Kainate receptors are ligand-gated ion channels, which belong to the family of ionotropic glutamate receptors (iGluRs), and are important modulators of synaptic transmission. Glutamate binding results in rapid channel activation and desensitization, but less is known about cases in which ligand binding occurs at only a subset of the four receptor subunits. This phenomenon is particularly relevant for heteromeric iGluRs that combine subunits with different glutamate affinities or when subunit-selective pharmacology is used. To further investigate how partial ligand occupancies drive gating, we focused on GluK2/GluK5 heteromers, which are one of the prevalent kainate receptor complexes in the central nervous system. New cryo-EM studies in the presence of two GluK5-selective agonists (5-iodowillardiine and AMPA) reveal different structural states, including a high-resolution structure of a partially occupied, pre-active state, which shows closed/open ligand binding domain (LBD) dimers as well as gating-associated interface changes and pore-linker repositioning. Depending on the conditions, other particles show partially and fully ruptured LBDs, which leads us to propose a modified model of stepwise iGluR activation and desensitization. Functional studies (fast-perfusion patch clamp measurements) further show that LBD mutations to a previously unrecognized “central cluster,” which is exclusively formed by the inter-dimer GluK5 subunits, have a strong impact on channel gating, including the uniquely slow deactivation kinetics seen in GluK5-containing receptors. Further work addresses the gating contributions of the asymmetric LBD-pore linkers, which differ between the GluK5 subunits in the pore-proximal positions and GluK2 subunits in the pore-distal positions. Overall, these data provide new insights into how the four iGluR subunits control gating of their common channel pore.
Reiner et al. (Sun,) studied this question.