Autophosphorylation of protein kinase C may require a high order of protein-phospholipid aggregates

Regulation of basal lateral membrane mobility and permeability to divalent cations by membrane associated-protein kinase C

Biological membrane stabilization is essential for maintenance of cellular homeostasis, functionality and appropriate response to various stimuli. Previous studies have showed that accumulation of PKCs in the cell membrane significantly downregulates the membrane fluidity and Ca²⁺ influxes through the membranes in activated cells. In addition, membrane-inserted form of PKCs has been found in a variety of resting mammalian cells and tissues. This study is aimed to investigate possible role of the endogenous membrane-associated PKCs in the modulation of basal membrane fluidity. Here, we showed that interfering PKC expression by chronic activation of PKC with phorbol myristate acetate (PMA) or shRNA targeting at PKCα lowered the levels of PKCα in cytosol, peripheral membrane and integral membrane pools, while short-term activation of PKC with PMA induced accumulation of PKCα in the membrane pool accompanied by a dramatic decrease in the cytosol fraction. The lateral membrane mobility increased or decreased in accordance with the abundance alterations in the membrane-associated PKCα by these treatments. In addition, membrane permeability to divalent cations including Ca²⁺, Mn²⁺ and Ba²⁺ were also potentiated or abrogated along with the changes in PKC expression on the plasma membrane. Membrane stabilizer ursodeoxycholate abolished both of the enhanced lateral membrane mobility and permeability to divalent cations due to PKCα deficiency, whereas Gö6983, a PKC antagonist, or Gd³⁺ and 2-aminoethyoxydipheyl borne, two Ca²⁺ channels blockers, showed no effect, suggesting that this PKC-related regulation is independent of PKC activation or a modulation of specific divalent cation channel. Thus, these data demonstrate that the native membrane-associated PKCα is involved in the maintenance of basal membrane stabilization in resting cells.

Amyloid-type fiber formation in control of enzyme action: interfacial activation of phospholipase A2

The lag-burst behavior in the action of phospholipase A(2) (PLA(2)) on 1,2-dipalmitoyl-sn-glycero-3-phosphocholine was investigated at temperatures slightly offset from the main phase transition temperature T(m) of this lipid, thus slowing down the kinetics of the activation process. Distinct stages leading to maximal activity were resolved using a combination of fluorescence parameters, including Förster resonance energy transfer between donor- and acceptor-labeled enzyme, fluorescence anisotropy, and lifetime, as well as thioflavin T fluorescence enhancement. We showed that the interfacial activation of PLA(2), evident after the preceding lag phase, coincides with the formation of oligomers staining with thioflavin T and subsequently with Congo red. Based on previous studies and our findings here, we propose a novel mechanism for the control of PLA(2), involving amyloid protofibrils with highly augmented enzymatic activity. Subsequently, these protofibrils form "mature" fibrils, devoid of activity. Accordingly, the process of amyloid formation is used as an on-off switch to obtain a transient burst in enzymatic catalysis.

Phosphorylation of connexin43 on serine368 by protein kinase C regulates gap junctional communication

Phorbol esters (e.g., TPA) activate protein kinase C (PKC), increase connexin43 (Cx43) phosphorylation, and decrease cell-cell communication via gap junctions in many cell types. We asked whether PKC directly phosphorylates and regulates Cx43. Rat epithelial T51B cells metabolically labeled with (32)P(i) yielded two-dimensional phosphotryptic maps of Cx43 with several phosphopeptides that increased in intensity upon TPA treatment. One of these peptides comigrated with the major phosphopeptide observed after PKC phosphorylation of immunoaffinity-purified Cx43. Purification of this comigrating peptide and subsequent sequencing indicated that the phosphorylated serine was residue 368. To pursue the functional importance of phosphorylation at this site, fibroblasts from Cx43(-/-) mice were transfected with either wild-type (Cx43wt) or mutant Cx43 (Cx43-S368A). Intercellular dye transfer studies revealed different responses to TPA and were followed by single channel analyses. TPA stimulation of T51B cells or Cx43wt-transfected fibroblasts caused a large increase in the relative frequency of approximately 50-pS channel events and a concomitant loss of approximately 100-pS channel events. This change to approximately 50-pS events was absent when cells transfected with Cx43-S368A were treated with TPA. These data strongly suggest that PKC directly phosphorylates Cx43 on S368 in vivo, which results in a change in single channel behavior that contributes to a decrease in intercellular communication.

Calcium and phosphatidylserine stimulate the self-association of conventional protein kinase C isoforms

Conflicting evidence exists as to whether "conventional" protein kinase C isoforms (cPKCs) function as monomers or oligomers. In this report, we demonstrate that purified cPKC isoforms can be rapidly cross-linked by the sulfhydryl-selective cross-linker bis(maleimido)hexane, but only in the presence of both Ca(2+) and phosphatidylserine; cross-linking was minimal in the presence of either of these activators alone. In addition, cross-linking of these cPKCs did not require Mg(2+) or ATP. Among the various phospholipids tested, phosphatidylserine was found to be the most effective in the promotion of cPKC self-association and for the stimulation of protein kinase activity toward the exogenous substrate histone. Phosphatidic acid and phosphatidylinositol were less effective in this regard, whereas phosphatidylcholine exhibited little ability to induce cPKC self-association or to stimulate kinase activity. An examination of the mechanism by which the cPKC isoforms self-associate in the presence of phospholipid/Ca(2+) revealed that this process occurred independently of phospholipid aggregation. Moreover, self-association was not inhibited by saturating the enzyme active site with a peptide substrate, suggesting that self-association is distinct from an enzyme-substrate interaction. Isoform-specific antibodies revealed that all cPKC isoforms (alpha, beta, and gamma) self-associate and that, in a mixture of cPKC isoforms, PKC-alpha forms primarily alpha-alpha homodimers. Besides cPKC interactions detected with purified enzyme, PKC-alpha also appeared capable of self-association in murine B82L fibroblasts that were treated with calcium ionophore, phorbol ester, or epidermal growth factor but not in untreated cells. Collectively, these data indicate that self-association occurs in parallel with cPKC activation, that self-association is not mediated by the substrate binding site, and, at least in the case of PKC-alpha, that the formation of isoform homodimers predominates.

The hydrophobic phosphorylation motif of conventional protein kinase C is regulated by autophosphorylation

BACKGROUND

A growing number of kinases are now known to be controlled by two phosphorylation switches, one on a loop near the entrance to the active site and a second on the carboxyl terminus. For the protein kinase C (PKC) family of enzymes, phosphorylation at the activation loop is mediated by another kinase but the mechanism for carboxy-terminal phosphorylation is still unclear. The latter switch contains two phosphorylation sites - one on a 'turn' motif and the second on a conserved hydrophobic phosphorylation motif - that are found separately or together in a number of other kinases.

RESULTS

Here, we investigated whether the carboxy-terminal phosphorylation sites of a conventional PKC are controlled by autophosphorylation or by another kinase. First, kinetic analyses revealed that a purified construct of the kinase domain of PKC betaII autophosphorylated on the Ser660 residue of the hydrophobic phosphorylation motif in an apparently concentration-independent manner. Second, kinase-inactive mutants of PKC did not incorporate phosphate at either of the carboxy-terminal sites, Thr641 or Ser660, when expressed in COS-7 cells. The inability to incorporate phosphate on the hydrophobic site was unrelated to the phosphorylation state of the other key phosphorylation sites: kinase-inactive mutants with negative charge at Thr641 and/or the activation-loop position were also not phosphorylated in vivo.

CONCLUSIONS

PKC betaII autophosphorylates at its conserved carboxy-terminal hydrophobic phosphorylation site by an apparently intramolecular mechanism. Expression studies with kinase-inactive mutants revealed that this mechanism is the only one responsible for phosphorylating this motif in vivo. Thus, conventional PKC autoregulates the carboxy-terminal phosphorylation switch following phosphorylation by another kinase at the activation loop switch.

Inhibitory properties of the regulatory domains of human protein kinase Calpha and mouse protein kinase Cepsilon

Two fusion proteins in which the regulatory domains of human protein kinase Calpha (Ralpha; amino acids 1-270) or mouse protein kinase Cepsilon (Repsilon; amino acids 1-385) were linked in frame with glutathione S-transferase (GST) were examined for their abilities to inhibit the catalytic activities of protein kinase Calpha (PKCalpha) and other protein kinases in vitro. Both GST-Ralpha and GST-Repsilon but not GST itself potently inhibited the activities of lipid-activated rat brain PKCalpha. In contrast, the fusion proteins had little or no inhibitory effect on the activities of the Ser/Thr protein kinases cAMP-dependent protein kinase, cGMP-dependent protein kinase, casein kinase II, myosin light chain kinase, and mitogen activated protein kinase or on the src Tyr kinase. GST-Ralpha and GST-Repsilon, on a molar basis, were 100-200-fold more potent inhibitors of PKCalpha activity than was the pseudosubstrate peptide PKC19-36. In addition, a GST-Ralpha fusion protein in which the first 32 amino acids of Ralpha were deleted (including the pseudosubstrate sequence from amino acids 19-31) was an effective competitive inhibitor of PKCalpha activity. The three GST-R fusion proteins also inhibited protamine-activated PKCalpha and proteolytically activated PKCalpha (PKM), two lipid-independent forms of PKCalpha; however, the IC50 values for inhibition were 1 order of magnitude greater than the IC50 values obtained in the presence of lipid. These results suggest that part of the inhibitory effect of the GST-R fusion proteins on lipid-activated PKCalpha may have resulted from sequestration of lipid activators. Nonetheless, as evidenced by their abilities to inhibit the lipid-independent forms of the enzyme, the GST-R fusion proteins also inhibited PKCalpha catalytic activity through direct interactions. These data indicate that the R domains of PKCalpha and PKCepsilon are specific inhibitors of protein kinase Calpha activity and suggest that regions of the R domain outside the pseudosubstrate sequence contribute to autoinhibition of the enzyme.

The gap-junction protein connexin 56 is phosphorylated in the intracellular loop and the carboxy-terminal region

The lens gap-junction protein, connexin 56, is modified by phosphorylation. Two-dimensional mapping of tryptic phosphopeptides of 32P-labeled connexin 56 from primary chicken-lens cultures showed that treatment with 12-O-tetradecanoylphorbol 13-acetate (TPA) induced an increase in phosphorylation of connexin 56 at specific constitutively phosphorylated sites. Treatment with 8-Br-cAMP or forskolin did not induce substantial changes in connexin 56 phosphorylation. Two phosphorylation sites within connexin 56, S493 and S118, were identified after HPLC purification and peptide sequencing of tryptic phosphopeptides from bacterially expressed connexin 56 fusion proteins phosphorylated by protein kinase C or protein kinase A in vitro. Comparisons of the two-dimensional maps of tryptic phosphopeptides from in vitro phosphorylated connexin 56 fusion proteins and in vivo phosphorylated connexin 56 showed that S493 and S118 were constitutively phosphorylated in lentoid-containing cultures, and that treatment with TPA induced an increase in phosphorylation of the peptides containing S118. It is suggested that phosphorylation of connexin 56 at S118 is involved in the TPA-induced decrease in intercellular communication and acceleration of connexin 56 degradation.

Formation of membrane domains during the activation of protein kinase C

The lateral membrane organization of phosphatidylserine, diacylglycerol, substrate, and Ca(2+)-dependent protein kinase C in large unilamellar vesicles was investigated by using fluorescence digital imaging microscopy. The formation of phosphatidylserine domains could be induced by either Ca2+, the MARCKS peptide, or protein kinase C. However, only Ca2+ could induce diacylglycerol to partition into the phosphatidylserine domains. In the complete protein kinase C assay mixture, two separate triple-labeling experiments demonstrated the colocalization of phosphatidylserine, protein kinase C, diacylglycerol, and the MARCKS peptide in domains. The amounts of all the labeled components in whole vesicles and in domains were measured at various concentrations of either phosphatidylserine, Ca2+, diacylglycerol, or the MARCKS peptide or with the addition of polylysine. The role of each component in forming membrane domains and in mediating the enzyme activity was analyzed. The results indicated that the inclusion of the MARCKS peptide in the domains, not just the binding of the substrate to vesicles, was especially important for PKC activity. The formation of PKC domains required the presence of DAG and Ca2+ at physiological ionic strength. The PKC activity was proportional to the amounts of PKC and substrate in the domains. The results also showed that the MARCKS peptide left the domains after being phosphorylated. A model for the activation of protein kinase C involving sequestering of the reaction components into membrane domains is proposed. The efficiency of the reaction was greatly increased by concentrating the activators, the enzyme, and the substrate into domains.

Molecular strategies for the dominant inhibition of protein kinase C

The regulatory (R) domain of PKC alpha fused to glutathione-S-transferase (GST-R alpha) competitively inhibited PKC activity associated with extracts of Y1 mouse adrenocortical tumor cells and the activities of several specific PKC isozymes. GST-R alpha did not inhibit the activities of cAMP-dependent protein kinase, cGMP-dependent protein kinase or calmodulin-dependent myosin light chain kinase. GST-R alpha inhibited PKC activities 20 times more potently than did a synthetic peptide corresponding to the pseudosubstrate sequence of PKC alpha. In intact yeast cells, the R domain prevented PKC beta-1-induced inhibition of growth and cytokinesis. These results indicate that the R domain of PKC alpha acts as a specific, dominant inhibitor of PKC activity, and suggest that the PKC alpha R domain may provide a useful genetic tool to assess the roles of PKC in various signal transduction processes.

Protein kinase C-alpha is multiply phosphorylated in response to phorbol ester stimulation of PC12 cells

The protein kinase C (PKC) family consists of a number of closely related isotypes, whose in vivo phosphorylation state is regulated in a dynamic fashion by the enzyme's activators. We have investigated here the changes in PKC phosphorylation in response to phorbol ester. Using a combination of hydroxylapatite chromatography and immunoblot with isotype-specific antibodies, we identified PKC-alpha, -delta, -epsilon, and -zeta as the isotypes expressed in PC12 cells. A two-dimensional immunoblot approach was then developed to measure the changes in the phosphorylation state of PKC-alpha before and after exposure of intact PC12 cells to phorbol ester. We found a pool of four differentially migrating PKC-alpha forms in untreated cells, which undergoes an acidic shift after phorbol ester. Furthermore, a similar shift in the two-dimensional immunoblot profile of PKC-alpha was the result of the enzyme autophosphorylation upon in vitro treatment with a combination of phosphatidylserine and phorbol ester, an effect which was enhanced by co-application of purified bovine lung cGMP-dependent protein kinase-I (PKG-I). These results demonstrate a multiple phosphorylation of PKC-alpha in untreated PC12 cells and suggest that various levels of autophosphorylation and trans-phosphorylation of this isoenzyme may occur in response to phorbol ester.

Regulatory domain of human protein kinase C alpha dominantly inhibits protein kinase C beta-I-regulated growth and morphology in Saccharomyces cerevisiae

This study demonstrates that the isolated regulatory (R) domain (amino acids 1-270) of human protein kinase C alpha (PKC alpha) is a potent inhibitor of PKC beta-I activity in a yeast expression system. The PKC alpha R domain fused to glutathione-S-transferase competitively inhibited the activity of yeast-expressed rat PKC beta-I in vitro (Ki = 0.2 microns) and was 400-fold more potent than a synthetic pseudosubstrate peptide corresponding to amino acids 19-36 from PKC alpha. In contrast, the fusion protein did not affect the activity of the purified catalytic subunit of cAMP-dependent protein kinase. The PKC alpha R domain (without glutathione-S-transferase [GST]) also was tested for its ability to inhibit PKC beta-I activity in vivo, in a yeast strain expressing rat PKC beta-I. Upon treatment with a PKC-activating phorbol ester, yeast cells expressing rat PKC beta-I were growth-inhibited and a fraction of the cells appeared as long chains. Coexpression of the R domain with rat PKC beta-I blocked the phorbol ester-induced inhibition of yeast cell growth and the phorbol ester-dependent alterations in yeast cell morphology. These results indicate that the R domain of PKC alpha acts as a dominant inhibitor of PKC activity in vivo and thus provides a useful genetic tool to assess the roles of PKC in various signal transduction processes.

Virtual kinetics: using statistical experimental design for rapid analysis of enzyme inhibitor mechanisms

Modern automated drug-screening can generate hundreds of inhibitor leads from diverse chemical sources in a short period of time. Traditional methods of inhibitor analysis are resource intensive and limit the number of inhibitors that can be analyzed for their mechanism of inhibition. This paper presents methods we have developed for rapid estimation of both potency and mechanism of potential inhibitor leads for a biochemically complex screening target (protein kinase C) using commercially available computer programs for statistical experimental design. Our findings indicate that, with careful choice of factor levels, statistical experimental design clearly identifies the various interactions of the assay components with inhibitors. Suitably plotted, the data can be used to examine the competitive nature of the inhibitor and can provide estimates of IC50 and Michaelis constants useful for planning further kinetic work. The techniques used are amenable to automation and should be useful for identifying inhibitors that may have only marginal potency, but exhibit desirable mechanistic profiles suitable for structural analoging efforts.

Selective phosphorylation of cationic polypeptide aggregated with phosphatidylserine/diacylglycerol/Ca2+/detergent mixed micelles by Ca(2+)-independent but not Ca(2+)-dependent protein kinase C isozymes

Mixed micelles containing Nonidet P40 (NP-40) (829 microM or 4.8 mM), phosphatidylserine (PS) (14.5 or 8 mol%), and 1,2-diacylglycerol (DG) (0.5 or 1 mol%) when preincubated with protein kinase C (PKC) assay mixture containing cationic substrate and CaCl2 (400 microM) formed aggregates in a time-, temperature-, and substrate concentration-dependent manner with a t1/2 approximately 3-12 min (22 degrees C). Concomitant with the formation of these aggregates there was a substantial loss of substrate phosphorylation catalyzed by the Ca(2+)-dependent PKC alpha, beta, and gamma but not the Ca(2+)-independent PKC, delta and epsilon. All cationic PKC substrates tested, neurogranin peptide analog, neurogranin, and histone III-S, formed aggregates with PS/DG/NP-40/Ca2+ mixed micelles in a time-dependent fashion. The poly(cationic-anionic) PKC substrate protamine sulfate also forms aggregates with the mixed micelles in the presence of Ca2+, but without affecting the substrate phosphorylation by the kinase. Under similar conditions, but at 4 degrees C, neither aggregation nor loss of cationic substrate phosphorylation was observed. Another nonionic detergent, octyl glucoside, behaved similarly to NP-40. Phosphatidylinositol (PI) and phosphatidylglycerol like PS, were effective in forming aggregates with NP-40/cationic polypeptide/DG/Ca2+ as monitored by light scattering, yet without affecting substrate phosphorylation. Phosphorylation of cationic substrates by M-kinase, derived from trypsinized PKC beta, was also greatly diminished by the aggregation. In contrast, [3H]phorbol 12,13-dibutyrate binding to PKC beta was unaffected. Formation of the aggregates that were selectively utilized by the Ca(2+)-independent PKCs was dependent on the ratio of cationic substrate to the number of mixed micelles.(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro activation of rat brain protein kinase C by polyenoic very-long-chain fatty acids

A variety of fatty acids including the cis-polyunsaturated very-long-chain fatty acids (VLCFA) (> 22 carbon atoms) common in retina, spermatozoa, and brain were examined for their ability to activate protein kinase C (PKC) purified from rat brain. Arachidonic [20:4(n-6)], eicosapentaenoic [20:5(n-3)], and docosahexaenoic [22:6(n-3)] acids as well as the VLCFA dotriacontatetraenoic [32:4(n-6)] and tetratriacontahexaenoic [34:6(n-3)] were equally capable of activating PKC in vitro with maximal activity being between 25 and 50 microM. The phorbol ester 12-O-tetradecanoylphorbol 13-acetate further enhanced the in vitro activation of PKC when added to the protein kinase assay system with the fatty acids. The fully saturated arachidic acid (20:0) was inactive in both assay systems. The potential significance of the in vitro activation of PKC by the VLCFA is discussed.

Autophosphorylation: a salient feature of protein kinases

Most protein kinases catalyze autophosphorylation, a process which is generally intramolecular and is modulated by regulatory ligands. Either serine/threonine or tyrosine serves as the phosphoacceptor, and several sites on the same kinase subunit are usually autophosphorylated. Autophosphorylation affects the functional properties of most protein kinases. Members of the protein kinase family exhibit diversity in the characteristics and functions of autophosphorylation, but certain common themes are emerging.

Induction of lipid-protein mismatch by xenobiotics with general membrane targets

Many membrane-active xenobiotics, such as organic solvents or general anesthetics, are without specific target sites. A common, but as yet ill-defined physical mechanism of action is usually assumed. Displacement of 'boundary' lipid activators from functional membrane proteins by the induction of lipid/protein mismatch is now established as a candidate common physical mechanism. Multiple binding site kinetics demonstrate that xenobiotics can be strongly inhibitory at realistic membrane concentrations of below 4 mol%. A general equation for inhibition is derived.

Protein kinase C and annexins: unusual calcium response elements in the cell

Protein kinase C and the annexins appear to share some unusual and potentially important membrane- and calcium-binding properties. While these proteins are calcium response elements, they are not calcium-binding proteins in the formal sense; at intracellular calcium concentrations, they only bind significant amounts of calcium when membranes or other suitable surfaces are present. The number of calcium ions bound per protein is large (> 8) and this stoichiometry, at the protein-membrane interface, may provide the large number of contact points needed for the very high-affinity interaction that is observed. The further ability of annexins and PKC to form structures with properties of integral membrane proteins may be important to provide a type of long-term cell signalling that produces a constitutively active kinase or ion channel activity. Selectivity for phospholipids in bilayer form is modest with respect to the acidic phospholipids but there is a surprising preference for phosphatidylethanolamine as the neutral phospholipid matrix. Along with other unusual properties, these proteins offer the potential for unique types of cell regulation events.