Role of asparagine-linked oligosaccharides in rhodopsin maturation and association with its molecular chaperone, NinaA

Xport-A functions as a chaperone by stabilizing the first five transmembrane domains of rhodopsin-1

Rhodopsin-1 (Rh1), the main photosensitive protein of Drosophila, is a seven-transmembrane domain protein, which is inserted co-translationally in the endoplasmic reticulum (ER) membrane. Biogenesis of Rh1 occurs in the ER, where various chaperones interact with Rh1 to aid in its folding and subsequent transport from the ER to the rhabdomere, the light-sensing organelle of the photoreceptors. Xport-A has been proposed as a chaperone/transport factor for Rh1, but the exact molecular mechanism for Xport-A activity upon Rh1 is unknown. Here, we propose a model where Xport-A functions as a chaperone during the biogenesis of Rh1 in the ER by stabilizing the first five transmembrane domains (TMDs) of Rh1.

Ecology-relevant bacteria drive the evolution of host antimicrobial peptides in Drosophila

Antimicrobial peptides are host-encoded immune effectors that combat pathogens and shape the microbiome in plants and animals. However, little is known about how the host antimicrobial peptide repertoire is adapted to its microbiome. Here, we characterized the function and evolution of the Diptericin antimicrobial peptide family of Diptera. Using mutations affecting the two Diptericins (Dpt) of Drosophila melanogaster, we reveal the specific role of DptA for the pathogen Providencia rettgeri and DptB for the gut mutualist Acetobacter. The presence of DptA- or DptB-like genes across Diptera correlates with the presence of Providencia and Acetobacter in their environment. Moreover, DptA- and DptB-like sequences predict host resistance against infection by these bacteria across the genus Drosophila. Our study explains the evolutionary logic behind the bursts of rapid evolution of an antimicrobial peptide family and reveals how the host immune repertoire adapts to changing microbial environments.

EMC is required for biogenesis of Xport-A, an essential chaperone of Rhodopsin-1 and the TRP channel

The ER membrane protein complex (EMC) is required for the biogenesis of a subset of tail anchored (TA) and polytopic membrane proteins, including Rhodopsin-1 (Rh1) and the TRP channel. To understand the physiological implications of EMC-dependent membrane protein biogenesis, we perform a bioinformatic identification of Drosophila TA proteins. From 254 predicted TA proteins, screening in larval eye discs identified two proteins that require EMC for their biogenesis: fan and Xport-A. Fan is required for male fertility in Drosophila and we show that EMC is also required for this process. Xport-A is essential for the biogenesis of both Rh1 and TRP, raising the possibility that disruption of Rh1 and TRP biogenesis in EMC mutants is secondary to the Xport-A defect. We show that EMC is required for Xport-A TMD membrane insertion and that EMC-independent Xport-A mutants rescue Rh1 and TRP biogenesis in EMC mutants. Finally, our work also reveals a role for Xport-A in a glycosylation-dependent triage mechanism during Rh1 biogenesis in the endoplasmic reticulum.

Knockdown of Dehydrodolichyl Diphosphate Synthase in the Drosophila Retina Leads to a Unique Pattern of Retinal Degeneration

Dehydrodolichyl diphosphate synthase (DHDDS) is a ubiquitously expressed enzyme that catalyzes cis-prenyl chain elongation to produce the poly-prenyl backbone of dolichol. It appears in all tissues including the nervous system and it is a highly conserved enzyme that can be found in all animal species. Individuals who have biallelic missense mutations in the DHDDS gene are presented with non-syndromic retinitis pigmentosa with unknown underlying mechanism. We have used the Drosophila model to compromise DHDDS ortholog gene (CG10778) in order to look for cellular and molecular mechanisms that, when defective, might be responsible for this retinal disease. The Gal4/UAS system was used to suppress the expression of CG10778 via RNAi-mediated-knockdown in various tissues. The resulting phenotypes were assessed using q-RT-PCR, transmission-electron-microscopy (TEM), electroretinogram, antibody staining and Western blot analysis. Targeted knockdown of CG10778-mRNA in the early embryo using the actin promoter or in the developing wings using the nub promoter resulted in lethality, or wings loss, respectively. Targeted expression of CG10778-RNAi using the glass multiple reporter (GMR)-Gal4 driver (GMR-DHDDS-RNAi) in the larva eye disc and pupal retina resulted in a complex phenotype: (a) TEM retinal sections revealed a unique pattern of retinal-degeneration, where photoreceptors R2 and R5 exhibited a nearly normal structure of their signaling-compartment (rhabdomere), but only at the region of the nucleus, while all other photoreceptors showed retinal degeneration at all regions. (b) Western blot analysis revealed a drastic reduction in rhodopsin levels in GMR-DHDDS-RNAi-flies and TEM sections showed an abnormal accumulation of endoplasmic reticulum (ER). To conclude, compromising DHDDS in the developing retina, while allowing formation of the retina, resulted in a unique pattern of retinal degeneration, characterized by a dramatic reduction in rhodopsin protein level and an abnormal accumulation of ER membranes in the photoreceptors cells, thus indicating that DHDDS is essential for normal retinal formation.

N-glycosylation of α1D-adrenergic receptor N-terminal domain is required for correct trafficking, function, and biogenesis

G protein-coupled receptor (GPCR) biogenesis, trafficking, and function are regulated by post-translational modifications, including N-glycosylation of asparagine residues. α1D-adrenergic receptors (α1D-ARs) - key regulators of central and autonomic nervous system function - contain two putative N-glycosylation sites within the large N-terminal domain at N65 and N82. However, determining the glycosylation state of this receptor has proven challenging. Towards understanding the role of these putative glycosylation sites, site-directed mutagenesis and lectin affinity purification identified N65 and N82 as bona fide acceptors for N-glycans. Surprisingly, we also report that simultaneously mutating N65 and N82 causes early termination of α1D-AR between transmembrane domain 2 and 3. Label-free dynamic mass redistribution and cell surface trafficking assays revealed that single and double glycosylation deficient mutants display limited function with impaired plasma membrane expression. Confocal microscopy imaging analysis and SNAP-tag sucrose density fractionation assays revealed the dual glycosylation mutant α1D-AR is widely distributed throughout the cytosol and nucleus. Based on these novel findings, we propose α1D-AR transmembrane domain 2 acts as an ER localization signal during active protein biogenesis, and that α1D-AR N-terminal glycosylation is required for complete translation of nascent, functional receptor.

Adaptation to constant light requires Fic-mediated AMPylation of BiP to protect against reversible photoreceptor degeneration

In response to environmental, developmental, and pathological stressors, cells engage homeostatic pathways to maintain their function. Among these pathways, the Unfolded Protein Response protects cells from the accumulation of misfolded proteins in the ER. Depending on ER stress levels, the ER-resident Fic protein catalyzes AMPylation or de-AMPylation of BiP, the major ER chaperone and regulator of the Unfolded Protein Response. This work elucidates the importance of the reversible AMPylation of BiP in maintaining the Drosophila visual system in response to stress. After 72 hr of constant light, photoreceptors of fic-null and AMPylation-resistant BiPT366A mutants, but not wild-type flies, display loss of synaptic function, disintegration of rhabdomeres, and excessive activation of ER stress reporters. Strikingly, this phenotype is reversible: photoreceptors regain their structure and function within 72 hr once returned to a standard light:dark cycle. These findings show that Fic-mediated AMPylation of BiP is required for neurons to adapt to transient stress demands.

The V-ATPase V1 subunit A1 is required for rhodopsin anterograde trafficking in Drosophila

Synthesis and maturation of the light sensor, rhodopsin, are critical for the maintenance of light sensitivity and for photoreceptor homeostasis. In Drosophila, the main rhodopsin, Rh1, is synthesized in the endoplasmic reticulum and transported to the rhabdomere through the secretory pathway. In an unbiased genetic screen for factors involved in rhodopsin homeostasis, we identified mutations in vha68-1, which encodes the vacuolar proton-translocating ATPase (V-ATPase) catalytic subunit A isoform 1 of the V1 component. Loss of vha68-1 in photoreceptor cells disrupted post-Golgi anterograde trafficking of Rh1, reduced light sensitivity, increased secretory vesicle pH, and resulted in incomplete Rh1 deglycosylation. In addition, vha68-1 was required for activity-independent photoreceptor cell survival. Importantly, vha68-1 mutants exhibited phenotypes similar to those exhibited by mutations in the V0 component of V-ATPase, vha100-1. These data demonstrate that the V1 and V0 components of V-ATPase play key roles in post-Golgi trafficking of Rh1 and that Drosophila may represent an important animal model system for studying diseases associated with V-ATPase dysfunction.

Light-Driven Processes Control Both Rhodopsin Maturation and Recycling in Mosquito Photoreceptors

Many invertebrates carry out a daily cycle of shedding and rebuilding of the photoreceptor's photosensitive rhabdomeric membranes. The mosquito Aedes aegypti shows a robust response, losing nearly all Aaop1 rhodopsin from the rhabdomeric membranes during the shedding process at dawn. Here, we made use of Aaop1 antibodies capable of distinguishing newly synthesized, glycosylated rhodopsin from mature nonglycosylated rhodopsin to characterize the fate of Aaop1 during the shedding and rebuilding processes. The rhabdomeric rhodopsin is moved into large cytoplasmic vesicles at dawn and is subsequently degraded during the standard 12 h daytime period. The endocytosed rhodopsin is trafficked back to the photosensitive membranes if animals are shifted back to dark conditions during the morning hours. During the daytime period, small vesicles containing newly synthesized and glycosylated Aaop1 rhodopsin accumulate within the cytoplasm. At dusk, these vesicles are lost as the newly synthesized Aaop1 is converted to the nonglycosylated form and deposited in the rhabdomeres. We demonstrate that light acts though a novel signaling pathway to block rhodopsin maturation, thus inhibiting the deglycosylation and rhabdomeric targeting of newly synthesized Aaop1 rhodopsin. Therefore, light controls two cellular processes responsible for the daily renewal of rhodopsin: rhodopsin endocytosis at dawn and inhibition of rhodopsin maturation until dusk.

SIGNIFICANCE STATEMENT

Organisms use multiple strategies to maximize visual capabilities in different light conditions. Many invertebrates show a daily cycle of shedding the photoreceptor's rhabdomeric membranes at dawn and rebuilding these during the following night. We show here that the Aedes aegypti mosquito possesses two distinct light-driven cellular signaling processes for modulating rhodopsin content during this cycle. One of these, endocytosis of rhabdomeric rhodopsin, has been described previously. The second, a light-activated cellular pathway acting to inhibit the anterograde movement of newly synthesized rhodopsin, is revealed here for the first time. The discovery of this cellular signaling pathway controlling a G-protein-coupled receptor is of broad interest due to the prominent role of this receptor family across all areas of neuroscience.

Transforming Lepidopteran Insect Cells for Improved Protein Processing and Expression

The lepidopteran insect cells used with the baculovirus expression vector system (BEVS) are capable of synthesizing and accurately processing foreign proteins. However, proteins expressed in baculovirus-infected cells often fail to be completely processed, or are not processed in a manner that meets a researcher's needs. This chapter discusses a metabolic engineering approach that addresses this problem. Basically, this approach involves the addition of new or enhancement of existing protein processing functions in established lepidopteran insect cell lines. In addition to improvements in protein processing, this approach has also been used to improve protein expression levels obtained with the BEVS. Methods for engineering cell lines and assessing their properties as improved hosts for the BEVS are detailed. Examples of lepidopteran insect cell lines engineered for improved protein N-glycosylation, folding/trafficking, and expression are described in detail.

Glycosylation of rhodopsin is necessary for its stability and incorporation into photoreceptor outer segment discs

Rhodopsin, a G-protein coupled receptor, most abundant protein in retinal rod photoreceptors, is glycosylated at asparagines-2 and 15 on its N-terminus. To understand the role of rhodopsin's glycosylation in vivo, we generated and characterized a transgenic mouse model that expresses a non-glycosylated form of rhodopsin. We show that lack of glycosylation triggers a dominant form of progressive retinal degeneration. Electron microscopic examination of retinas at postnatal day 17 revealed the presence of vacuolar structures that distorted rod photoreceptor outer segments and became more prominent with age. Expression of non-glycosylated rhodopsin alone showed that it is unstable and is regulated via ubiquitin-mediated proteasomal degradation at the base of outer segments. We observed similar vacuolization in outer segments of transgenic mice expressing human rhodopsin with a T17M mutation (hT17M), suggesting that the mechanism responsible for the degenerative process in mice expressing the non-glycosylated rhodopsin and the RHO(hT17M) mice is likely the cause of phenotype observed in retinitis pigmentosa patients carrying T17M mutation.

The Gos28 SNARE protein mediates intra-Golgi transport of rhodopsin and is required for photoreceptor survival

SNARE proteins play indispensable roles in membrane fusion events in many cellular processes, including synaptic transmission and protein trafficking. Here, we characterize the Golgi SNARE protein, Gos28, and its role in rhodopsin (Rh1) transport through Drosophila photoreceptors. Mutations in gos28 lead to defective Rh1 trafficking and retinal degeneration. We have pinpointed a role for Gos28 in the intra-Golgi transport of Rh1, downstream from α-mannosidase-II in the medial- Golgi. We have confirmed the necessity of key residues in Gos28's SNARE motif and demonstrate that its transmembrane domain is not required for vesicle fusion, consistent with Gos28 functioning as a t-SNARE for Rh1 transport. Finally, we show that human Gos28 rescues both the Rh1 trafficking defects and retinal degeneration in Drosophila gos28 mutants, demonstrating the functional conservation of these proteins. Our results identify Gos28 as an essential SNARE protein in Drosophila photoreceptors and provide mechanistic insights into the role of SNAREs in neurodegenerative disease.

Rhodopsin management during the light-dark cycle of Anopheles gambiae mosquitoes

The tropical disease vector mosquito Anopheles gambiae possesses 11 rhodopsin genes. Three of these, GPROP1, GPROP3, and GPROP4, encode rhodopsins with >99% sequence identity. We created antisera against these rhodopsins and used immunohistology to show that one or more of these rhodopsins are expressed in the major R1-6 photoreceptor class of the adult A.gambiae eye. Under dark conditions, rhodopsin accumulates within the light-sensitive rhabdomere of the photoreceptor. Light treatment, however, causes extensive movement of rhodopsin to the cytoplasmic compartment. Protein electrophoresis showed that the rhodopsin is present in two different forms. The larger form is an immature species that is deglycosylated during the posttranslational maturation process to generate the smaller, mature form. The immature form is maintained at a constant level regardless of lighting conditions. These results indicate that rhodopsin biosynthesis and movement into the rhabdomere occurs at a constant rate. In contrast, the mature form increases in abundance when animals are placed in dark conditions. Light-triggered internalization and protein degradation counteracts this rhodopsin increase and keeps rhabdomeric rhodopsin levels low in light conditions. The interplay of the constant maturation rate with light-triggered degradation causes rhodopsin to accumulate within the rhabdomere only in dark conditions. Thus, Anopheles photoreceptors possess a mechanism for adjusting light sensitivity through light-dependent control of rhodopsin levels and cellular location.

Phosphatidic acid phospholipase A1 mediates ER-Golgi transit of a family of G protein-coupled receptors

Cytosolic phosphatidic acid phospholipase A1 interacts with COPII protein family members and is required for the anterograde trafficking of GPCRs.

The coat protein II (COPII)–coated vesicular system transports newly synthesized secretory and membrane proteins from the endoplasmic reticulum (ER) to the Golgi complex. Recruitment of cargo into COPII vesicles requires an interaction of COPII proteins either with the cargo molecules directly or with cargo receptors for anterograde trafficking. We show that cytosolic phosphatidic acid phospholipase A1 (PAPLA1) interacts with COPII protein family members and is required for the transport of Rh1 (rhodopsin 1), an N-glycosylated G protein–coupled receptor (GPCR), from the ER to the Golgi complex. In papla1 mutants, in the absence of transport to the Golgi, Rh1 is aberrantly glycosylated and is mislocalized. These defects lead to decreased levels of the protein and decreased sensitivity of the photoreceptors to light. Several GPCRs, including other rhodopsins and Bride of sevenless, are similarly affected. Our findings show that a cytosolic protein is necessary for transit of selective transmembrane receptor cargo by the COPII coat for anterograde trafficking.

Mutations in four glycosyl hydrolases reveal a highly coordinated pathway for rhodopsin biosynthesis and N-glycan trimming in Drosophila melanogaster

As newly synthesized glycoproteins move through the secretory pathway, the asparagine-linked glycan (N-glycan) undergoes extensive modifications involving the sequential removal and addition of sugar residues. These modifications are critical for the proper assembly, quality control and transport of glycoproteins during biosynthesis. The importance of N-glycosylation is illustrated by a growing list of diseases that result from defects in the biosynthesis and processing of N-linked glycans. The major rhodopsin in Drosophila melanogaster photoreceptors, Rh1, is highly unique among glycoproteins, as the N-glycan appears to be completely removed during Rh1 biosynthesis and maturation. However, much of the deglycosylation pathway for Rh1 remains unknown. To elucidate the key steps in Rh1 deglycosylation in vivo, we characterized mutant alleles of four Drosophila glycosyl hydrolases, namely α-mannosidase-II (α-Man-II), α-mannosidase-IIb (α-Man-IIb), a β-N-acetylglucosaminidase called fused lobes (Fdl), and hexosaminidase 1 (Hexo1). We have demonstrated that these four enzymes play essential and unique roles in a highly coordinated pathway for oligosaccharide trimming during Rh1 biosynthesis. Our results reveal that α-Man-II and α-Man-IIb are not isozymes like their mammalian counterparts, but rather function at distinct stages in Rh1 maturation. Also of significance, our results indicate that Hexo1 has a biosynthetic role in N-glycan processing during Rh1 maturation. This is unexpected given that in humans, the hexosaminidases are typically lysosomal enzymes involved in N-glycan catabolism with no known roles in protein biosynthesis. Here, we present a genetic dissection of glycoprotein processing in Drosophila and unveil key steps in N-glycan trimming during Rh1 biosynthesis. Taken together, our results provide fundamental advances towards understanding the complex and highly regulated pathway of N-glycosylation in vivo and reveal novel insights into the functions of glycosyl hydrolases in the secretory pathway.

Retinal proteins as model systems for membrane protein folding

Experimental folding studies of membrane proteins are more challenging than water-soluble proteins because of the higher hydrophobicity content of membrane embedded sequences and the need to provide a hydrophobic milieu for the transmembrane regions. The first challenge is their denaturation: due to the thermodynamic instability of polar groups in the membrane, secondary structures in membrane proteins are more difficult to disrupt than in soluble proteins. The second challenge is to refold from the denatured states. Successful refolding of membrane proteins has almost always been from very subtly denatured states. Therefore, it can be useful to analyze membrane protein folding using computational methods, and we will provide results obtained with simulated unfolding of membrane protein structures using the Floppy Inclusions and Rigid Substructure Topography (FIRST) method. Computational methods have the advantage that they allow a direct comparison between diverse membrane proteins. We will review here both, experimental and FIRST studies of the retinal binding proteins bacteriorhodopsin and mammalian rhodopsin, and discuss the extension of the findings to deriving hypotheses on the mechanisms of folding of membrane proteins in general. This article is part of a Special Issue entitled: Retinal Proteins-You can teach an old dog new tricks.

Rhodopsin homeostasis and retinal degeneration: lessons from the fly

Rhodopsins (Rh) are G protein-coupled receptors that function as light-sensors in photoreceptors. In humans, Rh mutations cause retinitis pigmentosa (RP), a degenerative disease that ultimately results in blindness. Studies in Drosophila have provided many insights into basic Rh biology and have identified pathways that lead to retinal degeneration. It has been shown that, because Rh is very abundant in photoreceptors, its accumulation in numerous organelles induces severe stress and results in degeneration of these cells. Moreover, genetic lesions that affect proper activation of membrane-bound Rh lead to disruption in Ca(2+) homeostasis which also causes photoreceptor degeneration. We review here the molecular signals involved in Rh homeostasis and the mechanisms underlying retinal degeneration in flies, and discuss possible links to human diseases.

The N's and O's of Drosophila glycoprotein glycobiology

The past 25 years have seen significant advances in understanding the diversity and functions of glycoprotein glycans in Drosophila melanogaster. Genetic screens have captured mutations that reveal important biological activities modulated by glycans, including protein folding and trafficking, as well as cell signaling, tissue morphogenesis, fertility, and viability. Many of these glycan functions have parallels in vertebrate development and disease, providing increasing opportunities to dissect pathologic mechanisms using Drosophila genetics. Advances in the sensitivity of structural analytic techniques have allowed the glycan profiles of wild-type and mutant tissues to be assessed, revealing novel glycan structures that may be functionally analogous to vertebrate glycans. This review describes a selected set of recent advances in understanding the functions of N-linked and O-linked (non-glycosaminoglycan) glycoprotein glycans in Drosophila with emphasis on their relatedness to vertebrate organisms.

A role for the cytoplasmic DEAD box helicase Dbp21E2 in rhodopsin maturation and photoreceptor viability

The Dbp21E2 (DEAD box protein 21E2) is a member of a family of DEAD box helicases active in RNA processing and stability. The authors used genetic mosaics to identify mutants in Dbp21E2 that affect rhodopsin biogenesis and the maintenance of photoreceptor structure. Analysis of a green fluorescent protein (GFP)-tagged Rh1 rhodopsin construct placed under control of a heat shock promoter showed that Dbp21E21 fails to efficiently transport Rh1 from the photoreceptor cell body to the rhabdomere. Retinal degeneration is not dependent on the Rh1 transport defects. The authors also showed that GFP- and red fluorescent protein (RFP)-tagged Dbp21E2 proteins are localized to discrete cytoplasmic structures that are not associated with organelles known to be active in rhodopsin transport. The molecular genetic analysis described here reveals an unexpected role for the Dbp21E2 helicase and provides an experimental system to further characterize its function.

Biochemical and pharmacological study of N-linked glycosylation of the human serotonin 5-HT₇a receptor

The 5-hydroxytryptamine (5-HT)(7(a)) receptor is a G-protein-coupled receptor critically involved in human psychiatric and neurological disorders. In the present study, we evaluate the presence and the functional role of N-glycosylation of the human 5-HT(7) receptor. Western blot analysis of HEK293T cells transiently expressing the 5-HT(7(a)) receptor in the presence of tunicamycin gave rise to a band shift, indicating the existence of an N-glycosylated form of the 5-HT(7(a)) receptor. To further investigate this, we mutated the two predicted N-glycosylation sites (N5Q and N66Q) and compared the molecular mass of the immunoreactive bands with those of the wild-type receptor, indicating that both asparagines were N-glycosylated. The mutant receptors had the same binding affinity for [(3) H]5-CT and the same potency and efficacy with regard to 5-HT-induced activation of adenylyl cyclase. However, there was a reduction in maximal ligand binding for the single and double mutants compared to the wild-type receptor. Next, membrane labelling and immunocytochemical studies demonstrated that the N-glycosylation mutants were expressed at the cell surface. We conclude that N-glycosylation is not important for cell surface expression of the 5-HT(7) receptor.

XPORT-dependent transport of TRP and rhodopsin

TRP channels have emerged as key biological sensors in vision, taste, olfaction, hearing, and touch. Despite their importance, virtually nothing is known about the folding and transport of TRP channels during biosynthesis. Here, we identify XPORT (exit protein of rhodopsin and TRP) as a critical chaperone for TRP and its G protein-coupled receptor (GPCR), rhodopsin (Rh1). XPORT is a resident ER and secretory pathway protein that interacts with TRP and Rh1, as well as with Hsp27 and Hsp90. XPORT promotes the targeting of TRP to the membrane in Drosophila S2 cells, a finding that provides a critical first step toward solving a longstanding problem in the successful heterologous expression of TRP. Mutations in xport result in defective transport of TRP and Rh1, leading to retinal degeneration. Our results identify XPORT as a molecular chaperone and provide a mechanistic link between TRP channels and their GPCRs during biosynthesis and transport.

Molecular genetics of retinal degeneration: A Drosophila perspective

Inherited retinal degeneration in Drosophila has been explored for insights into similar processes in humans. Based on the mechanisms, I divide these mutations in Drosophila into three classes. The first consists of genes that control the specialization of photoreceptor cells including the morphogenesis of visual organelles  (rhabdomeres) that house the visual signaling proteins. The second class contains genes that regulate the activity or level of the major rhodopsin, Rh1, which is the light sensor and also provides a structural role for the maintenance of rhabdomeres. Some mutations in Rh1 (NinaE) are dominant due to constitutive activity or folding defects, like autosomal dominant retinitis pigmentosa (ADRP) in humans. The third class consists of genes that control the Ca ( 2+) influx directly or indirectly by promoting the turnover of the second messenger and regeneration of PIP 2, or mediate the Ca ( 2+) -dependent regulation of the visual response. These gene products are critical for the increase in cytosolic Ca ( 2+ ) following light stimulation to initiate negative regulatory events. Here I will focus on the signaling mechanisms underlying the degeneration in norpA, and in ADRP-type NinaE mutants that produce misfolded Rh1. Accumulation of misfolded Rh1 in the ER triggers the unfolded protein response (UPR), while endosomal accumulation of activated Rh1 may initiate autophagy in norpA. Both autophagy and the UPR are beneficial for relieving defective endosomal trafficking and the ER stress, respectively. However, when photoreceptors fail to cope with the persistence of these stresses, a cell death program is activated leading to retinal degeneration.

A Drosophila metallophosphoesterase mediates deglycosylation of rhodopsin

Oligosaccharide chains of newly synthesized membrane receptors are trimmed and modified to optimize their trafficking and/or signalling before delivery to the cell surface. For most membrane receptors, the functional significance of oligosaccharide chain modification is unknown. During the maturation of Rh1 rhodopsin, a Drosophila light receptor, the oligosaccharide chain is trimmed extensively. Neither the functional significance of this modification nor the enzymes mediating this process are known. Here, we identify a dmppe (Drosophila metallophosphoesterase) mutant with incomplete deglycosylation of Rh1, and show that the retained oligosaccharide chain does not affect Rh1 localization or signalling. The incomplete deglycosylation, however, renders Rh1 more sensitive to endocytic degradation, and causes morphological and functional defects in photoreceptors of aged dmppe flies. We further demonstrate that the dMPPE protein functions as an Mn(2+)/Zn(2+)-dependent phosphoesterase and mediates in vivo dephosphorylation of α-Man-II. Most importantly, the dephosphorylated α-Man-II is required for the removal of the Rh1 oligosaccharide chain. These observations suggest that the glycosylation status of membrane proteins is controlled through phosphorylation/dephosphorylation, and that MPPE acts as the phosphoesterase in this regulation.

Drosophila arf72A acts as an essential regulator of endoplasmic reticulum quality control and suppresses autosomal-dominant retinopathy

The eukaryotic endoplasmic reticulum operates multiple quality control mechanisms to ensure that only properly folded proteins are exported to their final destinations via the secretory pathway and those that are not are destroyed via the degradation pathway. However, molecular mechanisms underlying such regulated exportation to these distinct routes are unknown. In this article, we report the role of Drosophila arf72A--the fly homologue of the mammalian Arl1 - in the quality checks of proteins and in the autosomal-dominant retinopathy. ARF72A localizes to the Golgi membranes of Drosophila photoreceptor cells, consistent with mammalian Arl1 localization in cell culture systems. A loss of arf72A function changes the membrane character of the endoplasmic reticulum and shifts the membrane balance between the endoplasmic reticulum and the Golgi complex toward the Golgi complex, resulting in over-proliferated Golgi complexes and accelerated protein secretion. Interestingly, our study indicated that more ARF72A localized on the endoplasmic reticulum in the ninaE(D1) photoreceptor cell, a Drosophila model of autosomal-dominant retinitis pigmentosa, compared to that in the wild-type. In addition, arf72A loss was shown to rescue the ninaE(D1)-related membrane accumulation and the rhodopsin maturation defect, and suppress ninaE(D1)-triggered retinal degeneration, indicating that rhodopsin accumulated in the endoplasmic reticulum bypasses the quality checks. While previous studies of ARF small GTPases have focused on their roles in vesicular budding and transport between the specific organelles, our findings establish an additional function of arf72A in the quality check machinery of the endoplasmic reticulum distinguishing the cargoes for secretion from those for degradation.

Analysis of disease-linked rhodopsin mutations based on structure, function, and protein stability calculations

Retinitis pigmentosa (RP) refers to a heterogeneous group of inherited diseases that result in progressive retinal degeneration, characterized by visual field constriction and night blindness. A total of 103 mutations in rhodopsin are linked to RP to date, and the phenotypes range from severe to asymptomatic. To study the relation between phenotype and rhodopsin stability in disease mutants, we used a structure-based approach. For 12 of the mutants located at the protein-lipid interphase, we used the von Heijne water-membrane transfer scale, and we find that 9 of the mutations could affect membrane insertion. For 91 mutants, we used the protein design algorithm FoldX. The 3 asymptomatic mutations had no significant reduced stability, 2 were unsuitable for FoldX analysis since the structure was incorrect in this region, 63 mutations had a significant change in protein stability (>1.6 kcal/mol), and 23 mutations had energy change values under the prediction error threshold (<1.6 kcal/mol). Out of these 23, the disease-causing effect could be explained by the involvement in other functions (e.g., glycosylation motifs, the interface with arrestin and transducin, and the cilia-binding motif) for 19 mutants. The remaining 4 mutants were probably incorrectly associated with RP or have functionalities not discovered yet. For destabilizing mutations where clinical data were available, we found a highly significant correlation between FoldX energy changes and the average age of night blindness and between FoldX energy changes and daytime vision loss onset. Our detailed structural, functional, and energetic analysis provides a complete picture of the rhodopsin mutations and can guide mutation-specific therapies.

Autophagy-dependent rhodopsin degradation prevents retinal degeneration in Drosophila

Recent studies have demonstrated protective roles for autophagy in various neurodegenerative disorders, including the polyglutamine diseases; however, the role of autophagy in retinal degeneration has remained unclear. Accumulation of activated rhodopsin in some Drosophila mutants leads to retinal degeneration, and although it is known that activated rhodopsin is degraded in endosomal pathways in normal photoreceptor cells, the contribution of autophagy to rhodopsin regulation has remained elusive. This study reveals that activated rhodopsin is degraded by autophagy in collaboration with endosomal pathways to prevent retinal degeneration. Light-dependent retinal degeneration in the Drosophila visual system is caused by the knockdown or mutation of autophagy-essential components, such as autophagy-related protein 7 and 8 (atg-7/atg-8), or genes essential for PE (phosphatidylethanolamine) biogenesis and autophagosome formation, including Phosphatidylserine decarboxylase (Psd) and CDP-ethanolamine:diacylglycerol ethanolaminephosphotransferase (Ept). The knockdown of atg-7/8 or Psd/Ept produced an increase in the amount of rhodopsin localized to Rab7-positive late endosomes. This rhodopsin accumulation, followed by retinal degeneration, was suppressed by overexpression of Rab7, which accelerated the endosomal degradation pathway. These results indicate a degree of cross talk between the autophagic and endosomal/lysosomal pathways. Importantly, a reduction in rhodopsin levels rescued Psd knockdown-induced retinal degeneration. Additionally, the Psd knockdown-induced retinal degeneration phenotype was enhanced by Ppt1 inactivation, which causes infantile neuronal ceroid lipofuscinosis, implying that autophagy plays a significant role in its pathogenesis. Collectively, the current data reveal that autophagy suppresses light-dependent retinal degeneration in collaboration with the endosomal degradation pathway and that rhodopsin is a key substrate for autophagic degradation in this context.

Rhodopsin: the functional significance of asn-linked glycosylation and other post-translational modifications

Rhodopsin, the G-protein coupled receptor in retinal rod photoreceptors, is a highly conserved protein that undergoes several types of post-translational modifications. These modifications are essential to maintain the protein's structure as well as its proper function in the visual transduction cycle. Rhodopsin is N-glycosylated at Asn-2 and Asn-15 in its extracellular N-terminal domain. Mutations within the glycosylation consensus sequences of rhodopsin cause autosomal dominant retinitis pigmentosa, a disease that leads to blindness. Several groups have studied the role of rhodopsin's N-linked glycan chains in protein structure and function using a variety of approaches. These include the generation of a transgenic mouse model, study of a naturally occurring mutant animal model, in vivo pharmacological inhibition of glycosylation, and in vitro analyses using transfected COS-1 cells. These studies have provided insights into the possible role of rhodopsin glycosylation, but have yielded conflicting results.

Regulation of G-protein coupled receptor traffic by an evolutionary conserved hydrophobic signal

Plasma membrane (PM) expression of G-protein coupled receptors (GPCRs) is required for activation by extracellular ligands; however, mechanisms that regulate PM expression of GPCRs are poorly understood. For some GPCRs, such as alpha2c-adrenergic receptors (alpha(2c)-ARs), heterologous expression in non-native cells results in limited PM expression and extensive endoplasmic reticulum (ER) retention. Recently, ER export/retentions signals have been proposed to regulate cellular trafficking of several GPCRs. By utilizing a chimeric alpha(2a)/alpha(2c)-AR strategy, we identified an evolutionary conserved hydrophobic sequence (ALAAALAAAAA) in the extracellular amino terminal region that is responsible in part for alpha(2c)-AR subtype-specific trafficking. To our knowledge, this is the first luminal ER retention signal reported for a GPCR. Removal or disruption of the ER retention signal dramatically increased PM expression and decreased ER retention. Conversely, transplantation of this hydrophobic sequence into alpha(2a)-ARs reduced their PM expression and increased ER retention. This evolutionary conserved hydrophobic trafficking signal within alpha(2c)-ARs serves as a regulator of GPCR trafficking.

N-linked deglycosylated melanopsin retains its responsiveness to light

Melanopsin is an opsin expressed in the plasma membrane of retinal ganglion cells that mainly project to the circadian clock and thus is important for nonvisual responses to light. Rat melanopsin contains two potential sites (Asn31 and Asn35) for N-linked glycosylation in the N-terminal extracellular part. To investigate if melanopsin is N-linked glycosylated and whether N-bound glycans influence the response of melanopsin to light as evidenced by Fos mRNA induction, we transfected PC12 cells to stably express rat wild-type melanopsin or mutant melanopsin lacking both N-linked glycosylation sites. Immunoblotting for membrane-bound melanopsin from the PC12 cells transfected to express wild-type melanopsin disclosed two immunoreactive bands of 62 and 49 kDa. Removal of N-linked glycosylation by tunicamycin or PNGase F changed the 62 kDa band to a 55 kDa band, while the 49 kDa band corresponding to the core melanopsin protein was unaffected. Likewise, mutation of the two extracellular N-linked glycosylation sites gave a melanopsin size comparable to that of PNGase F or tunicamycin treatment (55 kDa). Further in vitro O-linked deglycosylation of wild-type or mutant melanopsin with O-glycosidase and neuraminidase converted the 55 kDa band to a 49 kDa band. Finally, neither in vivo N-linked deglycosylation nor mutations of the two N-linked glycosylation sites significantly affected melanopsin function measured by Fos induction after light stimulation. In conclusion, we have shown that heterologously expressed rat melanopsin is both N-linked and O-linked glycosylated and that N-linked glycosylation is not crucial for the melanopsin response to light.

N-glycan-mediated quality control in the endoplasmic reticulum is required for the expression of correctly folded delta-opioid receptors at the cell surface

A great majority of G protein-coupled receptors are modified by N-glycosylation, but the functional significance of this modification for receptor folding and intracellular transport has remained elusive. Here we studied these phenomena by mutating the two N-terminal N-glycosylation sites (Asn(18) and Asn(33)) of the human delta-opioid receptor, and expressing the mutants from the same chromosomal integration site in stably transfected inducible HEK293 cells. Both N-glycosylation sites were used, and their abolishment decreased the steady-state level of receptors at the cell surface. However, pulse-chase labeling, cell surface biotinylation, and immunofluorescence microscopy revealed that this was not because of intracellular accumulation. Instead, the non-N-glycosylated receptors were exported from the endoplasmic reticulum with enhanced kinetics. The results also revealed differences in the significance of the individual N-glycans, as the one attached to Asn(33) was found to be more important for endoplasmic reticulum retention of the receptor. The non-N-glycosylated receptors did not show gross functional impairment, but flow cytometry revealed that a fraction of them was incapable of ligand binding at the cell surface. In addition, the receptors that were devoid of N-glycans showed accelerated turnover and internalization and were targeted for lysosomal degradation. The results accentuate the importance of protein conformation-based screening before export from the endoplasmic reticulum, and demonstrate how the system is compromised when N-glycosylation is disrupted. We conclude that N-glycosylation of the delta-opioid receptor is needed to maintain the expression of fully functional and stable receptor molecules at the cell surface.

N-Glycosylation of the Drosophila neural protein Chaoptin is essential for its stability, cell surface transport and adhesive activity

Glycosylation of proteins can modulate their function in a striking variety of systems, including immune responses, neuronal activities and development. The Drosophila protein, Chaoptin (Chp), is essential for the development and maintenance of photoreceptor cells. This protein is heavily glycosylated, but the possible role of this glycosylation is not well-understood. Here we show that mutations introduced into about 1/3 of 16 potential N-linked glycosylation sites within Chp impaired its cell adhesive activities when expressed in Drosophila S2 cells. Mutation of 2/3 of the glycosylation sites resulted in a marked decrease in Chp protein abundance. These results suggest that N-linked glycosylation of Chp is essential for its stability and activity.

Cell-nonautonomous function of ceramidase in photoreceptor homeostasis

Neutral ceramidase, a key enzyme of sphingolipid metabolism, hydrolyzes ceramide to sphingosine. These sphingolipids are critical structural components of cell membranes and act as second messengers in diverse signal transduction cascades. Here, we have isolated and characterized functional null mutants of Drosophila ceramidase. We show that secreted ceramidase functions in a cell-nonautonomous manner to maintain photoreceptor homeostasis. In the absence of ceramidase, photoreceptors degenerate in a light-dependent manner, are defective in normal endocytic turnover of rhodopsin, and do not respond to light stimulus. Consistent with a cell-nonautonomous function, overexpression of ceramidase in tissues distant from photoreceptors suppresses photoreceptor degeneration in an arrestin mutant and facilitates membrane turnover in a rhodopsin null mutant. Furthermore, our results show that secreted ceramidase is internalized and localizes to endosomes. Our findings establish a role for a secreted sphingolipid enzyme in the regulation of photoreceptor structure and function.

Phototransduction and retinal degeneration in Drosophila

Drosophila visual transduction is the fastest known G-protein-coupled signaling cascade and has therefore served as a genetically tractable animal model for characterizing rapid responses to sensory stimulation. Mutations in over 30 genes have been identified, which affect activation, adaptation, or termination of the photoresponse. Based on analyses of these genes, a model for phototransduction has emerged, which involves phosphoinoside signaling and culminates with opening of the TRP and TRPL cation channels. Many of the proteins that function in phototransduction are coupled to the PDZ containing scaffold protein INAD and form a supramolecular signaling complex, the signalplex. Arrestin, TRPL, and G alpha(q) undergo dynamic light-dependent trafficking, and these movements function in long-term adaptation. Other proteins play important roles either in the formation or maturation of rhodopsin, or in regeneration of phosphatidylinositol 4,5-bisphosphate (PIP2), which is required for the photoresponse. Mutation of nearly any gene that functions in the photoresponse results in retinal degeneration. The underlying bases of photoreceptor cell death are diverse and involve mechanisms such as excessive endocytosis of rhodopsin due to stable rhodopsin/arrestin complexes and abnormally low or high levels of Ca2+. Drosophila visual transduction appears to have particular relevance to the cascade in the intrinsically photosensitive retinal ganglion cells in mammals, as the photoresponse in these latter cells appears to operate through a remarkably similar mechanism.

Transforming lepidopteran insect cells for improved protein processing

The lepidopteran insect cells used with the baculovirus expression vector system (BEVS) are capable of synthesizing and accurately processing foreign proteins. However, proteins expressed in baculovirus-infected cells often fail to be completely processed, or are not processed in a manner that meets a researcher's needs. This chapter discusses a metabolic engineering approach that addresses this problem. Basically, this approach involves the addition of new or enhancement of existing protein processing functions in established lepidopteran insect cell lines. Methods for engineering these cell lines and assessing their properties as improved hosts for the BEVS are detailed. Examples of lepidopteran insect cell lines engineered for improved protein N-glycosylation and trafficking are described.

Calnexin is essential for rhodopsin maturation, Ca2+ regulation, and photoreceptor cell survival

In sensory neurons, successful maturation of signaling molecules and regulation of Ca2+ are essential for cell function and survival. Here, we demonstrate a multifunctional role for calnexin as both a molecular chaperone uniquely required for rhodopsin maturation and a regulator of Ca2+ that enters photoreceptor cells during light stimulation. Mutations in Drosophila calnexin lead to severe defects in rhodopsin (Rh1) expression, whereas other photoreceptor cell proteins are expressed normally. Mutations in calnexin also impair the ability of photoreceptor cells to control cytosolic Ca2+ levels following activation of the light-sensitive TRP channels. Finally, mutations in calnexin lead to retinal degeneration that is enhanced by light, suggesting that calnexin's function as a Ca2+ buffer is important for photoreceptor cell survival. Our results illustrate a critical role for calnexin in Rh1 maturation and Ca2+ regulation and provide genetic evidence that defects in calnexin lead to retinal degeneration.

Rhodopsin maturation defects induce photoreceptor death by apoptosis: a fly model for RhodopsinPro23His human retinitis pigmentosa

rhodopsin mutations result in autosomal dominant retinitis pigmentosa (ADRP), the most frequent being Proline-23 substitution by histidine (RhoP23H). Although cellular and rodent animal models have been developed, the pathogenic mechanisms leading to RhoP23H-induced cell death are still poorly understood. For this, we have used a Drosophila model by introducing a mutation in the fly rhodopsin-1 gene (Rh1P37H) that corresponds to human RhoP23H. Rh1P37H transgenic flies show dominant photoreceptor degeneration that mimics age-, light-dependent and progressive ADRP. Moreover, we clarify the pathogenic mechanism of Rh1P37H mutation that acts as an antimorph. First, we show the dual-localization of mutant Rhodopsin since most of Rh1P37H accumulates in endoplasmic reticulum. Second, expression of mutant, mislocalized, Rhodopsin leads to cytotoxicity, via the activation of two stress-specific mitogen-activated protein kinases (MAPKs), p38 and JNK, which are known to control stress-induced apoptosis. In Rh1P37H flies, visual loss and degeneration are indeed accompanied by apoptotic features and prevented by expression of p35 apoptosis inhibitor. Finally, we show for the first time that properly localized, mutant, Rhodopsin is active. Thus, the development of a fly model that faithfully reproduces the human disease sheds light onto the molecular defects causing ADRP thereby making it possible to devise potential therapeutic approaches.

Regulation of peptide bond cis/trans isomerization by enzyme catalysis and its implication in physiological processes

In some cases, the slow rotational movement underlying peptide bond cis/trans isomerizations is found to control the biological activity of proteins. Peptide bond cis/trans isomerases as cyclophilins, Fk506-binding proteins, parvulins, and bacterial hsp70 generally assist in the interconversion of the polypeptide substrate cis/trans isomers, and rate acceleration is the dominating mechanism of action in cells. We present evidence disputing the hypothesis that some of the molecular properties of these proteins play an auxiliary role in enzyme function.

The cellular fate of mutant rhodopsin: quality control, degradation and aggresome formation

Mutations in the photopigment rhodopsin are the major cause of autosomal dominant retinitis pigmentosa. The majority of mutations in rhodopsin lead to misfolding of the protein. Through the detailed examination of P23H and K296E mutant opsin processing in COS-7 cells, we have shown that the mutant protein does not accumulate in the Golgi, as previously thought, instead it forms aggregates that have many of the characteristic features of an aggresome. The aggregates form close to the centrosome and lead to the dispersal of the Golgi apparatus. Furthermore, these aggregates are ubiquitinated, recruit cellular chaperones and disrupt the intermediate filament network. Mutant opsin expression can disrupt the processing of normal opsin, as co-transfection revealed that the wild-type protein is recruited to mutant opsin aggregates. The degradation of mutant opsin is dependent on the proteasome machinery. Unlike the situation with ΔF508-CFTR, proteasome inhibition does not lead to a marked increase in aggresome formation but increases the retention of the protein within the ER, suggesting that the proteasome is required for the efficient retrotranslocation of the mutant protein. Inhibition of N-linked glycosylation with tunicamycin leads to the selective retention of the mutant protein within the ER and increases the steady state level of mutant opsin. Glycosylation, however, has no influence on the biogenesis and targeting of wild-type opsin in cultured cells. This demonstrates that N-linked glycosylation is required for ER-associated degradation of the mutant protein but is not essential for the quality control of opsin folding. The addition of 9-cis-retinal to the media increased the amount of P23H, but not K296E, that was soluble and reached the plasma membrane. These data show that rhodopsin autosomal dominant retinitis pigmentosa is similar to many other neurodegenerative diseases in which the formation of intracellular protein aggregates is central to disease pathogenesis, and they suggest a mechanism for disease dominance.

Co- and posttranslational modification of the alpha(1B)-adrenergic receptor: effects on receptor expression and function

We have characterized the maturation, co- and posttranslational modifications, and functional properties of the alpha(1B)-adrenergic receptor (AR) expressed in different mammalian cells transfected using conventional approaches or the Semliki Forest virus system. We found that the alpha(1B)-AR undergoes N-linked glycosylation as demonstrated by its sensitivity to endoglycosidases and by the effect of tunicamycin on receptor maturation. Pulse-chase labeling experiments in BHK-21 cells demonstrate that the alpha(1B)-AR is synthesized as a 70 kDa core glycosylated precursor that is converted to the 90 kDa mature form of the receptor with a half-time of approximately 2 h. N-Linked glycosylation of the alpha(1B)-AR occurs at four asparagines on the N-terminus of the receptor. Mutations of the N-linked glycosylation sites did not have a significant effect on receptor function or expression. Surprisingly, receptor mutants lacking N-linked glycosylation migrated as heterogeneous bands in SDS-PAGE. Our findings demonstrate that N-linked glycosylation and phosphorylation, but not palmitoylation or O-linked glycosylation, contribute to the structural heterogeneity of the alpha(1B)-AR as it is observed in SDS-PAGE. The modifications found are similar in the different mammalian expression systems explored. Our findings indicate that the Semliki Forest virus system can provide large amounts of functional and fully glycosylated alpha(1B)-AR protein suitable for biochemical and structural studies. The results of this study contribute to elucidate the basic steps involved in the processing of G protein-coupled receptors as well as to optimize strategies for their overexpression.

Cyclophilin A mediates Vid22p function in the import of fructose-1,6-bisphosphatase into Vid vesicles

Fructose-1,6-bisphosphatase (FBPase) is synthesized in yeast during glucose starvation but is rapidly degraded in the vacuole following the addition of glucose. FBPase trafficking to the vacuole involves two distinct steps, import into intermediate transport vesicles (Vid vesicles) and Vid vesicle trafficking to the vacuole. FBPase import into Vid vesicles requires the VID22 gene. However, VID22 affects FBPase import indirectly through a cytosolic factor. To identify the required cytosolic component, wild type cytosol was fractionated and screened for proteins that complement Deltavid22 mutant cytosol using an in vitro assay that reproduces FBPase import into Vid vesicles. Cyclophilin A (Cpr1p) was identified as a cytosolic protein that mediates Vid22p function in FBPase import. Mutants lacking Cpr1p were defective in FBPase import. Furthermore, the addition of purified Cpr1p restored FBPase import in both the Deltacpr1 and the Deltavid22 mutants. The cyclosporin A binding pocket is important for Cpr1p function, since cyclosporin A binding-deficient mutants failed to complement FBPase import in Deltacpr1 and Deltavid22 mutants. The levels of Cpr1p were reduced in the Deltavid22 mutants, implying that the expression of Cpr1p is regulated by Vid22p. Our results suggest that Cpr1p mediates Vid22p function and is directly involved in the import of FBPase into Vid vesicles.

UDP-sugar transporter implicated in glycosylation and processing of Notch

Glycosylation modifies protein activities in various biological processes. Here, we report the functions of a novel UDP-sugar transporter (UST74C, an alternative name for Fringe connection (Frc)) localized to the Golgi apparatus in cellular signalling of Drosophila. Mutants in the frc gene exhibit phenotypes resembling wingless and Notch mutants. Both Fringe-dependent and Fringe-independent Notch pathways are affected, and both glycosylation and proteolytic maturation of Notch are defective in mutant larvae. The results suggest that changes in nucleotide-sugar levels can differently affect Wingless and two distinct aspects of Notch signalling.

Characterization of the primary photointermediates of Drosophila rhodopsin

Invertebrate opsins are unique among the visual pigments because the light-activated conformation, metarhodopsin, is stable following exposure to light in vivo. Recovery of the light-activated pigment to the dark conformation (or resting state) occurs either thermally or photochemically. There is no evidence to suggest that the chromophore becomes detached from the protein during any stage in the formation or recovery processes. Biochemical and structural studies of invertebrate opsins have been limited by the inability to express and purify rhodopsins for structure-function studies. In this study, we used Drosophila to produce an epitope-tagged opsin, Rh1-1D4, in quantities suitable for spectroscopic and photochemical characterization. When expressed in Drosophila, Rh1-1D4 is localized to the rhabdomere membranes, has the same spectral properties in vivo as wild-type Rh1, and activates the phototransduction cascade in a normal manner. Purified Rh1-1D4 visual pigment has an absorption maximum of the dark-adapted state of 474 nm, while the metarhodopsin absorption maximum is 572 nm. However, the metarhodopsin state is not stable as purified in dodecyl maltoside but decays with kinetics that require a double-exponential fit having lifetimes of 280 and 2700 s. We investigated the primary properties of the pigment at low temperature. At 70 K, the pigment undergoes a temperature-induced red shift to 486 nm. Upon illumination with 435 nm light, a photostationary state mixture is formed consisting of bathorhodopsin (lambda(max) = 545 nm) and isorhodopsin (lambda(max) = 462 nm). We also compared the spectroscopic and photochemical properties of this pigment with other vertebrate pigments. We conclude that the binding site of Drosophila rhodopsin is similar to that of bovine rhodopsin and is characterized by a protonated Schiff base chromophore stabilized via a single negatively charged counterion.