Phosphorylation of brain cytosol proteins. Effects of phospholipids and calmodulin

Characteristics of Ca2+/calmodulin- and Ca2+/phosphatidylserine-stimulated phosphoproteins in rat striatum

The characteristics of endogenous Ca2+/calmodulin (CaM)- and Ca2+/phosphatidylserine (PS)-stimulated phosphorylated proteins in the striatum of rat were partially determined and compared in this study. The Ca2+/CaM-dependent phosphoproteins were associated with serine and threonine residues. The sensitivity of these proteins for phosphorylation by Ca2+/CaM was not affected by pretreatment of tissue with Ca2+ chelating agent, EGTA or with non-ionic detergent, Triton X-114. Triton X-114 phase separation experiments revealed that these Ca2+/CaM-dependent phosphoproteins were partitioned in the detergent rich phase suggesting that they are integral proteins of the striatal membrane. On the other hand, the Ca2+/PS-dependent phosphorylated proteins were primarily associated with the serine residue. Phosphorylation of these proteins by Ca2+/PS were abolished after the treatment with EGTA or Triton X-114. These results suggest that Ca2+/PS-dependent striatal phosphoproteins are biochemically unstable in maintaining their state of phosphorylation.

Phenothiazines reduce ischemic damage to the central nervous system

We found substantial alterations in reactions catalyzed by calcium/phospholipid-dependent and calcium/calmodulin-dependent protein kinases during CNS ischemia which suggested that phenothiazines, drugs capable of inhibiting these reactions, might reduce neurologic damage. To test this hypothesis, we used chlorpromazine and trifluoperazine. Both drugs reduced neurologic function deficits relative to controls in a rabbit multiple cerebral embolism model and a rabbit spinal cord ischemia model. Chlorpromazine was effective despite reduction of blood pressure, and trifluoperazine did not alter blood pressure. These findings suggest that phenothiazines may be useful for preserving neurologic function when administered shortly after the onset of CNS ischemia.

Altered protein phosphorylation in intact rat cortical synaptosomes after in vivo administration of fluphenazine

Phenothiazines such as fluphenazine are able to inhibit calcium-stimulated protein kinases in vitro in both lysed and intact synaptosomes. In this study protein phosphorylation was assayed in intact synaptosomes isolated from the cerebral cortex of rats treated chronically (21 days, 10 mg/kg, i.p.) or acutely (1 hr, 10 mg/kg, i.p.) with fluphenazine. When intact synaptosomes from chronically treated animals were prelabeled with 32Pi, there were two effects on protein phosphorylation: an increase in the basal labeling of many phosphoproteins and a decrease in depolarization-evoked protein phosphorylation. Acute injections had even more pronounced effects, but the direction and nature of the effects were the same. No effects on K+-stimulated calcium entry or on protein phosphatase activity were detected. When lysed synaptosomes from chronically treated animals were labeled in the presence of [gamma-32P]ATP, a small decrease in calmodulin-dependent and cAMP-dependent protein phosphorylation was observed. The results suggest that two different in vivo mechanisms may underlie these effects, and these are discussed. We proposed that intact synaptosomes may be a good model in which to study the in vivo mechanisms of the action of fluphenazine since they appear to retain at least some effects of the drug after subcellular fractionation.

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.

Ca2+ and phospholipid-dependent protein kinase activity and phosphorylation of endogenous proteins in bovine adrenal medulla

Soluble and membrane fractions of bovine adrenal medulla contain several substrates for the Ca2+/phospholipid-dependent and cyclic AMP-dependent protein kinases. The phosphorylation of soluble proteins (36 and 17.7 kilodaltons) and a membrane protein (22.5 kilodaltons) showed an absolute requirement for the presence of both Ca2+ and phosphatidylserine; other substrates showed less stringent phosphorylation requirements and many of these proteins were specific for each of the protein kinases. The Ca2+/phospholipid-dependent phosphorylation was rapid, with effects seen as early as at 30 s of incubation. Measurement of enzyme activities with histone H1 as an exogenous substrate demonstrated that the Ca2+/phospholipid-dependent protein kinase was equally distributed between the soluble and membrane fractions whereas the cyclic AMP-dependent enzyme was predominantly membrane-bound in adrenal medulla and chromaffin cells. The activity of the soluble Ca2+/phospholipid-dependent protein kinase of adrenal medulla was found to be about 50% of the enzyme level present in rat brain, a tissue previously shown to contain a very high enzyme activity. These results suggest a prominent role for the Ca2+/phospholipid-dependent protein kinase in chromaffin cell function.

Is Ca2+-calmodulin-dependent protein phosphorylation in rat brain modulated by carboxylmethylation?

Calmodulin stimulation of protein kinase activity in calmodulin-depleted preparations of rat brain cytosol or synaptosomal membranes was attenuated by prior carboxylmethylation of the enzyme source with purified protein-O-carboxylmethyltransferase. Similarly, calmodulin stimulation of highly purified Ca2+-calmodulin-dependent protein kinase was reduced if the kinase was exposed to methylating conditions prior to addition of calmodulin. Biochemical and acidic sodium dodecyl sulfate-gel electrophoretic analyses indicated that all sources of protein kinase activity were substrates for methylation. The specific activity of methyl group incorporation into protein kinase increased with increasing purity of the preparation, reaching values of 1.72 pmol CH3/micrograms protein or 0.15-1.12 mol CH3/mol of holoenzyme. Analysis of ATP binding in cytosol with the use of the photoaffinity probe [32P]8-azido-ATP indicated that carboxylmethylation reduced ATP binding. These results suggest that carboxylmethylation of Ca2+-calmodulin protein kinase may modulate the activity of this enzyme in rat brain.

Partial characterization of endogenous phosphorylation conditions for hen brain cytosolic and membrane proteins

The optimal conditions for endogenous protein phosphorylation with 5 microM [gamma-32P]ATP, 10 mM MgCl2 in preparations containing synaptosomal cytosol or membranes (shocked crude mitochondrial fraction P2) from adult hen brains were determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, autoradiography and microdensitometry. Phosphate incorporation increased linearly with protein concentration from 75-125 micrograms/200 microliters in brain cytosol and was maximal at 75 micrograms/200 microliters in brain membranes. Optimal incubation times were 60-90 s for brain cytosol and 10-15 s for brain membranes. With the exception of the 20 kilodalton myelin basic protein in the membrane fraction, pH 6.5 is generally optimal. While temperature optima varied considerably with different bands, most of them were found between 35 and 45 degrees C. When identical preparations from hen and rat brain were co-electrophoresed, one of the most striking differences was that the enhancement of phosphorylation of a 55 kilodalton doublet, which may be tubulin, by addition of 50 microM Ca2+ was at least 3 times greater in rat than in hen brain cytosol. Another species difference was apparent in the membrane fractions in which the 20 kilodalton hen brain presumptive myelin basic protein (MBP) was phosphorylated to approximately the same extent as that of the 16 and 20 kilodalton rat brain MBPs combined.

Structural and chemical specificity of diacylglycerols for protein kinase C activation

The structural and chemical specificity of diacylglycerols, lipid components of the quaternary complex for protein kinase C activation, have been evaluated. The ether-linked analogs of the diacyl lipids, either dialkyl or alkyl acyl, were not effective activators of protein kinase C and thus had little influence on reducing the Ca++ requirement of the enzyme. Ester-linked compounds such as 1-palmitoyl-sn-2-butyrylglycerol were as effective as dioleoylglycerol in stimulating protein phosphorylation. Increasing the carbon number at the sn-2 position from two to four resulted in enhanced enzymatic activity, suggesting that the chain length at the secondary hydroxyl is also of importance. These data clearly establish the necessity of the sn-1 carbonyl group of ester-linked glycerolipids in the protein kinase C activation complex.

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

A soluble Ca2+/calmodulin-dependent protein kinase has been purified from rat brain to near homogeneity by using casein as substrate. The enzyme was purified by using hydroxylapatite adsorption chromatography, phosphocellulose ion-exchange chromatography, Sepharose 6B gel filtration, affinity chromatography using calmodulin-Sepharose 4B, and ammonium sulfate precipitation. On sodium dodecyl sulfate (NaDodSO4)-polyacrylamide gels, the purified enzyme consists of three protein bands: a single polypeptide of 51 000 daltons and a doublet of 60 000 daltons. Measurements of the Stokes radius by gel filtration (81.3 +/- 3.7 A) and the sedimentation coefficient by sucrose density sedimentation (13.7 +/- 0.7 S) were used to calculate a native molecular mass of 460 000 +/- 29 000 daltons. The kinase autophosphorylated both the 51 000-dalton polypeptide and the 60 000-dalton doublet, resulting in a decreased mobility in NaDodSO4 gels. Comparison of the phosphopeptides produced by partial proteolysis of autophosphorylated enzyme reveals substantial similarities between subunits. These patterns, however, suggest that the 51 000-dalton subunit is not a proteolytic fragment of the 60 000-dalton doublet. Purified Ca2+/calmodulin-dependent casein kinase activity was dependent upon Ca2+, calmodulin, and ATP X Mg2+ or ATP X Mn2+ when measured under saturating casein concentrations. Co2+, Mn2+, and La3+ could substitute for Ca2+ in the presence of Mg2+ and saturating calmodulin concentrations. In addition to casein, the purified enzyme displayed a broad substrate specificity which suggests that it may be a "general" protein kinase with the potential for mediating numerous processes in brain and possibly other tissues.

Acidic phospholipids stimulate the autophosphorylation of the catalytic subunit of cyclic AMP-dependent protein kinase

The autophosphorylation of the catalytic subunit of cAMP-dependent protein kinase was stimulated by the acidic phospholipids phosphatidic acid, phosphatidylserine and phosphatidylinositol. Other phospholipids (phosphatidyl-ethanolamine, phosphatidylcholine, sphingomyelin), acidic compounds (dextran sulfate, polyglutamic acid, chondroitin sulfate, hyaluronic acid) and calcium-calmodulin were essentially inactive. Sodium dodecyl sulfate also stimulated the catalytic subunit autophosphorylation, but other detergents (Triton X-100 and deoxycholic acid) did not. The combination of phosphatidic acid and sodium dodecyl sulfate was as effective as each agent alone, suggesting similar stimulation mechanisms. The data suggest that acidic membrane phospholipids might have a role in regulating the autophosphorylation of the catalytic subunit of cAMP-dependent protein kinase.