Protein phosphorylation in isolated plasma membranes and postsynaptic junctional structures from brain synapses

Histochemically demonstrable phosphotyrosine protein phosphatase in the rat hippocampal formation

Using o-phospho-L-tyrosine as substrate, a possible localization of phosphotyrosine protein phosphatase (PTPPase) activity was histochemically demonstrated in the rat hippocampal formation. The PTPPase activity was found in almost all layers of the hippocampal formation, with a high activity in the stratum moleculare. The activity was inhibited by vanadate and molybdate, but not by NaF and Zn2+. The activity was localized in the dendritic cytoplasm, particularly on the postsynaptic density, of hippocampal neurons.

Phosphorylation of synaptic proteins in chick forebrain: changes with development and passive avoidance training

We have used synaptic plasma membranes (SPMs) and postsynaptic densities (PSDs) to study protein phosphorylation at the synapse in the developing chick forebrain and in 1-day-old chick forebrain following training on a passive avoidance task. Endogenous phosphorylation patterns in SPMs and PSDs prepared by extraction with n-octylglucoside isolated from chick forebrain were investigated by labelling with [32P]ATP. The phosphoprotein components of the SPM and PSD fractions were separated using sodium dodecyl sulphate gradient polyacrylamide gel electrophoresis. Autoradiography and densitometry of the Coomassie Blue protein staining pattern revealed phosphate incorporation into several SPM components including those of molecular mass 52, 37, and 29 kilodaltons (kDa). Bands of similar molecular mass were not phosphorylated in PSD fractions. This difference in phosphorylation between SPMs and PSDs was not due to the detergent n-octylglucoside. In a developmental study in which SPM and PSD fractions were prepared from 1-day-old, 14-day-old, and 21-day-old chickens, the phosphorylation patterns of SPMs were similar throughout, but striking differences occurred in PSDs, both in the level of phosphorylation and in the components phosphorylated. A time-course study was carried out in which phosphorylation of SPMs and PSDs from 1-day-old chicks trained on a passive avoidance task was compared with patterns from control chicks trained on a water-coated bead and untrained chicks. In SPMs prepared from forebrains removed 10 mins following training, a consistent but nonsignificant decrease (-21%) in phosphorylation of a 52 kDa band occurred in chicks with passive avoidance training compared with water-trained and untrained chicks.(ABSTRACT TRUNCATED AT 250 WORDS)

Independent protein kinases associated with the rat cerebral synaptic junction: comparison with cyclic AMP-dependent and Ca2+/calmodulin-dependent protein kinases in the synaptic junction

Independent protein kinases in the synaptic junction (SJ) isolated from rat cerebrum were characterized. SJ showed a protein kinase activity, phosphorylating intrinsic proteins, even in the absence of cyclic AMP or Ca2+ plus calmodulin (CaM) exogenously added. The activity was affected neither by Ca2+ concentrations in the physiological fluctuation range nor by the addition of specific ligands such as glutamate, aspartate, acetylcholine, and concanavalin A. The activity was not due to cyclic AMP-dependent protein kinase in SJ, since the activity was not inhibited by an inhibitor protein for cyclic AMP-dependent protein kinase, and since synapsin I was not specifically phosphorylated whereas cyclic AMP-dependent kinase appeared to phosphorylate selectively the protein in SJ. Phosphorylation of SJ proteins by the independent kinases was about one-third of that of the Ca2+/CaM-dependent protein kinase intrinsic to SJ. The apparent Km for ATP was estimated to be 700 microM. Proteins of 16K Mr and 117K Mr were specifically phosphorylated under the basic condition (in the absence of the substances known to activate specifically protein kinases), as well as six other proteins both under the basic conditions and in the presence of Ca2+ and CaM. The phosphorylation of 150K Mr, 60K Mr, 51K Mr, and 16K Mr SJ proteins was enhanced after prephosphorylation of SJ proteins by intrinsic kinase in the presence of Ca2+ and CaM.(ABSTRACT TRUNCATED AT 250 WORDS)

Demonstration by phase-partitioning in Triton X-114 solutions that phosphoprotein B-50 (F-1) from rat brain is an integral membrane protein

The Triton X-114 phase separation technique was employed to fractionate phosphoproteins present in membrane fragments from rat brain. Membranes were labelled with [gamma-32P]ATP in media containing Ca2+, Ca2+ plus calmodulin or cyclic AMP, and then treated with Triton X-114. Phosphoproteins recovered in the detergent-insoluble fraction, aqueous and detergent phases were detected by SDS-polyacrylamide gel electrophoresis and autoradiography. Of the proteins solubilised by the detergent, a known substrate of protein kinase C, the B-50 phosphoprotein (45 kD; also known as F-1), partitioned quantitatively into the detergent-rich phase, making it very probable that this phosphoprotein is an integral membrane protein. The detergent-rich phase also contained an 80 kD phosphoprotein, which probably corresponds to the widespread acidic 87 kD substrate of protein kinase C.

Existence of a Ca2+-dependent K+ channel in synaptic membrane and postsynaptic density fractions isolated from canine cerebral cortex and cerebellum, as determined by apamin binding

Apamin, a 18-amino acid neurotoxin isolated from bee venom, is a specific blocker of one class of the Ca2+-dependent K+ channels. The monoiodo derivative of the toxin with high specific radioactivity (1600 Ci/mmol) has been used to study its binding to synaptic membrane (SM) and postsynaptic density (PSD) fractions isolated from cerebral cortex (CTX) and cerebellum (CL) of canine brains. The Bmax (30.2 fmol/mg protein) for CTX-PSD is about twice that for CTX-SM (17.3 fmol/mg protein), suggesting a concentration of the apamin receptor protein in CTX-PSD over CTX-SM fractions. The lower value of Bmax for CL-PSD (12.3 fmol/mg protein), and the higher Kd value (51 pM) than for CTX-SM (33 pM), CTX-PSD (24 pM), and CL-SM (39 pM), may reflect the disruptive effect of Triton X-100 on these thin structures. The values of Bmax and Kd for CTX-SM are similar to those (22.0 fmol/mg protein and 33 pM) for rat CTX-SM. Both Ca2+ and Na+ inhibit apamin binding to CTX-PSD with K0.5 values of 14 and 31 mM, respectively, while the optimum concentration of KCl for activation is 5 mM. All these values are similar to those found for rat synaptosomes. Covalent labeling of the apamin binding protein, using the non-cleavable cross-linker, disuccinimidyl suberate, reveals an apamin binding polypeptide of 27 kdaltons under reducing and denaturing conditions in both the CTX-SM and CTX-PSD preparations, similar to that (28 kdaltons) reported for rat CTX-SM fractions. Prior phosphorylation of isolated CTX-PSD had no effect on apamin binding, nor did apamin binding influence subsequent phosphorylation of CTX-PSD. Calmodulin, an intrinsic PSD protein, may not play a role in apamin binding to PSD, since addition of calmodulin, or removal of the calmodulin by EGTA treatment, resulted in no change in the binding capacity of the PSD. The apamin binding protein seems to be bound quite firmly in the CTX-PSD fraction since treatments with 0.5% deoxycholate, 1% N-lauroyl sarcosinate, 4 M guanidine-HCl, pH 7.0, 0.5 M KCl and 1.0 M KCl, could only remove the apamin-receptor complexes from CTX-PSD by 40, 55, 52, 12 and 15%, respectively. These results contrast with the findings that the two detergents mentioned solubilize 80-93% of the receptor from synaptosomal or synaptic membrane fractions, indicating that a good deal of the receptor in these fractions is membrane-bound and not connected to the PSD.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of phosphate incorporation into four brain phosphoproteins that are affected by experience

Various regulators of protein kinase activities were tested for their effects on the in vitro transfer of phosphate from [gamma-32P]ATP to four proteins of rat brain synaptic particulate preparations. One protein, of apparent molecular weight 44,000, accepted 32P in the presence of 8 mM EDTA and no added Mg2+. It was the major phosphoprotein of brain mitochondria. Its phosphorylation was inhibited by pyruvate and stimulated by K+, and it comigrated in electrophoretic gels with authentic alpha-subunit of pyruvate: lipoamide oxidoreductase (decarboxylating) (EC 1.2.4.1) from bovine heart. The major kinase acting on three proteins of apparent molecular weights 24,000, 21,000, and 19,000 was stimulated by Ca2+, by preincubation with phospholipase C, and by 12-tetradecanoyl 4-beta-phorbol 13-acetate. Phosphorylation of these lower-molecular-weight proteins was inhibited by ACTH1-24, by cyclic 3',5'-adenosine monophosphate, and by 50 microM trifluoperazine. The stimulatory effect of Ca2+ was antagonized by calmodulin. The kinase in question appears to be B-50 protein kinase or protein kinase C.

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.

Synaptic junctional glycoproteins are phosphorylated by cyclic-AMP-dependent protein kinase

Synaptic junctions (SJs) isolated from rat brain are associated with protein kinase activity and a unique complement of high molecular weight gglycoproteins. Incubation of SJs with [gamma-32P]A+ glycoproteins which were retained by concanavalin A agarose (con A+ glycoproteins). Three major (apparent mol. wt. 180 K, 130 K and 110 K) and 2 minor (apparent mol. wt. 230 K and 145 K) glycoproteins were identified in the con A+ fraction. Of these, GP180 incorporated the most 32P and GP145 was not labeled. Peptide mapping experiments showed that each molecular weight class of glycoprotein was associated with a unique set of phosphorylated peptides. Cyclic AMP stimulated the incorporation of 32P into total SJ proteins and con A+ lycoproteins by 38% and 58%, respectively. GP130 showed the greatest increase in labelling in the presence of cyclic AMP (198% of control levels) although incorporation into all 4 glycoproteins was increased. Cyclic AMP selectively stimulated the incorporation of 32P into only 2 of the 6 phosphorylated peptides derived from GP130. These studies demonstrate that endogenous glycoproteins serve as substrates for intrinsic SJ protein kinases and identify this reaction as a potential means of modifying postsynaptic membrane function.

Structural organization of filamentous proteins in postsynaptic density

Actin is one of the major protein constituents of the postsynaptic density (PSD), a characteristic structural entity subjacent to the postsynaptic membrane in excitatory synapses of the vertebrate central nervous system. In isolated purified PSD preparations, it is present to the extent of 29 +/- 2 micrograms/mg of total protein, 90% of which is in the filamentous (F-actin) form. Iodination by a discriminatory labeling technique demonstrates that actin is located on the surface of the PSD from which it can be stripped by treatment with a mixture of strong anionic detergents, leaving behind an insoluble core held together by disulfide bridges, consisting in part of tubulin and "PSD protein".

Regularity and differentiation within the structure of brain postsynaptic densities

Postsynaptic densities (PSDs) isolated from rat forebrain were examined in samples prepared for electron microscopy by negative staining. Two structurally distinct areas could be distinguished within each individual PSD, a peripheral zone composed of a planar array of spherical subunits with a mean diameter of 18 nm and one or more islands of fine granular material enclosed by the subunits. These enclosures correspond in distribution and size to 'holes' which are known to occur in PSDs in intact brain tissue. The spherical subunits can occur singly in the isolated PSD preparations showing that the structure of each individual subunit does not depend upon its interaction with others. These observations indicate that the PSD structure has both regular and differentiated features. These are considered in relation to the possible roles of the PSD in defining the postsynaptic locus and mediating the postsynaptic events of synaptic transmission.

Endogenous protein phosphorylation in rat brain mitochondria: occurrence of a novel ATP-dependent form of the autophosphorylated enzyme succinyl-CoA synthetase

When rat brain mitochondria are incubated with [gamma-32P]ATP, there is a rapid (10 s) phosphorylation of proteins designated E1 and F of M.W. 42,000 and 32,000, respectively. Although [gamma-32P]ATP was the preferred substrate for protein F, a small amount of labeling did occur with [gamma-32P]GTP. Phosphorylation of E1 was absolutely ATP-dependent. On the other hand, a 32,000 M.W. protein from rat liver mitoplasts (mitochondria devoid of an outer membrane) was highly phosphorylated when [gamma-32P]GTP was used but not at all phosphorylated within short time periods with [gamma-32P]ATP. Both the ATP-labeled brain phosphoprotein F and GTP-labeled liver protein migrated to identical positions on high-resolution two-dimensional polyacrylamide gels, and both contained acid-labile phosphoryl groups. Furthermore, both phosphoproteins were identified as the autophosphorylated subunit of succinyl-CoA synthetase (SCS, EC 6.2.1.4) by using antibody directed against purified GTP-dependent porcine SCS. However, immunotitration experiments with anti-porcine SCS revealed that ATP- and GTP-labeled protein F in brain differed in their interactions with antibody, suggesting that in rat brain mitochondria two different forms of the enzyme exist that are immunologically distinct and differ in substrate specificity. When mitochondrial preparations enriched in particular brain cell or subcellular types were examined, an unequal distribution of E1 and the two forms of protein F were observed. A brain subfraction containing neuronal cell body and glial mitochondria (CM) was found to contain E1 and approximately equal amounts of the ATP- and GTP-dependent forms of protein F. Light synaptic mitochondria (SM1) contained ATP-dependent protein F almost exclusively and were depleted in E1. Dense synaptic mitochondria (SM2) are rich in the ATP form of SCS but also contain low amounts of the GTP enzyme.

Function of a calmodulin in postsynaptic densities. II. Presence of a calmodulin-activatable protein kinase activity

Because the calmodulin in postsynaptic densities (PSDs) activates a cyclic nucleotide phosphodiesterase, we decided to explore the possibility that the PSD also contains a calmodulin-activatable protein kinase activity. As seen by autoradiographic analysis of coomassie blue-stained SDS polyacrylamide gels, many proteins in a native PSD preparation were phosphorylated in the presence of [gamma-(32)P]ATP and Mg(2+) alone. Addition of Ca(2+) alone to the native PSD preparation had little or no effect on phosphorylation. However, upon addition of exogenous calmodulin there was a general increase in background phosphorylation with a statistically significant increase in the phosphorylation of two protein regions: 51,000 and 62,000 M(r). Similar results were also obtained in sonicated or freeze thawed native PSD preparations by addition of Ca(2+) alone without exogenous calmodulin, indicating that the calmodulin in the PSD can activate the kinase present under certain conditions. The calmodulin dependency of the reaction was further strengthened by the observed inhibition of the calmodulin-activatable phosphorylation, but not of the Mg(2+)-dependent activity, by the Ca(2+) chelator, EGTA, which also removes the calmodulin from the structure (26), and by the binding to calmodulin of the antipsychotic drug chlorpromazine in the presence of Ca(2+). In addition, when a calmodulin-deficient PSD preparation was prepared (26), sonicated, and incubated with [gamma-(32)P]ATP, Mg(2+) and Ca(2+), one could not induce a Ca(2+)-stimulation of protein kinase activity unless exogenous calmodulin was added back to the system, indicating a reconstitution of calmodulin into the PSD. We have also attempted to identify the two major phosphorylated proteins. Based on SDS polyacrylamide gel electrophoresis, it appears that the major 51,000 M(r) PSD protein is the one that is phosphorylated and not the 51,000 M(r) component of brain intermediate filaments, which is a known PSD contaminant. In addition, papain digestion of the 51,000 M(r) protein revealed multiple phosphorylation sites different from those phosphorylated by the Mg(2+)-dependent kinase(s). Finally, although the calmodulin-activatable protein kinase may phosphorylate proteins I(a) and I(b), the cyclic AMP-dependent protein kinase, which definitely does phosphorylate protein I(a) and I(b) and is present in the PSD, does not phosphorylate the 51,000 and 62,000 M(r) proteins, because specific inhibition of this kinase has no effect on the levels of the phosphorylation of these latter two proteins.

In vivo phosphorylation following [32P]orthophosphate injection into neostriatum or hippocampus: selective and rapid labeling of electrophoretically separated brain proteins

Intracranial injections of [32P]orthophosphate readily label a number of brain phosphoproteins as resolved by polyacrylamide gel electrophoresis. The majority of these in vivo labeled phosphoproteins co-migrate with phosphoproteins that are labeled in vitro by incubation of brain membranes with [32P]ATP. Two of the major in vitro labeled phosphoproteins with apparent molecular weights of 47,000 (band F1) and 41,000 (band F2) are rapidly labeled in vivo. Since they are rapidly dephosphorylated in vitro, this suggests a high rate of phosphate turnover. The electrophoretic pattern of in vivo labeled phosphoproteins did not appear to be altered by the method of sacrifice (focused microwave irradiation, decapitation or liquid nitrogen immersion) or by the state of the animal at the time of labeling (awake or lightly anesthetized with pentobarbital). The reduction of phosphatase activity during tissue processing at 0 degree C may account for the similarities observed with different sacrifice methods. Removal of phospholipids or polynucleotides had little effect on the in vivo labeled 32P-containing bands. However, alkaline hydrolysis or protease treatment uniformly reduced the radioactivity in the labeled bands. These findings suggest that the 32P-containing bands consist of phosphoester linkages to serine or threonine residues. The present evidence emphasizes that previously characterized in vitro labeled brain phosphoproteins are, in fact, labeled in the awake, freely-moving animal.

Serotonin stimulates phosphorylation of protein I in the facial motor nucleus of rat brain

Protein I is one of the best candidates for a neuronal protein whose phosphorylation may have a functional role in synaptic activity. It is a substrate for both cyclic AMP-dependent and protein kinases, and these kinases show differential specificity for its multiple phosphorylation sites. Protein I is found exclusively in the central and peripheral nervous systems, and immunohistochemical and subcellular fractionation studies suggest an association primarily with synaptic vesicles. Using slices of rat cerebral cortex incubated in vitro, Protein I was phosphorylated both by agents which increase intracellular cyclic AMP and by agents causing Ca2+ influx, although not by any putative neurotransmitters or neuromodulators. We have now examined the facial motor nucleus and report here that serotonin produces a phosphorylation of Protein I when incubated with facial nucleus slices. Demonstration of a neurotransmitter-dependent alteration in the state of phosphorylation of a synapse-specific protein may be due to the relatively simple neuronal circuitry within the facial motor nucleus.