Identification of the residues on cyclic GMP-dependent protein kinase that are autophosphorylated in the presence of cyclic AMP and cyclic GMP

A substitution in cGMP-dependent protein kinase 1 associated with aortic disease induces an active conformation in the absence of cGMP

Type 1 cGMP-dependent protein kinases (PKGs) play important roles in human cardiovascular physiology, regulating vascular tone and smooth-muscle cell phenotype. A mutation in the human PRKG1 gene encoding cGMP-dependent protein kinase 1 (PKG1) leads to thoracic aortic aneurysms and dissections. The mutation causes an arginine-to-glutamine (RQ) substitution within the first cGMP-binding pocket in PKG1. This substitution disrupts cGMP binding to the pocket, but it also unexpectedly causes PKG1 to have high activity in the absence of cGMP via an unknown mechanism. Here, we identified the molecular mechanism whereby the RQ mutation increases basal kinase activity in the human PKG1α and PKG1β isoforms. Although we found that the RQ substitution (R177Q in PKG1α and R192Q in PKG1β) increases PKG1α and PKG1β autophosphorylation in vitro, we did not detect increased autophosphorylation of the PKG1α or PKG1β RQ variant isolated from transiently transfected 293T cells, indicating that increased basal activity of the RQ variants in cells was not driven by PKG1 autophosphorylation. Replacement of Arg-177 in PKG1α with alanine or methionine also increased basal activity. PKG1 exists as a parallel homodimer linked by an N-terminal leucine zipper, and we show that the WT chain in WT-RQ heterodimers partly reduces basal activity of the RQ chain. Using hydrogen/deuterium-exchange MS, we found that the RQ substitution causes PKG1β to adopt an active conformation in the absence of cGMP, similar to that of cGMP-bound WT enzyme. We conclude that the RQ substitution in PKG1 increases its basal activity by disrupting the formation of an inactive conformation.

Oxidation of cysteine 117 stimulates constitutive activation of the type Iα cGMP-dependent protein kinase

The type I cGMP-dependent protein kinase (PKG I) is an essential regulator of vascular tone. It has been demonstrated that the type Iα isoform can be constitutively activated by oxidizing conditions. However, the amino acid residues implicated in this phenomenon are not fully elucidated. To investigate the molecular basis for this mechanism, we studied the effects of oxidation using recombinant WT, truncated, and mutant constructs of PKG I. Using an in vitro assay, we observed that oxidation with hydrogen peroxide (H₂O₂) resulted in constitutive, cGMP-independent activation of PKG Iα. PKG Iα C42S and a truncation construct that does not contain Cys-42 (Δ53) were both constitutively activated by H₂O₂ In contrast, oxidation of PKG Iα C117S maintained its cGMP-dependent activation characteristics, although oxidized PKG Iα C195S did not. To corroborate these results, we also tested the effects of our constructs on the PKG Iα-specific substrate, the large conductance potassium channel (K_Ca 1.1). Application of WT PKG Iα activated by either cGMP or H₂O₂ increased the open probabilities of the channel. Neither cGMP nor H₂O₂ activation of PKG Iα C42S significantly increased channel open probabilities. Moreover, cGMP-stimulated PKG Iα C117S increased K_Ca 1.1 activity, but this effect was not observed under oxidizing conditions. Finally, we observed that PKG Iα C42S caused channel flickers, indicating dramatically altered K_Ca 1.1 channel characteristics compared with channels exposed to WT PKG Iα. Cumulatively, these results indicate that constitutive activation of PKG Iα proceeds through oxidation of Cys-117 and further suggest that the formation of a sulfur acid is necessary for this phenotype.

An N-terminally truncated form of cyclic GMP-dependent protein kinase Iα (PKG Iα) is monomeric and autoinhibited and provides a model for activation

The type I cGMP-dependent protein kinases (PKG I) serve essential physiological functions, including smooth muscle relaxation, cardiac remodeling, and platelet aggregation. These enzymes form homodimers through their N-terminal dimerization domains, a feature implicated in regulating their cooperative activation. Previous investigations into the activation mechanisms of PKG I isoforms have been largely influenced by structures of the cAMP-dependent protein kinase (PKA). Here, we examined PKG Iα activation by cGMP and cAMP by engineering a monomeric form that lacks N-terminal residues 1-53 (Δ53). We found that the construct exists as a monomer as assessed by whole-protein MS, size-exclusion chromatography, and small-angle X-ray scattering (SAXS). Reconstruction of the SAXS 3D envelope indicates that Δ53 has a similar shape to the heterodimeric RIα-C complex of PKA. Moreover, we found that the Δ53 construct is autoinhibited in its cGMP-free state and can bind to and be activated by cGMP in a manner similar to full-length PKG Iα as assessed by surface plasmon resonance (SPR) spectroscopy. However, we found that the Δ53 variant does not exhibit cooperative activation, and its cyclic nucleotide selectivity is diminished. These findings support a model in which, despite structural similarities, PKG Iα activation is distinct from that of PKA, and its cooperativity is driven by in trans interactions between protomers.

Simultaneous assessment of kinetic, site-specific, and structural aspects of enzymatic protein phosphorylation

Protein phosphorylation is a widespread process forming the mechanistic basis of cellular signaling. Up to now, different aspects, for example, site-specificity, kinetics, role of co-factors, and structure-function relationships have been typically investigated by multiple techniques that are incompatible with one another. The approach introduced here maximizes the amount of information gained on protein (complex) phosphorylation while minimizing sample handling. Using high-resolution native mass spectrometry on intact protein (assemblies) up to 150 kDa we track the sequential incorporation of phosphate groups and map their localization by peptide LC-MS/MS. On two model systems, the protein kinase G and the interplay between Aurora kinase A and Bora, we demonstrate the simultaneous monitoring of various aspects of the phosphorylation process, namely the effect of different cofactors on PKG autophosphorylation and the interaction of AurA and Bora as both an enzyme-substrate pair and physical binding partners.

Catalytic activity of cGMP-dependent protein kinase type I in intact cells is independent of N-terminal autophosphorylation

Although cGMP-dependent protein kinase type I (cGKI) is an important mediator of cGMP signaling and upcoming drug target, its in vivo-biochemistry is not well understood. Many studies showed that purified cGKI autophosphorylates multiple sites at its N-terminus. Autophosphorylation might be involved in kinase activation, but it is unclear whether this happens also in intact cells. To study cGKI autophosphorylation in vitro and in vivo, we have generated phospho-specific antisera against major in vitro-autophosphorylation sites of the cGKI isoforms, cGKIα and cGKIβ. These antisera detected specifically and with high sensitivity phospho-cGKIα (Thr58), phospho-cGKIα (Thr84), or phospho-cGKIβ (Thr56/Ser63/Ser79). Using these antisera, we show that ATP-induced autophosphorylation of cGKI in purified preparations and cell extracts did neither require nor induce an enzyme conformation capable of substrate heterophosphorylation; it was even inhibited by pre-incubation with cGMP. Interestingly, phospho-cGKI species were not detectable in intact murine cells and tissues, both under basal conditions and after induction of cGKI catalytic activity. We conclude that N-terminal phosphorylation, although readily induced in vitro, is not required for the catalytic activity of cGKIα and cGKIβ in vivo. These results will also inform screening strategies to identify novel cGKI modulators.

Nitration of tyrosine 247 inhibits protein kinase G-1α activity by attenuating cyclic guanosine monophosphate binding

The cGMP-dependent protein kinase G-1α (PKG-1α) is a downstream mediator of nitric oxide and natriuretic peptide signaling. Alterations in this pathway play a key role in the pathogenesis and progression of vascular diseases associated with increased vascular tone and thickness, such as pulmonary hypertension. Previous studies have shown that tyrosine nitration attenuates PKG-1α activity. However, little is known about the mechanisms involved in this event. Utilizing mass spectrometry, we found that PKG-1α is susceptible to nitration at tyrosine 247 and 425. Tyrosine to phenylalanine mutants, Y247F- and Y425F-PKG-1α, were both less susceptible to nitration than WT PKG-1α, but only Y247F-PKG-1α exhibited preserved activity, suggesting that the nitration of Tyr(247) is critical in attenuating PKG-1α activity. The overexpression of WT- or Y247F-PKG-1α decreased the proliferation of pulmonary artery smooth muscle cells (SMC), increased the expression of SMC contractile markers, and decreased the expression of proliferative markers. Nitrosative stress induced a switch from a contractile to a synthetic phenotype in cells expressing WT- but not Y247F-PKG-1α. An antibody generated against 3-NT-Y247 identified increased levels of nitrated PKG-1α in humans with pulmonary hypertension. Finally, to gain a more mechanistic understanding of how nitration attenuates PKG activity, we developed a homology model of PKG-1α. This model predicted that the nitration of Tyr(247) would decrease the affinity of PKG-1α for cGMP, which we confirmed using a [(3)H]cGMP binding assay. Our study shows that the nitration of Tyr(247) and the attenuation of cGMP binding is an important mechanism regulating in PKG-1α activity and SMC proliferation/differentiation.

Inhibitor-2 induced M-phase arrest in Xenopus cycling egg extracts is dependent on MAPK activation

The evolutionarily-conserved protein phosphatase 1 (PP1) plays a central role in dephosphorylation of phosphoproteins during the M phase of the cell cycle. We demonstrate here that the PP1 inhibitor inhibitor-2 protein (Inh-2) induces an M-phase arrest in Xenopus cycling egg extracts. Interestingly, the characteristics of this M-phase arrest are similar to those of mitogen-activated protein kinase (p42MAPK)-induced M-phase arrest. This prompted us to investigate whether Inh-2-induced M-phase arrest was dependent on activation of the p42MAPK pathway. We demonstrate here that MAPK activity is required for Inh-2-induced M-phase arrest, as inhibition of MAPK by PD98059 allowed cycling extracts to exit M phase, despite the presence of Inh-2. We next investigated whether Inh-2 phosphorylation by the MAPK pathway was required to induce an M-phase arrest. We discovered that while p90Rsk (a MAPK protein required for M-phase arrest) is able to phosphorylate Inh-2, this phosphorylation is not required for Inh-2 function. Overall, our results suggest a novel mechanism linking p42MAPK and PP1 pathways during M phase of the cell cycle.

Crystal structure of cGMP-dependent protein kinase reveals novel site of interchain communication

The cGMP-dependent protein kinase (PKG) serves as an integral component of second messenger signaling in a number of biological contexts including cell differentiation, memory, and vasodilation. PKG is homodimeric and large conformational changes accompany cGMP binding. However, the structure of PKG and the molecular mechanisms associated with protomer communication following cGMP-induced activation remain unknown. Here, we report the 2.5 Å crystal structure of a regulatory domain construct (aa 78-355) containing both cGMP binding sites of PKG Iα. A distinct and segregated architecture with an extended central helix separates the two cGMP binding domains. Additionally, a previously uncharacterized helical domain (switch helix) promotes the formation of a hydrophobic interface between protomers. Mutational disruption of this interaction in full-length PKG implicates the switch helix as a critical site of dimer communication in PKG biology. These results offer new structural insight into the mechanism of allosteric PKG activation.

cGMP-dependent protein kinases and cGMP phosphodiesterases in nitric oxide and cGMP action

To date, studies suggest that biological signaling by nitric oxide (NO) is primarily mediated by cGMP, which is synthesized by NO-activated guanylyl cyclases and broken down by cyclic nucleotide phosphodiesterases (PDEs). Effects of cGMP occur through three main groups of cellular targets: cGMP-dependent protein kinases (PKGs), cGMP-gated cation channels, and PDEs. cGMP binding activates PKG, which phosphorylates serines and threonines on many cellular proteins, frequently resulting in changes in activity or function, subcellular localization, or regulatory features. The proteins that are so modified by PKG commonly regulate calcium homeostasis, calcium sensitivity of cellular proteins, platelet activation and adhesion, smooth muscle contraction, cardiac function, gene expression, feedback of the NO-signaling pathway, and other processes. Current therapies that have successfully targeted the NO-signaling pathway include nitrovasodilators (nitroglycerin), PDE5 inhibitors [sildenafil (Viagra and Revatio), vardenafil (Levitra), and tadalafil (Cialis and Adcirca)] for treatment of a number of vascular diseases including angina pectoris, erectile dysfunction, and pulmonary hypertension; the PDE3 inhibitors [cilostazol (Pletal) and milrinone (Primacor)] are used for treatment of intermittent claudication and acute heart failure, respectively. Potential for use of these medications in the treatment of other maladies continues to emerge.

New nitric oxide donors based on ruthenium complexes

Nitric oxide (NO) donors produce NO-related activity when applied to biological systems. Among its diverse functions, NO has been implicated in vascular smooth muscle relaxation. Despite the great importance of NO in biological systems, its pharmacological and physiological studies have been limited due to its high reactivity and short half-life. In this review we will focus on our recent investigations of nitrosyl ruthenium complexes as NO-delivery agents and their effects on vascular smooth muscle cell relaxation. The high affinity of ruthenium for NO is a marked feature of its chemistry. The main signaling pathway responsible for the vascular relaxation induced by NO involves the activation of soluble guanylyl-cyclase, with subsequent accumulation of cGMP and activation of cGMP-dependent protein kinase. This in turn can activate several proteins such as K+ channels as well as induce vasodilatation by a decrease in cytosolic Ca2+. Oxidative stress and associated oxidative damage are mediators of vascular damage in several cardiovascular diseases, including hypertension. The increased production of the superoxide anion (O2-) by the vascular wall has been observed in different animal models of hypertension. Vascular relaxation to the endogenous NO-related response or to NO released from NO deliverers is impaired in vessels from renal hypertensive (2K-1C) rats. A growing amount of evidence supports the possibility that increased NO inactivation by excess O2- may account for the decreased NO bioavailability and vascular dysfunction in hypertension.

A multi-angular mass spectrometric view at cyclic nucleotide dependent protein kinases: in vivo characterization and structure/function relationships

Mass spectrometry has evolved in recent years to a well-accepted and increasingly important complementary technique in molecular and structural biology. Here we review the many contributions mass spectrometry based studies have made in recent years in our understanding of the important cyclic nucleotide activated protein kinase A (PKA) and protein kinase G (PKG). We both describe the characterization of kinase isozymes, substrate phosphorylation, binding partners and post-translational modifications by proteomics based methodologies as well as their structural and functional properties as revealed by native mass spectrometry, H/D exchange MS and ion mobility. Combining all these mass spectrometry based data with other biophysical and biochemical data has been of great help to unravel the intricate regulation of kinase function in the cell in all its magnificent complexity.

The hinge region operates as a stability switch in cGMP-dependent protein kinase I alpha

The molecular mechanism of cGMP-dependent protein kinase activation by its allosteric regulator cyclic-3',5'-guanosine monophosphate (cGMP) has been intensely studied. However, the structural as well as thermodynamic changes upon binding of cGMP to type I cGMP-dependent protein kinase are not fully understood. Here we report a cGMP-induced shift of Gibbs free enthalpy (DeltaDeltaGD) of 2.5 kJ.mol-1 as determined from changes in tryptophan fluorescence using urea-induced unfolding for bovine PKG Ialpha. However, this apparent increase in overall stability specifically excluded the N-terminal region of the kinase. Analyses of tryptic cleavage patterns using liquid chromatography-coupled ESI-TOF mass spectrometry and SDS/PAGE revealed that cGMP binding destabilizes the N-terminus at the hinge region, centered around residue 77, while the C-terminus was protected from degradation. Furthermore, two recombinantly expressed mutants: the deletion fragment Delta1-77 and the trypsin resistant mutant Arg77Leu (R77L) revealed that the labile nature of the N-terminus is primarily associated with the hinge region. The R77L mutation not only stabilized the N-terminus but extended a stabilizing effect on the remaining domains of the enzyme as well. These findings support the concept that the hinge region of PKG acts as a stability switch.

Cytosolic calcium concentration is reduced by photolysis of a nitrosyl ruthenium complex in vascular smooth muscle cells

The effect of the NO donors cis-[RuCl(bpy)(2)(NO)](PF(6)) (RUNOCL) and sodium nitroprusside (SNP) on the cytosolic Ca(2+) concentration ([Ca(2+)](c)) was studied in cells isolated from the rat aorta smooth muscle of cells isolated from the rat aorta smooth muscle. SNP is a metal nitrosyl complex made up of iron, cyanide groups, and a nitro moiety; the RUNOCL complex is made up of ruthenium and bipyridine ligands, with chloride and nitrosyl groups in the ruthenium axial positions. Rat aorta smooth muscle cells were loaded with fluo-3 acetoxymethyl ester (Fluo-3 AM) and imaged by a confocal scanning laser microscope excited with the 488 nm line of the argon ion laser. Fluorescence emission was measured at 510 nm. One of the NO donors, RUNOCL (100 micromol/L) or SNP (100 micromol/L), was then added to the cell chamber and the fluorescent intensity percentage (%IF) was measured after 240 s. RUNOCL reduced the %IF to 60.0+/-10.0% of the initial value. After treatment with the soluble guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazole[4,3-a]quinoxalin-1-one (ODQ) (10 micromol/L), the measurement of %IF was 81.0+/-5.0% (n=4). In the presence of tetraethylammonium (TEA) (1 mmol/L) the %IF was 79.0+/-6.4% (n=4). A combination of ODQ and TEA increased the %IF to 97.0+/-3.5% (n=4). As for SNP, it reduced the %IF to 81.4+/-4.7% (n=4), but this effect was inhibited by ODQ (%IF 94.0+/-3.6%; n=4) and TEA (%IF 88.0+/-2.1%; n=4). The combination of ODQ and TEA increased (%IF 92.0+/-2.8%; n=4). Taken together, these results indicate that both the new NO donor RUNOCL and SNP reduce [Ca(2+)](c). Our data also give evidence that soluble guanylyl cyclase and K(+) channels sensitive to TEA are involved in the mechanisms responsible for the reduction in [Ca(2+)](c) of the rat aorta smooth muscle cells.

A novel gain-of-function mutant of the cyclic GMP-dependent protein kinase egl-4 affects multiple physiological processes in Caenorhabditis elegans

cGMP-dependent protein kinases are key intracellular transducers of cell signaling. We identified a novel dominant mutation in the C. elegans egl-4 cGMP-dependent protein kinase (PKG) and show that this mutation causes increased normal gene activity although it is associated with a reduced EGL-4 protein level. Prior phenotypic analyses of this gain-of-function mutant demonstrated a reduced longevity and a reduced feeding behavior when the animals were left unperturbed. We characterize several additional phenotypes caused by increased gene activity of egl-4. These phenotypes include a small body size, reduced locomotion in the presence of food, a pale intestine, increased intestinal fat storage, and a decreased propensity to form dauer larvae. The multiple phenotypes of egl-4 dominant mutants are consistent with an instructive signaling role of PKG to control many aspects of animal physiology. This is among the first reported gain-of-function mutations in this enzyme of central physiological importance. In a genetic screen we have identified extragenic suppressors of this gain-of-function mutant. Thus, this mutant promises to be a useful tool for identifying downstream targets of PKG.

Probing noncovalent protein-ligand interactions of the cGMP-dependent protein kinase using electrospray ionization time of flight mass spectrometry

Nanoflow electrospray ionization time of flight mass spectrometry (ESI-TOF-MS) was used to study activation properties of the cGMP-dependent protein kinase (PKG). Our nanoflow ESI-TOF-MS analysis confirms that PKG mainly occurs as a 153 kDa homodimer and is able to bind four cGMP molecules, which is in agreement with the known stoichiometry. Binding order and stoichiometry of cGMP, the non-hydrolysable ATP analog beta,gamma-imidoadenosine 5'-triphosphate (AMPPNP) and Mn2+ for PKG were characterized as model for the active PKG-cGMP-ATP/Mg2+ complex. Already in the absence of cGMP, a noncovalent complex between PKG and two molecules of AMPPNP could be observed by ESI-TOF-MS. Binding of AMPPNP to PKG was strongly enhanced by the addition of MnCl2 to the spray solution. This is in agreement with binding of AMPPNP/Mn2+ in the ATP binding pocket of PKG since all protein kinases require a metal ion to accompany ATP in the ATP-binding pocket for proper positioning of the beta and gamma phosphates. Additionally, this finding could imply that within the inactive conformation of PKG, the autoinhibition-domain, when in contact with the substrate-docking domain, does not block the entrance to the ATP-binding site. In the presence of cGMP, less of the fully saturated PKG-(cGMP)4(AMPPNP/Mn2+)2 complex was observed, suggesting that the PKG-ATP interaction is weakened in the active conformation of PKG. Additionally, limited proteolysis in combination with native-ESI MS showed to be a useful tool to study the contact regions on the PKG-dimer and also allowed the rapid determination of the overall autophosphorylation status of the protein. These measurements indicated that autophosphorylation mainly occurs within the first 80 aminoterminal residues and involves in total 3-4 phosphates per subunit.

Activation of Aurora-A kinase by protein phosphatase inhibitor-2, a bifunctional signaling protein

Aurora-A kinase is necessary for centrosome maturation, for assembly and maintenance of a bipolar spindle, and for proper chromosome segregation during cell division. Aurora-A is an oncogene that is overexpressed in multiple human cancers. Regulation of kinase activity apparently depends on phosphorylation of Thr-288 in the T-loop. In addition, interactions with targeting protein for Xenopus kinesin-like protein 2 (TPX2) allosterically activate Aurora-A. The Thr-288 phosphorylation is reversed by type-1 protein phosphatase (PP1). Mutations in the yeast Aurora, Ipl1, are suppressed by overexpression of Glc8, the yeast homolog of phosphatase inhibitor-2 (I-2). In this study, we show that human I-2 directly and specifically stimulated recombinant human Aurora-A activity in vitro. The I-2 increase in kinase activity was not simply due to inhibition of PP1 because it was not mimicked by other phosphatase inhibitors. Furthermore, activation of Aurora-A was unaffected by deletion of the I-2 N-terminal PP1 binding motif but was eliminated by deletion of the I-2 C-terminal domain. Aurora-A and I-2 were recovered together from mitotic HeLa cells. Kinase activation by I-2 and TPX2 was not additive and occurred without a corresponding increase in T-loop phosphorylation. These results suggest that both I-2 and TPX2 function as allosteric activators of Aurora-A. This implies that I-2 is a bifunctional signaling protein with separate domains to inhibit PP1 and directly stimulate Aurora-A kinase.

Dimerization of cGMP-dependent protein kinase Ibeta is mediated by an extensive amino-terminal leucine zipper motif, and dimerization modulates enzyme function

All mammalian cGMP-dependent protein kinases (PKGs) are dimeric. Dimerization of PKGs involves sequences located near the amino termini, which contain a conserved, extended leucine zipper motif. In PKG Ibeta this includes eight Leu/Ile heptad repeats, and in the present study, deletion and site-directed mutagenesis have been used to systematically delete these repeats or substitute individual Leu/Ile. The enzymatic properties and quaternary structures of these purified PKG mutants have been determined. All had specific enzyme activities comparable to wild type PKG. Simultaneous substitution of alanine at four or more of the Leu/Ile heptad repeats ((L3A/L10A/L17A/I24A), (L31A/I38A/L45A/I52A), (L17A/I24A/L31A/I38A/L45A/I52A), and (L3A/L10A/L45A/I52A)) of the motif produces a monomeric PKG Ibeta. Mutation of two Leu/Ile heptad repeats can produce either a dimeric (L3A/L10A) or monomeric (L17A/I24A and L31A/I38A) PKG. Point mutation of Leu-17 or Ile-24 (L17A or I24A) does not disrupt dimerization. These results suggest that all eight Leu/Ile heptad repeats are involved in dimerization of PKG Ibeta. Six of the eight repeats are sufficient to mediate dimerization, but substitutions at some positions (Leu-17, Ile-24, Leu-31, and Ile-38) appear to have greater impact than others on dimerization. The Ka of cGMP for activation of monomeric mutants (PKG Ibeta (delta1-52) and PKG Ibeta L17A/I24A/L31A/I38A/L45A/I52A) is 2- to 3-fold greater than that for wild type dimeric PKG Ibeta, and there is a corresponding 2- to 3-fold increase in cGMP-dissociation rate of the high affinity cGMP-binding site (site A) of these monomers. These results indicate that dimerization increases sensitivity for cGMP activation of the enzyme.

Autophosphorylation of cGMP-dependent protein kinase type II

Cyclic nucleotides are shown to stimulate the autophosphorylation of type II cGMP-dependent protein kinase (cGK) on multiple sites. Mass spectrometric based analyses, using a quadrupole time-of-flight-mass spectrometry instrument revealed that cGMP stimulated the in vitro phosphorylation of residues Ser110 and Ser114, and, at a slow rate, of Ser126 and Thr109 or Ser117, all located in the autoinhibitory region. In addition Ser445 was found to be phosphorylated in a cGMP-dependent manner, whereas Ser110 and Ser97 were already prephosphorylated to a large extent in Sf9 cells. cGMP-dependent phosphorylation of cGK II was also demonstrated in intact COS-1 cells and intestinal epithelium. Substitution of most of the potentially autophosphorylated residues for alanines largely abolished the cGMP stimulation of the autophosphorylation. Prolonged autophosphorylation of purified recombinant cGK II in vitro resulted in a 40-50% increase in basal kinase activity, but its maximal cGMP-stimulated activity and the EC50 for cGMP remained unaltered. Mutation of the major phosphorylatable serines 110, 114, and 445 into "phosphorylation-mimicking" glutamates had no effect on the kinetic parameters of cGK II. However, replacing the slowly autophosphorylated residue Ser126 by Glu rendered cGK II constitutively active. These results show that the fast phase of cyclic nucleotide-stimulated autophosphorylation of cGK II has a relatively small feed forward effect on its activity, whereas the secondary phase, presumably involving Ser126 phosphorylation, may generate a constitutively active form of the enzyme.

cGMP-dependent protein kinase protects cGMP from hydrolysis by phosphodiesterase-5

The physiological effects of cGMP are largely determined by the activities of intracellular receptors, including cGMP-dependent protein kinase (PKG) and cGMP-binding cyclic nucleotide phosphodiesterases (PDEs), and the distribution of cGMP among these receptors dictates activity of the signalling pathway. In the present study, the effects of PKG-Ialpha or PKG-Ibeta on the rate of cGMP hydrolysis by the isolated PDE5 catalytic domain were examined. PKG-Ialpha strongly inhibited cGMP hydrolysis with an IC(50) value of 217 nM, which is similar to the physiological concentration of PKG in pig coronary artery reported previously. By contrast, PKG-Ibeta, which has lower affinity for cGMP than does PKG-Ialpha, inhibited cGMP hydrolysis with an IC(50) of approx. 1 microM. Inhibition by PKG-Ialpha was more effective than that by PKG-Ibeta, consistent with their relative affinities for cGMP. Autophosphorylation of PKGs increased their cGMP-binding affinities and their inhibitory effects on PDE5 hydrolysis of cGMP. Autophosphorylation of PKG-Ibeta increased its inhibitory potency on PDE5 hydrolysis of cGMP by 10-fold compared with a 2-fold increase upon autophosphorylation of PKG-Ialpha. The results indicate that cGMP bound to allosteric cGMP-binding sites of PKG is protected from hydrolysis by PDE5 and that persistent protection of cGMP by either non-phosphorylated or autophosphorylated PKGs may be a positive-feedback control to sustain cGMP signalling.

Activation of cGMP-dependent protein kinase by protein kinase C

The cGMP-dependent protein kinases (PKG) are emerging as important components of mainstream signal transduction pathways. Nitric oxide-induced cGMP formation by stimulation of soluble guanylate cyclase is generally accepted as being the most widespread mechanism underlying PKG activation. In the present study, PKG was found to be a target for phorbol 12-myristate 13-acetate (PMA)-responsive protein kinase C (PKC). PKG1alpha became phosphorylated in HEK-293 cells stimulated with PMA and also in vitro using purified components. PKC-dependent phosphorylation was found to activate PKG as measured by phosphorylation of vasodilator-stimulated phosphoprotein, and by in vitro kinase assays. Although there are 11 potential PKC substrate recognition sites in PKG1alpha, threonine 58 was examined due to its proximity to the pseudosubstrate domain. Antibodies generated against the phosphorylated form of this region were used to demonstrate phosphorylation in response to PMA treatment of the cells with kinetics similar to vasodilator-stimulated phosphoprotein phosphorylation. A phospho-mimetic mutation at this site (T58E) generated a partially activated PKG that was more sensitive to cGMP levels. A phospho-null mutation (T58A) revealed that this residue is important but not sufficient for PKG activation by PKC. Taken together, these findings outline a novel signal transduction pathway that links PKC stimulation with cyclic nucleotide-independent activation of PKG.

A conserved serine juxtaposed to the pseudosubstrate site of type I cGMP-dependent protein kinase contributes strongly to autoinhibition and lower cGMP affinity

Serines 64 and 79 are homologous residues that are juxtaposed to the autoinhibitory pseudosubstrate site in cGMP-dependent protein kinase type Ialpha and type Ibeta (PKG-Ialpha and PKG-Ibeta), respectively. Autophosphorylation of this residue is associated with activation of type I PKGs. To determine the role of this conserved serine, point mutations have been made in PKG-Ialpha (S64A, S64T, S64D, and S64N) and PKG-Ibeta (S79A). In wild-type PKG-Ialpha, basal kinase activity ratio (-cGMP/+cGMP) is 0.11, autophosphorylation increases this ratio 3-fold, and the K(a) and K(D) values for cGMP are 127 and 36 nm, respectively. S64A PKG-Ialpha basal kinase activity ratio increases 2-fold, cGMP binding affinity increases approximately 10-fold in both K(a) and K(D), and activation by autophosphorylation is slight. S64D and S64N mutants are nearly constitutively active in the absence of cGMP, cGMP binding affinity in each increases 18-fold, and autophosphorylation does not affect the kinase activity of these mutants. Mutation of the homologous site in PKG-Ibeta (S79A) increases the basal kinase activity ratio 2-fold and cGMP binding affinity 5-fold over that of wild-type PKG-Ibeta. The combined results demonstrate that a conserved serine juxtaposed to the pseudosubstrate site in type I PKGs contributes importantly to enzyme function by increasing autoinhibition and decreasing cGMP binding affinity.

Exploring the mechanisms of vascular smooth muscle tone with highly specific, membrane-permeable inhibitors of cyclic GMP-dependent protein kinase Ialpha

The structural similarity of cyclic GMP-dependent protein kinase (cGPK) and cyclic AMP-dependent protein kinase (cAPK) has made it difficult to study cGPK pathways independent of those mediated by cAPK, primarily due to the lack of potent and selective cGPK inhibitors. We recently reported a novel peptide library screen specifically designed to select for tight-binding peptides that identified selective inhibitors of cGPK [Proc Natl Acad Sci USA, 97 (2000) 14772]. Iterative deconvolution of octameric library arrays on paper identified the sequence LRK(5)H (W45). Binding of W45 to cGPK resulted in selective inhibition of the kinase, with K(i) values of 0.8 microM and 560 microM for cGPK and cAPK, respectively. Cellular internalization of highly charged W45 was accomplished by N-terminal fusion of membrane translocation sequences from either the human immunodeficiency virus tyrosine aminotransferase protein (47-59) DT-2 or from the Drosophila Antennapedia homeodomain (43-58) DT-3, respectively. For both fusion peptides, DT-2 and DT-3, we observed a potentiating effect with respect to the inhibitory potency, with K(i) values 40- to 80-fold lower than W45. Fluorescein-labeled DT-2 and DT-3 demonstrated rapid translocation through the cytosol and nuclei in a time-dependent manner using cultured cells and intact tissue samples (cerebral arteries). The physiological effects of DT-2 and DT-3 as selective cGPK inhibitors in smooth muscle were studied in small intact arteries. Nitric oxide, a cyclic GMP/cGPK activator, elicited a concentration-dependent dilation of isolated rat cerebral arteries, which was markedly inhibited by DT-2 and DT-3. Collectively, these results indicate that DT-2 and DT-3 effectively inhibit nitric oxide-induced vasodilation, further emphasizing the central role for cGPK in the modulation of vascular contractility.

Spatiotemporal dynamics of guanosine 3',5'-cyclic monophosphate revealed by a genetically encoded, fluorescent indicator

To investigate the dynamics of guanosine 3',5'-cyclic monophosphate (cGMP) in single living cells, we constructed genetically encoded, fluorescent cGMP indicators by bracketing cGMP-dependent protein kinase (cGPK), minus residues 1-77, between cyan and yellow mutants of green fluorescent protein. cGMP decreased fluorescence resonance energy transfer (FRET) and increased the ratio of cyan to yellow emissions by up to 1.5-fold with apparent dissociation constants of approximately 2 microM and >100:1 selectivity for cGMP over cAMP. To eliminate constitutive kinase activity, Thr(516) of cGPK was mutated to Ala. Emission ratio imaging of the indicators transfected into rat fetal lung fibroblast (RFL)-6 showed cGMP transients resulting from activation of soluble and particulate guanylyl cyclase, respectively, by nitric oxide (NO) and C-type natriuretic peptide (CNP). Whereas all naive cells tested responded to CNP, only 68% responded to NO. Both sets of signals showed large and variable (0.5-4 min) latencies. The phosphodiesterase (PDE) inhibitor 3-isobutyl-1-methylxanthine (IBMX) did not elevate cGMP on its own but consistently amplified responses to NO or CNP, suggesting that basal activity of guanylate cyclase is very low and emphasizing the importance of PDEs in cGMP recycling. A fraction of RFL cells showed slowly propagating tides of cGMP spreading across the cell in response to delocalized application of NO. Biolistically transfected Purkinje neurons showed cGMP responses to parallel fiber activity and NO donors, confirming that single-cell increases in cGMP occur under conditions appropriate to cause synaptic plasticity.

Identification of a conserved residue responsible for the autoinhibition of cGMP-dependent protein kinase Ialpha and beta

We isolated a constitutively active form of cGMP-dependent protein kinase Ialpha (cGK Ialpha) by PCR-driven random mutagenesis. The replacement of Ile-63 by Thr in the autoinhibitory domain results in the enhancement of autophosphorylation and the basal kinase activity in the absence of cGMP. The hydrophobicity at position 63 is essential for the inactive state of cGK Ialpha, and Ile-78 of cGK Ibeta is also required for the autoinhibitory property. Furthermore, cGK Ialpha (Ile-63-Thr) is constitutively active in vivo. These findings suggest that a conserved residue in the autoinhibitory domain was involved in the autoinhibition of both cGK Is.

Cyclic nucleotide-dependent protein kinases: intracellular receptors for cAMP and cGMP action

Intracellular cAMP and cGMP levels are increased in response to a variety of hormonal and chemical stimuli; these nucleotides play key roles as second messenger signals in modulating myriad physiological processes. The cAMP-dependent protein kinase and cGMP-dependent protein kinase are major intracellular receptors for these nucleotides, and the actions of these enzymes account for much of the cellular responses to increased levels of cAMP or cGMP. This review summarizes many studies that have contributed significantly to an improved understanding of the catalytic, regulatory, and structural properties of these protein kinases. These accumulated findings provide insights into the mechanisms by which these enzymes produce their specific physiological effects and are helpful in considering the actions of other protein kinases as well.

Delineation of selective cyclic GMP-dependent protein kinase Ialpha substrate and inhibitor peptides based on combinatorial peptide libraries on paper

Peptide libraries on cellulose paper have proven to be valuable tools for the a priori determination of substrate specificities of cyclic AMP- and cyclic GMP-dependent protein kinases (cAMP-kinase and cGMP-kinase) on the basis of octa-peptide sequences. Here, we report the extension of our peptide library screens to 12-mer and 14-mer peptide sequences, resulting in highly cGMP-kinase Ialpha selective peptides. The sequences TQAKRKKSLAMA-amide and TQAKRKKSLAMFLR-amide, with Km values for cGMP-kinase Ialpha of 0.7 and 0.26 microM and Vmax values of 11.5 and 10.9 micromol/min/mg, respectively, display a high specificity for this enzyme. Furthermore, replacing the phosphate acceptor residue serine with alanine in TQAKRKKSLAMA-amide resulted in the highly cGMP-kinase Ialpha selective inhibitor peptide TQAKRKKALAMA-amide, with inhibitor constants for cGMP-kinase Ialpha and cAMP-kinase of 7.5 microM and 750 microM, respectively. Selective cGMP-kinase inhibitors have the potential to play an important role in the elucidation of the distinct cellular functions of cGMP-kinase separate from those activated by cAMP-kinases, and, therefore, may play an important role as pharmaceutical targets. Molecular docking experiments of the most cGMP-kinase selective sequences on a molecular model of the catalytic domain of cGMP-kinase Ialpha suggest that they adopt unique conformations, which differ significantly from those observed for the cAMP-kinase-specific inhibitor PKI(5-24). Our results suggest that despite their structural similarities, cAMP-kinase and cGMP-kinase use distinct peptide substrate and inhibitor conformations, which could account for their unique substrate specificities. These findings are further supported by cAMP- and cGMP-kinase-selective inhibitor analogs with (D)-Ala residues at the inhibitory positions.

Phosphorylation and activation of cGMP-dependent protein kinase by Src

Using information obtained from experiments with peptide substrates of v-Src, a motif within the cGMP-binding domain of cGMP-dependent protein kinase (cGK) was identified as a potential phosphorylation site for v-Src. Here we show that the purified Ialpha isozyme of cGK is phosphorylated stoichiometrically and in a time-dependent manner by purified Src in vitro. The kinase activity of cGK is elevated approximately 4-fold (relative to autophosphorylated cGK) or 10-fold (relative to unphosphorylated cGK) upon tyrosine phosphorylation by Src. Phosphorylation of cGK by v-Src produces modest effects on the cGMP-binding properties and dissociation rates of cGK, and reduces the kact for cGMP. We hypothesize that the mechanism of activation may involve coupling of the cGMP binding domain to the catalytic domain.

Activation by autophosphorylation or cGMP binding produces a similar apparent conformational change in cGMP-dependent protein kinase

Binding of cyclic nucleotide to or autophosphorylation of cGMP-dependent protein kinase (PKG) activates this kinase, but the molecular mechanism of activation for either process is unknown. Activation of PKG by cGMP binding produces a conformational change in the enzyme (Chu, D.-M., Corbin, J. D., Grimes, K. A., and Francis, S. H. (1997) J. Biol. Chem. 272, 31922-31928; Zhao, J., Trewhella, J., Corbin, J., Francis, S., Mitchell, R., Brushia, R., and Walsh, D. (1997) J. Biol. Chem. 272, 39129-31936). In the present studies, activation of type Ibeta PKG by either autophosphorylation or cGMP-binding alone causes (i) an electronegative charge shift on ion exchange chromatography, (ii) a similar increase ( approximately 3.5 A) in the Stokes radius as determined by gel filtration chromatography, and (iii) a similar decrease in the mobility of the enzyme on native gel electrophoresis. Consistent with these results, cGMP binding increases the rate of phosphoprotein phosphatase-1 catalyzed dephosphorylation of PKG which is autophosphorylated only at Ser-63 (not activated); however, dephosphorylation of PKG that is highly autophosphorylated (activated) is not stimulated by cGMP. The combined results suggest that activation of PKG by either autophosphorylation or cGMP binding alone produces a similar apparent elongation of the enzyme, implying that either process activates the enzyme by a similar molecular mechanism.

Identification of the amino acid sequences responsible for high affinity activation of cGMP kinase Ialpha

The cGMP-dependent protein kinases (cGK) Ialpha and Ibeta have identical cGMP binding sites and catalytic domains. However, differences in their first 100 amino acids result in 15-fold different activation constants for cGMP. We constructed chimeras to identify those amino acid sequences that contribute to the high affinity cGK Ialpha and low affinity cGK Ibeta phenotype. The cGK Ialpha/Ibeta chimeras contained permutations of six amino-terminal regions (S1-S6) including the leucine zipper (S2), the autoinhibitory domain (S4), and the hinge domain (S5, S6). The exchange of S2 along with S4 switched the phenotype from cGK Ialpha to cGK Ibeta and vice versa, suggesting that the domains with the highest homology between the two isozymes determine their affinity for cGMP. The high affinity cGK Ialpha phenotype was also obtained by a specific substitution within the hinge domain. Chimeras with the sequence of cGK Ialpha in S5 and cGK Ibeta in S6 were activated at up to 6-fold lower cGMP concentrations than cGK Ialpha. Based on the activation constants of all chimeras constructed, empirical weighting factors have been calculated that quantitatively describe the contribution of the individual amino-terminal domains S1-S6 to the high affinity cGK Ialpha phenotype.

The catalytic domain of the cGMP-dependent protein kinase Ialpha modulates the cGMP-binding characteristics of its regulatory domain

The cGMP-dependent protein kinase Ialpha (PKG Ialpha) possesses two functional moieties, the regulatory and catalytic domains, which reside on a single polypeptide chain. Here we report on the influence of the catalytic domain on the binding of cGMP to the regulatory domain. A deletion mutant, delta352-670 of PKG Ialpha, lacking the catalytic domain, was constructed and expressed in E. coli. The purified 38 kDa mutant protein showed strong reactivity toward tryptic proteolysis at residue Arg77. Thus, a double deletion fragment delta1-77/352-670 PKG Ialpha, lacking the N-terminus, was also purified. Both proteins had functional cGMP binding, but differed kinetically from the wild-type protein. First the affinity constants for cGMP were modulated, second the constructs showed no signs of cooperative cGMP binding and third dimerization of the delta352-670 mutant was abolished. Our results provide evidence that the catalytic domain forms an intimate interaction with the regulatory domain and modulates the kinetics of cGMP binding.

Autophosphorylation of type Ibeta cGMP-dependent protein kinase increases basal catalytic activity and enhances allosteric activation by cGMP or cAMP

Autophosphorylation of purified bovine Ibeta isozyme of cGMP-dependent protein kinase (Ibeta cGK) in the presence of cGMP or cAMP increased basal kinase activity (-cGMP) as much as 4-fold and reduced the Ka for both cGMP and cAMP; maximum catalytic activity (+cGMP) was not altered. Autophosphorylation proceeded with at least two rate components. The faster rate correlated with phosphorylation of Ser-63. The slower rate, as well as the increase in basal kinase activity and decrease in Ka for cyclic nucleotides, correlated with phosphorylation of Ser-79. Autophosphorylation of either residue was an intramolecular reaction. Autophosphorylation of a proteolytically generated Ibeta cGK monomer lacking amino-terminal residues 1-64 increased basal activity (3-fold) and decreased Ka for cAMP (15-fold). This indicated that autophosphorylation of Ser-79 did not require dimeric cGK and that the phosphorylation of Ser-79 in the monomer was sufficient to alter enzymatic characteristics of Ibeta cGK. These studies suggested that increases in intracellular cGMP or cAMP could result in autophosphorylation of Ibeta cGK, which would increase basal kinase activity as well as the sensitivity of cGK to activation by cGMP or to cross-activation by cAMP. Autophosphorylation could also prolong the increased kinase activity after decline of the second messenger.

Stimulation of cGMP-dependent protein kinase I alpha by a peptide from its own sequence. An investigation by enzymology, circular dichroism and 1H NMR of the activity and structure of cGMP-dependent protein kinase I alpha-(546-576)-peptide amide

The structure of cGMP-dependent protein kinase I alpha-(546-576)-peptide amide (peptide-546) and its effects on cGMP-dependent protein kinase I alpha (G-kinase) have been studied. By primary sequence analysis and analogy to a peptide that stimulates protein kinase C, peptide-546 was predicted to form part of the protein/peptide binding site of G-kinase, and it was proposed that it would stimulate the enzyme by interaction with an autoinhibitory site. The portion of cAMP-dependent protein kinase analogous to peptide-546 forms part of the peptide substrate binding site, interacting with the peptide inhibitor residues Argp-2 and Phep-11 (where p is the pseudophosphorylation site), through residues at positions corresponding to Glu4, Pro10 and Ser13 in peptide-546. Peptide-546 is a reasonably potent G-kinase activator, increasing the turnover number with the peptide substrate Arg-Lys-Arg-Ser-Arg-Lys-Glu by about threefold with an activation constant that is about fivefold lower than the Km value of this peptide substrate. Peptide-546 does not appear to change the affinity of the enzyme for the above substrate, ATP or cGMP and does not affect the binding of [3H]cGMP to G-kinase. The activation does not seem to result from an interaction between peptide-546 and peptide substrates, and a kinetic scheme is proposed which is compatible with an action of peptide-546 on G-kinase independent of substrates. The activation is additive with that given by cGMP and causes the enzyme to enter a hitherto unrecognised superactive state. Peptide conformation has been monitored in mixed 2,2,2-trifluoroethanol/H2O solvents by circular dichroism: helical structure is observed in these mixtures when the 2,2,2-trifluoroethanol content is above 25%. The structure is lost only gradually on raising the temperature to 80 degrees C with no clear melting transition. Assignment of the resonances in the 1H-NMR spectrum has allowed the identification of elements of secondary structure from detected nuclear Overhauser effects. In particular, a helical segment from Met18 to Arg26 is observed. The four proline residues (Pro10, Pro11, Pro15 and Pro17) are all seen to be in the trans conformation, although additional, weaker peaks in the spectra may correspond to a minor conformer in which one or more of the prolines is in a cis conformation. The N-terminal residues are less structured but show some helical character.(ABSTRACT TRUNCATED AT 400 WORDS)

Autophosphorylation: a salient feature of protein kinases

Most protein kinases catalyze autophosphorylation, a process which is generally intramolecular and is modulated by regulatory ligands. Either serine/threonine or tyrosine serves as the phosphoacceptor, and several sites on the same kinase subunit are usually autophosphorylated. Autophosphorylation affects the functional properties of most protein kinases. Members of the protein kinase family exhibit diversity in the characteristics and functions of autophosphorylation, but certain common themes are emerging.

Cyclic nucleotide-dependent protein kinases

The actions of several hormones and neurotransmitters evoke signal transduction pathways which rapidly elevate the cytosolic concentrations of the intracellular messengers, cAMP and cGMP. The cyclic-nucleotide dependent protein kinases, cAMP-dependent protein kinase (PKA) and cGMP-dependent protein kinase (PKG), are the major intracellular receptors of cAMP and cGMP. These enzymes become active upon binding respective cyclic nucleotides and modulate a diverse array of biochemical events through the phosphorylation of specific substrate proteins. The focus of this review is to describe the progress made in understanding the structure and function of both PKA and PKG.

Molecular cloning and predicted full-length amino acid sequence of the type I beta isozyme of cGMP-dependent protein kinase from human placenta. Tissue distribution and developmental changes in rat

In this study we report the isolation and characterization of three overlapping cDNA clones for the type I beta isozyme of cGMP-dependent protein kinase (cGK) from human placenta libraries. The composite sequence was 3740 nucleotides long and contained 58 nucleotides from the 5'-noncoding region, an open reading frame of 2061 bases including the stop codon, and a 3'-noncoding region of 1621 nucleotides. The predicted full-length human type I beta cGK protein contained 686 amino acids including the initiator methionine, and had an estimated molecular mass of 77,803 Da. On comparison to the published amino acid sequence of bovine lung I alpha, human placenta I beta cGK differed by only two amino acids in the carboxyl-terminal region (amino acids 105-686). In contrast, the amino-terminal region of the two proteins was markedly different (only 36% similarity), and human I beta cGK was 16 amino acids longer. In a specific region in the amino-terminus (amino acids 63-75), 12 out of 13 amino acids of the human I beta cGK were identical to the partial amino acid sequence recently published for a new I beta isoform of cGK from bovine aorta. Northern blot analysis demonstrated a human I beta cGK mRNA, 7 kb in size, in human uterus and weakly in placenta. An mRNA of 7 kb was also observed in rat cerebellum, cerebrum, lung, kidney, and adrenal, whereas an mRNA doublet of 7.5 and 6.5 kb were observed in rat heart. Comparison of Northern and Western blot analyses demonstrated that the mRNA and protein for cerebellar cGK increased during the development of rats from 5 to 30 days old, whereas the 6.5 kb mRNA in rat heart declined.

The cDNA of the two isoforms of bovine cGMP-dependent protein kinase

cDNAs encoding the isoform I alpha of the cGMP-dependent protein kinase were isolated from a bovine trachea smooth muscle cDNA library constructed in lambda gt10. The deduced protein sequence is identical with the protein sequence obtained by Edman degradation of the bovine lung enzyme [(1984) Biochemistry 23, 4207-4218]. Alternate cDNA clones were isolated which code for a protein slightly different within the aminoterminal part from the known amino acid sequence. These alternate cDNAs contain the sequence of a peptide identified in the isoform I beta of cGMP-dependent protein kinase. Northern blot analysis of poly(A)+ RNA from bovine trachea smooth muscle indicated the presence of two different mRNA species of about 6.2 kb.

The amino terminus regulates binding to and activation of cGMP-dependent protein kinase

The allosteric regulation of binding to and the activation of cGMP-dependent protein kinase (cGMP kinase) was studied under identical conditions at 30 degrees C using three forms of cGMP-kinase which differed in the amino-terminal segment, e.g. native cGMP kinase, phosphorylated cGMP kinase which contained 1.4 +/- 0.4 mol phosphate/subunit and constitutively active cGMP kinase which lacked the amino-terminal dimerization domain. These three enzyme forms have identical kinetic constants, e.g. number of cGMP-binding sites, Km values for MgATP and the heptapeptide kemptide, and Vmax values. In the native enzyme, MgATP decreases the affinity for binding site 1. This effect is abolished by 1 M NaCl. In contrast, high concentrations of Kemptide increase the affinity of binding site 2 about fivefold. Under the latter conditions, identical Kd values of 0.2 microM were obtained for sites 1 and 2. Salt, MgATP and Kemptide do not affect the binding kinetics of the phosphorylated or the constitutively active enzyme, suggesting that allosteric regulation depends solely on the presence of a native amino-terminal segment. Cyclic GMP activates the native enzyme at Ka values which are identical with the Kd values for both binding sites. The activation of cGMP-dependent protein kinase is noncooperative but the Ka value depends on the substrate peptide concentration. These results show that the activity of cGMP kinase is primarily regulated by conformational changes within the amino-terminal domain.