Subunit dissociation and diffusion determine the subcellular localization of rod and cone transducins

Arrestins: A Small Family of Multi-Functional Proteins

The first member of the arrestin family, visual arrestin-1, was discovered in the late 1970s. Later, the other three mammalian subtypes were identified and cloned. The first described function was regulation of G protein-coupled receptor (GPCR) signaling: arrestins bind active phosphorylated GPCRs, blocking their coupling to G proteins. It was later discovered that receptor-bound and free arrestins interact with numerous proteins, regulating GPCR trafficking and various signaling pathways, including those that determine cell fate. Arrestins have no enzymatic activity; they function by organizing multi-protein complexes and localizing their interaction partners to particular cellular compartments. Today we understand the molecular mechanism of arrestin interactions with GPCRs better than the mechanisms underlying other functions. However, even limited knowledge enabled the construction of signaling-biased arrestin mutants and extraction of biologically active monofunctional peptides from these multifunctional proteins. Manipulation of cellular signaling with arrestin-based tools has research and likely therapeutic potential: re-engineered proteins and their parts can produce effects that conventional small-molecule drugs cannot.

Heterogeneity of G protein activation by the calcium-sensing receptor

The calcium-sensing receptor (CaSR) is a G protein-coupled receptor that plays a fundamental role in extracellular calcium (Ca2+e) homeostasis by regulating parathyroid hormone release and urinary calcium excretion. The CaSR has been described to activate all four G protein subfamilies (Gαq/11, Gαi/o, Gα12/13, Gαs), and mutations in the receptor that cause hyper/hypocalcaemia, have been described to bias receptor signalling. However, many of these studies are based on measurements of second messengers or gene transcription that occurs many steps downstream of receptor activation and can represent convergence points of several signalling pathways. Therefore, to assess CaSR-mediated G protein activation directly, we took advantage of a recently described NanoBiT G protein dissociation assay system. Our studies, performed in HEK293 cells stably expressing CaSR, demonstrate that Ca2+e stimulation activates all Gαq/11 family and several Gαi/o family proteins, although Gαz was not activated. CaSR stimulated dissociation of Gα12/13 and Gαs from Gβ-subunits, but this occurred at a slower rate than that of other Gα-subunits. Investigation of cDNA expression of G proteins in three tissues abundantly expressing CaSR, the parathyroids, kidneys and pancreas, showed Gα11, Gαz, Gαi1 and Gα13 genes were highly expressed in parathyroid tissue, indicating CaSR most likely activates Gα11 and Gαi1 in parathyroids. In kidney and pancreas, the majority of G proteins were similarly expressed, suggesting CaSR may activate multiple G proteins in these cells. Thus, these studies validate a single assay system that can be used to robustly assess CaSR variants and biased signalling and could be utilised in the development of new pharmacological compounds targeting CaSR.

Biological Role of Arrestin-1 Oligomerization

Members of the arrestin superfamily have great propensity of self-association, but the physiological significance of this phenomenon is unclear. To determine the biological role of visual arrestin-1 oligomerization in rod photoreceptors, we expressed mutant arrestin-1 with severely impaired self-association in mouse rods and analyzed mice of both sexes. We show that the oligomerization-deficient mutant is capable of quenching rhodopsin signaling normally, as judged by electroretinography and single-cell recording. Like wild type, mutant arrestin-1 is largely excluded from the outer segments in the dark, proving that the normal intracellular localization is not due the size exclusion of arrestin-1 oligomers. In contrast to wild type, supraphysiological expression of the mutant causes shortening of the outer segments and photoreceptor death. Thus, oligomerization reduces the cytotoxicity of arrestin-1 monomer, ensuring long-term photoreceptor survival.SIGNIFICANCE STATEMENT Visual arrestin-1 forms dimers and tetramers. The biological role of its oligomerization is unclear. To test the role of arrestin-1 self-association, we expressed oligomerization-deficient mutant in arrestin-1 knock-out mice. The mutant quenches light-induced rhodopsin signaling like wild type, demonstrating that in vivo monomeric arrestin-1 is necessary and sufficient for this function. In rods, arrestin-1 moves from the inner segments and cell bodies in the dark to the outer segments in the light. Nonoligomerizing mutant undergoes the same translocation, demonstrating that the size of the oligomers is not the reason for arrestin-1 exclusion from the outer segments in the dark. High expression of oligomerization-deficient arrestin-1 resulted in rod death. Thus, oligomerization reduces the cytotoxicity of high levels of arrestin-1 monomer.

Cul3-Klhl18 ubiquitin ligase modulates rod transducin translocation during light-dark adaptation

Adaptation is a general feature of sensory systems. In rod photoreceptors, light-dependent transducin translocation and Ca²⁺ homeostasis are involved in light/dark adaptation and prevention of cell damage by light. However, the underlying regulatory mechanisms remain unclear. Here, we identify mammalian Cul3-Klhl18 ubiquitin ligase as a transducin translocation modulator during light/dark adaptation. Under dark conditions, Klhl18^-/- mice exhibited decreased rod light responses and subcellular localization of the transducin α-subunit (Tα), similar to that observed in light-adapted Klhl18^+/+ mice. Cul3-Klhl18 promoted ubiquitination and degradation of Unc119, a rod Tα-interacting protein. Unc119 overexpression phenocopied Tα mislocalization observed in Klhl18^-/- mice. Klhl18 weakly recognized casein kinase-2-phosphorylated Unc119 protein, which is dephosphorylated by Ca²⁺ -dependent phosphatase calcineurin. Calcineurin inhibition increased Unc119 expression and Tα mislocalization in rods. These results suggest that Cul3-Klhl18 modulates rod Tα translocation during light/dark adaptation through Unc119 ubiquitination, which is affected by phosphorylation. Notably, inactivation of the Cul3-Klhl18 ligase and calcineurin inhibitors FK506 and cyclosporine A that are known immunosuppressant drugs repressed light-induced photoreceptor damage, suggesting potential therapeutic targets.

Farnesylation of the Transducin G Protein Gamma Subunit Is a Prerequisite for Its Ciliary Targeting in Rod Photoreceptors

Primary cilia are microtubule-based organelles, which protrude from the plasma membrane and receive a wide range of extracellular signals. Various cilia use G protein-coupled receptors (GPCRs) for the detection of these signals. For instance, vertebrate rod photoreceptors use their cilia (also called outer segments) as antennae detecting photons by GPCR rhodopsin. Rhodopsin recognizes incoming light and activates its G protein, transducin, which is composed of three subunits α, β, and γ. Similar to all G protein γ subunits, the transducin Gγ1 subunit undergoes C-terminal prenylation resulting in the addition of an isoprenoid farnesyl; however, the significance of this posttranslational modification is unclear. To study the role of the farnesyl group, we genetically introduced a mutant Gγ1 that lacked the prenylation site into the retinal photoreceptors of mice. The biochemical and physiological analyses of these mice revealed that mutant Gγ1 dimerizes with the endogenous transducin Gβ1 subunit and that the resulting Gβγ dimers display reduced hydrophobicity. Although mutant Gβγ dimers could form a heterotrimeric G protein, they could not mediate phototransduction. This deficiency was due to a strong exclusion of non-farnesylated Gβγ complexes from the cilia (rod outer segments). Our results provide the first evidence that farnesylation is required for trafficking of G-protein βγ subunits to the cilium of rod photoreceptors.

Cilia - The sensory antennae in the eye

Cilia are hair-like projections found on almost all cells in the human body. Originally believed to function merely in motility, the function of solitary non-motile (primary) cilia was long overlooked. Recent research has demonstrated that primary cilia function as signalling hubs that sense environmental cues and are pivotal for organ development and function, tissue hoemoestasis, and maintenance of human health. Cilia share a common anatomy and their diverse functional features are achieved by evolutionarily conserved functional modules, organized into sub-compartments. Defects in these functional modules are responsible for a rapidly growing list of human diseases collectively termed ciliopathies. Ocular pathogenesis is common in virtually all classes of syndromic ciliopathies, and disruptions in cilia genes have been found to be causative in a growing number of non-syndromic retinal dystrophies. This review will address what is currently known about cilia contribution to visual function. We will focus on the molecular and cellular functions of ciliary proteins and their role in the photoreceptor sensory cilia and their visual phenotypes. We also highlight other ciliated cell types in tissues of the eye (e.g. lens, RPE and Müller glia cells) discussing their possible contribution to disease progression. Progress in basic research on the cilia function in the eye is paving the way for therapeutic options for retinal ciliopathies. In the final section we describe the latest advancements in gene therapy, read-through of non-sense mutations and stem cell therapy, all being adopted to treat cilia dysfunction in the retina.

Separation of photoreceptor cell compartments in mouse retina for protein analysis

BACKGROUND

Light exposure triggers movement of certain signaling proteins within the cellular compartments of the highly polarized rod photoreceptor cell. This redistribution of proteins between the inner and outer segment compartments affects the performance and physiology of the rod cell. In addition, newly synthesized phototransduction proteins traverse from the site of their synthesis in the inner segment, through the thin connecting cilium, to reach their destination in the outer segment. Processes that impede normal trafficking of these abundant proteins lead to cell death. The study of movement and unique localization of biomolecules within the different compartments of the rod cell would be greatly facilitated by techniques that reliably separate these compartments. Ideally, these methods can be applied to the mouse retina due to the widespread usage of transgenic mouse models in the investigation of basic visual processes and disease mechanisms that affect vision. Although the retina is organized in distinct layers, the small and highly curved mouse retina makes physical separation of retinal layers a challenge. We introduce two peeling methods that efficiently and reliably isolate the rod outer segment and other cell compartments for Western blots to examine protein movement across these compartments.

METHODS

The first separation method employs Whatman^® filter paper to successively remove the rod outer segments from isolated, live mouse retinas. The second method utilizes Scotch^TM tape to peel the rod outer segment layer and the rod inner segment layer from lyophilized mouse retinas. Both procedures can be completed within one hour.

RESULTS

We utilize these two protocols on dark-adapted and light-exposed retinas of C57BL/6 mice and subject the isolated tissue layers to Western blots to demonstrate their effectiveness in detecting light-induced translocation of transducin (GNAT1) and rod arrestin (ARR1). Furthermore, we provide evidence that RGS9 does not undergo light-induced translocation.

CONCLUSIONS

These results demonstrate the effectiveness of the two different peeling protocols for the separation of the layered compartments of the mouse retina and their utility for investigations of protein compositions within these compartments.

Cone-like rhodopsin expressed in the all cone retina of the colubrid pine snake as a potential adaptation to diurnality

Colubridae is the largest and most diverse family of snakes, with visual systems that reflect this diversity, encompassing a variety of retinal photoreceptor organizations. The transmutation theory proposed by Walls postulates that photoreceptors could evolutionarily transition between cell types in squamates, but few studies have tested this theory. Recently, evidence for transmutation and rod-like machinery in an all cone retina has been identified in a diurnal garter snake (Thamnophis), and it appears that the rhodopsin gene at least may be widespread among colubrid snakes. However, functional evidence supporting transmutation beyond the existence of the rhodopsin gene remains rare. We examined the all cone retina of another colubrid, Pituophis melanoleucus, thought to be more secretive/burrowing than Thamnophis. We found that P. melanoleucus expresses two cone opsins (SWS1, LWS) and rhodopsin (RH1) within the eye. Immunohistochemistry localized rhodopsin to the outer segment of photoreceptors in the all-cone retina of the snake and all opsin genes produced functional visual pigments when expressed in vitro. Consistent with other studies, we found that P. melanoleucus rhodopsin is extremely blue-shifted. Surprisingly, P. melanoleucus rhodopsin reacted with hydroxylamine, a typical cone opsin characteristic. These results support the idea that the rhodopsin-containing photoreceptors of P. melanoleucus are the products of evolutionary transmutation from rod ancestors, and suggests that this phenomenon may be widespread in colubrid snakes. We hypothesize that transmutation may be an adaptation for diurnal, brighter-light vision, which could result in increased spectral sensitivity and chromatic discrimination with the potential for colour vision.

Autophagy supports survival and phototransduction protein levels in rod photoreceptors

Damage and loss of the postmitotic photoreceptors is a leading cause of blindness in many diseases of the eye. Although the mechanisms of photoreceptor death have been extensively studied, few studies have addressed mechanisms that help sustain these non-replicating neurons for the life of an organism. Autophagy is an intracellular pathway where cytoplasmic constituents are delivered to the lysosomal pathway for degradation. It is not only a major pathway activated in response to cellular stress, but is also important for cytoplasmic turnover and to supply the structural and energy needs of cells. We examined the importance of autophagy in photoreceptors by deleting the essential autophagy gene Atg5 specifically in rods. Loss of autophagy led to progressive degeneration of rod photoreceptors beginning at 8 weeks of age such that by 44 weeks few rods remained. Cone photoreceptor numbers were only slightly diminished following rod degeneration but their function was significantly decreased. Rod cell death was apoptotic but was not dependent on daily light exposure or accelerated by intense light. Although the light-regulated translocation of the phototransduction proteins arrestin and transducin were unaffected in rods lacking autophagy, Atg5-deficient rods accumulated transducin-α as they degenerated suggesting autophagy might regulate the level of this protein. This was confirmed when the light-induced decrease in transducin was abolished in Atg5-deficient rods and the inhibition of autophagy in retinal explants cultures prevented its degradation. These results demonstrate that basal autophagy is essential to the long-term health of rod photoreceptors and a critical process for maintaining optimal levels of the phototransduction protein transducin-α. As the lack of autophagy is associated with retinal degeneration and altered phototransduction protein degradation in the absence of harmful gene products, this process may be a viable therapeutic target where rod cell loss is the primary pathologic event.

The GPCR heterotetramer: challenging classical pharmacology

Two concepts are gaining increasing acceptance in G protein-coupled receptor (GPCR) pharmacology: (i) pre-coupling of GPCRs with their preferred signaling molecules, and (ii) GPCR oligomerization. This is begging for the introduction of new models such as GPCR oligomer-containing signaling complexes with GPCR homodimers as functional building blocks. This model favors the formation of GPCR heterotetramers - heteromers of homodimers coupled to their cognate G protein. The GPCR heterotetramer offers an optimal framework for a canonical antagonistic interaction between activated Gs and Gi proteins, which can simultaneously bind to their respective preferred receptors and to adenylyl cyclase (AC) catalytic units. This review addresses the current evidence for pre-coupling of the various specific components that provide the very elaborate signaling machinery exemplified by the Gs-Gi-AC-coupled GPCR heterotetramer.

An in vitro assay for entry into cilia reveals unique properties of the soluble diffusion barrier

The ciliary permeability barrier is mechanistically distinct from other cellular diffusion barriers and allows soluble proteins under ∼100 kD in size to enter cilia in the absence of active transport.

Specific proteins are concentrated within primary cilia, whereas others remain excluded. To understand the mechanistic basis of entry into cilia, we developed an in vitro assay using cells in which the plasma membrane was permeabilized, but the ciliary membrane was left intact. Using a diffusion-to-capture system and quantitative analysis, we find that proteins >9 nm in diameter (∼100 kD) are restricted from entering cilia, and we confirm these findings in vivo. Interference with the nuclear pore complex (NPC) or the actin cytoskeleton in permeabilized cells demonstrated that the ciliary diffusion barrier is mechanistically distinct from those of the NPC or the axon initial segment. Moreover, applying a mass transport model to this system revealed diffusion coefficients for soluble and membrane proteins within cilia that are compatible with rapid exploration of the ciliary space in the absence of active transport. Our results indicate that large proteins require active transport for entry into cilia but not necessarily for movement inside cilia.

Protein sorting, targeting and trafficking in photoreceptor cells

Vision is the most fundamental of our senses initiated when photons are absorbed by the rod and cone photoreceptor neurons of the retina. At the distal end of each photoreceptor resides a light-sensing organelle, called the outer segment, which is a modified primary cilium highly enriched with proteins involved in visual signal transduction. At the proximal end, each photoreceptor has a synaptic terminal, which connects this cell to the downstream neurons for further processing of the visual information. Understanding the mechanisms involved in creating and maintaining functional compartmentalization of photoreceptor cells remains among the most fascinating topics in ocular cell biology. This review will discuss how photoreceptor compartmentalization is supported by protein sorting, targeting and trafficking, with an emphasis on the best-studied cases of outer segment-resident proteins.

Functional comparison of rod and cone Gα(t) on the regulation of light sensitivity

The signaling cascades mediated by G protein-coupled receptors (GPCRs) exhibit a wide spectrum of spatial and temporal response properties to fulfill diverse physiological demands. However, the mechanisms that shape the signaling response of the GPCR are not well understood. In this study, we replaced cone transducin α (cTα) for rod transducin α (rTα) in rod photoreceptors of transgenic mice, which also express S opsin, to evaluate the role of Gα subtype on signal amplification from different GPCRs in the same cell; such analysis may explain functional differences between retinal rod and cone photoreceptors. We showed that ectopically expressed cTα 1) forms a heterotrimeric complex with rod Gβ(1)γ(1), 2) substitutes equally for rTα in generating photoresponses initiated by either rhodopsin or S-cone opsin, and 3) exhibited similar light-activated translocation as endogenous rTα in rods and endogenous cTα in cones. Thus, rTα and cTα appear functionally interchangeable. Interestingly, light sensitivity appeared to correlate with the concentration of cTα when expression is reduced below 35% of normal. However, quantification of endogenous cTα concentration in cones showed a higher level to rTα in rods. Thus, reduced sensitivity in cones cannot be explained by reduced coupling efficiency between the GPCR and G protein or a lower concentration of G protein in cones versus rods.

Evidence for multiple phototransduction pathways in a reef-building coral

Photosensitive behaviors and circadian rhythms are well documented in reef-building corals and their larvae, but the mechanisms responsible for photoreception have not been described in these organisms. Here we report the cloning, immunolocalization, and partial biochemical characterization of three opsins and four G proteins expressed in planulae of the Caribbean elkhorn coral, Acropora palmata. All three opsins (acropsins 1-3) possess conserved seven-pass transmembrane structure, and localize to distinct regions of coral planulae. Acropsin 1 was localized in the larval endoderm, while acropsin 2 was localized in solitary cells of the ectoderm. These rod-like cells displayed a remarkably polarized distribution, concentrated in the aboral end. We also cloned four A. palmata G protein alpha subunits. Three were homologs of vertebrate Gi, Go, and Gq. The fourth is presumably a novel G protein, which displays only 40% identity with the nearest known G protein, and we termed it Gc for "cnidarian". We show that Gc and Gq can be activated by acropsins in a light-dependent manner in vitro. This indicates that at least acropsins 1 and 3 can form functional photoreceptors and potentially may play a role in color preference during settlement, vertical positioning and other light-guided behaviors observed in coral larvae.

D2-like dopamine and β-adrenergic receptors form a signaling complex that integrates Gs- and Gi-mediated regulation of adenylyl cyclase

β-Adrenergic receptors (βAR) and D(2)-like dopamine receptors (which include D(2)-, D(3)- and D(4)-dopamine receptors) activate G(s) and G(i), the stimulatory and inhibitory heterotrimeric G proteins, respectively, which in turn regulate the activity of adenylyl cyclase (AC). β(2)-Adrenergic receptors (β(2)AR) and D(4)-dopamine receptors (D(4)DR) co-immunoprecipitated when co-expressed in HEK 293 cells, suggesting the existence of a signaling complex containing both receptors. In order to determine if these receptors are closely associated with each other, and with other components involved in G protein-mediated signal transduction, β(2)AR, D(4)DR, G protein subunits (Gα(i1) and the Gβ(1)γ(2) heterodimer) and AC were tagged so that bioluminescence resonance energy transfer (BRET) could be used to monitor their interactions. All of the tagged proteins retained biological function. For the first time, FlAsH-labeled proteins were used in BRET experiments as fluorescent acceptors for the energy transferred from Renilla luciferase-tagged donor proteins. Our experiments revealed that β(2)AR, D(4)DR, G proteins and AC were closely associated in a functional signaling complex in cellulo. Furthermore, BRET experiments indicated that although activation of G(i) caused a conformational change within the heterotrimeric protein, it did not cause the Gβγ heterodimer to dissociate from the Gα(i1) subunit. Evidence for the presence of a signaling complex in vivo was obtained by purifying βAR from detergent extracts of mouse brain with alprenolol-Sepharose and showing that the precipitate also contained both D(2)-like dopamine receptors and AC.

Phosphorylation of phosducin accelerates rod recovery from transducin translocation

PURPOSE

In rods saturated by light, the G protein transducin undergoes translocation from the outer segment compartment, which results in the uncoupling of transducin from its innate receptor, rhodopsin. We measured the kinetics of recovery from this adaptive cellular response, while also investigating the role of phosducin, a phosphoprotein binding transducin βγ subunits in its de-phosphorylated state, in regulating this process.

METHODS

Mice were exposed to a moderate rod-saturating light triggering transducin translocation, and then allowed to recover in the dark while free running. The kinetics of the return of the transducin subunits to the outer segments were compared in transgenic mouse models expressing full-length phosducin, and phosducin lacking phosphorylation sites serine 54 and 71, using Western blot analysis of serial tangential sections of the retina.

RESULTS

In mice expressing normal phosducin, transducin α and βγ subunits returned to the outer segments with a half-time (t(1/2)) of ∼24 and 29 minutes, respectively. In the phosducin phosphorylation mutants, the transducin α subunit moved four times slower, with t(1/2) ∼95 minutes, while the movement of transducin βγ was less affected.

CONCLUSIONS

We demonstrate that the recovery of rod photoreceptors from the ambient saturating levels of illumination, in terms of the return of the light-dispersed transducin subunits to the rod outer segments, occurs six times faster than reported previously. Our data also support the notion that the accumulation of transducin α subunit in the outer segment is driven by its re-binding to the transducin βγ dimer, because this process is accelerated significantly by phosducin phosphorylation.

Overexpression of GalNAc-transferase GalNAc-T3 promotes pancreatic cancer cell growth

O-linked glycans of secreted and membrane-bound proteins have an important role in the pathogenesis of pancreatic cancer by modulating immune responses, inflammation and tumorigenesis. A critical aspect of O-glycosylation, the position at which proteins are glycosylated with N-acetyl-galactosamine on serine and threonine residues, is regulated by the substrate specificity of UDP-GalNAc:polypeptide N-acetylgalactosaminyl-transferases (GalNAc-Ts). Thus, GalNAc-Ts regulate the first committed step in O-glycosylated protein biosynthesis, determine sites of O-glycosylation on proteins and are important for understanding normal and carcinoma-associated O-glycosylation. We have found that one of these enzymes, GalNAc-T3, is overexpressed in human pancreatic cancer tissues and suppression of GalNAc-T3 significantly attenuates the growth of pancreatic cancer cells in vitro and in vivo. In addition, suppression of GalNAc-T3 induces apoptosis of pancreatic cancer cells. Our results indicate that GalNAc-T3 is likely involved in pancreatic carcinogenesis. Modification of cellular glycosylation occurs in nearly all types of cancer as a result of alterations in the expression levels of glycosyltransferases. We report guanine the nucleotide-binding protein, α-transducing activity polypeptide-1 (GNAT1) as a possible substrate protein of GalNAc-T3. GalNAc-T3 is associated with O-glycosylation of GNAT1 and affects the subcellular distribution of GNAT1. Knocking down endogenous GNAT1 significantly suppresses the growth/survival of PDAC cells. Our results imply that GalNAc-T3 contributes to the function of O-glycosylated proteins and thereby affects the growth and survival of pancreatic cancer cells. Thus, substrate proteins of GalNAc-T3 should serve as important therapeutic targets for pancreatic cancers.

Photoreceptor signaling: supporting vision across a wide range of light intensities

For decades, photoreceptors have been an outstanding model system for elucidating basic principles in sensory transduction and biochemistry and for understanding many facets of neuronal cell biology. In recent years, new knowledge of the kinetics of signaling and the large-scale movements of proteins underlying signaling has led to a deeper appreciation of the photoreceptor's unique challenge in mediating the first steps in vision over a wide range of light intensities.

The functional cycle of visual arrestins in photoreceptor cells

Visual arrestin-1 plays a key role in the rapid and reproducible shutoff of rhodopsin signaling. Its highly selective binding to light-activated phosphorylated rhodopsin is an integral part of the functional perfection of rod photoreceptors. Structure-function studies revealed key elements of the sophisticated molecular mechanism ensuring arrestin-1 selectivity and paved the way to the targeted manipulation of the arrestin-1 molecule to design mutants that can compensate for congenital defects in rhodopsin phosphorylation. Arrestin-1 self-association and light-dependent translocation in photoreceptor cells work together to keep a constant supply of active rhodopsin-binding arrestin-1 monomer in the outer segment. Recent discoveries of arrestin-1 interaction with other signaling proteins suggest that it is a much more versatile signaling regulator than previously thought, affecting the function of the synaptic terminals and rod survival. Elucidation of the fine molecular mechanisms of arrestin-1 interactions with rhodopsin and other binding partners is necessary for the comprehensive understanding of rod function and for devising novel molecular tools and therapeutic approaches to the treatment of visual disorders.

Interaction of transducin with uncoordinated 119 protein (UNC119): implications for the model of transducin trafficking in rod photoreceptors

The key visual G protein, transducin undergoes bi-directional translocations between the outer segment (OS) and inner compartments of rod photoreceptors in a light-dependent manner thereby contributing to adaptation and neuroprotection of rods. A mammalian uncoordinated 119 protein (UNC119), also known as Retina Gene 4 protein (RG4), has been recently implicated in transducin transport to the OS in the dark through its interaction with the N-acylated GTP-bound transducin-α subunit (Gα(t1)). Here, we demonstrate that the interaction of human UNC119 (HRG4) with transducin is dependent on the N-acylation, but does not require the GTP-bound form of Gα(t1). The lipid specificity of UNC119 is unique: UNC119 bound the myristoylated N terminus of Gα(t1) with much higher affinity than a prenylated substrate, whereas the homologous prenyl-binding protein PrBP/δ did not interact with the myristoylated peptide. UNC119 was capable of interacting with Gα(t1)GDP as well as with heterotrimeric transducin (G(t)). This interaction of UNC119 with G(t) led to displacement of Gβ(1)γ(1) from the heterotrimer. Furthermore, UNC119 facilitated solubilization of G(t) from dark-adapted rod OS membranes. Consistent with these observations, UNC119 inhibited rhodopsin-dependent activation of G(t), but had no effect on the GTP-hydrolysis by Gα(t1). A model for the role of UNC119 in the IS→OS translocation of G(t) is proposed based on the UNC119 ability to dissociate G(t) subunits from each other and the membrane. We also found that UNC119 inhibited activation of G(o) by D2 dopamine receptor in cultured cells. Thus, UNC119 may play conserved inhibitory role in regulation of GPCR-G protein signaling in non-visual tissues.

Robust self-association is a common feature of mammalian visual arrestin-1

Arrestin-1 binds light-activated phosphorhodopsin and ensures rapid signal termination. Its deficiency in humans and mice results in prolonged signaling and rod degeneration. However, most of the biochemical studies were performed on bovine arrestin-1, which was shown to self-associate forming dimers and tetramers, although only the monomer binds rhodopsin. It is unclear whether self-association is a property of arrestin-1 in all mammals or a specific feature of bovine protein. To address this issue, we compared self-association parameters of purified human and mouse arrestin-1 with those of its bovine counterpart using multiangle light scattering. We found that mouse and human arrestin-1 also robustly self-associate, existing in a monomer-dimer-tetramer equilibrium. Interestingly, the combination of dimerization and tetramerization constants in these three species is strikingly different. While tetramerization of bovine arrestin-1 is highly cooperative (K(D,dim)(4) > K(D,tet)), K(D,dim) ∼ K(D,tet) in the mouse form and K(D,dim) ≪ K(D,tet) in the human form. Importantly, in all three species at very high physiological concentrations of arrestin-1 in rod photoreceptors, most of it is predicted to exist in oligomeric form, with a relatively low concentration of the free monomer. Thus, it appears that maintenance of low levels of the active monomer is the biological role of arrestin-1 self-association.

Diffusion and light-dependent compartmentalization of transducin

Diffusion and light-dependent compartmentalization of transducin are essential for phototransduction and light adaptation of rod photoreceptors. Here, transgenic Xenopus laevis models were designed to probe the roles of transducin/rhodopsin interactions and lipid modifications in transducin compartmentalization, membrane mobility, and light-induced translocation. Localization and diffusion of EGFP-fused rod transducin-α subunit (Gα(t1)), mutant Gα(t1) that is predicted to be N-acylated and S-palmitoylated (Gα(t1)A3C), and mutant Gα(t1) uncoupled from light-activated rhodopsin (Gα(t1)-Ctα(s)), were examined by EGFP-fluorescence imaging and fluorescence recovery after photobleaching (FRAP). Similar to Gα(t1), Gα(t1)A3C and Gα(t1)-Ctα(s) were correctly targeted to the rod outer segments in the dark, however the light-dependent translocation of both mutants was markedly impaired. Our analysis revealed a moderate acceleration of the lateral diffusion for the activated Gα(t1) consistent with the diffusion of the separated Gα(t1)GTP and Gβ(1)γ(1) on the membrane surface. Unexpectedly, the kinetics of longitudinal diffusion were comparable for Gα(t1)GTP with a single lipid anchor and heterotrimeric Gα(t1)β(1)γ(1) or Gα(t1)-Ctα(s)β(1)γ(1) with two lipid modifications. This contrasted the lack of the longitudinal diffusion of the Gα(t1)A3C mutant apparently caused by its stable two lipid attachment to the membrane and suggests the existence of a mechanism that facilitates axial diffusion of Gα(t1)β(1)γ(1).

Combining resonance energy transfer methods reveals a complex between the alpha2A-adrenergic receptor, Galphai1beta1gamma2, and GRK2

Traditionally, G-protein-coupled receptor (GPCR) interactions with their G proteins and regulatory proteins, GPCR kinases (GRKs) and arrestins, are described as sequential events involving rapid assemblies/disassemblies. To directly monitor the dynamics of these interactions in living cells, we combined two spectrally resolved bioluminescence and one fluorescence resonance energy transfer (RET) methods. The RET combination analysis revealed that stimulation of the α(2A)-adrenergic receptor (α(2A)AR) leads to the recruitment of GRK2 at a receptor still associated with the Gα(i1)β(1)γ(2) complex. The interaction kinetics of GRKs with Gγ(2) (2.8 ± 0.4 s) and α(2A)AR (5.2 ± 0.5 s) were similar to that of the receptor-promoted change in RET between Gα(i1) and Gγ(2) (5.2 ± 1.2 s), and persisted until the translocation of βarrestin2 to the receptor, indicating that GRK2 remains associated to the receptor/G-protein complex for longer periods than anticipated. Moreover, GRK2 or a kinase-deficient GRK2 mutant, but not GRK5, potentiated the receptor-promoted changes in RET between Gα(i1) and Gγ(2) and abrogated the α(2A)AR-stimulated calcium response, suggesting that the recruitment of GRK2 to the complex contributes to the structural rearrangement and functional regulation of the signaling unit, independently of the kinase activity. RET combination analysis revealed unanticipated dynamics in GPCR signaling and will be applicable to many biological systems.

Replacing the rod with the cone transducin subunit decreases sensitivity and accelerates response decay

Cone vision is less sensitive than rod vision. Much of this difference can be attributed to the photoreceptors themselves, but the reason why the cones are less sensitive is still unknown. Recent recordings indicate that one important factor may be a difference in the rate of activation of cone transduction; that is, the rising phase of the cone response per bleached rhodopsin molecule (Rh*) has a smaller slope than the rising phase of the rod response per Rh*, perhaps because some step between Rh* and activation of the phosphodiesterase 6 (PDE6) effector molecule occurs with less gain. Since rods and cones have different G-protein alpha subunits, and since this subunit (Talpha) plays a key role both in the interaction of G-protein with Rh* and the activation of PDE6, we investigated the mechanism of the amplification difference by expressing cone Talpha in rod Talpha-knockout rods to produce so-called GNAT2C mice. We show that rods in GNAT2C mice have decreased sensitivity and a rate of activation half that of wild-type (WT) mouse rods. Furthermore, GNAT2C responses recover more rapidly than WT responses with kinetic parameters resembling those of native mouse cones. Our results show for the first time that part of the difference in sensitivity and response kinetics between rods and cones may be the result of a difference in the G-protein alpha subunit. They also indicate more generally that the molecular nature of G-protein alpha may play an important role in the kinetics of G-protein cascades for metabotropic receptors throughout the body.

The pattern of expression of guanine nucleotide-binding protein beta3 in the retina is conserved across vertebrate species

Guanine nucleotide-binding protein beta3 (GNB3) is an isoform of the beta subunit of the heterotrimeric G protein second messenger complex that is commonly associated with transmembrane receptors. The presence of GNB3 in photoreceptors, and possibly bipolar cells, has been confirmed in murine, bovine and primate retinas [Lee RH, Lieberman BS, Yamane HK, Bok D, Fung BK (1992) J Biol Chem 267:24776-24781; Peng YW, Robishaw JD, Levine MA, Yau KW (1992) Proc Natl Acad Sci U S A 89:10882-10886; Huang L, Max M, Margolskee RF, Su H, Masland RH, Euler T (2003) J Comp Neurol 455:1-10]. Studies have indicated that a mutation in the GNB3 gene causes progressive retinopathy and globe enlargement (RGE) in chickens. The goals of this study were to (1) examine the expression pattern of GNB3 in wild-type and RGE mutant chickens, (2) characterize the types of bipolar cells that express GNB3 and (3) examine whether the expression of GNB3 in the retina is conserved across vertebrate species. We find that chickens homozygous for the RGE allele completely lack GNB3 protein. We find that the pattern of expression of GNB3 in the retina is highly conserved across vertebrate species, including teleost fish (Carassius auratus), frogs (Xenopus laevis), chickens (Gallus domesticus), mice (Mus musculata), guinea-pigs (Cavia porcellus), dogs (Canis familiaris) and non-human primates (Macaca fasicularis). Regardless of the species, we find that GNB3 is expressed by Islet1-positive cone ON-bipolar cells and by cone photoreceptors. In some vertebrates, GNB3-immunoreactivity was observed in both rod and cone photoreceptors. A protein-protein alignment of GNB3 across different vertebrates, from fish to humans, indicates a high degree (>92%) of sequence conservation. Given that analogous types of retinal neurons express GNB3 in different species, we propose that the functions and the mechanisms that regulate the expression of GNB3 are highly conserved.

Mechanistic basis for the failure of cone transducin to translocate: why cones are never blinded by light

The remarkable ability of our vision to function under ever-changing conditions of ambient illumination is mediated by multiple molecular mechanisms regulating the light sensitivity of rods and cones. One such mechanism involves massive translocation of signaling proteins, including the G-protein transducin, into and out of the light-sensitive photoreceptor outer segment compartment. Transducin translocation extends the operating range of rods, but in cones transducin never translocates, which is puzzling because cones typically function in much brighter light than rods. Using genetically manipulated mice in which the rates of transducin activation and inactivation were altered, we demonstrate that, like in rods, transducin translocation in cones can be triggered when transducin activation exceeds a critical level, essentially saturating the photoresponse. However, this level is never achieved in wild-type cones: their superior ability to tightly control the rates of transducin activation and inactivation, responsible for avoiding saturation by light, also accounts for the prevention of transducin translocation at any light intensity.

G proteins in reverse mode: receptor-mediated GTP release inhibits G protein and effector function

Active G protein-coupled receptors activate heterotrimeric Galphabetagamma proteins by catalyzing the exchange of GDP by GTP at the Galpha subunit. A paradoxical attenuation of G protein-activated inwardly rectifying potassium channels (GIRK) upon stimulation of native cells with high concentrations of agonist is known. However, a deactivation of activated G proteins by active receptors has not been experimentally studied in intact cells. We monitored GIRK currents and G(o) protein activation by means of fluorescence resonance energy transfer (FRET) in parallel. The results suggested that GIRK currents were paradoxically attenuated due to an inactivation of G(o) proteins by active alpha(2A)-adrenergic receptors. To study the mechanisms, G protein activation and receptor-G protein interactions were analyzed as a function of nucleotide type and nucleotide concentrations by means of FRET, while controlling intracellular nucleotides upon permeabilization of the cell membrane. Results suggested a receptor-catalyzed dissociation of GTP from activated heterotrimeric Galphabetagamma. Consequently, nucleotide-free G proteins were sequestrated in heterotrimeric conformation at the active receptor, thus attenuating downstream signaling in an agonist-dependent manner.

G protein subunit dissociation and translocation regulate cellular response to receptor stimulation

We examined the role of G proteins in modulating the response of living cells to receptor activation. The response of an effector, phospholipase C-β to M3 muscarinic receptor activation was measured using sensors that detect the generation of inositol triphosphate or diacylglycerol. The recently discovered translocation of Gβγ from plasma membrane to endomembranes on receptor activation attenuated this response. A FRET based G protein sensor suggested that in contrast to translocating Gβγ, non-translocating Gβγ subunits do not dissociate from the αq subunit on receptor activation leading to prolonged retention of the heterotrimer state and an accentuated response. M3 receptors with tethered αq induced differential responses to receptor activation in cells with or without an endogenous translocation capable γ subunit. G protein heterotrimer dissociation and βγ translocation are thus unanticipated modulators of the intensity of a cell's response to an extracellular signal.

Lipid second messengers and related enzymes in vertebrate rod outer segments

Rod outer segments (ROSs) are specialized light-sensitive organelles in vertebrate photoreceptor cells. Lipids in ROS are of considerable importance, not only in providing an adequate environment for efficient phototransduction, but also in originating the second messengers involved in signal transduction. ROSs have the ability to adapt the sensitivity and speed of their responses to ever-changing conditions of ambient illumination. A major contributor to this adaptation is the light-driven translocation of key signaling proteins into and out of ROS. The present review shows how generation of the second lipid messengers from phosphatidylcholine, phosphatidic acid, and diacylglycerol is modulated by the different illumination states in the vertebrate retina. Findings suggest that the light-induced translocation of phototransduction proteins influences the enzymatic activities of phospholipase D, lipid phosphate phosphatase, diacylglyceride lipase, and diacylglyceride kinase, all of which are responsible for the generation of the second messenger molecules.

Structure, function, and localization of Gβ5-RGS complexes

Members of the R7 subfamily of regulator of G protein signaling (RGS) proteins (RGS6, 7, 9, and 11) exist as heterodimers with the G protein beta subunit Gβ5. These protein complexes are only found in neurons and are defined by the presence of three domains: DEP/DHEX, Gβ5/GGL, and RGS. This article summarizes published work in the following areas: (1) the functional significance of structural organization of Gβ5-R7 complexes, (2) regional distribution of Gβ5-R7 in the nervous system and regulation of R7 family expression, (3) subcellular localization of Gβ5-R7 complexes, and (4) novel binding partners of Gβ5-R7 proteins. The review points out some contradictions between observations made by different research groups and highlights the importance of using alternative experimental approaches to obtain conclusive information about Gβ5-R7 function in vivo.

Functional interchangeability of rod and cone transducin alpha-subunits

Rod and cone photoreceptors use similar but distinct sets of phototransduction proteins to achieve different functional properties, suitable for their role as dim and bright light receptors, respectively. For example, rod and cone visual pigments couple to distinct variants of the heterotrimeric G protein transducin. However, the role of the structural differences between rod and cone transducin alpha subunits (Talpha) in determining the functional differences between rods and cones is unknown. To address this question, we studied the translocation and signaling properties of rod Talpha expressed in cones and cone Talpha expressed in rods in three mouse strains: rod Talpha knockout, cone Talpha GNAT2(cpfl3) mutant, and rod and cone Talpha double mutant rd17 mouse. Surprisingly, although the rod/cone Talpha are only 79% identical, exogenously expressed rod or cone Talpha localized and translocated identically to endogenous Talpha in each photoreceptor type. Moreover, exogenously expressed rod or cone Talpha rescued electroretinogram responses (ERGs) in mice lacking functional cone or rod Talpha, respectively. Ex vivo transretinal ERG and single-cell recordings from rd17 retinas treated with rod or cone Talpha showed comparable rod sensitivity and response kinetics. These results demonstrate that cone Talpha forms a functional heterotrimeric G protein complex in rods and that rod and cone Talpha couple equally well to the rod phototransduction cascade. Thus, rod and cone transducin alpha-subunits are functionally interchangeable and their signaling properties do not contribute to the intrinsic light sensitivity differences between rods and cones. Additionally, the technology used here could be adapted for any such homologue swap desired.

Immunohistochemical study of pig retinal development

PURPOSE

The pig eye is similar to the human eye in terms of anatomy, vasculature, and photoreceptor distribution, and therefore provides an attractive animal model for research into retinal disease. The purpose of this study was to characterize retinal histology in the developing and mature pig retina using antibodies to well established retinal cell markers commonly used in rodents.

METHODS

Eyes were enucleated from fetuses in the 9th week of gestation, 1 week old piglets and 6 months old adult animals. Eyeglobes were fixed and cryosectioned. A panel of antibodies to well established retinal markers was employed for immunohistochemistry. Fluorescently labeled secondary antibodies were used for signal detection, and images were acquired by confocal microscopy. Mouse retina at postnatal day (P) 5 was used as a reference for this study to compare progression of histogenesis. Most of the primary antibodies have previously been used on mouse tissue.

RESULTS

Most of the studied markers were detected in midgestation pig retina, and the majority had a similar distribution in pig as in P5 mouse retina. However, rhodopsin immunolabeling was detected in pig retina at midgestation but not in P5 mouse retina. Contrary to findings in all rodents, horizontal cells were Islet1-positive and cones were calbindin-immunoreactive in pig retina, as has also been shown for the primate retina. Recoverin and rhodopsin immunolabeling revealed an increase in the length of photoreceptor segments in 6 months, compared to 1 week old animals.

CONCLUSIONS

Comparison with the published data on human retina revealed similar marker distribution and histogenesis progression in the pig and human retina, supporting the pig as a valuable animal model for studies on retinal disease and repair. Furthermore, this study provides information about the dynamics of retinal histogenesis in the pig and validates a panel of antibodies that reliably detects developing and mature retinal cell phenotypes in the pig retina.

The retinal cGMP phosphodiesterase gamma-subunit - a chameleon

Intrinsically disordered proteins (IDPs) represent an emerging class of proteins (or domains) that are characterized by a lack of ordered secondary and tertiary structure. This group of proteins has recently attracted tremendous interest primarily because of a unique feature: they can bind to different targets due to their structural plasticity, and thus fulfill diverse functions. The inhibitory gamma-subunit (PDEgamma) of retinal PDE6 is an intriguing IDP, of which unique protein properties are being uncovered. PDEgamma critically regulates the turn on as well as the turn off of visual signaling through alternate interactions with the PDE6 catalytic core, transducin, and the regulator of G protein signaling RGS9-1. The intrinsic disorder of PDEgamma does not compromise, but rather, optimizes its functionality. PDEgamma "curls up" when free in solution but "stretches out" when binding with the PDE6 catalytic core. Conformational changes of PDEgamma also likely occur in its C-terminal PDE6-binding region upon interacting with transducin during PDE6 activation. Growing evidence shows that PDEgamma is also a player in non-phototransduction pathways, suggesting additional protein targets. Thus, PDEgamma is highly likely to be adaptive in its structure and function, hence a "chameleon".

Shuttling and translocation of heterotrimeric G proteins and Ras

Heterotrimeric G proteins (alphabetagamma) and Ras proteins are activated by cell-surface receptors that sense extracellular signals. Both sets of proteins were traditionally thought to be constrained to the plasma membrane and some intracellular membranes. Live-cell-imaging experiments have now shown that these proteins are mobile inside a cell, shuttling continually between the plasma membrane and intracellular membranes in the basal state, maintaining these proteins in dynamic equilibrium in different membrane compartments. Furthermore, on receptor activation, a family of G protein betagamma subunits translocates rapidly and reversibly to the Golgi and endoplasmic reticulum enabling direct communication between the extracellular signal and intracellular membranes. A member of the Ras family has similarly been shown to translocate on activation. Although the impact of this unexpected intracellular movement of signaling proteins on cell physiology is likely to be distinct, there are striking similarities in the properties of these two families of signal-transducing proteins.

The role of Gbetagamma subunits in the organization, assembly, and function of GPCR signaling complexes

The role of Gbetagamma subunits in cellular signaling has become well established in the past 20 years. Not only do they regulate effectors once thought to be the sole targets of Galpha subunits, but it has become clear that they also have a unique set of binding partners and regulate signaling pathways that are not always localized to the plasma membrane. However, this may be only the beginning of the story. Gbetagamma subunits interact with G protein-coupled receptors, Galpha subunits, and several different effector molecules during assembly and trafficking of receptor-based signaling complexes and not simply in response to ligand stimulation at sites of receptor cellular activity. Gbetagamma assembly itself seems to be tightly regulated via the action of molecular chaperones and in turn may serve a similar role in the assembly of specific signaling complexes. We propose that specific Gbetagamma subunits have a broader role in controlling the architecture, assembly, and activity of cellular signaling pathways.

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.

The translocation of signaling molecules in dark adapting mammalian rod photoreceptor cells is dependent on the cytoskeleton

In vertebrate rod photoreceptor cells, arrestin and the visual G-protein transducin move between the inner segment and outer segment in response to changes in light. This stimulus dependent translocation of signalling molecules is assumed to participate in long term light adaptation of photoreceptors. So far the cellular basis for the transport mechanisms underlying these intracellular movements remains largely elusive. Here we investigated the dependency of these movements on actin filaments and the microtubule cytoskeleton of photoreceptor cells. Co-cultures of mouse retina and retinal pigment epithelium were incubated with drugs stabilizing and destabilizing the cytoskeleton. The actin and microtubule cytoskeleton and the light dependent distribution of signaling molecules were subsequently analyzed by light and electron microscopy. The application of cytoskeletal drugs differentially affected the cytoskeleton in photoreceptor compartments. During dark adaptation the depolymerization of microtubules as well as actin filaments disrupted the translocation of arrestin and transducin in rod photoreceptor cells. During light adaptation only the delivery of arrestin within the outer segment was impaired after destabilization of microtubules. Movements of transducin and arrestin required intact cytoskeletal elements in dark adapting cells. However, diffusion might be sufficient for the fast molecular movements observed as cells adapt to light. These findings indicate that different molecular translocation mechanisms are responsible for the dark and light associated translocations of arrestin and transducin in rod photoreceptor cells.

Phosphorylation of GRK7 by PKA in cone photoreceptor cells is regulated by light

The retina-specific G protein-coupled receptor kinases, GRK1 and GRK7, have been implicated in the shutoff of the photoresponse and adaptation to changing light conditions via rod and cone opsin phosphorylation. Recently, we have defined sites of phosphorylation by cAMP-dependent protein kinase (PKA) in the amino termini of both GRK1 and GRK7 in vitro. To determine the conditions under which GRK7 is phosphorylated in vivo, we have generated an antibody that recognizes GRK7 phosphorylated on Ser36, the PKA phosphorylation site. Using this phospho-specific antibody, we have shown that GRK7 is phosphorylated in vivo and is located in the cone inner and outer segments of mammalian, amphibian and fish retinas. Using Xenopus laevis as a model, GRK7 is phosphorylated under dark-adapted conditions, but becomes dephosphorylated when the animals are exposed to light. The conservation of phosphorylation at Ser36 in GRK7 in these different species (which span a 400 million-year evolutionary period), and its light-dependent regulation, indicates that phosphorylation plays an important role in the function of GRK7. Our work demonstrates for the first time that cAMP can regulate proteins involved in the photoresponse in cones and introduces a novel mode of regulation for the retinal GRKs by PKA.

Electrostatic and lipid anchor contributions to the interaction of transducin with membranes: mechanistic implications for activation and translocation

The heterotrimeric G protein transducin is a key component of the vertebrate phototransduction cascade. Transducin is peripherally attached to membranes of the rod outer segment, where it interacts with other proteins at the membrane-cytosol interface. However, upon sustained activation by light, the dissociated G_tα and Gβ₁γ₁ subunits of transducin translocate from the outer segment to other parts of the rod cell. Here we used a computational approach to analyze the interaction strength of transducin and its subunits with acidic lipid bilayers, as well as the range of orientations that they are allowed to occupy on the membrane surface. Our results suggest that the combined constraints of electrostatics and lipid anchors substantially limit the rotational degrees of freedom of the membrane-bound transducin heterotrimer. This may contribute to a faster transducin activation rate by accelerating transducin-rhodopsin complex formation. Notably, the membrane interactions of the dissociated transducin subunits are very different from those of the heterotrimer. As shown previously, Gβ₁γ₁ experiences significant attractive interactions with negatively charged membranes, whereas our new results suggest that G_tα is electrostatically repelled by such membranes. We suggest that this repulsion could facilitate the membrane dissociation and intracellular translocation of G_tα. Moreover, based on similarities in sequence and electrostatic properties, we propose that the properties described for transducin are common to its homologs within the G_i subfamily. In a broader view, this work exemplifies how the activity-dependent association and dissociation of a G protein can change both the affinity for membranes and the range of allowed orientations, thereby modulating G protein function.

Unique transducins expressed in long and short photoreceptors of lamprey Petromyzon marinus

Lampreys represent the most primitive vertebrate class of jawless fish and serve as an evolutionary model of the vertebrate visual system. Transducin-alpha (G alpha(t)) subunits were investigated in lamprey Petromyzon marinus in order to understand the molecular origins of rod and cone photoreceptor G proteins. Two G alpha(t) subunits, G alpha(tL) and G alpha(tS), were identified in the P. marinus retina. G alpha(tL) is equally distant from cone and rod G proteins and is expressed in the lamprey's long photoreceptors. The short photoreceptor G alpha(tS) is a rod-like transducin-alpha that retains several unique features of cone transducins. Thus, the duplication of the ancestral transducin gene giving rise to rod transducins has already occurred in the last common ancestor of the jawed and jawless vertebrates.

Activation-dependent hindrance of photoreceptor G protein diffusion by lipid microdomains

The dynamics of G protein-mediated signal transduction depend on the two-dimensional diffusion of membrane-bound G proteins and receptors, which has been suggested to be rate-limiting for vertebrate phototransduction, a highly amplified G protein-coupled signaling pathway. Using fluorescence recovery after photobleaching (FRAP), we measured the diffusion of the G protein transducin alpha-subunit (Galpha(t)) and the G protein-coupled receptor rhodopsin on disk membranes of living rod photoreceptors from transgenic Xenopus laevis. Treatment with either methyl-beta-cyclodextrin or filipin III to disrupt cholesterol-containing lipid microdomains dramatically accelerated diffusion of Galpha(t) in its GTP-bound state and of the rhodopsin-Galphabetagamma(t) complex but not of rhodopsin or inactive GDP-bound Galphabetagamma. These results imply an activity-dependent sequestration of G proteins into cholesterol-dependent lipid microdomains, which limits diffusion and exclude the majority of free rhodopsin and the free G protein heterotrimer. Our data offer a novel demonstration of lipid microdomains in the internal membranes of living sensory neurons.

Interacting targets of the farnesyl of transducin gamma-subunit

G protein gamma-subunits are isoprenylated and carboxyl methylated at the C-terminal cysteine residue, and the set of the posttranslational modifications is indispensable for the function of the photoreceptor G protein transducin (Talpha/Tbetagamma). To explore farnesyl-mediated molecular interactions, we investigated molecular targets of a Tbetagamma analogue that was engineered to have a photoreactive farnesyl analogue, (3-azidophenoxy)geranyl (POG), covalently bound to the C-terminal cysteine of Tgamma. POG-modified Tbetagamma was further subjected to modification by methylation at the C-terminal carboxyl group, which copies a complete set of the known posttranscriptional modifications of Tbetagamma. Photoaffinity labeling experiment with the photoreactive Tbetagamma analogue in its free form indicated that the POG moiety of Tgamma interacted with Tbeta. In the trimeric Talpha/Tbetagamma complex, the POG moiety was cross-linked with Talpha in addition to concurrent affinity labeling of Tbeta. When photoreactive Tbetagamma was reconstituted with Talpha and light-activated rhodopsin (Rh*) in rod outer segment (ROS) membranes, the POG moiety interacted with not only Talpha and Tbeta but also Rh* and membrane phospholipids. The cross-linked phospholipid species was analyzed by ELISA employing a variety of lipid-binding probes, which revealed phosphatidylethanolamine (PE) and phosphatidylserine (PS) as selective phospholipids for POG interaction in the ROS membranes. These results demonstrate that the modifying group of Tgamma plays an active role in protein-protein and protein-membrane interactions and suggest that the farnesyl-PE/PS interaction may support the efficient formation of the signaling ternary complex between transducin and Rh*.

Dissociation of heterotrimeric g proteins in cells

Heterotrimeric G proteins dissociate into their component Galpha and Gbetagamma subunits when these proteins are activated in solution. Until recently, it has not been known if subunit dissociation also occurs in cells. The development of optical methods to study G protein activation in live cells has made it possible to demonstrate heterotrimer dissociation at the plasma membrane. However, subunit dissociation is far from complete, and many active [guanosine triphosphate (GTP)-bound] heterotrimers are intact in a steady state. This unexpectedly reluctant dissociation calls for inclusion of a GTP-bound heterotrimeric state in models of the G protein cycle and places renewed emphasis on the relation between subunit dissociation and effector activation.

Differential dissociation of G protein heterotrimers

Signalling by heterotrimeric G proteins is often isoform-specific, meaning certain effectors are regulated exclusively by one family of heterotrimers. For example, in excitable cells inwardly rectifying potassium (GIRK) channels are activated by G betagamma dimers derived specifically from G(i/o) heterotrimers. Since all active heterotrimers are thought to dissociate and release free G betagamma dimers, it is unclear why these channels respond primarily to dimers released by G(i/o) heterotrimers. We reconstituted GIRK channel activation in cells where we could quantify heterotrimer expression at the plasma membrane, GIRK channel activation, and heterotrimer dissociation. We find that G(oA) heterotrimers are more effective activators of GIRK channels than G(s) heterotrimers when comparable amounts of each are available. We also find that active G(oA) heterotrimers dissociate more readily than active G(s) heterotrimers. Differential dissociation may thus provide a simple explanation for G alpha-specific activation of GIRK channels and other G betagamma-sensitive effectors.

Mechanism of light-induced translocation of arrestin and transducin in photoreceptors: interaction-restricted diffusion

Many signaling proteins change their location within cells in response to external stimuli. In photoreceptors, this phenomenon is remarkably robust. The G protein of rod photoreceptors and rod transducin concentrates in the outer segments (OS) of these neurons in darkness. Within approximately 30 minutes after illumination, rod transducin redistributes throughout all of the outer and inner compartments of the cell. Visual arrestin concurrently relocalises from the inner compartments to become sequestered primarily within the OS. In the past several years, the question of whether these proteins are actively moved by molecular motors or whether they are redistributed by simple diffusion has been extensively debated. This review focuses on the most essential works in the area and concludes that the basic principle driving this protein movement is diffusion. The directionality and light dependence of this movement is achieved by the interactions of arrestin and transducin with their spatially restricted binding partners.

Signal transducing membrane complexes of photoreceptor outer segments

Signal transduction in outer segments of vertebrate photoreceptors is mediated by a series of reactions among multiple polypeptides that form protein-protein complexes within or on the surface of the disk and plasma membranes. The individual components in the activation reactions include the photon receptor rhodopsin and the products of its absorption of light, the three subunits of the G protein, transducin, the four subunits of the cGMP phosphodiesterase, PDE6 and the four subunits of the cGMP-gated cation channel. Recovery involves membrane complexes with additional polypeptides including the Na(+)/Ca(2+), K(+) exchanger, NCKX2, rhodopsin kinases RK1 and RK7, arrestin, guanylate cyclases, guanylate cyclase activating proteins, GCAP1 and GCAP2, and the GTPase accelerating complex of RGS9-1, G(beta5L), and membrane anchor R9AP. Modes of membrane binding by these polypeptides include transmembrane helices, fatty acyl or isoprenyl modifications, polar interactions with lipid head groups, non-polar interactions of hydrophobic side chains with lipid hydrocarbon phase, and both polar and non-polar protein-protein interactions. In the course of signal transduction, complexes among these polypeptides form and dissociate, and undergo structural rearrangements that are coupled to their interactions with and catalysis of reactions by small molecules and ions, including guanine nucleotides, ATP, Ca(2+), Mg(2+), and lipids. The substantial progress that has been made in understanding the composition and function of these complexes is reviewed, along with the more preliminary state of our understanding of the structures of these complexes and the challenges and opportunities that present themselves for deepening our understanding of these complexes, and how they work together to convert a light signal into an electrical signal.

Transducin gamma-subunit sets expression levels of alpha- and beta-subunits and is crucial for rod viability

Transducin is a prototypic heterotrimeric G-protein mediating visual signaling in vertebrate photoreceptor cells. Despite its central role in phototransduction, little is known about the mechanisms that regulate its expression and maintain approximately stoichiometric levels of the alpha- and betagamma-subunits. Here we demonstrate that the knock-out of transducin gamma-subunit leads to a major downregulation of both alpha- and beta-subunit proteins, despite nearly normal levels of the corresponding transcripts, and fairly rapid photoreceptor degeneration. Significant fractions of the remaining alpha- and beta-subunits were mislocalized from the light-sensitive outer segment compartment of the rod. Yet, the tiny amount of the alpha-subunit present in the outer segments of knock-out rods was sufficient to support light signaling, although with a markedly reduced sensitivity. These data indicate that the gamma-subunit controls the expression level of the entire transducin heterotrimer and that heterotrimer formation is essential for normal transducin localization. They further suggest that the production of transducin beta-subunit without its constitutive gamma-subunit partner sufficiently stresses the cellular biosynthetic and/or chaperone machinery to induce cell death.