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

Ca2+-independent protein kinase C Apl II mediates the serotonin-induced facilitation at depressed aplysia sensorimotor synapses

At nondepressed Aplysia sensory to motor synapses, serotonin (5-HT) facilitates transmitter release primarily through a protein kinase A pathway. In contrast, at depressed Aplysia sensory to motor synapses, 5-HT facilitates transmitter release primarily through a protein kinase C (PKC)-dependent pathway. It is known that only two phorbol ester-activated PKC isoforms, the Ca(2+)-dependent PKC Apl I and the Ca(2+)-independent PKC Apl II, exist in the Aplysia nervous system. For the first time, we have now been able to functionally determine which isoform of PKC is involved in a particular form of plasticity. We microinjected cultured sensorimotor pairs of neurons with various PKC constructs tagged with the enhanced green fluorescent protein as a reporter for successful plasmid expression. Our results demonstrate that short-term facilitation of depressed synapses is mediated by PKC Apl II. Dominant-negative PKC Apl II, but not dominant-negative PKC Apl I, disrupted the normal kinetics of 5-HT-induced facilitation by completely blocking its rapid onset. This effect was specific to depressed synapses, because dominant-negative PKC Apl II did not inhibit 5-HT-mediated facilitation of nondepressed synapses. Our results suggest that not only different signal transduction pathways but also different isoforms of a specific cascade may mediate physiological responses according to the state of a synapse.

Phorbol esters and related analogs regulate the subcellular localization of beta 2-chimaerin, a non-protein kinase C phorbol ester receptor

The novel phorbol ester receptor beta2-chimaerin is a Rac-GAP protein possessing a single copy of the C1 domain, a 50-amino acid motif initially identified in protein kinase C (PKC) isozymes that is involved in phorbol ester and diacylglycerol binding. We have previously shown that, like PKCs, beta2-chimaerin binds phorbol esters with high affinity in a phospholipid-dependent manner (Caloca, M. J., Fernandez, M. N., Lewin, N. E., Ching, D., Modali, R., Blumberg, P. M., and Kazanietz, M. G. (1997) J. Biol. Chem. 272, 26488-26496). In this paper we report that like PKC isozymes, beta2-chimaerin is translocated by phorbol esters from the cytosolic to particulate fraction. Phorbol esters also induce translocation of alpha1 (n)- and beta1-chimaerins, suggesting common regulatory mechanisms for all chimaerin isoforms. The subcellular redistribution of beta2-chimaerin by phorbol esters is entirely dependent on the C1 domain, as revealed by deletional analysis and site-directed mutagenesis. Interestingly, beta2-chimaerin translocates to the Golgi apparatus after phorbol ester treatment, as revealed by co-staining with the Golgi marker BODIPY-TR-ceramide. Structure relationship analysis of translocation using a series of PKC ligands revealed substantial differences between translocation of beta2-chimaerin and PKCalpha. Strikingly, the mezerein analog thymeleatoxin is not able to translocate beta2-chimaerin, although it very efficiently translocates PKCalpha. Phorbol esters also promote the association of beta2-chimaerin with Rac in cells. These data suggest that chimaerins can be positionally regulated by phorbol esters and that each phorbol ester receptor class has distinct pharmacological properties and targeting mechanisms. The identification of selective ligands for each phorbol ester receptor class represents an important step in dissecting their specific cellular functions.

Selective degradation of the PKC-epsilon isoform during cell death in AKR-2B fibroblasts

The protein kinase C (PKC) family of serine/threonine protein kinases is involved in intracellular signals that regulate growth, differentiation, and apoptosis. AKR-2B cells express the PKC isoforms alpha, gamma, epsilon, lambda, mu, und zeta (J. Hoppe, R. Schäfer, V. Hoppe, and A. Sachinidis, Cell Death Differ. 6, 546-556). Here we show that during serum starvation only PKC-epsilon was cleaved. An N-terminal fragment of 42 kDa remained associated with subcellular components, presumably the Golgi apparatus. The C-terminal part (catalytic domain) was further degraded and was no longer detectable in vivo. As published before, the activation of the DEVDase in AKR-2B cells is prevented by numerous agents like PDGF, TPA, and DEVD.cmk (R. Schäfer, D. Karbach, and J. Hoppe, Exp. Cell Res. 240, 28--39). All these agents completely prevented PKC-epsilon cleavage, indicating a tight correlation between DEVDase activity and PKC-epsilon cleavage. By using recombinant caspase-3 or highly purified DEVDase from cytosolic extracts we localized by Edman degradation the cleavage site in recombinant PKC-epsilon to asp383 in the hinge region between regulatory and catalytic domains. The corresponding tetrapeptide sequences SSPD and SATD for human and mouse PKC-epsilon, respectively, are unusual for caspase-3. Expression of the catalytic domain or of the cleavage-resistant mutant D383A had no effect on cell death in AKR-2B cells.

The WD protein Rack1 mediates protein kinase C and integrin-dependent cell migration

The scaffolding protein, Rack1, is a seven-WD-domain-containing protein that has been implicated in binding to integrin (β) subunit cytoplasmic domains and to members of two kinase families (src and protein kinase C, PKC) that mediate integrin bidirectional signaling. To explore the role of Rack1 in integrin function we have transfected this protein in Chinese hamster ovary (CHO) cells. We have observed no effect of Rack1 overexpression on inside-out signaling as the ligand binding properties of CHO cells also expressing constitutively active or inactive integrins were not affected. In contrast, we observed that cells stably or transiently overexpressing Rack1 had decreased migration compared to mock transfected cells. Stable Rack1 transfectants also demonstrated an increased number of actin stress fibers and focal contacts. These effects on motility and cytoskeletal organization did not appear to result from Rack1 inhibition of src function as downstream substrates of this kinase were phosphorylated normally. In addition, expression of an active src construct did not reverse the migratory deficit induced by Rack1 overexpression. On the other hand when we overexpressed a Rack1 variant with alanine substitutions in the putative PKC binding site in its third WD domain, we observed no deficit in migration. Thus the ability of Rack1 to bind, localize and stabilize PKC isoforms is likely to be involved in aspects of integrin outside-in signaling.

Localization and kinetics of protein kinase C-epsilon anchoring in cardiac myocytes

Protein kinase C-epsilon (PKC-epsilon) plays a central role in cardiac cell signaling, but mechanisms of translocation and anchoring upon activation are poorly understood. Conventional PKC isoforms rely on a rapid Ca2+-mediated recruitment to cell membranes, but this mechanism cannot be employed by PKC-epsilon or other PKC isoforms lacking a Ca2+-binding domain. In this study, we used recombinant green fluorescent protein (GFP) fusion constructs and confocal microscopy to examine the localization, kinetics, and reversibility of PKC-epsilon anchoring in permeabilized rat cardiac myocytes. PKC-epsilon-GFP bound with a striated pattern that co-localized with alpha-actinin, a marker of the Z-line of the sarcomere. Binding required activation of PKC and occurred slowly but reversibly with apparent rate constants of k(on) = 4.6 +/- 1.2 x 10(3) M(-1) x s(-1) and k(off) = 1.4 +/- 0.5 x 10(-3) s(-1) (t1/2 = 8 min) as determined by fluorescence recovery after photobleaching and by perfusion experiments. A truncated construct composed of the N-terminal 144-amino-acid variable region of PKC-epsilon (epsilonV1-GFP), but not an analogous N-terminal domain of PKC-delta, mimicked the Z-line decoration and slow binding rate of the full-length enzyme. These findings suggest that the epsilonV1 domain is important in determining PKC-epsilon localization and translocation kinetics in cardiac muscle. Moreover, PKC-epsilon translocation is not a diffusion-controlled binding process but instead may be limited by intramolecular conformational changes within the V1 domain. The k(off) for epsilonV1-GFP was two- to threefold faster than for full-length enzyme, indicating that other domains in PKC-epsilon contribute to anchoring by prolonging the bound state.

Use of functional proteomics to investigate PKC epsilon-mediated cardioprotection: the signaling module hypothesis

The characterization of biological processes on the basis of alterations in the cellular proteins, or "proteomic" analysis, is a powerful approach that may be adopted to decipher the signaling mechanisms that underlie various pathophysiological conditions, such as ischemic heart disease. This review represents a prospectus for the implementation of proteomic analyses to delineate the myocardial intracellular signaling events that evoke cardioprotection against ischemic injury. In concert with this, the manifestation of a protective phenotype has recently been shown to involve dynamic modulation of protein kinase C-epsilon (PKC epsilon) signaling complexes (Ping P, Zhang J, Pierce WM Jr, and Bolli R. Circ Res 88: 59--62, 2001). Accordingly, "the signaling module hypothesis" is formulated as a plausible mechanism by which multipurpose stress-activated proteins and signaling kinases may function collectively to facilitate the genesis of cardioprotection.

Signalling pathways regulating the dephosphorylation of Ser729 in the hydrophobic domain of protein kinase Cepsilon upon cell passage

We have recently demonstrated that in quiescent fibroblasts protein kinase C (PKC) epsilon(95) is phosphorylated at Ser(729), Ser(703), and Thr(566) and that upon passage of quiescent cells phosphorylation at Ser(729) is lost, giving rise to PKCepsilon(87). Ser(729) may be rephosphorylated later, suggesting cycling between PKCepsilon(87) and PKCepsilon(95). Here we show that the dephosphorylation at Ser(729) is insensitive to okadaic acid, calyculin, ascomycin C, and cyclosporin A, suggesting that dephosphorylation at this site is not mediated through protein phosphatases 1, 2A or 2B. We demonstrate that this dephosphorylation at Ser(729) requires serum and cell readhesion and is sensitive to rapamycin, PD98059, chelerythrine, and Ro-31-8220. These results suggest that the phosphorylation status of Ser(729) in the hydrophobic domain at Ser(729) is regulated independently of the phosphorylation status of other sites in PKCepsilon, by a mTOR-sensitive phosphatase. The mitogen-activated protein kinase pathway and PKC are also implicated in regulating the dephosphorylation at Ser(729).

PKCepsilon activation induces dichotomous cardiac phenotypes and modulates PKCepsilon-RACK interactions and RACK expression

Receptors for activated C kinase (RACKs) have been shown to facilitate activation of protein kinase C (PKC). However, it is unknown whether PKC activation modulates RACK protein expression and PKC-RACK interactions. This issue was studied in two PKCepsilon transgenic lines exhibiting dichotomous cardiac phenotypes: one exhibits increased resistance to myocardial ischemia (cardioprotected phenotype) induced by a modest increase in PKCepsilon activity (228 +/- 23% of control), whereas the other exhibits cardiac hypertrophy and failure (hypertrophied phenotype) induced by a marked increase in PKCepsilon activity (452 +/- 28% of control). Our data demonstrate that activation of PKC modulates the expression of RACK isotypes and PKC-RACK interactions in a PKCepsilon activity- and dosage-dependent fashion. We found that, in mice displaying the cardioprotected phenotype, activation of PKCepsilon enhanced RACK2 expression (178 +/- 13% of control) and particulate PKCepsilon-RACK2 protein-protein interactions (178 +/- 18% of control). In contrast, in mice displaying the hypertrophied phenotype, there was not only an increase in RACK2 expression (330 +/- 33% of control) and particulate PKCepsilon-RACK2 interactions (154 +/- 14% of control) but also in RACK1 protein expression (174 +/- 10% of control). Most notably, PKCepsilon-RACK1 interactions were identified in this line. With the use of transgenic mice expressing a dominant negative PKCepsilon, we found that the changes in RACK expression as well as the attending cardiac phenotypes were dependent on PKCepsilon activity. Our observations demonstrate that RACK expression is dynamically regulated by PKCepsilon and suggest that differential patterns of PKCepsilon-RACK interactions may be important determinants of PKCepsilon-dependent cardiac phenotypes.

Structural characterization of the C2 domain of novel protein kinase Cepsilon

Infrared spectroscopy (IR) and differential scanning calorimetry (DSC) were used to study the biophysical properties of the PKCepsilon-C2 domain, a C2 domain that possess special characteristics as it binds to acidic phospholipids in a Ca2+-independent manner and no structural information about it is available to date. When the secondary structure was determined by IR spectroscopy in H2O and D2O buffers, beta sheet was seen to be the major structural component. Spectroscopic studies of the thermal denaturation in D2O showed a broadening in the amide I' band starting at 45 degrees C. Curve fitting analysis of the spectra demonstrated that two components appear upon thermal denaturation, one at 1623 cm(-1) which was assigned to aggregation and a second one at 1645 cm(-1), which was assigned to unordered or open loop structures. A lipid binding assay has demonstrated that PKCepsilon-C2 domain has preferential affinity for PIP2 although it exhibits maximal binding activity for phosphatidic acid when 100 mol% of this negatively charged phospholipid was used. Thus, phosphatidic acid containing vesicles were used to characterize the effect of lipid binding on the secondary structure and thermal stability. These experiments showed that the secondary structure did not change upon lipid binding and the thermal stability was very high with no significant changes occurring in the secondary structure after heating. DSC experiments demonstrated that when the C2-protein was scanned alone, it showed a Tm of 49 degrees C and a calorimetric denaturation enthalpy of 144.318 kJ x mol(-1). However, when phoshatidic acid vesicles were included in the mixture, the transition disappeared and further IR experiments demonstrated that the protein structure was not modified under these conditions.

Phosphorylation cascades control the actions of ethanol on cell cAMP signalling

Our studies indicate that, in the presence of particular isoforms of adenylyl cyclase (i.e., type 7 AC), moderately intoxicating concentrations of ethanol will significantly potentiate transmitter-mediated activation of the cAMP signaling cascade. Activation of this signaling cascade may have important implications for the mechanisms by which ethanol produces intoxication, and/or for the mechanisms of neuroadaptation leading to tolerance to, and physical dependence on, ethanol. We initiated a series of studies to investigate the phosphorylation of AC7 by PKC, the role of this phosphorylation in modulating the sensitivity of AC7 to activation by Gsalpha, and the PKC isotype(s) involved in the phosphorylation of AC7. The T7 epitope-tagged AC7 expressed in Sf9 and HEK293 cells was found to be phosphorylated in vitro by the catalytic subunit of PKC. Treatment of AC7-transfected HEK293 cells with phorbol dibutyrate (PDBu) or ethanol increased the phosphorylation of AC7 and its responsiveness to Gsalpha. In human erythroleukemia (HEL) cells, which endogeneously express AC7, ethanol and PDBu increased AC activity stimulated by PGE(1). The potentiation by both PDBu and ethanol was found to be sensitive to the PKC delta-selective inhibitor, rottlerin. The potentiation of AC activity by ethanol in HEL cells was also selectively attenuated by the RACK inhibitory peptide specific for PKC delta, and by expression of the dominant negative, catalytically inactive, form of PKC delta. These data demonstrate that AC7 can be phosphorylated by PKC, leading to an increase in functional activity, and ethanol can potentiate AC7 activity through a PKC delta-mediated phosphorylation of AC7.

Associations of PKC isoforms with the cytoskeleton of B16F10 melanoma cells

Although PKC plays a major role in regulating the morphology and function of the cytoskeleton, little is known about in situ associations of specific isoforms with the cytoskeleton. We demonstrate that seven PKC isoforms are expressed in B16F10 melanoma cells and show different levels of induction by serum. Using cell cytoskeleton preparations (CSKs), confocal microscopy, and immunocytochemistry, all isoforms show specific patterns of localization to focal contact-like structures (alpha, delta), very small cytoplasmic granules/vesicles (all isoforms), dense ordered arrays of small granules in the perinuclear region (alpha, delta), granules/vesicles associated with a homogeneous framework in the cytoplasm adjacent to the nucleus (gamma), or irregular-shaped patches of granules at or near the nuclear perimeter (eta, theta). In addition, several isoforms are present as cytoplasmic granules/ vesicles in linear or curvilinear arrays (alpha, delta, epsilon, theta). When isoform localization is examined using 3.7% formaldehyde or methanol:acetone, the patterns of localization in CSKs are often difficult or impossible to detect, and many are described here for the first time. Double-labeling experiments with CSK demonstrate that PKC actin co-localizes with punctate alpha-rich particles above the nucleus, granules of epsilon throughout the cytoplasm, and with theta in irregular-shaped aggregates associated with the nucleus. Vimentin co-localizes with perinuclear granules of delta and beta(2), and alpha-tubulin co-localizes with theta in structures at or near the nuclear surface and in microtubules associated with the microtubule organizing center (MTOC). In summary, the present study demonstrates that seven PKC isoforms are endogenously expressed in B16F10 melanoma cells. These isoforms show various levels of induction by serum and specific patterns of association with various components of the detergent-resistant cell cytoskeleton.

Regulation of protein kinase C by the cytoskeletal protein calponin

Previous studies from this laboratory have shown that, upon agonist activation, calponin co-immunoprecipitates and co-localizes with protein kinase Cepsilon (PKCepsilon) in vascular smooth muscle cells. In the present study we demonstrate that calponin binds directly to the regulatory domain of PKC both in overlay assays and, under native conditions, by sedimentation with lipid vesicles. Calponin was found to bind to the C2 region of both PKCepsilon and PKCalpha with possible involvement of C1B. The C2 region of PKCepsilon binds to the calponin repeats with a requirement for the region between amino acids 160 and 182. We have also found that calponin can directly activate PKC autophosphorylation. By using anti-phosphoantibodies to residue Ser-660 of PKCbetaII, we found that calponin, in a lipid-independent manner, increased auto-phosphorylation of PKCalpha, -epsilon, and -betaII severalfold compared with control conditions. Similarly, calponin was found to increase the amount of (32)P-labeled phosphate incorporated into PKC from [gamma-(32)P]ATP. We also observed that calponin addition strongly increased the incorporation of radiolabeled phosphate into an exogenous PKC peptide substrate, suggesting an activation of enzyme activity. Thus, these results raise the possibility that calponin may function in smooth muscle to regulate PKC activity by facilitating the phosphorylation of PKC.

Receptor for activated C-kinase (RACK-1), a WD motif-containing protein, specifically associates with the human type I IFN receptor

The cytoplasmic domain of the human type I IFN receptor chain 2 (IFNAR2c or IFN-alphaRbetaL) was used as bait in a yeast two-hybrid system to identify novel proteins interacting with this region of the receptor. We report here a specific interaction between the cytoplasmic domain of IFN-alphaRbetaL and a previously identified protein, RACK-1 (receptor for activated C kinase). Using GST fusion proteins encoding different regions of the cytoplasmic domain of IFN-alphaRbetaL, the minimum site for RACK-1 binding was mapped to aa 300-346. RACK-1 binding to IFN-alphaRbetaL did not require the first 91 aa of RACK-1, which includes two WD domains, WD1 and WD2. The interaction between RACK-1 and IFN-alphaRbetaL, but not the human IFN receptor chain 1 (IFNAR1 or IFN-alphaRalpha), was also detected in human Daudi cells by coimmunoprecipitation. RACK-1 was shown to be constitutively associated with IFN-alphaRbetaL, and this association was not effected by stimulation of Daudi cells with type I IFNs (IFN-beta1b). RACK-1 itself did not become tyrosine phosphorylated upon stimulation of Daudi cells with IFN-beta1b. However, stimulation of cells with either IFN-beta1b or PMA did result in an increase in detectable immunofluorescence and intracellular redistribution of RACK-1.

Localization and activity of myosin light chain kinase isoforms during the cell cycle

Phosphorylation on Ser 19 of the myosin II regulatory light chain by myosin light chain kinase (MLCK) regulates actomyosin contractility in smooth muscle and vertebrate nonmuscle cells. The smooth/nonmuscle MLCK gene locus produces two kinases, a high molecular weight isoform (long MLCK) and a low molecular weight isoform (short MLCK), that are differentially expressed in smooth and nonmuscle tissues. To study the relative localization of the MLCK isoforms in cultured nonmuscle cells and to determine the spatial and temporal dynamics of MLCK localization during mitosis, we constructed green fluorescent protein fusions of the long and short MLCKs. In interphase cells, localization of the long MLCK to stress fibers is mediated by five DXRXXL motifs, which span the junction of the NH(2)-terminal extension and the short MLCK. In contrast, localization of the long MLCK to the cleavage furrow in dividing cells requires the five DXRXXL motifs as well as additional amino acid sequences present in the NH(2)-terminal extension. Thus, it appears that nonmuscle cells utilize different mechanisms for targeting the long MLCK to actomyosin structures during interphase and mitosis. Further studies have shown that the long MLCK has twofold lower kinase activity in early mitosis than in interphase or in the early stages of postmitotic spreading. These findings suggest a model in which MLCK and the myosin II phosphatase (Totsukawa, G., Y. Yamakita, S. Yamashiro, H. Hosoya, D.J. Hartshorne, and F. Matsumura. 1999. J. Cell Biol. 144:735-744) act cooperatively to regulate the level of Ser 19-phosphorylated myosin II during mitosis and initiate cytokinesis through the activation of myosin II motor activity.

Rab2 requires PKC iota/lambda to recruit beta-COP for vesicle formation

The small GTPase Rab2 initiates the recruitment of soluble components necessary for protein sorting and recycling from pre-Golgi intermediates. Our previous studies showed that Rab2 required protein kinase C (PKC) or a PKC-like protein to recruit beta-COP to membrane (Tisdale EJ, Jackson M. Rab2 protein enhances coatomer recruitment to pre-Golgi intermediates. J Biol Chem 1998;273: 17269-17277). We investigated the role of PKC in Rab2 function by first determining the active isoform that associates with membranes used in our assay. Western blot analysis detected three isoforms: PKC alpha, gamma and iota/lambda. A quantitative binding assay was used to measure recruitment of these kinases when incubated with Rab2. Only PKC iota/lambda translocated to membrane in a dose-dependent manner. Microsomes treated with anti-PKC iota/lambda lost the ability to bind beta-COP, suggesting that Rab2 requires PKC iota/lambda for beta-COP recruitment. The recruitment of beta-COP to membranes is not regulated by PKC iota/lambda kinase activity. However, PKC iota/lambda activity was necessary for Rab2-mediated vesicle budding. We found that the addition of either a kinase-deficient PKC iota/lambda mutant or atypical PKC pseudosubstrate peptide to the binding assay drastically reduced vesicle formation. These data suggest that Rab2 causes translocation of PKC iota/lambda to vesicular tubular clusters (VTCs), which promotes the recruitment of COPI to generate retrograde-transport vesicles.

RGS4 and RGS2 bind coatomer and inhibit COPI association with Golgi membranes and intracellular transport

COPI, a protein complex consisting of coatomer and the small GTPase ARF1, is an integral component of some intracellular transport carriers. The association of COPI with secretory membranes has been implicated in the maintenance of Golgi integrity and the normal functioning of intracellular transport in eukaryotes. The regulator of G protein signaling, RGS4, interacted with the COPI subunit beta'-COP in a yeast two-hybrid screen. Both recombinant RGS4 and RGS2 bound purified recombinant beta'-COP in vitro. Endogenous cytosolic RGS4 from NG108 cells and RGS2 from HEK293T cells cofractionated with the COPI complex by gel filtration. Binding of beta'-COP to RGS4 occurred through two dilysine motifs in RGS4, similar to those contained in some aminoglycoside antibiotics that are known to bind coatomer. RGS4 inhibited COPI binding to Golgi membranes independently of its GTPase-accelerating activity on G(ialpha). In RGS4-transfected LLC-PK1 cells, the amount of COPI in the Golgi region was considerably reduced compared with that in wild-type cells, but there was no detectable difference in the amount of either Golgi-associated ARF1 or the integral Golgi membrane protein giantin, indicating that Golgi integrity was preserved. In addition, RGS4 expression inhibited trafficking of aquaporin 1 to the plasma membrane in LLC-PK1 cells and impaired secretion of placental alkaline phosphatase from HEK293T cells. The inhibitory effect of RGS4 in these assays was independent of GTPase-accelerating activity but correlated with its ability to bind COPI. Thus, these data support the hypothesis that these RGS proteins sequester coatomer in the cytoplasm and inhibit its recruitment onto Golgi membranes, which may in turn modulate Golgi-plasma membrane or intra-Golgi transport.

Protein kinase C binding partners

Members of the protein kinase C family respond to second messengers and are involved in controlling a broad array of cellular functions. The overlapping specificity and promiscuity of these proteins has promoted the view that specific binding proteins constrain individual family members to create the appropriate specificity of action. It is speculated that such protein kinase C-regulator protein interactions affect substrate availability as well as exposure to allosteric activator(s) and that consequent interactions specify cellular location and impose integration with other signaling systems. These predicted features have been realized in the identification of many protein kinase C interacting proteins and examples of these are discussed.

Membrane targeting and cytoplasmic sequestration in the spatiotemporal localization of human protein kinase C alpha

In order to map the molecular determinants that dictate the subcellular localization of human protein kinase C alpha (hPKCalpha), full-length and deletion mutants of hPKCalpha were tagged with the green fluorescent protein (GFP) and transiently expressed in GH3B6 cells. We found that upon thyrotropin-releasing hormone (TRH) or phorbol 12-myristate 13-acetate stimulation, hPKCalpha-GFP was localized exclusively in regions of cell-cell contacts. Surprisingly, PKCalpha failed to translocate in single cells despite the presence of TRH receptors, as attested by the TRH-induced rise in intracellular calcium concentration in these cells. TRH-stimulated translocation of hPKCalpha-GFP from the cytoplasm to cell-cell contacts was a biphasic process: a fast (measured in seconds) and transient phase, followed by a slower (approximately 1 hour) and long lasting phase. The latter and the translocation induced by phorbol 12-myristate 13-acetate absolutely required the N-terminal V1 region. In contrast to the full-length hPKCalpha, the N-terminal regulatory domain alone or associated with the V3 hinge region was spontaneously and uniformly localized at the plasma membrane of single and apposed cells. However, treatment with the calcium chelator BAPTA/AM induced a differential cytoplasmic/nuclear redistribution of the regulatory domain, depending on its association with V3, which suggests the existence of a mechanism controlling the cytoplasmic sequestration of inactive hPKCalpha and involving the V3 region. By using other deletion mutants, we were able to map the sequence required for this sequestration to the C2+V3 regions. This work points to the existence of a complex interplay between membrane targeting and cytoplasmic sequestration in the control of the spatiotemporal localization of hPKCalpha.

Characterization of calreticulin as a protein interacting with protein kinase C

A protein kinase C (PKC)-binding protein was purified to homogeneity from the Triton-insoluble fraction from rat hepatocytes homogenates. The protein was identified as the mature calreticulin chain by N-terminal amino acid sequencing and by its immunoreactivity with anti-calreticulin antibody raised against the C-terminal KDEL (single-letter code) sequence. The calculated molecular mass was 46. 6 kDa but the protein migrates in SDS/PAGE as a doublet with apparent molecular masses of 60 and 55 kDa. Studies in vitro with purified calreticulin with the use of an overlay assay approach demonstrated that it binds to activated PKC isoenzymes expressed in rat hepatocytes. Phosphorylation of purified calreticulin with a PKC isoenzyme-specific immune complex kinase assay showed that it is also a very good substrate for all PKC isoforms in vitro. The treatment of intact cells with phorbol ester or with adrenaline (epinephrine) plus propranolol increased calreticulin phosphorylation, which was blocked by the pretreatment of cells with the PKC-specific inhibitor Ro 31-8220. The analysis of calreticulin immunoprecipitates from control or treated cells indicated that PKCalpha, PKCbeta, PKCtheta;, PKCzeta and PKCmu, but not PKCdelta or PKCepsilon, co-immunoprecipitated with calreticulin. Taken together, our results indicate that PKC interacts in vivo with calreticulin and suggest that they can operate in common signalling pathways.

Coat proteins regulating membrane traffic

This review focuses on the roles of coat proteins in regulating the membrane traffic of eukaryotic cells. Coat proteins are recruited to the donor organelle membrane from a cytosolic pool by specific small GTP-binding proteins and are required for the budding of coated vesicles. This review first describes the four types of coat complexes that have been characterized so far: clathrin and its adaptors, the adaptor-related AP-3 complex, COPI, and COPII. It then discusses the ascribed functions of coat proteins in vesicular transport, including the physical deformation of the membrane into a bud, the selection of cargo, and the targeting of the budded vesicle. It also mentions how the coat proteins may function in an alternative model for transport, namely via tubular connections, and how traffic is regulated. Finally, this review outlines the evidence that related coat proteins may regulate other steps of membrane traffic.

Targeting of PKA, PKC and protein phosphatases to cellular microdomains

The intracellular responses to many distinct extracellular signals involve the direction of broad-based protein kinases and protein phosphatases to catalyse quite specific protein phosphorylation/dephosphorylation events. It is now clear that such specificity is often achieved through subcellular targeting of distinct pools of kinase or phosphatase towards particular substrates at specific subcellular locations. Given the dynamic nature of protein phosphorylation reactions, coordinated control of both kinase and phosphatases is often required and complexes formed by common scaffold or targeting proteins exist to direct both kinase and phosphatase to the same subcellular location. In many cases more than one kinase or phosphatase is required and binding proteins which target more than one kinase or phosphatase have now been identified. This review summarizes recent findings relating to the concept of targeting PKA, PKC and the major serine/threonine phosphatases, PP1, PP2A and PP2B, through the formation of multi-enzyme signalling complexes.

The cytoplasmic coatomer protein COPI. A potential translational regulator

Expression of the asialoglycoprotein receptor (ASGR) by the human hepatocellular carcinoma cell lines HepG2 and HuH-7 in response to intracellular cGMP concentrations was previously shown to be regulated at the translational level (1). Stable transfection of COS-7 cells with deletion constructs encoding the asialoglycoprotein receptor H2b subunit localized the cGMP-responsive cis-acting element to the mRNA 5'-untranslated region. Resolution by anion exchange chromatography of an S-100 isolated from human liver resulted in the partial purification of an RNA-binding protein specific to this cis-acting element. Northwestern analysis using the 5'-untranslated region as probe indicated that a 140-kDa protein was the potential RNA-binding protein. Sequence of tryptic peptides suggested that the 140-kDa protein was the alpha-COP subunit of coatomer protein COPI, usually associated with trans-Golgi network membrane traffic. Immunoblot analysis confirmed the presence of alpha-COP in the Mono-Q fraction as well as that of a second coatomer subunit, beta-COP. Antibody induced gel retardation supershift confirmed the identification of the RNA-binding proteins as alpha- and beta-COP. Although the RNA recognition motif appears to reside solely in alpha-COP, antibody-induced supershift strongly indicated that the entire coatomer complex was the trans-acting factor. Depletion of S-100 with the antibody to beta-COP confirmed that the coatomer was the sole protein binding to the ASGR mRNA 5'-untranslated region in liver cytosol and responsible for inhibition of in vitro translation of the asialoglycoprotein receptor.

Sustained in vivo cardiac protection by a rationally designed peptide that causes epsilon protein kinase C translocation

Brief periods of cardiac ischemia trigger protection from subsequent prolonged ischemia (preconditioning). epsilon Protein kinase C (epsilonPKC) has been suggested to mediate preconditioning. Here, we describe an epsilonPKC-selective agonist octapeptide, psiepsilon receptor for activated C-kinase (psiepsilonRACK), derived from an epsilonPKC sequence homologous to its anchoring protein, epsilonRACK. Introduction of psiepsilonRACK into isolated cardiomyocytes, or its postnatal expression as a transgene in mouse hearts, increased epsilonPKC translocation and caused cardio-protection from ischemia without any deleterious effects. Our data demonstrate that epsilonPKC activation is required for protection from ischemic insult and suggest that small molecules that mimic this epsilonPKC agonist octapeptide provide a powerful therapeutic approach to protect hearts at risk for ischemia.

beta2-chimaerin is a novel target for diacylglycerol: binding properties and changes in subcellular localization mediated by ligand binding to its C1 domain

The members of the chimaerin family of Rac-GTPase-activating proteins possess a single C1 domain with high homology to those present in protein kinase C (PKC) isozymes. This domain in PKCs is involved in phorbol ester and diacylglycerol (DAG) binding. We previously have demonstrated that one of the chimaerin isoforms, beta2-chimaerin, binds phorbol esters with high affinity. In this study we analyzed the properties of beta2-chimaerin as a DAG receptor by using a series of conformationally constrained cyclic DAG analogues (DAG lactones) as probes. We identified analogs that bind to beta2-chimaerin with more than 100-fold higher affinity than 1-oleoyl-2-acetylglycerol. The potencies of these analogs approach those of the potent phorbol ester tumor promoters. The different DAG lactones show some selectivity for this novel receptor compared with PKCalpha. Cellular studies revealed that these DAG analogs induce translocation of beta2-chimaerin from cytosolic (soluble) to particulate fractions. Using green fluorescent protein-fusion proteins for beta2-chimaerin we determined that this novel receptor translocates to the perinuclear region after treatment with DAG lactones. Binding and translocation were prevented by mutation of the conserved Cys-246 in the C1 domain. The structural homology between the C1 domain of beta2-chimaerin and the C1b domain of PKCdelta also was confirmed by modeling analysis. Our results demonstrate that beta2-chimaerin is a high affinity receptor for DAG through binding to its C1 domain and supports the emerging concept that multiple pathways transduce signaling through DAG and the phorbol esters.

Coordinated movement of RACK1 with activated betaIIPKC

Protein kinase C (PKC) isozymes move upon activation from one intracellular site to another. PKC-binding proteins, such as receptors for activated C kinase (RACKs), play an important role in regulating the localization and diverse functions of PKC isozymes. RACK1, the receptor for activated betaIIPKC, determines the localization and functional activity of betaIIPKC. However, the mechanism by which RACK1 localizes activated betaIIPKC is not known. Here, we provide evidence that the intracellular localization of RACK1 changes in response to PKC activation. In Chinese hamster ovary cells transfected with the dopamine D2L receptor and in NG108-15 cells, PKC activation by either phorbol ester or a dopamine D2 receptor agonist caused the movement of RACK1. Moreover, PKC activation resulted in the in situ association and movement of RACK1 and betaIIPKC to the same intracellular sites. Time course studies indicate that PKC activation induces the association of the two proteins prior to their co-movement. We further show that association of RACK1 and betaIIPKC is required for the movement of both proteins. Our results suggest that RACK1 is a PKC shuttling protein that moves betaIIPKC from one intracellular site to another.

A novel nociceptor signaling pathway revealed in protein kinase C epsilon mutant mice

There is great interest in discovering new targets for pain therapy since current methods of analgesia are often only partially successful. Although protein kinase C (PKC) enhances nociceptor function, it is not known which PKC isozymes contribute. Here, we show that epinephrine-induced mechanical and thermal hyperalgesia and acetic acid-associated hyperalgesia are markedly attenuated in PKCepsilon mutant mice, but baseline nociceptive thresholds are normal. Moreover, epinephrine-, carrageenan-, and nerve growth factor- (NGF-) induced hyperalgesia in normal rats, and epinephrine-induced enhancement of tetrodotoxin-resistant Na+ current (TTX-R I(Na)) in cultured rat dorsal root ganglion (DRG) neurons, are inhibited by a PKCepsilon-selective inhibitor peptide. Our findings indicate that PKCepsilon regulates nociceptor function and suggest that PKCepsilon inhibitors could prove useful in the treatment of pain.

Involvement of protein kinase Cepsilon (PKCepsilon) in thyroid cell death. A truncated chimeric PKCepsilon cloned from a thyroid cancer cell line protects thyroid cells from apoptosis

The protein kinase C (PKC) family has been implicated in the regulation of apoptosis. However, the contribution of individual PKC isozymes to this process is not well understood. We reported amplification of the chromosome 2p21 locus in 28% of thyroid neoplasms, and in the WRO thyroid carcinoma cell line. By positional cloning we identified a rearrangement and amplification of the PKCepsilon gene, that maps to 2p21, in WRO cells. This resulted in the overexpression of a chimeric/truncated PKCepsilon (Tr-PKCepsilon) mRNA, coding for N-terminal amino acids 1-116 of the isozyme fused to an unrelated sequence. Expression of the Tr-PKCepsilon protein in PCCL3 cells inhibited activation-induced translocation of endogenous PKCepsilon, but its kinase activity was unaffected, consistent with a dominant negative effect of the mutant protein on activation-induced translocation of wild-type PKCepsilon and/or displacement of the isozyme to an aberrant subcellular location. Cell lines expressing Tr-PKCepsilon grew to a higher saturation density than controls. Moreover, cells expressing Tr-PKCepsilon were resistant to apoptosis, which was associated with higher Bcl-2 levels, a marked impairment in p53 stabilization, and dampened expression of Bax. These findings point to a role for PKCepsilon in apoptosis-signaling pathways in thyroid cells, and indicate that a naturally occurring PKCepsilon mutant that functions as a dominant negative can block cell death triggered by a variety of stimuli.

Characterization of the binding and phosphorylation of cardiac calsequestrin by epsilon protein kinase C

In this study, we report the cloning of the rat cardiac isoform of calsequestrin on the basis of its interaction with an epsilonprotein kinase C-unique sequence (epsilonV1) derived form the epsilonprotein kinase C regulatory domain. Calsequestrin binds activated epsilonprotein kinase C holoenzyme better than the inactive enzyme and nearly three times better than other protein kinase C isozymes. The interaction between epsilonprotein kinase C and calsequestrin is mediated by sequences in both the regulatory and kinase domains of the epsilonprotein kinase C. Finally, we show that calsequestrin is an epsilonprotein kinase C substrate in vitro and protein kinase C phosphorylation of calsequestrin leads to a decreased binding of epsilonprotein kinase C to calsequestrin.

Direct interaction in T-cells between thetaPKC and the tyrosine kinase p59fyn

The protein kinase C (PKC) family has been clearly implicated in T-cell activation as have several nonreceptor protein-tyrosine kinases associated with the T-cell receptor, including p59fyn. This report demonstrates that thetaPKC and p59fyn specifically interact in vitro, in the yeast two-hybrid system, and in T-cells. Further indications of direct interaction are that p59fyn potentiates thetaPKC catalytic activity and that thetaPKC is a substrate for tyrosine phosphorylation by p59fyn. This interaction may account for the localization of thetaPKC following T-cell activation, pharmacological disruption of which results in specific cell-signaling defects. The demonstration of a physical interaction between a PKC and a protein-tyrosine kinase expands the class of PKC-anchoring proteins (receptors for activated C kinases (RACKs)) and demonstrates a direct connection between these two major T-cell-signaling pathways.

Protein kinase C delta is essential for etoposide-induced apoptosis in salivary gland acinar cells

We have previously shown that parotid C5 salivary acinar cells undergo apoptosis in response to etoposide treatment as indicated by alterations in cell morphology, caspase-3 activation, DNA fragmentation, sustained activation of c-Jun N-terminal kinase, and inactivation of extracellular regulated kinases 1 and 2. Here we report that apoptosis results in the caspase-dependent cleavage of protein kinase C-delta (PKCdelta) to a 40-kDa fragment, the appearance of which correlates with a 9-fold increase in PKCdelta activity. To understand the function of activated PKCdelta in apoptosis, we have used the PKCdelta-specific inhibitor, rottlerin. Pretreatment of parotid C5 cells with rottlerin prior to the addition of etoposide blocks the appearance of the apoptotic morphology, the sustained activation of c-Jun N-terminal kinase, and inactivation of extracellular regulated kinases 1 and 2. Inhibition of PKCdelta also partially inhibits caspase-3 activation and DNA fragmentation. Immunoblot analysis shows that the PKCdelta cleavage product does not accumulate in parotid C5 cells treated with rottlerin and etoposide together, suggesting that the catalytic activity of PKCdelta may be required for cleavage. PKCalpha and PKCbeta1 activities also increase during etoposide-induced apoptosis. Inhibition of these two isoforms with Gö6976 slightly suppresses the apoptotic morphology, caspase-3 activation, and DNA fragmentation, but has no effect on the sustained activation of c-Jun N-terminal kinase or inactivation of extracellular regulated kinase 1 and 2. These data demonstrate that activation of PKCdelta is an integral and essential part of the apoptotic program in parotid C5 cells and that specific activated isoforms of PKC may have distinct functions in cell death.

The RACK1 signaling scaffold protein selectively interacts with the cAMP-specific phosphodiesterase PDE4D5 isoform

The WD-repeat protein receptor for activated C-kinase (RACK1) was identified by its interaction with the cyclic AMP-specific phosphodiesterase (PDE4) isoform PDE4D5 in a yeast two-hybrid screen. The interaction was confirmed by co-immunoprecipitation of native RACK1 and PDE4D5 from COS7, HEK293, 3T3-F442A, and SK-N-SH cell lines. The interaction was unaffected by stimulation of the cells with the phorbol ester phorbol 2-myristate 3-acetate. PDE4D5 did not interact with two other WD-repeat proteins, beta'-coatomer protein and Gsbeta, in two-hybrid tests. RACK1 did not interact with other PDE4D isoforms or with known PDE4A, PDE4B, and PDE4C isoforms. PDE4D5 and RACK1 interacted with high affinity (Ka approximately 7 nM) [corrected] when they were expressed and purified from Escherichia coli, demonstrating that the interaction does not require intermediate proteins. The binding of the E. coli-expressed proteins did not alter the kinetics of cAMP hydrolysis by PDE4D5 but caused a 3-4-fold change in its sensitivity to inhibition by the PDE4 selective inhibitor rolipram. The subcellular distributions of RACK1 and PDE4D5 were extremely similar, with the major amount of both proteins (70%) in the high speed supernatant (S2) fraction. Analysis of constructs with specific deletions or single amino acid mutations in PDE4D5 demonstrated that a small cluster of amino acids in the unique amino-terminal region of PDE4D5 was necessary for its interaction with RACK1. We suggest that RACK1 may act as a scaffold protein to recruit PDE4D5 and other proteins into a signaling complex.

PKCepsilon, via its regulatory domain and independently of its catalytic domain, induces neurite-like processes in neuroblastoma cells

To investigate the role of protein kinase C (PKC) isoforms in regulation of neurite outgrowth, PKCalpha, betaII, delta, and epsilon fused to enhanced green fluorescent protein (EGFP) were transiently overexpressed in neuroblastoma cells. Overexpression of PKCepsilon-EGFP induced cell processes whereas the other isoforms did not. The effect of PKCepsilon-EGFP was not suppressed by the PKC inhibitor GF109203X. Instead, process formation was more pronounced when the regulatory domain was introduced. Overexpression of various fragments from PKCepsilon regulatory domain revealed that a region encompassing the pseudosubstrate, the two C1 domains, and parts of the V3 region were necessary and sufficient for induction of processes. By deleting the second C1 domain from this construct, a dominant-negative protein was generated which suppressed processes induced by full-length PKCepsilon and neurites induced during retinoic acid- and growth factor-induced differentiation. As with neurites in differentiated neuroblastoma cells, processes induced by the PKCepsilon- PSC1V3 protein contained alpha-tubulin, neurofilament-160, and F-actin, but the PKCepsilon-PSC1V3-induced processes lacked the synaptic markers synaptophysin and neuropeptide Y. These data suggest that PKCepsilon, through its regulatory domain, can induce immature neurite-like processes via a mechanism that appears to be of importance for neurite outgrowth during neuronal differentiation.

Isolation, cloning, and characterization of a new mammalian coronin family member, coroninse, which is regulated within the protein kinase C signaling pathway

In order to understand the regulatory role of protein kinase C (PKC) in secretory epithelia, it is necessary to identify and characterize specific downstream targets. We previously identified one such protein in studies of gastric parietal cells. This protein was referred to as pp66 because it migrated with an apparent molecular mass of 66 kDa on SDS-polyacrylamide gels. The phosphorylation of pp66 is increased by the cholinergic agonist, carbachol, and by the PKC activator, phorbol-12-myristate-13-acetate, in a calcium-independent manner. In this study, we have purified pp66 to homogeneity and cloned the complete open reading frame. GenBankTM searches revealed a 45% homology with the Dictyostelium actin-binding protein, coronin, and approximately 67% homology with the previously cloned human and bovine coronin-like homologue, p57. pp66 appears to be most highly expressed in the gastrointestinal mucosa and in kidney and lung. Confocal microscopic studies of an enhanced green fluorescent protein fusion construct of pp66 in cultured parietal cells and in Madin-Darby canine kidney cells indicate that pp66 preferentially localizes in F-actin-rich regions. On the basis of our findings, we propose that pp66 may play an important, PKC-dependent role in regulating membrane/cytoskeletal rearrangements in epithelial cells. We have tentatively named this protein coroninse, because it appears to be highly expressed in secretory epithelia.

Atomic structure of clathrin: a beta propeller terminal domain joins an alpha zigzag linker

Clathrin triskelions form the lattice that organizes recruitment of proteins to coated pits and helps drive vesiculation of the lipid bilayer. We report the crystal structure at 2.6 A resolution of a 55 kDa N-terminal fragment from the 190 kDa clathrin heavy chain. The structure comprises the globular "terminal domain" and the linker that joins it to the end of a triskelion leg. The terminal domain is a seven-blade beta propeller, a structure well adapted to interaction with multiple partners, such as the AP-1 and AP-2 sorting adaptor complexes and the nonvisual arrestins. The linker is an alpha-helical zigzag emanating from the propeller domain. We propose that this simple motif may extend into the rest of the clathrin leg.

Molecular analysis of the interactions between protein kinase C-epsilon and filamentous actin

Protein kinase C-epsilon (PKC-epsilon) contains a putative actin binding motif that is unique to this individual member of the PKC gene family. We have used deletion mutagenesis to determine whether this hexapeptide motif is required for the physical association of PKC-epsilon and actin. Full-length recombinant PKC-epsilon, but not PKC-betaII, -delta, -eta, or -zeta, bound to filamentous actin in a phorbol ester-dependent manner. Deletion of PKC-epsilon amino acids 222-230, encompassing a putative actin binding motif, completely abrogated this binding activity. When NIH 3T3 cells overexpressing either PKC-epsilon or the deletion mutant of this isozyme were treated with phorbol ester only wild-type PKC-epsilon colocalized with actin in zones of cell adhesion. In binary reactions, it was possible to demonstrate that purified filamentous actin is capable of directly stimulating PKC-epsilon phosphotransferase activity. These and other findings support the hypothesis that a conformationally hidden actin binding motif in the PKC-epsilon sequence becomes exposed upon activation of this isozyme and functions as a dominant localization signal in NIH 3T3 fibroblasts. This protein-protein interaction is sufficient to maintain PKC-epsilon in a catalytically active conformation.

Protein kinases C: potential targets for intervention in diabetic nephropathy

Protein kinases C are a family of serine threonine protein kinases that play key roles in extracellular signal transduction. Inappropriate activation of protein kinase C has been implicated in the pathophysiology of many diseases, including diabetes mellitus. Indeed, protein kinase C activation may contribute not only to the pathogenesis of diabetic complications such as nephropathy and retinopathy, but also to insulin resistance. Growing awareness that protein kinase C isoforms subserve specific subcellular functions has led to the development of isoform-specific inhibitors, which may be useful investigational tools and therapeutic agents for attenuating the effects of inappropriate protein kinase C activity. Here we review the role played by protein kinases C in diabetic nephropathy and the recent progress that has been made to modulate its activity using specific inhibitors.

The role of C2 domains in Ca2+-activated and Ca2+-independent protein kinase Cs in aplysia

In the nervous system of the marine mollusk Aplysia there are two protein kinase C (PKC) isoforms, the Ca2+-activated PKC Apl I and the Ca2+-independent PKC Apl II. PKC Apl I, but not PKC Apl II is activated by a short-term application of the neurotransmitter serotonin. This may be explained by the fact that purified PKC Apl II requires a higher mole percentage of phosphatidylserine to stimulate enzyme activity than does PKC Apl I. In order to understand the molecular basis for this difference, we have compared the ability of lipids to interact with the purified kinases and with regulatory domain fusion proteins derived from the kinases using a variety of assays including kinase activity, phorbol dibutyrate binding, and liposome binding. We found that a C2 domain fusion protein derived from PKC Apl I binds to lipids constitutively, while a C2 domain fusion protein derived from PKC Apl II does not. In contrast, fusion proteins containing the C1 domains of PKC Apl I and PKC Apl II showed only small differences in lipid interactions. Thus, while the presence of a C2 domain assists lipid-mediated activation of PKC Apl I, it inhibits activation of PKC Apl II.

Activation of epsilon protein kinase C correlates with a cardioprotective effect of regular ethanol consumption

In addition to decreasing the incidence of myocardial infarction, recent epidemiological data suggest that regular alcohol consumption improves survival after myocardial infarction. We recently found that chronic ethanol exposure induces long-term protection against cardiac ischemia-reperfusion injury, which improves myocardial recovery after infarction. Furthermore, this cardioprotection by ethanol is mediated through myocyte adenosine A1 receptors. We now determine the role of protein kinase C (PKC) in ethanol's protective effect against ischemia-reperfusion injury. Using perfused hearts of ethanol-fed guinea pigs, we find that improved contractile recovery and creatine kinase release after ischemia-reperfusion are abolished by PKC inhibition with chelerythrine. Western blot analysis and immunofluorescence localization demonstrate that regular ethanol consumption causes sustained translocation (activation) of epsilonPKC, but not delta or alphaPKC. This same isozyme is directly implicated in ischemic preconditioning's protection against ischemia-reperfusion injury. Our findings suggest (i) that regular ethanol consumption induces long-term cardioprotection through sustained translocation of epsilonPKC and (ii) that PKC activity is necessary at the time of ischemia to mediate ethanol's protective effect against ischemia-reperfusion injury. Studying this selective effect of ethanol on epsilonPKC activation may lead to new therapies to protect against ischemia-reperfusion injury in the heart and other organ systems.

The extended protein kinase C superfamily

Members of the mammalian protein kinase C (PKC) superfamily play key regulatory roles in a multitude of cellular processes, ranging from control of fundamental cell autonomous activities, such as proliferation, to more organismal functions, such as memory. However, understanding of mammalian PKC signalling systems is complicated by the large number of family members. Significant progress has been made through studies based on comparative analysis, which have defined a number of regulatory elements in PKCs which confer specific location and activation signals to each isotype. Further studies on simple organisms have shown that PKC signalling paradigms are conserved through evolution from yeast to humans, underscoring the importance of this family in cellular signalling and giving novel insights into PKC function in complex mammalian systems.

Localization of atypical protein kinase C isoforms into lysosome-targeted endosomes through interaction with p62

An increasing number of independent studies indicate that the atypical protein kinase C (PKC) isoforms (aPKCs) are critically involved in the control of cell proliferation and survival. The aPKCs are targets of important lipid mediators such as ceramide and the products of the PI 3-kinase. In addition, the aPKCs have been shown to interact with Ras and with two novel proteins, LIP (lambda-interacting protein; a selective activator of lambda/iotaPKC) and the product of par-4 (a gene induced during apoptosis), which is an inhibitor of both lambda/iotaPKC and zetaPKC. LIP and Par-4 interact with the zinc finger domain of the aPKCs where the lipid mediators have been shown to bind. Here we report the identification of p62, a previously described phosphotyrosine-independent p56(lck) SH2-interacting protein, as a molecule that interacts potently with the V1 domain of lambda/iotaPKC and, albeit with lower affinity, with zetaPKC. We also show in this study that ectopically expressed p62 colocalizes perfectly with both lambda/iotaPKC and zetaPKC. Interestingly, the endogenous p62, like the ectopically expressed protein, displays a punctate vesicular pattern and clearly colocalizes with endogenous lambda/iotaPKC and endogenous zetaPKC. P62 colocalizes with Rab7 and partially with lamp-1 and limp-II as well as with the epidermal growth factor (EGF) receptor in activated cells, but not with Rab5 or the transferrin receptor. Of functional relevance, expression of dominant negative lambda/iotaPKC, but not of the wild-type enzyme, severely impairs the endocytic membrane transport of the EGF receptor with no effect on the transferrin receptor. These findings strongly suggest that the aPKCs are anchored by p62 in the lysosome-targeted endosomal compartment, which seems critical for the control of the growth factor receptor trafficking. This is particularly relevant in light of the role played by the aPKCs in mitogenic cell signaling events.