Characterization of the protein kinase activities of human platelet supernatant and particulate fractions.

After human platelets were lysed by freezing and thawing in the presence of EDTA, about 35% of the total cyclic AMP-dependent protein kinase activity was specifically associated with the particulate fraction. In contrast, Ca2+-activated phospholipid-dependent protein kinase was found exclusively in the soluble fraction. Photoaffinity labelling of the regulatory subunits of cyclic AMP-dependent protein kinase with 8-azido-cyclic [32P]AMP indicated that platelet lysate contained a 4-fold excess of 49 000-Da RI subunits over 55 000-Da RII subunits. The RI and RII subunits were found almost entirely in the particulate and soluble fractions respectively. Chromatography of the soluble fraction on DEAE-cellulose demonstrated a single peak of cyclic AMP-dependent activity with the elution characteristics and regulatory subunits characteristic of the type-II enzyme. A major enzyme peak containing Ca2+-activated phospholipid-dependent protein kinase was eluted before the type-II enzyme, but no type-I cyclic AMP-dependent activity was normally observed in the soluble fraction. The particulate cyclic AMP-dependent protein kinase and associated RI subunits were solubilized by buffers containing 0.1 or 0.5% (w/v) Triton X-100, but not by extraction with 0.5 M-NaCl, indicating that this enzyme is firmly membrane-bound, either as an integral membrane protein or via an anchor protein. DEAE-cellulose chromatography of the Triton X-100 extracts demonstrated the presence of both type-I cyclic AMP-dependent holoenzyme and free RI subunits. These results show that platelets contain three main protein kinase activities detectable with histone substrates, namely a membrane-bound type-I cyclic AMP-dependent enzyme, a soluble type-II cyclic AMP-dependent enzyme and Ca2+-activated phospholipid-dependent protein kinase, which was soluble in lysates containing EDTA.     

Modulation of Ca2+-activated, phospholipid-dependent protein kinase in platelets treated with a tumor-promoting phorbol ester

Incubation of human platelets with 12-0-tetradecanoylphorbol-13-acetate (TPA) caused a rapid decrease in soluble Ca2+, phospholipid-dependent protein kinase activity (protein kinase C) and an increase in protein kinase C associated with the particulate fraction. TPA also induced an increased activity of a Ca2+, phospholipid-independent protein kinase activity in both the soluble and the particulate fractions of platelets. This latter kinase eluted from DEAE cellulose columns at a higher salt concentration than protein kinase C, and was shown by Sephadex G-100 chromatography to have a MW of approx. 50,000 compared with an MW of 80,000 for protein kinase C. The data suggest that TPA treatment of platelets causes irreversible activation of protein kinase C by proteolysis of the enzyme to a form active in the absence of Ca2+ and phospholipid.     

Compartments of cyclic AMP and protein kinase in mammalian cardiomyocytes

We have studied the compartmentation of cyclic AMP action in purified ventricular cardiomyocytes prepared by collagenase perfusion of adult rabbit hearts. Incubation of purified adult myocytes with 1 microM isoproterenol causes rapid accumulation of intracellular cyclic AMP in both soluble (2.3 leads to 7.7 pmol/ mg of protein) and particulate (3.0 leads to 9.2) fractions of cell homogenates (3000 X g for 5 min), increases in the total activity and activity ratio of soluble cyclic AMP-dependent protein kinase (0.21 leads to 0.66), a decrease in protein kinase activity remaining in the particulate fraction (47 leads to 30%), and an increase in the activity ratio of glycogen phosphorylase (0.15 leads to 0.47). Incubation of myocytes with 10 microM prostaglandin E1 (PGE1) leads to a comparable increase in soluble cyclic AMP (2.3 leads to 5.8 pmol/mg of protein) and activation of soluble cyclic AMP-dependent protein kinase (0.21 leads to 0.39) but does not result in any change in cAMP or protein kinase in the particulate fraction and fails to cause an activation of glycogen phosphorylase. PGE1 does not inhibit the effects of isoproterenol; when myocytes are incubated with both isoproterenol and PGE1, the accumulation of cyclic AMP, activation of cAMP-dependent protein kinase and phosphorylase b leads to a conversion are equal to that achieved with isoproterenol alone. Perturbation of cellular calcium using the ionophore A23187, verapamil, or high or low extracellular calcium did not alter the ability of isoproterenol to cause activation of particulate cAMP-dependent protein kinase or influence the inability of PGE1 to do so. Activation of adenylate cyclase by forskolin (30 microM) caused immediate activation of both soluble and particulate cAMP-dependent protein kinase leading to rapid activation of phosphorylase. We conclude that the hormonally specific compartmentation of cyclic AMP and cAMP-dependent protein kinase that occurs in intact heart (Hayes, J. S., Brunton, L. L., Brown, J. H., Reese, J. B., and Mayer, S. E. (1979) Proc. Natl. Acad. Sci. U.S.A. 76, 1570-1574) is not explained on the basis of cellular heterogeneity but has a subcellular basis within the cardiomyocyte.     

Subcellular distribution of the different platelet proteins phosphorylated on exposure of intact platelets to ionophore A23187 or to prostaglandin E1. Possible role of a membrane phosphopolypeptide in the regulation of calcium-ion transport.

Exposure of 32P-labelled human platelets to ionophore A23187 results in an increased incorporation of 32P into polypeptides with apparent mol.wts. of 47 000 (P47) and 20 000 (P20), whereas exposure to prostaglandin E1 results in increased labelling of polypeptides with apparent mol.wts. of 24 000 (P24) and 22 000 (P22) [Haslam, Lynham & Fox (1979) Biochem. J. 178, 397-406]. Labelled platelets that had been incubated with ionophore A23187 or prostaglandin E1 were sonicated and rapidly separated into three fractions by differential centrifugation. Electron microscopy and measurement of marker enzymes indicated that the 1300-19 000 gav. particulate fraction was enriched in granules, mitochondria and plasma membranes, that the 19 000-90 000 gav. particulate fraction was enriched in both intracellular and plasma membranes and that the 90 000 gav. supernatant contained only soluble proteins. 32P-labelled phosphopolypeptide P47 was present almost exclusively in the 90 000 gav. supernatant, whereas phosphopolypeptide P20 was largely dephosphorylated under fractionation conditions that protected other phosphopolypeptides. 32P-labelled phosphopolypeptide P24 was enriched in both particulate fractions, but particularly in the 19 000-90 000 gav. fraction, and may therefore be present in both the intracellular and plasma membranes. Phosphopolypeptide P22 appeared to be similarly distributed. Both particulate fractions were capable of the ATP-dependent oxalate-stimulated uptake of Ca2+. When the 19 000-90 000 gav. membrane fraction was prepared from platelets that had been incubated with ionophore A23187, active uptake of Ca2+ did not occur, but when this fraction was isolated from platelets that had been exposed to prostaglandin E1, uptake of Ca2+ was significantly greater than observed with the corresponding membranes from control platelets. It is suggested that phosphorylation of polypeptide P24 (or P22) by a cyclic AMP-dependent protein kinase may promote the active transport of Ca2+ out of the platelet cytosol.     

Demonstration of protein kinase activities in the coronary artery of cattle]

Protein kinase activities were identified in a soluble and a particulate fraction from the A. coronaria of cattle. For both protein kinase activities Mg++ is essential. Protamine was used as a substrate of the protein kinase activity of the soluble fraction. The pH optimum of the protein kinase activity of the soluble fraction is around 6.5. The Km-value of the protein kinase for ATP is 1.9 +/- 0.4 - 10(-5) M. cAMP stimulates the protein kinase activity more effectively than cGMP. Ca++ cannot replace Mg++; monovalent cations (Na+ and K+) show no influence. The protein kinase activity of the fraction was determined via endogenous phosphorylation. By means of the cAMP-dependent particulate protein kinase 72 to 80 percent of the serine residues are phosphorylated. The pH optimum of the protein kinase activity of the particulate fraction lies around 7.0. The Km-value of the enzyme for ATP is 6.6 +/- 0.8 - 10(-5) M. cGMP stimulates the protein kinase of the particulate fraction better than cAMP. For the protein kinase activity of this fraction Ca++ replaces Mg++ in the endogenous phosphorylation but not in the exogenous phosphorylation (protamine). In the presence of Mg++ and in the additional presence of Na+ or K+, the protein kinase activity is suppressed in the endogenous phosphorylation whereas it is stimulated in the exogenous phosphorylation.     

Alterations in activities of cyclic nucleotide systems and in β-adrenergic receptor-mediated activation of cyclic amp-dependent protein kinase during progression and regression of isoproterenol-induced cardiac hypertrophy

Initial and transient increases in the basal levels of cyclic GMP in the heart were noted prior to cardiac hypertrophy in rats administered isoprotenol. Increased levels of cyclic AMP-phosphodiesterase (in both the soluble and particulate fractions) and stimulatory modulator of cyclic GMP-dependent protein kinase, however, were associated with the progression, or the state, of cardiomegaly, with their levels returning to the control values upon regression of the hypertrophy. The levels of cyclic GMP phosphodiesterase in the soluble fraction were lower, whereas those in the particulate fraction were higher, in the hypertrophied heart than the control. In cardiac hypertrophy, the maximal activity ratio (?cyclic AMP/+cyclic AMP) of cyclic AMP-dependent protein kinase in the incubated minced heart caused by isoproterenol was lower, whereas the concentration of isoproterenol required to increase the activit ratio half-maximally was higher than controls; The reduced responsiveness to the drug, however, was reversed when the hypertrophy regressed. These observations, taken collectively, appear to suggest that the desensitization of the β-adrenergic mechanism seen in the cardiac hypertrophy produced by repeated administration of isoproterenol is associated with adaptive modifications in certain parameters of the cyclic nucleotide systems.     

Alterations in activities of cyclic nucleotide systems and in beta-adrenergic receptor-mediated activation of cyclic AMP-dependent protein kinase during progression and regression of isoproterenol-induced cardiac hypertrophy.

Initial and transient increases in the basal levels of cyclic GMP in the heart were noted prior to cardiac hypertrophy in rats administered isoproterenol. Increased levels of cyclic AMP-phosphodiesterase (in both the soluble and particulate fractions) and stimulatory modulator of cyclic GMP-dependent protein kinase, however, were associated with the progression, or the state, of cardiomegaly, with their levels returning to the control values upon regression of the hypertrophy. The levels of cyclic GMP phosphodiesterase in the soluble fraction were lower, whereas those in the particulate fraction were higher, in the hypertrophied heart than the control. In cardiac hypertrophy, the maximal activity ratio(--cyclic AMP/+cyclic AMP) of cyclic AMP-dependent protein kinase in the incubated minced heart caused by isoproterenol was lower, whereas the concentration of isoproterenol required to increase the activity ratio half-maximally was higher than controls; the reduced responsiveness to the drug, however, was reversed when the hypertrophy regressed. These observations, taken collectively, appear to suggest that the desensitization of the beta-adrenergic mechanism seen in the cardiac hypertrophy produced by repeated administration of isoproterenol is associated with adaptive modifications in certain parameters of the cyclic nucleotide systems.     

Two separate tyrosine protein kinases in human platelets

Tyrosine protein kinase activities were detected in the cytosolic fraction (PC-TPK) and the particulate fraction (PM-TPK) in human platelets using the synthetic peptide, E11G1 (Glu-Asp-Ala-Glu-Tyr-Ala-Ala-Arg-Arg-Arg-Gly) as a substrate. PC-TPK and PM-TPK were different in substrate specificities, divalent cation requirements and apparent Mr values. These results strongly suggest that in platelets there exist at least two separate tyrosine protein kinases; one is present in cytosol and the other might be associated with membranes.     

Evidence that treatment of platelets with phorbol ester causes proteolytic activation of Ca2+-activated, phospholipid-dependent protein kinase

Incubation of human platelets with the phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (TPA) caused the accumulation of a protein kinase in the particulate fraction which was not dependent on Ca2+ and phosphatidylserine (Ptd-Ser). The Ca2+/Ptd-Ser-independent kinase eluted from DEAE-cellulose at a NaCl concentration of 0.18-0.22 M compared with 0.08 - 0.16 M for Ca2+/Ptd-Ser-dependent protein kinase (C-kinase). Formation of the Ca2+/Ptd-Ser-independent kinase in 12-O-tetradecanoylphorbol-13-acetate-treated platelets was blocked by leupeptin, an inhibitor of Ca2+-dependent neutral proteases. The Ca2+/Ptd-Ser-independent kinase and C-kinase both catalysed the same pattern of phosphorylation of smooth muscle myosin light chains and histone H1 as detected by one-dimensional or two-dimensional peptide mapping after tryptic digestion. The phosphorylation sites were different from those obtained with myosin light chain kinase or cAMP-dependent kinase. The Ca2+/Ptd-Ser-independent kinase and C-kinase had Mr values of about 50 000 and 77 000 respectively as determined by sucrose-gradient centrifugation. It was concluded that 12-O-tetradecanoylphorbol-13-acetate induces the proteolytic cleavage of C-kinase to a Ca2+/Ptd-Ser-independent form.     

Postnatal Age and Protein Tyrosine Phosphorylation at Synapses in the Developing Rat Brain

The relationship between postnatal age and protein tyrosine kinase activity in synaptosomes prepared from the rat forebrain was studied. Synaptosomal particulate and soluble fractions, as well as total homogenates, the cell soluble fraction, and P3, were prepared from rats ranging in postnatal age from 5 to 60 days and analyzed for (a) tyrosine kinase activity using polyglutamyltyrosine (4:1) as the substrate, (b) the presence of endogenous substrates for tyrosine phosphorylation using polyclonal antibodies specific for phosphotyrosine, and (c) levels of pp60src. Enzyme activity, expressed per milligram of protein, in the total homogenate, P3, and both the cell and synaptosomal soluble fractions was highest in the brains of young animals (postnatal days 5-10) and decreased thereafter to adult levels. In contrast, tyrosine kinase activity in the synaptosomal particulate fraction exhibited a unique biphasic developmental profile, increasing to maxima at postnatal days 10 and 20 before decreasing to adult values. Endogenous substrates for tyrosine phosphorylation were identified by incubating subcellular fractions with 2 mM ATP in the presence of sodium orthovanadate and probing nitrocellulose blots of proteins separated by gel electrophoresis with antiphosphotyrosine antibodies. Several phosphotyrosine-containing proteins were detected in the synaptosomal particulate and P3 fractions, including proteins of Mr 180K, 145K, 120K, 100K, 77K, 68K, 62K, 54K, 52K, and 42K. In the cell soluble fraction a protein doublet of Mr 54/52K and a 120K protein were the major phosphotyrosine-containing proteins. The 54/52K doublet was the major protein tyrosine kinase substrate in the synaptosomal soluble fraction.(ABSTRACT TRUNCATED AT 250 WORDS)     

Protein phosphorylation in human peripheral blood lymphocytes. Subcellular distribution and partial characterization of adenosine 3'':5''-cyclic monophosphate-dependent protein kinase.

Cytoplasmic and membrane fractions prepared from human peripheral-blood lymphocytes both contained cyclic AMP-dependent protein kinase activity and endogenous protein kinase substrates. Protein kinase activity in the particulate fractions was not eluted with 0.25 M-NaCl, suggesting that it was not derived from non-specifically absorbed soluble cytoplasmic protein kinase. Nor was the particulate protein kinase activity eluted by treatment with cyclic AMP, suggesting that the catalytic subunit is membrane-bound and arguing against cyclic AMP-induced translocation of particulate activity. Cyclic AMP-dependent protein-phosphorylating activity in the cytoplasmic fraction was highly sensitive to inhibition by Mn2+, and was co-eluted from DEAE-cellulose primarily with type-I rabbit skeletal-muscle kinase. Cyclic AMP-dependent phosphorylating activity in the plasma-membrane fractions was stimulated at low [Mn2+] and inhibited only at high [Mn2+]. When solubilized with Nonidet P-40, plasma-membrane protein kinase was co-eluted from DEAE-cellulose with type-II rabbit muscle kinase. These differences, together with the strong association of the particulate kinases with the particulate fraction, suggest the possibility of compartmentalized protein phosphorylation in intact lymphocytes.     

Structural properties of myosin from the particulate fraction of human blood platelets

Fractionation of human blood platelets has revealed that myosin, a contractile and mechanochemical protein, is present in both the soluble and particulate fraction. The aim of this study was to elucidate whether platelets contain more than one myosin isoform, especially in view of the fact that in other cellular systems (cardiac muscle, amoeba) several myosin isoenzymes were found. The particulate fraction was solubilized by Triton X-100, and the myosin was purified by the same procedure used for the cytoplasmic myosin. The final preparation contained, in addition to myosin, a 130-kDa polypeptide, which was observed also in myosin preparations obtained from the soluble fraction. The electrophoretic mobilities of the two myosins were identical under both dissociating and nondissociating conditions. Comparison of the molecular structure of the heavy chain of the two myosins by limited proteolysis with Staphylococcus aureus V8 protease showed that the proteolytic fragments of the two myosins were rather similar, with only minor alterations in the quantitative distribution of the products. Two-dimensional peptide mapping of the iodinated tryptic peptides of the myosin heavy chains indicated that at least one peptide is missing in the map of the particulate myosin, as compared to its soluble counterpart. According to the two-dimensional peptide map, the 130-kDa polypeptide seems to be a proteolytic fragment of the myosin heavy chain and most probably the rod portion of the molecule. The observed minor variations in the structure of myosins isolated from the soluble and the fractions of human platelets may reflect differences in their respective physiological functions.     

Effect of phorbol esters on the distribution and total activity of protein kinase C in the perfused rat heart

1. The perfused rat heart was treated with the tumour-promoter and protein kinase C activator, phorbol 12-myristate 13-acetate and the distribution of protein kinase C activity between cytosolic and particulate fractions determined. 2. Phorbol ester treatment led to a rapid loss of protein kinase C activity from the cytosol (t0.5 = 2 min) with a corresponding translocation into the particulate fraction. Translocated protein kinase C activity was tightly bound to the particulate fraction, could only be extracted with buffers containing 2% Triton X-100 and could therefore be misinterpreted as being down-regulated. 3. Claims of rapid down-regulation of protein kinase C activity by phorbol esters need to be supported by rigorous procedures for extraction of the particulate material.     

Contraction-associated translocation of protein kinase C in rat skeletal muscle

Electrical stimulation of the sciatic nerve of the anaesthetized rat in vivo led to a time-dependent translocation of protein kinase C from the muscle cytosol to the particulate fraction. Maximum activity of protein kinase C in the particulate fraction occurred after 2 min of intermittent short tetanic contractions of the gastrocnemius-plantaris-soleus muscle group and coincided with the loss of activity from the cytosol. Translocation of protein kinase C may imply a role for this kinase in contraction-initiated changes in muscle metabolism.     

Evidence for increased synthesis as well as increased degradation of protein kinase C after treatment of human osteosarcoma cells with phorbol ester

Phorbol 12-myristate 13-acetate (PMA) induces time-dependent changes in protein kinase C subcellular distribution and enzymatic activity in the human osteosarcoma cell line SaOS-2. Short (less than 60 min) incubations with PMA caused decreased cytosolic enzyme activity and a concomitant increase in particulate protein kinase; after 3 h, particulate protein kinase C activity also declined to reach less than 10% of basal activity by 24 h (Krug, E., and Tashjian, Jr., A. H., (1987) Cancer Res. 47, 2243-2246). In order to determine whether the loss in enzyme activity was due to decreased enzyme protein, Western blot analyses were performed using a polyclonal antibody against protein kinase C raised in rabbits. This approach confirmed the previously reported time-related changes: 80-kDa immunoreactive protein kinase C initially translocated from the cytosol to the particulate cell fraction and later disappeared completely from the particulate fraction. Loss of protein kinase C enzymatic activity thus results from actual loss of the 80-kDa protein; we found no evidence for generation of a calcium/phospholipid-independent protein kinase C-like form of the enzyme. Membrane association was confirmed by immunoprecipitation experiments using [35S]methionine-labeled cells. Brief exposure to PMA caused a marked loss in the [35S]methionine-labeled cytosolic protein kinase C band and an increase in the labeled particulate band. Protein kinase C immunoprecipitated from cells treated with PMA for 14 h displayed an increase in [35S]methionine label despite a greater than 80% loss of enzyme activity. The high specific radioactivity of the remaining 80-kDa protein leads us to conclude that long term treatment with PMA causes an increase in the rate of protein kinase C synthesis accompanied by a still greater increase in the rate of enzyme degradation in SaOS-2 cells.     

In vitro phosphorylation and purification of a nonstructural protein of bluetongue virus with affinity for single-stranded RNA.

A phosphorylated, nonstructural protein of bluetongue virus, NS2, is synthesized throughout the replication cycle in comparatively large amounts. The protein was detected in both the soluble and particulate fraction of the cytoplasm of infected cells. The particulate NS2 could be solubilized in 0.5 M NaCl. It was found that NS2 in the particulate fraction and immunoprecipitates of NS2 from the soluble protein fraction could be phosphorylated in vitro. It is not known whether the kinase involved is of cellular or viral origin, but after purification of NS2 by affinity chromatography on poly(U)-Sepharose it could still by phosphorylated in vitro without the addition of exogenous protein kinase. The affinity of NS2 for nucleic acid was also investigated. The protein was found to bind to single-stranded RNA. In the presence of purified bluetongue virus mRNA, NS2 formed a complex with an estimated S value of about 22S.     

Redistribution of protein kinase C during mitogenesis of human B lymphocytes

G0 human tonsillar B-lymphocytes were stimulated to divide by the polyclonal mitogen Staphylococcus Aureus Cowan strain 1 (SAC) and by the combined use of 12-O-tetradecanoyl phorbol-13-acetate (TPA) and the calcium ionophore ionomycin. The activities of protein kinase C, which requires Ca++ and phospholipid as co-factors, and a proteolytically cleaved form of this enzyme (protein kinase M), which is independent of calcium and phospholipid control, were determined in soluble and particulate fractions obtained from activated B cells. Treatment of G0 B cells with SAC or TPA together with ionomycin caused redistribution of protein kinase C from the soluble to the particulate fraction where the 80,000-Dalton protein kinase C was cleaved to give rise to a 50,000-Dalton form of the kinase which was also found in the cytoplasm. These data suggest that redistribution and proteolytic cleavage of protein kinase C are key signal transduction events in B cell mitogenesis.     

Rapid separation of particulate components and soluble cytoplasm of isolated rat-liver cells

A method is described for the rapid separation of mitochondria (plus other particulate components) from the soluble cytoplasm of isolated rat-liver cells. The cells were incubated briefly with a low concentration of digitonin. After rapid centrifugation, the pellet contained more than 90% of the total adenylate kinase and glutamate dehydrogenase activities and the supernatant at least 80% of the lactate dehydrogenase activity. About 60% of total adenine nucleotides in hepatocytes were found in the soluble cytoplasm. The ATP/ADP ratio in the particulate fraction 80 s after exposure to digitonin of hepatocytes metabolizing alanine was 2.0–2.4, and that in the soluble cytoplasm 6–19. In the presence of atractyloside, these values were 3.5–4.4 and 1.3–2.2, respectively.     

Enzymatic synthesis of poly-beta-hydroxybutyrate in Zoogloea ramigera.

The enzyme activity synthesizing poly-beta-hydroxybutyrate (PHB) was mainly localized in the PHB-containing particulate fraction of Zoogloea ramigera I-16-M, when it grew flocculatedly in a medium supplemented with glucose. On the other hand, the enzyme activity remained in the soluble fraction when the bacterium grew dispersedly in a glucose-starved medium. The soluble PHB synthase activity became associated with the particulate fraction as PHB synthesis was initiated on the addition of glucose to the dispersed culture. Conversely, the enzyme activity was released from the PHB-containing granules to the soluble fraction when the flocculated culture was kept incubated without supplementing the medium with glucose. PHB synthase was also incorporated into the newly formed PHB fraction when partially purified soluble PHB synthase was incubated with D(-)-beta-hydroxybutyryl CoA in vitro. Although attempts to solubilize the particulate enzyme were unsuccessful, and the soluble enzyme became extremely unstable in advanced stages of purification, both PHB synthases had the same strict substrate specificity for D(-)-beta-hydroxybutyryl CoA, and showed the same pH optimum at 7.0.     

Effect of membrane perturbation on protein kinase C activation: treatment with exogenous phospholipase C decreases translocation of enzyme to cellular membranes

Incubation of mouse epidermal HEL-37 cells with C. perfringens phospholipase C for 30 min caused a partial loss of protein kinase C activity after 30 min incubation. Essentially all the kinase activity was present in the cytosolic fraction of both control and treated cells, despite the continued hydrolysis of phospholipid by the exogenous phospholipase. At shorter incubation times with phospholipase C (5 and 10 min) an association of protein kinase with particulate material was observed, presumably reflecting the accumulation of diacylglycerol. It is proposed that further incubation with phospholipase C renders the plasma membrane unable to bind protein kinase C and that already bound enzyme is destroyed by proteolysis.