Allosteric Regulation and Spatial Distribution of Kainate Receptors Bound to Ancillary Proteins

A diverse range of accessory proteins regulates the behaviour of most ligand‐ and voltage‐gated ion channels. For glutamate receptor 6 (GluR6) kainate receptors, two unrelated proteins, concanavalin‐A (Con‐A) and postsynaptic density protein 95 (PSD‐95), bind to extra‐ and intracellular domains, respectively, but are reported to exert similar effects on GluR6 desensitization behaviour. We have tested the hypothesis that distinct allosteric binding sites control GluR6 receptors via a common transduction pathway. Rapid agonist application to excised patches revealed that neither Con‐A nor PSD‐95 affect the onset of desensitization. The rate of desensitization elicited by 10 m m l ‐glutamate was similar in control (τ fast = 5.5 ± 0.4 ms), Con‐A‐treated patches (τ fast = 6.1 ± 0.5 ms) and patches containing PSD‐95 and GluR6 receptors (τ fast = 4.7 ± 0.6 ms). Likewise, the time course of recovery from GluR6 desensitization was similar in both control and Con‐A conditions, whereas PSD‐95 accelerated recovery almost twofold. Peak and steady‐state (SS) dose‐response relationships to glutamate were unchanged by lectin treatment (e.g. control, EC 50(SS) = 31 ± 28 μ m vs Con‐A, EC 50(SS) = 45 ± 9 μ m , n = 6 ), suggesting that Con‐A does not convert non‐conducting channels with high agonist affinity into an open conformation. Instead, we demonstrate that the effects of Con‐A on macroscopic responses reflect a shift in the relative contribution of different open states of the channel. In contrast, the effect of PSD‐95 on recovery behaviour suggests that the association between kainate receptors and cytoskeletal proteins regulates signalling at glutamatergic synapses. Our results show that Con‐A and PSD‐95 regulate kainate receptors via distinct allosteric mechanisms targeting selective molecular steps in the transduction pathway.