Ala335 is essential for high-affinity cAMP-binding of both sites A and B of cAMP-dependent protein kinase type I

N 6-modified cAMP derivatives that activate protein kinase A also act as full agonists of murine HCN2 channels

cAMP acts as a second messenger in many cellular processes. Three protein types mainly mediate cAMP-induced effects: PKA, exchange protein directly activated by cAMP (Epac), and cyclic nucleotide-modulated channels (cyclic nucleotide-gated or hyperpolarization-activated and cyclic nucleotide-modulated (HCN) channels). Discrimination among these cAMP signaling pathways requires specific targeting of only one protein. Previously, cAMP modifications at position N ⁶ of the adenine ring (PKA) and position 2'-OH of the ribose (Epac) have been used to produce target-selective compounds. However, cyclic nucleotide-modulated ion channels were usually outside of the scope of these previous studies. These channels are widely distributed, so possible channel cross-activation by PKA- or Epac-selective agonists warrants serious consideration. Here we demonstrate the agonistic effects of three PKA-selective cAMP derivatives, N ⁶-phenyladenosine-3',5'-cyclic monophosphate (N ⁶-Phe-cAMP), N ⁶-benzyladenosine-3',5'-cyclic monophosphate (N ⁶-Bn-cAMP), and N ⁶-benzoyl-adenosine-3',5'-cyclic monophosphate (N ⁶-Bnz-cAMP), on murine HCN2 pacemaker channels. Electrophysiological characterization in Xenopus oocytes revealed that these derivatives differ in apparent affinities depending on the modification type but that their efficacy and effects on HCN2 activation kinetics are similar to those of cAMP. Docking experiments suggested a pivotal role of Arg-635 at the entrance of the binding pocket in HCN2, either causing stabilizing cation-π interactions with the aromatic ring in N ⁶-Phe-cAMP or N ⁶-Bn-cAMP or a steric clash with the aromatic ring in N ⁶-Bnz-cAMP. A reduced apparent affinity of N ⁶-Phe-cAMP toward the variants R635A and R635E strengthened that notion. We conclude that some PKA activators also effectively activate HCN2 channels. Hence, when studying PKA-mediated cAMP signaling with cAMP derivatives in a native environment, activation of HCN channels should be considered.

Dissecting the cAMP-inducible allosteric switch in protein kinase A RIalpha

The regulatory subunits of cAMP-dependent protein kinase (PKA) are the major receptors of cAMP in most eukaryotic cells. As the cyclic nucleotide binding (CNB) domains release cAMP and bind to the catalytic subunit of PKA, they undergo a major conformational change. The change is mediated by the B/C helix in CNB-A, which extends into one long helix that now separates the two CNB domains and docks onto the surface of the catalytic subunit. We explore here the role of three key residues on the B/C helix that dock onto the catalytic subunit, Arg226, Leu233, and Met 234. By replacing each residue with Ala, we show that each contributes significantly to creating the R:C interface. By also deleting the second CNB domain (CNB-B), we show furthermore that CNB-B is a critical part of the cAMP-induced conformational switch that dislodges the B/C helix from the surface of the catalytic subunit. Without CNB-B the K(a) for activation by cAMP increases from 80 to 1000 nM. Replacing any of the key interface residues with Ala reduces the K(a) to 25-40 nM. Leu233 and M234 contribute to a hydrophobic latch that binds the B/C helix onto the large lobe of the C-subunit, while Arg226 is part of an electrostatic switch that couples the B/C helix to the phosphate binding cassette where the cAMP docks.

High-throughput screening of holoprotein conformational stability by dynamic ligand exchange-affinity capillary electrophoresis

Dynamic ligand exchange-affinity capillary electrophoresis (DLE-ACE) is introduced as a convenient platform for assessing the conformational stability and relative affinity of a holoprotein to different ligands without off-line sample pretreatment, since ligand exchange and protein unfolding processes are integrated in-capillary during electromigration.

CK2-mediated phosphorylation of a type II regulatory subunit of cAMP-dependent protein kinase from the mollusk Mytilus galloprovincialis

Two isoforms of regulatory (R) subunit of cAMP-dependent protein kinase (PKA), named R(myt1) and R(myt2), were identified so far in the sea mussel Mytilus galloprovincialis. Out of them, only R(myt2) was phosphorylated in vitro by casein kinase 2 (CK2) using GTP as phosphate donor. CK2 catalytic subunit (CK2alpha) itself was sufficient to phosphorylate R(myt2), but phosphorylation was enhanced by the presence of the regulatory subunit CK2beta. Even in the absence of CK2, R(myt2) was phosphorylated to a certain extent when it was incubated with GTP. This basal phosphorylation was partially abolished by the known inhibitors apigenin and emodin, which suggests the presence of a residual amount of endogenous CK2 in the preparation of purified R subunit. CK2-mediated phosphorylation significantly decreases the ability of R(myt2) to inhibit PKA catalytic (C) subunit activity in the absence of cAMP. On the other hand, the sequence of several peptides obtained from the tryptic digestion of R(myt2) showed that mussel protein contains the signature sequence common to all PKA family members, within the "phosphate binding cassette" (PBC) A and B. Moreover, the degree of identity between the sequences of peptides from R(myt2), as a whole, and those from type II R subunits was 68-75%, but the global identity percentage with type I R subunits was only about 30%, so that R(myt2) can be classified as a type II R subunit.

Dynamic unfolding of a regulatory subunit of cAMP-dependent protein kinase by capillary electrophoresis: Impact of cAMP dissociation on protein stability

Characterization of the unfolding dynamics of a recombinant type IA regulatory subunit (RIalpha) of cyclic adenosine monophosphate (cAMP)-dependent protein kinase (cAPK) was examined by CE with UV detection. Electrophoretic separation of RIalpha by CE in a buffer devoid of cAMP resulted in rapid dissociation of the complex from the original sample due to the high negative mobility of the ligand relative to receptor. This process enabled in-capillary generation of cAMP-stripped RIalpha, which was used to estimate the apparent dissociation constant (Kd) of 0.6 +/- 0.2 microM. A comparison of RIalpha dynamic unfolding processes with urea denaturation was performed by CE with (i.e., RIalpha-cAMP) and without (i.e., cAMP-stripped RIalpha) excess cAMP in the buffer during electromigration. The presence of cAMP in the buffer confirmed greater stabilization of the protein, as reflected by a higher standard free energy change (DeltaG(U) degrees) of 10.1 +/- 0.5 kcal x mol(+1) and greater cooperativity in unfolding (m) of -2.30 +/- 0.11 kcal x mol(-1) M(-1). CE offers a rapid, yet versatile platform for probing the thermodynamics of cAPK and other types of receptor-ligand complexes in free solution.

A model for agonism and antagonism in an ancient and ubiquitous cAMP-binding domain

The cAMP-binding domain (CBD) is an ancient and conserved regulatory motif that allosterically modulates the function of a group of diverse proteins, thereby translating the cAMP signal into a controlled biological response. The main receptor for cAMP in mammals is the ubiquitous regulatory (R) subunit of protein kinase A. Despite the recognized significant potential for pharmacological applications of CBDs, currently only one group of competitive inhibitor antagonists is known: the (R(p))-cAMPS family of phosphorothioate cAMP analogs, in which the equatorial exocyclic oxygen of cAMP is replaced by sulfur. It is also known that the diastereoisomer (S(p))-cAMPS with opposite phosphorous chirality is a cAMP agonist, but the molecular mechanism of action of these analogs is currently not fully understood. Previous crystallographic and unfolding investigations point to the enhanced CBD dynamics as a key determinant of antagonism. Here, we investigate the (R(p))- and (S(p))-cAMPS-bound states of R(CBD-A) using a comparative NMR approach that reveals a clear chemical shift and dynamic NMR signature, differentiating the (S(p))-cAMPS agonist from the (R(p))-cAMPS antagonist. Based on these data, we have proposed a model for the (R(p)/S(p))-cAMPS antagonism and agonism in terms of steric and electronic effects on two main allosteric relay sites, Ile(163) and Asp(170), respectively, affecting the stability of a ternary inhibitory complex formed by the effector ligand, the regulatory and the catalytic subunits of protein kinase A. The proposed model not only rationalizes the existing data on the phosphorothioate analogs, but it will also facilitate the design of novel cAMP antagonists and agonists.

Isoforms of cAMP-dependent protein kinase in the bivalve mollusk Mytilus galloprovincialis: activation by cyclic nucleotides and effect of temperature

Two different isoforms of cAMP-dependent protein kinase (PKA) have been partially purified from the posterior adductor muscle and the mantle tissue of the sea mussel Mytilus galloprovincialis. The holoenzymes contain as regulatory subunit (R) the previously identified isoforms Rmyt1 and Rmyt2, and were named PKAmyt1 and PKAmyt2, respectively. Both cAMP and cGMP can activate these PKA isoforms completely, although they exhibit a sensitivity approximately 100-fold higher for cAMP than for cGMP. When compared to PKAmyt2, the affinity of PKAmyt1 for cAMP and cGMP is 2- and 3.5-fold higher, respectively. The effect of temperature on the protein kinase activity of both PKA isoforms was examined. Temperature changes did not affect significantly the apparent activation constants (Ka) for cAMP. However, the protein kinase activity was clearly modified and a remarkable difference was observed between both PKA isoforms. PKAmyt1 showed a linear Arrhenius plot over the full range of temperature tested, with an activation energy of 15.3+/-1.5 kJ/mol. By contrast, PKAmyt2 showed a distinct break in the Arrhenius plot at 15 degrees C; the activation energy when temperature was above 15 degrees C was 7-fold higher than that of lower temperatures (70.9+/-8.1 kJ/mol vs 10.6+/-6.5 kJ/mol). These data indicate that, above 15 degrees C, PKAmyt2 activity is much more temperature-dependent than that of PKAmyt1. This different behavior would be related to the different role that these isoforms may play in the tissues where they are located.

Dynamic interaction of cAMP with the Rap guanine-nucleotide exchange factor Epac1

Epac1 is a Rap-specific guanine-nucleotide exchange factor (GEF) which is activated by the binding of cAMP to a cyclic nucleotide monophosphate (cNMP)-binding domain. We investigated the equilibrium and dynamics of the interaction of cAMP and Epac1 using a newly designed fluorescence analogue of cAMP, 8-MABA-cAMP. We observed that the interaction of cAMP, measured by competition with 8-MABA-cAMP, with an isolated cNMP binding domain of Epac1 has an overall equilibrium constant (Kd) of 4 microM and that the kinetics of the interaction are highly dynamic. The binding properties of cAMP are apparently not affected when the catalytic domain is present, despite the fact that binding of cAMP results in activation of Epac1. This indicates that for the activation process, no appreciable binding energy is required. However, when bound to Rap1b, the apparent Kd of Epac to cAMP was about fivefold lower, suggesting that substrate interaction stabilizes cAMP binding. Since the fluorescent analogues used here were either less able or unable to induce activation of Epac1, we concluded that the binding of nucleotide to Epac and the activation of GEF activity are uncoupled processes and that thus appropriate cAMP analogues can be used as inhibitors of the Epac1-mediated signal transduction pathway of Rap.

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.

Fast and slow cyclic nucleotide-dissociation sites in cAMP-dependent protein kinase are transposed in type Ibeta cGMP-dependent protein kinase

Both cyclic GMP-dependent protein kinase (cGK) and cyclic AMP-dependent protein kinase (cAK) contain two distinct cyclic nucleotide-binding sites referred to as fast and slow sites based on cyclic nucleotide dissociation behavior. In cAK, the fast site lies amino-terminal to the slow site, and sequence homologies between cAK and cGK have suggested similar positioning for the sites in cGK. Recombinant human type Ibeta cGK (wild type (WT) cGK) was overexpressed, and the properties of purified WT cGK and native type Ibeta cGK were similar. cGK was mutated singly at Thr-193 (T193A, T193V, and T193S) and Thr-317 (T317A, T317V, and T317S), which have been predicted to provide cGMP specificity in the cGMP-binding sites of cGK; a double mutant (T193A/T317A) was produced also. Compared with WT cGK, half-maximal activation (Ka) of mutant cGKs by cGMP was increased 2- (T317A), 27- (T193A), or 63-fold (T193A/T317A), but the Ka for cAMP of these mutants was essentially unchanged. The T193A and T193V mutants had a large increase in the rate of the slow component of [3H]cGMP dissociation, but in the T317A and T317V mutants, there was no change in the slow component. The T193S and T317S mutants had only minor effects on [3H]cGMP dissociation, thus establishing the importance of the hydroxyl group of Thr-193 and -317 for cGMP binding to cGK. Thus, in type Ibeta cGK, the slow cGMP-binding site is identified as the amino-terminal site in contrast to the order assigned to the fast and slow cAMP-binding sites of cAK.

Active site mutations define the pathway for the cooperative activation of cAMP-dependent protein kinase

cAMP-dependent protein kinase (cAPK) is a heterotetramer containing two regulatory (R) and two catalytic (C) subunits. Each R-subunit contains two tandem cAMP-binding domains, and activation of cAPK is mediated by the cooperative, high affinity binding of cAMP to these two domains. Mutant R-subunits containing one intact high affinity cAMP-binding site and one defective site were used to define the pathway for activation and to delineate the unique roles that each cAMP-binding domain plays. Two mutations were introduced by replacing the essential Arg in each cAMP-binding site with Lys (R209K in Site A and R333K in Site B). Also, the double mutant (R209/333K) was constructed. Analysis of cAMP binding and dissociation and the apparent constants for holoenzyme activation and R- and C-subunit interaction, measured by analytical gel filtration and surface plasmon resonance, established the following: (1) For rR(R209K), occupancy of Site B is not sufficient to activate the holoenzyme; the low affinity Site A must also be occupied. In rR(R333K), Site A retains its high affinity for cAMP, but Site A cannot bind until the low affinity Site B is occupied. Thus, both mutants, for different reasons, have similar Ka's for activation that are approximately 20-fold higher than that of the wild-type holoenzyme. The double mutant with two defective sites is no worse than either single mutant. (2) Kinetic analysis of cAMP binding showed that the mutation in Site A or B abolishes high affinity cAMP binding to that site and slightly weakens the affinity of the adjacent site for cAMP. (3) In the presence of MgATP, both mutants rapidly form a stable holoenzyme even in the presence of cAMP in contrast to the wild-type R where holoenzyme forms slowly in vitro and requires dialysis. Regarding the mechanism of activation based on these and other mutants and from kinetic data, the following conclusions are reached: Site A provides the major contact site with the C-subunit; Site B is not essential for holoenzyme formation. Occupancy of Site A by cAMP mediates dissociation of the C-subunit. Site A is inaccessible to cAMP in the full length holoenzyme, while Site B is fully accessible. Access of cAMP to Site A is mediated by Site B. Thus Site B not only helps to shield Site A, it also provides the specific signal that "opens up" Site A. Finally, a nonfunctional Site A in the holoenzyme prevents stable binding of cAMP to Site B in the absence of subunit dissociation.

(RP)-cAMPS inhibits the cAMP-dependent protein kinase by blocking the cAMP-induced conformational transition

(RP)-cAMPS is known to inhibit competitively the cAMP-induced activation of cAMP-dependent protein kinase (PKA). The molecular nature of this inhibition, however, is unknown. By monitoring the intrinsic tryptophan fluorescence of recombinant type I regulatory subunit of PKA under unfolding conditions, a free energy value (delta GDH2O) of 8.23 +/- 0.22 kcal/mol was calculated. The cAMP-free form of the regulatory subunit was less stable with delta GDH2O = 6.04 +/- 0.05 kcal/mol. Native stability was recovered by treatment of the cAMP-free protein with either cAMP or (SP)-cAMPS but not with (RP)-cAMPS. Thus, (RP)-cAMPS binding to the regulatory subunit keeps the protein in a locked conformation, unable to release the catalytic subunit. This finding was further supported by demonstrating that holoenzyme formation was greatly accelerated only when bound cAMP was replaced with (RP)-cAMPS but not with cAMP or (SP)-cAMPS.