Molecular modeling and site-directed mutagenesis studies of a phorbol ester-binding site in protein kinase C

PMA treatment fosters rat retinal ganglion cell survival via TNF signaling

An insult can trigger a protective response or even cell death depending on different factors that include the duration and magnitude of the event and the ability of the cell to activate protective intracellular signals, including inflammatory cytokines. Our previous work showed that the treatment of Lister Hooded rat retinal cell cultures with 50 ng/mL phorbol 12-myristate 13-acetate (PMA), a protein kinase C activator, increases the survival of retinal ganglion cells (RGCs) kept in culture for 48 h after axotomy. Here we aim to analyze how PMA modulates the levels of TNF-α and IL-1β (both key inflammatory mediators) and the impact of this modulation on RGCs survival. We hypothesize that the increase in RGCs survival mediated by PMA treatment depends upon modulation of the levels of IL-1β and TNF-α. The effect of PMA treatment was assayed on cell viability, caspase 3 activation, TNF-α and IL-1β release and TNF receptor type I (TNFRI) and TNF receptor type II (TNFRII) levels. PMA treatment increases IL-1β and TNF-α levels in 15 min in culture and increases the release of both cytokines after 30 min and 24 h, respectively. Both IL-1β and TNF-α levels decrease after 48 h of PMA treatment. PMA treatment also induces an increase in TNFRII levels while decreasing TNFRI after 24 h. PMA also inhibited caspase-3 activation, and decreased ROS production and EthD-1/calcein ratio in retinal cell cultures leading to an increase in cell viability. The neutralization of IL-1β (anti-IL1β 0,1ng/mL), the neutralization of TNF-α (anti-TNF-α 0,1ng/mL) and the TNF-α inhibition using a recombinant soluble TNFRII abolished PMA effect on RGCs survival. These data suggest that PMA treatment induces IL1β and TNF-α release and modulation of TNFRI/TNFRII expression promoting RGCs survival after axotomy.

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.

Phorbol Rearrangements

An alternative procedure for isolation of 4β-phorbol from seeds of Croton tiglium has been developed, and an artifact containing a furan ring formed by rearrangement of 12,13,20- O-triacylated phorbol derivatives into (6b S,7 R,8 R,8a S)-2-(hydroxymethyl)-5,7,9,9-tetramethyl-3,7,8,9,9a,9b-hexahydrocyclopropa[3',4']benzo[1',2':3,4]cyclohepta[1,2- b]furan-6b,8,8a-triol (8a) has been characterized. A mechanism involving an oxidative rearrangement and a decarboxylation for formation of the artifact is proposed.

Design and synthesis of triarylacrylonitrile analogues of tamoxifen with improved binding selectivity to protein kinase C

The clinical selective estrogen receptor modulator tamoxifen is also a modest inhibitor of protein kinase C, a target implicated in several untreatable brain diseases such as amphetamine abuse. This inhibition and tamoxifen's ability to cross the blood brain barrier make it an attractive scaffold to conduct further SAR studies toward uncovering effective therapies for such diseases. Utilizing the known compound 6a as a starting template and guided by computational tools to derive physicochemical properties known to be important for CNS permeable drugs, the design and synthesis of a small series of novel triarylacrylonitrile analogues have been carried out providing compounds with enhanced potency and selectivity for PKC over the estrogen receptor relative to tamoxifen. Shortened synthetic routes compared to classical procedures have been developed for analogues incorporating a β-phenyl ring, which involve installing dialkylaminoalkoxy side chains first off the α and/or α' rings of a precursor benzophenone and then condensing the resultant ketones with phenylacetonitrile anion. A second novel, efficient and versatile route utilizing Suzuki chemistry has also been developed, which will allow for the introduction of a wide range of β-aryl or β-heteroaryl moieties and side-chain substituents onto the acrylonitrile core. For analogues possessing a single side chain off the α- or α'-ring, novel 2D NMR experiments have been carried out that allow for unambiguous assignment of E- and Z-stereochemistry. From the SAR analysis, one compound, 6c, shows markedly increased potency and selectivity for inhibiting PKC with an IC₅₀ of 80nM for inhibition of PKC protein substrate and >10μM for binding to the estrogen receptor α (tamoxifen IC₅₀=20μM and 222nM, respectively). The data on 6c provide support for further exploration of PKC as a druggable target for the treatment of amphetamine abuse.

Activation of nuclear factor κB (NF-κB) in prostate cancer is mediated by protein kinase C epsilon (PKCepsilon)

Protein kinase C ε (PKCε) has emerged as an oncogenic kinase and plays important roles in cell survival, mitogenesis and invasion. PKCε is up-regulated in most epithelial cancers, including prostate, breast, and lung cancer. Here we report that PKCε is an essential mediator of NF-κB activation in prostate cancer cells. A strong correlation exists between PKCε overexpression and NF-κB activation status in prostate cancer cells. Moreover, transgenic overexpression of PKCε in the mouse prostate causes preneoplastic lesions that display significant NF-κB hyperactivation. PKCε RNAi depletion or inhibition in prostate cancer cells diminishes NF-κB translocation to the nucleus with subsequent impairment of both activation of NF-κB transcription and induction of NF-κB responsive genes in response to the proinflammatory cytokine tumor necrosis factor α (TNFα). On the other hand, PKCε overexpression in normal prostate cells enhances activation of the NF-κB pathway. A mechanistic analysis revealed that TNFα activates PKCε via a C1 domain/diacylglycerol-dependent mechanism that involves phosphatidylcholine-phospholipase C. Moreover, PKCε facilitates the assembly of the TNF receptor-I signaling complex to trigger NF-κB activation. Our studies identified a molecular link between PKCε and NF-κB that controls key responses implicated in prostate cancer progression.

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.

Diacylglycerols, multivalent membrane modulators

Diacylglycerols are second messengers confined to biomembranes and, although relatively simple molecules from the structural point of view, they are able of triggering a surprisingly wide range of biological responses. Diacylglycerols are recognized by a well conserved protein motif, such as the C1 domain. This domain was observed for the first time in protein kinases C but is now known to be present in many other proteins. The effect of diacylglycerols is not limited to binding to C1 domains and they are able to alter the biophysical properties of biomembranes and hence modulate the activity of membrane associated proteins and also facilitate some processes like membrane fusion.

Identification of potent, selective protein kinase C inhibitors based on a phorbol skeleton

The elucidation of specific functions of protein kinase C (PKC) subtypes in physiological processes is an important challenge for the future development of new drug targets. Subtype-selective PKC agonists and antagonists are useful biological tools for this purpose. Most of the currently used PKC modulators elicit their activities through binding to the ATP binding site of PKC, which shares many features with other kinases. PKC modulators that target the PKC regulatory domain are considered to be advantageous in terms of selectivity, because the structure of the regulatory domain is intrinsic to each PKC subtype. In this paper, we describe the identification of new potent and conventional PKC-selective inhibitors that target the regulatory domain. The inhibitors contain a phorbol skeleton, a naturally occurring potent and selective PKC regulatory domain binder, with a perfluorinated alkyl group and a polyether hydrophilic chain on a terephthaloyl aromatic ring at the C12 position. Both of these substituents are essential for the potent inhibitory activity. Specifically, the binding affinity between PKC and the phorbol ester analogues was improved by an electron-deficient aromatic ring at C12. This finding cannot be explained by the previously proposed binding model and suggests a new binding mode between phorbol esters and PKC.

Comparison of the PKCalpha and the PKCepsilon C1b domains: identification of residues critical for PKCepsilon-mediated neurite induction

We showed earlier that over-expression of protein kinase C (PKC) epsilon induces neurite outgrowth. The effect is mediated by a region (PKCepsilonPSC1V3) encompassing the pseudosubstrate, the two C1 domains and part of the V3 region, and is independent of the catalytic activity of the enzyme. In this region, residues immediately N-terminal of the C1b domain are crucial for neurite outgrowth. However, in this study we show that the PKCepsilon C1b domain itself is necessary for neurite induction, since a mutant in which the PKCepsilon C1b domain has been replaced with the C1b domain from PKCalpha, PKCepsilonPSC1a(alphaC1b)V3 lacks neurite-inducing capacity. The molecular basis for the importance of the PKCepsilon C1b domain was investigated by mutation studies of the PKCalpha C1b domain. Point mutations were done in the PKCalpha C1b domain of the PKCepsilonPSC1a(alphaC1b)V3 construct, in which the PKCalpha residues were mutated into the corresponding residues in PKCepsilon. This highlighted residues in the C-terminal part of the primary sequence of the C1b domain, located in the base of the C1b domain, as important for neurite outgrowth. The mutations S48P, D32K and L49N all influenced neurite induction positively. Furthermore, the mutation of L49N alone was sufficient to make PKCepsilonPSC1a(alphaC1b)V3 neuritogenic in phorbol ester-stimulated cells, and mutation of this residue in full-length PKCepsilon into the corresponding residue in PKCalpha, N291L reduced the neurite-inducing effect of PKCepsilon. In conclusion, we have identified residues in the PKCepsilon C1b domain, in particular Asn49, that are essential for neurite outgrowth.

Identification of an amino acid residue in the protein kinase C C1b domain crucial for its localization to the Golgi network

Protein kinase C (PKC) isoforms have been reported to be targeted to the Golgi complex via their C1 domains. We have shown recently that the regulatory domain of PKC induces apoptosis in neuroblastoma cells and that this effect is correlated to Golgi localization via the C1b domain. This study was designed to identify specific residues in the C1 domains that mediate Golgi localization. We demonstrate that the isolated C1b domains from PKCalpha, -delta, -epsilon, -eta, and - are targeted to the Golgi complex, whereas the corresponding C1a domains localize throughout the cell. Sequence alignment showed that amino acid residues corresponding to Glu-246 and Met-267 in PKC are conserved among C1b but absent from C1a domains. Mutation of Met-267, but not of Glu-246, to glycine abolished the Golgi localization of the isolated C1b domain and the regulatory domain of PKC. The mutated PKC regulatory domain constructs lacking Golgi localization were unable to induce apoptosis, suggesting a direct correlation between Golgi localization and apoptotic activity of PKC regulatory domain. Mutation of analogous residues in the C1b domain of PKCepsilon abrogated its Golgi localization, demonstrating that this effect is not restricted to one PKC isoform. The abolished Golgi localization did not affect neurite induction by PKCepsilon. However, the PKCepsilon mutant did not relocate to the Golgi network in response to ceramide and ceramide did not suppress the neurite-inducing capacity of the protein. Thus, the specific mutations in the C1b domain influence both the localization and function of full-length PKCepsilon.

Ligand Structure-Activity Requirements and Phospholipid Dependence for the Binding of Phorbol Esters to Protein Kinase D

Although protein kinase D (PKD), like protein kinase C (PKC), possesses a C1 domain that binds phorbol esters and diacylglycerol, the structural differences from PKC within this and other domains of PKD imply differential regulation by lipids and ligands. We characterized the phorbol ester and phospholipid binding properties of a glutathione S-transferase-tagged full-length PKD and compared them with those of PKC-alpha and -delta. We found that PKD is a high-affinity phorbol ester receptor for a range of structurally and functionally divergent phorbol esters and analogs and showed both similarities and differences in structure-activity relations compared with the PKCs examined. In particular, PKD had lower affinity than PKC for certain diacylglycerol analogs, which might be caused by a lysine residue at the 22 position of the PKD-C1b domain in place of the tryptophan residue at this position conserved in the PKCs. The membrane-targeting domains in PKD are largely different from those in PKC; among these differences, PKD contains a pleckstrin homology (PH) domain that is absent in PKC. However, phosphatidylinositol-4,5-bisphosphate PIP2, a lipid ligand for some PH domains, reconstitutes phorbol 12,13-dibutyrate (PDBu) binding to PKD similarly as it does to PKC-alpha and -delta, implying that the PH domain in PKD may not preferentially interact with PIP2. Overall, the requirement of anionic phospholipids for the reconstitution of [3H]PDBu binding to PKD was intermediate between those of PKC-alpha and -delta. We conclude that PKD is a high-affinity phorbol ester receptor; its lipid requirements for ligand binding are approximately comparable with those of PKC but may be differentially regulated in cells through the binding of diacylglycerol to the C1 domain.

Synthesis and phorbol ester binding of the cysteine-rich domains of diacylglycerol kinase (DGK) isozymes. DGKgamma and DGKbeta are new targets of tumor-promoting phorbol esters

Diacylglycerol kinase (DGK) and protein kinase C (PKC) are two distinct enzyme families associated with diacylglycerol. Both enzymes have cysteine-rich C1 domains (C1A, C1B, and C1C) in the regulatory region. Although most PKC C1 domains strongly bind phorbol esters, there has been no direct evidence that DGK C1 domains bind phorbol esters. We synthesized 11 cysteine-rich sequences of DGK C1 domains with good sequence homology to those of the PKC C1 domains. Among them, only DGKgamma-C1A and DGKbeta-C1A exhibited significant binding to phorbol 12,13-dibutyrate (PDBu). Scatchard analysis of rat-DGKgamma-C1A, human-DGKgamma-C1A, and human-DGKbeta-C1A gave K(d) values of 3.6, 2.8, and 14.6 nm, respectively, suggesting that DGKgamma and DGKbeta are new targets of phorbol esters. An A12T mutation of human-DGKbeta-C1A enhanced the affinity to bind PDBu, indicating that the beta-hydroxyl group of Thr-12 significantly contributes to the binding. The K(d) value for PDBu of FLAG-tagged whole rat-DGKgamma (4.4 nm) was nearly equal to that of rat-DGKgamma-C1A (3.6 nm). Moreover, 12-O-tetradecanoylphorbol 13-acetate induced the irreversible translocation of whole rat-DGKgamma and its C1B deletion mutant, not the C1A deletion mutant, from the cytoplasm to the plasma membrane of CHO-K1 cells. These results indicate that 12-O-tetradecanoylphorbol 13-acetate binds to C1A of DGKgamma to cause its translocation.

Inhibition of protein kinase C by resveratrol

Evidence is emerging that resveratrol (RV), a polyphenolic phytoaxelin present in dietary sources including red wine, may protect against atherosclerosis and cardiovascular disease by enhancing the integrity of the endothelium. In this study, the possibility that such beneficial effects of RV may arise from a modulation of protein kinase C (PKC)-mediated signaling was investigated by determining the effects of RV on the in vitro activities of PKC isozymes. It was found that the Ca(2+)-dependent activities of membrane-associated PKCalpha induced by either phorbol ester or diacylglycerol were potently inhibited by RV, each with an IC(50) of approximately 2 microM. The inhibitory effect of RV was also observed for conventional PKCbetaI, whereas the activities of novel PKC epsilon and atypical PKCzeta were each unaffected. The inhibition of PKCalpha activity was found to be competitive with respect to phorbol ester concentration but noncompetitive with respect to Ca(2+) and phosphatidylserine concentrations, suggesting that the RV may compete for phorbol ester-binding to the C1 domains. Supporting this, it was found that RV bound to a fusion peptide containing the C1A and C1B domains of PKCalpha. Similar to the effects of diacylglycerol and phorbol ester, the interaction of RV with the C1 domains induced the association of PKCalpha with membrane lipid vesicles, although this did not result in activation. Overall, the results suggest that the inhibitory effect of RV on PKC activity, and therefore on the associated signaling networks, may, in part, underlie the mechanism(s) by which this agent exerts its beneficial effects on endothelial and cardiovascular function. Furthermore, the effects of RV on these signaling networks are predicted to differ according to the cellular localization and the regulating PKC isozyme.

The use of fluorescent phorbol esters in studies of protein kinase C-membrane interactions

The family of protein kinase C (PKC) isozymes belongs to a growing class of proteins that become active by associating with membranes containing anionic phospholipids, such as phosphatidylserine. Depending on the particular PKC isoform, this process is mediated by Ca(2+)-binding to a C2 domain and interaction of activators such as 1,2-diacyl-sn-glycerol or phorbol esters with tandem C1 domains. This cooperation between the C1 and C2 domains in inducing the association of PKC with lipid membranes provides the energy for a conformational change that consists of the release of a pseudosubstrate sequence from the active site, culminating in activation. Thus, the properties of the interactions of the C1 and C2 domains with membranes, both as isolated domains, and as modules in the full length PKC isoforms, have been the subject of intense scrutiny. Here, we review the findings of studies in which fluorescent phorbol esters have been utilized to probe the properties of the C1 domains of PKC with respect to the interaction with activators, the subsequent interaction with membranes, and the role of the activating conformational change that leads to activation.

Role of hydrophobic residues in the C1b domain of protein kinase C delta on ligand and phospholipid interactions

The C1 domains of conventional and novel protein kinase C (PKC) isoforms bind diacylglycerol and phorbol esters with high affinity. Highly conserved hydrophobic residues at or near the rim of the binding cleft in the second cysteine-rich domain of PKC-delta (PKC-deltaC1b) were mutated to probe their roles in ligand recognition and lipid interaction. [(3)H]Phorbol 12,13-dibutyrate (PDBu) binding was carried out both in the presence and absence of phospholipids to determine the contribution of lipid association to the ligand affinity. Lipid dependence was determined as a function of lipid concentration and composition. The binding properties of a high affinity branched diacylglycerol with lipophilicity similar to PDBu were compared with those of PDBu to identify residues important for ligand selectivity. As expected, Leu-20 and Leu-24 strongly influenced binding. Substitution of either by aspartic acid abolished binding in either the presence or absence of phosphatidylserine. Mutation of Leu-20 to Arg or of Leu-24 to Lys caused a dramatic (340- and 250-fold, respectively) reduction in PDBu binding in the presence of lipid but only a modest reduction in the weaker binding of PDBu observed in the absence of lipid, suggesting that the main effect was on C1 domain -phospholipid interactions. Mutation of Leu-20 to Lys or of Trp-22 to Lys had modest (3-fold) effects and mutation of Phe-13 to Tyr or Lys was without effect. Binding of the branched diacylglycerol was less dependent on phospholipid and was more sensitive to mutation of Trp-22 to Tyr or Lys, especially in the presence of phospholipid, than was PDBu. In terms of specific PKC isoforms, our results suggest that the presence of Arg-20 in PKC-zeta may contribute to its lack of phorbol ester binding activity. More generally, the results emphasize the interplay between the C1 domain, ligand, and phospholipid in the ternary binding complex.

The C4 hydroxyl group of phorbol esters is not necessary for protein kinase C binding

To investigate the role of the hydroxyl group at position 4 of the phorbol esters in protein kinase C (PKC) binding and function, 4beta-deoxy-phorbol-12,13-dibutyrate (4beta-deoxy-PDBu, 5a) and 4beta-deoxy-phorbol-13-acetate (6a) were synthesized from phorbol (1). The binding affinities of these 4beta-deoxy compounds (5a, 6a) to the 13 PKC isozyme C1 domains were quite similar to those of the corresponding 4beta-hydroxy compounds (4a, 4b), suggesting that the C4 hydroxyl group of phorbol esters is not necessary for PKC binding. Moreover, functional assays showed that 4beta-deoxy-PDBu (5a) exhibited biological activities (Epstein-Barr virus induction and superoxide generation) equally potent to those of PDBu (4a). These solution phase results differ from expectations based on the previously reported solid-phase structure of the complex of PKCdelta-C1B and phorbol-13-acetate (4b).

5-acyloxy-5-hydroxymethyltetrahydro-2-furancarboxylate as a novel template for protein kinase C (PKC) binding

A series of alkyl tetrahydrofuran-2-carboxylates (1-4) bearing a new set of three pharmacophoric groups were tested as protein kinase C (PKC) ligands. The compounds were synthesized from commercially available glycidyl 4-methoxyphenyl ether. The correlation between their binding affinities for PKC-alpha and a conformational fit to phorbol ester indicates they mimic a pharmacophore model comprising the C20-OH, C3-C=O and C9-OH rather than that including the C13-C=O moiety.

Chemistry and clinical biology of the bryostatins

Bryostatins are a class of antineoplastic compounds isolated from the bryozoans Bugula neritina. A wide range of scientific research is currently underway, studying different aspects of the bryostatins. In this review we try to summarize the latest findings, including all the topics involved, from marine biology to medicinal chemistry.

Selectivity analysis of 5-(arylthio)-2,4-diaminoquinazolines as inhibitors of Candida albicans dihydrofolate reductase by molecular dynamics simulations

A series of 5-(arylthio)-2,4-diaminoquinazolines are known as selective inhibitors of dihydrofolate reductase (DHFR) from Candida albicans. We have performed docking and molecular dynamics simulations of these inhibitors with C. albicans and human DHFR to understand the basis for selectivity of these agents. Study was performed on a selected set of 10 compounds with variation in structure and activity. Molecular dynamics simulations were performed at 300 K for 45 ps with equilibration for 10 ps. Trajectory data was analyzed on the basis of hydrogen bond interactions, energy of binding and conformational energy difference. The results indicate that hydrogen bonds formed between the compound and the active site residues are responsible for inhibition and higher potency. The selectivity index, i.e the ratio of I50 against human DHFR to I50 against fungal DHFR, is mainly determined by the conformation adapted by the compounds within the active site of two enzymes. Since the human DHFR active site is rigid, the compound is trapped in a higher energy conformation. This energy difference between the two conformations deltaE mainly governs the selectivity against fungal DHFR. The information generated from this analysis of potency and selectivity should be useful for further work in the area of antifungal research.

1,2-sn-Diacylglycerol in plant cells: Product, substrate and regulator

1,2-sn-Diacylglycerol (DAG) is a family of lipidic molecular species varying in the lengths and desaturation levels of acyl groups esterified at positions sn-1 and sn-2 of the glycerol backbone. In plant cells, DAG originating from plastid and from extraplastidial membranes have distinct molecular signatures, C18/C16 and C18/C18 structures, respectively. Under normal conditions, DAG is consumed nearly as fast as it is produced and is therefore a transient compound in the cell. In plants, DAG proved to be the most basic ingredient for cell membrane biogenesis and fat storage, but we still lack formal evidence to assert that DAG is also an intracellular messenger, as demonstrated for animals. From the biochemical and molecular comparisons of the best known DAG-manipulating proteins of prokaryotic and eukaryotic cells (phosphatidate phosphatases, diacylglycerol kinases, MGDG synthase, protein kinase C, etc.) this review aims to identify general rules driving DAG metabolism, and emphasizes its unique features in plant cells. DAG metabolism is an intricate network of local productions and utilizations: many isoenzymes can catalyse similar DAG modifications in distinct cell compartments or physiological processes. The enzymatic- or binding-specificity for DAG molecular species demonstrates that discrete DAG molecular subspecies fluxes are finely controlled (particularly for C18/C16 and C18/C18 structures in plastid membrane biogenesis). Eventually, this review stresses the diversity of structures and functioning of DAG-manipulating proteins. As a consequence, because DAG metabolism in plants is unique, the deciphering of genomic information cannot rely on homology searches using known prokaryotic, animal or yeast sequences, but requires sustained efforts in biochemical and molecular characterizations of plant DAG-manipulating proteins.

Interplay of C1 and C2 domains of protein kinase C-alpha in its membrane binding and activation

The regulatory domain of conventional protein kinase C (PKC) contains two membrane-targeting modules, the C2 domain that is responsible for Ca2+-dependent membrane binding of protein, and the C1 domain composed of two cysteine-rich zinc fingers (C1a and C1b) that bind diacylglycerols and phorbol esters. To understand the individual roles and the interplay of the C1 and C2 domains in the membrane binding and activation of PKC, we functionally expressed isolated C1 and C2 domains of PKC-alpha and measured their vesicle binding and monolayer penetration. Results indicate that the C2 domain of PKC-alpha is responsible for the initial Ca2+- and phosphatidylserine-dependent electrostatic membrane binding of PKC-alpha, whereas the C1 domain is involved in subsequent membrane penetration and diacylglycerol binding, which eventually lead to enzyme activation. To determine the roles of individual zinc fingers in the C1 domain, we also mutated hydrophobic residues in the C1a (Trp58 and Phe60) and C1b (Tyr123 and Leu125) domains of the native PKC-alpha molecule and measured the effects of mutations on vesicle binding, enzyme activity and monolayer penetration. Results show that the hydrophobic residues in the C1a domain are essential for the membrane penetration and activation of PKC-alpha, whereas those in the C1b domain are not directly involved in these processes. Based on these results in conjunction with our previous structure-function studies of the C2 domain (Medkova, M., and Cho, W. (1998) J. Biol. Chem. 273, 17544-17552), we propose a mechanism for the in vitro membrane binding and activation of conventional PKC that accounts for the temporal and spatial sequences of PKC activation.

Photoaffinity labeling of PKC isozymes by phorbol ester derivatives

Photoaffinity probes PPDA and PPTD, which have diazoacetyl and trifluorodiazopropionyl group at C-13 position in phorbol, respectively, were synthesized. Photoaffinity labeling of protein kinase C isozymes by both the probes resulted in specific cross-linking.

Intercellular communication, tumor promotion and non-genotoxic carcinogenesis: relationships based upon structural considerations

An SAR model for inhibition of metabolic cooperation (iMC) was developed. The structural and physicochemical features associated with the ability to cause iMC are primarily lipophilic moieties consistent with the possibility that they represent receptor-binding ligands. There are also significant parallels between the structural descriptors associated with iMC and those associated with tumor promotion and with carcinogenesis in rodents. Overall, the present study provides structural evidence that iMC is a feature associated with the carcinogenic process.