Autophosphorylation of the catalytic subunit of cAMP-dependent protein kinase

PKA Cβ: a forgotten catalytic subunit of cAMP-dependent protein kinase opens new windows for PKA signaling and disease pathologies

3',5'-cyclic adenosine monophosphate (cAMP) dependent protein kinase or protein kinase A (PKA) has served as a prototype for the large family of protein kinases that are crucially important for signal transduction in eukaryotic cells. The PKA catalytic subunits are encoded by the two major genes PRKACA and PRKACB, respectively. The PRKACA gene encodes two known splice variants, the ubiquitously expressed Cα1 and the sperm-specifically expressed Cα2. In contrast, the PRKACB gene encodes several splice variants expressed in a highly cell and tissue-specific manner. The Cβ proteins are called Cβ1, Cβ2, Cβ3, Cβ4 and so-called abc variants of Cβ3 and Cβ4. Whereas Cβ1 is ubiquitously expressed, Cβ2 is enriched in immune cells and the Cβ3, Cβ4 and their abc variants are solely expressed in neuronal cells. All Cα and Cβ splice variants share a kinase-conserved catalytic core and a C-terminal tail encoded by exons 2 through 10 in the PRKACA and PRKACB genes, respectively. All Cα and Cβ splice variants with the exception of Cα1 and Cβ1 are hyper-variable at the N-terminus. Here, we will discuss how the PRKACA and PRKACB genes have developed as paralogs that encode distinct and functionally non-redundant proteins. The fact that Cα and Cβ splice variant mutations are associated with numerous diseases further opens new windows for PKA-induced disease pathologies.

Cyclic AMP-Dependent Protein Kinase Exhibits Antagonistic Effects on the Replication Efficiency of Different Human Papillomavirus Types

Several types of widespread human papillomaviruses (HPVs) may induce the transformation of infected cells, provoking the development of neoplasms. Two main genera of HPVs are classified as mucosatropic alphapapillomaviruses and cutaneotropic betapapillomaviruses (α- and β-HPVs, respectively), and they both include high-risk cancer-associated species. The absence of antiviral drugs has driven investigations into the details of the molecular mechanisms of the HPV life cycle. HPV replication depends on the viral helicase E1 and the transcription factor E2. Their biological activities are controlled by numerous cellular proteins, including protein kinases. Here, we report that ubiquitously expressed cyclic AMP-dependent protein kinase A (PKA) differentially regulates the replication of α-HPV11, α-HPV18, and β-HPV5. PKA stimulates the replication of both α-HPVs studied but has a more profound effect on the replication of high-risk α-HPV18. However, the replication of β-HPV5 is inhibited by activated PKA in human primary keratinocytes and U2OS cells. We show that the activation of PKA signaling by different pharmacological agents induces the rapid proteasomal degradation of the HPV5 E2 protein, which in turn leads to the downregulation of E2-dependent transcription. In contrast, PKA-stimulated induction of HPV18 replication is the result of the downregulation of the E8^E2 transcript encoding a potent viral transcriptional inhibitor together with the rapid upregulation of E1 and E2 protein levels. IMPORTANCE Several types of human papillomaviruses (HPVs) are causative agents of various types of epithelial cancers. Here, we report that ubiquitously expressed cyclic AMP-dependent protein kinase A (PKA) differentially regulates the replication of various types of HPVs during the initial amplification and maintenance phases of the viral life cycle. The replication of the skin cancer-related pathogen HPV5 is suppressed, whereas the replication of the cervical cancer-associated pathogen HPV18 is activated, in response to elevated PKA activity. To inhibit HPV5 replication, PKA targets the viral transcriptional activator E2, inducing its rapid proteasomal degradation. PKA-dependent stimulation of HPV18 replication relies on the downregulation of another E2 gene product, E8^E2, which encodes a potent transcriptional repressor. Our findings highlight, for the first time, protein kinase-related mechanistic differences in the regulation of the replication of mucosal and cutaneous HPV types.

A tripartite cooperative mechanism confers resistance of the protein kinase A catalytic subunit to dephosphorylation

Phosphorylation of specific residues in the activation loops of AGC kinase group (protein kinase A, G, and C families) is required for activity of most of these kinases, including the catalytic subunit of PKA (PKAc). Although many phosphorylated AGC kinases are sensitive to phosphatase-mediated dephosphorylation, the PKAc activation loop uniquely resists dephosphorylation, rendering it "constitutively" phosphorylated in cells. Previous biophysical experiments and structural modeling have suggested that the N-terminal myristoylation signal and the C-terminal FXXF motif in PKAc regulate its thermal stability and catalysis. Here, using site-directed mutagenesis, molecular modeling, and in cell-free and cell-based systems, we demonstrate that substitutions of either the PKAc myristoylation signal or the FXXF motif only modestly reduce phosphorylation and fail to affect PKAc function in cells. However, we observed that these two sites cooperate with an N-terminal FXXW motif to cooperatively establish phosphatase resistance of PKAc while not affecting kinase-dependent phosphorylation of the activation loop. We noted that this tripartite cooperative mechanism of phosphatase resistance is functionally relevant, as demonstrated by changes in morphology, adhesion, and migration of human airway smooth muscle cells transfected with PKAc variants containing amino acid substitutions in these three sites. These findings establish that three allosteric sites located at the PKAc N and C termini coordinately regulate the phosphatase sensitivity of this enzyme. This cooperative mechanism of phosphatase resistance of AGC kinase opens new perspectives toward therapeutic manipulation of kinase signaling in disease.

Comprehensive Characterization of the Recombinant Catalytic Subunit of cAMP-Dependent Protein Kinase by Top-Down Mass Spectrometry

Reversible phosphorylation plays critical roles in cell growth, division, and signal transduction. Kinases which catalyze the transfer of γ-phosphate groups of nucleotide triphosphates to their substrates are central to the regulation of protein phosphorylation and are therefore important therapeutic targets. Top-down mass spectrometry (MS) presents unique opportunities to study protein kinases owing to its capabilities in comprehensive characterization of proteoforms that arise from alternative splicing, sequence variations, and post-translational modifications. Here, for the first time, we developed a top-down MS method to characterize the catalytic subunit (C-subunit) of an important kinase, cAMP-dependent protein kinase (PKA). The recombinant PKA C-subunit was expressed in Escherichia coli and successfully purified via his-tag affinity purification. By intact mass analysis with high resolution and high accuracy, four different proteoforms of the affinity-purified PKA C-subunit were detected, and the most abundant proteoform was found containing seven phosphorylations with the removal of N-terminal methionine. Subsequently, the seven phosphorylation sites of the most abundant PKA C-subunit proteoform were characterized simultaneously using tandem MS methods. Four sites were unambiguously identified as Ser10, Ser11, Ser18, and Ser30, and the remaining phosphorylation sites were localized to Ser2/Ser3, Ser358/Thr368, and Thr[215-224]Tyr in the PKA C-subunit sequence with a 20mer 6xHis-tag added at the N-terminus. Interestingly, four of these seven phosphorylation sites were located at the 6xHis-tag. Furthermore, we have performed dephosphorylation reaction by Lambda protein phosphatase and showed that all phosphorylations of the recombinant PKA C-subunit phosphoproteoforms were removed by this phosphatase.

The Molecular Basis for Specificity at the Level of the Protein Kinase a Catalytic Subunit

Assembly of multi enzyme complexes at subcellular localizations by anchoring- and scaffolding proteins represents a pivotal mechanism for achieving spatiotemporal regulation of cellular signaling after hormone receptor targeting [for review, see (1)]. In the 3' 5'-cyclic adenosine monophosphate (cAMP) dependent protein kinase (PKA) signaling pathway it is generally accepted that specificity is secured at several levels. This includes at the first level stimulation of receptors coupled to heterotrimeric G proteins which through stimulation of adenylyl cyclase (AC) forms the second messenger cAMP. Cyclic AMP has several receptors including PKA. PKA is a tetrameric holoenzyme consisting of a regulatory (R) subunit dimer and two catalytic (C) subunits. The R subunit is the receptor for cAMP and compartmentalizes cAMP signals through binding to cell and tissue-specifically expressed A kinase anchoring proteins (AKAPs). The current dogma tells that in the presence of cAMP, PKA dissociates into an R subunit dimer and two C subunits which are free to phosphorylate relevant substrates in the cytosol and nucleus. The release of the C subunit has raised the question how specificity of the cAMP and PKA signaling pathway is maintained when the C subunit no longer is attached to the R subunit-AKAP complex. An increasing body of evidence points toward a regulatory role of the cAMP and PKA signaling pathway by targeting the C subunits to various C subunit binding proteins in the cytosol and nucleus. Moreover, recent identification of isoform specific amino acid sequences, motifs and three dimensional structures have together provided new insight into how PKA at the level of the C subunit may act in a highly isoform-specific fashion. Here we discuss recent understanding of specificity of the cAMP and PKA signaling pathway based on C subunit subcellular targeting as well as evolution of the C subunit structure that may contribute to the dynamic regulation of C subunit activity.

cAMP-dependent protein kinase (PKA) complexes probed by complementary differential scanning fluorimetry and ion mobility-mass spectrometry

cAMP-dependent protein kinase (PKA) is an archetypal biological signaling module and a model for understanding the regulation of protein kinases. In the present study, we combine biochemistry with differential scanning fluorimetry (DSF) and ion mobility–mass spectrometry (IM–MS) to evaluate effects of phosphorylation and structure on the ligand binding, dynamics and stability of components of heteromeric PKA protein complexes in vitro. We uncover dynamic, conformationally distinct populations of the PKA catalytic subunit with distinct structural stability and susceptibility to the physiological protein inhibitor PKI. Native MS of reconstituted PKA R₂C₂ holoenzymes reveals variable subunit stoichiometry and holoenzyme ablation by PKI binding. Finally, we find that although a ‘kinase-dead’ PKA catalytic domain cannot bind to ATP in solution, it interacts with several prominent chemical kinase inhibitors. These data demonstrate the combined power of IM–MS and DSF to probe PKA dynamics and regulation, techniques that can be employed to evaluate other protein-ligand complexes, with broad implications for cellular signaling.

An Isoform-Specific Myristylation Switch Targets Type II PKA Holoenzymes to Membranes

Cyclic AMP-dependent protein kinase (PKA) is regulated in part by N-terminal myristylation of its catalytic (C) subunit. Structural information about the role of myristylation in membrane targeting of PKA has been limited. In mammalian cells there are four functionally non-redundant PKA regulatory subunits (RIα, RIβ, RIIα, and RIIβ). PKA is assembled as an inactive R2C2 holoenzyme in cells. To explore the role of N-myristylation in membrane targeting of PKA holoenzymes, we solved crystal structures of RIα:myrC and RIIβ2:myrC2, and showed that the N-terminal myristylation site in the myrC serves as a flexible "switch" that can potentially be mobilized for membrane anchoring of RII, but not RI, holoenzymes. Furthermore, we synthesized nanodiscs and showed by electron microscopy that membrane targeting through the myristic acid is specific for the RII holoenzyme. This membrane-anchoring myristylation switch is independent of A Kinase Anchoring Proteins (AKAPs) that target PKA to membranes by other mechanisms.

Akt kinase C-terminal modifications control activation loop dephosphorylation and enhance insulin response

The Akt protein kinase, also known as protein kinase B, plays key roles in insulin receptor signalling and regulates cell growth, survival and metabolism. Recently, we described a mechanism to enhance Akt phosphorylation that restricts access of cellular phosphatases to the Akt activation loop (Thr(308) in Akt1 or protein kinase B isoform alpha) in an ATP-dependent manner. In the present paper, we describe a distinct mechanism to control Thr(308) dephosphorylation and thus Akt deactivation that depends on intramolecular interactions of Akt C-terminal sequences with its kinase domain. Modifications of amino acids surrounding the Akt1 C-terminal mTORC2 (mammalian target of rapamycin complex 2) phosphorylation site (Ser(473)) increased phosphatase resistance of the phosphorylated activation loop (pThr(308)) and amplified Akt phosphorylation. Furthermore, the phosphatase-resistant Akt was refractory to ceramide-dependent dephosphorylation and amplified insulin-dependent Thr(308) phosphorylation in a regulated fashion. Collectively, these results suggest that the Akt C-terminal hydrophobic groove is a target for the development of agents that enhance Akt phosphorylation by insulin.

Role of N-terminal myristylation in the structure and regulation of cAMP-dependent protein kinase

The catalytic (C) subunit of cAMP-dependent protein kinase [protein kinase A (PKA)] is a major target of cAMP signaling, and its regulation is of fundamental importance to biological processes. One mode of regulation is N-myristylation, which has eluded structural and functional characterization so far because most crystal structures are of the non-myristylated enzyme, are phosphorylated on Ser10, and generally lack electron density for the first 13 residues. We crystallized myristylated wild-type (WT) PKA and a K7C mutant as binary (bound to a substrate peptide) and ternary [bound to a substrate peptide and adenosine-5'-(β,γ-imido)triphosphate] complexes. There was clear electron density for the entire N-terminus in the binary complexes, both refined to 2.0 Å, and K7C ternary complex, refined to 1.35 Å. The N-termini in these three structures display a novel conformation with a previously unseen helix from residues 1 to 7. The K7C mutant appears to have a more stable N-terminus, and this correlated with a significant decrease in the B-factors for the N-terminus in the myr-K7C complexes compared to the WT binary complex. The N-terminus of the myristylated WT ternary complex, refined to 2.0 Å, was disordered as in previous structures. In addition to a more ordered N-terminus, the myristylated K7C mutant exhibited a 53% increase in k(cat). The effect of nucleotide binding on the structure of the N-terminus in the WT protein and the kinetic changes in the K7C protein suggest that myristylation or occupancy of the myristyl binding pocket may serve as a site for allosteric regulation in the C-subunit.

Structure and function of the human sperm-specific isoform of protein kinase A (PKA) catalytic subunit Cα2

Protein kinase A (PKA) exists as several tissue-specific isoforms that through phosphorylation of serine and threonine residues of substrate proteins act as key regulators of a number of cellular processes. We here demonstrate that the human sperm-specific isoform of PKA named Cα2 is important for sperm motility and thus male fertility. Furthermore, we report on the first three-dimensional crystal structure of human apo Cα2 to 2.1 Å. Apo Cα2 displays an open conformation similar to the well-characterized apo structure of murine Cα1. The asymmetric unit contains two molecules and the core of the small lobe is rotated by almost 13° in the A molecule relative to the B molecule. In addition, a salt bridge between Lys72 and Glu91 was observed for Cα2 in the apo-form, a conformation previously found only in dimeric or ternary complexes of Cα1. Human Cα2 and Cα1 share primary structure with the exception of the amino acids at the N-terminus coded for by an alternative exon 1. The N-terminal glycine of Cα1 is myristoylated and this aliphatic chain anchors the N-terminus to an intramolecular hydrophobic pocket. Cα2 cannot be myristoylated and the crystal structure revealed that the equivalent hydrophobic pocket is unoccupied and exposed. Nuclear magnetic resonance (NMR) spectroscopy further demonstrated that detergents with hydrophobic moieties of different lengths can bind deep into this uncovered pocket. Our findings indicate that Cα2 through the hydrophobic pocket has the ability to bind intracellular targets in the sperm cell, which may modulate protein stability, activity and/or cellular localization.

The testis-specific Cα2 subunit of PKA is kinetically indistinguishable from the common Cα1 subunit of PKA

Background

The two variants of the α-form of the catalytic (C) subunit of protein kinase A (PKA), designated Cα1 and Cα2, are encoded by the PRKACA gene. Whereas Cα1 is ubiquitous, Cα2 expression is restricted to the sperm cell. Cα1 and Cα2 are encoded with different N-terminal domains. In Cα1 but not Cα2 the N-terminal end introduces three sites for posttranslational modifications which include myristylation at Gly1, Asp-specific deamidation at Asn2 and autophosphorylation at Ser10. Previous reports have implicated specific biological features correlating with these modifications on Cα1. Since Cα2 is not modified in the same way as Cα1 we tested if they have distinct biochemical activities that may be reflected in different biological properties.

Results

We show that Cα2 interacts with the two major forms of the regulatory subunit (R) of PKA, RI and RII, to form cAMP-sensitive PKAI and PKAII holoenzymes both in vitro and in vivo as is also the case with Cα1. Moreover, using Surface Plasmon Resonance (SPR), we show that the interaction patterns of the physiological inhibitors RI, RII and PKI were comparable for Cα2 and Cα1. This is also the case for their potency to inhibit catalytic activities of Cα2 and Cα1.

Conclusion

We conclude that the regulatory complexes formed with either Cα1 or Cα2, respectively, are indistinguishable.

Identification of Toxoplasma gondii cAMP dependent protein kinase and its role in the tachyzoite growth

Background

cAMP-dependent protein kinase (PKA) has been implicated in the asexual stage of the Toxoplasma gondii life cycle through assaying the effect of a PKA-specific inhibitor on its growth rate. Since inhibition of the host cell PKA cannot be ruled out, a more precise evaluation of the role of PKA, as well as characterization of the kinase itself, is necessary.

Methodology/Principal Finding

The inhibitory effects of two PKA inhibitors, H89, an ATP-competitive chemical inhibitor, and PKI, a substrate-competitive mammalian natural peptide inhibitor, were estimated. In the in vitro kinase assay, the inhibitory effect of PKI on a recombinant T. gondii PKA catalytic subunit (TgPKA-C) was weaker compared to that on mammalian PKA-C. In a tachyzoite growth assay, PKI had little effect on the growth of tachyzoites, whereas H89 strongly inhibited it. Moreover, T. gondii PKA regulatory subunit (TgPKA-R)-overexpressing tachyzoites showed a significant growth defect.

Conclusions/Significance

Our data suggest that PKA plays an important role in the growth of tachyzoites, and the inhibitory effect of substrate-competitive inhibitor PKI on T. gondii PKA was low compared to that of the ATP competitive inhibitor H89.

Yeast 3-phosphoinositide-dependent protein kinase-1 (PDK1) orthologs Pkh1-3 differentially regulate phosphorylation of protein kinase A (PKA) and the protein kinase B (PKB)/S6K ortholog Sch9

Pkh1, -2, and -3 are the yeast orthologs of mammalian 3-phosphoinositide-dependent protein kinase-1 (PDK1). Although essential for viability, their functioning remains poorly understood. Sch9, the yeast protein kinase B and/or S6K ortholog, has been identified as one of their targets. We now have shown that in vitro interaction of Pkh1 and Sch9 depends on the hydrophobic PDK1-interacting fragment pocket in Pkh1 and requires the complementary hydrophobic motif in Sch9. We demonstrated that Pkh1 phosphorylates Sch9 both in vitro and in vivo on its PDK1 site and that this phosphorylation is essential for a wild type cell size. In vivo phosphorylation on this site disappeared during nitrogen deprivation and rapidly increased again upon nitrogen resupplementation. In addition, we have shown here for the first time that the PDK1 site in protein kinase A is phosphorylated by Pkh1 in vitro, that this phosphorylation is Pkh-dependent in vivo and occurs during or shortly after synthesis of the protein kinase A catalytic subunits. Mutagenesis of the PDK1 site in Tpk1 abolished binding of the regulatory subunit and cAMP dependence. As opposed to PDK1 site phosphorylation of Sch9, phosphorylation of the PDK1 site in Tpk1 was not regulated by nitrogen availability. These results bring new insight into the control and prevalence of PDK1 site phosphorylation in yeast by Pkh protein kinases.

Protein kinase A type I activates a CRE-element more efficiently than protein kinase A type II regardless of C subunit isoform

Background

Protein kinase A type I (PKAI) and PKAII are expressed in most of the eukaryotic cells examined. PKA is a major receptor for cAMP and specificity is achieved partly through tissue-dependent expression and subcellular localization of subunits with different biochemical properties. In addition posttranslational modifications help fine tune PKA activity, distribution and interaction in the cell. In spite of this the functional significance of two forms of PKA in one cell has not been fully determined. Here we have tested the ability of PKAI and PKAII formed by expression of the regulatory (R) subunits RIα or RIIα in conjunction with Cα1 or Cβ2 to activate a co-transfected luciferace reporter gene, controlled by the cyclic AMP responsive element-binding protein (CREB) in vivo.

Results

We show that PKAI when expressed at equal levels as PKAII was significantly (p < 0.01) more efficient in inducing Cre-luciferace activity at saturating concentrations of cAMP. This result was obtained regardless of catalytic subunit identity.

Conclusion

We suggest that differential effects of PKAI and PKAII in inducing Cre-luciferace activity depend on R and not C subunit identity.

Analysis of protein kinase autophosphorylation using expressed protein ligation and computational modeling

Protein kinases represent a family of enzymes that are critical in cell signaling. One mechanism by which protein kinases are regulated is via autophosphorylation. In the studies described here, we have examined the mechanism of autophosphosphorylation at serine 338 in the regulation of protein kinase A (PKA). Expressed protein ligation allowed for the covalent linkage of an ATP moiety to a Ser mimic at this phosphorylation site. Using a combination of size exclusion chromatography, fluorescence nucleotide binding, kinase measurements, and limited proteolysis assays on this semisynthetic ATP-linked protein, we have obtained unique evidence for an intramolecular autophosphorylation mechanism in PKA regulation. Computational analysis provided a plausible model for a PKA conformation consistent with intramolecular phosphoryl transfer. This approach could be applied to other autoprocessing enzymes by exploiting appropriate transition state analogue motifs in the context of protein semisynthesis.

Inactive forms of the catalytic subunit of protein kinase A are expressed in the brain of higher primates

It is well documented that the beta-gene of the catalytic (C) subunit of protein kinase A encodes a number of splice variants. These splice variants are equipped with a variable N-terminal end encoded by alternative use of several exons located 5' to exon 2 in the human, bovine and mouse Cbeta gene. In the present study, we demonstrate the expression of six novel human Cbeta mRNAs that lack 99 bp due to loss of exon 4. The novel splice variants, designated CbetaDelta4, were identified in low amounts at the mRNA level in NTera2-N cells. We developed a method to detect CbetaDelta4 mRNAs in various cells and demonstrated that these variants were expressed in human and Rhesus monkey brain. Transient expression and characterization of the CbetaDelta4 variants demonstrated that they are catalytically inactive both in vitro against typical protein kinase A substrates such as kemptide and histone, and in vivo against the cAMP-responsive element binding protein. Furthermore, co-expression of CbetaDelta4 with the regulatory subunit (R) followed by kinase activity assay with increasing concentrations of cAMP and immunoprecipitation with extensive washes with cAMP (1 mm) and immunoblotting demonstrated that the CbetaDelta4 variants associate with both RI and RII in a cAMP-independent fashion. Expression of inactive C subunits which associate irreversibly with R may imply that CbetaDelta4 can modulate local cAMP effects in the brain by permanent association with R subunits even at saturating concentrations of cAMP.

Phosphorylation of serine residues in histidine-tag sequences attached to recombinant protein kinases: A cause of heterogeneity in mass and complications in function

High-level recombinant expression of protein kinases in eukaryotic cells or Escherichia coli commonly gives products that are phosphorylated by autocatalysis or by the action of endogenous kinases. Here, we report that phosphorylation occurred on serine residues adjacent to hexahistidine affinity tags (His-tags) derived from several commercial expression vectors and fused to overexpressed kinases. The result was observed with a variety of recombinant kinases expressed in either insect cells or E. coli. Multiple phosphorylations of His-tagged full-length Aurora A, a protein serine/threonine kinase, were detected by mass spectrometry when it was expressed in insect cells in the presence of okadaic acid, a protein phosphatase inhibitor. Peptide mapping by liquid chromatography-mass spectrometry detected phosphorylations on all three serine residues in an N-terminal tag, alpha-N-acetyl-MHHHHHHSSGLPRGS. The same sequence was also phosphorylated, but only at a low level, when a His-tagged protein tyrosine kinase, Pyk2 was expressed in insect cells and activated in vitro. When catalytic domains of Aurora A and several other protein serine/threonine kinases were expressed in E. coli, serines in the affinity tag sequence GSSHHHHHHSSGLVPRGS were also variably phosphorylated. His-Aurora A with hyperphosphorylation of the serine residues in the tag aggregated and resisted thrombin-catalyzed removal of the tag. Treatment with alkaline phosphatase partly restored sensitivity to thrombin. The same His-tag sequence was also detected bearing alpha-N-d-gluconoylation in addition to multiple phosphorylations. The results show that histidine-tag sequences can receive complicated posttranslational modification, and that the hyperphosphorylation and resulting heterogeneity of the recombinant fusion proteins can interfere with downstream applications.

Glucose-dependent activation of protein kinase A activity in Saccharomyces cerevisiae and phosphorylation of its TPK1 catalytic subunit

Protein kinase A (PKA), in yeast, plays a major role in controlling metabolism and gene expression in connection with the available nutrient conditions. We here measure, for the first time, a transient change in the in vivo PKA activity, along a cAMP peak produced by 100 mM glucose addition to glycerol-growing cells as well as a change in the phosphorylation state of its catalytic subunit (Tpk1p) following PKA activation. PKA activity was measured in situ in permeabilized cells, preserving its intracellular localization. Comparison of total PKA activity, measured in situ in permeabilized cells with data obtained from in vitro assays in crude extracts, underscores the inhibitory potency of the regulatory subunit within the cell. Tpk1p phosphorylation was detected through non-denaturing gel electrophoresis. Phosphorylation of Tpk1p increases its specificity constant toward kemptide substrate. The use of mutants of the cAMP pathway showed that phosphorylation depends on the activation of PKA via the G-protein coupled receptor pathway triggered by glucose. The phosphorylation state of Tpk1p was followed during the diauxic shift. Tpk1p phosphorylation is dynamic and reversible: its up-regulation correlates with a fully fermentative metabolism, while its down-regulation with stationary phase or respiratory metabolism. Reversible phosphorylation can thus be considered a new control mechanism possibly pointing to a fine-tuning of PKA activity in response to environmental conditions.

Dynamics of signaling by PKA

The catalytic and regulatory subunits of cAMP-dependent protein kinase (PKA) are highly dynamic signaling proteins. In its dissociated state the catalytic subunit opens and closes as it moves through its catalytic cycle. In this subunit, the core that is shared by all members of the protein kinase family is flanked by N- and C-terminal segments. Each are anchored firmly to the core by well-defined motifs and serve to stabilize the core. Protein kinases are not only catalysts, they are also scaffolds. One of their major functions is to bind to other proteins. In addition to its interactions with the N- and C- termini, the catalytic subunit interacts with its inhibitor proteins, PKI and the regulatory subunits. Both bind with subnanomolar affinity. To achieve this tight binding requires docking of a substrate mimetic to the active site cleft as well as a peripheral docking site. The peripheral site used by PKI is distinct from that used by RIalpha as revealed by a recent structure of a C:RIalpha complex. Upon binding to the catalytic subunit, the linker region of RIalpha becomes ordered. In addition, cAMP-binding domain A undergoes major conformational changes. RIalpha is a highly malleable protein. Using small angle X-ray scattering, the overall shape of the regulatory subunits and corresponding holoenzymes have been elucidated. These studies reveal striking and surprising isoform differences.

Consequences of lysine 72 mutation on the phosphorylation and activation state of cAMP-dependent kinase

General strategies to obtain inactive kinases have utilized mutation of key conserved residues in the kinase core, and the equivalent Lys72 in cAMP-dependent kinase has often been used to generate a "dead" kinase. Here, we have analyzed the consequences of this mutation on kinase structure and function. Mutation of Lys72 to histidine (K72H) generated an inactive enzyme, which was unphosphorylated. Treatment with an exogenous kinase (PDK-1) resulted in a mutant that was phosphorylated only at Thr197 and remained inactive but nevertheless capable of binding ATP. Ser338 in K72H cannot be autophosphorylated, nor can it be phosphorylated in an intermolecular process by active wild type C-subunit. The Lys72 mutant, once phosphorylated on Thr197, can bind with high affinity to the RIalpha subunits. Thus a dead kinase can still act as a scaffold for binding substrates and inhibitors; it is only phosphoryl transfer that is defective. Using a potent inhibitor of C-subunit activity, H-89, Escherichia coli-expressed C-subunit was also obtained in its unphosphorylated state. This protein is able to mature into its active form in the presence of PDK-1 and is able to undergo secondary autophosphorylation on Ser338. Unlike the H-89-treated wild type protein, the mutant protein (K72H) cannot undergo the subsequent cis autophosphorylation following phosphorylation at Thr197. Using these two substrates and mammalian-expressed PDK-1, we can elucidate a possible two-step process for the activation of the C-subunit: initial phosphorylation on the activation loop at Thr197 by PDK-1, or a PDK-1-like enzyme, followed by second cis autophosphorylation step at Ser338.

Enhanced dephosphorylation of cAMP-dependent protein kinase by oxidation and thiol modification

The catalytic subunit of cAMP-dependent protein kinase (PKA) is phosphorylated at threonine 197 and serine 338. Phosphorylation of threonine 197, located in the activation loop, is required for coordinating the active site conformation and optimal enzymatic activity. However, this phosphorylation has not been widely appreciated as a regulatory site because of the apparent constitutive nature of the phosphorylation and the general resistance of the kinase to phosphatase treatment. We demonstrate here that the observed resistance of the catalytic subunit to dephosphorylation is due, in part, to the presence of the highly nucleophilic cysteine 199 located proximal to the phosphate on threonine 197. Experiments performed in vitro demonstrated that mutation (cysteine 199 to alanine), oxidation, such as by glutathionylation or internal disulfide bond formation, or alkylation of the C-subunit enhanced its ability to be dephosphorylated. Furthermore, rephosphorylation of reduced C-subunit by PDK1 created a cycle whereby the inactive kinase could be reactivated. To demonstrate that thiol modification of PKA can lead to enhanced dephosphorylation in vivo, PC12 cells were treated with N-ethylmaleimide (NEM). Such treatment resulted in complete PKA inactivation and dephosphorylation of threonine 197. This effect of NEM was contingent upon prior treatment of the cells with PKA activators, demonstrating the resistance of the holoenzyme to thiol alkylation-mediated dephosphorylation. Our results also demonstrated that NEM treatment of PC12 cells enhanced the dephosphorylation of the protein kinase Calpha activation loop, suggesting a common mechanism of regulation among members of the AGC family of kinases.

Characterization of the cAMP-dependent protein kinase catalytic subunit Cγ expressed and purified from sf9 cells

The Cgamma and Calpha subunits of the cAMP-dependent protein kinase (PKA) contain 350 amino acids that are highly homologous (83% amino acid sequence), with 91% homology within the catalytic domain (a.a. 40-300). Unlike Cgamma, the Calpha subunit has been readily purified and characterized as a recombinant protein in vitro, in intact cells, and in vivo. This report describes for the first time the expression, purification, and characterization of Cgamma. The expression of active Cgamma was eukaryote-specific, from mammalian and insect cells, but not bacteria. Active recombinant Cgamma was optimally expressed and purified to homogeneity from Sf9 cells with a 273-fold increase in specific activity and a 21% recovery after sequential CM-Sepharose and Sephacryl S-300 chromatography. The specific activity of pure Cgamma was 0.31 and 0.81 U/mg with kemptide and histone as substrates, respectively. Physical characterization showed Cgamma had a lower apparent molecular weight and Stokes radii than Calpha, suggesting differences in tertiary structures. Steady-state kinetics demonstrated that like Calpha and Cbeta, Cgamma phosphorylates substrates requiring basic amino acids at P-3 and P-2. However, Cgamma generally exhibited a lower Km and Vmax than Calpha for peptide substrates tested. Cgamma also exhibited a distinct pseudosubstrate specificity showing inhibition by homogeneous preparations of RIalpha and RIIalpha-subunits, but not by pure recombinant protein kinase inhibitors PKIalpha and PKIbeta, PKA-specific inhibitors. These studies suggest that Cgamma and Calpha exhibit differences in structure and function in vitro, supporting the hypothesis that functionally different C-subunit isozymes could diversify and/or fine-tune cAMP signal transduction downstream of PKA activation.

Characterization of protein kinase A phosphorylation: multi-technique approach to phosphate mapping

A multi-technique approach to identification and mapping of phosphorylation on protein kinase A (PKA) is described. X-ray crystallography revealed phosphorylation at T197 and S338 while mass spectrometry (MS) on the intact protein suggested phosphorylation at three sites. Tryptic digestion, followed by MS, confirmed the presence of three phosphates. However, metal affinity treatment of the digest prior to MS revealed the presence of a fourth phosphopeptide. Subsequent analysis of the digests using liquid chromatography (LC) coupled with quadrupole ion trap (QIT) MS confirmed phosphorylation at S10 and S338 and suggested phosphorylation at S139 and T195/197. Unfortunately, identification of pS139 was inconclusive due to low signal intensity and early elution in reversed-phase LC while poor MS/MS data prevented localization of the phosphate to T195 or T197. Phosphopeptide modification with ethanethiol, followed by LC QIT-MS/MS, identified four phosphopeptides in a single experiment. In addition, the fragmentation data provided significantly more sequence information than data obtained from unmodified peptides. Data from this study suggested that PKA was completely phosphorylated at S10, T197, and S338 and partially phosphorylated at S139. These results illustrate that critical information can be lost unless multiple MS techniques are used for identification and validation of phosphorylation.

Minor participation of cAMP on the protein kinase phosphorylation of mitochondrial and cytosolic fractions from Ascaris suum: a comparative study with porcine heart muscle

In contrast to porcine heart muscle in which cAMP effectively activated the phosphorylation of cytosolic proteins, cAMP exerted a minor effect on the phosphorylation of proteins from the soluble fraction of Ascaris suum muscle. Similarly, cAMP did not enhance the kinase activity in the mitochondrial membranes from porcine heart and A. suum, although major differences in protein phosphorylation were observed between both fractions. However, cAMP-dependent protein kinases (PKA) were evidenced in the parasitic soluble mitochondrial fraction, since the phosphorylation of histone IIA and kemptide was augmented in this fraction, in the presence of cAMP. An increase in the phosphorylation of exogenously added A. suum phosphofructokinase was also obtained when cAMP was added to the parasite soluble mitochondrial fraction. The phosphorylation of phosphofructokinase by this fraction was inhibited when kemptide and cAMP were included in the reaction mixture, suggesting substrate competition for the same PKA. Although PKI (6-22), a reported inhibitor of the catalytic subunit of mammalian cAMP-dependent PKAs, did not affect the endogenous phosphorylation of proteins in the various A. suum fractions, an inhibition on the phosphorylation of exogenously added kemptide and phosphofructokinase was observed when PKI (6-22) was incubated with the parasite mitochondrial soluble fraction.

Phosphorylation and flexibility of cyclic-AMP-dependent protein kinase (PKA) using (31)P NMR spectroscopy

Cell signaling pathways rely on phosphotransfer reactions that are catalyzed by protein kinases. The protein kinases themselves are typically regulated by phosphorylation and concurrent structural rearrangements, both near the catalytic site and elsewhere. Thus, physiological function requires posttranslational modification and deformable structures. A prototypical example is provided by cyclic AMP-dependent protein kinase (PKA). It is activated by phosphorylation, is inhomogeneously phosphorylated when expressed in bacteria, and exhibits a wide range of dynamic properties. Here we use (31)P nuclear magnetic resonance (NMR) spectroscopy to characterize the phosphorylation states and to estimate the flexibility of the phosphorylation sites of 2-, 3-, and 4-fold phosphorylated PKA. The phosphorylation sites Ser10, Ser139, Thr197, and Ser338 are assigned to individual NMR resonances, assisted by complexation with AMP-PNP and dephosphorylation with alkaline phosphatase. Rotational diffusion correlation times estimated from resonance line widths show progressively increasing flexibilities for phosphothreonine 197, phosphoserines 139 and 338, and disorder at phosphoserine 10, consistent with crystal structures of PKA. However, because the apparent rotational diffusion correlation time fitted for phosphothreonine 197 of the activation loop is longer than the overall PKA rotational diffusion time, microsecond to millisecond time scale conformational exchange effects involving motions of phosphothreonine 197 are probable. These may represent "open"-"closed" transitions of the uncomplexed protein in solution. These data represent direct measurements of flexibilities also associated with functional properties, such as ATP binding and membrane association, and illustrate the applicability of (31)P NMR for functional and dynamic characterization of protein kinase phosphorylation sites.

Influence of myristoylation, phosphorylation, and deamidation on the structural behavior of the N-terminus of the catalytic subunit of cAMP-dependent protein kinase

A number of isoenzymes of the catalytic subunit of cAMP-dependent protein kinase arise through posttranslational modifications of the enzyme outside the catalytic domain; the biological significance of these is not yet fully clear. A clustering of sites for such modification exists at the N-terminus of the protein, where myristoylation (of Gly1), phosphorylation (at Ser10), and deamidation of Asn2 have been observed. As the first two are known to govern membrane binding and thus subcellular compartmentalization in some proteins, it was of interest to see whether the local structure of the N-terminus was being influenced by one or more of these modifications. A series of synthetic peptides mimicing the 16 N-terminal residues of the catalytic subunit Calpha was produced covering the full range of possible modifications, singly and in combination, and tested for possible effects on local structure by measuring the circular dichroism under varying polarity. It was found that myristoylation and phosphorylation modify the structure in this region in opposite ways and in a manner designed to amplify the action of a potential myristoyl/electrostatic switch. To what extent deamidation of Asn2 may oppose a potential membrane binding is unknown. Deamidation, however, had no effect on the structure of the peptide either alone or in combination with acylation and/or phosphorylation, suggesting that the change of the nuclear/cytoplasmic disribution in cells caused by deamidation [Pepperkok et al. (2000) J. Cell Biol. 148, 715-726] is due to a more complex signaling mechanism. The structural implications of the data are discussed.

Phosphorylation and activation of brain aromatic L-amino acid decarboxylase by cyclic AMP-dependent protein kinase

Aromatic L-amino acid decarboxylase (AAAD), an enzyme required for the synthesis of catecholamines, indoleamines, and trace amines, is rapidly activated by cyclic AMP-dependent pathways in striatum and midbrain in vivo, suggesting enzyme phosphorylation. We now report that the catalytic subunit of cyclic AMP-dependent protein kinase (PKA) directly phosphorylated AAAD immunoprecipitated from homogenates prepared from the mouse striatum and midbrain in vitro. Under the same phosphorylation conditions, the catalytic subunit of PKA also phosphorylated a recombinant AAAD protein expressed in Escherichia coli transfected with an AAAD cDNA isolated from the bovine adrenal gland. The PKA-induced AAAD phosphorylation of immunoprecipitates from striatum and midbrain was time and concentration dependent and blocked by a specific PKA peptide inhibitor. Incubation of the catalytic subunit of PKA with striatal homogenates increased enzyme activity by approximately 20% in a time- and concentration-dependent manner. Moreover, incubation of the catalytic subunit of PKA with recombinant AAAD increased activity by approximately 70%. A direct phosphorylation of AAAD protein by PKA might underlie the cyclic AMP-induced rapid and transient activation of AAAD in vivo.

The catalytic subunit of cAMP-dependent protein kinase: prototype for an extended network of communication

The protein kinase catalytic core in essence comprises an extended network of interactions that link distal parts of the molecule to the active site where they facilitate phosphoryl transfer from ATP to protein substrate. This review defines key sequence and structural elements, describes what is currently known about the molecular interactions, and how they are involved in catalysis.

Phosphorylation and activation of cAMP-dependent protein kinase by phosphoinositide-dependent protein kinase

Although phosphorylation of Thr-197 in the activation loop of the catalytic subunit of cAMP-dependent protein kinase (PKA) is an essential step for its proper biological function, the kinase responsible for this reaction in vivo has remained elusive. Using nonphosphorylated recombinant catalytic subunit as a substrate, we have shown that the phosphoinositide-dependent protein kinase, PDK1, expressed in 293 cells, phosphorylates and activates the catalytic subunit of PKA. The phosphorylation of PKA by PDK1 is rapid and is insensitive to PKI, the highly specific heat-stable protein kinase inhibitor. A mutant form of the catalytic subunit where Thr-197 was replaced with Asp was not a substrate for PDK1. In addition, phosphorylation of the catalytic subunit can be monitored immunochemically by using antibodies that recognize Thr-197 phosphorylated enzyme but not unphosphorylated enzyme or the Thr197Asp mutant. PDK1, or one of its homologs, is thus a likely candidate for the in vivo PKA kinase that phosphorylates Thr-197. This finding opens a new dimension in our thinking about this ubiquitous protein kinase and how it is regulated in the cell.

Partial activation of the insulin receptor kinase domain by juxtamembrane autophosphorylation

Increased enzymatic activity of receptor tyrosine kinases occurs after trans-phosphorylation of one or two tyrosines in the activation loop, located near the catalytic cleft. Partial activation of the insulin receptor's kinase domain was observed at dilute concentrations of kinase, suggesting that cis-autophosphorylation was occurring. Autophosphorylation during partial activation mapped to the juxtamembrane (JM) tyrosines and not to activation loop tyrosines. Furthermore, a double JM Tyr-to-Phe mutant kinase (JMY2F) did not undergo partial activation but catalyzed substrate phosphorylation at a very low rate. Steady-state kinetics of peptide phosphorylation were determined with and without JM autophosphorylation. The JMY2F mutant was used to prevent concurrent cis-autophosphorylation and therefore to approximate the basal state apoenzyme in the kinetic analysis. Partial activation was dominated by a decreased Michaelis constant for peptide substrate, from KM,PEP >/= 2.5 mM in the basal state to 0.2 mM in the partially activated state; the KM,ATP remained virtually unchanged at approximately 1 mM, and kcat increased from 180 to 600 min-1. The high KM,PEP suggests weak binding of peptide substrates to the apoenzyme. This was confirmed by Ki > 1 mM for peptide substrates used as inhibitors of JM autophosphorylation. The absence of comparably large changes in kcat and KM,ATP suggests that the JM region is primarily a strong barrier to the peptide entry step of trans-phosphorylation reactions. The JM region therefore functions as an intrasteric inhibitor in the basal state of the insulin receptor's kinase domain.

A conserved deamidation site at Asn 2 in the catalytic subunit of mammalian cAMP-dependent protein kinase detected by capillary LC-MS and tandem mass spectrometry

The N-terminal sequence myr-Gly-Asn is conserved among the myristoylated cAPK (protein kinase A) catalytic subunit isozymes Calpha, Cbeta, and Cgamma. By capillary LC-MS and tandem MS, we show that, in approximately one third of the Calpha and Cbeta enzyme populations from cattle, pig, rabbit, and rat striated muscle, Asn 2 is deamidated to Asp 2. This deamidation accounts for the major isoelectric variants of the cAPK C-subunits formerly called CA and CB. Deamidation also includes characteristic isoaspartate isomeric peptides from Calpha and Cbeta. Asn 2 deamidation does not occur during C-subunit preparation and is absent in recombinant myristoylated Calpha (rCalpha) from Escherichia coli. Deamidation appears to be the exclusive pathway for introduction of an acidic residue adjacent to the myristoylated N-terminal glycine, verified by the myristoylation negative phenotype of an rCalpha(Asn 2 Asp) mutant. This is the first report thus far of a naturally occurring myr-Gly-Asp sequence. Asp 2 seems to be required for the well-characterized (auto)phosphorylation of the native enzyme at Ser 10. Our results suggest that the myristoylated N terminus of cAPK is a conserved site for deamidation in vivo. Comparable myr-Gly-Asn sequences are found in several signaling proteins. This may be especially significant in view of the recent knowledge that negative charges close to myristic acid in some proteins contribute to regulating their cellular localization.

Protein engineering of the HMG-CoA reductase of Pseudomonas mevalonii. Construction of mutant enzymes whose activity is regulated by phosphorylation and dephosphorylation

The activity of Pseudomonas mevalonii HMG-CoA reductase (EC 1.1.1.88) is not regulated by phosphorylation, presumably due to the absence of a suitable target serine and protein kinase recognition motif. We have engineered P. mevalonii HMG-CoA reductase to a form whose activity, like that of mammalian HMG-CoA reductases, is regulated by phosphorylation/dephosphorylation. We substituted serine for arginine 387, the residue that corresponds to the regulatory serine of the HMG-CoA reductases of higher eukaryotes. A recognition motif for cAMP-dependent protein kinase was added by replacing leucine 384 by histidine (enzyme L384H/R387S) and also valine 391 by leucine (enzyme L384H/R387S/V391L). The activity of P. mevalonii HMG-CoA reductase mutant enzymes L384H/R387S and L384H/R387S/V391L was attenuated by phosphorylation. Restoration of activity accompanied subsequent dephosphorylation catalyzed by lambda protein phosphatase. Incorporation and subsequent release of phosphate paralleled the attenuation and restoration of catalytic activity. Incorporation of 0.5 mol of phosphate per subunit was accompanied by an approximately 50% decrease in initial activity. As in the analogous Syrian hamster mutant enzyme S871D, P. mevalonii mutant enzyme R387D exhibited 10% wild-type activity, suggesting that the attenuation of activity that accompanies phosphorylation results at least in part from the introduction of negative charge. Engineering of P. mevalonii HMG-CoA reductase to forms whose activity is reversibly regulated by phosphorylation/dephosphorylation provides an attractive model for future structure-based mechanistic studies. Solution of the X-ray structure of phosphorylated and dephosphorylated forms of engineered P. mevalonii HMG-CoA reductase should then reveal interactions of the active site phosphoseryl residue that result in attenuation of catalytic activity.

Synergistic binding of nucleotides and inhibitors to cAMP-dependent protein kinase examined by acrylodan fluorescence spectroscopy

We have engineered an acrylodan-modified derivative of the catalytic subunit of cyclic AMP-dependent protein kinase (cAPK) whose fluorescence emission signal has allowed the synergistic binding between nucleotides and physiological inhibitors of cAPK to be examined (Whitehouse, S., and Walsh, D. A. (1983) J. Biol. Chem. 258, 3682-3692). In the presence of the regulatory subunit, RI, the affinity of cAPK for adenosine, ADP, AMPPNP (adenosine 5'-(beta, gamma-imino)triphosphate), or ATP was 5-, 50-, 120-, and 15,000-fold enhanced, while in the presence of the heat-stable inhibitor protein of cAPK (PKI), there was a 3-, 20-, 33-, and 2000-fold enhancement in the binding of these nucleotides, respectively. A short inhibitor peptide, PKI-(14-22), enhanced the binding of ADP to the same degree as did full-length PKI (20-fold) but, in contrast, did not significantly enhance the binding of ATP or AMPPNP. The full binding synergism between PKI and either ATP (2000-fold) or AMPPNP (33-fold) to cAPK could, however, be mimicked by a longer peptide, PKI-(5-24), suggesting that the PKI NH2 terminus (residues 5-13) is most likely critical. Since this region is remote from the ATP gamma-phosphate, the binding synergism must arise through an extended network communication mechanism between the PKI NH2 terminus and the ATP binding site.

Phosphorylation of threonine 638 critically controls the dephosphorylation and inactivation of protein kinase Calpha

BACKGROUND

It has been widely reported that multisite phosphorylation plays an essential role in the regulation of protein kinases. However, our understanding of how these events modify protein function in vitro and in vivo is poorly understood. Protein kinase C (PKC) affords an interesting example of how phosphorylation control is coupled to effector control. PKC is acutely regulated by the second messenger diacylglycerol; however, it is also known to undergo multisite phosphorylation. Previously, we and others have shown that one site in the 'activation loop' of PKCalpha (a threonine residue at position 497; T497) and PKCbeta (T500) is essential for the catalytic competence of these proteins. More recently, a carboxy-terminal site (T638 in PKCalpha) has been implicated. In this report, we investigate the role of this site and its interaction with the catalytic core site.

RESULTS

We have analyzed mutant PKCalpha proteins, in which amino-acid substitutions were made at the T638 site, and shown that phosphorylation at this site affects the conformation of the protein, as judged by thermal stability, and sensitivity to oxidation, trypsin and phosphatase treatment. This supersensitivity to dephosphorylation in vitro was also seen in an agonist-dependent context in vivo. We have also shown that phosphorylation of this site is not essential for catalytic activity of the purified protein. The molecular basis of the control operating through the T638 site was provided by the evidence of a functional interaction with the previously described catalytic core site, T497. This inter-relationship was further established by the demonstration that the E497 mutant protein had a thermal instability and phosphatase supersensitivity similar to that of the A638 and E638 mutants.

CONCLUSIONS

The T638 phosphorylation site is not required for the catalytic function of PKCalpha per se, but serves to control the duration of activation by regulating the rate of dephosphorylation and inactivation of the protein. This is achieved through the cooperative interaction between the T638 and T497 sites; if either of these residues is not phosphorylated, the protein is supersensitive to phosphatase action. This model of PKCalpha function is likely to be of general significance to the protein kinase superfamily, where similarly juxtaposed sites exist. We conclude that dephosphorylation of PKCalpha, and, by inference, other protein kinases, is regulated by multisite phosphorylation.

Dephosphorylation of catalytic subunit of cAMP-dependent protein kinase at Thr-197 by a cellular protein phosphatase and by purified protein phosphatase-2A

Thr-197 phosphate is essential for optimal activity of the catalytic (C) subunit of cAMP-dependent protein kinase enzyme, and, in the C subunit crystal structure, it is buried in a cationic pocket formed by the side chains of His-87, Arg-165, Lys-189, and Thr-195. Because of its apparent role in stabilizing the active conformation of C subunit and its resistance to several phosphatases, the phosphate on Thr-197 has been assumed to be metabolically stable. We now show that this phosphate can be removed from C subunit by a protein phosphatase activity extracted from S49 mouse lymphoma cells or by purified protein phosphatase-2A (PP-2A) with concomitant loss of enzymatic activity. By anion-exchange chromatography, inhibitor sensitivity, and relative activity against glycogen phosphorylase a and C subunit as substrates, the cellular phosphatase resembled a multimeric form of PP-2A. PP-1 was ineffective against native C subunit, but it was able to dephosphorylate Thr-197 in urea-treated C subunit. Accessibility of Thr-197 phosphate to the cellular phosphatase was enhanced by storage of C subunit in a phosphate-free buffer or by inclusion of modest concentrations of urea in the reactions and was reduced by salt concentrations in the physiological range and/or by amino-terminal myristoylation. It is concluded that a multimeric form of PP-2A or a closely related enzyme from cell extracts is capable of removing the Thr-197 phosphate from native C subunit in vitro and could account for significant turnover of this phosphate in intact cells.

Heat-stable inhibitor protein derived peptide substrate analogs: phosphorylation by cAMP-dependent and cGMP-dependent protein kinases

The phosphorylation of substrate peptides derived from PKI, the heat-stable inhibitor protein of the cAMP-dependent protein kinase (PKA), has been studied with both PKA and the cGMP-dependent protein kinase (PKG) using a variety of substitution and deletion analogs. On the basis of Km, kcat and kcat/Km values, (Ser21)PKI alpha(14-22) amide (numbering based upon native PKI alpha) is the most effective peptide substrate yet discovered for either kinase, although other peptides, while phosphorylated considerably less efficiently by PKG, are more specific. Although the inhibitory peptide corresponding to this sequence (i.e., with an Ala at position 21) is a much more potent inhibitor of PKA than of PKG (approximately 250-fold), PKG actually exhibits a 60% higher kcat than does PKA with the (Ser21)PKI alpha(14-22) amide substrate peptide, with only a 20-fold higher Km value. The two key PKI residues within this peptide which were found to be essential for substrate activity with both kinases were Arg18 (P-3) and Ile22 (P+1). The Arg19 (P-2) residue, which contributes significantly to both PKI-based peptide inhibitors and substrates of PKA, was only a more minor contributor to PKG substrate efficacy. Of particular note, the Phe10 (P-11) residue, which contributes very substantially to high affinity binding of both PKI and longer PKI peptide inhibitors, neither positively nor negatively affects the kinetics of either PKA or PKG with PKI-based substrates.(ABSTRACT TRUNCATED AT 250 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.