Synaptic plasma membrane glycoproteins: molecular identification of lectin receptors

Synaptic plasma membranes isolated from rat cerebral cortex reacted with concanavalin (con A), wheat germ agglutinin (WGA), Lens culinaris phytohaemagglutinin (LcH), and Ricinus commounus agglutinin (RCA). Competition studies indicated that specific topographical relationships exist between receptors for con A and LcH and for WGA AND LcH. Reaction of [125I]con A with synaptic membrane proteins following polyacrylamide gel electrophoresis identified eight distinct molecular weight classes of glycoproteins possessing receptor activity for con A, ranging in apparent molecular weight from 27 000 to 165 000. Each of these also reacted to varying degrees with LcH, WGA, and RCA, indicating that a diverse population of carbohydrate moieties is associated with each molecular-weight class of glycoprotein. Following gel electrophoresis, competition between lectins did not occur, suggesting that each lectin reacts with a distinct group of carbohydrates and that specific relationships between these groups are destroyed by the solubilization and electrophoretic procedure. Synaptic junctional complexes isolated by Triton-X-100 extraction of synaptic membranes exhibited a simplified glycoprotein composition with only three major molecular weight classes of glycoproteins possessing receptor activity for con A being present.