An interface of interaction between photoreceptor cGMP phosphodiesterase catalytic subunits and inhibitory gamma subunits

A truncated form of rod photoreceptor PDE6 β-subunit causes autosomal dominant congenital stationary night blindness by interfering with the inhibitory activity of the γ-subunit

Autosomal dominant congenital stationary night blindness (adCSNB) is caused by mutations in three genes of the rod phototransduction cascade, rhodopsin (RHO), transducin α-subunit (GNAT1), and cGMP phosphodiesterase type 6 β-subunit (PDE6B). In most cases, the constitutive activation of the phototransduction cascade is a prerequisite to cause adCSNB. The unique adCSNB-associated PDE6B mutation found in the Rambusch pedigree, the substitution p.His258Asn, leads to rod photoreceptors desensitization. Here, we report a three-generation French family with adCSNB harboring a novel PDE6B mutation, the duplication, c.928-9_940dup resulting in a tyrosine to cysteine substitution at codon 314, a frameshift, and a premature termination (p.Tyr314Cysfs*50). To understand the mechanism of the PDE6β1-314fs*50 mutant, we examined the properties of its PDE6-specific portion, PDE6β1-313. We found that PDE6β1-313 maintains the ability to bind noncatalytic cGMP and the inhibitory γ-subunit (Pγ), and interferes with the inhibition of normal PDE6αβ catalytic subunits by Pγ. Moreover, both truncated forms of the PDE6β protein, PDE6β1-313 and PDE6β1-314fs*50 expressed in rods of transgenic X. laevis are targeted to the phototransduction compartment. We hypothesize that in affected family members the p.Tyr314Cysfs*50 change results in the production of the truncated protein, which binds Pγ and causes constitutive activation of the phototransduction thus leading to the absence of rod adaptation.

Effect of the ILE86TER mutation in the γ subunit of cGMP phosphodiesterase (PDE6) on rod photoreceptor signaling

The light-dependent decrease in cyclic guanosine monophosphate (cGMP) in the rod outer segment is produced by a phosphodiesterase (PDE6), consisting of catalytic α and β subunits and two inhibitory γ subunits. The molecular mechanism of PDE6γ regulation of the catalytic subunits is uncertain. To study this mechanism in vivo, we introduced a modified Pde6g gene for PDE6γ into a line of Pde6g(tm1)/Pde6g(tm1) mice that do not express PDE6γ. The resulting ILE86TER mice have a PDE6γ that lacks the two final carboxyl-terminal Ile(86) and Ile(87) residues, a mutation previously shown in vitro to reduce inhibition by PDE6γ. ILE86TER rods showed a decreased sensitivity and rate of activation, probably the result of a decreased level of expression of PDE6 in ILE86TER rods. More importantly, they showed a decreased rate of decay of the photoresponse, consistent with decreased inhibition of PDE6 α and β by PDE6γ. Furthermore, ILE86TER rods had a higher rate of spontaneous activation of PDE6 than WT rods. Circulating current in ILE86TER rods that also lacked both guanylyl cyclase activating proteins (GCAPs) could be increased several fold by perfusion with 100μM of the PDE6 inhibitor 3-isobutyl-1-methylxanthine (IBMX), consistent with a higher rate of dark PDE6 activity in the mutant photoreceptors. In contrast, IBMX had little effect on the circulating current of WT rods, unlike previous results from amphibians. Our results show for the first time that the Ile(86) and Ile(87) residues are necessary for normal inhibition of PDE6 catalytic activity in vivo, and that increased basal activity of PDE can be partially compensated by GCAP-dependent regulation of guanylyl cyclase.

Structural requirements of the photoreceptor phosphodiesterase gamma-subunit for inhibition of rod PDE6 holoenzyme and for its activation by transducin

The central enzyme of the visual transduction cascade, cGMP phosphodiesterase (PDE6), is regulated by its gamma-subunit (Pgamma), whose inhibitory constraint is released upon binding of activated transducin. It is generally believed that the last four or five C-terminal amino acid residues of Pgamma are responsible for blocking catalysis. In this paper, we showed that the last 10 C-terminal residues (Pgamma78-87) are the minimum required to completely block catalysis. The kinetic mechanism of inhibition by the Pgamma C terminus depends on which substrate is undergoing catalysis. We also discovered a second mechanism of Pgamma inhibition that does not require this C-terminal region and that is capable of inhibiting up to 80% of the maximal cGMP hydrolytic rate. Furthermore, amino acids 63-70 and/or the intact alpha2 helix of Pgamma stabilize binding of C-terminal Pgamma peptides by 100-fold. When PDE6 catalytic subunits were reconstituted with portions of the Pgamma molecule and tested for activation by transducin, we found that the C-terminal region (Pgamma63-87) by itself could not be displaced but that transducin could relieve inhibition of certain Pgamma truncation mutants. Our results are consistent with two distinct mechanisms of Pgamma inhibition of PDE6. One involves direct interaction of the C-terminal residues with the catalytic site. A second regulatory mechanism may involve binding of other regions of Pgamma to the catalytic domain, thereby allosterically reducing the catalytic rate. Transducin activation of PDE6 appears to require interaction with both the C terminus and other regions of Pgamma to effectively relieve its inhibitory constraint.

The retinal cGMP phosphodiesterase gamma-subunit - a chameleon

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

The Inhibitory γ Subunit of the Rod cGMP Phosphodiesterase Binds the Catalytic Subunits in an Extended Linear Structure

The unique feature of rod photoreceptor cGMP phosphodiesterase (PDE6) is the presence of inhibitory subunits (Pgamma), which interact with the catalytic heterodimer (Palphabeta) to regulate its activity. This uniqueness results in an extremely high sensitivity and sophisticated modulations of rod visual signaling where the Pgamma/Palphabeta interactions play a critical role. The quaternary organization of the alphabetagammagamma heterotetramer is poorly understood and contradictory patterns of interaction have been previously suggested. Here we provide evidence that supports a specific interaction, by systematically and differentially analyzing the Pgamma-binding regions on Palpha and Pbeta through photolabel transfer from various Pgamma positions throughout the entire molecule. The Pgamma N-terminal Val16-Phe30 region was found to interact with the Palphabeta GAFa domain, whereas its C terminus (Phe73-Ile87) interacted with the Palphabeta catalytic domain. The interactions of Pgamma with these two domains were bridged by its central Ser40-Phe50 region through interactions with GAFb and the linker between GAFb and the catalytic domain, indicating a linear and extended interaction between Pgamma and Palphabeta. Furthermore, a photocross-linked product alphabetagamma(gamma) was specifically generated by the double derivatized Pgamma, in which one photoprobe was located in the polycationic region and the other in the C terminus. Taken together the evidence supports the conclusion that each Pgamma molecule binds Palphabeta in an extended linear interaction and may even interact with both Palpha and Pbeta simultaneously.

Inhibitory effects of furoquinoline alkaloids fromMelicope confusa andDictamnus albus against human phosphodiesterase 5 (hPDE5A)in vitro

Eight furoquinoline alkaloids were purified from two plants belonging to the Rutaceae family. Kokusaginine, skimmianine, evolitrine, and confusameline were purified from Melicope confusa, and haplopine, robustine, dictamine, and gamma-fagarine from Dictamnus albus. In this study, the eight furoquinoline alkaloids were examined for inhibitory potency against human phosphodiesterase 5 (hPDE5A) in vitro. DNA encoding the catalytic domain of human PDE5A was amplified from the mRNA of T24 cells by RT-PCR and was fused to GST in an expression vector. GST-tagged PDE5A was then purified by glutathione affinity chromatography and used in inhibition assays. Of the eight alkaloids, gamma-fagarine was the most potent inhibitor of PDE5A, and its single methoxy group at the C-8 position was shown to be critical for inhibitory activity. These results clearly illustrate the relationship between PDE5A inhibition and the methoxy group position in furoquinoline alkaloids.

Direct interaction of the inhibitory gamma-subunit of Rod cGMP phosphodiesterase (PDE6) with the PDE6 GAFa domains

Retinal rod and cone cGMP phosphodiesterases (PDE6 family) function as the effector enzyme in the vertebrate visual transduction cascade. The activity of PDE6 catalytic subunits is controlled by the Pgamma-subunits. In addition to the inhibition of cGMP hydrolysis at the catalytic sites, Pgamma is known to stimulate a noncatalytic binding of cGMP to the regulatory GAFa-GAFb domains of PDE6. The latter role of Pgamma has been attributed to its polycationic region. To elucidate the structural basis for the regulation of cGMP binding to the GAF domains of PDE6, a photoexcitable peptide probe corresponding to the polycationic region of Pgamma, Pgamma-21-45, was specifically cross-linked to rod PDE6alphabeta. The site of Pgamma-21-45 cross-linking was localized to Met138Gly139 within the PDE6alpha GAFa domain using mass spectrometric analysis. Chimeras between PDE5 and cone PDE6alpha', containing GAFa and/or GAFb domains of PDE6alpha' have been generated to probe a potential role of the GAFb domains in binding to Pgamma. Analysis of the inhibition of the PDE5/PDE6alpha' chimeras by Pgamma supported the role of PDE6 GAFa but not GAFb domains in the interaction with Pgamma. Our results suggest that a direct binding of the polycationic region of Pgamma to the GAFa domains of PDE6 may lead to a stabilization of the noncatalytic cGMP-binding sites.

The positive role of the carboxyl terminus of the gamma subunit of retinal cGMP-phosphodiesterase in maintaining phosphodiesterase activity in vivo

The inhibitory rod cyclic GMP-phosphodiesterase gamma subunit, PDEgamma, is a key component of the photoresponse and is required to support rod integrity. Pdeg(tm1)/Pdeg(tm1) mice that lack PDEgamma due to a targeted disruption of the gene encoding PDEgamma, (Pdeg) suffer from a very rapid and severe photoreceptor degeneration. Previously, deletions in the carboxyl-terminal domain of PDEgamma blocked its ability to inhibit trypsin-activated PDE activity, in vitro. In other words, these mutations eliminated PDEgamma's control on the catalytic activity of PDEalpha and PDEbeta. To study the in vivo effects resulting from the deletion of the last seven amino acids of the PDEgamma carboxyl terminal, this PDEgamma allele (Del7C) was introduced as a transgene Pdeg(tm1)/Pdeg(tm1) mice. These animals could only synthesize transgenic mutant PDEgamma. The mutant retinas were expected to display a higher basal level of PDE activity and lower cGMP levels in light and darkness than the PDEgamma knockout mice, which would allow the rescue of their photoreceptors. Instead, our results showed that the Del7C transgene could not complement the Pdeg(tm1)/Pdeg(tm1) mutant for photoreceptor survival. In fact, animals carrying the Del7C transgene have low PDE activity as well as reduced PDEalpha and PDEbeta content.

The Catalytic and GAF Domains of the Rod cGMP Phosphodiesterase (PDE6) Heterodimer Are Regulated by Distinct Regions of Its Inhibitory γ Subunit

The central effector of visual transduction in retinal rod photoreceptors, cGMP phosphodiesterase (PDE6), is a catalytic heterodimer (alphabeta) to which low molecular weight inhibitory gamma subunits bind to form the nonactivated PDE holoenzyme (alphabetagamma(2)). Although it is known that gamma binds tightly to alphabeta, the binding affinity for each gamma subunit to alphabeta, the domains on gamma that interact with alphabeta, and the allosteric interactions between gamma and the regulatory and catalytic regions on alphabeta are not well understood. We show here that the gamma subunit binds to two distinct sites on the catalytic alphabeta dimer (K(D)(1) < 1 pm, K(D)(2) = 3 pm) when the regulatory GAF domains of bovine rod PDE6 are occupied by cGMP. Binding heterogeneity of gamma to alphabeta is absent when cAMP occupies the noncatalytic sites. Two major domains on gamma can interact independently with alphabeta with the N-terminal half of gamma binding with 50-fold greater affinity than its C-terminal, inhibitory region. The N-terminal half of gamma is responsible for the positive cooperativity between gamma and cGMP binding sites on alphabeta but has no effect on catalytic activity. Using synthetic peptides, we identified regions of the amino acid sequence of gamma that bind to alphabeta, restore high affinity cGMP binding to low affinity noncatalytic sites, and retard cGMP exchange with both noncatalytic sites. Subunit heterogeneity, multiple sites of gamma interaction with alphabeta, and positive cooperativity of gamma with the GAF domains are all likely to contribute to precisely controlling the activation and inactivation kinetics of PDE6 during visual transduction in rod photoreceptors.

In vivo studies of the gamma subunit of retinal cGMP-phophodiesterase with a substitution of tyrosine-84

The inhibitory rod cGMP phosphodiesterase gamma subunit (PDEgamma) is a major component of the photoresponse and is required to support rod integrity. Pdeg(tm1)/Pdeg(tm1) mice (which lack PDEgamma owing to a targeted disruption of the Pdeg gene) suffer from a very rapid and severe photoreceptor degeneration. The Y84G (Tyr(84)-->Gly) allele of PDEgamma has previously been shown in experiments carried out in vitro to reduce the regulatory control of the PDE catalytic core (PDEalphabeta) exerted by the wild-type gamma subunit. To determine the effects of this mutation on in vivo function, the murine opsin promoter was used to direct expression to the photoreceptors of +/Pdeg(tm1) mice of a mutant Y84G and a wild-type PDEgamma control transgene. The transgenic mice were crossed with Pdeg(tm1)/Pdeg(tm1) mice to generate animals able to synthesize only the transgenic PDEgamma. Our results showed that wild-type PDEgamma and Y84G transgenes could complement the Pdeg(tm1)/Pdeg(tm1) mutant for photoreceptor survival. The mutation caused a significant biochemical defect in PDE activation by transducin. However, the Y84G mutation did not fully eliminate the control of PDEgamma on the PDE catalytic core in vivo; the expression of the mutant subunit was associated with only a 10-fold reduction in the amplitude of the a-wave and a 1.5-fold decrease in the b-wave of the corneal electroretinogram. Unexpectedly, the mutation caused a much 'milder' phenotype in vivo than was predicted from the biochemical assays in vitro.

Mechanism of transducin activation of frog rod photoreceptor phosphodiesterase. Allosteric interactiona between the inhibitory gamma subunit and the noncatalytic cGMP-binding sites

The rod photoreceptor phosphodiesterase (PDE) is unique among all known vertebrate PDE families for several reasons. It is a catalytic heterodimer (alphabeta); it is directly activated by a G-protein, transducin; and its active sites are regulated by inhibitory gamma subunits. Rod PDE binds cGMP at two noncatalytic sites on the alphabeta dimer, but their function is unclear. We show that transducin activation of frog rod PDE introduces functional heterogeneity to both the noncatalytic and catalytic sites. Upon PDE activation, one noncatalytic site is converted from a high affinity to low affinity state, whereas the second binding site undergoes modest decreases in binding. Addition of gamma to transducin-activated PDE can restore high affinity binding as well as reducing cGMP exchange kinetics at both sites. A strong correlation exists between cGMP binding and gamma binding to activated PDE; dissociation of bound cGMP accompanies gamma dissociation from PDE, whereas addition of either cGMP or gamma to alphabeta dimers can restore high affinity binding of the other molecule. At the active site, transducin can activate PDE to about one-half the turnover number for catalytic alphabeta dimers completely lacking bound gamma subunit. These results suggest a mechanism in which transducin interacts primarily with one PDE catalytic subunit, releasing its full catalytic activity as well as inducing rapid cGMP dissociation from one noncatalytic site. The state of occupancy of the noncatalytic sites on PDE determines whether gamma remains bound to activated PDE or dissociates from the holoenzyme, and may be relevant to light adaptation in photoreceptor cells.

Cyclic nucleotide phosphodiesterases: relating structure and function

Cyclic nucleotide phosphodiesterases (PDEs) comprise a superfamily of metallophosphohydrolases that specifically cleave the 3',5'-cyclic phosphate moiety of cAMP and/or cGMP to produce the corresponding 5'-nucleotide. PDEs are critical determinants for modulation of cellular levels of cAMP and/or cGMP by many stimuli. Eleven families of PDEs with varying selectivities for cAMP or cGMP have been identified in mammalian tissues. Within these families, multiple isoforms are expressed either as products of different genes or as products of the same gene through alternative splicing. Regulation of PDEs is important for controlling myriad physiological functions, including the visual response, smooth muscle relaxation, platelet aggregation, fluid homeostasis, immune responses, and cardiac contractility. PDEs are critically involved in feedback control of cellular cAMP and cGMP levels. Activities of the various PDEs are highly regulated by a panoply of processes, including phosphorylation events, interaction with small molecules such as cGMP or phosphatidic acid, subcellular localization, and association with specific protein partners. The PDE superfamily continues to be a major target for pharmacological intervention in a number of medically important maladies.

The molecular biology of cyclic nucleotide phosphodiesterases

Recent progress in the field of cyclic nucleotides has shown that a large array of closely related proteins is involved in each step of the signal transduction cascade. Nine families of adenylyl cyclases catalyze the synthesis of the second messenger cAMP, and protein kinases A, the intracellular effectors of cAMP, are composed of four regulatory and three catalytic subunits. A comparable heterogeneity has been discovered for the enzymes involved in the inactivation of cyclic nucleotide signaling. In mammals, 19 different genes encode the cyclic nucleotide phosphodiesterases (PDEs), the enzymes that hydrolyze and inactivate cAMP and cGMP. This is only an initial level of complexity, because each PDE gene contains several distinct transcriptional units that give rise to proteins with subtle structural differences, bringing the number of the PDE proteins close to 50. The molecular biology of PDEs in Drosophila and Dictyostelium has shed some light on the role of PDE diversity in signaling and development. However, much needs to be done to understand the exact function of these enzymes, particularly during mammalian development and cell differentiation. With the identification and mapping of regulatory and targeting domains of the PDEs, modularity of the PDE structure is becoming an established tenet in the PDE field. The use of different transcriptional units and exon splicing of a single PDE gene generates proteins with different regulatory domains joined to a common catalytic domain, therefore expanding the array of isoforms with subtle differences in properties and sensitivities to different signals. The physiological context in which these different isoforms function is still largely unknown and undoubtedly will be a major area of expansion in the years to come.

Activation of the cAMP-specific phosphodiesterase PDE4D3 by phosphorylation. Identification and function of an inhibitory domain

Splicing variants of type 4 phosphodiesterases (PDE4) are regulated by phosphorylation. In these proteins, a conserved region is located between the amino-terminal domain, which is the target for phosphorylation, and the catalytic domain. Previous studies have indicated that nested deletions encompassing this region cause an increase in catalytic activity, suggesting this domain exerts an inhibitory constraint on catalysis. Here, we have further investigated the presence and function of this domain. A time-dependent increase in hydrolytic activity was observed when PDE4D3 from FRTL-5 cells was incubated with the endoproteinase Lys-C. The activation was abolished by protease inhibitors and was absent when a phosphorylated enzyme was used. Western blot analysis with PDE4D-specific antibodies indicated the Lys-C treatment separates the catalytic domain of PDE4D3 from the inhibitory domain. Incubation with antibodies recognizing an epitope within this domain caused a 3- to 4-fold increase in activity of native or recombinant PDE4D3. Again, PDE activation by these antibodies had properties similar to, and not additive with, the activation by protein kinase A phosphorylation. An interaction between the inhibitory domain and both regulatory and catalytic domains of PDE4D3 was detected by the yeast two-hybrid system. Mutations of Ser54 to Ala in the regulatory domain decreased or abolished this interaction, whereas mutations of Ser54 to the negatively charged Asp strengthened it. These data strongly support the hypothesis that an inhibitory domain is present in PDE4D and that phosphorylation of the regulatory domain causes activation of the enzyme by modulating the interaction between inhibitory and catalytic domains.

Delineation of two functionally distinct gammaPDE binding sites on the bovine retinal cGMP phosphodiesterase by a mutant gammaPDE subunit

The gamma subunit of the retinal cGMP phosphodiesterase (gammaPDE) acts as an inhibitor of phosphodiesterase (PDE) catalytic activity and mediates enzyme regulation by the alpha subunit of the GTP-binding protein transducin (alphaT). In this work, we describe a full length, doubly point-mutated gamma subunit, C68S, Y84C gammaPDE, which binds to PDE with increased affinity but has a decreased ability to inhibit the enzyme. Fluorescence studies monitoring the competition between wild-type gammaPDE and the C68S, Y84C gammaPDE mutant suggest that the mutant gammaPDE binds with high affinity to only half of the total sites occupied by wild-type gammaPDE. Competition studies between wild-type gammaPDE and the mutant further suggest that the wild-type protein is able to fully inhibit PDE activity even when the mutant gammaPDE occupies its high-affinity binding site on PDE. Taken together, our findings are consistent with a model in which there are two distinguishable binding sites for gammaPDE on the PDE enzyme but that only one of the two sites mediates PDE inhibition.

Probing domain functions of chimeric PDE6alpha'/PDE5 cGMP-phosphodiesterase

Chimeric cGMP phosphodiesterases (PDEs) have been constructed using components of the cGMP-binding PDE (PDE5) and cone photoreceptor phosphodiesterase (PDE6alpha') in order to study structure and function of the photoreceptor enzyme. A fully functional chimeric PDE6alpha'/PDE5 enzyme containing the PDE6alpha' noncatalytic cGMP-binding sites, and the PDE5 catalytic domain has been efficiently expressed in the baculovirus/High Five cell system. The catalytic properties of this chimera were practically indistinguishable from those of PDE5, whereas the noncatalytic cGMP binding was similar to that of native purified PDE6alpha'. The inhibitory gamma subunit of PDE6 (Pgamma) enhanced the affinity of cGMP binding at noncatalytic sites of native PDE6alpha' by approximately 6-fold. The polycationic region of Pgamma, Pgamma-24-45, was mainly responsible for this effect, while the inhibitory domain of Pgamma, Pgamma-63-87, was ineffective. On the contrary, Pgamma failed to inhibit catalytic activity of the chimeric PDE6alpha'/PDE5 or to modulate its noncatalytic cGMP binding. Substitutions of Ala residues for the conserved Asn, Asn193 or Asn402, in the two N(K/R)XD-like motifs of the chimeric PDE noncatalytic cGMP-binding sites, each led to a loss of the noncatalytic cGMP binding. Our data suggest that both putative noncatalytic sites of PDE6alpha' are important for binding of cGMP, and that the two binding sites are coupled. Furthermore, mutation Asn402 --> Ala resulted in an approximately 10-fold increase of the Km value for cGMP, indicating that occupation of the noncatalytic cGMP- binding sites of PDE6alpha' may regulate catalytic properties of the enzyme.

The gamma-subunit of the rod photoreceptor cGMP-binding cGMP-specific PDE is expressed in mouse lung

The type 6 phosphodiesterase (PDE-6) from retinal rod photoreceptors is an alpha beta gamma 2 heterotetramer. The alpha- and beta-subunits contain catalytic sites for cGMP hydrolysis, whereas the gamma-subunits (P gamma) serve as a protein inhibitor of the enzyme. P gamma is believed to be expressed only in photoreceptors. Using RT-PCR, we have amplified the complete coding sequence for P gamma from mouse lung RNA. The expression of P gamma in this tissue may be related to its ability to interact the type 5 phosphodiesterase (PDE-5), which is the predominant cGMP binding protein in lung. We therefore suggest that P gamma may have a wider signaling role in mammalian cells than previously appreciated.

Probing functional interfaces of rod PDE gamma-subunit using scanning fluorescent labeling

In the dark, the activity of the rod cGMP phosphodiesterase (PDE) catalytic alpha- and beta-subunits (P alpha beta) is inhibited by two gamma-subunits (P gamma). On light stimulation of the photoreceptor cells, the GTP-bound alpha-subunit of visual G-protein transducin (GtaGTP) displaces the P gamma-subunits from their inhibitory sites on P alpha beta, leading to the effect or enzyme activation. We designed a number of P gamma mutants, each with a single cysteine residue evenly distributed at a different position along the P gamma polypeptide chain. These cysteine residues served as sites for the introduction of the environmentally sensitive fluorescent probe, 3-(bromoacetyl)-7-diethyl aminocoumarin (BC). Analysis of the interactions of P alpha beta and Gta with the fluorescently labeled P gamma mutants suggests two distinct functional interfaces of P gamma. The P alpha beta/P gamma interface is formed essentially by the C-terminus of P gamma and by the N-terminal portion of the P gamma polycationic region, P gamma-24-45, whereas the P gamma/Gta interface includes the C-terminal portion of P gamma-24-45 and the region surrounding P gamma Cys68. Such functional organization of P gamma may represent an important element for the PDE activation mechanism during transduction of visual signals.

Mutational analysis of the Asn residue essential for RGS protein binding to G-proteins

Members of the RGS family serve as GTPase-activating proteins (GAPs) for heterotrimeric G-proteins and negatively regulate signaling via G-protein-coupled receptors. The recently resolved crystal structure of RGS4 bound to Gialpha1 suggests two potential mechanisms for the GAP activity of RGS proteins as follows: stabilization of the Gialpha1 switch regions by RGS4 and the catalytic action of RGS4 residue Asn128. To elucidate a role of the Asn residue for RGS GAP function, we have investigated effects of the synthetic peptide corresponding to the Galpha binding domain of human retinal RGS (hRGSr) containing the key Asn at position 131, and we have carried out mutational analysis of Asn131. Synthetic peptide hRGSr-(123-140) retained its ability to bind the AlF4--complexed transducin alpha-subunit, Gtalpha.AlF4-, but failed to elicit stimulation of Gtalpha GTPase activity. Wild-type hRGSr stimulated Gtalpha GTPase activity by approximately 10-fold with an EC50 value of 100 nM. Mutant hRGSr proteins with substitutions of Asn131 by Ser and Gln had a significantly reduced affinity for Gtalpha but were capable of substantial stimulation of Gtalpha GTPase activity, 80 and 60% of Vmax, respectively. Mutants hRGSr-Leu131, hRGSr-Ala131, and hRGSr-Asp131 were able to accelerate Gtalpha GTPase activity only at very high concentrations (>10 microM) which appears to correlate with a further decrease of their affinity for transducin. Two mutants, hRGSr-His131 and hRGSr-Delta131, had no detectable binding to transducin. Mutational analysis of Asn131 suggests that the stabilization of the G-protein switch regions rather than catalytic action of the Asn residue is a key component for the RGS GAP action.

Photoreceptor phosphodiesterase: interaction of inhibitory gamma subunit and cyclic GMP with specific binding sites on catalytic subunits

The photoreceptor phosphodiesterase (PDE6) is the central effector enzyme in the phototransduction cascade of photoreceptor cells. It is the only known PDE isoform the activity of which is regulated by interaction with a heterotrimeric G protein. The rod PDE6 holoenzyme is a tetrameric protein consisting of two large catalytic alpha and beta subunits and two small gamma subunits, which serve as potent inhibitors of PDE6. In dark-adapted photoreceptors, the gamma subunits maintain PDE6 activity at a low level. When exposed to light the visual pigment rhodopsin activates the retinal G protein, transducin, leading to release of the inhibitory action of the gamma subunits. In addition to the active sites where cGMP is hydrolyzed, the alpha and beta catalytic subunits have high-affinity, noncatalytic cGMP binding sites. These noncatalytic sites do not directly regulate cGMP catalysis at the active site, but rather can modulate the affinity with which the gamma subunits bind to the catalytic subunits. This article describes a number of experimental approaches that have recently been developed for studying the interactions between catalytic and inhibitory subunits of PDE6, as well as the dynamics of cGMP binding to and dissociation from the PDE6 noncatalytic sites.

The regulation of the cGMP-binding cGMP phosphodiesterase by proteins that are immunologically related to gamma subunit of the photoreceptor cGMP phosphodiesterase

The cGMP phosphodiesterase from retinal rods (PDE-6) is an alphabetagamma2 heterotetramer. The alpha and beta subunits contain catalytic sites for cGMP hydrolysis, whereas the gamma subunits serve as a protein inhibitor of the enzyme. Visual excitation of photoreceptors enables the activated GTP-bound form of the G-protein transducin to remove the inhibitory action of the gamma subunit, thereby triggering PDE-6 activation. The type 5 phosphodiesterase (PDE-5) isoform shares a number of similar characteristics with PDE-6, including binding of cGMP to noncatalytic sites, the cyclic nucleotide specificity, and inhibitor sensitivities. Although the functional role of PDE-5 remains unclear, it has been shown to be activated by protein kinase A (PKA) (Burns, F., Rodger, I. W. & Pyne, N. J. (1992) Biochem. J. 283, 487-491). Here we report that both the recombinant gamma subunit and a peptide corresponding to amino acids 24-46 in this protein inhibited the activation of PDE-5 by PKA. Furthermore, immunoblotting airway smooth muscle membranes with a specific antibody against amino acids 24-46 of the PDE-6 gamma subunit identified two major immunoreactive small molecular mass proteins of 14 and 18 kDa (p14 and p18). These appear to form a complex with PDE-5, because PDE activity was immunoprecipitated using antibody against the PDE-6 gamma subunit. p14 and p18 were also substrates for phosphorylation by a unidentified kinase that was stimulated by a pertussis toxin-sensitive G-protein. Phosphorylation of p14/p18 in membranes treated with guanine nucleotides correlated with a concurrent reduction in the activation of PDE-5 by PKA. We suggest that p14 and p18 share an epitope common to PDE-6 gamma and that this region may interact with PDE-5 to prevent its activation by PKA.

The gamma subunit of rod cGMP-phosphodiesterase blocks the enzyme catalytic site

Cyclic GMP phosphodiesterase (PDE) is the effector enzyme in the visual transduction cascade of vertebrate photoreceptor cells. In the dark, the activity of the enzyme catalytic alpha and beta subunits (Palphabeta) is inhibited by two gamma subunits (Pgamma). Previous results have established that approximately 5-7 C-terminal residues of Pgamma comprise the inhibitory domain. To study the interaction between the Pgamma C-terminal region and Palphabeta, the Pgamma mutant (Cys68 --> Ser, and the last 4 C-terminal residues replaced with cysteine, Pgamma-1-83Cys) was labeled with the fluorescent probe 3-(bromoacetyl)-7-diethylaminocoumarin (BC) at the cysteine residue (Pgamma-1-83BC). Pgamma-1-83BC was a more potent inhibitor of PDE activity than the unlabeled mutant, suggesting that the fluorescent probe in part substitutes for the Pgamma C terminus in PDE inhibition. HoloPDE (Palphabetagamma2) had no effect on the Pgamma-1-83BC fluorescence, but the addition of Palphabeta to Pgamma-1-83BC resulted in an approximately 8-fold maximal fluorescence increase. A Kd for the Pgamma-1-83BC-Palphabeta interaction was 4.0 +/- 0.5 nM. Zaprinast, a specific competitive inhibitor of PDE, effectively displaced the Pgamma-1-83BC C terminus from its binding site on Palphabeta (IC50 = 0.9 microM). cGMP and its analogs, 8-Br-cGMP and 2'-butyryl-cGMP, also competed with the Pgamma-1-83BC C terminus for binding to Palphabeta. Our results provide new insight into the mechanism of PDE inhibition by showing that Pgamma blocks the binding of cGMP to the PDE catalytic site.

Binding of transducin to light-activated rhodopsin prevents transducin interaction with the rod cGMP phosphodiesterase gamma-subunit

In photoreceptor cells of vertebrates, the GTP-bound alpha-subunit of rod G-protein, transducin (G(t alpha)), interacts with the cGMP phosphodiesterase inhibitory gamma-subunit (Pgamma) to activate the effector enzyme. The GDP-bound G(t alpha) can also bind the Pgamma subunit, albeit with a lower affinity than G(t alpha)GTP. In this work, interactions between G(t alpha)GDP and Pgamma or Pgamma-24-45Cys labeled with the fluorescent probe 3-(bromoacetyl)-7-(diethylamino)coumarin (PgammaBC, Pgamma-24-45BC) have been investigated. Addition of G(t alpha)GDP to PgammaBC produced approximately a 6-fold maximal increase in the probe fluorescence, while the fluorescence of Pgamma-24-45BC was enhanced by 2.3-fold. The Kd's for the G(t alpha)GDP binding to PgammaBC and Pgamma-24-45BC were 75 +/- 8 nM and 400 +/- 110 nM, respectively. The G(t betagamma) subunits had no notable effect on the binding of G(t alpha)GDP to PgammaBC or Pgamma-24-45BC, suggesting that Pgamma and G(t betagamma) bind to G(t alpha)GDP noncompetitively. The G(t alpha betagamma) interaction with the fluorescently labeled Pgamma was effectively blocked in the light-activated rhodopsin (R*)-G(t alpha betagamma) complex. Furthermore, addition of excess Pgamma or Pgamma-24-45 prevented binding of G(t alpha betagamma) to R*, indicating that the R* and Pgamma binding surfaces on G(t alpha betagamma) may overlap. It is likely that R* has a binding site within the alpha3-beta5 region of G(t alpha), which is a proposed site of G(t alpha)GDP binding to Pgamma-24-45. Alternatively, R* may induce conformational changes of the G(t alpha) alpha3-beta5 region such that the resulting structural changes alter the adjacent consensus sequence for the guanine ring binding of GDP/GTP(NKXD), and lead to a reduction in the affinity of G-protein for guanine nucleotides.