Auxiliary subunits keep AMPA receptors compact during activation and desensitization.

Jelena Baranovic,Andrew Jr Plested

doi:10.7554/elife.40548

Abstract

Signal transduction at vertebrate excitatory synapses involves the rapid activation of AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazole propionate) receptors, glutamate-gated ion channels whose four subunits assemble as a dimer-of-dimers. Technical advances in cryo-electron microscopy brought a slew of full-length structures of AMPA receptors, on their own and in combination with auxiliary subunits. These structures indicate that dimers might undergo substantial lateral motions during gating, opening up the extracellular layer along the central twofold symmetry axis. We used bifunctional methanethiosulfonate cross-linkers to calibrate the conformations found in functional AMPA receptors in the presence and absence of the auxiliary subunit Stargazin. Our data indicate that extracellular layer of AMPA receptors can get trapped in stable, opened-up conformations, especially upon long exposures to glutamate. In contrast, Stargazin limits this conformational flexibility. Thus, under synaptic conditions, where brief glutamate exposures and the presence of auxiliary proteins dominate, extracellular domains of AMPA receptors likely stay compact during gating.

Highlights

AMPA-type glutamate receptors are found at excitatory synapses throughout the mammalian brain, where they convert glutamate release into membrane depolarization
The results showed that AMPA receptors can undergo large structural changes but these movements require time and are much reduced by partner proteins
The rupture of the LBD intra-dimer interface is a structural hallmark of AMPA receptor desensitization, as shown by biophysical studies based on the structures of isolated ligand binding domains (Sun et al, 2002; Armstrong et al, 2006)

Summary

Introduction

AMPA-type glutamate receptors are found at excitatory synapses throughout the mammalian brain, where they convert glutamate release into membrane depolarization. Previous studies have shown that an AMPA receptor can go through dramatic changes in its structure, with the different subunits being able to spread apart widely These experiments had to be conducted when the proteins were isolated from membranes and held in a cocktail of activating or deactivating molecules for hours. Auxiliary subunits restrict the conformational ensemble, maintaining more compact arrangements of the LBD layer, without much separation of the two LBD dimers This kinetic classification suggests AMPA receptors at synapses probably have similar, compact geometries regardless of their instantaneous gating state

Results

Discussion

Materials and methods

Funding Funder