Ankyrin-rich membrane spanning protein as a novel modulator of transient receptor potential vanilloid 1-function in nociceptive neurons

BACKGROUND

The ion channel TRPV1 is mainly expressed in small diameter dorsal root ganglion (DRG) neurons, which are involved in the sensation of acute noxious thermal and chemical stimuli. Direct modifications of the channel by diverse signalling events have been intensively investigated, but little is known about the composition of modulating macromolecular TRPV1 signalling complexes. Here, we hypothesize that the novel adaptor protein ankyrin-rich membrane spanning protein/kinase D interacting substrate (ARMS) interacts with TRPV1 and modulates its function in rodent DRG neurons.

METHODS

We used immunohistochemistry, electrophysiology, microfluorimetry and immunoprecipitation experiments to investigate TRPV1 and ARMS interactions in DRG neurons and transfected cells.

RESULTS

We found that TRPV1 and ARMS are co-expressed in a subpopulation of DRG neurons. ARMS sensitizes TRPV1 towards capsaicin in transfected HEK 293 cells and in mouse DRG neurons in a PKA-dependent manner. Using a combination of functional imaging and immunocytochemistry, we show that the magnitude of the capsaicin response in DRG neurons depends not only on TRPV1 expression, but on the co-expression of ARMS alongside TRPV1.

CONCLUSION

These data indicate that ARMS is an important component of the signalling complex regulating the sensitivity of TRPV1.

SIGNIFICANCE

The study identifies ARMS as an important component of the signalling complex regulating the sensitivity of excitatory ion channels (TRPV1) in peripheral sensory neurons (DRG neurons) and transfected cells.

Biomolecular interaction analysis in functional proteomics

To understand the function of highly complex eukaryotic tissues like the human brain, in depth knowledge about cellular protein networks is required. Biomolecular interaction analysis (BIA), as a part of functional proteomics, aims to quantify interaction patterns within a protein network in detail. We used the cAMP dependent protein kinase (PKA) as a model system for the binding analysis between small natural ligands, cAMP and cAMP analogues, with their physiological interaction partner, the regulatory subunit of PKA. BIA comprises a variety of methods based on physics, biochemistry and molecular biology. Here we compared side by side real time SPR (surface plasmon resonance, Biacore), a bead based assay (AlphaScreen), a fluorescence based method (Fluorescence polarisation) and ITC (isothermal titration calorimetry). These in vitro methods were complemented by an in cell reporter assay, BRET(2) (bioluminescence resonance energy transfer), allowing to test the effects of cAMP analogues in living cells.

Regulation of anchoring of the RIIα regulatory subunit of PKA to AKAP95 by threonine phosphorylation of RIIα: implications for chromosome dynamics at mitosis

CDK1 phosphorylates the A-kinase regulatory subunit RIIα on threonine 54 (T54) at mitosis, an event proposed to alter the subcellular localization of RIIα. Using an RIIα-deficient leukemic cell line (Reh) and stably transfected Reh cell clones expressing wild-type RIIα or an RIIα(T54E) mutant, we show that RIIα associates with chromatin-bound A-kinase anchoring protein AKAP95 at mitosis and that this interaction involves phosphorylation of RIIα on T54. During interphase, both RIIα and RIIα(T54E) exhibit a centrosome-Golgi localization, whereas AKAP95 is intranuclear. At mitosis and in a mitotic extract, most RIIα, but not RIIα(T54E), co-fractionates with chromatin, onto which it associates with AKAP95. This correlates with T54 phosphorylation of RIIα. Disrupting AKAP95-RIIα anchoring or depleting RIIα from the mitotic extract promotes premature chromatin decondensation. In a nuclear reconstitution assay that mimics mitotic nuclear reformation, RIIα is threonine dephosphorylated and dissociates from AKAP95 prior to assembly of nuclear membranes. Lastly, the Reh cell line exhibits premature chromatin decondensation in vitro, which can be rescued by addition of wild-type RIIα or an RIIα(T54D) mutant, but not RIIα(T54E, A, L or V) mutants. Our results suggest that CDK1-mediated T54 phosphorylation of RIIα constitutes a molecular switch controlling anchoring of RIIα to chromatin-bound AKAP95, where the PKA-AKAP95 complex participates in remodeling chromatin during mitosis.

CDK1-mediated phosphorylation of the RIIα regulatory subunit of PKA works as a molecular switch that promotes dissociation of RIIα from centrosomes at mitosis

Protein kinase A regulatory subunit RIIα is tightly bound to centrosomal structures during interphase through interaction with the A-kinase anchoring protein AKAP450, but dissociates and redistributes from centrosomes at mitosis. The cyclin B-p34cdc2 kinase (CDK1) has been shown to phosphorylate RIIα on T54 and this has been proposed to alter the subcellular localization of RIIα. We have made stable transfectants from an RIIα-deficient leukemia cell line (Reh) that expresses either wild-type or mutant RIIα (RIIα(T54E)). When expressed, RIIα detaches from centrosomes at mitosis and dissociates from its centrosomal location in purified nucleus-centrosome complexes by incubation with CDK1 in vitro. By contrast, centrosomal RIIα(T54E) is not redistributed at mitosis, remains mostly associated with centrosomes during all phases of the cell cycle and cannot be solubilized by CDK1 in vitro. Furthermore, RIIα is solubilized from particular cell fractions and changes affinity for AKAP450 in the presence of CDK1. D and V mutations of T54 also reduce affinity for the N-terminal RII-binding domain of AKAP450, whereas small neutral residues do not change affinity detected by surface plasmon resonance. In addition, only RIIα(T54E) interacts with AKAP450 in a RIPA-soluble extract from mitotic cells. Finally, microtubule repolymerization from mitotic centrosomes of the RIIα(T54E) transfectant is poorer and occurs at a lower frequency than that of RIIα transfectants. Our results suggest that T54 phosphorylation of RIIα by CDK1 might serve to regulate the centrosomal association of PKA during the cell cycle.

Human phosphatidylinositol 4-kinase isoform PI4K92. Expression of the recombinant enzyme and determination of multiple phosphorylation sites

Human phosphatidylinositol 4-kinase, isoform PI4K92, was expressed as His6 tagged protein in Sf9 cells reaching a level of approximately 5% of cellular protein. The enzyme can be purified nearly to homogeneity in a single step by absorption/desorption on Ni/nitriloacetic acid agarose magnetic beads. High Km values in the millimolar range for ATP and PtdIns as well as only a moderate inhibition by adenosine and a sensitivity to Wortmannin (IC50 approximately 300 nM) characterize the enzyme as a type 3 PI4K. The enzyme produces PtdIns4P as product. The isolated enzyme is a phosphoprotein, additionally phosphate is incorporated by incubation with ATP/Mg or ATP/Mn. Phosphorylation sites were mapped by MALDI-MS and LC-MS/MS at the following positions: S258, T263, S266, S277, S294, T423, S496, T504. Accordingly, a stretch of 81 amino acids between the common and the C-terminal catalytic domain was designated phosphorylation domain.

Neurobeachin: A protein kinase A-anchoring, beige/Chediak-higashi protein homolog implicated in neuronal membrane traffic

We describe the identification and initial characterization of neurobeachin, a neuron-specific multidomain protein of 327 kDa with a high-affinity binding site (K(d), 10 nm) for the type II regulatory subunit of protein kinase A (PKA RII). Neurobeachin is peripherally associated with pleomorphic tubulovesicular endomembranes near the trans sides of Golgi stacks and throughout the cell body and cell processes. It is also found in a subpopulation of synapses, where it is concentrated at the postsynaptic plasma membrane. In live cells, perinuclear neurobeachin is dispersed by brefeldin A (BFA) within 1 min, and in permeabilized cells a recruitment of neurobeachin from cytosol to Golgi-near membranes is stimulated by GTPgammaS and prevented by brefeldin A. Spots of neurobeachin recruitment are close to but distinct from recruitment sites of COP-I, AP-1, and AP-3 coat proteins involved in vesicle budding. These observations indicate that neurobeachin binding to membranes close to the trans-Golgi requires an ADP-ribosylation factor-like GTPase, possibly in association with a novel type of protein coat. A neurobeachin isoform that does not bind RII, beige-like protein (BGL), is expressed in many tissues. Neurobeachin, BGL, and approximately 10 other mammalian gene products share a characteristic C-terminal BEACH-WD40 sequence module, which is also present in gene products of invertebrates, plants, protozoans, and yeasts, thus defining a new protein family. The prototype member of this family of BEACH domain proteins, lysosomal trafficking regulator (LYST), is deficient in genetic defects of protein sorting in lysosome biogenesis (the beige mouse and Chediak-Higashi syndrome). Neurobeachin's subcellular localization, its coat protein-like membrane recruitment, and its sequence similarity to LYST suggest an involvement in neuronal post-Golgi membrane traffic, one of its functions being to recruit protein kinase A to the membranes with which it associates.

Analysis of A-kinase anchoring protein (AKAP) interaction with protein kinase A (PKA) regulatory subunits: PKA isoform specificity in AKAP binding

Compartmentalization of cAMP-dependent protein kinase (PKA) is in part mediated by specialized protein motifs in the dimerization domain of the regulatory (R)-subunits of PKA that participate in protein-protein interactions with an amphipathic helix region in A-kinase anchoring proteins (AKAPs). In order to develop a molecular understanding of the subcellular distribution and specific functions of PKA isozymes mediated by association with AKAPs, it is of importance to determine the apparent binding constants of the R-subunit-AKAP interactions. Here, we present a novel approach using surface plasmon resonance (SPR) to examine directly the association and dissociation of AKAPs with all four R-subunit isoforms immobilized on a modified cAMP surface with a high level of accuracy. We show that both AKAP79 and S-AKAP84/D-AKAP1 bind RIIalpha very well (apparent K(D) values of 0.5 and 2 nM, respectively). Both proteins also bind RIIbeta quite well, but with three- to fourfold lower affinities than those observed versus RIIalpha. However, only S-AKAP84/D-AKAP1 interacts with RIalpha at a nanomolar affinity (apparent K(D) of 185 nM). In comparison, AKAP95 binds RIIalpha (apparent K(D) of 5.9 nM) with a tenfold higher affinity than RIIbeta and has no detectable binding to RIalpha. Surface competition assays with increasing concentrations of a competitor peptide covering amino acid residues 493 to 515 of the thyroid anchoring protein Ht31, demonstrated that Ht31, but not a proline-substituted peptide, Ht31-P, competed binding of RIIalpha and RIIbeta to all the AKAPs examined (EC(50)-values from 6 to 360 nM). Furthermore, RIalpha interaction with S-AKAP84/D-AKAP1 was competed (EC(50) 355 nM) with the same peptide. Here we report for the first time an approach to determine apparent rate- and equilibria binding constants for the interaction of all PKA isoforms with any AKAP as well as a novel approach for characterizing peptide competitors that disrupt PKA-AKAP anchoring.

Surface plasmon resonance studies prove the interaction of skeletal muscle sarcoplasmic reticular Ca(2+) release channel/ryanodine receptor with calsequestrin

A high affinity molecular interaction is demonstrated between calsequestrin and the sarcoplasmic reticular Ca(2+) release channel/ryanodine receptor (RyR) by surface plasmon resonance. K(D) values of 92 nM and 102 nM for the phosphorylated and dephosphorylated calsequestrin have been determined, respectively. Phosphorylation of calsequestrin seems not to influence this high affinity interaction, i.e. calsequestrin might always be bound to RyR. However, the phosphorylation state of calsequestrin determines the amount of Ca(2+) released from the lumen. Dephosphorylation of approximately 1% of the phosphorylated calsequestrin could be enough to activate the RyR channel half-maximally, as we have shown previously [Szegedi et al., Biochem. J. 337 (1999) 19].

Study of the subunit interactions in myosin phosphatase by surface plasmon resonance

The interactions of the catalytic subunit of type 1 protein phosphatase (PP1c) and the N-terminal half (residues 1-511) of myosin phosphatase target subunit 1 (MYPT1) were studied. Biotinylated MYPT1 derivatives were immobilized on streptavidin-biosensor chips, and binding parameters with PP1c were determined by surface plasmon resonance (SPR). The affinity of binding of PP1c was: MYPT11-296 > MYPT11-38 > MYPT123-38. No binding was detected with MYPT11-34, suggesting a critical role for residues 35-38, i.e. the PP1c binding motif. Binding of residues 1-22 was inferred from: a higher affinity binding to PP1c for MYPT11-38 compared to MYPT123-38, as deduced from SPR kinetic data and ligand competition assays; and an activation of the myosin light chain phosphatase activity of PP1c by MYPT11-38, but not by MYPT123-38. Residues 40-296 (ankyrin repeats) in MYPT11-296 inhibited the phosphorylase phosphatase activity of PP1c (IC50 = 0.2 nM), whereas MYPT11-38, MYPT123-38 or MYPT11-34 were without effect. MYPT140-511, which alone did not bind to PP1c, showed facilitated binding to the complexes of PP1c-MYPT11-38 and PP1c-MYPT123-38. The inhibitory effect of MYPT140-511 on the phosphorylase phosphatase activity of PP1c also was increased in the presence of MYPT11-38. The binding of MYPT1304-511 to complexes of PP1c and MYPT11-38, or MYPT11-296, was detected by SPR. These results suggest that within the N-terminal half of MYPT1 there are at least four binding sites for PP1c. The essential interaction is with the PP1c-binding motif and the other interactions are facilitated in an ordered and cooperative manner.

Dissection of the nucleotide and metal-phosphate binding sites in cAMP-dependent protein kinase

The catalytic (C) subunit of cAMP-dependent protein kinase (cAPK) is more stable by several criteria when it is part of a holoenzyme complex. By measuring the thermal stability of the free C subunit in the presence and absence of nucleotides and/or divalent metal ions, it was found that most of the stabilizing effects associated with the type I holoenzyme could be attributed to the nucleotide. The specific requirements for this enhanced stability were further dissected: Adenosine stabilized the C subunit up to 5 degrees C; however, divalent cations (i.e., Mg2+, Ca2+, and Mn2+) do not increase heat stability in combination with adenosine and adenine (1). Divalent cations as well as ATP and ADP have no effect by themselves (2). The enhanced stability derived from both ATP and ADP requires divalent cations. MnATP (12 degrees C) shows a much stronger effect than CaATP (7 degrees C) and MgATP (5 degrees C) (3). In the holoenzyme complex or the protein kinase inhibitor/C subunit complex, metal/ATP is also required for enhanced stability; neither the RI or RII subunits nor PKI alone stabilize the C subunit significantly (4). For high thermal stability, the occupation of the second, low-affinity metal-binding site is necessary (5). From these results, we concluded that the adenine moiety works independently from the metal-binding sites, stabilizing the free C subunit by itself. When the beta- and gamma-phosphates are present, divalent metals are required for positioning these phosphates, and two metals are required to achieve thermostability comparable to adenosine alone. The complex containing two metals is the most stable. A comparison of several conformations of the C subunit derived from different crystal structures is given attributing open and closed forms of the C subunit to less and more thermostable enzymes, respectively.

Functional expression and characterisation of a new human phosphatidylinositol 4-kinase PI4K230

By constructing DNA probes we have identified and cloned a human PtdIns 4-kinase, PI4K230, corresponding to a mRNA of 7.0 kb. The cDNA encodes a protein of 2044 amino acids. The C-terminal part of ca. 260 amino acids represents the catalytic domain which is highly conserved in all recently cloned PtdIns 4-kinases. N-terminal motifs indicate multiple heterologous protein interactions. Human PtdIns 4-kinase PI4K230 expressed in vitro exhibits a specific activity of 58 micromol mg-1min-1. The enzyme expressed in Sf9 cells is essentially not inhibited by adenosine, it shows a high Km for ATP of about 300 microM and it is half-maximally inactivated by approximately 200 nM wortmannin. These data classify this enzyme as type 3 PtdIns 4-kinase. Antibodies raised against the N-terminal part moderately activate and those raised against the C-terminal catalytic domain inhibit the enzymatic activity. The coexistence of two different type 3 PtdIns 4-kinases, PI4K92 and PI4K230, in several human tissues, including brain, suggests that these enzymes are involved in distinct basic cellular functions.

Recombinant human peroxisomal targeting signal receptor PEX5. Structural basis for interaction of PEX5 with PEX14

Import of matrix proteins into peroxisomes requires two targeting signal-specific import receptors, Pex5p and Pex7p, and their binding partners at the peroxisomal membrane, Pex13p and Pex14p. Several constructs of human PEX5 have been overexpressed and purified by affinity chromatography in order to determine functionally important interactions and provide initial structural information. Sizing chromatography and electron microscopy suggest that the two isoforms of the human PTS1 receptor, PEX5L and PEX5S, form homotetramers. Surface plasmon resonance analysis indicates that PEX5 binds to the N-terminal fragment of PEX14-(1-78) with a very high affinity in the low nanomolar range. Stable complexes between recombinant PEX14-(1-78) and both the full-length and truncated versions of PEX5 were formed in vitro. Analysis of these complexes revealed that PEX5 possesses multiple binding sites for PEX14, which appear to be distributed throughout its N-terminal half. Coincidentally, this part of the molecule is also responsible for oligomerization, whereas the C-terminal half with its seven tetratricopeptide repeats has been reported to bind PTS1-proteins. A pentapeptide motif that is reiterated seven times in PEX5 is proposed as a determinant for the interaction with PEX14.

PrKX is a novel catalytic subunit of the cAMP-dependent protein kinase regulated by the regulatory subunit type I

The human X chromosome-encoded protein kinase X (PrKX) belongs to the family of cAMP-dependent protein kinases. The catalytically active recombinant enzyme expressed in COS cells phosphorylates the heptapeptide Kemptide (LRRASLG) with a specific activity of 1.5 micromol/(min.mg). Using surface plasmon resonance, high affinity interactions were demonstrated with the regulatory subunit type I (RIalpha) of cAMP-dependent protein kinase (KD = 10 nM) and the heat-stable protein kinase inhibitor (KD = 15 nM), but not with the type II regulatory subunit (RIIalpha, KD = 2.3 microM) under physiological conditions. Kemptide and autophosphorylation activities of PrKX are strongly inhibited by the RIalpha subunit and by protein kinase inhibitor in vitro, but only weakly by the RIIalpha subunit. The inhibition by the RIalpha subunit is reversed by addition of nanomolar concentrations of cAMP (Ka = 40 nM), thus demonstrating that PrKX is a novel, type I cAMP-dependent protein kinase that is activated at lower cAMP concentrations than the holoenzyme with the Calpha subunit of cAMP-dependent protein kinase. Microinjection data clearly indicate that the type I R subunit but not type II binds to PrKX in vivo, preventing the translocation of PrKX to the nucleus in the absence of cAMP. The RIIalpha subunit is an excellent substrate for PrKX and is phosphorylated in vitro in a cAMP-independent manner. We discuss how PrKX can modulate the cAMP-mediated signal transduction pathway by preferential binding to the RIalpha subunit and by phosphorylating the RIIalpha subunit in the absence of cAMP.

Stepwise subunit interaction changes by mono- and bisphosphorylation of cardiac troponin I

Four phosphorylation degrees of cardiac troponin I (cTnI) have been characterized, namely, a dephospho, a bisphospho, and two monophospho states. Here we describe for the first time a role of the monophosphorylated forms. We have investigated the interaction between the cardiac troponin subunits dependent on the phosphorylation state of cTnI by surface plasmon resonance (SPR) spectroscopy. The monophosphorylated forms were generated by mutating each of the two serine residues, located in human cTnI at positions 22 and 23, to alanine. Association and dissociation rate constants of binary (cTnI-cTnT and cTnI-cTnC) and ternary (cTnI/cTnC complex-cTnT) complexes were determined. Mono- and consecutive bisphosphorylation of cTnI gradually reduces the affinity to cTnC and cTnT by lowering the association rate constants; the dissociation rate constants remain unchanged. Phosphorylation also affects formation of the ternary complexes; however, in this instance, association rate constants are constant, and dissociation rate constants are enhanced. A model of cardiac troponin is presented describing an induction of distinct conformational changes by mono- and bisphosphorylation of cTnI.

A stable alpha-helical domain at the N terminus of the RIalpha subunits of cAMP-dependent protein kinase is a novel dimerization/docking motif

The RIalpha subunit of cAMP-dependent protein kinase is maintained as an asymmetric dimer by a dimerization motif at the N terminus. Based on resistance to proteolysis and expression as a discrete domain in Escherichia coli, this motif is defined as residues 12-61. This motif is chemically, kinetically, and thermally stable. The two endogenous interchain disulfide bonds between Cys16 and Cys37 in RIalpha are extremely resistant to reduction even in 8 M urea, indicating that they are well shielded from the reducing environment of the cell. The disulfide bonds were present in recombinant RIalpha as well as when the dimerization domain alone was expressed in E. coli, emphasizing the unusual stability of this motif and the disulfide bonds. Although 100 mM dithiothreitol was sufficient to reduce the disulfide bonds, it did not abolish dimerization. In addition, a stable dimer also still formed when Cys37 was replaced with His, confirming unambiguously the original antiparallel alignment of the disulfide bonds. Thus, both in vitro and in vivo, disulfide bonds are not required for dimerization. Circular dichroism of the dimerization domain indicated a high content of a thermostable alpha-helix. Based on the CD data, trypsin resistance of the fragment, location of the disulfide bonds, and amphipathic helix predictions, potential models are discussed. A new alignment of the dimerization domains of RI, RII, and cGMP-dependent protein kinase elucidates fundamental similarities as well as significant differences among these three domains.

Importance of the A-helix of the catalytic subunit of cAMP-dependent protein kinase for stability and for orienting subdomains at the cleft interface

All eukaryotic protein kinases share a conserved catalytic core. In the catalytic (C) subunit of cAMP-dependent protein kinase (cAPK) this core is preceded by a myristylation motif followed by a long helix with Trp 30 at the end of this A-helix filling a hydrophobic cavity between the two lobes of the core. To understand the importance of the A-helix, the myristylation motif (delta 1-14) as well as the entire N-terminal segment (delta 1 -39) were deleted. In addition, Trp 30 was replaced with both Tyr and Ala. All proteins were overexpressed in E. coli and purified to homogeneity. rC(delta 1-14), rC(W30Y), and rC(W30A) all had reduced thermostability, but were catalytically indistinguishable from wild-type C. Based on Surface Plasmon Resonance, all three also formed stable holoenzyme complexes with the RI-subunit, although the appKds were reduced by more than 10-fold due to decrease in the association rate. Surprisingly, however, the holoenzymes were even more thermostable than wild-type holoenzyme. To obtain active enzyme, it was necessary to purify rC(delta 1-39) as a fusion protein with glutathione-S-transferase (GST-rC(delta 1-39), although its thermostability (Tm) was decreased by 12.5 degrees C, was catalytically similar to wild-type C and was inhibited by both the type I and II R-subunits and the heat-stable protein kinase inhibitor (PKI). The Tm for holoenzyme II formed with GST-rC(delta 1-39) was 16.5 degrees C greater than the Tm for free GST-rC(delta 1-39), and the Ka(cAMP) was increased nearly 10-fold. These mutants point out striking and unanticipated differences in how the RI and RII subunits associate with the C-subunit to form a stable holoenzyme and indicate, furthermore, that this N-terminal segment, far from the active site cleft, influences those interactions. The importance of the A-helix and Trp 30 for stability correlates with its location at the cleft interface where it orients the C-helix in the small lobe and the activation loop in the large so that these subdomains are aligned in a way that allows for correct configuration of residues at the active site. This extensive network of contacts that links the A-helix directly to the active site in cAPK is compared to other kinases whose crystal structures have been solved.

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.

Expression of a chimeric, cGMP-sensitive regulatory subunit of the cAMP-dependent protein kinase type I alpha

To study the fluctuations of cGMP in living cells through changes of energy transfer of dissociable fluorescence labeled subunits, we constructed a cGMP-sensitive probe by combining the N-terminus of the type I regulatory subunit of cAMP-dependent protein kinase (PKA) with the cGMP binding sites of cGMP-dependent protein kinase I alpha (PKG). This chimeric regulatory subunit retained PKA-like dimerization and PKG-compatible cGMP binding constants (Kd = 53 nM) for both binding sites. High affinity interaction with the PKA catalytic subunit was verified by Surface Plasmon Resonance (Kd = 3.15 nM). Additionally, the chimera inhibits the formation of wild-type holoenzyme with an apparent Ki of 1.05 nM. Furthermore, cGMP dissociated the mutant holoenzyme with an apparent activation constant of 146 nM. Thus, our construct provides all the requirements needed to investigate changes in intracellular cGMP concentrations.

Regulatory subunit of protein kinase A: structure of deletion mutant with cAMP binding domains

In the molecular scheme of living organisms, adenosine 3',5'-monophosphate (cyclic AMP or cAMP) has been a universal second messenger. In eukaryotic cells, the primary receptors for cAMP are the regulatory subunits of cAMP-dependent protein kinase. The crystal structure of a 1-91 deletion mutant of the type I alpha regulatory subunit was refined to 2.8 A resolution. Each of the two tandem cAMP binding domains provides an extensive network of hydrogen bonds that buries the cyclic phosphate and the ribose between two beta strands that are linked by a short alpha helix. Each adenine base stacks against an aromatic ring that lies outside the beta barrel. This structure provides a molecular basis for understanding how cAMP binds cooperatively to its receptor protein, thus mediating activation of the kinase.

Crosstalk between domains in the regulatory subunit of cAMP-dependent protein kinase: influence of amino terminus on cAMP binding and holoenzyme formation

The regulatory (R) subunit of cAMP-dependent protein kinase os an asymmetric multidomain protein with a dimerization domain at the N-terminus, an autoinhibitors site, and two cAMP binding domains at the C-terminus. Activation of the tetrameric holoenzyme is mediated by the cooperative binding of cAMP to the two cAMP binding sites. To better understand how the various domains influence each other, the N-terminus (delta 1-91) up to the autoinhibitor site was deleted. Not only did this monomeric deletion mutant, purified from Escherichia coli, still bind cAMP and the catalytic (C) subunit with high affinity, holoenzyme formation was actually accelerated by at least 50-fold. MgATP also was not required for rapid reassociation of (delta 1-91)R(cAMP)2 and C. The Kd(cAMP) and the Ka(cAMP) were similar to those for holoenzyme formed with full-length R; however, cooperatively was lost. Thus the N-terminus, either by inter- or intraprotomer contacts, not only impedes holoenzyme formation but also influences the cooperative binding of cAMP. The 1-91 deletion also renders the remaining fragment resistant to proteolytic degradation. Finally, unlike full-length R, the mutant protein can migrate freely into the nucleus. Surface plasmon resonance studies for the first time enabled direct measurements of the association and dissociation rate constants both for the intact R and for (delta 1-91)R. Both displayed very fast on-rates (1 x 10(-5) M-1 s-1 and 1.1 x 10(-5) M-1 s-1, respectively) and extremely slow off-rates (2.3 x 10(5) M-1 and 4.3 x 10(5) M-1, respectively). Thus, unlike the heat-stable protein kinase inhibitor, the region preceding the autoinhibitor site in R does not contribute in a quantitatively significant way to the high-affinity binding of C.

Physiological inhibitors of the catalytic subunit of cAMP-dependent protein kinase: effect of MgATP on protein-protein interactions

The catalytic (C) subunit of cAMP-dependent protein kinase interacts with two classes of inhibitors. The regulatory (R) subunits, types I and II, associate to form an inactive holoenzyme complex that is activated in response to cAMP. The C-subunit is also inhibited by small heat-stable protein kinase inhibitors (PKI's). Inhibition by both PKI and RI-subunit requires the synergistic high-affinity binding of MgATP. The stabilizing effect of ATP was quantitated by using analytical gel chromatography. Both the type I holoenzyme and the C.PKI complex in the presence of MgATP show apparent Kd's for subunit association that are below 0.1 nM, while in the absence of MgATP the apparent Kd's are 125 nM and 2.3 microM, respectively, for the two complexes. In the absence of MgATP both complexes also can be dissociated readily and, hence, activated by salt-induced dissociation. Under physiological salt concentrations, salt-induced dissociation would be substantial in the absence of the high-affinity binding of MgATP. In both complexes, the ATPase activity of the free C-subunit is abolished. The off rates for MgATP also indicate that the type I holoenzyme is more stable than the C.PKI complex. The off rate (t1/2) for MgATP from the C.PKI complex is 17 min, while the off rate for the type I holoenzyme is 11.7 h. When the C.PKI complex is incubated with RI-subunit in the presence or absence of MgATP, the C-subunit preferentially reassociates with the RI-subunit, forming holoenzyme. In contrast, free PKI cannot compete for the C-subunit when it is part of a holoenzyme complex.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of the catalytic subunit of cAMP-dependent protein kinase in Escherichia coli: multiple isozymes reflect different phosphorylation states

The catalytic subunit of mouse cAMP-dependent protein kinase expressed in Escherichia coli was separated into three distinct species using Mono-S ion exchange chromatography. These isoenzymes corresponded to three isoelectric variants with pIs of 6.4 (30%), 7.2 (60%) and 8.2 (10%). The Stokes' radius of each form was 27.7, 27.1 and 26.3 A respectively. Using electrospray mass spectroscopy the differences between the isozymes were shown to be due to phosphorylation, with each form differing by 80 mass units corresponding to a single phosphate. The fully phosphorylated recombinant enzyme contained four phosphates while the dominant isozyme contained only three. Since the enzyme is not phosphorylated when active site mutations are introduced into the C-subunit, these phosphates are incorporated in an autocatalytic mechanism and are not due to E. coli protein kinases. When the recombinant enzyme was compared with the mammalian porcine heart enzyme significant differences in post-translational modifications were observed. The mammalian enzyme could also be separated into two isozymes. However, in contrast to the recombinant enzyme, the mammalian isozymes displayed an identical mass of 40 840. This correlated with two different post-translational modifications: two phosphates and an N-terminal myristyl moiety. The importance of post-translational modifications, and in particular the phosphorylation state, for the expression of eukaryotic proteins in E. coli is discussed.

Regulation-defective mutants of type I cAMP-dependent protein kinase. Consequences of replacing arginine 94 and arginine 95

The type I alpha regulatory subunits of cAMP-dependent protein kinase contain an autoinhibitor site, Arg94-Arg-Gly-Ala-Ile, which serves as a pseudosubstrate. To evaluate their contribution to subunit association, Arg94 and Arg95, key determinants for peptide recognition, were replaced singly and in tandem with Ala, Glu, and His. Unlike substrate peptides in which replacement of either arginine leads to an increase in Km of approximately 3 orders of magnitude, replacement of either arginine causes only a maximal 20-fold decrease in subunit association. Replacement of both arginine residues with alanine, however, generates a regulatory subunit that can no longer recombine with the catalytic subunit under physiological conditions when the regulatory subunit is saturated with cAMP. To evaluate more fully the specific consequences of replacing these 2 arginine residues, a rapid gel filtration chromatographic method was developed so that subunit affinity could be measured independently of assaying for catalytic activity. The R94,95A mutant shows a Kd(app) = 677 nM, representing an increase of greater than 3 orders of magnitude compared with the native subunits in the presence of MgATP. In the absence of MgATP, the Kd(app) for native regulatory subunit was 125 nM, whereas the Kd(app) for the R94,R95A mutant regulatory subunit was determined to 2.87 microM. When this mutant holoenzyme is assayed at microM concentrations, no activity is observed, whereas below microM, activity is observed because of cAMP-independent subunit dissociation.

cAMP-dependent protein kinase defines a family of enzymes

The structure of the recombinant mouse catalytic subunit of cAMP-dependent protein kinase is reviewed with particular emphasis on the overall features and specific amino acids that are shared by all members of the eukaryotic protein kinase family. The crystal structure of a ternary complex containing both MgATP and a twenty-residue inhibitor peptide defines the precise role of the conserved residues that are clustered at the active site. In addition to catalysing the post-translational modification of other proteins, the catalytic subunit is itself subject to covalent modifications. It is a phosphoprotein and is also myristylated at its amino terminus. The enzyme when crystallized in the presence of detergent shows a detergent molecule bound to an acyl pocket that is presumably occupied by the myristyl moiety in the mammalian enzyme. When expressed in E. coli, the catalytic subunit is autophosphorylated at four sites. Two stable phosphates at Ser338 and Thr197 interact with multiple protein side chains thus explaining why they are inaccessible to phosphatases. Although all substrates and inhibitors of the catalytic subunit share a general minimum consensus sequence, the high affinity binding of protein inhibitors such as the regulatory subunits and the heat stable protein kinase inhibitors require additional determinants that lie beyond the consensus site. These two physiological inhibitors of the catalytic subunit appear to use different sites to achieve high-affinity binding.

Excitation-contraction coupling in a pre-vertebrate twitch muscle: the myotomes of Branchiostoma lanceolatum

The segmented trunk muscle (myotome muscle) of the lancelet (Branchiostoma lanceolatum), a pre-vertebrate chordate, was studied in order to gain information regarding the evolution of excitation-contraction (EC) coupling. Myotome membrane vesicles could be separated on isopycnic sucrose gradients into two main fractions, probably comprising solitary microsomes and diads of plasma membrane and sarcoplasmic reticulum, respectively. Both fractions bound the dihydropyridine PN 200/110 and the phenylalkylamine (-)D888 (devapamil) while specific ryanodine binding was observed in the diad preparation only. Pharmacological effects on Ca2+ currents measured under voltage-clamp conditions in single myotome fibers included a weak block by the dihydropyridine nifedipine and a shift of the voltage dependences of inactivation and restoration to more negative potentials by (-)D888. After blocking the Ca2+ current by cadmium in voltage-clamped single fibers, the contractile response persisted and a rapid intramembrane charge movement could be demonstrated. Both responses exhibited a voltage sensitivity very similar to the one of the voltage-activated Ca2+ channels. Our biochemical and electrophysiological results indicate that the EC coupling mechanism of the protochordate myotome cell is similar to that of the vertebrate skeletal muscle fiber: Intracellular Ca2+ release, presumably taking place via the ryanodine receptor complex, is under control of the cell membrane potential. The sarcolemmal Ca2+ channels might serve as voltage sensors for this process.