Summary: A new study has revealed how the brain’s predominant kainate receptor, the GluK2/GluK5 heteromer, opens and remains active. Combining cryo-electron microscopy with rapid patch-clamp electrophysiology, researchers show that ligand binding at just the GluK5 subunits is sufficient to lock the shared ion channel pore into a persistently open, non-desensitizing state. This discovery clarifies long-standing questions about subunit interplay in ionotropic glutamate receptors and identifies structural features that could be exploited for targeted synaptic therapies.
Key Facts
- Four-subunit architecture: Ionotropic glutamate receptors (iGluRs) are tetrameric ion channels. Each of the four subunits contributes a glutamate binding site and together they form a single central ion pore.
- The activation puzzle: It was previously assumed that GluK2 subunits—because of their closer coupling to the central pore—are the primary drivers of channel opening. This study demonstrates that ligand binding at the two GluK5 subunits alone can activate the channel and maintain it in an open state.
- Structural cross-talk: High-resolution cryo-EM structures reveal that when the GluK5 ligand binding domains (LBDs) are occupied, they mechanically displace adjacent GluK2 subunits. This inter-subunit movement transmits force to the pore-forming region and opens the channel despite GluK5’s less favorable positioning.
- Partial occupancy avoids desensitization: Functional data show that partial ligand occupancy—binding to a subset rather than all four subunits—activates the receptor without triggering the large-scale rearrangement that causes desensitization. Full occupancy of all four binding sites is required to induce the desensitized, collapsed state.
- Unique GluK5-GluK5 interface: The authors identified a previously unseen close interaction between opposing GluK5 subunits. This contact is absent in other kainate or AMPA receptor assemblies and contributes to distinct kinetic properties.
- Slow deactivation: The GluK5-GluK5 interaction correlates with an unusually slow deactivation of GluK2/GluK5 receptors—approximately ten times slower than many other kainate receptors—prolonging synaptic currents.
- Physiological and therapeutic relevance: As one of the most abundant kainate receptor types in the human brain, GluK2/GluK5 plays modulatory roles at synapses. Differences in glutamate affinity between GluK2 and GluK5 make partial-occupancy states physiologically plausible, generating long-lasting, non-desensitizing currents and making this complex an attractive target for drug development aimed at synaptic modulation.
Source: RUB
Although the basic structure of ionotropic glutamate receptors has been established for years, the precise mechanism by which individual subunits communicate to open and close the shared ion pore remained unclear. Each subunit contains its own ligand binding site, but how asymmetric ligand occupancy influences global channel gating has been difficult to resolve until now.
The research focused on the GluK2/GluK5 kainate receptor heteromer, composed of two GluK2 and two GluK5 subunits. Electrophysiological recordings using the GluK5-selective agonist 5-iodowillardiine showed that occupancy of only the GluK5 sites was sufficient to produce a stable, non-desensitizing open state in these receptors.
Cryo-electron microscopy conducted in Professor Joshua Levitz’s laboratory resolved the structural basis of this response. Structures captured under partially occupied conditions show LBD rearrangements and inter-dimer motions that propagate to pore-linking elements. These asymmetric conformational changes explain how GluK5 binding can drive pore opening by mechanically influencing neighboring GluK2 subunits.
Importantly, the cryo-EM models demonstrate that the collapse associated with desensitization—an extensive rearrangement of LBD interfaces—occurs only when all four ligand sites are filled. In partially occupied states, interface contacts remain intact, supporting a stable pre-active conformation and sustained ion flow.
The identification of a discrete GluK5-GluK5 interaction site further clarifies why these heteromers deactivate slowly. Mutational and functional studies in the paper link this interface to prolonged current decay, suggesting a direct structural determinant of the receptor’s prolonged signaling profile.
Key Questions Answered:
A: GluK5 subunits are not as closely positioned to the central pore as GluK2 subunits, so they were not expected to drive channel opening. The study shows that GluK5 ligand binding mechanically displaces adjacent GluK2 proteins, which in turn open the pore.
A: Desensitization requires full occupancy of all four binding sites. If only some sites—such as the two GluK5 sites—are occupied, the receptor can remain active without undergoing the structural collapse that causes inactivation. This leads to prolonged currents that could significantly affect synaptic signaling and plasticity.
A: The researchers identified a unique GluK5-GluK5 interface that slows deactivation. That distinct structural feature provides a precise target for designing selective ligands or modulators that tune receptor kinetics and synaptic effects without broadly affecting other iGluR families.
Editorial Notes:
- This article was edited by a Neuroscience News editor.
- Journal paper reviewed in full.
- Additional context added by our staff.
About this neuroscience research news
Author: Meike Driessen
Source: RUB
Contact: Meike Driessen – RUB
Image: The image is credited to Neuroscience News
Original Research: Open access. “Structures of Partially Occupied Hetero-Tetramers Provide Insight Into Kainate Receptor Activation and Desensitizations” by Nandish K. Khanra, Alexa Strauss, Laura Moreno Wasielewski, Sophie Lenze, Joel Meyerson, Andreas Reiner & Joshua Levitz. Nature Communications. DOI: 10.1038/s41467-026-72226-w
Abstract
Structures of Partially Occupied Hetero-Tetramers Provide Insight Into Kainate Receptor Activation and Desensitization
Kainate receptors (KARs) mediate and modulate synaptic transmission by rapidly activating and desensitizing in response to glutamate. Under physiological and pharmacological conditions, agonist binding may occur on only a subset of subunits within these tetramers. To decipher how partial versus full occupancy shapes function and conformation, the authors report cryo-EM structures of the GluK2/GluK5 hetero-tetramer captured with GluK5-selective agonists (including 5-iodowillardiine and AMPA). High-resolution pre-active structures featuring closed/open LBD dimers with intact interfaces reveal gating-associated reshaping of interfaces, inter-dimer movements, and pore-linker repositioning driven by asymmetric agonist binding. Mutations at LBD interfaces—especially within the central GluK5 cluster and the GluK2–GluK5 inter-dimer interface—highlight the role of inter-dimer contacts in controlling receptor behavior, including the distinct slow deactivation of GluK5-containing receptors. The presence, absence, or progressive rupture of LBD interfaces under varying ligand conditions supports a revised stepwise model of iGluR activation and desensitization.