Distinct functional roles of subunits within the heteromeric kainate receptor

Kainate receptors (KARs) have been implicated in a number of neurological disorders, including epilepsy. KARs are tetrameric, composed of a combination of GluK1-GluK5 subunits. We examined the contribution of GluK2 and GluK5 subunits to activation and desensitization of the heteromeric receptor. Heteromeric GluK2/K5 receptors expressed in HEK-293T cells showed markedly higher glutamate sensitivity than GluK2 homomers and did not desensitize at low glutamate concentrations. Mutation of residue E738 in GluK2 substantially lowered its glutamate sensitivity. However, heteromeric KARs containing this mutant GluK2 [GluK2(E738D)] assembled with wild-type GluK5 showed no change in glutamate EC(50) compared with wild-type heteromeric KARs. Instead, higher concentrations of glutamate were required to produce desensitization. This suggested that, within the heteromeric receptor, glutamate binding to the high-affinity GluK5 subunit alone was sufficient for channel activation but not desensitization, whereas agonist binding to the low-affinity GluK2 subunit was not necessary to open the channel but instead caused the channel to enter a closed, desensitized state. To test this hypothesis in wild-type receptors, we used the competitive antagonist kynurenate, which has higher affinity for the GluK2 than the GluK5 subunit. Coapplication of kynurenate with glutamate to heteromeric receptors reduced the onset of desensitization without affecting the peak current response, consistent with our hypothesis. Our results suggest that GluK2 and GluK5 subunits can be individually activated within the heteromeric receptor and that these subunits serve dramatically different functional roles.