Chemical cross-linking of cyclic AMP-dependent protein kinase and its dissimilar subunits

Previous kinetic studies have demonstrated that the activation of cyclic AMP-dependent protein kinase by cyclic AMP involves the formation of a ternary complex of cyclic AMP, the regulatory subunit (R) and the catalytic subunit (C). It is suggested that only this ternary complex breaks down to liberate the enzymically active catalytic subunit. We have performed cross-linking experiments with the holoenzyme and its dissimilar subunits in the presence of MgATP and various concentrations of cyclic AMP. Results from these cross-linking studies indicate that regulatory subunits exist as dimers in the native form. Moreover, dissociation of the holoenzyme or the reconstituted enzyme is promoted by cyclic AMP, and the effect of MgATP is to stabilize the enzyme in the tetrameric form. The success in cross-linking the regulatory and the catalytic subunits of protein kinase with the lysine-specific bifunctional cross-linking reagent dimethyl suberimidate may be attributed to the presence of a large number of lysine residues in the enzyme.     

Determination of binding parameters of cyclic AMP and its analogs to cyclic AMP-dependent protein kinase by the fluorescent probe method.

The method for determination of dissociation constants for cyclic AMP and its analogs bound to cyclic AMP-dependent protein kinase from pig brain is described. The technique for measuring the binding parameters of the ligands is based on the changes in the fluorescent spectrum of etheno cyclic AMP once it is bound to protein kinase. The dissociation constants for a number of nonfluorescent cyclic AMP analogs were determined in the competitive displacement of etheno cyclic AMP by these analogs. The number of cyclic AMP-binding sites in the pig brain protein kinase was found to be 2.2; no cooperativity was observed upon binding. The holoenzyme complex (Mr = 180,000) of the protein kinase under study was established to have the stoichiometry of R2C2 type under native conditions.     

Protein kinase from Mucor rouxii

Summary Cyclic AMP binding to Mucor rouxii protein kinase holoenzyme and free regulatory subunit was measured by the classical membrane filtration technique and by equilibrium dialysis. The results obtained demonstrate that the filtration method can be used without loss of any cyclic AMP binding site. Both methods unambiguously demonstrate that the number of molecules of cyclic AMP bound to the holoenzyme are half of those bound to the regulatory subunit. This result suggests that unshielding of new cyclic AMP binding sites occurs upon dissociation of the ternary complex holoenzyme-cyclic AMP.     

The autophosphorylation reaction in the mechanism of activation of pig brain cyclic AMP-dependent protein kinase

Autophosphorylation of cyclic AMP-dependent protein kinase (ATP: protein phosphotransferase, EC 2.7.1.37) was shown to occur via an intramolecular mechanism: the regulatory subunit undergoes phosphorylation only within the holoenzyme. The phospho form of the catalytic subunit has the capacity to phosphorylate the regulatory subunit. The phosphotransferase reaction and the reaction of autophosphorylation were found to proceed with the involvement of the same active site. The activation constant of phospho- and dephosphoprotein kinase under the influence of cyclic AMP and the dissociation constant of the cyclic AMP complex with phospho- and dephospho forms of the holoenzyme were estimated. Autophosphorylation was demonstrated to lead to almost complete dissociation of the holoenzyme under the influence of cyclic AMP. Circular dichroism spectra of the phosphorylated and non-phosphorylated forms of protein kinase were studied. The relative content of the secondary structure elements in proteins was estimated and conformational changes were detected in the enzyme upon its interaction with cycli AMP. The anti-conformation of the cyclic nucleotide fixed in the complex with the phospho form of the regulatory subunit is suggested.     

The use of affinity chromatography in purification of cyclic nucleotide receptor proteins.

Biospecific affinity chromatography has been used to purify specific cyclic AMP and cyclic GMP receptor proteins. Several variables are important for successful purification of the cyclic AMP receptor protein, the most critical being the length of the aliphatic spacer side arm. 8-(2-Aminoethyl)-amino-cyclic AMP coupled to the aliphatic spacer side arm. 8-(2-Aminoethyl)-amino-cyclic AMP coupled to agarose specifically retains the cyclic AMP receptor protein by interaction with the immobilized nucleotide. Binding of the cyclic AMP receptor subunit of cyclic AMP-dependent protein kinase to the immobilized nucleotide results in dissociation of the catalytic protein phosphokinase subunit which is not retained. The retained cyclic AMP receptor protein is subsequently eluted by cyclic AMP. Homogeneous cyclic AMP receptor protein prepared from rabbit skeletal muscle by affinity chromatography has been characterized. The molecular weight of the native protein as determined by analytical ultracentrifugation and polyacrylamide gel electrophoresis at varying acrylamide concentrations is 76 800 and 82 000, respectively. The protein is asymmetric with frictional and axial ratios of 1.64 and 12. SDS and urea polyacrylamide gel electrophoresis indicate that the native cyclic AMP receptor is composed of two identical subunits of 42 700 molecular weight. The native protein dimer binds 2 moles of cyclic AMP per mole of protein and is active in suppressing activity of isolated catalytic subunits of cyclic AMP-dependent protein kinase. Cyclic GMP receptor protein from bovine lung has been purified using the same affinity chromatography media. Since cyclic nucleotide binding to cyclic GMP-dependent protein kinase does not result in dissociation of regulatory receptor and catalytic phosphotransferase subunits, the cyclic GMP-dependent protein kinase holoenzyme is retained on the column and can be subsequently specifically eluted with cyclic GMP.     

Altered regulation of cyclic AMP-dependent protein kinase in a mouse lymphoma cell line.

The ability of cyclic AMP to inhibit growth, cause cytolysis and induce synthesis of cyclic AMP-phosphodiesterase in S49.1 mouse lymphoma cells is deficient in cells selected on the basis of their resistance to killing by 2 mM dibutyryl cyclic AMP. The properties of the cyclic AMP-dependent protein kinase (ATP:protein phosphotransferase, EC 2.7.1.37) in the cyclic AMP-sensitive (S) and cyclic AMP-resistant (R) lymphoma cells were comparatively studied. The cyclic AMP-dependent protein kinase activity or R cells cytosol exhibits an apparent Ka for activation by cyclic AMP 100-fold greater than that of the enzyme from the parental S cells. The free regulatory and catalytic subunits from both S and R kinase are thermolabile, when associated in the holoenzyme the two subunits are more stable to heat inactivation in R kinase than in S kinase. The increased heat stability of R kinase is observed however only for the enzyme in which the catalytic and cyclic AMP-binding activities are expressed at high cyclic AMP concentrations (10(-5)--10(-4) M), the activities expressed at low cyclic AMP concentrations (10(-9)--10(-6) M) being thermolabile. The regulatory subunit of S kinase can be stabilized against heat inactivation by cyclic AMP binding both at 2-10(-7) and 10(-5) M cyclic AMP concentrations. In contrast, the regulatory subunit-cyclic AMP complex from R kinase is stable to heat inactivation only when formed in the presence of high cyclic AMP concentrations (10(-5)M). The findings indicate that the transition from a cyclic AMP-sensitive to a cyclic AMP-resistant lymphoma cell phenotype is related to a structural alteration in the regulatory subunit of the cyclic AMP-dependent protein kinase which has affected the protein's affinity for cyclic AMP and its interaction with the catalytic subunit.     

Polymeric structure of the cyclic AMP-dependent protein kinase from the dimorphic fungus Mucor rouxii and purification of its catalytic subunit

Summary The polymeric structure of the cyclic AMP-dependent protein kinase (E.C.2.7.1.37) from the dimorphic fungus Mucor rouxii was analyzed through studies of gel filtration and sucrose gradient centrifugation of the holoenzyme and its subunits and by photoaffinity labeling of the regulatory subunit. It was demonstrated that it is a tetramer composed by two regulatory subunits (R) of mol. wt. 75 000 and two catalytic subunits (C) of mol. wt. 41 000 forming a holoenzyme R₂C₂ of mol. wt. 242 000. Frictional coefficients of 1.55 and 1.62 for the holoenzyme and for the regulatory dimer, respectively, indicate a significant degree of dimensional asymmetry in both molecules. A procedure for the purification of the catalytic subunit of the kinase is presented. The holoenzyme could be bound to a cyclic AMP-agarose column and the catalytic subunit could be eluted by 0.5 M NaCl, well resolved from the bulk of protein. This particular behaviour of the holoenzyme in cyclic AMP-agarose chromatography allowed the inclusion of this step in the purification of the catalytic subunit and corroborated that the holoenzyme was not dissociated by cyclic AMP alone. The isolated catalytic subunit displays Michaelis-Menten behaviour towards kemptide, protamine and histone and is inhibited by sulfhydryl reagents, indicating that the molecule has at least one cysteine residue essential for enzyme activity. The catalytic activity of the isolated C subunit is inactivated by the mammalian protein kinase inhibitor, and is inhibited by the regulatory subunit from homologous and heterologous sources. In general, the properties of the catalytic subunit suggest a structural similarity between Mucor and mammalian C subunits.Abbreviations C catalytic subunit monomer of protein kinase - R regulatory subunit monomer of protein kinase - 8-N₃-cyclic AMP 8-azido-cylic AMP - SDS sodium dodecyl sulfate - Pipes piperazine-N,N-bis(2-ethanesulfonic acid) See AcknowledgementsCareer Investigators from the CONICET     

Evidence that rabbit muscle protein kinase has two kinetically distinct binding sites for adenosine 3' ; 5'-cyclic monophosphate.

Two distinct populations of binding sites for cyclic AMP are associated with the regulatory moity of cyclic AMP dependent protein kinase (E.C. 2.7.1.37), as judged from the kinetics of the interaction between the nucleotide and the binding protein. The two types of sites were present at the proportion 1:1. The rate of dissociation of bound cyclic AMP was more rapid for one type of site than for the other type. High ionic strength accentuated the difference in the rate of dissociation of cyclic AMP from the two sites.The two binding sites and protein kinase activity copurified during the entire procedure for preparation of protein kinase holoenzyme. The kinetic properties of each of the two sites and the proportion between them was the same in a highly purified preparation of the regulatory moiety of protein kinase and in binding protein freshly prepared in the presence of protease-inhibitor.     

Cyclic AMP-dependent pig brain protein kinase: subunit structure, mechanism of autophosphorylation and holoenzyme dissociation under cyclic AMP action

Some properties of cyclic AMP-dependent pig brain protein kinase were studied. The holoenzyme was shown to exist in solution in the form of a tetramer complex R2C2 with mol. weight of 180 000. The limited proteolysis of the regulatory subunit caused the formation of a fragment with mol. weight of 35 000, capable of independent binding of 3H-cyclic AMP and containing a site, which can be phosphorylated in the autophosphorylation reaction. Autophosphorylation of the holoenzyme led to an increase in the degree of dissociation of the former into individual subunits under the effect of cyclic AMP. The ability of the phosphoform of the catalytic subunit was demonstrated. The autophosphorylation process and the phosphotransferase reaction involve the same active site of the catalytic subunit.     

Autophosphorylation of cyclic AMP-dependent protein kinase from pig brain

Autophosphorylation of cyclic AMP-dependent pig brain protein kinase has been detected. Up to 1,5 moles of gamma-32P are transferred from [gamma-32P]ATP to the dimer of the regulatory subunit. The autophosphorylation reaction is Mg2+-dependent and occurs at a high rate: more than 50% of the radioactive label is incorporated during the first minute of incubation at 30 degrees. The pH dependence of this reaction differs from that of the phosphotransferase reaction. The phosphoholoenzyme is more sensitive to cyclic AMP than the dephosphoholoenzyme; however, both forms bind up to 2 moles of 3H-cyclic AMP per 1 mole of the holoenzyme. The activation and dissociation constants for both forms of the holoenzyme have been calculated. The autophosphorylation reaction has been shown to occur via an intramolecular mechanism; the phosphorylation of the regulatory subunit can occur only within the holoenzyme. The increase in the concentration of cyclic AMP causes the latter to produce an inhibitory effect on autophosphorylation. The regulatory action of autophosphorylation on cyclic AMP-dependent protein kinases is discussed.     

A kinetic study of cyclic adenosine 3':5'-monophosphate binding and mode of activation of protein kinase from Drosophila melanogaster embryos.

Cyclic AMP-dependent protein kinase and its regulatory subunit were isolated from Drosophila melanogaster embryos. The profiles of cyclic AMP binding by these proteins were significantly different. In order to explain such a difference and to find the mode of enzyme activation by cyclic AMP, a kinetic study of cyclic AMP binding was carried out. First, the association rate constant k1 and dissociation rate constant k-1 in the cyclic AMP-regulatory subunit interaction at 0 degrees C were estimated to be 2.3 X 10(6)M-1s-1 and 1.1 X 10(-3)s-1, respectively. Secondly, the three possible modes of enzyme activation by cyclic AMP were mathematically considered and could be described by a unique formula: r=APt + BQt (A + B=1) in which the parameters A, B, P, and Q are equivalent to rate constants in the sense that the rate constants are simply expressed by these parameters. Thirdly, the values of the parameters and subsequently the values of rate constants involved in the possible mechanisms were evaluated using a curve-fitting technique and compared with experimental observation. It was then found that the following mechanism was the only one which fitted the experimental observations. Namely, RC + L k3 equilibrium k-3 LRC k4 equilibrium k-4 RL + C where R, C, and L represent the regulatory and catalytic subunits and cyclic AMP as a ligand. Thus, our results indicate that in the presence of cyclic AMP the active enzyme (C) is released from a ternary intermediate which is the primary product of the cyclic AMP-holoenzyme interaction. The estimated values of the rate constants are: k3=3.5 X 10(6)M-1s-1;k-3=7.3 X 10(-1)s-1;and k4=3.8 X 10(-2)s. These estimates indicate that the reaction LRC leads to RL + C is relatively slow and limits the rate of the overall reaction. By comparing k-3 and k4, it is apparent that a large part of newly formed ternary intermediate reverts to the holoenzyme.     

Interaction of cyclic adenosine 3':5'-monophosphate with protein kinase. Equilibrium binding models.

A number of potential models for the interaction of cyclic AMP with protein kinase (RC or R2C2) have been examined. These include: Model 1, the simultaneous binding of cyclic AMP and release of C (catalytic subunit) from an independent RC protomer; Model 2, dissociation of an independent RC protomer prior to cyclic AMP binding to R (regulatory subunit); Model 3, cyclic AMP binding to RC prior to the dissociation of C; Model 4, random binding of cyclic AMP and dissociation of C with an interaction factor alpha less than 1; Model 5, release of 2C concomitant with the binding of one cyclic AMP to R2C2 followed by binding of the second cyclic AMP to the vacant R subunit; and Model 6, the simultaneous binding of cyclic AMP and release of C from one RC protomer resulting in a greater "affinity" of the other RC protomer for cyclic AMP, i.e., a cooperative version of Model 1. All the above models yield [cyclic AMP]0.5 values that increase with increasing protein concentration and Hill plots with average slopes equal to or less than 1.0 in the usual experimental range (10 to 90% of saturation). The Hill plots can be nonlinear, but for each model the exact shape of the plot changes in a characteristic (diagnostic) manner with changing protein concentration. Skeletal muscle protein kinase yields relatively linear Hill plots with napp values greater than 1.0. Consequently, Models 1 to 6 are not likely candidates. However, Model 2 is an excellent alternative model for proteins that display "negative cooperativity" with respect to the binding of a ligand. The properties of several "linear", "tetrahedral", and "all-or-nothing" cooperative models have also been examined. These include Models 7, A, B, and C and 8, A, B, and C which are cooperative versions of Models 2 and 3, respectively, and Model 9, a cooperative version of random Model 4. Model 9 is the most general model from which all others can be derived. Models 9 and 7, A, B, and C in which the prior dissociation of C greatly enhances or is an absolute requirement for cyclic AMP binding to R, are likely candidates for skeletal muscle protein kinase. All four of these models are capable of yielding Hill plots with average slopes greater than 1, and napp values that decrease with increasing protein concentration (in agreement with published data). In addition, in all four models the tight binding of MgATP to R2C2 yields decreased napp values and increased [cyclic AMP]0.5 values (also consistent with published data).     

Reconstitution of types I and II adenosine cyclic 3',5'-phosphate dependent protein kinase

Fluorescence intensity and anisotropy measurements using the fluorescent adenosine cyclic 3',5'-phosphate (cAMP) analogue 1,N6-ethenoadenosine cyclic 3',5'-phosphate (epsilon-cAMP) are sensitive to the dissociation of epsilon-cAMP which occurs when either the type I or the type II regulatory subunit (RI or RII) of cAMP-dependent protein kinase associates with the catalytic subunit. Studies using epsilon-cAMP show that MgATP has opposite effects on the reconstitution of both types of protein kinase: MgATP strongly stabilizes the type I holoenzyme while it slightly destabilizes the type II holoenzyme. The synthetic substrate Kemptide has a small inhibitory effect on the reconstitution of both holoenzymes when tested at 10 microM concentration. The protein kinase inhibitor has a larger effect which is especially pronounced in the reassociation of the type I enzyme. The diminished relative ability of the type I regulatory subunit to compete with the protein kinase inhibitor suggests that the combined effects of the two opposing equilibria (epsilon-cAMP and catalytic subunit binding) are different for the two types of regulatory subunits. Displacement experiments show that cAMP and epsilon-cAMP bind about equally well to the type I subunit. Slow conformational changes accompanying the binding of epsilon-cAMP by both regulatory subunits are greatly accelerated with the holoenzymes, suggesting that dissociation of the holoenzymes occurs via ternary complexes. The time courses of epsilon-cAMP binding also show the heterogeneity of binding characteristics of RII. The 37 000-dalton fragment of type II subunit retains the epsilon-cAMP binding properties of the native subunit. However, only a fraction of the fragment preparation (approximately 32% estimated from sedimentation measurements) binds the catalytic subunit well, suggesting heterogeneity of cleavage.     

Further studies on cyclic adenosine 3':5'-monophosphate protein kinase from dimorphic fungus Mucor rouxii

In this paper, cyclic adenosine-3′:5′-monophosphate-dependent protein kinase from yeast-like cells of Mucor rouxii is characterized. A scheme of partial purification is described together with K_m for ATP (15 μm), histone (0.2 mg/ml), half-maximal activation constant for cyclic AMP (30 nm), and dissociation constant for the binding of cyclic AMP (40 nm). This enzyme is similar to type II protein kinases in two main aspects: the elution position in DEAE-cellulose chromatography and the readiness of its reassociation. But it has a singular characteristic: it does not dissociate completely with cyclic AMP alone (even at concentrations as high as 0.3 mm) unless histone or NaCl is present. NaCl displays several roles: helps dissociation, prevents inactivation of the catalytic subunit, inhibits enzyme activity, and does not prevent reassociation as occurs with type II protein kinases. Once the holoenzyme is dissociated, cyclic AMP is essential to maintain the enzyme in the dissociated state.     

Sensing Domain Dynamics in Protein Kinase A-I�� Complexes by Solution X-ray Scattering

The catalytic (C) and regulatory (R) subunits of protein kinase A are exceptionally dynamic proteins. Interactions between the R- and C-subunits are regulated by cAMP binding to the two cyclic nucleotide-binding domains in the R-subunit. Mammalian cells express four different isoforms of the R-subunit (RIα, RIβ, RIIα, and RIIβ) that all interact with the C-subunit in different ways. Here, we investigate the dynamic behavior of protein complexes between RIα and C-subunits using small angle x-ray scattering. We show that a single point mutation in RIα, R333K (which alters the cAMP-binding properties of Domain B) results in a compact shape compared with the extended shape of the wild-type R·C complex. A double mutant complex that disrupts the interaction site between the C-subunit and Domain B in RIα, RIα_ABR333K·C(K285P), results in a broader P(r) curve that more closely resembles the P(r) profiles of wild-type complexes. These results together suggest that interactions between RIα Domain B and the C-subunit in the RIα·C complex involve large scale dynamics that can be disrupted by single point mutations in both proteins. In contrast to RIα·C complexes. Domain B in the RIIβ·C heterodimer is not dynamic and is critical for both inhibition and complex formation. Our study highlights the functional differences of domain dynamics between protein kinase A isoforms, providing a framework for elucidating the global organization of each holoenzyme and the cross-talk between the R- and C-subunits.     

Dissociation and reassociation of the phosphorylated and nonphosphorylated forms of adenosine 3':5' -monophosphate-dependent protein kinase from bovine cardiac muscle.

Adenosine 3':5' -monophosphate (cyclic AMP) -dependent protein kinase from bovine heart muscle catalyzes the phosphorylation of its regulatory, cyclic AMP-binding subunit. Phosphorylation enhances net dissociation of the enzyme by cyclic AMP. Chromatography on omega-aminohexyl-agarose was used to study the effects of phosphorylation on cyclic AMP binding and subunit dissociation and reassociation. This method permitted rapid separation of the catalytic subunit from the cyclic AMP -binding protein and holoenzyme. Phospho- and dephosphoprotein kinases were found to dissociate to the same extent at any given concentration of cyclic AMP and completely at saturation. At equilibrium, the amount of cyclic AMP bound was the same for both forms of enzyme and was directly proportional to the degree of dissociation of the holoenzyme. In the absence of cyclic AMP, phospho- and dephospho-cyclic AMP-binding proteins reassociated completely with the catalytic subunit. However, the rate of reassociation of the dephospho-cyclic AMP-binding protein was at least 5 times greater than the phospho-cyclic AMP-binding protein. Retardation of reassociation was directly proportional to the extent of phosphorylation. We conclude that the degree to which the cyclic AMP-binding protein is phosphorylated markedly affects its intrinsic ability to combine with the catalytic subunit to regenerate the inactive cyclic nucleotide-dependent kinase and that the state of phosphorylation of this subunit may be important in detemining the proportion of dissociated (active) and reassociated (inactive) protein kinase at any given time.     

Low molecular weight cyclic AMP binding protein isolated from the extract of human tonsillar lymphocytes

A protein fraction of molecular weight 33,000-36,000 accounted for about 40% of the cyclic AMP binding capacity of the cytoplasmic extract of human tonsillar lymphocytes. This cyclic AMP binding fraction (designated as R' protein [10]) proved to be a proteolytic fragment of the regulatory subunit of the cyclic AMP-dependent protein kinase. The Scatchard plot of cyclic AMP binding by the isolated R' fraction indicated positive cooperativity. 50% saturation of the cyclic AMP binding sites was achieved at about 4 . 10(-9) M cyclic AMP. An upward concave curve was obtained in the Scatchard plot of cyclic GMP binding by the R' protein. These results strongly suggest that more than one molecule of cyclic nucleotide can be bound by one molecule of the R' protein. The R' protein could not be detected in the physiological salt extract of isolated nuclei in which type I cyclic AMP-dependent protein kinase was the dominating isoenzyme (according to the terminology used by Corbin, S.D., Keely, S.L. and Park, C.R. (1975) J. Biol. Chem. 250, 218-225). The cytoplasm of cells contained a higher amount of type II than type I regulatory subunit. In the cytoplasm the predominant part of RII was present in the dissociated state in all preparations, while when the RII was found in the nucleus it was mainly in the holoenzyme form. The R' protein presumably from the dissociated type II regulatory subunit.     

Adenosine cyclic 3',5'-monophosphate-dependent protein kinase from human platelets.

A single cyclic AMP-dependent protein kinase (EC 2.7.1.37) has been isolated from human platelets by using DEAE-cellulose ion-exchange chromatography and Sephadex G-150 gel filtration. The molecular weight of the protein kinase was estimated to be 86 490. In the presence of cyclic AMP, the protein kinase could be dissociated into a catalytic subunit of molecular weight 50 000, and either one regulatory subunit of molecular weight 110 000 or two regulatory subunits of molecular weights 110 000 and 38 100, depending on the pH used. Recombination of either of the regulatory subunits with the catalytic subunit restored cyclic AMP-dependency in the catalytic subunit. The apparent Km for ATP in the presence of 10 muM Mg2+ was 4 muM (plus cyclic AMP) and 4.3 muM (minus cyclic AMP). The concentration of cyclic AMP needed for half-maximal stimulation of the protein kinase was 0.172 muM and apparent dissociation constants of 3.7 nM (absence of MgATP) and 0.18 muM (presence of MgATP) were exhibited by the "protein kinase-cyclic AMP complex". The enzyme required Mg2+ for maximum activity and showed a pH optimum of 6.2 with histone as substrate. In addition to four major endogenous platelet protein acceptors of apparent molecular weights 45 000, 28000, 18 500, and 11 100, the platelet protein kinase also phosphorylated the exogenous acceptor proteins thrombin, collagen and histone, all capable of inducing platelet aggregation. Prothrombin, a nonaggregating agent, was not phosphorylated.     

Effects of glucagon and insulin on the cyclic AMP binding capacity of hepatocyte cyclic AMP-dependent protein kinase

Extracts obtained from rat hepatocytes incubated with saline, glucagon or insulin were electrophoresed on polyacrylamide gels and then assayed for cyclic (³H)AMP binding capacity. Analysis of the binding patterns demonstrated that glucagon dissociated a holoenzyme of cyclic AMP-dependent protein kinase in a dose-dependent manner. The increase in free regulatory subunits and, hence, in free catalytic subunits explains the activation of this enzyme by glucagon in the liver. Insulin decreased both the amount of cyclic (³H)AMP bound to the holoenzyme and the capacity of the enzyme to be dissociated when the extracts were incubated with increasing concentrations of this cyclic nucleotide. We propose that these insulin-induced effects are determined by an inhibition of the cyclic AMP binding capacity of this protein kinase. This mechanism could account for the inactivation of cyclic AMP-dependent protein kinase that insulin causes in the liver.Abbreviations cAMP (cyclic AMP), Adenosine 3,5 monophosphate - (³H)cAMP cyclic (³H)AMP - MIX 1-methyl-3-isobutylxanthine     

Binding of the cyclic AMP receptor protein of Escherichia coli to RNA polymerase.

Fluorescence polarization studies were used to study the interaction of a fluorescein-labelled conjugate of the Escherichia coli cyclic AMP receptor protein (F-CRP) and RNA polymerase. Under conditions of physiological ionic strength, F-CRP binds to RNA polymerase holoenzyme in a cyclic AMP-dependent manner; the dissociation constant was about 3 microM in the presence of cyclic AMP and about 100 microM in its absence. Binding to core RNA polymerase under the same conditions was weak (Kdiss. approx. 80-100 microM) and independent of cyclic AMP. Competition experiments established that native CRP and F-CRP compete for the same binding site on RNA polymerase holoenzyme and that the native protein binds about 3 times more strongly than does F-CRP. Analytical ultracentrifuge studies showed that CRP binds predominantly to the monomeric rather than the dimeric form of RNA polymerase.