Rapid in vitro conformational changes of the catalytic site of PKC alpha assessed by FIM-1 fluorescence

F-actin does not modulate the initial steps of the protein kinase C activation process in living nerve cells

Actin is a major substrate for protein kinase C (PKC) and PKC is considered a modulator of the actin network. In addition in vitro studies (Biochemistry 39 (2000) 271) have suggested that all PKC isoforms bind to actin during the process of activation of the enzyme. To test the physiological significance of such a coupling we used living PC12 cells and primary cultures of cerebellar granule cells. When PC12 cells were treated with either latrunculin B, which impairs actin polymerization, or phalloidin, which stabilizes actin filaments, we observed a significant reduction of the [Ca2+]i response revealed by Fura-2 fluorescence, while the PKC conformational changes followed by Fim-1 fluorescence were unaffected. The responses induced either by cell depolarization or muscarinic receptor activation were similarly affected by the toxin treatment of PC12 cells. In cerebellar granule cells the [Ca2+]i response induced by KCl depolarization was increased by latrunculin treatment, whereas no effect was observed on the PKC response. Latrunculin had no effect on the NMDA-induced responses in these cells. Finally we also show that the response induced by a long-lasting depolarization, which mimics stimulation leading to neuronal plasticity, was not significantly altered by latrunculin or phalloidin treatment of the cells. These results suggest that the actin network is not involved in the initial steps of the PKC activation process in living nerve cells.

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

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

Regulation of PKC alpha activity by C1-C2 domain interactions

In this study, the role of interdomain interactions involving the C1 and C2 domains in the mechanism of activation of PKC was investigated. Using an in vitro assay containing only purified recombinant proteins and the phorbol ester, 4 beta-12-O-tetradecanoylphorbol-13-acetate (TPA), but lacking lipids, it was found that PKC alpha bound specifically, and with high affinity, to a alpha C1A-C1B fusion protein of the same isozyme. The alpha C1A-C1B domain also potently activated the isozyme in a phorbol ester- and diacylglycerol-dependent manner. The level of this activity was comparable with that resulting from membrane association induced under maximally activating conditions. Furthermore, it was found that alpha C1A-C1B bound to a peptide containing the C2 domain of PKC alpha. The alpha C1A-C1B domain also activated conventional PKC beta I, -beta II, and -gamma isoforms, but not novel PKC delta or -epsilon. PKC delta and -epsilon were each activated by their own C1 domains, whereas PKC alpha, -beta I, -beta II, or -gamma activities were unaffected by the C1 domain of PKC delta and only slightly activated by that of PKC epsilon. PKC zeta activity was unaffected by its own C1 domain and those of the other PKC isozymes. Based on these findings, it is proposed that the activating conformational change in PKC alpha results from the dissociation of intra-molecular interactions between the alpha C1A-C1B domain and the C2 domain. Furthermore, it is shown that PKC alpha forms dimers via inter-molecular interactions between the C1 and C2 domains of two neighboring molecules. These mechanisms may also apply for the activation of the other conventional and novel PKC isozymes.

Effects of ethanol on protein kinase C activity induced by filamentous actin

Protein kinase C (PKC) can be activated by interaction with filamentous actin (F-actin) in the absence of membrane lipids (S.J. Slater, S.K. Milano, B.A. Stagliano, K.J. Gergich, J.P. Curry, F.J. Taddeo and C.D. Stubbs, Biochemistry 39 (2000) 271-280). Here, the effects of ethanol on the F-actin-induced activities of a panel of PKC isoforms consisting of 'conventional' (cPKC) alpha, betaI, gamma, 'novel' (nPKC) delta, epsilon and 'atypical' (aPKC) zeta were investigated using purified PKC and F-actin. Ethanol was found to inhibit the Ca2+- and phorbol ester-dependent activities of cPKCalpha and betaI, and the Ca2+- and phorbol ester-independent activity of cPKCgamma, whereas the activities of nPKCdelta, epsilon and aPKCzeta were unaffected. Although the activities of cPKCalpha and betaI induced by saturating levels of phorbol ester were inhibited by ethanol, the binding of these isozymes to F-actin was unaffected within the same phorbol ester concentration range. Conversely, within submaximal levels of phorbol ester, cPKCalpha and betaI activities were unaffected by ethanol whereas binding to F-actin was inhibited. The potency of the inhibition of F-actin-induced cPKCbetaI activity increased with n-alkanol chain length up to n-hexanol, after which it declined. The results indicate that PKC activities associated with F-actin, and therefore cellular processes involving the actin cytoskeleton, are potential targets for ethanol action. The effects of ethanol on these processes may differ according to the particular regulating PKC isoform, its intracellular localization and the presence of activators and cofactors.

Reversible protein kinase C activation in PC12 cells: effect of NGF treatment

Although protein kinase C (PKC) is a key enzyme in the signal transduction process, there is little information on the mechanism leading to PKC activation in living cells. Using a new fluorescence imaging method, we studied this mechanism and correlated PKC conformational changes with intracellular Ca2+ concentration. PC12 cells were simultaneously loaded with Fura-2-AM and Fim-1, two fluorescent probes, which recognize Ca2+ and PKC, respectively. KCl and carbachol (an agonist to muscarinic receptors) applications induced dose-dependent increases of fluorescence for both probes. Both Ca2+ and PKC responses were observed within seconds following KCl or carbachol application, and were reversible upon stimulus withdrawal. PKC activation kinetics was slightly more rapid than the Ca2+ response after KCl application. After nerve growth factor (NGF) treatment of the cells, the amplitude of the KCl-induced PKC responses was larger indicating an increase in the activated PKC-pool in these cells. This difference between control and NGF-treated cells was not observed following carbachol application, suggesting the involvement of different PKC pools. While the Ca2+ response uniformly occurred in the cytosol, the PKC response displayed a patch pattern with higher intensities in the peripheral zone near the plasma membrane. This heterogeneous distribution of PKC activation sites was similar to the immunocytological localization of Ca2+-dependent and independent PKC isoforms, which suggested that at least several PKC isoforms interacted with intracellular elements. Upon repeated stimulation, the PKC response rapidly desensitized.