Characterization of PKC2, a gene encoding a second protein kinase C isotype of Saccharomyces cerevisiae

Background: Protein kinase C (PKC) has attracted considerable attention over the past decade, primarily because of its presumed role in cellular growth control and tumourigenesis. Mammalian cells express at least 10 different isozymes of PKC; it is this complexity that has made elucidating the precise functions of PKC: so difficult. The identification of PKC homologues in organisms such as Drosophila, Xenopus, Dictyostelium, Aplysia and Caenorhabditis indicates that the enzyme is evolutionarily conserved, and this has stimulated our search for counterparts in the yeast Saccharomyces cerevisiae, in which powerful genetic analyses can be used. To date, only one PKC homologue, PKC1, has been identified in yeast and no biochemical activity has been definitively ascribed to the encoded protein. This, and the inability to identify other PKC homologues in yeast by DNA hybridization, has led to doubts about the existence of PKC isozymes in yeast. We have taken the approach of screening yeast expression libraries with anti-PKC antibodies in an attempt to identify further homologues.Results: We have identified a novel PKC isozyme, Pkc2p, encoded by the gene PKC2. We report here the sequence of PKC2 and a comparison showing its similarity to other PKCs. Phylogenetic analysis suggests that all known PKC genes, including PKC2, originated from a common ancestor. Disruption of the PKC2 protein-coding region, deleting the entire catalytic domain of the encoded enzyme, is not lethal to yeast growing on rich media. However, the pkc2 mutant, unlike wild-type strains, fails to grow on minimal media containing limited concentrations of amino acids. This implicates Pkc2p in the response of yeast cells to amino-acid starvation.Conclusion: We have shown that yeast cells do express more than one PKC isozyme, by identifying and characterizing a novel PKC gene PKC2, the product of which may be involved in the cellular response to amino-acid starvation.     

Protein kinase C controls activation of the DNA integrity checkpoint

The protein kinase C (PKC) superfamily plays key regulatory roles in numerous cellular processes. Saccharomyces cerevisiae contains a single PKC, Pkc1, whose main function is cell wall integrity maintenance. In this work, we connect the Pkc1 protein to the maintenance of genome integrity in response to genotoxic stresses. Pkc1 and its kinase activity are necessary for the phosphorylation of checkpoint kinase Rad53, histone H2A and Xrs2 protein after deoxyribonucleic acid (DNA) damage, indicating that Pkc1 is required for activation of checkpoint kinases Mec1 and Tel1. Furthermore, Pkc1 electrophoretic mobility is delayed after inducing DNA damage, which reflects that Pkc1 is post-translationally modified. This modification is a phosphorylation event mediated by Tel1. The expression of different mammalian PKC isoforms at the endogenous level in yeast pkc1 mutant cells revealed that PKCδ is able to activate the DNA integrity checkpoint. Finally, downregulation of PKCδ activity in HeLa cells caused a defective activation of checkpoint kinase Chk2 when DNA damage was induced. Our results indicate that the control of the DNA integrity checkpoint by PKC is a mechanism conserved from yeast to humans.     

Molecular analysis reveals localization of Saccharomyces cerevisiae protein kinase C to sites of polarized growth and Pkc1p targeting to the nucleus and mitotic spindle

The catalytic activity and intracellular localization of protein kinase C (PKC) are both highly regulated in vivo. This family of kinases contains conserved regulatory motifs, i.e., the C1, C2, and HR1 domains, which target PKC isoforms to specific subcellular compartments and restrict their activity spatially. Saccharomyces cerevisiae contains a single PKC isozyme, Pkc1p, which contains all of the regulatory motifs found in mammalian PKCs. Pkc1p localizes to sites of polarized growth, consistent with its main function in maintaining cell integrity. We dissected the molecular basis of Pkc1p localization by expressing each of its domains individually and in combinations as green fluorescent protein fusions. We find that the Rho1p-binding domains, HR1 and C1, are responsible for targeting Pkc1p to the bud tip and cell periphery, respectively. We demonstrate that Pkc1p activity is required for its normal localization to the bud neck, which also depends on the integrity of the septin ring. In addition, we show for the first time that yeast protein kinase C can accumulate in the nucleus, and we identify a nuclear exit signal as well as nuclear localization signals within the Pkc1p sequence. Thus, we propose that Pkc1p shuttles in and out of the nucleus and consequently has access to nuclear substrates. Surprisingly, we find that deletion of the HR1 domain results in Pkc1p localization to the mitotic spindle and that the C2 domain is responsible for this targeting. This novel nuclear and spindle localization of Pkc1p may provide a molecular explanation for previous observations that suggest a role for Pkc1p in regulating microtubule function.     

The synthetic genetic network around PKC1 identifies novel modulators and components of protein kinase C signaling in Saccharomyces cerevisiae

Budding yeast Saccharomyces cerevisiae contains one protein kinase C (PKC) isozyme encoded by the essential gene PKC1. Pkc1 is activated by the small GTPase Rho1 and plays a central role in the cell wall integrity (CWI) signaling pathway. This pathway acts primarily to remodel the cell surface throughout the normal life cycle and upon various environmental stresses. The pathway is heavily branched, with multiple nonessential branches feeding into and out of the central essential Rho1-Pkc1 module. In an attempt to identify novel components and modifiers of CWI signaling, we determined the synthetic lethal genetic network around PKC1 by using dominant-negative synthetic genetic array analysis. The resulting mutants are hypersensitive to lowered Pkc1 activity. The corresponding 21 nonessential genes are closely related to CWI function: 14 behave in a chemical-genetic epistasis test as acting in the pathway, and 6 of these genes encode known components. Twelve of the 21 null mutants display elevated CWI reporter activity, consistent with the idea that the pathway is activated by and compensates for loss of the gene products. Four of the 21 mutants display low CWI reporter activity, consistent with the idea that the pathway is compromised in these mutants. One of the latter group of mutants lacks Ack1(Ydl203c), an uncharacterized SEL-1 domain-containing protein that we find modulates pathway activity. Epistasis analysis places Ack1 upstream of Pkc1 in the CWI pathway and dependent on the upstream Rho1 GTP exchange factors Rom2 and Tus1. Overall, the synthetic genetic network around PKC1 directly and efficiently identifies known and novel components of PKC signaling in yeast.     

Increased amounts of HMG-CoA reductase induce "karmellae": a proliferation of stacked membrane pairs surrounding the yeast nucleus

Overproduction of the enzyme 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase in yeast resulted in striking morphological effects on the structure of intracellular membranes. Specifically, stacks of paired membranes closely associated with the nuclear envelope were observed in strains that over-produced the HMG1 isozyme, one of two isozymes for HMG-CoA reductase in yeast. These nuclear-associated, paired membranes have been named "karmellae." In strains that overproduced the HMG1 isozyme, HMG-CoA reductase was present in the karmellar layers. At mitosis, karmellae were asymmetrically segregated: the mother cells inherited all of the karmellae and the daughter cells inherited none. A membranous structure of different morphology was occasionally found in cells that overproduced the HMG2 isozyme. These observations further establish the existence of cellular mechanisms that monitor the levels of membrane proteins and compensate for changes in these levels by inducing synthesis of particular types of membrane.     

Identification of the sequences in HMG-CoA reductase required for karmellae assembly.

In all eukaryotic cells that have been examined, specific membrane arrays are induced in response to increased levels of the ER membrane protein, HMG-CoA reductase. Analysis of these inducible membranes has the potential to reveal basic insights into general membrane assembly. Yeast express two HMG-CoA reductase isozymes, and each isozyme induces a morphologically distinct proliferation of the endoplasmic reticulum. The isozyme encoded by HMG1 induces karmellae, which are long stacks of membranes that partially enclose the nucleus. In contrast, the isozyme encoded by HMG2 induces short stacks of membrane that may be associated with the nucleus, but are frequently present at the cell periphery. To understand the molecular nature of the different cellular responses to Hmg1p and Hmg2p, we mapped the region of Hmg1p that is needed for karmellae assembly. For this analysis, a series of exchange alleles was examined in which a portion of the Hmg2p membrane domain was replaced with the corresponding Hmg1p sequences. Results of this analysis indicated that the ER lumenal loop between predicted transmembrane domains 6 and 7 was both necessary and sufficient for karmellae assembly, when present in the context of an HMG-CoA reductase membrane domain. Immunoblotting experiments ruled out the simple possibility that differences in the amounts of the various chimeric HMG-CoA reductase proteins was responsible for the altered cellular responses. Our results are consistent with the hypothesis that each yeast isozyme induces or organizes a qualitatively different organization of ER membrane.     

Phorbol 12-myristate 13-acetate induces protein kinase ceta-specific proliferative response in astrocytic tumor cells

Protein kinase C (PKC) activation has been implicated in cellular proliferation in neoplastic astrocytes. The roles for specific PKC isozymes in regulating this glial response, however, are not well understood. The aim of this study was to characterize the expression of PKC isozymes and the role of PKC-eta expression in regulating cellular proliferation in two well characterized astrocytic tumor cell lines (U-1242 MG and U-251 MG) with different properties of growth in cell culture. Both cell lines expressed an array of conventional (alpha, betaI, betaII, and gamma) and novel (theta and epsilon) PKC isozymes that can be activated by phorbol myristate acetate (PMA). Another novel PKC isozyme, PKC-eta, was only expressed by U-251 MG cells. In contrast, PKC-delta was readily detected in U-1242 MG cells but was present only at low levels in U-251 MG cells. PMA (100 nm) treatment for 24 h increased cell proliferation by over 2-fold in the U-251 MG cells, whereas it decreased the mitogenic response in the U-1242 MG cells by over 90%. When PKC-eta was stably transfected into U-1242 MG cells, PMA increased cell proliferation by 2.2-fold, similar to the response of U-251 MG cells. The cell proliferation induced by PMA in both the U-251 MG and U-1242-PKC-eta cells was blocked by the PKC inhibitor bisindolylmaleimide (0.5 micrometer) and the MEK inhibitor, PD 98059 (50 micrometer). Transient transfection of wild type U-251 with PKC-eta antisense oligonucleotide (1 micrometer) also blocked the PMA-induced increase in [(3)H]thymidine incorporation. The data demonstrate that two glioblastoma lines, with functionally distinct proliferative responses to PMA, express different novel PKC isozymes and that the differential expression of PKC-eta plays a determining role in the different proliferative capacity.     

Resveratrol preferentially inhibits protein kinase C-catalyzed phosphorylation of a cofactor-independent, arginine-rich protein substrate by a novel mechanism.

Resveratrol, a polyphenolic natural product abundantly present in grape skins, is a candidate cancer chemopreventive agent that antagonizes each stage of carcinogenesis and inhibits protein kinase C (PKC), a key mediator of tumor promotion. While resveratrol has been shown to antagonize both isolated and cellular forms of PKC, the weak inhibitory potency observed against isolated PKC cannot account for the reported efficacy of the polyphenol against PKC in cells. In this report, we analyze the mechanism of PKC inhibition by resveratrol. Our results indicate that resveratrol has a broad range of inhibitory potencies against purified PKC that depend on the nature of the substrate and the cofactor dependence of the phosphotransferase reaction. Resveratrol weakly inhibited the Ca2+/phosphatidylserine-stimulated activity of a purified rat brain PKC isozyme mixture (IC(50) = 90 microM) by competition with ATP (K(i) = 55 microM). Consistent with the kinetic evidence for a catalytic domain-directed mechanism, resveratrol inhibited the lipid-dependent activity of PKC isozymes with divergent regulatory domains similarly, and it was even more effective in inhibiting a cofactor-independent catalytic domain fragment (CDF) of PKC generated by limited proteolysis. This suggested that regulatory features of PKC might impede resveratrol inhibition of the enzyme. To explore this, we examined the effects of resveratrol on PKC-catalyzed phosphorylation of the cofactor-independent substrate protamine sulfate, which is a polybasic protein that activates PKC by a novel mechanism. Resveratrol potently inhibited protamine sulfate phosphorylation (IC(50) = 10 microM) by a mechanism that entailed antagonism of the activation of PKC by protamine sulfate and did not involve competition with either substrate. On the basis of the presence of PKC isozymes at subcellular sites rich in polybasic proteins, it has been proposed that certain endogenous polybasic PKC substrates may activate PKC in cells by the same mechanism as protamine sulfate. Our results suggest that antagonism by resveratrol of the phosphorylation of cellular PKC substrates that resemble protamine sulfate in their interactions with PKC may contribute to the efficacy of resveratrol against PKC in cells.     

Identification and functional characterization of protein kinase C isozymes in platelets and HEL cells.

To determine if selective activation of individual isozymes of protein kinase C (PKC) might explain the apparently divergent effects of PKC stimulation on platelets, we purified and characterized the isozymes from both platelets and human erythroleukemia (HEL) cells, a cell line that has many features of megakaryocytes. Two peaks of platelet PKC activity were resolved by hydroxylapatite chromatography; immunoblot analysis revealed that these two peaks represented the alpha and beta isozymes of PKC. In contrast, HEL cells produced only a single peak that contained the beta isozyme. None of the other PKC isozymes were detected in these fractions. The cytosol of platelets and HEL cells, however, were both found to contain the PKC-delta isozyme. Northern hybridization analyses and mRNA amplification by the polymerase chain reaction demonstrated the presence of mRNA encoding the alpha, beta, and delta PKC isozymes in platelets, but only the beta and delta isozymes in HEL cells. Phorbol myristate acetate (PMA), thrombin, or an endoperoxide analog induced the phosphorylation of the 47-kDa substrate of PKC (pleckstrin) found in platelets and HEL cells; preincubation of either HEL cells or platelets with PMA reduced the intracellular Ca2+ rise induced by thrombin. Thus, although both HEL cells and platelets contain PKC-beta and the recently described PKC-delta isozymes, the widely distributed alpha isozyme of PKC is absent in HEL cells; however, isozymes other than PKC-alpha are sufficient for some PMA-mediated functions that are similar to those seen in stimulated platelets.     

Identification of a novel alpha-amylase by expression of a newly cloned human amy3 cDNA in yeast

A novel amylase gene (amy3) that differs in nucleotide sequence from salivary amylase gene (amy1) and pancreatic amylase gene (amy2) has been described [Tomita et al., Gene 76 (1989) 11-18], but whether this gene can ever code for an active enzyme has not been shown. We prepared cDNA of this gene from an mRNA obtained from lung carcinoid tissue, and expressed it in Saccharomyces cerevisiae under the control of an acid phosphatase promoter. The product was secreted into culture media, and showed enzymatic activity, demonstrating that this novel alpha-amylase gene (amy3) can code for a functional isozyme. We purified this enzyme, and compared its biological properties with those of salivary and pancreatic human amylases similarly expressed in yeast. We observed that the novel amylase isozyme is more heat-sensitive than others, and that its substrate specificity is different from the other two isozymes.     

Baker's yeast as a tool for the development of antifungal kinase inhibitors--targeting protein kinase C and the cell integrity pathway

Today, the yeast Saccharomyces cerevisiae is probably the best-studied eukaryotic organism. This review first focuses on the signaling process which is mediated by the unique yeast protein kinase C (Pkc1p) and a downstream mitogen-activated protein kinase (MAPK) cascade. This pathway ensures cellular integrity by sensing cell surface stress and controlling cell wall biosynthesis and progression through the cell cycle. The domain structure of Pkc1p is conserved from yeast to humans. A yeast system for heterologous expression of specific domains in a chimeric yeast/mammalian PKC enzyme ("domain shuffling") is depicted. It is also proposed how this system could be employed for the study of protein kinase inhibitors in high-throughput screens. Moreover, a reporter assay that allows a quantitative readout of the activity of the cell integrity signaling pathway is introduced. Since a variety of protein kinases take part in the signal transduction, this broadens the range of targets for potential inhibitors.     

Oxidant-induced S-glutathiolation inactivates protein kinase C-alpha (PKC-alpha): a potential mechanism of PKC isozyme regulation

Protein kinase C (PKC) isozymes are subject to inactivation by reactive oxygen species (ROS) through as yet undefined oxidative modifications of the isozyme structure. We previously reported that Cys-containing, Arg-rich peptide-substrate analogues spontaneously form disulfide-linked complexes with PKC isozymes, resulting in isozyme inactivation. This suggested that PKC might be inactivated by oxidant-induced S-glutathiolation, i.e., disulfide linkage of the endogenous molecule glutathione (GSH) to PKC. Protein S-glutathiolation is a reversible oxidative modification that has profound effects on the activity of certain enzymes and binding proteins. To directly examine whether PKC could be inactivated by S-glutathiolation, we used the thiol-specific oxidant diamide because its oxidant activity is restricted to induction of disulfide bridge formation. Diamide weakly inactivated purified recombinant cPKC-alpha, and this was markedly potentiated to nearly full inactivation by 100 microM GSH, which by itself was without effect on cPKC-alpha activity. Diamide inactivation of cPKC-alpha and its potentiation by GSH were both fully reversed by DTT. Likewise, GSH markedly potentiated diamide inactivation of a PKC isozyme mixture purified from rat brain (alpha, beta, gamma, epsilon, zeta) in a DTT-reversible manner. GSH potentiation of diamide-induced cPKC-alpha inactivation was associated with S-glutathiolation of the isozyme. cPKC-alpha S-glutathiolation was demonstrated by the DTT-reversible incorporation of [(35)S]GSH into the isozyme structure and by an associated change in the migration position of cPKC-alpha in nonreducing SDS-PAGE. Diamide treatment of NIH3T3 cells likewise induced potent, DTT-reversible inactivation of cPKC-alpha in association with [(35)S] S-thiolation of the isozyme. Taken together, the results indicate that PKC isozymes can be oxidatively inactivated by S-thiolation reactions involving endogenous thiols such as GSH.     

Domain shuffling as a tool for investigation of protein function: substitution of the cysteine-rich region of Raf kinase and PKC eta for that of yeast Pkc1p

With the completion of the sequences of entire genomes, the need for functional characterisation of proteins and their domains is becoming acute. Conserved regions within proteins often share overlapping functions but despite this conservation may fulfil quite different tasks in different species. In this work, we investigated the cysteine-rich motif (C1 domain) of yeast protein kinase C (Pkc1p) as a model to establish a test system for domain function. C1 domains activate kinases through binding of either diacylglycerol and/or phosphatidylserine, as in many members of the protein kinase C (PKC) family, or by binding small GTPases, as in Raf kinase. In contrast to other members of the protein kinase C superfamily, Pkc1p of Saccharomyces cerevisiae is activated via binding of the small G-protein Rho1p to its C1 domain. We developed a system for domain shuffling to establish the function of C1 domains from human Raf kinase and rat PKC eta in yeast. Only the C1 domain from Raf kinase enabled the chimeric enzyme to bind Rho1p when substituted for the native yeast domain. Accordingly, a chimeric Pkc1p carrying the C1 from Raf kinase, but not that from PKC eta, was able to partially complement the phenotypes of a yeast pkc1 deletion mutant. We interpret these data as further evidence that interaction with a small GTPase is the main regulatory function of the C1 domain in yeast.     

Analysis of a novel gene and beta-galactosidase isozyme from a psychrotrophic Arthrobacter isolate.

We have characterized a new psychrotrophic Arthrobacter isolate which produces beta-galactosidase isozymes. When DNA from this isolate was transformed into an Escherichia coli host, we obtained three different fragments, designated 12, 14, and 15, each encoding a different beta-galactosidase isozyme. The beta-galactosidase produced from fragment 12 was of special interest because the protein subunit was smaller (about 71 versus 116 kDa) than those typically encoded by the lacZ family. The isozyme encoded by fragment 12 was purified, and its activity and thermostability were examined. Although the enzyme is highly specific towards beta-D-galactoside substrates, its levels in the isolate do not increase in cells grown with lactose. Nucleotide sequence determination showed that the gene encoding isozyme 12 is not similar to the other members of the lacZ family but has regions similar to beta-galactosidase isozymes from Bacillus stearothermophilus and B. circulans. Addition of the isozyme 12 sequence to the database made it possible to examine these enzymes as possible members of a new, separate family. Our analysis of this new family showed some conserved amino acids corresponding to the lacZ acid-base catalytic region but no homology with the nucleophilic region. On the basis of these comparisons, we designated this a new lacG family.     

Studies with transfected and permeabilized RBL-2H3 cells reveal unique inhibitory properties of protein kinase C gamma.

To characterize protein kinase C (PKC) gamma, an isozyme found exclusively in brain and spinal cord, its cDNA was introduced into basophilic RBL-2H3 cells that lack this isozyme. The expression of PKC gamma significantly attenuated antigen-induced responses including hydrolysis of inositol phospholipids, increase in cytosolic calcium, and secretion of granules but enhanced antigen-induced release of arachidonic acid. Instead of a sustained increase in cytosolic calcium, antigen now induced calcium oscillations; possibly as a consequence of suppression of the phospholipase C activity and incomplete emptying of internal calcium stores. In addition, PKC gamma appeared to inhibit activation of other PKC isozymes because phorbol 12-myristate 13-acetate failed to act synergistically with the Ca(2+)-ionophore on secretion. This was confirmed in other studies where PKC gamma was shown to suppress the transduction of stimulatory signals by other isozymes of PKC on provision of these isozymes to PKC-depleted permeabilized cells. The studies in total indicated that only PKC gamma was capable of inhibiting both early and distal signals for secretion including those signals transduced by endogenous isozymes of PKC.     

Use of a novel method to find substrates of protein kinase C delta identifies M2 pyruvate kinase

Protein kinase C (PKC) family members have been implicated in numerous cellular processes. However, identifying the substrates of each PKC isozyme remains a challenge. Here, we describe a method using two-dimensional (2D) isoelectric focusing gel electrophoresis to identify substrates of delta PKC (deltaPKC) in MCF-7 breast carcinoma cells. We show that M2 pyruvate kinase is a substrate of deltaPKC, and further characterize the interaction between M2 pyruvate kinase and deltaPKC in MCF-7 cells by immunoprecipitation. deltaPKC activation in vitro or in cells did not appear to alter the enzyme activity or polymerization of M2 pyruvate kinase.     

The identification of a functional interaction between PKC and topoisomerase II

Topoisomerase II plays an essential role in the segregation of chromosomes during cell division. It is also a major component of the nuclear matrix. Proteins that interact with and regulate this essential enzyme are of great interest. To investigate the role of proteins interacting with the N-terminal domain of the Saccharomyces cerevisiae topoisomerase II, we used a yeast two-hybrid protein interaction screen. We identified an interaction between the catalytic domain of the yeast protein kinase 1 enzyme (Pkc1) and the N-terminal domain of the S. cerevisiae topoisomerase II. The S. cerevisiae Pkc1 is the homologue of the mammalian calcium dependent PKC.