The coatomer protein beta'-COP, a selective binding protein (RACK) for protein kinase Cepsilon

At the crossroads of myocardial signaling: the role of Z-discs in intracellular signaling and cardiac function

Understanding the molecular interactions among components of cardiac Z-discs and their role in signaling has become pivotal in explaining long- and short-term regulation of cardiac function. In striated muscle, the ends of the thin filaments from opposing sarcomeres overlap and are cross-linked by an elaborate array of proteins to form a highly ordered, yet dynamic network that is the Z-disc. We review here a current picture of the function and structure of the Z-disc of mammalian cardiac myocytes. We emphasize provocative findings that advance new theories about the place of cardiac Z-discs in myocardial intra- and intercellular signaling in myocardial physiology and pathology. Relatively new approaches, especially yeast two-hybrid screens, immunoprecipitation, and pull down assays, as well as immunohistochemical analysis have significantly altered previous views of the protein content of the Z-disc. These studies have generally defined domain structure and binding partners for Z-disc proteins, but the functional significance of the binding network and of the domains in cardiac cell biology remains an unfolding story. Yet, even at the present level of understanding, perceptions of potential functions of the Z-disc proteins are expanding greatly and leading to new and exciting experimental approaches toward mechanistic understanding. The theme of the following discussion of these Z-disc proteins centers on their potential to function not only as a physical anchor for myofilament and cytoskeletal proteins, but also as a pivot for reception, transduction, and transmission of mechanical and biochemical signals.

Myofilament anchoring of protein kinase C-epsilon in cardiac myocytes

Regulatory proteins on muscle filaments are substrates for protein kinase C (PKC) but mechanisms underlying activation and translocation of PKC to this non-membrane compartment are poorly understood. Here we demonstrate that the epsilon isoform of PKC (ϵ-PKC) activated by arachidonic acid (AA) binds reversibly to cardiac myofibrils with an EC50 of 86 nM. Binding occurred near the Z-lines giving rise to a striated staining pattern. The delta isoform of PKC (δ-PKC) did not bind to cardiac myofibrils regardless of the activator used, and the alpha isoform (α-PKC) bound only under strong activating conditions. Three established PKC anchoring proteins, filamentous actin (F-actin), the LIM domain protein Cypher-1, and the coatamer protein β′-COP were each tested for their involvement in cytoskeletal anchoring. F-actin bound ϵ-PKC selectively over δ-PKC and α-PKC, but this interaction was readily distinguishable from cardiac myofilament binding in two ways. First, the F-actin/ϵ-PKC interaction was independent of PKC activation, and second, the synthetic hexapeptide LKKQET derived from the C1 region of ϵ-PKC effectively blocked ϵ-PKC binding to F-actin, but was without effect on its binding to cardiac myofilaments. Involvement of Cypher-1 was ruled out on the basis of its absence from detergent-skinned myofibrils that bound ϵ-PKC, despite its presence in intact cardiac myocytes. The ϵ-PKC translocation inhibitor peptide EAVSLKPT reduced activated ϵ-PKC binding to cardiac myofibrils in a concentration dependent manner, suggesting that a RACK2 or a similar protein plays a role in ϵ-PKC anchoring in cardiac myofilaments.

A Critical Intramolecular Interaction for Protein Kinase Cϵ Translocation

Disruption of intramolecular interactions, translocation from one intracellular compartment to another, and binding to isozyme-specific anchoring proteins termed RACKs, accompany protein kinase C (PKC) activation. We hypothesized that in inactive epsilonPKC, the RACK-binding site is engaged in an intramolecular interaction with a sequence resembling its RACK, termed psiepsilonRACK. An amino acid difference between the psiepsilonRACK sequence in epsilonPKC and its homologous sequence in epsilonRACK constitutes a change from a polar non-charged amino acid (asparagine) in epsilonRACK to a polar charged amino acid (aspartate) in epsilonPKC. Here we show that mutating the aspartate to asparagine in epsilonPKC increased intramolecular interaction as indicated by increased resistance to proteolysis, and slower hormone- or PMA-induced translocation in cells. Substituting aspartate for a non-polar amino acid (alanine) resulted in binding to epsilonRACK without activators, in vitro, and increased translocation rate upon activation in cells. Mathematical modeling suggests that translocation is at least a two-step process. Together our data suggest that intramolecular interaction between the psiepsilonRACK site and RACK-binding site within epsilonPKC is critical and rate limiting in the process of PKC translocation.

alpha3beta1 integrin-CD151, a component of the cadherin-catenin complex, regulates PTPmu expression and cell-cell adhesion

The beta1 family of integrins has been primarily studied as a set of receptors for the extracellular matrix. In this paper, we define a novel role for alpha3beta1 integrin in association with the tetraspanin CD151 as a component of a cell-cell adhesion complex in epithelial cells that directly stimulates cadherin-mediated adhesion. The integrin-tetraspanin complex affects epithelial cell-cell adhesion at the level of gene expression both by regulating expression of PTPmu and by organizing a multimolecular complex containing PKCbetaII, RACK1, PTPmu, beta-catenin, and E-cadherin. These findings demonstrate how integrin-based signaling can regulate complex biological responses at multiple levels to determine cell morphology and behavior.

Protein kinase C epsilon and gamma: involvement in formalin-induced nociception in neonatal rats

The central nervous system undergoes dynamic changes as it matures. However, until recently, very little was known about the impact of these changes on pain and analgesia. This study tested the hypothesis that the epsilon and gamma isozymes of protein kinase C (PKC) contribute to formalin-induced nociception in an age-dependent manner. Expression of epsilon and gamma PKC and the contributions of these isozymes in formalin-induced nociception was examined in postnatal day 7, 15, and 21 rats. epsilonPKC expression in dorsal root ganglion neurons and gammaPKC expression in lamina II of the spinal cord increased from the first to the third postnatal week. Coupling immunohistochemical and Western analysis, translocation of epsilonPKC followed intraplantar formalin in all ages. In contrast, formalin-induced gammaPKC translocation was observed only in postnatal day 21 rats. Behaviorally, intrathecal administration of the epsilonPKC-specific inhibitor (epsilonV1-2) attenuated phase 1 and phase 2 formalin behaviors at all ages. In contrast, intrathecal administration of the gammaPKC-specific inhibitor (gammaV5-3) attenuated only phase 2 responses in postnatal day 15 and 21 rats. Functionally, inhibition of epsilonPKC decreased capsaicin-stimulated release of glutamate and calcitonin gene-related peptide in spinal cords isolated from postnatal day 7 rats. These results suggest that epsilonPKC age independently mediates inflammatory pain produced by intraplantar formalin. In contrast, gammaPKC contributes to formalin-induced nociception in an age-dependent manner. Identifying the molecular mechanisms responsible for age-specific patterns of nociception is necessary for the rational development of novel therapeutic strategies for treating pediatric pain.

State-specific monoclonal antibodies identify an intermediate state in epsilon protein kinase C activation

Evaluation of the activation state of protein kinase C (PKC) isozymes relies on analysis of subcellular translocation. A monoclonal antibody, 14E6, specific for the activated conformation of epsilonPKC, was raised using the first variable (V1) domain of epsilonPKC as the immunogen. 14E6 binding is specific for epsilonPKC and is greatly increased in the presence of PKC activators. Immunofluorescence staining by 14E6 of neonatal rat primary cardiac myocytes and the NG108-15 neuroblastoma glioma cell line, NG108-15/D2, increases rapidly following cell activation and is localized to new subcellular sites. However, staining of translocated epsilonPKC with 14E6 is transient, and the epitope disappears 30 min after activation of NG-108/15 cells by a D2 receptor agonist. In contrast, subcellular localization associated with activation, as determined by commercially available polyclonal antibodies, persists for at least 30 min. In vitro, epsilonRACK, the receptor for activated epsilonPKC, inhibits 14E6 binding to epsilonPKC, suggesting that the 14E6 epitope is lost or hidden when active epsilonPKC binds to its RACK. Therefore, the 14E6 antibody appears to identify a transient state of activated but non-anchored epsilonPKC. Moreover, binding of 14E6 to epsilonPKC only after activation suggests that lipid-dependent conformational changes associated with epsilonPKC activation precede binding of the activated isozyme to its specific RACK, epsilonRACK. Further, monoclonal antibody 14E6 should be a powerful tool to study the pathways that control rapid translocation of epsilonPKC from cytosolic to membrane localization on activation.

Differential effects of prenyl pyrophosphates on the phosphatase activity of phosphotyrosyl protein phosphatase

Phosphotyrosyl protein phosphatase (PTPase) 1B was purified from human placenta. Immunoprecipitation analysis revealed that the isolated PTPase 1B appears as a complex with the receptor for protein kinase C (RACK1) and protein kinase C (PKC)delta. The abilities of PTPase 1B and PKCdelta to associate with RACK1 were reconfirmed by an in vitro reconstitution experiment. The E. coli expressed and biotinylated mice-RACK1-encoded fusion protein was capable of recruiting PTPase 1B and PKCdelta in the antibiotin immunoprecipitate as a complex of PTPase 1B/RACK1/PKCdelta. Thus PTPase 1B enzyme preparation was subjected to further purification by selective binding of PTPase 1B onto PEP(Taxol) affinity column in the absence of ATP. The purified PTPase 1B enzyme exihibited dose-dependent phosphatase activity towards [gamma-(32)P]-ATP labeled mice beta-tubulin-encoded fusion protein. The dephosphorylation reaction with PTPase 1B was enhanced with geranylgeranyl pyrophosphate, but not with farnesyl pyrophosphate. Interestingly, additional incubation of the purified PTPase 1B enzyme preparation with RACK1, geranylgeranyl pyrophosphate failed to modulate the dephosphorylation activity of PTPase 1B. In contrast, the enhancement effect of farnesyl pyrophosphate on the kinase activity of PKCdelta was sustained in the presence of RACK1. That is, farnesyl pyrophosphate may function as a signal to induce the kinase activity of PKCdelta in PTPase 1B/RACK1/PKCdelta complex but geranylgeranyl pyrophosphate may not for PTPase 1B. J. Exp. Zool. 301A:307-316, 2004.

Ki-1/57 interacts with RACK1 and is a substrate for the phosphorylation by phorbol 12-myristate 13-acetate-activated protein kinase C

Ki-1/57, the 57-kDa human protein antigen recognized by the CD30 antibody Ki-1, is a cytoplasmic and nuclear protein that is phosphorylated on serine and threonine residues. When isolated from the Hodgkin's lymphoma analogous cell line L540 Ki-1/57 co-immunoprecipitated with a Thr/Ser protein kinase activity. It has been also found to interact with hyaluronic acid and has therefore been termed intracellular IHABP4 (hyaluronan-binding protein 4). Recent studies demonstrated, however, that Ki-1/57 engages in specific interaction with the chromo-helicase-DNA-binding domain protein 3, a nuclear protein involved in chromatin remodeling and transcription regulation. We used the yeast two-hybrid system to find proteins interacting with Ki-1/57 and identified the adaptor protein RACK1 (receptor of activated kinase 1). Next, we confirmed this interaction in vitro and in vivo, performed detailed mapping studies of the interaction sites of Ki-1/57 and RACK-1, and demonstrated that Ki-1/57 also co-precipitates with protein kinase C (PKC) when isolated from phorbol 12-myristate 13-acetate (PMA)-activated L540 tumor cells and is a substrate for PKC phosphorylation in vitro and in vivo. Interestingly, the interaction of Ki-1/57 with RACK1 is abolished upon activation of L540 cells with PMA, which results in the phosphorylation of Ki-1/57 and its exit from the nucleus. Taken together, our data suggest that Ki-1/57 forms a stable complex with RACK-1 in unstimulated cells and upon PMA stimulation gets phosphorylated on threonine residues located at its extreme C terminus. These events associate Ki-1/57 with the RACK1/PKC pathway and may be important for the regulation of its cellular functions.

The simultaneous production of phosphatidic acid and diacylglycerol is essential for the translocation of protein kinase Cepsilon to the plasma membrane in RBL-2H3 cells

To evaluate the role of the C2 domain in protein kinase Cepsilon (PKCepsilon) localization and activation after stimulation of the IgE receptor in RBL-2H3 cells, we used a series of mutants located in the phospholipid binding region of the enzyme. The results obtained suggest that the interaction of the C2 domain with the phospholipids in the plasma membrane is essential for anchoring the enzyme in this cellular compartment. Furthermore, the use of specific inhibitors of the different pathways that generate both diacylglycerol and phosphatidic acid has shown that the phosphatidic acid generated via phospholipase D (PLD)-dependent pathway, in addition to the diacylglycerol generated via phosphoinosite-phospholipase C (PLC), are involved in the localization of PKCepsilon in the plasma membrane. Direct stimulation of RBL-2H3 cells with very low concentrations of permeable phosphatidic acid and diacylglycerol exerted a synergistic effect on the plasma membrane localization of PKCepsilon. Moreover, the in vitro kinase assays showed that both phosphatidic acid and diacylglycerol are essential for enzyme activation. Together, these results demonstrate that phosphatidic acid is an important and essential activator of PKCepsilon through the C2 domain and locate this isoenzyme in a new scenario where it acts as a downstream target of PLD.

Localization of functional endothelin receptor signaling complexes in cardiac transverse tubules

Endothelin-1 (ET-1) is an autocrine factor in the mammalian heart important in enhancing cardiac performance, protecting against myocardial ischemia, and initiating the development of cardiac hypertrophy. The ETA receptor is a seven-transmembrane G-protein-coupled receptor whose precise subcellular localization in cardiac muscle is unknown. Here we used fluorescein ET-1 and 125I-ET-1 to provide evidence for ET-1 receptors in cardiac transverse tubules (T-tubules). Moreover, the ETA receptor and downstream effector phospholipase C-beta 1 were co-localized within T-tubules using standard immunofluorescence techniques, and protein kinase C (PKC)-epsilon-enhanced green fluorescent protein bound reversibly to T-tubules upon activation. Localized photorelease of diacylglycerol further suggested compartmentation of PKC signaling, with release at the myocyte "surface" mimicking the negative inotropic effects of bath-applied PKC activators and "deep" release mimicking the positive inotropic effect of ET-1. The functional significance of T-tubular ET-1 receptors was further tested by rendering the T-tubule lumen inaccessible to bath-applied ET-1. Such "detubulated" cardiac myocytes showed no positive inotropic response to 20 nM ET-1, despite retaining both a nearly normal twitch response to field stimulation and a robust positive inotropic response to 20 nm isoproterenol. We propose that ET-1 enhances myocyte contractility by activating ETA receptor-phospholipase C-beta 1-PKC-epsilon signaling complexes preferentially localized in cardiac T-tubules. Compartmentation of ET-1 signaling complexes may explain the discordant effects of ET-1 versus bath applied PKC activators and may contribute to both the specificity and diversity of the cardiac actions of ET-1.

Hyperalgesic priming in the rat demonstrates marked sexual dimorphism

In male rats, carrageenan (CAR)-induced inflammation or exposure to a selective protein kinase C epsilon (PKC epsilon ) agonist (psi epsilon RACK) produces prolongation of the hyperalgesia induced by a subsequent exposure to an inflammatory mediator, a phenomenon referred to as hyperalgesic priming. Since many chronic inflammatory conditions are sexually dimorphic, we tested the hypothesis that hyperalgesic priming is sexually dimorphic. Prior injection of CAR or psi epsilon RACK produced a prolongation of the hyperalgesia induced by a subsequent injection of prostaglandin E(2), from less than 3 h to greater than 24 h, but only in male rats. In ovariectomized female rats priming with CAR and psi epsilon RACK produced hyperalgesic priming effects similar to that observed in the male rat, and this effect was reversed by estrogen replacement. While gonadectomy in males had no effect on CAR and psi epsilon RACK induced hyperalgesic priming, female phenotype was observed following implantation of estrogen in males. Thus, mechanisms mediating the development of hyperalgesic priming produced by inflammation are suppressed by estrogen. This regulation of priming by estrogen appears to occur at or downstream of the activation of PKC epsilon.

PICK1, an anchoring protein that specifically targets protein kinase Calpha to mitochondria selectively upon serum stimulation in NIH 3T3 cells

PICK1 binds to protein kinase Calpha (PKCalpha) through the carboxylate-binding loop in its PDZ (PSD95/Disc-large/ZO-1) domain and the C terminus of PKCalpha. We have previously shown that PICK1 modulates the catalytic activity of PKC selectively toward the antiproliferative gene TIS21. To investigate whether PICK1 plays a role in targeting activated PKCalpha to a particular intracellular compartment in addition to regulating PKC activity, we examine the localization of PICK1 and PKCalpha in response to various stimuli. Double staining with organelle markers and anti-rPICK1 antibodies reveals that PICK1 is associated with mitochondria but not with endoplasmic reticulum or Golgi in NIH 3T3 cells. Deletion of the PDZ domain impairs the mitochondria localization of PICK1, whereas mutations in the carboxylate-binding loop do not have an effect, suggesting that PICK1 can bind PKCalpha and mitochondria simultaneously. Upon serum stimulation, PICK1 translocates and displays a dense ring-like structure around the nucleus, where it still associates with mitochondria. A substantial portion of PKCalpha is concomitantly found in the condense perinuclear region. The C terminal-deleted PKCalpha fails to translocate and remains a diffuse cytoplasmic distribution, indicating that a direct interaction between PICK1 and PKCalpha is required for PKCalpha anchoring to mitochondria. 12-O-Tetradecanoylphorbol-13-acetate stimulation, in contrast, causes translocation of PKCalpha to the plasma membrane, whereas the majority of PICK1 remains in a cytoplasmic punctate pattern. Deletion at the C terminus of PKCalpha has no effect on 12-O-tetradecanoylphorbol-13-acetate-induced translocation. These findings indicate a previously unidentified role for PICK1 in anchoring PKCalpha to mitochondria in a ligand-specific manner.

Farnesyl pyrophosphate promotes and is essential for the binding of RACK1 with beta-tubulin

Receptors for activated C kinase (RACKs) are a group of protein kinase C (PKC) binding proteins that have been shown to be crucial in the translocation and subsequent functioning of PKC on activation. RACK1 isolated from BALB/3T3 cells transformed with S-ras(Q61K) exhibits receptor activity for PKCgamma as competent as that of RACK1 from BALB/3T3 cells without transformation. However, the ability of RACK1 from transformed cells to bind with beta-tubulin peptide specific for Taxol (PEPtaxol) is defective. Interestingly, when farnesyl pyrophosphate was added at the submicrogram level, the association between RACK1 and PEPtaxol was enhanced significantly in a dosage-dependent manner. A parallel finding for the enhanced effect of farnesyl pyrophosphate on tubulin binding was established with mice RACK1 expressed in vitro. On the other hand, geranylgeranyl pyrophosphate, and retinoic acid failed to modulate the binding between RACK1 and tubulin. The dissociation of RACK1 and tubulin was not effective at damaging the binding between RACK1 and membrane receptor integrin beta1 in transformed cells. These findings indicate that depletion of farnesyl pyrophosphate provides a mechanism to seal PKC signaling on the membrane with immobile RACK1 and to divert cells to aberrant growth, such as transformation.

Human malaria parasites display a receptor for activated C kinase ortholog

Receptors for activated C kinases (RACKs) are scaffold proteins that anchor diverse signaling proteins and are involved in modulating cell cycle. We report the cloning and cellular localization of a RACK ortholog (PfRACK) in the human malaria parasite Plasmodium falciparum. The full-length transcript obtained by 3(') and 5(') RACE has 1.4 kbp with a predicted ORF of 972 bp, coding for a protein with 323 residues of 35.8 kDa molecular weight and pI 6.38. PfRACK has 59% and 60% identity at the amino acid level to Chlamydomonas reinhardtii and Danio rerio RACKs, respectively, presenting seven WD40 motifs and retaining the conserved domains in repeats III (DVFSVSF) and VI (STINSLCF) that are important for PKC binding. Semi-quantitative RT-PCR revealed that PfRACK is constitutively expressed in the intraerythrocytic stages of P. falciparum. Using confocal microscopy, PfRACK was immunolocalized in all parasite stages, being conspicuously spread throughout the schizont. The high similarity of PfRACK to those previously described in other organisms, as well as its constitutive expression in Plasmodium asexual stages, suggests that it might play a key role in the regulatory processes of malaria parasite life cycle.

Direct binding of syndecan-4 cytoplasmic domain to the catalytic domain of protein kinase C alpha (PKC alpha) increases focal adhesion localization of PKC alpha

Syndecan-4 is a transmembrane heparan sulfate proteoglycan that acts as a coreceptor with integrins in focal adhesion formation. The central region of syndecan-4 cytoplasmic domain (4V; LGKKPIYKK) binds phosphatidylinositol 4,5-bisphosphate, and together they regulate protein kinase C alpha (PKC alpha) activity. Syndecan 4V peptide directly potentiates PKC alpha activity, leading to "superactivation" of the enzyme, apparently through an interaction with its catalytic domain. We now have performed yeast two-hybrid and in vitro binding assays to determine the interaction sites between 4V and PKC alpha. Full-length PKC alpha weakly interacted with 4V by yeast two-hybrid assays, but PKC alpha constructs that lack the pseudosubstrate region or constructs of the whole catalytic domain interacted more strongly. A mutated 4V sequence (4V(YF): LGKKPIFKK) did not interact with PKC alpha, indicating that tyrosine 192 in the syndecan-4 cytoplasmic domain might be critical for this interaction. Further assays identified a novel interaction site in the C terminus of the catalytic domain of PKC alpha (amino acid sequence 513-672). This encompasses the autophosphorylation sites, which are implicated in activation and stability. Yeast two-hybrid data were confirmed by in vitro binding and coimmunoprecipitation assays. The interaction of syndecan-4 with PKC alpha appears unique since PKC delta and epsilon did not interact with 4V in yeast two-hybrid assays or coimmunoprecipitate with syndecan-4. Finally, overexpression of syndecan-4 in rat embryo fibroblast cells, but not expression of the YF mutant, increased PKC alpha localization to focal adhesions. The data support a mechanism where syndecan-4 binds PKC alpha and localizes it to focal adhesions, whose assembly may be regulated by the kinase.

Ethanol differentially enhances hippocampal GABA A receptor-mediated responses in protein kinase C gamma (PKC gamma) and PKC epsilon null mice

Ethanol intoxication results partly from actions of ethanol at specific ligand-gated ion channels. One such channel is the GABA(A) receptor complex, although ethanol's effects on GABA(A) receptors are variable. For example, we found that hippocampal neurons from selectively bred mice and rats with high hypnotic sensitivity to ethanol have increased GABA(A) receptor-mediated synaptic responses during acute ethanol treatment compared with mice and rats that display low behavioral sensitivity to ethanol. Here we investigate whether specific protein kinase C (PKC) isozymes modulate hypnotic and GABA(A) receptor sensitivity to ethanol. We examined acute effects of ethanol on GABA(A) receptor-mediated inhibitory postsynaptic currents (IPSCs) in mice lacking either PKCgamma (PKCgamma(-/-)) or PKCepsilon (PKCepsilon(-/-)) isozymes and compared the results to those from corresponding wild-type littermates (PKCgamma(+/+) and PKCepsilon(+/+)). GABA(A) receptor-mediated IPSCs were evoked in CA1 pyramidal neurons by electrical stimulation in stratum pyramidale, and the responses were recorded in voltage-clamp mode using whole-cell patch recording techniques. Ethanol (80 mM) enhanced the IPSC response amplitude and area in PKCgamma(+/+) mice, but not in the PKCgamma(-/-) mice. In contrast, ethanol markedly potentiated IPSCs in the PKCepsilon(-/-) mice, but not in PKCepsilon(+/+) littermates. There was a positive correlation between ethanol potentiation of IPSCs and the ethanol-induced loss of righting reflex such that mice with larger ethanol-induced increases in GABA(A) receptor-mediated IPSCs also had higher hypnotic sensitivity to ethanol. These results suggest that PKCgamma and PKCepsilon signaling pathways reciprocally modulate both ethanol enhancement of GABA(A) receptor function and hypnotic sensitivity to ethanol.

Protein kinase C-epsilon-null mice have decreased hypoxic pulmonary vasoconstriction

PKC contributes to regulation of pulmonary vascular reactivity in response to hypoxia. The role of individual PKC isozymes is less clear. We used a knockout (null, -/-) mouse to test the hypothesis that PKC-epsilon is important in acute hypoxic pulmonary vasoconstriction (HPV). We asked whether deletion of PKC-epsilon would decrease acute HPV in adult C57BL6xSV129 mice. In isolated, salt solution-perfused lung, reactivity to acute hypoxic challenges (0% and 3% O(2)) was compared with responses to angiotensin II (ANG II) and KCl. PKC-epsilon -/- mice had decreased HPV, whereas responses to ANG II and KCl were preserved. Inhibition of nitric oxide synthase (NOS) with nitro-l-arginine augmented HPV in PKC-epsilon +/+ but not -/- mice. Inhibition of Ca(2+)-gated K(+) channels (K(Ca)) with charybdotoxin and apamin did not enhance HPV in -/- mice relative to wild-type (+/+) controls. In contrast, the voltage-gated K(+) channel (K(V)) antagonist 4-aminopyridine increased the response of -/- mice beyond that of +/+ mice. This suggested that increased K(V) channel expression could contribute to blunted HPV in PKC-epsilon -/- mice. Therefore, expression of the O(2)-sensitive K(V) channel subunit Kv3.1b (100-kDa glycosylated form and 70-kDa core protein) was compared in whole lung and pulmonary artery smooth muscle cell (PASMC) lysates from +/+ and -/- mice. A subtle increase in Kv3.1b was detected in -/- vs. +/+ whole lung lysates. A much greater rise in Kv3.1b expression was found in -/- vs. +/+ PASMC. Thus deletion of PKC-epsilon blunts murine HPV. The decreased response could not be attributed to a general loss in vasoreactivity or derangements in NOS or K(Ca) channel activity. Instead, the absence of PKC-epsilon allows increased expression of K(V) channels (like Kv3.1b) to occur in PASMC, which likely contributes to decreased HPV.

Cloning, expression and genomic structure of a novel human GNB2L1 gene, which encodes a receptor of activated protein kinase C (RACK)

During a large-scale screen of a human fetal brain cDNA library, a novel human gene GNB2L1 encoding a novel RACK (receptor of activated protein kinase C) protein was isolated and sequenced. The cDNA is 1142 bp long and has a predicted open reading frame encoding 316 aa. The predicted protein shows higher similarity to rat RACK1 and many RACK proteins of different organisms including Drosophila, C. elegans, mouse, rat, human, C. fasciculata, zebrafish, A. thaliana, S. cerevisiae and so on, suggesting it is conserved during evolution. The gene was mapped to human chromosome 5q35.3, the telomer position of chromosome 5q, in which the disease gene for early-onset primary congenital lymphedema was mapped. Also, 5q35.3 is a frequently reported location for cytogenetic and molecular abnormalities in renal cell carcinomas. The gene has 8 exons and 7 introns. It is expressed ubiquitously in many human tissues detected by northern blot analysis and RT-PCR.

Stability of actin cytoskeleton and PKC-delta binding to actin regulate NKCC1 function in airway epithelial cells

Activation of airway epithelial Na-K-2Cl cotransporter (NKCC)1 requires increased activity of protein kinase C (PKC)-delta, which localizes predominantly to the actin cytoskeleton. Prompted by reports of a role for actin in NKCC1 function, we studied a signaling mechanism linking NKCC1 and PKC. Stabilization of actin polymerization with jasplakinolide increased activity of NKCC1, whereas inhibition of actin polymerization with latrunculin B prevented hormonal activation of NKCC1. Protein-protein interactions among NKCC1, actin, and PKC-delta were verified by Western blot analysis of immunoprecipitated proteins. PKC-delta was detected in immunoprecipitates of NKCC1 and vice versa. Actin was also detected in immunoprecipitates of NKCC1 and PKC-delta. Pulldown of endogenous actin revealed the presence of NKCC1 and PKC-delta. Binding of recombinant PKC-delta to NKCC1 was not detected in overlay assays. Rather, activated PKC-delta bound to actin, and this interaction was prevented by a peptide encoding deltaC2, a C2-like domain based on the amino acid sequence of PKC-delta. deltaC2 also blocked stimulation of NKCC1 function by methoxamine. Immunofluorescence and confocal microscopy revealed PKC-delta in the cytosol and cell periphery. Merged images of cells stained for actin and PKC-delta indicated colocalization of PKC-delta and actin at the cell periphery. The results indicate that actin is critical for the activation of NKCC1 through a direct interaction with PKC-delta.

Regulation of membrane-bound PKC in adult cardiac ventricular myocytes

Activation of protein kinase C (PKC) is thought to involve translocation to the particulate fraction. The present study demonstrates a membrane-associated, inactive pool of PKC in adult rat ventricular myocytes. Membranes were isolated from stimulated (phorbol 12-myristate 13-acetate (PMA), endothelin-1 (ET-1)) or control myocytes and PKC activity determined in the absence (active PKC) or presence (total PKC) of PMA. An inactive, PMA-responsive, pool of PKC was detected. In intact myocytes, PMA or ET-1 induced a translocation of PKC epsilon from the cytosol into the particulate fraction. In contrast, ET-1 decreased both total and active PKC in the membranes: this decrease was associated with a loss of PKC epsilon immunoreactivity. PMA increased the amount of membrane-associated, inactive PKC. Our results demonstrate the presence of a membrane-associated pool of PKC in cardiac myocytes that is differentially modulated by ET-1 or PMA.

Effects of prenyl pyrophosphates on the binding of PKCgamma with RACK1

Receptors for activated C kinase (RACKs) are a group of PKC binding proteins that have been shown to mediate isoform-selective functions of PKC and to be crucial in the translocation and subsequent functioning of the PKC isoenzymes on activation. RACK1 cDNA from the shrimp Penaeus japonicus was isolated by homology cloning. The hepatopancreas cDNA from this shrimp was found to encode a 318-residue polypeptide whose predicted amino acid sequence shared 91% homology with human G(beta2)-like proteins. Expression of the cDNA of shrimp RACK1 in vitro yielded a 45-kDa polypeptide with positive reactivity toward the monoclonal antibodies against RACK1 of mammals. The shrimp RACK1 was biotinylated and used to compare the effects of geranylgeranyl pyrophosphate and farnesyl pyrophosphate on its binding with PKCgamma in anti-biotin-IgG precipitates. PKCgammas were isolated from shrimp eyes and mouse brains. Both enzyme preparations were able to inhibit taxol-induced tubulin polymerization. Interestingly, when either geranylgeranyl pyrophosphate or farnesyl pyrophosphate was reduced to the submicrogram level, the recruitment activity of RACK1 with purified PKCgamma was found to increase dramatically. The activation is especially significant for RACK1 and PKCgamma from different species. The observation implies that the deprivation of prenyl pyrophosphate might function as a signal for RACK1 to switch the binding from the conventional isoenzymes of PKC (cPKC) to the novel isoenzymes of PKC (nPKC). A hydrophobic binding pocket for geranylgeranyl pyrophosphate in RACK1 is further revealed via prenylation with protein geranylgeranyl transferase I of shrimp P. japonicus.

A role for PKC-epsilon in Fc gammaR-mediated phagocytosis by RAW 264.7 cells

Protein kinase C (PKC) plays a prominent role in immune signaling, and the paradigms for isoform selective signaling are beginning to be elucidated. Real-time microscopy was combined with molecular and biochemical approaches to demonstrate a role for PKC- epsilon in Fc gamma receptor (Fc gammaR)-dependent phagocytosis. RAW 264.7 macrophages were transfected with GFP-conjugated PKC isoforms, and GFP movement was followed during phagocytosis of fluorescent IgG-opsonized beads. PKC- epsilon, but not PKC-delta, concentrated around the beads. PKC- epsilon accumulation was transient; apparent as a "flash" on target ingestion. Similarly, endogenous PKC- epsilon was specifically recruited to the nascent phagosomes in a time-dependent manner. Overexpression of PKC- epsilon, but not PKC-alpha, PKC-delta, or PKC-gamma enhanced bead uptake 1.8-fold. Additionally, the rate of phagocytosis in GFP PKC- epsilon expressors was twice that of cells expressing GFP PKC-delta. Expression of the regulatory domain ( epsilon RD) and the first variable region ( epsilon V1) of PKC- epsilon inhibited uptake, whereas the corresponding PKC-delta region had no effect. Actin polymerization was enhanced on expression of GFP PKC- epsilon and epsilon RD, but decreased in cells expressing epsilon V1, suggesting that the epsilon RD and epsilon V1 inhibition of phagocytosis is not due to effects on actin polymerization. These results demonstrate a role for PKC- epsilon in Fc gammaR-mediated phagocytosis that is independent of its effects on actin assembly.

Identification of PSF as a protein kinase Calpha-binding protein in the cell nucleus

Protein kinase C (PKC) isoforms are present in the cell nucleus in diverse cell lines and tissues. Since little is known about proteins interacting with PKC inside the cell nucleus, we used Neuro-2a neuroblastoma cells, in which PKCalpha is present in the nucleus, to screen for nuclear binding partners for PKC. Applying overlay assays, we detected several nuclear proteins which bind to PKCalpha. Specificity of binding was shown by its dependence on PKC activation by phorbol ester, calcium, and phosphatidylserine. The PKC-binding proteins were partially purified and analyzed by microsequencing and mass spectrometry. Four proteins could be identified: PTB-associated splicing factor (PSF), p68 RNA helicase, and the heterogeneous nuclear ribonucleoprotein (hnRNP) proteins A3 and L. In the case of PSF, binding to PKC could also be demonstrated in a GST-pull-down assay using GST-PKCalpha, expressed in insect cells. Phosphorylation experiments revealed that PSF is a weak in vitro substrate for PKCalpha.

Myelin biogenesis: vesicle transport in oligodendrocytes

Intracellular trafficking of membranes plays an essential role in the biogenesis and maintenance of myelin. The requisite proteins and lipids are transported from their sites of synthesis to myelin via vesicles. Vesicle transport is tightly coordinated with synthesis of lipids and proteins. To maintain the structural and functional organization of oligodendrocytes it is essential synchronize the various pathways of vesicle transport and to coordinate vesicle transport with reorganization of cytoskeleton. The systems that regulate the targeting of protein to myelin by vesicle transport are now being described. Here we review the current knowledge of these systems including those involved in (a) protein folding, (b) protein sorting and formation of carrier vesicles, (c) vesicle transport along elements of the cytoskeleton, and (d) vesicle targeting/fusion.

Opposing effects of PKCalpha and PKCepsilon on basolateral membrane dynamics in intestinal epithelia

PKC is a critical effector of plasma membrane dynamics, yet the mechanism and isoform-specific role of PKC are poorly understood. We recently showed that the phorbol ester PMA (100 nM) induces prompt activation of the novel isoform PKCepsilon followed by late activation of the conventional isoform PKCalpha in T84 intestinal epithelia. PMA also elicited biphasic effects on endocytosis, characterized by an initial stimulatory phase followed by an inhibitory phase. Activation of PKCepsilon was shown to be responsible for stimulation of basolateral endocytosis, but the role of PKCalpha was not defined. Here, we used detailed time-course analysis as well as selective activators and inhibitors of PKC isoforms to infer the action of PKCalpha on basolateral endocytosis. Inhibition of PKC by the selective conventional PKC inhibitor Gö-6976 (5 microM) completely blocked the late inhibitory phase and markedly prolonged the stimulatory phase of endocytosis measured by FITC-dextran uptake. The PKCepsilon-selective agonist carbachol (100 microM) induced prolonged stimulation of endocytosis devoid of an inhibitory phase. Actin disassembly caused by PMA was completely blocked by Gö-6850 but not by Gö-6976, implicating PKCepsilon as the key isoform responsible for actin disruption. The Ca2+ agonist thapsigargin (5 microM) induced early activation of PKC when added simultaneously with PMA. This early activation of PKCalpha blocked the ability of PMA to remodel basolateral F-actin and abolished the stimulatory phase of basolateral endocytosis. Activation of PKCalpha stabilizes F-actin and thereby opposes the effect of PKCepsilon on membrane remodeling in T84 cells.

C2 domains of protein kinase C isoforms alpha, beta, and gamma: activation parameters and calcium stoichiometries of the membrane-bound state

The independently folding C2 domain motif serves as a Ca(2+)-dependent membrane docking trigger in a large number of Ca(2+) signaling pathways. A comparison was initiated between three closely related C2 domains from the conventional protein kinase C subfamily (cPKC, isoforms alpha, beta, and gamma). The results reveal that these C2 domain isoforms exhibit some similarities but are specialized in important ways, including different Ca(2+) stoichiometries. In the absence of membranes, Ca(2+) affinities of the isolated C2 domains are similar (2-fold difference) while Hill coefficients reveal cooperative Ca(2+) binding for the PKC beta C2 domain but not for the PKC alpha or PKC gamma C2 domain (H = 2.3 +/- 0.1 for PKC beta, 0.9 +/- 0.1 for PKC alpha, and 0.9 +/- 0.1 for PKC gamma). When phosphatidylserine-containing membranes are present, Ca(2+) affinities range from the sub-micromolar to the micromolar (7-fold difference) ([Ca(2+)](1/2) = 0.7 +/- 0.1 microM for PKC gamma, 1.4 +/- 0.1 microM for PKC alpha, and 5.0 +/- 0.2 microM for PKC beta), and cooperative Ca(2+) binding is observed for all three C2 domains (Hill coefficients equal 1.8 +/- 0.1 for PKC beta, 1.3 +/- 0.1 for PKC alpha, and 1.4 +/- 0.1 for PKC gamma). The large effects of membranes are consistent with a coupled Ca(2+) and membrane binding equilibrium, and with a direct role of the phospholipid in stabilizing bound Ca(2+). The net negative charge of the phospholipid is more important to membrane affinity than its headgroup structure, although a slight preference for phosphatidylserine is observed over other anionic phospholipids. The Ca(2+) stoichiometries of the membrane-bound C2 domains are detectably different. PKC beta and PKC gamma each bind three Ca(2+) ions in the membrane-associated state; membrane-bound PKC alpha binds two Ca(2+) ions, and a third binds weakly or not at all under physiological conditions. Overall, the results indicate that conventional PKC C2 domains first bind a subset of the final Ca(2+) ions in solution, and then associate weakly with the membrane and bind additional Ca(2+) ions to yield a stronger membrane interaction in the fully assembled tertiary complex. The full complement of Ca(2+) ions is needed for tight binding to the membrane. Thus, even though the three C2 domains are 64% identical, differences in Ca(2+) affinity, stoichiometry, and cooperativity are observed, demonstrating that these closely related C2 domains are specialized for their individual functions and contexts.

Delineation of RAID1, the RACK1 interaction domain located within the unique N-terminal region of the cAMP-specific phosphodiesterase, PDE4D5

BACKGROUND

The cyclic AMP specific phosphodiesterase, PDE4D5 interacts with the beta-propeller protein RACK1 to form a signaling scaffold complex in cells. Two-hybrid analysis of truncation and mutant constructs of the unique N-terminal region of the cAMP-specific phosphodiesterase, PDE4D5 were used to define a domain conferring interaction with the signaling scaffold protein, RACK1.

RESULTS

Truncation and mutagenesis approaches showed that the RACK1-interacting domain on PDE4D5 comprised a cluster of residues provided by Asn-22/Pro-23/Trp-24/Asn-26 together with a series of hydrophobic amino acids, namely Leu-29, Val-30, Leu-33, Leu-37 and Leu-38 in a 'Leu-Xaa-Xaa-Xaa-Leu' repeat. This was done by 2-hybrid analyses and then confirmed in biochemical pull down analyses using GST-RACK1 and mutant PDE4D5 forms expressed in COS cells. Mutation of Arg-34, to alanine, in PDE4D5 attenuated its interaction with RACK1 both in 2-hybrid screens and in pull down analyses. A 38-mer peptide, whose sequence reflected residues 12 through 49 of PDE4D5, bound to RACK1 with similar affinity to native PDE4D5 itself (Ka circa 6 nM).

CONCLUSIONS

The RACK1 Interaction Domain on PDE4D5, that we here call RAID1, is proposed to form an amphipathic helical structure that we suggest may interact with the C-terminal beta-propeller blades of RACK1 in a manner akin to the interaction of the helical G-gamma signal transducing protein with the beta-propeller protein, G-beta.

Protein kinase C epsilon-dependent regulation of cystic fibrosis transmembrane regulator involves binding to a receptor for activated C kinase (RACK1) and RACK1 binding to Na+/H+ exchange regulatory factor

Protein kinase C (PKC) regulation of cystic fibrosis transmembrane regulator (CFTR) chloride function has been demonstrated in several cell lines, including Calu-3 cells that express native, wild-type CFTR. We demonstrated previously that PKC epsilon was required for cAMP-dependent CFTR function. The goal of this study was to determine whether PKC epsilon interacts directly with CFTR. Using overlay assay, immunoprecipitation, pulldown and binding assays, we show that PKC epsilon does not bind to CFTR, but does bind to a receptor for activated C kinase (RACK1), a 37-kDa scaffold protein, and that RACK1 binds to Na(+)/H(+) exchange regulatory factor (NHERF1), a binding partner of CFTR. In vitro binding assays demonstrate dose-dependent binding of PKC epsilon to RACK1 which is inhibited by an 8-amino acid peptide based on the sequence of the sixth Trp-Asp repeat in RACK1 or by an 8-amino acid sequence in the V1 region of PKC epsilon, epsilon V1-2. A 4-amino acid sequence INAL (70-73) expressed in CFTR shares 50% homology to the RACK1 inhibitory peptide, but it does not bind PKC epsilon. NHERF1 and RACK1 bind in a dose-dependent manner. Immunofluorescence and confocal microscopy of RACK1 and CFTR revealed colocalization of the proteins to the apical and lateral regions of Calu-3 cells. The results indicate the RACK1 binds PKC epsilon and NHERF1, thus serving as a scaffold protein to anchor the enzyme in proximity to CFTR.

PKC epsilon is associated with myosin IIA and actin in fibroblasts

Proteins coimmunoprecipitating with protein kinase C (PKC) epsilon in fibroblasts were identified through matrix-assisted laser desorption/ionisation time of flight mass spectrometry (MALDI TOF m/s). This method identified myosin IIA in PKC epsilon immunoprecipitates, as well as known PKC epsilon binding proteins, actin, beta'Cop and cytokeratin. Myosin is not a substrate for PKC epsilon. Immunofluorescence analysis showed that PKC epsilon is colocalised with actin and myosin in actomyosin stress fibers in fibroblasts. Inhibitors of PKC and myosin ATPase activity, as well as microfilament-disrupting drugs, all inhibited spreading of fibroblasts after passage, suggesting a role for a PKC epsilon-actin-myosin complex in cell spreading.

Intracellular transport mechanisms of signal transducers

Recent discoveries have revolutionized our conceptions of enzyme-substrate specificity in signal transduction pathways. Protein kinases A and C are localized to discreet subcellular regions, and this localization changes in an isozyme-specific manner upon activation, a process referred to as translocation. The mechanisms for translocation involve interactions of soluble kinases with membrane-bound anchor proteins that recognize individual kinase isoenzymes and their state of activation. Recently, modulation of kinase-anchor protein interactions has been used to specifically regulate, positively or negatively, the activity of C kinase isozymes. Also described in this review is a role for the Rab family of small G proteins in regulating subcellular protein trafficking. The pathophysiological significance of disrupted subcellular protein transport in cell signaling and the potential therapeutic utility of targeted regulation of these events are in the process of being characterized.

Nitric oxide (NO) induces nitration of protein kinase Cepsilon (PKCepsilon ), facilitating PKCepsilon translocation via enhanced PKCepsilon -RACK2 interactions: a novel mechanism of no-triggered activation of PKCepsilon

Activation of protein kinase C (PKC) epsilon by nitric oxide (NO) has been implicated in the development of cardioprotection. However, the cellular mechanisms underlying the activation of PKCepsilon by NO remain largely unknown. Nitration of protein tyrosine residues has been shown to alter functions of a variety of proteins, and NO-derived peroxynitrite is known as a strong nitrating agent. In this investigation, we demonstrate that NO donors promote translocation and activation of PKCepsilon in an NO- and peroxynitrite-dependent fashion. NO induces peroxynitrite-mediated tyrosine nitration of PKCepsilon in rabbit cardiomyocytes in vitro, and nitrotyrosine residues were also detected on PKCepsilon in vivo in the rabbit myocardium preconditioned with NO donors. Furthermore, coimmunoprecipitation of PKCepsilon and its receptor for activated C kinase, RACK2, illustrated a peroxynitrite-dependent increase in PKCepsilon-RACK2 interactions in NO donor-treated cardiomyocytes. Moreover, using an enzyme-linked immunosorbent assay-based protein-protein interaction assay, PKCepsilon proteins treated with the peroxynitrite donor SIN-1 exhibited enhanced binding to RACK2 in an acellular environment. Our data demonstrate that post-translational modification of PKCepsilon by NO donors, namely nitration of PKCepsilon, facilitates its interaction with RACK2 and promotes translocation and activation of PKCepsilon. These findings offer a plausible novel mechanism by which NO activates the PKC signaling pathway.

Isozyme-specific inhibitors and activators of protein kinase C

We describe here the methods we have used to generate selective peptide inhibitors and activators of PKC-mediated signaling. These approaches should be applicable to any signaling event that is dependent on protein-protein interaction. Furthermore, targeting downstream enzymes in signal transduction has been notoriously difficult as there are often families of related enzymes in each cell. The approaches we have used overcame this difficulty and may prove useful not only in basic research, but also in drug discovery.

Selective enhanced phosphorylation of shrimp beta-tubulin by PKC-delta with PEP(taxol), a synthetic peptide encoding the taxol binding region

Beta-tubulin cDNA from the shrimp Penaeus japonicus was isolated by homology cloning. Expression of cDNA in Escherichia coli yielded a 55 kDa polypeptide, positive for monoclonal antibodies against mammalian beta-tubulin. Autoradiography demonstrated the bacterially expressed hepatopancreas beta-tubulin of P. japonicus is specifically phosphorylated by the delta isoenzyme of protein kinase C (PKC-delta) purified from the plasma membrane of the shrimp heart, in the presence of the receptor for activated PKC (RACK), but not in its absence. Purified shrimp heart PKC-delta is able to phosphorylate bacterially expressed shrimp beta-tubulin without the presence of Ca(++), but requires Mg(++). The kinase activity of purified PKC-delta on bacterially expressed beta-tubulin was enhanced by incubation with PEP(taxol), a synthetic peptide encoding the taxol-binding region of beta-tubulin. In other words, PEP(taxol) modulates the kinase activity of PKC-delta through RACK.

Chimaerins, novel non-protein kinase C phorbol ester receptors, associate with Tmp21-I (p23): evidence for a novel anchoring mechanism involving the chimaerin C1 domain

The regulation and function of chimaerins, a family of "non-protein kinase C" (PKC) phorbol ester/diacylglycerol receptors with Rac-GAP activity, is largely unknown. In a search for chimaerin-interacting proteins, we isolated Tmp21-I (p23), a protein localized at the perinuclear Golgi area. Remarkably, phorbol esters translocate beta2-chimaerin to the perinuclear region and promote its association with Tmp21-I in a PKC-independent manner. A deletional analysis revealed that the C1 domain in chimaerins is required for the interaction with Tmp21-I, thereby implying a novel function for this domain in protein-protein associations in addition to its role in lipid and phorbol ester binding. Our results support the emerging concept that multiple pathways transduce signaling by phorbol esters and revealed that, like PKC isozymes, chimaerins are subject to a positional regulation. In this setting, Tmp21-I serves as an anchoring protein that determines the intracellular localization of these novel phorbol ester receptors.

Thrombin differentiates normal lung fibroblasts to a myofibroblast phenotype via the proteolytically activated receptor-1 and a protein kinase C-dependent pathway

Myofibroblasts are ultrastructurally and metabolically distinctive fibroblasts that express smooth muscle (SM)-alpha actin and are associated with various fibrotic lesions. The present study was undertaken to investigate the myofibroblast phenotype that appears after activation of normal lung fibroblasts by thrombin. We demonstrate that thrombin induces smooth muscle-alpha actin expression and rapid collagen gel contraction by normal lung fibroblasts via the proteolytically activated receptor-1 and independent of transforming growth factor-beta pathway. Using antisense oligonucleotides we demonstrate that a decreased level of PKCepsilon abolishes SM-alpha actin expression and collagen gel contraction induced by thrombin in normal lung fibroblasts. Inhibition of PKCepsilon translocation also abolishes thrombin-induced collagen gel contraction, SM-alpha actin increase, and its organization by normal lung fibroblasts, suggesting that activation of PKCepsilon is required for these effects. In normal lung fibroblasts PKCepsilon binds to SM-alpha actin after thrombin treatment, but in activated fibroblasts derived from scleroderma lung they associate even in untreated cells. This suggests that SM-alpha actin may serve as a substrate for PKCepsilon in lung fibroblasts when activated by thrombin. We propose that thrombin differentiates normal lung fibroblasts to a myofibroblast phenotype via a PKC-dependent pathway. Thrombin-induced differentiation of normal lung fibroblasts to a myofibroblast phenotype resembles the phenotype observed in scleroderma lung fibroblasts. Therefore, we conclude that chronic exposure to thrombin after microvascular injury leads to activation of normal lung fibroblasts and to the appearance of a myofibroblast phenotype in vivo. Our study provides novel, compelling evidence that thrombin is an important mediator of the interstitial lung fibrosis associated with scleroderma.

Adaptor proteins in protein kinase C-mediated signal transduction

Spatial and temporal organization of signal transduction is essential in determining the speed and precision by which signaling events occur. Adaptor proteins are key to organizing signaling enzymes near their select substrates and away from others in order to optimize precision and speed of response. Here, we describe the role of adaptor proteins in determining the specific function of individual protein kinase C (PKC) isozymes. These isozyme-selective proteins were called collectively RACKs (receptors for activated C-kinase). The role of RACKs in PKC-mediated signaling was determined using isozyme-specific inhibitors and activators of the binding of each isozyme to its respective RACK. In addition to anchoring activated PKC isozymes, RACKs anchor other signaling enzymes. RACK1, the anchoring protein for activated betaIIPKC, binds for example, Src tyrosine kinase, integrin, and phosphodiesterase. RACK2, the epsilonPKC-specific RACK, is a coated-vesicle protein and thus is involved in vesicular release and cell-cell communication. Therefore, RACKs are not only adaptors for PKC, but also serve as adaptor proteins for several other signaling enzymes. Because at least some of the proteins that bind to RACKs, including PKC itself, regulate cell growth, modulating their interactions with RACKs may help elucidate signaling pathways leading to carcinogenesis and could result in the identification of novel therapeutic targets.

Structure of the C2 domain from novel protein kinase Cepsilon. A membrane binding model for Ca(2+)-independent C2 domains

Protein kinase Cepsilon (PKCepsilon) is a member of the novel PKCs which are activated by acidic phospholipids, diacylglycerol and phorbol esters, but lack the calcium dependence of classical PKC isotypes. The crystal structures of the C2 domain of PKCepsilon, crystallized both in the absence and in the presence of the two acidic phospholipids, 1,2-dicaproyl-sn-phosphatidyl-l-serine (DCPS) and 1,2-dicaproyl-sn-phosphatidic acid (DCPA), have now been determined at 2.1, 1.7 and 2.8 A resolution, respectively. The central feature of the PKCepsilon-C2 domain structure is an eight-stranded, antiparallel, beta-sandwich with a type II topology similar to that of the C2 domains from phospholipase C and from novel PKCdelta. Despite the similar topology, important differences are found between the structures of C2 domains from PKCs delta and epsilon, suggesting they be considered as different PKC subclasses. Site-directed mutagenesis experiments and structural changes in the PKCepsilon-C2 domain from crystals with DCPS or DCPA indicate, though phospholipids were not visible in these structures, that loops joining strands beta1-beta2 and beta5-beta6 participate in the binding to anionic membranes. The different behavior in membrane-binding and activation between PKCepsilon and classical PKCs appears to originate in localized structural changes, which include a major reorganization of the region corresponding to the calcium binding pocket in classical PKCs. A mechanism is proposed for the interaction of the PKCepsilon-C2 domain with model membranes that retains basic features of the docking of C2 domains from classical, calcium-dependent, PKCs.

PICK1 targets activated protein kinase Calpha to AMPA receptor clusters in spines of hippocampal neurons and reduces surface levels of the AMPA-type glutamate receptor subunit 2

The PICK1 protein interacts in neurons with the AMPA-type glutamate receptor subunit 2 (GluR2) and with several other membrane receptors via its single PDZ domain. We show that PICK1 also binds in neurons and in heterologous cells to protein kinase Calpha (PKCalpha) and that the interaction is highly dependent on the activation of the kinase. The formation of PICK1-PKCalpha complexes is strongly induced by TPA, and PICK1-PKCalpha complexes are cotargeted with PICK1-GluR2 complexes to spines, where GluR2 is found to be phosphorylated by PKC on serine 880. PICK1 also reduces the plasma membrane levels of the GluR2 subunit, consistent with a targeting function of PICK1 and a PKC-facilitated release of GluR2 from the synaptic anchoring proteins ABP and GRIP. This work indicates that PICK1 functions as a targeting and transport protein that directs the activated form of PKCalpha to GluR2 in spines, leading to the activity-dependent release of GluR2 from synaptic anchor proteins and the PICK1-dependent transport of GluR2 from the synaptic membrane.

Binding specificity for RACK1 resides in the V5 region of beta II protein kinase C

Identification of selective anchoring proteins responsible for specialized localization of specific signaling proteins has led to the identification of new inhibitors of signal transduction, inhibitors of anchoring protein-ligand interactions. RACK1, the first receptor for activated C kinase identified in our lab, is a selective anchoring protein for betaII protein kinase C (betaIIPKC). We previously found that at least part of the RACK1-binding site resides in the C2 domain of betaIIPKC (Ron, D., Luo, J., and Mochly-Rosen, D. (1995) J. Biol. Chem. 270, 24180-24187). Here we show that the V5 domain also contains part of the RACK1-binding site in betaIIPKC. In neonatal rat cardiac myocytes, the betaIIV5-3 peptide (amino acids 645-650 in betaIIPKC) selectively inhibited phorbol 12-myristate 13-acetate (PMA)-induced translocation of betaIIPKC and not betaIPKC. In addition, the betaIIV5-3 peptide inhibited cardiac myocyte hypertrophy in PMA-treated cells. Interestingly, betaIV5-3 (646-651 in betaIPKC), a selective translocation inhibitor of betaIPKC, also inhibited PMA-induced cardiac myocyte hypertrophy, demonstrating that both betaI- and betaIIPKC are essential for this cardiac function. Therefore, the betaIIV5 domain contains part of the RACK1-binding site in betaIIPKC; a peptide corresponding to this site is a selective inhibitor of betaIIPKC and, hence, enables the identification of betaIIPKC-selective functions.

Diminished alpha1-adrenergic-mediated contraction and translocation of PKC in senescent rat heart

Myocardial reserve function declines with aging due in part to reduced alpha- and beta-adrenergic receptor (AR)-mediated contractile augmentation. Whereas specific age-associated deficits in beta-AR signaling have been identified, it is not known which components of the alpha1-AR signaling cascade, e.g., protein kinase C (PKC) and associated anchoring proteins (receptors for activated C kinase; RACKs), underlie deficits in alpha1-AR contractile function with aging. We therefore assessed cardiac contraction (dP/dt) in Langendorff perfused hearts isolated from adult (5 mo) and senescent (24 mo) Wistar rats following maximal alpha1-AR stimulation with phenylephrine (PE), and we measured the subcellular distribution of PKCalpha and PKCepsilon, and their respective anchoring proteins RACK1 and RACK2 by Western blotting. The maximum dP/dt response to PE (10(-5) M) was significantly reduced by 41% in 24-mo-old vs. 5-mo-old (P < 0.01). Inhibitory effects of PKC blockade (chelerythrine; 10 microM) on dP/dt following alpha1-AR stimulation with PE observed in adult hearts were absent in 24-mo-old hearts (P < 0.01). In 5-mo-old hearts, PE elicited reductions in soluble PKCalpha and PKCepsilon levels, while increasing particulate PKCalpha and PKCepsilon levels to a similar extent. In contrast, soluble PKCalpha and PKCepsilon levels in 24-mo-old hearts were increased in response to PE; particulate PKCepsilon and PKCalpha were unchanged or reduced and associated with significant reductions in particulate RACK1 and RACK2. The results indicate, for the first time, that selective translocation of PKCalpha and PKCepsilon in response to alpha1-AR stimulation is disrupted in the senescent myocardium. That age-related reductions in particulate RACK1 and RACK2 levels were also observed provide evidence that alterations in PKC-anchoring proteins may contribute to impaired PKC translocation and defective alpha1-AR contraction in the aged rat heart.

Cutting edge: protein kinase C beta expression is critical for export of Il-2 from T cells

Protein kinase C (PKC) plays an integral part in T cell activation and IL-2 secretion. We investigated the role of a particular PKC isoform, PKCbeta, in IL-2 production and secretion. The T cell lymphoma line HuT 78 secretes IL-2 in response to the phorbol ester PMA. A PKCbeta-deficient clone of HuT 78, K-4, did not secrete IL-2 in response to PMA stimulation. As assessed by RT-PCR, K-4 expressed mRNA for IL-2 following PMA activation, and intracellular IL-2 protein was detected by immunofluorescence. An enhanced green fluorescent protein-linked PKCbeta construct was microinjected into K-4 cells, which were then stimulated with PMA; those cells that expressed PKCbeta could secrete IL-2, as determined by an in situ immunofluorescent assay. This study demonstrates that PKCbeta is not necessary for transcription of the IL-2 gene or translation of mRNA to protein, but that expression of this PKC isoform is critical to the export of IL-2 molecules from T cells.

Localization, anchoring, and functions of protein kinase C isozymes in the heart

Although protein kinase C (PKC) was identified more than 20 years ago, and is involved in a wide variety of essential cellular processes, assigning specific roles to each PKC isozyme has proved difficult. Results over the last few years have suggested that much of the specificity of activated PKC isozymes is attributed to their subcellular localization bringing them into close proximity to a subset of substrates. Our laboratory has taken advantage of the importance of PKC localization and studied the way in which PKC isozymes are anchored. We have identified PKC anchoring proteins (RACKs or Receptors for Activated C Kinase) and used information about interaction sites between PKC isozymes and their respective RACKs to design peptides which modulate translocation of specific PKC isozymes to the functional site. These isozyme-specific peptides can be delivered into isolated or cultured cells or expressed in transgenic mice to determine the role of specific PKC isozymes in particular functions. Here we will describe the isozymes-specific peptide activators and inhibitors that we have developed and the specific functions of each isozyme in cardiac ventricular tissue.

Evidence for a role of protein kinase C in FGF signal transduction in the developing chick limb bud

In developing limbs, numerous signaling molecules have been identified but less is known about the mechanisms by which such signals direct patterning. We have explored signal transduction pathways in the chicken limb bud. A cDNA encoding RACK1, a protein that binds and stabilizes activated protein kinase C (PKC), was isolated in a screen for genes induced by retinoic acid (RA) in the chick wing bud. Fibroblast growth factor (FGF) also induced RACK1 and such induction of RACK1 expression was accompanied by a significant augmentation in the number of active PKC molecules and an elevation of PKC enzymatic activity. This suggests that PKCs mediate signal transduction in the limb bud. Application of chelerythrine, a potent PKC inhibitor, to the presumptive wing region resulted in buds that did not express sonic hedgehog (Shh) and developed into wings that were severely truncated. This observation suggests that the expression of Shh depends on PKCs. Providing ectopic SHH protein, RA or ZPA grafts overcome the effects of blocking PKC with chelerythrine and resulted in a rescue of the wing morphology. Taken together, these findings suggest that the responsiveness of Shh to FGF is mediated, at least in part, by PKCs.

Protein kinase C-delta C2-like domain is a binding site for actin and enables actin redistribution in neutrophils

Neutrophils play a key role in host-defence mechanisms against invading pathogens, using their capacity to migrate, engulf micro-organisms and produce toxic radicals. Protein kinase C (PKC) isotypes are important intracellular regulators of these processes in neutrophils. PKC isotypes themselves are controlled by interactions with lipids, Ca(2+) and proteins. The C2-like domain of PKC-delta (deltaC2) has been identified as a protein-interaction domain in this PKC isotype. In the present paper we have investigated the contribution of protein interactions at this domain to the regulation/function of PKC-delta in neutrophils. Using affinity chromatography we identified actin as a deltaC2 binding partner in these cells. Fluorescein-labelled deltaC2, microinjected into immobilized neutrophils, interacts with filamentous actin (F-actin) inside the cell. PKC-delta co-localizes with F-actin in neutrophils, in lamellipodia at the leading edge of the cell. Stimulation with phorbol ester or IgG-opsonized Staphylococcus aureus results in co-ordinated redistribution of PKC-delta and F-actin, and a PKC-delta inhibitor inhibits these changes. Microinjection of deltaC2 also inhibits F-actin redistribution. Thus PKC-delta binds to F-actin through its C2 domain, and these interactions are important in regulating actin redistribution in neutrophils.

Compartmentation of G protein-coupled signaling pathways in cardiac myocytes

There is a large body of functional data that supports the existence of subcellular compartmentation of the components of cyclic AMP action in the heart. Data from isolated perfused hearts and from purified ventricular myocytes imply a fixed and hormone-specific spatial relationship amongst components of cyclic AMP synthesis, response, and degradation. Available data demonstrate that within a cardiac myocyte, not all cyclic AMP gains access to all cyclic AMP-dependent protein kinase (PKA), that not all PKA interacts with all possible cellular substrates of PKA, and that only a subset of the myocyte's phosphodiesterases (PDEs) may degrade cyclic AMP after a given synthetic stimulus. Molecular mechanisms contributing to compartmentation are being discovered: localization of receptors, G proteins, and adenylyl cyclases in caveolar versus noncaveolar regions of the sarcolemma; localization of PKA by A-kinase anchoring proteins; localization of PKA substrates, PDE isoforms, and phosphoprotein phosphatases in discrete subcellular regions; and differential regulation of multiple isoforms of adenylyl cyclase, phosphoprotein phosphatase, and PDE in distinct subcellular compartments.

The interaction of Src and RACK1 is enhanced by activation of protein kinase C and tyrosine phosphorylation of RACK1

RACK1 is an intracellular receptor for the serine/ threonine protein kinase C. Previously, we demonstrated that RACK1 also interacts with the Src protein-tyrosine kinase. RACK1, via its association with these protein kinases, may play a key role in signal transduction. To further characterize the Src-RACK1 interaction and to analyze mechanisms by which cross-talk occurs between the two RACK1-linked signaling kinases, we identified sites on Src and RACK1 that mediate their binding, and factors that regulate their interaction. We found that the interaction of Src and RACK1 is mediated, in part, by the SH2 domain of Src and by phosphotyrosines in the sixth WD repeat of RACK1, and is enhanced by serum or platelet-derived growth factor stimulation, protein kinase C activation, and tyrosine phosphorylation of RACK1. To the best of our knowledge, this is the first report of tyrosine phosphorylation of a member of the WD repeat family of proteins. We think that tyrosine phosphorylation of these proteins is an important mechanism of signal transduction in cells.