Guanine nucleotide regulation of inositol phospholipid hydrolysis and CD3-antigen phosphorylation in permeabilized T lymphocytes

T lymphocyte activation signals

Activation of T lymphocytes results in immediate biochemical changes including increases in intracellular calcium levels, activation of protein kinase C (PKC) and changes in tyrosine phosphorylation. In T cells recent studies have indicated that activation of the guanine nucleotide-binding proteins p21ras is mediated by PKC, which suggests that the p21ras proteins may regulate intracellular signalling events downstream of PKC. The p21ras proteins can be activated in T cells by signals generated by triggering of the T cell antigen receptor (TCR), the CD2 antigen and the interleukin 2 receptor. Experiments using a PKC pseudosubstrate inhibitor indicate that PKC does not mediate TCR-induced activation of p21ras. These results imply that an alternative signal transduction pathway not involving PKC can regulate the activity of p21ras proteins in T cells.

Modulation of intracellular Ca(2+) concentration by vitamin B12 in rat thymocytes

We have studied several novel effects of vitamin B12 (cyanocobalamin) on cellular Ca(2+) homeostasis in rat thymocytes. We determined the effect of various concentrations of vitamin B12 on intracellular Ca(2+) concentration ([Ca(2+)]i) and parameters of Ca(2+)in signaling using the fluorescent dye Fura-2. The basal [Ca(2+)]i in Ca(2+)-containing media was 115 +/- 5 nM but in vitamin B12 (10 nM)-treated thymocytes [Ca(2+)]i was decreased to 60 +/- 15 nM (mean +/- SEM) during the first 5 min. The decline in [Ca(2+)]i was accompanied by an increase in the endoplasmic reticulum Ca(2+) store, presumably as a result of Ca-ATPase activation. At the same time 100 nM-10 mM B12 induced the accumulation of Ca(2+) in mitochondria. Somewhat higher concentrations of B12 (1-10 microM) had no effect on [Ca(2+)]i. A further increase in B12 concentration with range from 50 microM to 1 mM caused a dose-dependent elevation of [Ca(2+)]i from the basal level (115 +/- 5 nM) up to 200 +/- 50 nM in thymocytes, and this elevation was partially blocked in Ca(2+)-free media. This high concentration of vitamin B12 caused a gradual decrease of endoplasmic reticulum Ca(2+) stores by means of Ca-ATPase inhibition. The B12-induced increase in [Ca(2+)]i was not observed after depletion of intracellular Ca(2+) stores, induced by addition of 2',5'-di(tert-butyl)-1,4-benzohydroquinone (BHQ), an inhibitor of endoplasmic reticulum Ca (2+)-ATPase, concanavalin A, or arachidonic acid. These studies show that vitamin B12 regulates [Ca(2+)]i via several different mechanisms at different B12 concentrations. Participation of G proteins and calmodulin activity in B12-mediated [Ca(2+)]i increase is discussed.

Defective activation of p21ras in peripheral blood mononuclear cells from patients with insulin dependent diabetes mellitus

We previously reported that a decreased TCR mediated activity of the GTP-GDP binding p21ras protooncogene is associated with prediabetes in non-obese diabetic (NOD) mice. Furthermore, prevention of autoimmune diabetes is associated with reversal of the p21ras signaling defect in NOD T cells. Based on these animal studies we determined the activation of p21ras in PBMC from patients with Insulin Dependent Diabetes Mellitus (IDDM), Non-Insulin Dependent Diabetes Mellitus (NIDDM) and normal healthy controls. Stimulation by PHA induced a decrease of 3.7 +/- 1.4% and an increase of 2.44 +/- 2.3%, p < 0.02 and 2.6 +/- 1.6%,p < 0.003 in the basal unstimulated p21ras activity in the IDDM, NIDDM and normal control groups, respectively. Expression of p21ras and its regulatory elements, the GTPase activating protein p120ras-GAP and the guanine nucleotide releasing factor (GNRF) hSOS, was comparable in the three groups. The in vitro proliferative response to PHA was comparable in the IDDM and control groups: stimulation index (SI) of 8.6 +/- 2.5 and 9.4 +/- 3.5 respectively, p < 0.44. No correlations were found in the IDDM patients between the degree of p21ras activation and the mitogen induced in vitro proliferative response or the various clinical parameters including age, gender, disease duration, daily insulin requirements and metabolic control. Taken together these data indicate that PBMC from IDDM patients are characterized by a persistent impairment in the activation of their p21ras. They also suggest that p21ras stimulated activity is a sensitive and independent parameter of PBMC activation in these patients.

c-Src is required for stimulation of gelsolin-associated phosphatidylinositol 3-kinase

We have shown that osteopontin binding to integrin alphav beta3 in osteoclasts stimulates gelsolin-associated phosphatidylinositol (PtdIns) 3-hydroxyl kinase (PI 3-kinase), leading to increased levels of gelsolin-bound PtdIns 3,4-P2, PtdIns 4,5-P2, and PtdIns 3, 4,5-P3, uncapping of barbed end actin, and actin filament formation. Inhibition of PI 3-kinase activity by wortmannin blocks osteopontin stimulation of actin filament formation, suggesting that activation of gelsolin-associated PI 3-kinase is an important pathway in cytoskeletal regulation. To study the mechanism of gelsolin-associated PI 3-kinase activation, we analyzed anti-gelsolin immunoprecipitates for the association of protein kinases. We demonstrated that c-Src co-immunoprecipitates with gelsolin, and that osteopontin stimulates its activity. Elimination of osteopontin-stimulated Src activity associated with gelsolin through antisense oligodeoxynucleotides blocked the stimulation of PI 3-kinase activity associated with gelsolin and the gelsolin-dependent cytoskeletal reorganization induced by osteopontin, including increased F-actin levels. In addition, treatment of osteoclasts with antisense oligonucleotides to Src reduced bone resorption. Our results demonstrate that osteopontin stimulates gelsolin-associated Src, leading to increased gelsolin-associated PI 3-kinase activity and PtdIns 3,4,5-P3 levels, which facilitate actin filament formation, osteoclast motility, and bone resorption.

Calcium-dependent immediate-early gene induction in lymphocytes is negatively regulated by p21Ha-ras

The induction of immediate-early (IE) response genes, such as egr-1, c-fos, and c-jun, occurs rapidly after the activation of T lymphocytes. The process of activation involves calcium mobilization, activation of protein kinase C (PKC), and phosphorylation of tyrosine kinases. p21(ras), a guanine nucleotide binding factor, mediates T-cell signal transduction through PKC-dependent and PKC-independent pathways. The involvement of p21(ras) in the regulation of calcium-dependent signals has been suggested through analysis of its role in the activation of NF-AT. We have investigated the inductions of the IE genes in response to calcium signals in Jurkat cells (in the presence of activated p21(ras)) and their correlated consequences. The expression of activated p21(ras) negatively regulated the induction of IE genes by calcium ionophore. This inhibition of calcium-activated IE gene induction was reversed by treatment with cyclosporin A, suggesting the involvement of calcineurin in this regulation. A later result of inhibition of this activation pathway by p21(ras) was down-regulation of the activity of the transcription factor AP-1 and subsequent coordinate reductions in IL-2 gene expression and protein production. These results suggest that p2l(ras) is an essential mediator in generating not only positive but also negative modulatory mechanisms controlling the competence of T cells in response to inductive stimulations.

The novel cyclic dinucleotide 3'-5' cyclic diguanylic acid binds to p21ras and enhances DNA synthesis but not cell replication in the Molt 4 cell line

1. The effect of the novel, naturally occurring nucleotide 3'-5' cyclic diguanylic acid (c-di-GMP) on the lymphoblastoid Molt 4 cell line was studied. When exposed to this guanine nucleotide. Molt 4 cells exhibited a marked increase in [3H]thymidine incorporation, up to 200-fold at 50 microM c-di-GMP. Correspondingly, the DNA content of the treated cells was 9-fold higher than untreated cells. Stimulation of [3H]thymidine incorporation into the cells was time- and concentration-dependent. This effect was specific and was not observed with GMP or cyclic GMP, nor with the unhydrolysable GTP analogues, guanosine 5'-[gamma-thio]triphosphate and guanosine 5'-[beta gamma-imido]-triphosphate. C-di-GMP entrance into the cells was experimentally verified and occurred without using any means of cell permeabilization. SDS/PAGE analysis of cells exposed to [32P]c-di-GMP, followed by autoradiography, revealed the labelling of three low-molecular-mass proteins at 18-27 kDa. The labelling is highly specific to c-di-GMP and its extent was not affected by other guanine nucleotides. 2. One of the c-di-GMP-binding proteins was found to be the p21ras protein, by immunoprecipitation with the anti-Ras monoclonal antibody Y13-259. The effects described appear to be unique for c-di-GMP and, taken together, raise the possibility that an irreversible binding of this guanine nucleotide to the growth-promoting p21ras protein results in a fixed active conformation of this protein affecting DNA synthesis. Strikingly, although at 48 h of growth markedly high DNA levels were found in Molt 4 cells treated with c-di-GMP, this guanine nucleotide had no effect on cell replication during this period. Thus Molt 4 cells exposed to c-di-GMP enter the S phase uncoordinated with their overall replication rate.

Streptolysin O-permeabilized cell system for studying trans-acting activities of exogenous nuclear proteins

Efficient transfer of exogenous proteins into culture animal cells can be achieved by Streptolysin O (SLO) permeabilization of plasma membranes. We used this method to establish an in vitro transcription system for early response genes. The promoters of many early response genes contain an essential DNA motif known as the Serum Response Element (SRE). Recent data has shown that this DNA sequence is recognized by Serum Response Factor (SRF) and its associated proteins. Our initial experiments showed that HeLa nuclear extracts induced the transcription of the c-fos gene in serum-starved murine fibroblasts which were permeabilized by either physical method (glass beads) or cytolytic pore-forming protein (SLO). Plasma membrane permeabilization presumably permits passive diffusion of macromolecules into target cells and we showed that a truncated SRF expressed in bacteria was translocated into the nucleus within 30 minutes after SLO permeabilization. HeLa crude extracts were fractionated in order to identify the active nuclear factors. SRF was purified by binding to Wheat Germ Agglutinin (WGA)-agarose but the active factors remained in the WGA-unbound fractions. Our results demonstrate that this permeabilized cell in vitro transcription system is simple, efficient and can be used to test crude nuclear fractions as well as purified proteins expressed in bacteria; it will be an useful tool for the reproduction of transcriptional regulation on chromatin templates in vitro as well as the investigation of the biochemical functions of specific transcription factors or signal transduction effectors.

Thymic T cell anergy in autoimmune nonobese diabetic mice is mediated by deficient T cell receptor regulation of the pathway of p21ras activation

Thymic T cell anergy, as manifested by thymocyte proliferative unresponsiveness to antigens expressed in the thymic environment, is commonly believed to mediate the acquisition of immunological self-tolerance. However, we previously found that thymic T cell anergy may lead to the breakdown of tolerance and predispose to autoimmunity in nonobese diabetic (NOD) mice. Here, we show that NOD thymic T cell anergy, as revealed by proliferative unresponsiveness in vitro after stimulation through the T cell receptor (TCR), is associated with defective TCR-mediated signal transduction along the PKC/p21ras/p42mapk pathway of T cell activation. PKC activity is reduced in NOD thymocytes. Activation of p21ras is deficient in quiescent and stimulated NOD T cells, and this is correlated with a significant reduction in the tyrosine phosphorylation of p42mapk, a serine/threonine kinase active downstream of p21ras. Treatment of NOD T cells with a phorbol ester not only enhances their p21ras activity and p42mapk tyrosine phosphorylation but also restores their proliferative responsiveness. Since p42mapk activity is required for progression through to S phase of the cell cycle, our data suggest that reduced tyrosine phosphorylation of p42mapk in stimulated NOD T cells may abrogate its activity and elicit the proliferative unresponsiveness of these cells.

G-proteins are not directly involved in the CD3-antigen-mediated production of inositol phosphates in HPB-ALL T-leukaemia cells expressing phospholipase C isoforms gamma 1 and beta 3

The possible involvement of G-proteins in T cell antigen-receptor complex (TCR)-mediated inositol phosphate production was investigated in HPB-ALL T-cells, which were found to express the phospholipase C gamma 1 and beta 3 isoforms. Cross-linking the CD3 antigen on streptolysin-O-permeabilized cells stimulated a dose-dependent increase in inositol phosphate production, as did addition of guanosine 5'-[gamma-thio]triphosphate (GTP[S]) or vanadate, a phosphotyrosine phosphatase inhibitor. It was possible, therefore, that the CD3-antigen-mediated production of inositol phosphates was either via a G-protein-dependent mechanism or by stimulation of protein tyrosine phosphorylation. The CD3-induced inositol phosphate production was potentiated by addition of vanadate, but not by addition of GTP[S]. Guanosine 5'-[beta-thio]diphosphate (GDP[S]) inhibited the rise in inositol phosphates induced by GTP[S], vanadate or cross-linking the CD3 antigen. The increase in protein tyrosine phosphorylation stimulated by vanadate or the OKT3 monoclonal antibody was not observed in the presence of GDP[S], showing that in permeabilized HPB-ALL cells, GDP[S] inhibits the actions of tyrosine kinases as well as G-protein function. Addition of either ADP[S] or phenylarsine oxide inhibited CD3- and vanadate-mediated increases in both tyrosine phosphorylation and inositol phosphate production, but did not inhibit GTP[S]-stimulated inositol phosphate production. On the other hand, pretreatment of cells with phorbol 12,13-dibutyrate inhibited subsequent GTP[S]-stimulated inositol phosphate production but did not inhibit significantly inositol phosphate production stimulated by either OKT3 F(ab')2 fragments or vanadate. Our results are consistent with the CD3 antigen stimulating inositol phosphate production by increasing the level of protein tyrosine phosphorylation, but not by activating a G-protein.

Inositol-lipid-specific phospholipase C isoenzymes and their differential regulation by receptors

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A 68-kD GTP-binding protein associated with the T cell receptor complex

The identity of the guanine nucleotide-binding protein (G protein) involved in T cell activation pathways remains unclear. We identified a 68-kD GTP-binding protein associated with the T cell receptor (TCR)/CD3 complex using immunoprecipitation and GTP-affinity labeling techniques. Proteins coimmunoprecipitated with the TCR/CD3 complex in digitonin lysate of a human leukemic T cell line, MOLT 16, were incubated with alpha-[32P]GTP and irradiated with ultraviolet rays to covalently link the labeled GTP to GTP-binding proteins. They were then analyzed by electrophoresis. The 68-kD protein exhibited nucleotide specificity for GTP-binding and was insensitive to cholera and pertussis toxins. The 68-kD GTP-binding protein could be coimmunoprecipitated with the TCR/CD3 complex but not with other surface molecules such as major histocompatibility complex class I and lymphocyte function associated-1, which do not cause rapid Ca2+ mobilization. These suggest that the 68-kD GTP-binding protein is specifically associated with the TCR/CD3 complex.

Role of protein kinase C in T-cell antigen receptor regulation of p21ras: evidence that two p21ras regulatory pathways coexist in T cells

T-lymphocyte activation via the antigen receptor complex (TCR) results in accumulation of p21ras in the active GTP-bound state. Stimulation of protein kinase C (PKC) can also activate p21ras, and it has been proposed that the TCR effect on p21ras occurs as a consequence of TCR regulation of PKC. To test the role of PKC in TCR regulation of p21ras, a permeabilized cell system was used to examine TCR regulation of p21ras under conditions in which TCR activation of PKC was blocked, first by using a PKC pseudosubstrate peptide inhibitor and second by using ionic conditions that prevent phosphatidyl inositol hydrolysis and hence diacylglycerol production and PKC stimulation. The data show that TCR-induced p21ras activation is not mediated exclusively by PKC. Thus, in the absence of PKC stimulation, the TCR was still able to induce accumulation of p21ras-GTP complexes, and this stimulation correlated with an inactivation of p21ras GTPase-activating proteins. The protein tyrosine kinase inhibitor herbimycin could prevent the non-PKC-mediated, TCR-induced stimulation of p21ras. These data indicate that two mechanisms for p21ras regulation coexist in T cells: one PKC mediated and one not. The TCR can apparently couple to p21ras via a non-PKC-controlled route that may involve tyrosine kinases.

Electroporation of CTL clones: a useful method to investigate signalling pathways leading to the expression of effector functions

Signal transduction mechanisms leading to effector functions in mouse cytolytic T lymphocyte (CTL) clones were studied following the introduction of exogenous molecules by electroporation. Conditions were defined in which the application of an electric pulse permeabilized the CTL without affecting functions such as antigen-dependent or antibody-mediated cytotoxicity. When a non-permeant Ca2+ chelator such as EGTA was added in the external medium during the electric pulse, it inhibited subsequent target cell cytolysis carried out in the presence of external Ca2+, thereby indicating the efficiency of EGTA uptake. Results obtained in this system, using a 13 amino-acid protein kinase C (PKC) pseudo-substrate peptide, indicated that it selectively inhibited cytolysis, whereas a substrate peptide with one amino-acid substitution was not inhibitory. This suggests that the technique could be used to study the signal transduction mechanisms of CTL clones which lead to the expression of effector functions.

Role of protein kinase C in T-cell antigen receptor regulation of p21ras: evidence that two p21ras regulatory pathways coexist in T cells

T-lymphocyte activation via the antigen receptor complex (TCR) results in accumulation of p21ras in the active GTP-bound state. Stimulation of protein kinase C (PKC) can also activate p21ras, and it has been proposed that the TCR effect on p21ras occurs as a consequence of TCR regulation of PKC. To test the role of PKC in TCR regulation of p21ras, a permeabilized cell system was used to examine TCR regulation of p21ras under conditions in which TCR activation of PKC was blocked, first by using a PKC pseudosubstrate peptide inhibitor and second by using ionic conditions that prevent phosphatidyl inositol hydrolysis and hence diacylglycerol production and PKC stimulation. The data show that TCR-induced p21ras activation is not mediated exclusively by PKC. Thus, in the absence of PKC stimulation, the TCR was still able to induce accumulation of p21ras-GTP complexes, and this stimulation correlated with an inactivation of p21ras GTPase-activating proteins. The protein tyrosine kinase inhibitor herbimycin could prevent the non-PKC-mediated, TCR-induced stimulation of p21ras. These data indicate that two mechanisms for p21ras regulation coexist in T cells: one PKC mediated and one not. The TCR can apparently couple to p21ras via a non-PKC-controlled route that may involve tyrosine kinases.

Increased intracellular cyclic AMP inhibits inositol phospholipid hydrolysis induced by perturbation of the T cell receptor/CD3 complex but not by G-protein stimulation. Association with protein kinase A-mediated phosphorylation of phospholipase C-gamma 1

Modulation of inositol phospholipid (InsPL) hydrolysis in response to increasing intracellular concentrations of cyclic AMP (cAMP) was studied in a murine T helper type II (Th2) lymphocyte clone, 8-5-5. Intact 8-5-5 cells produced maximal amounts of cAMP in response to prostaglandin E2 (PGE2), cholera toxin (CTx) or 7 beta-deacetyl-7 beta-(gamma-N-methylpiperazino)butyryl forskolin (dmpb-forskolin). cAMP generation reached a plateau after 5 min of treatment with dmpb-forskolin (300 microM) or PGE2 (1 microM), but required 60 min of treatment with CTx (1 microgram/ml). Preincubation of 8-5-5 cells with 1 microM-PGE2 or 300 microM-dmpb-forskolin (10 min at 37 degrees C) or with 1 microgram of CTx/ml (60 min at 37 degrees C) completely inhibited InsPL hydrolysis induced by perturbation of the T cell receptor (TCR)/CD3 complex with the monoclonal antibody 145.2C11. Preincubation with the cAMP analogue 8-bromo-cyclic AMP (8-Br-cAMP) also inhibited InsPL hydrolysis. Tetanolysin-permeabilized 8-5-5 cells produced cAMP in response to PGE2, dmpb-forskolin and guanosine 5'-[gamma-thio]triphosphate (GTP[S]), a non-cell-permeating, non-hydrolysable analogue of GTP that directly activates G-proteins. No inhibition of TCR/CD3-induced InsPL hydrolysis was observed under these conditions. InsPL hydrolysis was also unaffected when permeabilized cells were incubated with up to 10 mM-8-Br-cAMP, suggesting that permeabilized cells lost (a) soluble effector molecule(s) involved in mediating the inhibitory effect observed in intact cells. Treatment of 8-5-5 cells with dmpb-forskolin or CTx prior to permeabilization resulted in inhibition of TCR/CD3-induced InsPL hydrolysis, but did not affect InsPL hydrolysis induced via G-protein stimulation with GTP[S]. Treatment of permeabilized 8-5-5 cells with purified cAMP-dependent protein kinase (PKA) resulted in inhibition of TCR/CD3- but not GTP[S]-induced InsPL hydrolysis. This effect was associated with phosphorylation of phospholipase (PLC)-gamma 1 in the absence of phosphorylation of components of the TCR/CD3 complex. These results suggest that PKA-mediated phosphorylation of PLC may regulate TCR/CD3-induced InsPL hydrolysis.

G proteins in T cell signal transduction

This review summarises recent data on G protein implication in receptor signalling in T cells. The data show that PPI-specific PLC in T cell membranes is under G protein control. Some evidence indicates that a G protein couples PLC to TCR. Differences are revealed between the effects induced by direct G protein activators, such as GTP gamma S or AlF4-, and TCR ligands, which imply that TCR ligands may trigger some G-protein-independent signals. An analysis of the conflicting results on the action of PTX and CTX, one of the main tools in studying G proteins, has shown that the toxins produce both G protein-dependent and independent effects. The G protein which couples PLC to TCR appears insensitive to both PTX and CTX. Some findings suggest G protein involvement in signalling induced by interleukins; however, in this case the effector molecules implicate often remain unknown. Scarce data on G protein involvement in signalling from differentiation antigens, on direct G protein regulation of ion channels, and on identification of G proteins in T cells, are also discussed.

A role for guanine-nucleotide-binding proteins in mediating T-cell-receptor coupling to inositol phospholipid hydrolysis in a murine T-helper (type II) lymphocyte clone

Perturbation of the T-cell receptor (TCR) complex is followed by the rapid hydrolysis of inositol phospholipids (InsPL) by phospholipase C (PLC), producing diacylglycerol and inositol phosphates, which act as second messengers in signal transduction. The mechanism coupling the TCR to InsPL hydrolysis is not clearly defined, and no information is available on this mechanism in the CD4+ helper subset of T-lymphocytes (Th). We have tested the hypothesis that guanine-nucleotide-binding proteins (G-proteins) may couple the TCR to PLC in a murine Th type II (Th2) cell clone. Cell permeabilization with streptolysin O (SLO) or tetanolysin (TL) was used to allow membrane-impermeable nucleotides access to intracellular sites of action. Exposure of permeabilized Th2 cells to guanosine 5'-[gamma-thio]triphosphate (GTP gamma S), a non-hydrolysable GTP analogue, resulted in a 2.1-2.5-fold increase in inositol phosphate generation. Similarly, perturbation of the TCR with the monoclonal antibody 145.2C11 (directed against the epsilon-chain of the CD3 component of the TCR) resulted in a 3.1-4.2-fold increase in InsPL hydrolysis by permeabilized cells. Both lysins were similarly effective in allowing GTP gamma S induction of InsPL hydrolysis, but TL-permeabilized cells responded better to TCR perturbation than SLO-treated cells. A role for G-proteins in TCR coupling to PLC was further supported by the inhibition of TCR-induced InsPL hydrolysis by guanosine 5'-[beta-thio]diphosphate (GDP beta S), a guanine nucleotide analogue that inhibits G-protein function. ATP was required for TCR-mediated InsPL hydrolysis, and potentiated GTP gamma S-induced hydrolysis. Other nucleotides (i.e. CTP, GDP, GTP, ITP) did not affect the response. These data indicate that G-proteins may contribute to the regulation of PLC activation in Th2 cells, coupling it to the TCR.

Stimulation of p21ras upon T-cell activation

External signals that control the activity of proteins encoded by the ras proto-oncogenes have not previously been characterized. It is now shown that stimulation of the antigen receptor of T lymphocytes causes a rapid activation of p21ras. The mechanism seems to involve a decrease in the activity of GAP, the GTPase-activating protein, on stimulation of protein kinase C. In lymphocytes, p21ras may therefore be an important mediator of the action of protein kinase C.

A method for measuring protein kinase C activity in permeabilized T lymphocytes by using peptide substrates. Evidence for multiple pathways of kinase activation

Activation of protein kinase C (PKC) in human T lymphocytes is an immediate consequence of mitogenic signalling via the antigen-receptor complex and CD2 antigen. In order to investigate further the signal-transduction pathways which result in PKC activation, we have established a novel PKC assay system using streptolysin-O-permeabilized T cells. Known peptide substrates of PKC were introduced into permeabilized cells in the presence of [gamma-32P]ATP, 3 mM-Mg2+ and 150 nM free Ca2+. The peptide found to have the lowest background phosphorylation had the sequence Pro-Leu-Ser-Arg-Thr-Leu-Ser-Val-Ala-Ala-Lys-Lys (peptide GS), and the phosphorylation of the peptide was increased up to 6-fold by direct activation of PKC with phorbol 12,13-dibutyrate. Induction of PKC activation with the UCHT1 antibody against the CD3 antigen, or with phytohaemagglutinin (PHA) or guanosine 5'-[gamma-thio]triphosphate (GTP[S]), increased peptide-GS phosphorylation by 2-3 fold. The specificity of PKC action on peptide GS was demonstrated by blocking increases in phosphorylation with a pseudosubstrate peptide PKC inhibitor. PKC activation by this technique could be detected within 1 min of adding external ligand. Dose-response curves revealed that PHA-induced production of inositol phosphates correlated closely with PKC activities, whereas only a partial correlation between these parameters was observed with GTP[S]. Our data are consistent with the presence of more than one G-protein-mediated pathway of PKC regulation in T cells. The quantitative PKC assay system described is both simple and reproducible, and its potential application to a wide range of cell types should prove useful in further investigations of PKC activation mechanisms.