Identification of glycoproteins and proteins at synapses in the central nervous system

Synaptic plasma membranes, synaptic junctions, and postsynaptic densities have been isolated from rat brain, the proteins resolved by polyacrylamide gel electrophoresis, and the glycoproteins identified. The synaptic junction is composed of a spectrum of polypeptides which range in Mr from 13,000 to 250,000. The overall pattern is similar to synaptic plasma memranes; however, the relative proportions of the polypeptides are distinctive. The postsynaptic density fraction consists primarily of one band with an Mr of 52,000. Polypeptides with an Mr of 55,000, and another five of higher Mr, make up the remaining protein. The polypeptides of the postsynaptic density fraction must be reduced with mercaptoethanol in order to permeate the polyacrylamide gel. Therefore, postsynaptic density proteins are cross-linked by disulfide bonds into supramacromolecular aggregates. Glycoproteins which bind concanavalin A were identified in synaptic junctions by studying the binding of 123I-concanavalin A directly to the polypeptides resolved on the polyacrylamide gels. Only four bands, each with an Mr greater than 95,000, bind concanavalin A. In contrast, the pattern of concanavalin A-binding polypeptides in synaptic plasma membranes is distinctive and more complex. In the postsynaptic density fraction, most of the concanavalin A binding occurs to a glyco-component which migrates at the dye front. These data, together with previous cytochemical data using concanavalin A-ferritin conjugates, indicate a limited and select group of high Mr concanavalin A glycoproteins resides within the synaptic cleft of asymmetric type synapses. Whereas a select group of polypeptides bind concanavalin A, all polypeptides resolved in the synaptic junction fraction are glycoproteins and contain galactosyl or galactosyl-like residues, since they label with tritiated borohydride following galactose oxidase treatment. This suggests that the carbohydrate composition of individual glycoproteins is different.