Rhodopsin kinase prepared from bovine rod disk membranes quenches light activation of cGMP phosphodiesterase in a reconstituted system

Rhodopsin kinase was extracted into a buffer containing 200 mM KCl and no MgCl2. The activity of the enzyme was stabilized with the use of a mixture of protease inhibitors, aprotinin, benzamidine, leupeptin, and pepstatin. The extract consisted of three major proteins of molecular weight (Mr) 65,000, 56,000, and 37,000, of which the Mr 65,000 protein was identified with the kinase activity since preparations containing the other proteins had no kinase activity and the Mr 65,000 protein was phosphorylated when the extract was incubated with ATP. A reconstituted cGMP phosphodiesterase (PDE) system consisting of peripheral protein-depleted rod disk membranes (RDM), GTP binding protein (G-protein), and PDE was used to test the effectiveness of the rhodopsin kinase preparation in mediating the ATP-dependent quench of light activation of PDE. In the absence of kinase, light-activated PDE activity lasted several minutes. In its presence, ATP and to a lesser extent GTP quenched the activation about as rapidly as in rod disk membranes. The influence of kinase was unaffected by increasing G-protein or PDE content of the reconstituted system but was slowed down by brighter flashes, showing that quench was caused by the inactivation of bleached rhodopsin and not of PDE or G-protein.     

Characterization of transducin from bovine retinal rod outer segments. Use of monoclonal antibodies to probe the structure and function of the subunit

A panel of monoclonal antibodies has been developed against the T alpha, T beta and T gamma subunits of bovine transducin. Two anti-T alpha antibodies from this panel (TF15 and TF16) and a third one (4A) against frog T alpha (Witt, P. L., Hamm, H. E., and Bownds, M. D. (1984) J. Gen. Physiol. 84, 251-263) were characterized. Each of these monoclonal antibodies recognizes a different region of T alpha and has a specific effect on the function of transducin. The binding of TF15 is reversibly enhanced by treating T alpha with either 1 M guanidinium chloride or, to a smaller extent, by the removal of bound guanine nucleotide. Its epitope is located in a 12-kDa tryptic fragment containing the binding site for the guanine moiety of GTP. Taken together, these results support previous observations that the conformation of T alpha is modulated by the occupancy of the guanine nucleotide binding site. In contrast to TF15, TF16 recognizes only the native form of T alpha. Its epitope resides within the central portion of the T alpha molecule. While T alpha-bound TF16 does not inhibit either pertussis toxin-catalyzed ADP-ribosylation, rhodopsin binding, or transducin subunit interaction, it blocks both the light-activated uptake of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) and the GTP-dependent elution of transducin from photolyzed rhodopsin. These effects are unlikely to be caused by the occupation of the guanine nucleotide binding site by TF16 because this antibody quantitatively precipitates T alpha-GTP gamma S. We propose that bound TF16 locks T alpha in a conformation that prevents the entrance of guanine nucleotide and favors T beta gamma association. In contrast to TF16, the epitope of 4A was mapped to the amino-terminal region of T alpha. This monoclonal antibody blocks pertussis toxin-catalyzed ADP-ribosylation, GTP gamma S uptake, and T alpha-T beta gamma association. Moreover, the binding site for 4A becomes inaccessible when transducin binds to photolyzed rhodopsin. These results suggest that the inhibitory effects of 4A are due to a simultaneous steric blockage of both the interaction of T alpha with T beta gamma and their binding to photolyzed rhodopsin. The results obtained from these studies are correlated with the structure and function of T alpha.     

Amplification of phosphodiesterase activation is greatly reduced by rhodopsin phosphorylation

In the vertebrate rod outer segment (ROS), the light-dependent activation of a GTP-binding protein (G-protein) and phosphodiesterase (PDE) is quenched by a process that requires ATP [Liebman, P.A., & Pugh, E.N. (1979) Vision Res. 19, 375-380]. The ATP-dependent quenching mechanism apparently requires the phosphorylation of photoactivated rhodopsin (Rho*); however, a 48-kilodalton protein (48K protein) has also been proposed to participate in the inactivation process. Purified species of phosphorylated rhodopsin containing 0, 2, or greater than or equal to 4 (high) phosphates per rhodopsin (PO4/Rho) were reconstituted into phosphatidylcholine (PC) vesicles and reassociated with a hypotonic extract from isotonically washed disk membranes that were depleted of 48K protein; PDE activation, in response to bleaching from 0.01% to 15% of the rhodopsin present, was measured. PDE activity was reduced by at least 30% at high fractional rhodopsin bleaches and by greater than 80% at low fractional rhodopsin bleaches in high PO4/Rho samples when compared to the activity measured in O PO4/Rho controls. A phosphorylation level of 2 PO4/Rho produced PDE activities that were intermediate between O PO4/Rho and high PO4/Rho samples at low bleaches, but were identical with the O PO4/Rho samples at high rhodopsin bleaches. Rhodopsin phosphorylation is thus capable of producing a graded inhibition of light-stimulated PDE activation over a limited range of (near physiological) bleach levels. This effect become less pronounced as the bleach levels approach those that saturate PDE activation. These results are consistent with increasing levels of phosphorylation, producing a reduction of the binding affinity of G-protein for Rho*.     

Effects of fluoride on retinal rod outer segment cGMP phosphodiesterase and G-protein

The effects of fluoride on ROS phosphodiesterase and G-protein have been studied using membrane-free extracts. When G-protein was present NaF, at millimolar concentrations, stimulated PDE activity however, in a G-protein free extract, cGMP hydrolysis was inhibited by high fluoride concentrations. Fluoride was also found to profoundly inhibit the ability of G-protein to bind a GTP analogue, GTP gamma S, both in the presence and absence of rhodopsin. Aluminium greatly modified these effects of fluoride on PDE and G-protein. The possibility that fluoride activates PDE through its effect on G-protein is discussed.     

The receptor-bound "empty pocket" state of the heterotrimeric G-protein alpha-subunit is conformationally dynamic

Heterotrimeric G-protein activation by a G-protein-coupled receptor (GPCR) requires the propagation of structural signals from the receptor-interacting surfaces to the guanine nucleotide-binding pocket. To probe conformational changes in the G-protein alpha-subunit (G(alpha)) associated with activated GPCR (R*) interactions and guanine nucleotide exchange, high-resolution solution NMR methods are being applied in studying signaling of the G-protein, transducin, by light-activated rhodopsin. Using these methods, we recently demonstrated that an isotope-labeled G(alpha) reconstituted heterotrimer forms functional complexes under NMR experimental conditions with light-activated, detergent-solubilized rhodopsin and a soluble mimic of R*, both of which trigger guanine nucleotide exchange [Ridge, K. D., et al. (2006) J. Biol. Chem. 281, 7635-7648]. Here, it is shown that both light-activated rhodopsin and the soluble mimic of R form trapped intermediate complexes with a GDP-released "empty pocket" state of the heterotrimer in the absence of GTP (or GTPgammaS). In contrast to guanine nucleotide-bound forms of G(alpha), the NMR spectra of the GDP-released, R-bound empty pocket state of G(alpha) display severe line broadening suggestive of a dynamic intermediate state. Interestingly, the conformation of a GDP-depleted, Mg(2+)-bound state of G(alpha) generated in a manner independent of R* does not exhibit a similar degree of line broadening but rather appears structurally similar to the GDP/Mg(2+)-bound form of the protein. Taken together, these results suggest that R*-mediated changes in the receptor-interacting regions of G(alpha), and not the absence of bound guanine nucleotide, are the predominant factors which dictate G(alpha) conformation and dynamics in this R*-bound state of the heterotrimer.     

Extracellular cGMP phosphodiesterase related to the rod outer segment phosphodiesterase isolated from bovine and monkey retinas

A phosphodiesterase (PDE) has been characterized in the interphotoreceptor matrix (IPM) of light-adapted fresh bovine retinas. It is obtained through a gentle rinsing of the retinal surface under conditions where the light-activated rod outer segment (ROS) enzyme remains attached. The enzyme has an apparent native molecular weight of 350 000 by gel filtration and appears as a doublet at Mr 47 000 and 45 000 on sodium dodecyl sulfate-polyacrylamide gels. It has an apparent Km value for cGMP of 33 microM and an apparent Km value for cAMP of 2200 microM. It is activated 3-6-fold by protamine and over 40-fold by trypsin. Protamine has no effect on the Km for cGMP while trypsin decreases the Km for cGMP by a factor of 2. The enzyme occurs in at least two forms as evidenced by two distinct peaks of activity after gel electrophoresis under nondenaturing conditions. A heat-stable inhibitor is tightly bound to the enzyme. The inhibitor obtained from the IPM PDE inhibits 98% of the activity of the trypsin-activated ROS PDE: conversely, the inhibitor obtained by boiling the ROS PDE completely inhibits the trypsin-activated IPM enzyme. A high-affinity monoclonal antibody to the active site of the ROS PDE, ROS 1 [Hurwitz, R., Bunt-Milan, A.H., & Beavo, J. (1984) J. Biol. Chem. 259, 8612-8618], quantitatively absorbs the IPM PDE. These observations indicate a clear relationship between these two PDEs even though their location, sizes, and specific functions in the retina appear to be distinct.     

Mechanism of action of monoclonal antibodies that block the light activation of the guanyl nucleotide-binding protein, transducin

Seven monoclonal antibodies to the alpha subunit (G alpha) of the frog photoreceptor guanyl nucleotide-binding protein (transducin or G-protein) have been characterized as to their effect on G-protein function, and this has been correlated in the accompanying paper (Deretic, D., and Hamm, H. E. (1987) J. Biol. Chem. 262, 10839-10847) with the antibody-binding sites on G alpha tryptic fragments. Antibodies 4A, 7A, 7B, 7C, and 7D are members of a class of antibodies that block G-protein activation by light and therefore also block activation of the cGMP phosphodiesterase. All these blocking antibodies also block the interaction of G-protein with rhodopsin as measured by the light-scattering "binding signal," and as measured by the stabilization of meta-rhodopsin II by bound G-protein (extra-meta-rhodopsin II). The antibodies (or Fab fragments) also solubilize G alpha beta gamma from the membrane in the dark under isosmotic conditions and thus interfere with G alpha interaction with the membrane. Antibody 4A also blocks the extra-meta-rhodopsin II generated by G-protein-rhodopsin interaction in detergent solubilized membranes. Thus, even in the absence of phospholipids, antibody 4A blocks G-protein-rhodopsin interaction. Therefore, we suggest that the antibodies recognize a region of G alpha involved with binding to rhodopsin. An alternative hypothesis is that this antigenic site is a region of interaction between the alpha and beta gamma subunits, disruption of this interaction leading to removal of both the alpha and beta gamma subunits from the membrane and blocking interaction with rhodopsin. This does not seem to be the case because the antibodies immunoprecipitate the alpha beta gamma complex, and not just the alpha subunit. Other antibodies, 4C and 4H, do not block phosphodiesterase activation, the light-scattering signal, extra-meta-rhodopsin II formation, or interaction with the membrane in the dark and therefore recognize other sites on G alpha.     

Rhodopsin-G-protein interactions monitored by resonance energy transfer

Resonance energy transfer measurements were implemented to monitor the specific interactions between G-protein and rhodopsin in phospholipid vesicles reconstituted with the purified proteins. Fluorescently labeled G-protein was extracted from bleached rod outer segments (ROS) reacted with several sulfhydryl reagents: N-(1-pyrenyl)maleimide (P), monobromobimane (B), 7-(diethylamino)-3-(4-maleimidylphenyl)-4-methylcoumarin (C), and N-(4-anilino-1-naphthyl)maleimide (A). Limited labeling of ROS, resulting in the modification of less than a single -SH residue per G-protein molecule and less than 0.2 residue per rhodopsin, did not impair the specific in situ interactions between rhodopsin and G-protein. This was demonstrated by preservation of their light-activated tight association and Gpp(NH)p binding and their fast dissociation with excess GTP. The distribution of fluorescent label among the three subunits of G-protein revealed a highly reactive -SH group in the gamma subunit accessible to labeling when G-protein was bound specifically to bleached rhodopsin. Recombination of purified fluorescent derivatives of G-protein with purified rhodopsin reconstituted in lipid vesicles restored the light-activated Gpp(NH)p binding to a level comparable to that measured with unlabeled G-protein. Similar observations were obtained with ROS depleted of peripheral proteins. Likewise, modification of up to two -SH groups per rhodopsin molecule with the fluorescent reagents did not affect the functional recombination of G-protein with rhodopsin in reconstituted lipid vesicles or in depleted ROS. Interactions between rhodopsin and G-protein were monitored by resonance energy transfer measurements, with the following fluorescent conjugates as donor/acceptor couples: P-rhodopsin/C-G-protein, P-rhodopsin/B-G-protein, and P-G-protein/C-rhodopsin.(ABSTRACT TRUNCATED AT 250 WORDS)     

The dynamics of phosphodiesterase activation in rods and cones

Phototransduction starts with the activation of a rhodopsin (respectively, coneopsin) molecule, located in the outer segment of rod (respectively, cone) photoreceptors. The subsequent amplification pathway proceeds via the G-protein transducin to the activation of phosphodiesterase (PDE), a G-protein coupled effector enzyme. In this article, we study the dynamics of PDE activation by constructing a Markov model that is based on the underlying chemical reactions including multiple rhodopsin phosphorylations. We derive explicit equations for the mean and the variance of activated PDE. Our analysis reveals that a low rhodopsin lifetime variance is neither necessary nor sufficient to achieve reliable PDE activation. The numerical simulations show that during the rising phase the variability of PDE activation is much lower compared to the recovery phase, and this property depends crucially on the transducin activation rates. Furthermore, we find that the dynamics of the activation process greatly differs depending on whether rhodopsin or PDE deactivation limits the recovery of the photoresponse. Finally, our simulations for cones show that only very few PDEs are activated by an excited photopigment, which might explain why in S-cones no single photon response can be observed.     

Localization of binding sites for carboxyl terminal specific anti-rhodopsin monoclonal antibodies using synthetic peptides

The binding sites for four monoclonal antibodies, rho 1D4, rho 3C2, rho 3A6, and rho 1C5, have been localized within the C-terminal region of bovine rhodopsin: Asp18'-Glu-Ala16'-Ser-Thr-Thr-Val12'-Ser-Lys-Thr-Gl u8'-Thr-Ser-Gln-Val4'-Ala-Pr o -Ala1'. Antibody binding sites were localized by using synthetic C-terminal peptides in conjunction with solid-phase competitive inhibition assays and limited proteolytic digestion of rhodopsin in conjunction with electrophoretic immunoblotting techniques. Binding of the rho 1D4 and rho 3C2 antibodies to immobilized rhodopsin was inhibited with peptides of length 1'-8' and longer. Antibody rho 1D4 binding was not inhibited by peptides 2'-13' or 3'-18', indicating that the C-terminal alanine residue of rhodopsin was required. Similar competitive inhibition studies indicated that the antibody rho 3A6 required peptides of length 1'-12' and longer whereas rho 1C5 required peptide 1'-18'. Peptide 3'-18' was as effective as 1'-18' in inhibiting rho 3A6 binding to rhodopsin, but replacement of glutamic acid in position 8' with glutamine abolished competition. This substitution had little effect on the binding of antibody rho 1C5. Thus, Glu8' was essential for rho 3A6 binding but not for the binding of the rho 1C5 antibody. Cleavage of the seven amino acid C-terminus from rhodopsin and further cleavage to F1 (Mr 25 000) and F2 (Mr 12 000) fragments with Staphylococcus aureus V8 protease abolished binding of rho 1D4 antibody to the membrane-bound rhodopsin fragments.(ABSTRACT TRUNCATED AT 250 WORDS)     

A dominant-negative Galpha mutant that traps a stable rhodopsin-Galpha-GTP-betagamma complex

Residues comprising the guanine nucleotide-binding sites of the α subunits of heterotrimeric (large) G-proteins (Gα subunits), as well as the Ras-related (small) G-proteins, are highly conserved. This is especially the case for the phosphate-binding loop (P-loop) where both Gα subunits and Ras-related G-proteins have a conserved serine or threonine residue. Substitutions for this residue in Ras and related (small) G-proteins yield nucleotide-depleted, dominant-negative mutants. Here we have examined the consequences of changing the conserved serine residue in the P-loop to asparagine, within a chimeric Gα subunit (designated αT*) that is mainly comprised of the α subunit of the retinal G-protein transducin and a limited region from the α subunit of Gi1. The αT*(S43N) mutant exhibits a significantly higher rate of intrinsic GDP-GTP exchange compared with wild-type αT*, with light-activated rhodopsin (R*) causing only a moderate increase in the kinetics of nucleotide exchange on αT*(S43N). The αT*(S43N) mutant, when bound to either GDP or GTP, was able to significantly slow the rate of R*-catalyzed GDP-GTP exchange on wild-type αT*. Thus, GTP-bound αT*(S43N), as well as the GDP-bound mutant, is capable of forming a stable complex with R*. αT*(S43N) activated the cGMP phosphodiesterase (PDE) with a dose-response similar to wild-type αT*. Activation of the PDE by αT*(S43N) was unaffected if either R* or β1γ1 alone was present, whereas it was inhibited when R* and the β1γ1 subunit were added together. Overall, our studies suggest that the S43N substitution on αT* stabilizes an intermediate on the G-protein activation pathway consisting of an activated G-protein-coupled receptor, a GTP-bound Gα subunit, and the β1γ1 complex.     

Purification and characteristics of photoreceptor light-activated guanosinetriphosphatase

We describe a reconstitution of light-activated vertebrate photoreceptor GTPase and a purification of the GTP-binding protein (G protein), which is a component of the GTPase and modulates the light-activated phosphodiesterase (PDE) enzyme system. Rod outer segments (ROS) of bull frogs were treated with ethylenediaminetetraacetic acid (EDTA), and the GTPase and PDE fractions were solubilized (EDTA supernatant). When the EDTA supernatant and EDTA-treated membrane fraction (EDTA-washed membranes) were recombined, light-dependent GTPase activity appeared. In the reconstituted system, the Km for GTP as substrate was 0.5 microM; the optimum pH was 7.5-8.0. The isoelectric point of GTPase in EDTA supernatant was 4.8. G protein was purified 400-fold from ROS, and the molecular weight of G protein was determined to be 40 000 by polyacrylamide gel electrophoresis. The amount of G protein in ROS was calculated as at least 1 molecule per 400 rhodopsin molecules. By recombining (in the presence or absence of GTP) purified G protein, PDE, H fraction (an additional component of GTPase), and illuminated or unilluminated EDTA-washed membranes (as a source of rhodopsin), we showed that illuminated rhodopsin, G protein, PDE, and GTP are the minimum requirements for light-dependent PDE activity. We discuss the significance of these findings in the regulation of the light-activated GTPase and PDE activities, especially with regard to the mechanism of activation.     

Phosphorylation in sealed rod outer segments: effects of cyclic nucleotides

Rod outer segments (ROS) from rat were purified on Percoll gradients. These ROS had intact plasma membranes since they were impermeable to small molecules. Protein phosphorylation in the purified ROS was studied after the plasma membrane was disrupted by freeze/thawing. [gamma-32P]ATP was used as phosphate donor. ATP concentration, time, temperature, and light or dark adaptation were varied in the assays. The 32P-labeled proteins were separated by polyacrylamide gel electrophoresis and autoradiographed. Rhodopsin was the dominant phosphorylated protein, and the addition of adenosine cyclic 3',5'-phosphate (cAMP) or guanosine cyclic 3',5'-phosphate (cGMP) (10(-4) M) did not qualitatively alter the ROS phosphorylation pattern. The only cyclic nucleotide effect we could establish in these experiments was the inhibition of rhodopsin phosphorylation by cGMP. This inhibition did not appear to be competitive with ATP since cAMP was much less inhibitory than cGMP and the phosphorylation in the presence of cGMP reached a plateau at a much lower level than in control conditions. Hypotheses implying an involvement of protein phosphorylation/dephosphorylation in dark adaptation have been formulated [Miller, J. A., & Paulsen, R. (1975) J. Biol. Chem. 250, 4427-4432; Kuhn, H., McDowell, J. H., Leser, K. H., & Bader, S. (1977) Biophys. Struct. Mech. 3, 175-180]; we suggest that cGMP may control this process through the modulation of the extent of inhibition of phosphorylation of the visual pigment.     

Stimulation of rhodopsin phosphorylation by guanine nucleotides in rod outer segments

Porcine rod outer segment (ROS) proteins were phosphorylated in the presence of [gamma-32P]ATP and Mg2+, separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and detected by autoradiography. The phosphorylation of rhodopsin, the major protein-staining band (Mr approximately 34 000-38 000), was markedly and specifically increased by exposure of rod outer segments to light; various guanine nucleotides (10 microM) including GMP, GDP, and GTP also specifically increased rhodopsin phosphorylation (up to 5-fold). Adenine nucleotides (cyclic AMP, AMP, and ADP at 10 microM) and 8-bromo-GMP (10 microM) or cyclic 8-bromo-GMP (10 microM) had no detectable stimulatory effect on rhodopsin phosphorylation. GTP increased the phosphorylation of rhodopsin at concentrations as low as 100 nM, and guanosine 5'-(beta, gamma-imidotriphosphate), a relatively stable analogue of GTP, was nearly as effective as GTP. Maximal stimulation of rhodopsin phosphorylation by GTP was observed at 2 microM. GMP and GDP were less potent than GTP. Both cyclic GMP and GMP were converted to GTP during the time period of the protein phosphorylation reaction, suggestive of a GTP-specific effect. Transphosphorylation of guanine nucleotides by [32P]ATP and subsequent utilization of [32P]GTP as a more effective substrate were ruled out as an explanation for the guanine nucleotide stimulation. With increasing concentrations of ROS proteins, the phosphorylation of rhodopsin was nonlinear, whereas in the presence of GTP (2 microM) linear increases in rhodopsin phosphorylation as a function of added ROS protein were observed. These results suggest that GTP stimulates the phosphorylation of rhodopsin by ATP and that a GTP-sensitive inhibitor (or regulator) of rhodopsin phosphorylation may be present in ROS.     

Constraints on the conformation of the cytoplasmic face of dark-adapted and light-excited rhodopsin inferred from antirhodopsin antibody imprints

Rhodopsin is the best-understood member of the large G protein-coupled receptor (GPCR) superfamily. The G-protein amplification cascade is triggered by poorly understood light-induced conformational changes in rhodopsin that are homologous to changes caused by agonists in other GPCRs. We have applied the "antibody imprint" method to light-activated rhodopsin in native membranes by using nine monoclonal antibodies (mAbs) against aqueous faces of rhodopsin. Epitopes recognized by these mAbs were found by selection from random peptide libraries displayed on phage. A new computer algorithm, FINDMAP, was used to map the epitopes to discontinuous segments of rhodopsin that are distant in the primary sequence but are in close spatial proximity in the structure. The proximity of a segment of the N-terminal and the loop between helices VI and VIII found by FINDMAP is consistent with the X-ray structure of the dark-adapted rhodopsin. Epitopes to the cytoplasmic face segregated into two classes with different predicted spatial proximities of protein segments that correlate with different preferences of the antibodies for stabilizing the metarhodopsin I or metarhodopsin II conformations of light-excited rhodopsin. Epitopes of antibodies that stabilize metarhodopsin II indicate conformational changes from dark-adapted rhodopsin, including rearrangements of the C-terminal tail and altered exposure of the cytoplasmic end of helix VI, a portion of the C-3 loop, and helix VIII. As additional antibodies are subjected to antibody imprinting, this approach should provide increasingly detailed information on the conformation of light-excited rhodopsin and be applicable to structural studies of other challenging protein targets.     

cGMP suppresses GTPase activity of a portion of transducin equimolar to phosphodiesterase in frog rod outer segments. Light-induced cGMP decreases as a putative feedback mechanism of the photoresponse.

In rod photoreceptor cells, the light response is triggered by an enzymatic cascade that causes cGMP levels to fall: excited rhodopsin (Rho*)----rod G-protein (transducin, Gt)----cGMP-phosphodiesterase (PDE). This results in the closure of plasma membrane channels that are gated by cGMP. PDE activation by Gt occurs when GDP bound to the alpha-subunit of Gt (Gt alpha) is exchanged with free GTP. The interaction of Gt alpha-GTP with the gamma-subunits of PDE releases their inhibitory action and causes cGMP hydrolysis. Inactivation is thought to be caused by subsequent hydrolysis of Gt alpha-GTP by an intrinsic Gt-GTPase activity. Here we report that there are two portions of Gt in frog rod outer segments (ROS) expressing different rates of GTP hydrolysis: 19.5 +/- 3 mmol of Gt/mol of Rho, equivalent to that amount which participates in PDE activation, hydrolyzing GTP at a rate of approximately 0.6 turnover/s ("fast") and the remaining Gt (80.5 +/- 3 mmol/mol Rho) hydrolyzing GTP at a rate of 0.058 +/- 0.009 turnover/s. Fast GTPase activity is abolished in the presence of cGMP. This effect occurs over the physiological range of cGMP concentration changes in ROS, half-saturating at approximately 2 microM and saturating at 5 microM cGMP. cGMP-dependent suppression of GTPase is specific for cGMP; cAMP in millimolar concentration does not affect GTPase, while the poorly hydrolyzable cGMP analogue, 8-bromo-cGMP, mimics the effect. GTPase regulation by cGMP is not affected by Ca2+ over the concentration range 5-500 nM, which spans the physiological changes in cytoplasmic Ca2+ in rod cells. We suggest that the fast cGMP-sensitive GTPase activity is a property of the Gt that activates PDE. In this model, cGMP serves not only as a messenger of excitation but also modulates GTPase activity, thereby mediating negative feedback regulation of the pathway via PDE turnoff: a light-dependent decrease in cGMP accelerates the hydrolysis of GTP bound to Gt, resulting in the rapid inactivation of PDE.     

Reduced rhodopsin phosphorylation during retinal dystrophy

During inherited retinal dystrophy in Irish Setter dogs, decreased activity of cGMP phosphodiesterase (PDE) results in high cGMP levels and retinal degeneration (1-3). This defect could be in PDE itself, or in its interactions with other proteins of the rod outer segment. We report herein that when retinas from 8-week-old dogs were phosphorylated with gamma-32P-ATP, and separated on SDS-PAGE, phosphorylation of rd dog rhodopsin was reduced. When rd retinas were mixed with normal dog retinas, phosphorylation of the latter was inhibited. Since rd-mediated inhibition was prevented by 1 mM NaF, the results suggest that the cause of reduced rd phosphorylation is increased phosphatase activity. Together, these results demonstrate that decreased phosphorylation of rhodopsin due to increased phosphatase activity is a fundamental biochemical change which may partially account for the degenerative process and loss of visual acuity during inherited retinal dystrophy.     

Reconstitution of rhodopsin and the cGMP cascade in polymerized bilayer membranes

The successful reconstitution of rhodopsin, the rod outer segment (ROS) G protein, and the ROS phosphodiesterase (PDE) into partially polymerized bilayer membranes is described. Purified bovine rhodopsin (Rh) was inserted into performed partially polymerized lipid vesicles. Sonicated vesicles composed of approximately equal moles of dioleoylphosphatidylcholine (DOPC) (or 1-palmitoyl-2-oleoyl-phosphatidylcholine) and 1,2-bis(octadeca-2,4-dienoyl)phosphatidylcholine (DENPC) were photolyzed with 254-nm light to polymerize the DENPC and form domains of DOPC and polyDENPC in the vesicle wall. Rh-octyl glucoside (OG) micelles were slowly added to the vesicle suspension to give 15 mM OG (below the OG critical micelle concentration). The suspension was incubated and then dialyzed and purified on a sucrose gradient. Ultracentrifugation revealed a major Rh-lipid band which was harvested and found to contain a 100 +/- 10 phosphatidylcholine to rhodopsin ratio (Rh-polyDENPC/DOPC). The orientation of Rh in the membrane was determined by limited proteolytic digestion of Rh and by competitive inhibition of monoclonal antibody binding to solubilized disk membranes. Results were compared with control membranes of Rh-DOPC (1:43) prepared by insertion and Rh-phospholipid membranes prepared by detergent dialysis. Visual inspection of thermolysin proteolytic patterns of Rh indicates one major population cleaved at the carboxy terminus, as is found in disk membranes with an asymmetric arrangement of Rh. In contrast, proteolysis of a Rh-egg PC/PE (1:50/50) membrane (detergent dialysis) produced two Rh populations, which indicates a symmetric arrangement of Rh. The Rh-polyDENPC/DOPC (1:100) membranes were allowed to compete with solubilized, immobilized disk membranes for the monoclonal antibody R2-15 (specific for the amino-terminal region of Rh). They were intermediate between the asymmetric ROS disk membranes and the symmetric dialysis membranes in their ability to bind the R2-15 monoclonal antibody. The data indicate approximately 80% of the Rh's in Rh-polyDENPC/DOPC are in the normal orientation found in disks. These Rh-containing polymerized bilayer membranes demonstrated functionality as determined by chemical regeneration, kinetic spectrophotometry, and cGMP cascade reconstitution experiments. In the latter experiments the peripheral proteins, ROS G protein and PDE, bound with comparable efficiency to both the polymerized PC bilayers and egg PC bilayers. Thus the biocompatibility of the phosphatidylcholine membrane surface was maintained after polymerization of DENPC.     

Rod outer segment structure influences the apparent kinetic parameters of cyclic GMP phosphodiesterase

Cyclic GMP hydrolysis by the phosphodiesterase (PDE) of retinal rod outer segments (ROS) is a key amplification step in phototransduction. Definitive estimates of the turnover number, kcat, and of the Km are crucial to quantifying the amplification contributed by the PDE. Published estimates for these kinetic parameters vary widely; moreover, light-dependent changes in the Km of PDE have been reported. The experiments and analyses reported here account for most observed variations in apparent Km, and they lead to definitive estimates of the intrinsic kinetic parameters in amphibian rods. We first obtained a new and highly accurate estimate of the ratio of holo-PDE to rhodopsin in the amphibian ROS, 1:270. We then estimated the apparent kinetic parameters of light-activated PDE of suspensions of disrupted frog ROS whose structural integrity was systematically varied. In the most severely disrupted ROS preparation, we found Km = 95 microM and kcat = 4,400 cGMP.s-1. In suspensions of disc-stack fragments of greater integrity, the apparent Km increased to approximately 600 microM, though kcat remained unchanged. In contrast, the Km for cAMP was not shifted in the disc stack preparations. A theoretical analysis shows that the elevated apparent Km of suspensions of disc stacks can be explained as a consequence of diffusion with hydrolysis in the disc stack, which causes active PDEs nearer the center of the stack to be exposed to a lower concentration of cyclic GMP than PDEs at the disc stack rim. The analysis predicts our observation that the apparent Km for cGMP is elevated with no accompanying decrease in kcat. The analysis also predicts the lack of a Km shift for cAMP and the previously reported light dependence of the apparent Km for cGMP. We conclude that the intrinsic kinetic parameters of the PDE do not vary with light or structural integrity, and are those of the most severely disrupted disc stacks.