On the Mechanisms of α-Amino-3-hydroxy-5-methylisoxazole-4-propionic Acid (AMPA) Receptor Binding to Glutamate and Kainate

Michael K Fenwick,Robert E Oswald

doi:10.1074/jbc.m109.086371

Abstract

The alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) subtype of ionotropic glutamate receptors mediates much of the fast excitatory neurotransmission in the central nervous system. The ability of these receptors to shape such responses appears to be due in part to dynamic processes induced by agonists in the ligand-binding domain. Previous studies employing fluorescence spectroscopy and whole cell recording suggest that agonist binding is followed by sequential transitions to one or more distinct conformational states. Here, we used hydrogen-deuterium exchange to determine the mechanisms of binding of glutamate and kainate (full and partial agonists, respectively) to a soluble ligand-binding domain of GluR2. Our results provide a structural basis for sequential state models of agonist binding and the free energy changes of the associated state-to-state transitions. For glutamate, a multi-equilibrium binding reaction was discerned involving distinct ligand docking, domain isomerization, and lobe-locking steps. In contrast, kainate binding involves a simpler dock-isomerization process in which the isomerization equilibrium is shifted dramatically toward open domain conformations. In light of increasing evidence that the stability, in addition to the extent, of domain closure is a critical component of the channel activation mechanism, the differences in domain opening and closing equilibria detected for glutamate and kainate should be useful structural measures for interpreting the markedly different current responses evoked by these agonists.

Highlights

Large extracellular domains, a membrane-spanning ion channel, and a C-terminal intracellular region [4, 7,8,9,10,11,12]
This suggests that amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor function is dependent on the extent of lobe closure caused by the agonist and on the lobe closing and opening dynamics associated with agonist binding and dissociation [19]
According to Equation 2, these deactivation rates are dependent on agonist binding affinity and application time because increases in either of these variables increase the probability that the binding cleft of S1S2 is closed and populating S*G2 or S*G3 [18]

Summary

Introduction

Large extracellular domains, a membrane-spanning ion channel, and a C-terminal intracellular region [4, 7,8,9,10,11,12]. Among the full agonists examined, those like glutamate, which are less potent and which bind to S1S2 with lower affinity, produce higher rates of channel deactivation and resensitization [18], despite inducing the same degree of cleft closure in S1S2 This suggests that AMPA receptor function is dependent on the extent of lobe closure caused by the agonist and on the lobe closing and opening dynamics associated with agonist binding and dissociation [19]. Previous studies have argued that small thermal fluctuations are not expected to lead to successful reactions but rather that separations of at least a few angstroms are required [25, 26] Based on this view, domain opening and agonist dissociation events were inferred from the detection of hydrogen exchange at key sites protected through ligand binding and lobe closure

Objectives

Results

Conclusion