Residues in the second cysteine-rich region of protein kinase C delta relevant to phorbol ester binding as revealed by site-directed mutagenesis

Single-residue mutation in protein kinase C toggles between cancer and neurodegeneration

Conventional protein kinase C (cPKC) isozymes tune the signaling output of cells, with loss-of-function somatic mutations associated with cancer and gain-of-function germline mutations identified in neurodegeneration. PKC with impaired autoinhibition is removed from the cell by quality-control mechanisms to prevent the accumulation of aberrantly active enzyme. Here, we examine how a highly conserved residue in the C1A domain of cPKC isozymes permits quality-control degradation when mutated to histidine in cancer (PKCβ-R42H) and blocks down-regulation when mutated to proline in the neurodegenerative disease spinocerebellar ataxia (PKCγ-R41P). Using FRET-based biosensors, we determined that mutation of R42 to any residue, including lysine, resulted in reduced autoinhibition as indicated by higher basal activity and faster agonist-induced plasma membrane translocation. R42 is predicted to form a stabilizing salt bridge with E655 in the C-tail and mutation of E655, but not neighboring E657, also reduced autoinhibition. Western blot analysis revealed that whereas R42H had reduced stability, the R42P mutant was stable and insensitive to activator-induced ubiquitination and down-regulation, an effect previously observed by deletion of the entire C1A domain. Molecular dynamics (MD) simulations and analysis of stable regions of the domain using local spatial pattern (LSP) alignment suggested that P42 interacts with Q66 to impair mobility and conformation of one of the ligand-binding loops. Additional mutation of Q66 to the smaller asparagine (R42P/Q66N), to remove conformational constraints, restored degradation sensitivity. Our results unveil how disease-associated mutations of the same residue in the C1A domain can toggle between gain- or loss-of-function of PKC.

Single-residue mutation in protein kinase C toggles between cancer and neurodegeneration

Conventional protein kinase C (PKC) isozymes tune the signaling output of cells, with loss-of-function somatic mutations associated with cancer and gain-of-function germline mutations identified in neurodegeneration. PKC with impaired autoinhibition is removed from the cell by quality-control mechanisms to prevent accumulation of aberrantly active enzyme. Here, we examine how a single residue in the C1A domain of PKCβ, arginine 42 (R42), permits quality-control degradation when mutated to histidine in cancer (R42H) and blocks downregulation when mutated to proline in the neurodegenerative disease spinocerebellar ataxia (R42P). Using FRET-based biosensors, we determined that mutation of R42 to any residue, including lysine, resulted in reduced autoinhibition as indicated by higher basal activity and faster agonist-induced plasma membrane translocation. R42 is predicted to form a stabilizing salt bridge with E655 in the C-tail and mutation of E655, but not neighboring E657, also reduced autoinhibition. Western blot analysis revealed that whereas R42H had reduced stability, the R42P mutant was stable and insensitive to activator-induced ubiquitination and downregulation, an effect previously observed by deletion of the entire C1A domain. Molecular dynamics (MD) simulations and analysis of stable regions of the domain using local spatial pattern (LSP) alignment suggested that P42 interacts with Q66 to impair mobility and conformation of one of the ligand-binding loops. Additional mutation of Q66 to the smaller asparagine (R42P/Q66N), to remove conformational constraints, restored degradation sensitivity to that of WT. Our results unveil how disease-associated mutations of the same residue in the C1A domain can toggle between gain- or loss-of-function of PKC.

Insertion Depth Modulates Protein Kinase C-δ-C1b Domain Interactions with Membrane Cholesterol as Revealed by MD Simulations

Protein kinase C delta (PKC-δ) is an important signaling molecule in human cells that has both proapoptotic as well as antiapoptotic functions. These conflicting activities can be modulated by two classes of ligands, phorbol esters and bryostatins. Phorbol esters are known tumor promoters, while bryostatins have anti-cancer properties. This is despite both ligands binding to the C1b domain of PKC-δ (δC1b) with a similar affinity. The molecular mechanism behind this discrepancy in cellular effects remains unknown. Here, we have used molecular dynamics simulations to investigate the structure and intermolecular interactions of these ligands bound to δC1b with heterogeneous membranes. We observed clear interactions between the δC1b-phorbol complex and membrane cholesterol, primarily through the backbone amide of L250 and through the K256 side-chain amine. In contrast, the δC1b-bryostatin complex did not exhibit interactions with cholesterol. Topological maps of the membrane insertion depth of the δC1b-ligand complexes suggest that insertion depth can modulate δC1b interactions with cholesterol. The lack of cholesterol interactions suggests that bryostatin-bound δC1b may not readily translocate to cholesterol-rich domains within the plasma membrane, which could significantly alter the substrate specificity of PKC-δ compared to δC1b-phorbol complexes.

Comparison of the ligand binding site of C1 domains: a molecular dynamics simulation study of the C1 domain-phorbol 13-acetate-membrane system

C1 domains are lipid-binding structural units of about 50 residues. Typical C1 domains associate with the plasma membrane and bind to diacylglycerol/phorbol ester during the activation of the proteins containing these domains. Although the overall structure of the C1 domains are similar, there are differences in their primary sequence and in the orientation of the ligand/lipid binding residues. To gain structural insights into the ligand/lipid binding, we performed molecular docking of phorbol 13-acetate into the C1 domain and 1.0 μs molecular dynamics simulation on the C1 domain-ligand-lipid ternary system for PKCθ C1A, PKCδ C1B, PKCβII C1B, PKCθ C1B, Munc13-1 C1, and βII-Chimaerin C1. We divided these C1 domains into three types based on the orientations of Gln-27 and Trp/Tyr-22. In type 1, Trp/Tyr-22 is outside and Gln-27 is inside the ligand binding pocket. In type 2, both Trp/Tyr-22 and Gln-27 are outside the ligand binding pocket, and in type 3, Trp/Tyr-22 is inside and Gln-27 is outside the pocket. The type 1 C1 domains showed higher ligand binding and higher membrane binding with a shorter distance between the C1 domain and the membrane than the type 2 and type 3. For ligand binding, Pro-11 plays a major role in the type 1 and 2, and Gly-23 in the type 1 and type 3 C1 domains. This study elucidates the role of Gln-27, Trp-22, Pro-11 and Gly-23 in ligand/lipid binding in typical C1 domains and bears significance in developing selective modulators of C1 domain-containing proteins.Communicated by Ramaswamy H. Sarma.

An intact zinc finger motif of the C1B domain is critical for stability and activity of diacylglycerol kinase-ε

Diacylglycerol kinase-ε (DGKε) phosphorylates DAG to phosphatidic acid with unique specificity toward 18:0/20:4 DAG (SAG). SAG is a typical backbone of phosphatidylinositol and its derivatives, therefore DGKε activity is crucial for the turnover of these signaling lipids. Malfunction of DGKε contributes to several pathophysiological conditions, including atypical hemolytic uremic syndrome (aHUS) linked with DGKE mutations. In the present study we analyzed the role of a zinc finger motif of the C1B domain of DGKε, as some aHUS-linked mutations affect this ill-defined part of the kinase. For this, we introduce a novel fluorescent assay for determination of DGKε activity which relies on the use of NBD-SAG in mixed micelles as a substrate, followed by TLC separation of NBD-phosphatidic acid formed. The assay reliably determines the activity of purified human GST-DGKε, also endogenous DGKε or overexpressed mouse DGKε-Myc in cell lysates, homogenates, and kinase immunoprecipitates. Using the above assay we found that four amino acids, Cys135, Cys138, His161 and Cys164, forming the zinc finger motif in the C1B domain are required for the DGKε-Myc activity and stability. Substitution of any of these amino acids with Ala or Trp in DGKε-Myc abolished its activity and led to its proteasomal degradation, possibly assisted by Hsp70/90/40 chaperones. Inhibition of the 26S proteasome prevented the degradation but the mutated proteins were inactive. The present data on the deleterious effect of the zinc finger motif disruption contribute to the understanding of the DGKε-linked aHUS, as the Cys164Trp substitution in mouse DGKε corresponds to the Cys167Trp one in human DGKε found in some aHUS patients.

Structural anatomy of Protein Kinase C C1 domain interactions with diacylglycerol and other agonists

Diacylglycerol (DAG) is a versatile lipid whose 1,2-sn-stereoisomer serves both as second messenger in signal transduction pathways that control vital cellular processes, and as metabolic precursor for downstream signaling lipids such as phosphatidic acid. Effector proteins translocate to available DAG pools in the membranes by using conserved homology 1 (C1) domains as DAG-sensing modules. Yet, how C1 domains recognize and capture DAG in the complex environment of a biological membrane has remained unresolved for the 40 years since the discovery of Protein Kinase C (PKC) as the first member of the DAG effector cohort. Herein, we report the high-resolution crystal structures of a C1 domain (C1B from PKCδ) complexed to DAG and to each of four potent PKC agonists that produce different biological readouts and that command intense therapeutic interest. This structural information details the mechanisms of stereospecific recognition of DAG by the C1 domains, the functional properties of the lipid-binding site, and the identities of the key residues required for the recognition and capture of DAG and exogenous agonists. Moreover, the structures of the five C1 domain complexes provide the high-resolution guides for the design of agents that modulate the activities of DAG effector proteins.

Impaired respiratory burst contributes to infections in PKCδ-deficient patients

Patients with autosomal recessive protein kinase C δ (PKCδ) deficiency suffer from childhood-onset autoimmunity, including systemic lupus erythematosus. They also suffer from recurrent infections that overlap with those seen in patients with chronic granulomatous disease (CGD), a disease caused by defects of the phagocyte NADPH oxidase and a lack of reactive oxygen species (ROS) production. We studied an international cohort of 17 PKCδ-deficient patients and found that their EBV-B cells and monocyte-derived phagocytes produced only small amounts of ROS and did not phosphorylate p40phox normally after PMA or opsonized Staphylococcus aureus stimulation. Moreover, the patients' circulating phagocytes displayed abnormally low levels of ROS production and markedly reduced neutrophil extracellular trap formation, altogether suggesting a role for PKCδ in activation of the NADPH oxidase complex. Our findings thus show that patients with PKCδ deficiency have impaired NADPH oxidase activity in various myeloid subsets, which may contribute to their CGD-like infectious phenotype.

Probing the Diacylglycerol Binding Site of Presynaptic Munc13-1

Munc13-1 is a presynaptic active zone protein that acts as a master regulator of synaptic vesicle priming and neurotransmitter release in the brain. It has been implicated in the pathophysiology of several neurodegenerative diseases. Diacylglycerol and phorbol ester activate Munc13-1 by binding to its C1 domain. The objective of this study is to identify the structural determinants of ligand binding activity of the Munc13-1 C1 domain. Molecular docking suggested that residues Trp-588, Ile-590, and Arg-592 of Munc13-1 are involved in ligand interactions. To elucidate the role of these three residues in ligand binding, we generated W588A, I590A, and R592A mutants in full-length Munc13-1, expressed them as GFP-tagged proteins in HT22 cells, and measured their ligand-induced membrane translocation by confocal microscopy and immunoblotting. The extent of 1,2-dioctanoyl-sn-glycerol (DOG)- and phorbol ester-induced membrane translocation decreased in the following order: wild type > I590A > W588A > R592A and wild type > W588A > I590A > R592A, respectively. To understand the effect of the mutations on ligand binding, we also measured the DOG binding affinity of the isolated wild-type C1 domain and its mutants in membrane-mimicking micelles using nuclear magnetic resonance methods. The DOG binding affinity decreased in the following order: wild type > I590A > R592A. No binding was detected for W588A with DOG in micelles. This study shows that Trp-588, Ile-590, and Arg-592 are essential determinants for the activity of Munc13-1 and the effects of the three residues on the activity are ligand-dependent. This study bears significance for the development of selective modulators of Munc13-1.

Structural insights into C1-ligand interactions: Filling the gaps by in silico methods

Protein Kinase C isoenzymes (PKCs) are the key mediators of the phosphoinositide signaling pathway, which involves regulated hydrolysis of phosphatidylinositol (4,5)-bisphosphate to diacylglycerol (DAG) and inositol-1,4,5-trisphosphate. Dysregulation of PKCs is implicated in many human diseases making this class of enzymes an important therapeutic target. Specifically, the DAG-sensing cysteine-rich conserved homology-1 (C1) domains of PKCs have emerged as promising targets for pharmaceutical modulation. Despite significant progress, the rational design of the C1 modulators remains challenging due to difficulties associated with structure determination of the C1-ligand complexes. Given the dearth of experimental structural data, computationally derived models have been instrumental in providing atomistic insight into the interactions of the C1 domains with PKC agonists. In this review, we provide an overview of the in silico approaches for seven classes of C1 modulators and outline promising future directions.

Transcriptome sequencing and whole genome expression profiling of hexaploid sweetpotato under salt stress

BACKGROUND

Purple-fleshed sweetpotato (PFSP) is one of the most important crops in the word which helps to bridge the food gap and contribute to solve the malnutrition problem especially in developing countries. Salt stress is seriously limiting its production and distribution. Due to lacking of reference genome, transcriptome sequencing is offering a rapid approach for crop improvement with promising agronomic traits and stress adaptability.

RESULTS

Five cDNA libraries were prepared from the third true leaf of hexaploid sweetpotato at seedlings stage (Xuzi-8 cultivar) treated with 200 mM NaCl for 0, 1, 6, 12, 48 h. Using second and third generation technology, Illumina sequencing generated 170,344,392 clean high-quality long reads that were assembled into 15,998 unigenes with an average length 2178 base pair and 96.55% of these unigenes were functionally annotated in the NR protein database. A number of 537 unigenes failed to hit any homologs which may be considered as novel genes. The current results indicated that sweetpotato plants behavior during the first hour of salt stress was different than the other three time points. Furthermore, expression profiling analysis identified 4, 479, 281, 508 significantly expressed unigenes in salt stress treated samples at the different time points including 1, 6, 12, 48 h, respectively as compared to control. In addition, there were 4, 1202, 764 and 2195 transcription factors differentially regulated DEGs by salt stress at different time points including 1, 6, 12, 48 h of salt stress. Validation experiment was done using 6 randomly selected unigenes and the results was in agree with the DEG results. Protein kinases include many genes which were found to play a vital role in phosphorylation process and act as a signal transductor/ receptor proteins in membranes. These findings suggest that salt stress tolerance in hexaploid sweetpotato plants may be mainly affected by TFs, PKs, Protein Detox and hormones related genes which contribute to enhance salt tolerance.

CONCLUSION

These transcriptome sequencing data of hexaploid sweetpotato under salt stress conditions can provide a valuable resource for sweetpotato breeding research and focus on novel insights into hexaploid sweetpotato responses to salt stress. In addition, it offers new candidate genes or markers that can be used as a guide to the future studies attempting to breed salt tolerance sweetpotato cultivars.

It Takes Two to Tango: Activation of Protein Kinase D by Dimerization

The recent discovery and structure determination of a novel ubiquitin-like dimerization domain in protein kinase D (PKD) has significant implications for its activation. PKD is a serine/threonine kinase activated by the lipid second messenger diacylglycerol (DAG). It is an essential and highly conserved protein that is implicated in plasma membrane directed trafficking processes from the trans-Golgi network. However, many open questions surround its mechanism of activation, its localization, and its role in the biogenesis of cargo transport carriers. In reviewing this field, the focus is primarily on the mechanisms that control the activation of PKD at precise locations in the cell. In light of the new structural findings, the understanding of the mechanisms underlying PKD activation is critically evaluated, with particular emphasis on the role of dimerization in PKD autophosphorylation, and the provenance and recognition of the DAG that activates PKD.

Identification of a truncated β1-chimaerin variant that inactivates nuclear Rac1

β1-chimaerin belongs to the chimaerin family of GTPase-activating proteins (GAPs) and is encoded by the CHN2 gene, which also encodes the β2- and β3-chimaerin isoforms. All chimaerin isoforms have a C1 domain that binds diacylglycerol as well as tumor-promoting phorbol esters and a catalytic GAP domain that inactivates the small GTPase Rac. Nuclear Rac has emerged as a key regulator of various cell functions, including cell division, and has a pathological role by promoting tumorigenesis and metastasis. However, how nuclear Rac is regulated has not been fully addressed. Here, using several approaches, including siRNA-mediated gene silencing, confocal microscopy, and subcellular fractionation, we identified a nuclear variant of β1-chimaerin, β1-Δ7p-chimaerin, that participates in the regulation of nuclear Rac1. We show that β1-Δ7p-chimaerin is a truncated variant generated by alternative splicing at a cryptic splice site in exon 7. We found that, unlike other chimaerin isoforms, β1-Δ7p-chimaerin lacks a functional C1 domain and is not regulated by diacylglycerol. We found that β1-Δ7p-chimaerin localizes to the nucleus via a nuclear localization signal in its N terminus. We also identified a key nuclear export signal in β1-chimaerin that is absent in β1-Δ7p-chimaerin, causing nuclear retention of this truncated variant. Functionally analyses revealed that β1-Δ7p-chimaerin inactivates nuclear Rac and negatively regulates the cell cycle. Our results provide important insights into the diversity of chimaerin Rac-GAP regulation and function and highlight a potential mechanism of nuclear Rac inactivation that may play significant roles in pathologies such as cancer.

Redox regulation of protein kinase C by selenometabolites and selenoprotein thioredoxin reductase limits cancer prevention by selenium

The cancer-preventive mechanism of selenium should address the way low concentrations of selenometabolites react with cellular targets without being diffused from the sites of generation, the way selenium selectively kills tumor cells, and the intriguing U-shaped curve that is seen with dietary supplementation of selenium and cancer prevention. Protein kinase C (PKC), a receptor for tumor promoters, is well suited for this mechanism. Due to the catalytic redox cycle, low concentrations of methylselenol, a postulated active metabolite of selenium, react with the tumor-promoting lipid hydroperoxide bound to PKC to form methylseleninic acid (MSA), which selectively reacts with thiol residues present within the vicinity of the PKC catalytic domain to inactivate it. Given that lipid hydroperoxide levels are high in promoting cells, PKC inactivation selectively leads to death in these cells. A biphasic effect of MSA in inducing cell death was observed in certain prostate cancer cell lines; lower concentrations of MSA induced cell death, while higher concentrations failed to do so. Lower concentrations of selenium inactivate more sensitive antiapoptotic isoenzymes of PKC (ε and α), sparing less sensitive proapoptotic isoenzymes (PKCδ and PKCζ). Higher concentrations of selenium also inactivate proapoptotic isoenzymes and consequently make tumor cells resistant to apoptosis. Due to a high-affinity binding of thioredoxin to the PKC catalytic domain, this thiol oxidation is explicitly reversed by thioredoxin reductase (TXNRD), a selenoprotein. Therefore, overexpression of TXNRD in advanced tumor cells could make them resistant to selenium-induced death. Conceivably, this mechanism, at least in part, explains why selenium prevents cancer only in certain cases.

Imbalance in Protein Thiol Redox Regulation and Cancer-Preventive Efficacy of Selenium

Although several experimental studies showed cancer-preventive efficacy of supplemental dietary selenium, human clinical trials questioned this efficacy. Identifying its molecular targets and mechanism is important in understanding this discrepancy. Methylselenol, the active metabolite of selenium, reacts with lipid hydroperoxides bound to protein kinase C (PKC) and is oxidized to methylseleninic acid (MSA). This locally generated MSA selectively inactivates PKC by oxidizing its critical cysteine sulfhydryls. The peroxidatic redox cycle occurring in this process may explain how extremely low concentrations of selenium catalytically modify specific membrane-bound proteins compartmentally separated from glutathione and selectively induce cytotoxicity in promoting cells. Mammalian thioredoxin reductase (TR) is itself a selenoenzyme with a catalytic selenocysteine residue. Together with thioredoxin (Trx), it catalyzes reduction of selenite and selenocystine by NADPH generating selenide which in the presence of oxygen redox cycles producing reactive oxygen species. Trx binds with high affinity to PKC and reverses PKC inactivation. Therefore, established tumor cells overexpressing TR and Trx may escape the cancer-preventive actions of selenium. This suggests that in some cases, certain selenoproteins may counteract selenometabolite actions. Lower concentrations of selenium readily inactivate antiapoptotic PKC isoenzymes e and a which have a cluster of vicinal thiols, thereby inducing apoptosis. Higher concentrations of selenium also inactivate proapoptotic enzymes such as proteolytically activated PKCd fragment, holo-PKCz, caspase-3, and c-Jun N-terminal kinase, which all have a limited number of critical cysteine residues and make tumor cells resistant to selenium-induced apoptosis. This may explain the intriguing U-shaped curve that is seen with dietary selenium intake and the extent of cancer prevention.

Structural Basis for the Failure of the C1 Domain of Ras Guanine Nucleotide Releasing Protein 2 (RasGRP2) to Bind Phorbol Ester with High Affinity

The C1 domain represents the recognition module for diacylglycerol and phorbol esters in protein kinase C, Ras guanine nucleotide releasing protein (RasGRP), and related proteins. RasGRP2 is exceptional in that its C1 domain has very weak binding affinity (Kd = 2890 ± 240 nm for [(3)H]phorbol 12,13-dibutyrate. We have identified four amino acid residues responsible for this lack of sensitivity. Replacing Asn(7), Ser(8), Ala(19), and Ile(21) with the corresponding residues from RasGRP1/3 (Thr(7), Tyr(8), Gly(19), and Leu(21), respectively) conferred potent binding affinity (Kd = 1.47 ± 0.03 nm) in vitro and membrane translocation in response to phorbol 12-myristate 13-acetate in LNCaP cells. Mutant C1 domains incorporating one to three of the four residues showed intermediate behavior with S8Y making the greatest contribution. Binding activity for diacylglycerol was restored in parallel. The requirement for anionic phospholipid for [(3)H]phorbol 12,13-dibutyrate binding was determined; it decreased in going from the single S8Y mutant to the quadruple mutant. The full-length RasGRP2 protein with the mutated C1 domains also showed strong phorbol ester binding, albeit modestly weaker than that of the C1 domain alone (Kd = 8.2 ± 1.1 nm for the full-length protein containing all four mutations), and displayed translocation in response to phorbol ester. RasGRP2 is a guanyl exchange factor for Rap1. Consistent with the ability of phorbol ester to induce translocation of the full-length RasGRP2 with the mutated C1 domain, phorbol ester enhanced the ability of the mutated RasGRP2 to activate Rap1. Modeling confirmed that the four mutations helped the binding cleft maintain a stable conformation.

Vitamin A as PKC Co-factor and Regulator of Mitochondrial Energetics

For the past century, vitamin A has been considered to serve as a precursor for retinoids that facilitate vision or as a precursor for retinoic acid (RA), a signaling molecule that modulates gene expression. However, vitamin A circulates in plasma at levels that far exceed the amount needed for vision or the synthesis of nanomolar levels of RA, and this suggests that vitamin A alcohol (i.e. retinol) may possess additional biological activity. We have pursued this question for the last 20 years, and in this chapter, we unfold the story of our quest and the data that support a novel and distinct role for vitamin A (alcohol) action. Our current model supports direct binding of vitamin A to the activation domains of serine/threonine kinases, such as protein kinase C (PKC) and Raf isoforms, where it is involved in redox activation of these proteins. Redox activation of PKCs was first described by the founders of the PKC field, but several hurdles needed to be overcome before a detailed understanding of the biochemistry could be provided. Two discoveries moved the field forward. First, was the discovery that the PKCδ isoform was activated by cytochrome c, a protein with oxidoreduction activity in mitochondria. Second, was the revelation that both PKCδ and cytochrome c are tethered to p66Shc, an adapter protein that brings the PKC zinc-finger substrate into close proximity with its oxidizing partner. Detailed characterization of the PKCδ signalosome complex was made possible by the work of many investigators. Our contribution was determining that vitamin A is a vital co-factor required to support an unprecedented redox-activation mechanism. This unique function of vitamin A is the first example of a general system that connects the one-electron redox chemistry of a heme protein (cytochrome c) with the two-electron chemistry of a classical phosphoprotein (PKCδ). Furthermore, contributions to the regulation of mitochondrial energetics attest to biological significance of vitamin A alcohol action.

Novel N-pyrimidin-4-yl-3-amino-pyrrolo [3, 4-C] pyrazole derivatives as PKC kinase inhibitors: a patent evaluation of US2015099743 (A1)

INTRODUCTION

Protein kinase Cβ (PKCβ) is a member of the PKC family of serine/threonine kinases that has been implicated in the pathophysiology of diabetic complications. Developing small molecule drugs targeting this PKC isozyme is a rational approach for treating these disease states. PKCβ belongs to the conventional class of PKC and contains both regulatory and kinase domain. Numerous compounds of different chemical classes were designed targeting the kinase domain, but achieved very limited success in clinical trials.

AREAS COVERED

This patent application reports the synthesis of about 100 new N-pyrimidin-4-yl-3-amino-pyrolo [3, 4-C] pyrazole derivatives and their competitive inhibition constant (Ki) for protein kinase C βII (PKCβII), one of the two splice variants of PKCβ. The compounds compete with ATP at the kinase domain of PKCβII, and inhibit with high potency having Ki values in the 0.1-181 nM range. The compounds are claimed to be selective towards PKCβI, PKCβII and PKCα over other protein kinases. Several routes of administration of these compounds are discussed for possible treatment of diabetes and related diseases.

EXPERT OPINION

This is an important effort toward developing PKC-based drugs for diabetic complications. Further biological evaluations of these compounds are required before proceeding toward clinical trails.

Structural insights into the interactions of phorbol ester and bryostatin complexed with protein kinase C: a comparative molecular dynamics simulation study

Protein kinase C (PKC) isozymes are important regulatory enzymes that have been implicated in many diseases, including cancer, Alzheimer's disease, and in the eradication of HIV/AIDS. Given their potential clinical ramifications, PKC modulators, e.g. phorbol esters and bryostatin, are also of great interest in the drug development. However, structural details on the binding between PKC and its modulators, especially bryostatin - the highly potent and non-tumor promoting activator for PKCs, are still lacking. Here, we report the first comparative molecular dynamics study aimed at gaining structural insight into the mechanisms by which the PKC delta cys2 activator domain is used in its binding to phorbol ester and bryostatin-1. As anticipated in the phorbol ester binding, hydrogen bonds are formed through the backbone atoms of Thr242, Leu251, and Gly253 of PKC. However, the opposition of H-bond formation between Thr242 and Gly253 may cause the phorbol ester complex to become less stable when compared with the bryostatin binding. For the PKC delta-bryostatin complex, hydrogen bonds are formed between the Gly253 backbone carbonyl and the C30 carbomethoxy substituent of the ligand. Additionally, the indole Nε1 of the highly homologous Trp252 also forms an H-bond to the C20 ester group on bryostatin. Backbone fluctuations also suggest that this latter H-bond formation may abrogate the transient interaction between Trp252 and His269, thus dampening the fluctuations observed on the nearby Zn(2+)-coordinating residues. This new dynamic fluctuation dampening model can potentially benefit future design of new PKC modulators.

Alcohol binding in the C1 (C1A+C1B) domain of protein kinase C epsilon

BACKGROUND

Alcohol regulates the expression and function of protein kinase C epsilon (PKCε). In a previous study we identified an alcohol binding site in the C1B, one of the twin C1 subdomains of PKCε (Das et al., Biochem. J., 421, 405-13, 2009).

METHODS

In this study, we investigated alcohol binding in the entire C1 domain (combined C1A and C1B) of PKCε. Fluorescent phorbol ester, SAPD and fluorescent diacylglycerol (DAG) analog, dansyl-DAG were used to study the effect of ethanol, butanol, and octanol on the ligand binding using fluorescence resonance energy transfer (FRET). To identify alcohol binding site(s), PKCεC1 was photolabeled with 3-azibutanol and 3-azioctanol, and analyzed by mass spectrometry. The effects of alcohols and the azialcohols on PKCε were studied in NG108-15 cells.

RESULTS

In the presence of alcohol, SAPD and dansyl-DAG showed different extent of FRET, indicating differential effects of alcohol on the C1A and C1B subdomains. Effects of alcohols and azialcohols on PKCε in NG108-15 cells were comparable. Azialcohols labeled Tyr-176 of C1A and Tyr-250 of C1B. Inspection of the model structure of PKCεC1 reveals that these residues are 40Å apart from each other indicating that these residues form two different alcohol binding sites.

CONCLUSIONS

The present results provide evidence for the presence of multiple alcohol-binding sites on PKCε and underscore the importance of targeting this PKC isoform in developing alcohol antagonists.

The many hats of protein kinase Cδ: one enzyme with many functions

A large number of protein substrates are phosphorylated by each protein kinase under physiological and pathological conditions. However, it remains a challenge to determine which of these phosphorylated substrates of a given kinase is critical for each cellular response. Genetics enabled the generation of separation-of-function mutations that selectively cause a loss of one molecular event without affecting others, thus providing some tools to assess the importance of that one event for the measured physiological response. However, the genetic approach is laborious and not adaptable to all systems. Furthermore, pharmacological tools of the catalytic site are not optimal due to their non-selective nature. In the present brief review, we discuss some of the challenges in drug development that will regulate the multifunctional protein kinase Cδ (PKCδ).

Toward a biorelevant structure of protein kinase C bound modulators: design, synthesis, and evaluation of labeled bryostatin analogues for analysis with rotational echo double resonance NMR spectroscopy

Protein kinase C (PKC) modulators are currently of great importance in preclinical and clinical studies directed at cancer, immunotherapy, HIV eradication, and Alzheimer's disease. However, the bound conformation of PKC modulators in a membrane environment is not known. Rotational echo double resonance (REDOR) NMR spectroscopy could uniquely address this challenge. However, REDOR NMR requires strategically labeled, high affinity ligands to determine interlabel distances from which the conformation of the bound ligand in the PKC-ligand complex could be identified. Here we report the first computer-guided design and syntheses of three bryostatin analogues strategically labeled for REDOR NMR analysis. Extensive computer analyses of energetically accessible analogue conformations suggested preferred labeling sites for the identification of the PKC-bound conformers. Significantly, three labeled analogues were synthesized, and, as required for REDOR analysis, all proved highly potent with PKC affinities (∼1 nM) on par with bryostatin. These potent and strategically labeled bryostatin analogues are new structural leads and provide the necessary starting point for projected efforts to determine the PKC-bound conformation of such analogues in a membrane environment, as needed to design new PKC modulators and understand PKC-ligand-membrane structure and dynamics.

Identification and characterization of PKCγ, a kinase associated with SCA14, as an amyloidogenic protein

Amyloid assemblies are associated with a wide range of human disorders, including Alzheimer's and Parkinson's diseases. Here, we identify protein kinase C (PKC) γ, a serine/threonine kinase mutated in the neurodegenerative disease spinocerebellar ataxia type 14 (SCA14), as a novel amyloidogenic protein with no previously characterized amyloid-prone domains. We found that overexpression of PKCγ in cultured cells, as well as in vitro incubation of PKCγ without heat or chemical denaturants, causes amyloid-like fibril formation of this protein. We also observed that SCA14-associated mutations in PKCγ accelerate the amyloid-like fibril formation both in cultured cells and in vitro. We show that the C1A and kinase domains of PKCγ are involved in its soluble dimer and aggregate formation and that SCA14-associated mutations in the C1 domain cause its misfolding and aggregation. Furthermore, long-term time-lapse imaging indicates that aggregates of mutant PKCγ are highly toxic to neuronal cells. Based on these findings, we propose that PKCγ could form amyloid-like fibrils in physiological and/or pathophysiological conditions such as SCA14. More generally, our results provide novel insights into the mechanism of amyloid-like fibril formation by multi-domain proteins.

Interfacial partitioning of a loop hinge residue contributes to diacylglycerol affinity of conserved region 1 domains

Conventional and novel isoenzymes of PKC are activated by the membrane-embedded second messenger diacylglycerol (DAG) through its interactions with the C1 regulatory domain. The affinity of C1 domains to DAG varies considerably among PKCs. To gain insight into the origin of differential DAG affinities, we conducted high-resolution NMR studies of C1B domain from PKCδ (C1Bδ) and its W252Y variant. The W252Y mutation was previously shown to render C1Bδ less responsive to DAG (Dries, D. R., Gallegos, L. L., and Newton, A. C. (2007) A single residue in the C1 domain sensitizes novel protein kinase C isoforms to cellular diacylglycerol production. J. Biol. Chem. 282, 826-830) and thereby emulate the behavior of C1B domains from conventional PKCs that have a conserved Tyr at the equivalent position. Our data revealed that W252Y mutation did not perturb the conformation of C1Bδ in solution but significantly reduced its propensity to partition into a membrane-mimicking environment in the absence of DAG. Using detergent micelles doped with a paramagnetic lipid, we determined that both the residue identity at position 252 and complexation with diacylglycerol influence the geometry of C1Bδ-micelle interactions. In addition, we identified the C-terminal helix α1 of C1Bδ as an interaction site with the head groups of phosphatidylserine, a known activator of PKCδ. Taken together, our studies (i) reveal the identities of C1Bδ residues involved in interactions with membrane-mimicking environment, DAG, and phosphatidylserine, as well as the affinities associated with each event and (ii) suggest that the initial ligand-independent membrane recruitment of C1B domains, which is greatly facilitated by the interfacial partitioning of Trp-252, is responsible, at least in part, for the differential DAG affinities.

Charge density influences C1 domain ligand affinity and membrane interactions

The C1 domain, which represents the recognition motif on protein kinase C for the lipophilic second messenger diacylglycerol and its ultrapotent analogues, the phorbol esters, has emerged as a promising therapeutic target for cancer and other indications. Potential target selectivity is markedly enhanced both because binding reflects ternary complex formation between the ligand, C1 domain, and phospholipid, and because binding drives membrane insertion of the C1 domain, permitting aspects of the C1 domain surface outside the binding site, per se, to influence binding energetics. Here, focusing on charged residues identified in atypical C1 domains which contribute to their loss of ligand binding activity, we showed that increasing charge along the rim of the binding cleft of the protein kinase C δ C1 b domain raises the requirement for anionic phospholipids. Correspondingly, it shifts the selectivity of C1 domain translocation to the plasma membrane, which is more negatively charged than internal membranes. This change in localization is most pronounced in the case of more hydrophilic ligands, which provide weaker membrane stabilization than do the more hydrophobic ligands and thus contributes an element to the structure-activity relations for C1 domain ligands. Coexpressing pairs of C1-containing constructs with differing charges each expressing a distinct fluorescent tag provided a powerful tool to demonstrate the effect of increasing charge in the C1 domain.

The molecular basis for the pharmacokinetics and pharmacodynamics of curcumin and its metabolites in relation to cancer

This review addresses the oncopharmacological properties of curcumin at the molecular level. First, the interactions between curcumin and its molecular targets are addressed on the basis of curcumin's distinct chemical properties, which include H-bond donating and accepting capacity of the β-dicarbonyl moiety and the phenylic hydroxyl groups, H-bond accepting capacity of the methoxy ethers, multivalent metal and nonmetal cation binding properties, high partition coefficient, rotamerization around multiple C-C bonds, and the ability to act as a Michael acceptor. Next, the in vitro chemical stability of curcumin is elaborated in the context of its susceptibility to photochemical and chemical modification and degradation (e.g., alkaline hydrolysis). Specific modification and degradatory pathways are provided, which mainly entail radical-based intermediates, and the in vitro catabolites are identified. The implications of curcumin's (photo)chemical instability are addressed in light of pharmaceutical curcumin preparations, the use of curcumin analogues, and implementation of nanoparticulate drug delivery systems. Furthermore, the pharmacokinetics of curcumin and its most important degradation products are detailed in light of curcumin's poor bioavailability. Particular emphasis is placed on xenobiotic phase I and II metabolism as well as excretion of curcumin in the intestines (first pass), the liver (second pass), and other organs in addition to the pharmacokinetics of curcumin metabolites and their systemic clearance. Lastly, a summary is provided of the clinical pharmacodynamics of curcumin followed by a detailed account of curcumin's direct molecular targets, whereby the phenotypical/biological changes induced in cancer cells upon completion of the curcumin-triggered signaling cascade(s) are addressed in the framework of the hallmarks of cancer. The direct molecular targets include the ErbB family of receptors, protein kinase C, enzymes involved in prostaglandin synthesis, vitamin D receptor, and DNA.

Identification of the activator-binding residues in the second cysteine-rich regulatory domain of protein kinase Cθ (PKCθ)

PKC (protein kinase C) θ is predominantly expressed in T-cells and is critically involved in immunity. Design of PKCθ-selective molecules to manage autoimmune disorders by targeting its activator-binding C1 domain requires the knowledge of its structure and the activator-binding residues. The C1 domain consists of twin C1 domains, C1A and C1B, of which C1B plays a critical role in the membrane translocation and activation of PKCθ. In the present study we determined the crystal structure of PKCθC1B to 1.63 Å (1 Å=0.1 nm) resolution, which showed that Trp(253) at the rim of the activator-binding pocket was orientated towards the membrane, whereas in PKCδC1B the homologous tryptophan residue was orientated away from the membrane. This particular orientation of Trp(253) affects the size of the activator-binding pocket and the membrane affinity. To further probe the structural constraints on activator-binding, five residues lining the activator-binding site were mutated (Y239A, T243A, W253G, L255G and Q258G) and the binding affinities of the PKCθC1B mutants were measured. These mutants showed reduced binding affinities for phorbol ester [PDBu (phorbol 12,13-dibutyrate)] and diacylglycerol [DOG (sn-1,2-dioctanoylglycerol), SAG (sn-1-stearoyl 2-arachidonyl glycerol)]. All five full-length PKCθ mutants exhibited reduced phorbol-ester-induced membrane translocation compared with the wild-type. These results provide insights into the PKCθ activator-binding domain, which will aid in future design of PKCθ-selective molecules.

Methods for studying oxidative regulation of protein kinase C

The protein kinase C (PKC) family of isoenzymes may be a crucial player in transducing H2O2-induced signaling in a wide variety of physiological and pathophysiological processes. PKCs contain unique structural features that make them highly susceptible to oxidative modification. Depending on the site of oxidation and the extent to which it is modified, PKC can be either activated or inactivated by H2O2. The N-terminal regulatory domain contains zinc-binding, cysteine-rich motifs that are readily oxidized by H2O2. When oxidized, the autoinhibitory function of the regulatory domain is compromised, and as a result, PKC is activated in a lipid cofactor-independent manner. The C-terminal catalytic domain contains several reactive cysteine residues, which when oxidized with a higher concentration of H2O2 leads to an inactivation of PKC. Here, we describe the methods used to induce oxidative modification of purified PKC isoenzymes by H2O2 and the methods to assess the extent of this modification. Protocols are given for isolating oxidatively activated PKC isoenzymes from cells treated with H2O2. Furthermore, we describe the methods used to assess indirect regulation of PKC isoenzymes by determining their cytosol to membrane or mitochondrial translocation and tyrosine phosphorylation of PKCδ in response to sublethal levels of H2O2. Finally, as an example, we describe the methods used to demonstrate the role of H2O2-mediated cell signaling of PKCɛ in green tea polyphenol-induced preconditioning against neuronal cell death caused by oxygen-glucose deprivation and reoxygenation, an in vitro model for cerebral ischemic/reperfusion injury.

Reactive cysteine in the structural Zn(2+) site of the C1B domain from PKCα

Structural cysteine-rich Zn(2+) sites that stabilize protein folds are considered to be unreactive. In this article, we identified a reactive cysteine residue, Cys151, in a treble-clef zinc finger with a Cys(3)His coordination sphere. The protein in question is the C1B domain of Protein Kinase Cα (PKCα). Mass-tagging cysteine assays of several C1B variants were employed to ascertain the site specificity of the covalent modification. The reactivity of Cys151 in C1B also manifests itself in the structural dynamics of the Zn(2+) coordination sphere where the Sγ of Cys151 alternates between the Zn(2+)-bound thiolate and free thiol states. We used NMR-detected pH titrations, ZZ-exchange spectroscopy, and residual dipolar coupling (RDC)-driven structure refinement to characterize the two exchanging conformations of C1B that differ in zinc coordination. Our data suggest that Cys151 serves as an entry point for the reactive oxygen species that activate PKCα in a process involving Zn(2+) release.

Molecular basis for failure of "atypical" C1 domain of Vav1 to bind diacylglycerol/phorbol ester

C1 domains, the recognition motif of the second messenger diacylglycerol and of the phorbol esters, are classified as typical (ligand-responsive) or atypical (not ligand-responsive). The C1 domain of Vav1, a guanine nucleotide exchange factor, plays a critical role in regulation of Vav activity through stabilization of the Dbl homology domain, which is responsible for exchange activity of Vav. Although the C1 domain of Vav1 is classified as atypical, it retains a binding pocket geometry homologous to that of the typical C1 domains of PKCs. This study clarifies the basis for its failure to bind ligands. Substituting Vav1-specific residues into the C1b domain of PKCδ, we identified five crucial residues (Glu(9), Glu(10), Thr(11), Thr(24), and Tyr(26)) along the rim of the binding cleft that weaken binding potency in a cumulative fashion. Reciprocally, replacing these incompatible residues in the Vav1 C1 domain with the corresponding residues from PKCδ C1b (δC1b) conferred high potency for phorbol ester binding. Computer modeling predicts that these unique residues in Vav1 increase the hydrophilicity of the rim of the binding pocket, impairing membrane association and thereby preventing formation of the ternary C1-ligand-membrane binding complex. The initial design of diacylglycerol-lactones to exploit these Vav1 unique residues showed enhanced selectivity for C1 domains incorporating these residues, suggesting a strategy for the development of ligands targeting Vav1.

S-nitrosylation Inhibits protein kinase C-mediated contraction in mouse aorta

S-nitrosylation is a ubiquitous protein modification in redox-based signaling and forms S-nitrosothiol from nitric oxide (NO) on cysteine residues. Dysregulation of (S)NO signaling (nitrosative stress) leads to impairment of cellular function. Protein kinase C (PKC) is an important signaling protein that plays a role in the regulation of vascular function, and it is not known whether (S)NO affects PKC's role in vascular reactivity. We hypothesized that S-nitrosylation of PKC in vascular smooth muscle would inhibit its contractile activity. Aortic rings from male C57BL/6 mice were treated with auranofin or 1-chloro-2,4-dinitrobenzene (DNCB) as pharmacological tools, which lead to stabilize S-nitrosylation, and propylamine propylamine NONOate (PANOate) or S-nitrosocysteine (CysNO) as NO donors. Contractile responses of aorta to phorbol-12,13-dibutyrate, a PKC activator, were attenuated by auranofin, DNCB, PANOate, and CysNO. S-nitrosylation of PKCα was increased by auranofin or DNCB and CysNO as compared with control protein. Augmented S-nitrosylation inhibited PKCα activity and subsequently downstream signal transduction. These data suggest that PKC is inactivated by S-nitrosylation, and this modification inhibits PKC-dependent contractile responses. Because S-nitrosylation of PKC inhibits phosphorylation and activation of target proteins related to contraction, this posttranslational modification may be a key player in conditions of decreased vascular reactivity.

A systematic screen for protein-lipid interactions in Saccharomyces cerevisiae

Protein-metabolite networks are central to biological systems, but are incompletely understood. Here, we report a screen to catalog protein-lipid interactions in yeast. We used arrays of 56 metabolites to measure lipid-binding fingerprints of 172 proteins, including 91 with predicted lipid-binding domains. We identified 530 protein-lipid associations, the majority of which are novel. To show the data set's biological value, we studied further several novel interactions with sphingolipids, a class of conserved bioactive lipids with an elusive mode of action. Integration of live-cell imaging suggests new cellular targets for these molecules, including several with pleckstrin homology (PH) domains. Validated interactions with Slm1, a regulator of actin polarization, show that PH domains can have unexpected lipid-binding specificities and can act as coincidence sensors for both phosphatidylinositol phosphates and phosphorylated sphingolipids.

Probing the determinants of diacylglycerol binding affinity in the C1B domain of protein kinase Cα

C1 domains are independently folded modules that are responsible for targeting their parent proteins to lipid membranes containing diacylglycerol (DAG), a ubiquitous second messenger. The DAG binding affinities of C1 domains determine the threshold concentration of DAG required for the propagation of signaling response and the selectivity of this response among DAG receptors in the cell. The structural information currently available for C1 domains offers little insight into the molecular basis of their differential DAG binding affinities. In this work, we characterized the C1B domain of protein kinase Cα (C1Bα) and its diagnostic mutant, Y123W, using solution NMR methods and molecular dynamics simulations. The mutation did not perturb the C1Bα structure or the sub-nanosecond dynamics of the protein backbone, but resulted in a >100-fold increase in DAG binding affinity and a substantial change in microsecond timescale conformational dynamics, as quantified by NMR rotating-frame relaxation-dispersion methods. The differences in the conformational exchange behavior between wild type and Y123W C1Bα were localized to the hinge regions of ligand-binding loops. Molecular dynamics simulations provided insight into the identity of the exchanging conformers and revealed the significance of a particular residue (Gln128) in modulating the geometry of the ligand-binding site. Taken together with the results of binding studies, our findings suggest that the conformational dynamics and preferential partitioning of the tryptophan side chain into the water-lipid interface are important factors that modulate the DAG binding properties of the C1 domains.

Fluorescent-responsive synthetic C1b domains of protein kinase Cδ as reporters of specific high-affinity ligand binding

Protein kinase C (PKC) is a critical cell signaling pathway involved in many disorders such as cancer and Alzheimer-type dementia. To date, evaluation of PKC ligand binding affinity has been performed by competitive studies against radiolabeled probes that are problematic for high-throughput screening. In the present study, we have developed a fluorescent-based binding assay system for identifying ligands that target the PKC ligand binding domain (C1 domain). An environmentally sensitive fluorescent dye (solvatochromic fluorophore), which has been used in multiple applications to assess protein-binding interactions, was inserted in proximity to the binding pocket of a novel PKCδ C1b domain. These resultant fluorescent-labeled δC1b domain analogues underwent a significant change in fluorescent intensity upon ligand binding, and we further demonstrate that the fluorescent δC1b domain analogues can be used to evaluate ligand binding affinity.

Rac GTPase-activating protein (Rac GAP) α1-Chimaerin undergoes proteasomal degradation and is stabilized by diacylglycerol signaling in neurons

α1-Chimaerin is a neuron-specific member of the Rho GTPase-activating protein family that selectively inactivates the small GTPase Rac. It is known to regulate the structure of dendrites and dendritic spines. We describe here that under basal conditions α1-chimaerin becomes polyubiquitinated and undergoes rapid proteasomal degradation. This degradation is partly dependent on the N-terminal region that is unique to this isoform. Mimicking diacylglycerol (DAG) signaling with a phorbol ester stabilizes endogenous α1-chimaerin against degradation and causes accumulation of the protein. The stabilization requires phorbol ester binding via the C1 domain of the protein and is independent of PKC activity. In addition, overexpression of a constitutively active Rac1 mutant is sufficient to cause an accumulation of α1-chimaerin through a phospholipase C-dependent mechanism, showing that endogenous DAG signaling can also stabilize the protein. These results suggest that signaling via DAG may regulate the abundance of α1-chimaerin under physiological conditions, providing a new model for understanding how its activity could be controlled.

A novel cross-talk in diacylglycerol signaling: the Rac-GAP beta2-chimaerin is negatively regulated by protein kinase Cdelta-mediated phosphorylation

Although the family of chimaerin Rac-GAPs has recently gained significant attention for their involvement in development, cancer, and neuritogenesis, little is known about their molecular regulation. Chimaerins are activated by the lipid second messenger diacylglycerol via their C1 domain upon activation of tyrosine kinase receptors, thereby restricting the magnitude of Rac signaling in a receptor-regulated manner. Here we identified a novel regulatory mechanism for beta2-chimaerin via phosphorylation. Epidermal growth factor or the phorbol ester phorbol 12-myristate 13-acetate caused rapid phosphorylation of beta2-chimaerin on Ser(169) located in the SH2-C1 domain linker region via protein kinase Cdelta, which retained beta2-chimaerin in the cytosol and prevented its C1 domain-mediated translocation to membranes. Furthermore, despite the fact that Ser(169) phosphorylation did not alter intrinsic Rac-GAP activity in vitro, a non-phosphorylatable beta2-chimaerin mutant was highly sensitive to translocation, and displayed enhanced association with activated Rac, enhanced Rac-GAP activity, and anti-migratory properties when expressed in cells. Our results not only revealed a novel regulatory mechanism that facilitates Rac activation, but also identified a novel mechanism of cross-talk between diacylglycerol receptors that restricts beta2-chimaerin relocalization and activation.

Protein kinase C: poised to signal

Nestled at the tip of a branch of the kinome, protein kinase C (PKC) family members are poised to transduce signals emanating from the cell surface. Cell membranes provide the platform for PKC function, supporting the maturation of PKC through phosphorylation, its allosteric activation by binding specific lipids, and, ultimately, promoting the downregulation of the enzyme. These regulatory mechanisms precisely control the level of signaling-competent PKC in the cell. Disruption of this regulation results in pathophysiological states, most notably cancer, where PKC levels are often grossly altered. This review introduces the PKC family and then focuses on recent advances in understanding the cellular regulation of its diacylglycerol-regulated members.

cAMP-response element-binding protein (CREB) controls MSK1-mediated phosphorylation of histone H3 at the c-fos promoter in vitro

The rapid induction of the c-fos gene correlates with phosphorylations of histone H3 and HMGN1 by mitogen- and stress-activated protein kinases. We have used a cell-free system to dissect the mechanism by which MSK1 phosphorylates histone H3 within the c-fos chromatin. Here, we show that the reconstituted c-fos chromatin presents a strong barrier to histone H3 phosphorylation by MSK1; however, the activators (serum response factor, Elk-1, cAMP-response element-binding protein (CREB), and ATF1) bound on their cognate sites recruit MSK1 to phosphorylate histone H3 at Ser-10 within the chromatin. This activator-dependent phosphorylation of histone H3 is enhanced by HMGN1 and occurs preferentially near the promoter region. Among the four activators, CREB plays a predominant role in MSK1-mediated phosphorylation of histone H3, and the phosphorylation of Ser-133 in CREB is essential for this process. Mutational analyses of MSK1 show that its N-terminal inhibition domain is critical for the kinase to phosphorylate chromatin-embedded histone H3 in a CREB-dependent manner, indicating the presence of an intricate regulatory network for MSK1-mediated phosphorylation of histone H3.

Synthesis of protein kinase Cdelta C1b domain by native chemical ligation methodology and characterization of its folding and ligand binding

The C1b domain of protein kinase Cdelta (PKCdelta), a potent receptor for ligands such as diacylglycerol and phorbol esters, was synthesized by utilizing native chemical ligation. With this synthetic strategy, the domain was efficiently constructed and shown to have high affinity ligand binding and correct folding. The C1b domain has been utilized for the development of novel ligands for the control of phosphorylation by PKC family members. This strategy will pave the way for the efficient construction of C1b domains modified with fluorescent dyes, biotin, etc.

PKCϵ has an alcohol-binding site in its second cysteine-rich regulatory domain

Alcohols regulate the expression and function of PKC (protein kinase C), and it has been proposed that an alcohol-binding site is present in PKCα in its C1 domain, which consists of two cysteine-rich subdomains, C1A and C1B. A PKCϵ-knockout mouse showed a significant decrease in alcohol consumption compared with the wild-type. The aim of the present study was to investigate whether an alcohol-binding site could be present in PKCϵ. Here we show that ethanol inhibited PKCϵ activity in a concentration-dependent manner with an EC50 (equilibrium ligand concentration at half-maximum effect) of 43 mM. Ethanol, butanol and octanol increased the binding affinity of a fluorescent phorbol ester SAPD (sapintoxin-D) to PKCϵC1B in a concentration-dependent manner with EC50 values of 78 mM, 8 mM and 340 μM respectively, suggesting the presence of an allosteric alcohol-binding site in this subdomain. To identify this site, PKCϵC1B was photolabelled with 3-azibutanol and 3-azioctanol and analysed by MS. Whereas azibutanol preferentially labelled His236, Tyr238 was the preferred site for azioctanol. Inspection of the model structure of PKCϵC1B reveals that these residues are 3.46 Å (1 Å=0.1 nm) apart from each other and form a groove where His236 is surface-exposed and Tyr238 is buried inside. When these residues were replaced by alanine, it significantly decreased alcohol binding in terms of both photolabelling and alcohol-induced SAPD binding in the mutant H236A/Y238A. Whereas Tyr238 was labelled in mutant H236A, His236 was labelled in mutant Y238A. The present results provide direct evidence for the presence of an allosteric alcohol-binding site on protein kinase Cϵ and underscore the role of His236 and Tyr238 residues in alcohol binding.

Photoincorporation of azialcohol to the C1B domain of PKCdelta is buffer dependent

Protein kinase C (PKC) is a signal transducing protein that has been implicated in binding alcohol and anesthetics. The alcohol and anesthetic binding of protein kinase C delta C1B domain has been determined previously by photolabeling and mass spectrometry [J. Das, G.H. Addona, W.S. Sandberg, S.S. Husain, T. Stehle, K.W. Miller, Identiffcation of a general anesthetic binding site in the diacylglycerol-binding domain of protein kinase C delta, J. Biol. Chem. 279 (2004) 37964-37972]. Here we studied photoincorporation of 3-azioctanol, a photoactive analog of octanol into PKC delta C1B in two buffer systems containing tris and hepes. The extent of photoincorporation was higher in hepes compared to tris as determined by high performance liquid chromatography and mass spectrometric analysis. The results are explained on the basis of the presence of number of primary hydroxyl and amino groups in tris and hepes molecules that could affect the binding of alcohol molecules to protein. This observation will be useful in selecting buffer system for biochemical studies on PKC.

Locally generated methylseleninic acid induces specific inactivation of protein kinase C isoenzymes: relevance to selenium-induced apoptosis in prostate cancer cells

In this study, we show that methylselenol, a selenometabolite implicated in cancer prevention, did not directly inactivate protein kinase C (PKC). Nonetheless, its oxidation product, methylseleninic acid (MSA), inactivated PKC at low micromolar concentrations through a redox modification of vicinal cysteine sulfhydryls in the catalytic domain of PKC. This modification of PKC that occurred in both isolated form and in intact cells was reversed by a reductase system involving thioredoxin reductase, a selenoprotein. PKC isoenzymes exhibited variable sensitivity to MSA with Ca(2+)-dependent PKC isoenzymes (alpha, beta, and gamma) being the most susceptible, followed by isoenzymes delta and epsilon. Other enzymes tested were inactivated only with severalfold higher concentrations of MSA than those required for PKC inactivation. This specificity for PKC was further enhanced when MSA was generated within close proximity to PKC through a reaction of methylselenol with PKC-bound lipid peroxides in the membrane. The MSA-methylselenol redox cycle resulted in the catalytic oxidation of sulfhydryls even with nanomolar concentrations of selenium. MSA inhibited cell growth and induced apoptosis in DU145 prostate cancer cells at a concentration that was higher than that needed to inhibit purified PKC alpha but in a range comparable with that required for the inhibition of PKC epsilon. This MSA-induced growth inhibition and apoptosis decreased with a conditional overexpression of PKC epsilon and increased with its knock-out by small interfering RNA. Conceivably, when MSA is generated within the vicinity of PKC, it specifically inactivates PKC isoenzymes, particularly the promitogenic and prosurvival epsilon isoenzyme, and this inactivation causes growth inhibition and apoptosis.

Characterization of the differential roles of the twin C1a and C1b domains of protein kinase C-delta

Classic and novel protein kinase C (PKC) isozymes contain two zinc finger motifs, designated "C1a" and "C1b" domains, which constitute the recognition modules for the second messenger diacylglycerol (DAG) or the phorbol esters. However, the individual contributions of these tandem C1 domains to PKC function and, reciprocally, the influence of protein context on their function remain uncertain. In the present study, we prepared PKCdelta constructs in which the individual C1a and C1b domains were deleted, swapped, or substituted for one another to explore these issues. As isolated fragments, both the deltaC1a and deltaC1b domains potently bound phorbol esters, but the binding of [(3)H]phorbol 12,13-dibutyrate ([(3)H]PDBu) by the deltaC1a domain depended much more on the presence of phosphatidylserine than did that of the deltaC1b domain. In intact PKCdelta, the deltaC1b domain played the dominant role in [(3)H]PDBu binding, membrane translocation, and down-regulation. A contribution from the deltaC1a domain was nonetheless evident, as shown by retention of [(3)H]PDBu binding at reduced affinity, by increased [(3)H]PDBu affinity upon expression of a second deltaC1a domain substituting for the deltaC1b domain, and by loss of persistent plasma membrane translocation for PKCdelta expressing only the deltaC1b domain, but its contribution was less than predicted from the activity of the isolated domain. Switching the position of the deltaC1b domain to the normal position of the deltaC1a domain (or vice versa) had no apparent effect on the response to phorbol esters, suggesting that the specific position of the C1 domain within PKCdelta was not the primary determinant of its activity.

Identification of an autoinhibitory mechanism that restricts C1 domain-mediated activation of the Rac-GAP alpha2-chimaerin

Chimaerins are a family of GTPase activating proteins (GAPs) for the small G-protein Rac that have gained recent attention due to their important roles in development, cancer, neuritogenesis, and T-cell function. Like protein kinase C isozymes, chimaerins possess a C1 domain capable of binding phorbol esters and the lipid second messenger diacylglycerol (DAG) in vitro. Here we identified an autoinhibitory mechanism in alpha2-chimaerin that restricts access of phorbol esters and DAG, thereby limiting its activation. Although phorbol 12-myristate 13-acetate (PMA) caused limited translocation of wild-type alpha2-chimaerin to the plasma membrane, deletion of either N- or C-terminal regions greatly sensitize alpha2-chimaerin for intracellular redistribution and activation. Based on modeling analysis that revealed an occlusion of the ligand binding site in the alpha2-chimaerin C1 domain, we identified key amino acids that stabilize the inactive conformation. Mutation of these sites renders alpha2-chimaerin hypersensitive to C1 ligands, as reflected by its enhanced ability to translocate in response to PMA and to inhibit Rac activity and cell migration. Notably, in contrast to PMA, epidermal growth factor promotes full translocation of alpha2-chimaerin in a phospholipase C-dependent manner, but not of a C1 domain mutant with reduced affinity for DAG (P216A-alpha2-chimaerin). Therefore, DAG generation and binding to the C1 domain are required but not sufficient for epidermal growth factor-induced alpha2-chimaerin membrane association. Our studies suggest a role for DAG in anchoring rather than activation of alpha2-chimaerin. Like other DAG/phorbol ester receptors, including protein kinase C isozymes, alpha2-chimaerin is subject to autoinhibition by intramolecular contacts, suggesting a highly regulated mechanism for the activation of this Rac-GAP.

PKC gamma mutations in spinocerebellar ataxia type 14 affect C1 domain accessibility and kinase activity leading to aberrant MAPK signaling

Spinocerebellar ataxia type 14 (SCA14) is a neurodegenerative disorder caused by mutations in the neuronal-specific protein kinase C gamma (PKCgamma) gene. Since most mutations causing SCA14 are located in the PKCgamma C1B regulatory subdomain, we investigated the impact of three C1B mutations on the intracellular kinetics, protein conformation and kinase activity of PKCgamma in living cells. SCA14 mutant PKCgamma proteins showed enhanced phorbol-ester-induced kinetics when compared with wild-type PKCgamma. The mutations led to a decrease in intramolecular FRET of PKCgamma, suggesting that they ;open' PKCgamma protein conformation leading to unmasking of the phorbol ester binding site in the C1 domain. Surprisingly, SCA14 mutant PKCgamma showed reduced kinase activity as measured by phosphorylation of PKC reporter MyrPalm-CKAR, as well as downstream components of the MAPK signaling pathway. Together, these results show that SCA14 mutations located in the C1B subdomain ;open' PKCgamma protein conformation leading to increased C1 domain accessibility, but inefficient activation of downstream signaling pathways.

Selective binding of phorbol esters and diacylglycerol by individual C1 domains of the PKD family

The PKD (protein kinase D) family are novel DAG (diacylglycerol) receptors. The twin C1 domains of PKD, designated C1a and C1b, have been shown to bind DAG or phorbol esters. However, their ligand-binding activities and selectivities have not been fully characterized. Here, binding activities of isolated C1a, C1b and intact C1a-C1b domains to DAG and phorbol esters were analysed. The isolated C1b domains of PKD isoforms bind [(3)H]PDBu ([20-(3)H]phorbol 12, 13-dibutyrate) with similar high affinities, while they exhibit weaker affinities towards a synthetic DAG analogue, DOG (1,2-dioctanoyl-sn-glycerol), as compared to the control. Mutating a conserved lysine residue at position 22 to tryptophan in C1b of PKD3 fully restores its affinity to DOG, indicating that this residue accounts for its weaker affinity to DOG. In contrast, the non-consensus residues in the isolated C1a domain of PKD mainly contribute to maintaining the protein's structural fold, since converting these residues in C1a of PKD3 to those in PKD1 or PKD2 drastically reduces the maximal number of active receptors, while only minimally impacting ligand-binding activities. Moreover, ligand-binding activities of C1a and C1b are sensitive to the structural context in an intact C1a-C1b domain and exhibit unique patterns of ligand selectivity. C1a and C1b in the intact C1a-C1b of PKD1 are opposite in selectivity for PDBu and DOG. In contrast, C1a of PKD3 exhibits 48-fold higher affinity to DOG as compared to C1b, although both domains bind PDBu with equivalent affinities. Accordingly, mutating C1a of a full-length PKD3-GFP greatly reduces DOG-induced plasma membrane translocation, but does not affect that induced by PMA. In summary, individual C1 domains of PKD isoforms differ in ligand-binding activity and selectivity, implying isoform-selective regulation of PKD by phorbol esters and DAG.

A direct redox regulation of protein kinase C isoenzymes mediates oxidant-induced neuritogenesis in PC12 cells

In this study, we have used the PC12 cell model to elucidate the mechanisms by which sublethal doses of oxidants induce neuritogenesis. The xanthine/xanthine oxidase (X/XO) system was used for the steady state generation of superoxide, and CoCl(2) was used as a representative transition metal redox catalyst. Upon treatment of purified protein kinase C (PKC) with these oxidants, there was an increase in its cofactor-independent activation. Redox-active cobalt competed with the redoxinert zinc present in the zinc-thiolates of the PKC regulatory domain and induced the oxidation of these cysteine-rich regions. Both CoCl(2) and X/XO induced neurite outgrowth in PC12 cells, as determined by an overexpression of neuronal marker genes. Furthermore, these oxidants induced a translocation of PKC from cytosol to membrane and subsequent conversion of PKC to a cofactor-independent form. Isoenzyme-specific PKC inhibitors demonstrated that PKCepsilon plays a crucial role in neuritogenesis. Moreover, oxidant-induced neurite outgrowth was increased with a conditional overexpression of PKCepsilon and decreased with its knock-out by small interfering RNA. Parallel with PKC activation, an increase in phosphorylation of the growth-associated neuronal protein GAP-43 at Ser(41) was observed. Additionally, there was a sustained activation of extracellular signal-regulated kinases 1 and 2, which was correlated with activating phosphorylation (Ser(133)) of cAMP-responsive element-binding protein. All of these signaling events that are causally linked to neuritogenesis were blocked by antioxidant N-acetylcysteine (both L and D-forms) and by a variety of PKC-specific inhibitors. Taken together, these results strongly suggest that sublethal doses of oxidants induce neuritogenesis via a direct redox activation of PKCepsilon.