Light-induced dephosphorylation of a 33K protein in rod outer segments of rat retina

Deletion of Protein Phosphatase 2A Accelerates Retinal Degeneration in GRK1- and Arr1-Deficient Mice

Purpose

Light detection in retinal rod photoreceptors is initiated by activation of the visual pigment rhodopsin. A critical, yet often-overlooked, step enabling efficient perception of light is rhodopsin dephosphorylation mediated by protein phosphatase 2A (PP2A). PP2A deficiency has been reported to impair rhodopsin regeneration after phosphorylation by G protein receptor kinase 1 (GRK1) and binding of arrestin (Arr1), thereby delaying rod dark adaptation. However, its effects on the viability of photoreceptors in the absence of GRK1 and Arr1 remain unclear. Here, we investigated the effects of PP2A deficiency in the absence of GRK1 or Arr1, both of which have been implicated in Oguchi disease, a form of night blindness.

Methods

Rod-specific mice lacking the predominant catalytic Cα-subunit of PP2A were crossed with the Grk1^−/− or Arr1^−/− strains to obtain double knockout lines. Rod photoreceptor viability was analyzed in histological cross-sections of the retina stained with hematoxylin and eosin, and rod function was evaluated by ex vivo electroretinography.

Results

PP2A deficiency alone did not impair photoreceptor viability up to 12 months of age. Retinal degeneration was more pronounced in rods lacking GRK1 compared to rods lacking Arr1, and degeneration was accelerated in both Grk1^−/− or Arr1^−/− strains where PP2A was also deleted. In Arr1^−/− mice, rod maximal photoresponse amplitudes were reduced by 80% at 3 months, and this diminution was enhanced further with concomitant PP2A deficiency.

Conclusions

These results suggest that although PP2A is not required for the survival of rods, its deletion accelerates the degeneration induced by the absence of either GRK1 or Arr1.

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.

Dephosphorylation by protein phosphatase 2A regulates visual pigment regeneration and the dark adaptation of mammalian photoreceptors

Resetting of G-protein-coupled receptors (GPCRs) from their active state back to their biologically inert ground state is an integral part of GPCR signaling. This "on-off" GPCR cycle is regulated by reversible phosphorylation. Retinal rod and cone photoreceptors arguably represent the best-understood example of such GPCR signaling. Their visual pigments (opsins) are activated by light, transduce the signal, and are then inactivated by a GPCR kinase and arrestin. Although pigment inactivation by phosphorylation is well understood, the enzyme(s) responsible for pigment dephosphorylation and the functional significance of this reaction remain unknown. Here, we show that protein phosphatase 2A (PP2A) acts as opsin phosphatase in both rods and cones. Elimination of PP2A substantially slows pigment dephosphorylation, visual chromophore recycling, and ultimately photoreceptor dark adaptation. These findings demonstrate that visual pigment dephosphorylation regulates the dark adaptation of photoreceptors and provide insights into the role of this reaction in GPCR signaling.

Absence of synaptic regulation by phosducin in retinal slices

Phosducin is an abundant photoreceptor protein that binds G-protein βγ subunits and plays a role in modulating synaptic transmission at photoreceptor synapses under both dark-adapted and light-adapted conditions in vivo. To examine the role of phosducin at the rod-to-rod bipolar cell (RBC) synapse, we used whole-cell voltage clamp recordings to measure the light-evoked currents from both wild-type (WT) and phosducin knockout (Pd^−/−) RBCs, in dark- and light-adapted retinal slices. Pd^−/−RBCs showed smaller dim flash responses and steeper intensity-response relationships than WT RBCs, consistent with the smaller rod responses being selectively filtered out by the non-linear threshold at the rod-to-rod bipolar synapse. In addition, Pd^−/− RBCs showed a marked delay in the onset of the light-evoked currents, similar to that of a WT response to an effectively dimmer flash. Comparison of the changes in flash sensitivity in the presence of steady adapting light revealed that Pd^−/− RBCs desensitized less than WT RBCs to the same intensity. These results are quantitatively consistent with the smaller single photon responses of Pd^−/− rods, owing to the known reduction in rod G-protein expression levels in this line. The absence of an additional synaptic phenotype in these experiments suggests that the function of phosducin at the photoreceptor synapse is abolished by the conditions of retinal slice recordings.

Protein phosphatase 2A dephosphorylates CaBP4 and regulates CaBP4 function

PURPOSE

CaBP4 is a neuronal Ca(2+)-binding protein that is expressed in the retina and in the cochlea, and is essential for normal photoreceptor synaptic function. CaBP4 is phosphorylated by protein kinase C zeta (PKCζ) in the retina at serine 37, which affects its interaction with and modulation of voltage-gated Ca(v)1 Ca(2+) channels. In this study, we investigated the potential role and functional significance of protein phosphatase 2A (PP2A) in CaBP4 dephosphorylation.

METHODS

The effect of protein phosphatase inhibitors, light, and overexpression of PP2A subunits on CaBP4 dephosphorylation was measured in in vitro assays. Pull-down experiments using retinal or transfected HEK293 cell lysates were used to investigate the association between CaBP4 and PP2A subunits. Electrophysiologic recordings of cotransfected HEK293 cells were performed to analyze the effect of CaBP4 dephosphorylation in modulating Ca(v)1.3 currents.

RESULTS

PP2A inhibitors, okadaic acid (OA), and fostriecin, but not PP1 selective inhibitors, NIPP-1, and inhibitor 2, block CaBP4 dephosphorylation in retinal lysates. Increased phosphatase activity in light-dependent conditions reverses phosphorylation of CaBP4 by PKCζ. In HEK293 cells, overexpression of PP2A enhances the rate of dephosphorylation of CaBP4. In addition, inhibition of protein phosphatase activity by OA increases CaBP4 phosphorylation and potentiates the modulatory effect of CaBP4 on Ca(v)1.3 Ca(2+) channels in HEK293T cells.

CONCLUSIONS

This study provides evidence that CaBP4 is dephosphorylated by PP2A in the retina. Our findings reveal a novel role for protein phosphatases in regulating CaBP4 function in the retina, which may fine tune presynaptic Ca(2+) signals at the photoreceptor synapse.

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.

The phosducin-like protein PhLP1 impacts regulation of glycoside hydrolases and light response in Trichoderma reesei

BACKGROUND

In the biotechnological workhorse Trichoderma reesei (Hypocrea jecorina) transcription of cellulase genes as well as efficiency of the secreted cellulase mixture are modulated by light. Components of the heterotrimeric G-protein pathway interact with light-dependent signals, rendering this pathway a key regulator of cellulase gene expression.

RESULTS

As regulators of heterotrimeric G-protein signaling, class I phosducin-like proteins, are assumed to act as co-chaperones for G-protein beta-gamma folding and exert their function in response to light in higher eukaryotes. Our results revealed light responsive transcription of the T. reesei class I phosducin-like protein gene phlp1 and indicate a light dependent function of PhLP1 also in fungi. We showed the functions of PhLP1, GNB1 and GNG1 in the same pathway, with one major output being the regulation of transcription of glycoside hydrolase genes including cellulase genes in T. reesei. We found no direct correlation between the growth rate and global regulation of glycoside hydrolases, which suggests that regulation of growth does not occur only at the level of substrate degradation efficiency.Additionally, PhLP1, GNB1 and GNG1 are all important for proper regulation of light responsiveness during long term exposure. In their absence, the amount of light regulated genes increased from 2.7% in wild type to 14% in Δphlp1. Besides from the regulation of degradative enzymes, PhLP1 was also found to impact on the transcription of genes involved in sexual development, which was in accordance with decreased efficiency of fruiting body formation in Δphlp1. The lack of GNB1 drastically diminished ascospore discharge in T. reesei.

CONCLUSIONS

The heterotrimeric G-protein pathway is crucial for the interconnection of nutrient signaling and light response of T. reesei, with the class I phosducin-like protein PhLP1, GNB1 and GNG1 acting as important nodes, which influence light responsiveness, glycoside hydrolase gene transcription and sexual development.

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.

Phosducin regulates transmission at the photoreceptor-to-ON-bipolar cell synapse

The rate of synaptic transmission between photoreceptors and bipolar cells has been long known to depend on conditions of ambient illumination. However, the molecular mechanisms that mediate and regulate transmission at this ribbon synapse are poorly understood. We conducted electroretinographic recordings from dark- and light-adapted mice lacking the abundant photoreceptor-specific protein phosducin and found that the ON-bipolar cell responses in these animals have a reduced light sensitivity in the dark-adapted state. Additional desensitization of their responses, normally caused by steady background illumination, was also diminished compared with wild-type animals. This effect was observed in both rod- and cone-driven pathways, with the latter affected to a larger degree. The underlying mechanism is likely to be photoreceptor specific because phosducin is not expressed in other retina neurons and transgenic expression of phosducin in rods of phosducin knock-out mice rescued the rod-specific phenotype. The underlying mechanism functions downstream from the phototransduction cascade, as evident from the sensitivity of phototransduction in phosducin knock-out rods being affected to a much lesser degree than b-wave responses. These data indicate that a major regulatory component responsible for setting the sensitivity of signal transmission between photoreceptors and ON-bipolar cells is confined to photoreceptors and that phosducin participates in the underlying molecular mechanism.

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.

Phosphorylation of the Ca2+-binding protein CaBP4 by protein kinase C zeta in photoreceptors

CaBP4 is a calmodulin-like neuronal calcium-binding protein that is crucial for the development and/or maintenance of the cone and rod photoreceptor synapse. Previously, we showed that CaBP4 directly regulates Ca(v)1 L-type Ca2+ channels, which are essential for normal photoreceptor synaptic transmission. Here, we show that the function of CaBP4 is regulated by phosphorylation. CaBP4 is phosphorylated by protein kinase C zeta (PKCzeta) at serine 37 both in vitro and in the retina and colocalizes with PKCzeta in photoreceptors. CaBP4 phosphorylation is greater in light-adapted than dark-adapted mouse retinas. In electrophysiological recordings of cells transfected with Ca(v)1.3 and CaBP4, mutation of the serine 37 to alanine abolished the effect of CaBP4 in prolonging the Ca2+ current through Ca(v)1.3 channel, whereas inactivating mutations in the CaBP4 Ca2+-binding sites strengthened Ca(v)1.3 modulation. These findings demonstrate how light-stimulated changes in CaBP4 phosphorylation and Ca2+ binding may regulate presynaptic Ca2+ signals in photoreceptors.

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.

Light-driven translocation of signaling proteins in vertebrate photoreceptors

The dynamic localization of proteins within cells is often determined by environmental stimuli. In retinal photoreceptors, light exposure results in the massive translocation of three key signal transduction proteins, transducin, arrestin and recoverin, into and out of the outer segment compartment where phototransduction takes place. This phenomenon has rapidly taken the center stage of photoreceptor cell biology, thanks to the introduction of new quantitative and transgenic approaches. Here, we discuss evidence that intracellular protein translocation contributes to adaptation of photoreceptors to diurnal changes in ambient light intensity and summarize the current debate on whether it is driven by diffusion or molecular motors.

Photosensory transduction in unicellular eukaryotes: A comparison between related ciliates Blepharisma japonicum and Stentor coeruleus and photoreceptor cells of higher organisms

Blepharisma japonicum and Stentor coeruleus are related ciliates, conspicuous by their photosensitivity. They are capable of avoiding illuminated areas in the surrounding medium, gathering exclusively in most shaded places (photodispersal). Such behaviour results mainly from motile photophobic response occurring in ciliates. This light-avoiding response is observed during a relatively rapid increase in illumination intensity (light stimulus) and consists of cessation of cell movement, a period of backward movement (ciliary reversal), followed by a forward swimming, usually in a new direction. The photosensitivity of ciliates is ascribed to their photoreceptor system, composed of pigment granules, containing the endogenous photoreceptor -- blepharismin in Blepharisma japonicum, and stentorin in Stentor coeruleus. A light stimulus, applied to both ciliates activates specific stimulus transduction processes leading to the electrical changes at the plasma membrane, correlated with a ciliary reversal during photophobic response. These data indicate that both ciliates Blepharisma japonicum and Stentor coeruleus, the lower eukaryotes, are capable of transducing the perceived light stimuli in a manner taking place in some photoreceptor cells of higher eukaryotes. Similarities and differences concerning particular stages of light transduction in eukaryotes at different evolutional levels are discussed in this article.

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.

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.

Subcellular localization of phosducin in rod photoreceptors

Phosducin (Pd) is a 28-kD phosphoprotein whose expression in retina appears limited to photoreceptor cells. Pd binds to the β,γ subunits of transducin (Gt). Their binding affinity is markedly diminished by Pd phosphorylation. While Pd has long been regarded as a candidate for the regulation of Gt, the molecular details of Pd function remain unclear. This gap in understanding is due in part to a lack of precise information concerning the total amount and subcellular localization of rod Pd. While earlier studies suggested that Pd was a rod outer segment (ROS) protein, recent findings have demonstrated that Pd is distributed throughout the rod. In this report, the subcellular distribution and amounts of rat Pd are quantified with immunogold electron microscopy. After light or dark adaptation, retinal tissues were fixedin situand prepared for ultrathin sectioning and immunogold labeling. Pd concentrations were analyzed over the entire length of the rod. The highest Pd labeling densities were found in the rod synapse. Less intense Pd staining was observed in the ellipsoid and myoid regions, while minimal labeling densities were found in the ROS and the rod nucleus. In contrast with rod Gt, no evidence was found for light-dependent movement of Pd between inner and outer segments. There is a relative paucity of Pd in the ROS as compared with the large amounts of Gtfound there. This does not support the earlier idea that Pd could modulate Gtactivity by controlling its concentration. On the other hand, the presence of Pd in the nucleus is consistent with its possible role as a regulator of transcription. The functions of Pd in the ellipsoid and myoid regions remain unclear. The highest concentration of Pd was found at the rod synapse, consistent with a suggested role for Pd in the regulation of synaptic function.

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.

Light-induced dephosphorylation of a 65-kDa protein in the cyanobacterium Synechocystis sp. PCC6803

We found that a 65-kDa protein (p65) of Synechocystis sp. PCC 6803 is dephosphorylated in a light-dependent manner. In darkness, p65 was specifically phosphorylated and then completely dephosphorylated within 2 min upon exposure to high-intensity light. The phosphorylation of p65 recurred after 8 hours incubated in the dark following light exposure. Green (540-560 nm) and red (660 nm) light dephosphorylated p65 efficiently, with the efficiency being greater with green light. These results suggest that p65 is a novel substrate involved in the quantity and quality of light-dependent dephosphorylation in cyanobacteria.

Molecular evolution of proteins involved in vertebrate phototransduction

Vision is one of the most important senses for vertebrates. As a result, vertebrates have evolved a highly organized system of retinal photoreceptors. Light triggers an enzymatic cascade, called the phototransduction cascade, that leads to the hyperpolarization of photoreceptors. It is expected that a systematic comparison of phototransduction cascades of various vertebrates can provide insights into the diversity of vertebrate photoreceptors and into the evolution of vertebrate vision. However, only a few attempts have been made to compare each phototransduction protein participating in this cascade. Here, we determine phylogenetic trees of the vertebrate phototransduction proteins and compare them. It is demonstrated that vertebrate opsin sequences fall into five fundamental subfamilies. It is speculated that this is crucial for the diversity of the spectral sensitivity observed in vertebrate photoreceptors and provides the vertebrates with the molecular tools to discriminate the color of incident light. Other phototransduction proteins can be classified into only a few subfamilies. Cones generally share isoforms of phototransduction proteins that are different from those found in rods. The difference in sensitivity to light between rods and cones is likely due to the difference in the molecular properties of these isoforms. The phototransduction proteins seem to have co-evolved as a system. Switching the expression of these isoforms may characterize individual vertebrate photoreceptors.

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.

G proteins and phototransduction

Phototransduction is the process by which a photon of light captured by a molecule of visual pigment generates an electrical response in a photoreceptor cell. Vertebrate rod phototransduction is one of the best-studied G protein signaling pathways. In this pathway the photoreceptor-specific G protein, transducin, mediates between the visual pigment, rhodopsin, and the effector enzyme, cGMP phosphodiesterase. This review focuses on two quantitative features of G protein signaling in phototransduction: signal amplification and response timing. We examine how the interplay between the mechanisms that contribute to amplification and those that govern termination of G protein activity determine the speed and the sensitivity of the cellular response to light.

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.

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.

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.

Functional roles of the two domains of phosducin and phosducin-like protein

Phosducin and phosducin-like protein regulate G protein signaling pathways by binding the betagamma subunit complex (Gbetagamma) and blocking Gbetagamma association with Galpha subunits, effector enzymes, or membranes. Both proteins are composed of two structurally independent domains, each constituting approximately half of the molecule. We investigated the functional roles of the two domains of phosducin and phosducin-like protein in binding retinal G(t)betagamma. Kinetic measurements using surface plasmon resonance showed that: 1) phosducin bound G(t)betagamma with a 2. 5-fold greater affinity than phosducin-like protein; 2) phosphorylation of phosducin decreased its affinity by 3-fold, principally as a result of a decrease in k(1); and 3) most of the free energy of binding comes from the N-terminal domain with a lesser contribution from the C-terminal domain. In assays measuring the association of G(t)betagamma with G(t)alpha and light-activated rhodopsin, both N-terminal domains inhibited binding while neither of the C-terminal domains had any effect. In assays measuring membrane binding of G(t)betagamma, both the N- and C-terminal domains inhibited membrane association, but much less effectively than the full-length proteins. This inhibition could only be described by models that included a change in G(t)betagamma to a conformation that did not bind the membrane. These models yielded a free energy change of +1.5 +/- 0.25 kcal/mol for the transition from the G(t)alpha-binding to the Pd-binding conformation of G(t)betagamma.

Unconventional myosin VIIA is a novel A-kinase-anchoring protein

To gain an insight into the cellular function of the unconventional myosin VIIA, we sought proteins interacting with its tail region, using the yeast two-hybrid system. Here we report on one of the five candidate interactors we identified, namely the type I alpha regulatory subunit (RI alpha) of protein kinase A. The interaction of RI alpha with myosin VIIA tail was demonstrated by coimmunoprecipitation from transfected HEK293 cells. Analysis of deleted constructs in the yeast two-hybrid system showed that the interaction of myosin VIIA with RI alpha involves the dimerization domain of RI alpha. In vitro binding assays identified the C-terminal "4.1, ezrin, radixin, moesin" (FERM)-like domain of myosin VIIA as the interacting domain. In humans and mice, mutations in the myosin VIIA gene underlie hereditary hearing loss, which may or may not be associated with visual deficiency. Immunohistofluorescence revealed that myosin VIIA and RI alpha are coexpressed in the outer hair cells of the cochlea and rod photoreceptor cells of the retina. Our results strongly suggest that myosin VIIA is a novel protein kinase A-anchoring protein that targets protein kinase A to definite subcellular sites of these sensory cells.

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.

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.

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.

Isolation and investigation of canine phosducin as a candidate for canine generalized progressive retinal atrophies

A subtractive cDNA cloning strategy was used to isolate canine retina-specific genes. Canine phosducin cDNA was cloned from a canine subtracted retinal cDNA library and was analysed as a candidate for canine generalized progressive retinal atrophies (gPRA). Canine phosducin cDNA is 1230 bp in length encoding 245 amino acids. The nucleotide and amino acid sequences of canine phosducin are highly conserved when compared with those of five other mammalian species, namely human, cat, cow, rat, and mouse. Northern blot analysis demonstrated that the mRNA transcript for phosducin was approximately 1.3 kb in size and was present in canine retina, but showed no visible signals in 13 other canine tissues. The phosducin gene was examined for polymorphisms in a total of 101 pedigree dogs of eight breeds, including normal, obligate gPRA carriers, and gPRA-affected dogs, by single-stranded conformation polymorphisms (SSCP) analysis. Polymorphisms in the phosducin gene were detected only in the 3' untranslated region of the gene in two breeds of dogs: allelic heterozygous polymorphisms in miniature poodles suffering from one form of gPRA (progressive rod-cone degeneration, prcd), and a different polymorphism in a single normal Irish wolfhound. The polymorphisms of phosducin in prcd-affected miniature poodles did not segregate with the autosomal recessive form of gPRA. Heterozygous inheritance of the polymorphisms suggests that phosducin is very unlikely to carry the mutation causing prcd, so phosducin was probably excluded as a candidate for prcd-affected miniature poodles in this study.

Phosducin induces a structural change in transducin beta gamma

BACKGROUND

Phosducin binds tightly to the beta gamma subunits (Gt beta gamma) of the heterotrimeric G protein transducin, preventing Gt beta gamma reassociation with Gt alpha-GDP and thereby inhibiting the G-protein cycle. Phosducin-like proteins appear to be widely distributed and may play important roles in regulating many heterotrimeric G-protein signaling pathways.

RESULTS

The 2.8 A crystal structure of a complex of bovine retinal phosducin with Gt beta gamma shows how the two domains of phosducin cover one side and the top of the seven-bladed beta propeller of Gt beta gamma. The binding of phosducin induces a distinct structural change in the beta propeller of Gt beta gamma, such that a small cavity opens up between blades 6 and 7. Electron density in this cavity has been assigned to the farnesyl moiety of the gamma subunit.

CONCLUSIONS

beta gamma subunits of heterotrimeric G proteins can exist in two distinct conformations. In the R (relaxed) state, corresponding to the structure of the free beta gamma or the structure of beta gamma in the alpha beta gamma heterotrimer, the hydrophobic farnesyl moiety of the gamma subunit is exposed, thereby mediating membrane association. In the T (tense) state, as observed in the phosducin-Gt beta gamma structure, the farnesyl moiety of the gamma subunit is effectively buried in the cavity formed between blades 6 and 7 of the beta subunit. Binding of phosducin to Gt beta gamma induces the formation of this cavity, resulting in a switch from the R to the T conformation. This sequesters beta gamma from the membrane to the cytosol and turns off the signal-transduction cascade. Regulation of this membrane association/dissociation switch of Gt beta gamma by phosducin may be a general mechanism for attenuation of G protein coupled signal transduction cascades.

Phosphorylation of rod outer segment proteins modulates phosphatidylethanolamine N-methyltransferase and phospholipase A2 activities in photoreceptor membranes

The activities of enzymes involved in lipid metabolism--phospholipase A2 (PLA2) and phosphatidylethanolamine N-methyltransferase (PE N-MTase)--were found to be differently affected by pre-incubation of rod outer segments (ROS) under protein phosphorylating or dephosphorylating conditions. Exposure to cAMP-dependent protein kinase (PKA), under dark or light conditions, produced a significant increase in PE N-MTase activity, whereas PLA2 activity decreased. Under standard protein kinase C (PKC) phosphorylating conditions in light, PE N-MTase activity was stimulated and PLA2 activity was not affected. When the assays were performed in the dark, both enzymatic activities were unaffected when compared to the corresponding controls. Incubation of ROS membranes in light in the presence of PKC activators phorbol 12,13-dibutyrate (PDBu) and dioctanoylglycerol (DOG) resulted in the same pattern of changes in enzyme activities as described for standard PKC phosphorylating condition. Pre-incubation of membranes with the PKC inhibitor H-7 reduced the stimulation of PDBu on PE N-MTase activity, and had no effect on PLA2 activity in ROS membranes incubated with the phorbol ester. Pre-treatment of isolated ROS with alkaline phosphatase resulted in decreased PE N-MTase activity and produced a significant stimulation of PLA2 activity under dark as well as under light conditions when compared to the corresponding controls. These findings suggest that ROS protein phosphorylation and dephosphorylation modulates PE N-MTase and PLA2 activities in isolated ROS, and that these activities are independently and specifically modulated by particular kinases. Furthermore, dephosphorylation of ROS proteins has the opposite effect to that produced by protein phosphorylation on the enzymes studied.

Phosducin expression in NG 108-15 hybrid cells enhances prostaglandin E1 stimulated adenylate cyclase activity

Phosducin (Phd), a cytosolic protein, has been proposed to compete with certain receptor kinases for Gbetagamma of heterotrimeric G proteins, and may inhibit GTPase activity of G alpha s. These suggestions together with the enhancing effect of Phd on odorant-induced cAMP accumulation let us assume a stimulatory action of the protein on intracellular signaling. Therefore, this investigation was designed to examine the excitatory effect of PGE1 on signal transmission in neuroblastoma x glioma hybrid cells (NG 108-15) overexpressing Phd. The neuronal cells stably expressing Phd were found to display a 3 to 4-fold increased sensitivity to PGE1 as compared to wild type cells, using cAMP accumulation as measure. Examination of membranes prepared from Phd-overexpressing cells revealed an elevated GTPase activity as indicated by the formation of 32Pi upon PGE1 challenge. This activity was inhibited by exogenous Phd. In addition, receptor independent stimulation of adenylate cyclase by forskolin reveals an increased formation of cAMP in Phd expressing cells, which is accompanied by an increased binding of [3H]forskolin. The findings let us propose that Phd elevates intracellular levels of functional G alpha s which accounts for the increased response to PGE1.

Analysis of the T beta gamma-binding domain of MEKA/phosducin

MEKA/phosducin, a 33 kDa phosphoprotein in the photoreceptor cell, associates with transducin beta gamma (T beta gamma) with its N-terminal domain (N-terminal 105 amino acids of MEKA), and translocates T beta gamma from the photoreceptor disc membrane to the soluble fraction. The present study further localized the T beta gamma-binding domain to aa 17-105 of MEKA, and showed that the activity of MEKA to translocate T beta gamma depends on the domain. A series of deletion mutant MEKA proteins were prepared to investigate the domain of MEKA which binds to and translocates T beta gamma. Both binding and translocation activities were not impaired by the deletion of the N-terminal 16 amino acids of MEKA, but completely abolished by further deletion to 42Val. Although anti-MEKA serum inhibited the T beta gamma-MEKA association, the antiserum absorbed with a recombinant peptide corresponding to aa 17-105 of MEKA did not, confirming that aa 17-105 of MEKA directly interacts with T beta gamma.

Phosducin and betagamma-transducin interaction I: effects of post-translational modifications

The interaction between phosducin and betagamma-transducin plays regulatory roles in light adaptation of photoreceptors. Both phosducin and betagamma-transducin undergo post-translational modifications, with phosducin modified by phosphorylation and the gamma subunit of betagamma-transducin by farnesylation and carboxylmethylation. In this study we exploited the electrophoretic mobilities of these native proteins to develop a micro binding assay and examined the effects of post-translational modifications on binding affinities. It was found that decarboxylmethylation of gamma-transducin increased the mobility of betagamma-transducin during native gel electrophoresis, but decreased the apparent affinity for phosducin by about 2-fold. Phosphorylation of phosducin by protein kinase A increased the mobility but decreased the apparent affinity for betagamma-transducin by at least 3-fold.

Modulation of the calcium sensitivity of bovine retinal rod outer segment guanylyl cyclase by sodium ions and protein kinase A

Guanylyl cyclases (GC, EC 4.6.1.2) serve as receptors that produce cGMP in response to ligand binding. The production of cGMP is essential for the ability of retinal photoreceptor cells to restore the dark state after photoexcitation. GC activity is enhanced in rod outer segments (ROS) by a decrease in the cytosolic free Ca2+ concentration. We recently developed a new real-time assay to measure initial rates of ROS GC activity with much improved precision [Wolbring, G. & P. P. M. Schnetkamp (1995) Biochemistry 34, 4689-4695]. With this assay we examined the Ca2+ sensitivity of ROS GC, and we report here that protein kinase A-mediated phosphorylation and Na+ cause significant shifts in the IC50 for Ca2+ of the particulate guanylyl cyclase from bovine retinal rod outer segments. The IC50 for Ca2+ ranged between 30 and 270 nM Ca2+ dependent on the presence of Na+, choline, cAMP, cGMP, 8-bromo-cAMP, 8-bromo-cGMP, or the catalytic subunit of protein kinase A.

Regulation of the kinetics of phosducin phosphorylation in retinal rods

Phosducin (Pd) is a widely expressed phosphoprotein that regulates G-protein (G) signaling. Unphosphorylated Pd binds to Gbetagamma subunits and blocks their interaction with Galpha. This binding sequesters Gbetagamma and inhibits both receptor-mediated activation of Galpha and direct interactions between Gbetagamma and effector enzymes. When phosphorylated by cAMP-dependent protein kinase, Pd does not affect these functions of Gbetagamma. To further understand the role of Pd in regulating G-protein signaling in retinal rod photoreceptor cells, we have measured the abundance of Pd in rods and examined factors that control the rate of Pd phosphorylation. Pd is expressed at a copy number comparable to that for the rod G-protein, transducin (Gt). The ratio of rhodopsin (Rho) to Pd is 15. 5 +/- 3.5 to 1. The rate of Pd phosphorylation in rod outer segment preparations was dependent on [cAMP]. K1/2 for cAMP was 0.56 +/- 0. 09 microM, and the maximal rate of phosphorylation was approximately 500 pmol PO4 incorporated/min/nmol Rho. In the presence of Gtbetagamma this rate was decreased approximately 50-fold. From these data, one can estimate a t1/2 of approximately 3 min for the rephosphorylation of Pd in rods during the recovery period after a light response. This relatively slow rephosphorylation of the Pd.Gtbetagamma complex may provide a period of molecular memory in which sensitivity to further light stimuli is reduced as a result of sequestration of Gtbetagamma by Pd.