Existence of endogenous substrate proteins for Ca2+/calmodulin-dependent and Ca2+/phospholipid-dependent protein kinases in rat adrenal glomerulosa cells

In rat adrenal glomerulosa cells, endogenous substrate proteins for Ca2+/calmodulin (CaM)-dependent protein kinase (glomerulosa CaM kinase) and Ca2+/phospholipid-dependent protein kinase (protein kinase C) were investigated. In a 105,000 g-supernatant fraction (cytosol), the Mr 100,000 protein was phosphorylated in the presence of calcium (calculated free Ca2+ concentration, 460 microM) alone or calcium and CaM, and the phosphorylation of this protein was completely inhibited by the CaM antagonists pimozide (500 microM) and melittin (5 microM) in the presence of calcium alone, respectively. These results indicate that the Mr 100,000 protein is a major substrate for glomerulosa CaM kinase, and considerable amounts of endogenous CaM might be present in the cytosol. In the presence of phospholipids (the micelles of 8 micrograms of phosphatidyl serine and 1 microgram of diacylglycerol), at least twelve proteins of Mr 127,000, 80,000, 70,000, 36,000, 35,000, 33,000, 32,000, 30,000, 27,000, 22,000, 19,000 and 17,000 were phosphorylated, and the phosphorylation of these proteins was enhanced by the addition of calcium, indicating that these proteins are substrates for protein kinase C. No endogenous protein phosphorylation was found in a 105,000 g-particulate fraction. Thus, these findings demonstrate that adrenal glomerulosa cells have specific substrate proteins for glomerulosa CaM kinase and protein kinase C, respectively.     

Non-requirement of calcium on protamine phosphorylation by calcium-activated, phospholipid-dependent protein kinase

Phosphorylation of clupeine sulfate by purified rat brain calcium-activated, phospholipid-dependent protein kinase (protein kinase C) was studied. In the absence of Ca2+, phosphatidylserine and diolein markedly stimulated its phosphorylation. However Ca2+ did not stimulate but inhibit this phosphorylation about 30% in the presence of phospholipids. Random polymer (Arg, Ser) 3:1 and (Lys, Ser) 3:1 could be phosphorylated by protein kinase C. In the presence of phospholipids Ca2+ is not needed for the phosphorylation of polymer (Arg, Ser) 3:1, while Ca2+ is necessary for polymer (Lys, Ser) 3:1. Non-requirement of Ca2+ on clupeine phosphorylation by protein kinase C is briefly discussed.     

Activation of protein kinase in the bovine corpus luteum by phospholipid and Ca2+.

A new species of protein kinase has been identified in cytosol preparations from bovine corpora lutea. Enzyme activity required the simultaneous presence of Ca2+ and phospholipid, and was also enhanced by glyceryl dioleate. Phosphatidylserine was the most effective phospholipid for stimulating histone phosphorylation. Other phospholipids capable of supporting enzymic activity were, in order of decreasing activity, phosphatidylinositol, phosphatidic acid, cardiolipin and phosphatidylglycerol. Several other phospholipids tested were ineffective. A cyclic AMP-dependent protein kinase was also present in the luteal cytosol. This enzyme activity was eliminated by protein kinase inhibitor without affecting the Ca2+- and phospholipid-stimulated activity. Lysine-rich histone (IIIS) was a much better substrate than type-IIA histone for Ca2+- and phospholipid-dependent phosphorylation. Ca2+ and phospholipid also enhanced phosphorylation of endogenous luteal cytosol protein. Calmodulin, alone or in the presence of Ca2+, was unable to increase phosphorylation. Trifluoperazine inhibited protein kinase activity stimulated by Ca2+ and phospholipid. These data suggest that a phospholipid-sensitive, Ca2+-dependent protein kinase may provide an important link between hormonally-induced changes in phospholipid metabolism and corpus-luteum function.     

Phosphorylation of troponin in the heart and skeletal muscle by Ca2+-phospholipid-dependent protein kinase

The phosphorylation of the whole troponin complex and of the cardiac and skeletal troponin components by Ca2+-phospholipid-dependent protein kinase was studied. The activity of enzyme isolated from rat brain by ion-exchange chromatography on DEAE-Sephadex and by affinity chromatography on phosphatidylserine immobilized on polyacrylamide gel was shown to be completely dependent on Ca2+ and phospholipids and was equal to 0.4-0.6 mumol of phosphate/min.mg protein with histone H1 as substrate. The resulting preparation of Ca2+-phospholipid-dependent protein kinase was able to phosphorylate the isolated troponin I; the amount of phosphate transferred per mol of cardiac and skeletal troponin I was equal to 1.1 and 0.4, respectively. The maximal degree of phosphorylation of isolated troponin T by Ca2+-phospholipid-dependent protein kinase was 0.6 mol of phosphate per mol of troponin T both for skeletal and cardiac proteins. The rate and degree of phosphorylation were independent of the initial level of troponin T phosphorylation. Ca2+-phospholipid-dependent protein kinase did not phosphorylate the first serine residue of troponin T, i.e., the site which was phosphorylated in the highest degree after isolation of troponin T from skeletal muscles. The data obtained and the fact that the rate and degree of phosphorylation of troponins I and T within the whole troponin complex are 10-20 times less than those for isolated components provide little evidence for the participation of protein kinase C in troponin phosphorylation in vivo.     

Ca2+-dependent hydrophobic-interaction chromatography. Isolation of a novel Ca2+-binding protein and protein kinase C from bovine brain.

Several bovine brain proteins have been found to interact with a hydrophobic chromatography resin (phenyl-Sepharose CL-4B) in a Ca2+-dependent manner. These include calmodulin, the Ca2+/phospholipid-dependent protein kinase (protein kinase C) and a novel Ca2+-binding protein that has now been purified to electrophoretic homogeneity. This latter protein is acidic (pI 5.1) and, like calmodulin and some other high-affinity Ca2+-binding proteins, exhibits a Ca2+-dependent mobility shift on sodium dodecyl sulphate/polyacrylamide-gel electrophoresis, with an apparent Mr of 22 000 in the absence of Ca2+ and Mr 21 000 in the presence of Ca2+. This novel calciprotein is distinct from known Ca2+-binding proteins on the basis of Mr under denaturing conditions, Cleveland peptide mapping and amino acid composition analysis. It may be a member of the calmodulin superfamily of Ca2+-binding proteins. This calciprotein does not activate two calmodulin-dependent enzymes, namely cyclic nucleotide phosphodiesterase and myosin light-chain kinase, nor does it have any effect on protein kinase C. It may be a Ca2+-dependent regulatory protein of an as-yet-undefined enzymic activity. The Ca2+/phospholipid-dependent protein kinase is also readily purified by Ca2+-dependent hydrophobic-interaction chromatography followed by ion-exchange chromatography, during which it is easily separated from calmodulin. A preparation of protein kinase C that lacks contaminating kinase or phosphatase activities is thereby obtained rapidly and simply. Such a preparation is ideal for the study of phosphorylation reactions catalysed in vitro by protein kinase C.     

A Protein Modulator Stimulates C Kinase-Dependent Phosphorylation of a 90K Substrate in Synaptic Membranes

We have identified and partially purified an acidic, heat-stable, noncalmodulin protein from bovine brain cytosol that stimulates Ca2+-dependent phosphorylation of an Mr 90K substrate in crude rat brain synaptic membranes. We show that this modulator of phosphorylation (MOP) enhances Ca2+- and phospholipid-dependent protein kinase (C kinase) phosphorylation of this 90K substrate. The 90K substrate is a higher Mr form of an 87K substrate that is a major C kinase substrate in rat brain. The Ca2+-dependent phosphorylation of both substrates is inhibited by the Ca2+-binding proteins S-100 and calmodulin. Both substrates yield phosphopeptide fragments of Mr 9K and 13K after limited proteolysis with V8 protease. Two-dimensional polyacrylamide gel electrophoresis reveals that they have similar acidic isoelectric points (pI 5.0). MOP enhances Ca2+-dependent phosphorylation of the 90K substrate whereas the phosphorylation of 87K is diminished. This reciprocal relationship suggests that the mobility of the 87K substrate in sodium dodecyl sulfate-polyacrylamide gels is decreased to 90K with increasing phosphorylation. MOP may be a novel protein modulator of C kinase-mediated phosphorylation in the nervous system.     

Ca2+-phospholipid-dependent phosphorylation of vesicular preparations of myocardial sarcolemma

A Ca2+-phospholipid-dependent protein kinase C was isolated from the soluble fraction of bovine brain, using hydrophobic chromatography on phenyl-Sepharose CL-4B and high performance liquid chromatography on a Mono Q column. The enzyme had a specific activity of 822 nmol 32P/mg protein/min with histone H1 as a substrate. Phosphorylation of pig myocardium sarcolemma protein substrates was stimulated by Ca2+ and phosphatidylserine; the optimal concentrations of these compounds were 10(-4) M and 200 micrograms/ml, respectively. The value of Km(app) for Ca2+ was 3.10(-6) M. An addition of exogenous dioleine increased the enzyme affinity for Ca2+ which led to a decrease of Ca2+ concentration necessary for the maximal activation to occur. The optimal concentration of ATP needed for sarcolemmal preparation phosphorylation was 0.3-0.4 mM, which seems to be due to the high activity of sarcolemmal ATPases. The proteins phosphorylated in sarcolemmal preparations were identified, using SDS polyacrylamide gel electrophoresis with subsequent autoradiography. The 250, 140, 67, 58, 25 and 11 kD proteins appeared to be phosphorylated in the greatest degree. Since in myocardial sarcolemma protein kinase C predominantly phosphorylates the same proteins as does the cAMP-dependent protein kinase, it was assumed that protein kinase C can also play a role in the regulation of Ca2+-transporting systems of sarcolemma.     

Inhibition by adriamycin of calmodulin-sensitive and phospholipid-sensitive calcium-dependent phosphorylation of endogenous proteins from heart.

Adriamycin, a lipid-interacting anti-cancer agent, was found to inhibit phospholipid-sensitive Ca2+-dependent phosphorylation of endogenous proteins from the cytosol of the guinea-pig heart. The drug, unexpectedly, also inhibited phosphorylation of separate endogenous proteins in the cardiac cytosol and membranes catalysed by the calmodulin-sensitive species of Ca2+-dependent protein kinase. In both phosphorylation systems, the inhibition by adriamycin was reversed by either phospholipid (phosphatidylserine or cardiolipin) or calmodulin respectively. Adriamycin also inhibited phosphorylation of histone (exogenous protein) catalysed by purified cardiac phospholipid-sensitive Ca2+-dependent protein kinase, but not that by cyclic AMP-dependent and cyclic GMP-dependent protein kinases. It appears that Ca2+-dependent protein phosphorylation systems, regulated either by phospholipid or calmodulin, may represent hitherto unrecognized sites of action of adriamycin. It remains to be seen whether inhibition by adriamycin of these systems is related to the severe cardiotoxicity, the major adverse effect of the drug that limits its clinical usefulness.     

Further characterization of a protein kinase-substrate complex precipitable with Ca2+ from the cytosol fraction of AH-66 hepatoma cells

The properties of a protein kinase-substrate complex precipitated with Ca2+ from the cytosol of AH-66 hepatoma cells were characterized. The endogenous phosphorylation reaction of the complex was little affected by addition of histone, cyclic nucleotides, Ca2+-calmodulin, or Ca2+-phospholipid but was increased about two-fold by addition of casein. The complex contained several phosphate acceptor proteins with molecular weights ranging from 74,000 to 13,000 as analyzed by two-dimensional gel electrophoresis. These phosphate acceptor proteins were specifically concentrated in the complex. The protein kinase in the complex was purified by successive chromatography and proved to be casein kinase 2.     

Ganglioside-modulated protein phosphorylation. Partial purification and characterization of a ganglioside-stimulated protein kinase in brain

Gangliosides have profound modulatory effects on protein phosphorylation in brain. A protein kinase activated directly by gangliosides has been partially purified from the particulate fractions of guinea pig brain through extraction with nonionic detergent, ion-exchange chromatography, hydrophobic chromatography, and gel filtration. This novel ganglioside-stimulated protein kinase is distinct from cAMP-dependent, Ca2+/calmodulin-dependent, and Ca2+/phospholipid-dependent protein kinases. The partially purified kinase preparation could undergo ganglioside-stimulated autophosphorylation of a major phosphoprotein with Mr corresponding to 68,000. It also could phosphorylate exogenous substrates such as the synthetic peptide Leu-Arg-Arg-Ala Ser-Leu-Gly. The requirement of gangliosides for the activation of kinase activity is dose-dependent and specific. Among the various gangliosides tested, GT1b and GD1a were found to be the most potent activators, whereas GD1b and GM1 were slightly less effective. The activation process is rapid and does not require the presence of Ca2+, suggesting that the stimulatory effect of gangliosides is not mediated through limited proteolysis or Ca2+-glycolipid complexes. Although the exact physiological significance of the ganglioside-stimulated protein kinase is not known at present, it is possible that certain functions related to gangliosides in the nervous system are mediated through the activation of this novel enzyme.     

Progesterone-inhibited phosphorylation of an unique Mr 48,000 protein in the plasma membrane of Xenopus laevis oocytes

The meiotic maturation of Xenopus laevis oocytes is induced in vitro by progesterone which interacts at the cell surface level. A cell-free membrane preparation (P-10,000) incorporated 32P from [gamma-32P]ATP, mostly into two proteins, Mr approximately 56,000 and approximately 48,000 (as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis). Progesterone, added in vitro, specifically inhibited the phosphorylation of the Mr approximately 48,000 protein (named p48). Half-maximal inhibition of p48 phosphorylation occurred with progesterone approximately 8 microM, in good correlation with hormone concentration inducing oocyte maturation. The effect was not due to stimulation of protein phosphatase activity. The potent maturation inducers testosterone and deoxycorticosterone also inhibited p48 phosphorylation, whereas biologically inactive steroids or cholesterol did not. p48 phosphorylation was not affected by cAMP, cGMP, polyamines, calmodulin, and phospholipids + diolein. EGTA had a stimulatory effect which was reversed by added Ca2+. The inhibitory effects of progesterone and Ca2+ were additive, suggesting two distinct sites of action. Phospho-p48 was not detected in yolk platelets, microsomes, and cytosol of oocytes. Contrary to p48 itself, the p48 kinase activity was loosely associated with P-10,000. Progesterone inhibited p48 phosphorylation produced by either cytosol or exogenous pure catalytic subunit of cAMP-dependent protein kinase. Conversely, phosphorylation of casein and histones by protein kinase activity present in P-10,000 was not modified by progesterone. It is then suggested that progesterone regulates p48 phosphorylation by affecting the protein substrate in the membrane, rather than by inhibiting the protein kinase enzyme itself. The data demonstrate a direct effect (not mediated by change of protein synthesis) of steroids on p48 phosphorylation in the plasma membrane, and they suggest that this protein could be implicated in the initial action of progesterone on oocyte maturation.     

Rat liver glucocorticoid receptor isolated by affinity chromatography is not a Mg2+- or Ca2+-dependent protein kinase

The glucocorticoid hormone receptor (92 kDa), purified 9000-fold from rat liver cytosol by steroid affinity chromatography and DEAE-Sephacel chromatography, was assayed for the presence of protein kinase activity by incubations with [gamma-32P]ATP and the photoaffinity label 8-azido-[gamma-32P]ATP. Control preparations isolated by affinity chromatography in the presence of excess steroid to prevent the receptor from binding to the affinity matrix were assayed for kinase activity in parallel. The receptor was not labeled by the photoaffinity label under photoactivation conditions in the presence of Ca2+ or Mg2+. A Mg2+-dependent protein kinase (48 kDa) that could be photoaffinity labeled with 8-azido-ATP copurified with the receptor. This kinase was also present in control preparations. The kinase could phosphorylate several minor contaminants present in the receptor preparation, including a protein (or proteins) of similar molecular weight to the receptor. The phosphorylation of 90-92-kDa proteins was independent of the state of transformation or steroid-binding activity of the receptor. These experiments provide direct evidence that neither the glucocorticoid receptor nor the 90-92-kDa non-steroid-binding protein associated with the molybdate-stabilized glucocorticoid receptor possesses intrinsic Ca2+- or Mg2+-dependent protein kinase activity.     

Protein phosphorylation and secretion in digitonin-permeabilized adrenal chromaffin cells. Effects of micromolar Ca2+, phorbol esters, and diacylglycerol

The effects of phorbol esters, dioctanoylglycerol (DiC8), and micromolar Ca2+ on protein phosphorylation and catecholamine secretion in digitonin-treated chromaffin cells were investigated. [gamma-32P]ATP was used as a substrate for phosphorylation in the permeabilized cells. 12-O-Tetradecanoylphorbol-13-acetate (TPA) enhanced Ca2+-dependent catecholamine secretion from digitonin-permeabilized cells. The enhancement required MgATP. Only those phorbol esters which activate protein kinase C in vitro enhanced both catecholamine secretion and protein phosphorylation. DiC8, which activates protein kinase C in vitro and mimics phorbol ester effects in situ, also enhanced both catecholamine secretion and protein phosphorylation. Preincubation of intact cells with TPA or DiC8 was necessary for maximal effects on both catecholamine secretion and protein phosphorylation in subsequently digitonin-treated chromaffin cells. The TPA-induced enhancement of protein phosphorylation was almost entirely Ca2+-independent, whereas DiC8-induced enhancement of protein phosphorylation was mainly Ca2+-dependent. Micromolar Ca2+ alone also enhanced the phosphorylation of a large number of proteins. Most of the proteins phosphorylated in response to TPA or potentiated by DiC8 in combination with Ca2+ were also phosphorylated by micromolar Ca2+ in the absence of exogenous protein kinase C activators. In intact cells, 1,1-dimethyl-4-phenylpiperazinium (DMPP) induced Ca2+-dependent phosphorylation of at least 17 proteins which were detected by two-dimensional gel electrophoresis. All of the proteins phosphorylated upon incubation with 1,1-dimethyl-4-phenylpiperazinium were phosphorylated upon incubation with micromolar Ca2+ in digitonin-treated cells. These results demonstrate that TPA- or DiC8-enhanced Ca2+-dependent catecholamine secretion is associated with enhanced protein phosphorylation which is probably mediated by protein kinase C and that activation of protein kinase C modulates catecholamine secretion from digitonin-treated chromaffin cells.     

Characteristics of the cell surface antigen, p72, associated with a variety of human tumours and mitogen-stimulated T-lymphoblasts

We recently purified two closely related 33 kDa proteins from rat hepatic cytosol, designated bile acid binder I and II, which selectively bind bile acids with comparable affinity as glutathione S-transferase B. This work has now been extended to human liver in which we have identified a similar cytosolic binding activity in the 30-40 kDa fraction from gel filtration. Subsequent chromatofocusing and hydroxyapatite chromatography resulted in the isolation of a homogeneous monomeric protein of 36 kDa. The binding affinity of this protein for lithocholate using the displacement of 1-anilino-8-naphthalenesulfonate (ANS) was 0.1 microM, whereas human hepatic glutathione S-transferases purified from glutathione affinity chromatography demonstrated no competitive displacement of ANS.     

Depolarization-induced ERK phosphorylation depends on the cytosolic Ca2+ level rather than on the Ca2+ channel subtype of chromaffin cells

The contribution of Ca2+ entry through different voltage-activated Ca2+ channel (VACC) subtypes to the phosphorylation of extracellular signal regulated kinase (ERK) was examined in bovine adrenal-medullary chromaffin cells. High K+ depolarization (40 mM, 3 min) induced ERK phosphorylation, an effect that was inhibited by specific mitogen-activated protein kinase kinase inhibitors. By using selective inhibitors, we observed that depolarization-induced ERK phosphorylation completely depended on protein kinase C-alpha (PKC-alpha), but not on Ca2+/calmodulin-dependent protein kinase nor cyclic AMP-dependent protein kinase. Blockade of L-type Ca2+ channels by 3 microm furnidipine, or blockade of N channels by 1 micromomega-conotoxin GVIA reduced ERK phosphorylation by 70%, while the inhibition of P/Q channels by 1 micromomega-agatoxin IVA only caused a 40% reduction. The simultaneous blockade of L and N, or P/Q and N channels completely abolished this response, yet 23% ERK phosphorylation remained when L and P/Q channels were simultaneously blocked. Confocal imaging of cytosolic Ca2+ elevations elicited by 40 mm K+, showed that Ca2+ levels increased throughout the entire cytosol, both in the presence and the absence of Ca2+ channel blockers. Fifty-eight percent of the fluorescence rise depended on Ca2+ entering through N channels. Thus, ERK phosphorylation seems to depend on a critical level of Ca2+ in the cytosol rather than on activation of a given Ca2+ channel subtype.     

Ca2+– and Calmodulin-Dependent Phosphorylation of Microtubule-Associated Protein 2 and t Factor, and Inhibition of Microtubule Assembly

Microtubule-associated proteins (MAPs) were phosphorylated by a Ca2+- and calmodulin-dependent protein kinase from rat brain cytosol. The maximal amount of phosphate incorporated into MAPs was 25 nmol of phosphate/mg protein. A Ka value of the enzyme for calmodulin was 57.0 nM, with MAPs as substrates. Among MAPs, MAP2 and tau factor were phosphorylated in a Ca2+- and calmodulin-dependent manner. The phosphorylation of MAPs led to an inhibition of microtubule assembly in accordance with its degree. This reaction was dependent on addition of the enzyme, Ca2+, and calmodulin, and had a greater effect on the initial rate of microtubule assembly rather than on the final extent. The critical tubulin concentration for microtubule assembly was unchanged by the MAPs phosphorylation. Therefore assembly and disassembly of brain microtubule are regulated by the Ca2+- and calmodulin-dependent protein kinase that requires only a nanomolar concentration of calmodulin for activation.     

Binding of ions and hydrophobic probes to alpha-lactalbumin and kappa-casein as determined by analytical affinity chromatography

The technique of analytical affinity chromatography was extended to characterize binding of ions and hydrophobic probes to proteins. Using the immobilized protein mode of chromatography, alpha-lactalbumin and kappa-casein were covalently attached to 200-nm-pore-diameter controlled-pore glass beads and accommodated for high-performance liquid chromatography. The existence of a high affinity binding site (Kdiss = 0.16 microM) (site I) for calcium ion in alpha-lactalbumin was confirmed by chromatography of [45Ca2+]. In addition, chromatography of the hydrophobic probes, 1-(phenylamino)-8-naphthalene-sulfonate (ANS)2 and 4,4'-bis[1-(phenylamino)-8-naphthalenesulfonate (bis-ANS) indicated that Ca2+ bound to a second site (presumably the zinc site or site II) with weaker affinity. Dissociation constants obtained for apo-alpha-lactalbumin were about 80 microM for ANS and 4.7 microM for bis-ANS in the absence of sodium ion. Addition of Ca2+ initially caused a reduction in surface hydrophobicity (lowered affinity for the probe dyes) followed by an increase at higher Ca2+ concentrations (greater than 0.5 mM), suggesting that occupancy of site II restores an apo-like conformation to the protein. Moreover, the effect of Zn2+ was similar to that observed in the higher Ca2+ concentration range, whereas Na+ apparently bound to site I. A calcium binding site of moderate affinity also exists in kappa-casein (Kdiss = 15.6 microM). A cluster of negative charges, probably including the orthophosphate group, most likely comprise this binding site. By preventing self-association, analytical affinity chromatography permits microscale characterization of ligand equilibria in proteins that are unaffected by protein-protein interactions.     

Tubulin-Associated Calmodulin-Dependent Kinase: Evidence for an Endogenous Complex of Tubulin with a Calcium-Calmodulin-Dependent Kinase

A Ca2+ -calmodulin kinase that phosphorylates tubulin and microtubule-associated proteins as major substrates has been purified and characterized from brain cytoplasm. It is important to determine if cytoskeletal proteins are major natural substrates for this kinase system. This report demonstrates that a significant fraction of brain cytosolic calmodulin-dependent kinase activity exists in tight association with tubulin in the form of a stable complex. The tubulin-calmodulin kinase complex displayed an apparent molecular weight on gel filtration of approximately 1.8 X 10(6) daltons. The specific activity of tubulin kinase in the complex was enriched over 20-fold in comparison with brain cytosol. Although purified tubulin alone did not adhere to a calmodulin column, the tubulin associated with the calmodulin kinase complex did bind specifically to the calmodulin affinity resin. The kinase activity was shown to be tightly associated in complex with tubulin by (1) copurification, (2) isolation on gel filtration chromatography, (3) isolation on ion-exchange chromatography, and (4) binding to calmodulin. The kinase complexed with tubulin was identical to the previously purified kinase as judged by several criteria including (1) subunit molecular weights, (2) isoelectric points, (3) autophosphorylation characteristics, (4) calmodulin binding properties, (5) kinetic parameters of tubulin phosphorylation, (6) phosphoamino acid phosphorylation sites on alpha- and beta-tubulin, and (7) identical subunit 125I-tryptic peptide maps. The results indicate that a significant fraction of this previously purified calmodulin kinase is endogenously associated with tubulin in brain cytoplasm and may play a role in mediating some of the effects of calcium on neuronal function.     

Calcium/Ganglioside-Dependent Protein Kinase Activity in Rat Brain Membrane

The effects of gangliosides on phosphorylation were studied in rat brain membrane. Gangliosides stimulated phosphorylation only in the presence of Ca2+ with major phosphoproteins of 45,000, 50,000, 60,000, and 80,000 daltons and high-molecular-weight species. In addition, gangliosides inhibited the phosphorylation of three proteins with molecular weights of 15,000, 20,000, and 78,000 daltons. The two low-molecular-weight proteins comigrated with rat myelin basic proteins. Ganglioside stimulation was dependent on the formation of a Ca2+-ganglioside complex since the calcium salt of gangliosides stimulated phosphorylation maximally. Disialo and trisialo gangliosides were more potent stimulators of kinase activity than the monosialo GM1 X GD1a was the most potent activator tested. Asialo-GM1, cerebroside, sialic acid, neuraminyllactose, sulfatide, and the acidic phospholipids phosphatidylserine and phosphatidylinositol did not stimulate kinase activity. The Ca2+-dependent, ganglioside-stimulated phosphorylation was qualitatively similar to the pattern for calmodulin-dependent phosphorylation. However, while calmodulin-dependent kinase activity was inhibited with an IC50 of 10 microM trifluoperazine, ganglioside-stimulated kinase was inhibited with an IC50 of 200 microM trifluoperazine. These results indicate that gangliosides have complex effects on membrane-associated kinase activities and suggest that Ca2+-ganglioside complexes are potent stimulators of membrane kinase activity.     

Diacylglycerol-activated, calcium/phospholipid-dependent protein kinase (protein kinase C) activity in bovine thyroid

Bovine thyroid 100,000 X g supernatant contained diacylglycerol-activated, calcium/phospholipid-dependent protein kinase (protein kinase C). The protein kinase C was partially purified using ion-exchange chromatography and characterized. Substrate specificity studies revealed that the enzyme was most active when histone F1 was used as substrate. The thyroid protein kinase C was not stimulated by Ca2+ or phosphatidylserine (PS), but was stimulated by the combination of the two by 570%. Diolein stimulated the kinase by increasing its sensitivity to Ca2+. Other phospholipids could not substitute for PS and were ineffective in stimulating the protein kinase C in the absence of diolein. However, in the presence of diolein some of the other phospholipids were stimulatory albeit not to the extent of PS. Quercitin, a protein kinase C inhibitor in other systems, inhibited the thyroid enzyme in a dose-related manner. Protein kinase C could also be demonstrated using endogenous thyroid proteins as substrate. Separation of these 32P-labelled proteins by electrophoresis and subsequent autoradiography revealed that three proteins were phosphorylated by the protein kinase C of approximate molecular weights 60,000, 45,000, and less than 29,000. These results offer a possible mechanism by which Ca2+ and/or diacylglycerol effects may be mediated in thyroid.