Putative 51,000-Mr protein marker for postsynaptic densities is virtually absent in cerebellum

Cerebrum and cerebellum contain numerous asymmetric synapses characterized by the presence of a postsynaptic thickening prominently stained by phosphotungstic acid and other electron-dense stains suitable for electron microscopy. A 51,000-Mr protein, copurified in postsynaptic density-enriched fractions from cerebrum, is considered to be a well established marker for the postsynaptic density. On the basis of two criteria, our studies demonstrate that the 51,000-Mr protein marker for postsynaptic densities is virtually absent in cerebellum, First, it is present in negligible amounts in deoxycholate-insoluble fractions from cerebellum but abundant in parallel fractions from cerebrum. Secondly, the 51,000-Mr protein, which binds 125I-calmodulin after SDS PAGE is readily visualized in membrane samples from cerebrum but is virtually undetectable in cerebellar samples. It is apparent that these results require reexamination of the role of the 51,000-Mr protein in postsynaptic density structures.

Structural heterogeneity and subcellular distribution of nicotinic synapse-associated proteins

Peripheral membrane proteins (Mr = 43,000) are associated with Torpedo membranes highly enriched in nicotinic receptor. These 43,000-dalton proteins are not required for ion translocation or other known receptor functions, but they have been implicated in constraint of the nicotinic receptor within the plane of the membrane bilayer. Sodium dodecyl sulfate-polyacrylamide electrophoresis allows partial resolution of the 43,000-dalton band into a doublet. We have carried out further analysis using two-dimensional gel electrophoresis, which reveals the existence of at least seven Coomassie blue-staining spots in the isoelectric focusing dimension. Peptide maps of the individual spots serve to elucidate the observed electrophoretic complexity. Three different membrane-bound proteins, designated v1, v2, and v3, were identified on the basis of their characteristic peptide maps which show no apparent homology in amino acid sequence. Two of these proteins, v1 and v2, are resolved into multiple spots in the isoelectric focusing dimension, but each group of isoelectric focusing variants has nearly identical peptide fingerprints. Of relevance to the putative role of these proteins in synaptic or receptor supramolecular structures is the observation that only v1 is exclusively membrane bound, and co-purifies with receptor whereas both v2 and v3 are also prominent proteins of the cytoplasm and are depleted from membrane fractions most enriched in receptor. These proteins may interact in the formation or maintenance of synaptic and nicotinic receptor supramolecular structures.

Affinity partitioning of membranes. Evidence for discrete membrane domains containing cholinergic receptor

Subsynaptic membrane domains from Torpedo californica electroplax contain nicotinic cholinergic receptor molecules at densities as high as 20,000 micrometers-2. Intense homogenization of the electroplax releases membrane fragments enriched in nicotinic receptor from basal lamina and other synaptic cleft and presynaptic elements. Ideally, preparations of membrane fragments, highly enriched in nicotinic receptor, should approach 125I-alpha-bungarotoxin-specific binding activities near the levels observed after receptor dispersal in detergents and subsequent affinity chromatography. We report the application of affinity partitioning, combined with multiple extraction techniques, to yield preparations of virtually homogeneous membranes enriched in nicotinic receptor alpha, beta, gamma, and delta subunits as well as the 43,000-dalton peripheral protein subunit. The countercurrent distribution technique serves to resolve three populations of receptor-containing membranes. One fraction is refractory to affinity partitioning and may represent aggregates of receptor-rich membranes with fragments derived from nonsynaptic membranes. The second and third fractions contain membrane fragments derived from the subsynaptic membrane and are highly enriched in nicotinic receptor (5.1 to 7.8 nmol of alpha-bungarotoxin binding sites/mg of protein). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of all three fractions indicates that alpha, beta, and gamma subunits are present in stable stoichiometric ratios, while the 43,000-dalton peripheral protein content varies by 33% between the fractions. However, removal of 90% of the 43,000-dalton component by mild alkali treatment does not result in conversion of one fraction into the other. The combination of affinity partitioning and counter-current distribution techniques utilized in this study should prove useful in the resolution of a variety of subcellular particles that contain specific binding sites.

An 125I-labeled binding probe for the muscarinic cholinergic receptor

We describe the synthesis and characterization of [125I]3-quinuclidinyl-(3-iodo-4-hydroxy-benzilate), a binding probe for the muscarinic cholinergic receptor of high specific radioactivity. The binding isotherm of this 125I-labeled compound to a rat synaptic plasma membrane-enriched fraction consists of two components: a 'specific' component which is saturable and closely fits hyperbolic binding to a single class of sites as evaluated by Scatchard analysis (Kd = 1.5 to 3 nM), and a linear component which may be measured directly by preincubating membranes in 0.1 microM 3-quinuclidinyl-benzilate, the most potent muscarinic antagonist known. The specific binding of [3-3H]3-quinuclidinyl-benzilate and [125I]3-quinuclidinyl-(3-iodo-4-hydroxy-benzilate) to rat brain subcellular fractions is parallel in myelin, synaptic plasma membrane and mitochondrial fractions with a 3--4-fold enrichment observed in synaptic plasma membrane over crude mitochondrial fractions. The concentrations of muscarinic antagonists required to block [125I]3-quinuclidinyl-(3-iodo-4-hydroxy-benzilate) binding parallel that reported for tritiated binding probes and are consistent with physiological measurements of their dissociation constants. Because of the high specific radioactivity of [125I]3-quinuclidinyl-(3-iodo-4-hydroxy-benzilate), this iodinated binding probe should prove useful in assaying low levels of muscarinic receptor in tissue culture and other biological sources.

Affinity partitioning of membranes. Cholinergic receptor-containing membranes from Torpedo californica

Affinity partitioning has been employed in the purification of membranes rich in cholinergic receptor from Torpedo californica electric organs. The procedure involves a modification of poly(ethylene oxide)-dextran aqueous phase partitioning systems where a ligand selective for the receptor is conjugated to the poly(ethylene oxide). Specific partitioning of the receptor-containing membranes into the poly(ethylene oxide)-rich phase occurs when bis-alpha,omega-trimethylamino poly(ethylene oxide) or bis-rho-tri-methylammonium phenylamino poly(ethylene oxide) was added to the phase system in low mole ratio. bis-alpha,omega-Methylamino poly(ethylene oxide), which should impart equivalent interfacial electromotive potential to the system but bind poorly to the receptor sites, was much less effective in producing phase distribution changes. The ligand-polymer-dependent phase distribution shifts were blocked by bisquaternary methonium ligands at concentrations consistent with their relative affinities for the cholinergic receptor. Titration or receptor sites with cobra alpha-toxin decreased the phase distribution changes in a linear fashion up to the point of stoichiometry. These observations are consistent with the phase distribution changes being consequent to ligand-polymer association with the pharmacologically important site on the receptor. The affinity partitioning procedure, when employed following an initial purification of the membranes by differential and density gradient centrifugation, yields membrane preparations with a high degree of morphological uniformity and a specific activity between 2.9 and 4.6 nmol of bound cobra alpha-toxin/mg of protein.

Affinity partitioning. A method for purification of proteins using specific polymer-ligands in aqueous polymer two-phase systems

We describe a method, called affinity partitioning, for the purification of proteins containing specific ligand binding receptor sites. This method adds specificity to the procedures for protein purification with aqueous polymer two-phase systems by introduction of a polymer derivative, coupled to an appropriate ligand. The addition of a polymer-ligand that partitions predominantly into one phase shifts the protein that binds this substance to the same phase. By performing countercurrent distribution in the presence of a polymer-ligand, the protein that binds the polymer-ligand can be separated from a heterogenous mixture. One example of affinity paritioning used dextran as the polymer-ligand. Dextran was chosen since it is a constituent of the most commonly used system for partitioning proteins. In a dextran-poly(ethylene oxide) system, concanavalin A bound dextran and partitioned predominantly into the dextran-rich phase. The addition of the specific competitor, D-mannose, displaced the partition coefficient toward unity, while the application of L-fucose, a noncompetitor, had little effect. Application of affinity partitioning to the purification of another protein required the synthesis of a specific polymer-ligand. To study this we synthesized dinitrophenyl-poly-(ethylene oxide), which binds specifically to S-23 myeloma protein. Addition of dinitrophenyl-poly(ethylene oxide) to the dextran-poly(ethylene oxide) phase system shifted the S-23 myeloma protein into the poly(ethylene oxide)-rich phase. epsilon-N-dinitrophenyl-L-lysine, by competing with binding of dinitrophenyl-poly(ethylene oxide), antagonized the latter's effect on the partition coefficient of S-23 myeloma protein. By adding various amounts of dinitrophenyl-poly-(ethylene oxide), we correlated the partition coefficient with concentration of polymer-ligand. A model of the action of polymer-ligand derivatives on the partition coefficient, derived from thermodynamic considerations, was found to be consistent with the experimental data relating the concentration of polymer-ligand and partition coefficient. Affinity partitioning should prove to be a useful complement to affinity chromatography in the purification of mixtures of proteins. Since cells and subcellular particles may be purified with aqueous polymer two-phase systems, affinity partitioning might be applied to their fractionation by using polymer-ligands specific for unique surface receptors.