Solubilization, partial purification, and reconstitution of glutamate- and N-methyl-D-aspartate-activated cation...

L-Glutamate-activated cation channel proteins from rat brain synaptic membranes were solubilized, partially purified, and reconstituted into liposomes. Optimal conditions for solubilization and reconstitution included treatment of the membranes with nonionic detergents in the presence of neutral phospholipids plus glycerol. The affinity batch chromatography procedure described previously [Chen et al. (1988) J. Biol. Chem. 263, 417-427] was used to obtain a fraction enriched in glutamate-binding proteins. Quench-flow procedures were developed to characterize the rapid kinetics of ion flux induced by receptor agonists. [14C]Methylamine, a cation that permeates through the open channel of both vertebrate and invertebrate glutamate receptors, was used to measure the activity of glutamate receptor-ion channel complexes in reconstituted liposomes. L-Glutamate caused an increase in the rate of [14C]methylamine influx into liposomes reconstituted with either solubilized membrane proteins or partially purified glutamate-binding proteins. The increase in methylamine influx was dependent on the concentration of L-glutamic acid with an estimated Kact for L-glutamate equal to 0.2 microM for synaptic membrane proteins and 0.32 microM for purified proteins. Of the major glutamate receptor agonists, only N-methyl-D-aspartate activated cation fluxes in liposomes reconstituted with glutamate-binding proteins. Glutamate-activated methylamine flux was completely inhibited by the N-methyl-D-aspartate receptor antagonist 2-amino-5-phosphonopentanoic acid. In liposomes reconstituted with glutamate-binding proteins, N-methyl-D-aspartate- or glutamate-induced influx of Na+ led to a transient increase in the influx of the lipid-permeable anion probe S14CN-. Electrophoretic analysis of partially purified proteins reconstituted in liposomes indicated enrichment of several bands, the most prominent being those of molecular size equal to approximately 69, 60, 35, and 25 kDa. Antibodies raised against the purified 71- and 63-kDa glutamate-binding proteins reacted strongly with the approximately 69-kDa band of reconstituted proteins and markedly decreased the initial rate of glutamate-activated cation flux. These results indicate the functional reconstitution of N-methyl-D-aspartate-sensitive glutamate receptors and the role of the approximately 69-kDa protein in the function of these ion channels.