Cytoplasmic domain of rhodopsin is essential for post-Golgi vesicle formation in a retinal cell-free system

Deciphering the role of cytoplasmic domain of Channelrhodopsin in modulating the interactome and SUMOylome of Chlamydomonas reinhardtii

Translocation of channelrhodopsins (ChRs) is mediated by the intraflagellar transport (IFT) machinery. However, the functional role of the network involving photoreceptors, IFT and other proteins in controlling algal ciliary motility is still not fully delineated. In the current study, we have identified two important motifs at the C-terminus of ChR1, VXPX and LKNE. VXPX is a known ciliary targeting sequence in animals, and LKNE is a well-known SUMOylation motif. To the best of our knowledge, this study gives prima facie insight into the role of SUMOylation in Chlamydomonas. We prove that VMPS of ChR1 is important for interaction with GTPase CrARL11. We show that SUMO motifs are present in the C-terminus of putative ChR1s from green algae. Performing experiments with n-Ethylmaleimide (NEM) and Ubiquitin-like protease 1 (ULP-1) we show that SUMOylation may modulate ChR1 protein in Chlamydomonas. Experiments with 2D08, a known sumoylation blocker, increased the concentration of ChR1 protein. Finally, we show the endogenous SUMOylated proteins (SUMOylome) of C. reinhardtii, identified by using immunoprecipitation followed by nano-LC-MS/MS detection. This report establishes a link between evolutionarily conserved SUMOylation, and ciliary machinery for the maintenance and functioning of cilia across the eukaryotes. Our enriched SUMOylome of C. reinhardtii comprehends the proteins related to ciliary development and, photo-signaling, along with orthologue(s) associated to human ciliopathies as SUMO targets.

Rab GTPases in cilium formation and function

Cilia are microtubule-based organelles extending from a basal body at the surface of eukaryotic cells. Cilia regulate cell and fluid motility, sensation and developmental signaling, and ciliary defects cause human diseases (ciliopathies) affecting the formation and function of many tissues and organs. Over the past decade, various Rab and Rab-like membrane trafficking proteins have been shown to regulate cilia-related processes such as basal body maturation, ciliary axoneme extension, intraflagellar transport and ciliary signaling. In this review, we provide a comprehensive overview of Rab protein ciliary associations, drawing on findings from multiple model systems, including mammalian cell culture, mice, zebrafish, C. elegans, trypanosomes, and green algae. We also discuss several emerging mechanistic themes related to ciliary Rab cascades and functional redundancy.

Rab GTPases in cilium formation and function

Cilia are microtubule-based organelles extending from a basal body at the surface of eukaryotic cells. Cilia regulate cell and fluid motility, sensation and developmental signaling, and ciliary defects cause human diseases (ciliopathies) affecting the formation and function of many tissues and organs. Over the past decade, various Rab and Rab-like membrane trafficking proteins have been shown to regulate cilia-related processes such as basal body maturation, ciliary axoneme extension, intraflagellar transport and ciliary signaling. In this review, we provide a comprehensive overview of Rab protein ciliary associations, drawing on findings from multiple model systems, including mammalian cell culture, mice, zebrafish, C. elegans, trypanosomes, and green algae. We also discuss several emerging mechanistic themes related to ciliary Rab cascades and functional redundancy.

The Arf GEF GBF1 and Arf4 synergize with the sensory receptor cargo, rhodopsin, to regulate ciliary membrane trafficking

The small GTPase Arf4 and the Arf GTPase-activating protein (GAP) ASAP1 cooperatively sequester sensory receptor cargo into transport carriers targeted to primary cilia, but the input that drives Arf4 activation in this process remains unknown. Here, we show, by using frog retinas and recombinant human proteins, that during the carrier biogenesis from the photoreceptor Golgi/trans-Golgi network (TGN) a functional complex is formed between Arf4, the Arf guanine nucleotide exchange factor (GEF) GBF1 and the light-sensing receptor, rhodopsin. Rhodopsin and Arf4 bind the regulatory N-terminal dimerization and cyclophillin-binding (DCB)-homology upstream of Sec7 (HUS) domain of GBF1. The complex is sensitive to Golgicide A (GCA), a selective inhibitor of GBF1 that accordingly blocks rhodopsin delivery to the cilia, without disrupting the photoreceptor Golgi. The emergence of newly synthesized rhodopsin in the endomembrane system is essential for GBF1-Arf4 complex formation in vivo Notably, GBF1 interacts with the Arf GAP ASAP1 in a GCA-resistant manner. Our findings indicate that converging signals on GBF1 from the influx of cargo into the Golgi/TGN and the feedback from Arf4, combined with input from ASAP1, control Arf4 activation during sensory membrane trafficking to primary cilia.

Small GTPases Rab8a and Rab11a Are Dispensable for Rhodopsin Transport in Mouse Photoreceptors

Rab11a and Rab8a are ubiquitous small GTPases shown as required for rhodopsin transport in Xenopus laevis and zebrafish photoreceptors by dominant negative (dn) disruption of function. Here, we generated retina-specific Rab11a (^retRab11a) and Rab8a (^retRab8a) single and double knockout mice to explore the consequences in mouse photoreceptors. Rhodopsin and other outer segment (OS) membrane proteins targeted correctly to OS and electroretinogram (ERG) responses in all three mutant mouse lines were indistinguishable from wild-type (WT). Further, AAV (adeno-associated virus)-mediated expression of dnRab11b in ^retRab11a^-/- retina, or expression of dnRab8b in ^retRab8a^-/- retina did not cause OS protein mislocalization. Finally, a ^retRab8a^-/- retina injected at one month of age with AAVs expressing dnRab11a, dnRab11b, dnRab8b, and dnRab10 (four dn viruses on Rab8a^-/- background) and harvested three months later exhibited normal OS protein localization. In contrast to results obtained with dnRab GTPases in Xenopus and zebrafish, mouse Rab11a and Rab8a are dispensable for proper rhodopsin and outer segment membrane protein targeting. Absence of phenotype after expression of four dn Rab GTPases in a Rab8a^-/- retina suggests that Rab8b and Rab11b paralogs maybe dispensable as well. Our data thus demonstrate significant interspecies variation in photoreceptor membrane protein and rhodopsin trafficking.

Aberrant protein trafficking in retinal degenerations: The initial phase of retinal remodeling

Retinal trafficking proteins are involved in molecular assemblies that govern protein transport, orchestrate cellular events involved in cilia formation, regulate signal transduction, autophagy and endocytic trafficking, all of which if not properly controlled initiate retinal degeneration. Improper function and or trafficking of these proteins and molecular networks they are involved in cause a detrimental cascade of neural retinal remodeling due to cell death, resulting as devastating blinding diseases. A universal finding in retinal degenerative diseases is the profound detection of retinal remodeling, occurring as a phased modification of neural retinal function and structure, which begins at the molecular level. Retinal remodeling instigated by aberrant trafficking of proteins encompasses many forms of retinal degenerations, such as the diverse forms of retinitis pigmentosa (RP) and disorders that resemble RP through mutations in the rhodopsin gene, retinal ciliopathies, and some forms of glaucoma and age-related macular degeneration (AMD). As a large majority of genes associated with these different retinopathies are overlapping, it is imperative to understand their underlying molecular mechanisms. This review will discuss some of the most recent discoveries in vertebrate retinal remodeling and retinal degenerations caused by protein mistrafficking.

Biology and therapy of inherited retinal degenerative disease: insights from mouse models

Retinal neurodegeneration associated with the dysfunction or death of photoreceptors is a major cause of incurable vision loss. Tremendous progress has been made over the last two decades in discovering genes and genetic defects that lead to retinal diseases. The primary focus has now shifted to uncovering disease mechanisms and designing treatment strategies, especially inspired by the successful application of gene therapy in some forms of congenital blindness in humans. Both spontaneous and laboratory-generated mouse mutants have been valuable for providing fundamental insights into normal retinal development and for deciphering disease pathology. Here, we provide a review of mouse models of human retinal degeneration, with a primary focus on diseases affecting photoreceptor function. We also describe models associated with retinal pigment epithelium dysfunction or synaptic abnormalities. Furthermore, we highlight the crucial role of mouse models in elucidating retinal and photoreceptor biology in health and disease, and in the assessment of novel therapeutic modalities, including gene- and stem-cell-based therapies, for retinal degenerative diseases.

Cone Health and Retinoids

Cones are photoreceptor cells used for bright light and color vision. Retinoids are vitamin A derivatives, one of which is the 11-cis aldehyde form that serves as the chromophore for both cone and rod visual pigments. In the visual disease, Type 2 Leber congenital amaurosis (LCA2), 11-cis-retinal generation is inhibited or abolished. Work by others has shown that patients with LCA2 have symptoms consistent with degenerating cones. In mouse models for LCA2, early cone degeneration is readily apparent: cone opsins and other proteins associated with the outer segment are delocalized and cell numbers decline rapidly within the first month. Rods would appear normal morphologically and functionally, if not for the absence of chromophore. Supplementation of mouse models of LCA2 with cis-retinoids has been shown to slow loss of cone photoreceptor cells if mice were maintained in darkness. Thus, 11-cis-retinal appears not only to have a role in the light response reaction but also to promote proper trafficking of the cone opsins and maintain viable cones.

Rhodopsin Trafficking and Mistrafficking: Signals, Molecular Components, and Mechanisms

Rhodopsin is a seven-transmembrane G protein-coupled receptor (GPCR) and is the main component of the photoreceptor outer segment (OS), a ciliary compartment essential for vision. Because the OSs are incapable of protein synthesis, rhodopsin must first be synthesized in the inner segments (ISs) and subsequently trafficked across the connecting cilia to the OSs where it participates in the phototransduction cascade. Rapid turnover of the OS necessitates a high rate of synthesis and efficient trafficking of rhodopsin to the cilia. This cilia-targeting mechanism is shared among other ciliary-localized GPCRs. In this review, we will discuss the process of rhodopsin trafficking from the IS to the OS beginning with the trafficking signals present on the protein. Starting from the endoplasmic reticulum and the Golgi apparatus within the IS, we will cover the molecular components assisting the biogenesis and the proper sorting. We will also review the confirmed binding and interacting partners that help target rhodopsin toward the connecting cilium as well as the cilia-localized components which direct proteins into the proper compartments of the OS. While rhodopsin is the most critical and abundant component of the photoreceptor OS, mutations in the rhodopsin gene commonly lead to its mislocalization within the photoreceptors. In addition to covering the trafficking patterns of rhodopsin, we will also review some of the most common rhodopsin mutants which cause mistrafficking and subsequent death of photoreceptors. Toward the goal of understanding the pathogenesis, three major mechanisms of aberrant trafficking as well as putative mechanisms of photoreceptor degeneration will be discussed.

Rab proteins: the key regulators of intracellular vesicle transport

Vesicular/membrane trafficking essentially regulates the compartmentalization and abundance of proteins within the cells and contributes in many signalling pathways. This membrane transport in eukaryotic cells is a complex process regulated by a large and diverse array of proteins. A large group of monomeric small GTPases; the Rabs are essential components of this membrane trafficking route. Most of the Rabs are ubiquitously expressed proteins and have been implicated in vesicle formation, vesicle motility/delivery along cytoskeleton elements and docking/fusion at target membranes through the recruitment of effectors. Functional impairments of Rabs affecting transport pathways manifest different diseases. Rab functions are accompanied by cyclical activation and inactivation of GTP-bound and GDP-bound forms between the cytosol and membranes which is regulated by upstream regulators. Rab proteins are characterized by their distinct sub-cellular localization and regulate a wide variety of endocytic, transcytic and exocytic transport pathways. Mutations of Rabs affect cell growth, motility and other biological processes.

Molecular complexes that direct rhodopsin transport to primary cilia

Rhodopsin is a key molecular constituent of photoreceptor cells, yet understanding of how it regulates photoreceptor membrane trafficking and biogenesis of light-sensing organelles, the rod outer segments (ROS) is only beginning to emerge. Recently identified sequence of well-orchestrated molecular interactions of rhodopsin with the functional networks of Arf and Rab GTPases at multiple stages of intracellular targeting fits well into the complex framework of the biogenesis and maintenance of primary cilia, of which the ROS is one example. This review will discuss the latest progress in dissecting the molecular complexes that coordinate rhodopsin incorporation into ciliary-targeted carriers with the recruitment and activation of membrane tethering complexes and regulators of fusion with the periciliary plasma membrane. In addition to revealing the fundamental principals of ciliary membrane renewal, recent advances also provide molecular insight into the ways by which disruptions of the exquisitely orchestrated interactions lead to cilia dysfunction and result in human retinal dystrophies and syndromic diseases that affect multiple organs, including the eyes.

Crag is a GEF for Rab11 required for rhodopsin trafficking and maintenance of adult photoreceptor cells

Transport of newly synthesized Rhodopsin upon light stimulation in adult Drosophila photoreceptors is mediated by a Crag/Rab11-dependent vesicular trafficking process.

Rhodopsins (Rhs) are light sensors, and Rh1 is the major Rh in the Drosophila photoreceptor rhabdomere membrane. Upon photoactivation, a fraction of Rh1 is internalized and degraded, but it remains unclear how the rhabdomeric Rh1 pool is replenished and what molecular players are involved. Here, we show that Crag, a DENN protein, is a guanine nucleotide exchange factor for Rab11 that is required for the homeostasis of Rh1 upon light exposure. The absence of Crag causes a light-induced accumulation of cytoplasmic Rh1, and loss of Crag or Rab11 leads to a similar photoreceptor degeneration in adult flies. Furthermore, the defects associated with loss of Crag can be partially rescued with a constitutive active form of Rab11. We propose that upon light stimulation, Crag is required for trafficking of Rh from the trans-Golgi network to rhabdomere membranes via a Rab11-dependent vesicular transport.

Animals sense light through receptors called Rhodopsins. These proteins are typically localized to stacked membranes in photoreceptors. In flies, upon light exposure, Rhodopsin undergoes conformational changes and becomes active as metarhodopsin. Metarhodopsin then initiates a signaling cascade that activates the photoreceptor cell. To deactivate the light response, metarhodopsin is converted back into Rhodopsin by absorption of another photon of light. Under certain conditions, metarhodopsin cannot be converted back to Rhodopsin, and it is then endocytosed and degraded. Rhodopsin then needs to be synthesized and delivered back to the membrane stacks. Here, we show that the Calmodulin-binding protein Crag is required for the delivery of newly made Rhodopsin to the membrane stacks. Loss of Crag leads to the accumulation of Rhodopsin in the cytosol, followed by shrinkage of membrane stack volume, and, eventually, photoreceptor cell degeneration. We also show that Crag activates a target protein, Rab11, which mediates the vesicular transport of Rhodospin to the membrane. Finally, we document that the human homolog of Crag, DENND4A, is able to rescue the loss of Crag in flies, suggesting that DENND4A functions in a similar process in vertebrates.

Tubby is required for trafficking G protein-coupled receptors to neuronal cilia

Background

Tubby is the founding member of the tubby-like family of proteins. The naturally occurring tubby mutation in mice causes retinitis pigmentosa, hearing loss and obesity. Tubby has been proposed to function as an accessory factor in ciliary trafficking. We directly examined a role for tubby in ciliary trafficking in vivo.

Methods

We used immunofluoresence labeling to examine the subcellular localization of rhodopsin, somatostatin receptor 3 (SSTR3) and melanin concentrating hormone receptor 1 (MCHR1), all of which are G protein-coupled receptors (GPCR), in the retina and brain of wild type (WT) and tubby mutant mice.

Results

In tubby mouse retina, rhodopsin is not fully transported across the connecting cilia to the outer segments with ensuing photoreceptor degeneration. In the tubby mouse brain, SSTR3 and MCHR1 fail to localize at the neuronal primary cilia in regions where these receptors play critical roles in neural signaling. The tubby mutant does not manifest a generalized defect in ciliogenesis or protein trafficking.

Conclusions

Tubby plays a critical role in trafficking select GPCRs to the cilia. This role is reminiscent of tubby-like proteins 1 and 3, which have been proposed to facilitate trafficking of rhodopsin and select GPCRs in photoreceptors and the developing neural tube, respectively. Thus tubby-like proteins may be generally involved in transciliary trafficking of GPCRs.

Ciliary signaling cascades in photoreceptors

For being a polarized neuron and having a sensory cilium, photoreceptors attract remarkable attention. This is due their highly polarized structure and active visual signal transduction cascades and for the enrichment of complex networks of proteins in the cilium. Structural and functional maintenance of the photoreceptor sensory cilium, also called outer segment, ensures that light signal is received and relayed appropriately to the brain. Any perturbations in the protein content of the outer segment result in photoreceptor dysfunction, degeneration and eventually, blindness. This review focuses on the importance of photoreceptor sensory cilium to carry out signal transduction cascade for vision.

The role of mislocalized phototransduction in photoreceptor cell death of retinitis pigmentosa

Most of inherited retinal diseases such as retinitis pigmentosa (RP) cause photoreceptor cell death resulting in blindness. RP is a large family of diseases in which the photoreceptor cell death can be caused by a number of pathways. Among them, light exposure has been reported to induce photoreceptor cell death. However, the detailed mechanism by which photoreceptor cell death is caused by light exposure is unclear. In this study, we have shown that even a mild light exposure can induce ectopic phototransduction and result in the acceleration of rod photoreceptor cell death in some vertebrate models. In ovl, a zebrafish model of outer segment deficiency, photoreceptor cell death is associated with light exposure. The ovl larvae show ectopic accumulation of rhodopsin and knockdown of ectopic rhodopsin and transducin rescue rod photoreceptor cell death. However, knockdown of phosphodiesterase, the enzyme that mediates the next step of phototransduction, does not. So, ectopic phototransduction activated by light exposure, which leads to rod photoreceptor cell death, is through the action of transducin. Furthermore, we have demonstrated that forced activation of adenylyl cyclase in the inner segment leads to rod photoreceptor cell death. For further confirmation, we have also generated a transgenic fish which possesses a human rhodopsin mutation, Q344X. This fish and rd10 model mice show photoreceptor cell death caused by adenylyl cyclase. In short, our study indicates that in some RP, adenylyl cyclase is involved in photoreceptor cell death pathway; its inhibition is potentially a logical approach for a novel RP therapy.

Immunocytochemical evidence of Tulp1-dependent outer segment protein transport pathways in photoreceptor cells

Tulp1 is a protein of unknown function exclusive to rod and cone photoreceptor cells. Mutations in the gene cause autosomal recessive retinitis pigmentosa in humans and photoreceptor degeneration in mice. In tulp1-/- mice, rod and cone opsins are mislocalized, and rhodopsin-bearing extracellular vesicles accumulate around the inner segment, indicating that Tulp1 is involved in protein transport from the inner segment to the outer segment. To investigate this further, we sought to define which outer segment transport pathways are Tulp1-dependent. We used immunohistochemistry to examine the localization of outer segment proteins in tulp1-/- photoreceptors, prior to retinal degeneration. We also surveyed the condition of inner segment organelles and rhodopsin transport machinery proteins. Herein, we show that guanylate cyclase 1 and guanylate cyclase activating proteins 1 and 2 are mislocalized in the absence of Tulp1. Furthermore, arrestin does not translocate to the outer segment in response to light stimulation. Additionally, data from the tulp1-/- retina adds to the understanding of peripheral membrane protein transport, indicating that rhodopsin kinase and transducin do not co-transport in rhodopsin carrier vesicles and phosphodiesterase does not co-transport in guanylate cyclase carrier vesicles. These data implicate Tulp1 in the transport of selective integral membrane outer segment proteins and their associated proteins, specifically, the opsin and guanylate cyclase carrier pathways. The exact role of Tulp1 in outer segment protein transport remains elusive. However, without Tulp1, two rhodopsin transport machinery proteins exhibit abnormal distribution, Rab8 and Rab11, suggesting a role for Tulp1 in vesicular docking and fusion at the plasma membrane near the connecting cilium.

Trafficking to the ciliary membrane: how to get across the periciliary diffusion barrier?

The primary cilium organizes numerous signal transduction cascades, and an understanding of signaling receptor trafficking to cilia is now emerging. A defining feature of cilia is the periciliary diffusion barrier that separates the ciliary and plasma membranes. Although lateral transport through this barrier may take place, polarized exocytosis to the base of the cilium has been the prevailing model for delivering membrane proteins to cilia. Key players for this polarized exocytosis model include the GTPases Rab8 and Rab11, the exocyst, and possibly the intraflagellar tranport machinery. In turn, the sorting of membrane proteins to cilia critically relies on the recognition of ciliary targeting signals by sorting machines such as the BBSome coat complex or the GTPase Arf4. Finally, some proteins need to exit from cilia, and ubiquitination may regulate this step. The stage is now set to dissect the interplay between signaling and regulated trafficking to and from cilia.

Q344ter mutation causes mislocalization of rhodopsin molecules that are catalytically active: a mouse model of Q344ter-induced retinal degeneration

Q344ter is a naturally occurring rhodopsin mutation in humans that causes autosomal dominant retinal degeneration through mechanisms that are not fully understood, but are thought to involve an early termination that removed the trafficking signal, QVAPA, leading to its mislocalization in the rod photoreceptor cell. To better understand the disease mechanism(s), transgenic mice that express Q344ter were generated and crossed with rhodopsin knockout mice. Dark-reared Q344ter(rho+/-) mice exhibited retinal degeneration, demonstrating that rhodopsin mislocalization caused photoreceptor cell death. This degeneration is exacerbated by light-exposure and is correlated with the activation of transducin as well as other G-protein signaling pathways. We observed numerous sub-micrometer sized vesicles in the inter-photoreceptor space of Q344ter(rho+/-) and Q344ter(rho-/-) retinas, similar to that seen in another rhodopsin mutant, P347S. Whereas light microscopy failed to reveal outer segment structures in Q344ter(rho-/-) rods, shortened and disorganized rod outer segment structures were visible using electron microscopy. Thus, some Q344ter molecules trafficked to the outer segment and formed disc structures, albeit inefficiently, in the absence of full length wildtype rhodopsin. These findings helped to establish the in vivo role of the QVAPA domain as well as the pathways leading to Q344ter-induced retinal degeneration.

Assay for in vitro budding of ciliary-targeted rhodopsin transport carriers

Primary cilia and cilia-derived sensory organelles are cell's antennas that contain sensory receptors and signal transduction modules. Defects in the expression and targeting of ciliary proteins to this specialized cellular compartment lead to human disorders collectively known as ciliopathies. To examine the molecular basis for the ciliary targeting of the light receptor rhodopsin, we have developed a cell-free assay that reconstitutes its packaging into the specific post-Golgi rhodopsin transport carriers (RTCs). This assay accurately reproduces the in vivo process of carrier budding, while allowing examination of individual components of the macromolecular complexes, thus providing insight into a more general mechanism for the regulation of ciliary membrane targeting. Examples are shown for the use of this assay in rhodopsin trafficking. The cell-free assay is applicable to other ciliary-targeted sensory molecules.

What drives cell morphogenesis: a look inside the vertebrate photoreceptor

Vision mediating photoreceptor cells are specialized light-sensitive neurons in the outer layer of the vertebrate retina. The human retina contains approximately 130 million of such photoreceptors, which enable images of the external environment to be captured at high resolution and high sensitivity. Rod and cone photoreceptor subtypes are further specialized for sensing light in low and high illumination, respectively. To enable visual function, these photoreceptors have developed elaborate morphological domains for the detection of light (outer segments), for changing cell shape (inner segments), and for communication with neighboring retinal neurons (synaptic terminals). Furthermore, rod and cone subtypes feature unique morphological variations of these specialized characteristics. Here, we review the major aspects of vertebrate photoreceptor morphology and key genetic mechanisms that drive their formation. These mechanisms are necessary for cell differentiation as well as function. Their defects lead to cell death.

Molecular mechanisms of rhodopsin retinitis pigmentosa and the efficacy of pharmacological rescue

Variants of rhodopsin, a complex of 11-cis retinal and opsin, cause retinitis pigmentosa (RP), a degenerative disease of the retina. Trafficking defects due to rhodopsin misfolding have been proposed as the most likely basis of the disease, but other potentially overlapping mechanisms may also apply. Pharmacological therapies for RP must target the major disease mechanism and contend with overlap, if it occurs. To this end, we have explored the molecular basis of rhodopsin RP in the context of pharmacological rescue with 11-cis retinal. Stable inducible cell lines were constructed to express wild-type opsin; the pathogenic variants T4R, T17M, P23A, P23H, P23L, and C110Y; or the nonpathogenic variants F220L and A299S. Pharmacological rescue was measured as the fold increase in rhodopsin or opsin levels upon addition of 11-cis retinal during opsin expression. Only Pro23 and T17M variants were rescued significantly. C110Y opsin was produced at low levels and did not yield rhodopsin, whereas the T4R, F220L, and A299S proteins reached near-wild-type levels and changed little with 11-cis retinal. All of the mutant rhodopsins exhibited misfolding, which increased over a broad range in the order F220L, A299S, T4R, T17M, P23A, P23H, P23L, as determined by decreased thermal stability in the dark and increased hydroxylamine sensitivity. Pharmacological rescue increased as misfolding decreased, but was limited for the least misfolded variants. Significantly, pathogenic variants also showed abnormal photobleaching behavior, including an increased ratio of metarhodopsin-I-like species to metarhodopsin-II-like species and aberrant photoproduct accumulation with prolonged illumination. These results, combined with an analysis of published biochemical and clinical studies, suggest that many rhodopsin variants cause disease by affecting both biosynthesis and photoactivity. We conclude that pharmacological rescue is promising as a broadly effective therapy for rhodopsin RP, particularly if implemented in a way that minimizes the photoactivity of the mutant proteins.

Regulation of the formation and trafficking of vesicles from Golgi by PCH family proteins during chemotaxis

Previous study demonstrated that WASP localizes on vesicles during Dictyostelium chemotaxis and these vesicles appear to be preferentially distributed at the leading and trailing edge of migrating cells. In this study, we have examined the role of PCH family proteins, Nwk/Bzz1p-like protein (NLP) and Syndapin-like protein (SLP), in the regulation of the formation and trafficking of WASP-vesicles during chemotaxis. NLP and SLP appear to be functionally redundant and deletion of both nlp and slp genes causes the loss of polarized F-actin organization and significant defects in chemotaxis. WASP and NLP are colocalized on vesicles and interactions between two molecules via the SH3 domain of NLP/SLP and the proline-rich repeats of WASP are required for vesicle formation from Golgi. Microtubules are required for polarized trafficking of these vesicles as vesicles showing high directed mobility are absent in cells treated with nocodazole. Our results suggest that interaction of WASP with NLP/SLP is required for the formation and trafficking of vesicles from Golgi to the membrane, which might play a central role in the establishment of cell polarity during chemotaxis.

Ciliary targeting motif VxPx directs assembly of a trafficking module through Arf4

Dysfunctions of primary cilia and cilia-derived sensory organelles underlie a multitude of human disorders, including retinal degeneration, yet membrane targeting to the cilium remains poorly understood. Here, we show that the newly identified ciliary targeting VxPx motif present in rhodopsin binds the small GTPase Arf4 and regulates its association with the trans-Golgi network (TGN), which is the site of assembly and function of a ciliary targeting complex. This complex is comprised of two small GTPases, Arf4 and Rab11, the Rab11/Arf effector FIP3, and the Arf GTPase-activating protein ASAP1. ASAP1 mediates GTP hydrolysis on Arf4 and functions as an Arf4 effector that regulates budding of post-TGN carriers, along with FIP3 and Rab11. The Arf4 mutant I46D, impaired in ASAP1-mediated GTP hydrolysis, causes aberrant rhodopsin trafficking and cytoskeletal and morphological defects resulting in retinal degeneration in transgenic animals. As the VxPx motif is present in other ciliary membrane proteins, the Arf4-based targeting complex is most likely a part of conserved machinery involved in the selection and packaging of the cargo destined for delivery to the cilium.

Transport of truncated rhodopsin and its effects on rod function and degeneration

PURPOSE

Most transgenic animal models of retinal degeneration caused by rhodopsin mutations express the rhodopsin transgene on a wild-type (WT) genetic background. Previous studies have demonstrated that one mechanism of retinal degeneration is rhodopsin overexpression. To study the effect of C-terminal truncation of rhodopsin without the confounding factors of overexpression, several lines of transgenic mice were generated that expressed a C-terminal rhodopsin mutation on rhodopsin-knockout backgrounds.

METHODS

Two lines of transgenic mice, expressing different levels of C-terminal truncated rhodopsin (S334ter) were mated with heterozygous rhodopsin-knockout (rho+/-) mice to express S334ter rhodopsin on a background with reduced endogenous rhodopsin expression. S334ter mice were mated to homozygous knockout (rho-/-) mice to examine the effect of S334ter rhodopsin on a null rhodopsin background. S334ter rhodopsin expression was estimated by Western blot. Retinal function was assessed by ERG and retinal degeneration by histopathology and morphometry. C-terminal rhodopsin sorting and trafficking was examined by fluorescence immunocytochemistry with detection by electron microscope.

RESULTS

Expression of S334ter truncated rhodopsin at low levels in the presence of decreased total rhodopsin in rods (S334ter, rho+/-) increased the rate of rod cell death in comparison to rho+/- littermates. In addition, S334ter rhodopsin prolonged the recovery time of the rod ERG to a light flash and diminished the a-wave amplitudes in comparison to their (rho+/-) littermates. Photoreceptors of S334ter mice on a homozygous rhodopsin-knockout background (S334ter+, rho-/-) had a fraction of mutant rhodopsin localized to the ciliary membranes.

CONCLUSIONS

Expression of S334ter rhodopsin without overexpression of total opsin in the rod photoreceptor decreased rod cell contribution to the ERG and compromised rod cell survival in adult mice. The increased cell death may be a consequence of C-terminal truncated rhodopsin mislocalization in membranes of the inner segment. Another possible pathologic mechanism is prolonged activation of phototransduction from the presence of mutant rhodopsin in the outer segment lacking the normal C-terminal binding sites for shutoff by arrestin and phosphorylation. These results suggest that rhodopsin lacking a C-terminal trafficking signal can be transported to the rod outer segment without cotransporting with full-length rhodopsin.

A role for rhodopsin in a signal transduction cascade that regulates membrane trafficking and photoreceptor polarity

This review summarizes the most recent progress in the understanding of the role of rhodopsin C-terminal domain in the regulation of intracellular trafficking and photoreceptor morphogenesis. A proposed cascade of molecular interactions, initiated by the rhodopsin C-terminal sequence VXPX-COOH during trafficking from the Golgi/TGN in retinal photoreceptors, is relayed by the small GTPase ARF4 to the downstream effectors. One of the candidates for an ARF4 effector is the ARF-GAP ASAP1, which may function as a subunit of, or form a novel protein coat involved in trafficking from the TGN and in cytoskeletal remodeling, whose assembly is regulated by the binding of ARF4 to rhodopsin, and whose function is essential for the polarized trafficking toward the ROS.

Pharmacologic rescue of conformationally-defective proteins: implications for the treatment of human disease

The process of quality control in the endoplasmic reticulum involves a variety of mechanisms which ensure that only correctly folded proteins enter the secretory pathway. Among these are conformation-screening mechanisms performed by molecular chaperones that assist in protein folding and prevent non-native (or misfolded) proteins from interacting with other misfolded proteins. Chaperones play a central role in the triage of newly formed proteins prior to their entry into the secretion, retention, and degradation pathways. Despite this stringent quality control mechanism, gain- or loss-of-function mutations that affect protein folding in the endoplasmic reticulum can manifest themselves as profound effects on the health of an organism. Understanding the molecular, cellular, and energetic mechanisms of protein routing could prevent or correct the structural abnormalities associated with disease-causing misfolded proteins. Rescue of misfolded, "trafficking-defective", but otherwise functional, proteins is achieved by a variety of physical, chemical, genetic, and pharmacological approaches. Pharmacologic chaperones (or "pharmacoperones") are template molecules that may potentially arrest or reverse diseases by inducing mutant proteins to adopt native-type-like conformations instead of improperly folded ones. Such restructuring leads to a normal pattern of cellular localization and function. This review focuses on protein misfolding and misrouting related to various disease states and describes promising approaches to overcoming such defects. Special attention is paid to the gonadotropin-releasing hormone receptor, since there is a great deal of information about this receptor, which has recently emerged as a particularly instructive model.

Rhodopsin C terminus, the site of mutations causing retinal disease, regulates trafficking by binding to ADP-ribosylation factor 4 (ARF4)

The maintenance of photoreceptor cell polarity is compromised by the rhodopsin mutations causing the human disease autosomal dominant retinitis pigmentosa. The severe form mutations occur in the C-terminal sorting signal of rhodopsin, VXPX-COOH. Here, we report that this sorting motif binds specifically to the small GTPase ARF4, a member of the ARF family of membrane budding and protein sorting regulators. The effects of blocking ARF4 action were functionally equivalent to the effects of blocking the rhodopsin C-terminal sorting signal. ARF4 was essential for the generation of post-Golgi carriers targeted to the rod outer segments of retinal photoreceptors. Thus, the severe retinitis pigmentosa alleles that affect the rhodopsin sorting signal interfere with interactions between ARF4 and rhodopsin, leading to aberrant trafficking and initiation of retinal degeneration.

Misrouted cell surface receptors as a novel disease aetiology and potential therapeutic target: the case of hypogonadotropic hypogonadism due to gonadotropin-releasing hormone resistance

Molecules that are incorrectly folded or defectively assembled are recognised by cellular quality control mechanisms. This leads such conformationally abnormal molecules to intracellular retention and eventual degradation. A number of diseases caused by mutations that interfere with proper processing and intracellular trafficking of key cell surface proteins have been described. These include a particular variant of hypogonadotropic hypogonadism, which results from mislocalisation of the gonadotropin-releasing hormone (GnRH) receptor. It has been shown recently that membrane expression and function of misfolded GnRH receptor mutants can be rescued by a peptidomimetic antagonist of GnRH (IN3) that permeates into the cell and reaches the abnormally manufactured nascent receptor, stabilising a conformation compatible with cell-surface transport and reversing intracellular retention. This approach seems applicable for the development of defined therapeutic strategies for an array of diseases caused by incorrectly routed cell surface or secreted proteins.

The carboxyl-terminal domain is essential for rhodopsin transport in rod photoreceptors

The role of the carboxyl-terminal domain in rhodopsin transport was investigated using transgenic mice expressing a rhodopsin truncation mutant lacking the terminal 15 amino acids (S334ter). It was previously shown that S334ter translocates to the outer segment in the presence of endogenous rhodopsin. We now show that in the absence of endogenous rhodopsin S334ter mis-localizes to the plasma membrane and fails to reconstitute outer segment structures. Surprisingly, this mis-localization does not affect photoreceptor cell survival. These results provide further evidence on the important role of the COOH-terminal domain in rhodopsin trafficking and demonstrate an absolute requirement of this domain for correct vectorial transport of rhodopsin in rod photoreceptors.

Photoreceptor renewal: a role for peripherin/rds

Visual transduction begins with the detection of light within the photoreceptor cell layer of the retina. Within this layer, specialized cells, termed rods and cones, contain the proteins responsible for light capture and its transduction to nerve impulses. The phototransductive proteins reside within an outer segment region that is connected to an inner segment by a thin stalk rich in cytoskeletal elements. A unique property of the outer segments is the presence of an elaborate intracellular membrane system that holds the phototransduction proteins and provides the requisite lipid environment. The maintenance of normal physiological function requires that these postmitotic cells retain the unique structure of the outer segment regions--stacks of membrane saccules in the case of rods and a continuous infolding of membrane in the case of cones. Both photoreceptor rod and cone cells achieve this through a series of coordinated steps. As new membranous material is synthesized, transported, and incorporated into newly forming outer segment membranes, a compensatory shedding of older membranous material occurs, thereby maintaining the segment at a constant length. These processes are collectively referred to as ROS (rod outer segment) or COS (cone outer segment) renewal. We review the cellular and molecular events responsible for these renewal processes and present the recent but compelling evidence, drawn from molecular genetic, biochemical, and biophysical approaches, pointing to an essential role for a unique tetraspanning membrane protein, called peripherin/rds, in the processes of disk morphogenesis.

A functional rhodopsin-green fluorescent protein fusion protein localizes correctly in transgenic Xenopus laevis retinal rods and is expressed in a time-dependent pattern

To study rhodopsin biosynthesis and transport in vivo, we engineered a fusion protein (rho-GFP) of bovine rhodopsin (rho) and green fluorescent protein (GFP). rho-GFP expressed in COS-1 cells bound 11-cis retinal, generating a pigment with spectral properties of rhodopsin (A(max) at 500 nm) and GFP (A(max) at 488 nm). rho-GFP activated transducin at 50% of the wild-type activity, whereas phosphorylation of rho-GFP by rhodopsin kinase was 10% of wild-type levels. We expressed rho-GFP in the rod photoreceptors of Xenopus laevis using the X. laevis principal opsin promoter. Like rhodopsin, rho-GFP localized to rod outer segments, indicating that rho-GFP was recognized by membrane transport mechanisms. In contrast, a rho-GFP variant lacking the C-terminal outer segment localization signal distributed to both outer and inner segment membranes. Confocal microscopy of transgenic retinas revealed that transgene expression levels varied between cells, an effect that is probably analogous to position-effect variegation. Furthermore, rho-GFP concentrations varied along the length of individual rods, indicating that expression levels varied within single cells on a daily or hourly basis. These results have implications for transgenic models of retinal degeneration and mechanisms of position-effect variegation and demonstrate the utility of rho-GFP as a probe for rhodopsin transport and temporal regulation of promoter function.

Identification of an outer segment targeting signal in the COOH terminus of rhodopsin using transgenic Xenopus laevis

Mislocalization of the photopigment rhodopsin may be involved in the pathology of certain inherited retinal degenerative diseases. Here, we have elucidated rhodopsin's targeting signal which is responsible for its polarized distribution to the rod outer segment (ROS). Various green fluorescent protein (GFP)/rhodopsin COOH-terminal fusion proteins were expressed specifically in the major red rod photoreceptors of transgenic Xenopus laevis under the control of the Xenopus opsin promoter. The fusion proteins were targeted to membranes via lipid modifications (palmitoylation and myristoylation) as opposed to membrane spanning domains. Membrane association was found to be necessary but not sufficient for efficient ROS localization. A GFP fusion protein containing only the cytoplasmic COOH-terminal 44 amino acids of Xenopus rhodopsin localized exclusively to ROS membranes. Chimeras between rhodopsin and alpha adrenergic receptor COOH-terminal sequences further refined rhodopsin's ROS localization signal to its distal eight amino acids. Mutations/deletions of this region resulted in partial delocalization of the fusion proteins to rod inner segment (RIS) membranes. The targeting and transport of endogenous wild-type rhodopsin was unaffected by the presence of mislocalized GFP fusion proteins.

Towards the proteome of the rhodopsin-bearing post-Golgi compartment of retinal photoreceptor cells

Polarized sorting of rhodopsin in retinal rod photoreceptor cells is mediated by post-Golgi carrier membranes that bud from the trans-Golgi network and fuse with the specialized domain of the plasma membrane in the rod inner segment. The identity of the majority of the resident proteins of this organelle still remains elusive, despite multifaceted approaches to study this compartment. In the present study we have taken a proteomic approach to the analysis of the post-Golgi carriers. First, we modified the previously established fractionation protocols in order to achieve greater purity of the isolated membranes. Specifically, the new fractionation scheme depleted the post-Golgi fraction of cytosolic proteins that were the most abundant contaminants complicating analysis of two-dimensional (2-D) gel profiles in our previous preparations. The isolated membranes were subjected to 2-D gel electrophoresis, immunoblotting and microsequencing. This analysis showed that the improved subcellular fractionation yielded a fraction highly enriched in rhodopsin-bearing post-Golgi carrier membranes. Two-dimensional mapping revealed 29 proteins that are preferentially found in this fraction and therefore represent candidates for post-Golgi membrane-specific proteins. This preparation of rhodopsin-bearing post-Golgi carriers is a first step towards the proteomics of this important organelle.

Genetic evidence for selective transport of opsin and arrestin by kinesin-II in mammalian photoreceptors

To test whether kinesin-II is important for transport in the mammalian photoreceptor cilium, and to identify its potential cargoes, we used Cre-loxP mutagenesis to remove the kinesin-II subunit, KIF3A, specifically from photoreceptors. Complete loss of KIF3A caused large accumulations of opsin, arrestin, and membranes within the photoreceptor inner segment, while the localization of alpha-transducin was unaffected. Other membrane, organelle, and transport markers, as well as opsin processing appeared normal. Loss of KIF3A ultimately caused apoptotic photoreceptor cell death similar to a known opsin transport mutant. The data suggest that kinesin-II is required to transport opsin and arrestin from the inner to the outer segment and that blocks in this transport pathway lead to photoreceptor cell death as found in retinitis pigmentosa.

Rhodopsin trafficking and its role in retinal dystrophies

We review the sorting/targeting steps involved in the delivery of rhodopsin to the outer segment compartment of highly polarized photoreceptor cells. The transport of rhodopsin includes (1) the sorting/budding of rhodopsin-containing vesicles at the trans-Golgi network, (2) the directional translocation of rhodopsin-bearing vesicles through the inner segment, and (3) the delivery of rhodopsin across the connecting cilium to the outer segment. Several independent lines of evidence suggest that the carboxyl-terminal, cytoplasmic tail of rhodopsin is involved in the post-Golgi trafficking of rhodopsin. Inappropriate subcellular targeting of naturally occurring rhodopsin mutants in vivo leads to photoreceptor cell death. Thus, the genes encoding mutations in the cellular components involved in photoreceptor protein transport are likely candidate genes for retinal dystrophies.

Single-cysteine substitution mutants at amino acid positions 306-321 in rhodopsin, the sequence between the cytoplasmic end of helix VII and the palmitoylation sites: sulfhydryl reactivity and transducin activation reveal a tertiary structure

As sensors for structure at the cytoplasmic face of rhodopsin, single-cysteine substitution mutants have been previously studied in the regions connecting helices III and IV and helices V and VI. In this paper we report on single-cysteine substitution mutants at amino acid positions 306-321, comprising the cytoplasmic sequence between helix VII and the palmitoylation sites in rhodopsin. The cysteine opsin mutants were expressed in COS-1 cells and on treatment with 11-cis-retinal all formed the characteristic rhodopsin chromophore. Cysteines at positions 306-316 and 319 reacted in the dark with the thiol-specific reagent 4, 4'-dithiodipyridine (4-PDS) but showed a wide variation in reactivity. Cysteines at positions 317, 318, 320, and 321 showed no reaction with 4-PDS. The mutants on illumination also showed wide variations in activating GT. The mutant Y306C showed almost no GT activation, I307C and N310C were poor, and the activity of the mutants M309C, F313C, and M317C was also reduced relative to WT. The results suggest that the region comprising amino acids 306-321 is a part of a tertiary structure and that specific amino acids in this region on light-activation participate in the interaction with GT.

Evectins: vesicular proteins that carry a pleckstrin homology domain and localize to post-Golgi membranes

We have identified two vesicular proteins, designated evectin (evt)-1 and -2. These proteins are approximately 25 kDa in molecular mass, lack a cleaved N-terminal signal sequence, and appear to be inserted into membranes through a C-terminal hydrophobic anchor. They also carry a pleckstrin homology domain at their N termini, which potentially couples them to signal transduction pathways that result in the production of lipid second messengers. evt-1 is specific to the nervous system, where it is expressed in photoreceptors and myelinating glia, polarized cell types in which plasma membrane biosynthesis is prodigious and regulated; in contrast, evt-2 is widely expressed in both neural and nonneural tissues. In photoreceptors, evt-1 localizes to rhodopsin-bearing membranes of the post-Golgi, an important transport compartment for which specific molecular markers have heretofore been lacking. The structure and subcellular distribution of evt-1 strongly implicate this protein as a mediator of post-Golgi trafficking in cells that produce large membrane-rich organelles. Its restricted cellular distribution and genetic locus make it a candidate gene for the inherited human retinopathy autosomal dominant familial exudative vitreoretinopathy and suggest that it also may be a susceptibility gene for multiple sclerosis.

Mutagenesis studies of human red opsin: trp-281 is essential for proper folding and protein-retinal interactions

Human red and green opsins contain a strikingly large number of tryptophan residues. These tryptophans are highly conserved among all red and green opsins. To investigate possible roles of these tryptophans in folding and structure, we have systematically replaced each tryptophan of human red opsin. When expressed in COS cells, wild-type red opsin undergoes N-linked glycosylation, forms a light-sensitive pigment with absorption maximum at 560 nm upon reconstitution with 11-cis-retinal, and is transported to the plasma membrane. We used the extent of glycosylation, pigment generation, and intracellular localization of mutant red opsins as our criteria for assessing the effect of substitution. Replacement of eight tryptophans, Trp-59, Trp-90, Trp-149, Trp-152, Trp-183, Trp-191, Trp-195, and Trp-243, with Phe or Ala did not affect the wild-type phenotype significantly. However, replacement of Trp-5 and Trp-51 in the putative N-terminal domain and Trp-142, Trp-177, Trp-179, and Trp-281 in the transmembrane domain with Phe had profound effects, indicating that these substitutions affected red opsin folding. Judged by the severity of the effects, we propose that Trp-5, Trp-51, Trp-177, and Trp-281 are important for red opsin folding. Although substitution of Trp-281 with Phe and Cys did not permit normal glycosylation and transport, substitution with Tyr and His permitted these processes but resulted in blue-shifted pigment. Thus, polar aromatics appear to substitute for Trp-281 to allow red opsin folding. The large spectral shift indicates that Trp-281 is essential for the proper interaction of the protein with 11-cis-retinal.

Rab11 is required for trans-golgi network-to-plasma membrane transport and a preferential target for GDP dissociation inhibitor

The rab11 GTPase has been localized to both the Golgi and recycling endosomes; however, its Golgi-associated function has remained obscure. In this study, rab11 function in exocytic transport was analyzed by using two independent means to perturb its activity. First, expression of the dominant interfering rab11S25N mutant protein led to a significant inhibition of the cell surface transport of vesicular stomatitis virus (VSV) G protein and caused VSV G protein to accumulate in the Golgi. On the other hand, the expression of wild-type rab11 or the activating rab11Q70L mutant had no adverse effect on VSV G transport. Next, the membrane association of rab11, which is crucial for its function, was perturbed by modest increases in GDP dissociation inhibitor (GDI) levels. This led to selective inhibition of the trans-Golgi network to cell surface delivery, whereas endoplasmic reticulum-to-Golgi and intra-Golgi transport were largely unaffected. The transport inhibition was reversed specifically by coexpression of wild-type rab11 with GDI. Under the same conditions two other exocytic rab proteins, rab2 and rab8, remained membrane bound, and the transport steps regulated by these rab proteins were unaffected. Neither mutant rab11S25N nor GDI overexpression had any impact on the cell surface delivery of influenza hemagglutinin. These data show that functional rab11 is critical for the export of a basolateral marker but not an apical marker from the trans-Golgi network and pinpoint rab11 as a sensitive target for inhibition by excess GDI.

Post-Golgi trafficking of rhodopsin in retinal photoreceptors

Rod outer segment renewal in retinal rod photoreceptors is mediated by polarised sorting of rhodopsin, and its associated proteins and lipids, on post-Golgi vesicles that bud from the trans-Golgi network and fuse with the specialised domain of the plasma membrane in the rod inner segment. This domain surrounds the cilium that connects the inner segment and the rod outer segment to which mature rhodopsin is delivered. The intracellular sorting machinery that regulates budding, targeting and fusion of rhodopsin carrier vesicles has been studied using multiple means including a newly developed cell-free assay that reconstitutes vesicle budding. These studies have revealed an essential role for small GTP-binding protein rab6, as well as the carboxyl-terminal domain of rhodopsin, in the formation of post-Golgi vesicles. In this report their role in post-Golgi trafficking of rhodopsin and the maintenance of photoreceptor cell polarity and health is discussed.

Effect of vitamin A supplementation on rhodopsin mutants threonine-17 --> methionine and proline-347 --> serine in transgenic mice and in cell cultures

A therapeutic effect of vitamin A supplementation on the course of photoreceptor degeneration, previously reported for patients with retinitis pigmentosa, was tested in two transgenic mouse models of this disease, each carrying a dominant rhodopsin mutation. The threonine-17 --> methionine (T17M) mutation is a class II rhodopsin mutation, characterized by a thermal instability/folding defect and minimal regeneration with the chromophore. The proline-347 --> serine (P347S) mutation belongs to class I, comprised of a smaller number of mutations that exhibit no recognized biochemical abnormality in vitro. In the present study, each of the two mouse models was fed a diet containing 2.5 mg of vitamin A palmitate (control) or 102.5 mg of vitamin A palmitate (high vitamin A) per kilogram of diet. Dark-adapted, full-field electroretinograms showed that the high vitamin A diet significantly reduced the rate of decline of a-wave and b-wave amplitudes in the T17M mice but had no significant effect on the decline of electroretinogram amplitude in the P347S mice. Correspondingly, histologic evaluation revealed that the treatment was associated with significantly longer photoreceptor inner and outer segments and a thicker outer nuclear layer in the T17M mice but had no effect on photoreceptor morphology in the P347S mice. In a separate series of experiments, the instability defect of the T17M mutant opsin expressed in vitro was partially alleviated by inclusion of 11-cis-retinal in the culture media. These results show that vitamin A supplementation slows the rate of photoreceptor degeneration caused by a class II rhodopsin mutation. Vitamin A supplementation may confer therapeutic benefit by stabilizing mutant opsins through increased availability of the chromophore.

The cytoplasmic tail of rhodopsin acts as a novel apical sorting signal in polarized MDCK cells

All basolateral sorting signals described to date reside in the cytoplasmic domain of proteins, whereas apical targeting motifs have been found to be lumenal. In this report, we demonstrate that wild-type rhodopsin is targeted to the apical plasma membrane via the TGN upon expression in polarized epithelial MDCK cells. Truncated rhodopsin with a deletion of 32 COOH-terminal residues shows a nonpolar steady-state distribution. Addition of the COOH-terminal 39 residues of rhodopsin redirects the basolateral membrane protein CD7 to the apical membrane. Fusion of rhodopsin's cytoplasmic tail to a cytosolic protein glutathione S-transferase (GST) also targets this fusion protein (GST-Rho39Tr) to the apical membrane. The targeting of GST-Rho39Tr requires both the terminal 39 amino acids and the palmitoylation membrane anchor signal provided by the rhodopsin sequence. The apical transport of GST-Rho39Tr can be reversibly blocked at the Golgi complex by low temperature and can be altered by brefeldin A treatment. This indicates that the membrane-associated GST-Rho39Tr protein may be sorted along a yet unidentified pathway that is similar to the secretory pathway in polarized MDCK cells. We conclude that the COOH-terminal tail of rhodopsin contains a novel cytoplasmic apical sorting determinant. This finding further indicates that cytoplasmic sorting machinery may exist in MDCK cells for some apically targeted proteins, analogous to that described for basolaterally targeted proteins.

Regulation of sorting and post-Golgi trafficking of rhodopsin by its C-terminal sequence QVS(A)PA

Several mutations that cause severe forms of the human disease autosomal dominant retinitis pigmentosa cluster in the C-terminal region of rhodopsin. Recent studies have implicated the C-terminal domain of rhodopsin in its trafficking on specialized post-Golgi membranes to the rod outer segment of the photoreceptor cell. Here we used synthetic peptides as competitive inhibitors of rhodopsin trafficking in the frog retinal cell-free system to delineate the potential regulatory sequence within the C terminus of rhodopsin and model the effects of severe retinitis pigmentosa alleles on rhodopsin sorting. The rhodopsin C-terminal sequence QVS(A)PA is highly conserved among different species. Peptides that correspond to the C terminus of bovine (amino acids 324-348) and frog (amino acids 330-354) rhodopsin inhibited post-Golgi trafficking by 50% and 60%, respectively, and arrested newly synthesized rhodopsin in the trans-Golgi network. Peptides corresponding to the cytoplasmic loops of rhodopsin and other control peptides had no effect. When three naturally occurring mutations: Q344ter (lacking the last five amino acids QVAPA), V345M, and P347S were introduced into the frog C-terminal peptide, the inhibitory activity of the peptides was no longer detectable. These observations suggest that the amino acids QVS(A)PA comprise a signal that is recognized by specific factors in the trans-Golgi network. A lack of recognition of this sequence, because of mutations in the last five amino acids causing autosomal dominant retinitis pigmentosa, most likely results in abnormal post-Golgi membrane formation and in an aberrant subcellular localization of rhodopsin.

Vesicle transport: more work for the Rabs?

The Rab family of small GTP-binding proteins has long been implicated in the docking and fusion of transport vesicles with their target membranes. Recent evidence, however, suggests that some Rabs may play an additional role in transport vesicle formation.

Folding and cell surface expression of the vasopressin V2 receptor: requirement of the intracellular C-terminus

We characterized truncations of the human vasopressin V2 receptor to determine the role of the intracellular C-terminus (comprising about 44 amino acids) in receptor function and cell surface expression. In contrast to the wild-type receptor, the naturally occurring mutant R337X failed to confer specific [3H]AVP binding to transfected cells. In addition, no vasopressin-sensitive adenylyl cyclase was detectable in membrane preparations of these cells. Laser scanning microscopy revealed that c-myc epitope- or green fluorescent protein-tagged R337X mutant receptors were retained within the endoplasmic reticulum. Increasing the number of C-terminal residues (truncations after codons 348, 354 and 356) restored G protein coupling, but revealed a length-dependent reduction of cell surface expression. Replacement of positively charged residues within the C-terminus by glutamine residues also decreased cell surface expression. A chimeric V2 receptor with the C-terminus replaced by that of the beta2-adrenergic receptor did not bind [3H]AVP and was retained within the cell. These data suggest that residues in the N-terminal part of the C-terminus are necessary for correct folding and that C-terminal residues are important for efficient cell surface expression.

Rab proteins and post-Golgi trafficking of rhodopsin in photoreceptor cells

Polarized sorting of rhodopsin in retinal rod photoreceptors is mediated by post-Golgi vesicles that bud from the trans-Golgi network and fuse with the specialized domain of the plasma membrane in the rod inner segment. This domain surrounds the cilium that connects the inner segment and the rod outer segment to which mature rhodopsin is delivered. To dissect the sorting machinery that regulates budding, targeting, and fusion of rhodopsin carrier vesicles, their GTP-binding protein composition has been studied using multiple means including high-resolution two-dimensional gel electrophoresis and [32P]GTP overlays of renatured proteins. These studies indicate a succession on rhodopsin-bearing vesicles of rab6, rab11, rab3 and rab8, all members of the small GTP-binding protein family of the known regulators of membrane trafficking. In this review the role of rab proteins in post-Golgi trafficking of rhodopsin is discussed.

Two new Ypt GTPases are required for exit from the yeast trans-Golgi compartment

Small GTPases of the Ypt/rab family are involved in the regulation of vesicular transport. These GTPases apparently function during the targeting of vesicles to the acceptor compartment. Two members of the Ypt/rab family, Ypt1p and Sec4p, have been shown to regulate early and late steps of the yeast exocytic pathway, respectively. Here we tested the role of two newly identified GTPases, Ypt31p and Ypt32p. These two proteins share 81% identity and 90% similarity, and belong to the same protein subfamily as Ypt1p and Sec4p. Yeast cells can tolerate deletion of either the YPT31 or the YPT32 gene, but not both. These observations suggest that Ypt31p and Ypt32p perform identical or overlapping functions. Cells deleted for the YPT31 gene and carrying a conditional ypt32 mutation exhibit protein transport defects in the late exocytic pathway, but not in vacuolar protein sorting. The ypt31/ 32 mutant secretory defect is clearly downstream from that displayed by a ypt1 mutant and is similar to that of sec4 mutant cells. However, electron microscopy revealed that while sec4 mutant cells accumulate secretory vesicles, ypt31/32 mutant cells accumulate aberrant Golgi structures. The ypt31/32 phenotype is epistatic to that of a sec1 mutant, which accumulates secretory vesicles. Together, these results indicate that the Ypt31/32p GTPases are required for a step that occurs in the trans-Golgi compartment, between the reactions regulated by Ypt1p and Sec4p. This step might involve budding of vesicles from the trans-Golgi. Alternatively, Ypt31/32p might promote secretion indirectly, by allowing fusion of recycling vesicles with the trans-Golgi compartment.