1) by circular dichroism suggest that this soluble protein contains substantial polyproline II (PPII) type left-handed helices. Urea and thermal titrations cause gradual and reversible decrease in PPII content. The absence of sharp melting in urea and thermal titrations suggests that there is no long range cooperativity among the PPII helices. Studies with solid phase and surface plasmon resonance assays indicate that TP1 interacts with actin and some but not all cloned nebulin fragments with high affinity. Interestingly, Ca(2+)/calmodulin and Ca(2+)/S100 abolish nebulin/PEVK interaction. We suggest that in aqueous solution, PEVK is an open and flexible chain of relatively stable structural folds of the polyproline II type. PEVK region of titin may be involved in interfilament association with thin filaments in a calcium/calmodulin-sensitive manner. This adhesion may modulate titin extensibility and elasticity.

Molecular cloning of the crustacean DD4 cDNA encoding a Ca(2+)-binding protein

A cDNA, named DD4, was identified in the prawn Penaeus japonicus in a search for genes that were expressed during calcification of the crustacean exoskeleton. DD4 transcripts were detected in the epidermal cells underlying the exoskeleton specifically during the postmolt stage, when the calcification takes place. In the DD4 cDNA an open reading frame of 542 amino acids was found. The deduced protein was acidic and proline-rich, and exhibited similarity to the Drosophila Ca(2+)-binding protein calphotin in the amino acid sequence and composition. The DD4 cDNA was expressed in Escherichia coli to characterize Ca(2+)-binding of the encoded protein, and Ca(2+) was found to bind to a central segment of 186 amino acids. The DD4 protein is suggested to play a role in the calcification of the crustacean exoskeleton.

A hydrophobic peptide (VAP-peptide) of the silkworm, Bombyx mori: structure, expression and an enhancing function of diapause hormone activity

We have recently identified a unique lipophilic peptide (VAP-peptide) with diapause egg inducing activity in the silkworm, Bombyx mori (Imai et al., 1996). The cloning and sequencing of cDNA encoding VAP-peptide have demonstrated that the deduced amino acid sequence consisted of 84 amino acid residues, from which the mature VAP-peptide of 68 amino acid residues was released by cleaving a signal sequence. Searches of the GenBank data base revealed no significant sequence similarity to other proteins including diapause hormone (DH). VAP-peptide gene was selectively expressed just before and at adult eclosion in the head and the thorax not in the abdomen. By a Western blot analysis, VAP-peptide was also localized in the head and the thorax of adults. The purified recombinant VAP-peptide could not induce diapause eggs even when injected at a high dose of 10 nmol/pupa. Whereas, injection of a mixture of VAP-peptide and DH clearly decreased a half-maximum dose (ED50 value) and a threshold dose (TD value) of DH, and these values decreased according to increasing molar ratios of VAP-peptide to DH. Thus, the VAP-peptide is concluded to be an endogenous protein acting as a potent enhancer of DH activity through interaction with DH.

Vitamin A, visual pigments, and visual receptors in Drosophila

The fly visual system has served for decades as a model for receptor spectral multiplicity and vitamin A utilization. A diverse armamentarium of structural techniques has dovetailed with convenient electrophysiology, photochemistry, genetics, and molecular biology in Drosophila to facilitate recent progress, which is reviewed here. New data are also presented. Ultrastructure of retinula cells of carotenoid-deprived flies shows that organelles associated with protein biosynthesis, i.e., rough endoplasmic reticulum and Golgi apparatus, are present, while organelles associated with rhabdomere turnover, i.e., multivesicular bodies (MVBs), are rare. Ultrastructure and morphometry suggest that retinoic acid-rearing stimulates membrane export and rhabdomere buildup, even though functional rhodopsin is missing. Confocal microscopy suggests that RH4, one of the ultraviolet rhodopsins, may reside in the previously-described pale fluorescent R7 cells with RH3 in the yellow fluorescent R7 cells.

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.

A Drosophila photoreceptor cell-specific protein, calphotin, binds calcium and contains a leucine zipper

The calphotin protein, encoded by the calphotin (cap) gene, is expressed in the soma and axons of all Drosophila photoreceptor cells. It is expressed early in photo-receptor cell development, at the time when cell-type decisions are being made. Expression of calphotin is not altered by the glass mutation, which blocks photoreceptor cell development. The calphotin protein binds calcium and contains a long C-terminal leucine zipper. Potential implications of these properties are discussed.