Regulation of retinal cGMP cascade by phosducin in bovine rod photoreceptor cells. Interaction of phosducin and transducin

Design and Implementation of a Smart Traffic Signal Control System for Smart City Applications

Infrastructure supporting vehicular network (V2X) capability is the key factor to the success of smart city because it enables many smart transportation services. In order to reduce the traffic congestion and improve the public transport efficiency, many intelligent transportation systems (ITS) need to be developed. In this paper, a smart traffic signal control (STSC) system is designed and implemented, it supports several smart city transportation applications including emergency vehicle signal preemption (EVSP), public transport signal priority (TSP), adaptive traffic signal control (ATSC), eco-driving supporting, and message broadcasting. The roadside unit (RSU) controller is the core of the proposed STSC system, where the system architecture, middleware, control algorithms, and peripheral modules are detailed discussed in this paper. It is compatible with existed traffic signal controller so that it can be fast and cost-effectively deployed. A new traffic signal scheme is specially designed for the EVSP scenario, it can inform all the drivers near the intersection regarding which direction the emergency vehicle (EV) is approaching, smoothing the traffic flow, and enhancing the safety. EVSP scenario and the related control algorithms are implemented in this work; integration test and field test are performed to demonstrate the STSC system.

Chaperones and retinal disorders

Defects in protein folding and trafficking are a common cause of photoreceptor degeneration, causing blindness. Photoreceptor cells present an unusual challenge to the protein folding and transport machinery due to the high rate of protein synthesis, trafficking and the renewal of the outer segment, a primary cilium that has been modified into a specialized light-sensing compartment. Phototransduction components, such as rhodopsin and cGMP-phosphodiesterase, and multimeric ciliary transport complexes, such as the BBSome, are hotspots for mutations that disrupt proteostasis and lead to the death of photoreceptors. In this chapter, we review recent studies that advance our understanding of the chaperone and transport machinery of phototransduction proteins.

Structural Basis for the 14-3-3 Protein-Dependent Inhibition of Phosducin Function

Phosducin (Pdc) is a conserved phosphoprotein that, when unphosphorylated, binds with high affinity to the complex of βγ-subunits of G protein transducin (G_tβγ). The ability of Pdc to bind to G_tβγ is inhibited through its phosphorylation at S54 and S73 within the N-terminal domain (Pdc-ND) followed by association with the scaffolding protein 14-3-3. However, the molecular basis for the 14-3-3-dependent inhibition of Pdc binding to G_tβγ is unclear. By using small-angle x-ray scattering, high-resolution NMR spectroscopy, and limited proteolysis coupled with mass spectrometry, we show that phosphorylated Pdc and 14-3-3 form a complex in which the Pdc-ND region 45-80, which forms a part of Pdc's G_tβγ binding surface and contains both phosphorylation sites, is restrained within the central channel of the 14-3-3 dimer, with both 14-3-3 binding motifs simultaneously participating in protein association. The N-terminal part of Pdc-ND is likely located outside the central channel of the 14-3-3 dimer, but Pdc residues 20-30, which are also involved in G_tβγ binding, are positioned close to the surface of the 14-3-3 dimer. The C-terminal domain of Pdc is located outside the central channel and its structure is unaffected by the complex formation. These results indicate that the 14-3-3 protein-mediated inhibition of Pdc binding to G_tβγ is based on steric occlusion of Pdc's G_tβγ binding surface.

The physiological roles of phosducin: from retinal function to stress-dependent hypertension

In the time since its discovery, phosducin's functions have been intensively studied both in vivo and in vitro. Phosducin's most important biochemical feature in in vitro studies is its binding to heterotrimeric G protein βγ-subunits. Data on phosducin's in vivo relevance, however, have only recently been published but expand the range of biological actions, as shown both in animal models as well as in human studies. This review gives an overview of different aspects of phosducin biology ranging from structure, phylogeny of phosducin family members, posttranscriptional modification, biochemical features, localization and levels of expression to its physiological functions. Special emphasis will be placed on phosducin's function in the regulation of blood pressure. In the second part of this article, findings concerning cardiovascular regulation and their clinical relevance will be discussed on the basis of recently published data from gene-targeted mouse models and human genetic studies.

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.

Function of phosducin-like proteins in G protein signaling and chaperone-assisted protein folding

Members of the phosducin gene family were initially proposed to act as down-regulators of G protein signaling by binding G protein betagamma dimers (Gbetagamma) and inhibiting their ability to interact with G protein alpha subunits (Galpha) and effectors. However, recent findings have over-turned this hypothesis by showing that most members of the phosducin family act as co-chaperones with the cytosolic chaperonin complex (CCT) to assist in the folding of a variety of proteins from their nascent polypeptides. In fact rather than inhibiting G protein pathways, phosducin-like protein 1 (PhLP1) has been shown to be essential for G protein signaling by catalyzing the folding and assembly of the Gbetagamma dimer. PhLP2 and PhLP3 have no role in G protein signaling, but they appear to assist in the folding of proteins essential in regulating cell cycle progression as well as actin and tubulin. Phosducin itself is the only family member that does not participate with CCT in protein folding, but it is believed to have a specific role in visual signal transduction to chaperone Gbetagamma subunits as they translocate to and from the outer and inner segments of photoreceptor cells during light-adaptation.

Phosducin regulates the expression of transducin betagamma subunits in rod photoreceptors and does not contribute to phototransduction adaptation

For over a decade, phosducin's interaction with the betagamma subunits of the G protein, transducin, has been thought to contribute to light adaptation by dynamically controlling the amount of transducin heterotrimer available for activation by photoexcited rhodopsin. In this study we directly tested this hypothesis by characterizing the dark- and light-adapted response properties of phosducin knockout (Pd- / -) rods. Pd- / - rods were notably less sensitive to light than wild-type (WT) rods. The gain of transduction, as measured by the amplification constant using the Lamb-Pugh model of activation, was 32% lower in Pd- / - rods than in WT rods. This reduced amplification correlated with a 36% reduction in the level of transducin betagamma-subunit expression, and thus available heterotrimer in Pd- / - rods. However, commonly studied forms of light adaptation were normal in the absence of phosducin. Thus, phosducin does not appear to contribute to adaptation mechanisms of the outer segment by dynamically controlling heterotrimer availability, but rather is necessary for maintaining normal transducin expression and therefore normal flash sensitivity in rods.

Compartment-specific phosphorylation of phosducin in rods underlies adaptation to various levels of illumination

Phosducin is a major phosphoprotein of rod photoreceptors that interacts with the Gbetagamma subunits of heterotrimeric G proteins in its dephosphorylated state. Light promotes dephosphorylation of phosducin; thus, it was proposed that phosducin plays a role in the light adaptation of G protein-mediated visual signaling. Different functions, such as regulation of protein levels and subcellular localization of heterotrimeric G proteins, transcriptional regulation, and modulation of synaptic transmission have also been proposed. Although the molecular basis of phosducin interaction with G proteins is well understood, the physiological significance of light-dependent phosphorylation of phosducin remains largely hypothetical. In this study we quantitatively analyzed light dependence, time course, and subcellular localization of two principal light-regulated phosphorylation sites of phosducin, serine 54 and 71. To obtain physiologically relevant data, our experimental model exploited free-running mice and rats subjected to controlled illumination. We found that in the dark-adapted rods, phosducin phosphorylated at serine 54 is compartmentalized predominantly in the ellipsoid and outer segment compartments. In contrast, phosducin phosphorylated at serine 71 is present in all cellular compartments. The degree of phosducin phosphorylation in the dark appeared to be less than 40%. Dim light within rod operational range triggers massive reversible dephosphorylation of both sites, whereas saturating light dramatically increases phosphorylation of serine 71 in rod outer segment. These results support the role of phosducin in regulating signaling in the rod outer segment compartment and suggest distinct functions for phosphorylation sites 54 and 71.

Distribution and Translocation of Photoreceptor Gβγ-phosducin System in Medaka Retina‡

Unlike other vertebrates, teleosts have rod- and cone-specific phosducins (PD-R and PD-C) in the retina. To evaluate the teleost Gbetagamma-PD systems, we isolated cDNAs encoding medaka Gbeta1 and GbetaC, which selectively expressed rods and cones. Immunohistochemical studies showed that the strong reactivity of GbetaC but not PD-C was detected in cone outer segments. In rod outer segments (ROS), PD-R reactivity was stronger in light-adapted retina than in dark-adapted retina. Western blot analyses of fractions torn from the cryosections showed that the PD-R concentration was low in dark-adapted ROS. It is suggested that PD-R is translocated to ROS and effectively downregulates the phototransduction cascade in light-adapted rods.

Differential modification of phosducin protein in degenerating rd1 retina is associated with constitutively active Ca2+/calmodulin kinase II in rod outer segments

Retinitis pigmentosa comprises a heterogeneous group of incurable progressive blinding diseases with unknown pathogenic mechanisms. The retinal degeneration 1 (rd1) mouse is a retinitis pigmentosa model that carries a mutation in a rod photoreceptor-specific phosphodiesterase gene, leading to rapid degeneration of these cells. Elucidation of the molecular differences between rd1 and healthy retinae is crucial for explaining this degeneration and could assist in suggesting novel therapies. Here we used high resolution proteomics to compare the proteomes of the rd1 mouse retina and its congenic, wild-type counterpart at postnatal day 11 when photoreceptor death is profound. Over 3000 protein spots were consistently resolved by two-dimensional gel electrophoresis and subjected to a rigorous filtering procedure involving computer-based spot analyses. Five proteins were accepted as being differentially expressed in the rd1 model and subsequently identified by mass spectrometry. The difference in one such protein, phosducin, related to an altered modification pattern in the rd1 retina rather than to changed expression levels. Additional experiments showed phosducin in healthy retinae to be highly phosphorylated in the dark- but not in the light-adapted phase. In contrast, rd1 phosducin was highly phosphorylated irrespective of light status, indicating a dysfunctional rd1 light/dark response. The increased rd1 phosducin phosphorylation coincided with increased activation of calcium/calmodulin-activated protein kinase II, which is known to utilize phosducin as a substrate. Given the increased rod calcium levels present in the rd1 mutation, calcium-evoked overactivation of this kinase may be an early and long sought for step in events leading to photoreceptor degeneration in the rd1 mouse.

The phosducin-like protein PhLP1 is essential for G{beta}{gamma} dimer formation in Dictyostelium discoideum

Phosducin proteins are known to inhibit G protein-mediated signaling by sequestering Gbetagamma subunits. However, Dictyostelium discoideum cells lacking the phosducin-like protein PhLP1 display defective rather than enhanced G protein signaling. Here we show that green fluorescent protein (GFP)-tagged Gbeta (GFP-Gbeta) and GFP-Ggamma subunits exhibit drastically reduced steady-state levels and are absent from the plasma membrane in phlp1(-) cells. Triton X-114 partitioning suggests that lipid attachment to GFP-Ggamma occurs in wild-type cells but not in phlp1(-) and gbeta(-) cells. Moreover, Gbetagamma dimers could not be detected in vitro in coimmunoprecipitation assays with phlp1(-) cell lysates. Accordingly, in vivo diffusion measurements using fluorescence correlation spectroscopy showed that while GFP-Ggamma proteins are present in a complex in wild-type cells, they are free in phlp1(-) and gbeta(-) cells. Collectively, our data strongly suggest the absence of Gbetagamma dimer formation in Dictyostelium cells lacking PhLP1. We propose that PhLP1 serves as a cochaperone assisting the assembly of Gbeta and Ggamma into a functional Gbetagamma complex. Thus, phosducin family proteins may fulfill hitherto unsuspected biosynthetic functions.

Alterations of ciliate phosducin phosphorylation in Blepharisma japonicum cells

We have previously reported that motile photophobic response in ciliate Blepharisma japonicum correlates with dephosphorylation of a cytosolic 28 kDa phosphoprotein (PP28) exhibiting properties similar to those of phosducin. Here we demonstrate in in vivo phosphorylation assay that the light-elicited dephosphorylation of the PP28 is significantly modified by cell incubation with substances known to modulate protein phosphatase and kinase activities. Immunoblot analyses showed that incubation of ciliates with okadaic acid and calyculin A, potent inhibitors of type 1 or 2A protein phosphatases, distinctly increased phosphorylation of PP28 in dark-adapted cells and markedly weakened dephosphorylation of the ciliate phosducin following cell illumination. An enhancement of PP28 phosphorylation was also observed in dark-adapted ciliates exposed to 8-Br-cAMP and 8-Br-cGMP, slowly hydrolysable cyclic nucleotide analogs and 3-isobutyryl-1-methylxanthine (IBMX), a non-specific cyclic nucleotide phosphodiesterase (PDEs) inhibitor. Only slight changes in light-evoked dephosphorylation levels of PP28 were observed in cells treated with the cyclic nucleotide analogs and IBMX. Incubation of ciliates with H 89 or KT 5823, highly selective inhibitor of cAMP-dependent protein kinase (PKA) and cGMP-dependent protein kinase (PKG), respectively, decreased PP28 phosphorylation levels in dark-adapted cells, whereas the extent of light-evoked dephosphorylation of the phosphoprotein was only slightly influenced. Cell treatment with higher Ca2+ concentration together with ionophore A23187 in culture medium resulted in marked increase in PP28 phosphorylation levels, while quite an opposite effect was observed in cells exposed to Ca2+ chelators, EGTA or BAPTA/AM as well as calmodulin antagonists, such as trifluoperazine (TFP), W-7 or calmidazolium. Light-dependent dephosphorylation was not considerably affected by these treatments. The experimental findings presented here suggest that an endogenous light-dependent protein kinase-phosphatase system may be engaged in the alteration of phosducin phosphorylation in ciliate B. japonicum thereby to modulate the cell motile photophobic behavior.

Phosducin-like protein acts as a molecular chaperone for G protein betagamma dimer assembly

Phosducin-like protein (PhLP) is a widely expressed binding partner of the G protein betagamma subunit dimer (Gbetagamma). However, its physiological role is poorly understood. To investigate PhLP function, its cellular expression was blocked using RNA interference, resulting in inhibition of Gbetagamma expression and G protein signaling. This inhibition was caused by an inability of nascent Gbetagamma to form dimers. Phosphorylation of PhLP at serines 18-20 by protein kinase CK2 was required for Gbetagamma formation, while a high-affinity interaction of PhLP with the cytosolic chaperonin complex appeared unnecessary. PhLP bound nascent Gbeta in the absence of Ggamma, and S18-20 phosphorylation was required for Ggamma to associate with the PhLP-Gbeta complex. Once Ggamma bound, PhLP was released. These results suggest a mechanism for Gbetagamma assembly in which PhLP stabilizes the nascent Gbeta polypeptide until Ggamma can associate, resulting in membrane binding of Gbetagamma and release of PhLP to catalyze another round of assembly.

Structure of the complex between the cytosolic chaperonin CCT and phosducin-like protein

The three-dimensional structure of the complex formed between the cytosolic chaperonin CCT (chaperonin containing TCP-1) and phosducin (Pdc)-like protein (PhLP), a regulator of CCT activity, has been solved by cryoelectron microscopy. Binding of PhLP to CCT occurs through only one of the chaperonin rings, and the protein does not occupy the central folding cavity but rather sits above it through interactions with two regions on opposite sides of the ring. This causes the apical domains of the CCT subunits to close in, thus excluding access to the folding cavity. The atomic model of PhLP generated from several atomic structures of the homologous Pdc fits very well with the mass of the complex attributable to PhLP and predicts the involvement of several sequences of PhLP in CCT binding. Binding experiments performed with PhLP/Pdc chimeric proteins, taking advantage of the fact that Pdc does not interact with CCT, confirm that both the N- and C-terminal domains of PhLP are involved in CCT binding and that several regions suggested by the docking experiment are indeed critical in the interaction with the cytosolic chaperonin.

Phosphorylation of teleost phosducins and its effect on the affinity to G-protein beta gamma subunits

Phosducin (PD) is a regulatory protein involved in the phototransduction cascade of vertebrate photoreceptor cells. We have previously demonstrated that there are rod- and cone-specific PDs (OlPD-R and OlPD-C) in the retina of the teleost fish, medaka (Oryzias latipes) [FEBS Lett. 502 (2001) 117]. A 6x His affinity precipitation assay revealed that phosphorylation by either protein kinase A (PKA) or Ca(2+)/calmodulin-dependent kinase II (CaMKII) reduced the affinity of recombinant medaka PDs to endogenous medaka G-protein beta gamma subunits (Gbetagamma). These results suggest that the affinity of medaka PDs to Gbetagamma is regulated by cAMP and Ca(2+) concentrations as also found for mammalian PDs. However, we found a specific difference in the phosphorylation patterns between recombinant OlPD-R and OlPD-C, which resulted in different affinities to Gbetagamma. These differences may affect the light/dark-adaptation between medaka rods and cones.

Site-specific phosphorylation of phosducin in intact retina. Dynamics of phosphorylation and effects on G protein beta gamma dimer binding

Phosducin (Pdc) is a G protein beta gamma dimer (G beta gamma) binding protein, highly expressed in retinal photoreceptor and pineal cells, yet whose physiological role remains elusive. Light controls the phosphorylation of Pdc in a cAMP and Ca(2+)-dependent manner, and phosphorylation in turn regulates the binding of Pdc to G(t)beta gamma or 14-3-3 proteins in vitro. To directly examine the phosphorylation of Pdc in intact retina, we prepared antibodies specific to the three principal phosphorylation sites (Ser-54, Ser-73, and Ser-106) and measured the kinetics of phosphorylation/dephosphorylation during light/dark adaptation and the subsequent effects on G(t)beta gamma binding. Ser-54 phosphorylation increased slowly (t((1/2)) approximately 90 min) during dark adaptation to approximately 70% phosphorylated and decreased rapidly (t((1/2)) approximately 2 min) during light adaptation to less than 20% phosphorylated. Ser-73 phosphorylation increased much faster during dark adaptation (t((1/2)) approximately 3 min) to approximately 50% phosphorylated and decreased more slowly during light adaptation (t((1/2)) approximately 9 min) to less than 20% phosphorylated. The Ca(2+) chelator BAPTA-AM blocked Ser-54 phosphorylation during dark adaptation but had no effect on Ser-73 phosphorylation. In contrast, Ser-106 was not phosphorylated in either the light or dark. Importantly, G beta gamma binding to Pdc was enhanced by Ca(2+) chelation and the binding kinetics closely paralleled those of Ser-54 dephosphorylation, indicating that Ser-54 phosphorylation controls G(t)beta gamma binding in vivo. These results suggest a pivotal role of Ser-54 and Ser-73 phosphorylation in determining the interactions of Pdc with its binding partners, G(t)beta gamma and 14-3-3 protein, which may regulate the light-dependent translocation of the photoreceptor G protein.

Phosducin facilitates light-driven transducin translocation in rod photoreceptors. Evidence from the phosducin knockout mouse

Phosducin is a photoreceptor-specific protein known to interact with the beta gamma subunits of G proteins. In pursuit of the function of phosducin, we tested the hypothesis that it regulates the light-driven translocation of G protein transducin from the outer segments of rod photoreceptors to other compartments of the rod cell. Transducin translocation has been previously shown to contribute to rod adaptation to bright illumination, yet the molecular mechanisms underlying the translocation phenomenon remain unknown. In this study we provide two major lines of evidence in support of the role of phosducin in transducin translocation. First, we have demonstrated that transducin beta gamma subunits interact with phosducin along their entire intracellular translocation route, as evident from their co-precipitation in serial tangential sections from light-adapted but not dark-adapted retinas. Second, we generated a phosducin knockout mouse and found that the degree of light-driven transducin translocation in the rods of these mice was significantly reduced as compared with that observed in the rods of wild type animals. In knockout animals the translocation of transducin beta gamma subunits was affected to a larger degree than the translocation of the alpha subunit. We also found that the amount of phosducin in rods is sufficient to interact with practically all of the transducin present in these cells and that the subcellular distribution of phosducin is consistent with that of a soluble protein evenly distributed throughout the entire rod cytoplasm. Together, these data indicate that phosducin binding to transducin beta gamma subunits facilitates transducin translocation. We suggest that the mechanism of phosducin action is based on the reduction of transducin affinity to the membranes of rod outer segments, achieved by keeping the transducin beta gamma subunits apart from the alpha subunit. This increased solubility of transducin would make it more susceptible to translocation from the outer segments.

Mutation and activation of Galpha s similarly alters pre- and postsynaptic mechanisms modulating neurotransmission

Constitutive activation of Galphas in the Drosophila brain abolishes associative learning, a behavioral disruption far worse than that observed in any single cAMP metabolic mutant, suggesting that Galphas is essential for synaptic plasticity. The intent of this study was to examine the role of Galphas in regulating synaptic function by targeting constitutively active Galphas to either pre- or postsynaptic cells and by examining loss-of-function Galphas mutants (dgs) at the glutamatergic neuromuscular junction (NMJ) model synapse. Surprisingly, both loss of Galphas and activation of Galphas in either pre- or postsynaptic compartment similarly increased basal neurotransmission, decreased short-term plasticity (facilitation and augmentation), and abolished posttetanic potentiation. Elevated synaptic function was specific to an evoked neurotransmission pathway because both spontaneous synaptic vesicle fusion frequency and amplitude were unaltered in all mutants. In the postsynaptic cell, the glutamate receptor field was regulated by Galphas activity; based on immunocytochemical studies, GluRIIA receptor subunits were dramatically downregulated (>75% decrease) in both loss and constitutive active Galphas genotypes. In the presynaptic cell, the synaptic vesicle cycle was regulated by Galphas activity; based on FM1-43 dye imaging studies, evoked vesicle fusion rate was increased in both loss and constitutively active Galphas genotypes. An important conclusion of this study is that both increased and decreased Galphas activity very similarly alters pre- and postsynaptic mechanisms. A second important conclusion is that Galphas activity induces transynaptic signaling; targeted Galphas activation in the presynapse downregulates postsynaptic GluRIIA receptors, whereas targeted Galphas activation in the postsynapse enhances presynaptic vesicle cycling.

Ubiquitylation of the transducin betagamma subunit complex. Regulation by phosducin

G proteins (Galphabetagamma) are essential signaling molecules, which dissociate into Galpha and Gbetagamma upon activation by heptahelical membrane receptors. We have identified the betagamma subunit complex of the photoreceptor-specific G protein, transducin (T), as a target of the ubiquitin-proteasome pathway. Ubiquitylated species of the transducin gamma-subunit (Tgamma) but not the alpha- or beta-subunits were assembled de novo in bovine photoreceptor preparations. In addition, Tgamma was exclusively ubiquitylated when Tbetagamma was dissociated from Talpha. Ubiquitylation of Tbetagamma on Tgamma was selectively catalyzed by human ubiquitin-conjugating enzymes UbcH5 and UbcH7 and was coincident with degradation of the entire Tbetagamma subunit complex in vitro by a mechanism requiring ATP and the proteasome. We also show that Tbetagamma association with phosducin, a photoreceptor-specific protein of unknown physiological function, blocks Tbetagamma ubiquitylation and subsequent degradation. Phosphorylation of phosducin by Ca(2+)/calmodulin-dependent protein kinase II, which inhibits phosducin-Tbetagamma complex formation, completely restored Tbetagamma ubiquitylation and degradation. We conclude that Tbetagamma is a substrate of the ubiquitin-proteasome pathway and suggest that phosducin serves to protect Tbetagamma following the light-dependent dissociation of Talphabetagamma.

Regulation of angiotensin II-induced G protein signaling by phosducin-like protein

Phosducin-like protein (PhLP) is a broadly expressed member of the phosducin (Pd) family of G protein betagamma subunit (Gbetagamma)-binding proteins. Though PhLP has been shown to bind Gbetagamma in vitro, little is known about its physiological function. In the present study, the effect of PhLP on angiotensin II (Ang II) signaling was measured in Chinese hamster ovary cells expressing the type 1 Ang II receptor and various amounts of PhLP. Up to 3.6-fold overexpression of PhLP had no effect on Ang II-stimulated inositol trisphosphate (IP(3)) formation, whereas further increases caused an abrupt decrease in IP(3) production with half-maximal inhibition occurring at 6-fold PhLP overexpression. This threshold level for inhibition corresponds to the cellular concentration of cytosolic chaperonin complex, a recently described binding partner that preferentially binds PhLP over Gbetagamma. Results of pertussis toxin sensitivity, GTPgammaS binding, and immunoprecipitation experiments suggest that PhLP inhibits phospholipase Cbeta activation by dual mechanisms: (i) steric blockage of Gbetagamma activation of PLCbeta and (ii) interference with Gbetagamma-dependent cycling of G(q)alpha by the receptor. These results suggest that G protein signaling may be regulated through controlling the cellular concentration of free PhLP by inducing its expression or by regulating its binding to the chaperonin.

Distribution of rod- and cone-specific phosducins in retinas of non-mammalian vertebrates

In mammalian retinas, it has been believed that just one kind of phosducin (PD) commonly exists in both rods and cones. However, we have previously reported that there are rod- and cone-specific PDs (OlPD-R and OlPD-C) in medaka (Oryzias latipes) retina [FEBS Lett., 502, 117-121, 2001]. To clarify the distribution and evolution of these photoreceptor type-specific PDs, we investigated PDs of another teleost and a reptile. Immunohistochemical and Western blot analyses using anti-medaka PD antisera demonstrated that two kinds of PDs are expressed in zebrafish (Danio rerio) photoreceptor cells. Our study is suggestive that teleosts generally possess rod- and cone-specific PDs. We isolated a cDNA encoding putative PD (PmlPD) of a diurnal gecko (Phelsuma madagascariensis longinsulae). Because diurnal gecko possesses a pure-cone retina, it was expected that PmlPD would be expressed in cones. Molecular phylogenetic analysis demonstrated that PmlPD was more closely related to mammalian PDs than teleost cone-specific PDs, suggesting that the rod- and cone-specific subtype of teleost PDs have arisen after the teleost-tetrapod divergence.

Regulatory interaction of phosducin-like protein with the cytosolic chaperonin complex

Phosducin and phosducin-like protein (PhLP) bind G protein betagamma subunits and regulate their activity. This report describes a previously uncharacterized binding partner unique to PhLP that was discovered by coimmunoprecipitation coupled with mass spectrometric identification. Chaperonin containing tailless complex polypeptide 1 (CCT), a cytosolic chaperone responsible for the folding of many cellular proteins, binds PhLP with a stoichiometry of one PhLP per CCT complex. Unlike protein-folding substrates of CCT, which interact only in their nonnative conformations, PhLP binds in its native state. Native PhLP competes directly for binding of protein substrates of CCT and thereby inhibits CCT activity. Overexpression of PhLP inhibited the ability of CCT to fold newly synthesized beta-actin by 80%. These results suggest that the interaction between PhLP and CCT may be a means to regulate CCT-dependent protein folding or alternatively, to control the availability of PhLP to modulate G protein signaling.

Hypoviruses and chestnut blight: exploiting viruses to understand and modulate fungal pathogenesis

Fungal viruses are considered unconventional because they lack an extracellular route of infection and persistently infect their hosts, often in the absence of apparent symptoms. Because mycoviruses are limited to intracellular modes of transmission, they can be considered as intrinsic fungal genetic elements. Such long-term genetic interactions, even involving apparently asymptomatic mycoviruses, are likely to have an impact on fungal ecology and evolution. One of the clearest examples supporting this view is the phenomenon of hypovirulence (virulence attenuation) observed for strains of the chestnut blight fungus, Cryphonectria parasitica, harboring members of the virus family Hypoviridae. The goal of this chapter is to document recent advances in hypovirus molecular genetics and to provide examples of how that progress is leading to the identification of virus-encoded determinants responsible for altering fungal host phenotype, insights into essential and dispensable elements of hypovirus replication, revelations concerning the role of G-protein signaling in fungal pathogenesis, and new avenues for enhancing biological control potential.

Identification of rod- and cone-specific phosducins in teleost retinas

Phosducin (PD) is a regulatory protein of vertebrate phototransduction cascades. In mammalian retina, it has been thought that only one kind of PD commonly exists in both rods and cones. However, we have found two kinds of PD (OIPD-R and OIPD-C) in the retina of a teleost, medaka (Oryzias latipes). In situ hybridization and immunohistochemical analysis demonstrated that OIPD-R and -C are selectively expressed in rods and cones, respectively. The antiserum against medaka PDs recognized two kinds of proteins in bluegill (Lepomis macrochirus) retina. These results suggest that rod- and cone-specific PDs exist in teleost retinas, probably creating differences in light adaptation between rods and cones.

Regulation of cAMP by light and dopamine receptors is dysfunctional in photoreceptors of dystrophic retinal degeneration slow(rds) mice

cAMP levels in dark and light were studied in dystrophic retinal degeneration slow(rds) mice, which carry a mutation in the rds/peripherin gene. cAMP levels were measured in vivo, in freshly isolated retinas, and in vitro in the presence of glutamate, which confines light modulation to photoreceptors. Experiments were conducted on young animals, when significant numbers of viable photoreceptor cells are present. In vivo levels of cAMP are higher in illuminated rds/rds retinas than levels measured in normal BALB/c retinas. Light-evoked down-regulation of cAMP levels was observed in vitro in normal photoreceptors. These measurements were made in the presence of the cyclic nucleotide phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine; therefore, they reflect an inhibition of cAMP formation. In contrast, light had no effect on cAMP formation in photoreceptors of mutant mice, measured under identical conditions. Thus, elevated levels of cAMP in rds/rds retinas in vivo result from abnormalities in cAMP synthesis in the mutant photoreceptor cells. In addition to regulation by light, cAMP formation in photoreceptor cells is regulated by dopamine, acting through dopamine D4 receptors. A dopamine D2/D4 receptor agonist, quinpirole, reduced cAMP levels in dark-adapted normal retinas in vitro, but not in rds/rds retinas. Our data indicate that alterations in a signal transduction pathway that leads to inhibition of adenylyl cyclase might underlie the abnormalities in cAMP levels in mutant rds/rds retinas. Heterozygous rds/+ photoreceptors demonstrated a normal pattern of light-evoked and quinpirole-mediated down-regulation of cAMP. Thus, partial expression of the normal phenotype is sufficient to render normal characteristics of cAMP regulation to the photoreceptors of the heterozygous mouse. The data obtained in the present study might be relevant to the understanding of photoreceptor pathology of patients with peripherin/rds mutations.

Modulation of the G protein regulator phosducin by Ca2+/calmodulin-dependent protein kinase II phosphorylation and 14-3-3 protein binding

Phototransduction is a canonical G protein-mediated cascade of retinal photoreceptor cells that transforms photons into neural responses. Phosducin (Pd) is a Gbetagamma-binding protein that is highly expressed in photoreceptors. Pd is phosphorylated in dark-adapted retina and is dephosphorylated in response to light. Dephosphorylated Pd binds Gbetagamma with high affinity and inhibits the interaction of Gbetagamma with Galpha or other effectors, whereas phosphorylated Pd does not. These results have led to the hypothesis that Pd down-regulates the light response. Consequently, it is important to understand the mechanisms of regulation of Pd phosphorylation. We have previously shown that phosphorylation of Pd by cAMP-dependent protein kinase moderately inhibits its association with Gbetagamma. In this study, we report that Pd was rapidly phosphorylated by Ca(2+)/calmodulin-dependent kinase II, resulting in 100-fold greater inhibition of Gbetagamma binding than cAMP-dependent protein kinase phosphorylation. Furthermore, Pd phosphorylation by Ca(2+)/calmodulin-dependent kinase II at Ser-54 and Ser-73 led to binding of the phosphoserine-binding protein 14-3-3. Importantly, in vivo decreases in Ca(2+) concentration blocked the interaction of Pd with 14-3-3, indicating that Ca(2+) controls the phosphorylation state of Ser-54 and Ser-73 in vivo. These results are consistent with a role for Pd in Ca(2+)-dependent light adaptation processes in photoreceptor cells and also suggest other possible physiological functions.

Effects of differential sulfhydryl group-specific labeling on the rhodopsin and guanine nucleotide binding activities of transducin

The role of transducin sulfhydryl groups was examined by chemical modification with four different reagents: 4-acetamido-4'-maleimidyl-stilbene-2, 2' disulfonic acid (AMDA); 4-vinyl pyridine (VP); 2-nitro-5-thiocyano benzoic acid (NTCBA); and 2, 5-dimethoxystilbene-4'-maleimide (DM). All these compounds rapidly inhibited the [3H]GMPpNp-binding activity of transducin stimulated by photoexcited rhodopsin (R*). Sedimentation experiments showed that the labeling of transducin with AMDA or VP hindered its binding to R* while NTCBA-modified transducin was capable of interacting with the photoreceptor protein. In contrast, DM-labeled transducin precipitated even in the absence of R*. Photoactivated rhodopsin was capable of protecting against the observed AMDA and NTCBA inhibition in transducin function, but not against the inactivation caused by VP or DM. These results suggest the existence of different functional cysteines on transducin that are located in the proximity of the interaction site with the photoreceptor protein, near the guanine nucleotide binding site, or in regions involved in the structural changes taking place upon protein activation. With the use of these reagents, transducin appears to be "frozen" in various conformational stages of its cycle, providing conditions for studying two of the initial steps of the visual process: the light-dependent binding of transducin to rhodopsin and the transducin guanine nucleotide exchange reaction.

Vertebrate photoreceptors

The basis of the duplex theory of vision is examined in view of the dazzling array of data on visual pigment sequences and the pigments they form, on the microspectrophotometry measurements of single photoreceptor cells, on the kinds of photoreceptor cascade enzymes, and on the electrophysiological properties of photoreceptors. The implications of the existence of five distinct visual pigment families are explored, especially with regard to what pigments are in what types of photoreceptors, if there are different phototransduction enzymes associated with different types of photoreceptors, and if there are electrophysiological differences between different types of cones.

The pharmacology of phosducin

The discovery of phosducin (Phd) in photoreceptor cells of the retina and the further identification of phosducin-like proteins (PhdLP) emphasizes the existence of a family of proteins characterized as cytosolic regulators of G protein functions. The individual members represent phosphoproteins with distinct tissue distributions whose highest concentrations were in the retina and the pineal gland, while lower levels were reported for tissues such as liver, spleen, striated muscle, and the brain. Several functions of Phd and PhdLP have been suggested, but their most important ability appears to be their high affinity sequestration with G betagamma subunits of heterotrimeric G proteins. This finding suggests that neutralization of G betagamma by Phd effectively impedes G protein-mediated signal transmission, since G alpha cannot reassemble with G betagamma to provide a functional G protein trimer (G alphabetagamma). Thus, it is the scavenger quality of Phd that is hypothesized to diminish intracellular communication simply by reducing the number of G proteins. An additional important function of Phd relates to the inhibition of G alpha subunits' inherent GTPase. The ability of Phd to directly bind G alpha subunits is probably of minor significance as the affinity between both proteins is low. In general, similar mechanisms have been reported for PhdLPs. In the majority of investigations concerning the interference of Phd with physiological mechanisms, the dark/light adaptation of retinal photoreceptor cells has been the most frequently studied aspect of Phd. More recently, Phd was associated with the adenylyl cyclase of olfactory cilia, as in the presence of the phosphoprotein an increased concentration of cAMP is observed. This finding is in line with the experimental outcome of permanent cell lines transfected to overexpress Phd, which exhibit sensitization to excitatory acting PGE(1), and isoproterenol, respectively. Furthermore, Phd was found to effectively slow down the mechanism of internalization of G protein-coupled opioid receptors. Pathophysiological processes associated with Phd were found for certain eye diseases. Experimental evidence suggests the development of retinal inflammation as a consequence of an autoimmunization process triggered by Phd or shorter fragments thereof. Thus, our present knowledge regarding the functions of members of the Phd family is limited currently to their control of G protein-mediated intracellular signal transmission, the process of endocytosis, and certain autoimmune diseases of the uvea and the pineal gland. However, recent information regarding the presence of certain members of the Phd family in the cell nucleus may bear new insights into the function of these compounds.

The Mouse Clock Locus: Sequence and Comparative Analysis of 204 Kb from Mouse Chromosome 5

The Clock gene encodes a basic helix-loop-helix (bHLH)–PAS transcription factor that regulates circadian rhythms in mice. We previously cloned Clock in mouse and human using a battery of behavioral and molecular techniques, including shotgun sequencing of two bacterial artificial chromosome (BAC) clones. Here we report the finished sequence of a 204-kb region from mouse chromosome 5. This region contains the complete loci for the Clock andTpardl (pFT27) genes, as well as the 3′ partial locus of the Neuromedin U gene; sequence analysis also suggests the presence of two previously unidentified genes. In addition, we provide a comparative genomic sequence analysis with the syntenic region from human chromosome 4. Finally, a new BAC transgenic line indicates that the genomic region that is sufficient for rescue of the Clock mutant phenotype is no greater than 120 kb and tightly flanks the 3′ end of the Clockgene.

[The sequence data reported in this paper have been submitted to the GenBank data library under accession no. AF146793.]

Regulation of exocytosis in chromaffin cells by phosducin-like protein, a protein interacting with G protein betagamma subunits

Phosducin and related proteins have been identified as ubiquitous regulators of signalling mediated by betagamma subunits of trimeric G proteins. To explore a role for phosducin in regulated exocytosis, we have examined the distribution and putative function of phosducin-like protein (PhLP) in adrenal medullary chromaffin cells. The full-length cDNA encoding the short splice variant of PhLP (PhLPs) was cloned from cultured chromaffin cells. Native PhLPs was found associated with plasma membranes and detected in the subplasmalemmal area of resting chromaffin cells by confocal immunofluorescence analysis. Stimulation with secretagogues triggered a massive redistribution of PhLPs into the cytoplasm. When microinjected into individual chromaffin cells, recombinant PhLPs inhibited catecholamine secretion evoked by a depolarizing concentration of K+ without affecting calcium mobilization. Thus, PhLPs may participate directly in the regulation of calcium-evoked exocytosis.

Modulation of CRX transactivation activity by phosducin isoforms

Phosducin (Phd) and Phd-like proteins (PhLPs) selectively bind guanine nucleotide protein (G protein) betagamma subunits (Gbetagamma), while Phd-like orphan proteins (PhLOPs) lack the major functional domain for the binding of Gbetagamma. A retina- and pineal gland-specific transcription factor, cone-rod homeobox (CRX), was identified by a yeast two-hybrid screen using PhLOP1 as the bait. Direct protein-protein interactions between Phd or PhLOP1 and CRX were demonstrated using a beta-galactosidase quantitative assay in the yeast two-hybrid system and were confirmed by an in vitro binding assay and a glutathione S-transferase (GST) pull-down assay. To determine if the interaction with Phd or PhLOP1 affected CRX transactivation, a 120-bp interphotoreceptor retinoid binding protein (IRBP) promoter-luciferase reporter construct containing a CRX consensus element (GATTAA) was cotransfected into either COS-7 or retinoblastoma Weri-Rb-1 cells with expression constructs for CRX and either Phd or PhLOP1. Phd and PhLOP1 inhibited the transcriptional activation activity of CRX by 50% during transient cotransfection in COS-7 cells and by 70% in Weri-Rb-1 cells and COS-7 cells stably transfected with CRX. Phd inhibited CRX transactivation in a dose-dependent manner. Whereas Phd is a cytoplasmic phosphoprotein, coexpression of Phd with CRX results in Phd being localized both in the cytoplasm and nucleus. By contrast, PhLOP1 is found in the nucleus even without CRX coexpression. To address the physiological relevance of these potential protein interacting partners, we identified immunoreactive proteins for Phd and CRX in retinal cytosolic and nuclear fractions. Immunohistochemical analysis of bovine retinas reveals colocalization of Phd isoforms with CRX predominantly in the inner segment of cone cells, with additional costaining in the outer nuclear layer and the synaptic region. Our findings demonstrate that both Phd and PhLOP1 interact directly with CRX and that each diminishes the transactivation activity of CRX on the IRBP promoter. A domain that interacts with CRX is found in the carboxyl terminus of the Phd isoforms. Phd antibody-immunoreactive peptides are seen in light-adapted mouse retinal cytosolic and nuclear extracts. Neither Phd nor PhLOP1 affected CRX binding to its consensus DNA element in electrophoretic mobility shift assays. A model that illustrates separate functional roles for interactions between Phd and either SUG1 or CRX is proposed. The model suggests further a mechanism by which Phd isoforms could inhibit CRX transcriptional activation.

Functional analysis of Plp1 and Plp2, two homologues of phosducin in yeast

Mammalian phosducins are known to bind G protein betagamma subunits in vitro, and are postulated to regulate their signaling function in vivo. Here we describe two homologues of phosducin in yeast, called PLP1 and PLP2. Both gene products were cloned, expressed, and purified as glutathione S-transferase fusions. Of the two isoforms, Plp1 bound most preferentially to Gbetagamma. Binding was enhanced by pheromone stimulation and by the addition of GTPgammaS, conditions that favor dissociation of Gbetagamma from Galpha. Gene disruption mutants and gene overexpression plasmids were prepared and analyzed for changes in signaling and nonsignaling phenotypes. Haploid spore products bearing the plp2Delta mutant failed to grow, suggesting that PLP2 is an essential gene. Cell viability was not restored by a mutation in STE7 that blocks signaling downstream of the G protein. Haploid products bearing the plp1Delta mutant were viable and exhibited a 6-7% increase in pheromone-mediated gene induction. Cells overexpressing PLP1 or PLP2 exhibited a 70-80% decrease in gene induction but no change in pheromone-mediated growth arrest. These data indicate that phosducin can selectively regulate early signaling events following pheromone stimulation and has an essential role in cell growth independent of its regulatory role in cell signaling.

Cyclic adenosine monophosphate signaling in the rat vomeronasal organ: role of an adenylyl cyclase type VI

The present study indicates that male rat urinary components in female rat vomeronasal organ microvillar preparations not only induce a rapid and transient IP(3) signal, but in addition, the level of cAMP decreases with a delayed and sustained time course. This decrease seems to be a consequence of the preceding activation of the phosphoinositol pathway rather than the result of an enhanced phosphodiesterase activity or an inhibition of adenylyl cyclase (AC) via Galpha(i) or Galpha(o). This notion is supported by the finding that activation of the endogenous protein kinase C suppresses basal as well as forskolin-induced cAMP formation. Furthermore, it was observed that elevated levels of calcium inhibit cAMP formation in rat VNO microvillar preparations. These properties of cAMP signaling in the VNO of rats may be mediated by a calcium- and protein kinase C-inhibited AC VI subtype, which is localized in microvillar preparations of the VNO.

The carboxyl terminal domain of phosducin functions as a transcriptional activator

In previous work, we identified a set of phosducin (Phd) isoforms with unknown function including the phosducin (Phd)-like orphan protein 1 (PhLOP1), an amino terminal truncated isoform of the retinal Phd lacking the Gbetagamma binding domain. To investigate the potential biological function of PhLOP1, PhLOP1 was fused at its amino terminus with the DNA binding domain (BD) of the yeast transcriptional factor, GAL4, and used as bait in a yeast two-hybrid screen. Two potential functional protein partners were identified during the screen: SUG1, a subunit of the 26S proteasome and a putative transcriptional mediator, and CRX, a retina- and pineal-specific transcription factor. Upon localizing the interacting domain of PhLOP1 with one of the new partners, SUG1, we found that a domain of 40 amino acids at the carboxyl terminus of Phd and PhLOP1 had intrinsic transcriptional activation activity in yeast. The transactivation activity was further confirmed in mammalian cells. This region contains an acidic domain that has been shown to be involved in the function of several transcriptional activators. In addition, we showed that Phd is cytoplasmic while PhLOP1 is localized predominantly to the nucleus when fused to an enhanced green fluorescent protein (EGFP) and transiently expressed in transfected cells, suggesting that PhLOP1 may play a distinct functional role in transcriptional regulation independent of the known Phd interaction/regulation of Gbetagamma transduction.

Identification of bdm-1, a gene involved in G protein beta-subunit function and alpha-subunit accumulation

Targeted disruption of Galpha and Gbeta genes has established the requirement of an intact G protein signaling pathway for optimal execution of several important physiological processes, including pathogenesis, in the chestnut blight fungus Cryphonectria parasitica. We now report the identification of a G protein signal transduction component, beta disruption mimic factor-1, BDM-1. Disruption of the corresponding gene, bdm-1, resulted in a phenotype indistinguishable from that previously observed after disruption of the Gbeta subunit gene, cpgb-1. The BDM-1 deduced amino acid sequence contained several significant clusters of identity with mammalian phosducin, including a domain corresponding to a highly conserved 11-amino acid stretch that has been implicated in binding to the Gbetagamma dimer and two regions of defined Gbeta/phosducin contact points. Unlike the negative regulatory function proposed for mammalian phosducin, the genetic data presented in this report suggest that BDM-1 is required for or facilitates Gbeta function. Moreover, disruption of either bdm-1 or cpgb-1 resulted in a significant, posttranscriptional reduction in the accumulation of CPG-1, a key Galpha subunit required for a range of vital physiological processes.

PHR1 encodes an abundant, pleckstrin homology domain-containing integral membrane protein in the photoreceptor outer segments

We cloned human and murine cDNAs of a gene (designated PHR1), expressed preferentially in retina and brain. In both species, PHR1 utilizes two promoters and alternative splicing to produce four PHR1 transcripts, encoding isoforms of 243, 224, 208, and 189 amino acids, each with a pleckstrin homology domain at their N terminus and a transmembrane domain at their C terminus. Transcript 1 originates from a 5'-photoreceptor-specific promoter with at least three Crx elements ((C/T)TAATCC). Transcript 2 originates from the same promoter but lacks exon 7, which encodes 35 amino acids immediately C-terminal to the pleckstrin homology domain. Transcripts 3 and 4 originate from an internal promoter in intron 2 and either include or lack exon 7, respectively. In situ hybridization shows that PHR1 is highly expressed in photoreceptors, with lower expression in retinal ganglion cells. Immunohistochemistry localizes the PHR1 protein to photoreceptor outer segments where chemical extraction studies confirm it is an integral membrane protein. Using a series of PHR1 glutathione S-transferase fusion proteins to perform in vitro binding assays, we found PHR1 binds transducin betagamma subunits but not inositol phosphates. This activity and subcellular location suggests that PHR1 may function as a previously unrecognized modulator of the phototransduction pathway.