What does it take to gate AMPA receptors?

In vivo, agonist binding to the open conformation of the ligand-binding domain initiates the process of gating in ionotropic glutamate receptors. Arguably, an alternative manner to gate the receptors exists, which requires a point mutation in the most-conserved sequence motif in the second transmembrane domain. Originally, this mutation occurred spontaneously in the orphan glutamate receptor subunit delta2, causing the ataxic phenotype of lurcher mice.(1) In the absence of a ligand that could initiate gating at this orphan subunit, the introduction of the lurcher mutation led to spontaneous currents through delta2-lurcher channels.(1) Introduction of the corresponding mutation into the AMPA receptor GluR1 induced a number of aberrant gating properties.(2-5) Among those, glutamate potency was highly increased, and competitive antagonists suddenly behaved as partial agonists.(2,5) We reported that the introduction of delta2 amino acids in the domain preceding the first transmembrane domain in GluR1 resulted in a mutant receptor that displayed all characteristics of lurcher-typical gating. We proposed that lurcher-like mutations work to enhance gating by destabilizing the closed state of the receptor. As a result, no or minimal conformational changes in the ligand-binding domain are sufficient for gating, explaining, respectively, why spontaneous currents occur and competitive antagonists act as partial agonists in lurcher-like channels. Strikingly, a similar conversion of antagonists upon coexpression of glutamate receptors with TARPs has recently been reported.(6,7) We take this as indication that the actual mechanism of action might be very similar, and that both lurcher-like mutations and TARPs work as 'gating enhancers'.