Different structural requirements for functional ion pore transplantation suggest different gating mechanisms of NMDA and kainate receptors

Abstract

Although considerable progress has been made in characterizing the physiological function of the high‐affinity kainate (KA) receptor subunits KA1 and KA2, no homomeric ion channel function has been shown. An ion channel transplantation approach was employed in this study to directly test if homomerically expressed KA1 and KA2 pore domains are capable of conducting currents. Transplantation of the ion pore of KA1 or KA2 into GluR6 generated perfectly functional ion channels that allowed characterization of those electrophysiological and pharmacological properties that are determined exclusively by the ion pore of KA1 or KA2. This demonstrates for the first time that KA1 and KA2 ion pore domains are intrinsically capable of conducting ions even in homomeric pore assemblies. NMDA receptors, similar to KA1‐ or KA2‐containing receptors, function only as heteromeric complexes. They are composed of NR1 and NR2 subunits, which both are non‐functional when expressed homomerically. In contrast to NR1, the homomeric NR2B ion pore failed to translate ligand binding into pore opening when transplanted into GluR6. Similarly, heteromeric coexpression of the ion channel domains of both NR1 and NR2 inserted into GluR6 failed to produce functional channels. Therefore, we conclude that the mechanism underlying the ion channel opening in the obligatorily heterotetrameric NMDA receptors differs significantly from that in the facultatively heterotetrameric α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazole propionate and KA receptors.

Assembly Stoichiometry of the GluK2/GluK5 Kainate Receptor Complex

“…These “released” GluK5 receptors are non-functional, but it is not known why. Transplanting the pore sequence of GluK5 into the background of GluK2 gives small currents, suggesting that GluK5 itself could be permeation competent (Villmann et al, 2008). Furthermore, it is known that GluK5 subunits harbor a functional ligand binding domain, with high affinity for both kainate and glutamate (Barberis et al, 2008; Herb et al, 1992).…”

The Importance of TM3-4 Loop Subdomains for Functional Reconstitution of Glycine Receptors by Independent Domains

“…The strategy to reassemble functional receptor proteins from non-functional modules takes advantage of the domain architecture of ion channels. Domain swapping experiments between different ion channel subunits have been shown to preserve receptor function in various receptor families, such as the glycine receptor with the prokaryotic GLIC or different glutamate receptors (24,25). For GlyR, nonsense and frameshift mutations have been described in patients suffering from hyperekplexia, a rare neuromotor disorder (20,26).…”

Glycine receptor mutants of the mouse: what are possible routes of inhibitory compensation?

“…In addition, the pore domain represents high homology to the ion channel domain of K + channels (Villmann and Becker, 2007). Several studies on domain swapping have shown that homologous domains can be exchanged between different receptor subclasses without dramatic changes in ion channel properties (Strutz et al, 2002; Hoffmann et al, 2006; Villmann et al, 2008). Reconstitution of functionality from independent folding domains seemed to be a chance to gain-of-function for truncated receptor proteins.…”