Calcium-regulated phosphorylation in synaptosomal cytosol: dependence on calmodulin

Calcium-Stimulated Protein Phosphorylation in Synaptic Membranes

Synaptic membranes from rat brain contain several calcium-requiring protein kinase (PK) activities with different substrate specificities: (a) an activity (CaH-PK) effective at high concentrations of Ca2+ ion in the absence of Mg2+ (active on class F substrates); (b) a (Ca + Mg)-PK activity that is mediated by Ca2+ ion in the presence of Mg2+ (active on class B substrates); (c) (Ca-CaM)-PK activities that exhibit simultaneous requirements for both Ca2+ ion and CaM (for class C and D substrates). Also described are three activities (d-f) that do not require Ca2+ ion: (d) a Mg-PK activity in which the presence of Ca2+ causes the inhibition of phosphorylation (active on class A substrates); (e) an activity affecting a diverse group of substrates (class E substrates), the phosphorylation of which occurs in the presence of Mg2+ ion alone (Mg-PK activity) and is unaffected by the addition of Ca2+ ion and CaM, the substrates of which show different responses to several types of inhibitors; and, finally, (f) the previously well characterized cAMP-dependent PK activities. Several of the substrates of these kinases have been identified in a fairly unambiguous manner: among them are P43 (class A), as the alpha subunit of pyruvate dehydrogenase; P54 (class B), as the presynaptic protein B50; and the doublet P75-P80, as proteins IA and IB of Ueda and Greengard. The most interesting activity is that requiring both Ca2+ and CaM. The half-maximal stimulation (K0.5) for Ca2+ in the presence of CaM was found to be 1.0 microM Ca2+F in untreated membranes. There is little change in this value on prior EGTA extraction of the membranes, which removes the bulk of its Ca2+ and reduces its residual CaM by greater than or equal to 50%. The apparent K0.5 for CaM in the presence of excess Ca2+ ion was found to equal 0.4 microgram per reaction mixture (8 micrograms/ml) or 1.35 micrograms per reaction mixture (27 micrograms/ml), for the untreated and EGTA-treated membranes, respectively.

Binding of pEL98 protein, an S100-related calcium-binding protein, to nonmuscle tropomyosin

The cDNA coding for mouse fibroblast tropomyosin isoform 2 (TM2) was placed into a bacterial expression vector to produce a fusion protein containing glutathione-S-transferase (GST) and TM2 (GST/TM2). Glutathione-Sepharose beads bearing GST/TM2 were incubated with [35S]methionine-labeled NIH 3T3 cell extracts and the materials bound to the fusion proteins were analyzed to identify proteins that interact with TM2. A protein of 10 kD was found to bind to GST/TM2, but not to GST. The binding of the 10-kD protein to GST/TM2 was dependent on the presence of Ca2+ and inhibited by molar excess of free TM2 in a competition assay. The 10-kD protein-binding site was mapped to the region spanning residues 39-107 on TM2 by using several COOH-terminal and NH2-terminal truncation mutants of TM2. The 10-kD protein was isolated from an extract of NIH 3T3 cells transformed by v-Ha-ras by affinity chromatography on a GST/TM2 truncation mutant followed by SDS-PAGE and electroelution. Partial amino acid sequence analysis of the purified 10-kD protein, two-dimensional polyacrylamide gel analysis and a binding experiment revealed that the 10-kD protein was identical to a calcium-binding protein derived from mRNA named pEL98 or 18A2 that is homologous to S100 protein. Immunoblot analysis of the distribution of the 10-kD protein in Triton-soluble and -insoluble fractions of NIH 3T3 cells revealed that some of the 10-kD protein was associated with the Triton-insoluble cytoskeletal residue in a Ca(2+)-dependent manner. Furthermore, immunofluorescent staining of NIH 3T3 cells showed that some of the 10-kD protein colocalized with nonmuscle TMs in microfilament bundles. These results suggest that some of the pEL98 protein interacts with microfilament-associated nonmuscle TMs in NIH 3T3 cells.

Autophosphorylation of neuronal calcium/calmodulin-stimulated protein kinase II

A unique feature of neuronal calcium/calmodulin-stimulated protein kinase II (CaM-PK II) is its autophosphorylation. A number of sites are involved and, depending on the in vitro conditions used, three serine and six threonine residues have been tentatively identified as autophosphorylation sites in the alpha subunit. These sites fall into three categories. Primary sites are phosphorylated in the presence of calcium and calmodulin, but under limiting conditions of temperature, ATP, Mg2+, or time. Secondary sites are phosphorylated in the presence of calcium and calmodulin under nonlimiting conditions. Autonomous sites are phosphorylated in the absence of calcium and calmodulin after initial phosphorylation of Thr-286. Mechanisms that lead to a decrease in CaM-PK II autophosphorylation include the thermolability of the enzyme and the activity of protein phosphatases. A range of in vitro inhibitors of CaM-PK II autophosphorylation have recently been identified. Autophosphorylation of CaM-PK II leads to a number of consequences in vitro, including generation of autonomous activity and subcellular redistribution, as well as alterations in conformation, activity, calmodulin binding, substrate specificity, and susceptibility to proteolysis. It is established that CaM-PK II is autophos-phorylated in neuronal cells under basal conditions. Depolarization and/or activation of receptors that lead to an increase in intracellular calcium induces a marked rise in the autophosphorylation of CaM-PK II in situ. The incorporation of phosphate is mainly found on Thr-286, but other sites are also phosphorylated at a slower rate. One consequence of the increase in CaM-PK II autophosphorylation in situ is an increase in the level of autonomous kinase activity. It is proposed that the formation of an autonomous enzyme is only one of the consequences of CaM-PK II autophosphorylation in situ and that some of the other consequences observed in vitro will also be seen. CaM-PK II is involved in the control of neuronal plasticity, including neurotransmitter release and long-term modulation of postreceptor events. In order to understand the function of CaM-PK II, it will be essential to ascertain more fully the mechanisms of its autophosphorylation in situ, including especially the sites involved, the consequences of this autophosphorylation for the kinase activity, and the relationships between the state of CaM-PK II autophosphorylation and the physiological events within neurons.

Regulation of norepinephrine uptake by adenine nucleotides and divalent cations: role for extracellular protein phosphorylation

This study examined the hypothesis that ATP, released together with norepinephrine (NE) from brain noradrenergic nerve terminals, may serve as a cosubstrate for an extracellular protein phosphorylation system that regulates the reuptake of the transmitter, NE. The possible regulation of high-affinity uptake (uptake 1) of [3H]NE by divalent cations and ATP, both of which are involved in protein phosphorylation, was examined in rat cerebral cortical synaptosomes. A marked inhibition of uptake 1 by 5'-adenylylimidodiphosphate [App(NH)p], a nonhydrolyzable, competitive antagonist of ATP, was observed. A similar inhibition of uptake was observed when Ca2+ and Mg2+ were both omitted from the incubation medium. App(NH)p distinguished the actions of Ca2+ from those of Mg2+: Ca2+-stimulated uptake 1 was blocked by App(NH)p; Mg2+-stimulated uptake was not. In parallel experiments, the patterns of protein phosphorylation in crude and purified preparations of synaptosomes were examined under conditions similar to those used in uptake assays. A striking correlation was found between the inhibition of uptake 1, by either App(NH)p or Ca-omission, and inhibition of the phosphorylation of one specific, 39,000-dalton, Ca2+-dependent, protein component in synaptosomes. This 39K protein was distinct from the alpha subunit of pyruvate dehydrogenase, a mitochondrial protein of similar electrophoretic mobility. These findings are consistent with the possibility that an ectokinase on synaptosomes utilizes extracellular ATP and Ca2+ in phosphorylating a protein(s) associated with the regulation of NE uptake.

Acceptors for cyclic AMP-dependent and calcium ion-dependent protein kinases in rat brain cytosol fractions: a comparison of occluded (synaptosomal) cytosol with non-occluded cytosol

Endogenous protein phosphorylation patterns were compared in occluded and non-occluded cytosol fractions prepared from rat forebrain. The occluded fraction was taken as representative of synaptosomal cytosol. One- and two-dimensional autoradiographs revealed the presence in non-occluded cytosol of a substrate for cAMP- and Ca2+/calmodulin-dependent protein kinase activities of Mr 300kD, corresponding to phosphorylated microtubule-associated protein-2 (MAP-2); this protein was absent in occluded cytosol. In contrast, a major substrate for protein kinase C was observed exclusively in occluded cytosol after phosphorylation under basal conditions. However, after phosphorylation in the presence of exogenous lipids, approximately equal amounts of the 82kD substrate were detected in both fractions, suggesting that protein kinase C in the occluded fraction was present in a partially activated state. Other minor differences in phosphorylation patterns between the two fractions were observed.

Phosphorylation of synaptic membrane glycoproteins: the effects of Ca2+ and calmodulin

Synaptic membranes were incubated with [gamma-32P]ATP, and glycoproteins were isolated by affinity chromatography on concanavalin A agarose. Glycoproteins accounted for 1.5-2.5% of the total 32P incorporated into synaptic membrane proteins. Ca2+ and calmodulin enhanced the phosphorylation of synaptic membrane glycoproteins approximately threefold. In the presence of Ca2+ and calmodulin, the rate of glycoprotein dephosphorylation was also increased three- to four-fold. Gel electrophoretic analysis identified several synaptic membrane glycoproteins that incorporated 32P, with the most highly labeled glycoprotein under basal phosphorylating conditions having an apparent Mr of 205,000 (gpiii). Ca2+ and calmodulin produced a marked increase in the phosphorylation of a glycoprotein with an apparent Mr of 180,000 (gpiv) and lesser increases in the labeling of three other glycoproteins. Membranes that had been labeled with [gamma-32P]ATP were extracted with Triton X-100 under conditions that yield a detergent-insoluble residue enriched in postsynaptic structures. The Triton X-100 insoluble residue accounted for 20-25% of the 32P associated with synaptic membrane glycoproteins. Gpiv and other glycoproteins, the phosphorylation of which was stimulated by calmodulin, were located exclusively in the Triton X-100 insoluble residue, whereas gpiii and other calmodulin-insensitive glycoproteins partitioned predominantly into the Triton X-100-soluble fraction. Phosphopeptide maps and phosphoamino acid analysis of gpiv isolated from synaptic membranes and a postsynaptic glycoprotein of apparent Mr of 180,000 (gp180) isolated from synaptic junctions indicated that the former protein was identical to the previously identified postsynaptic-specific gp180. In addition to phosphoserine and phosphothreonine, gpiv also contained phosphotyrosine, identifying it as a substrate for tyrosine-protein kinase as well as for Ca2+/calmodulin-dependent protein kinase.

Regulation of reactivated elongation in lysed cell models of teleost retinal cones by cAMP and calcium

Teleost retinal cones elongate in the dark and contract in the light. In isolated retinas of the green sunfish Lepomis cyanellus, cone myoids undergo microtubule-dependent elongation from 5 to 45 micron. We have previously shown that cone contraction can be reactivated in motile models of cones lysed with Brij-58. Reactivated contraction is both actin and ATP dependent, activated by calcium, and inhibited by cAMP. We report here that we have obtained reactivated cone elongation in lysed models prepared by the same procedures. Reactivated elongation is ATP dependent, activated by cAMP, and inhibited by calcium. The rate of reactivated elongation is proportional to the cAMP concentration between 10 microM and 0.5 mM, but is constant between 10 microM and 1.0 mM Mg-ATP. No elongation occurs if cAMP or Mg-ATP concentration is less than or equal to 5 microM. Mg-ATP is required for both cAMP-dependent and cAMP-independent processes, suggesting that Mg-ATP is required both for a regulatory process entailing cAMP-dependent phosphorylation and for a force-producing process. Free calcium concentrations greater than or equal to 10(-7) reduce the elongation rate by 78% or more, completely inhibiting elongation at 10(-5) M. This inhibition is not due to competition from calcium-activated contraction. Cytochalasin D blocks reactivated contraction, but does not abolish calcium inhibition of reactivated elongation. Thus calcium directly affects the elongation mechanism. Calcium inhibition is calmodulin dependent. The calmodulin inhibitor trifluoperazine abolishes calcium inhibition of elongation. Furthermore, calcium blocks elongation only if present during the lysis step; subsequent calcium addition has no effect. However, if calcium plus exogenous calmodulin are subsequently added, elongation is again inhibited. Thus calcium inhibition appears to require a soluble calmodulin which is lost shortly after lysis.

Synchronous exocytosis in Paramecium cells involves very rapid (less than or equal to 1 s), reversible dephosphorylation of a 65-kD phosphoprotein in exocytosis-competent strains

Synchronous exocytosis in Paramecium cells involves the rapid (less than or equal to 1 s) dephosphorylation of a 65-kD phosphoprotein, which, after a lag phase of approximately 5 s, is reversed within approximately 20 s. Exocytosis inhibitors suppress this reaction; stimulatory and inhibitory effects are dose dependent. The dephosphorylation of the 65-kD phosphoprotein occurs only in exocytosis-competent strains, but not in mutant strains that cannot carry out membrane fusion, or that are devoid of secretory organelles or cannot transport them to the cell membrane. Since under all conditions analyzed the transient dephosphorylation of the 65-kD phosphoprotein strictly parallels the actual amount of exocytosed organelles, this process might be involved in exocytosis performance, perhaps in its initiation.

Changes in in vitro brain and spinal cord protein phosphorylation after a single oral administration of tri-o-cresyl phosphate to hens

The effect of a single oral 750 mg/kg dose of tri-o-cresyl phosphate (TOCP) on the endogenous phosphorylation of brain and spinal cord proteins was assessed in hens during the development of and recovery from delayed neurotoxicity. Crude membrane and cytosolic fractions were prepared from the brains and spinal cords of control and TOCP-treated hens at 1, 7, 14, 21, 35, and 55 days after treatment. Brain and spinal cord protein phosphorylation with [gamma-32P]ATP was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), autoradiography, and microdensitometry. TOCP administration conferred calcium and calmodulin dependence on the phosphorylation of a few brain cytosolic proteins and caused an increase in the phosphorylation of a number of other cytosolic and membrane proteins. This effect of TOCP was large in magnitude, and its time course reflected the onset of and recovery from the signs of ataxia and paralysis associated with delayed neurotoxicity in the hen. The molecular weights (Mr) and maximal phosphorylation (percent of control) for the most prominently affected bands were as follows: brain cytosol--50K (183%), 55K (575%), 60K (529%), 65K (273%), and 70K (548%); brain membranes--50K (622%) and 60K (697%); and spinal cord cytosol--20K (182%). The role of endogenous phosphorylation reactions in and their potential usefulness as biochemical indicators of delayed neurotoxicity are being explored further.

Ca2+/calmodulin-dependent microtubule-associated protein 2 kinase: broad substrate specificity and multifunctional potential in diverse tissues

In previous studies, we described a soluble Ca2+/calmodulin-dependent protein kinase which is the major Ca2+/calmodulin-dependent microtubule-associated protein 2 (MAP-2) kinase in rat brain [Schulman, H. (1984) J. Cell Biol. 99, 11-19; Kuret, J. A., & Schulman, H. (1984) Biochemistry 23, 5495-5504]. We now demonstrate that this protein kinase has broad substrate specificity. Consistent with a multifunctional role in cellular physiology, we show that in vitro the enzyme can phosphorylate numerous substrates of both neuronal and nonneuronal origin including vimentin, ribosomal protein S6, synapsin I, glycogen synthase, and myosin light chains. We have used MAP-2 to purify the enzyme from rat lung and show that the brain and lung kinases have nearly indistinguishable physical and biochemical properties. A Ca2+/calmodulin-dependent protein kinase was also detected in rat heart, rat spleen, and in the ring ganglia of the marine mollusk Aplysia californica. Partially purified MAP-2 kinase from each of these three sources displayed endogenous phosphorylation of a 54 000-dalton protein. Phosphopeptide analysis reveals a striking homology between this phosphoprotein and the 53 000-dalton autophosphorylated subunit of the major rat brain Ca2+/calmodulin-dependent protein kinase. The enzymes phosphorylated MAP-2, synapsin I, and vimentin at peptides that are identical with those phosphorylated by the rat brain kinase. This enzyme may be a multifunctional Ca2+/calmodulin-dependent protein kinase with a widespread distribution in nature which mediates some of the effects of Ca2+ on microtubules, intermediate filaments, and other cellular constituents in brain and other tissues.

Depolarisation-dependent protein phosphorylation and dephosphorylation in rat cortical synaptosomes is modulated by calcium

The effect of calcium on protein phosphorylation was investigated using intact synaptosomes isolated from rat cerebral cortex and prelabelled with 32Pi. For nondepolarised synaptosomes a group of calcium-sensitive phosphoproteins were maximally labelled in the presence of 0.1 mM calcium. The phosphorylation of these proteins was slightly decreased in the presence of strontium and absent in the presence of barium, consistent with the decreased ability of these cations to activate calcium-stimulated protein kinases. Addition of calcium alone to synaptosomes prelabelled in its absence increased phosphorylation of a number of proteins. On depolarisation in the presence of calcium certain of the calcium-sensitive phosphoproteins were further increased in labelling above nondepolarised levels. These increases were maximal and most sustained after prelabelling at 0.1 mM calcium. On prolonged depolarisation at this calcium concentration a slow decrease in labelling was observed for most phosphoproteins, whereas a greater rate and extent of decrease occurred at higher calcium concentrations. At 2.5 mM calcium a rapid and then a subsequent slow dephosphorylation was observed, indicating two distinct phases of dephosphorylation. Of all the phosphoproteins normally stimulated by depolarisation, only phosphoprotein 59 did not exhibit the rapid phase of dephosphorylation at high calcium concentrations. Replacing calcium with strontium markedly decreased the extent of change observed on depolarisation whereas barium decreased phosphorylation changes even further. Taken together these data suggest that an influx of calcium into synaptosomes initially activates protein phosphorylation, but as the levels of intrasynaptosomal calcium rise protein dephosphorylation predominates. Other phosphoproteins were dephosphorylated immediately on depolarisation in the presence of calcium. The fine control of protein phosphorylation levels exerted by calcium supports the idea that the synaptosomal phosphoproteins could play a role in modulating events such as neurotransmitter release in the nerve terminal.

Biochemical and immunochemical evidence that the "major postsynaptic density protein" is a subunit of a calmodulin-dependent protein kinase

By three criteria, two biochemical and one immunochemical, the major postsynaptic density protein (mPSDp) is indistinguishable from the 50-kilodalton (kDa) alpha subunit of a brain calmodulin-dependent protein kinase. First, the two proteins comigrate on NaDodSO4/polyacrylamide gels. Second, iodinated tryptic peptide maps of the two are identical. Finally, a monoclonal antibody (6G9) that was raised against the protein kinase binds on immunoblots to a single 50 kDa band in crude brain homogenates and to both the alpha subunit of the purified kinase and the mPSDp from postsynaptic density fractions. The purified kinase holoenzyme also contains a 60-kDa subunit termed beta. A comparison of the peptide map of beta with the maps of 60-kDa proteins from the postsynaptic density fraction suggests that beta is present there but is not the only protein present in this molecular weight range. These results indicate that the calmodulin-dependent protein kinase is a major constituent of the postsynaptic density fraction and thus may be a component of type I postsynaptic densities.

S-100-mediated inhibition of brain protein phosphorylation

The effects of the glial-specific, calcium-binding, S-100 protein on brain membrane and supernatant protein phosphorylation were assessed. S-100 concentrations as low as 5 micrograms/ml caused a marked inhibition of the phosphorylation of a soluble brain protein having a molecular weight of 73,000 daltons (73K). This protein was designated the S-100 protein-modulated phosphoprotein (SMP). Half-maximal inhibition of the phosphorylation of SMP by S-100 was obtained at concentrations of 12 micrograms/ml (0.57 microM). The inhibition of SMP phosphorylation by S-100 was calcium-dependent, with a calculated calcium Ka of 2.0 +/- 0.3 microM. SMP phosphorylation was also inhibited by calmodulin, but only partially and with a much lower potency. The inhibition of SMP phosphorylation by S-100 was not inhibited by fluphenazine, whereas the effect of calmodulin was. SMP was found in many brain areas, with the highest levels seen in the corpus callosum. Various peripheral tissues, such as kidney; liver; and pineal, pituitary, and adrenal glands, did not contain detectable SMP levels. At higher S-100 concentrations, greater than 10 micrograms/ml, the phosphorylation of several other soluble proteins was markedly inhibited. These proteins have molecular weights of 56K, 50K, and 47K. The phosphorylation of these proteins was enhanced by calmodulin. These data suggest that the S-100 protein may function to modulate the phosphorylation of brain proteins in a manner analogous to (although in a reciprocal fashion) that of calmodulin.

Effect of oral administration of tri-o-cresyl phosphate on in vitro phosphorylation of membrane and cytosolic proteins from chicken brain

The effects of a single oral dose of 750 mg/kg tri-o-cresyl phosphate (TOCP) on the endogenous phosphorylation of specific brain proteins were assessed in male adult chickens following the development of delayed neurotoxicity. Phosphorylation of crude synaptosomal (P2) membrane and synaptosomal cytosolic proteins was assayed in vitro by using [gamma-32P]ATP as phosphate donor. Following resolution of brain proteins by sodium dodecyl sulfate polyacrylamide gel electrophoresis, specific protein phosphorylation was detected by autoradiography and quantified by microdensitometry. TOCP administration enhanced the phosphorylation of both cytosolic (Mr 65,000 and 55,000) and membrane (20,000) proteins by as much as 146% and 200%, respectively.

Calcium/calmodulin-dependent phosphorylation of vimentin in rat sertoli cells

Ca2+-dependent protein phosphorylation and the role of calmodulin in this process was investigated in subcellular fractions of primary cultures of rat Sertoli cells. Significant Ca2+/calmodulin-dependent protein phosphorylation in Sertoli cells was restricted to the cytosol fraction. The calmodulin dependence of these effects was confirmed by using the calmodulin inhibitor trifluoperazine. One of the Ca2+/calmodulin-dependent phosphoproteins was identified as the intermediate filament protein vimentin, based on the following criteria: (i) migration pattern in two-dimensional polyacrylamide gels, (ii) Ca2+/calmodulin-dependent phosphorylation of a 58-kilodalton protein present in detergent-insoluble intermediate filament protein extract of Sertoli cells, and (iii) peptide mapping of the phosphoprotein. These data support a role for Ca2+/calmodulin-dependent protein phosphorylation in the modulation of Sertoli cell cytoskeletal components.

Purification and characterization of a Ca2+- and calmodulin-dependent protein kinase from rat brain

A Ca2+- and calmodulin-dependent protein kinase was purified from rat brain cytosol fraction to apparent homogeneity at approximately 800-fold and with a 5% yield. The purified enzyme had a molecular weight of 640,000 as determined by gel filtration analysis on Sephacryl S-300 and a sedimentation coefficient of 15.3 S by sucrose density gradient centrifugation, and resulted in a single protein band of MW 49,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. These results suggest that the native enzyme has a large molecular weight and consists of 11 to 14 identical subunits. The purified enzyme exhibited Km values of 109 and 30 microM for ATP and chicken gizzard myosin light chain, respectively, and Ka values of 12 nM and 1.9 microM for brain calmodulin and Ca2+, respectively. In addition to myosin light chain, myelin basic protein, casein, arginine-rich histone, microtubule protein, and synaptosomal proteins were phosphorylated by the enzyme in a CA2+- and calmodulin-dependent manner. The purified enzyme was phosphorylated without the addition of the catalytic subunits of cyclic AMP-dependent protein kinase. Our findings indicate that there is a multifunctional Ca2+- and calmodulin-dependent protein kinase in the brain and that this enzyme may regulate the reactions of various endogenous proteins.

Calmodulin stimulation of 45Ca2+ transport and protein phosphorylation in cholinergic synaptic vesicles

Cholinergic synaptic vesicles isolated from the electric organ of Torpedo californica exhibit ATP-dependent uptake of 45Ca2+ that is stimulated by exogenous calmodulin. ATP-independent uptake also occurs, but it is only weakly stimulated by calmodulin. Saturating calmodulin decreased the Michaelis constant for ATP-dependent 45Ca2+ uptake from 52 +/- 0.4 to 12 +/- 0.2 microM and increased the maximal velocity from 3.4 +/- 0.3 to 5.2 +/- 0.5 nmol/mg of protein per min. The dose-response curve for calmodulin-dependent stimulation showed a maximal increase of 3.5-fold in the uptake rate; 0.2 microM calmodulin gave half-maximal stimulation. The activity of the vesicle-associated ATPase was unaffected. Incubation of vesicles with [gamma-32P]ATP and Ca2+ resulted in phosphorylation of four polypeptides of molecular weights about 64,000, 58,000, 54,000, and 41,000 when calmodulin was added. Vesicles that were previously phosphorylated and purified exhibited 2-fold enhanced ATP-independent uptake of 45Ca2+. Cyclic AMP could not substitute for calmodulin. The calcium transport system of the cholinergic synaptic vesicle is regulated by a calcicalmodulin-dependent protein kinase that is vesicle-associated.

Calcium and cyclic GMP regulation of light-sensitive protein phosphorylation in frog photoreceptor membranes

In frog photoreceptor membranes, light induces a dephosphorylation of two small proteins and a phosphorylation of rhodopsin. The level of phosphorylation of the two small proteins is influenced by cyclic GMP. Measurement of their phosphorylation as a function of cyclic GMP concentration shows fivefold stimulation as cyclic GMP is increased from 10(-5) to 10(-3) M. This includes the concentration range over which light activation of a cyclic GMP phosphodiesterase causes cyclic GMP levels to fall in vivo. Cyclic AMP does not affect the phosphorylations. Calcium ions inhibit the phosphorylation reactions. Calcium inhibits the cyclic GMP-stimulated phosphorylation of the small proteins as its concentration is increased from 10(-6) to 10(-3) M, with maximal inhibition of 70% being observed. Rhodopsin phosphorylation is not stimulated by cyclic nucleotides, but is inhibited by calcium, with 50% inhibition being observed as the Ca++ concentration is increased from 10(-9) to 10(-3) M. A nucleotide binding site appears to regulate rhodopsin phosphorylation. Several properties of the rhodopsin phosphorylation suggest that it does not play a role in a rapid ATP-dependent regulation of the cyclic GMP pathway. Calcium inhibition of protein phosphorylation is a distinctive feature of this system, and it is suggested that Ca++ regulation of protein phosphorylation plays a role in the visual adaptation process. Furthermore, the data provide support for the idea that calcium and cyclic GMP pathways interact in regulating the light-sensitive conductance.