Immunolocalization of membrane-associated CTP:phosphocholine cytidylyltransferase in phosphatidylcholine-deficient Chinese hamster ovary cells.

The location of CTP:phosphocholine cytidylyltransferase in Chinese hamster ovary (CHO) cells made deficient in phosphatidylcholine was determined by immunofluorescence techniques. A rabbit polyclonal antibody was raised against a synthetic peptide corresponding to the amino-terminal 17 amino acid residues of rat liver cytidylyltransferase. The antibody recognized both native and denatured cytidylyltransferase from both rat liver and CHO cells. CHO cells were treated with phospholipase C to alter the lipid composition of the plasma membrane and to elicit translocation of cytidylyltransferase from the less active soluble pool to an activated membrane fraction. Visualization of cytidylyltransferase by indirect immunofluorescence revealed staining of the nuclear envelope in phospholipase C-treated cells but not in untreated cells. CHO cells were also starved for choline and supplemented with a choline analogue to provide an alternative technique of rendering the cells phosphatidylcholine-deficient. Although this treatment should affect different cellular membranes than those affected by phospholipase C treatment, cytidylyltransferase still translocated to the nuclear envelope, as shown by indirect immunofluorescence. These results indicate that activated, membrane-bound cytidylyltransferase is associated with the nuclear membrane and suggest that the nuclear membrane may be a site of de novo phosphatidylcholine synthesis.     

Intracellular processing of cytidylyltransferase in Krebs II cells during stimulation of phosphatidylcholine synthesis. Evidence that a plasma membrane modification promotes enzyme translocation specifically to the endoplasmic reticulum

After a 3-h incubation of Krebs II ascitic cells in the presence of phospholipase C from Clostridium welchii under nonlytic conditions, the incorporation of [3H] choline into phosphatidylcholine was increased 1.7-fold as compared to untreated cells. The total amounts of phosphatidylcholine, phosphatidylethanolamine, and sphingomyelin were unchanged up to 3 h of incubation. The limiting step in phosphatidylcholine biosynthesis was the formation of CDP-choline catalyzed by CTP:choline-phosphate cytidylyltransferase (EC 2.7.7.15) as monitored by the decrease in phosphocholine labeling following phospholipase C treatment of cells prelabeled with [3H]choline. The specific activity of homogenate cytidylyltransferase was increased about 1.6-fold in phospholipase C-treated cells. Specific activity of the membrane fraction was increased 2-fold, whereas cytosolic specific activity decreased in phospholipase C-treated cells. The activation of cytidylyltransferase was concomitant with translocation of the enzyme from the cytosol to the membrane fraction. The latter was further fractionated using a Percoll gradient that allowed an efficient separation between endoplasmic reticulum and other subcellular membranes. In control cells, particulate cytidylyltransferase activity co-migrated with the endoplasmic reticulum and ribosome markers and not with the plasma membrane. Also, in treated cells, the stimulation of cytidylyltransferase activity occurred at the endoplasmic reticulum level and did not involve either the external cell membrane or other cellular organelles including the Golgi apparatus, lysosomes, or mitochondria. Thus, our results demonstrate that a stimulus acting on the plasma membrane promotes the translocation of the soluble form of cytidylyltransferase specifically to the endoplasmic reticulum.     

Regulation of CTP:phosphocholine cytidylyltransferase activity and phosphorylation in rat hepatocytes: lack of effect of elevated cAMP levels.

Immunoprecipitation of 32P-labeled CTP:phosphocholine cytidylyltransferase from freshly isolated rat hepatocytes followed by trypsin digestion and two-dimensional peptide mapping revealed multiple phosphorylation sites. Treatment of the hepatocytes with 0.5 mM of the cAMP analog, 8-(4-chlorophenylthio)-adenosine 3':5'-monophosphate or elevation of intracellular cAMP levels by cholera toxin activated the cAMP-dependent protein kinase activity in intact cells. Despite the activation of cAMP-dependent protein kinase no change in the rate of [3H]choline incorporation into phosphatidylcholine was detected. In addition, the activity of cytidylyltransferase in total cell homogenates and its distribution between soluble and particulate fractions remained unchanged. Comparison of peptide maps of 32P-labeled cytidylyltransferase obtained from control and cholera-toxin-treated hepatocytes did not reveal any differences in the phosphorylation state of cytidylyltransferase. Furthermore, only [32P]phosphoserine residues were detected following phosphoamino acid analysis. We conclude that cytidylyltransferase activity is not altered solely by the activation of the cAMP-dependent kinase in fresh hepatocytes.     

Regulation of phosphatidylcholine biosynthesis in mammalian cells. II. Effects of phospholipase C treatment on the activity and subcellular distribution of CTP:phosphocholine cytidylyltransferase in Chinese hamster ovary and LM cell lines

CTP:phosphocholine cytidylyltransferase was located in both the cytosolic and particulate fractions from Chinese hamster ovary cells. The activity of the cytosolic form of the enzyme was greatly enhanced by incubation with sonicated preparations of several different lipids, although incubations with either phosphatidylcholine or 1,2-sn-diolein did not increase activity. The activation of the cytidylyltransferase in Chinese hamster ovary cells treated with phospholipase C from Clostridium perfringens occurred with a concomitant shift in the subcellular distribution of the enzyme from cytosolic to particulate fractions. This shift was rapid and did not require protein synthesis. Removal of phospholipase C from the cell cultures resulted in a return to basal levels of incorporation of [3H]choline into phosphatidylcholine, a decrease in the activity of cytidylyltransferase, and a loss of the membrane-bound form of the enzyme. Similar experiments with LM cells, which are resistant to exogenous phospholipase C, showed no change in subcellular distribution of cytidylyltransferase, suggesting that the activation of CTP:phosphocholine cytidylyltransferase required a change in membrane phospholipid composition. The results presented are discussed in terms of a mechanism of regulation of phosphatidylcholine production involving monitoring of membrane phospholipid composition.     

Growth factor stimulation of phospholipase C-gamma 1 activity. Comparative properties of control and activated enzymes.

We demonstrated previously tyrosine phosphorylation-dependent modulation of phospholipase C-gamma 1 (PLC-gamma 1) catalytic activity (Nishibe, S., Wahl, M. I., Hernandez-Sotomayor, S. M. T., Tonks, N. K., Rhee, S. G., and Carpenter, G. (1990) Science 250, 1253-1256). The increase in PLC-gamma 1 catalytic activity in A-431 cells occurs rapidly, with maximal activation 5 min after epidermal growth factor (EGF) stimulation. Certain other growth factors (fibroblast growth factor, platelet-derived growth factor) also stimulate PLC-gamma 1 catalytic activity, whereas insulin does not. A similar increase in PLC-gamma 1 specific activity (2-3-fold) was observed in both soluble (cytosol) and particulate (membrane) preparations from EGF-treated cells. Tyrosine-phosphorylated PLC-gamma 1 was detected in both cytosol and membrane fractions in lysates from EGF-treated A-431 cells, but the proportion of tyrosine-phosphorylated PLC-gamma 1 was higher in the cytosol (approximately 50%) than in the membrane (approximately 20%). Because a micellar concentration of the non-ionic detergent Triton X-100 allows detection of the tyrosine phosphorylation-dependent increase in PLC-gamma 1 catalytic activity in this assay, we evaluated the kinetic properties of PLC-gamma 1, immunoprecipitated from cytosol of control or EGF-treated cells, using substrate, phosphatidylinositol 4,5-bisphosphate (PtdIns 4,5-P2), solubilized in Triton X-100 at various molar ratios. The behavior of the control enzyme differed from the EGF-activated enzyme with respect to both Ks and Km. The control enzyme has a 7.5-fold higher Ks value than the activated enzyme (1.5 mM as compared with 0.22 mM). Activation by EGF is also a positive allosteric modifier of PLC-gamma 1-catalyzed PtdIns 4,5-P2 hydrolysis, i.e. the activated enzyme displayed apparent Michalis-Menton kinetics, with a Km of 0.6 mol fraction PtdIns 4,5-P2, whereas the control enzyme displayed sigmoidal kinetics with respect to PtdIns 4,5-P2 hydrolysis. At low substrate mol fractions (e.g. 0.07), the reaction velocity of the control enzyme was 4-fold lower than the activated enzyme. However, at a high substrate mol fraction (e.g. 0.33), the estimated maximal reaction velocities (Vmax) for both forms of PLC-gamma 1 were equivalent. PLC-gamma 1 activity from both control and EGF-treated cells was stimulated by increasing nanomolar Ca2+ concentrations. Although the catalytic activity of PLC-gamma 1 from EGF-treated cells was greater than control PLC-gamma 1 at every Ca2+ concentration tested, the relative stimulation of activity was markedly greater at Ca2+ concentrations above approximately 300 nM.     

Bovine aortic endothelial cell transglutaminase. Enzyme characterization and regulation of activity.

Bovine aortic endothelial cells contain Ca2+-dependent tissue-type transglutaminase. Its activity in these cells was high, with apparent Km and Vmax. values with respect to putrescine of 0.203 mM and 18.5 nmol/min per mg of protein, and its activity was inhibited by the three competitive inhibitors dansylcadaverine, spermine and methylamine. The molecular mass of endothelial cell transglutaminase estimated by gel filtration chromatography was 88 kDa and it was immunoprecipitated by rabbit monospecific antiserum raised against rat liver transglutaminase. Its enzymic activity rose when the cell cultures reached confluence, and was further increased when their proliferation was arrested (synchronized at G0/G1 phase). Most of the enzymic activity was found in the 15,000 g soluble fraction, with only 4-22% of the activity found in the particulate fraction, depending on the state of cell proliferation. Examination of these cellular fractions by SDS/polyacrylamide-gel electrophoresis and immunoblotting revealed that at confluence endothelial cells have accumulated transglutaminase antigen in their 15,000 g particulate fraction. A series of experiments demonstrated the existence of a latent transglutaminase form in non-proliferating cells, and suggested that this might involve the formation of an inhibitory complex. Treatment of cell lysates and the 15,000 g particulate fraction with high salt concentration showed a significant increase in transglutaminase activity. Mixing experiments using the 100,000 g particulate fraction or purified rat liver transglutaminase on one hand and the cytosolic fraction on the other showed dose-dependent inhibition of the transglutaminase activity of the latter. It is concluded that endothelial cells contain a particulate fraction-residing inhibitor of transglutaminase which interacts via ionic interaction with the enzyme.     

Freezing injury and phospholipid degradation in vivo in woody plant cells: I. Subcellular localization of phospholipase d in living bark tissues of the black locust tree (robinia pseudoacacia L.)

The subcellular localization of phospholipase D in homogenates of living bark tissues of the black locust tree (Robinia pseudoacacia L.) was examined and found in both soluble and particulate fractions. At least some of the soluble enzyme was considered to be compartmentalized in vacuoles. Considerable amounts of phospholipase D seemed to be tightly bound on several membranes such as endoplasmic reticulum, tonoplast, and a membrane associated with potassium-stimulated ATPase (pH 6.1). The mitochondrial fraction banding at the 40 to 43% (w/w) sucrose layer, however, had the lowest specific activity. The soluble and the particulate phospholipase D were considered to be similar in nature. It is possible that the particulate enzyme, as a part, may be derived from the coexisting nonvesiculated materials visualized in the electron micrograph of each membrane fraction. An involvement of the soluble or the presumed membrane-bound phospholipase D in phospholipid degradation in vivo during freezing at sublethal temperatures was discussed with special reference to freezing injury of plant cells.     

Direct activation of phospholipase A2 by GTP-binding protein in human peripheral polymorphonuclear leukocytes.

In human peripheral polymorphonuclear leukocyte (PMN), 10% of PLA2 activity was found in the particulate fraction. In the particulate fraction, the activity of phospholipase A2 was enhanced 270% by 100 microM guanosine 5'-[gamma-thio]triphosphate, a hydrolysis-resistant analog of GTP. In the soluble fraction, such enhancement was not observed. Guanosine 5'-[beta-thio]diphosphate (2 mM), which irreversibly inactivates GTP-binding protein, blocked the enhancement in the particulate fraction. Membrane-binding phospholipase A2 activity of PMN would thus appear to be regulated directly by GTP-binding protein.     

Regulation of phosphatidic acid phosphohydrolase activity during stimulation of human polymorphonuclear leukocytes.

Phosphatidic acid phosphohydrolase (PPH) activity was determined in human polymorphonuclear leukocytes (PMNs) by measuring the hydrolysis of [32P]phosphatidic acid (PA) added to cell sonicates. Enzyme activity was localized primarily to a soluble fraction. Soluble and particulate activities required magnesium and were inhibited by calcium, N-ethylmaleimide, sphingosine, and propranolol. The activity in unstimulated PMNs was 0.64 +/- 0.11 nmol of PA hydrolyzed.mg protein-1.min-1 in particulate and 4.20 +/- 0.42 in soluble fractions. Stimulation of PMNs with 1 microM f-Met-Leu-Phe (FMLP) for 10 min caused a slight decrease in soluble activity and a small increase in the activity of particulate fractions. Preincubation with 10 microM cytochalasin B for 5 min before FMLP stimulation markedly enhanced both of these changes. The effect of FMLP plus cytochalasin B was rapid (less than 10 s), whereas the calcium ionophore A23187 (1 microM) and phorbol myristate acetate (100 ng/ml) caused slower and smaller changes in enzyme activity. These results indicate that after chemoattractant stimulation; PPH activity decreases in the soluble fraction and increases in the particulate fraction suggesting that PPH may participate in signal transduction in the PMN.     

Identification of type-2 phosphatidic acid phosphohydrolase (PAPH-2) in neutrophil plasma membranes

Plasma membrane phosphatidic acid phosphohydrolase (PAPH) plays an important role in signal transduction by converting phosphatidic acid to diacylglycerol. PAPH-2, a Mg²⁺-independent, detergent-dependent enzyme involved in cellular signal transduction, is reportedly absent from the plasma membranes of neutrophilic leukocytes, a cell that responds to metabolic stimulation with abundant phospholipase -dependent diacylglycerol generation. The present study was designed to resolve this discrepancy, focusing on the influence of cellular disruption techniques, detergenta availability and cation sensitivity on the apparent distribution of PAPH in neutrophil sub-cellular fractions. The results clearly indicate the presence of two distinct types of PAPH within the particulate and cytosolic fractions of disrupted cells. Unlike the cytosolic enzyme, the particulate enzyme was not potentiated by magnesium and was strongly detergent-dependent. The soluble and particulate enzymes displayed dissimilar pH profiles. Separation of neutrophil particulate material into fractions rich in plasma membranes, specific granules and azurophilic granules by high speed discontinuous density gradient centrifugation revealed that the majority of the particulate activity was confined to plasma membranes. This activity was not inhibited by pretreatment with n-ethyl-maleimide in concentrations as high as 25 mM. PAPH activity recovered in the cytosolic fraction of disrupted neutrophils was almost completely inhibited by 5.0 mM n-ethylmaleimide. We conclude that resting neutrophils possess n-ethylmaleimide-resistant PAPH (type 2) within their plasma membranes. This enzyme may markedly influence the kinetics of cell activation by metabolizing second messengers generated as a result of activation of plasma membrane phospholipase D.     

Phosphatidylinositol-cleaving activity in smooth muscle from rat vas deferens

• 1.1. Phosphatidylinositol-cleaving activity was studied in subcellular fractions from smooth muscle of rat vas deferens.
• 2.2. In the presence of calcium ions and deoxycholate most of the endogenous phosphatidylinositol was broken-down in 60 min, whilst the other phospholipids were stable.
• 3.3. The enzymatic activity responsible for this breakdown catalyses a phospholipase C-type cleavage of the glycerol-phosphate bond, the water soluble products from exogenous [³²P]-labelled phosphatidylinositol being d-myoinositol 1:2-cyclic phosphate (702-80%) and d-myoinositol 1-phosphate (202-30%).
• 4.4. Activity was abolished by 1 mM ethanedioxybis(ethylamine)tetra-acetate (EGTA) and in the presence of deoxycholate both the soluble and total particulate fractions showed maximum activity at pH 6.52-6.8. The soluble fraction showed a second peak of activity at pH 5.52-5.8 that was independent of deoxycholate; this was not observed in the particulate fraction.
• 5.5. About two-thirds of the activity was soluble. The remaining activity was particulate, with a preferential concentration in the microsomal fraction.

     

Kinetic and biochemical properties of CTP:choline-phosphate cytidylyltransferase from the rat brain

In order to investigate the mechanisms involved in some brain disorders at the membrane level, we studied the kinetics and biochemical properties of brain CTP:choline-phosphate cytidylyltransferase (EC 2.7.7.15), the rate-limiting enzyme of the two-step biosynthesis of phosphatidylcholine. This enzyme catalyzes the biosynthesis of CDPcholine from choline phosphate and CTP. We found that its subcellular localization (mainly in microsomal and cytosolic fractions) was different from that of phosphatidylethanolamine N-methyltransferase (EC 2.1.1.17), the enzyme of the alternative pathway for phosphatidylcholine synthesis. CTP:choline-phosphate cytidylyltransferase showed a Km of 10 mM for CTP and 0.3 mM for choline phosphate and exhibited a random mechanism. CDPcholine, the reaction product, was a competitive inhibitor of choline phosphate and CTP utilization and had a Ki of 0.090 mM. Both particulate and soluble enzymes required Mg2+ and exhibited an optimal pH at about 7. Cytosolic activity was enhanced by addition of unsaturated fatty acids or phospholipids extracted from brain membranes. Such an enhancement was increased with the centrifugation time used for preparing the soluble enzyme.     

Insulin-receptor phosphotyrosyl-protein phosphatases.

Calmodulin-dependent protein phosphatase has been proposed to be an important phosphotyrosyl-protein phosphatase. The ability of the enzyme to attack autophosphorylated insulin receptor was examined and compared with the known ability of the enzyme to act on autophosphorylated epidermal-growth-factor (EGF) receptor. Purified calmodulin-dependent protein phosphatase was shown to catalyse the complete dephosphorylation of phosphotyrosyl-(insulin receptor). When compared at similar concentrations, 32P-labelled EGF receptor was dephosphorylated at greater than 3 times the rate of 32P-labelled insulin receptor; both dephosphorylations exhibited similar dependence on metal ions and calmodulin. Native phosphotyrosyl-protein phosphatases in cell extracts were also characterized. With rat liver, heart or brain, most (75%) of the native phosphatase activity against both 32P-labelled insulin and EGF receptors was recovered in the particulate fraction of the cell, with only 25% in the soluble fraction. This subcellular distribution contrasts with results of previous studies using artificial substrates, which found most of the phosphotyrosyl-protein phosphatase activity in the soluble fraction of the cell. Properties of particulate and soluble phosphatase activity against 32P-labelled insulin and EGF receptors are reported. The contribution of calmodulin-dependent protein phosphatase activity to phosphotyrosyl-protein phosphatase activity in cell fractions was determined by utilizing the unique metal-ion dependence of calmodulin-dependent protein phosphatase. Whereas Ni2+ (1 mM) markedly activated the calmodulin-dependent protein phosphatase, it was found to inhibit potently both particulate and soluble phosphotyrosyl-protein phosphatase activity. In fractions from rat liver, brain and heart, total phosphotyrosyl-protein phosphatase activity against both 32P-labelled receptors was inhibited by 99.5 +/- 6% (mean +/- S.E.M., 30 observations) by Ni2+. Results of Ni2+ inhibition studies were confirmed by other methods. It is concluded that in cell extracts phosphotyrosyl-protein phosphatases other than calmodulin-dependent protein phosphatase are the major phosphotyrosyl-(insulin receptor) and -(EGF receptor) phosphatases.     

Angiotensin II but not potassium induces subcellular redistribution of protein kinase C in bovine adrenal glomerulosa cells

The distribution of calcium-activated, phospholipid-dependent protein kinase (protein kinase C) between cytosol and membrane fractions was examined in bovine adrenal glomerulosa cells treated with angiotensin II or potassium. Protein kinase C was isolated from cytosol and from detergent-solubilized particulate fractions by DEAE-cellulose chromatography. A major peak of activity for both the soluble and particulate forms of adrenal glomerulosa protein kinase C was eluted at 0.05-0.09 M NaCl. The soluble and particulate forms were found to constitute about 95 and 5%, respectively, of the total enzyme activity in unstimulated cells. A second peak of kinase activity was eluted with 0.15-0.19 M NaCl, which was not dependent on the presence of phospholipids. Exposure of isolated cells for 20 min to 10(-8) M angiotensin II resulted in a decrease in cytosolic activity to 30-40% of control values, and in a corresponding increase in protein kinase C activity associated with the particulate fraction. This hormone-induced redistribution was found to be dose-dependent with an ED50 of 2 nM for angiotensin II, and it occurred rapidly, reaching a plateau within 5-10 min. It was prevented by the specific antagonist [Sar1,Ala8]angiotensin II. By contrast, stimulation with 12 mM KCl did not change the subcellular distribution of protein kinase C activity. These results suggest that redistribution of protein kinase C represents an early step in the post-receptor activation cascade following angiotensin II, but not potassium stimulation of adrenal glomerulosa cells.     

Phosphatidylcholine synthesis in castor bean endosperm: the localization and control of CTP: choline-phosphate cytidylyltransferase activity

The reaction catalyzed by CTP:choline-phosphate cytidylyltransferase (EC 2.7.7.15) has been postulated to be a control reaction in the synthesis of phosphatidylcholine (PtdCho) in many animal tissues and some plants. In 3-day-old castor bean (Ricinus communis L. var. Hale) endosperm the majority of cytidylyltransferase activity resided in a 12,000gav 10-min pellet. Following density-gradient fractionation, 60 to 70% of the enzyme activity was associated with the endoplasmic reticulum (ER) fraction, with the remainder in the particulate fraction being in an unidentified membrane band (band A), less than occurred in the soluble fractions. The properties and kinetics of the forward and reverse reactions are described. About 40% of the total ER activity could be solubilized by treatment of the fraction with 0.32 M KCl, which resulted in a threefold increase in the specific activity of the enzyme. The Michaelis constants of the solubilized enzyme were similar to those of the ER activity. The activity of the solubilized enzyme was stimulated 35% by addition of phosphatidylglycerol or phosphatidylinositol to the assay. Addition of a number of other phospholipids to the incubation medium caused only a small change in activity (+/- 10%) but the enzyme could be stimulated up to 60% by the addition of 0.01-1 mM sodium oleate. A combination of 0.25 mM PtdCho with oleate in the assay resulted in additional stimulation at all concentrations of oleate. Oleate had no effect on the ER activity. These results are discussed in relation to the regulation of cytidylyltransferase activity in plants.     

A lectin-binding glycoprotein of Mr 135,000 associated with basal keratinocytes in pig epidermis.

Pig epidermis separated by 1 M-CaCl2 treatment was homogenized and separated into three fractions by filtration through nylon mesh and high-speed centrifugation. Lectin-binding glycoproteins were isolated from urea/deoxycholate/mercaptoethanol extracts of the residue fraction that resisted filtration, from deoxycholate extracts of the particulate material in the filtrate and from the soluble fraction. Concanavalin A, Ricinus communis (castor bean) agglutinin 1, peanut (Arachis hypogaea) agglutinin and Ulex europaeus (gorse) agglutinin-binding glycoproteins in the three epidermal fractions were analysed by SDS/polyacrylamide-gel electrophoresis. A major neuraminidase-sensitive glycoprotein component of the particulate fraction of Mr 135,000 was strongly bound by concanavalin A and Ricinus communis agglutinin 1, but only weakly by peanut and Ulex europaeus agglutinins. This glycoprotein was not detected in the residue or soluble fractions of the epidermis, indicating that it had only a limited distribution within the tissue. The 135,000-Mr glycoprotein was one of two major glycoprotein antigens in the particulate fraction. Rabbits immunized with total particulate glycoproteins produced antibodies directed mainly against 135,000- and 110,000-Mr components. Monospecific antibodies were obtained from guinea pigs immunized with the 135,000-Mr glycoprotein band excised from polyacrylamide gels. Indirect immunofluorescence with the use of affinity-purified antibodies showed that the 135,000-Mr glycoprotein was present at the surface of cells in the basal layer of the epidermis as well as at that of other stratified epithelia. It was not present on differentiating cells in the suprabasal layers of the epithelium, suggesting an important role in the attachment or proliferative functions of basal cells in stratified epithelia. Metabolic labelling studies with skin explants cultured in the presence of D-[3H]glucosamine showed that this basal-cell glycoprotein was synthesized by cultured tissue. The major D-[3H]glucosamine-labelled glycoprotein component in the residue and particulate fractions of cultured epidermis had an Mr of 135,000, was immunoprecipitated by rabbit antisera raised against particulate epidermal glycoproteins and was bound by concanavalin A. The labelling of this glycoprotein with D-[3H]glucosamine was sensitive to tunicamycin, indicating that the basal-cell glycoprotein contained N-glycosidically linked oligosaccharides.     

Resolution of the phosphoinositide-specific phospholipase C isolated from porcine lymphocytes into multiple species. Partial purification of two isoenzymes.

Phospholipase C isolated from porcine mesenteric lymph node lymphocytes was distributed between the soluble and particulate fractions. Enzyme activity was found predominantly in the soluble fraction with optimal activity at pH 5.5. Gel filtration chromatography of the soluble phospholipase C revealed that it was composed of multiple species of enzyme activity. The activity associated with the particulate fraction had optimal activity at pH 7.0, as also did one of the species of soluble phospholipase C. Cellulose phosphate chromatography resolved the major soluble form into two species designated PLC-A and PLC-B. Both phenyl-Sepharose chromatography and hydroxyapatite chromatography purified these species still further. PLC-A and PLC-B demonstrated similar activities against phosphatidylinositol with a pH optimum near 5.5. The phospholipase C activities were abolished against this substrate by the addition of 1 mM-EDTA. When assayed in the presence of Ca2+-EDTA buffers providing a range of Ca2+ free concentrations, both enzymes exhibited optimal activity near 10(-3) M free Ca2+, but PLC-B was inhibited above this concentration more than PLC-A. PLC-B exhibited markedly lower activity against phosphatidylinositol 4,5-bisphosphate, suspended as liposomes of the pure phospholipid, than did PLC-A.     

Elevated phosphatidylinositol kinase activity in Rous sarcoma virus-transformed cells. Lack of evidence for enzyme translocation

Phosphatidylinositol kinase (E.C. 2.7.1.67) activity of rat fibroblasts transformed by Rous sarcoma virus (RSV) was measured and compared with immunoprecipitated protein tyrosine kinase activity associated with pp60v-src. Both enzyme activities were elevated in the particulate fractions from wild-type RSV-transformed cells and cells transformed by a temperature-sensitive mutant of RSV when grown at the permissive temperature. The presence of the non-ionic detergent Nonidet P-40 in the phosphatidylinositol kinase assays stimulated the soluble and particulate forms of the enzyme to different degrees but did not affect the relative differences between transformed and untransformed cells. Our results indicate that phosphatidylinositol kinase activity is a good correlate of RSV transformation and suggest a functional relationship between pp60v-src and phosphatidylinositol kinase.     

Lipocortins are major substrates for protein kinase C in extracts of human neutrophils.

We have identified two major proteins in human neutrophils that are phosphorylated in vitro by protein kinase C (PKC) as lipocortins III and a fragment of a lipocortin-like 68-kDa protein. In electroporated cells, the 68-kDa protein was phosphorylated during stimulation of the cells with either FMLP or PMA. Lipocortins are of interest because of their Ca2(+)- and phospholipid-dependent actin binding properties and ability to inhibit phospholipase A2. Two crude fractions of enzymes and proteins exposed to [gamma-32]PATP in the presence of Ca2+, Mg2+, phosphatidylserine and 1,2-oleoyl-acetyl-rac-glycerol were analyzed by gel electrophoresis and autoradiography. A number of proteins in a detergent-free fraction, including proteins at 36 and 32 kDa, were phosphorylated in the presence of these cofactors. In contrast, only two major proteins (35 and 32 kDa) were phosphorylated in a detergent-extracted fraction. Phosphorylation of the 36, 35, and 32 kDa proteins required the presence of Ca2+, Mg2+, and phosphatidylserine in our soluble fraction and detergent extract, indicating PKC-dependent phosphorylation. The 32-kDa protein phosphorylated in both the soluble fraction and detergent extract was identified as lipocortin III by immunoprecipitation with a cross-reactive antibody that recognized lipocortin I and comparison of cyanogen bromide (CNBr) cleavage patterns of this protein with a lipocortin III standard. The 68-kDa protein was identified as a lipocortin VI-like protein by immunoprecipitation with anti-calelectrin. Additionally, the CNBr cleavage pattern of the 68-kDa protein was similar to that of the 36-kDa protein phosphorylated in our soluble fraction. Autoradiograms of the 68- and 36-kDa fragments immunoprecipitated from our soluble fraction with anticalelectrin and cleaved with CNBr showed that both of these proteins were phosphorylated in this sample. Because phosphorylation is known to change the functional characteristics of the lipocortins, the potential exists to link PKC and lipocortins in neutrophils to regulation of granulemembrane interactions or mediation of inflammation.     

In vivo phosphorylation of clathrin-coated vesicle proteins from rat reticulocytes

We have studied the in vivo phosphorylation of clathrin-coated vesicle proteins from rat reticulocytes. The major 32P-labeled polypeptides of clathrin-coated vesicles isolated from metabolically labeled cells were the the 165-, 100-110-, and 50-kDa polypeptides of the assembly protein, the clathrin beta-light chain, and to a lesser extent the clathrin alpha-light chain. The phosphorylation of the assembled (particulate) and unassembled (soluble) pools of clathrin and assembly protein was compared by immunoprecipitating the respective protein complexes from particulate and soluble cell fractions. Although all the phosphorylated polypeptides were present in both fractions, the extent of labeling was protein and fraction specific: the apparent specific activities of the assembly protein 50-kDa polypeptide and clathrin light chain were higher in the unassembled pool, whereas those of the 100-110-kDa polypeptides were higher in the assembled pool. The amino acids and polypeptide fragments labeled in vivo appeared similar to those labeled in vitro.