Demonstration of the transmembrane nature of the acetylcholine receptor by labeling with anti-receptor antibodies

Antibodies raised in rabbits to Triton-solubilized, purified acetylcholine receptor from Torpedo californica were used to immunospecifically label intact T. californica electroplaque membrane vesicles attached to cover slips and oriented with the extracellular face of the synaptic membrane facing outward. Hemocyanin conjugated to Protein A was then used as a marker, making the antibody binding visible at the electron microscope level. Parallel labeling experiments were performed on vesicles attached to cover slips and sheared by sonication, leaving their cytoplasmic faces fully exposed to the labeling solution. While differences in antibody populations among different rabbits were observed, antigenic determinants of the receptor were present on both faces of the membrane, with those on the extracellular side more numerous than those on the cytoplasmic side, demonstrating the transmembrane nature of the receptor molecule.

Subcellular distribution in cerebral cortex of two proteins phosphorylated by a cAMP-dependent protein kinase

The subcellular distribution of Proteins Ia and Ib, two proteins which serve as specific substrates for protein kinases present in mammalian brain, was studied in the dog cerebral cortex. Proteins Ia and Ib were found to be most highly enriched in synaptic vesicle fractions; they were also present in postsynaptic density and synaptic membrane fractions in significant amounts. Proteins Ia and Ib present in the synaptic vesicle fraction appear to be similar, if not identical, to those present in the postsynaptic density fraction as judged by several criteria: (a) the ability to serve as substrate for cAMP-dependent protein kinase, (b) electrophoretic mobility in the presence of sodium dodecyl sulfate, (c) extractability with NH4Cl or EGTA, and (d) fragmentation to electrophoretically similar peptides by a purified Staphylococcus aureus protease. In addition, the postsynaptic density fraction has been found to contain cAMP-dependent Protein Ia and Protein Ib kinase activity. The subcellular localization of Proteins Ia and Ib suggests a role for these proteins in the physiology of the synapse.

A study of the muscarinic receptor by gel electrophoresis

1 Muscarinic receptors from the brain of rat, guinea-pig and frog have been labelled with tritiated propylbenzilylcholine mustard ([3H]-PrBCM). 2 After solubilisation and sodium dodecyl sulphate (SDS)-polycarylamide gel electrophoresis a single radiolabelled peak was seen which was completely suppressed when labelling was carried out in the presence of 10(-6) M atropine. The results suggest the presence of a single polypeptide chain with a molecular weight of similar to or approximately 80,000. 3 In the case of intestinal smooth muscle multiple labelled peaks were found. Precautions against proteolysis led to the identification of a major peak with a molecular weight in the region of 80,000.

Protein and glycoprotein composition of subsynaptosomal fractions. Implications for Exocytosis and recycling of synaptic vesicles

Highly enriched fractions of synaptic junctional complexes, synaptic vesicles, and coated vesicles were isolated from rat forebrains and compared, along with synaptosomal plasma membrane and its nonjunctional components, by discontinuous sodium dodecylsulfate-polyacrylamide gel electrophoresis. When stained for proteins with Coomassie blue, the gels all contained the same protein bands, only in different relative amounts. Mixing experiments revealed no additional bands. However, gel patterns of each fraction were not only quantitatively but also qualitatively different when stained for carbohydrate. These observations suggested that some of the protein bands may vary in their degree of glycosylation among the various synaptic fractions. Within the limits of resolution of the methods used here, these results are consistent with the morphological process of synaptic vesicle exocytosis and recycling but suggest the possibility of a reversible modification of certain membrane glycoproteins as they pass through the various membrane compartments.

Actin-like and tubulin-like proteins in synaptic junctional complexes

Synaptic junctional complexes (SJCs), isolated by a procedure which preserves presynaptic dense projections (PDPs) contain as their major component a polypeptide (P55) which comigrates with tubulin on sodium dodecyl sulfate - polyacrylamide gels and another major polypeptide (P45) which comigrates with muscle actin. We report here the characterization of P55 and P45 by two-dimensional polyacrylamide gel electrophoresis and by peptide mapping of the radioiodinated polypeptides. Our results indicate that P55 and P45 are homologous with tubulin and actin respectively, in terms of molecular weight, protein charge, and primary structure. Comparison of the sodium dodecyl sulfate - polyacrylamide gel electrophoresis patterns of SJCs and postsynaptic densities (PSDs) indicates that P55 and P45 are associated mainly with PDPs but are also found in the PSD.

14-3-2 protein in rat brain

The distribution of the 14-3-2 protein in rat brain was investigated by immuno-electron microscopy using antiserum to the protein conjugated with peroxidase. 14-3-2 was demonstrated in the nuclear membrane, the endoplasmic reticular membranes and in the plasma membrane of nerve cells. The protein was also localized to the presynaptic densities and to the pre- and postsynaptic membranes. It could not be demonstrated in the membranes of the Golgi complex, inner membrane of mitochondria or in the nucleoplasm of neurons. No 14-3-2 protein was found in astrocytes, oligodendrocytes or in non-neuroectodermal tissue elements.

Synaptosomal plasma membrane glycoproteins. II. Isolation of fucosyl-glycoproteins by affinity chromatography on the Ulex europeus lectin specific for L-fucose

After solubilization in sodium dodecyl sulphate, almost 90% of synaptosomal plasma membrane glycoproteins were separated from the bulk of synaptosomal plasma membrane proteins by sequential affinity chromatography on two immobilized lectins: concanavalin A and the Ulex europeus lectin specific for L-fucose. Four fractions were obtained and their sugar composition and electrophoretic patterns were determined. Fucosyl-glycoproteins contain more than 26% of protein and 85% of the protein-bound sugar of synaptosomal plasma membrane; hence they constitute a major class of glycoproteins in these membranes. The presence of some glucose in glycoproteins fractions obtained after affinity chromatography on the two lectins suggests that this sugar could be a structural component of some brain glycoproteins. Polyacrylamide gel electrophoresis revealed at least 28 major bands in fucosylglycoprotein fractions, and 11 in other fractions. Several of these major bands appear to contain more than one glycoprotein each. This heterogeneity appears to be mostly the result of the heterogeneity of the neuronal population in the central nervous system. Microheterogeneity of glycoprotein sugar chains and possible contamination of synaptosomal plasma membranes play, in our opinion, only a minor role.

Localization of acetylcholine receptors in central synapses

The localization of cholinergic receptors in brain synaptosomes and in synapses of the midbrain reticular formation and hypothalamic preoptic nucleus has been demonstrated by means of a horseradish peroxidase-alpha-bungarotoxin (HRP-alpha-Btx) conjugate. Only a small proportion of the total number of synapses was reactive. Axon terminals of reactive synapses contained primarily small clear vesicles, while synapses characterized by large numbers of dense core vesicles were unreactive. Toxin-binding sites were found to occur in a thickened zone of the postsynaptic surface. This procedure can be employed to study the regional distribution and localization of nicotinic receptor sites in the central nervous system.

Adenosine 3':5'-monophosphate-regulated phosphoprotein system of neuronal membranes. I. Solubilization, purification, and some properties of an endogenous phosphoprotein

An endogenous substrate for adenosine 3':5'-monophosphate-dependent protein kinase has been solubilized, and purified about 5,000-fold to apparent homogeneity, from a particulate fraction of bovine cerebral cortex enriched in synaptic membranes. This endogenous substrate, referred to as Protein I, is apparently specific to nervous tissue, and is composed of two types of polypeptides, present in a proportion of 1 (Protein Ia, 86,000 daltons) to 2 (Protein Ib, 80,000 daltons). In the presence of cAMP-dependent Protein I kinase purified from the same membrane fractions, Proteins Ia and Ib incorporated 0.83 and 0.81 mol of phosphate into serine/mol of peptide, respectively. Proteins Ia and Ib have similar amino acid compositions and have isoelectric points of 10.3 and 10.2, respectively. Both types of polypeptide have a relatively high content of glycine and proline, and both are degraded to a peptide of 48,000 daltons by highly purified collagenase, suggesting that Proteins Ia and Ib contain some sequences similar to those observed in collagen. The sedimentation coefficient of Protein Ia and Protein Ib was determined to be 2.9 S. The data suggest that both Protein Ia and Protein Ib have an elongated shape.

Membrane associated particles of the presynaptic active zone in rat spinal cord. A morphometric analysis

The distribution of membrane-associated particles in the presynaptic membrane was investigated in the spinal cord of unanesthetized and anesthetized rats by freeze etching electron microscopy. Both 'external' face (EF) and 'protoplasmic' face (PF) were examined. Particles were classified according to size in two categories: small particles (diameter 5.0-8.7 nm) and large particles (diameter 8.7-13.7 nm). The presynaptic region was subdivided into an active and a surrounding zone, depending on specific ultrastructural criteria. The density of large particles in the PF was found to be significantly higher in the active as compared with the surrounding zone in both unanesthetized and anesthetized rats. Thus, the presence of large particles represents an important feature of the active zone. Considerably more large particles were found in the waking than in the barbiturized state. This difference is paralleled by a vast increase of vesicle attachment sites in the presynaptic membrane of unanesthetized animals reported by Streit et al. and confirmed in the present study. It is suggested that the two phenomena could be interrelated and that the large particles may represent calcium channels and thus provide the morphological substrate for the mechanism of excitation-secretion coupling.

Synaptic membrane protein D2 in the cerebrospinal fluid of manic-melancholic patients

The synaptic membrane protein D2 was measured in the cerebrospinal fluid (CSF) of manic-melancholic patients. The concentration of D2 increased with the age of the patients until about 35 years of age. No difference was found between the D2-concentration in CSF from a control group compared with different manic-melancholic subgroups. The D2-concentration in CSF collected from the patients during depression or mania was compared with CSF collected from the same patients when their moods were normalized. In the case of the depressed patients, we found that the D2-concentration increased slightly when the mood was normalized.

The buoyant density of synaptic plasma membranes from the cerebral cortex of neonatal rats

A fraction enriched in synaptic plasma membranes was prepared from neonatal (5-6 day old) rat cerebral cortex. The procedure was based on a method used to prepare synaptic plasma membranes from adult cerebral cortex. Critical steps were monitored by electron microscopy. Synaptic plasma membranes from neonatal cerebral cortex sedimented as a broad peak between 0.9 M and 1.1 M sucrose. In contrast, the majority of adult synaptic plasma membranes have been reported to sediment to 1.2 M sucorose. The activities of various enzyme markers were determined in subfractions of neonatal preparations in order to estimate contamination. The specific activities of these markers indicated substantial contamination of the neonatal synaptic plasma membrane fractions by microsomes and glia.

Effects of electroshock and drugs administered in vivo on protein kinase activity in rat brain

The effect of electroshock and treatment with reserpine, amphetamine or lithium chloride on protein kinase activity in synaptic membrane fragments prepared from rat brain was investigated. Naive rats subjected to electroshock procedures showed significant increases in both basal and cyclic AMP-stimulated activity irrespective of whether the treatment was sham, acute or chronic. These increases did not occur in animals which had been tamed by daily handling for 15 days prior to treatment, suggesting that the response was induced by the stress of an unfamiliar situation. Administration of lithium chloride and reserpine caused a small but significant increase in the stimulated activity. Doses of d-amphetamine of 5 mg/kg had no effect on either basal or stimulated activity, but higher doses (up to 15 mg/kg) resulted in a pronounced increase in both activities, which may have been related to drug-induced stress.