Distribution of guanylate cyclase within photoreceptor outer segments

The myosin-tail homology domain of centrosomal protein 290 is essential for protein confinement between the inner and outer segments in photoreceptors

Mutations in the centrosomal protein 290 (CEP290) gene cause various ciliopathies involving retinal degeneration. CEP290 proteins localize to the ciliary transition zone and are thought to act as a gatekeeper that controls ciliary protein trafficking. However, precise roles of CEP290 in photoreceptors and pathomechanisms of retinal degeneration in CEP290-associated ciliopathies are not sufficiently understood. Using conditional Cep290 mutant mice, in which the C-terminal myosin-tail homology domain of CEP290 is disrupted after the connecting cilium is assembled, we show that this domain is essential for protein confinement between the inner and the outer segments. Upon disruption of the myosin-tail homology domain, inner segment plasma membrane proteins, including syntaxin 3 (STX3), synaptosome-associated protein 25 (SNAP25), and interphotoreceptor matrix proteoglycan 2 (IMPG2), rapidly accumulated in the outer segment. In contrast, localization of endomembrane proteins was not altered. Trafficking and confinement of most outer segment-resident proteins appeared to be unaffected or only minimally affected in Cep290 mutant mice. One notable exception was rhodopsin (RHO), which severely mislocalized to inner segments during the initial stage of degeneration. Similar mislocalization phenotypes were observed in Cep290^rd16 mice. These results suggest that a failure of protein confinement at the connecting cilium and consequent accumulation of inner segment membrane proteins in the outer segment, along with insufficient RHO delivery, is part of the disease mechanisms that cause retinal degeneration in CEP290-associated ciliopathies. Our study provides insights into the pathomechanisms of retinal degenerations associated with compromised ciliary gates.

Protein and Signaling Networks in Vertebrate Photoreceptor Cells

Vertebrate photoreceptor cells are exquisite light detectors operating under very dim and bright illumination. The photoexcitation and adaptation machinery in photoreceptor cells consists of protein complexes that can form highly ordered supramolecular structures and control the homeostasis and mutual dependence of the secondary messengers cyclic guanosine monophosphate (cGMP) and Ca²⁺. The visual pigment in rod photoreceptors, the G protein-coupled receptor rhodopsin is organized in tracks of dimers thereby providing a signaling platform for the dynamic scaffolding of the G protein transducin. Illuminated rhodopsin is turned off by phosphorylation catalyzed by rhodopsin kinase (GRK1) under control of Ca²⁺-recoverin. The GRK1 protein complex partly assembles in lipid raft structures, where shutting off rhodopsin seems to be more effective. Re-synthesis of cGMP is another crucial step in the recovery of the photoresponse after illumination. It is catalyzed by membrane bound sensory guanylate cyclases (GCs) and is regulated by specific neuronal Ca²⁺-sensor proteins called guanylate cyclase-activating proteins (GCAPs). At least one GC (ROS-GC1) was shown to be part of a multiprotein complex having strong interactions with the cytoskeleton and being controlled in a multimodal Ca²⁺-dependent fashion. The final target of the cGMP signaling cascade is a cyclic nucleotide-gated (CNG) channel that is a hetero-oligomeric protein located in the plasma membrane and interacting with accessory proteins in highly organized microdomains. We summarize results and interpretations of findings related to the inhomogeneous organization of signaling units in photoreceptor outer segments.

The interaction network of rhodopsin involving the heterotrimeric G-protein transducin and the monomeric GTPase Rac1 is determined by distinct binding processes

The monomeric G-protein Rac1, a member of the family of Rho/Rac/Cdc42 GTPases, is involved in light-induced photoreceptor degeneration, but its specific role remains elusive. In particular, reports on Rac1 interacting with the visual pigment rhodopsin are puzzling and need a more quantitative examination. We probed the presence of Rac1 in rod outer segments by immunohistochemical staining of bovine retinae and western blot analysis of isolated rod outer segments. Rac1 was present throughout the whole retina except in the outer and inner nuclear layers, but was strongly expressed in photoreceptor cells. Rac1 was distributed in three different fractions of rod outer segments: one fraction was soluble in detergents, a second fraction cosegregated with lipid rafts, and a third fraction was associated with lipid bilayer free axonemal/cytoskeletal structures. We also investigated the interaction between rhodopsin and Rac1 by using surface plasmon resonance spectroscopy under dark and light conditions. Biophysical interaction studies revealed that Rac1 could interact with rhodopsin, but in a light-independent manner, and kinetic analysis indicated that binding of Rac1 occurred with lower affinity and speed than the association of transducin and rhodopsin. Thus, in dark-adapted rod cells, Rac1 cannot compete with transducin for binding to rhodopsin, and signalling can proceed normally. Instead, the concentration of transducin has to drop significantly so that Rac1 can bind to rhodopsin; in the outer segment, this occurs only under intense illumination, when transducin is translocated to the inner segment.

Structural and functional protein network analyses predict novel signaling functions for rhodopsin

Proteomic analyses, literature mining, and structural data were combined to generate an extensive signaling network linked to the visual G protein-coupled receptor rhodopsin. Network analysis suggests novel signaling routes to cytoskeleton dynamics and vesicular trafficking.

Using a shotgun proteomic approach, we identified the protein inventory of the light sensing outer segment of the mammalian photoreceptor.

These data, combined with literature mining, structural modeling, and computational analysis, offer a comprehensive view of signal transduction downstream of the visual G protein-coupled receptor rhodopsin.

The network suggests novel signaling branches downstream of rhodopsin to cytoskeleton dynamics and vesicular trafficking.

The network serves as a basis for elucidating physiological principles of photoreceptor function and suggests potential disease-associated proteins.

Photoreceptor cells are neurons capable of converting light into electrical signals. The rod outer segment (ROS) region of the photoreceptor cells is a cellular structure made of a stack of around 800 closed membrane disks loaded with rhodopsin (Liang et al, 2003; Nickell et al, 2007). In disc membranes, rhodopsin arranges itself into paracrystalline dimer arrays, enabling optimal association with the heterotrimeric G protein transducin as well as additional regulatory components (Ciarkowski et al, 2005). Disruption of these highly regulated structures and processes by germline mutations is the cause of severe blinding diseases such as retinitis pigmentosa, macular degeneration, or congenital stationary night blindness (Berger et al, 2010).

Traditionally, signal transduction networks have been studied by combining biochemical and genetic experiments addressing the relations among a small number of components. More recently, large throughput experiments using different techniques like two hybrid or co-immunoprecipitation coupled to mass spectrometry have added a new level of complexity (Ito et al, 2001; Gavin et al, 2002, 2006; Ho et al, 2002; Rual et al, 2005; Stelzl et al, 2005). However, in these studies, space, time, and the fact that many interactions detected for a particular protein are not compatible, are not taken into consideration. Structural information can help discriminate between direct and indirect interactions and more importantly it can determine if two or more predicted partners of any given protein or complex can simultaneously bind a target or rather compete for the same interaction surface (Kim et al, 2006).

In this work, we build a functional and dynamic interaction network centered on rhodopsin on a systems level, using six steps: In step 1, we experimentally identified the proteomic inventory of the porcine ROS, and we compared our data set with a recent proteomic study from bovine ROS (Kwok et al, 2008). The union of the two data sets was defined as the ‘initial experimental ROS proteome'. After removal of contaminants and applying filtering methods, a ‘core ROS proteome', consisting of 355 proteins, was defined.

In step 2, proteins of the core ROS proteome were assigned to six functional modules: (1) vision, signaling, transporters, and channels; (2) outer segment structure and morphogenesis; (3) housekeeping; (4) cytoskeleton and polarity; (5) vesicles formation and trafficking, and (6) metabolism.

In step 3, a protein-protein interaction network was constructed based on the literature mining. Since for most of the interactions experimental evidence was co-immunoprecipitation, or pull-down experiments, and in addition many of the edges in the network are supported by single experimental evidence, often derived from high-throughput approaches, we refer to this network, as ‘fuzzy ROS interactome'. Structural information was used to predict binary interactions, based on the finding that similar domain pairs are likely to interact in a similar way (‘nature repeats itself') (Aloy and Russell, 2002). To increase the confidence in the resulting network, edges supported by a single evidence not coming from yeast two-hybrid experiments were removed, exception being interactions where the evidence was the existence of a three-dimensional structure of the complex itself, or of a highly homologous complex. This curated static network (‘high-confidence ROS interactome') comprises 660 edges linking the majority of the nodes. By considering only edges supported by at least one evidence of direct binary interaction, we end up with a ‘high-confidence binary ROS interactome'. We next extended the published core pathway (Dell'Orco et al, 2009) using evidence from our high-confidence network. We find several new direct binary links to different cellular functional processes (Figure 4): the active rhodopsin interacts with Rac1 and the GTP form of Rho. There is also a connection between active rhodopsin and Arf4, as well as PDEδ with Rab13 and the GTP-bound form of Arl3 that links the vision cycle to vesicle trafficking and structure. We see a connection between PDEδ with prenyl-modified proteins, such as several small GTPases, as well as with rhodopsin kinase. Further, our network reveals several direct binary connections between Ca2+-regulated proteins and cytoskeleton proteins; these are CaMK2A with actinin, calmodulin with GAP43 and S1008, and PKC with 14-3-3 family members.

In step 4, part of the network was experimentally validated using three different approaches to identify physical protein associations that would occur under physiological conditions: (i) Co-segregation/co-sedimentation experiments, (ii) immunoprecipitations combined with mass spectrometry and/or subsequent immunoblotting, and (iii) utilizing the glycosylated N-terminus of rhodopsin to isolate its associated protein partners by Concanavalin A affinity purification. In total, 60 co-purification and co-elution experiments supported interactions that were already in our literature network, and new evidence from 175 co-IP experiments in this work was added. Next, we aimed to provide additional independent experimental confirmation for two of the novel networks and functional links proposed based on the network analysis: (i) the proposed complex between Rac1/RhoA/CRMP-2/tubulin/and ROCK II in ROS was investigated by culturing retinal explants in the presence of an ROCK II-specific inhibitor (Figure 6). While morphology of the retinas treated with ROCK II inhibitor appeared normal, immunohistochemistry analyses revealed several alterations on the protein level. (ii) We supported the hypothesis that PDEδ could function as a GDI for Rac1 in ROS, by demonstrating that PDEδ and Rac1 co localize in ROS and that PDEδ could dissociate Rac1 from ROS membranes in vitro.

In step 5, we use structural information to distinguish between mutually compatible (‘AND') or excluded (‘XOR') interactions. This enables breaking a network of nodes and edges into functional machines or sub-networks/modules. In the vision branch, both ‘AND' and ‘XOR' gates synergize. This may allow dynamic tuning of light and dark states. However, all connections from the vision module to other modules are ‘XOR' connections suggesting that competition, in connection with local protein concentration changes, could be important for transmitting signals from the core vision module.

In the last step, we map and functionally characterize the known mutations that produce blindness.

In summary, this represents the first comprehensive, dynamic, and integrative rhodopsin signaling network, which can be the basis for integrating and mapping newly discovered disease mutants, to guide protein or signaling branch-specific therapies.

Orchestration of signaling, photoreceptor structural integrity, and maintenance needed for mammalian vision remain enigmatic. By integrating three proteomic data sets, literature mining, computational analyses, and structural information, we have generated a multiscale signal transduction network linked to the visual G protein-coupled receptor (GPCR) rhodopsin, the major protein component of rod outer segments. This network was complemented by domain decomposition of protein–protein interactions and then qualified for mutually exclusive or mutually compatible interactions and ternary complex formation using structural data. The resulting information not only offers a comprehensive view of signal transduction induced by this GPCR but also suggests novel signaling routes to cytoskeleton dynamics and vesicular trafficking, predicting an important level of regulation through small GTPases. Further, it demonstrates a specific disease susceptibility of the core visual pathway due to the uniqueness of its components present mainly in the eye. As a comprehensive multiscale network, it can serve as a basis to elucidate the physiological principles of photoreceptor function, identify potential disease-associated genes and proteins, and guide the development of therapies that target specific branches of the signaling pathway.

Targeting of mouse guanylate cyclase 1 (Gucy2e) to Xenopus laevis rod outer segments

Photoreceptor guanylate cyclase (GC1) is a transmembrane protein and responsible for synthesis of cGMP, the secondary messenger of phototransduction. It consists of an extracellular domain, a single transmembrane domain, and an intracellular domain. It is unknown how GC1 targets to the outer segments where it resides. To identify a putative GC1 targeting signal, we generated a series of peripheral membrane and transmembrane constructs encoding extracellular and intracellular mouse GC1 fragments fused to EGFP. The constructs were expressed in Xenopus laevis rod photoreceptors under the control of the rhodopsin promoter. We examined the localization of GFP-GC1 fusion proteins containing the complete GC1 sequence, or partial GC1 sequences, which were membrane-associated via either the GC1 transmembrane domain or the rhodopsin C-terminal palmitoyl chains. Full-length GFP-GC1 targeted to the rod outer segment disk rims. As a group, fusion proteins containing the entire cytoplasmic domain of GC1 targeted to the OS, whereas other fusion proteins containing portions of the cytoplasmic or the extracellular domains did not. We conclude that GC1 likely has no single linear peptide-based OS targeting signal. Our results suggest targeting is due to either multiple weak signals in the cytoplasmic domain of GC1, or co-transport to the OS with an accessory protein.

Interaction of retinal guanylate cyclase with the alpha subunit of transducin: potential role in transducin localization

Vertebrate phototransduction is mediated by cGMP, which is generated by retGC (retinal guanylate cyclase) and degraded by cGMP phosphodiesterase. Light stimulates cGMP hydrolysis via the G-protein transducin, which directly binds to and activates phosphodiesterase. Bright light also causes relocalization of transducin from the OS (outer segments) of the rod cells to the inner compartments. In the present study, we show experimental evidence for a previously unknown interaction between G(alphat) (the transducin alpha subunit) and retGC. G(alphat) co-immunoprecipitates with retGC from the retina or from co-transfected COS-7 cells. The retGC-G(alphat) complex is also present in cones. The interaction also occurs in mice lacking RGS9 (regulator of G-protein signalling 9), a protein previously shown to associate with both G(alphat) and retGC. The G(alphat)-retGC interaction is mediated primarily by the kinase homology domain of retGC, which binds GDP-bound G(alphat) stronger than the GTP[S] (GTPgammaS; guanosine 5'-[gamma-thio]triphosphate) form. Neither G(alphat) nor G(betagamma) affect retGC-mediated cGMP synthesis, regardless of the presence of GCAP (guanylate cyclase activating protein) and Ca2+. The rate of light-dependent transducin redistribution from the OS to the inner segments is markedly accelerated in the retGC-1-knockout mice, while the migration of transducin to the OS after the onset of darkness is delayed. Supplementation of permeabilized photoreceptors with cGMP does not affect transducin translocation. Taken together, these results suggest that the protein-protein interaction between G(alphat) and retGC represents a novel mechanism regulating light-dependent translocation of transducin in rod photoreceptors.

Intraflagellar transport and the sensory outer segment of vertebrate photoreceptors

Analysis of the other segments of rod and cone photoreceptors in vertebrates has provided a rich molecular understanding of how light absorbed by a visual pigment can result in changes in membrane polarity that regulate neurotransmitter release. These events are carried out by a large group of phototransduction proteins that are enriched in the outer segment. However, the mechanisms by which phototransduction proteins are sequestered in the outer segment are not well defined. Insight into those mechanisms has recently emerged from the findings that outer segments arise from the plasma membrane of a sensory cilium, and that intraflagellar transport (IFT), which is necessary for assembly of many types of cilia and flagella, plays a crucial role. Here we review the general features of outer segment assembly that may be common to most sensory cilia as well those that may be unique to the outer segment. Those features illustrate how further analysis of photoreceptor IFT may provide insight into both IFT cargo and the role of alternative IFT kinesins.

Guanylate cyclase-activating proteins and retina disease

Detailed biochemical, structural and physiological studies of the role of Ca2(+)-binding proteins in mammalian retinal neurons have yielded new insights into the function of these proteins in normal and pathological states. In phototransduction, a biochemical process that is responsible for the conversion of light into an electrical impulse, guanylate cyclases (GCs) are regulated by GC-activating proteins (GCAPs). These regulatory proteins respond to changes in cytoplasmic Ca2+ concentrations. Disruption of Ca2+ homeostasis in photoreceptor cells by genetic and environmental factors can result ultimately in degeneration of these cells. Pathogenic mutations in GC1 and GCAP1 cause autosomal recessive Leber congenital amaurosis and autosomal dominant cone dystrophy, respectively. This report provides a recent account of the advances, challenges, and possible future prospects of studying this important step in visual transduction that transcends to other neuronal Ca2+ homeostasis processes.

Protein networks and complexes in photoreceptor cilia

Vertebrate photoreceptor cells are ciliated sensory cells specialized for single photon detection. The photoreceptor outer segment corresponds to the ciliary shaft of a prototypic cilium. In the outer segment compartment, the ciliary membrane is highly modified into membranous disks which are enveloped by the plasma membrane in rod cells. At these outer segment disks, the visual transduction cascade--a prototypical G-protein coupled receptor transduction pathway is arranged. The light sensitive outer segments are linked by the socalled connecting cilium with the inner segment, the photoreceptor compartment which contains all organelles necessary for cell metabolism. The connecting cilium correlates with the transition zone, the short junction between the basal body and the axoneme of a prototypic cilium. The connecting cilium and the calycal processes, including the periciliary ridge complex, as well as the basal body complex are in close functional association with each other. In the latter ciliary compartments, the export and import from/into the outer segment of the photoreceptor cell are controlled and regulated. In all subciliary compartments, proteins are arranged in functional multiprotein complexes. In the outer segment, signaling components are arranged into complexes which provide specificity and speed for the signaling and serve in adaptation. Centrin-G-protein complexes may regulate the light driven translocation of the visual G-protein transducin through the connecting cilium. Intraflagellar transport (IFT) complexes may serve in intersegmental exchange of molecules. The import/export of molecules is thought to be regulated by proteins arranged in networks at the basal body complex. Proteins of the interactome related to the human Usher syndrome are localized in the connecting cilium and may participate in the ciliary transport, but are also arranged at interfaces between the inner segment and the connecting cilium where they probably control the cargo handover between the transport systems of the inner segment and these of the cilium. Furthermore, USH protein complexes may further provide mechanical stabilization to membrane specializations of the calycal processes and the connecting cilium. The protein complex in which the retinitis pigmentosa GTPase regulator (RPGR) participates in the ciliary compartments also plays a key role in the function and maintenance of photoreceptor cells. It further associates through the presumed scaffolding protein RPGRIP1 with the nephrocystin protein network. Although many of these proteins have been also found in prototypic cilia or primary cilia, the arrangements of the proteins in complexes can be specific for vertebrate photoreceptor cells. Defects of proteins in these complexes lead to photoreceptor cell death and retinal degeneration, underlying syndromic and non-syndromic blindness.

The proteome of the mouse photoreceptor sensory cilium complex

Primary cilia play critical roles in many aspects of biology. Specialized versions of primary cilia are involved in many aspects of sensation. The single photoreceptor sensory cilium (PSC) or outer segment elaborated by each rod and cone photoreceptor cell of the retina is a classic example. Mutations in genes that encode cilia components are common causes of disease, including retinal degenerations. The protein components of mammalian primary and sensory cilia have not been defined previously. Here we report a detailed proteomics analysis of the mouse PSC complex. The PSC complex comprises the outer segment and its cytoskeleton, including the axoneme, basal body, and ciliary rootlet, which extends into the inner segment of photoreceptor cells. The PSC complex proteome contains 1968 proteins represented by three or more unique peptides, including approximately 1500 proteins not detected in cilia from lower organisms. This includes 105 hypothetical proteins and 60 proteins encoded by genes that map within the critical intervals for 23 inherited cilia-related disorders, increasing their priority as candidate genes. The PSC complex proteome also contains many cilia proteins not identified previously in photoreceptors, including 13 proteins produced by genes that harbor mutations that cause cilia disease and seven intraflagellar transport proteins. Analyses of PSC complexes from rootletin knock-out mice, which lack ciliary rootlets, confirmed that 1185 of the identified PSC complex proteins are derived from the outer segment. The mass spectrometry data, benchmarked by 15 well characterized outer segment proteins, were used to quantify the copy number of each protein in a mouse rod outer segment. These results reveal mammalian cilia to be several times more complex than the cilia of unicellular organisms and open novel avenues for studies of how cilia are built and maintained and how these processes are disrupted in human disease.

Progressive cone and cone-rod dystrophies: phenotypes and underlying molecular genetic basis

The cone and cone-rod dystrophies form part of a heterogeneous group of retinal disorders that are an important cause of visual impairment in children and adults. There have been considerable advances made in recent years in our understanding of the pathogenesis of these retinal dystrophies, with many of the chromosomal loci and causative genes having now been identified. Mutations in 12 genes, including GUCA1A, peripherin/RDS, ABCA4 and RPGR, have been described to date; and in many cases detailed functional assessment of the effects of the encoded mutant proteins has been undertaken. This improved knowledge of disease mechanisms has raised the possibility of future treatments for these disorders, for which there are no specific therapies available at the present time.

Mutation in the Gene GUCA1A, Encoding Guanylate Cyclase-Activating Protein 1, Causes Cone, Cone-Rod, and Macular Dystrophy

PURPOSE

To determine the underlying molecular genetic basis of a retinal dystrophy identified in a 4-generation family and to examine the phenotype and the degree of intrafamilial variability.

DESIGN

Prospective case series.

PARTICIPANTS

Six affected individuals from a nonconsanguineous British family.

METHODS

Detailed ophthalmologic examination, color fundus photography, autofluorescence imaging, and electrophysiologic assessment were performed. Blood samples were taken for DNA extraction, and mutation screening of GUCA1A, the gene encoding guanylate cyclase-activating protein 1 (GCAP1), was undertaken.

RESULTS

All affected subjects complained of mild photophobia and reduced central and color vision. Onset was between the third and fifth decade, with subsequent gradual deterioration of visual acuity and color vision. Visual acuity ranged between 6/9 and counting fingers. Color vision was either absent or markedly reduced along all 3 color axes. A range of macular appearances was seen, varying from mild retinal pigment epithelial disturbance to extensive atrophy. Electrophysiologic testing revealed a range of electrophysiologic abnormalities: isolated cone electroretinography abnormalities, reduced cone and rod responses (with cone loss greater than rod), and isolated macular dysfunction. The 4 coding exons of GUCA1A were screened for mutations in affected and unaffected family members. A single transition, A319G, causing a nonconservative missense substitution, Tyr99Cys, segregated uniquely in all affected subjects.

CONCLUSIONS

The Tyr99Cys GUCA1A mutation has been previously shown to cause autosomal dominant progressive cone dystrophy. This is the first report of this mutation also causing both cone-rod dystrophy and isolated macular dysfunction. The phenotypic variation described here exemplifies the intrafamilial heterogeneity of retinal dysfunction that can be observed in persons harboring the same mutation and chromosomal segment.

Mouse models to study GCAP functions in intact photoreceptors

In photoreceptor cells cGMP is the second messenger that transduces light into an electrical response. Regulation of cGMP synthesis by Ca2+ is one of the key mechanisms by which Ca2+ exerts negative feedback to the phototransduction cascade in the process of light adaptation. This Ca2+ feedback to retinal guanylyl cyclases (Ret-GCs) is conferred by the guanylate cyclase-activating proteins (GCAPs). Mutations in GCAP1 that disrupt the Ca2+ regulation of Ret-GCs in vitro have been associated with severe human vision disorders. This chapter focuses on recent data obtained from biochemical and electrophysiological studies of GCAP1/GCAP2 knockout mice and other GCAP transgenic mice, addressing: 1. the quantitative aspects of the Ca2+-feedback to Ret-GCs in regulating the light sensitivity and adaptation in intact rods; 2. functional differences between GCAP1 and GCAP2 in intact rod photoreceptors; and 3. whether GCAP mutants with impaired Ca2+ binding lead to retinal disease in vivo by constitutive activation of Ret-GCs and elevation of intracellular cGMP, as predicted from in vitro studies.

Identification of novel molecular components of the photoreceptor connecting cilium by immunoscreens

The connecting cilium of photoreceptor cells is the only intracellular link between the morphologically, functionally and biochemically different compartments of the inner and outer segments. The non-motile modified cilium plays an important role in the organization and the function of photoreceptor cells, namely in delivery and turnover of enzymes and substrates of the visual transduction cascade, and the photosensitive membranes of the outer segment. The protein components of the cilium participate in the intracellular transport through the cilium, in the outer segment disk morphogenesis and in the maintenance of discrete membrane domains. In order to identify yet unknown cytoskeletal components of the connecting cilium, a combined biochemical and molecular biological strategy was applied. For this purpose, antibodies were raised against proteins of photoreceptor cell axonemes. Using this AX-4-antiserum, a rat retina cDNA expression library was immunoscreened and clones encoding partial sequences of (i) already known photoreceptor specific proteins; (ii) ubiquitously expressed proteins; (iii) clones with homologies to retinal ESTs; and (iv) clones coding for cytoskeletal proteins were isolated. Further analysis revealed that these candidate clones have homologies to Drosophila flightless I, mouse APC-binding protein EB2, human microtubule associated protein 4 (MAP4), human centrin 3, human cytoplasmic dynein intermediate chain 2C, and human dynamitin.The immunoscreening approach used here was successfully applied to isolate genes encoding yet unknown cytoskeletal proteins of photoreceptor cell axonemes. The obtained information will provide further insight into the role of the connecting cilium in photoreceptor cell function.

S100B and S100A1 proteins in bovine retina:their calcium-dependent stimulation of a membrane-bound guanylate cyclase activity as investigated by ultracytochemistry

The Ca2(+)-binding proteins of the EF-hand type, S100B and S100A1, were detected in the outer segment of bovine retina photoreceptors where they are localized to disc membranes, as investigated by immunofluorescence and immunogold cytochemistry. S100B and S100A1 stimulate a membrane-bound guanylate cyclase activity associated with photoreceptor disc membranes in dark-adapted retina in a Ca2(+)-dependent manner, although with different Ca2+ requirements, as investigated by an ultracytochemical approach. Other retinal cell types express S100B and S100A1 as well. S100B is detected in the outer limiting membrane, fine cell processes in the outer nuclear layer and the outer plexiform layer, cell bodies in the inner nuclear layer and the ganglion cell layer, and the inner limiting membrane, whereas S100A1 has a more discrete distribution. S100B and S100A1 also stimulate a membrane-bound guanylate cyclase activity in photoreceptor cell bodies and Muller cells, but their effect appears independent of the light- or dark-adapted state of the retina and is observed at relatively high Ca2+ concentrations. These data represent the ultrastructural counterpart of recent biochemical observations implicating S100B and, possibly, S100A1 in the Ca2(+)-dependent stimulation of a photoreceptor membrane-bound guanylate cyclase activity [T. Duda, R. M. Goraczniak and R. K. Sharma (1996) Molecular characterization of S100A1-S1000B protein in retina and its activation mechanism of bovine photoreceptor guanylate cyclast. Biochemistry 35, 6263-6266; A. Margulis, N. Pozdnyakov and A. Sitaramayya (1996) Activation of bovine photoreceptor guanylate cyclast by S100 proteins. Biochem. Biophys. Res. Commun. 218, 243-247]. Our data suggest that at least S100B may take part in the regulation of a membrane-bound guanylate cyclase-based signalling pathway in both photoreceptors and Muller cells.

Activation of retinal guanylyl cyclase-1 by Ca2+-binding proteins involves its dimerization

Retinal guanylyl cyclase-1 (retGC-1), a key enzyme in phototransduction, is activated by guanylyl cyclase-activating proteins (GCAPs) if [Ca2+] is less than 300 nM. The activation is believed to be essential for the recovery of photoreceptors to the dark state; however, the molecular mechanism of the activation is unknown. Here, we report that dimerization of retGC-1 is involved in its activation by GCAPs. The GC activity and the formation of a 210-kDa cross-linked product of retGC-1 were monitored in bovine rod outer segment homogenates, GCAPs-free bovine rod outer segment membranes and recombinant bovine retGC-1 expressed in COS-7 cells. In addition to recombinant bovine GCAPs, constitutively active mutants of GCAPs that activate retGC-1 in a [Ca2+]-independent manner and bovine brain S100b that activates retGC-1 in the presence of approximately 10 microM [Ca2+] were used to investigate whether these activations take place through a similar mechanism, and whether [Ca2+] is directly involved in the dimerization. We found that a monomeric form of retGC-1 ( approximately 110 kDa) was mainly observed whenever GC activity was at basal or low levels. However, the 210-kDa product was increased whenever the GC activity was stimulated by any Ca2+-binding proteins used. We also found that [Ca2+] did not directly regulate the formation of the 210-kDa product. The 210-kDa product was detected in a purified GC preparation and did not contain GCAPs even when the formation of the 210-kDa product was stimulated by GCAPs. These data strongly suggest that the 210-kDa cross-linked product is a homodimer of retGC-1. We conclude that inactive retGC-1 is predominantly a monomeric form, and that dimerization of retGC-1 may be an essential step for its activation by active forms of GCAPs.

Identification of a domain in guanylyl cyclase-activating protein 1 that interacts with a complex of guanylyl cyclase and tubulin in photoreceptors

The membrane-bound guanylyl cyclase in rod photoreceptors is activated by guanylyl cyclase-activating protein 1 (GCAP-1) at low free [Ca2+]. GCAP-1 is a Ca2+-binding protein and belongs to the superfamily of EF-hand proteins. We created an oligopeptide library of overlapping peptides that encompass the entire amino acid sequence of GCAP-1. Peptides were used in competitive screening assays to identify interaction regions in GCAP-1 that directly bind the guanylyl cyclase in bovine photoreceptor cells. We found four regions in GCAP-1 that participate in regulating guanylyl cyclase. A 15-amino acid peptide located adjacent to the second EF-hand motif (Phe73-Lys87) was identified as the main interaction domain. Inhibition of GCAP-1-stimulated guanylyl cyclase activity by the peptide Phe73-Lys87 was completely relieved when an excess amount of GCAP-1 was added. An affinity column made from this peptide was able to bind a complex of photoreceptor guanylyl cyclase and tubulin. Using an anti-GCAP-1 antibody, we coimmunoprecipitated GCAP-1 with guanylyl cyclase and tubulin. Complex formation between GCAP-1 and guanylyl cyclase was observed independent of [Ca2+]. Our experiments suggest that there exists a tight association of guanylyl cyclase and tubulin in rod outer segments.

Calcium binding, but not a calcium-myristoyl switch, controls the ability of guanylyl cyclase-activating protein GCAP-2 to regulate photoreceptor guanylyl cyclase

Guanylyl cyclase-activating protein 2 (GCAP-2) is a recoverin-like calcium-binding protein that regulates photoreceptor guanylyl cyclase (RetGC) (Dizhoor, A. M., and Hurley, J. B. (1996) J. Biol. Chem. 271, 19346-19350). It was reported that myristoylation of a related protein, GCAP-1, was critical for its affinity for RetGC (Frins, S., Bonigk, W., Muller, F., Kellner, R., and Koch, K.-W. (1996) J. Biol. Chem. 271, 8022-8027). We demonstrate that the N terminus of GCAP-2, like those of other members of the recoverin family of Ca2+-binding proteins, is fatty acylated. However, unlike other proteins of this family, more GCAP-2 is present in the membrane fraction at low Ca2+ than at high Ca2+ concentrations. We investigated the role of the N-terminal fatty acyl residue in the ability of GCAP-2 to regulate RetGCs. Myristoylated or nonacylated GCAP-2 forms were expressed in Escherichia coli. Wild-type GCAP-2 and the Gly2 --> Ala2 GCAP-2 mutant, which is unable to undergo N-terminal myristoylation, were also expressed in mammalian HEK293 cells. We found that compartmentalization of GCAP-2 in photoreceptor outer segment membranes is Ca2+- and ionic strength-sensitive, but it does not require the presence of the fatty acyl group and does not necessarily directly reflect GCAP-2 interaction with RetGC. The lack of myristoylation does not significantly affect the ability of GCAP-2 to stimulate RetGC. Nor does it affect the ability of the Ca2+-loaded form of GCAP-2 to compete with the GCAP-2 mutant that constitutively activates RetGC. We conclude that while Ca2+ binding plays a major regulatory role in GCAP-2 function, it does not operate through a calcium-myristoyl switch similar to the one found in recoverin.

Two eye guanylyl cyclases are expressed in the same photoreceptor cells and form homomers in preference to heteromers

We recently described two eye guanylyl cyclases (GC-E and GC-F) that contain an apparent extracellular domain potentially capable of binding ligands (Yang, R.-B., Foster, D. C., Garbers, D. L., and Fülle, H.-J. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 602-606). Here, Northern and Western analyses showed that both cyclases are expressed in the retina and enriched in photoreceptor outer segments. By the use of specific GC-E and GC-F antibodies coupled to different sized gold particles both cyclases were colocalized within the same photoreceptor cells raising the possibility of homomeric and/or heteromeric interactions. A point mutant of GC-E (D878A) was constructed and expressed; it contained no detectable cyclase activity but acted in a dominant negative fashion to abolish the activity of native GC-E and GC-F in coexpression studies. These results suggested that GC-E and GC-F could form either homomers or heteromers, at least when overexpressed in COS-7 cells. Immunoprecipitation with GC-E and GC-F antibody followed by Western analysis confirmed that both homomers and heteromers could be formed. However, similar experiments using retina or outer segments revealed that a vast majority of GC-E and GC-F were precipitated as homomers in the eye. Therefore, like other members of the membrane guanylyl cyclase subfamily, GC-E and GC-F appear to preferentially form homomers.

Localization of guanylate cyclase-activating protein 2 in mammalian retinas

Guanylate cyclase-activating proteins (GCAP1 and GCAP2) are thought to mediate the intracellular stimulation of guanylate cyclase (GC) by Ca2+, a key event in recovery of the dark state of rod photoreceptors after exposure to light. GCAP1 has been localized to rod and cone outer segments, the sites of phototransduction, and to photoreceptor synaptic terminals and some cone somata. We used in situ hybridization and immunocytochemistry to localize GCAP2 in human, monkey, and bovine retinas. In human and monkey retinas, the most intense immunolabeling with anti-GCAP2 antibodies was in the cone inner segments, somata, and synaptic terminals and, to a lesser degree, in rod inner segments and inner retinal neurons. In bovine retina, the most intense immunolabeling was in the rod inner segments, with weaker labeling of cone myoids, somata, and synapses. By using a GCAP2-specific antibody in enzymatic assays, we confirmed that GCAP1 but not GCAP2 is the major component that stimulates GC in bovine rod outer segment homogenates. These results suggest that although GCAP1 is involved in the Ca2+-sensitive regulation of GC in rod and cone outer segments, GCAP2 may have non-phototransduction functions in photoreceptors and inner retinal neurons.