Localization and Characterization of Concanavalin a Receptors in the Synaptic Cleft

At the synapse the specialized preand postsynaptic membranes are connected via a structure called the synaptic junction. In order to understand the nature of the synaptic junction or connection, it is necessary to determine which molecules are associated with the surfaces of the specialized preand postsynaptic membranes and which are present in the cleft itself. Cytochemical evidence indicates the possible presence of carbohydrates in the cleft. The cleft area stains with ruthenium red and periodic acid-Schiff stain (Rambourg and Leblond, 1969; Bondareff, 1967). Recently Matus et al. (1973) suggested that concanavalin A (Con A) binding sites may be present in the synaptic cleft. Since Con A binds to mannopyranosyl or glucopyranosyl receptors, the cleft would appear to contain these sugars or closely related molecules. At present it is not known whether Con A sites are soluble or membrane bound and nothing is known about the type of molecule to which the lectin binds. In this paper we describe the organization of Con A binding sites in the synaptic cleft of isolated synaptosomes and synaptic junctional complexes (SJC) by the use of ferritin-Con A conjugates and quantitate .the binding and nature of the receptors by the use of radioactive Con A. We confirm the observations of Matus et al. (1973) and demonstrate that within the cleft a portion of the Con A binding sites are present on the external surface of the postsynaptic membrane overlying the postsynaptic density (PSD). These sites appear to be mainly glycoproteins. In contrast to the surface membrane the underlying PSD binds very little, if any, Con A conjugate.