Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Transducin”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 181 records · Page 10Linked to original sources
PubMed1997

Transducin-alpha C-terminal peptide binding site consists of C-D and E-F loops of rhodopsin.

The binding of heterotrimeric GTP-binding proteins (G-proteins) to serpentine receptors involves several independent contacts. We have deduced the points of interaction between mutant bovine rhodopsins and alphat-(340-350), a peptide corresponding to the C terminus of the alpha subunit (alphat) of bovine retinal G-protein, transducin. Direct binding of alphat-(340-350) to rhodopsin stabilizes the activated metarhodopsin II state (M II), consequently uncoupling the rhodopsin-transducin interaction. This peptide action requires two segments on the cytoplasmic domain of rhodopsin: the Tyr136-Val137-Val138-Val139 sequence on the C-D loop and the Glu247-Lys248-Glu249-Val250-Thr251 sequence on the E-F loop. We propose that a tertiary interaction of these two loop regions forms a pocket for binding the alphat C terminus of the transducin during light transduction in vivo. In most G-proteins, the C termini of alpha subunits are important for interaction with receptors, and, in several serpentine receptors, regions similar to those in rhodopsin are essential for G-protein activation, indicating that the interaction described here may be a generally applicable mode of G-protein binding in signal transduction.

Amino Acid Sequence↗
PubMed1997

Functional coupling of a human retinal metabotropic glutamate receptor (hmGluR6) to bovine rod transducin and rat Go in an in vitro reconstitution system.

The cDNA encoding hmGluR6, appended with a 15-amino acid antibody epitope (1D4), was transiently transfected in COS-7 cells. The receptor was purified from COS cell membranes using an antibody affinity column. The purified receptor was then reconstituted into lipid vesicles, and its ability to activate either transducin, the rod photoreceptor-specific GTP-binding protein, or the alpha subunit of Go was assayed in vitro using a guanosine 5'-3-O-(thio)triphosphate binding assay. Activation of both transducin and Go was observed. The rate of Go activation was 18-fold greater than the rate of transducin activation. This indicates that the coupling of mGluR6 to Go is more efficient and suggests that Go may be involved in coupling to mGluR6 in ON-bipolar cells.

Amino Acid Sequence↗
PubMed1999

Roles of the transducin alpha-subunit alpha4-helix/alpha4-beta6 loop in the receptor and effector interactions.

The visual GTP-binding protein, transducin, couples light-activated rhodopsin (R*) with the effector enzyme, cGMP phosphodiesterase in vertebrate photoreceptor cells. The region corresponding to the alpha4-helix and alpha4-beta6 loop of the transducin alpha-subunit (Gtalpha) has been implicated in interactions with the receptor and the effector. Ala-scanning mutagenesis of the alpha4-beta6 region has been carried out to elucidate residues critical for the functions of transducin. The mutational analysis supports the role of the alpha4-beta6 loop in the R*-Gtalpha interface and suggests that the Gtalpha residues Arg310 and Asp311 are involved in the interaction with R*. These residues are likely to contribute to the specificity of the R* recognition. Contrary to the evidence previously obtained with synthetic peptides of Gtalpha, our data indicate that none of the alpha4-beta6 residues directly or significantly participate in the interaction with and activation of phosphodiesterase. However, Ile299, Phe303, and Leu306 form a network of interactions with the alpha3-helix of Gtalpha, which is critical for the ability of Gtalpha to undergo an activational conformational change. Thereby, Ile299, Phe303, and Leu306 play only an indirect role in the effector function of Gtalpha.

3',5'-Cyclic-GMP Phosphodiesterases↗
PubMed2004

Rhodopsin activation exposes a key hydrophobic binding site for the transducin alpha-subunit C terminus.

Conformational changes enable the photoreceptor rhodopsin to couple with and activate the G-protein transducin. Here we demonstrate a key interaction between these proteins occurs between the C terminus of the transducin alpha-subunit (G(Talpha)) and a hydrophobic cleft in the rhodopsin cytoplasmic face exposed during receptor activation. We mapped this interaction by labeling rhodopsin mutants with the fluorescent probe bimane and then assessed how binding of a peptide analogue of the G(Talpha) C terminus (containing a tryptophan quenching group) affected their fluorescence. From these and other assays, we conclude that the G(Talpha) C-terminal tail binds to the inner face of helix 6 in a retinal-linked manner. Further, we find that a "hydrophobic patch" comprising key residues in the exposed cleft is required for transducin binding/activation because it enhances the binding affinity for the G(Talpha) C-terminal tail, contributing up to 3 kcal/mol for this interaction. We speculate the hydrophobic interactions identified here may be important in other GPCR signaling systems, and our Trp/bimane fluorescence methodology may be generally useful for mapping sites of protein-protein interaction.

Animals↗
PubMed1996

Persistent activation of transducin by bleached rhodopsin in salamander rods.

The hydrolysis-resistant GTP analogue GTP-gamma-S was introduced into rods isolated from the retina of the salamander Ambystoma tigrinum to study the origin of the persistent excitation induced by intense bleaching illumination. Dialysis of a dark-adapted rod with a whole-cell patch pipette containing 2 mM GTP-gamma-S resulted in a gradual decrease in circulating current. If the rod was first bleached and its sensitivity allowed to stabilize for at least 30 min, then dialysis with GTP-gamma-S produced a much faster current decay. The circulating current could be restored by superfusion with the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine, suggesting that the decay in current originated from persistent excitation of the phosphodiesterase by transducin bound to GTP-gamma-S. We conclude that the persistent excitation which follows bleaching is likely to involve the GTP-binding protein transducin, which mediates the normal photoresponse. This observation suggests that a form of rhodopsin which persists long after bleaching can activate transducin much as does photoisomerized rhodopsin, although with considerably lower gain.

1-Methyl-3-isobutylxanthine↗
PubMed1988

Total synthesis and expression of a gene for the alpha-subunit of bovine rod outer segment guanine nucleotide-binding protein (transducin).

To facilitate structure-function studies by site-specific mutagenesis, we have synthesized a gene for the alpha-subunit of the bovine rod outer segment (ROS) guanine nucleotide-binding protein (transducin). The gene codes for the native amino acid sequence and contains, by design, 38 unique restriction sites which are uniformly spaced. This enables mutagenesis in any part of the gene by restriction fragment replacement. The gene is 1076 base pairs in length. It was constructed from 44 synthetic oligonucleotides which were joined enzymatically in vitro into four fragments which were cloned. The synthetic transducin gene and cDNA encoding transducin were expressed to similar levels in monkey kidney cells (COS-1) using a vector in which transcription was under the control of the adenovirus major late promoter.

Amino Acid Sequence↗
PubMed1986

Identification and characterization of the 35-kDa beta subunit of guanine-nucleotide-binding proteins by an antiserum raised against transducin.

Antisera were raised against the retinal guanine-nucleotide-binding protein (N-protein), transducin, purified from bovine rod outer segments. Sera obtained after repeated injections of antigen recognized all transducin subunits (alpha, beta and gamma). One antiserum, tested for cross-reactivity with non-retinal N-proteins, was found to cross-react with the beta subunits of the ubiquitously occurring N-proteins, Ns and Ni, but not with their respective alpha and gamma subunits. The antiserum also cross-reacted with the beta subunit of the recently identified N-protein, No, which has been found in high abundance in the central nervous system. These data support the similarity of the beta subunits of the N-proteins identified so far. Purification of N-proteins from porcine cerebral cortex without the use of activating ligands yielded fractions containing the isolated alpha subunit of No, free beta gamma complex, Ni, No and fractions containing both N-proteins in various proportions. The purity of the preparations was at least 80% as judged by Coomassie-blue-stained SDS gels. No pure Ns was obtained. Use of the transducin antibody during the course of the purification revealed that the beta subunits coeluted from a gel filtration column largely with the alpha subunits of Ni and No but were hardly detectable in fractions that were able to reconstitute Ns activity into membranes of an Ns-deficient cell line (S49 cyc- lymphoma cells). This indicates that in the central nervous system the concentrations of Ni and No are of magnitudes higher than that of Ns. Two-dimensional gel electrophoresis of N-proteins, purified from porcine cerebral cortex, resulted in the resolution of two major peptides in the 35-kDa region, which differed in their pI values and were identified as beta subunits by the use of the antiserum. Identical results were achieved using crude cholate extracts from membranes of the same tissue instead of purified proteins. The occurrence of different beta subunits may be explained by posttranslational N-protein modification.

Animals↗
PubMed1994

Activation of the retinal cGMP-specific phosphodiesterase by the GDP-loaded alpha-subunit of transducin.

The interaction of the GDP-bound form of the alpha-subunit of transducin (T alpha GDP) with the cGMP-specific phosphodiesterase, the effector enzyme in the visual system, has been studied. T alpha GDP is demonstrated to be able to activate the phosphodiesterase: (a) the basal activity in suspensions of dark-adapted retinal rod outer segments, examined in the absence of GTP, was found to be inhibited by binding of transducin to activated rhodopsin (Rh*) and by the complex of the beta- and gamma-subunits of transducin (T beta gamma); (b) purified T alpha GDP is able to activate phosphodiesterase in the presence of membranes; (c) no activation is obtained either with holotransducin (T alpha GDP T beta gamma) or with T alpha GDP in the presence of excess T beta gamma to prevent dissociation of TGDP. The maximal level of phosphodiesterase activation reached with T alpha GDP (about 1500 mol cGMP/mol phosphodiesterase-1.s-1) is similar to that obtained through the 'classical' activation by T alpha GTP whereas the apparent affinity of T alpha GDP for phosphodiesterase (Kd about 50 microM) is much lower than that of T alpha GTP. Our data suggest that GTP hydrolysis itself does not inactivate T alpha. The role of T beta gamma to sequester T alpha is therefore of critical importance for phosphodiesterase inactivation. Our results support observations on the regulation of adenylyl cyclase by G-proteins, which suggested the ability of the free alpha-subunits loaded with GDP to activate their effectors.

3',5'-Cyclic-GMP Phosphodiesterases↗
PubMed1986

Detection of alpha-transducin in retinal rods but not cones.

The distribution in chicken retina of the alpha subunit of transducin, the guanine nucleotide--binding protein that couples light-dependent activation of rhodopsin with activation of guanosine 3',5'-monophosphate phosphodiesterase, was determined with the aid of a specific antiserum. alpha-Transducin was found in rod photoreceptor cells but was not detected in cones. These results show that rods and cones differ with respect to alpha-transducin content and suggest that the processes of phototransduction may differ correspondingly in rods and cones.

Animals↗
PubMed1992

Heterogeneity of the retinal G-protein transducin from frog rod photoreceptors. Biochemical identification and characterization of new subunits.

Transducin, a retinal G-protein, has been shown to exist as heterotrimers of alpha (39,000), beta (36,000), and gamma (approximately 7,000) subunits. Blue Sepharose CL-6B column chromatography of a transducin preparation extracted with a metal-free, low salt buffer containing GTP showed three distinct alpha and two distinct beta gamma activities in frog (Rana catesbeiana) rod outer segment. The binding of a hydrolysis-resistant GTP analog in these alpha fractions was proportional to the amount of the M(r) 39,000 protein. The first alpha was eluted in a complex with an inhibitory subunit of cGMP phosphodiesterase, but alpha subunits in the second and the third fractions were not complexed with any proteins. Two-dimensional gel electrophoresis and characterization with regard to the interaction with the inhibitory subunit of cGMP phosphodiesterase suggested that the first and the second alpha s were the same protein; however, the third alpha showed different characters as follows. We designated alpha in the first two fractions as alpha 1, and alpha in the third fraction as alpha 2. Nonlinear regression analysis for the binding of a hydrolysis-resistant GTP analog to both alpha subunits revealed a single class of GTP binding sites with an apparent stoichiometry of 1 mol of GTP/mol of alpha. Compared with alpha 1, alpha 2 required larger amounts of rhodopsin and beta gamma for the binding of a hydrolysis-resistant GTP analog. alpha 2 also showed less binding with the inhibitory subunit of cGMP phosphodiesterase. Both alpha 1 and alpha 2 complexed with beta gamma or beta delta (described below) were substrates for pertussis toxin-dependent ADP-ribosylation. The protein profiles of two beta gamma fractions revealed that the main fraction was composed of a beta gamma complex; however, the second active fraction was composed of beta complexed with delta (M(r) 12,000). Compared with beta gamma, beta delta stimulated GTP binding to alpha 1 at approximately 10-fold higher concentration. Two-dimensional gel electrophoresis revealed five beta and two gamma isoforms in beta gamma. Only one beta isoform was present in beta delta. The diversity of transducin subunits may reflect different signaling pathways in visual signal transduction.

3',5'-Cyclic-GMP Phosphodiesterases↗
PubMed1992

Lack of association of Ca(2+)-calmodulin with the beta gamma-subunits of the photo-receptor G protein (transducin).

We previously reported that the beta gamma-subunit of transducin (T beta gamma) is composed of two components, T beta gamma-1 and T beta gamma-2 with distinctive gamma-subunits, T gamma-1 and T gamma-2, respectively. T beta gamma-2 enhances GTP binding to the alpha-subunit of transducin (T alpha) in the presence of a photobleaching intermediate of rhodopsin, while T beta gamma-1 is an inactive component with little enhancement ability (Fukada, Y., Ohguro, H., Saito, T., Yoshizawa, T., and Akino, T. (1989) J. Biol. Chem. 264: 5937-5943). To further elucidate the functional differences between T beta gamma-1 and T beta gamma-2, we examined the association of T beta gamma s with Ca(2+)-calmodulin, and the effect of Ca2+ on binding of GTP to T alpha in the presence of either T beta gamma-1 or T beta gamma-2. Ca2+ had no effect on the GTP binding activity of transducin and T beta gamma s could not associate with Ca(2+)-calmodulin, indicating that the relationship of T beta gamma with Ca(2+)-calmodulin of is different from that of the brain G protein.

Animals↗
PubMed2006

Mutation R238E in transducin-alpha yields a GTPase and effector-deficient, but not dominant-negative, G-protein alpha-subunit.

PURPOSE: Certain forms of inherited and light-induced retinal degenerations are believed to involve excessive phototransduction signaling. A dominant-negative mutant of the visual G-protein, transducin, would represent a major tool in designing potential therapeutical strategies for this group of visual diseases. We thought to further investigate a novel mutant of the transducin-alpha subunit, R238E, that was recently reported to be a dominant-negative inhibitor of the rhodopsin/transducin/PDE visual system. METHODS: The R238E substitution was introduced into a tranducin-like chimeric Gtalpha*-subunit. The nucleotide-bound state of the Gtalpha*R238E mutant was assessed using the trypsin-protection assay. The ability of the Gtalpha*R238E mutant to interact with Gtbetagamma, couple to photoexcited rhodopsin (R*), and undergo R*-stimulated guanine nucleotide exchange was examined by a GTPgammaS binding assay. The GTPase activity of the mutant Gtalpha* and its interaction with RGS proteins was characterized in the steady-state and single turnover measurements of GTP hydrolysis. A binding assay utilizing the fluorescently-labeled gamma-subunit of PDE6 (Pgamma) was employed to monitor the effector function of Gtalpha*R238E. RESULTS: The Gtalpha*R238E mutant bound GDP and was capable of the AlF4--induced activational conformational change. The capacity of Gtalpha*R238E to couple to R* in the presence of Gtbetagamma was similar to that of Gtalpha*. However, the mutant GTPase activity was markedly impaired. This defect was further exacerbated by the diminished interactions of Gtalpha*R238E with the GAP proteins, RGS9 and RGS16. Another consequence of the mutation was the reduction in Gtalpha*R238E's affinity for Pgamma. CONCLUSIONS: Transducin mutant Gtalpha*R238E exists in a nucleotide-bound state and is fully capable of activational coupling to R*. This mutation results in a significant impairment of Gtalpha*'s ability to hydrolyze GTP and interact with the inhibitory subunit of PDE6. This phenotype is entirely inconsistent with that of a dominant-negative inhibitor as recently reported.

Animals↗
PubMed1990

Analysis of genes coding for S-antigen, interstitial retinol binding protein, and the alpha-subunit of cone transducin in patients with retinitis pigmentosa.

We screened 526 unrelated patients with autosomal dominant, autosomal recessive, or simplex retinitis pigmentosa for evidence of mutations of the genes encoding S-antigen (S-Ag), interstitial retinol binding protein (IRBP), and the alpha-subunit of cone-specific transducin. Restriction fragment length polymorphisms (RFLPs) were identified at each of these loci. Within each set of patients with a particular genetic type of retinitis pigmentosa, RFLP alleles at each of these loci showed no departure from Hardy-Weinberg equilibrium. No gene deletions or rearrangements could be detected in any patient. Furthermore, in each of six pedigrees (one autosomal dominant, one autosomal recessive, three Usher's syndrome type I, and one Laurence-Moon-Bardet-Biedl syndrome) there was no co-segregation of the disease with alleles determined by RFLPs at the locus for S-antigen. At the IRBP locus, lack of co-segregation was seen in one autosomal dominant, two autosomal recessive, and three Usher's syndrome type I pedigrees. Finally, one pedigree with autosomal recessive retinitis pigmentosa showed no co-segregation of the disease with alleles at the locus for the alpha-subunit of the cone-specific transducin. These data support the idea that the genes coding for S-Ag, IRBP, and the alpha-subunit of the cone-specific transducin do not play an etiologic role in the families with retinitis pigmentosa so far studied.

Alleles↗
PubMed1990

Interactions between the subunits of transducin and cyclic GMP phosphodiesterase in Rana catesbiana rod photoreceptors.

In bullfrog (Rana catesbiana) rods the activity of cyclic GMP (cGMP) phosphodiesterase was stimulated 10 times by washing disc membranes with an isotonic, GTP-containing buffer. This stimulation was maintained following hydrolysis of GTP and after removal of guanine nucleotides. At least 60-70% of the inhibitory gamma subunit of cGMP phosphodiesterase (P gamma) was physically released from membranes by these washing procedures. When cGMP phosphodiesterase was activated by a hydrolysis-resistant GTP analogue, P gamma was found in the supernatant complexed with the transducin alpha subunit (T alpha) using three chromatography systems. When GTP was used to activate cGMP phosphodiesterase, P gamma was also found in the supernatant complexed with GDP.T alpha. This complex was also isolated using the same three chromatography systems, indicating that P gamma remained tightly bound to T alpha even after bound GTP was hydrolyzed. Interaction with the beta,gamma subunits of transducin, which remained associated with disc membranes, was required for the release of P gamma from the GDP.T alpha complex, which resulted in the deactivation of active cGMP phosphodiesterase. We conclude that during activation of cGMP phosphodiesterase, P gamma is complexed with T alpha (both GTP and GDP forms) in the supernatant and that, following GTP hydrolysis, beta,gamma subunits of transducin are necessary for the release of P gamma from the complex and the resulting inactivation of cGMP phosphodiesterase in frog photoreceptors.

3',5'-Cyclic-GMP Phosphodiesterases↗
PubMed1986

Structural and functional characterization of guanyl nucleotide-binding proteins using monoclonal antibodies to the alpha-subunit of transducin.

Transducin, the GTP-binding protein of the retinal light-sensitive phosphodiesterase system, and Gs and Gi, regulatory proteins of the hormone-sensitive adenylate cyclase, are members of a family of guanyl nucleotide-binding proteins termed G proteins that are important in signal transduction. To probe relationships within this family of G proteins, monoclonal antibodies were prepared against the alpha-subunit of bovine transducin (T alpha). Three of four monoclonal antibodies were specific for T alpha and did not cross-react with other G proteins. One, MAB1, cross-reacted strongly with the alpha-subunit of Gi (Gi alpha) purified from rabbit liver and, to a lesser extent, with the alpha-subunit of Go (Go alpha) purified from bovine brain and the proto-oncogene product H-ras p21. All four monoclonal antibodies recognized epitopes on a 23-kDa tryptic peptide fragment of T alpha which is derived from the N-proximal region. The three monoclonal antibodies that recognized only T alpha inhibited rhodopsin-stimulated GTP binding and hydrolysis by transducin, whereas MAB1 had no significant effect in these assays. These studies demonstrate that, within the 23-kDa tryptic peptide of T alpha, there is a domain(s) unique to T alpha that is involved in GTP binding and hydrolysis and another domain which is highly conserved in T alpha and to a lesser extent in other G proteins. Prior studies have identified regions involved in nucleotide binding and hydrolysis that are homologous in all G proteins. The observations reported here are consistent with the conclusion that the G proteins may have in addition unique regions involved in these functions.

Amino Acid Sequence↗
PubMed1985

Mechanism of inhibition of transducin GTPase activity by fluoride and aluminum.

Transducin, the guanyl nucleotide-binding protein of the retinal light-activated cGMP phosphodiesterase system, is structurally and functionally similar to the inhibitory and stimulatory guanyl nucleotide-binding proteins, Gi and Gs, of the adenylate cyclase complex. All are heterotrimers composed of alpha, beta, and gamma subunits. Gs and Gi can be activated by NaF with AlCl3 as well as by agonists acting through specific receptors. The effects of NaF and AlCl3 on transducin were investigated in a reconstituted system consisting of the purified subunits of transducin (T alpha, T beta, gamma) and rhodopsin. NaF noncompetitively inhibited the GTPase activity of T alpha in a concentration- and time-dependent manner. Inhibition by NaF was enhanced synergistically by AlCl3 which alone only slightly inhibited GTPase activity. None of the other anions tested reproduced the effect of fluoride. Fluoride inhibited [3H]guanosine 5'-(beta, gamma-imido)triphosphate binding to T alpha and release of bound GDP. The ADP-ribosylation of T alpha by pertussis toxin and binding of T alpha to rhodopsin, both of which are enhanced in the presence of T beta gamma, were inhibited by NaF and AlCl3. These findings are consistent with the hypothesis that fluoride enhances the dissociation of T alpha from T beta gamma, resulting in the inhibition of GTP-GDP exchange, and therefore, GTP hydrolysis.

Adenosine Diphosphate Ribose↗
PubMed1988

The intrinsic fluorescence of the alpha subunit of transducin. Measurement of receptor-dependent guanine nucleotide exchange.

We have made use of the enhancement of the intrinsic fluorescence of the alpha subunit of transducin (alpha T), which accompanies guanine nucleotide exchange, to follow the reconstituted interactions between pure rhodopsin and pure transducin in phospholipid vesicles. When the pure alpha T.GDP complex is added to lipid vesicles containing rhodopsin and the beta gamma T complex, a light- and guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S)-dependent enhancement of the fluorescence emission of alpha T is observed. When GTP is substituted for GTP gamma S, a similar enhancement of the intrinsic fluorescence of alpha T occurs; however, this enhancement is transient and precedes a fluorescence decay which is complete in 2-5 min. The fact that the fluorescence decay is specifically induced by GTP and is not observed either with nonhydrolyzable GTP analogs or with NaF (plus AlCl3) indicates that the decay represents GTP hydrolysis in alpha T. The dose-response profiles for the effects of the beta gamma T complex on the rate and extent of the GTP gamma S-stimulated fluorescence enhancement of alpha T have also been examined. The addition of relatively low levels of beta gamma T to these reconstituted systems can promote the GTP gamma S-stimulated enhancement of the fluorescence of multiple alpha T subunits with half-maximal enhancement occurring at alpha T:beta gamma T ratios of 150:1. These findings are consistent with earlier suggestions (Fung, B. K.-K. (1983) J. Biol. Chem. 258, 10495-10502) that the beta gamma T subunit dissociates from alpha T as a result of the GDP-GTP exchange reaction and thus can act catalytically to promote the activation of a number of inactive alpha T species. However, the dependence of the rate of the GTP gamma S-stimulated fluorescence enhancement on beta gamma T is complex and cannot be explained adequately by simple models where alpha T-beta gamma T interactions (or rhodopsin-transducin interactions) are rate-limiting for the rhodopsin-stimulated activation of the alpha T subunits. Overall, the results reported here demonstrate that fluorescence spectroscopy can be used to monitor directly a receptor-catalyzed activation-deactivation cycle of a GTP-binding protein within a lipid milieu.

Animals↗
PubMed1987

Transducin interactions with rhodopsin. Evidence for positive cooperative behavior.

Transducin and rhodopsin belong to a family of guanine nucleotide-binding (G) protein-coupled receptor systems that provide signal transduction mechanisms resulting in numerous metabolic responses in a variety of cell types. A simple, direct binding assay has been developed to investigate the molecular interactions between transducin and rhodopsin. The binding curves generated by these studies are sigmoidal, indicating an allosteric response. The Scatchard plots of this data display an asymptotic, bell-shaped character representative of the positive cooperative behavior. A Hill coefficient, nH = 1.92, was determined and found to be in close agreement with previous kinetic studies of allosterism described for rhodopsin's catalytic mechanism (Wessling-Resnick, M., and Johnson G.L. (1987) J. Biol. Chem. 262, 3697-3705). The value for Kd app was determined to be 0.05 microM. Bmax values obtained from the binding studies suggest that oligomeric complexes of rhodopsin may be involved in interactions with transducin to form multiple high affinity binding sites for the G protein. The positive cooperative behavior demonstrated in this investigation can provide insight into the molecular basis for regulation of other G protein-coupled receptor systems.

Allosteric Regulation↗