Lectin-induced inhibition of desensitization of the kainate receptor GluR6depends on the activation state and can be mediated by a single native or ectopic N-linked carbohydrate side chain

The ionotropic glutamate receptor GluR6 exhibits strongly and rapidly desen sitizing current responses. Treatment of heterologically expressed GluR6 wi th the lectin concanavalin A (ConA) in Xenopus oocytes as well as in human embryonic kidney-293 cells results in a considerable increase of the steady -state current, presumably by inhibiting receptor desensitization. In the p resent study, we investigated the molecular basis of this effect using a sy stematic mutagenesis approach. We found that although N-glycosylation is an absolute prerequisite for the tectin-mediated inhibition of desensitizatio n, no single one of the nine extracellular consensus sites for N-glycosylat ion of GluR6 is required. Rather, each of the nine N-linked carbohydrate si de chains is independently capable of modulatory interaction with the lecti n. Moreover, even artificially introduced N-glycosylation sites can substit ute for native sites. Thus, the specific site of the lectin binding does no t appear to be important for its desensitization-inhibiting action. Further more, we show that the extent of the receptor's ConA sensitivity depends on its state of activation, because the desensitized GluR6 exhibits significa ntly lower lectin sensitivity than the nondesensitized receptor. We conclud e that binding of ConA "locks" the receptor in the activatable state, there by inhibiting conformational changes required to shift the receptor to the desensitized state.