PLASTICITY OF AGONIST BINDING-SITES IN HETEROOLIGOMERS OF THE UNITARYGLUTAMATE-RECEPTOR SUBUNIT XENU1

Two subunits from Xenopus, XenNR1G and the ''short'' subunit XenU1, ha ve previously been coexpressed to form a unitary (NMDA/non-NMDA type) glutamate receptor. We now show that an antibody to XenNR1G or an anti body to XenU1 precipitates the binding sites of both XenNR1G and XenU1 , with the recombinant subunits or with solubilised Xenopus brain memb ranes, i.e., the combination occurs in vivo. The expressed XenU1 subun its are in the cell membrane and oriented correctly. XenU1 binds not o nly kainate with high affinity (K-D 1.2 nM at 25 degrees C), but also the glycine site antagonist 5,7-dichlorokynurenic acid (DCKA). DCKA GT P, or GTP gamma S displaces competitively all of the bound [H-3]kainat e, but glycine has no effect. The results suggest that a common bindin g site for kainate, DCKA, and GTP can exist on XenU1, In the XenNR1G/X enU1 complex, the kainate affinity is lowered eightfold, whereas the D CKA affinity is considerably increased (K-D 147 nM). Only 18% of the b inding to the complex has the properties of the NMDA receptor glycine site, the rest being due to switching of the high-affinity kainate sit e of XenU1 (low-affinity DCKA) to a high-affinity DCKA (low-affinity k ainate) conformation. Surprisingly, a mammalian NR2 subunit can also c ombine with XenU1, and this introduces similar reciprocal changes in t he binding of kainate and DCKA, The combined evidence suggests a commo n basic mode of agonist site formation in different subunit types of t he ionotropic glutamate receptors.