A synaptic membrane glycine-, glutamate- and thienylcyclohexylpiperidine-binding protein: Isolation and immunochemical characterization

Antibodies raised against a 43 kDa component of a complex of synaptic membrane proteins with ligand binding sites characteristic of glutamate/N-methyl-D-aspartate (NMDA) receptors, were used previously to clone a cDNA For a glycine-, glutamate-, and thienylcyclohexylpirperidine (TCP)-binding protein, pGlyBP (Kumar ei al., Biochem. Biophys. Res. Commun. 216, 390-398, 1995). In the present studies, the antibodies were shown to label a 60 kDa protein, in synaptic membranes, that was relatively hydrophilic as demonstrated by its predominant separation in the detergent-depleted phase of proteins solubilized with Triton X-114. A 55-60 kDa protein was purified From rat brain synaptic membranes by chromatographic separation through matrices derivatized with 5,7-di-chlorokynurenic acid (5,7-DCK) followed by chromatography on a matrix derivatized with 8-hydroxyquinoline (8-OHQ). The isolated fractions were highly enriched in strychnine-insensitive [H-3]glycine; NMDA- and glutamate-sensitive L-[H-3]glutamate, and MK-801-sensitive [H-3]TCP binding sites. The purified protein bound [H-3]glycine with a stoichiometry of 1.1-1.2 mol glycine per mol protein and exhibited both high (K-D = 280 nM) and low affinity (K-D = 30 mu M) glycine binding sites. Glycine binding was inhibited by D-serine and R-(+)-3-amino-1-hydroxypyrrolidin-2-one (R-(+)-HA-966). The K-D values for high and low affinity Sites of glycine binding as well as those for the inhibition by R-(+)-HA-966 were very similar to the K(D)s for glycine binding to the expressed pGlyBP. Both L-glutamate and glycine activated [H-3]TCP binding to the isolated proteins, but with relatively low affinity. The anti-43 kDa antibodies reacted strongly with the 55-60 kDa protein. Based on these results, it appears that the 60 kDa glycoprotein in brain synaptic membranes described in the present study is the same protein as the cloned pGlyBP. Copyright (C) 1996 Elsevier Science Ltd