Expression in Escherichia coli and characterization of the heat-stable inhibitor of the cAMP-dependent protein kinase

PKIB, a Novel Target for Cancer Therapy

The serine-threonine kinase protein kinase A (PKA) is a cyclic AMP (cAMP)-dependent intracellular protein with multiple roles in cellular biology including metabolic and transcription regulation functions. The cAMP-dependent protein kinase inhibitor β (PKIB) is one of three known endogenous protein kinase inhibitors of PKA. The role of PKIB is not yet fully understood. Hormonal signaling is correlated with increased PKIB expression through genetic regulation, and increasing PKIB expression is associated with decreased cancer patient prognosis. Additionally, PKIB impacts cancer cell behavior through two mechanisms; the first is the nuclear modulation of transcriptional activation and the second is the regulation of oncogenic AKT signaling. The limited research into PKIB indicates the oncogenic potential of PKIB in various cancers. However, some studies suggest a role of PKIB in non-cancerous disease states. This review aims to summarize the current literature and background of PKIB regarding cancer and related issues. In particular, we will focus on cancer development and therapeutic possibilities, which are of paramount interest in PKIB oncology research.

The M-phase regulatory phosphatase PP2A-B55δ opposes protein kinase A on Arpp19 to initiate meiotic division

Oocytes are held in meiotic prophase for prolonged periods until hormonal signals trigger meiotic divisions. Key players of M-phase entry are the opposing Cdk1 kinase and PP2A-B55δ phosphatase. In Xenopus, the protein Arpp19, phosphorylated at serine 67 by Greatwall, plays an essential role in inhibiting PP2A-B55δ, promoting Cdk1 activation. Furthermore, Arpp19 has an earlier role in maintaining the prophase arrest through a second serine (S109) phosphorylated by PKA. Prophase release, induced by progesterone, relies on Arpp19 dephosphorylation at S109, owing to an unknown phosphatase. Here, we identified this phosphatase as PP2A-B55δ. In prophase, PKA and PP2A-B55δ are simultaneously active, suggesting the presence of other important targets for both enzymes. The drop in PKA activity induced by progesterone enables PP2A-B55δ to dephosphorylate S109, unlocking the prophase block. Hence, PP2A-B55δ acts critically on Arpp19 on two distinct sites, opposing PKA and Greatwall to orchestrate the prophase release and M-phase entry.

Multi-state recognition pathway of the intrinsically disordered protein kinase inhibitor by protein kinase A

In the nucleus, the spatiotemporal regulation of the catalytic subunit of cAMP-dependent protein kinase A (PKA-C) is orchestrated by an intrinsically disordered protein kinase inhibitor, PKI, which recruits the CRM1/RanGTP nuclear exporting complex. How the PKA-C/PKI complex assembles and recognizes CRM1/RanGTP is not well understood. Using NMR, SAXS, fluorescence, metadynamics, and Markov model analysis, we determined the multi-state recognition pathway for PKI. After a fast binding step in which PKA-C selects PKI's most competent conformations, PKI folds upon binding through a slow conformational rearrangement within the enzyme's binding pocket. The high-affinity and pseudo-substrate regions of PKI become more structured and the transient interactions with the kinase augment the helical content of the nuclear export sequence, which is then poised to recruit the CRM1/RanGTP complex for nuclear translocation. The multistate binding mechanism featured by PKA-C/PKI complex represents a paradigm on how disordered, ancillary proteins (or protein domains) are able to operate multiple functions such as inhibiting the kinase while recruiting other regulatory proteins for nuclear export.

A Stapled Peptide Mimic of the Pseudosubstrate Inhibitor PKI Inhibits Protein Kinase A

Kinases regulate multiple and diverse signaling pathways and misregulation is implicated in a multitude of diseases. Although significant efforts have been put forth to develop kinase-specific inhibitors, specificity remains a challenge. As an alternative to catalytic inhibition, allosteric inhibitors can target areas on the surface of an enzyme, thereby providing additional target diversity. Using cAMP-dependent protein kinase A (PKA) as a model system, we sought to develop a hydrocarbon-stapled peptide targeting the pseudosubstrate domain of the kinase. A library of peptides was designed from a Protein Kinase Inhibitor (PKI), a naturally encoded protein that serves as a pseudosubstrate inhibitor for PKA. The binding properties of these peptide analogs were characterized by fluorescence polarization and surface plasmon resonance, and two compounds were identified with K_D values in the 500-600 pM range. In kinase activity assays, both compounds demonstrated inhibition with 25-35 nM IC₅₀ values. They were also found to permeate cells and localize within the cytoplasm and inhibited PKA activity within the cellular environment. To the best of our knowledge, these stapled peptide inhibitors represent some of the highest affinity binders reported to date for hydrocarbon stapled peptides.

Phosphorylation of ARPP19 by protein kinase A prevents meiosis resumption in Xenopus oocytes

During oogenesis, oocytes are arrested in prophase and resume meiosis by activating the kinase Cdk1 upon hormonal stimulation. In all vertebrates, release from prophase arrest relies on protein kinase A (PKA) downregulation and on the dephosphorylation of a long-sought but still unidentified substrate. Here we show that ARPP19 is the PKA substrate whose phosphorylation at serine 109 is necessary and sufficient for maintaining Xenopus oocytes arrested in prophase. By downregulating PKA, progesterone, the meiotic inducer in Xenopus, promotes partial dephosphorylation of ARPP19 that is required for the formation of a threshold level of active Cdk1. Active Cdk1 then initiates MPF autoamplification loop that occurs independently of both PKA and ARPP19 phosphorylation at serine 109 but requires the Greatwall-dependent phosphorylation of ARPP19 at serine 67. Therefore, ARPP19 stands at a crossroads in the meiotic M-phase control network by integrating differential effects of PKA and Greatwall, two essential kinases for meiosis resumption.

Parkinson-related LRRK2 mutation R1441C/G/H impairs PKA phosphorylation of LRRK2 and disrupts its interaction with 14-3-3

Leucine-rich repeat kinase 2 (LRRK2) is a multidomain protein implicated in Parkinson disease (PD); however, the molecular mechanism and mode of action of this protein remain elusive. cAMP-dependent protein kinase (PKA), along with other kinases, has been suggested to be an upstream kinase regulating LRRK2 function. Using MS, we detected several sites phosphorylated by PKA, including phosphorylation sites within the Ras of complex proteins (ROC) GTPase domain as well as some previously described sites (S910 and S935). We systematically mapped those sites within LRRK2 and investigated their functional consequences. S1444 in the ROC domain was confirmed as a target for PKA phosphorylation using ROC single-domain constructs and through site-directed mutagenesis. Phosphorylation at S1444 is strikingly reduced in the major PD-related LRRK2 mutations R1441C/G/H, which are part of a consensus PKA recognition site ((1441)RASpS(1444)). Furthermore, our work establishes S1444 as a PKA-regulated 14-3-3 docking site. Experiments of direct binding to the three 14-3-3 isotypes gamma, theta, and zeta with phosphopeptides encompassing pS910, pS935, or pS1444 demonstrated the highest affinities to phospho-S1444. Strikingly, 14-3-3 binding to phospho-S1444 decreased LRRK2 kinase activity in vitro. Moreover, substitution of S1444 by alanine or by introducing the mutations R1441C/G/H, abrogating PKA phosphorylation and 14-3-3 binding, resulted in increased LRRK2 kinase activity. In conclusion, these data clearly demonstrate that LRRK2 kinase activity is modulated by PKA-mediated binding of 14-3-3 to S1444 and suggest that 14-3-3 interaction with LRRK2 is hampered in R1441C/G/H-mediated PD pathogenesis.

The phosphorylation of ARPP19 by Greatwall renders the auto-amplification of MPF independently of PKA in Xenopus oocytes

Entry into mitosis or meiosis relies on the coordinated action of kinases and phosphatases that ultimately leads to the activation of Cyclin-B-Cdk1, also known as MPF for M-phase promoting factor. Vertebrate oocytes are blocked in prophase of the first meiotic division, an arrest that is tightly controlled by high PKA activity. Re-entry into meiosis depends on activation of Cdk1, which obeys a two-step mechanism: a catalytic amount of Cdk1 is generated in a PKA and protein-synthesis-dependent manner; then a regulatory network known as the MPF auto-amplification loop is initiated. This second step is independent of PKA and protein synthesis. However, none of the molecular components of the auto-amplification loop identified so far act independently of PKA. Therefore, the protein rendering this process independent of PKA in oocytes remains unknown. Using a physiologically intact cell system, the Xenopus oocyte, we show that the phosphorylation of ARPP19 at S67 by the Greatwall kinase promotes its binding to the PP2A-B55δ phosphatase, thus inhibiting its activity. This process is controlled by Cdk1 and has an essential role within the Cdk1 auto-amplification loop for entry into the first meiotic division. Moreover, once phosphorylated by Greatwall, ARPP19 escapes the negative regulation exerted by PKA. It also promotes activation of MPF independently of protein synthesis, provided that a small amount of Mos is present. Taken together, these findings reveal that PP2A-B55δ, Greatwall and ARPP19 are not only required for entry into meiotic divisions, but are also pivotal effectors within the Cdk1 auto-regulatory loop responsible for its independence with respect to the PKA-negative control.

Effect of metal ions on high-affinity binding of pseudosubstrate inhibitors to PKA

Conformational control of protein kinases is an important way of modulating catalytic activity. Crystal structures of the C (catalytic) subunit of PKA (protein kinase A) in complex with physiological inhibitors and/or nucleotides suggest a highly dynamic process switching between open and more closed conformations. To investigate the underlying molecular mechanisms, SPR (surface plasmon resonance) was used for detailed binding analyses of two physiological PKA inhibitors, PKI (heat-stable protein kinase inhibitor) and a truncated form of the R (regulatory) subunit (RIalpha 92-260), in the presence of various concentrations of metals and nucleotides. Interestingly, it could be demonstrated that high-affinity binding of each pseudosubstrate inhibitor was dependent only on the concentration of divalent metal ions. At low micromolar concentrations of Mg2+ with PKI, transient interaction kinetics with fast on- and off-rates were observed, whereas at high Mg2+ concentrations the off-rate was slowed down by a factor of 200. This effect could be attributed to the second, low-affinity metal-binding site in the C subunit. In contrast, when investigating the interaction of RIalpha 92-260 with the C subunit under the same conditions, it was shown that the association rate rather than the dissociation rate was influenced by the presence of high concentrations of Mg2+. A model is presented, where the high-affinity interaction of the C subunit with pseudosubstrate inhibitors (RIalpha and PKI) is dependent on the closed, catalytically inactive conformation induced by the binding of a nucleotide complex where both of the metal-binding sites are occupied.

Zinc regulates the ability of Cdc25C to activate MPF/cdk1

Zn(2+) is an essential micronutrient for the growth and development of multicellular organisms, as Zn(2+) deficiencies lead to growth retardation and congenital malformations (Vallee, BL, Falchuk, KH. 1993. Physiol Rev., 73:79-118). At the cellular level Zn(2+) depravation results in proliferation defects in many cell types (Vallee, BL, Falchuk, KH. 1993. Physiol Rev., 73:79-118), however the molecular pathways involved remain poorly defined. Here we show that the transition metal chelator TPEN (N,N,N',N'-tetrakis(2-pyridylmethyl) ethylene diamine) blocks the G2/M transition of the meiotic cell cycle by inhibiting Cdc25C-cdk1 activation. ICP-MS analyses reveal that Cdc25C is a Zn(2+)-binding metalloprotein, and that TPEN effectively strips Zn(2+) away from the enzyme. Interestingly, although apo-Cdc25C (Zn(2+)-deficient) remains fully catalytically active, it is compromised in its ability to dephosphorylate and activate MPF/cdk1. Thus, Zn(2+) is an important regulator of Cdc25C function in vivo. Because of the conserved essential role of the Cdc25C-cdk1 module in the eukaryotic cell cycle, these studies provide fundamental insights into cell cycle regulation.

Protein kinase inhibitor peptide (PKI): a family of endogenous neuropeptides that modulate neuronal cAMP-dependent protein kinase function

Signal transduction cascades involving cAMP-dependent protein kinase are highly conserved among a wide variety of organisms. Given the universal nature of this enzyme it is not surprising that cAMP-dependent protein kinase plays a critical role in numerous cellular processes. This is particularly evident in the nervous system where cAMP-dependent protein kinase is involved in neurotransmitter release, gene transcription, and synaptic plasticity. Protein kinase inhibitor peptide (PKI) is an endogenous thermostable peptide that modulates cAMP-dependent protein kinase function. PKI contains two distinct functional domains within its amino acid sequence that allow it to: (1) potently and specifically inhibit the activity of the free catalytic subunit of cAMP-dependent protein kinase and (2) export the free catalytic subunit of cAMP-dependent protein kinase from the nucleus. Three distinct PKI isoforms (PKIalpha, PKIbeta, PKIgamma) have been identified and each isoform is expressed in the brain. PKI modulates neuronal synaptic activity, while PKI also is involved in morphogenesis and symmetrical left-right axis formation. In addition, PKI also plays a role in regulating gene expression induced by cAMP-dependent protein kinase. Future studies should identify novel physiological functions for endogenous PKI both in the nervous system and throughout the body. Most interesting will be the determination whether functional differences exist between individual PKI isoforms which is an intriguing possibility since these isoforms exhibit: (1) cell-type specific tissue expression patterns, (2) different potencies for the inhibition of cAMP-dependent protein kinase activity, and (3) expression patterns that are hormonally, developmentally and cell-cycle regulated. Finally, synthetic peptide analogs of endogenous PKI will continue to be invaluable tools that are used to elucidate the role of cAMP-dependent protein kinase in a variety of cellular processes throughout the nervous system and the rest of the body.

Purification and structural study of the beta form of human cAMP-dependent protein kinase inhibitor

The beta form of human cAMP-dependent protein kinase inhibitor (human PKIbeta), a novel heat-stable protein, was isolated with high yield using a bacterial expression system. Assays of PKI activity demonstrated that purified PKIbeta inhibits the catalytic subunit of cAMP-dependent protein kinase. FTIR, Raman spectroscopy and CD experiments implied that human PKIbeta contained only small amounts of alpha-helix and beta-structures, but large amounts of random coil and turn structures, which may explain its high thermostability. The details of its conformational changes in response to heat were studied by CD experiments for the first time, revealing that the protein unfolded at high temperature and refolded when decreased to room temperature.

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.

Regulation of the G2/M transition in oocytes of xenopus tropicalis

The molecular events regulating hormone-induced oocyte activation and meiotic maturation are probably best understood in Xenopus laevis. In X. laevis, progesterone activates the G2-arrested oocyte, induces entry into M phase of meiosis I (MI) and resumption of the meiotic cell cycles, and leads to the formation of a mature, fertilizable egg. Oocytes of Xenopus tropicalis offer several practical advantages over those of X. laevis, including faster and more synchronous meiotic cell cycle progression, less seasonal variability, and the availability of transgenic approaches. Previous work found several similarities in the pathways regulating oocyte maturation in the two species. Here, we report several additional ones that are conserved in X. tropicalis. (1). Injection of Mos mRNA into G2-arrested oocytes activates the MAP kinase cascade and induces the G2/MI transition. (2). Injection of the beta subunit of the kinase CK2 (a negative regulator of Mos and oocyte activation) delays the G2/MI transition. (3). Elevating PKA activity blocks progesterone-induced maturation; repressing PKA activity induces entry into MI in the absence of progesterone. (4). LF (anthrax lethal factor), which cleaves certain MAP kinase kinases, strongly reduces both the rate and extent of entry into MI. In contrast to the one previously reported major difference between oocytes of the two species, we find that injection of egg cytoplasm ("MPF activity") into G2-arrested X. tropicalis oocytes induces entry into meiosis I even when protein synthesis is blocked, just as it does in oocytes of X. laevis. These results indicate that much of what we have learned from studies of X. laevis oocytes holds for those of X. tropicalis, and suggest that X. tropicalis oocytes offer a good experimental system for investigating certain questions that require a rapid, synchronous progression through the G2/meiosis I transition.

G2 arrest in Xenopus oocytes depends on phosphorylation of cdc25 by protein kinase A

Xenopus oocytes, which are arrested in G(2) of meiosis I, contain complexes of cyclin B-cdc2 (M phase-promoting factor) that are kept repressed by inhibitory phosphorylations on cdc2 at Thr-14 and Tyr-15. Progesterone induces a cytoplasmic signaling pathway that leads to activation of cdc25, the phosphatase that removes these phosphorylations, catalyzing entry into M phase. It has been known for 25 years that high levels of cAMP and protein kinase A (PKA) are required to maintain the G(2) arrest and that a drop in PKA activity is required for M phase-promoting factor activation, but no physiological targets of PKA have been identified. We present evidence that cdc25 is a critical target of PKA. (i) In vitro, cdc25 Ser-287 serves as a major site of phosphorylation by PKA, resulting in sequestration by 14-3-3. (ii) Endogenous cdc25 is phosphorylated on Ser-287 in oocytes and dephosphorylated in response to progesterone just before cdc2 dephosphorylation and M-phase entry. (iii) High PKA activity maintains phosphorylation of Ser-287 in vivo, whereas inhibition of PKA by its heat-stable inhibitor (PKI) induces dephosphorylation of Ser-287. (iv) Overexpression of mutant cdc25 (S287A) bypasses the ability of PKA to maintain oocytes in G(2) arrest. These findings argue that cdc25 is a physiologically relevant target of PKA in oocytes. In the early embryonic cell cycles, Ser-287 is phosphorylated during interphase and dephosphorylated just before cdc2 activation and mitotic entry. Thus, in addition to its role in checkpoint arrest, cdc25 Ser-287 serves as a site for regulation during normal, unperturbed cell cycles.

What does it mean to be natively unfolded?

Natively unfolded or intrinsically unstructured proteins constitute a unique group of the protein kingdom. The evolutionary persistence of such proteins represents strong evidence in the favor of their importance and raises intriguing questions about the role of protein disorders in biological processes. Additionally, natively unfolded proteins, with their lack of ordered structure, represent attractive targets for the biophysical studies of the unfolded polypeptide chain under physiological conditions in vitro. The goal of this study was to summarize the structural information on natively unfolded proteins in order to evaluate their major conformational characteristics. It appeared that natively unfolded proteins are characterized by low overall hydrophobicity and large net charge. They possess hydrodynamic properties typical of random coils in poor solvent, or premolten globule conformation. These proteins show a low level of ordered secondary structure and no tightly packed core. They are very flexible, but may adopt relatively rigid conformations in the presence of natural ligands. Finally, in comparison with the globular proteins, natively unfolded polypeptides possess 'turn out' responses to changes in the environment, as their structural complexities increase at high temperature or at extreme pH.

The polo-like kinase Plx1 is required for activation of the phosphatase Cdc25C and cyclin B-Cdc2 in Xenopus oocytes

In the Xenopus oocyte system mitogen treatment triggers the G(2)/M transition by transiently inhibiting the cAMP-dependent protein kinase (PKA); subsequently, other signal transduction pathways are activated, including the mitogen-activated protein kinase (MAPK) and polo-like kinase pathways. To study the interactions between these pathways, we have utilized a cell-free oocyte extract that carries out the signaling events of oocyte maturation after addition of the heat-stable inhibitor of PKA, PKI. PKI stimulated the synthesis of Mos and activation of both the MAPK pathway and the Plx1/Cdc25C/cyclin B-Cdc2 pathway. Activation of the MAPK pathway alone by glutathione S-transferase (GST)-Mos did not lead to activation of Plx1 or cyclin B-Cdc2. Inhibition of the MAPK pathway in the extract by the MEK1 inhibitor U0126 delayed, but did not prevent, activation of the Plx1 pathway, and inhibition of Mos synthesis by cycloheximide had a similar effect, suggesting that MAPK activation is the only relevant function of Mos. Immunodepletion of Plx1 completely inhibited activation of Cdc25C and cyclin B-Cdc2 by PKI, indicating that Plx1 is necessary for Cdc25C activation. In extracts containing fully activated Plx1 and Cdc25C, inhibition of cyclin B-Cdc2 by p21(Cip1) had no significant effect on either the phosphorylation of Cdc25C or the activity of Plx1. These results demonstrate that maintenance of Plx1 and Cdc25C activity during mitosis does not require cyclin B-Cdc2 activity.

Why are "natively unfolded" proteins unstructured under physiologic conditions?

"Natively unfolded" proteins occupy a unique niche within the protein kingdom in that they lack ordered structure under conditions of neutral pH in vitro. Analysis of amino acid sequences, based on the normalized net charge and mean hydrophobicity, has been applied to two sets of proteins: small globular folded proteins and "natively unfolded" ones. The results show that "natively unfolded" proteins are specifically localized within a unique region of charge-hydrophobicity phase space and indicate that a combination of low overall hydrophobicity and large net charge represent a unique structural feature of "natively unfolded" proteins.

Serine-53 at the tip of the glycine-rich loop of cAMP-dependent protein kinase: role in catalysis, P-site specificity, and interaction with inhibitors

The glycine-rich loop, one of the most important motifs in the conserved protein kinase catalytic core, embraces the entire nucleotide, is very mobile, and is exquisitely sensitive to what occupies the active site cleft. Of the three conserved glycines [G(50)TG(52)SFG(55) in cAMP-dependent protein kinase (cAPK)], Gly(52) is the most important for catalysis because it allows the backbone amide of Ser(53) at the tip of the loop to hydrogen bond to the gamma-phosphate of ATP [Grant, B. D. et al. (1998) Biochemistry 37, 7708]. The structural model of the catalytic subunit:ATP:PKI((5)(-)(24)) (heat-stable protein kinase inhibitor) ternary complex in the closed conformation suggests that Ser(53) also might be essential for stabilization of the peptide substrate-enzyme complex via a hydrogen bond between the P-site carbonyl in PKI and the Ser(53) side-chain hydroxyl [Bossemeyer, D. et al. (1993) EMBO J. 12, 849]. To address the importance of the Ser(53) side chain in catalysis, inhibition, and P-site specificity, Ser(53) was replaced with threonine, glycine, and proline. Removal of the side chain (i.e., mutation to glycine) had no effect on the steady-state phosphorylation of a peptide substrate (LRRASLG) or on the interaction with physiological inhibitors, including the type-I and -II regulatory subunits and PKI. However, this mutation did affect the P-site specificity; the glycine mutant can more readily phosphorylate a P-site threonine in a peptide substrate (5-6-fold better than wild-type). The proline mutant is compromised catalytically with altered k(cat) and K(m) for both peptide and ATP and with altered sensitivity to both regulatory subunits and PKI. Steric constraints as well as restricted flexibility could account for these effects. These combined results demonstrate that while the backbone amide of Ser(53) may be required for efficient catalysis, the side chain is not.

Is the manganese stabilizing 33 kDa protein of photosystem II attaining a 'natively unfolded' or 'molten globule' structure in solution?

This study compares the properties of the extrinsic 33 kDa subunit acting as 'manganese stabilizing protein' (MSP) of the water oxidizing complex with characteristic features of proteins that are known to attain a 'natively unfolded' or a 'molten globule' structure. The analysis leads to the conclusion that the MSP in solution is most likely a 'molten globule' with well defined compact regions of beta structure. The possible role of these structural peculiarities of MSP in solution for its function as important constituent of the WOC is discussed.

Role of regulatory subunits and protein kinase inhibitor (PKI) in determining nuclear localization and activity of the catalytic subunit of protein kinase A

Regulation of protein kinase A by subcellular localization may be critical to target catalytic subunits to specific substrates. We employed epitope-tagged catalytic subunit to correlate subcellular localization and gene-inducing activity in the presence of regulatory subunit or protein kinase inhibitor (PKI). Transiently expressed catalytic subunit distributed throughout the cell and induced gene expression. Co-expression of regulatory subunit or PKI blocked gene induction and prevented nuclear accumulation. A mutant PKI lacking the nuclear export signal blocked gene induction but not nuclear accumulation, demonstrating that nuclear export is not essential to inhibit gene induction. When the catalytic subunit was targeted to the nucleus with a nuclear localization signal, it was not sequestered in the cytoplasm by regulatory subunit, although its activity was completely inhibited. PKI redistributed the nuclear catalytic subunit to the cytoplasm and blocked gene induction, demonstrating that the nuclear export signal of PKI can override a strong nuclear localization signal. With increasing PKI, the export process appeared to saturate, resulting in the return of catalytic subunit to the nucleus. These results demonstrate that both the regulatory subunit and PKI are able to completely inhibit the gene-inducing activity of the catalytic subunit even when the catalytic subunit is forced to concentrate in the nuclear compartment.

Two well-defined motifs in the cAMP-dependent protein kinase inhibitor (PKIalpha) correlate with inhibitory and nuclear export function

The heat stable inhibitor of cAMP-dependent protein kinase (PKIalpha) contains both a nuclear export signal (NES) and a high affinity inhibitory region that is essential for inhibition of the catalytic subunit of the kinase. These functions are sequentially independent. Two-dimensional NMR spectroscopy was performed on uniformly [15N]-labeled PKIalpha to examine its structure free in solution. Seventy out of 75 residues were identified, and examination of the CaH chemical shifts revealed two regions of upfield chemical shifts characteristic of alpha-helices. When PKIalpha was fragmented into two functionally distinct peptides for study at higher concentrations, no significant alterations in chemical shifts or secondary structure were observed. The first ordered region, identified in PKIalpha (1-25), contains an alpha-helix from residues 1-13. This helix extends by one turn the helix observed in the crystal structure of a PKIalpha (5-24) peptide bound to the catalytic subunit. The second region of well-defined secondary structure, residues 35-47, overlaps with the nuclear export signal in the PKIalpha (26-75) fragment. This secondary structure consists of a helix with a hydrophobic face comprised of Leu37, Leu41, and Leu44, followed by a flexible turn containing Ile46. These four residues are critical for nuclear export function. The remainder of the protein in solution appears relatively unstructured, and this lack of structure surrounding a few essential and well-defined signaling elements may be characteristic of a growing family of small regulatory proteins that interact with protein kinases.

A cytosolic activity distinct from crm1 mediates nuclear export of protein kinase inhibitor in permeabilized cells

The leucine-rich nuclear export signal (NES) is used by a variety of proteins to facilitate their delivery from the nucleus to the cytoplasm. One of the best-studied examples, protein kinase inhibitor (PKI), binds to the catalytic subunit of protein kinase A in the nucleus and mediates its rapid export to the cytoplasm. We developed a permeabilized cell assay that reconstitutes nuclear export mediated by PKI, and we used it to characterize the cytosolic factors required for this process. The two-step assay involves an import phase and an export phase, and quantitation is achieved by digital fluorescence microscopy. During the import phase, a fluorescent derivative of streptavidin is imported into the nuclei of digitonin-permeabilized HeLa cells. During the export phase, biotinylated PKI diffuses into the nucleus, binds to fluorescent streptavidin, and mediates export of the complex to the cytoplasm. Nuclear export of the PKI complex is cytosol dependent and can be stimulated by addition of the purified NES receptor, Crm1. HeLa cell cytosol treated with N-ethylmaleimide (NEM) or phenyl-Sepharose to inactivate or deplete Crm1, respectively, is still fully active in the PKI export assay. Significantly, the export activity can be depleted from cytosol by preadsorption with a protein conjugate that contains a functional NES. These data indicate that cytosol contains an export activity that is distinct from Crm1 and is likely to correspond to an NES receptor.

Two novel brain-specific splice variants of the murine Cbeta gene of cAMP-dependent protein kinase

We have previously characterized two murine cAMP-dependent protein kinase catalytic subunit genes, Calpha and Cbeta1. Targeted disruption of the Cbeta1 promoter revealed two splice variants of the Cbeta catalytic subunit gene (designated Cbeta2 and Cbeta3) that continue to be expressed. These variants arise from unique promoters and are brain-specific. Cbeta2 is expressed in several discrete areas in the limbic system. These include the lateral septum, the bed nucleus of the stria terminalis, the ventral medial hypothalamus, and the amygdala. In the neocortex, expression is highest in cortical areas such as the prefrontal and insular cortex that are associated limbic structures. By contrast, Cbeta1 is most highly expressed in the cortex and hippocampus and is also present in all non-neuronal tissues examined. Cbeta3 is expressed at very low levels with wide distribution throughout the brain. Both the Cbeta2 and Cbeta3 variants are enzymatically active and induce gene expression in transient transfections with a cAMP response element-reporter construct. This activity is inhibited by protein kinase A regulatory subunits, the protein kinase inhibitor, and the chemical inhibitor H-89. We also demonstrate that Cbeta1 is myristoylated at the amino terminus like the Calpha isoform, but neither Cbeta2 nor Cbeta3 is myristoylated. The discrete expression of Cbeta variants in the brain suggests specific functional roles in neuronal signaling.

Hepatitis C virus NS5A protein is phosphorylated in vitro by a stably bound protein kinase from HeLa cells and by cAMP-dependent protein kinase A-alpha catalytic subunit

Hepatitis C virus (HCV) has a positive-strand RNA genome that codes for a polyprotein precursor, which is processed co- and post-translationally by cellular and viral proteinases into three structural and at least six non-structural (NS) proteins. The NS5A protein, expressed in mammalian cells, exists in two phosphorylated forms of 56-kDa and 58-kDa. In this study, we provide evidence for a stable association between NS5A and a protein kinase from HeLa cells and hepatocellular carcinoma (HepG2) cells by co-immunoprecipitation and by affinity to immobilized glutathione-S-transferase (GST)-NS5A fusion protein produced in E. coli. This protein kinase could phosphorylate in vitro the native NS5A on serine residues, (GST)-NS5A, histone H1, and casein as substrates. In addition, the GST-NS5A was also phosphorylated in vitro by the cAMP-dependent protein kinase A-alpha catalytic subunit.

Protein kinase C-mediated phosphorylation of HIV-I nef in human cell lines

Stable human cell lines expressing the human immunodeficiency virus type I (HIV-I) Nef protein from inducible promoters were used to analyze the phosphorylation status of Nef in vivo. Nef phosphorylation in both HeLa and Jurkat cells was stimulated by phorbol ester treatment. Phosphoamino acid analysis revealed a predominance of phosphoserine with a small proportion of phosphothreonine. Treatment of cells with selective protein kinase inhibitors revealed that Nef phosphorylation was markedly reduced by bisindolylmaleimide, an inhibitor of protein kinase C, but was unaffected by inhibitors of mitogen-activated protein kinase kinase or cAMP-dependent kinase. These data implicate protein kinase C in Nef phosphorylation in vivo, and thus confirm and extend earlier in vitro data. Phosphorylation of a nonmyristoylated Nef mutant was impaired, suggesting that membrane targeting of Nef was required for phosphorylation. This was expected given that activated protein kinase C translocates from the cytosol to the plasma membrane. However, analysis of the subcellular localization of phosphorylated wild-type Nef revealed that both the cytosolic and membrane-associated pools of Nef were phosphorylated to an equivalent extent. Thus the significance of myristoylation for Nef function may be in influencing protein conformation, although these data could be explained by a transient and dynamic interaction between myristoylated Nef and the plasma membrane.

Cloning and characterization of a novel serine/threonine protein kinase expressed in early Xenopus embryos

We have cloned from a Xenopus ovary cDNA library a novel protein kinase gene whose expression peaks in the oocyte and unfertilized egg, begins to decrease gradually after fertilization, and disappears during the gastrulation stage of embryogenesis. The cloned gene, termed XEEK1 (for Xenopus egg and embryo kinase), encodes a protein with a predicted molecular mass of 49 kDa. Bacterially expressed XEEK1 migrates at 57 kDa upon polyacrylamide gel electrophoresis analysis, and a XEEK1-specific antibody recognizes a protein of 57 kDa in Xenopus oocyte and egg extracts. The XEEK1 kinase domain shares 35% identity (approximately 65% similarity) with the yeast SNF1 kinase and related kinases. However, expression of XEEK1 does not complement a snf1 deletion mutation in yeast, which suggests that it is probably not a Xenopus homolog of SNF1. Recombinant XEEK1 protein autophosphorylates on threonine residues in vitro in a reaction that prefers Mn2+ to Mg2+ ions. Site-directed mutagenesis of the conserved lysine residue (Lys-81) within the kinase domain to isoleucine totally abolishes kinase activity, and threonine 192 has been identified as the autophosphorylation site. This site is distinct from the conserved threonine (Thr-215 in XEEK1) present in the protein kinase activation loop that is the site of autophosphorylation for many protein kinases. XEEK1 is a substrate for the cyclic AMP-dependent protein kinase both in vitro and in vivo, suggesting a possible mode of regulation of XEEK1. An immunoprecipitate of oocyte/egg extracts with anti-XEEK1 serum contains a protein of approximately 155 kDa that may be a substrate and/or a regulatory component of the kinase.

Identification of a signal for rapid export of proteins from the nucleus

Active nuclear import of protein is controlled by nuclear localization signals (NLSs), but nuclear export is not understood well. Nuclear trafficking of the catalytic (C) subunit of cAMP-dependent protein kinase (cAPK) is critical for regulation of gene expression. The heat-stable inhibitor (PKl) of cAPK contains a nuclear export signal (NES) that triggers rapid, active net extrusion of the C-PKl complex from the nucleus. This NES (residues 35-49), fused or conjugated to heterologous proteins, was sufficient for rapid nuclear export. Hydrophobic residues were critical. The NES is a slightly weaker signal than the SV40 NLS. A sequence containing only residues 37-46, LALKLAGLDI, is also sufficient for nuclear export. This is an example of a protein-based NES having no obvious association with RNA. A similar sequence, LQLPPLERLTL, from Rev, an RNA-binding protein of HIV-1, also is an NES.

The expression and intracellular distribution of the heat-stable protein kinase inhibitor is cell cycle regulated

The heat-stable protein kinase inhibitor (PKI) is a potent and specific inhibitor of the catalytic (C) subunit of the cAMP-dependent protein kinase. We report the isolation of a polyclonal antibody raised to purified recombinant PKI alpha. Using this antibody, the intracellular distribution of endogenous PKI alpha was assessed by immunostaining. The PKI alpha expression and intracellular distribution varied as a function of cell cycle progression. PKI alpha expression appeared low in serum-starved cells and in cells in G1 and increased as cells progressed through S phase. Its distribution became increasingly nuclear as cells entered G2/M. Nuclear levels of PKI alpha remained high through cell division and decreased again as cells reentered G1. The cell cycle regulated expression and nuclear distribution suggests a specific role for PKI alpha in the nucleus during the G2/M phases of the cell cycle. Consistent with this, microinjection of PKI alpha antibody into serum-starved cells prevented their subsequent cell cycle progression. Similarly, overexpression of C subunit in cells arrested at the G1/S boundary prevented their subsequent division. Together these results support the idea that PKI alpha plays an important role in the inhibition of nuclear C subunit activity required for cell cycle progression, although a determination of the relative amounts of endogenous nuclear PKI and C-subunit will be required to substantiate this hypothesis.

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)

Association of a heat-stable inhibitor protein with cyclic-3',5'-AMP-dependent protein kinase from the nematode Ascaris suum: purification and characterization of the inhibitor

An inhibitor protein of the catalytic subunit of the cyclic 3',5'-AMP-dependent protein kinase from the nematode Ascaris suum was isolated and characterized. The molecular weight of the inhibitor was estimated as 28,000 by electrophoresis under denaturing conditions and as 30,000 by gel permeation chromatography on Superose 12. The Trypsin-labile inhibitor was resistant to short incubations (less than or equal to 5 min) at temperatures up to 95 degrees C and at pH 3. It affected the protein kinase from Ascaris and bovine heart with almost the same affinity, and inhibition was not relieved by the presence of cAMP and cGMP. However, the inhibition was antagonized by low concentrations of heparin. Unlike in mammalian tissues, the concentration of the inhibitor was sufficiently high to exert at least 90% inhibition of the protein kinase activity in Ascaris muscle. Therefore, the inhibitor may play a role in cellular regulation in the nematode.

Optimizing protein folding to the native state in bacteria

A correctly folded protein is usually both active and soluble. This review focuses on novel ways to improve the folding of recombinant proteins during production in bacteria and includes a few tips for refolding proteins. Major results in correlating protein primary structure with proper folding and stability, and the production of viral antigens and antibodies in bacteria are also discussed.