Cholecystokinin induces a decrease in Ca2+ current in snail neurons that appears to be mediated by protein kinase C

Three distinct classes of protein kinases have been shown to regulate Ca2+ current in excitable tissues. Cyclic AMP-dependent protein kinase mediates the action of noradrenaline on the Ca2+ current of cardiac muscle cells. Cyclic GMP-dependent protein kinase mediates the serotonin-induced modulation of the Ca2+ current in identified snail neurons. The Ca2+/diacylglycerol-dependent protein kinase (protein kinase C) has also been found to regulate Ca2+ currents of neurons. However, no neurotransmitter has yet been shown to regulate Ca2+ current through the activation of protein kinase C. We now report that cholecystokinin, a widely occurring neuropeptide which is present in molluscan neuron, modulates the Ca2+ current in identified neurons of the snail Helix aspersa, and that this effect appears to be mediated by protein kinase C. Specifically, sulphated cholecystokinin octapeptide 26-33 (CCK8), activators of protein kinase C, and intracellular injection of protein kinase C, all shorten the Ca2+-dependent action potential and decrease the amplitude of the Ca2+ current in these cells. All these effects are not reversible within the duration of the experiments. Moreover, intracellular injections of low concentrations of protein kinase C, which are ineffective by themselves, enhance the effectiveness of low concentrations of CCK8 on the Ca2+ current.

cGMP-dependent protein kinase enhances Ca2+ current and potentiates the serotonin-induced Ca2+ current increase in snail neurones

Protein phosphorylation catalysed by cyclic AMP-dependent, Ca2+/calmodulin-dependent and Ca2+/diacylglycerol-dependent protein kinases is important both in the modulation of synaptic transmission and in the regulation of neuronal membrane permeability (for reviews see refs 5-7). However, there has previously been no evidence for the involvement of cyclic GMP-dependent protein kinase (cGMP-PK) in the regulation of neuronal function. Serotonin induces an increase of Ca2+ current in a group of identified ventral neurones of the snail Helix aspersa. This effect is probably mediated by cGMP because it is mimicked by the intracellular injection of cGMP or the application of zaprinast, an inhibitor of cGMP-dependent phosphodiesterase. We have now found that the effect of either serotonin or zaprinast on the Ca2+ current is potentiated by the intracellular injection of cGMP-PK. Moreover, the intracellular injection of activated cGMP-PK (cGMP-PK + 1 microM cGMP) greatly enhances the Ca2+ current of the identified ventral neurones seen in the absence of serotonin. These results indicate that cGMP-PK has a physiological role in the control of the membrane permeability of these neurones.

Intracellular injection of cGMP-dependent protein kinase results in increased input resistance in neurons of the mammalian motor cortex

Purified, cyclic GMP-dependent protein kinase (cGPK) was pressure-injected into neurons of the precruciate cortex of awake cats. Input resistances increased within seconds after injection and remained elevated for 2 min or longer. The increases were larger when cGPK was injected in a mixture with 10 microM cGMP than when injected alone. Injections of heat-inactivated cGPK, with or without 10 microM cGMP, failed to produce increases in input resistance. The present results indicate that injection of activated cGPK into neurons of the mammalian motor cortex can mimic actions of extracellularly applied acetylcholine and intracellularly applied cGMP, the latter in 100-fold higher concentrations than those used here, in neurons of the same cortical areas.

Protein phosphorylation in cultured endothelial cells

We have investigated the protein phosphorylation systems present in cultured bovine aortic and pulmonary artery endothelial cells. The cells contain cyclic AMP-dependent protein kinase, three calcium/calmodulin-dependent protein kinases, protein kinase C, and at least one tyrosine kinase. No cyclic GMP-dependent protein kinase activity was found. The cells also contained numerous substrates for cyclic AMP-dependent protein kinase and protein kinase C. Fewer substrates were found for the calcium/calmodulin-dependent protein kinases. There was little difference between either protein kinase activities or substrates when pulmonary artery endothelium was compared to aortic endothelium grown under similar culture conditions. It is likely that these various protein kinases and their respective substrate proteins are involved in mediating several of the actions of the hormones and drugs which affect the vascular endothelium.

Autophosphorylation reversibly regulates the Ca2+/calmodulin-dependence of Ca2+/calmodulin-dependent protein kinase II

Ca2+/calmodulin-dependent protein kinase II contains two subunits, alpha (Mr 50,000) and beta (Mr 60,000/58,000), both of which undergo Ca2+/calmodulin-dependent autophosphorylation. In the present study, we have studied the mechanism of this autophosphorylation reaction and its effect on the activity of the enzyme. Both subunits are autophosphorylated through an intramolecular mechanism. Using synapsin I as substrate, Ca2+/calmodulin-dependent protein kinase II, in its unphosphorylated form, was totally dependent on Ca2+ and calmodulin for its activity. Preincubation of the enzyme with Ca2+, calmodulin, and ATP, under conditions where autophosphorylation of both subunits occurred, converted the enzyme to one that was only partially dependent on Ca2+ and calmodulin for its activity. No change in the total activity, measured in the presence of Ca2+ and calmodulin, was observed. The nonhydrolyzable ATP analog adenosine 5'-[beta, gamma-imido] triphosphate did not substitute for ATP in the preincubation. Moreover, dephosphorylation of autophosphorylated Ca2+/calmodulin-dependent protein kinase II with protein phosphatase 2A resulted in an enzyme that was again totally dependent on Ca2+ and calmodulin for its activity. We propose that autophosphorylation and dephosphorylation reversibly regulate the Ca2+ and calmodulin requirement of Ca2+/calmodulin-dependent protein kinase II.

cAMP increases junctional conductance and stimulates phosphorylation of the 27-kDa principal gap junction polypeptide

Membrane-permeant cAMP derivatives (dibutyryl- and 8-bromo-cAMP) increase gap-junctional conductance within minutes when applied to voltage-clamped pairs of rat hepatocytes. Glucagon also increases junctional conductances, but the response has a more rapid onset and is more rapidly reversible. The glucagon effect can be prevented by intracellular injection of the protein inhibitor of the cAMP-dependent protein kinase (Walsh inhibitor), indicating that the catalytic subunit of cAMP-dependent protein kinase is directly involved. The 27-kDa major gap junction polypeptide is phosphorylated when liver cells dissociated into small groups are incubated with 32P. Addition of 8-bromo-cAMP to cells increases the incorporation of 32P into the 27-kDa junctional protein. Serine is the amino acid residue that is phosphorylated. When isolated liver gap junctions are incubated in the presence of catalytic subunit of the cAMP-dependent protein kinase, the 27-kDa gap junction polypeptide is phosphorylated with low stoichiometry on serine. The rapid increases in gap junctional conductance caused by agents that elevate cAMP and phosphorylation of the gap junction protein by cAMP-dependent protein kinase suggest that cAMP-dependent phosphorylation of the gap junction channel modulates the conductance of liver gap junctions.

PCPP-260, a Purkinje cell-specific cyclic AMP-regulated membrane phosphoprotein of Mr 260,000

The present study reports the existence of Purkinje cell-specific phosphoprotein, Mr 260,000 (PCPP-260), a neuronal membrane phosphoprotein, in cerebellar Purkinje cells. PCPP-260, which on sodium dodecyl sulfate-polyacrylamide gel electrophoresis has an apparent molecular mass of 260,000 Da, has been found to be phosphorylated in particulate preparations by endogenous or added exogenous cyclic AMP-dependent protein kinase, but not by cyclic GMP-dependent, calcium/calmodulin-dependent or calcium/phospholipid-dependent protein kinases. The protein has been found in high concentrations in all mammalian cerebella so far analyzed, including human cerebellum. One-and two-dimensional electrophoretic and peptide mapping analyses of proteins in other brain regions show that a closely related 265,000 Da phosphoprotein also exists, albeit in low concentrations, outside the cerebellum. Analysis of cerebella from mutant mice, deficient in either Purkinje cells or in granule cells, indicates that PCPP-260 within the cerebellum is restricted to Purkinje cells. Furthermore, subcellular fractionation of rat cerebella indicates that the protein is an integral membrane protein. The cAMP-regulated phosphorylation of PCPP-260 is presumably involved in membrane functions important to Purkinje cells.

Identification of calmodulin-dependent protein kinase III and its major Mr 100,000 substrate in mammalian tissues

A major substrate, Mr 100,00 (100 kDa), for a Ca2+/calmodulin (CaM)-dependent protein kinase found in many mammalian tissues has been purified from rat pancreas. The purified substrate was used to identify and partially purify a CaM-dependent protein kinase (CaM kinase III) from rat pancreas. The physical properties and substrate specificity of CaM kinase III were distinct from those of all known CaM-dependent protein kinases. Only CaM kinase III was able to phosphorylate the 100-kDa protein; synapsin I, phosphorylase b, myosin light chain, and histone were poor substrates for this enzyme. Polyclonal antibodies, raised against the purified 100-kDa protein, recognized the protein in a variety of mammalian tissues and cell lines. Immunoassay revealed that the 100-kDa protein made up 0.3-1.7% of the total cytosolic protein in these samples. Analysis of CaM kinase III revealed that the enzyme had a similar widespread tissue distribution. These results demonstrate the existence of a fifth CaM-dependent protein phosphorylation system present in high levels in animal cells.

DARPP-32, a dopamine- and adenosine 3':5'-monophosphate-regulated neuronal phosphoprotein. II. Comparison of the kinetics of phosphorylation of DARPP-32 and phosphatase inhibitor 1

DARPP-32 (dopamine- and cyclic AMP-regulated phosphoprotein, Mr = 32,000) is a cytosolic neuronal phosphoprotein enriched in dopamine-innervated brain regions which, in its phosphorylated form, acts as an inhibitor of protein phosphatase 1. We have compared the phosphorylation of purified DARPP-32 with that of purified phosphatase inhibitor 1, a widespread inhibitor of protein phosphatase 1. Purified cyclic AMP-dependent protein kinase and cyclic GMP-dependent protein kinase each catalyzed the maximal incorporation of 0.9-1.1 mol of [32P]phosphate/mol of DARPP-32 or phosphatase inhibitor 1, with phosphorylation occurring on threonine residues. Evidence for the existence of a single phosphorylation site in each substrate protein was obtained by two-dimensional thin-layer phosphopeptide mapping of thermolytic digests. Initial rate studies of the phosphorylation of DARPP-32 yielded an apparent Km of 2.4 microM and a kcat of 2.7 S-1 for the catalytic subunit of cyclic AMP-dependent protein kinase, and an apparent Km of 5.4 microM and a kcat of 2.3 S-1 for cyclic GMP-dependent protein kinase. These in vitro results are compatible with a physiological role for the phosphorylation of DARPP-32 by either protein kinase in vivo. Similar studies with phosphatase inhibitor 1 yielded an apparent Km of 5.0 microM and a kcat of 1.4 S-1 for the catalytic subunit of cyclic AMP-dependent protein kinase, and an apparent Km of 25.0 microM and a kcat of 1.2 S-1 for cyclic GMP-dependent protein kinase. A synthetic nonapeptide, corresponding to the phosphorylation site of DARPP-32, was phosphorylated with apparent Km values of 1.12 mM and 1.86 mM and kcat values of 0.22 S-1 and 3.4 S-1 for cyclic AMP-dependent and cyclic GMP-dependent protein kinase, respectively.

Calcium/phospholipid-dependent protein kinase (protein kinase C) phosphorylates and activates tyrosine hydroxylase

Protein kinase C, purified to homogeneity, was found to phosphorylate and activate tyrosine hydroxylase that had been partially purified from pheochromocytoma PC 12 cells. These actions of protein kinase C required the presence of calcium and phospholipid. This phosphorylation of tyrosine hydroxylase reduced the Km for the cofactor 6-methyltetrahydropterine from 0.45 mM to 0.11 mM, increased the Ki for dopamine from 4.2 microM to 47.5 microM, and produced no change in the Km for tyrosine. Little or no change in apparent Vmax was observed. These kinetic changes are similar to those seen upon activation of tyrosine hydroxylase by cAMP-dependent protein kinase. Two-dimensional phosphopeptide maps of tyrosine hydroxylase were identical whether the phosphorylation was catalyzed by protein kinase C or by the catalytic subunit of cAMP-dependent protein kinase. Both protein kinases phosphorylated serine residues. The results suggest that protein kinase C and cAMP-dependent protein kinase phosphorylate the same site(s) on tyrosine hydroxylase and activate tyrosine hydroxylase by the same mechanism.

Localization in mammalian brain of G-substrate, a specific substrate for guanosine 3',5'-cyclic monophosphate-dependent protein kinase

The regional and cellular distribution of G-substrate, a 23,000-dalton protein substrate specific for guanosine 3',5'-cyclic monophosphate-dependent protein kinase, has been examined in mammalian brain using immunoprecipitation, radioimmunoassay, and peptide-mapping techniques. In rabbit brain, G-substrate was found to be highly concentrated in the cerebellum. The concentration of G-substrate in cerebellar cytosol was 27.2 pmol/mg. The concentrations of G-substrate in cortex, hippocampus, and caudate were only 1 to 2% of that found in cerebellum. Studies of neurological mutant mice lacking either Purkinje cells (PCD and nervous) or granule cells (weaver) suggested that, within the cerebellum, G-substrate is localized almost exclusively in Purkinje cells. A phosphoprotein present in noncerebellar brain regions, which co-migrated with G-substrate on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, was shown by peptide mapping to consist predominantly of phosphatase inhibitor-1. Phosphatase inhibitor-1, a potent inhibitor of protein phosphatase-1, is known to share several physicochemical properties with G-substrate. In contrast to the results obtained with G-substrate, the concentration of phosphatase inhibitor-1 was significantly lower in cerebellum than in other major brain regions. These and other data suggest that G-substrate may be a Purkinje cell-specific protein phosphatase inhibitor.

Mammalian brain phosphoproteins as substrates for calcineurin

Calcineurin, a Ca2+/calmodulin-dependent phosphoprotein phosphatase found in several tissues, is highly concentrated in mammalian brain. In an attempt to identify endogenous brain substrates for calcineurin, kinetic analyses of the dephosphorylation of several well-characterized phosphoproteins purified from brain were performed. The proteins studied were: G-substrate, a substrate for cyclic GMP-dependent protein kinase; DARPP-32, a substrate for cyclic AMP-dependent protein kinase; Protein K.-F., a substrate for a cyclic nucleotide- and Ca2+-independent protein kinase; and synapsin I, a substrate for cyclic AMP-dependent (site I) and a Ca2+/calmodulin-dependent protein kinase (site II). Calcineurin dephosphorylated each of these proteins in a Ca2+/calmodulin-dependent manner. Similar Km values were obtained for each substrate: G-substrate, 3.8 microM; DARPP-32, 1.6 microM; Protein K.-F., approximately 3 microM (S0.5); synapsin I (site I), 7.0 microM; synapsin I (site II), 4.4 microM. However, significant differences were obtained for the maximal rates of dephosphorylation. The kcat values were: G-substrate, 0.41 s-1; DARPP-32, 0.20 s-1; Protein K.-F., 0.7 s-1; synapsin I (site I), 0.053 s-1; synapsin I (site II), 0.040 s-1. Comparisons of the catalytic efficiency (kcat/Km) for each substrate indicated that DARPP-32, G-substrate, and Protein K.-F. are all potential substrates for calcineurin in vivo.

Inhibition by calmodulin of calcium/phospholipid-dependent protein phosphorylation

Calmodulin was previously found to inhibit the Ca2+/phospholipid-dependent phosphorylation of an endogenous substrate, called the 87-kilodalton protein, in a crude extract prepared from rat brain synaptosomal cytosol. We investigated the mechanism of this inhibition, using Ca2+/phospholipid-dependent protein kinase and the 87-kilodalton protein, both of which had been purified to homogeneity from bovine brain. Rabbit brain calmodulin and some other Ca2+-binding proteins inhibited the phosphorylation of the 87-kilodalton protein by this kinase in the purified system. Calmodulin also inhibited the Ca2+/phospholipid-dependent phosphorylation of H1 histone, synapsin I, and the delta subunit of the acetylcholine receptor, with use of purified components. These results suggest that calmodulin may be a physiological regulator of Ca2+/phospholipid-dependent protein kinase.

The amino acid sequence of rabbit skeletal muscle calmodulin

The amino acid sequence of calmodulin which can be extracted from rabbit skeletal muscle with low ionic strength buffer and presumably activates myosin light chain kinase has been determined. It is a single polypeptide chain of 148 residues with a blocked N terminus. The sequence of the N terminal tripeptide and residues 98 and 99 were not determined unequivocally nor were the amide assignments of residues 48, 50, 58 and 60. The protein is otherwise identical with the subunit of phosphorylase kinase and bovine uterus calmodulin and very similar to all other mammalian calmodulins.

DARPP-32, a dopamine- and adenosine 3':5'-monophosphate-regulated phosphoprotein enriched in dopamine-innervated brain regions. II. Purification and characterization of the phosphoprotein from bovine caudate nucleus

DARPP-32 is a neuronal phosphoprotein of Mr = 32,000, originally identified in rat brain (Walaas, S.I., D.W. Aswad, and P. Greengard (1983) Nature 301: 69-72). This protein has now been identified in bovine caudate nucleus cytosol and purified 435-fold to apparent homogeneity as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The purification procedure involved acid extraction at pH 2, CM-cellulose chromatography, DEAE-cellulose chromatography, hydroxylapatite chromatography, and gel filtration on Ultrogel AcA 44. The purified catalytic subunit of cAMP-dependent protein kinase catalyzed the incorporation of 0.96 mol of phosphate/mol of purified DARPP-32. Phosphorylation occurred exclusively on threonine. The isoelectric point of dephospho-DARPP-32 was 4.7, and that of phospho-DARPP-32 was 4.6. The amino acid composition showed a high content of glutamate/glutamine and proline, and a low content of hydrophobic amino acids. DARPP-32 was found to have a Stokes radius of 34 A and a sedimentation coefficient of 2.05 S, indicating that it exists as an elongated monomer.

Cyclic GMP-dependent protein phosphorylation in mammalian brain

Many of the actions of cyclic AMP (cAMP) and cyclic GMP (cGMP) are believed to be mediated via the phosphorylation of specific substrate proteins by cAMP-dependent and cGMP-dependent protein kinases. cGMP-regulated protein phosphorylation systems are less widely distributed than cAMP-regulated systems. This conclusion is supported by studies of the distribution, within the mammalian brain, of cGMP-dependent protein kinase and of G substrate, the only specific substrate protein for this enzyme yet found in the nervous system. cGMP-dependent protein kinase and G substrate are both highly enriched in the cerebellum. Within the cerebellum both proteins are concentrated in Purkinje cells. The localization of these and other components of the cGMP-dependent protein phosphorylation system in Purkinje cells suggests an important and selective role for cGMP in this neuron. The functional significance of cGMP-dependent protein kinase and G substrate in the Purkinje cell is currently being investigated. That study should be greatly facilitated by the recent preparation and characterization of serum antibodies highly selective for either the phospho or dephospho form of G substrate.

Regional distribution of calcium- and cyclic adenosine 3':5'-monophosphate-regulated protein phosphorylation systems in mammalian brain. II. Soluble systems

The regional distribution of phosphoproteins whose phosphorylation is regulated either by cyclic AMP or by calcium in combination with calmodulin or phospholipid has been investigated in soluble preparations from rat CNS. About 40 distinct phosphoproteins were observed. These cytosolic phosphoproteins exhibited widely different patterns of regional distribution. Based upon distribution patterns, we have divided these phosphoproteins into three categories: category A, phosphoproteins found in all parts of the CNS in approximately equal amounts; category B, phosphoproteins which are widely distributed within the CNS, but which show large regional variations; and category C, phosphoproteins which show a highly restricted regional distribution. We have tentatively interpreted the results on cytosolic phosphoproteins in the following way: some are present in all or nearly all brain cells, others are present only in certain classes of brain cells, and still others have an even more limited distribution, being present in only a single type of brain cell. The regional distribution of soluble protein kinase activity was also studied. Calcium/phospholipid-dependent protein kinase and calcium/calmodulin-dependent protein kinase had marked regional distributions. Cyclic AMP-dependent protein kinase was more evenly distributed throughout the CNS. This investigation thus demonstrates striking differences in the regional distribution of cytosolic protein phosphorylation systems in mammalian brain. These regional differences may reflect highly specific functional roles for certain of these protein phosphorylation systems. Similar conclusions concerning particulate protein phosphorylation systems are described in the preceding paper.

Regional distribution of calcium- and cyclic adenosine 3':5'-monophosphate-regulated protein phosphorylation systems in mammalian brain. I. Particulate systems

The regional distribution of phosphoproteins whose phosphorylation is regulated either by cyclic AMP or by calcium in combination with calmodulin or phospholipid has been investigated in particulate preparations from rat CNS. About 30 distinct phosphoproteins were observed. These phosphoproteins exhibited widely different patterns of regional distribution. Based upon distribution patterns, we have divided these phosphoproteins into three categories: category A, phosphoproteins found in all parts of the CNS in approximately equal amounts; category B, phosphoproteins which are widely distributed within the CNS but show large regional variations; and category C, phosphoproteins which show a highly restricted regional distribution. We have tentatively interpreted the results on particulate phosphoproteins in the following way: some are present in all or nearly all brain cells, others are present only in certain classes of brain cells, and still others have an even more limited distribution, being present in only a single type of brain cell. The regional distribution of particulate protein kinase activity was also examined. Calcium/calmodulin-dependent protein kinase activity had a marked regional distribution, whereas cyclic AMP-dependent protein kinase activity was more evenly distributed. Calcium/phospholipid-dependent protein kinase activity was barely detectable under the experimental conditions used. This investigation thus demonstrates striking differences in the regional distribution of particulate protein phosphorylation systems in mammalian brain. These regional differences may reflect highly specific functional roles for certain of these protein phosphorylation systems. Similar conclusions concerning cytosolic protein phosphorylation systems are described in the accompanying paper.

Cyclic nucleotide-dependent protein kinases and some major substrates in the rat cerebellum after neonatal X-irradiation

The levels of cAMP-dependent protein kinase (type I), or cGMP-dependent protein kinase, or protein I, and of a 23,000 MW substrate for the cGMP-dependent protein kinase were measured in cerebella from normal rats and in the cerebella from rats in which a selective loss of interneurons in the cerebellar cortex had been produced by X-irradiation. A decrease was observed in the concentrations of cAMP-dependent protein kinase and of protein I, whereas an increase was observed in the concentrations of cGMP-dependent protein kinase and of the 23,000 MW substrate. The data, taken together with the results of other studies, support the interpretation that cAMP-dependent protein kinase and protein I are distributed throughout the cerebellum, but that cGMP-dependent protein kinase and the 23,000 MW substrate are highly concentrated in Purkinje cells.

Calcium/phospholipid regulates phosphorylation of a Mr "87k" substrate protein in brain synaptosomes

Depolarization-induced calcium influx into rat cerebral cortex synaptosomes increased the phosphorylation of several synaptosomal proteins as examined by 32Pi incorporation. A phosphopeptide mapping technique involving NaDodSO4/polyacrylamide gels has been used to show that phosphorylation of a Mr 87,000 substrate protein is stimulated by depolarization-induced calcium influx. Phosphorylation of this Mr 87,000 substrate occurred in synaptosomal cytosol and was markedly stimulated by calcium/phosphatidylserine. Calmodulin inhibited this phosphorylation reaction. This substrate for calcium/phospholipid-dependent protein kinase is enriched in and appears to be specific to neurons.

Intracellular injection of t he catalytic subunit of cyclic AMP-dependent protein kinase simulates facilitation of transmitter release underlying behavioral sensitization in Aplysia

It has been difficult to establish whether cyclic AMP-mediated protein phosphorylation in nerve cells plays a specific role in synaptic transmission. This difficulty can be overcome in higher invertebrates because their large neurons allow the injection of protein molecules into the cell. We have used intracellular injection to study whether protein phosphorylation is involved in the mechanism of sensitization, a simple form of learning. Sensitization of the gill-withdrawal reflex in Aplysia involves enhancement of transmitter release by presynaptic facilitation at a particular set of synaptic connections between identified sensory neurons and their follower cells. We have found that injection of the catalytic subunit of cyclic AMP-dependent protein kinase (ATP:protein phosphotransferase, EC 2.7.1.37) purified from bovine heart mimics the action of the natural transmitter and of serotonin, the putative transmitter, by simulating the physiological changes that accompany presynaptic facilitation. Intracellular injection of the kinase into a sensory cell (i) broadens the action potential in the presence of tetraethylammonium, indicating an increase in Ca2+ current, (ii) decreases the input conductance of the cell, presumably as a result of a decrease in the K+ current, and (iii) increases the amount of transmitter released by terminals of the sensory cell onto follower neurons.

Microinjection of catalytic subunit of cyclic AMP-dependent protein kinase enhances calcium action potentials of bag cell neurons in cell culture

We have found that the calcium action potentials of bag cell neurons from the abdominal ganglion of Aplysia may be enhanced by intracellular microinjection of the catalytic subunit of cyclic AMP-dependent protein kinase (ATP:protein phosphotransferase, EC 2.7.1.37). The catalytic subunit was purified from bovine heart and shown to be effective in stimulating the phosphorylation of bag cell proteins in homogenates at concentrations of 10-50 nM. Intracellular injection into isolated bag cell neurons maintained in primary culture was through pressure applied to microelectrodes filled at the tip with catalytic subunit (5-22 muM). In 11 of 16 injected cells, both the slope of the rising phase and the height of the action potentials evoked by a constant depolarizing current were markedly enhanced relative to the pre-injection control (mean increases, 73% and 35%, respectively). This effect could occur with no change in resting potential or in the latency of the action potential from the onset of the depolarizing pulse. The effect was observed with enzyme dissolved in three different salt solutions (Na phosphate, K phosphate, or KCl). In two experiments, tetrodotoxin (50 muM) added to the extracellular medium had no effect on the enhanced action potentials. Subsequent addition of the calcium antagonist Co(2+), however, diminished or abolished the spikes. In more than half of the experiments, the injection of catalytic subunit was accompanied by an increase in the input resistance of the cells as measured by applying small hyperpolarizing current pulses. In three experiments, subthreshold oscillations in membrane potential resulted from the injections. Control injections (24 cells), carried out either with carrier medium alone or with heat-inactivated enzyme preparations, did not produce spike enhancement, increased input resistance, or oscillations. Our data suggest that the stimulation of intracellular protein phosphorylation by the catalytic subunit of cyclic AMP-dependent protein kinase enhances the excitability of bag cell neurons by modifying calcium and potassium channels or currents.

The preparation of calmodulins from barley (Hordeum sp.) and basidiomycete fungi

1. Calmodulin-like proteins were purified from the fruiting bodies of higher (basidiomycete) fungi and barley (Hordeum sp.) shoots. 2. These calmodulins have electrophoretic mobilities on 10% (w/v) polyacrylamide gels at pH 8.3 in the presence of 6 M-urea and at pH 8.3 in the presence of 0.1% sodium dodecyl sulphate similar to that of bovine brain calmodulin. They interacted with rabbit skeletal-muscle troponin I in the presence of Ca2+. 3. Barley and fungal calmodulins activated myosin light-chain kinase and phosphodiesterase in the presence of Ca2+, although the amounts needed were at least an order of magnitude greater than is required to produce the same effect with mammalian calmodulin. 4. Amino acid analyses indicated a number of differences from the mammalian protein, most notably the absence of trimethyl-lysine. 5. By using 125I-labelled calmodulin, a small amount of calmodulin-binding protein was detected in homogenates of barley and fungi. 6. No protein corresponding to calmodulin could be found in Escherichia coli or yeast, although a relatively high concentration of a protein that bound calmodulin was detected in E. coli by this technique.

Calmodulin and myosin light-chain kinase of rabbit fast skeletal muscle

1. It is confirmed that myosin light-chain kinase is a protein of mol.wt. about 80,000 that is inactive in the absence of calmodulin. 2. In the presence of 1 mol of calmodulin/mol of kinase 80-90% of the maximal activity is obtained. 3. Crude preparations of the whole light-chain fraction of rabbit fast-skeletal-muscle myosin contain enough calmodulin to activate the enzyme. A method for the preparation of calmodulin-free P light chain is described. 4. A procedure is described for the isolation of calmodulin from rabbit fast skeletal muscle. 5. Rabbit fast-skeletal-muscle calmodulin is indistinguishable from bovine brain calmodulin in its ability to activate myosin light-chain kinase. The other properties of these two proteins are also very similar. 6. Rabbit fast-skeletal-muscle troponin C was about 10% as effective as calmodulin as activator for myosin light-chain kinase. 7. By chromatography on a Sepharose-calmodulin affinity column evidence was obtained for the formation of a Ca2+-dependent complex between calmodulin and myosin light-chain kinase. 8. Troponin I from rabbit fast skeletal muscle and histone IIAS were phosphorylated by fully activated myosin light-chain kinase at about 1% of the rate of the P light chain.

The equilibrium constant and the reversibility of the reaction catalysed by nicotinamide-adenine dinucleotide kinase from pigeon liver

The reversibility of the NAD+ kinase reaction was established, and the kinetic parameters of the rate equation in the reverse direction were determined. The equilibrium constant of the reaction was determined by using the purified pigeon liver enzyme and radioactively labelled nicotinamide nucleotides. The relationship between kinetic parameters of the forward and reverse reactions is in reasonable agreement with the measured equilibrium constant. As expected from the proposed mechanism of action, the enzyme does not catalyse isotope exchange between NAD+ and NADP+ in the absence of ADP and ATP. Although homogeneous as judged by polyacrylamide-gel electrophoresis, the enzyme preparation exhibits ADP/ATP isotope-exchange activity which could not be separated from NAD+ kinase activity, but kinetic evidence suggests that this is probably due to a contaminant.