NORMAL GENOMIC

• Kuwayama H, Snippe H, Derks M, Roelofs J, Van Haastert PJ
• Identification and characterization of DdPDE3, a cGMP-selective phosphodiesterase from Dictyostelium.
• Biochem J. 2001; 353: 635-44
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• In Dictyostelium cAMP and cGMP have important functions as first and second messengers in chemotaxis and development. Two cyclic-nucleotide phosphodiesterases (DdPDE 1 and 2) have been identified previously, an extracellular dual-specificity enzyme and an intracellular cAMP-specific enzyme (encoded by the psdA and regA genes respectively). Biochemical data suggest the presence of at least one cGMP-specific phosphodiesterase (PDE) that is activated by cGMP. Using bioinformatics we identified a partial sequence in the Dictyostelium expressed sequence tag database that shows a high degree of amino acid sequence identity with mammalian PDE catalytic domains (DdPDE3). The deduced amino acid sequence of a full-length DdPDE3 cDNA isolated in this study predicts a 60 kDa protein with a 300-residue C-terminal PDE catalytic domain, which is preceded by approx. 200 residues rich in asparagine and glutamine residues. Expression of the DdPDE3 catalytic domain in Escherichia coli shows that the enzyme has Michaelis-Menten kinetics and a higher affinity for cGMP (K(m)=0.22 microM) than for cAMP (K(m)=145 microM); cGMP does not stimulate enzyme activity. The enzyme requires bivalent cations for activity; Mn(2+) is preferred to Mg(2+), whereas Ca(2+) yields no activity. DdPDE3 is inhibited by 3-isobutyl-1-methylxanthine with an IC(50) of approx. 60 microM. Overexpression of the DdPDE3 catalytic domain in Dictyostelium confirms these kinetic properties without indications of its activation by cGMP. The properties of DdPDE3 resemble those of mammalian PDE9, which also shows the highest sequence similarity within the catalytic domains. DdPDE3 is the first cGMP-selective PDE identified in lower eukaryotes.

• Lin CS, Lau A, Tu R, Lue TF
• Expression of three isoforms of cGMP-binding cGMP-specific phosphodiesterase (PDE5) in human penile cavernosum.
• Biochem Biophys Res Commun. 2000; 268: 628-35
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• Inhibition of cGMP-specific phosphodiesterase type V (PDE5) has been shown to improve penile erection in patients with erectile dysfunction. We report here the cloning of three PDE5 isoforms from human penile tissues. Two of the isoforms were identical to PDE5A1 and PDE5A2, respectively, which had been isolated from nonpenile tissues. The third isoform was novel and hence called PDE5A3. The deduced amino acid sequence of PDE5A3 was the same as the C-terminal 823-residue sequence of PDE5A1 and PDE5A2. While PDE5A1 and A2 isoforms were expressed in all tissues examined, the A3 isoform was confined to tissues with a smooth muscle or cardiac muscle component. When expressed in COS-7 cells, PDE5A1, A2, and A3 isoforms had similar cGMP-catalytic activities with K(m) of 6.2, 5.75, and 6.06 microM, respectively. Their cGMP-catalytic activities were inhibited by zaprinast with IC(50) values of 3.2 microM, 1.3 microM, and 1.6 microM, respectively, and by sildenafil with IC(50) of 28, 14, and 13 nM, respectively.

• Omori K, Kotera J
• [Structure and characteristics of cGMP phosphodiesterase family]
• Seikagaku. 1999; 71: 541-6

• Fisher DA, Smith JF, Pillar JS, St Denis SH, Cheng JB
• Isolation and characterization of PDE9A, a novel human cGMP-specific phosphodiesterase.
• J Biol Chem. 1998; 273: 15559-64
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• We have cloned and characterized the first human isozyme in a new family of cyclic nucleotide phosphodiesterases, PDE9A. By sequence homology in the catalytic domain, PDE9A is almost equidistant from all eight known mammalian PDE families but is most similar to PDE8A (34% amino acid identity) and least like PDE5A (28% amino acid identity). We report the cloning of human cDNA encoding a full-length protein of 593 amino acids, including a 261-amino acid region located near the C terminus that is homologous to the approximately 270-amino acid catalytic domain of other PDEs. PDE9A is expressed in all eight tissues examined as a approximately 2. 0-kilobase mRNA, with highest levels in spleen, small intestine, and brain. The full-length PDE9A was expressed in baculovirus fused to an N-terminal 9-amino acid FLAG tag. Kinetic analysis of the baculovirus-expressed enzyme shows it to be a very high affinity cGMP-specific PDE with a Km of 170 nM for cGMP and 230 microM for cAMP. The Km for cGMP makes PDE9A one of the highest affinity PDEs known. The Vmax for cGMP (4.9 nmol/min/microg recombinant enzyme) is about twice as fast as that of PDE4 for cAMP. The enzyme is about twice as active in vitro in 1-10 mM Mn2+ than in the same concentration of Mg2+ or Ca2+. PDE9A is insensitive (up to 100 microM) to a variety of PDE inhibitors including rolipram, vinpocetine, SKF-94120, dipyridamole, and 3-isobutyl-1-methyl-xanthine but is inhibited (IC50 = 35 microM) by zaprinast, a PDE5 inhibitor. PDE9A lacks a region homologous to the allosteric cGMP-binding regulatory regions found in the cGMP-binding PDEs: PDE2, PDE5, and PDE6.

• Turko IV, Francis SH, Corbin JD
• Potential roles of conserved amino acids in the catalytic domain of the cGMP-binding cGMP-specific phosphodiesterase.
• J Biol Chem. 1998; 273: 6460-6
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• The known mammalian 3':5'-cyclic nucleotide phosphodiesterases (PDEs) contain a conserved region located toward the carboxyl terminus, which constitutes a catalytic domain. To identify amino acids that are important for catalysis, we introduced substitutions at 23 conserved residues within the catalytic domain of the cGMP-binding cGMP-specific phosphodiesterase (cGB-PDE; PDE5). Wild-type and mutant proteins were compared with respect to Km for cGMP, kcat, and IC50 for zaprinast. The most dramatic decrease in kcat was seen with H643A and D754A mutants with the decrease in free energy of binding (DeltaDeltaGT) being about 4.5 kcal/mol for each, which is within the range predicted for loss of a hydrogen bond involving a charged residue. His643 and Asp754 are conserved in all known PDEs and are strong candidates to be directly involved in catalysis. Substitutions of His603, His607, His647, Glu672, and Asp714 also produced marked changes in kcat, and these residues are likely to be important for efficient catalysis. The Y602A and E775A mutants exhibited the most dramatic increases in Km for cGMP, with calculated DeltaDeltaGT of 2.9 and 2.8 kcal/mol, respectively, that these two residues are important for cGMP binding in the catalytic site. Zaprinast is a potent competitive inhibitor of cGB-PDE, but the key residues for its binding differ significantly from those that bind cGMP.

• Stacey P, Rulten S, Dapling A, Phillips SC
• Molecular cloning and expression of human cGMP-binding cGMP-specific phosphodiesterase (PDE5).
• Biochem Biophys Res Commun. 1998; 247: 249-54
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• A human PDE5 cDNA has been isolated which contains an open reading frame encoding an 875 amino acid, 100,012 Da polypeptide, the expression of which yields a protein of the predicted size and is capable of hydrolyzing cGMP. The deduced amino acid sequence is very similar (95%) to that of bovine PDE5, and comprises a conserved cGMP-binding domain and catalytic domain. Northern analysis reveals a major and minor transcript of approximately 9 kb and approximately 8 kb respectively, thus indicating the existence of at least two splice variants, the major form being readily detected in bladder, colon, lung, pancreas, placenta, prostate, small intestine, and stomach.

• Kotera J, Fujishige K, Akatsuka H, Imai Y, Yanaka N, Omori K
• Novel alternative splice variants of cGMP-binding cGMP-specific phosphodiesterase.
• J Biol Chem. 1998; 273: 26982-90
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• After our recent findings that the amino-terminal portion of rat cGMP-binding, cGMP-specific phosphodiesterase (cGB-PDE) differs from those of bovine and human cGB-PDEs, we found two forms of canine cGB-PDE cDNAs (CFPDE5A1 and CFPDE5A2) in canine lung. Each contained a distinct amino-terminal sequence, CFPDE5A1, possessing an amino-terminal portion with sequence similar to those of bovine and human, and CFPDE5A2, having one similar to that of rat. Other portions coding for the cGMP binding domains and the catalytic domain were conserved. Both CFPDE5A1 and CFPDE5A2 transcripts were detected in the cerebellum, hippocampus, retina, lung, heart, spleen, and thoracic artery. CFPDE5A1 transcripts were particularly abundant in the pylorus, whereas CFPDE5A2 transcripts were quite low in this tissue. CFPDE5A1 and CFPDE5A2 expressed in COS-7 cells had cGMP Km values of 2.68 and 1.97 microM, respectively, and both were inhibited by a low concentration of a cGB-PDE inhibitor, Zaprinast. Both CFPDE5A1 and CFPDE5A2 bound cGMP to their allosteric cGMP binding domains, and this cGMP binding was stimulated by 3-isobutyl-1-methylxanthine. Thus, two types of alternative splice variants of canine cGB-PDE have been identified and shown to have similar biological properties in vitro.

• Seno K et al.
• A possible role of RGS9 in phototransduction. A bridge between the cGMP-phosphodiesterase system and the guanylyl cyclase system.
• J Biol Chem. 1998; 273: 22169-72
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• In the current concept of phototransduction, the concentration of cGMP in retinal rod outer segments is controlled by the balance of two enzyme activities: cGMP phosphodiesterase (PDE) and guanylyl cyclase (GC). However, no protein directly mediates these two enzyme systems. Here we show that RGS9, which is suggested to control PDE activity through regulation of transducin GTPase activity (He, W., Cowan, C. W., and Wensel, T. G. (1998) Neuron 20, 95-102), directly interacts with GC. When proteins in the Triton X-100-insoluble fraction of bovine rod outer segments were isolated by two-dimensional gel electrophoresis and binding of GC to these proteins was examined using a GC-specific antibody, proteins (55 and 32 kDa) were found to interact with GC. However, the activity of GC bound to the 55-kDa protein was not detected. This observation was elucidated by the finding that the 55-kDa protein inhibited GC activity in a dose-dependent manner. Amino acid sequence showed that five peptides derived from the 55-kDa protein were identical to corresponding peptides of RGS9. Together with other biochemical characterization of the 55-kDa protein, these observations indicate that the 55-kDa protein is RGS9 and that RGS9 inhibits GC. RGS9 may serve as a mediator between the PDE and GC systems.

• Rosman GJ, Martins TJ, Sonnenburg WK, Beavo JA, Ferguson K, Loughney K
• Isolation and characterization of human cDNAs encoding a cGMP-stimulated 3',5'-cyclic nucleotide phosphodiesterase.
• Gene. 1997; 191: 89-95
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• Human cyclic GMP-stimulated 3',5'-cyclic nucleotide phosphodiesterase (PDE2A3) cDNAs were cloned from hippocampus and fetal brain cDNA libraries. A 4.2-kb composite DNA sequence constructed from overlapping cDNA clones encodes a 941 amino acid protein with a predicted molecular mass of 105,715 Da. Extracts prepared from yeast expressing the human PDE2A3 hydrolyzed both cyclic AMP (cAMP) and cyclic GMP (cGMP). This activity was inhibited by EHNA, a selective PDE2 inhibitor, and was stimulated three-fold by cGMP. Human PDE2A is expressed in brain and to a lesser extent in heart, placenta, lung, skeletal muscle, kidney and pancreas. The human PDE2A3 differs from the bovine PDE2A1 and rat PDE2A2 proteins at the amino terminus but its amino-terminal sequence is identical to the bovine PDE2A3 sequence. The different amino termini probably arise from alternative exon splicing of the PDE2A mRNA.

• Berger AL, Cerione RA, Erickson JW
• Real time conformational changes in the retinal phosphodiesterase gamma subunit monitored by resonance energy transfer.
• J Biol Chem. 1997; 272: 2714-21
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• 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 order to characterize conformational changes in the 87-amino acid gammaPDE subunit that may accompany the activation of the holoenzyme, gammaPDE was labeled with the fluorescent probes 5-iodoacetamidofluorescein and eosin-5-isothiocyanate for use in resonance energy transfer measurements. 5-Iodoacetamidofluorescein specifically labeled a cysteine residue at position 68 and served as a resonance energy transfer donor. The site of modification of eosin-5-isothiocyanate, which served as the resonance energy transfer acceptor, was determined to be within the first seven residues of the amino terminus of gammaPDE. Energy transfer between the labeled sites on free, unbound gammaPDE indicated that they were separated by a distance of 63 A, consistent with a random conformation. Upon binding the catalytic alphabeta subunits of the PDE, the distance between the two probes on gammaPDE increased to 77 A. Binding of the labeled gammaPDE by alphaT.guanosine 5'-3-O-(thio)triphosphate did not affect the distance between the probes under conditions where the PDE was activated. These data are consistent with the view that the binding of activated alphaT to gammaPDE, which is essential for the stimulation of PDE activity, does not impart significant alterations in the tertiary structure of the gammaPDE molecule. They also support a model for PDE activation that places active alphaT in a complex with the holoenzyme.

• Natochin M, Artemyev NO
• An interface of interaction between photoreceptor cGMP phosphodiesterase catalytic subunits and inhibitory gamma subunits.
• J Biol Chem. 1996; 271: 19964-9
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• Cyclic guanosine 5'-monophosphate (cGMP) phosphodiesterase (PDE) regulates the level of cGMP on transduction of a visual signal in vertebrate photoreceptor cells. Two identical inhibitory PDE gamma subunits (Pgammas) block catalytic activity of PDE-alpha and -beta subunits (Palphabeta) in the dark. The primary regions of Pgamma involved in the interaction with Palphabeta are a central polycationic region, Pgamma-24-45, and a C-terminal region of Pgamma. Recently, we have shown that the C-terminal region of Pgamma, which is the major Pgamma inhibitory domain, blocks PDE activity by binding to the catalytic site of PDE (Artemyev, N. O., Natochin, M., Busman, M., Schey, K. L., and Hamm, H. E. (1996) Proc. Natl. Acad. Sci. U. S. A. 93, 5407-5412). Here, we localize the site on the rod cGMP PDE alpha subunit that binds to the central polycationic domain of Pgamma. This site is located within a region that links a second noncatalytic cGMP binding site with the catalytic domain of PDE. A polypeptide coresponding to this region, Palpha-461-553, expressed as a glutathione S-transferase fusion protein in Escherichia coli and isolated after cleavage of the fusion protein with thrombin, blocks inhibition of PDE activity by Pgamma. In addition, Palpha-461-553 binds to the Pgamma-24-45 region (Kd, 7 microM), as measured by a fluorescent increase in a Pgamma-24-45Cys peptide labeled with 3-(bromoacetyl)-7-diethylaminocoumarin. The Palpha-461-553 region was further characterized by using a set of synthetic peptides. A peptide corresponding to residues 517-541 of Palpha (Palpha-517-541) effectively suppressed inhibition of PDE activity by Pgamma and bound to Pgamma-24-45Cys labeled with 3-(bromoacetyl)-7-diethylaminocoumarin (Kd, 22 microM). Palpha-517-541 also competes with the activated rod G-protein alpha-subunit for binding to Pgamma labeled with lucifer yellow vinyl sulfone. This suggests that light activation of rod PDE by the G-protein transducin involves competition between transducin alpha-guanosine 5'-triphosphate and Palpha-517-541 for binding to the Pgamma-24-45 region. Based on the results, we propose a linear model of interactions between catalytic and inhibitory PDE subunits.

• Feshchenko EA, Andreeva SG, Suslova VA, Smirnova EV, Zagranichny VE, Lipkin VM
• Human cone-specific cGMP phosphodiesterase alpha' subunit: complete cDNA sequence and gene arrangement.
• FEBS Lett. 1996; 381: 149-52
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• Four independent phage clones containing the fragments of cone-specific cGMP phosphodiesterase (PDE) alpha' subunit (PDE-alpha') cDNA were isolated from the human cDNA library. The screening of the genomic library resulted in isolation of four independent phage clones with the fragments of human cone PDEalpha' gene including 5'-flanking region and exons ranged from 1 to 14 (overall 32 kilobases). Structural studies of the clones made it possible to establish the complete human cone PDEalpha' cDNA structure (3455 base pairs). The encoding polypeptide consists of 858 amino acid residues with a calculated molecular mass of 99169 Da. The deduced amino acid sequence displays high homology to the earlier analyzed catalytic alpha, beta and alpha' subunits of bovine, human, chicken and mouse photoreceptor PDEs.

• Artemyev NO, Natochin M, Busman M, Schey KL, Hamm HE
• Mechanism of photoreceptor cGMP phosphodiesterase inhibition by its gamma-subunits.
• Proc Natl Acad Sci U S A. 1996; 93: 5407-12
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• cGMP phosphodiesterase (PDE) is the key effector enzyme of vertebrate photoreceptor cells that regulates the level of the second messenger, cGMP. PDE consists of catalytic alpha and beta subunits (Palpha and Pbeta) and two inhibitory gamma subunits (Pgamma) that block PDE activity in the dark. The major inhibitory region has been localized to the C terminus of Pgamma. The last C-terminal residues -IleIle form an important hydrophobic domain critical for the inhibition of PDE activity. In this study, mutants of Pgamma were designed for cross-linking experiments to identify regions on Palpha and Pbeta subunits that bind to the Pgamma C terminus. In one of the mutants, the cysteine at position 68 was substituted with serine, and the last four C-terminal residues of Pgamma were replaced with a single cysteine. This mutant, Pgamma83Cys, was labeled with photoprobe 4-(N-maleimido) benzophenone (MBP) at the cysteine residue. The labeled Pgamma83CysMBP mutant was a more potent inhibitor of PDE activity than the unlabeled mutant, indicating that the hydrophobic MBP probe mimics the Pgamma hydrophobic C terminus. A specific, high-yield cross-linking of up to 70% was achieved between the Pgamma83CysMBP and PDE catalytic subunits. Palpha and the N-terminally truncated Pbeta (lacking 147 aa residues) cross-linked to Pgamma83CysMBP with the same efficiency. Using mass spectrometric analysis of tryptic fragments from the cross-linked PDE, we identified the site of cross-linking to aa residues 751-763 of Palpha. The corresponding region of Pbeta, Pbeta-749-761, also may bind to the Pgamma C terminus. Our data suggest that Pgamma blocks PDE activity through the binding to the catalytic site of PDE, near the NKXD motif, a consensus sequence for interaction with the guanine ring of cGMP.

• Turko IV, Haik TL, McAllister-Lucas LM, Burns F, Francis SH, Corbin JD
• Identification of key amino acids in a conserved cGMP-binding site of cGMP-binding phosphodiesterases. A putative NKXnD motif for cGMP binding.
• J Biol Chem. 1996; 271: 22240-4
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• cGMP-binding phosphodiesterases contain two kinetically distinct cGMP-binding sites (a and b), and each site contains a conserved N(K/R)XnFX3DE sequence. N276A, K277A, K277R, D289A, and E290A mutants in the N276KX7FX3DE290 sequence of site a (higher affinity site) of bovine cGMP-binding, cGMP-specific phosphodiesterase (cGB-PDE or PDE5A) were expressed in High Five cells and purified. The cGMP-binding affinities of three mutants [K277A (Kd approximately 12 microM), D289A (Kd approximately 24 microM), and N276A (Kd approximately 60 microM)] were decreased in comparison with wild-type enzyme (Kd = 1.3 microM), which suggested an important role for Asn276, Lys277, and Asp289 in cGMP binding. These residues could be presented as a putative NKXnD motif, and their functions were predicted based on analogy with the canonical NKXD motif in GTP-binding proteins. No marked differences in catalytic functions such as specific activity, Km for cGMP, and IC50 for zaprinast or 3-isobutyl-1-methylxanthine were found among wild-type and mutant cGB-PDEs. This suggested that cGMP binding to site a does not influence the catalytic properties of cGB-PDE.

• Erickson JW, Mittal R, Cerione RA
• Use of resonance energy transfer to determine the proximity of the guanine nucleotide binding site of transducin relative to a conformationally-sensitive site on the gamma subunit of the cyclic GMP phosphodiesterase.
• Biochemistry. 1995; 34: 8693-700
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• In this work, we have used resonance energy transfer to determine the relative positions of a reactive cysteine residue on the gamma subunit of the retinal cyclic GMP phosphodiesterase (gamma PDE) and a reactive lysine residue on the alpha subunit of transducin (alpha T). The single cysteine residue on gamma PDE (residue 68) is located at a site that is sensitive to the binding of both the inactive and active forms of alpha T. This is demonstrated by the finding that the addition of an alpha T-GDP complex to a gamma PDE subunit labeled with the environmentally-sensitive probe 2-(4-maleimidoanilino)naphthalene-6-sulfonate (MIANS) results in an enhancement in the MIANS fluorescence. The alpha TGDP-induced fluorescence enhancement is dose-dependent and yields an apparent Kd value of approximately 3 microM. Activation of alpha TGDP by aluminum fluoride, when bound to the MIANS-labeled gamma PDE (M-gamma PDE), then results in a quenching of the MIANS fluorescence. The aluminum fluoride-induced change in M-gamma PDE fluorescence occurs on a time scale identical to that observed for changes in the intrinsic alpha T fluorescence that correspond to activating conformational changes in the alpha T "switch II" region. These results suggest that the induction of the activated state of the alpha T subunit results in a change in conformation close to cysteine 68 in gamma PDE.(ABSTRACT TRUNCATED AT 250 WORDS)

• Francis SH, Colbran JL, McAllister-Lucas LM, Corbin JD
• Zinc interactions and conserved motifs of the cGMP-binding cGMP-specific phosphodiesterase suggest that it is a zinc hydrolase.
• J Biol Chem. 1994; 269: 22477-80
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• cGMP-binding cGMP-specific phosphodiesterase (cG-BPDE) binds tightly to a Zn(2+)-chelate column (Francis, S. H., and Corbin, J. D. (1988) Methods Enzymol. 159, 722-729). Using three different approaches, Zn2+ is now shown to bind to cG-BPDE, and the Kd is determined to be approximately 0.5 microM, with a binding stoichiometry of approximately 3 mol of Zn2+/mol of monomer. A similar concentration range of Zn2+ (0.05-1 microM Zn2+) also supports phosphodiesterase (PDE) catalytic activity. The Zn2+ binding to cG-BPDE is not diminished by, nor is catalysis supported by, relatively high concentrations of Cu2+, Cd2+, Ca2+, or Fe2+. Neither cGMP nor 3-isobutyl-1-methylxanthine affects Zn2+ binding under the conditions used. Mn2+, Co2+, or Mg2+ supports catalysis, but only at significantly higher concentrations (4-, 15-, and 250-fold, respectively) than that required for Zn2+. Two tandem amino acid sequences, which are conserved in the catalytic domains of all characterized mammalian PDEs, resemble the single sequence motif that has been shown to coordinate Zn2+ in the catalytic sites of Zn2+ hydrolases such as thermolysin.

• Li TS, Volpp K, Applebury ML
• Bovine cone photoreceptor cGMP phosphodiesterase structure deduced from a cDNA clone.
• Proc Natl Acad Sci U S A. 1990; 87: 293-7
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• A full-length cDNA clone encoding the alpha' subunit of cGMP phosphodiesterase (PDE) from bovine cone photoreceptors was selected by probing a retinal library with a DNA fragment encoding the catalytic core of the rod cGMP PDE alpha subunit. Identity of the clone was confirmed by comparing its deduced sequence with cone PDE peptide sequences determined by Charbonneau et al. [Charbonneau, H., Prusti, R. K., LeTrong, H., Sonnenburg, W. K., Mullaney, P. J., Walsh, K. A. & Beavo, J. A. (1990) Proc. Natl. Acad. Sci. USA, pp. 288-292]. The cone PDE alpha' and the rod PDE alpha and beta subunits are encoded by distinct genes. cGMP PDE subunits share a common ancestry with cAMP PDEs and cyclic nucleotide-binding proteins. Sequence comparisons predict the presence of a catalytic core and possible secondary sites for noncatalytic cGMP binding. The presence of a C-terminal CAAX (Cys-aliphatic-aliphatic-Xaa) motif suggests the cone enzyme may be posttranslationally modified by proteolysis, methylation, and isoprenylation.

• Gurevich VV et al.
• Bovine rhodopsin: amino acid substitutions Asp-83----Asn and Glu-134----Gln prevent activation of cyclic GMP phosphodiesterase.
• Biomed Sci. 1990; 1: 527-30
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• Two bovine rhodopsin mutants with substitutions of negatively charged residues within transmembrane domains II and III by uncharged ones (Asp-83----Asn and Glu-134----Gln) were constructed. Both mutants stimulated transducin GTPase with slightly lowered efficiency, but were completely unable to activate cyclic GMP phosphodiesterase.