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

Enhanced tyrosine phosphorylation of the 2B subunit of the N-methyl-D-aspartate receptor in long-term potentiation

Both serine/threonine and tyrosine phosphorylation of receptor proteins have been implicated in the process of long-term potentiation (LTP), but there has been no direct demonstration of a change in receptor phosphorylation after LTP induction. We show that, after induction of LTP in the dentate gyrus of anesthetized adult rats, there is an increase in the tyrosine phosphorylation of the 2B subunit of the N-methyl-D-aspartate (NMDA) receptor (NR2B), as well as several other unidentified proteins. Tyrosine phosphorylation of NR2B was measured in two ways: binding of antiphosphotyrosine antibodies (PY20) to glycoprotein(s) of 180 kDa (GP180) purified on Con A-Sepharose and binding of anti-NR2B antibodies to tyrosine-phosphorylated proteins purified on PY20-agarose. Three hours after LTP induction, anti-NR2B binding to tyrosine phosphorylated proteins, expressed as a ratio of tetanized to control dentate (Tet/Con), was 2.21 +/- 0.50 and PY20 binding to GP180 was 1.68 +/- 0.16. This increase in the number of tyrosine phosphorylated NR2B subunits occurred without a change in the total number of NR2B subunits. When the induction of LTP was blocked by pretreatment of the animal with the NMDA receptor antagonist MK801, the increase in PY20 binding to GP180 was also blocked (Tet/Con = 1.09 +/- 0.26). The increased PY20 binding to GP180 was also apparent 15 min after LTP induction (Tet/Con = 1.41 +/- 0.16) but not detectable 5 min after LTP induction (Tet/Con = 1.01 +/- 0.19). These results suggest that tyrosine phosphorylation of the NMDA receptor contributes to the maintenance of LTP.

Molecular characterisation and structural relationship of the synapse-enriched glycoproteins gp65 and gp55

gp65 and gp55 are glycoprotein components of CNS synapses that are recognised by a single monoclonal antibody, SMgp65. This antibody has now been used to investigate the molecular properties of these two glycoproteins and the structural relationship between them. Both gp65 and gp55 occur in most brain regions as doublets of apparent molecular masses of 63 and 67 kDa, and 52 and 57 kDa, respectively. Striatal samples, however, are enriched in a novel gp65 isoform of 69 kDa. Removal of oligosaccharide residues from gp65 and gp55 with trifluoromethanesulphonic acid shows that gp65 and gp55 are composed of single polypeptide chains of 40 and 28 kDa, respectively. Removal of sialic acid residues with neuraminidase lowers the apparent molecular mass of both glycoproteins by 5-6 kDa. Triton X-114 phase partitioning and alkaline extraction of synaptic membranes indicate that both gp65 and gp55 are integral membrane glycoproteins. Treatment of synaptic membranes with phosphatidylinositol-specific phospholipase C does not solubilise either glycoprotein. One-dimensional peptide and epitope maps obtained by digestion of gp65 and gp55 with endoproteinase lys C or subtilisin are consistent with a close structural relationship between the two molecules. Tryptic digestion of samples enriched in gp65 and/or gp55 results in the formation of a novel immunoreactive 53-kDa species that is resistant to further trypsin degradation except in the presence of 0.1% (wt/vol) sodium dodecyl sulphate. Trypsin treatment of cultures of forebrain neurones in situ lowers the apparent molecular mass of gp65 to 53 kDa.(ABSTRACT TRUNCATED AT 250 WORDS)

Calmodulin-dependent protein kinase II. Multifunctional roles in neuronal differentiation and synaptic plasticity

One of the most important mechanisms for regulating neuronal functions is through second messenger cascades that control protein kinases and the subsequent phosphorylation of substrate proteins. Ca2+/calmodulin-dependent protein kinase II (CaM-kinase II) is the most abundant protein kinase in mammalian brain tissues, and the alpha-subunit of this kinase is the major protein and enzymatic molecule of synaptic junctions in many brain regions. CaM-kinase II regulates itself through a complex autophosphorylation mechanism whereby it becomes calcium-independent following its initial activation. This property has implicated CaM-kinase II as a potential molecular switch at central nervous system (CNS) synapses. Recent studies have suggested that CaM-kinase II is involved in many diverse phenomena such as epilepsy, sensory deprivation, ischemia, synapse formation, synaptic transmission, long-term potentiation, learning, and memory. During brain development, the expression of CaM-kinase II at both protein and mRNA levels coincides with the active periods of synapse formation and, therefore, factors regulating the genes encoding kinase subunits may play a role in the cell-to-cell recognition events that underlie neuronal differentiation and the establishment of mature synaptic functions. Recent findings have demonstrated that the mRNA encoding the alpha-subunit of CaM-kinase II is localized in neuronal dendrites. Current speculation suggests that the localized translation of dendritic mRNAs encoding specific synaptic proteins may be responsible for producing synapse-specific changes associated with the processing, storage, and retrieval of information in neural networks.

Electroconvulsive treatment induces a rapid and transient increase in tyrosine phosphorylation of a 40-kilodalton protein associated with microtubule-associated protein 2 kinase activity

Recent studies have identified protein tyrosine phosphorylation as a major intracellular signaling pathway. However, little is known about regulation of this signaling pathway in neuronal systems. To help identify changes in levels of protein tyrosine phosphorylation in brain, we have utilized specific anti-phosphotyrosine antibodies to detect phosphotyrosine-containing proteins by immunoblotting techniques. We have found that electroconvulsive treatment induces a selective increase in tyrosine phosphorylation of a soluble 40-kDa protein. The rise is rapid and transient, reaching maximal levels at 1-2 min and returning to basal levels by 8 min. The phosphotyrosine-containing 40-kDa protein is most prominent in hippocampus, smaller in neocortex, and not detected in brainstem or cerebellum. A phosphotyrosine-containing 42-kDa protein present in several cell types has recently been identified as a serine/threonine phosphotransferase, referred to as microtubule-associated protein 2 kinase. Comparison of the levels of tyrosine phosphorylation of the 40-kDa protein and microtubule-associated protein 2 kinase activity during column chromatography of hippocampal extracts demonstrates that the phosphotyrosine-containing 40-kDa protein and microtubule-associated protein 2 co-purify. Moreover, the tyrosine phosphorylation of the 40-kDa protein and microtubule-associated protein 2 kinase activity are increased to a similar extent following electroconvulsive treatment. These findings suggest that the phosphotyrosine-containing 40-kDa protein identified in brain is closely related to microtubule-associated protein 2 kinase.

Synaptic protein tyrosine kinase: partial characterization and identification of endogenous substrates

The subcellular distribution of protein tyrosine kinase in rat forebrain was determined using [Val5]-angiotensin II as exogenous substrate. Enzyme activity was present in each of the fractions analyzed and was enriched in synaptic membranes (SMs) and the synaptosomal soluble fraction (2.2- and 2.5-fold over the homogenate, respectively). SMs also phosphorylated polyglutamyltyrosine (pGT; molar ratio of 4:1), the Vmax for angiotensin and pGT phosphorylation being 26.3 +/- 1.6 and 142 +/- 4 pmol/min/mg, respectively. Extraction of SMs with several different detergents resulted in enhanced enzyme activity and the solubilization of 33-37% of the angiotensin and 43-70% of the pGT-phosphorylating activity. Isolated postsynaptic densities (PSDs) contained tyrosine kinase and phosphorylated angiotensin and pGT. The Vmax values for angiotensin and pGT phosphorylation by PSDs were 17 +/- 5 and 23 +/- 1 pmol/min/mg, respectively. Six putative endogenous substrates for SM tyrosine kinase, with molecular weights of 205K, 180K, 76K, 60K, 50K, and 45K, were identified. Each of these proteins, except p76, was phosphorylated in the detergent-insoluble residue obtained following the extraction of SMs with Triton X-100 as well as in PSDs, indicating that the postsynaptic apparatus is an active site of tyrosine phosphorylation. The phosphorylation of p76 was localized to the Triton X-100 extract and also occurred in the synaptosomal soluble fraction. The results indicate that tyrosine kinase and its substrates are located in both pre- and postsynaptic compartments and suggest a role for this enzyme in synaptic function.

The binding of a monoclonal antibody reactive with pp60v-src to the rat CNS both in vitro and in vivo: evidence that the epitope is present intracellularly as well as being associated with a number of antigenically related polypeptides located externally in the plasma membrane only in the synaptic region

A monoclonal antibody, F4, has been produced which reacts with an epitope possessing an unusual subcellular distribution. It binds to the external surface of the neuronal plasma membrane only in the region of the synapse. This is evidenced by binding of F4 to presynaptic terminals in unfixed cultures of rat cerebellum and to preparations of unfixed synaptosomes. In addition, much larger amounts of the epitope are present intracellularly. In fixed nervous tissue, the epitope is found in many neurons, and is associated mainly with presynaptic plasma membranes, synaptic vesicles, postsynaptic densities (cerebral cortex and hippocampus, but not cerebellum), rough endoplasmic reticulum, and the Golgi apparatus. The epitope is especially abundant in large neurons (e.g. pyramidal cells). Similar amounts of epitope are present in the chromaffin cells of the adrenal medulla. It is also expressed in ependymal cells in the brain, and in epithelial cells present in ducts of the medulla, but not cortex, of the kidney. However, the epitope is not found in glial cells in the brain, or in either liver, spleen, skeletal muscle, or testes. F4 is not species specific, as it binds to postmortem adult human cerebral cortex and neonatal cerebellum in a manner as described for the rat. It also binds to homogenates of brains of fish, chicken and mouse. The appearance of the epitope during development of the cerebellum in vivo and in vitro occurs in parallel with the differentiation of neurons and formation of synapses, though small amounts are also present in neuronal precursor cells. The F4 antibody can detect nanogram amounts of pp60v-src on immunodots. The strength of this reaction is high enough that F4 can be used to demonstrate pp60v-src-like immunoreactivity in Rous Sarcoma virus-transformed chick embryo fibroblasts. However, present evidence suggests that it may be premature to assign the immunocytochemical reactivity of F4 in the brain exclusively to pp60c-src. This conclusion is based on the fact that F4 reacts with several polypeptides from synaptic plasma membranes on Western blots of renaturing, two-dimensional gels that are dissimilar in size to pp60c-src, and from the fact that it can cross-react, albeit weakly, with several other serine protein kinases in an immunodot assay. Appreciation of this cross-reactivity, and of the evolutionary conservation of the epitope, as well as its sensitivity to denaturation, has led to our working hypothesis that F4 binds to a conformational epitope present on several polypeptides that may be most perfectly represented by some aspect of the catalytic domain of tyrosine protein kinases.

Developmental changes in calmodulin-kinase II activity at brain synaptic junctions: alterations in holoenzyme composition

Synaptic junctions (SJs) from rat forebrain were isolated at increasing postnatal ages and examined for endogenous protein kinase activities. Our studies focused on the postnatal maturation of the multifunctional protein kinase designated Ca2+/calmodulin-dependent protein kinase II (CaM-kinase II). This kinase is comprised of a major 50-kilodalton (kDa) and a minor 60-kDa subunit. Experiments examined the developmental properties of CaM-kinase II associated with synaptic plasma membranes (SPMs) and synaptic junctions (SJs), as well as the holoenzyme purified from cytosolic extracts. Large developmental increases in CaM-kinase II activity of SJ fractions were observed between postnatal days 6 and 20; developmental changes were examined for a number of properties including (a) autophosphorylation, (b) endogenous substrate phosphorylation, (c) exogenous substrate phosphorylation, and (d) immunoreactivity. Results demonstrated that forebrain CaM-kinase II undergoes a striking age-dependent change in subunit composition. In early postnatal forebrain the 60-kDa subunit constitutes the major catalytic and immunoreactive subunit of the holoenzyme. The major peak of CaM-kinase II activity in SJ fractions occurred at approximately postnatal day 20, a time near the end of the most active period of in vivo synapse formation. Following this developmental age, CaM-kinase II continued to accumulate at SJs; however, its activity was not as highly activated by Ca2+ plus calmodulin.

Involvement of sialic acid in high-affinity uptake of dopamine by synaptosomes from rat brain

The high-affinity, sodium-dependent uptake of dopamine (DA) was inhibited by the pretreatment of synaptosomes with neuraminidase from Vibrio cholerae. The inhibition was of a non-competitive type, resulting in a 40% decrease of Vmax. Neither basal nor depolarization-stimulated release of DA was affected. Treatment of synaptosomes with neuraminidase caused a 48% loss of sialic acid from the lipid-bound pool and a 80% decrease in the protein-bound fraction. The inhibition of DA uptake was found to be related linearly to the loss of sialic acid from the protein pool. It is postulated that a sialic acid moiety is involved in DA transport across the synaptosomal membrane.

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)

Characterization of fucosyl oligosaccharides associated with synaptic membrane and synaptic junctional glycoproteins

Rats were administered [3H]fucose by intracranial injection and synaptic membranes (SMs) isolated 18 h later. Oligosaccharides associated with SM glycoproteins were prepared by hydrazinolysis and analyzed by a combination of affinity chromatography on concanavalin A (Con A)-agarose, ion-exchange chromatography on DEAE-cellulose, and gel permeation chromatography. Most (94%) of the [3H]fucose-labelled oligosaccharides were present in the fraction that did not bind to Con A. Of these 41% did not bind to DEAE-cellulose, indicating the absence of negatively charged groups and the remainder were resolved into four fractions of increasing acidity. Gel permeation chromatography of the fractions from the DEAE-cellulose column suggested that the major oligosaccharides corresponded to fucosylated triantennary structures containing varying amounts of sialic acid although more highly branched structures containing peripheral branches lacking one or more sugars may also have been present. Comparison of fucosyl oligosaccharides associated with SMs prepared from 10- and 28-day-old animals indicated that although the general oligosaccharide content was similar at both ages, membranes from younger animals were characterized by an increase in the proportion of highly acidic structures. Fucosylated glycans derived from synaptic junctional (SJ) glycoproteins were also characterized by a greater percentage of highly acidic structures than SMs. The results indicate that SMs and SJs are characterized by specific complements of fucosylated glycoprotein oligosaccharides.

Characterization of gp 50, a major glycoprotein present in rat brain synaptic membranes, with a monoclonal antibody

Several cell lines secreting monoclonal antibodies (Mabs) against a major forebrain synaptic membrane (SM) glycoprotein, gp 50, have been raised. Western blots show that the Mabs react with a polypeptide doublet of Mrs 49 and 45 kDa. These polypeptides exist solely in a concanavalin A (Con A) binding form. Removal of the Con A receptors by digestion with endo-beta-N-acetylglucosaminidase H (endo H) lowers the Mrs of the glycoprotein doublet to 36.5 and 34 kDa. Western blots of 2D polyacrylamide gels indicate that gp 50 exists in several isoforms. Solid phase radioimmunoassay (RIA) and Western blots of brain subcellular fractions show the antigenic material to be concentrated in the SM fraction, but to be present in much lower amounts in synaptic junctions and postsynaptic densities. Gp 50 appears to be brain specific. Regional distribution studies show that it is present in all brain regions but is two-fold concentrated in cerebellum, brainstem and midbrain compared to forebrain. Immunocytochemical studies of several brain regions show that gp 50-like immunoreactivity is neuron specific and is concentrated in selected neuronal species, particularly granule cells. In both cerebellar and hippocampal granule cells gp 50-like immunoreactivity is localized in the perikarya and primary dendrites. Though immunocytochemistry did not show staining of synaptic regions this may be due to masking of the reactive epitope. The results are discussed in terms of the molecular properties of gp 50 and its subcellular localization in brain tissue.

Characterization of Ca2+/calmodulin-dependent protein kinase associated with rat cerebral synaptic junction: substrate specificity and effect of autophosphorylation

The Ca2+/calmodulin (CaM)-dependent protein kinase associated with rat cerebral synaptic junction (SJ) was characterized, using the SJ fraction as the enzyme preparation, to clarify the functional significance of the enzyme in situ. The protein kinase was greatly activated in the presence of micromolar concentrations of both Ca2+ and calmodulin (EC50 for Ca2+, 1.0 microM; that for CaM, 100 nM). The Km for ATP was 150 microM. SJ proteins were phosphorylated without a lag time, and the phosphorylation reached its maximum within 2-10 min at 25 degrees C. The endogenous substrates consisted of four major (160K, 120K, 60K, and 51K Mr) and 10 minor proteins. Compared with the endogenous substrate phosphorylation, the phosphorylation of exogenously added proteins (myosin light chains from chicken muscle, casein, arginine-rich histone, microtubule-associated protein-2, tau-protein, and tubulin) was weak, although they are expected to be good substrates for the soluble form of the Ca2+/CaM-dependent protein kinase. Autophosphorylation of the enzyme in SJ inhibited its activity and did not alter the subcellular distribution of the enzyme.

Involvement of fodrin-binding proteins in the structure of the neuronal postsynaptic density and regulation by phosphorylation

Novel polypeptides with Mr values about 140,000 bind fodrin and spectrin and are enriched in the postsynaptic density (PSD) compared to other tissues or subcellular fractions. 125I-fodrin binding to these polypeptides is competed for by unlabeled spectrin. These polypeptides are distinct from ankyrin and its proteolytic fragments and from band 4.1 which also bind fodrin. Phosphorylation of PSDs by the endogenous calmodulin-dependent protein kinase markedly reduces 125I-fodrin binding to the transblotted preparation. Such an event may play a regulatory role in governing protein-protein interactions among elements of the PSD.

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.

Calmodulin-dependent protein phosphorylation in synaptic junctions

Synaptic junctions (SJs) from rat forebrain were examined for Ca2+/calmodulin (CaM)-dependent kinase activity and compared to synaptic plasma membrane (SPM) and postsynaptic density (PSD) fractions. The kinase activity in synaptic fractions was examined for its capacity to phosphorylate endogenous proteins or exogenous synapsin I, in the presence or absence of Ca2+ plus CaM. When assayed for endogenous protein phosphorylation, SJs contained approximately 25-fold greater amounts of Ca2+/CAM-dependent kinase activity than SPMs, and fivefold more activity than PSDs. When kinase activities were measured by phosphorylation of exogenous synapsin I, SJs contained fourfold more activity than SPMs, and 10-fold more than PSDs. The phosphorylation of SJ proteins of 60- and 50-kilodalton (major PSD protein) polypeptides were greatly stimulated by Ca2+/CaM; levels of phosphorylation for these proteins were 23- and 17-fold greater than basal levels, respectively. Six additional proteins whose phosphorylation was stimulated 6-15-fold by Ca2+/CAM were identified in SJs. These proteins include synapsin I, and proteins of 240, 207, 170, 140, and 54 kilodaltons. The 54-kilodalton protein is a highly phosphorylated form of the major PSD protein and the 170-kilodalton component is a cell-surface glycoprotein of the postsynaptic membrane that binds concanavalin A. The CaM-dependent kinase in SJ fractions phosphorylated endogenous phosphoproteins at serine and/or threonine residues. Ca2+-dependent phosphorylation in SJ fractions was strictly dependent on exogenous CaM, even though SJs contained substantial amounts of endogenous CaM (15 micrograms CaM/mg SJ protein). Exogenous CaM, after being functionally incorporated into SJs, was rapidly removed by sequential washings. These observations suggest that the SJ-associated CaM involved in regulating Ca2+-dependent protein phosphorylation may be in dynamic equilibrium with the cytoplasm. These findings indicate that a brain CaM-dependent kinase(s) and substrate proteins are concentrated at SJs and that CaM-dependent protein phosphorylation may play an important role in mechanisms that underlie synaptic communication.

Phosphorylation of the postsynaptic density glycoprotein gp180 by Ca2+/calmodulin-dependent protein kinase

Postsynaptic densities (PSDs) were prepared by the aqueous two-phase extraction of synaptic membranes in the presence of n-octyl glucoside. Incubation of postsynaptic densities with [gamma-32P]ATP resulted in the incorporation of 32P into a range of proteins. Isolation of glycoproteins from 32P-labelled PSDs by affinity chromatography on concanavalin A-agarose identified the postsynaptic glycoprotein of apparent Mr 180,000 (gp180) as a substrate for endogenous protein kinase(s). When the phosphorylation reaction was performed in the presence of Ca2+ and calmodulin, there was an overall 13-fold increase in the phosphorylation of PSD proteins. The largest effects of calmodulin were associated with two proteins of molecular weights 51,000 and 60,000, which showed average calmodulin-dependent increases in phosphorylation of 68-fold. The phosphorylation of gp180 was increased 7.5-fold in the presence of calmodulin. Fifty percent of maximum phosphorylation of proteins and glycoproteins occurred with a free Ca2+ concentration of 0.3 X 10(-6) M. The amounts 12.6 micrograms/ml and 9.1 micrograms/ml of calmodulin were required for 50% of maximum phosphorylation of proteins and glycoproteins, respectively. Peptide mapping experiments identified three major phosphorylation sites in gp180. The phosphorylation of all three sites was increased in the presence of calmodulin. Phosphoamino acid analysis of gp180 revealed that [32P]phosphoserine and [32P]phosphothreonine were both produced during the phosphorylation reaction, with phosphoserine being the predominant product. The phosphorylation of both amino acids was increased in the presence of calmodulin. [32P]phosphotyrosine was also identified as a product of the phosphorylation of gp180.

In vivo phosphorylation of the postsynaptic density glycoprotein gp180

Rats received intraventricular injections of [32P]PO4 and were killed after 30 min for the preparation of postsynaptic densities (PSDs). Gel electrophoretic analysis identified a number of PSD proteins that incorporated 32P under these conditions. Major proteins that were labelled with 32P had Mr of 185,000, 165,000, 140,000, 92,000, and 51,000. Of these p185, p165, and p140 were also labelled when PSDs were incubated with [gamma-32P]ATP in vitro. In contrast p92 and p51 were relatively poorly labelled under in vitro conditions. Analysis of glycoproteins isolated by chromatography on concanavalin A (Con A)-agarose demonstrated that greater than 70-80% of the 32P present in the glycoproteins eluted from Con A-agarose with alpha-methyl-D-mannopyranoside (Con A+ glycoproteins) was associated with the PSD specific glycoprotein gp180 following both in vivo and in vitro labelling. Phosphopeptide maps and phosphoamino acid analysis of gp180 indicated that similar sites were labelled in vitro and in vivo. Analysis of the subcellular distribution of glycoproteins that incorporated 32P during in vivo labelling demonstrated that gp180 was highly concentrated in PSDs, in accord with the previously suggested exclusive association of this glycoprotein with postsynaptic structures.

Phosphorylation of the postsynaptic density glycoprotein gp180 by endogenous tyrosine kinase

Incubation of postsynaptic densities (PSDs) with [gamma-32P]adenosine triphosphate (ATP) results in the phosphorylation of a number of proteins. Of these, phosphoproteins with apparent molecular weights (Mr) of 51,000, 180,000, 300,000, 320,000 and 370,000 contain 32P which is resistant to digestion with hot KOH suggesting the presence of [32P]phosphotyrosine residues. Phosphoamino acid analysis of total 32P-labelled PSDs identified [32P]phosphotyrosine as well as phosphoserine and phosphothreonine as products of the phosphorylation reaction. The PSD-specific glycoprotein gp180 was isolated from 32P-labelled PSDs and shown to contain [32P]phosphotyrosine. The results identify tyrosine kinase as a component of purified PSDs and gp180 as an endogenous substrate for this enzyme.

Autophosphorylation of Calmodulin-Kinase II in Synaptic Junctions Modulates Endogenous Kinase Activity

Previous studies have purified from brain a Ca2+/calmodulin-dependent protein kinase II (designated CaM-kinase II) that phosphorylates synapsin I, a synaptic vesicle-associated phosphoprotein. CaM-kinase II is composed of a major Mr 50K polypeptide and a minor Mr 60K polypeptide; both bind calmodulin and are phosphorylated in a Ca2+/calmodulin-dependent manner. Recent studies have demonstrated that the 50K component of CaM-kinase II and the major postsynaptic density protein (mPSDp) in brain synaptic junctions (SJs) are virtually identical and that the CaM-kinase II and SJ 60K polypeptides are highly related. In the present study the photoaffinity analog [alpha-32P]8-azido-ATP was used to demonstrate that the 60K and 50K polypeptides of SJ-associated CaM-kinase II each bind ATP in the presence of Ca2+ plus calmodulin. This result is consistent with the observation that these proteins are phosphorylated in a Ca2+/calmodulin-dependent manner. Experiments using 32P-labeled peptides obtained by limited proteolysis of 60K and 50K polypeptides from SJs demonstrated that within each kinase polypeptide the same peptide regions contain both autophosphorylation and 125I-calmodulin binding sites. These results suggested that the autophosphorylation of CaM-kinase II could regulate its capacity to bind calmodulin and, thus, its capacity to phosphorylate substrate proteins. By using 125I-calmodulin overlay techniques and sodium dodecyl sulfate-polyacrylamide gel electrophoresis we found that phosphorylated 50K and 60K CaM-kinase II polypeptides bound more calmodulin (50-70%) than did unphosphorylated kinase polypeptides. Levels of in vitro CaM-kinase II activity in SJs were measured by phosphorylation of exogenous synapsin I. SJs containing highly phosphorylated CaM-kinase II displayed greater activity in phosphorylating synapsin I (300% at 15 nM calmodulin) relative to control SJs that contained unphosphorylated CaM-kinase II. The CaM-kinase II activity in phosphorylated SJs was indistinguishable from control SJs at saturating calmodulin concentrations (300-1,000 nM). These findings show that the degree of autophosphorylation of CaM-kinase II in brain SJs modulates its in vitro activity at low and possibly physiological calmodulin concentrations; such a process may represent a mechanism of regulating this kinase's activity at CNS synapses in situ.

Evidence that a cerebellum-enriched, synaptic junction glycoprotein is related to fodrin and resists extraction with triton in a calcium-dependent manner

Subcellular fractions from rat cerebellum and other tissues were examined for the presence of a 240K glycoprotein, designated GP-A. Previous results have shown that GP-A is enriched in cerebellum synaptic junction (SJ) fractions when compared to parent synaptic plasma membrane (SPM) fractions and is not detected in forebrain SPM or SJ fractions. In the present studies, GP-A was not detected in myelin, mitochondria, purified nuclei, or cytosolic fractions from cerebellum, but was present in microsomal fractions. GP-A is partially soluble in the non-ionic detergent Triton X-100 and is completely soluble when cerebellum SPMs are treated with the ionic detergent N-lauryl sarcosinate. The solubilization of GP-A from cerebellum membranes was shown to be a function of bound calcium ions, e.g., pretreating SPMs with 100 microM-1mM Ca2+ decreased the solubility of GP-A in Triton by approximately threefold. GP-A is a major concanavalin A (Con A)-binding glycoprotein in cerebellum SJ fractions and migrates on sodium dodecyl sulfate (SDS) gels with a slower relative mobility than the 235K/230K fodrin doublet. Comparisons between purified fodrin and the 235K/230K doublet in cerebellum and forebrain synaptic fractions by two-dimensional peptide mapping indicated that they were identical. The Con A-binding property of GP-A was exploited to purify it by affinity chromatography with agarose-Con A. Peptide mapping comparisons between affinity-purified GP-A and GP-A in SPM and SJ fractions indicated that GP-A in synaptic fractions is apparently homogeneous. Peptide map comparisons between GP-A and 235K fodrin poly-peptide indicated that these two synaptic components are highly related (50% of their respective peptides are shared). The 235K fodrin polypeptide in SJs reacted with anti-fodrin antisera on Western blots; however, GP-A failed to cross-react. These observations, together with results from previous studies, indicate that GP-A is highly enriched in cerebellum compared to other neuronal and nonneural tissues. Moreover, GP-A is enriched in SJs relative to SPM fractions, is related to fodrin, and is most likely a cell-surface glycoprotein at asymmetric synapses in cerebellum. GP-A may be involved in neuronal recognition or synaptic transmission in the cerebellum. The important role of calcium in synaptic transmission, together with the decreased solubility of GP-A in Triton that results from micromolar concentrations of calcium, suggest that GP-A may play a role in stabilizing cerebellar synaptic junctions.

Developmental changes in the oligosaccharide composition of synaptic junctional glycoproteins

Synaptic junctions (SJs) were isolated from the forebrains of rats ranging in postnatal age from 10 days to greater than 1 year. SJ glycoproteins that react with Concanavalin A (Con A) were isolated by chromatography on Con A-agarose and separated by gel electrophoresis. The concentrations of the major SJ Con A binding (Con A+) glycoproteins (apparent Mr 180,000, 130,000, and 110,000) increased between 10 and 28 days, with GP180 and GP110 showing greater relative increases than GP130. Con A binding oligosaccharides associated with 10-day SJs were sensitive to digestion with endoglycosidase C11 and alpha-mannosidase, indicating that they were of the high-mannose type, as previously shown for 28-day SJs. Con A+ oligosaccharides from rats of increasing postnatal age were analyzed by chromatography on Biogel P-4. Two major oligosaccharides, containing five and eight mannose residues, were present in SJs of all ages examined. During development the ratio of man5 to man8 oligosaccharides increased, so that man5 constituted the predominant species in 28-day and adult SJs. Peptide mapping experiments showed that GP180, GP130, and GP110 were each associated with a unique polypeptide composition. Little or no change in peptide composition of the major SJ glycoproteins occurred during development.

Identification of intrinsic sialidase and sialoglycoprotein substrates in rat brain synaptic junctions

Sialidase activity associated with rat brain synaptic junctions (SJ) and synaptic membranes (SM) was determined. Both fractions released sialic acid from exogenous glycopeptides and gangliosides. SJ accounted for 5-10% of the total sialidase activity recovered from SM following extraction with Triton X-100, and the specific activity of SJ sialidase was 60% of that of the parent SM fraction. Intrinsic SJ sialidase hydrolysed 12-15% of the sialic acid associated with endogenous SJ glycoproteins. Sialic acid residues associated with SJ glycoproteins were labelled with sodium borotritide and SJ proteins fractionated by affinity chromatography on concanavalin A-agarose. SJ glycoproteins that reacted with concanavalin A (con A+ glycoproteins) accounted for 25% of the total SJ [3H]sialic acid. Intrinsic SJ sialidase hydrolysed 20% of the [3H]sialic acid associated with these glycoproteins. Each molecular weight class of con A+ glycoprotein previously shown to be a specific component of the postsynaptic apparatus contained sialic acid and was acted on by intrinsic SJ sialidase.

Concanavalin A receptors associated with rat brain synaptic junctions are high mannose-type oligosaccharides

Glycoproteins were isolated from a rat brain synaptic junction fraction by affinity chromatography on Concanavalin A-agarose. The isolated glycoproteins were digested with pronase and radiolabeled with 125I-Bolton Hunter reagent, and 125I-Concanavalin A-binding glycopeptides were isolated by chromatography on Concanavalin A-agarose. Treatment of the 125I-Concanavalin A-binding glycopeptides with either alpha-mannosidase or endo-beta-N-acetylglucosaminidase-C11 abolished their interaction with Concanavalin A. The pronase digest was reacted with endo-beta-N-acetylglucosaminidase-C11 and released oligosaccharides were reduced with NaB3H4. Following affinity chromatography on Concanavalin A-agarose, Concanavalin A-binding [3H]oligosaccharides were chromatographed on Biogel P4. Two major oligosaccharides corresponding to standard carbohydrates containing eight and five mannose residues were identified. Treatment of these oligosaccharides with alpha-mannosidase converted them to smaller saccharides having a mobility on Biogel P4 columns equal to the standard disaccharide mannose-beta-1-4-N'-acetylglucosamine. These results demonstrate that the Concanavalin A receptor activity associated with CNS synaptic junctions resides in asparagine-linked oligosaccharides of the high-mannose type.