Summary: A new study has decoded the gating mechanism of the brain’s most prominent kainate receptor. Investigators evaluated the GluK2/GluK5 kainate receptor heteromer, a complex composed of two GluK2 and two GluK5 subunits that functions as a glutamate-activated ion channel.
Using cryo-electron microscopy and fast patch-clamp recordings, the team upended long-held neuroscientific assumptions by proving that ligand binding at only the GluK5 subunits is sufficient to force the shared ion channel pore into a permanently open, non-desensitizing state. This structural cross-talk provides a precise blueprint to engineer targeted therapeutics for synaptic modulation.
Key Facts
- The Subunit Mystery: Ionotropic glutamate receptors (iGluRs) consist of four subunits forming a shared central ion channel pore, with each subunit containing an individual glutamate binding site. Exactly how these subunits coordinate their shapes to open and close the pore has long remained a mystery.
- The GluK5 Activation Paradox: Historically, neuroscientists assumed that only the GluK2 subunits mediated receptor activation because they are more tightly coupled to the central ion channel pore. However, fast patch-clamp measurements using the agonist 5-iodowillardiine proved that ligand binding exclusively at the two GluK5 subunits is entirely sufficient to put the receptor into a permanently open state.
- Unexpected Structural Cross-Talk: High-resolution cryo-electron microscopy unmasked the physical reason behind this paradox: ligand binding at the structurally less-favorable GluK5 subunits triggers a mechanical movement in the adjacent GluK2 subunits, forcing the common pore open.
- Evading Receptor Inactivation: The structural data confirmed that partial occupancy of the four subunits activates the receptor without triggering desensitization. The extensive restructuring responsible for shutting the channel down (desensitization) is only initiated when all four subunits are fully occupied by ligands.
- The GluK5-GluK5 Interaction Site: The team discovered a close physical interaction between opposing GluK5 subunits, a unique structural signature completely absent in other kainate or related AMPA receptor complexes.
- The Ten-Fold Deactivation Delay: This unique interaction site directly causes the unusually slow deactivation speed of the GluK2/GluK5 receptor, making its closure roughly ten times slower than competing kainate receptors.
- Physiological Implications & Drug Targets: As the most abundant kainate receptor in the human brain, GluK2/GluK5 primarily exerts a modulatory influence on synapses. Because GluK2 and GluK5 subunits possess different affinities for glutamate, partial occupancy can generate long-lasting, non-desensitizing electrical currents, making this complex a high-priority target for future neurological drugs.
Source: RUB
The structure of ionotropic glutamate receptors (iGluRs), which function as glutamate-activated ion channels in the membrane of neurons, has been known for many years. All iGluRs consist of four subunits that form a shared ion channel pore.
Each subunit has a glutamate binding site. However, it remains largely unknown how glutamate binding affects individual subunits and how the subunits act together to cause opening and closing of their common pore.
The research team investigated this mechanism for a special glutamate receptor complex, the so-called GluK2/GluK5 kainate receptor heteromer, which consists of two GluK2 and two GluK5 subunits.
One initial observation was that ligand binding at just the two GluK5 subunits is sufficient to cause receptor activation. Using fast patch-clamp measurements, Laura Moreno Wasielewski, one of the study’s first authors, was able to show that 5-iodowillardiine, an agonist that only binds at the two GluK5 subunits, puts the receptors into a permanently open state.
“This is remarkable,” explains Laura Moreno Wasielewski, “since it had been assumed that only the GluK2 subunits may mediate activation, as they are more closely coupled to the ion channel pore.”
Structural biology studies show details
Cryo-electron microscopy studies conducted in the laboratory of Professor Joshua Levitz in the United States provided further insights into the gating mechanism of this receptor complex. The structures revealed that ligand binding at the GluK5 subunits causes a movement of the adjacent GluK2 subunits.
“This was unexpected; however, it explains why the GluK5 subunits are able to open the channel pore, although they are structurally less favorably positioned to do so,” summarizes Andreas Reiner.
The structures also confirmed that partial occupancy of the four subunits, which is sufficient to cause receptor activation, does not yet elicit the extensive restructuring which is responsible for the subsequent inactivation (desensitization) of the receptors. The latter is only observed when all four subunits are occupied.
The structures revealed also another surprising detail: A close interaction between the opposing GluK5 subunits was observed, which is a unique feature not being seen in other kainate or related AMPA receptor complexes. In accompanying patch-clamp measurements the researchers found that this interaction also plays an important functional role.
“This interaction site appears to affect the unusually slow deactivation that is seen for GluK2/GluK5 receptors, which is around ten times slower than in other kainate receptors,” Laura Moreno Wasielewski summarizes her findings.
Function in the nervous system
How the receptor’s unusual properties contribute to neuronal function remains to be investigated. The GluK2/GluK5 receptor complex is known to primarily exert a modulatory influence on synapses. This may make the receptor also an interesting target for therapeutic purposes, especially since it appears to be the most common kainate receptor in the human brain.
Since GluK2 and GluK5 subunits have different affinities for glutamate, the partially occupied states that were investigated in this study could be of actual physiological significance, as they could cause long-lasting, non-desensitizing currents, which are rather unusual.
“So far, it is also unclear to which extent the slow deactivation of this receptor heteromer contributes to synaptic signals. The GluK5-GluK5 interactions we have identified here, now give us the possibility to address this experimentally,” explains Andreas Reiner.
The obtained structural information could also enable the future development of specific drugs that are tailored to this particular receptor.
Key Questions Answered:
A: Because they seemed poorly positioned to do the job. Scientists historically assumed that only GluK2 subunits could open the channel because they are much more closely coupled to the central pore. The study proved that when a ligand binds to GluK5, it mechanically pushes the neighboring GluK2 subunits to open the gate anyway.
A: It depends on how many slots are filled. The receptor only undergoes the massive structural collapse that shuts it down (desensitization) when all four slots are full. Because GluK2 and GluK5 have different affinities for glutamate, partial filling leaves the channel permanently open, creating unusually long-lasting currents.
A: By targeting a unique structural feature. The researchers found a specific physical interaction between opposing GluK5 subunits that exists nowhere else in related receptor families. This unique interaction site slows channel closing ten-fold, giving chemists a precise target to design highly specific, tailored drugs.
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) are critical mediators and modulators of synaptic transmission which undergo rapid activation and desensitization upon binding of the neurotransmitter glutamate.
Under various physiological and pharmacological conditions agonist binding likely occurs to only a subset of subunits within these tetrameric receptors, motivating an analysis of the functional and conformational effects of partial versus complete ligand occupancy.
Here we report cryo-EM structures of the GluK2/GluK5 hetero-tetramer under partially-occupied conditions using 5-iodowillardiine and AMPA as GluK5-selective agonists. High-resolution pre-active state structures containing closed/open ligand binding domain (LBD) dimers with intact interfaces reveal gating-associated interface reshaping, inter-dimer motions, and pore-linker repositioning in response to asymmetric agonist binding.
Interfacial LBD mutations to a central cluster formed by the GluK5 subunits and to an inter-dimer interface between GluK2 and GluK5 subunits, highlight the roles of interactions between LBD dimers in controlling receptor function, including the distinct slow deactivation of GluK5-containing receptors.
Finally, the absence or presence of intact, partially, and fully ruptured LBD interfaces under different ligand conditions allows us to propose a revised model of stepwise ionotropic glutamate receptor activation and desensitization.
